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Functional analysis of selected hox homeobox genes in hematopoiesis Thorsteinsdottir, Unnur 1997

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F U N C T I O N A L A N A L Y S I S O F S E L E C T E D H O X H O M E O B O X G E N E S IN H E M A T O P O I E S I S by UNNUR THORSTEINSDOTTIR B.Sc, University of Iceland, 1984 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES Genetics Programme We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA October 3, 1997 © Unnur Thorsteinsdottir, 1997 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. 1 further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department The University of British Columbia Vancouver, Canada DE-6 (2/88) ABSTRACT Hematopoiesis is an ordered process of differentiation and proliferation, leading to the generation of mature blood cells of multiple distinctive lineages from totipotent hematopoietic stem cells. Regulation of this impressive machinery occurs at multiple levels, involving both extrinsic and intrinsic regulators. The members of the Hox homeobox gene family of transcription factors, known for their roles in determination of cell identity and pattern formation in a variety of structures during embryogenesis, have recently come under scrutiny as important regulators of hematopoiesis. In normal hematopoietic cells Hox genes have been shown to be expressed at levels that vary both as a function of the stage of differentiation of the cells, and of the specific Hox gene examined. Thus, some genes like HOXB3 and HOXB4 are preferentially expressed in the most primitive fraction of hematopoietic cells, and others like HOXA10 and HOXB9 show a broader range of expression with downregulation at later stages of hematopoietic differentiation. Furthermore, our group has recently shown that retroviral overexpression of HOXB4 in murine hematopoietic cells causes selective expansion of primitive hematopoietic cells, most profoundly the totipotent hematopoietic stem cell (HSC), without altering either myeloid or lymphoid differentiation or predisposing to leukemia. Taken together, these results strongly implicate Hox genes in the regulation of early hematopoietic cells. The major objective of this thesis was to investigate further the possible roles of Hox genes in the regulation of hematopoietic cell growth and differentiation, and to determine whether these roles are Hox gene-specific. The strategy taken was to independently engineer the overexpression of selected Hox genes in murine bone marrow cells. Two Hox genes were chosen, HOXB3 and HOXA10, based on their divergent expression patterns in hematopoietic cells. The subsequent effects of these manipulations on the proliferation and differentiation of various populations of myeloid and lymphoid cells were then analyzed in a transplantation model and various in vitro cultures. Overexpression of either of these two Hox genes was found to impact on ii both the processes of proliferation and differentiation of hematopoietic cells, involving multiple lineages. Specifically, overexpression of HOXA10 led to enhanced formation of megakaryocytic progenitors in vivo and in vitro, diminished numbers of macrophage and B lymphoid progenitors, and the generation of myeloid leukemias in a significant proportion of mice 5 to 8 months after transplantation. In contrast, overexpression of HOXB3 led to an almost complete block in the thymic production of CD4+CD8+ T lymphocytes accompanied by an expansion of yS-TCR + thymocytes, impaired B lymphoid development, and enhanced myelopoiesis leading to a myeloproliferative disorder. The hematopoietic perturbations generated by overexpression of HOXB3and HOXAWwere thus strikingly different from each other and also from those previously reported for the overexpression of HOXB4, which did not detectably perturb myeloid, B or T cell differentiation but did induce expansion of myeloid and lymphoid progenitor cells, and enhanced up to 47-fold the regeneration of the HSC. This thesis work was also aimed at delineating further the effects of HOXB4 overexpression on the regenerative potential of HSC. For that purpose, the size and the clonal composition of the regenerated pool of HSCs in mice transplanted with bone marrow cells overexpressing HOXB4 was analyzed at various time points (16 to 52 weeks) after transplantation. These studies, in addition to confirming our initial observation that HOXB4 overexpression enhances the regeneration of HSC following transplantation, show that this effect is long-lasting, and that the expansion of HSCs appears both to be controlled and to involve multiple HSCs. Taken together, these results presented in this thesis add to the recognition of Hox genes as important regulators of hematopoiesis. These results suggest a role for Hox genes in the regulation of proliferation of the most primitive hematopoietic cells, and in lineage-commitments and proliferation of myeloid and lymphoid progenitor cells. Furthermore, these results also point to Hox gene-specific roles in the regulation of hematopoiesis. iii TABLE OF CONTENTS T I T L E A B S T R A C T ii T A B L E O F C O N T E N T S iv L I S T O F T A B L E S A N D F I G U R E S viii A B B R E V I A T I O N S xi A C K N O W L E D G M E N T S xiv C H A P T E R 1. I n t r o d u c t i o n 1.1 Overview 1 1.2 Organization and regulation of the hematopoietic system 2 1.2.1 Developmental and hierarchal organization of hematopoiesis 2 1.2.2 Definition and properties of early hematopoietic cell types 5 1.2.2.1 The totipotent hematopoietic stem cell (HSC) 5 1.2.2.2 Properties of the HSC 10 1.2.2.3 Myeloid and erythroid progenitor cells 13 1.2.2.4 Lymphoid progenitor cells 15 1.2.3 Regulation of hematopoiesis 17 1.2.3.1 Regulation of hematopoiesis by external factors 18 1.2.3.2 Regulation of hematopoiesis by intracellular factors 20 1.3 Homeodomain-containing proteins 26 1.3.1 Classification, chromosomal organization and evolution of the homeobox genes 26 1.3.2 Important conserved properties of Hox genes 30 1.3.3 Hox genes and embryonic development 32 1.3.4 The structure and functional specificity of Hox proteins 34 1.3.5 Regulation of Hox gene expression 38 1.3.6 Target genes regulated by Hox proteins 41 1.4 Hox genes and hematopoiesis 43 1.4.1 Hox gene expression in human and murine hematopoietic cell lines and human leukemias 43 1.4.2 Hox gene expression in normal hematopoietic cells 44 1.4.3 Hox gene functions in normal hematopoietic cells 48 1.5 Thesis Objectives 52 C H A P T E R 2. M a t e r i a l s a n d m e t h o d s 2.1 Generation of retroviruses and viral assays...; 54 2.1.1 Recombinant retroviral vectors 54 2.1.2 Generation of viral producer cells 54 2.1.3 Viral titering and helper virus assays * 55 2.2 Hematopoietic cell cultures and assays 56 2.2.1 Mice 56 2.2.2 Viral infection of murine bone marrow cells 56 2.2.3 Transplantation of retrovirally transduced bone marrow 57 2.2.4 In vitro clonogenic progenitor assays 57 2.2.5 Day 12 CFU-S assay 58 2.2.6 Competitive repopulating unit (CRU) assay 58 2.2.7 Culturing of Sca+Lin-WGA+ cells 59 2.2.8 Morphological evaluations of bone marrow, spleen and peripheral blood cells from transplanted mice 59 2.3 Antibodies, flow cytometry and cell sorting 60 2.4 Molecular analysis 62 2.4.1 Southern blot analysis 62 2.4.2 Northern blot analysis 63 2.4.3 Western blot analysis 64 2.4.4 cDNA generation, amplification and analysis 64 2.5 Statistical Methods 65 C H A P T E R 3 . O v e r e x p r e s s i o n o f H O X A 1 0 i n m u r i n e h e m a t o p o i e t i c c e l l s p e r t u r b s b o t h m y e l o i d a n d l y m p h o i d d i f f e r e n t i a t i o n a n d l e a d s t o a c u t e m y e l o i d l e u k e m i a 3.1 Introduction 67 3.2 Results 68 3.2.1 Retroviral-mediated transduction of HOXA10 to murine bone marrow cells 68 3.2.2 Altered colony formation in vitro of myeloid progenitor cells overexpressing HOXA10 69 3.2.3 HOXA10 overexpression increases the maintenance of day 12 CFU-S in vitro 71 3.2.4 Expansion in vivo of myeloid progenitor cells overexpressing HOXA10 and their altered colony formation in vitro 72 3.2.5 Overexpression of HOXA10 in vivo is not permissive for pre-B lymphoid colony formation 79 3.2.6 Acute myeloid leukemia arises in recipients of HOXA10-transduced bone marrow cells 81 3.3 Discussion 86 C H A P T E R 4. O v e r e x p r e s s i o n o f H O X B 3 i n h e m o p o i e t i c c e l l s c a u s e s d e f e c t i v e l y m p h o i d d e v e l o p m e n t a n d p r o g r e s s i v e m y e l o p r o l i f e r a t i o n 4.1 Introduction 92 4.2 Results 93 4.2.1 cDNA cloning of a HOXB3 transcript from hemopoietic cells and generation of a HOXB3 retroviral vector 93 4.2.2 Hemopoietic reconstitution of recipients of HOXB3-transduced bone marrow cells 93 4.2.3 Recipients of HOXB3-transduced cells show increased granulo-poiesis intheir bone marrow and spleen 97 4.2.4 Recipients of HOXB3-transcuded cells have defective T cell development 101 4.2.5 Altered B cell development by overexpression of HOXB3 105 4.3 Discussion 107 C H A P T E R 5. E n h a n c e d p o l y c l o n a l r e g e n e r a t i o n o f h e m a t o p o i e t i c s t e m c e l l s o v e r e x p r e s i n g H O X B 4 f o l l o w i n g b o n e m a r r o w t r a n s p l a n t a t i o n 5.1 Introduction 112 5.2 Results 114 5.2.1 Experimantal model 114 5.2.2 Enhanced regeneration of CRU cells in mice transplanted with HOXB4-transduced bone marrow cells 117 5.2.3 Polyclonal hematopoietic regeneration by HOXB4-transduced cells both at the levels of HSC and mature end cells 119 5.2.4 HOXB4-induced expansion in vivo of clonogenic progenitor cells... 123 5.2.5 Enhanced regeneration of HOXB4-transduced CRU cells is not reflected in the number of regenerated Sca1+Lin_WGA+ cells 126 5.2.6 Lack of HOXB4 effects on mature end cell output 127 5.3 Discussion 129 vi C H A P T E R 6 G e n e r a l d i s c u s s i o n a n d c o n c l u s i o n 6.1 Overexpression of Hox genes perturbs hematopoietic development at multiple stages involving multiple lineages 135 6.2 Overexpression of Hox genes: phenotype vs function 138 6.3 Hox genes and hematopoiesis 141 6.4 Future directions 143 C H A P T E R 7 R e f e r e n c e s 146 vii LIST OF TABLES AND FIGURES C h a p t e r 1 Figure 1.1 Schematic representation of the organization of the hematopoietic system 3 Figure 1.2 Simplified representation of B and T cell development 16 Figure 1.3 Schematic representation indicating the position of essential function of some of the transcription factors known to be active in hematopoiesis 23 Figure 1.4 Alignment of vertebrate Hox complexes into paralogous groups and comparison with Drosophila HOM-C 29 Figure 1.5 Schematic representaion of Hox proteins domain organization and their DNAbinding sequences 35 Figure 1.6 Expression of Hox genes in human bone marrow cells 46 C h a p t e r 3 Figure 3.1 Structure and expression of the HOXA10 retrovirus used in this study 69 Figure 3.2 Effects of HOXA10 and HOXB4 overexpression on the relative frequency of various colony types generated in vitro, immediately following retroviral infection of bone marrow cells 70 Figure 3.3 Effect of overexpression of HOXA10 on the recovery of day 12 CFU-S in vitro 72 Figure 3.4 Southern blot analysis of DNA isolated from hematopoietic tissue of HOXA10 and neo mice 74 Figure 3.5 Norhtern and Western blot analyses to demonstrate high levels of viral derived HOXA10 messages and protein in reconstituted HOXAIOmice 75 Table 3.1 Body weight and hematological parameters in neo and HOXAIOmice 76 Table 3.2 HOXA10 mice have increased numbers of myeloid clonogenic progenitor cells when compared to neo control mice 77 Table 3.3 HOXA10 mice have greatly increased numbers of bone marrow megakaryocyte/blast and blast colony forming cells (CFC) and decreased numbers of unilineage macrophage CFC 78 Table 3.4 Sca+Lin-WGA+ cells overexpressing HOXA10 have increased potential to generate megakaryocyte-containing colonies in vitro 79 viii Table 3.5 Overexpression of HOXA10 is into permissive for lymphoid pre-B colony formation 80 Figure 3.6 Survival of HOXA10 mice compared to that of neo control mice and mice transplanted with HOXB4-transduced bone marrow cells using Kaplan-Meyer curves 82 Figure 3.7 Wright-Geimsa staining of cytospin preparations of HOXA10-transduced myeloid colonies (A-C) and of hematopoietic tissues from a representative leukemic HOXA10 mouse (D-F) 83 Figure 3.8 Southern blot analysis to determine: (A) the intensity of the proviral signal in bone marrow, spleen and thymus of HOXA10 and neo mice; and (B) the clonality of the leukemias in HOXAIOmice 85 C h a p t e r 4 Figure 4.1 Structure and expression of the HOXB3 and neo control retroviruses used in this study 94 Figure 4.2 Overview of experimental design 95 Figure 4.3 Donor-derived hemopoietic reconstitution of HOXB3 and neo mice.. 96 Figure 4.4 Hemopoietic parameters of neo and HOXB3 mice 98 Figure 4.5 Flow cytometric analysis of various hematopoietic populations 99 Table 4.1 Evaluation by limiting dilution analysis of competitive long-term repopulating cells (CRU) in primary mice transplanted with HOXB3 or neo control transduced bone marrow cells, 18 weeks after transplantation 100 Figure 4.6 Cellularity of thymuses and their subpopulations in individual neo and HOXB3 mice 102 Figure 4.7 Cytoflourometric analysis of thymocyte subpopulations 103 Figure 4.8 RT-PCR analysis purified thymic subpopulations 104 C h a p t e r 5 Figure 5.1 Structure of the HOXB4 and control neo retroviruses and the experimantal outline 116 Table 5.1 CRU numbers regenerated in primary recipients of neo- or HOXB4- transduced bone marrow cells 118 ix Figure 5.2 Southern blot analysis of proviral integration patterns in primary neo and HOXB4 mice sacrificed 32 weeks (A) or 52 weeks (B) post transplantation and their secondary recipients from the CRU assay 121 Figure 5.3 Northern blot analyses to detect expression of the transduced HOXB4 gene in bone marrow of HOXB4 mice 123 Figure 5.4 Effects of HOXB4 overexpression on the expansion of myeloid and pre-B colony forming cells following bone marrow transplantation.... 125 Table 5.2 Regeneration of Seal+Lin _WGA+cells in neo and HOXB4 mice 126 Table 5.3 Hematological parameters in HOXB4 and neo mice, 16-52 weeks post transplantation 127 Table 5.4 Absolute numbers of various phenotypically defined hematopoietic populations in neo and HOXB4 mice 32 weeks after transplantation 128 C h a p t e r 6 Figure 6.1 Schematic representation of the effects observed on various hematopoietic lineages with overexpression of HOXB3, HOXB4orHOXA10 137 x ABBREVIATIONS apTCR alfa beta T cell receptor bp base pair BFU-E burst forming unit-erythroid BFU-Mk burst forming unit-megakaryocyte bHLH basic helix loop helix BMT bone marrow transplantation BSA bovine serum albumin cDNA complementary deopyribonucleic acid y8 TCR gamma delta T cell receptor CFU colony forming unit CFU-E colony forming unit-erytthroid CFU-GEMM CFU- granulocyte, erythrocyte, macrophage, megakaryocyte CFU-GM CFU- granulocyte, macrophage CFU-Mk CFU-megakaryocyte CFU-S colony forming unit-spleen cGy centiGray CLL chronic lymphocytic leukemia CRU competitive repopulation unit dpp decapentaplegic Dhh desert hedgehod EPO erythropoietin ES embryonic stem (cell) FCS fetal calf serum 5-FU 5-flourouracil G-SCF granulocyte-colony stimulating factor xi GM-CSF gransulcyte, macrophage-colony stimulating factor HGF hematopoietic growth factor hh hedgehog HSC hematopoietic stem cell HTH helix turn helix HXM hypoxanthine-xanthine-mycophenoloc acid IAP intracisternal A particle IL interleukin Ihh indian hedgehog KCI potassium chloride LTC long-term culture LTC-IC long-term culture-initiating cell LTR long terminal repeat P-ME b-mercaptoethanol MIP-1a macrophage inhibitory protein-1 alpha rnRNA messenger ribonucleic acid MSCV murine stem cell virus NCS newborn calf serum OSM oncostatin-M Pc-G polycomb group genes PHA phytohemagglutinin pol II RNA polymerase II R-PE R-phycoerythrin RNA ribonucleic acid RU repopulation unit S.D. standard deviation SCCM spleen cell conditioned medium SDS sodium dodecyl sulfate Shh sonic hedgehog TGF-pM transforming growth factor -beta 1 TPO thrombopoietin trx-G trithorax group genes U units ACKNOWLEDGEMENT I am very grateful to my supervisor Dr. Keith Humphries for the opportunity to do graduate training in his laboratory and for his enthusiastic support and tireless guidance throughout this project. I would also like to thank Dr. Connie Eaves for her invaluable help and support throughout this project. Thanks to all members of Keith's laboratory (Margaret, Patty, Jana, Rob, Cheryl, Jennifer, Sue, Nick, Thilo and Sharlene) for their help and many pleasant unforgettable moments. A very special thanks to Dr. Guy Sauvageau, with whom I have worked in close collaboration, for his genuine passion for science and all the stimulating discussions. I would also like to thank Wieslava Dragowska, Gayle Thornbury, Maya St-Clair, and Fred Jenson for their valuable assistance and Drs. Connie Eaves, Frank Jirik and Fumio Takei for serving on my graduate committee. Finally, above all I would like to thank my parents for teaching me the value of education and knowledge and for their years of constant support and caring, and my husband and son for their love and understanding. C o n t r i b u t i o n o f c o l l a b o r a t o r s t o t h e w o r k p r e s e n t e d i n t h i s t h e s i s . In the studies presented in Chapter 3, Dr. Guy Sauvageau's contribution to the work involving construction and production of the retroviruses used, and cytological evaluation of hematopoietic cells derived from HOXA10 and neo mice, is gratefully acknowledged. Dr. Margaret R. Hough and Ms. Wieslawa Dragowska provided assistance with flow cytometric analysis and purification of cells, respectively. xiv In the studies presented in Chapter 4, Dr. Guy Sauvageau's contribution to the work involving the construction and production of retroviruses used, cytological evaluation of hematopoietic cells derived from HOXB3 and neo mice, and flow cytometric analysis, and Dr. Margaret R. Hough's additional assistance with flow cytometric analysis, are gratefully acknowledged. In the studies presented in Chapter 5, Dr. Margaret R. Hough and Dr. Guy Sauvageau provided valuable assistance with flow cytometric analysis and cytological evaluation of hematopoietic cells derived from HOXB4 and neo mice, respectively. xv Chapter 1 Introduction 1.1 Overview Hematopoiesis is the process by which blood cells of multiple distinct lineages are produced throughout life. The mature products of this process have highly specialized and essential functions including transport of carbon dioxide and oxygen by erythrocytes, blood clotting by platelets, mediation of innate immunity by macrophages, granulocytes and natural killer cells, and mediation of antigen-specific immunity by B lymphocytes (humoral) and T lymphocytes (cellular). Large numbers of these cells are found in the circulation and because most of these cells have relatively short life spans (a few days to weeks) they need to be constantly replenished. Thus for example it has been estimated that about 1 trillion mature blood cells are produced in adult man every day, including 200 billion erythrocytes and 60 billion neutrophilic leukocytes. The ultimate burden of the life-long production of these mature elements is carried out by a small population of primitive cells, predominantly residing in the bone marrow, which has the capacity for multilineage differentiation and self-renewal divisions i.e. is self-maintaining. The control of this highly complex and dynamic process occurs at multiple levels by both positive and negative regulators, which under normal conditions assure the regulated production of all types of blood cells to meet changing physiological needs. As yet, very little is known about the genetic mechanisms that bring about the self-renewal or differentiation outcomes of early hematopoietic cell divisions, although much progress has been made in identifying a variety of cytokines that can regulate the cycling status of these cells. Accumulating evidence, from a number of recent studies, is now pointing to transcription factors such as SCL/tal-1, Ikaros, AML-1 and 1 GATA-2 as key components of the intrinsic processes involved in lineage determination and the subsequent lineage-specific differentiation of early hematopoietic cells. Among transcriptional regulators is the Hox homeobox family of genes, initially described in the fruit fly as genetic elements essential for specification of cell identity and pattern formation of a number of embryonic structures. Strikingly, Hox gene functions have been found to apply to most if not all multicellular organisms, albeit with very different end results, reflecting the "flexibility" of Hox genes in orchestrating tissue specification. Hox genes have recently been shown to be expressed in normal hematopoietic cells, at levels that vary as a function of the stage of differentiation of the cells and of the specific Hox gene examined. By analogy of hematopoiesis to other developmental programs it was hypothesized that this differential expression pattern of Hox genes might reflect roles for Hox genes in the regulation of hematopoiesis. As a test of this hypothesis, the research described in this thesis was aimed at analyzing possible functional roles of Hox genes in hematopoiesis. Toward that end the normal expression patterns of three Hox genes were disturbed by engineering their overexpression in murine bone marrow cells, using retroviral gene transfer. The subsequent effects of these manipulations on the behavior of hematopoietic cells both in vitro and in vivo were then analyzed. This introduction is divided into three main parts that review current understanding of three separated but related topics central to this thesis. These are: (1) the organization and regulation of the hematopoietic system; (2) the Hox homeobox genes; and (3) the involvement of Hox genes in hematopoiesis. 1.2 Organization and Regulation of the Hematopoietic System 1.2.1 D e v e l o p m e n t a l a n d h i e r a r c h i c a l o r g a n i z a t i o n o f h e m a t o p o i e s i s In mammals, hematopoietic cells originate from the mesodermal layer of the embryo. The first known site of hematopoiesis is extra-embryonic, in the yolk sac blood islands (known as primitive hematopoiesis). A few days later, the appearance of 2 hematopoietic cells in an intraembryonic site, the so-called aortic-gonadal-mesonephros (AGM) region, has recently been described (Godin et al., 1993; Huyhn et al., 1995). Hematopoiesis then shifts to the fetal liver (definitive hematopoiesis) and finally to the bone marrow where hematopoiesis continues throughout adult life (Zon, 1995). Evidence favoring the AGM region as the source of definitive hematopoiesis rather than the extra-embryonic yolk sac, as previously thought, has recently been reported (Cumano et al., 1996; Medvinsky and Dzierzak, 1996). ® Totipotent HSC Potential for Self- Prolif-renewal eration Lymphoid ® ® ® ® End cells Multi-potential progenitors Uni-potential progenitors CRU LTC-IC Day 12 CFU-S In vitro colony assays Morph-ology Figure 1.1 Schematic representation of the organization of the hematopoietic system Some of the assays used to detect various populations of hematopoietic cells are listed in bold. HSC, hematopoietic stem cells,: CRU, competitive repopulation unit assay; LTC-IC, in vitro assay that detects cells that initiate long-term cultures; day 12 CFU-S, colony-forming-unit spleen cells that generate nodules on the spleen 12 days after inoculum. The relative potential for self-renewal and proliferation of various populations of hematopoietic cells is shown to the right The cells of the hematopoietic system can been subdivided into myeloid and lymphoid cells as their pathways are thought to diverge early on in hematopoietic differentiation (Figure 1.1), although the existence of "mixed" lymphoid and myeloid pathway (macrophage and B lymphoid) has been described both in bone marrow and fetal liver cells (Cumano et al., 1992; Ohara et al., 1991). All of the myeloid cells are produced in the bone marrow, which include the erythroid, granulocytic and megakaryocytic lineages (Lichtman, 1981). The cells of the lymphoid lineages which give rise to T and B lymphocytes and natural killer (NK) cells, however, are produced and/or found to varying degrees in bone marrow, spleen, thymus and lymph nodes. The differentiation of all of these different cell types is a multistep process usually spanning many cell divisions, which has allowed each lineage to be visualized as a series of distinct stages. Cells at these different stages have been distinguished by differences in proliferation and differentiation potential, response to different regulators and cell surface antigenic profiles (Figure 1.1). The most primitive stage is that represented by hematopoietic cells with unrestricted proliferation and differentiation potentials. These primitive cells comprise a very small proportion of the total population of hematopoietic cells and their ability to sustain life-long hematopoiesis is associated with a capacity to undergo self-renewal divisions and therefore avoid exhaustion of the stem cell pool. Cells arising from these stem cells by the process of differentiation to specific lineages, are commonly referred to as hematopoietic progenitors (Figure 1.1). When these progenitor cells divide, they differentiate into morphologically recognizable cells, with very limited proliferative potential (3-5 cell divisions), that are the immediate precursors of specific types of mature blood cells. This hierarchical organization of hematopoiesis is now widely accepted, with the emergence of key questions of how this process is regulated and what are the actual changes in gene expression associated with the commitment of totipotent stem cells to particular blood cells lineages. Pivotal to these studies is the availability of assays and purification schemes to quantitate and functionally characterize early hematopoietic cells. In the following section some of these methods will be discussed, as well as what they have taught us about the properties of these early cells. 4 1.2 .2 D e f i n i t i o n a n d p r o p e r t i e s o f e a r l y h e m a t o p o i e t i c c e l l t y p e s 1 .2 .2 .1 T h e t o t i p o t e n t h e m a t o p o i e t i c s t e m c e l l ( H S C ) The existence of very primitive cells with self-renewal and lympho-myeloid differentiation potential was first clearly shown almost three decades ago. By using radiation induced chromosomal abnormalities and bone marrow transplantation, Wu et al. showed that both myeloid and lymphoid cells in transplanted mice could be derived from a common cell (Wu et al., 1968). This observation has since been confirmed by other approaches, including the use of retroviruses to introduce unique clonal markers by their random integration into the genome, and hence demonstrated common proviral integrants in both lymphoid and myeloid cells in mice transplanted with retrovirally infected cells (Capel et al., 1989; Dick et al., 1985; Keller et al., 1985). The existence of the HSC is thus a well established concept however, the quantitation of these cells has been more problematic. Quantitation of HSC - One of the first assays developed for primitive hematopoietic cells was the in vivo spleen-colony-forming unit (CFU-S) assay (Till and McCulloch, 1961). This assay detects cells that have the ability to form macroscopic nodules of hematopoietic cells on the spleen 8-12 days after their intravenous injection into myeloablated recipients. These CFU-S cells were initially considered to be HSCs because they share many properties that are attributed to HSCs. These include their ability to generate large clonal populations (105-107 cells), containing progeny of multiple hematopoietic lineages, thus demonstrating the high differentation and proliferation potential of individual CFU-S (Becker et al., 1963; Wu et al., 1968). Importantly, many CFU-S were found to possess the ability for self-renewal as progeny CFU-S were frequently detected in spleen colonies by injection into secondary recipients (Siminovitch et al., 1963). Although many of these cells possess the ability to differentiate into erythrocytes as well as all cells of the myeloid lineage, their lymphoid potential remained controversial (Wu et al., 1968; Lala and Johnson, 1978; 5 Paige et al., 1979), until recently (Lepault et al., 1993). The validity of the CFU-S assay to detect HSC with long-term reconstituting potential was also complicated by the discovery of functional heterogeneity among cells detectable as CFU-S, with some of these cells capable of only unilineage differentiation and lacking demonstrable self-renewal potential (Magli et al., 1982). Further assays have been developed that are based on the operational definition of an HSC as a cell type that can sustain both myeloid and lymphoid differentiation in vivo for a prolonged period of time. With the use of such assays and techniques that can physically separate functionally distinct primitive hematopoietic cells (see below) it was demonstrated that most CFU-S cells in normal adult bone marrow are committed myeloid progenitors, as they can be physically separated from more primitive cells with long-term lympho-myeloid repopulating potential (Jones et al., 1990). Nevertheless, the CFU-S assay can detect primitive progenitor cells and has thus played a key role in the development of concepts of primitive cell organization and regulation. Two of the most rigorous and now widely used assays to quantitate the murine HSC are those developed by Harrison et. al and Szilvassy et. al (Harrison et al., 1993; Szilvassy et al., 1990). Both of these are functional assays based on the ability of the HSC to regenerate and sustain hematopoiesis in myeloablated or genetically anemic mice (i.e. W/Wv) for long periods. Their designs, however, differ and so do, their potential applications. The method developed by Harrison (for review see Harrison et al., 1993) is based on the principles of "competitive repopulation" and binomial distribution. In this assay two populations of hematopoietic cells, distinguished from each other by phenotypic, genetic or biochemical markers, are compared in their ability to repopulate both lymphoid and myeloid compartments for long periods after their transplantation into myeloablated mice. The first of these populations is called the "competitor" and normally consists of a fixed number (usually 1-2x106) of fresh bone marrow cells that 6 serves as a standard for repopulating potential. The second population, the "donor", is of unknown HSC content. Various numbers of "donor" cells are then injected along with "competitor" cells into myeloablated mice and the relative contribution of the two populations to hematopoiesis then measured. The frequency of HSC, here called "repopulating units" (RU), in the "donor" population is then calculated by the formula RU=%(C)/(100-%), where the % is the measured percentage of "donor" hematopoiesis and C is the number of fresh "competitor" cells used. In contrast to the CRU assay which uses limit dilution principles (see below), this assay uses high cell numbers and thus reduces the demand for short-term repopulation imposed on the "tested1 population. This assay is therefore particularly suited for the comparison of the relative numbers of RU in two different populations of hematopoietic cells (Harrison et al., 1993). Although highly rigorous, the potential drawbacks of this assay are its requirement for a relatively long incubation period (>3 months). The Competitive Repopulating Unit (CRU) assay developed by Szilvassy et. al combines limiting dilution and "competitive repopulation" principles to quantitate absolute frequencies of HSC, which go by the operational name CRU cells. As originally described, a population of cells being analyzed, the "test" cells, which are uniquely distinguished by genetically determined markers (Szilvassy et al., 1990; Rebel et al., 1994), are injected at various dilutions into multiple myeloablated recipients along with a fixed number of cells that serve the purpose of ensuring short-term survival of the mice and provide a standardized level of "competition", independent of the "test" cell source. Recipients that are defined to be repopulated by at least one "test" cell derived CRU are those where a significant proportion (>5%) of their mature lymphoid and myeloid cells are of "test" cell origin. According to Poisson statistics, the number of "test" cells which has a 63% chance of repopulating a recipient would contain one CRU. The frequency of CRU cells in the "test" cell population can then be determined by estimating that value from the proportion of negative mice obtained for each "test" cell dilution (i.e., where a negative mouse is one 7 that does not meet the repopulating criteria). Using this method the frequency of CRU cells in normal bone marrow has been estimated to be 1 per 104 cells (Szilvassy et al., 1990). CRU frequencies determined five weeks after transplantation have been shown to be virtually identical to those obtained at later time points (at least up to 6 months), indicating that this assay can be used to evaluate true long-term repopulating cells after little more than a month. Originally, a source of cells with relatively reduced long-term repopulating ability was used to ensure survival of the mice. However, more recently, it was found that 105 normal adult mouse bone marrow cells could be substituted, with alterations of the sensitivity of the assay. (Rebel et al., 1994, see also chapter 4 and 5 of this thesis). The main advantages of this assay are that it allows rigorous but relatively quick (5-6 weeks) quantitation of HSC, and that since it is built around limiting dilution principles it allows quantitation of individual HSCs independent of their proliferative behavior (beyond the minimum threshold required for their detection). In vitro assays to detect primitive hematopoietic cells have also been developed. The best characterized of these is the long-term culture initiating cell (LTC-IC) assay, which detects cells that are capable of initiating and sustaining myeloid clonogenic progenitor production for at least 6 weeks under long-term culture conditions on a pre-established stromal layer (Sutherland et al., 1990). In humans this assay has thus far been the best available assay to quantitate the earliest hematopoietic cells. Recently, however, with the use of SCID mice the developmant of an in vivo assay to quantitate human lympho-myeloid repopulating cells, has been proposed (Conneally et al., 1996). Phenotyping and purification of HSC - Both the low frequency of HSC in hematopoietic tissue and the use of indirect functional assays as the only means to detect and quantitate these cells has driven interest in methods to physically isolate a pure population of HSCs as a way to quantitate HSC by phenotype and to faciliate their molecular and functional characterization. 8 Numerous strategies to enrich for these cells have been developed. These include those that are based on physical differences between HSC and more mature cells (e.g. buoyant density and/or size), which have led to the characterization of HSC as relatively small, low density cells with an undifferentiated blast cell-like morphology (Jones et al., 1990; Orlic et al., 1993). The most commonly used methods are, however, those that have combined the use of flow cytometry and the capacity of HSC to exclude the vital mitochondrial dye rhodamin-123 (Rh-123) (Ploemacher and Brons, 1988; Spangrude and Johnson, 1990), to bind lectins such as wheat germ agglutinin (WGA) (Ploemacher et al., 1993; Rebel et al., 1994; Visser et al., 1984) and by their expression of cell-surface antigens that can be recognized by monoclonal antibodies (Morrison and Weissman, 1994; Spangrude et al., 1988; Szilvassy and Cory, 1993) Although some differences occur between different mouse strains (Spangrude and Brooks, 1993), the following surface phenotype is usually associated with the adult murine HSCs: they express high levels of the Ly-6A/E (Sca-1) (Spangrude et al., 1988) and H-2K antigens (Szilvassy and Cory, 1993), and WGA (Rebel et al., 1994), low levels of the receptors c-kit (Katayama et al., 1993; Li and Johnson, 1995) and Thy-1 (Spangrude et al., 1988), and are largely negative for markers expressed on terminally differentiated cells (Lin-) such as B220, CD3, CD4, CD8 (lymphocytes), Mac-1, Gr-1 (myeloid) and TER-119 (erythrocytes) (Spangrude et al., 1988). It is important to note that none of the known surface markers that are associated with HSC are restricted in their expression to HSCs, and as a result the stem cell surface phenotype is defined by the combination of those markers that are expressed and those that are not expressed on these cells. Cell sorting based on these criteria, now allows the enrichment of murine HSC by 500 to 1000-fold (Morrison and Weissman, 1994). Thus for example, with the use of the CRU assay to quantitate the enrichment factor, the frequencies of HSCs in a population of cells with the Sca-1 +Lin_WGA+ phenotype is 1 per 30 cells (Rebel et al., 1994) and frequencies up to 1 per 5 cells have been achieved with Sca-1+Thy-9 1.1 loLin-Rhdull cells (Spangrude et al., 1995). The enrichment factors stated above are likely an underestimation of the real frequencies of HSC in these purified populations, since they don't take into account seeding efficiency (i.e. homing to appropriate site for further growth) and their stimulation to generate mature blood cells as opposed to them either remaining or becoming quiescent or executing predominantly self-renewal divisions. Despite the significant advances in the purification of murine HSC, the application of the HSC phenotype has its restrictions. This is largely due to the fact that no surface markers that are exclusive to HSCs have been identified to date. This is best exemplified by studies that compared the frequencies of HSCs in the bone marrow population with the candidate HSC phenotype, before and after either culture (Rebel et al., 1994) or transplantation (Spangrude et al., 1995) of these cells. Both of these manipulations can result in great expansion of cells with the original HSC candidate phenotype, but without concurrent expansion in HSC numbers as measured by functional assays. The availability both of assays to identify and quantitate HSC and of schemes to purify these cells, has opened a way for answering some key questions regarding the properties and regulation of HSC. These include their cycling status and utilization in steady state hematopoiesis, their potential for self-renewal divisions and their possible heterogeneity as a function of source, some of which will be discussed in the following section. 1 .2 .2 .2 P r o p e r t i e s o f t h e H S C One characteristic that can distinguish HSC from more mature progenitor cells is their cycling status. Under homeostatic conditions in vivo many HSC are quiescent or cycling very slowly, as demonstrated by their relative resistance to killing by the cytotoxic drug 5-fluorouracil (5-FU), in contrast to the much higher proportion of progenitor cells killed by the same treatment (Hodgson and Bradley, 1979; Lemer and 10 Harrison, 1990). Interestingly, the 5-FU treatment also appears to recruit HSC into active cell cycle, since they become highly sensitive to a second dose of 5-FU, given 3-5 days later (Harrison and Lemer, 1991). Several studies using retroviral marking of HSCs have demonstrated convincingly the ability of HSCs to undergo self-renewal divisions both in vitro (Fraser et al., 1992) and in vivo (Jordan and Lemischka, 1990; Keller and Snodgrass, 1990; Lemischka et al., 1986). The potential of HSC to execute self-renewal divisions is, however, not fully resolved. For example, given the right conditions do HSC have unlimited ability to undergo self-renewal divisions or do these cells possess an inherent limit for such divisions? In support of the latter possibility are observations that have shown that following bone marrow transplantation the HSC pool is not found to regenerate to levels >10% of normal pre-transplantation values, despite regeneration of bone marrow cellularity and progenitor cell numbers to normal pre-transplantation levels (Harrison and Astle, 1982; Harrison et al., 1978; Harrison et al., 1990; Mauch and Hellman, 1989; Pawliuk et al., 1996). Even when mice are transplanted with a cell dose representing one half of the total bone marrow of a mouse, the HSC pool is not fully regenerated. Interestingly, no difference in HSCs content has been found between bone marrow cells from old (2 to 2.5 years) and young mice, suggesting that if there are limits to HSC self-renewal, those limits are at least not reached in the normal life span of a mouse. Other studies using retroviral marking, have indicated that at least some HSC can undergo a very high number of self-renewal divisions (Keller and Snodgrass, 1990; Pawliuk et al., 1996). Keller and Snodgrass showed that a single HSC could completely reconstitute all lineages of a primary recipient for more than 10 months and also almost completely that of a secondary recipient for another 17 months. Pawliuk et. al were able to put a numerical value on this process and showed that at least some HSC can be amplified ~370-fold following transplantation. These studies therefore suggest that the substantial loss of marrow repopulating potential associated with transplantation is not simply a reflection of HSC fostering low 11 inherent potential for self-renewal divisions, and that other constrains might be involved. One possibility is that as a consequence of HSCs regeneration of more differentiated cells under the conditions operative in the post-transplant recipients, it may result in loss of their subsequent ability to execute self-renewal divisions. Others have suggested (Pawliuk et al., 1996) involvement of negative regulatory feedback mechanisms imposed in vivo by more mature cells as a possible mechanism that could prematurely inhibit HSC expansion. These might involve the production of factors such as MIP-1a and TGF-p1 which have been shown to decrease the proportion of primitive hematopoietic cells in cycle (Cashman et al., 1992; Eaves et al., 1991; Hatzfeld et al., 1991; Ruscetti et al., 1991). In the above discussion HSCs have been considered more or less as a homogeneous population. However, functionally defined HSCs appear heterogeneous by several criteria. HSCs are heterogeneous in their expression of surface markers. For example, the Sca-1 +Thy1.1 loLin- cells which are highly enriched for HSCs, can be further divided into subgroups based on differential expression of c-kit or CD4 antigens. Although the levels of HSCs in each of these subgroups are different, they all contain HSCs. Furthermore, HSCs are also found in other populations (e.g. Sca-1 -Lin+). albeit at low frequencies (Rebel et al., 1994). HSCs in a given hematopoietic population are also likely heterogeneous in their proliferative potential. Experiments using retroviral tagging have revealed that some HSCs only contribute to hematopoiesis for the first ~6 months post transplantation whereas others contribute for the duration of the recipient's life (Jordan and Lemischka, 1990; Keller and Snodgrass, 1990). Whether this heterogeneity represents true intrinsic differences in their proliferative potential or differences in the expression of this potential due to stochastic commitment versus self-renewal events, is, however, not clear. HSC from fetal liver have been shown to manifest both higher competitive repopulating and expansion potential than their adult bone marrow counterparts (Pawliuk et al., 1996; Rebel et al., 1996), indicating ontogeny related differences in the proliferative potential of HSC. 12 Heterogeneity in the behavior of HSC from different mouse strains, has also been documented. Transplantation experiments carried out with bone marrow cells from allophenic mice created by mixing C57BL/6 and DBA/2 early embryonic cells, demonstrated that cells of DBA/2 origin dominated in the early phase of regeneration, whereas cells of C57BL/6 origin dominated the late phase (Van Zant et al., 1992; Van Zant et al., 1991). Subsequent experiments demonstrated that a larger proportion of progenitor cells from DBA/2 mice were actively cycling and responded more rapidly to Steel factor, whereas C57BL/6 mice had a larger pool of candidate HSCs with a primitive phenotype (Rhduli). These differences might at least in part explain the different engrafment pattern of these two populations of cells (Phillips et al., 1992). Genotypic differences in HSC frequencies have also been documented (Muller-Sieburg and Riblet, 1996). Taking advantage of this difference, it was shown by genetic analysis that HSC frequencies are determined by multiple genes of which two loci appear to have major quantitative effects (Muller-Sieburg and Riblet, 1996). 1 .2 .2 .3 M y e l o i d a n d e r y t h r o i d p r o g e n i t o r c e l l s A variety of more differentiated progeny of HSC can be detected by their ability to form colonies in semi-solid medium (usually containing either methylcellulose or agar) supplemented with appropriate nutrients and growth factors, either provided by poorly defined "conditioned" cell medium or as pure recombinant growth factors (for review see Eaves, 1995). Different classes of these progenitor cells can be distinguished by their growth factor requirements, the time required for their formation of a mature colony and the types of mature cells that it generates. The period that elapses from the time of plating until morphologically mature cells are detected, and the number and types of cells in each mature colony can be used to infer the developmental stage of the progenitor cell. Thus, large colonies containing cells from multiple lineages, originate from more primitive cells and small colonies of a single cell type from later progenitors. These assays have played an instrumental role in the characterization of cytokines that act at various stages of hematopoietic development. They have also 13 been central to studies aimed at dissecting the nature of .the regulatory processes, extrinsic versus intrinsic, that govern the survival, proliferation and differentiation of hematopoietic progenitor cells (see below 1.2.3.1, for more discussion) The vast majority of progenitors detected in these cultures are uni- or bi-potential. These include progenitors that give rise to colonies which contain granulocytes and macrophages, and are called granulocyte-macrophage colony-forming units or CFU-GM. These CFU-GM cells have a broad range of proliferative potentials, as reflected in different sizes of the GM colonies that can be produced. More differentiated progeny of CFU-GM cells are detected in these cultures as CFU-G and CFU-M. Two erythroid restricted progenitors have been characterized: a more primitive progenitor, the burst-forming units-erythroid or BFU-E, and a more mature progenitor, the colony-forming unit-erythroid or CFU-E. In the megakaryocyte lineage two progenitor types have also been identified, the more primitive BFU-Mk and more mature CFU-Mk. In steady state hematopoiesis, the majority of these uni- and bi-potential progenitors appear to be actively cycling as they are highly susceptible to killing by cytotoxic drugs such as 5-FU (Van Zant, 1984). Progenitors that are more resistant to the effects of 5-FU and have multilineage and high proliferative potential can also be detected in vitro by their colony formation. Depending on the semi-solid medium and the growth factors provided, these progenitors are detected by their formation of colonies containing a mixture of granulocytes, erythrocytes, macrophages and megakaryocytes (CFU-GEMM) (Humphries et al., 1981; Johnson and Metcalf, 1977); very large macrophage colonies (high proliferative potential colony-forming cells, HPP-CFC) (Bradley and Hodgson, 1979); or colonies consisting of undifferentiated blast cells (blast colony-forming cells) (Nakahata and Ogawa, 1982). Such progenitors can also exhibit limited self-renewal ability (Humphries et al., 1981; Nakahata and Ogawa, 1982). 14 1 .2 .2 .4 L y m p h o i d p r o g e n i t o r c e l l s It has been proposed that both T and B cells originate from a common progenitor cell (Figure 1.1). However, the existence of such a cell type in the bone marrow has not been convincingly demonstrated. The strongest evidence for the existence of such a progenitor is an intrathymic cell population with the Thy-1loSca-2+c-kit+Lin-CD4l° surface phenotype, which has both T and B cell developmental potential, but very reduced myeloid potential (Matsuzaki et al., 1993; Wu et al., 1991; Wu et al., 1991). Two in vitro assays that detect primitive B cell progenitors have recently been developed (Saffran et al., 1992; Suda et al., 1989), but no such assays has yet been described for progenitors with T lymphoid potential. Nevertheless, early stages of B and T cell development have been well characterized, mainly on the basis of the changes in expression of cell surface markers, and immunoglobulin (Ig) and T cell receptor (TCR) gene rearangements, that occure during these processes (Figure 1.2). The various stages of B cell development have been characterized by the surface expression of developmetally regulated antigens, the rearrangement status and surface expression of the Ig genes, and by responsiveness to interleukin-7 (outlined in Figure 1.2). These early steps in B cell development largely take place in the bone marrow, with further development of immature and mature B cells taking place in the periphery (spleen and lymph nodes). The main site of T cell development is the thymus. The thymus is repopulated, as described above, with progenitor cells that have both T and B cell developmental potential (Figure 1.2). Following commitment of thymus repopulating cells to the T cell lineage, T cells develop through various stages characterized by expression of the CD4 and CD8 antigens, the TCR, the CD3 complex (the TCR associated complex), and a variety of other surface molecules such as CD44, heat stable antigen (HSA) and CD25 (IL-2 receptor oc-chain (IL-2Ra)) (Figure 1.2) (reviewed in Godfrey and Zlotnik, 1993; Zlotnik and Moore, 1995)). The majority of thymocytes develop along the 15 pathway leading to the generation of cells expressing oq3 TCR along with CD4 and CD8. A smaller proportion develops into T cells expressing y5 TCR and lacking expression of CD4 and CD8. These pathways are thought to diverge early in T cell development, at the so-called pre-T cell stage (Figure 1.2). The actual events that determine which pathway is selected are, however, not clear, but might involve transcriptional silencing of the TCR y genes in the a(3 T cell precursors cells (Haas and Tonegawa, 1992). Surface antigens Growth properties Immature Immature Pro-B cells pre-B cells Pre-B cells B cells Mature B cells B220+ Thy-1 l 0 B220+ CD43+ Stromal dependent B220+ B220+ B220+ CD43 1 0 lgM+ lgM+lgD+or lgG+ Stromal independent IL-7 responsive B DN < § > - • < § > - • ( § > CD44+ CD44- CD44-CD25+ CD25+ CD25-c-kit+ c-kit-DP SP ® TCR YS/CD3+ CD4+ / ^ v ' CD8-\m) TCRa(3/CD3+ CD4+ \ CD8+ > TCRa(3/CD3 l 0 CD4-CD8+ TCRa(3/CD3+ Figure 1.2 Simplified representation of B and T cell development (A) Various stages in B cells development that have been characterized based on expression of developmentally regulated antigens and the growth requirements of the cells i.e. whether their growth is dependent on stromal cells and/or IL-7 or not. (B) Stages in T cell development as characterized by differential surface expression of number of antigens that lead either to the generation of a(3 T cell receptor (TCR) T cells or y8 TCR T cells. DN, double negative; DP, double positive; and SP, single positive. 16 1.2 .3 R e g u l a t i o n o f h e m a t o p o i e s i s As discussed above, the process of mature blood cell production must be tightly controlled. Similarly, the entry of mature cells into the circulation, their localization to the appropriate tissue, as well as their functional activation must also be regulated. Regulation of this complex process can be simplistically viewed as the combined effects of external influences (composed of both humoral factors and cell-cell or cell-matrix interactions) and intracellular signaling events with consequent transcriptional factor regulation, that ultimately lead to changes in gene expression of multiple effector molecules (Orkin, 1995). Although there is abundant evidence supporting the role of molecules from each of these "control levels" in the regulation of hematopoiesis, their relative importance and interactions are not fully resolved. The production of mature blood cells takes several cell cycles to complete. Thus, there must be mechanisms in place at the single cell level that assure an appropriate balance between the processes of differentiation, proliferation and maintenance of cell viability. Both viability and proliferation of hematopoietic cells are regulated by number of cytokines (for review see Ogawa, 1993). In vivo the size of a progenitor population can thus be regulated by the availability of hematopoietic growth factors which determine both the fate (survival or death) and amplification (by proliferation) of the progenitor population. The more primitive that a given progenitor population is, the greater is the impact of its altered behavior. Therefore fine tuning of mature cell output can thus be achieved by modulation in viability and proliferation of later cells whereas more dramatic alterations in mature cell production (e.g. blood loss) would require altered behavior of more primitive cells. Although much progress has been made in identifying a variety of cytokines that can regulate the cycling status of hematopoietic progenitor cells the genetic mechanisms that are responsible for their lineage-commitment and differentiation are largely unknown. It has been proposed that differentiation of hematopoietic cells, in contrast to regulation of their viability and 17 proliferation, is largely determined by intrinsic factors due to the observed apparent stochastic nature of this process (Fairbairn et al., 1993; Ogawa, 1993). 1.2.3 .1 R e g u l a t i o n o f h e m a t o p o i e s i s b y e x t e r n a l f a c t o r s Hematopoietic growth factors (HGF) currently represent the most extensively described external regulators of hematopoiesis (see Metcalf, 1993 and Ogawa, 1993, for detailed review). To date more than 25 HGF have been identified. These include the hematopoietic colony-stimulating factors (G-CSF, M-CSF and GM-CSF), the interleukins (IL-1 to IL-17), the hematopoietic inhibitors (TGF-p and MIP-1cc), the "stem cell" factors (LIF, Steel factor and flk2/flt3 ligand), and erythropoietin (Epo) and thrombopoietin (TPO). The regulation of hematopoiesis by HGFs has largely been studied using either hematopoietic cell lines or the combined use of primary hematopoietic cells and the in vitro colony forming assays described above. In several of these studies HGFs have been shown to be necessary for proliferation and survival of hematopoietic cells (Metcalf, 1993; Ogawa, 1993), whereas their effects on lineage commitment have only been demonstrated in limited studies (Metcalf, 1991). The majority of the HGFs, with the exception of Epo and G-CSF, appear to have considerable overlap in their function. This is evidenced both by the ability of different HGFs to support growth of the same type of progenitor cells and by the ability of a given HGF to act on different types of progenitors. For some of these HGFs the nature of this redundancy may be explained by their receptors sharing a common subunit, that is involved in the initiation of the intracellular signal. Thus, for example the IL-3, IL-5 and GM-CSF share a common (3 subunit and the ability to stimulate eosinophil proliferation. Similarly, IL-6, IL-11, LIF and OSM which share pleiotropic activity also share a common (3 subunit in their respective receptors. An interesting concept that has developed from these HGF studies, is that primitive hematopoietic progenitor cells can only be stimulated to proliferate in the presence of two (or more) HGFs, whereas more mature progenitors can proliferate in response to a 18 single HGF although the presence of additional factors may have synergistic effects (Metcalf, 1993). Even though primitive hematopoietic cells can be recruited into proliferation by the combination of various cytokines, net expansion of HSCs has been difficult to demonstrate (Bodine et al., 1992; Li and Johnson, 1994). These difficulties may imply the existence of as yet unidentified HGFs acting on HSCs. They have also raised the possibility that HSC self-renewal may predominantly be regulated by other mechanisms, such as signaling through adhesion molecules and/or by some poorly understood "internal regulators". The roles of some HGFs in hematopoiesis have also been studied in mice lacking the expression of a particular HGF or its receptor as a result of inactivation by gene targeting in embryonic stem cells. In mice where either Epo or the Epo receptor (EpoR) were inactivated, the generation of erythroid BFU-E and CFU-E progenitors was not affected, indicating that Epo-mediated signaling is not necessary for commitment to the erythroid lineage (Wu et al., 1995). However, terminal differentiation of these progenitor cells was blocked in these mice. Interestingly, this block could be overcome in vitro by TPO, underscoring the redundancy between HGFs (Kieran et al., 1996). Of the HGF receptors known to date, one, the flk2/flt3 receptor, shows expression that is largely confined to primitive hematopoietic cells (Matthews et al., 1991; Palacios and Nishikawa, 1992). Homozygous mutant mice for the flk2/flt3 receptor are viable, and the only detectable hematological abnormalities in these mice are diminished numbers of pro- and pre-B cells (Mackarehtschian et al., 1995), indicating that the flk2/flt3 ligand is not essential or solely responsible for hematopoietic development in mice. HGFs with the potential to inhibit primitive hematopoietic proliferation have also emerged as important regulators of hematopoiesis (Graham et al., 1990). Of these, TGFpl and MIP-1a have been the best characterized, and were shown to decrease the proportion of primitive hematopoietic progenitors that are cycling, both in vitro and 19 in vivo (Cashman et al., 1992; Eaves et al., 1991; Hatzfeld et al., 1991; Jacobsen et al., 1994; Lord et al., 1992). The responses of hematopoietic cells to the external cues provided by their microenvironment thus depends on their repertoire of expressed HGF receptor types and other membrane-bound regulators, such as adhesion molecules. Upon activation, these receptors transmit signals (e.g. proliferative/inhibitory or survival), through signal transduction processes, which initiate a myriad of cellular responses including the activation of nuclear transcriptional regulators. Nuclear factors in turn activate multiple effector genes that elicit the appropriate cellular responses directed by the external stimulus. Clear connections have now been established between external factors and regulation of many nuclear factors involved in proliferation (e.g. c-fos, c-jun, c-myc), survival (c-myc) and mature end cell functions (e.g. STATs) of hematopoietic cells. The roles of external factors in the regulation of nuclear factors involved in lineage-commitment of hematopoietic cells are, however, not clear. Nevertheless, whether regulated by external factors or by some poorly understood intrinsic mechanisms, transcriptional regulators are central to the processes of proliferation, differentiation and survival of hematopoietic cells. The next section will thus focus on the current understanding of the molecules involved in transcriptional regulation of hematopoiesis. 1 .2 .3 .2 R e g u l a t i o n o f h e m a t o p o i e s i s b y i n t r a c e l l u l a r f a c t o r s Appropriate transcriptional regulation of a given eukaryotic gene depends on contributions from a variety of factors. RNA polymerase II (pol II) is the enzyme responsible for transcription of the messenger RNA. For pol II to bind DNA and initiate transcription it must be part of a multimeric protein complex called the basal transcription machinery, which in addition to pol II contains general transcription factors called, TFIID, TFIIA, TFIIB, TFIIE, TFIIF, TFIIH and TFIIJ. This complex binds to DNA at the core promoter (usually the TATA (TATAAA) box) which spans the DNA 20 sequence from -35 to +35 relative to the transcription start site (for review see Ernst and Smale, 1995). The ability of pol II to initiate transcription at a defined frequency is influenced by DNA-binding proteins called transcription factors. These factors bind to DNA at multiple regulatory regions, some of which are close to the basal transcription machinery (promoters) while others can be very distant (enhancers), and can mediate either transcriptional activation or repression. These proteins modulate pol II activity by directly interacting with the basal transcription machinery, and their interactions can, in turn, be altered either by the presence of other transcription factors bound to nearby DNA at the same time or by interactions between these transcription factors (for review see Ernst and Smale, 1995). Although the main focus in studies involving transcription regulation has been the transcription factors themselves, growing evidence indicates that other regulatory mechanisms involving chromatin structure and specialized chromatin elements, also play a crucial role (Ernst and Smale, 1995; Grunstein, 1990; Peterson and Tamkun, 1995; Tamkun, 1995). In eukaryotes DNA is packed with proteins into a chromatin structure which can present a barrier against binding to DNA of both the basal transcription machinery and transcription factors. In S. cerevisiae, modulations of chromatin proteins (histones) have been shown to result in alterations in transcription (Felsenfeld, 1992). The most convincing evidence for a dynamic role of chromatin in the regulation of gene expression has come from genetic and biochemical studies in Drosophila and S. cerevisiae. These studies have identified multiprotein complexes that can alter chromatin structure and either enhance (inS. cerevisiae the SWI-SNF complex, in Drosophila the trithorax group) or decrease transcriptional activity (in Drosophila the Polycomb group) (Peterson and Tamkun, 1995; Tamkun, 1995). In Drosophila and mice, both the trithorax and Polycomb group proteins have been implicated as important transcriptional regulators of the Hox homeobox genes (see for more detail in section 1.3.5). DNA cis-acting regions such as silencer and locus control regions have also been shown to be involved in transcriptional regulation which, at 21 least in some cases, can be mediated through alteration in chromatin structure (Felsenfeld, 1992; Sawada et al., 1994; Siu et al., 1994) Transcription factors have been grouped into families according to the structure of their DNA-binding domain. Several classes of DNA binding domains have been described including the zinc finger, basic leucine zipper, basic helix-loop-helix (bHLH), helix-turn-helix (HTH), paired, runt homology and many other domains (Ernst and Smale, 1995). Some of these broad classes have been further subdivided. For example the majority of proteins in the HTH family constitute a subfamily of homeodomain proteins, which can be further divided into multiple subgroups based on differences in their homeodomains and on additional conserved motifs within these proteins (see below in 1.3.1) (DeRobertis, 1994). In addition to the DNA binding domain, transcriptional activators contain a "transcriptional activation domain" which can interact with the basal transcriptional machinery. Transcriptional repressors, conversely, can act either by steric hindrance mechanisms or by active inhibitory mechanisms through protein-protein interaction with the basal transcription machinery or with transcriptional activators (Hanna-Rose and Hansen, 1996). To date a number of transcription factors have been identified that play critical (non-redundant) roles in hematopoietic cell lineage commitment, proliferation and survival. The majority of these factors were originally identified either by their aberrant expression in leukemias (Nichols and Nimer, 1992), or by their binding to cis-regulatory DNA sequences of lineage-specific genes (Georgopoulos et al., 1992; Tsai et al., 1989). Of these factors, none shows absolute specificity for hematopoietic tissue (Shivdasani and Orkin, 1996). Thus, like in other differentiation systems, transcription factors appear to regulate specific gene expression during hematopoietic development by their combinatorial actions. The functional roles of these transcription factors in hematopoiesis have mainly been investigated through their inactivation by gene targeting methods in embryonic 22 stem cells, and in some cases by their forced overexpression. Several of these transcription factors are essential for normal hematopoietic development, as their absence can lead to a block in normal hematopoietic development. Depicted in figure 1.3 are the positions of essential function for selected transcription factors involved in hematopoietic development, as suggested from the above mentioned studies. Some of these factors like the bHLH domain-containing tal-1/SCL and the LIM domain-containing rbtn2/LM02 proteins must act very early in hematopoietic development or be essential at later time points in multible lineages, as mice lacking these factors lack all lineages of both primitive and definitive hematopoiesis (Porcher et al., 1996; Shivdasani and Orkin, 1996; Warren et al., 1994). The tal-1/SCL and rbtn2/LM02 proteins have been shown to interact in vivo and form a heterocomplex (Valge-Archer et al., 1994; Wadman et al., 1994), which together with the similarities in the phenotypes of the tal-1/SCL and the rbtn2/LM02 null mice suggest, that they might act together in transcriptional regulation. Both of these early acting transcription factors are also expressed at later stages of hematopoietic development, however, their functional role at these later stages could not be evaluated using this experimental system. None of the target genes regulated by either tal-1/SCL or rbtn2/LM02 have been identified to date. 23 HSC ® GATA-1 EKLF tal-1/SCL rbtn2/LM02 (primitive) ® Erythrocytes Nf-E2 Tcell B cells Figure 1.3 Schematic representation indicating the position of essential function of some transcription factors known to be active in hematopoiesis Positioning of each gene product is based on the earliest block observed in hematopoieis resulting from its absence. The various transcription factors are shown in italics. Adapted from (Shivdasani and Orkin, 1996). Other transcription factors whose inactivation also affects multiple hematopoietic lineages include the AML-1, GATA-2, PU.1, c-myb and Ikaros proteins. A possible role for the runt homology domain AML-1 in the initiation of definitive hematopoiesis has been suggested, as mice lacking AML-1 have normal primitive hematopoiesis but completly lack definitive hematopoiesis (Okuda et al., 1996). AML-1 has been implicated in the regulation of a number of both myeloid and lymphoid-specific genes (e.g. TCR, GM-CSF, M-CSF and IL-3), as the AML-1 core DNA binding motive has been shown to be essential for tissue specific expression of those genes (Shivdasani and Orkin, 1996). Absence of the zinc finger protein GATA-2 appears to impair the proliferation of the HSC rather than its differentiation (Tsai et al., 1994). This is evidenced by greatly reduced ability (as opposed to complete absence) of GATA-2-I-cells to contribute to all hematopoietic lineages. Mice that lack functional c-myb protein have a phenotype similar to that of the GATA-2-/- mice, with the exception of the megakaryocyte lineage which appears to develop normally in the absence of c-myb (Mucenski et al., 1991). Both GATA-2 and c-myb are normally expressed in primitive hematopoietic cells and then downregulated as these cells differentiate (Thompson 24 and Ramsay, 1995; Yamomoto et al., 1990). In contrast to the inactivation of GATA-2 and c-myb, their forced overexpression in progenitor cells [c-myb in myeloid and GATA-2 in erythroid progenitor cells), promotes their proliferation and blocks differentiation (Briegel et al., 1993; Yamomoto et al., 1990). Mice null mutant for the ets family member PU. 1 lack cells of the granulocytic, monocytic and B cell lineages - the cells in which PU.1 is normally expressed (McKercher et al., 1996). Interestingly, a separate line of PU. 1 knockout mice, derived using a different targeting vector, has a more severe phenotype with additional defects in cells of the T and erythroid lineages (Scott et al., 1994). In monocytic and granulocytic differentiation a number of PU.1 target genes have been identified, and based on the known functional roles of these targets it appears that PU. 1 is crucial for terminal rather than early monocytic and granulocytic differentiation (Olson et al., 1995). The Ikaros gene gives rise to six different proteins, by means of differential splicing, that are differentially expressed in lymphoid cells wherein they are thought to regulate the expression of a number of lineage specific genes (Georgopoulos et al., 1992). Mice lacking Ikaros function display a complete absence of all cells of the lymphoid lineages (T, B and NK), whereas both progenitor and mature cells of the myeloid and erythroid lineages were increased (Georgopoulos et al., 1994). Based on these results it has been proposed that Ikaros acts as a developmental switch in HSCs, with Ikaros expression driving lymphoid rather than myeloid differetiation. Other factors appear to be more lineage-specific in action such as GATA-1, EKLF, NF-E2, Pax5 and E2A, as their absence affects only one hematopoietic lineage (Nuez et al., 1995; Perkins et al., 1995; Pevny et al., 1991; Shivdasani et al., 1995b; Urbanek et al., 1994; Weiss et al., 1994; Zhuang et al., 1994) (Figure 1.3). GATA-1 is perhaps the best studied of the transcription factors known to be active in hematopoietic cells. Absence of GATA-1 blocks hematopoietic differentiation at the proerythroblast stage followed by apoptosis of these cells, suggesting that the functional role of GATA-1 is to permit survival and maturation of erythroid progenitor cells by preventing apoptosis 25 (Weiss and Orkin, 1995). A role for GATA-1 in lineage selection has also been suggested because its forced overexpression can reprogram two different myelomonocytic cell lines, one into megakaryocytic differentiation (Visvader et al., 1992), and the other into megakaryocytic, erythrocytic or eosinophilic differentiation (Kulessa et al., 1995). Remarkably, in the latter case the intracellular level of GATA-1 affected the differentiation outcome (high megakaryocytic vs. low eosinophilic). The ability of one transcription factor to reprogram the lineage commitments of hematopoietic cells lines, has suggested the existence of crosstalk between the regulatory networks involved in lineage choice. This idea is further supported by recent evidence that the transcription factor mafB (a member of the AP-1 superfamily), whose expression is restricted to myelomonocytic cells, can inhibit erythroid differentiation by interacting with Ets-1 and thereby repressing Ets-1 mediated gene activation of erythroid specific genes (Sieweke et al., 1996). In principle, mafB and GATA-1 may thus serve two complementary functions: GATA-1 would maintain a megakaryocytic/eosinophilic/erythroid phenotype and suppress a myeloid phenotype, whereas mafB would maintain a myeloid phenotype and suppress the erythroid phenotype. The phenotype of the Ikaros null mice would also suggest the existence of such lineage crosstalk, however, reprogramming of primitive cells with myeloid/erythroid potential to the lymphoid lineage by forced expression of Ikaros has not been demonstrated. Our current knowledge of the transcription factors that are involved in regulation of hematopoiesis is far from complete. Several lines of evidence are now suggesting that factors that are highly conserved through evolution and play an important role in embryonic development could also be active in adult tissues with continuing developmental potential (Krumlauf, 1994; Orkin, 1996). Among such genes are the homeodomain proteins, including the 39 members of the Hox homeobox family. The possible functional role for these genes in hematopoiesis had only recently come under scrutiny at the time this thesis work was initiated. These initial studies had 26 revealed that many of these genes are expressed in hematopoietic cells lines and primitive subpopulations of normal hematopoietic cells (review in Lawrence et al., 1996). Such findings, and added evidence of function, prompted the work in this thesis to further resolve Hox gene roles in hematopoiesis. 1.3 Homeodomain-Containing Proteins 1.3.1 C l a s s i f i c a t i o n , c h r o m o s o m a l o r g a n i z a t i o n a n d e v o l u t i o n o f t h e h o m e o b o x g e n e s The term homeobox gene arose from an earlier genetic term, the homeotic mutation, described some 102 years ago by Drosophila geneticists to qualify morphological variations transforming something "... into the likeness of something else" (Bateson, 1894), as exemplified by generation of a leg in place of the antennae and halteres instead of wings (Lawrence and Morata, 1994). Many years later (1978), Lewis identified a homeotic gene complex in Drosophila, called Bithorax, that determined the development of middle and posterior body parts of the fruit fly (Lewis, 1978). Shortly thereafter another homeotic complex, Antennapedia, was similarly found to control segmental development of the more anterior structure of the fruit fly (Kaufman, 1983). These complexes were both located on Drosophila chromosome 3, and by "chromosomal walking" on this chromosome the first homeotic gene called Antennapedia (Antp) was cloned (Garber et al., 1983). Using the cDNA of the Antp gene, other homeotic genes of the Antennapedia and Bithorax complexes were found to cross-hybridize to a highly-conserved repetitive sequence located at the 3' end of the Antp gene (McGinnis et al., 1984). Subsequently, this sequence was found to encode for a 60 amino acid domain that was given the name homeodomain. In addition to being conserved among Drosophila homeotic genes, this domain was also found to be highly conserved among metazoans, including vertebrates (McGinnis et al., 1984; McGinnis et al., 1984; Scott and Weiner, 1984). The homeodomain forms a helix-turn-helix DNA binding domain, and the homeoproteins are found to act as transcription factors (Hoey and Levine, 1988; Levine and Hoey, 1988; Thali et al., 27 1988). With this new knowledge about homeotic genes, the concept of determination during embryonic development became less abstract and more molecular, and suggested that the principles of genetic control of development, with some variations, could be applied to all multicellular organisms (Lawrence and Morata, 1994). Homeobox genes in insects and vertebrates can be divided into two broad categories: those that belong to the homeotic complex and are related to the Antp gene of Drosophila, and those which do not cause homeotic transformation and contain a divergent homeodomain. The latter group contains numerous genes that have been classified into more than 12 different classes and families including: Pax homeodomain genes, which contain a so-called paired box domain and homeodomain (Stuart and Gruss, 1996); POU domain genes that are defined by the presence of a bipartite DNA binding domain consisting of a 70 amino acid POU domain and a 60 amino acid POU-specific homeodomain and include members like the ubiquitously expressed Oct-1 protein and the pituitary-specific factor Pit-1, responsible for murine dwarfism (Rosenfeld, 1991); L//W-homeodomain genes, which contain two tandemly arranged cystein-rich LIM motifs and include the Lmx1 gene that patterns the dorsal-ventral axis of developing vertebrate limbs (Sanchez-Garcia and Rabbits, 1994) ; Msx family of genes that are expressed at numerous sites in the developing mouse embryo and which are essential for teeth formation and for normal craniofacial bone development (Davidson, 1995); the exd/Pbx family originally identified for the involvement of one of its members (PBX1) in a translocation associated with human leukemia (Kamps et al., 1990) and later for their role as Hox gene co-factors (see below) (Chan and Mann, 1996; Chang et al., 1996); and the Hlx class identified by their expression in hematopoietic cells (Deguchi et al., 1991). In the fruit fly there are 8 homeotic homeobox genes, found in two separate clusters -the Antennapedia (ANT-C) and Bithorax (BX-C) complexes- that together form a larger complex termed Homeotic complex (HOM-C) (Figure 1.4) (Lawrence and Morata, 1994). To date 39 Antp like genes have been identified in vertebrates and they 28 are collectively called Hox genes (Krumlauf, 1994). These genes are organized in 4 clusters, called A, B, C and D, found on separate chromosomes, each containing 9-11 different Hox genes (Figure 1.4). The genomic structure and organization of Hox and HOM-C share many intriguing similarities. In addition to organization into clusters, the relative chromosomal position of HOM-C/Hox genes is highly conserved, suggesting that the Hox clusters arose by duplication of a common ancestral cluster, which is as yet, unidentified (Krumlauf, 1994). Drosophila, HOM-C Bithorax Abd-B Abd-A Ubx Antennapedia Antp Scr Dfd pb lab v/^j-aa—no-Vertebrates, Hox HoxA -|JT3 mouse Hoxa-13 human HOXA13 HoxB -fb"i3 Paralogous 13 12 11 10 groups Late Low RA response Posterior Early High RA response Anterior Mouse embryo Figure 1.4. Alignment of vertebrate Hox complexes into paralogous groups and comparison with Drosophila HOM-C. The Drosophila HOM-C is at the top, and the hashed marks between Antp and Ubx indicate the junction where the ANT-C and BX-C split. The 13 paralogous groups are labeled at the bottom. Examples of current nomenclature for mouse and human Hox genes is given for the paralogous group gene 1 and 13 in the Hox A cluster. In places where no gene is found, a gap is left. Shown is the vertical homology relationship that exists between the Drosophila HOM-C and paralogous group Hox genes: group 1, labial (lab); group 2, proboscipedia (pb); group 4, Deformed (Dfd); group 5, Sex combs reduced (Scr), and groups 9-13, Abdominal B (Abd-B). At the sequence level, the homology between group 5 genes and Scr is not strong, but there are strong functional similarities that are the bases for this assignment. Antennapedia (Antp), Ultrabithorax, (Ubx) and Abdominal-B (Abd-B) genes have no vertebrate homologues. At the bottom is indicated the anterior-posterior, temporal and RA response colinearity in Hox gene expression and the expression boundaries of selected HoxB cluster genes along the A-P axis of an early mouse embryo. Adapted from, Krumlauf, 1994. 29 Figure 1.4 summarizes the chromosomal organization and homology relationships between the four vertebrate Hox clusters and HOM-C genes. Alignments of genes are made on the basis of multiple domains of sequence identity, in addition to the homeodomain itself, and on the relative position of the genes within each complex. There are 13 different sets of genes with shared properties, and they are termed paralogous groups 1 to 13 (Boncinelli et al., 1989). The nomenclature of Hox genes was initially confusing since each gene was labeled according to its discovery. A new nomenclature based on the name of the cluster and the paralogous group that a particular gene is in, was coined at the third homeobox workshop (Scott, 1992). The old and new nomenclature of each Hox gene has been reported elsewhere (Scott, 1992) and only the new nomenclature is shown in Figure 1.4 and used in this thesis. The names of human Hox genes are written with upper case letters, to be distinguishable from their murine homologs, which are represented by lower case letters (Figure 1.4). Homeotic genes have also been extensively studied in other animals such as nematodes (C. eleganse) (Salser and Kenyon, 1994) and annelides (Leech) (Shankland, 1994) where they are found to share both functional and structural similarities with those of vertebrates and insects. Throughout this thesis homeotic genes will thus collectively be termed Hox genes. 1.3 .2 I m p o r t a n t c o n s e r v e d p r o p e r t i e s o f Hox g e n e s . A distinguishing hallmark of the Hox complexes both in Drosophila and vertebrates is the correlation between the physical order of genes along the chromosomes and their expression/function along the anterior-posterior (A-P) axis of the embryo. This property has been referred to as colinearity (Krumlauf, 1994). There is a spatial colinearity, which refers to the ordered array of spatially restricted expression domains along the A-P axis in embryonic tissue such as paraaxial mesoderm, surface ectoderm, neural tube and hindbrain segments (Burke et al., 1995; Gaunt, 1991; Graham et al., 1989; 30 Kessel and Gruss, 1991; Wilkinson et al., 1989). In vertebrates there is also a temporal colinearity based upon the time of appearance of expression during embryogenesis (Dekker et al., 1993; Izpisua-Belmonte et al., 1991). Retinoic acid is known to induce vertebrate Hox gene expression, and there is a colinear sensitivity in the level and time of responses of Hox genes to retinoic acid (Dekker et al., 1993). When these properties are all related to the clustered organization of the Hox complexes, the genes located at the extreme 3' end of each cluster ( i.e. paralog group 1 genes) are activated earliest, have the most anterior boundary of expression and display the highest sensitivity to retinoic acid (Figure 1.4). Paralog group genes with more 5' locations show progressively later expression, a more posterior location of their anterior expression boundaries, and reduced response to retinoic acid. The conservation of this colinearity of Hox genes in evolution and in diverse vertebrate embryonic structures suggests that it is an important component for their function. The mechanisms underlying these expression patterns, however, remain poorly understood. Another conserved property of Hox genes is an apparent functional hierarchy. This phenomenon was first discovered in Drosophila, where more posteriorly expressed genes appeared to suppress the effects of more anteriorly expressed genes. Initially this effect, termed "phenotypic suppression", was thought to be caused by transcriptional regulation, as more posterior gene products can repress transcription of more anteriorly expressed genes (Struhl and White, 1985). However, this effect was later found to be postranscriptional, as Hox proteins expressed at high levels and ubiquitously (under the control of a heat shock promoter) only altered Drosophila development of body parts anterior to their normal expression domains (Gonzales-Reyes and Morata, 1990; Mann and Hogness, 1990). In vertebrates, a similar functional hierarchy is observed, where posteriorly expressed genes suppress the effects of Hox genes expressed more anteriorly, and is called "posterior prevalence". Evidence for existence of posterior prevalence in mice comes from two types of 31 studies: gain-of-function studies where ectopically expressed Hox genes predominantly gave phenotypes anterior to their normal endogenous expression domain, and loss-of-function studies which gave phenotypes only in a rostral part of the endogeneous expression domain of the inactivated gene (Duboule and Morata, 1994; Krumlauf, 1994). 1.3.3 Hox g e n e s a n d e m b r y o n i c d e v e l o p m e n t The Hox genes have been most extensively studied during embryogenesis, where they determine cell identity and pattern formation of a variety of structures such as the anterior-posterior segmentation of early embryos; the limbs (wings, legs), the skeleton and the nervous system (Krumlauf, 1993; Krumlauf, 1994; Lawrence and Morata, 1994; Tabin, 1995). Different animals initiate their embryonic development leading to anterior-posterior segmentation, in a diverse manner (Kenyon, 1994). No matter how they initiate development, as these embryos establish their body plans and begin to undergo morphogenesis a conserved Hox expression pattern appears along their anterior-posterior (A-P) axis, that parallels their relative 3' to 5' location in the Hox clusters (Figure 1.4). Because this Hox expression is position specific, one might imagine that this pattern would be established by localized cell-extrinsic signals. In Drosophila that is essentially what happens (Lawrence and Morata, 1994; St Johnston and Nusslein-Volhard, 1992). A molecular cascade initiated by a homeodomain transcription factor, bicoid, leads to localized expression pattern of the gap (e.g. hunchback, orthodental, giant, Kruppel and knirps) and pair-rule (e.g. hairy, paired and even-skipped) segmentation genes along the A-P axis. It is this expression pattern that allocates cells to the 14 A-P parasegments and activates different Hox genes in a 3' to 5' manner along the A-P axis. Like in flies the Hox gene expression in vertebrates is strongly influenced by the position of a cell within the embryo (Lawrence and Morata, 1994; Riddle et al., 1993). However, vertebrates do not appear to have homologs of many 32 Drosophila gap and pair rule genes, suggesting that different molecules are involved in setting up the initial Hox expression pattern in vertebrates (Kenyon, 1994). Interestingly, in leeches and C. elegans the initiation of Hox expression patterns along the A-P axis appears not to be position dependent, but rather determined by temporal-and lineage-specific control systems, respectively (Cowing and Kenyon, 1996; Nardelli-Haefliger et al., 1994). After parasegment formation in the Drosophila embryo, the control of Hox genes within individual segments is turned over to the segment polarity genes, which consist of the hedgehog (hh) and wingless [wg) signalling system, and decapentaplegic (dpp) which is a member of the transforming growth factor f5 (TGFft) superfamily of genes (Lawrence et al., 1996; Lawrence and Morata, 1994; Perrimon, 1994). The products of the hh, wg and the dpp genes are secreted proteins that play key roles in instructing cells about their fate within the parasegments and their functional role appears to be more widely conserved than that of the gap and pair rule genes (Patel et al., 1989; Shankland, 1994; Tear et al., 1990). A number of vertebrate homologs of these secreted proteins have been identified including, Sonic hedgehog (Shh), Desert hedgehog (Dhh) and Indian hedgehog (ihh), which are the homologs of hh (Ingham, 1995); the members of the Wnt gene family (to date 14 members have been identified) which are the homologs of wg (Nusse and Varmus, 1992); and the bone morphogenetic proteins (BMPs) which comprise a large family (>20 members) and are the homologs of dpp (Hogan, 1996). These secreted proteins are expressed in many vertebrate embryonic tissues, and a number of recent studies have indicated that these proteins are the extracellular signals central to the formation and organization of a number of embryonic tissues (Chiang et al., 1996; Concordet and Ingham, 1995; Fietz et al., 1994; Hogan, 1996; McMahon and Bradley, 1990; Parr and McMahon, 1995; Stark et al., 1994). In vertebrates, direct interactions betwen Hox genes and these secreted proteins have not, yet, been demonstrated. However, in vertebrate limb development Hox genes have been shown to act in pathways that are both upstream 33 and downstream of SHH signalling, which is essential for limb development (Charite et al., 1994; Cohn and Tickle, 1996; Niswander et al., 1994; Riddle et al., 1993). In the last decade, the field of embryology has taken a major leap with the identification of many of the molecules and the principles applied in the formation of a variety of embryonic structures both in insects and vertebrates. These studies are indicating that many of these molecules as well as the principles applied, are shared, with some variations, both between different animals and different embryonic structures in the same species. Thus, for example, the known molecular steps in wing development in the fruit fly show a striking resemblance to those involved in vertebrate limb development (Lawrence and Struhl, 1996; Riddle, 1995; Vogel et al., 1995). Central to this genetic machinery are secreted molecules of the hedgehog, Wnt and BMP families which act as morphogens, as well as transcription factors such as Hox and others, many of which contain homeodomains, which give sets of cells their genetic addresses to determine their fates and responses to secreted morphogens (Lawrence and Struhl, 1996). 1.3 .4 T h e s t r u c t u r e a n d f u n c t i o n a l s p e c i f i c i t y o f H o x p r o t e i n s In vitro DNA binding studies have shown that the homeodomain and its flanking sequences are essential for DNA binding and likely therefore the transcriptional activity of Hox proteins (Desplan et al., 1988; Hoey and Levine, 1988). The crystal structure of three homeodomains (from engrailed, MATa2 and Antp ) all complexed with DNA, have been characterized (Billeter et al., 1993; Kissinger et al., 1990; Wolberger et al., 1991). All three structures were very similar to each other and showed that the homeodomain consists of three a-helices and an N-terminal arm that is without secondary structure (Figure 1.5). The first and the second helices are separated by a loop, and the second and third helices, together with a 4 amino-acid turn which separates them, form a motif similar to the helix-turn-helix structure found in prokaryotic transcriptional regulators. From in 34 vitro DNA binding studies Hox proteins have been shown to bind to a very similar 'core' DNA sequence (Figure 1.5) (Ekker et al., 1994; Ekker et al., 1991; Kalionis and O'Farrell, 1993). The homeodomain crystal structure studies showed that the homeodomain contact with this "core" DNA sequence is mediated both by the third a -helix, which sits in the major groove of DNA, and the N-terminal arm that makes base specific contact in the minor groove (Figure 1.5). COOH N-arm a helix 1 a helix 2 a helix 3 I I T-N A-T-G/T-G/A 5 , Hox DNA "core" binding sequence 3 T- G - A -T - N -N A-T- G/T- G/A (1) (2) (3) (4) (5) (6) Hox/pbx(exd) DNA "core" (7) (8) (9) (io) minor groove binding sequence major groove Figure 1.5. Schematic representation of Hox protein domain organization and their DNA binding sequences. Positions of conserved domains shared by all Hox proteins in the same paralogous group (shaded) or by subsets of Hox genes (striped) are indicated. The structure of the homeodomain is shown and its contacts to DNA. "Core" DNA sequences are indicated which are preferred by Hox proteins alone or in complexes with pbx/exd. Abbrevations: HP (T), conserved hexapeptide (tryptophan) motif. During embryogenesis Hox proteins act with great biological specificity, presumably by their ability to regulate different sets of target genes. It has thus been one of the central issues in developmental biology to understand how Hox proteins achieve their functional specificity, and yet bind in vitro to a very similar 'core' DNA sequence with similar affinity (Ekker et al., 1994; Ekker et al., 1991; Kalionis and O'Farrell, 1993). Although this mystery has not been fully solved, recent molecular and genetic studies have started to give some answers. In a number of studies, mouse or chicken Hox genes (i.e. Hoxb-1, Hoxb-6, Hoxb-9, Hoxb-4 and Hoxa-5) have been shown to be able to replace the function of their 35 corresponding Drosophila homologs (i.e. lab, Antp, Abd-B, Dfd and Scr, respectively), indicating a functional phylogenic conservation between Hox genes in the same paralogous group (Bachiller et al., 1994; Lutz et al., 1996; Malicki et al., 1990; McGinnis et al., 1990; Zhao et al., 1993). These studies and others involving chimeric or truncated Hox proteins (Chan and Mann, 1993; Furukubo-Tokunaga et al., 1993; Gibson et al., 1990; Lin and McGinnis, 1992; Mann and Hogness, 1990; Phelan et al., 1994; Zeng et al., 1993; Zhao et al., 1996), have shown that Hox genes functional specificity is in large part mediated both by homeodomain and by conserved motif N-terminal to the homeodomain, which in paralogous groups 1-8 is termed the hexapeptide (also pentapeptide or "YPWM") motif (Lutz et al., 1996; Zhao et al., 1993). This hexapeptide motif is not found in proteins from paralogous groups 9-13; however, a distinctive conserved tryptophan-containing motif is found at a similar location in proteins from paralogous groups 9 and 10 (Figure 1.5) (Chang et al., 1996). These studies also indicated that of the sequences in the homeodomain, the N-terminal arm is the most critical in mediating functional specificity. Taken together, these studies showed that Hox in vivo specificity is mediated both by conserved residues that directly bind DNA and also non-DNA binding residues, suggesting that protein-protein interaction could be important in mediating Hox functional specificity. In 1990, a Drosophila gene called extradenticle (exd) was identified by its ability, when mutated, to modify Drosophila Hox in vivo functions without altering the expression patters of Hox proteins (Peifer and Wieschaus, 1990). This suggested that exd could act as an Hox protein co-factor that might play a role mediating Hox specificity. Exd encodes a protein with a divergent homeodomain (see above), that has been highly conserved throughout evolution (Flegel et al., 1993; Rauskolb et al., 1993). In vertebrates the exd homologs are the pbx genes {pbxl, pbx2 and pbx3) which were originally identified from the involvement of the PBX1 gene in the translocation t(1;19), frequently observed in child pre-B acute lymphoblastic leukemia (Kamps et al., 1990; Nourse et al., 1990). Consistent with the co-factor model, 36 Drosophila and vertebrate Hox proteins were found to cooperatively bind DNA in vitro with pbx or exd, and with higher affinity than when in monomeric form (Chan et al., 1994; Chang et al., 1995; Lu et al., 1995; Phelan et al., 1995; Popperl et al., 1995; van Dijk and Murre, 1994). These studies also identified that, in a majority of cases, the hexapeptide or the conserved tryptophan-containing motive of Hox proteins is essential for successful cooperative binding (Chang et al., 1995; Lu et al., 1995; Phelan et al., 1995). From a number of in vitro DNA binding studies (Chan et al., 1996; Chang et al., 1996; Lu and Kamps, 1996) and in vivo functional analysis (Chan et al., 1996; Popperl et al., 1995) the following model has been proposed to describe how interactions between exd/pbx and Hox proteins can contribute to Hox protein functioal specificity. In this model the pbx/exd and Hox homeodomains are oriented as head-to-tail heterodimers with the centers of their binding sites only 4 bp apart. The N-terminal arm of the Hox homeodomains is in the middle of the complex and interacts with a base pair in the minor groove. It is these two bases (5 and 6) (Figure 1.5), that determine which pbx/exd-Hox complexes are capable of binding with high affinity. Interestingly, the binding specificity of the Hox gene complexed with pbx/exd, correlates with its relative position in the Hox cluster; base number 5 is increasingly preferred as G by Hox proteins towards the 3' end of the clusters, as T at the 5' end and as A throughout the middle of the cluster (Chang et al., 1996). The formation of a heterodimer complex between pbx/exd and Hox proteins thus induces stable conformational changes in the Hox protein, which allows different Hox proteins to bind to with high affinity and discriminate between DNA motifs with only 1-2 bp differences. Furthermore, the sequence specificity of a Hox protein monomer can be different from its specificity as heteromer with pbx/exd. Interactions of Hox proteins with pbx/exd co-factors are unlikely to provide all Hox proteins with their functional specificity. Other co-factors are likely to be involved, such as the pbx-like protein Meis-1 which was recently found to be co-activated with Hoxa-7 37 and Hoxa-9 in retroviral insertion-induced murine myeloid leukemias (Nakamura et al., 1996b). Similarily, some studies have indicated that protein-protein interaction between Hox proteins, could also modulate their target selections (Zappavigna et al., 1994). Another level of regulatory control may involve post-translational modifications in response to extracellular signals. Indeed, recently it has been shown that during Drosophila embryonic midgut development, the subcellular localization (nuclear vs. cytoplasmic) of exd is controlled by two secreted proteins, dpp and wg which themselves are thought to act as morphogens that direct the midgut development (Mann and Abu-Shaar, 1996). 1.3 .5 R e g u l a t i o n o f Hox g e n e e x p r e s s i o n The molecular mechanisms that regulate Hox gene expression are poorly defined. The striking feature of Hox genes is the conservation of their cluster organization and the relative position of each gene within the clusters. This, combined with the colinearity in their expression, suggests that their proper regulation is dependent on their position within the cluster (Krumlauf, 1994). In support of this idea it has been shown that different Hox genes can share promoters and that some of their regulatory elements are interspersed in the complex (Simeone et al., 1988; van der Hoeven et al., 1996; Whiting et al., 1991). It was thus surprising to discover that for many Hox genes {Hoxa-4, Hoxa-7, and Hoxb-3) their proper expression pattern could be recapitulated using a small region of their respective complex in transgenic mice (Behringer et al., 1993; Puschel et al., 1990; Sham et al., 1992). However, in many cases (Hoxb-6, Hoxb-7, Hoxd-9 and Hoxd-11), the transgenic expression pattern did not fully reproduce the corresponding endogenous Hox expression (Eid et al., 1993; Gerard et al., 1996; van der Hoeven et al., 1996; Vogels et al., 1993). By using gene transpositions, where some 5' Hoxd genes along with their promoters were relocated within the Hoxd cluster, it was recently shown that Hox genes can be regulated both in a complex-dependent and in a gene-dependent manner (van der Hoeven et al., 1996). The complex-dependent mechanism appears to predominate early in 38 development to coordinate colinear Hox gene expression, but appears to give way to a more gene dependent mechanism later in development (van der Hoeven et al., 1996). The factors that regulate Hox gene expression are still poorly defined. In Drosophila, as described earlier, transcription factors of the gap and pair rule families set up the initial Hox gene expression pattern, but a similar role for vertebrate homologs of these factors has not been established, despite repeated attempts (Kenyon, 1994; Krumlauf, 1994; Lawrence and Morata, 1994). In vertebrates, retinoic acid (RA) is a candidate for regulation of Hox genes (through its nuclear receptors), as Hox genes can respond to RA in a colinear fashion and RA can induce alterations in Hox expression in a wide variety of vertebrates in embryogenesis (Krumlauf, 1994). Furthermore, RA response elements have been found in Hoxa-1, Hoxb-1 and .Hoxd-4 genes, and most significantly, this element is essential for normal Hoxb-1 expression (Langston and Gudas, 1992; Marshall et al., 1994; Ogura and Evans, 1995; Ogura and Evans, 1995; Popperl and Featherstone, 1993). Hox genes have also been shown to regulate their own expression in both auto-and a cross-regulatory manner (Bienz, 1994; Chouinard and Kaufman, 1991; Popperl et al., 1995; Zappavigna et al., 1994). During development of the hindbrain into segmented structures called rhombomeres, Hox genes are thought to play roles both in rhombomere segmentation and in segment identification (Lumsden and Krumlauf, 1996). There, in addition to RA, two candidate genes have been identified as regulators of Hox gene expression: kreisler, a b-Zip member of the c-maf proto-oncogene family (Frohman et al., 1993; McKay et al., 1994) and Krox-20, a zinc finger gene which directly regulates the activity of both Hoxa-2 and Hoxb-2 (Nonchev et al., 1996; Sham et al., 1993). However, in human hematopoietic cell lines with erythroid-megakaryocytic potential, HOXB2 is apparently not regulated by Krox-20, but here another zinc finger transcription factor, GATA-1, has been implicated in its regulation (Vieille-Grosjean and Huber, 1995). 39 The gap and pair rule genes that set up the Hox expression pattern along the A-P axis in Drosophila are only expressed transiently and thus distinct regulators are needed to maintain the Hox gene expression pattern. In Drosophila, two groups of genes of high genetic complexity have been described as such regulators: the Poly comb-group (Pc-G) and the trithorax-group {trx-G) (Simon, 1995). The Pc-G proteins form multimeric complexes that maintain transcriptional repression of Hox genes initially turned off, presumably by induction of heterochromatin formation (Jurgens, 1985; Landecker et al., 1994; Orlando and Paro, 1995; Simon, 1995). In contrast, the trx-G proteins, which also form multimeric complexes, maintain the expression of Hox genes initially turned on, by promoting an open chromatin structure by counteracting the repressive effects of chromatin components (Simon, 1995; Tamkun, 1995). Genetic studies in Drosophila have indicated that Pc-G and trx-G proteins are not only dedicated to Hox gene regulation, but rather are general transcriptional regulators (Orlando and Paro, 1995). In vertebrates, genes that are homologues of some of the Pc-G and trx-G genes have been identified. The Pc-G homologue bmi-1, first identified as an oncogene inducing murine T-cell lymphomas (Haupt et al., 1991; van Lohuizen et al., 1991), has both been overexpressed and inactivated in transgenic mice (Alkema et al., 1995; van der Lugt et al., 1994). The effects of these manipulations indicated that bmi-1 does negatively regulate Hox gene expression in vertebrates and interestingly severe hematopoietic defects were observed in the bmi-1 null mutant mice. In vertebrates bmi-1 might have an additional role, as its oncogenic potential does not appear to correlate with its transcriptional suppression activity (Cohen et al., 1996). Mel-18, another vertebrate Pc-G homologue has also been shown to negatively regulate transcription, but in contrast to bmi-1 has a tumor suppressor activity (Kanno et al., 1995). Several vertebrate homologs of the trx-G genes have also been identified, one of which is the MLL [HRX or ALL-1) gene originally identified for its rearrangement in human acute leukemias (Gu et al., 1992; Tkachuk et al., 1992). Inactivation of the Mil 40 gene in mice causes embryonic lethality and absence of Hox gene expression, and in Mil heterozygous mice the expression boundaries of Hox genes were shifted posteriorly, consistent with a role for M//as a positive regulator of Hox gene expression (Yu et al., 1995). The heterozygous Mil mice were documented to have some defects in erythroid and B lymphoid cells, which were however not fully characterized (Yu et al., 1995). 1.3 .6 T a r g e t g e n e s r e g u l a t e d b y H o x p r o t e i n s The identification of Hox target genes is fundamental for our understanding of how Hox genes function in regulating cell identity. However, this field of Hox research has not yet been very fruitful. In Drosophila, the best characterized targets are the Hox genes themselves (Bienz, 1994; Chouinard and Kaufman, 1991; Zheng et al., 1994) and this can be mediated by both direct and indirect pathways. The indirect pathways are frequently associated with the secreted proteins dpp and wg (see above), that can both mediate auto- (Ubx and lab) and cross-(abdA -> Ubx and Ubx -> lab) regulatory signals (Bienz, 1994). The dpp- and wg- mediated auto-regulation of lab in the developing midgut was recently shown to involve the translocation of the Hox co-factor exd from the cytoplasm into the nucleus (Mann and Abu-Shaar, 1996). Interestingly, Hox proteins can also regulate dpp and wg expression. The Ubx protein has been shown to bind to dpp cis-regulatory elements and directly activate dpp expression in the developing midgut (Capovilla et al., 1994); for full dpp activation the exd protein was also necessary (Sun et al., 1995). The abd-A protein has also been implicated as a direct regulator of dpp, by suppressing its expression (Capovilla et al., 1994). Wg expression has also been shown to be regulated by Hox proteins, but these interactions are thought to be indirect (Bienz, 1994). In vertebrates, interactions of Hox genes with members of the BMP and the Writ families (dpp and wg homologs) have not been well documented although the ability of a single homeodomain binding site in the regulatory region of the Wnt-1 gene to 41 spatially restrict Wnt-1 expression in the developing brain has been reported (Her et al., 1995). Vertebrate Hox genes have also been shown to auto- and cross-regulate each other (Popperl et al., 1995; Zappavigha et al., 1994) and because of the conservation of BMP and Wnt members in evolution (see above), it is likely that vertebrate Hox genes could also interact both directly and indirectly with the members of these two families. Other known Hox targets in Drosophila, also have a function in cell-cell interaction. These including the connectine gene (Gould and White, 1992), which encodes a cell adhesion molecule involved in the innervation of muscles (Nose et al., 1992) and the scabrous gene, which appears to produce a secreted protein involved in cellular communication during neurogenesis (Graba et al., 1992). Likely, vertebrate Hox targets also include those involved in cell-cell interaction. The mouse homologue of the Drosophila tumor suppressor gene l(2)gl, which shows homology to the cadherin family of cell-adhesion molecules, is controlled in vivo by Hoxc-8 (Tomotsune et al., 1993). Others, such as Hoxb-8, Hoxb-9 and Hoxc-6 have been shown to bind to the promoter of the gene encoding the neural cell adhesion molecule (N-CAM) and to modulate its expression (Hirsch et al., 1990; Hirsch et al., 1991; Jones et al., 1993; Jones et al., 1992) and the Hoxd-9 was found to bind to the L-CAM enhancer (Goomer et al., 1994). Most recently the first vertebrate cytokine regulated by Hox proteins was identified where constitutive expression of HOXB7 in human melanoma cells was shown to directly activate basic fibroblast growth factor (FGF) (Care et al., 1996). Null mutant mice have been generated for many Hox genes. An interesting feature of the phenotypes of these mutants is that they are less severe than that of the Drosophila null mutants, and that what would be predicted based on their expression patterns (Krumlauf, 1994). This has tentatively been explained with the argument that paralogous genes (which can be up to four) (Figure 1.1.) may compensate for the loss of one gene, due to both the highly similar structures and expression patterns of paralogous genes. To analyze possible interactions between paralogous genes, 42 double Hox null mutant mice have been generated for paralogous groups 3 and 4 (Condie and Capecchi, 1994; Horan et al., 1995). The phenotypes of these mice indicate that paralogous genes function cooperatively in specifying cell identity along the axial skeleton, and with functional redundancy. Whether this functional redundancy reflects their regulation of the same target genes or their action through parallel pathways is, however, not clear. 1.4 Hox Genes and Hematopoiesis 1.4.1 Hox g e n e e x p r e s s i o n i n h u m a n a n d m u r i n e h e m a t o p o i e t i c c e l l l i n e s a n d h u m a n l e u k e m i a s Numerous studies have now shown that, in addition to embryonic tissue, Hox genes are expressed in mammalian adult tissues, both normal and neoplastic. These include normal and neoplastic skin (melanoma) (Bieberich et al., 1992; Care et al., 1996; Scott et al., 1994); normal kidney and renal cell carcinomas (Barba et al., 1993), normal and neoplastic human colonic mucosa (De Vita et al., 1993); normal and neoplastic mammary glands (Friedmann et al., 1994); testis (Watrin and Wolgemuth, 1993); and normal and neoplastic hematopoietic cells (see below). Initial reports of Hox gene expression in blood cells involved the demonstration of expression in immortalized hematopoietic cell lines of both human (Lawrence et al., 1993; Lowney et al., 1991; Magli et al., 1991; Mathews et al., 1991; Shen et al., 1989; Taniguchi et al., 1995; Vieille-Grosjean et al., 1992) and murine origin (Kongsuwan et al., 1988; Lonai et al., 1987). Taken together, these studies showed that the majority of HoxA, HoxB and HoxC cluster genes were expressed in hematopoietic cell lines, but that HoxD cluster gene expression, with a rare exception (Taniguchi et al., 1995), could not be detected in the hematopoietic cell lines surveyed. These studies also showed that many Hox genes were expressed in an apparent lineage-specific fashion, with HoxB and some HoxC cluster genes (11-13) primarily expressed in cell lines with erythroid features and HoxA cluster genes predominantly expressed in cell lines with myeloid features. Therefore the "paralogous" Hox gene expression patterns seen in 43 embryonic development were not obvious in hematopoietic cell lines, but rather that whole clusters (or large regions of a cluster) were turned on or off in a lineage-specific manner. Expression of several of the HoxA, HoxB and HoxC cluster genes could also be detected in cell lines with B- or T-lymphoid potential (Lawrence et al., 1993; Petrini et al., 1992; Vieille-Grosjean et al., 1992), and the expression of the HOXC4 gene appeared to be restricted to lymphoid cell lines (Lawrence et al., 1993). A number of studies have reported the expression of specific Hox genes in certain types of human leukemias. For example, HOXC4 expression is limited to lymphoid leukemias (Celetti et al., 1993; Lawrence et al., 1993), and conversely the HOXA10 gene is strongly expressed in myeloid leukemias but silent in lymphoid leukemias (Lawrence et al., 1995). A block of HOXB genes is also expressed in acute myeloid leukemias, but switched off in chronic myelogenous leukemias (Celetti et al., 1993). 1.4.2 Hox g e n e e x p r e s s i o n i n n o r m a l h e m a t o p o i e t i c c e l l s The expression studies of Hox genes in hematopoietic cell lines were extended to normal human bone marrow cells, where expression of a number of Hox genes was detected using RNase protection, thus indicating that Hox gene expression was not simply a hallmark of transformed cells (Lawrence et al., 1993; Lowney et al., 1991; Mathews et al., 1991). Systematic analyses of Hox gene expression in different hematopoietic lineages and at different stages of hematopoietic differentiation was, however, hampered by the difficulties in obtaining high numbers of "pure", functionally different subpopulations of hematopoietic cells, and by the low levels of expression of Hox genes in normal hematopoietic cells. However, with the recent advances in fractionation of bone marrow cells into functionally distinct population using cell surface specific antibodies and flow cytometry (Lansdorp and Dragowska, 1992), combined with reverse transcription polymerase chain reaction (RT-PCR) procedure, more systematic analyses of Hox gene expression in normal hematopoietic cells became possible. 44 Three laboratories have analyzed the expression of Hox genes in the fraction of human bone marrow cells that expresses the CD34 antigen (CD34+), which contains the majority if not all hematopoietic progenitor cells, including the most primitive hematopoietic cells (Sauvageau et al., 1994). These studies detected expression of 9 of the 11 HOXA genes (Moretti et al., 1994; Sauvageau et al., 1994), 8 of the 9 HOXB genes (Giampaolo et al., 1994; Sauvageau et al., 1994), and 4 of 9 HOXC genes in CD34+ cells, but expression of HOXD genes was not detected (Figure 1.6). In the most detailed of these studies, Sauvageau et al. further analyzed the expression of several of the HOXA and HOXB cluster genes in functionally distinct subpopulations of CD34+ cells i.e. those enriched for CFU-GM and BFU-E progenitors, and more primitive stem cells, the LTC-IC, where two patterns of expression were observed. Specific genes, primarily located at the 5' end of each cluster (e.g. HOXA 10, HOX9), showed essentially invariant expression in all subpopulations, whereas a second group of genes, located towards the 3' side of the clusters (e.g. HOXB3, HOXB4, HOXA4), were expressed at their highest levels in the LTC-IC enriched subpopulation and then sharply downregulated in later cell populations (Sauvageau et al., 1994) (Figure 1.6). This study also suggested that genes located throughout the clusters, were preferentially expressed in primitive bone marrow cells as expression of both HOXB3 and HOXA10 genes were virtually extinguished in the more differentiated CD34" cells (Figure 1.6). Furthermore, the apparent lineage-restricted expression of particular Hox clusters observed in hematopoietic cell lines (i.e. HOXB cluster genes in erythroid cell lines and HOXA cluster genes in myeloid cell lines), could not be detected in this study. Rather there appeared to be "reverse colinearity" in expression, where the majority of Hox genes are expressed in the most primitive hematopoietic cells and then downregulated as cells become progressively more differentiated, with the 3' genes being downregulated earlier than 5' located genes. 45 A 5' HoxA -O HoxB-f HoxC 3' HoxDH Paralogous 13 12 11 10 groups B 8 5 4 3 2 1 HOXA1 25 I r - 2 0 I r—15 o> X CD C/> v> O 3 CD34-Mature cells 5 0 CD34+ I CFU-GM and BFU-E enriched Progenitor cells CD34+ LTC- IC enriched Stem cells Figure 1.6. Expression of Hox genes in human bone marrow cells. (A) Hox genes that are expressed in human hematopoietic cells are shown as black or striped boxes. (B) Graphic representation of the expression of two 3' and one 5' located Hox genes (shown as striped boxes in(A)) in subpopulations of human bone marrow cells that are enriched for functionally distinct cells. The expression levels are shown as relative to actin expression in each subpopulation and the expression of each of the Hox genes in the most primitive subpopulation, the LTC-IC enriched. Much less data exists on the expression pattern of Hox genes in normal murine hematopoietic cells. Expression of a number of Hoxa, Hoxb and Hoxc cluster genes has been demonstrated in the murine yolk sac (Palis et al., 1994). Expression of one of these genes, Hoxb-6, has been analyzed in more detail by RT-PCR, and which showed that it is also expressed in fetal liver, and in embryonic and adult CFU-E and BFU-E progenitor cells, but not in murine stem cell enriched cell fractions (Rich and Zimmermann, 1995). In the stem cell enriched fraction of human bone marrow (LTC-IC), HOXB6 was one of the few Hox genes for which expression could not be detected, suggesting a conservation in Hox gene expression between human and murine hematopoietic cells (Sauvageau et al., 1994). This conservation is further supported by recent unpublished RT-PCR data where the expression of Hoxb-3, Hoxb-4 and Hoxa-10 also appears to be restricted to early cells, as their expression could only be 46 detected in the primitive Sca1+Lin" cells but not in more mature Sca1"Lin+ cells, derived either from murine fetal liver or adult bone marrow cells (Pinneault and Humphries, unpublished). Hox gene expression during the various stages of normal B and T lymphoid development has not been systematically analyzed either in mice or humans. Human HOXC4 expression appears to be restricted to B and T lymphocytes, where it is activated during early to intermediate stages of both T and B cell development (Lawrence et al., 1993). In mature resting B, T and NK lymphocytes Hox genes are apparently not expressed with the exception of HOXB7 in CD8+ T cells (Care et al., 1994; Inamori et al., 1993; Petrini et al., 1992). However, upon mitogenic activation (phytohemaglutinin (PHA) or IL-2/IL1 (3 stimulation) the expression of the HoxB cluster genes is activated both in T cells and NK cells (Care et al., 1994; Petrini et al., 1992; Quaranta et al., 1996). Interestingly, this activation both in T and NK cells appears colinear as in embryonic development, with early activation of 3' located genes and then sequential later activation of 5' genes (Care et al., 1994; Quaranta et al., 1996). Limited information is now also available on the expression patterns of the known Hox co-factors, pbx-1, -2 and -3, in hematopoietic cells. Expression of all three PBX genes has been documented in human fetal thymuses and spleens, and in human adult peripheral blood mononuclear cells, whereas in adult human thymuses PBX-2 and -3 are expressed but not PBX-1 (Monica et al., 1991). Similarly, PBX-1 expression has not been detected in human B and T lymphoid cell lines representing various developmental stages or in cell lines with monocytic potential, in contrast to expression of PBX-2 and -3 in all of these cell lines (Monica et al., 1991). Other divergent homeobox genes are also expressed in hematopoietic cells. These include genes such as HLX (previously called HB24), first identified due to its expression in mitogen-stimulated human B lymphocytes and later in CD34+ human bone marrow cells, but not in more differentiated cells CD34 - (Deguchi and Kehrl, 47 1991; Deguchi et al., 1991) and the HEX gene whose expression appears to be restricted to the hematopoietic system (Bedford et al., 1993). 1 .4 .3 . Hox g e n e f u n c t i o n s i n n o r m a l h e m a t o p o i e t i c c e l l s Direct evidence for Hox gene function in hematopoiesis as a result of modulation of their expression, has only recently become available. Thus for example only four reports (see below) were published when the research presented in this thesis was initiated (Perkins et al., 1990; Shen et al., 1992; Takeshita et al., 1993; Wu et al., 1992). Three studies have described the use of antisense oligonucleotides to down-regulate the mRNA levels of specific Hox genes in normal hematopoietic cells (Care et al., 1994; Takeshita et al., 1993; Wu et al., 1992). Treatment of murine bone marrow with antisense nucleotides against Hoxb-7 lead to four-fold reduction in the formation of CFU-GM, whereas BFU-E and CFU-Mk were unaffected (Wu et al., 1992). Effects of antisense oligonucleotides directed against HOXC6 in human bone marrow were examined, and shown to suppress the formation of CFU-E without affecting either earlier BFU-E or myeloid progenitors (Takeshita et al., 1993). In mature T or NK cells, the HOXB cluster genes are activated in a 3' to 5' manner upon mitogen stimulation (see above), and treatment of these cells with antisense oligonucleotides against HOXB2 or HOXB4 severely inhibits the proliferation of both the T and NK cells, suggesting an important role for Hox genes in proliferation of these cells (Care et al., 1994; Quaranta et al., 1996). In the NK cell study, HOXB gene induction was not observed when cells were treated with cytokines that either stimulated NK cell activation (IL-12) or survival (stem cell factor), further supporting the proliferative role of Hox genes in these cells. Potential drawbacks of the antisense oligonucleotide approach, however, are the uncertainty in the level of mRNA suppression (in some cases only -50%) and the specificity of the oligonucleotides used. 48 Most recently, analyses of possible hematological defects in mice with targeted distributions of specific Hox genes have been initiated (Lawrence et al., 1996). Mice lacking a functional Hoxa-9 gene have defects in both granulocytic and lymphocytic pathways (Lawrence et al., 1996). These mice, which are otherwise healthy and fertile, have reduced numbers of peripheral blood granulocytes and lymphocytes, smaller spleens and thymuses, and reduced numbers of bone marrow myeloid and pre-B progenitor cells. However, their numbers of more primitive hematopoietic cells (i.e. CFU-S and LTC-IC ), were not altered, indicating that absence of the Hoxa-9 affected hematopoiesis primarily at the level of the committed progenitors and not at earlier stages. Expression of specific Hox genes has also been modulated by overexpression, either in hematopoietic cell lines or in normal hematopoietic cells. In the human erythroid cell line K562, overexpression of HOXB6 caused reduction in erythroid features as evidenced by decreased globin synthesis and surface glycophorin expression (Shen et al., 1992). The HOXB7 gene has been overexpressed in the human myelomonocytic HL-60 cell line (Lill et al., 1995). Normally HOXB7 expression is not detected in undifferentiated HL-60 cells; however, upon stimulation that induces monocytic differentiation its expression is readily detected, but not when induced to differentiate into the granulocytic lineage (Lill et al., 1995). Interestingly, overexpression of HOXB7 blocks granulocytic differentiation, whereas monocytic differentiation is unaffected (Lill et al., 1995). In the murine myelomonocytic leukemic cell line WEHI-3B transposition of endogenous retroviral like elements into both the Hoxb-8 and the IL-3 loci results in constitutive expression of both genes (Perkins et al., 1990). Since enforced expression of IL-3 alone does not render normal hematopoietic cells malignant but rather induces myeloproliferation (Wong et al., 1989), it was suggested that concomitant expression of Hoxb-8 with IL-3 might provoke the malignant phenotype of the WEHI-3B cell line. To analyze the possible role of Hoxb-8 in leukemogenesis, Hoxb-8 was overexpressed, either with IL-3 or alone, in murine 49 bone marrow cells using retrovirus-mediated gene transfer (Perkins et al., 1990; Perkins and Cory, 1993). Concomitant overexpression of Hoxb-8 and IL-3 was fully transforming, as mice transplanted with bone marrow cells that had been infected with retrovirus containing both genes developed fulminant polyclonal leukemias ~3 weeks after transplantation (Perkins et al., 1990). Overexpression of Hoxb-8 alone, however, enhanced the self-renewal ability of myeloid progenitor cells as evidenced by their enhanced replating ability and by the generation of non-tumorigenic myeloid cell lines in the presence of high concentrations of IL-3 (Perkins and Cory, 1993). Mice reconstituted with bone marrow cells overexpressing Hoxb-8 were free of leukemia for at least 7 months, but -20% eventually developed myeloid leukemias, indicating that Hoxb-8 alone was not fully leukemogenic. Interestingly, in some cases the leukemias were associated with activation or rearrangement of the IL-3 gene (Perkins and Cory, 1993). As described in the last section, HOXB4 is one of the Hox genes found to be preferentially expressed in the most primitive subpopulation (LTC-IC enriched) of human bone marrow cells. Our group has recently retrovirally overexpressed this gene in murine bone marrow cells (Sauvageau et al., 1995). Serial transplantation studies revealed a greatly enhanced ability of /-/OXS4-transduced bone marrow cells to regenerate the HSC (here CRU) compartment, resulting in 47-fold higher numbers of CRU both in primary and secondary recipients, compared to serially passaged neo-infected control cells. This enhanced regeneration of HOXS4-transduced CRU, brought the CRU pool in primary recipients slightly above normal pretransplantation levels and that of secondary recipients near to normal levels. Myeloid and lymphoid pre-B clonogenic progenitor cells were also increased in recipients of HOXB4-transduced bone marrow cells (5- and 2-fold, respectively) compared to that of control neo mice. However, despite enhanced expansion of both CRU and clonogenic progenitor cells the relative numbers of the various types of in vitro myeloid clonogenic progenitors (CFU-GM, BFU-E and CFU-GEMM) in primary and secondary recipients of 50 H0XB4-transduced bone marrow were the same as in recipients of neo-control cells. In addition, total bone marrow and spleen cellularity of recipients of HOXB4-transduced bone marrow cells, as well as their peripheral blood differential counts, were all within normal range. Thus, despite a marked effect of overexpression of HOXB4 on the number of CRU and myeloid and lymphoid clonogenic progenitors, their was no gross effect on lineage determination nor evidences of consequent expansion of later cell types. Most recently reported during the course of this thesis, Hox genes have been directly implicated in the pathogenesis of human leukemias (Borrow et al., 1996; Nakamura et al., 1996a). In the t(7;11)(p15;p15) translocation, which is recurrently observed in a subset of acute myeloid leukemias and in rare cases of chronic myeloid leukemias, the N-terminal half of the nucleoporin gene NUP98 is found to be fused in frame with most of the coding region of the HOXA9 gene. Intriguingly, murine Hoxa9 and its near neighbor Hoxa7 have also been implicated in myeloid leukemias associated with retroviral insertional activation in the BXH-2 mouse line (Nakamura et al., 1996b). The functions of some of the divergent homeobox genes in hematopoiesis have also been analyzed. Enforced expression of the Hlx/HLX genes could modify the phenotype of several murine and human hematopoietic cell lines (Allen and Adams, 1993; Deguchi et al., 1992), as well as disturb T cell development in transgenic mice (Allen et al., 1995; Deguchi et al., 1993). Although Hlx null mice suffer from severe embryonic anemia, they have no intrinsic defect in their hematopoietic cells, and, rather, the anemia is caused by an inadequate embryonic liver microenvironment (Hentsch et al., 1996). Like PBX1, another divergent homeobox gene, HOX11, was also identified for its involvement in a leukemia associated with a translocation (Hatano et al., 1991). In the t(10;14) translocation associated with human T cell acute lymphoblastic leukemia, the promoter of the T cell receptor 5 gene is juxtaposed to a region upstream of the HOX11 gene resulting in its ectopic expression. The murine 51 H0X11 homolog, Tlx-1, has been overexpressed in murine bone marrow cells by retroviral mediated gene transfer and generated IL-3 dependent immortal myeloid cell lines (Hawley et al., 1994). Interestingly, mice with targeted distribution of the Tlx-1 gene were asplenic (Roberts et al., 1994) which was later found to be caused by apoptotic death of mesoderm-derived spleen progenitors, suggesting that Txl-1's oncogenic potential could be due to enhanced cell survival (Dear et al., 1995). 1.5 Thesis Objectives As reviewed in previous sections, several lines of evidence are now pointing to Hox genes as important regulators of growth and differentiation of hematopoietic cells. The work presented in this thesis was initiated to test the hypothesis that individual Hox genes may play unique roles in regulating of hematopoiesis and to gain further insights into the nature of these roles. The approach taken was influenced by two main observations. The first one was derived from our initial demonstration, that retroviral overexpression of HOXB4 in murine hematopoietic cells can selectively enhance the expansion of primitive cell populations, most profoundly the HSC, which suggested that HOXB4 might be an important natural regulator of HSC regenerative potential. The second was the apparent stage-dependent expression of Hox genes in hematopoietic cells, with some genes like HOXB3 and HOXB4 being preferentially expressed in the most primitive subpopulation of human CD34+ hematopoietic cells, and others likeHOX/4 70 and HOXB9 showing essentially invariant expression in all CD34+ subpopulations with downregulation at later stages of hematopoietic differentiation when cells become CD34". Together these results raised the interesting hypothesis that specific Hox genes play distinctive roles in the regulation of different aspects of hematopoiesis. The first objective of this thesis was to test the hypothesis that different Hox genes might have unique effets on the regulation of hematopoietic cell proliferation and differentiation. The strategy taken was to engineer the overexpression in murine bone 52 marrow cells of previously untested Hox genes. Two such Hox genes were chosen, HOXB3 and HOXA10, based on their divergent expression patterns in hematopoietic cells (see above). Based on the known expression patters of these two genes it was hypothesized that overexpression of HOXB3 would primarily effect the properities of the most primitive hematopoietic cells, similar to HOXB4, whereas overexpression of HOXA10 might effect a broader range of cell types. The subsequent effects of these genetic manipulations on the proliferation and differentiation of various populations of myeloid and lymphoid cells were then analyzed in a transplantation model and various in vitro cultures. The results of these studies are presented in Chapter 3 and 4. The second objective of this thesis work was aimed at delineating further the effects of HOXB4 overexpression on the expansion of HSC. In our initial studies we demonstrated by serial transplantation that HOXS4-transduced bone marrow cells had greatly enhanced potential to regenerate the HSC compartment resulting in ~50-fold higher numbers of HSC in both primary and secondary recipients compared to serially passaged neo-infected cells. The aim of the studies described in this thesis was to determine the long-term effects of overexpression of HOXB4 on the HSC pool in steady state hematopoiesis i.e. whether these cells would continue to expand or become exhausted, or if their expansion would be subjected to environmental control mechanisms. For that purpose, the size and the clonal composition of the regenerated pool of HSCs in mice transplanted with bone marrow cells overexpressing HOXB4 was analyzed at various time points (16 to 52 weeks) after transplantation. The results from these studies are presented in Chapter 5. 53 Chapter 2 Materials and methods 2.1 Generation of Retroviruses and Viral Assays 2.1 .1 R e c o m b i n a n t r e t r o v i r a l v e c t o r s The human HOXB4 cDNA used in the experiments presented in Chapters 3 and 5 was isolated from human fetal liver cells (Piverali et al., 1990); the human HOXA10 cDNA used in experiments presented in Chapter 3 was isolated from the human myeloid cell line ML3 (Lowney et al., 1991); and the HOXB3 cDNA used in experiments presented in Chapter 4 was isolated from a cDNA library generated from CD34+ human bone marrow cells (Sauvageau et al., 1997). These cDNAs were individually cloned, by blunt end ligation, into the murine stem cell virus (MSCV) 2.1 retroviral vector (Hawley et al., 1992) (kindly provided by Dr. R. Hawley; Sunnybrook Research Institute, Toronto, Ontario) at a polylinker site immediately 5' to a murine pgk promoter-neo cassette, using standard procedures (Davis et al., 1994). The HOXB4 cDNA, encompassing the complete coding sequence, was isolated as a BamHI fragment from a plasmid (kindly provided by Dr. E. Boncinelli, Ospedale S. Faffaele, Milan, Italy) and subcloned at the Xba\ site in the polylinker. The HOXA10 and HOXB3 cDNAs, encompassing the complete coding sequences, were isolated as EcoRI and Kpn\ fragments, respectively, and subcloned at the Hpal site in the polylinker. 2 . 1 . 2 G e n e r a t i o n o f v i r a l p r o d u c e r c e l l s The ecotropic cell line, GP+E-86 (Markowitz et al., 1988) and the amphotropic cell line, GP+ envAM12 (Markowitz et al., 1988), were used to generate helper-free recombinant retroviruses. These cell lines were maintained in HXM medium which consists of Dulbecco's modified Eagle medium (DMEM; StemCell Technologies, 54 Vancouver, British Columbia), 10% heat-inactivated (55°C for 30 minutes) newborn calf serum (Gibco/BRL Canada, Burlington, Ontario), hypoxanthine (15 mg/ml; Sigma Chemical Co., St. Louis, MO), xanthine (250 mg/ml; Sigma), and mycophenolic acid (25 mg/ml; Sigma). Purified plasmid vector DNA (10-14 |j,g), i.e. the MSCV 2.1 (control), MSCV 2A-HOXB4, MSCV 2A-HOXA10 or MSCV 2A-HOXB3 were introduced into the GP+E-86 and the GP+envAM-12 packaging cell lines, using the calcium phosphate (CaP0 4) transfection technique. Virus-containing supernatants were harvested 24-48 hours after the transfection, filtered, and then used to cross-infect the ecotropic and amphotropic packaging cells, transfected with the same plasmid vector, in the presence of 6 fj,g/ml polybrene (Sigma). Infected cells were then selected in 1 mg/ml of the neomycin analog G418 (Gibco/BRL), to obtain a polyclonal population of amphotropic or ecotropic viral producer cells. To increase the viral titer of these cells, filtered supernatant from ecotropic and amphotropic virus producing cells harboring the same retroviral construct were used to cross-infect these same cells 4-6 times. The viral producer cells were then maintained in HXM medium supplemented with 1 mg/ml G418. 2 . 1 . 3 V i r a l t i t e r i n g a n d h e l p e r v i r u s a s s a y s Viral titers of the GP+E-86-MSCV-pgk-neo, GP+E-86-MSCV-HOXB4-pgk-neo, GP+E-86-MSCV-HOX4 70-pgk-neo and GP+E-86-MSCV-HOXB3-pgk-neo viral producer cells (generating viruses hereafter called neo, HOXB4, HOXA10 and HOXB3, respectively) were determined by assaying various dilutions of filtered viral supernatant for the transfer of neomycin resistance to NIH-3T3 cells (American Type Culture Collection (ATCC), Rockville, MD) (Cone and Mulligan, 1984). The viral titers of the neo viral producer cells were 3-5x106 colony forming unit (CFU)/ml, and 3-5x105 for HOXB4, HOXA10 and HOXB3 viral producer cells. Absence of helper virus generation in the HOXB4, HOXA10, and HOXB3 viral producer cells was verified by failure to serially transfer virus conferring G418 resistance to NIH-3T3 cell (Cone and Mulligan, 1984). 55 2.2 Hematopoietic Cell Cultures and Assays 2 .2 .1 M i c e Mice used as recipients were 7 to 12 week old male or female (C57BI/6J x C3H/HeJ)F1 ((B6C3)F1) and used as donors were (C57BI/6Ly-Pep3b x C3H/HeJ)F1 ((PepC3)F1) mice. The (B6C3)F1 and (PepC3)F1 mice are phenotypically distinguishable by their cell surface expression of different allelic forms of the Ly5 locus; (B6C3)F1 are homozygous for the Ly5.2 allotype and (PepC3)F1 are heterozygous for the Ly5.1/I_y5.2 allotypes. These mice were bred from parental strain breeders originally obtained from the Jackson Laboratories (Bar Harbor, MA) and maintained in microisolator cages and provided with sterilized food and acidified water in the animal facility of the British Columbia Cancer Research Center. 2 . 2 . 2 V i r a l i n f e c t i o n o f m u r i n e b o n e m a r r o w c e l l s Bone marrow cells used for retroviral infection were isolated by flushing femurs and tibias of (PepC3)F1 (Ly5.1/Ly5.2) mice, injected intravenously 4 days previously with 150 mg/kg body weight of 5-fluorouracil (5-FU), with DMEM 2% fetal calf serum (FCS) (StemCells Technologies), using a 21 gauge needle. Single cell suspensions of 1-5 x 105 bone marrow cells/ml were then incubated in DMEM containing 15% FCS, 6 ng/ml murine interleukin-3 (mlL-3), 100 ng/ml murine Steel factor (mSF) and 10 ng/ml human IL-6 (hlL-6) for 48 hours at 37°C in 5% C 0 2 . All cells were then harvested and plated on monolayers of irradiated viral producer cells (1500 cGy X-ray), using identical medium with the addition of 6 |j.g/ml polybrene (Sigma), and cultured at 37°C for additional 48 hours. Loosely adherent and non-adherent bone marrow cells were recovered from the co-cultures by repeated washing of dishes, using Hank's balanced salt solution (StemCells Technologies) containing 2% FCS, and then counted using a hemocytometer. All growth factors, unless otherwise specified, were used as diluted supernatants from transfected COS cells as prepared in the Terry Fox Laboratory. 56 2 . 2 . 3 T r a n s p l a n t a t i o n o f r e t r o v i r a l l y t r a n s d u c e d b o n e m a r r o w For bone marrow transplantation procedures, lethally irradiated (950cGy, 110cGy/min., 1 3 7 C s gamma-rays) (B6C3)F1 (Ly5.2) recipients were injected intravenously with 2 x 10^ bone marrow cells derived from (PepC3)F1 (Ly5.1/Ly5.2) immediately after their co-cultivation with neo, HOXB4, HOXA10 or HOXB3- viral producer cells. These mice are hereafter called neo, HOXB4, HOXA10 and HOXB4 mice, respectively. Donor-derived repopulation in recipients was assessed using flow cytometry, from the proportion of leukocytes in bone marrow, thymus, spleen and peripheral blood, which expressed the Ly5.1 allelic form of the Ly5-locus. 2 . 2 . 4 In vitro c l o n o g e n i c p r o g e n i t o r a s s a y s For myeloid clonogenic progenitor assays, cells were cultured at 37°C and in 5% C 0 2 on 35mm petri dishes (Greiner, Germany ) in a 1.1 ml mixture of 0.8% methylcellulose in alpha medium supplemented with 30% FCS, 1% bovine serum albumin (BSA),10~ 4 M R-mercaptoethanol (B-ME), 3 U/ml human urinary erythropoietin (hEpo) (StemCells Technologies) and 2% spleen cell conditioned medium (SCCM) (StemCells Technologies), in the presence or absence of 1.4 mg/ml of G418. Bone marrow cells harvested after co-cultivation with viral producer cells were plated at a concentration of 1-2 x 10 3 cells/dish, whereas bone marrow cells from neo, HOXB4, HOXA10, HOXB3 mice were plated at a concentration of 4 x 10^ cells/dish. Spleen cells from neo mice were plated at 3-10 x 10 6 cells/dish and from HOXB4, HOXA 10 and HOXB3 mice at a concentration 3-10 x 10^ cells/dish. Colonies were scored on day 10-12 of incubation as derived from CFU-M, CFU-GM, BFU-E or CFU-GEMM according to standard criteria (Humphries et al., 1981). In some experiments, identification of colony types was confirmed by Wright-Geimsa staining of cytospin preparations of colonies and in some instances the classifacations of colony type as megakaryocyte/blast cell colony was confirmed using the megakaryocytic specific, GPIIb/llla (CD41) surface antigen and flow cytometry. For pre-B clonogenic progenitor assays, bone marrow cells from neo, 57 H0XB4, HOXA10 and H0XB3 mice were plated at a cell concentration of 5-10 x 104 cell/dish, in 0.8% methylcellulose in alpha medium supplemented with 30% FCS, 10"4 M B-ME and 0.2 ng/ml of IL-7 with or without 1.4 mg/ml G418. Pre-B colonies were scored on day 7 of incubation. 2 . 2 . 5 D a y 12 C F U - S a s s a y In chapter 3 the day 12 CFU-S assay was utilized. HOXA10- or neo-transduced bone marrow cells were injected into lethally irradiated recipients either immediately after retroviral infection, or after 1 week of culture, at an initial density of 1-5 x 10 5 cells/ml in medium containing 30% FCS, 1% BSA, 10"4 M (3-ME, 3 U/ml of hEpo, 2% SCCM with or without 1.4 mg/ml of G418. The number of cells that each mouse received was adjusted to give 10-15 macroscopic spleen colonies. Untransplanted lethally irradiated mice were tested in each experiment for endogenous CFU-S surviving irradiation and consistently gave no spleen colonies. Twelve days after injection, animals were sacrificed by cervical dislocation and the number of macroscopic colonies on the spleen were evaluated after fixation in Telleyesniczky's solution. In certain cases, prior to fixation, well isolated spleen colonies were excised with scalpel blade, cut open and cells were gently spread on a microscopic slide for cytological evaluation after Wright-Geimsa stain. 2 . 2 . 6 C o m p e t i t i v e R e p o p u l a t i n g U n i t ( C R U ) a s s a y Bone marrow cells from neo, HOXB4, HOXA 10 or HOXB3 mice that had been transplanted earlier with transduced cells derived from (PepC3)F1 (Ly5.1/Ly5.2) mice, were injected at different dilutions into lethally irradiated (B6C3)F1 (Ly5.2) mice (5-7 recipients per group/dilution), together with a life sparing dose of 1 x 105 competitor bone marrow cells from (B6C3)F1 (Ly5.2) mice. The level of lymphoid and myeloid repopulation with Ly5.1+ donor-derived cells in these secondary recipients was evaluated >13 weeks later by flow cytometric analysis of peripheral blood as 58 described (Rebel et al., 1994) Recipients with > 1% donor (Ly5.1+) derived peripheral blood lymphoid and myeloid leukocytes as determined by the side scatter distribution of Ly5.1 + cells (i.e. lymphoid low side scatter; myeloid high side scatter), were considered to be repopulated by at least one lympho-myeloid repopulating (CRU) cell. CRU frequency in the test cell population was then calculated by applying Poison statistics to the proportion of negative recipients at different dilutions as described previously (Szilvassy et al., 1990). 2 . 2 . 7 C u l t u r i n g o f S c a 1 + L i r r W G A + c e l l s In Chapter 3, Sca1+Lin"WGA+ cells were purified from bone marrow of neo and HOXA10 mice (see below, 2.3 ) and cultured as single cells in serum free medium. After purification of Sca1+Lin"WGA+ cells, they were re-sorted and deposited directly into wells of 96-well plates using an automatic cell deposition attachment to FACStar* (Becton Dickinson). Single cells were cultured in Iscove's modified Dulbecco's medium containing 10 mg/ml BSA, 10 mg/ml bovine insulin, 0.2 mg/ml transferrin, 10"4 M (3-ME and 40 u.g/ml low density liporotein (LDL) (serum free medium), supplemented with the following growth factors: 20ng/ml mlL-3, 10ng/ml hlL-6, 5ng/ml hlL-7, 25ng/ml hlL-11, 3u/ml hEpo, 50ng/ml mSF, 10ng/ml hG-CSF and 1.4 mg/ml G418 for selection of transduced cells. Following G418 selection, 9-15 days later, those wells containing >10 cells were scored by visual inspection for the presence of megakaryocytes. Visual scoring criteria were validated by Wright-Geimsa staining of cytospin preparations and by flow cytometry for expression of the megakaryocytic specific, GPIIb/llla (CD41) surface antigen, using D9 mAb (kindly provided by Dr. K.A. Ault, Maine Medical Center Research Institute, South Portland, ME, USA). 2 . 2 . 8 M o r p h o l o g i c a l e v a l u a t i o n s o f b o n e m a r r o w , s p l e e n a n d p e r i p h e r a l b l o o d c e l l s f r o m t r a n s p l a n t e d m i c e At various times after transplantation, peripheral blood cell counts and hematocrits of neo , HOXB4, HOXA10 and HOXB3 mice were determined using a Coulter CBC5. 59 Differential counts of bone marrow, spleen and peripheral blood cells from neo, HOXB4, HOXA10 and HOXB3 mice, that were sacrificed or that became terminally ill were performed on Wright-Geimsa stained cytospin preparations. 2.3 Antibodies, Flow Cytometry and Cell Sorting Various hematopoietic populations in bone marrow, spleens and thymuses of neo, HOXB4, HOXA10 and HOXB3 mice were analyzed by flow cytometry at different times post transplantation. In some of the HOXB3 mice (Chapter 4) the lymph nodes were also analysed. A single cell suspension of bone marrow was prepared by injecting Hanks-HEPES buffered salt solution containing 2% fetal calf serum (FCS) and 0.1% sodium azide (Sigma) (HFN) into femurs to flush out cells, followed by gentle disaggregation through a 21 gauge needle. Cells were released from the thymus, spleen and lymph nodes by disruption through a fine steel mesh. To lyse erythrocytes, cell suspensions were treated with 0.165 M NH4CI (StemCells Technologies) and washed once with HFN. Cells were stained with primary antibodies in HFN on ice for 40 minutes, washed twice with HFN and resuspended in HFN containing 1jxg/ml propidium iodide (Sigma). Flow cytometric analysis was performed using a FACSort or FACStar flow cytometer equipped with PC LYSISII software. Monoclonal antibodies (mAbs) were titered and used as described (Hough et al., 1996; Hough et al., 1994; Rebel et al., 1994). The following mAb were used in this thesis.work: fluorescein isothiocyanate (FITC)-conjugated anti-CD4, anti-CD8, anti-y8-T cell receptor (TCR), anti-CD43 (S7), anti-Gr-1, anti-Ly5.1 (A20-1.7) (kindly provided by Dr. G. Spangrude, Salt Lake City, Utha) and anti-wheat germ agglutinin (WGA); phycoerythrine- (PE) conjugated anti-IL-2 receptor a chain (IL-2Roc), anti-CD4, anti-CD8, and anti-B220; and biotinylated anti-B220, anti-Ly-1, anti Gr-1 and anti-ap-TCR mAb all of which were purchased from Pharmingen. HSA was detected by a cyanine 5-succinimidylester (Cy5)-labeled M1/69 mAb purified from the TIB125 hybridoma (American Type Culture Collection, Rockville, MD). The Sca-1 antigen was detected with Cy5-conjugated E13161-7 mAb. The anti-Mac-1 mAb was purified from the M1/70.15.11 hybridoma 60 (American Type Culture Collection, Rockville, MD) and either labeled with FITC or biotin. FITC-labeled mouse anti-IgM, and PE-labeled mouse anti-lgD were purchased from Southern Biotechnology (Birmingham, AL). PE and FITC conjugated Streptavidin were purchased from Jackson Immuno Research Laboratories (Westgrove, PA). Bone marrow and spleen cells from neo, HOXB4, HOXA10 and HOXB3 mice were stained with anti-Ly5.1, anti-Mac-1, anti-Gr-1, anti-B220, anti-CD43, anti-IgM and anti-lgD mAbs, and their spleen cells in addition with anti-CD4 and anti-CD8 mAbs. Thymic cells were stained with anti-Ly5.1, anti-CD4 and anti-CD8. Thymic cells from HOXB3 mice were further analyzed using anti-ap-TCR, anti-y8-TCR, anti-IL-2Rcc, M/169, anti-B220 and anti-Mac-1. For some of the HOXB3 mice (see Chapter 4) analyzed at 14 weeks after transplantation (n=6), staining of their thymic cells was performed as described elsewhere (Hugo et al., 1993). A single cell suspension of 106 cells prepared in PBS 2 % FCS, 0.1 % NaN 3 (PBSWB) was deposited in microculture wells. After centrifugation, the pellet was resuspended in 10 ui of a blocking cocktail containing 5 |j,g/ml of human gamma globulin (Sigma) diluted in supernatant from the hybridoma 2.4G2, which produces a mAb against the Fc Rllb/lll (Unkeless, 1979), and incubated for 5 min at 23°C. A cocktail of 40 \i\ PBSWB containing FITC-conjugated anti-TCR (GL3-1.4; (Gorski et al., 1993)), PE-conjugated anti-CD4 (GK.15; Gibco BRL), and RED613-conjugated anti-CD8 (Gibco BRL), biotinylated-anti-TcRa (H57-597: (Kubo et al., 1989)) mAbs at the appropriate concentrations was added. After a 25 minute incubation at 4°C, the cells were washed three times and resuspended in 50 jxl of a PBSWB with streptavidin-conjugated RED670 (Gibco BRL). The cells were subsequently incubated for an additional 30 minute period, washed and resuspended in PBSWB to allow analysis on a Coulter XLTM flow cytometer equipped with a 488 nm laser and fluorescence detectors at 525, 575, 620 and 670 nm. For all cytofluorometric analyses done at 14 weeks post-transplantation, a minimum of 150,000 events were acquired. 61 As presented in Chapter 4, CD4 and CD8 thymic subpopulations i.e. double negative CD4"CD8" cells, double positive CD4+CD8+ cells and single positive CD4+CD8" and CD4-CD8+ cells were purified from thymuses of neo and HOXB3 mice using PE-anti-CD4 and FITC-labeled anti-CD-8. For each cell population 10,000 cells were harvested and a proportion re-analyzed for purity (>95%) As presented in Chapters 3 and 5, the proportion of Sca1+Lin_WGA+ cells in bone marrow of neo, HOXB4 and HOXA10 was evaluated. Bone marrow cells were first stained with biotinylated mAb directed against the following lineage (Lin) markers: B220, Ly-1, Gr-1 and Mac-1. After two washes cells were stained simultaneously with Cy5-labeled anti-Sca-1, FITC-labeled anti-WGA and PE-labeled streptavidin. The Sca1+Lin"WGA+ cells from neo and HOXA10 mice (Chapter 3) were purified from their bone marrow and, after re-sorting, cultured as single cells (see above, 2.2.7). 2.4 Molecular Analysis 2.4 .1 S o u t h e r n b l o t a n a l y s i s High-molecular weight DNA was isolated from bone marrow, spleen and thymic cells of neo, HOXB4, HOXA10 and HOXB3 mice using the DNAzol reagent (Canadian Life Technologies, Burlington, Ontario), then precipitated with 95% ethanol and washed twice in 80% ethanol. DNA was then dissolved in 1xTE (10 mM Tris ph7.5, 1 mM EDTA ph8.0), and 10-20 jag of DNA digested with various restriction enzymes at 37°C for 12-16 hours. The digested DNA was separated on a 0.9% agarose gel and the gel then treated for 15 minutes with 0.1 M HCI solution, followed by a 30-40 minutes treatment with denaturing solution containing 0.5 M NaOH, 1.5 M NaCI. The DNA was then transferred to nylon membrane (Zeta-Probe; Bio-Rad Laboratories, Richmond CA) overnight in 10 x SSC by standard blotting method. The membranes were then baked at 80°C for 1 hour and then pre-hybridized at 65° for 2 hours in 4.4 x SSC, 7.5% formamide (Gibco/BRL), 7.5% dextral sulfate (Sigma), 0.75% sodium dodecylsulphate (SDS) (Gibco/BRL), 1.5 mM EDTA (Sigma), 0.75% skim milk and 370mg/ml of salmon 62 sperm DNA (Sigma). A radioactive probe was then added to the pre-hybridization solution and the membranes hybridized for 20 hours at 65°C. Probes were labeled with 3 2 P-dCTP (3000 Ci/mmol; ICN Biomedical INC.Costa Mesa, CA) by random priming and purified on a Sephadex-G50 (Pharmacia) column before hybridization. Following hybridization membranes were washed 4 times at 65°C for 30 minutes each time, in 0.3 x SSC, 0.1% SDS and 1 mg/ml sodium pyrophosphate. Autoradiography was performed with Kodak XAR-5 film and an intensifying screen, at -70°C for 1-5 days. For re-probing, membranes were stripped by washing for 30-40 minutes in a 1% SDS solution at 100°C. Kpn\, which cuts once in the retroviral long terminal repeats (LTR's ), was used for releasing the integrated proviruses in DNA isolated from hematopoietic tissue of neo, HOXB4, HOXA10 and HOXB3 mice (Ssti was used for some of the DNA isolated from neo mice in Chapter 3). For release of DNA fragment(s) specific for the proviral integration site(s), DNA was cut with restriction enzymes that only cut once in the provirus. For that purpose, DNA isolated from neo mice was cut with EcoR\ or BamH\ (Chapter 5), and Hind\\\ (Chapter 3), from HOXB4 mice with EcoPA or BamH\ (Chapter 5), and from HOXA10 mice with Hind\\\ (Chapters 3). Probes used were a Xho\/Sal\ fragment of pMC1 neo (Thomas and Capecchi, 1987) (Chapters 3,4 and 5), a 1.8 kb genomic Kpn\l Hind\\\ fragment of the murine SH2-containing inositol phoshatase (SHIP) gene (Damen et al., 1996)(Chapters 3 and 5), a Kpn\/Mse\ fragment of pXM(ER)-190 which releases the full-length erythropoietin receptor (EpoR) cDNA (Chapter 3), (kindly provided by Dr. A. D'Andrea) and a 307 bp cDNA fragment 3' to HOXB3 homeodomain obtained by Apa\ I SamHI restriction digest (Chapter 4) 2 . 4 . 2 N o r t h e r n b l o t a n a l y s i s Total cellular RNA was isolated from various hematopoietic tissues of neo, HOXB4, HOXA10 and HOXB3 mice using TRIzol reagent (Gibco/BRL), precipitated out using isopropanol and then washed twice with 75% ethanol. RNA was then dissolved in 63 sterile water and 5-10 mg RNA separated on a 1% formaldehyde/agarose gel. Following treatment of the gel with 10 x SSC for 30-40 minutes, the RNA was transferred to nylon membrane (Zeta Probe) overnight in 10 x SSC . The membrane was baked for 1 hour at 80°C and then pre-hybridized for 1-2 hours in 50% formamide, 0.5 M NaH 2P04/Na 2HP04ph 7.2,5% SDS, 1 mg/ml BSA, followed by 18-20 hour hybridization after addition of 3 2P-labeled probe. Probes used were, Xho\/Sal\ fragment of pMCIneo (Chapters 3 and 4), full length human HOXB4 cDNA (Chapter 5), a 300 bp cDNA fragment from a region 3' to the homeodomain of murine HOXA10 obtained by PCR (Chapter 3), and the 2.0 kb Pst\ chicken B-actin fragment (Chapters 3, 4 and 5) 2 . 4 . 3 W e s t e r n b l o t a n a l y s i s To detect the HOXA 10 protein, neo and HOXA10 viral producer cells, and bone marrow cells of untransplanted, neo and HOXA10 mice were lysed in phosphate solubilization buffer (PSB; 50 mM HEPES, 100 mM NaF, 10 mM Sodium pyrophosfate, 2 mM Sodium vanadate, 2 mM EDTA, 2 mM Na 2Mo042H 20 ph7.35, 1% NP40 and protease inhibitors PMSF, Leupeptin and Aprotinin). After electrophoresis of the whole cells lysate in a 10% polyacrylamide gel, proteins were transferred to nitrocellulose and probed with a whole rabbit antisera to a synthetic oligopeptide derived from the C-terminal, non-homeodomain region of HOXA10. The blot was then treated with a goat anti-rabbit IgG second antibody followed by incubation with ECL detection reagent (Amersham Corp., Arlington Heights, IL) 2 . 4 . 4 c D N A g e n e r a t i o n , a m p l i f i c a t i o n a n d a n a l y s i s Reverse transcription and amplification of total messenger RNA isolated from the purified subpopulation of thymic cells from neo and HOXB3 mice (Chapter 4) was done exactly as previously reported (Sauvageau et al., 1994). Thymocytes purified by cell sorting (10,000) were pelleted and lysed in 100 jxl of 5M guanidinium isothiocyanate solution. Nucleic acids were then precipitated, and subsequent synthesis of the cDNA done with a 60 mer 64 primer containing a 3' poly thymidine stretch as described (Brady et al., 1990). A short poly-adenosine tail was added to the 3' end of the first strand cDNA using terminal deoxynucleotidyl transferase and the second strand synthesis and subsequent PCR amplification done by the polymerase chain reaction with the same primer, but at a higher concentration, as used during the reverse transcription (Sauvageau et al., 1994). Amplified total cDNA was size fractionated on a 1% agarose gel, transferred to nylon membranes and hybridized as described above (section 2.4.1). Probes used were the Xho\/Sal\ fragment of pMCIneo and a 307 bp cDNA fragment 3' to HOXB3 homeodomain obtained by Apal I BamHI restriction digest. 2.5 Statistical Methods Through out this thesis, results are expressed as mean ± standard deviation from the mean, as calculated using Student's t-Test: Two-Sample Assuming Equal Variances, provided in Statistical Analysis Tools in the Microsoft Excel program. The Student's t-Test: Two-Sample Assuming Equal Variances was also used to determine whether two sample means where equal (the null hypothesis was correct or should be rejected). P value of <0.05 was pre-chosen as a significant difference between two means. Estimated survival of transplanted mice were based on Kaplan-Meier curves, a product-limit method estimating the survivorship function (Kaplan and Meier, !958), where the survival at various time points post transplantation is calculated based on the size of each group of mice when an animal died or became terminally ill. 6 5 Chapter 3 Overexpression of HOXA10 in murine hematopoietic cells perturbs both myeloid and lymphoid differentiation and leads to acute myeloid leukemia1 Mhe material presented in this Chapter is essentially as described in: U. Thorsteinsdottir, G. Sauvageau, M.R. Hough, W. Dragowska, P.M. Lansdorp, H.J. Lawrence, C. Largman, and R.K. Humphries (1997). Overexpression of HOXA10 In Murine Hematopoietic Cells Perturbs Both Myeloid and Lymphoid Differentiation, and Leads to Acute Myeloid Leukemia. Mol. Biol. Cell 17: 495-505. 66 3.1 Introduction As discussed in Chapter 1, members of the HoxA, HoxB and HoxC cluster genes have been shown to be preferentially expressed in the CD34+ fraction of human bone marrow cells, which contains most if not all of the hematopoietic progenitor cells (Sauvageau et al., 1994). Further detailed analyses of the expression of several of these Hox genes in functionally distinct subpopulations of CD34+ cells revealed two patterns of expression: specific genes, primarily located at the 5' end of the clusters (e.g. HOXA10,HOXB9 ) showed essentially invariant expression in these subpopulations, whereas a second group of genes located towards the 3' side of the clusters (e.g. HOXB3, HOXB4) were expressed at their highest levels in the subpopulation containing the most primitive hematopoietic cells and then sharply downregulated in later cell populations (Sauvageau et al., 1995). By retrovirally overexpressing in murine bone marrow one of the 3' located genes, HOXB4, our group has recently shown that its overexpression selectively enhances the proliferative potential of primitive hematopoietic cells, most profoundly HSC, without detectable effects on hematopoietic differentiation (Sauvageau et al., 1995). On the basis of these results it was hypothesized that these expression patterns might reflect different functional activities of Hox genes in hematopoiesis. As an initial test of this hypothesis, I have examined both the in vitro and in vivo behavior of primary murine hematopoietic cells which were engineered by retroviral gene transfer to overexpress HOXA10. Here, I demonstrate that the effects of HOXA10 overexpression stand in sharp contrast to those of HOXB4. Overexpression of HOXA10 is found to profoundly perturb differentiation of bone marrow progenitors with megakaryocytic and monocytic potentials, is not permissive for B-cell development and enhances proliferation of hematopoietic progenitor cells. Furthermore, a significant proportion of recipients of HOXA 70-transduced bone marrow cells eventually develop acute myeloid leukemia. 67 3.2 Results 3 . 2 . 1 R e t r o v i r a l - m e d i a t e d t r a n s d u c t i o n o f HOXA10 t o m u r i n e b o n e m a r r o w c e l l s For this study I used a full length HOXA10 cDNA isolated from the human myeloid cell line ML3 which encodes a 55 kDa protein (Lowney et al., 1991). This HOXA10 cDNA was chosen because it represents the most abundant HOXA10 transcript found in human bone marrow cells (Lowney et al., 1991). The HOXA 70 cDNA was inserted into the murine stem cell virus (MSCV) 2.1 retroviral vector (Hawley et al., 1992) 5' to a phosphoglycerate kinase promoter (PGK)-driven neo gene such that HOXA10 was expressed from the viral enhancer and promoter sequences within the long terminal repeat (LTR) (Figure 3.1A). The LTR sequences in this MSCV vector have been shown previously to give high and long-term expression in primitive murine hematopoietic cells and their mature progeny of both myeloid and lymphoid lineage (Hawley et al., 1994; Pawliuk and Humphries, Unpublished data; Sauvageau et al., 1995). Integrity of the HOXA10 retrovirus was verified by Northern and Western blot analysis which detected the two expected HOXA 70-containing viral mRNAs (due to the splice donor and acceptor sites in the MSCV 2.1 vector) (Hawley et al., 1994) and the HOXA10 protein in the viral producer cells, respectively (Figure 3.1 B and 3.1 C). For some parts of this study I used in addition an MSCV retroviral vector containing the HOXB4 cDNA under the control of the viral LTR, which is desribed in Chapters 2 and 5. To investigate the effects of HOXA 10 overexpression in primary hematopoietic cells, bone marrow cells from mice injected 4 days previously with 5-fluorouracil (5-FU) were co-cultivated with neo or HOXA10 viral producer cells for 48 hours, and the subsequent effects of deregulated expression of HOXA 10 on the behavior of hematopoietic cells was analysed both in vitro and in vivo. 68 SD SA Kp 1 ' HOXA10 pgk-neo H O X A 1 0 4.6 kb 1.3 kb Ss I I pgk-neo neo 2.7 kb 1.3 kb B neo < x o < X o ^JJJl H O X A 1 0 o o c < X O 4.6 kb 3.9 kb — 1.3 kb — 97 kDa — 69 kDa — 46 kDa Figure 3.1. Structure and expression of the HOXA10 retrovirus used in this study. (A) Diagrammatic representation of the integrated HOXA10- and neo-proviruses. Expected size of the full-length viral transcripts and those initiated from the internal PGK promoter are shown. Two viral transcripts are generated from each virus due to the splice donor and acceptor sites in the MSCV 2.1 vector (B) Northern blot analysis of total RNA isolated from the HOXA 10-viral producer cells. The membrane was hybridized with a probe specific for neo that detects both viral transcripts (a 4.6 kb full-length and 3.9 kb spliced RNA) and the 1.3 kb neo transcript initiated from the PGK promoter (left) and subsequently with a full-length HOXA10 cDNA probe that detects both viral transcripts (right). (C) Western blot analysis of whole cell lysate from neo- and HOXA10-v'\ra\ producer cells. The blot was probed with polyclonal antisera directed against a HOXA10 synthetic oligopeptide. The arrow indicates the band for the HOXA10 protein migrating at the expected size of 55 kDa. 3 . 2 . 2 A l t e r e d c o l o n y f o r m a t i o n in vitro o f m y e l o i d p r o g e n i t o r c e l l s o v e r e x p r e s s i n g HOXA10 To test whether overexpression of HOXA10 would affect the ability of committed myeloid progenitor cells to complete their differentiation in vitro, neo- and HOXA10-69 transduced bone marrow cells were plated, immediately after retroviral infection, in methylcellulose cultures for myeloid colony formation. The efficiency of retroviral infection, as assessed by the proportion of G418 resistant clonogenic cells, varied between experiments and viral producer cells, and was 50±28% and 30±3% for neo-and /-/OX470-retrovirus, respectively (mean±SD, from 2 independent experiments). in CD 'c o o o * (5 I neo I I HOXB4 • HOXA 10 GEMM G/GM M0 Mega/Blast Figure 3.2 Effects of HOXA10 and HOXB4 overexpression on the relative frequency of various colony types generated in vitro, immediately following retroviral infection of bone marrow cells. On day 12-13, well isolated G418 resistant colonies were randomly picked (n=74 for neo, n=76 for HOXB4 and n=77 for HOXA10) and examined after Wright staining. Results are expressed as mean±SD from two independent experiments. The various colony types (GEMM, G/GM and M0) generated from neo- and HOXS4-transduced cells were not significantly different, p>0.5. Megakaryocyte/blast colonies were significantly increased in HOXA 10 cultures compared to neo cultures, p<0.01; and G/GM and GEMM colonies were significantly decreased in HOXA10 cultures compared to neo cultures, p<0.05. Although no M0 colonies were detected in HOXA10 cultures they were not significantly decreased compared to neo cultures as those numbers were both low and varied greatly between experiments (student t-test). The total number of colonies generated was similar for both types of infected cells (-80 colonies/1000 cells). However, there was a striking difference between the cellular constituents of neo- and HOXA 70-transduced colonies as revealed by cytological examinations of Wright-Geimsa stained G418 resistant colonies (Figure 3.2). About 45% of the progenitors transduced with HOXA10 generated large colonies containing megakaryocytes and blast cells (Figure 3.7A ), a colony type not detected 70 among neo-transduced colonies. The generation of this unique colony type in HOXA10 cultures was accompanied by a proportional reduction in multilineage GEMM, granulocyte-macrophage and granulocyte colonies; moreover, no unilineage macrophage colonies could be detected among G418 resistant {I.e.HOXA 10-transduced) colonies. In contrast to this altered myeloid differentiation mediated by HOXA 10, overexpression of HOXB4 did not alter the proportion of various colony types generated in vitro (Figure 3.2). 3 . 2 . 3 HOXA10 o v e r e x p r e s s i o n i n c r e a s e s t h e m a i n t e n a n c e o f d a y 1 2 C F U - S in vitro. In our previous study on the effects of overexpression of HOXB4 in murine bone marrow cells, we observed a more than 2 log enhancement in the recovery of multipotent myeloid progenitor cells, day 12 CFU-S, after a 7 day culture period following retroviral infection with HOXB4 (Sauvageau et al., 1995). To assess whether HOXA10 overexpression had a similar effect, the day 12 CFU-S content of neo- and HOXA 70-infected bone marrow cells were measured immediately after retroviral infection and again after 7 days in liquid culture supplemented with growth factors and 1.4 mg/ml G418 for selection of transduced cells. The day 12 CFU-S frequencies measured immediately after retroviral infection were similar for neo-and HOXA10-transduced bone marrow cells (Figure 3.3, day 0). However after maintaining these cells for 7 days in liquid cultures the day 12 CFU-S content of cultures initiated with HOXA 10-transduced cells showed a net increase to 200% of input values, whereas levels in neo control cultures had decreased to -5% of input (Fig 3.3, day 7). Thus HOXA10 like HOXB4 can reverse the decline in day 12 CFU-S numbers normally observed under these culture conditions, suggesting that overexpression of HOXA10 can affect processes involved in the generation or maintenance of cells with day 12 CFU-S ability. 71 10000 -cr Starting cells post in vitro culture (day 0) (day 7) Figure 3.3. Effect of overexpression of HOXA10 on the recovery of day 12 CFU-S in vitro. The CFU-S content was assessed immediately after co-cultivation (day 0) and also after 7 days in liquid culture. Results (mean±SD) are expressed as day 12 CFU-S numbers per 10 5 starting day 0 cells from 2 independent experiments, except for day 0 values for HOXA10 which represent 1 experiment (neo white and HOXA10 black bars). Cytological examination of Wright-Geimsa stained cell preparations from day 12 spleen colonies, showed the same proportion of differentiated erythroid (>90%) and myeloid elements for both HOXA10- and neo-transduced colonies (data not shown), indicating that overexpression of HOXA10 did not overtly alter the pattern of terminal differentiation of day 12 CFU-S during spleen colony formation in vivo. 3 . 2 . 4 E x p a n s i o n in vivo o f m y e l o i d p r o g e n i t o r c e l l s o v e r e x p r e s s i n g HOXA10 a n d t h e i r a l t e r e d c o l o n y f o r m a t i o n in vitro To delineate further the effects of HOXA10 overexpression on hematopoiesis, we analysed long-term myeloid and lymphoid reconstitution in lethally irradiated mice transplanted with HOXA 10- or neo-transduced bone marrow cells (hereafter called HOXA 10 and neo mice, respectively). At 8-15 weeks post transplantation, hematopoietic regeneration in either neo or HOXA10 mice was essentially completely donor derived, as >85% of bone marrow, thymic and peripheral blood leukocytes were 72 of transplant origin (Ly5.1+). Moreover, the intensities and patterns of proviral signals seen upon Southern blot analysis of DNA from bone marrow and thymus of these mice, were consistent with high level, polyclonal reconstitution by transduced cells in both neo and HOXA10 mice (Figure 3.4A and 3.4B). Expression of the transduced HOXA10 cDNA was readily detected in total bone marrow cells of HOXA10 mice, at both the RNA and protein levels by Northern and Western blot analyses, respectively (Figure 3.5A and 3.5B). Endogenous HoxaW expression in contrast, was below the detection level, using these same methods, in either neo, HOXAW or normal untransplanted mice (Figure 3.5A and 3.5B). 73 Figure 3.4. Southern blot analysis of DNA isolated from hematopoietic tissue of HOXA10 and neo mice. (A) Demonstration of the presence of intact integrated HOXA10 and neo proviruses. DNA from bone marrow of HOXA 10 mice was digested with Kpn\ and that of neo mouse with Ssrl to release the integrated HOXA10 (4.6 kb) and neo (2.7 kb) proviral fragments, respectively. The blots were hybridized with a neo specific probe to detect the proviruses and then subsequently with a probe specific for murine EpoR to provide a single gene copy control for loading. Exposure times were equivalent for both probes (-35 hour). (B) Analysis of proviral integration patterns in DNA isolated from bone marrow and thymic cells from these same HOXA10 and neo mice. DNA was digested with HindW which cuts the integrated provirus once, generating DNA fragments unique to each integration site. The membranes were hybridized with a probe specific for neo to detect proviral fragments, and subsequently with a probe specific for murine EpoR to provide a single gene copy control for loading. Exposure times were 72 hours for the neo probe and 24 hours for the EpoR probe. The numbers assigned to various lanes identifies a specific mouse. The HOXA10 mouse number 1 was sacrificed 8 weeks post transplantation whereas other HOXA10 mice and the neo mouse, were sacrificed 15 weeks post transplantation. Abbrev. BM, bone marrow; T, thymus. 74 B un-Tx neo HOXA10 CM T - CM CM CD T CO CO CO CD CD CD CD CD HoxalO • t un-Tx neo HOXA10 CM T- OJ T-2 2 2 2 2 2 2 co co m m m m m 4.6 kb 3.9 kb 2.7 kb 1.3 kb 4.6 kb 3.9 kb 66 kDa 55 kDa - 46 kDa • 30 kDa Actln -1.2 kb Figure 3.5. Northern and Western blot analyses to demonstrate high levels of viral derived HOXA10 messages and protein in reconstituted HOXA10 mice. (A) Northern blot analysis of total RNA (10ug) isolated from the bone marrow of normal-, neo-and HOXA10-m\ce. The membranes were hybridized with a probe specific for neo that detects both viral transcripts in neo (2.7 kb) and HOXA10 mice (4.6 kb full-length and 3.9 kb spliced RNA) and the 1.3 kb neo transcript initiated from the PGK promoter, and subsequently with a probe specific for the murine HoxaWior detection of both the endogenous HoxalO messages and the HOXA 10 viral transcripts. The endogenous 2.0 or 2.5 kb murine HoxalO messages were not detected in any of the mice. As a control for loading the membranes were also hybridized with a probe specific for actin. Exposure times were equivalent for both HoxalO and actin probes (-48 hours). (B) Western blot analysis of whole cell lysates from normal-,neo- and HOXA10-m\ce. The blot was probed with a polyclonal antisera directed against a HOXA10 synthetic oligopeptide. The arrow shows the band for the HOXA10 protein migrating at the expected size of 55 kDa. Each number assigned to various lanes identifies a specific mouse (not the same as in figure 5). HOXA10 mice number 1 and 2 were leukemic whereas 3 and 4 were not. Abbrev. BM, bone marrow; un-Tx, normal untransplanted mouse. 75 When analysed 8-15 weeks post transplantaton in contrast to neo control mice, HOXA10 mice showed weight loss and signs of hematological abnormalities (Table 3.1). Although their peripheral blood and bone marrow nucleated cell numbers were within normal range, HOXA10 mice had moderate splenomegaly and mild anemia (Table 3.1). Table 3.1 Body weight and hematological parameters in neo and HOXA10 mice Peripheral blood parameters3 Mice Body weightb Spleen weight3 (g) (g) WBC/femura (x107) RBC (x109/ml) Hb (g/di) WBC (x106/ml) neo 31.5±3.0 0.11±0.01 1.9±0.5 8.6±1.0 15.811.5 8.312.3 HOXA 10 26.5+1.3° 0.17±0.06 d 2.1+0.2 7 .5 i1 .1 e 13.811.8 11.315.6 Results shown represents meaniSD. an=7 for neo mice and n=9 for HOXA 10 mice, 8-15 weeks post transplantation bn=11 mice sex and age matched, 15 weeks post transplantation cSignificantly less than neo control, p<0.03 dSignificantly greater than neo control, p<0.03 eSignificantly less than neo control, p<0.05 The numbers of myeloid progenitor cells were significantly increased in HOXA10 mice when compared to neo control mice 15 weeks post transplantation (Table 3.2). This was most pronounced in the spleen, with on average a 9-fold increase over that found in neo mice (Table 3.2). Preferential expansion of HOXA 10-transduced progenitor cells over that of non-transduced was indicated by a higher proportion of G418-resistant myeloid colonies detected from the HOXA10 mice compared to that of neo control mice (Table 3.2), despite lower gene transfer in the initial transplanted bone marrow inoculum for the HOXA10 (67.5±4% and 42.5±18% for neo and HOXA10 respectively). For some of these HOXA10 mice their splenic nucleated cell counts were also found to be elevated, which in all cases was due to an increase in 76 cells expressing both the Mac-1 and Gr-1 antigens as determined by flow cytometric analyses. Table 3.2. HOXA10 mice have increased numbers of myeloid clonogenic progenitor cells when compared to neo control mice. Mice Number of myeloid CFC (103)/femur (%G418r) Number of myeloid CFC (103)/spleen (%G418 r) neo 44.0+10.5 4.0+2.0 (53.0+19) (62±22) HOXA10 75.0±37 a 37.0±22 b (62.0+13) (82.0+20) Results are shown as mean±SD of the number of myeloid clonogenic progenitor cells in bone marrow and spleen of n=6 neo and n=8 HOXA10 mice, 15 weeks post transplantation. Abbrev. CFC, colony forming cell aSignificantly higher than in neo control mice, p<0.05 bSignificantly higher than in neo control mice, p<0.005 Myeloid clonogenic progenitors of HOXA 10 mice remained growth factor dependent for in vitro colony formation when analysed 8 weeks post transplantation. However, in the presence of added growth factors, -45% of the HOXA 10-transduced progenitor cells from bone marrow of these mice generated a unique colony type containing megakaryocytes and blast cells (Figure 3.7B, Table 3.3), representing at least a 35-fold increase in the absolute numbers of this progenitor type compared to neo control mice. Curiously, despite this high frequency in bone marrow of HOXA10 mice of a progenitor cell with potential to differentiate in vitro into megakaryocytes, inspection of bone marrow and peripheral blood smears from HOXA 10 mice revealed no gross increase in megakaryocyte or platelet counts. In addition to a high proportion of megakaryocyte and blast cell colonies, another -20% of colonies from HOXA10 mice were small (-100 cells), and contained mainly highly granular blast cells and very few differentiated myeloid elements, a colony type not detected among the neo-transduced colonies (Figure 3.7C, Table 3.3). Furthermore, no unilineage 77 macrophage colonies could be detected among the HOX/4 70-transduced colonies, which in contrast represented about 30% of colonies generated from bone marrow of neo mice. Table 3.3 HOXA10 mice have greatly increased numbers of bone marrow megakaryocyte/blast and blast colony forming cells (CFC) and decreased numbers of unilineage macrophage CFC. Number of G418 r CFC / femur, 10 3 Mice Total CFC GEMM/BFU-E 3 G/GM M Mega/Blast Blast neo 16.910.8 5.511.5 6.512 4.713 <0.33 <0.33 HOXA10 26.211.3 3.812 5.012.5 <0.46 11.511 5.511 Fold difference 1.6 0.7 0.7 0.1 36.5 16.0 HOXA10/neo On day 12-13 well isolated colonies were randomly picked and analysed by Wright- Geimsa staining (n=50 for neo and n=58 for HOXA10) . Results shown represent meanlSD of the number of various G418 resistant colony types generated from 2 mice in each group. a BF U-E colonies represented only 0.5% of analysed colonies and therefore were combined with GEMM Abbrev. GEMM, granulocyte-erythrocyte-macrophage-megakaryocyte; G/GM, granulocyte/granulo-cyte-macrophage; M, macrophage; Mega/Blast, megakaryocyte-blast cells. Interestingly, macrophage colonies could be detected in the absence of G418 in cultures initiated with bone marrow cells from HOXA 10 mice (data not shown), indicating the presence of non-transduced macrophage progenitor cells capable of normal differentiation in these mice. Thus the block in macrophage colony formation due to overexpression of HOXA10 appears to be intrinsic to transduced cells rather than attributable to accessory cells. 78 Table 3.4 Sca1 + L in 'WGA + cells overexpressing HOXA10 have increased potential to generate megakaryocyte-containing colonies in vitro Mice Sca1+Lin-WGA+ %G418 r G418 r Seal +LirrWGA+ cells (103) cells (103) /femur, Seal +Lin"WGA + cells /femur generating megakaryocyte containing colonies, (% of total G418 r colonies) HOXA10 30.0+0.3 57.5+9 6.6+2.3a (38±6) neo 29.0±9.9 30.0+10 1.3±0.5 (15±4) Sca1 + Lin"WGA + cells purified from bone marrow cells of neo (n=2) and HOXA10 (n=2) mice 8 weeks post transplantation, were seeded as single cells/well and following G418 selection, 9-15 days later, those wells containing >10 cells (n=296 for HOXA10 and n=186 for neo mice) were scored for the presence of megakaryocytes. Results shown represents mean+SD. aSignificantly different from neo control, p<0.05 Similarly, the induction of megakaryocytic differentiation also appears to be intrinsic to HOXA 70-transduced progenitor cells. When cells with the primitive phenotype Sca1+Lin-WGA+ (purified from bone marrow of HOXA10 ,neo and untransplanted mice) were cultured as single cells in serum free liquid cultures supplemented with various growth factors, /-/OX4 70-transduced Sca1+Lin-WGA+ cells showed increased potential to generate megakaryocyte containing colonies, both compared to neo-transduced and untransduced Sca1+Lin-WGA+ cells (Table 3.4). Thus, overexpression of HOXA10 in vivo, like that of HOXB4 (Sauvageau et al., 1995) induces expansion of myeloid progenitor cells, but in contrast to HOXB4, alters their normal differentiation. 3 . 2 . 5 O v e r e x p r e s s i o n o f HOXA10 in vivo i s n o t p e r m i s s i v e f o r p r e - B l y m p h o i d c o l o n y f o r m a t i o n . While transduced myeloid progenitor cells were increased in numbers in HOXA10 mice, total pre-B colony forming cells were slightly reduced in number (~2-fold) and virtually none were derived from transduced cells (G418-resistant). Overall, 79 transduced pre-B progenitors in HOXA10 mice were some 22-fold lower in absolute number compared to those in neo control mice (Table 3.5). Table 3.5. Overexpression of HOXA10 is not permissive for lymphoid pre-B colony formation. Mice Number of pre-B %G418 r progenitors(103)/femur pre-B progenitors neo 10+5.5 43±10 HOXA10 5.5+4 4+4.5a Results are expressed as mean+SD of the number and the % G418 resistant colonies generated from bone marrow of neo (n=5) and HOXA10 (n=5) mice, 8 and 14 weeks post transplantation. aSignificantly less than in neo control mice, p<0.0005 To further demonstrate the inability of HOXA 7 0-t ran sduced cells to contribute to B-lymphopoiesis, the intensity of the proviral signal in DNA isolated from spleen, in which mature B-cells normally constitute the majority of cells (-2/3), was compared to that from bone marrow and thymus. For that purpose only those HOXA10 mice were analysed (n=4) in which the proportion of splenic B- and myeloid cells were within normal range, as determined by flow cytometric analyses (B220+ cells, 60-62% and Mac-1+, 3-4%). In contrast to neo control mice, where equal proviral signals were detected in all three tissues, the strength of the proviral signal from spleen cells in HOXA10 mice was on average only 1/3 of that in bone marrow and thymus, consistent with a minimal contribution of marked cells from the B cell fraction (Figure 3.8A). Flow cytometric analysis of various B cell populations in bone marrow and spleen of neo and HOXA10 mice showed that the absolute numbers of pro-B, pre-B and mature B cells were within normal range in HOXA10 mice (data not shown). Thus the regeneration and differentiation of B-cells in HOXA10 mice from untransduced cells 80 (estimated to be >50% of bone marrow cells in the transplant inoculum, see above) appeared unaffected and compensated for impaired B-cell differentiation by transduced cells . HOXA10 mice when analysed 8-15 weeks post transplantation had normal thymic size, and by using antibodies against CD4 and CD8, were found to have normal numbers and relative frequencies of thymocyte subpopulations (data not shown). The contribution of transduced cells to thymic regeneration in these HOXA10 mice was shown by Southern blot analysis that detected proviral signals, which in most cases was comparable to levels seen in bone marrow (Figure 3.5B and 3.8A). Overexpression of HOXA 10 thus appears to have no gross effect on T lymphoid development. 3 . 2 . 6 A c u t e m y e l o i d l e u k e m i a a r i s e s i n r e c i p i e n t s o f HOXA 7 0 - t r a n s d u c e d b o n e m a r r o w c e l l s . In two independent experiments, a total of 23 mice were transplanted with HOXA10-transduced bone marrow cells. Of these, 11 animals were sacrificed 8-15 weeks post-transplantation but the remaining 12 were monitored for a prolonged period of time to test whether overexpression of HOXA10 would eventually lead to a disease state. Nine of these HOXA 10 mice (from both transplantation experiments) became terminally ill and died 19-48 weeks post-transplantation; 3 remained alive at 52 weeks post transplantation (Figure 3.6). The diminished survival of HOXA10 mice contrasts with mice transplanted with /-/OXS4-transduced bone marrow cells, for which survival was not different from that of the neo control mice (Figure 3.6). 81 125 100-CO > Z5 50 H 25 4 a = 23 b = 21 c = 22 a = 12 b = 15 c = 8 a=7 b=12 c = 5 HOXA 10 T o o CM O CO O a=3 b = 5 c = 2 ..j neo H0XB4 \ o in o co Weeks post-transplantation Figure 3.6. Survival of HOXA10 mice compared to that of neo control mice and mice transplanted with HOX84-transduced bone marrow cells using Kaplan-Meier estimates. Survival at various time points post transplantation was calculated based on the size of each group of mice when an animal died or became terminally ill. The size of each group at various time points post transplantation is shown in the upper part of the graph where a=HOXA10, b=HOXB4 and c=neo mice. Survival of HOXA10 mice was statistically different from that of neo control mice and HOXB4 mice, p<0.05. In all cases where documentation was possible (n=7), acute myeloid leukemia (AML) was the cause of death of the HOXA10 mice. These mice had elevated white blood cell counts (>50,000/MJ), were profoundly anemic and some had hind limb paralysis, likely caused by leukemic infiltration of lumbo-sacral roots. Cytological examination revealed large numbers of blast cells in bone marrow, spleen and lymph nodes (Figure 3.7 D-F). For the two mice tested, the leukemia was readily transplantable to both irradiated (n=5) and non-irradiated (n=5) recipients, which developed a fulminant leukemia 3-5 weeks after intravenous injection of 2 x 1 0 6 bone marrow cells. 82 Figure 3.7 Wright-Geimsa staining of cytospin preparations from HOXA 7 0-transduced myeloid colonies (A-C) and from hematopoietic tissues from a representative leukemic HOXA10 mouse (D-F) (A and B) Representative megakaryocyte (a) and blast cell (b) colonies picked from methylcellulose cultures initiated with cells obtained either immediately following retroviral infection of 5-FU bone marrow cells (A) or with cells recovered from bone marrow of HOXA10 mice (B). (C) Smaller colony observed only in methylcellulose cultures initiated with bone marrow cells from HOXA10 mice containing immature blast-like myeloid cells (c) together with few differentiated granulocytic elements (d). D to F show peripheral blood, bone marrow and lymph nodes, respectively, from a representative HOXA 10 mouse that developed acute myeloid leukemia. Note the presence of leukemic blasts together with a few mature granulocytic elements in each of these three organs. Magnification: x600. 83 Southern blot analysis of hematopoietic tissue from leukemic HOXA10 mice showed that the leukemic cells contained the HOXA 10-pro virus, and that the leukemia appeared mono-or bi-clonal (Figure 3.8B). Similarly, Northern and Western blot analysis of bone marrow cells from two leukemic mice detected high levels of the full length retrovirally driven HOXA10 messages (4.6 kb and 3.9 kb) and the HOXA10 protein (Figure 3.5A and 3.5B, respectively). The mono-or bi-clonality of the leukemias, together with their delayed onset, suggests that overexpression of HOXA 10 is in itself not fully transforming and that additional mutation(s) is (are) required for leukemic transformation. Further evidence for the leukemogenic nature of overexpression of HOXA10 was obtained by transplanting bone marrow cells from an apparently healthy primary HOXA 10 mouse (15 weeks post transplantation) into lethally irradiated secondary recipients. By 8 to 12 weeks post-transplantation all of the secondary recipients (n=17) developed AML which in all cases was found to be caused by the same HOX/WO-transduced leukemic clone, as evident by Southern blot analysis (Figure 3.8B). This clone was also found to be the dominant HOXA 10-transduced clone in the bone marrow of the primary donor mouse (Figure 3.8B) indicating the existence of a preleukemic state in this mouse. Interestingly, the primitive Sca1+Lin-WGA+ bone marrow fraction of that same primary HOXA10 mouse was not as leukemogenic. Of the 2 secondary recipients which received HOXA10-transduced Sca1+Lin-WGA+ cells, the AML developed after a longer latency period (34 and 45 weeks) and interestingly in the one mouse analysed, the leukemia originated from a different HOX470-transduced clone (Figure 3.8B). Although overexpression of HOXA 10 in non leukemic primary recipients did not render clonogenic cells growth factor independent, all three leukemic clones tested showed growth factor independent growth, in cultures containing fetal calf serum. 84 H O X A 1 0 ^ ^ J c o h - e g " co v ° v ° v ° ^ x ° v ^ 0 ? g l O C D j j r ^ l ^ ^ C N C M ^ " " • C - ^ ^ - ^ ^ - C - ^ ^ ^ m a H eg ih £ m W__H ID W H ^ ^ J J ' J A ? ^ J vg> -g> ^ ^ ^ V ^ J ' v g ' v . o V 4.6 kb c d — I C D — i m m m c a m c Q c o e r ) 2.7 kb M » « 1 2 k b SHIP — — 2 5 k b 4 kb 2 k b Hind III EcoR I Figure 3.8. Southern blot analysis to determine: (A) the intensity of the proviral signal in bone marrow, spleen and thymus of HOXA 10 and neo mice; and (B) the clonality of the leukemias in HOXA10 mice (A) Southern blot analysis of the intensity of the proviral signal in bone marrow, spleen and thymus of HOXA10 and neo mice. DNA was digested with Kpn\, which releases the integrated HOXA10 (4.6 kb) and neo (2.7 kb) proviral fragments. The blot was hybridized to a probe specific for neo to detect the proviruses and subsequently to a probe specific for the murineS/-//P gene as a single copy gene control for loading. Digitized images of the autoradiograms were obtained by densitometric scanning with a Computing Densitometer (Molecular Dynamics) and the signal intensity for each lane then analysed by ImageQuaNT (4.1). Each number assigned to individual lanes identifies a specific mouse. HOXA10 and neo mice were sacrificed 8 (number 6 and 3) or 14 weeks post transplantation. (B) Southern blot analysis of proviral integration sites in DNA isolated from leukemic HOXA10 mice. DNA was digested with either Hind\\\ or EcoRI that each cut the integrated provirus once, generating DNA fragments unique to each integration site. The membranes were hybridized with a probe specific for neo to identify proviral fragments. Abbrev. BM, bone marrow; S, spleen; T, thymus; LN, lymph node; 1°°, primary mouse; 2°°, secondary mouse; SLW, Sca1 +Lin"WGA + bone marrow cells; TBM, total bone marrow. 85 3.3 Discussion In this study I have shown that retroviral overexpression of one of the 5' located Hox genes, HOXA 10, perturbs the differentiation of both myeloid and B-lymphoid progenitor cells and eventually leads to the generation of acute myeloid leukemia. These effects are quite distinct from those previosly observed for the 3' located HOXB4 gene, whose overexpression enhanced the expansion of primitive hematopoietic cells but neither altered hematopoietic differentiation nor predisposed to leukemia. Taken together, these data add further functional evidence for involvement of Hox genes in key hematopoietic developmental processes in a Hox gene-specific manner. A striking effect of overexpression of HOXA10 was the enhanced generation in vitro of colonies containing megakaryocytes and blast cells and the suppression of macrophage colony formation. This effect was seen both for the transduced progenitors recovered from the regenerated bone marrow of HOXA10 mice and immediately following HOXA10 infection of committed progenitor cells in the 5-FU bone marrow, thus arguing for a direct effect of HOXA 10 on differentiation of committed myeloid progenitor cells. The detection of non-transduced macrophage progenitors in bone marrow of HOXA10 mice further supports the idea that the observed block in macrophage colony formation is an intrinsic property of HOXA10-transduced cells rather than attributed to accessory cells. The induction of megakaryocytic differentiation also appears to be intrinsic to HOXA 10-transduced progenitor cells, since HOXA 70-transduced Sca1+Lin-WGA+ cells, when cultured as single cells in liquid culture showed, a similar increase in megakaryocyte generation. Interestingly, although colonies containing megakaryocytes were generated in high frequency from /-/OX470-transduced progenitor cells in vitro, I did not detect any gross increase in megakaryocyte or platelet numbers in HOXA10 reconstituted mice. This suggests that the probability of HOXA 70-mediated megakaryocytic induction is 86 increased when growth factor concentrations are above normal physiological levels, as they are in vitro, or alternatively, that HOXA 10 induced megakaryocytic differentiation could be inhibited in vivo by some unknown negative regulatory mechanisms either not present or functional in vitro. The transcriptional control of megakaryocytic development remains largely unknown. Of the lineage-specific transcription factors known to be expressed in the megakaryocytic lineage e.g. GATA-1 and -2, Tal-1/SCL and NF-E2, only NF-E2 has been shown to be essential for normal megakaryocytic differentiation, where it is needed for completion of megakaryocytic maturation and platelet generation (Pevny et al., 1991; Shivdasani et al., 1995a; Shivdasani et al., 1995b; Tsai et al., 1994). A functional role for GATA-1 during early stages of megakaryocytic development has been inferred, since forced overexpression of GATA-1 is able to reprogram both murine and avian myeloid cell lines to differentiate into the megakaryocytic lineage (Kulessa et al., 1995; Visvader et al., 1995). Although HOXA10 expression has not been detected in the limited studies of cell lines which have both megakaryocytic and erythroid differentiation potential (Lowney et al., 1991; Magli et al., 1991; Vieille-Grosjean et al., 1992), the results presented in this study implicate Hox genes as potentially important regulators of megakaryocytic differentiation. A more detailed assessment of Hox gene expression patterns, particularly HOXA10, during megakaryocytic differentiation will now be of considerable interest as will attempts to identify effects subsequent to Hox gene knockout. Although HOXA 10 expression during megakaryocytic differentiation has not been assessed, its expression during other stages of myeloid differentiation has been analysed in some detail (Lawrence et al., 1995; Sauvageau et al., 1994). HOXA10 expression was found to be highest in subpopulations of human bone marrow enriched for LTC-IC, CFU-GM and BFU-E cells, then down-regulated ~6-fold with progression to bone marrow CD34- cells (Sauvageau et al., 1994) and undetectable in mature circulating monocytes and granulocytes (Lawrence et al., 1995). Thus the 87 suppression of macrophage colony formation and the appearance of colonies containing immature myeloid blast cells when HOXA10 is overexpressed suggest that the observed down-regulation of HOXA10 as myeloid cells mature is a critical event for normal myeloid differentiation. The second major effect of overexpression of HOXA10 was an impairment in early B cell development, as reflected by virtual absence of HOXA 70-transduced pre-B clonogenic progenitor cells in bone marrow of HOXA10 mice. Non-transduced B cells in HOXA10 mice appeared to develop normally and compensated for the absence of transduced B cells; thus the effect of HOXA 10 again appears to be intrinsic to transduced cells. This finding for HOXA 10 contrasts with the increase in pre-B progenitor cells induced by overexpression of HOXB4, in mice transplanted with HOXS4-transduced bone marrow cells shown previously (Sauvageau et al., 1995) and presented in Chapter 5, thus indicating differential effects of Hox proteins on B-lymphopoiesis. Using an RT/PCR based approach, the murine HoxalO message, can not be detected in FACS-purified B cell subpopulations from normal (B6C3)F1 mice, representing both early and late stages of B cell development (Sauvageau et al., Unpublished data). These results are in agreement with earlier studies, where HOXA10 expression could not be detected in cell lines of pre-B or mature B cell origin nor in leukemic cells from patients with pre-B cell acute lymphoid leukemia (ALL), B-cell ALL or B-cell chronic lymphoid leukemia (CLL) (Celetti et al., 1993; Lawrence et al., 1995; Vieille-Grosjean et al., 1992). The observed block in B-cell development when HOXA10 is retrovirally overexpressed suggests that HOXA 10 target genes may interfere with normal B-cell development, and thus that downregulation of HOXA10 is important following commitment to the B-cell lineage. Since many Hox genes are expressed during hematopoiesis, it is also possible that HOXA10 might be mimicking the overexpression of or blocking the action of another Hox gene normally involved in B cell development. 88 A major consequence of HOXA10 overexpression in transplanted mice was a high frequency of acute myeloid leukemia. The relatively long latency period (>19 weeks) and the mono- or bi-clonality of the leukemias suggest that other complementing mutation(s) is(are) needed for dominant outgrowth of a single cell. The hallmark of leukemia is a block in the normal differentiation program coupled with unrestrained growth which leads to clonal expansion of immature blast cells. Like HOXA 10, overexpression of HOXB4 clearly leads to expansion of primitive hematopoietic cells as evident by increased progenitor numbers in transplanted mice and enhanced recovery of day 12 CFU-S following in vitro culture ((Sauvageau et al., 1995) and Chapter 5). Overexpression of HOXB4 however, unlike HOXA10 does not predispose to leukemia. This difference may relate to the absence of HOXB4 induced differentiation changes in contrast to profound effects of HOXA10 on differentiation. The effects of HOXA10 are thus reminiscent of that reported for the nuclear oncoprotein c-myb which like HOXA 10 is normally predominantly expressed in immature hematopoietic cells and is thought to contribute to leukemogenesis by blocking differentiation but maintaining proliferation (Thompson and Ramsay, 1995). Perkins et. al., using experimental strategies similar to those applied here for HOXA10, have shown that Hoxb8 also has leukemogenic potential, and at least in some cases, the onset of the leukemias may have been triggered by autocrine growth factor production (Perkins and Cory, 1993). Interestingly, some of the HOXA 10 leukemic clones were also found to be capable of factor-independent growth. The recently observed co-activation of the PBX-1 related Meis-1 gene with Hoxa7 or Hoxa9 in myeloid leukemias in BXH-2 mice (Nakamura et al., 1996b) raises the possibility that the leukemogenic effect of HOXA10 may also involve Hox protein co-factors. The results presented in this study, together with the extensive literature showing that HOXA 10 expression is largely confined to primitive cells of the myeloid lineage, suggest that under normal physiological conditions HOXA 10 is likely involved in 89 processes of hematopoietic lineage commitment and differentiation, playing a positive role in megakaryopoiesis but negatively regulating monocytic and B-cell development. These results add to the recognition of Hox genes as important regulators of hematopoiesis and point to Hox gene-specific effects that likely reflect their regulation of different target genes during hematopoietic development. Further resolution of the Hox gene "code" and the molecular processes that they affect during hematopoietic development remains an important challenge. 90 Chapter 4 Overexpression of HOXB3 in hemopoietic cells causes defective lymphoid development and progressive myeloproliferation2 ^The material presented in this Chapter is essentially as described in: G. Sauvageau, U. Thorsteinsdottir, M.R. Hough, P. Hugo, H.J. Lawrence- C. Largman, and R.K. Humphries (1997) Overexpression of HOXB3 In Hemopoietic Cells Causes Defective Lymphoid Development And Progressive Myeloproliferation. Immunity. 6: 13-22 91 4.1 Introduction In the last Chapter it was shown that retroviral overexpression of one of the 5' located Hox genes, HOXA 10, perturbs the differentiation of both myeloid and B-lymphoid progenitor cells and eventually leads to the generation of acute myeloid leukemia. These effects are quite distinct from those previously reported for the 3' located HOXB4 gene, whose overexpression enhanced the expansion of primitive hematopoietic cells, but neither altered hematopoietic differentiation nor predisposed to leukemia (Sauvageau et al., 1995). Together these results suggest that during hematopoiesis Hox genes can regulate different target genes, in both lineage- and stage-specific manners, which might be reflected in their normal expression pattern. To explore this concept further, the effects of overexpression of a third Hox gene, the 3' located HOXB3 gene, were analyzed in a murine transplantation model. The HOXB3 gene was selected based on its distinct expression pattern in human bone marrow cells, where its expression is strictly restricted to LTC-IC enriched fraction of CD34+, which represents less than 1% of all nucleated cells found in the bone marrow (Giampaolo et al., 1994; Sauvageau et al., 1994). In contrast to findings for HOXB4, overexpression of HOXB3 did not cause expansion of the most primitive hematopoietic cells (i.e. HSCs or multipotential progenitor cells). However, HOXB3 severely impaired both B and T lymphoid development, and enhanced proliferation of bi-potential CFU-GM progenitor cells. 92 4.2 Results 4.2 .1 c D N A c l o n i n g o f a HOXB3 t r a n s c r i p t f r o m h e m o p o i e t i c c e l l s a n d g e n e r a t i o n o f a HOXB3 r e t r o v i r a l v e c t o r As is observed with many homeobox genes (Magli et al., 1991), HOXB3 has several transcripts, some of which may be tissue specific (Sham et al., 1992). In an effort to identify a HOXB3 transcript that is normally expressed in primitive blood cells, a 3.2 kb human HOXB3 cDNA was isolated from a CD34+ bone marrow expression library. The predicted coding region of this HOXB3 cDNA (GeneBank accession # U59298) shares > 97% identity at the protein level over the coding region to the mouse Hoxb-3 previously isolated from embryonic tissue (Sham et al., 1992). The HOXB3 coding sequence was subcloned into the MSCV 2.1 retroviral vector 5' to a phosphoglycerate kinase promoter (pgk)-driven neo gene such that HOXB3 expression was under the control of the regulatory elements in the viral long-terminal repeat (LTR) (Figure 4.1 A). Integrity of this virus was shown both by the expression of the expected 4.1 kb LTR-derived full-length HOXB3 message in reconstituted hemopoietic tissue of mice transplanted with HOX63-transduced bone marrow (Figure 4.1B) and by the presence of an unrearranged 4.1 kb proviral fragment detected by Southern blot analysis of genomic DNA isolated from bone marrow and thymus of such mice (Figure 4.3B). 4 . 2 . 2 H e m o p o i e t i c r e c o n s t i t u t i o n o f r e c i p i e n t s o f H O X S 3 - t r a n s d u c e d b o n e m a r r o w c e l l s High-titer, helper-free viral producer cell lines for either the HOXB3 or a neo-control retrovirus were used to infect murine bone marrow cells as summarized in Figure 4.2. Immediately following retroviral infection, a proportion of the bone marrow cells was plated under G418 selection in methylcellulose cultures supplemented with hemopoietic growth factors that support proliferation and differentiation of myeloid and erythroid progenitors (Figure 4.2). The gene transfer efficiencies as assessed by 93 recovery of G418 resistant clonogenic progenitors were from 15 to 50% in the 3 experiments. Analyses of the cellular content of HOXB3-transduced colonies (i.e. G418 resistant) showed all types of myeloid and/or erythroid colonies, suggesting that HOXB3 overexpression was permissive for in vitro differentiation of already committed myeloid progenitor cells (data not shown). A 4.1 kb 1.3 kb i • i LTR HOXB3 pgk-Neo LTR 2.7 kb 1.3 kb LTR pgk-Neo LTR 4.1 kb -2.7 kb _ 1.3 kb - • 2.0 kb _ [3-act in n e o Figure 4.1 Structure and expression of the HOXB3 and neo control retroviruses used in this study (A) Diagrammatic representation of the integrated HOXB3 (upper) and neo (lower) proviruses. (B) Northern blot analysis of 5 ug of total RNA extracted from splenocytes of mice 18 weeks after transplantation with neo- (left lane) or HOXB3- (right lane) transduced bone marrow cells. The membrane was sequentially hybridized to a probe specific for neo and p-actin and exposed for 48 hours. Two signals were obtained with the neo probe. The upper band represent long terminal repeat (LTR)-derived message of 2.7 and 4.1 kb for the neo and HOXB3 provirus respectively and the lower band common to both constructs represents a 1.3 kb signal originating from the internal pgk promoter. 94 Evaluation of gene transfer to clonogenic progenitors 5-FU Pre-stimulation (IL-3 + IL-6 + Steel) Viral infection (co-cultivation) 2d Methylcellulose A Determination of colony phenotype harvest 950 cGy 14-30 wks Evaluation of various hematopoietic cell compartment Transplantation in recipients (Ly5.2+) Figure 4.2 Overview of experimental design To explore in more detail the possibility that HOXB3 might affect hemopoiesis, reconstitution of lymphoid and myeloid populations in lethally irradiated mice transplanted with HOXB3-transduced bone marrow cells was examined. In 3 separate transplantation experiments, recipient mice (Ly5.2+) were injected with a life-sparing dose (2 x 105 cells) of congenic Ly5.1+ bone marrow cells immediately following their infection with HOXB3 or neo retroviruses, as detailed in Figure 4.2. Based on the CRU quantitation in a similar population of cells (Sauvageau et al., 1995), it was estimated that each recipient received between 40 to 100 CRU cells, of which approximately 1/5 to 1/2 were estimated to be transduced, given the gene transfer efficiency to clonogenic progenitor cells in the transplant inoculum (see above). Donor-derived (i.e., Ly5.1+) reconstitution of hemopoietic tissues was assessed 14 to 30 weeks after transplantation by FACS analysis (Figure 4.3A). Hemopoietic regeneration in either HOXB3 or neo mice was essentially completely donor-derived, as >80% of splenic, thymic and peripheral blood leukocytes were of donor origin (Ly5.1+, Figure 4.3A). The slightly lower levels that were detected in bone marrow of neo mice presumably reflect the presence of cells of the erythroid lineage which are negative for the expression of Ly5.1, rather than incomplete chimerism (Figure 4.3A). Furthermore, Southern blot analysis of DNA isolated from bone marrow and thymuses of these mice indicated a significant contribution by transduced cells to this 95 regeneration, as evidenced by detection of the expected neo or HOXB3 proviral signals at levels comparable to that of the endogenous HOXB3 gene (Figure 4 . 3 B ) . A PBL BM S P L THY B B T B T B B T B T T T T Figure 4.3 Donor-derived hemopoietic reconstitution of HOXB3 and neo mice (A) FACS analysis of proportion of donor-derived (Ly5.1+) cells in various hemopoietic tissues of HOXB3 (white bars) and neo (black bars) mice. Results shown are mean+SD for n > 6 mice (B) Southern blot analysis of DNA isolated from bone marrow and thymuses of HOXB3 (B3-1 to B3-6) or neo (neo-1 and neo-2) mice, sacrificed 18 to 25 weeks after transplantation. DNA was digested with Kpnl which cuts once in each long-terminal repeat (LTR) of the integrated neo or HOXB3 proviruses to release proviral fragments of 2.7 kb (neo) or 4.1 kb (HOXB3). Donor-derived reconstitution (i.e., %Ly5.1) of the mice shown in this Figure varied between 86% to 98% for HOXB3 mice and between 78 to 94% for neo controls. The membrane was sequentially hybridized to probes specific for neo (specific activity -30 x 10 6 DPM, 70 hours exposure time) and HOXB3 (specific activity not measured, exposure time: 24 hours). The endogenous HOXB3 signal is detected at 1.4 kb. Abbrev. PBL, peripheral blood leukocytes; BM or B, bone marrow; SPL, spleen; THY or T, thymus. 96 4 . 2 . 3 R e c i p i e n t s o f H OXB 3 - t r a n s d u c e d c e l l s s h o w i n c r e a s e d g r a n u l o p o i e s i s i n t h e i r b o n e m a r r o w a n d s p l e e n When analyzed 18-27 weeks after transplantation, age and sex-matched neo or HOXB3 mice had similar peripheral blood values (hemoglobin and white blood cells, data not shown) and bone marrow cellularity (Figure 4.4A). HOXB3 mice however showed a significant reduction in thymic cellularity and a gradual increase in spleen size over time (Figure 4.4A and 4.4B). FACS analysis (Figure 4.5) and cytological examination (not shown) of spleen and bone marrow cells documented a consistent increase in mature granulocytes in these tissues. Increased numbers of peripheral blood granulocytes were also observed in some HOXB3 mice (Figure 4.5D, e.g., HOXB3#~\ versus HOXB3#2). The absolute numbers of in vitro myeloid colony forming cells in bone marrow and spleen of HOXB3 mice were increased 3-fold (Figure 4.4C), consistent with the increase in bone marrow and spleen mature granulocytes in these two tissues. Significantly, 50-100% of these progenitors were G418 resistant, indicating that retrovirally transduced cells contributed substantially to these cell pools (Figure 4.4C). Of the G418-resistant myeloid progenitors present in the bone marrow of these mice, between 10 to 100% generated small colonies (<100-200 cells) containing a mixture of greater than 95% mature and immature granulocytic cells, a colony type rarely detected in cultures initiated with cells from neo mice. The cellular content of larger G418-resistant colonies generated from bone marrow of HOXB3 mice was similarly evaluated. Although all expected colony types were found (i.e., CFU-GEMM, CFU-GM, CFU-M, CFU-G, CFU-Basophils-Eosinophil, BFU-E), the majority of these colonies were either restricted to the granulocytic and/or macrophage lineages (46 of 54 large colonies examined from 3 mice). Thus only one colony out of 54 was of mixed lineage (CFU-GEMM), in contrast to 8 of 21 such colonies from neo mice. 97 A 70-q c 03 60-j D) . k. O 50-: a • 40-| o a> X 3 0-j E • 3 C 2 0 - j "55 1 0-^  O 0 -B U) 0 . 8 H 0.6H N "35 § 0.4 a * 0.2 -| BM SPL THY 14-25 >25 weeks post-transplantation | 100 J E o <D CO ° - o u? x 1 0 -i O re o H G418 resistant Q not G418 resistant X • B3 neo BM myeloid B3 neo SPL myeloid B3 neo BM pre-B cells Figure 4.4 Hemopoietic parameters of neo and HOXB3 mice (A) Bone marrow, spleen and thymic cellularity of HOXB3 (white bars, n=13) and neo mice (black bars, n=8) 18-27 weeks after transplantation. (B) Increments in spleen weight in HOXB3 mice (black bars) with time. (C) Myeloid and pre-B lymphoid clonogenic progenitor content in bone marrow and spleen of neo (n=5) and HOXB3 (n=8) mice. All results shown (A,B and C) represent mean values +SD. **, significantly decreased compared to neo control, p < 0.005; *, significantly increased compared to neo control, p<0.05 (two-tailed student T test with unequal variance) Abbrev.: BM, bone marrow; SPL, spleen; THY, thymus. 98 A Bone Marrow NEO B Spleen \ 1 2 5 • !^>rtiW-?-A 25 i 5 19 JO C L y m p h N o d e s N E O 84 | 30 w A i t 49 "I' 14 H 0 X B 3 #1 L X HOXB3 #2 93 Ly5.1 I 52 " I I 25 T i l 52 | I A 8 3 1 I 50 1 | . 8 4 GR-1 11 .. ^ 5 JL: • CD43 IgM HOXB3 #1 85 Ly5.1 56 B220 IgM 28 '4'': " 6 HOXB3 #2 I 87 61 A... -A, 57 CO NEO ^ ^ 5 3 Hi Wi m .V...' J'.'.'i'v D P B L NEO m28 I 34 HOXB3 #1 59 HOXB3 #2 I 75 Ly5.1 -l-,.,m^*V^W] i n , . . 1 • T^ 'lMll"l1Mw Mac-1 h! 2 K. 5 32 v s . . K. 25 24 IgM .. .12 IgM 19 at*?" '• CD8 H 0 X B 3 #1 ..if;1- ' ;# 2 5 Ly5.1 24 # 1 1 CD8 I 15 Mac-1 . 10 .;.;-;0;Vw • i i 11 y 3 HOXB3 #2 # 4° i u ! . I . . . ^ w / . - V CD8 Figure 4.5 Flow cytometric analysis of various hematopoietic populations Results from flow cytometric analysis of cells isolated form bone marrow (A) spleen (B) lymph nodes (C) and peripheral blood leukocytes (D) of mice transplanted 18 weeks earlier with HOXB3- or neo-transduced cells. HOXB3 #1 and HOXB3 #2 are 2 representative mice with moderate and severe phenotypes respectively. Numbers in boxes represent percentages of live cells found in this region. Lower number of cells were collected for analysis of lymph node populations (C). 99 Together, these data suggest that HOXB3 caused the expansion of more mature uni or bi-lineage myeloid progenitors but had minimal effect on the proliferation of earlier, multipotent cell types. This possibility was further assessed by limit dilution analysis to quantitate the number of long-term repopulating cells present in HOXB3 mice using the competitive repopulating unit (CRU) assay (Szilvassy et al., 1990). By 18 weeks post-transplantation, CRU frequencies in recipients of neo or HOXB3 transduced marrow were similar and, as expected for experiments using transduced marrow, had only regenerated to approximately 5-8% of normal levels (Table 4.1). Thus HOXB3 overexpression, in contrast to that of HOXB4 described previously (Sauvageau et al., 1995) and in Chapter 5, does not appear to enhance the regenerative potential of primitive long term repopulating cells. Table 4.1 Evaluation by limiting dilution analysis of competitive long-term repopulating cells (CRU) in primary mice transplanted with HOXB3 or neo control-transduced bone marrow cells, 18 weeks after transplantation3. Number of cells injected into neo-transduced bone marrow HQXB3transduced bone marrow secondary recipients recipients recipients 54,000 11/11 440,000 4/4 110,000 1/5 54,000 2/9 27,500 0/5 9,000 1/13 CRU frequency per 10 5 cells (95% CI) 0.43 (0.28-0.99) 0.72 (0.5-1.0) Relative to normal (%)D 5.1% 8.5% aResults are expressed as number of mice repopulated with donor-derived cells (Ly5.1+) over control. DNormal pretransplantation CRU values from n=2,16 week old (PebC3)F1 mice = 8.5 (5.5-13.3) CRU/10 5 bone marrow cells. Abbrev. CI, confident interval 100 4 . 2 . 4 R e c i p i e n t s o f H O X B 3 - t r a n s d u c e d c e l l s h a v e d e f e c t i v e T c e l l d e v e l o p m e n t HOXB3 mice had significantly smaller thymuses than age and sex matched neo control recipients (Figure 4.4A) with corresponding abnormalities in the numbers and proportions of various subpopulations detected by flow cytometry (Figure 4.6). All recipients of HOXB3Anfected cells had abnormal CD4/CD8 profiles as evident by a 24-fold decrease in the proportion of CD4+CD8+(double positive) cells and a 6-fold decrease in a single positive cell population (CD4-CD8+, Figure 4.6). In contrast, the proportion of CD4-CD8- (double-negative) and CD4 l oCD8- thymocytes were increased 3 and 2 fold, respectively (Figure 4.6 and data not shown). Although there was some heterogeneity among mice analyzed, in most mice (11 of 14) the majority of thymocytes were either CD4-CD8- or CD4 l 0CD8". These cells expressed little or no B220 or Mad (in general < 3% each) and, morphologically, were free of contaminating myeloid cells as assessed by cytospin preparations (data not shown). The dominant thymic CD4_CD8" and CD4 l 0CD8 _ populations in these mice were heat-stable antigen-positive (HSAni) and expressed variable levels of CD25 (IL-2Ra, Figure 4.7 and data not shown). Curiously, the majority of these cells were y5 TCR +, whereas few were of the a(3 lineage (Figure 4.7). There was a tendency for mice analyzed at later times (i.e., >25 weeks post transplantation) to have a more severe thymic phenotype with one mouse analyzed at 14 weeks post transplantation having over 90% of CD4-CD8" or CD4 | 0 CD8" thymocytes (data not shown). HOXB3 overexpression was thus associated with an apparent block in thymocyte maturation from the double negative to the double positive stages coupled with expansion of the CD4l0CD8-y5TCR+ and CD4-CD8-y8TCR+ thymocytes (Figure 4.7). 101 • in E 100 + 10 -1 -T o o o : i o + + o 8 O O o 43-9 o o + 8 1 o o * o _a o o 0.1 - 9 Ratio 4.0 0.34 1.28 0.26 24 1.4 6.2 neo IHOXB3 (p<0.001) (p=0.1) (p=0.5) (p=0.05) (p<0.001) (p=0.4) (p=0.01) Figure 4.6 Celluiarity of thymuses and their subpopulations in individual neo and HOXB3 mice Results shown are celluiarity of thymuses and their subpopulations for individual neo (cross) and HOXB3 (circles) mice sacrificed between 18 to 27 weeks after transplantation. Mean values are shown as horizontal lines. Ratios of mean values for each thymic population in neo and HOXB3 mice are indicated below with p values (two-tailed student T test with unequal variance). One HOXB3 recipient was omitted from this Figure because it had excessive values including 5.5 x 10 7 y8 T cells (see discussion). Three mice analyzed at 25-27 weeks post transplantation had >80% 78 T cells in their thymus but as their exact thymic celluiarity is not available, these mice were not included in this Figure. Contaminant Mac-1 or B220-positive cells were below 5% in all thymic samples used for these analyses. 102 N E O C D 4 C D 8 " 'A " 1 , 1 2 0 1 I 4 6 m • /i . 71 H O X B 3 M o d e r a t e P h e n o t y p e 2 2 8 4 1 7 J 3 6 JSi*^ H O X B 3 S e v e r e P h e n o t y p e C D 4 L y 5 . 1 a ( 3 - T C R y 5 - T C R I L - 2 R H S A Figure 4.7 Cytofluorometric analysis of thymocytes subpopulations Thymocytes analysed were isolated from 1 representative neo mouse and 2 representative HOXB3 mice with moderate and severe thymic phenotype. CD4 and CD8 profiles of total thymocytes are shown in the scatterplot (left). Each of the histograms (right) illustrates a FACS profile of 5,000 CD4" CD8" (double negative) cells analyzed for their expression of: ocp and yS-TCR, IL-2 receptor (CD25) and the heat stable antigen (HSA). Results for all neo and HOXB3 mice analyzed (including these) are presented in Figure 6. 103 Although the intensity of the proviral signal seen on Southern blot analysis of DNA extracted from whole thymuses of HOXB3 mice was consistent with high level reconstitution by transduced cells (Figure 4.3B, all thymuses > 80% CD4 l o /-CD8" T cells), RT/PCR analysis was also performed to assess proviral expression in various thymic subpopulations of neo and HOXB3 mice. In contrast to neo control mice in which strong to moderate retroviral derived signals were detected in all subpopulations, for HOXB3 mice, retroviral-derived HOXB3 mRNA were only detected at high levels in double-negative thymocytes, at low to moderate levels in CD4+CD8" (mostly CD4 l0CD8+) and was not detectable in CD8+CD4+ and CD8+CD4" cells (Figure 4.8). These results thus suggest that HOXB3 overexpression is not compatible with progression from double negative to the double positive stage of thymic development and further indicate that the few double-positive thymocytes and their progeny found in some HOXB3 mice were either derived from untransduced cells or HOXB3 transduced cells in which proviral expression was down regulated. To assess whether the perturbations in thymic development in HOXB3 mice extended into the periphery, T cells in peripheral blood, spleen and lymph nodes were analyzed by flow cytometry (Figure 4.5). In all HOXB3 mice analyzed (n=6) there were normal distributions of CD4+ and CD8+ single positive cells and essentially no CD4+CD8+ T cells. Although these single positive cells were donor-derived, whether they arose from transduced or non-transduced cells was not ascertained. 104 N e o 3 - a c t i n Figure 4.8 RT-PCR analysis purified thymic subpopulations Total RNA was isolated from different thymic subpopulations,10,000 cells/sample, that were previously purified by FACS from thymuses o\HOXB3 or neo mice (n=4 separate mice, only 1 representative of each is shown). The blot was sequentially hybridized with a probe specific for p-actin (lower panel) and neo (upper panel). Hybridization to a probe specific to HOXB3 gave results superimposable to those obtained with the neo probe (data not shown). Note that double-positive (CD4+CD8+) and single positive CD4~CD8+ cells from /-/OXS3-recipients do not express the retroviral-derived transcripts. Exposure times are 30 minutes for neo and 1 hour for actin. 4 . 2 . 5 A l t e r e d B c e l l d e v e l o p m e n t b y o v e r e x p r e s s i o n o f HOXB3 To detect possible effects of HOXB3 overexpression on B cell development, bone marrow IL-7-responsive B lymphoid clonogenic progenitor numbers were measured. While the absolute numbers of these progenitors in HOXB3 mice were within the normal range ( -7000 per femur, Figure 4.4C) only a small proportion were G418 resistant. This contrasts with both the high proportion of transduced myeloid progenitors in these same HOXB3 mice and that of pre-B progenitors in neo-control mice (Figure 4.4C). Furthermore, the rare G418 resistant colonies derived from the N e o H O X B 3 %% %%%% M M I mm*m 4 105 HOXB3 mice were only approximately one tenth of normal size. Together these results suggest that HOXB3 overexpression inhibits early B cell development. To characterize further possible B cell alterations in HOXB3 mice, phenotypically distinct B cell populations were evaluated by flow cytometry using a combination of monoclonal antibodies (anti- CD45R, CD43, IgM and IgD) (Figure 4.5). Normal proportions of the B cell subpopulations were seen in the peripheral blood and lymph nodes of HOXB3 mice (Figure 4.5, profiles from 2 of 5 HOXB3 recipients analyzed 18 weeks after transplantation); their proportions were somewhat reduced in the spleen but taking into consideration the increase in splenic cellularity, the absolute number of B cells in this organ was within normal range. Abnormalities were apparent however in the bone marrow where total B cells were consistently reduced (Figure 4.5A) with marked variations (moderate phenotype, HOXB3#1 and severe phenotype, HOXB3#2). HOXB3 mice manifested a 2-10 fold reduction in total bone marrow B cells (B220+) with all B cell populations being affected in those mice showing a more severe phenotype (HOXB3#2, Figure 4.5). 106 4.3 Discussion Overexpression of HOXB3 had multiple effects on hemopoiesis as assessed in a bone marrow transplantation model. Specifically, overexpression of HOXB3 leads to an almost complete block in the thymic production of CD4+CD8+ T lymphocytes accompanied by an expansion of y8-TCR+ thymocytes, impaired B lymphoid development and enhanced myelopoiesis leading to a myeloproliferative disorder. HOX63-induced alterations were particularly prominent in thymocyte development. The generation of mature ocf3-T cells were blocked by overexpression of HOXB3, as evidenced by reductions in double positive and single positive thymocytes. Furthermore, the few double positive thymocytes and their progeny found in thymuses of HOXB3 mice were negative for retroviral-derived HOXB3 expression representing either selection for transduced cells in which there was spontaneous downregulation of the LTR driven expression or contributions from untransduced cells. In contrast to the block in a(3-T cell maturation, the y8+ thymocytes in HOXB3 mice were expanded (on average ~4 fold). This reached the extreme in one HOXB3 mouse which had a large thymus containing >97% HSA+ y8 T cells. As we did not rule out the possibility that these cells had acquired pre-neoplastic properties as a result of additional somatic mutations, this mouse was not included in Figure 4.6. The expanded y8-TCR+ thymocytes in HOXB3 mice were distributed equally between CD4_CD8~ and CD4 l 0CD8 _ populations and consisted predominantly of cells expressing high levels of HSA, suggesting that they represent an immature subset of y5 T cells (Suda and Zlotnik, 1993). Together these results indicate that overexpression of HOXB3 has different effects on precursors of the a(3 TCR lineage than on those of the y8 TCR lineage, blocking the maturation of the former whereas inducing expansion of the latter. Whether these different effects are reflecting normal HOXB3 expression patterns in these cells is 107 currently unknown. While we have shown using an RT/PCR approach that the murine Hoxb-3 is normally expressed at very low levels in FACS purified double negative, double positive and mature single positive thymocytes (Sauvageau et al., Unpublished data), its expression pattern in subsets of double negative cells is currently unknown. Further characterization of HOXB3 expression in these subsets will be of interest since this population contains both mature y5 T cells and precursors of both y5 and ocp lineages (Dudley et al., 1995). It is curious that the defect in thymocyte development is not compensated by the non-transduced donor cells. One possible explanation for this is that HOXB3-transduced progenitors have a superior thymus-repopulating ability and therefore occupy the majority of the limited number of thymic niches (Spangrude and Scollay, 1990). Preferential niche occupation could also occur as a result of other mechanisms such as self-renewal advantages of /-/OX63-transduced cells or active suppression of normal thymocyte development by HOXB3 transduced cells. This thymic defect does not appear to be due to replacement by myeloid cells, since no myeloid infiltration is seen cytologically and fewer than 5% Mac-1+ cells are seen on FACS analysis of the thymuses isolated from HOXB3 mice. B lymphoid development was also impaired by HOXB3 overexpression. This was apparent from the virtual absence of transduced pre-B colony forming cells in bone marrow of HOXB3 mice and small colony size from the few such progenitors detected. However, in contrast to the findings in the thymus, the absolute number of pre-B colony forming cells in HOXB3 mice were within normal range presumably reflecting competitive repopulation from non-transduced cells. Some overall reduction in B cell content of the bone marrow was however seen, likely reflecting infiltration and displacement by the expanding granulocytic populations (Figure 4.5). The defects in B and T cell development described in this manuscript have not been reported with any other molecules, including overexpression of another 108 homeobox gene Hlx, which caused a release of immature CD4+CD8+ T cells in the periphery (Allen et al., 1995). HOXB3 mice also developed a myeloproliferative disorder as evidenced by splenomegaly, a marked accumulation of myeloid clonogenic progenitors with granulocyte and/or macrophage differentiation potential and by granulocytic infiltration of bone marrow and other hemopoietic organs. For all HOXB3 mice analyzed (n=4) this myeloproliferative disorder was readily transplantable to secondary recipients (data not shown). Most HOXB3 mice were sacrificed before 30 weeks and, in this time frame, progression of the myeloproliferative disorder to frank leukemia was not observed except for one mouse. Bone marrow clonogenic progenitor cells from this mouse were all G418 resistant and showed growth factor independence in the presence of fetal calf serum. Studies to assess characterization of this leukemia and further leukemic transformations that might develop in HOXB3 mice, are currently underway. Although incomplete, these data nonetheless support the accumulating evidence of Hox gene involvement in both murine and human leukemic transformation (Borrow et al., 1996; Nakamura et al., 1996a; Nakamura et al., 1996b; Perkins et al., 1990; Thorsteinsdottir et al., 1996). The results presented in this chapter stand in contrast to the findings previously described (Sauvageau et al., 1995) and presented in Chapter 5, that overexpression of HOXB4 did not detectably perturb myeloid, B or T cell differentiation but however induced expansion of myeloid and lymphoid progenitor cells and enhanced up to 47 fold the regeneration of the most primitive hemopoietic repopulating cell. These results also differ from findings presented in Chapter 3, that overexpression of HOXA10 in murine bone marrow cells leads to enhanced formation of megakaryotic progenitors in vivo and in vitro, diminished numbers of macrophage and B lymphoid progenitors and generation of myeloid leukemias in a significant proportion of mice 5 to 8 months after transplantation. These findings thus demonstrate very distinctive effects that results from enforced expression of different Hox genes in marrow cells. These observations 109 combined with previous study by Sauvageau et. al, which showed that Hox mRNA levels differ significantly between various purified subpopulations of bone marrow cells suggest that, as in embryonic development, Hox genes are important regulators of early hemopoietic developmental processes. 110 Chapter 5 Enhanced polyclonal regeneration of hematopoietic stem cells overexpressing HOXB4 following bone marrow transplantations ^The materia! presented in this Chapter is essentially as described in: U. Thorsteinsdottir, G. Sauvageau, M. R. Hough and R. Keith Humphries. (1997) Enhanced polyclonal regeneration of hematopoietic stem cells overexpressing HOXB4 following bone marrow transplantation. Manuscript in preparation. 111 5.1 Introduction As discussed in detail in Chapter 1, overexpression of HOXB4 in murine bone marrow cells was found to greatly enhance the regeneration of the HSC compartment following bone marrow transplantation, without detectable effects on hematopoietic differentiation, or on mature end cell output (Sauvageau et al., 1995). These results thus implicated HOXB4 as a potential regulator of self-renewal of very early but not late hematopoietic cells. In these studies, the assessment of the effects of overexpression of HOXB4 were done at 20 weeks post transplantation, and at that time point the HSC compartment was regenerated slightly above normal pretransplantation levels. By serially transplanting these bone marrow cells, HOXB4-transduced HSC were demonstrated to still harbor extensive potential for expansion, as evidenced by their -1000-fold expansion in secondary recipients, which again brought the HSC pool in those mice near to pretransplantation levels. Based on these results, I hypothesized that this greatly enhanced expansion potential of HOXB4-transduced HSC would only be expressed in a highly proliferative environment, such as in the early phase of hematopoietic regeneration, whereas in steady state hematopoiesis their expansion would be constrained. The HSC pool in recipients of HOX84-transduced bone marrow cells might thus plateau, presumably at pretransplantation levels, but would not become exhausted nor necessarily continue to expand. To test this hypothesis, I designed experiments to analyze the size of the regenerated pool of HSC in mice transplanted with HOXS4-transduced bone marrow cell, as a function of time after transplantation for a period extending up to a year after transplantation. Another unresolved issue from the initial studies on the effects of HOXB4 was the actual contribution to this expansion by individual HSC, i.e was HOXB4 primarily acting on one or few HSC, or on a broad spectrum of HSCs. In this study I have addressed that issue by analyzing the degree of polyclonality in the regenerated pool of HSC concurrently with the quantitation of the size of the pool. 112 The results presented in this chapter confirm our previous finding that overexpression of HOXB4 enhances the expansion of HSC following bone marrow transplantation and, moreover, show that in vivo expansion of HOXB4-transduced HSCs is subjected to environmental control mechanisms, as the pool size of HOXB4-transduced HSCs eventually stabilizes when it reaches that typical of normal (pre-transplant) mice. Increased HOXB4 expression also appeared to have a pronounced effect on many HSCs, as the regenerated pool of HOXS4-transduced HSC in transplanted mice was highly polyclonal. Furthermore, detailed analysis of various later cell populations in these mice strongly suggests that overexpression of HOXB4 does not overtly alter myeloid or lymphoid differentiation, nor lead to dominant outgrowth of any type of hematopoietic cell. 113 5.2 Results 5.2 .1 E x p e r i m e n t a l m o d e l To study the effects of increased and extended expression of HOXB4 on hematopoiesis, the HOXB4 cDNA isolated from human fetal liver cells (Piverali et al., 1990) was introduced into murine bone marrow cells using retroviral-mediated gene transfer. The murine stem cell virus (MSCV) 2.1 retroviral vector was used (Hawley et al., 1992) in which the HOXB4 cDNA was inserted 5' to the phosphoglycerate kinase promoter (PGK)-driven neo gene such that the HOXB4 expression was under the control of the regulatory elements within the MSCV 2.1 long terminal repeat (LTR) (Figure 5.1 A). The LTR sequences in this MSCV vector have been shown previously to give high and long-term expression both in primitive murine hematopoietic cells and their mature progeny (Hawley et al., 1994; Pawliuk and Humphries, Unpublished data; Sauvageau et al., 1995). The MSCV PGK-neo vector lacking the HOXB4 cDNA was used as a control (Hawley et al., 1992). The generation of high titer, polyclonal and helper-virus free HOXS4-viral producer cells, was carried out as described in Chapter 2. Integrity of the /-/OXS4-retrovirus was verified both by the detection of unrearranged 3.9 kb proviral fragment by Southern blot analysis of DNA isolated from bone marrow of HOXB4 mice (Figure 5.1 C), and by Northern blot analysis of RNA isolated from the same tissue, which detected the full length HOXB4 LTR-driven mRNA (Figure 5.3). To assess the possible effects of HOXB4 overexpression on hematopoietic regeneration, lethally irradiated mice were transplanted with HOXB4- or neo-transduced bone marrow cells. Cohorts of mice from three independent transplantation experiments (hereafter called HOXB4 and neo mice, respectively), were assessed for regeneration of various hematopoietic compartments at different times after transplantation, beginning as soon as 16 weeks and as late as 52 weeks after transplantation (Figure 5.1 B). In an effort to achieve high retroviral transduction to primitive hematopoietic cells, bone marrow cells from mice treated 4 days previously 114 with 5-fluorouracil (5-FU) were co-cultivated with HOXB4- or neo- viral producer cells for 48 hours prior to transplantation. Recovered cells were transplanted without pre-selection into lethally irradiated recipients at a dose of 2x105 cells/recipient, estimated to contain -30-40 HSCs (Sauvageau et al., 1995). The gene transfer efficiencies to the transplanted bone marrow as assessed by the proportion of G418 resistant clonogenic cells varied between experiments, and were 30 to 58% and 70 to 74% for HOXB4- and neo-transduced cells, respectively (Figure 5.1 B). Assuming a retroviral infection efficiency of HSCs no greater than that of clonogenic progenitor cells, each recipient would have received an estimated maximum of 15-30 transduced (neo or HOXB4) HSCs, plus an approximately equal number of non-transduced HSCs. 115 Kp LTR E B V Kp 1 I HOXB4 PGK-neo LTR Kp 1 SD E N SA 1 B / Kp 1 LTR l PGK-neo LTR 3.9kb 1.3kb 2.7kb 1.3kb HOXB4 % G418 resistant CFC Exp. 1 Exp. 2 Exp. 3 neo • 70 HOXB4 • 58 neo ° 74 j HOXB4 = 47 neo • 70 1 HOXB4 = 30 Transplantation Analysis of hematopoietic regeneration; time post Tx and number of mice 20 weeks neo | H0XB4 (n=3) (n-3) 16 weeks neo | HOXB4 (n-3) (n-3) 52 weeks | H0XB4 (n-2) 32 weeks | HOXB4 (n-2) 41 weeks neo I HOXB4 (n-2) (n-2) HOXB4 32-A 32-B 32-A 32-B 42-A 52-A 52-B B T B T B T B T B T B T B T • • • • • M M 3.9 kb 2.7 kb 3.9 kb HOXB4 1.3 kb Figure 5.1. Structure of the HOXB4 and control neo retroviruses and the experimental outline (A) Diagrammatic representation of the integrated HOXB4 and neo proviruses. Expected size of the full-length viral transcripts and also those initiated from the PGK promoter are shown, as are the sites for the various restriction enzymes used in this study. (B) Experimental outline showing the number of HOXB4 and neo mice from 3 transplantation experiments that were used in this study, and the time post transplantation when they were analysed. Also shown is the initial gene transfer to the transplanted bone marrow inoculum received by neo and HOXB4 mice in these transplantations. (C) Southern blot analysis of DNA isolated from bone marrow and thymus of some of the neo (sacrificed 32 weeks post -transplantation) and HOXB4 mice (sacrificed 32, 41 and 52 weeks post transplantation) used in this study ,to demonstrate the presence of the integrated provirus. DNA was cut with Kpn\, which releases the neo (2.7 kb) and the HOXB4 (3.9 kb) proviruses, and the blot was successively hybridized to probes specific for the neo and HOXB4 genes (full length HOXB4 cDNA was used as a probe). The endogenous murine HOXB4 is detected at 1.3 kb by the HOXB4 probe and provides a single gene copy control of loading. In some of the HOXB4 mice, in addition to the full length HOXB4 provirus, a weaker 2.7 kb proviral signal is detected with the neo probe but not with the HOXB4 probe. This probably represents the loss of the HOXB4 gene from a few of the integrated proviruses. Abbrev. CFC, colony forming cells; B, bone marrow: T, thymus. 116 5 . 2 . 2 E n h a n c e d r e g e n e r a t i o n o f C R U c e l l s i n m i c e t r a n s p l a n t e d w i t h H 0 X S 4 - t r a n s d u c e d b o n e m a r r o w c e l l s Hematopoietic regeneration in both neo and HOXB4 mice, from all 3 transplantation experiments, was essentially completely donor-derived as >85% of bone marrow, spleen, thymic and peripheral blood leukocytes were of transplant origin (Ly5.1+) at all times analyzed. A major contribution by transduced cells to this reconstitution was evident by Southern blot analysis that readily detected the neo- or the HOXB4-proviruses in the bone marrow and thymuses of these mice (Figure 5.1C). To determine the effects of HOXB4 overexpression on the size of the regenerated pool of HSCs as a function of time, HSC numbers in bone marrow of neo and HOXB4 mice were quantitated using the CRU assay (described in detail in Chapter 1) at the various time points outlined in Figure 5.1 B. The CRU numbers in bone marrow of neo control mice were quantitated 16, 20 and 32 weeks post transplantation. At all of these time points the CRU pool was reconstituted to levels <10% of normal pre-transplantation CRU values (Table 5.1). This low regeneration of CRU numbers after transplantation is consistent with previous studies by others following transplantation of non-retrovirally infected normal or 5-FU-treated adult bone marrow cells, where HSC levels did not regenerate above 10% of normal pre-transplantation values, as assessed either by the CRU assay or Harrison's Competitive repopulating assay (described in Chapter 1) (Harrison and Astle, 1982; Harrison et al., 1990; Pawliuk et al., 1996). 117 Table 5.1. CRU numbers regenerated in primary recipients of neo- or HOXB4 transduced bone marrow cells neo -transduced bone marrow recipients /-/OXB4-transduced bone marrow recipients Weeks post Tx Total CRU/femur % of normal3 (95% CI) levels Total CRU/femur % of normal a (95% CI) levels Experiment 1 2 0 b 60 (40-100) 3.5 2800 (2000-4000) 168 5 2 c ND 1062 (465-1616) 68 Experiment 2 16 b 151 (66-346) 9 1110 (759-1606) 67 3 2 d 128 (42-390) 7.5 1300 (556-2963) 78 Experiment 3 4 1 e ND 1428 (1112-1837) 86 aNormal pretransplantation CRU values from n=2, 16 week old (PebC3)F1 mice=1667(1042-2667) CRU/femur b At both these time points CRU numbers where evaluated in bone marrow cells pooled from three neo and \hreeHOXB4 mice C C R U numbers were evaluated in n=1 HOXB4 mouse d C R U numbers were evaluated in n=1 neo and in individually in n=2 HOXB4 mice, and their values were then averaged e C R U numbers were evaluated in individual n=2 HOXB4 mice, and their values were then averaged Abbrev. Tx, transplantation; CI, confident interval CRU numbers in HOXB4 mice from all three transplantation experiments, in contrast, were regenerated to levels near to or slightly above normal pretransplantation values, or with on average some 14-fold greater numbers of CRU than in neo control mice (Table 5.1). In both experiments 1 and 2 the frequency of CRU cells in HOXB4 mice were similar at the early ( 20 or 16 weeks) and late time points (32 or 52 weeks) after transplantation (Table 5.1). This suggests that the enhanced expansion of CRU cells occurs relatively early during hematopoietic regeneration, with stabilization in pool size at normal levels. Similarly, in HOXB4 mice from 118 transplantation experiment 3, which were analyzed 41 weeks post transplantation, the CRU frequency reached that of normal unmanipulated mice (Table 5.1). 5 . 2 . 3 P o l y c l o n a l h e m a t o p o i e t i c r e g e n e r a t i o n b y W O X B 4 - t r a n s d u c e d c e l l s b o t h a t t h e l e v e l s o f C R U a n d m a t u r e e n d c e l l s Hematopoietic regeneration by transduced cells in neo and HOXB4 recipients was further assessed by Southern blot analyses of proviral integration sites (clonality) in DNA isolated from hematopoietic tissues of neo and HOXB4 recipients initially transplanted with neo or WOXS4-transduced bone marrow cells (primary recipients), and those which were transplanted with various dilutions of bone marrow from these primary recipients for the CRU quantitation (secondary recipients). In the bone marrow, spleen and thymus of primary HOXB4 recipients, sacrificed 32 and 52 weeks post transplantation, multiple proviral integrations could be detected (Figure 5.2A and B). Since proviral signals of different intensities could be detected in the same tissues, these multiple integrations most likely reflect polyclonal regeneration by HOXB4-transduced HSCs, rather than multiple proviral integrations into the same HSC. This interpretation was further supported by clonal analysis of the secondary transplant recipients (see below). Thus, even as late as 52 weeks post transplantation, the hematopoietic regeneration in the primary HOXB4 recipients was polyclonal and without apparent dominance of any /-/OXB4-transduced clone. As the quantitation of CRU cells in primary mice was based upon injecting CRU cells at a limiting dilution into secondary recipients, Southern blot analysis of proviral integrations in thymus (lymphoid) and bone marrow (myeloid) of these secondary recipients could be used to analyze further the clonality of the regenerated pool of transduced lympho-myeloid repopulating (CRU) cells in primary HOXB4 and neo mice. Of the 10 secondary recipients (32-1 to 32-10) that were positive for lympho-myeloid repopulation by donor cells from one of the primary HOXB4 mice sacrificed 32 weeks post transplantation, 8 had detectable HOXB4 proviral integration in their bone marrow and thymus (Figure 5.2A). In the case of the two "negative" mice (32-5 and 32-119 10), which both had low donor repopulation (Ly5.1+ PBL, 4 and 5 %), the expression of HOXB4 could however be detected by Northern blot analysis of their bone marrow (Figure 5.3), suggesting that this provided a more sensitive measure of the presence of HOXB4 than the Southern blot analysis. The one mouse analyzed (32-11) that was negative for donor derived repopulation was also negative for proviral integration by Southern blot analysis (Figure 5.2A). There was thus complete concordance between detection of donor derived hematopoietic regeneration and contribution to regeneration by HOXB4 transduced cells in secondary mice. In essence, all donor-derived regeneration in the primary mouse must thus have been derived from HOXB4-transduced stem cells. Of the secondary mice analyzed, several, including those which were estimated to receive between 1 and 2 CRU cells (mice 32-6, 32-8 and 32-9), had a common proviral integration in their bone marrow and thymus, thus confirming the lympho-myeloid repopulating potential of the regenerated /-/OXB4-transduced CRU cells (Figure 5.2A). Self-renewal of /-/OXB4-transduced CRU cells was also demonstrated by detecting a common lympho-myeloid repopulating clone in all of the secondary recipients (32-1 to 32-4) receiving high cell dose (43 CRU/mouse) and in one mouse (32-6) receiving fewer CRU cells (2 CRU/mouse). Interestingly, this clone could not be detected in bone marrow, spleen or thymus of the primary HOXB4 mouse (Figure 5.2A), indicating that this cell, despite extensive self-renewal division, did not contribute significantly to bone marrow, spleen or thymic repopulation in this primary HOXB4 recipient. The detection of 3 different HOXS4-transuced CRU clones, and at least 7 others with either lymphoid or myeloid potential in these secondary HOXB4 mice, together with detection of other /-/OXB4-transduced CRU clones in the primary recipient, strongly suggest that the enhanced CRU regeneration in the primary HOXB4 mouse sacrificed 32 weeks post transplantation was a polyclonal event. 120 leo HOXB4 2° mice 2" mice 2" mice 2" mice 1x106cells 1° mouse SVxIOScells 34x104cells 1.4x104cells (-5 CRU) (-43 CRU) (-2CRU) (-1 CRU) 32 32-1 32-2 32 32-1 32-2 32-3 32-4 32-5 32-6 32-7 32-8 32-9 32-10 32-11 B S T B T B T B S T B T B T B T B T B T B T B T B T B T B T B T 12 kb 5 ; f s a -5kb •4kb •3kb • 2kb S H I P - - « » « » • W — m • - m — w w • — — — — — - — mm 25 kb HOXB4 2° mice 2° mice 2 mice 1° mouse 6.7x10s cells 3.4x104 06115 1.4X104 cells (-39 CRU) (-2 CRU) (-1 CRU) 52 52-1 52-2 52-3 52 4 52-5 52-6 52-7 52-B 52-9 B S T B T B T T B T B T B T B T B T B T is K -5 kb • 4 kb • 2 kb S H I P „ ^ I t w M M M W M W M M M — 11 kb Figure 5.2 Southern blot analysis of proviral integration patterns in primary neo and HOXB4 mice sacrificed 32 weeks (A) or 52 weeks (B) post transplantation and their secondary recipients from the CRU assay. DNA was digested with eamHI (A) or EcoRI (B), that cuts the integrated provirus once, generating a DNA fragment specific for each proviral integration site. The membranes were first hybridized to a neo specific probe for detection of proviral fragments and subsequently with a probe specific for the SHIP gene to provide a single copy control of loading. Exposure times were 48 hours for the neo and SHIP probes. To demonstrate that the proviral fragments contained the HOXB4 cDNA the blots were also hybridized with full-length HOXB4 cDNA probe, which generated the same proviral banding pattern as the neo probe (data not shown). Each mouse is identified with a specific number derived from the time that the primary recipient was sacrificed and above that number are indicated the number of bone marrow cells received by each secondary recipient and the estimated CRU content of each inoculum. Percentage of donor derived leukocytes (Ly5.1+) in the peripheral blood of the secondary neo mice was: 32-1(44% ), 32-1(18%) and that of secondary HOXB4 mice: 32-1(60%), 32-2(76%), 32-3(83%), 32-4(52%), 32-5(4%), 32-6(23%), 32-7(18%), 32-8(9%), 32-9(51%), 32-10(5%), 32-11(0%), 52-1(5%), 52-2(66%), 52-3(28%), 52-4(47%), 52-5( 5%), 52-6(36%), 52-7(47%), 52-8(20%) and 52-9(50%). Abbrev. B, bone marrow; T, thymus. 121 To determine if the polyclonality of the CRU pool was maintained with time in primary HOXB4 mice, proviral integration was also assessed in DNA isolated from bone marrow and thymus of those secondary HOXB4 mice regenerated with bone marrow cells from the primary HOXB4 mouse sacrificed 52 weeks post transplantation (Figure 5.2B). In all of the 9 secondary HOXB4 recipients that were scored positive for donor derived regeneration, proviral integration(s) were detected in their bone marrow and/or thymus (Figure 5.2B). Of those, 5 had a common proviral pattern in their bone marrow and thymus (mice 52-2, 52-4, 52-6, 52-8, and 52-9 (bone marrow signal very faint)), indicating again the lympho-myeloid repopulating potential of the regenerated CRU cells. Interestingly, the proviral integrations in the regenerated pool of HOXB4-transduced repopulating cells in this primary HOXB4 mouse were more complex than in the HOXB4 mouse analyzed 32 weeks post transplantation (Figure 5.2A and 5.2B). This was evident both by the numbers of transduced clones detected, where a common /-/OXS4-transduced clone was only detected in thymuses of two secondary mice (i.e. mice 52-2 and 52-3) and by the complexity of proviral banding patterns of individual clones (Figure 5.2B). This difference is likely a reflection of higher gene transfer to the transplant inoculum received initially by the HOXB4 mice analyzed at 52 weeks than by those analysed 32 weeks after transplantation (Figure 5.1 B). Furthermore, these results, together with polyclonal regeneration of bone marrow, spleen and thymus of the primary HOXB4 mouse by transduced totipotent cells, indicate that even as late as 52 weeks after transplantation, the CRU pool is highly polyclonal. By Northern blot analysis, the HOXB4 message could readily be detected in bone marrow of both primary HOXB4 recipients at 41 weeks post transplantation, and in secondary recipients 13 weeks after transplantation of bone marrow cells from primary HOXB4 recipients sacrificed 32 or 52 weeks after transplantation (Figure 5.3). Thus, the HOXB4 message was clearly expressed at high levels in HOXB4 recipients, without any evidence for promoter shutdown, for more than a year after 122 transplantation. As seen in Figure 5.3, the strength of the HOXB4 message varied between the mice analyzed, and, as expected roughly correlated with their levels of donor derived repopulation (%Ly5.1). 1 ° HOXB4 2° H0XB4 32 w. post Tx 2° H0XB4 52 w. post Tx 41 w. post Tx HOXB4 Actin 141-A 141-B | 32-1 | 32-2 132-3 132-4 132-5 132-6 132-7 32-8 32-10 52-1 52-2 52-3 52-4 52-5 52-6 52-7 • 3.9 kb •1.3 kb Figure 5.3 Northern blot analyses to detect expression of the transduced HOXB4 gene in bone marrow of HOXB4 mice Total RNA (~5u.g) was isolated from bone marrow of primary HOXB4 mice sacrificed 41 weeks post transplantation and those secondary recipients that received varying numbers of bone marrow cells from the primary HOXB4 mice sacrificed 32 and 52 weeks post transplantation. The blots were sequentially hybridized to HOXB4 specific probes to detect the viral transcript (3.9 kb) and to probe specific for actin to assess for loading. Each number assigned to individual lanes identifies a specific mouse which can also be identified in Figure 2 on the Southern blot analysis. The percentage donor derived repopulation in the secondary HOXB4 mice analysed were: 32-1(60%), 32-2(76%), 32-3(83%), 32-4(52%), 32-5(4%), 32-6(23%), 32-7(18%), 32-8(9%), 32-9(51%), 32-10(5%), 52-1(5%), 52-2(66%), 52-3(28%), 52-4(47%), 52-5( 5%), 52-6(36%) and 52-7(47%). 5 .2 .5 W O X 6 4 - i n d u c e d e x p a n s i o n in vivo o f c l o n o g e n i c p r o g e n i t o r c e l l s Elevated numbers of myeloid clonogenic progenitors were also detected in HOXB4 mice compared to control neo mice (Figure 5.4B). This was most prominent in the 123 spleen. At all time points, myeloid progenitors were significantly increased in spleens of HOXB4 mice, compared to those of neo control mice, with the greatest expansion documented being ~40-fold over control (Figure 5.4). In mice from all three transplantations, the splenic progenitor numbers increased with time after transplantation, and the magnitude of this expansion appeared to correlate with the initial HOXB4 retroviral transduction efficiency (Figure 5.4 and Figure 5.1 B). The bone marrow myeloid progenitor numbers were also increased in HOXB4 mice compared to neo control mice, but this increase was not statistically significant except in mice analyzed 20 weeks post transplantation (Figure 5.4). In both the bone marrow and spleen of HOXB4 mice, the majority of myeloid progenitors were G418 resistant (68+18% and 75+30%, respectively), likely reflecting preferential expansion of HOXB4-transduced cells over untransduced cells. Overall, the total body increase in myeloid progenitor numbers was less than 2-fold in the majority of the mice analyzed, with the exception of those analyzed 20 weeks post transplantation, where the increase was ~5-fold. Overexpression of HOXB4 was not found to cause preferential expansion of any one of the different types of myeloid progenitor (i.e. CFU-M, CFU-GM, BFU-E, and CFU-GEMM), as transduced progenitors from HOXB4 mice generated the various in vitro colony types with the same relative frequencies as those derived from neo control mice (data not shown). Evaluation of bone marrow pre-B lymphoid colony forming cells at various times after transplantation showed equivalent numbers in HOXB4 and neo mice at an early time point (16 weeks after transplantation), but elevated numbers (2 to 3-fold) in HOXB4 mice at later time points (32 and 41 weeks post transplantation) (Figure 5.4). As seen for the myeloid progenitor cells, a major proportion of pre-B progenitor cells in HOXB4 mice were G418 resistant (HOXB4 mice=59±9% vs. neo mice=37±10%), consistent with their preferential derivation from HOXB4-transduced cells. 124 1000000 100000 -A 10000H 1000 1000000 100000H 10000H 1000 100000 E2 neo • HOXB4 20 52 16 32 41 Experiment 1 Experiment 2 Experiment 3 20 52 Experiment 1 16 32 41 Experiment 2 Experiment 3 10000-^  1000 20 52 Experiment 1 16 32 Experiment 2 41 Experiment 3 Weeks post transplantation Figure 5.4 Effects of HOXB4 overexpression on the expansion of myeloid and pre-B colony forming cells following bone marrow transplantation Results shown are means ±SD of the numbers of in vitro myeloid colony forming cells in bone marrow (top) and spleen (middle) and of pre-B colony forming cells in bone marrow (bottom) of individual neo and HOXB4 mice, at various time points after transplantation. The number of neo and HOXB4 mice analyzed at each time point are shown in Figure 1B. Bone marrow myeloid colony forming cells in HOXB4 mice were only significantly increased over that of neo control mice in experiment 1 at 20 weeks post transplantation (p<0.05). Number of splenic myeloid colony forming cells were significantly increased at all time points in HOXB4 mice (p<0.05). Pre-B colony forming cells were not significantly increased in HOXB4 mice at any time point. Abbrev. CFC, colony forming cell. 125 5 . 2 . 5 E n h a n c e d r e g e n e r a t i o n o f H O X S 4 - t r a n s d u c e d C R U c e l l s i s n o t r e f l e c t e d i n t h e n u m b e r o f t h e r e g e n e r a t e d S c a 1 + L i n " W G A + c e l l s Murine bone marrow cells that express the Ly6A/E antigen (Sca1+), but are negative for various lineage markers (B220, Ly-1, Gr-1 and Mac-1) and bind to lectin wheat germ agglutinin (WGA+), have previously been shown to be enriched ~400-fold for cells with long term lympho-myeloid repopulating potential when compared to unfractionated bone marrow (Rebel et al., 1994). To analyze whether the enhanced regeneration of CRU cells in HOXB4 mice was reflected in the size of the regenerated pool of Sca1+Lin-WGA+ cells, the numbers of these cells were measured in bone marrow of HOXB4 and neo mice 32 and 52 weeks post-transplantation. In both neo and HOXB4 mice, >90% of Sca1+Lin- cells were donor-derived (Ly5.1+). Interestingly, the proportion and absolute number of Sca1+Lin-WGA+ cells were higher in the bone marrow of neo mice than in HOXB4 mice and thus in contrast to the CRU numbers (Table 5.2). This suggests that HOXB4 expansion of CRU is not tightly coupled to expansion of phenotype. Table 5.2 Regeneration of Sca1 +Lin"WGA + cells in neo and HOXB4 mice3 neo HOXB4 32 weeks post Tx 32 weeks post Tx 52 weeks post Tx %Sca1+Lin-WGA+ cells in total bone marrow 0.073±0.0042 0.015+0.0014 0.041+0.024 Number of Seal + L in 'WGA + cells/femur 15450±5020 2810+580b 7230+2079 CRU numbers/femur 128 1300 1062 aResults shown are means +SD for n=2 neo mice and n=2 HOXB4 mice 32 and 52 weeks post transplantation bSignificantly lower than in neo control mice, p<0.05, students t-test Abbrev. Tx, transplantation; #, number 126 5 .2 .6 L a c k o f HOXB4 e f f e c t s o n m a t u r e e n d c e l l o u t p u t In contrast to increases in both CRU and clonogenic progenitor numbers, the peripheral blood white and red blood cell counts in HOXB4 mice analyzed at early and late time points were within the normal range (Table 5.3). The same was true for their femoral, thymic and splenic nucleated cell counts, with the exception of the two mice analyzed 52 weeks post transplantation, which had moderate splenomegaly (Table 5.3). Table 5.3 Hematological parameters in HOXB4 and neo mice 16-52 weeks post transplantation3 Peripheral blood Weeks post Tx Mice TNC/femur TNC/thymus TNC/spleen RBC WBC (x107) (x107) (x108) (x109/ml) (x106/ml) Experiment 1 20 neo (n=3) 1.9+0.2 ND 3.3±0.5 7.9+0.4 7.8±2.5 HOXB4 (n=3) 2.0±o.2 ND 3.6±0.6 7.6±1 8.0+2.2 52 HOXB4 (n=2) 1.95+0.6 4.5+0.5 7.7+5.0 7.7+1.0 8.2+2.5 Experiment 2 16 neo (n=3) 2.4±0.7 9.1+0.5 HOXB4{n=3) 1.8±0.2 8.0+1.5 3.1+0.5 ND 3.2+0.25 ND ND ND 32 neo (n=2) HOXB4 (n=2) 2.2+0.5 2.0+0.7 6.0+1.0 6.4+1.6 2.7±0.2 2.8±0.1 7.6±0.5 7.7±0.75 7.5±0.6 8.0+1.4 Experiment 3 41 neo (n=2) 1.9+0.5 HOXB4 (n=2) 2.1+0.1 ND 2.2+0.7 ND ND 2.45±0.6 ND ND ND aResults shown are mean±SD for the indicated number of neo and HOXB4 mice. Abbrev. Tx, transplantation; TNC, total nucleated cell counts; RBC, red blood cells; WBC, white blood cells To analyze further the possible effects of HOXB4 overexpression on the cellular constitution of hematopoietic organs, the absolute numbers and proportions of various lymphoid and myeloid cell populations in bone marrow, spleen and thymus of HOXB4 127 and neo mice were assesed by flow cytometric analysis. As shown in Table 5.4, the absolute numbers of bone marrow B and myeloid cell subpopulations, and of splenic B, T and myeloid cell subpopulations in HOXB4 mice analyzed 32 weeks post transplantation, were all comparable to that of neo control mice, as were their thymic CD4 and CD8 subpopulations. Similar results were also obtained for the HOXB4 mice analyzed 16 and 52 weeks post transplantation except for the two HOXB4 mice sacrificed 52 weeks after transplantation with splenomegaly. These mice had an elevation in all splenic B,T and myeloid cell subpopulations assessed, consistent with the increase in their spleen celluiarity (i.e. ~2- and ~5-fold). Table 5.4 Absolute numbers of various phenotypically defined hematopoietic populations in neo and HOXB4 mice 32 weeks after transplantation3 Mice Bone marrow B220+ (x106) B220+/CD43+ (x106) B220+/lgM+ (x106) Mac1 + (x106) neo 7.6+1.5 1.6±0.4 3.2+0.7 5.0±0.5 HOXB4 6.4+0.3 1.5+0.1 2.5+0.4 5.2+1.0 Spleen B220+ (X107) lgM+/lgD+ (x107) CD3+ (X107) Mac1 + (x107) neo 12.4±2.0 7.5±1.5 3.3±0.4 0.9+0.1 HOXB4 13.2±2.0 8.5±3.0 3.0+0.1 1.1+0.1 Thymus CD4-CD8" (x107) CD4+CD8+ (x107) CD4+CD8" (x107) CD4"CD8+ (x107) neo 0.24+0.11 7.8±0.5 0.83+0.05 0.17+0.08 HOXB4 0.19+0.04 6.9±1.7 0.62±0.14 0.10±0.07 aResults shown are mean±SD for n=2neo and n=2 HOXB4 mice. 128 5.3 Discussion This study confirmes our earlier findings that overexpression of HOXB4 enhances the regeneration of the HSC compartment following transplantation. Moreover, measure of the numbers of regenerated HSC at various times after transplantation, revealed stabilization at normal pre-transplantation levels, indicating that in vivo the expansion of /-/OX84-transduced HSC can be regulated by in vivo control mechanisms. HOXB4 was also demonstrated to act on multiple HSCs, as the regenerated pool of HOXB4-transduced HSCs in mice transplanted with /-/OXS4-transduced bone marrow cells was highly polyclonal, even as late as one year after transplantation. Furthermore, detailed analysis of various later cell populations in these mice strongly suggests that overexpression of HOXB4 does not overtly alter myeloid or lymphoid differentiation, nor lead to dominant outgrowth of any type of hematopoietic cells. Previous studies have shown that there is only limited recovery of the HSC pool in mice transplanted with normal bone marrow cells, in contrast to complete regeneration of progenitor cells and more mature cells to normal levels (Harrison and Astle, 1982; Harrison et al., 1990; Pawliuk et al., 1996). This has been demonstrated both by competitive repopulating studies, where the competitive repopulating potential of transplanted bone marrow was found to be reduced ~ 10-fold relative to normal bone marrow (Harrison and Astle, 1982; Harrison et al., 1990), and more recently by limit dilution analysis, which showed that the CRU numbers following single transplantation only recover to -10% of pretransplantation levels (Pawliuk et al., 1996). It has been suggested that the transplantable HSCs may fail to fully regenerate the HSC compartment because of their inherently limited capacity for self-renewal and/or the sustained proliferative stress imposed on these cells during the early phase of hematopoietic regeneration, resulting in stimuli that could favor differentiation rather than self-renewal responses and/or as a result of negative feedback mechanisms that prematurely inhibit their expansion (Harrison et al., 1990; Pawliuk et al., 1996). In mice 129 transplanted with H0XB4-transduced bone marrow cells, there is in contrast, regeneration of HSC (CRU) numbers to normal pre-transplantation levels, when analyzed over periods extending from 16 to 52 weeks after transplantation. Although mice in these experiments received a transplant inocula of which likely <50% of total CRU were transduced, analysis of the regenerated pool of CRU cells indicated that the vast majority of the CRU cells in these HOXB4 mice were positive for the presence of the HOXB4 provirus. This was evidenced by detecting the HOXB4 provirus in all of the secondary recipients of bone marrow cells from these primary transplant recipients, including those that were estimated to have received 1 to 2 CRU cells. Thus, an enhanced regeneration of CRU in HOXB4 mice appeared to be an intrinsic property of /-/OXB4-transduced CRU, conferring on them the capacity to outcompete the regeneration of untransduced CRU in the post-transplant period. Moreover, clonal analysis indicated that this property was not limited to selected CRUs, as hematopoiesis in HOXB4 recipients was highly polyclonal. One explanation for the enhanced regeneration /-/OXB4-transduced CRU is that the initial CRU pool, prior to their expansion in the mice, was larger in the HOXB4 recipient than in the neo control mice, due to overexpression of HOXB4 enhancing either the recruitment of CRUs in the transplanted inocula or the seeding efficiency of CRU cells to appropriate locations following transplantation. To evaluate the contribution of such factors requires knowledge about the exact numbers of CRU cells received by each recipient, so the expansion per CRU can be estimated. Although the number of CRU received by primary recipients was not evaluated either in this study or in our previous study when HOXB4-transduced bone marrow was serially transplanted (Sauvageau et al., 1995), the numbers received by secondary recipients in that study were known. There secondary neo and HOXB4 recipients were transplanted with cell doses estimated to contain equivalent numbers of cells detectable as CRU (2-5 CRU per mouse), thus eliminating differences due to HOXB4 enhancing recruitment of CRU. When analyzed 16 weeks after transplantation, the expansion per CRU in secondary 130 H0XB4 recipients was estimated to be ~50-fold higher than in neo control recipient, which is much greater expansion than could be accounted for by increased seeding efficiency alone (Sauvageau et al., 1995). Other possibilities are that HOXB4 enhances the probability of HSCs to execute self-renewal divisions or increases the self-renewal potential (i.e. allows for more self-renewal divisions) and/or proliferation (i.e. increased number of cell divisions) of HSCs. One way to determine whether HOXB4 enhances the probability of CRU cells to undergo self-renewal divisions, would be to analyze the size of the CRU pool at very early time points after transplantation, when the CRU regeneration is in log phase. If HOXB4 increases the self-renewal probability of CRU cells, /-/OXB4-transduced CRU would expand their numbers faster than control CRU cells. Another explanation for the enhanced regeneration of HOXB4-transduced HSC is that overexpression of HOXB4 reduces the sensitivity of HSC to negative feedback mechanisms that might normally prematurely inhibit the expansion of HSC. However, the enhanced ability of /-/OXB4-transduced progenitor cells to generate secondary colonies in vitro upon replating as we previously reported (Sauvageau et al., 1995), suggest that more likely explanations are those discussed above, involving enhanced HSC self-renewal. In our previous study on the effects of overexpression of HOXB4 we showed by serial transplantation that the CRU regenerated in primary recipients retained the capacity for extensive regeneration upon further transplantation into secondary recipients. In both primary and secondary HOXB4 recipients the CRU levels reached near normal values or with an overall -2500-fold greater net expansion over serially transplanted CRU in neo control recipients (Sauvageau et al., 1995). These serial transplantation studies thus demonstrated that the HOXB4-Xransduced CRU cells in the primary HOXB4 recipients still harbored extensive expansion potential. To assess how this enhanced expansion potential of HOXB4-transduced CRU cells would affect their behavior in steady state hematopoiesis, the CRU frequency in two cohorts of primary HOXB4 recipients were measured at early (16 or 20 week) and late (32 or 52 131 weeks) time points after transplantation. Interestingly, the CRU frequency was similar in both cohorts of mice and at both time points, with apparent stabilization in the size of the CRU pool around normal adult mouse marrow levels. Thus, the population of /-/OXB4-transduced CRU cells did not continue to expand nor become exhausted in the primary HOXB4 recipients. These data point to the existence of regulatory mechanisms that can control the size of the CRU pool and which overexpression of HOXB4 does not override. Shutdown of HOXB4 expression in CRU cells could explain the observed stabilization in the size of the CRU pool. The detection of the HOXB4 message in bone marrow of secondary recipients repopulated with as few as ~1 CRU cells originating from primary HOXB4 recipients sacrificed 32 or 52 weeks after transplantation, strongly indicates that the HOXB4 message was being expressed at high levels in CRU cells as late as 52 weeks post transplantation, and promoter shutdown is thus an unlikely explanation for the stabilization in the CRU pool. In mice, the number of HSCs remains constant throughout their adult live, indicating that during steady state in vivo hematopoiesis the proliferation of HSCs is controlled (Harrison and Astle, 1982; Harrison and Lemer, 1991; Ogden and Micklem, 1976). These likely involve mechanisms that keep the majority of HSC out of cycle, as many of these cells are highly resistant to cytotoxic drugs such as 5-FU (Harrison and Lemer, 1991). The stabilization of the CRU pool in HOXB4 mice at normal levels, suggests that although CRU cells overexpressing HOXB4 have enhanced regenerative potential, their ability to respond to this regulatory mechanism may not be altered. However, as the cycling status of CRU cells in HOXB4 recipients is currently unknown, other regulatory mechanisms acting to maintain stable levels of /-/QXB4-transduced CRU cells cannot be ruled out. Overexpression of HOXB4 also promoted expansion of intermediate types of progenitors in HOXB4 mice to levels above normal values. Whether this expansion was due to an increased input from more primitive cells or increased division of progenitor cells prior to reaching mature end cell stage, is not clear. However, the 132 latter possibility is suggested by our previous observation that progenitor cells overexpressing HOXB4 replate better in vitro than control neo infected progenitors (Sauvageau et al., 1995). Despite profound effects on the expansion of primitive hematopoietic cells, overexpression of HOXB4 did not alter the ability of these cells to complete normal differentiation programs in vitro or in vivo to produce mature cells of both lymphoid and myeloid lineages, nor did HOXB4 appear to promote preferential expansion along any hematopoietic lineage or lead to leukemia despite evidences of persistent HOXB4 expression for at least 52 weeks. Limited information exists about the downstream targets regulated by Hox genes. Of those identified to date, the majority appear to be either growth factors (e.g. decapentaplegic (dpp), a member of the TGFp superfamily, and basic fibroblast growth factor) (Capovilla et al., 1994; Care et al., 1996; Her et al., 1995) or cell adhesion molecules (e.g. connectin, N-CAM and L-CAM) (Goomer et al., 1994; Gould and White, 1992; Jones et al., 1992). Although Hox proteins have not yet been shown to directly regulate hematopoietic adhesion molecules or growth factors, the evidence cited above and the important roles that both adhesion molecules and growth factors play in hematopoiesis makes them interesting candidates for further study. The effects of HOXB4 overexpression during hematopoietic regeneration can be viewed as more "restricted" than those generated by overexpression of either HOXA10 or HOXB3 that were presented in Chapters 3 and 4. The effects of HOXB4 are predominantly seen on the expansion of HSCs during the early phase of hematopoietic regeneration, despite LTR-driven expression in most other hematopoietic cells and for a more extended period (>1 year post transplantation). It could thus be hypothesized that targets "available" to HOXB4 were restricted to primitive hematopoietic cells and that these targets might increase the sensitivity of these cells to some form of "stem cell expanding factor(s)" present only during the 133 early phase of hematopoietic regeneration. The ability of HOXB4 to enhance the regeneration of HSC, without altering their ability to differentiate normally, makes HOXB4 a useful tool to gain insight into the molecular processes that regulate proliferation of early hematopoietic cells. 134 Chapter 6 General Discussion and Conclusions 6.1 Overexpression of Hox genes perturbs hematopoietic development at multiple stages and involving multiple lineages The transcriptional control of hematopoiesis is not well characterized. To identify transcription factors that might play an instructive role in regulation of hematopoiesis, various approaches have been applied. Many of the factors now known to be active in hematopoietic cells have been identified either by their aberrant expression in hematological malignancies (Nichols and Nimer, 1992) or by detection of their ability to bind to cis-regulatory sequences of various lineage specific genes (Georgopoulos et al.,,1992; Tsai et al., 1989). By analogy of hematopoiesis to other developmental programs, another fruitful approach has been to focus on factors highly conserved in evolution and that play central roles in tissue specification in other developmental systems. Taking this approach, it has been demonstrated that at least 20 of the 39 members of the Hox homeobox gene family, which are best known for their roles in specifying cell identity and pattern formation in a number of embryonic tissues, are expressed in normal hematopoietic cells. In hematopoietic cells, Hox gene expression appears to be largely confined to early cells (Giampaolo et al., 1994; Sauvageau et al., 1994), although a reactivation in expression of some of these genes has been described upon mitogenic activation of mature NK and T cells (Care et al., 1994; Quaranta et al., 1996). In early hematopoietic cells, Hox genes display distinctive expression patterns, with genes located at the 3' side of the Hox clusters being preferentially expressed in the most primitive subpopulation of bone marrow cells, while others, primarily located at the 5' end show a broader range of expression with downregulation at later stages of hematopoietic differentiation (Sauvageau et al., 1994). Functional roles for Hox genes in hematopoiesis is further strengthened by 135 studies where their normal expression has been altered. Thus, for example, overexpression of Hoxb-8 in murine hematopoietic cells enhances the generation of non-leukemic myeloid cell lines (Perkins and Cory, 1993), and our group has recently shown that overexpression of the 3' located HOXB4 selectively enhances the expansion of primitive hematopoietic cells, most profoundly the HSC (Sauvageau et al., 1995). The main goals pursued in this thesis were to provide further insight into the possible functional roles of Hox genes in hematopoiesis, and to determine whether there were Hox gene-specific roles. The initial strategy taken was to retrovirally overexpress, in murine bone marrow cells, two selected Hox genes, HOXB3 and HOXA 10, that were chosen based on their divergent expression patterns in hematopoietic cells. The subsequent effects of these manipulations on the behavior of hematopoietic cells were then analyzed both in a transplantation model and in various culture systems. Manipulations of both these genes resulted in demonstrable perturbations in hematopoiesis, and each gene generated a unique phenotype. These effects are summarized in figure 6.1, in relation to those reported for the overexpression of HOXB4. In sharp contrast to HOXB4, both HOXB3 and HOXA10 failed to enhance the regeneration of HSC, but rather induced expansion of more restricted progenitor cells and altered myeloid and lymphoid differentiation, albeit in different manners. Specifically, overexpression of HOXA 10 lead to enhanced formation of progenitor cells with megakaryocytic potential, was non-permissive for macrophage and B-lymphoid differentiation and predisposed to leukemic transformation (Thorsteinsdottir et al., 1996). Overexpression of HOXB3, in contrast, blocked thymic production of CD4+CD8+ T lymphocytes and enhanced expansion of Y8-thymocytes, suppressed early B-lymphoid development, and led to a myeloproliferative disorder. (Sauvageau et al., 1996). Taken together, these results, combined with both the effects of overexpression of HOXB4 and the extensive literature showing that Hox gene expression is largely confined to early hematopoietic 136 cells, strongly indicate that Hox genes are involved in regulation of both proliferation and differentiation of early hematopoietic cells of multiple hematopoietic lineages. Furthermore, they point to Hox gene-specific effects, which likely reflects their regulation of different target genes during hematopoietic development. T cells Ty5-T cells HOXB3 lymphoid precursor common? / . ap-T cells HOXB4 HOXB3 HSC B cells pre B . . mature B cells HOXB3 and HOXA10 expansion of "megakaryotic progenitors uncontrolled myeloproliferation Others HOXA10 CFU-M Figure 6.1. Schematic representation of the effects observed on various hematopoietic lineages with overexpression of HOXB3, HOXB4 or HOXA10 The second aim of the work presented in this thesis was to delineate further the nature of effects of HOXB4 overexpression on the regenerative potential of HSC. For that purpose, the size and the clonal composition of the regenerated pool of HSCs in mice transplanted with bone marrow cells overexpressing HOXB4 was analyzed at various time points (16 to 52 weeks) after transplantation. The results from these studies confirmed our previous observation that HOXB4 overexpression enhances the regeneration of HSC following transplantation, and demonstrated for the first time that this effect involves multiple HOXS4-transduced HSCs, and that in vivo expansion of HOXS4-transduced HSCs is ultimately subject to endogenous control mechanisms 137 that limits their further expansion. Furthermore, detailed analysis of various later cell populations in these mice (as late as 52 weeks after transplantation) strongly suggests that overexpression of HOXB4 does not overtly alter myeloid or lymphoid differentiation, nor lead to dominant outgrowth of any type of hematopoietic cell. 6.2 Overexpression of Hox genes: phenotype vs function The striking differences in the phenotypes generated by overexpression of HOXB4, HOXB3 and HOXA10, strongly suggest that under normal physiological conditions these genes play different functional roles in hematopoiesis. However, it becomes more problematic to infer from the phenotype generated by their overexpression, the exact stage(s) of hematopoiesis where these genes, under normal physiological conditions, have functional roles. This is due in part to limitations that are inherent in the forced overexpression approach taken. The LTR in the MSCV 2.1 retrovirus used to drive the forced expression of HOXB3, HOXB4 and HOXA10 has been shown previously to direct high and long-term expression in primitive murine hematopoietic cells, as well as in their mature progeny (Pawliuk and Humphries, Unpublished data). Because Hox genes are expressed at relatively low levels under normal physiological conditions, their forced expression directed from the LTR thus both increases their levels in cells where they are normally expressed and extends their persistent expression into cell types were they are normally not expressed. Thus in order to draw conclusions from the perturbations generated by their overexpression about the stage(s) of hematopoiesis where these Hox genes have physiological roles, the knowledge of their normal expression patterns becomes essential. Although Hox gene expression in hematopoietic cells has been analyzed in some details, the picture is far from complete. In the case of HOXB4, the picture is perhaps the clearest. Its overexpression causes selective expansion of primitive hematopoietic cells, and thus appears to primarily affect the behavior of very primitive cell types where HOXB4 is normally expressed, 138 whereas later cell types appear unaffected. From these data, a role for HOXB4 in the regulation of differentiation of the earliest hematopoietic cells can thus be implied. In contrast to HOXB4, both HOXB3 and HOXA10 overexpression affect multiple stages and lineages of hematopoiesis, some of which have not been analyzed for Hox gene expression. Thus, for example, although overexpression of HOXA10 greatly enhances the generation of progenitor cells with megakaryocytic potential, a role for HOXA10 in normal megakaryopoiesis is not clear, because analysis of Hox gene expression in these progenitor cells has been hampered by the difficulties in obtaining relatively pure populations of these cells. In contrast to the megakaryocytic lineage, expression of HOXA10 in the granulocytic and monocytic lineages is better characterized, where HOXA10 expression is confined to progenitor cells of these lineages and then sharply downregulated as they differentiate. Thus both the apparent block in monocyte differentiation and the high frequency generation of myeloid leukemias in recipients of HOXA10 transduced bone marrow cells, suggest that downregulation of HOXA10 expression is critical for progenitor cells of granulocytic and monocytic lineages to go through the final stages of differentiation. Overexpression of HOXA10 also appears to block B lymphopoiesis, from at least the stage of pre-B progenitor cells, without detectable concommitant expansion of any B cell progenitor. These data, combined with the lack of Hoxa-10 expression both in purified murine B cell subpopulations representing early and late stages of B cell development, and in B lymphoid cell lines, suggest that HOXA10 might negatively regulate commitment of progenitor cells to the B cell lineage. However, this assumption needs further clarification by using different experimental models (e.g. transgenic mice or cell line with lympho-myeloid potential), where both the earliest stage of B cell development affected by overexpression of HOXA10 and its effect on molecules known to be essential for B lymphopoiesis (e.g. E2A, Pax5 and Rag-1) could be assessed. Although HOXB3 expression in various subpopulations of CD34+ human bone marrow cells is similar to that of HOXB4, the results presented in this thesis suggest different roles for these two genes in primitive hematopoietic cells. However, due to limitations of the experimental system used in 139 these experiments, this difference was not characterized further. The most striking effect of overexpression of HOXB3 was an almost complete block in the thymic production of oc(3 CD4+CD8+ T lymphocytes accompanied by an expansion of y§-TCR+ thymocytes. However, a more detailed analysis of HOXB3 expression in various thymocyte populations is needed, to evaluate the possible physiological role of HOXB3 in the development of a(3 and y8 T cells. Similarly, a possible physiological role for HOXB3 in B cell development needs further clarification. As for HOXA10, the earliest stage of B cell development affected by HOXB3 overexpression needs to be assessed, as well as normal HOXB3 expression at various stages of B cell development. Some properties of Hox genes add a level of complexity that needs to be considered when interpreting these overexpression data. As discussed in Chapter 1, a functional redundancy has been suggested between Hox genes, particularly those that belong to the same paralogous group. In hematopoietic cells many members of the Hox family are active, including some paralogs of HOXB3 and HOXB4, however expression of HOXA10 paralogs has not been detected. It can thus not be ruled out that some of the effects observed when Hox genes are overexpressed could be caused by them either mimicking or blocking the action of another Hox gene. However, the strikingly different phenotypes generated when HOXB3, HOXB4 or HOXA10 are overexpressed at least argues against the possibility that redundancies might involve Hox genes from different paralogous groups. It is believed that Hox gene expression can, at least in some cases, be auto- and/or cross regulated by other Hox genes (Zappavigna et al., 1994 ).Thus, for example, it has been shown that Hox genes located at the 3' end of the Hox clusters may regulate the expression of more 5' located genes (Care et al., 1994; Quaranta et al., 1996). The effects of overexpression of a particular Hox gene could thus also be attributed to its activation of more 5' located genes. The different phenotypes generated by overexpression of HOXB3 and its next neighbor HOXB4, however, would argue against this possibility. Furthermore, it 140 has recently been reported that embryonic stem cells overexpressing HOXB4 do not up-regulate the expression of a more 5' located Hox gene, Hoxb-8, when grown in conditions that allow differentiation along hematopoietic lineages (Helgason et al., 1996). Another property of Hox genes that was discussed in Chapter 1 is their interaction with co-factors, such as the pbx proteins, which have at least in some instances been shown to be important for their functional specificity. It has also been proposed that these co-factors could be shared by different Hox genes, although the extent of this sharing is far from clear. In that scenario, overexpression of a particular Hox gene thus could have a dominant negative effect by binding all available co-factor molecules and therefore preventing other Hox genes, sharing that same co-factor, from activating their target genes. Although this possibility cannot be ruled out, the different effects of overexpression of HOXB3, HOXB4 and HOXA10 genes would favor other mechanisms. 6.3 Hox genes and hematopoiesis Together, the results presented in this thesis, combined with earlier functional studies where altered Hox gene expressions were also found to produce effects on differentiation or proliferation of hematopoietic cells (Care et al., 1994; Giampaolo et al., 1994; Lill et al., 1995; Perkins and Cory, 1993; Sauvageau et al., 1994; Shen et al., 1992; Wu et al., 1992), and the demonstration that at least 20 of the 39 members of the Hox gene family are expressed in hematopoietic cells in an apparent stage-dependent manner (Giampaolo et al., 1994; Sauvageau et al., 1994), point to the existence of a complex "Hox code" in hematopoietic cells that may be deterministic for their growth and differentiation. The big question thus arises: What is the biological function of Hox genes in hematopoiesis? This question may be addressed by inferring their biological function in hematopoiesis from what is known about their functions in other developmental systems. Both in Drosophila and vertebrate embryonic development, Hox genes are thought to confer positional information along various body axes thereby directing morphogenesis (Krumlauf, 1994; Lawrence and Struhl, 1996). 141 However, the nature of their mechanisms of action, that is how they can 'identify" and "transform" the morphology of sets of cells, is currently largely unknown. In Drosophila, the prevalent view is that Hox genes can select for different developmental programs, for example by activating or repressing incompletely overlapping sets of genes, thereby leading to the formation of different structures, such as legs or antennae (Lawrence and Struhl, 1996). Based on the evolutionary conservation of both Hox gene organization and at least some of their properties (see Chapter 1 for more detail), similar mechanisms of action would be suggested for Drosophila and vertebrate Hox genes. Homeotic transformations in vertebrates have been best documented within the vertebral column, where they result either in the appearance of additional vertebra or in a transition from one morphological type to another (Krumlauf, 1994). It has been proposed that these transformations are unlikely the results of selection of alternative developmental pathways but rather result from different control of growth rates within the developing vertebra. Based on these observations, a model has been proposed whereby sequential activation of Hox genes along the anterior-posterior axis with the generation of the Hox gene "rainbow", determines the growth rates of cells with in each segment and thus the patterning of that structure (Duboule, 1995). In this scenario, Hox genes can be viewed as having a general function as biological clocks in developmental programs, where ordered and time dependent series of sequential transcriptional events need to be carefully orchestrated. It is not immediately obvious that the hematopoietic system requires positional information or that it has any "axes". However, some analogy can be drawn between hematopoiesis and embryogenesis. First, the regulation of Hox gene expression in hematopoietic cells has striking parallels with the colinear Hox gene expression during embryogenesis. Second, the adult bone marrow has a three-dimensional structure and organization, within which hematopoietic cells interact with stromal cells and migrate as they differentiate. Third, hematopoiesis is an ordered and time-dependent process of differentiation and proliferation leading to the generation of 142 mature blood cells. It is thus possible to imagine that Hox genes might confer positional information to hematopoietic cells, mediated by interactions with stromal cells or regulatory molecules secreted by these cells, which would coordinate their subsequent growth and migration as they differentiate. The pattern of Hox gene expression in a given cell type could thus be viewed as some sort of barometer which "informs" the cell where it is exactly positioned with respect to state of development, and thus insure that the subsequent behavior of that cell would be according to that "location". As discussed in Chapter 1, limited information exists about the downstream targets regulated by Hox genes. Interestingly, however, of those few identified the majority appear to be either growth factors (e.g. decapentaplegic (dpp), a member of the TGF(3 superfamily, and basic fibroblast growth factor) (Capovilla et al., 1994; Care et al., 1996; Her et al., 1995) or cell adhesion molecules (e.g. connectin, N-CAM and L-CAM) (Goomer et al., 1994; Gould and White, 1992; Jones et al., 1992). Given the important roles that both growth factors and cell-adhesion molecules play in hematopoiesis, it is tempting to speculate that Hox gene targets in hematopoietic cells could be either growth factors or cell adhesion molecules (previously identified or unidentified). 6 . 4 . Future directions In hematopoietic cells expression of Hox genes has mainly been analyzed in leukemic cell lines and CD34+ subpopulations of normal human bone marrow, whereas limited data exists on their expression in murine hematopoietic cells. An important next step would thus be to systematically analyze Hox gene expression in adult murine hematopoietic cells at various stages of both myeloid and lymphoid development. Furthermore, their expression in similar subpopulations at various stages during ontogeny, needs to be resolved as an initial step in assessing the role of Hox gene in embryonic hematopoiesis. 143 The functional roles of Hox genes in hematopoiesis have been analyzed either by overexpression or inactivation of specific Hox genes. Of these approches, overexpression studies have been more "fruitful" with dramatic perturbation in hematopoiesis observed (as described in this thesis). In contrast, "knockout studies" have revealed more subtle perturbations (Lawrence et al., 1996; Lawrence et al., 1996). This discrepancy can be explained at least in part by the functional redundancy amongst Hox genes of the same paralogous group, which would thus complement for the absence of one of its members. Recently, with the growing list of single Hox gene knockouts, a generation of double and even triple Hox gene knockout, by mating of single knockouts, is now possible. Detailed analyses of possible hematological abnormalities in these double or triple knockout mice are likely to be informative in resolving the role of Hox genes in hematopoiesis. Another interesting avenue to explore, is the role of individual Hox gene clusters in hematopoiesis. Expression of Hox genes from three clusters i.e. HoxA, HoxB and HoxC, has been detected in hematopoietic cells, or specifically, 9 of the 11 HoxA genes, 8 of the 10 HoxB genes and 3 of the 9 HoxC genes. An important issue to resolve is whether any one of these clusters are essential for hematopoiesis in general or in any of the hematopoietic lineages. With the use of the loxP/Cre recombinase system it is now possible to "knockout" an entire Hox gene cluster in a tissue specific manner (Gu et al., 1994). For that purpose loxP sites could be inserted on both ends of a Hox cluster (A, B or C cluster) in embryonic stem cells with the subsequent generation of mice homozygous for these loxP insertions. These mice would then be mated with transgenic mice which direct expression of the Cre recombinase in hematopoietic cells. The result would be deletion of the Hox cluster between the loxP sites only in cells expressing the Cre recombinase (i.e. hematopoietic populations) and thus eliminate problems due to early embryonic lethality. Transgenic mice which express Cre recombinase at various stages of B and T lymphoid development have 144 been generated. However, mice directing Cre expression in myeloid cells would need to be generated. As discussed in Chapter 1 there is now ample evidence suggesting that, in addition to Hox genes, many of the molecules and principles applied during morphogenesis of various embryonic structures in diverse animals, are highly conserved. Central to this machinery are secreted molecules of the hedgehog, Wnt and BMPs families of genes, some of which have been found to act in pathways downstream or upstream of Hox genes. In a recent report (Van Den Berg et al., 1996), expression of some members of the Wnt family in human fetal liver stromal cells was described. This report also described the expression of selected frizzled family members, the receptors for Wnt proteins, on CD34+ human bone marrow cells. Interestingly, co-cultivation of human bone marrow cells on stromal cells overexpressing some of these Wnt genes resulted in greatly increased clonogenic progenitor output (20-30-fold increase in CFU-GEMM), compared to control. Based on this data it would thus be very interesting to determine whether Hox genes could be acting in pathways either upstream or downstream of Wnt genes or their receptors. 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