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Regulatory elements within repeated elements : a case study of NAIP transcriptional innovation

University of British Columbia

Neuronal Apoptosis Inhibitory Protein (NAIP, also known as BIRC1) is a member of the conserved Inhibitor of Apoptosis Protein (IAP) family. However, it is no longer principally considered an apoptosis inhibitor since its domain structure and functions in innate immunity also warrant inclusion in the Nod-Like Receptor (NLR) superfamily. Lineage-specific rearrangement and expansion of this locus have yielded different copy numbers among primates and rodents, providing an interesting case study in which to study transcriptional regulatory changes by a rapidly evolving gene. In the first stage of my thesis, I show that NAIP has multiple promoters sharing no similarity between human and rodents. Moreover, I demonstrate that multiple, domesticated long terminal repeats (LTRs) of endogenous retroviral (ERV) elements provide NAIP promoter function in human, mouse and rat. In human, an LTR serves as a tissue-specific promoter active primarily in testis. However, in rodents, our evidence indicates that an ancestral LTR common to all rodent genes is the major, constitutive promoter for these genes and that a second LTR found in two of the mouse genes is a minor promoter. Thus, independently acquired LTRs have assumed regulatory roles for orthologous genes, a remarkable evolutionary scenario. It is also demonstrated that 5’ flanking regions of IAP family genes as a group, in both human and mouse, are enriched for LTR insertions compared to average genes. In the second stage of my thesis, I demonstrate that several of the human NAIP paralogues are expressed, and that novel transcripts arise from both internal and upstream transcription start sites. Remarkably, two internal start sites initiate within Alu short interspersed element (SINE) retrotransposons, and a third novel transcription start site exists within the final intron of the GUSBP1 gene, upstream of only two NAIP copies. One Alu functions alone as a promoter in transient assays, while the other likely combines with upstream L1 sequences to form a composite promoter. The novel transcripts encode shortened open reading frames and I show that corresponding proteins are translated in a number of cell lines and primary tissues, in some cases above the level of full length NAIP. Interestingly, some NAIP isoforms lack their caspase-sequestering motifs, indicating that they have novel functions. My results support an important role for transposable elements in NAIP evolution, particularly as transcriptional regulatory modules, and illustrate a fascinating example of regulatory innovations adopted by a rapidly evolving gene.

Thesis/Dissertation

application/pdf

2009-08-17

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/12271

Genetics

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/