UBC Theses and Dissertations
Assessment of a potential therapeutic target in the Hedgehog pathway for the eradication of primitive chronic myeloid leukemia cells Turner, Kelly Anne
Chronic myeloid leukemia (CML) is a hematological malignancy characterized by the presence of a novel fusion oncoprotein called BCR-ABL1 in a hematopoietic stem cell. BCR-ABL1 has constitutively active tyrosine kinase activity and deregulates many intracellular signaling pathways contributing to cancer formation, maintenance, and progression. The consistent genetic aberration of BCR-ABL1 in CML led to the development of the first molecularly-targeted cancer therapy called imatinib (IM), which revolutionized the treatment of early phase CML. However, IM is not curative, and 40% of patients with advanced CML experience primary intolerance or acquire resistance to IM. In addition, leukemic stem cells (LSCs) are relatively insensitive to IM and do not exclusively rely on BCR-ABL1 for survival. Therefore, it is important to investigate alternative pathways that are critical for LSC maintenance, to develop a strategy to eradicate them. The Hedgehog (HH) pathway, and particularly the protein Smoothened (SMO), has recently been described to be essential for CML LSCs in a mouse model. I hypothesized that the HH pathway was critical for the survival of CML stem/progenitor cells, and that dual inhibition of BCR-ABL1 and SMO would be superior to either alone in killing CML LSCs. I used a variety of biological and molecular assays to investigate expression changes of several HH pathway-associated genes and the functionality of different leukemic cell subsets from primary CML patient samples. I observed that HH pathway genes SMO and GLI2 were upregulated in CML compared with healthy bone marrow controls, and were more highly expressed in CD34⁺ cells from IM non-responders as compared with responders. In addition, these genes were most highly expressed in the stem cell-enriched Lin-CD34⁺CD38ˉ subpopulation in IM non-responders compared with progenitors and mature leukemic cells in the same patients. I also observed that CD34⁺ IM non-responder cells were more sensitive to SMO inhibition compared with responders in terms of viability, apoptosis, re-plating potential, and colony-forming ability following long-term culture. Taken together, my results support the hypothesis that the HH pathway is more critical for primitive CML cells from IM non-responders, and may represent a mechanism by which drug-resistant cells evade eradication by TKIs.
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