Role of BCR/ABL gene-expression levels in determining the phenotype and imatinib sensitivity of transformed human hematopoietic cells

Increased levels of Bcr-Abl expression in chronic myelogenous leukemia (CIVIL) cells are associated with disease progression and imatinib (IM) resistance. However, it is not clear if these associations are a direct result of elevated Bcr-Abl expression. We used a human transduction model of CML to directly investigate the role of varying Bcr-Abl expression levels in determining the phenotype and IM sensitivity of hematopoietic cells. CD34(+) cells were transduced with vectors coexpressing Bcr-Abl and GFP, and cells expressing low and high levels of GFP and Bcr-Abl (BA(lo) and BA(hi)) were selected. BA(hi) cells demonstrated enhanced activation of downstream proliferative and antiapoptotic signaling and enhanced proliferation and survival compared to BA(lo) cells. Freshly isolated BA(hi) CD34(+) cells and cell lines demonstrated increased IM-mediated growth inhibition likely reflecting Bcr-Abl dependence for growth and survival. CD34(+) cells expressing BCR/ABL kinase-mutant genes demonstrated resistance to IM-mediated inhibition of proliferation and viability, which was not enhanced by increased expression of BCR/ABL kinase-mutant genes. We conclude that Bcr-Abl overexpression results in increased proliferation and antiapoptotic signaling in CD34(+) cells, but may not play a direct role in IM resistance in progenitor cells expressing either wildtype or mutant BCR/ABL genes.