BCR-ABL stimulates mutagenic homologous DNA double-strand break repair via the DNA-end-processing factor CtIP

Expression of BCR-ABL oncoprotein in chronic myeloid leukemia (CML) promotes neoplastic transformation of hematopoietic stem cells through modulation of diverse pathways. CML is a multistep disease, which evolves as a chronic phase and progresses to blast crisis. This progression has been associated with the appearance and accumulation of new cytogenetic anomalies and mutations. The mechanisms underlying the genomic instability promoted by BCR-ABL remain obscure. Through comparative analysis of different DNA double-strand break (DSB) repair mechanisms as a function of the BCR-ABL status in human megakaryocytic and CML cell lines, we found that BCR-ABL upregulates error-prone DSB repair pathways [single-strand annealing (SSA) and non-homologous end joining] rather than the high-fidelity mechanism of homologous recombination. Intriguingly, expression analysis of DSB repair pathway choice determining factors revealed increased levels of the protein CtIP in BCR-ABL-positive cells, particularly in response to irradiation. Moreover, treatment with the BCR-ABL kinase inhibitor, Imatinib Mesylate, abolished CtIP accumulation. When we silenced CtIP expression in cells with functional BCR-ABL, SSA enhancement by BCR-ABL was completely abrogated. Importantly, we also provide evidence that BCR-ABL stimulates DSB end resection, which is mediated by CtIP. Briefly, BCR-ABL promotes mutagenic DSB repair with the DSB end-processing protein CtIP acting as the key mediator downstream of BCR-ABL.