The RALGEF Pathway Contributes to the Pathogenesis of Chronic Myelogenous Leukemia by BCR/ABL

Selected inhibitors of the BCR/ABL tyrosine kinase have shown a remarkable clinical activity in patients with chronic myelogenous leukemia (CML). However, these drugs do not completely eradicate leukemic cells and drug resistance emerges. Identification of additional contributors to the pathogenesis of CML remains to be important for developing strategies for overcoming resistance to BCR/ABL kinase inhibitors and for eradicating leukemic cells. We and others have previously shown that expression of BCR/ABL in mouse bone marrow cells by retroviral transduction and transplantation (BMT) efficiently induces a CML-like myeloproliferative disorder (MPD). Using this BMT model for CML, we have found that the Grb2 SH2 binding site Y177 of BCR/ABL, which is critical for BCR/ABL to activate the RAS pathway, is required for the induction of CML-like disease by BCR/ABL. We have further shown that oncogenic NRAS (NRASG12D) can rescue the defect of the Y177F mutant BCR/ABL in the induction of CML-like disease. To further elucidate the pathways downstream of NRAS responsible for this “rescue” effect, we utilized three well-characterized NRAS effector mutants, G12D/T35S (activating RAF but is impaired in binding PI3K and RALGEF), G12D/E37G (activating RALGEF but not RAF or PI3K), and G12D/Y40C (activating PI3K but not RAF or RALGEF). None of the three NRAS effector mutants could induce hematological malignancies by themselves, but all were capable of “rescuing” the CML defect of BCR/ABL^Y177F to some extent. Interestingly, the NRAS G12D/E37G effector mutant was the most efficient. We further examined the role of RALGEF pathway by introducing an activated RALGDSCAAX into our BCR/ABL^Y177F mouse model. RALGDS-CAAX was able to cooperate with BCR/ABL^Y177F to induce a CML-like MPD in nearly half of mice whereas RALGDSCAAX alone did not induce diseases in any mice. As a complement to this study, we also introduced dominant negative RAL A or RAL B in the background of BCR/ABL. This resulted in a significant delay in disease onset. Taken together, this data sheds light on the importance of the RALGEF pathway in BCR/ABL-mediated CML induction and supports the notion of inhibition of this pathway as a potential therapeutic strategy.