Abstract: The MLL?AF4 (MA4) fusion gene is the genetic hallmark of an aggressive infant pro?B-acute lymphoblastic leukemia (B-ALL). Our understanding of MA4-mediated transformation is very limited. Whole-genome sequencing studies revealed a silent mutational landscape, which contradicts the aggressive clinical outcome of this hematologic malignancy. Only RAS mutations were recurrently detected in patients and found to be associated with poorer outcome. The absence of MA4-driven B-ALL models further questions whether MA4 acts as a single oncogenic driver or requires cooperating mutations to manifest a malignant phenotype. We explored whether KRAS activation cooperates with MA4 to initiate leukemia in cord blood derived CD34þ hematopoietic stem/progenitor cells (HSPC). Clonogenic and differentiation/proliferation assays demonstrated that KRAS activation does not cooperate with MA4 to immortalize CD34þ HSPCs. Intrabone marrow transplantation into immunodeficient mice further showed that MA4 and KRASG12V alone or in combination enhanced hematopoietic repopulation without impairing myeloid?lymphoid differentiation, and that mutated KRAS did not cooperate with MA4 to initiate leukemia. However, KRAS activation enhanced extramedullary hematopoiesis of MA4-expressing cell lines and CD34þ HSPCs that was associated with leukocytosis and central nervous system infiltration, both hallmarks of infant t(4;11)þ B-ALL. Transcriptional profiling of MA4-expressing patients supported a cell migration gene signature underlying the mutant KRAS-mediated phenotype. Collectively, our findings demonstrate that KRAS affects the homeostasis of MA4-expressing HSPCs, suggesting that KRAS activation in MA4þ B-ALL is important for tumor maintenance rather than initiation.

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