c-Myc is a target of RNA-binding motif protein 15 in the regulation of adult hematopoietic stem cell and megakaryocyte development

RNA-binding motif protein 15 (RBM15) is involved in the RBM15-megakaryoblastic leukemia 1 fusion in acute megakaryoblastic leukemia. Although Rbm15 has been reported to be required for B-cell differentiation and to inhibit myeloid and megakaryocytic expansion, it is not clear what the normal functions of Rbm15 are in the regulation of hematopoietic stem cell (HSC) and megakaryocyte development. In this study, we report that Rbm15 may function in part through regulation of expression of the proto-oncogene c-Myc. Similar to c-Myc knockout (c-Myc-KO) mice, long-term (LT) HSCs are significantly increased in Rbm15-KO mice due to an apparent LT-HSC to short-term HSC differentiation defect associated with abnormal HSC-niche interactions caused by increased N-cadherin and beta(1) integrin expression on mutant HSCs. Both serial transplantation and competitive reconstitution capabilities of Rbm15-KO LT-HSCs are greatly compromised. Rbm15-KO and c-Myc-KO mice also share related abnormalities in megakaryocyte development, with mutant progenitors producing increased, abnormally small low-ploidy megakaryocytes. Consistent with a possible functional interplay between Rbm15 and c-Myc, the megakaryocyte increase in Rbm15-KO mice could be partially reversed by ectopic c-Myc. Thus, Rbm15 appears to be required for normal HSC-niche interactions, for the ability of HSCs to contribute normally to adult hematopoiesis, and for normal megakaryocyte development; these effects of Rbm15 on hematopoiesis may be mediated at least in part by c-Myc. (Blood. 2009; 114: 2087-2096)