Projects in the lab focus on molecular mechanisms that regulate early hematopoiesis and may be dysfunctional in leukemogenesis. Specifically, we are using primary cells as well as murine and human embryonic stem cells to study RBM15 and MKL1, two genes that are fused in the t(1;22) translocation associated with Acute Megakaryoblastic Leukemia AMKL). We are studying the roles of RBM15 and MKL1 in normal myelopoiesis and leukemogenesis.
We have shown that RBM15 is downregulated as hematopoietic stem cells differentiate down the myeloid lineage such that megakaryoblasts express low levels of RBM15. When RBM15 is overexpressed, it prevents myeloid differentiation, and when RBM15 is inhibited or deleted, myeloid differentiation is enhanced, and there is a loss of hematopoietic stem and progenitor cell self-renewal. RBM15 is a member of the spen family of proteins that share a C-terminal SPOC domain that bind to the nuclear corepressor complex. Consistent with other members of the SPOC domain family that can affect Notch signaling, we have shown that RBM15 represses Notch induced Hes1 promoter activity. RBM15 coimmunoprecipitates with RBPJ, a critical transcription factor in the Notch signaling pathway. Thus, RBM15 plays a role in hematopoiesis by maintaining myeloid cells in an undifferentiated state, and this activity is mediated by inhibition of Notch signaling.
MKL1, identified at the C-terminus of the t(1;22) translocation specific to acute megakaryoblastic leukemia, is highly expressed in differentiated muscle cells and promotes muscle differentiation by activating serum response factor (SRF). The Krause laboratory has shown that MKL1 expression is upregulated during murine and human megakaryocytic differentiation, and that enforced overexpression of MKL1 enhances megakaryocytic differentiation. When the Human Erythroleukemia (HEL) cell line is induced to differentiate with TPA, overexpression of MKL1 results in an increased number of megakaryocytes with a concurrent increase in ploidy. MKL1 overexpression also promotes thrombopoietin-induced megakaryocytic differentiation of primary human CD34+ cells. The effect of MKL1 is abrogated when SRF is knocked down, suggesting that MKL1 acts through SRF. Consistent with these findings in human cells, knock out of MKL1 in mice leads to reduced platelet counts, and reduced ploidy in bone marrow megakaryocytes. Thus, MKL1 promotes physiological maturation of human and murine megakaryocytes.
Ongoing work on RBM15 is focused on the mechanisms by which it affects notch signaling, its RNA binding properties, ands its ability to bind to other nuclear proteins to affect transcription. Ongoing work on MKL1 is focused on the mechanisms by which MKL1 promotes megakaryocytopoiesis.
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