Essential Roles of Transcription Factor MEF2D in the Maintenance of MLL-Rearranged Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is an aggressive disease with uncontrolled proliferation of myeloid progenitor cells and a failure of proper cell differentiation. Chromosomal translocations of the KMT2A (MLL) gene are found in about 10% AMLs affecting both children and adults. Patients harboring MLL rearrangement (MLL-r) have a high relapse rate and poor overall survival, emphasizing an unmet need to understand MLL-r AML pathogenesis and develop new therapeutic means. Through a comparative analysis of super-enhancers in AML cell lines and primary samples, we discovered that MLL-r leukemia cells preferentially gain super-enhancer signals at the MEF2D gene. MEF2D is aberrantly activated in MLL-r AML patient samples and predicts poor disease outcomes, implying a potential oncogenic role of MEF2D. Indeed, depletion of MEF2D inhibits human MLL-r leukemia growth and induces profound differentiation. Mechanistically, MEF2D directly represses CEBPE, a master regulator of myeloid differentiation. CEBPE depletion could rescue the growth defect and cell differentiation induced by MEF2D loss. We also demonstrated that MEF2D is positively regulated by HOXA9, and downregulation of MEF2D is an important mechanism for DOT1L inhibitor-induced anti-leukemia effects. Interestingly, MEF2C, a target of MLL fusion oncoproteins, is by far the most studied MEF2 family member with essential roles in MLL-r AML. Our genetic, biochemical, and integrative genomic data shows that MEF2D and MEF2C, both upregulated in MLL-r AML, interact with each other, colocalize genome-widely, and co-regulate target gene expression. We are currently investigating the molecular mechanisms underlying the interdependent function between MEF2 paralogs in MLL-r AML. Collectively, our findings suggest that MEF2D is a novel transcriptional dependency in MLL-r AML and uncover the MEF2-CEBPE axis as a crucial transcriptional mechanism regulating leukemia cell self-renewal and differentiation block.