SRP343463 PRJNA775622 GSE186709 A molecular switch between mammalian MLL complexes dictates response to Menin-MLL inhibition [RNA-seq] The chromatin adaptor Menin interacts with oncogenic fusion proteins encoded by MLL1-rearrangements (MLL1-r), and small molecules that disrupt these associations are currently in clinical trials for the treatment of leukemia. Here, we delineate a molecular switch between the MLL1-Menin and MLL3/4-UTX chromatin modifying complexes that dictates response to Menin-MLL inhibitors. We show that Menin safeguards leukemia cell fitness by impeding binding of the histone demethylase UTX at a subset of non-canonical target gene promoters. Disrupting the interaction between Menin and MLL1 leads to UTX-dependent transcriptional activation of genes with tumor suppressive function. We show that this epigenetic mechanism is operative in murine and human models of AML, and clinical responses to Menin-MLL inhibition in primary human leukemia are accompanied by induction of tumor suppressive gene expression at Menin-UTX targets. These findings shed light on the context-dependent and often antagonistic roles that chromatin regulators exhibit in development and disease and provide mechanistic insight for rational design of targeted epigenetic therapies. Overall design: Using RNA-Seq followed by gene expression analyses we determined genes that change in expression upon treatment with Menin-MLL inhibitors in leukemia cell lines (days 0 and 4 post treatment) in triplicates of samples. GSE186709 parent_bioproject PRJNA775621