MEF2C phosphorylation is required for chemotherapy resistance in acute myeloid leukemia

HIGHLIGHTS MEF2C S222 phosphorylation is a specific marker of chemotherapy resistance in diagnostic AML patient specimens. MEF2C S222 phosphorylation is dispensable for normal hematopoiesis in mice, as established using genome editing in vivo , but is required for MLL-AF9 induced leukemogenesis. MARK kinases specifically phosphorylate MEF2C S222, potentiating its transcriptional activity. Chemical inhibition of MARK-induced MEF2C phosphorylation overcomes chemotherapy resistance of and exhibits selectivity toxicity against MEF2C-activated human AML cells. SUMMARY In acute myeloid leukemia, chemotherapy resistance remains prevalent and poorly understood. Using functional proteomics of patient AML specimens, we identified MEF2C S222 phosphorylation as a specific marker of primary chemoresistance. We found that Mef2c S222A/S222A knock-in mutant mice engineered to block MEF2C phosphorylation exhibited normal hematopoiesis, but were resistant to leukemogenesis induced by MLL-AF9 . MEF2C phosphorylation was required for leukemia stem cell maintenance, and induced by MARK kinases in cells. Treatment with the selective MARK inhibitor MRT199665 caused apoptosis of MEF2C-activated human AML cell lines and primary patient specimens, but not those lacking MEF2C phosphorylation. These findings identify kinase-dependent dysregulation of transcription factor control as a determinant of therapy response in AML, with immediate potential for improved diagnosis and therapy for this disease.