MEF2C phosphorylation is required for chemotherapy resistance in acute myeloid leukemia

Brown FC1,2 and Kentsis A1

  1. Memorial Sloan Kettering Cancer Center, NY, USA.
  2. Monash University, Melbourne, Australia.

In acute myeloid leukemia, chemotherapy resistance remains highly prevalent, representing the major barrier to cure in children and adults alike. We sought to investigate molecular mechanisms that may explain primary chemotherapy resistance in AML using targeted genomic sequencing and high-resolution mass spectrometry proteomics. This analysis identified aberrant phosphorylation of MEF2C S222 in primary chemoresistant human AML specimens, and using an affinity-purified antibody, established its prevalence and prognostic significance in a cohort of 47 patients, spanning the major biologic subtypes of human AML. MEF2C is a transcription factor required for hematopoietic cell fate determination. We found that Mef2cS222A/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 in both MLL-rearranged and non-MLL-rearranged mouse AML in vivo, and chemotherapy resistance and leukemogenicity of human AML cells. Transcriptome and chromatin analysis of gene expression changes and composition of MEF2C transcriptional complexes showed assembly of an active MEF2C transactivation complex specifically induced by MEF2C phosphorylation in leukemia cells. MARK3 activation was sufficient to cause MEF2C phosphorylation, leading to enhanced transcription of MEF2 response elements in cells. Notably, treatment with the selective MARK inhibitor MRT199665 caused apoptosis and conferred enhanced sensitivity to cytarabine 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.