MEF2C phosphorylation is required for chemotherapy resistance in acute myeloid leukemia

Abstract

HIGHLIGHTSMEF2C 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.SUMMARYIn 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 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, 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.

Genetic mechanisms of primary chemotherapy resistance in pediatric acute myeloid leukemia: A report from the TARGET initiative

“…This suggests that diverse genetic subtypes of induction failure AML may engage common gene expression programs. This notion is consistent with the recent study implicating epigenetic signaling by the transcription factor MEF2C in AML chemotherapy resistance (9).…”

“…Recent study of primary chemotherapy resistance in a cohort of 107 children and adults with AML using targeted gene sequencing demonstrated that few patients exhibited specific individual mutations associated with primary chemotherapy resistance and failure of induction chemotherapy (8). In addition, at least for some patients, chemotherapy resistance is caused by the epigenetic activation of the transcription factor MEF2C (9)(10). This suggests that there are additional genetic or molecular mechanisms mediating primary chemotherapy resistance in pediatric and adult AML.…”

The Salt-Inducible Kinase inhibitor YKL-05-099 suppresses MEF2C function and acute myeloid leukemia progressionin vivo

“…Knockout studies in mice indicate that Mef2c is essential in the normal lymphoid and megakaryocytic lineages, but is largely dispensable for myelopoiesis and for hematopoietic stem cell self-renewal [10][11][12][13] . Insertional mutagenesis screens performed in mice first revealed a leukemogenic function of MEF2C 14 , which was later shown to be overexpressed in a variety of human myeloid and lymphoid cancers in association with poor clinical outcomes [15][16][17][18][19][20][21] . The MLL-rearranged subtypes of leukemia exemplify a group of leukemias with high MEF2C expression.…”

“…Importantly, it has been shown that MLL fusion AML cells are addicted to continuous MEF2C expression for their growth and viability 15,22 . The powerful nature of MEF2C addiction in MLL-rearranged AML has been most convincingly demonstrated in the hypomorphic Mef2c S222A/S222A mouse strain, which lacks any detectable developmental abnormalities, but is entirely resistant to leukemic transformation by the MLL-AF9 oncoprotein 21 . Collectively, these genetic experiments validate MEF2C as a vulnerability in AML cells and an attractive target for therapy.…”

“…Each class IIa HDAC can be phosphorylated by several different kinases, such as calmodulin-dependent protein kinase (CaMK) and Salt-Inducible Kinases (SIKs), at conserved serine residues to promote their interaction with 14-3-3 proteins, which function to sequester HDAC proteins in the cytoplasm 23,27,28 . In addition, MEF2C can be directly phosphorylated by MARK kinases at S222 to promote its transcriptional function 21 . Through such mechanisms, kinase signaling pathways are able to control MEF2C function in a variety of cellular contexts 23,24,27 .…”