Role of Protein Arginine Methyltransferase 9 in Acute Myeloid Leukemia Maintenance

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Project Summary Acute myeloid leukemia (AML) is a deadly disease. Although cytogenetic and molecular risk-adapted approaches can guide treatment, overall outcomes remain poor. Currently available cell cycle-based chemotherapy or targeted therapy, such as use of tyrosine kinase inhibitors (TKIs), cannot eliminate all leukemia clones. Mounting evidence suggests that remaining cells that depend on oncoproteins for survival become a source of relapse. Many of these oncoproteins, such as Mcl-1, STAT5, and c-Myc, are short-lived, and their high levels in tumors are likely due to aberrant activation of the translation machinery, a hallmark of cancer. Thus, understanding mechanisms underlying dysregulated translation is necessary to antagonize leukemia persistence. We recently found that higher expression levels of PRMT9, the most recently defined symmetric- dimethylarginine (SDMA)-forming enzyme, are associated with decreased overall AML patient survival. Our proteomics and mutagenesis analysis revealed a novel mechanism whereby PRMT9 catalyzes methylation of translation elongation factor eEF1A1, linking PRMT9 to active translation and promoting AML maintenance. Accordingly, PRMT9 inhibition blocked AML cell survival/proliferation in an eEF1A1 methylation-dependent manner, while sparing normal hematopoietic cells. Inhibition of PRMT9/eEF1A signaling indeed decreased protein biosynthesis, reducing levels of the short-lived oncoproteins Mcl-1, c-Myc, and STAT5 and significantly extending leukemic mouse survival. Thus, we hypothesize that PRMT9 is critical for AML pathogenesis and that PRMT9-mediated eEF1A methylation promotes mRNA translation and protein synthesis to enable AML maintenance. To test this hypothesis, we will: 1) define the function of PRMT9-mediated eEF1A arginine methylation in AML pathogenesis using a MLL-AF9-related doubly-hit AML mouse model and a newly developed PRMT9 conditional knockout mouse; 2) define molecular mechanisms underlying PRMT9/eEF1A signaling in AML by testing whether eEF1A methylation alters its GTPase activity by enhancing GTP/GDP binding and assessing whether PRMT9 deficiency induces global translation changes in AML based on ribosome profiling sequencing; 3) determine combined effects of PRMT9 deletion or inhibition with daunorubicin/cytarabine or AC220 treatments on AML eradication in murine and human AML models. Our studies should reveal functional interaction between PRMT9 and eEF1A and indicate how arginine methylation of highly conserved eEF1A R166 governs translation in cancer cells. These studies will close the knowledge gap relevant to how leukemia cells acquire a survival/growth advantage through aberrant activity of the translation machinery and may show how combining targeting of PRMT9 with current treatments could represent a more effective strategy to eliminate AML cells.