PRMT1-mediated FLT3 arginine methylation is required for maintenance of FLT3-ITD+ Acute Myeloid Leukemia

The presence of FLT3-ITD mutations in patients with acute myeloid leukemia (AML) is associated with poor clinical outcome. FLT3 tyrosine kinase inhibitors (TKIs), although effective in kinase ablation, do not eliminate FLT3-ITD+ leukemia stem cells (LSCs) which are potential sources of disease relapse, prompting us to ask whether FLT3-ITD protein regulates the AML LSCs survival through a kinase-independent mechanism. Here, we show that expression of PRMT1, the primary type I arginine methyltransferase, significantly increases in LSC-enriched CD34+CD38- populations relative to normal counterparts. Genetic PRMT1 depletion blocked AML CD34+ cell survival, and had more potent effects in AML cells from patients harboring FLT3-ITD. Our genome wide analysis of gene expression and PRMT1 conditional KO mouse study confirmed that PRMT1 preferentially cooperates with FLT3-ITD contributing to AML cell maintenance. Mechanistically, PRMT1 catalyzed FLT3-ITD protein methylation at arginines 972/973, and PRMT1 promoted leukemia cell growth in a FLT3 methylation-dependent manner. Moreover, effects of FLT3-ITD methylation in AML cells were in part due to crosstalk with FLT3-ITD phosphorylation at tyrosine 969 (Y969). Importantly, FLT3 methylation persisted in FLT3-ITD+ AML cells following TKI (AC220) treatment, indicating that methylation occurs independently of kinase activity. Finally, in both patient-derived xenograft (PDX) and murine AML models, combined administration of AC220 with a type I PRMT inhibitor (MS023) enhanced elimination of FLT3-ITD+ AML relative to AC220 treatment alone. Our study demonstrates that PRMT1-mediated FLT3 methylation promotes LSC activity and suggests that combining PRMT1 inhibition with FLT3 TKI treatment could be a promising approach to selectively target FLT3-ITD+ LSCs. Overall design: We want to explore whether PRMT1 function is more relevant to the pathogenesis of FLT3-ITD+ than that of FLT3-WT AML. To do so, we transduced two AML cell lines (MV4-11 and THP-1) plus one FLT3-ITD-transformed CB CD34+ cells along with shCtrl or shPRMT1 and performed RNA sequencing (RNA-Seq). Six Groups: Group1: FLT3-ITD+ CB cells transduced with lentiviral vector targeting non-silence squence (control ShRNA); Group2:FLT3-ITD+ CB cells transduced with lentiviral vector containing interference squence targeting PRMT1 (PRMT1 shRNA); Group 3: MV4-11 cells transduced with lentiviral vector targeting non-silence squence (control ShRNA); Group 4: MV4-11 cells transduced with lentiviral vector containing interference squence targeting PRMT1 (PRMT1 shRNA);Group5: THP-1 cells transduced with lentiviral vector targeting non-silence squence (control ShRNA); Group6:THP-1 cells transduced with lentiviral vector containing interference squence targeting PRMT1 (PRMT1 SHRNA). Expression profiling data in PRMT1 KD FLT3-ITD+ Cord blood cells, MV4-11 and THP-1 cell lines were compared to corresponding control shRNA transduced cells by RNA-seq technology