Inhibition of PRMT1 Mediated FLT3 Arginine Methylation As a Potent Therapeutic Strategy for MLL-r ALL

Mixed-lineage leukemia-rearranged (MLL-r) ALL, seen in 70% of infant ALL, has a dismal prognosis compared to those with wild type MLL1 gene. Transcriptional profiling has identified Fms-like receptor tyrosine kinase 3 (FLT3) as one of the most significantly upregulated genes in MLL-r ALL. The highly expressed FLT3 protein is activated by the autocrine ligand, making the kinase a therapeutic target. FLT3 tyrosine kinase inhibitors (TKIs) such as PKC412, although effective in kinase inhibition, partially impair survival of MLL-r ALL cells and clinical trial results are not promising, promoting us to ask whether FLT3 regulates the ALL cells survival also through a kinase-independent mechanism.

Herein, we report the finding of dimethylated arginines on FLT3, detected through mass spectrometry analysis of a MLL-r ALL specimen and a MLL-r ALL line SEM. The most conserved and enriched of dimethylated arginines are residues R972/R973. Using home-made arginine methylation (R-Me) antibody, we found that PRMT1, which is responsible for most type I arginine methyltransferases activity, catalyzes FLT3 methylation. Immunoblot (IB) analysis validated the expression of FLT3 R-Me in MLL-r ALL samples (6 out of 6) and MLL-r ALL lines (4 out of 4). Analysis of the GEO dataset (GSE13204) revealed that PRMT1 mRNA levels are increased in MLL-r ALL relative to normal cells (MLL-r, n=70 vs. normal, n=73, p<0.0001). We studied FLT3 R-Me biological function using two approaches that specifically blocked FLT3 methylation levels: cells expressing FLT3 methylation deficient construct (R972/973K, arginine [R] to lysine [K]) exhibited reduced survival (BaF3: FLT3-WT 98.5±0.11% vs. R972/973K 71.5±0.53%, p=0.0004); knockdown of PRMT1 in SEM cells also had an inhibitory effect (siCtrl 95.1±0.1% vs. siPRMT1 74.7±0.5%, p=0.0007). Moreover, the type I arginine methyltransferase inhibitor MS023 (5 µM) treatment markedly induced apoptosis of primary ALL cells but spared normal counterparts from healthy donors (ALL: vehicle 10.4±0.4% vs. MS023 23.7±0.8%, n=4; p<0.0001; normal CD19⁺: 8.3±0.3% vs. 8.2±0.1%, n=3, p=0.86). Interestingly, inhibition of FLT3 methylation decreased FLT3 phosphorylation at tyrosine 969 (Y969) but not Y589/591 or Y842. Expression of R972/973K decreased FLT3 downstream signaling like phospho-STAT5 and -AKT to a greater extent than that of Y969F mutant (Y to phenylalanine [F] substitution, mimics loss of Y phosphorylation). Next, FLT3 WT, R972/973K or Y969F transduced primary MLL-r ALL cells were transplanted into NSGS mice for analysis of leukemia development (n=6/group). Mice transplanted with FLT3 Y969F MLL-r ALL had longer survival relative to FLT-WT injected animals (p=0.0031), and the median survival was further extended in mice injected with R972/973K mutant compared with FLT3 Y969F MLL-r ALL (p=0.0007). Additionally, PKC412 treatment alone did not alter FLT3 R-Me, and high FLT3 methylation level in SEM cells was not affected by FLT3 ligand stimulation, confirming that the function of R-Me is independent of FLT3 phosphorylation. Importantly, we observed that the combination of MS023 with PKC412 significantly induced a higher rate of apoptosis in primary MLL-r ALL cells compared with each drug alone (control, 10±0.43%, MS023, 21.1±1.2%, PKC412, 21.5±0.11%, combination, 39.8±2.9%, PKC412 vs combination, p<0.01, n=4). We further tested the effects of in vivo administration of MS023 plus PKC412 on primary MLL-r ALL cells xenografted in NSGS mice. Following engraftment >1% in peripheral blood, mice were subdivided into four groups and treated with vehicle, PKC412 (100 mg/kg, i.g.), MS023 (80 mg/kg, i.p, bid), or the combination (n=7/group) for 4 weeks. The BM tumor burden of CD45⁺ CD19⁺ cells was reduced in single drug-treated mice cohorts, with further reduction after combination treatment (vehicle, 94.4±0.5%, PKC412, 50.2±6.3%, MS023, 55.6±4.5%, combination, 30.7±4.9%, PKC412 vs. combination, p<0.001). Secondary transplantation of BM cells from mice receiving combination treatment resulted in significantly reduced BM engraftment at 16 weeks compared to PKC412 treatment alone (PKC412, 62.2±4.9%, combination, 8.4±5.1%, n=5, p<0.0001), indicating reduced leukemia initiating capacity.

Our results support further exploring the molecular function of FLT3 R-Me. We will determine whether PRMT1 and FLT3 methylation are potential druggable targets in MLL-r ALL.