Targeting PRMT1 As a Novel Curative Therapy for Acute Myeloid Leukemia

Protein arginine methyltransferase 1 (PRMT1) is dynamically regulated in normal hematopoiesis and constitutively high expression of PRMT1 is associated with low survival rate in acute myeloid leukemia (AML) (Zhang et al. 2015). PRMT1 has been demonstrated for maintaining cell proliferation of AML1-ETO, MLL fusion initiated AML. Therefore, targeting PRMT1 may be a novel curative therapy for AML. In this report, we will use leukemia mouse models to address how to target protein arginine methyltransferase 1 as a novel therapy for treating acute megakaryocytic leukemia (AMKL).

Pediatric AMKL is a fatal disease with poor prognosis. So far chemotherapy is still the first line therapy for AMKL, yet gene-specific therapy is not available. Chromosomal translocation t(1;22)(p13;q13), which generates the fusion protein RBM15-MKL1 (aka OTT-MAL), is exclusively found in pediatric AMKL patients. Knock-in mice that express Rbm15-MKL1 from endogenous Rbm15 locus develops leukemia with low penetrance and long latency, suggesting that additional genetic or epigenetic changes are required for the rapid leukemia progression. Previous study by Mercher et al. showed that addition of mutant thrombopoietin receptor tyrosine kinase c-MPL^W515L to Rbm15-MKL1 translocation leads to aggressive leukemia development in mice. We further demonstrated that forced PRMT1 expression bypassed the need for c-MPL^W515L to allow Rbm15-MKL1 cells to proliferate in a cytokine-independent fashion in cell culture. Viability of PRMT1-expressing cells as well as the c-MPL^W515L expressing cells is highly sensitive to the treatment of the PRMT1 inhibitor MS203, indicating that the survival of transformed cells is dependent on PRMT1. Co-expression of PRMT1 and Rbm15-MKL1 in cord blood cells can sustain the long term culture while RBM15-MKL1 expressing cord blood cells will die quickly. Based on these results, we rationalize that PRMT1 is a key factor that cooperates with Rbm15-MKL1 to mediate malignant transformation and aggressive leukemia development. Inhibition of PRMT1 activity would have therapeutic effect on OTT-MAL-initiated leukemia. In this vein, we administered daily i.p. injection of PRMT1 inhibitor to mice that were transferred either c-MPL^W515L- or PRMT1-expressing Rbm15-MKL1 cells. Inhibitor-treated group has greatly improved survival time. Vehichle-treated mice suffered from fast weight loss, early disease onset and then died within 21-23 days post transplantation while most inhibitor-treated mice remained alive with stable body weight. PRMT1 inhibitor treatment significantly blocks leukemia-associated splenomegaly. Flow cytometry analysis showed that inhibitor-treated mice have notably less leukemia cells in bone marrow and in peripheral blood, compared to their control counterparts. More than half of control mice developed rear limb paralysis and visible blood clots were found on their spine upon dissection. Two and four weeks post the last injection we analyzed the peripheral blood of surviving inhibitor-treated mice to detect residual disease and found that the percentage of leukemic cells was very low, in some cases completely undetectable. Up to 3 months those mice remained healthy and no signs of relapse without further MS203 treatment. How the PRMT1 inhibitor MS203 blocks leukemia progression with PDX models of Down syndrome AMKL will be presented.

At the molecular level, we performed metabolomics analysis with mass spectrometry and Seahorse analyzer, and discovered that PRMT1 reprogrammed metabolism in leukemia cells through upregulation of glycolysis pathway and downregulation of fatty acid oxidation pathway. We reported that PRMT1 controls RBM15 protein stability by triggering methylation-dependent ubiquitilation (Zhang et al. 2015). In this report, we further demonstrated that RBM15 is directly responsible for controlling the mRNA stability of key metabolic enzymes. Thus we reasoned that the PRMT1-RBM15 axis is a key axis in controlling metabolism during leukemia progression. Based on these findings, we further tested whether we can use weight-controlling drugs such as etomoxir for treating leukemia. In summary, copious expression of PRMT1 in AML makes PRMT1 an encouraging new target for curative therapy, The PRMT1 inhibitor MS203 which has few side effects in mice will be a strong candidate for future clinical trials.