PRMT1 Activates Leukemic Stem Cell Program in MLL-Rearranged Leukemia

In MLL-rearranged leukemia, the Mixed Lineage Leukemia (MLL) gene undergoes chromosomal translocation that results in the loss of C-terminal histone methyltransferase SET domain, whereas the N-terminal of MLL gene fuses in-frame with one of the 60 identified partner genes. The resultant MLL fusion proteins lead to a characteristic aberrant gene expression pattern in human acute myeloid and lymphoblastic leukemia. Epigenetic dysregulation mediated by MLL fusion proteins has been suggested to be a key event in MLL-rearranged leukemia. It has been demonstrated that MLL-EEN/PRMT1 oncogenic complex induces transformation of primary myeloid progenitors via introduction of aberrant H4R3me2 at target Hoxloci. PRMT1 is the predominant protein arginine methyltransferase in mammals and is responsible for over 85% of arginine methylation activity in mammalian cells. Dysregulation of PRMT1 has been implicated in different cancers such as leukemia, suggesting the expression of PRMT1 is positively correlated with cancer progression and clinical parameters. Nevertheless, the leukemogenic role of PRMT1 in the establishment of leukemic stem cell (LSC) remains unclear.

Previously we have demonstrated that a MLL fusion protein, MLL-EEN, can strongly enhance the self-renewal ability of murine primary hematopoietic cells through multiple rounds of replating assays. We have created a conditional Mll-Een invertor mouse model (Mll^Een/+) in which the expression of fusion protein is restricted to hematopoietic progenitors. Immunophenotypic analysis demonstrated a significant increase in the immature myeloid cell population (c-kit⁺Mac-1⁺) in bone marrow of Mll^Een/+ mice, suggesting that the expression of Mll-Een induces the development of acute myeloid leukemia. We have also established an Mll-Een expressing cell line from the bone marrow of Mll^Een/+ mouse. These leukemic cells can persistently form colonies and they also demonstrated deregulation of Hox genes, which is frequently observed in human leukemia cases. The leukemogenicity of Mll-Een is closely associated with Prmt1, which was demonstrated through knockdown of Prmt1. Strikingly, we discovered a subpopulation of CD41⁺Mll-Een expressing cells, which showed enhanced self-renewal ability in the serial colony forming assays. The percentage of CD41⁺ leukemic cells is reduced once Prmt1 was knocked down, suggesting that Prmt1 is crucial in the maintenance of this subpopulation of cells. In addition, the CD41⁺ cells showed enhanced expression of genes associated with hematopoietic stem cell (HSC) activities (Bmi-1, Runx1, Tal-1 and Lmo2), implying that part of the HSC transcriptional program has been re-activated in these cells. We therefore speculate that the CD41⁺ cells may represent a group of MLL leukemic cells that harbors strong stem cell features, and presumably functions as LSCs. The CD41⁺ leukemic cells will be further characterized by their LSC functions and CD41 can potentially serve as a novel LSC marker in MLL-rearranged leukemia. Taken together, studies on the role of PRMT1 can provide novel insights on the establishment of LSC and the development of effective clinical treatment for MLL-rearranged leukemia.