in the bone marrow
Investigators have discovered a new mechanism of megakaryocyte differentiation, according to a paper published in eLife.
They found that overexpression of the methyltransferase enzyme PRMT1 in acute megakaryocytic leukemia blocks megakaryocyte differentiation by downregulating levels of the RNA-binding protein RBM15.
The team therefore believes that targeting PRMT1 could restore megakaryocyte differentiation in this malignancy.
They also think their findings could lead to new approaches for researching and treating other hematologic malignancies and solid tumors.
Xinyang Zhao, PhD, of the University of Alabama at Birmingham, and his colleagues began this study looking at PRMT1, which attaches a methyl group onto specific arginine amino acid residues of target proteins.
The investigators screened for proteins that were tagged with methyl groups by PRMT1 and selected one of them—RBM15—for further study. RBM15 was of interest because a mutant fusion of RBM15 and MKL1 proteins is associated with acute megakaryoblastic leukemia.
The team discovered that when a cell’s PRMT1 levels are high, a greater proportion of RBM15 is tagged with methyl groups on certain arginine residues. This tagging causes a ligase called CNOT4 to mark RBM15 with another tag, ubiquitin, which marks the protein for transport to the cell’s garbage removal machinery.
The methyl-tagged RBM15 proteins rapidly disappear, even though the amount of RBM15 messenger RNA does not change. Thus, the expression levels of PRMT1 inversely affect the amount of RBM15.
When the concentration of RBM15 is low, megakaryocytic progenitor cells cannot move forward to differentiation. But when the concentration of RBM15 is high enough, the progenitor cells differentiate into mature megakaryocytes.
The investigators also found that RBM15 binds to intron regions of the pre-messenger RNA for genes known to be important in megakaryocyte differentiation, including 3 transcription factors—RUNX1, GATA1, and TAL1—that are important for normal and abnormal hematopoiesis.
And RBM15 appears to recruit the splicing factor SF3B1 to correctly splice exons. When RBM15 is low, one or more exons are not correctly spliced.
The team said this is a new mechanism for cell differentiation, initiated by methylation of RNA-binding proteins.
“The regulation of alternative splicing by RBM15 through SF3B1 is an exciting and novel pathway that clearly participates in the decision of a megakaryocyte to grow or differentiate,” said John Crispino, PhD, of the Northwestern University Feinberg School of Medicine in Chicago, Illinois, who was not involved in this study.
“These findings suggest that modulation of RBM15 activity by suppressing PRMT1 activity may change the splicing pattern of megakaryocytic tumor cells and facilitate their differentiation.”
The investigators also believe RBM15 may have broader functions in cells. They found that RBM15 binds directly to the pre-messenger RNA of 1257 genes. Among them are genes involved in metabolic regulation.
In agreement with this finding, the team discovered that overexpression of PRMT1 or reduced expression of RBM15 enhances the creation of more mitochondria.
The investigators have further identified metabolic pathways regulated by PRMT1 in leukemia cells. They said these data, in a manuscript under preparation, will further link tumorigenesis to metabolic pathways.
The team also noted that SF3B1 contains mutations in more than 70% of myelodysplastic syndrome patients and 20% of chronic lymphocytic leukemia patients, and mutated SF3B1 appears in other hematologic malignancies as well.
So the investigators believe that understanding the PRMT1-RBM15 axis can shed new light on SF3B1-mutated hematologic malignancies and may lead to targeting PRMT1 as a novel therapy for myelodysplastic syndromes. The team is already testing PRMT1 inhibitors.
