Polycomb-like 2 (Mtf2/Pcl2) is Required for Epigenetic Regulation of Hematopoiesis

Polycomb proteins are epigenetic regulators that are critical in mediating gene repression at critical stages during development. Core and accessory proteins make up the Polycomb Repressive Complex 2 (PRC2), which is responsible for trimethylation of lysine 27 on histone 3 (H3K27me3), leading to maintenance of chromatin compaction and sustained gene repression. Classically, Polycomb accessory proteins are often thought of as having minor roles in fine-tuning the repressive action of PRC2. Their actions have often been attributed to chromatin recognition, targeting to specific loci and enhancing methyltransferase activity. In our previous work in mouse embryonic stem cells (ESCs), we showed that Polycomb-like 2 (Mtf2/Pcl2) is critical for PRC2-mediated regulation of stem cell self-renewal through feed-forward control of the pluripotency network. In moving beyond the ESC model system, we sought to interrogate the role of Mtf2 in vivo by creating a gene-targeted knockout mouse model. Surprisingly, we discovered a tissue-specific role for Mtf2 in controlling erythroid maturation and hematopoietic stem cell self-renewal. Via its regulation of other PRC2 members, Mtf2 is critical for global H3K27me3 methylation at promoter-proximal sites in developing erythroblasts. Thus, Mtf2 is required for proper maturation of erythroblasts. Loss of Mtf2 also reduces HSC self-renewal leading to stem cell pool exhaustion. Additionally, misregulation of Mtf2 in leukemia models contributes to massive leukemic blast expansion at the expense of leukemic stem cell self-renewal. In the developing hematopoietic system, Mtf2 functions as a core complex member, controlling epigenetic regulation of self-renewal and maturation of both stem and committed cells