Role of Polycomb Group Protein Mel18 in Hematopoietic Stem Cell Maintenance

Abstract

Polycomb group (PcG) proteins are epigenetic gene silencers that have been implicated in stem cell maintenance and cancer development. Genetic and biochemical studies indicate that Polycomb group proteins exist in at least two protein complexes, Polycomb repressive complex 2 (PRC2) and Polycomb repressive complex 1 (PRC1). PRC2 complex deposits the mono-, di-, and tri- methylation on histone 3 lysine 27, whereas PRC1 introduces mono-ubiquitination on histone 2A lysine 119 (H2AK119ub1). PRC1 can also regulate 3D chromatin structure. Bmi1 (PCGF4) and Mel18 (PCGF2) are two major homologs of the PCGF subunit within the canonical PRC1 complex. While Bmi1 is a positive regulator of hematopoietic stem cells (HSCs) and leukemia stem cells (LSCs) self-renewal, the role of Mel18 in normal and malignant hematopoiesis is not fully understood. Based upon our previous studies and the literature, I hypothesized that Mel18 inhibits HSC self-renewal and proliferation but promotes HSC senescence. To test my hypothesis, I examined HSC behavior in Mel18 conditional knockout mice (Mel18f/f-Mx1Cre+). I found that acute deletion of Mel18 enhances the repopulating potential of HSCs and increases the number of functional HSCs, without affecting HSC homing. Loss of Mel18 decreased hematopoietic stem and progenitor cell (HSPCs) senescence. In addition, loss of Mel18 promotes cell cycle progression in HSPCs. Therefore, I demonstrated that loss of Mel18 enhances the repopulating potential of HSCs, promotes cell cycle progression in HSCs, but reduces HSC senescence. Mechanistically, loss of Mel18 increases the chromatin accessibility to genes important for HSC self-renewal and ex vivo expansion such as homeobox gene Hoxb4. CUT&RUN sequencing assays revealed that loss of Mel18 reduces the H2AK119ub1 enrichment at the promoter regions of Cdk4 and Cdk6, leading to their enhanced expression in HSPCs. Furthermore, I identified a Mel18-specific chromatin loop at the S100a9 locus, a gene important for senescence and inflammation, using Hi-C assays, Collectively, these findings demonstrated that Mel18 plays an important role in hematopoietic stem cell maintenance. Mel18 inhibits HSC self-renewal and proliferation but promotes HSC senescence through modulating histone modifications, chromatin accessibility, and 3D chromatin structure in HSCs.