The role of Polycomb-mediated epigenetic regulation in embryonic stem cell differentiation

Abstract

During mammalian development, a single founding cell must produce all of the different types of cells in the adult organism. What are the regulatory mechanisms required to coordinate the necessary gene expression networks for this process? Polycomb group (PcG) proteins are epigenetic regulators necessary for proper differentiation of cells and for mammalian development. Notably, faulty regulation of PRC2 has been associated with a broad range of cancers, suggesting that it has a critical role in maintaining cell identity. Polycomb Repressive Complex 2 (PRC2) catalyzes the posttranslational histone modification H3K27me3, a histone modification associated with transcriptional repression. Although PRC2 has critical functions in lineage commitment and in mediating cell fate transitions, it has proved difficult to study its precise role in these processes since complete loss of H3K27me3 leads to an inability of embryonic stem cells (ESCs) to properly undergo directed differentiation in vitro. PRC2 functions with additional regulators and regulatory pathways, including PRC1, accessory PcG subunits, and DNA methylation, among others; however, we know little about how they work together to coordinate gene expression programs during lineage commitment. Thus, dissecting the function of PRC2 is critical to improve our understanding of mammalian development and disease. Here, we analyzed gene expression and DNA methylation levels in several PRC2 mutant ESC lines that maintained varying levels of H3K27me3. We found that while a partial reduction of H3K27me3 levels allowed for proper temporal activation of lineage genes during directed differentiation of ESCs to spinal motor neurons (SMNs), genes that function to specify other lineages failed to be repressed, suggesting that PRC2 activity is necessary for regulating lineage fidelity. We also found that H3K27me3 is antagonistic to DNA methylation in cis. Thus, these data suggest a role for PRC2 in coordinating gene repression while protecting against inappropriate promoter DNA methylation during differentiation. Our work provides novel insights into the functional relationship between two distinct epigenetic regulatory mechanisms, as mediated by PRC2 and DNA methylation, in regulating lineage decisions during development.