Posttranslational modifications of histone proteins have emerged as central regulators of gene expression. Through the factors that install, interpret, and erase them, histone marks control access to the genome, establishing chromatin environments that either support or counteract transcription. The histone methyltransferase Polycomb Repressive Complex 2 (PRC2) is a key mediator of gene repression all throughout development and adulthood. It is often misregulated in disease states such as cancer. The overarching goal of the Action is to advance our understanding of the mechanisms by which Polycomb Repressive Complex (PRC) 2 regulates expression of developmental genes in embryonic stem cells (ESCs) and during differentiation. Specifically, we ask how PRC2 is regulated in a context-dependent manner to achieve both terminal repression as well as poising of genes, keeping them in a plastic state for subsequent activation at the so-called bivalent domains. We hypothesise that the interplay and cooperation of PRC2 with other chromatin regulators and its ability to modify nucleosomes in both symmetric and asymmetric fashion are central to determining whether PRC2 acts to repress or to poise genes. Given its role in development and diseases, insight into the mechanisms underlying PRC2 function will inform future therapeutic approaches for regenerative medicine and for treatment of cancer and other diseases.
Work package 1 of this project examines how PRC2 cooperates with other histone modifiers and chromatin organisers at enhancers and promoters to achieve poising of developmental genes. These studies will enable us to appreciate how the pivotal PRC2 module interfaces with other players in a complex system of chromatin regulators, contributing to a much-needed integrated view of chromatin regulation. In work package 2 we assess how the generation of asymmetric nucleosomes by PRC2 is regulated by PRC2-intrinsic catalytic properties and through interactions with other chromatin modifiers, especially at bivalent domains. Work package 3 aims to generate a systems biology-informed quantitative view of PRC2 recruitment and function at bivalent domains. Together, these studies aim to determine how PRC2 achieves both full repression and transcriptional poising in a context-dependent manner, crucial functions for proper embryonic development.
Through this project, we determined how PRC2 regulates chromatin structure and uncovered novel mechanisms by which the complex controls expression of developmental genes in embryonic stem cells and during differentiation. We set up a novel system for the quantitative study of bivalent domain function and applied it to assess how bivalency achieves poising of developmental genes. We further found a novel level of regulation for the histone acetyltransferase CBP/p300, a key antagonist of PRC2-mediated gene repression. Its TAZ2 domain interacts with DNA, thereby achieving specificity towards H3K27. Together, these findings shed light on how developmental genes are repressed, poised, and activated during embryonic stem cell differentiation.
