EpiSwitch

The DNA methylation machinery and Polycomb group proteins are two highly conserved chromatin modification pathways associated with epigenetically inherited transcriptional repression. Despite similar features, their activities are generally mutually exclusive in mammalian genomes. Moreover, during mouse embryogenesis, both cytosine DNA methylation (5meC) and Polycomb-deposited histone H3 lysine 27 trimethylation (H3K27me3) undergo dramatic reprogramming: as pluripotent embryonic stem cells (ESCs) prepare for lineage commitment, the genome becomes highly DNA methylated, while initially broad H3K27me3 domains become largely restricted to silent 5meC-free gene promoters. Recently, using a culture-based model of ESC differentiation, we demonstrated that a subset of genes requires 5meC deposition in order to ensure proper gene activation. The explanation for this counterintuitive observation is that 5meC relieves Polycomb-mediated silencing in these cases. This emerging regulatory interplay may partially explain the epigenomic reorganization that occurs in development. Nevertheless, how this “epigenetic switch” occurs and its implications later in development remain unclear. This project will elucidate these questions with three aims: 1) investigating the role of a variant Polycomb complex that our preliminary data implicate in the epigenetic switch, 2) an unbiased genetic screen to discover novel mediators of 5meC-dependent activation and 3) development of an “epigenome-edited” mouse model to ascertain the developmental repercussions of epigenetic switching. DNA methylation and Polycomb are both essential for mammalian development, and misregulation of each pathway is associated with myriad human pathologies, notably several cancers. The research proposed here will achieve major insights into their interplay.