Targeting Polycomb to Pericentric Heterochromatin in Embryonic Stem Cells Reveals a Role for H2AK119u1 in PRC2 Recruitment

Abstract

SummaryThe mechanisms by which the major Polycomb group (PcG) complexes PRC1 and PRC2 are recruited to target sites in vertebrate cells are not well understood. Building on recent studies that determined a reciprocal relationship between DNA methylation and Polycomb activity, we demonstrate that, in methylation-deficient embryonic stem cells (ESCs), CpG density combined with antagonistic effects of H3K9me3 and H3K36me3 redirects PcG complexes to pericentric heterochromatin and gene-rich domains. Surprisingly, we find that PRC1-linked H2A monoubiquitylation is sufficient to recruit PRC2 to chromatin in vivo, suggesting a mechanism through which recognition of unmethylated CpG determines the localization of both PRC1 and PRC2 at canonical and atypical target sites. We discuss our data in light of emerging evidence suggesting that PcG recruitment is a default state at licensed chromatin sites, mediated by interplay between CpG hypomethylation and counteracting H3 tail modifications.

Short sequences can efficiently recruit histone H3 lysine 27 trimethylation in the absence of enhancer activity and DNA methylation

“…They extend previous studies that already suggested a role for CpG dinucleotides in PRC2 recruitment (22,23) but are also more comprehensive and provide novel mechanistic details. Interestingly, a recent study suggests that in vitro PRC2 activity is not directly inhibited by DNA methylation (38). It is thus possible that the antagonistic effect of DNA methylation on H3K27me3 deposition that we observe in vivo is dependent on additional factors, possibly the presence of methyl-CpG-binding proteins (39).…”

Genome-wide redistribution of H3K27me3 is linked to genotoxic stress and defective growth

“…The fact that redistribution of H3K27me3 is deleterious in Neurospora is somewhat surprising; however, a similar observation may have been difficult to make in other experimental systems. In plants and animals, loss of heterochromatin components leads to similar redistribution of H3K27me3 (17,(58)(59)(60)(61)(62), but in these systems, depletion of H3K27me3 from Pc-target regions is sufficient to cause poor growth and developmental defects (74,75). Thus, in many organisms, it would be difficult to determine if the phenotypic defects displayed by H3K9me3-deficient strains were caused by loss of H3K27me3 from native sites or by gain of H3K27me3 in heterochromatin domains.…”

“…H3K9me3, HP1, and 5mC have all been implicated in preventing H3K27me3 redistribution to heterochromatin in animals, and reduction of 5mC leads to redistribution of H3K27me3 in plants (17,(58)(59)(60)(61)(62)71). These observations highlight the need for additional studies to fully elucidate the complex functional and regulatory relationships between heterochromatin and Pc system components.…”

“…However, the mechanisms that control Pc complexes are not fully understood. In some situations, H3K27me3-independent recruitment of PRC1 can occur, leading to subsequent PRC2 recruitment and deposition of H3K27me3 (17,18). Pc components are absent from the model yeasts Saccharomyces cerevisiae and Schizosaccharyomyces pombe, whereas PRC1 appears to be absent from all fungi (19).…”

Au cœur d’une complexité biologique

“…Il a été établi à partir de plusieurs études montrant l'implication de PRC1 dans le recrutement de PRC2. En effet, un défaut d'expression de PRC1 entraîne une diminution de liaison de PRC2 à l'ADN, et l'adressage forcé des protéines de PRC1 à la chromatine se traduit par l'apposition de la marque H2AK119ub1 à l'origine du recrutement de PRC2 [19,20]. Des expériences de chromatographie d'affinité (ou pull down 3 ), réalisées sur des extraits nucléaires d'embryons de drosophile ou de cellules ES murines, ont révélé que les composants de PRC2 étaient, respectivement, fortement associés avec l'H2AK118ub et l'H2AK119ub.…”