SummaryPolycomb group (PcG) proteins are epigenetic modifiers involved in controlling gene repression. Organized within multiprotein complexes, they regulate developmental genes in multiple cell types and tissue contexts, including embryonic and adult stem cells, and are essential for cell fate transitions and proper development. Here, we summarize recent breakthroughs that have revealed the diversity of PcG complexes acting in different cell types and genomic contexts. Intriguingly, it appears that particular PcG proteins have specific functions in embryonic development, in pluripotent stem cells and in reprogramming somatic cells into a pluripotent-like state. Finally, we highlight recent results from analyzing PcG protein functions in multipotent stem cells, such as neural, hematopoietic and epidermal stem cells. Key words: Polycomb, Stem cells, Transcription, Differentiation, Self-renewal IntroductionAlthough stem cells were discovered decades ago (Till and McCulloch, 1961;Spangrude et al., 1988), their potential as model cells for studying cell differentiation, tissue homeostasis and regeneration has only recently begun to be realized. In particular, embryonic stem cells (ESCs), which are pluripotent cells capable of giving rise to all cell types of the embryo (Boiani and Schöler, 2005), provide a valuable tool for studying embryonic development in vitro.Several transcription factors have been identified as master regulators of pluripotent and multipotent stem cells (Niwa, 2007). Increasing evidence suggests that epigenetic modifications additionally play a crucial role in regulating stem cell characteristics. Among the chromatin modifiers, Polycomb group (PcG) proteins function as gene repressors and are involved in the regulation of stem cell characteristics (Simon and Kingston, 2009). The PcG was originally described as a set of genes responsible for controlling proper body segmentation in Drosophila (Lewis, 1978). During Drosophila embryonic development, PcG proteins repress the homeobox genes of the Hox cluster, thereby determining the proper activation of homeotic genes (Schuettengruber and Cavalli, 2009). The function of PcG proteins as repressors of developmental genes is strongly conserved in mammals (Morey and Helin, 2010). Here, we discuss the latest insights into PcG-mediated epigenetic regulation in stem cells and embryonic development. Molecular activities of PcG complexesIn mammals, PcG proteins are found in several multiprotein complexes (Simon and Kingston, 2009), the best characterized of which are Polycomb repressive complexes 1 and 2 (PRC1 and PRC2) (Margueron and Reinberg, 2011). As epigenetic modifiers, PcG complexes promote gene repression via particular chromatin modifications and compaction (Fig. 1).Here, we provide a brief overview of the molecular mechanisms by which PcG complexes regulate gene expression; for further details, we refer the reader to recent reviews (Lanzuolo and Orlando, 2012;Simon and Kingston, 2013). At the molecular level, PRC2 is responsible for di-and tri-me...

The Polycomb repressive complex 1 (PRC1) is required for decisions of stem cell fate. In mouse embryonic stem cells (ESCs), two major variations of PRC1 complex, defined by the mutually exclusive presence of Cbx7 or RYBP, have been identified. Here, we show that although the genomic localization of the Cbx7- and RYBP-containing PRC1 complexes overlaps in certain genes, it can also be mutually exclusive. At the molecular level, Cbx7 is necessary for recruitment of Ring1B to chromatin, whereas RYBP enhances the PRC1 enzymatic activity. Genes occupied by RYBP show lower levels of Ring1B and H2AK119ub and are consequently more highly transcribed than those bound by Cbx7. At the functional level, we show that genes occupied by RYBP are primarily involved in the regulation of metabolism and cell-cycle progression, whereas those bound by Cbx7 predominantly control early-lineage commitment of ESCs. Altogether, our results indicate that different PRC1 subtypes establish a complex pattern of gene regulation that regulates common and nonoverlapping aspects of ESC pluripotency and differentiation.

Polycomb group proteins have long been linked to the occurrence of different forms of cancer. Polycomb proteins form at least two distinct complexes, the Polycomb-repressive complexes 1 and 2 (PRC1 and PRC2). Some of the PRC complex subunits have been found to be overexpressed in a variety of different tumors. Epigenetic perturbations are likely to be the cause for transcriptional misregulation of tumor suppressor genes and of certain cell fates. It is especially critical for stem cells that their potential to self-renewal and to differentiate is tightly controlled and properly orchestrated. Misregulation of Polycomb protein levels often leads to either a block or unscheduled activation of developmental pathways, thereby enhancing the proliferation capability of a cell. The consequences of this misregulation have been linked to the establishment of cancer stem cells, which can produce tumors through a combination of increased self-renewal and the lack of complete cellular differentiation. Cancer stem cells are believed to persist within tumors and to elicit relapse and metastasis. In this review, we recapitulate the roles of Polycomb proteins in stem cell biology, and the impact their misregulation can have on cancer.

MH designed and performed experiments and interpreted results.. L.A. designed and performed the in vitro experiments, M.A.M., designed and performed the in vivo experiments, L.C-E., the hydroxymethylation and EdU stainings, G.B, experiments with small molecule inhibitors. G.V. and E.A.M. prepared and analysed WGBS and RRHP libraries, analysed RNAseq and interpreted corresponding bionformatic analyses related. N.A., A.R. and S.J.F. performed experiments with β1 integrin model and interpreted results of the p21 models. J.v.d.A. and A.H.B. performed DamID-seq experiments. B.F-C helped on the in vivo analysis. R.A.C. helped on bioinformatics analyses. R.L.M. provided the R26Fucci2a line. F.A. and M.Z.G. performed the live imaging of ductal cells.