The Polycomb group (PcG) encodes an evolutionarily conserved set of chromatin-modifying proteins that are thought to maintain cellular transcriptional memory by stably silencing gene expression1. In mouse embryos that are mutated for the PcG protein Eed, X-chromosome inactivation (XCI) is not stably maintained in extra-embryonic tissues2. Eed is a component of a histone-methyltransferase complex that is thought to contribute to stable silencing in undifferentiated cells due to its enrichment on the inactive X-chromosome in cells of the early mouse embryo and in stem cells of the extra-embryonic trophectoderm lineage3,4,5,6,7,8. Here, we demonstrate that the inactive X-chromosome in Eed−/− trophoblast stem cells and in cells of the trophectoderm-derived extra-embryonic ectoderm in Eed−/− embryos remain transcriptionally silent, despite lacking the PcG-mediated histone modifications that normally characterize the facultative heterochromatin of the inactive X-chromosome. Whereas undifferentiated Eed−/− trophoblast stem cells maintained XCI, reactivation of the inactive X-chromosome occurred when these cells were differentiated. These results indicate that PcG complexes are not necessary to maintain transcriptional silencing of the inactive X-chromosome in undifferentiated stem cells. Instead, PcG proteins seem to propagate cellular memory by preventing transcriptional activation of facultative heterochromatin during differentiation.
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We are grateful to T. Jenuwein for sharing the anti-H3-3mK27 antibody; and to K. Worringer, S. Mlynarczyk-Evans and A. Anderson for supplying Xist, Hprt, Mecp2 and Pgk1 FISH templates and probes. We also acknowledge S. Mlynarczyk-Evnas for technical advice on FISH. S.K. is a recipient of an American Cancer Society postdoctoral fellowship. This work was funded by a National Institutes of Health grant to T.M.
The authors declare no competing financial interests.
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lncRNA-Induced Spread of Polycomb Controlled by Genome Architecture, RNA Abundance, and CpG Island DNA
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