Journal
NATURE CELL BIOLOGY
Volume 8, Issue 2, Pages 195-U66Publisher
NATURE PUBLISHING GROUP
DOI: 10.1038/ncb1351
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- NICHD NIH HHS [R37 HD024462] Funding Source: Medline
- NIGMS NIH HHS [R01 GM063671] Funding Source: Medline
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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 expression(1). In mouse embryos that are mutated for the PcG protein Eed, X-chromosome inactivation (XCI) is not stably maintained in extra-embryonic tissues(2). 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 lineage(3-8). 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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