Expression and ChIP-seq analyses of embryonic stem cells, extraembryonic endoderm stem cells, and trophoblast stem cells

Bivalent histone domains have been proposed to contribute to pluripotency in embryonic stem cells, suggesting an epigenetic mechanism may regulate stem cell behavior in general. Here we compare histone modifications in two other stem cells derived from the blastocyst. We show that extraembryonic stem cells have little repressive lysine 27 trimethylation and few bivalent domains. Thus, bivalent domains are not a common mechanism for maintaining the undifferentiated state in blastocyst-derived stem cells and alternative mechanisms must mediate transcriptional repression in extraembryonic cells. We show that lysine 9 trimethylation, but not DNA methylation, is likely to fulfill this role. Intriguingly, although we do detect bivalent domains in pluripotent cells in the early mouse embryo, the epigenetic status of extraembryonic cells does not entirely reflect their in vitro stem cell counterparts. Therefore, differences in epigenetic regulation between lineage progenitors in vivo and in vitro may arise during selection for self-renewal in vitro.