During mouse embryogenesis, reversion of imprinted X chromosome inactivation in the pluripotent inner cell mass of the female blastocyst is initiated by the repression of Xist from the paternal X chromosome. Here we report that key factors supporting pluripotency-Nanog, Oct3/4, and Sox2-bind within Xist intron 1 in undifferentiated embryonic stem (ES) cells. Whereas Nanog null ES cells display a reversible and moderate up-regulation of Xist in the absence of any apparent modification of Oct3/4 and Sox2 binding, the drastic release of all three factors from Xist intron 1 triggers rapid ectopic accumulation of Xist RNA. We conclude that the three main genetic factors underlying pluripotency cooperate to repress Xist and thus couple X inactivation reprogramming to the control of pluripotency during embryogenesis.
Abstract Background Delimiting distinct chromatin domains is essential for temporal and spatial regulation of gene expression. Within the X-inactivation centre region ( Xic ), the Xist locus, which triggers X-inactivation, is juxtaposed to a large domain of H3K27 trimethylation (H3K27me3). Results We describe here that developmentally regulated transcription of Tsix , a crucial non-coding antisense to Xist , is required to block the spreading of the H3K27me3 domain to the adjacent H3K4me2-rich Xist region. Analyses of a series of distinct Tsix mutations suggest that the underlying mechanism involves the RNA Polymerase II accumulating at the Tsix 3'-end. Furthermore, we report additional unexpected long-range effects of Tsix on the distal sub-region of the Xic , involved in Xic - Xic trans-interactions. Conclusion These data point toward a role for transcription of non-coding RNAs as a developmental strategy for the establishment of functionally distinct domains within the mammalian genome.
