X-chromosome inactivation (XCI), which is achieved by chromosome coating with Xist RNA and repressive chromatin modifications, results in the silencing of one of the two X chromosomes in female cells. In early mouse XX embryos, the paternal X chromosome is silenced during imprinted XCI. However, in pre-implantation mouse embryos, inner cell mass (ICM) cells reverse this imprinted XCI in order to initiate random XCI later in development. Now, Terry Magnuson and colleagues show that, during this reversal process, gene transcription from the inactive-X precedes the loss of Xist coating (p. 2049). By analysing the kinetics of X-chromosome activity in developing ICMs, the researchers show that reactivation of gene expression from the inactive-X can occur in the presence of both Xist coating and repressive histone H3K27me3 marks, suggesting that X-linked gene transcription and Xist coating are uncoupled during this period. This finding, they propose, alters our perception of the reversal process and supports the existence of Xist-independent silencing mechanisms in the mouse embryo.

© 2011.
2011