X-INACTIVATION AND HISTONE H4 ACETYLATION IN EMBRYONIC STEM-CELLS

In female mammalian cells, dosage compensation for X-linked genes is a chieved by the transcriptional silencing, early in development, of man y genes on just one of the two X chromosomes. Several properties disti nguish the inactive X (Xi) from its active counterpart (Xa). These inc lude expression of Xist, a gene located in the X-inactivation center ( Xic), late replication, differential methylation of selected CpG islan ds and underacetylation of histone H4. The relationship between these properties and transcriptional silencing remains unclear. Female mouse embryonic stem (ES) cells have two active X chromosomes, one of which is inactivated as cells differentiate in culture. We describe here th e use of these cells in studying the sequence of events leading to X-i nactivation. By immunofluorescent labeling of metaphase chromosome spr eads from ES cells with antibodies to acetylated H4, we show that an u nderacetylated X chromosome appears only after 4 days of differentiati on, and only in female cells. The frequency of cells with an underacet ylated X reaches a maximum by Day 6. In undifferentiated cells, H4 in centric heterochromatin is acetylated to the same extent as that in eu chromatin but has become relatively underacetylated, as in adult cells , by Day 4 of differentiation (i.e., when deacetylation of Xi is first seen). The overall deacetylation of Xi follows Xist expression and th e first appearance of a single, late-replicating X, both of which occu r on Day 2. It also follows the silencing of X-linked genes. Levels of mRNA from four such genes, Hprt, G6pd, Rps4, and Pgk-1, had all falle n by approximately 50% (relative to the autosomal gene Aprt) by Days 2 -4. The results show that properties that characterize Xi are put in p lace in a set order over several days. H4 deacetylation occupies a def ined place within this sequence, suggesting that it is an intrinsic pa rt of the X-inactivation process. The stage at which a completely deac etylated Xi is first seen suggests that deacetylation may be necessary for the maintenance of silencing but is not required for its initiati on. Nor is it required for, or an immediate consequence of, late repli cation. However, we note that selective deacetylation of H4 on specifi c genes would not be detected by the microscopical approach we have us ed and that such selective deacetylation may still be part of the sile ncing process. (C) 1996 Academic Press, Inc.