Mitotically stable association of polycomb group proteins Eed and Enx1 with the inactive X chromosome in trophoblast stem cells

X inactivation in female mammals is one of the best studied examples of heritable gene silencing and provides an important model for studying maintenance of patterns of gene expression during differentiation and development [1]. The process is initiated by a cis-acting RNA, the X inactive specific transcript (Xist). Xist RNA is thought to recruit silencing complexes to the inactive X, which then serve to establish and maintain the inactive state in all subsequent cell divisions [2-4]. Most lineages undergo random X inactivation, there being an equal probability of either the maternally (Xm) or paternally (Xp) inherited X chromosome being inactivated in a given cell [5]. In the extraembryonic trophectoderm and primitive endoderm lineages of mouse embryos, however, there is imprinted X inactivation of Xp [6]. This process is also Xist dependent [7]. A recent study has shown that imprinted X inactivation in trophectoderm is not maintained in embryonic ectoderm development (eed) mutant mice [8]. Here we show that Eed and a second Polycomb group protein, Enx1, are directly localized to the inactive X chromosome in XX trophoblast stem (TS) cells. The association of Eed/Enx1 complexes is mitotically stable, suggesting a mechanism for the maintenance of imprinted X inactivation in these cells.