Mitotically Stable Association of Polycomb Group Proteins Eed and Enx1 with the Inactive X Chromosome in Trophoblast Stem Cells

Abstract

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. 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. 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. In the extraembryonic trophectoderm and primitive endoderm lineages of mouse embryos, however, there is imprinted X inactivation of Xp. This process is also Xist dependent. A recent study has shown that imprinted X inactivation in trophectoderm is not maintained in embryonic ectoderm development (eed) mutant mice. 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.

LHP1, the Arabidopsis homologue of HETEROCHROMATIN PROTEIN1, is required for epigenetic silencing of FLC

“…However, the continued association of VRN1 with chromosomes all the way through mitosis was unexpected. Most Polycomb, Polycomb-associated proteins, and transcription factors have been found to be displaced from their recognition sequences during mitosis (43-47); however, some do remain, including Drosophila GAGA factor and Pipsqueak, which function as sequence-specific binding proteins and are involved in recruitment of Polycomb complexes to the Polycomb response element and high-mobility-group proteins, which bind DNA non-sequence-specifically (48)(49)(50)(51)(52). Knowledge of VRN1 interactors may help elucidate why it remains associated during mitosis and the significance of this finding for epigenetic stability of FLC silencing.…”

Genome imprinting regulated by the mouse Polycomb group protein Eed

“…We recently showed that loss of function of Eed in the mouse results in reactivation of the imprinted X chromosome in extraembryonic lineages 12 . Additionally, the Eed and Ezh2 proteins have been reported to co-localize with XIST on the imprinted X chromosome of mouse trophoblast stem cells 13 . Thus, we investigated whether Eed is also required for epigenetic regulation of autosomal imprinted loci.…”

A Continuity of X-Chromosome Silence from Gamete to Zygote

“…Both are controlled by a cis-acting X-inactivation center (Xic) (Brown et al 1991), a region that contains several noncoding RNA genes including Xist (Brockdorff et al 1992;Brown et al 1992) and the complementary Tsix gene (Lee et al 1999). During imprinted XCI, silencing is associated with the spread of Xist RNA along the paternal X chromosome (Mak et al 2002;Huynh and Lee 2003), while activity on the maternal X chromosome is preserved by expression of Tsix (Lee 2000;Sado et al 2001). Random XCI is also ini-tiated by Xist expression and blocked by Tsix.…”

“…In the extraembryonic tissues, as represented by trophoblast stem (TS) cells, the gradient of silencing along the X P disappears and all genes tested along the X P exhibit complete silencing (Huynh and Lee 2003). A more global pattern of inactivation in postimplantation cells may naturally follow from increased recruitment of heterochromatic factors such as the polycomb group proteins (Mak et al 2002(Mak et al , 2004Okamoto et al 2004), macroH2A1 (Costanzi et al 2000), and DNA methylation, all of which serve to "lock in" the inactive state. Thus, imprinted XCI is characterized by two distinct phases-one in the preimplantation embryo, where dosage compensation favors maternal transcription without precluding leaky paternal expression, and one in the postimplantation embryo, where the imperfection of the earlier form gives way to a globalized paternal silencing mechanism.…”

“…There is evidence to suggest that the partial extent of inactivation in preimplantation embryos may reflect the spreading of Xist along the X P . Xist RNA only partially coats metaphase X chromosomes of 8-to 16-cell-stage blastomeres, whereas Xist RNA coats the entire length of the chromosome in TS and somatic cells (Matsui et al 2001;Mak et al 2002;Huynh and Lee 2003). While these findings imply a role for Xist, whether Xist actually plays a role in preimplantation XCI has yet to be investigated.…”