Evidence of Xist RNA-independent initiation of mouse imprinted X-chromosome inactivation

SummaryXX female mammals undergo transcriptional silencing of most genes on one of their two X-chromosomes to equalize X-linked gene dosage with XY males in a process referred to as X-chromosome inactivation (XCI). XCI is a paradigm of epigenetic regulation1. Once enacted in individual cells of the early female embryo, XCI is stably transmitted such that most descendant cells maintain silencing of that X-chromosome2. In eutherian mammals, XCI is thought to be triggered by the expression of the non-coding Xist RNA from the future inactive-X (Xi)3,4,5; Xist RNA in turn is proposed to recruit protein complexes that bring about heterochromatinization of the Xi6,7. Here we test whether imprinted XCI, which results in preferential inactivation of the paternal X-chromosome (Xp), occurs in mouse embryos inheriting an Xp lacking Xist. We find that silencing of Xp-linked genes can initiate in the absence of paternal Xist; Xist is, however, required to stabilize silencing of the Xp. Xp-linked gene silencing associated with mouse imprinted XCI, therefore, can initiate in the embryo independently of Xist RNA

Differentiation-dependent Requirement of Tsix long non-coding RNA in Imprinted X-chromosome Inactivation

Imprinted X-inactivation is a paradigm of mammalian transgenerational epigenetic regulation resulting in silencing of genes on the paternally-inherited X-chromosome. The pre-programmed fate of the X-chromosomes is thought to be controlled in cis by the parent-of-origin-specific expression of two long non-coding RNAs, Tsix and Xist, in mice. Exclusive expression of Tsix from the maternal–X has implicated it as the instrument through which the maternal germline prevents inactivation of the maternal–X in the offspring. Here, we show that Tsix is dispensable for inhibiting Xist and X-inactivation in the early embryo and in cultured stem cells of extra-embryonic lineages. Tsix is instead required to prevent Xist expression as trophectodermal progenitor cells differentiate. Despite induction of wild-type Xist RNA and accumulation of histone H3-K27me3, many Tsix-mutant X-chromosomes fail to undergo ectopic X-inactivation. We propose a novel model of lncRNA function in imprinted X-inactivation that may also apply to other genomically imprinted loci

Evidence of Xist RNA-independent initiation of mouse imprinted X-chromosome inactivation

XX female mammals undergo transcriptional silencing of most genes on one of their two X-chromosomes to equalize X-linked gene dosage with XY males in a process referred to as X-chromosome inactivation (XCI). XCI is a paradigm of epigenetic regulation1. Once enacted in individual cells of the early female embryo, XCI is stably transmitted such that most descendant cells maintain silencing of that X-chromosome2. In eutherian mammals, XCI is thought to be triggered by the expression of the non-coding Xist RNA from the future inactive-X (Xi)3,4,5; Xist RNA in turn is proposed to recruit protein complexes that bring about heterochromatinization of the Xi6,7. Here we test whether imprinted XCI, which results in preferential inactivation of the paternal X-chromosome (Xp), occurs in mouse embryos inheriting an Xp lacking Xist. We find that silencing of Xp-linked genes can initiate in the absence of paternal Xist; Xist is, however, required to stabilize silencing of the Xp. Xp-linked gene silencing associated with mouse imprinted XCI, therefore, can initiate in the embryo independently of Xist RNA. In the mouse, Xist RNA expression is first detected at the late two-cell stage of embryogenesis exclusively from the Xp8. During subsequent stages of preimplantation development Xist RNA spreads from its site of synthesis to eventually coat most of th