Polycomb Proteins Targeted by a Short Repeat RNA to the Mouse X Chromosome

TLDR

X‑chromosome inactivation balances dosage between sexes and is initiated by the long noncoding RNA Xist, but the mechanism by which Xist induces silencing remains unknown. We identified a 1.6‑kb RNA element, RepA, within Xist that directly recruits the Polycomb complex PRC2 via its Ezh2 subunit, and showed that RepA and PRC2 are essential for Xist up‑regulation and spreading of silencing, with Tsix RNA antagonizing this interaction.

Abstract

To equalize X-chromosome dosages between the sexes, the female mammal inactivates one of her two X chromosomes. X-chromosome inactivation (XCI) is initiated by expression of Xist, a 17-kb noncoding RNA (ncRNA) that accumulates on the X in cis. Because interacting factors have not been isolated, the mechanism by which Xist induces silencing remains unknown. We discovered a 1.6-kilobase ncRNA (RepA) within Xist and identified the Polycomb complex, PRC2, as its direct target. PRC2 is initially recruited to the X by RepA RNA, with Ezh2 serving as the RNA binding subunit. The antisense Tsix RNA inhibits this interaction. RepA depletion abolishes full-length Xist induction and trimethylation on lysine 27 of histone H3 of the X. Likewise, PRC2 deficiency compromises Xist up-regulation. Therefore, RepA, together with PRC2, is required for the initiation and spread of XCI. We conclude that a ncRNA cofactor recruits Polycomb complexes to their target locus.