Xist RNA targets selective autosomal genes to modulate expression in a Repeat B-dependent manner [RNA-seq]

Xist, a pivotal player in X chromosome inactivation (XCI), has long been perceived as a cis-acting long noncoding RNA that binds exclusively to the inactive X chromosome (Xi). However, Xist’s ability to diffuse under select circumstances has long been documented, leading us to suspect that Xist RNA may have targets and functions beyond the Xi. Here, using female mouse embryonic stem cells (ES) and mouse embryonic fibroblasts (MEF) as a model, we demonstrate that Xist RNA indeed can spread beyond the Xi. However, its binding is limited to ~100 genes in differentiating ES and MEF cells. The target genes are diverse in function but are unified by their active chromatin status. Although active, Xist binding is associated with a downregulation of gene expression. Unlike on the Xi, Xist binding does not lead to full silencing and also does not spread beyond the target gene. Deleting Xist’s Repeat B (RepB) domain reduces Xist’s diffusion away from the Xi and inhibits Xist’s localization to autosomal targets. Nonetheless, Xist transcript lacking RepB fails to down-modulate autosomal gene expression. Altogether, our findings reveal Xist targets beyond the Xi and identify Repeat B as a crucial domain for its in-trans function.