Human XIST RNA acts early to condense architecture which facilitates A-repeat density-dependent initiation of gene silencing

XIST RNA triggers gene silencing chromosome-wide and transforms a euchromatic chromosome into a condensed Barr body. XIST is heavily studied in mouse ES cells, but here an inducible iPSC system allows analysis of initial steps in human chromosome silencing, revealing key points not known in either system. XIST RNA distribution was examined relative to biochemical and transcriptional changes directly within architecture of individual chromosome territories. Within a few hours of induction, XIST transcripts distribute as a large “sparse zone” and a smaller “dense zone”, which, importantly, exhibit different effects on chromatin. Very sparse transcripts immediately trigger bright staining for H2AK119ub and CIZ1, a structural matrix protein. In contrast, H3K27me3 enrichment comes hours later and is much more restricted to the smaller dense RNA zone, which enlarges as the chromosome condenses. Importantly, silencing of several genes examined occurred well after architectural condensation, suggesting a possibly separable step. Surprisingly, we show the small A-repeat fragment of XIST can alone silence endogenous genes; however, results indicate this requires high local RNA density for effective histone deacetylation. Results support a concept whereby XIST RNA acts directly to condense the chromosome territory, comprised largely of non-coding DNA, which facilitates a required step to initiate gene silencing by the A-repeat. Hence, compacted architecture is not a consequence of collective gene silencing, but an early step required for chromosome-wide gene silencing. ### Competing Interest Statement The authors have declared no competing interest.