# Deletion of Xist repeat B disrupts cell cycle and asymmetric cell division through Usp9x hyperactivation in mice

## TL;DR

Deleting a specific part of the Xist RNA in mice disrupts cell division and growth by overactivating a protein called Usp9x.

## Contribution

The study reveals a new role for Xist repeat B in maintaining XCI and regulating cell proliferation and differentiation.

## Key findings

- Loss of Xist repeat B leads to reduced H3K27me3 and H2AK119ub on the X chromosome and partial reactivation of X-linked genes.
- Usp9x overdosage causes centrosome amplification and chromosome instability, affecting asymmetric cell division and differentiation.

## Abstract

During X chromosome inactivation (XCI), Xist RNA establishes silencing by coating the chromosome in cis and binding diverse proteins to promote formation of a heterochromatic domain. However, Xist repeat B role beyond initiation of XCI remains unclear. Here, we find that loss of Xist repeat B in female mice allows survival and leads to a small body size persisting throughout life. Epigenetic and transcriptomic analyses reveal low levels of H3K27me3 and H2AK119ub occupancy on the X chromosome, except in certain CpG island regions, and partial reactivation of X-linked genes on the inactive X across multiple tissues. Notably, overdosage of Usp9x promotes centrosome amplification and chromosome instability. We further demonstrate that Usp9x overdosage alters asymmetric cell division, thereby affecting the process of cell differentiation. Thus, Xist repeat B is necessary for gene-specific silencing during XCI maintenance and impacts cell proliferation and differentiation during development. This provides insights into repeat B importance in maintaining XCI.

Graphical Abstract