XIST and the mechanism of X inactivation

X inactivation normally occurs early in development, so it is not an easy time to study in humans!  Many groups study mice as an alternative – and in mice Embryonic Stem cells (ES cells) from females have two active X chromosomes, and one undergoes silencing.  People have isolated female human ES cells, or induced pluripotency from differentiated somatic cells (iPS cells), but these usually have an active and an inactive X chromosome – and the inactivation is unstable.  Therefore to study how human XIST functions, we have developed a system where we can induce XIST expression.

When Thomas was in the lab he examined which regions of XIST are involved in localization of XIST, and the recruitment of the heterochromatic features.  He published two studies that reveal each region of XIST contributes to the recruitment of features, and that unlike in early mouse development, two important aspects – the heterochromatic marks laid down by PRC1 and PRC2 – rely on different regions.

Dixon-McDougall T, Brown CJ. 2021. Independent domains for recruitment of PRC1 and PRC2 by human XIST. PLoS Genet. 2021 PMID: 33750950.

Dixon-McDougall T, Brown CJ. 2022.  Multiple distinct domains of human XIST are required to coordinate gene silencing and subsequent heterochromatin formation.  Epigenetics & Chromatin 15 (1), 1-18. PMID: 35120578 

Maria-Jose is now following these studies by making small constructs fusing some of the critical domains – and even including some from mice!

This work is posted on bioRxiv pending peer review for publication in a journal:

Derivation of a minimal functional XIST by combining human and mouse interaction domains

In a previous paper we looked at what happens when XIST is expressed from nine different locations (only one of which was on the X chromosome).  While it functioned partially at all of these sites, recruitment of the heterochromatic features SMCHD1, macroH2A, H3K27me3, and H4K20me1 occurred independently of each other in an integration site-dependent manner.

Kelsey AD, Yang C, Leung D, Minks J, Baldry SEL, Bogutz AB, Lefebvre L, Brown CJ. 2015. Impact of flanking chromosomal sequences on localization and silencing by the human non-coding RNA XIST. Genome Biology.PMID:26429547

We have also examined the regulation of the XIST gene itself.

Chapman AG, Cotton AM, Kelsey AD, Brown CJ. 2014 Differentially methylated CpG island within human XIST mediates alternative P2 transcription and YY1 binding. BMC Genet. 2014 Sep 9;15(1):89. PMID: 25200388