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.
In a recent 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
Thomas in the lab is looking at which regions of XIST are involved in localization of XIST, and the recruitment of the heterochromatic features. He wrote a review about the many factors involved in X inactivation:
Dixon-McDougall, T and Brown CJ. 2016. The Making of a Barr Body: The Mosaic of Factors that eXIST on the Mammalian Inactive X Chromosome. Biochem Cell Biol. 2016 Feb;94(1):56-70. PMID: 26283003
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