Exploring the timing and dynamics of X chromosome inactivation during the specification of human trophectoderm cells

In placental mammals, X-linked gene expression is balanced between females (XX) and males (XY) through a process termed “X Chromosome Inactivation” (XCI). This event leads to the almost complete transcriptional silencing of one of the two X chromosomes during early development of female embryos. The master regulator of XCI initiation is Xist (“X-Inactive Specific Transcript”), a long non-coding RNA able to trigger in cis a cascade of epigenetic modifications that ultimately leads to the X chromosome-wide transcriptional repression. Despite XCI being essential, both mouse and primate non-human embryos showed crucial divergences regarding when and how XCI is set in the embryonic and extraembryonic lineages. As for humans, the timing and mechanisms involved in triggering XCI remain elusive. By using human Embryonic Stem Cells (hESCs) in a naïve-like state of pluripotency, the present study aimed to characterize XIST role for XCI and its requirement during the specification of early trophectoderm (TE) ex vivo. We could determine that XCI is initiated during TE specification and that XIST is essential for the process. Moreover, disruption of XIST expression does not seem to dramatically impair TE specification. This suggests that XCI is not strictly required for the acquisition of TE identity.