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Cantone Reversal of X chromosome inactivation

degradation during differentiation [48] . Second, it directly blastocyst stages [60,61,103] . However, whether bi-
binds Xist in a Tsix-dependent manner. Interestingly, allelic accumulation of XIST leads to dampening of
depletion of Tsix during iPSC reprogramming does X-linked gene expression remains to be determined.
not compromise Xi reactivation neither iPSC colony Interestingly, a human-specific long non-coding RNA,
formation [52,98] , although it affects Xist repression and named XACT, has been shown to localize to either
Xi reactivation in the mouse embryonic epiblast [102] . one or both X chromosomes in human embryos and
has been hypothesized to compete with XIST for
iPSC reprogramming studies also gave further binding along the chromosome [103,107] . Notably, XIST
mechanistic insights in the role of Xist during RNA signal has a dispersed nuclear localization in
Xi reactivation [52] . Kinetic studies during iPSC human embryos suggesting that it might be delocalized
reprogramming showed that loss of Xist RNA in from Xi chromatin or bind only some chromosomal
Nanog positive cells precedes bi-allelic expression of domains [60,103] .
X-linked genes, suggesting that Xist repression might
be required for Xi gene reactivation. Supporting this Xi reactivation in human pluripotent cells
hypothesis, constitutive Xist expression decreased Human pluripotent stem cells (i.e. hESCs and
the reactivation of Xi genes within Nanog positive hiPSCs) are epigenetically unstable ex vivo and
cells without affecting reprogramming, whereas Xist heterogeneous in their XCI state that varies between
depletion did not alter Xi reactivation events neither different cell lines and also among cells of the same
its kinetics. This suggests that Xist repression might culture [53-59,108,109] . Three different classes of hESCs
be necessary but insufficient for Xi gene reactivation. have been originally described based on XIST
Notably, bi-allelic expression of X-linked genes was nuclear pattern and the transcription of CotI repeats
enhanced upon combined deletion of Xist and inhibition before and after differentiation [54] . Class I hESCs
of DNA methylation, thus supporting the hypothesis that were characterized by the absence of XIST RNA
Xi reactivation requires both Xist silencing and DNA signal and the presence of CotI transcripts from
demethylation. both X chromosomes. Upon differentiation of these
hESCs, XIST was upregulated and coated one of
REPROGRAMMING-MEDIATED Xi the two X chromosomes whereas CotI transcription
REACTIVATION IN HUMAN within the XIST-coated domain ceased. Collectively
these results suggested that class I hESC retain
two transcriptionally active X chromosomes and are
Recent studies in human embryos have shown able re-initiate XCI. XIST-coating and CotI exclusion,
substantial differences between human and mouse however, were shown to occur spontaneously
XCI [60,61,103] [Figure 1A and C]. Differences concern when class I cells were maintained in culture, thus
both the status of the female X chromosomes in pre- generating XaXi XIST hESCs (class II). Finally,
+
implantation embryos and the factors involved in the class III hESCs were observed upon progressive
regulation of XCI. It has been shown that the two loss of XIST-coating in class II cultures. Notably,
female X chromosomes are transcriptionally active loss of XIST nuclear domain was associated with
throughout human pre-implantation development and promoter DNA demethylation and transcriptional
both in embryonic and extraembryonic lineages [60] . reactivation of some Xi genes [53,56] . However, these
This contrasts with mouse embryos where several class III hESCs could not re-establish XIST expression
rounds of Xi reactivation/inactivation take place in neither Xi gene silencing upon differentiation and
the epiblast cells that give rise to the embryo proper were considered subject to an “erosion” of dosage
[Figure 1B], whereas imprinted XCI of the paternally- compensation [53,54,59,110] . Interestingly, Xi reactivation
inherited chromosome is maintained in extraembryonic occurring during erosion was shown to localize
cells [104] . Random XCI initiates in mouse epiblast preferentially within chromosomal domains enriched
cells around the time of implantation and is essential with H3K27me3 [107] . These H3K27me3 domains
for proper development [63,64,105,106] , while its precise are lost upon erosion whereas H3K9me3-enriched
timing is unknown in human [60] . Furthermore, a recent domains, which spatially segregate along the human
single cell RNA-seq study showed a progressive Xi, remain unaffected. Surprisingly, loss of XIST is
downregulation of both X chromosomes beyond not directly associated with Xi gene reactivation as it
the zygote genome activation stage through human occurs afterwards. Reactivation and accumulation of
preimplantation development, suggesting that a XACT RNA along the Xi was instead observed ahead
different dosage compensation mechanisms might of XIST loss and Xi gene reactivation [Figure 2A] and
be in place ahead of XCI [61] . Consistently with this was indeed hypothesized to trigger the reorganization
hypothesis, it has been observed that human XIST of Xi heterochromatin domains and consequent
RNA coats both X chromosome by the morula and gene reactivation. As XACT coats the active X
6 Journal of Translational Genetics and Genomics ¦ Volume 1 ¦ November 16, 2017