Therian female mammalian cells have two X chromosomes, while males have only one, requiring X-chromosome inactivation (XCI) for sex-chromosome dosage compensation.
[1] XCR is an emerging topic of interest for multiple reasons:[1] In 1959 Susumu Ohno showed that the two X chromosomes of mammals were different: one appeared similar to the autosomes; the other was condensed and heterochromatic.
[10] However, newer investigations using modern technologies have found evidence that Xist RNA is involved in the maintenance of XCI.
For example, Zhang, et al. found that “Deleting Xist results in a loss of nucleolar association and an inability to maintain Xi heterochromatin, leading to Xi reactivation at the single gene level.”[11] Unlike XCI, XCR lacks an appropriate in vitro system for study, limiting current research to mouse embryos and in vitro stem cell reprogramming.
In early mouse embryos, the paternal X chromosome is already partially silenced at the zygote stage by imprinted XCI, suggesting that sex-chromosome dosage compensation exists from conception.
[18] Because only one X chromosome is expressed, genetic mutations that lead to cancer are dominant when they occur on the Xa, and silent on the Xi.
[13] Factors implicated in XCI maintenance include origin recognition complex 2 (Orc2), heterochromatin protein 1 (HP1a),[35] macroH2A1,[36] and Bmil.
[18] Deleting Xist RNA, demethylating DNA, and inhibiting histone hypoacetylation together increases reactivation in primary mouse embryonic fibroblasts on the gene level in a stochastic manner.
[13] For example, a mutation of the Polycomb protein EED leads to a lack of Xist RNA coating on the Xi and X reactivation in differentiated trophoblast stem cells.
[50] However, in X-linked disorders where the healthy cells are insufficient to restore wild-type function, X chromosome reactivation may be a potential therapy.
By reactivating the Xi in diseased cells, the previously unexpressed wild-type allele can regain some level of expression and restore function.
For example, female mouse embryos that inherited a paternal germline Xist deletion had trophoblast cells where both X chromosomes were fully expressed, resulting in a lethal phenotype.
[57] Pre-XCI Xist deletion in mouse zygotes can be tolerated to the extent that the embryos can be carried to term, but none survive past weaning as a result of issues with organ development.
[58] However, these concerns may be mitigated by the fact that applications of X reactivation are more focused on post-XCI cells, past the early developmental stages.
[62] In developing a treatment to reactivate the Xi, screens to identify single factors that inhibit XCI have listed many potential pharmacological targets to consider for further study.
[63][64] However, XCI is complex and maintained by a diverse set of proteins[35] and multiple mechanisms, such as DNA methylation, histone hypoacetylation, and the role of Xist RNA.