Origin and evolution of X chromosome inactivation

Evolution of the mammalian sex chromosomes heavily impacts on the expression of X-encoded genes, both in marsupials and placental mammals. The loss of genes from the Y chromosome forced a two-fold upregulation of dose sensitive X-linked homologues. As a corollary, female cells would experience a lethal dose of X-linked genes, if this upregulation was not counteracted by evolution of X chromosome inactivation (XCI) that allows for only one active X chromosome per diploid genome. Marsupials rely on imprinted XCI, which inactivates always the paternally inherited X chromosome. In placental mammals, random XCI (rXCI) is the predominant form, inactivating either the maternal or paternal X. In this review, we discuss recent new insights in the regulation of XCI. Based on these findings, we propose an X inactivation center (Xic), composed of a cis-Xic and trans-Xic that encompass all elements and factors acting to control rXCI either in cis or in trans. We also highlight that XCI may have evolved from a very small nucleation site on the X chromosome in the vicinity of the Sox3 gene. Finally, we discuss the possible evolutionary road maps that resulted in imprinted XCI and rXCI as observed in present day mammals.     

Paternal RLIM/Rnf12 is a survival factor for milk-producing alveolar cells

In female mouse embryos, somatic cells undergo a random form of X chromosome inactivation (XCI), whereas extraembryonic trophoblast cells in the placenta undergo imprinted XCI, silencing exclusively the paternal X chromosome. Initiation of imprinted XCI requires a functional maternal allele of the X-linked gene Rnf12, which encodes the ubiquitin ligase Rnf12/RLIM. We find that knockout (KO) of Rnf12 in female mammary glands inhibits alveolar differentiation and milk production upon pregnancy, with alveolar cells that lack RLIM undergoing apoptosis as they begin to differentiate. Genetic analyses demonstrate that these functions are mediated primarily by the paternal Rnf12 allele due to nonrandom maternal XCI in mammary epithelial cells. These results identify paternal Rnf12/RLIM as a critical survival factor for milk-producing alveolar cells and, together with population models, reveal implications of transgenerational epigenetic inheritance.     

The role of the X chromosome in mammalian extra embryonic development

Accumulating evidence points to the importance of the X chromosome for trophoblast development. In rodents, the extraembryonic cell lineage differs from somatic tissues in that X chromosome inactivation is imprinted, preferentially silencing the paternal X chromosome. As a consequence, trophoblast development is extremely susceptible to deviations from normal X inactivation and is impaired in situations of increased and reduced X-linked gene dosage. Mouse mutants have also shown that maintenance of X chromosome silencing in extraembryonic tissues requires a special set of heterochromatin proteins. Moreover, the X chromosome has been implicated in causing several malformations of the placenta. The observed importance of the X chromosome for placental development can be explained by the presence of many trophoblast-expressed genes, especially in the proximal and central regions. Given that the placenta represents a postzygotic barrier to reproduction, evolutionary constraints may be responsible for the presence of placental genes on the X chromosome that are often co-expressed in brain and testis.     

The dynamics of imprinted X inactivation during preimplantation development in mice

In the mouse, there are two forms of X chromosome inactivation (XCI), random XCI in the fetus and imprinted paternal XCI, which is limited to the extraembryonic tissues. While the mechanism of random XCI has been studied extensively using the in vitro XX ES cell differentiation system, imprinted XCI during early embryonic development has been less well characterized. Recent studies of early embryos have reported unexpected findings for the paternal X chromosome (Xp). Imprinted XCI may not be linked to meiotic silencing in the male germ line but rather to the imprinted status of the Xist gene. Furthermore, the Xp becomes inactivated in all cells of cleavage-stage embryos and then reactivated in the cells of the inner cell mass (ICM) that form the epiblast, where random XCI ensues.     

Silence of the fathers: early X inactivation

X chromosome inactivation is the mammalian answer to the dilemma of dosage compensation between males and females. The study of this fascinating form of chromosome-wide gene regulation has yielded surprising insights into early development and cellular memory. In the past few months, three papers reported unexpected findings about the paternal X chromosome (X(p)). All three studies agree that the X(p) is imprinted to become inactive earlier than ever suspected during embryonic development. Although apparently incomplete, this early form of inactivation insures dosage compensation throughout development. Silencing of the X(p) persists in cells of extraembryonic tissues, but it is erased and followed by random X inactivation in cells of the embryo proper. These findings challenge several aspects of the current view of X inactivation during early development and may have profound impact on studies of pluripotency and epigenetics.     

Imprint switching for non-random X-chromosome inactivation during mouse oocyte growth

In mammals, X-chromosome inactivation occurs in all female cells, leaving only a single active X chromosome. This serves to equalise the dosage of X-linked genes in male and female cells. In the mouse, the paternally derived X chromosome (X(P)) is imprinted and preferentially inactivated in the extraembryonic tissues whereas in the embryonic tissues inactivation is random. To investigate how X(P) is chosen as an inactivated X chromosome in the extraembryonic cells, we have produced experimental embryos by serial nuclear transplantation from non-growing (ng) oocytes and fully grown (fg) oocytes, in which the X chromosomes are marked with (1) an X-linked lacZ reporter gene to assay X-chromosome activity, or (2) the Rb(X.9)6H translocation as a cytogenetic marker for studying replication timing. In the extraembryonic tissues of these ng/fg embryos, the maternal X chromosome (X(M)) derived from the ng oocyte was preferentially inactivated whereas that from the fg oocyte remained active. However, in the embryonic tissues, X inactivation was random. This suggests that (1) a maternal imprint is set on the X(M) during oocyte growth, (2) the maternal imprint serves to render the X(M) resistant to inactivation in the extraembryonic tissues and (3) the X(M) derived from an ng oocyte resembles a normal X(P).     

X-chromosome inactivation in monkey embryos and pluripotent stem cells

Inactivation of one X chromosome in female mammals (XX) compensates for the reduced dosage of X-linked gene expression in males (XY). However, the inner cell mass (ICM) of mouse preimplantation blastocysts and their in vitro counterparts, pluripotent embryonic stem cells (ESCs), initially maintain two active X chromosomes (XaXa). Random X chromosome inactivation (XCI) takes place in the ICM lineage after implantation or upon differentiation of ESCs, resulting in mosaic tissues composed of two cell types carrying either maternal or paternal active X chromosomes. While the status of XCI in human embryos and ICMs remains unknown, majority of human female ESCs show non-random XCI. We demonstrate here that rhesus monkey ESCs also display monoallelic expression and methylation of X-linked genes in agreement with non-random XCI. However, XIST and other X-linked genes were expressed from both chromosomes in isolated female monkey ICMs indicating that ex vivo pluripotent cells retain XaXa. Intriguingly, the trophectoderm (TE) in preimplantation monkey blastocysts also expressed X-linked genes from both alleles suggesting that, unlike the mouse, primate TE lineage does not support imprinted paternal XCI. Our results provide insights into the species-specific nature of XCI in the primate system and reveal fundamental epigenetic differences between in vitro and ex vivo primate pluripotent cells.     

Disruption of imprinted X inactivation by parent-of-origin effects at Tsix

In marsupials and in extraembryonic tissues of placental mammals, X inactivation is imprinted to occur on the paternal chromosome. Here, we find that imprinting is controlled by the antisense Xist gene, Tsix. Tsix is maternally expressed and mice carrying a Tsix deletion show normal paternal but impaired maternal transmission. Maternal inheritance occurs infrequently, with surviving progeny showing intrauterine growth retardation and reduced fertility. Transmission ratio distortion results from disrupted imprinting and postimplantation loss of mutant embryos. In contrast to effects in embryonic stem cells, deleting Tsix causes ectopic X inactivation in early male embryos and inactivation of both X chromosomes in female embryos, indicating that X chromosome counting cannot override Tsix imprinting. These results highlight differences between imprinted and random X inactivation but show that Tsix regulates both. We propose that an imprinting center lies within Tsix.     

Heterogeneous X inactivation in trophoblastic cells of human full-term female placentas

In female mammalian cells, one of the two X chromosomes is inactivated to compensate for gene-dose effects, which would be otherwise doubled compared with that in male cells. In somatic lineages in mice, the inactive X chromosome can be of either paternal or maternal origin, whereas the paternal X chromosome is specifically inactivated in placental tissue. In human somatic cells, X inactivation is mainly random, but both random and preferential paternal X inactivation have been reported in placental tissue. To shed more light on this issue, we used PCR to study the methylation status of the polymorphic androgen-receptor gene in full-term human female placentas. The sites investigated are specifically methylated on the inactive X chromosome. No methylation was found in microdissected stromal tissue, whether from placenta or umbilical cord. Of nine placentas for which two closely apposed samples were studied, X inactivation was preferentially maternal in three, was preferentially paternal in one, and was heterogeneous in the remaining five. Detailed investigation of two additional placentas demonstrated regions with balanced (1:1 ratio) preferentially maternal and preferentially paternal X inactivation. No differences in ratio were observed in samples microdissected to separate trophoblast and stromal tissues. We conclude that methylation of the androgen receptor in human full-term placenta is specific for trophoblastic cells and that the X chromosome can be of either paternal or maternal origin.     

De novo DNA methylation independent establishment of maternal imprint on X chromosome in mouse oocytes

In female mouse embryos, the paternal X chromosome (Xp) is preferentially inactivated during preimplantation development and trophoblast differentiation. This imprinted X-chromosome inactivation (XCI) is partly due to an activating imprint on the maternal X chromosome (Xm), which is set during oocyte growth. However, the nature of this imprint is unknown. DNA methylation is one candidate, and therefore we examined whether disruptions of the two de novo DNA methyltransferases in growing oocytes affect imprinted XCI. We found that accumulation of histone H3 lysine-27 trimethylation, a hallmark of XCI, occurs normally on the Xp, and not on the Xm, in female blastocysts developed from the mutant oocytes. Furthermore, the allelic expression patterns of X-linked genes including Xist and Tsix were unchanged in preimplantation embryos and also in the trophoblast. These results show that a maternal disruption of the DNA methyltransferases has no effect on imprinted XCI and argue that de novo DNA methylation is dispensable for Xm imprinting. This underscores the difference between imprinted XCI and autosomal imprinting.     

Imprinted X-chromosome inactivation: enlightenment from embryos in vivo

There are two forms of X chromosome inactivation (XCI) in the laboratory mouse, random XCI in the fetus and imprinted paternal XCI limited to the extraembryonic tissues supporting the fetal life in utero. Imprinted XCI has been studied extensively because it takes place first in embryogenesis and it may hold clues to the mechanism of control of XCI in general and to the evolution of random' XCI. Classical microscopic and biochemical studies of embryos in vivo provide a basis for interpreting the multifaceted information yielded by various inventive approaches and for planning further experiments.     

哺乳动物X染色体失活机制

哺乳动物X染色体连锁基因的剂量平衡,是通过雌性胚胎发育早期随机或印记失活一条X染色体来实现的,这是一个复杂的过程,包括：启动、计数、选择、维持等一系列的步骤。X染色体失活中心是X染色体失活的主控开关座位,调节X失活的早期事件,失活发生后,X染色体的失活状态可稳定地存在并传递给后代,这一过程涉及基因组印记的形成。此外,在雄性动物,精原细胞减数分裂早期也存在着短暂的X染色体失活现象。现对哺乳动物X染色体失活机制的最新进展进行综述。