Loss of Xist imprinting in diploid parthenogenetic preimplantation embryos

X-chromosome inactivation: a hypothesis linking ontogeny and phylogeny

In mammals, sex is determined by differential inheritance of a pair of dimorphic chromosomes: the gene-rich X chromosome and the gene-poor Y chromosome. To balance the unequal X-chromosome dosage between the XX female and XY male, mammals have adopted a unique form of dosage compensation in which one of the two X chromosomes is inactivated in the female. This mechanism involves a complex, highly coordinated sequence of events and is a very different strategy from those used by other organisms, such as the fruitfly and the worm. Why did mammals choose an inactivation mechanism when other, perhaps simpler, means could have been used? Recent data offer a compelling link between ontogeny and phylogeny. Here, we propose that X-chromosome inactivation and imprinting might have evolved from an ancient genome-defence mechanism that silences unpaired DNA.

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.

Reactivation of the Paternal X Chromosome in Early Mouse Embryos

It is generally accepted that paternally imprinted X inactivation occurs exclusively in extraembryonic lineages of mouse embryos, whereas cells of the embryo proper, derived from the inner cell mass (ICM), undergo only random X inactivation. Here we show that imprinted X inactivation, in fact, occurs in all cells of early embryos and that the paternal X is then selectively reactivated in cells allocated to the ICM. This contrasts with more differentiated cell types where X inactivation is highly stable and generally irreversible. Our observations illustrate that an important component of genome plasticity in early development is the capacity to reverse heritable gene silencing decisions.

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 primate embryo gene expression resource: a novel resource to facilitate rapid analysis of gene expression patterns in non-human primate oocytes and preimplantation stage embryos

Detailed molecular studies of preimplantation stage development in a suitable nonhuman primate model organism have been inhibited due to the cost and scarcity of embryos. To circumvent these limitations, we have created a new resource for the research community, designated as the Primate Embryo Gene Expression Resource (PREGER). The PREGER sample collection currently contains over 160 informative samples of oocytes, obtained from various sized antral follicles, and embryos obtained through a variety of different protocols. The PREGER makes it possible to undertake quantitative gene-expression studies in rhesus monkey oocytes and embryos through simple and cost-effective hybridization-based methods. The PREGER also makes available other molecular tools to facilitate nonhuman primate embryology. We used PREGER here to compare the temporal expression patterns of five housekeeping mRNAs and three transcription factor mRNAs between mouse and rhesus monkey. We observed noticeable differences in temporal expression patterns between species for some mRNAs, but clear similarities for others. Our results also provide new information related to genome activation and the effects of embryo culture conditions on gene expression in primate embryos. These results provide one illustration of how the PREGER can be employed to obtain novel insight into primate embryogenesis.

Epigenetic dynamics of imprinted X inactivation during early mouse development

The initiation of X-chromosome inactivation is thought to be tightly correlated with early differentiation events during mouse development. Here, we show that although initially active, the paternal X chromosome undergoes imprinted inactivation from the cleavage stages, well before cellular differentiation. A reversal of the inactive state, with a loss of epigenetic marks such as histone modifications and polycomb proteins, subsequently occurs in cells of the inner cell mass (ICM), which give rise to the embryo-proper in which random X inactivation is known to occur. This reveals the remarkable plasticity of the X-inactivation process during preimplantation development and underlines the importance of the ICM in global reprogramming of epigenetic marks in the early embryo.

Micromanipulating dosage compensation: understanding X-chromosome inactivation through nuclear transplantation

Nuclear transfer (NT) studies have provided insight into the functional importance of epigenetic alteration of the X chromosomes during X-inactivation. Uniparental embryos created by NT have been informative as to the time and location at which the imprint controlling extraembryonic X-inactivation is established. Experiments with female somatic cells, have demonstrated that the inactive X chromosome (Xi) is reactivated after NT, leading to random X-inactivation in the embryonic lineages of cloned embryos. However, in the extraembryonic lineages of clones, epigenetic information from the donor cell nucleus persists, leading to preferential inactivation of the donor cell's inactive X in the placenta of cloned animals. These results suggest epigenetic information established during embryonic X-inactivation is functionally equivalent to the gametic imprint.

Genomic Imprinting: Intricacies of Epigenetic Regulation in Clusters

An intriguing characteristic of imprinted genes is that they often cluster in large chromosomal domains, raising the possibility that gene-specific and domain-specific mechanisms regulate imprinting. Several common features emerged from comparative analysis of four imprinted domains in mice and humans: (a) Certain genes appear to be imprinted by secondary events, possibly indicating a lack of gene-specific imprinting marks; (b) some genes appear to resist silencing, predicting the presence of cis-elements that oppose domain-specific imprinting control; (c) the nature of the imprinting mark remains incompletely understood. In addition, common silencing mechanisms are employed by the various imprinting domains, including silencer elements that nucleate and propagate a silent chromatin state, insulator elements that prevent promoter-enhancer interactions when hypomethylated on one parental allele, and antisense RNAs that function in silencing the overlapping sense gene and more distantly located genes. These commonalities are reminiscent of the behavior of genes subjected to, and the mechanisms employed in, dosage compensation.

Xist RNA and the mechanism of X chromosome inactivation

Dosage compensation in mammals is achieved by the transcriptional inactivation of one X chromosome in female cells. From the time X chromosome inactivation was initially described, it was clear that several mechanisms must be precisely integrated to achieve correct regulation of this complex process. X-inactivation appears to be triggered upon differentiation, suggesting its regulation by developmental cues. Whereas any number of X chromosomes greater than one is silenced, only one X chromosome remains active. Silencing on the inactive X chromosome coincides with the acquisition of a multitude of chromatin modifications, resulting in the formation of extraordinarily stable facultative heterochromatin that is faithfully propagated through subsequent cell divisions. The integration of all these processes requires a region of the X chromosome known as the X-inactivation center, which contains the Xist gene and its cis-regulatory elements. Xist encodes an RNA molecule that plays critical roles in the choice of which X chromosome remains active, and in the initial spread and establishment of silencing on the inactive X chromosome. We are now on the threshold of discovering the factors that regulate and interact with Xist to control X-inactivation, and closer to an understanding of the molecular mechanisms that underlie this complex process.

Establishment of histone h3 methylation on the inactive X chromosome requires transient recruitment of Eed-Enx1 polycomb group complexes

Previous studies have implicated the Eed-Enx1 Polycomb group complex in the maintenance of imprinted X inactivation in the trophectoderm lineage in mouse. Here we show that recruitment of Eed-Enx1 to the inactive X chromosome (Xi) also occurs in random X inactivation in the embryo proper. Localization of Eed-Enx1 complexes to Xi occurs very early, at the onset of Xist expression, but then disappears as differentiation and development progress. This transient localization correlates with the presence of high levels of the complex in totipotent cells and during early differentiation stages. Functional analysis demonstrates that Eed-Enx1 is required to establish methylation of histone H3 at lysine 9 and/or lysine 27 on Xi and that this, in turn, is required to stabilize the Xi chromatin structure.

X-chromosome inactivation: closing in on proteins that bind Xist RNA

X inactivation is the developmentally regulated silencing of a single X chromosome in XX female mammals. In recent years, the Xist gene has been revealed as the master regulatory switch controlling this process. Parental imprinting and/or counting mechanisms ensure that Xist is expressed only on the inactive X chromosome. Chromosome silencing then results from the accumulation of the Xist RNA silencing signal, in cis, over the entire length of the X chromosome. A key issue has been to identify the factors that interact with Xist RNA to initiate heritable gene silencing. This review discusses recent progress that has put this goal in sight.

Xist expression and macroH2A1.2 localisation in mouse primordial and pluripotent embryonic germ cells

The molecular mechanism underlying X chromosome inactivation in female mammals involves the non-coding RNAs Xist and its antisense partner Tsix. Prior to X inactivation, these RNAs are transcribed in an unstable form from all X chromosomes, both in the early embryo and in undifferentiated embryonic stem (ES) cells. Upon differentiation, the expression of these unstable transcripts from all alleles is silenced, and Xist RNA becomes stabilised specifically on the inactivating X chromosome. This pattern of expression is then maintained throughout subsequent somatic cell divisions. Once established, the inactive state of the X chromosome is remarkably stable, the only natural case of reactivation occurring in XX primordial germ cells (PGCs) when they enter the genital ridge. To gain insight into the X reactivation process, we have analysed Xist gene expression using RNA FISH in PGCs and also in PGC-derived embryonic germ (EG) cells. XX EG cells were shown to express unstable Xist/Tsix from both X chromosomes. In contrast, no unstable Xist/Tsix transcripts were detected in XX PGCs at any stage. Instead, a proportion of XX PGCs isolated from the genital ridge between 11.5 and 13.5 dpc (the period during which X chromosome reactivation occurs) showed an accumulation of stable Xist RNA on one X. The number of these cells decreased progressively and was nearly extinguished by 13.5 dpc. As a late marker for the inactive state, we analysed localisation of the histone H2A variant macroH2A1.2. Although macroH2A1.2 expression was observed in PGCs, no significant localisation to the inactive X was detected at any stage. We discuss these results in the context of understanding X chromosome reactivation.

Imprinted X inactivation in eutherians: a model of gametic execution and zygotic relaxation

In mammals, X-chromosome inactivation (XCI) ensures equal expression of X-linked genes in XX and XY individuals by transcriptionally silencing one X-chromosome in female cells. In this review, we discuss an imprinted form of X-inactivation in which the paternal X (Xp) is preferentially silenced. Believed to be the ancestral mechanism of dosage compensation in mammals, imprinted X-inactivation can still be observed in modern-day marsupials and in the extraembryonic tissues of some eutherians such as the mouse. Recent experiments have addressed the nature of the gametic imprint and focused on the regulatory interaction between the noncoding RNA gene, Xist, and its antisense partner, Tsix. Our review of the literature has inspired an unconventional view of imprinted XCI in mice. First, the evidence strongly argues that imprinted XCI is inabsolute, so that a stochastic number of extraembryonic cells escape imprinting. Second, contrary to conventional thinking, we would like to consider the possibility that the paternal X might actually be transmitted to the zygote as a pre-inactivated chromosome. In this model, the gamete initiates and establishes imprinted XCI, while the zygote maintains the pre-established pattern of gametic inactivation. Finally, we hypothesize that the inabsolute nature of imprinting is caused by imperfect zygotic maintenance. We propose that the mouse represents a transitional stage in the evolution of random XCI from an absolutely imprinted mechanism.