An ectopic human XIST gene can induce chromosome inactivation in postdifferentiation human HT-1080 cells

De novo DNA methylation is dispensable for the initiation and propagation of X chromosome inactivation

Xist (X-inactive specific transcript) plays a crucial role in X-inactivation. This non-coding RNA becomes upregulated on the X chromosome that is to be inactivated upon differentiation. Previous studies have revealed that although maintenance-type DNA methylation is not essential for X-inactivation to occur, it is required for the stable repression of Xist in differentiated cells. However, it is unknown whether differential de novo methylation at the Xist promoter, which is mediated by Dnmt3a and/or Dnmt3b, is a cause or a consequence of monoallelic expression of Xist. We show that Xist expression is appropriately regulated in the absence of Dnmt3a and Dnmt3b and that a single X chromosome undergoes proper inactivation in mutant females. Our results indicate that a mechanism(s) other than DNA methylation plays a principal role in initiating X-inactivation. We also demonstrate that delayed upregulation of Xist does not induce X-inactivation, consistent with a crucial developmental window for the chromosomal silencing.

Inheritance of a pre-inactivated paternal X chromosome in early mouse embryos

In mammals, dosage compensation ensures equal X-chromosome expression between males (XY) and females (XX) by transcriptionally silencing one X chromosome in XX embryos. In the prevailing view, the XX zygote inherits two active X chromosomes, one each from the mother and father, and X inactivation does not occur until after implantation. Here, we report evidence to the contrary in mice. We find that one X chromosome is already silent at zygotic gene activation (2-cell stage). This X chromosome is paternal in origin and exhibits a gradient of silencing. Genes close to the X-inactivation centre show the greatest degree of inactivation, whereas more distal genes show variable inactivation and can partially escape silencing. After implantation, imprinted silencing in extraembryonic tissues becomes globalized and more complete on a gene-by-gene basis. These results argue that the XX embryo is in fact dosage compensated at conception along much of the X chromosome. We propose that imprinted X inactivation results from inheritance of a pre-inactivated X chromosome from the paternal germ line.

Beyond sense: the role of antisense RNA in controlling Xist expression

The Xist RNA is a critical component of X inactivation, and Tsix is a non-coding antisense RNA to the Xist gene. We review the data from mouse that demonstrates that Tsix serves to regulate Xist expression. TSIX antisense transcripts have also been detected in humans, but without a manipulatable system to study the inactivation process in humans it remains unknown whether these antisense transcripts are functional in regulating human XIST. After a review of the differences between the human and mouse antisense, we discuss how the question of whether or not the human TSIX is functional impacts models of Tsix function.

Barring gene expression after XIST: maintaining facultative heterochromatin on the inactive X

X chromosome inactivation refers to the developmentally regulated process of silencing gene expression from all but one X chromosome per cell in female mammals in order to equalize the levels of X chromosome derived gene expression between the sexes. While much attention has focused on the genetic and epigenetic events early in development that initiate the inactivation process, it is also important to understand the events that ensure maintenance of the inactive state through subsequent cell divisions. Gene silencing at the inactive X chromosome is irreversible in somatic cells and is achieved through the formation of facultative heterochromatin (visible as the Barr body) that is remarkably stable and faithfully preserved. Here we review the many features of inactive X chromatin in terminally differentiated cells and address the highly redundant mechanisms of maintaining the inactive X chromatin.

Repositioning of muscle-specific genes relative to the periphery of SC-35 domains during skeletal myogenesis

Previous studies have shown that in a given cell type, certain active genes associate with SC-35 domains, nuclear regions rich in RNA metabolic factors and excluded from heterochromatin. This organization is not seen for all active genes; therefore, it is important to determine whether and when this locus-specific organization arises during development and differentiation of specific cell types. Here, we investigate whether gene organization relative to SC-35 domains is cell type specific by following several muscle and nonmuscle genes in human fibroblasts, committed but proliferative myoblasts, and terminally differentiated muscle. Although no change was seen for other loci, two muscle genes (Human beta-cardiac myosin heavy chain and myogenin) became localized to the periphery of an SC-35 domain in terminally differentiated muscle nuclei, but not in proliferative myoblasts or in fibroblasts. There was no apparent change in gene localization relative to either the chromosome territory or the heterochromatic compartment; thus, the gene repositioning seemed to occur specifically with respect to SC-35 domains. This gene relocation adjacent to a prominent SC-35 domain was recapitulated in mouse 3T3 cells induced into myogenesis by introduction of MyoD. Results demonstrate a cell type-specific reorganization of specific developmentally regulated loci relative to large domains of RNA metabolic factors, which may facilitate developmental regulation of genome expression.

Characterization of expression at the human XIST locus in somatic, embryonal carcinoma, and transgenic cell lines

X inactivation requires XIST, a functional RNA that is expressed exclusively from, and localizes to, the inactive X in female somatic cells. In mouse, low-level unstable transcription of Xist is observed prior to the time of inactivation, and an antisense transcript, Tsix, is a critical regulator of early Xist expression. To examine the presence and impact of an antisense transcript in humans we have characterized the extent of sense and antisense transcription in human somatic, transgenic, and embryonal carcinoma (EC) cell lines. Downstream antisense expression at the human XIST locus was not detected in somatic cells, but was detected in the EC line N-Tera2D1 and in somatic cells with an ectopic XIST locus. Presence of the antisense did not disrupt the stability or localization of the sense transcript. We have also identified additional sense transcripts in EC and female somatic cells and demonstrate that the 5' flanking JPX/ENOX gene is expressed from both the active and the inactive X chromosome in somatic cell hybrids, delimiting the extent of inactive X-specific transcriptional control in somatic cells. These analyses reveal similarities to and differences from the murine Xist and Tsix transcripts and generate a complex picture of developmentally regulated transcription through the region.

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.

SETting the stage. Eed-Enx1 leaves an epigenetic signature on the inactive X chromosome

Despite evidence implicating the Polycomb group protein, Eed (embryonic ectoderm development protein) in imprinted X inactivation, a similar role in random X inactivation in the embryo has remained an open question. Brockdorff and colleagues now report that Eed, along with its binding partner Enx1, transiently associates with the inactive X chromosome (Xi) and likely contributes to the epigenetic signature and long-term stability of the Xi heterochromatin.