Skip to main content

Xist localization and function: new insights from multiple levels


In female mammals, one of the two X chromosomes in each cell is transcriptionally silenced in order to achieve dosage compensation between the genders in a process called X chromosome inactivation. The master regulator of this process is the long non-coding RNA Xist. During X-inactivation, Xist accumulates in cis on the future inactive X chromosome, triggering a cascade of events that provoke the stable silencing of the entire chromosome, with relatively few genes remaining active. How Xist spreads, what are its binding sites, how it recruits silencing factors and how it induces a specific topological and nuclear organization of the chromatin all remain largely unanswered questions. Recent studies have improved our understanding of Xist localization and the proteins with which it interacts, allowing a reappraisal of ideas about Xist function. We discuss recent advances in our knowledge of Xist-mediated silencing, focusing on Xist spreading, the nuclear organization of the inactive X chromosome, recruitment of the polycomb complex and the role of the nuclear matrix in the process of X chromosome inactivation.


X chromosome inactivation (XCI) is the mechanism that has evolved in eutherian mammals to ensure dosage compensation between XX (female) and XY (male) individuals. Dosage compensation depends on the efficient silencing of genes on one of the two X chromosomes in each cell of the female early in development. This process is crucially dependent on a specific locus on the X — the X inactivation center (XIC) — which includes, among other genetic elements, the Xist gene, which is necessary for the process of XCI [1]. Xist encodes a 17-kb long non-coding RNA (lncRNA) that, despite being capped, spliced and poly-adenylated, is retained in the nucleus.

In mouse, XCI occurs in two different fashions. During early embryogenesis, the paternal X is preferentially inactivated (imprinted XCI). At the blastula stage, in the cells of the inner cell mass, this imprinted XCI is reverted, and each chromosome in such cells has an equal chance to be inactivated (random XCI). Initiation of XCI is associated with the monoallelic upregulation of Xist and its spreading and coating in cis of the presumptive inactive X (initiation phase of XCI). This triggers a cascade of events, including the acquisition of repressive chromatin modifications, exclusion of RNA polymerase II (Pol II) and removal of active histone marks, histone exchange and DNA methylation. These events act in concert to ensure the stable repression of the entire chromosome and the maintenance of the silent state (maintenance phase of XCI) [25].

Although many studies have described various aspects of the underlying XCI mechanism, we are far from having a complete understanding of the process, especially at the molecular level. For example, we currently still do not have definitive answers to questions such as how Xist triggers silencing, how it recruits chromatin remodelers or how the silent state is maintained.

Here, we review recent progress in the field, pointing out the strengths, weaknesses and inconsistencies of recent findings. In particular, we highlight recent evidence indicating that chromosomal topology, nuclear organization, and chromatin accessibility all have key roles in the XCI process [6].

Xist spreading and nuclear organization of the inactive X chromosome

Two recently published studies have shed light on Xist spreading and localization [7, 8] (and are commented upon elsewhere [9, 10]). Taking advantage of labeled probes complementary to Xist, pulldowns of Xist-associated chromatin at different stages of XCI were obtained and analyzed by next-generation DNA sequencing [capture hybridization analysis of RNA targets (CHART) and RNA antisense purification-sequencing (RAP-Seq); Box 1]. The studies cover both the initiation phase [recapitulated in differentiating female embryonic stem cells (ESCs) and male inducible-Xist ESCs], and the maintenance phase of XCI (studied in fully differentiated female fibroblasts; Box 1). Importantly, the different experimental systems used were complementary, compensating for the potential limitations of each system. For instance, in the male inducible-Xist cell lines used by Engreitz and colleagues [7], Xist upregulation can be both more rapid and intense than that occurring at the endogenous Xist locus. It is also possible that early time-points in the inducible systems correspond to relatively late time-points in differentiating female ESC lines [11, 12]. Finally, Xist upregulation in the inducible system is both well synchronized and relatively homogeneous [11], whereas ex vivo differentiation of ESC systems is often both asynchronous and non-homogeneous [6].

The principle result from both papers is the observation that Xist localization is initially restricted to a few discrete genomic locations, before extending more broadly over the entire chromosome. Xist coating starts within silent gene-dense regions and then proceeds to spread to active genes on the entire presumptive Xi. Both studies [7, 8] also confirmed that Xist accumulation at active genes requires the presence of Xist A-repeats, a class of structurally conserved repeats, previously identified as necessary for Xist-mediated silencing [12].

The two studies concur to show that, once spread, Xist is associated with gene-rich, open chromatin regions (high-affinity sites), which are enriched for the presence of short interspersed nuclear elements (SINEs) and anti-correlated with the presence of long interspersed nuclear elements (LINEs) and lamin interaction sites [7, 8]. These observations are in keeping with older cytological evidence suggesting a strong association of Xist localization with G-light bands (gene-rich regions), with gene-poor regions representing, predominantly, Xist low-affinity sites [13, 14] (Fig. 1a).

Fig. 1
figure 1

Models of the localization and spreading of Xist. a Three-dimensional spreading model of Xist localization. Xist might use close-proximity sites for its initial spreading (left and middle panels) before accumulating over the whole chromosome. At the final stages of spreading, Xist shows the highest enrichment at gene-rich regions (right panel). b Linear model of Xist spreading showing a classical representation of Xist decorating G-light bands on metaphase chromosomes

Strikingly, both papers highlight a strong dependence of Xist localization on the relative positioning of the Xist locus (XIC). Engreitz and colleagues [7] elegantly showed that moving the inducible Xist integration site from its endogenous location to that of the Hprt locus leads to a substantial change in the initial contact sites of Xist accumulation. The new contact sites correlate with the high-throughput chromosome conformation capture (Hi-C) interactions of the new locus [1517] (Box 1). This strongly suggests that Xist exploits genomic proximity and topology in order to spread in cis, rather than depends on the presence of particular consensus sequences, as occurs in Caenorhabditis elegans [18] or in Drosophila [19]. Interestingly, the studies used XIC-centered HiC datasets obtained from a male cell line as a reference for identifying regions that are in close proximity, suggesting that the initial steps do not depend on female-specific identifiers [16, 17].

The observation that early localization sites depend on the location of Xist and appear to be exclusively position dependent implies that, at least initially, the first contact sites are not necessarily high-affinity ones. After this first, proximity-driven, accumulation of Xist, Xist then spreads to other target sites. This secondary spreading might be a consequence of the initial contacts, chromosome reorganization or intrinsic site affinity. Given that Xist spreading is likely to be a dynamic process, permissive sites might be those at which Xist is more stably retained [7, 13] (Fig. 1a). This model contrasts strongly with the classic model that proposes linear spreading of Xist along the X chromosome from the XIC [13, 14] (Fig. 1b).

Through nuclear architecture and topology studies of the X chromosome using the circular chromosome conformation capture technique (4C), Splinter and colleagues showed that, upon differentiation of female ESCs, the inactive X chromosome (Xi) loses the specific interactions between loci that are typical of the active X chromosome (Xa) [20]. This could be due to Xist binding sites differing in individual differentiated cells [7]. Differential Xist binding in individual cells might result in a loss of specific 4C signal at the level of the overall cell population upon Xist-induced chromatin remodeling. Noticeably, CHART and Xist-centered HiC profiles obtained at early time-points in differentiation do show some degree of overlap. However, while Xist profiles obtained by CHART and RAP-Seq are broad and diffuse [7, 8], the Xist-locus profile generated using 4C and HiC is quite sharp, suggesting only a few genomic locations are in close contact with the XIC [20]. This apparent discrepancy could reflect the preference of the latter technique for picking up those genomic sites that interact most frequently or are in closest proximity in most of the cells, while the CHART and RAP-Seq profiles more typically represent an ‘average’ signal of Xist contact sites within the overall population of cells [7, 8, 15, 16, 20]. Other differences might be reflections of different experimental protocols — Splinter and colleagues [20], for instance, used a differentiation protocol that enriches for a single specific cell lineage [neuronal precursor cells (NPCs)], whereas Simon et al. [8] and Engreitz et al. [7] used withdrawal of leukemia inhibitory factor (LIF) and differentiation by retinoic acid (RA), respectively. The latter two conditions are known to lead to the differentiation of a heterogeneous mixture of different cell types (Box 1).

Interestingly, Splinter et al. [20] and Minajigi et al. [21], provided evidence for the need for continuous Xist expression in order to achieve proper folding of the Xi. Indeed, a conditional deletion of Xist was shown to be associated with a reshaping of the topology of the Xi into an Xa-like conformation. Minajigi et al. also suggest a role for the cohesin complex in keeping Xa topologically associated domains (TADs) in place. Such conformation changes might explain the slightly higher rate of reactivation of X-linked genes in Xist-deficient cells observed by the Jaenisch group in the maintenance phase of XCI, which is otherwise thought to be Xist independent [22]. As Xist seems to interact directly with the lamin B receptor (LBR), a protein mediating the interaction between chromatin and lamin B [21, 23], this interaction could be a necessary intermediate in keeping the Xi in the proximity of the nuclear envelope, where heterochromatin is tethered, thereby reinforcing or stabilizing the Xi conformation and gene silencing [24] (Table 1).

Table 1 Factors involved in X chromosome inactivation

An alternative method that has been applied to studying Xist localization and Xi topology is fluorescence microscopy. Smeets et al. [25] and Cerase et al. [26] have studied Xist localization by super-resolution three dimensional structured illumination microscopy (3D-SIM) [27], a technique that allows specimen imaging at sub-diffraction resolution (resolution limit ~100 nm; Box 1). Their findings challenge the idea of a broad distribution of Xist along the Xi and suggest that Xist, even when fully spread, might be in contact with only a limited number of genomic sites at any one time. Smeets and colleagues [25] have reported a discrete number of Xist foci (approximately 100 per cell) in fully differentiated and differentiating female ESCs. As each focus might represent multiple Xist molecules, the results are compatible with earlier estimates of the number of Xist molecules, which are in the range of 300 to 1000 per cell [28, 29]. The apparent disagreement with the results of chromosome-wide Xist profiling obtained by RAP and sequencing by capture hybridization analysis of RNA targets (CHART-seq) could reflect differences between the analysis of pooled and single cells. For example, it is clearly possible that Xist localizes to relatively few genomic locations at any one time in a given cell and yet appears as a broad domain when population-based sequencing approaches are used. An alternative explanation could be that single RNA molecules cannot be detected by RNA fluorescence in situ hybridization [25].

Xist-mediated Polycomb recruitment and gene silencing

A much-debated aspect of XCI is the link between Xist spreading and recruitment of Polycomb protein. The most widely accepted model predicts a direct recruitment of Polycomb by Xist RNA (Fig. 2a). This interaction has been reported to be mediated by the structurally conserved Xist RepA domain, which would interact directly with polycomb repressive complex 2 (PRC2) [3033]. In agreement with this model, Engreitz et al. [7] and Simon et al. [8] found linear correlations between Xist and PRC2 localization and between Xist and PRC2-mediated tri-methylation of lysine 27 of histone H3 (H3K27me3). This agrees with previous mapping studies of PRC2 on the X chromosome that suggested a broad overall distribution of PRC2 and H3K27me3 [3436] following accumulation at discrete sites (CpG islands) [36]. Most of the studies supporting a direct recruitment model [3033] have, however, exploited in vitro biochemical approaches such as band-shift assays and RNA immunoprecipitation (RNA-IP) approaches (Box 1). Such techniques are notoriously prone to false-positive results, reflecting non-specific interactions between RNA and proteins.

Fig. 2
figure 2

Direct and indirect models of recruitment of PRC2 by Xist RNA. a In the direct model, Xist localization brings PRC2 onto the chromatin by direct recruitment (upper panel). The PRC2 complex then places the H3K27me3 mark on the chromatin (middle panel), and this is followed by chromatin remodeler recruitment and chromatin compaction (lower panel). b In the indirect model, Xist interacts with gene-dense regions (upper panel) and induces chromatin changes (middle panel; i.e. histone deacetylation induced by Hdac3, chromatin compaction, eviction of RNA polymerase II). These changes might, in turn, recruit PRC1 or PRC2 and remodeler complexes (lower panel). H3K27me2-3 dimethylated or trimethylated histone 3 lysine 27, PRC1 polycomb repressive complex 1, PRC2 polycomb repressive complex 2

An almost equally well-represented body of evidence argues against the direct-interaction model. For instance, some studies show that Xist upregulation clearly precedes PRC2 recruitment in early mouse development [37, 38], whereas others demonstrate that a RepA deletion mutant version of Xist (∆Arep) is still fully able to recruit PRC2 and H3K27me3 [3740]. Still-other observations suggest that Xist expression in fully differentiated cells is not sufficient to recruit PRC2. The absence of PRC2 recruitment, upon Xist induction, is not related to the expression level of this complex, as the PRC2 complex is often expressed in such differentiated cells [41].

More-recent observations made by Cerase and colleagues [26] using a 3D-SIM approach showed that the bulk of Xist RNA and PRC2/PRC1 complexes are clearly spatially separated. This finding argues strongly against the direct-interaction model. The study used a mouse male ESC carrying an inducible Xist transgene (Xist-TG) inserted on chromosome 17 and capable of Xist upregulation under undifferentiated conditions. Whilst Xist spreading and localization could be influenced by this autosomal context [25, 26, 42, 43] and by the type of undifferentiated culture conditions used, the main findings of this paper have been confirmed using a fully differentiated female cell line [26].

The above study argues that the observed distance between Xist RNA and Polycomb proteins is likely to preclude direct interaction between the PRC complex and Xist (Fig. 2a). This interpretation is supported by results from Smeets et al. [25], who showed that Xist localizes to the inter-chromatin/perichromatin regions (IC/PR), a non-DAPI-dense area that displays poor overlap with H3K27me3 domains (correlating with DAPI-dense chromatin compartments) (Box 1). Both lines of evidence suggest an alternative model not only of how Xist might recruit Polycomb proteins, but also more generally of the role of chromatin remodelers in X inactivation (Fig. 2b). According to the model, in agreement with Engreitz et al. [7], Xist would initially interact with gene-dense silent regions, possibly partially marked by H3K27me3. It would then induce histone deacetylation, chromatin compaction and the exclusion of Pol II and the basal transcription machinery from nearby active regions [21, 23, 44]. Silenced, compacted chromatin would, in turn, recruit PRC2 and/or chromatin remodelers (indirect model; Fig. 2b) [23, 45, 46]. In accordance with a two-step recruitment model of chromatin factors by Xist, PRC2 initially would only accumulate at approximately 100 to 150 sites before spreading broadly along the X chromosome [36].

Interestingly, Simon and colleagues showed that, when Xist was stripped off the chromatin of female mouse embryonic fibroblasts (MEFs) using complementary locked nucleic acids (LNAs), the kinetics of re-attachment differed from that of the de novo kinetics [8]. This suggests that Xist could also function by priming the chromatin, possibly making it a better substrate for Xist re-spreading after cell division or for recruitment of repressive complexes (for example, by histone deacetylation) [8, 26, 39]. Such an interpretation is, however, in at least partial disagreement with a study from Ng and colleagues, where Xist re-spreading events were observed to occur with the same kinetics as those seen in the initial round of Xist spreading [47].

Very recently, indirect recruitment of PRC2 by Xist has been confirmed by two independent studies [23, 44], which used biotinylated probes complementary to Xist to pull down Xist-associated proteins for mass spectrometry analysis. While McHugh and colleagues used UV crosslinking conditions coupled with mass spectrometry under denaturing conditions (RAP-MS) [23], Chu and colleagues relied on formaldehyde crosslinking followed by mass spectrometry in non-denaturing conditions (ChIRP-MS) [44) (Box 1). The former technique allows recovery only of direct RNA–protein interactors, whereas the latter also allows recovery of proteins in the same complex or in close proximity that are not interacting directly with Xist [26]. The stringent conditions used by McHugh and colleagues allowed the specific isolation of ten bona fide Xist direct interactors. By contrast, Chu and colleagues found 81 proteins that directly or indirectly associate with Xist. Although Chu et al. reported a possible direct interaction with the PRC1 complex [44], neither study listed members of the PRC2 complex as Xist interactors. McHugh et al. suggest that PRC2 recruitment is a consequence of histone H3 deacetylation by Hdac3, part of the NCoR repressive complex, and exclusion of Pol II [23, 48]. They also suggest that the silencing mediator for the retinoic acid receptor and thyroid hormone receptor/nuclear receptor co-repressor (SMRT/NCoR) complex is recruited to the inactivating X via SMRT- and HDAC-associated repressor complex/Msx2-interacting protein (SHARP/Spen), which itself directly binds to Xist RNA [23, 44] (Fig. 2b). Both knockdown of Hdac3 and of SHARP/Spen have similar negative effects on PRC2 recruitment to the inactive X and gene silencing. Chu et al. also suggest that heterogeneous nuclear ribonucleoprotein K (HnrnpK), an heterogeneous nuclear ribonucleoprotein similar to Saf-A, but from which it differs in both binding sites and specificity, might have a direct role in Polycomb recruitment [44].

The results from another very recent proteomics paper [21] contrast with the findings of McHugh et al. and Chu et al. The authors, while using an approach similar to that used by McHugh et al. [22], describe 80 to 250 proteins interacting with Xist at any one time. Among these they were able to identify RbAp46/RbAp48 proteins as direct interactors with Xist. While these proteins are part of the repressive complex PRC2, it should be noted that they are also part of both the Nurd and Sin3 complexes [49].

Finally, roles have been proposed for Jarid2 and Pcl2, two non-canonical subunits of PRC2 [50], in mediating the recruitment of the PRC2 complex to the Xi [51, 52]. Knockdown and knockout experiments have shown that PRC2 recruitment on the X is impaired in the absence or reduction of these two PRC2 cofactors, whereas Xist upregulation itself seems to be unaffected. It should be noted that neither study [52, 53] allowed for discrimination between direct versus indirect PRC2 recruitment. Two interesting reviews have addressed the issue of Xist-mediated PRC2 recruitment in detail [54, 55].

Nuclear scaffolding and XCI

Important examples of other putative Xist-interacting proteins that could be involved in Xist spreading and silencing include nuclear scaffold proteins [25, 5660] (Table 1). The nuclear scaffold (also known as nuclear matrix) is a stable, proteinaceous structure that remains after treating cell nuclei with high-salt buffers, detergents and nucleases and might provide the framework for chromatin organization. In particular, scaffold-attachment or matrix-attachment regions (SARs or MARs) could be mediating the interaction between DNA and the matrix proteins in a highly regulated fashion.

Fackelmayer and coworkers were the first to describe the enrichment of Saf-A, a nuclear matrix protein, in the Xi territory [56, 57]. Hasegawa and colleagues [58] subsequently showed that Saf-A is necessary for Xist localization in both neuroblasts and fully differentiated MEFs. They also showed, using UV crosslinking conditions and RNA-IP, that Xist and Saf-A might interact directly [58]. It should be noted that the UV crosslinking experiments performed by Hasegawa and colleagues are less prone to artifacts than are band-shift assays and non-crosslinked-formaldehyde IPs. This is because UV crosslinking between RNA and proteins is only effective over short distances [61] (Box 1). Nevertheless, nucleic-acid–protein and protein–protein interactions — involving not only Xist and Saf-A, but also other components — cannot be formally excluded. Smeet and colleagues [25], using a GFP–Saf-A fusion protein, have confirmed the enrichment of Saf-A on the Xi and have provided additional evidence of a direct interaction between Xist and the Saf-A protein. Using a 3D-SIM approach, they evaluated the average distance between Saf-A and Xist signals. Measured distances fall below the resolution limits of the technique, implying at least some degree of interaction. There is no reason to believe that the use of formaldehyde-fixed cells calls into question the observed interaction as Xist–matrix binding appears relatively stable [62, 63]. The results of Smeet and colleagues suggest that the Saf-A protein that is enriched on the Xi could be post-translationally modified (Fig. 3a), an observation in possible agreement with the lack of recognition of Saf-A on the Xi by certain antibodies against Saf-A [25]. The possibility of post-transcriptional modifications of the scaffold is of interest in the light of results concerning Xist spreading in cis and fuels speculation about the idea that Xist might interact with modified matrix proteins on the presumptive inactive X and that this would provide the mechanism for restricting Xist RNA spreading to the chromosome from which it was transcribed [64].

Fig. 3
figure 3

Possible role of scaffold proteins in X chromosome inactivation. a The binding of Xist to modified scaffold proteins induces the reorganization of the chromatin, as in (b), where Xist-mediated silencing is maintained by the nuclear scaffold. Genes to be silenced are dragged towards the nuclear matrix, preventing engagement of transcription factors at regulatory sites. CCCTC-binding factor (CTCF) might serve as a barrier to prevent Xist-induced chromatin reorganization. LINEs long interspersed nuclear elements

Chu et al. [44], McHugh et al. [23] and Minajigi et al. [21] all identified Saf-A as an interactor with Xist, using RAP-MS and ChIRP-MS, and confirmed the role of Saf-A in Xist localization on the inactive X, substantiating the previous findings of Hasegawa and colleagues [58]. Although the authors suggest a direct role for Saf-A in Xist-mediated silencing, as Xist silencing is affected both by a loss of Xist localization and by compaction, other possibilities must be considered [23, 44].

It is tempting to push this model slightly further and hypothesize that Xist interacts directly with modified scaffold proteins [63], mediating a chromosome-wide reorganization of the chromosome [40, 65]. Interestingly, facultative scaffold/matrix attachment regions (S/MARs) are enriched in open chromatin regions and in gene bodies, where Xist accumulates [7, 8, 66] (Box 1; Table 1). As Simons et al. and Engreitz et al. have shown that Xist does not accumulate on the gene body of escapee genes, which are genes that avoid being silenced by Xist, and active genes at early stages of XCI, we could further speculate that Xist needs to access the gene bodies to achieve full gene silencing. Under such a model, Xist would accumulate on S/MAR-enriched loci, interacting with post-translationally modified Saf-A, triggering the relocation of active genes close to repeat-dense regions (for example, LINE-rich and lamin-bound regions) [40, 66], in agreement with the model proposed by Chaumeil and colleagues [40] (Fig. 3a, b). Moving active genes into the proximity of the compacted/repeat-rich region of the Xi would prevent access of the transcription factors to regulatory regions of genes, resulting in silencing [35]. Escapee genes, through a looping of the chromatin outside and away from the repeat-dense region, would be protected from silencing [40, 67]. CCCTC-binding factor (CTCF) is one factor that has been reported to have a role in the organization of chromosomal domains that efficiently escape XCI [67] (Table 1). The Smeets et al. paper [25], however, challenges the common view of a compacted central area of the Xi, with active genes arranged at the periphery of the Xi [40, 63]. Instead, Smeets and colleagues suggest a honeycombed structure, with the center of the Xi marked by pockets of reduced compaction, which are permissive for transcription [25]. Differences in the proposed structures likely reflect differences in microscope resolution.

A very recent paper [68] also hints at a possible role of the nuclear matrix in more-general chromatin organization, suggesting that the interaction of non-coding RNAs (ncRNAs) and the nuclear matrix might be a more widespread phenomenon. The authors showed an enrichment of C0T-1 RNA in euchromatic regions that directly interact with the nuclear matrix. Such interspersed repeat RNA, which mostly consists of truncated 5′ L1 elements, could serve either to recruit transcription factors or to act as a platform for opening up the chromatin. We are tempted to speculate that Xist competes with C0t-1 RNA for scaffold attachment sites in this context, triggering a release of structural euchromatic RNA, a collapse of chromatin, and triggering silencing [68] (Fig. 4a, b). Smeets and colleagues offer a similar potential explanation for the repressive function of Xist, linked to Xist coating of the Xi, which would prompt a collapse of interchromatin channels that, in turn, impedes access of Pol II and basal transcription factors to the chromatin. The absence of transcription would, in turn, trigger the recruitment of repressive complexes (for example, PRC2/PRC1 and DNA methyltransferases), inducing further silencing (Fig. 4a, b). For more information about the role of scaffolding in XCI, we refer the reader to two recent reviews [69, 70].

Fig. 4
figure 4

A speculative model of Xist function. The central part of the diagram shows a nucleus, with the active (Xa) and the inactive (Xi) chromosomal territories highlighted in green and yellow, respectively (gray indicates the chromosomal territories of other chromosomes). Magnified views of the Xi (right) and the Xa (left) territories are shown. The following model is based on the observations of Smeets and colleagues [25]. Coating with Xist RNA might cause a collapse of open chromatin channels, and this, in turn, might block the access of transcription factors and RNA polymerase II (RNA Pol II) to gene-regulatory elements. Alternatively, Xist might compete with C0t-1 RNA, and removal of this class of RNA could, in turn, lead to chromosome compaction [68]

Concluding remarks

Here, we have discussed several notable advances in the field of Xist biology. The articles under review represent important contributions to our understanding of the mechanism(s) of Xist silencing, especially in relation to four main areas for which there are outstanding gaps in knowledge: (1) Xist spreading; (2) Xi nuclear organization; (3) Polycomb/chromatin remodeler recruitment and gene silencing; and (4) Xist–matrix interactions.

The very recent papers of McHugh et al. [23], Chu et al. [44] and Minajigi et al. [21] have critically shed light on the previously elusive Xist-interacting proteins and how Xist might both establish gene silencing by Hdac3-mediated histone deacetylation and reinforce gene silencing by tethering the inactive X chromosome to the nuclear periphery through interaction with the lamin B receptor (LBR) and topoisomerase remodeling of the Xi.

However, many unresolved questions remain. For example, currently available data do not allow determination of whether different Xist splicing variants have a similar function and spreading pattern compared with those of the full-length Xist RNA that is most often exploited experimentally.

Several Xist splice variants have been described, including two major forms [7173]. More recently a RepA variant of Xist RNA that seems to mimic the full-length version has also been reported [30]. Use of the male ESC Xist-inducible system, which exploits a mature form of Xist, and differentiating female ESC lines, in which Xist transcription is subject to splicing, might therefore not necessarily be completely interchangeable experimentally. A possible way to make the systems more comparable would be to include the relatively small introns of Xist in the inducible constructs. This could be particularly informative in the light of eventual Xist post-transcriptional modifications, which are not necessarily confined to exons.

The regulation of Xist has been shown to be tightly regulated by pluripotency factors [6, 74, 75], and the pluripotent state of ESCs is known to be highly sensitive to culture conditions. If, as seems likely, silencing initiation and Xist spreading are sensitive to quantitative variations in Xist RNA levels, ex vivo culture conditions might also crucially impact inactivation parameters. For example, ESCs grown in 2 inhibitor (2i) medium culture conditions are known to be closer to the ‘ground zero’ state of pluripotency [76], to have a more homogeneous composition [76] and to show different transcriptional profiles compared with those of cells grown under conventional LIF and serum conditions [7779]. Both differentiation and upregulation of XIC lncRNAs clearly occur much faster using such 2i cultured cells, but whether the underlying mechanisms differ or remain unchanged still needs to be clarified. Only through standardization of ESC culturing (for example, by consistent use of 2i culturing conditions) and differentiation protocols (for example, using NPC differentiation) will direct comparison of data obtained in different experiments be possible.

Finally, given that the initial Xist spreading is likely to vary between individual cells, validation at the single-cell level, including single-cell RAP/CHART experiments, will be key towards obtaining a more thorough characterization and a better knowledge of Xist early dynamics. Such approaches would be expected to facilitate the identification of causal correlations between possible chromatin states and specific modifications of Xist binding sites.



two-cell embryonic stage

2i medium:

2 inhibitor medium


three dimensional structured illumination microscopy


circular chromosome conformation capture


alpha thalassemia/mental retardation syndrome X-linked


capture hybridization analysis of RNA targets


chromatin isolation by RNA purification-mass spectrometry


crosslinking and immunoprecipitation


CCCTC-binding factor




embryoid body


embryonic stem cell


high-throughput chromosome conformation capture


heterogeneous nuclear ribonucleoprotein K


histone 3 lysine 27 trimethylation




long non-coding RNA


lamin B receptor


leukemia inhibitory factor


long interspersed nuclear element


mouse embryonic fibroblast


non-coding RNA


neuronal precursor cell

Pol II:

RNA polymerase II


retinoic acid


RNA antisense purification-sequencing


RNA antisense purification-mass spectrometry


repeat A region of Xist RNA


RNA immunoprecipitation

Pol II:

RNA polymerase II


polycomb repressive complex 1/polycomb repressive complex 2

SAF-A (hnrnpU):

scaffold attachment factor A (heterogeneous ribonucleoprotein U)


special AT-rich sequence-binding protein-1


short interspersed nuclear element


structural maintenance of chromosome hinge domain 1


SMRT- and HDAC-associated repressor complex/Msx2-interacting protein


scaffold/matrix attachment region


silencing mediator for retinoic acid receptor and thyroid hormone receptor/nuclear receptor co-repressor


topologically associated domain


Wilms’ tumor-associated protein


active X chromosome


X chromosome inactivation


inactive X chromosome


X inactivation center


inactive X-specific transcript


Xist transgene


Yin-Yang 1


  1. Penny GD, Kay GF, Sheardown SA, Rastan S, Brockdorff N. Requirement for Xist in X chromosome inactivation. Nature. 1996;379:131–7.

    Article  CAS  PubMed  Google Scholar 

  2. Augui S, Nora EP, Heard E. Regulation of X-chromosome inactivation by the X-inactivation centre. Nat Rev Genet. 2011;12:429–42.

    Article  CAS  PubMed  Google Scholar 

  3. Lee JT, Bartolomei MS. X-inactivation, imprinting, and long noncoding RNAs in health and disease. Cell. 2013;152:1308–23.

    Article  CAS  PubMed  Google Scholar 

  4. Morey C, Avner P. The demoiselle of X-inactivation: 50 years old and as trendy and mesmerising as ever. PLoS Genet. 2011;7:e1002212.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  5. Matarazzo MR, Cerase A, D'Esposito M. Building up the inactive X chromosome. Biol Cell. 2008;100:63–70.

    Article  CAS  PubMed  Google Scholar 

  6. Pollex T, Heard E. Recent advances in X-chromosome inactivation research. Curr Opin Cell Biol. 2012;24:825–32.

    Article  CAS  PubMed  Google Scholar 

  7. Engreitz JM, Pandya-Jones A, McDonel P, Shishkin A, Sirokman K, Surka C, et al. The Xist lncRNA exploits three-dimensional genome architecture to spread across the X chromosome. Science. 2013;341:1237973.

    Article  PubMed Central  CAS  PubMed  Google Scholar 

  8. Simon MD, Pinter SF, Fang R, Sarma K, Rutenberg-Schoenberg M, Bowman SK, et al. High-resolution Xist binding maps reveal two-step spreading during X-chromosome inactivation. Nature. 2013;504:465–9.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  9. Leung KN, Panning B. X-inactivation: Xist RNA uses chromosome contacts to coat the X. Curr Biol. 2014;24:R80–2.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  10. Dimond A, Fraser P. Molecular biology. Long noncoding RNAs Xist in three dimensions. Science. 2013;341:720–1.

    Article  CAS  PubMed  Google Scholar 

  11. Wutz A, Jaenisch R. A shift from reversible to irreversible X inactivation is triggered during ES cell differentiation. Mol Cell. 2000;5:695–705.

    Article  CAS  PubMed  Google Scholar 

  12. Wutz A, Rasmussen TP, Jaenisch R. Chromosomal silencing and localization are mediated by different domains of Xist RNA. Nat Genet. 2002;30:167–74.

    Article  CAS  PubMed  Google Scholar 

  13. Duthie SM, Nesterova TB, Formstone EJ, Keohane AM, Turner BM, Zakian SM, et al. Xist RNA exhibits a banded localization on the inactive X chromosome and is excluded from autosomal material in cis. Hum Mol Genet. 1999;8:195–204.

    Article  CAS  PubMed  Google Scholar 

  14. Smith KP, Byron M, Clemson CM, Lawrence JB. Ubiquitinated proteins including uH2A on the human and mouse inactive X chromosome: enrichment in gene rich bands. Chromosoma. 2004;113:324–35.

    Article  CAS  PubMed  Google Scholar 

  15. Nora EP, Lajoie BR, Schulz EG, Giorgetti L, Okamoto I, Servant N, et al. Spatial partitioning of the regulatory landscape of the X-inactivation centre. Nature. 2012;485:381–5.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  16. Dixon JR, Selvaraj S, Yue F, Kim A, Li Y, Shen Y, et al. Topological domains in mammalian genomes identified by analysis of chromatin interactions. Nature. 2012;485:376–80.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  17. Lieberman-Aiden E, van Berkum NL, Williams L, Imakaev M, Ragoczy T, Telling A, et al. Comprehensive mapping of long-range interactions reveals folding principles of the human genome. Science. 2009;326:289–93.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  18. Csankovszki G, McDonel P, Meyer BJ. Recruitment and spreading of the C. elegans dosage compensation complex along X chromosomes. Science. 2004;303:1182–5.

    Article  CAS  PubMed  Google Scholar 

  19. Straub T, Grimaud C, Gilfillan GD, Mitterweger A, Becker PB. The chromosomal high-affinity binding sites for the Drosophila dosage compensation complex. PLoS Genet. 2008;4, e1000302.

    Article  PubMed Central  CAS  PubMed  Google Scholar 

  20. Splinter E, de Wit E, Nora EP, Klous P, van de Werken HJ, Zhu Y, et al. The inactive X chromosome adopts a unique three-dimensional conformation that is dependent on Xist RNA. Genes Dev. 2011;25:1371–83.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  21. Minajigi A, Froberg JE, Wei C, Sunwoo H, Kesner B, Colognori D, et al. A comprehensive Xist interactome reveals cohesin repulsion and an RNA-directed chromosome conformation. Science. 2015;349:6245.

    Article  CAS  Google Scholar 

  22. Csankovszki G, Nagy A, Jaenisch R. Synergism of Xist RNA, DNA methylation, and histone hypoacetylation in maintaining X chromosome inactivation. J Cell Biol. 2001;153:773–84.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  23. McHugh CA, Chen CK, Chow A, Surka CF, Tran C, McDonel P, et al. The Xist lncRNA interacts directly with SHARP to silence transcription through HDAC3. Nature. 2015;521:232–6.

    Article  CAS  PubMed  Google Scholar 

  24. Zhang LF, Huynh KD, Lee JT. Perinucleolar targeting of the inactive X during S phase: evidence for a role in the maintenance of silencing. Cell. 2007;129:693–706.

    Article  CAS  PubMed  Google Scholar 

  25. Smeets D, Markaki Y, Schmid VJ, Kraus F, Tattermusch A, Cerase A, et al. Three-dimensional super-resolution microscopy of the inactive X chromosome territory reveals a collapse of its active nuclear compartment harboring distinct Xist RNA foci. Epigenetics Chromatin. 2014;7:8.

    Article  PubMed Central  PubMed  Google Scholar 

  26. Cerase A, Smeets D, Tang YA, Gdula M, Kraus F, Spivakov M, et al. Spatial separation of Xist RNA and polycomb proteins revealed by superresolution microscopy. Proc Natl Acad Sci U S A. 2014;111:2235–40.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  27. Schermelleh L, Carlton PM, Haase S, Shao L, Winoto L, Kner P, et al. Subdiffraction multicolor imaging of the nuclear periphery with 3D structured illumination microscopy. Science. 2008;320:1332–6.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  28. Buzin CH, Mann JR, Singer-Sam J. Quantitative RT-PCR assays show Xist RNA levels are low in mouse female adult tissue, embryos and embryoid bodies. Development. 1994;120:3529–36.

    CAS  PubMed  Google Scholar 

  29. Sun BK, Deaton AM, Lee JT. A transient heterochromatic state in Xist preempts X inactivation choice without RNA stabilization. Mol Cell. 2006;21:617–28.

    Article  CAS  PubMed  Google Scholar 

  30. Zhao J, Sun BK, Erwin JA, Song JJ, Lee JT. Polycomb proteins targeted by a short repeat RNA to the mouse X chromosome. Science. 2008;322:750–6.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  31. Kaneko S, Li G, Son J, Xu CF, Margueron R, Neubert TA, et al. Phosphorylation of the PRC2 component Ezh2 is cell cycle-regulated and up-regulates its binding to ncRNA. Genes Dev. 2010;24:2615–20.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  32. Maenner S, Blaud M, Fouillen L, Savoye A, Marchand V, Dubois A, et al. 2-D structure of the A region of Xist RNA and its implication for PRC2 association. PLoS Biol. 2010;8:e1000276.

    Article  PubMed Central  CAS  PubMed  Google Scholar 

  33. Kanhere A, Viiri K, Araújo CC, Rasaiyaah J, Bouwman RD, Whyte WA, et al. Short RNAs are transcribed from repressed polycomb target genes and interact with polycomb repressive complex-2. Mol Cell. 2010;38:675–88.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  34. Marks H, Chow JC, Denissov S, Françoijs KJ, Brockdorff N, Heard E, et al. High-resolution analysis of epigenetic changes associated with X inactivation. Genome Res. 2009;19:1361–73.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  35. Calabrese JM, Sun W, Song L, Mugford JW, Williams L, Yee D, et al. Site-specific silencing of regulatory elements as a mechanism of X inactivation. Cell. 2012;151:951–63.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  36. Pinter SF, Sadreyev RI, Yildirim E, Jeon Y, Ohsumi TK, Borowsky M, et al. Spreading of X chromosome inactivation via a hierarchy of defined Polycomb stations. Genome Res. 2012;22:1864–76.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  37. Okamoto I, Otte AP, Allis CD, Reinberg D, Heard E. Epigenetic dynamics of imprinted X inactivation during early mouse development. Science. 2004;303:644–9.

    Article  CAS  PubMed  Google Scholar 

  38. Mak W, Nesterova TB, de Napoles M, Appanah R, Yamanaka S, Otte AP, et al. Reactivation of the paternal X chromosome in early mouse embryos. Science. 2004;303:666–9.

    Article  CAS  PubMed  Google Scholar 

  39. Kohlmaier A, Savarese F, Lachner M, Martens J, Jenuwein T, Wutz A. A chromosomal memory triggered by Xist regulates histone methylation in X inactivation. PLoS Biol. 2004;2, E171.

    Article  PubMed Central  PubMed  Google Scholar 

  40. Chaumeil J, Le Baccon P, Wutz A, Heard E. A novel role for Xist RNA in the formation of a repressive nuclear compartment into which genes are recruited when silenced. Genes Dev. 2006;20:2223–37.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  41. Savarese F, Flahndorfer K, Jaenisch R, Busslinger M, Wutz A. Hematopoietic precursor cells transiently reestablish permissiveness for X inactivation. Mol Cell Biol. 2006;26:7167–77.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  42. Popova BC, Tada T, Takagi N, Brockdorff N, Nesterova TB. Attenuated spread of X-inactivation in an X;autosome translocation. Proc Natl Acad Sci U S A. 2006;103:7706–11.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  43. Tang YA, Huntley D, Montana G, Cerase A, Nesterova TB, Brockdorff N. Efficiency of Xist-mediated silencing on autosomes is linked to chromosomal domain organisation. Epigenetics Chromatin. 2010;3:10.

    Article  PubMed Central  CAS  PubMed  Google Scholar 

  44. Chu C, Zhang QC, da Rocha ST, Flynn RA, Bharadwaj M, Calabrese JM, et al. Systematic discovery of Xist RNA binding proteins. Cell. 2015;161:404–16.

    Article  CAS  PubMed  Google Scholar 

  45. Yuan W, Wu T, Fu H, Dai C, Wu H, Liu N, et al. Dense chromatin activates Polycomb repressive complex 2 to regulate H3 lysine 27 methylation. Science. 2012;337:971–5.

    Article  CAS  PubMed  Google Scholar 

  46. Riising EM, Comet I, Leblanc B, Wu X, Johansen JV, Helin K. Gene silencing triggers polycomb repressive complex 2 recruitment to CpG islands genome wide. Mol Cell. 2014;55:347–60.

    Article  CAS  PubMed  Google Scholar 

  47. Ng K, Daigle N, Bancaud A, Ohhata T, Humphreys P, Walker R, et al. A system for imaging the regulatory noncoding Xist RNA in living mouse embryonic stem cells. Mol Biol Cell. 2011;22:2634–45.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  48. Perissi V, Jepsen K, Glass CK, Rosenfeld MG. Deconstructing repression: evolving models of co-repressor action. Nat Rev Genet. 2010;11:109–23.

    Article  CAS  PubMed  Google Scholar 

  49. Adcock IM, Ford P, Ito K, Barnes PJ. Epigenetics and airways disease. Respir Res. 2006;7:21.

    Article  PubMed Central  CAS  PubMed  Google Scholar 

  50. Schwartz YB, Pirrotta V. A new world of Polycombs: unexpected partnerships and emerging functions. Nat Rev Genet. 2013;14:853–64.

    Article  CAS  PubMed  Google Scholar 

  51. Casanova M, Preissner T, Cerase A, Poot R, Yamada D, Li X, et al. Polycomblike 2 facilitates the recruitment of PRC2 Polycomb group complexes to the inactive X chromosome and to target loci in embryonic stem cells. Development. 2011;138:1471–82.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  52. da Rocha ST, Boeva V, Escamilla-Del-Arenal M, Ancelin K, Granier C, Matias NR, et al. Jarid2 is implicated in the initial Xist-induced targeting of PRC2 to the inactive X chromosome. Mol Cell. 2014;53:301–16.

    Article  CAS  PubMed  Google Scholar 

  53. Kaneko S, Bonasio R, Saldaña-Meyer R, Yoshida T, Son J, Nishino K, et al. Interactions between JARID2 and noncoding RNAs regulate PRC2 recruitment to chromatin. Mol Cell. 2014;53:290–300.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  54. Brockdorff N. Chromosome silencing mechanisms in X-chromosome inactivation: unknown unknowns. Development. 2011;138:5057–65.

    Article  CAS  PubMed  Google Scholar 

  55. Brockdorff N. Noncoding RNA, and Polycomb recruitment. RNA. 2013;19:429–42.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  56. Helbig R, Fackelmayer FO. Scaffold attachment factor A (SAF-A) is concentrated in inactive X chromosome territories through its RGG domain. Chromosoma. 2003;12:173–82.

    Article  CAS  Google Scholar 

  57. Fackelmayer FO. A stable proteinaceous structure in the territory of inactive X chromosomes. J Biol Chem. 2005;280:1720–3.

    Article  CAS  PubMed  Google Scholar 

  58. Hasegawa Y, Brockdorff N, Kawano S, Tsutui K, Tsutui K, Nakagawa S. The matrix protein hnRNP U is required for chromosomal localization of Xist RNA. Dev Cell. 2010;19:469–76.

    Article  CAS  PubMed  Google Scholar 

  59. Agrelo R, Souabni A, Novatchkova M, Haslinger C, Leeb M. Komnenovic, et al. SATB1 defines the developmental context for gene silencing by Xist in lymphoma and embryonic cells. Dev Cell. 2009;16:507–16.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  60. Jeon Y, Lee JT. YY1 tethers Xist RNA to the inactive X nucleation center. Cell. 2011;146:119–33.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  61. Ascano M, Hafner M, Cekan P, Gerstberger S, Tuschl T. Identification of RNA-protein interaction networks using PAR-CLIP. Wiley Interdiscip Rev RNA. 2012;3:159–77.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  62. Schmiedeberg L, Skene P, Deaton A, Bird A. A temporal threshold for formaldehyde crosslinking and fixation. PloS One. 2009;4:e4636.

    Article  PubMed Central  CAS  PubMed  Google Scholar 

  63. Clemson CM, McNeil JA, Willard HF, Lawrence JB. XIST RNA paints the inactive X chromosome at interphase: evidence for a novel RNA involved in nuclear/chromosome structure. J Cell Biol. 1996;132:259–75.

    Article  CAS  PubMed  Google Scholar 

  64. Hall LL, Byron M, Pageau G, Lawrence JB. AURKB-mediated effects on chromatin regulate binding versus release of XIST RNA to the inactive chromosome. J Cell Biol. 2009;186:491–507.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  65. Clemson CM, Hall LL, Byron M, McNeil J, Lawrence JB. The X chromosome is organized into a gene-rich outer rim and an internal core containing silenced nongenic sequences. Proc Natl Acad Sci U S A. 2006;103:7688–93.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  66. Gluch A, Vidakovic M, Bode J. Scaffold/matrix attachment regions (S/MARs): relevance for disease and therapy. Handb Exp Pharmacol. 2008;186:67–103.

    Article  CAS  PubMed  Google Scholar 

  67. Filippova GN, Cheng MK, Moore JM, Truong JP, Hu YJ, Nguyen DK, et al. Boundaries between chromosomal domains of X inactivation and escape bind CTCF and lack CpG methylation during early development. Dev Cell. 2005;8:31–42.

    Article  CAS  PubMed  Google Scholar 

  68. Hall LL, Carone DM, Gomez AV, Kolpa HJ, Byron M, Mehta N, et al. Stable C0T-1 repeat RNA is abundant and is associated with euchromatic interphase chromosomes. Cell. 2014;156:907–19.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  69. Tattermusch A, Brockdorff N. A scaffold for X chromosome inactivation. Hum Genet. 2011;130:247–53.

    Article  CAS  PubMed  Google Scholar 

  70. Hasegawa Y, Nakagawa S. Revisiting the function of nuclear scaffold/matrix binding proteins in X chromosome inactivation. RNA Biol. 2011;8:735–9.

    Article  CAS  PubMed  Google Scholar 

  71. Royce-Tolland ME et al. The A-repeat links ASF/SF2-dependent Xist RNA processing with random choice during X inactivation. Nat Struct Mol Biol. 2010;17:948–54.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  72. Ciaudo C, Bourdet A, Cohen-Tannoudji M, Dietz HC, Rougeulle C, Avner P. Nuclear mRNA degradation pathway(s) are implicated in Xist regulation and X chromosome inactivation. PLoS Genet. 2006;2:e94.

    Article  PubMed Central  CAS  PubMed  Google Scholar 

  73. Memili E, Hong YK, Kim DH, Ontiveros SD, Strauss WM. Murine Xist RNA isoforms are different at their 3′ ends: a role for differential polyadenylation. Gene. 2001;266:131–7.

    Article  CAS  PubMed  Google Scholar 

  74. Navarro P, Chambers I, Karwacki-Neisius V, Chureau C, Morey C, Rougeulle C, et al. Molecular coupling of Xist regulation and pluripotency. Science. 2008;321:1693–5.

    Article  CAS  PubMed  Google Scholar 

  75. Gontan C, Achame EM, Demmers J, Barakat TS, Rentmeester E, van IJcken W, et al. RNF12 initiates X-chromosome inactivation by targeting REX1 for degradation. Nature. 2012;485:386–90.

    Article  CAS  PubMed  Google Scholar 

  76. Marks H, Kalkan T, Menafra R, Denissov S, Jones K, Hofemeister H, et al. The transcriptional and epigenomic foundations of ground state pluripotency. Cell. 2012;149:590–604.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  77. Silva J, Nichols J, Theunissen TW, Guo G, van Oosten AL, Barrandon O, et al. Nanog is the gateway to the pluripotent ground state. Cell. 2009;138:722–37.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  78. Ying QL, Wray J, Nichols J, Batlle-Morera L, Doble B, Woodgett J, et al. The ground state of embryonic stem cell self-renewal. Nature. 2008;453:519–23.

    Article  CAS  PubMed  Google Scholar 

  79. Silva J, Barrandon O, Nichols J, Kawaguchi J, Theunissen TW, Smith A. Promotion of reprogramming to ground state pluripotency by signal inhibition. PLoS Biol. 2008;6:e253.

    Article  PubMed Central  CAS  PubMed  Google Scholar 

  80. Plath K, Fang J, Mlynarczyk-Evans SK, Cao R, Worringer KA, Wang H, et al. Role of histone H3 lysine 27 methylation in X inactivation. Science. 2003;300:131–5.

    Article  CAS  PubMed  Google Scholar 

  81. Silva J, Mak W, Zvetkova I, Appanah R, Nesterova TB, Webster Z, et al. Establishment of histone H3 methylation on the inactive X chromosome requires transient recruitment of Eed-Enx1 polycomb group complexes. Dev Cell. 2003;4:481–95.

    Article  CAS  PubMed  Google Scholar 

  82. Blackledge NP, Farcas AM, Kondo T, King HW, McGouran JF, Hanssen LL, et al. Variant PRC1 complex-dependent H2A ubiquitylation drives PRC2 recruitment and polycomb domain formation. Cell. 2014;157:1445–59.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  83. Schoeftner S, Sengupta AK, Kubicek S, Mechtler K, Spahn L, Koseki H, et al. Recruitment of PRC1 function at the initiation of X inactivation independent of PRC2 and silencing. EMBO J. 2006;25:3110–22.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  84. Nechanitzky R, Davila A, Savarese F, Fietze S, Grosschedl R. Satb1 and Satb2 are dispensable for X chromosome inactivation in mice. Dev Cell. 2012;23:866–71.

    Article  CAS  PubMed  Google Scholar 

  85. Makhlouf M, Ouimette JF, Oldfield A, Navarro P, Neuillet D, Rougeulle C. A prominent and conserved role for YY1 in Xist transcriptional activation. Nat Commun. 2014;5:4878.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  86. Gendrel AV, Apedaile A, Coker H, Termanis A, Zvetkova I, Godwin J, et al. Smchd1-dependent and -independent pathways determine developmental dynamics of CpG island methylation on the inactive X chromosome. Dev Cell. 2012;23:265–79.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  87. Navarro P, Oldfield A, Legoupi J, Festuccia N, Dubois A, Attia M, et al. Molecular coupling of Tsix regulation and pluripotency. Nature. 2010;468:457–60.

    Article  CAS  PubMed  Google Scholar 

  88. Payer B, Rosenberg M, Yamaji M, Yabuta Y, Koyanagi-Aoi M, Hayashi K, et al. Tsix RNA and the germline factor, PRDM14, link X reactivation and stem cell reprogramming. Mol Cell. 2013;52:805–18.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  89. Sarma K, Cifuentes-Rojas C, Ergun A, Del Rosario A, Jeon Y, White F, et al. ATRX directs binding of PRC2 to Xist RNA and Polycomb targets. Cell. 2014;159:869–83.

    Article  CAS  PubMed  Google Scholar 

  90. Bailey JA, Carrel L, Chakravarti A, Eichler EE. Molecular evidence for a relationship between LINE-1 elements and X chromosome inactivation: the Lyon repeat hypothesis. Proc Natl Acad Sci U S A. 2000;97:6634–9.

    Article  CAS  PubMed Central  PubMed  Google Scholar 

  91. Chow JC, Ciaudo C, Fazzari MJ, Mise N, Servant N, Glass JL, et al. LINE-1 activity in facultative heterochromatin formation during X chromosome inactivation. Cell. 2010;141:956–69.

    Article  CAS  PubMed  Google Scholar 

Download references


We apologize to all those colleagues whose work could not be discussed and cited in this review owing to space limitations. We thank, in particular, Kristina Cavalletti-Havas, Sara Buonomo, Christophe Lancrin and members of the Avner group for critical reading of the manuscript. PA and AC were funded by EMBL grant to PA, GP was funded by a Welcome Trust Grant, and AT was funded by an SDW fellowship and by MRC grant number 1036324.

Author information

Authors and Affiliations


Corresponding authors

Correspondence to Andrea Cerase or Philip Avner.

Additional information

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

PA and AC conceived this project. PA, AC, GP and AT designed the figures and wrote the paper.

Rights and permissions

Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (, which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver ( applies to the data made available in this article, unless otherwise stated.

Reprints and Permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Cerase, A., Pintacuda, G., Tattermusch, A. et al. Xist localization and function: new insights from multiple levels. Genome Biol 16, 166 (2015).

Download citation

  • Received:

  • Accepted:

  • Published:

  • DOI: