Explore the vast range of titles available.

During mammalian X-chromosome inactivation, the Xist long noncoding RNA coats the future inactive X chromosome and recruits polycomb repressive complex 2 to a nucleation site, but how Xist spreads silencing across the entire X chromosome is unclear; here high-resolution maps of Xist binding sites across the X chromosome are generated and show that Xist does not spread across the inactive X chromosome uniformly but in two steps, initially targeting gene-rich islands before later spreading to intervening gene-poor domains. Mapping Xist long noncoding RNA During mammalian X-chromosome inactivation (XCI), the Xist long noncoding RNA coats the future inactive X (Xi) and recruits polycomb repressive complex 2 (PRC2) to a nucleation site, but how Xist spreads silencing across the entire X chromosome is unclear. Here, Jeannie Lee and colleagues generate high-resolution maps of Xist binding sites across the X chromosome using a technique termed CHART-seq. By following four early developmental stages in female mouse cells undergoing XCI, they show that Xist does not spread across the Xi uniformly but in two steps — initially targeting gene-rich islands and later spreading to intervening gene-poor domains. The Xist long noncoding RNA (lncRNA) is essential for X-chromosome inactivation (XCI), the process by which mammals compensate for unequal numbers of sex chromosomes 1 , 2 , 3 . During XCI, Xist coats the future inactive X chromosome (Xi) 4 and recruits Polycomb repressive complex 2 (PRC2) to the X-inactivation centre ( Xic ) 5 . How Xist spreads silencing on a 150-megabases scale is unclear. Here we generate high-resolution maps of Xist binding on the X chromosome across a developmental time course using CHART-seq. In female cells undergoing XCI de novo , Xist follows a two-step mechanism, initially targeting gene-rich islands before spreading to intervening gene-poor domains. Xist is depleted from genes that escape XCI but may concentrate near escapee boundaries. Xist binding is linearly proportional to PRC2 density and H3 lysine 27 trimethylation (H3K27me3), indicating co-migration of Xist and PRC2. Interestingly, when Xist is acutely stripped off from the Xi in post-XCI cells, Xist recovers quickly within both gene-rich and gene-poor domains on a timescale of hours instead of days, indicating a previously primed Xi chromatin state. We conclude that Xist spreading takes distinct stage-specific forms. During initial establishment, Xist follows a two-step mechanism, but during maintenance, Xist spreads rapidly to both gene-rich and gene-poor regions.

A hallmark of chromosome organization is the partition into transcriptionally active A and repressed B compartments, and into topologically associating domains (TADs). Both structures were regarded to be absent from the inactive mouse X chromosome, but to be re-established with transcriptional reactivation and chromatin opening during X-reactivation. Here, we combine a tailor-made mouse iPSC reprogramming system and high-resolution Hi-C to produce a time course combining gene reactivation, chromatin opening and chromosome topology during X-reactivation. Contrary to previous observations, we observe A/B-like compartments on the inactive X harbouring multiple subcompartments. While partial X-reactivation initiates within a compartment rich in X-inactivation escapees, it then occurs rapidly along the chromosome, concomitant with downregulation of Xist . Importantly, we find that TAD formation precedes transcription and initiates from Xist-poor compartments. Here, we show that TAD formation and transcriptional reactivation are causally independent during X-reactivation while establishing Xist as a common denominator. Both A/B compartments and TADs are thought to be absent from the inactive X chromosome, but to be re-established with transcriptional reactivation and chromatin opening during X-reactivation. Here, the authors characterise gene reactivation, chromatin opening and chromosome topology during X-reactivation, observe A/B-like compartments on the inactive X that guide TAD formation independently of transcription during X-reactivation.

The mammalian inactive X-chromosome (Xi) is structurally distinct from all other chromosomes and serves as a model for how the 3D genome is organized. The Xi shows weakened topologically associated domains and is instead organized into megadomains and superloops directed by the noncoding loci, Dxz4 and Firre . Their functional significance is presently unclear, though one study suggests that they permit Xi genes to escape silencing. Here, we find that megadomains do not precede Xist expression or Xi gene silencing. Deleting Dxz4 disrupts the sharp megadomain border, whereas deleting Firre weakens intra-megadomain interactions. However, deleting Dxz4 and/or Firre has no impact on Xi silencing and gene escape. Nor does it affect Xi nuclear localization, stability, or H3K27 methylation. Additionally, ectopic integration of Dxz4 and Xist is not sufficient to form megadomains on autosomes. We conclude that Dxz4 and megadomains are dispensable for Xi silencing and escape from X-inactivation. The mammalian inactive X-chromosome (Xi) is organized into megadomains and superloops directed by the noncoding loci, Dxz4 and Firre . Here the authors provide evidence that megadomains do not precede Xist expression or Xi gene silencing, and suggest that Dxz4 , Firre , and megadomains are dispensable for Xi silencing and escape from X-inactivation.

In addition to balancing X-chromosome dosage between males and females via X inactivation, mammals also balance dosage of X chromosomes and autosomes. Allele-specific chromatin immunoprecipitation with deep sequencing (ChIP-seq) analyses now show that the active X chromosome is upregulated at the level of both transcription initiation and elongation. Dosage compensation in mammals occurs at two levels. In addition to balancing X-chromosome dosage between males and females via X inactivation, mammals also balance dosage of Xs and autosomes. It has been proposed that X-autosome equalization occurs by upregulation of Xa (active X). To investigate mechanism, we perform allele-specific ChIP-seq for chromatin epitopes and analyze RNA-seq data. The hypertranscribed Xa demonstrates enrichment of active chromatin marks relative to autosomes. We derive predictive models for relationships among Pol II occupancy, active mark densities and gene expression, and we suggest that Xa upregulation involves increased transcription initiation and elongation. Enrichment of active marks on Xa does not scale proportionally with transcription output, a disparity explained by nonlinear quantitative dependencies among active histone marks, Pol II occupancy and transcription. Notably, the trend of nonlinear upregulation also occurs on autosomes. Thus, Xa upregulation involves combined increases of active histone marks and Pol II occupancy, without invoking X-specific dependencies between chromatin states and transcription.

Recent efforts in mapping spatial genome organization have revealed three evocative and conserved structural features of the inactive X in female mammals. First, the chromosomal conformation of the inactive X reveals a loss of topologically associated domains (TADs) present on the active X. Second, the macrosatellite emerges as a singular boundary that suppresses physical interactions between two large TAD-depleted \"megadomains.\" Third, reaches across several megabases to form \"superloops\" with two other X-linked tandem repeats, and , which also loop to each other. Although all three structural features are conserved across rodents and primates, deletion of mouse and human orthologs of and from the inactive X have revealed limited impact on X chromosome inactivation (XCI) and escape In contrast, loss of or SMCHD1 have been shown to impair TAD erasure and gene silencing on the inactive X. In this perspective, we summarize these results in the context of new research describing disruption of X-linked tandem repeats , and discuss their possible molecular roles through the lens of evolutionary conservation and clinical genetics. As a null hypothesis, we consider whether the conservation of some structural features on the inactive X may reflect selection for X-linked tandem repeats on account of necessary - and -regulatory roles they may play on the active X, rather than the inactive X. Additional hypotheses invoking a role for X-linked tandem repeats on X reactivation, for example in the germline or totipotency, remain to be assessed in multiple developmental models spanning mammalian evolution.

In mammals, monoallelic gene expression can result from X-chromosome inactivation, genomic imprinting, and random monoallelic expression (RMAE). Epigenetic regulation of RMAE is not fully understood. Here we analyze allelic imbalance in chromatin state of autosomal genes using ChIP-seq in a clonal cell line. We identify approximately 3.7% of autosomal genes that show significant differences between chromatin states of two alleles. Allelic regulation is represented among several functional gene categories including histones, chromatin modifiers, and multiple early developmental regulators. Most cases of allelic skew are produced by quantitative differences between two allelic chromatic states that belong to the same gross type (active, silent, or bivalent). Combinations of allelic states of different types are possible but less frequent. When different chromatin marks are skewed on the same gene, their skew is coordinated as a result of quantitative relationships between these marks on each individual allele. Finally, combination of allele-specific densities of chromatin marks is a quantitative predictor of allelic skew in gene expression.

Gene expression is controlled by coordinated action of many epigenetic mechanisms including covalent histone modifications. Although numerous recurrent patterns of colocalized histone modifications have been associated with specific gene expression states, interrelationships between individual modifications are largely unknown. Here, we analyze quantitative relationships between colocalized histone marks during embryonic stem cell (ESC) differentiation and find that, for autosomal genes, these densities follow bimodal patterns. Analysis of repressive H3K27me3 and activating H3K4me3 modifications reveals the expected anticorrelation between them at active promoters but an unexpected positive correlation at inactive promoters. The two trends connect in a region corresponding to bivalent genes. Interestingly, this region is characterized by maximal H3K27 methylation. Resolving gene bivalency during ESC differentiation does not conform to the expected model of two marks as counteracting and competing forces. Although activated genes acquire H3K4me3 and lose H3K27me3, repressed genes lose H3K4me3 without gaining H3K27me3. The behavior of X-linked genes also deviates from expected models. Allele-specific analysis of chromatin modifications during X-chromosome inactivation (XCI) suggests that the silencing machinery focuses on active genes and depletion of H3K4me3 and that H3K27me3 is most significant during establishment of gene silencing. Our analysis reveals nontrivial relationships between H3K4me3 and H3K27me3, reveals unique aspects of gene bivalency, and demonstrates that XCI does not conform neatly to autosomal models.

Pluripotency and the X chromosome During both stem cell differentiation and X chromosome inactivation, the process that silences one female X chromosome to ensure gene dosage parity between the sexes, chromatin undergoes epigenetic reprogramming to lock in a new state. Reprogramming of differentiated cells into iPS cells also causes reactivation of the inactivated X chromosome, and it has been proposed that the pluripotency factor Oct4 links both processes. In this study, Donohoe et al . find that Oct4 regulates X-chromosome inactivation by triggering X-chromosome pairing and counting. Oct4 interacts with both noncoding RNA genes ( Tsix and Xite ) and proteins (Ctcf and Yy1). This work shows that there is a complex network involved in epigenetic reprogramming of the X chromosome in stem cells. During both stem cell differentiation and X-chromosome inactivation (XCI) of mouse embryonic stem cells, chromatin undergoes epigenetic reprogramming. XCI and cell differentiation are tightly coupled, with the blocking of one process compromising the other. The pluripotency factor, Oct4, is now shown to regulate XCI, and is the first identified factor that links both processes. Pluripotency of embryonic stem (ES) cells is controlled by defined transcription factors 1 , 2 . During differentiation, mouse ES cells undergo global epigenetic reprogramming, as exemplified by X-chromosome inactivation (XCI) in which one female X chromosome is silenced to achieve gene dosage parity between the sexes 3 , 4 , 5 . Somatic XCI is regulated by homologous X-chromosome pairing 6 , 7 and counting 8 , 9 , 10 , and by the random choice of future active and inactive X chromosomes. XCI and cell differentiation are tightly coupled 11 , as blocking one process compromises the other 8 , 12 and dedifferentiation of somatic cells to induced pluripotent stem cells is accompanied by X chromosome reactivation 2 . Recent evidence suggests coupling of Xist expression to pluripotency factors occurs 13 , but how the two are interconnected remains unknown. Here we show that Oct4 (also known as Pou5f1) 14 lies at the top of the XCI hierarchy, and regulates XCI by triggering X-chromosome pairing and counting. Oct4 directly binds Tsix and Xite , two regulatory noncoding RNA genes of the X-inactivation centre 15 , 16 , and also complexes with XCI trans-factors, Ctcf and Yy1 (ref. 17 ), through protein–protein interactions. Depletion of Oct4 blocks homologous X-chromosome pairing and results in the inactivation of both X chromosomes in female cells. Thus, we have identified the first trans-factor that regulates counting, and ascribed new functions to Oct4 during X-chromosome reprogramming.

New analyses reveal that TERRA transcripts arising from the subtelomeric pseudoautosomal (PAR) region of sex chromosomes nucleate pairing of X alleles in mouse ES cells. In mammals, homologous chromosomes rarely pair outside meiosis. One exception is the X chromosome, which transiently pairs during X-chromosome inactivation (XCI). How two chromosomes find each other in 3D space is not known. Here, we reveal a required interaction between the X-inactivation center (Xic) and the telomere in mouse embryonic stem (ES) cells. The subtelomeric, pseudoautosomal regions (PARs) of the two sex chromosomes (X and Y) also undergo pairing in both female and male cells. PARs transcribe a class of telomeric RNA, dubbed PAR-TERRA, which accounts for a vast majority of all TERRA transcripts. PAR-TERRA binds throughout the genome, including to the PAR and Xic. During X-chromosome pairing, PAR-TERRA anchors the Xic to the PAR, creating a 'tetrad' of pairwise homologous interactions (Xic–Xic, PAR–PAR, and Xic–PAR). Xic pairing occurs within the tetrad. Depleting PAR-TERRA abrogates pairing and blocks initiation of XCI, whereas autosomal PAR-TERRA induces ectopic pairing. We propose a 'constrained diffusion model' in which PAR-TERRA creates an interaction hub to guide Xic homology searching during XCI.

Mammalian sex chromosomes encode homologous X/Y gene pairs that were retained on the Y chromosome in males and escape X chromosome inactivation (XCI) in females. Inferred to reflect X/Y pair dosage sensitivity, monosomy X is a leading cause of miscarriage in humans with near full penetrance. This phenotype is shared with many other mammals but not the mouse, which offers sophisticated genetic tools to generate sex chromosomal aneuploidy but also tolerates its developmental impact. To address this critical gap, we generated X-monosomic human induced pluripotent stem cells (hiPSCs) alongside otherwise isogenic euploid controls from male and female mosaic samples. Phased genomic variants in these hiPSC panels enable systematic investigation of X/Y dosage-sensitive features using in vitro models of human development. Here, we demonstrate the utility of these validated hiPSC lines to test how X/Y-linked gene dosage impacts a widely used model for human syncytiotrophoblast development. While these isogenic panels trigger a GATA2/3- and TFAP2A/C-driven trophoblast gene circuit irrespective of karyotype, differential expression implicates monosomy X in altered levels of placental genes and in secretion of placental growth factor (PlGF) and human chorionic gonadotropin (hCG). Remarkably, weighted gene coexpression network modules that significantly reflect these changes are also preserved in first-trimester chorionic villi and term placenta. Our results suggest monosomy X may skew trophoblast cell type composition and function, and that the combined haploinsufficiency of the pseudoautosomal region likely plays a key role in these changes.