Deletion of Xist repeat B disrupts cell cycle and asymmetric cell division through Usp9x hyperactivation in mice [II]
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ABSTRACT: During X-inactivation (XCI), Xist RNA establishes silencing by coating the chromosome in cis and binding diverse proteins to promote formation of a heterochromatic domain. However, Xist repeat B role beyond initiation of XCI remains unclear. Here, we find that loss of Xist repeat B in female mice allows survival and leads to a small body size persisting throughout life. Epigenetic and transcriptomic analyses reveal low levels of H3K27me3 and H2AK119ub occupancy on the X chromosome, except in certain CpG island regions, and partial reactivation of X-linked genes on the inactive X across multiple tissues. Notably, overdosage of Usp9x promotes centrosome amplification and chromosome instability, increasing the propensity of tumorigenesis. We further demonstrate that Usp9x overdosage leads to abnormalities in asymmetric cell division, thereby affecting the process of cell differentiation. Thus, Xist repeat B is necessary for gene-specific silencing during XCI maintenance and impacts cell proliferation and differentiation during development. This provides insights into repeat B importance in maintaining XCI.
Project description:During X-inactivation (XCI), Xist RNA establishes silencing by coating the chromosome in cis and binding diverse proteins to promote formation of a heterochromatic domain. However, Xist repeat B role beyond initiation of XCI remains unclear. Here, we find that loss of Xist repeat B in female mice allows survival and leads to a small body size persisting throughout life. Epigenetic and transcriptomic analyses reveal low levels of H3K27me3 and H2AK119ub occupancy on the X chromosome, except in certain CpG island regions, and partial reactivation of X-linked genes on the inactive X across multiple tissues. Notably, overdosage of Usp9x promotes centrosome amplification and chromosome instability, increasing the propensity of tumorigenesis. We further demonstrate that Usp9x overdosage leads to abnormalities in asymmetric cell division, thereby affecting the process of cell differentiation. Thus, Xist repeat B is necessary for gene-specific silencing during XCI maintenance and impacts cell proliferation and differentiation during development. This provides insights into repeat B importance in maintaining XCI.
Project description:Xist represents a paradigm for long non-coding RNA function in epigenetic regulation, although how it mediates X-chromosome inactivation (XCI) remains largely unexplained. Multiple Xist-RNA binding proteins have recently been identified, including SPEN/SHARP, whose knockdown has been associated with deficient XCI at multiple loci. Here we demonstrate that SPEN is a key orchestrator of XCI in vivo and unravel its mechanism of action. We show that SPEN is essential for initiating gene silencing on the X chromosome in preimplantation mouse embryos and embryonic stem cells. On the other hand, SPEN is dispensable for maintenance of XCI in neural progenitor cells, although it significantly dampens expression of genes that escape from XCI. During initiation of XCI, we show by live-cell imaging and CUT&RUN approaches that SPEN is immediately recruited to the X chromosome upon Xist up-regulation, where it is targeted to enhancers and promoters of actively transcribed genes. SPEN rapidly disengages from chromatin once silencing is accomplished, implying a need for active transcription to tether it to chromatin. We define SPEN’s SPOC (SPEN paralog and ortholog C-terminal) domain as a major effector of SPEN’s gene silencing function, and show that artificial tethering of SPOC to Xist RNA is sufficient to mediate X-linked gene silencing. We identify SPOC’s protein partners which include NCOR/SMRT, the m6A RNA methylation machinery, the NuRD complex, RNA polymerase II and factors involved in regulation of transcription initiation and elongation. We propose that SPEN acts as a molecular integrator for initiation of XCI, bridging Xist RNA with the transcription machinery as well as nucleosome remodelers and histone deacetylases, at active enhancers and promoters.
Project description:X-chromosome inactivation (XCI) is a global silencing mechanism by which XX and XY mammals equalize X-linked gene dosages. XCI begins with an establishment phase during which Xist RNA spreads and induces de novo heterochromatinization across a female X chromosome, and is followed by a maintenance phase when multiple epigenetic pathways lock down the inactive X (Xi) state. Involvement of Polycomb repressive complexes 1 and 2 in XCI has been intensively studied, but with conflicting conclusions regarding their recruitment and role in Xi silencing. Here we reveal that establishment of XCI has two phases and reconcile the roles that Xist Repeats A and B play in gene silencing and Polycomb recruitment. Repeat A initiates both processes, whereas Repeat B bolsters or stabilizes them thereafter. Once established, XCI no longer requires Repeat A during maintenance. These findings integrate disparate studies and present a unified view of Xist?s role in Polycomb-mediated silencing.
Project description:Many large noncoding RNAs (lncRNAs) regulate chromatin, but the mechanisms by which they localize to genomic targets remain unexplored. Here we investigate the localization mechanisms of Xist during X-chromosome inactivation (XCI), a paradigm of lncRNA-mediated chromatin regulation. During the maintenance of XCI, Xist binds broadly across the X-chromosome. During initiation of XCI, Xist initially transfers to distal regions across the X-chromosome that are not defined by specific sequences. Instead, Xist identifies these regions by exploiting the three-dimensional conformation of the X-chromosome. Xist initially accumulates on the periphery of actively transcribed regions and requires its silencing domain to spread across active regions. This suggests a model where Xist coats the entire X-chromosome by searching in three dimensions, modifying chromosome structure, and spreading to newly accessible locations. We examined the genomic localization of the Xist lncRNA using RNA Antisense Purification (RAP) in multiple cell contexts: 1) differentiated female cells (MLFs); 2) a time-course of Xist localization in male embryonic stem (ES) cells where the endogenous Xist promoter is replaced by a tet-inducible one (pSM33); 3) a time-course of Xist localization in differentiating female ES cells (F1 2-1); and 4) wild-type (delXF6) and A-repeat deletion (delSXC9) Xist transgenes incorporated into the Hprt locus under the control of a tet-inducible promoter.
Project description:During development, transcriptional and chromatin modification changes co-occur but the order and causality of events often remain unclear. We explore the interrelationship of these processes using the paradigm of X-chromosome inactivation (XCI). We initiate XCI in female, mouse embryonic stem cells by inducing Xist expression and monitor changes in transcription and chromatin by allele-specific TT-seq and ChIP-seq respectively. An unprecedented temporal resolution enabled identification of the earliest chromatin alterations during XCI. We demonstrate that HDAC3 interacts with both NCOR1 and NCOR2 and is pre-bound on the X chromosome where it deacetylates histones to promote efficient gene silencing. We also reveal the choreography of polycomb accumulation following Xist RNA coating, with PRC1-associated H2AK119Ub preceding PRC2-associated H3K27me3. Furthermore, polycomb-associated marks accumulate initially at large, intergenic domains and then spreads into genes but only in the context of gene silencing. Our results provide the hierarchy of chromatin events during XCI and demonstrate that some chromatin changes play key roles in mediating transcriptional silencing.
Project description:Xist is indispensable for X chromosome inactivation (XCI) in female mammalian cells. However, how Xist RNA directs chromosome-wide transcriptional inactivation of the X chromosome is largely unknown. Here, to study chromosome inactivation by Xist, we generated a system where ectopic Xist expression can be induced from several genomic contexts in aneuploid mouse ES cells. We found that ectopic Xist expression from any location on the X chromosome faithfully recapitulated endogenous XCI, showing the potency of Xist to initiate XCI. Genes that escape XCI remain consistently transcriptionally active upon ectopic XCI, regardless of their position relative to Xist transgenes, and the enrichment of CTCF at their promoters is implicated in directing XCI escape. Xist expression from autosomes facilitates their transcriptional silencing to different degrees, and gene density in proximity of the Xist transcription locus plays a central role in determining the efficiency of gene inactivation. We also show that the enrichment of LINE elements together with a specific chromatin environment facilitates Xist-mediated silencing of both X-linked and autosomal genes. These findings provide new insights into the epigenetic mechanisms that mediate XCI and identify genomic features that promote Xist-mediated chromosome-wide gene inactivation
Project description:During X chromosome inactivation (XCI), in female mammals, gene silencing is initiated by a long-noncoding RNA, called Xist. Xist RNA accumulation also allows for the deposition of specific chromatin marks. Among others, PRC2-dependent H3K27me3 and PrSet7-dependent H4K20me1 become enriched at the inactive X chromosome. However, the dynamics of this process in relation to Xist RNA accumulation remains unknown as is the molecular mechanism allowing for H4K20me1 enrichment. To follow XCI dynamics in living cells, we developed a genetically encoded, H3K27me3-specific intracellular antibody, or H3K27me3-mintbody. By combining it with live-imaging of H4K20me1, the X chromosome and Xist RNA we uncover striking similarities in the accumulation dynamics of H3K27me3 and H4K20me1. Further ChIP-seq analysis confirmed concurrent accumulation of H4K20me1 and H3K27me3 during XCI albeit with distinct genomic distributions. Xist B and C repeat mutant, which can silence the X but does not allow for H3K27me3 enrichment, also showed lack of H4K20me1 deposition. Thus, both H3K27me3 and H4K20me1 are deposited at the inactive X through a common molecular mechanism but are dispensable for the initiation of gene silencing.
Project description:At initiation of X chromosome inactivation (XCI), Xist is monoallelically upregulated from the future inactive X (Xi) chromosome, overcoming repression by its antisense transcript Tsix. Xist recruits various chromatin remodelers, amongst them SPEN, which are involved in silencing of X-linked genes in cis and establishment of the Xi. Here, we show that SPEN plays an important role in the initiation of XCI. Spen null female mouse embryonic stem cells (ESCs) are defective in Xist upregulation upon differentiation. We find that Xist-mediated SPEN recruitment to the Xi chromosome happens very early in XCI, and that SPEN-mediated silencing of the Tsix promoter is required for Xist upregulation. Accordingly, failed Xist upregulation in Spen-/- ESCs can be rescued by concomitant removal of Tsix. These findings indicate that SPEN is not only required for the establishment of the Xi, but is also crucial in the initiation of the XCI process.
Project description:At initiation of X chromosome inactivation (XCI), Xist is monoallelically upregulated from the future inactive X (Xi) chromosome, overcoming repression by its antisense transcript Tsix. Xist recruits various chromatin remodelers, amongst them SPEN, which are involved in silencing of X-linked genes in cis and establishment of the Xi. Here, we show that SPEN plays an important role in the initiation of XCI. Spen null female mouse embryonic stem cells (ESCs) are defective in Xist upregulation upon differentiation. We find that Xist-mediated SPEN recruitment to the Xi chromosome happens very early in XCI, and that SPEN-mediated silencing of the Tsix promoter is required for Xist upregulation. Accordingly, failed Xist upregulation in Spen-/- ESCs can be rescued by concomitant removal of Tsix. These findings indicate that SPEN is not only required for the establishment of the Xi, but is also crucial in the initiation of the XCI process.
Project description:Female human ESC-lines can carry active X-chromosomes (Xa) or an XIST-RNA- coated inactive X-chromosome (Xi XIST+ ). Additionally, many ESC-lines have abnormal X-chromosome-inactivation (XCI)-states where the Xi no longer expresses XIST-RNA and has transcriptionally active regions (eroded Xi=Xe). The fate of each XCI-state upon differentiation is unclear because individual lines often contain a mixture of XCI-states. Here, we established homogeneous XiXa, XeXa, and XaXa ESC-lines. We found that these lines were unable to initiate XIST-expression and X-chromosome- wide silencing upon differentiation indicating that the ESC XCI-state is maintained in differentiated cells. Consequently, differentiated XeXa and XaXa cells displayed higher levels of X-linked gene-expression than XiXa cells. Although global transcriptional compensation between X-chromosomes and autosomes is not required for female ESC-differentiation, the degree of X-chromosome- silencing influences differentiation efficiencies. Our data suggest that the Xi XIST+ Xa state is inherent to human ESCs and that all other XCI-states, including XaXa, are abnormal and arise during ESC-derivation or maintenance.