The Ftx Noncoding Locus Controls X Chromosome Inactivation Independently of Its RNA Products

Long non coding RNA function in epigenetic memory with a particular emphasis on genomic imprinting and X chromosome inactivation

Cells preserve and convey certain gene expression patterns to their progeny through the mechanism called epigenetic memory. Epigenetic memory, encoded by epigenetic markers and components, determines germline inheritance, genomic imprinting, and X chromosome inactivation. First discovered long non coding RNAs were implicated in genomic imprinting and X-inactivation and these two phenomena clearly demonstrate the role of lncRNAs in epigenetic memory regulation. Undoubtedly, lncRNAs are well-suited for regulating genes in close proximity at imprinted loci. Due to prolonged association with the transcription site, lncRNAs are able to guide chromatin modifiers to certain locations, thereby enabling accurate temporal and spatial regulation. Nevertheless, the current state of knowledge regarding lncRNA biology and imprinting processes is still in its nascent phase. Herein, we provide a synopsis of recent scientific advancements to enhance our comprehension of lncRNAs and their functions in epigenetic memory, with a particular emphasis on genomic imprinting and X chromosome inactivation, thus gaining a deeper understanding of the role of lncRNAs in epigenetic regulatory networks.

Sex-specific Associations of Gene Expression with Alzheimer's Disease Neuropathology and Ante-mortem Cognitive Performance

The biological mechanisms underlying the increased prevalence of Alzheimer's disease (AD) in women remain undefined. While previous case/control studies have identified sex-biased molecular pathways, the sex-specific relationships between gene expression and AD endophenotypes, particularly involving sex chromosomes, are underexplored. With bulk transcriptomic data across 3 brain regions from 767 decedents, we investigated sex-specific associations between gene expression and post-mortem β-amyloid and tau, as well as antemortem longitudinal cognition. Among 23,118 significant gene associations, 10% were sex-specific, with 73% of these identified in females and primarily associated with tau tangles and longitudinal cognition (90%). Notably, four X-linked genes, MCF2, HDAC8, FTX, and SLC10A3, demonstrated significant sex differences in their associations with AD endophenotypes (i.e., significant sex × gene interaction). Our results also uncovered sex-specific biological pathways, including a female-specific role of neuroinflammation and neuronal development, underscoring the importance of sex-aware analyses to advance precision medicine approaches in AD.

Syntenic lncRNA locus exhibits DNA regulatory functions with sequence evolution

Syntenic long non-coding RNAs (lncRNAs) often show limited sequence conservation across species, prompting concern in the field. This study delves into functional signatures of syntenic lncRNAs between humans and zebrafish. Syntenic lncRNAs are highly expressed in zebrafish, with ∼90 % located near protein-coding genes, either in sense or antisense orientation. During early zebrafish development and in human embryonic stem cells (H1-hESC), syntenic lncRNA loci are enriched with cis-regulatory repressor signatures, influencing the expression of development-associated genes. In later zebrafish developmental stages and specific human cell lines, these syntenic lncRNA loci function as enhancers or transcription start sites (TSS) for protein-coding genes. Analysis of transposable elements (TEs) in syntenic lncRNA sequences revealed intriguing patterns: human lncRNAs are enriched in simple repeat elements, while their zebrafish counterparts show enrichment in LTR elements. This sequence evolution likely arises from post-rearrangement mutations that enhance DNA elements or cis-regulatory functions. It may also contribute to vertebrate innovation by creating novel transcription factor binding sites within the locus. This study highlights the conserved functionality of syntenic lncRNA loci through DNA elements, emphasizing their conserved roles across species despite sequence divergence.

KDM6A facilitates Xist upregulation at the onset of X inactivation

BACKGROUND

X chromosome inactivation (XCI) is a female-specific process in which one X chromosome is silenced to balance X-linked gene expression between the sexes. XCI is initiated in early development by upregulation of the lncRNA Xist on the future inactive X (Xi). A subset of X-linked genes escape silencing and thus have higher expression in females, suggesting female-specific functions. One of these genes is the highly conserved gene Kdm6a, which encodes a histone demethylase that removes methyl groups at H3K27 to facilitate gene expression. KDM6A mutations have been implicated in congenital disorders such as Kabuki Syndrome, as well as in sex differences in development and cancer.

METHODS

Kdm6a was knocked out (KO) using CRISPR/Cas9 gene editing in hybrid female mouse embryonic stem (ES) cells derived either from a 129 × Mus castaneus (cast) cross or a BL6 x cast cross. In one of the lines a transcriptional stop signal inserted in Tsix results in completely skewed X silencing upon differentiation. The effects of both homozygous and heterozygous Kdm6a KO on Xist expression during the onset of XCI were measured by RT-PCR and RNA-FISH. Changes in gene expression and in H3K27me3 enrichment were investigated using allele-specific RNA-seq and Cut&Run, respectively. KDM6A binding to the Xist gene was characterized by Cut&Run.

RESULTS

We observed impaired upregulation of Xist and reduced coating of the Xi during early stages of differentiation in Kdm6a KO cells, both homozygous and heterozygous, suggesting a threshold effect of KDM6A. This was associated with aberrant overexpression of genes from the Xi after differentiation, indicating loss of X inactivation potency. Consistent with KDM6A having a direct role in Xist regulation, we found that the histone demethylase binds to the Xist promoter and KO cells show an increase in H3K27me3 at Xist, consistent with reduced expression.

CONCLUSIONS

These results reveal a novel female-specific role for the X-linked histone demethylase, KDM6A in the initiation of XCI through histone demethylase-dependent activation of Xist during early differentiation. X chromosome inactivation is a female-specific mechanism that evolved to balance sex-linked gene dosage between females (XX) and males (XY) by silencing one X chromosome in females. X inactivation begins with the upregulation of the long noncoding RNA Xist on the future inactive X chromosome. While most genes become silenced on the inactive X chromosome some genes escape inactivation and thus have higher expression in females compared to males, suggesting that escape genes may have female-specific functions. One such gene encodes the histone demethylase KDM6A which function is to turn on gene expression by removing repressive histone modifications. In this study, we investigated the role of KDM6A in the regulation of Xist expression during the onset of X inactivation. We found that KDM6A binds to the Xist gene to remove repressive histone marks and facilitate its expression in early development. Indeed, depletion of KDM6A prevents upregulation of Xist due to abnormal persistence of repressive histone modifications. In turn, this results in aberrant overexpression of genes from the inactive X chromosome. Our findings point to a novel mechanism of Xist regulation during the initiation of X inactivation, which may lead to new avenues of treatment to alleviate congenital disorders such as Kabuki syndrome and sex-biased immune disorders where X-linked gene dosage is perturbed.

Sex-specific Associations of Gene Expression with Alzheimer's Disease Neuropathology and Ante-mortem Cognitive Performance

The biological mechanisms underlying women's increased Alzheimer's disease (AD) prevalence remain undefined. Previous case/control studies have identified sex-biased molecular pathways, but sex-specific relationships between gene expression and AD endophenotypes, particularly sex chromosomes, are underexplored. With bulk transcriptomic data across 3 brain regions from 767 decedents, we investigated sex-specific associations between gene expression and post-mortem β-amyloid and tau as well as antemortem longitudinal cognition. Of 23,118 significant gene associations, 10% were significant in one sex and not the other (sex-specific). Most sex-specific gene associations were identified in females (73%) and associated with tau tangles and longitudinal cognition (90%). Four X-linked genes, MCF2, HDAC8, FTX, and SLC10A3, demonstrated significant sex differences in their associations with AD endophenotypes (i.e., significant sex x gene interaction). Our results also uncovered sex-specific biological pathways, including a female-specific role of neuroinflammation and neuronal development, reinforcing the potential for sex-aware analyses to enhance precision medicine approaches in AD.

Loss of XIST lncRNA unlocks stemness and cellular plasticity in ovarian cancer

Plasticity, a key hallmark of cancer, enables cells to transition into different states, driving tumor heterogeneity. This cellular plasticity is associated with cancer progression, treatment resistance, and relapse. Cancer stem cells (CSCs) play a central role in this process, yet the molecular factors underlying cancer cell stemness remain poorly understood. In this study, we explored the role of XIST (X-inactive specific transcript) long noncoding RNA in ovarian cancer stemness and plasticity through in silico and in vitro analyses. We found that XIST is significantly down-regulated in ovarian tumors, with low XIST expression linked to a higher stemness index and lower overall survival. Knocking down XIST in ovarian cancer cells enhanced stemness, particularly increasing mesenchymal-like CSCs, and under hypoxic conditions, it promoted epithelial-like CSC markers. Our findings suggest that XIST loss leads to CSC enrichment and cellular plasticity in ovarian cancer, pointing to potential therapeutic targets for patients with low XIST expression.

SCAR-6 elncRNA locus epigenetically regulates PROZ and modulates coagulation and vascular function

In this study, we characterize a novel lncRNA-producing gene locus that we name Syntenic Cardiovascular Conserved Region-Associated lncRNA-6 (scar-6) and functionally validate its role in coagulation and cardiovascular function. A 12-bp deletion of the scar-6 locus in zebrafish (scar-6gib007Δ12/Δ12) results in cranial hemorrhage and vascular permeability. Overexpression, knockdown and rescue with the scar-6 lncRNA modulates hemostasis in zebrafish. Molecular investigation reveals that the scar-6 lncRNA acts as an enhancer lncRNA (elncRNA), and controls the expression of prozb, an inhibitor of factor Xa, through an enhancer element in the scar-6 locus. The scar-6 locus suppresses loop formation between prozb and scar-6 sequences, which might be facilitated by the methylation of CpG islands via the prdm14-PRC2 complex whose binding to the locus might be stabilized by the scar-6 elncRNA transcript. Binding of prdm14 to the scar-6 locus is impaired in scar-6gib007Δ12/Δ12 zebrafish. Finally, activation of the PAR2 receptor in scar-6gib007Δ12/Δ12 zebrafish triggers NF-κB-mediated endothelial cell activation, leading to vascular dysfunction and hemorrhage. We present evidence that the scar-6 locus plays a role in regulating the expression of the coagulation cascade gene prozb and maintains vascular homeostasis.

Female-bias in systemic lupus erythematosus: How much is the X chromosome to blame?

Systemic lupus erythematosus (SLE or lupus) is an immune-mediated disease associated with substantial medical burden. Notably, lupus exhibits a striking female bias, with women having significantly higher susceptibility compared to men, up to 14-fold higher in some ethnicities. Supernumerary X chromosome syndromes, like Klinefelter (XXY) and Triple X syndrome (XXX), also present higher SLE prevalence, whereas Turner syndrome (XO) displays lower prevalence. Taken together, SLE prevalence in different X chromosome dosage sceneries denotes a relationship between the number of X chromosomes and the risk of developing lupus. The dosage of X-linked genes, many of which play roles in the immune system, is compensated between males and females through the inactivation of one of the two X chromosomes in female cells. X-chromosome inactivation (XCI) initiates early in development with a random selection of which X chromosome to inactivate, a choice that is then epigenetically maintained in the daughter cells. This process is regulated by the X-Inactive-Specific Transcript (XIST), encoding for a long non-coding RNA, exclusively expressed from the inactive X chromosome (Xi). XIST interacts with various RNA binding proteins and chromatin modifiers to form a ribonucleoprotein (RNP) complex responsible for the transcriptional silencing and heterochromatinization of the Xi. This ensures stable silencing of most genes on the X chromosome, with only a few genes able to escape this process. Recent findings suggest that the molecular components involved in XCI, or their dysregulation, contribute to the pathogenesis of lupus. Indeed, nonrandom XCI, elevated gene escape from XCI, and the autoimmune potential of the XIST RNP complex have been suggested to contribute to auto-immune diseases, such as lupus. This review examines these current hypotheses concerning how this dosage compensation mechanism might impact the development of lupus, shedding light on potential mechanisms underlying the pathogenesis of the disease.

Stepwise de novo establishment of inactive X chromosome architecture in early development

X chromosome inactivation triggers a dramatic reprogramming of transcription and chromosome architecture. However, how the chromatin organization of inactive X chromosome is established de novo in vivo remains elusive. Here, we identified an Xist-separated megadomain structure (X-megadomains) on the inactive X chromosome in mouse extraembryonic lineages and extraembryonic endoderm (XEN) cell lines, and transiently in the embryonic lineages, before Dxz4-delineated megadomain formation at later stages in a strain-specific manner. X-megadomain boundary coincides with strong enhancer activities and cohesin binding in an Xist regulatory region required for proper Xist activation in early embryos. Xist regulatory region disruption or cohesin degradation impaired X-megadomains in extraembryonic endoderm cells and caused ectopic activation of regulatory elements and genes near Xist, indicating that cohesin loading at regulatory elements promotes X-megadomains and confines local gene activities. These data reveal stepwise X chromosome folding and transcriptional regulation to achieve both essential gene activation and global silencing during the early stages of X chromosome inactivation.

Multifaceted role of CTCF in X-chromosome inactivation

Therian female mammals compensate for the dosage of X-linked gene expression by inactivating one of the X-chromosomes. X-inactivation is facilitated by the master regulator Xist long non-coding RNA, which coats the inactive-X and facilitates heterochromatinization through recruiting different chromatin modifiers and changing the X-chromosome 3D conformation. However, many mechanistic aspects behind the X-inactivation process remain poorly understood. Among the many contributing players, CTCF has emerged as one of the key players in orchestrating various aspects related to X-chromosome inactivation by interacting with several other protein and RNA partners. In general, CTCF is a well-known architectural protein, which plays an important role in chromatin organization and transcriptional regulation. Here, we provide significant insight into the role of CTCF in orchestrating X-chromosome inactivation and highlight future perspectives.

Compensation of gene dosage on the mammalian X

Changes in gene dosage can have tremendous evolutionary potential (e.g. whole-genome duplications), but without compensatory mechanisms, they can also lead to gene dysregulation and pathologies. Sex chromosomes are a paradigmatic example of naturally occurring gene dosage differences and their compensation. In species with chromosome-based sex determination, individuals within the same population necessarily show 'natural' differences in gene dosage for the sex chromosomes. In this Review, we focus on the mammalian X chromosome and discuss recent new insights into the dosage-compensation mechanisms that evolved along with the emergence of sex chromosomes, namely X-inactivation and X-upregulation. We also discuss the evolution of the genetic loci and molecular players involved, as well as the regulatory diversity and potentially different requirements for dosage compensation across mammalian species.

RNA-Independent Regulatory Functions of lncRNA in Complex Disease

During the metagenomics era, high-throughput sequencing efforts both in mice and humans indicate that non-coding RNAs (ncRNAs) constitute a significant fraction of the transcribed genome. During the past decades, the regulatory role of these non-coding transcripts along with their interactions with other molecules have been extensively characterized. However, the study of long non-coding RNAs (lncRNAs), an ncRNA regulatory class with transcript lengths that exceed 200 nucleotides, revealed that certain non-coding transcripts are transcriptional "by-products", while their loci exert their downstream regulatory functions through RNA-independent mechanisms. Such mechanisms include, but are not limited to, chromatin interactions and complex promoter-enhancer competition schemes that involve the underlying ncRNA locus with or without its nascent transcription, mediating significant or even exclusive roles in the regulation of downstream target genes in mammals. Interestingly, such RNA-independent mechanisms often drive pathological manifestations, including oncogenesis. In this review, we summarize selective examples of lncRNAs that regulate target genes independently of their produced transcripts.

An emerging link between lncRNAs and cancer sex dimorphism

The prevalence and progression of cancer differ in males and females, and thus, sexual dimorphism in tumor development directly impacts clinical research and medicine. Long non-coding RNAs (lncRNAs) are increasingly recognized as important players in gene expression and various cellular processes, including cancer development and progression. In recent years, lncRNAs have been implicated in the differences observed in cancer incidence, progression, and treatment responses between men and women. Here, we present a brief overview of the current knowledge regarding the role of lncRNAs in cancer sex dimorphism, focusing on how they affect epigenetic processes in male and female mammalian cells. We discuss the potential mechanisms by which lncRNAs may contribute to sex differences in cancer, including transcriptional control of sex chromosomes, hormonal signaling pathways, and immune responses. We also propose strategies for studying lncRNA functions in cancer sex dimorphism. Furthermore, we emphasize the importance of considering sex as a biological variable in cancer research and the need to investigate the role lncRNAs play in mediating these sex differences. In summary, we highlight the emerging link between lncRNAs and cancer sex dimorphism and their potential as therapeutic targets.

Spatial orchestration of the genome: topological reorganisation during X-chromosome inactivation

Genomes are organised through hierarchical structures, ranging from local kilobase-scale cis-regulatory contacts to large chromosome territories. Most notably, (sub)-compartments partition chromosomes according to transcriptional activity, while topologically associating domains (TADs) define cis-regulatory landscapes. The inactive X chromosome in mammals has provided unique insights into the regulation and function of the three-dimensional (3D) genome. Concurrent with silencing of the majority of genes and major alterations of its chromatin state, the X chromosome undergoes profound spatial rearrangements at multiple scales. These include the emergence of megadomains, alterations of the compartment structure and loss of the majority of TADs. Moreover, the Xist locus, which orchestrates X-chromosome inactivation, has provided key insights into regulation and function of regulatory domains. This review provides an overview of recent insights into the control of these structural rearrangements and contextualises them within a broader understanding of 3D genome organisation.

Xist Deletion in B Cells Results in Systemic Lupus Erythematosus Phenotypes

Systemic lupus erythematosus (SLE) is an autoimmune disease preferentially observed in females. X-linked gene expression in XX females is normalized to that of XY males by X-Chromosome Inactivation (XCI). However, B cells from female SLE patients and mouse models of SLE exhibit mislocalization of Xist RNA, a critical regulator of XCI, and aberrant expression of X-linked genes, suggesting that impairment of XCI may contribute to disease. Here, we find that a subset of female mice harboring a conditional deletion of Xis t in B cells ("Xist cKO") spontaneously develop SLE phenotypes, including expanded activated B cell subsets, disease-specific autoantibodies, and glomerulonephritis. Moreover, pristane-induced SLE-like disease is more severe in Xist cKO mice. Activated B cells from Xist cKO mice with SLE phenotypes have increased expression of proinflammatory X-linked genes implicated in SLE. Together, this work indicates that impaired XCI maintenance in B cells directly contributes to the female-bias of SLE.

Altered X-chromosome inactivation predisposes to autoimmunity

In mammals, males and females show marked differences in immune responses. Males are globally more sensitive to infectious diseases, while females are more susceptible to systemic autoimmunity. X-chromosome inactivation (XCI), the epigenetic mechanism ensuring the silencing of one X in females, may participate in these sex biases. We perturbed the expression of the trigger of XCI, the noncoding RNA Xist, in female mice. This resulted in reactivation of genes on the inactive X, including members of the Toll-like receptor 7 (TLR7) signaling pathway, in monocyte/macrophages and dendritic and B cells. Consequently, female mice spontaneously developed inflammatory signs typical of lupus, including anti-nucleic acid autoantibodies, increased frequencies of age-associated and germinal center B cells, and expansion of monocyte/macrophages and dendritic cells. Mechanistically, TLR7 signaling is dysregulated in macrophages, leading to sustained expression of target genes upon stimulation. These findings provide a direct link between maintenance of XCI and female-biased autoimmune manifestations and highlight altered XCI as a cause of autoimmunity.

A Comparative Analysis of Mouse Imprinted and Random X-Chromosome Inactivation

The mammalian sexes are distinguished by the X and Y chromosomes. Whereas males harbor one X and one Y chromosome, females harbor two X chromosomes. To equalize X-linked gene expression between the sexes, therian mammals have evolved X-chromosome inactivation as a dosage compensation mechanism. During X-inactivation, most genes on one of the two X chromosomes in females are transcriptionally silenced, thus equalizing X-linked gene expression between the sexes. Two forms of X-inactivation characterize eutherian mammals, imprinted and random. Imprinted X-inactivation is defined by the exclusive inactivation of the paternal X chromosome in all cells, whereas random X-inactivation results in the silencing of genes on either the paternal or maternal X chromosome in individual cells. Both forms of X-inactivation have been studied intensively in the mouse model system, which undergoes both imprinted and random X-inactivation early in embryonic development. Stable imprinted and random X-inactivation requires the induction of the Xist long non-coding RNA. Following its induction, Xist RNA recruits proteins and complexes that silence genes on the inactive-X. In this review, we present a current understanding of the mechanisms of Xist RNA induction, and, separately, the establishment and maintenance of gene silencing on the inactive-X by Xist RNA during imprinted and random X-inactivation.

Orchestrating Asymmetric Expression: Mechanisms behind Xist Regulation

Compensation for the gene dosage disequilibrium between sex chromosomes in mammals is achieved in female cells by repressing one of its X chromosomes through a process called X chromosome inactivation (XCI), exemplifying the control of gene expression by epigenetic mechanisms. A critical player in this mechanism is Xist, a long, non-coding RNA upregulated from a single X chromosome during early embryonic development in female cells. Over the past few decades, many factors involved at different levels in the regulation of Xist have been discovered. In this review, we hierarchically describe and analyze the different layers of Xist regulation operating concurrently and intricately interacting with each other to achieve asymmetric and monoallelic upregulation of Xist in murine female cells. We categorize these into five different classes: DNA elements, transcription factors, other regulatory proteins, long non-coding RNAs, and the chromatin and topological landscape surrounding Xist.

GATA transcription factors drive initial Xist upregulation after fertilization through direct activation of long-range enhancers

X-chromosome inactivation (XCI) balances gene expression between the sexes in female mammals. Shortly after fertilization, upregulation of Xist RNA from one X chromosome initiates XCI, leading to chromosome-wide gene silencing. XCI is maintained in all cell types, except the germ line and the pluripotent state where XCI is reversed. The mechanisms triggering Xist upregulation have remained elusive. Here we identify GATA transcription factors as potent activators of Xist. Through a pooled CRISPR activation screen in murine embryonic stem cells, we demonstrate that GATA1, as well as other GATA transcription factors can drive ectopic Xist expression. Moreover, we describe GATA-responsive regulatory elements in the Xist locus bound by different GATA factors. Finally, we show that GATA factors are essential for XCI induction in mouse preimplantation embryos. Deletion of GATA1/4/6 or GATA-responsive Xist enhancers in mouse zygotes effectively prevents Xist upregulation. We propose that the activity or complete absence of various GATA family members controls initial Xist upregulation, XCI maintenance in extra-embryonic lineages and XCI reversal in the epiblast.

Emerging roles of long non-coding RNA FTX in human disorders

Long non-coding RNAs (lncRNAs) are involved the progression of cancerous and non-cancerous disorders via different mechanism. FTX (five prime to xist) is an evolutionarily conserved lncRNA that is located upstream of XIST and regulates its expression. FTX participates in progression of various malignancy including gastric cancer, glioma, ovarian cancer, pancreatic cancer, and retinoblastoma. Also, FTX can be involved in the pathogenesis of non-cancerous disorders such as endometriosis and stroke. FTX acts as competitive endogenous RNA (ceRNA) and via sponging various miRNAs, including miR-186, miR-200a-3p, miR-215-3p, and miR-153-3p to regulate the expression of their downstream target. FTX by targeting various signaling pathways including Wnt/β-catenin, PI3K/Akt, SOX4, PDK1/PKB/GSK-3β, TGF-β1, FOXA2, and PPARγ regulate molecular mechanism involved in various disorders. Dysregulation of FTX is associated with an increased risk of various disorders. Therefore, FTX and its downstream targets may be suitable biomarkers for the diagnosis and treatment of human malignancies. In this review, we summarized the emerging roles of FTX in human cancerous and non-cancerous cells.

Landscape of semi-extractable RNAs across five human cell lines

Phase-separated membraneless organelles often contain RNAs that exhibit unusual semi-extractability using the conventional RNA extraction method, and can be efficiently retrieved by needle shearing or heating during RNA extraction. Semi-extractable RNAs are promising resources for understanding RNA-centric phase separation. However, limited assessments have been performed to systematically identify and characterize semi-extractable RNAs. In this study, 1074 semi-extractable RNAs, including ASAP1, DANT2, EXT1, FTX, IGF1R, LIMS1, NEAT1, PHF21A, PVT1, SCMH1, STRG.3024.1, TBL1X, TCF7L2, TVP23C-CDRT4, UBE2E2, ZCCHC7, ZFAND3 and ZSWIM6, which exhibited consistent semi-extractability were identified across five human cell lines. By integrating publicly available datasets, we found that semi-extractable RNAs tend to be distributed in the nuclear compartments but are dissociated from the chromatin. Long and repeat-containing semi-extractable RNAs act as hubs to provide global RNA-RNA interactions. Semi-extractable RNAs were divided into four groups based on their k-mer content. The NEAT1 group preferred to interact with paraspeckle proteins, such as FUS and NONO, implying that RNAs in this group are potential candidates of architectural RNAs that constitute nuclear bodies.

KDM6A facilitates Xist upregulation at the onset of X inactivation

X chromosome inactivation (XCI) is a female-specific process in which one X chromosome is silenced to balance X-linked gene expression between the sexes. XCI is initiated in early development by upregulation of the lncRNA Xist on the future inactive X (Xi). A subset of X-linked genes escape silencing and thus have higher expression in females, suggesting female-specific functions. One of these genes is the highly conserved gene Kdm6a , which encodes a histone demethylase that removes methyl groups at H3K27 to facilitate gene expression. Here, we investigate the role of KDM6A in the regulation of Xist . We observed impaired upregulation of Xist during early stages of differentiation in hybrid mouse ES cells following CRISPR/Cas9 knockout of Kdm6a . This is associated with reduced Xist RNA coating of the Xi, suggesting diminished XCI potency. Indeed, Kdm6a knockout results in aberrant overexpression of genes from the Xi after differentiation. KDM6A binds to the Xist promoter and knockout cells show an increase in H3K27me3 at Xist . These results indicate that KDM6A plays a role in the initiation of XCI through histone demethylase-dependent activation of Xist during early differentiation.

Regulatory principles and mechanisms governing the onset of random X-chromosome inactivation

X-chromosome inactivation (XCI) has evolved in mammals to compensate for the difference in X-chromosomal dosage between the sexes. In placental mammals, XCI is initiated during early embryonic development through upregulation of the long noncoding RNA Xist from one randomly chosen X chromosome in each female cell. The Xist locus must thus integrate both X-linked and developmental trans-regulatory factors in a dosage-dependent manner. Furthermore, the two alleles must coordinate to ensure inactivation of exactly one X chromosome per cell. In this review, we summarize the regulatory principles that govern the onset of XCI. We go on to provide an overview over the factors that have been implicated in Xist regulation and discuss recent advances in our understanding of how Xist's cis-regulatory landscape integrates information in a precise fashion.

Gender Differences in the Pathogenesis and Risk Factors of Hepatocellular Carcinoma

Several chronic liver diseases are characterized by a clear gender disparity. Among them, hepatocellular carcinoma (HCC) shows significantly higher incidence rates in men than in women. The different epidemiological distribution of risk factors for liver disease and HCC only partially accounts for these gender differences. In fact, the liver is an organ with recognized sexual dysmorphism and is extremely sensitive to the action of androgens and estrogens. Sex hormones act by modulating the risk of developing HCC and influencing its aggressiveness, response to treatments, and prognosis. Furthermore, androgens and estrogens are able to modulate the action of other factors and cofactors of liver damage (e.g., chronic HBV infection, obesity), significantly influencing their carcinogenic power. The purpose of this review is to examine the factors related to the different gender distribution in the incidence of HCC as well as the pathophysiological mechanisms involved, with particular reference to the central role played by sex hormones.

The compleX balancing act of controlling X-chromosome dosage and how it impacts mammalian germline development

In female mammals, the two X chromosomes are subject to epigenetic gene regulation in order to balance X-linked gene dosage with autosomes and in relation to males, which have one X and one Y chromosome. This is achieved by an intricate interplay of several processes; X-chromosome inactivation and reactivation elicit global epigenetic regulation of expression from one X chromosome in a stage-specific manner, whilst the process of X-chromosome upregulation responds to this by fine-tuning transcription levels of the second X. The germline is unique in its function of transmitting both the genetic and epigenetic information from one generation to the next, and remodelling of the X chromosome is one of the key steps in setting the stage for successful development. Here, we provide an overview of the complex dynamics of X-chromosome dosage control during embryonic and germ cell development, and aim to decipher its potential role for normal germline competency.

Species-specific regulation of XIST by the JPX/FTX orthologs

X chromosome inactivation (XCI) is an essential process, yet it initiates with remarkable diversity in various mammalian species. XIST, the main trigger of XCI, is controlled in the mouse by an interplay of lncRNA genes (LRGs), some of which evolved concomitantly to XIST and have orthologues across all placental mammals. Here, we addressed the functional conservation of human orthologues of two such LRGs, FTX and JPX. By combining analysis of single-cell RNA-seq data from early human embryogenesis with various functional assays in matched human and mouse pluripotent stem- or differentiated post-XCI cells, we demonstrate major functional differences for these orthologues between species, independently of primary sequence conservation. While the function of FTX is not conserved in humans, JPX stands as a major regulator of XIST expression in both species. However, we show that different entities of JPX control the production of XIST at various steps depending on the species. Altogether, our study highlights the functional versatility of LRGs across evolution, and reveals that functional conservation of orthologous LRGs may involve diversified mechanisms of action. These findings represent a striking example of how the evolvability of LRGs can provide adaptative flexibility to constrained gene regulatory networks.

Generation of blastoids from human parthenogenetic stem cells

Parthenogenetic embryos derive their genomes entirely from the maternal genome and lack paternal imprint patterns. Many achievements have been made in the study of genomic imprinting using human parthenogenetic embryonic stem cells (hPg-ESCs). However, due to developmental defects and ethical limits, a comprehensive understanding of parthenogenetic embryonic development is still lacking. Here, we generated parthenogenetic blastoids (hPg-EPSCs blastoids) from hPg-ESC-derived extended pluripotent stem cells (hPg-EPSCs) using our previously published two-step induction protocol. Morphology, specific marker expression and single-cell transcriptome analysis showed that hPg-EPSCs blastoids contain crucial cell lineages similar to blastoids (hBp-EPSCs blastoids) generated from human biparental EPSCs (hBp-EPSCs). Single-cell RNA-seq compared the expression of genes related to imprinting and X chromosome inactivation in hPg-EPSCs blastoids and hBp-EPSCs blastoids. In conclusion, we generated parthenogenetic blastoids, which will potentially promote the study of genomic imprinting in embryonic development and uncover the influence of parental origin bias on human development and pathological mechanisms.

Multi-hallmark long noncoding RNA maps reveal non-small cell lung cancer vulnerabilities

Long noncoding RNAs (lncRNAs) are widely dysregulated in cancer, yet their functional roles in cancer hallmarks remain unclear. We employ pooled CRISPR deletion to perturb 831 lncRNAs detected in KRAS-mutant non-small cell lung cancer (NSCLC) and measure their contribution to proliferation, chemoresistance, and migration across two cell backgrounds. Integrative analysis of these data outperforms conventional "dropout" screens in identifying cancer genes while prioritizing disease-relevant lncRNAs with pleiotropic and background-independent roles. Altogether, 80 high-confidence oncogenic lncRNAs are active in NSCLC, which tend to be amplified and overexpressed in tumors. A follow-up antisense oligonucleotide (ASO) screen shortlisted two candidates, Cancer Hallmarks in Lung LncRNA 1 (CHiLL1) and GCAWKR, whose knockdown consistently suppressed cancer hallmarks in two- and three-dimension tumor models. Molecular phenotyping reveals that CHiLL1 and GCAWKR control cellular-level phenotypes via distinct transcriptional networks. This work reveals a multi-dimensional functional lncRNA landscape underlying NSCLC that contains potential therapeutic vulnerabilities.

Effects of N6-Methyladenosine Modification on Cancer Progression: Molecular Mechanisms and Cancer Therapy

N⁶-methyladenosine (m⁶A) is a major internal epigenetic modification in eukaryotic mRNA, which is dynamic and reversible. m⁶A is regulated by methylases (“writers”) and demethylases (“erasers”) and is recognized and processed by m⁶A-binding proteins (“readers”), which further regulate RNA transport, localization, translation, and degradation. It plays a role in promoting or suppressing tumors and has the potential to become a therapeutic target for malignant tumors. In this review, we focus on the mutual regulation of m⁶A and coding and non-coding RNAs and introduce the molecular mechanism of m⁶A methylation involved in regulation and its role in cancer treatment by taking common female malignant tumors as an example.

Activation of Xist by an evolutionarily conserved function of KDM5C demethylase

XX female and XY male therian mammals equalize X-linked gene expression through the mitotically-stable transcriptional inactivation of one of the two X chromosomes in female somatic cells. Here, we describe an essential function of the X-linked homolog of an ancestral X-Y gene pair, Kdm5c-Kdm5d, in the expression of Xist lncRNA, which is required for stable X-inactivation. Ablation of Kdm5c function in females results in a significant reduction in Xist RNA expression. Kdm5c encodes a demethylase that enhances Xist expression by converting histone H3K4me2/3 modifications into H3K4me1. Ectopic expression of mouse and human KDM5C, but not the Y-linked homolog KDM5D, induces Xist in male mouse embryonic stem cells (mESCs). Similarly, marsupial (opossum) Kdm5c but not Kdm5d also upregulates Xist in male mESCs, despite marsupials lacking Xist, suggesting that the KDM5C function that activates Xist in eutherians is strongly conserved and predates the divergence of eutherian and metatherian mammals. In support, prototherian (platypus) Kdm5c also induces Xist in male mESCs. Together, our data suggest that eutherian mammals co-opted the ancestral demethylase KDM5C during sex chromosome evolution to upregulate Xist for the female-specific induction of X-inactivation.

Inversion of a topological domain leads to restricted changes in its gene expression and affects interdomain communication

The interplay between the topological organization of the genome and the regulation of gene expression remains unclear. Depletion of molecular factors (e.g. CTCF) underlying topologically associating domains (TADs) leads to modest alterations in gene expression, whereas genomic rearrangements involving TAD boundaries disrupt normal gene expression and can lead to pathological phenotypes. Here, we targeted the TAD neighboring that of the noncoding transcript Xist, which controls X-chromosome inactivation. Inverting 245 kb within the TAD led to expected rearrangement of CTCF-based contacts but revealed heterogeneity in the 'contact' potential of different CTCF sites. Expression of most genes therein remained unaffected in mouse embryonic stem cells and during differentiation. Interestingly, expression of Xist was ectopically upregulated. The same inversion in mouse embryos led to biased Xist expression. Smaller inversions and deletions of CTCF clusters led to similar results: rearrangement of contacts and limited changes in local gene expression, but significant changes in Xist expression in embryos. Our study suggests that the wiring of regulatory interactions within a TAD can influence the expression of genes in neighboring TADs, highlighting the existence of mechanisms of inter-TAD communication.

Long noncoding RNA XIST: Mechanisms for X chromosome inactivation, roles in sex-biased diseases, and therapeutic opportunities

Sexual dimorphism has been reported in various human diseases including autoimmune diseases, neurological diseases, pulmonary arterial hypertension, and some types of cancers, although the underlying mechanisms remain poorly understood. The long noncoding RNA (lncRNA) X-inactive specific transcript (XIST) is involved in X chromosome inactivation (XCI) in female placental mammals, a process that ensures the balanced expression dosage of X-linked genes between sexes. XIST is abnormally expressed in many sex-biased diseases. In addition, escape from XIST-mediated XCI and skewed XCI also contribute to sex-biased diseases. Therefore, its expression or modification can be regarded as a biomarker for the diagnosis and prognosis of many sex-biased diseases. Genetic manipulation of XIST expression can inhibit the progression of some of these diseases in animal models, and therefore XIST has been proposed as a potential therapeutic target. In this manuscript, we summarize the current knowledge about the mechanisms for XIST-mediated XCI and the roles of XIST in sex-biased diseases, and discuss potential therapeutic strategies targeting XIST.

Mechanisms of Choice in X-Chromosome Inactivation

Early in development, placental and marsupial mammals harbouring at least two X chromosomes per nucleus are faced with a choice that affects the rest of their lives: which of those X chromosomes to transcriptionally inactivate. This choice underlies phenotypical diversity in the composition of tissues and organs and in their response to the environment, and can determine whether an individual will be healthy or affected by an X-linked disease. Here, we review our current understanding of the process of choice during X-chromosome inactivation and its implications, focusing on the strategies evolved by different mammalian lineages and on the known and unknown molecular mechanisms and players involved.

Zfp57 Exerts Maternal and Sexually Dimorphic Effects on Genomic Imprinting

Zfp57 has both maternal and zygotic functions in mouse. It maintains genomic imprinting at most known imprinted regions and controls allelic expression of the target imprinted genes in mouse embryos. The DNA methylation imprint at many imprinting control regions (ICRs) is lost when both maternal and zygotic Zfp57 are absent in Zfp57 maternal–zygotic mutant mouse embryos. Interestingly, we found that DNA methylation at a few ICRs was partially lost without maternal Zfp57 in Zfp57 heterozygous mouse embryos derived from Zfp57 homozygous female mice. This suggests that maternal Zfp57 is essential for the maintenance of DNA methylation at a small subset of imprinted regions in mouse embryos. This maternal effect of Zfp57 was applied to allelic expression switch as well as expression levels of the corresponding imprinted genes. It is rather surprising that DNA methylation imprint was affected differently at Rasgrf1 and AK008011 imprinted regions in the female or male Zfp57 maternal–zygotic mutant embryos, with more significant loss of DNA methylation observed in the male mutant embryos. Loss of ZFP57 resulted in gender-specific differences in allelic expression switch and expression level changes of some imprinted genes in female or male mutant embryos. These results indicate maternal and sexually dimorphic effects of ZFP57 on genomic imprinting in mouse.

Capture-C: a modular and flexible approach for high-resolution chromosome conformation capture

Chromosome conformation capture (3C) methods measure the spatial proximity between DNA elements in the cell nucleus. Many methods have been developed to sample 3C material, including the Capture-C family of protocols. Capture-C methods use oligonucleotides to enrich for interactions of interest from sequencing-ready 3C libraries. This approach is modular and has been adapted and optimized to work for sampling of disperse DNA elements (NuTi Capture-C), including from low cell inputs (LI Capture-C), as well as to generate Hi-C like maps for specific regions of interest (Tiled-C) and to interrogate multiway interactions (Tri-C). We present the design, experimental protocol and analysis pipeline for NuTi Capture-C in addition to the variations for generation of LI Capture-C, Tiled-C and Tri-C data. The entire procedure can be performed in 3 weeks and requires standard molecular biology skills and equipment, access to a next-generation sequencing platform, and basic bioinformatic skills. Implemented with other sequencing technologies, these methods can be used to identify regulatory interactions and to compare the structural organization of the genome in different cell types and genetic models.

Gene regulation in time and space during X-chromosome inactivation

X-chromosome inactivation (XCI) is the epigenetic mechanism that ensures X-linked dosage compensation between cells of females (XX karyotype) and males (XY). XCI is essential for female embryos to survive through development and requires the accurate spatiotemporal regulation of many different factors to achieve remarkable chromosome-wide gene silencing. As a result of XCI, the active and inactive X chromosomes are functionally and structurally different, with the inactive X chromosome undergoing a major conformational reorganization within the nucleus. In this Review, we discuss the multiple layers of genetic and epigenetic regulation that underlie initiation of XCI during development and then maintain it throughout life, in light of the most recent findings in this rapidly advancing field. We discuss exciting new insights into the regulation of X inactive-specific transcript (XIST), the trigger and master regulator of XCI, and into the mechanisms and dynamics that underlie the silencing of nearly all X-linked genes. Finally, given the increasing interest in understanding the impact of chromosome organization on gene regulation, we provide an overview of the factors that are thought to reshape the 3D structure of the inactive X chromosome and of the relevance of such structural changes for XCI establishment and maintenance.

The lncRNAs at X Chromosome Inactivation Center: Not Just a Matter of Sex Dosage Compensation

Non-coding RNAs (ncRNAs) constitute the majority of the transcriptome, as the result of pervasive transcription of the mammalian genome. Different RNA species, such as lncRNAs, miRNAs, circRNA, mRNAs, engage in regulatory networks based on their reciprocal interactions, often in a competitive manner, in a way denominated “competing endogenous RNA (ceRNA) networks” (“ceRNET”): miRNAs and other ncRNAs modulate each other, since miRNAs can regulate the expression of lncRNAs, which in turn regulate miRNAs, titrating their availability and thus competing with the binding to other RNA targets. The unbalancing of any network component can derail the entire regulatory circuit acting as a driving force for human diseases, thus assigning “new” functions to “old” molecules. This is the case of XIST, the lncRNA characterized in the early 1990s and well known as the essential molecule for X chromosome inactivation in mammalian females, thus preventing an imbalance of X-linked gene expression between females and males. Currently, literature concerning XIST biology is becoming dominated by miRNA associations and they are also gaining prominence for other lncRNAs produced by the X-inactivation center. This review discusses the available literature to explore possible novel functions related to ceRNA activity of lncRNAs produced by the X-inactivation center, beyond their role in dosage compensation, with prospective implications for emerging gender-biased functions and pathological mechanisms.

Distal and proximal cis-regulatory elements sense X chromosome dosage and developmental state at the Xist locus

Developmental genes such as Xist, which initiates X chromosome inactivation, are controlled by complex cis-regulatory landscapes, which decode multiple signals to establish specific spatiotemporal expression patterns. Xist integrates information on X chromosome dosage and developmental stage to trigger X inactivation in the epiblast specifically in female embryos. Through a pooled CRISPR screen in differentiating mouse embryonic stem cells, we identify functional enhancer elements of Xist at the onset of random X inactivation. Chromatin profiling reveals that X-dosage controls the promoter-proximal region, while differentiation cues activate several distal enhancers. The strongest distal element lies in an enhancer cluster associated with a previously unannotated Xist-enhancing regulatory transcript, which we named Xert. Developmental cues and X-dosage are thus decoded by distinct regulatory regions, which cooperate to ensure female-specific Xist upregulation at the correct developmental time. With this study, we start to disentangle how multiple, functionally distinct regulatory elements interact to generate complex expression patterns in mammals.

Lesson of regulatory anatomy: Using integrated functional genomics to dissect the X-inactivation center

Gjaltema et al. (2021) perform systematic screens to identify the long-sought cis-regulatory elements of Xist. They discover that distal elements give Xist a boost as cells exit pluripotency, while proximal elements restrict Xist expression to cells with two X chromosomes.

RIF1 and KAP1 differentially regulate the choice of inactive versus active X chromosomes

The onset of random X chromosome inactivation in mouse requires the switch from a symmetric to an asymmetric state, where the identities of the future inactive and active X chromosomes are assigned. This process is known as X chromosome choice. Here, we show that RIF1 and KAP1 are two fundamental factors for the definition of this transcriptional asymmetry. We found that at the onset of differentiation of mouse embryonic stem cells (mESCs), biallelic up-regulation of the long non-coding RNA Tsix weakens the symmetric association of RIF1 with the Xist promoter. The Xist allele maintaining the association with RIF1 goes on to up-regulate Xist RNA expression in a RIF1-dependent manner. Conversely, the promoter that loses RIF1 gains binding of KAP1, and KAP1 is required for the increase in Tsix levels preceding the choice. We propose that the mutual exclusion of Tsix and RIF1, and of RIF1 and KAP1, at the Xist promoters establish a self-sustaining loop that transforms an initially stochastic event into a stably inherited asymmetric X-chromosome state.

LncRNA FTX promotes the malignant progression of colorectal cancer by regulating the miR-214-5p-JAG1 axis

Background

Long non-coding RNAs (lncRNAs) have recently been found to be vital regulators of various cancers, including colorectal cancer (CRC). It has been previously reported that the dysregulated expression of lncRNA Five prime to Xist (FTX) is involved in carcinogenesis. However, the role of lncRNA FTX in the progression of CRC is still unclear.

Methods

Fluorescence in situ hybridization (FISH) was used to detect the expression of lncRNA FTX and miR-214-5p in CRC tissues. Cell Counting Kit-8 assay, transwell assay, wound-healing assay, and proliferation assay were used to explore the function of lncRNA FTX in CRC cells. Quantitative real-time polymerase chain reaction (qRT-PCR), western blotting, and luciferase reporter assay were used to confirm the relationship between lncRNA FTX and miR-214-5p-jagged canonical Notch ligand 1 (JAG1). We further explored the role of lncRNA FTX in vivo using xenograft tumor assay.

Results

lncRNA FTX was found to be upregulated in CRC tissues by FISH. The downregulation of endogenous lncRNA FTX expression inhibited CRC cell proliferation, migration, and invasion. Mechanistically, lncRNA FTX sequestered miR-214-5p and thus released its repression on JAG1, driving the malignant progression of CRC.

Conclusions

These findings give rise to a new perspective, the lncRNA FTX-miR-214-5p-JAG1 regulatory axis, in exploring the cancer-promoting mechanism of lncRNA FTX in CRC.

First female with Allan-Herndon-Dudley syndrome and partial deletion of X-inactivation center

Allan-Herndon-Dudley is an X-linked recessive syndrome caused by pathogenic variants in the SLC16A2 gene. Clinical manifestations are a consequence of impaired thyroid metabolism and aberrant transport of thyroid hormones to the brain. Carrier females are generally asymptomatic and may show subtle symptoms of the disease. We describe a female with a complete Allan-Herndon-Dudley phenotype, carrying a de novo 543-kb deletion of the X chromosome. The deletion encompasses exon 1 of the SLC16A2 gene and JPX and FTX genes; it is known that the latter two genes participate in the X-inactivation process upregulating XIST gene expression. Subsequent studies in the patient demonstrated the preferential expression of the X chromosome with the JPX and FTX deletion.

Revisiting the consequences of deleting the X inactivation center

Mammalian cells equalize X-linked dosages between the male (XY) and female (XX) sexes by silencing one X chromosome in the female sex. This process, known as "X chromosome inactivation" (XCI), requires a master switch within the X inactivation center (Xic). The Xic spans several hundred kilobases in the mouse and includes a number of regulatory noncoding genes that produce functional transcripts. Over three decades, transgenic and deletional analyses have demonstrated both the necessity and sufficiency of the Xic to induce XCI, including the steps of X chromosome counting, choice, and initiation of whole-chromosome silencing. One recent study, however, reported that deleting the noncoding sequences of the Xic surprisingly had no effect for XCI and attributed a sufficiency to drive counting to the coding gene, Rnf12/Rlim Here, we revisit the question by creating independent Xic deletion cell lines. Multiple independent clones carrying heterozygous deletions of the Xic display an inability to up-regulate Xist expression, consistent with a counting defect. This defect is rescued by a second site mutation in Tsix occurring in trans, bypassing the defect in counting. These findings reaffirm the essential nature of noncoding Xic elements for the initiation of XCI.

Linking Chromosomal Silencing With Xist Expression From Autosomal Integrated Transgenes

Xist is the master regulator of X-Chromosome Inactivation (XCI), the mammalian dosage compensation mechanism that silences one of the two X chromosomes in a female cell. XCI is established during early embryonic development. Xist transgene (Tg) integrated into an autosome can induce transcriptional silencing of flanking genes; however, the effect and mechanism of Xist RNA on autosomal sequence silencing remain elusive. In this study, we investigate an autosomal integration of Xist Tg that is compatible with mouse viability but causes male sterility in homozygous transgenic mice. We observed ectopic Xist expression in the transgenic male cells along with a transcriptional reduction of genes clustered in four segments on the mouse chromosome 1 (Chr 1). RNA/DNA Fluorescent in situ Hybridization (FISH) and chromosome painting confirmed that Xist Tg is associated with chromosome 1. To determine the spreading mechanism of autosomal silencing induced by Xist Tg on Chr 1, we analyzed the positions of the transcriptionally repressed chromosomal sequences relative to the Xist Tg location inside the cell nucleus. Our results show that the transcriptionally repressed chromosomal segments are closely proximal to Xist Tg in the three-dimensional nucleus space. Our findings therefore support a model that Xist directs and maintains long-range transcriptional silencing facilitated by the three-dimensional chromosome organization.

Chromosome compartments on the inactive X guide TAD formation independently of transcription during X-reactivation

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.

Dosage Sensing, Threshold Responses, and Epigenetic Memory: A Systems Biology Perspective on Random X-Chromosome Inactivation

X-chromosome inactivation ensures dosage compensation between the sexes in mammals by randomly choosing one out of the two X chromosomes in females for inactivation. This process imposes a plethora of questions: How do cells count their X chromosome number and ensure that exactly one stays active? How do they randomly choose one of two identical X chromosomes for inactivation? And how do they stably maintain this state of monoallelic expression? Here, different regulatory concepts and their plausibility are evaluated in the context of theoretical studies that have investigated threshold behavior, ultrasensitivity, and bistability through mathematical modeling. It is discussed how a twofold difference between a single and a double dose of X-linked genes might be converted to an all-or-nothing response and how mutually exclusive expression can be initiated and maintained. Finally, candidate factors that might mediate the proposed regulatory principles are reviewed.

Long noncoding RNA network: Novel insight into hepatocellular carcinoma metastasis (Review)

Hepatocellular carcinoma (HCC) is one of the most common, aggressive malignancies with poor prognosis and high mortality. Although great progress has been made in recent decades, overall survival of HCC patients remains unsatisfactory due to high recurrence and metastasis. Accordingly, understanding and clarifying the underlying molecular mechanisms of metastasis has become increasingly important. Recently, accumulated reports have supported that long noncoding RNAs (lncRNAs) are dysregulated in HCC and are involved in various pivotal biological processes, including metastasis. The aim of this review was to investigate the dysregulation of lncRNAs in HCC and their function as oncogenes or tumour suppressors. Furthermore, reciprocal regulatory networks between lncRNAs and various molecules that were identified in HCC metastasis, including regulating epithelial-mesenchymal transition (EMT), controlling metastasis-associated genes, and regulating tumour angiogenesis were examined. Numerous reports and information on lncRNAs may help identify lncRNAs that are potential novel diagnostic markers, prognostic markers and therapeutic targets.

Chd8 regulates X chromosome inactivation in mouse through fine-tuning control of Xist expression

Female mammals achieve dosage compensation by inactivating one of their two X chromosomes during development, a process entirely dependent on Xist, an X-linked long non-coding RNA (lncRNA). At the onset of X chromosome inactivation (XCI), Xist is up-regulated and spreads along the future inactive X chromosome. Contextually, it recruits repressive histone and DNA modifiers that transcriptionally silence the X chromosome. Xist regulation is tightly coupled to differentiation and its expression is under the control of both pluripotency and epigenetic factors. Recent evidence has suggested that chromatin remodelers accumulate at the X Inactivation Center (XIC) and here we demonstrate a new role for Chd8 in Xist regulation in differentiating ES cells, linked to its control and prevention of spurious transcription factor interactions occurring within Xist regulatory regions. Our findings have a broader relevance, in the context of complex, developmentally-regulated gene expression.

Chromatin Regulator SPEN/SHARP in X Inactivation and Disease

Simple Summary

Carcinogenesis is a multistep process involving not only the activation of oncogenes and disabling tumor suppressor genes, but also epigenetic modulation of gene expression. X chromosome inactivation (XCI) is a paradigm to study heterochromatin formation and maintenance. The double dosage of X chromosomal genes in female mammals is incompatible with early development. XCI is an excellent model system for understanding the establishment of facultative heterochromatin initiated by the expression of a 17,000 nt long non-coding RNA, known as Xinactivespecifictranscript (Xist), on the X chromosome. This review focuses on the molecular mechanisms of how epigenetic modulators act in a step-wise manner to establish facultative heterochromatin, and we put these in the context of cancer biology and disease. An in depth understanding of XCI will allow a better characterization of particular types of cancer and hopefully facilitate the development of novel epigenetic therapies.

Abstract

Enzymes, such as histone methyltransferases and demethylases, histone acetyltransferases and deacetylases, and DNA methyltransferases are known as epigenetic modifiers that are often implicated in tumorigenesis and disease. One of the best-studied chromatin-based mechanism is X chromosome inactivation (XCI), a process that establishes facultative heterochromatin on only one X chromosome in females and establishes the right dosage of gene expression. The specificity factor for this process is the long non-coding RNA Xinactivespecifictranscript (Xist), which is upregulated from one X chromosome in female cells. Subsequently, Xist is bound by the corepressor SHARP/SPEN, recruiting and/or activating histone deacetylases (HDACs), leading to the loss of active chromatin marks such as H3K27ac. In addition, polycomb complexes PRC1 and PRC2 establish wide-spread accumulation of H3K27me3 and H2AK119ub1 chromatin marks. The lack of active marks and establishment of repressive marks set the stage for DNA methyltransferases (DNMTs) to stably silence the X chromosome. Here, we will review the recent advances in understanding the molecular mechanisms of how heterochromatin formation is established and put this into the context of carcinogenesis and disease.