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Sakamuri A, Visniauskas B, Kilanowski-Doroh I, McNally AB, Imulinde A, Kamau A, Sengottaian D, McLachlan J, Anguera M, Mauvais-Jarvis F, Lindsey SH, Ogola BO. Testosterone deficiency promotes arterial stiffening independent of sex chromosome complement. Biol Sex Differ 2024; 15:46. [PMID: 38845040 PMCID: PMC11155160 DOI: 10.1186/s13293-024-00624-0] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 10/02/2023] [Accepted: 05/28/2024] [Indexed: 06/09/2024] Open
Abstract
BACKGROUND Sex hormones and sex chromosomes play a vital role in cardiovascular disease. Testosterone plays a crucial role in men's health. Lower testosterone level is associated with cardiovascular and cardiometabolic diseases, including inflammation, atherosclerosis, and type 2 diabetes. Testosterone replacement is beneficial or neutral to men's cardiovascular health. Testosterone deficiency is associated with cardiovascular events. Testosterone supplementation to hypogonadal men improves libido, increases muscle strength, and enhances mood. We hypothesized that sex chromosomes (XX and XY) interaction with testosterone plays a role in arterial stiffening. METHODS We used four core genotype male mice to understand the inherent contribution of sex hormones and sex chromosome complement in arterial stiffening. Age-matched mice were either gonadal intact or castrated at eight weeks plus an additional eight weeks to clear endogenous sex hormones. This was followed by assessing blood pressure, pulse wave velocity, echocardiography, and ex vivo passive vascular mechanics. RESULTS Arterial stiffening but not blood pressure was more significant in castrated than testes-intact mice independent of sex chromosome complement. Castrated mice showed a leftward shift in stress-strain curves and carotid wall thinning. Sex chromosome complement (XX) in the absence of testosterone increased collagen deposition in the aorta and Kdm6a gene expression. CONCLUSION Testosterone deprivation increases arterial stiffening and vascular wall remodeling. Castration increases Col1α1 in male mice with XX sex chromosome complement. Our study shows decreased aortic contractile genes in castrated mice with XX than XY sex chromosomes.
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Affiliation(s)
- Anil Sakamuri
- Vascular Biology Center and Department of Medicine, Medical College of Georgia at Augusta University, Augusta, GA, USA
| | | | | | | | - Ariane Imulinde
- Department of Pharmacology, Tulane University, New Orleans, LA, USA
| | - Anne Kamau
- Vascular Biology Center and Department of Medicine, Medical College of Georgia at Augusta University, Augusta, GA, USA
| | - Divya Sengottaian
- Vascular Biology Center and Department of Medicine, Medical College of Georgia at Augusta University, Augusta, GA, USA
| | - John McLachlan
- Department of Pharmacology, Tulane University, New Orleans, LA, USA
| | - Montserrat Anguera
- Division of Rheumatology, Department of Medicine, Perelman School of Medicine at the University of Pennsylvania, Philadelphia, PA, USA
| | - Franck Mauvais-Jarvis
- Tulane Center of Excellence in Sex-Based Biology & Medicine, New Orleans, LA, USA
- Southeast Louisiana Veterans Healthcare System Medical Center, New Orleans, LA, USA
- Deming Department of Medicine, Section of Endocrinology and Metabolism, Tulane University, New Orleans, LA, USA
| | - Sarah H Lindsey
- Department of Pharmacology, Tulane University, New Orleans, LA, USA
- Tulane Center of Excellence in Sex-Based Biology & Medicine, New Orleans, LA, USA
| | - Benard O Ogola
- Vascular Biology Center and Department of Medicine, Medical College of Georgia at Augusta University, Augusta, GA, USA.
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Luchsinger-Morcelle SJ, Gribnau J, Mira-Bontenbal H. Orchestrating Asymmetric Expression: Mechanisms behind Xist Regulation. EPIGENOMES 2024; 8:6. [PMID: 38390897 PMCID: PMC10885031 DOI: 10.3390/epigenomes8010006] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/20/2023] [Revised: 01/19/2024] [Accepted: 01/22/2024] [Indexed: 02/24/2024] Open
Abstract
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.
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Affiliation(s)
| | - Joost Gribnau
- Department of Developmental Biology, Erasmus MC, University Medical Center, 3015 GD Rotterdam, The Netherlands
| | - Hegias Mira-Bontenbal
- Department of Developmental Biology, Erasmus MC, University Medical Center, 3015 GD Rotterdam, The Netherlands
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Achiro JM, Tao Y, Gao F, Lin CH, Watanabe M, Neumann S, Coppola G, Black DL, Martin KC. Aging differentially alters the transcriptome and landscape of chromatin accessibility in the male and female mouse hippocampus. Front Mol Neurosci 2024; 17:1334862. [PMID: 38318533 PMCID: PMC10839115 DOI: 10.3389/fnmol.2024.1334862] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/07/2023] [Accepted: 01/02/2024] [Indexed: 02/07/2024] Open
Abstract
Aging-related memory impairment and pathological memory disorders such as Alzheimer's disease differ between males and females, and yet little is known about how aging-related changes in the transcriptome and chromatin environment differ between sexes in the hippocampus. To investigate this question, we compared the chromatin accessibility landscape and gene expression/alternative splicing pattern of young adult and aged mouse hippocampus in both males and females using ATAC-seq and RNA-seq. We detected significant aging-dependent changes in the expression of genes involved in immune response and synaptic function and aging-dependent changes in the alternative splicing of myelin sheath genes. We found significant sex-bias in the expression and alternative splicing of hundreds of genes, including aging-dependent female-biased expression of myelin sheath genes and aging-dependent male-biased expression of genes involved in synaptic function. Aging was associated with increased chromatin accessibility in both male and female hippocampus, especially in repetitive elements, and with an increase in LINE-1 transcription. We detected significant sex-bias in chromatin accessibility in both autosomes and the X chromosome, with male-biased accessibility enriched at promoters and CpG-rich regions. Sex differences in gene expression and chromatin accessibility were amplified with aging, findings that may shed light on sex differences in aging-related and pathological memory loss.
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Affiliation(s)
- Jennifer M. Achiro
- Department of Biological Chemistry, David Geffen School of Medicine, UCLA, Los Angeles, CA, United States
| | - Yang Tao
- Department of Biological Chemistry, David Geffen School of Medicine, UCLA, Los Angeles, CA, United States
| | - Fuying Gao
- Department of Psychiatry and Biobehavioral Sciences, Semel Institute for Neuroscience and Human Behavior, David Geffen School of Medicine, UCLA, Los Angeles, CA, United States
| | - Chia-Ho Lin
- Department of Microbiology, Immunology and Molecular Genetics, UCLA, Los Angeles, CA, United States
| | - Marika Watanabe
- Department of Biological Chemistry, David Geffen School of Medicine, UCLA, Los Angeles, CA, United States
| | - Sylvia Neumann
- Department of Biological Chemistry, David Geffen School of Medicine, UCLA, Los Angeles, CA, United States
| | - Giovanni Coppola
- Department of Psychiatry and Biobehavioral Sciences, Semel Institute for Neuroscience and Human Behavior, David Geffen School of Medicine, UCLA, Los Angeles, CA, United States
| | - Douglas L. Black
- Department of Microbiology, Immunology and Molecular Genetics, UCLA, Los Angeles, CA, United States
| | - Kelsey C. Martin
- Department of Biological Chemistry, David Geffen School of Medicine, UCLA, Los Angeles, CA, United States
- Department of Psychiatry and Biobehavioral Sciences, Semel Institute for Neuroscience and Human Behavior, David Geffen School of Medicine, UCLA, Los Angeles, CA, United States
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Matsumoto A, Kano S, Kobayashi N, Matsuki M, Furukawa R, Yamagishi H, Yoshinari H, Nakata W, Wakabayashi H, Tsuda H, Watanabe K, Takahashi H, Yamagata T, Matsumura T, Osaka H, Mori H, Iwamoto S. Unfavorable switching of skewed X chromosome inactivation leads to Menkes disease in a female infant. Sci Rep 2024; 14:440. [PMID: 38172222 PMCID: PMC10764769 DOI: 10.1038/s41598-023-50668-2] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/05/2023] [Accepted: 12/22/2023] [Indexed: 01/05/2024] Open
Abstract
Menkes disease is an X-linked disorder of copper metabolism caused by mutations in the ATP7A gene, and female carriers are usually asymptomatic. We describe a 7-month-old female patient with severe intellectual disability, epilepsy, and low levels of serum copper and ceruloplasmin. While heterozygous deletion of exons 16 and 17 of the ATP7A gene was detected in the proband, her mother, and her grandmother, only the proband suffered from Menkes disease clinically. Intriguingly, X chromosome inactivation (XCI) analysis demonstrated that the grandmother and the mother showed skewing of XCI toward the allele with the ATP7A deletion and that the proband had extremely skewed XCI toward the normal allele, resulting in exclusive expression of the pathogenic ATP7A mRNA transcripts. Expression bias analysis and recombination mapping of the X chromosome by the combination of whole genome and RNA sequencing demonstrated that meiotic recombination occurred at Xp21-p22 and Xq26-q28. Assuming that a genetic factor on the X chromosome enhanced or suppressed XCI of its allele, the factor must be on either of the two distal regions derived from her grandfather. Although we were unable to fully uncover the molecular mechanism, we concluded that unfavorable switching of skewed XCI caused Menkes disease in the proband.
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Affiliation(s)
- Ayumi Matsumoto
- Division of Human Genetics, Center for Molecular Medicine, Jichi Medical University, Shimotsuke, Tochigi, Japan
- Department of Pediatrics, Jichi Medical University, Shimotsuke, Tochigi, Japan
| | - Shintaro Kano
- Department of Radiology, Jichi Medical University, Shimotsuke, Tochigi, Japan
| | - Natsumi Kobayashi
- Division of Human Genetics, Center for Molecular Medicine, Jichi Medical University, Shimotsuke, Tochigi, Japan
| | - Mitsuru Matsuki
- Department of Radiology, Jichi Medical University, Shimotsuke, Tochigi, Japan
| | - Rieko Furukawa
- Department of Radiology, Jichi Medical University, Shimotsuke, Tochigi, Japan
| | - Hirokazu Yamagishi
- Department of Pediatrics, Jichi Medical University, Shimotsuke, Tochigi, Japan
| | - Hiroki Yoshinari
- Department of Pediatrics, Jichi Medical University, Shimotsuke, Tochigi, Japan
| | - Waka Nakata
- Department of Radiology, Jichi Medical University, Shimotsuke, Tochigi, Japan
| | - Hiroko Wakabayashi
- Division of Human Genetics, Center for Molecular Medicine, Jichi Medical University, Shimotsuke, Tochigi, Japan
| | - Hidetoshi Tsuda
- Division of Human Genetics, Center for Molecular Medicine, Jichi Medical University, Shimotsuke, Tochigi, Japan
| | - Kazuhisa Watanabe
- Division of Human Genetics, Center for Molecular Medicine, Jichi Medical University, Shimotsuke, Tochigi, Japan
| | - Hironori Takahashi
- Department of Obstetrics and Gynecology, Jichi Medical University, Shimotsuke, Tochigi, Japan
| | - Takanori Yamagata
- Department of Pediatrics, Jichi Medical University, Shimotsuke, Tochigi, Japan
| | - Takayoshi Matsumura
- Division of Human Genetics, Center for Molecular Medicine, Jichi Medical University, Shimotsuke, Tochigi, Japan
- Division of Cardiovascular Medicine, Department of Medicine, Jichi Medical University, Shimotsuke, Tochigi, Japan
| | - Hitoshi Osaka
- Department of Pediatrics, Jichi Medical University, Shimotsuke, Tochigi, Japan
| | - Harushi Mori
- Department of Radiology, Jichi Medical University, Shimotsuke, Tochigi, Japan
| | - Sadahiko Iwamoto
- Division of Human Genetics, Center for Molecular Medicine, Jichi Medical University, Shimotsuke, Tochigi, Japan.
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5
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Sakamuri A, Visniauskas B, Kilanowski-Doroh I, McNally A, Imulinde-Sugi A, Kamau A, Sengottaian D, McLachlan J, Anguera M, Mauvais-Jarvis F, Lindsey S, Ogola BO. Testosterone Deficiency Promotes Arterial Stiffening Independent of Sex Chromosome Complement. RESEARCH SQUARE 2023:rs.3.rs-3370040. [PMID: 37886462 PMCID: PMC10602149 DOI: 10.21203/rs.3.rs-3370040/v1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/28/2023]
Abstract
Background Testosterone plays a vital role in men's health. Lower testosterone level is associated with cardiovascular and cardiometabolic diseases, including inflammation, atherosclerosis, and type 2 diabetes. Testosterone replacement is beneficial or neutral to men's cardiovascular health. Testosterone deficiency is associated with cardiovascular events. Testosterone supplementation to hypogonadal men improves libido, increases muscle strength, and enhances mood. We hypothesized that sex chromosomes (XX and XY) interaction with testosterone plays a role in arterial stiffening. Methods We used four core genotype male mice to understand the inherent contribution of sex hormones and sex chromosome complement in arterial stiffening. Age-matched mice were either gonadal intact or castrated for eight weeks, followed by an assessment of blood pressure, pulse wave velocity, echocardiography, and ex vivo passive vascular mechanics. Results Arterial stiffening but not blood pressure was more significant in castrated than testes-intact mice independent of sex chromosome complement. Castrated mice showed a leftward shift in stress-strain curves and carotid wall thinning. Sex chromosome complement (XX) in the absence of testosterone increased collagen deposition in the aorta and Kdm6a gene expression. Conclusion Testosterone deprivation increases arterial stiffening and vascular wall remodeling. Castration increases Col1α1 in male mice with XX sex chromosome complement. Our study shows decreased aortic contractile genes in castrated mice with XX than XY sex chromosomes.
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Affiliation(s)
| | | | | | | | | | - Anne Kamau
- Augusta University Medical College of Georgia
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6
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Yang Q, Lo TW, Brejc K, Schartner C, Ralston EJ, Lapidus DM, Meyer BJ. X-chromosome target specificity diverged between dosage compensation mechanisms of two closely related Caenorhabditis species. eLife 2023; 12:e85413. [PMID: 36951246 PMCID: PMC10076027 DOI: 10.7554/elife.85413] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/07/2022] [Accepted: 03/21/2023] [Indexed: 03/24/2023] Open
Abstract
An evolutionary perspective enhances our understanding of biological mechanisms. Comparison of sex determination and X-chromosome dosage compensation mechanisms between the closely related nematode species Caenorhabditis briggsae (Cbr) and Caenorhabditis elegans (Cel) revealed that the genetic regulatory hierarchy controlling both processes is conserved, but the X-chromosome target specificity and mode of binding for the specialized condensin dosage compensation complex (DCC) controlling X expression have diverged. We identified two motifs within Cbr DCC recruitment sites that are highly enriched on X: 13 bp MEX and 30 bp MEX II. Mutating either MEX or MEX II in an endogenous recruitment site with multiple copies of one or both motifs reduced binding, but only removing all motifs eliminated binding in vivo. Hence, DCC binding to Cbr recruitment sites appears additive. In contrast, DCC binding to Cel recruitment sites is synergistic: mutating even one motif in vivo eliminated binding. Although all X-chromosome motifs share the sequence CAGGG, they have otherwise diverged so that a motif from one species cannot function in the other. Functional divergence was demonstrated in vivo and in vitro. A single nucleotide position in Cbr MEX can determine whether Cel DCC binds. This rapid divergence of DCC target specificity could have been an important factor in establishing reproductive isolation between nematode species and contrasts dramatically with the conservation of target specificity for X-chromosome dosage compensation across Drosophila species and for transcription factors controlling developmental processes such as body-plan specification from fruit flies to mice.
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Affiliation(s)
- Qiming Yang
- Howard Hughes Medical InstituteBerkeleyUnited States
- Department of Molecular and Cell Biology, University of California, BerkeleyBerkeleyUnited States
| | - Te-Wen Lo
- Howard Hughes Medical InstituteBerkeleyUnited States
- Department of Molecular and Cell Biology, University of California, BerkeleyBerkeleyUnited States
| | - Katjuša Brejc
- Howard Hughes Medical InstituteBerkeleyUnited States
- Department of Molecular and Cell Biology, University of California, BerkeleyBerkeleyUnited States
| | - Caitlin Schartner
- Howard Hughes Medical InstituteBerkeleyUnited States
- Department of Molecular and Cell Biology, University of California, BerkeleyBerkeleyUnited States
| | - Edward J Ralston
- Howard Hughes Medical InstituteBerkeleyUnited States
- Department of Molecular and Cell Biology, University of California, BerkeleyBerkeleyUnited States
| | - Denise M Lapidus
- Howard Hughes Medical InstituteBerkeleyUnited States
- Department of Molecular and Cell Biology, University of California, BerkeleyBerkeleyUnited States
| | - Barbara J Meyer
- Howard Hughes Medical InstituteBerkeleyUnited States
- Department of Molecular and Cell Biology, University of California, BerkeleyBerkeleyUnited States
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Combinatorial clustering of distinct DNA motifs directs synergistic binding of Caenorhabditis elegans dosage compensation complex to X chromosomes. Proc Natl Acad Sci U S A 2022; 119:e2211642119. [PMID: 36067293 PMCID: PMC9477397 DOI: 10.1073/pnas.2211642119] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022] Open
Abstract
Diverse regulatory mechanisms balance X-chromosome gene expression between sexes in mammals, fruit flies, and nematodes (XY/XO males and XX females/hermaphrodites). We identify DNA motifs on X that recruit dosage compensation complexes (DCCs) in nematode hermaphrodites to reduce X-chromosome expression. Recruitment sites on X, but not regions on autosomes, contain diverse combinations of different motifs or multiple copies of one motif. DCC binding studies in vivo and in vitro of wild-type and mutant X-recruitment sites validate motif usage. We find that clustering of motifs in different combinations with appropriate orientation and spacing promotes synergy in DCC binding, thereby triggering DCC assembly specifically along X. We demonstrate how regulatory complexes can be recruited across an entire chromosome to control its gene expression. Organisms that count X-chromosome number to determine sex utilize dosage compensation mechanisms to balance X-gene expression between sexes. Typically, a regulatory complex is recruited to X chromosomes of one sex to modulate gene expression. A major challenge is to determine the mechanisms that target regulatory complexes specifically to X. Here, we identify critical X-sequence motifs in Caenorhabditis elegans that act synergistically in hermaphrodites to direct X-specific recruitment of the dosage compensation complex (DCC), a condensin complex. We find two DNA motifs that collaborate with a previously defined 12-bp motif called MEX (motif enriched on X) to mediate binding: MEX II, a 26-bp X-enriched motif and Motif C, a 9-bp motif that lacks X enrichment. Inserting both MEX and MEX II into a new location on X creates a DCC binding site equivalent to an endogenous recruitment site, but inserting only MEX or MEX II alone does not. Moreover, mutating MEX, MEX II, or Motif C in endogenous recruitment sites with multiple different motifs dramatically reduces DCC binding in vivo to nearly the same extent as mutating all motifs. Changing the orientation or spacing of motifs also reduces DCC binding. Hence, synergy in DCC binding via combinatorial clustering of motifs triggers DCC assembly specifically on X chromosomes. Using an in vitro DNA binding assay, we refine the features of motifs and flanking sequences that are critical for DCC binding. Our work reveals general principles by which regulatory complexes can be recruited across an entire chromosome to control its gene expression.
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Meyer BJ. The X chromosome in C. elegans sex determination and dosage compensation. Curr Opin Genet Dev 2022; 74:101912. [PMID: 35490475 DOI: 10.1016/j.gde.2022.101912] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/21/2021] [Revised: 03/17/2022] [Accepted: 03/24/2022] [Indexed: 11/16/2022]
Abstract
Abnormalities in chromosome dose can reduce organismal fitness and viability by disrupting the balance of gene expression. Unlike imbalances in chromosome dose that cause pathologies, differences in X-chromosome dose that determine sex are well tolerated. Dosage compensation mechanisms have evolved in diverse species to balance X-chromosome gene expression between sexes. Mechanisms underlying nematode X-chromosome counting to determine sex revealed how small quantitative differences in molecular signals are translated into dramatically different developmental fates. Mechanisms underlying X-chromosome dosage compensation revealed the interplay between chromatin modification and three-dimensional chromosome structure imposed by an X-specific condensin complex to regulate gene expression over vast chromosomal territories. In a surprising twist of evolution, this dosage-compensation condensin complex also regulates lifespan and tolerance to proteotoxic stress.
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Affiliation(s)
- Barbara J Meyer
- Department of Molecular and Cell Biology, Howard Hughes Medical Institute, University of California, 16 Barker Hall, Berkeley, CA 94720-3204, USA.
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Mechanisms of Choice in X-Chromosome Inactivation. Cells 2022; 11:cells11030535. [PMID: 35159344 PMCID: PMC8833938 DOI: 10.3390/cells11030535] [Citation(s) in RCA: 13] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/17/2021] [Revised: 01/30/2022] [Accepted: 01/31/2022] [Indexed: 12/04/2022] Open
Abstract
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.
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Gene regulation in time and space during X-chromosome inactivation. Nat Rev Mol Cell Biol 2022; 23:231-249. [PMID: 35013589 DOI: 10.1038/s41580-021-00438-7] [Citation(s) in RCA: 85] [Impact Index Per Article: 42.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 11/18/2021] [Indexed: 12/21/2022]
Abstract
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.
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