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Mutzel V, Schulz EG. Dosage Sensing, Threshold Responses, and Epigenetic Memory: A Systems Biology Perspective on Random X-Chromosome Inactivation. Bioessays 2021; 42:e1900163. [PMID: 32189388 DOI: 10.1002/bies.201900163] [Citation(s) in RCA: 21] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/10/2019] [Revised: 01/27/2020] [Indexed: 02/06/2023]
Abstract
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.
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Affiliation(s)
- Verena Mutzel
- Otto Warburg Laboratories, Max Planck Institute for Molecular Genetics, Berlin, 14195, Germany
| | - Edda G Schulz
- Otto Warburg Laboratories, Max Planck Institute for Molecular Genetics, Berlin, 14195, Germany
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Mutzel V, Okamoto I, Dunkel I, Saitou M, Giorgetti L, Heard E, Schulz EG. A symmetric toggle switch explains the onset of random X inactivation in different mammals. Nat Struct Mol Biol 2019; 26:350-360. [PMID: 30962582 PMCID: PMC6558282 DOI: 10.1038/s41594-019-0214-1] [Citation(s) in RCA: 27] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/15/2018] [Accepted: 03/07/2019] [Indexed: 12/31/2022]
Abstract
Gene-regulatory networks control establishment and maintenance of alternative gene expression states during development. A particular challenge is the acquisition of opposing states by two copies of the same gene, as it is the case in mammals for Xist at the onset of random X-chromosome inactivation (XCI). The regulatory principles that lead to stable mono-allelic expression of Xist remain unknown. Here, we uncovered the minimal Xist regulatory network, by combining mathematical modeling and experimental validation of central model predictions. We identified a symmetric toggle switch as the basis for random mono-allelic Xist up-regulation, which reproduces data from several mutant, aneuploid and polyploid murine cell lines with various Xist expression patterns. Moreover, this toggle switch explains the diversity of strategies employed by different species at the onset of XCI. In addition to providing a unifying conceptual framework to explore X-chromosome inactivation across mammals, our study sets the stage for identifying the molecular mechanisms required to initiate random XCI.
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Affiliation(s)
- Verena Mutzel
- Otto Warburg Laboratories, Max Planck Institute for Molecular Genetics, Berlin, Germany
| | - Ikuhiro Okamoto
- Department of Anatomy and Cell Biology, Graduate School of Medicine, Kyoto University, Kyoto, Japan.,Japan Science and Technology (JST), Exploratory Research for Advanced Technology (ERATO), Kyoto, Japan
| | - Ilona Dunkel
- Otto Warburg Laboratories, Max Planck Institute for Molecular Genetics, Berlin, Germany
| | - Mitinori Saitou
- Institute for the Advanced Study of Human Biology (ASHBi), Kyoto University, Kyoto, Japan.,Department of Anatomy and Cell Biology, Graduate School of Medicine, Kyoto University, Kyoto, Japan.,Center for iPS Cell Research and Application (CiRA), Kyoto University, Kyoto, Japan
| | - Luca Giorgetti
- Friedrich Miescher Institute for Biomedical Research, Basel, Switzerland
| | - Edith Heard
- Institut Curie, PSL Research University, CNRS UMR3215, INSERM U934, Paris, France.,European Molecular Biology Laboratory (EMBL), Directors' research unit, Heidelberg, Germany
| | - Edda G Schulz
- Otto Warburg Laboratories, Max Planck Institute for Molecular Genetics, Berlin, Germany.
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Choosing the Active X: The Human Version of X Inactivation. Trends Genet 2017; 33:899-909. [PMID: 28988701 DOI: 10.1016/j.tig.2017.09.005] [Citation(s) in RCA: 30] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/18/2017] [Revised: 08/29/2017] [Accepted: 09/12/2017] [Indexed: 01/30/2023]
Abstract
Humans and rodents differ in how they carry out X inactivation (XI), the mammalian method to compensate for the different number of X chromosomes in males and females. Evolutionary changes in staging embryogenesis and in mutations within the XI center alter the process among mammals. The mouse model of XI is predicated on X counting and subsequently choosing the X to 'inactivate'. However, new evidence suggests that humans initiate XI by protecting one X in both sexes from inactivation by XIST, the noncoding RNA that silences the inactive X. This opinion article explores the question of how the active X is protected from silencing by its own Xist locus, and the possibility of different solutions for mouse and human.
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Abstract
X chromosome inactivation (XCI) reduces the number of actively transcribed X chromosomes to one per diploid set of autosomes, allowing for dosage equality between the sexes. In eutherians, the inactive X chromosome in XX females is randomly selected. The mechanisms for determining both how many X chromosomes are present and which to inactivate are unknown. To understand these mechanisms, researchers have created X chromosome mutations and transgenes. Here, we introduce a new model of X chromosome inactivation that aims to account for the findings in recent studies, to promote a re-interpretation of existing data and to direct future experiments.
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Affiliation(s)
- Joshua Starmer
- Department of Genetics and the Carolina Center for the Genome Sciences, and University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA
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Gartler SM, Varadarajan KR, Luo P, Norwood TH, Canfield TK, Hansen RS. Abnormal X: autosome ratio, but normal X chromosome inactivation in human triploid cultures. BMC Genet 2006; 7:41. [PMID: 16817970 PMCID: PMC1526452 DOI: 10.1186/1471-2156-7-41] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/15/2006] [Accepted: 07/03/2006] [Indexed: 11/10/2022] Open
Abstract
BACKGROUND X chromosome inactivation (XCI) is that aspect of mammalian dosage compensation that brings about equivalence of X-linked gene expression between females and males by inactivating one of the two X chromosomes (Xi) in normal female cells, leaving them with a single active X (Xa) as in male cells. In cells with more than two X's, but a diploid autosomal complement, all X's but one, Xa, are inactivated. This phenomenon is commonly thought to suggest 1) that normal development requires a ratio of one Xa per diploid autosomal set, and 2) that an early event in XCI is the marking of one X to be active, with remaining X's becoming inactivated by default. RESULTS Triploids provide a test of these ideas because the ratio of one Xa per diploid autosomal set cannot be achieved, yet this abnormal ratio should not necessarily affect the one-Xa choice mechanism for XCI. Previous studies of XCI patterns in murine triploids support the single-Xa model, but human triploids mostly have two-Xa cells, whether they are XXX or XXY. The XCI patterns we observe in fibroblast cultures from different XXX human triploids suggest that the two-Xa pattern of XCI is selected for, and may have resulted from rare segregation errors or Xi reactivation. CONCLUSION The initial X inactivation pattern in human triploids, therefore, is likely to resemble the pattern that predominates in murine triploids, i.e., a single Xa, with the remaining X's inactive. Furthermore, our studies of XIST RNA accumulation and promoter methylation suggest that the basic features of XCI are normal in triploids despite the abnormal X:autosome ratio.
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Affiliation(s)
- Stanley M Gartler
- Department of Medicine, Division of Medical Genetics,, University of Washington, Seattle, WA 98195, USA
- Department of Genome Sciences, University of Washington, Seattle, WA 98195, USA
| | - Kartik R Varadarajan
- Department of Medicine, Division of Medical Genetics,, University of Washington, Seattle, WA 98195, USA
| | - Ping Luo
- Department of Medicine, Division of Medical Genetics,, University of Washington, Seattle, WA 98195, USA
- Department of Pathology, University of Washington, Seattle, WA 98195, USA
| | - Thomas H Norwood
- Department of Pathology, University of Washington, Seattle, WA 98195, USA
| | - Theresa K Canfield
- Department of Medicine, Division of Medical Genetics,, University of Washington, Seattle, WA 98195, USA
| | - R Scott Hansen
- Department of Medicine, Division of Medical Genetics,, University of Washington, Seattle, WA 98195, USA
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Abstract
Dosage compensation in mammals is achieved by the transcriptional inactivation of one X chromosome in female cells. From the time X chromosome inactivation was initially described, it was clear that several mechanisms must be precisely integrated to achieve correct regulation of this complex process. X-inactivation appears to be triggered upon differentiation, suggesting its regulation by developmental cues. Whereas any number of X chromosomes greater than one is silenced, only one X chromosome remains active. Silencing on the inactive X chromosome coincides with the acquisition of a multitude of chromatin modifications, resulting in the formation of extraordinarily stable facultative heterochromatin that is faithfully propagated through subsequent cell divisions. The integration of all these processes requires a region of the X chromosome known as the X-inactivation center, which contains the Xist gene and its cis-regulatory elements. Xist encodes an RNA molecule that plays critical roles in the choice of which X chromosome remains active, and in the initial spread and establishment of silencing on the inactive X chromosome. We are now on the threshold of discovering the factors that regulate and interact with Xist to control X-inactivation, and closer to an understanding of the molecular mechanisms that underlie this complex process.
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Affiliation(s)
- Kathrin Plath
- Department of Biochemistry & Biophysics, University of California San Francisco, San Francisco, California 94143, USA.
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Moore T, Hurst LD, Reik W. Genetic conflict and evolution of mammalian X-chromosome inactivation. DEVELOPMENTAL GENETICS 1995; 17:206-11. [PMID: 8565327 DOI: 10.1002/dvg.1020170305] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
The existence of parentally imprinted gene expression in the somatic tissues of mammals and plants can be explained by a theory of intragenomic genetic conflict, which is a logical extension of classical parent-offspring conflict theory. This theory unites conceptually the phenomena of autosomal imprinting and X-chromosome inactivation. We argue that recent experimental studies of X-chromosome inactivation and androgenetic development address previously published predictions of the conflict theory, and we discuss possible explanations for the occurrence of random X-inactivation in the somatic tissues of eutherians.
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Affiliation(s)
- T Moore
- Department of Development and Signalling, Babraham Institute, Cambridge, United Kingdom
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Webb S, de Vries TJ, Kaufman MH. The differential staining pattern of the X chromosome in the embryonic and extraembryonic tissues of postimplantation homozygous tetraploid mouse embryos. Genet Res (Camb) 1992; 59:205-14. [PMID: 1380938 DOI: 10.1017/s0016672300030494] [Citation(s) in RCA: 26] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022] Open
Abstract
(C57BL x CBA)F1 hybrid female mice were mated with hemizygous Rb(X.2)2Ad males to distinguish the paternal X chromosome. Homozygous tetraploids were produced by blastomere fusion at the 2-cell stage, and 161 of these were transferred to recipients and analysed on the 10th day of gestation. 59 implants contained resorptions and 76 contained either an embryo and/or extraembryonic membranes. 38 (20, XXXX and 18, XXYY) were analysed to investigate their X-inactivation pattern. Embryonic and yolk sac endodermally- and mesodermally-derived samples were analysed by G-banding and by Kanda analysis. In the XX and XY controls, the predicted pattern of X-inactivation was observed, though 12.2% of metaphases in the XX series displayed no X-inactivation. In the XY series the Y chromosome was seen in a high proportion of metaphases. In the XXXX tetraploids, 8 cell lineages were recognized with regard to their X-inactivation pattern, though most belonged to the following 3 categories: (XmXm)XpXp, Xm(XmXp)Xp and XmXm(XpXp). The other categories were only rarely encountered. In the embryonic and mesodermally-derived tissue the ratio of these groups was close to 1:2:1, whereas in the endodermally-derived tissue it was 1:4.11:4.88, due to preferential paternal X-inactivation. A significant but small proportion of all 3 tissues analysed displayed no evidence of X-inactivation. Indirect evidence suggests that this represents a genuine group because of the high efficiency of the Kanda staining. The presence of the Xm(XmXp)Xp category is consistent with the expectation that X-inactivation occurs randomly in 2 of the 4 X chromosomes present. The presence of small numbers of preparations with no evidence of X-inactivation and other unexpected categories suggests that these are probably selected against during development.
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Affiliation(s)
- S Webb
- Department of Anatomy, University Medical School, Edinburgh, U.K
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Tada T, Takagi N. Early development and X-chromosome inactivation in mouse parthenogenetic embryos. Mol Reprod Dev 1992; 31:20-7. [PMID: 1562323 DOI: 10.1002/mrd.1080310105] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
Early development and X-chromosome inactivation were studied in ethanol-induced mouse parthenogenones. About 24% of oocytes transferred to 0.5-day pseudopregnant recipients successfully implanted. However, only 49%, 20%, and 16% of implanted parthenogenones survived 5, 6, and 7 days later, respectively. Abnormal development was evident in every parthenogenone as early as 5 days after activation with the degenerating polar trophectoderm. These embryos were destined to become either small disorganized embryos or embryonic ectoderm vesicles bounded by the visceral endoderm. Only 2 of 51 representative 6- to 8-day parthenogenones sectioned had morphology of the normal egg cylinder, although growth retardation was evident. Spontaneous LT/Sv parthenogenones shared similar morphological features. In late blastocysts, the frequency of cells with an apparently inactivated X chromosome was lower in parthenogenones than in fertilized embryos. The failure of X-inactivation in the trophectoderm seems to contribute to the defective development of parthenogenones.
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Affiliation(s)
- T Tada
- Research Center for Molecular Genetics, Hokkaido University, Sapporo, Japan
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Speirs S, Cross JM, Kaufman MH. The pattern of X-chromosome inactivation in the embryonic and extra-embryonic tissues of post-implantation digynic triploid LT/Sv strain mouse embryos. Genet Res (Camb) 1990; 56:107-14. [PMID: 2272499 DOI: 10.1017/s0016672300035175] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022] Open
Abstract
Spontaneously cycling LT/Sv strain female mice were mated to hemizygous Rb(X.2)2Ad males in order to facilitate the distinction of the paternal X chromosome, and the pregnant females were autopsied at about midday on the tenth day of gestation. Out of a total of 222 analysable embryos recovered, 165 (74.3%) were diploid and 57 (25.7%) were triploid. Of the triploids, 26 had an XXY and 31 an XXX sex chromosome constitution. Both embryonic and extra-embryonic tissue samples from the triploids were analysed cytogenetically by G-banding and by the Kanda technique to investigate their X-inactivation pattern. The yolk sac samples were separated enzymatically into their endodermally-derived and mesodermally-derived components, and these were similarly analysed, as were similar samples from a selection of control XmXp diploid embryos. In the case of the XmXmY digynic triploid embryos, a single darkly-staining Xm chromosome was observed in 485 (82.9%) out of 585, 304 (73.3%) out of 415, and 165 (44.7%) out of 369 metaphases from the embryonic, yolk sac mesodermally-derived and yolk sac endodermally-derived tissues, respectively. The absence of a darkly staining X-chromosome in the other metaphase spreads could either indicate that both X-chromosomes present were active, or that the Kanda technique had failed to differentially stain the inactive X-chromosome(s) present. In the case of the XmXmXp digynic triploid embryos, virtually all of the tissues analysed comprised two distinct cell lineages, namely those with two darkly-staining X-chromosomes, and those with a single darkly staining X-chromosome.(ABSTRACT TRUNCATED AT 250 WORDS)
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Affiliation(s)
- S Speirs
- Department of Anatomy, University Medical School, Edinburgh, UK
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Lyon MF. X-chromosome inactivation as a system of gene dosage compensation to regulate gene expression. PROGRESS IN NUCLEIC ACID RESEARCH AND MOLECULAR BIOLOGY 1989; 36:119-30. [PMID: 2499905 DOI: 10.1016/s0079-6603(08)60166-x] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
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