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Aksit MA, Yu B, Roelen BAJ, Migeon BR. Silencing XIST on the future active X: Searching human and bovine preimplantation embryos for the repressor. Eur J Hum Genet 2024; 32:399-406. [PMID: 35585273 PMCID: PMC10999447 DOI: 10.1038/s41431-022-01115-9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/12/2021] [Revised: 03/28/2022] [Accepted: 04/26/2022] [Indexed: 11/09/2022] Open
Abstract
X inactivation is the means of equalizing the dosage of X chromosomal genes in male and female eutherian mammals, so that only one X is active in each cell. The XIST locus (in cis) on each additional X chromosome initiates the transcriptional silence of that chromosome, making it an inactive X. How the active X in both males and females is protected from inactivation by its own XIST locus is not well understood in any mammal. Previous studies of autosomal duplications suggest that gene(s) on the short arm of human chromosome 19 repress XIST on the active X. Here, we examine the time of transcription of some candidate genes in preimplantation embryos using single-cell RNA sequencing data from human embryos and qRT-PCR from bovine embryos. The candidate genes assayed are those transcribed from 19p13.3-13.2, which are widely expressed and can remodel chromatin. Our results confirm that XIST is expressed at low levels from the future active X in embryos of both sexes; they also show that the XIST locus is repressed in both sexes when pluripotency factors are being upregulated, during the 4-8 cell and morula stages in human and bovine embryos - well before the early blastocyst (E5) when XIST on the inactive X in females starts to be upregulated. Our data suggest a role for DNMT1, UHRF1, SAFB and SAFB2 in XIST repression; they also exclude XACT and other 19p candidate genes and provide the transcriptional timing for some genes not previously assayed in human or bovine preimplantation embryos.
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Affiliation(s)
- Melis A Aksit
- McKusick Nathans Department of Genetic Medicine and Johns Hopkins University, School of Medicine, Baltimore, MD, USA
| | - Bo Yu
- Farm Animal Health, Department of Population Health Sciences, and Utrecht University, 3584CM, Utrecht, The Netherlands
| | - Bernard A J Roelen
- Embryology, Anatomy and Physiology, Department of Clinical Sciences, Faculty of Veterinary Medicine, Utrecht University, 3584CM, Utrecht, The Netherlands
| | - Barbara R Migeon
- McKusick Nathans Department of Genetic Medicine and Johns Hopkins University, School of Medicine, Baltimore, MD, USA.
- The Department of Pediatrics, Johns Hopkins University, School of Medicine, Baltimore, MD, USA.
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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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Migeon BR, Beer MA, Bjornsson HT. Embryonic loss of human females with partial trisomy 19 identifies region critical for the single active X. PLoS One 2017; 12:e0170403. [PMID: 28403217 PMCID: PMC5389809 DOI: 10.1371/journal.pone.0170403] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/07/2016] [Accepted: 01/04/2017] [Indexed: 11/18/2022] Open
Abstract
To compensate for the sex difference in the number of X chromosomes, human females, like human males have only one active X. The other X chromosomes in cells of both sexes are silenced in utero by XIST, the Inactive X Specific Transcript gene, that is present on all X chromosomes. To investigate the means by which the human active X is protected from silencing by XIST, we updated the search for a key dosage sensitive XIST repressor using new cytogenetic data with more precise resolution. Here, based on a previously unknown sex bias in copy number variations, we identify a unique region in our genome, and propose candidate genes that lie within, as they could inactivate XIST. Unlike males, the females who duplicate this region of chromosome 19 (partial 19 trisomy) do not survive embryogenesis; this preimplantation loss of females may be one reason that more human males are born than females.
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Affiliation(s)
- Barbara R. Migeon
- McKusick Nathans Institute of Genetic Medicine, Baltimore, MD, United States of America
- Department of Pediatrics, Johns Hopkins University, School of Medicine, Baltimore, MD, United States of America
- * E-mail:
| | - Michael A. Beer
- McKusick Nathans Institute of Genetic Medicine, Baltimore, MD, United States of America
- Department of Biomedical Engineering, Johns Hopkins University School of Medicine, Baltimore, MD, United States of America
| | - Hans T. Bjornsson
- McKusick Nathans Institute of Genetic Medicine, Baltimore, MD, United States of America
- Department of Pediatrics, Johns Hopkins University, School of Medicine, Baltimore, MD, United States of America
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Migeon BR, Pappas K, Stetten G, Trunca C, Jacobs PA. X inactivation in triploidy and trisomy: the search for autosomal transfactors that choose the active X. Eur J Hum Genet 2007; 16:153-62. [PMID: 17971834 DOI: 10.1038/sj.ejhg.5201944] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
Only one X chromosome functions in diploid human cells irrespective of the sex of the individual and the number of X chromosomes. Yet, as we show, more than one X is active in the majority of human triploid cells. Therefore, we suggest that (i) the active X is chosen by repression of its XIST locus, (ii) the repressor is encoded by an autosome and is dosage sensitive, and (iii) the extra dose of this key repressor enables the expression of more than one X in triploid cells. Because autosomal trisomies might help locate the putative dosage sensitive trans-acting factor, we looked for two active X chromosomes in such cells. Previously, we reported that females trisomic for 18 different human autosomes had only one active X and a normal inactive X chromosome. Now we report the effect of triplication of the four autosomes not studied previously; data about these rare trisomies - full or partial - were used to identify autosomal regions relevant to the choice of active X. We find that triplication of the entire chromosomes 5 and 11 and parts of chromosomes 1 and 19 is associated with normal patterns of X inactivation, excluding these as candidate regions. However, females with inherited triplications of 1p21.3-q25.3, 1p31 and 19p13.2-q13.33 were not ascertained. Thus, if a single key dose-sensitive gene induces XIST repression, it could reside in one of these locations. Alternatively, more than one dosage-sensitive autosomal locus is required to form the repressor complex.
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Affiliation(s)
- Barbara R Migeon
- The McKusick - Nathans Institute of Genetic Medicine, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.
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Migeon BR, Lee CH, Chowdhury AK, Carpenter H. Species differences in TSIX/Tsix reveal the roles of these genes in X-chromosome inactivation. Am J Hum Genet 2002; 71:286-93. [PMID: 12023758 PMCID: PMC379161 DOI: 10.1086/341605] [Citation(s) in RCA: 97] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/16/2002] [Accepted: 05/02/2002] [Indexed: 11/03/2022] Open
Abstract
Transcriptional silencing of the human inactive X chromosome is induced by the XIST gene within the human X-inactivation center. The XIST allele must be turned off on one X chromosome to maintain its activity in cells of both sexes. In the mouse placenta, where X inactivation is imprinted (the paternal X chromosome is always inactive), the maternal Xist allele is repressed by a cis-acting antisense transcript, encoded by the Tsix gene. However, it remains to be seen whether this antisense transcript protects the future active X chromosome during random inactivation in the embryo proper. We recently identified the human TSIX gene and showed that it lacks key regulatory elements needed for the imprinting function of murine Tsix. Now, using RNA FISH for cellular localization of transcripts in human fetal cells, we show that human TSIX antisense transcripts are unable to repress XIST. In fact, TSIX is transcribed only from the inactive X chromosome and is coexpressed with XIST. Also, TSIX is not maternally imprinted in placental tissues, and its transcription persists in placental and fetal tissues, throughout embryogenesis. Therefore, the repression of Xist by mouse Tsix has no counterpart in humans, and TSIX is not the gene that protects the active X chromosome from random inactivation. Because human TSIX cannot imprint X inactivation in the placenta, it serves as a mutant for mouse Tsix, providing insights into features responsible for antisense activity in imprinted X inactivation.
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Affiliation(s)
- Barbara R Migeon
- McKusick-Nathans Institute of Genetic Medicine, The Johns Hopkins University School of Medicine, 600 North Wolfe Street, Baltimore, MD 21287, USA.
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Abstract
There are numerous reports describing the pathology of the fetus and placenta in triploidy. Although gonadal pathology is described in many of these reports, consistent changes have not been noted nor is it clear whether genital ambiguity can be considered part of the triploid phenotype. We present a case of triploidy of probable diandric origin, in which there were dysgenetic gonads with abnormal seminiferous tubules, nodules of undifferentiated stroma, and focal absence of the tunica albuginea. As this finding was distinctly unusual in our experience of triploid gonadal pathology, we reviewed the gonadal histology in 51 fetal and infant triploids examined in our autopsy/embryopathology laboratory. The gonads were compared to age-matched normal controls to determine if there was a specific gonadal pathology associated with triploidy and if there was any correlation of this pathology with parental origin of the triploidy. Our review of the triploid gonads indicated that while minor, nonspecific changes were not uncommon, overtly dysgenetic gonads, as observed in the index case, are rare.
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Affiliation(s)
- D E McFadden
- Department of Pathology, Children's and Women's Health Centre of British Columbia and University of British Columbia, 4480 Oak Street, Vancouver, British Columbia, Canada V6H 3V4
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Abstract
Hypomelanosis of Ito is a sporadic multisystem disorder known to be associated in many cases with chromosomal mosaicism. While no particular pattern is generally evident for the specific chromosomes involved in such patients, a subgroup of female patients exists in whom the common factor is the presence of a balanced, constitutional X;autosome translocation, with a cytogenetic breakpoint in the pericentromeric region of the X. It is argued here that the phenotype in these cases results not from the interruption of X linked genes but from the presence of mosaic functional disomy of X sequences above the breakpoint.
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Affiliation(s)
- E Hatchwell
- Wessex Regional Genetics Service, Princess Anne Hospital, Southampton, UK
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Abstract
Mammalian X-chromosome inactivation results in dosage compensation for X-linked genes. More than 30 years after its discovery, the molecular bases of this inactivation are being revealed. Multiple mechanisms are responsible for the initiation of this developmental event and the maintenance of the inactive state. Somatic cellular mosaicism, which is the genetic consequence of X-chromosome inactivation, has a profound influence on the phenotype of mammalian females.
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Affiliation(s)
- B R Migeon
- Center for Medical Genetics, Johns Hopkins University, Baltimore, MD 21287-3914
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