Making sense (and antisense) of the X inactivation center.
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Animals↗
Biomedical subjects
L Carrel
Publications and source records attributed to L Carrel.
A recurrent RNA-splicing mutation in the SEDL gene causes X-linked spondyloepiphyseal dysplasia tarda.
Spondyloepiphyseal dysplasia tarda (SEDL) is a genetically heterogeneous disorder characterized by mild-to-moderate short stature and early-onset osteoarthritis. Both autosomal and X-linked forms have been described. Elsewhere, we have reported the identification of the gene for the X-linked recessive form, which maps to Xp22.2. We now report characterization of an exon-skipping mutation (IVS3+5G-->A at the intron 3 splice-donor site) in two unrelated families with SEDL. Using reverse transcriptase (RT)-PCR, we demonstrated that the mutation resulted in elimination of the first 31 codons of the open reading frame. The mutation was not detected in 120 control X chromosomes. Articular cartilage from an adult who had SEDL and carried this mutation contained chondrocytes with abundant Golgi complexes and dilated rough endoplasmic reticulum (ER). RT-PCR experiments using mouse/human cell hybrids revealed that the SEDL gene escapes X inactivation. Homologues of the SEDL gene include a transcribed retropseudogene on chromosome 19, as well as expressed genes in mouse, rat, Drosophila melanogaster Caenorhabditis elegans, and Saccharomyces cerevisiae. The latter homologue, p20, has a putative role in vesicular transport from ER to Golgi complex. These data suggest that SEDL mutations may perturb an intracellular pathway that is important for cartilage homeostasis.
Adult↗
Molecular evidence for a relationship between LINE-1 elements and X chromosome inactivation: the Lyon repeat hypothesis.
X inactivation is a chromosome-specific form of genetic regulation in which thousands of genes on one homologue become silenced early in female embryogenesis. Although many aspects of X inactivation are now understood, the spread of the X inactivation signal along the entire length of the chromosome remains enigmatic. Extending the Gartler-Riggs model [Gartler, S. M. & Riggs, A. D. (1983) Annu. Rev. Genet. 17, 155-190], Lyon recently proposed [Lyon, M. F. (1998) Cytogenet. Cell Genet. 80, 133-137] that a nonrandom organization of long interspersed element (LINE) repetitive sequences on the X chromosome might be responsible for its facultative heterochromatization. In this paper, we present data indicating that the LINE-1 (L1) composition of the human X chromosome is fundamentally distinct from that of human autosomes. The X chromosome is enriched 2-fold for L1 repetitive elements, with the greatest enrichment observed for a restricted subset of LINE-1 elements that were active <100 million years ago. Regional analysis of the X chromosome reveals that the most significant clustering of these elements is in Xq13-Xq21 (the center of X inactivation). Genomic segments harboring genes that escape inactivation are significantly reduced in L1 content compared with X chromosome segments containing genes subject to X inactivation, providing further support for the association between X inactivation and L1 content. These nonrandom properties of L1 distribution on the X chromosome provide strong evidence that L1 elements may serve as DNA signals to propagate X inactivation along the chromosome.
Base Pairing↗
A first-generation X-inactivation profile of the human X chromosome.
In females, most genes on the X chromosome are generally assumed to be transcriptionally silenced on the inactive X as a result of X inactivation. However, particularly in humans, an increasing number of genes are known to "escape" X inactivation and are expressed from both the active (Xa) and inactive (Xi) X chromosomes; such genes reflect different molecular and epigenetic responses to X inactivation and are candidates for phenotypes associated with X aneuploidy. To identify genes that escape X inactivation and to generate a first-generation X-inactivation profile of the X, we have evaluated the expression of 224 X-linked genes and expressed sequence tags by reverse-transcription-PCR analysis of a panel of multiple independent mouse/human somatic cell hybrids containing a normal human Xi but no Xa. The resulting survey yields an initial X-inactivation profile that is estimated to represent approximately 10% of all X-linked transcripts. Of the 224 transcripts tested here, 34 (three of which are pseudoautosomal) were expressed in as many as nine Xi hybrids and thus appear to escape inactivation. The genes that escape inactivation are distributed nonrandomly along the X; 31 of 34 such transcripts map to Xp, implying that the two arms of the X are epigenetically and/or evolutionarily distinct and suggesting that genetic imbalance of Xp may be more severe clinically than imbalance of Xq. A complete X-inactivation profile will provide information relevant to clinical genetics and genetic counseling and should yield insight into the genomic and epigenetic organization of the X chromosome.
Animals↗
Heterogeneous gene expression from the inactive X chromosome: an X-linked gene that escapes X inactivation in some human cell lines but is inactivated in others.
In mammalian females, most genes on one X chromosome are transcriptionally silenced as a result of X chromosome inactivation. Whereas it is well established that some X-linked genes "escape" X inactivation and are expressed from both active (Xa) and inactive (Xi) X chromosomes, most models for the chromosomal control of X-linked gene expression assume that the X inactivation status of a given gene is constant among different females within a population. In this report, we test the expression of human X-linked genes in primary cell lines from females with complete nonrandom X inactivation, by using transcribed polymorphisms to distinguish Xa and Xi expression. Six X-linked genes used to document this assay system showed monoallelic expression in all informative cell lines, consistent with X inactivation. However, a novel pattern of expression was observed for another gene, REP1; monoallelic expression, indicating inactivation, was detected in some lines, whereas biallelic expression, indicating escape from inactivation, was detected in others. Furthermore, levels of Xi expression varied among cell lines that expressed REP1. The cellular basis of Xi expression was examined by expression assays in single cells. These data indicate that REP1 is expressed from the Xi in all cells, but that the level of expression relative to Xa levels is reduced. These findings suggest that Xi gene expression is under a previously unsuspected level of genetic or epigenetic control, likely involving local or regional changes in chromatin organization that determine whether a gene escapes or is subject to X inactivation.
Alleles↗
Germ cell development in the XXY mouse: evidence that X chromosome reactivation is independent of sexual differentiation.
Prior to entry into meiosis, XX germ cells in the fetal ovary undergo X chromosome reactivation. The signal for reactivation is thought to emanate from the genital ridge, but it is unclear whether it is specific to the developing ovary. To determine whether the signals are present in the developing testis as well as the ovary, we examined the expression of X-linked genes in germ cells from XXY male mice. To facilitate this analysis, we generated XXY and XX fetuses carrying X chromosomes that were differentially marked and subject to nonrandom inactivation. This pattern of nonrandom inactivation was maintained in somatic cells but, in XX as well as XXY fetuses, both parental alleles were expressed in germ cell-enriched cell populations. Because testis differentiation is temporally and morphologically normal in the XXY testis and because all germ cells embark upon a male pathway of development, these results provide compelling evidence that X chromosome reactivation in fetal germ cells is independent of the somatic events of sexual differentiation. Proper X chromosome dosage is essential for the normal fertility of male mammals, and abnormalities in germ cell development are apparent in the XXY testis within several days of X reactivation. Studies of exceptional germ cells that survive in the postnatal XXY testis demonstrated that surviving germ cells are exclusively XY and result from rare nondisjunctional events that give rise to clones of XY cells.
Animals↗
Counting on Xist.
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Dosage Compensation, Genetic↗
The mouse Mid1 gene: implications for the pathogenesis of Opitz syndrome and the evolution of the mammalian pseudoautosomal region.
We have recently reported isolation of the gene responsible for X-linked Opitz G/BBB syndrome, a defect of midline development. MID1 is located on the distal short arm of the human X chromosome (Xp22. 3) and encodes a novel member of the B box family of zinc finger proteins. We have now cloned the murine homolog of MID1 and performed preliminary expression studies during development. Mid1 expression in undifferentiated cells in the central nervous, gastrointestinal and urogenital systems suggests that abnormal cell proliferation may underlie the defect in midline development characteristic of Opitz syndrome. We have also found that Mid1 is located within the mouse pseudoautosomal region (PAR) in Mus musculus , while it seems to be X-specific in Mus spretus. Therefore, Mid1 is likely to be a recent acquisition of the M. musculus PAR. Genetic and FISH analyses also demonstrated a high frequency of unequal crossovers in the murine PAR, creating spontaneous deletion/duplication events involving Mid1. These data provide evidence for the first time that genetic instability of the PAR may affect functionally important genes. In addition, we show that MID1 is the first example of a gene subject to X-inactivation in man while escaping it in mouse. These data contribute to a better understanding of the molecular content and evolution of the rodent PAR.
Abnormalities, Multiple↗
The UTX gene escapes X inactivation in mice and humans.
We recently have identified a ubiquitously transcribed mouse Y chromosome gene, Uty , which encodes a tetratricopeptide repeat (TPR) protein. A peptide derived from the UTY protein confers H-Y antigenicity on male cells. Here we report the characterization of a widely transcribed X-linked homologue of Uty , called Utx , which maps to the proximal region of the mouse X chromosome and which detects a human X-linked homologue at Xp11.2. Given that Uty is ubiquitously transcribed, we assayed for Utx expression from the inactive X chromosome (Xi) in mice and found that Utx escapes X chromosome inactivation. Only Smcx and the pseudoautosomal Sts gene on the mouse X chromosome have been reported previously to escape inactivation. The human UTX gene was also found to be expressed from Xi. We discuss the significance of these data for our understanding of dosage compensation of X-Y homologous genes in humans and mice.
Amino Acid Sequence↗
[Sudden death under anticoagulation for pulmonary embolism].
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Aged↗
Expression of genes from the human active and inactive X chromosomes.
X-chromosome inactivation results in the cis-limited inactivation of many, but not all, of the genes on one of the pair of X chromosomes in mammalian females. In addition to the genes from the pseudoautosomal region, which have long been anticipated to escape inactivation, genes from several other regions of the human X chromosome have now been shown to escape inactivation and to be expressed from both the active and inactive X chromosomes. The growing number of genes escaping inactivation emphasizes the need for a reliable system for assessing the inactivation status of X-linked genes. Since many features of the active or inactive X chromosome, including transcriptional activity, are maintained in rodent/human somatic-cell hybrids, such hybrids have been used to study the inactivation process and to determine the inactivation status of human X-linked genes. In order to assess the fidelity of inactivation status in such hybrids, we have examined the expression of 33 X-linked genes in eight mouse/human somatic-cell hybrids that contain either the human active (three hybrids) or inactive X (five hybrids) chromosome. Inactivation of nine of these genes had previously been demonstrated biochemically in human cells, and the expression of these genes only in hybrids retaining an active X, but not in those retaining an inactive X, confirms that expression in hybrids reflects expression in human cells. Although the majority of genes tested showed consistent patterns of expression among the active X hybrids or inactive X hybrids, surprisingly, 5 of the 33 genes showed heterogeneous expression among the hybrids, demonstrating a significantly higher rate of variability than previously reported for other genes in either human somatic cells or mouse/human somatic-cell hybrids. These data suggest that at least some X-linked genes may be under additional levels of epigenetic regulation not previously recognized and that somatic-cell hybrids may provide a useful approach for studying these chromosomal phenomena.
Animals↗
An assay for X inactivation based on differential methylation at the fragile X locus, FMR1.
We describe an assay analyzing methylation at the fragile X mental retardation gene, FMR1, to examine patterns of random or non-random X chromosome inactivation. Digestion of genomic DNA with the methylation-sensitive enzyme HpaII cleaves two restriction sites near the CGG repeat of the FMR1 gene if they are unmethylated on the active X chromosome, but fails to digest these sites on the inactive chromosome. Subsequent PCR using primers that flank the sites and the variable CGG repeat within the FMR1 gene amplifies alleles only on undigested, methylated inactive X chromosomes. Amplification of the hypervariable CGG repeat distinguishes alleles in heterozygous samples, while the relative ratio of alleles within a HpaII-digested sample reflects the randomness or non-randomness of inactivation. To demonstrate that methylation of the HpaII sites within the amplified FMR1 fragment correlates strictly with the activity state of the X chromosome, we have tested the validity of this assay by comparing DNA from normal males and females, as well as DNA from mouse/human somatic cell hybrids carrying either active or inactive human X chromosomes. The data demonstrate that this assay provides a reliable means of assessing the inactivation status of X chromosomes in individuals with X-linked disorders or X chromosome abnormalities.
Alleles↗
A novel gene, DXS8237E, lies within 20 kb upstream of UBE1 in Xp11.23 and has a different X inactivation status.
A novel X-linked gene, DXS8237E, was isolated from human fetal brain cDNA, and its 3' end was mapped to within 20 kb upstream of UBE1 in Xp11.23. A 1.3-kb cDNA for DXS8237E detects homologous sequences in other mammals and a 3-kb mRNA that is widely expressed in human cell lines and mouse tissues. Sequence analysis indicated that the 1.3-kb cDNA contains the 3' end of the DXS8237E gene, but the sequence shows no significant homology to known genes. DXS8237E was shown to be subject to X inactivation in five somatic cell hybrids that contain an inactive human X chromosome but no active homologue. Since UBE1 escapes X inactivation, DXS8237E and UBE1 are the closest mapped genes with discordant X inactivation profiles. Sequences in the vicinity of these two genes may be important determinants of X inactivation status.
Animals↗
X inactivation analysis and DNA methylation studies of the ubiquitin activating enzyme E1 and PCTAIRE-1 genes in human and mouse.
Previously reported data on the X inactivation status of the ubiquitin activating enzyme E1 (UBE1) gene have been contradictory, and the issue has remained unsettled. Here we present three lines of evidence that UBE1 is expressed from the inactive X chromosome and therefore escapes X inactivation. First, by RNA in situ hybridization, UBE1 RNA is detected from both the active and inactive X chromosomes in human female fibroblasts. Second, UBE1 is expressed in a large panel of somatic cell hybrids retaining inactive human X chromosomes, including two independent hybrids that did not require UBE1 expression for survival. And third, sites at the 5' end of UBE1 are unmethylated on both active and inactive X chromosomes, consistent with the gene escaping inactivation. In order to address whether other genes that escape inactivation map to the same region of the X chromosome, we have also examined the expression of genes mapping adjacent to UBE1. The gene for PCTAIRE-1 (PCTK1) maps within 5 kb of UBE1 and similarly escapes X inactivation by the somatic cell hybrid assay, whereas six other genes that are within 1 Mb of UBE1 in Xp11.23 are silenced on the inactive X chromosome. Comparative mapping studies of the homologous loci in mouse establish that Ube1-x and Pctk1 are also within close physical proximity on the murine X chromosome, and expression studies of the Pctk1 gene determine that, similar to Ube1-x, it is subject to X inactivation in mouse. Methylation of CpG residues at restriction sites at the 5' end of both genes on the murine inactive X chromosome is consistent with both genes being subject to X inactivation in mouse, in contrast to their expression status in humans.
Animals↗
Tissue and lineage-specific variation in inactive X chromosome expression of the murine Smcx gene.
To understand how gene expression patterns are established on the inactive X chromosome during development, we have studied the murine gene Smcx, which is expressed from both the active and inactive mouse X chromosomes. In all tissues assayed, Smcx only partially escapes X inactivation, with expression levels from the inactive X allele approximately 30-65% that of the active X allele. Additionally, inactive X expression levels differed between extraembryonic and embryonic tissues and among different tissues from newborn and adult mice. Imprinted extraembryonic tissue had the lowest levels of inactive X Smcx expression, whereas the highest levels were in heart. These data suggest that the chromosomal basis of X inactivation differs among tissues, perhaps reflecting differences in the timing or regulation of inactivation in these cell lineages.
Animals↗
The DXS423E gene in Xp11.21 escapes X chromosome inactivation.
The DXS423E gene has been localized to Xp11.21 and is expressed in somatic cell hybrids retaining either the human active or inactive X chromosome, demonstrating that DXS423E escapes X chromosome inactivation. The XE169 (DXS1272E or SMCX) gene that escapes X chromosome inactivation is also located in Xp11.21-11.22 and maps within the same YAC as DXS423E. Thus the DXS423E and XE169 genes define a new region in the proximal short arm of the X chromosome that is not subject to X chromosome inactivation, supporting a regional basis for escape from inactivation.
Base Sequence↗
The mouse Sb1.8 gene located at the distal end of the X chromosome is subject to X inactivation.
The mouse homolog of the human DXS423E (SB1.8) gene has been isolated by screening a mouse cDNA library. Like its human counterpart, the mouse Sb1.8 gene is X-linked, as shown by Southern blot analysis and by in situ hybridization to metaphase chromosomes. Sb1.8 was sublocalized to band F of the mouse X chromosome, distal to Alas2 and proximal to DXPas1, which confirms a region of conservation between band Xp11.21-p11.22 in human and band XF in mouse. In situ hybridization also showed that the Smcx (Xe169) gene maps near Sb1.8 in band F. The Sb1.8 gene was shown to be highly conserved in mammals; partial DNA sequence analysis indicates 92% identity between the mouse and human genes. In contrast to the human DXS423E gene, the mouse Sb1.8 gene is subject to X inactivation, as shown by restriction enzyme and sequence analysis of mRNA from mice with Searle's translocation (T(X;16)16H). Absence of Sb1.8 expression from the inactive mouse X chromosome in vitro was confirmed by analysis of a cell line (Hobmski) in which the M. spretus X chromosome is inactivated. The Sb1.8 gene is a new member of a group of genes that escape X inactivation in human, but are inactivated in mouse.
Animals↗
A novel transmembrane transporter encoded by the XPCT gene in Xq13.2.
To study the regulation and chromosomal basis of X chromosome inactivation, we have physically characterized the region in Xq13.2 known to contain the X inactivation center (XIC), a locus required in cis for inactivation to occur. Here, we report a novel gene isolated by positional cloning in this region. The gene (previously identified as DXS128E) encodes a predicted 67 kDa protein containing twelve hydrophobic transmembrane domains, characteristic of a family of transporter proteins. Presence of an N-terminal PEST domain, consisting mainly of proline/glutamic acid repeats, suggests that the protein may be rapidly or conditionally degraded. We designate this gene XPCT for X-linked PEST-containing transporter. Expression studies suggest that XPCT is subject to X chromosome inactivation, being expressed only from the active X, despite mapping within 600 kb of the XIST gene which is expressed exclusively from the inactive X. Thus, a chromosomal switch in inactivation pattern occurs between these two genes on the X chromosome.
Amino Acid Sequence↗