Partial characterization of the X chromosome of Diuraphis noxia (Hemiptera: Aphididae)

We present the first sequencing results after separation of the X chromosome of Diuraphis noxia (Kurdjumov) (Hemiptera: Aphididae), the largest known X chromosome described to date, using flow cytometry. The X chromosome of D. noxia is 0.1824 pg (1C) and an estimated 178.4 Mb (1C) in size. Mapping confirmed that the X chromosome contains 13,799 protein coding genes, but with a slight bias towards GC richness when compared to the complete D. noxia genome.     

DNA methylation analysis of ade novo balanced X;13 translocation in a girl with abnormal phenotype: evidence for functional duplication of the whole short arm of the X chromosome

We report on a 13-month-old girl showing dysmorphic features and a delay in psychomotor development. She was diagnosed with a balancedde novo translocation 46, X, t(X;13)(p11. 2;p13) and non-random inactivation of the X chromosome. FISH analysis, employing the X chromosome centromere andXIST-region-specific probes, showed that theXIST locus was not involved in the translocation. Selective inactivation of paternal X, which was involved in translocation, was revealed by the HUMARA assay. The pattern of methylation of 5 genes located within Xp, which are normally silenced on an inactive X chromosome, corresponded to an active (unmethylated) X chromosome. These results revealed that in our proband the X chromosome involved in translocation (Xt) was preferentially inactivated. However, genes located on the translocated Xp did not includeXIST. This resulted in functional Xp disomy, which most probably accounts for the abnormal phenotype in our patient.     

Expression of X-linked genes in androgenetic,gynogenetic, and normal mouse preimplantation embryos

A quantitative RT-PCR approach has been used to examine the expression of a number of X-linked genes during preimplan-tation development of normal mouse embryos and in androgenetic and gynogenetic mouse embryos. The data reveal moderately reduced expression of the Prps1, Hprt, and Pdha1 mRNAs in androge-netic eight-cell and morula stage embryos, but not in androgenetic blastocysts. Pgk1 mRNA abundance was severely reduced in androgenones at the eight-cell and morula stages and remained reduced, but to a lesser degree, in androgenetic blastocysts. These data indicate that paternally inherited X chromosomes are at least partially repressed in androgenones, as they are in normal XX embryos, and that the degree of this repression is chromosome position-dependent or gene-dependent. Gynogenetic embryos expressed elevated amounts of some mRNAs at the morula and blas-tocyst stages, indicative of a delay in dosage compensation that may be chromosome position-dependent. The Xist RNA was expressed at a greater abundance in androgenones than in gynogenones at the eight-cell and morula stages, consistent with previous studies. Xist expression was observed in both and rogenones and gynogenones at the blas-tocyst stage. We conclude that the developmental arrest in early androgenones may be, in part, due to reduced expression of essential X-linked genes, particularly those near the X inactivation center, where as the developmental defects of gyno-genones and parthenogenones, by contrast, may be partially due to overexpression of X-linked genes in extraembryonic tissues, possibly those far-thest away from the X inactivation center. © 1995 Wiley-Liss, Inc.     

Targeting of >1.5 Mb of Human DNA into the Mouse X Chromosome Reveals Presence of cis-Acting Regulators of Epigenetic Silencing

Regulatory sequences can influence the expression of flanking genes over long distances, and X chromosome inactivation is a classic example of cis-acting epigenetic gene regulation. Knock-ins directed to the Mus musculus Hprt locus offer a unique opportunity to analyze the spread of silencing into different human DNA sequences in the identical genomic environment. X chromosome inactivation of four knock-in constructs, including bacterial artificial chromosome (BAC) integrations of over 195 kb, was demonstrated by both the lack of expression from the inactive X chromosome in females with nonrandom X chromosome inactivation and promoter DNA methylation of the human transgene in females. We further utilized promoter DNA methylation to assess the inactivation status of 74 human reporter constructs comprising >1.5 Mb of DNA. Of the 47 genes examined, only the PHB gene showed female DNA hypomethylation approaching the level seen in males, and escape from X chromosome inactivation was verified by demonstration of expression from the inactive X chromosome. Integration of PHB resulted in lower DNA methylation of the flanking HPRT promoter in females, suggesting the action of a dominant cis-acting escape element. Female-specific DNA hypermethylation of CpG islands not associated with promoters implies a widespread imposition of DNA methylation during X chromosome inactivation; yet transgenes demonstrated differential capacities to accumulate DNA methylation when integrated into the identical location on the inactive X chromosome, suggesting additional cis-acting sequence effects. As only one of the human transgenes analyzed escaped X chromosome inactivation, we conclude that elements permitting ongoing expression from the inactive X are rare in the human genome.     

Comparative analysis of the DXPas34 regulatory region in rodents

Mouse X chromosome inactivation center contains the DXPas34 minisatellite locus which plays an important role in expression regulation of the Tsix and Xist genes, involved into female dosage compensation. Comparative analysis of the DXPas34 locus from mouse, rat, and four common vole species revealed similar organization of this region in the form of tandem repeat blocks. A search for functionally important elements in this locus showed that all the species examined carried the conservative motif monomers, which could be involved in regulation of X inactivation.     

X-chromosome inactivation: molecular mechanisms from the human perspective

X-chromosome inactivation is an epigenetic process whereby one X chromosome is silenced in mammalian female cells. Since it was first proposed by Lyon in 1961, mouse models have been valuable tools to uncover the molecular mechanisms underlying X inactivation. However, there are also inherent differences between mouse and human X inactivation, ranging from sequence content of the X inactivation center to the phenotypic outcomes of X-chromosome abnormalities. X-linked gene dosage in males, females, and individuals with X aneuploidies and X/autosome translocations has demonstrated that many human genes escape X inactivation, implicating cis-regulatory elements in the spread of silencing. We discuss the potential nature of these elements and also review the elements in the X inactivation center involved in the early events in X-chromosome inactivation.     

Time of X Chromosome Inactivation in Retinal Melanocytes of the Mouse

SEX-LINKED genes in mammals tend to produce a mosaic phenotype in the heterozygous female, which Lyon¹ explained by assuming that, at an unidentified stage in the female embryo, one of the two X chromosomes in each cell becomes inactive in a random manner and remains so in all the descendants of this cell. Russell and Bangham² gave a similar explanation, but maintained that the inactivation did not involve the entire X chromosome and Grüneberg³ has argued that the inactivation is partial, but occurs in both the X chromosomes (see ref. 4).     

X Chromosome Inactivation during Drosophila Spermatogenesis

Genes with male- and testis-enriched expression are under-represented on the Drosophila melanogaster X chromosome. There is also an excess of retrotransposed genes, many of which are expressed in testis, that have “escaped” the X chromosome and moved to the autosomes. It has been proposed that inactivation of the X chromosome during spermatogenesis contributes to these patterns: genes with a beneficial function late in spermatogenesis should be selectively favored to be autosomal in order to avoid inactivation. However, conclusive evidence for X inactivation in the male germline has been lacking. To test for such inactivation, we used a transgenic construct in which expression of a lacZ reporter gene was driven by the promoter sequence of the autosomal, testis-specific ocnus gene. Autosomal insertions of this transgene showed the expected pattern of male- and testis-specific expression. X-linked insertions, in contrast, showed only very low levels of reporter gene expression. Thus, we find that X linkage inhibits the activity of a testis-specific promoter. We obtained the same result using a vector in which the transgene was flanked by chromosomal insulator sequences. These results are consistent with global inactivation of the X chromosome in the male germline and support a selective explanation for X chromosome avoidance of genes with beneficial effects late in spermatogenesis.     

非编码的mRNA

非编码的mRNA是近年发现的一类不含典型ORF的mRNA.目前已发现或克隆的这类基因主要有：H19基因,XIST基因,XLSIRT基因,His-1基因,bic基因,rox1和rox2基因等.它们与胚胎发育,肿瘤发生及X染色体失活密切相关.     

Somatic damage to the X chromosome of the nematode Caenorhabditis elegans induced by gamma radiation

Summary Wild-type male embryos and young larvae of the nematode Caenorhabditis elegans were more sensitive than wild-type hermaphrodites to inactivation by gamma rays; wild-type males have one X chromosome per cell (XO), whereas wild-type hermaphrodites have two (XX). Furthermore, after transformation into fertile hermaphrodites by a her-1 mutation, XO animals were more radiosensitive than XX her-1 animals; and XX animals transformed into fertile males by a tra-1 mutation did not show increased radiosensitivity. It is concluded that wild-type males are more radiosensitive than wild-type hermaphrodites because they have one X chromosome rather than two, and the predominant mode of inactivation of XO animals involves damage to the single X chromosome. No sex-specific differences in survival were observed after UV irradiation.     

Pigmentary mosaicism in hypomelanosis of Ito

We report on a female with mental and motor retardation, facial dysmorphism, abnormal pigmentation reminiscent to hypomelanosis of Ito (HI), and karyotypic mosaicism involving a small supernumerary marker chromosome. The marker chromosome was defined by fluorescence in situ hybridisation (FISH) as a ring X chromosome with breakpoints in the juxtacentromeric region. FISH analysis showed that the ring does not include the XIST locus at the X-inactivation centre and, therefore, may not be subject to X inactivation. X-inactivation studies with the HUMARA (human androgen receptor) and FMR1 assay showed a skewed X-inactivation pattern (85:15) with preferential inactivation of the paternal X chromosome. These results are discussed with respect to the role of functional disomy of Xp in the pathogenesis of HI. Received: 16 February 1998 / Accepted: 17 July 1998     

X chromosome inactivation during Drosophila spermatogenesis

Genes with male- and testis-enriched expression are under-represented on the Drosophila melanogaster X chromosome. There is also an excess of retrotransposed genes, many of which are expressed in testis, that have “escaped” the X chromosome and moved to the autosomes. It has been proposed that inactivation of the X chromosome during spermatogenesis contributes to these patterns: genes with a beneficial function late in spermatogenesis should be selectively favored to be autosomal in order to avoid inactivation. However, conclusive evidence for X inactivation in the male germline has been lacking. To test for such inactivation, we used a transgenic construct in which expression of a lacZ reporter gene was driven by the promoter sequence of the autosomal, testis-specific ocnus gene. Autosomal insertions of this transgene showed the expected pattern of male- and testis-specific expression. X-linked insertions, in contrast, showed only very low levels of reporter gene expression. Thus, we find that X linkage inhibits the activity of a testis-specific promoter. We obtained the same result using a vector in which the transgene was flanked by chromosomal insulator sequences. These results are consistent with global inactivation of the X chromosome in the male germline and support a selective explanation for X chromosome avoidance of genes with beneficial effects late in spermatogenesis.     

An embryonic story: Analysis of the gene regulative network controlling Xist expression in mouse embryonic stem cells

In mice, dosage compensation of X‐linked gene expression is achieved through the inactivation of one of the two X‐chromosomes in XX female cells. The complex epigenetic process leading to X‐inactivation is largely controlled by Xist and Tsix, two non‐coding genes of opposing function. Xist RNA triggers X‐inactivation by coating the inactive X, while Tsix is critical for the designation of the active X‐chromosome through cis‐repression of Xist RNA accumulation. Recently, a plethora of trans‐acting factors and cis‐regulating elements have been suggested to act as key regulators of either Xist, Tsix or both; these include ubiquitous factors such as Yy1 and Ctcf, developmental proteins such as Nanog, Oct4 and Sox2, and X‐linked regulators such as Rnf12. In this paper we summarise recent advances in our knowledge of the regulation of Xist and Tsix in embryonic stem (ES) and differentiating ES cells.     

Localization of a gene that escapes inactivation to the X chromosome proximal short arm: implications for X inactivation.

The process of mammalian X chromosome inactivation results in the inactivation of most, but not all, genes along one or the other of the two X chromosomes in females. On the human X chromosome, several genes have been described that "escape" inactivation and continue to be expressed from both homologues. All such previously mapped genes are located in the distal third of the short arm of the X chromosome, giving rise to the hypothesis of a region of the chromosome that remains noninactivated during development. The A1S9T gene, an X-linked locus that complements a mouse temperature-sensitive defect in DNA synthesis, escapes inactivation and has now been localized, in human-mouse somatic cell hybrids, to the proximal short arm, in Xp11.1 to Xp11.3. Thus, A1S9T lies in a region of the chromosome that is separate from the other genes known to escape inactivation and is located between other genes known to be subject to X inactivation. This finding both rules out models based on a single chromosomal region that escapes inactivation and suggests that X inactivation proceeds by a mechanism that allows considerable autonomy between different genes or regions on the chromosome.     

Polymorphic X-chromosome inactivation of the human TIMP1 gene.

X inactivation silences most but not all of the genes on one of the two X chromosomes in mammalian females. The human X chromosome preserves its activation status when isolated in rodent/human somatic-cell hybrids, and hybrids retaining either the active or inactive X chromosome have been used to assess the inactivation status of many X-linked genes. Surprisingly, the X-linked gene for human tissue inhibitor of metalloproteinases (TIMP1) is expressed in some but not all inactive X-containing somatic-cell hybrids, suggesting that this gene is either prone to reactivation or variable in its inactivation. Since many genes that escape X inactivation are clustered, we examined the expression of four genes (ARAF1, ELK1, ZNF41, and ZNF157) within approximately 100 kb of TIMP1. All four genes were expressed only from the active X chromosome, demonstrating that the factors allowing TIMP1 expression from the inactive X chromosome are specific to the TIMP1 gene. To determine if this variable inactivation of TIMP1 is a function of the hybrid-cell environment or also is observed in human cells, we developed an allele-specific assay to assess TIMP1 expression in human females. Expression of two alleles was detected in some female cells with previously demonstrated extreme skewing of X inactivation, indicating TIMP1 expression from the inactive chromosome. However, in other cells, no expression of TIMP1 was observed from the inactive X chromosome, suggesting that TIMP1 inactivation is polymorphic in human females.