X chromosome

About: X chromosome is a research topic. Over the lifetime, 9862 publications have been published within this topic receiving 407354 citations. The topic is also known as: GO:0000805 & chrX.

Characterization of Genes Encoding Translation Initiation Factor eIF-2γ in Mouse and Human: Sex Chromosome Localization, Escape from X-Inactivation and Evolution

Abstract: interval of the mouse Y chromosome is criti-cal for spermatogenesis and expression of the male-specific minor transplantation antigen H-Y. Severalgenes have been mapped to this interval and each hasa homologue on the X chromosome. Four, Zfy1, Zfy2,Ube1y and Dffry , are expressed specifically in the testisand their X homologues are not transcribed from theinactive X chromosome. A further two, Smcy and Uty,are ubiquitously expressed and their X homologuesescape X-inactivation. Here we report the identificationof another gene from this region of the mouse Ychromosome. It encodes the highly conserved euka-ryotic translation initiation factor eIF-2γ. In the mousethis gene is ubiquitously expressed, has an X chromo-some homologue which maps close to Dmd and es-capes X-inactivation. The coding regions of the X andY genes show 86% nucleotide identity and encode pu-tative products with 98% amino acid identity. In hu-mans, the eIF-2γ structural gene is located on the Xchromosome at Xp21 and this also escapes X-inactiva-tion. However, there is no evidence of a Y copy of thisgene in humans. We have identified autosomal retro-posons of eIF-2 γ in both humans and mice and an addi-tional retroposon on the X chromosome in somemouse strains. Ark blot analysis of eutherian and meta-therian genomic DNA indicates that X-Y homologuesare present in all species tested except simian pri-mates and kangaroo and that retroposons are com-mon to a wide range of mammals. These results shedlight on the evolution of X-Y homologous genes.INTRODUCTIONGenes controlling the functions of spermatogenesis, Spy (1), andexpression of the male-specific minor transplantation antigenH-Y, Hya, (2) have been mapped to a region of the short arm ofthe mouse Y chromosome (∆Sxr

Non-random inactivation of X chromosome in the rat yolk sac

Abstract: THE Lyon hypothesis postulates that the mammalian female is a natural mosaic for clones of cells with either the maternally derived X chromosome (Xm) or the paternally derived one (Xp) which is randomly inactivated early in development1,2. We have presented evidence for the dominance of the inactive Xp in extraembryonic regions of 7.5- and 8.5-d mouse embryos heterozygous for Cattanach's translocation3 in which the two X chromosomes could be readily identified4. Though we presumed that this might not be an exceptional phenomenon restricted to mice bearing this X-autosome translocation, this has been difficult to confirm because of the lack of a system suitable for experiments. Here we report further cytological evidence that the inactive X is predominantly paternal in the yolk sac of the laboratory rat.

Two types of sites required for meiotic chromosome pairing in Caenorhabditis elegans.

Abstract: Previous studies have shown that isolated portions of Caenorhabditis elegans chromosomes are not equally capable of meiotic exchange. These results led to the proposal that a homolog recognition region (HRR), defined as the region containing those sequences enabling homologous chromosomes to pair and recombine, is localized near one end of each chromosome. Using translocations and duplications we have localized the chromosome I HRR to the right end. Whereas the other half of chromosome I did not confer any ability for homologs to pair and recombine, deficiencies in this region dominantly suppressed recombination to the middle of the chromosome. These deletions may have disrupted pairing mechanisms that are secondary to and require an HRR. Thus, the processes of pairing and recombination appear to utilize at least two chromosomal elements, the HRR and other pairing sites. For example, terminal sequences from other chromosomes increase the ability of free duplications to recombine with their normal homologs, suggesting that telomere-associated sequences, homologous or nonhomologous, play a role in facilitating meiotic exchange. Recombination can also initiate at internal sites separated from the HRR by chromosome rearrangement, such as deletions of the unc-54 region of chromosome I. When crossing over was suppressed in a region of chromosome I, compensatory increases were observed in other regions. Thus, the presence of the HRR enabled recombination to occur but did not determine the distribution of the crossover events. It seems most likely that there are multiple initiation sites for recombination once homolog recognition has been achieved.

Loss of the Inactive X Chromosome and Replication of the Active X in BRCA1-Defective and Wild-type Breast Cancer Cells

Abstract: In females, X chromosome inactivation (XCI) begins with the expression of the XIST gene from the X chromosome destined to be inactivated (Xi) and the coating of XIST RNA in cis. It has recently been reported that this process is supported by the product of the BRCA1 tumor suppressor gene and that BRCA1-/- cancers show Xi chromatin structure defects, thus suggesting a role of XCI perturbation in BRCA1-mediated tumorigenesis. Using a combined genetic and epigenetic approach, we verified the occurrence of XCI in BRCA1-/- and BRCA1wt breast cancer cell lines. It was ascertained that the Xi was lost in all cancer cell lines, irrespective of the BRCA1 status and that more than one active X (Xa) was present. In addition, no epigenetic silencing of genes normally subjected to XCI was observed. We also evaluated XIST expression and found that XIST may be occasionally transcribed also from Xa. Moreover, in one of the BRCA1wt cell line the restoring of XIST expression using a histone deacetylase inhibitor, did not lead to XCI. To verify these findings in primary tumors, chromosome X behavior was investigated in a few BRCA1-associated and BRCA1-not associated primary noncultured breast carcinomas and the results mirrored those obtained in cancer cell lines. Our findings indicate that the lack of XCI may be a frequent phenomenon in breast tumorigenesis, which occurs independently of BRCA1 status and XIST expression and is due to the loss of Xi and replication of Xa and not to the reactivation of the native Xi.

X Inactive-Specific Transcript (Xist) Expression and X Chromosome Inactivation in the Preattachment Bovine Embryo

Abstract: Expression of the X inactive-specific transcript (Xist) is thought to be essential for the initiation of X chromosome inactivation and dosage compensation during female embryo development. In the present study, we analyzed the patterns of Xist transcription and the onset of X chromosome inactivation in bovine preattachment embryos. Reverse transcription-polymerase chain reaction (RT-PCR) revealed the presence of Xist transcripts in all adult female somatic tissues evaluated. In contrast, among the male tissues examined, Xist expression was detected only in testis. No evidence for Xist transcription was observed after a single round of RT-PCR from pools of in vitro-derived embryos at the 2- to 4-cell stage. Xist transcripts were detected as a faint amplicon at the 8-cell stage initially, and consistently thereafter in all stages examined up to and including the expanded blastocyst stage. Xist transcripts, however, were subsequently detected from the 2-cell stage onward after nested RT-PCR. Preferential [3H]thymidine labeling indicative of late replication of one of the X chromosomes was noted in female embryos of different developmental ages as follows: 2 of 7 (28.5%) early blastocysts, 6 of 13 (46.1%) blastocysts, 8 of 11 (72.1%) expanded blastocysts, and 14 of 17 (77.7%) hatched blastocysts. These results suggest that Xist expression precedes the onset of late replication in the bovine embryo, in a pattern compatible with a possible role of bovine Xist in the initiation of X chromosome inactivation.