Showing papers on "X hyperactivation published in 2001"

Skewed X inactivation in X-linked disorders.

Abstract: X chromosome inactivation is a process by which the dosage of proteins transcribed from genes on the X chromosome is equalized between males (XY) and females (XX) through the silencing of most genes on one of the two X chromosomes in females. Although the choice of which of the two X's is inactivated is entirely random, not all women have a 50:50 ratio of cells with one or the other X chromosomes active. A number of different mechanisms lead to extremely skewed ratios and this can result in expression of the phenotype of X-linked recessive disorders in females. Nonrandom X inactivation patterns are also associated with selective female survival in male-lethal X-linked dominant disorders or with variable severity of the phenotype in women carrying X-linked dominant mutations. These features are important for the study and gene identification of X-linked disorders and for counseling of affected families.

Painting of fourth, a chromosome-specific protein in Drosophila

Abstract: Chromosome-specific gene regulation is known thus far only as a mechanism to equalize the transcriptional activity of the single male X chromosome with that of the two female X chromosomes In Dros

Absence of correlation between late-replication and spreading of X inactivation in an X;autosome translocation.

Abstract: We have analysed the spread of X inactivation in an individual with an unbalanced 46,X,der(X)t(X;10)(q26.3;q23.3) karyotype. Despite being trisomic for the region 10q23.3-qter, both the proband and her aunt with the same karyotype presented only with secondary amenorrhoea and lacked any features normally associated with trisomy of distal 10q. Cytogenetic and molecular studies showed that the derivative X;10 chromosome was exclusively inactive. Transcribed polymorphisms were identified in five genes contained within the translocated region of chromosome 10 and were used to perform allele-specific transcription studies. We showed that four of the genes studied are inactive on the derivative chromosome, directly demonstrating the spread of X inactivation over some 30 Mb of autosomal DNA. However, the most distal gene examined remained active, indicating that this spreading was incomplete. In contrast to the gene expression data, replication timing studies showed no spreading of late replication into the translocated portion of 10q. We conclude that silencing of autosomal genes by X inactivation can occur without a delay in the replication timing of the surrounding chromatin. Our findings support the hypothesis that autosomal chromatin lacks certain features present on the X chromosome that are required for the effective spread and/or maintenance of X inactivation.

The X chromosome and the ovary.

Abstract: X chromosome abnormalities are the leading identifiable cause of premature ovarian failure (POF). POF-related abnormalities range from the complete absence of one X chromosome to assorted deletions and translocations to mutations in specific genes. The diversity of X chromosome abnormalities associated with POF indicates that the disorder is genetically heterogeneous. Potential molecular mechanisms include both dominant and recessive mutations in X-linked genes as well as nonspecific chromosome effects that impair meiosis. A list of candidate X-linked POF genes is emerging from molecular studies of X chromosome abnormalities, data from the Human Genome Project and related functional genomics projects, and the results of gene targeting experiments in mice. Mutational analysis of candidate genes in a large number of women with idiopathic POF is needed to determine which of these genes contribute to the cause of this disorder.

Equality of the sexes: mammalian dosage compensation.

Abstract: X chromosome inactivation achieves dosage equivalence for most X-linked genes between the two X chromosomes in females and the single X chromosome in males. In this article the evidence for random inactivation of an X chromosome is reviewed, along with the exceptions that result in nonrandom inactivation. Another exception to X chromosome inactivation is the presence of genes that escape inactivation and are expressed from both the active and inactive X chromosomes. The phenotypic consequences of such expression from the inactive X chromosome are discussed. The major players in the process of inactivation are presented. Initiation of inactivation requires the functional RNA, XIST, and the subsequent stable inactivation of the X chromosome relies upon the recruitment of many other factors, the majority of which are generally associated with heterochromatin.