X chromosome

Abstract: Ohno and Hauschka1 showed that in female mice one chromosome of mammary carcinoma cells and of normal diploid cells of the ovary, mammary gland and liver was heteropyknotic. They interpreted this chromosome as an X-chromosome and suggested that the so-called sex chromatin was composed of one heteropyknotic X-chromosome. They left open the question whether the heteropyknosis was shown by the paternal X-chromosome only, or the chromosome from either parent indifferently.

Abstract: X-linked spinal and bulbar muscular atrophy (Kennedy's disease) is an adult-onset form of motorneuron disease which may be associated with signs of androgen insensitivity. We have now investigated whether the androgen receptor gene on the proximal long arm of the X chromosome is a candidate gene for this disease. In patient samples we found androgen receptor gene mutations with increased size of a polymorphic tandem CAG repeat in the coding region. These amplified repeats were absolutely associated with the disease, being present in 35 unrelated patients and none of 75 controls. They segregated with the disease in 15 families, with no recombination in 61 meioses (the maximum log likelihood ratio (lod score) is 13.2 at a recombination rate of 0). The association is unlikely to be due to linkage disequilibrium, because 11 different disease alleles were observed. We conclude that enlargement of the CAG repeat in the androgen receptor gene is probably the cause of this disorder.

Abstract: The initiation of male development in mammals requires one or more genes on the Y chromosome. A recently isolated gene, termed SRY in humans and Sry in mouse, has many of the genetic and biological properties expected of a Y-located testis-determining gene. It is now shown that Sry on a 14-kilobase genomic DNA fragment is sufficient to induce testis differentiation and subsequent male development when introduced into chromosomally female mouse embryos.

Abstract: The male-specific region of the Y chromosome, the MSY, differentiates the sexes and comprises 95% of the chromosome's length. Here, we report that the MSY is a mosaic of heterochromatic sequences and three classes of euchromatic sequences: X-transposed, X-degenerate and ampliconic. These classes contain all 156 known transcription units, which include 78 protein-coding genes that collectively encode 27 distinct proteins. The X-transposed sequences exhibit 99% identity to the X chromosome. The X-degenerate sequences are remnants of ancient autosomes from which the modern X and Y chromosomes evolved. The ampliconic class includes large regions (about 30% of the MSY euchromatin) where sequence pairs show greater than 99.9% identity, which is maintained by frequent gene conversion (non-reciprocal transfer). The most prominent features here are eight massive palindromes, at least six of which contain testis genes.

Abstract: In female mammals, most genes on one X chromosome are silenced as a result of X-chromosome inactivation. However, some genes escape X-inactivation and are expressed from both the active and inactive X chromosome. Such genes are potential contributors to sexually dimorphic traits, to phenotypic variability among females heterozygous for X-linked conditions, and to clinical abnormalities in patients with abnormal X chromosomes. Here, we present a comprehensive X-inactivation profile of the human X chromosome, representing an estimated 95% of assayable genes in fibroblast-based test systems. In total, about 15% of X-linked genes escape inactivation to some degree, and the proportion of genes escaping inactivation differs dramatically between different regions of the X chromosome, reflecting the evolutionary history of the sex chromosomes. An additional 10% of X-linked genes show variable patterns of inactivation and are expressed to different extents from some inactive X chromosomes. This suggests a remarkable and previously unsuspected degree of expression heterogeneity among females.