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.

The molecular genetics of the corneal dystrophies--current status.

Abstract: The pertinent literature on inherited corneal diseases is reviewed in terms of the chromosomal localization and identification of the responsible genes. Disorders affecting the cornea have been mapped to human chromosome 1 (central crystalline corneal dystrophy, familial subepithelial corneal amyloidosis, early onset Fuchs dystrophy, posterior polymorphous corneal dystrophy), chromosome 4 (Bietti marginal crystalline dystrophy), chromosome 5 (lattice dystrophy types 1 and IIIA, granular corneal dystrophy types 1, 2 and 3, Thiel-Behnke corneal dystrophy), chromosome 9 (lattice dystrophy type II), chromosome 10 (Thiel-Behnke corneal dystrophy), chromosome 12 (Meesmann dystrophy), chromosome 16 (macular corneal dystrophy, fish eye disease, LCAT disease, tyrosinemia type II), chromosome 17 (Meesmann dystrophy, Stocker-Holt dystrophy), chromosome 20 (congenital hereditary endothelial corneal dystrophy types I and II, posterior polymorphous corneal dystrophy), chromosome 21 (autosomal dominant keratoconus) and the X chromosome (cornea verticillata, cornea farinata, deep filiform corneal dystrophy, keratosis follicularis spinulosa decalvans, Lisch corneal dystrophy). Mutations in nine genes (ARSC1, CHST6, COL8A2, GLA, GSN, KRT3, KRT12, M1S1and TGFBI [BIGH3]) account for some of the corneal diseases and three of them are associated with amyloid deposition in the cornea (GSN, M1S1, TGFBI) including most of the lattice corneal dystrophies (LCDs) [LCD types I, IA, II, IIIA, IIIB, IV, V, VI and VII] recognized by their lattice pattern of linear opacities. Genetic studies on inherited diseases affecting the cornea have provided insight into some of these disorders at a basic molecular level and it has become recognized that distinct clinicopathologic phenotypes can result from specific mutations in a particular gene, as well as some different mutations in the same gene. A molecular genetic understanding of inherited corneal diseases is leading to a better appreciation of the pathogenesis of these conditions and this knowledge has made it imperative to revise the classification of inherited corneal diseases.

Close linkage of fragile X-mental retardation syndrome to haemophilia B and transmission through a normal male

Abstract: The fragile X-mental retardation syndrome is defined by a moderate to severe mental retardation associated with a cytogenetic marker, a fragile site localized on the long arm of the X chromosome at band Xq 27. This syndrome has recently been recognized as one of the major causes of genetically determined mental retardation, and as one of the most important X-linked diseases with respect to its frequency (analogous to that of Duchenne muscular dystrophy or of haemophilia A) and severity. In the absence of treatment, genetic screening for this disease would seem particularly important. Prenatal diagnosis is now feasible although difficult and detection of heterozygous carriers is only possible in approximately 50% of cases. The recent demonstration of genetic linkage between the glucose 6-phosphate dehydrogenase (G6PD)-colour blindness cluster (at Xq28) and the fragile X locus has suggested that the fragile site is indeed the site of the mutation. We show here that the fragile X and haemophilia B loci are closely linked, using as genetic marker a polymorphism of the coagulation factor IX gene. Our study of a large family has demonstrated transmission through a phenotypically normal male, a feature previously described in retrospective analysis of a few other fragile X pedigrees. Restriction polymorphisms associated with the factor IX gene should be useful for analysing this peculiar aspect of the genetics of the fragile X syndrome, and for genetic screening of the disease.

An apparent excess of sex- and reproduction related genes on the human X chromosome

Abstract: We describe here the results of a search of Mendelian inheritance in man, GENDIAG and other sources which suggest that, in comparison with autosomes 1, 2, 3, 4 and 11, the X chromosome may contain a significantly higher number of sex- and reproduction-related (SRR) genes. A similar comparison between X-linked entries and a subset of randomly chosen entries from the remaining autosomes also indicates an excess of genes on the X chromosome with one or more mutations affecting sex determination (e.g. DAX1), sexual differentiation (e.g. androgen receptor) or reproduction (e.g. POF1). A possible reason for disproportionate occurrence of such genes on the X chromosome could be that, during evolution, the 'choice' of a particular pair of homomorphic chromosomes for specialization as sex chromosomes may be related to the number of such genes initially present in it or, since sex determination and sexual dimorphism are often gene dose-dependent processes, the number of such genes necessary to be regulated in a dose-dependent manner. Further analysis of these data shows that XAR, the region which has been added on to the short arm of the X chromosome subsequent to eutherian-marsupial divergence, has nearly as high a proportion of SRR genes as XCR, the conserved region of the X chromosome. These observations are consistent with current hypotheses on the evolution of sexually antagonistic traits on sex chromosomes and suggest that both XCR and XAR may have accumulated SRR traits relatively rapidly because of X linkage.

X inactivation in mammalian testis is correlated with inactive X–specific transcription

Abstract: X chromosome inactivation occurs twice during the mammalian life cycle. In females one of the two X chromosomes of somatic nuclei is inactive, while in males the solitary X chromosome is inactivated during germ cell development. Despite the different properties of the inactivated chromosomes of females and males, the molecular initiation of inactivation may be the same. X inactive–specific transcripts, XIST, are produced from somatic inactivated X chromosomes. We demonstrate here the existence of XIST transcripts in testes of man and mouse. Inactivation of X chromosomes in males, as in females, may thus be mediated through XIST. Conceivably, the silencing of X–linked genes is the price paid for the evolution of successful mechanisms of chromosomal sex determination.