Functional disomies of the X chromosome influence the cell selection and hence the X inactivation pattern in females with balanced X-autosome translocations: a review of 122 cases

Xq-Yq interchange resulting in supernormal X-linked gene expression in severely retarded males with 46,XYq- karyotype

The critical importance of dosage compensation is underscored by a novel human syndrome ("XYXq syndrome") in which we have detected partial X disomy, demonstrated supernormal gene expression resulting from the absence of X inactivation, and correlated this overexpression with its phenotypic consequences. Studies of three unrelated boys with 46,XYq- karyotypes and anomalous phenotypes (severe mental retardation, generalized hypotonia and microcephaly) show the presence of a small portion of distal Xq on the long arm of the Y derivative. Cells from these boys exhibit twice-normal activity of glucose-6-phosphate dehydrogenase, a representative Xq28 gene product. In all three cases, the presence of Xq DNA on a truncated Y chromosome resulted from an aberrant Xq-Yq interchange occurring in the father's germline.

Partial X chromosome trisomy with functional disomy of Xp due to failure of X inactivation

A 5-month-old girl with mild phenotypic abnormalities, developmental delay, and seizures was found to have the de novo karyotype 46,XX,-13,+der(13)t(X;13)(p21.2;p11.1). The partial trisomy of Xp21.2-->pter was confirmed with fluorescence in situ hybridization, using an X chromosome painting probe and several cosmid and YAC probes for Xp sequences. Replication banding showed that one of the structurally normal X chromosomes was late-replicating, but that the Xp segment of the der(13) was early-replicating in all cells examined. Since segments of the X chromosome separated from the X inactivation center in Xq13.2 cannot undergo X inactivation, the result is functional disomy of distal Xp. As the loss of short arm material from chromosome 13 is not considered to be clinically significant, the genomic imbalance of Xp expressed in this patient most likely accounts for her abnormal phenotype.

The molecular biology of Norrie's disease

The Norrie's disease gene has been accurately located on the short arm of the X chromosome. The methodology underlying this achievement and the structure of the three-exon gene is described in this review article. The clinical implications of these recent advances are discussed. Allelic variants of Norrie's disease and the phenomenon of females affected by X-linked disease are also discussed.

Premature ovarian failure and ovarian dysgenesis associated with balanced and unbalanced X-6 translocations, respectively: implications for the investigation of ovarian failure

This study reports the effect of an inherited (X;6) translocation which has not previously been described. The proband was intellectually delayed and had ovarian dysgenesis. Karyotyping revealed an unbalanced karyotype: 46,X,der(X)t(X;6)(q22; p11.2)*. Her mother was shown to be a carrier of an apparently balanced translocation between the X chromosome and chromosome 6: 46,X,t(X;6)(q22;p11.2). This finding in the mother raises to 7 the number of cases reported which involve a break within the X chromosome 'critical region', at band Xq22, without causing ovarian dysgenesis, although it was associated with premature ovarian failure. These cases aim to highlight to clinical specialists the range of gonadal and other phenotypic anomalies (apart from those associated with Turner syndrome) which can occur due to partial deletions of the X chromosome. These findings have implications for the investigation of both ovarian dysgenesis and premature ovarian failure.

Isolation of a yeast artificial chromosome contig spanning the X chromosomal translocation breakpoint in a patient with Rett syndrome

Rett syndrome is a neurodevelopmental disorder observed exclusively in females. A de novo X;3 translocation was detected in a patient (TH) with Rett syndrome. The X chromosomal breakpoint maps to Xp21.3 between the distal end of the Duchenne muscular dystrophy (DMD) gene and the DXS28 (C7) locus. To determine if this translocation caused the Rett syndrome in this patient, our efforts focused on mapping and cloning of the X chromosomal breakpoint in this patient. Toward these goals, we generated a set of radiation-reduced hybrid cell lines for the short arm of the X chromosome to use as a source for region-specific markers. Using Alu-PCR, 13 new DNA markers were isolated from a radiation-reduced hybrid, which retained both DMD and DXS28. These markers were localized within Xp21 using DNA from males with various interstitial deletions in this region. Two new markers, K23-2p and K23b-1, were found to be closer flanking markers to the X chromosomal breakpoint than DMD and DXS28. Long range restriction mapping using K23-2p and K23b-1 determined that the maximum distance between them was 800 kb. Several of the new markers were developed into sequence tagged-sites and were used to isolate yeast artificial chromosome (YAC) clones. A total of 22 YAC clones was isolated and characterized; these YACs were then developed into 3 large contigs in the Xp21.3 region. This effort resulted in the cloning of the region containing the X chromosomal translocation breakpoint of the Rett syndrome patient in a 170-kb YAC clone.