An anonymous single copy X-chromosome clone, pTAK8, identifies a frequent RFLP at Xp11-q12(HGM8 no. DXS146)

X-linked retinitis pigmentosa: new map studies of XLRP2, and a possible human centromere effect

A new large Danish family with X-linked retinitis pigmentosa was studied for linkage analysis. Carrier diagnosis was performed using full-field electro-retinogram combined with a careful fundus examination. Multipoint linkage analysis, employing DNA markers from the proximal short arm of the X chromosome and the cytogenetic centromere marker, revealed the highest location score distally to DXS255 and proximal to the ornithine carbamoyl transferase locus. In comparison with the first Danish family that we studied, the pericentromeric recombination fraction was increased; it is speculated that the observed difference in genetic distances from the centromere in the 2 Danish families is correlated with a difference in the size and location of the centromeric heterochromatin.

A highly polymorphic dinucleotide repeat on the proximal short arm of the human X chromosome: linkage mapping of the synapsin I/A-raf-1 genes

A compound (AC)n repeat located 1,000 bp downstream from the human synapsin I gene and within the last intron of the A-raf-1 gene has been identified. DNA data-base comparisons of the sequences surrounding the repeat indicate that the synapsin I gene and the A-raf-1 gene lie immediately adjacent to each other, in opposite orientation. PCR amplification of this synapsin I/A-raf-1 associated repeat by using total genomic DNA from members of the 40 reference pedigree families of the Centre d'Etude du Polymorphisme Humaine showed it to be highly polymorphic, with a PIC value of .84 and a minimum of eight alleles. Because the synapsin I gene has been mapped previously to the short arm of the human X chromosome at Xp11.2, linkage analysis was performed with markers on the proximal short arm of the X chromosome. The most likely gene order is DXS7SYN/ARAF1TIMPDXS255DXS146, with a relative probability of 5 x 10(8) as compared with the next most likely order. This highly informative repeat should serve as a valuable marker for disease loci mapped to the Xp11 region.

Linkage relationships of the Wiskott-Aldrich syndrome to 10 loci in the pericentromeric region of the human X chromosome

Twelve families with Wiskott-Aldrich syndrome (WAS) were studied by linkage analysis using 10 polymorphic marker loci from the X-chromosome pericentromeric region. The results confirm close linkage of WAS to the DXS14, DXS7, TIMP, and DXZ1 loci and are consistent with previous data suggesting that WAS maps to the proximal Xp and is flanked by the DXS14 and DXS7 loci. The strongest linkage (Z = 10.19 at theta = 0.00) was found to be between WAS and the hypervariable DXS255 locus, a marker locus already mapped between DXS7 and DXS14 and which was informative for all meioses included in this analysis. Linkage of the WAS to two pericentromeric Xq loci, DXS1 and PGK1, was also established. On the basis of these results, accurate predictive testing should now be feasible in the majority of WAS families.

Physical fine-mapping of a deletion spanning the Norrie gene

Norrie disease (ND), atrophia bulborum hereditaria, is caused by a gene defect on the proximal short arm of the X-chromosome. As shown by us and others, microdeletions spanning the DXS7 locus are not uncommon in this disorder, and there is recent evidence that, at least in some of the Norrie deletion patients, the monoamine oxidase (MAO) A and B genes are deleted as well. Molecular hybridization experiments with 19 cloned DNA fragments have enabled us to construct a preliminary long-range restriction map around DXS77, DXS7, MAO-A and MAO-B, and to localize the distal end point of an ND deletion between DXS77 and DXS7.

X-linked severe combined immunodeficiency: localization within the region Xq13.1-q21.1 by linkage and deletion analysis

X-linked severe combined immunodeficiency (SCID) (McKusick 30040; IMD4) is a disease of unknown pathogenesis characterized by severe and persistent infections from early in life that are due to absence of both cellular and humoral immune function. Although the disease has been provisionally mapped to proximal Xq, high lethality and lack of a carrier test have limited the number of scorable meioses. We performed linkage analysis in six new kindreds with X-linked SCID, using a random pattern of T-cell X inactivation to rule out the carrier state in at-risk women. Our linkage results, combined with analysis of Xq interstitial deletions, confirmed the regional assignment of X-linked SCID, narrowed the boundaries within which this locus lies to Xq13.1-q21.1, and established the locus order DXS159-(PGK1, SCID)-DXS72-DXS3, defining flanking markers for prenatal diagnosis and carrier testing.

Linkage studies do not confirm the cytogenetic location of incontinentia pigmenti on Xp11

Linkage studies have been performed in 5 incontinentia pigmenti (IP) families totaling 29 potentially informative meioses. Ten probes of the Xp arm were used, six of them were precisely localized on the X chromosome, using hamster X human somatic cell hybrids containing a broken X chromosome derived from an incontinentia pigmenti patient carrying an X;9 translocation [46,XX,t(X;9)(p11.21;q34)]. The following order for probes is proposed: pter - (DXS7, DXS146, DXS255) - IP1 - (DXS14, DXS90) - DXS106 - qter. The negative lod scores obtained exclude the possibility that in the families studied, the gene for IP is located in Xp11 or in the major part of the Xp arm.

The gene for incontinentia pigmenti: failure of linkage studies using DNA probes to confirm cytogenetic localization

Probes for restriction fragment length polymorphisms mapping between Xp21 and Xq22.3 have been used in a linkage study of incontinentia pigmenti (IP). Six independent sporadic cases of disorders resembling IP with X-autosome translocations involving the same X chromosome breakpoint (Xp11) have been reported. These observations suggest that the IP gene may be located in the Xp11 chromosomal region. However, the linkage study with DNA probes has failed to confirm this localisation.

X-linked lymphoproliferative disease: linkage studies using DNA probes

Linkage studies have been carried out with 28 X-linked polymorphic probes to try to locate the gene for X-linked lymphoproliferative disease (XLP). DNA from three families has been analysed, including three affected boys among 21 family members. None of the probes tested has been found to be linked to XLP. However, the data are recorded for the use of other workers on this rare disease.

Close linkage between X-linked ectodermal dysplasia and a cloned DNA sequence detecting a two allele restriction fragment length polymorphism in the region Xp11-q12

EDA (ectodermal dysplasia, anhidrotic) is an X-linked recessive disorder characterized by hypohidrosis, hypoor anodontia, and hypotrichosis. A possible linkage between the gene for EDA and a number of restriction fragment length polymorphisms (RFLPs) spread over the X chromosome was investigated in two Danish families segregating EDA. No recombination between the gene for EDA and our probe pTAK8, which detects a two allele polymorphism in the region Xp11-q12, was found in nine informative meiotic events (seven of which are phase known), giving a maximal lod score of 2.41 at a recombination fraction of 0.00. This juxtacentromeric location of the gene for EDA agrees well with the linkage data obtained with the other markers used in this study.