An interstitial duplication of the X chromosome in a male allows physical fine mapping of probes from the Xq13-q22 region

Summary An insertional translocation into the proximal long arm of the X chromosome in a boy showing muscular hypotony, growth retardation, psychomotor retardation, cryptorchidism, and Pelizaeus-Merzbacher disease (PMD) was identified as a duplication of the Xq21–q22 segment by employing DNA probes. With densitometric scanning for quantitation of hybridization signals, 15 Xq probes were assigned to the duplicated region. Analysis of the duplication allowed us to dissect the X-Y homologous region physically at Xq21 and to refine the assignments of the loci for DXYS5, DXYS12, DXYS13, DXS94, DXS95, DXS96, DXS111, and DXS211. Furthermore, we demonstrated the presence of two different DXYS13, and DXS17 alleles in genomic DNA of our patient, suggesting that the duplication resulted from a meiotic recombination event involving the two maternal X chromosomes.     

Physical fine mapping of genes underlying X-linked deafness and non fra(X)-X-linked mental retardation at Xq21

Summary Linkage studies and cytogenetically visible deletions associated with nonspecific X-linked mental retardation (XLMR) and a specific form of deafness (DFN3) have indicated that the genes responsible for these disorders are located at Xq21. Using DNA probes from this region, we have studied several overlapping deletions spanning different parts of Xq21. This has enabled us to assign the DFN3 gene and a gene for nonspecific XLMR to an interval that encompasses the locus DXS232 and that is flanked by DXS26 and DXS121.     

Exclusion mapping of the X-linked dominant chondrodysplasia punctata/ichthyosis/cataract/short stature (Happle) syndrome: possible involvement of an unstable pre-mutation

Summary Homology with the mouse bare patches mutant suggests that the gene for the X-linked dominant chondrodysplasia punctata / ichthyosis / cataract / short stature syndrome (Happle syndrome) is located in the human Xq28 region. To test this hypothesis, we performed a linkage study in three families comprising a total of 12 informative meioses. Multiple recombinations appear to exclude the Xq28 region as the site of the gene. Surprisingly, multiple crossovers were also found with 26 other markers spread along the rest of the X chromosome. Two-point linkage analysis and analysis of recombination chromosomes seem to exclude the gene from the entire X chromosome. Three different mechanisms are discussed that could explain the apparent exclusion of an X-linked gene from the X chromosome by linkage analysis: (a) different mutations on the X chromosome disturbing X inactivation, (b) metabolic interference, i.e. allele incompatibility of an X-linked gene, and (c) an unstable pre-mutation that can become silent in males. We favour the last explanation, as it would account for the unexpected sex ratio (MF) of 1.21 among surviving siblings, and for the striking clinical variability of the phenotype, including stepwise increases in disease expression in successive generations.     

Toward early diagnosis of myotonic dystrophy: construction and characterization of a somatic cell hybrid with a single human der(19) chromosome

We have constructed a somatic cell hybrid line, designated 908K1, with a single human der(19) chromosome on a Chinese hamster background by employing conventional as well as microcell-mediated cell fusion techniques. The der(19) chromosome comprises the 19p13.1----q13.2 segment, as well as the distal (Xq24----qter) portion of the X chromosome long arm, and is stably retained by HAT selection. Extensive characterization of this hybrid line and comparison with other somatic cell hybrids has enabled us to regionally assign PGK2 to the distal short arm of chromosome 19 and to narrow down the assignments of CYP1, TGFB, and ERCC1 on 19q. Moreover, a cosmid library has been constructed from this microcell hybrid. By screening this library, as well as a chromosome 19-enriched library obtained elsewhere, 14 single-copy probes have been isolated that map on the 19p13.1----q13.2 segment, and 5 probes were assigned to the distal Xq. It is anticipated that these probes will be useful for the diagnosis of myotonic dystrophy and fra(X) mental retardation.     

Genetic and physical mapping of a novel region close to the fragile X site on the human X chromosome

We report the isolation and characterization of a novel DNA marker (1A1) in Xqter in the region of the fragile X. Genetic studies in families segregating for the fragile X syndrome suggest that 1A1 lies between the disease mutation and the distal locus, DXS52. Studies in normal and fragile X families show that 1A1 is tightly linked to DXS52 (Zmax = 17.20; theta max = 0.03) and F8 (Zmax = 7.01; theta max = 0.08). Multipoint mapping of families supports the order Xcen-DXS105-FRAXA-1A1-DXS52-(F8, DXS115)-Xqter. Pulsed-field gel electrophoresis (PFGE) studies demonstrate that 1A1 defines a new region of at least 2 Mb of DNA not physically linked to DXS52 or F8, thus extending the physical map of Xq27-qter to over 4 Mb. Complex partial digestion PFGE patterns, probably due to differing degrees of methylation, are observed with 1A1 in unrelated normal and fragile-X-positive individuals, whereas other distal markers give uniform digestion profiles. Physical data suggest that 1A1 lies in a region less CpG rich than other distal markers in Xq27-qter.     

Analysis of a second family with hereditary non-chromaffin paragangliomas locates the underlying gene at the proximal region of chromosome 11q

The gene for autosomal, dominantly inherited, non-chromaffin paragangliomas has previously been mapped at 11q23-qter by linkage analysis of a single family. In the present study, we have used genetic markers from 11q for the analysis of two distantly related pedigrees with the same disorder. Linkage analysis and haplotyping indicate that the gene underlying the disorder in the present family is located on chromosome 11q proximal to the tyrosinase gene locus (11q14–q21). Closely linked markers are the human homologue of the murine INT2 protooncogene and the anonymous DNA marker D11S527. A maximum lod score of 5.4 (=0.0) has been obtained for linkage between the disorder and the chromosomal region defined by these markers. The human INT2 gene can be regarded as a candidate for the disorder on the basis of its expression pattern during embryogenesis in the mouse. However, haplotype analysis indicates that this gene is probably not the predisposing genetic factor in the present family.     

X-linked myoclonus epilepsy explained as a maternally inherited mitochondrial disorder

A family with myoclonus epilepsy has been described previously as suffering from an X-linked disorder, because at least four males were affected, and only mild and variable symptoms were seen in some female carriers. In this family, we have now identified a mitochondrial AG (8344) heteroplasmic point mutation. This point mutation has been described in families with maternally inherited myoclonus epilepsy and ragged red fibers. The degree of severity of the disorder in the different family members was reflected in the relative quantity of mutated mitochondrial DNA. It is concluded that the mode of inheritance in this family is not X-linked but maternal.     

A High-Resolution Interval Map of the q21 Region of the Human X Chromosome

In a previous study, we have developed a panel of chromosomal rearrangements for the physical mapping of the q13-q21 region of the human X chromosome (Philippe et al., Genomics 17: 147-152, 1993). Here, we report the physical localization of 36 additional polymorphic markers by polymerase chain reaction analysis. The high density of chromosomal breakpoints in Xq21 allows us to map 58 DNA loci in 22 intervals. As a result, this segment of the X chromosome is saturated with approximately three sequence tagged sites per megabase of DNA, which will facilitate the construction of a YAC contig of this region.     

Toward a complete linkage map of the human X chromosome: regional assignment of 16 cloned single-copy DNA sequences employing a panel of somatic cell hybrids.

Closely linked restriction fragment length polymorphisms (RFLPs) are potentially useful as diagnostic markers of genetic defects, and, in principle, RFLPs can be employed to construct a complete linkage map of the human genome. On the X chromosome, linkage studies are particularly rewarding because in man more than 120 X-linked genes are known. Thus, it is probable that each X-specific RFLP will be of use as a genetic marker of one or several X-linked disorders. To facilitate the search for closely linked RFLPs, we have regionally assigned 16 cloned DNA sequences to various portions of the human X chromosome, employing a large panel of somatic cell hybrids. These probes have been used to correlate genetic and physical distances on Xp, and it can be extrapolated from these data that the number and distribution of available Xq sequences will also suffice to span the long arm of the X chromosome.