Automated fluorescent genotyping detects 10% of cryptic subtelomeric rearrangements in idiopathic syndromic mental retardation.

Recent studies have shown that cryptic unbalanced subtelomeric rearrangements contribute to a significant proportion of idiopathic syndromic mental retardation cases. Using a fluorescent genotyping based strategy, we found a 10% rate of cryptic subtelomeric rearrangements in a large series of 150 probands with severe idiopathic syndromic mental retardation and normal RHG-GTG banded karyotype. Fourteen children were found to carry deletions or duplications of one or more chromosome telomeres and two children had uniparental disomy. This study clearly shows that fluorescent genotyping is a sensitive and cost effective method that not only detects cryptic subtelomeric rearrangements but also provides a unique opportunity to detect uniparental disomies. We suggest giving consideration to systematic examination of subtelomeric regions in the diagnostic work up of patients with unexplained syndromic mental retardation.

The molecular basis of X-linked spondyloepiphyseal dysplasia tarda.

The X-linked form of spondyloepiphyseal dysplasia tarda (SEDL), a radiologically distinct skeletal dysplasia affecting the vertebrae and epiphyses, is caused by mutations in the SEDL gene. To characterize the molecular basis for SEDL, we have identified the spectrum of SEDL mutations in 30 of 36 unrelated cases of X-linked SEDL ascertained from different ethnic populations. Twenty-one different disease-associated mutations now have been identified throughout the SEDL gene. These include nonsense mutations in exons 4 and 5, missense mutations in exons 4 and 6, small (2-7 bp) and large (>1 kb) deletions, insertions, and putative splicing errors, with one splicing error due to a complex deletion/insertion mutation. Eight different frameshift mutations lead to a premature termination of translation and account for >43% (13/30) of SEDL cases, with half of these (7/13) being due to dinucleotide deletions. Altogether, deletions account for 57% (17/30) of all known SEDL mutations. Four recurrent mutations (IVS3+5G-->A, 157-158delAT, 191-192delTG, and 271-275delCAAGA) account for 43% (13/30) of confirmed SEDL cases. The results of haplotype analyses and the diverse ethnic origins of patients support recurrent mutations. Two patients with large deletions of SEDL exons were found, one with childhood onset of painful complications, the other relatively free of additional symptoms. However, we could not establish a clear genotype/phenotype correlation and therefore conclude that the complete unaltered SEDL-gene product is essential for normal bone growth. Molecular diagnosis can now be offered for presymptomatic testing of this disorder. Appropriate lifestyle decisions and, eventually, perhaps, specific SEDL therapies may ameliorate the prognosis of premature osteoarthritis and the need for hip arthroplasty.

A novel automated strategy for screening cryptic telomeric rearrangements in children with idiopathic mental retardation.

Cryptic unbalanced subtelomeric rearrangements are known to cause a significant proportion of idiopathic mental retardation in childhood. Because of the limited sensitivity of routine analyses, the cytogenetic detection of such rearrangements requires molecular techniques, namely FISH and comparative genomic hybridisation (CGH). An alternative approach consists in using genetic markers to detect segmental aneusomy. Here, we describe a new strategy based upon automated fluorescent genotyping to search for non mendelian segregation of telomeric microsatellites. A total of 29 individuals belonging to 24 unrelated families were screened and three abnormal patterns of segregation were detected (two rearrangements and one parental disomy). This study gives strong support to the view that cryptic telomeric rearrangements significantly contribute to idiopathic mental retardation and demonstrates that fluorescent genotyping is a very sensitive and cost-effective method to detect deletions, duplications and uniparental disomies.

Genomic rearrangement in NEMO impairs NF-kappaB activation and is a cause of incontinentia pigmenti. The International Incontinentia Pigmenti (IP) Consortium.

Familial incontinentia pigmenti (IP; MIM 308310) is a genodermatosis that segregates as an X-linked dominant disorder and is usually lethal prenatally in males. In affected females it causes highly variable abnormalities of the skin, hair, nails, teeth, eyes and central nervous system. The prominent skin signs occur in four classic cutaneous stages: perinatal inflammatory vesicles, verrucous patches, a distinctive pattern of hyperpigmentation and dermal scarring. Cells expressing the mutated X chromosome are eliminated selectively around the time of birth, so females with IP exhibit extremely skewed X-inactivation. The reasons for cell death in females and in utero lethality in males are unknown. The locus for IP has been linked genetically to the factor VIII gene in Xq28 (ref. 3). The gene for NEMO (NF-kappaB essential modulator)/IKKgamma (IkappaB kinase-gamma) has been mapped to a position 200 kilobases proximal to the factor VIII locus. NEMO is required for the activation of the transcription factor NF-kappaB and is therefore central to many immune, inflammatory and apoptotic pathways. Here we show that most cases of IP are due to mutations of this locus and that a new genomic rearrangement accounts for 80% of new mutations. As a consequence, NF-kappaB activation is defective in IP cells.

Genetic mapping of Xp22.12-p22.31, with a refined localization for spondyloepiphyseal dysplasia (SEDL).

Spondyloepiphyseal dysplasia tarda (SEDL) is an X-linked recessive disorder characterized in affected males by short stature resulting from a growth defect of the vertebral bodies. We have extended our earlier studies by analyzing 15 families with newly identified microsatellite DNA markers; analysis of recombination events with these markers indicates that the gene responsible for SEDL is located in Xp22 between DXS 16 and DXS 987 on an interval spanning approximately 2 Mb.

Fine deletion mapping of the p22 region of the human X chromosome using a radiation-induced hybrid panel.

Radiation-induced somatic cell hybrids containing fragments of the human X chromosome were constructed. A panel of 17 hybrids was selected with the help of known markers in the Xp22 region. These hybrids identified 11 different breakpoints between Xp22.2 and Xp21.3. Eight markers were located in eight of the nine corresponding intervals, resulting in the following physical map: tel...DXS89-DXS278-DXS85-(DXS1224, DXS16)-(GLRA2, DXS987)-DXS207-(DXS-197, DXS1053)-(DXS43, DXS1195)-(DXS1229, DXS-999)-(DXS1052, DXS92, DXS274)-(DXS41, DXS1226)-DXS1198-DXS28...cen.

The gene for the familial form of incontinentia pigmenti (IP2) maps to the distal part of Xq28.

Linkage data for familial incontinentia pigmenti (IP2) and 17 X chromosomal markers are reported. The linkage previously found between IP2 and the F8C locus is confirmed (Z max = 11.85 at theta = 0.028). Linkage is established with distal markers DXS1108 (Z max = 10.06 at theta = 0.00) and DXYS154 (Z = 9.07 at theta = 0.019). Multipoint analysis supports the distal localization of the IP2 gene with respect to the F8C locus.

The gene for spondyloepiphyseal dysplasia (SEDL) maps to Xp22 between DXS16 and DXS92.

Previous linkage studies in X-linked spondyloepiphyseal dysplasia (SEDL) placed the gene in the region Xp22.2-p22.1 by linkage to DXS41. Here we have extended our earlier studies by analyzing 15 families with 13 markers from the Xp22 region. Pairwise linkage analysis revealed significant linkage of the SEDL to 8 markers from the Xp22.2-Xp22.1 region. Maximum lod scores were obtained with DXS207, tau max = 9.16 at theta max = 0.021 with confidence limits of 0.00-0.09, and DXS197, tau max = 7.98 at theta max = 0.00 with confidence limits of 0.00-0.06. The study of one recombinant in family 4 indicated that DXS 41 is more likely proximal to DXS92 than distal. Multipoint linkage results and analysis of recombination events indicated that the mutation responsible for SEDL is located in Xp22 between DXS 16 and DXS 92.

Isolation and characterisation of five new anonymous X-specific probes. One of them detecting a TaqI RFLP.

The authors isolated five single-copy X-specific probes from an X-enriched library. These probes were regionally localized on the X chromosome by using somatic hybrid cell lines obtained from patients carrying different X-autosome translocations. Three clones were located between Xq23 and Xq26, the two others were mapped between Xp21 and Xp11.2. Analyses with different restriction enzymes indicated that one of them detects a TaqI polymorphism with a polymorphism information content (PIC) of 0.28.

Alport syndrome: a genetic study of 31 families.

Thirty one families with Alport syndrome including 3 families with associated syndromes were studied. The location of the COL4A5 gene, responsible for the Alport syndrome, was determined by linkage analysis with eight probes of the Xq arm and by a radiation hybrid panel. Concordant data indicated the localization of the Alport gene between DXS17 and DXS11. Four deletions and one single base mutation of the COL4A5 gene were detected. Homogeneity tests failed to show any evidence of genetic heterogeneity superimposed on clinical heterogeneity for ophthalmic signs and end-stage renal disease age.

Linkage relationship between incontinentia pigmenti (IP2) and nine terminal X long arm markers.

Linkage data for familial incontinentia pigmenti (IP2) and nine X chromosomal markers are reported. Previously found linkage between IP2 and the DXS52 locus is confirmed with the maximum lod score of 6.19 at a recombination fraction of 0.03. Linkage is also established with loci DXS134, DXS15 and DXS33. Multipoint analysis allows us to localize the IP2 locus outside a block of seven linked markers of the Xq28 region.

[An example of detection of heterozygotes and antenatal diagnosis in four families with anhidrotic ectodermal dysplasia]. / Un exemple de diagnostic d'hétérozygotes et de diagnostic anténatal dans quatre familles de dysplasie ectodermique anhidrotique.

Anhidrotic ectodermal dysplasia is an X-linked inherited skin disorder; only affected males exhibit the complete syndrome, whereas females may have a few mild features. The gene involved in this disease is located in the proximal area of the long arm of the X chromosome, in the q13 position. Molecular analysis is very helpful for calculating the risk of transmission in sisters with normal phenotypes and affected individuals (family 1 provides an example), but cannot solve all problems (example of family 4). The best results are obtained when there are two informative markers, each located on either side of and very close to the mutant gene. Molecular analysis can also be applied to chorionic villi sampled at the tenth week of gestation in order to achieve antenatal diagnosis in male fetuses in high risk families. Until recently, antenatal diagnosis could be performed only at the twentieth week of gestation by the demonstration of inadequate development of skin glands in skin biopsy specimens sampled under fetoscopy. Family 2 provides an example of antenatal diagnosis and highlights the risk of error that always exists in molecular analysis studies.

The gene for incontinentia pigmenti is assigned to Xq28.

A linkage study of eight families with incontinentia pigmenti (IP) has been performed, and linkage to site DXS52 has been established. We suggest that the IP locus lies in the Xq terminal region on the long arm of the X chromosome.

Incontinentia pigmenti: Xp breakpoint is not the same in a case of r(X) and in X/autosome translocations.

X-specific DNA probes were used to characterize the r(X) of a 45,X/46,X,r(X) female patient with Incontinentia pigmenti. It was found to be of maternal origin. Breakpoints were shown to be in or distal to p11.22 and between q12.2 and q13.1. When considering all known cases of Incontinentia pigmenti and X rearrangements at least four different break sites on the X have been shown.

Spondyloepiphyseal dysplasia tarda: linkage with genetic markers from the distal short arm of the X chromosome.

A linkage study of six families with spondyloepiphyseal dysplasia tarda (SEDL) has been performed. A linkage to site DXS41 (theta = 0.08; z = 3.07) and DXS92 (theta = 0.05; z = 2.95) has been established. We propose that the SEDL locus lies on the distal part of the short arm of the X chromosome.

Linkage studies in X-linked Alport's syndrome.

Four kindreds segregating for Alport's syndrome (ASLN) compatible with a X-linked inheritance were studied for linkage with polymorphic markers of the human X chromosome. No recombinant was observed between the ASLN locus and the DXS101 and DXS94 loci, the maximum lod scores were z = 3.93 and 3.50 respectively. Linkage data between the ASLN locus and the other genetic markers used in the present study are in keeping with the assignment of the mutation to the proximal Xq arm.