Hh Ropers

Association between X-linked mixed deafness and mutations in the POU domain gene POU3F4

Deafness with fixation of the stapes (DFN3) is the most frequent X-linked form of hearing impairment. The underlying gene has been localized to a 500-kilobase segment of the Xq21 band. Here, it is reported that a candidate gene for this disorder, Brain 4 (POU3F4), which encodes a transcription factor with a POU domain, maps to the same interval. In five unrelated patients with DFN3 but not in 50 normal controls, small mutations were found that result in truncation of the predicted protein or in nonconservative amino acid substitutions. These findings indicate that POU3F4 mutations are a molecular cause of DFN3.

Molecular basis of choroideremia ( CHM ) : mutations involving the rab escort protein - 1 (REP - 1) gene

Choroideremia (CHM) is an X‐linked recessive eye disease that results from mutations involving the Rab escort protein‐1 (REP‐1) gene. In 18 patients deletions of different sizes have been found. Two females suffering from CHM were reported to have translocations that disrupt the REP‐1 gene. In 22 patients, small mutations have been identified. Interestingly, these are all nonsense, frameshift or splice‐site mutations; with one possible exception, missense mutations have not been found. This comprises all the known mutations in the disease. Hum Mutat 9:110–117, 1997.

Systematic characterisation of disease associated balanced chromosome rearrangements by FISH: cytogenetically and genetically anchored YACs identify microdeletions and candidate regions for mental retardation genes

Disease associated balanced chromosome rearrangements (DBCRs) have been instrumental in the isolation of many disease genes. To facilitate the molecular cytogenetic characterisation of DBCRs, we have generated a set of >1200 non-chimeric, cytogenetically and genetically anchored CEPH YACs, on average one per 3 cM, spaced over the entire human genome. By fluorescence in situ hybridisation (FISH), we have performed a systematic search for YACs spanning translocation breakpoints. Patients with DBCRs and either syndromic or non-syndromic mental retardation (MR) were ascertained through the Mendelian Cytogenetics Network (MCN), a collaborative effort of, at present, 270 cytogenetic laboratories throughout the world. In this pilot study, we have characterised 10 different MR associated chromosome regions delineating candidate regions for MR. Five of these regions are narrowed to breakpoint spanning YACs, three of which are located on chromosomes 13q21, 13q22, and 13q32, respectively, one on chromosome 4p14, and one on 6q25. In two out of six DBCRs, we found cytogenetically cryptic deletions of 3-5 Mb on one or both translocation chromosomes. Thus, cryptic deletions may be an important cause of disease in seemingly balanced chromosome rearrangements that are associated with a disease phenotype. Our region specific FISH probes, which are available to MCN members, can be a powerful tool in clinical cytogenetics and positional cloning.

A duplication/paracentric inversion associated with familial X-linked deafness (DFN3) suggests the presence of a regulatory element more than 400 kb upstream of the POU3F4 gene

X-linked deafness with stapes fixation (DFN3) is caused by mutations in the POU3F4 gene at Xq21.1. By employing pulsed field gel electrophoresis (PFGE) we identified a chromosomal aberration in the DNA of a DFN3 patient who did not show alterations in the open reading frame (ORF) of POU3F4. Southern blot analysis indicated that a DNA segment of 150 kb, located 170 kb proximal to the POU3F4 gene, was duplicated. Fluorescence in situ hybridization (FISH) analysis, PFGE, and detailed Southern analysis revealed that this duplication is part of a more complex rearrangement including a paracentric inversion involving the Xq21.1 region, and presumably the Xq21.3 region. Since at least two DFN3-associated minideletions are situated proximal to the duplicated segment, the inversion most likely disconnects the POU3F4 gene from a regulatory element which is located at a distance of at least 400 kb upstream of the POU3F4 gene.

Localization of the gene (or genes) for a syndrome with X-linked mental retardation, ataxia, weakness, hearing impairment, loss of vision and a fatal course in early childhood

Linkage analysis is described in a family with X-linked mental retardation, ataxia, weakness, floppiness, delayed motor development, absence of deep tendon reflexes, hearing impairment and loss of vision (MIM no. 301835). The disease has a fatal course due to the susceptibility of the patients to infections, especially of the respiratory tract. Clinical signs indicate impairment of the posterior columns, peripheral motor and sensory neurons and the second and eighth cranial nerves and/or their nuclei. The involvement of the posterior columns of the spinal cord is further suggested by the almost complete absence of myelinated fibers therein. We localized the responsible gene(s) to Xq21.33–q24 between DXS1231 and DXS1001 with a maximum lod score of 6.97. The proteolipid protein gene, which codes for two myelin proteins of the central nervous system and is located in this region, was considered as a candidate gene for this disorder. However, no mutations were found in the protein-coding part of this gene.

Yeast artificial chromosome cloning of the Xq13.3-q21.31 region and fine mapping of a deletion associated with choroideremia and nonspecific mental retardation

Microscopically detectable deletions and X;autosome translocations have previously facilitated the construction of a high-resolution interval map of the Xq21 region. Here, we have generated three yeast artificial chromosome contigs spanning approximately 7 megabases of the Xq13.3-q21.31 region. In addition, a novel deletion associated with choroideremia and mental retardation was identified and mapped in detail. The proximal deletion endpoint was positioned between the loci DXS995 and DXS232, which enabled us to confirm the critical region for a locus involved in mental retardation. The distal deletion endpoint is situated in the Xq21.33 band, which allowed us to refine the order of several markers in this region.

Molecular cytogenetic characterisation of a complex 46,XY,t(7;8;11;13) chromosome rearrangement in a patient with Moebius syndrome

Editor—Carriers of de novo balanced reciprocal translocations and inversions have an increased risk of approximately 6% for developing multiple congenital abnormalities (MCA) and/or mental retardation (MR), compared to a 2-3% risk overall in newborn populations.1 2 Cytogenetically cryptic deletions or physical disruption or inactivation of a gene(s) in one or both breakpoint regions may account for the observed phenotypes.3 4 It seems plausible to assume that the risk for MCA/MR may be even higher in carriers of de novo complex chromosome rearrangements (CCRs), which involve at least three different chromosomes and breakpoint regions. Extreme cases involving up to seven chromosomes and 10 breakpoints have been described.5-7Indeed, most reported CCRs are associated with MCA/MR.8 9In addition, they have been found in infertile men10 and in women suffering from multiple miscarriages.11 12 The complex nature of CCRs renders karyotype interpretation by classical chromosome banding alone difficult. In many cases fluorescence in situ hybridisation (FISH) will be the best method to delineate the underlying chromosome rearrangements.7 13Here we have applied conventional FISH with chromosome painting probes and region specific large insert clones, comparative genomic hybridisation (CGH),14 15 and spectral karyotyping (SKY)16 17 to an apparently balanced and very complex rearrangement in a profoundly retarded patient with Moebius syndrome (MBS, MIM 157900).18 MBS is characterised by congenital paralysis of the seventh cranial nerve leading to facial diplegia.19 Other cranial nerves may also be affected. In addition, orofacial and limb malformations, defects of the musculoskeletal system, and MR may occur. This patient, who has classical Moebius syndrome, has been reported previously.4 He was the third child born to a 35 year old father and a 32 year old mother. Because of paresis of the facial muscles as a …

Localisation of a gene for non-specific X linked mental retardation (MRX46) to Xq25-q26.

We report linkage data on a new large family with non-specific X linked mental retardation (MRX), using 24 polymorphic markers covering the entire X chromosome. We could assign the underlying disease gene, denoted MRX46, to the Xq25-q26 region. MRX46 is tightly linked to the markers DXS8072, HPRT, and DXS294 with a maximum lod score of 5.12 at theta=0. Recombination events were observed with DXS425 in Xq25 and DXS984 at the Xq26-Xq27 boundary, which localises MRX46 to a 20.9 cM (12 Mb) interval. Several X linked mental retardation syndromes have been mapped to the same region of the X chromosome. In addition, the localisation of two MRX genes, MRX27 and MRX35, partially overlaps with the linkage interval obtained for MRX46. Although an extension of the linkage analysis for MRX35 showed only a minimal overlap with MRX46, it cannot be excluded that the same gene is involved in several of these MRX disorders. On the other hand, given the considerable genetic heterogeneity in MRX, one should be extremely cautious in using interfamilial linkage data to narrow down the localisation of MRX genes. Therefore, unless the underlying gene(s) is characterised by the analysis of candidate genes, MRX46 can be considered a new independent MRX locus.

A new X linked neurodegenerative syndrome with mental retardation, blindness, convulsions, spasticity, mild hypomyelination, and early death maps to the pericentromeric region

We report on a family with an X linked neurodegenerative disorder consisting of mental retardation, blindness, convulsions, spasticity, and early death. Neuropathological examination showed mild hypomyelination. By linkage analysis, the underlying genetic defect could be assigned to the pericentromeric region of the X chromosome with a maximum lod score of 3.30 at θ=0.0 for the DXS1204 locus with DXS337 and PGK1P1 as flanking markers.

Molecular cloning of Xp11 breakpoints in two unrelated mentally retarded females with X;autosome translocations

Mental retardation is a very common and extremely heterogeneous disorder that affects about 3% of the human population. Its molecular basis is largely unknown, but many loci have been mapped to the X chromosome. We report on two mentally retarded females with X;autosome translocations and breakpoints in Xp11, viz., t(X;17)(p11;p13) and t(X;20)(p11;q13). (Fiber-) FISH analysis assigned the breakpoints to different subbands, Xp11.4 and Xp11.23, separated by approximately 8 Mb. High-resolution mapping of the X- chromosome breakpoints using Southern blot hybridization resulted in the isolation of breakpoint-spanning genomic subclones of 3 kb and 0.5 kb. The Xp11.4 breakpoint is contained within a single copy sequence, whereas the Xp11.23 breakpoint sequence resembles an L1 repetitive element. Several expressed sequences map close to the breakpoints, but none was found to be inactivated. Therefore, mechanisms other than disruption of X-chromosome genes likely cause the phenotypes.