CDH23 mutation and phenotype heterogeneity: a profile of 107 diverse families with Usher syndrome and nonsyndromic deafness.

Usher syndrome type I is characterized by congenital hearing loss, retinitis pigmentosa (RP), and variable vestibular areflexia. Usher syndrome type ID, one of seven Usher syndrome type I genetic localizations, have been mapped to a chromosomal interval that overlaps with a nonsyndromic-deafness localization, DFNB12. Mutations in CDH23, a gene that encodes a putative cell-adhesion protein with multiple cadherin-like domains, are responsible for both Usher syndrome and DFNB12 nonsyndromic deafness. Specific CDH23 mutational defects have been identified that differentiate these two phenotypes. Only missense mutations of CDH23 have been observed in families with nonsyndromic deafness, whereas nonsense, frameshift, splice-site, and missense mutations have been identified in families with Usher syndrome. In the present study, a panel of 69 probands with Usher syndrome and 38 probands with recessive nonsyndromic deafness were screened for the presence of mutations in the entire coding region of CDH23, by heteroduplex, single-strand conformation polymorphism, and direct sequence analyses. A total of 36 different CDH23 mutations were detected in 45 families; 33 of these mutations were novel, including 18 missense, 3 nonsense, 5 splicing defects, 5 microdeletions, and 2 insertions. A total of seven mutations were common to more than one family. Numerous exonic and intronic polymorphisms also were detected. Results of ophthalmologic examinations of the patients with nonsyndromic deafness have found asymptomatic RP-like manifestations, indicating that missense mutations may have a subtle effect in the retina. Furthermore, patients with mutations in CDH23 display a wide range of hearing loss and RP phenotypes, differing in severity, age at onset, type, and the presence or absence of vestibular areflexia.

Reading disability and chromosome 6p21.3: evaluation of MOG as a candidate gene.

Linkage analysis has localized a gene influencing specific reading disability (dyslexia) to 6p21.3. The myelin oligodendrocyte glycoprotein (MOG) gene, which maps to this region, was selected as a candidate. Myelin oligodendrocyte glycoprotein is a membrane protein, a member of the immunoglobin superfamily, that is found on the outermost lamellae of mature myelin. Although the exact function of this protein is unknown, its presence in the central nervous system and the hypothesized relationship between dyslexia and temporal processing rate as well as a suggested relationship with intelligence made this gene a candidate for dyslexia. Analysis of the coding exons and adjacent splice sites in a subset of 22 children with dyslexia from 10 sibships found a missense mutation in exon 4 in 2 of the sibships. This change from the published sequence also occurred in 86 of 96 random controls, making it considerably less frequent in this small sample of individuals with dyslexia. Subsequent typing of this single nucleotide polymorphism (SNP) in 74 nuclear families in which at least one child had reading disability showed no significant difference in frequency from the controls, however. Sib-pair linkage analysis with these families did not show significant linkage with the SNP nor with a separate polymorphic dinucleotide repeat marker in the MOG gene (MOG31/32), but association analysis identified two alleles of MOG31/32 that were associated with reading disability phenotypes with a low level of significance. Thus, although alleles in the MOG gene may be in linkage disequilibrium with a locus that contributes to reading disability, it is unlikely that the MOG gene itself is involved.

Human connexin 30 (GJB6), a candidate gene for nonsyndromic hearing loss: molecular cloning, tissue-specific expression, and assignment to chromosome 13q12.

Mutations in connexin 26 are responsible for approximately 20% of genetic hearing loss and 10% of all childhood hearing loss. However, only about 75% of the mutations predicted to be in Cx26 are actually observed. While this may be due to mutations in noncoding regulatory regions, an alternative hypothesis is that some cases may be due to mutations in another gene immediately adjacent to Cx26. Another gap junction gene, connexin 30 (HGMW-approved symbol GJB6), is found to lie on the same PAC clone that hybridizes to chromosome 13q12. Human connexin 26 and connexin 30 are expressed in the same cells of the cochlea. Cx26 and Cx30 share 77% identity in amino acid sequence but Cx30 has an additional 37 amino acids at its C-terminus. These considerations led us to hypothesize that mutations in Cx30 might also be responsible for hearing loss. Eight-eight recessive nonsyndromic hearing loss families from both American and Japanese populations were screened for mutations. In addition, 23 dominant hearing loss families and 6 singleton families presumed to be recessive were tested. No significant mutation has been found in the dominant or recessive families.

Novel mutation in the KCNQ4 gene in a large kindred with dominant progressive hearing loss.

Analysis of genotyping of a five-generation American family with nonsyndromic dominant progressive hearing loss indicated linkage to the DFNA2 locus on chromosome 1p34. This kindred consists of 170 individuals, of which 51 are affected. Pure tone audiograms, medical records, and blood samples were obtained from 36 family members. Linkage analysis with five microsatellite markers spanning the region around DFNA2 produced a lod score of 6.6 for the marker MYCL1 at straight theta = 0.0. Hearing loss in this family showed a very similar pattern as the first reported American family with the same linkage. High frequency hearing loss was detectable as early as 3 years of age, and progressed to severe to profound loss by the fourth decade. Using intronic primers, we screened the coding region of the KCNQ4 gene. Heteroduplex analysis followed by direct sequencing identified a T-->C transition at position 842, which would produce an L281S amino acid substitution. The observed mutation was shown to segregate completely with affected status in this family. The L281 residue is significantly conserved among the other members of the voltage-gated K(+) channel genes superfamily. Hydrophobicity analysis indicated that L281S substitution would lower formation of the beta structure at the P region of this ion channel. Mutation analysis of KCNQ4 was also performed on 80 unrelated probands from families with recessive or dominant nonsyndromic hearing loss. None of these cases showed a truncated mutation in KCNQ4.

Novel mutations in the connexin 26 gene (GJB2) that cause autosomal recessive (DFNB1) hearing loss.

Mutations in the connexin 26 (Cx26) gene (GJB2) are associated with the type of autosomal recessive nonsyndromic neurosensory deafness known as "DFNB1." Studies indicate that DFNB1 (13q11-12) causes 20% of all childhood deafness and may have a carrier rate as high as 2. 8%. This study describes the analysis of 58 multiplex families each having at least two affected children diagnosed with autosomal recessive nonsyndromic deafness. Twenty of the 58 families were observed to have mutations in both alleles of Cx26. Thirty-three of 116 chromosomes contained a 30delG allele, for a frequency of .284. This mutation was observed in 2 of 192 control chromosomes, for an estimated gene frequency of .01+/-.007. The homozygous frequency of the 30delG allele is then estimated at .0001, or 1/10,000. Given that the frequency of all childhood hearing impairment is 1/1,000 and that half of that is genetic, the specific mutation 30delG is responsible for 10% of all childhood hearing loss and for 20% of all childhood hereditary hearing loss. Six novel mutations were also observed in the affected population. The deletions detected cause frameshifts that would severely disrupt the protein structure. Three novel missense mutations, Val84Met, Val95Met, and Ser113Pro, were observed. The missense mutation 101T-->C has been reported to be a dominant allele of DFNA3, a dominant nonsyndromic hearing loss. Data further supporting the finding that this mutation does not cause dominant hearing loss are presented. This allele was found in a recessive family segregating independently from the hearing-loss phenotype and in 3 of 192 control chromosomes. These results indicate that 101T-->C is not sufficient to cause hearing loss.