The search of a genetic basis for noise-induced hearing loss (NIHL).

BACKGROUND AND AIM: Knowledge about the genetic factors responsible for noise-induced hearing loss (NIHL) is still limited. This study investigated whether genetic factors are associated or not to susceptibility to NIHL. SUBJECTS AND METHODS: The family history and genotypes were studied for candidate genes in 107 individuals with NIHL, 44 with other causes of hearing impairment and 104 controls. Mutations frequently found among deaf individuals were investigated (35delG, 167delT in GJB2, Δ(GJB6- D13S1830), Δ(GJB6- D13S1854) in GJB6 and A1555G in MT-RNR1 genes); allelic and genotypic frequencies were also determined at the SNP rs877098 in DFNB1, of deletions of GSTM1 and GSTT1 and sequence variants in both MTRNR1 and MTTS1 genes, as well as mitochondrial haplogroups. RESULTS: When those with NIHL were compared with the control group, a significant increase was detected in the number of relatives affected by hearing impairment, of the genotype corresponding to the presence of both GSTM1 and GSTT1 enzymes and of cases with mitochondrial haplogroup L1. CONCLUSION: The findings suggest effects of familial history of hearing loss, of GSTT1 and GSTM1 enzymes and of mitochondrial haplogroup L1 on the risk of NIHL. This study also described novel sequence variants of MTRNR1 and MTTS1 genes.

Role of the mitochondrial mutations, m.827A>G and the novel m.7462C>T, in the origin of hearing loss.

Samples from 30 deaf probands exhibiting features suggestive of syndromic mitochondrial deafness or from families with maternal transmission of deafness were selected for investigation of mutations in the mitochondrial genes MT-RNR1 and MT-TS1. Patients with mutation m.1555A>G had been previously excluded from this sample. In the MT-RNR1 gene, five probands presented the m.827A>G sequence variant, of uncertain pathogenicity. This change was also detected in 66 subjects of an unaffected control sample of 306 Brazilian individuals from various ethnic backgrounds. Given its high frequency, we consider it unlikely to have a pathogenic role on hereditary deafness. As to the MT-TS1 gene, one proband presented the previously known pathogenic m.7472insC mutation and three probands presented a novel variant, m.7462C>T, which was absent from the same control sample of 306 individuals. Because of its absence in control samples and association with a family history of hearing impairment, we suggest it might be a novel pathogenic mutation.

Novel OTOF mutations in Brazilian patients with auditory neuropathy.

The OTOF gene encoding otoferlin is associated with auditory neuropathy (AN), a type of non-syndromic deafness. We investigated the contribution of OTOF mutations to AN and to non-syndromic recessive deafness in Brazil. A test for the Q829X mutation was carried out on a sample of 342 unrelated individuals with non-syndromic hearing loss, but none presented this mutation. We selected 48 cases suggestive of autosomal recessive inheritance, plus four familial and seven isolated cases of AN, for genotyping of five microsatellite markers linked to the OTOF gene. The haplotype analysis showed compatibility with linkage in 11 families (including the four families with AN). Samples of the 11 probands from these families and from seven isolated cases of AN were selected for an exon-by-exon screening for mutations in the OTOF gene. Ten different pathogenic variants were detected, among which six are novel. Among the 52 pedigrees with autosomal recessive inheritance (including four familial cases of AN), mutations were identified in 4 (7.7%). Among the 11 probands with AN, seven had at least one pathogenic mutation in the OTOF gene. Mutations in the OTOF gene are frequent causes of AN in Brazil and our results confirm that they are spread worldwide.

Prevalence of GJB2 (connexin-26) and GJB6 (connexin-30) mutations in a cohort of 300 Brazilian hearing-impaired individuals: implications for diagnosis and genetic counseling.

OBJECTIVE: Hereditary nonsyndromic deafness is an autosomal recessive condition in about 80% of cases, and point mutations in the GJB2 gene (connexin 26) and two deletions in the GJB6 gene (connexin 30), del(GJB6-D13S1830) and del(GJB6-D13S1854), are reported to account for 50% of recessive deafness. Aiming at establishing the frequencies of GJB2 mutations and GJB6 deletions in the Brazilian population, we screened 300 unrelated individuals with hearing impairment, who were not affected by known deafness related syndromes. METHODS: We firstly screened the most frequently reported mutations, c.35delG and c.167delT in the GJB2 gene, and del(GJB6-D13S1830) and del(GJB6-D13S1854) in the GJB6 gene, through specific techniques. The detected c.35delG and c.167delT mutations were validated by sequencing. Other mutations in the GJB2 gene were screened by single-strand conformation polymorphism and the coding region was sequenced when abnormal patterns were found. RESULTS: Pathogenic mutations in GJB2 and GJB6 genes were detected in 41 individuals (13.7%), and 80.5% (33/41) presented these mutations in homozygosis or compound heterozygosis, thus explaining their hearing defect. The c.35delG in the GJB2 gene was the most frequent mutation (37/300; 12.4%), detected in 23% familial and 6.2% the sporadic cases. The second most frequent mutation (1%; 3/300) was the del(GJB6-D13S1830), always found associated with the c.35delG mutation. Nineteen different sequence variations were found in the GJB2 gene. In addition to the c.35delG mutation, nine known pathogenic alterations were detected c.167delT, p.Trp24X, p.Val37Ile, c.176_191del16, c.235delC, p.Leu90Pro, p.Arg127His, c.509insA, and p.Arg184Pro. Five substitutions had been previously considered benign polymorphisms: c.-15C>T, p.Val27Ile, p.Met34Thr, p.Ala40Ala, and p.Gly160Ser. Two previously reported mutations of unknown pathogenicity were found (p.Lys168Arg, and c.684C>A), and two novel substitutions, p.Leu81Val (c.G241C) and p.Met195Val (c.A583G), both in heterozygosis without an accompanying mutation in the other allele. None of these latter four variants of undefined status was present in a sample of 100 hearing controls. CONCLUSIONS: The present study demonstrates that mutations in the GJB2 gene and del(GJB6 D13S1830) are important causes of hearing impairment in Brazil, thus justifying their screening in a routine basis. The diversity of variants in our sample reflects the ethnic heterogeneity of the Brazilian population.

Is autosomal recessive deafness associated with oculocutaneous albinism a "coincidence syndrome"?

Hearing impairment is frequently found associated with pigmentary disorders in many syndromes. However, total oculocutaneous albinism (OCA) associated with deafness has been described only once, by Ziprkowski and Adam (Arch Dermatol 89:151-155, 1964) in an inbred family. A syndrome associating deafness and OCA was suggested by the authors, but two separate recessive genes segregating in this inbred group were also proposed later by Fraser (OMIM # 220900). Combined deafness and total OCA were also observed by us in a family originally reported to be nonconsanguineous but in which haplotyping showed evidence of a common ancestry: the proband was affected by both diseases, one of his sisters had only OCA and another sister had only deafness. Both the proband and his deaf sister were found to be homozygotes for the 35delG mutation (GJB2 gene), the most frequent cause of hereditary deafness. Linkage analysis with markers close to the four known OCA loci excluded linkage to OCA1, OCA2, and OCA3, and homozygosity in markers near OCA4 locus was observed. Sequencing of the corresponding gene (MATP) revealed a c.1121delT mutation, which leads to a stop codon at position 397 (L374fsX397). Clearly, the combined occurrence of deafness and albinism in this pedigree was due to mutations in two different genes, showing autosomal recessive inheritance. We speculate that the putative syndrome reported by Ziprkowski and Adam might have resulted from the co-occurrence of autosomal recessive deafness and albinism in the same pedigree, as suggested by Fraser.

Atualizacao em genetica da surdez nao-sindromica / Atualization in genetics of nonsyndromic hereditary deafness

A surdez nao-sindromica e uma condicao altamente heterogenea, com inumeros genes de locos diferentes interferindo no desenvolvimento e na fisiologia da audicao...