title: 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.
abstract: 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.
authors.affiliation: Departamento de Genética e Biologia Evolutiva, Centro de Estudos do Genoma Humano, Instituto de Biociências, Universidade de São Paulo, São Paulo, SP, Brazil.
authors.forename: Ana Carla
authors.initials: AC
authors.lastname: Batissoco
authors.forename: Ronaldo Serafim
authors.initials: RS
authors.lastname: Abreu-Silva
authors.forename: Maria Cristina Célia
authors.initials: MC
authors.lastname: Braga
authors.forename: Karina
authors.initials: K
authors.lastname: Lezirovitz
authors.forename: Valter
authors.initials: V
authors.lastname: Della-Rosa
authors.forename: Tabith
authors.initials: T
authors.lastname: Alfredo
authors.forename: Paulo Alberto
authors.initials: PA
authors.lastname: Otto
authors.forename: Regina Célia
authors.initials: RC
authors.lastname: Mingroni-Netto
mesh_terms: D000483:Alleles; D019943:Amino Acid Substitution; D001938:Brazil; D015331:Cohort Studies; D000072259:Connexin 26; D000075302:Connexin 30; D017630:Connexins / Q000235:genetics*; D005787:Gene Frequency; D005817:Genetic Counseling; D005838:Genotype; D034381:Hearing Loss / Q000175:diagnosis / Q000235:genetics* / Q000503:physiopathology; D006579:Heterozygote; D006801:Humans; D009154:Mutation; D018807:Polymorphism, Single-Stranded Conformational; D012720:Severity of Illness Index
chemical_list: D000075302:Connexin 30; D017630:Connexins; C000607042:GJB2 protein, human; C491020:GJB6 protein, human; D000072259:Connexin 26