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Audiological profiles and gjb2, gjb6 mutations: A retrospective study on genetic and clinical data from 2003 to 2008

In many world populations, mutations in the GJB2 gene (codifying for Connexin 26) are the most common causes of autosomal recessive non-syndromic hearing loss and account for approximately 50% of cases. To date, more than 100 (dominant or recessive) mutations have been identified (The Connexin Deafness Homepage, 2009) and differences in frequency and distribution across the world are significant. In European and American Caucasian populations, the 35delG is the most common mutation found to account for nearly 70% of the pathological alleles. Mutations in the GJB6 gene (codifying for Connexin 30) can co-occur in some cases (we refer to the 342-kb truncating deletion, named as GJB6-D13S1830). Thus, these mutations have been associated with autosomal recessive and non-syndromic hearing loss, mostly as biallelic/digenic inheritance of the Cx26 and/or Cx30. Our objective in this study was to describe audiological features and genotypes in patients with GJB2 and/or GJB6 mutations. We performed a retrospective study on a deaf cohort of 566 patients who underwent specific genetic tests for Connexin 26 and 30; the latter was investigated in 385 cases. GJB2 mutations were found in 162 patients and GJB6 mutations in five. The most common mutation of GJB2 was 35delG, a truncating (T) mutation, although we also found other types of truncating (nine genotypes) and non-truncating (NT) (11 genotypes) mutations, such as M34T, L90P, R184P, IVS1 + 1G→A, V37I, and E47X. Even if more than 70% of patients with biallelic/digenic mutations exhibit a severe/profound hearing impairment (even between the simple heterozygotes), mild/moderate deafness is also possible. Other interesting clinical data, such as atypical history or phenotype, were considered. In accordance with the literature, all categories of HL were found. The severe-profound HL was predominant especially in T/T, T/NT forms. The 35delG is the most common mutation that we found, especially in profound hearing impairment. Mild-moderate HL was identified overall among NT/NT forms. Our findings confirm the importance of newborn screening, and the evaluation of genetic mutations to define genotype/phenotype correlation and clinical or audiological features useful to early diagnosis and improvement of therapeutic protocols.