Prevalence of the GJB2 mutations and the del(GJB6-D13S1830) mutation in Brazilian patients with deafness
Introduction
Surveys show that more than 70 million people worldwide have hearing loss that affects normal communication. In developed countries, the incidence of congenital severe hearing impairment is 1 in 1000 births, half of which can be attributed to genetic factors (Marazita et al., 1993). Genetic heterogeneity and environmental factors have impaired identification of the genes causing deafness until recently (Mustapha et al., 2001). In Brazil, most cases of hearing loss are due to environmental factors, such as congenital infections (mainly rubella), perinatal anoxia and meningitis (Simões and Maciel-Guerra, 1992).
About 70% of cases of hereditary pre-lingual deafness belong to the non-syndromic form and are believed to result from a sensorineural (cochlear) defect. Within the non-syndromic hearing loss category, 75–80% of cases of congenital pre-lingual deafness are inherited in an autosomal recessive way, followed by dominant (15–20%) and X-linked (1–1.5%) manners (Nance, 2003). Deafness is an extremely genetically heterogeneous disorder, shown by the fact that 33 loci for recessive non-syndromic hearing loss (recessive locus has the prefix “DFNB”) and 39 loci for dominant non-syndromic hearing loss have been mapped to date (update regularly on the Hereditary Hearing Loss Homepage (HHH); http://dnalab-www.uia.ac.be/dnalab/hhh/index.html.
The most common mutation associated with DFNB1 hearing loss, responsible for most (up to 85%) of the mutants alleles in Europe–Mediterranean populations is a deletion of a guanine from a series of six guanines extending from nucleotide position 30–35 (35delG) in the coding region of the connexin 26 gene, GJB2. This leads to a frameshift and a resulting stop codon at position 13 (Zelante et al., 1997). Many studies from various parts of the world have documented the incidence of GJB2 mutations in the deaf population. These include France, Italy, Spain, UK, the United States, Israel, and most recently, Lebanon, Greece, Austria, China, Brazil, and the Iranian and Palestinian populations (Kelley et al., 1998; Green et al., 1999; Sobe et al., 2000; Mustapha et al., 2001; Frei et al., 2002; Liu et al., 2002; Najmabadi et al., 2002; Oliveira et al., 2002; Pampanos et al., 2002; Shahin et al., 2002).
In several European countries, the prevalence of the 35delG mutation has been estimated to 2–4% of the population with normal hearing (Estivill et al., 1998; Gasparini et al., 2000). In Brazil, the 35delG carrier frequency in the white population with normal hearing in the Southeast region (1 in 51) (Oliveira et al., 2004) was similar to that in the overall European population (Gasparini et al., 1997). In fact, in a previous study performed in Brazil, six 35delG heterozygotes were identified among 620 randomly selected neonates; a 35delG carrier rate of 0.97% (1 in 103), showing that this mutation in not rare in the Brazilian population (Sartorato et al., 2000). In spite of the fact that all patients studied were Caucasian, the composition of the Brazilian population is difficult to be established, due to its high ethnic composition of Caucasian, African and Amerindian origin.
It is worth noticing that for several deaf patients, a Cx26 mutation was detected on one allele only, indicating either the existence of another Cx26 mutation in the gene unexplored region or the possible complication of another connexin gene for a digenic origin of the hearing loss, which could be related to the putative formation of heteromeric connexons or heterotypic channels, such as Cx26 and Cx32 or as Cx26 and Cx30 (Ahmad et al., 1999; Kumar, 1999; Lautermann et al., 1999; Marziano et al., 2003). These cases accounted for 10–42% of all deaf subjects with a least one GJB2 mutation (Wilcox et al., 2000). Perhaps, these findings could be also attributed to others mutations that might exist in the DFNB1 locus, but not in the GJB2 gene, which could provide an explanation for the high proportion of the heterozygotes deaf subjects. Recently, this hypothesis received experimental support by the finding of a novel class of mutations in the DFNB1 locus, which does not affect GJB2, but truncates the neighboring GJB6 gene, which encodes Cx30 (Grifa et al., 1999; Lerer et al., 2001; del Castillo et al., 2002; Pallares-Ruiz et al., 2002). In one study, the deletion breakpoint junction was isolated and sequenced, revealing the loss of DNA segment of approximately 342 Kb, with one breakpoint inside the GJB6 coding region. This deletion, named del(GJB6-D13S1830) was the accompanying mutation in about 50% of these deaf GJB2 patients (del Castillo et al., 2002).
In this study, we assessed the prevalence of GJB2 mutations in Brazilian patients with non-syndromic sensorineural hearing loss and we determined the types of mutations in this population. Based on the association found between GJB2 monoallelic mutations and the del(GJB6-D13S1830) mutation (Grifa et al., 1999; Lerer et al., 2001; del Castillo et al., 2002; Pallares-Ruiz et al., 2002;), we also investigated the contribution of this deletion to hearing impairment in this population.
Section snippets
Subjects and methods
From March 2002 to June 2002, the study was conducted on 33 unrelated probands with congenital non-syndromic sensorineural hearing loss, referred to from the Otorhinolaryngology Service of Medical School of São José do Rio Preto (FAMERP), São Paulo, Brazil. Deaf subjects aged 1–37 years (mean 24.2), 23 males and 10 females, monitored at least twice a year since the diagnosis in the same clinical and audiological institution (FAMERP), were included in the study. Written informed consent was
Results
Mutations in the GJB2 gene were found in nine of the unrelated probands in this studied population. Five subjects of this hearing impairment population were homozygotes for the most frequent mutation, the deletion named 35delG. This mutation accounted in 14 out of 66 of the GJB2 analyzed alleles, and in 14 out of 16 of the GJB2 mutated alleles. Four heterozygotes for the 35delG mutation were identified, but the second GJB2 mutation was detected in only one of them (35delG/V37I) by direct
Discussion
Even with this study that has been carried out with a small number of samples, mutations in the GJB2 gene were found in nine of unrelated probands in this study and, the 35delG mutation accounted in fourteen of the mutated alleles. Our results are in accordance with a previous study in which the 35delG mutation was found in 16 of the mutated alleles in Brazilian patients with non-syndromic hearing loss resulting from mutations in the GJB2 gene (Oliveira et al., 2002) and are also in accordance
Acknowledgements
We would like to thank the families who participated in this work. We further thank Camila Andréa Oliveira and Fabiana Alexandrino from UNICAMP, and Carla Renata Graça, Ellen Caroline Toledo do Nascimento and Érika Cristina Pavarino Bertelli, from FAMERP for their help and support in this work.
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