A exposição contínua ao ruído de alta intensidade é o fator ambiental mais importante como causa de problemas auditivos em adultos. Esses tipos de perdas crônicas e irreversíveis causadas pelo ruído são chamados de Perdas Auditivas Induzidas por Ruído (PAIR). O objetivo desse estudo foi estudar a influência de fatores genéticos na susceptibilidade à PAIR. Para atingir esse objetivo comparamos uma amostra de indivíduos com PAIR e de indivíduos sem PAIR que trabalharam expostos ao ruído em relação à etnia, à história familial de perda auditiva, idade, tempo de exposição ao ruído, tabagismo e alcoolismo social. Para verificar a possível contribuição de fatores genéticos, testamos a presença de mutações conhecidas como causas freqüentes de surdez. As mutações testadas foram 35delG e 167delT no gene GJB2, as deleções Δ(GJB6-D13S1830) e Δ(GJB6-D13S1854) no gene GJB6 e A1555G (RFLP) no gene MT-RNR1. Determinamos as freqüências alélicas e genotípicas de um polimorfismo no gene GJB2 (SNP RS877098) e dos polimorfismos do tipo presença/deleção dos genes GSTM1 e GSTT1. Também verificamos a ocorrência e a freqüência de variações nas seqüências dos genes mitocondriais MT-RNR1 e MT-TS1, dois genes mitocondriais importantes como causa de surdez de herança materna. Nossa amostra constituiu-se de 107 indivíduos que apresentavam audiometrias sugestivas de PAIR (grupo PAIR), 44 indivíduos afetados por perdas de audição com curvas audiométricas que não eram sugestivas de PAIR (grupo PANO) e 104 indivíduos com audição normal (grupo NORMAL). Nossos resultados apontaram aumento significativo no número de parentes afetados por problemas de audição no grupo PAIR. O tabagismo, a idade e o tempo de exposição ao ruído também influenciaram significativamente na manifestação da PAIR. Aparentemente, não houve contribuição das mutações associadas à manifestação de surdez, 35delG e 167delT no gene GJB2, Δ(GJB6-D13S1830) e Δ(GJB6-D13S1854) no gene GJB6 e A1555G no gene MT-RNR1. Não houve diferença significativa nas freqüências dos alelos do SNP RS87098 (gene GJB2) entre os afetados e os não afetados. Observamos um aumento significativo do genótipo que corresponde a presença dos dois genes das enzimas GSTM1 e GSTT1 entre os indivíduos do grupo PAIR, sugerindo possível papel dessas enzimas relacionadas a proteção contra espécies reativas de oxigênio na etiologia da PAIR. Não observamos associação significativa entre nenhuma das 54 variantes de seqüências do DNA mitocondrial averiguadas nos genes MT-RNR1 e MT-TS1 (32 já previamente descritas e 22 detectadas nesse estudo) e a ocorrência de PAIR. Não observamos associação significativa da PAIR com o número total de variantes de seqüência do DNA mitocondrial observado em cada indivíduo. Não foi detectada associação significativa com os haplótipos constituídos por pares de variantes de seqüência do DNA mitocondrial. A comparação entre a concentração de peróxidos e de grupos sulfidril no soro de 15 indivíduos com PAIR com amostras de 15 indivíduos sem PAIR não revelou diferenças significativas. Em resumo, nosso estudo evidenciou a influência da história familial de perda auditiva na probabilidade de manifestação da PAIR e o possível papel das enzimas GSTT1 e GSTM1 na susceptibilidade a essa condição. Nossos achados reforçam a idéia de que a susceptibilidade à PAIR possa ser determinada por fatores genéticos. / Chronic exposure to loud noise is the most important environmental cause of hearing impairment among adults. Chronic and irreversible hearing loss due to exposure to noise is named Noise Induced Hearing Loss (NIHL). The aim of this study was to investigate the influence of genetic factors in the susceptibility to NIHL. We compared individuals with and without NIHL regarding ethnic origin, familial history of hearing loss, age, noise exposure time, alcohol consumption and smoking habits. In order to investigate genetic factors associated to NIHL we screened frequent deafness causative mutations. The investigated mutations were 35delG and 167delT in the GJB2 gene, Δ(GJB6- D13S1830) and Δ(GJB6- D13S1854) in the GJB6 gene and A1555G in the MT-RNR1 gene. Allelic and genotypic frequencies were determined for the SNP RS877098 in the GJB2 gene, and for the polymorphic deletions of GSTM1 and GSTT1 genes. We also investigated the frequency of variants in the mitochondrial genes MT-RNR1 and MT-TS1, which are known to harbor many hearing loss causative mutations. Our sample comprised 107 individuals with suggestive NIHL audiograms, 44 individuals with hearing impairment and non-suggestive NIHL audiograms, and 104 normal hearing individuals. A significant increase in the number of relatives affected by hearing impairment was detected in the NIHL group, when compared to the normal hearing group. Smoking habits, age and noise exposure time significantly affected the probability of NIHL. We did not detected any effect of the deafness-causing mutations 35delG and 167delT in the GJB2 gene, Δ (GJB6- D13S1830) and Δ (GJB6- D13S1854) in the GJB6 gene, and A1555G in the MT-RNR1 gene. There was no significant difference in allelic and genotypic frequencies of SNP RS87098 (gene GJB2), but the presence of the two genes encoding GSTM1 and GSTT1 enzymes was increased in the NIHL group. We did not detect any significant association of any of the 54 sequence variants in the mitochondrial genes MT-RNR1 and MT-TS1 (32 previously described and 22 novel) with the occurrence of NIHL. No significant associations were observed between NIHL and either the total number of sequence variants detected in each individual or haplotypes (combinations of two variants). The comparison of peroxides and sulfhydryl groups concentrations in serum from 15 individuals with NIHL and 15 individuals without NIHL did not show significant differences. In conclusion, our study demonstrated a significant effect of family history of hearing loss on the probability of presenting NIHL and pointed to a possible role of GSTT1 and GSTM1 enzymes on the susceptibility to this condition. These findings reinforce the idea that susceptibility to NIHL has a genetic basis.

The aetiological diagnosis of recessive non-syndromic hearing loss poses a challenge owing to marked heterogeneity and the lack of identifying clinical features. The finding that up to 50% of recessive non-syndromal genetic hearing loss among Caucasians was due to mutations in GJB2, the gene encoding Connexin 26 (Cx26) was a breakthrough, whose value as a diagnostic tool has been limited by the significant variation in the prevalence of deafness genes and loci among population groups. The significant association of the GJB6-D13S1830 deletion among individuals with one mutant GJB2 allele highlighted the need to explore population specific genetic mutations for NSHL. Although data from Sub-Saharan Africa is limited, reported studies found a high prevalence of R143W GJB2 mutation among Ghanaian, the 35delG mutation in 5 out of 139 Sudanese and a low prevalence of GJB2 variations among 385 Kenyan deaf children. The mutation spectrum of Waardenburg Syndrome (WS) in Africans has not been documented. During a visit to a School for the Deaf in the Limpopo Province of South Africa in 1997, it was noted that a high number of students came from Nzhelele sub-district. All had childhood onset hearing loss with no associated anomalies or disorders. The question arose as to whether there was a high-risk area for deafness in the Limpopo Province and what the aetiology of this hearing loss was.The main aim of this study was to investigate the role of GJB2, the GJB6-D13S1830 deletion, and the four common mitochondrial mutations, A1555G, A3243G, A7511C and A7445G, in the African hearing-impaired population of Limpopo province in South Africa, and to identify the mutation spectrum of the deafness genes found. The type and degree of hearing loss in this hearing impaired population would also be assessed. Secondly, this study sought to identify the mutations in a sibling pair with 2 clinical WS and to use the findings in a future study to establish the mutation spectrum of WS in the African population of the Limpopo province and of South Africa in general. The study was designed as a two phase study, in which phase 1 was used for hypothesis formulation and phase 2 was for hypothesis testing. While phase 1 was a descriptive retrospective case study, phase 2 was a combination of sample survey and prospective descriptive case study. In phase 1, demographic data of 361 students in two schools of the deaf in the Limpopo province was analyzed for evidence of areas of high risk populations for deafness in the province. In phase 2, a group of 182 individuals with genetic non-syndromic hearing loss (NSHL) and two siblings with clinical WS from two schools for the Deaf in the Limpopo Province of South Africa were investigated. A thorough clinical examination, audiological evaluation and urinalysis were done. Mutational screening was carried out in all 184 subjects using genomic DNA using single-strand conformation polymorphism (SSCP), multiplex polymerase chain reaction (PCR), and direct sequencing for GJB2, and Restriction Fragment-Length Polymorphism (PCR–RFLP) analysis for GJB6, and SSCP, hetero-duplex analysis, and direct sequencing of the first 8 exons of PAX3 and all of MITF for Waarenburg syndrome. Data analysis was by geographical mapping, frequency tables, tests of association with calculation of odds ratios, and binary logistic regression analysis using STATA and GIS mapping systems. The results indicate that there seem to be areas of genuine populations at risk for hearing loss in the Limpopo province of South Africa, namely Mutale and parts of Makhado and Thulamela municipalities. In Thulamela (NP343) wards 11-15, 26-30 and 31-35, and in Mutale (NP 344) wards 6-10, together accounted for 67 (18%) of participants in phase 1, and 33 (18%) of the participants in phase 2 of the study. Mutale municipality in the Vhembe 3 district gave with a projected prevalence of at least 13.14 deaf children per 100,000 African population attending the local school for the deaf. The observed hearing loss is a genetic, non-syndromic form, which is mainly severe and severe to profound, although without any clear defining configuration or shape. It is a stable, non-progressive and prelingual form of hearing loss, implying that this may be a recessive form of deafness. No identifiable environmental confounding factors or associations were identified. The deafness is not linked the common known auditory gene mutations in GJB2, the GJB6-D13S1830 deletion, or the common mitochondrial mutations A1555G, A3243G, A7511C and A7445G. Severe and profound levels of hearing loss were found in 22.8% and 75% of the cohort respectively, with the majority exhibiting flat (70.1%) or sloping (23.4%) audiograms that were commonly symmetrical (81.5%). However, as indicated, there was no clear pattern in the audiological findings overall. None of the 184 hearing impaired individuals exhibited any of the reported disease causing mutations of GJB2, including 35delG. There was, however, a high prevalence of two variants, the C>T variant at position g.3318-15 and the C>T variant at position g.3318-34, occurring in 21.4% and 46.2% of the deaf cohort respectively. The same variants were found to occur in 35% and 42.6% of a normal hearing control group (n = 63) respectively, indicating that these variations are polymorphisms. In three subjects (1.63% of the cohort), a T>A homozygous variation at position g.3318-6 was detected. Its significance in the causation of NSSNHL is yet to be determined. The GJB6-D13S1830 deletion was not detected in any of the participants. None of the four mitochondrial mutations screened for were found. 4 These results indicate that GJB2 is not a significant deafness gene in the African population of the Limpopo Province of South Africa and that significant genes for non-syndromic recessive hearing loss in this population are yet to be found. The geographical clustering of deafness found in this study, combined with the lack of identifiable common associated clinical features among the subjects of this study (excluding the WS sibling pair), suggests that these subjects have a genetic recessive non-syndromal type of hearing loss. In the context of historical and cultural evidence of consanguinity in this population, a founder effect cannot be ruled out. A rare mutation, R223X, previously identified only once out of 470 WS patients, was identified in the PAX3 gene among the WS sibling pair. A novel silent change GGG>GGT at amino acid 293, was also identified. These identical findings document, for the first time, a molecular defect in WS in an African sibling pair, and confirm WS Type I in this family, which could be found in other WS type I South Africans in the Limpopo Province of South Africa. The current study demonstrated that parents of genetically hearing impaired children in these areas are able to detect hearing loss at an early age, with over 60% suspecting their children’s hearing loss below 6 months of age. A child-centered management model encompassing all the areas relevant to childhood deafness/hearing impairment, which takes into consideration the prevailing logistical and financial constraints of the available healthcare system, is proposed. The implementation of this model requires a paradigm shift from the current fragmented model of service delivery to a cohesive patient-centered approach, based on concrete data from appropriate community based research, in which all the relevant parties communicate and share resources. 5 It would achieve the goals of early detection and intervention, as well as inclusive education for all. The relevant health and education policies are already in place and the posts funded. Equitable implementation of these policies would require appropriate community based research, as well as improved communication and consultation between the various stakeholders to ensure an efficient and affordable quality healthcare service for all hearing impaired South Africans.