High frequency hearing loss correlated with mutations in the GJB2 gene

Genetic hearing impairment affects approximately 1/2000 live births. Mutations in one gene, GJB2, coding for connexin 26 cause 10%-20% of all genetic sensorineural hearing loss. Mutation analysis in the GJB2 gene and audiology were performed on 106 families presenting with at least one child with congenital hearing loss. The families were recruited from a hospital-based multidisciplinary clinic, which functions to investigate the aetiology of sensorineural hearing loss in children and which serves an ethnically diverse population. In 74 families (80 children), the aetiology was consistent with non-syndromic recessive hearing loss. Six different connexin 26 mutations, including one novel mutation, were identified. We show that GJB2 mutations cause a range of phenotypes from mild to profound hearing impairment and that loss of hearing in the high frequency range (4000-8000 Hz) is a characteristic feature in children with molecularly diagnosed connexin 26 hearing impairment. We also demonstrate that this type of audiology and high frequency hearing loss is found in a similar-sized group of deaf children in whom a mutation could only be found in one of the connexin 26 alleles, suggesting connexin 26 involvement in the aetiology of hearing loss in these cases. In our study of the M34T mutation, only compound heterozygotes exhibited hearing loss, suggesting autosomal recessive inheritance.     

Progressive hearing loss,and recurrent sudden sensorineural hearing loss associated with GJB2 mutations--phenotypic spectrum and frequencies of GJB2 mutations in Austria

Mutations of GJB2 (encoding connexin 26) are the most common cause of hearing loss (HL) in different populations, and a broad spectrum of GJB2 mutations has been identified. We screened 204 consecutive patients with non-syndromic sensorineural hearing loss for GJB2 mutations. Causative GJB2mutations were identified in 31 (15.2%) patients, and two common mutations, c.35delG and L90P (c.269T>C), accounted for 72.1% and 9.8% of GJB2 disease alleles. In four additional patients (2.0%) only one recessive GJB2 mutation was identified, making genetic counselling difficult. No genotype-phenotype correlation was established. We found, however, that homozygotes for truncating mutations were more likely to have a more severe degree of HL compared with other genotypes. Moreover, we showed by co-segregation studies that L90P is a GJB2 disease allele, and that compound heterozygotes for L90P and any recessive mutation share a mild to moderate phenotype. GJB2-associated HL was linked with progressive HL or with recurrent sudden sensorineural hearing loss (SSNHL) in three of 15 cases being analysed retrospectively. We extended the phenotypic spectrum of GJB2-related disease and recommend GJB2 mutation screening also in cases of progressive HL, and recurrent SSNHL. In addition, a carrier frequency of 1/110 (0.9%) for the most common Caucasian mutation in this gene, c.35delG, was determined in 1,212 blood donors from West-Austria, supporting the prevailing hypothesis of a Mediterranean founder mutation. Based on population and patient data, an overall GJB2 mutation carrier frequency of 1.3% was estimated for West-Austria.     

Autosomal-dominant osteopetrosis in Chuvashiya

A genetic epidemiological study of osteopetrosis was carried out in Chuvashiya. The major signs of this disorder are severe anemia developed in the prenatal or early postnatal life, hepatosplenomegaly, and a progressive loss of sight and hearing. Osteopetrosis showed the autosomal recessive inheritance with a somewhat increased proportion of affected patients in families. The lowest estimate of osteopetrosis frequency in Chuvashiya was 0.00026, one affected patient per 3879 newborns. The osteopetrosis gene occurred at a frequency of 0.016; the proportion of heterozygotes was 3.15%. The gene was shown to be evenly distributed throughout the republic.     

Autosomal Recessive Osteopetrosis in Chuvashiya

A genetic epidemiological study of osteopetrosis was carried out in Chuvashiya. The major signs of this disorder are severe anemia developed in the prenatal or early postnatal life, hepatosplenomegaly, and a progressive loss of sight and hearing. Osteopetrosis showed the autosomal recessive inheritance with a somewhat increased proportion of affected patients in families. The lowest estimate of osteopetrosis frequency in Chuvashiya was 0.00026, one affected patient per 3879 newborns. The osteopetrosis gene occurred at a frequency of 0.016; the proportion of heterozygotes was 3.15%. The gene was shown to be evenly distributed throughout the republic.     

De novo mutation of the connexin 26 gene associated with dominant non-syndromic sensorineural hearing loss

Mutations of the connexin 26 (Cx26) gene cause isolated recessive or dominant hearing loss or both sensorineural hearing impairment and keratoderma. We have identified the first de novo mutation of the Cx26 gene, R75 W, in a sporadic case of isolated profound hearing loss. R75 W has been previously observed in association with hearing impairment and keratoderma in one family and is thus thought to cause both syndromic and non-syndromic hearing loss. This case illustrates the risk of a possible erroneous diagnosis of autosomal recessive hearing loss in a sporadic case.     

High frequency of 35delG GJB2 mutation and absence of del(GJB6-D13S1830) in Greek Cypriot patients with nonsyndromic hearing loss

Mutations in the GJB2 (Connexin 26) gene are responsible for more than half of all cases of prelingual, recessive, inherited, nonsyndromic deafness in Europe. This paper presents a mutation analysis of the GJB2 and GJB6 (Connexin 30) genes in 30 Greek Cypriot patients with sensorineural nonsyndromic hearing loss compatible with recessive inheritance. Ten of the patients (33.3%) had the 35delG mutation in the GJB2 gene. Moreover, 9 of these were homozygous for the 35delG mutation, whereas 1 patient was in the compound heterozygous state with the disease causing E47X nonsense mutation. Another patient with severe sensorineural hearing loss was heterozygous for the V153I missense mutation. Finally, no GJB6 mutations or the known del(GJB6-D13S1830) were identified in any of the investigated Greek Cypriot nonsyndromic hearing loss patients. This work confirms that the GJB2 35delG mutation is an important pathogenic mutation for hearing loss in the Greek Cypriot population. This finding will be used toward the effective diagnosis of nonsyndromic hearing loss, improve genetic counseling, and serve as a potential therapeutic platform in the future for the affected patients in Cyprus.     

Phenotype and genotype of deaf patients with combined genomic and mitochondrial inheritance models

In most studies, sensorineural hearing loss is reported as a single-gene disease with autosomal dominant or autosomal recessive or with X-linked or maternal inheritance. It is uncommon that the hearing impairment is caused by a combined inheritance model including genomic and mitochondrial models. Here, we report six patients with sensorineural hearing loss caused by co-existing mutations in GJB2 or SLC26A4 and the mitochondrial gene. And there was no significant difference in hearing phenotypes between the six patients and the controls. The results indicate the complicated genetic etiology of, and may impact the diagnostic strategy for, hereditary hearing impairment. All patient siblings will carry mitochondrial DNA A1555G or C1494T mutations, and 25% of siblings may carry the same homozygous or compound heterozygote mutations in GJB2 or SLC26A4. Although this combined inheritance is not common in the Chinese deaf population (0.10%), our findings will have great impact in genetic counseling and risk prediction for deafness.     

A novel insertion-induced frameshift mutation of the SLC26A4 gene in a Korean family with Pendred syndrome

Pendred syndrome (PS) is an autosomal recessive disorder characterized by congenital bilateral sensorineural hearing loss, goiter, and incomplete iodide organification. Patients with PS also have structural anomalies of the inner ear such as enlarged vestibular aqueducts (EVA) and Mondini's malformation. The goiter, which is a major clinical manifestation of PS, usually develops around adolescence. PS is caused by biallelic mutations of the SLC26A4 gene, while nonsyndromic bilateral EVA is associated with zero or one SLC26A4 mutant allele. We report here a Korean family including a young female with PS who had goiter and progressive, fluctuating sensorineural hearing loss that could be partially recovered by oral steroid treatment. Genetic investigation revealed compound heterozygous mutations for p.R677AfsX11, a novel frameshift mutation, and p.H723R in the SLC26A4 gene. Our findings provide detailed information regarding the distribution of mutant alleles for PS and may serve as a foundation for studies to comprehend the genetic portion of syndromic hearing loss.     

The c.242G>A mutation in LRTOMT gene is responsible for a high prevalence of deafness in the Moroccan population

Congenital hearing impairment (HI) affects one in 1,000 newborns and has a genetic cause in 50?% of the cases. Autosomal recessive non-syndromic hearing impairment is responsible for 70–80?% of all hereditary cases of HI. Recently, it has been demonstrated that, mutations of LRTOMT are associated with profound nonsyndromic hearing impairment at the DFNB63 locus. The objective of this study is to evaluate the carrier frequency of c.242G>A mutation in LRTOMT gene and define the contribution of this gene in the etiology of deafness in Moroccan population. We screened 105 unrelated Moroccan families with non-syndromic HI and 120 control individuals for mutation in the exon 8 of the LRTOMT gene, by sequencing and PCR-RFLP. The Homozygous c.242G>A mutation was found in 8.75?% of the families tested and in 4.16?% of control in the heterozygous state. Our results show that after the GJB2 gene mutation in LRTOMT gene is the second cause of congenital hearing impairment in Moroccan patients. This finding should facilitate diagnosis of congenital deafness of the affected subjects in Morocco.     

Mutations in mitochondrial tRNA genes: non-linkage with syndromes of Wolfram and chronic progressive external ophthalmoplegia.

We have recently identified a point mutation in the mitochondrially encoded tRNA(Leu(UUR)) gene which associates with a combination of type II diabetes mellitus and sensorineural hearing loss in a large pedigree. To extend this finding to other syndromes which exhibit a combination of diabetes mellitus and hearing loss we have sequenced all mitochondrial tRNA genes from two patients with the Wolfram syndrome, a rare congenital disease characterized by diabetes mellitus, deafness, diabetes insipidus and optic atrophy. In each patient, a single different mutation was identified. One is an A to G transition mutation at np 12,308 in tRNA(Leu(CUN)) gene in a region which is highly conserved between species during evolution. This mutation has been described by Lauber et al. (1) as associating with chronic progressive external ophthalmoplegia (CPEO). The other is a C to T transition mutation at np 15,904 in tRNA(Thr) gene. Both mutations are also present in the general population (frequency tRNA(Leu(CUN)) mutation 0.16, tRNA(Thr) mutation 0.015). These findings suggest that evolutionarily conserved regions in mitochondrial tRNA genes can exhibit a significant polymorphism in humans, and that the mutation at np 12,308 in the tRNA(Leu(CUN)) gene is unlikely to be associated with CPEO and Wolfram syndrome.     

Delayed detection of congenital hearing loss in high risk infants.

OBJECTIVE--To examine the methods used to investigate children at high risk of congenital hearing impairment, and to see whether the introduction of evoked response audiometry has reduced the mean age at which hearing loss is identified. DESIGN--Clinicians who notified children to the national congenital rubella surveillance programme were asked retrospectively to complete a questionnaire examining the methods used to identify hearing impairment and the age at testing in two consecutive five year cohorts. The presence or absence of hearing loss was confirmed by obtaining the results of audiometric evaluations and, whenever possible, a recent pure tone audiogram. SETTING--The United Kingdom. PATIENTS--Children notified to the national congenital rubella surveillance programme and born in 1978-87 in whom IgM specific for rubella was detected shortly after birth. MAIN OUTCOME MEASURES--The age at which hearing loss was identified and the degree of loss in decibels at 250, 500, 1000, 2000, and 4000 Hz measured by pure tone audiometry. RESULTS--61 (52%) Of 117 children born in 1978-82 had a hearing impairment of 40 dB or greater in both ears. The mean loss was 93 dB. In the following five years 75 (47%) of 159 children had impaired hearing, their mean loss being 96 dB. The age at which the hearing loss was confirmed decreased from 11.6 to 9.8 months as a result of earlier auditory evoked response testing. Nevertheless, only eight (13%) of the children with hearing impairment born in 1978-82 and 16 (21%) of those born in 1983-7 had these tests performed in the first six months of life. CONCLUSIONS--Unacceptable delays in identifying hearing loss occurred in this high risk group because of failure to arrange auditory evoked response testing in early infancy. Evoked response audiometry is sensitive and specific and should be undertaken within the first few months of life for all infants known to be at risk of sensorineural hearing loss.     

Connexin-30 deletion analysis in connexin-26 heterozygotes

Mutations in the Connexin-26 gene (Cx 26, GJB2) are the most common cause of hereditary nonsyndromic sensorineural hearing loss (SNHL). DNA analysis of the Cx 26 gene in deaf or hard-of-hearing individuals frequently demonstrates heterozygosity despite the fact that most mutations are known to be recessive. A 342-kb deletion in a gene adjacent to Cx 26, the Connexin-30 gene (Cx 30, GJB6), has been reported to cause deafness in the homozygous state or in combination with heterozygous mutations in Cx 26 (digenic inheritance). We have analyzed deaf or hard-of-hearing Cx 26 heterozygotes and individuals with no mutations in Cx 26 for this Cx 30 deletion. We found that 4/20 (20%) of the Cx 26 heterozygotes are heterozygous for this deletion and that no individuals were homozygous for the Cx 30 deletion. Cx 30 deletion analysis is recommended for all individuals with nonsyndromic SNHL following Cx 26 sequencing that does not demonstrate two recessive mutations.     

Paternal isodisomy for chromosome 7 is compatible with normal growth and development in a patient with congenital chloride diarrhea.

Uniparental disomy for maternal chromosome 7 has been described in three patients with recessive disorders. Short stature in each of these patients has been explained by the effect of imprinting of growth-related genes on maternal chromosome 7. Alternatively, although less likely, all these patients may be homozygous for a rare recessive mutation. Here we report both paternal isodisomy for chromosome 7 and normal growth in a patient with a recessive disorder, congenital chloride diarrhea. She had inherited only paternal alleles at 10 loci and was homozygous for another 10 chromosome 7 loci studied. Her physical status and laboratory tests were normal except for a mild high-frequency sensorineural hearing loss. As the patient has normal stature, it is likely that the paternal chromosome 7 lacks the suggested maternal imprinting effect on growth. Paternal isodisomy for human chromosome 7 may have no phenotypic effect on growth.     

Novel mutations in the connexin 26 gene (GJB2) that cause autosomal recessive (DFNB1) hearing loss.

Mutations in the connexin 26 (Cx26) gene (GJB2) are associated with the type of autosomal recessive nonsyndromic neurosensory deafness known as "DFNB1." Studies indicate that DFNB1 (13q11-12) causes 20% of all childhood deafness and may have a carrier rate as high as 2. 8%. This study describes the analysis of 58 multiplex families each having at least two affected children diagnosed with autosomal recessive nonsyndromic deafness. Twenty of the 58 families were observed to have mutations in both alleles of Cx26. Thirty-three of 116 chromosomes contained a 30delG allele, for a frequency of .284. This mutation was observed in 2 of 192 control chromosomes, for an estimated gene frequency of .01+/-.007. The homozygous frequency of the 30delG allele is then estimated at .0001, or 1/10,000. Given that the frequency of all childhood hearing impairment is 1/1,000 and that half of that is genetic, the specific mutation 30delG is responsible for 10% of all childhood hearing loss and for 20% of all childhood hereditary hearing loss. Six novel mutations were also observed in the affected population. The deletions detected cause frameshifts that would severely disrupt the protein structure. Three novel missense mutations, Val84Met, Val95Met, and Ser113Pro, were observed. The missense mutation 101T-->C has been reported to be a dominant allele of DFNA3, a dominant nonsyndromic hearing loss. Data further supporting the finding that this mutation does not cause dominant hearing loss are presented. This allele was found in a recessive family segregating independently from the hearing-loss phenotype and in 3 of 192 control chromosomes. These results indicate that 101T-->C is not sufficient to cause hearing loss.     

Application of deafness diagnostic screening panel based on deafness mutation/gene database using invader assay

Despite progress in identification of deafness genes, clinical application has lagged due to the genetic heterogeneity of deafness. We designed and tested a comprehensive and simple diagnostic strategy to simultaneously detect deafness gene mutations based on a mutation/gene database followed by Invader assay screening of 41 known mutations of nine known deafness genes. Three hundred thirty-eight Japanese patients with congenital or childhood-onset (up to age 10) bilateral sensorineural hearing loss participated in this study. A total of 100 (29.6%) subjects had at least one mutation in GJB2, SLC26A4, and/or the mitochondrial 12S rRNA, indicating that these are the three major causative genes in Japanese deafness patients. The present study demonstrated that the Invader assay has excellent sensitivity and accuracy, and its application to deafness mutation screening will greatly improve medical management and facilitate extensive genetic counseling for hearing impairment.     

Non-syndromic hearing loss associated with enlarged vestibular aqueduct is caused by PDS mutations

Enlarged vestibular aqueduct (EVA), known as the most common form of inner ear abnormality, has recently been of particular genetic interest because this anomaly is inherited in a recessive manner. The locus for non-syndromic sensorineural hearing loss with EVA has been mapped to the same chromosomal region, 7q31, as the Pendred syndrome locus. In the present study, seven mutations in the PDS gene (PDS), the gene responsible for Pendred syndrome, have been found in families of non-syndromic sensorineural hearing loss with EVA. One family is homozygous, three families are compound heterozygotes, and two families are heterozygous but with no other mutation detected. The present results provide evidence that mutations in PDS cause both syndromic and non-syndromic hearing loss. Received: 21 October 1998 / Accepted: 5 December 1998     

Extremely low penetrance of deafness associated with the mitochondrial 12S rRNA mutation in 16 Chinese families: implication for early detection and prevention of deafness

Mutations in mitochondrial DNA (mtDNA) have been found to be associated with sensorineural hearing loss. We report here the clinical, genetic, and molecular characterization of 16 Chinese pedigrees (a total of 246 matrilineal relatives) with aminoglycoside-induced impairment. Clinical evaluation revealed the variable phenotype of hearing impairment including audiometric configuration in these subjects, although these subjects share some common features: being bilateral and sensorineural hearing impairment. Strikingly, these Chinese pedigrees exhibited extremely low penetrance of hearing loss, ranging from 4% to 18%, with an average of 8%. In particular, nineteen of 246 matrilineal relatives in these pedigrees had aminoglycoside-induced hearing loss. Mutational analysis of the mtDNA in these pedigrees showed the presence of homoplasmic 12S rRNA A1555G mutation, which has been associated with hearing impairment in many families worldwide. The extremely low penetrance of hearing loss in these Chinese families carrying the A1555G mutation strongly supports the notion that the A1555G mutation itself is not sufficient to produce the clinical phenotype. Children carrying the A1555G mutation are susceptible to the exposure of aminoglycosides, thereby inducing or worsening hearing impairment, as in the case of these Chinese families. Using those genetic and molecular approaches, we are able to diagnose whether children carry the ototoxic mtDNA mutation. Therefore, these data have been providing valuable information and technology to predict which individuals are at risk for ototoxicity, to improve the safety of aminoglycoside therapy, and eventually to decrease the incidence of deafness.     

Molecular analysis of the TMPRSS3 gene in Moroccan families with non-syndromic hearing loss

Autosomal recessive non-syndromic hearing impairment (ARNSHI) is the most common type of inherited hearing impairment, accounting for approximately 80% of inherited prelingual hearing impairment. Hearing loss is noted to be both phenotypically and genetically heterogeneous. Mutations in the TMPRSS3 gene, which encodes a transmembrane serine protease, are known to cause autosomal recessive non-syndromic hearing impairment DFNB8/10. In order to elucidate if the TMPRSS3 gene is responsible for ARNSHI in 80 Moroccan families with non-syndromic hearing impairment, the gene was sequenced using DNA samples from these families. Nineteen TMPRSS3 variants were found, nine are located in the exons among which six are missense and three are synonymous. The 10 remaining variations are located in non-coding regions. Missense variants analysis show that they do not have a significant pathogenic effect on protein while pathogenicity of some variant remains under discussion. Thus we show that the TMPRSS3 gene is not a major contributor to non-syndromic deafness in the Moroccan population.     

X-linked recessive inheritance of sensorineural hearing loss expressed during adolescence.

Eight males in 4 generations with hearing impairment were observed in a kindred; family transmission suggested X-linked recessive inheritance. In previously reported cases of X-linked sensorineural hearing loss, hearing impairment was usually severe to profound and was either present at birth or manifested by 5 years of age.In the present cases, rapid onset of a bilateral sensorineural hearing loss occurred during adolescence and did not generally progress beyond moderate impairment. Significant deterioration in seech production did not usually result.