Molecular analysis of hearing loss associated with enlarged vestibular aqueduct in the mainland Chinese: a unique SLC26A4 mutation spectrum

It has been shown that mutations in the SLC26A4 gene are involved in syndromic deafness characterized by congenital sensorineural hearing impairment and goitre (Pendred's syndrome), as well as in congenital isolated deafness (DFNB4), both of which are associated with enlarged vestibular aqueduct (EVA). The prevalence of SLC26A4 mutations in Pendred's syndrome is clearly established in many ethnic groups, but the data from Mainland Chinese patients with deafness and EVA remain poor. In this report, 15 patients from 13 unrelated Chinese families with deafness and EVA were analyzed for SLC26A4 using direct sequencing. A total of 15 pathogenic mutations were observed in 11 unrelated families, 4 of which were novel. One mutation, IVS7-2A>G, was most common, accounting for 22.3% (5/22) of all the mutant alleles, and H723R was infrequent. To date, a total of 23 mutations have been reported among the Chinese, 13 of which were unique. In conclusion, EVA could be a radiological marker for SLC26A4 analysis among Mainland Chinese hearing-loss patients, and the SLC26A4 mutation spectrum in the Chinese was different from other reported populations.     

A distinct spectrum of SLC26A4 mutations in patients with enlarged vestibular aqueduct in China

There is a worldwide interest in studying SLC26A4 mutations that are responsible for enlarged vestibular aqueduct (EVA) in different ethnic background and populations. The spectrum of SLC26A4 mutations in Chinese population is yet to be fully characterized. In this study, all the 21 exons of SLC26A4 were screened in 107 Chinese patients with hearing loss associated with EVA or both EVA and Mondini dysplasia (MD), taken from six multiplex and 95 simplex families. The two types of control populations consisted of 84 normal-hearing subjects and 46 sensorineural hearing loss subjects without inner ear malformations. Biallelic mutations were found in 12 patients from multiplex families and 84 patients (88.4%) from the simplex families. In addition, monoallelic variant was detected in nine patients in the remaining 11 simplex families. Overall, up to 97.9% patients were found having at least one possible pathogenic variant in SLC26A4 , with most having biallelic variants consistent with recessive inheritance of this disorder. A total of 40 mutations including 25 novel mutations were identified in the Chinese patients but were not detected in all the controls except for one normal subject. For the Chinese mutation spectrum of SLC26A4 gene, IVS7-2A>G mutation was the most common form accounting for 57.63% (102/177) of all the mutant alleles.     

Distribution and frequencies of PDS (SLC26A4) mutations in Pendred syndrome and nonsyndromic hearing loss associated with enlarged vestibular aqueduct: a unique spectrum of mutations in Japanese

Molecular diagnosis makes a substantial contribution to precise diagnosis, subclassification, prognosis, and selection of therapy. Mutations in the PDS (SLC26A4) gene are known to be responsible for both Pendred syndrome and nonsyndromic hearing loss associated with enlarged vestibular aqueduct, and the molecular confirmation of the PDS gene has become important in the diagnosis of these conditions. In the present study, PDS mutation analysis confirmed that PDS mutations were present and significantly responsible in 90% of Pendred families, and in 78.1% of families with nonsyndromic hearing loss associated with enlarged vestibular aqueduct. Furthermore, variable phenotypic expression by the same combination of mutations indicated that these two conditions are part of a continuous category of disease. Interestingly, the PDS mutation spectrum in Japanese, including the seven novel mutations revealed by this study, is very different from that found in Caucasians. Of the novel mutations detected, 53% were the H723R mutation, suggesting a possible founder effect. Ethnic background is therefore presumably important and should be noted when genetic testing is being performed. The PDS gene mutation spectrum in Japanese may be representative of those in Eastern Asian populations and its elucidation is expected to facilitate the molecular diagnosis of a variety of diseases.     

Screening of SLC26A4 (PDS) gene in Pendred's syndrome: a large spectrum of mutations in France and phenotypic heterogeneity

Sensorineural hearing defect and goiter are common features of Pendred's syndrome. The clinical diagnosis of Pendred's syndrome remains difficult because of the lack of sensitivity and specificity of the thyroid signs. The identification of PDS as the causative gene allowed molecular screening and enabled a re-evaluation of the syndrome to identify potential diagnostic characteristics. This report presents the clinical and genotypic findings of 30 French families, for whom a diagnosis of Pendred's syndrome had been made. Twenty-seven families had at least one mutated allele. Twenty-eight different mutations were identified, 11 of which had never been previously reported. The main clinical characteristics were: early hearing loss, fluctuation in terms of during deafness evolution, and the presence of an enlarged vestibular aqueduct.     

Extremely discrepant mutation spectrum of <Emphasis Type="Italic">SLC26A4</Emphasis> between Chinese patients with isolated Mondini deformity and enlarged vestibular aqueduct

Background

Mutations in SLC26A4 cause Pendred syndrome (hearing loss with goiter) or DFNB4 (non-syndromic hearing loss with inner ear malformation, such as enlarged vestibular aqueduct or Mondini deformity). The relationship between mutations in SLC26A4 and Mondini deformity without enlarged vestibular aqueduct has not been studied in any Chinese deaf population. The purpose of this study was to assess whether mutations in the SLC26A4 gene cause Mondini deformity without an enlarged vestibular aqueduct (isolated Mondini deformity) in a Chinese population.

Methods

In total, 144 patients with sensorineural hearing loss were included and subjected to high-resolution temporal bone CT. Among them, 28 patients with isolated Mondini dysplasia (MD group), 50 patients with enlarged vestibular aqueduct with Mondini dysplasia (EVA with MD group), 50 patients with enlarged vestibular aqueduct without Mondini dysplasia (EVA group), and 16 patients with other types of inner ear malformations (IEM group) were identified. The coding exons of SLC26A4 were analyzed in all subjects.

Results

DNA sequence analysis of SLC26A4 was performed in all 144 patients. In the different groups, the detection rate of the SLC26A4 mutation differed. In the isolated MD group, only one single allelic mutation in SLC26A4 was found in one patient (1/28, 3.6%). In the EVA with MD group, biallelic and monoallelic SLC26A4 mutations were identified in 46 patients (46/50, 92.0%) and three patients (3/50, 6.0%), respectively. Also, in the EVA group, biallelic and monoallelic SLC26A4 mutations were identified in 46 patients (46/50, 92.0%) and three patients (3/50, 6.0%), respectively. These percentages were identical to those in the EVA plus MD group. Only two patients carried monoallelic mutations of the SLC26A4 gene in the IEM group (2/16, 12.5%). There were significant differences in the frequency of SLC26A4 mutation among the groups (P < 0.001). The detection rate of SLC26A4 mutation in the isolated MD group was significantly lower than in the EVA group (with or without MD; P < 0.001), and there was no significant difference in the detection rate of SLC26A4 between the MD group and IEM group (P > 0.5).

Conclusion

Although mutations in the SLC26A4 gene were frequently found in Chinese EVA patients with and without MD, there was no evidence to show a relationship between isolated MD and the SLC26A4 gene in the Chinese population examined. Hearing impairment in patients with isolated MD may be caused by factors other than mutations in the SLC26A4 gene.

     

Molecular heterogeneity in two families with auditory pigmentary syndromes: the role of neuroimaging and genetic analysis in deafness

We report two cases in which the probands presented with deafness and a family history of a dominantly inherited auditory pigmentary syndrome, yet the cause of deafness in each proband was not associated with the pigmentary abnormalities but was a result of mutations in SLC26A4, the gene mutated in Pendred's syndrome. The first case is a young woman with congenital sensorineural hearing loss and a family history of piebaldism. Despite showing no pigmentary abnormalities, the proband was found to harbor the same KIT mutation as her relatives affected by piebaldism, as well as two mutations in the SLC26A4 gene. In the second case, 2-year-old identical twin boys born to deaf parents presented with congenital sensorineural deafness and an extensive maternal family history of Waardenburg's syndrome type I (WSI). Their father had recessively inherited deafness associated with dilated vestibular aqueducts and a clinical diagnosis of Pendred's syndrome was made in him, which was confirmed molecularly. As the twin boys did not have features of WSI, both the mother and children were tested for mutations in SLC26A4 which showed the mother to be a carrier of a single mutation and both boys to be compound heterozygotes, illustrating pseudodominant inheritance of the condition.     

Searching for evidence of DFNB2

Deafness is the most common form of sensory impairment in humans, affecting about 1 in 1,000 births in the United States. Of those cases with genetic etiology, approximately 80% are nonsyndromic and recessively inherited. Mutations in several unconventional myosins, members of a large superfamily of actin-associated molecular motors, have been found to cause hearing loss in both humans and mice. Mutations in the human unconventional Myosin VIIa (MYO7A), located at 11q13.5, are reported to be responsible for both syndromic and nonsyndromic deafness. MYO7A mutations are responsible for Usher syndrome type Ib, the most common genetic subtype of Usher I. Usher I is clinically characterized by congenital profound deafness, progressive retinal degeneration called retinitis pigmentosa (RP), and vestibular areflexia. Although a wide spectrum of MYO7A mutations have been identified in Usher Ib patients, four mutations have been reported to cause DFNB2, a recessive deafness without retinal degeneration, and one mutation has been implicated in a single case of dominant nonsyndromic hearing loss (DFNA11). Our study attempts to ascertain additional DFNB2 families to investigate the disparate nonsyndromic phenotype and alleged causative mutations. Data from both linkage and heterogeneity analyses on 36 selected autosomal recessive nonsyndromic deafness (RNSD) families, all previously excluded by mutational analysis from GJB2 (Cx26), the leading cause of nonsyndromic deafness, showed no evidence of DFNB2 within the sample. These negative results and the isolated reports of DFNB2 bring into question whether certain MYO7A mutations produce nonsyndromic recessive hearing loss.     

Spectrum and Frequency of SLC26A4 Mutations Among Czech Patients with Early Hearing Loss with and without Enlarged Vestibular Aqueduct (EVA)

Mutations in SLC26A4 cause Pendred syndrome (PS) – hearing loss with goitre – or DFNB4 – non‐syndromic hearing loss (NSHL) with inner ear abnormalities such as Enlarged Vestibular Aqueduct (EVA) or Mondini Dysplasia (MD). We tested 303 unrelated Czech patients with early hearing loss (298 with NSHL and 5 with PS), all GJB2‐negative, for SLC26A4 mutations and evaluated their clinical and radiological phenotype. Among 115 available HRCT/MRI scans we detected three MD (2.6%), three Mondini‐like affections (2.6%), 16 EVA (13 bilateral – 19.2% and 15.6% respectively) and 61 EVA/MD‐negative scans (73.4%). We found mutation(s) in 26 patients (8.6%) and biallelic mutations in eight patients (2.7%) out of 303 tested. In 18 of 26 (69%) patients, no second mutation could be detected even using MLPA. The spectrum of SLC26A4 mutations in Czech patients is broad without any prevalent mutation. We detected 21 different mutations (four novel). The most frequent mutations were p.Val138Phe and p.Leu445Trp (18% and 8.9% of pathogenic alleles respectively). Among 13 patients with bilateral EVA, six patients (50%) carry biallelic mutations. In EVA ‐negative patients no biallelic mutations were found but 4.9% had monoallelic mutations. SLC26A4 mutations are present mostly in patients with EVA/MD and/or progressive HL and those with affected siblings.     

Molecular screening of patients with nonsyndromic hearing loss from Nanjing city of China

Hearing loss is the most frequent sensory disorder involving a multitude of factors,and at least 50% of cases are due to genetic etiology.To further characterize the molecular etiology of hearing loss in the Chinese population,we recruited a total of 135 unrelated patients with nonsyndromic sensorineural hearing loss (NSHL) for mutational screening of GJB2,GJB3,GJB6,SLC26A4,SLC26A5 IVS2-2A>G and mitochondrial 12SrRNA,tRNA Ser(UCN) by PCR amplification and direct DNA sequencing.The carrier frequencies of deafness-causing mutations in these patients were 35.55% in GJB2,3.70% in GJB6,15.56% in SLC26A4 and 8.14% in mitochondrial 12SrRNA,respectively.The results indicate the necessity of genetic screening for mutations of these causative genes in Chinese population with nonsyndromic hearing loss.     

A mutational analysis of the SLC26A4 gene in Spanish hearing-impaired families provides new insights into the genetic causes of Pendred syndrome and DFNB4 hearing loss

Pendred syndrome (PS) and DFNB4, a non-syndromic sensorineural hearing loss with enlargement of the vestibular aqueduct (EVA), are caused by mutations in the SLC26A4 gene. Both disorders are recessive, and yet only one mutated SLC26A4 allele, or no mutations, are identified in many cases. Here we present the genetic characterization of 105 Spanish patients from 47 families with PS or non-syndromic EVA and 20 families with recessive non-syndromic hearing loss, which segregated with the DFNB4 locus. In this cohort, two causative SLC26A4 mutations could be characterized in 18 families (27%), whereas a single mutated allele was found in a patient with unilateral hearing loss and EVA in the same ear. In all, 24 different causative mutations were identified, including eight novel mutations. The novel p.Q514K variant was the most prevalent mutation in SLC26A4, accounting for 17% (6/36) of the mutated alleles identified in this study, deriving from a founder effect. We also characterized a novel multiexon 14 kb deletion spanning from intron 3 to intron 6 (g.8091T_22145Cdel). This study also revealed the first case of a de novo recessive mutation p.Q413P causing PS that arose in the proband's paternal allele, the maternal one carrying the p.L445W. The relevance of our results for genetic diagnosis of PS and non-syndromic EVA hearing loss is discussed.     

SLC26A4 variations among Graves' hyper-functioning thyroid gland

Deleterious mutations of SLC26A4 cause Pendred syndrome (PS), an autosomal recessive disorder comprising goitre and deafness with enlarged vestibular aqueducts (EVA), and nonsyndromic hearing loss (NSHL). However, the SLC26A4 hyperactivity was recently associated with the emergence of autoimmune thyroid diseases (AITD) and asthma among human and mouse model. Here, by direct sequencing, we investigate the sequences of the 20 coding exons (2 to 21) of SLC26A4 and their flanking intron-exon junctions among patients affected with Graves' disease (GD) hyperthyroidism. Ten mono-allelic variants were identified, seven of which are intronic and previously unreported. Two, c.898A>C (p.I300L) and c.1061T>C (p.F354S), of the three exonic variants are non synonymous. The p.F354S variant is already described to be involved in PS or NSHL inheritances. The exploration by PCR-RFLP of p.I300L and p.F354S variants among 132 GD patients, 105 Hashimoto thyroiditis (HT), 206 Healthy subjects and 102 families with NSHL have shown the presence of both variants. The p.F354S variation was identified both among patients (1~HT and 3 GD) and healthy subjects (n=5). Whereas, the p.I300L variant was identified only in GD patients (n=3). Our studies provide evidence of the importance of systematic analysis of SLC26A4 gene sequences on models other than deafness. This approach allows the identification of new variants and the review of the pathogenic effects of certain mono-allelic variants reported responsible for PS and NSHL development.     

Association of SLC26A4 muations with clinical features and thyroid function in deaf infants with enlarged vestibular aqueduct

Pendred syndrome and non-syndromic recessive deafness associated with enlarged vestibular aqueduct (NSRD with EVA) are caused by mutations in the SLC26A4 (PDS) gene. Unlike NSRD with EVA, Pendred syndrome is characterized by goiter, which may be present after early adulthood. However, the clinical diagnosis of these two disorders is difficult in deaf children. Expression of the SLC26A4 gene may be responsible for iodide transport in the thyroid as well as for formation and function of the inner ear. Here, we analyzed the SLC26A4 gene and performed thyroid function tests (FT3, FT4, TSH, and Thyroglobulin) on six congenitally deaf infants (mean age 2.7 years) with EVA. Mutation of the SLC26A4 gene was identified in five patients: four were compound heterozygous (H723R/919−2A>G, H723R/IVS15+5G>A, H723R/R581S, IVS7-2A>G/IVS8+1G>A), the fifth had a frameshift mutation (322delC). All the patients demonstrated an elevation of serum thyroglobulin level. FT3 level was elevated in four of the five patients. The patient who did not have a detectable gene mutation showed normal thyroid function. We conclude that the mutations in the SLC26A4 gene identified here are highly associated with high serum thyroglobulin levels in congenital and deafness infants. These mutations may be of value for the diagnosis of Pendred syndrome and NSRD with EVA.     

Identification of five new mutations of PDS/SLC26A4 in Mediterranean families with hearing impairment

Pendred syndrome is an autosomal‐recessive disorder characterized by congenital sensorineural hearing loss combined with goiter. This disorder may account for up to 10% of cases of hereditary deafness. The disease gene (PDS/SLC26A4) has been mapped to chromosome 7q22‐q31 and encodes a chloride‐iodide transport protein. Mutations in this gene are also a cause of non‐syndromic autosomal recessive hearing impairment (DFNB4). We have analyzed the PDS/SLC26A4 gene in Spanish and Italian families and we have detected five new mutations (X871M, T132I, IVS1‐2A>G, Y556H and 406del5). © 2001 Wiley‐Liss, Inc.     

Pendred syndrome, DFNB4, and PDS/SLC26A4 identification of eight novel mutations and possible genotype-phenotype correlations

Mutations in PDS (SLC26A4) cause both Pendred syndrome and DFNB4, two autosomal recessive disorders that share hearing loss as a common feature. The hearing loss is associated with temporal bone abnormalities, ranging from isolated enlargement of the vestibular aqueduct (dilated vestibular aqueduct, DVA) to Mondini dysplasia, a complex malformation in which the normal cochlear spiral of 2(1/2) turns is replaced by a hypoplastic coil of 1(1/2) turns. In Pendred syndrome, thyromegaly also develops, although affected persons usually remain euthyroid. We identified PDS mutations in the proband of 14 of 47 simplex families (30%) and nine of 11 multiplex families (82%) (P=0.0023). In all cases, mutations segregated with the disease state in multiplex families. Included in the 15 different PDS allele variants we found were eight novel mutations. The two most common mutations, T416P and IVS8+1G>A, were present in 22% and 30% of families, respectively. The finding of PDS mutations in five of six multiplex families with DVA (83%) and four of five multiplex families with Mondini dysplasia (80%) implies that mutations in this gene are the major genetic cause of these temporal anomalies. Comparative analysis of phenotypic and genotypic data supports the hypothesis that the type of temporal bone anomaly may depend on the specific PDS allele variant present.     

Genetic basis of hearing loss associated with enlarged vestibular aqueducts in Koreans

Sensorineural hearing loss associated with enlargement of the vestibular aqueduct (EVA) can be associated with mutations of the SLC26A4 gene. In western populations, less than one-half of the affected individuals with EVA have two mutant SLC26A4 alleles, and EVA is frequently caused by unknown genetic or environmental factors alone or in combination with a single SLC26A4 mutation as part of a complex trait. In this study, we ascertained 26 Korean probands with EVA and performed nucleotide sequence analysis to detect SLC26A4 mutations. All subjects had bilateral EVA, and 20 of 26 were sporadic (simplex) cases. Fourteen different mutations were identified, including nine novel mutations. Five mutations were recurrent and accounted for 80% of all mutant alleles, providing a basis for the design and interpretation of cost-efficient mutation detection algorithms. Two mutant alleles were identified in 21 (81%), one mutant allele was detected in three (11%), and zero mutant allele was detected in two (8%) of 26 probands. The high proportion of Korean probands with two SLC26A4 mutations may reflect a reduced frequency of other genetic or environmental factors causing EVA in comparison to western populations.     

Two Iranian families with a novel mutation in GJB2 causing autosomal dominant nonsyndromic hearing loss

Mutations in GJB2, encoding connexin 26 (Cx26), cause both autosomal dominant and autosomal recessive nonsyndromic hearing loss (ARNSHL) at the DFNA3 and DFNB1 loci, respectively. Most of the over 100 described GJB2 mutations cause ARNSHL. Only a minority has been associated with autosomal dominant hearing loss. In this study, we present two families with autosomal dominant nonsyndromic hearing loss caused by a novel mutation in GJB2 (p.Asp46Asn). Both families were ascertained from the same village in northern Iran consistent with a founder effect. This finding implicates the D46N missense mutation in Cx26 as a common cause of deafness in this part of Iran mandating mutation screening of GJB2 for D46N in all persons with hearing loss who originate from this geographic region.     

Borate transporter SLC4A11 mutations cause both Harboyan syndrome and non-syndromic corneal endothelial dystrophy

Harboyan syndrome, or corneal dystrophy and perceptive deafness (CDPD), consists of congenital corneal endothelial dystrophy and progressive perceptive deafness, and is transmitted as an autosomal recessive trait. CDPD and autosomal recessive, non-syndromic congenital hereditary endothelial corneal dystrophy (CHED2) both map at overlapping loci at 20p13, and mutations of SLC4A11 were reported recently in CHED2. A genotype study on six families with CDPD and on one family with either CHED or CDPD, from various ethnic backgrounds (in the seventh family, hearing loss could not be assessed because of the proband's young age), is reported here. Novel SLC4A11 mutations were found in all patients. Why some mutations cause hearing loss in addition to corneal dystrophy is presently unclear. These findings extend the implication of the SLC4A11 borate transporter beyond corneal dystrophy to perceptive deafness.     

Origins and frequencies of SLC26A4 (PDS) mutations in east and south Asians: global implications for the epidemiology of deafness

Recessive mutations of SLC26A4 (PDS) are a common cause of Pendred syndrome and non-syndromic deafness in western populations. Although south and east Asia contain nearly one half of the global population, the origins and frequencies of SLC26A4 mutations in these regions are unknown. We PCR amplified and sequenced seven exons of SLC26A4 to detect selected mutations in 274 deaf probands from Korea, China, and Mongolia. A total of nine different mutations of SLC26A4 were detected among 15 (5.5%) of the 274 probands. Five mutations were novel and the other four had seldom, if ever, been identified outside east Asia. To identify mutations in south Asians, 212 Pakistani and 106 Indian families with three or more affected offspring of consanguineous matings were analysed for cosegregation of recessive deafness with short tandem repeat markers linked to SLC26A4. All 21 SLC26A4 exons were PCR amplified and sequenced in families segregating SLC26A4 linked deafness. Eleven mutant alleles of SLC26A4 were identified among 17 (5.4%) of the 318 families, and all 11 alleles were novel. SLC26A4 linked haplotypes on chromosomes with recurrent mutations were consistent with founder effects. Our observation of a diverse allelic series unique to each ethnic group indicates that mutational events at SLC26A4 are common and account for approximately 5% of recessive deafness in south Asians and other populations.     

Gap-junction channels dysfunction in deafness and hearing loss

Gap-junction channels connect the cytoplasm of adjacent cells, allowing the diffusion of ions and small metabolites. They are formed at the appositional plasma membranes by a family of related proteins named connexins. Mutations in connexins 26, 31, 30, 32, and 43 have been associated with nonsyndromic or syndromic deafness. The majority of these mutations are inherited in an autosomal recessive manner, but a few of them have been associated with dominantly inherited hearing loss. Mutations in the connexin26 gene (GJB2) are the most common cause of genetic deafness. This review summarizes the most relevant and recent information about different mutations in connexin genes found in human patients, with emphasis on GJB2. The possible effects of the mutations on channel expression and function are discussed, in addition to their possible physiologic consequences for inner ear physiology. Finally, we propose that connexin channels (gap junctions and hemichannels) may be targets for age-related hearing loss induced by oxidative damage.