Unresolved questions regarding human hereditary deafness.

Human hearing loss is a common neurosensory disorder about which many basic research and clinically relevant questions are unresolved. This review on hereditary deafness focuses on three examples considered at first glance to be uncomplicated, however, upon inspection, are enigmatic and ripe for future research efforts. The three examples of clinical and genetic complexities are drawn from studies of (i) Pendred syndrome/DFNB4 (PDS, OMIM 274600), (ii) Perrault syndrome (deafness and infertility) due to mutations of CLPP (PRTLS3, OMIM 614129), and (iii) the unexplained extensive clinical variability associated with TBC1D24 mutations. At present, it is unknown how different mutations of TBC1D24 cause non-syndromic deafness (DFNB86, OMIM 614617), epilepsy (OMIM 605021), epilepsy with deafness, or DOORS syndrome (OMIM 220500) that is characterized by deafness, onychodystrophy (alteration of toenail or fingernail morphology), osteodystrophy (defective development of bone), mental retardation, and seizures. A comprehensive understanding of the multifaceted roles of each gene associated with human deafness is expected to provide future opportunities for restoration as well as preservation of normal hearing.

Progressive irreversible hearing loss is caused by stria vascularis degeneration in an Slc26a4-insufficient mouse model of large vestibular aqueduct syndrome.

Hearing loss of patients with enlargement of the vestibular aqueduct (EVA) can fluctuate or progress, with overall downward progression. The most common detectable cause of EVA is mutations of SLC26A4. We previously described a transgenic Slc26a4-insufficient mouse model of EVA in which Slc26a4 expression is controlled by doxycycline administration. Mice that received doxycycline from conception until embryonic day 17.5 (DE17.5; doxycycline discontinued at embryonic day 17.5) had fluctuating hearing loss between 1 and 6 months of age with an overall downward progression after 6 months of age. In this study, we characterized the cochlear functional and structural changes underlying irreversible hearing loss in DE17.5 mice at 12 months of age. The endocochlear potential was decreased and inversely correlated with auditory brainstem response thresholds. The stria vascularis was thickened and edematous in ears with less severe hearing loss, and thinned and atrophic in ears with more severe hearing loss. There were pathologic changes in marginal cell morphology and gene expression that were not observed at 3 months. We conclude that strial dysfunction and degeneration are the primary causes of irreversible progressive hearing loss in our Slc26a4-insufficient mouse model of EVA. This model of primary strial atrophy may be used to explore the mechanisms of progressive hearing loss due to strial dysfunction.

Segregation of enlarged vestibular aqueducts in families with non-diagnostic SLC26A4 genotypes.

BACKGROUND: Hearing loss with enlarged vestibular aqueduct (EVA) can be inherited as an autosomal recessive trait caused by bi-allelic mutations of SLC26A4. However, many EVA patients have non-diagnostic SLC26A4 genotypes with only one or no detectable mutant alleles. METHODS AND RESULTS: In this study, the authors were unable to detect occult SLC26A4 mutations in EVA patients with non-diagnostic genotypes by custom comparative genomic hybridisation (CGH) microarray analysis or by sequence analysis of conserved non-coding regions. The authors sought to compare the segregation of EVA among 71 families with two (M2), one (M1) or no (M0) detectable mutant alleles of SLC26A4. The segregation ratios of EVA in the M1 and M2 groups were similar, but the segregation ratio for M1 was significantly higher than in the M0 group. Haplotype analyses of SLC26A4-linked STR markers in M0 and M1 families revealed discordant segregation of EVA with these markers in eight of 24 M0 families. CONCLUSION: The results support the hypothesis of a second, undetected SLC26A4 mutation that accounts for EVA in the M1 patients, in contrast to non-genetic factors, complex inheritance, or aetiologic heterogeneity in the M0 group of patients. These results will be helpful for counselling EVA families with non-diagnostic SLC26A4 genotypes.

Do mutations of the Pendred syndrome gene, SLC26A4, confer resistance to asthma and hypertension?

BACKGROUND AND AIMS: Mutations of SLC26A4 cause Pendred syndrome, an autosomal recessive disorder comprising goitre and deafness with enlarged vestibular aqueducts (EVA). Recent studies in mouse models implicate Slc26a4 in the pathogenesis of asthma and hypertension. We hypothesise that asthma and hypertension are less prevalent among humans with SLC26A4 mutations. METHODS: We reviewed medical histories and SLC26A4 genotypes for 80 individuals with EVA and 130 of their unaffected family members enrolled in a study of EVA. We used Fisher's exact test to compare the prevalence of asthma and hypertension among groups of subjects with zero, one, or two mutant alleles of SLC26A4. RESULTS: Although none of the 21 subjects with two mutant alleles of SLC26A4 had asthma or hypertension, there were no statistically significant differences in the prevalence of asthma or hypertension among subjects with zero, one, or two mutant alleles. CONCLUSION: There might be a protective effect of SLC26A4 mutations for asthma and hypertension but our study is statistically underpowered to detect this effect. Study sizes of at least 1125 and 504 individuals will be needed for 80% power to detect an effect at alpha = 0.05 for asthma and hypertension, respectively. Our hypothesis merits a larger study since it has implications for potential strategies to treat hearing loss by manipulating SLC26A4 expression or function.

Identities and frequencies of mutations of the otoferlin gene (OTOF) causing DFNB9 deafness in Pakistan.

Mutations in OTOF, encoding otoferlin, cause non-syndromic recessive hearing loss. The goal of our study was to define the identities and frequencies of OTOF mutations in a model population. We screened a cohort of 557 large consanguineous Pakistani families segregating recessive, severe-to-profound, prelingual-onset deafness for linkage to DFNB9. There were 13 families segregating deafness consistent with linkage to markers for DFNB9. We analyzed the genomic nucleotide sequence of OTOF and detected probable pathogenic sequence variants among all 13 families. These include the previously reported nonsense mutation p.R708X and 10 novel variants: 3 nonsense mutations (p.R425X, p.W536X, and p.Y1603X), 1 frameshift (c.1103_1104delinsC), 1 single amino acid deletion (p.E766del) and 5 missense substitutions of conserved residues (p.L573R, p.A1090E, p.E1733K, p.R1856Q and p.R1939W). OTOF mutations thus account for deafness in 13 (2.3%) of 557 Pakistani families. This overall prevalence is similar, but the mutation spectrum is different from those for Western populations. In addition, we demonstrate the existence of an alternative splice isoform of OTOF expressed in the human cochlea. This isoform must be required for human hearing because it encodes a unique alternative C-terminus affected by some DFNB9 mutations.

A locus for autosomal dominant progressive non-syndromic hearing loss, DFNA27, is on chromosome 4q12-13.1.

We ascertained a large North American family, LMG2, segregating progressive, non-syndromic, sensorineural hearing loss. A genome-wide scan identified significant evidence for linkage (maximum logarithm of the odds (LOD) score = 4.67 at theta = 0 for D4S398) to markers in a 5.7-cM interval on chromosome 4q12-13.1. The DFNA27 interval spans 8.85 Mb and includes at least 61 predicted and 8 known genes. We sequenced eight genes and excluded them as candidates for the DFNA27 gene.

Haplogroup analysis supports a pathogenic role for the 7510T>C mutation of mitochondrial tRNA(Ser(UCN)) in sensorineural hearing loss.

We ascertained a large North American family, LMG309, with matrilineal transmission of non-syndromic, progressive sensorineural hearing loss (SNHL). There was no history of aminoglycoside exposure, and penetrance was complete. We sequenced the entire mitochondrial genome and identified the previously reported 7510T>C transition in the tRNA(Ser(UCN)) gene. The 7510T>C was homoplasmic in all affected members. The LMG309 mitochondrial sequence belongs to an unnamed subgroup of mitochondrial haplogroup H. We demonstrate that the previously reported Spanish family S258 carries 7510T>C on a different mitochondrial sub-haplogroup, H1. We did not detect 7510T>C among 79 Caucasian haplogroup H control samples, including 11 from sub-haplogroup H1 and one from the same sub-haplogroup as LMG309. Our results provide strong genetic evidence that 7510T>C is a pathogenic mutation that causes non-syndromic SNHL.

Identities, frequencies and origins of TMC1 mutations causing DFNB7/B11 deafness in Pakistan.

Non-syndromic deafness is genetically heterogeneous. We previously reported that mutations of transmembrane channel-like gene 1 (TMC1) cause non-syndromic recessive deafness at the DFNB7/B11 locus on chromosome 9q13-q21 in nine Pakistani families. The goal of this study was to define the identities, origins and frequencies of TMC1 mutations in an expanded cohort of 557 large Pakistani families segregating recessive deafness. We screened affected family members for homozygosity at short-tandem repeats flanking known autosomal recessive (DFNB) deafness loci, followed by TMC1 sequence analysis in families segregating deafness linked to DFNB7/B11. We identified 10 new families segregating DFNB7/B11 deafness and TMC1 mutations, including three novel alleles. Overall, 9 different TMC1 mutations account for deafness in 19 (3.4%) of the 557 Pakistani families. A single mutation, p.R34X, causes deafness in 10 (1.8%) of the families. Genotype analysis of p.R34X-linked markers indicates that it arose from a common founder. We also detected p.R34X among normal control samples of African-American and northern European origins, raising the possibility that p.R34X and other mutations of TMC1 are prevalent contributors to the genetic load of deafness across a variety of populations and continents.

A novel mutation at the DFNA36 hearing loss locus reveals a critical function and potential genotype-phenotype correlation for amino acid-572 of TMC1.

We ascertained a North American Caucasian family (LMG248) segregating autosomal dominant, non-syndromic, post-lingual, progressive sensorineural hearing loss. The hearing loss begins in the second decade of life and initially affects high frequencies. It progresses to profound deafness at all frequencies by the fourth or fifth decade. The phenotype co-segregates with short-tandem repeat markers flanking the TMC1 gene at the DFNA36 locus on chromosome 9q31-q21. The affected individuals carry a novel missense substitution, p.D572H (c.G1714C), of the TMC1 gene. This mutation is at the same nucleotide and amino acid position as the only other reported DFNA36 mutation, p.D572N (c.G1714A). Our observations implicate a critical function for amino acid-572 for wild-type TMC1 function or the pathogenesis of DFNA36 hearing loss. The slower progression of hearing loss associated with p.D572H, in comparison with that caused by p.D572N, may reflect a correlation of DFNA36 phenotype with TMC1 genotype.

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.

Mutations of ESPN cause autosomal recessive deafness and vestibular dysfunction.

We mapped a human deafness locus DFNB36 to chromosome 1p36.3 in two consanguineous families segregating recessively inherited deafness and vestibular areflexia. This phenotype co-segregates with either of two frameshift mutations, 1988delAGAG and 2469delGTCA, in ESPN, which encodes a calcium-insensitive actin-bundling protein called espin. A recessive mutation of ESPN is known to cause hearing loss and vestibular dysfunction in the jerker mouse. Our results establish espin as an essential protein for hearing and vestibular function in humans. The abnormal vestibular phenotype associated with ESPN mutations will be a useful clinical marker for refining the differential diagnosis of non-syndromic 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.

Auditory dysfunction in Stickler syndrome.

OBJECTIVES: To characterize the natural history and possible mechanisms of hearing loss in Stickler syndrome (OMIM 108300; or hereditary progressive arthro-ophthalmopathy) and to determine if the auditory phenotype is a useful discriminating feature for the differential diagnosis of this group of disorders. DESIGN: Multifamily study. SETTING: Outpatient audiology and otolaryngology clinics at the Warren Grant Magnuson Clinical Center of the National Institutes of Health, Rockville, Md. SUBJECTS: Forty-six affected individuals from 29 different families segregating Stickler syndrome. INTERVENTIONS: Clinical audiologic and otolaryngological examinations were performed on all individuals, including pure-tone audiometry, speech audiometry, and middle ear immittance testing. Otoacoustic emissions, auditory brainstem response, infrared video electronystagmography, and temporal bone computed tomography were performed on a subset of participants. RESULTS: The hearing loss was most often sensorineural in adults, and approximately 28 (60%) of the 46 adult patients had 2 or more thresholds greater than the corresponding 95th percentile values for an age-matched, otologically normal population. The hearing loss most often affected high frequencies (4000-8000 Hz) and was generally no more progressive than that due to age-related hearing loss. Type A(D) tympanograms (classification using the Jerger model), indicating hypermobile middle ear systems, were observed in 21 (46%) of the 46 affected individuals. Computed tomography of the temporal bones revealed no inner ear malformations in 19 affected individuals. CONCLUSIONS: The hypermobile middle ear systems observed in ears with normal-appearing tympanic membranes represent a novel finding for Stickler syndrome and are likely to be a useful diagnostic feature for this disorder. The overall sensorineural hearing loss in type I Stickler syndrome is typically mild and not significantly progressive. It is less severe than that reported for types II and III Stickler syndrome linked to COL11A2 (OMIM 120290) and COL11A1 (OMIM 120280) mutations, respectively, or the closely related Marshall syndrome. This difference will be a useful discriminatory feature in the differential diagnosis of this group of disorders.

Mutations of the protocadherin gene PCDH15 cause Usher syndrome type 1F.

Human chromosome 10q21-22 harbors USH1F in a region of conserved synteny to mouse chromosome 10. This region of mouse chromosome 10 contains Pcdh15, encoding a protocadherin gene that is mutated in ames waltzer and causes deafness and vestibular dysfunction. Here we report two mutations of protocadherin 15 (PCDH15) found in two families segregating Usher syndrome type 1F. A Northern blot probed with the PCDH15 cytoplasmic domain showed expression in the retina, consistent with its pathogenetic role in the retinitis pigmentosa associated with USH1F.