Tomoko Makishima

Dominant and recessive deafness caused by mutations of a novel gene, TMC1, required for cochlear hair-cell function.

Positional cloning of hereditary deafness genes is a direct approach to identify molecules and mechanisms underlying auditory function. Here we report a locus for dominant deafness, DFNA36, which maps to human chromosome 9q13–21 in a region overlapping the DFNB7/B11 locus for recessive deafness. We identified eight mutations in a new gene, transmembrane cochlear-expressed gene 1 (TMC1), in a DFNA36 family and eleven DFNB7/B11 families. We detected a 1.6-kb genomic deletion encompassing exon 14 of Tmc1 in the recessive deafness (dn) mouse mutant, which lacks auditory responses and has hair-cell degeneration. TMC1 and TMC2 on chromosome 20p13 are members of a gene family predicted to encode transmembrane proteins. Tmc1 mRNA is expressed in hair cells of the postnatal mouse cochlea and vestibular end organs and is required for normal function of cochlear hair cells.

Mechanotransduction in mouse inner ear hair cells requires transmembrane channel–like genes

Inner ear hair cells convert the mechanical stimuli of sound, gravity, and head movement into electrical signals. This mechanotransduction process is initiated by opening of cation channels near the tips of hair cell stereocilia. Since the identity of these ion channels is unknown, and mutations in the gene encoding transmembrane channel-like 1 (TMC1) cause hearing loss without vestibular dysfunction in both mice and humans, we investigated the contribution of Tmc1 and the closely related Tmc2 to mechanotransduction in mice. We found that Tmc1 and Tmc2 were expressed in mouse vestibular and cochlear hair cells and that GFP-tagged TMC proteins localized near stereocilia tips. Tmc2 expression was transient in early postnatal mouse cochlear hair cells but persisted in vestibular hair cells. While mice with a targeted deletion of Tmc1 (Tmc1(Δ) mice) were deaf and those with a deletion of Tmc2 (Tmc2(Δ) mice) were phenotypically normal, Tmc1(Δ)Tmc2(Δ) mice had profound vestibular dysfunction, deafness, and structurally normal hair cells that lacked all mechanotransduction activity. Expression of either exogenous TMC1 or TMC2 rescued mechanotransduction in Tmc1(Δ)Tmc2(Δ) mutant hair cells. Our results indicate that TMC1 and TMC2 are necessary for hair cell mechanotransduction and may be integral components of the mechanotransduction complex. Our data also suggest that persistent TMC2 expression in vestibular hair cells may preserve vestibular function in humans with hearing loss caused by TMC1 mutations.

The highly conserved DAD1 protein involved in apoptosis is required for N-linked glycosylation

Background: The tsBN7 cell line is one of the temperature‐sensitive mutants for cell proliferation which have been isolated from the BHK21 cell line derived from the golden hamster. It has a mutation in the DAD1 gene encoding a 12.5 kDa highly conserved protein through evolution, and enters apoptosis at the restrictive temperature due to this mutation.

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.

Topology of transmembrane channel-like gene 1 protein.

Mutations of transmembrane channel-like gene 1 (TMC1) cause hearing loss in humans and mice. TMC1 is the founding member of a family of genes encoding proteins of unknown function that are predicted to contain multiple transmembrane domains. The goal of our study was to define the topology of mouse TMC1 expressed heterologously in tissue culture cells. TMC1 was retained in the endoplasmic reticulum (ER) membrane of five tissue culture cell lines that we tested. We used anti-TMC1 and anti-HA antibodies to probe the topologic orientation of three native epitopes and seven HA epitope tags along full-length TMC1 after selective or complete permeabilization of transfected cells with digitonin or Triton X-100, respectively. TMC1 was present within the ER as an integral membrane protein containing six transmembrane domains and cytosolic N- and C-termini. There is a large cytoplasmic loop, between the fourth and fifth transmembrane domains, with two highly conserved hydrophobic regions that might associate with or penetrate, but do not span, the plasma membrane. Our study is the first to demonstrate that TMC1 is a transmembrane protein. The topologic organization revealed by this study shares some features with that of the shaker-TRP superfamily of ion channels.

Multiple quantitative trait loci modify cochlear hair cell degeneration in the Beethoven (Tmc1Bth) mouse model of progressive hearing loss DFNA36

Dominant mutations of transmembrane channel-like gene 1 (TMC1) cause progressive sensorineural hearing loss in humans and Beethoven (Tmc1Bth/+) mice. Here we show that Tmc1Bth/+ mice on a C3HeB/FeJ strain background have selective degeneration of inner hair cells while outer hair cells remain structurally and functionally intact. Inner hair cells primarily function as afferent sensory cells, whereas outer hair cells are electromotile amplifiers of auditory stimuli that can be functionally assessed by distortion product otoacoustic emission (DPOAE) analysis. When C3H-Tmc1Bth/Bth is crossed with either C57BL/6J or DBA/2J wild-type mice, F1 hybrid Tmc1Bth/+ progeny have increased hearing loss associated with increased degeneration of outer hair cells and diminution of DPOAE amplitudes but no difference in degeneration of inner hair cells. We mapped at least one quantitative trait locus (QTL), Tmc1m1, for DPOAE amplitude on chromosome 2 in [(C/B)F1 × C]N2-Tmc1Bth/+ backcross progeny, and three other QTL on chromosomes 11 (Tmc1m2), 12 (Tmc1m3), and 5 (Tmc1m4) in [(C/D)F1 × C]N2-Tmc1Bth/+ progeny. The polygenic basis of outer hair cell degeneration in Beethoven mice provides a model system for the dissection of common, complex hearing loss phenotypes, such as presbycusis, that involve outer hair cell degeneration in humans.

Analysis of auditory phenotype and karyotype in 200 females with Turner syndrome.

Objectives: Turner syndrome is the most common sex chromosome disorder in females, and is caused by a total or partial deletion of one X chromosome. The purpose of this study was to describe the auditory phenotype in a large group of individuals with Turner Syndrome, with analysis focusing on hearing loss and age, as well as the phenotypic relationship to karyotype variation. Design: Our analysis of auditory function was part of a large-scale, natural history study in which clinical and genetic factors related to Turner syndrome were examined. This ascertainment avoids the bias inherent in studies of patients referred to audiology or otolaryngology specialty clinics. Analysis included data from 200 females with Turner syndrome ranging in age from 7 to 61 yr (mean = 27.9 yr). Results: We observed hearing loss in approximately one-half of females with Turner syndrome, and report on a common, previously unlabeled audiometric configuration found in 24% of ears tested. Our cross-sectional design revealed an observable deterioration in hearing loss above the averaged rate of age-related hearing loss seen in an otologically screened, standardized population. Karyotype analysis revealed air conduction thresholds that were significantly poorer in the 46, XdelXp and 46, XiXq groups than in the 46, XdelXq group. Conclusions: This natural history study provides a more representative description of the auditory phenotype associated with Turner syndrome than previous studies that may have been biased by the method of ascertainment. Correlative analysis of Turner syndrome-specific hearing loss features with karyotype revealed that air conduction threshold elevations are associated with loss of the p arm of chromosome X. Our cross-sectional data indicate a loss of hearing sensitivity at an accelerated rate beyond a normal age-related decline, which warrants continued audiologic monitoring in all females with Turner syndrome regardless of a history of normal hearing.

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.

Nonsyndromic hearing loss DFNA10 and a novel mutation of EYA4: Evidence for correlation of normal cardiac phenotype with truncating mutations of the Eya domain

Dominant, truncating mutations of eyes absent 4 (EYA4) on chromosome 6q23 can cause either nonsyndromic hearing loss DFNA10 or hearing loss with dilated cardiomyopathy (DCM). It has been proposed that truncations of the C‐terminal Eya domain cause DFNA10 whereas upstream truncations of the N‐terminal variable region cause hearing loss with DCM. Here we report an extended family co‐segregating autosomal dominant, postlingual‐onset, progressive, sensorineural hearing loss (SNHL) with a novel frameshift mutation, 1490insAA, of EYA4. The 1490insAA allele is predicted to encode a truncated protein with an intact N‐terminal variable region, but lacking the entire C‐terminal Eya domain. Clinical studies including electrocardiography, echocardiography, and magnetic resonance imaging (MRI) of the heart in nine affected family members revealed no DCM or associated abnormalities and confirmed their nonsyndromic phenotype. These are the first definitive cardiac evaluations of DFNA10 hearing loss to support a correlation of EYA4 mutation position with the presence or absence of DCM. These results will facilitate the counseling of patients with these phenotypes and EYA4 mutations. Published 2007 Wiley‐Liss, Inc.

Otolaryngologic markers for the early diagnosis of Turner syndrome

OBJECTIVE To identify and characterize otolaryngologic markers for the early diagnosis of Turner syndrome (TS). STUDY DESIGN Prospective cohort survey. METHODS SETTING Clinical Center of the National Institutes of Health (NIH). PATIENTS Ninety-one females, 7-61 years old (average=28.7 y), enrolled in a multidisciplinary study of karyotype-phenotype correlations in TS. MAIN OUTCOME MEASURES Age at diagnosis, X chromosome karyotype, history of chronic or recurrent otitis media (OM), sensorineural hearing loss (SNHL), palate dysmorphism, pinna deformity, pterygium colli, low posterior hairline, low-set ears, and micrognathia. RESULTS Sixty-nine (76%) patients had a history of chronic or recurrent OM, 62 (68%) had a dysmorphic palate, 57 (63%) had SNHL, and 90 (99%) had one or more of these findings. 83 (91%; average age at diagnosis=9.4 y) had one or more external craniofacial signs: pinna abnormalities, pterygium colli, low-set ears, micrognathia or a low posterior hairline. Eight patients (average age at diagnosis=13.2 y) had no external craniofacial signs, although seven (88%) of these eight patients had a history of chronic or recurrent OM, dysmorphic palate or SNHL. The age at diagnosis was not significantly different between groups with or without external craniofacial signs (P=0.126). CONCLUSIONS PATIENTS with mild or incompletely penetrant TS phenotypes often present with otitis media, hearing loss, or both before the diagnosis of TS is established. Palatal dysmorphism, including ogival morphology, is another otolaryngologic marker for TS. Prompt recognition of these manifestations of TS could hasten its diagnosis and appropriate medical care.