Genetics of deafness

Phenotypic differences in the inner ears of CBA/CaJ and C57BL/6J mice carrying missense and single base pair deletion mutations in the Cdh23 gene

Different mutations in the cadherin 23 (CDH23) gene in different genetic backgrounds have been linked to either syndromic or nonsyndromic forms of deafness in humans. We previously reported a progressive hearing loss (HL) mouse model, the Cdh23^erl/erl mouse, which carries a 208T > C mutation causing an amino acid substitution at S70P in C57BL/6J mice. To investigate the differences in Cdh23 mutation-related HL in different genetic backgrounds, we used the CRISPR/Cas9 system to generate homozygous mice in the CBA/CaJ background that have the same base pair missense mutation (208T > C) (Cdh23^erl2/erl2 ) as Cdh23^erl/erl mice in the C57BL/6J background or a single base pair deletion (235G) (Cdh23^V2J2/V2J2 ) in the Cdh23 gene at exon 5. The two mutant mice exhibit hearing impairment across a broad range of frequencies. The progression of HL in Cdh23^erl2/erl2 mice is slower than that in Cdh23^erl/erl mice. We also found structural abnormalities in the stereocilia of cochlear hair cells in Cdh23^erl2/erl2 and Cdh23^V2J2/V2J2 mice. Cdh23^V2J2/V2J2 mice show signs of vestibular dysfunction in open field behavior and swimming tests. In addition, we observed hair bundle defects in vestibular hair cells in Cdh23^V2J2/V2J2 mice. Our results suggest an interaction between the erl locus and the C57BL/6J background that exacerbates HL in Cdh23^erl/erl mice. Moreover, our study confirms that the Cdh23 gene is essential for normal hearing and balance. These two novel mutant mouse strains provide excellent models for studying CDH23 mutation-related deafness in humans.

Basilar membrane and tectorial membrane stiffness in the CBA/CaJ mouse

The mouse has become an important animal model in understanding cochlear function. Structures, such as the tectorial membrane or hair cells, have been changed by gene manipulation, and the resulting effect on cochlear function has been studied. To contrast those findings, physical properties of the basilar membrane (BM) and tectorial membrane (TM) in mice without gene mutation are of great importance. Using the hemicochlea of CBA/CaJ mice, we have demonstrated that tectorial membrane (TM) and basilar membrane (BM) revealed a stiffness gradient along the cochlea. While a simple spring mass resonator predicts the change in the characteristic frequency of the BM, the spring mass model does not predict the frequency change along the TM. Plateau stiffness values of the TM were 0.6 ± 0.5, 0.2 ± 0.1, and 0.09 ± 0.09 N/m for the basal, middle, and upper turns, respectively. The BM plateau stiffness values were 3.7 ± 2.2, 1.2 ± 1.2, and 0.5 ± 0.5 N/m for the basal, middle, and upper turns, respectively. Estimations of the TM Young's modulus (in kPa) revealed 24.3 ± 25.2 for the basal turns, 5.1 ± 4.5 for the middle turns, and 1.9 ± 1.6 for the apical turns. Young's modulus determined at the BM pectinate zone was 76.8 ± 72, 23.9 ± 30.6, and 9.4 ± 6.2 kPa for the basal, middle, and apical turns, respectively. The reported stiffness values of the CBA/CaJ mouse TM and BM provide basic data for the physical properties of its organ of Corti.

A novel mutation in the TECTA gene in a Chinese family with autosomal dominant nonsyndromic hearing loss

TECTA-related deafness can be inherited as autosomal-dominant nonsyndromic deafness (designated DFNA) or as the autosomal-recessive version. The α-tectorin protein, which is encoded by the TECTA gene, is one of the major components of the tectorial membrane in the inner ear. Using targeted DNA capture and massively parallel sequencing (MPS), we screened 42 genes known to be responsible for human deafness in a Chinese family (Family 3187) in which common deafness mutations had been ruled out as the cause, and identified a novel mutation, c.257-262CCTTTC>GCT (p. Ser86Cys; p. Pro88del) in exon 3 of the TECTA gene in the proband and his extended family. All affected individuals in this family had moderate down-sloping hearing loss across all frequencies. To our knowledge, this is the second TECTA mutation identified in Chinese population. This study demonstrates that targeted genomic capture, MPS, and barcode technology might broaden the availability of genetic testing for individuals with undiagnosed DFNA.

Cochlear implant considerations in children with additional disabilities

Early identification and management of disabilities in children are essential to reduce long-term developmental sequelae. Many of the causes of hearing loss also produce cognitive delays resulting in a large number of children with both deafness and developmental disabilities. Children who have hearing loss and additional disabilities require complex, individualized therapy to maximize their long-term quality of life. Hearing loss is often detected early because of widespread newborn hearing screening programs and the decision for cochlear implantation in children presenting with multiple medical and developmental disorders is still evolving. This article will review the literature regarding cochlear implant considerations in children with additional developmental disabilities in areas of family perception, speech and language development, cognitive development including adaptive behavior and intelligence, communication and functional skills, auditory outcomes, quality of life outcomes, predictors of outcomes and realistic expectations after cochlear implantation.

Cognitive outcomes and familial stress after cochlear implantation in deaf children with and without developmental delays

OBJECTIVE

The benefits of cochlear implantation for children with developmental delays (DD) often are unclear. We compared cognition, adaptive behavior, familial stress, and communication in children with and without DD.

STUDY DESIGN

Retrospective review.

SETTING

Two tertiary care pediatric hospitals.

PATIENTS

Two hundred four children who underwent cochlear implantation assessed before and more than 1 year after implantation.

MAIN OUTCOME MEASURES

The Mullen Scales of Early Learning (MSEL), vineland adaptive behavior scales (VABS), Parental Stress Index, and Preschool Language Scale.

RESULTS

We developed a specific definition of DD for hearing-impaired children based upon diagnostic and statistical manual of mental disorders, fourth edition, criteria for mental retardation; 60 children met the criteria for DD, and 144 children did not. Before implantation, multiple linear regression demonstrated that children with DD had lower scores in every domain of the MSEL and VABS (p < 0.05), but no differences in any domains of the parental stress index and preschool language scale (p > 0.1) compared with children without DD. After implantation, children without DD demonstrated significant improvements in intelligence as measured by the MSEL and age-appropriate improvements in adaptive behavior as evaluated by the VABS, and their familial stress levels were not increased after cochlear implantation. In contrast, children with DD underwent implantation at a later age and demonstrated less comprehensive developmental improvements after cochlear implantation and higher stress levels. However, when the age differences were taken into account using multiple linear regression analyses, the differences between the 2 cohorts were reduced.

CONCLUSION

These data indicate that our definition of DD is a reliable method of stratifying deaf children. Although children with DD have a normal developmental rate of adaptive behavior after cochlear implantation, their developmental rate of intelligence is lower, and they have higher stress levels than children without DD. However, our data suggest that if children with DD could be implanted as early as children without DD, their intelligence and stress outcomes would be improved.

Quantitative X-ray tomography of the mouse cochlea

Imaging with hard X-rays allows visualizing cochlear structures while maintaining intrinsic qualities of the tissue, including structure and size. With coherent X-rays, soft tissues, including membranes, can be imaged as well as cells making use of the so-called in-line phase contrast. In the present experiments, partially coherent synchrotron radiation has been used for micro-tomography. Three-dimensional reconstructions of the mouse cochlea have been created using the EM3D software and the volume has been segmented in the Amira Software Suite. The structures that have been reconstructed include scala tympani, scala media, scala vestibuli, Reissner's membrane, basilar membrane, tectorial membrane, organ of Corti, spiral limbus, spiral ganglion and cochlear nerve. Cross-sectional areas of the scalae were measured. The results provide a realistic and quantitative reconstruction of the cochlea.

Comprehensive diagnostic battery for evaluating sensorineural hearing loss in children

OBJECTIVE

Selection of diagnostic tests for children with sensorineural hearing loss (SNHL) is influenced by clinical suspicion. Testing results reported in the literature are similarly biased. We evaluate the usefulness of a comprehensive diagnostic battery for each child.

STUDY DESIGN

Retrospective review.

SETTING

Tertiary care university hospital.

PATIENTS

A total of 270 children referred for severe to profound SNHL between January 2002 and June 2009.

INTERVENTIONS

Results of the following were reviewed: magnetic resonance imaging, computed tomography, renal ultrasound, electrocardiography, fluorescent treponemal antibody absorption test, connexin 26 sequencing, genetic consultation, and ophthalmologic consultation.

MAIN OUTCOME MEASURE

Diagnostic yield of each test was determined.

RESULTS

Each diagnostic test or consultation was completed by at least 95% of patients for whom it was ordered. Magnetic resonance imaging revealed abnormalities explaining SNHL in 24% of patients. Computed tomography showed inner ear anomalies in 18% of patients. Biallelic connexin 26 mutations were found in 15%. Renal ultrasound found anomalies in 4% of patients. Electrocardiography found 1% of patients with prolonged QT intervals. Fluorescent treponemal antibody absorption test result was positive in 0.5%. Genetic consultation found a genetic cause for hearing loss in 25%. Ophthalmologic consultation found abnormalities associated with hearing loss in 8%.

CONCLUSION

Diagnostic radiologic imaging is the highest yielding test for evaluating children with SNHL. Connexin 26 sequencing identifies a nearly nonoverlapping subset of children compared with imaging. Specialty consultations, particularly from a clinical geneticist, can improve diagnostic yield. Other tests, although of lower diagnostic yield for SNHL, can identify important diseases that significantly affect patient health.

Deficient forward transduction and enhanced reverse transduction in the alpha tectorin C1509G human hearing loss mutation

Most forms of hearing loss are associated with loss of cochlear outer hair cells (OHCs). OHCs require the tectorial membrane (TM) for stereociliary bundle stimulation (forward transduction) and active feedback (reverse transduction). Alpha tectorin is a protein constituent of the TM and the C1509G mutation in alpha tectorin in humans results in autosomal dominant hearing loss. We engineered and validated this mutation in mice and found that the TM was shortened in heterozygous Tecta(C1509G/+) mice, reaching only the first row of OHCs. Thus, deficient forward transduction renders OHCs within the second and third rows non-functional, producing partial hearing loss. Surprisingly, both Tecta(C1509G/+) and Tecta(C1509G/C1509G) mice were found to have increased reverse transduction as assessed by sound- and electrically-evoked otoacoustic emissions. We show that an increase in prestin, a protein necessary for electromotility, in all three rows of OHCs underlies this phenomenon. This mouse model demonstrates a human hearing loss mutation in which OHC function is altered through a non-cell-autonomous variation in prestin.

Does a pleiotropic gene explain deafness and blue irises in white cats?

The prevalence of deafness is high in cat populations in which the dominant white gene is segregating. The objective of this study was to investigate whether there is a gene that is responsible for deafness as well as for blue eyes and to establish a plausible mode of inheritance. For this purpose, data from an experimental colony with deaf cats were analyzed. The hearing status was determined by acoustically evoked brain stem responses (BAER). Complex segregation analyses were conducted to find out the most probable mode of inheritance using maximum likelihood procedures. The prevalence of deafness and partial hearing in the experimental colony was 67% and 29%, respectively. The results of the bivariate segregation analysis support the hypothesis of a pleiotropic major gene segregating for deafness and blue iris colour. The high heritability coefficients for both traits, 0.55 and 0.75 respectively, indicate that beside the major gene there is an important influence of polygenic effects.

A quantitative survey of gravity receptor function in mutant mouse strains

The purpose of this research was to identify vestibular deficits in mice using linear vestibular evoked potentials (VsEPs). VsEP thresholds, peak latencies, and peak amplitudes from 24 strains with known genetic mutations and 6 inbred background strains were analyzed and descriptive statistics generated for each strain. Response parameters from mutant homozygotes were compared with heterozygote and/or background controls and all strain averages were contrasted to normative ranges. Homozygotes of the following recessive mutations had absent VsEPs at the ages tested: Espn(je), Atp2b2dfw-2J, Spnb4qv-lnd2J, Spnb4qv-3J, Myo7ash1, Tmie(sr), Myo6sv, jc, Pcdh15av-J, Pcdh15av-2J, Pcdh15av-3J, Cdh23v-2J, Sans(js), hr, Kcne1pkr and Pou3f4del. These results suggest profound gravity receptor deficits for these homozygotes, which is consistent with the structural deficits that have been documented for many of these strains. Homozygotes of Catna2cdf, Grid2ho4J, Wnt1sw, qk, and Mbpshi strains and heterozygotes of Grid2lc had measurable VsEPs but one or more response parameters differed from the respective control group (heterozygote or background strain) or were outside normal ranges. For example, qk and Mbpshi homozygotes showed significantly prolonged latencies consistent with the abnormal myelin that has been described for these strains. Prolonged latencies may suggest deficits in neural conduction; elevated thresholds suggest reduced sensitivity, and reduced amplitudes may be suggestive for reduced neural synchrony. One mutation, Otx1jv, had all VsEP response parameters within normal limits--an expected finding because the abnormality in Otxljv is presumably restricted to the lateral semicircular canal. Interestingly, some heterozygote groups also showed abnormalities in one or more VsEP response parameters, suggesting that vestibular dysfunction, although less severe, may be present in some heterozygous animals.

Age-related changes in cochlear endolymphatic potassium and potential in CD-1 and CBA/CaJ mice

The CD-1 mouse strain is known to have early onset of hearing loss that is progressive with aging. We sought to determine whether a disturbance of K+ homeostasis and pathological changes in the cochlear lateral wall were involved in the age-related hearing loss (AHL) of CD-1 as compared to the CBA/CaJ strain which has minimal AHL. In the present study, the endocochlear potential (EP) and endolymphatic K+ concentration ([K+]e) were measured in both strains of mice with double-barrel microelectrodes at "young" (1-2 mo) and "old" (5-9 mo) ages. CBA/CaJ mice displayed no changes with aging in EP and [K+]e of the basal turn. In the apical turn, there was a small positive shift of the EP (10 mV) with aging under both normoxic and acute anoxic conditions (-EP), without any change of [K+]e. Further, there were no obvious pathological changes in the lateral wall of CBA/CaJ mice. By contrast, old CD-1 mice displayed a significantly reduced [K+]e by 30% in both basal and apical turns with no significant changes in normoxic EP. The -EP in the apical turn was significantly reduced in magnitude by 6 mV. A severe loss of cells with aging was observed in the region of type IV fibrocytes of the apical and basal turns and of type II fibrocytes in the basal turn. A complete degeneration of organ of Corti was also observed at the basal turn of old CD-1 mice, as well as a basalward decline of spiral ganglion neuron density. The pathological changes in spiral ligament of CD-1 mice were similar to those of an inbred mouse strain C57BL/6J that expresses an AHL gene (ahl) and might be a primary etiology of AHL of CD-1 mice. These findings have ramifications for our understanding of AHL and for interpretation of genetic mutations in a CD-1 background.

Genetic testing for hearing loss: different motivations for the same outcome

The recent discoveries of genes involved in deafness open new options for families and individuals with hearing loss. Our aim was to learn if parents of children with hearing loss will benefit from these new possibilities. A total of 139 parents answered questionnaires aimed at evaluating their intentions as well as their reasons to opt for or against genetic testing and prenatal diagnosis for hearing loss. A very high interest (87%) in genetic testing was found among Israeli Jewish parents of deaf and hard-of-hearing children. Although the Jewish population in Israel is very diverse in its religious beliefs, this high interest was similar across all religious sectors (secular, traditional, orthodox, and ultraorthodox); however, some of the reasons for undertaking such a test were very different between them. Reasons related to family planning and prenatal diagnosis were significantly less important to parents from the ultraorthodox sector, but the possibility to utilize genetic testing for matchmaking the children with hearing loss and their hearing siblings was an important factor in motivating them to undertake the test. Parents from all religious sectors wished testing would shed light on the cause of the hearing loss. We conclude that genetic testing would be welcomed by a wide range of communities, including those that usually do not apply for genetic counseling and testing, if it is offered in accordance with their cultural norms and beliefs.

Breed differences in cochlear integrity in adult, commercially raised chickens

Two types of chickens are commercially available. Broiler birds are bred to develop quickly for meat production, while egg layers are bred to attain a smaller adult size. Because we have observed breed differences in the response of central auditory neurons to cochlear ablation in adult birds [Edmonds et al. (1999) Hear. Res. 127, 62-76], we examined cochleae from the two breeds for differences in integrity. We evaluated cochlear hair cell structure using scanning electron microscopy and cochlear hair cell function using distortion product otoacoustic emissions (DPOAEs) and the auditory brainstem response. We observed striking breed differences in cochlear integrity in adult but not hatchling birds. In adult broiler birds, all cochleae showed damage, encompassing at least the basal 29% of the cochlea. In 15 of 18 broiler ears, damage was observed throughout the basal 60% of the cochlea. In contrast, cochleae from egg layer adults were largely normal. Two thirds of egg layer ears showed no anatomical abnormalities, while in the remainder cochlear damage was seen within the basal 48% of the cochlea. DPOAEs recorded from egg layer birds showed loss of high frequency emissions in every ear for which the cochlea displayed anatomical damage. Average sound pressure levels in both commercial facilities were 90 dB, suggesting these two breeds may exhibit differential susceptibility to noise damage.

Nonsyndromic hereditary hearing loss

Like many areas of medicine, the rapid advances in genetics and molecular biology are revolutionizing our understanding of hearing and balance disorders. Dramatic progress has been made in identifying deafness genes in the past few years. These genes encode proteins of diverse function, including transcription factors, cytoskeletal and extracellular matrix components, and ion channels. The diversity of the genes so far identified is testimony to the complexities of auditory development and function and the power of genetic approaches. In what is about to become the postgenomic era, the study of the proteins encoded by these genes will advance our understanding of auditory development and function and lead to innovative approaches toward the treatment of patients with hearing disorders.

Cochlear dimensions obtained in hemicochleae of four different strains of mice: CBA/CaJ, 129/CD1, 129/SvEv and C57BL/6J

Because homologies between mice and human genomes are well established and hereditary abnormalities are similar in both, mice present a valuable animal model to study hereditary hearing disorders in humans. One of the manifestations of hereditary hearing disorders might be in the structure of cochlear elements, such as the gross morphology of the cochlea. Cochlear dimensions, however, are one factor that determines inner ear mechanics and thus hearing function. Therefore, gross cochlear dimension might be important when different strains of mice are compared regarding their hearing. Although several studies have examined mouse inner ear structures on a sub-cellular level, only few have studied cochlear gross morphology. Moreover, the sparse data available were acquired from fixed and dehydrated tissue. Dehydration, however, produces severe distortion of gel-like cochlear structures such as the tectorial membrane and the basilar membrane hyaline matrix. In this study, the hemicochlea technique, which allows fresh mouse cochlear material to be viewed from a radial perspective, was used to provide an itemized study of the dimensions of gross cochlear structures in four mouse strains (CBA/CaJ, 129/SvEv, 129/CD1 and C57BL/6J). Except for the CBA/CaJ, these strains are known to possess genes for age-related hearing loss. The measurements showed no major differences among the four strains. However, when compared with previous data, the thickness measures of the basilar membrane were up to 10 times larger. Such differences are likely to result from the different techniques used to process the material. The hemicochlea technique eliminates much of the distortion caused by dehydration, which was present in previous experiments.

Epidemiological studies on hearing impairment with reference to genetic factors in Sichuan, China

Hearing impairment is the most common disorder of sensorineural function and is an economically and socially important cause of human morbidity. A large-scale epidemiological survey of hearing loss was conducted with 126,876 unselected subjects (63,741 male and 63,135 female) from Sichuan, China. The overall prevalence of hearing loss was 3.28% (4,164 of 126,876), and the prevalence increased with age, reaching 12.8% (1,465 of 11,421) at 60 years of age. In 73.03% of all cases (3,041 of 4,164), the hearing loss was sensorineural, and in 20.39% (849 of 4,164), it was conductive; the remaining cases (6%) were mixed hearing loss. Bilateral loss was found in 74.5% of cases (3,103 of 4,164). In 63.79% of cases (2,656 of 4,164), the degree of hearing loss was less than 55 dB hearing level (HL), and in 5.67% of cases (236 of 4,164), it was greater than 90 dB HL. The prevalence of hearing loss in childhood (<15 years of age) was 0.67% (227 of 34,157), of which 57.7% of cases were conductive and 38.8% were sensorineural. The prevalence of genetic hearing loss was 0.28% (349 of 126,876). Persons who lived in the flatlands appeared to have a higher prevalence than those who lived in the hills. Several ethnic groups, including Tibetans, the Yi, and the Lisu, had a higher prevalence of hearing loss. Presbycusis, otitis media, and genetic factors were the most commonly recognized causes of hearing impairment overall, but otitis media and genetic factors were the main causes of hearing loss in children. Causes for the observed differences in prevalence and etiologic factors between China and industrialized countries will be discussed. In China, infections and genetic factors appear to be of major importance as causes of hearing loss.

Effects of PMCA2 mutation on DPOAE amplitudes and latencies in deafwaddler mice

The deafwaddler (dfw) mouse mutant is caused by a spontaneous mutation in the gene that encodes a plasma membrane Ca(2+) ATPase (type 2), PMCA2 (Street et al., 1998. Nat. Genet. 19, 390-394), which is expressed in cochlear and vestibular hair cells. Distortion product otoacoustic emission (DPOAE) amplitudes and latencies were examined in control mice, deafwaddler mutants, and controls treated with the drug furosemide. Furosemide causes a transient reduction of DPOAEs (Mills et al., 1993. J. Acoust. Soc. Am. 94, 2108-2122). We wanted to determine whether DPOAEs obtained in furosemide-treated mice were similar or different from results obtained in +/dfw mice. DPOAE amplitude and phase were measured as a function of f(2)/f(1) ratio. These data were converted into waveforms using inverse fast Fourier transform, and their average latency was used to estimate DPOAE group delay. Homozygous deafwaddlers did not produce DPOAEs. Heterozygous deafwaddlers (+/dfw) had increased DPOAE thresholds and reduced amplitudes at high frequencies, compared to controls. To the extent that DPOAEs depend on functional outer hair cells (OHCs), abnormal DPOAEs in +/dfw mice suggest that PMCA2 is important for OHC function at high frequencies. Similar to the effects of furosemide, the mutation reduced DPOAEs for low-level stimuli; in contrast to furosemide, the mutation altered DPOAEs elicited by high levels.

Dorsal patterning defects in the hindbrain, roof plate and skeleton in the dreher (dr(J)) mouse mutant

dreher is a spontaneous mouse mutation in which adult animals display a complex phenotype associated with hearing loss, neurological, pigmentation and skeletal abnormalities. During early embryogenesis, the neural tube of dreher mutants is abnormally shaped in the region of the rhomboencephalon, due to problems in the formation of a proper roof plate over the otic hindbrain. We have studied the expression of Hox/lacZ transgenic mouse strains in the dreher background and shown that primary segmentation of the neural tube is not altered in these mutants, although correct morphogenesis is affected resulting in misshapen rhombomeres. Neural crest derivatives from rhombomere 6, such as the glossopharyngeal ganglion, are defective, and the dorsal neural tube marker Wnt1 is absent from this segment. Selected trunk neural crest populations are also altered, as there is a lack of pigmentation in the thoracic region of mutant mice. Skeletal defects include abnormal cranial bones of neural crest origin, and improper fusion of the dorsal aspects of cervical and thoracic vertebrae. Taken together, the gene affected in the dreher mutant is responsible for correct patterning of the dorsal-most cell types of the neural tube, that is, the neural crest and the roof plate, in the hindbrain region. Axial skeletal defects could reflect inductive influence of the dorsal neural tube on proper fusion of the neural arches. It is possible that a common precursor population for both neural crest and roof plate is the cellular target of the dreher mutation.

A transgenic insertional inner ear mutation on mouse chromosome 1

OBJECTIVES/HYPOTHESIS

To clone and characterize the integration site of an insertional inner ear mutation, produced in one of fourteen transgenic mouse lines. The insertion of the transgene led to a mutation in a gene(s) necessary for normal development of the vestibular labyrinth.

STUDY DESIGN

Molecular genetic analysis of a transgene integration site.

METHODS

Molecular cloning, Southern and northern blotting, DNA sequencing and genetic database searching were the methods employed.

RESULTS

The integration of the transgene resulted in a dominantly inherited waltzing phenotype and in degeneration of the pars superior. During development, inner ear fluid homeostasis was disrupted. The integration consisted of the insertion of a single copy of the transgene. Flanking DNA was cloned, and mapping indicated that the genomic DNA on either side of the transgene was not contiguous in the wild-type mouse. Localization of unique markers from the two flanks indicated that both were in the proximal region of mouse chromosome 1. However, in the wild-type mouse the markers were separated by 6.3 cM, indicating a sizable rearrangement. Analysis of the mutant DNA indicated that the entire region between the markers was neither deleted nor simply inverted.

CONCLUSIONS

These results are consistent with a complex rearrangement, including at least four breakpoints and spanning at least 6.3 cM, resulting from the integration of the transgene. This genomic rearrangement disrupted the function of one or more genes critical to the maintenance of fluid homeostasis during development and the normal morphogenesis of the pars superior.

Useful residual hearing despite radiological findings suggestive of anacusis

A severe malformation of the inner ear, often referred to as severe labyrinthine dysplasia or common cavity deformity, consists of an absent or dilated cochlear basal coil, wide communication with the vestibule and a tapered internal acoustic meatus and can be associated with absent hearing. We discuss two children with severe labyrinthine dysplasia as shown by computed tomography (CT) scans and, in the first case, an absent VIIIth nerve bilaterally shown by magnetic resonance imaging (MRI). In 1995, both cases were precluded from cochlear implantation, on the basis of the absent VIIIth nerve (first case) and increased risk of CSF leak during operation (second case). However, audiometric results and vocalization patterns of both children suggested the presence of some residual hearing function, while recently reported specific surgical techniques have been found to be safe and effective in the cochlear implantation of the common cavity deformity. The management of such cases should be decided on the grounds of a full audiological assessment in conjunction with the radiological features, in the light of current surgical trends shown to be safe and effective.

Gene transfer into the mammalian inner ear using HSV-1 and vaccinia virus vectors

The introduction of foreign genes into cells has become an effective means of achieving intracellular expression of foreign proteins, both for therapeutic purposes and for experimental manipulation. Gene delivery to the nervous system has been extensively studied, primarily using viral vectors. However, to date less work has focused on gene delivery to the inner ear, and existing studies have primarily used adenovirus and adeno-associated virus. Using two recombinant viral vectors, herpes simplex type 1 (HSV-1), and vaccinia virus, bearing the Escherichia coli lacZ gene, we tested gene delivery to the guinea pig cochlea in vivo with beta-galactosidase staining as an assay. The HSV-1 and vaccinia virus vectors were both found to infect and elicit transgene expression successfully in many cells in the guinea pig cochlea, including cells in the organ of Corti. These data demonstrate the feasibility of gene delivery to the inner ear using these two viral vectors. Such techniques may facilitate study of the auditory systems, and might be used to develop gene therapy strategies for some forms of hearing loss.

Myosins and deafness

The discovery in the past few years of a huge diversity within the myosin superfamily has been coupled with an understanding of the role of these motor proteins in various cellular functions. Extensive studies have revealed that myosin isoforms are not only involved in muscle contraction but also in crucial functions of many specialized mammalian cells such as melanocytes, kidney and intestinal brush border microvilli, nerve growth cones or inner ear hair cells. A search for genes involved in the pathology of human genetic deafness resulted in identification of three novel myosins: myosin VI, myosin VIIA and, very recently, myosin XV. The structure, tissue and cellular distribution of these myosin isoforms, as well as mutations detected within their genes that have been found to affect the hearing process, are described in this review.

Assessment of hearing in 80 inbred strains of mice by ABR threshold analyses

The common occurrence of hearing loss in both humans and mice, and the anatomical and functional similarities of their inner ears, attest to the potential of mice being used as models to study inherited hearing loss. A large-scale, auditory screening project is being undertaken at The Jackson Laboratory (TJL) to identify mice with inherited hearing disorders. To assess hearing sensitivity, at least five mice from each inbred strain had auditory brainstem response (ABR) thresholds determined. Thus far, we have screened 80 inbred strains of mice; 60 of them exhibited homogeneous ABR threshold values not significantly different from those of the control strain CBA/CaJ. This large database establishes a reliable reference for normal hearing mouse strains. The following 16 inbred strains exhibited significantly elevated ABR thresholds before the age of 3 months: 129/J, 129/ReJ, 129/SvJ, A/J, ALR/LtJ, ALS/LtJ, BUB/BnJ, C57BLKS/J, C57BR/cdJ, C57L/J, DBA/2J, I/LnJ, MA/MyJ, NOD/LtJ, NOR/LtJ, and SKH2/J. These hearing impaired strains may serve as models for some forms of human non-syndromic hearing loss and aid in the identification of the underlying genes.

Molecular markers for cell types of the inner ear and candidate genes for hearing disorders

To identify genes expressed in the vertebrate inner ear, we have established an assay that allows rapid analysis of the differential expression pattern of mRNAs derived from an auditory epithelium-specific cDNA library. We performed subtractive hybridization to create an enriched probe, which then was used to screen the cDNA library. After digoxigenin-labeled antisense cRNAs had been transcribed from hybridization-positive clones, we conducted in situ hybridization on slides bearing cryosections of late embryonic chicken heads, bodies, and cochleae. One hundred and twenty of the 196 clones analyzed encode 12 proteins whose mRNAs are specifically or highly expressed in the chicken's inner ear; the remainder encode proteins that occur more widely. We identified proteins that have been described previously as expressed in the inner ear, such as beta-tectorin, calbindin, and type II collagen. A second group of proteins abundant in the inner ear includes five additional types of collagens. A third group, including Coch-5B2 and an ear-specific connexin, comprises proteins whose human equivalents are candidates to account for hearing disorders. This group also includes proteins expressed in two unique cell types of the inner ear, homogene cells and cells of the tegmentum vasculosum.

Mutations in a plasma membrane Ca2+-ATPase gene cause deafness in deafwaddler mice

Hearing loss is the most common sensory deficit in humans. Because the auditory systems of mice and humans are conserved, studies on mouse models have predicted several human deafness genes and identified new genes involved in hearing. The deafwaddler (dfw) mouse mutant is deaf and displays vestibular/motor imbalance. Here we report that the gene encoding a plasma membrane Ca2+-ATPase type 2 pump (Atp2b2, also known as Pmca2) is mutated in dfw. An A-->G nucleotide transition in dfw DNA causes a glycine-to-serine substitution at a highly conserved amino-acid position, whereas in a second allele, dfw2J, a 2-base-pair deletion causes a frameshift that predicts a truncated protein. In the cochlea, the protein Atp2b2 is localized to stereocilia and the basolateral wall of hair cells in wild-type mice, but is not detected in dfw2J mice. This indicates that mutation of Atp2b2 may cause deafness and imbalance by affecting sensory transduction in stereocilia as well as neurotransmitter release from the basolateral membrane. These mutations affecting Atp2b2 in dfw and dfw2J are the first to be found in a mammalian plasma membrane calcium pump and define a new class of deafness genes that directly affect hair-cell physiology.

A novel type of myosin encoded by the mouse deafness gene shaker-2

The mouse recessive deafness mutation, shaker-2(sh-2), represents a plausible model for an autosomal recessive form of human non-syndromic genetic deafness, DFNB3. Here we report the use of a positional cloning approach to show that the gene mutated in sh-2 mice encodes a novel type of unconventional myosin. A G-to-A transition changing cysteine to tyrosine in the conserved actin binding domain is detected in sh-2 but absent in laboratory strains and wild mice belonging to different mouse subspecies and species. This suggests that the novel myosin gene is a strong candidate for DFNB3.

Detection of transcripts for delayed rectifier potassium channels in the Xenopus laevis inner ear

Reverse transcriptase polymerase chain reaction (RT-PCR) was used to amplify sequences for delayed rectifier potassium (drk) channel transcripts in Xenopus laevis inner ear and brain. We used degenerate primers that spanned a region between the N-terminal cytoplasmic portion and a region located between the S2 and S3 transmembrane domains of the potassium channel protein. When inner ear total RNA or brain mRNA was used as a template for RT-PCR, a unique product of the expected size (approximately 560 bp) was observed as a single band after electrophoresis on agarose gels. The PCR product from reactions using X. laevis genomic DNA as template was similarly sized, indicating a lack of introns in this region. The RT-PCR products from inner ear and brain were isolated, cloned, and sequenced. Sequence analysis showed that the X. laevis inner ear and brain clones were identical. Sequence alignments of the cloned RT-PCR products with posted GenBank sequences established that the drk sequences from X. laevis inner ear and brain share highest identity with larval X. laevis brain, mouse, rat, and human Kv2 sequences. Positive signals were obtained from inner ear and brain mRNA in Northern dot blots hybridized with digoxigenin labeled probes from the inner ear clone. Taken together, results provide evidence for the expression of Kv2 sequences in the X. laevis inner ear and brain.

FH proteins as cytoskeletal organizers

Regulation of cell shape is a poorly understood yet central issue in cell biology. Recent experiments indicate that FH proteins link cellular signalling pathways to changes in cell shape. Members of the FH protein family play essential roles in cytokinesis and in driving alterations in cell polarity. This review discusses the structure and function of these proteins and examines the evidence that they interact specifically with Rho GTPases and profilin to organize the actin-based cytoskeleton.

A genetic screen for mutations that disrupt an auditory response in Drosophila melanogaster

Hearing is one of the last sensory modalities to be subjected to genetic analysis in Drosophila melanogaster. We describe a behavioral assay for auditory function involving courtship among groups of males triggered by the pulse component of the courtship song. In a mutagenesis screen for mutations that disrupt the auditory response, we have recovered 15 mutations that either reduce or abolish this response. Mutant audiograms indicate that seven mutants reduced the amplitude of the response at all intensities. Another seven abolished the response altogether. The other mutant, 5L3, responded only at high sound intensities, indicating that the threshold was shifted in this mutant. Six mutants were characterized in greater detail. 5L3 had a general courtship defect; courtship of females by 5L3 males also was affected strongly. 5P1 males courted females normally but had reduced success at copulation. 5P1 and 5N18 showed a significant decrement in olfactory response, indicating that the defects in these mutations are not specific to the auditory pathway. Two other mutants, 5M8 and 5N30, produced amotile sperm although in 5N30 this phenotype was genetically separable from the auditory phenotype. Finally, a new adult circling behavior phenotype, the pirouette phenotype, associated with massive neurodegeneration in the brain, was discovered in two mutants, 5G10 and 5N18. This study provides the basis for a genetic and molecular dissection of auditory mechanosensation and auditory behavior.

Trapping genes expressed in the developing mouse inner ear

Identification of the genes involved in the development of the mouse inner ear and developmental studies of mice that bear mutations in these genes is an important approach to understanding genetically determined human auditory dysfunction. Towards this end, we initiated a gene trap screen designed to simultaneously mark and mutate genes in mouse embryonic stem cells by the insertion of a lacZ reporter gene. Expression of beta-galactosidase in gene trap cell lines was monitored both before and after the addition of factors that are known to affect inner ear development. Gene trap cell lines that expressed beta-galactosidase under one or more culture conditions were used to create chimeric mouse embryos for studies of reporter gene expression in vivo. A high proportion of these gene trap insertions were expressed in the developing inner ear, suggesting that this strategy provides an effective means of identifying genes that may be involved in inner ear development or function.

Unconventional myosins in inner-ear sensory epithelia

To understand how cells differentially use the dozens of myosin isozymes present in each genome, we examined the distribution of four unconventional myosin isozymes in the inner ear, a tissue that is particularly reliant on actin-rich structures and unconventional myosin isozymes. Of the four isozymes, each from a different class, three are expressed in the hair cells of amphibia and mammals. In stereocilia, constructed of cross-linked F-actin filaments, myosin-Ibeta is found mostly near stereociliary tips, myosin-VI is largely absent, and myosin-VIIa colocalizes with crosslinks that connect adjacent stereocilia. In the cuticular plate, a meshwork of actin filaments, myosin-Ibeta is excluded, myosin-VI is concentrated, and modest amounts of myosin-VIIa are present. These three myosin isozymes are excluded from other actin-rich domains, including the circumferential actin belt and the cortical actin network. A member of a fourth class, myosin-V, is not expressed in hair cells but is present at high levels in afferent nerve cells that innervate hair cells. Substantial amounts of myosins-Ibeta, -VI, and -VIIa are located in a pericuticular necklace that is largely free of F-actin, squeezed between (but not associated with) actin of the cuticular plate and the circumferential belt. Our localization results suggest specific functions for three hair-cell myosin isozymes. As suggested previously, myosin-Ibeta probably plays a role in adaptation; concentration of myosin-VI in cuticular plates and association with stereociliary rootlets suggest that this isozyme participates in rigidly anchoring stereocilia; and finally, colocalization with cross-links between adjacent stereocilia indicates that myosin-VIIa is required for the structural integrity of hair bundles.

Mutations in the myosin VIIA gene cause non-syndromic recessive deafness

Genetic hearing impairment affects around 1 in every 2,000 births. The bulk (approximately 70%) of genetic deafness is non-syndromic, in which hearing impairment is not associated with any other abnormalities. Over 25 loci involved in non-syndromic deafness have been mapped and mutations in connexin 26 have been identified as a cause of non-sydromic deafness. One locus for non-syndromic recessive deafness, DFNB2 (ref. 4), has been localized to the same chromosomal region, 11q14, as one of the loci, USH1B, underlying the recessive deaf-blind syndrome. Usher syndrome type 1b, which is characterized by profound congenital sensorineural deafness, constant vestibular dysfunction and prepubertal onset of retinitis pigmentosa. Recently, it has been shown that a gene encoding an unconventional myosin, myosin VIIA, underlies the mouse recessive deafness mutation, shaker-1 (ref. 5) as well as Usher syndrome type 1b. Mice with shaker-1 demonstrate typical neuroepithelial defects manifested by hearing loss and vestibular dysfunction but no retinal pathology. Differences in retinal patterns of expression may account for the variance in phenotype between shaker-1 mice and Usher type 1 syndrome. Nevertheless, the expression of MYO7A in the neuroepithelium suggests that it should be considered a candidate for non-syndromic deafness in the human population. By screening families with non-syndromic deafness from China, we have identified two families carrying MYO7A mutations.