Embryonic and Neonatal Mouse Cochleae Are Susceptible to Zika Virus Infection

Congenital Zika Syndrome (CZS) is caused by vertical transmission of Zika virus (ZIKV) to the gestating human fetus. A subset of CZS microcephalic infants present with reduced otoacoustic emissions; this test screens for hearing loss originating in the cochlea. This observation leads to the question of whether mammalian cochlear tissues are susceptible to infection by ZIKV during development. To address this question using a mouse model, the sensory cochlea was explanted at proliferative, newly post-mitotic or maturing stages. ZIKV was added for the first 24 h and organs cultured for up to 6 days to allow for cell differentiation. Results showed that ZIKV can robustly infect proliferating sensory progenitors, as well as post-mitotic hair cells and supporting cells. Virus neutralization using ZIKV-117 antibody blocked cochlear infection. AXL is a cell surface molecule known to enhance the attachment of flavivirus to host cells. While Axl mRNA is widely expressed in embryonic cochlear tissues susceptible to ZIKV infection, it is selectively downregulated in the post-mitotic sensory organ by E15.5, even though these cells remain infectible. These findings may offer insights into which target cells could potentially contribute to hearing loss resulting from fetal exposure to ZIKV in humans.

A New Pathogenic Variant in POU3F4 Causing Deafness Due to an Incomplete Partition of the Cochlea Paved the Way for Innovative Surgery

Incomplete partition type III (IP-III) is a relatively rare inner ear malformation that has been associated with a POU3F4 gene mutation. The IP-III anomaly is mainly characterized by incomplete separation of the modiolus of the cochlea from the internal auditory canal. We describe a 71-year-old woman with profound sensorineural hearing loss diagnosed with an IP-III of the cochlea that underwent cochlear implantation. Via targeted sequencing with a non-syndromic gene panel, we identified a heterozygous c.934G > C p. (Ala31Pro) pathogenic variant in the POU3F4 gene that has not been reported previously. IP-III of the cochlea is challenging for cochlear implant surgery for two main reasons: liquor cerebrospinalis gusher and electrode misplacement. Surgically, it may be better to opt for a shorter array because it is less likely for misplacement with the electrode in a false route. Secondly, the surgeon has to consider the insertion angles of cochlear access very strictly to avoid misplacement along the inner ear canal. Genetic results in well describes genotype-phenotype correlations are a strong clinical tool and as in this case guided surgical planning and robotic execution.

Single nucleotide polymorphisms in DNA glycosylases: From function to disease

Oxidative stress is associated with a growing number of diseases that span from cancer to neurodegeneration. Most oxidatively induced DNA base lesions are repaired by the base excision repair (BER) pathway which involves the action of various DNA glycosylases. There are numerous genome wide studies attempting to associate single-nucleotide polymorphisms (SNPs) with predispositions to various types of disease; often, these common variants do not have significant alterations in their biochemical function and do not exhibit a convincing phenotype. Nevertheless several lines of evidence indicate that SNPs in DNA repair genes may modulate DNA repair capacity and contribute to risk of disease. This overview provides a convincing picture that SNPs of DNA glycosylases that remove oxidatively generated DNA lesions are susceptibility factors for a wide disease spectrum that includes besides cancer (particularly lung, breast and gastrointestinal tract), cochlear/ocular disorders, myocardial infarction and neurodegenerative disorders which can be all grouped under the umbrella of oxidative stress-related pathologies.

Variation in genes related to cochlear biology is strongly associated with adult-onset deafness in border collies

Domestic dogs can suffer from hearing losses that can have profound impacts on working ability and quality of life. We have identified a type of adult-onset hearing loss in Border Collies that appears to have a genetic cause, with an earlier age of onset (3–5 years) than typically expected for aging dogs (8–10 years). Studying this complex trait within pure breeds of dog may greatly increase our ability to identify genomic regions associated with risk of hearing impairment in dogs and in humans. We performed a genome-wide association study (GWAS) to detect loci underlying adult-onset deafness in a sample of 20 affected and 28 control Border Collies. We identified a region on canine chromosome 6 that demonstrates extended support for association surrounding SNP Chr6.25819273 (p-value = 1.09×10⁻¹³). To further localize disease-associated variants, targeted next-generation sequencing (NGS) of one affected and two unaffected dogs was performed. Through additional validation based on targeted genotyping of additional cases (n = 23 total) and controls (n = 101 total) and an independent replication cohort of 16 cases and 265 controls, we identified variants in USP31 that were strongly associated with adult-onset deafness in Border Collies, suggesting the involvement of the NF-κB pathway. We found additional support for involvement of RBBP6, which is critical for cochlear development. These findings highlight the utility of GWAS–guided fine-mapping of genetic loci using targeted NGS to study hereditary disorders of the domestic dog that may be analogous to human disorders.

The domestic dog offers a unique opportunity to study complex disorders similar to those seen in humans, but within the context of the much simpler genetic backgrounds of pure breeds, which represent closed populations. We performed a whole-genome search for genetic risk factors of adult-onset deafness in the Border Collie, a breed of herding dog that relies on acute hearing to perceive and respond to commands while working. Adult-onset deafness in Border Collies typically begins in early adulthood and is similar to age-related hearing loss in humans. This earlier onset has particular impact on the utility of working Border Collies and the livelihoods of their owners, and it appears to have a genetic cause. We identified three genetic variants that were strongly associated with adult-onset deafness in a sample of 405 Border Collies. These variants are located in two genes that have previously been linked to deafness, one involved in ear development and another that appears to mitigate tissue damage in the ear. These results provide new insight regarding genetic risk factors for age-related hearing loss in both dogs and humans.

Cochlear gene therapy

PURPOSE OF REVIEW

This review highlights recent advances in cochlear gene therapy over the past several years. Cochlear gene therapy has undergone tremendous advances over the past decade. Beginning with some groundbreaking work in 2005 documenting hair cell regeneration using virally mediated delivery of the mouse atonal 1 gene, gene therapy is now being explored as a possible treatment for a variety of causes of hearing loss.

RECENT FINDINGS

Recent advances in cochlear gene therapy include improved methods of gene delivery with a better delineation of viral vectors that are suitable for this purpose, additional improvements in hair cell regeneration, and directed research toward autoimmune hearing loss, ototoxicity, spiral ganglion survival, and genetic forms of hearing loss.

SUMMARY

If successful, cochlear gene therapy will dramatically alter our ability to treat a variety of forms of acquired and genetic hearing loss.

Selective cochlear degeneration in mice lacking the F-box protein, Fbx2, a glycoprotein-specific ubiquitin ligase subunit

Little is known about the role of protein quality control in the inner ear. We now report selective cochlear degeneration in mice deficient in Fbx2, a ubiquitin ligase F-box protein with specificity for high-mannose glycoproteins (Yoshida et al., 2002). Originally described as a brain-enriched protein (Erhardt et al., 1998), Fbx2 is also highly expressed in the organ of Corti, in which it has been called organ of Corti protein 1 (Thalmann et al., 1997). Mice with targeted deletion of Fbxo2 develop age-related hearing loss beginning at 2 months. Cellular degeneration begins in the epithelial support cells of the organ of Corti and is accompanied by changes in cellular membrane integrity and early increases in connexin 26, a cochlear gap junction protein previously shown to interact with Fbx2 (Henzl et al., 2004). Progressive degeneration includes hair cells and the spiral ganglion, but the brain itself is spared despite widespread CNS expression of Fbx2. Cochlear Fbx2 binds Skp1, the common binding partner for F-box proteins, and is an unusually abundant inner ear protein. Whereas cochlear Skp1 levels fall in parallel with the loss of Fbx2, other components of the canonical SCF (Skp1, Cullin1, F-box, Rbx1) ubiquitin ligase complex remain unchanged and show little if any complex formation with Fbx2/Skp1, suggesting that cochlear Fbx2 and Skp1 form a novel, heterodimeric complex. Our findings demonstrate that components of protein quality control are essential for inner ear homeostasis and implicate Fbx2 and Skp1 as potential genetic modifiers in age-related hearing loss.

Vertical corneal striae in families with autosomal dominant hearing loss: DFNA9/COCH

PURPOSE

Investigation of a possible association between vertical corneal striae and mutations in the COCH gene, observed in four DFNA9 families with autosomal dominant hearing loss and vestibular dysfunction.

DESIGN

Prospective case series.

METHODS

Ophthalmologic examinations with photography of the cornea after instillation of fluorescein were performed in 98 family members with 61 mutation carriers of four DFNA9 families at the Radboud University Nijmegen Medical Centre. Families 1 and 2 harbor the Pro51Ser mutation, and families 3 and 4 harbor the Gly88Glu and the Gly87Trp mutation, respectively. Statistical analysis was performed to find an association between the vertical corneal striae and the COCH mutation for each family and to test whether the four families were different in this respect.

RESULTS

The vertical corneal striae were exclusively visible after instillation of fluorescein. They caused minor problems, as dry eye symptoms, and were not present in the general Dutch ophthalmologic population. The striae were present from an age of 47 years in 32 individuals, of whom 27 individuals had a COCH mutation. Statistical analysis on the striae and the COCH mutations showed a significant association in families 1, 2, and 3 (P = .0006), but not in family 4 (P = .63).

CONCLUSIONS

Data analysis demonstrated a significant association between vertical corneal striae and the Pro51Ser and Gly88Glu mutations in the COCH gene in DFNA9 families 1, 2, and 3 with cochleovestibular dysfunction. Our findings suggest that the vertical corneal striae and cochleovestibular dysfunction may be caused by the same COCH mutations.

How to prevent a stapes gusher

A stapes gusher is the result of a congenital inner ear anomaly showing at tone audiometry a conductive or mixed hearing loss. The conductive part of the hearing loss could lead to the thought to explore the middle ear. The congenital origin should lead to a high resolution. CT-scanning to evaluate a widening of the internal acoustic canal. Repeated audiometry could show especially a large conductive impairment in the lowest frequencies with a closure of the airbone gap at 2 khz and a high sensorineural high frequency loss at 4 and 8 khz. Contralateral stapedial reflexes may be present. Since the x-recessive mixed deafness syndrome (DFN3) frequently involves males with an early childhood hearing impairment, clinical suspicion should be high. When stapes surgery is considered a precise medical history is essential regarding on the start of the hearing impairment. A continuous suspicion will guide to the audiological, radiological and molecular genetic clues to trace the correct diagnosis before embarking on stapes surgery.

Defective carbohydrate metabolism in mice homozygous for the tubby mutation

Tub is a member of a small gene family, the tubby-like proteins (TULPs), with predominant expression in neurons. Mice carrying a mutation in Tub develop retinal and cochlear degeneration as well as late-onset obesity with insulin resistance. During behavioral and metabolic testing, we found that homozygous C57BL/6J-Tub(tub) mice have a lower respiratory quotient than C57BL/6J controls before the onset of obesity, indicating that tubby homozygotes fail to activate carbohydrate metabolism and instead rely on fat metabolism for energy needs. In concordance with this, tubby mice show higher excretion of ketone bodies and accumulation of glycogen in the liver. Quantitation of liver mRNA levels shows that, during the transition from light to dark period, tubby mice fail to induce glucose-6-phosphate dehydrogenase (G6pdh), the rate-limiting enzyme in the pentose phosphate pathway that normally supplies NADPH for de novo fatty acid synthesis and glutathione reduction. Reduced G6PDH protein levels and enzymatic activity in tubby mice lead accordingly to lower levels of NADPH and reduced glutathione (GSH), respectively. mRNA levels for the lipolytic enzymes acetyl-CoA synthetase and carnitine palmitoyltransferase are increased during the dark cycle and decreased during the light period, and several citric acid cycle genes are dysregulated in tubby mice. Examination of hypothalamic gene expression showed high levels of preproorexin mRNA leading to accumulation of orexin peptide in the lateral hypothalamus. We hypothesize that abnormal hypothalamic orexin expression leads to changes in liver carbohydrate metabolism and may contribute to the moderate obesity observed in tubby mice.

Vestibular deterioration precedes hearing deterioration in the P51S COCH mutation (DFNA9): an analysis in 74 mutation carriers

OBJECTIVES

To analyze cochleovestibular impairment features in P51S COCH mutation carriers (n = 22) in a new, large Dutch family and to compare the results to those obtained in previously identified similar mutation carriers (n = 52). To evaluate age-related features between progressive hearing and vestibular impairment of all mutation carriers (n = 74).

STUDY DESIGN

Family study.

METHODS

Regression analysis was performed in relation to age to outline the development of hearing thresholds, speech recognition scores, and vestibulo-ocular reflex time constant as the key vestibular response parameter.

RESULTS

Pure tone thresholds, phoneme recognition scores, and vestibular responses of the mutation carriers in the new family were essentially similar to those previously established in all other mutation carriers. Hearing started to deteriorate in all mutation carriers from 43 years of age onwards, whereas deterioration of vestibular function started from age 34.

CONCLUSION

Vestibular impairment starts earlier, progresses more rapidly, and, eventually, is more complete than hearing impairment in P51S COCH mutation carriers.

p27Kip1 deficiency causes organ of Corti pathology and hearing loss

p27(Kip1) (p27) has been shown to inhibit several cyclin-dependent kinase molecules and to play a central role in regulating entry into the cell cycle. Once hair cells in the cochlea are formed, p27 is expressed in non-sensory cells of the organ of Corti and prevents their re-entry into the cell cycle. In one line of p27 deficient mice (p27(-/-)), cell division in the organ of Corti continues past its normal embryonic time, leading to continual production of cells in the organ of Corti. Here we report on the structure and function of the inner ear in another line of p27 deficient mice originating from the Memorial Sloan-Kettering Cancer Center. The deficiency in p27 expression of these mice is incomplete, as they retain expression of amino acids 52-197. We determined that mice homozygote for this mutation had severe hearing loss and their organ of Corti exhibited an increase in the number of inner and outer hair cells. There also was a marked increase in the number of supporting cells, with severe pathologies in pillar cells. These data show similarities between this p27(Kip1) mutation and another, previously reported null allele of this gene, and suggest that reducing the inhibition on the cell cycle in the organ of Corti leads to pathology and dysfunction. Manipulations to regulate the time and place of p27 inhibition will be necessary for inducing functionally useful hair cell regeneration.

Audiometric, Vestibular, and Genetic Aspects of a DFNA9 Family with a G88E COCH Mutation

OBJECTIVES

To perform genetic analysis and to analyze cochleovestibular impairment features in a newly identified Dutch family with nonsyndromic autosomal dominant hearing impairment (DFNA9).

STUDY DESIGN

Genetic analysis was performed using microsatellite markers and single nucleotide polymorphisms. Audiometric data were collected and analyzed longitudinally. Results were compared with those obtained in previously identified P51S COCH mutation carriers (n = 74). Special attention was also given to a comparison of age-related features such as progressive hearing loss and vestibular impairment.

SETTING

Tertiary referral center.

PATIENTS

G88E COCH mutation carriers from a Dutch family.

MAIN OUTCOME MEASURES

The study of clinical features of a DFNA9 family carrying a G88E COCH mutation and to compare this to the symptoms of those carrying a P51S/COCH mutation.

RESULTS

Pure-tone thresholds, phoneme recognition scores, and vestibular responses of the G88E mutation carriers were essentially similar to those previously established in the P51S mutation carriers. Hearing started to deteriorate in G88E mutation carriers from age 46 to 49 years and onward, whereas deterioration of vestibular function started from approximately age 46 years. In the P51S mutation carriers, vestibular impairment started earlier, at approximately age 34 years. However, the difference in age of onset with the G88E mutation carriers was not significant. Remarkably, the proportion of patients who developed complete vestibular areflexia within the age range of 40 to 56 years was significantly lower for the G88E mutation carriers than for the P51S mutation carriers.

CONCLUSION

Apart from a significantly lower frequency of vestibular areflexia between the ages of 40 and 56 years, there are no phenotypic differences between carriers of the G88E and P51S mutations in the COCH gene.

Breed-dependent susceptibility to acute sound exposure in young chickens

Commercially available chickens fall into two categories: egg layers and broilers. Durham et al. (Hear. Res. 166 (2002) 82-95) showed that despite similar noisy living environments, cochleae of most adult broilers show extensive damage, while cochleae of adult egg layers are largely normal. This finding suggests that egg layers and broilers differ in their susceptibility to noise damage. Here, we evaluate breed differences in susceptibility to acoustic trauma. Young egg layers and broilers (10-17 weeks) were exposed to a 1500Hz pure tone (120dB SPL; 24h) and killed 24 or 72h later. Cochleae were prepared for scanning electron microscopy and photomicrographs of the cochlear surface were used to determine location and severity of damage. Cochleae were grouped based upon damage severity (moderate or severe). While location and area of damage were similar between both breeds at each recovery time, cochlear damage at 72h was more extensive than at 24h. We found no quantitative breed differences within either damage category or recovery time. However, more egg layers (25/27) than broilers (16/32) displayed severely damaged cochleae. Our findings conflict with those reported by Durham et al. (2002). Our results identify a breed-dependent difference in susceptibility to acute sound exposure, with young egg layers displaying increased sensitivity.

Temporal bone histopathology in dominantly inherited audiovestibular syndrome

OBJECTIVE

To describe the clinical and pathologic features of a new dominantly inherited audiovestibular syndrome.

METHODS

History, examination, and audiometric testing in the proband, brother, and son; quantitative rotational testing in the proband and son; histopathology of the cochlea and vestibular labyrinth in the proband; sequencing candidate genes COCH and MYO7A in the brother and son.

RESULTS

Affected family members developed slowly progressive hearing loss beginning in their late 30s and progressive imbalance in their early 70s. Three of four affected had brief (minutes) episodes of vertigo typically occurring a few times per year. Auditory and vestibular function testing documented a slowly progressive loss of auditory and vestibular function. Postmortem examination showed a loss of hair cells in the cochlea and vestibular receptor organs. There were no cellular infiltrates or acidophilic deposits. No mutations were found in the COCH or MYO7A genes.

CONCLUSIONS

This dominantly inherited audiovestibular syndrome results in a selective loss of hair cells in the auditory and vestibular end organs. Finding the causative gene could have important implications for understanding the pathophysiology of presbycusis and dysequilibrium of aging.

Mouse homolog of KIAA0143 protein: hearing deficit induces specific changes of expression in auditory brainstem neurons

Hearing deficit induced by mechanical cochlear damage, intense noise or ototoxic drugs produces a variety of structural and functional changes in the inner ear and the auditory brainstem. In the present study, we identified a novel gene that has activity dependent plasticity in the superior olivary complex by using suppression subtractive hybridization. We cloned a gene that encodes mouse homolog of KIAA0143 protein, one derived from a series of unidentified human genes. This gene termed mKIAA0143 shows differential expression of mRNA in the lateral superior olive between mice with hearing deficit and those with normal hearing ability. The mRNA thus obtained encodes a unique membrane-bound protein that consists of 819 amino acids. The gene locus was mapped using genomic DNA databases to the mouse chromosome 15D1. Green fluorescent protein-tagged mKIAA0143 was expressed in COS-1 cells. It was amply seen in the cellular plasma membrane.

Hearing loss following Gata3 haploinsufficiency is caused by cochlear disorder

Patients with HDR syndrome suffer from hypoparathyroidism, deafness, and renal dysplasia due to a heterozygous deletion of the transcription factor GATA3. Since GATA3 is prominently expressed in both the inner ear and different parts of the auditory nervous system, it is not clear whether the deafness in HDR patients is caused by peripheral and/or central deficits. Therefore, we have created and examined heterozygous Gata3 knockout mice. Auditory brainstem response (ABR) thresholds of alert heterozygous Gata3 mice, analyzed from 1 to 19 months of age, showed a hearing loss of 30 dB compared to wild-type littermates. Neither physiological nor morphological abnormalities were found in the brainstem, cerebral cortex, the outer or the middle ear. In contrast, cochleae of heterozygous Gata3 mice showed significant progressive morphological degeneration starting with the outer hair cells (OHCs) at the apex and ultimately affecting all hair cells and supporting cells in the entire cochlea. Together, these findings indicate that hearing loss following Gata3 haploinsufficiency is peripheral in origin and that this defect is detectable from early postnatal development and maintains through adulthood.

Myo15 function is distinct from Myo6, Myo7a and pirouette genes in development of cochlear stereocilia

The unconventional myosin genes Myo15, Myo6 and Myo7a are essential for hearing in both humans and mice. Despite the expression of each gene in multiple organs, mutations result in identifiable phenotypes only in auditory or ocular sensory organs. The pirouette (pi) mouse also exhibits deafness and an inner ear pathology resembling that of Myo15 mutant mice and thus may be functionally related to Myo15. In order to investigate possible interactions between Myo15 and Myo6, Myo7a, and the gene affected in pirouette, we crossed Myo15(sh2/sh2) mice to the three other mutant mouse strains. Hearing in doubly heterozygous mice was similar to age-matched singly heterozygous animals, indicating that partial deficiency for both Myo15 and one of these other deafness genes does not reduce hearing. Viable double mutants were obtained from each cross, indicating that potential overlapping functions between these genes in other organs are not essential for viability. All critical cell types of the cochlear sensory epithelium were present in double mutant mice and cochlear stereocilia exhibited a superimposition of single mutant phenotypes. These data suggest that the function of Myo15 is distinct from that of Myo6, Myo7a or pi in development and/or maintenance of stereocilia.

Cross-sectional analysis of hearing threshold in relation to age in a large family with cochleovestibular impairment thoroughly genotyped for DFNA9/COCH

Hearing threshold was analyzed for each frequency in relation to age in 88 members of a large Dutch family with cochleovestibular impairment caused by a P51S mutation in the COCH gene within the DFNA9 locus (chromosome 14q12-13). The participants in this study were 34 mutation carriers and 54 relatives without the mutation (control subjects). A sigmoidal dose-response curve with a variable slope was used to fit the mutation carriers' threshold-on-age data. Progression started at about 40 years of age and only lasted for some 20 to 25 years; the associated average progression was 2.9 dB/y for all frequencies. However, some hearing impairment was already present before, predominantly at the high frequencies. The mean thresholds in the young mutation carriers (< 33 years of age) were significantly higher (by 4 to 13 dB) than those in age-matched controls at 2 to 8 kHz. Presumably, mutation carriers have a congenital, stable offset threshold (10 to 29 dB) at these frequencies, and develop progression later in life.

[Protective and rescue effects of transgenic bFGF/GFP expression mediated by cationic liposome on gentamicin-induced guinea pig cochlear toxicity]

OBJECTIVE

To observe the expression of cationic stearylamine (SA) liposome mediated basic fibroblast growth factor/green fluorescence protein (bFGF/GFP) gene in the cochlea of guinea pig, and evaluate the protection and rescue action of bFGF against the damage caused by gentamicin.

METHODS

Thirty-six guinea pigs were divided into 3 experimental groups. Prevention group with inoculation of SA-bFGF/GFP complexes through the round window of right ear and injection of gentamicin 150 mg.kg(-1).d(-1) one day after for 8 days and rescue group with injection of gentamicin for 8 days and infusion of SA-bFGF/GFP complexes in the same way on the ninth day, and control group with only injection of gentamicin for 8 days. Auditory brainstem responses (ABR) was measured prior to and after the administration and before the animals were killed respectively. The animals were killed after the experiment and ABR test, and specimens and slices of chochleae were made to examine the absence of outer and inner hair cells. The expression of GFP in cochlea shown by green fluorescence was observed with fluorescent microscopy.

RESULTS

Fluorescent microscopy showed green fluorescence in the cochleae of guinea pigs in prevention and rescue groups. There was no significant difference in ABR threshold between the left and right ears of animals in each group before and after experiment, among both ears of animals in the 3 groups before experiment, and between prevention and rescue group groups before killing (all P > 0.05). However, the ABR thresholds in prevention group and rescue group were significantly lower than that in control group before the animals were killed (P < 0.01 and P < 0.05). The average number of lost outer hair cells was 5 106 +/- 299 cells and 5 605 +/- 109 cells in prevention group and rescue group respectively, without a significant difference between them (P > 0.05). The average amount of missing inner hair cells was 301 +/- 64 cells and 487 +/- 92 cells in prevention group and rescue group respectively, without a significant difference between them (P > 0.05), and significantly lower than that in control group (1 062 +/- 67, P < 0.01 and P < 0.05). The average amount of missing outer hair cells in control group was 6 248 +/- 119 cells, significantly higher than that in prevention group (5 106 +/- 299, P < 0.01) and that in rescue group (5 605 +/- 109, P < 0.05).

CONCLUSION

SA-liposome mediated bFGF/GFP gene, which was perfused in one ear, can be expressed highly in both cochleae of the guinea pig, and may protect and rescue cochlea against gentamicin ototoxicity.

Mpv37 mouse strain--a model for the relationship between the kidney and the inner ear

Using the Mpv17-negative mouse strain, which developed inner ear and kidney dysfunction, we confirm a strong relationship between the kidney and the inner ear. Both organs have specialized epithelia involved in active ion transport, which are separated from the vessels by a basement membrane of similar composition. Our recent results indicate that the glomerular and the stria vascularis basement membrane are simultaneously affected in early stages. Concomitant deposits of IgG during the progressive development of the disease support the idea of a shared antigen. Understanding the pattern of the development of the degeneration will provide a basis towards understanding the essential role of the Mpv17 protein in the structures of both organs and may provide a basis for future therapeutic intervention.

Electrophysiological characteristics of inferior colliculus neurons in mutant mice with hereditary absence of cochlear outer hair cells

One strain of homozygous Kit(W-v) mice (formerly known as W(v)/W(v)) lack 98% of the cochlear outer hair cells (OHCs) from birth. Inner hair cells (IHCs) and supporting cells develop normally. Thus, this strain is an attractive model to study the effect of complete OHC absence on central frequency representation. Frequency threshold curves were recorded along the tonotopic axis of inferior colliculus (IC) in mutant and control mice of the genetic background strain (C57BL/6J) and a different outbred strain (NMRI/wild mouse hybrids) known to be free of any cochlear pathology. The average threshold level of neurons in the mutants was 100 dB sound pressure level, 60 dB higher than in C57BL/6J and NMRI mice. Their tuning curves lacked the sharply tuned tip. In the C57BL/6J mice, although younger than four months, abnormal tuning curves were found for about 30% of the neurons, especially in the high frequency range. No abnormal tuning curves were found in the NMRI mice. The bandwidth of the tuning curves, measured at 10 dB above threshold, was on average 1.27 octaves in mutants, 0.62 octaves in C57BL/6J mice, and 0.34 octaves in NMRI mice. The range for the high cut-off frequency of the tuning curves at 10 dB above threshold was 6.4-61.1 kHz in the NMRI and 7-59.5 kHz in C57BL/6J. In the mutants, the range was limited to 11.1-41.7 kHz. The tonotopic gradient based on the cut-off frequency was less steep in the IC of the mutants than in both control groups.

Hereditary cochleovestibular dysfunction due to a COCH gene mutation (DFNA9): a follow-up study of a family

Cochleovestibular impairment was evaluated, in relation to age, in a longitudinal follow-up study on a Dutch family with a DFNA9 trait caused by a Pro51Ser mutation in the COCH gene on chromosome 14q12-q13. Fourteen cases were genotyped. The onset age of progressive impairment reported by the mutation carriers was between age 35 and 45 years. Pure-tone thresholds deteriorated by about 2-7 dB per year (mean 3.8 dB per year) in a variable, often asymmetrical, fashion. One mutation carrier developed recurrent episodes of vertigo accompanied by nausea and vomiting, resembling Ménière's disease. Two others developed special susceptibility for motion sickness and appeared to have a hyperactive vestibulo-ocular reflex. More advanced stages of vestibular impairment, i.e. vestibular hyporeflexia and complete vestibular areflexia, were eventually found in a number of cases. DFNA9/COCH should be considered as a possible cause in patients developing combined progressive cochlear and vestibular impairment, or suspected hereditary Ménière-like disease, from around middle age.

Delayed inner ear maturation and neuronal loss in postnatal Igf-1-deficient mice

Insulin-like growth factor-1 (IGF-1) has been shown to play a key role during embryonic and postnatal development of the CNS, but its effect on a sensory organ has not been studied in vivo. Therefore, we examined cochlear growth, differentiation, and maturation in Igf-1 gene knock-out mice at postnatal days 5 (P5), P8, and P20 by using stereological methods and immunohistochemistry. Mutant mice showed reduction in size of the cochlea and cochlear ganglion. An immature tectorial membrane and a significant decrease in the number and size of auditory neurons were also evident at P20. IGF-1-deficient cochlear neurons showed increased caspase-3-mediated apoptosis, along with aberrant expression of the early neural markers nestin and Islet 1/2. Cochlear ganglion and fibers innervating the sensory cells of the organ of Corti presented decreased levels of neurofilament and myelin P(0) in P20 mouse mutants. In addition, an abnormal synaptophysin expression in the somata of cochlear ganglion neurons and sensory hair cells suggested the persistence of an immature pattern of synapses distribution in the organ of Corti of these animals. These results demonstrate that lack of IGF-1 in mice severely affects postnatal survival, differentiation, and maturation of the cochlear ganglion cells and causes abnormal innervation of the sensory cells in the organ of Corti.

Ultrastructural and physiological defects in the cochlea of the Mpv17 mouse strain

Ultrastructural investigations were performed in young (approximately 2 months) and old (7 months) Mpv17-negative and wild-type mice. The onset, the severity and the pattern of the degeneration significantly differed between both mice strains. In the wild-type mouse strain the degenerative changes of the cochlear structures were similar to the aging pattern described for other species. In contrast, the Mpv17 mutants showed degenerative changes of the cochlear structures already at the age of 2 months. The degenerative changes were patchy arranged throughout the entire length of the cochlea and involved the organ of Corti as well as the stria vascularis epithelia with alterations of the basement membrane of the capillaries. The severe sensorineural hearing loss and degenerative changes of the cochlear structures indicate that cochlear structures, especially the outer hair cells and the intermediate cells of the stria vascularis, are vulnerable to the missing Mpv17 gene product.

Hearing impairment in 18q deletion syndrome

The 18q-syndrome is associated with hearing impairment in 50-80 per cent of cases. The hearing loss may be sensorineural or conductive. A high proportion of cases are associated with narrow or stenosed external auditory canals. This may be a useful clinical pointer to the syndrome. Two cases with impaired hearing are presented in this paper including one case with complex external ear and middle ear malformations. The clinical and audiological features in each case are described.

Progressive cochleovestibular impairment caused by a point mutation in the COCH gene at DFNA9

OBJECTIVES

Analysis of phenotype-genotype correlation.

STUDY DESIGN

Family study.

METHODS

Auditory and vestibulo-ocular functions were examined in a Dutch family with autosomal dominantly inherited sensorineural hearing impairment caused by a 208C > T mutation in the COCH gene, located in chromosome 14q12-q13 (DFNA9). Linear regression analysis of individual longitudinal hearing threshold data (n = 11) on age was performed.

RESULTS

Fifteen of the 16 genetically affected persons could be evaluated. They all developed hearing and vestibular impairment symptoms--and in many cases also cardiovascular disease--in the fourth to fifth decade. At the low frequencies (0.25-2 kHz), hearing loss started at the age of about 40 years and showed an average annual progression of approximately 3 dB, finally resulting in profound hearing losses. In two exceptional cases, annual progression attained levels of up to 24 dB. At the high frequencies (4-8 kHz), the average threshold increased from about 50 dB at the age of 35 years to about 120 dB at the age of 75 years (which amounts to 1.8 dB annual threshold increase). All affected individuals tested showed normal ocular motor functions. The patients older than 46 years generally showed absence of the vestibulo-ocular reflex, but their cervico-ocular reflex was enhanced compared with normal subjects, whereas those aged 40 to 46 years showed either severe vestibular hyporeflexia or unilateral caloric areflexia.

CONCLUSION

These findings suggest a gradual development of cochleovestibular impairment caused by the new mutation found.

Chromosomal localization of a gene responsible for vestibulocochlear defects of BUS/Idr mice: identification as an allele of waltzer

Mice of the bustling mutant strain BUS/Idr have vestibulocochlear defects. bus/bus homozygotes, but not heterozygotes, are hyperactive and display an abnormal behavior such as circling, head bobbing and head tilting. To characterize BUS mice further, the auditory brain-stem response of the mutant was examined. In +/bus heterozygotes as well as control animals, the auditory brain-stem response was developmentally first recorded as early as 11 days of age and heterozygous and normal adults showed typical auditory brain-stem responses with five peaks in a threshold of 40-45 dB SPL. In contrast, bus/bus homozygotes showed no auditory brain-stem response at any age in response to stimuli up to 130 dB SPL, indicating that they are deaf throughout life. Linkage analysis revealed that the responsible gene, originally designated as bus, maps on chromosome 10, 1.09+/-0.9 cM distal to D10Mit127 and D10Mit59, and 0.72+/-0.51 cM proximal to three markers, D10Mit48, D10Mit112 and D10Mit258, at a site indistinguishable from that of the Albany waltzer, v(A/b). The results of allelism tests between BUS and Albany waltzer indicated that bus is allelic with v(Alb). From these data, we propose here that the bus mutation could represent another allele of waltzer, now designated v(bus).

On the relationships of high-frequency hearing loss and cochlear pathology to the acoustic startle response (ASR) and prepulse inhibition of the ASR in the BXD recombinant inbred series

The measurement of the acoustic startle response (ASR) and prepulse inhibition (PPI) of the ASR in many inbred strains of mice, including C57BL/6 and DBA/2, may be complicated by age-related high-frequency hearing loss (HFHL) and the associated cochlear pathology. Willott and Erway (1998) have recently reported on the age-related changes of the acoustic brain response in the BXD recombinant inbred (RI) series. Based on these data, the RI series was divided into three groups: juvenile-, intermediate-, and adult-onset HFHL. Each of these groups was tested using paradigms which varied the frequency or intensity of the auditory startle and prepulse stimuli. The results obtained in adolescent mice (6-8 weeks) demonstrate that ASR performance is independent of HFHL; there was no group-dependent decline in the ASR amplitudes for high-frequency stimuli. The expected effect of HFHL on PPI is to increase the salience of the still-audible tones. In response to a white-noise prepulse stimulus, the PPI in the juvenile-onset group (which shows marked HFHL at 6 weeks) was similar to that in the adult-onset group. However, when the prepulse stimulus was a pure tone, the juvenile group showed a decrease in salience across all frequencies tested (5-20 kHz). The data point out the need for carefully constructing auditory tasks in the BXD RI series, to avoid the confounding effects of HFHL.

Glycine immunoreactivity and receptor binding in the cochlear nucleus of C57BL/6J and CBA/CaJ mice: effects of cochlear impairment and aging

Glycinergic neurons in the cochlear nucleus (CN) of C57BL/6J (C57) and CBA/CaJ (CBA) mice were studied by using immunocytochemical and receptor-binding techniques. Adult C57 mice exhibit progressive cochlear pathology as they age, whereas aging CBA mice retain good hearing. In the CN of old C57 mice (18 months) with severe hearing loss, the number of glycine-immunoreactive neurons decreased significantly. The number (Bmax) of strychnine-sensitive glycine receptors (GlyR) decreased significantly in the dorsal CN of old C57 mice. Significant effects were not observed in the CN of middle-aged C57 mice (with less-severe hearing loss) or in very old CBA mice (which do not exhibit severe hearing loss). The data suggest that the combination of severe hearing loss and old age results in deficits in one or more inhibitory glycinergic circuits in the CN.

Pathology of the cochlea following a spontaneous mutation in DBA/2 mice

The DBA/2 strain of mice usually presents with noise-induced epileptic seizures and hearing disorders. After a spontaneous mutation a strain with early hearing loss and circling behaviour was produced. This strain presents with clinical symptoms found in diseases connected to inner ear disorders. These animals do not suffer from periodical disorders, however, but have functional disturbances continuously and can therefore serve as an animal model for diseases originating from both parts of the inner ear. The genetic inheritance appears to be autosomal recessive. Offspring showed circling behaviour and severe pathology in the vestibular part of the inner ear. In the present study pathology of the cochlear part of the inner ear was visualized using conventional microscopical techniques. The content of actin and fodrin was labelled immunohistochemically, and hearing was assessed with auditory brainstem recordings. After 1 month the animals showed deterioration of the cochlear part of the inner ear. At 6 months no organ of Corti remained and the animals were deaf. Transmission and scanning electron microscopy revealed severe apical hair cell changes. The content of alpha-actinin and fodrin in the DBA/2 mouse was already fainter than that in age-matched CBA control mice at the age of 1 month. Labelling of antibodies against fodrin increased in the supporting cells of the older animals, probably owing to the replacement of hair cells.

Association of mitochondrial DNA deletions and cochlear pathology: a molecular biologic tool

The purpose of these experiments was to develop a method of isolation, amplification, and identification of cochlear mitochondrial DNA (mtDNA) from minute quantities of tissue. Additionally, studies were designed to detect mtDNA deletions (mtDNA del) from the cochlea that previously have been amplified from other organ systems and tissues. MtDNA del have been associated with many pathologies, including neurological disorders, sensorineural hearing loss, ischemia, cardiomyopathies, and aging. DNA was extracted from rat and human tissues, and polymerase chain reaction was used to amplify mtDNA sequences. A 360 base pair (bp) cytochrome-b gene product and the highly conserved ND1-16S ribosomal ribonucleic acid regions found only in mtDNA were amplified from all tissues. Preliminary studies have identified a 4834 bp mtDNA del in aged rats and a corresponding 4977 bp mtDNA del in aged humans. Additionally, preliminary results in human archival temporal bone studies reveal the presence of the 4977-bp mtDNA deletion in two out of three patients with presbycusis. The deletion was not evident in age-matched control patients without a history of presbycusis. This technique of mtDNA identification makes it possible to investigate specific mtDNA defects from a single cochlea, promoting the study of hereditary hearing loss and presbycusis at a molecular biologic level.

Inherited hearing defects in mice

Mouse mutants with hearing impairment are useful for elucidating the pathological processes underlying auditory system defects, as well as for understanding the normal process of auditory development and sensory transduction. Deaf mouse mutants are also valuable for identifying the responsible genes by positional cloning, and are used to expedite the search for genes involved in human deafness. The distribution of candidate genes for deafness across the mouse genome is presented, together with a summary of the key features of the mutants involved. Genetic defects affecting hearing can be grouped into broad categories according to their pathological features. These categories include middle ear defects, morphogenetic inner ear defects, central auditory system defects, peripheral neural defects, neuroepithelial defects, cochleo-saccular defects, and late onset hearing loss. The biological features and molecular basis of each type of hearing impairment are described. Finally, the effects of mutations in orthologous genes involved in the auditory system in humans and mice are compared.

A new autosomal dominant syndrome of idiopathic progressive vestibulo-cochlear dysfunction with middle-age onset

A kindred is described with progressive autosomal dominant vestibulo-cochlear dysfunction resulting in instability in the dark, head movement dependent oscillopsia and hearing loss. The first symptoms appeared in the 4th decade and progressed to vestibular areflexia, presumably in the 5th decade and to almost total deafness in the 6th-7th decade of life. The history was negative for other neurological, otological or infectious diseases, or the use of neuro-ototoxic drugs. The affected subjects showed remarkable compensation for the loss of vestibular function.

Aberrant frequency tuning and early stereociliary derangement in genetic inner ear disease

We have developed an auditory brainstem response (ABR) based method for frequency selective forward masking tuning curve technique. Normative data from CBA/CBA mice are compared with tuning curves (TCs) from heterozygote dancer mice which in most cases initially had a normal ABR threshold. The sharpest tuning in normal mice was obtained at 10 kHz (Q10 = 7.6) and 12 kHz (Q10 = 10.9). With increasing age the mutants showed a broadening of the TC tip before ABR threshold impairment occurred. Morphological analysis of the cochleae from dancer mutants revealed an early disarray of hair cell stereocilia which became increasingly more severely affected with advancing age. We conclude that the broadening of the TC tip is an early finding in the genetically induced cochlear susceptibility which can be correlated to stereociliary dysfunction.