Histopathology and molecular genetics of hearing loss in the human

Application of New Materials in Auditory Disease Treatment

Auditory diseases are disabling public health problems that afflict a significant number of people worldwide, and they remain largely incurable until now. Driven by continuous innovation in the fields of chemistry, physics, and materials science, novel materials that can be applied to hearing diseases are constantly emerging. In contrast to conventional materials, new materials are easily accessible, inexpensive, non-invasive, with better acoustic therapy effects and weaker immune rejection after implantation. When new materials are used to treat auditory diseases, the wound healing, infection prevention, disease recurrence, hair cell regeneration, functional recovery, and other aspects have been significantly improved. Despite these advances, clinical success has been limited, largely due to issues regarding a lack of effectiveness and safety. With ever-developing scientific research, more novel materials will be facilitated into clinical use in the future.

Temporal bone histopathology in MELAS syndrome

OBJECTIVES

Describe the histopathology of the temporal bones in MELAS (myopathy, encephalopathy, lactic acidosis, and stroke-like episodes) syndrome. The syndrome results from a known point mutation in mitochondrial DNA.

METHODS

Histopathology analysis of a pair of temporal bones from the oldest surviving MELAS syndrome temporal bone donor. Histopathologic findings were correlated with known premortem clinical data.

RESULTS

The inner ears showed severe but incomplete atrophy of the stria vascularis for the length of the cochleae. In contrast, the organ of Corti and inner hair cells appeared intact with some loss of outer hair cells. Other than moderate loss at the basal turn, spiral ganglion cells numbers were normal. The vestibular neuroepithelium was mostly normal with the exception of moderate degeneration of the macula sacculi and partial collapse of the saccular wall on the right. The cerebral cortex had infarct-like lesions with adjacent gliosis.

CONCLUSION

This is an analysis of the oldest patient with MELAS syndrome to date, an addition to only two previously published patients. It supports the notion that hearing loss is a result of dysfunction of the stria vascularis and not loss of hair cells or neurons. Patterns of vestibular pathology are in agreement to in-vivo measurements. These findings support auditory rehabilitation with cochlear implants and may be relevant to hearing loss due to other mitochondrial mutations.

LEVEL OF EVIDENCE

4.

Cochlear histopathology in human genetic hearing loss: State of the science and future prospects

Sensorineural hearing loss (SNHL) is an extraordinarily common disability, affecting 466 million people across the globe. Half of these incidents are attributed to genetic mutations that disrupt the structure and function of the cochlea. The human cochlea's interior cannot be imaged or biopsied without damaging hearing; thus, everything known about the morphologic correlates of hereditary human deafness comes from histopathologic studies conducted in either cadaveric human temporal bone specimens or animal models of genetic deafness. The purpose of the present review is to a) summarize the findings from all published histopathologic studies conducted in human temporal bones with known SNHL-causing genetic mutations, and b) compare the reported phenotypes of human vs. mouse SNHL caused by the same genetic mutation. The fact that human temporal bone histopathologic analysis has been reported for only 22 of the nearly 200 identified deafness-causing genes suggests a great need for alternative and improved techniques for studying human hereditary deafness; in light of this, the present review concludes with a summary of promising future directions, specifically in the fields of high resolution cochlear imaging, intracochlear fluid biopsy, and gene therapy.

Resolving complexity in mitochondrial disease: Towards precision medicine

Mitochondrial diseases, caused by mutations in either the nuclear or mitochondrial genomes (mtDNA), are the most common form of inherited neurometabolic disorders. They are remarkably heterogeneous, both in their clinical presentation and genetic etiology, presenting challenges for diagnosis, clinical management and elucidation of molecular mechanism. The multifaceted nature of these diseases, compounded by the unique characteristics of mitochondrial genetics, cement their space in the field of complex disease. In this review we examine the m.3243A>G variant, one of the most prevalent mitochondrial DNA mutations, using it as an exemplar to demonstrate the challenges presented by these complex disorders. Disease caused by m.3243A>G is one of the most phenotypically diverse of all mitochondrial diseases; we outline known causes of this heterogeneity including mtDNA heteroplasmy, mtDNA copy number and nuclear genetic factors. We consider the impact that this has in the clinic, discussing the personalized management of common manifestations attributed to this pathogenic mtDNA variant, including hearing impairment, diabetes mellitus, myopathy, cardiac disease, stroke-like episodes and gastrointestinal disturbances. Future research into this complex disorder must account for this heterogeneity, benefitting from the use of large patient cohorts to build upon current clinical expertise. Through multi-disciplinary collaboration, the complexities of this mitochondrial disease can be addressed with the variety of diagnostic, prognostic, and treatment approaches that are moulded to best fit the needs of each individual patient.

In Vivo Electrocochleography in Hybrid Cochlear Implant Users Implicates TMPRSS3 in Spiral Ganglion Function

Cochlear implantation, a surgical method to bypass cochlear hair cells and directly stimulate the spiral ganglion, is the standard treatment for severe-to-profound hearing loss. Changes in cochlear implant electrode array design and surgical approach now allow for preservation of acoustic hearing in the implanted ear. Electrocochleography (ECochG) was performed in eight hearing preservation subjects to assess hair cell and neural function and elucidate underlying genetic hearing loss. Three subjects had pathogenic variants in TMPRSS3 and five had pathogenic variants in genes known to affect the cochlear sensory partition. The mechanism by which variants in TMPRSS3 cause genetic hearing loss is unknown. We used a 500-Hz tone burst to record ECochG responses from an intracochlear electrode. Responses consist of a cochlear microphonic (hair cell) and an auditory nerve neurophonic. Cochlear microphonics did not differ between groups. Auditory nerve neurophonics were smaller, on average, in subjects with TMPRSS3 deafness. Results of this proof-of-concept study provide evidence that pathogenic variants in TMPRSS3 may impact function of the spiral ganglion. While ECochG as a clinical and research tool has been around for decades, this study illustrates a new application of ECochG in the study of genetic hearing and deafness in vivo.

Syndromic Hearing Loss: A Brief Review of Common Presentations and Genetics

Congenital hearing loss is one of the most common birth defects worldwide, with around 1 in 500 people experiencing some form of severe hearing loss. While over 400 different syndromes involving hearing loss have been described, it is important to be familiar with a wide range of syndromes involving hearing loss so an early diagnosis can be made and early intervention can be pursued to maximize functional hearing and speech-language development in the setting of verbal communication. This review aims to describe the presentation and genetics for some of the most frequently occurring syndromes involving hearing loss, including neurofibromatosis type 2, branchio-oto-renal syndrome, Treacher Collins syndrome, Stickler syndrome, Waardenburg syndrome, Pendred syndrome, Jervell and Lange-Nielsen syndrome, Usher syndromes, Refsum disease, Alport syndrome, MELAS, and MERRF.

Hearing impairment in MELAS: new prospective in clinical use of microRNA, a systematic review

AIM

To evaluate the feasibility of microRNAs (miR) in clinical use to fill in the gap of current methodology commonly used to test hearing impairment in MELAS patients.

MATERIAL AND METHOD

A literature review was performed using the following keywords, i.e., MELAS, Hearing Loss, Hearing Impairment, Temporal Bone, Otoacustic Emission (OTOAE), Auditory Brain Response (ABR), and microRNA. We reviewed the literature and focused on the aspect of the temporal bone, the results of electrophysiological tests in human clinical studies, and the use of miR for detecting lesions in the cochlea in patients with MELAS.

RESULTS

In patients with MELAS, Spiral Ganglions (SG), stria vascularis (SV), and hair cells are damaged, and these damages affect in different ways various structures of the temporal bone. The function of these cells is typically investigated using OTOAE and ABR, but in patients with MELAS these tests provide inconsistent results, since OTOAE response is absent and ABR is normal. The normal ABR responses are unexpected given the SG loss in the temporal bone. Recent studies in humans and animals have shown that miRs, and in particular miRs 34a, 29b, 76, 96, and 431, can detect damage in the cells of the cochlea with high sensitivity. Studies that focus on the temporal bone aspects have reported that miRs increase is correlated with the death of specific cells of the inner ear. MiR - 9/9* was identified as a biomarker of human brain damage, miRs levels increase might be related to damage in the central auditory pathways and these increased levels could identify the damage with higher sensitivity and several months before than electrophysiological testing.

CONCLUSION

We suggest that due to their accuracy and sensitivity, miRs might help monitor the progression of SNHL in patients with MELAS.

Blood Flow in the Ears of Patients Receiving Cochlear Implants

We measured cochlear blood flow (CBF) in 55 patients who received cochlear implants, using a laser-Doppler probe placed over the site of drilling in the cochlear bony wall. The subjects included 29 patients with congenital deafness of unknown cause, 8 with idiopathic progressive sensorineural hearing loss, 4 with postmeningitic deafness, 3 with Waardenburg's syndrome, 3 with congenital cytomegalovirus infection, and 8 whose deafness had other causes. There was a wide range of CBF values in patients with congenital deafness of unknown cause. In the patients with idiopathic progressive sensorineural hearing loss, the CBF was significantly lower in patients more than 40 years old. Intracochlear calcification following meningitis appears to be associated with a reduced CBF.

Induction of neuronal-like phenotype in human mesenchymal stem cells by overexpression of Neurogenin1 and treatment with neurotrophins

AIM OF THE STUDY

The induced expression of the transcription factors neurogenin1 (Neurog1) or neuronal differentiation 1 (NeuroD1) has previously been shown to initiate neuronal differentiation in embryonic stem cells (ESC). Human bone marrow-derived mesenchymal stem cells (hBMSCs) are ethically non-controversial stem cells. However, they are not pluripotent. In cochlear implantation, regeneration or replacement of lost spiral ganglion neurons may be a measure for the improvement of implant function. Thus, the aim of the study was to investigate whether the expression of Neurog1 or NeuroD1 is sufficient for induction of neuronal differentiation in hBMSCs.

MATERIALS AND METHODS

Human BMSCs were transduced with lentivirus expressing NeuroD1 or Neuorg1. Transduced cells were then treated with small molecules that enhanced neuronal differentiation. Markers of neuronal differentiation were evaluated.

RESULTS

Using quantitative reverse transcription PCR, the up-regulation of transcription factors expressed by developing primary auditory neurons, such as BRN3a (POU4F1) and GATA3, was quantified after induction of Neurog-1 expression. In addition, the expression of the receptor NTRK2 was induced by treatment with its specific ligand BDNF. The induction of expression of the vesicular glutamate transporter 1 was identified on gene and protein level. NeuroD1 seemed not sufficient to induce and maintain neuronal differentiation.

CONCLUSIONS

Induction of neuronal differentiation by overexpression of Neurog1 initiated important steps for the development of glutamatergic neurons such as the spiral ganglion neurons. However, it seems not sufficient to maintain the glutamatergic spiral ganglion neuron-like phenotype.

Evaluation of serum antioxidants in age-related hearing loss

BACKGROUND

Age-related hearing loss (ARHL) has been linked to the shift in the pro-oxidant/antioxidant ratio. Our objectives were to assess serum levels of retinol and zinc among the elderly individuals and to correlate the levels with hearing threshold.

METHODS

Prospective study of apparently healthy individuals aged ≥60 years of age. Participants had complete clinical history, physical examination and pure tone average conducted. Blood samples were collected for determination of serum levels of retinol and zinc. Mann-Whitney U test was used to compare retinol and zinc values. Pearson's correlation test was used to determine the relationship between hearing threshold and serum levels of retinol and zinc.

RESULTS

Among 126 elderly participants with mean age 67 ± 2.7 years; the mean pure tone average for air conduction was 29.3 ± 1.6 dBHL while the mean bone conduction was 36.5 ± 1.8 dBHL. The median values of serum retinol and zinc levels in the elderly participants who had hearing loss in the speech frequencies were 52 and 83.3 μg/L, respectively, while among participants with normal hearing threshold, values were 50 and 89.9 μg/L, respectively (p = 0.59 and 0.99, respectively). For the high frequencies, the median value of serum retinol and zinc levels among the elderly participants with normal hearing threshold was 70.3 and 99.9 μg/L, while among those with hearing loss, it was 46.9 and 83.2 μg/L, respectively (p = 0.000 and 0.005, respectively).

CONCLUSION

Serum retinol and zinc levels were significantly lower among elderly with hearing loss involving the high frequencies. This is added evidence to extant literature on the possible role of antioxidants in the development of ARHL and suggests further study on the effect of antioxidants supplementation in the control of ARHL which is presently controversial and inconclusive.

Audio profiles in mitochondrial deafness m.1555A>G and m.3243A>G show distinct differences

BACKGROUND

Hearing loss is one of the most common symptoms of mitochondrial disorders. However, audiological phenotypes associated with different molecular defects in mtDNA are not yet well characterized.

MATERIAL AND METHODS

A large cohort of 1499 nonconsanguineous patients aged 5-40 years with hearing loss of unknown etiology was screened for mutations in mtDNA. For further analysis, patients harboring m.1555A>G and m.3243A>G were selected. Hearing status of the patients was assessed by pure tone audiometry. Patterns of audiograms (hearing threshold levels at each examined frequency) were statistically compared among the carriers of the m.1555A>G and the m.3243A>G mutations.

RESULTS

We identified 20 patients positive for m.1555A>G mutation and 16 patients positive for m.3243A>G change. The frequency of the above transitions was calculated in our cohort as 1.33% and 1.06%, respectively. Seventeen affected family members carrying the mutations were included into the study. Typical shape of the audiograms in patients with m.1555A>G mutation presented a ski-slope pattern, whereas the audiometric curves among the m.3243A>G individuals had a pantonal shape (a flat curve) with slight downward sloping at the higher frequencies. The differences were statistically significant. The onset of hearing loss was noted earlier among m.1555A>G than m.3243A>G patients (12.5 and 26 years, respectively). Aminoglycoside administration was declared in both groups in 11 and 4 cases respectively, and caused abrupt hearing deterioration in all cases.

CONCLUSIONS

A pattern of audiogram in patients with mitochondrial deafness may suggest a localization of mtDNA mutation. The pathogenesis of the audiometric differences needs further study.

Generation of induced neurons by direct reprogramming in the mammalian cochlea

Primary auditory neurons (ANs) in the mammalian cochlea play a critical role in hearing as they transmit auditory information in the form of electrical signals from mechanosensory cochlear hair cells in the inner ear to the brainstem. Their progressive degeneration is associated with disease conditions, excessive noise exposure and aging. Replacement of ANs, which lack the ability to regenerate spontaneously, would have a significant impact on research and advancement in cochlear implants in addition to the amelioration of hearing impairment. The aim of this study was to induce a neuronal phenotype in endogenous non-neural cells in the cochlea, which is the essential organ of hearing. Overexpression of a neurogenic basic helix-loop-helix transcription factor, Ascl1, in the cochlear non-sensory epithelial cells induced neurons at high efficiency at embryonic, postnatal and juvenile stages. Moreover, induced neurons showed typical properties of neuron morphology, gene expression and electrophysiology. Our data indicate that Ascl1 alone or Ascl1 and NeuroD1 is sufficient to reprogram cochlear non-sensory epithelial cells into functional neurons. Generation of neurons from non-neural cells in the cochlea is an important step for the regeneration of ANs in the mature mammalian cochlea.

Genetics of peripheral vestibular dysfunction: lessons from mutant mouse strains

BACKGROUND

A considerable amount of research has been published about genetic hearing impairment. Fifty to sixty percent of hearing loss is thought to have a genetic cause. Genes may also play a significant role in acquired hearing loss due to aging, noise exposure, or ototoxic medications. Between 1995 and 2012, over 100 causative genes have been identified for syndromic and nonsyndromic forms of hereditary hearing loss. Mouse models have been extremely valuable in facilitating the discovery of hearing loss genes and in understanding inner ear pathology due to genetic mutations or elucidating fundamental mechanisms of inner ear development.

PURPOSE

Whereas much is being learned about hereditary hearing loss and the genetics of cochlear disorders, relatively little is known about the role genes may play in peripheral vestibular impairment. Here we review the literature with regard to genetics of vestibular dysfunction and discuss what we have learned from studies using mutant mouse models and direct measures of peripheral vestibular neural function.

RESULTS

Several genes are considered that when mutated lead to varying degrees of inner ear vestibular dysfunction due to deficits in otoconia, stereocilia, hair cells, or neurons. Behavior often does not reveal the inner ear deficit. Many of the examples presented are also known to cause human disorders.

CONCLUSIONS

Knowledge regarding the roles of particular genes in the operation of the vestibular sensory apparatus is growing, and it is clear that gene products co-expressed in the cochlea and vestibule may play different roles in the respective end organs. The discovery of new genes mediating critical inner ear vestibular function carries the promise of new strategies in diagnosing, treating, and managing patients as well as predicting the course and level of morbidity in human vestibular disease.

An epidemiological study on children with syndromic hearing loss

OBJECTIVES

To study the epidemiological factors in children with syndromic hearing loss.

STUDY DESIGN

Interview based prospective study.

SETTINGS

Govt. ENT Hospital, AYJNIHH, NIMH-SEC, and Schools for the Deaf-in Hyderabad and SecunderabadPatients: Children aged below 14 years, with hearing loss, their parents/guardians.

INTERVENTION(S)

The study revealed type and degree of hearing impairment. In high risk groups genetic counseling was offered.

RESULTS

Epidemiological studies were carried out in 743 children below 14 years with hearing impairment and 138 (18.57%) were found to have syndromic deafness. Majority of the children with hearing loss have an association of ocular abnormality (22.46%, n=31) followed by skeletal anomalies 14.49% (n=20) and dental anomalies (10.86%). We observed 24 cases (3.21%) with genetically well recognized syndromes.

CONCLUSION

Data is generated on epidemiological and etiology of Hearing Impairment. Hearing Impairment is due to both environmental and genetic causes. Environmental factors in 17 (13.77%), genetically inherited 21 cases (15.22%) and the cause is not known in the remaining cases. Such a data is required in order to offer genetic counseling to reduce the genetic burden.

Postlingual hearing loss as a mitochondrial 3243A>G mutation phenotype

Background

The prevalence of isolated hearing loss (HL) associated with the m.3243A>G mutation is unknown. The aim of this study was to assess the frequency and heteroplasmy level of the m.3243A>G mutation in a large group of Polish patients with postlingual bilateral sensorineural HL of unidentified cause.

Methodology/Principal Findings

A molecular search was undertaken in the archival blood DNA of 1482 unrelated patients with isolated HL that had begun at ages between 5 and 40 years. Maternal relatives of the probands were subsequently investigated and all carriers underwent audiological tests. The m.3243A>G mutation was found in 16 of 1482 probands (an incidence of 1.08%) and 18 family members. Of these 34 individuals, hearing impairment was detected in 29 patients and the mean onset of HL was at 26 years. Some 42% of the identified m.3243A>G carriers did not develop multisystem symptomatology over the following 10 years. Mean heteroplasmy level of m.3243A>G was lowest in blood at a level of 14% and highest in urine at 58%. These values were independent of the manifested clinical severity of the disease.

Conclusions

A single m.3243A>G carrier can usually be found among each 100 individuals who have postlingual hearing loss of unknown cause. Urine samples are best for detecting the m.3243A>G mutation and diagnosing mitochondrially inherited hearing loss.

Prediction of cochlear implant performance by genetic mutation: the spiral ganglion hypothesis

BACKGROUND

Up to 7% of patients with severe-to-profound deafness do not benefit from cochlear implantation. Given the high surgical implantation and clinical management cost of cochlear implantation (>$1 million lifetime cost), prospective identification of the worst performers would reduce unnecessary procedures and healthcare costs. Because cochlear implants bypass the membranous labyrinth but rely on the spiral ganglion for functionality, we hypothesize that cochlear implant (CI) performance is dictated in part by the anatomic location of the cochlear pathology that underlies the hearing loss. As a corollary, we hypothesize that because genetic testing can identify sites of cochlear pathology, it may be useful in predicting CI performance.

METHODS

29 adult CI recipients with idiopathic adult-onset severe-to-profound hearing loss were studied. DNA samples were subjected to solution-based sequence capture and massively parallel sequencing using the OtoSCOPE(®) platform. The cohort was divided into three CI performance groups (good, intermediate, poor) and genetic causes of deafness were correlated with audiometric data to determine whether there was a gene-specific impact on CI performance.

RESULTS

The genetic cause of deafness was determined in 3/29 (10%) individuals. The two poor performers segregated mutations in TMPRSS3, a gene expressed in the spiral ganglion, while the good performer segregated mutations in LOXHD1, a gene expressed in the membranous labyrinth. Comprehensive literature review identified other good performers with mutations in membranous labyrinth-expressed genes; poor performance was associated with spiral ganglion-expressed genes.

CONCLUSIONS

Our data support the underlying hypothesis that mutations in genes preferentially expressed in the spiral ganglion portend poor CI performance while mutations in genes expressed in the membranous labyrinth portend good CI performance. Although the low mutation rate in known deafness genes in this cohort likely relates to the ascertainment characteristics (postlingual hearing loss in adult CI recipients), these data suggest that genetic testing should be implemented as part of the CI evaluation to test this association prospectively.

Myosin light-chain kinase is necessary for membrane homeostasis in cochlear inner hair cells

The structural homeostasis of the cochlear hair cell membrane is critical for all aspects of sensory transduction, but the regulation of its maintenance is not well understood. In this report, we analyzed the cochlear hair cells of mice with specific deletion of myosin light chain kinase (MLCK) in inner hair cells. MLCK-deficient mice showed impaired hearing, with a 5- to 14-dB rise in the auditory brainstem response (ABR) thresholds to clicks and tones of different frequencies and a significant decrease in the amplitude of the ABR waves. The mutant inner hair cells produced several ball-like structures around the hair bundles in vivo, indicating impaired membrane stability. Inner hair cells isolated from the knockout mice consistently displayed less resistance to hypoosmotic solution and less membrane F-actin. Myosin light-chain phosphorylation was also reduced in the mutated inner hair cells. Our results suggest that MLCK is necessary for maintaining the membrane stability of inner hair cells.

Successful cochlear implantation in a patient with MNGIE syndrome

Abstract A 28-year-old woman with mitochondrial neurogastrointestinal encephalomyopathy (MNGIE syndrome) undergoing evaluation for multichannel cochlear implantation is described. The case history, diagnosis of mitochondrial disease, and assessment of the benefits of cochlear implantation are documented. The hearing level with cochlear implant and speech recognition were improved significantly for this patient. MNGIE syndrome is a rare congenital disorder of mitochondrial DNA (mt-DNA). It is crucial for the otolaryngologist to have awareness of MNGIE syndrome and other mitochondrial encephalomyopathies when patients present with sensorineural hearing loss (SNHL). Cochlear implantation can be recommended to patients with MNGIE syndrome and satisfactory results can be achieved.

Quo vadis, hair cell regeneration?

Hearing loss is a global health problem with profound socioeconomic impact. We contend that acquired hearing loss is mainly a modern disorder caused by man-made noise and modern drugs, among other causes. These factors, combined with increasing lifespan, have exposed a deficit in cochlear self-regeneration that was irrelevant for most of mammalian evolution. Nevertheless, the mammalian cochlea has evolved from phylogenetically older structures, which do have the capacity for self-repair. Moreover, nonmammalian vertebrates can regenerate auditory hair cells that restore sensory function. We will offer a critical perspective on recent advances in stem cell biology, gene therapy, cell cycle regulation and pharmacotherapeutics to define and validate regenerative medical interventions for mammalian hair cell loss. Although these advances are promising, we are only beginning to fully appreciate the complexity of the many challenges that lie ahead.

[Otoneurological manifestations associated with antiretroviral therapy]

Ototoxicity and antiretroviral therapy seem to be associated. The aim of this study was to evaluate this possible correlation. Evaluations were carried out on 779 medical records from HIV-infected patients who were being regularly followed up, of whom 162 were being treated with antiretroviral therapy and 122 were untreated (controls). The patients undergoing treatment were older (mean: 42 years), had had serological confirmation for longer times (80 months) and had smaller viral loads (P = 0.00). CD4+ was similar between the groups (P = 0.60). In the treated group, three cases (1.8%) of idiopathic hearing loss and two (1.3%) of otosclerosis-related hearing loss were observed, which both started after antiretroviral therapy. No statistical difference relating to idiopathic hearing loss was found between the groups. While descriptive studies consider possible ototoxicity associated with antiretroviral therapy, this possible adverse effect was not related to the antiretroviral therapy in this study. Conversely, otosclerosis might have been correlated with antiretroviral therapy. This issue deserves to be studied.

A comparison of vestibular and auditory phenotypes in inbred mouse strains

The purposes of this research were to quantify gravity receptor function in inbred mouse strains and compare vestibular and auditory function for strain- and age-matched animals. Vestibular evoked potentials (VsEPs) were collected for 19 inbred strains at ages from 35 to 389 days old. On average, C57BL/6J (35 to 190 days), BALB/cByJ, C3H/HeSnJ, CBA/J, and young LP/J mice had VsEP thresholds comparable to normal. Elevated VsEP thresholds were found for elderly C57BL/6J, NOD.NONH2(kb), BUB/BnJ, A/J, DBA/2J, NOD/LtJ, A/WySnJ, MRL/MpJ, A/HeJ, CAST/Ei, SJL/J, elderly LP/J, and CE/J. These results suggest that otolithic function varies among inbred strains and several strains displayed gravity receptor deficits by 90 days old. Auditory brainstem response (ABR) thresholds were compared to VsEP thresholds for 14 age-matched strains. C57BL/6J mice (up to 190 days) showed normal VsEPs with normal to mildly elevated ABR thresholds. Four strains (BUB/BnJ, NOD/LtJ, A/J, elderly LP/J) had significant hearing loss and elevated VsEP thresholds. Four strains (DBA/2J, A/WySnJ, NOD.NONH2(kb), A/HeJ) had elevated VsEP thresholds (including absent VsEPs) with mild to moderate elevations in ABR thresholds. Three strains (MRL/MpJ, Ce/J, SJL/J) had significant vestibular loss with no concomitant hearing loss. These results suggest that functional change in one sensory system does not obligate change in the other. We hypothesize that genes responsible for early onset hearing loss may affect otolithic function, yet the time course of functional change may vary. In addition, some genetic mutations may produce primarily gravity receptor deficits. Potential genes responsible for selective gravity receptor impairment demonstrated herein remain to be identified.

Cochlear implantation in patients with MELAS syndrome

MELAS syndrome (mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes) is a rare congenital disorder of mitochondrial DNA (mt-DNA). Patients with this syndrome may present to the otolaryngologist with sensorineural hearing loss (SNHL) that is genetic in origin. Mitochondrial cytopathies can present with a variety of symptoms, but they occasionally present with SNHL as their first manifestation. Two cases of MELAS patients who responded well to cochlear implantation are presented. A review of the literature is also carried out focusing mainly on diagnosis, anesthetic considerations and management of these patients.

The MELAS syndrome. Review of the literature: the role of the otologist

The mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) syndrome is a rare congenital disorder of mitochondrial DNA (mt-DNA). Patients with this syndrome may present to the otolaryngologist with sensorineural hearing loss (SNHL), which is genetic in origin. A high index of suspicion is required because this hearing loss is part of a syndrome for which early diagnosis and intervention is required.

Age-related hearing loss: current research

PURPOSE OF REVIEW

Significant changes in population demographics with respect to age have taken place, and this pattern is expected to continue. The aging of the population underscores the importance of finding ways to improve the quality of life of the elderly. Most of the elderly population, however, suffers from progressive hearing loss: 60% of people older than 70 years have hearing loss of at least 25 dB. Age-related hearing loss affects the quality of life, not only of the elderly but also of their families and loved ones.

RECENT FINDINGS

The research goal in this field is to elucidate the mechanisms involved in age-related hearing loss and the molecular basis of normal and impaired auditory function, with the aim of developing preventative therapies. During the past few years, extraordinary progress has been made in the identification of genes that contribute to deafness. Additionally, inbred strains of mice have proven to be useful models to identify specific factors relevant to age-related hearing loss. A detailed description of the pathology exhibited by inbred mice that exhibit age-related hearing loss is helping to identify the specific structures and cell types affected by age-related hearing loss. A summary of current research efforts is presented. This review focuses on studies using inbred mice.

SUMMARY

By defining the molecular basis of normal and impaired auditory function, therapies can be developed to ameliorate the effects of aging in the auditory system.

Mutation in Mitochondrial DNA as a Cause of Presbyacusis

Much of the hearing loss that occurs in old age is likely to be due to the long-term deterioration of the mitochondria in the different structures of the cochlea. The current review surveys some of the basic information on mitochondria and mitochondrial DNA, as a background to their possible involvement in presbyacusis. It is likely that oxygen radicals damage mitochondrial DNA and other components of the mitochondria, such as their proteins and lipids. This further compromises both oxidative phosphorylation and the repair processes in mitochondria, setting up a vicious cycle of degradation. Evidence is presented from inherited point mutations on the possibly most critical sites for mutations in mitochondrial DNA associated with hearing loss. It is suggested that random sorting and clonal expansion of mutations both maintain the integrity of the pool of mitochondrial DNA molecules and give rise to the apoptosis that leads to loss of vulnerable cells, and hence to deafness. It is moreover suggested that apoptosis of the vulnerable cells of the inner ear may to some extent be preventable, or at least delayed.

Mouse models for human deafness: current tools for new fashions

Mouse models are one of the major tools used for discovery and characterization of genes for non-syndromic deafness in humans. The similarities between the mouse and human genomes, and between the physiology and morphology of their auditory systems, are striking. This article describes the latest mouse models, including spontaneous, 'knockout' and ENU (N-ethyl-N-nitrosourea)-induced mutants, and the recent discovery of modifier genes that are involved in mouse deafness; this discovery is leading the search for genetic modifiers for human disorders.