The mitochondrial A3243G mutation involves the peripheral vestibule as well as the cochlea

Auditory and vestibular function in mitochondrial patients harbouring the m.3243A>G variant

Hearing impairment is a frequent clinical feature in patients with mitochondrial disease harbouring the pathogenic variant, m.3243A>G. However, auditory neural dysfunction, its perceptual consequences and implications for patient management are not established. Similarly, the association with vestibular impairment has not yet been explored. This case-control study investigated in 12 adults with genetically confirmed m.3243A>G adults [9 females; 45.5 ± 16.3 years (range 18-66); 47.1 ± 21.5 hearing level, dB] compared with 12 age, sex and hearing level-matched controls with sensory (cochlear level) hearing loss [9 females; 46.6 ± 11.8 years (range 23-59); 47.7 ± 25.4 hearing level, dB]. Participants underwent a battery of electroacoustic, electrophysiologic and perceptual tests, which included pure tone audiometry, otoacoustic emissions, auditory brainstem responses, auditory temporal processing measures, monaural/binaural speech perception, balance and vestibular testing and self-reported questionnaires (dizziness and hearing disability). Our findings showed evidence of auditory neural abnormality and perceptual deficits greater than expected for cochlear pathology. Compared with matched controls with sensory hearing loss, adults with mitochondrial disease harbouring m.3243A>G had abnormal electrophysiologic responses from the VIII nerve and auditory brainstem (P = 0.005), an impaired capacity to encode rapidly occurring acoustic signal changes (P = 0.005), a reduced ability to localize sound sources (P = 0.028) and impaired speech perception in background noise (P = 0.008). Additionally, vestibular dysfunction (P = 0.011), greater perceived dizziness (P = 0.001) and reduced stance time (balance, P = 0.009) were also seen in participants with m.3243A>G mitochondrial disease when compared with matched counterparts. This pilot study revealed that auditory evaluation including evoked potential responses from the auditory nerve/brainstem and speech perception in noise tests should form an important part of the management for individuals with m.3243A>G-related mitochondrial disease. Those presenting with hearing impairment and symptoms concerning balance and dizziness should undergo vestibular testing and appropriate management.

Age-dependent structural reorganization of utricular ribbon synapses

In mammals, spatial orientation is synaptically-encoded by sensory hair cells of the vestibular labyrinth. Vestibular hair cells (VHCs) harbor synaptic ribbons at their presynaptic active zones (AZs), which play a critical role in molecular scaffolding and facilitate synaptic release and vesicular replenishment. With advancing age, the prevalence of vestibular deficits increases; yet, the underlying mechanisms are not well understood and the possible accompanying morphological changes in the VHC synapses have not yet been systematically examined. We investigated the effects of maturation and aging on the ultrastructure of the ribbon-type AZs in murine utricles using various electron microscopic techniques and combined them with confocal and super-resolution light microscopy as well as metabolic imaging up to 1 year of age. In older animals, we detected predominantly in type I VHCs the formation of floating ribbon clusters, mostly consisting of newly synthesized ribbon material. Our findings suggest that VHC ribbon-type AZs undergo dramatic structural alterations upon aging.

Structural and functional diversity of mitochondria in vestibular/cochlear hair cells and vestibular calyx afferents

Mitochondria supply energy in the form of ATP to drive a plethora of cellular processes. In heart and liver cells, mitochondria occupy over 20% of the cellular volume and the major need for ATP is easily identifiable - i.e., to drive cross-bridge recycling in cardiac cells or biosynthetic machinery in liver cells. In vestibular and cochlear hair cells the overall cellular mitochondrial volume is much less, and mitochondria structure varies dramatically in different regions of the cell. The regional demands for ATP and cellular forces that govern mitochondrial structure and localization are not well understood. Below we review our current understanding of the heterogeneity of form and function in hair cell mitochondria. A particular focus of this review will be on regional specialization in vestibular hair cells, where large mitochondria are found beneath the cuticular plate in close association with the striated organelle. Recent findings on the role of mitochondria in hair cell death and aging are covered along with potential therapeutic approaches. Potential avenues for future research are discussed, including the need for integrated computational modeling of mitochondrial function in hair cells and the vestibular afferent calyx.

Long-Term Progression and Rapid Decline in Hearing Loss in Patients with a Point Mutation at Nucleotide 3243 of the Mitochondrial DNA

Patients with m.3243A>G mutation of mitochondrial DNA develop bilaterally symmetric sensorineural hearing loss. However, it is unclear how fast their hearing loss progresses over time, and whether they experience rapid progression of hearing loss. In the present study, we conducted a long-term hearing evaluation in patients with MELAS or MIDD who harbored the m.3243A>G mutation of mitochondrial DNA. A retrospective chart review was performed on 15 patients with this mutation who underwent pure-tone audiometry at least once a year for more than two years. The mean follow-up period was 12.8 years. The mean progression rate of hearing loss was 5.5 dB per year. Hearing loss progressed rapidly to be profoundly deaf in seven patients during the observation period. Heteroplasmy and age-corrected heteroplasmy levels correlated with the age of onset of hearing loss. These results indicate that patients with m.3243A>G mutation have a gradual progression of hearing loss in the early stages and rapid decline in hearing to be profoundly deaf in approximately half of the patients. Although it is possible to predict the age of onset of hearing loss from heteroplasmy and age-corrected heteroplasmy levels, it is difficult to predict whether and when the rapid hearing loss will occur.

Audiological and Vestibular Findings in Subjects with MELAS Syndrome

OBJECTIVES

The mitochondrial DNA (mtDNA) point mutation m.3243A>G is known to express the following two syndromes among others: maternally inherited diabetes and deafness (MIDD) and mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS). Sensorineural hearing loss (SNHL) is the most frequent symptom in individuals harboring the m.3243A>G mutation. However, dysfunction of the vestibular organs has been scarcely examined. Therefore, the present study aimed to study the impact of the m.3243A>G mutation on the inner ear.

MATERIALS AND METHODS

A total of 8 subjects harboring the blood-verified m.3243A>G mutation underwent thorough audiological and vestibular examinations, including tone and speech audiometry, video head impulse test (vHIT), ocular and cervical vestibular-evoked myogenic potential (oVEMP and cVEMP), and full otoneurological examination. The subjects also answered a Dizziness Handicap Inventory (DHI) questionnaire.

RESULTS

SNHL was identified in all the 8 subjects, with a mean pure-tone average-4 (PTA-4) of 59 dB. Speech discrimination score (n=7) ranged from 24% to 100% (mean 74%), and vHIT (n=42) detected pathology in nine lateral semicircular canals (SCCs), five posterior SCCs, and one anterior SCC, whereas three measurements were inconclusive. All oVEMPs (n=14 ears) were absent, nine cVEMPs were absent, and two were inconclusive. Based on the DHI scores, 6 subjects reported none to mild dizziness, 1 reported moderate, and 1 reported severe dizziness.

CONCLUSION

Our study population had pathological findings from every audiological and vestibular end organs. The results indicated that the pathological findings originated from within the end organs themselves and not within the superior and inferior vestibular or cochlear nerve.

Why study inner ear hair cell mitochondria?

In several systems of the body (muscle, liver, nerves), new studies have examined the internal structure of mitochondria and brought to light striking new findings about how mitochondria are constructed and how their structure affects cell function. In the inner ear field, however, we have little structural knowledge about hair cell and supporting cell mitochondria, and virtually none about mitochondrial subtypes or how they function in health and disease. The need for such knowledge is discussed in this short review.

Effects of Ageing on the Mitochondrial Genome in Rat Vestibular Organs

Background:

Deterioration in vestibular function occurs with ageing and is linked to age-related falls. Sensory hair cells located in the inner ear vestibular labyrinth are critical to vestibular function. Vestibular hair cells rely predominantly on oxidative phosphorylation (OXPHOS) for ener-gy production and contain numerous mitochondria. Mitochondrial DNA (mtDNA) mutations and perturbed energy production are associated with the ageing process.

Objective:

We investigated the effects of ageing on mtDNA in vestibular hair and support cells, and vestibular organ gene expression, to better understand mechanisms of age-related vestibular deficits.

Methods:

Vestibular hair and supporting cell layers were microdissected from young and old rats, and mtDNA was quantified by qPCR. Additionally, vestibular organ gene expression was analysed by microarray and gene set enrichment analyses.

Results:

In contrast to most other studies, we found no evidence of age-related mtDNA deletion mu-tations. However, we found an increase in abundance of major arc genes near the mtDNA control re-gion. There was also a marked age-related reduction in mtDNA copy number in both cell types. Ves-tibular organ gene expression, gene set enrichment analysis showed the OXPHOS pathway was down regulated in old animals.

Conclusion:

Given the importance of mtDNA to mitochondrial OXPHOS and hair cell function, our findings suggest the vestibular organs are potentially on the brink of an energy crisis in old animals

Progression of Peripheral Vestibular Dysfunctions in Patients With a Mitochondrial A3243G Mutation

OBJECTIVE

To evaluate the progression of peripheral vestibular dysfunction in patients with an A-to-G point mutation at nucleotide pair 3243 in mitochondrial DNA (A3243G mutation).

STUDY DESIGN

Retrospective patient series.

SETTING

Tertiary referral center.

PATIENTS

Six unrelated patients with an A3243G mutation (four mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes; and two maternally inherited diabetes and deafness; five females and one male; average age 41 ± 11.3 yr old), who underwent repeated vestibular examinations, were included (average interval between the first and second examinations: 5.7 ± 2.1 yr).

INTERVENTION

Diagnostic.

MAIN OUTCOME MEASURE

Results of caloric testing and cervical vestibular evoked myogenic potential testing in response to air-conducted sound were analyzed.

RESULTS

All the patients except one, who was already completely deaf in both ears at the first examination, showed progression of hearing loss (average 4.0 ± 6.3 dB per year). Five of the six patients had vestibular symptoms at the first examination. The other patient developed dizziness later. Caloric responses were abnormal in four patients at the first examination and in five patients at the second examination. Cervical vestibular evoked myogenic potential responses were bilaterally absent in three patients at the first examination and in five patients at the second examination.

CONCLUSIONS

The A3243G mutation causes progression of peripheral vestibular dysfunction as well as that of hearing loss.

Repeated Attacks of Dizziness Caused by a Rare Mitochondrial Encephalomyopathy

Cases of dizziness caused by multiple sclerosis are commonly reported, but those caused by mitochondrial encephalomyopathy have been rarely reported. Particularly, the description of eye nystagmography (ENG) using caloric and optokinetic nystagmus tests has not been reported to date. We encountered the case of a 40-year-old woman with mitochondrial encephalomyopathy who visited us with the chief complaint of dizziness. At first, we considered multiple sclerosis based on the magnetic resonance imaging (MRI) findings and dizziness. Repeated attacks of dizziness and serum lactic acid levels suggested mitochondrial encephalomyopathy. A muscle biopsy confirmed the diagnosis. ENG findings suggested central vestibular disorder of the cerebellum and brainstem. This case suggests that we should not rule out the differential diagnosis of a very rare mitochondrial encephalomyopathy in patients who experience dizziness with MRI findings indicative of multiple sclerosis.

Bilateral Vestibulopathy Aggravates Balance and Gait Disturbances in Sensory Ataxic Neuropathy, Dysarthria, and Ophthalmoparesis: A Case Report

In patients with a triad of sensory ataxic neuropathy, dysarthria, and ophthalmoparesis (SANDO), the presenting features are mainly ataxia or ptosis. SANDO patients often have impaired balance and gait, which is not surprising considering the combination of sensory ataxic neuropathy, and additional symptoms like cerebellar ataxia and limb girdle weakness. We describe a SANDO patient who noticed an increasingly impaired balance and gait, without any dizziness. Neurological investigation revealed an external ophthalmeplegia and a cerebellar ataxia; the head impulse test was not reliable because of eye movement disorders. The caloric reflex tests showed lack of responses on both sides, compatible with severe bilateral vestibulopathy. Making the diagnosis of bilateral vestibulopathy in SANDO patients may have implications for the management of the patient, because specific vestibular rehabilitation can improve gaze and postural stability.

Charles Darwin's mitochondria

Charles Darwin's long-term illness has been the subject of much speculation. His numerous symptoms have led to conclusions that his illness was essentially psychogenic in nature. These diagnoses have never been fully convincing, however, particularly in regard to the proposed underlying psychological background causes of the illness. Similarly, two proposed somatic causes of illness, Chagas disease and arsenic poisoning, lack credibility and appear inconsistent with the lifetime history of the illness. Other physical explanations are simply too incomplete to explain the range of symptoms. Here, a very different sort of explanation will be offered. We now know that mitochondrial mutations producing impaired mitochondrial function may result in a wide range of differing symptoms, including symptoms thought to be primarily psychological. Examination of Darwin's maternal family history supports the contention that his illness was mitochondrial in nature; his mother and one maternal uncle had strange illnesses and the youngest maternal sibling died of an infirmity with symptoms characteristic of mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS syndrome), a condition rooted in mitochondrial dysfunction. Darwin's own symptoms are described here and are in accord with the hypothesis that he had the mtDNA mutation commonly associated with the MELAS syndrome.