Frequency and clinical features of patients with sensorineural hearing loss associated with the A3243G mutation of the mitochondrial DNA in otorhinolaryngic clinics

Role of Oxidative Stress and Antioxidants in Acquired Inner Ear Disorders

Oxygen metabolism in the mitochondria is essential for biological activity, and reactive oxygen species (ROS) are produced simultaneously in the cell. Once an imbalance between ROS production and degradation (oxidative stress) occurs, cells are damaged. Sensory organs, especially those for hearing, are constantly exposed during daily life. Therefore, almost all mammalian species are liable to hearing loss depending on their environment. In the auditory pathway, hair cells, spiral ganglion cells, and the stria vascularis, where mitochondria are abundant, are the main targets of ROS. Excessive generation of ROS in auditory sensory organs is widely known to cause sensorineural hearing loss, and mitochondria-targeted antioxidants are candidates for treatment. This review focuses on the relationship between acquired hearing loss and antioxidant use to provide an overview of novel antioxidants, namely medicines, supplemental nutrients, and natural foods, based on clinical, animal, and cultured-cell studies.

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.

Audiologic and genetic features of the A3243G mtDNA mutation

BACKGROUND

Mitochondrial mutations have been shown to be responsible for syndromic and nonsyndromic hearing impairment.

AIM

To assess the genotypic-phenotypic correlation of mitochondrial DNA mutations in three generations of a single family.

METHODS

A single family with maternally inherited diabetes and hearing loss was recruited. Genomic DNA was subject to polymerase chain reaction-restriction fragment length polymorphism analysis (ApaI) for A3243G mutation detection and confirmation with direct DNA sequencing. The degree of heteroplasmy for the A3243G mutation in blood DNA samples was quantified. In addition, we reviewed audiological data of A3243G-associated hearing loss cases from the literature to provide details of audiologic features.

RESULTS

Six of 11 family members were recruited. All affected members harbored the A3243G mutation. Four of six members had diabetes. Five of five affected members demonstrated hearing loss ranging from mild to severe. The degree of heteroplasmy ranged from 5.51% to 27.74%.

CONCLUSIONS

Patients with a greater percentage of heteroplasmy have a trend toward more severe phenotypic presentations. Hearing loss is bilateral, sensorineural, and symmetric. The main audiogram shapes found were sloping. Additional studies are necessary to clarify the relationship between degree of heteroplasmy and phenotypic presentation.

Large-scale screening of mitochondrial DNA mutations among Iranian patients with prelingual nonsyndromic hearing impairment

Hereditary hearing impairment (HI) is a genetically heterogeneous disorder caused by mutations either in nuclear DNA (nDNA) or in mitochondrial DNA (mtDNA). The nDNA mutations account for the majority of prelingual nonsyndromic HI (NSHI). The present survey was conducted to screen for known pathogenic mtDNA mutations including A1555G, A3243G, C1494T, and A7445G to provide an accurate estimate of their prevalence in prelingual NSHI for the first time in the Iranian subpopulations. One thousand unrelated probands with NSHI (including both GJB2-negative and GJB2 heterozygote cases) and 1000 healthy matched controls were investigated using the PCR/RFLP method followed by DNA sequencing to confirm the observed mtDNA mutations. Two of the studied mutations, namely A3243G and A7445G, were each found in a single family (a frequency of 0.1% for each). Mutation screening for A3243G followed by DNA sequencing led to the identification of G3316A substitution, with no prior link to HI. Surprisingly, screening for A3243G in the studied population identified 6 cases (0.6%) in probands and 10 (1%) in normal subjects. A1555G, the most common mtDNA mutation associated with deafness in other populations, was not found in the studied samples. To conclude, our findings indicate G3316A as a nonpathogenic variant in the prelingual NSHI subpopulations of Iran and suggest that mtDNA mutations do not play a major role in the etiology of NSHI in Iran.

Analysis of mtDNA A3243G mutation frequency in Hungary

The A3243G mutation in the mitochondrial tRNALeu (UUR) gene is one of the most common causes of mitochondrial DNA related disorders. Originally it was described in MELAS syndrome (Mitochondrial Encephalomyopathy, Lactic acidosis, Stroke-like episodes), later it had been found to be associated with various phenotypes. In our study the mutation frequency of the A3243G mtDNA mutation was investigated in patients with maternal sensoneural hearing loss, stroke-like episodes, ataxia and myopathy with undetermined etiology. We screened 631 Hungarian patients in North-East, South-West and Central Hungary between 1999 and 2008 for this mutation. The mtDNA analysis was performed from blood and/or muscle tissue. The A3243G substitution was present in 6 patients in heteroplasmic form. The segregation analysis detected 8 further cases. The frequency of the A3243G mutation was 2.22% in the investigated patients. The A3243G mutation frequency in Hungary does not differ significantly from other countries using similar patient selection criteria, however in Finland a higher mutation rate was found. In studies investigated the mutation frequency of this mutation in diabetes mellitus similarly wide variety was detected as well. We conclude that the study design has a huge impact on the result of the genetic epidemiological investigation analyzing the mutation frequency of the A3243G mutation due to the broad clinical phenotype and the different mutation load in different tissues.

Role of mitochondrial variation in maternally inherited diabetes and deafness syndrome

Maternally inherited diabetes and deafness syndrome is caused by the mitochondrial deoxyribonucleic acid mutation 3243 A>G (where A = adenine and G = guanine). The degree of heteroplasmy of the mitochondrial deoxyribonucleic acid may correlate with the rate of progression of the hearing loss. This has important implications for counselling patients with this pathology. Cochlear implantation is a successful method of rehabilitation for patients with hearing loss as part of this syndrome.

Genetic, pathogenetic, and phenotypic implications of the mitochondrial A3243G tRNALeu(UUR) mutation

Mitochondrial disorders are frequently caused by mutations in mitochondrial genes and usually present as multisystem disease. One of the most frequent mitochondrial mutations is the A3,243G transition in the tRNALeu(UUR) gene. The phenotypic expression of the mutation is variable and comprises syndromic or non-syndromic mitochondrial disorders. Among the syndromic manifestations the mitochondrial encephalopathy, lactacidosis, and stroke-like episode (MELAS) syndrome is the most frequent. In single cases the A3,243G mutation may be associated with maternally inherited diabetes and deafness syndrome, myoclonic epilepsy and ragged-red fibers (MERRF) syndrome, MELAS/MERRF overlap syndrome, maternally inherited Leigh syndrome, chronic external ophthalmoplegia, or Kearns-Sayre syndrome. The wide phenotypic variability of the mutation is explained by the peculiarities of the mitochondrial DNA, such as heteroplasmy and mitotic segregation, resulting in different mutation loads in different tissues and family members. Moreover, there is some evidence that additional mtDNA sequence variations (polymorphisms, haplotypes) influence the phenotype of the A3,243G mutation. This review aims to give an overview on the actual knowledge about the genetic, pathogenetic, and phenotypic implications of the A3,243G mtDNA mutation.

Accurate consent for insertion and later removal of grommets

Each year in the UK over 30 000 patients undergo insertion of grommets. The grommet insertion may cause many problems like persistent otorrhoea, scarred drum, retraction pockets and retention. The grommets may be extruded from the middle ears by the normal epithelial migration mechanism once they have served their purpose. These may become infected and require removal. We have analysed the Department of Health Hospital Episode Statistics relating to the insertion and removal of grommets (ventilation tubes). We have shown that 7.6 per cent of patients who have grommets inserted will have grommets removed.

Mutational analysis of the mitochondrial tRNALeu(UUR) gene in Tunisian patients with mitochondrial diseases

The mitochondrial tRNA(Leu(UUR)) gene (MTTL) is a hot spot for pathogenic mutations that are associated with mitochondrial diseases with various clinical features. Among these mutations, the A3243G mutation was associated with various types of mitochondrial multisystem disorders, such as MIDD, MELAS, MERRF, PEO, hypertrophic cardiomyopathy, and a subtype of Leigh syndrome. We screened 128 Tunisian patients for the A3243G mutation in the mitochondrial tRNA(Leu(UUR)) gene. This screening was carried out using PCR-RFLP with the restriction endonuclease ApaI. None of the 128 patients or the 100 controls tested were found to carry the mitochondrial A3243G mutation in the tRNA(Leu(UUR)) gene in homoplasmic or heteroplasmic form. After direct sequencing of the entire mitochondrial tRNA(Leu(UUR)) gene and a part of the mitochondrial NADH dehydrogenase 1, we found neither mutations nor polymorphisms in the MTTL1 gene in the tested patients and controls, and we confirmed the absence of the A3243G mutation in this gene. We also found a T3396C transition in the ND1 gene in one family with NSHL which was absent in the other patients and in 100 controls. Neither polymorphisms nor other mutations were found in the mitochondrial tRNA(Leu(UUR)) gene in the tested patients.

Prevalence of the mitochondrial DNA A1555G mutation in sensorineural deafness patients in island Southeast Asia

A mtDNA A1555G base substitution in a highly conserved region of the 12S rRNA gene has been reported to be the main cause of aminoglycoside induced deafness. This mutation is found in approximately 3% of Japanese and 0.5-2.4% of European sensorineural deafness patients. We report a high prevalence (5.3%) of the A1555G mutation in sensorineural deafness patients in Sulawesi (Indonesia). Our result confirms the importance of determining the prevalence of the mtDNA A1555G mutation in different populations, and the need for mutation detection before the administration of aminoglycoside antibiotics.

A review of cochlear implantation in mitochondrial sensorineural hearing loss

OBJECTIVE

Mitochondrial sensorineural hearing loss (SNHL) may be nonsyndromic (occurring in isolation), associated with the A1555G mutation in the MTRNR1 gene. Mitochondrial SNHL may also be syndromic, associated with the A3243G point mutation in the MTTL1 gene. In syndromic cases-mitochondrial encephalopathy, lactic acidosis, and strokelike episodes (MELAS), maternally inherited diabetes and deafness, Kearns-Sayre syndrome, and chronic progressive external ophthalmoplegia-the SNHL compounds already existing disabilities. The genetic basis for mitochondrial SNHL and postulated sites of pathologic changes are discussed.

DATA SOURCES

Sources used were relevant clinical and basic science publications.

STUDY SELECTION

A search of the entire databases of Medline and Web of Science, using various subject headings and free-text terms, was used to identify patients with mitochondrial disease having cochlear implants.

DATA EXTRACTION

The data from publications were critically reviewed and tabulated to assess implantation outcomes.

DATA SYNTHESIS

The data were not amenable to formal meta-analysis or valid data summarization, other than descriptive statistics.

CONCLUSIONS

There is an increasing awareness of the prevalence of mitochondrial SNHL and its progressive nature. High-risk candidates warrant genetic testing and family screening. Correlating the data for mitochondrial SNHL as a treatable entity is important, and the authors present an overview of these patients successfully rehabilitated by cochlear implantation.