Marcus Deschauer, Tobias Müller, Thomas Wieser, Wilhelm Schulte-Mattler, Malte Kornhuber, Stephan Zierz. Hearing Impairment Is Common in Various Phenotypes of the Mitochondrial DNA A3243G Mutation. Arch Neurol. 2001;58(11):1885–1888. doi:10.1001/archneur.58.11.1885
To determine whether there are common symptoms within different phenotypes of the mitochondrial DNA A3243G mutation.
A series of 52 adults with mitochondrial encephalomyopathies and their symptomatic relatives were screened for the A3243G mutation using restriction enzyme analysis. In addition to clinical examination, patients with the mutation underwent audiometry.
The A3243G mutation was identified in 16 patients (10 index patients and 6 symptomatic relatives). Six of these patients presented with strokelike episodes and met the classical criteria of MELAS syndrome (mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes), and one had MELAS/MERRF (myoclonic epilepsy with ragged-red fibers) overlap syndrome. Two patients presented with strokelike episodes but did not meet the classical criteria of MELAS. Predominant features of the 8 other patients were myopathy with hearing loss and diabetes mellitus (n = 1), chronic progressive external ophthalmoplegia (n = 1), diabetes mellitus with hearing loss (n = 1), painful muscle stiffness with hearing loss (n = 1), cardiomyopathy (n = 1), diabetes mellitus (n = 1), and hearing loss (n = 2). In 11 of 16 patients, hearing impairment was obvious on clinical examination. Furthermore, all 5 patients with normal hearing on clinical examination showed subclinical hearing loss; in 4, hearing loss was more pronounced than age-related hearing impairment and in 1, hearing loss can be age related as well.
A variety of phenotypes represent the variable multisystemic involvement of the A3243G mutation. Less than half of the patients presented with MELAS. Hearing impairment, the most common symptom, was clinically or subclinically relevant in 15 (94%) of 16 patients.
THE MITOCHONDRIAL DNA transfer RNALeu(UUR) A3243G mutation was first described in patients with MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes) syndrome in 1990 by Goto et al.1 During the past 10 years, numerous other phenotypes of the A3243G mutation have been reported: chronic progressive external ophthalmoplegia,2 diabetes mellitus and deafness,3 cardiomyopathy,4 painful muscle stiffness,5 MERRF (myoclonic epilepsy with ragged-red fibers) syndrome,6 and dystonia.7 The A3243G mutation is regarded as the most frequent mitochondrial point mutation. In a Finnish population, prevalence was estimated to be greater than 16 in 100 000.8
We studied 16 patients with the A3243G mutation to determine whether there are common symptoms among these highly variable phenotypes.
A series of 52 adults with mitochondrial encephalomyopathies was identified by clinical examination by an experienced neurologist, histological and biochemical analysis of muscle biopsy specimens, and measurement of lactate levels at rest and after bicycle exercise (30 W for 15 minutes).
These patients and their symptomatic relatives were screened for the mitochondrial A3243G mutation. Polymerase chain reaction amplification was determined according to the methods of Yamamoto9 after preparation of DNA samples from skeletal muscle or blood. The presence of the mutation led to an ApaI restriction site, and the fragment of 330 base pairs (bp) was cut into 213 and 117 bp. Fragments were separated on a 6% polyacrylamide gel, stained with ethidium bromide, and visualized on a UV transilluminator. After photography, the negative film was scanned and intensities of bands were measured using Image Quant software (Molecular Dynamics, Sunnyvale, Calif). The amount of mutant DNA was expressed as the percentage of total ApaI-cleaved material.
Screening for deletions of mitochondrial DNA (mtDNA) was carried out by Southern blot analysis on muscle DNA using standard procedures.10 Mitochondrial DNA was linearized using the restriction enzyme BamHI. Hybridization was performed using a probe ranging from nucleotide position 15149 to 14831 created by the Expand Long Template PCR System (Roche Diagnostics, Mannheim, Germany).
Pure-tone audiometry was done using standard methods, including air and bone conduction measurement, and patients underwent otological examination. Mean hearing loss was calculated over the frequencies 0.5, 1.0, 2.0, and 4.0 kHz. Mean high-frequency hearing loss was calculated over 4.0, 6.0, and 8.0 kHz. Age-corrected hearing impairment was defined as the measured hearing loss minus the mean loss in an otologically unscreened population of the same age and sex using data reported in the National Study of Hearing.11
All patients were informed of the research character of the investigations and gave their consent.
The A3243G mutation was identified in 16 patients (10 index patients and 6 symptomatic family members). Six patients met the classical criteria of MELAS syndrome according to Hirano et al,12 and one of these patients presented with MELAS/MERRF (myoclonic epilepsy with ragged-red fibers) overlap syndrome. Two patients (patients 6 and 10) presented with strokelike episodes but did not meet the classical criteria of MELAS. Predominant features of the 8 other patients were myopathy with hearing loss and diabetes mellitus (n = 1), chronic progressive external ophthalmoplegia (n = 1), diabetes mellitus with hearing loss (n = 1), painful muscle stiffness with hearing loss (n = 1), cardiomyopathy (n = 1), diabetes mellitus (n = 1), and hearing loss (n = 2). Symptoms of the patients are shown in Table 1.
The most frequent symptom was hearing loss, found in 11 of 16 patients on clinical examination. In all 5 patients with normal hearing on clinical examination, audiometry revealed subclinical hearing impairment. In 4 of these patients (patients 2a, 4, 6, and 7a), mean hearing loss (0.5, 1.0, 2.0, and 4.0 kHz) was at least 20 dB and mean high-frequency hearing loss (4.0, 6.0, and 8.0 kHz) was at least 30 dB in the better ear. Age-corrected mean hearing loss and age-corrected mean high-frequency hearing loss were present in these 4 patients. Patient 1a showed hearing loss that can be related to age as well (Table 2).
In all 12 patients who underwent audiometry, air and bone conduction thresholds were similar, as seen in sensorineural hearing loss. Otological examination showed no other causes of hearing impairment.
The second most frequent symptom in our patients was at least one strokelike episode in 8 of 16 patients, followed by dementia, present in 7 patients, and by seizures and diabetes mellitus, each being present in 6 patients (Table 3).
Hearing was analyzed in a control group of 18 patients with deletions of mtDNA (15 patients with single deletions and 3 patients with multiple deletions). Hearing loss on clinical examination was found in 2 of these patients. Five patients with normal hearing on clinical examination underwent audiometry, and all showed normal findings on audiograms (mean hearing loss always <15 dB).
In our study, a variety of phenotypes represent variable multisystemic involvement. Less than half of the patients (38%) presented with classical MELAS syndrome, consistent with a study by Hammans et al,6 who found MELAS syndrome in 10 (28%) of 36 patients with the A3243G mutation. Therefore, the A3243G mutation, also called MELAS mutation, is not specific for MELAS, and the term MELAS mutation can be misleading.
The most common symptom in the study by Hammans et al was limb weakness (often mild) in 22 of 36 patients. In a meta-analysis by Chinnery et al,13 myopathy was found in 53% of patients with the A3243G mutation. Limb weakness was also present in 4 (25%) of 16 patients in the present study.
The most consistent symptom in our study was hearing impairment. As early as clinical examination (whispered voice and finger rub), hearing impairment was the most frequent symptom, found in 11 of 16 patients. All 5 patients with normal hearing on clinical examination showed subclinical hearing impairment in the audiogram. In 4 of these patients, hearing loss was more pronounced than the expected mean hearing impairment of that age group.
In contrast, only 2 of 18 patients with deletions of mtDNA showed hearing loss on clinical examination. Furthermore, all 5 patients with deletions and normal hearing on clinical examination who underwent audiometry had no subclinical hearing loss.
Results of the present study suggest that sensorineural hearing loss is a frequent symptom of the A3243G mutation and that it is not as prominent in patients with mitochondrial disorders owing to deletions of mtDNA.
Our finding of hearing impairment in 11 of 16 patients with the A3243G mutation on clinical examination is consistent with the study by Hammans et al6 showing deafness in 20 of 36 patients by means of clinical examination. In the meta-analysis by Chinnery et al,13 deafness was a clinical feature in 44% of patients with the A3243G mutation. Subsequently, Chinnery et al14 examined 23 patients with mitochondrial encephalomyopathies using audiometry. Hearing loss was present in 8 of 10 patients with the A3243G mutation, and in 5 of 8 patients with deletions of mtDNA. Similar to patients with the A3243G mutation in the study by Chinnery et al,14 we also had patients with predominantly high-frequency hearing impairment (patients 2a and 7a) and profound hearing impairment across all frequencies (patients 8 and 9c). High-frequency hearing impairment was found predominantly in our patients with subclinical hearing loss, whereas patients with hearing impairment across all frequencies had manifest hearing loss.
Damian et al15 investigated a large family with the A3243G mutation and found clinical deafness in 18 of 21 symptomatic family members. Also, in a pedigree with the A3243G mutation described by Mosewich et al,16 hearing loss was the most consistent feature in 5 of 6 symptomatic family members. Hammans et al6 showed that there is clustering of phenotypes within families. Thus, the frequency of a symptom in one pedigree is not representative of the general frequency of this symptom.
Chinnery et al13 postulated that deafness is more frequent in individuals with lower levels of A3243G mutation in muscle. However, in their subsequent study,14 severity of hearing loss correlated with percentage level of mutated DNA in skeletal muscle. We found subclinical hearing impairment also in patients with low levels of mutant DNA (patients 1a, 4, and 6) and in a patient with high levels (patient 7a). Manifest hearing loss was also found in patients with low levels of heteroplasmy in blood (patients 1b, 5b, and 7b) and in a patient with high levels (patient 2b). Moreover, we confirmed the observation that hearing impairment is prominent in oligosymptomatic patients (patients 5b and 7b).6,14,15
The observation that even very low levels of heteroplasmy in some of our patients can be symptomatic is consistent with recent studies showing that there seems to be no threshold. Chinnery et al17 used magnetic resonance spectroscopy in a patient with the A3243G mutation to show that 6% mutant DNA in muscle leads to reduced mitochondrial adenosine triphosphate production. Schröder et al18 found that even low degrees of heteroplasmy of mtDNA deletions result in biochemical abnormalities of the respiratory chain.
The pathogenesis leading to different phenotypes is still unknown. Variability might be explained by tissue-specific differences in the level of mutated mtDNA and threshold effects, affected by nuclear genetic factors. Chinnery et al14 postulated that hearing deficit has a cochlear origin because they did not find any central abnormalities of the auditory pathway. Stria vascularis and hair cells are highly metabolically active and might be particularly vulnerable to mitochondrial dysfunction.
In patients with suspected mitochondrial disorders, audiometry could add a diagnostic hint to the multisystemic involvement typical of mitochondrial disorders. It can be speculated that there is a relation between vulnerability of the inner ear in patients with the A3243G mutation and susceptibility to aminoglycoside ototoxic effects in patients carrying the mitochondrial A1555G mutation.19 This has to be considered in the decision to treat patients with the A3243G mutation with aminoglycosides.
Accepted for publication November 7, 2000.
Corresponding author and reprints: Marcus Deschauer, MD, Department of Neurology, Martin-Luther-Universität Halle-Wittenberg, Ernst-Grube-Str. 40, D-06097 Halle, Germany (e-mail: firstname.lastname@example.org).