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Hearing Impairment Is Common in Various Phenotypes of the Mitochondrial DNA A3243G Mutation
Marcus Deschauer, MD;
Tobias Müller, MD;
Thomas Wieser, MD;
Wilhelm Schulte-Mattler, MD;
Malte Kornhuber, MD;
Stephan Zierz, MD
Arch Neurol. 2001;58:1885-1888.
ABSTRACT
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Objective To determine whether there are common symptoms
within different phenotypes of the mitochondrial DNA A3243G mutation.
Design 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.
Results 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.
Conclusions 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.
INTRODUCTION
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.
PATIENTS AND METHODS
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.
RESULTS
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.
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Table 1. Synopsis of 16 Patients With the A3243G Mutation*
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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).
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Table 2. Auditory Characterization of 16 Patients With the A3243G Mutation*
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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).
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Table 3. Frequency of Symptoms and of the MELAS Syndrome in Patients With the A3243G Mutation*
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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).
COMMENT
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.
AUTHOR INFORMATION
Accepted for publication November 7, 2000.
From the Department of Neurology, Martin-Luther-Universität Halle-Wittenberg, Halle/Saale, Germany.
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: marcus.deschauer{at}medizin.uni-halle.de).
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