Download Title: A novel MFN2 mutation causing Charcot-Marie

Case report
A novel MFN2 mutation causing Charcot-Marie-Tooth type 2A disease in a
Chinese family
CHING Chor Kwan1 (MSc, MBChB), LAU Kwok Kwong2 (FRCP, FHKAM
(Medicine)),
YU
Kwok
Wai1
(BSc),
CHAN
Yan
Wo
Albert1
(MD,
FHKAM(Pathology)) and MAK Miu Chloe1 (PhD, FHKAM(Pathology))
1
Department of Pathology, Princess Margaret Hospital, Hong Kong, China
2
Department of Medicine and Geriatrics, Princess Margaret Hospital, Hong Kong,
China
*Correspondence to: Dr MAK Miu Chloe, Department of Pathology, Princess
Margaret Hospital, Hong Kong, China
Tel: (852)29901882
Fax: (852)29901883
Email: [email protected]
Keywords: Charcot-Marie-Tooth type 2A; MFN2; Hong Kong Chinese
Word Count: 1680 (text)
Charcot-Marie-Tooth disease (CMT), also known as hereditary motor and sensory
neuropathies, comprises a genetically heterogeneous group of inherited peripheral
neuropathies. Clinically it is characterized by progressive distal weakness, muscle
atrophy, distal sensory loss and loss of deep tendon reflexes. Following
electrophysiological criteria, CMT is divided into two main forms: the primarily
demyelinating neuropathy CMT1 with severely decreased nerve conduction velocity
(NCV) (<38 m/s), and CMT2, the primarily axonal form with normal or slightly
reduced NCV (>38 m/s) but decreased amplitudes.1 CMT2A, an autosomal dominant
disease caused by mitofusin 2 gene (MFN2) mutations, is the most common type of
CMT2, accounting for up to 33% of familial CMT2 cases.2 We report a patient with
clinical diagnosis of CMT2 caused by a novel MFN2 mutation. To our knowledge,
this is the first case of genetically confirmed CMT2A in Chinese.
CASE REPORT
The patient is a 31-year-old Hong Kong Chinese woman who first presented with
steppage gait at 9 years old. She developed progressive distal muscle weakness and
atrophy in lower limbs and upper limbs. Neurological examination at the age of 31
years revealed atrophy of distal lower and upper limbs muscles, severe weakness of
distal lower limbs, and mild paresis over proximal lower limbs and distal upper limbs.
She had impaired sensation to pinprick up to the knees, and the sensation was nearly
absent from ankles downward. There was no sensory disturbance in upper limbs. Her
proprioception and vibration sensation was normal. Ankle jerks were absent, and deep
tendon reflexes of the upper extremities were also diminished. Other clinical features,
such as optic atrophy, hearing loss, or pyramidal signs, were not found. Laboratory
tests, including vitamin B12 level, were all unremarkable. Visual evoked potentials
which tested the intracerebral visual pathway were normal. Nerve conduction studies
showed low compound muscle action potential and preserved motor NCV in median
and ulnar nerves. Sural sensory nerve action potential was absent on left side and
decreased on right side. The compound muscle action potential of peroneal and tibial
nerves was not demonstrable. These findings were compatible with those of axonal
sensorimotor peripheral neuropathy. In view of the typical clinical and
electrophysiological features and the early age of onset, CMT2 was suspected in the
patient.
Molecular genetic studies were performed after obtaining informed consent of the
patient. Genomic DNA was extracted from peripheral blood samples. The 19 coding
exons and the flanking introns of the MFN2 gene were amplified by polymerase chain
reaction. Direct sequence analysis of the MFN2 gene revealed a novel heterozygous
missense mutation, MFN2 NM_014874.2:c.828G>C (p.Gln276His), in the patient
(Figure 1). The novel mutation was not found in 300 chromosomes from healthy
controls using real-time amplification refractory mutation system polymerase chain
reaction (Figure 2). Primer sequences and protocols are available upon request. The
amino acid substitution was predicted to affect protein function using the algorithm
tool SIFT (Sorting Intolerant From Tolerant). The amino acid glutamine at position
276 is also conserved among different species. Hence, the mutation is conceivably
disease causing. The patient initially did not recall any family history of neuropathy.
Family screening was performed for her parents after obtaining informed consent. Her
mother was surprisingly found to carry the same heterozygous mutation. She enjoyed
good past health. She only described recent onset mild distal lower limb weakness,
which was managed as osteoarthritis by a general practitioner. After the mutation
detection in the mother, electrophysiological study was performed and was normal.
On further inquiry, it was noted that the proband’s deceased maternal grandfather also
had a history of limping gait but the age of onset was unclear. The proband’s five
siblings were asymptomatic and were not available for medical consultation and
genetic testing at the time of writing. Her two minor children were completely
asymptomatic. Presymptomatic genetic analysis was not recommended by us for her
children after genetic counseling because of their young age and the lack of curative
therapy for CMT2A.
DISCUSSION
CMT is one of the most common inherited neuromuscular disorders, with a
prevalence of about 1 in 2500.3 Today, at least 36 loci and 28 genes have been linked
to CMT.3 The demyelinating CMT1 is the most common form, and among this group,
about 70% of the cases are due to CMT1A caused by duplication of the PMP22 gene;
the X-linked CMTX associated with GJB1/Cx32 mutations with either demyelinating
or axonal phenotype accounts for about 12% of all CMT cases.3 The axonal CMT2 is
very genetically heterogeneous, and there are more than 10 subtypes. CMT2A
associated with MFN2 mutations accounts for about one-third of CMT2 cases.3 In
addition to the classical demyelinating and axonal CMT, there are also autosomal
dominant intermediate CMT (DI-CMT) with NCV overlapped those of CMT1 and
CMT2 (NCV 25 – 45 m/s). Two genes and one additional locus have been identified
for this group. To further complicate the picture, mutations in a few genes can cause
both demyelinating and axonal neuropathies with NCV ranged from severely reduced
to normal levels. For example, mutations in the MPZ, NEFL, and GDAP1 genes,
which are related to CMT1B, CMT1F, and CMT4A respectively, can also give rise to
an axonal phenotype with normal or slightly decreased NCV. Besides, NCV of
X-linked CMT affected females caused by GJB1/Cx32 mutations are often within the
CMT2 range. What’s more, the distinction between CMT and either distal hereditary
motor neuropathies or hereditary sensory and autonomic neuropathies may not be
always straight forward when either motor or sensory loss predominates.
Initially CMT2A was reported to be related to KIF1B gene mutation in a single
Japanese family.4 However, no further KIF1B mutations were found in other CMT2A
patients. Investigation of other genes in this locus finally identified MFN2 as the
primary gene mutated in CMT2A.5 Mitofusin 2 encoded by MFN2 is a large
mitochondrial outer membrane GTPase, with a large N-terminal and a smaller
C-terminal domain exposed in the cytoplasm. This protein plays an important role in
mediating mitochondrial fusion. It has been postulated that defects in mitochondrial
fusion, oxidative phosphorylation and mitochondrial transport could be implicated in
the pathology of CMT2A.6 Peripheral nerves are particularly involved because the
energy demands are high and the long axons would be mostly affected by the
mitochondrial transport defect.6 In addition to the classical motor and sensory
peripheral neuropathy, extra clinical features, including tremor, scoliosis, optic
atrophy, sensorineural hearing loss, pyramidal signs, cognitive impairment,
parkinsonism and stroke, have also been described in MFN2 mutations.2,7,8 The age of
onset of CMT2A is variable, ranging from 1 to 45 years in one study with 29
probands.2 The clinical features were markedly different for the early onset group
(<10 years) and the late onset group (≥10 years), with a more severe phenotype and
more progressive disease course in the former.2,9
We have identified a novel missense mutation, MFN2 NM_014874.2:c.828G>C
(p.Gln276His). This novel mutation is located in the region linking the GTPase
domain and the first coiled-coil region of mitofusin 2. A known mutation with
different amino acid substitution at the same codon [MFN2 NM_014874.2:c.827A>G
(p.Gln276Arg)] has been reported to be associated with CMT2 and optic atrophy,2,10
though visual impairment is not observed in our patient. A significant proportion of
MFN2 mutations have been shown to be de novo (up to 34%), implying that this gene
may be prone to spontaneous mutations.2,8,9,11 Phenotypic heterogeneity with
intrafamilial variability and incomplete penetrance has also been reported for MFN2
mutations.7,12 In our case, the patient’s mother who carried the same heterozygous
mutation only developed mild symptoms recently at her fifties with normal
electrophysiological study, suggesting noteworthy intrafamilial variability in this
family. The patient’s deceased maternal grandfather might also be affected by
CMT2A in view of his unexplained limping gait.
MFN2 mutations have been described in different ethnic groups, although population
data is still limited. In an American study, MFN2 mutations were detected in 23% of
13 unrelated probands with CMT2.12 MFN2 mutation was found to be the cause of
CMT2 in just one out of 18 families in the Netherlands (5.6%).13 In a study of
Japanese population, seven of 81 unrelated patients with axonal or unclassified CMT
carried MFN2 mutations (8.6%).14 Higher frequencies were revealed in two Korean
studies of CMT2, with MFN2 mutations detected in 24.2% of 62 families and 33.3%
of 12 families respectively.9,15 The different diagnostic yields in these studies could
also be attributed by the variable selection criteria of cases for genetic testing.
CMT is suspected when there are typical clinical phenotype, characteristic nerve
conduction studies findings and positive family history, after excluding common
causes of polyneuropathy like diabetes mellitus, vitamin B12 deficiency and
monoclonal gammopathies. Genetic testing is of paramount importance for
confirming diagnosis, classification and also genetic counseling. The selection of
genetic test should be guided by the clinical phenotype, inheritance pattern,
electrophysiological studies and occasionally sural nerve biopsy if available. Since
CMT2A is the most common type of CMT2, genetic testing for patients suspected of
autosomal dominant hereditary axonal neuropathy should begin with MFN2. A
suggested decision algorithm for selecting genetic tests in evaluation of suspected
hereditary neuropathies has been published recently.3 For those with axonal
neuropathy and autosomal dominant inheritance pattern, MFN2 mutations should be
screened first, followed by MPZ mutations which can give rise to both CMT1 and
CMT2.3 Given that a significant proportion of mutations are de novo, genetic testing
should also be considered for apparently sporadic cases if suspicion of axonal CMT is
high. For these patients, it has been suggested to start with MFN2 and also
GJB1/Cx32, which is the causative gene of CMTX1 with both demyelinating and
axonal phenotypes, and MPZ gene analysis as the second tier testing.3
In conclusion, we have identified a novel mutation, MFN2 NM_014874.2:c.828G>C
(p.Gln276His), in a family with CMT2 phenotype. Intrafamilial variability was
observed in this family. This is the first report of MFN2 mutation in Chinese. Clinical
information, inheritance pattern, nerve conduction studies and molecular analysis are
indispensable for diagnosis of different types of CMT. De novo MFN2 mutations are
not uncommon, and considerable phenotypic heterogeneity within a family could be
present, thus screening of the gene for apparently sporadic cases with typical clinical
phenotype should still be warranted.
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Figure 1. DNA sequencing electrophoretogram of the MFN2 gene shows the mutation
MFN2 NM_014874.2:c.828G>C (p.Gln276His) (arrow) of the index patient in the
sense direction.
Figure 2. Screening of the novel mutation MFN2 NM_014874.2:c.828G>C
(p.Gln276His) in healthy controls using real-time amplification refractory mutation
system polymerase chain reaction. Amplification plots of the index patient, the normal
controls and water blank using primers specific for the mutant (A) and the wild type
(B). Reaction with cycle threshold <30 is considered to be positive. The index patient
sample was positive with both mutant specific and wild type specific primers;
whereas the healthy controls samples were positive with the wild type specific
primers only (arrows).