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Axonal neuropathy with optic atrophy is caused by mutations in mitofusin 2.

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Axonal Neuropathy with Optic Atrophy Is
Caused by Mutations in Mitofusin 2
Stephan Züchner, MD,1 Peter De Jonghe, MD, PhD,2,3 Albena Jordanova, PhD,2,4,5 Kristl G. Claeys, MD, PhD,2,3
Velina Guergueltcheva, MD,4 Sylvia Cherninkova, MD, PhD,4 Steven R. Hamilton, MD,6 Greg Van Stavern, MD,7
Karen M. Krajewski, MS,8 Jeffery Stajich, PA-C,1 Ivajlo Tournev, MD, DSc,4 Kristien Verhoeven, PhD,2
Christine T. Langerhorst, MD,11 Marianne de Visser, MD, PhD,9 Frank Baas, MD, PhD,9,10 Thomas Bird, MD,12
Vincent Timmerman, PhD,2 Michael Shy, MD,8 and Jeffery M. Vance, MD, PhD1
Objective: Charcot-Marie-Tooth (CMT) neuropathy with visual impairment due to optic atrophy has been designated as
hereditary motor and sensory neuropathy type VI (HMSN VI). Reports of affected families have indicated autosomal
dominant and recessive forms, but the genetic cause of this disease has remained elusive. Methods:Here, we describe six
HMSN VI families with a subacute onset of optic atrophy and subsequent slow recovery of visual acuity in 60% of the
patients. Detailed clinical and genetic studies were performed. Results: In each pedigree, we identified a unique mutation
in the gene mitofusin 2 (MFN2). In three families, the MFN2 mutation occurred de novo; in two families the mutation
was subsequently transmitted from father to son indicating autosomal dominant inheritance. Interpretation: MFN2 is a
mitochondrial membrane protein that was recently reported to cause axonal CMT type 2A. It is intriguing that MFN2
shows functional overlap with optic atrophy 1 (OPA1), the protein underlying the most common form of autosomal
dominant optic atrophy, and mitochondrial encoded oxidative phosphorylation components as seen in Leber’s hereditary
optic atrophy. We conclude that autosomal dominant HMSN VI is caused by mutations in MFN2, emphasizing the
important role of mitochondrial function for both optic atrophies and peripheral neuropathies.
Ann Neurol 2006;59:276 –281
Approximately 1 in 2,500 individuals are diagnosed
with Charcot-Marie-Tooth neuropathy (CMT), making peripheral neuropathies one of the most common
hereditary diseases.1 Historically, CMT neuropathies
have been divided by neuropathological and electrophysiological criteria into demyelinating “CMT1” and
axonal “CMT2.”2 However, in some pedigrees, CMT
is known to be associated with a variety of additional
symptoms, such as spastic paraparesis, optic atrophy,
cranial nerve involvement, glaucoma, or neutropenia.2
Repeated clinical observation of these features has lead
to several specifically designated subtypes of CMT. Axonal CMT with optic atrophy is referred to as HMSN
VI.3,4 Autosomal dominant and autosomal recessive
traits have been described for HMSN VI.5,6 The number of reported multigenerational families with HMSN
VI is small; therefore, a genome-wide linkage screen
has never been attempted. The most common type of
pure autosomal dominant optic atrophy is caused by
mutations in the gene optic atrophy 1 (OPA1).7,8 Thus,
genes with a functional or structural relation to OPA1
are potential candidates for HMSN VI. We recently
reported that mitofusin 2 (MFN2) mutations cause the
most common form of autosomal dominant axonal
CMT disease, CMT2A.9 OPA1 and MFN2 are both
dynamin-like GTPases and have complementary functions in fusing the inner and outer mitochondrial
membranes respectively.10,11 Mutations in OPA1 and
mitofusins have been shown to disturb the balance between fusion and fission events in the highly dynamic
mitochondrial network.12 Thus, MFN2 is an excellent
candidate gene for HSMN VI.
We identified six nuclear HMSN VI families of European and African American origin. The probands developed an unusually severe form of axonal neuropathy
with early age of onset and experienced a subacute de-
From the 1Center for Human Genetics, and Department of Psychiatry and Behavioral Sciences, Duke University Medical Center,
Durham, NC; 2Molecular Genetics Department, Flanders Interuniversity Institute for Biotechnology, University of Antwerp; 3Division
of Neurology, University Hospital Antwerp, Antwerp, Belgium;
4
Department of Neurology and 5Laboratory of Molecular Pathology, Sofia Medical University, Sofia, Bulgaria; 6Neuroophthalmology Unit, Neuroscience Institute, Swedish Medical Center, Seattle, WA; 7Kresge Eye Institute and 8Department of
Neurology, Wayne State University School of Medicine, Detroit,
MI; 9Department of Neurology, 10Neurogenetics Laboratory, and
11
Department of Ophthalmology, Academic Medical Center, Am-
sterdam, the Netherlands; and 12Department of Neurology, University of Washington, Geriatric Research Center, VA Medical Center,
Seattle, WA.
276
Received Sep 14, 2005, and in revised form Dec 6. Accepted for
publication Dec 12, 2005.
Published online Jan 23, 2006 in Wiley InterScience
(www.interscience.wiley.com). DOI: 10.1002/ana.20797
Address correspondence to Dr Züchner, Center For Human Genetics, Duke University Medical Center, 595 LaSalle Street, Box 3445
DUMC, Durham, NC 27710. E-mail: szuchner@duke.edu
© 2006 American Neurological Association
Published by Wiley-Liss, Inc., through Wiley Subscription Services
Table. Clinical and Genetic Data of Hereditary Motor and Sensory neuropathy VI Families
Family/No.
of Affected
Median Motor
NCV/Sensory
NCV (m/sec)
MFN2
Mutation
Exon
CMT
CMT-5/2
c.280C3T
p.R94W
4
No response/47 Proximal weakness,
less sensory involvement
8; 3
CMT-52/2
c.617C3T
p.T206I
de-novo
7
No response/no Proximal weakness,
response
paraspinal paresis, mild hearing
loss
3; 3
CMT-273/1
c.827A3G
p.Q276R
9
Upper and lower
extremities severely involved,
wheelchair
bound
Upper and lower
extremities severely involved,
wheelchair
bound
Lower extremities
involved, gait
problems
Pes cavus, steppage
gait
10
CMT-506/1
c.1081C3T 11
p.H361Y
de-novo
No response/no Proximal weakness
response
1
CMT-11/3
c.1090C3T 11
p.R364W
Upper and lower
extremities severely involved,
wheelchair
bound
Upper and lower
extremities severely involved,
wheelchair
bound
Upper and lower
extremities severely involved,
wheelchair
bound
No response/no Proximal weakness,
response
vocal cord paresis, scoliosis
1; 1; 1
DUK2158/1 c.1252C3T 12
p.R418X
de-novo
45.6/no response
53/54
Additional
Symptoms
Onset CMT
(yr)
Proximal weakness,
vocal cord paralysis
1
Optic Atrophy
Onset (yr)
Imaging
Subacute onset, bilateral, se- 50; 40
vere deterioration (20/
400), central scotoma,
color vision partial recuperation,
Subacute onset, bilateral, se- 19; 20
vere deterioration (20/
200), central scotoma, partial recuperation
Cranial MRI
unremarkable
Mild course (20/40), bilateral, visual-evoked potentials prolonged full recuperation (20/20)
Subacute onset, bilateral, severe deterioration (20/
400), central scotoma,
color vision, full recuperation (20/30)
Slow onset, bilateral, severe
deterioration (4/20, 6/60);
VEP weak and prolonged,
No recuperation
20
Cranial MRI
unremarkable
19
Cranial MRI
unremarkable
Slow onset, bilateral, severe
deterioration (20/400);
VEP prolonged, No recuperation
10; 10; 10
5
Cranial CT
(contrast)
unremarkable
-
Cranial MRI:
Increased
T2 signal
in both
cerebellar
peduncles
CMT ⫽ Charcot-Marie-Tooth; AOO ⫽ age of onset; OA ⫽ optic atrophy; NCV ⫽ nerve conduction velocity; CT ⫽ computed tomography;
MRI ⫽ magnetic resonance imaging; VEP ⫽ visual evolved potentials.
crease of visual acuity later in life. We detected six
unique MFN2 mutations in these HMSN VI patients
not present in controls. The identification of the first
gene underlying HMSN VI has implications for both
neurological and ophthalmological practices. It also
raises difficult issues for genetic and prognostic counseling, because certain MFN2 mutations might predict
the later development of severe optic atrophy in affected individuals.
Patients and Methods
Patients
Six nuclear families with moderate to severe axonal CMT
and a history of visual impairment were selected. The families CMT-5, CMT-506, DUK2158 were ascertained in the
United States. CMT-5 was of African American origin; all
other subjects were of European descent. Families CMT-52,
CMT-273, and CMT-11 were identified in Belgium, Bulgaria, and the Netherlands, respectively. Controls were unrelated spouses of CMT families. All samples were collected
with informed consent. The study was approved by each collaborator’s institutional review board or equivalent.
Clinical Examinations
All individuals included in this study were examined by a
neurologist and an ophthalmologist. The axonal type of
CMT was confirmed by electrophysiological studies and in
one patient (DUK2158) by a sural nerve biopsy examination. At least one affected individual per family underwent
cranial magnetic resonance imaging or cranial computer tomography with contrast, except CMT-11.
Mutation Screening
All exons of the MFN2 gene were screened by direct DNA
sequencing. Polymerase chain reaction (PCR) primers used
in this study were previously reported.9 PCRs followed standard protocols. Amplified DNA samples were directly sequenced by applying the BigDye Terminator reaction kit
(Applied Biosystems, Foster City, CA) and then subjected to
an ABI3730 sequencer. The Sequencher software (Gene
Codes, Ann Arbor, MI) was used to assemble the sequence
trace files and to compare the results with the MFN2 reference sequence (GenBank accession no. NM_014874).
Results
Neurological Characterization
The 10 affected individuals included in this study all
had a very early onset of peripheral neuropathy (mean
age, 2.1 ⫾ 3.2 years; range, 1–10 years). The symptoms
severely worsened over the course of the disease, and all
but one patient became wheelchair bound. Patients exhibited marked distal weakness, muscle atrophy, and
foot deformities. In all but family CMT-273, the upper
extremities were severely involved (Table). On neurological examination, we found distal hypoesthesia for light
touch, pinprick, vibration sense, and position sense of
toes and fingers. However, motor symptoms were the
patients primary complaints. All patients had lower limb
or generalized areflexia. All affected individuals but family CMT-273 developed proximal weakness with some
(CMT-11) showing contractures of the major joints,
scapula alatae, lumbar hyperlordosis, and scoliosis. Two
Züchner et al: Mutations in MFN2 Cause HMSN VI
277
families (CMT-11, DUK2158) developed a hoarse voice
and a vocal cord paresis. The individuals in CMT-11
also showed signs of respiratory insufficiency. In CMT52, mild hearing loss was observed, which has not been
described previously for MFN2 mutations. Patient’s
electrophysiological studies all showed nerve conduction
velocities greater than 38m/sec unless they were unobtainable because of atrophy.
Cranial magnetic resonance imaging (MRI) or computer tomography (CT) was unremarkable in all but one
patient. The MRI of the index patient of DUK2158
showed an increased T2 signal in both cerebellar peduncles. Periventricular changes were not present.
Ophthalmological Characterization
All individuals studied developed bilateral optic atrophy during the course of their CMT disease. Visual
deterioration started between early childhood and the
fifth decade (mean age, 19 years ⫾ 14.5 years; range,
5–50 years). Most individuals experienced subacute deterioration of visual acuity. Within several months, visual acuity dropped to as low as 20/400 bilaterally (see
Table). A central scotoma was detected in most patients. Also, most affected individuals reported color vision defects. Ishihara color plate testing scored low in
those patients tested (4/17–7/17). Ophthalmological
examination shows pale optic discs in all individuals.
The macula, peripheral retina, and blood vessels appeared normal. The anterior segment of the eye was
normal and the intraocular pressure was not increased.
Visual-evoked potentials showed decreased amplitudes and a delayed pattern. Over the course of several
years, most individuals (4 of 6 families; 6 of 10 individuals) experienced recovery of visual acuity, color vision, and the central scotoma to normal or near normal
levels (see Table). However, bilaterally pale optic discs
remained in all patients and also visual-evoked potentials were abnormal in three individuals after recovery.
The individuals with onset of visual loss in childhood
(CMT-11 and DUK2158) did not report a subacute
onset of visual impairment but rather a prolonged decline of visual acuity. Some could not point out the exact age of onset of the visual impairment; this might in
part be attributed to the young age of these patients.
Interestingly, these two early-onset families also did not
improve their visual acuity over time. Thus, early age of
onset might predict a poorer outcome of the optic atrophy. In contrast, the two affected individuals of Family
CMT-5 developed optic atrophy only in their forties
and fifties. After subacute bilateral onset, visual acuity
partially recuperated within a few years.
Mutation Analysis
Direct sequencing of all coding exons of the gene
MFN2 showed unique missense and nonsense mutations in all six families (see Table). The identified mu-
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February 2006
tations completely cosegregated with the disease status
in the small pedigrees (Fig 1). The axonal CMT phenotype was fully penetrant. The mutations were not
present in 400 control chromosomes. The segregation
pattern suggested autosomal dominant inheritance
(CMT-52 and CMT-11). In at least three families we
found de novo mutations (CMT-52, CMT-506, and
DUK2158).
In CMT-5, we identified a missense mutation;
c.280C3 T, p.R94W. CMT-52 carried a de novo missense mutation; c.617C3 T, p.T206I. In CMT-273,
we detected a c.827A3 G, p.Q276R change. The patient in family CMT-506 had a de novo missense mutation; c.1081C3 T, p.H361Y. In family CMT-11, we
identified a new missense mutation, c.1090C3 T,
p.R364W. DUK2158 carried a de novo nonsense mutation; c.1252C3 T, p.R418X.
Mutations in the mitochondrial genome were excluded in CMT-5, CMT-506, and DUK2158.
Discussion
Optic atrophy is a severe condition with deterioration
of visual acuity and limited therapeutic options. Clinical variants range from subacute episodes to chronic
progressive courses. Not surprisingly, all of our patients
initially were diagnosed clinically with Leber’s hereditary optic atrophy (OMIM#535000), but mitochondrial mutations were subsequently excluded in three of
six families. In six HMSN VI families, we found five
unique missense mutations (R94W, T206I, Q276R,
H361Y, and R364W) and one nonsense mutation in
MFN2 (R418X) (see Fig 1, Table). These mutations
have not been reported for “pure” CMT2A with the
exception of R94W.9,13,14 The R94W change in
CMT-5 was associated with late onset of optic atrophy
(see Table). We have previously identified the R94W
mutation as a de novo occurrence in a patient with severe and early onset CMT2A (DUK2173).9 This patient is now in their late thirties and currently has no
visual complaints; however, he was not available for
ophthalmological examination.
In Family CMT-273, we found evidence for incomplete penetrance of visual impairment, whereas the
CMT phenotype was expressed in all mutation carriers
(see Fig 1). Compared with the other HMSN VI families, CMT-273 had a milder phenotype for both axonal CMT and optic atrophy. The family lives in a
remote area of Bulgaria and only the index patient was
available for ophthalmological testing. We think that
CMT-273 is part of a clinical spectrum of HMSN VI
phenotypes that ranges from most severe to subclinical
changes. Moreover, incomplete penetrance has been reported previously in HMSN VI.15
In three nuclear families (CMT-5, CMT-52, and
CMT-11), all patients showed very similar interfamilial
clinical courses, suggesting a close genotype–phenotype
Fig 1. Summary of identified pedigrees and associated MFN2 mutations. (A–F) Pedigree and the identified mutation compared to
a healthy control. The numbers below affected individuals (solid symbols) provide the age of onset in years for Charcot-MarieTooth (CMT)/optic atrophy. (A, D) Ophthalmological examination revealed bilaterally pale optic discs (arrows). (B) Goldman visual fields showed central scotoma in both eyes (black and gray central area). (C) Two brothers were equally severe affected by
CMT. Note the prominent muscular wasting in the forearms and hands.
Züchner et al: Mutations in MFN2 Cause HMSN VI
279
correlation, although the number of affected individuals was limited.
The male patient from Family DUK2158 carried a
nonsense mutation (c.1252C3 T, p.R418X), which
presumably resulted in a truncated MFN2 protein lacking the transmembrane domain and fzo_mitofusin domain at the C-terminal end. Loss of the anchoring transmembrane domain will result in dislocation of MFN2
from the mitochondrial membrane. The other missense
mutations were located within the GTPase domain and
C-terminal of the transmembrane domain (Fig 2). All
MFN2 mutations were well conserved throughout different species (see Fig 2B). However, the location of the
mutations did not provide much insight into why these
mutations were associated with optic atrophy.
Examining for functional overlap between MFN2
and other known genes leading to optic atrophy could
reveal more insight into this question. The dynaminlike GTPase MFN2 (CMT type 2A) belongs to the
same protein family as OPA1 (autosomal dominant
optic atrophy 1), and both factors act in concert in
mediating the fusion of mitochondrial double membranes.16 In addition, mutations in the mitochondrial
oxidative phosphorylation complexes I and III are frequent causes for Leber’s hereditary optic atrophy.17,18
It has been shown that the mitochondrial inner membrane potential but not a functional respiratory chain is
required by MFN2 to mediate fusion of mitochondria.19 Nevertheless, MFN2 function recently has been
linked with the regulation of expression of nuclearencoded mitochondrial oxidative phosphorylation complexes.20 Thus, although our results underscore the
Fig 2. Location of MFN2 mutations in hereditary motor and sensory neuropathy type VI, conservation in different species, and illustration of mitochondrial defects causing optic atrophy. (A) Identified mutations in relation to the exons of MFN2 (black numbered boxes) and in relation to protein domains (Cc ⫽ coiled coil; TM ⫽ transmembrane domain). (B) The identified mutations
represent highly conserved protein residues in MFN2. (C) Molecular basis of the three mitochondrial diseases leading to optic atrophy. All proteins are in contact with the inner mitochondrial membrane, which is essential for the function of mitochondrial oxidative phosphorylation complexes and ATP production.
280
Annals of Neurology
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February 2006
central role of mitochondrial dysfunction for the development of optic atrophy (see Fig 2),21,22 currently no
specific mitochondrial function appears to be the key
factor leading to this symptom.
Spontaneous recovery of vision is not a feature of
autosomal dominant optic atrophy due to OPA1 mutations.22 However, in Leber’s hereditary optic atrophy
patients, improved visual acuity has been reported.23 In
our sample, 6 of 10 (60%) patients experienced significant recovery of their visual acuity to normal or near
normal levels within several years (see Table). Thus,
recovery of visual acuity might delineate a specific but
not yet recognized subtype of HMSN VI with functional deficits in the optic nerve similar to those in
Leber’s hereditary optic atrophy. We speculate that
some nerve fibers bundles in the optic nerve might
have a greater potential to regenerate and therefore restore visual acuity over a prolonged time. Degeneration
and regeneration of nerve fibers are prominent histopathological features in the peripheral nerve of axonal
CMT forms, such as CMT2A.
Finally, since Bird and Griep found 4 of 25 subjects
with CMT having abnormal visual-evoked responses,24
the prevalence of HMSN VI might actually be higher
than expected. Interestingly, and in contrast with pure
axonal CMT type 2A, three of six mutations occurred de
novo. This could explain why reports on HMSN VI are
rare and mostly confined to small pedigrees. In knowing
the underlying gene for HMSN VI, we were able to
identify many affected families not previously reported.
We anticipate that our findings will stimulate genetic
screening of unresolved patients with clinical symptoms
of both the peripheral nervous system and optic nerve.
The project was supported by donations from CMT families to the
Center for Human Genetics, by the NIH (NS26630, J.V.) and additional NINDS grants (J.V.), the Fund for Scientific Research–Flanders, the Medical Foundation Queen Elisabeth, the University of
Antwerp, the Interuniversity Attraction Poles program of the Belgian
Federal Science Office (V.T., P.D.J.), the Netherlands Organization
for Health Research and Development, and by VA Research Funds,
(F.B.) A.J. received visiting research fellowships from the POD and
NATO/FWO. K.V. is a postdoctoral fellow of the FWO, Belgium.
We thank the patients and their family members for their participation in this study. We also thank Dr T. L. Young for a critical
review of the manuscript. We appreciate the technical assistance of
E. De Vriendt for DNA sequencing.
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