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Clinical and genetic study of autosomal recessive cerebellar ataxia type 1.

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Clinical and Genetic Study of Autosomal
Recessive Cerebellar Ataxia Type 1
Nicolas Dupré, MD, MSc,1,2 François Gros-Louis, PhD,2 Nicolas Chrestian, MD,1 Steve Verreault, MD, MSc,1
Denis Brunet, MD,1 Danielle de Verteuil, BSc,3 Bernard Brais, MD, PhD,3 Jean-Pierre Bouchard, MD,1
and Guy A. Rouleau, MD, PhD2
Objective: Define the phenotype and genotype of a cluster of families with a relatively pure cerebellar ataxia referred to as
autosomal recessive cerebellar ataxia type 1 (ARCA-1).
Methods: We ascertained 64 probands and affected members of 30 French-Canadian families all showing similar clinical features
and originating from the same region of Quebec. After informed consent, we performed detailed clinical history, neurological
examination, brain imaging, nerve conduction studies, and SYNE1 mutation detection of all available subjects.
Results: Based on the cases examined, ARCA-1 is a cerebellar syndrome characterized by recessive transmission, middle-age
onset (mean, 31.60; range, 17– 46 years), slow progression and moderate disability, significant dysarthria, mild oculomotor
abnormalities, occasional brisk reflexes in the lower extremities, normal nerve conduction studies, and diffuse cerebellar atrophy
on imaging. We identified a total of seven mutations in our population, thereby providing evidence of genotypic heterogeneity.
Patients with different mutations did not show significant phenotypic heterogeneity.
Interpretation: We identified a cluster of French-Canadian families with a new recessive ataxia of relatively pure cerebellar type
caused by mutations in SYNE1. The function of SYNE1 is thus critical in the maintenance of cerebellar structure in humans.
We expect that this disease will be a common cause of middle-age-onset recessive ataxia worldwide.
Ann Neurol 2007;62:93–98
The hereditary ataxias can be divided based on their
mode of inheritance into autosomal dominant, autosomal recessive, X-linked, and mitochondrial ataxias.
These disease categories share the prototypic feature of
impaired walking, although they usually present a variety of other neurological symptoms such as pyramidal
signs, peripheral neuropathy, extrapyramidal signs, cognitive loss, or retinopathy. The autosomal recessive
ataxias are also a heterogeneous group of disorders
composed mainly of Friedreich’s ataxia, ataxia telangiectasia, ataxia with vitamin E deficiency, autosomal recessive spastic ataxia of Charlevoix–Saguenay (ARSACS),
abetalipoproteinemia, and ataxia with oculomotor
apraxia types 1 and 2.1
Over the past decade, we have identified a large cluster of French-Canadian families whose ancestors originate mostly from the same region of the Province of
Quebec in Canada. This region, located in southeastern Quebec near the US border, is called Beauce. The
affected individuals in these families all share similar
clinical characteristics that define this new disease entity we named ARCA-1, also known as recessive ataxia
of Beauce.2,3 Genome-wide linkage and fine-mapping
analysis on selected families with ARCA-1 established a
minimum candidate interval of about 0.5Mb on chromosome 6q containing a single causal gene (SYNE1).4
Spanning over 0.5Mb genomic DNA, SYNE1 is one of
the biggest genes in the human genome formed of 147
exons that encodes a 27,652kb messenger RNA and an
8,797 amino-acid-long protein. To date, five different
truncating mutations within SYNE1 have been described.4 In this article, we report two novel mutations,
also leading to premature termination of the protein,
and define the full clinical and molecular spectrum of
ARCA-1.
From the 1Faculty of Medicine, Laval University, Department of
Neurological Sciences, Centre Hospitalier Affilié Universitaire de
Québec–Enfant-Jésus, Quebec City; 2Center for the Study of Brain
Diseases, Université de Montréal, Centre Hospitalier de l’Université
de Montréal (Notre-Dame); and 3Laboratory of Neurogenetics of
Motion, Center for the Study of Brain Diseases, Université de Montréal, Centre Hospitalier de l’Université de Montréal (Notre-Dame),
Montréal, Québec, Canada.
Received Dec 22, 2006, and in revised form Feb 8, 2007. Accepted
for publication Mar 2, 2007.
Subjects and Methods
Subjects were referred to the study protocol by their treating
physician based on a preliminary assessment consistent with
the core features of ARCA-1 (ataxia and dysarthria of
middle-age onset with cerebellar atrophy) and a recessive
family history (parents unaffected). They were informed of
the procedures entailed in the protocol and signed, before
their participation, a consent form approved by the local eth-
Published online May 14, 2007, in Wiley InterScience
(www.interscience.wiley.com). DOI: 10.1002/ana.21143
Address correspondence to Dr Dupre, Department of Neurological
Sciences, CHAUQ–Enfant-Jésus, 1401, 18th Street, Quebec City,
QC, Canada, G1J 1Z4. E-mail: nicolas.dupre@cha.quebec.qc.ca
© 2007 American Neurological Association
Published by Wiley-Liss, Inc., through Wiley Subscription Services
93
ics review boards (Centre Hospitalier de l’Université de
Montréal and Centre Hospitalier Affilié Universitaire de
Québec). All 64 available affected and unaffected members
of the 30 recruited families underwent a thorough neurological examination and were examined independently by at
least two neurologists. We also used an assessment scale5 to
grade severity of symptoms: dysarthria (0 ⫽ no impairment;
1 ⫽ mild dysarthria but comprehensible; 2 ⫽ moderate dysarthria with interruption in flow; 3 ⫽ severe dysarthria and
incomprehensible, very difficult to understand; 4 ⫽ completely unintelligible); dysmetria (0 ⫽ no impairment; 1 ⫽
mild dysmetria but reaches the target; 2 ⫽ moderate dysmetria, reaches target after several attempts; 3 ⫽ severe dysmetria, short of target after many attempts; 4 ⫽ cannot use
hands); gait (0 ⫽ normal; 1 ⫽ stance width increased,
mildly unstable gait but can walk without support; 2 ⫽
moderately unstable gait and needs support for walking; 3 ⫽
unable to walk, needs the assistance of two persons; 4 ⫽
wheelchair bound). Twenty-two subjects underwent electrophysiological studies including compound motor action potentials of median, ulnar, tibial, and peroneal nerves, as well
as sensory nerve action potentials of median, ulnar, radial,
and sural nerves, with standard values for filters, stimulus
duration, and electrode positioning. Brain imaging with
magnetic resonance imaging was performed on 50 affected
subjects.
On receipt of informed consent, blood samples were obtained from affected individuals in 30 families. DNA was
extracted from peripheral blood by standard methods.6 For
mutation screening, a set of 154 polymerase chain reaction
(PCR) primer pairs were designed from genomic DNA to
amplify each exon of the SYNE1 gene, including the flanking
splice sites and the untranslated regions.4 Products were
PCR-amplified, checked on agarose gels, and then sequenced
using the forward primers for all of the amplicons. Each fragment containing mutations was PCR-amplified a second
time and sequenced with the reverse primer to confirm that
the identified mutations were not due to PCR artifact.
Results
Sample Case
This 42-year-old woman originates from the Beauce region in Quebec. On her initial visit, she mentioned
that around the age of 30 she started noticing that her
speech was slurred. During the same period, she also
noticed some mild walking impairment. As she advanced in her thirties, the slurred speech and walking
impairment progressed. Strangers would sometimes
wonder if she had been drinking alcohol, and her gait
was becoming more wide based with a tendency to fall
if she did not pay more attention than usual during her
movements. In addition, her hands sometimes felt
clumsier. On neurological examination at age 42, she
had normal funduscopy, normal ocular saccades and
pursuit, and no nystagmus. She had significant cerebellar dysarthria. Strength was normal throughout, with
no spasticity. Reflexes were normal in the upper and
lower limbs, with down-going toes. Sensory examination was normal to pain, temperature, vibration, pro-
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prioception, and light touch. Gait was broad based.
There was significant dysmetria on finger-to-nose and
heel-to-shin, as well as cerebellar hypotonia (Holmes’
sign) in the upper limbs. Magnetic resonance imaging
demonstrated diffuse cerebellar atrophy, and nerve
conduction studies were normal. Genetic testing confirmed that she was homozygous for the g.306434A⬎G
mutation.
Overall, the ARCA1 phenotype consists of a middleage onset disease that presents with either dysarthria,
cerebellar ataxia, or both coincidentally (Table 1 and
Fig 1). Over time, all patients develop significant dysarthria and ataxia, with other associated features such
as dysmetria, brisk lower extremity tendon reflexes, and
minor abnormalities in saccade and smooth pursuit.
None of the subjects evaluated showed extrapyramidal
signs, cognitive loss, retinopathy, cardiomyopathy, sensory abnormalities, or autonomic disturbances. The
disease progresses slowly and evolves into a moderate
degree of disability. There appears to be no effect on
life expectancy. Nerve conduction studies performed
on 22 affected individuals were always normal, showing therefore no sign of a peripheral sensory or motor
neuropathy in this disease. Single-fiber electromyography was also normal when performed on one subject,
suggesting preserved function at the neuromuscular
junction. Imaging findings on 50 affected individuals
invariably showed marked diffuse cerebellar atrophy
(Fig 2). On detailed review of imaging in 38 affected
subjects, there was no cerebral cortical atrophy; no
midbrain, pontine, or bulbar atrophy; no atrophy of
inferior olives; and no white matter changes.
Genetic analysis demonstrated seven mutations (Table 2 and Fig 3), including five previously described
mutations4 and two novel truncating mutations
(g.409218C⬎T and g.281100-281101delTG). The
most frequent mutation, g.306434A⬎G, was present
homozygously in 20 of 64 subjects (31.2%). This same
mutation was present heterozygously with the
g.310067A⬎G mutation in 11 of 64 (17.2%), with
the g.247012A⬎T mutation in 6 of 64 (9.4%), with
the g.426494C⬎T mutation in 1 of 64 (1.6%), and
with the g.334338-334342delATTTG mutation in 4
of 64 (6.2%). Of these 64 subjects, there was 1 (1.6%)
g.310067A⬎G homozygote, 3 (4.7%) g.247012A⬎T
homozygotes, and 1 (1.6%) g.247012A⬎T/
g.409218C⬎T heterozygote. Finally, 6 of 64 (9.4%)
carried a known mutation on one chromosome and an
unknown mutation on the other chromosome, whereas
in 9 (14.0%) the mutation was not found using the
technique described earlier and in 2 (3.1%) genetic
testing could not be performed. The g.306434A⬎G
mutation was present on 50.8% of chromosomes
tested, whereas mutations remain to be identified in
close to one-fifth of carrier chromosomes. We performed genotype–phenotype correlation studies by seg-
Table 1. Clinical Results of Patients with ARCA-1 (n ⴝ 64)
Sex
Male: 37
(58%)
Female: 27
(42%)
First complaint
Dysarthria: 8
(12.5%)
Ataxia: 40
(62.5%)
Both: 16
(25%)
Age of onset (dysarthria), yr
Mean: 34.79
SD: 7.62
Range: 17–50
Age of onset (ataxia), yr
Mean: 31.60
SD: 7.81
Range: 17–45
Age at evaluation, yr
Mean: 45.36
SD: 10.71
Range: 24–69
Duration at evaluation, yr
Mean: 14.33
SD: 9.77
Range: 3–40
Dysarthria
Mean: 1.78
Range: 0–3
Dysdiadochokinesis
Mean: 1.26
Range: 0–2
Dysmetria
Mean: 1.44
Range: 0–3
Gait
Mean: 1.48
Range: 0–3
Assessment scale
Neurological examination
Dysarthria
64 (100%)
Nystagmus
6 (9.4%)
Slow saccades
20 (31.2%)
Abnormal pursuit
28 (43.8%)
Brisk reflexes in the lower limbs
21 (32.8%)
Ankle clonus and/or Babinski sign
Dysmetria on finger-to-nose
4 (6.2%)
58 (90.6%)
Dysmetria on heal-to-shin
58 (90.6%)
Ataxia
63 (98.4%)
SD ⫽ standard deviation.
regating based on the most common genotypes
(g.306434A⬎G homozygotes and g.306434A⬎G/
g.310067A⬎G heterozygotes) using the following parameters: age of onset (dysarthria, ataxia, overall), disease duration, and eye movement abnormalities. This
analysis gave no statistically significant differences. In
Fig 1. Age of onset by decade.
addition, when we analyzed the clinical data of patients
bearing the less common genotypes, we were unable to
demonstrate that they showed any atypical clinical features.
Discussion
ARCA-1 is thus a new recessive relatively pure cerebellar ataxia that is caused by various mutations in
SYNE1. ARCA-1 shows relative homogeneity of the
phenotype, despite being caused by more than seven
different mutations. The age of onset does not vary
significantly in function of given mutations like in
trinucleotide repeat disorders, whereas the disease
shows little associated features accompanying the core
symptoms of dysarthria and dysmetria. SYNE1 encodes
a protein of about 8,797 amino acid residues
(⬎1,000kDa).4 The protein contains two N-terminal
actin-binding regions that comprise tandem paired
calponin-homology domains, a transmembrane domain, multiple spectrin repeats, and a C-terminal Klarsicht domain. Although SYNE1 is expressed in multiple tissues, its greatest level in the central nervous
system of mice is in the cell bodies of the Purkinje cells
Dupré et al: SYNE1-Related Cerebellar Ataxia
95
Fig 2. Magnetic resonance imaging of a 43-year-old autosomal
recessive cerebellar ataxia type 1 (ARCA-1) patient after 5
years of disease evolution. Sagittal T1 shows marked diffuse
cerebellar atrophy with no cerebral cortical atrophy and no
midbrain, pontine, or bulbar atrophy.
and in neurons of the olivary region of the brainstem,
whereas in humans it is also expressed predominantly
in the cerebellum; it is not expressed in glial cells. In
the peripheral nervous system, SYNE1 is involved in
anchoring specialized myonuclei underneath the neuromuscular junctions.4 It was found in a muscle biopsy
of an ARCA-1 patient that fewer myonuclei come to
lie beneath the neuromuscular junction, although this
has no consequences clinically, electrophysiologically,
or ultrastructurally. SYNE1 is part of the spectrin family of structural proteins that share a common function
of linking the plasma membrane to the actin cytoskeleton. This family also includes dystrophin (Duchenne’s
and Becker’s muscular dystrophies),7 SPTBN2 (spinocerebellar ataxia type 5),8 PLEKHG4 (16q-autosomal
dominant cerebellar ataxia),9 and Spnb4.10
The closest description of ARCA-1 phenotype in the
literature is Holmes’ hereditary ataxia. Holmes’ type of
hereditary ataxia11 was described a century ago (1907)
in a family of eight siblings, with four of them presenting with middle-age-onset dysarthria, ataxia, and hypogonadism (not present in ARCA-1). The inheritance
pattern was most likely autosomal recessive, because
the parents were not affected. Autopsy of one affected
case showed diffuse cerebellar atrophy with no pontine
or olivary involvement. To our knowledge, no linkage
or gene defect responsible for this type of ataxia has
been reported as yet.
ARSACS is the most common of all autosomal recessive ataxias in Quebec with more than 300 affected
individuals. ARSACS patients exhibit early-onset signs
of spasticity in the lower limbs usually observed at gait
initiation (12–18 months).12 The clinical picture noticed by parents from early childhood is always that of
a gait ataxia with a tendency to fall. Nerve conduction
studies demonstrate signs of progressive axonal sensorimotor neuropathy. Ataxia with oculomotor apraxia
type 2 is also present in Quebec, where more than 10
families have been described. It is characterized mainly
by cerebellar atrophy, axonal sensorimotor neuropathy,
and increased serum ␣-fetoprotein.13 The main differences clinically between these other recessive ataxias
common in Quebec and ARCA-1 is the earlier age of
onset, the greater degree of disability, and the associated peripheral neuropathy.
Of interest, 16q-autosomal dominant cerebellar
ataxia is characterized by an age of onset older than 55
years and sensorineuronal hearing impairment,9 which
Table 2. Known Mutations Causing ARCA-1
Variantsa
Exons/Introns
Protein Changes
Total Carrier Chromosomes
Tested in the Patient Population
(n ⴝ 124)
g.306434A⬎G
Intron 81
Premature stop at position 5244
63 (50.8%)
g.310067A⬎G
Intron 84
Premature stop at position 5402
13 (10.5%)
g.247012A⬎T
Exon 56
R2906X
16 (12.9%)
g.426494C⬎T
Exon 126
Q7640X
2 (1.6%)
g.334338-334342delATTTG
Exon 93
Premature stop at position 5880
4 (3.2%)
g.409218C⬎T
Exon 118
Q7386X
1 (0.8%)
g.281100-281101delTG
Exon 71
4077X
1 (0.8%)
Unknown
24 (19.3%)
a
Variants were named according to the genomic DNA sequence NM_033071; nucleotide “A” from the ATG initiation codon is referred
as 1. Allele frequencies of variants were 0 of 380 French-Canadian control chromosomes.
96
Annals of Neurology
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Fig 3. SYNE1 identified truncating mutations. (A) Sequence traces of healthy individual (top) and ARCA-1 patients (bottom)
showing the novel detected mutations within exons 71 and 118 of SYNE1. (B) Protein structure of SYNE-1. Known ARCA-1
disease causing mutations are in black, and newly identified mutations lie within boxes. Light gray region defines areas rich in
predicted spectrin repeats; gray boxes at the N-terminal part of the protein correspond to calponin-homology domains involved in
actin binding; dark gray box indicates the C-terminal nuclear envelope binding domains containing sequences homologous to the
Drosophila protein Klarsicht.
is different from what we found in ARCA-1. However,
there are similarities in that it is also a relatively pure
cerebellar syndrome caused by mutations in PLEKHG4, also part of the spectrin family of structural
proteins. On pathology, 16q-autosomal dominant cerebellar ataxia shows peculiar degeneration of Purkinje
cells that undergo shrinkage and are surrounded by
amorphous material. Spinocerebellar ataxia type 5 is
characterized by a slowly progressive cerebellar syndrome beginning mostly in the third decade.14,15 The
most consistent clinical feature is downbeat nystagmus,
whereas other common features included gait, stance,
and limb ataxia; dysarthria; intention tremor and resting tremor; impaired smooth pursuit; and gaze-evoked
nystagmus. Symptom progression is slow, and all patients remain ambulatory despite disease duration of up
to 30 years. Magnetic resonance imaging shows atrophy of the cerebellar vermis and hemispheres. Again,
this other ataxia caused by mutations in a spectrin family protein shows striking similarities with ARCA-1 for
the predominant cerebellar involvement, middle-age
onset, relatively slow progression, and moderate degree
of disability.
In conclusion, ARCA-1, spinocerebellar ataxia type
5, and 16q-autosomal dominant cerebellar ataxia taken
together allow us to define a new category of hereditary
ataxias related to the spectrin family of structural proteins. Despite significant genetic heterogeneity, this
category of ataxia shares many common clinical features. We expect that this new category of inherited
ataxias may be more frequent than previously thought,
mainly through the contribution of ARCA-1, because
we have encountered important genetic heterogeneity
even within a homogeneous founder population. We
speculate that a significant proportion of yet undiagnosed recessive or “sporadic” ataxias may be due to
SYNE1 mutations, which would have great repercussions on our ability to diagnose more precisely these
ataxia types in specialized clinics worldwide.
This work was supported by the National Ataxia Foundation,
(G.A.R.), the Canadian Genetic Disease Network, (G.A.R.), the Canadian Institute of Health Research (F.G.L., N.D.) and the Association des Ataxies Familiales (D.V.).
F. Gosselin and M. Plante performed blood collection of patients
and obtained their consent. Finally, we also thank the family members who participated in this study.
Dupré et al: SYNE1-Related Cerebellar Ataxia
97
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