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Missense CACNA1A Mutation Causing Episodic Ataxia Type 2

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Missense CACNA1A Mutation Causing Episodic Ataxia Type 2 OBSERVATION Missense CACNA1A Mutation Causing Episodic Ataxia Type 2 Christian Denier, MD; Anne Ducros, MD, PhD; Alexandra Durr, MD, PhD; Bruno Eymard, MD; Be´ne´dicte Chassande, MD; Elisabeth Tournier-Lasserve, MD Objectives: To characterize the nature of CACNA1A mu- Results: A CACNA1A missense mutation, Glu 1757 Lys, tation in a previously unreported family with episodic ataxia was identified. It was absent in 200 control chromo- type 2 (EA2) and to better delineate EA2 clinical features. somes. It is predicted to result in an amino acid substi- tution at a highly phylogenetically conserved position, Background: Episodic ataxia type 2 is an autosomal domi- within a domain that plays a major role in the function nant disorder characterized by the recurrence of acetazol- of the channel. amide-responsive spells of cerebellar ataxia, usually start- ing during childhood or adolescence. The mutated gene, Conclusions: The Glu 1757 Lys missense mutation is CACNA1A, is located on chromosome 19 and encodes likely to be pathogenic, causing episodic ataxia within a the a1A subunit voltage-dependent calcium channel. So family whose phenotype is indistinguishable from EA2 far, most CACNA1A mutations detected in patients with except for a slightly later age of onset. These data strongly EA2 have led to a truncated CACNA1A protein, whereas suggest that additional work is needed to fully establish missense mutations cause familial hemiplegic migraine. genotype/phenotype correlations for CACNA1A muta- tions. Methods: All 47 exons of CACNA1A were screened by a combination of single-strand conformer polymor- phism and sequencing analysis. Arch Neurol. 2001;58:292-295 UTOSOMAL dominant epi- permanent gait ataxia. In a minority of pa- sodic ataxias are a clini- tients, permanent symptoms are severe, cally and genetically het- and some patients may be wheelchair con- erogeneous group of fined.2,8 Cerebral imaging often reveals a conditions characterized vermian cerebellar atrophy. Aby recurrent paroxysmal attacks of cer- ebellar ataxia starting during childhood or For editorial comment adolescence. Episodic ataxia type 1, a con- see page 179 dition characterized by the association of brief ataxic spells and interictal myokym- Episodic ataxia type 2 is caused by mu- ias, is caused by mutations within a voltage- tations within the a1A subunit of a P/Q- gated potassium channel gene, KCNA1. Epi- type voltage-dependent calcium channel sodic ataxia type 2 (EA2) is an autosomal gene, CACNA1A.6 P/Q type channels, which Institut National de la Sante´ dominant paroxysmal cerebellar ataxia, are expressed throughout the brain and at et de la Re´che´rche Me´dicale characterized by acetazolamide-respon- the neuromuscular junction, are impli- (INSERM) EPI 99-21, Faculte´ sive recurrent attacks of unsteadiness, lack cated in the control of membrane excit- de Me´decine Lariboisie`re of limb coordination, and dysarthria, of- ability and neurotransmitter release.10 So far, (Drs Denier, Ducros and ten provoked by emotional or physical 11 EA2 mutations have been reported, most Tournier-Lasserve), INSERM stress.1 Other symptoms during attacks in- of them leading to a truncated CACNA1A U289 (Dr Durr), Service de clude vertigo or dizziness, visual distur- protein.6-9 Interestingly, distinct types of Neurologie du Pr J-Y, Delattre, bances (diplopia or oscillopsia), and head- CACNA1A mutations have been reported Hoˆpital de la Salpeˆtrie`re, ache.1-9 Attacks last from several minutes to in other autosomal dominant neurologi- (Drs Eymard and Chassande), and Laboratoire de a few hours or days. Clinical onset occurs cal conditions. Chromosome 19–linked fa- Cytoge´ne´tique, Hoˆpital usually during childhood or adoles- milial hemiplegic migraine is caused by mis- 1-9 6,11-13 Lariboisie`re cence. Findings of interictal neurologi- sense mutations. Small expansions of (Dr Tournier-Lasserve), cal examination usually disclose a gaze- the CAG repeat located within the 39 cod- Paris, France. evoked nystagmus and sometimes a mild ing end of CACNA1A cause spinocerebel- (REPRINTED) ARCH NEUROL / VOL 58, FEB 2001 WWW.ARCHNEUROL.COM 292 ©2001 American Medical Association. All rights reserved. Downloaded From: https://jamanetwork.com/ on 09/26/2021 lar ataxia type 6,14 a late-onset, moderate to severe pro- gressive cerebellar ataxia, without paroxysmal event. I 1 2 However, these strong genotype/phenotype correlations may not be absolute. Two families with a permanent pro- II gressive cerebellar ataxia, associated with paroxysmal ataxic 1 234 5 6 episodes, and 1 family with pure episodic ataxia were shown to harbor CAG repeat expansions.15-16 More recently, a III 4 2 4 2 CACNA1A missense mutation was shown to cause a se- 1 2 vere progressive cerebellar ataxia with early onset in sev- 17 IV eral members of a family. 123 We report herein a missense CACNA1A mutation causing episodic ataxia within a family whose pheno- Figure 1. Family pedigree. Squares indicate male family members; circles, female; open symbols, unaffected individuals; solid symbols, affected; type is indistinguishable from EA2 except for a slightly diamond-shaped open symbols with enclosed numbers, number of later onset. unaffected children whose sex was not specified. REPORT OF CASES cal examination results disclosed a gaze-evoked nystagmus and a mild statokinetic cerebellar ataxia. PEDIGREE OTHER AFFECTED FAMILY MEMBERS This family included 4 symptomatic members (Figure 1, individuals I-2, II-1, II-5, and III-2). Detailed clinical in- The proband reported that his father (individual II-1, Fig- formation was obtained from patient III-2 and 2 of his 3 ure 1) experienced similar attacks of episodic ataxia since sons who were clinically examined. Clinical informa- his 40s, with an average frequency initially close to twice tion regarding patients I-2, II-1, II-5 was obtained from a month and increasing with age. Attacks usually lasted patient III-2 (Figure 1). 2 hours, precipitated by physical exercise or emotional stress, and disappeared with sleep. Acetazolamide treat- PROBAND ment had never been tried. He had not developed any progressive severe ataxia or gait disorder by the time of Proband III-2 (Figure 1), a 53-year-old man, experi- his death at age 76 years (of prostate cancer). The pro- enced recurrent episodes of paroxysmal cerebellar ataxia band’s grandmother (Figure 1, I-2) and paternal aunt (Fig- since he was 40 years old. His medical history was un- ure 1, II-5) also exhibited paroxysmal attacks of gener- remarkable except for a strabismus, which needed sur- alized ataxia with late onset (after age 30 years ) and gical repair at age 22 years. Ataxic spells were strongly without permanent severe gait disorders. stereotyped. Onset was sudden with brief bilateral par- The proband had 3 sons, aged 12 (Figure 1, IV-1), esthesias in upper and lower extremities, diffuse weak- 20 (Figure 1, IV-2), and 21 (Figure 1, IV-3) years. All 3 ness, and heat sensations rapidly followed by general- were asymptomatic. Findings from clinical examina- ized ataxic symptoms. Attacks always included severe tion of patients IV-1 and IV-2 were normal. Patient IV-3 truncal and limb ataxia with dysarthria, vertigo, and os- was not examined. cillopsia and diplopia sometimes associated with nau- sea, vomiting, and blurred vision. The patient reported GENETIC ANALYSIS headaches fulfilling International Headache Society cri- teria for migraine without aura, both during and be- Samples of DNA from the proband (Figure 1, III-2) and tween ataxic spells. Duration of ataxic episodes usually 2 of his 3 sons (Figure 1, IV-1 and IV-2) were extracted ranged from half an hour to 4 hours. They were precipi- from peripheral blood using standard procedures. In ad- tated by emotional and physical stress and spontane- dition, DNA samples from 100 unrelated healthy sub- ously resolved with rest or sleeping. jects (white French individuals, to match the proband This patient suffered from 3 to 4 attacks per month, family) were also available for the study. All 47 exons of up to 1 per day in stressful periods. He first presented to CACNA1A were screened using a combination of single- us in 1996 at age 50. Findings of interictal examination strand conformer polymorphism18 and sequencing analy- disclosed an isolated, gaze-evoked nystagmus. The re- sis as previously described.11 mainder of his neurological examination results were normal. Brain magnetic resonance imaging revealed a RESULTS moderate vermian cerebellar atrophy. Findings of elec- troencephalographic and electromyelographic studies A missense mutation was identified within exon 35 in were normal. The patient began treatment with 250 mg the proband’s DNA. This G→A substitution at codon 1757 of acetazolamide twice a day, and reported a marked de- (GAA→AAA; Figure 2 and Figure 3) leads to the re- crease in severity and frequency of attacks (once a month) placement of a highly conserved preexisting glutamic acid during 1 year, but no improvement on isolated mi- for a lysine. This mutation was absent in the panel of 200 graine episodes. In 1997, he stopped treatment during 2 normal chromosomes, as well as in the 2 asymptomatic months and experienced an outbreak of attacks; fre- sons of the proband (Figure 1, IV-1 and IV-2). The num- quency of these attacks decreased with the reinstate- ber of CAG repeats at the 39 coding end of the gene was ment of treatment. Two years later, interictal neurologi- in the normal range (7/13). (REPRINTED) ARCH NEUROL / VOL 58, FEB 2001 WWW.ARCHNEUROL.COM 293 ©2001 American Medical Association. All rights reserved. Downloaded From: https://jamanetwork.com/ on 09/26/2021 Domain I Domain II Domain III Domain IV Extracellular Membrane S1 S2 S3 S4 S5 S6 S1 S2 S3 S4 S5 S6 S1 S2 S3 S4 S5 S6 S1 S2 S3 S4 S5 S6 Intracellular N C Figure 2. Schematic structure of the a1A calcium channel subunit coded by CACNA1A and location of the missense episodic ataxia type 2–causing mutation.
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