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A novel recurrent mutation in ATP1A3 causes CAPOS syndrome Demos, Michelle K; van Karnebeek, Clara D; Ross, Colin J; Adam, Shelin; Shen, Yaoqing; Zhan, Shing H; Shyr, Casper; Horvath, Gabriella; Suri, Mohnish; Fryer, Alan; Jones, Steven J; Friedman, Jan M Jan 28, 2014

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RESEARCH Open AccessA novel recurrent mutation in ATP1A3 causesCAPOS syndromeMichelle K Demos1*†, Clara DM van Karnebeek2†, Colin JD Ross3,4,5,6,7, Shelin Adam5, Yaoqing Shen8,Shing Hei Zhan8, Casper Shyr6, Gabriella Horvath2, Mohnish Suri9, Alan Fryer10, Steven JM Jones5,8,11,Jan M Friedman5,7,12* and the FORGE Canada ConsortiumAbstractBackground: We undertook genetic analysis of three affected families to identify the cause of dominantly-inheritedCAPOS (cerebellar ataxia, areflexia, pes cavus, optic atrophy and sensorineural hearing loss) syndrome.Methods: We used whole-exome sequencing to analyze two families affected with CAPOS syndrome, including theoriginal family reported in 1996, and Sanger sequencing to assess familial segregation of rare variants identified inthe probands and in a third, apparently unrelated family with CAPOS syndrome.Results: We found an identical heterozygous missense mutation, c.2452G > A (p.(Glu818Lys)), in the Na+/K+ ATPaseα3 (ATP1A3) gene in the proband and his affected sister and mother, but not in either unaffected maternalgrandparent, in the first family. The same mutation was also identified in the proband and three other affectedmembers of the second family and in all three affected members of the third family. This mutation was not foundin more than 3600 chromosomes from unaffected individuals.Conclusion: Other mutations in ATP1A3 have previously been demonstrated to cause rapid-onset dystonia-parkinsonism(also called dystonia-12) or alternating hemiplegia of childhood. This study shows that an allelic mutation in ATP1A3produces CAPOS syndrome.Keywords: CAPOS syndrome, Cerebellar ataxia, Optic atrophy, Sensorineural hearing loss, ATP1A3BackgroundCerebellar ataxia, areflexia, pes cavus, optic atrophy, andsensorineural hearing loss (CAPOS) syndrome (OMIM601338) is a rare neurological disorder, which to date hasonly been reported in a single family. In 1996, Nicolaideset al. [1] described CAPOS syndrome in a brother andsister and their mother, all of whom were normal until theypresented with a relapsing and partially remitting, early-onset cerebellar ataxia following a febrile illness. Other fea-tures included progressive optic atrophy and sensorineuralhearing loss, generalized hypotonia, areflexia and pes cavuswithout evidence of a peripheral neuropathy on neuro-physiological studies. All three patients shared these keyfeatures, although the severity and number of ataxicrelapses varied. The mode of inheritance was thought to beautosomal dominant or mitochondrial. Extensive investiga-tions failed to identify a cause, and the authors believedthat the neurological disorder affecting this family probablyrepresented a “new” “ataxia plus” syndrome. No otherpatients with CAPOS syndrome have been reported in thesubsequent 17 years.We have identified two additional families with CAPOSsyndrome and reassessed the original family. We heredescribe the clinical features and natural history of thisdisorder and report a novel heterozygous missense muta-tion of the ATP1A3 gene that causes CAPOS syndromein all ten affected members of these three apparentlyunrelated families.MethodsWe obtained informed consent and assent, when appro-priate, from participating family members. Ethical review* Correspondence: mdemos@cw.bc.ca; jan.friedman@ubc.ca†Equal contributors1Division of Neurology, Department of Pediatrics, University of BritishColumbia and BC Children’s Hospital, Vancouver, BC V6H 3N1, Canada5Department of Medical Genetics, University of British Columbia, Vancouver,BC V5Z 4H4, CanadaFull list of author information is available at the end of the article© 2014 Demos et al.; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the CreativeCommons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, andreproduction in any medium, provided the original work is properly cited. The Creative Commons Public Domain Dedicationwaiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwisestated.Demos et al. Orphanet Journal of Rare Diseases 2014, 9:15http://www.ojrd.com/content/9/1/15and approval according to the Finding Of Rare diseaseGEnes (FORGE) Canada Consortium and University ofBritish Columbia were also obtained. Clinical details ofFamily 1, originally described in 1996 [1], were updatedby A.F. in 2013.We performed whole exome sequencing on DNA fromthe probands in Families 1 (Figure 1A, Family 1 III-1)and 2 (Figure 1A, Family 2 II-2) on an Illumina HiSeq2000 after SureSelect Target enrichment with an Agilent50 Mb Human All Exon Kit (Agilent Technologies Inc.,USA) using the manufacturers’ protocols and othermethods previously described [2]. Illumina's GA Pipelinewas used to remove sequencing reads that failed chastityfiltering, and the remaining reads were mapped to theFigure 1 Pedigrees and Sanger sequencing results in three families with CAPOS syndrome. A) Pedigees. Family 1 was initially described byNicolaides et al. [1] Individuals with CAPOS syndrome are indicated by filled pedigree symbols, and unaffected individuals, by empty symbols.B) Sanger sequencing results in affected and unaffected family members. A portion of the Sanger sequencing trace is shown for each individualwho was tested, with the nucleotides at Ch19:47,166,267, corresponding to ATP1A3 position c.2452 on the minus strand, indicated by a verticalblue line. Each affected individual is heterozygous for the variant T (corresponding to c.2452A) and the reference C (corresponding to c.2452G)nucleotides. C) Conservation of Na+/K+ ATPase α3 protein sequence in the region of the mutation. The E818K mutation found in all three CAPOSfamilies is shown in red type; the location of the E815K loss-of-function mutation, which is associated with alternating hemiplegia of childhood, isshown in blue type. This segment of the Na+/K+ ATPase α3 protein is highly conserved.Demos et al. Orphanet Journal of Rare Diseases 2014, 9:15 Page 2 of 9http://www.ojrd.com/content/9/1/15human genome reference sequence (HG18) using BWA[3]. Duplicate reads and reads with a mapping score of0 were removed, and the remaining aligned reads wereexported to pileup format. Variants were identifiedusing SAMtools [4]. We filtered out variants with a qual-ity score below 10 at varFilter parameter −D 1000 forsingle nucleotide variants and varFilter parameters −D1000, −d 2 and − l 30 for indels. We annotated thefiltered variants as “known” or “novel”, depending onwhether they had been previously reported in dbSNP(version 129 or 130) [5,6] or in the 1000 Genomes Project[7,8]. We also determined how many times the variant hadpreviously been observed in our in-house database of 1834normal germline genomes sequenced at Canada’s MichaelSmith Genome Sciences Centre.ResultsTable 1 provides an overview of the clinical features ofthese three, apparently unrelated families. Additionalfile 1: Table S1 summarizes the results of investigationsperformed in the families.Family 1Following normal development, the male proband(Figure 1A, Family 1 III-1) of this Caucasian UnitedKingdom family experienced three episodes of lethargy,hypotonia, and ataxia during acute febrile illnesses,beginning at age 16 months. The most severe episodeoccurred at age 4.5 years, when he was comatose for aweek. Neurologic sequelae at 6 years of age includedmarkedly ataxic gait, poor hand coordination, truncalhypotonia, nystagmus, dysarthria, optic atrophy with ab-sent visual evoked potentials and moderate bilateral sen-sorineural deafness; the latter two features had developedat age 3 years.On re-examination at age 22 years, his phenotype wasslightly more severe than that described at age 6 years.He had suffered no acute episodes since age 4.5 yearsbut had slow progression of all symptoms since thattime. His balance remained poor, and, although hewalked unaided, he could not walk on uneven surfaces.He read with the assistance of an electronic magnifierand wore hearing aids. His cognition was normal, and hewas studying at university.His 49-year old mother (Figure 1A, Family 1 II-1) waswell until the age of 18 months when she developedacute ataxia during a febrile illness. She experienced nofurther acute episodes but has shown a progressive coursewith visual impairment, deafness and loss of balance. Atage 31 years she had profound bilateral sensorineuralhearing loss, bilateral optic atrophy with horizontal nys-tagmus, mild cerebellar ataxia, pes cavus, and absent deeptendon reflexes. Since then, her walking has progressivelydeteriorated: she cannot walk on uneven surfaces and hasrequired a walking stick for the past 2 years. She is nowregistered blind and requires hearing aids for profound bilat-eral hearing loss. Her visual evoked potentials and brainstemauditory evoked potentials are absent. Electromyogramshowed motor unit loss and innervation changes.The proband’s 20-year old sister (Figure 1A, Family 1III-2) presented with an ataxic episode with markedhypotonia, nystagmus and areflexia at age 9 months dur-ing a non-specific febrile illness. She recovered but wasnoted to have optic atrophy shortly after her acuteneurological symptoms resolved. She has not had anysubsequent ataxic episodes. She has developed sen-sorineural deafness, but the severity is less than thatof her 22-year old brother. She can walk on uneven sur-faces but tends to fall when running. Although registeredas partially sighted, she does not use any visual aids. Shehas hearing aids for mild-moderate hearing loss. She isalso studying at university.The proband’s mother’s parents (Figure 1A, Family 1 I-1and I-2) both have normal neurological examinations.There are two maternal sibs who were re-evaluated re-cently, and are both unaffected at ages 50 and 46 years,respectively.Family 2The proband of Family 2 (Figure 1A, Family 2 II-2) isnow a 14 year-old girl who was well until she developedataxic encephalopathy with a febrile illness at age 5years. She had a reduced level of consciousness for thefirst two weeks and slow recovery afterwards with dys-arthria, dysphagia, dysmetria and truncal and gait ataxia.She did not fully recover and was unable to walk withoutsupport for one year following the episode. She has hadno further episodes, but progressive optic atrophy andsensorineural hearing loss developed, and 9 years latershe wears hearing aids for bilateral moderate-to-severeupsloping sensorineural hearing loss. She has bilateraloptic atrophy, horizontal end-gaze nystagmus, and 6/46visual acuity. The patient has mild dysarthria, and,although her gait has deteriorated and she is areflexic,she is still able to walk unaided. She has had mild leftventricular enlargement on cardiac evaluation since earlychildhood. Her cognition was assessed to be low averageto average at age 10 years, and she has had attention andschool difficulties.Her two siblings and father show similar features buttheir clinical courses have been milder. The proband’solder brother (Figure 1A, Family 2 II-1), who is now 15years old, presented at 9 months of age during a febrileillness. He had two further episodes of ataxic encephal-opathy with febrile illnesses at 2.5 and 7 years of age;transient limitation of abduction or adduction in one orboth eyes was also noted during these episodes. He hadmild residual balance difficulties following the episodes,Demos et al. Orphanet Journal of Rare Diseases 2014, 9:15 Page 3 of 9http://www.ojrd.com/content/9/1/15but his cerebellar ataxia is currently minimal, with onlymild difficulties with tandem gait and standing on one foot.He is areflexic but does not have pes cavus. Sensorineuralhearing loss was identified at age 7 years, but his subse-quent audiological assessments have been stable, with amild upsloping sensorineural hearing loss bilaterally. Heuses an FM amplification device. He has bilateral optic discatrophy and horizontal nystagmus, with visual acuity of6/15 in the right eye and 6/18 in the left eye. He has no aca-demic difficulties. He has mild left ventricular enlargementof the heart that has been stable since early childhood.The proband’s younger brother (Figure 1A, Family 2II-3), now aged 10 years, presented at 18 months of agewith an acute ataxic episode triggered by a febrile illness.He has had no subsequent acute episodes but has mildresidual balance difficulties, minimal cerebellar ataxia,and absent reflexes. He had eye movement difficultiesduring his acute episode, with a residual left intermittentesotropia that improved with patching. He also has bilat-eral optic disc atrophy and horizontal nystagmus, withvisual acuity of 6/15 bilaterally. He developed a moderateupsloping bilateral sensorineural hearing loss and nowwears hearing aids. He has no academic problems.The father (Figure 1A, Family 2 I-2), who is of FrenchCanadian descent, is currently 43 years of age. When hewas 6 months old, he developed fever, generalized hypo-tonia and weakness, areflexia, vertical nystagmus, andlimited abduction of both eyes. He recovered but had asimilar episode with fever at age 4 years. Optic disc atro-phy and sensorineural hearing loss were also identifiedat that time. He now has profound sensorineural hearingloss and has had a cochlear implant. He is legally blind.Table 1 Clinical features of 10 patients from three families with CAPOS syndromeFamily 1 (Previouslyreported by Nicolaideset al., 1996 [1])Family 2 Family 3Subject II-1 III-1a III-2 I-2 II-1 II-2a II-3 I-1a II-1 II-2Current age 49 yr 22 yr 20 yr 43 yr 15 yr 14 yr 10 yr 40 yr 15 yr 11 yrEpisodes of ataxic encephalopathy and/or weaknessAge of onset 18 m 16 m 9 m 6 m 9 m 5 yr 18 m 3 yr 1 yr 3 yrNumber of episodes 1 3 1 2 3 1 1 3 3 2Episodes triggered by febrile illness + + + + + + + + + +Age of last episode 18 m 4 yr 9 m 4 yr 7 yr 5 yr 18 m 25 yr 18 m 5 yrAbnormal eye movements - - - + + - + + + -Dysphagia - - - - + - - +b - -Seizures - - - - +c - - - - -Symptoms present at most recent examinationAge at most recent exam 49 yr 22 yr 20 yr 42 yr 14 yr 13 yr 9 yr 39 yr 14 yr 10 yrCerebellar ataxia + + + + + + + + + +Areflexia + + + + + + + + + +Pes cavus + + - + - - - - - -Optic atrophy + + + + + + + + + +Sensorineural hearing loss + + + + + + + + + +Dystonia - - - +d - - - - - -Urinary symptoms - - - - - - - +e - -Autonomic dysfunction - - - - - - - - - -Cognitive dysfunction - - - - +f - - - - -Autistic traits - - - - - - - - +g +gCardiac arrhythmia - - - +h - - - - - -Abbreviations used: m, months; yr, years; +, present; -, absent or not noted.aProband.bMild persistent dysphagia was noted by age 32 years.cBrief focal seizures (unilateral arm jerking) developed in association with first episode of fever and ataxic encephalopathy.dPatient developed cervical dystonia responsive to onabotulinumtoxinA injections at age 32 years.eUrinary urgency and frequency were noted by age 32 years.fPatient was assessed at age 10 years for school difficulties and found to have low average to average IQ and reduced attention skills.gSiblings II-1 and II-2 of family 3 were noted to have repetitive behaviours and social difficulties at age 10 and 4 years, respectively.hPatient was diagnosed with Wolff-Parkinson-White syndrome at age 24 years.Demos et al. Orphanet Journal of Rare Diseases 2014, 9:15 Page 4 of 9http://www.ojrd.com/content/9/1/15He walks unaided but has mild dysmetria and ataxicgait, areflexia and pes cavus. He was treated for Wolff-Parkinson-White syndrome at age 24 years. He devel-oped cervical dystonia with dystonic tremor at age 32years and has benefited from onabotulinumtoxinA injec-tions since 38 years of age. Sural nerve biopsy performedat age 34 years revealed findings consistent with a mild-moderate axonal neuropathy.Family 3The proband of Family 3 (Figure 1A, Family 3 I-1), wholives in the United Kingdom and is of Caucasian descent,is a 40-year old woman who presented at age 3 yearswith a febrile illness and associated weakness and ataxia,which resolved after 4 months. At age 11 years, she hadanother acute episode, during which she became coma-tose and was diagnosed with encephalitis. She made afull intellectual recovery but was left with poor vision inassociation with optic atrophy, severe bilateral sensori-neural hearing loss and ataxia. After another febrile ill-ness at age 25 years, she developed generalized weaknessand worsening of her hearing, vision and ataxia. Clinicalre-evaluation at age 27 years showed optic atrophy withimpaired vision (acuity 6/60 in the left eye and 6/36 inthe right eye) with pendular nystagmus in all directionsof gaze and superimposed square wave jerks. She had se-vere bilateral sensorineural hearing loss, mild dysarthria,moderate ataxia and absent deep tendon reflexes but nopes cavus. Her cognition was normal. The neurologicalfindings were unchanged at 32 years of age, but shereported swallowing difficulties owing to slow movementof her lips and tongue, and increased urinary urgencyand frequency, which responded well to treatment withoxybutynin. She had abnormal visual evoked potentialsbilaterally, but her electroretinogram was normal.The proband’s mother died at age 50 years of “heartproblems” but was otherwise said to have enjoyed goodhealth; autopsy was not performed. The proband’s fatherhas no signs of neurologic or systemic disease.The proband’s 15-year old daughter (Figure 1A, Family3 II-1) was well until the age of 1 year when she pre-sented with a febrile episode associated with generalizedweakness, floppiness and areflexia. She made a full re-covery after 5 weeks. She developed similar, but milder,problems following another febrile illness at the age of18 months. At age 3 years she was found to have in-creased latencies of her visual evoked potentials andbrain-stem auditory evoked potentials. Bilateral mildsensorineural hearing loss was diagnosed at 7 years ofage, and optic disc pallor was noted at age 7.5 years. Atage 10 years, she had an upper respiratory infection anddeveloped acute onset of strabismus, which took severalweeks to resolve. Although her academic progress wasreported to be normal, repetitive behaviors and socialdifficulties resulted in a diagnosis of autism spectrumdisorder at age 10.5 years.The proband's son (Figure 1A, Family 3 II-2), who isnow 11 years old, developed profound weakness, hypo-tonia and areflexia during a febrile illness at age 3 years.He recovered slowly over 6 months, but his brain stemauditory evoked potentials were abnormal and by age3.5 years he had optic disc pallor and visual acuity of 6/12 in both eyes. He had an ataxic gait and was areflexic.At age 5 years he had another acute febrile episode withweakness, hypotonia and altered sensorium that resolvedafter 3 days. Bilateral mild low frequency sensorineuralhearing loss was diagnosed at age 5.8 years and has beenstable since then. He has also been diagnosed withautism spectrum disorder.Genetic studiesWe used whole exome sequencing to identify heterozy-gous variants that were present in the same gene in theprobands of both Family 1 and Family 2, were rare inthe normal population, and were predicted to causenon-synonymous changes in protein-coding regions orto interfere with splicing. The results of this analysis aresummarized in Table 2. Candidate variants were vali-dated using PCR and standard Sanger sequencing in allseven affected and four unaffected members of CAPOSFamilies 1 and 2.Only one novel heterozygous missense variant wasdemonstrated in all affected members of both familiesand absent in the two unaffected spouses tested. Thisvariant, which was the same in both families (Additionalfile 2: Figure S1), occurred at position 47,166,267 (C > T)of chromosome 19 (NCBI36/HG18) and correspondsto c.2452G > A (p.(Glu818Lys)) (Ensembl transcriptENST00000302102, version 5)[9] in the ATP1A3 (sodium/potassium-transporting ATPase subunit α3) gene (OMIM182350) encoded on the opposite strand. We then analyzedATP1A3 by targeted Sanger sequencing in Family 3 anddemonstrated the same c.2452G >A mutation in all threeaffected individuals. Figure 1B shows the ATP1A3 Sangersequencing results for Families 1–3.We found the same ATP1A3 c.2452G >A mutation in allthree CAPOS families studied. This observation raised thepossibility that the mutation might exhibit a common ori-gin through unaffected carrier antecedents. However, theATP1A3 c.2452G >A mutation in the affected mother (II-1)in Family 1 was not inherited from either of her unaffectedparents (Figure 1B, Family 1 I-1 or I-2) and, therefore, musthave arisen de novo. (Non-paternity was excluded by testingall six Family 1 members shown in Figure 1A with IlluminaOMNIExpress whole genome genotyping arrays)(Illumina,Inc., San Diego, Calfiornia, USA).The parents of the affected adults in Families 2 and 3were not available for mutation testing, so we performedDemos et al. Orphanet Journal of Rare Diseases 2014, 9:15 Page 5 of 9http://www.ojrd.com/content/9/1/15genome-wide SNP genotyping on Illumina OMNIEx-press arrays using standard protocols to determine thehaplotype on which the mutation arose. We found thatboth families shared a region of more than 2 Mb con-taining 35 informative SNPs surrounding the mutation(Additional file 3: Figure S2). Although new mutationsare most likely in all three families, we cannot rule out thepossibility of a remote relationship between Families 2 and3 with a common ancestral ATP1A3 c.2452G >A mutationand incomplete penetrance.The heterozygous ATP1A3 c.2452G > A variant foundin all 10 affected individuals in these three CAPOS fam-ilies was not observed in more than 1834 unaffectedindividuals who had undergone whole exome or wholegenome sequencing.DiscussionATP1A3 encodes the catalytic α3 subunit of Na+/K+ATPase, an integral membrane protein responsible forestablishing and maintaining electrochemical gradientsacross the plasma membrane. The c.2452G > A mutationsubstitutes a positively-charged lysine for a negatively-charged glutamate in the C-terminus cation transportingdomain of the Na+/K+ ATPase α3 protein. SIFT [10]predicts this change to be damaging with high confi-dence (score = 0), and Mutation Taster [11] predicts it tobe disease-causing (probability = 1.00). PhyloP [12] indi-cates that the affected nucleotide is highly conserved(score = 2.17), and this is also apparent by inspection of theamino acid sequence in the altered region of the protein(Figure 1C).Na+/K+ ATPase uses adenosine triphosphate (ATP) topump Na+ ions out of cells and K+ ions into cells [13,14].These gradients are involved in regulating neurotransmit-ter reuptake and the electrical excitability of nerve andmuscle. In addition, Na+/K+ ATPase plays a key role in sev-eral signal transduction pathways. We have not studied theeffect of the c.2452G >A mutation on Na+/K+ ATPasefunction, but our observation of exactly the same missensechange in association with the same, extremely rare pheno-type as a result of at least two, and probably three, separatemutational events is consistent with a gain of function.Studies of other characteristic phenotypes that are causedby identical recurrent mutations of other genes have oftenTable 2 Summary statistics of whole exome sequencing in two unrelated patients with CAPOS syndromeFamily 1, subject III-1 Family 2, subject II-2Total reads 108,841,666 104,389,738Chastity-passed reads 103,868,186 102,337,954Reads aligned with mapping quality ≥10 92,611,315 91,501,102Average exome read deptha 69x 65xNon-synonymous single nucleotide variants 10,911 11,254Splice-site single nucleotide variants 517 524Coding insertions/deletionsb 805/416 810/501Non-silent variantsc not in dbSNP 129 or 130 2,259 2,209Noveld heterozygous, autosomal variants 390 224Genes with novel non-identical heterozygous variants in both probands 9eGenes with novel identical heterozygous variants in both probands 3fVariant segregating in all 10 affected family members tested in three CAPOS families ATP1A3chr19:47,166,267C > T (hg18)gc.2452G > Ahp.Glu818LysaAverage read depth of exons annotated using Ensembl 54 (including potential PCR duplicates) calculated as (sum of the number of reads aligned per site for allexonic sites) / (total number of exonic sites).bSupported by ≥7 reads.cIncludes non-synonymous single nucleotide variants, splice-site single nucleotide variants (within 2 bases of exon boundaries), and small, coding insertions anddeletions as annotated using Ensembl 54 gene models.dNot previously reported in dbSNP129 or 130, 1000 Genomes Project, or 1834 non-cancer genomes collected in Canada’s Michael Smith Genome Sciences Centre’slocal database.eNon-identical variants of 6 of these genes were called with sufficient quality in both probands to warrant confirmation by Sanger sequencing. Variants of thefollowing genes were tested but did not segregate with the disease in either family: WDR26, C12orf56, LMO7, and DNAH17. A variant of SIGLEC1 segregated asexpected in Family 2, but the variant found in Family 1 did not segregate as expected. A variant of MUC16 segregated as expected in Family 1, but neither of twodifferent variants of this gene found in Family 2 segregated as expected.fOnly one of these variants (c.2452G > A of ATP1A3) was called with sufficient quality in both probands to warrant confirmation by Sanger sequencing. This variantshowed the expected segregation pattern with the disease in both families.gLocation in hg19 is chr19:42,474,427C > T.hAnnotation is on Ensembl transcript ENST00000302102, version 5.Demos et al. Orphanet Journal of Rare Diseases 2014, 9:15 Page 6 of 9http://www.ojrd.com/content/9/1/15shown that the responsible mutations produce a gain offunction e.g., [15-17].Although the c.2452G > A mutation has not beenreported before, different mutations of ATP1A3 areknown to cause two other autosomal dominant neuro-logical diseases: rapid-onset dystonia-parkinsonism [18-20](DYT12; OMIM: 128235) and alternating hemiplegia ofchildhood [21,22] (AHC; OMIM: 614820). These two dis-orders have somewhat overlapping clinical features butare generally considered to be distinct [22-24]. Moreover,the ATP1A3 mutations reported in the two conditionsall differ, and most alternating hemiplegia of childhoodmutations occur de novo, while rapid-onset dystonia-parkinsonism mutations are more often inherited [21-24].This genotype-phenotype correlation also extends toCAPOS syndrome in that the mutation we observed hasnot been seen in either of the other conditions associatedwith ATP1A3 mutations, and the clinical features ofCAPOS syndrome, although somewhat overlapping, aredistinct from those of rapid-onset dystonia-parkinsonismTable 3 Comparison of clinical features in alternating hemiplegia of childhood (AHC), rapid-onset dystonia-parkinsonism(DYT12) and CAPOS syndrome (adapted and modified from Rosewich et al. [22])Alternating hemiplegiaof childhoodRapid-onset dystonia-parkinsonismCAPOSsyndromeUsual age of onset 0 – 18 m 4 – 55 yr 6 m – 5 yrOnset triggerEmotional stress ?a + ?aExercise ?a + ?aHypo/Hyperthermia + + +bBathing + Not reported -Alcohol ?a + ?aNeurological symptomsAtaxic encephalopathy episodes - - +Hemiplegic episodes + - -Quadriplegic or paretic episodes + - +Dystonia + + −/+Dysarthria + + −/+Drooling + + −/+Reduced facial expression −/+ + -Mutism −/+ + -Rostrocaudal gradient + + -Ataxic gait −/+ + +Bradykinesia −/+ + -Seizures −/+ −/+ −/+Choreoathetosis + - -Abnormal eye movements during episodes + −/+ −/+Areflexia - - +Pes cavus - - −/+Optic atrophy/visual loss - - +Sensorineural hearing loss - - +Developmental delay or intellectual disability + −/+ −/+Clinical courseAbrupt onset + + +Polyphasic with slow progression of non-paroxysmal symptoms + - −/+Mono- or biphasic with slow progression of neurological symptoms - + −/+Abbreviations used: m, months; yr, years; +, feature usually occurs; +/−, feature may occur; -, feature not described.aCannot assess in infancy or early childhood.bHyperthermia only.Demos et al. Orphanet Journal of Rare Diseases 2014, 9:15 Page 7 of 9http://www.ojrd.com/content/9/1/15or alternating hemiplegia of childhood. In addition, all ofthe ATP1A3 mutations associated with DYT12 or AHCthat have been assessed functionally produce loss of func-tion [18,20,21], while the recurrent c.2452G > A variantfound in the three CAPOS syndrome families presentedhere has characteristics of a gain-of-function mutation, asdiscussed above.Table 3 provides an overview of the phenotypic simi-larities and differences between the three conditions.CAPOS syndrome, DYT12 and AHC all can be inheritedas autosomal dominant traits, and all three are charac-terized by variable expressivity. All may exhibit acuteonset of neurological symptoms in childhood in associ-ation with a febrile illness, but the predominant neuro-logical manifestations differ – ataxic encephalopathy inCAPOS syndrome, dystonia/parkinsonism in DYT12,and transient episodes of hemiplegia and other symp-toms including dystonia in AHC.In contrast to AHC and DYT12, in which the dystonicsymptoms are often asymmetric and progress in a ros-trocaudal gradient, dystonia is an uncommon feature inCAPOS syndrome. The clinical features that progress inthis disorder are more generalized and symmetric; theyinclude progressive gait ataxia and loss of vision andhearing. Bulbar symptoms, which typically occur in AHCand DYT12, are uncommon in CAPOS syndrome.Optic atrophy and sensorineural hearing loss have notbeen reported in DYT12 or AHC but are frequent fea-tures of CAPOS syndrome. These symptoms in ourpatients are slowly progressive over time. The recentdemonstration that the Na+/K+-ATPase α3 subunit playsa critical role in anchoring retinoschisin, the protein in-volved in X-linked juvenile retinoschisis, to photorecep-tor and bipolar cells of the retina in a mouse model iscompatible with involvement of ATP1A3 in visual function[25]. The role of the Na+/K+-ATPase α3 subunit in hearingis unknown, but ATP1A3 is abundantly expressed in mem-branes of spiral ganglion somata, type I afferent terminalscontacting inner hair cells and medial efferent terminalscontacting the outer hair cells of the cochlea [26].Unaffected carriers have been reported for other ATP1A3missense mutations in rapid-onset dystonia-parkinsonism[18,19,27], but there is no evidence for incomplete pene-trance in CAPOS syndrome. We demonstrated a de novoorigin for the ATP1A3 c.2452G >A mutation in Family 1(Figure 1B), and the clinical and family histories are mostcompatible with separate, recurrent de novo mutations inFamilies 2 and 3 as well.We conclude that a heterozygous c.2452G > A muta-tion in ATP1A3 causes CAPOS syndrome in ten affectedindividuals in three different families. Clinically, CAPOSsyndrome is characterized by acute onset of ataxic enceph-alopathy with febrile illness in childhood, partial recoveryand subsequent slow progression. Testing for ATP1A3mutations should be considered in other patients withfeatures of CAPOS syndrome or with other paroxysmaland progressive forms of early-onset dystonia, weaknessor ataxia.Additional filesAdditional file 1: Table S1. Investigations performed on 10 patientsfrom three families with CAPOS syndrome.Additional file 2: Figure S1. Visualization of read alignments supportingthe ATP1A3 mutation in the libraries from each of the two probands. Upperpanel: Family 1 subject III-1. Lower panel: Family 2 subject II-2. Readalignments to hg18 stored in BAM files were manually examined, and thealignment image was exported using Integrated Genome Viewer [28,29]. Theheterozygous C > T mutation at chromosome 19:47,166,267 was corroboratedby 22 out of 41 reads in Family 1 subject III-I and by 34 out of 59 reads inFamily 2 subject II-2.Additional file 3: Figure S2. Haplotyping results in Families 2 and 3 inthe region flanking the ATP1A3 mutation.Competing interestsThe authors declare that they have no competing interests.Authors’ contributionsThe study was conceived and designed by MKD, CDMvK, SA, SJMJ, and JMF.Data were acquired by MKD, CDMvK, CJDR, SA, GH, MS, AF, and SJMJ. Datawere analysed and interpreted by MKD, CDMvK, CJDR, YS, SHZ, CS, GH, MS,AF, SJMJ, and JMF. The study was supervised by SJMJ and JMF. The manuscriptwas prepared by MKD and CDMvK and critically revised for important intellectualcontent by JMF. All authors approved the manuscript.AcknowledgementsFORGE Canada (Finding of Rare Disease Genes in Canada) Consortium SteeringCommittee: Kym Boycott (leader, University of Ottawa), Jan Friedman (co-leader,University of British Columbia), Jacques Michaud (co-leader, Université deMontréal), Francois Bernier (University of Calgary), Michael Brudno (University ofToronto), Bridget Fernandez (Memorial University), Bartha Knoppers (McGillUniversity), Mark Samuels (Université de Montréal), and Steve Scherer (Universityof Toronto).This work was supported by Genome Canada and Genome British Columbiathrough the FORGE Canada Consortium, by the British Columbia ClinicalGenomics Network (funded by the Michael Smith Foundation for HealthResearch), and by the Rare Disease Foundation. Open Access publicationcosts were paid by Omics2TreatID (awarded to CvK by the CanadianInstitutes of Health Research).Author details1Division of Neurology, Department of Pediatrics, University of BritishColumbia and BC Children’s Hospital, Vancouver, BC V6H 3N1, Canada.2Division of Biochemical Diseases, Department of Pediatrics, University ofBritish Columbia and BC Children’s Hospital, Vancouver, BC V6H 3N1, Canada.3Division of Translational Therapeutics, Department of Pediatrics, Universityof British Columbia and BC Children’s Hospital, Vancouver, BC V6H 3N1,Canada. 4The Pharmaceutical Outcomes Programme, University of BritishColumbia, Vancouver, BC V5Z 4H4, Canada. 5Department of MedicalGenetics, University of British Columbia, Vancouver, BC V5Z 4H4, Canada.6Centre for Molecular Medicine and Therapeutics, Vancouver, BC V5Z 4H4,Canada. 7Child & Family Research Institute, Vancouver, BC V5Z 4H4, Canada.8Canada's Michael Smith Genome Sciences Centre, British Columbia CancerAgency, Vancouver, BC V5Z 4S6, Canada. 9Department of Clinical Genetics,Nottingham University Hospitals National Health Service Trust, NottinghamNG5 1PB, UK. 10Department of Clinical Genetics, Royal Liverpool Children’sHospital, Liverpool L12 2AP, UK. 11Department of Molecular Biology andBiochemistry, Simon Fraser University, Burnaby, BC V5A 1S6, Canada.12Medical Genetics Research Unit, Child & Family Research Insitute, Box 153,4500 Oak Street, Vancouver, BC V6H 3N1, Canada.Demos et al. 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Orphanet Journal of Rare Diseases 2014 9:15.Submit your next manuscript to BioMed Centraland take full advantage of: • Convenient online submission• Thorough peer review• No space constraints or color figure charges• Immediate publication on acceptance• Inclusion in PubMed, CAS, Scopus and Google Scholar• Research which is freely available for redistributionSubmit your manuscript at www.biomedcentral.com/submitDemos et al. Orphanet Journal of Rare Diseases 2014, 9:15 Page 9 of 9http://www.ojrd.com/content/9/1/15

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