- Email: [email protected]
Episodic ataxia associated with a de novo SCN2A mutation
e u r o p e a n j o u r n a l o f p a e d i a t r i c n e u r o l o g y 2 0 ( 2 0 1 6 ) 7 7 2 e7 7 6
Official Journal of the European Paediatric Neurology Society
Case Study
Episodic ataxia associated with a de novo SCN2A mutation Emma L. Leach a, Clara D.M. van Karnebeek b,c,d, Katelin N. Townsend a,c, Maja Tarailo-Graovac d, Juliette Hukin e, William T. Gibson a,c,* a
Department of Medical Genetics, University of British Columbia, Vancouver, Canada Division of Biochemical Diseases, Department of Pediatrics, British Columbia Children's Hospital, Vancouver, Canada c Child and Family Research Institute, British Columbia Children's Hospital, Vancouver, Canada d Centre for Molecular Medicine and Therapeutics, University of British Columbia, Vancouver, Canada e Division of Neurology, Department of Pediatrics, British Columbia Children's Hospital, Vancouver, Canada b
article info
abstract
Article history:
Introduction: Episodic ataxia (EA) is characterized by paroxysmal attacks of ataxia
Received 27 November 2015
interspersed by asymptomatic periods. Dominant mutations or copy number variants in
Received in revised form
CACNA1A are a well-known cause of EA.
4 May 2016
Clinical presentation: This boy presented with clinical features of episodic ataxia, and also
Accepted 26 May 2016
showed cerebellar atrophy, hypotonia, autism and global developmental delay at age 4 years. Acetazolamide prevented further episodes of ataxia, dystonia and encephalopathy.
Keywords:
Extensive biochemical and genetic tests were unrevealing; whole exome sequencing found
Episodic ataxia
a previously unreported variant in SCN2A, proven to be de novo and predicted to be protein-
SCN2A
damaging.
Acetazolamide
Conclusion: Considered alongside previous reports of episodic ataxia in SCN2A mutation-
Cerebellar atrophy
positive patients, our case further illustrates the genetic heterogeneity of episodic ataxia.
Treatment
In addition, this case suggests that acetazolamide may be an effective treatment for some
Channelopathy
aspects of the phenotype in a broader range of channelopathy-related conditions. © 2016 European Paediatric Neurology Society. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The emergence of whole exome sequencing (WES) in research and clinical contexts has proven that many sporadic
conditions, including seizure disorders, have a broader range of genetic causes than was previously suspected. Similarly, genes previously thought to cause unique disorders when mutated have been shown to cause disorders in which they had not previously been implicated.
* Corresponding author. Child & Family Research Institute, Room A4-182, 950 West 28th Avenue, Vancouver, BC V5Z 4H4, Canada. Fax: þ1 604 875 2373. E-mail address: [email protected] (W.T. Gibson). http://dx.doi.org/10.1016/j.ejpn.2016.05.020 1090-3798/© 2016 European Paediatric Neurology Society. Published by Elsevier Ltd. All rights reserved.
e u r o p e a n j o u r n a l o f p a e d i a t r i c n e u r o l o g y 2 0 ( 2 0 1 6 ) 7 7 2 e7 7 6
Episodic ataxia type 2 (EA2) is characterized by paroxysmal attacks of ataxia.1,2 Attacks typically onset in childhood or early adolescence, and range in frequency from annually to several times per week, with asymptomatic periods of variable duration. Episodes can include other neurological manifestations such as vertigo, nausea, headache, dysarthria, diplopia, tinnitus, dystonia, hemiplegia, and seizures.1,3 Maintenance treatment with oral acetazolamide (Diamox®) effectively prevents or reduces the frequency and severity of attacks.1,2 Heterozygous variants in the voltage-gated calcium channel encoding CACNA1A account for over 95% of EA2 cases.1 However, Hirose et al.4 described a large family with autosomal dominant acetazolamide-responsive EA2 wherein no CACNA1A mutations were identifiable, supporting genetic heterogeneity in the EA2 phenotype and suggesting that other causative genes remain to be identified.3 Indeed, more recently, additional genes have been associated with the EA2 clinical presentation. A 2 year-old boy with periodic attacks of ataxic gait, repeated vomiting, and abnormal eye movement was diagnosed with EA2 and treated with acetazolamide; he was found to have glucose transporter type 1 deficiency attributed to a de novo frameshift insertion in the SLC2A1 gene.5 Conroy et al.6 also proposed UBR4 may be associated with a form of episodic ataxia characterized by periods of unsteadiness, generalized weakness and slurred speech in early childhood. As genome sequencing technology yields additional genes associated with discrete clinical phenotypes, so does it also identify new phenotypes associated with genes implicated previously in other disorders. SCN2A (MIM 182390) encodes the alpha subunit of voltagegated, type II sodium channels, highly expressed in the brain. Pathogenic SCN2A mutations are also known to cause benign familial neonatal-infantile epilepsy,7 and generalised epilepsy with febrile seizures plus (GEFSþ).8 However, recent studies have associated coding variants in SCN2A with a broader phenotypic spectrum, including severe infantile-onset epilepsy,9 intellectual disability without seizures,10 and autism.11,12 SCN2A mutations have also been identified among patients with episodic ataxia.13,14 Liao et al.13 described a patient with neonatal-onset seizures and variable episodes of ataxia, myoclonia, headache, and back pain associated with a de novo missense mutation (p.Ala263Val) in SCN2A. Three additional patients with a similar phenotype to the patient described by Liao et al.13 were also found to harbour SCN2A mutations.14 The de novo p.Ala263Val mutation previously reported was identified in one of the additional cases, and the two patients were found to have a novel de novo p.Arg1882Gly mutation, as well as a paternally inherited p.Gly1522Ala variant in one case.14 In vitro studies showed gain-of-function effects for both de novo mutations, and predicted neuronal hyperexcitability.13,14 Ogiwara et al.15 identified two de novo SCN2A mutations in a cohort of SCN1A-negative intractable epilepsy cases. Recently, Nakamura et al.16 identified SCN2A mutations in 15 of 328 patients with Ohtahara syndrome, West syndrome or unclassified early-onset epileptic encephalopathies and recommended that SCN2A molecular analysis should be considered for children with different epileptic conditions.
773
Here we describe a novel SCN2A variant associated with acetazolamide-responsive episodic ataxia identified through WES, and thus broaden the clinical spectrum of this genetic sodium channelopathy.
2.
Case report
Parents provided informed consent for publication of this report. The family was enrolled into both the GARD study (protocol H09-01228) and TIDEX Study (protocol H12-00067), each approved by the Clinical Research Ethics Board of the University of British Columbia, Vancouver Canada. This 5-year old boy, born after an unremarkable pregnancy and delivery with normal birth weight and normal Apgar scores, presented acutely to our centre at 11 months of age with a history of minor head injury due to a fall forward out of his stroller that was accompanied by decreased level of consciousness, vomiting and possible seizures. Though there had been steady gains in motor skills with no regression prior to his first episode, his developmental profile had not been entirely typical. Specifically, he was not yet standing independently, cruising, nor engaged in canonical babbling, and would not respond to his name being called, despite normal hearing and normal interest in other sounds. While in hospital, he was markedly hypotonic and had encephalopathy. He also experienced episodes of intermittent limb stiffness and dystonic posturing, with arching of his back, rightward deviation of his eyes and head, and concomitant nystagmus. EEG on the day of presentation was normal, thus these episodes were interpreted as dystonic posturing. Head CT and cerebrospinal fluid (CSF) glucose were normal. He was initially treated with phenytoin and diazepam, but developed choreoathetoid movements. These were attributed to phenytoin toxicity, so because of his normal EEG the phenytoin was stopped. MRI shortly after admission demonstrated cerebellar oedema with T2 hyperintensity, and bilateral arachnoid cysts of the middle cranial fossa (Fig. 1). Biochemical investigations (performed according to the algorithm for treatable inborn errors of metabolism in developmentally and intellectually delayed patients17) in blood (plasma amino acids, homocysteine, copper, ceruloplasmin), urine (glycosaminoglycans, oligosaccharides, organic acids, purines, pyrimidines and creatine metabolites) and CSF (protein, glucose, lactate), did not yield significant abnormalities (Supplementary Table 1). His encephalopathy, dystonia and ataxia gradually decreased over the next five days, though he had not completely returned to his baseline level of developmental functioning. He was discharged on day five on no medications, much improved clinically with residual mild clumsiness and central hypotonia. Presumptive diagnosis at that time was post-infectious encephalitis. The family history is positive for migraines without hemiplegia or any neurologic deficit in his mother; as well as migraine with aura in a second cousin. His father experienced adult-onset seizures secondary to two separate oligodendrogliomas. There was no other reported family history of migraine, ataxia, epilepsy or autism. At 17 months of age, he again presented after another unprovoked fall with minor head injury, having manifested an
774
e u r o p e a n j o u r n a l o f p a e d i a t r i c n e u r o l o g y 2 0 ( 2 0 1 6 ) 7 7 2 e7 7 6
Fig. 1 e Left: Coronal T2-weighted MR image at 11 months of age shows swollen hyperintense cerebellar vermis and hemispheres. Right: Coronal T2-weighted MR image at 3 years, 7 months of age shows normal cerebellar signal. There is prominence of the cerebellar folia in keeping with mild cerebellar atrophy.
episode of stiffness in all four limbs lasting less than a minute, with acute decrease in level of consciousness. On admission, he had encephalopathy, intermittent vomiting, ataxia, low axial tone, hyperreflexia, and intermittent dystonic posturing. EEG was again normal on day two of admission. He gradually improved and by day ten was fit for discharge, though he had residual ataxia. Metabolic workup was again normal (Supplementary Table 1). An acylcarnitine profile showed low free carnitine, so he was started on oral 250 mg L-carnitine. Repeat brain MRI showed mild cerebellar oedema, less severe than the previous scan. Given the recurrence of acute encephalopathy, hypotonia, dystonia and ataxia following minor head injury, a channelopathy related to the CACNA1A gene was considered the probable diagnosis and he was started empirically on 62.5 mg acetazolamide twice daily (5.2 mg/kg at that time). His encephalopathy, dystonia and ataxia improved markedly within 12 h of starting acetazolamide, and he has not suffered any subsequent acute episodes of encephalopathy, dystonia, seizures or ataxia. The dose has been increased commensurate with growth and also following a couple of minor ataxia spells 15 months post-initial presentation; he is presently on 100 mg (approximately 5 mg/kg) BID. Even though CACNA1A mutations had been ruled out with molecular sequencing, acetazolamide was continued given its observed benefit (as attested by the severity of previous events when not taking acetazolamide, and the resolution of other, minor events when the dose was increased). Neither the parents nor the treating physician were willing to consider an empirical re-challenge of acetazolamide withdrawal. Follow-up MRIs of his brain at 2.5 years of age showed bilateral atrophy of the cerebellar hemispheres and vermis, which initially progressed and then stabilized within one year (Fig. 1). At age 3 years, our patient was assessed and found to meet DSM-V criteria for an autism spectrum disorder. He was noted
to have deficits in social communication and social interaction in combination with atypical patterns of behaviour. He also was found to have global developmental delay, functioning at an 11-month level. Clinical-grade Sanger sequencing of CACNA1A to investigate his possible diagnosis of EA2 did not identify any rare variants in that gene. As well, glucose-1 transporter deficiency syndrome (GLUT1-DS) was ruled out by normal CSF glucose studies and clinical-grade Sanger sequencing of SLC2A1. Chromosome microarray analysis identified no significant deletions larger than 200 kb and no duplications larger than 400 kb. At 4 years of age he was evaluated at the TIDE Complex Diagnostic Clinic18 in our institution to elucidate the genetic basis for his intellectual and developmental disabilities, central hypotonia, and autism spectrum disorder. Based on previous genetic testing, the remaining differential diagnosis included episodic ataxia types 1, 5 or 6 (KCNA2, CACNB4, SLC1A3), primary coenzyme Q deficiency, pyruvate dehydrogenase deficiency and mitochondrial diseases. Clinical WES at GeneDx in 2012 targeted known human disease genes in both the nuclear and mitochondrial genomes (mean depth of coverage 206 on Agilent SureSelect XT2 All Exon V4 kit). No sequence changes were identified in any of the known genes associated with any of the types of episodic ataxia. Two candidate rare missense variants were identified, one each in KCNQ3 (hg19 8:133,141,789C>T (p.R780H)) and SCN2A (hg19 2:166,245,217G>A (p.G1634D)). The KCNQ3 variant is rare in the Exome Aggregation Consortium (ExAC) data set (frequency ¼ 2.443e-05), however it is predicted to be benign at the protein level by multiple programs (PROVEAN score for p.R780H in KCNQ3 was 0.5, considered “neutral”). The SCN2A variant is novel, not reported in ExAC or other healthy population databases. Further, it is predicted damaging (PROVEAN Score for p.G1634D in SCN2A was 6.1, with anything below 2.5 considered to be deleterious).
e u r o p e a n j o u r n a l o f p a e d i a t r i c n e u r o l o g y 2 0 ( 2 0 1 6 ) 7 7 2 e7 7 6
775
Fig. 2 e A) Family pedigree. B) Sanger sequencing traces with an arrow indicating the location of interest. Traces show that the SCN2A variant (hg19 2:166,245,217G>A (p.G1634D)) is de novo in the index and that the KCNQ3 variant (hg19 8:133,141,789C>T (p.R780H)) was inherited from the mother. C) Multi-sequence alignment showing high conservation of the Gly (depicted in blue) at position 1634 in SCN2A. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Research-grade Sanger sequencing of the variants in SCN2A and KCNQ3 was done to confirm the presence of the variants identified by WES in the proband, and to assess segregation with disease. Parental testing indicated that the KCNQ3 variant was maternally-inherited, and that the SCN2A variant was de novo (Fig. 2). Based on the de novo inheritance and in silico prediction of deleterious impact on protein function, the SCN2A variant was concluded as most likely causative of the proband's phenotype.
3.
Discussion
This case expands the differential diagnosis of episodic ataxia with a treatable condition e a de novo SCN2A mutation responsive to acetazolamide. In conjunction with previous literature reports,13e16 evidence from our patient suggests that episodic ataxia may extend the SCN2A phenotypic spectrum, and that SCN2A mutations should be considered in the differential diagnosis of episodic ataxia, acute encephalopathy and cerebellar oedema following minor head injury. Combined with the previous reports of patients with SCN2A mutations presenting with episodic ataxia13,14 and its phenotypic fit with the autism and moderate developmental delay, we consider the SCN2A variant in this patient to be pathogenic based on its de novo status and its predicted high functional impact. A maternally-inherited, likely benign KCNQ3 variant was also identified in the patient but deemed to be an unlikely cause, given its inheritance from a mother without ataxia, intellectual disability or autism, and its predicted neutrality. Since both genes affect neuronal
excitability, it is possible that the two variants may have an epistatic interaction, such that the phenotype associated with both variants might be more severe than with the SCN2A variant alone. Functional studies to elucidate the combined effect on transmembrane conductance of both variants fall outside the scope of this study. Acetazolamide is a carbonic anhydrase inhibitor used to treat a variety of conditions, including hypokalemic periodic paralysis, altitude sickness, epilepsy, benign intracranial hypertension as well as episodic ataxia.2 The mechanism by which acetazolamide relieves symptoms associated with EA2 is not well understood, though it has been suggested that acetazolamide may correct abnormal intracellular pH levels in the cerebellum.2,19 Trials of acetazolamide in two patients with neonatal-onset seizures and episodic ataxia attributed to SCN2A mutations have been reported.13,14 In the first patient, acetazolamide appeared to improve the patient's seizures, however there was no effect on episodic ataxia.13 In the second patient, a trial with acetazolamide had no effect.14 The application of acetazolamide to treat apparently unrelated conditions suggests that its mode of action is not confined to calcium voltage-gated channels, but instead acts indirectly via another pathway. Given the therapeutic response of our patient, acetazolamide may be effective for a broader range of ataxias and channelopathies, and might be considered in the treatment of episodic ataxia in SCN2A patients.
Conflict of interest statement The authors declare no conflict of interest.
776
e u r o p e a n j o u r n a l o f p a e d i a t r i c n e u r o l o g y 2 0 ( 2 0 1 6 ) 7 7 2 e7 7 6
Acknowledgements We would like to thank Dr. Michael Sargent for his expert interpretation of neuroimaging. Salary funding to WTG provided by the BC Children's Hospital Foundation through a CFRI Clinician-Scientist Award. Operating funds to WTG's lab provided by CIHR MOP-119595. CvK is the recipient of a Michael Smith Foundation for Health Research Award. We gratefully acknowledge the support of the BC Children's Hospital Foundation (1st Collaborative Area of Innovation: Treatable Intellectual Disability Endeavour in British Columbia, www.tidebc.org) and the BC Children's Hospital BioBank.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ejpn.2016.05.020.
references
1. Spacey S. Episodic ataxia type 2. 2003 Feb 24 [Updated 2011 Dec 8]. In: Pagon RA, Adam MP, Ardinger HH, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2014. Available from: http://www. ncbi.nlm.nih.gov/books/NBK1501/. 2. Strupp M, Zwergal A, Brandt T. Episodic ataxia type 2. Neurotherapeutics 2007;4:267e73. 3. Jen J, Kim GW, Baloh RW. Clinical spectrum of episodic ataxia type 2. Neurology 2004;62(1):17e22. 4. Hirose H, Arayama T, Takita J, et al. A family of episodic ataxia type 2: no evidence of genetic linkage to the CACNA1A gene. Int J Mol Med 2003;11(2):187e9. 5. Ohshiro-Sasaki A, Shimbo H, Takano K, Wada T, Osaka H. A three-year-old boy with glucose transporter type 1 deficiency syndrome presenting with episodic ataxia. Pediatr Neurol 2014;50(1):99e100.
6. Conroy J, McGettigan P, Murphy R, et al. A novel locus for episodic ataxia: UBR4 the likely candidate. Eur J Hum Genet 2014;22(4):505e10. 7. Berkovic SF, Heron SE, Giordano L, et al. Benign familial neonatal-infantile seizures: characterization of a new sodium channelopathy. Ann Neurol 2004;55(4):550e7. 8. Sugawara T, Tsurubuchi Y, Agarwala KL, et al. A missense mutation of the Naþ channel aII subunit gene Nav1.2 in a patient with febrile and afebrile seizures causes channel dysfunction. Proc Natl Acad Sci U S A 2001;98:6384e9. 9. Matalon D, Goldberg E, Medne L, Marsh ED. Confirming an expanded spectrum of SCN2A mutations: a case series. Epileptic Disord 2014;16(1):13e8. 10. Rauch, et al. Range of genetic mutations associated with severe non-syndromic sporadic intellectual disability: an exome sequencing study. Lancet 2012;380(9854):1674e82. 11. Sanders SJ, Murtha MT, Gupta AR, et al. De novo mutations revealed by whole exome sequencing are strongly associated with autism. Nature 2012;485(7397):237e41. 12. Tavassoli T, Kolevzon A, Wang AT, et al. De novo SCN2A splice site mutation in a boy with Autism spectrum disorder. BMC Med Genet 2014;15:35. 13. Liao Y, Anttonen AK, Liukkonen E, et al. SCN2A mutation associated with neonatal epilepsy, late-onset episodic ataxia, myoclonus, and pain. Neurology 2010;75(16):1454e8. 14. Schwarz N, Hahn A, Bast T, et al. Mutations in the sodium channel gene SCN2A cause neonatal epilepsy with late-onset episodic ataxia. J Neurol 2016;263(2):334e43. 15. Ogiwara I, Ito K, Sawaishi Y, et al. De novo mutations of voltage-gated sodium channel aII gene SCN2A in intractable epilepsies. Neurology 2009;73:1046e53. 16. Nakamura K, Kato M, Osaka H, et al. Clinical spectrum of SCN2A mutations expanding to Ohtahara syndrome. Neurology 2013;81(11):992e8. 17. van Karnebeek CDM, Shevell M, Zschocke J, et al. The metabolic evaluation of the child with an intellectual developmental disorder: diagnostic algorithm for identification of treatable causes and new digital resource. Mol Genet Metab 2014;111:428e38. 18. van Karnebeek CD, Murphy T, Giannasi W, et al. Diagnostic value of a multidisciplinary clinic for intellectual disability, TIDE Complex Diagnostic Clinic at B.C. Children's Hospital. Can J Neurol Sci 2014;41:333e45. 19. Bain PG, O'Brien MD, Keevil SF, Porter DA. Familial periodic cerebellar ataxia: a problem of cerebellar intracellular pH homeostasis. Ann Neurol 1992;31(2):147e54.
Official Journal of the European Paediatric Neurology Society
Case Study
Episodic ataxia associated with a de novo SCN2A mutation Emma L. Leach a, Clara D.M. van Karnebeek b,c,d, Katelin N. Townsend a,c, Maja Tarailo-Graovac d, Juliette Hukin e, William T. Gibson a,c,* a
Department of Medical Genetics, University of British Columbia, Vancouver, Canada Division of Biochemical Diseases, Department of Pediatrics, British Columbia Children's Hospital, Vancouver, Canada c Child and Family Research Institute, British Columbia Children's Hospital, Vancouver, Canada d Centre for Molecular Medicine and Therapeutics, University of British Columbia, Vancouver, Canada e Division of Neurology, Department of Pediatrics, British Columbia Children's Hospital, Vancouver, Canada b
article info
abstract
Article history:
Introduction: Episodic ataxia (EA) is characterized by paroxysmal attacks of ataxia
Received 27 November 2015
interspersed by asymptomatic periods. Dominant mutations or copy number variants in
Received in revised form
CACNA1A are a well-known cause of EA.
4 May 2016
Clinical presentation: This boy presented with clinical features of episodic ataxia, and also
Accepted 26 May 2016
showed cerebellar atrophy, hypotonia, autism and global developmental delay at age 4 years. Acetazolamide prevented further episodes of ataxia, dystonia and encephalopathy.
Keywords:
Extensive biochemical and genetic tests were unrevealing; whole exome sequencing found
Episodic ataxia
a previously unreported variant in SCN2A, proven to be de novo and predicted to be protein-
SCN2A
damaging.
Acetazolamide
Conclusion: Considered alongside previous reports of episodic ataxia in SCN2A mutation-
Cerebellar atrophy
positive patients, our case further illustrates the genetic heterogeneity of episodic ataxia.
Treatment
In addition, this case suggests that acetazolamide may be an effective treatment for some
Channelopathy
aspects of the phenotype in a broader range of channelopathy-related conditions. © 2016 European Paediatric Neurology Society. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The emergence of whole exome sequencing (WES) in research and clinical contexts has proven that many sporadic
conditions, including seizure disorders, have a broader range of genetic causes than was previously suspected. Similarly, genes previously thought to cause unique disorders when mutated have been shown to cause disorders in which they had not previously been implicated.
* Corresponding author. Child & Family Research Institute, Room A4-182, 950 West 28th Avenue, Vancouver, BC V5Z 4H4, Canada. Fax: þ1 604 875 2373. E-mail address: [email protected] (W.T. Gibson). http://dx.doi.org/10.1016/j.ejpn.2016.05.020 1090-3798/© 2016 European Paediatric Neurology Society. Published by Elsevier Ltd. All rights reserved.
e u r o p e a n j o u r n a l o f p a e d i a t r i c n e u r o l o g y 2 0 ( 2 0 1 6 ) 7 7 2 e7 7 6
Episodic ataxia type 2 (EA2) is characterized by paroxysmal attacks of ataxia.1,2 Attacks typically onset in childhood or early adolescence, and range in frequency from annually to several times per week, with asymptomatic periods of variable duration. Episodes can include other neurological manifestations such as vertigo, nausea, headache, dysarthria, diplopia, tinnitus, dystonia, hemiplegia, and seizures.1,3 Maintenance treatment with oral acetazolamide (Diamox®) effectively prevents or reduces the frequency and severity of attacks.1,2 Heterozygous variants in the voltage-gated calcium channel encoding CACNA1A account for over 95% of EA2 cases.1 However, Hirose et al.4 described a large family with autosomal dominant acetazolamide-responsive EA2 wherein no CACNA1A mutations were identifiable, supporting genetic heterogeneity in the EA2 phenotype and suggesting that other causative genes remain to be identified.3 Indeed, more recently, additional genes have been associated with the EA2 clinical presentation. A 2 year-old boy with periodic attacks of ataxic gait, repeated vomiting, and abnormal eye movement was diagnosed with EA2 and treated with acetazolamide; he was found to have glucose transporter type 1 deficiency attributed to a de novo frameshift insertion in the SLC2A1 gene.5 Conroy et al.6 also proposed UBR4 may be associated with a form of episodic ataxia characterized by periods of unsteadiness, generalized weakness and slurred speech in early childhood. As genome sequencing technology yields additional genes associated with discrete clinical phenotypes, so does it also identify new phenotypes associated with genes implicated previously in other disorders. SCN2A (MIM 182390) encodes the alpha subunit of voltagegated, type II sodium channels, highly expressed in the brain. Pathogenic SCN2A mutations are also known to cause benign familial neonatal-infantile epilepsy,7 and generalised epilepsy with febrile seizures plus (GEFSþ).8 However, recent studies have associated coding variants in SCN2A with a broader phenotypic spectrum, including severe infantile-onset epilepsy,9 intellectual disability without seizures,10 and autism.11,12 SCN2A mutations have also been identified among patients with episodic ataxia.13,14 Liao et al.13 described a patient with neonatal-onset seizures and variable episodes of ataxia, myoclonia, headache, and back pain associated with a de novo missense mutation (p.Ala263Val) in SCN2A. Three additional patients with a similar phenotype to the patient described by Liao et al.13 were also found to harbour SCN2A mutations.14 The de novo p.Ala263Val mutation previously reported was identified in one of the additional cases, and the two patients were found to have a novel de novo p.Arg1882Gly mutation, as well as a paternally inherited p.Gly1522Ala variant in one case.14 In vitro studies showed gain-of-function effects for both de novo mutations, and predicted neuronal hyperexcitability.13,14 Ogiwara et al.15 identified two de novo SCN2A mutations in a cohort of SCN1A-negative intractable epilepsy cases. Recently, Nakamura et al.16 identified SCN2A mutations in 15 of 328 patients with Ohtahara syndrome, West syndrome or unclassified early-onset epileptic encephalopathies and recommended that SCN2A molecular analysis should be considered for children with different epileptic conditions.
773
Here we describe a novel SCN2A variant associated with acetazolamide-responsive episodic ataxia identified through WES, and thus broaden the clinical spectrum of this genetic sodium channelopathy.
2.
Case report
Parents provided informed consent for publication of this report. The family was enrolled into both the GARD study (protocol H09-01228) and TIDEX Study (protocol H12-00067), each approved by the Clinical Research Ethics Board of the University of British Columbia, Vancouver Canada. This 5-year old boy, born after an unremarkable pregnancy and delivery with normal birth weight and normal Apgar scores, presented acutely to our centre at 11 months of age with a history of minor head injury due to a fall forward out of his stroller that was accompanied by decreased level of consciousness, vomiting and possible seizures. Though there had been steady gains in motor skills with no regression prior to his first episode, his developmental profile had not been entirely typical. Specifically, he was not yet standing independently, cruising, nor engaged in canonical babbling, and would not respond to his name being called, despite normal hearing and normal interest in other sounds. While in hospital, he was markedly hypotonic and had encephalopathy. He also experienced episodes of intermittent limb stiffness and dystonic posturing, with arching of his back, rightward deviation of his eyes and head, and concomitant nystagmus. EEG on the day of presentation was normal, thus these episodes were interpreted as dystonic posturing. Head CT and cerebrospinal fluid (CSF) glucose were normal. He was initially treated with phenytoin and diazepam, but developed choreoathetoid movements. These were attributed to phenytoin toxicity, so because of his normal EEG the phenytoin was stopped. MRI shortly after admission demonstrated cerebellar oedema with T2 hyperintensity, and bilateral arachnoid cysts of the middle cranial fossa (Fig. 1). Biochemical investigations (performed according to the algorithm for treatable inborn errors of metabolism in developmentally and intellectually delayed patients17) in blood (plasma amino acids, homocysteine, copper, ceruloplasmin), urine (glycosaminoglycans, oligosaccharides, organic acids, purines, pyrimidines and creatine metabolites) and CSF (protein, glucose, lactate), did not yield significant abnormalities (Supplementary Table 1). His encephalopathy, dystonia and ataxia gradually decreased over the next five days, though he had not completely returned to his baseline level of developmental functioning. He was discharged on day five on no medications, much improved clinically with residual mild clumsiness and central hypotonia. Presumptive diagnosis at that time was post-infectious encephalitis. The family history is positive for migraines without hemiplegia or any neurologic deficit in his mother; as well as migraine with aura in a second cousin. His father experienced adult-onset seizures secondary to two separate oligodendrogliomas. There was no other reported family history of migraine, ataxia, epilepsy or autism. At 17 months of age, he again presented after another unprovoked fall with minor head injury, having manifested an
774
e u r o p e a n j o u r n a l o f p a e d i a t r i c n e u r o l o g y 2 0 ( 2 0 1 6 ) 7 7 2 e7 7 6
Fig. 1 e Left: Coronal T2-weighted MR image at 11 months of age shows swollen hyperintense cerebellar vermis and hemispheres. Right: Coronal T2-weighted MR image at 3 years, 7 months of age shows normal cerebellar signal. There is prominence of the cerebellar folia in keeping with mild cerebellar atrophy.
episode of stiffness in all four limbs lasting less than a minute, with acute decrease in level of consciousness. On admission, he had encephalopathy, intermittent vomiting, ataxia, low axial tone, hyperreflexia, and intermittent dystonic posturing. EEG was again normal on day two of admission. He gradually improved and by day ten was fit for discharge, though he had residual ataxia. Metabolic workup was again normal (Supplementary Table 1). An acylcarnitine profile showed low free carnitine, so he was started on oral 250 mg L-carnitine. Repeat brain MRI showed mild cerebellar oedema, less severe than the previous scan. Given the recurrence of acute encephalopathy, hypotonia, dystonia and ataxia following minor head injury, a channelopathy related to the CACNA1A gene was considered the probable diagnosis and he was started empirically on 62.5 mg acetazolamide twice daily (5.2 mg/kg at that time). His encephalopathy, dystonia and ataxia improved markedly within 12 h of starting acetazolamide, and he has not suffered any subsequent acute episodes of encephalopathy, dystonia, seizures or ataxia. The dose has been increased commensurate with growth and also following a couple of minor ataxia spells 15 months post-initial presentation; he is presently on 100 mg (approximately 5 mg/kg) BID. Even though CACNA1A mutations had been ruled out with molecular sequencing, acetazolamide was continued given its observed benefit (as attested by the severity of previous events when not taking acetazolamide, and the resolution of other, minor events when the dose was increased). Neither the parents nor the treating physician were willing to consider an empirical re-challenge of acetazolamide withdrawal. Follow-up MRIs of his brain at 2.5 years of age showed bilateral atrophy of the cerebellar hemispheres and vermis, which initially progressed and then stabilized within one year (Fig. 1). At age 3 years, our patient was assessed and found to meet DSM-V criteria for an autism spectrum disorder. He was noted
to have deficits in social communication and social interaction in combination with atypical patterns of behaviour. He also was found to have global developmental delay, functioning at an 11-month level. Clinical-grade Sanger sequencing of CACNA1A to investigate his possible diagnosis of EA2 did not identify any rare variants in that gene. As well, glucose-1 transporter deficiency syndrome (GLUT1-DS) was ruled out by normal CSF glucose studies and clinical-grade Sanger sequencing of SLC2A1. Chromosome microarray analysis identified no significant deletions larger than 200 kb and no duplications larger than 400 kb. At 4 years of age he was evaluated at the TIDE Complex Diagnostic Clinic18 in our institution to elucidate the genetic basis for his intellectual and developmental disabilities, central hypotonia, and autism spectrum disorder. Based on previous genetic testing, the remaining differential diagnosis included episodic ataxia types 1, 5 or 6 (KCNA2, CACNB4, SLC1A3), primary coenzyme Q deficiency, pyruvate dehydrogenase deficiency and mitochondrial diseases. Clinical WES at GeneDx in 2012 targeted known human disease genes in both the nuclear and mitochondrial genomes (mean depth of coverage 206 on Agilent SureSelect XT2 All Exon V4 kit). No sequence changes were identified in any of the known genes associated with any of the types of episodic ataxia. Two candidate rare missense variants were identified, one each in KCNQ3 (hg19 8:133,141,789C>T (p.R780H)) and SCN2A (hg19 2:166,245,217G>A (p.G1634D)). The KCNQ3 variant is rare in the Exome Aggregation Consortium (ExAC) data set (frequency ¼ 2.443e-05), however it is predicted to be benign at the protein level by multiple programs (PROVEAN score for p.R780H in KCNQ3 was 0.5, considered “neutral”). The SCN2A variant is novel, not reported in ExAC or other healthy population databases. Further, it is predicted damaging (PROVEAN Score for p.G1634D in SCN2A was 6.1, with anything below 2.5 considered to be deleterious).
e u r o p e a n j o u r n a l o f p a e d i a t r i c n e u r o l o g y 2 0 ( 2 0 1 6 ) 7 7 2 e7 7 6
775
Fig. 2 e A) Family pedigree. B) Sanger sequencing traces with an arrow indicating the location of interest. Traces show that the SCN2A variant (hg19 2:166,245,217G>A (p.G1634D)) is de novo in the index and that the KCNQ3 variant (hg19 8:133,141,789C>T (p.R780H)) was inherited from the mother. C) Multi-sequence alignment showing high conservation of the Gly (depicted in blue) at position 1634 in SCN2A. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Research-grade Sanger sequencing of the variants in SCN2A and KCNQ3 was done to confirm the presence of the variants identified by WES in the proband, and to assess segregation with disease. Parental testing indicated that the KCNQ3 variant was maternally-inherited, and that the SCN2A variant was de novo (Fig. 2). Based on the de novo inheritance and in silico prediction of deleterious impact on protein function, the SCN2A variant was concluded as most likely causative of the proband's phenotype.
3.
Discussion
This case expands the differential diagnosis of episodic ataxia with a treatable condition e a de novo SCN2A mutation responsive to acetazolamide. In conjunction with previous literature reports,13e16 evidence from our patient suggests that episodic ataxia may extend the SCN2A phenotypic spectrum, and that SCN2A mutations should be considered in the differential diagnosis of episodic ataxia, acute encephalopathy and cerebellar oedema following minor head injury. Combined with the previous reports of patients with SCN2A mutations presenting with episodic ataxia13,14 and its phenotypic fit with the autism and moderate developmental delay, we consider the SCN2A variant in this patient to be pathogenic based on its de novo status and its predicted high functional impact. A maternally-inherited, likely benign KCNQ3 variant was also identified in the patient but deemed to be an unlikely cause, given its inheritance from a mother without ataxia, intellectual disability or autism, and its predicted neutrality. Since both genes affect neuronal
excitability, it is possible that the two variants may have an epistatic interaction, such that the phenotype associated with both variants might be more severe than with the SCN2A variant alone. Functional studies to elucidate the combined effect on transmembrane conductance of both variants fall outside the scope of this study. Acetazolamide is a carbonic anhydrase inhibitor used to treat a variety of conditions, including hypokalemic periodic paralysis, altitude sickness, epilepsy, benign intracranial hypertension as well as episodic ataxia.2 The mechanism by which acetazolamide relieves symptoms associated with EA2 is not well understood, though it has been suggested that acetazolamide may correct abnormal intracellular pH levels in the cerebellum.2,19 Trials of acetazolamide in two patients with neonatal-onset seizures and episodic ataxia attributed to SCN2A mutations have been reported.13,14 In the first patient, acetazolamide appeared to improve the patient's seizures, however there was no effect on episodic ataxia.13 In the second patient, a trial with acetazolamide had no effect.14 The application of acetazolamide to treat apparently unrelated conditions suggests that its mode of action is not confined to calcium voltage-gated channels, but instead acts indirectly via another pathway. Given the therapeutic response of our patient, acetazolamide may be effective for a broader range of ataxias and channelopathies, and might be considered in the treatment of episodic ataxia in SCN2A patients.
Conflict of interest statement The authors declare no conflict of interest.
776
e u r o p e a n j o u r n a l o f p a e d i a t r i c n e u r o l o g y 2 0 ( 2 0 1 6 ) 7 7 2 e7 7 6
Acknowledgements We would like to thank Dr. Michael Sargent for his expert interpretation of neuroimaging. Salary funding to WTG provided by the BC Children's Hospital Foundation through a CFRI Clinician-Scientist Award. Operating funds to WTG's lab provided by CIHR MOP-119595. CvK is the recipient of a Michael Smith Foundation for Health Research Award. We gratefully acknowledge the support of the BC Children's Hospital Foundation (1st Collaborative Area of Innovation: Treatable Intellectual Disability Endeavour in British Columbia, www.tidebc.org) and the BC Children's Hospital BioBank.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ejpn.2016.05.020.
references
1. Spacey S. Episodic ataxia type 2. 2003 Feb 24 [Updated 2011 Dec 8]. In: Pagon RA, Adam MP, Ardinger HH, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2014. Available from: http://www. ncbi.nlm.nih.gov/books/NBK1501/. 2. Strupp M, Zwergal A, Brandt T. Episodic ataxia type 2. Neurotherapeutics 2007;4:267e73. 3. Jen J, Kim GW, Baloh RW. Clinical spectrum of episodic ataxia type 2. Neurology 2004;62(1):17e22. 4. Hirose H, Arayama T, Takita J, et al. A family of episodic ataxia type 2: no evidence of genetic linkage to the CACNA1A gene. Int J Mol Med 2003;11(2):187e9. 5. Ohshiro-Sasaki A, Shimbo H, Takano K, Wada T, Osaka H. A three-year-old boy with glucose transporter type 1 deficiency syndrome presenting with episodic ataxia. Pediatr Neurol 2014;50(1):99e100.
6. Conroy J, McGettigan P, Murphy R, et al. A novel locus for episodic ataxia: UBR4 the likely candidate. Eur J Hum Genet 2014;22(4):505e10. 7. Berkovic SF, Heron SE, Giordano L, et al. Benign familial neonatal-infantile seizures: characterization of a new sodium channelopathy. Ann Neurol 2004;55(4):550e7. 8. Sugawara T, Tsurubuchi Y, Agarwala KL, et al. A missense mutation of the Naþ channel aII subunit gene Nav1.2 in a patient with febrile and afebrile seizures causes channel dysfunction. Proc Natl Acad Sci U S A 2001;98:6384e9. 9. Matalon D, Goldberg E, Medne L, Marsh ED. Confirming an expanded spectrum of SCN2A mutations: a case series. Epileptic Disord 2014;16(1):13e8. 10. Rauch, et al. Range of genetic mutations associated with severe non-syndromic sporadic intellectual disability: an exome sequencing study. Lancet 2012;380(9854):1674e82. 11. Sanders SJ, Murtha MT, Gupta AR, et al. De novo mutations revealed by whole exome sequencing are strongly associated with autism. Nature 2012;485(7397):237e41. 12. Tavassoli T, Kolevzon A, Wang AT, et al. De novo SCN2A splice site mutation in a boy with Autism spectrum disorder. BMC Med Genet 2014;15:35. 13. Liao Y, Anttonen AK, Liukkonen E, et al. SCN2A mutation associated with neonatal epilepsy, late-onset episodic ataxia, myoclonus, and pain. Neurology 2010;75(16):1454e8. 14. Schwarz N, Hahn A, Bast T, et al. Mutations in the sodium channel gene SCN2A cause neonatal epilepsy with late-onset episodic ataxia. J Neurol 2016;263(2):334e43. 15. Ogiwara I, Ito K, Sawaishi Y, et al. De novo mutations of voltage-gated sodium channel aII gene SCN2A in intractable epilepsies. Neurology 2009;73:1046e53. 16. Nakamura K, Kato M, Osaka H, et al. Clinical spectrum of SCN2A mutations expanding to Ohtahara syndrome. Neurology 2013;81(11):992e8. 17. van Karnebeek CDM, Shevell M, Zschocke J, et al. The metabolic evaluation of the child with an intellectual developmental disorder: diagnostic algorithm for identification of treatable causes and new digital resource. Mol Genet Metab 2014;111:428e38. 18. van Karnebeek CD, Murphy T, Giannasi W, et al. Diagnostic value of a multidisciplinary clinic for intellectual disability, TIDE Complex Diagnostic Clinic at B.C. Children's Hospital. Can J Neurol Sci 2014;41:333e45. 19. Bain PG, O'Brien MD, Keevil SF, Porter DA. Familial periodic cerebellar ataxia: a problem of cerebellar intracellular pH homeostasis. Ann Neurol 1992;31(2):147e54.