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Department of Medical Genetics, Medical School, Clinical Center, University of Pécs, Pécs, HungarySzentágothai Research Center, University of Pécs, Pécs, Hungary
Department of Medical Genetics, Medical School, Clinical Center, University of Pécs, Pécs, HungarySzentágothai Research Center, University of Pécs, Pécs, Hungary
Department of Medical Genetics, Medical School, Clinical Center, University of Pécs, Pécs, HungarySzentágothai Research Center, University of Pécs, Pécs, Hungary
Corresponding author at: Department of Medical Genetics, Medical School, Clinical Center, University of Pécs, H-7624 Pécs, Szigeti street 12., Hungary.
Department of Medical Genetics, Medical School, Clinical Center, University of Pécs, Pécs, HungarySzentágothai Research Center, University of Pécs, Pécs, Hungary
Department of Medical Genetics, Medical School, Clinical Center, University of Pécs, Pécs, HungarySzentágothai Research Center, University of Pécs, Pécs, Hungary
Hungarian patients presenting Dravet syndrome were screened for SCN1A mutation.
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Fifteen novel point mutations of SCN1A gene were found with Sanger sequencing.
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Gross gene deletion were identified in three patients with Dravet syndrome.
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Clear relationship between phenotype and genotype could not be found.
Abstract
Purpose
The vast majority of mutations responsible for epilepsy syndromes such as genetic epilepsy with febrile seizures plus (GEFS+) and Dravet syndrome (DS) occur in the gene encoding the type 1 alpha subunit of neuronal voltage-gated sodium channel (SCN1A).
Methods
63 individuals presenting with either DS or GEFS + syndrome phenotype were screened for SCN1A gene mutation using Sanger sequencing and multiplex ligation-dependent probe amplification (MLPA).
Results
Our research study identified 15 novel pathogen mutations in the SCN1A gene of which 12 appeared to be missense mutations with addition of two frameshift-deletions and one in-frame deletion. The distribution of clinical phenotypes in patients carrying SCN1A mutations was as follows: twelve patients had classical DS, three patients had GEFS + syndrome and two relatives of DS patients were suffering from febrile seizures.
Conclusions
Our study highlights the phenotypic and genotypic heterogeneities of DS and GEFS + with the important aim of gaining a deeper understanding of SCN1A-related disorders. This study also represents the first genetic analysis of the SCN1A gene in a Hungarian cohort with the DS and GEFS + syndrome phenotype.
Epilepsy, conventionally classified as idiopathic by etiology in up to 70 % of the cases, has started to reveal its genetic roots with the advent of widely available genetic testing [
Revised terminology and concepts for organization of seizures and epilepsies: report of the ILAE Commission on Classification and Terminology, 2005-2009.
]. The majority of the genes identified as disease causing mutation encode ion channels or receptors including voltage-gated sodium, potassium, calcium and chloride channels; additionally, receptors for acetylcholine and γ-amino butyric acid (GABA) [
]. The most widely investigated association is between SCN1A mutations (OMIM 182389) with Dravet syndrome (DS) (OMIM 607208) and the genetic epilepsy with febrile seizure plus syndrome (GEFS+) (OMIM 604403) [
DS (previously known as severe myoclonic epilepsy in infancy, SMEI) is one of the most common epileptic encephalopathies of infancy. The incidence of DS is about 1 in 20,000–40,000 live births [
]. It is characterized by febrile and afebrile, generalized and unilateral, clonic or tonic-clonic seizures that are often prolonged and occur in the first year of life in an otherwise healthy infant. The condition may be later associated with myoclonus, atypical absences and partial seizures. Some children, however, do not develop myoclonic symptoms and will have a milder form of DS [
]. The seizures are frequently triggered by fever and refractory to antiepileptic treatment. The convulsions present commonly as febrile status epilepticus during the first year of life [
]. Brain magnetic resonance imaging (MRI) shows no pathological structural alterations in most cases and electroencephalogram (EEG) typically remains normal at the onset of the disease [
]. Valproate, stiripentol, topiramate and bromide are the mainstay of treatment while sodium channel blocking anticonvulsants have been shown to aggravate the seizures and therefore must be avoided [
]. Non-epileptic manifestations such as intellectual disability and ataxia may appear with age.
Alterations in the gene encoding the type 1 alpha subunit of neuronal voltage-gated sodium channel (SCN1A) are responsible for the symptoms in about 70–80% of the cases [
]. The majority of changes are de novo mutations; however, DS shows an autosomal dominant inheritance pattern in 10% of the cases. In the inherited cases, relatives carrying the same SCN1A mutation as the patient often develop milder forms of epilepsy consistent with the phenotypic spectrum of GEFS+, or might even stay unaffected suggesting the role of additional genes in disease severity [
]. Missense and truncating mutations are found at approximately equal frequencies in DS, while GEFS + is largely associated with missense mutations. SCN1A was first described as an epilepsy-causing gene in 2001 and more than 1200 mutations have been identified so far [
The type 1 alpha subunit of the voltage-gated sodium channel is one of the four isoforms of the mammalian voltage-gated sodium channel alpha subunits that are expressed at high levels in the central nervous system (CNS) [
]. The SCN1A gene harbours 26 exons and encodes a 2009 amino acid-containing transmembrane protein which is a critical component of the voltage-gated sodium channels in the CNS [
]. If the alpha subunit, the core component of the transmembrane protein, does not function normally, the sodium channel can remain closed which, in turn, stops synaptic signal propagation to the next neuron [
]. Murine models suggest that the primary effect of both GEFS + and DS mutations is to decrease GABAergic inhibitory neuron activity, contributing to seizure generation in these patients. Impaired function of cerebellar GABAergic and inhibitory Purkinje cells may explain the observed ataxia and cognitive impairment in these patients [
The aim of this study was to investigate the mutational spectrum of the SCN1A gene in Hungarian patients with DS and GEFS + syndrome phenotype.
2. Materials and methods
Between January 2012 and December 2017 a total of 183 Hungarian individuals with fever-triggered and/or pharmacoresistant epilepsy were referred for genetic examination to the Department of Medical Genetics, University of Pécs. A total of 63 patients met the clinical and EEG characteristics diagnostic criteria of DS as per Guerrini and Oguni. DNA samples were examined for SCN1A gene mutations using Sanger sequencing analysis and multiplex ligation-dependent probe amplification (MLPA) [
]. In cases where an SCN1A mutation was found, segregation analysis was performed to determine the mutation’s de novo or inherited origin. Written informed consent was obtained from all subjects. The collection and usage of DNA samples, and management of data followed the Helsinki Declaration of 1975 and also satisfying the Hungarian legal requirements of genetic examination, research and biobanking.
Genomic DNA was extracted from peripheral blood cells of the patients using the E.Z.N.A. Blood DNA Maxi Kit (Omega Bio-tek, USA) according to the protocol of the manufacturer. As the first stage of screening, mutation analysis was performed of the SCN1A gene (sequence reference: NM_001165963, NP_001159435) by direct sequencing. Exons 1–26 were amplified by PCR. Primers were designed in our laboratory (primer sequences and PCR conditions are available upon request in our institute). Sequencing was performed with the same primers as those applied for the PCR amplification using a BigDye Terminator v1.1 Cycle Sequencing Kit (Applied Biosystems) according to the manufacturer’s instructions. Interpretation of the results was performed with the help of Mutation Taster (http://www.mutationtaster.org), PolyPhen-2 (http://genetics.bwh.harvard.edu/pph2) PROVEAN (http://provean.jcvi.org/index.php) and Mutation Assessor (http://mutationassessor.org/r3) prediction software. The identified mutations were validated on a second sample obtained from the patients. During the second stage of screening, 33 sequence-negative patients were tested with MLPA method. MLPA was performed with the SALSA MLPA Kit P-137 Probemix (MRC-Holland, Netherlands) in accordance with the manufacturer’s instructions. All identified variants were classified by the standards and guidelines set by the American College of Medical Genetics and Genomics (ACMG) standards and guidelines [
Standards and guidelines for the interpretation of sequence variants: a joint consensus recommendation of the American College of Medical Genetics and Genomics and the Association for Molecular Pathology.
A total of 12 previously described SCN1A alterations (in 15 patients and three relatives) and 15 previously unknown pathogenic mutations (in 15 patients and two relatives) were identified by Sanger sequencing analysis. MLPA testing detected gross deletions of the SCN1A gene in three additional patients. Altogether, different types of SCN1A mutations were identified in 33 patients from our cohort. The inheritance patterns could not be determined in all cases as parental samples were not available in some families. The mutations proved to be inherited in six cases. The parents either have developed GEFS + syndrome (Patient 1b) or suffered from febrile seizures in childhood (Patient 3c, 21b, 24b). A couple of parents have remained unaffected (Patient 5b, 8b).
Among the previously described mutations, a recurrent, missense mutation in exon 17 was detected (p.Thr1174Ser) in three non-consanguineous patients and in the mother of one patient without any neurological symptoms (Patients 4, 5/a, b, 20) According to the study by Cestéle et al. (2013) the functional effects of this mutation are divergent. They reported a three-generation family segregating this mutation where three affected individuals had developed febrile seizures and/or focal occipital epilepsy while two family members suffered from typical familial hemiplegic migraine [
]. In exon 19, the recurrent truncating mutation p.Arg1245* was identified in two non-consanguineous patients presenting with classical DS phenotype (Patients 6, 7). A familial truncating mutation in exon 10 was detected in a father and in his two daughters. While the girls were affected by classical DS, the father merely had developed childhood febrile seizures (Patients 3/a, b, c). A mother with GEFS+, and his son with classical DS phenotype were harbouring a splice region variant (Patients 1/a, b). A frameshift causing deletion and a missense mutation associated with GEFS + phenotype (Patients 11, 13) and five other different, previously known missense alterations associated with classical DS were also found in our cohort (Patients 2, 8/a, 9, 10, 12). Missense and nonsense mutations, one splice region variant and one frameshift-causing deletion were detected so far (Table 1).
Table 1Previously described mutations in the SCN1A gene.
Patient
Location
Sequence change
Amino acid change
Mutation type
Phenotype
1/a
intron 5-6
c.473 + 5 het G-A
–
splice site
DS
1/b
intron 5-6
c.473 + 5 het G-A
–
splice site
GEFS+
2
exon 9
c.1277A > G
p.Tyr426Cys
missense
DS
3/a
exon 10
c.1624 C > T
p.Arg542*
nonsense
DS
3/b
exon 10
c.1624 C > T
p.Arg542*
nonsense
DS
3/c
exon 10
c.1624 C > T
p.Arg542*
nonsense
FS
4
exon 17
c.3521C > G
p.Thr1174Ser
missense
GEFS+
5/a
exon 17
c.3521C > G
p.Thr1174Ser
missense
DS
5/a
exon 21
c.4219C > T
p.Arg1407*
nonsense
DS
5/b
exon 17
c.3521C > G
p.Thr1174Ser
missense
sine morbo
6
exon 19
c.3733C > T
p.Arg1245*
nonsense
DS
7
exon 19
c.3733C > T
p.Arg1245*
nonsense
DS
8/a
exon 20
c.3924A > T
p.Glu1308Asp
missense
DS
8/b
exon 20
c.3924A > T
p.Glu1308Asp
missense
sine morbo
9
exon 25
c.4793A > T
p.Tyr1598Phe
missense
DS
10
exon 26
c.4934 G > A
p.Arg1645Gln
missense
DS
11
exon 26
c.5189 T > C
p.Leu1730Pro
missense
GEFS+
12
exon 26
c.5264A > G
p.Asp1755Gly
missense
DS
13
exon 26
c.5536-5539delAAAC
p.Lys1846SerfsX11
frameshift
GEFS+
Sequence reference: NM_001165963 NP_001159435.
Patient 1/b is the mother of Patient 1/a; Patient 3/c is the father of Patient 3/a and 3/b; Patient 5/b is the mother of Patient 5/a; Patient 8/b is the mother of Patient 8/a.
Three patients carried two different mutations simultaneously: two known mutations in one case (Patient 5/a), one known and one previously undescribed variants in the other patient (Patient 20), and another patient with two novel SCN1A mutations were identified (Patient 15).
Among the novel, previously undescribed SCN1A mutations 12 missense variants, two frameshift causing and one in-frame deletions were identified (Table 2). A missense variant in the last exon of SCN1A was detected in monozygotic twins with classic DS phenotype (Patients 26/a, b). One patient with a novel frameshift causing deletion and another with a missense mutation had a mild type of DS as they both appeared to have near normal cognitive ability with satisfactory seizure-control at school-age (Patients 19, 20). One previously undescribed frameshift causing and one in-frame deletion in patients with DS phenotype (Patients 16, 18) and seven other, novel missense mutations associated with DS phenotype were also found in our cohort (Patients 15, 17, 21/a, 22, 25, 27). Three novel missense mutations associated with GEFS + phenotype were also identified (Patients 14, 23, 24/a). Two family members were detected as carriers of a pathogen SCN1A mutation having had childhood febrile seizures only (Patients 21/b, 24/b). Using the ACMG guidelines for the interpretation of sequence variants, 2 of 15 novel variants were classified as “pathogenic”, 12 were classified as “likely pathogenic” and one remained a variant of uncertain significance [
Standards and guidelines for the interpretation of sequence variants: a joint consensus recommendation of the American College of Medical Genetics and Genomics and the Association for Molecular Pathology.
The diagnosis of DS was confirmed with MLPA method in three additional patients. Phenotypes of two of them were not significantly different from those with point mutations (Patients 28, 30). On the other hand, the third patient (Patient 29) with a large heterozygous deletion of exon 1–17 ha d an unexpectedly mild DS phenotype. Table 4 contains relevant information about our MLPA-positive patients.
Table 4Patients with SCN1A CNVs detected by MLPA.
Patient
Genotype
Phenotype
28
heterozygous whole gene deletion
DS
29
heterozygous exon 1-17 deletion
DS#
30
heterozygous exon 1-2 deletion
DS
##Mild type of the disease with near normal cognitive ability and satisfactory seizure-control.
]. Noting the disease’s association with fever, Claes et al. screened for mutations in SCN1A which had been known to cause GEFS + syndrome. Seven de novo mutations were eventually found by the group, and SCN1A has become one of the most relevant epilepsy genes since. Today DS is described as “a prototype of an epileptic encephalopathy” [
Among the 63 patients with DS or GEFS + phenotype investigated in this study pathogen alterations of the SCN1A gene have been confirmed in 33 patients (52,4%) and in five symptomless relatives. Twelve previously described mutations were detected in 18 participants and 15 novel mutations were found in 17 individuals. Most of the identified mutations proved to be missense mutations that probably alter but do not abolish the ion channel’s function [
]. Among the discovered mutations there were only two previously known, recurrent mutations that were identified in more than one patient in our cohort (p.Thr1174Ser and p.Arg1245*). In accordance with our data, previous observations also show that out of more than 1200 reported SCN1A mutations, only 18% are recurrent [
]. Apart from the 30 patients with SCN1A point mutations, MLPA method revealed three cases of SCN1A gene deletion. The frequency of MLPA-detected anomalies were 9,09% in our cohort which is similar to that published in the study of Marini et al. (2009) [
]. Based on literature data, the average frequency of MLPA-detected deletions and duplications is approximately 10–12% among SCN1A-mutation negative patients; therefore we recommend this method as a second-tier of screening [
]. Our patients with confirmed gross deletions of SCN1A do not show any phenotypic difference compared to those with point mutations: febrile and/or afebrile seizures and intellectual disability remain the characteristic features in both groups. Despite the large heterozygous deletion, interestingly, the phenotype appeared to be a milder form of DS in one case. In fact, evidence shows that an SCN1A deletion combined with SCN2A and SCN3A deletions produce a more severe phenotype than DS with earlier onset and progressive microcephaly. On the other hand, disease associated with SCN1A and SCN9A deletions produces milder DS phenotypes [
In this paper the authors aimed to further understand the genotypic and phenotypic diversity of SCN1A gene-related disorders. Phenotype exhibited large variability in our patient cohort, and we could not detect any strong correlation between genotype and phenotype. As shown by Fig. 1. The detected mutations cannot be localized to a hot spot region of the gene. As Brunklaus et al. speculate, the phenotype might not necessarily be determined only by the SCN1A protein itself, but by a number of auxiliary proteins. [
]. Recently, Sadleir et al. identified a recurrent SCN1A missense variant in exon 5 (c.677C > T, p.Thr226Met) in 8 children with an early onset epileptic encephalopathy much more severe than DS. The disorder is characterized by an earlier age of onset, profound developmental impairment and a distinctive hyperkinetic movement disorder [
Nav1.1 localizes to axons of parvalbumin-positive inhibitory interneurons: a circuit basis for epileptic seizures in mice carrying an Scn1a gene mutation.
]. DS as a channelopathy causes widespread Nav1.1 dysfunction throughout the brain which, in turn contributes to the encephalopathy. This already-vulnerable system may be susceptible to secondary aggravating events such as status epilepticus.
The early clinical diagnosis of DS may be difficult because the typical clustering of symptoms become apparent only during follow-up. An infant with prolonged febrile seizures and a confirmed SCN1A mutation has an SCN1A gene-related disorder. Due to the lack of consistent genotype-phenotype correlations it is unpredictable whether the disorder may lead to the evolution toward GEFS + syndrome or DS.
Recently, Cetica et al. reported that age of seizure onset in individuals with SCN1A mutations is a stronger predictor of outcome than the type of mutation [
]. It is assumed that early recognition and treatment to control prolonged/repeated seizures in the first year of life might limit the progression to epileptic encephalopathy [
One of the limitations of our study is that some mutations may pass undetected using conventional techniques such as Sanger sequencing. Resequencing of SCN1A – negative patients by a targeted next generation sequencing (NGS) panel comprising different epilepsy genes, including SCN1A or by whole exome sequencing (WES) could further refine our results [
]. Using an NGS gene panel these patients could also obtain an accurate diagnosis.
In the future, we are also planning to investigate genes that may influence the severity of the GEFS + and DS phenotype. This could further advance our understanding of the genotype-phenotype correlations in patients with pathogenic SCN1A mutations.
We hope that better understanding of the pathophysiology of this epilepsy syndrome will allow more adequate therapy and better outcome in the future.
5. Conclusions
This work represents the first genetic analysis of the SCN1A gene in a large Hungarian cohort with DS or GEFS + syndrome phenotype. Fifteen novel point mutations of SCN1A gene were identified among the 12 previously described mutations. Some cases proved to be familial. Three patients with DS phenotype harboured gross gene deletion of SCN1A.
Funding
This work was supported by the grant of the Hungarian Science FoundationNKFIH 119540, from the grant of GINOP-2.3.2-15-2016-00039 and from the grant of EFOP3.6.1-16-2016-00004.
Conflicts of interest
None declared.
Ethical approval
According the Hungarian legislation, the genetic examination process was done by the guidelines of the XXI/2008; the study design was approved by the HRB National Ethics Committee. Approval for the study was also provided by the Regional Research Ethics Committee of the Medical School University of Pécs. Record number: 7473-2018PTE.
Author contributions
KH designed the study. JZ, AS and JB carried out the analysis. ÁT, KH and MC collected the data and interpreted the results. ÁT and AF wrote the first version of the article. BM and KH critically revised the manuscript. All authors reviewed and edited the manuscript and approved the final version.
Acknowledgements
On behalf of the whole research team we would like to thank all the patients’ relatives who took part in this investigation. Furthermore, acknowledgement should be given to Zs. Siegler, M. Hegyi, R. Jakus, A. Fogarasi, B. Rosdy, M. Mellár, J. Janszky, M. Tóth, L. Liptai, E. Bereg, J. Cservenyák, I. György and M. Kassay for referring the patients to our institute.
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