The Role Of Scn1a Gene Variations In The Pathogenesis Of Familial Febrile Seizures

Backgrounds and Objective Febrile seizures (FS) are the most commonseizure disorder of early childhood, affecting approximately2%to5%of children. FSusually occurs between the age of3months and5years, with a peak incidence at18months, and onset after6years of age is uncommon. FS may be sporadic or familial;the latter is familial FS with a heritability of76%, which suggesting that geneticcomponent contributes to the pathogenesis of this disorder. The genetic mode of FSwas thought to be complex, showing autosomal dominant, autosomal recessive, andpolygenic inheritance. Twelve susceptibility loci or disease-causing genes of familialFS have been mapped or identified since1996, indicating that familial FS is a highlyheterogeneous disease.A small proportion of children suffering from FS may have seizures with feverbeyond6years of age, with or without subsequent afebrile generalized seizures,which is known as generalized epilepsy with febrile seizures plus (GEFS+). FS is themost common clinical phenotype of GEFS+. Previous studies have shown thatmutations of the voltage-gated sodium channel α1subunit gene (SCN1A) are the mostcommon genetic cause of GEFS+; moreover, the SCN1A is the most clinicallyrelevant among all of the known epilepsy genes. So far, however, the role of theSCN1A gene variations in the pathogenesis of familial FS has been seldom reported,and the existing research results are in debate. In view of the above considerations, weintend to carry out the SCN1A gene sequencing analysis for family members offamilial FS, in order to find pathogenic gene variations, which is help to clarify therelevance between familial FS and the SCN1A gene variations, further enrich thegenetic pathogenesis of familial FS, and provide a new clue for the prevention and treatment of familial FS.Methods From January2013to December2013, FS patients (both inpatientsand outpatients) were enrolled as research subjects. The diagnosis of all patients isconsistent with the definition of febrile seizures issued by the US National Institutesof Health (NIH) and the International League Against Epilepsy (ILAE). Patients withdevelopmental delay and/or intellectual disability were excluded from the study.Pedigrees of FS were screened by family history inquiry. The study protocol wasapproved by the Medical Ethics Committee of The First Affiliated Hospital of BengbuMedical College. After informed consent had been obtained, peripheral venous bloodsamples were collected from the probands and their available family members.Genomic DNA was extracted from the blood samples, and all26coding exons andexon-intron boundaries of the human SCN1A gene were amplified by polymerasechain reaction. All polymerase chain reaction products were subsequently sequenced.For proband who had variations in the SCN1A gene, genomic DNA of his availablefamily members was performed corresponding sequence analysis. Two hundredage-matched healthy children were served as normal controls. All subjects wereChinese Han in origin.Results Eight pedigrees with familial FS were enrolled. Among8probands,there were7boys and1girl, aged1to4years, with a median age of2.5years.Twenty FS patients were found in these families, clinical phenotype is simple-type FS,and seizure type is generalized tonic-clonic seizures. A total of33sequence variationsin the SCN1A gene were identified in these families. Of these sequence variations, onewas a missense mutation; the remaining sequence variations were previouslysubmitted as single nucleotide polymorphisms (SNPs). A c.2650G>A heterozygousmissense mutation in exon15of the SCN1A gene was found in the proband of family4, which was inherited from his father who had seizures with fever in early childhood.The c.2650G>A mutation was absent in the chromosomes of all200normal controls.To the best of our knowledge, The SCN1A c.2650G>A mutation has neither beenreported in the NCBI SNP database nor in the literature to date. The c.2650G>A mutation changes a highly conserved glycine at amino acid884in the SCN1A proteinto a serine (p.Gly884Ser). Protein sequence analysis showed that the p.Gly884Ser islocated between the4th and5th transmembrane segment of the homologous domain IIof voltage-gated sodium channel α1subunit (DIIS4-S5), which belongs to the outsideregion of ion-pore region (S5-S6). Analysis using PolyPhen-2, prediction program ofpoint mutation, showed that the p.Gly884Ser mutation is predicted to be probablydamaging to the structure and function of the SCN1A protein with a score of0.989(sensitivity:0.72; specificity:0.97). This result was confirmed by SIFT (score:0.04).There is a certain gap of the three-dimensional (3D) structures between wild-type andp.Gly884Ser mutant-type SCN1A protein by homology modeling usingSWISS-MODEL workspace. Among the32SNPs identified, the mutated-allelefrequency of21polymorphisms is higher than60%, of which that of16polymorphisms is100%. The rs3812718polymorphism is an increased risk factor ofepilepsy with FS and epilepsy, especially in Caucasian; the rs2298771polymorphismis an increased risk factor of epilepsy in north Indian population.Conclusions and study implications By analyzing the sequencingresults of SCN1A gene in the8familial FS, we draw the following conclusions:(1)We report for the first time to our knowledge that the c.2650G>A mutation in theSCN1A gene is associated with familial FS in Han Chinese in North Anhui Province,China.(2) The SNPs of the SCN1A gene may be risk factors for familial FS in HanChinese in North Anhui Province, China.(3) Familial FS and GEFS+share a commonmolecular pathogenesis, namely, SCN1A gene mutation is a common pathogeneticbasis of both disorder.The present study lays a molecular genetic basis for further exploration of therole of SCN1A p.Gly884Ser mutant in the pathogenesis of familial FS, and also lays afoundation for further analysis of the relevance between SNPs of the SCN1A gene andfamilial FS, which has an important theoretical value to further clarifying thepathogenesis of this disorder. Moreover, the present study has potentially clinicalapplication prospects, such as genetic counseling, genetic diagnosis, disease screening, and even the establishment of intervention.