Molecular genetics of febrile seizures

Febrile seizures (FSs) represent the most common form of childhood seizures, occurring in 2-5% of infants in Europe and North America and in 6-9% in Japan. It has been recognized that there is a significant genetic component for susceptibility to this type of seizure. Six susceptibility FS loci have been identified on chromosomes 8q13-q21 (FEB1), 19p (FEB2), 2q23-q24 (FEB3), 5q14-q15 (FEB4), 6q22-q24 (FEB5), and 18p11 (FEB6). Furthermore, mutations in the voltage-gated sodium channel alpha-1, alpha-2 and beta-1 subunit genes (SCN1A, SCN2A and SCN1B) and the GABA(A) receptor gamma-2 subunit gene (GABRG2) have been identified in families with a clinical subset of seizures termed "generalized epilepsy with febrile seizure plus (GEFS+)". However, the causative genes have not been identified in most patients with FSs or GEFS+. Common forms of FSs are genetically complex disorders believed to be influenced by variations in several susceptibility genes. Recently, several association studies in FSs have been reported, but the results vary among different groups and no consistent or convincing FS susceptibility genes have emerged. To find a true association, larger sample size and newer methodologic refinements are recommended.     

Generalized epilepsy with febrile seizures plus: mutation of the sodium channel subunit SCN1B

Generalized epilepsy with febrile seizures plus (GEFS(+)) is an important childhood genetic epilepsy syndrome with heterogeneous phenotypes, including febrile seizures (FS) and generalized epilepsies of variable severity. Forty unrelated GEFS(+) and FS patients were screened for mutations in the sodium channel beta-subunits SCN1B and SCN2B, and the second GEFS(+) family with an SCN1B mutation is described here. The family had 19 affected individuals: 16 with typical GEFS(+) phenotypes and three with other epilepsy phenotypes. Site-specific mutation within SCN1B remains a rare cause of GEFS(+), and the authors found no evidence to implicate SCN2B in this syndrome.     

Generalized epilepsy with febrile seizures plus (GEFS+): clinical spectrum in seven Italian families unrelated to SCN1A, SCN1B, and GABRG2 gene mutations

PURPOSE: We describe seven Italian families with generalized epilepsy with febrile seizures plus (GEFS+), in which mutations of SCN1A, SCN1B, and GABRG2 genes were excluded and compare their clinical spectrum with that of previously reported GEFS+ with known mutations. METHODS: We performed a clinical study of seven families (167 individuals). The molecular study included analysis of polymerase chain reaction (PCR) fragments of SCN1A and SCN1B exons by denaturing high-performance liquid chromatography (DHPLC) and direct sequencing of GABRG2 in all families. We excluded SCN1A, SCN1B, and GABRG2 genes with linkage analysis in a large pedigree and directly sequenced SCN2A in a family with neonatal-infantile seizures onset. We compared the epilepsy phenotypes observed in our families with those of GEFS+ families harboring mutations of SCN1A, SCN1B, and GABRG2 and estimated the percentage of mutations of these genes among GEFS+ cases by reviewing all published studies. RESULTS: Inheritance was autosomal dominant with 69% penetrance. Forty-one individuals had epilepsy: 29 had a phenotype consistent with GEFS+; seven had idiopathic generalized epilepsy (IGE); in three, the epilepsy type could not be classified; and two were considered phenocopies. Clinical phenotypes included FS+ (29.2%), FS (29.2%), IGE (18.2%), FS+ with focal seizures (13%) or absence seizures (2.6%), and FS with absence seizures (2.6%). Molecular study of SCN1A, SCN2A, SCN1B, and GABRG2 did not reveal any mutation. Results of our study and literature review indicate that mutations of SCN1A, SCN2A, SCN1B, and GABRG2 in patients with GEFS+ are rare. CONCLUSIONS: The most frequently observed phenotypes matched those reported in families with mutations of the SCN1A, SCN1B, and GABRG2 genes. IGE and GEFS+ may overlap in some families, suggesting a shared genetic mechanism. The observation that 13% of affected individuals had focal epilepsy confirms previously reported rates and should prompt a reformulation of the "GEFS+" concept to include focal epileptogenesis.     

A deletion in SCN1B is associated with febrile seizures and early-onset absence epilepsy

Generalized epilepsy with febrile seizures plus (GEFS+) is a clinically and genetically heterogeneous syndrome with childhood onset, characterized by febrile seizures (FS) and a variety of afebrile epileptic seizure types. The authors performed a mutational analysis of SCN1B on 74 unrelated probands with GEFS+, FS, or FS plus (FS+). In a family with FS+ and early-onset absence epilepsy, a mutation was identified that predicts a deletion of five amino acids in the extracellular immunoglobulin-like domain of SCN1B and potential loss of function. SCN1B mutations are associated with GEFS+ and may have a role in the elicitation of absence seizures.     

Generalized epilepsy with febrile seizures plus: further heterogeneity in a large family

BACKGROUND: Generalized epilepsy with febrile seizures plus (GEFS(+)) is a recently described benign childhood-onset epileptic syndrome with autosomal dominant inheritance. The most common phenotypes are febrile seizures (FS) often with accessory afebrile generalized tonic-clonic seizures (GTCS, FS(+)). In about one third, additional seizure types occur, such as absences, myoclonic, or atonic seizures. So far, three mutations within genes encoding subunits of neuronal voltage-gated Na(+) channels have been found in GEFS(+) families, one in SCN1B (beta(1)-subunit) and two in SCN1A (alpha-subunit). METHODS: The authors examined the phenotypic variability of GEFS(+) in a five-generation German family with 18 affected individuals. Genetic linkage analysis was performed to exclude candidate loci. RESULTS: Inheritance was autosomal dominant with a penetrance of about 80%. A variety of epilepsy phenotypes occurred predominantly during childhood. Only four individuals showed the FS or FS(+) phenotype. The others presented with different combinations of GTCS, tonic seizures, atonic seizures, and absences, only in part associated with fever. The age at onset was 2.8 +/- 1.3 years. Interictal EEG recordings showed rare, 1- to 2-second-long generalized, irregular spike-and-wave discharges of 2.5 to 5 Hz in eight cases and additional focal parietal discharges in one case. Linkage analysis excluded the previously described loci on chromosomes 2q21-33 and 19q13. All other chromosomal regions containing known genes encoding neuronal Na(+) channel subunits on chromosomes 3p21-24, 11q23, and 12q13 and described loci for febrile convulsions on chromosomes 5q14-15, 8q13-21, and 19p13.3 were also excluded. CONCLUSION: These results indicate further clinical and genetic heterogeneity in GEFS(+).     

Partial and generalized epilepsy with febrile seizures plus and a novel SCN1A mutation.

BACKGROUND: Generalized epilepsy with febrile seizures plus (GEFS+) is an autosomal dominant syndrome characterized by febrile seizures (FS) and a variety of afebrile generalized seizure types. GEFS+ has previously been linked to mutations in two genes encoding the voltage-gated sodium channel alpha-subunit (SCN1A) and beta1-subunit (SCN1B). We studied a large family with FS and partial as well as generalized seizure types. METHODS: All but two living affected family members were interviewed and examined. Information on deceased affected family members was sought. EEG for 11 affected family members and one unaffected family member were obtained. Genetic linkage analysis and mutation screening of SCN1A were performed on blood samples from 16 affected individuals and their first-degree relatives. RESULTS: There were 27 affected family members; 18 were alive at the time of the study. All affected family members had FS; seven had FS only, and 19 also had afebrile seizures. Eleven individuals continued to have FS beyond 6 years of age. FS were complex in 12 family members, usually with prolonged duration. The index patient had right temporal lobe epilepsy and hippocampal sclerosis. Four other patients had strong historical evidence of temporal lobe epilepsy, and three others had nonlocalizing evidence of partial epilepsy. Pedigree analysis indicated autosomal dominant transmission. All affected individuals who were tested and one asymptomatic individual had a sodium channel mutation of SCN1A, an A-->C transversion at nucleotide 3809 resulting in the substitution of lysine 1270 by threonine in the D3/S2 segment (designated as K1270T). CONCLUSIONS: Our findings indicate that partial epilepsy preceded by FS can be associated with sodium channel mutations and may represent a variant of GEFS+.     

Novel de novo splice-site mutation of SCN1A in a patient with partial epilepsy with febrile seizures plus

This report describes a 4-year-old male patient experienced prolonged febrile seizures after 1 year of age, multiple febrile seizures and complex partial seizures with secondary generalization. The gene encoding voltage-gated sodium channel alpha1-subunit: SCN1A analysis revealed a heterozygous de novo one-point mutation (IVS16+2 T>C) at a splice-acceptor site. This mutation was inferred to cause truncation of the alpha1-subunit molecule and, thereby, a loss of channel function. To date, truncation mutation has been found exclusively in patients with severe myoclonic epilepsy in infancy (SMEI), although only missense mutations have been found in generalized epilepsy with febrile seizures plus (GEFS+), partial epilepsy with FS+, FS+, and FS. The patient's phenotype is consistent with that of partial epilepsy with FS+, rather than SMEI, including borderline SMEI (SMEB). We present the first case report of partial epilepsy with FS+ associated with a truncation mutation of SCN1A. The possibility exists for concomitant involvement of multiple genes other than SCN1A for seizure phenotypes.     

Idiopathic epilepsies with seizures precipitated by fever and SCN1A abnormalities

PURPOSE: SCN1A is the most clinically relevant epilepsy gene, most mutations lead to severe myoclonic epilepsy of infancy (SMEI) and generalized epilepsy with febrile seizures plus (GEFS+). We studied 132 patients with epilepsy syndromes with seizures precipitated by fever, and performed phenotype-genotype correlations with SCN1A alterations. METHODS: We included patients with SMEI including borderline SMEI (SMEB), GEFS+, febrile seizures (FS), or other seizure types precipitated by fever. We performed a clinical and genetic study focusing on SCN1A, using dHPLC, gene sequencing, and MLPA to detect genomic deletions/duplications on SMEI/SMEB patients. RESULTS: We classified patients as: SMEI/SMEB = 55; GEFS+= 26; and other phenotypes = 51. SCN1A analysis by dHPLC/sequencing revealed 40 mutations in 37 SMEI/SMEB (67%) and 3 GEFS+ (11.5%) probands. MLPA showed genomic deletions in 2 of 18 SMEI/SMEB. Most mutations were de novo (82%). SMEB patients carrying mutations (8) were more likely to have missense mutations (62.5%), conversely SMEI patients (31) had more truncating, splice site or genomic alterations (64.5%). SMEI/SMEB with truncating, splice site or genomic alterations had a significantly earlier age of onset of FS compared to those with missense mutations and without mutations (p = 0.00007, ANOVA test). None of the remaining patients with seizures precipitated by fever carried SCN1A mutations. CONCLUSION: We obtained a frequency of 71%SCN1A abnormalities in SMEI/SMEB and of 11.5% in GEFS+ probands. MLPA complements DNA sequencing of SCN1A increasing the mutation detection rate. SMEI/SMEB with truncating, splice site or genomic alterations had a significantly earlier age of onset of FS. This study confirms the high sensitivity of SCN1A for SMEI/SMEB phenotypes.     

Phenotypes and genotypes in epilepsy with febrile seizures plus

In the last several years, mutations of sodium channel genes, SCN1A, SCN2A, and SCN1B, and GABA(A) receptor gene, GABRG2 were identified as causes of some febrile seizures related epilepsies. In 19 unrelated Japanese families whose probands had febrile seizures plus or epilepsy following febrile seizures plus, we identified 2 missense mutations of SCN1A to be responsible for the seizure phenotypes in two FS+ families and another mutation of SCN2A in one family. The combined frequency of SCN1A, SCN2A, SCN1B, SCN2B, and GABRG2 mutations in Japanese patients with FS+ was 15.8%. One family, which had R188W mutation in SCN2A, showed digenic inheritance, and another modifier gene was thought to take part in the seizure phenotype. The phenotypes of probands were FS+ in 5, FS+ and partial epilepsy in 10, FS+ and generalized epilepsy in 3, and FS+ and unclassified epilepsy in 1. We proposed the term epilepsy with febrile seizures plus (EFS+), because autosomal-dominant inheritance in EFS+ might be rare, and most of EFS+ display a complex pattern of inheritance, even when it appears to be an autosomal-dominant inheritance. There is a possibility of simultaneous involvement of multiple genes for seizure phenotypes.     

Genetic screening of Scandinavian families with febrile seizures and epilepsy or GEFS+

Background –  Mutations in the three genes SCN1A , SCN1B and GABRG2 , all encoding subunits of ion channels, have been known to cause generalized epilepsy with febrile seizures plus (GEFS+) in families of different origin.
Objective –  To study the occurrence of mutations in these genes in families with GEFS+ or a GEFS+ resembling phenotype of Scandinavian origin.
Material and methods –  We performed linkage analysis in 19 Scandinavian families with a history of febrile seizures (FS) and epilepsy or GEFS+. Where linkage could not be excluded, the genes of interest were sequenced.
Results –  We identified only one mutation in SCN1A , which seems to be a rare variant with no functional consequence.
Conclusion –  This suggests that mutations in these three genes are not a prevalent cause of familial cases of FS and epilepsy or GEFS+ in Scandinavia.     

Clinical and genetic analysis of a new multigenerational pedigree with GEFS+ (Generalized Epilepsy with Febrile Seizures Plus)

Febrile seizures affect 2-5% of all children younger than 6 years. A small proportion of children with febrile seizures later develop epilepsy. The syndrome of generalized epilepsy with febrile seizures plus (GEFS+) is a heterogeneous disorder characterized by febrile seizures that may persist beyond age 6 years and nonfebrile seizures. Several genes have been localized for FS by linkage analysis, and three GEFS+ genes (SCN1A, SCN1B, GABRG2) have been identified. We identified a large multigenerational family with GEFS+ in France. All affected members had FSs. Among them, seven had other types of epileptic seizures including FSs after age 6 years, nonfebrile generalized seizures, or partial seizures later in life. Genetic linkage study excluded the candidate genes and loci for FS and GEFS+, thus proving the existence of a new GEFS+ genetic locus underlying the phenotype observed in this family.     

A novel SCN1A mutation in a cytoplasmic loop in intractable juvenile myoclonic epilepsy without febrile seizures

Generalised (genetic) epilepsy with febrile seizures plus (GEFS+) is a familial epilepsy syndrome with various phenotypes. The majority of individuals with GEFS+ have generalised seizure types, in addition to febrile seizures (FS) or febrile seizures plus (FS+), defined as either continued FS after 6 years of age or afebrile seizures following FS. A 27‐year‐old man with no history of FS/FS+ experienced intractable generalised convulsive seizures. The patient's father had a history of similar seizures during puberty and the patient's siblings had only FS. No individual in the family had both generalised seizures and FS/FS+, although GEFS+ might be considered to be present in the family. Analysis of SCN1A, a sodium channel gene, revealed a novel mutation (c.3250A>T [S1084C]) in the cytoplasmic loop 2 of SCN1A in both the patient and his father. Most previously reported SCN1A mutations in GEFS+ patients are located in the conserved homologous domains of SCN1A, whereas mutations in the cytoplasmic loops are very rare. SCN1A gene analysis is not commonly performed in subjects with generalised seizures without FS. SCN1A mutation may be a clinically‐useful genetic marker in order to distinguish GEFS+ patients from those with classic idiopathic generalised epilepsy, even if they present an atypical clinical picture.     

Genetic screening of two Tunisian families with generalized epilepsy with febrile seizures plus

Background and purpose:  Febrile Seizure can be associated with heterogeneous epilepsy phenotypes regrouped in a syndrome called generalized epilepsy with febrile seizures plus (GEFS+). The aim of this report is to search for the gene responsible for GEFS+ in two affected Tunisian families.
Methods:  Microsatellite marker analysis was performed on the known FS and GEFS+ loci. According to the results obtained by statistical analyses, GABRG2 on GEFS+3 locus and SCN1A on GEFS+2 locus were considered as two of the potential candidate genes and were tested for mutations by direct sequencing.
Results and conclusions:  The mutation analysis and statistical test of the GABRG2 gene revealed a disease association with rs211014 in intron 8 (χ² = 5.25, P =  0.021). A sequencing analysis of the SCN1A gene was performed for the two tested families and showed a known mutation (c.1811G>A) and a putative disease-associated haplotype in only one family. Our results support that SCN1A is the responsible gene for GEFS+ in one of the two studied Tunisian families and suggest a positive association of an intronic SNP in the GABRG2 gene in both families.     

SCN1A mutations in focal epilepsy with auditory features: widening the spectrum of GEFS plus

Aims. Epilepsy with auditory features (EAF) is a focal epilepsy syndrome characterized by prominent auditory ictal manifestations. Two main genes, LGI1 and RELN, have been implicated in EAF, but the genetic aetiology remains unknown in half of families and most sporadic cases. We previously described a pathogenic SCN1A missense variant (p.Thr956Met) segregating in a large family in which the proband and her affected daughter had EAF, thus satisfying the minimum requirement for diagnosis of autosomal dominant EAF (ADEAF). However, the remaining eight affected family members had clinical manifestations typically found in families with genetic epilepsy with febrile seizures plus (GEFS+). We aimed to investigate the role/impact of SCN1A mutations in EAF. Methods. We detailed the phenotype of this family and report on SCN1A screening in a cohort of 29 familial and 52 sporadic LGI1 variant‐negative EAF patients. Results. We identified two possibly pathogenic missense variants (p.Tyr790Phe and p.Thr140Ile) in sporadic patients (3.8%) showing typical EAF and no antecedent febrile seizures. Both p.Thr956Met and p.Tyr790Phe were previously described in unrelated patients with epilepsies within the GEFS+ spectrum. Conclusion. SCN1A mutations may be involved in EAF within the GEFS+ spectrum, however, the role of SCN1A in EAF without features that lead to a suspicion of underlying GEFS+ remains unclear and should be elucidated in future studies.     

Lack of SCN1A mutations in familial febrile seizures

PURPOSE: Mutations in the voltage-gated sodium channel subunit gene SCN1A have been associated with febrile seizures (FSs) in autosomal dominant generalized epilepsy with febrile seizures plus (GEFS+) families and severe myoclonic epilepsy of infancy. The present study assessed the role of SCN1A in familial typical FSs. METHODS: FS families were selected throughout a collaborative study of the Italian League Against Epilepsy. For each index case, the entire coding region of SCN1A was screened by denaturant high-performance liquid chromatography. DNA fragments showing variant chromatograms were subsequently sequenced. RESULTS: Thirty-two FS families accounting for 91 affected individuals were ascertained. Mutational analysis detected a single coding variant (A3169G) on exon 16. The extended analysis of all family members and 78 normal controls demonstrated that A3169G did not contribute to the FS phenotype. CONCLUSIONS: Our study demonstrated that SCN1A is not frequently involved in common FSs and suggested the involvement of specific FS genes.     

A case report of a family with overlapping features of autosomal dominant febrile seizures and GEFS+

Familial febrile seizures occur in both generalized epilepsy with febrile seizures plus (GEFS+) and autosomal dominant febrile seizures (ADFS). The literature largely separates families with GEFS+ from those with ADFS. However, there is clinical overlap, and families with ADFS also include individuals with afebrile seizures. The phenotypic spectrum of GEFS+ is broader now than when first described, resulting in unclear boundaries between these two familial syndromes. The purpose of this report is to highlight the phenotypic similarities of GEFS+ and ADFS. A multigenerational family with febrile and afebrile seizures is described and the clinical features are compared to those of previously reported GEFS+ and ADFS families. This family meets the requirements for both ADFS and the broader definition of GEFS+. Linkage analysis has shown no clear linkage to known febrile seizure or GEFS+ loci. Despite locus heterogeneity, identified mutations in reported GEFS+ have so far all been in sodium channel or γ-aminobutyric acid (GABA)–receptor genes, with other modifier genes postulated to affect individual phenotypes. The two mutations identified in families with ADFS are in genes implicated in GEFS+, SCN1A , and GABRG2 . We conclude that it is inappropriate to separate GEFS+ and ADFS at present given the clinical and genotypic overlap.     

全面性癫癎伴热性惊厥附加症的临床和遗传学特点(附9家系报告)

目的探讨全面性癫癎伴热性惊厥附加症(GEFS+)的临床和遗传学特点。方法回顾性分析9个GEFS+家系的临床资料。结果本组9例先证者中男7例,女2例;起病年龄1~3岁;发作类型均为全面性强直-阵挛发作(GTCS)。其中,1例为GTCS,4例为热性惊厥(FS),4例为热性惊厥附加症(FS+)。脑电图检查示6例有典型疒间样波,1例有θ波,2例无异常。9个GEFS+家系184人中共有患者45例,男32例,女13例;男性发病比率(34.4%)明显高于女性发病比率(14.3%)(P<0.05)。其中,2例为GTCS,39例为FS,4例为FS+。家系系谱图分析显示,17例患者(37.8%)父母一方或双方患病,符合常染色体显性遗传;28例患者(62.2%)父母不发病或发病情况不详,但其近亲中至少有2例或以上患者。结论 GEFS+的脑电图不一定有异常波。GEFS+具有遗传异质性,本组中男性显著多于女性,不完全符合常染色体显性遗传。     

Dravet syndrome or genetic (generalized) epilepsy with febrile seizures plus?

Dravet syndrome and genetic epilepsy with febrile seizures plus (GEFS+) can both arise due to mutations of SCN1A, the gene encoding the alpha 1 pore-forming subunit of the sodium channel. GEFS+ refers to a familial epilepsy syndrome where at least two family members have phenotypes that fit within the GEFS+ spectrum. The GEFS+ spectrum comprises a range of mild to severe phenotypes varying from classical febrile seizures to Dravet syndrome. Dravet syndrome is a severe infantile onset epilepsy syndrome with multiple seizure types, developmental slowing and poor outcome. More than 70% of patients with Dravet syndrome have mutations of SCN1A; these include both truncation and missense mutations. In contrast, only 10% of GEFS+ families have SCN1A mutations and these comprise missense mutations. GEFS+ has also been associated with mutations of genes encoding the sodium channel beta 1 subunit, SCN1B, and the GABA_A receptor gamma 2 subunit, GABRG2. The phenotypic heterogeneity that is characteristic of GEFS+ families is likely to be due to modifier genes. Interpretation of the significance of a SCN1A missense mutation requires a thorough understanding of the phenotypes in the GEFS+ spectrum whereas a de novo truncation mutation is likely to be associated with a severe phenotype. Early recognition of Dravet syndrome is important as aggressive control of seizures may improve developmental outcome.     

Patients with a sodium channel alpha 1 gene mutation show wide phenotypic variation

We investigated the roles of mutations in voltage-gated sodium channel alpha 1 subunit gene (SCN1A) in epilepsies and psychiatric disorders. The SCN1A gene was screened for mutations in three unrelated Japanese families with generalized epilepsy with febrile seizure plus (GEFS+), febrile seizure with myoclonic seizures, or intractable childhood epilepsy with generalized tonic-clonic seizures (ICEGTC). In the family with GEFS+, one individual was affected with panic disorder and seizures, and another individual was diagnosed with Asperger syndrome and seizures. The novel mutation V1366I was found in all probands and patients with psychiatric disorders of the three families. These results suggest that SCN1A mutations may confer susceptibility to psychiatric disorders in addition to variable epileptic seizures. Unidentified modifiers may play critical roles in determining the ultimate phenotype of patients with sodium channel mutations.     

Evidence favoring genetic heterogeneity for febrile convulsions

PURPOSE: Two large Canadian kindreds appearing to segregate febrile convulsions as an autosomal dominant trait were evaluated for linkage to three known FC loci, as well as other epilepsy loci. METHODS: Members of the two families were genotyped with microsatellite markers linked to the previously identified febrile convulsion loci, FEB1, FEB2, and GEFS+, and we performed two-point linkage analyses by assuming an autosomal dominant mode of inheritance. RESULTS: We report the exclusion of the FC trait in our families to FEB1 on 8q13-21 and to a second febrile convulsion locus on 19p13. Furthermore, we also excluded the GEFS+ locus on 19q13.1 as the cause of febrile convulsions in both kindreds. Microsatellite markers linked to juvenile myoclonic epilepsy (EJM1), benign neonatal familial convulsions EBN1 and EBN2, autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE), idiopathic generalized epilepsy (EGI), progressive myoclonic epilepsy of Unverricht-Lundborg (EPM1), and partial epilepsy with auditory features (EPT), were also excluded as potential loci linked to the FC trait in our families. CONCLUSIONS: These findings favor considerable genetic heterogeneity for febrile convulsions.