Autosomal dominant epilepsy with febrile seizures plus with missense mutations of the (Na+)-channel alpha 1 subunit gene, SCN1A

Electroclinical Phenotype-Genotype Homogeneity in Drug-Resistant "Generalized" Tonic-Clonic Seizures of Early Childhood

PURPOSE

Children with refractory focal to bilateral tonic-clonic seizures, despite normal high-resolution imaging, are often not subjected to genetic tests due to the costs involved and instead undergo multimodality presurgical evaluation targeted at delineating a focal onset. The objective of this study was to ascertain genotype-phenotype correlations in this group of patients.

METHOD

An online hospital database search was conducted for children who presented in 2019 with drug-resistant epilepsy dominated by nonlateralizing focal-onset/rapid generalized (bilateral) tonic-clonic seizures (GTCS), subjected to presurgical evaluation and subsequent genetic testing due to absence of a clear focus hypothesis.

RESULTS

Phenotypic homogeneity was apparent in 3 children who had onset in infancy with drug-resistant GTCS (predominantly unprovoked and occasionally fever provoked) and subsequent delayed development. 3-Tesla magnetic resonance imaging (MRI) scans were negative and video EEG documented a homogeneous pattern of multifocal and/or generalized epileptiform discharges with phenomenology favoring probable focal-onset/generalized-onset bilateral tonic-clonic seizures. All 3 tested positive for SCN1A gene variants (heterozygous missense substitution variants in 2 children, one of which was novel and a novel duplication in one that led to frameshift and premature truncation of the protein), suggestive of SCN1A-mediated epilepsy. This electroclinical profile constituted 3 out of 25 patients with SCN1A-epilepsy phenotypes at our center.

CONCLUSIONS

These cases suggest that children with early-onset drug-resistant "generalized" epilepsy are likely to have a genetic basis although the presentation may not be typical of Dravet syndrome. Hence, genetic testing for SCN1A variants is recommended in children with drug-resistant MRI negative focal-onset/generalized-onset bilateral tonic-clonic seizures before subjecting them to exhaustive presurgical workup and to guide appropriate treatment and prognostication.

Structural determination of human Nav1.4 and Nav1.7 using single particle cryo-electron microscopy

Voltage-gated sodium (Na_v) channels are responsible for the initiation and propagation of action potentials. Their abnormal functions are associated with numerous diseases, such as epilepsy, cardiac arrhythmia, and pain syndromes. Therefore, these channels represent important drug targets. Even in the post-resolution revolution era, a lack of structural information continues to impede structure-based drug discovery. The limiting factor for the structural determination of Na_v channels using single particle cryo-electron microscopy (cryo-EM) resides in the generation of sufficient high-quality recombinant proteins. After extensive trials, we have been successful in determining a series of high-resolution structures of Na_v channels, including Na_vPaS from American cockroach, Na_v1.4 from electric eel, and human Na_v1.1, Na_v1.2, Na_v1.4, Na_v1.5, and Na_v1.7, with distinct strategies. These structures established the framework for understanding the electromechanical coupling and disease mechanism of Na_v channels, and for facilitating drug discovery. Here, we exemplify these methods with two specific cases, human Na_v1.4 and Na_v1.7, which may shed light on the structural determination of other membrane proteins.

SCN1A and Its Related Epileptic Phenotypes

Epilepsy is one of the most common neurological disorders, with a lifetime incidence of 1 in 26. Approximately two-thirds of epilepsy has a substantial genetic component in its etiology. As a result, simultaneous screening for mutations in multiple genes and performing whole exome sequencing (WES) are becoming very frequent in the clinical evaluation of children with epilepsy. In this setting, mutations in voltage-gated sodium channel (SCN) α-subunit genes are the most commonly identified cause of epilepsy, with sodium channel genes (i.e., SCN1A, SCN2A, SCN8A) being the most frequently identified causative genes. SCN1A mutations result in a wide spectrum of epilepsy phenotypes ranging from simple febrile seizures to Dravet syndrome, a severe epileptic encephalopathy. In case of mutation of SCN1A, it is also possible to observe behavioral alterations, such as impulsivity, inattentiveness, and distractibility, which can be framed in an attention deficit hyperactivity disorder (ADHD) like phenotype. Despite more than 1,200 SCN1A mutations being reported, it is not possible to assess a clear phenotype–genotype correlations. Treatment remains a challenge and seizure control is often partial and transitory.

Mutations in the sodium channel genes SCN1A, SCN3A, and SCN9A in children with epilepsy with febrile seizures plus(EFS+)

PURPOSE

To explore disease-causing gene mutations of epilepsy with febrile seizures plus (EFS+) in Southern Chinese Han population.

METHODS

Blood samples and clinical data were collected from 49 Southern Han Chinese patients with EFS+. Gene screening was performed using whole-exome sequencing and panel sequencing for 485 epilepsy-related genes. The pathogenicity of variants was evaluated based on ACMG scoring and assessment of clinical concordance.

RESULTS

We identified 10 putatively causative sodium channel gene variants in 49 patients with EFS+, including 8 variants in SCN1A (R500Q appeared twice), one in SCN3A and one in SCN9A. All these missense mutations were inherited from maternal or paternal and were evaluated to be of uncertain significance according to ACMG. The clinical features of patients were in concordance with the EFS+ phenotype of the mutated SCN1A, SCN3A and SCN9A gene. The clinical phenotypes of 11 probands with these gene variants included febrile seizures plus (FS+, n=7), Dravet Syndrome (n=3), FS+ with focal seizures (n=1). Three probands with SCN1A variants (R500Q located in the non-voltage areas, or G1711D in the pore-forming domain) developed severe Dravet syndrome. The affected individuals with the other 6 SCN1A variants located outside the pore-forming domain showed mild phenotypes. Novel SCN3A variant ((D1688Y) and SCN9A variant (R185H) were identified in two probands respectively and both of the probands had FS+.

CONCLUSION

The SCN1A, SCN3A, and SCN9A gene mutations might be a pathogenic cause of EFS+ in Southern Chinese Han population.

GABRG2 Deletion Linked to Genetic Epilepsy with Febrile Seizures Plus Affects the Expression of GABAA Receptor Subunits and Other Genes at Different Temperatures

Mutations in γ-aminobutyric acid A receptor (GABA_A) subunits and sodium channel genes, especially GABRG2 and SCN1A, have been reported to be associated with febrile seizures (FS) and genetic epilepsy with febrile seizures plus (GEFS+). GEFS+ is a well-known family of epileptic syndrome with autosomal dominant inheritance in children. Its most common phenotypes are febrile seizures often with accessory afebrile generalized tonic-clonic seizures, febrile seizures plus (FS+), severe epileptic encephalopathy, as well as other types of generalized or localization-related seizures. However, the pathogenesis of febrile seizures remains largely unknown. Here, we generated a GABRG2 gene knockout cell line (HT22^GABRG2KO) by applying the CRISPR/Cas9-mediated genomic deletion in HT-22 mouse hippocampal neuronal cell line to explore the function of GABRG2 in vitro. With mRNA-seq, we found significant changes in the expression profiles of several epilepsy-related genes when GABRG2 was knockout, some of them showing temperature-induced changes as well. Kyoto Encyclopedia Gene and Genomic (KEGG) analysis revealed a significant alteration in the MAPK and PI3K-Akt signaling pathways. We also observed an up-regulation of the matrix metalloproteinases (MMPs) family after GABRG2 knockout. Furthermore, the significant decrease in expression of GABRA1 and CACNA1A (but not others) with an increase in temperature is a novel finding. In summary, mutations in the GABA_A receptor can lead to a decrease in numbers of receptors, which may cause the impairment of GABAergic pathway signaling. This data has been the first time to reveal that GABRG2 mutations would affect the function of other genes, and based on this finding we hope this work would also provide a new direction for the research of GABRG2 in GEFS+. It also may provide a molecular basis for the severity of epilepsy, and guide the clinical medication for the treatment of the epilepsy focused on the function on GABA_A receptors, which, might be a new strategy for genetic diagnosis and targeted treatment of epilepsy.

A case of Dravet Syndrome with a newly defined mutation in the SCN1A gene

Dravet syndrome is a catastrophic progressive epileptic syndrome. De novo loss of function mutations on the SCN1A gene coding voltage-gated sodium channels are responsible. Disruption of the triggering of hippocampal GABAergic interneurons is assumed as the cause of fall in the seizure threshold. A ten-year-old boy first presented at age 10 months with febrile-clonic seizures, which began when he was aged 8 months. Electroencephalography was found as normal. Phenobarbital was initiated because of long-lasting seizures. However, his seizures continued and the therapy was replaced with valproic acid. On follow-up, different antiepileptics were used, which were stopped due to inefficiency or adverse effects. SCN1A gene analysis was performed and a heterozygous c.4018delC mutation was identified. This new frame-shift mutation resulting from an early stop-codon is thought to be the cause of the disease. Finally, he was prescribed valproic acid and stiripentol. For patients with fever-triggered, treatment-resistant seizures, and delayed psychomotor development, Dravet syndrome should be considered. Genetic diagnosis is important for treatment and follow-up.

FOXD3 inhibits SCN2A gene transcription in intractable epilepsy cell models

The expression of sodium voltage-gated channel alpha subunit 2 (SCN2A) is closely related to the development of epilepsy. This study investigated regulatory element of the SCN2A gene involved in epilepsy. An intractable epilepsy cell model was constructed using hippocampal primary neurons and the SH-SY5Y cell line. SCN2A protein and gene expression in cells as well as the level of lactic acid dehydrogenase (LDH) in the cell culture supernatants was detected. Potential regulatory factors of SCN2A and its upstream regulatory elements were identified using the dual-luciferase reporter assay. Finally, the role of the hypothetical transcription factor in epilepsy was examined by using its small interfering RNA (siRNA). Results found that levels of LDH and expression of the hypothetical transcription factor, Forkhead box D3 (FOXD3), was both increased in the model cells, whereas that of SCN2A was decreased. The results of dual-luciferase reporter assays revealed that an upstream region of SCN2A gene spanning from nucleotides -1617 to -1470 was a transcription factor binding region and a trans-acting factor role of FOXD3 was identified in the core region (GGCAAAATTAT). Then the FOXD3 binding site was further verified by the chromatin immunoprecipitation (ChIP) assay and electrophoretic mobility shift assay (EMSA). After SH-SY5Y cells were transfected with FOXD3 siRNA, the release of LDH into culture supernatants and the LDH expression levels in cells were significantly decreased. SCN2A expression in model cells was increased by knockdown of FOXD3. Therefore, this study demonstrated that FOXD3 is a trans-acting factor of SCN2A, and this mechanism may play a role in cell injury after epilepsy.

SCN1A clinical spectrum includes the self-limited focal epilepsies of childhood

INTRODUCTION

Amongst autosomal dominant genetic epilepsy with febrile seizures plus (GEFS+) families, SCN1A variants are the most common genetic cause. Initially regarded as a generalized form of epilepsy, the GEFS+ spectrum is now known to include some focal epilepsies, but it is generally not conceptualized as extending to the self-limited focal epilepsies of childhood, such as Panayiotopoulos syndrome. There are, however, three reports of SCN1A variants in Panayiotopoulos syndrome. We describe the variable clinical phenotypes that include the self-limited focal epilepsies of childhood, present in a large GEFS+ family, segregating a heterozygous SCN1A missense variant.

MATERIAL AND METHODS

Electro-clinical details on all putatively affected family members were sought and blood samples were taken for genetic analysis. Two individuals were chosen for SCN1A testing. All 26 exons and exon-intron junctions were amplified, sequenced and analyzed. This was followed by pedigree segregation analysis of the variant identified.

RESULTS

A pathogenic heterozygous SCN1A (c.2624C>A; p.Thr875Lys) variant was identified. Sixteen of the 18 variant positive family members were affected (88% penetrance): 8 with febrile seizures, 2 febrile seizures plus, 1 unclassified seizures and 5 with self-limited focal epilepsy of childhood. Of these, one was diagnosed with atypical childhood epilepsy with centrotemporal spikes and four with Panayiotopoulos syndrome.

DISCUSSION

By characterizing the heterogeneous clinical phenotypes in a large, SCN1A mutation positive GEFS+ family, we conclude that the GEFS+ spectrum can extend to the self-limited focal epilepsies of childhood, including Panayiotopoulos syndrome, and in turn highlight the complex genotype-phenotype correlations associated with SCN1A-related epilepsies.

From focal epilepsy to Dravet syndrome--Heterogeneity of the phenotype due to SCN1A mutations of the p.Arg1596 amino acid residue in the Nav1.1 subunit

OBJECTIVE

The aim of this study was to analyze the intra-/interfamilial phenotypic heterogeneity due to variants at the highly evolutionary conservative p.Arg1596 residue in the Nav1.1 subunit.

MATERIALS/PARTICIPANTS

Among patients referred for analysis of the SCN1A gene one recurrent, heritable mutation was found in families enrolled into the study. Probands from those families even clinically diagnosed with atypical Dravet syndrome (DS), generalized epilepsy with febrile seizures plus (GEFS+), and focal epilepsy, had heterozygous p.Arg1596 His/Cys missense substitutions, c.4787G>T and c.4786C>T in the SCN1A gene.

METHOD

Full clinical evaluation, including cognitive development, neurological examination, EEGs, MRI was performed in probands and affected family members in developmental age. The whole SCN1A gene sequencing was performed for all probands. The exon 25, where the identified missense substitutions are localized, was directly analyzed for the other family members.

RESULTS

Mutation of the SCN1A p.1596Arg was identified in three families, in one case substitution p.Arg1596Cys and in two cases p.Arg1596His. Both mutations were previously described as pathogenic and causative for DS, GEFS+ and focal epilepsy. Spectrum of phenotypes among presented families with p.Arg1596 mutations shows heterogeneity ranged from asymptomatic cases, through FS and FS+ to GEFS+/Panayiotopoulos syndrome and epilepsies with and without febrile seizures, and epileptic encephalopathy such as DS. Phenotypes differ among patients displaying both focal and generalized epilepsies. Some patients demonstrated additionally Asperger syndrome and ataxia.

CONCLUSION

Clinical picture heterogeneity of the patients carrying mutation of the same residue indicates the involvement of the other factors influencing the SCN1A gene mutations' penetrance.

Mutant GABA(A) receptor subunits in genetic (idiopathic) epilepsy

The γ-aminobutyric acid receptor type A (GABAA receptor) is a ligand-gated chloride channel that mediates major inhibitory functions in the central nervous system. GABAA receptors function mainly as pentamers containing α, β, and either γ or δ subunits. A number of antiepileptic drugs have agonistic effects on GABAA receptors. Hence, dysfunctions of GABAA receptors have been postulated to play important roles in the etiology of epilepsy. In fact, mutations or genetic variations of the genes encoding the α1, α6, β2, β3, γ2, or δ subunits (GABRA1, GABRA6, GABRB2, GABRB3, GABRG2, and GABRD, respectively) have been associated with human epilepsy, both with and without febrile seizures. Epilepsy resulting from mutations is commonly one of following, genetic (idiopathic) generalized epilepsy (e.g., juvenile myoclonic epilepsy), childhood absence epilepsy, genetic epilepsy with febrile seizures, or Dravet syndrome. Recently, mutations of GABRA1, GABRB2, and GABRB3 were associated with infantile spasms and Lennox-Gastaut syndrome. These mutations compromise hyperpolarization through GABAA receptors, which is believed to cause seizures. Interestingly, most of the insufficiencies are not caused by receptor gating abnormalities, but by complex mechanisms, including endoplasmic reticulum (ER)-associated degradation, nonsense-mediated mRNA decay, intracellular trafficking defects, and ER stress. Thus, GABAA receptor subunit mutations are now thought to participate in the pathomechanisms of epilepsy, and an improved understanding of these mutations should facilitate our understanding of epilepsy and the development of new therapies.

Genetic generalized epilepsies

In the International League Against Epilepsy's most recent revision of classification and terminology, the term idiopathic epilepsy, previously used to describe those epilepsies whose cause was unknown, but presumed genetic, has been removed. It has been replaced by the term genetic epilepsy, only to be used to describe epilepsy in which the etiology has a known or presumed genetic defect in which seizures are the core symptom of the disorder. The purpose of this article was to review the electroclinical spectrum of those epilepsies that would fall under this new designation of genetic epilepsies in the context of specific generalized epilepsy syndromes providing an update in the clinical, electroencephalographic, and genetic findings in these syndromes.

Genetics of idiopathic epilepsies

One of the most exciting areas in epilepsy has been the explosion in our understanding of the genetics of the epilepsies over the last decade. Built on a long history of careful clinical genetic studies of the epilepsies, the relatively recent discovery of epilepsy genes has enabled insights into pathways causing seizure disorders. A variety of mutational mechanisms can cause epilepsy resulting from different, and sometimes surprising, molecular processes such as copy number variation within the genome. The majority of known epilepsy genes encode ion channel subunits leading many of the genetic epilepsies to be regarded as channelopathies. Understanding how dysfunction of a mutant protein leads to hyperexcitability is key to understanding the pathophysiology of this group of serious and common childhood disorders. The architecture of the common genetic epilepsies following complex inheritance, where multiple genes are involved, is also beginning to be unraveled. The clinical approach to understanding the genetics of the epilepsies has matured and requires a detailed family history of seizures together with delineation of the child's epilepsy syndrome. Recognition of specific genetic epilepsy syndromes enables optimal treatment and prognostic and genetic counseling.

Coexistence of idiopathic generalized epilepsy among surgically treated patients with drug-resistant temporal lobe epilepsy

INTRODUCTION

Failure to identify the association antiepileptic drug (AED)-resistant temporal lobe epilepsy (TLE) with idiopathic generalized epilepsy (IGE) can interfere with decision for anterior temporal lobectomy (ATL) and prediction of post-ATL seizure outcome.

METHODS

Out of the 664 consecutive patients who underwent ATL between March 1995 and December 2007, 12 (1.8%) had coexisting IGE. The decision for ATL was made after a thorough discussion in the multidisciplinary patient management conference based upon the concordance between the clinical, electroencephalographic and magnetic resonance imaging data. All of them underwent epilepsy surgery for AED-resistant TLE.

RESULTS

In seven of the 12 patients, IGE was not identified until post-ATL. During a median follow-up period of 8.5 years, 8 of our 12 patients were seizure-free; the remaining 4 patients only had infrequent myoclonus. In two them, AEDs were discontinued; others were on montherapy for IGE.

CONCLUSIONS

Our study highlights the rare association of IGE with TLE, the most common AED-resistant focal epilepsy syndrome. As the seizure outcome following ATL is similar in AED-resistant TLE patients with and without IGE, their co-existence is not a contraindication for ATL. Future studies should explore the molecular genetic basis of the rare association between these two epilepsy syndromes.

Sodium channel SCN1A and epilepsy: mutations and mechanisms

Mutations in a number of genes encoding voltage-gated sodium channels cause a variety of epilepsy syndromes in humans, including genetic (generalized) epilepsy with febrile seizures plus (GEFS+) and Dravet syndrome (DS, severe myoclonic epilepsy of infancy). Most of these mutations are in the SCN1A gene, and all are dominantly inherited. Most of the mutations that cause DS result in loss of function, whereas all of the known mutations that cause GEFS+ are missense, presumably altering channel activity. Family members with the same GEFS+ mutation often display a wide range of seizure types and severities, and at least part of this variability likely results from variation in other genes. Many different biophysical effects of SCN1A-GEFS+ mutations have been observed in heterologous expression systems, consistent with both gain and loss of channel activity. However, results from mouse models suggest that the primary effect of both GEFS+ and DS mutations is to decrease the activity of GABAergic inhibitory neurons. Decreased activity of the inhibitory circuitry is thus likely to be a major factor contributing to seizure generation in patients with GEFS+ and DS, and may be a general consequence of SCN1A mutations.

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.

A novel inherited mutation in the voltage sensor region of SCN1A is associated with Panayiotopoulos syndrome in siblings and generalized epilepsy with febrile seizures plus

We report 2 families harboring a novel SCN1A mutation, one of whom had Panayiotopoulos syndrome and the other a phenotype consistent with generalized epilepsy with febrile seizures plus. Two siblings had recurrent episodes of autonomic status epilepticus with focal features consistent with the diagnosis of Panayiotopoulos syndrome. Both have the SCN1A mutation p.Phe218Leu. The mutation was present in their father who has never had a seizure. The same mutation was identified in a child diagnosed with intractable childhood epilepsy with generalized tonic clonic seizures. From the age of 5, he developed complex focal seizures associated with left hippocampal sclerosis. The mutation was present in his mother, aged 25, who had febrile seizures and developed generalized tonic clonic seizures and his sister who had 1 febrile seizure. Our findings suggest that SCN1A mutations may cause susceptibility to an idiopathic focal epilepsy phenotype, the final phenotype depending on other (genetic or nongenetic) factors.

A catalog of SCN1A variants

Over the past 10 years mutations in voltage-gated sodium channels (Na(v)s) have become closely associated with inheritable forms of epilepsy. One isoform in particular, Na(v)1.1 (gene symbol SCN1A), appears to be a superculprit, registering with more than 330 mutations to date. The associated phenotypes range from benign febrile seizures to extremely serious conditions, such as Dravet's syndrome (SMEI). Despite the wealth of information, mutational analyses are cumbersome, owing to inconsistencies among the Na(v)1.1 sequences to which different research groups refer. Splicing variability is the core problem: Na(v)1.1 co-exists in three isoforms, two of them lack 11 or 28 amino acids compared to full-length Na(v).1.1. This review establishes a standardized nomenclature for Na(v)1.1 variants so as to provide a platform from which future mutation analyses can be started without need for up-front data normalization. An online resource--SCN1A infobase--is introduced.

Haptoglobin HP2-2 genotype, alpha-thalassaemia and acute seizures in children living in a malaria-endemic area

Polymorphisms of the haptoglobin (HP) gene and deletions in alpha-globin gene (alpha-thalassaemia) are common in malaria-endemic Africa. The same region also has high incidence rates for childhood acute seizures. The haptoglobin HP2-2 genotype has been associated with idiopathic generalized epilepsies and altered iron metabolism in children with alpha-thalassaemia can potentially interfere with neurotransmission and increase the risk of seizures. We investigated the hypothesis that the HP2-2 genotype and the common African alpha-globin gene deletions are associated with the increased risk of seizures. 288 children aged 3-156 months admitted with acute seizures to Kilifi District Hospital (Kenya), were matched for ethnicity to an equal number of community controls. The proportion of cases (72/288 [25.0%]) and controls (80/288 [27.8%]) with HP2-2 genotype was similar, p=0.499. The allele frequency of HP2 gene in cases (49.3%) and controls (48.6%) was also similar, p=0.814. Similarly, we found no significant difference between the proportion of cases (177/267 [66.3%]) and controls (186/267 [69.7%]) with deletions in alpha-globin gene (p=0.403). Among cases, HP2-2 polymorphism and deletions in alpha-globin gene were neither associated with changes in the type, number or duration of seizures nor did they affect outcome. We conclude that the HP2-2 polymorphism and deletions in alpha-globin gene are not risk factors for acute seizures in children. Future studies should examine other susceptibility genes.

Novel de novo mutation of a conserved SCN1A amino-acid residue (R1596)

We report on the case of a 6-year-old boy with epilepsy involving febrile seizures and unprovoked generalized tonic clonic seizures. Genetic testing revealed a novel de novo mutation in the SCN1A gene (C>T 4786, R1596C). The epilepsy phenotype is within the spectrum of generalized epilepsy with febrile seizures plus. However, de novo mutations are more commonly reported in cases of severe myoclonic epilepsy of infancy, and are less often reported in generalized epilepsy with febrile seizures plus. The clinical utility of specific genetic testing in this case is discussed, as are criteria for determining the pathologic significance of novel DNA variants. In this case, the wild type of residue (R1596) is well-conserved across evolution from bacteria to humans, providing support for the hypothesis that this mutation causes epilepsy.

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.

Sudden death in toddlers associated with developmental abnormalities of the hippocampus: a report of five cases

Sudden unexplained death in childhood (SUDC) is the sudden death of a child older than 1 year of age that remains unexplained after review of the clinical history, circumstances of death, and autopsy with appropriate ancillary testing. We report here 5 cases of SUDC in toddlers that we believe define a new entity associated with hippocampal anomalies at autopsy. All of the toddlers died unexpectedly during the night, apparently during sleep. Within 48 hours before death, 2 toddlers had fever, 3 had a minor upper respiratory tract infection, and 3 experienced minor head trauma. There was a history of febrile seizures in 2 (40%) and a family history of febrile seizures in 2 (40%). Hippocampal findings included external asymmetry and 2 or more microdysgenetic features. The incidence of certain microdysgenetic features was substantially increased in the temporal lobes of these 5 cases compared with the temporal lobes of 39 (control) toddlers with the causes of death established at autopsy (P < 0.01). We propose that these 5 cases define a potential subset of SUDC whose sudden death is caused by an unwitnessed seizure arising during sleep in the anomalous hippocampus and producing cardiopulmonary arrest. Precipitating factors may be fever, infection, and/or minor head trauma. Suggested risk factors are a history of febrile seizures and/or a family history of febrile seizures. Future studies are needed to confirm these initial findings and to define the putative links between sudden death, hippocampal anomalies, and febrile seizures in toddlers.

SCN1A mutations and epilepsy

SCN1A is part of the SCN1A-SCN2A-SCN3A gene cluster on chromosome 2q24 that encodes for alpha pore forming subunits of sodium channels. The 26 exons of SCN1A are spread over 100 kb of genomic DNA. Genetic defects in the coding sequence lead to generalized epilepsy with febrile seizures plus (GEFS+) and a range of childhood epileptic encephalopathies of varied severity (e.g., SMEI). All published mutations are collated. More than 100 novel mutations are spread throughout the gene with the more debilitating usually de novo. Some clustering of mutations is observed in the C-terminus and the loops between segments 5 and 6 of the first three domains of the protein. Functional studies so far show no consistent relationship between changes to channel properties and clinical phenotype. Of all the known epilepsy genes SCN1A is currently the most clinically relevant, with the largest number of epilepsy related mutations so far characterized.

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.

Clinical spectrum of mutations in SCN1A gene: severe myoclonic epilepsy in infancy and related epilepsies

Severe myoclonic epilepsy in infancy (SMEI) manifests very frequent generalized tonic-clonic seizures (GTC), accompanied by myoclonic seizures, absences and partial seizures [Dravet, C., 1978. Les épilepsie grave de l'enfant. Vie Méd. 8, 543-548; Dravet, C., Roger, J., Bureau, M., Dalla Bernardina, B., 1982. Myoclonic epilepsies in childhood. In: Akimoto, H., Kazamatsuri, H., Seino, M., Ward, A. (Eds.), Advances in Epileptology. Raven Press, New York, pp. 135-140; Dravet, C., Bureau, M., Oguni, H., Fukuyama, Y., Cokar, O., 2002. Severe myoclonic epilepsy of infancy (Dravet syndrome). In: Roger, J., Bureau, M., Dravet, C., Genton, P., Tassinari, C.A., Wolf, P. (Eds.), Epileptic Syndromes in Infancy, Childhood and Adolescence, third ed. John Libbey, London, pp. 81-103]. However, there is a group of severe epilepsy that has many characteristics common to SMEI except for myoclonic seizures. We reported this group of epilepsy as intractable childhood epilepsy with GTC (ICEGTC) [Watanabe, M., Fujiwara, T., Yagi, K., Seino, M., Higashi, T., 1989b. Intractable childhood epilepsy with generalized tonic-clonic seizures. J. Jpn. Epil. Soc. 7, 96-105 (in Japanese); Fujiwara, T., Watanabe, M., Takahashi, Y., Higashi, T., Yagi, K., Seino, M., 1992. Long-term course of childhood epilepsy with intractable grand mal seizures. Jpn. J. Psychiatr. Neurol. 46, 297-302]. Recently, mutations of the neuronal voltage-gated sodium channel alphasubunit type 1 gene (SCN1A) have been found in SMEI [Claes, L., Del-Favero, J., Ceulemans, B., Lagae, L., Van Broeckhoven, C., De Jonghe, P., 2001, De novo mutations in the sodium-channel gene SCN1A cause severe myoclonic epilepsy of infancy. Am. J. Hum. Genet. 68, 327-1332]. Mutations in SCN1A are found in both SMEI and ICEGTC at high rates of 70-81%. The loci of the mutations seen in ICEGTC are quite similar to those found in SMEI, suggesting a genotypic continuity between these entities. The clinical spectrum of epilepsies harboring SCN1A mutations may be consisted of various phenotypes with GEFS+ on the mildest end and SMEI on the severest end of the spectrum.

Sodium channel mutations in epilepsy and other neurological disorders

Since the first mutations of the neuronal sodium channel SCN1A were identified 5 years ago, more than 150 mutations have been described in patients with epilepsy. Many are sporadic mutations and cause loss of function, which demonstrates haploinsufficiency of SCN1A. Mutations resulting in persistent sodium current are also common. Coding variants of SCN2A, SCN8A, and SCN9A have also been identified in patients with seizures, ataxia, and sensitivity to pain, respectively. The rapid pace of discoveries suggests that sodium channel mutations are significant factors in the etiology of neurological disease and may contribute to psychiatric disorders as well.

Inherited disorders of voltage-gated sodium channels

A variety of inherited human disorders affecting skeletal muscle contraction, heart rhythm, and nervous system function have been traced to mutations in genes encoding voltage-gated sodium channels. Clinical severity among these conditions ranges from mild or even latent disease to life-threatening or incapacitating conditions. The sodium channelopathies were among the first recognized ion channel diseases and continue to attract widespread clinical and scientific interest. An expanding knowledge base has substantially advanced our understanding of structure-function and genotype-phenotype relationships for voltage-gated sodium channels and provided new insights into the pathophysiological basis for common diseases such as cardiac arrhythmias and epilepsy.

Sodium channel dysfunction in intractable childhood epilepsy with generalized tonic-clonic seizures

Mutations in SCN1A, the gene encoding the brain voltage-gated sodium channel alpha(1) subunit (Na(V)1.1), are associated with genetic forms of epilepsy, including generalized epilepsy with febrile seizures plus (GEFS+ type 2), severe myoclonic epilepsy of infancy (SMEI) and related conditions. Several missense SCN1A mutations have been identified in probands affected by the syndrome of intractable childhood epilepsy with generalized tonic-clonic seizures (ICEGTC), which bears similarity to SMEI. To test whether ICEGTC arises from molecular mechanisms similar to those involved in SMEI, we characterized eight ICEGTC missense mutations by whole-cell patch clamp recording of recombinant human SCN1A heterologously expressed in cultured mammalian cells. Two mutations (G979R and T1709I) were non-functional. The remaining alleles (T808S, V983A, N1011I, V1611F, P1632S and F1808L) exhibited measurable sodium current, but had heterogeneous biophysical phenotypes. Mutant channels exhibited lower (V983A, N1011I and F1808L), greater (T808S) or similar (V1611F and P1632S) peak sodium current densities compared with wild-type (WT) SCN1A. Three mutations (V1611F, P1632S and F1808L) displayed hyperpolarized conductance-voltage relationships, while V983A exhibited a strong depolarizing shift in the voltage dependence of activation. All mutants except T808S had hyperpolarized shifts in the voltage dependence of steady-state channel availability. Three mutants (V1611F, P1632S and F1808L) exhibited persistent sodium current ranging from approximately 1-3% of peak current amplitude that was significantly greater than WT-SCN1A. Several mutants had impaired slow inactivation, with V983A showing the most prominent effect. Finally, all of the functional alleles exhibited reduced use-dependent channel inhibition. In summary, SCN1A mutations associated with ICEGTC result in a wide spectrum of biophysical defects, including mild-to-moderate gating impairments, shifted voltage dependence and reduced use dependence. The constellation of biophysical abnormalities for some mutants is distinct from those previously observed for GEFS+ and SMEI, suggesting possible, but complex, genotype-phenotype correlations.

A missense mutation in SCN1A in brothers with severe myoclonic epilepsy in infancy (SMEI) inherited from a father with febrile seizures

Severe myoclonic epilepsy in infancy (SMEI) is an age-dependent epileptic encephalopathy occurring in the first year of life and is one of the intractable epilepsies. Heterozygous mutations in the voltage-gated sodium channel alpha subunit type1 gene (SCN1A) are frequently identified in patients with SMEI; two-thirds of these mutations are truncation mutations (non-sense and frameshift), and one-third are missense mutations. Although most reported SMEI cases arise as sporadic mutations, close relatives of SMEI patients have also been shown to manifest other types of epilepsies at a higher rate than that in the general population. Here, we report a familial case of SMEI, in which two brothers were affected with SMEI while their father had previously experienced simple febrile seizures. A gene-based analysis identified a novel missense mutation in the SCN1A gene (c.5138G>A, S1713N) in both brothers and in their father. Clinically, both siblings showed failure in locomotion, an impairment of the sleep-wake cycle after late infancy, and the subsequent appearance of frontal foci. The similarity in clinical manifestations in both brothers suggests that the impairment of elements of the brainstem, particularly aminergic neurons, develops after late infancy in SMEI. However, the siblings differed in age at onset of SMEI and of myoclonic seizures, as well as in the severity of speech delay. Our molecular and clinical findings suggest that different genetic backgrounds and/or environmental factors may critically affect the clinical features of patients with SCN1A mutations, consistent with the heterogeneity prevalent in this disorder.

Genetics of idiopathic epilepsies

PURPOSE

To search for clues to molecular genetics of common idiopathic epilepsy syndromes. Genetic defects have been identified recently in certain inherited epilepsy syndromes in which the phenotypes are similar to those of common idiopathic epilepsies.

METHODS

Mutations identified as the causes of inherited idiopathic epilepsies were reviewed.

RESULTS

Mutations of the genes encoding two subunits of the neuronal nicotinic acetylcholine receptor were found in autosomal dominant nocturnal frontal lobe epilepsy. Mutations of two K(+)-channel genes were identified in benign familial neonatal convulsions. Mutations of the genes encoding several subunits of the voltage-gated Na(+)-channel and gamma-aminobutyric acid (GABA)(A) receptor also were identified as the underlying causes of various epilepsy syndromes, such as autosomal dominant epilepsy with febrile seizures plus, benign familial neonatal infantile seizures, and autosomal dominant juvenile myoclonic epilepsy. Mutations within the same gene may result in different epilepsy phenotypes. Thus, the Na(+) channel, GABA(A) receptor, and their auxiliaries may be involved in the pathogenesis of various types of epilepsy. Some forms of juvenile myoclonic epilepsy, idiopathic generalized epilepsy, and absence epilepsy may result from mutations of Ca(2+) channels. Mutations of the Cl(-) channel have been recently found to be associated with a certain type of epilepsy. The recent discovery that mutations of LGI1, a gene encoding a nonchannel molecule, are associated with autosomal partial epilepsy with auditory features may provide a new insight into our understanding of the genetics of idiopathic epilepsy.

CONCLUSIONS

These findings suggest the involvement of brain channelopathies in the pathogenesis of certain types of idiopathic epilepsy.

Clinical and genetic aspects of idiopathic epilepsies in childhood

The identification of the first genes associated with idiopathic epilepsy has been an important breakthrough in the field of epilepsy research. In almost all cases these genes were found to encode components of voltage- or ligand-gated ion channels or functionally related structures. For many other idiopathic syndromes, there is linkage evidence to one or more chromosomes, but the genes have not yet been identified. Identification of the responsible genes and their gene products will further increase the knowledge of the pathogenic mechanisms involved in epilepsy, and will hopefully facilitate the development of drug targets for the effective treatment of epilepsy. This review gives an overview of the clinical characteristics and an update of genetic research of those idiopathic childhood epilepsies for which genes have been identified and the monogenic idiopathic childhood epilepsies for which mapping data are available.

Seizure phenotypes of a family with missense mutations in SCN2A

The seizure phenotypes of a Japanese family with missense mutations in SCN2A are described. The proband of the family had partial epilepsy after febrile seizures plus. He had three missense mutations of SCN2A (R19K, R188W, and R524Q). The R188W mutation was suggested by electrophysiologic studies to be the main disease mutation. However, it is suggested that the penetrance rate of this pedigree is extremely low, or that other genes may have modified the phenotype of the proband.

Fever, genes, and epilepsy

About 13% of patients with epilepsy have a history of febrile seizures (FS). Studies of familial forms suggest a genetic component to the epidemiological link. Indeed, in certain monogenic forms of FS, for which several loci have been reported, some patients develop epilepsy with a higher risk than in the general population. Patients with generalised epilepsy with febrile seizures plus (GEFS+) can have typical and isolated FS, FS lasting more beyond age 6 years, and subsequent afebrile (typically generalised) seizures. Mutations associated with GEFS+ were identified in genes for subunits of the voltage-gated sodium channel and the gamma2 subunit of the ligand-gated GABAA receptor. Screening for these genes in patients with severe myoclonic epilepsy in infancy showed de novo mutations of the alpha1 subunit of the voltage-gated sodium channel. Antecedent FS are commonly observed in temporal-lobe epilepsy (TLE). In sporadic mesial TLE-characterised by the sequence of complex FS in childhood, hippocampal sclerosis, and refractory temporal-lobe seizures-association studies suggested the role of several susceptibility genes. Work on some large pedigrees also suggests that FS and temporal-lobe seizures may have a common genetic basis, whether hippocampus sclerosis is present or not. The molecular defects identified in the genetic associations of FS and epileptic seizures are very attractive models to aid our understanding of epileptogenesis and susceptibility to seizure-provoking factors, especially fever.

Clinical and electroencephalographic characteristics of children with febrile seizures plus

OBJECTIVES

Febrile seizures plus (FS+) are attracting attention for their corresponding genetic abnormalities, and are defined as febrile seizures (FS) continuing beyond 6 years of age (late FS) or those associated with afebrile seizures. We tried to elucidate their clinical and EEG characteristics as compared with those of children having only FS.

SUBJECTS AND METHODS

We reviewed clinical records in a pediatric neurology clinic to identify 31 patients with FS+ (group FS+) and 51 with only FS (group FS). Their family history of seizures, clinical features and EEG findings were compared.

RESULTS

A family history of seizures was noted in 14 patients (45.2%) of group FS+ and in 24 (47.1%) of group FS. In group FS+, 19 patients had late FS, 11 had afebrile seizures, and the remaining one had both types of seizures. Two patients had seizures induced by TV/video-game as well, and another suffered from absences. Epileptic EEG abnormalities, which included diffuse spike-waves and focal spikes, were noted in 13 patients (41.9%) of group FS+ and 12 (23.5%) of group FS.

CONCLUSIONS

The clinical and EEG characteristics of the children having FS+ were diverse, without significant differences from those with FS except for the seizures types.

A nonsense mutation of the sodium channel gene SCN2A in a patient with intractable epilepsy and mental decline

Mutations, exclusively missense, of voltage-gated sodium channel alpha subunit type 1 (SCN1A) and type 2 (SCN2A) genes were reported in patients with idiopathic epilepsy: generalized epilepsy with febrile seizures plus. Nonsense and frameshift mutations of SCN1A, by contrast, were identified in intractable epilepsy: severe myoclonic epilepsy in infancy (SMEI). Here we describe a first nonsense mutation of SCN2A in a patient with intractable epilepsy and severe mental decline. The phenotype is similar to SMEI but distinct because of partial epilepsy, delayed onset (1 year 7 months), and absence of temperature sensitivity. A mutational analysis revealed that the patient had a heterozygous de novo nonsense mutation R102X of SCN2A. Patch-clamp analysis of Na(v)1.2 wild-type channels and the R102X mutant protein coexpressed in human embryonic kidney 293 cells showed that the truncated mutant protein shifted the voltage dependence of inactivation of wild-type channels in the hyperpolarizing direction. Analysis of the subcellular localization of R102X truncated protein suggested that its dominant negative effect could arise from direct or indirect cytoskeletal interactions of the mutant protein. Haploinsufficiency of Na(v)1.2 protein is one plausible explanation for the pathology of this patient; however, our biophysical findings suggest that the R102X truncated protein exerts a dominant negative effect leading to the patient's intractable epilepsy.

Données récentes sur l’implications des canaux ioniques dans les formes familiales d’épilepsies généralisées idiopathiques associées ou non à des convulsions fébriles

Major advances have recently been made in the understanding of the genetic bases of monogenic inherited epilepsies. For several idiopathic epilepsies, mutations in genes encoding subunits of ion channels or ligand receptors have been demonstrated. This is the case for some generalized idiopathic epilepsies and generalized epilepsies associated with febrile seizures. In this Article, we review the recent clinical and genetic data of these forms of epilepsy.

Monogenic idiopathic epilepsies

Major advances have recently been made in our understanding of the genetic bases of monogenic inherited epilepsies. Direct molecular diagnosis is now possible in numerous inherited symptomatic epilepsies. Progress has also been spectacular with respect to several idiopathic epilepsies that are caused by mutations in genes encoding subunits of ion channels or neurotransmitter receptors. Although these findings concern only a few families and sporadic cases, their potential importance is great, because these genes are implicated in a wide range of more common epileptic disorders and seizure types as well as some rare syndromes. Functional studies of these mutations, while leading to further progress in the neurobiology of the epilepsies, will help to refine genotype-phenotype relations and increase our understanding of responses to antiepileptic drugs. In this article, we review the clinical and genetic data on most of the idiopathic human epilepsies and epileptic contexts in which the association of epilepsy and febrile convulsions is genetically determined.

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.

Mutations of Neuronal Voltage-gated Na+ Channel alpha1 Subunit Gene SCN1A in Core Severe Myoclonic Epilepsy in Infancy (SMEI) and in Borderline SMEI (SMEB)

PURPOSE

Severe myoclonic epilepsy in infancy (SMEI) is a distinct epilepsy syndrome. Patients with borderline SMEI (SMEB) are a subgroup with clinical features similar to those of core SMEI but are not necessarily consistent with the accepted diagnostic criteria for core SMEI. The aim of this study was to delineate the genetic correlation between core SMEI and SMEB and to estimate the frequency of mutations in both phenotypes.

METHODS

We examined 96 healthy volunteers and 58 unrelated individuals whose clinical features were consistent with either core SMEI (n = 31) or SMEB (n = 27). We screened for genetic abnormalities within exons and their flanking introns of the genes encoding major subunits of the Na+ channels (SCN1A, SCN2A, SCN1B, and SCN2B) by using a direct sequencing method.

RESULTS

In both core SMEI and SMEB, various mutations of SCN1A including nonsense and missense mutations were identified, whereas no mutations of SCN2A, SCN1B, and SCN2B were found within the regions examined. All mutations were heterozygous and not found in 192 control chromosomes. Mutations were identified in 26 (44.8%) of the 58 individuals and were more frequent (p < 0.05) in core SMEI (19 of 31) than in SMEB (seven of 27), as assessed by the continuity-adjusted chi2 test. Mutations resulting in a molecular truncation were found only in core SMEI. Among the mutations, two missense mutations were found in both core SMEI and SMEB.

CONCLUSIONS

Our findings confirm that SMEB is part of the SMEI spectrum and may expand the recognition of SMEI and suggest other responsible or modifying genes.

Epilepsy-associated dysfunction in the voltage-gated neuronal sodium channel SCN1A

Mutations in SCN1A, the gene encoding the brain voltage-gated sodium channel alpha1 subunit (NaV1.1), are associated with at least two forms of epilepsy, generalized epilepsy with febrile seizures plus (GEFS+) and severe myoclonic epilepsy of infancy (SMEI). We examined the functional properties of four GEFS+ alleles and one SMEI allele using whole-cell patch-clamp analysis of heterologously expressed recombinant human SCN1A. One previously reported GEFS+ mutation (I1656M) and an additional novel allele (R1657C), both affecting residues in a voltage-sensing S4 segment, exhibited a similar depolarizing shift in the voltage dependence of activation. Additionally, R1657C showed a 50% reduction in current density and accelerated recovery from slow inactivation. Unlike three other GEFS+ alleles that we recently characterized, neither R1657C nor I1656M gave rise to a persistent, noninactivating current. In contrast, two other GEFS+ mutations (A1685V and V1353L) and L986F, an SMEI-associated allele, exhibited complete loss of function. In conclusion, our data provide evidence for a wide spectrum of sodium channel dysfunction in familial epilepsy and demonstrate that both GEFS+ and SMEI can be associated with nonfunctional SCN1A alleles.

Is phenotype difference in severe myoclonic epilepsy in infancy related to SCN1A mutations?

We classified 28 patients with severe myoclonic epilepsy in infancy (SME) according to the presence or absence of myoclonic seizures and/or atypical absences. Eleven of the patients had myoclonic seizures and/or atypical absences, and we refer to this condition as 'typical SME (TSME)'. Seventeen of the patients had only segmental myoclonias, and we refer to this condition as 'borderline SME (BSME)'. We then analyzed the electroclinical and genetic characteristics of these two groups. Ten of the 11 TSME patients had a photoparoxysmal response at some time during their clinical course, while none of the BSME patients showed this response. TSME and BSME showed a significant difference in regard to gender ratio: female dominance in TSME and male dominance in BSME (P=0.008). The detection rate of the voltage-gated sodium channel alpha1-subunit (SCN1A) gene mutations was 72.7 and 88.2% in TSME and BSME, respectively. There was no difference in the type or rate of mutation between TSME and BSME. We conclude that TSME and BSME show distinct differences in photoparoxysmal response and gender, which might be caused by some genetic mechanism(s) other than the SCN1A gene mutation.

The genetics of febrile seizures and related epilepsy syndromes

Febrile seizures (FS) may represent the most common seizure disorder in childhood and are known to be associated with putative genetic predispositions. Nevertheless, molecular genetic approaches toward understanding FS have been just initiated this decade. Recently, several genetic loci for FS have been mapped thereby assuring the genetic heterogeneity of FS. However, the exact molecular mechanisms of FS are yet to be elucidated. Genetic defects have been recently identified in autosomal dominant epilepsy with FS plus or generalized epilepsy with FS plus. The underlying mutations were found in genes encoding several Na+ channel subunits and the gamma2 subunit of gamma amino-butyric acid (GABA)A receptors in the brain. Furthermore, both channels are also associated with severe myoclonic epilepsy in infancy, where the seizure attacks often begin with prolonged FS and are precipitated by fever even afterwards. Na+ channels are associated with other temperature-sensitive disorders, and GABA(A) receptors are known to play an important role in the pathogenesis of FS. These lines of evidence suggest the involvement of various Na+ channels, GABA(A) receptors and additional auxiliary proteins in the pathogenesis of frequent FS and even in simple FS. This hypothesis may facilitate our understanding of the genetic background of FS.

Nav1.1 channels with mutations of severe myoclonic epilepsy in infancy display attenuated currents

Severe myoclonic epilepsy in infancy (SMEI) is characterized by intractable febrile and afebrile seizures, severe mental decline, and onset during the first year of life. Nonsense, frameshift, and missense mutations of SCN1A gene encoding the voltage-gated Na(+) channel alpha-subunit type I (Na(v)1.1) have been identified in patients with SMEI. Here, we performed whole-cell patch-clamp analyses on HEK293 cells expressing human Na(v)1.1 channels bearing SMEI nonsense and missense mutations. The mutant channels showed remarkably attenuated or barely detectable inward sodium currents. Our findings indicate that SMEI mutations lead to loss-of-function and may contribute to the development of SMEI phenotypes.

Genetics of epilepsy: current status and perspectives

Epilepsy affects more than 0.5% of the world's population and has a large genetic component. The most common human genetic epilepsies display a complex pattern of inheritance and the susceptibility genes are largely unknown. However, major advances have recently been made in our understanding of the genetic basis of monogenic inherited epilepsies. Progress has been particularly evident in familial idiopathic epilepsies and in many inherited symptomatic epilepsies, with the discovery that mutations in ion channel subunits are implicated, and direct molecular diagnosis of some phenotypes of epilepsy is now possible. This article reviews recent progress made in molecular genetics of epilepsy, focusing mostly on idiopathic epilepsy, and some types of myoclonus epilepsies. Mutations in the neuronal nicotinic acetylcholine receptor alpha4 and beta2 subunit genes have been detected in families with autosomal dominant nocturnal frontal lobe epilepsy, and those of two K(+) channel genes were identified to be responsible for underlying genetic abnormalities of benign familial neonatal convulsions. The voltage-gated Na(+) -channel (alpha1,2 and beta1 subunit), and GABA receptor (gamma2 subunit) may be involved in the pathogenesis of generalized epilepsy with febrile seizure plus and severe myoclonic epilepsy in infancy. Mutations of Ca(2+)-channel can cause some forms of juvenile myoclonic epilepsy and idiopathic generalized epilepsy. Based upon these findings, pathogenesis of epilepsy as a channelopathy and perspectives of molecular study of epilepsy are discussed.

Significant correlation of the SCN1A mutations and severe myoclonic epilepsy in infancy

To investigate the possible correlation between genotype and phenotype of epilepsy, we analyzed the voltage-gated sodium channel alpha1-subunit (SCN1A) gene, beta1-subunit (SCN1B) gene, and gamma-aminobutyric acid(A) receptor gamma2-subunit (GABRG2) gene in DNAs from peripheral blood cells of 29 patients with severe myoclonic epilepsy in infancy (SME) and 11 patients with other types of epilepsy. Mutations of the SCN1A gene were detected in 24 of the 29 patients (82.7%) with SME, although none with other types of epilepsy. The mutations included deletion, insertion, missense, and nonsense mutations. We could not find any mutations of the SCN1B and GABRG2 genes in all patients. Our data suggested that the SCN1A mutations were significantly correlated with SME (p<.0001). As we could not find SCN1A mutations in their parents, one of critical causes of SME may be de novo mutation of the SCN1A gene occurred in the course of meiosis in the parents.

Genetic abnormalities underlying familial epilepsy syndromes

Genetic defects have been recently identified in certain inherited epilepsy syndromes in which the phenotypes are similar to common idiopathic epilepsies. Mutations in the neuronal nicotinic acetylcholine receptor 4 and 2 subunit genes have been detected in families with autosomal dominant nocturnal frontal lobe epilepsy. Both receptors are components of neuronal acetylcholine receptor, a ligand-gated ion channel in the brain. Furthermore, mutations of two K+-channel genes were also identified as the underlying genetic abnormalities of benign familial neonatal convulsions. Mutations in the voltage-gated Na+-channel 1, 2 and 1 and the gamma aminobutyric acid (GABAA) receptor 2 subunit genes were found as a cause of generalized epilepsy with febrile seizures plus, a clinical subset of febrile convulsions. Na+-channels, GABAA receptor and their auxiliaries may be involved in the pathogenesis of this subtype and even in simple febrile convulsions. Mutation of a voltage-gated K+-channel gene can cause partial seizures associated with periodic ataxia type 1 and some forms of juvenile myoclonic epilepsy and idiopathic generalized epilepsy can result from mutations of a Ca2+-channel. This line of evidence suggests the involvement of channels expressed in the brain in the pathogenesis of certain types of epilepsy. Our working hypothesis is to view certain idiopathic epilepsies as disorders of ion channels, i.e. 'channelopathies'. Such hypothesis should provide a new insight to our understanding of the genetic background of epilepsy.