The voltage-gated potassium channel KCNQ2 in Taiwanese children with febrile convulsions

A KCNQ2 E515D mutation associated with benign familial neonatal seizures and continuous spike and waves during slow-wave sleep syndrome in Taiwan

BACKGROUND/PURPOSE

Pediatric epilepsy caused by a KCNQ2 gene mutation usually manifests as benign familial neonatal seizures (BFNS) during the 1^st week of life. However, the exact mechanism, phenotype, and genotype of the KCNQ2 mutation are unclear.

METHODS

We studied the KCNQ2 genotype from 75 nonconsanguineous patients with childhood epilepsy without an identified cause (age range: from 2 days to 18 years) and from 55 healthy adult controls without epilepsy. KCNQ2 mutation variants were transfected into HEK293 cells to investigate what functional changes they induced.

RESULTS

Four (5%) of the patients had the E515D KCNQ2 mutation, which the computer-based PolyPhen algorithm predicted to be deleterious. Their seizure outcomes were favorable, but three had an intellectual disability. Two patients with E515D presented with continuous spikes and waves during slow-wave sleep (CSWS), and the other two presented with BFNS. We also analyzed 10 affected family members with the same KCNQ2 mutation: all had epilepsy (8 had BFNS and 2 had CSWS). A functional analysis showed that the recordings of the E515D currents were significantly different (p<0.05), which suggested that channels with KCNQ2 E515D variants are less sensitive to voltage and require stronger depolarization to reach opening probabilities than those with the wild type or N780T (a benign polymorphism).

CONCLUSION

KCNQ2 mutations can cause various phenotypes in children: they lead to BFNS and CSWS. We hypothesize that patients with the KCNQ2 E515D mutation are susceptible to seizures.

Genetic background of febrile seizures

Febrile seizures (FSs) occur in children older than 1 month and without prior afebrile seizures in the absence of a central nervous system infection or acute electrolyte imbalance. Their pathogenesis is multifactorial. The most relevant familial studies evidence an occurrence rate ranging from 10% to 46% and median recurrence rate of 36% in children with positive familial history for FS. The main twin studies demonstrated a higher concordance rate in monozygotic twins with FS than in dizygotic ones. Linkage studies have proposed 11 chromosomal locations responsible to FS attributed to FEB1 to FEB11. Population-based association studies have shown at least one positive association for 14 of 41 investigated genes with FS. The proinflammatory cytokine interleukin 1β (IL-1β) was the most investigated and also gene associated with susceptibility to FS. A possible role in the overlapping of epilepsy and FS was found for 16 of 36 investigated genes. SCN1A, IL-1β, CHRNA4, and GABRG2 were the most commonly involved genes in this context. The genetic background of FS involves the regulation of different processes, including individual and familial susceptibility, modulation of immune response, and neuronal excitability and interactions with exogenous agents such as viruses.

Interleukin (IL)-1beta, IL-1 receptor antagonist, IL-6, IL-8, IL-10, and tumor necrosis factor alpha gene polymorphisms in patients with febrile seizures

Inflammation and genetics may play a role in the pathogenesis of febrile seizures (FSs). We aimed to test whether interleukin-1beta (IL-1beta), IL-1 receptor antagonist (IL-1 Ra), IL-6 promoter, IL-8, IL-10, or tumor necrosis factor (TNF) gene polymorphisms could be used as markers of susceptibility to FSs. An association study was performed among a cohort of 104 patients with FSs and 143 normal control subjects. There was no significant difference between patients and controls in the distribution of allele frequencies of the IL-1beta promoter, IL-1beta exon 5, IL-6 promoter, IL-8, IL-10, or TNF-alpha gene polymorphisms. In contrast, the IL-1 Ra-I homozygote was more frequent in patients with FSs than in healthy controls (93.2% vs. 83.92%, chi(2)=4.51, P=0.034). In addition, individuals homozygous for the IL-1 Ra-I genotype were more than twice as likely to develop FSs than individuals heterozygous for the IL-1 Ra-I/II genotype (OR, 2.63, 95% CI: 1.08-6.39; chi(2)=4.55, P=0.033). We conclude that the IL-1 Ra gene might be one of the useful markers for predicting susceptibility to FSs.

Genetic susceptibility to febrile seizures: case-control association studies

OBJECTIVE

A genetic predisposition to febrile seizures (FS) has long been recognized. The inheritance appears to be polygenic in small families or sporadic cases of FS encountered in daily clinical practice. To determine whether candidate genes are responsible for the susceptibility to FS, we have performed genetic association studies in FS patients and controls.

METHODS

The single-nucleotide polymorphisms (SNPs) of genes involved in immune response (interleukin (IL) 1B), endocannabinoid signaling (CNR1), acid-base balance (SLC4A3, SLC9A1, SLC9A3), gap junction channel (CX43), and GABA(A) receptor trafficking (PRIP1) were examined in 249 FS patients (186 simple and 63 complex FS) and 225 controls.

RESULTS

There were no significant differences in the allele frequencies of the SNPs between controls and all FS, simple FS, and complex FS patients. When the simple FS patients were divided into two groups according to either having (familial) or not having a family history of FS in close relatives (sporadic), there was a significant association between IL1B -511 SNP and sporadic simple FS (p=0.003).

CONCLUSIONS

These data suggest that cytokine genes may act as enhancers or attenuators of FS susceptibility. Genetic association study may be an effective approach to understanding the molecular basis of FS at least in a subgroup of patients.

Association of idiopathic generalized epilepsy with polymorphisms in the neuronal nicotinic acetylcholine receptor subunits

Idiopathic generalized epilepsy (IGE) refers to a common group of epilepsies, and genetic factors play an important role in the pathogenesis of these disorders. Mutations in CHRNA4 and CHRNB2 are associated with some cases of familial epilepsies classified as autosomal-dominant nocturnal frontal lobe epilepsies. We aimed to evaluate whether polymorphisms of CHRNA4 and CHRNB2 are associated with IGE. A total of 75 children with IGE and 80 normal control subjects were included in the study. Each genetic polymorphism was typed by polymerase chain reaction (PCR)-based restriction analysis. The genotypes and allelic frequencies of each polymorphism were compared between the IGE patients and controls. The results showed that genotype and allelic frequency for CHRNB2 did not differ significantly between the groups. However, the genotype proportion of the CHRNA4 (Ser543Ser) gene in both groups was significantly different (P<0.0001). The T allele frequency was significantly higher (P=0.0126) in patients with IGE compared to healthy controls. The odds ratio (OR) for developing IGE in individuals with the CHRNA4 (Ser543Ser)-T homozygote was 4.9 (95% confidence interval (CI), 1.71-14.04) compared to individuals with two copies of the CHRNA4 (Ser543Ser)-C allele. This study demonstrates that the CHRNA4 gene may be one of the susceptibility factors for IGE.

Association of GABRG2 polymorphisms with idiopathic generalized epilepsy

Missense mutations in the gamma2 subunit of gamma-aminobutyric acid (GABA) receptor gene have recently been described in families with idiopathic generalized epilepsies. This study aimed to evaluate whether polymorphisms of the gamma2 subunit of the GABA receptor gene are associated with idiopathic generalized epilepsies. A total of 77 children with idiopathic generalized epilepsies and 83 normal control subjects were included in the study. Polymerase chain reaction was used to identify the C/T and A/G polymorphisms of the gamma2 subunit of the GABA receptor gene on chromosome 5q33. Genotypes and allelic frequencies in both groups were compared. The gamma2 subunit of the GABA receptor (nucleotide position 3145 in intron G-> A) gene in both groups was not significantly different. In contrast, the gamma2 subunit of GABA receptor (SNP211037)-C allele frequency in patients with idiopathic generalized epilepsies was significantly higher than that in healthy control subjects (P = 0.002). The odds ratio for developing idiopathic generalized epilepsies in individuals with the gamma2 subunit of the GABA receptor (SNP211037)-C/C genotype was 3.61 compared with individuals with the gamma2 subunit of the GABA receptor (SNP211037)-T/T genotype. These data suggest that the gamma2 subunit of the GABA receptor gene might be one of the susceptibility factors for idiopathic generalized epilepsies.

Febrile seizures in southern Chinese children: incidence and recurrence

This study investigates the incidence, recurrence, and risk factors of febrile seizures in southern Chinese children. A retrospective study of a 5-year period (March 1998 through February 2003) was conducted for all children admitted with first febrile seizure to a university teaching hospital of Hong Kong, serving a population of 31,700 under 6 years. A total of 565 Chinese children (329 males, 236 females) were identified with mean age of 2.1 +/- 1.1 years. The annual incidence was 0.35%. Among them 16% (91/565) had complex febrile seizures. Family history of febrile and afebrile seizures was present in 17.5% and 2.7% respectively. The mean follow-up period was 2.33 +/- 1.69 years. Altogether 103 children (18%) had recurrence, and the cumulative rates by 1, 2, and 3 years were 12.7%, 18.7%, and 20.5% respectively. Three significant factors were identified for higher risk of recurrence: early age of onset, family history of febrile seizure, and complex febrile seizure. The incidence of first febrile seizure in Chinese children is low compared with the Western world and relatively similar to mainland China. Recurrence is also lower despite similarities in the predictive factors. Further epidemiologic and genetic studies will be necessary to confirm and explain this interethnic variation.

Genetic Association Studies in Epilepsy: "The Truth Is Out There"

Success has been achieved in identifying many mutations in rare monogenic epilepsy syndromes by using linkage analysis, but dissecting the genetic basis of common epilepsy syndromes has proven more difficult. Common epilepsies are genetically complex disorders believed to be influenced by variation in several susceptibility genes. Association studies can theoretically identify these genes, but despite more than 50 association studies in epilepsy, no consistent or convincing susceptibility genes have emerged, leading to scepticism about the association-study approach. We review the results of existing association studies in focal epilepsies, generalized epilepsies, febrile seizures, and epilepsy pharmacogenetics. By using an illustrative example, we discuss how methodologic issues of sample size, selection of appropriate controls, population stratification, and significance thresholds can lead to bias and false-positive associations; the importance of biologic plausibility also is emphasized. Newer methodologic refinements for association studies, such as use of two control groups, genomic control, haplotyping, and use of two independent datasets, are discussed. A summary of existing guidelines and a checklist for planning and appraising such association studies in epilepsy is presented. We remain cautiously optimistic that with methodologic refinements and multicenter collaborations with large sample sizes, association studies will ultimately be useful in dissecting the genetic basis of common epilepsy syndromes.

Association of the nicotinic receptor beta 2 subunit and febrile seizures

The nicotinic acetylcholine receptors are members of a superfamily of ligand-gated ion channels that mediate fast signal transmission at synapses. Mutations in neuronal nicotinic acetylcholine receptor beta 2 subunit have been associated with autosomal dominant nocturnal frontal lobe epilepsies. A major challenge is to establish whether the monogenic epilepsy gene also contributes to the common epilepsies. Because febrile seizures represent the majority of childhood seizures, and a genetic predisposition, we investigated the possibility that the nicotinic acetylcholine receptor beta 2 subunit might be involved in the etiology of febrile seizures. Children were divided into two groups: those with febrile seizures (group 1; n = 104) and control patients (group 2; n = 83). Polymerase chain reaction was used to identify the G/C and T/C polymorphisms of the nicotinic acetylcholine receptor beta 2 subunit gene, which is mapped on chromosome 1. Genotypes and allelic frequencies for nicotinic acetylcholine receptor beta 2 subunit gene polymorphisms in both groups were compared. The results indicated that genotypes and allelic frequencies in both groups were not significantly different. These data suggest that nicotinic acetylcholine receptor beta 2 subunit polymorphisms are not a useful marker for prediction of the susceptibility to febrile seizures.

Association analysis of gamma 2 subunit of gamma- aminobutyric acid type A receptor polymorphisms with febrile seizures

An alternation of gamma-aminobutyric acid (GABA)-ergic neurotransmission has been implicated as an etiologic factor in epileptogenesis. Missense mutations in the GABRG2 gene, which encodes the gamma2 subunit of central nervous GABAA receptors, have recently been described in one family with childhood absence epilepsy and febrile seizures (FSs). FSs represent the majority of childhood seizures and have a genetic predisposition. It is not known, however, whether polymorphisms in those genes involved in familial epilepsies also contribute to the pathogenesis of FSs. By performing an association study, we used single-nucleotide polymorphisms to investigate the distribution of genotypes of GABRG2 in patients with FSs. A total of 104 children with FSs and 83 normal control subjects were included in the study. PCR was used to identify the C/T and A/G polymorphisms of the GABRG2 gene on chromosome 5q33. Genotypes and allelic frequencies for the GABRG2 gene polymorphisms in both groups were compared. The GABRG2 (nucleotide position 3145 in intron G-->A) gene in both groups was not significantly different. In contrast, the number of individuals with the GABRG2 (SNP211037)-C/C genotype in patients with FSs was significantly greater compared with that in healthy control subjects (p = 0.017), and the GABRG2 (SNP211037)-C allele frequency in patients with FSs was significantly higher than that in healthy control subjects (p = 0.009). The odds ratio for developing FSs in individuals with the GABRG2 (SNP211037)-C/C genotype was 2.56 compared with individuals with the GABRG2 (SNP211037)-T/T genotype. These data suggest that the GABRG2 gene might be one of the susceptibility factors for FSs.

The lack of association between febrile convulsions and polymorphisms in SCN1A

Febrile convulsions (FCs) represent the majority of childhood seizures, and patients have a genetic predisposition to their development. The genetic susceptibility to FCs seems to involve multiple genes in most instances. Recent studies provided evidence that mutations in SCN1A represent the most frequent cause of generalized epilepsy with febrile seizures plus an autosomal-dominant epilepsy syndrome. SCN1A mutations alter channel inactivation, resulting in persistent inward sodium current. It is not known if polymorphisms in those genes involved in familial epilepsies also contribute to the pathogenesis of FCs. By performing an association study, we used single nucleotide polymorphisms to investigate the distribution of genotypes of SCN1A in patients with FCs. A total of 104 Taiwanese children with FCs and 83 normal control subjects were included in the study. Polymerase chain reaction was used to identify the A/G polymorphism of the SCN1A gene. The results showed that genotypes and allelic frequencies for the SCN1A gene polymorphisms in both groups were not significantly different. These data suggest that the SCN1A gene might not be one of the susceptibility factors for FCs. Pure FCs and febrile convulsions associated with idiopathic generalized epilepsy may not share a common genetic etiology.