A novel GABRG2 mutation associated with febrile seizures

Genetic mechanisms in idiopathic epilepsies

Idiopathic epilepsies are considered to be genetically determined. The inheritance can be monogenic and the detected mutation considered sufficient to cause the phenotype. In contrast, when the inheritance is complex, the epileptic phenotype is determined by several minor genetic defects that are much more difficult to discover. In recent years, an increasing number of mutations, mainly associated with rare monogenic idiopathic epilepsy syndromes, have been identified in genes encoding subunits of voltage- or ligand-gated ion channels. A few mutations have also been found in the frequent classical forms of idiopathic generalized epilepsies which are thought to follow a complex genetic trait, for example, in absence or juvenile myoclonic epilepsies. Functional studies characterizing the molecular defects of the mutant channels point to an important role of GABAergic synaptic inhibition in the pathophysiology of idiopathic epilepsies. As a result of genetic and functional investigations, not only will the pathophysiology of epilepsy be better understood, but newly discovered genes and pathophysiological pathways may also determine novel targets for pharmacotherapy, as has been shown for the anticonvulsant drug retigabine, which enhances the activity of neuronal KCNQ potassium channels.

Sexually dimorphic expression of KCC2 and GABA function

GABA(A) receptors have an age-adapted function in the brain. During early development, they mediate depolarizing effects, which result in activation of calcium-sensitive signaling processes that are important for the differentiation of the brain. In more mature stages of development and in adults, GABA(A) receptors acquire their classical hyperpolarizing signaling. The switch from depolarizing to hyperpolarizing GABA(A)-ergic signaling is triggered through the developmental shift in the balance of chloride cotransporters that either increase (i.e. NKCC1) or decrease (i.e. KCC2) intracellular chloride. The maturation of GABA(A) signaling follows sex-specific patterns, which correlate with the developmental expression profiles of chloride cotransporters. This has first been demonstrated in the substantia nigra, where the switch occurs earlier in females than in males. As a result, there are sensitive periods during development when drugs or conditions that activate GABA(A) receptors mediate different transcriptional effects in males and females. Furthermore, neurons with depolarizing or hyperpolarizing GABA(A)-ergic signaling respond differently to neurotrophic factors like estrogens. Consequently, during sensitive developmental periods, GABA(A) receptors may act as broadcasters of sexually differentiating signals, promoting gender-appropriate brain development. This has particular implications in epilepsy, where both the pathophysiology and treatment of epileptic seizures involve GABA(A) receptor activation. It is important therefore to study separately the effects of these factors not only on the course of epilepsy but also design new treatments that may not necessarily disturb the gender-appropriate brain development.

What's new in: "genetics in childhood epilepsy"

In recent years, different mutations in genes that control the excitability of neurons have been described in idiopathic childhood epilepsies. Most commonly, sodium/potassium channelopathies and GABA-receptor mutations are involved. Major progress has been made in the field of idiopathic generalised epilepsies associated with febrile seizures (GEFS+). It now is becoming clear that mutations should not only be looked for in familial cases, but also in sporadic cases, especially in infants and young children with unexplained severe epileptic encephalopathies. Many studies also define 'epilepsy susceptibility genes', which contribute to one's individual genetic vulnerability to develop epilepsy. It should be realized, however, that in the most common idiopathic benign childhood epilepsies (benign rolandic and occipital epilepsies), major breakthroughs are still awaited. In addition, a better clinical description of the epileptic phenotypes is needed to explain more precisely the genotypic and phenotypic heterogeneity. Genetic studies are nowadays becoming a necessary diagnostic step in the evaluation of idiopathic childhood epilepsies, not only in familial cases, but also in sporadic cases.

SCN1A, SCN1B, and GABRG2 gene mutation analysis in Chinese families with generalized epilepsy with febrile seizures plus

Generalized epilepsy with febrile seizures plus (GEFS+; MIM#604233) is a familial epilepsy syndrome characterized by phenotypic and genetic heterogeneity. It was associated with mutations in the neuronal voltage-gated sodium channel subunit gene (SCN1A, SCN2A, SCN1B) and ligand-gated gamma aminobutyric acid receptors genes (GABRG2, GABRD). We investigated the roles of SCN1A, SCN1B, and GABRG2 mutations in the etiology of Chinese GEFS+ families. Genomic deoxyribonucleic acid (DNA) was extracted from peripheral blood lymphocytes of 23 probands and their family members. The sequences of SCN1A, SCN1B, and GABRG2 genes were analyzed by polymerase chain reaction (PCR) and direct sequencing. The major phenotypes of affected members in the 23 GEFS+ families exhibited FS and FS+, whereas rare phenotypes afebrile generalized tonic-clonic seizures (AGTCS), myoclonic-astatic epilepsy (MAE), and partial seizures were also observed. A novel SCN1A mutation, p.N935H, was identified in one family and another novel mutation in GABRG2, p.W390X, in another family. However, no SCN1B mutation was identified. The combined frequency of SCN1A, SCN1B, and GABRG2 mutations was 8.7% (2/23), extending the distribution of SCN1A and GABRG2 mutations to Chinese GEFS+ families. There were still unidentified genes contributing to the pathogenesis of GEFS+.

Epilepsy, E/I Balance and GABA(A) Receptor Plasticity

GABA_A receptors mediate most of the fast inhibitory transmission in the CNS. They form heteromeric complexes assembled from a large family of subunit genes. The existence of multiple GABA_A receptor subtypes differing in subunit composition, localization and functional properties underlies their role for fine-tuning of neuronal circuits and genesis of network oscillations. The differential regulation of GABA_A receptor subtypes represents a major facet of homeostatic synaptic plasticity and contributes to the excitation/inhibition (E/I) balance under physiological conditions and upon pathological challenges. The purpose of this review is to discuss recent findings highlighting the significance of GABA_A receptor heterogeneity for the concept of E/I balance and its relevance for epilepsy. Specifically, we address the following issues: (1) role for tonic inhibition, mediated by extrasynaptic GABA_A receptors, for controlling neuronal excitability; (2) significance of chloride ion transport for maintenance of the E/I balance in adult brain; and (3) molecular mechanisms underlying GABA_A receptor regulation (trafficking, posttranslational modification, gene transcription) that are important for homoeostatic plasticity. Finally, the relevance of these findings is discussed in light of the involvement of GABA_A receptors in epileptic disorders, based on recent experimental studies of temporal lobe epilepsy (TLE) and absence seizures and on the identification of mutations in GABA_A receptor subunit genes underlying familial forms of epilepsy.

GABA(A) receptors in normal development and seizures: friends or foes?

GABA(A) receptors have an age-adapted function in the brain. During early development, they mediate excitatory effects resulting in activation of calcium sensitive signaling processes that are important for the differentiation of the brain. In more mature stages of development and in adults, GABA(A) receptors transmit inhibitory signals. The maturation of GABA(A) signaling follows sex-specific patterns, which appear to also be important for the sexual differentiation of the brain. The inhibitory effects of GABA(A) receptor activation have been widely exploited in the treatment of conditions where neuronal silencing is necessary. For instance, drugs that target GABA(A) receptors are the mainstay of treatment of seizures. Recent evidence suggests however that the physiology and function of GABA(A) receptors changes in the brain of a subject that has epilepsy or status epilepticus.This review will summarize the physiology of and the developmental factors regulating the signaling and function of GABA(A) receptors; how these may change in the brain that has experienced prior seizures; what are the implications for the age and sex specific treatment of seizures and status epilepticus. Finally, the implications of these changes for the treatment of certain forms of medically refractory epilepsies and status epilepticus will be discussed.

How do mutant Nav1.1 sodium channels cause epilepsy?

Voltage-gated sodium channels comprise pore-forming alpha subunits and auxiliary beta subunits. Nine different alpha subtypes, designated Nav1.1-Nav1.9 have been identified in excitable cells. Nav1.1, 1.2 and 1.6 are major subtypes in the adult mammalian brain. More than 200 mutations in the Nav1.1 alpha subtype have been linked to inherited epilepsy syndromes, ranging in severity from the comparatively mild disorder Generalized Epilepsy with Febrile Seizures Plus to the epileptic encephalopathy Severe Myoclonic Epilepsy of Infancy. Studies using heterologous expression and functional analysis of recombinant Nav1.1 channels suggest that epilepsy mutations in Nav1.1 may cause either gain-of-function or loss-of-function effects that are consistent with either increased or decreased neuronal excitability. How these diverse effects lead to epilepsy is poorly understood. This review summarizes the data on sodium channel mutations and epilepsy and builds a case for the hypothesis that most Nav1.1 mutations have their ultimate epileptogenic effects by reducing Nav1.1-mediated whole cell sodium currents in GABAergic neurons, resulting in widespread loss of brain inhibition, an ideal background for the genesis of epileptic seizures.

Reduced cortical inhibition in a mouse model of familial childhood absence epilepsy

Mutations in the GABA(A) receptor gamma2 subunit are associated with childhood absence epilepsy and febrile seizures. To understand better the molecular basis of absence epilepsy in man, we developed a mouse model harboring a gamma2 subunit point mutation (R43Q) found in a large Australian family. Mice heterozygous for the mutation demonstrated behavioral arrest associated with 6-to 7-Hz spike-and-wave discharges, which are blocked by ethosuximide, a first-line treatment for absence epilepsy in man. Seizures in the mouse showed an abrupt onset at around age 20 days corresponding to the childhood nature of this disease. Reduced cell surface expression of gamma2(R43Q) was seen in heterozygous mice in the absence of any change in alpha1 subunit surface expression, ruling out a dominant-negative effect. GABA(A)-mediated synaptic currents recorded from cortical pyramidal neurons revealed a small but significant reduction that was not seen in the reticular or ventrobasal thalamic nuclei. We hypothesize that a subtle reduction in cortical inhibition underlies childhood absence epilepsy seen in humans harboring the R43Q mutation.

Fever, febrile seizures and epilepsy

Seizures induced by fever (febrile seizures) are the most common type of pathological brain activity in infants and children. These febrile seizures and their potential contribution to the mechanisms of limbic (temporal lobe) epilepsy have been a topic of major clinical and scientific interest. Key questions include the mechanisms by which fever generates seizures, the effects of long febrile seizures on neuronal function and the potential contribution of these seizures to epilepsy. This review builds on recent advances derived from animal models and summarizes our current knowledge of the mechanisms underlying febrile seizures and of changes in neuronal gene expression and function that facilitate the enduring effects of prolonged febrile seizures on neuronal and network excitability. The review also discusses the relevance of these findings to the general mechanisms of epileptogenesis during development and points out gaps in our knowledge, including the relationship of animal models to human febrile seizures and epilepsy.

The GABAA receptor alpha1 subunit epilepsy mutation A322D inhibits transmembrane helix formation and causes proteasomal degradation

A form of autosomal dominant juvenile myoclonic epilepsy is caused by a nonconservative missense mutation, A322D, in the GABAA receptor alpha1 subunit M3 transmembrane helix. We reported previously that the A322D mutation reduced total and surface alpha1(A322D) subunit protein and that residual alpha1(A322D) subunit resided in the endoplasmic reticulum. Here, we demonstrate that the reduction in alpha1(A322D) expression results from rapid endoplasmic reticulum-associated degradation of the alpha1(A322D) subunit through the ubiquitin-proteasome system. We provide direct evidence that the alpha1(A322D) subunit misfolds and show that in at least 33% of alpha1(A322D) subunits, M3 failed to insert into the lipid bilayer. We constructed a series of mutations in the M3 domain and empirically determined the apparent free energy cost (DeltaGapp) of membrane insertion failure, and we show that the DeltaGapp correlated directly with the recently elucidated transmembrane sequence code (DeltaGLep). These data provide a biochemical mechanism for the pathogenesis of this epilepsy mutation and demonstrate that DeltaGLep predicts the efficiency of lipid partitioning of a naturally occurring protein's transmembrane domain expressed in vivo. Finally, we calculated the DeltaDeltaGLep for 277 known transmembrane missense mutations associated with Charcot-Marie-Tooth disease, diabetes insipidus, retinitis pigmentosa, cystic fibrosis, and severe myoclonic epilepsy of infancy and showed that the majority of these mutations also are likely to destabilize transmembrane domain membrane insertion, but that only a minority of the mutations would be predicted to be as destabilizing as the A322D mutation.

Channelopathies in idiopathic epilepsy

Approximately 70% of all patients with epilepsy lack an obvious extraneous cause and are presumed to have a predominantly genetic basis. Both familial and de novo mutations in neuronal voltage-gated and ligand-gated ion channel subunit genes have been identified in autosomal dominant epilepsies. However, patients with dominant familial mutations are rare and the majority of idiopathic epilepsy is likely to be the result of polygenic susceptibility alleles (complex epilepsy). Data on the identity of the genes involved in complex epilepsy is currently sparse but again points to neuronal ion channels. The number of genes and gene families associated with epilepsy is rapidly increasing and this increase is likely to escalate over the coming years with advances in mutation detection technologies. The genetic heterogeneity underlying idiopathic epilepsy presents challenges for the rational selection of therapies targeting particular ion channels. Too little is currently known about the genetic architecture of the epilepsies, and genetic testing for the known epilepsy genes remains costly. Pharmacogenetic studies have yet to explain why 30% of patients do not respond to the usual antiepileptic drugs. Despite this, the recognition that the idiopathic epilepsies are a group of channelopathies has, to a limited extent, explained the therapeutic action of the common antiepileptic drugs and has assisted clinical diagnosis of some epilepsy syndromes.

Lack of evidence of an allelic association of a functional GABRB3 exon 1a promoter polymorphism with idiopathic generalized epilepsy

PURPOSE

Mutation screening and linkage disequilibrium mapping of the gene encoding the GABA(A) beta(3) subunit (GABRB3) identified a common genetic variant in the exon 1a promoter region (C-allele of rs4906902) which displayed a reduced transcriptional activity and showed a strong allelic association with childhood absence epilepsy (CAE). The present population-based association study tested whether the C-allele of rs4906902 confers susceptibility to CAE or other common syndromes of idiopathic generalized epilepsy (IGE) in a German sample.

METHODS

Seven hundred and eighty unrelated German IGE patients (250 CAE, 123 juvenile absence epilepsy, 303 juvenile myoclonic epilepsy (JME), 104 epilepsy with generalized tonic-clonic seizures on awakening) and 559 healthy population controls were genotyped for the single nucleotide polymorphism (SNP) rs4906902.

RESULTS

The frequency of the risk-conferring C-allele did not differ significantly between CAE patients (f(C)=0.190) and controls (f(C)=0.183; P=0.376, one-tailed). Similarly, no evidence for an allelic association was found for 373 patients with idiopathic absence epilepsy, 303 JME patients, and the entire IGE sample (P>0.77, two-tailed).

CONCLUSION

Our study failed to replicate an association of the common GABRB3 exon 1a promoter SNP rs4906902 with CAE. Moreover, the present results do not provide evidence that the common functional C-variant confers a substantial epileptogenic effect to a broad spectrum of IGE syndromes in the German population.