Mutant GABA(A) receptor gamma2-subunit in childhood absence epilepsy and febrile seizures

A mutation in the GABAA receptor α1 subunit linked to human epilepsy affects channel gating properties

A genetic component is associated with the development of many forms of epilepsy. Recently, mutations in the GABAA receptor have been linked to several inherited epilepsies. One of these mutations is a non-conservative change of alanine to aspartate in the third transmembrane domain of the alpha1 subunit. To determine the functional consequences of this alteration, mutated alpha subunits were transiently transfected along with wild-type beta3 and gamma2L subunits into HEK-293T cells. The mutated alpha1(A294D) subunit reduced GABA sensitivity of the receptor, increased the deactivation rate and slowed desensitization. The mutation caused a reduction in channel open time but no change in single channel conductance. Studies with additional mutants, altering the charge and/or size of the side-chain, indicated that both size and hydrophobicity of the residue at this location influence channel gating. The effects on GABA sensitivity, deactivation rate and channel open time are consistent with a reduced efficacy of channel gating, and would be expected to decrease GABAergic neurotransmission. The alpha1 subtype is the most widely expressed of the alpha subunits, with expression increasing throughout development. Therefore, production of the mutated subunit could cause global hyperexcitability throughout the brain, leading to generalized seizures with juvenile onset.

Clinical correlations of mutations in the SCN1A gene: from febrile seizures to severe myoclonic epilepsy in infancy

Mutations in the alpha-subunit of the first neuronal sodium channel gene SCN1A have been described in isolated patients with severe myoclonic epilepsy in infancy or Dravet syndrome and in families with generalized epilepsy with febrile seizures plus. To find phenotype/genotype correlations, we reviewed all published cases of mutations in SCN1A in addition to four new patients reported here. A total of 60 mutations were observed. Approximately 52% (31/60) are truncating mutations correlating with de novo cases of classical Dravet syndrome in 32 of 34 (94%) patients. Missense mutations in the pore-forming part constitute 27% (16/60) and correspond to a classical type in 12 of 16 (75%) patients. Missense mutations in the voltage sensor were present in 12% (7/60) and correlate with a clinical picture ranging from febrile seizures plus to severe myoclonic epilepsy in infancy. Outside these regions missense mutations are rare and account for only 10% (6/60), corresponding mostly with febrile seizures plus. These results illustrate that the clinical spectrum of SCN1A mutations ranges from febrile seizures, febrile seizures plus, over a milder type to the classical form of severe myoclonic epilepsy in infancy, and confirm the clinical experience that severe myoclonic epilepsy in infancy is the most severe form on this spectrum.

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.

Pharmacological plasticity of GABA(A) receptors at dentate gyrus synapses in a rat model of temporal lobe epilepsy

In the lithium-pilocarpine model (Li-pilocarpine) of temporal lobe epilepsy, GABA(A) receptor-mediated inhibitory postsynaptic currents (GABA(A) IPSCs) were recorded in dentate gyrus granule cells (GCs) from adult rat hippocampal slices. The properties of GABA(A) IPSCs were compared before and after superfusion of modulators in control conditions (Li-saline rats) and in Li-pilocarpine rats 24-48 h and 3-5 months (epileptic rats) after status epilepticus (SE). The mean peak amplitude of GABA(A) IPSCs increased by about 40% over Li-saline values in GCs 24-48 h after SE and remained higher in epileptic rats. In Li-pilocarpine rats, studied at 24-48 h after SE, diazepam (1 microm) lost 65% of its effectiveness at increasing the half-decay time (T(50%)) of GABA(A) miniature IPSCs (mIPSCs). Diazepam had no effects on mIPSC T(50%) in epileptic rats. The benzodiazepine ligand flumazenil (1 microm), acting as an antagonist in Li-saline rats, exhibited a potent inverse agonistic effect on GABA(A) mIPSCs of GCs from Li-pilocarpine rats 24-48 h and 3-5 months after SE. The neurosteroid allopregnanolone (100 nm), which considerably prolonged GABA(A) mIPSCs in Li-saline rats, totally lost its effect in rats studied 24-48 h after SE. However, this decrease in effectiveness was transient and was totally restored in epileptic rats. In addition to the up-regulation in the number of receptors at individual GC synapses, we propose that these 'epileptic' GABA(A) receptors possess benzodiazepine binding sites with altered allosteric properties. The failure of benzodiazepine and neurosteroid to potentiate inhibition early after SE may be a critical factor in the development of epileptogenesis and occurrence of seizures.

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.

Inherited Channelopathies Associated with Epilepsy

Ion channels are critical for neuronal excitability and provide important targets for anticonvulsant drugs. In the past few years, several monogenetic epilepsies have been linked to mutations in genes encoding either voltage-gated or ligand-gated channels. The recognition that certain epilepsy syndromes are "channelopathies" initiates a new era in understanding the molecular pathophysiology of seizure disorders. This review summarizes recent advances related to this exciting area of investigation.

Association between variation in the human KCNJ10 potassium ion channel gene and seizure susceptibility

PURPOSE

Our research program uses genetic linkage and association analysis to identify human seizure sensitivity and resistance alleles. Quantitative trait loci mapping in mice led to identification of genetic variation in the potassium ion channel gene Kcnj10, implicating it as a putative seizure susceptibility gene. The purpose of this work was to translate these animal model data to a human genetic association study.

METHODS

We used single stranded conformation polymorphism (SSCP) electrophoresis, DNA sequencing and database searching (NCBI) to identify variation in the human KCNJ10 gene. Restriction fragment length polymorphism (RFLP) analysis, SSCP and Pyrosequencing were used to genotype a single nucleotide polymorphism (SNP, dbSNP rs#1130183) in KCNJ10 in epilepsy patients (n = 407) and unrelated controls (n = 284). The epilepsy group was comprised of patients with refractory mesial temporal lobe epilepsy (n = 153), childhood absence (n = 84), juvenile myoclonic (n = 111) and idiopathic generalized epilepsy not otherwise specified (IGE-NOS, n = 59) and all were of European ancestry.

RESULTS

SNP rs#1130183 (C > T) alters amino acid 271 (of 379) from an arginine to a cysteine (R271C). The C allele (Arg) is common with conversion to the T allele (Cys) occurring twice as often in controls compared to epilepsy patients. Contingency analysis documented a statistically significant association between seizure resistance and allele frequency, Mantel-Haenszel chi square = 5.65, d.f. = 1, P = 0.017, odds ratio 0.52, 95% CI 0.33-0.82.

CONCLUSION

The T allele of SNP rs#1130183 is associated with seizure resistance when common forms of focal and generalized epilepsy are analyzed as a group. These data suggest that this missense variation in KCNJ10 (or a nearby variation) is related to general seizure susceptibility in humans.

Genetic influences on myoclonic and absence seizures

OBJECTIVE

To examine the relationship between genotype and phenotype in idiopathic generalized epilepsies (IGEs) using a novel approach that focuses on seizure type rather than syndrome.

METHODS

The authors evaluated whether the genetic effects on myoclonic seizures differ from the genetic effects on absence seizures. For this purpose, they studied 34 families containing 2 or more members with IGEs and assessed whether the number of families concordant for seizure type exceeded that expected by chance. The authors performed a similar analysis to examine the genetic contributions to juvenile myoclonic epilepsy (JME), juvenile absence epilepsy (JAE), and childhood absence epilepsy (CAE).

RESULTS

The observed number of families concordant for seizure type (myoclonic, absence, or both) was greater than expected (20 vs 7.51; p < 0.0001). The observed number of families concordant for syndrome was greater than expected when JME was compared with absence epilepsies (JAE+CAE) (17 vs 11.9; p < 0.012) but not when JAE was compared with CAE (8 vs 6.82; p = 0.516).

CONCLUSIONS

These results provide evidence for distinct genetic effects on absence and myoclonic seizures, suggesting that examining the two seizure types separately would be useful in linkage studies of idiopathic generalized epilepsies. The approach presented here can also be used to discover other clinical features that could direct division of epilepsies into groups likely to share susceptibility 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.

Expression of the KCl cotransporter KCC2 parallels neuronal maturation and the emergence of low intracellular chloride

Fast synaptic inhibition in the adult central nervous system (CNS) is mediated by GABA and glycine. During early development GABA acts as an excitatory neurotransmitter, which is deemed to be important for the maturation of the CNS. During development GABAergic responses undergo a switch from excitatory to inhibitory. This switch is correlated with upregulation of KCC2, the neuronal isoform of the potassium-chloride cotransporter family. KCC2 lowers the intraneuronal chloride concentration below its electrochemical equilibrium. KCC2 activity is thought to depend on phosphorylation by endogenous tyrosine kinases. Here, we analyzed the expression pattern of KCC2 during murine embryonic and postnatal development by in situ hybridization and Western blot analysis. KCC2 expression paralleled neuronal differentiation and preceded the decline of the GABA reversal potential (EGABA) in spinal cord motoneurons and hippocampal pyramidal cells. The adult inhibitory response to GABA was established earlier in the spinal cord than in the hippocampus. Phosphorylated KCC2 protein was already present early in development when the functional GABA switch had not yet occurred. Thus, tyrosine-phosphorylation seems to be less important than the transcriptional upregulation of KCC2.

Recent Developments in the Quest for Myoclonic Epilepsy Genes

Understanding the latest advances in the molecular genetics of the epilepsies is important, as it provides a basis for comprehending the new practice of epileptology. Epilepsies have traditionally been classified and subtyped on the basis of clinical and neurophysiologic concepts. However, the complexity and variability of phenotypes and overlapping clinical features limit the resolution of phenotype-based classification and confound epilepsy nosology. Identification of tightly linked epilepsy DNA markers and discovery of epilepsy-causing mutations provide a basis for refining the classification of epilepsies. Recent discoveries regarding the genetics surrounding certain epilepsy types (including Lafora's progressive myoclonic epilepsy, the severe myoclonic epilepsy of infancy of Dravet, and idiopathic generalized epilepsies) may be the beginning of a better understanding of how rare Mendelian epilepsy genes and their genetic architecture can explain some complexities of the common epilepsies.

Nonsyndromic seizure disorders: epilepsy and the use of the internet to advance research

The progress in understanding the genetics of nonsyndromic epilepsy is the direct result of dramatic advances made by the Human Genome Project. The development of thousands of precisely mapped genetic markers and the nearly complete sequencing of the entire human genome in 2001 allowed genetic researchers in epilepsy to identify many loci and genes as causal in inherited idiopathic epilepsy. This substantial increase in information has required the development of accurate and online bioinformatic databases. Only the Internet can enable such large amounts of precise DNA sequence information to be transferred to researchers. Along with the construction of these databases has been the development of efficient search algorithms for specific DNA sequences and genetic information. This article summarizes the effect that this burst of new genomic information has had on research aimed at discovering the underlying genetic factors for nonsyndromic epilepsy. Many of the web sites important to epilepsy gene discovery are listed and discussed in this article, including sites with extensive information on genetic markers, genetic analysis, gene sequence, gene expression, gene mutations, and DNA sequence variation. Continued acquisition of information on naturally occurring DNA sequence variants will greatly help research directed towards understanding the genetic susceptibility of the common, nonsyndromic epilepsies and will lead to the promise of personalized medicine.

Significant behavioral disturbances in succinic semialdehyde dehydrogenase (SSADH) deficiency (gamma-hydroxybutyric aciduria)

We report two adult patients with succinic semialdehyde dehydrogenase deficiency, manifesting as gamma-hydroxybutyric aciduria. For both, the clinical presentation included significant behavioral disturbances and psychosis (hallucinations, disabling anxiety, aggressive behavior, and sleep disorder), leading to multiple therapeutic attempts. Intervention with benzodiazepines appeared most efficacious, resulting in decreased aggression and agitation and improvement in anxiety. A review of 56 published and unpublished studies of SSADH-deficient patients revealed that 42% manifested behavioral disturbances, whereas 13% (predominantly adults) displayed psychotic symptomatology. To explore the potential biochemical basis of these behavioral abnormalities, we studied cerebrospinal fluid derived from 13 patients, which revealed significantly elevated GHB (65- to 230-fold), high free and total GABA (up to threefold), and low glutamine. Although within the control range, homovanillic and 5-hydroxyindoleacetic acids (end products of dopamine and serotonin metabolism, respectively) showed a significant linear correlation with increasing GHB concentration, suggesting enhanced dopamine and serotonin turnover. We conclude that elevated GABA combined with low glutamine suggest disruption of the glial-neuronal glutamine/GABA/glutamate shuttle necessary for replenishment of neuronal neurotransmitters, whereas altered dopamine and serotonin metabolism may be causally linked to the hyperkinetic movement disorders and behavioral disturbances seen in SSADH-deficient patients.

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.

Evidence for distinct genetic influences on generalized and localization-related epilepsy

PURPOSE

Determining the existence of syndrome-specific genetic factors in epilepsy is essential for phenotype definition in genetic linkage studies, and informs research on basic mechanisms. Analysis of concordance of epilepsy syndromes in families has been used to assess shared versus distinct genetic influences on generalized epilepsy (GE) and localization-related epilepsy (LRE). However, it is unclear how the results should be interpreted in relation to specific genetic hypotheses.

METHODS

To assess evidence for distinct genetic influences on GE and LRE, we examined concordance of GE and LRE in 63 families containing multiple individuals with idiopathic or cryptogenic epilepsy, drawn from the Epilepsy Family Study of Columbia University. To control for the number of concordant families expected by chance, we used a permutation test to compare the observed number with the number expected from the distribution of individuals with GE and LRE in the study families.

RESULTS

Of the families, 62% were concordant for epilepsy type, and 38% were discordant. In all analyses, the proportion of concordant families was significantly greater than expected.

CONCLUSIONS

This suggests that some genetic influences predispose specifically to either GE or LRE. Because of the ascertainment bias resulting from the selection of families containing multiple individuals with epilepsy, we could not test whether there are also shared genetic influences on these two epilepsy subtypes. Population-based studies will be needed to explore these results further.

Evidence for a major susceptibility locus at 11q22.1-23.3 has been detected in a large Chinese family with pure grand mal epilepsy

Pure grand mal epilepsy (PGME) is a common subtype of idiopathic generalized epilepsy (IGE) with an unclear mode of inheritance. Several studies with the multiple families have provided evidence for the disorder to be linked to chromosome 8q24 and 8p. In this work, we performed an autosomal-wide scan linkage analysis using microsatellite markers in a large Chinese family with PGME and found seven markers with likelihood of odds (LOD), scores >/=1.0 (theta=0) in chromosome 11q22.1-23.3. The highest LOD score for two-point and multi-point linkage analysis are 1.99 (theta=0) at marker D11S4159 and 2.18 between markers D11S1782 and D11S3178, respectively, which reached the level of a suggested positive linkage LOD score (Z>/=1.9), under an autosomal dominant manner of inheritance with a penetrance of 65% but no significant positive LOD score (Z>/=3.3) was found after high density of microsatellite markers used in the regions. Obviously, our data do not support the linkage of the disease to chromosome 8q24 and 8p but implicate that chromosome 11q22.1-23.3 may be a new locus linked to PGME, which indicates the existence of genetic heterogeneity in the disorder.

The genetics of human epilepsy

In recent years genetic discoveries have shown the central role of ion channels in the pathophysiology of idiopathic epilepsies. Uncommon epilepsy syndromes that have monogenic inheritance are associated with mutations in genes that encode subunits of voltage-gated and ligand-gated ion channels. For voltage-gated ion channels, mutations of Na(+), K(+) and Cl(-) channels are associated with forms of generalized epilepsy and infantile seizure syndromes. Ligand-gated ion channels, such as nicotinic acetylcholine receptors and GABA receptor subunits, are associated with specific syndromes of frontal and generalized epilepsies, respectively. Striking features are the variable epilepsy phenotypes that are associated with the known gene mutations and the genetic heterogeneity that underlies all known monogenic syndromes. Mutations in two genes that do not encode ion channels have been identified in the idiopathic human epilepsies. The heterogeneity of mutations described to date has precluded the development of simple diagnostic tests, but advances in the next few years are likely to have an impact on both the clinical diagnosis and the treatment of epilepsies.

Natural history of absence epilepsy in children

Absence seizures may be seen in a variety of epileptic syndromes in childhood. Identification of the specific syndrome is important to determine medical prognosis. With childhood absence epilepsy, approximately two thirds of children can be expected to enter long-term remission, while in juvenile absence epilepsy, seizure control is often achieved, however, lifelong treatment is usually required. Other absence syndromes have a poorer prognosis, with lower rates of seizure control and remission. Psychosocial outcome is often poor, even in patients with more benign forms of absence epilepsy. Remission of epilepsy does not preclude psychosocial morbidity.

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.

Association of the neuronal nicotinic acetylcholine receptor subunit alpha4 polymorphisms with febrile convulsions

PURPOSE

The alpha4-subunit gene of the neuronal nicotinic acetylcholine receptor (CHRNA4) has been identified as the first gene underlying an idiopathic partial epilepsy syndrome in human autosomal-dominant nocturnal frontal lobe epilepsy. Studies provided evidence that the protein coded by CHRNA4 is one of the most abundant subunits of the neuronal nicotinic acetylcholine receptors in mammalian brains, and mutations of CHRNA4 seem to cause neuronal excitation. The CHRNA4 gene may have a role in the development of febrile convulsions (FCs), the majority of childhood seizures. This study assessed the distribution of genotypes of CHRNA4 in patients with FCs.

METHODS

A total of 102 children with FCs and 80 normal control subjects were included in the study. Polymerase chain reaction was used to identify the C/T polymorphism of the CHRNA4 gene. Genotypes and allelic frequencies for the CHRNA4 gene polymorphisms in both groups were compared.

RESULTS

The number of individuals with heterozygous CHRNA4 (Ser543Ser)-C/T genotype was significantly greater (60.8% vs. 32.5%; p = 0.001), and the CHRNA4 (Ser543Ser)-T allele frequency was significantly higher (p = 0.001), in patients with FCs compared with healthy controls. The odds ratio for developing FCs in individuals with the CHRNA4 (Ser543Ser)-CT genotype was 3.77 compared with individuals with two copies of the CHRNA4 (Ser543Ser)-C allele.

CONCLUSIONS

This study demonstrated an association between the CHRNA4 gene and FCs. Individuals with the T allele had a higher incidence of FCs. These data suggest that the CHRNA4 gene or a closely linked gene might be one of the susceptibility factors for FCs.

Exploring new gene discoveries in idiopathic generalized epilepsy

Most epilepsies are categorized under the umbrella term "idiopathic;" these seizure disorders lack a known cause. New genetic technologies are rapidly identifying specific genes responsible for idiopathic generalized epilepsies (IGEs) and are gradually taking the "I" out of "IGE." Ion channel (both voltage- and receptor-mediated) mutations have been linked to a variety of epilepsies considered idiopathic. Gene errors alter excitability in various ways, depending on the mutation, the regional network, and the stage of brain development. The majority of mutations prolong depolarization, favor repetitive firing, and alter neurotransmitter release or postsynaptic sensitivity at central synapses, but the reason for specific seizure types is unclear. Further analyses of these gene mutations and their effects on the developing brain are providing critical clues in the search to explain the origin of "idiopathic" epilepsy.

The genetic and molecular basis of epilepsy

In the past decade, studies of large families in which epilepsy has been inherited in an autosomal dominant fashion have revealed several mutated genes, most of which encode ion channel subunits. Despite these exciting findings, only a few families with similar phenotypes have mutations in these known genes. More frustrating has been the genetic research into idiopathic epilepsies with complex inheritance. Although these forms are more common than those with Mendelian inheritance, their unknown mode of inheritance, phenotypic heterogeneity and the uncertainty of the genetic overlap among syndrome subtypes have hampered gene mapping. New techniques of molecular analysis could help the dissection of genes for epilepsies with complex inheritance. Hopefully, in the near future, successful genetic studies will make possible the discovery of new and more-targeted anti-epileptic drugs.

Generalized epilepsy with febrile seizures plus type 2 mutation W1204R alters voltage-dependent gating of Na(v)1.1 sodium channels

Nine mutations that cause generalized epilepsy with febrile seizures plus have been identified in the SCN1A gene encoding the alpha subunit of the Na(v)1.1 voltage-gated sodium channel. The functional properties of two of these mutations (T875M and R1648H) have previously been described. T875M was shown to enhance slow inactivation, while R1648H dramatically accelerated recovery from inactivation. In this report, we have cloned, expressed and characterized the functional effects of a third generalized epilepsy with febrile seizures plus mutation, W1204R (Am J Hum Genet 68 (2001) 866). The mutation was cloned into the orthologous rat channel, rNa(v)1.1, and at the same time a single base pair insertion at base 120 in the original rNa(v)1.1 clone was corrected. The level of expression of the corrected wild-type rNa(v)1.1 was approximately 1000-fold higher than that of the original clone and comparable to that achieved with other neuronal sodium channels expressed in Xenopus oocytes. The properties of the W1204R mutant in the corrected rNa(v)1.1 were determined in the absence and presence of the beta1 subunit in Xenopus oocytes. The W1204R mutation resulted in approximately 11 mV hyperpolarized shifts in the voltage-dependence of activation and steady-state inactivation when expressed as an alpha subunit alone. When the channels were coexpressed with the beta1 subunit, the hyperpolarized shifts were still present but smaller, approximately 5 mV in magnitude. All other properties that we examined were comparable for the mutant and wild-type channels. The negative shift in activation would increase channel excitability, whereas the negative shift in inactivation would decrease excitability. The negative shifts in both properties also shifted the window current, which is the voltage region in which sodium channels can continue to open because some percentage of channels are activated and not all of the channels are inactivated. The shift in window current for the W1204R mutation could result in hyperexcitability because the neuron's potential is more likely to reach the more negative range. These results demonstrate that a third SCN1A mutation that causes generalized epilepsy with febrile seizures plus 2 alters the properties of the sodium channel in a different manner than the previous two mutations that were studied. The diversity in functional effects for these three mutations indicates that a similar clinical phenotype can result from very different underlying sodium channel abnormalities.

Failure to find causal mutations in the GABA(A)-receptor gamma2 subunit (GABRG2) gene in Japanese febrile seizure patients

Recently, mutations in the GABA(A)-receptor gamma2 subunit (GABRG2) gene were identified in two families with generalized epilepsy with febrile seizures plus (GEFS+) and two families with childhood absence epilepsy (CAE) and febrile seizures (FS). We tested the hypothesis that genetic variations in the GABRG2 gene confer susceptibility to FS in the Japanese population. We performed a systematic search for mutations in 94 unrelated Japanese patients with FS and detected six variants (-158C>T, 315C>T, 588T>C, IVS5-55C>T, IVS7+20G>A, and IVS7-141T>A). No non-synonymous mutation was detected. We genotyped three exonic polymorphisms and performed a case control study and a transmission disequilibrium test using 55 independent complete trios with FS and 106 control subjects. None of these polymorphic alleles were significantly associated with FS. Our results indicate that genomic variations of GABRG2 are not likely to be substantially involved in the etiology of FS in the Japanese population.

De novo SCN1A mutations are a major cause of severe myoclonic epilepsy of infancy

Severe myoclonic epilepsy of infancy (SMEI or Dravet syndrome) is a rare disorder occurring in young children often without a family history of a similar disorder. The earliest disease manifestations are usually fever-associated seizures. Later in life, patients display different types of afebrile seizures including myoclonic seizures. Arrest of psychomotor development occurs in the second year of life and most patients become ataxic. Patients are resistant to antiepileptic drug therapy. Recently, we described de novo mutations of the neuronal sodium channel alpha-subunit gene SCN1A in seven isolated SMEI patients. To investigate the contribution of SCN1A mutations to the etiology of SMEI, we examined nine additional SMEI patients. We observed eight coding and one noncoding mutation. In contrast to our previous study, most mutations are missense mutations clustering in the S4-S6 region of SCN1A. These findings demonstrate that de novo mutations in SCN1A are a major cause of isolated SMEI.

Formation and plasticity of GABAergic synapses: physiological mechanisms and pathophysiological implications

gamma-Aminobutyric acid(A) (GABA(A)) receptors mediate most of the fast inhibitory neurotransmission in the CNS. They represent a major site of action for clinically relevant drugs, such as benzodiazepines and ethanol, and endogenous modulators, including neuroactive steroids. Alterations in GABA(A) receptor expression and function are thought to contribute to prevalent neurological and psychiatric diseases. Molecular cloning and immunochemical characterization of GABA(A) receptor subunits revealed a multiplicity of receptor subtypes with specific functional and pharmacological properties. A major tenet of these studies is that GABA(A) receptor heterogeneity represents a key factor for fine-tuning of inhibitory transmission under physiological and pathophysiological conditions. The aim of this review is to highlight recent findings on the regulation of GABA(A) receptor expression and function, focusing on the mechanisms of sorting, targeting, and synaptic clustering of GABA(A) receptor subtypes and their associated proteins, on trafficking of cell-surface receptors as a means of regulating synaptic (and extrasynaptic) transmission on a short-time basis, on the role of endogenous neurosteroids for GABA(A) receptor plasticity, and on alterations of GABA(A) receptor expression and localization in major neurological disorders. Altogether, the findings presented in this review underscore the necessity of considering GABA(A) receptor-mediated neurotransmission as a dynamic and highly flexible process controlled by multiple mechanisms operating at the molecular, cellular, and systemic level. Furthermore, the selected topics highlight the relevance of concepts derived from experimental studies for understanding GABA(A) receptor alterations in disease states and for designing improved therapeutic strategies based on subtype-selective drugs.

Clinical characteristics and inheritance of idiopathic epilepsy in Vizslas

Medical record, seizure survey, and telephone interview information was obtained for 29 Vizslas with idiopathic epilepsy (IE), 74 unaffected siblings, and 41 parents to determine the common clinical characteristics and most likely mode of inheritance. IE was diagnosed on the basis of the age of seizure onset, laboratory results, and neurologic examination findings. Computerized tomography (CT) or magnetic resonance imaging (MRI) scan with cerebrospinal fluid (CSF) analysis was required for the inclusion of dogs with an age of seizure onset of < 6 months or > 5 years. Simple segregation analysis was performed with an ascertainment correction and chi-square analysis. IE appeared to be familial in these pedigrees, with 79% of affected Vizslas exhibiting partial onset seizures. Partial seizure signs included a combination of limb tremors, staring, pupillary dilatation, or salivation without loss of consciousness in > 50% of the dogs with partial signs. The estimated segregation frequency of P = .22 (95% CI, P = .08 to .36) was consistent with autosomal recessive inheritance; however, polygenic inheritance could not be excluded as a possibility. Simulated linkage with FASTSLINK estimated that the average logarithm of odds (LOD) score would be 3.23 with a 10-centimorgan (cM) whole-genome scan for these families, indicating that these families would be useful for a whole-genome scan to potentially find the chromosomal segment(s) containing the epilepsy gene or genes. We conclude that IE in Vizslas appears to be primarily a partial onset seizure disorder that may be inherited as an autosomal recessive trait.

Unraveling monogenic channelopathies and their implications for complex polygenic disease

Ion channels are a large family of >400 related proteins representing >1% of our genetic endowment; however, ion-channel diseases reflect a relatively new category of inborn error. They were first recognized in 1989, with the discovery of cystic fibrosis transmembrane conductance regulator, and rapidly advanced as positional and functional studies converged in the dissection of components of the action potential of excitable tissues. Although it remains true that diseases of excitable tissue still most clearly illustrate this family of disease, ion-channel disorders now cover the gamut of medical disciplines, causing significant pathology in virtually every organ system, producing a surprising range of often unanticipated symptoms, and providing valuable targets for pharmacological intervention. Many of the features shared among the monogenic ion-channel diseases provide a general framework for formulating a foundation for considering their intrinsically promising role in polygenic disease. Since an increasingly important approach to the identification of genes underlying polygenic disease is to identify "functional candidates" within a critical region and to test their disease association, it becomes increasingly important to appreciate how these ion-channel mechanisms can be implicated in pathophysiology.

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.

No evidence of GABRG2 mutations in severe myoclonic epilepsy of infancy

Severe myoclonic epilepsy of infancy (SMEI) has been long suspected to have a genetic origin. Recently mutations in the gene encoding a voltage-gated alpha-1 sodium channel subunit-SCN1A-have been identified as a common cause of SMEI. Moreover, a mutation in the gene encoding the gamma2 subunit of the GABA(A) receptor-GABRG2-has been described in a GEFS+ family with a member affected by SMEI. In order to further investigate the role of GABRG2 in the pathogenesis of SMEI, we have screened for mutations 53 SMEI patients who resulted negative for SCN1A mutations. Mutational screening of GABRG2 genes was performed by denaturing high performance liquid chromatography (DHPLC) and direct sequencing of DNA fragments showing a variant chromatogram. Twenty-nine variant chromatograms were identified corresponding to seven different nucleotide variants. None of them leads to an amino acid change or obvious protein dysfunction. No difference in allele frequency was observed for the SMEI patients compared to a control population indicating that these variants are not involved in SMEI. Our study demonstrates that GABRG2 is not a commonly involved in the etiology of SMEI and suggests that other and yet unidentified genes are involved in the syndrome

Age-dependent modulation of hippocampal excitability by KCNQ-channels

Recently, mutations of KCNQ2 or KCNQ3, members of the KCNQ-related K(+)-channel (KCNQ-channel) family, were identified as cause of benign familial neonatal convulsions (BFNC). However, the exact pathogenic mechanisms of age-dependent development and spontaneous remission of BFNC remain to be elucidated. To clarify the age-dependent etiology of BFNC, we determined age-dependent functional switching of KCNQ-channels, GABAergic- and glutamatergic-transmission in rat hippocampus. The effects of inhibitors of KCNQ-channel, GABA- and glutamate-receptors on propagation of neuronal-excitability and neurotransmitter release were determined by 64-channel multielectrode-dish (MED64), whole-cell recording, in vitro release technique and in vivo microdialysis biosensor, using rat hippocampus from day of birth (P0) to postnatal-day 56 (P56). Inhibition of KCNQ-channels enhanced depolarization-induced glutamate and GABA releases during P0-P7, but not during P14-P28. Inhibition of KCNQ-channels magnified neuronal-excitability propagation from P0 to P14: maximal at P3, but this effect disappeared by P28. GABA(A)-receptor inhibition surprisingly reduced neuronal-excitability propagation during P0-P3, but not at P7. AMPA/glutamate-receptors inhibition reduced propagation of neuronal-excitability throughout the study period. KCNQ-channels inhibition shortened spike-frequency adaptation, but this stimulation was more predominant during P<7 than P>14. During the first week of life, KCNQ-channels performed as a predominant inhibitory system, whereas after this period GABAergic-transmission switched from excitatory to inhibitory function. Contrary, glutamatergic-transmission has acquired as excitatory function from P0. These findings suggest that the pathogenic mechanisms of age-dependent development and spontaneous remission of BFNC are, at least partially, associated with the interaction between age-dependent reduction of inhibitory KCNQ-channel activity and age-dependent functional switching of the GABAergic-system from excitatory to inhibitory action in neonatal CNS.

Genetic disruption of cortical interneuron development causes region- and GABA cell type-specific deficits, epilepsy, and behavioral dysfunction

The generation of properly functioning circuits during brain development requires precise timing of cell migration and differentiation. Disruptions in the developmental plan may lead to neurological and psychiatric disorders. Neocortical circuits rely on inhibitory GABAergic interneurons, the majority of which migrate from subcortical sources. We have shown that the pleiotropic molecule hepatocyte growth factor/scatter factor (HGF/SF) mediates interneuron migration. Mice with a targeted mutation of the gene encoding urokinase plasminogen activator receptor (uPAR), a key component in HGF/SF activation and function, have decreased levels of HGF/SF and a 50% reduction in neocortical GABAergic interneurons at embryonic and perinatal ages. Disruption of interneuron development leads to early lethality in most models. Thus, the long-term consequences of such perturbations are unknown. Mice of the uPAR-/- strain survive until adulthood, and behavior testing demonstrates that they have an increased anxiety state. The uPAR-/- strain also exhibits spontaneous seizure activity and higher susceptibility to pharmacologically induced convulsions. The neocortex of the adult uPAR-/- mouse exhibits a dramatic region- and subtype-specific decrease in GABA-immunoreactive interneurons. Anterior cingulate and parietal cortical areas contain 50% fewer GABAergic interneurons compared with wild-type littermates. However, interneuron numbers in piriform and visual cortical areas do not differ from those of normal mice. Characterization of interneuron subpopulations reveals a near complete loss of the parvalbumin subtype, with other subclasses remaining intact. These data demonstrate that a single gene mutation can selectively alter the development of cortical interneurons in a region- and cell subtype-specific manner, with deficits leading to long-lasting changes in circuit organization and behavior.

Persistent abnormality detected in the non-ictal electroencephalogram in primary generalised epilepsy

OBJECTIVES

Gamma oscillations (30-100 Hz gamma electroencephalographic (EEG) activity) correlate with high frequency synchronous rhythmic bursting in assemblies of cerebral neurons participating in aspects of consciousness. Previous studies in a kainic acid animal model of epilepsy revealed increased intensity of gamma rhythms in background EEG preceding epileptiform discharges, leading the authors to test for intensified gamma EEG in humans with epilepsy.

METHODS

64 channel cortical EEG were recorded from 10 people with primary generalised epilepsy, 11 with partial epilepsy, and 20 controls during a quiescent mental state. Using standard methods of EEG analysis the strength of EEG rhythms (fast Fourier transformation) was quantified and the strengths of rhythms in the patient groups compared with with controls by unpaired t test at 1 Hz intervals from 1 Hz to 100 Hz.

RESULTS

In patients with generalised epilepsy, there was a threefold to sevenfold increase in power of gamma EEG between 30 Hz and 100 Hz (p<0.01). Analysis of three unmedicated patients with primary generalised epilepsies revealed an additional 10-fold narrow band increase of power around 35 Hz-40 Hz (p<0.0001). There were no corresponding changes in patients with partial epilepsy.

CONCLUSIONS

Increased gamma EEG is probably a marker of the underlying ion channel or neurotransmitter receptor dysfunction in primary generalised epilepsies and may also be a pathophysiological prerequisite for the development of seizures. The finding provides a new diagnostic approach and also links the pathophysiology of generalised epilepsies to emerging concepts of neuronal correlates of consciousness.

Cellular biology of epileptogenesis

The ionic currents that underlie the mechanisms of epileptogenesis have been systematically characterised in different experimental preparations. The recent elucidation of the molecular structures of most membrane channels and receptors has enabled structure-function analyses in both physiological and pathophysiological conditions. The neurophysiological and biomolecular features of epileptogenic mechanisms that putatively account for human epilepsies are summarised in this review. Particular emphasis is given to epilepsies that are associated with genetically determined alterations of ligand-gated and voltage-gated ion channels. Changes in ionic currents that flow through sodium, potassium, and calcium channels can lead to different types of epilepsies. Inherited or acquired changes that alter the function of receptors for acetylcholine, glutamate, and gamma-aminobutryic acid are also involved. better understanding of the role of these epileptogenic mechanisms will promote new advances in the development of selective and targeted antiepileptic drugs.

Molecular biology and ontogeny of gamma-aminobutyric acid (GABA) receptors in the mammalian central nervous system

gamma-Aminobutyric acid (GABA) is the predominant inhibitory neurotransmitter in the mammalian central nervous system. After release from nerve terminals, GABA binds to at least two classes of postsynaptic receptors (ie, GABAA and GABAB), which are nearly ubiquitous in the brain. GABAA receptors are postsynaptic heteropentameric complexes that display unique physiologic and pharmacologic properties based on subunit composition. Activation of GABAA receptors in mature neurons results in membrane hyperpolarization, which is mediated principally by inward chloride flux, whereas in early stages of brain development, GABAA receptor activation causes depolarization of the postsynaptic membrane. GABA, receptors reside both presynaptically and postsynaptically, exist as heterodimers and are coupled to voltage-dependent ion channels through interactions with heterotrimeric G proteins. This review summarizes the molecular biology and ontogeny of GABAA and GABAB receptors, highlighting some of their putative roles during normal brain development as well as in disease states such as epilepsy.

Altered kinetics and benzodiazepine sensitivity of a GABAA receptor subunit mutation [gamma 2(R43Q)] found in human epilepsy

The gamma-aminobutyric acid type A (GABA(A)) receptor mediates fast inhibitory synaptic transmission in the CNS. Dysfunction of the GABA(A) receptor would be expected to cause neuronal hyperexcitability, a phenomenon linked with epileptogenesis. We have investigated the functional consequences of an arginine-to-glutamine mutation at position 43 within the GABA(A) gamma(2)-subunit found in a family with childhood absence epilepsy and febrile seizures. Rapid-application experiments performed on receptors expressed in HEK-293 cells demonstrated that the mutation slows GABA(A) receptor deactivation and increases the rate of desensitization, resulting in an accumulation of desensitized receptors during repeated, short applications. In Xenopus laevis oocytes, two-electrode voltage-clamp analysis of steady-state currents obtained from alpha(1)beta(2)gamma(2) or alpha(1)beta(2)gamma(2)(R43Q) receptors did not reveal any differences in GABA sensitivity. However, differences in the benzodiazepine pharmacology of mutant receptors were apparent. Mutant receptors expressed in oocytes displayed reduced sensitivity to diazepam and flunitrazepam but not the imidazopyridine zolpidem. These results provide evidence of impaired GABA(A) receptor function that could decrease the efficacy of transmission at inhibitory synapses, possibly generating a hyperexcitable neuronal state in thalamocortical networks of epileptic patients possessing the mutant subunit.

Expression of human epileptic temporal lobe neurotransmitter receptors in Xenopus oocytes: An innovative approach to study epilepsy

Poly(A(+)) RNA was extracted from the temporal lobe (TL) of medically intractable epileptic patients which underwent surgical TL resection. Injection of this mRNA into Xenopus oocytes led to the expression of ionotropic receptors for gamma-aminobutyric acid (GABA), kainate (KAI) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). Membrane currents elicited by GABA inverted polarity at -15 mV, close to the oocyte's chloride equilibrium potential, were inhibited by bicuculline, and were potentiated by pentobarbital and flunitrazepam. These basic characteristics were also displayed by GABA currents elicited in oocytes injected with mRNAs isolated from human TL glioma (TLG) or from mouse TL. However, the GABA receptors expressed by the epileptic TL mRNA exhibited some unusual properties, consisting in a rapid current run-down after repetitive GABA applications and a large EC(50) (125 microM). AMPA alone evoked very small or nil currents, whereas KAI induced larger currents. Nevertheless, upon cyclothiazide treatment, AMPA elicited substantial currents that, like the KAI currents, were inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Furthermore, the glutamate receptor 5 (GluR5) agonist, ATPA, failed to evoke an obvious current although both RT-PCR and Western blot analyses showed GluR5 expression in the epileptic TL. Oocytes injected with mouse TL or human TLG mRNAs generated KAI and AMPA currents similar to those evoked in oocytes injected with epileptic TL mRNA but, in contrast to these, the mouse TL and human TLG oocytes were also responsive to ATPA. Our findings are in accord with the concept that both a depression of GABA inhibition and a dysfunction of the KAI-receptor system maintain a high neuronal excitability that results in epileptic seizures.

Mutation screen of the GABA(A) receptor gamma 2 subunit gene in Chinese patients with childhood absence epilepsy

Childhood absence epilepsy (CAE) is considered to be a genetic disease, the genes responsible for which have not yet been identified. To investigate whether or not GABA(A) receptor gamma 2 subunit gene (GABRG2) is the susceptibility gene for CAE in the Chinese population, we screened 68 CAE patients of Han ethnicity from North China for mutations in the nine exons of GABRG2. Although we found no mutation in the exons of GABRG2, we did identify two single nucleotide polymorphisms (SNPs) in exon 3 and exon 5. Using the two SNPs as markers, we carried out a transmission/disequilibrium test (TDT) in 68 trios with CAE. TDT results showed that there were no significant discrepancies between the CAE patients and 'internal controls' in allele frequencies of the two SNPs. We postulate that the GABRG2 gene might be neither a susceptibility gene for CAE nor in linkage disequilibrium with disease-predisposing sites in the Chinese population.

Mechanisms of GABAA receptor assembly and trafficking: implications for the modulation of inhibitory neurotransmission

Fast synaptic inhibition in the brain is largely mediated by ionotropic GABA receptors, which can be subdivided into GABAA and GABAC receptors based on pharmacological and molecular criteria. GABAA receptors are important therapeutic targets for a range of sedative, anxiolytic, and hypnotic agents and are implicated in several diseases including epilepsy, anxiety, depression, and substance abuse. In addition, modulating the efficacy of GABAergic neurotransmission may play a key role in neuronal plasticity. Recent studies have begun to reveal that the accumulation of ionotropic GABAA receptors at synapses is a highly regulated process that is facilitated by receptor-associated proteins and other cell-signaling molecules. This review focuses on recent experimental evidence detailing the mechanisms that control the assembly and transport of functional ionotropic GABAA receptors to cell surface sites, in addition to their stability at synaptic sites. These regulatory processes will be discussed within the context of the dynamic modulation of synaptic inhibition in the central nervous system (CNS).

Genetic and environmental factors in febrile seizures: a Danish population-based twin study

The relative importance of genetic and environmental factors in the etiology of febrile seizures was estimated using a large, unselected population-based twin sample. A total of 34,076 twins (aged 12-41 years), recruited from the Danish Twin Registry, were screened for febrile seizures by questionnaire. Information was obtained from 11,872 complete pairs. Concordance rates, odds ratios and correlations were used to assess the degree of similarity in monozygotic (MZ) and dizygotic (DZ) twins. Model fitting and estimation of heritability (proportion of the population variance attributable to genetic variation) were performed using standard biometrical methods. Significantly higher probandwise concordance rates were found for MZ compared with DZ twins (0.36 and 0.12, P < 0.01). Odds ratios and correlations showed a similar pattern. An etiological model including additive genetic effects and individual-specific environmental factors provided the best fit to the data with a heritability for febrile seizures of 70% (95% CI: 61-77%). The remaining 30% of the variation could be attributed to individual-specific environmental factors. In conclusion, this study has confirmed a major impact of genetic factors in the etiology of febrile seizures. Future studies aimed at identifying the specific genetic factors and environmental exposures involved in determining febrile seizure risk are clearly warranted.

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.

Two different mechanisms of disinhibition produced by GABAA receptor mutations linked to epilepsy in humans

The first mutations of the GABA(A) receptor channel linked to familial epilepsy in humans were reported recently (Baulac et al., 2001; Wallace et al., 2001). Preliminary functional analysis of alpha1beta2gamma2 GABA(A) receptors expressed in Xenopus oocytes suggested that the gamma2 subunit R43Q mutation abolished current enhancement by the benzodiazepine, diazepam, and that the gamma2 subunit K289M mutation decreased current amplitudes. We used single-channel recording and concentration jump techniques applied to outside out patches to evaluate the impact of these mutations on GABA(A) receptor channel function of the highly conserved rat ortholog subunits expressed in human embryonic kidney cells. When coexpressed with alpha1 and beta3 subunits, no differences were observed between wild-type and mutant GABA(A) receptor current activation rates or rates or extent of desensitization during prolonged (400 msec) GABA application (1 mm). Although deactivation after brief (5 msec) or prolonged (400 msec) GABA application was unaltered by the R43Q mutation, deactivation (a correlate of IPSC duration) was accelerated for the K289M mutation. Faster deactivation was likely a consequence of altered gating, because single-channel openings had shorter mean duration. Interestingly, the R43Q mutation did not alter diazepam potentiation. It did, however, substantially decrease current amplitude, which was not caused by decreased single-channel conductance or open time, suggesting reduced surface expression of functional receptors. The two gamma2 subunit mutations likely produce disinhibition and familial epilepsy by distinct mechanisms, suggesting that maintenance of neuronal inhibition depends not only on the peak amplitude of IPSCs, but also on their time course.

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.

Proconvulsant-induced seizures in alpha(4) nicotinic acetylcholine receptor subunit knockout mice

The genetic basis of a number of epilepsy syndromes has been identified but the precise mechanism whereby these mutations produce seizures is unknown. Three mutations of the alpha(4) subunit of the neuronal nicotinic acetylcholine receptor (nAChR) have been identified in autosomal dominant nocturnal frontal lobe epilepsy. In vitro studies of two mutations suggest an alteration of receptor function resulting in decreased ion channel current flow. We investigated the response of alpha(4) nAChR subunit knockout mice to the gamma-aminobutyric acid (GABA) receptor antagonists; pentylenetetrazole (PTZ) and bicuculline (BIC), the glutamate receptor agonist kainic acid (KA), the glycine receptor antagonist strychnine and the K(+) channel blocker 4-aminopyridine (4-AP). Mutant (Mt) mice had a greater sensitivity to PTZ and BIC, with an increase in major motor seizures and seizure-related deaths. Furthermore, Mt mice were more sensitive to KA and strychnine, but the effects were much smaller compared to those seen with the GABA receptor antagonists. Paradoxically, Mt mice appeared to be relatively protected from 4-AP-induced major motor seizures and death. The results show that a functional deletion of the alpha(4) nAChR subunit in vivo is associated with a major increase in sensitivity to GABA receptor blockers.