Association analysis of exonic variants of the gene encoding the GABAB receptor and idiopathic generalized epilepsy

Epigenetic Regulation of GABAergic Neurotransmission and Neurosteroid Biosynthesis in Alcohol Use Disorder

BACKGROUND

Alcohol use disorder (AUD) is a chronic relapsing brain disorder. GABAA receptor (GABAAR) subunits are a target for the pharmacological effects of alcohol. Neurosteroids play an important role in the fine-tuning of GABAAR function in the brain. Recently, we have shown that AUD is associated with changes in DNA methylation mechanisms. However, the role of DNA methylation in the regulation of neurosteroid biosynthesis and GABAergic neurotransmission in AUD patients remains under-investigated.

METHODS

In a cohort of postmortem brains from 20 male controls and AUD patients, we investigated the expression of GABAAR subunits and neurosteroid biosynthetic enzymes and their regulation by DNA methylation mechanisms. Neurosteroid levels were quantified by gas chromatography-mass spectrometry.

RESULTS

The α 2 subunit expression was reduced due to increased DNA methylation at the gene promoter region in the cerebellum of AUD patients, a brain area particularly sensitive to the effects of alcohol. Alcohol-induced alteration in GABAAR subunits was also observed in the prefrontal cortex. Neurosteroid biosynthesis was also affected with reduced cerebellar expression of the 18kDa translocator protein and 3α-hydroxysteroid dehydrogenase mRNAs. Notably, increased DNA methylation levels were observed at the promoter region of 3α-hydroxysteroid dehydrogenase. These changes were associated with markedly reduced levels of allopregnanolone and pregnanolone in the cerebellum.

CONCLUSION

Given the key role of neurosteroids in modulating the strength of GABAAR-mediated inhibition, our data suggest that alcohol-induced impairments in GABAergic neurotransmission might be profoundly impacted by reduced neurosteroid biosynthesis most likely via DNA hypermethylation.

Genetic susceptibility in Juvenile Myoclonic Epilepsy: Systematic review of genetic association studies

BACKGROUND

Several genetic association investigations have been performed over the last three decades to identify variants underlying Juvenile Myoclonic Epilepsy (JME). Here, we evaluate the accumulating findings and provide an updated perspective of these studies.

METHODOLOGY

A systematic literature search was conducted using the PubMed, Embase, Scopus, Lilacs, epiGAD, Google Scholar and Sigle up to February 12, 2016. The quality of the included studies was assessed by a score and classified as low and high quality. Beyond outcome measures, information was extracted on the setting for each study, characteristics of population samples and polymorphisms.

RESULTS

Fifty studies met eligibility criteria and were used for data extraction. With a single exception, all studies used a candidate gene approach, providing data on 229 polymorphisms in or near 55 different genes. Of variants investigating in independent data sets, only rs2029461 SNP in GRM4, rs3743123 in CX36 and rs3918149 in BRD2 showed a significant association with JME in at least two different background populations. The lack of consistent associations might be due to variations in experimental design and/or limitations of the approach.

CONCLUSIONS

Thus, despite intense research evidence established, specific genetic variants in JME susceptibility remain inconclusive. We discussed several issues that may compromise the quality of the results, including methodological bias, endophenotype and potential involvement of epigenetic factors.

PROSPERO REGISTRATION NUMBER

CRD42016036063.

Generalized Epilepsy and Myoclonic Seizures in 22q11.2 Deletion Syndrome

Prompted by the observations of juvenile myoclonic epilepsy (JME) in 22q11.2 deletion syndrome (22q11DS) and recurrent copy number variants in genetic generalized epilepsy (GGE), we searched for further evidence supporting a possible correlation of 22q11DS with GGE and with myoclonic seizures. Through routine diagnostics, we identified 3 novel individuals with the seemingly uncommon combination of 22q11DS and JME. We subsequently screened the literature for reports focussing on the epilepsy phenotype in 22q11DS. We additionally screened a database of 173 22q11DS patients and identified a fourth individual with JME as well as 2 additional cases with GGE. We describe 6 novel and 22 published cases with co-occurrence of 22q11DS and GGE. In many patients, GGE was associated with myoclonic seizures allowing for a diagnosis of JME in at least 6 individuals. Seventeen of the 173 22q11DS cases (10%) had a diagnosis of either focal or generalized epilepsy. In these cases, focal epilepsy could often be attributed to syndrome-associated hypocalcaemia, cerebral bleeds, or structural brain anomalies. However, the cause of GGE remained unclear. In this study, we describe and review 28 individuals with 22q11DS and GGE (especially JME), showing that both disorders frequently co-occur. Compared to the reported prevalence of 15-21%, in our case series only 10% of 22q11DS individuals were found to have epilepsy, often GGE. Since 22q11.2 does not contain convincing GGE candidate genes, we discuss the possibility of an aetiological correlation through a possibly disturbed interaction with the GABA_B receptor.

Intrathecal baclofen in patients with persistent vegetative state: 2 hypotheses

Sporadic cases of recovery from persistent vegetative state (PVS) after administration of intrathecal baclofen (ITB) have been reported without giving any possible explanation for its paradoxical effect. We summarize our recent findings on 5 patients with PVS treated with ITB and make some speculations on the mechanisms responsible for the observed clinical improvement. The patients developed spasticity and were judged eligible for ITB therapy. Two weeks after pump implantation, patients began to show a clinical improvement that, at the end of the 6 months' follow-up, was stable in all but 1 patient, ranging from a mere increased alertness to a full recovery of consciousness, as revealed by changes of the Coma Recovery Scale-Revised (CRS-R) score. Our findings suggest that ITB might favor a variable degree of clinical improvement. A proposal for a pharmacodynamic explanation of this effect has not been formally put forward. We hypothesize 2 possible mechanisms: first, a modulation confined to spinal cord segmental activities and to neuronal centripetal outputs reaching the cortex; and second, a modulation of sleep-wake cycles that, although present, may be dysregulated and interfere with alertness and awareness. Although our research is confined to a few subjects, it provides follow-up information by means of the CRS-R that is a validated standardized neurobehavioral instrument expressly designed for use in patients with PVS. Our observations indicate that further systematic investigation of the mechanisms and the putative clinical applications of ITB should be undertaken.

Association of GABA(B)R1 receptor gene polymorphism with obstructive sleep apnea syndrome

OBJECTIVE

GABA(B)R (gamma-amino butyric acid B receptor)-mediated neurotransmission has been implicated in the pathophysiology of a variety of neuropsychiatric disorders. GABA(B)R1 gene variants were identified by single-strand conformation analysis. The nucleotide exchanges cause a substitution of alanine to valine in exon 1a1 (Ala20Val), a substitution of glycine to serine in exon 7 (Gly489Ser) and a silent C to G nucleotide exchange encoding the amino acid phenylalanine in exon 11 (Phe658Phe). The significance of GABA(B)R1a gene polymorphism in obstructive sleep apnea syndrome (OSAS) as well as the association of these polymorphisms with the polysomnography findings in OSAS patients are not known. In this study, we aimed to assess the significance of 3 different GABA(B)R1 gene polymorphisms (Ala20Val, Gly489Ser and Phe658Phe) in OSAS.

METHODS

Seventy-five patients (23 female and 52 male) with OSAS and 99 healthy volunteers (51 female, 48 male) were included in the study to assess Ala20Val, Gly489Ser and Phe658Phe polymorphisms of the GABA(B)R1 gene.

RESULTS

For the Ala20Val variants, there was no significant difference between the genotypes and allele frequencies of the patients and controls, nor between both genders (p > 0.05). For Phe658Phe polymorphism, there was no significant difference between genotypes and allele frequencies of the patients and controls (p > 0.05). However, the C/C genotype was overrepresented and the T/C genotype was less frequent in male than female patients (p = 0.03). The C/C genotype was overrepresented and the T/C genotype was less frequent in male patients than male controls (p = 0.01). For GABA(B)R1-Gly489Ser polymorphism, all of the patients and controls had G/G genotype. The apnea arousal index scores of the male patients with C/C genotype were significantly higher than in the patients with C/T genotype (p = 0.01). The percent total sleep time in non-REM 1 scores of the male patients with T/T genotype were significantly higher than in the patients with T/C genotype (p = 0.021). The percent total sleep time in non-REM 2 scores of the female patients with C/C genotype were significantly higher than in the patients with C/T genotype (p = 0.006).

CONCLUSION

The Ala20Val polymorphism of the GABA(B)R1 gene may be associated with OSAS, whereas Gly489Ser polymorphism does not seem to be involved in OSAS. The C/C variant of the Phe658Phe polymorphism GABA(B)R1 gene seems associated with the occurrence of OSAS and is also associated with some sleep related parameters (apnea arousal index and percent total sleep time in non-REM) recorded by polysomnography.

The polymorphism GABABR1 T1974C[rs29230] of the GABAB receptor gene is not associated with the diagnosis of alcoholism or alcohol withdrawal seizures

As the inhibitory neurotransmitter gamma aminobutyric acid (GABA) modulates ethanol consumption, alcohol withdrawal symptoms and seizure generation by interacting with the GABAB receptor, the genes encoding for the GABAB receptor can be considered as candidate genes for alcoholism and alcohol withdrawal seizures (AWS). As the polymorphism GABABR1 T1974C[rs29230] of the GABAB receptor gene had been associated with alcoholism and EEG abnormalities in prior studies, the present examination investigated if the polymorphism is associated with the diagnosis of alcoholism or AWS. After genotyping the allele and genotype frequencies of a group of alcoholics with a history of AWS (n = 69) were compared with the results of a group of alcoholics with only mild withdrawal symptoms (n = 97). Additionally a group of healthy controls (n = 101) was compared with individuals with the diagnosis of alcoholism (n = 220). As no significant differences were found between the compared groups, this study gave no further evidence for GABABR1 T1974C[rs29230] as a candidate for alcoholism or AWS.

Successive alterations of hippocampal gamma-aminobutyric acid B receptor subunits in a rat model of febrile seizure

Febrile seizure (FS) is a frequently encountered seizure type in childhood. Changes of brain function following FS have clinical importance. The recently identified gamma-aminobutyric acid B receptor (GABA(B)R) is a metabotropic receptor of GABA. In this study, we used a rat model of recurrent FS to investigate the changes of GABA(B)R1a and GABA(B)R2 subunits in hippocampus after recurrent FS by using Western blot, quantitative RT-PCR, double immunofluorescence, in situ hybridization and immunoprecipitation/Western blot. After treatment of hyperthermia and the presence of induced seizures once every 2 days for 10 times, GABA(B)R1a and GABA(B)R2 subunits in hippocampus were decreased after 24 h of the last treatment. The decrease of GABA(B)R1a lasted for 15 days but that of GABA(B)R2 persisted for more than 30 days. The binding of GABA(B)R1a to GABA(B)R2 in hippocampus was also decreased significantly after 24 h of the last treatment and lasted for more than 30 days. In situ hybridization showed that GABA(B)R1a mRNA was significantly decreased in dentate gyrus, and GABA(B)R2 mRNA was considerably reduced in CA3 region. In H10 and FS1 groups in which hyperthermia treatment was the same but no (H10 group) or only one seizure (FS(1) group) was induced, the decrease of GABA(B)R1a and GABA(B)R2 subunits and the reduced binding capability between GABA(B)R1a and GABA(B)R2 subunits were also detected but with less severity, and the time recovering from these abnormalities was shorter. We conclude that GABA(B)R1a and GABA(B)R2 subunits and the binding of the 2 subunits decrease in hippocampus for a relatively long period of time after recurrent FS in immature rats. These changes may result in long-lasting imbalance of excitation/inhibition function in hippocampus, and are derived from the consequences of recurrent febrile seizures.

Lack of association between a GABA receptor 1 gene polymorphism and temporal lobe epilepsy

PURPOSE

Recently a coding nonsynonymous single-nucleotide polymorphism (SNP; G1465A) in the GABBR1 gene was reported to be associated with the incidence and severity of temporal lobe epilepsy (TLE). To clarify the role of this polymorphism in TLE, we attempted to replicate this study.

METHODS

We genotyped 188 unrelated patients with TLE (110 women, 78 men) and 259 controls of middle European descent by a restriction-length polymerase chain reaction (PCR) assay.

RESULTS

Only two (0.5%) patients and none of the controls exhibited the heterozygous A/G genotype, which was previously reported to be overrepresented among patients as compared with controls.

CONCLUSIONS

Although our study was sufficiently powered, we could not replicate the original association. Potential reasons for this failure could lie in subtle genetic differences between the studied populations or differences in the TLE phenotypes.

Is variation in the GABA(B) receptor 1 gene associated with temporal lobe epilepsy?

PURPOSE

gamma-Aminobutyric acid (GABA)-receptor genes are prime candidates for a role in seizure susceptibility. An association between the c.1465G-->A variant in the GABA(B) receptor 1 gene (GABBR1) and susceptibility to temporal lobe epilepsy (TLE) has been reported in an Italian cohort. We sought to replicate this association in an independent Australian cohort.

METHODS

The 234 patients with TLE and 164 healthy controls were genotyped for the c.1465G-->A variant, by using inclusion criteria identical to those of the first study.

RESULTS

The c.1465G-->A variant was found in one TLE patient and one control subject. Genotype and allele frequencies did not differ between groups.

CONCLUSIONS

We did not replicate the reported associations between the c.1465G-->A variant and susceptibility to TLE. We suggest that the initial positive association may be due to undetected population stratification; the importance of genomic control is emphasized. Population-specific effects also may play a role, and we highlight the need to demonstrate an in vitro functional effect to give biologic meaning to any proposed association.

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.

Molecular structure and physiological functions of GABA(B) receptors

GABA(B) receptors are broadly expressed in the nervous system and have been implicated in a wide variety of neurological and psychiatric disorders. The cloning of the first GABA(B) receptor cDNAs in 1997 revived interest in these receptors and their potential as therapeutic targets. With the availability of molecular tools, rapid progress was made in our understanding of the GABA(B) system. This led to the surprising discovery that GABA(B) receptors need to assemble from distinct subunits to function and provided exciting new insights into the structure of G protein-coupled receptors (GPCRs) in general. As a consequence of this discovery, it is now widely accepted that GPCRs can exist as heterodimers. The cloning of GABA(B) receptors allowed some important questions in the field to be answered. It is now clear that molecular studies do not support the existence of pharmacologically distinct GABA(B) receptors, as predicted by work on native receptors. Advances were also made in clarifying the relationship between GABA(B) receptors and the receptors for gamma-hydroxybutyrate, an emerging drug of abuse. There are now the first indications linking GABA(B) receptor polymorphisms to epilepsy. Significantly, the cloning of GABA(B) receptors enabled identification of the first allosteric GABA(B) receptor compounds, which is expected to broaden the spectrum of therapeutic applications. Here we review current concepts on the molecular composition and function of GABA(B) receptors and discuss ongoing drug-discovery efforts.

GABA, gamma-hydroxybutyric acid, and neurological disease

gamma-Aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the central nervous system. GABA is converted from glutamic acid by the action of glutamic acid decarboxylase (GAD) of which two isoforms exist GAD65 and GAD67. GABA then is broken down, both within the cell and in the synaptic cleft by GABA transaminase to form succinic semialdehyde. In turn, succinic semialdehyde is converted either to succinic acid by succinic semialdehyde dehydrogenase or into gamma-hydroxybutyric acid (GHB) by succinic semialdehyde reductase. Because GABA modulates the majority of inhibition that is ongoing in the brain, perturbations in GABAergic inhibition have the potential to result in seizures. Therefore, the most common disorder in which GABA is targeted as a treatment is epilepsy. However, other disorders such as psychiatric disease, spasticity, and stiff-person syndrome all have been related to disorders of GABAergic function in the brain. This review covers the roles of GABAergic neurotransmission in epilepsy, anxiety disorders, schizophrenia, stiff-person syndrome, and premenstrual dysphoric disorder. In the final section of this review, the GABA metabolite GHB is discussed in terms of its physiological significance and its role in epilepsy, sleep disorders, drug and alcohol addiction, and an inborn error of GABA metabolism, succinic semialdehyde dehydrogenase deficiency.

Up-regulation of GABA(B) receptor mRNA and protein in the hippocampus of cocaine- and lidocaine-kindled rats

To evaluate the effect of GABA(B) receptor in drug-kindled seizures, the gene expression of GABA(B) receptor in cocaine- and lidocaine-kindled rats was examined in this study. Rats were injected (i.p.) daily with cocaine (55 mg/kg) or lidocaine (65 mg/kg) until they experienced a motor seizure (kindling). After kindling, rats received a 1-day, 10-day, or 30-day drug washout period. The rats in the 1-day washout group were killed after the washout. Those in the 10-day and 30-day groups were challenged either with drug or saline, and killed 24 h later. Control rats were injected and challenged with saline. GABA(B)R1a, 1b and R2 mRNAs in discrete regions of brain were detected by in situ hybridization; GABA(B)R1a protein level was measured by Western blotting. Ninety percent of the cocaine-treated rats and 100% of the lidocaine-treated rats were kindled by day 12. Those rats responded to the challenge cocaine or lidocaine with a motor seizure after the 10-day and 30-day washout. GABA(B) receptor mRNA and protein levels in the hippocampus were significantly increased after the 1-day and 10-day washout, but not the 30-day washout. In addition, the levels in drug-treated and drug-challenged rats were significantly greater than those in drug-treated and saline-challenged rats after the 10-day washout. Those data suggest that changes of GABA(B) receptor gene expression could be a factor underlying the development of drug-kindled seizure, but not a necessary component for the maintenance of this phenomenon.

The gene encoding GABBR1 is not associated with childhood absence epilepsy in the Chinese Han population

Childhood absence epilepsy (CAE) is considered to be a genetic disease, but the genes responsible for it have not yet been identified. To investigate whether or not the GABBR1 gene is a susceptibility gene for CAE in the Chinese Han population, we systematically screened all the 22 exons and nearby intron regions of the gene and found 12 single nucleotide polymorphisms (SNPs). Using four SNPs as markers, we conducted a case-control study in 96 CAE patients and 96 normal controls. There were no significant discrepancies between the cases and controls in allele and phenotype frequencies of the four SNPs. There were still no significant differences in haplotype distributions between the cases and controls. We postulate that the GABBR1 gene might not be a susceptibility gene for CAE at least in the Chinese population.

GABA(B) receptor 1 polymorphism (G1465A) is associated with temporal lobe epilepsy

BACKGROUND

Dysfunction of gamma-aminobutyric acid (GABA) (B) receptors has been implicated in the pathogenesis of temporal lobe epilepsy (TLE).

OBJECTIVE

To evaluate the genetic contribution of cloned human GABA(B) receptors to TLE.

METHODS

The authors genotyped 141 patients (78 women and 63 men; mean age = 49.1 +/- 18.0 years) with nonlesional TLE and 372 age- and sex-matched normal individuals for the known polymorphism G1465A in the human GABA(B) receptor 1 [GABA(B[1])] gene.

RESULTS

There was a highly significant overrepresentation of the G1465A heterozygote in patients with TLE compared with controls. The A/G genotype was found in 17% of the 141 patients with TLE and in only 0.5% of the 372 controls (p < 0.0001). The authors also found that patients carrying the A allele had a significantly higher risk (p = 0.003, OR = 6.47, 95% CI = 2.02 to 20.76) of developing drug-resistant TLE. Furthermore, the age at onset of seizures tended to be lower in patients with A/G genotype, but the difference was not significant.

CONCLUSIONS

The results of this study indicate that the GABA(B[1]) polymorphism (G1465A) confers a highly increased susceptibility to TLE. Moreover, it seems to influence the severity of this common epileptic disorder.

Association of EEG coherence and an exonic GABA(B)R1 gene polymorphism

The GABA(B) receptor 1 gene is mapped to chromosome 6p21.3 within the HLA class I region close to the HLA-F gene. Susceptibility loci for epilepsy and schizophrenia have been mapped in this region. Based on pharmacological evidence, it has been suggested that GABA(B) receptors may play a crucial role in the synchronization of EEG oscillations, which in turn can be abnormal in neuropsychiatric disorders. In the present study, the hypothesis was tested, whether three exonic variants of the gene encoding the human GABA(B) receptor (GABA(B)R1) modify cortical synchronization measured as scalp-recorded EEG-coherence. Two principal components of EEG coherence (frontal coherence, parietotemporal coherence) were investigated in 104 healthy subjects during three conditions: resting EEG, activated EEG, and event-related EEG. No significant associations were found between the frontal coherence component and any polymorphism or between the parietotemporal coherence component and the exon 1a1 polymorphism. However, parietotemporal coherence showed statistically highly significant associations across all three experimental conditions with exon 7 and trend associations with exon 11. The results provide evidence that the translated polymorphism of exon 7 may be functionally meaningful and impact cortical EEG oscillations. Since variations of EEG coherence have been described for several neuropsychiatric disorders, the present association should be tested in clinical samples using EEG coherence as an intermediate phenotype.

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.

Association analysis of an (AC)n repeat polymorphism in the GABA(B) receptor gene and schizophrenia

The human gamma-aminobutyric acid type B (GABA(B)) receptor gene is a candidate gene for schizophrenia due to its chromosomal location and neurobiologic roles. In the present study, association analyses of genetic polymorphisms of the GABA(B) receptor gene with schizophrenia were carried out in 102 unrelated schizophrenic patients and 100 healthy controls, using a polymerase chain reaction-based, single-strand conformational polymorphism analysis. Although the Ala20Val and Gly489Ser mutations were not found in our samples, we found a novel polymorphism of (AC)n dinucleotide repeats located approximately 1.6 kb upstream from the translational start site. No significant difference in allele frequencies was found between controls and patients with schizophrenia (P = 0.0587) using the Monte Carlo method. Significant differences were found between controls and patients with continuous-course schizophrenia (P = 0.0019), and between controls and patients with a positive family history of psychoses (P = 0.0015). These differences, however, were not significant after Bonferroni correction. These data did not support our hypothesis that polymorphisms of the GABA(B) receptor gene may confer vulnerability for schizophrenia.

Childhood absence epilepsy: genes, channels, neurons and networks

Childhood absence epilepsy is an idiopathic, generalized non-convulsive epilepsy with a multifactorial genetic aetiology. Molecular-genetic analyses of affected human families and experimental models, together with neurobiological investigations, have led to important breakthroughs in the identification of candidate genes and loci, and potential pathophysiological mechanisms for this type of epilepsy. Here, we review these results, and compare the human and experimental phenotypes that have been investigated. Continuing efforts and comparisons of this type will help us to elucidate the multigenetic traits and pathophysiology of this form of generalized epilepsy.

Identification of epilepsy genes in human and mouse

The development of molecular markers and genomic resources has facilitated the isolation of genes responsible for rare monogenic epilepsies in human and mouse. Many of the identified genes encode ion channels or other components of neuronal signaling. The electrophysiological properties of mutant alleles indicate that neuronal hyperexcitability is one cellular mechanism underlying seizures. Genetic heterogeneity and allelic variability are hallmarks of human epilepsy. For example, mutations in three different sodium channel genes can produce the same syndrome, GEFS+, while individuals with the same allele can experience different types of seizures. Haploinsufficiency for the sodium channel SCN1A has been demonstrated by the severe infantile epilepsy and cognitive deficits in heterozygotes for de novo null mutations. Large-scale patient screening is in progress to determine whether less severe alleles of the genes responsible for monogenic epilepsy may contribute to the common types of epilepsy in the human population. The development of pharmaceuticals directed towards specific epilepsy genotypes can be anticipated, and the introduction of patient mutations into the mouse genome will provide models for testing these targeted therapies.

C-terminal interaction is essential for surface trafficking but not for heteromeric assembly of GABA(b) receptors

Assembly of fully functional GABA(B) receptors requires heteromerization of the GABA(B(1)) and GABA(B(2)) subunits. It is thought that GABA(B(1)) and GABA(B(2)) undergo coiled-coil dimerization in their cytoplasmic C termini and that assembly is necessary to overcome GABA(B(1)) retention in the endoplasmatic reticulum (ER). We investigated the mechanism underlying GABA(B(1)) trafficking to the cell surface. We identified a signal, RSRR, proximal to the coiled-coil domain of GABA(B(1)) that when deleted or mutagenized allows for surface delivery in the absence of GABA(B(2)). A similar motif, RXR, was recently shown to function as an ER retention/retrieval (ERR/R) signal in K(ATP) channels, demonstrating that G-protein-coupled receptors (GPCRs) and ion channels use common mechanisms to control surface trafficking. A C-terminal fragment of GABA(B(2)) is able to mask the RSRR signal and to direct the GABA(B(1)) monomer to the cell surface, where it is functionally inert. This indicates that in the heteromer, GABA(B(2)) participates in coupling to the G-protein. Mutagenesis of the C-terminal coiled-coil domains in GABA(B(1)) and GABA(B(2)) supports the possibility that their interaction is involved in shielding the ERR/R signal. However, assembly of heteromeric GABA(B) receptors is possible in the absence of the C-terminal domains, indicating that coiled-coil interaction is not necessary for function. Rather than guaranteeing heterodimerization, as previously assumed, the coiled-coil structure appears to be important for export of the receptor complex from the secretory apparatus.

Epileptogenesis and enhanced prepulse inhibition in GABA(B1)-deficient mice

The recent cloning of two GABA(B) receptor subunits, GABA(B1) and GABA(B2), has raised the possibility that differences in GABA(B) receptor subunit composition may give rise to pharmacologically or functionally distinct receptors. If present, such molecular diversity could permit the selective targeting of GABA(B) receptor subtypes specifically involved in pathologies such as drug addiction, spasticity, pain, and epilepsy. To address these issues we have developed a GABA(B1) subunit knockout mouse using gene targeting techniques. In the brains of GABA(B1) null mice, all pre- and postsynaptic GABA(B) receptor function was absent demonstrating that the GABA(B1) subunit is essential for all GABA(B) receptor-mediated mechanisms. Despite this, GABA(B1) null mice appeared normal at birth, although by postnatal week four their growth was retarded and they developed a generalized epilepsy that resulted in premature death. In addition, GABA(B1) heterozygote animals showed enhanced prepulse inhibition responses compared to littermate controls, suggesting that GABA(B1) deficient mice exhibit increased sensorimotor gating mechanisms. These data suggest that GABA(B) receptor antagonists may be of benefit in the treatment of psychiatric and neurological disorders in which attentional processing is impaired.

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

Epilepsies affect at least 2% of the population at some time in life, and many forms have genetic determinants. We have found a mutation in a gene encoding a GABA(A) receptor subunit in a large family with epilepsy. The two main phenotypes were childhood absence epilepsy (CAE) and febrile seizures (FS). There is a recognized genetic relationship between FS and CAE, yet the two syndromes have different ages of onset, and the physiology of absences and convulsions is distinct. This suggests the mutation has age-dependent effects on different neuronal networks that influence the expression of these clinically distinct, but genetically related, epilepsy phenotypes. We found that the mutation in GABRG2 (encoding the gamma2-subunit) abolished in vitro sensitivity to diazepam, raising the possibility that endozepines do in fact exist and have a physiological role in preventing seizures.

GABA(B(1)) mRNA expression in hippocampal sclerosis associated with human temporal lobe epilepsy

GABA(B) receptors act to inhibit neurotransmitter release from presynaptic terminals, and mediate the late inhibitory postsynaptic potential. Studies of GABA(B) receptor function in rodent models of temporal lobe epilepsy (TLE) suggest that GABA(B) receptor expression and/or function may be perturbed. GABA(B(1)) mRNA levels were investigated in 10 hippocampal resection samples obtained at surgery from intractable hippocampal sclerosis (HS) associated TLE patients and five neurologically normal post-mortem (PM) control samples. In situ hybridisation with a 35S-dATP-labelled oligonucleotide was carried out to measure mRNA levels, along with three-dimensional cell counting, for assessment of neuronal density in hippocampal subregions. GABA(B(1)) mRNA was significantly up-regulated in the subiculum of HS samples as compared with PM controls. When adjusted for the characteristic neuronal density changes observed in HS, GABA(B(1)) mRNA was significantly up-regulated in CA1, hilus and dentate gyrus granule cell layer of HS samples as compared with PM controls. The possibility of increased GABA(B(1)) expression suggests that changes in GABA(B) receptor mechanisms may be involved in the pathogenesis of human HS-associated TLE.

Variation of the genes encoding the human glutamate EAAT2, serotonin and dopamine transporters and Susceptibility to idiopathic generalized epilepsy

Several interacting genetic factors are likely to be involved in the epileptogenesis of idiopathic generalized epilepsies (IGE). Neurotransmitter transporters play a central role in the fine tuning of neurotransmission by removal of released neurotransmitters from the synaptic cleft. The present association study tested the hypotheses that variation of the genes encoding neurotransmitter transporters confers susceptibility to IGE. The genotypes of 133 German IGE subjects and 223 ethnically matched controls were assessed for DNA polymorphisms of genes encoding the glutamate (EAAT2), the serotonin (SERT), and dopamine (DAT) transporters. To increase genetic homogeneity, a subgroup of 76 patients with idiopathic absence epilepsy (IAE) was analyzed separately. We found no evidence for an allelic association of either the silent G603A substitution polymorphism in exon 5 of the EAAT2 gene or the regulatory promoter polymorphism of the SERT gene with either IGE or IAE. The frequency of the nine-copy allele of the 40 base pair repeat polymorphism in the 3' un pop popd region of the DAT gene was significantly increased in the IGE patients (chi2 = 4.11, degrees of freedom (d.f.) = 1, P = 0.043) and, in particular, in the IAE patients (chi2 = 7.81, d.f. = 1, P = 0.005) compared with the controls. The present findings strengthen previous evidence that genetic variation of the DAT gene modulates neuronal network excitability and contributes to the epileptogenesis of IAE.

Coding and noncoding variation of the human calcium-channel beta4-subunit gene CACNB4 in patients with idiopathic generalized epilepsy and episodic ataxia

Inactivation of the beta4 subunit of the calcium channel in the mouse neurological mutant lethargic results in a complex neurological disorder that includes absence epilepsy and ataxia. To determine the role of the calcium-channel beta4-subunit gene CACNB4 on chromosome 2q22-23 in related human disorders, we screened for mutations in small pedigrees with familial epilepsy and ataxia. The premature-termination mutation R482X was identified in a patient with juvenile myoclonic epilepsy. The R482X protein lacks the 38 C-terminal amino acids containing part of an interaction domain for the alpha1 subunit. The missense mutation C104F was identified both in a German family with generalized epilepsy and praxis-induced seizures and in a French Canadian family with episodic ataxia. These coding mutations were not detected in 255 unaffected control individuals (510 chromosomes), and they may be considered candidate disease mutations. The results of functional tests of the truncated protein R482X in Xenopus laevis oocytes demonstrated a small decrease in the fast time constant for inactivation of the cotransfected alpha1 subunit. Further studies will be required to evaluate the in vivo consequences of these mutations. We also describe eight noncoding single-nucleotide substitutions, two of which are present at polymorphic frequency, and a previously unrecognized first intron of CACNB4 that interrupts exon 1 at codon 21.

Genetics of the epilepsies

Molecular genetic analysis of mendelian epilepsies in humans and mice has revealed a diversity of underlying genes in symptomatic epilepsies associated with disordered brain development and neuronal survival. In contrast, the idiopathic mendelian epilepsies have emerged as a new category of channelopathies. New epilepsy loci have been mapped and one new epilepsy gene isolated. Functional analysis of epilepsy genes is providing new insights into the pathways that lead from mutant gene to hyperexcitable neurones. The major challenge for the future is the analysis of genetic epilepsies with complex inheritance.