Susceptibility genes for complex epilepsy

Relation between serum amylin level and epilepsy

Background

Epilepsy is a neurological disorder characterized by convulsions. Identification of biological pathways underlying epilepsy and novel genes may shed light on the pathogenesis of epilepsy as well as new targets for treatment.

Objectives

Amylin is cosecreted with insulin from the pancreatic β-cells in a pulsatile manner as a response to nutrient stimuli. In vitro studies have shown the neurotoxicity potential of amylin. We aimed to investigate serum amylin levels between epilepsy patients and a healthy control group.

Subjects and methods

For this study, 45 patients with epilepsy and 60 healthy controls were enrolled. Routine blood analysis and electroencephalography scan were performed for all participants. Five cc venous blood sample was collected from each participant. Sera were isolated and stored at − 80 °C until the time of amylin analysis with the enzyme-linked immunosorbent assay.

Results

Gender distribution of the two groups was as follows: 44.4% males and 55.6% females among epileptic patients and 53.3% males and 46.7% females for control subjects.

Body mass index was 23.09 ± 3.99 kg/m2 for epileptic patients and 26.29 ± 4.83 kg/m2 for controls, with a statistically significantly higher body mass index in control subjects (p ˂ 0.001). With regard to serum amylin levels, a statistically significant difference was observed between the two groups (p ˂ 0.001). The median serum amylin concentration was 226.62 ng/ml (69.49–6961.19 (min–max)) for epileptic patients and 103.66 ng/ml (37.42–607.11 (min–max)) for controls (p ˂ 0.001).

Conclusion

In the present study, a significant difference was observed between patient and control groups in serum amylin concentrations, which were considerably higher in epileptic patients.

Connexin 43 Differentially Regulates Epileptiform Activity in Models of Convulsive and Non-convulsive Epilepsies

The influence of astrocytic cell networks on neuronal network activity is an emerging issue in epilepsy. Among the various mechanisms by which astrocytes modulate neuronal function, synchronization of astrocytes via gap junction channels is widely considered to be a crucial mechanism in epileptic conditions, contributing to the synchronization of the neuronal cell networks, possibly inducing recurrent epileptiform activity. Here, we explored whether modulation of astrocytic gap junctions could alter epileptic seizures in different types of epilepsy. Opening of gap junctions by trimethylamine intensifies seizure-like events (SLEs) in the low-[Mg2+] in vitro model of temporal lobe epilepsy, while alleviates seizures in the in vivo WAG/Rij rat model of absence epilepsy. In contrast, application of the gap junction blocker carbenoxolone prevents the appearance of SLEs in the low-[Mg2+] epilepsy model, but aggravates seizures in non-convulsive absence epilepsy, in vivo. Pharmacological dissection of neuronal vs. astrocytic connexins shows that astrocytic Cx43 contribute to seizure formation to a significantly higher extent than neuronal Cx36. We conclude that astrocytic gap junctions are key players in the formation of epileptiform activity and we provide a scheme for the different mode of action in the convulsive and non-convulsive epilepsy types.

Sociodemographic risk factors for febrile seizures: A school-based study from Izmir, Turkey

PURPOSE

Despite the fact that socioeconomic and environmental factors of a population are changing over time, there are few studies focusing on the effects of sociodemographic factors on the prevalence of febrile seizures (FS). This study was designed to find out the prevalence of FS and to investigate the effect of socio-cultural and economic factors on this prevalence among the Turkish school children.

METHODS

A school-based, cross-sectional study was conducted in first and second-class children. Data were collected through a questionnaire from the parents who agreed to be involved in the study. The survey had questions about some socioeconomic and demographic features of the children and febrile seizure episodes.

RESULTS

3806 children and parent pairs accepted to participate in the survey. Febrile seizure prevalence was 4.8%. It was found that the prevalence of FS was significantly associated with the chronic illnesses of a child that requires continuous medication, developmental delay of a child, NICU history, gestational hypertension history of a mother, and lower educational level of a mother. Recurrence of FS was observed in 32.9% of children. Children whose first FS was seen below the 39 °C had 1.9 times more recurrence risk.

CONCLUSION

FS prevalence rate has declined from 9.7% to 4.3% in our study population within ten years. It was thought that advancing healthcare systems in our country might be decreased the prevalence. Our study enabled us to find out sociodemographic risk factors of FS, but further studies are needed in order to confirm the effect of sociodemographic factors on FS prevalence.

Deleterious Rare Variants Reveal Risk for Loss of GABAA Receptor Function in Patients with Genetic Epilepsy and in the General Population

Genetic epilepsies (GEs) account for approximately 50% of all seizure disorders, and familial forms include mutations in single GABA_A receptor subunit genes (GABRs). In 144 sporadic GE cases (GECs), exome sequencing of 237 ion channel genes identified 520 GABR variants. Among these variants, 33 rare variants in 11 GABR genes were present in 24 GECs. To assess functional risk of variants in GECs, we selected 8 variants found in GABRA, 3 in GABRB, and 3 in GABRG and compared them to 18 variants found in the general population for GABRA1 (n = 9), GABRB3 (n = 7), and GABRG2 (n = 2). To identify deleterious variants and gain insight into structure-function relationships, we studied the gating properties, surface expression and structural perturbations of the 32 variants. Significant reduction of GABA_A receptor function was strongly associated with variants scored as deleterious and mapped within the N-terminal and transmembrane domains. In addition, 12 out of 17 variants mapped along the β+/α- GABA binding interface, were associated with reduction in channel gating and were predicted to cause structural rearrangements of the receptor by in silico simulations. Missense or nonsense mutations of GABRA1, GABRB3 and GABRG2 primarily impair subunit biogenesis. In contrast, GABR variants affected receptor function by impairing gating, suggesting that different mechanisms are operating in GABR epilepsy susceptibility variants and disease-causing mutations. The functional impact of single GABR variants found in individuals with sporadic GEs warrants the use of molecular diagnosis and will ultimately improve the treatment of genetic epilepsies by using a personalized approach.

Polymorphism in ion channel genes of Dirofilaria immitis: Relevant knowledge for future anthelmintic drug design

Dirofilaria immitis, a filarial parasite, causes cardiopulmonary dirofilariasis in dogs, cats and wild canids. The macrocyclic lactone (ML) class of drugs has been used to prevent heartworm infection. There is confirmed ML resistance in D. immitis and thus there is an urgent need to find new anthelmintics that could prevent and/or control the disease. Targeting ion channels of D. immitis for drug design has obvious advantages. These channels, present in the nematode nervous system, control movement, feeding, mating and respond to environmental cues which are necessary for survival of the parasite. Any new drug that targets these ion channels is likely to have a motility phenotype and should act to clear the worms from the host. Many of the successful anthelmintics in the past have targeted these ion channels and receptors. Knowledge about genetic variability of the ion channel and receptor genes should be useful information for drug design as receptor polymorphism may affect responses to a drug. Such information may also be useful for anticipation of possible resistance development. A total of 224 ion channel genes/subunits have been identified in the genome of D. immitis. Whole genome sequencing data of parasites from eight different geographical locations, four from ML-susceptible populations and the other four from ML-loss of efficacy (LOE) populations, were used for polymorphism analysis. We identified 1762 single nucleotide polymorphic (SNP) sites (1508 intronic and 126 exonic) in these 224 ion channel genes/subunits with an overall polymorphic rate of 0.18%. Of the SNPs found in the exon regions, 129 of them caused a non-synonymous type of polymorphism. Fourteen of the exonic SNPs caused a change in predicted secondary structure. A few of the SNPs identified may have an effect on gene expression, function of the protein and resistance selection processes.

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In the Dirofilaria immitis genome, 126 ion channel genes were identified.

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Within 126 ion channel genes, 1762 polymorphic loci were identified.

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Fourteen exonic SNPs caused a change in predicted secondary structure.

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SNPs may effect gene expression, protein function or resistance selection.

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D. immitis populations have low genetic variability among ion channel genes.

Reduced tonic inhibition after stroke promotes motor performance and epileptic seizures

Stroke survivors often recover from motor deficits, either spontaneously or with the support of rehabilitative training. Since tonic GABAergic inhibition controls network excitability, it may be involved in recovery. Middle cerebral artery occlusion in rodents reduces tonic GABAergic inhibition in the structurally intact motor cortex (M1). Transcript and protein abundance of the extrasynaptic GABAA-receptor complex α4β3δ are concurrently reduced (δ-GABAARs). In vivo and in vitro analyses show that stroke-induced glutamate release activates NMDA receptors, thereby reducing KCC2 transporters and down-regulates δ-GABAARs. Functionally, this is associated with improved motor performance on the RotaRod, a test in which mice are forced to move in a similar manner to rehabilitative training sessions. As an adverse side effect, decreased tonic inhibition facilitates post-stroke epileptic seizures. Our data imply that early and sometimes surprisingly fast recovery following stroke is supported by homeostatic, endogenous plasticity of extrasynaptic GABAA receptors.

Mutations in the mammalian target of rapamycin pathway regulators NPRL2 and NPRL3 cause focal epilepsy

OBJECTIVE

Focal epilepsies are the most common form observed and have not generally been considered to be genetic in origin. Recently, we identified mutations in DEPDC5 as a cause of familial focal epilepsy. In this study, we investigated whether mutations in the mammalian target of rapamycin (mTOR) regulators, NPRL2 and NPRL3, also contribute to cases of focal epilepsy.

METHODS

We used targeted capture and next-generation sequencing to analyze 404 unrelated probands with focal epilepsy. We performed exome sequencing on two families with multiple members affected with focal epilepsy and linkage analysis on one of these.

RESULTS

In our cohort of 404 unrelated focal epilepsy patients, we identified five mutations in NPRL2 and five in NPRL3. Exome sequencing analysis of two families with focal epilepsy identified NPRL2 and NPRL3 as the top candidate-causative genes. Some patients had focal epilepsy associated with brain malformations. We also identified 18 new mutations in DEPDC5.

INTERPRETATION

We have identified NPRL2 and NPRL3 as two new focal epilepsy genes that also play a role in the mTOR-signaling pathway. Our findings show that mutations in GATOR1 complex genes are the most significant cause of familial focal epilepsy identified to date, including cases with brain malformations. It is possible that deregulation of cellular growth control plays a more important role in epilepsy than is currently recognized.

Comprehensive identification of essential pathways and transcription factors related to epilepsy by gene set enrichment analysis on microarray datasets

Epilepsy is a common chronic neurological disorder characterized by seizures or convulsions, and is known to affect patients with primary brain tumors. The etiology of epilepsy is superficially thought to be multifactorial; however, the genetic factors which may be involved in the pathogenesis of seizures have not yet been elucidated, particularly at the pathway level. In the present study, in order to systematically investigate the gene regulatory networks involved in epilepsy, we employed a microarray dataset from the public database library of Gene Expression Omnibus (GEO) associated with tumor-induced epileptogenesis and applied gene set enrichment analysis (GSEA) on these data sets and performed candidate transcription factor (TF) selection. As a result, 68 upregulated pathways, including the extracellular matrix (ECM)-receptor interaction (P=0.004) and peroxisome proliferator-activated receptor (PPAR) signaling pathways (P=0.045), as well as 4 downregulated pathways, including the GnRH signaling pathway (P=0.029) and gap junction (P=0.034) were identified as epileptogenesis-related pathways. The majority of these pathways identified have been previously reported and our results were in accordance with those reports. However, some of these pathways identified were novel. Finally, co-expression networks of the related pathways were constructed with the significant core genes and TFs, such as PPAR-γ and phosphatidylethanolamine-binding protein. The results of our study may contribute to the improved understanding of the molecular mechanisms of epileptogenesis on a genome-wide level.

The role of T-type calcium channel genes in absence seizures

The thalamic relay neurons, reticular thalamic nucleus, and neocortical pyramidal cells form a circuit that sustains oscillatory burst firing, and is regarded as the underlying mechanism of absence seizures. T-type calcium channels play a key role in this circuit. Here, we review the role of T-type calcium channel genes in the development of absence seizures, and emphasize gain or loss of function mutations, and other variations that alter both quantity and quality of transcripts, and methylation status of isoforms of T-type calcium channel proteins might be of equal importance in understanding the pathological mechanism of absence seizures.

Broad phenotypic heterogeneity due to a novel SCN1A mutation in a family with genetic epilepsy with febrile seizures plus

Genetic (generalized) epilepsy with febrile seizures plus is a familial epilepsy syndrome with marked phenotypic heterogeneity ranging from simple febrile seizure to severe phenotypes. Here we report on a large Israeli family with genetic (generalized) epilepsy with febrile seizures plus and 14 affected individuals. A novel SCN1A missense mutation in exon 21 (p.K1372E) was identified in all affected individuals and 3 unaffected carriers. The proband had Dravet syndrome, whereas febrile seizure plus phenotypes were present in all other affected family members. Simple febrile seizures were not observed. Phenotypes were found at both extremes of the genetic (generalized) epilepsy with febrile seizures plus spectrum and distribution of phenotypes suggested modifying familial, possibly genetic factors. We suggest that families with extreme phenotype distributions can represent prime candidates for the identification of genetic or environmental modifiers.

Unraveling the genetics of common epilepsies: approaches, platforms, and caveats

With no known intervention to prevent or cure epilepsy, treatment is primarily symptomatic and requires long-term administration of medications to suppress seizure occurrence. Current antiepileptic drugs (AEDs) are ineffective in one-third of patients (Kwan and Brodie, 2000). Such therapeutic inadequacy is largely due to our insufficient understanding of the basic molecular pathophysiological processes that underlie epileptogenesis. Breakthroughs are needed in the identification of new molecular targets that will translate to novel intervention approaches. Discovering genetic variants that increase the susceptibility to disease is a promising avenue to identifying such targets. However, early candidate gene-based studies in epilepsy proved ineffective in identifying genetic risk factors for the non-Mendelian, complex epilepsies, which represent >95% of clinically encountered epilepsy. Furthermore, genome-wide association studies (GWAS) of epilepsy patients have been largely negative, with the exception of several putative susceptibility loci discovered in Han Chinese focal epilepsy and European Caucasian GGE patients (Kasperaviciute et al., 2010; Guo et al., 2012; Consortium et al., 2012). Results of these GWAS suggest that, similar to other common diseases, associations with common single nucleotide variants (SNV) appear likely to account for a small fraction of the heritability of epilepsy, thus fuelling the effort to also search for alternative genetic contributors, with a recent increased emphasis on rare variants with larger effects (Manolio et al., 2009). It is possible that both common and rare variants contribute to an increased susceptibility to common epilepsy syndromes (Mulley et al., 2005). We review the approaches that have been taken to identify genetic risk markers of the common epilepsy syndromes, the experimental platforms, and their caveats. We discuss current technologies and analytical frameworks that might expedite the discovery of these variants by leveraging advances in microarray-based, high-throughput, genotyping technology, and complementary interdisciplinary expertise of study teams including the need for meta-analyses under global collaborative frameworks. We briefly discuss the analytical options made available through rapid advances in sequencing and other genomic technologies.

Glyoxalase 1 and its substrate methylglyoxal are novel regulators of seizure susceptibility

PURPOSE

Epilepsy is a complex disease characterized by a predisposition toward seizures. There are numerous barriers to the successful treatment of epilepsy. For instance, current antiepileptic drugs have adverse side effects and variable efficacies. Furthermore, the pathophysiologic basis of epilepsy remains largely elusive. Therefore, investigating novel genes and biologic processes underlying epilepsy may provide valuable insight and enable the development of new therapeutic agents. We previously identified methylglyoxal (MG) as an endogenous γ-aminobutyric acid (GABAA ) receptor agonist. Here, we investigated the role of MG and its catabolic enzyme, glyoxalase 1 (GLO1), in seizures.

METHODS

We pretreated mice with MG before seizure induction with picrotoxin or pilocarpine and then assessed seizures behaviorally or by electroencephalography (EEG). We then investigated the role of GLO1 in seizures by treating mice with a pharmacologic inhibitor of GLO1 before seizure induction with pilocarpine and measured subsequent seizure phenotypes. Next, we explored the genetic relationship between Glo1 expression and seizures. We analyzed seizure phenotypes among C57BL/6J × DBA/2J (BXD) recombinant inbred (RI) mice with differential Glo1 expression. Lastly, we investigated a causal role for Glo1 in seizures by administering pilocarpine to transgenic (Tg) mice that overexpress Glo1.

KEY FINDINGS

Pretreatment with MG attenuated pharmacologically-induced seizures at both the behavioral and EEG levels. GLO1 inhibition, which increases MG concentration in vivo, also attenuated seizures. Among BXD RI mice, high Glo1 expression was correlated with increased seizure susceptibility. Tg mice overexpressing Glo1 displayed reduced MG concentration in the brain and increased seizure severity.

SIGNIFICANCE

These data identify MG as an endogenous regulator of seizures. Similarly, inhibition of GLO1 attenuates seizures, suggesting that this may be a novel therapeutic approach for epilepsy. Furthermore, this system may represent an endogenous negative feedback loop whereby high metabolic activity increases inhibitory tone via local accumulation of MG. Finally, Glo1 may contribute to the genetic architecture of epilepsy, as Glo1 expression regulates both MG concentration and seizure severity.

Exon-disrupting deletions of NRXN1 in idiopathic generalized epilepsy

PURPOSE

Neurexins are neuronal adhesion molecules located in the presynaptic terminal, where they interact with postsynaptic neuroligins to form a transsynaptic complex required for efficient neurotransmission in the brain. Recently, deletions and point mutations of the neurexin 1 (NRXN1) gene have been associated with a broad spectrum of neuropsychiatric disorders. This study aimed to investigate if NRXN1 deletions also increase the risk of idiopathic generalized epilepsies (IGEs).

METHODS

We screened for deletions involving the NRXN1 gene in 1,569 patients with IGE and 6,201 controls using high-density oligonucleotide microarrays.

KEY FINDINGS

We identified exon-disrupting deletions of NRXN1 in 5 of 1,569 patients with IGE and 2 of 6,201 control individuals (p = 0.0049; odds ratio (OR) 9.91, 95% confidence interval (CI) 1.92-51.12). A complex familial segregation pattern in the IGE families was observed, suggesting that heterozygous NRXN1 deletions are susceptibility variants. Intriguingly, we identified a second large copy number variant in three of five index patients, supporting an involvement of heterogeneous susceptibility alleles in the etiology of IGE.

SIGNIFICANCE

We conclude that exon-disrupting deletions of NRXN1 represent a genetic risk factor in the genetically complex predisposition of common IGE syndromes.

Naturally occurring carboxypeptidase A6 mutations: effect on enzyme function and association with epilepsy

Carboxypeptidase A6 (CPA6) is a member of the A/B subfamily of M14 metallocarboxypeptidases that is expressed in brain and many other tissues during development. Recently, two mutations in human CPA6 were associated with febrile seizures and/or temporal lobe epilepsy. In this study we screened for additional CPA6 mutations in patients with febrile seizures and focal epilepsy, which encompasses the temporal lobe epilepsy subtype. Mutations found from this analysis as well as CPA6 mutations reported in databases of single nucleotide polymorphisms were further screened by analysis of the modeled proCPA6 protein structure and the functional role of the mutated amino acid. The point mutations predicted to affect activity and/or protein folding were tested by expression of the mutant in HEK293 cells and analysis of the resulting CPA6 protein. Common polymorphisms in CPA6 were also included in this analysis. Several mutations resulted in reduced enzyme activity or CPA6 protein levels in the extracellular matrix. The mutants with reduced extracellular CPA6 protein levels showed normal levels of 50-kDa proCPA6 in the cell, and this could be converted into 37-kDa CPA6 by trypsin, suggesting that protein folding was not greatly affected by the mutations. Interestingly, three of the mutations that reduced extracellular CPA6 protein levels were found in patients with epilepsy. Taken together, these results provide further evidence for the involvement of CPA6 mutations in human epilepsy and reveal additional rare mutations that inactivate CPA6 and could, therefore, also be associated with epileptic phenotypes.

Epi4K: gene discovery in 4,000 genomes

A major challenge in epilepsy research is to unravel the complex genetic mechanisms underlying both common and rare forms of epilepsy, as well as the genetic determinants of response to treatment. To accelerate progress in this area, the National Institute of Neurological Disorders and Stroke (NINDS) recently offered funding for the creation of a “Center without Walls” to focus on the genetics of human epilepsy. This article describes Epi4K, the collaborative study supported through this grant mechanism and having the aim of analyzing the genomes of a minimum 4,000 subjects with highly selected and well-characterized epilepsy.

Extrasynaptic GABA(A) receptors: their function in the CNS and implications for disease

Over the past two decades, research has identified extrasynaptic GABA(A) receptor populations that enable neurons to sense the low ambient GABA concentrations present in the extracellular space in order to generate a form of tonic inhibition not previously considered in studies of neuronal excitability. The importance of this tonic inhibition in regulating states of consciousness is highlighted by the fact that extrasynaptic GABA(A) receptors (GABA(A)Rs) are believed to be key targets for anesthetics, sleep-promoting drugs, neurosteroids, and alcohol. The neurosteroid sensitivity of these extrasynaptic GABA(A)Rs may explain their importance in stress-, ovarian cycle-, and pregnancy-related mood disorders. Moreover, disruptions in network dynamics associated with schizophrenia, epilepsy, and Parkinson's disease may well involve alterations in the tonic GABA(A)R-mediated conductance. Extrasynaptic GABA(A)Rs may therefore present a therapeutic target for treatment of these diseases, with the potential to enhance cognition and aid poststroke functional recovery.

The Role of Seizure-Related SEZ6 as a Susceptibility Gene in Febrile Seizures

Sixty cases of febrile seizures from a Chinese cohort had previously been reported with a strong association between variants in the seizure-related (SEZ) 6 gene and febrile seizures. They found a striking lack of genetic variation in their controls. We found genetic variation in SEZ6 at similar levels at the same DNA sequence positions in our 94 febrile seizure cases as in our 96 unaffected controls. Two of our febrile seizure cases carried rare variants predicted to have damaging consequences. Combined with some of the variants from the Chinese cohort, these data are compatible with a role for SEZ6 as a susceptibility gene for febrile seizures. However, the polygenic determinants underlying most cases of febrile seizures with complex inheritance remain to be determined.

Genetically complex epilepsies, copy number variants and syndrome constellations

Epilepsy is one of the most common neurological disorders, with a prevalence of 1% and lifetime incidence of 3%. There are numerous epilepsy syndromes, most of which are considered to be genetic epilepsies. Despite the discovery of more than 20 genes for epilepsy to date, much of the genetic contribution to epilepsy is not yet known. Copy number variants have been established as an important source of mutation in other complex brain disorders, including intellectual disability, autism and schizophrenia. Recent advances in technology now facilitate genome-wide searches for copy number variants and are beginning to be applied to epilepsy. Here, we discuss what is currently known about the contribution of copy number variants to epilepsy, and how that knowledge is redefining classification of clinical and genetic syndromes.

Mutation detection in candidate genes for benign familial infantile seizures on a novel locus

Benign familial infantile seizures (BFIS) is an autosomal dominant epileptic syndrome characterized by afebrile partial seizures with or without secondary generalized tonic-clonic seizures beginning at three to ten months of age. Genetic studies have revealed three susceptibility chromosomal loci on 19q12-q13.1, 16p12-q12 and 2q24. Previously we described the novel locus on 1p36.12-p35.1 for a Chinese family affected with BFIS, and below is a subsequent mutation analysis of candidate genes for the mapped chromosome region. Forty-five genes were selected and subjected to mutation analysis. Thirty-six nucleotide variants were found, none of which led to pathogenic changes, thereby were identified as nucleotide polymorphisms. The analyses suggest those candidate genes that were detected might not be involved in the epileptogenesis of pure BFIS, at least in the Chinese family we studied.

Genetic testing in the epilepsies--report of the ILAE Genetics Commission

In this report, the International League Against Epilepsy (ILAE) Genetics Commission discusses essential issues to be considered with regard to clinical genetic testing in the epilepsies. Genetic research on the epilepsies has led to the identification of more than 20 genes with a major effect on susceptibility to idiopathic epilepsies. The most important potential clinical application of these discoveries is genetic testing: the use of genetic information, either to clarify the diagnosis in people already known or suspected to have epilepsy (diagnostic testing), or to predict onset of epilepsy in people at risk because of a family history (predictive testing). Although genetic testing has many potential benefits, it also has potential harms, and assessment of these potential benefits and harms in particular situations is complex. Moreover, many treating clinicians are unfamiliar with the types of tests available, how to access them, how to decide whether they should be offered, and what measures should be used to maximize benefit and minimize harm to their patients. Because the field is moving rapidly, with new information emerging practically every day, we present a framework for considering the clinical utility of genetic testing that can be applied to many different syndromes and clinical contexts. Given the current state of knowledge, genetic testing has high clinical utility in few clinical contexts, but in some of these it carries implications for daily clinical practice.

Role of voltage-gated calcium channels in epilepsy

It is well established that idiopathic generalized epilepsies (IGEs) show a polygenic origin and may arise from dysfunction of various types of voltage- and ligand-gated ion channels. There is an increasing body of literature implicating both high- and low-voltage-activated (HVA and LVA) calcium channels and their ancillary subunits in IGEs. Cav2.1 (P/Q-type) calcium channels control synaptic transmission at presynaptic nerve terminals, and mutations in the gene encoding the Cav2.1 alpha1 subunit (CACNA1A) have been linked to absence seizures in both humans and rodents. Similarly, mutations and loss of function mutations in ancillary HVA calcium channel subunits known to co-assemble with Cav2.1 result in IGE phenotypes in mice. It is important to note that in all these mouse models with mutations in HVA subunits, there is a compensatory increase in thalamic LVA currents which likely leads to the seizure phenotype. In fact, gain-of-function mutations have been identified in Cav3.2 (an LVA or T-type calcium channel encoded by the CACNA1H gene) in patients with congenital forms of IGEs, consistent with increased excitability of neurons as a result of enhanced T-type channel function. In this paper, we provide a broad overview of the roles of voltage-gated calcium channels, their mutations, and how they might contribute to the river that terminates in epilepsy.

Dravet syndrome

"Dravet syndrome" (DS) previously named severe myoclonic epilepsy of infancy (SMEI), or epilepsy with polymorphic seizures, is a rare disorder characterized by an early, severe, generalized, epileptic encephalopathy.DS is characterized by febrile and afebrile seizures beginning in the 1st year of life followed by different types of seizures (either focal or generalized), which are typically resistant to antiepileptic drugs. A developmental delay from the 2nd to 3rd year of life becomes evident, together with motor disturbances and personality disorders.Beside the classic syndrome, there are milder cases which have been called severe myoclonic epilepsy borderline (SMEB).DS is caused by a mutation in the neuronal sodium channel gene, SCN1A , that is also mutated in generalized epilepsy with FS+ (GEFS+).

Sodium and potassium channel dysfunctions in rare and common idiopathic epilepsy syndromes

Mutations in the SCN1A gene are found in up to 80% of individuals with severe myoclonic epilepsy of infancy (SMEI), and mutations in KCNQ2 and KCNQ3 were identified in benign familial neonatal convulsions (BFNC) as well as in single families with Rolandic epilepsy (RE) and idiopathic generalized epilepsies (IGE). This paper summarizes recent findings concerning sodium (SCN1A) and potassium channel (KCNQ2 and KCNQ3) dysfunctions in the pathogenesis of rare and common idiopathic epilepsies (IE). SMEI, severe idiopathic generalized epilepsy of infancy (SIGEI), and myoclonic-astatic epilepsy (MAE) are rare IE. Because of some semeiologic overlap, a comparative analysis of the SCN1A gene performed in 20 patients with MAE and in 18 with SIGEI. This revealed mutations in three subjects with SIGEI only. Since BFNC are over-represented in families with RE, a mutational analysis was performed in 58 families with RE with and without BNFC. This revealed functionally relevant mutations in two index cases with BNFC, and three missense mutations (one resulting in a significantly reduced potassium current amplitude) in three patients with RE, but without BNFC. One KCNQ3 missense variant was also detected in eight out of 455 IGE patients but not in 454 controls, and a silent KCNQ2-SNP was found over-represented in both epilepsy samples. These findings confirm that mutations in the SCN1A gene are mainly involved in the pathogenesis of SMEI, rarely in that of SIGEI, and are commonly not found in patients with MAE. They also demonstrate that sequence variations of the KCNQ2 and KCNQ3 genes may contribute to the etiology of common IE syndromes.

Neurobiological and physiological mechanisms of fever-related epileptiform syndromes

Febrile seizures (FS) are the most common type of convulsive events in children. FS have been extensively studied using animal models, where rat and mice pups are placed in a hyperthermic environment. Such work has largely focused on the consequences rather than on the mechanisms of experimental febrile seizures (eFS). We have recently shown that eFS are preceded by a dramatic rise in the rate of respiration. The consequent respiratory alkalosis affecting the brain and increasing neuronal excitability is a direct cause of the eFS [1]. If a similar mechanism contributes to human FS and other fever-related epileptiform syndromes, a number of factors operating at the molecular, cellular and systems level that have not been previously thought to be involved in their etiology must be considered. These include physiological and pathophysiological factors affecting CO(2) chemosensitivity as well as cellular and systemic mechanisms of acid-base regulation. Furthermore, a critical role for brain pH in FS points to novel types of susceptibility genes, which include genes coding pH-sensitive target proteins (e.g. neuronal ion channels) and pH-regulatory proteins. We will discuss these novel ideas and putative therapies based on them.

Cation-chloride cotransporters and neuronal function

Recent years have witnessed a steep increase in studies on the diverse roles of neuronal cation-chloride cotransporters (CCCs). The versatility of CCC gene transcription, posttranslational modification, and trafficking are on par with what is known about ion channels. The cell-specific and subcellular expression patterns of different CCC isoforms have a key role in modifying a neuron's electrophysiological phenotype during development, synaptic plasticity, and disease. While having a major role in controlling responses mediated by GABA(A) and glycine receptors, CCCs also show close interactions with glutamatergic signaling. A cross-talk among CCCs and trophic factors is important in short-term and long-term modification of neuronal properties. CCCs appear to be multifunctional proteins that are also involved in shaping neuronal structure at various stages of development, from stem cells to synaptogenesis. The rapidly expanding work on CCCs promotes our understanding of fundamental mechanisms that control brain development and functions under normal and pathophysiological conditions.

Chipping away at the common epilepsies with complex genetics: the 15q13.3 microdeletion shows the way

The idiopathic epilepsies are genetically heterogeneous with more than 50 clinical classifications. They are characterized by episodic seizures arising from erratic neuronal discharge in susceptible individuals. The most common predisposing genetic cause is the recently discovered chromosome 15q13.3 microdeletion. Other disorders previously attributed to the same lesion include autism, intellectual disability and schizophrenia. This phenotypic spectrum is most easily imagined as a contiguous gene syndrome with idiopathic generalized epilepsy as the most common clinical manifestation. Expressivity of the microdeletion in carriers is too variable for antenatal prediction of phenotype to be possible; however, when it is detected in living affected cases, it can be taken as the major predisposing cause for the observed phenotype. The discovery of this small 15q13.3 lesion barely scratches the surface that conceals what we ultimately need to know about the molecular genetic mechanisms behind the common epilepsies with complex genetics, but it provides valuable insight into how to proceed toward that goal.

The role of neuronal GABA(A) receptor subunit mutations in idiopathic generalized epilepsies

Rare GABA(A) receptor gamma2 and alpha1 subunit mutations of pathogenic effect have been described segregating in families with "monogenic" epilepsies. We now report globally on the genetic variation contained within all 16 neuronal GABA(A) receptor subunit genes from the one patient cohort. The cohort consists of GEFS(+), FS, and IGE subgroups as either sporadic cases or index cases from small families, with one index case from one large IGE family. The rarity of mutations and coding variation in general across all of the subunits suggests a low tolerance for mutations affecting GABA mediated neuronal inhibition. Characterization of the broader channelopathy load associated with susceptibility to these common epilepsies mostly with complex genetics will need to be expanded beyond the family of GABA(A) receptor subunits to all families of neuronal ion channels and their interacting molecules by systematic mutation detection associated with functional investigation of their naturally occurring genetic variations.

Forty Years From Markers to Genes

There have been incredible advances made in human genetics over the past 40 years. I have set out in the next few pages to describe just some of these changes and to illustrate how they unfolded through my own experiences.

Epilepsy in childhood and adolescence

INTRODUCTION

More than 3% of the total population will suffer from epilepsy at some time during their lives. One quarter of newly diagnosed cases worldwide are children. With optimal treatment, around 70% will enter remission. In the long run, antiepileptic drugs can be discontinued in almost half of affected individuals. Most patients with epilepsy undergo normal cognitive development. This relatively good prognosis contrasts with persistent public stigma surrounding the condition.

METHODS

selective literature review.

RESULTS

A classification of epileptic seizures and epileptic syndromes is provided by the International League Against Epilepsy (ILAE). Epilepsies are mainly split into symptomatic forms (i.e. those with a recognizable cause) and idiopathic forms, which result from a genetic predisposition and imply no other pathology aside from epilepsy. The classification of epilepsy syndromes is based on assumed etiology, and seizure symptomatology. Drug therapy is usually started after a second unprovoked seizure. The best possible seizure control, continuation of a successful school career, and preservation of a stable family situation are equally important therapeutic aims.

Homozygosity mapping in a family presenting with schizophrenia, epilepsy and hearing impairment

Homozygosity mapping within consanguineous families is a powerful method of localising genes for autosomal recessive disease. We investigated a family from Punjab, Pakistan, a region where consanguineous marriages are frequent. The parents have no detectable clinical disorders. However, five out of six children present with schizophrenia, epilepsy or hearing impairment either alone or in combination. This unusual phenotype in several offspring of first cousins is strongly suggestive of a rare, Mendelian recessive disorder. Two genome-wide scans initially using low-density microsatellites, and subsequently high-density SNP markers were used to map homozygous-by-descent regions in affected individuals. Candidate genes within these loci were subsequently screened for mutations. Homozygosity analysis and inbreeding coefficients were investigated to give an estimate of consanguinity. Two putative disease loci were mapped to 22q12.3-q13.3 and 2p24.3. The candidate locus on chromosome 2p24 overlaps with a deafness locus, DFNB47, linked to autosomal recessive hearing impairment, while positive findings reported for affective psychosis and schizophrenia cluster in a region of 4-5 cM on 22q13.1 within our second candidate locus. Sequence analysis of three candidate genes (KCNF1 (2p); ATF4, CACNG2 (22q)) did not reveal any exonic mutations. Inbreeding coefficients calculated for each family member support a very high degree of ancestral and recent inbreeding. The screening of other candidate genes located within these newly identified disease intervals on Chr2p24.3 and 22q12.3-q13.3 may lead to the discovery of causative variants, and consequent disrupted molecular pathways associated with this rare phenotype.

5-HT2C and GABAB receptors influence handling-induced convulsion severity in chromosome 4 congenic and DBA/2J background strain mice

Progress towards elucidating the underlying genetic variation for susceptibility to complex central nervous system (CNS) hyperexcitability states has just begun. Genetic mapping analyses suggest that a gene(s) on mid-chromosome 4 has pleiotropic effects on multiple CNS hyperexcitability states in mice, including alcohol and barbiturate withdrawal and convulsions elicited by chemical and audiogenic stimuli. We recently identified Mpdz within this chromosomal region as a gene that influences alcohol and barbiturate withdrawal convulsions. Mpdz encodes the multi-PDZ domain protein (MPDZ). Currently, there is limited information available about the mechanism by which MPDZ influences drug withdrawal and/or other CNS hyperexcitability states, but may involve its interaction with 5-HT2C and/or GABAB receptors. One of the most useful tools we have developed thus far is a congenic strain that possesses a segment of chromosome 4 from the C57BL/6J (donor) mouse strain superimposed on a genetic background that is >99% from the DBA/2J strain. The introduced segment spans the Mpdz gene. Here, we demonstrate that handling-induced convulsions are less severe in congenic vs. background strain mice in response to either a 5-HT2C receptor antagonist (SB242084) or a GABAB receptor agonist (baclofen), but not a GABAA receptor channel blocker (pentylenetetrazol). These data suggest that allelic variation in Mpdz, or a linked gene, influences SB242084- and baclofen-enhanced convulsions. Our results are consistent with the hypothesis that Mpdz's effects on CNS hyperexcitability, including alcohol and barbiturate withdrawal, involve MPDZ interaction with 5-HT2C and/or GABAB receptors. However, additional genes reside within the congenic interval and may also influence CNS hyperexcitability.

Extended spectrum of idiopathic generalized epilepsies associated withCACNA1Hfunctional variants

OBJECTIVE

The relationship between genetic variation in the T-type calcium channel gene CACNA1H and childhood absence epilepsy is well established. The purpose of this study was to investigate the range of epilepsy syndromes for which CACNA1H variants may contribute to the genetic susceptibility architecture and determine the electrophysiological effects of these variants in relation to proposed mechanisms underlying seizures.

METHODS

Exons 3 to 35 of CACNA1H were screened for variants in 240 epilepsy patients (167 unrelated) and 95 control subjects by single-stranded conformation analysis followed by direct sequencing. Cascade testing of families was done by sequencing or single-stranded conformation analysis. Selected variants were introduced into the CACNA1H protein by site-directed mutagenesis. Constructs were transiently transfected into human embryo kidney cells, and electrophysiological data were acquired.

RESULTS

More than 100 variants were detected, including 19 novel variants leading to amino acid changes in subjects with phenotypes including childhood absence, juvenile absence, juvenile myoclonic and myoclonic astatic epilepsies, as well as febrile seizures and temporal lobe epilepsy. Electrophysiological analysis of 11 variants showed that 9 altered channel properties, generally in ways that would be predicted to increase calcium current.

INTERPRETATION

Variants in CACNA1H that alter channel properties are present in patients with various generalized epilepsy syndromes. We propose that these variants contribute to an individual's susceptibility to epilepsy but are not sufficient to cause epilepsy on their own. The genetic architecture is dominated by rare functional variants; therefore, CACNA1H would not be easily identified as a susceptibility gene by a genome-wide case-control study seeking a statistical association.

A polygenic heterogeneity model for common epilepsies with complex genetics

Approximately 40% of epilepsy has a complex genetic basis with an unknown number of susceptibility genes. The effect of each susceptibility gene acting alone is insufficient to account for seizure phenotypes, but certain numbers or combinations of variations in susceptibility genes are predicted to raise the level of neuronal hyperexcitability above a seizure threshold for a given individual in a given environment. Identities of susceptibility genes are beginning to be determined, initially by translation of knowledge gained from gene discovery in the monogenic epilepsies. This entrée into idiopathic epilepsies with complex genetics has led to the experimental validation of susceptibility variants in the first few susceptibility genes. The genetic architecture so far emerging from these results is consistent with what we have designated as a polygenic heterogeneity model for the epilepsies with complex genetics.

Bipolar disorder and epilepsy: a bidirectional relation? Neurobiological underpinnings, current hypotheses, and future research directions

A number of studies have demonstrated that affective disorders in epilepsy represent a common psychiatric comorbidity; however, most of the classic neuropsychiatric literature focuses on depression, which is actually prominent, but little is known about bipolar depression, and very little about mania, in epilepsy. Biochemical, structural, and functional abnormalities in primary bipolar disorder could also occur secondary to seizure disorders. The kindling paradigm, invoked as a model for understanding seizure disorders, has also been applied to the episodic nature of bipolar disorder. In bipolar patients, changes in second-messenger systems, such as G-proteins, phosphatidylinositol, protein kinase C, myristoylated alanine-rich C kinase substrate, or calcium activity have been described, along with changes in c-fos expression. Common mechanisms at the level of ion channels might include the antikindling and the calcium-antagonistic and potassium outward current-modulating properties of antiepileptic drugs. All these lines of research appear to be converging on a richer understanding of neurobiological underpinnings between bipolar disorder and epilepsy. Mania, which is the other side of the coin in affective disorders, may represent a privileged window into the neurobiology of mood regulation and the neurobiology of epilepsy itself. Future research on intracellular mechanisms might become decisive for a better understanding of the similarities between these two disorders.

A multicenter study of BRD2 as a risk factor for juvenile myoclonic epilepsy

PURPOSE

Although complex idiopathic generalized epilepsies (IGEs) are recognized to have a significant genetic component, as yet there are no known common susceptibility variants. It has recently been suggested that variation in the BRD2 gene confers increased risk of juvenile myoclonic epilepsy (JME), which accounts for around a quarter of all IGE. Here we examine the association between the candidate causal SNP (the promoter variant rs3918149) and JME in five independent cohorts comprising in total 531 JME cases and 1,390 healthy controls.

METHODS

The strongest candidate causal variant from the original report (rs3918149) was genotyped across all five cohorts. In an effort to identify novel candidate causal polymorphisms, previously unscreened regions of UTR were resequenced.

RESULTS

We observed a significant effect in a small sample recruited in Britain (genotype p = 0.001, allele p = 0.001), a borderline significant effect in a sample recruited in Ireland and no association in larger samples of German, Australian, and Indian populations. There was no association with other common forms of epilepsy or any other clear candidate casual variants in or near the BRD2 region.

CONCLUSIONS

The replication of an effect in the British cohort and suggestive evidence from that recruited in Ireland but lack of replication from the larger German, Australian, and Indian cohorts is surprising and difficult to explain. Further replication in carefully matched populations is required. Results presented here do not, however, support a strong effect for susceptibility to JME across populations of European descent.

Computational analysis of the R85C and R85H epilepsy mutations in Na+ channel β1 subunits

Mutations in Na+ channels cause a variety of epilepsy syndromes. Analysis of these mutations shows a range of simultaneous functional consequences, each of which may increase or decrease membrane excitability, making it difficult to predict the combined effect on neuron firing. This may be addressed by building mathematical models of Na+ channel gating and using them in neuron models to predict responses to natural stimuli. The R85C and R85H mutations of the beta1 subunit cause generalized epilepsy syndromes in humans, and an experimental study showed that these mutations shift steady-state activation in the negative direction, which predicts increased excitability, and shift fast inactivation in the negative direction, which predicts decreased excitability. In addition, the R85C also shifts slow inactivation in the negative direction. To predict changes in neuron excitability resulting from these contradictory effects we built Na+ channel models based on our earlier data and on new measurements of the rate of slow inactivation over a range of potentials. Use of these Na+ channel models in simple neuron models revealed that both mutations cause an increase in excitability but the R85H mutation was more excitable. This is due to differences in steady-state slow inactivation and to subtle differences in fast kinetics captured by the model fitting process. To understand the effect of changes in different gating processes and to provide a simple guide for interpreting changes caused by mutations, we performed a sensitivity analysis. Using the wild-type model we shifted each activation curve by +/-5 mV or altered gating rates up or down by 20%. Excitability was most sensitive to changes in voltage dependence of activation, followed by voltage dependence of inactivation and then slow inactivation. By contrast, excitability was relatively insensitive to gating rates.

Channelopathies in idiopathic epilepsy

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

NEDD4-2 as a potential candidate susceptibility gene for epileptic photosensitivity

Photosensitive seizures occur most commonly in childhood and adolescence, usually as a manifestation of complex idiopathic generalized epilepsies (IGEs). Molecular mechanisms underlying this condition are yet to be determined because no susceptibility genes have been identified. The NEDD4-2 (Neuronally Expressed Developmentally Downregulated 4) gene encodes a ubiquitin protein ligase proposed to regulate cell surface levels of several ion channels, receptors and transporters involved in regulating neuronal excitability, including voltage-gated sodium channels (VGSCs), the most clinically relevant of the epilepsy genes. The regulation of NEDD4-2 in vivo involves complex interactions with accessory proteins in a cell type specific manner. We screened NEDD4-2 for mutations in a cohort of 253 families with IGEs. We identified three NEDD4-2 missense changes in highly conserved residues; S233L, E271A and H515P in families with photosensitive generalized epilepsy. The NEDD4-2 variants were as effective as wild-type NEDD4-2 in downregulating the VGSC subtype Na(v)1.2 when assessed in the Xenopus oocyte heterologous expression system showing that the direct interaction with the ion channel was not altered by these variants. These data raise the possibility that photosensitive epilepsy may arise from defective interaction of NEDD4-2 with as yet unidentified accessory or target proteins.