Genes and mutations in human idiopathic epilepsy

Genomic evidence for the suitability of Göttingen Minipigs with a rare seizure phenotype as a model for human epilepsy

Epilepsy is a complex genetic disorder that affects about 2% of the global population. Although the frequency and severity of epileptic seizures can be reduced by a range of pharmacological interventions, there are no disease-modifying treatments for epilepsy. The development of new and more effective drugs is hindered by a lack of suitable animal models. Available rodent models may not recapitulate all key aspects of the disease. Spontaneous epileptic convulsions were observed in few Göttingen Minipigs (GMPs), which may provide a valuable alternative animal model for the characterisation of epilepsy-type diseases and for testing new treatments. We have characterised affected GMPs at the genome level and have taken advantage of primary fibroblast cultures to validate the functional impact of fixed genetic variants on the transcriptome level. We found numerous genes connected to calcium metabolism that have not been associated with epilepsy before, such as ADORA2B, CAMK1D, ITPKB, MCOLN2, MYLK, NFATC3, PDGFD, and PHKB. Our results have identified two transcription factor genes, EGR3 and HOXB6, as potential key regulators of CACNA1H, which was previously linked to epilepsy-type disorders in humans. Our findings provide the first set of conclusive results to support the use of affected subsets of GMPs as an alternative and more reliable model system to study human epilepsy. Further neurological and pharmacological validation of the suitability of GMPs as an epilepsy model is therefore warranted.

The Spontaneous Incidence of Neurological Clinical Signs in Preclinical Species Using Cage-side Observations or High-definition Video Monitoring: A Retrospective Analysis

When conducting toxicology studies, the interpretation of drug-related neurological clinical signs such as convulsions, myoclonus/myoclonic jerks, tremors, ataxia, and salivation requires an understanding of the spontaneous incidence of those observations in commonly used laboratory animal species. The spontaneous incidence of central nervous system clinical signs in control animals from a single facility using cage-side observations or high definition video monitoring was retrospectively analyzed. Spontaneous convulsions were observed at low incidence in Beagle dogs and Sprague-Dawley rats but were not identified in cynomolgus monkeys and Göttingen minipigs. Spontaneous myoclonic jerks and muscle twitches were observed at low incidence in Beagle dogs, cynomolgus monkeys, and Sprague-Dawley rats but were not seen in Göttingen minipigs. Spontaneous ataxia/incoordination was identified in all species and generally with a higher incidence when using video monitoring. Salivation and tremors were the two most frequent spontaneous clinical signs and both were observed in all species. Data from the current study unveil potential limitations when using control data obtained from a single study for toxicology interpretation related to low incidence neurological clinical signs while providing historical control data from Beagle dogs, cynomolgus monkeys, Sprague-Dawley rats, and Göttingen minipigs.

Rodent Models of Audiogenic Epilepsy: Genetic Aspects, Advantages, Current Problems and Perspectives

Animal models of epilepsy are of great importance in epileptology. They are used to study the mechanisms of epileptogenesis, and search for new genes and regulatory pathways involved in the development of epilepsy as well as screening new antiepileptic drugs. Today, many methods of modeling epilepsy in animals are used, including electroconvulsive, pharmacological in intact animals, and genetic, with the predisposition for spontaneous or refractory epileptic seizures. Due to the simplicity of manipulation and universality, genetic models of audiogenic epilepsy in rodents stand out among this diversity. We tried to combine data on the genetics of audiogenic epilepsy in rodents, the relevance of various models of audiogenic epilepsy to certain epileptic syndromes in humans, and the advantages of using of rodent strains predisposed to audiogenic epilepsy in current epileptology.

Epilepsy in patients with familial hemiplegic migraine

OBJECTIVE

The coexistence of epilepsy in familial hemiplegic migraine (FHM) has not been reviewed systematically. We investigated the associations of epilepsy in patients with FHM with CACNA1A, ATP1A2, SCN1A or PRRT2 mutations along with clinical and genetic data.

MATERIALS AND METHODS

We performed a search in the PubMed bibliographic database and the Cochrane Library was screened for eligible studies, from April 1997 to December 2020. Additionally, Online Mendelian Inheritance in Man (OMIM) was searched for mutations in the CACNA1A, ATP1A2, SCN1A and PRRT2 genes. Brief reports, letters, and original articles about FHM and epilepsy were included in the review if their mutations and clinical course of diseases were identified.

RESULTS

Of the included patients with FHM whose information could be accessed, there were 28 families and 195 individuals, 78 of whom had epilepsy; 30 patients had focal epilepsy and 30 patients had generalized epilepsy. All mutations except ATP1A2, which could not be evaluated due to insufficient data, revealed first epilepsy then HM. In 60 patients for whom the epilepsy prognosis was evaluated, only 3.5% of patients were drug-resistant, and the remainder had a self-limited course or responded to anti-epileptic drug treatment.

CONCLUSION

Mutations in all three and possibly four FHM genes can cause epilepsy. Contrary to our expectations, the well-known epilepsy gene SCN1A mutations are not the leading cause; the highest number of cases associated with epilepsy belongs to the ATP1A2 mutation. Drug-resistant forms of epilepsy are rare in all FHM mutations, and this information is important for counseling patients.

Antiepileptogenic Effect of Retinoic Acid

Retinoic acid, a metabolite of vitamin A, acts through either genomic or nongenomic actions. The genomic action of retinoids exerts effects on gene transcription through interaction with retinoid receptors such as retinoic acid receptors (RARα, β, and γ) and retinoid X receptors (RXRα, β, and γ) that are primarily concentrated in the amygdala, pre-frontal cortex, and hippocampal areas in the brain. In response to retinoid binding, RAR/RXR heterodimers undergo major conformational changes and orchestrate the transcription of specific gene networks. Previous experimental studies have reported that retinoic acid exerts an antiepileptogenic effect through diverse mechanisms, including the modulation of gap junctions, neurotransmitters, long-term potentiation, calcium channels and some genes. To our knowledge, there are no previous or current clinical trials evaluating the use of retinoic acid for seizure control.

Eukaryotic Elongation Factor 2 Kinase a Pharmacological Target to Regulate Protein Translation Dysfunction in Neurological Diseases

Two major processes tightly regulate protein synthesis, the initiation of mRNA translation and elongation phase that mediates the movement of ribosomes along the mRNA. The elongation phase is a high energy-consuming process, and is mainly regulated by the eukaryotic elongation factor 2 kinase (eEF2K) activity that phosphorylates and inhibits eEF2, the only known substrate of the kinase. eEF2K activity is closely regulated by several signaling pathways because the translation elongation phase strongly influences the cellular energy demand and can change the expression of specific proteins in different tissues. An increasing number of recent findings link eEF2k over activation to an array of human diseases, such as atherosclerosis, pulmonary arterial hypertension, progression of solid tumors, and some major neurological disorders. Several neurological studies suggest that eEF2K is a valuable target in treating epilepsy, depression and major neurodegenerative diseases. Despite eEF2k is an ubiquitous and conserved protein, it has been proved that its deletion does not affect development in animal models and in general cell viability. Therefore, it is possible to postulate that inhibiting its function may not cause serious side effects. In addition, eEF2K is a peculiar kinase molecularly different from most of other mammalian kinases and new compounds that inhibit eEF2K should not necessarily interfere with other important protein kinases. In this review we will critically summarize the evidence supporting the role of the altered eEF2K/eEF2 pathway in defined neurological diseases and its implications in curing these diseases in animal models, and possibly in humans, by targeting eEF2K activity.

Role of Kv7.2/Kv7.3 and M1 muscarinic receptors in the regulation of neuronal excitability in hiPSC-derived neurons

The Kv7 family of voltage-dependent non-inactivating potassium channels is composed of five members, of which four are expressed in the CNS. Kv7.2, 7.3 and 7.5 are responsible for the M-current, which plays a critical role in the regulation of neuronal excitability. Stimulation of M₁ muscarinic acetylcholine receptor, M₁ receptor, increases neuronal excitability by suppressing the M-current generated by the Kv7 channel family. The M-current modulation via M₁ receptor is well-described in in vitro assays using cell lines and in native rodent tissue. However, this mechanism was not yet reported in human induced pluripotent stem cells (hiPSC) derived neurons. In the present study, we investigated the effects of both agonists and antagonists of Kv7.2/7.3 channel and M₁ receptor in hiPSC derived neurons and in primary rat cortical neuronal cells. The role of M₁ receptors in the modulation of neuronal excitability could be demonstrated in both rat primary and hiPSC neurons. The M₁ receptors agonist, xanomeline, increased neuronal excitability in both rat cortical and the hiPSC neuronal cells. Furthermore, M₁ receptor agonist-induced neuronal excitability in vitro was reduced by an agonist of Kv7.2/7.3 in both neuronal cells. These results show that hiPSC derived neurons recreate the modulation of the M-current by the muscarinic receptor in hiPSC neurons similarly to rat native neurons. Thus, hiPSC neurons could be a useful human-based cell assay for characterization of drugs that affect neuronal excitability and/or induce seizure activity by modulation of M₁ receptors or inhibition of Kv7 channels.

Effects of MTHFR and ABCC2 gene polymorphisms on antiepileptic drug responsiveness in Jordanian epileptic patients

Background

Epilepsy is one of the most common neurological diseases with unclear etiology where its genetic background and treatment regime still need further exploration.

Objectives

This study designed to evaluate the pharmacogenomics of MTHFR and ABCC2 genes, and their association with epilepsy susceptibility among Jordanian population.

Methods

A case-control study was conducted on Jordanian cohort of 296 epileptic patients and 299 healthy individuals. Custom platform array was used to genotype the genetic polymorphisms within MTHFR (rs1801133) and ABCC2 (rs717620, rs3740066, rs2273697) genes.

Results

This study revealed a significant genetic association of MTHFR rs1801133 polymorphism with susceptibility to generalized in general and generalized tonic-clonic epilepsy (GTCE)(p=0.018 and 0.01, respectively). Regarding ABCC2 gene, rs717620 was of linkage with generalized and GTCE subtypes (p=0.045 and 0.048, respectively), while rs717620 was associated with poor responder patients (p=0.036) with no linkage of the ABCC2 haplotypes.

Conclusions

MTHFR and ABCC2 polymorphisms showed an association with either epilepsy types in general or subtypes and treatment response among Jordanian population. This study also suggested that these gene polymorphisms have an important role in epilepsy development and drug effectiveness and could be of a great impact in the era of epilepsy diagnosis and treatment.

Metabolic, Reproductive, and Neurologic Abnormalities in Agpat1-Null Mice

Defects in the biosynthesis of phospholipids and neutral lipids are associated with cell membrane dysfunction, disrupted energy metabolism, and diseases including lipodystrophy. In these pathways, the 1-acylglycerol-3-phosphate O-acyltransferase (AGPAT) enzymes transfer a fatty acid to the sn-2 carbon of sn-1-acylglycerol-3-phosphate (lysophosphatidic acid) to form sn-1, 2-acylglycerol-3-phosphate [phosphatidic acid (PA)]. PA is a precursor for key phospholipids and diacylglycerol. AGPAT1 and AGPAT2 are highly homologous isoenzymes that are both expressed in adipocytes. Genetic defects in AGPAT2 cause congenital generalized lipodystrophy, indicating that AGPAT1 cannot compensate for loss of AGPAT2 in adipocytes. To further explore the physiology of AGPAT1, we characterized a loss-of-function mouse model (Agpat1-/-). The majority of Agpat1-/- mice died before weaning and had low body weight and low plasma glucose levels, independent of plasma insulin and glucagon levels, with reduced percentage of body fat but not generalized lipodystrophy. These mice also had decreased hepatic messenger RNA expression of Igf-1 and Foxo1, suggesting a decrease in gluconeogenesis. In male mice, sperm development was impaired, with a late meiotic arrest near the onset of round spermatid production, and gonadotropins were elevated. Female mice showed oligoanovulation yet retained responsiveness to gonadotropins. Agpat1-/- mice also demonstrated abnormal hippocampal neuron development and developed audiogenic seizures. In summary, Agpat1-/- mice developed widespread disturbances of metabolism, sperm development, and neurologic function resulting from disrupted phospholipid homeostasis. AGPAT1 appears to serve important functions in the physiology of multiple organ systems. The Agpat1-deficient mouse provides an important model in which to study the contribution of phospholipid and triacylglycerol synthesis to physiology and diseases.

Dorsoventral differences in Kv7/M-current and its impact on resonance, temporal summation and excitability in rat hippocampal pyramidal cells

In rodent hippocampi, the connections, gene expression and functions differ along the dorsoventral (D-V) axis. CA1 pyramidal cells show increasing excitability along the D-V axis, although the underlying mechanism is not known. In the present study, we investigated how the M-current (IM ), caused by Kv7/M (KCNQ) potassium channels, and known to often control neuronal excitability, contributes to D-V differences in intrinsic properties of CA1 pyramidal cells. Using whole-cell patch clamp recordings and the selective Kv7/M blocker 10,10-bis(4-pyridinylmethyl)-9(10H)-anthracenone dihydrochloride (XE991) in hippocampal slices from 3- to 4-week-old rats, we found that: (i) IM had a stronger impact on subthreshold electrical properties in dorsal than ventral CA1 pyramidal cells, including input resistance, temporal summation of artificial synaptic potentials, and M-resonance; (ii) IM activated at more negative potentials (left-shifted) and had larger peak amplitude in the dorsal than ventral CA1; and (iii) the initial spike threshold (during ramp depolarizations) was elevated, and the medium after-hyperpolarization and spike frequency adaptation were increased (i.e. excitability was lower) in the dorsal rather than ventral CA1. These differences were abolished or reduced by application of XE991, indicating that they were caused by IM . Thus, it appears that IM has stronger effects in dorsal than in ventral rat CA1 pyramidal cells because of a larger maximal M-conductance and left-shifted activation curve in the dorsal cells. These mechanisms may contribute to D-V differences in the rate and phase coding of position by CA1 place cells, and may also enhance epileptiform activity in ventral CA1.

Role of the hippocampus in Nav1.6 (Scn8a) mediated seizure resistance

SCN1A mutations are the main cause of the epilepsy disorders Dravet syndrome (DS) and genetic epilepsy with febrile seizures plus (GEFS+). Mutations that reduce the activity of the mouse Scn8a gene, in contrast, are found to confer seizure resistance and extend the lifespan of mouse models of DS and GEFS+. To investigate the mechanism by which reduced Scn8a expression confers seizure resistance, we induced interictal-like burst discharges in hippocampal slices of heterozygous Scn8a null mice (Scn8a(med/+)) with elevated extracellular potassium. Scn8a(med/+) mutants exhibited reduced epileptiform burst discharge activity after P20, indicating an age-dependent increased threshold for induction of epileptiform discharges. Scn8a deficiency also reduced the occurrence of burst discharges in a GEFS+ mouse model (Scn1a(R1648H/+)). There was no detectable change in the expression levels of Scn1a (Nav1.1) or Scn2a (Nav1.2) in the hippocampus of adult Scn8a(med/+) mutants. To determine whether the increased seizure resistance associated with reduced Scn8a expression was due to alterations that occurred during development, we examined the effect of deleting Scn8a in adult mice. Global Cre-mediated deletion of a heterozygous floxed Scn8a allele in adult mice was found to increase thresholds to chemically and electrically induced seizures. Finally, knockdown of Scn8a gene expression in the adult hippocampus via lentiviral Cre injection resulted in a reduction in the number of EEG-confirmed seizures following the administration of picrotoxin. Our results identify the hippocampus as an important structure in the mediation of Scn8a-dependent seizure protection and suggest that selective targeting of Scn8a activity might be efficacious in patients with epilepsy.

LGI1: from zebrafish to human epilepsy

Mutations in the LGI1 gene predispose to autosomal dominant lateral temporal lobe epilepsy, a rare hereditary form with incomplete penetrance and associated with acoustic auras. LGI1 is not a structural component of an ion channel like most epilepsy-related genes, but is a secreted protein. Mutant null mice exhibit early-onset seizures, and electrophysiological analysis shows abnormal synaptic transmission. LGI1 binds to ADAM23 on the presynaptic membrane and ADAM22 on the postsynaptic membrane, further implicating it in regulating the strength of synaptic transmission. Patients with limbic encephalitis show autoantibodies against LGI1 and develop seizures, supporting a role for LGI1 in synapse transmission in the post developmental brain. LGI1, however, also seems to be involved in aspects of neurite development and dendritic pruning, suggesting an additional role in corticogenesis. LGI1 is also involved in cell movement and suppression of dendritic outgrowth in in vitro systems, possibly involving actin cytoskeleton dynamics. Expression patterns in embryonic development correspond to areas of neuronal migration. Loss of LGI1 expression also impacts on myelination of the central and peripheral nervous systems. In zebrafish embryos, knockdown of lgi1a leads to a seizure-like behavior and abnormal brain development, providing a system to study its role in early embryogenesis. Despite being implicated in a role in both synapse transmission and neuronal development, how LGI1 predisposes to epilepsy is still largely unknown. It appears, however, that LGI1 may function differently in a cell context-specific manner, implying a complex involvement in brain development and function that remains to be defined.

The molecular biology of genetic-based epilepsies

Epilepsy is one of the most common neurological disorders characterized by abnormal electrical activity in the central nervous system. The clinical features of this disorder are recurrent seizures, difference in age onset, type, and frequency, leading to motor, sensory, cognitive, psychic, or autonomic disturbances. Since the discovery of the first monogenic gene mutation in 1995, it is proposed that genetic factor plays an important role in the mechanism of epilepsy. Genes discovered in idiopathic epilepsies encode for ion channel or neurotransmitter receptor proteins, whereas syndromes with epilepsy as a main feature are caused by genes that are involved in functions such as cortical development, mitochondrial function, and cell metabolism. The identification of these monogenic epilepsy-causing genes provides new insight into the pathogenesis of epilepsies. Although most of the identified gene mutations present a monogenic inheritance, most of idiopathic epilepsies are complex genetic diseases exhibiting a polygenic or oligogenic inheritance. This article reviews recent genetic and molecular progresses in exploring the pathogenesis of epilepsy, with special emphasis on monogenic epilepsy-causing genes, including voltage-gated channels (Na(+), K(+), Ca(2+), Cl(-), and HCN), ligand-gated channels (nicotinic acetylcholine and GABAA receptors), non-ion channel genes as well as the mitochondrial DNA genes. These progresses have improved our understanding of the complex neurological disorder.

Candidate genes for idiopathic epilepsy in four dog breeds

Background

Idiopathic epilepsy (IE) is a naturally occurring and significant seizure disorder affecting all dog breeds. Because dog breeds are genetically isolated populations, it is possible that IE is attributable to common founders and is genetically homogenous within breeds. In humans, a number of mutations, the majority of which are genes encoding ion channels, neurotransmitters, or their regulatory subunits, have been discovered to cause rare, specific types of IE. It was hypothesized that there are simple genetic bases for IE in some purebred dog breeds, specifically in Vizslas, English Springer Spaniels (ESS), Greater Swiss Mountain Dogs (GSMD), and Beagles, and that the gene(s) responsible may, in some cases, be the same as those already discovered in humans.

Results

Candidate genes known to be involved in human epilepsy, along with selected additional genes in the same gene families that are involved in murine epilepsy or are expressed in neural tissue, were examined in populations of affected and unaffected dogs. Microsatellite markers in close proximity to each candidate gene were genotyped and subjected to two-point linkage in Vizslas, and association analysis in ESS, GSMD and Beagles.

Conclusions

Most of these candidate genes were not significantly associated with IE in these four dog breeds, while a few genes remained inconclusive. Other genes not included in this study may still be causing monogenic IE in these breeds or, like many cases of human IE, the disease in dogs may be likewise polygenic.

Epilepsy genetics--past, present, and future

Human epilepsy is a common and heterogeneous condition in which genetics play an important etiological role. We begin by reviewing the past history of epilepsy genetics, a field that has traditionally included studies of pedigrees with epilepsy caused by defects in ion channels and neurotransmitters. We highlight important recent discoveries that have expanded the field beyond the realm of channels and neurotransmitters and that have challenged the notion that single genes produce single disorders. Finally, we project toward an exciting future for epilepsy genetics as large-scale collaborative phenotyping studies come face to face with new technologies in genomic medicine.

Associated proteins: The universal toolbox controlling ligand gated ion channel function

Ligand gated ion channels are integral multimeric membrane proteins that can detect with high sensitivity the presence of a specific transmitter in the extracellular space and transduce this signal into an ion flux. While these receptors are widely expressed in the nervous system, their expression is not limited to neurons or their postsynaptic targets but extends to non-neuronal cells where they participate in many physiological responses. Cells have developed complex regulatory mechanisms allowing for the precise control and modulation of ligand gated ion channels. In this overview the roles of accessory subunits and associated proteins in these regulatory mechanisms are reviewed and their relevance illustrated by examples at different ligand gated ion channel types, with emphasis on nicotinic acetylcholine receptors. Dysfunction of ligand gated ion channels can result in neuromuscular, neurological or psychiatric disorders. A better understanding of the precise function of associated proteins and how they impact on ligand gated ion channels will provide new therapeutic opportunities for clinical intervention.

Absence epilepsy in apathetic, a spontaneous mutant mouse lacking the h channel subunit, HCN2

Analysis of naturally occurring mutations that cause seizures in rodents has advanced understanding of the molecular mechanisms underlying epilepsy. Abnormalities of I(h) and h channel expression have been found in many animal models of absence epilepsy. We characterized a novel spontaneous mutant mouse, apathetic (ap/ap), and identified the ap mutation as a 4 base pair insertion within the coding region of Hcn2, the gene encoding the h channel subunit 2 (HCN2). We demonstrated that Hcn2(ap) mRNA is reduced by 90% compared to wild type, and the predicted truncated HCN2(ap) protein is absent from the brain tissue of mice carrying the ap allele. ap/ap mice exhibited ataxia, generalized spike-wave absence seizures, and rare generalized tonic-clonic seizures. ap/+ mice had a normal gait, occasional absence seizures and an increased severity of chemoconvulsant-induced seizures. These findings help elucidate basic mechanisms of absence epilepsy and suggest HCN2 may be a target for therapeutic intervention.

Zincergic innervation of the forebrain distinguishes epilepsy-prone from epilepsy-resistant rat strains

Zinc is released from a subset of cerebral cortical neurons whereupon it exerts a powerful modulatory influence on excitatory and inhibitory neurotransmission. A number of studies have suggested that alterations in the regulation of zinc may contribute to the genesis of epilepsy. Here, we tested this hypothesis by examining the distribution of zinc-containing axon terminals in rats selectively bred for an innate susceptibility (FAST) or resistance (SLOW) to the development of kindling-induced seizures. Zinc was stained histochemically and levels of staining were quantitatively assessed. We found that the levels of synaptic zinc were significantly lower in the SLOW rats throughout the telencephalon. This relative reduction was most pronounced in limbic cortices where levels were less than 30% of FAST rats. These results suggest that innate differences in the homeostatic regulation of synaptic zinc, particularly in limbic cortices, may underlie differences in epileptogenicity.

Cellular and network mechanisms of genetically-determined absence seizures

The absence epilepsies are characterized by recurrent episodes of loss of consciousness associated with generalized spike-and-wave discharges, with an abrupt onset and offset, in the thalamocortical system. In the absence of detailed neurophysiological studies in humans, many of the concepts regarding the pathophysiological basis of absence seizures are based on studies in animal models. Each of these models has its particular strengths and limitations, and the validity of findings from these models for the human condition cannot be assumed. Consequently, studies in different models have produced some conflicting findings and conclusions. A long-standing concept, based primarily from studies in vivo in cats and in vitro brain slices, is that these paroxysmal electrical events develop suddenly from sleep-related spindle oscillations. More specifically, it is proposed that the initial mechanisms that underlie absence-related spike-and-wave discharges are located in the thalamus, involving especially the thalamic reticular nucleus. By contrast, more recent studies in well-established, genetic models of absence epilepsy in rats demonstrate that spike-and-wave discharges originate in a cortical focus and develop from a wake-related natural corticothalamic sensorimotor rhythm. In this review we integrate recent findings showing that, in both the thalamus and the neocortex, genetically-determined, absence-related spike-and-wave discharges are the manifestation of hypersynchronized, cellular, rhythmic excitations and inhibitions that result from a combination of complex, intrinsic, synaptic mechanisms. Arguments are put forward supporting the hypothesis that layer VI corticothalamic neurons act as 'drivers' in the generation of spike-and-wave discharges in the somatosensory thalamocortical system that result in corticothalamic resonances particularly initially involving the thalamic reticular nucleus. However an important unresolved question is: what are the cellular and network mechanisms responsible for the switch from physiological, wake-related, natural oscillations into pathological spike-and-wave discharges? We speculate on possible answers to this, building particularly on recent findings from genetic models in rats.

Novel anticonvulsant drugs targeting voltage-dependent ion channels

Epilepsy is one of the most common neurological disorders with a prevalence of 0.5-1%. About two-thirds of epilepsy patients respond well to anticonvulsant pharmacotherapy and become seizure free. There is a third who remain pharmacoresistant, demonstrating the pressing need for novel treatment options that could be drugs with a different mechanism of action compared with those that are currently in clinical use. During the past, many new substances have been screened for blocking or activating effects on specific ion channels, particularly those that are not targets for currently used antiepileptic drugs. This review provides an overview of new anticonvulsant compounds targeting voltage-dependent ion channels.

Genetic and environmental interactions determine seizure susceptibility in epileptic EL mice

Gene identification has progressed rapidly for monogenic epilepsies, but complex gene-environmental interactions have hindered progress in gene identification for multifactorial epilepsies. We analyzed the role of environmental risk factors in the inheritance of multifactorial idiopathic generalized epilepsy in the EL mouse. Seizure susceptibility was evaluated in the EL (E) and seizure-resistant ABP/LeJ (A) parental mouse strains and in their AEF1 and AEF2 hybrid offspring using a handling-induced seizure test. The seizure test was administered in three environments (environments I, II and III) that differed with respect to the number of seizure tests administered (one test or four tests) and the age of the mice when tested (young or old). The inheritance of seizure susceptibility appeared dominant after repetitive seizure testing in young or old mice, but recessive after a single test in old mice. Heritability was high (0.67-0.77) in each environment. Significant quantitative trait loci (QTL) that were associated with environments I and III (repetitive testing) were found on chromosomes 2 and 9 and colocalized with previously mapped El2 and El4, respectively. The El2 QTL found in environment I associated only with female susceptibility. A novel QTL, El-N, for age-dependent predisposition to seizures was found on proximal chromosome 9 only in environment II. The findings indicate that environmental risk factors determine the genetic architecture of seizure susceptibility in EL mice and suggest that QTL for complex epilepsies should be defined in terms of the environment in which they are expressed.

Prenatal exposure to thalidomide, altered vasculogenesis, and CNS malformations

Malformations of cortical development (MCD) result from abnormal neuronal positioning during corticogenesis. MCD are believed to be the morphological and perhaps physiological bases of several neurological diseases, spanning from mental retardation to autism and epilepsy. In view of the fact that during development, an appropriate blood supply is necessary to drive organogenesis in other organs, we hypothesized that vasculogenesis plays an important role in brain development and that E15 exposure in rats to the angiogenesis inhibitor thalidomide would cause postnatal MCD. Our results demonstrate that thalidomide inhibits angiogenesis in vitro at concentrations that result in significant morphological alterations in cortical and hippocampal regions of rats prenatally exposed to this vasculotoxin. Abnormal neuronal development was associated with vascular malformations and a leaky blood-brain barrier. Protein extravasation and uptake of fluorescent albumin by neurons, but not glia, was commonly associated with abnormal cortical development. Neuronal hyperexcitability was also a hallmark of these abnormal cortical regions. Our results suggest that prenatal vasculogenesis is required to support normal neuronal migration and maturation. Altering this process leads to failure of normal cerebrovascular development and may have a profound implication for CNS maturation.

Genetics of Idiopathic Generalized Epilepsies

The idiopathic generalized epilepsies (IGEs) are considered to be primarily genetic in origin. They encompass a number of rare mendelian or monogenic epilepsies and more common forms which are familial but manifest as complex, non-mendelian traits. Recent advances have demonstrated that many monogenic IGEs are ion channelopathies. These include benign familial neonatal convulsions due to mutations in KCNQ2 or KCNQ3, generalized epilepsy with febrile seizures plus due to mutations in SCN1A, SCN2A, SCN1B, and GABRG2, autosomal-dominant juvenile myoclonic epilepsy (JME) due to a mutation in GABRA1 and mutations in CLCN2 associated with several IGE sub-types. There has also been progress in understanding the non-mendelian IGEs. A haplotype in the Malic Enzyme 2 gene, ME2, increases the risk for IGE in the homozygous state. Five missense mutations have been identified in EFHC1 in 6 of 44 families with JME. Rare sequence variants have been identified in CACNA1H in sporadic patients with childhood absence epilepsy in the Chinese Han population. These advances should lead to new approaches to diagnosis and treatment.

Canine epilepsy: what can we learn from human seizure disorders?

Epilepsy is a common neurological disorder in both dogs and humans. It is refractory to therapy in approximately one-third of canine patients, and even with the advent of new antiepileptic drugs for humans, appropriate treatment options in dogs remain limited. The pathogenesis and pathophysiology of epilepsy is being studied extensively in both human patients and rodent models of experimental epilepsy at the cellular and molecular level, but very little is known about the aetiologies of epilepsies in dogs. In this review, canine epilepsy will be discussed with reference to the human epilepsies and experimental epilepsy research. There is much work to be done in order to classify canine seizure types and breed-specific epileptic syndromes, particularly with reference to electroencephalographic abnormalities and possible genetic abnormalities. The review considers the appropriate use of antiepileptic drugs: phenobarbitone and potassium bromide are effective in most canine patients, although dosing regimes need to be carefully tailored to the individual, with serum concentration measurement. However, a significant proportion of patients remains refractory to these drugs. Work is currently underway to test the efficacy of newer antiepileptic drugs in the treatment of canine epilepsy, and preliminary data suggest that human drugs such as levetiracetam and gabapentin are of benefit in dogs with refractory epilepsy.

The relationship between the pharmacology of antiepileptic drugs and human gene variation: an overview

Individual differences in clinical responsiveness to antiepileptic drugs are due to a complex interaction between environmental factors and genetic variation. Considerable interest has arisen in exploiting advances in molecular genetics to improve drug therapy for epilepsy and many other diseases; however, practical application of pharmacogenetics has been difficult to realize. Attempts to define gene variants that are associated with therapeutic (or adverse) effects of antiepileptic drugs rely currently on the prior identification of candidate genes and the subsequent evaluation of the distribution of allelic variants between individuals who have a "good" versus a "poor" clinical response. Many factors can adversely affect interpretation of such data, and careful consideration must be given to the design of genetic association studies involving candidate genes. Candidate genes may be identified in a number of ways; however, for studies of drugs, application of knowledge derived from basic pharmacology can suggest focused and testable hypotheses that are based on the fundamental principles of drug action. Thus, studies of genetic variation as they relate to proteins involved in antiepileptic drug kinetics and dynamics will identify key polymorphisms in endogenous molecules that determine degrees of drug efficacy and toxicity. Delineation of these effects in the coming years will promote enhanced success in the treatment of epilepsy.

Ring chromosome 20 with nonconvulsive status epilepticus: electroclinical correlation of a rare epileptic syndrome

The electroclinical features of two Thai women with ring chromosome 20 and nonconvulsive status epilepticus (NCSE) were studied. Both have also had generalized tonic-clonic seizures and complex partial seizures of varying frequencies since adolescence. Their intellectual functions were normal. Twenty-four-hour video/EEG telemetry recorded during the NCSE showed fluctuating consciousness between overt unresponsiveness and normal awareness. The EEG consisted of long-lasting generalized rhythmic 3-5 Hz sharp or slow waves with a few spikes, lasting several days. Despite the continuous discharges, the patients had relatively subtle clinical episodes of seizures, during which they were sometimes responsive to verbal stimuli. Intravenous antiepileptic drugs (AED) had little effect on the rhythmic EEG. No lesion in their MRIs contributed to NCSE. Ring chromosome 20 was found in 20% of female karyotype in both patients [46,XX,r(20) (p13 q13)/46,XX] but were negative in four healthy siblings. Oral AEDs decreased more than 75% of the overt CPS episodes in both patients at 22 and 26 months of follow-up but had no effect on the natural history of electrical NCSE. The patients' daily activities were minimally affected by the ongoing electrical discharges. These are the first two cases reported of ring chromosome 20 with NCSE in Thailand. Our patients present a rather benign and pharmacologically responsive course probably because of the low percentage of r(20) mosaicism. The electroclinical correlations in our cases raise the possibility that the mechanism of continuous rhythmic waves in this syndrome may be unrelated to epilepsy. Assessing the severity of this syndrome using both clinical seizures and EEG is crucial.

Drug Resistance in Epilepsy: Putative Neurobiologic and Clinical Mechanisms

Drug-resistant epilepsy with uncontrolled severe seizures despite state-of-the-art medical treatment continues to be a major clinical problem for up to one in three patients with epilepsy. Although drug resistance may emerge or remit in the course of epilepsy or its treatment, in most patients, drug resistance seems to be continuous and to occur de novo. Unfortunately, current antiepileptic drugs (AEDs) do not seem to prevent or to reverse drug resistance in most patients, but add-on therapy with novel AEDs is able to exert a modest seizure reduction in as many as 50% of patients in short-term clinical trials, and a few become seizure free during the trial. It is not known why and how epilepsy becomes drug resistant, while other patients with seemingly identical seizure types can achieve seizure control with medication. Several putative mechanisms underlying drug resistance in epilepsy have been identified in recent years. Based on experimental and clinical studies, two major neurobiologic theories have been put forward: (a) removal of AEDs from the epileptogenic tissue through excessive expression of multidrug transporters, and (b) reduced drug-target sensitivity in epileptogenic brain tissue. On the clinical side, genetic and clinical features and structural brain lesions have been associated with drug resistance in epilepsy. In this article, we review the laboratory and clinical evidence to date supporting the drug-transport and the drug-target hypotheses and provide directions for future research, to define more clearly the role of these hypotheses in the clinical spectrum of drug-resistant epilepsy.

Advances in the pathophysiology of developmental epilepsies

Pediatric epilepsies display unique characteristics that differ significantly from epilepsy in adults. The immature brain exhibits a decreased seizure threshold and an age-specific response to seizure-induced brain injury. Many idiopathic epilepsy syndromes and symptomatic epilepsies commonly present during childhood. This review highlights recent advances in the pathophysiology of developmental epilepsies. Cortical development involves maturational regulation of multiple cellular and molecular processes, such as neurogenesis, neuronal migration, synaptogenesis, and expression of neurotransmitter receptors and ion channels. These normal developmental changes of the immature brain also contribute to the increased risk for seizures and unique responses to seizure-induced brain injury in pediatric epilepsies. Recent technological advances, especially in genetics and imaging, have yielded exciting discoveries about the pathophysiology of specific pediatric epilepsy syndromes, such as the emergence of channelopathies as the cause of many idiopathic epilepsies and identification of malformations of cortical development as a major source of symptomatic epilepsies in children.