Ion channels and the genetic contribution to epilepsy

TRPM2 ion channel is involved in the aggravation of cognitive impairment and down regulation of epilepsy threshold in pentylenetetrazole-induced kindling mice

Epilepsy is one of the most common neurological conditions. Recent findings suggest that one of the mechanisms promoting its existence is calcium influx. The transient receptor potential melastatin type 2 channel (TRPM2) is a Ca²⁺-permeable cation channel that contributes to cell apoptosis; its possible signaling pathway is the PARP1/BNIP3/AIF/Endo G pathway that may be related to epilepsy. The aim of this study was to investigate the TRPM2 channel's involvement in epilepsy and how it works. We also explored the possible role of the TRPM2 channel on cognitive ability and emotion in epilepsy. To accomplish our goals, we used different animal epilepsy models to study the effect of the TRPM2 channel on epilepsy. The results showed that the knockout (KO) of the TRPM2 gene might play a protective role in epilepsy. Considering the advantages attributed to pentylenetetrazole (PTZ)-induced kindling mouse model, we used the model for the following assessments: 1. to observe changes in cognition and anxiety between wild type (WT) mice and TRPM2-KO mice with the recognition of new things trial and elevated plus-maze; 2. to determine the expression of apoptosis-associated proteins (PARP1, BNIP3, AIF, and Endo G) using Reverse transcription-polymerase chain reaction (RT-PCR) and Western blot; 3. to observe neurons pathologic damages and astrocyte activation in each group. The main findings of our study were: (a) TRPM2-KO had a protective effect on epilepsy; (b) TRPM2-KO improved spatial memory deficits overtime during epilepsy, but it did not improve anxiety; (c) the protective effect probably occurred via the PARP1 downstream signaling pathway; (d) TRPM2-KO could ameliorate epilepsy-induced hippocampal pathological damages and weaken astrocyte activation. These findings may provide a new approach for the treatment of epilepsy and early intervention.

Regulation of nicotinic receptor trafficking by the transmembrane Golgi protein UNC-50

Nicotinic acetylcholine receptors (AChRs) are pentameric ligand-gated ion channels that mediate fast synaptic transmission at the neuromuscular junction (NMJ). After assembly in the endoplasmic reticulum (ER), AChRs must be transported to the plasma membrane through the secretory apparatus. Little is known about specific molecules that mediate this transport. Here we identify a gene that is required for subtype-specific trafficking of assembled nicotinic AChRs in Caenorhabditis elegans. unc-50 encodes an evolutionarily conserved integral membrane protein that localizes to the Golgi apparatus. In the absence of UNC-50, a subset of AChRs present in body-wall muscle are sorted to the lysosomal system and degraded. However, the trafficking of a second AChR type and of GABA ionotropic receptors expressed in the same muscle cells is not affected in unc-50 mutants. These results suggest that, in addition to ER quality control, assembled AChRs are sorted within the Golgi system by a mechanism that controls the amount of cell-surface AChRs in a subtype-specific way.

Altered calcium/calmodulin kinase II activity changes calcium homeostasis that underlies epileptiform activity in hippocampal neurons in culture

Epilepsy is characterized by the occurrence of spontaneous recurrent epileptiform discharges (SREDs) in neurons. A decrease in calcium/calmodulin-dependent protein kinase II (CaMK-II) activity has been shown to occur with the development of SREDs in a hippocampal neuronal culture model of acquired epilepsy, and altered calcium (Ca(2+)) homeostasis has been implicated in the development of SREDs. Using antisense oligonucleotides, this study was conducted to determine whether selective suppression of CaMK-II activity, with subsequent induction of SREDs, was associated with altered Ca(2+) homeostasis in hippocampal neurons in culture. Antisense knockdown resulted in the development of SREDs and a decrease in both immunocytochemical staining and enzyme activity of CaMK-II. Evaluation of [Ca(2+)](i) using Fura indicators revealed that antisense-treated neurons manifested increased basal [Ca(2+)](i), whereas missense-treated neurons showed no change in basal [Ca(2+)](i). Antisense suppression of CaMK-II was also associated with an inability of neurons to restore a Ca(2+) load. Upon removal of oligonucleotide treatment, CaMK-II suppression and Ca(2+) homeostasis recovered to control levels and SREDs were abolished. To our knowledge, the results demonstrate the first evidence that selective suppression of CaMK-II activity results in alterations in Ca(2+) homeostasis and the development of SREDs in hippocampal neurons and suggest that CaMK-II suppression may be causing epileptogenesis by altering Ca(2+) homeostatic mechanisms.

Epilepsy-induced microarchitectural changes in the brain

Our understanding of the pathogenesis of the neuropathology of epilepsy has been challenged by a need to separate the "lesions" that cause epilepsy from the "lesions" that are produced by the epilepsy. Significant clinical, genetic, pathologic, and experimental studies of Ammon horn sclerosis (AHS) suggest that AHS is the result and cause of seizures. The data support the idea that seizures cause alterations in cell numbers, cell shape, and organization of neuronal circuitry, thus setting up an identifiable seizure-genic focus. As such, AHS represents a slowly progressive lesion and a search for the cause of the initiating seizure has led to the identification of ion channel mutations. In this report, the neuropathology of other conditions associated with intractable epilepsy is considered, suggesting that in them similar epilepsy-produced alterations in microarchitecture can be observed. The idea is important to define the optimum time for epilepsy surgery and the underlying etiology of these seizure-genic lesions.

Evidence that injury-induced changes in hippocampal neuronal calcium dynamics during epileptogenesis cause acquired epilepsy

Alterations in hippocampal neuronal Ca(2+) and Ca(2+)-dependent systems have been implicated in mediating some of the long-term neuroplasticity changes associated with acquired epilepsy (AE). However, there are no studies in an animal model of AE that directly evaluate alterations in intracellular calcium concentration ([Ca(2+)](i)) and Ca(2+) homeostatic mechanisms (Ca(2+) dynamics) during the development of AE. In this study, Ca(2+) dynamics were evaluated in acutely isolated rat CA1 hippocampal, frontal, and occipital neurons in the pilocarpine model by using [Ca(2+)](i) imaging fluorescence microscopy during the injury (acute), epileptogenesis (latency), and chronic-epilepsy phases of the development of AE. Immediately after status epilepticus (SE), hippocampal neurons, but not frontal and occipital neurons, had significantly elevated [Ca(2+)](i) compared with saline-injected control animals. Hippocampal neuronal [Ca(2+)](i) remained markedly elevated during epileptogenesis and was still elevated indefinitely in the chronic-epilepsy phase but was not elevated in SE animals that did not develop AE. Inhibiting the increase in [Ca(2+)](i) during SE with the NMDA channel inhibitor MK801 was associated in all three phases of AE with inhibition of the changes in Ca(2+) dynamics and the development of AE. Ca(2+) homeostatic mechanisms in hippocampal neurons also were altered in the brain-injury, epileptogenesis, and chronic-epilepsy phases of AE. These results provide evidence that [Ca(2+)](i) and Ca(2+)-homeostatic mechanisms are significantly altered during the development of AE and suggest that altered Ca(2+) dynamics may play a role in the induction and maintenance of AE and underlie some of the neuroplasticity changes associated with the epileptic phenotype.

Identification of a monogenic locus (jams1) causing juvenile audiogenic seizures in mice

Epilepsy is a debilitating disease with a strong genetic component. Positional cloning has identified a few genes for rare monogenic epilepsy syndromes; however, the genetics of common human epilepsies are too complex to be analyzed easily by current techniques. Mouse models of epilepsy can further this analysis by eliminating genetic background heterogeneity and enabling the production of sufficient numbers of offspring. Here, we report that Black Swiss mice have a heretofore unrecognized specific susceptibility to audiogenic seizures. These seizures are characterized by wild running, loss of righting reflex, and tonic flexion and extension, and are followed by a postictal period. The susceptibility to these seizures is developmentally regulated, peaking at 21 d of age and nearly disappearing by adulthood. Interestingly, both the susceptibility to seizures and their developmental regulation appear unrelated to hearing thresholds in the Black Swiss strain and backcrossed progeny. Genetic mapping and linkage analysis of hybrid mice localize the seizure gene, jams1 (juvenile audiogenic monogenic seizures), to a 1.6 +/- 0.5 centimorgan (cM) region on mouse chromosome 10, delimited by the gene basigin (Bsg) and marker D10Mit140. Interestingly, the majority of the critical region is syntenic to a region on human chromosome 19p13.3 implicated in a familial form of juvenile febrile convulsions. Cloning the gene for audiogenic seizures in these mice may provide important insight into the fundamental mechanisms for developmentally regulated human epilepsy syndromes.

Epilepsy genetics

BACKGROUND

Epilepsy genetics is a complex and rapidly expanding field that involves the contributions and collaborative efforts of geneticists, molecular biologists, epidemiologists, and clinicians.

REVIEW SUMMARY

This article first provides a background review of basic terminology and contributions from the fields of epilepsy, genetics, and genetic epidemiology. It further describes the evidence for the genetic basis of epilepsy and enumerates the linkage findings and epilepsy susceptibility genes reported to date. It then addresses existing knowledge about the genetics of electroencephalogram abnormalities and their relationship to clinical epilepsy. Finally, it gives guidelines for genetic counseling, with attention to patients' frequently asked questions.

CONCLUSIONS

This review provides a framework for understanding current and future investigations in the field of epilepsy genetics and a way to assist and educate individuals and families living with epilepsy.

Identification of epilepsy genes in human and mouse

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

Genetic variation of the human glycine receptor subunit genes GLRA3 and GLRB and susceptibility to idiopathic generalized epilepsies

Alterations of glycine receptor alpha1 and beta subunit genes have been associated with hypertonic motor disorders in both mice and humans. Mutations in genes encoding other ligand- and voltage-gated ion channels have been identified in rare monogenic forms of idiopathic generalized epilepsies (IGE). We tested the hypothesis that allelic variants of the glycine receptor subunit genes, GLRA3 and GLRB, both localized on chromosome 4q, confer susceptibility to common subtypes of IGE. Mutation screening was carried out in index patients of 14 IGE families. No pathogenic mutation was found, but two intronic polymorphisms were detected in the GLRB gene, and four intronic, three exonic, and one 3'-UTR polymorphisms were identified for the GLRA3 gene. Subsequent screening for exonic and 3'-UTR polymorphisms in GLRA3 showed no statistical difference between a group of sporadic IGE patients (n = 104) and a control group (n = 141). The genotype frequencies for exonic and 3'-UTR polymorphisms in GLRA3 showed no statistically significant difference between IGE patients (n = 104) and an ethnically matched control group (n = 141). Thus, no association between IGE and alterations in GLRA3 or GLRB genes could be detected, indicating that both genes do not play a major causative role in the epileptogenesis of common IGE subtypes. Still, these novel single nucleotide polymorphisms may be useful markers for candidate gene analyses of other disorders.

Long-term alteration of calcium homeostatic mechanisms in the pilocarpine model of temporal lobe epilepsy

The pilocarpine model of temporal lobe epilepsy is an animal model that shares many of the clinical and pathophysiological characteristics of temporal lobe or limbic epilepsy in humans. This model of acquired epilepsy produces spontaneous recurrent seizure discharges following an initial brain injury produced by pilocarpine-induced status epilepticus. Understanding the molecular mechanisms mediating these long lasting changes in neuronal excitability would provide an important insight into developing new strategies for the treatment and possible prevention of this condition. Our laboratory has been studying the role of alterations in calcium and calcium-dependent systems in mediating some of the long-term neuroplasticity changes associated with epileptogenesis. In this study, [Ca(2+)](i) imaging fluorescence microscopy was performed on CA1 hippocampal neurons acutely isolated from control and chronically epileptic animals at 1 year after the induction of epileptogenesis with two different fluorescent dyes (Fura-2 and Fura-FF) having high and low affinities for [Ca(2+)](i). The high affinity Ca(2+) indicator Fura-2 was utilized to evaluate [Ca(2+)](i) levels up to 900 nM and the low affinity indicator Fura-FF was employed for evaluating [Ca(2+)](i) levels above this range. Baseline [Ca(2+)](i) levels and the ability to restore resting [Ca(2+)](i) levels after a brief exposure to several glutamate concentrations in control and epileptic neurons were evaluated. Epileptic neurons demonstrated a statistically significantly higher baseline [Ca(2+)](i) level in comparison to age-matched control animals. This alteration in basal [Ca(2+)](i) levels persisted up to 1 year after the induction of epileptogenesis. In addition, the epileptic neurons were unable to rapidly restore [Ca(2+)](i) levels to baseline following the glutamate-induced [Ca(2+)](i) loads. These changes in Ca(2+) regulation were not produced by a single seizure and were not normalized by controlling the seizures in the epileptic animals with anticonvulsant treatment. Peak [Ca(2+)](i) levels in response to different concentrations of glutamate were the same in both epileptic and control neurons. Thus, glutamate produced the same initial [Ca(2+)](i) load in both epileptic and control neurons. Characterization of the viability of acutely isolated neurons from control and epileptic animals utilizing standard techniques to identify apoptotic or necrotic neurons demonstrated that epileptic neurons had no statistically significant difference in viability compared to age-matched controls. These results provide the first direct measurement of [Ca(2+)](i) levels in an intact model of epilepsy and indicate that epileptogenesis in this model produced long-lasting alterations in [Ca(2+)](i) homeostatic mechanisms that persist for up to 1 year after induction of epileptogenesis. These observations suggest that altered [Ca(2+)](i) homeostatic mechanisms may underlie some aspects of the epileptic phenotype and contribute to the persistent neuroplasticity changes associated with epilepsy.

Skeletal muscle sodium channel is affected by an epileptogenic beta1 subunit mutation

The syndrome of generalized epilepsy with febrile seizures plus type 1 (GEFS+) has been associated to the gene SCN1B coding for the sodium channel beta1 subunit (Wallace, R. H. et al. (1998) Nature Genetics 19, 366-370). In patients, a mutation of the cysteine 121 to trpyptophane (C121W) would cause a lack of modulatory activity of the beta1 subunit on sodium channels expressed in the brain, rendering neurons hyperexcitable. We have confirmed that the normal beta1-modulation of type-IIA adult brain alpha subunits (BIIA) expressed in frog oocytes is defective in C121W. We observed that the mixture of wild-type and mutant beta1 subunits is less effective than wild-type alone, suggesting that the mutant beta1 subunit does bind the alpha subunit. However, we also observed a similar lack of modulation by C121W of the in adult skeletal muscle alpha subunit (SkM1). This finding is in contrast with the simple idea that the mutational effect observed in the oocyte expression system is the principal physiopathological correlate of GEFS+, because no skeletal muscle symptoms have been reported in GEFS+ patients. We conclude that the manifestation of the pathological phenotype is conditioned by the presence of susceptibility genes and/or that the frog oocyte expression system is inadequate for the study of the mutant beta1 subunit physiopathology.

Epilepsy genes: the link between molecular dysfunction and pathophysiology

Our understanding of the genetic basis of epilepsy is progressing at a rapid pace. Gene mutations causing several of the inherited epilepsies have been mapped, and several more are likely to be added in coming years. In this review, we summarize the available information on the genetic basis of human epilepsies and epilepsy syndromes, emphasizing how genetic defects may correlate with the pathophysiological mechanisms of brain hyperexcitability. Mutations leading to epilepsy have been identified in genes encoding voltage- and ligand-gated ion channels (benign familial neonatal convulsions, autosomal dominant nocturnal frontal lobe epilepsy, generalized epilepsy with febrile seizures "plus"), neurotransmitter receptors (Angelman syndrome), the molecular cascade of cellular energy production (myoclonic epilepsy with ragged red fibers), and proteins without a known role in neuronal excitability (Unverricht-Lundborg disease). Gene defects can lead to epilepsy by altering multiple and diverse aspects of neuronal function.

Migrating partial seizures in infancy: two new cases

Two infants presented at 3 weeks and 3 months of age with intractable partial seizures. Extensive investigations failed to identify an underlying cause. There was no response to antiepileptic drug therapy and no developmental progress following the onset of the seizures. In both infants there was a distinctive pattern of seizures that arose independently from multiple regions of both hemispheres. Interictal electroencephalograms revealed multifocal epileptiform activity. The infants died aged 9 and 12 months. One underwent postmortem examination, which was normal with no hippocampal sclerosis. These infants fulfill the diagnostic criteria of the syndrome of migrating partial seizures in infancy described by Coppola and colleagues in 1995.

Opposite effects of T- and L-type Ca(2+) channels blockers in generalized absence epilepsy

The role of the T-type Ca(2+) channel blocker, ethosuximide, the L-type Ca(2+) channel blocker, nimodipine and L-type Ca(2+) channel opener, BAY K8644 (1,4 Dihydro-2, 6-dimethyl-5-nitro-4-[trifluoromethyl)-phenyl]-3-pyridine carboxylic acid methyl ester), was investigated on spike-wave discharges in WAG/Rij rats. This strain is considered as a genetic model for generalized absence epilepsy. A dose-dependent decrease in the number of spike-wave discharges was found after i.c.v. ethosuximide, an increase after i.p. nimodipine and a decrease after i.c.v. BAY K8644. BAY K8644 was also able to antagonise the effects of nimodipine. Preliminary data were obtained with two conotoxins, MVIIC and GVIA, which block P/Q-type and N-type Ca(2+) channels, respectively. Only after i.c.v. administration of omega-conotoxin GVIA were the number and duration of spike-wave discharges reduced, but animals showed knock-out lying. The latter suggests behavioural or toxic effects and that the decrease in spike-wave activity cannot unequivocally be attributed to blockade of N-type Ca(2+) channels. It can be concluded that T- and L-type Ca(2+) channel blockers show opposite effects on spike-wave discharges. Furthermore, these effects are difficult to explain in terms of a model for spindle burst activity in thalamic relay cells proposed by McCormick and Bal [Sleep and arousal: thalamocortical mechanisms.

Induction of spontaneous recurrent epileptiform discharges causes long-term changes in intracellular calcium homeostatic mechanisms

Calcium and calcium-dependent systems have been long implicated in the induction of epilepsy. We have previously observed that intracellular calcium ([Ca2+]i) levels remain elevated in cells undergoing epileptogenesis in the hippocampal neuronal culture (HNC) model. In this study, we employed the hippocampal neuronal culture (HNC) model of in vitro 'epilepsy' which produces spontaneous recurrent epileptiform discharges (SREDs) for the life of the neurons in culture to investigate alterations in [Ca2+]i homeostatic mechanisms that may be associated with the 'epileptic' phenotype. [Ca2+]i imaging fluorescence microscopy was performed on control and 'epileptic' neurons with two different fluorescent dyes ranging from high to low affinities for [Ca2+]i. We measured baseline [Ca2+]i levels and the ability to restore resting [Ca2+]i levels after a brief 2-min exposure to the excitatory amino acid glutamate in control neurons and neurons with SREDs. Neurons manifesting SREDs had statistically significantly higher baseline [Ca2+]i levels that persisted for the life of the culture. In addition, the 'epileptic' phenotype was associated with an inability to rapidly restore [Ca2+]i levels to baseline following a glutamate induced [Ca2+]i load. The use of the low affinity dye Fura-FF demonstrated that the difference in restoring baseline [Ca2+]i levels was not due to saturation of the high affinity dye Indo-1, which was utilized for evaluating the [Ca2+]i kinetics at lower [Ca2+]i levels. Peak [Ca2+]i levels in response to glutamate were the same in both 'epileptic' and control neurons. While [Ca2+]i levels recovered in approximately 30 min in control cells, it took more than 90 min to reach baseline levels in cells manifesting SREDs. Alterations of [Ca2+]i homeostatic mechanisms observed with the 'epileptic' phenotype were shown to be independent of the presence of continuous SREDs and persisted for the life of the neurons in culture. Epileptogenesis was shown not to affect the degree or duration of glutamate induced neuronal depolarization in comparing control and 'epileptic' neurons. The results indicate that epileptogenesis in this in vitro model produced long-lasting alterations in [Ca2+]i regulation that may underlie the 'epileptic' phenotype and contribute to the persistent neuroplasticity changes associated with epilepsy.

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

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