Genetic dissection of the common epilepsies

Cytokine Polymorphism and HLA Genotyping in Patients with Temporal Lobe Epilepsy Related to Hippocampal Sclerosis

Objective

Hippocampal sclerosis (HS) is the most common pathological substrate associated with mesial temporal lobe epilepsy (MTLE), where inflammatory processes are known to play an increasingly important role in the pathogenesis. To further investigate the role of the immune system, both cytokine gene polymorphisms and human leukocyte antigen (HLA) genotyping in patients with MTLE-HS were investigated.

Methods

The DNA samples of 100 patients with MTLE-HS and 201 healthy individuals were genotyped for cytokines (IL-6,IL-10, TNF-α, TGF-β1 and IFN-γ) and HLA using polymerase chain reaction (PCR)-SSP and SSO methods. The results were statistically analyzed in patient and healthy control groups and then according to the presence of febrile seizures (FS) in the patient group.

Results

Analysis of cytokine genotyping did not reveal any significant difference between patients with MTLE-HS and controls and patients with or without FS. However, the HLA DRB1*13 allele was found to be more frequent in the patient population after Bonferroni correction.

Conclusion

This study suggests the possible role of HLA in the pathogenesis of MTLE-HS, although it failed to show any relationship with the cytokine system. However, data regarding the role of HLA are still lacking, and further studies are necessary to verify our results.

Susceptibility to seizure-induced excitotoxic cell death is regulated by an epistatic interaction between Chr 18 (Sicd1) and Chr 15 (Sicd2) loci in mice

Seizure-induced cell death is believed to be regulated by multiple genetic components in addition to numerous external factors. We previously defined quantitative trait loci that control susceptibility to seizure-induced cell death in FVB/NJ (susceptible) and C57BL/6J (resistant) mice. Two of these quantitative trait loci assigned to chromosomes 18 (Sicd1) and 15 (Sicd2), control seizure-induced cell death resistance. In this study, through the use of a series of novel congenic strains containing the Sicd1 and Sicd2 congenic strains and different combinations of the Sicd1 or Sicd2 sub region(s), respectively, we defined these genetic interactions. We generated a double congenic strain, which contains the two C57BL/6J differential segments from chromosome 18 and 15, to determine how these two segments interact with one another. Phenotypic comparison between FVB-like littermates and the double congenic FVB.B6-Sicd1/Sicd2 strain identified an additive effect with respect to resistance to seizure-induced excitotoxic cell death. It thus appears that C57BL/6J alleles located on chromosomes 18 and 15 interact epistatically in an additive manner to control the extent of seizure-induced excitotoxic cell death. Three interval-specific congenic lines were developed, in which either segments of C57BL/6J Chr 18 or C57BL/6J Chr 15 were introduced in the FVB/NJ genetic background, and progeny were treated with kainate and examined for the extent of seizure-induced cell death. All of the interval-specific congenic lines exhibited reduced cell death in both area CA3 and the dentate hilus, associated with the C57BL/6J phenotype. These experiments demonstrate functional interactions between Sicd1 and Sicd2 that improve resistance to seizure-induced excitotoxic cell death, validating the critical role played by gene-gene interactions in excitotoxic cell death.

A variant of KCC2 from patients with febrile seizures impairs neuronal Cl- extrusion and dendritic spine formation

Genetic variation in SLC12A5 which encodes KCC2, the neuron-specific cation-chloride cotransporter that is essential for hyperpolarizing GABAergic signaling and formation of cortical dendritic spines, has not been reported in human disease. Screening of SLC12A5 revealed a co-segregating variant (KCC2-R952H) in an Australian family with febrile seizures. We show that KCC2-R952H reduces neuronal Cl(-) extrusion and has a compromised ability to induce dendritic spines in vivo and in vitro. Biochemical analyses indicate a reduced surface expression of KCC2-R952H which likely contributes to the functional deficits. Our data suggest that KCC2-R952H is a bona fide susceptibility variant for febrile seizures.

Genetics of temporal lobe epilepsy

The most common partial epilepsy, temporal lobe epilepsy (TLE) consists of a heterogeneous group of seizure disorders originating in the temporal lobe. TLE had been thought to develop as a result of acquired structural problems in the temporal lobe. During the past two decades, there has been growing evidence of the important influence of genetic factors, and familial and non-lesional TLE have been increasingly described. Here, we focus on the genetics of TLE and review related genes which have been studied recently. Although its molecular mechanisms are still poorly understood, TLE genetics is a fertile field, awaiting more research.

Pediatric epilepsy: Five new things

Epilepsy in children can be very different from epilepsy in adults, both in seizure type and epilepsy syndrome. The goal in treating children is seizure freedom, no treatment side effects, and function that is no different from the general population. In a significant percentage of patients, this goal remains unachievable, but many aspects of epilepsy are becoming clearer. This review will highlight 5 areas where progress is being made to achieve these goals in pediatric epilepsy. Specific research animal models are being developed to reflect the unique features of different pediatric epilepsies. As genetic syndromes are better identified, for some patients this has led to improved treatment. New advances in drug therapy have led to 3 new medications approved for children. More effective drug choices can now be recommended due to comparative drug trials and better overall care of children can be provided due to awareness of the comorbidities of epilepsy.

Galanin receptor 1 deletion exacerbates hippocampal neuronal loss after systemic kainate administration in mice

Background

Galanin is a neuropeptide with a wide distribution in the central and peripheral nervous systems and whose physiological effects are mediated through three G protein-coupled receptor subtypes, GalR1, GalR2, and GalR3. Several lines of evidence indicate that galanin, as well as activation of the GalR1 receptor, is a potent and effective modulator of neuronal excitability in the hippocampus.

Methodology/Principal Findings

In order to test more formally the potential influence of GalR1 on seizure-induced excitotoxic cell death, we conducted functional complementation tests in which transgenic mice that exhibit decreased expression of the GalR1 candidate mRNA underwent kainate-induced status epilepticus to determine if the quantitative trait of susceptibility to seizure-induced cell death is determined by the activity of GalR1. In the present study, we report that reduction of GalR1 mRNA via null mutation or injection of the GalR1 antagonist, galantide, prior to kainate-induced status epilepticus induces hippocampal damage in a mouse strain known to be highly resistant to kainate-induced neuronal injury. Wild-type and GalR1 knockout mice were subjected to systemic kainate administration. Seven days later, Nissl and NeuN immune- staining demonstrated that hippocampal cell death was significantly increased in GalR1 knockout strains and in animals injected with the GalR1 antagonist. Compared to GalR1-expressing mice, GalR1-deficient mice had significantly larger hippocampal lesions after status epilepticus.

Conclusions/Significance

Our results suggest that a reduction of GalR1 expression in the C57BL/6J mouse strain renders them susceptible to excitotoxic injury following systemic kainate administration. From these results, GalR1 protein emerges as a new molecular target that may have a potential therapeutic value in modulating seizure-induced cell death.

Neuronal voltage-gated ion channels are genetic modifiers of generalized epilepsy with febrile seizures plus

Mutations in the neuronal voltage-gated sodium channel genes SCN1A and SCN2A are associated with inherited epilepsies, including genetic epilepsy with febrile seizures plus (GEFS+) and Dravet syndrome (severe myoclonic epilepsy of infancy). The clinical presentation and severity of these epilepsies vary widely, even in people with the same mutation, suggesting the action of environmental or genetic modifiers. To gain support for the hypothesis that genetic modifiers can influence clinical presentation in patients with SCN1A-derived GEFS+, we used mouse models to study the effect of combining the human GEFS+ mutation SCN1A-R1648H with SCN2A, KCNQ2, and SCN8A mutations. Knock-in mice heterozygous for the R1648H mutation (Scn1a(RH/+)) have decreased thresholds to induced seizures and infrequent spontaneous seizures, whereas homozygotes display spontaneous seizures and premature lethality. Scn2a(Q54) transgenic mice have a mutation in Scn2a that results in spontaneous, adult-onset partial motor seizures, and mice carrying the Kcnq2-V182M mutation exhibit increased susceptibility to induced seizures, and rare spontaneous seizures as adults. Combining the Scn1a-R1648H allele with either Scn2a(Q54) or Kcnq2(V182M/+) results in early-onset, generalized tonic-clonic seizures and juvenile lethality in double heterozygous mice. In contrast, Scn8a mutants exhibit increased resistance to induced seizures. Combining the Scn1a-R1648H and Scn8a-med-jo alleles restores normal thresholds to flurothyl-induced seizures in Scn1a(RH/+) heterozygotes and improved survival of Scn1a(RH/RH) homozygotes. Our results demonstrate that variants in Scn2a, Kcnq2, and Scn8a can dramatically influence the phenotype of mice carrying the Scn1a-R1648H mutation and suggest that ion channel variants may contribute to the clinical variation seen in patients with monogenic epilepsy.

Whole-genome linkage scan for epilepsy-related photosensitivity: a mega-analysis

Photoparoxysmal response (PPR) is considered to be a risk factor for idiopathic generalised epilepsy (IGE) and it has a strong genetic basis. Two genome-wide linkage studies have been published before and they identified loci for PPR at 6p21, 7q32, 13q13, 13q31 and 16p13. Here we combine these studies, augmented with additional families, in a mega-analysis of 100 families. Non-parametric linkage analysis identified three suggestive peaks for photosensitivity, two of which are novel (5q35.3 and 8q21.13) and one has been found before (16p13.3). We found no evidence for linkage at four previously detected loci (6p21, 7q32, 13q13 and 13q31). Our results suggest that the different family data sets are not linked to a shared locus. Detailed analysis showed that the peak at 16p13 was mainly supported by a single subset of families, while the peaks at 5q35 and 8q21 had weak support from multiple subsets. Family studies clearly support the role of PPR as a risk factor for IGE. This mega-analysis shows that distinct loci seem to be linked to subsets of PPR-positive families that may differ in subtle clinical phenotypes or geographic origin. Further linkage studies of PPR should therefore include in-depth phenotyping to make appropriate subsets and increase genetic homogeneity.

Epilepsy research 150 years after Darwin's theory of evolution

On February 12, 2009, we commemorated the 200th anniversary of Charles Darwin's birth and the 150th anniversary of the publication of the ûrst edition of the "On the origin of species". Only in the sixth edition of the Origin Darwin explicitly stated that natural selection applied to the brain as to all other organs and contemporary epilepsy research plays an interesting role in this scenario. Epilepsy affects approximately 3 percent of the general population and is a complex disease. At least 11 genes have now been described for human epilepsy and over 50 more genes have been identified in animal models of epilepsy. The complex gene to gene interactions and gene-environment interactions may account for epilepsy susceptibility and antiepileptic drug response. Darwin's thoughts on evolution are relevant to understand these gene interactions, contributing to current development of new treatments and prevention of chronic diseases, such as epilepsy.

A genome-wide association study of hypertension and blood pressure in African Americans

The evidence for the existence of genetic susceptibility variants for the common form of hypertension (“essential hypertension”) remains weak and inconsistent. We sought genetic variants underlying blood pressure (BP) by conducting a genome-wide association study (GWAS) among African Americans, a population group in the United States that is disproportionately affected by hypertension and associated complications, including stroke and kidney diseases. Using a dense panel of over 800,000 SNPs in a discovery sample of 1,017 African Americans from the Washington, D.C., metropolitan region, we identified multiple SNPs reaching genome-wide significance for systolic BP in or near the genes: PMS1, SLC24A4, YWHA7, IPO7, and CACANA1H. Two of these genes, SLC24A4 (a sodium/potassium/calcium exchanger) and CACNA1H (a voltage-dependent calcium channel), are potential candidate genes for BP regulation and the latter is a drug target for a class of calcium channel blockers. No variant reached genome wide significance for association with diastolic BP (top scoring SNP rs1867226, p = 5.8×10⁻⁷) or with hypertension as a binary trait (top scoring SNP rs9791170, p = 5.1×10⁻⁷). We replicated some of the significant SNPs in a sample of West Africans. Pathway analysis revealed that genes harboring top-scoring variants cluster in pathways and networks of biologic relevance to hypertension and BP regulation. This is the first GWAS for hypertension and BP in an African American population. The findings suggests that, in addition to or in lieu of relying solely on replicated variants of moderate-to-large effect reaching genome-wide significance, pathway and network approaches may be useful in identifying and prioritizing candidate genes/loci for further experiments.

Despite intense research, the genetic risk factors for essential hypertension and blood pressure (BP) regulation have not been identified with consistency. We conducted a genome wide association scan using over 800,000 genetic markers in an African American sample of 1,017 adults in the Washington, D.C., area of the United States. We found evidence to suggest that genetic variants in several genes, including PMS1, SLC24A4, YWHA7, IPO7, and CACANA1H, are significantly associated with systolic BP levels. From our previous knowledge of human physiology, two of these genes have potential roles to play in BP regulation. The evidence for genetic variants influencing diastolic BP levels and hypertension status was weaker and inconclusive. To our knowledge, this is the first study that has used a genome-wide association approach to study hypertension and BP in an African American population, a minority group that experiences hypertension more frequently and more severely than other population groups in the United States. The findings will be useful to other researchers seeking to advance our understanding of the genetic factors that influence BP with the hope that these insights will eventually translate to new and better treatment options for hypertension in African Americans and other global populations.

ApoE epsilon4 is not associated with posictal confusion in patients with mesial temporal lobe epilepsy with hippocampal sclerosis

A previous report found an association between ApoE isoforms and postictal confusion in medically intractable temporal lobe epilepsy (TLE). We performed a molecular epidemiology study in an independent sample of 77 TLE patients. We failed to replicate the original allelic association between ApoE epsilon4 allele and postictal confusion in our population (chi(2)=1.67; d.f.=1; p=0.2). Thus, the association between ApoE epsilon4 allele and postictal confusion still needs to be fully investigated in different and independent populations.

Curing epilepsy: progress and future directions

During the past decade, substantial progress has been made in delineating clinical features of the epilepsies and the basic mechanisms responsible for these disorders. Eleven human epilepsy genes have been identified and many more are now known from animal models. Candidate targets for cures are now based upon newly identified cellular and molecular mechanisms that underlie epileptogenesis. However, epilepsy is increasingly recognized as a group of heterogeneous syndromes characterized by other conditions that co-exist with seizures. Cognitive, emotional and behavioral co-morbidities are common and offer fruitful areas for study. These advances in understanding mechanisms are being matched by the rapid development of new diagnostic methods and therapeutic approaches. This article reviews these areas of progress and suggests specific goals that once accomplished promise to lead to cures for epilepsy.

Genome-wide association studies in neurological disorders

BACKGROUND

During the past decade, the genetic causes of monogenic forms of disease have been successfully defined; this work has helped the progression of basic scientific investigation into many disorders, and has helped to characterise several molecular biological processes. An important goal of genetic research is to extend this work and define genetic risk factor loci for complex disorders. The aim is for these data not only to offer further basic understanding of the disease process, but also to provide the opportunity to obtain genetic risk assessments that could be generalised to the public.

RECENT DEVELOPMENTS

The development of resources such as the Human Genome Project and the International Human Haplotype Map Project, coupled with technological advances in ultra-high-throughput genotyping, have provided the basis for genome-wide association studies (GWAS). This approach has been successful for several complex disorders in a short time. Although GWAS are still a new method, these studies have been used for a small number of neurological disorders and, despite varied results for these conditions, GWAS can usefully show the power and limitations of this approach. WHERE NEXT?: GWAS have the potential to show and emphasise common genetic variability associated with disease. However, a challenge of this approach is that large sample series and considerable resources are required. One important consideration will be the interpretation of the results of GWAS in a clinically meaningful way and to discern the implications for all therapy areas, including neurological disorders; this challenge will require specialised skills and resources from both the medical and the scientific communities.

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.

Possible role of the innate immunity in temporal lobe epilepsy

PURPOSE

Temporal lobe epilepsy (TLE) is a multifactorial disease often involving the hippocampus. So far the etiology of the disease has remained elusive. In some pharmacoresistant TLE patients the hippocampus is surgically resected as treatment. To investigate the involvement of the immune system in human TLE, we performed large-scale gene expression profiling on this human hippocampal tissue.

METHODS

Microarray analysis was performed on hippocampal specimen from TLE patients with and without hippocampal sclerosis and from autopsy controls (n = 4 per group). We used a common reference pool design to perform an unbiased three-way comparison between the two patient groups and the autopsy controls. Differentially expressed genes were statistically analyzed for significant overrepresentation of gene ontology (GO) classes.

RESULTS

Three-way analysis identified 618 differentially expressed genes. GO analysis identified immunity and defense genes as most affected in TLE. Particularly, the chemokines CCL3 and CCL4 were highly (>10-fold) upregulated. Other highly affected gene classes include neuropeptides, chaperonins (protein protection), and the ubiquitin/proteasome system (protein degradation).

DISCUSSION

The strong upregulation of CCL3 and CCL4 implicates these chemokines in the etiology and pathogenesis of TLE. These chemokines, which are mainly expressed by glia, may directly or indirectly affect neuronal excitability. Genes and gene clusters identified here may provide targets for developing new TLE therapies and candidates for genetic research.

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.

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.

Novel antiseizure drug mechanisms

The amount of new knowledge being generated regarding brain mechanisms in general, and epileptic mechanisms in particular, is enormous. Anticonvulsant drugs are ineffective in approximately a third of people with epilepsy. To our knowledge, strategies for preventing epilepsy after an initial insult are nonexistent. In this review, we briefly examine some recent novel concepts for preventing seizures, which might lead to enhanced anticonvulsant drug therapy. We start with some known seizure mechanisms that have yet to yield widely used anticonvulsant drugs, including potassium channels, chloride cotransporters, extracellular space constriction, gap junctions and magnesium. Pharmacoresistance is then discussed, focusing on the upregulation of drug-resistance proteins (a concept with significant therapeutic appeal) and the drug-target hypothesis. Two further areas that hold great promise for future therapeutics are sex hormones and inflammatory processes. The genetics of epilepsy are currently being elaborated, providing potential novel anticonvulsant targets. Prevention being better than a cure, we discuss epileptogenesis and its treatment. Given the astounding progress of neuroscience research, one hopes for many new therapeutics for our intractable epileptic patients.

Exploration of the Genetic Architecture of Idiopathic Generalized Epilepsies

PURPOSE

Idiopathic generalized epilepsy (IGE) accounts for approximately 20% of all epilepsies and affects about 0.2% of the general population. The etiology of IGE is genetically determined, but the complex pattern of inheritance suggests an involvement of a large number of susceptibility genes. The objective of the present study was to explore the genetic architecture of common IGE syndromes and to dissect out susceptibility loci predisposing to absence or myoclonic seizures.

METHODS

Genome-wide linkage scans were performed in 126 IGE-multiplex families of European origin ascertained through a proband with idiopathic absence epilepsy or juvenile myoclonic epilepsy. Each family had at least two siblings affected by IGE. To search for seizure type-related susceptibility loci, linkage analyses were carried out in family subgroups segregating either typical absence seizures or myoclonic and generalized tonic-clonic seizures on awakening.

RESULTS

Nonparametric linkage scans revealed evidence for complex and heterogeneous genetic architectures involving linkage signals at 5q34, 6p12, 11q13, 13q22-q31, and 19q13. The signal patterns differed in their composition, depending on the predominant seizure type in the families.

CONCLUSIONS

Our results are consistent with heterogeneous configurations of susceptibility loci associated with different IGE subtypes. Genetic determinants on 11q13 and 13q22-q31 seem to predispose preferentially to absence seizures, whereas loci on 5q34, 6p12, and 19q13 confer susceptibility to myoclonic and generalized tonic-clonic seizures on awakening.