Genome scan of idiopathic generalized epilepsy: Evidence for major susceptibility gene and modifying genes influencing the seizure type

Molecular genetics of febrile seizures

Febrile seizures (FSs) represent the most common form of childhood seizures, occurring in 2-5% of infants in Europe and North America and in 6-9% in Japan. It has been recognized that there is a significant genetic component for susceptibility to this type of seizure. Six susceptibility FS loci have been identified on chromosomes 8q13-q21 (FEB1), 19p (FEB2), 2q23-q24 (FEB3), 5q14-q15 (FEB4), 6q22-q24 (FEB5), and 18p11 (FEB6). Furthermore, mutations in the voltage-gated sodium channel alpha-1, alpha-2 and beta-1 subunit genes (SCN1A, SCN2A and SCN1B) and the GABA(A) receptor gamma-2 subunit gene (GABRG2) have been identified in families with a clinical subset of seizures termed "generalized epilepsy with febrile seizure plus (GEFS+)". However, the causative genes have not been identified in most patients with FSs or GEFS+. Common forms of FSs are genetically complex disorders believed to be influenced by variations in several susceptibility genes. Recently, several association studies in FSs have been reported, but the results vary among different groups and no consistent or convincing FS susceptibility genes have emerged. To find a true association, larger sample size and newer methodologic refinements are recommended.

Concepts in classification and their relevance to epilepsy

The classification of the epilepsies was advanced in order to formalize efforts to identify coherent clinical entities and to develop a standardized set of diagnostic terminology for facilitating communication among workers in the field world-wide. The classification initially was, and still is today, based primarily on description and expert opinion. In the era of genomics, neuroimaging and tremendous technological and scientific advances in the neurosciences, it is time to introduce scientific principles and standards into the classification of the epilepsies. Phylogenetic systematics provides an initial model worth studying in this context. While the classification of species cannot be directly applied to the classification of epilepsy syndromes, three general points can be appreciated. (1) In evolutionary biology, there is an operationalized definition of the end point (a species). There is no such definition of a syndrome. (2) There are rules and criteria for the type of evidence and how it is evaluated to determine whether an entity does or does not represent a separate species. There are currently no such rules or criteria for epilepsy syndromes. (3) There is an underlying model (evolution) that generates the diversity among species. With the possible and only partial exception of the idiopathic generalized epilepsies, there are no models to explain the diversity among the epilepsies. Previously held beliefs and convictions must, in time, give way to dispassionate examination of testable scientific hypotheses examined with rigorously collected scientific evidence. Although current classification is in need of revision, it has also come into common use and has great practical utility. Any changes to its form and to the terminology of the classification must be made only after careful deliberation and broad consensus is reached. The change(s) should be based on agreed upon scientific criteria and processes. Any changes should represent major improvements and not merely incremental steps.

Susceptibility genes for complex epilepsy

Common idiopathic epilepsies are, clinically and genetically, a heterogeneous group of complex seizure disorders. Seizures arise from periodic neuronal hyperexcitability of unknown cause. The genetic component is mostly polygenic, where each susceptibility gene in any given individual is likely to represent a small component of the total heritability. Two susceptibility genes have been so far identified, where genetic variation is associated with experimentally demonstrated changes in ion channel properties, consistent with seizure susceptibility. Rare variants and a polymorphic allele of the T-type calcium channel CACNA1H and a polymorphic allele and a rare variant of the GABA(A) receptor delta subunit gene have differential functional effects. We speculate that these and other as yet undiscovered susceptibility genes for complex epilepsy could act as 'modifier' loci, affecting penetrance and expressivity of the mutations of large effect in those 'monogenic' epilepsies with simple inheritance that segregate through large families. Discovery of epilepsy-associated ion channel defects in these rare families has opened the door to the discovery of the first two susceptibility genes in epilepsies with complex genetics. The susceptibility genes so far detected are not commonly involved in complex epilepsy suggesting the likelihood of considerable underlying polygenic heterogeneity.

Phenotype definition in epilepsy

Phenotype definition consists of the use of epidemiologic, biological, molecular, or computational methods to systematically select features of a disorder that might result from distinct genetic influences. By carefully defining the target phenotype, or dividing the sample by phenotypic characteristics, we can hope to narrow the range of genes that influence risk for the trait in the study population, thereby increasing the likelihood of finding them. In this article, fundamental issues that arise in phenotyping in epilepsy and other disorders are reviewed, and factors complicating genotype-phenotype correlation are discussed. Methods of data collection, analysis, and interpretation are addressed, focusing on epidemiologic studies. With this foundation in place, the epilepsy subtypes and clinical features that appear to have a genetic basis are described, and the epidemiologic studies that have provided evidence for the heritability of these phenotypic characteristics, supporting their use in future genetic investigations, are reviewed. Finally, several molecular approaches to phenotype definition are discussed, in which the molecular defect, rather than the clinical phenotype, is used as a starting point.

Complex inheritance and parent-of-origin effect in juvenile myoclonic epilepsy

BACKGROUND

Juvenile myoclonic epilepsy (JME) is an idiopathic generalized epilepsy (IGE) with complex inheritance. Previous studies have suggested maternal inheritance and female excess in IGEs but have not been specific for JME. We investigated evidence for maternal inheritance, female excess and patterns of familial seizure risk in a well-characterized sample of JME families.

METHODS

We ascertained 89 families through a JME proband and 50 families through a non-JME IGE proband. JME families were divided into those with and without evidence of linkage to the EJM1 susceptibility locus on chromosome 6. We analyzed transmission in 43 multigenerational families, calculated the adjusted sex ratio for JME, and looked for evidence of seizure specific risk in 806 family members.

RESULTS

We found evidence for preferential maternal transmission in both EJM1-linked and unlinked families (2.7:1), evidence even more marked when potential selection factors were excluded. The adjusted female: male risk ratio was very high in JME (RR=12.5; 95% CI: 1.9-83.7). Absence seizures in JME probands increased the overall risk of seizures in first degree relatives (15.8% vs. 7.0%, P=0.011), as well as first-degree relatives' specific risk of absence seizures (6% vs. 1.6%, P=0.01), but not myoclonic seizures.

CONCLUSIONS

We have confirmed the finding of maternal inheritance in JME, which is not restricted to JME families linked to the EJM1 locus. The striking female excess in JME may relate to anatomical and/or endocrine sexual dimorphism in the brain. Evidence for independent inheritance of absence and myoclonic seizures in JME families reinforces a model in which combinations of loci confer susceptibility to the component seizure types of IGE.

Familial clustering of seizure types within the idiopathic generalized epilepsies

OBJECTIVE

To examine the genetic relationships among epilepsies with different seizure types--myoclonic, absence, and generalized tonic-clonic--within the idiopathic generalized epilepsies (IGEs).

BACKGROUND

Careful phenotype definition in the epilepsies may allow division into groups that share susceptibility genes. Examination of seizure type, a phenotypic characteristic less complex than IGE syndrome, may help to define more homogeneous subgroups.

METHODS

Using the approach that found evidence of distinct genetic effects on myoclonic vs absence seizures in families from the Epilepsy Family Study of Columbia University, the authors examined an independent sample of families from Australia and Israel. They also examined the familial clustering of generalized tonic-clonic seizures (GTCs) within the IGEs in two combined data sets. Families were defined as concordant if all affected members had the same type of seizure or IGE syndrome, as appropriate for the analysis performed.

RESULTS

The proportion of families concordant for myoclonic vs absence seizures was greater than expected by chance in the Australian families. In addition, GTCs clustered in families with IGEs to a degree greater than expected by chance.

CONCLUSIONS

These results provide additional evidence for distinct genetic effects on myoclonic vs absence seizures in an independent set of families and suggest that there is a genetic influence on the occurrence of generalized tonic-clonic seizures within the idiopathic generalized epilepsies.

A locus for generalized tonic-clonic seizure susceptibility maps to chromosome 10q25-q26

Inheritance patterns in twins and multiplex families led us to hypothesize that two loci were segregating in subjects with juvenile myoclonic epilepsy (JME), one predisposing to generalized tonic-clonic seizures (GTCS) and a second to myoclonic seizures. We tested this hypothesis by performing genome-wide scan of a large family (Family 01) and used the results to guide analyses of additional families. A locus was identified in Family 01 that was linked to GTCS (10q25-q26). Model-based multipoint analysis of the 10q25-q26 locus showed a logarithm of odds (LOD) score of 2.85; similar results were obtained with model-free analyses (maximum nonparametric linkage [NPL] of 2.71; p = 0.0019). Analyses of the 10q25-q26 locus in 10 additional families assuming heterogeneity revealed evidence for linkage in four families; model-based and model-free analyses showed a heterogeneity LOD (HLOD) of 2.01 (alpha = 0.41) and maximum NPL of 2.56 (p = 0.0027), respectively, when all subjects with GTCS were designated to be affected. Combined analyses of all 11 families showed an HLOD of 4.04 (alpha = 0.51) and maximum NPL score of 4.20 (p = 0.000065). Fine mapping of the locus defined an interval of 4.45Mb. These findings identify a novel locus for GTCS on 10q25-q26 and support the idea that distinct loci underlie distinct seizure types within an epilepsy syndrome such as JME.

Pathophysiology of mental illness: A view from the fourth ventricle

The concept of mental disorders as diseases of the brain is as old as the ancient Greek philosopher-physicians. However, for thousands of years, the majority of doctors, as well as laypersons, held strongly to the belief that epilepsy and "madness" (i.e., schizophrenia)--the major disorders discussed in this paper--were caused by demonic "possession." As always, the theory of causation led to apposite therapies: Cast out the devil by whatever means necessary. Later, more enlightened views of etiology led to less punitive "cures," which, to modern sensibilities, still seem barbaric. The 20th century saw the introduction of medications that provide symptomatic relief, if not cures, for seizure disorders and schizophrenia. In this paper, we consider the etiology of absence (petit mal) epilepsy and schizophrenia based on the pathophysiology underlying the shared symptom of impaired sustained attention. We emphasize the role of abnormal functioning of brainstem structures in the region of the fourth ventricle, whether caused by genetic or environmental factors or a combination thereof. Our theorizing relies on the findings of Lindsley, Magoun, and Moruzzi, who delineated the role of the brainstem reticular activating system in sleep, wakefulness, and consciousness. It also relies on the work of Penfield, Jasper, and Gloor, who sought to illuminate the role of brainstem-thalamus-cortical dysfunction in idiopathic generalized epilepsies. We consider evidence from recent studies that emphasize the phasic attentional functions supported by brainstem structures in the region of the fourth ventricle, and possible genetic links among disorders in which impaired attention is a prominent symptom.

A new EF-hand containing gene EFHC2 on Xp11.4: Tentative evidence for association with juvenile myoclonic epilepsy

Genetic factors play a major role in the etiology of idiopathic generalized epilepsies (IGE). An oligogenic or polygenic predisposition is suspected in the majority of families with common IGE syndromes. It has been hypothesized that some IGE genes might increase the general level of neuronal excitability while others specify the age of onset and the seizure type. The EFHC1 gene on 6p12-p11 was previously described as the first susceptibility gene for juvenile myoclonic epilepsy (JME). EFHC1 codes for a protein of unknown function that is characterized by Ca2+-binding EF-hand motifs and DM10 domains. We have now cloned the brain-expressed paralog EFHC2 (Xp11.3) and carried out an association study of six single nucleotide polymorphisms (SNPs) in a large sample of 654 German IGE patients and 662 population controls. A tentative association was found between the amino acid exchange S430Y in exon 9 of EFHC2 and 97 male JME patients (chi2=4.705, d.f.=1, P=0.030; OR=2.17; 95-CI: 1.06-4.43). The allelic association was even stronger for the 81 males with "classical" JME (JME without absence seizures) (chi2=6.06, d.f.=1, P=0.014; OR=2.46; 95-CI: 1.18-5.13). An association with the gonosomal gene EFHC2 would be in accordance with the observed preponderance of maternal inheritance in JME maternal inheritance of JME. Independent replication studies are needed to further analyse the tentative association described here.

Supportive evidence for an allelic association of the human KCNJ10 potassium channel gene with idiopathic generalized epilepsy

PURPOSE

Quantitative trait loci (QTL) mapping in mice revealed a seizure-related QTL (Szs1), for which the inward-rectifying potassium channel Kcnj10 is the most compelling candidate gene. Association analysis of the human KCNJ10 gene identified a common KCNJ10 missense variation (Arg271Cys) that influences susceptibility to focal and generalized epilepsies. The present replication study tested the initial finding that the KCNJ10 Cys271 allele is associated with seizure resistance to common syndromes of idiopathic generalized epilepsy (IGE).

METHODS

The study sample comprised 563 German IGE patients and 660 healthy population controls. To search for seizure type-specific effects, two IGE subgroups were formed, comprising 258 IGE patients with typical absences (IAE group) and 218 patients with juvenile myoclonic epilepsy (JME group). A TaqMan nuclease assay was used to genotype the KCNJ10 single nucleotide polymorphism c.1037C > T (dbSNP: rs1130183) that alters amino acid at position 271 from arginine to cysteine.

RESULTS

Replication analysis revealed a significant decrease of the Cys271 allele frequency in 446 IGE patients compared to controls (chi2 = 3.52, d.f. = 1, P = 0.030, one-sided; OR(Cys271+) = 0.69; 95% CI: 0.50-0.95). Among the IGE subgroups, lack of the Cys271 allele was accentuated in the JME group (chi2 = 5.20, d.f. = 1, P = 0.011, one-sided).

CONCLUSION

Our results support previous evidence that the common KCNJ10 Arg271Cys missense variation influences seizure susceptibility of common IGE syndromes.

Malic enzyme 2 may underlie susceptibility to adolescent-onset idiopathic generalized epilepsy

Idiopathic generalized epilepsy (IGE) is a class of genetically determined, phenotypically related epilepsy syndromes. Linkage analysis identified a chromosome 18 locus predisposing to a number of adolescent-onset IGEs. We report a single-nucleotide polymorphism (SNP) association analysis of the region around the marker locus with the high LOD score. This analysis, which used both case-control and family-based association methods, yielded strong evidence that malic enzyme 2 (ME2) is the gene predisposing to IGE. We also observed association among subgroups of IGE syndromes. An ME2-centered nine-SNP haplotype, when present homozygously, increases the risk for IGE (odds ratio 6.1; 95% confidence interval 2.9-12.7) compared with any other genotype. Both the linkage analysis and the association analysis support recessive inheritance for the locus, which is compatible with the fact that ME2 is an enzyme. ME2 is a genome-coded mitochondrial enzyme that converts malate to pyruvate and is involved in neuronal synthesis of the neurotransmitter gamma-aminobutyric acid (GABA). The results suggest that GABA synthesis disruption predisposes to common IGE and that clinical seizures are triggered when mutations at other genes, or perhaps other insults, are present.

Clinical and genetic aspects of idiopathic epilepsies in childhood

The identification of the first genes associated with idiopathic epilepsy has been an important breakthrough in the field of epilepsy research. In almost all cases these genes were found to encode components of voltage- or ligand-gated ion channels or functionally related structures. For many other idiopathic syndromes, there is linkage evidence to one or more chromosomes, but the genes have not yet been identified. Identification of the responsible genes and their gene products will further increase the knowledge of the pathogenic mechanisms involved in epilepsy, and will hopefully facilitate the development of drug targets for the effective treatment of epilepsy. This review gives an overview of the clinical characteristics and an update of genetic research of those idiopathic childhood epilepsies for which genes have been identified and the monogenic idiopathic childhood epilepsies for which mapping data are available.

Genome-wide linkage scan of epilepsy-related photoparoxysmal electroencephalographic response: evidence for linkage on chromosomes 7q32 and 16p13

Photoparoxysmal response (PPR) is an abnormal visual sensitivity of the brain in reaction to intermittent photic stimulation. It is an epilepsy-related electroencephalographic trait with high prevalence in idiopathic epilepsies, especially in common idiopathic generalized epilepsies (IGEs), such as childhood absence epilepsy and juvenile myoclonic epilepsy. This degree of co-morbidity suggests that PPR may be involved in the predisposition to IGE. The identification of genes for PPR would, therefore, aid the dissection of the genetic basis of IGE. Sixteen PPR-multiplex families were collected to conduct a genome-wide linkage scan using broad (all PPR types) and narrow (exclusion of PPR types I and II and the occipital epilepsy cases) models of affectedness for PPR. We found an empirical genome-wide significance for parametric (HLOD) and non-parametric (NPL) linkage (Pgw(HLOD)=0.004 and Pgw(NPL)=0.01) for two respective chromosomal regions, 7q32 at D7S1804 (HLOD=3.47 with alpha=1, P(NPL)=3.39x10(-5)) and 16p13 at D16S3395 (HLOD=2.44 with alpha=1, P(NPL)=7.91x10(-5)). These two genomic regions contain genes that are important for the neuromodulation of cortical dynamics and may represent good targets for candidate-gene studies. Our study identified two susceptibility loci for PPR, which may be related to the underlying myoclonic epilepsy phenotype present in the families studied.

Autosomal dominant early childhood seizures associated with chondrocalcinosis and a mutation in the ANKH Gene

We describe the pattern of early childhood seizures within a family with autosomal dominant chondrocalcinosis (CCAL, which causes adult-onset arthritis). All affected family members with CCAL experienced seizures in early childhood, usually, but not always, associated with fever. Similarities exist to the syndrome of generalized epilepsy with febrile seizures plus (GEFS+). A mutation within the ANKH gene on chromosome 5p has been found previously in this family; other patients with familial CCAL (but without seizures) have mutations in the same gene. ANKH codes for a transmembrane protein involved in the regulation of extracellular pyrophosphate ion levels, although its precise mechanism of action remains unclear. It is highly expressed in the brain, and its expression may be influenced by seizure activity. The mutation within this family creates a premature initiation codon, adding four amino acids to the N-terminus of the protein. We postulate that this may lead to a gain of function, causing seizure susceptibility as well as chondrocalcinosis. Mutations within this gene may underlie other forms of genetic epilepsy and febrile seizures.

Convulsive disorder and genetic polymorphism. Association of idiopathic generalized epilepsy with haptoglobin polymorphism

Haptoglobin is a polymorphic protein that is well known for its hemoglobin (Hb)-binding property. The protein shows gross differences in molecular size among genotypes, resulting in different degrees of diffusion in central nervous system tissue. Since the breakdown of erythrocytes in the intracerebral fluid results in Hb-mediated free OH radical formation, lipid peroxidation, and increased neuronal excitability, a differential diffusion of haptoglobin phenotypes in the intracerebral fluid might result in a different degree of protection from oxidative damage. We have studied two samples of children with idiopathic generalized epilepsy from two different Italian populations. In both samples the haptoglobin *1/*1 genotype is much less represented in epileptic children than in controls. These observations suggest that subjects carrying the Hp*1/*1 genotype, that has the lowest molecular size and diffuses more readily in the interstitial cerebral fluid, are more protected against idiopathic generalized epilepsy than those with other haptoglobin genotypes.

Genes and mutations in human idiopathic epilepsy

Thirteen genes have already been identified in human idiopathic epilepsies since 1995, but they account only for a minority of all epilepsy cases. Most of these genes are associated with rare monogenic epilepsy syndromes, but some of them contribute to the common epilepsy subtypes. The questions remains to be answered how many more epilepsy genes exist in brain. Idiopathic epilepsies are common neurological disorders, and it can therefore be expected that the total number of genes associated with an increased seizure susceptibility is much higher than 13. Most of the known genes code for either voltage-gated or ligand gated ion channels, but recently two epilepsy genes have been found which do not fit into the concept of epilepsies as channelopathies. It can therefore be suspected that more than one pathogenetic concept exists in epileptogenesis.

Monogenic idiopathic epilepsies

Major advances have recently been made in our understanding of the genetic bases of monogenic inherited epilepsies. Direct molecular diagnosis is now possible in numerous inherited symptomatic epilepsies. Progress has also been spectacular with respect to several idiopathic epilepsies that are caused by mutations in genes encoding subunits of ion channels or neurotransmitter receptors. Although these findings concern only a few families and sporadic cases, their potential importance is great, because these genes are implicated in a wide range of more common epileptic disorders and seizure types as well as some rare syndromes. Functional studies of these mutations, while leading to further progress in the neurobiology of the epilepsies, will help to refine genotype-phenotype relations and increase our understanding of responses to antiepileptic drugs. In this article, we review the clinical and genetic data on most of the idiopathic human epilepsies and epileptic contexts in which the association of epilepsy and febrile convulsions is genetically determined.

MRS shows syndrome differentiated metabolite changes in human-generalized epilepsies

OBJECTIVE

While it is generally accepted that the thalamo-cortical loop is abnormal in idiopathic generalized epilepsy (IGE), it is uncertain whether this loop is similarly affected among different IGE syndromes. We recently demonstrated reduced frontal lobe levels of N-acetyl aspartate (NAA) in patients with juvenile myoclonic epilepsy (JME). The present follow-up study investigates if similar or other types of changes exist in subjects with pure primarily generalized tonic clonic epilepsy (GTCS).

METHOD

Twenty patients with GTCS, 26 patients with JME, and 10 matched healthy controls were investigated with quantitative single voxel MR spectroscopy (MRS) measurements of NAA, choline (Cho), creatine (Cr), and myo-inositol (mI) at 1.5 T scanner. The voxels were placed over the right cerebellum, right thalamus, prefrontal, occipital cortex, and over a spherical phantom above the subject's head.

RESULTS

Patients with JME had reduced frontal lobe NAA (mmol/l) in relation to controls (9.8 +/- 1.1 vs. 10.8 +/- 0.7, P = 0.01), as well as GTCS patients (9.8 +/- 1.1 vs. 10.6 +/- 0.7, P = 0.007), whose values were normal. Patients with GTCS, on the other hand, showed significantly lower thalamic NAA than controls (9.7 +/- 1.0 vs. 10.8 +/- 0.9, P = 0.002), and both groups of patients had reduced thalamic Cho, and mI; [CHO: 2.0 +/- 0.4 (control) vs. 1.61 +/- 0.3 (JME) P = 0.001, and vs. 1.57 +/- 0.3 (GTCS) P = 0.0005; MI: 4.8 +/- 1.5 (control) vs. 3.3 +/- 1.4 (JME) P = 0.003, and vs. 3.2 +/- 1.5 (GTCS), P = 0.002]. No other regional changes were observed.

CONCLUSION

The present MRS data emphasize the involvement of thalamus in IGE. They also show partly differentiated alterations within the thalamo-cortical loop in JME vs. GTCS. The various clinical expressions of IGE may, thus, be associated with more localized neuroanatomical substrates than generally believed.

GABA, gamma-hydroxybutyric acid, and neurological disease

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

Nonsyndromic seizure disorders: epilepsy and the use of the internet to advance research

The progress in understanding the genetics of nonsyndromic epilepsy is the direct result of dramatic advances made by the Human Genome Project. The development of thousands of precisely mapped genetic markers and the nearly complete sequencing of the entire human genome in 2001 allowed genetic researchers in epilepsy to identify many loci and genes as causal in inherited idiopathic epilepsy. This substantial increase in information has required the development of accurate and online bioinformatic databases. Only the Internet can enable such large amounts of precise DNA sequence information to be transferred to researchers. Along with the construction of these databases has been the development of efficient search algorithms for specific DNA sequences and genetic information. This article summarizes the effect that this burst of new genomic information has had on research aimed at discovering the underlying genetic factors for nonsyndromic epilepsy. Many of the web sites important to epilepsy gene discovery are listed and discussed in this article, including sites with extensive information on genetic markers, genetic analysis, gene sequence, gene expression, gene mutations, and DNA sequence variation. Continued acquisition of information on naturally occurring DNA sequence variants will greatly help research directed towards understanding the genetic susceptibility of the common, nonsyndromic epilepsies and will lead to the promise of personalized medicine.

Common and unique susceptibility loci in Graves and Hashimoto diseases: results of whole-genome screening in a data set of 102 multiplex families

The autoimmune thyroid diseases (AITDs), comprising Graves disease (GD) and Hashimoto thyroiditis (HT), develop as a result of a complex interaction between predisposing genes and environmental triggers. Previously, we identified six loci that showed evidence for linkage with AITD in a data set of 56 multiplex families. The goals of the present study were to replicate/reject the previously identified loci before fine mapping and sequencing the candidate genes in these regions. We performed a whole-genome linkage study in an expanded data set of 102 multiplex families with AITD (540 individuals), through use of 400 microsatellite markers. Seven loci showed evidence for linkage to AITD. Three loci, on chromosomes 6p, 8q, and 10q, showed evidence for linkage with both GD and HT (maximum multipoint heterogeneity LOD scores [HLOD] 2.0, 3.5, and 4.1, respectively). Three loci showed evidence for linkage with GD: on 7q (HLOD 2.3), 14q (HLOD 2.1), and 20q (LOD 3.3, in a subset of the families). One locus on 12q showed evidence of linkage with HT, giving an HLOD of 3.4. Comparison with the results obtained in the original data set showed that the 20q (GD-2) and 12q (HT-2) loci continued to show evidence for linkage in the expanded data set; the 6p and 14q loci were located within the same region as the previously identified 6p and 14q loci (AITD-1 and GD-1, respectively), but the Xq (GD-3) and 13q (HT-1) loci were not replicated in the expanded data set. These results demonstrated that multiple genes may predispose to GD and HT and that some may be common to both diseases and some are unique. The loci that continue to show evidence for linkage in the expanded data set represent serious candidate regions for gene identification.

BRD2 (RING3) is a probable major susceptibility gene for common juvenile myoclonic epilepsy

Juvenile myoclonic epilepsy (JME) is a common form of generalized epilepsy that starts in adolescence. A major JME susceptibility locus (EJM1) was mapped to chromosomal region 6p21 in three independent linkage studies, and association was reported between JME and a microsatellite marker in the 6p21 region. The critical region for EJM1 is delimited by obligate recombinants at HLA-DQ and HLA-DP. In the present study, we found highly significant linkage disequilibrium (LD) between JME and a core haplotype of five single-nucleotide-polymorphism (SNP) and microsatellite markers in this critical region, with LD peaking in the BRD2 (RING3) gene (odds ratio 6.45; 95% confidence interval 2.36-17.58). DNA sequencing revealed two JME-associated SNP variants in the BRD2 (RING3) promoter region but no other potentially causative coding mutations in 20 probands from families with positive LOD scores. BRD2 (RING3) is a putative nuclear transcriptional regulator from a family of genes that are expressed during development. Our findings strongly suggest that BRD2 (RING3) is EJM1, the first gene identified for a common idiopathic epilepsy. These findings also suggest that abnormalities of neural development may be a cause of common idiopathic epilepsy, and the findings have implications for the generalizability of proposed pathogenetic mechanisms, derived from diseases that show Mendelian transmission, to their complex counterparts.

The genetics of human epilepsy

In recent years genetic discoveries have shown the central role of ion channels in the pathophysiology of idiopathic epilepsies. Uncommon epilepsy syndromes that have monogenic inheritance are associated with mutations in genes that encode subunits of voltage-gated and ligand-gated ion channels. For voltage-gated ion channels, mutations of Na(+), K(+) and Cl(-) channels are associated with forms of generalized epilepsy and infantile seizure syndromes. Ligand-gated ion channels, such as nicotinic acetylcholine receptors and GABA receptor subunits, are associated with specific syndromes of frontal and generalized epilepsies, respectively. Striking features are the variable epilepsy phenotypes that are associated with the known gene mutations and the genetic heterogeneity that underlies all known monogenic syndromes. Mutations in two genes that do not encode ion channels have been identified in the idiopathic human epilepsies. The heterogeneity of mutations described to date has precluded the development of simple diagnostic tests, but advances in the next few years are likely to have an impact on both the clinical diagnosis and the treatment of epilepsies.

The genetic and molecular basis of epilepsy

In the past decade, studies of large families in which epilepsy has been inherited in an autosomal dominant fashion have revealed several mutated genes, most of which encode ion channel subunits. Despite these exciting findings, only a few families with similar phenotypes have mutations in these known genes. More frustrating has been the genetic research into idiopathic epilepsies with complex inheritance. Although these forms are more common than those with Mendelian inheritance, their unknown mode of inheritance, phenotypic heterogeneity and the uncertainty of the genetic overlap among syndrome subtypes have hampered gene mapping. New techniques of molecular analysis could help the dissection of genes for epilepsies with complex inheritance. Hopefully, in the near future, successful genetic studies will make possible the discovery of new and more-targeted anti-epileptic drugs.

Phenotypic features of familial febrile seizures: case-control study

OBJECTIVE

To identify phenotypic features of febrile seizures that can be used to reduce heterogeneity and thereby increase power in linkage analysis.

BACKGROUND

Despite exciting discoveries in several rare pedigrees, the genetic basis of common febrile seizures remains a mystery. The major drawback of studying common febrile seizure disorder is etiologic and genetic heterogeneity. A linkage sample must therefore be classified a priori on phenotypic criteria likely to reflect genetically homogeneous subgroups.

METHODS

Eighty-three cases (children with one or more febrile seizure plus first-degree family history of febrile seizures) and 101 controls (children with one or more febrile seizure but no first-degree family history of febrile seizures) were compared for association of phenotypic features in an unmatched case-control design. Odds ratios were calculated using univariate and multivariate methods.

RESULTS

Recurrent febrile seizures was the only phenotypic feature significantly associated with first-degree family history of febrile seizures (OR 2.1, 95% CI 1.15 to 3.88). First-degree family history and later occurrence of afebrile seizures (OR 3.47, 95% CI 0.94 to 12.78) were independently associated with recurrent febrile seizures. Complex features did not show familial aggregation.

CONCLUSIONS

The authors suggest recurrent and afebrile seizures as criteria on which to subgroup a linkage sample. These subgroups will not be evident at the time of the initial febrile seizure. Meticulous and prospective collection of phenotypic and family data are recommended.

Childhood absence epilepsy and febrile seizures: a family with a GABA(A) receptor mutation

Although several genes for idiopathic epilepsies from families with simple Mendelian inheritance have been found, genes for the common idiopathic generalized epilepsies, where inheritance is complex, presently are elusive. We studied a large family with epilepsy where the two main phenotypes were childhood absence epilepsy (CAE) and febrile seizures (FS), which offered a special opportunity to identify epilepsy genes. A total of 35 family members had seizures over four generations. The phenotypes comprised typical CAE (eight individuals); FS alone (15), febrile seizures plus (FS(+)) (three); myoclonic astatic epilepsy (two); generalized epilepsy with tonic-clonic seizures alone (one); partial epilepsy (one); and unclassified epilepsy despite evaluation (two). In three remaining individuals, no information was available. FS were inherited in an autosomal dominant fashion with 75% penetrance. The inheritance of CAE in this family was not simple Mendelian, but suggestive of complex inheritance with the involvement of at least two genes. A GABA(A) receptor gamma2 subunit gene mutation on chromosome 5 segregated with FS, FS(+) and CAE, and also occurred in individuals with the other phenotypes. The clinical and molecular data suggest that the GABA(A) receptor subunit mutation alone can account for the FS phenotype. An interaction of this gene with another gene or genes is required for the CAE phenotype in this family. Linkage analysis for a putative second gene contributing to the CAE phenotype suggested possible loci on chromosomes 10, 13, 14 and 15. Examination of these loci in other absence pedigrees is warranted.

Exploration of a putative susceptibility locus for idiopathic generalized epilepsy on chromosome 8p12

PURPOSE

A recent genome-wide scan revealed a major susceptibility locus for idiopathic generalized epilepsies (IGEs) in the chromosomal region 8p12 in 32 IGE families without members with juvenile myoclonic epilepsy (JME). This study explored the presence of an IGE locus in the chromosomal region 8p12.

METHODS

Our study included 176 multiplex families of probands with common IGE syndromes. Parametric and nonparametric multipoint linkage analyses were carried out between the IGE trait and six microsatellite polymorphisms encompassing the putative susceptibility locus. To explore the associated phenotype-genotype relation, two distinct subgroups of families were selected by the presence (n = 64) or absence (n = 112) of a family member with JME. To adjust the phenotypic spectrum toward adolescent-onset IGEs, a third subgroup of 28 families without JME was chosen through an IGE proband with seizure onset at age 10-20 years.

RESULTS

Parametric and nonparametric multipoint linkage analyses provided no evidence for linkage between IGE and markers encompassing the putative IGE locus in the chromosomal region 8p12. Furthermore, we found no hint of linkage along the candidate region in any of the three family subgroups.

CONCLUSIONS

We failed to provide evidence for a major IGE locus in the chromosomal region 8p12. On the contrary, these parametric linkage results provide strong evidence against linkage across the candidate region under a broad range of genetic models. If there is a susceptibility locus for IGE in the chromosomal region 8p12, then the size of the effect or the proportion of linked families is too small to detect linkage in the investigated family sample.

Convulsive disorder and the genetics of signal transduction; a study of a low molecular weight protein tyrosine phosphatase in a pediatric sample

Recent studies point to an involvement of kinases and phosphatases in ionic channel regulation and in physiopathologic mechanisms leading to convulsive disorders. Acid phosphatase locus 1 (ACP1), also named cytosolic low molecular weight phosphotyrosine phosphatase, is a highly polymorphic phosphatase that is especially abundant in the central nervous system and is known to be involved in several signal transduction pathways. We studied ACP1 in 122 children with idiopathic generalized tonic-clonic seizures, 80 children with febrile convulsions, and 417 controls from the population of Rome. Low activity phenotypes of ACP1 (*A/*A and *A/*B) were found to be over-represented while high activity phenotypes (*C/*C and *B/*C) were under-represented in generalized seizures cases compared to controls (P < 0.005). No significant difference was observed between febrile convulsion cases and controls. These observations suggest a protective role of the high activity ACP1 phenotypes against seizures in children.

Association of the 867Asp variant of the human anion exchanger 3 gene with common subtypes of idiopathic generalized epilepsy

Genetic factors play a major role in the etiology of idiopathic generalized epilepsies (IGE). Our recent genome-wide search revealed suggestive evidence for a susceptibility locus for common IGE syndromes in the chromosomal region 2q36. The gene encoding the anion exchanger isoform 3 (AE3; gene symbol: SLC4A3) has been mapped to this candidate region. AE3 is prominently expressed in the brain and performs an electroneutral exchange of chloride and bicarbonate. To study the potential role of AE3 in the epileptogenesis of IGE, we performed a mutation analysis of the AE3 coding region, including the adjacent exon/intron boundaries, and the 5'-untranslated region in 16 IGE probands of families linked to chromosome 2q36 (cumulative two-point lod score: Z=5.32 at D2S371). Three exonic sequence variants were found: exon 17: 2600C/A, Ala867Asp; exon 21: 3391C/T, Leu1131Leu; exon 23: 3771G/A, 3'-UTR. Our subsequent population-based association study of the Ala867Asp substitution polymorphism revealed a significant increase of the 867Asp variant in 366 unrelated German IGE patients compared with 183 German control subjects (chi(2)=5.37, df=1, P=0.021). Consistently, the transmission disequilibrium test (TDT) of 121 parent-child trios showed a significant preferential transmission of the 867Asp allele (McNemar chi(2)=5.81, df=1, P=0.016). Our results support the hypothesis that variation of the AE3 gene confers a common but small susceptibility effect to the etiology of a broad spectrum of IGE syndromes.

Clinical expression and EEG features of patients with juvenile myoclonic epilepsy (JME) from North India

We aimed to characterize the clinical profile, EEG features and response to treatment of juvenile myoclonic epilepsy (JME) patients. We studied 103 JME probands with a standard protocol recording age of onset, type, frequency of seizures, EEG data, detailed family history and response to treatment in a superspeciality university hospital in New Delhi. The mean age of onset of disease was 14.01 +/- 3.14 years with a male to female ratio of 1.19 : 1. The myoclonic jerks were present in all the probands, generalized tonic-clonic seizures (GTCS) were present in 75.72% and 11.65% probands had absence seizures. The incidence of febrile convulsion (FC) was higher (9.7%) in our JME probands reflecting some ethnic variation or ascertainment bias. There was a considerable delay (of approximately 5.26 +/- 4.61 years) in the diagnosis of JME in our probands because most of the patients were referred from private physicians who were possibly not familiar with this epileptic syndrome in this part of the world or thought it was a milder variety of GTCS. The family history was positive in 25.24% JME probands among first- and second-degree relatives. An interictal EEG was found to be abnormal in 81 (78.64%) patients with the predominant abnormality being generalized polyspike and wave (PSW) discharges (39.80% probands). The majority of patients (80.58%) showed a good response to treatment with valproate alone. There was a subset of patients (11.65%) who required the addition of other antiepileptic drugs (AEDs) for control of GTCS: 7.76% of JME patients were diagnosed as cases of GTCS by private practitioners before they were registered in our study and their seizures were well controlled on other AEDs (without valproate) prescribed by the referring physicians (carbamazepine-4, phenytoin-2, clobazam-2). It is concluded that the clinical features and EEG data of JME probands were comparable to reports from other parts of the world except for the fact that the incidence of FC was higher in our JME patients. There was a delay in the diagnosis of JME due to unfamiliarity with the epileptic syndrome among private practitioners in this part of the world. There was a subset of JME patients who had complete seizure control on other AEDs besides valproate.

No evidence for a susceptibility locus for idiopathic generalized epilepsy on chromosome 5 in families with typical absence seizures

A recent genome-wide scan revealed suggestive evidence for two susceptibility loci for idiopathic generalized epilepsy (IGE) in the chromosomal regions 5p15 and 5q14-q22 in families with typical absence seizures. The present replication study tested the validity of the tentative IGE loci on chromosome 5. Our study included 99 multiplex families in which at least one family member had typical absence seizures. Parametric and non-parametric multipoint linkage analyses were carried out between the IGE trait and 23 microsatellite polymorphisms covering the entire region of chromosome 5. Multipoint parametric heterogeneity lod scores < -2 were obtained along chromosome 5 when a proportion of linked families greater than 50% was assumed under recessive inheritance and > 60% under dominant inheritance. Furthermore, non-parametric multipoint linkage analyses revealed no hint of linkage throughout the candidate region (P > 0.05). Accordingly, we failed to support previous evidence for common IGE loci on chromosome 5. If there is a susceptibility locus for IGE on chromosome 5 then the size of the effect or the proportion of linked families is too small to detect linkage in the investigated family sample.

The novel EPTP repeat defines a superfamily of proteins implicated in epileptic disorders

Recent studies suggest that mutations in the LGI1/Epitempin gene cause autosomal dominant lateral temporal epilepsy. This gene encodes a protein of unknown function, which we postulate is secreted. The LGI1 protein has leucine-rich repeats in the N-terminal sequence and a tandem repeat (which we named EPTP) in its C-terminal region. A redefinition of the C-terminal repeat and the application of sensitive sequence analysis methods enabled us to define a new superfamily of proteins carrying varying numbers of the novel EPTP repeats in combination with various extracellular domains. Genes encoding proteins of this family are located in genomic regions associated with epilepsy and other neurological disorders.

No evidence for a susceptibility locus for idiopathic generalized epilepsy on chromosome 18q21.1

A recent genome-wide scan showed strong evidence for a major locus for common syndromes of idiopathic generalized epilepsy (IGE) at the marker D18S474 on chromosome 18q21.1 (LOD score 4.5/5.2 multipoint/two-point). The present replication study tested the presence of an IGE locus in the chromosomal region 18q21.1. Our linkage study included 130 multiplex families of probands with common IGE syndromes. Eleven microsatellite polymorphisms encompassing a candidate region of 30 cM on either side of the marker D18S474 were genotyped. The two-point homogeneity LOD score for D18S474 showed strong evidence against linkage at the original linkage peak (Z = -18.86 at theta(m = f) = 0.05), assuming a recessive mode of inheritance with 50% penetrance. Multipoint parametric heterogeneity LOD scores < -2 were obtained along the candidate region when proportions of linked families greater than 35% were assumed under recessive inheritance. Furthermore, non-parametric multipoint linkage analyses showed no hint of linkage throughout the candidate region (P > 0.19). Accordingly, we failed to support evidence for a major IGE locus in the chromosomal region 18p11-18q23. If there is a susceptibility locus for IGE in this region then the size of the effect or the proportion of linked families is too small to detect linkage in the investigated family sample.

Childhood absence epilepsy: genes, channels, neurons and networks

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

Decreased motor unit number estimates in juvenile myoclonic epilepsy

In this study, motor unit number estimate (MUNE) analysis with McComa's technique was used to detect any change in lower motor neuron count in juvenile myoclonic epilepsy (JME). The study included 10 JME patients, 8 idiopathic generalized epilepsy (IGE) patients, 7 patients with mesial temporal sclerosis (MTS), and 15 normal subjects. All the patients and normal subjects were subjected to MUNE analysis on the abductor pollicis brevis and tibialis anterior muscles, as well as needle electromyography and nerve conduction studies. Electromyographic and nerve conduction studies were normal in all groups. MUNEs in the JME group and were significantly lower than those of normal subjects (P < 0.001). MUNEs of IGE and MTS patients also tended to be lower than that of normal subjects with no significant difference from normal subjects. MUNEs of abductor pollicis brevis and tibialis anterior muscles were 114 +/- 24 and 90 +/- 15 for normal subjects, 59 +/- 18 and 50 +/- 23 for JME patients, 91 +/- 22 and 75 +/- 19 for IGE patients, and 84 +/- 42 and 80 +/- 29 for MTS patients respectively. It may be reasonable to suggest that a genetic origin is responsible for a tendency to contract epilepsy, and the disorganization of lower motor neurons may be shared in JME.

Genetics of epilepsy

Understanding the molecular biology of epilepsy is a challenge for modern science. Epilepsy results from alternations in fundamental mechanisms of brain and membrane function. Although an understanding of the mode of inheritance and the etiology of genetic epilepsy syndromes forms the basis for genetic counseling, the development of specific therapies will come from knowing the basic mechanisms of epilepsy. Defining the genes causing epilepsy requires an unambiguous definition of seizure phenotype, along with the stability of that trait, an unremitting clinical course, and an abundance of clinical material. This article reviews the task of defining the genetics of epilepsy and discusses genetic methodology, idiopathic generalized and localization-related partial epilepsies, neuronal migration disorders, progressive myoclonus epilepsies, molecular biology of epileptogenesis, and future research.

Four new families with autosomal dominant partial epilepsy with auditory features: clinical description and linkage to chromosome 10q24

PURPOSE

Autosomal dominant partial epilepsy with auditory features (ADPEAF) is a rare form of nonprogressive lateral temporal lobe epilepsy characterized by partial seizures with auditory disturbances. The gene predisposing to this syndrome was localized to a 10-cM region on chromosome 10q24. We assessed clinical features and linkage evidence in four newly ascertained families with ADPEAF, to refine the clinical phenotype and confirm the genetic localization.

METHODS

We genotyped 41 individuals at seven microsatellite markers spanning the previously defined 10-cM minimal genetic region. We conducted two-point linkage analysis with the ANALYZE computer package, and multipoint parametric and nonparametric linkage analyses as implemented in GENEHUNTER2.

RESULTS

In the four families, the number of individuals with idiopathic epilepsy ranged from three to nine. Epilepsy was focal in all of those with idiopathic epilepsy who could be classified. The proportion with auditory symptoms ranged from 67 to 100%. Other ictal symptoms also were reported; of these, sensory symptoms were most common. Linkage analysis showed a maximum 2-point LOD score of 1.86 at (theta=0.0 for marker D10S603, and a maximum multipoint LOD score of 2.93.

CONCLUSIONS

These findings provide strong confirmation of linkage of a gene causing ADPEAF to chromosome 10q24. The results suggest that the susceptibility gene has a differential effect on the lateral temporal lobe, thereby producing the characteristic clinical features described here. Molecular studies aimed at the identification of the causative gene are underway.

A novel gene in the chromosomal region for juvenile myoclonic epilepsy on 6p12 encodes a brain-specific lysosomal membrane protein

Juvenile myoclonic epilepsy (JME) is the most frequent and, hence, most important form of hereditary grand mal epilepsy. Genetic linkage, haplotype, and recombination analyses have indicated that 6p11-12 (EJM1) is one of the candidate regions harboring a gene responsible for JME. In efforts to identify a gene responsible for JME, we identified several expressed sequences in the EJM1 critical region. Here we report the identification and characterization of a gene, named C6orf33, in the EJM1 region. Northern blot analysis showed that C6orf33 is predominantly expressed in brain but in mice, testis shows additional transcripts. C6orf33 is predicted to encode a novel approximately 40-kDa membrane protein, LMPB1, that defines a novel protein family by having highly conserved orthologs in eukaryotes and three putative paralogs in human. Biochemical and immunocytochemical studies revealed that LMPB1 is indeed an integral membrane protein that targets to lysosomal structures. LMPB1 may be involved in specialized lysosomal functions that are unique to brain and testis, and the C6orf33 gene is of interest as a candidate for EJM1.

Inherited epilepsies

Epilepsy may be acquired or inherited. At least one half of epilepsy is genetic in origin; this figure is likely higher in children regardless of whether seizures are generalized or partial. Inherited epilepsies are classified as benign, cryptogenic, or symptomatic depending on associated clinical, electrographic, and neuroimaging features. To date, genetic mutations in the idiopathic inherited epilepsies affect channel function within the central nervous system; genes underlying symptomatic epilepsies are more heterogeneous. Accurate diagnosis of an inherited epilepsy syndrome provides useful prognostic information; it also may help guide diagnostic evaluation, including request for specific gene testing. In the near future, the relationship between genetic defect and response to specific anticonvulsants may also be better defined.