Genetic influences on myoclonic and absence seizures

Familial aggregation of seizure outcomes in four familial epilepsy cohorts

OBJECTIVE

To assess the possible effects of genetics on seizure outcome by estimating the familial aggregation of three outcome measures: seizure remission, history of ≥4 tonic-clonic seizures, and seizure control for individuals taking antiseizure medication.

METHODS

We analyzed families containing multiple persons with epilepsy in four previously collected retrospective cohorts. Seizure remission was defined as being 5 and 10 years seizure-free at last observation. Total number of tonic-clonic seizures was dichotomized at <4 and ≥4 seizures. Seizure control in patients taking antiseizure medication was defined as no seizures for 1, 2, and 3 years. We used Bayesian generalized linear mixed-effects model (GLMM) to estimate the intraclass correlation coefficient (ICC) of the family-specific random effect, controlling for epilepsy type, age at epilepsy onset, and age at last data collection as fixed effects. We analyzed each cohort separately and performed meta-analysis using GLMMs.

RESULTS

The combined cohorts included 3644 individuals with epilepsy from 1463 families. A history of ≥4 tonic-clonic seizures showed strong familial aggregation in three separate cohorts and meta-analysis (ICC .28, 95% confidence interval [CI] .21-.35, Bayes factor 8 × 1016). Meta-analyses did not reveal significant familial aggregation of seizure remission (ICC .08, 95% CI .01-.17, Bayes factor 1.46) or seizure control for individuals taking antiseizure medication (ICC .13, 95% CI 0-.35, Bayes factor 0.94), with heterogeneity among cohorts.

SIGNIFICANCE

A history of ≥4 tonic-clonic seizures aggregated strongly in families, suggesting a genetic influence, whereas seizure remission and seizure control for individuals taking antiseizure medications did not aggregate consistently in families. Different seizure outcomes may have different underlying biology and risk factors. These findings should inform the future molecular genetic studies of seizure outcomes.

Quantitative analysis of phenotypic elements augments traditional electroclinical classification of common familial epilepsies

OBJECTIVE

Classification of epilepsy into types and subtypes is important for both clinical care and research into underlying disease mechanisms. A quantitative, data-driven approach may augment traditional electroclinical classification and shed new light on existing classification frameworks.

METHODS

We used latent class analysis, a statistical method that assigns subjects into groups called latent classes based on phenotypic elements, to classify individuals with common familial epilepsies from the Epi4K Multiplex Families study. Phenotypic elements included seizure types, seizure symptoms, and other elements of the medical history. We compared class assignments to traditional electroclinical classifications and assessed familial aggregation of latent classes.

RESULTS

A total of 1120 subjects with epilepsy were assigned to five latent classes. Classes 1 and 2 contained subjects with generalized epilepsy, largely reflecting the distinction between absence epilepsies and younger onset (class 1) versus myoclonic epilepsies and older onset (class 2). Classes 3 and 4 contained subjects with focal epilepsies, and in contrast to classes 1 and 2, these did not adhere as closely to clinically defined focal epilepsy subtypes. Class 5 contained nearly all subjects with febrile seizures plus or unknown epilepsy type, as well as a few subjects with generalized epilepsy and a few with focal epilepsy. Family concordance of latent classes was similar to or greater than concordance of clinically defined epilepsy types.

SIGNIFICANCE

Quantitative classification of epilepsy has the potential to augment traditional electroclinical classification by (1) combining some syndromes into a single class, (2) splitting some syndromes into different classes, (3) helping to classify subjects who could not be classified clinically, and (4) defining the boundaries of clinically defined classifications. This approach can guide future research, including molecular genetic studies, by identifying homogeneous sets of individuals that may share underlying disease mechanisms.

Phenotypic analysis of 303 multiplex families with common epilepsies

Gene identification in epilepsy has mainly been limited to large families segregating genes of major effect and de novo mutations in epileptic encephalopathies. Many families that present with common non-acquired focal epilepsies and genetic generalized epilepsies remain unexplained. We assembled a cohort of 'genetically enriched' common epilepsies by collecting and phenotyping families containing multiple individuals with unprovoked seizures. We aimed to determine if specific clinical epilepsy features aggregate within families, and whether this segregation of phenotypes may constitute distinct 'familial syndromes' that could inform genomic analyses. Families with three or more individuals with unprovoked seizures were studied across multiple international centres. Affected individuals were phenotyped and classified according to specific electroclinical syndromes. Families were categorized based on syndromic groupings of affected family members, examined for pedigree structure and phenotypic patterns and, where possible, assigned specific familial epilepsy syndromes. A total of 303 families were assembled and analysed, comprising 1120 affected phenotyped individuals. Of the 303 families, 117 exclusively segregated generalized epilepsy, 62 focal epilepsy, and 22 were classified as genetic epilepsy with febrile seizures plus. Over one-third (102 families) were observed to have mixed epilepsy phenotypes: 78 had both generalized and focal epilepsy features within the same individual (n = 39), or within first or second degree relatives (n = 39). Among the genetic generalized epilepsy families, absence epilepsies were found to cluster within families independently of juvenile myoclonic epilepsy, and significantly more females were affected than males. Of the 62 familial focal epilepsy families, two previously undescribed familial focal syndrome patterns were evident: 15 families had posterior quadrant epilepsies, including seven with occipito-temporal localization and seven with temporo-parietal foci, and four families displayed familial focal epilepsy of childhood with multiple affected siblings that was suggestive of recessive inheritance. The findings suggest (i) specific patterns of syndromic familial aggregation occur, including newly recognized forms of familial focal epilepsy; (ii) although syndrome-specificity usually occurs in multiplex families, the one-third of families with features of both focal and generalized epilepsy is suggestive of shared genetic determinants; and (iii) patterns of features observed across families including pedigree structure, sex, and age of onset may hold clues for future gene identification. Such detailed phenotypic information will be invaluable in the conditioning and interpretation of forthcoming sequencing data to understand the genetic architecture and inter-relationships of the common epilepsy syndromes.

Familial aggregation of focal seizure semiology in the Epilepsy Phenome/Genome Project

OBJECTIVE

To improve phenotype definition in genetic studies of epilepsy, we assessed the familial aggregation of focal seizure types and of specific seizure symptoms within the focal epilepsies in families from the Epilepsy Phenome/Genome Project.

METHODS

We studied 302 individuals with nonacquired focal epilepsy from 149 families. Familial aggregation was assessed by logistic regression analysis of relatives' traits (dependent variable) by probands' traits (independent variable), estimating the odds ratio for each symptom in a relative given presence vs absence of the symptom in the proband.

RESULTS

In families containing multiple individuals with nonacquired focal epilepsy, we found significant evidence for familial aggregation of ictal motor, autonomic, psychic, and aphasic symptoms. Within these categories, ictal whole body posturing, diaphoresis, dyspnea, fear/anxiety, and déjà vu/jamais vu showed significant familial aggregation. Focal seizure type aggregated as well, including complex partial, simple partial, and secondarily generalized tonic-clonic seizures.

CONCLUSION

Our results provide insight into genotype-phenotype correlation in the nonacquired focal epilepsies and a framework for identifying subgroups of patients likely to share susceptibility genes.

Chromosome loci vary by juvenile myoclonic epilepsy subsyndromes: linkage and haplotype analysis applied to epilepsy and EEG 3.5-6.0 Hz polyspike waves

Juvenile myoclonic epilepsy (JME), the most common genetic epilepsy, remains enigmatic because it is considered one disease instead of several diseases. We ascertained three large multigenerational/multiplex JME pedigrees from Honduras with differing JME subsyndromes, including Childhood Absence Epilepsy evolving to JME (CAE/JME; pedigree 1), JME with adolescent onset pyknoleptic absence (JME/pA; pedigree 2), and classic JME (cJME; pedigree 3). All phenotypes were validated, including symptomatic persons with various epilepsies, asymptomatic persons with EEG 3.5-6.0 Hz polyspike waves, and asymptomatic persons with normal EEGs. Two-point parametric linkage analyses were performed with 5185 single-nucleotide polymorphisms on individual pedigrees and pooled pedigrees using four diagnostic models based on epilepsy/EEG diagnoses. Haplotype analyses of the entire genome were also performed for each individual. In pedigree 1, haplotyping identified a 34 cM region in 2q21.2-q31.1 cosegregating with all affected members, an area close to 2q14.3 identified by linkage (Z max = 1.77; pedigree 1). In pedigree 2, linkage and haplotyping identified a 44 cM cosegregating region in 13q13.3-q31.2 (Z max = 3.50 at 13q31.1; pooled pedigrees). In pedigree 3, haplotyping identified a 6 cM cosegregating region in 17q12. Possible cosegregation was also identified in 13q14.2 and 1q32 in pedigree 3, although this could not be definitively confirmed due to the presence of uninformative markers in key individuals. Differing chromosome regions identified in specific JME subsyndromes may contain separate JME disease-causing genes, favoring the concept of JME as several distinct diseases. Whole-exome sequencing will likely identify a CAE/JME gene in 2q21.2-2q31.1, a JME/pA gene in 13q13.3-q31.2, and a cJME gene in 17q12.

Translation of genetic findings to clinical practice in juvenile myoclonic epilepsy

It has been estimated that JME (juvenile myoclonic epilepsy), when compared to other adult epilepsy syndromes, is most likely to have a genetic cause. However, decades of research have not brought us closer to finding a single 'JME gene' that is important on a population basis. Is this due in part to the genetic complexity of the syndrome, the cryptic nature of the genes of effect, or perhaps because JME is not one condition at all but many? Before we can begin to harness the power of next-generation sequencing techniques, we must first reduce JME down to lacunae of homogeneity--using increasingly more sophisticated phenotyping tools. The current technological advances in gene sequencing have been used to dramatic effect to identify single gene causes in rare syndromes and identify risk variants in malignancies. Filtering the variety of the human exome or genome down into a handful of biologically plausible candidates now relies on a pipeline of biostatistics, software, and functional analyses. It is simply unacceptable to return uncertain findings to the clinical domain and, therefore, it is crucial that pathogenicity is fully determined before families receive genetic counseling and test results.

How should we be searching for genes for common epilepsy? A critique and a prescription

Despite enormous data collection and analysis efforts, the genetic influences on common epilepsies remain mostly unknown. We propose that reasons for the lack of progress can be traced to three factors: (1) A reluctance to consider fine-grained phenotype definitions based on extensive and carefully collected clinical data; (2) the pursuit of genetic analysis methods that are popular but poorly conceived and are inadequate to the task of resolving the problems inherent in common disease studies; (3) preconceived ideas about the genetic mechanisms that cause epilepsy (which we have discussed elsewhere). We propose a paradigm for finding epilepsy-related loci and alleles that has proven successful in other common diseases.

Effects of SCN1A and GABA receptor genetic polymorphisms on carbamazepine tolerability and efficacy in Chinese patients with partial seizures: 2-year longitudinal clinical follow-up

AIMS

To investigate the tolerability and efficacy of carbamazepine treatment in patients with partial-onset seizures and the association with polymorphisms in the sodium channel α-subunit type 1 (SCN1A), and gamma-aminobutyric acid (GABA) receptor genes among the Chinese Han population.

METHODS

448 patients were genotyped for single nucleotide polymorphisms selected of the SCN1A and GABA-receptor genes. Monotherapy with carbamazepine (CBZ) was administered to the patients. The effectiveness of CBZ treatment was evaluated with regard to efficacy by the decrease in seizures and tolerability by retention rates.

RESULTS

SCN1A rs3812718 A/G with CBZ tolerability (P= 0.038) throughout 24 months of clinical follow-up and the GABRA1 rs2290732 A/G were significantly associated with CBZ tolerability (P= 0.001). The maintenance dose and serum level of CBZ in AA genotype carriers of rs3812718 A/G were significantly higher than those of GG genotype carriers between 3 and 12 months of follow-up. The proportion of AA genotype carriers of rs2298771 A/G with seizure free was significantly higher than that of AG+GG genotype carriers from 3 months to 15 months of follow-up (P < 0.05).

CONCLUSION

rs3812718 A/G and rs2290732 A/G polymorphisms affected the tolerability of CBZ. rs2298771 A/G was associated with efficacy of CBZ treatment.

Genome-wide linkage meta-analysis identifies susceptibility loci at 2q34 and 13q31.3 for genetic generalized epilepsies

PURPOSE

Genetic generalized epilepsies (GGEs) have a lifetime prevalence of 0.3% with heritability estimates of 80%. A considerable proportion of families with siblings affected by GGEs presumably display an oligogenic inheritance. The present genome-wide linkage meta-analysis aimed to map: (1) susceptibility loci shared by a broad spectrum of GGEs, and (2) seizure type-related genetic factors preferentially predisposing to either typical absence or myoclonic seizures, respectively.

METHODS

Meta-analysis of three genome-wide linkage datasets was carried out in 379 GGE-multiplex families of European ancestry including 982 relatives with GGEs. To dissect out seizure type-related susceptibility genes, two family subgroups were stratified comprising 235 families with predominantly genetic absence epilepsies (GAEs) and 118 families with an aggregation of juvenile myoclonic epilepsy (JME). To map shared and seizure type-related susceptibility loci, both nonparametric loci (NPL) and parametric linkage analyses were performed for a broad trait model (GGEs) in the entire set of GGE-multiplex families and a narrow trait model (typical absence or myoclonic seizures) in the subgroups of JME and GAE families.

KEY FINDINGS

For the entire set of 379 GGE-multiplex families, linkage analysis revealed six loci achieving suggestive evidence for linkage at 1p36.22, 3p14.2, 5q34, 13q12.12, 13q31.3, and 19q13.42. The linkage finding at 5q34 was consistently supported by both NPL and parametric linkage results across all three family groups. A genome-wide significant nonparametric logarithm of odds score of 3.43 was obtained at 2q34 in 118 JME families. Significant parametric linkage to 13q31.3 was found in 235 GAE families assuming recessive inheritance (heterogeneity logarithm of odds = 5.02).

SIGNIFICANCE

Our linkage results support an oligogenic predisposition of familial GGE syndromes. The genetic risk factor at 5q34 confers risk to a broad spectrum of familial GGE syndromes, whereas susceptibility loci at 2q34 and 13q31.3 preferentially predispose to myoclonic seizures or absence seizures, respectively. Phenotype- genotype strategies applying narrow trait definitions in phenotypic homogeneous subgroups of families improve the prospects of disentangling the genetic basis of common familial GGE syndromes.

EEG-LORETA endophenotypes of the common idiopathic generalized epilepsy syndromes

OBJECTIVE

We tested the hypothesis that the cortical areas with abnormal local EEG synchronization are dissimilar in the three common idiopathic generalized epilepsy (IGE) phenotypes: IGE patients with absence seizures (ABS), juvenile myoclonic epilepsy (JME) and epilepsy with generalized tonic-clonic seizures exclusively (EGTCS).

PATIENTS AND METHODS

Groups of unmedicated ABS, JME and EGTCS patients were investigated. Waking EEG background activity (without any epileptiform potentials) was analyzed by a source localization method, LORETA (Low Resolution Electromagnetic Tomography). Each patient group was compared to a separate, age-matched group of healthy control persons. Voxel-based, normalized broad-band (delta, theta, alpha, and beta) and very narrow band (VNB, 1Hz bandwidth, from 1 to 25Hz) LORETA activity (=current source density, A/m(2)) were computed for each person. Group comparison included subtraction (average patient data minus average control data) and group statistics (multiple t-tests, where Bonferroni-corrected p<0.05 values were accepted as statistically significant).

RESULTS

Statistically not significant main findings were: overall increased delta and theta broad band activity in the ABS and JME groups; decrease of alpha and beta activity in the EGTCS group. Statistically significant main findings were as follows. JME group: bilaterally increased theta activity in posterior (temporal, parietal, and occipital) cortical areas; bilaterally increased activity in the medial and basal prefrontal area in the 8Hz VNB; bilaterally decreased activity in the precuneus, posterior cingulate and superior parietal lobule in the 11Hz and 21-22Hz VNBs. ABS group: bilaterally increased theta activity emerged in the basal prefrontal and medial temporal limbic areas. Decreased activity was found at 19-21Hz in the right postcentral gyrus and parts of the right superior and medial temporal gyri. EGTCS group: decreased activity was found in the frontal cortex and the postcentral gyrus at 10-11Hz, increased activity in the right parahippocampal gyrus at 16-18Hz.

DISCUSSION

Increased theta activity in the posterior parts of the cortex is the endophenotype for JME. Increased theta activity in the fronto-temporal limbic areas is the endophenotype for ABS. Statistically not significant findings might indicate diffuse biochemical abnormality of the cortex in JME and ABS.

SIGNIFICANCE

EEG-LORETA endophenotypes may correspond to the selective propensity to generate absence and myoclonic seizures in the ABS and JME syndromes.

Opportunities and challenges for genome sequencing in the clinic

Human genome sequencing technology is developing rapidly. These developments are providing exciting opportunities for genetic mapping of human traits, ranging from accelerated discovery of mutations underlying relatively simple Mendelian disorders to more genetically complex human diseases. This chapter outlines the development of whole-genome sequencing in a historical context of genetic mapping and explores the impact that sequencing is having on gene discovery study design. Using the example of epilepsy, the authors outline the opportunities and barriers for the translation of genetic predictors from discovery to the clinic. Finally, the authors discuss the practical challenges of actual implementation of whole-genome sequencing to the clinic.

Importance of genetic factors in the occurrence of epilepsy syndrome type: a twin study

Although there is strong evidence that genetic factors contribute to risk for epilepsy, their role in the determination of syndrome type is less clear. This study was undertaken to address this question. Information related to epilepsy was obtained from twins included in 455 monozygotic and 868 dizygotic pairs ascertained from population-based twin registries in Denmark, Norway and the United States. Syndrome type was determined based on medical record information and detailed clinical interviews and classified using the International Classification Systems for the Epilepsies and Epileptic Syndromes. Concordance rates were significantly increased in monozygotic versus dizygotic pairs for all major syndrome groups except localization-related cryptogenic epilepsy. Among generalized epilepsies, genetic factors were found to play an important role in the determination of childhood absence, juvenile absence, juvenile myoclonic, and idiopathic generalized epilepsy; and to a lesser degree for epilepsies with grand mal seizures on awakening. Among localization-related epilepsies, genetic factors contributed to risk for localization-related idiopathic and symptomatic syndromes overall, but did not appear to play an important role in determining risk for frontal, occipital or temporal lobe epilepsy. These results suggest that, while genetic factors contribute to risk for major syndrome types, determined when possible, their contribution to risk for localization-related syndrome sub-types, as defined by specific focality, may be modest.

Benign myoclonic epilepsy of infancy evolving to Jeavons syndrome

Benign myoclonic epilepsy of infancy is a rare idiopathic generalized epileptic syndrome occurring below the age of 3 years. Although benign outcome is presumed, some recent studies suggest less favorable outcome. A 14-year-old boy had a history of repeated episodes of myoclonic jerks of the shoulders and upper limbs in infancy (age 5 months). An ictal electroencephalogram indicated generalized spike-wave discharges associated with the myoclonic seizures, and the diagnosis of benign myoclonic epilepsy of infancy was made. Valproate treatment resulted in control of the myoclonic seizures, and the drug was withdrawn when the patient was 5 years of age. At the age of 10, he presented with episodes of eyelid jerks associated with brief lapses in concentration triggered by sunlight. Electroencephalography revealed photosensitivity and a pattern of eye-closure sensitivity. These features were compatible with the diagnosis of eyelid myoclonia with absences, or Jeavons syndrome. Lamotrigine eliminated the seizures. The evolution of benign myoclonic epilepsy of infancy to eyelid myoclonia with absences has been reported in one other case. A possible continuum of myoclonic epileptic syndromes, mediated by a common genetic abnormality, suggests the need for longer monitoring of patients with benign myoclonic epilepsy of infancy.

Phenotypic concordance in 70 families with IGE-implications for genetic studies of epilepsy

INTRODUCTION

A crucial issue in the genetic analysis of idiopathic generalized epilepsy (IGE) is deciding on the phenotypes that are likely to give the greatest power to detect predisposing variants. A complex inheritance pattern and unclear nature of the genotype-phenotype correlation makes this task difficult. In the absence of much definitive genetic information to clarify this correlation, we inferred the putative effects of predisposing genes by studying the clustering of various phenotypic features, both clinical and electrophysiological, within families.

METHODS

We examined the distribution of clinical features among relatives of a proband in 70 French-Canadian families with a minimum of two affected individuals with a clear diagnosis of IGE and then, using concordance analysis, identified the relative genetic influences on IGE syndrome, seizure type, age-at-onset, and EEG features.

RESULTS

The mean number of affected individuals with IGE per family was three. One-third of relatives had the same syndrome as the proband. 16-22.5% of relatives of a proband with one of the absence syndromes had juvenile myoclonic epilepsy (JME). Conversely, 27% of relatives of probands with JME had an absence syndrome. 15% of relatives displayed the exact constellation of seizure types as the proband. Concordance analysis demonstrated greater clustering within families of IGE syndrome, seizure type, and age-at-onset than would be expected by chance. Significant concordance was not evident for EEG features.

DISCUSSION

There was a large degree of clinical heterogeneity present within families. However we found evidence for clustering of a number of clinical features. Further refinement of the phenotypes used in genetic studies of complex IGE is necessary for progress to be made.

Multicentre search for genetic susceptibility loci in sporadic epilepsy syndrome and seizure types: a case-control study

BACKGROUND

The Epilepsy Genetics (EPIGEN) Consortium was established to undertake genetic mapping analyses with augmented statistical power to detect variants that influence the development and treatment of common forms of epilepsy.

METHODS

We examined common variations across 279 prime candidate genes in 2717 case and 1118 control samples collected at four independent research centres (in the UK, Ireland, Finland, and Australia). Single nucleotide polymorphism (SNP) and combined set-association analyses were used to examine the contribution of genetic variation in the candidate genes to various forms of epilepsy.

FINDINGS

We did not identify clear, indisputable common genetic risk factors that contribute to selected epilepsy subphenotypes across multiple populations. Nor did we identify risk factors for the general all-epilepsy phenotype. However, set-association analysis on the most significant p values, assessed under permutation, suggested the contribution of numerous SNPs to disease predisposition in an apparent population-specific manner. Variations in the genes KCNAB1, GABRR2, KCNMB4, SYN2, and ALDH5A1 were most notable.

INTERPRETATION

The underlying genetic component to sporadic epilepsy is clearly complex. Results suggest that many SNPs contribute to disease predisposition in an apparently population-specific manner. However, subtle differences in phenotyping across cohorts, combined with a poor understanding of how the underlying genetic component to epilepsy aligns with current phenotypic classifications, might also account for apparent population-specific genetic risk factors. Variations across five genes warrant further study in independent cohorts to clarify the tentative association.

Allelic association of a truncation mutation of the KCNMB3 gene with idiopathic generalized epilepsy

The gene encoding the beta3-subunit regulatory subunit (KCNMB3) of large conductance calcium-sensitive potassium (BK) channels represents a positional and functional candidate gene for idiopathic generalized epilepsy (IGE). A single base pair deletion in exon 4 of KCNMB3 (delA750) alters/truncates the terminal 21 amino acids of the 3-subunit and affects channel inactivation of the beta3b-isoform. The present association study tested whether the KCNMB3 delA750 mutation confers susceptibility to common IGE syndromes. In total, 592 unrelated German IGE patients and 462 healthy population controls were genotyped for the delA750 truncation mutation. The frequency of the delA750 mutation was significantly increased in the IGE patients (7.9%) compared to that in the controls (5.5%; P = 0.016, one-sided; OR = 1.52; 95%-CI: 1.05-2.21). The increase of the delA750 frequency was accentuated in 312 patients exhibiting typical absence seizures (8.8%, P = 0.005, one-sided; OR = 1.72; 95%-CI: 1.13-2.62) relatively to that observed in the 237 patients with myoclonic seizures on awakening (7.2%; P = 0.11, one-sided; OR = 1.36; 95%-CI: 0.85-2.19), when compared with controls. The present results suggest that the functional KCNMB3 beta3b-truncation confers a common epileptogenic effect preferentially to the ictogenesis of typical absence seizures.

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.

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.

Analysis of genetically complex epilepsies

During the last decade, great progress has been made in the discovery of genes that influence risk for epilepsy. However, these gene discoveries have been in epilepsies with Mendelian modes of inheritance, which comprise only a tiny fraction of all epilepsy. Most people with epilepsy have no affected relatives, suggesting that the great majority of all epilepsies are genetically complex: multiple genes contribute to their etiology, none of which has a major effect on disease risk. Gene discovery in the genetically complex epilepsies is a formidable task. It is unclear which epilepsy phenotypes are most advantageous to study, and chromosomal localization and mutation detection are much more difficult than in Mendelian epilepsies. Association studies are very promising for the identification of complex epilepsy genes, but we are still in the earliest stages of their application in the epilepsies. Future studies should employ very large sample sizes to ensure adequate statistical power, clinical phenotyping methods of the highest quality, designs and analytic techniques that control for population stratification, and state-of-the-art molecular methods. Collaborative studies are essential to achieve these goals.

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.

Genetic Epidemiology of Epilepsy or What Do We Tell Families?

Over the past few decades, epidemiologic and molecular research has transformed the field of epilepsy genetics. In this review, we discuss the ways in which accumulating evidence on the genetics of epilepsy and febrile seizures can inform health care practitioners advising patients and families with epilepsy. We will review the epidemiologic data from twin and family studies, and illustrate how it can be used to guide genetic counseling. Although there have been many exciting advances in the last few decades-both molecular and epidemiologic-what we have learned has not appreciably changed what we tell families, and what we tell them can remain reassuring.

Genetics of Idiopathic Generalized Epilepsies

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

Association analysis of the Arg220His variation of the human gene encoding the GABA delta subunit with idiopathic generalized epilepsy

PURPOSE

Mutation analysis of the gene encoding the GABA delta subunit (GABRD) identified a common missense variation (c.659G>A; Arg220His) of which the His220 allele displayed decreased GABA(A) alpha(1)beta(2)delta receptor current amplitudes. The present association study tested whether the functional GABRD His220 allele confers susceptibility to common syndromes of idiopathic generalized epilepsy (IGE).

METHODS

Five hundred and sixty two unrelated German IGE patients and 664 healthy population controls were genotyped for the c.659G>A polymorphism in exon 6 of the GABRD gene.

RESULTS

His220 allele frequencies did not differ significantly between IGE patients (2.3%) and the controls (2.8%; P=0.46). Likewise, no evidence for an allelic association was found with juvenile myoclonic epilepsy (n=218; 2.8%; P=0.97) or idiopathic absence epilepsy (n=260; 2.3%; P=0.56).

CONCLUSION

Our results provide no evidence that the functional GABRD His220 allele mediates a substantial susceptibility effect to common IGE syndromes in the German population.

Association Analysis of Malic Enzyme 2 Gene Polymorphisms with Idiopathic Generalized Epilepsy

PURPOSE

Linkage disequilibrium mapping revealed allelic and haplotypic associations between single-nucleotide polymorphisms (SNPs) of the gene encoding the malic enzyme 2 (ME2) and adolescent-onset idiopathic generalized epilepsy (IGE). Homozygote carriers of the associated ME2 haplotype had a sixfold higher risk of IGE compared with any other genotype. The present population-based association study tested whether genetic variation of the ME2 gene confers susceptibility to common IGE syndromes in the German population.

METHODS

The study included 666 German healthy control subjects and 660 German IGE patients (IGE group), of which 416 patients had an age at onset in adolescence (IGEado group). Genotyping was performed for six SNPs and one dinucleotide repeat polymorphism, all located in the ME2 region.

RESULTS

Neither allele nor genotype frequencies of any ME2 polymorphism differed significantly between the controls and the IGE groups (p > 0.22). No hint of an association of the putative risk-conferring haplotype was seen, when present homozygously, in both IGE groups compared with controls (p > 0.18).

CONCLUSIONS

These results do not support previous evidence that genetic variation of the ME2 gene predisposes to common IGE syndromes. Thus if a recessively inherited ME2 mutation is present, then the size of the epileptogenic effect might be too small or not frequent enough to detect it in the present IGE sample.

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.

Mechanisms, genetics, and pathogenesis of juvenile myoclonic epilepsy: review

PURPOSE OF REVIEW

This review addresses the mechanisms, genetics and pathogenesis of juvenile myoclonic epilepsy (JME, Janz syndrome).

RECENT FINDINGS

Although JME is a well defined clinical syndrome among the idiopathic generalized epilepsies (IGEs), recent studies suggest that JME is distinct from other IGE syndromes and must be considered separately for the purposes of genetic studies. Clinical, morphological and metabolic data suggest a preferential role for frontal regions in this syndrome. However, JME is clinically and genetically heterogeneous. Although several major genes for JME have been identified and pathogenetic mechanisms suggested based on these findings, these genes account for only a small proportion of JME cases, suggesting multifactorial or complex inheritance in most. The roles played by other major genes, susceptibility genes and environmental factors in the pathogenesis of JME remain to be defined.

SUMMARY

JME is clinically and genetically heterogeneous and should be considered separately from other IGE syndromes. Proposed mechanisms, such as those involving microdysgenesis or altered neuronal inhibition, may be related to different genetic abnormalities in different patients. Major genes account for relatively few cases, and most cases appear to involve multifactorial or complex inheritance.

Genetic liability to epilepsy in Kerala State, India

BACKGROUND

Familial clustering is common in epilepsies, but pedigree patterns suggest a multi-factorial inheritance. Genetic liability for multi-factorial inheritance is population specific and such data are not available for the population of Kerala or other states in south India.

OBJECTIVES

In this study, we have attempted to determine the genetic liability to epilepsy based on an adult population of this state.

MATERIAL AND METHODS

Pedigrees were recorded for probands who reported to the Kerala Registry of Epilepsy and Pregnancy. In order to obtain a genetically matched sample for comparison and estimation of empiric risks, we have used the family history of the spouse except when the spouse was proband's relative. The ILAE criteria were followed for diagnosis and classification of epilepsy.

RESULTS

Data were collected on 18,419 family members of 505 probands with epilepsy (82 men and 423 women) and 10,231 family members of spouses (control). The frequency of epilepsy in first and second-degree relatives of the spouses was comparable to the population frequency (0.5%), justifying the use of this sample as control. Positive family history was observed in 22.2% of probands and 8.24% of controls (Odd's Ratio 3.2, 95% Confidence Interval 2.12-4.73). An affected first-degree relative was observed in 7.5% of probands. The corresponding figure for GE, LRE and other epileptic syndromes were 10.2%, 5.8% and 5.12%, respectively. The segregation ratio for Juvenile Myoclonic Epilepsy (JME) (1:19) was higher than that for other types of Generalized Epilepsy (GE) (1:24) and Localization Related Epilepsy (LRE) (1:52). Prevalence of epilepsy among the first-degree relatives (1.96%) was greater than the square root of the population frequency (0.51%) and was higher than that for second-degree (1.24%) and third-degree (0.64%) relatives for the probands. Probands had higher parental consanguinity (13.07%) compared to controls (6.64%). The above factors support a complex inheritance. Genetic liability to epilepsy (heritability) is greater for GE (0.6) and significantly higher for JME (0.7) compared to LRE (0.4). A limitation of this study is that the inferences are based on a predominantly adult female proband sample but no gender specific differences were identified.

CONCLUSIONS

The observations of this study indicate complex inheritance and the liability values are useful for genetic counseling in the local population. Further studies involving more individuals from younger age group and male gender are envisaged.