Brain, Vol. 126, No. 1, 230-240,
January 2003
© 2003 Guarantors of Brain
doi: 10.1093/brain/awg018
Childhood absence epilepsy and febrile seizures: a family with a GABAA receptor mutation
1 Epilepsy Research Institute, The University of Melbourne, Austin and Repatriation Medical Centre, 2 Royal Children’s Hospital, Melbourne and 3 Centre for Medical Genetics, Department of Cytogenetics and Molecular Genetics, Women’s and Children’s Hospital, North Adelaide, Australia
Correspondence to: Professor Samuel F. Berkovic, Director, Epilepsy Research Institute, Level 1, Neurosciences Building, Austin and Repatriation Medical Centre, Banksia Street, West Heidelberg, Victoria 3081, Australia E-mail: s.berkovic{at}unimelb.edu.au
Received March 18, 2002. Revised June 16, 2002. Accepted August 20, 2002.
 |
Summary |
|---|
 Top Summary IntroductionMethodsResultsDiscussionReferences |
|---|
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 GABAA receptor
2 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 GABAA 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. Keywords: childhood absence epilepsy; epilepsy; GABAA receptor; genetics; linkage analysis
Abbreviations: CAE= childhood absence epilepsy; FS = febrile seizures; GEFS+ = generalized epilepsy with febrile seizures plus; GTCS = generalized tonic–clonic seizures; IGE = idiopathic generalized epilepsy; JME = juvenile myoclonic epilepsy; MAE = myoclonic–astatic epilepsy
|
Introduction |
|---|
|
TopSummaryIntroductionMethodsResultsDiscussionReferences |
|---|
Childhood absence epilepsy (CAE) is the prototype of idiopathic generalized epilepsy (IGE) accounting for 2–8% of patients with epilepsy (Livingston et al., 1965
; Cavazzuti, 1980; Olsson, 1988). Absences have an age-dependent expression, beginning in childhood and often resolving spontaneously by mid-adolescence. In 10–15% of CAE patients, there is a history of earlier febrile seizures (FS) (Livingston et al., 1965; Penry et al., 1975; Dieterich et al., 1985; Loiseau, 1992; Berkovic, 1997). Afebrile generalized tonic–clonic seizures (GTCS) occur in 40% of individuals, usually after the onset of absences, and may continue into adolescent or adult life (Livingston et al., 1965; Dieterich et al., 1985; Olsson, 1988).
CAE is genetically determined. Estimates of the risk of seizures in first-degree relatives of IGE probands vary from 4 to 10% (Metrakos and Metrakos, 1961; Doose et al., 1973; Annegers et al., 1982; Tsuboi, 1989). Within families of CAE probands, only about a quarter of affected relatives have the CAE phenotype, about half have other IGE subsyndromes and about a quarter have FS alone (Italian League Against Epilepsy Genetic Collaborative Group, 1993; Bianchi and the Italian League Against Epilepsy Genetic Collaborative Group, 1995). Although a dominant mode of inheritance was suggested initially (Metrakos and Metrakos, 1961), clinical analysis of twins and families, and molecular analyses strongly suggests that CAE and related IGE are inherited in a complex manner with involvement of more than one gene (Lennox and Lennox, 1960; Doose et al., 1973; Andermann, 1982; Greenberg et al., 1988a, 1992; Janz et al., 1992; Italian League Against Epilepsy Collaborative Group, 1993; Bianchi and the Italian League Against Epilepsy Collaborative Group, 1995; Berkovic et al., 1998).
A number of loci have been claimed for CAE. The first unconfirmed locus on chromosome 1 was reported in a Mexican family with a phenotype of CAE evolving into juvenile myoclonic epilepsy (JME) (Westling et al., 1996). A locus on chromosome 8q24 has been reported in a large Indian family with a phenotype of persisting CAE and GTCS (Fong et al., 1998). An earlier suggestion of a chromosome 8q locus, centromeric to the region indicated in the Indian family (Fong et al., 1998), was reported by Zara et al. (1995) in families with mixed IGE subsyndromes. However re-analysis of these data using non-parametric linkage analysis did not produce a significant LOD score on chromosome 8 (Kruglyak et al., 1996). Two other studies were performed in mixed IGE families looking at a possible locus on chromosome 8q, with opposing results (Sander et al., 1998; Durner et al., 1999). Sander et al. (2000) recently reported a genome-wide search for susceptibility loci in 130 families with mixed IGE and found evidence for epilepsy susceptibility loci on chromosome 14q23, 2q36 and 3q36. Similarly, Durner et al. (2001) performed a genome scan on 91 families comprising probands with JME, juvenile absence epilepsy (JAE) and epilepsy with GTCS alone. Analysis including all 91 families showed strong evidence for a chromosome 18 locus, common to all IGE. Together with the locus on chromosome 18, the analysis of subsets of the 91 families showed other positive LOD scores to other chromosomes depending on the IGE subsyndrome. Families with JME showed linkage to the previously reported chromosome 6 locus (Greenberg et al., 1988b), and families with JAE and GTCS alone contributed to a positive LOD score on chromosome 8 (Durner et al., 2001). Grouping families with at least one member with absence seizures, two new putative loci were also reported in two regions of chromosome 5 (Durner et al., 2001). A single case of absence epilepsy with episodic ataxia type II had a mutation in the calcium channel CACNA1A gene; whether this is relevant to his absences remains uncertain (Jouvenceau et al., 2001).
We recently described a GABAA receptor 2 subunit gene (GABRG2) mutation in a family with classical CAE associated with FS (Wallace et al., 2001a). The mutation results in substitution of a highly conserved arginine for glutamine (R43Q) in the first of two benzodiazepine-binding domains of the protein, causing loss of benzodiazepine sensitivity in vitro. No other families with mutations in the benzodiazepine-binding domains have been described to date (Wallace et al., 2001a).
We now present a detailed clinical and genetic analysis of this large pedigree that includes eight individuals with CAE. This permits an understanding of the phenotype–genotype correlation of this human GABAA receptor mutation. Further, using the power of this large family, we explored the complex genetics of CAE and identified other putative loci.
|
Methods |
|---|
|
TopSummaryIntroductionMethodsResultsDiscussionReferences |
|---|
Ascertainment of the family
The family was ascertained via two different probands (Fig. 1). The first proband (IV-28) was referred by Drs L. Shield and M. Mackay, Royal Children’s Hospital, Melbourne, when she presented with onset of daily absences at age 4 years. The second proband (III-15), aged 35 years, was identified through the First Seizure Clinic at the Austin and Repatriation Medical Centre (ARMC), Melbourne. This study was undertaken with the approval of the Human Research Ethics Committee of the ARMC. All participating individuals and parents/guardians in the case of minors gave informed consent.
|
Clinical evaluation
Information was obtained on 220 family members from an outbred family, of whom 35 were reputed to have a history of seizures. Fifty-nine members within generations II–IV (Fig. 1) underwent a detailed personal interview using a validated seizure questionnaire (Reutens et al., 1992
). Extensive genealogical information was obtained. Previous medical records of affected individuals were reviewed where possible. Fifty members of the family were personally examined and underwent neurological examination. Forty-eight individuals had a 21-channel EEG recording. In all the subjects, the EEG was recorded during wakefulness, photic stimulation and 3 min of hyperventilation. Blood samples were taken for genetic analysis.
Classification of epilepsy phenotypes
Seizure types and epilepsy syndromes were classified according to the International Classification of Seizures (Commission on Classification and Terminology of the International League Against Epilepsy, 1981) and Epilepsy Syndromes (Commission on Classification and Terminology of the International League Against Epilepsy, 1989) for each affected individual. Children with FS that persisted beyond 6 years of age or were associated with afebrile GTCS were designated as having febrile seizures plus (FS+) (Scheffer and Berkovic, 1997). Those individuals who had a history of seizures, but insufficient information to make an epilepsy syndrome diagnosis, were designated as having unclassified epilepsy. Those individuals in whom the history of seizures could not be verified by an eye-witness account were also considered as having unclassified epilepsy.
Genetic analysis
Mutational analysis of GABRG2(R43Q) on additional family members (I-2, II-2, II-3, III-2 and IV-16) not included in our initial report was performed by direct sequencing as previously described (Wallace et al., 2001a).
We also re-analysed our whole genome screen of this family, with only individuals with CAE considered affected. All other individuals with seizures were classified as ‘unknown’ for this analysis. The analysis was performed following our digenic model of IGEs (S. F. Berkovic, C. Marini, R. H. Wallace, F. L. Phillips, J. C. Mulley, I. E. Scheffer, unpublished results). This model predicts that the IGE subsyndromes are determined by a combination of mutations in two ion channel subunit genes. In its basic form, the digenic model is a simple two-locus heterogeneity model. The digenic model incorporates the presence of a number of loci and the IGE subsyndromes are due to various combinations of two such loci. Having found one gene segregating with individuals with CAE, FS and FS+, we predicted that a second locus would be largely restricted to those with the CAE phenotype.
After obtaining whole-genome screening data (provided by the Australian Genome Research Facility) from the newly ascertained individuals listed above, we performed a CAE affected-only analysis on the entire family. Individuals with FS or FS+ were coded as unknown in the linkage analysis (performed using FASTLINK v4.0). Two-point LOD scores were calculated assuming 50% penetrance and a 2% phenocopy rate. Additional markers were genotyped in regions that could not be excluded.
|
Results |
|---|
|
TopSummaryIntroductionMethodsResultsDiscussionReferences |
|---|
Genealogy
Ancestors of the family migrated to Australia from England and Ireland in the 18th century. All living members of the family resided in Melbourne, rural Victoria or New South Wales. There was no consanguinity in the family. From the genealogical and clinical information obtained, an extensive pedigree was constructed (Fig. 1).
Epilepsy phenotypes
Thirty-five individuals had a history of seizures. Their clinical details are presented in Table 1.
|
Childhood absence epilepsy (n = 8, see Fig. 1)
The first proband and seven other individuals of two consecutive generations had CAE (III-2, III-8, III-18, III-20, IV-17, IV-25, IV-27 and IV-28). Six were females and two were males, aged from 3 to 44 years. The mean age of onset of absences was 37 months, ranging from 10 months to 6 years (median 33 months). Typical absences were described with loss of awareness and staring for 5 to 10 s occurring many times per day. Individual IV-25 had daily absences with loss of awareness, occasional oral automatisms, infrequent falls and incontinence; duration was <10 s. Individual III-8 had absences with tonic/atonic components and marked clinical photosensitivity lasting <1 min. The mean age of offset of absences was 6 years (range 3–12 years). Individual IV-27 aged 4.5 years had ongoing absences.
All but two individuals (III-2 and III-8) had FS prior to the onset of absences. FS were simple, lasting <5 min in all. The mean number of FS was seven (ranging from one to 25); the median was four. One child (IV-17) had a febrile seizure lasting 2–3 min, affecting predominantly the left side, followed by a left Todd’s paresis. Four individuals (III-2, III-8, III-20 and IV-17) also had later afebrile GTCS petering out by 14 years of age. Three members (III-2, III-8 and IV-17) had learning problems.
Typical 3 Hz spike–wave activity was documented on EEG studies of five out of eight members with a clinical diagnosis of CAE (III-2, III-8, III-18, IV-27 and IV-28) (Fig. 2). One individual aged 8 years (IV-25) had bilateral centro-temporal spikes; at the time of the study, her absences were controlled on sodium valproate and could not be elicited by hyperventilation. Two subjects (III-20 and IV-17) had normal EEG recordings at the time of the study (37 and 5 years of age, respectively) and were in remission; earlier records were unavailable.
|
Of the eight CAE cases, all but one (III-2) had the GABRG2(R43Q) mutation. Interestingly, individual III-2 did not have FS.
Febrile seizures alone (n = 15, see Fig. 1)
There were five females and 10 males whose mean age was 21 years (range 6–41 years) at the time of the study. The mean number of FS was four, ranging from one to 20 (median two). The mean age of onset for FS was 14 months (median12 months), and that for offset was 26 months (median 18 months).
EEG was performed in 11 cases (aged from 5 to 38 years, mean 19, median 14) and was normal in 10; only individual III-24 showed left temporal sharp waves. Of the 15 cases with FS alone, 10 underwent DNA testing and all had the GABRG2(R43Q) mutation.
In addition to these 15 individuals, 11 others had seizures with fever, including six with later CAE (see above), three with FS+, one with myoclonic–astatic epilepsy (MAE) and one with IGE with GTCS only (see below).
Febrile seizures plus (FS+) (n = 3, see Fig. 1)
A 39-year-old woman (III-11) had four FS between the ages of 2 and 7 years. A 2-year-old boy (IV-29) had clusters of 10 or more GTCS, associated with illness. The majority of GTCS were febrile, but some were afebrile. As he was only 2 years old at the time of the study, his epilepsy syndrome may not have fully evolved. A 15-year-old girl (IV-19) had a total of nine GTCS, five with fever and four without, between the ages of 11 months and 5 years 6 months.
EEG was normal in two and showed left temporal epileptiform activity in one (III-11). All three had the GABRG2(R43Q) mutation.
Myoclonic–astatic epilepsy (MAE) (n = 2, see Fig. 1)
A 41-year-old-man (II-2) had 
10 FS from 6 to 12 months. Daily absence seizures with head nods began at 2 years, followed 3 years later by GTCS and occasional jerks in the morning. He was treated originally with phenobarbital and then with phenytoin and carbamazepine. He had 50 GTCS over a period of 20 years. His epilepsy was controlled at age 29 when sodium valproate was introduced. He had normal development but had difficulty in secondary school. Apart from borderline intellectual function, and facial coarseness, ataxia and nystagmus probably due to chronic phenytoin intoxication, his neurological examination was unremarkable. The report of an EEG performed at the age of 13 years described spikes during photic stimulation; the actual record was destroyed. The EEG at 41 years showed a slow background with bitemporal independent epileptiform activity. The GABRG2(R43Q) mutation was present.
A 14-year-old boy (IV-16) had seizure onset at 8 months with episodes of staring and unresponsiveness for a few seconds occurring 25–30 times a day. A month later, these episodes became associated with generalized stiffening, eyes rolling back, grey colour of the face and incontinence followed by falling. The episodes lasted <1 min and then he would resume his activities immediately. They occurred 5 times per day. The introduction of nitrazepam stopped the attacks. At around 2 years, marked photosensitivity became evident and staring and drop attacks would only occur in front of the television. At 6 years, myoclonic jerks and GTCS, provoked by photic stimulation, occurred. His seizures were controlled from 9 years on sodium valproate. He had normal early developmental milestones. Concerns regarding learning were evident from kindergarten. WISC III at 10 years showed a borderline intellect with significant difficulties in the verbal domain. His EEGs showed generalized 2–3 Hz spike–waves during rest, and brief discharges of generalized polyspike–waves during rest and photic stimulation. The GABRG2(R43Q) mutation was present.
Generalized epilepsy with tonic–clonic seizures alone (n = 1, see Fig. 1)
A 40-year-old woman (II-3) had 3–4 FS at 1 year of age. She was well until 13 years, when afebrile tonic–clonic seizures occurred. Prior to the seizure, she would look vague, and on one occasion was witnessed having manual automatisms. Auras, isolated absences and myoclonus were denied. She had several seizures over a period of 12 years. The last one was at the age of 25 years, and she remains on phenytoin and sodium valproate. Her EEG at the age of 7 years showed a 2.5–3 Hz generalized spike–wave during hyperventilation. She had the GABRG2(R43Q) mutation.
Partial epilepsy (n = 1, see Fig. 1)
A 55-year-old woman (II-6) developed complex partial seizures of temporal lobe origin at 8 years. Occasional secondary generalized seizures occurred. No details of her early childhood history could be obtained, so the occurrence of FS or absences was not known. Her course was complicated by a chronic paranoid psychosis that required frequent hospitalization and ultimate institutionalization. Her EEG showed epileptiform spikes over the right temporal region. An MRI showed several small foci in the cerebral white matter, most probably due to chronic small vessel disease. No epileptogenic lesions, including hippocampal sclerosis, were identified. She had the GABRG2(R43Q) mutation.
Unclassified epilepsy (n = 5, see Fig. 1)
A 36-year-old woman (III-15, second proband) had two GTCS at the age of 35 years. Her EEG within 24 h of the seizures did not show epileptiform abnormalities and her MRI was normal. Her mother was alive and confirmed that childhood seizures did not occur. The GABRG2(R43Q) mutation was present.
A 30-year-old woman (III-13) had three GTCS, two at 14 years of age and the third at 24 years. Several EEGs including a sleep study failed to show any epileptiform activity; her MRI was normal. The GABRG2(R43Q) mutation was absent.
Three family members (I-4, II-17 and II-18) in the first and second generations were identified as having a history of seizures. Insufficient information was available to diagnose an epilepsy syndrome and they were considered unaffected for the analysis.
EEG studies in unaffected individuals (n = 23)
EEG studies were also performed on 23 clinically unaffected family members (nine married-in individuals) including seven children aged 3–15 years (mean 8 years) and 16 adults aged 28–67 years (mean 45 years). A 3-year-old boy (IV-14) showed 2.5–3 Hz generalized spike–waves. He did not have the GABRG2(R43Q) mutation. Of the adults, only a married-in individual (III-28) showed generalized spike–wave discharges during photic stimulation without clinical correlate. She did not have the GABRG2(R43Q) mutation. None of the other family members had epileptiform discharges.
Genetic analysis
Genetic analysis of the pedigree was not consistent with X-linked or mitochondrial inheritance, as male to male transmission occurred. Autosomal recessive inheritance was also excluded, as affected members were found in three consecutive generations. FS appeared to have an autosomal dominant mode of inheritance with 75% penetrance. Bilineal inheritance was observed in one branch of the family (II-14 and II-15 offspring); however, this could not be investigated further because individuals II-17 and II-18 did not consent to the study. Two married-in individuals (III-21 and III-28) had a family history of seizures.
Genotyping and identification of GABRG2 mutation
Family members (I-2, II-2, II-3, III-2 and IV-16) not included in our initial report (Wallace et al., 2001a) were genotyped and examined for the GABRG2 mutation. Their mutation status is shown in Fig. 1 and discussed under the phenotypes described above. Of those with the GABRG2 mutation, 64% had FS (alone or as a precursor of other syndromes), 21% had CAE and 15% had a GEFS+ phenotype.
Affected-only analysis for CAE
Genome-wide linkage analysis was performed on the whole family assuming only individuals with CAE were affected (see Methods). Under this model, several regions where linkage to CAE could not be excluded (LOD scores >1.0) were detected (Table 2). After further genotyping in selected regions and re-analysis, no single region of the genome was found to be linked to every CAE case in this pedigree. The original linkage to chromosome 5q was detected and, as mentioned above, III-2 was a recombinant and did not carry the GABRG2(R43Q) mutation. A LOD score of 1.97 was obtained on chromosome 13 and one recombinant (IV-17) was present (Table 2 and Fig. 3A). Three other possible loci were identified where the LOD score was >1, on chromosomes 10, 14 and 15, and all three had two recombinants (Table 2 and Fig, 3B).
|
|
|
Discussion |
|---|
|
TopSummaryIntroductionMethodsResultsDiscussionReferences |
|---|
This large family, where a pathogenic GABAA receptor subunit gene defect has been found, provides insights into the molecular relationships of FS, FS+ and CAE. The size of this family has also allowed us to probe the difficult area of the complex genetics of CAE. Not only do we have strong evidence that the GABAA receptor gene contributes to this phenotype, but we have tentative evidence for a second locus.
Febrile seizures and GABRG2 mutation
FS are the most common form of childhood seizures, affecting 2–5% of all children (Hauser et al., 1985; Johnson et al., 1996). FS have a major genetic component with dominant inheritance in some families, but complex inheritance is probably operative in the majority of cases (Rich et al., 1987; Johnson et al., 1996). Three putative loci have been published for FS on chromosomes 8q13–q21, 19p and 5q14–q15 (Wallace et al., 1996; Johnson et al., 1998; Nakayama et al., 2000). No specific genes were identified until we reported a mutation in GABRG2 on chromosome 5q32–34 in this family where FS is inherited as an autosomal dominant trait with 75% penetrance (Wallace et al., 2001a). All 10 tested individuals with FS alone in our family had the mutation. Additionally, all 11 individuals with FS as a precursor to other syndromes had the mutation.
Generalized epilepsy with febrile seizure plus (GEFS+)
GEFS+ was described recently as a familial syndrome with a heterogeneous spectrum of phenotypes including FS, FS+ and MAE (Scheffer and Berkovic, 1997). Mutations have been found in the genes encoding ß and 
subunits of the neuronal sodium channel genes (SCN1B, SCN1A and SCN2A) in a number of large pedigrees (Wallace et al., 1998, 2001b, 2002; Escayg et al., 2000, 2001; Sugawara et al., 2001).
Simultaneously with our description of the GABRG2(R43Q) mutation in this kindred, a French family with GEFS+ was also found to have a different GABRG2 mutation (Baulac et al., 2001). Interestingly, in our pedigree, there were three individuals with FS+ and two with mild MAE, confirming that GABRG2 mutations can also contribute to GEFS+ phenotypes.
These clinical and molecular findings thus challenge us to ask what is the relationship between classical FS and GEFS+, and how this family should be classified. In GEFS+ families, individuals with a phenotype indistinguishable from classical FS are observed, and some have sodium channel mutations. However, the distinguishing clinical feature of previously described GEFS+ families is the common occurrence of the more extended phenotypes such as FS+, where seizures with fever extend beyond age 6 years and/or are associated with afebrile tonic–clonic seizures (Scheffer and Berkovic, 1997). In published families, individuals with the extended phenotypes account for 55–89% of cases with identified mutations within each family (Scheffer and Berkovic 1997; Baulac et al., 1999; Moulard et al., 1999; Singh et al., 1999; Escayg et al., 2001; Wallace et al., 2001b). In the present family, amongst those with the mutation, the proportion of those with extended phenotypes was 24% (five extended phenotypes, versus 10 with classical FS plus six with classical FS evolving into CAE).
One interpretation is that the GABRG2(R43Q) mutation is a gene for classical FS although it also contributes to the GEFS+ spectrum, presumably due to modifying genes. An alternative explanation is that the mutation is fundamentally for GEFS+ but where the predominant phenotype is simple FS.
Our recent findings of a small GEFS+ family (Harkin et al., 2002) with a truncation mutation in GABRG2 may clarify this dilemma. The mutation in the present family affects the extracellular benzodiazepine-binding domain, with no impairment of currents to GABA in vitro (Wallace et al., 2001a), whereas the mutations in the French and Australian GEFS+ families are located in the extracellular and intracellular loops of the mature 2 subunit and cause severe reduction of the current in response to GABA (Baulac et al., 2001; Harkin et al., 2002). Thus, we hypothesize that mutations in the benzodiazepine-binding domains predominantly cause classical FS, while mutations that reduce the response to GABA lead to GEFS+. A precedent for this is the phenotype–genotype correlation with the calcium channel subunit CACNA1A, where mutations at different sites cause a spectrum of overlapping phenotypes of migraine, ataxia and even seizures (Ophoff et al., 1998). Further analysis of GABA receptor genes in carefully characterized families may clarify the clinical and molecular relationships of GEFS+, IGE and classical FS phenotypes.
Childhood absence epilepsy: a specific phenotype?
In this large family, eight individuals had CAE. The phenotype was classical CAE with pyknoleptic absences affecting more girls (six out of eight), associated with typical 3 Hz spike and wave, and remission before puberty. Unusual features were the age of onset of absences before 4 years in five out of eight cases and the earlier occurrence of FS in six.
Although CAE is a well-recognized entity, it does not have crisp phenotypic boundaries and it is unclear whether CAE, as currently defined, comprises one or a number of entities (Berkovic et al., 1987). CAE may overlap with JAE or evolve into JME. Similarly, subtypes have been posited including a form with only self-remitting absence seizures, a subtype with absences and GTCS, a form with absences, GTCS and myoclonic jerks, and also eyelid myoclonia with typical absence epilepsy (EMA) (Appleton et al., 1993; Fong et al., 1998). Doose and colleagues described two types of absence epilepsy of childhood onset: one beginning before 5 years, associated with other seizure types such as myoclonic–astatic seizures, often refractory and with developmental delay and, a second with absences alone, later onset and more favourable course (Doose and Baier, 1973; Doose et al., 1989). The molecular and neurobiological distinction of these clinically posited subtypes presently is unknown.
The genetic relationship between FS and CAE is undoubted. Most studies show that FS occurs in 10–15% of CAE cases (Livingston et al., 1965; Penry et al., 1975; Dieterich et al., 1985; Loiseau, 1992; Berkovic, 1997). In multiplex families with CAE and other IGEs, 25% of affected relatives have FS alone (Italian League Against Epilepsy Genetic Collaborative Group, 1993; Bianchi and the Italian League Against Epilepsy Genetic Collaborative Group, 1995). In our family, the frequency of FS was unusually high in relatives. Also, the observation that six out of eight CAE patients had preceding FS was unusual.
This family also sheds light on the relationship of GEFS+ to classical IGE. Previously we described two large GEFS+ families where single individuals had typical IGE; those members subsequently were found not to have sodium channel mutations causing GEFS+, suggesting that the clinical association was fortuitous (Wallace et al., 1998, 2001b). In the present family, however, GEFS+ phenotypes of FS+ and MAE were associated with the GABRG2(R43Q) mutation, suggesting that there is a genetic relationship between IGE and GEFS+ phenotypes in some circumstances. This overlap is supported further by the finding of the mutation in II-13 who had IGE with preceding FS, and in III-8 with CAE without prior FS.
Genetic basis for CAE
The GABRG2(R43Q) mutation was found in seven out of eight CAE subjects in this pedigree. The causative role of mutations in GABRG2 for CAE is also supported by the recent finding of a small German family with CAE and FS in which a splice site mutation of the GABRG2 gene was found (Kananura et al., 2002). One CAE individual from our family (III-2), who did not have preceding FS, did not have the mutation. She is from a distant branch of the family and was only recently assessed, and thus was not included in the initial linkage study. Presumably, other genes were responsible for her phenotype.
Unlike FS, here inherited in an autosomal dominant fashion, CAE has complex inheritance. This large pedigree allowed testing of a digenic model. We hypothesized that the CAE phenotype in seven individuals of this family was due to the combination of the GABRG2(R43Q) mutation with a second gene segregating with the CAE phenotype. In the individual III-2 lacking the GABRG2(R43Q) mutation, the putative second gene segregating with CAE is combined with another mutation with the equivalent effect of the GABRG2(R43Q) mutation. The unusual density of CAE in this pedigree raised the possibility that we might identify a single second locus.
The affected-only analysis of CAE failed to reveal a single locus segregating with CAE. The most promising finding from a linkage point of view was on chromosome 13q where there was only a single recombinant (IV-17). However, this region is not known to harbour ion channel subunit genes, and no other obvious candidates have emerged. Weaker linkage evidence for a region on 15q was found where there were two recombinants (III-2 and III-8). This region is of interest because it includes other GABAA subunit genes, but mutation screening has been negative to date. Haplotype analysis of the locus on chromosome 14q22–q23 also revealed two recombinants. The GPHN gene, which codes for gephyrin, maps to this region on chromosome 14q. Gephyrin is essential for clustering and localizing GABA receptors in the plasma membrane (Essrich et al., 1998) and therefore considered a candidate for family members with CAE that map to chromosome 14q22–q23.
The clinical and molecular analysis of this family suggests that the GABRG2(R43Q) mutation is a cause of FS and may contribute to GEFS+. There is evidence that CAE is explained by an oligogenic or perhaps digenic model (Greenberg et al., 1988a, 1992; S. F. Berkovic, C. Marini, R. H. Wallace, F. L. Phillips, J. C. Mulley, I. E. Scheffer, unpublished results). One of those genes in this family is GABRG2, but even in such a high density pedigree there may be a complex interplay of multiple mutations. Nevertheless, this family has provided clues to the putative location of other CAE genes that require further evaluation in other pedigrees.
|
Acknowledgements |
|---|
We wish to thank the family for their participation in this study, Jan Barchett, Ann Godsil, Joanne Atkinson, Louise Feiler, Josie Curatolo, Yvonne Torn-Broers, Gayathri Parasivam, Kathryn Crossland, the EEG staff at the Royal Children’s Hospital and Robert Schultz for technical assistance, Dr Lloyd Shield and Dr Mark Mackay for the referral of the first proband, Dr Mark Newton (First Seizure Clinic, ARMC) for the referral of the second proband, and finally the clinicians who provided previous information on individuals studied. Functional studies of the GABRG2 mutation were kindly performed by Dr Steve Petrou, David Williams, Rekha Panchal and David Bowser, Department of Physiology, University of Melbourne. The study was supported by National Health and Medical Research Council, Women’s and Children’s Hospital Foundation, Bionomics Limited and a University of Melbourne Scholarship (to C.M.).
|
References |
|---|
|
TopSummaryIntroductionMethodsResultsDiscussionReferences |
|---|
Andermann E. Multifactorial inheritance of generalized and focal epilepsy. In: Anderson VE, Hauser WA, Penry JK, Sing CF, editors. Genetic basis of the epilepsies. New York: Raven Press; 1982. p. 129–45.
Annegers JF, Hauser WA, Anderson VE, Kurland LT. The risk of seizure disorders among relatives of patients with childhood onset epilepsy. Neurology 1982; 32: 174–79.
Appleton RE, Panayiotopoulos CP, Acomb BA, Beirne M. Eyelid myoclonia with typical absences: an epilepsy syndrome. J Neurol Neurosurg Psychiatry 1993; 56: 1312–16.[Abstract]
Baulac S, Gourfinkel-An I, Picard F, Rosenberg-Bourgin M, Prudhomme JF, Baulac M, et al. A second locus for familial generalized epilepsy with febrile seizures plus maps to chromosome 2q21–q33. Am J Hum Genet 1999; 65: 1078–85.[CrossRef][ISI][Medline]
Baulac S, Huberfeld G, Gourfinkel-An I, Mitropoulou G, Beranger A, Prud’homme JF, et al. First genetic evidence of GABA(A) receptor dysfunction in epilepsy: a mutation in the 2 subunit gene. Nature Genet 2001; 28: 46–8.[CrossRef][ISI][Medline]
Berkovic SF. Childhood absence epilepsy and juvenile absence epilepsy. In: Wyllie E, editor. The treatment of epilepsy: principles and practice. 2nd edn. Baltimore (MD): Williams & Wilkins; 1997. p. 461–6.
Berkovic SF, Andermann F, Andermann E, Gloor P. Concepts of absence epilepsies: discrete syndromes or biological continuum? [Review]. Neurology 1987; 37: 993–1000.
Berkovic SF, Howell RA, Hay DA, Hopper JL. Epilepsies in twins: genetics of the major epilepsy syndromes. Ann Neurol 1998; 43: 435–45.[CrossRef][ISI][Medline]
Bianchi A, the Italian League Against Epilepsy Collaborative Group. Study of concordance of symptoms in families with absence epilepsies. In: Duncan JS, Panayiotopoulos CP, editors. Typical absences and related epileptic syndromes. London: Churchill Communications; 1995. p. 328–37.
Cavazzuti GB. Epidemiology of different types of epilepsy in school age children of Modena, Italy. Epilepsia 1980; 21: 57–62.[ISI][Medline]
Commission on Classification and Terminology of the International League Against Epilepsy. Proposal for revised clinical and electroencephalographic classification of epileptic seizures. Epilepsia 1981; 22: 489–501. [ISI][Medline]
Commission on Classification and Terminology of the International League Against Epilepsy. Proposal for revised classification of epilepsies and epileptic syndromes. Epilepsia 1989; 30: 389–99. [ISI][Medline]
Dieterich E, Baier WK, Doose H, Tuxhorn I, Fichsel H. Long term follow-up of childhood epilepsy with absences. I. Epilepsy with absences at onset. Neuropediatrics 1985;16: 149–54.
Doose H, Baier WK. Absences. In: Beck-Mannagetta G, Anolerson EV, Doose H, Janz D, editors. Genetics of the epilepsies. Berlin: Spring-Verlag; 1989. p. 34–42.
Doose H, Gerken H, Horstmann T, Volzke E. Genetic factors in spike–wave absences. Epilepsia 1973; 14: 57–75.[ISI][Medline]
Durner M, Zhou G, Fu D, Abreu P, Shinnar S, Resor SR, et al. Evidence for linkage of adolescent-onset idiopathic generalized epilepsy to chromosome 8—and genetic heterogeneity. Am J Hum Genet 1999; 64: 1411–19.[CrossRef][ISI][Medline]
Durner M, Keddache MA, Tomasini L, Shinnar S, Resor SR, Cohen J, et al. Genome scan of idiopathic generalized epilepsy: evidence for major susceptibility gene and modifying genes influencing the seizure type. Ann Neurol 2001; 49: 328–35.[CrossRef][ISI][Medline]
Escayg A, MacDonald BT, Meisler MH, Baulac S, Huberfeld G, An-Gourfinkel I, et al. Mutations of SCN1A, encoding a neuronal sodium channel, in two families with GEFS+ 2. Nature Genet 2000; 24: 343–45.[CrossRef][ISI][Medline]
Escayg A, Heils A, MacDonald BT, Haug K, Sander T, Meisler MH. A novel SCN1A mutation associated with generalized epilepsy with febrile seizure plus—and prevalence of variants in patients with epilepsy. Am J Hum Genet 2001; 68: 866–73.[CrossRef][ISI][Medline]
Essrich C, Lorez M, Benson JA, Fritschy JM, Luscher B. Postsynaptic clustering of major GABAA receptor subtypes requires the 2 subunit and gephyrin. Nat Neurosci 1998; 1: 563–71.[CrossRef][ISI][Medline]
Fong GC, Shah PU, Gee MN, Serratosa JM, Castroviejo IP, Khan S, et al. Childhood absence epilepsy with tonic–clonic seizures and electroencephalogram 3–4-Hz spike and multispike–slow wave complexes: linkage to chromosome 8q24. Am J Hum Genet 1998; 63: 1117–29.[CrossRef][ISI][Medline]
Greenberg DA, Delgado-Escueta AV, Maldonado HM, Widelitz H. Segregation analysis of juvenile myoclonic epilepsy. Genet Epidemiol Suppl 1988a; 5: 81–94.
Greenberg DA, Delgado-Escueta AV, Widelitz H, Sparkes RS, Treiman L, Maldonado HM, et al. Juvenile myoclonic epilepsy (JME) may be linked to the BF and HLA loci on human chromosome 6. Am J Med Genet 1988b; 31: 185–92.[CrossRef][ISI][Medline]
Greenberg DA, Durner M, Delgado-Escueta AV. Evidence for multiple gene loci in the expression of the common generalized epilepsies. [Review]. Neurology 1992; 42 (4 Suppl 5): 56–62.[ISI][Medline]
Harkin LA, Bowser DN, Dibbens LM, Singh R, Phillips F, Wallace RH, et al. Truncation of the GABA(A)-receptor gamma2 subunit in a family with generalized epilepsy with febrile seizures plus. Am J Hum Genet 2002; 70: 530–6.[CrossRef][ISI][Medline]
Hauser WA, Annegers JF, Anderson VE, Kurland LT. The risk of seizure disorders among relatives of children with febrile convulsions. Neurology 1985; 35: 1268–73.
Italian League Against Epilepsy Genetic Collaborative Group. Concordance of clinical forms of epilepsy in families with several affected members. Epilepsia 1993; 34: 819–26. [CrossRef][ISI][Medline]
Janz D, Beck-Mannagetta G, Sander T. Do idiopathic generalized epilepsies share a common susceptibility gene? Neurology 1992; 42 (4 Suppl 5): 48–55.[ISI][Medline]
Johnson WG, Kugler SL, Stenroos ES, Meulener MC, Rangwalla I, Johnson TW, et al. Pedigree analysis in families with febrile seizures. Am J Med Genet 1996; 61: 345–52.[CrossRef][ISI][Medline]
Johnson EW, Dubovsky J, Rich SS, O’Donovan CA, Orr HT, Anderson VE, et al. Evidence for a novel gene for familial febrile convulsions, FEB2, linked to chromosome 19p in an extended family from the Midwest. Hum Mol Genet 1998; 7: 63–7.
Jouvenceau A, Eunson LH, Spauschus A, Ramesh V, Zuberi SM, Kullmann DM, et al. Human epilepsy with dysfunction of the brain P/Q-type calcium channel. Lancet 2001; 358: 801–7.[CrossRef][ISI][Medline]
Kananura C, Haug K, Sander T, Runge U, Gu W, Hallmann K, et al. A splice-site mutation in GABRG2 associated with childhood absence epilepsy and febrile convulsion. Arch Neurol 2002; 59: 1137–41.
Kruglyak L, Daly MJ, Reeve-Daly MP, Lander ES. Parametric and nonparametric linkage analysis: a unified multipoint approach. Am J Hum Genet 1996; 58: 1347–63.[ISI][Medline]
Lennox WG, Lennox MA. Epilepsy and related disorders. Boston (MA): Little, Brown; 1960. p. 548–74.
Livingston S, Torres I, Pauli LL, Rider RV. Petit mal epilepsy. Results of a prolonged follow-up study of 117 patients. J Am Med Assoc 1965; 194: 113–8.
Loiseau P. Childhood absence epilepsy. In: Roger J, Dravet C, Bureau M, Dreifuss FE, Wolf P, editors. Epileptic syndromes in infancy, childhood and adolescence. 2nd edn. London: John Libbey; 1992. p. 135–50.
Metrakos K, Metrakos JD. Genetics of convulsive disorders. II. Genetic and electroencephalographic studies in centrencephalic epilepsy. Neurology 1961; 11: 474–83.
Moulard B, Guipponi M, Chaigne D, Mouthon D, Buresi C, Malafosse A. Identification of a new locus for generalized epilepsy with febrile seizures plus (GEFS+) on chromosome 2q24–q33. Am J Hum Genet 1999; 65: 1396–400.[CrossRef][ISI][Medline]
Nakayama J, Hamano K, Iwasaki N, Nakahara S, Horigome Y, Saitoh H, et al. Significant evidence for linkage of febrile seizures to chromosome 5q14–q15. Hum Mol Genet 2000; 9: 87–91.
Olsson I. Epidemiology of absence epilepsy. Acta Paediatr Scand 1988; 77: 860–6.[ISI][Medline]
Ophoff RA, Terwindt GM, Frants RR, Ferrari MD. P/Q-type Ca2+ channel defects in migraine, ataxia and epilepsy. [Review]. Trends Pharmacol Sci 1998; 19: 121–7.[CrossRef][Medline]
Penry JK, Porter RJ, Dreifuss FE. Simultaneous recording of absence seizures with video tape and electroencephalography. A study of 374 seizures in 48 patients. Brain 1975; 98: 427–40.
Reutens DA, Howell RA, Gebert KE, Berkovic SF. Validation of a questionnaire for clinical seizure diagnosis. Epilepsia 1992;33: 1065–71.
Rich SS, Annegers JF, Hauser WA, Anderson VE. Complex segregation analysis of febrile convulsions. Am J Hum Genet 1987; 41: 249–57.[ISI][Medline]
Sander T, Kretz R, Schulz H, Sailer U, Bauer G, Scaramelli A, et al. Replication analysis of a putative susceptibility locus (EG1) for idiopathic generalized epilepsy on chromosome 8q24. Epilepsia 1998; 39: 715–20.[CrossRef][ISI][Medline]
Sander T, Schulz H, Saar K, Gennaro E, Riggio MC, Bianchi A, et al. Genome search for susceptibility loci of common generalised epilepsies. Hum Mol Genet 2000; 9: 1465–72.
Scheffer IE, Berkovic SF. Generalized epilepsy with febrile seizures plus. A genetic disorder with heterogeneous clinical phenotypes. Brain 1997; 120: 479–90.
Singh R, Scheffer IE, Crossland K, Berkovic SF. Generalized epilepsy with febrile seizures: a common childhood onset genetic epilepsy syndrome. Ann Neurol 1999; 45: 75–81.[CrossRef][ISI][Medline]
Sugawara T, Tsurubuchi Y, Agarwala KL, Ito M, Fukuma G, Mazaki-Miyazaki E, et al. A missense mutation of the Na+ channel II subunit gene Nav 1.2 in a patient with febrile and afebrile seizures causes channel dysfunction. Proc Natl Acad Sci USA 2001; 98: 6384–89.
Tsuboi T. Genetic risk in offspring of epileptic parents. In: Beck-Mannagetta G, Anderson EV, Doose H, Janz D. editors. Genetics of the epilepsies. Berlin: Springer-Verlag; 1989. p. 111–18.
Wallace RH, Berkovic SF, Howell RA, Sutherland GR, Mulley JC. Suggestion of a major gene for familial febrile convulsions mapping to 8q13–21. J Med Genet 1996; 33: 308–12.[Abstract]
Wallace RH, Wang DW, Singh R, Scheffer IE, George AL Jr, Phillips HA, et al. Febrile seizures and generalized epilepsy associated with a mutation in the Na+ channel ß1 subunit gene SCN1B. Nature Genet 1998; 19: 366–70.[CrossRef][ISI][Medline]
Wallace RH, Marini C, Petrou S, Harkin LA, Bowser DN, Panchal RG, et al. Mutant GABA(A) receptor 2- subunit in childhood absence epilepsy and febrile seizures. Nature Genet 2001a; 28: 49–52.[CrossRef][ISI][Medline]
Wallace RH, Scheffer IE, Barnett S, Richards M, Dibbens L, Desai RR, et al. Neuronal sodium-channel 1-subunit mutations in generalized epilepsy with febrile seizure plus. Am J Hum Genet 2001b; 68: 859–65.[CrossRef][ISI][Medline]
Wallace RH, Scheffer IE, Parasivam G, Barnett S, Wallace GB, Sutherland GR et al. Generalized epilepsy with febrile seizures plus: mutation of the sodium channel subunit SCN1B. Neurology 2002; 58: 1426–9
Westling B, Weissbecker K, Serratosa JM, Jara-Prado A, Alsonso ME, Cordova S, et al. Evidence for linkage of juvenile myoclonic epilepsy with absence to chromosome 1p [abstract]. Am Hum Genet 1996; 59 Suppl: A241.
Zara F, Bianchi A, Avanzini G, Di Donato S, Castellotti B, Patel PI, et al. Mapping of genes predisposing to idiopathic generalized epilepsy. Hum Mol Genet 1995; 4: 1201–7.
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  S. Baulac, I. Gourfinkel-An, P. Couarch, C. Depienne, A. Kaminska, O. Dulac, M. Baulac, E. LeGuern, and R. Nabbout A Novel Locus for Generalized Epilepsy With Febrile Seizures Plus in French Families Arch Neurol, July 1, 2008; 65(7): 943 - 951. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. Fedi, S. F. Berkovic, R. A. L. Macdonell, J. M. Curatolo, C. Marini, and D. C. Reutens Intracortical Hyperexcitability in Humans with a GABAA Receptor Mutation Cereb Cortex, March 1, 2008; 18(3): 664 - 669. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 E. Eugene, C. Depienne, S. Baulac, M. Baulac, J. M. Fritschy, E. Le Guern, R. Miles, and J. C. Poncer GABAA Receptor {gamma}2 Subunit Mutations Linked to Human Epileptic Syndromes Differentially Affect Phasic and Tonic Inhibition J. Neurosci., December 19, 2007; 27(51): 14108 - 14116. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 H. O. Tan, C. A. Reid, F. N. Single, P. J. Davies, C. Chiu, S. Murphy, A. L. Clarke, L. Dibbens, H. Krestel, J. C. Mulley, et al. Reduced cortical inhibition in a mouse model of familial childhood absence epilepsy PNAS, October 30, 2007; 104(44): 17536 - 17541. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
X.-B. Liu, J. Coble, G. van Luijtelaar, and E. G. Jones Reticular nucleus-specific changes in {alpha}3 subunit protein at GABA synapses in genetically epilepsy-prone rats PNAS, July 24, 2007; 104(30): 12512 - 12517. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 R. Nabbout, S. Baulac, I. Desguerre, N. Bahi-Buisson, C. Chiron, M. Ruberg, O. Dulac, and E. LeGuern New locus for febrile seizures with absence epilepsy on 3p and a possible modifier gene on 18p Neurology, April 24, 2007; 68(17): 1374 - 1381. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 T. Bessaih, L. Bourgeais, C. I. Badiu, D. A. Carter, T. I. Toth, D. Ruano, B. Lambolez, V. Crunelli, and N. Leresche Nucleus-Specific Abnormalities of GABAergic Synaptic Transmission in a Genetic Model of Absence Seizures J Neurophysiol, December 1, 2006; 96(6): 3074 - 3081. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 T. G. Hales, T. Z. Deeb, H. Tang, K. A. Bollan, D. P. King, S. J. Johnson, and C. N. Connolly An Asymmetric Contribution to {gamma}-Aminobutyric Type A Receptor Function of a Conserved Lysine within TM2-3 of {alpha}1, beta2, and {gamma}2 Subunits J. Biol. Chem., June 23, 2006; 281(25): 17034 - 17043. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|