Evaluation of a putative major susceptibility locus for juvenile myoclonic epilepsy on chromosome 15q14

A 5.6-Mb deletion in 15q14 in a boy with speech and language disorder, cleft palate, epilepsy, a ventricular septal defect, mental retardation and developmental delay

We report a male patient with speech and language disorder, cleft palate, epilepsy, a ventricular septal defect, mental retardation and developmental delay. Characteristic facial features include low-set ears, a beak-like nose, a prominent nasal bridge, a long philtrum, a narrow forehead, a long face, a pointed chin and dental position abnormalities. Array-comparative genomic hybridization (CGH) analysis demonstrated the presence of a 5.6-Mb deletion in 15q14 (chromosome 15: 3,18,33,000-3,74,77,000bp). The present case provides the evidence that 15q14 deletion outside the region encompassing the CHRNA7 gene can cause generalized epilepsy, and a locus in 15q14 is associated with speech and language disorder.

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.

Candidate gene analysis of the human metabotropic glutamate receptor type 4 (GRM4) in patients with juvenile myoclonic epilepsy

Hereditary factors play a major role in the genetically complex etiology of juvenile myoclonic epilepsy (JME). Linkage studies in families of JME probands suggest a susceptibility locus (EJM1) for idiopathic generalized epilepsy (IGE) in the chromosomal region 6p21.3 near the HLA region. The gene encoding the metabotropic glutamate receptor type 4 (GRM4) has been localized within the EJM1-region and represents a high-ranking candidate gene. Therefore, we have sequenced the coding regions and regulatory GRM4 sequences in 20 IGE probands who were derived from families of JME probands providing positive linkage evidence to the HLA-DQ locus. Our mutation analysis detected three synonymous exonic single nucleotide polymorphisms (SNP; exon-7: c.1455T > C, exon-8: c.2002A > G, exon-10: c.2733C > T), one SNP in the 3'-untranslated region (c.2890A > G), and two intronic SNPs (intron-3: IVS3 + 2732A > G, intron-7: IVS7 + 39C > T). None of the identified SNPs was likely to affect receptor function or gene expression. The population-based association study did not show significant differences in the allele and genotype frequencies of the common c.1455T > C SNP between 144 German JME probands and 144 healthy population controls (P > 0.84). Likewise, the family-based transmission disequilibrium test did not indicate a preferential transmission of exon-7 SNP alleles in 31 informative parent-child transmissions (P = 0.86). Our results provide no evidence that genetic variation of the GRM4 gene confers susceptibility to JME-related IGE syndromes.

Evaluation of the positional candidate gene CHRNA7 at the juvenile myoclonic epilepsy locus (EJM2) on chromosome 15q13-14

A previous study of 34 nuclear pedigrees segregating juvenile myoclonic epilepsy (JME) gave significant evidence of linkage with heterogeneity to marker loci on chromosome 15q13-14 close to the candidate gene CHRNA7 (Hum. Mol. Genet. 6 (1997) 1329). The aim of this work was to further evaluate the putative aetiological role of CHRNA7 in JME within the 34 families originally described, and to assess the contribution of this locus to a broader phenotype of idiopathic generalised epilepsy (IGE). Multipoint linkage analysis and intrafamilial association studies were performed with microsatellite markers that encompass both CHRNA7 and its partial duplication (CHRFAM7A). A maximum HLOD of 3.45 [alpha=0.58; (Zall=2.88, P=0.0008)] was observed 8 cM distal to D15S1360, a CHRNA7 intragenic marker. Significant exclusion lod scores were obtained across the region in 12 mixed phenotype JME/IGE families. Mutation screening of the CHRNA7 gene (and consequently exons 5-10 of CHRFAM7A) and its putative promoter sequence identified a total of 13 sequence variants across 23 of 34 JME-affected families. Two variants (c.1354G>A and c.1466C>T) are predicted to result in amino acid changes and one (IVS9+5G>A) is predicted to result in aberrant transcript splicing. However, none of the variants alone appeared either necessary or sufficient to cause JME in the families in which they occurred. In conclusion, linkage analyses continue to support the existence of a locus on chromosome 15q13-14 that confers susceptibility to JME but not to a broader IGE phenotype. Causal sequence variants in the positional candidate CHRNA7 have not been identified but the presence of multiple segmental duplications in this region raises the possibility of undetected disease-causing genomic rearrangements.

Absence of alpha7-containing neuronal nicotinic acetylcholine receptors does not prevent nicotine-induced seizures

Nicotine is the primary addictive component in tobacco, and at relatively low doses it affects cardiovascular responses, locomotor activity, thermoregulation, learning, memory, and attention. At higher doses nicotine produces seizures. The mechanisms underlying the convulsive effects of nicotine are not known, but studies conducted on a number of inbred strains of mice have indicated a positive correlation between the number of alpha-bungarotoxin (alpha-BTX) binding sites in the hippocampus and the sensitivity to nicotine-induced seizures. Because alpha7-containing neuronal nicotinic acetylcholine receptors (nAChRs) represent the major binding site for alpha-BTX, mice lacking the alpha7 nAChR subunit were predicted to be less sensitive to the convulsive effects of nicotine. To test this hypothesis, we injected nicotine intraperitoneally in alpha7 mutant mice and found that the dose-response curve for nicotine-induced seizures was similar in the alpha7 +/+, alpha7 +/- and alpha7 -/- mice. The retained sensitivity to the convulsant effects of nicotine could not be explained by the presence of cholinergic compensatory mechanisms such as increases in mRNA levels for other nAChR subunits, or changes in binding levels or affinity for nicotinic ligands such as epibatidine and nicotine. These findings indicate that alpha7 may not be necessary for the mechanisms underlying nicotine-induced seizures.

Spike-and-wave discharges of absence seizures as a transformation of sleep spindles: the continuing development of a hypothesis

OBJECTIVES

This review aims to offer a critical account of recent scientific developments relevant to the hypothesis which Pierre Gloor proposed in the 1970s for the generation of spike and wave discharges (SWDs) of primary generalized absence seizures.

RESULTS

According to this hypothesis SWDs develop in the same circuits, which normally generate sleep spindles, by an initially cortical transformation of one every two or more spindle waves to a 'spike' component of SWDs, while the next one or more spindle waves are eliminated and replaced by a slow negative wave. This hypothesis was based on experiments in feline generalized penicillin epilepsy showing the possibility of transition from spindles to SWDs, when cortical neurons become hyper-responsive to thalamocortical volleys, which normally induce spindles, and thus engage feedback cortical inhibition, rebound excitation, recurrent intracortical dissemination of excitation during the 'spike' and strong excitation of thalamus for further augmentation of a brain wide synchronous oscillation. In the 1980s, electrophysiological studies in vitro and in vivo revealed the basic features of spindle rhythm generation by neurons in nucleus reticularis thalami and thalamocortical-corticothalamic oscillatory reverberations.

CONCLUSIONS

In the light of this knowledge, experimental studies in several genetic and pharmacological animal models of absence seizures, clinical observations and theoretical studies in computer models have considered, tested, modified and challenged this hypothesis. It may still be found useful in the era of dynamic digital EEG analysis of SWDs and its current sources.

Genetics of the epilepsies

Molecular genetic analysis of mendelian epilepsies in humans and mice has revealed a diversity of underlying genes in symptomatic epilepsies associated with disordered brain development and neuronal survival. In contrast, the idiopathic mendelian epilepsies have emerged as a new category of channelopathies. New epilepsy loci have been mapped and one new epilepsy gene isolated. Functional analysis of epilepsy genes is providing new insights into the pathways that lead from mutant gene to hyperexcitable neurones. The major challenge for the future is the analysis of genetic epilepsies with complex inheritance.

Neuronal nicotinic receptors in human epilepsy

Autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) is a rare monogenic idiopathic partial epilepsy characterized by clusters of frontal lobe motor seizures during sleep. Recently, it has been shown that mutations of the chromosome-20q-located neuronal nicotinic acetylcholine receptor alpha4-subunit (CHRNA4) are associated with ADNFLE in some families, but that other families are not linked to this locus. Both CHRNA4 mutations (Ser248Phe and 776ins3) identified so far are found in the pore-forming second transmembrane region of the gene. Electrophysiological studies showed that mutations in this functional important part of the receptor subunit have a profound effect on the permeability for calcium ions. Interestingly, the Ser248Phe mutation was found again in a second ADNFLE family. Haplotype analysis excluded a founder effect and showed that Ser248Phe occurred independently twice. This provides the possibility to study the effect of the same mutation on different genetic backgrounds. Several attempts have been made to identify additional genes responsible for ADNFLE. But despite some positive linkage results including the CHRNA3-CHRNA5-CHRNB2 cluster on chromosome 15q24, no further mutations have been found so far. The mutation screening of functionally important parts of CHRNA5 in 12 ADNFLE patients did not support a causative role of this nicotinic acetylcholine receptor subunit.