Searching for human epilepsy genes: a progress report

Benign familial neonatal epilepsy with mutations in two potassium channel genes

The significant progress made over the past year in understanding the basis for a form of neonatal seizures can be attributed to the successful positional cloning of two new voltage-gated potassium channel genes. Expression studies have increased our understanding of the biology of these channels and their role in epilepsy.

Ion channels and the genetic contribution to epilepsy

Recent application of genetic analysis to rare, hereditary epilepsies has resulted in the identification of mutations in genes encoding ion channels or functionally related proteins in several human and animal syndromes. Reviewed here are selected human and murine epilepsies that result from ion channel mutations. In humans, three autosomal-dominant disorders--benign familial neonatal convulsions, nocturnal frontal lobe epilepsy, and "generalized epilepsy with febrile seizures plus"--result from mutations affecting voltage-sensitive potassium channels, a central nicotinic acetylcholine receptor, and a voltage-sensitive sodium channel, respectively. In mice, four genetically distinct, autosomal-recessive models of absence epilepsy are caused by mutations in genes encoding three types of calcium channel subunits and a sodium-hydrogen ion exchanger. These findings suggest that variation in genes encoding ion channels could determine susceptibility to common human epilepsies.

Potassium ion channels and human disease: phenotypes to drug targets?

A significant difficulty faced by the pharmaceutical industry is the initial identification and selection of macromolecular targets upon which de novo drug discovery programs can be initiated. A drug target should have several characteristics: known biological function; robust assay systems for in vitro characterization and high-throughput screening; and be specifically modified by and accessible to small molecular weight compounds in vivo. Ion channels have many of these attributes and can be viewed as suitable targets for small molecule drugs. Potassium (K+) ion channels form a large and diverse gene family responsible for critical functions in numerous cell types, tissues and organs. Recent discoveries, facilitated by genomics technologies combined with advanced biophysical characterization methods, have identified novel K+ channels that are involved in important physiologic processes, or mutated in human inherited disease. These findings, coupled with a rapidly growing body of information regarding modulatory channel subunits and high resolution channel structures, are providing the critical information necessary for validation of K+ channels as drug targets.

A pore mutation in a novel KQT-like potassium channel gene in an idiopathic epilepsy family

Epileptic disorders affect about 20-40 million people worldwide, and 40% of these are idiopathic generalized epilepsies (IGEs; ref. 1). Most of the IGEs that are inherited are complex, multigenic diseases. To address basic mechanisms for epilepsies, we have focused on one well-defined class of IGEs with an autosomal-dominant mode of inheritance: the benign familial neonatal convulsions (BFNC; refs 2,3). Genetic heterogeneity of BFNC has been observed. Two loci, EBN1 and EBN2, have been mapped by linkage analysis to chromosome 20q13 (refs 5,6) and chromosome 8q24 (refs 7,8), respectively. By positional cloning, we recently identified the gene for EBN1 as KCNQ2 (ref. 9). This gene, a voltage-gated potassium channel, based on homology, is a member of the KQT-like family. Here we describe an additional member, KCNQ3. We mapped this new gene to chromosome 8, between markers D8S256 and D8S284 on a radiation hybrid map. We screened KCNQ3 for mutations in the large BFNC family previously linked to chromosome 8q24 in the same marker interval. We found a missense mutation in the critical pore region in perfect co-segregation with the BFNC phenotype. The same conserved amino acid is also mutated in KVLQT1 (KCNQ1) in an LQT patient. KCNQ2, KCNQ3 and undiscovered genes of the same family of K+ channels are strong candidates for other IGEs.

A novel potassium channel gene, KCNQ2, is mutated in an inherited epilepsy of newborns

Idiopathic generalized epilepsies account for about 40% of epilepsy up to age 40 and commonly have a genetic basis. One type is benign familial neonatal convulsions (BFNC), a dominantly inherited disorder of newborns. We have identified a sub-microscopic deletion of chromosome 20q13.3 that co-segregates with seizures in a BFNC family. Characterization of cDNAs spanning the deleted region identified one encoding a novel voltage-gated potassium channel, KCNQ2, which belongs to a new KQT-like class of potassium channels. Five other BFNC probands were shown to have KCNQ2 mutations, including two transmembrane missense mutations, two frameshifts and one splice-site mutation. This finding in BFNC provides additional evidence that defects in potassium channels are involved in the mammalian epilepsy phenotype.

Linkage mapping of benign familial infantile convulsions (BFIC) to chromosome 19q

Benign familial infantile convulsions (BFIC) are an autosomal-dominant epileptic syndrome characterized by an age of onset within the first year of life. Although they were first reported in families of Italian descent, BFIC have also been described in non-Italian families. We have mapped the BFIC gene to chromosome 19 by linkage analysis in five Italian families with a maximum two-point lod score of 6.36 at D19S114; maximum multipoint lod scores > 8 were obtained for the interval D19S250-D19S245. BFIC are therefore the third idiopathic partial epileptic syndrome to be mapped on the human genome.

Epilepsies with single gene inheritance

Single gene disorders offer the best opportunity for identification of genetic linkage and of abnormal genes. Epilepsies with single gene inheritance include symptomatic epilepsies where there is associated diffuse brain dysfunction, and idiopathic epilepsies where seizures are the major neurological abnormality. There are over 200 single gene symptomatic epilepsies; most are rare. Gene identification has been achieved in a number of these conditions but these important advances have not yet led to a better understanding of epileptogenesis, because of the associated brain disease. Idiopathic single gene epilepsies include benign familial neonatal convulsions, where genetic linkage to chromosomes 20q and 8q has been found in different families, and benign familial infantile convulsions where linkage is presently unknown. Recently, four autosomal dominant partial epilepsies have been described. In autosomal dominant nocturnal frontal lobe epilepsy a genetic defect in the alpha 4 subunit of the nicotinic acetylcholine receptor was found in one family. This is the first genetic defect described in an idiopathic epilepsy. The other three syndromes are autosomal dominant partial epilepsy with variable foci, autosomal dominant rolandic epilepsy with speech dyspraxia, and familial temporal lobe epilepsy. In the latter condition, linkage to chromosome 10q has been reported in one family, but the genetic defect is unknown. It is likely that other idiopathic single gene epilepsies will be identified. Molecular genetic study of these disorders is likely to lead to discovery of other epilepsy genes. This will lead to an improved understanding of human epileptogenesis with implications for clinical diagnosis, genetic counselling, pharmacological therapy and possibly prevention of epilepsy.

Evidence of a third locus for benign familial convulsions

Two autosomal dominant forms of benign idiopathic epilepsy of early life have been described: benign neonatal familial convulsions and benign infantile familial convulsions. Herein we describe a pedigree with familial convulsions in which the age of onset is intermediate between that seen in these two disorders. Two genes responsible for benign neonatal familial convulsions have been mapped to chromosome 20q and to chromosome 8q. Previously, the chromosome 20q benign neonatal familial convulsions locus had been excluded in this pedigree. Further linkage analysis in our laboratory revealed that the chromosome 8 benign neonatal familial convulsions locus also is not responsible for seizures in this pedigree. These results indicate that there are at least three loci responsible for autosomal dominant benign epilepsies of early life.

Pontine reticular formation neurons exhibit a premature and precipitous increase in acoustic responses prior to audiogenic seizures in genetically epilepsy-prone rats

The genetically epilepsy-prone rat (GEPR-9) exhibits elevated seizure sensitivity and audiogenic seizures (AGS). The pontine reticular formation (PRF) is implicated in the neuronal network for AGS in the GEPR-9. The present study examined PRF neuronal firing and convulsive behavior simultaneously in the GEPR-9. Chronically implanted microwire electrodes in PRF allowed single neuronal responses and behavior to be examined in freely-moving rats. PRF neurons in the GEPR-9 exhibit precipitous intensity-evoked increases at a significantly lower (approx. 15 dB SPL) intensity than normal Sprague-Dawley rats. PRF neurons in the GEPR-9 also exhibit increased auditory response latencies. At the onset of AGS (wild running) the firing rate of PRF neurons increased, and the rate of PRF firing increased dramatically as the tonic phase of the seizure began. During post-ictal depression the rate of PRF neuronal firing slowed, gradually returning to normal. This pattern of PRF periseizural neuronal firing changes differ dramatically in pattern and temporal characteristics from those previously observed in inferior colliculus (IC). The IC serves as the AGS initiation site. IC neurons show extensive firing increases prior to and during the initial wild running, silence during the tonic and post-ictal phases, and gradual recovery of responses thereafter. The changes in PRF neuronal firing pattern suggest that the PRF may play a major role in the generation of the tonic phase of AGS. The premature onset of the precipitous rise in PRF neuronal firing suggests that the influence of the IC on PRF neurons may be magnified in association with AGS susceptibility. The PRF neuronal firing increases observed in the present study coupled with previous observation of AGS blockade by PRF microinjections in the GEPR-9 further support an important role of the PRF in the propagation of AGS in the GEPR-9. The mechanisms of PRF firing elevation may also be relevant in other seizure models in which the brain-stem reticular formation is implicated.

Genetic linkage studies in familial frontal epilepsy: exclusion of the human chromosome regions homologous to the El-1 mouse locus

Familial frontal epilepsy has been recently described in six pedigrees. All families reported show autosomal dominant inheritance with incomplete penetrance. Affected individuals develop predominantly nocturnal seizures with frontal lobe semiology. In 1959, a genetic mouse model for partial epilepsy, the El mouse, was reported. In the El mouse, a major seizure susceptibility gene, El-1, segregates in an autosomal dominant fashion and has been localized to a region distal to the centromere of mouse ch 9. Comparative genetic maps between man and mouse have been used to predict the location of several human disease genes. The El-1 locus in the mouse is homologous to human chromosomes 3p23-p21.2, 3p11.2-q11.2, 3q21-q25.3, 6p12-q12 and 15q24. Polymorphic microsatellite markers covering these candidate regions were used for genotyping individuals in the three larger families ascertained, one of which is French-Canadian and two are Australian. Significant negative two-point and multipoint lod scores were obtained separately for each family, thus excluding linkage with the candidate regions on chromosomes 3, 6 and 15.