Ion channels and epilepsy in man and mouse

Inherited disorders of voltage-gated ion channels are a recently recognized etiology of epilepsy in the developing and mature central nervous system. Two human epilepsy syndromes, benign familial neonatal convulsions and generalized epilepsy with febrile seizures plus, represent K+ and Na+ channelopathies, and other newly defined syndromes have now been mapped to chromosomal regions that are rich in ion channel genes. Experimental mouse models promise a resolution of their intriguing pathophysiology, which includes a diverse array of cellular phenotypes consistent with the differential contributions of individual channels to excitability in neural networks.

Independent occurrence of the CHRNA4 Ser248Phe mutation in a Norwegian family with nocturnal frontal lobe epilepsy

PURPOSE

To describe the clinical features of a family from Northern Norway in which autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) is associated with a Ser248Phe amino acid exchange in the second transmembrane domain of the neuronal nicotinic acetylcholine receptor alpha4 subunit (CHRNA4). We also tested for evidence of a de novo mutation or founder effect by comparing haplotypes with the original Australian family where the Ser248Phe mutation was first described.

METHODS

Clinical details were obtained from 19 family members. Personal interviews and genetic analysis were carried out in 17. Parts of the coding region of CHRNA4 were sequenced, and two known polymorphisms (bp555/FokI, bp594/CfoI) were typed by restriction analysis.

RESULTS

Eleven individuals had ADNFLE. The haplotypes of the mutation-carrying alleles of affected individuals from the Northern Norwegian and the Australian ADNFLE family are different. The phenotypic expressions are remarkably similar.

CONCLUSIONS

The Ser248Phe mutation occurred independently in both families. Given the rarity of the disease, this suggests that not only the position of a mutation in the coding sequence but also the type of an amino acid exchange is important for the etiology of ADNFLE. The phenotypic similarity of these two families with different genetic backgrounds suggests that the Ser248Phe mutation largely determines the phenotype, with relatively little influence of other background genes.

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.

Mutation analysis of the inwardly rectifying K(+) channels KCNJ6 (GIRK2) and KCNJ3 (GIRK1) in juvenile myoclonic epilepsy

Genetic factors play a major role in the etiology of idiopathic generalized epilepsy. However, in most syndromes, especially the common ones, multiple genetic factors seem to be involved. Mutations in K(+) channel genes have previously found to be associated with epilepsy both in humans and in mice. The weaver mice phenotype, characterized by ataxia, tremor, male infertility, and tonic-clonic seizures, is caused by a point mutation in the inwardly rectifier K(+) channel gene KCNJ6 (GIRK2). A knockout mouse model deprived of functional KCNJ6 protein is susceptible to spontaneous and provoked seizures without showing the histological signs of neuronal cell death found in the weaver mouse. Thus, the KCNJ6 gene seems to play an important role in seizure control. We therefore performed a mutation analysis of KCNJ6 and the related KCNJ3 gene in 38 patients with juvenile myoclonic epilepsy (JME). Two novel same-sense nucleotide exchanges were identified, but none of these changed the coding sequence. These results do not support a major role for the KCNJ6/KCNJ3 heteromeric receptor in the etiology of JME. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 96:8-11, 2000

A KCNQ2 splice site mutation causing benign neonatal convulsions in a Scottish family

Benign familial neonatal convulsions (BFNC) are one of the rare idiopathic epilepsies with autosomal dominant mode of inheritance. Two voltage-gated potassium channels, KCNQ2 on chromosome 20q13.3 and KCNQ3 on 8q24, have been recently identified as the genes responsible for BFNC. Here we describe a large family with BFNC in which we found a previously undescribed mutation in the KCNQ2 gene. A 1187(+2)T/G nucleotide exchange affects the conserved donor splice site motif in intron 9. This mutation can be predicted to give rise to aberrant splicing of the primary transcript. There was a wide range of clinical manifestations in this family. An unusual clinical feature is the occurrence of partial seizures in later life with corresponding focal neurological deficits.

Mutation screening of the CHRNA4 and CHRNB2 nicotinic cholinergic receptor genes in Alzheimer's disease

Potential genomic changes leading to decreased nicotine binding, crucial for cognitive dysfunction in Alzheimer's disease (AD), have not yet been studied. A search for mutations of the genes coding for the most widely distributed nicotinic receptor subtype alpha4beta2 (CHRNA4/CHRNB2) has been performed in AD patients by screening the coding regions of both genes by single strand conformation analysis and heteroduplex analysis of fibroblast-derived genomic DNA. Polymorphisms in CHRNA4, none of which led to amino acid changes in the predicted sequence, were found in three patients. Although the other receptor subunits have yet to be screened, it appears likely that the reduction of nicotine binding sites in AD is not due to genomic changes.

REVIEW ■ : The Genetic Basis of Epilepsy: Mutant Alleles of Ligand- and Voltage-Gated Ion Channels

Idiopathic epilepsies are a heterogenous group of conditions characterized by different types of seizures, ages of onset, and EEG features. By definition, idiopathic epilepsies show no underlying cause other than a possible inherited predisposition. For most syndromes, especially the common ones, the mode of inheritance is complex rather than monogenetic. However, some rare idiopathic epilepsies show an autosomal dominant mode of inheritance, and genetic studies have been most successful in these diseases. Thus far, genes underlying the rare syndromes of autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE), benign familial neonatal convulsions (BFNC), and generalized epilepsy with febrile seizures plus (GEFS+) have been cloned. The first genes that were identified code for different types of ion channel proteins. Thus, it is likely that ion channels play an important role in the etiology of complex inherited forms of idiopathic epilepsies. A detailed under standing of the pathophysiological implications of these results could have important implications for the de velopment of target-specific antiepileptic drugs. NEUROSCIENTIST 5:295-301, 1999

A reduced K+ current due to a novel mutation in KCNQ2 causes neonatal convulsions

Benign familial neonatal convulsions (BFNC) is a rare dominantly inherited epileptic syndrome characterized by frequent brief seizures within the first days of life. The disease is caused by mutations in one of two recently identified voltage-gated potassium channel genes, KCNQ2 or KCNQ3. Here, we describe a four-generation BFNC family carrying a novel mutation within the distal, unconserved C-terminal domain of KCNQ2, a 1-bp deletion, 2513delG, in codon 838 predicting substitution of the last seven and extension by another 56 amino acids. Three family members suffering from febrile but not from neonatal convulsions do not carry the mutation, confirming that febrile convulsions and BFNC are of different pathogenesis. Functional expression of the mutant channel in Xenopus oocytes revealed a reduction of the potassium current to 5% of the wild-type current, but the voltage sensitivity and kinetics were not significantly changed. To find out whether the loss of the last seven amino acids or the C-terminal extension because of 2513delG causes the phenotype, a second, artificial mutation was constructed yielding a stop codon at position 838. This truncation increased the potassium current by twofold compared with the wild type, indicating that the pathological extension produces the phenotype, and suggesting an important role of the distal, unconserved C-terminal domain of this channel. Our results indicate that BFNC is caused by a decreased potassium current impairing repolarization of the neuronal cell membrane, which results in hyperexcitability of the central nervous system.

Mutated nicotinic receptors responsible for autosomal dominant nocturnal frontal lobe epilepsy are more sensitive to carbamazepine

PURPOSE

The recent linkage between a genetically transmissible form of epilepsy (ADNFLE) and mutations within the alpha4 subunit, one component of the major brain neuronal nicotinic acetylcholine receptor (nAChR), raises the question of the role of this receptor in epileptogenesis. Although acting by different mechanisms, the two genetic alterations so far identified both render the nAChR less efficient. In view of the high sensitivity of ADNFLE to carbamazepine (CBZ), we studied the effects of this drug and of valproate (VPA) on the human alpha4beta2 nAChR and its mutations.

METHODS

The alpha4beta2 nAChRs from control and mutant alpha4 subunits were reconstituted in Xenopus oocytes and investigated by using a dual-electrode voltage clamp technique. Acetylcholine (ACh)-evoked currents recorded in the absence or presence of antiepileptic drugs (AEDs) were studied to analyze the mode of action of these compounds.

RESULTS

ACh-evoked currents at the human alpha4beta2 nAChR were readily and reversibly inhibited by approximately 100 microM CBZ. This compound was found to be a noncompetitive inhibitor of the nAChR, which probably acts by entering the channel and causing a blockade by steric hindrance. Dose-response inhibition curves determined on the control receptor and on ADNFLE-mutant receptors showed a greater sensitivity of the mutants to CBZ, with median inhibitory concentrations (IC50s) in the range of the antiepileptic plasma levels of CBZ. In contrast, VPA had nearly no effect on control and mutant nAChRs.

CONCLUSIONS

CBZ inhibits the neuronal alpha4beta2 nAChRs at pharmacologic concentrations, with ADNFLE mutants displaying about threefold higher sensitivity to this compound. The increased sensitivity of these mutant receptors supports the hypothesis that the antiepileptic activity of CBZ can, at least to some extent, be attributed to the nAChR inhibition.

Idiopathic epilepsies with a monogenic mode of inheritance

Idiopathic epilepsies account for approximately 40% of all epileptic diseases. For a long time, it has been known that genetic factors play a major role in the etiology of these diseases. Although oligogenic or polygenic inheritance is suspected in most of the common syndromes, a few rare idiopathic epilepsies are single-gene disorders. They offer a chance to identify candidate genes that also may be involved in epilepsies with complex inheritance. In recent years, major progress has been made regarding the analysis of genetic factors in idiopathic epilepsy. For the first time, gene defects could be linked to two idiopathic epilepsies. Mutations in the CHRNA4 gene, which codes for the alpha4 subunit of the neuronal nicotinic acetylcholine receptor, lead to autosomal dominant nocturnal frontal lobe epilepsy, a rare idiopathic partial epilepsy syndrome. Two highly homologous voltage-gated potassium channels, KCNQ2 and KCNQ3, were found to be mutated in benign familial neonatal convulsions.

Gene defects in idiopathic epilepsy

Idiopathic epilepsies are mainly due to genetic factors. In most cases the mode of inheritance is either oligogenic or multifactorial. Only a few rare idiopathic epilepsies are single gene disorders. Monogenic epilepsies offer the chance to identify genes/gene families which might also be involved in the aetiology of common forms of the disease. The genetic basis of two monogenic epilepsies have recently been identified: autosomal dominant nocturnal frontal lobe epilepsy and benign familial neonatal convulsions.

The voltage gated potassium channel KCNQ2 and idiopathic generalized epilepsy

Mutations in the voltage gated potassium channel gene KCNQ2 and the homologous gene KCNQ3 have been found to cause a rare monogenic subtype of idiopathic generalized epilepsy, the benign familial neonatal convulsions. Recently, the heteromeric KCNQ2/KCNQ3 channel was found to contribute to the native M-current, one of the most important regulators of neuronal excitability. By performing a systematic mutation scan of the coding region and an association study involving a frequent Thr752Asn substitution polymorphism, we, therefore, investigated whether allelic variation of the KCNQ2 gene confers susceptibility to common subtypes of idiopathic generalized epilepsy. Our results do not provide evidence that allelic variation of the KCNQ2 gene contributes a common and relevant effect to the pathogenesis of common subtypes of idiopathic generalized epilepsy.

Structural and mutational analysis of KCNQ2, the major gene locus for benign familial neonatal convulsions

Mutations in the voltage-gated potassium channel gene KCNQ2 on chromosome 20q13.3 are responsible for benign familial neonatal convulsions (BFNC), a rare monogenic idiopathic epilepsy. Here we report the determination of the detailed genomic structure of KCNQ2, and use of this information in mutational analysis. There are at least 18 exons, occupying more than 50 kb of genomic DNA. Several formerly unknown polymorphisms and splice variants as well as a new single base pair deletion mutation of unusual localization are described. In addition to facilitating more effective mutation detection among BFNC patients, the results presented here provide the basis for analysing the role of KCNQ2 in other types of epilepsy.

Autosomal dominant nocturnal frontal lobe epilepsy: an electroclinical study of a Norwegian family with ten affected members

PURPOSE

The aim of the study was to describe in detail the electroclinical findings associated with a mutation in the acetylcholine receptor in a Norwegian family with autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE). Furthermore, we compared the clinical features associated with this mutation with those of an Australian family with a different mutation at the same locus, as well as with those of eight Italian families with ADNFLE and without a verified mutation in this gene.

METHODS

We obtained medical records from all of the 10 known affected members of the Norwegian family. A personal interview and a clinical neurologic examination were carried out in six of them. Interictal and ictal scalp EEG recordings were obtained in eight and three, respectively, computed tomography/magnetic resonance imaging (CT/MRI) in five, and blood samples for genetic analysis in seven individuals. The clinical features after an insertion of a leucine residue in the alpha4 subunit of the neuronal nicotinic acetylcholine receptor are examined. Furthermore, the clinical features that accompany this insertion and the clinical features associated with a missense mutation (Ser248Phe) in the same gene were compared.

RESULTS

All the affected individuals had a seizure semiology consistent with frontal lobe seizures. Their seizures started in childhood (mean age, 8 years) and were often misinterpreted as benign nocturnal parasomnias, nocturnal paroxysmal dystonia, or a psychiatric disorder. The affected family members were of normal intellect and showed no abnormalities at neurologic and neuroradiologic examinations. Interictal scalp EEG registrations were mostly normal, ictal scalp EEG registrations in three individuals revealed left frontal low-voltage epileptiform discharges in two, and only shallow arousal preceding the attack in one. Although the seizure susceptibility varied among the affected individuals, the epilepsy course was mostly benign.

CONCLUSIONS

Patients with ADNFLE, either with the 776ins3 mutation or the Ser248Phe mutation, and those without any recognized mutation in the acetylcholine receptor, have strikingly homogeneous phenotypes, and it seems difficult to separate them on clinical grounds.

The structures of the human neuronal nicotinic acetylcholine receptor beta2- and alpha3-subunit genes (CHRNB2 and CHRNA3)

The alpha4-subunit gene (CHRNA4) of the neuronal nicotinic acetylcholine receptor (nAChR) subunit family has recently been identified in two families as the gene responsible for autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE), a rare monogenic idiopathic epilepsy. As a result of this finding, other subunits of the neuronal nAChR gene family are being considered as candidate genes for ADNFLE in families not linked to CHRNA4 and for other idiopathic epilepsies. Alpha4-subunits often assemble together with beta2-subunits (gene symbol CHRNB2) to build heteromeric nAChRs. The gene encoding another abundant AChR subunit, the alpha3-subunit gene (CHRNA3), is present with those encoding two other subunits, CHRNB4 and CHRNA5, in a gene cluster whose functional role is still unclear. Here we provide the information on the genomic structures of both the CHRNB2 and the CHRNA3 genes that is necessary for comprehensive mutational analyses, and we refine the genomic assignment of CHRNB2 on chromosome 1.

Properties of neuronal nicotinic acetylcholine receptor mutants from humans suffering from autosomal dominant nocturnal frontal lobe epilepsy

1. Physiological and pharmacological properties of the human neuronal alpha4beta2 nicotinic AChR and mutants found in patients suffering from autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) were studied. 2. Investigations of nicotinic AChRs reconstituted in Xenopus oocytes with the control or mutated alpha4 subunits revealed that both mutation S248F as well as the Leucine insertion (776ins3) result in major but different changes in the physiological and pharmacological properties of the receptors. 3. Mutation S248F causes a decrease in apparent affinity to ACh of about 7 fold. In addition, this receptor already desensitizes during exposure to agonist concentration 3000 times lower than the control. 4. 776ins3 provokes a 10 fold increase of apparent ACh affinity, an increase in the IC50 caused by prolonged ACh exposures and a slowing down of the response decay. 5. At saturating ACh concentration cells expressing the S248F mutant display average currents that are about five times smaller than control. 6. When measured at very low concentration, agonist sensitivities of the control and mutated receptors to ACh, nicotine and epibatidine exhibit differences that match those observed for higher agonist concentrations. 7. Mutation 776ins3 increases the apparent efficacy to cytisine. 8. Data presented herein suggest that mutation S248F mainly affects the desensitization properties of the receptor while the leucine insertion (776ins3) increases the probability of transition to the active state. Although these mutations differentially affect the receptor properties they both result in reduced permeability to calcium and enhanced desensitization sensitivity that might account for the ADNFLE phenotype.

New insights into the molecular and genetic mechanisms underlying idiopathic epilepsies

For many years, idiopathic epilepsies have been known to have a strong genetic background. In most subtypes, the mode of inheritance appears to be complex, with only some rare idiopathic epilepsies being monogenic disorders. Thus far, several gene loci have been reported for the common subtypes, such as juvenile myoclonic epilepsy, but the results of linkage studies in independent samples have often been conflicting. Recently, the gene defects underlying two monogenic epilepsies, autosomal dominant nocturnal frontal lobe epilepsy and benign familial neonatal convulsions, have been identified. Both diseases are caused by ion channel mutations, a similarity which may shed light on the understanding of the basic mechanisms of epileptogenesis.

[Autosomal dominant nocturnal frontal lobe epilepsy. An electroclinical and genetic description of a Norwegian family with ten affected members]

We describe a Norwegian family with clusters of brief nocturnal motor seizures with hyperkinetic or tonic manifestations. Seizures started in childhood. Neurological examination and neuroimaging were normal. Interictal EEG registrations were mostly normal, ictal EEG registrations disclosed left frontal epileptiform discharges in two of three patients examined and just shallow arousal preceding the attack in one of the three patients. Segregation analysis indicated an autosomal dominant inheritance pattern, and the patients were subsequently diagnosed as having autosomal dominant nocturnal frontal lobe epilepsy, a disorder first described in 1995. A missense mutation in the gene for the alpha-4 subunit of the neuronal nicotinergic acetylcholine receptor was recently described in an Australian family with this disorder. Our Norwegian family proved to have a novel insertion mutation (776ins3) in the same gene. This mutation affects the second transmembrane domain (M2) which forms the critical section of the ion channel. This is the first case of idiopathic partial epilepsy where the underlying molecular defect has been found. The fact that a dysfunction of the nicotinergic acetylcholine receptor may give rise to frontal epileptic seizures was surprising and may shed new light on the basic mechanisms of epileptogenesis. Manipulations of the cholinergic system may open up a new therapeutic approach.

A potassium channel mutation in neonatal human epilepsy

Benign familial neonatal convulsions (BFNC) is an autosomal dominant epilepsy of infancy, with loci mapped to human chromosomes 20q13.3 and 8q24. By positional cloning, a potassium channel gene (KCNQ2) located on 20q13.3 was isolated and found to be expressed in brain. Expression of KCNQ2 in frog (Xenopus laevis) oocytes led to potassium-selective currents that activated slowly with depolarization. In a large pedigree with BFNC, a five-base pair insertion would delete more than 300 amino acids from the KCNQ2 carboxyl terminus. Expression of the mutant channel did not yield measurable currents. Thus, impairment of potassium-dependent repolarization is likely to cause this age-specific epileptic syndrome.

An insertion mutation of the CHRNA4 gene in a family with autosomal dominant nocturnal frontal lobe epilepsy

Autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) is the first, and to date only, idiopathic epilepsy for which a specific mutation has been found. A missense mutation in the critical M2 domain of the alpha4 subunit of the neuronal nicotinic acetylcholine receptor (CHRNA4) has been recently identified in one large Australian pedigree. Here we describe a novel mutation in the M2 domain of the CHRNA4 gene in a Norwegian family. Three nucleotides (GCT) were inserted at nucleotide position 776 into the coding region for the C-terminal end of the M2 domain. Physiological investigations of the receptor reconstituted with the mutated CHRNA4 subunit reveal that this insertion does not prevent the receptor function but increases its apparent affinity for ACh. In addition, this mutant receptor shows a significantly lower calcium permeability that, at the cellular level, may correspond to a loss of function. Comparison of the two mutations identified so far in families with ADNFLE illustrates that different mutations can result in similar phenotypes.

Neuronal nicotinic acetylcholine receptor alpha 4 subunit (CHRNA4) and panic disorder: an association study

Anxiety disorders have been reported to be associated with low-voltage EEG (LVEEG). Some cases with LVEEG (approximately 1/3) have been linked to chromosome 20q13.2q13.3. In the same chromosomal region, the gene for the neuronal nicotinic acetylcholine receptor alpha 4 subunit (CHRNA4) has been located. We therefore tested the hypothesis that polymorphisms in the CHRNA4 gene show an allelic association with panic disorder. We examined the allele frequencies of three different CHRNA4 polymorphisms in patients with panic disorder and in healthy controls. No significant differences in the allele frequencies of these three polymorphisms were noted. This study does not support an association between panic disorder and the CHRNA4 gene.

A missense mutation in the neuronal nicotinic acetylcholine receptor alpha 4 subunit is associated with autosomal dominant nocturnal frontal lobe epilepsy

Epilepsy affects at least 2% of the population at some time in their lives. The epilepsies are a heterogeneous group of disorders, many with an inherited component. Although specific genes have been identified in a few rare diseases causing seizures as part of a more diffuse brain disorder, the molecular pathology of the common idiopathic epilepsies is still unknown. Linkage has been reported for some generalised epilepsy syndromes, but only very recently for familial partial epilepsy syndromes. Autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) is a partial epilepsy causing frequent, violent, brief seizures at night, usually beginning in childhood. The gene for ADNFLE maps to chromosome 20q13.2-q13.3 in one large Australian kindred. The neuronal nicotinic acetylcholine receptor alpha 4 subunit (CHRNA4) maps to the same region of 20q (ref. 12) and the gene is expressed in all layers of the frontal cortex. We screened affected family members for mutations within CHRNA4 and found a missense mutation that replaces serine with phenylalanine at codon 248, a strongly conserved amino acid residue in the second transmembrane domain. The mutation is present in all 21 available affected family members and in four obligate carriers, but not in 333 healthy control subjects.