Genetics of epilepsy: current status and perspectives

Epilepsy affects more than 0.5% of the world's population and has a large genetic component. The most common human genetic epilepsies display a complex pattern of inheritance and the susceptibility genes are largely unknown. However, major advances have recently been made in our understanding of the genetic basis of monogenic inherited epilepsies. Progress has been particularly evident in familial idiopathic epilepsies and in many inherited symptomatic epilepsies, with the discovery that mutations in ion channel subunits are implicated, and direct molecular diagnosis of some phenotypes of epilepsy is now possible. This article reviews recent progress made in molecular genetics of epilepsy, focusing mostly on idiopathic epilepsy, and some types of myoclonus epilepsies. Mutations in the neuronal nicotinic acetylcholine receptor alpha4 and beta2 subunit genes have been detected in families with autosomal dominant nocturnal frontal lobe epilepsy, and those of two K(+) channel genes were identified to be responsible for underlying genetic abnormalities of benign familial neonatal convulsions. The voltage-gated Na(+) -channel (alpha1,2 and beta1 subunit), and GABA receptor (gamma2 subunit) may be involved in the pathogenesis of generalized epilepsy with febrile seizure plus and severe myoclonic epilepsy in infancy. Mutations of Ca(2+)-channel can cause some forms of juvenile myoclonic epilepsy and idiopathic generalized epilepsy. Based upon these findings, pathogenesis of epilepsy as a channelopathy and perspectives of molecular study of epilepsy are discussed.     

Nicotinic acetylcholine receptors in human genetic disease

Nicotinic acetylcholine receptors represent a family of ligand-gated ion channels that are widely expressed in the central and peripheral nervous systems. To date, 16 genes encoding subunits of mammalian nicotinic acetylcholine receptors have been identified. The various subunits form homomeric or heteromeric receptor proteins, allowing for a complex and adaptable system of nicotinic neurotransmission. Mutations of nicotinic receptor genes can cause Mendelian disorders, most importantly congenital myasthenic syndromes, multiple pterygium syndromes, and nocturnal frontal lobe epilepsies. Haploinsufficiency of CHRNA7 predisposes to neuropsychiatric phenotypes in 15q13.3 deletion syndrome. The role of various nicotinic receptor genes is also discussed for complex disorders such as addiction, schizophrenia, Alzheimer disease, and Parkinson disease.     

Mechanisms of human inherited epilepsies

It is just over a decade since the discovery of the first human epilepsy associated ion channel gene mutation. Since then mutations in at least 25 different genes have been described, although the strength of the evidence for these genes having a pathogenic role in epilepsy varies. These discoveries are allowing us to gradually begin to unravel the molecular basis of this complex disease. In the epilepsies, virtually all the established genes code for ion channel subunits. This has led to the concept that the idiopathic epilepsies are a family of channelopathies. This review first introduces the epilepsy syndromes linked to mutations in the various genes. Next it collates the genetic and functional analysis of these genes. This part of the review is divided into voltage-gated channels (Na+, K+, Ca2+, Cl(-) and HCN), ligand-gated channels (nicotinic acetylcholine and GABA(A) receptors) and miscellaneous proteins. In some cases significant advances have been made in our understanding of the molecular and cellular deficits caused by mutations. However, the link between molecular deficit and clinical phenotype is still unknown. Piecing together this puzzle should allow us to understand the underlying pathology of epilepsy ultimately providing novel therapeutic strategies to complete the clinic-bench-clinic cycle.     

Genetic mapping of a major susceptibility locus for juvenile myoclonic epilepsy on chromosome 15q

The epilepsies are a group of disorders characterised by recurrent seizures caused by episodes of abnormal neuronal hyperexcitability involving the brain. Up to 60 million people are affected worldwide and genetic factors may contribute to the aetiology in up to 40% of patients. The most common human genetic epilepsies display a complex pattern of inheritance. These are categorised as idiopathic in the absence of detectable structural or metabolic abnormalities. Juvenile myoclonic epilepsy (JME) is a distinctive and common variety of familial idiopathic generalised epilepsy (IGE) with a prevalence of 0.5- 1.0 per 1000 and a ratio of sibling risk to population prevalence (lambda(s)) of 42. The molecular genetic basis of these familial idiopathic epilepsies is entirely unknown, but a mutation in the gene CHRNA4, encoding the alpha4 subunit of the neuronal nicotinic acetylcholine receptor (nAChR), was recently identified in a rare Mendelian variety of idiopathic epilepsy. Chromosomal regions harbouring genes for nAChR subunits were therefore tested for linkage to the JME trait in 34 pedigrees. Significant evidence for linkage with heterogeneity was found to polymorphic loci encompassing the region in which the gene encoding the alpha7 subunit of nAChR (CHRNA7) maps on chromosome 15q14 (HLOD = 4.4 at alpha = 0.65; Z(all) = 2.94, P = 0.0005). This major locus contributes to genetic susceptibility to JME in a majority of the families studied.      

Ion channels and epilepsy

Ion channels provide the basis for the regulation of excitability in the central nervous system and in other excitable tissues such as skeletal and heart muscle. Consequently, mutations in ion channel encoding genes are found in a variety of inherited diseases associated with hyper- or hypoexcitability of the affected tissue, the so-called 'channelopathies.' An increasing number of epileptic syndromes belongs to this group of rare disorders: Autosomal dominant nocturnal frontal lobe epilepsy is caused by mutations in a neuronal nicotinic acetylcholine receptor (affected genes: CHRNA4, CHRNB2), benign familial neonatal convulsions by mutations in potassium channels constituting the M-current (KCNQ2, KCNQ3), generalized epilepsy with febrile seizures plus by mutations in subunits of the voltage-gated sodium channel or the GABA(A) receptor (SCN1B, SCN1A, GABRG2), and episodic ataxia type 1-which is associated with epilepsy in a few patients--by mutations within another voltage-gated potassium channel (KCNA1). These rare disorders provide interesting models to study the etiology and pathophysiology of disturbed excitability in molecular detail. On the basis of genetic and electrophysiologic studies of the channelopathies, novel therapeutic strategies can be developed, as has been shown recently for the antiepileptic drug retigabine activating neuronal KCNQ potassium channels.     

A major role of the nicotinic acetylcholine receptor gene CHRNA2 in autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) is unlikely

Autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) is known to be caused by mutations in the transmembrane regions of the neuronal nicotinic acetylcholine receptor (nAChR) genes CHRNA4 and CHRNB2. A third nAChR gene, CHRNA2, has been recently reported as mutated in an Italian family with nocturnal frontal lobe epilepsy, nocturnal wandering and ictal fear. We have now evaluated the role of CHRNA2 in families with "classical" ADNFLE. Mutation screening was performed in 47 families by amplification and subsequent sequencing of part of CHRNA2 exon 6 containing transmembrane regions 1-3. Detected variants were tested in a case-control association study. No mutations were identified in the parts of CHRNA2 that contribute to the ion pore. Sequencing identified a novel synonymous nucleotide exchange (c.771C/T) that was also present in two of 78 controls and is therefore likely to be non-pathogenic. The absence of mutations in our sample of 47 families renders a major role of CHRNA2 in ADNFLE unlikely.     

儿童失神癫痫易感基因的研究

尽管近年来发现有少数非离子通道编码基因参与人类特发性癫痫（idiopathic eplepsies，IE），但更多的遗传学研究证实，离子通道在IE的遗传病理机制中起核心作用。离子通道基因突变是一些罕见类型的单基因遗传IE的常见病因，被称为通道病。但离子通道基因突变仅能解释IE的少数家系或散发病例，更大的难题来自于对复杂遗传IE的研究，它们未知的遗传模式、表型异质性和综合征亚型间不确定的遗传背景重叠限制了遗传图谱的绘制。失神癫痫是常见的IE亚型，呈复杂遗传方式。现共发现有11个基因与失神癫痫有关联，其中有4种编码神经元钙通道亚单位。因此钙通道基因是失神癫痫的重要候选基因。失神癫痫钙通道基因的遗传学研究可能是复杂遗传IE病因研究的最佳切入点，并有利于最终阐明失神癫痫的分子机制。     

New insights into the molecular and genetic mechanisms underlying idiopathic epilepsies

Steinlein OK. 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 epileptogenesk.     

GABRD encoding a protein for extra- or peri-synaptic GABAA receptors is a susceptibility locus for generalized epilepsies

A major challenge in understanding complex idiopathic generalized epilepsies has been the characterization of their underlying molecular genetic basis. Here, we report that genetic variation within the GABRD gene, which encodes the GABAA receptor delta subunit, affects GABA current amplitude consistent with a model of polygenic susceptibility to epilepsy in humans. We have found a GABRD Glu177Ala variant which is heterozygously associated with generalized epilepsy with febrile seizures plus. We also report an Arg220His allele in GABRD which is present in the general population. Compared with wild-type receptors, alpha1beta2Sdelta GABAA receptors containing delta Glu177Ala or Arg220His have decreased GABAA receptor current amplitudes. As GABAA receptors mediate neuronal inhibition, the reduced receptor current associated with both variants is likely to be associated with increased neuronal excitability. Since delta subunit-containing receptors localize to extra- or peri-synaptic membranes and are thought to be involved in tonic inhibition, our results suggest that alteration of this process may contribute to the common generalized epilepsies.     

Neuroimaging and neurogenetics of epilepsy in humans

In the past decade, several genetic mutations have been associated with different forms of familial focal and generalized epilepsies. Most of these genes encode ion-channel subunits. Based on neurophysiological in vitro and in vivo animal studies, substantial progress has been made in understanding the functional consequences of gene defects associated with epilepsies. However, the knowledge transition from animal studies to patients carrying a mutation, or even suffering from a nonfamilial form of epilepsy, is very limited. This review will illustrate how neuroimaging studies in humans may help to bridge the gap between genotype and phenotype. We will be presenting examples of familial focal (autosomal dominant nocturnal frontal lobe epilepsy), idiopathic generalized epilepsies (severe myoclonic epilepsy of infancy). Such studies will help to better understand functional consequences of genetic alterations and may contribute to a better phenotype characterization.     

The nicotinic acetylcholine receptor CHRNA5/A3/B4 gene cluster: Dual role in nicotine addiction and lung cancer

More than 1 billion people around the world smoke, with 10 million cigarettes sold every minute. Cigarettes contain thousands of harmful chemicals including the psychoactive compound, nicotine. Nicotine addiction is initiated by the binding of nicotine to nicotinic acetylcholine receptors, ligand-gated cation channels activated by the endogenous neurotransmitter, acetylcholine. These receptors serve as prototypes for all ligand-gated ion channels and have been extensively studied in an attempt to elucidate their role in nicotine addiction. Many of these studies have focused on heteromeric nicotinic acetylcholine receptors containing α4 and β2 subunits and homomeric nicotinic acetylcholine receptors containing the α7 subunit, two of the most abundant subtypes expressed in the brain. Recently however, a series of linkage analyses, candidate-gene analyses and genome-wide association studies have brought attention to three other members of the nicotinic acetylcholine receptor family: the α5, α3 and β4 subunits. The genes encoding these subunits lie in a genomic cluster that contains variants associated with increased risk for several diseases including nicotine dependence and lung cancer. The underlying mechanisms for these associations have not yet been elucidated but decades of research on the nicotinic receptor gene family as well as emerging data provide insight on how these receptors may function in pathological states. Here, we review this body of work, focusing on the clustered nicotinic acetylcholine receptor genes and evaluating their role in nicotine addiction and lung cancer.     

α-Bungarotoxin-acetylcholine receptors in the chick ciliary ganglion during development

[¹²⁵I]α-Bungarotoxin binds in a saturable and practically irreversible fashion to membrane-associated receptors in the ciliary ganglion of the chick embryo and young chick. Nicotinic but not muscarinic ligands are potent inhibitors of toxin binding. Thus, the binding site for toxin in the ganglion appears to have the properties of a nicotinic acetylcholine receptor. The ontogeny of the receptors was studied from day 7in ovo to day 70 after hatching. When expressed per ganglion, the developmental pattern of the receptors may be divided into three stages: a peak of high receptor level (days 7–15in ovo) with a maximum at day 12in ovo, a period of constant level (days 15in ovo to day 8 after hatching) and a second increase in receptor number (days 8–50 after hatching).Light-microscope autoradiography suggests that different developmental changes in receptor distribution occur in the two neuronal populations of the ganglion (i.e. the ciliary and choroid neurons). While choroid neurons show an adult-like receptor distribution from late embryonic stages, the ciliary neurons do not fully develop their distribution pattern until after hatching.These findings are discussed in relation to morphological and physiological events which are known to occur in the ganglion during development, in particular those concerning synaptic transmission.     

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.      

Properties of mutated murine alpha4beta2 nicotinic receptors linked to partial epilepsy

We characterized, by electrophysiological methods, two biophysical properties of murine recombinant alpha4beta2 nicotinic acetylcholine receptors (nAChR) bearing a mutation (alpha4:+L264alpha4:beta2 or alpha4:S252Falpha4:beta2) linked to autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE). Sensitivity to acetylcholine (ACh) was increased by the S252F substitution expressed in heterozygosis (alpha4:S252Falpha4:beta2) but was markedly reduced when this mutation was expressed in homozygosis (S252Falpha4:beta2). ACh sensitivity was not altered by the +L264 insertion. Moreover, receptor desensitization was significantly increased by both mutations expressed in heterozygosis. These results are in general agreement to those of rat and human recombinant receptors bearing the same mutations, thus contributing to validate the use of knock-in mice harboring ADNFLE mutations as models to study this pathology.     

Mutations affecting GABAergic signaling in seizures and epilepsy

The causes of epilepsies and epileptic seizures are multifactorial. Genetic predisposition may contribute in certain types of epilepsies and seizures, whether idiopathic or symptomatic of genetic origin. Although these are not very common, they have offered a unique opportunity to investigate the molecular mechanisms underlying epileptogenesis and ictogenesis. Among the implicated gene mutations, a number of GABA_A receptor subunit mutations have been recently identified that contribute to several idiopathic epilepsies, febrile seizures, and rarely to certain types of symptomatic epilepsies, like the severe myoclonic epilepsy of infancy. Deletion of GABA_A receptor genes has also been linked to Angelman syndrome. Furthermore, mutations of proteins controlling chloride homeostasis, which indirectly defines the functional consequences of GABA_A signaling, have been identified. These include the chloride channel 2 (CLCN2) and the potassium chloride cotransporter KCC3. The pathogenic role of CLCN2 mutations has not been clearly demonstrated and may represent either susceptibility genes or, in certain cases, innocuous polymorphisms. KCC3 mutations have been associated with hereditary motor and sensory polyneuropathy with corpus callosum agenesis (Andermann syndrome) that often manifests with epileptic seizures. This review summarizes the recent progress in the genetic linkages of epilepsies and seizures to the above genes and discusses potential pathogenic mechanisms that contribute to the age, sex, and conditional expression of these seizures in carriers of these mutations.     

Tandem pore domain K+‐channel TASK‐3 (KCNK9) and idiopathic absence epilepsies

Recently, the gene coding for the tandem pore domain K⁺‐channel TASK‐3 (KCNK9) has been localized to the chromosomal region 8q24. Because mutations in ion channel genes have been recognized as an important factor in the etiology of abnormal neuronal excitability, TASK‐3 is an interesting candidate gene for epilepsies linked to 8q24. We therefore performed a mutation analysis of the TASK‐3 gene in 65 patients with childhood and juvenile absence epilepsy. Only one silent nucleotide exchange (636C/T) was detected in exon 2 of the TASK‐3 coding region. No evidence for an allelic association was found between the exon 2 polymorphism and absence epilepsy. Accordingly, genetic variation of the TASK‐3 coding region does not play a major role in the etiology of idiopathic absence epilepsies. © 2002 Wiley‐Liss, Inc.     

Lophotoxin: Selective blockade of nicotinic transmission in autonomic ganglia by a coral neurotoxin

Lophotoxin is a diterpene lactone isolated from gorgonian corals. The toxin has previously been shown to bind with high affinity to an acetylcholine recognition site located on skeletal muscle nicotinic receptors, producing an essentially irreversible blockade of neuromuscular transmission. Lophotoxin has also been shown to block nicotinic transmission in autonomie ganglia of the frog and in ileal strips of guinea pig and rabbit. The effects of lophotoxin have now been examined on neuronal nicotinic receptors in autonomie ganglia of the chick and rat. Low concentrations of lophotoxin (1 μM) produce a blockade of neuronal nicotinic transmission which is partially reversed by 3–5 h of washing out the toxin. The blockade produced by higher concentrations of lophotoxin (up to 32μM) is not reversed during a similar washout period. Prior exposure tod-tubocurarine, a competitive nicotinic antagonist, can partially protect ganglia against exposure to lophotoxin. In contrast the local anesthetic QX-314, a noncompetitive nicotinic antagonist, does not protect ganglia against lophotoxin exposure. Lophotoxin binds to a site in ganglia identified by [¹²⁵I]κ-bungarotoxin which appears to be on the neuronal nicotinic receptor. Intracellular recordings reveal that lophotoxin has no effect on either muscarinic responses or on responses to γ-aminobutyrate in autonomic ganglia. Passive and active membrane properties of the neurons are unaffected by lophotoxin except for the blockade of nicotinic responses.

It is concluded that lophotoxin is a selective, high-affinity antagonist at the neuronal nicotinic receptor. The long-term nature of the blockade with lophotoxin suggests that the toxin will be of considerable value as a probe for characterizing the ganglionic nicotinic receptor.     

The role of neuronal GABAA receptor subunit mutations in idiopathic generalized epilepsies

Rare GABA_A receptor γ2 and α1 subunit mutations of pathogenic effect have been described segregating in families with “monogenic” epilepsies. We now report globally on the genetic variation contained within all 16 neuronal GABA_A receptor subunit genes from the one patient cohort. The cohort consists of GEFS⁺, FS, and IGE subgroups as either sporadic cases or index cases from small families, with one index case from one large IGE family. The rarity of mutations and coding variation in general across all of the subunits suggests a low tolerance for mutations affecting GABA mediated neuronal inhibition. Characterization of the broader channelopathy load associated with susceptibility to these common epilepsies mostly with complex genetics will need to be expanded beyond the family of GABA_A receptor subunits to all families of neuronal ion channels and their interacting molecules by systematic mutation detection associated with functional investigation of their naturally occurring genetic variations.     

A nerve stump dependent appearance of junctional and perijunctional acetylcholine receptors in organ culture

A rapid, nerve stump dependenl appearance of junctional and perijunctional acetylcholine receptors occurred after 42 h following the introduction of rat diaphragm into organ culture. Approximately2 × 10⁶ or more acetylcholine receptors (assayed by [¹²⁵I]alphabungarotoxin hinding) appeared at the endplate within a 2 h period, between 42 and 44 h, while only approximately2.5 × 10³ acetylcholine receptors appeared in an equivalent area of extrajunctional membrane during the same time. Autoradiographic studies confirmed that most of the new acetylcholine receptor sites appeared at the endplate, although a small perijunctional component was also detected. The presence of a long nerve stump prevented the increase in the number of receptors specific to the endplate region, but had no effect on the appearance of extrajunctional acetylcholine receptors. Pharmacological studies showed that the nerve stump effect did not involve nerve impulse transmission, or the interaction of nicotinic acetylcholine receptors with acetylcholine. The findings suggest that junctional and extrajunctional acetylcholine receptors are controlled by different neuronal mechanisms and that junctional receptors are subject to     

Characterization and distribution of acetylcholine receptors and acetylcholinesterase during electric organ development in Torpedo marmorata

The changes that occur in the distribution and properties of the nicotinic acetylcholine receptor and acetylcholinesterase during the development of the electric organ of Torpedo marmorata have been investigated. At early stages of development, both proteins are distributed diffusely over the myotube surface and with differentiation of the myotubes into electrocytes, they become increasingly restricted to the ventral cell surface. This process occurs before axons contact the electrocytes. The concentrations of the acetylcholine receptor and acetylcholinesterase remain at rather low and stable levels during these developmental changes. The acetylcholine receptor concentration begins to increase rapidly as soon as electromotor axons begin to contact the electrocytes. No significant differences in the subunit composition, affinity for d-tubocurarine, isoelectric point or immunochemical properties of the embryonic acetylcholine receptors were detected when they were compared to the receptors of the adult electric organ. The onset of receptor accumulation occurs before the increase in the amount of 17 S acetylcholinesterase, suggesting that increases in acetylcholine receptor and in acetylcholinesterase are not regulated by the same mechanisms. The various molecular forms of acetylcholinesterase undergo characteristic changes during development. Sequential extraction of the esterase forms indicates that their interaction with cellular compartments changes during development. The solubility properties of the esterase forms suggest that most of the 17 S and 13 S acetylcholinesterase become strongly associated with cellular components via ionic interactions and that a hydrophobic 6 S form begins to accumulate at later embryonic stages when the number of mature presynaptic terminals is beginning to show its rapid phase of increase.The results show that events concerning the distribution and some properties of essential components of the electromotor synapse occur during early embryonic development before synaptogenesis, whereas the sequential accumulation of acetylcholine receptor and esterase begins after the axons start contacting the electrocytes. This suggests that the ingrowing nerves exert some regulative influence on the metabolic state of the electrocytes during development.