American Journal of Medical Genetics (Semin. Med. Genet.) 106:146±159 (2001) ARTICLE

Ion Channels and Epilepsy

HOLGER LERCHE, KARIN JURKAT-ROTT, AND FRANK LEHMANN-HORN*

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 GABAA receptor (SCN1B, SCN1A, GABRG2), and episodic ataxia type 1Ðwhich is associated with epilepsy in a few patientsÐby 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. ß 2001 Wiley-Liss, Inc.

KEY WORDS: ion channel; epilepsy; genetics; electrophysiology; patch clamp

INTRODUCTION between neurons: Axonal conduction is mitters, such as acetylcholine (ACh). mediated by action potentials and signal With regard to these basic principles, Epileptic seizures are induced by abnor- transduction from cell to cell by synaptic two distinct and structurally conserved mal focal or generalized synchronized transmission. Since ion channels provide classes of ion channels emerged during electrical discharges within the central the basis for these processes, any muta- evolution, the voltage-gated and the nervous system (CNS). The equilibrium tion-induced channel malfunction may ligand-gated channels [Hille, 1992]. in communication between neurons is directly alter brain excitability and can regulated by a network of excitatory and induce epileptic seizures. inhibitory circuits. Both enhancement Ion channels are membrane-span- Ion channels are of excitatory and impairment of inhibi- ning proteins forming selective pores for membrane-spanning tory mechanisms will disturb this equili- Na‡,K‡,Cl,orCa2‡ ions. During brium, which may result in epileptic action potentials a precise control of ion proteins forming selective discharges. There are two basic mechan- channel gating is mediated by membrane pores for Na‡,K‡, isms underlying the electrophysiological voltage, during synaptic transmission Cl,orCa2‡ ions. excitability of and the communication by the binding of speci®c neurotrans-

Since these two are the only channel Holger Lerche is a neurophysiologist and clinical neurologist in the Departments of Applied types that so far have been shown to Physiology and Neurology, University of Ulm, Germany. Main research interests are the genetics, be affected by mutations causing epi- pathophysiology, and therapy of inherited neurological diseases; in particular, inherited forms of lepsy, other classes of ionic channels, epilepsy and the relationship to molecular mechanisms of ion channel gating. Karin Jurkat-Rott is in the Department of Applied Physiology, University of Ulm, Germany. e.g., those regulated by intracellular ions Research focus: Physiology and pathophysiology of cellular excitation and muscle excitation- such as Ca2‡, by nucleotides, or by cell contraction coupling; genetics and pathogenesis of hereditary muscle and channel diseases with volume, will not be considered in this respect to skeletal muscle and the central nervous system; data bases on diagnostic criteria. Frank Lohmann-Horn is in the Department of Applied Physiology, University of Ulm, Germany. article. Research focus: Physio(patho)logy of cellular excitation, particularly structure±function relation- Over the last 10±15 years, the ships of ligand- and voltage-dependent ion channels, etiology and pathogenesis of hereditary ion combination of electrophysiological and channel diseases in neurology. Grant sponsor: the Deutsche Forschungsgemeinschaft; Grant number: DFG Le1030/5-1; Grant genetic studies has revealed an increasing sponsor: the Bundesministerium fuÈ r Bildung und Forschung (BMBF) / InterdisziplinaÈ res Zentrum fuÈr number of inherited diseases associated Klinische Forschung (IZKF) Ulm, projects B1 and B8. with mutations in ion channel encoding *Correspondence to: Frank Lehmann-Horn, Department of Applied Physiology, University of Ulm, D-89069 Ulm, Germany. E-mail: [email protected] genes. The ®rst of these so-called ion DOI 10.1002/ajmg.1582 channel disorders or `channelopathies'

ß 2001 Wiley-Liss, Inc. ARTICLE AMERICAN JOURNAL OF MEDICAL GENETICS (SEMIN. MED. GENET.) 147 were found in skeletal muscle, the therapy of the more common forms of four to eight positively charged residues myotonias and periodic paralyses, caused epilepsy. conferring voltage dependence to the by mutations in voltage-gated Na‡,Cl, channel protein and the S5±S6 loops or Ca2‡ channels. Subsequently, several form the major part of the ionic pore STRUCTURE AND disorders of the CNS, the episodic with the selectivity ®lter (Figs. 3, 5, 7). FUNCTION OF ataxias, familial hemiplegic migraine, VOLTAGE-GATED There are three main conforma- spinocerebellar ataxia type 6, startle CATION CHANNELS tional states of voltage-gated channels, a disease, and several epileptic syndromes, closed, an open, and an inactivated state. were identi®ed as belonging to the Voltage-gated K‡,Na‡, and Ca2‡ At the resting potential the channels are growing family of channelopathies channels consist of several subunits, a in the closed and activatable state. Upon [Lehmann-Horn and Jurkat-Rott, main a-subunit constituting both the membrane depolarization, the voltage 1999, 2000; Ptacek, 1999; Cannon, gating and permeation machinery of the sensors move outward opening the 2000]. The current review will channel and one or more smaller sub- `activation gate' of the channel on a focus on the pathophysiological mecha- units with modifying functions, called b, time-scale of milliseconds by a yet- nisms of the epileptic channelopathies g,ord. The a-subunits have a common unknown mechanism, and with sus- in man. We will start with a short tetrameric structure of homologous tained depolarization the channels inac- overview of the structure and function domains (I±IV) each with six transmem- tivate spontaneously by closing of a of voltage- and ligand-gated ion chan- brane segments (S1±S6). Whereas K‡ different, `inactivation' gate. Upon nels, then summarize the clinical, channels are constituted by four identical membrane repolarization, inactivated genetic, and pathophysiological con- domains, the about fourfold longer channels remain refractory to further cepts of the known epileptic channel genes of Na‡- and Ca2‡-channel a- openings for a certain period deter- syndromes and ®nally discuss the impli- subunits encode four homologous but mined by the time needed for recovery cations of the general contribution of distinct domains. In all voltage-gated from inactivation (Fig. 1). Typical mod- mutated ion channels to the genetics and cation channels, the S4 segments contain ifying properties of the smaller b-, g-, or

Figure 1. The three main conformational states of voltage-gated ion channels. From a closed state at the hyperpolarized resting membrane potential, channels open upon depolarization during an action potential via outward movement of the voltage sensors that open the activation gate. Some channels, such as the voltage-gated Na‡ channel, inactivate spontaneously via closing of a different, inactivation gate, when depolarization is maintained. From the inactivated state they can only recover upon repolarization of the cell membrane before they are ready for another opening. 148 AMERICAN JOURNAL OF MEDICAL GENETICS (SEMIN. MED. GENET.) ARTICLE d-subunits are the regulation of the neuronal voltage-gated Ca2‡ channels physiological properties is known amount of functional protein in the are the regulation of transmitter release [Chandy and Gutman, 1995; Leh- membrane or minor alterations of the in presynaptic nerve-terminals. mann-Horn and Jurkat-Rott, 1999]. kinetics or voltage dependence of chan- The different subunits, in particular For example, there are inactivating nel gating [Lehmann-Horn and Jurkat- the channel a-subunits, are expressed (e.g., KCNA1) and noninactivating Rott, 1999; Catterall, 2000; Siegelbaum tissue speci®cally. For example, there are (e.g., KCNQ1-5) K‡ channels and large and Koester, 2000]. several genes encoding different Na‡ differences in the kinetics of activation The time course of depolarization channel a-subunits (SCN1A-SCN11A) and inactivation have been described. and repolarization during an action that are expressed in skeletal muscle potential is conveyed by the gating of (SCN4A), heart muscle (SCN5A) or STRUCTURE AND voltage-dependent Na‡ and K‡ chan- neuronal tissue; four of these subunits FUNCTION OF LIGAND- nels: Activation of the Na‡ inward (SCN1A, SCN2A, SCN3A, and GATED ION CHANNELS current mediates the steep depolarizing SCN8A) are considered to be respon- phase, whereas fast inactivation of Na‡ sible for the sodium current in brain Ligand-gated channels are a group of ion channels and activation of the outward [Goldin et al., 2000]. The tissue speci- channels activated by different neuro- K‡ current are responsible for mem- ®city explains why there are Na‡ transmitters such as acetylcholine brane repolarization. Consequently, dis- channel disorders with symptoms (ACh), g-amino-butyric-acid (GABA), ruption of fast Na‡ channel inactivation restricted to skeletal or heart muscle glycine, glutamate, or nucleotides. They or a decrease in K‡ conductance leads to (myotonia or cardiac arrhythmia), or to are also composed of several subunits, slowed or incomplete repolarization of the CNS (febrile and afebrile seizures). usually four or ®ve. In contrast to the the cell membrane, resulting in hyper- Whereas the relatively few different Na‡ voltage-gated cation channels, all sub- excitability and spontaneous series of channels are structurally and function- units have a similar structure, with two to action potentials. Both are the most ally highly conserved among each other, four transmembrane segments (M1±4, common disease-causing mechanisms in a large variety of different voltage-gated Fig. 2). They form a channel complex the channelopathies. Main functions of K‡ channel types with distinct electro- with each subunit contributing equally

Figure 2. Proposed structure of the nicotinic acetylcholine receptor with mutations found in patients with autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE, see text). Each of the ®ve subunits contains four transmembrane regions (M1±M4), the M2 segments line the channel pore and the long extracellular N-terminal part of the a-subunit contains the binding site for acetylcholine. ARTICLE AMERICAN JOURNAL OF MEDICAL GENETICS (SEMIN. MED. GENET.) 149

to the ion conducting central pore for- 2a12b21g2 [Mehta and Ticku, 1999; S248F and the 776ins3 mutations also med by the M2 segments (Fig. 2). The Sieghart et al., 1999]. revealed mechanisms that increase the pore is not as selective as in the voltage- activity of the channel. A use-dependent gated channels and permeable either to potentiation to repetitive ACh-exposi- AUTOSOMAL DOMINANT cations, as in excitatory ACh or gluta- tions, absent in wild-type receptors, was NOCTURNAL FRONTAL mate receptors, or to anions, such as in LOBE EPILEPSY (ADNFLE) found for both mutations, and the inhibitory GABA or glycine receptors. 776ins3 mutation revealed a 10-fold This disorder is characterized by fre- increase in ACh-sensitivity [Steinlein quent brief seizures with hyperkinetic or et al., 1997a; Bertrand et al., 1998; Ligand-gated channels are tonic manifestations occurring typically Figl et al., 1998]. On the other hand, in clusters at night. Ictal video-electro- Ca2‡ permeability was reduced for a group of ion channels encephalographic studies revealed both receptors [Bertrand et al., 1998]. activated by different partial seizures originating from the First studies of the mutations in the neurotransmitters such as frontal lobe. The onset is usually in b2-subunit revealed only functional childhood, inheritance is autosomal alterations that enhance channel activity. acetylcholine (ACh), dominant, and the penetrance ap- The V287L mutation showed a pro- c-amino-butyric-acid proximately 70±80% [Scheffer et al., found slowing of desensitization kine- 1995; Picard et al., 2000]. ADNFLE tics [De Fusco et al., 2000] and V287M (GABA), glycine, has often been misdiagnosed as par- showed a 10-fold increase in ACh- glutamate, or nucleotides. oxysmal nocturnal dyskinesia, sleep sensitivity [Phillips et al., 2001]. Ca2‡ disorders such as night terrors or night- permeability for V287L was normal. mares, or hysteria [Scheffer et al., 1994]. Thus, pathomechanisms that enhance The binding sites for transmitters are In a large Australian family, linkage was the activity of the nAChR seem to located in long extracellular loops. found to chromosome 20q13.2 [Phillips predominate. A disease-causing hyper- Similar to the voltage-gated chan- et al., 1995] and subsequently a mutation activity of the channel is also sup- nels, there are three main con forma- was identi®ed in the gene CHRNA4 ported by a study showing a threefold tional states of the ligand-gated channels: encoding the a4-subunit of a neuronal increase in sensitivity to block by closed, open, and desensitized. Binding nicotinic acetylcholine receptor carbamazepine of mutant nAChR, sug- of the transmitter opens the channel (nAChR), being the ®rst ion channel gesting that the good therapeutic from the closed state and during constant mutation in an inherited form of response of ADNFLE patients to this presence of the transmitter desensiti- epilepsy [Steinlein et al., 1995]. Two drug is at least in part due to carbama- zation will occur. Only after removal more mutations were found in zepine block of the mutant channel of the transmitter can the channel CHRNA4 [Steinlein et al., 1997a; [Picard et al., 1999]. recover from desensitization and sub- Hirose et al., 1999; Phillips et al., 2000] sequently be available for another and recently, two groups identi®ed opening [Kandel and Siegelbaum, mutations in CHRNB2 [De Fusco ADNFLE has often been 2000]. et al., 2000; Phillips et al., 2001], the

Neuronal nicotinic ACh receptors gene encoding the b2-subunit of neuro- misdiagnosed as paroxysmal (nAChR) have a pentameric structure of nal nAChR, located on chromosome 1. nocturnal dyskinesia, sleep two a- and three b-subunits (Fig. 2). All mutations described so far reside in disorders such as night terrors Eight a-(a2±9) and three b-(b2±4) sub- one of the M2 transmembrane segments unit isoforms are known to be expressed lining the ion conducting pore of the or nightmares, or hysteria. differentially in brain. Most abundantly ligand-gated channel (Fig. 2). found in all brain areas are the a4- and Functional expression of some of b2-subunits encoded by the genes the known mutations in Xenopus oocytes How these changes in the electro- CHRNA4 and CHRNB2, which are or human embryonic kidney (HEK) physiological properties of the nAChR both affected in autosomal dominant cells revealed different effects on gating induce frontal lobe seizures remains to nocturnal frontal lobe epilepsy [Bertrand of heteromeric a4b2 channels. The ®rst be elucidated. Both the a4- and b2- and Changeux, 1999]. GABA receptors two studies of the S248F mutation subunits are expressed abundantly in belong to the same family of ligand- postulated a decrease of the overall nearly all brain tissues without speci®city gated channels having the same penta- channel activity by enhanced desensiti- to the frontal lobe or to projections meric structure. There are several zation, slowed recovery from desensiti- into this region [Bertrand and Chan- different subunit classes of GABAA zation, reduced single channel geux, 1999]. Also, the nocturnal occur- receptors (a1±6, b1±3, g1±3, d, E, p, conductance, and reduced permeability rence of the seizures is dif®cult to 2‡ r1±3). The subunit composition most for Ca ions [Weiland et al., 1996; explain. Transgenic mice generated with abundantly found in brain is probably Kuryatov et al., 1997]. Further studies of either a knock-out or knock-in of the 150 AMERICAN JOURNAL OF MEDICAL GENETICS (SEMIN. MED. GENET.) ARTICLE

a4-subunit were not reported to develop chromosome 20 [Leppert et al., 1989] Steinlein, 1999; Lerche et al., 1999; seizures [Ross et al., 2000; Labarca et al., and a second locus on chromosome 8 Miraglia del Giudice et al., 2000] and 2001]. has been described [Lewis et al., 1993]. KCNQ3 (8q24) [Charlier et al., 1998; Hirose et al., 2000], have been identi®ed (Fig. 3). The KCNQ gene family encodes BENIGN FAMILIAL BFNC is a rare dominantly ‡ NEONATAL CONVULSIONS delayed recti®er K channels that are (BFNC) inherited epileptic syndrome mainly expressed in heart muscle characterized by frequent (KCNQ1), in the CNS (KCNQ2±5), BFNC is a rare dominantly inherited the inner ear (KCNQ4), and skeletal epileptic syndrome characterized by brief seizures within the muscle (KCNQ5) [reviewed by Jentsch, frequent brief seizures within the ®rst ®rst days of life that typically 2000]. They are activated upon depolar- days of life that typically disappear ization of the cell membrane and con- spontaneously after weeks to months. disappear spontaneously tribute to the repolarizing phase of the Neurological examination, interictal after weeks to months. action potential. Mutations in four of the EEG, and development of these children ®ve genes identi®ed cause inherited are usually normal. The risk of recurring diseases. KCNQ1 mutations cause car- seizures later in life is about 15%. The Subsequently, mutations in two novel diac arrhythmia in the long QT syn- penetrance is as high as 85% [Ronen voltage-gated potassium channel genes, drome [Wang et al., 1996], KCNQ2 et al., 1993; Plouin, 1994]. The disease KCNQ2 (20q13.3) [Biervert et al., and KCNQ3 mutations cause epileptic was ®rst mapped to the long arm of 1998; Singh et al., 1998; Biervert and seizures in BFNC (see above), and

Figure 3. Proposed structure of the voltage-gated K‡ channels KCNQ2 and KCNQ3 containing mutations causing benign familial neonatal convulsions (BFNC, see text). The channels are built of six transmembrane segments (S1±S6), the S4 segments containing positively charged residues conferring voltage-dependence to the channel protein and the P-loops between S5 and S6 forming the ion conducting pore. The long cytoplasmic C-terminus is a particular feature of all KCNQ K‡ channels and most probably mediates the formation of heteromeric KCNQ2/3 channels (see text). Mutations are located within two hot spots: in the pore and in the C-terminus. ARTICLE AMERICAN JOURNAL OF MEDICAL GENETICS (SEMIN. MED. GENET.) 151

Figure 4. Functional consequences of a BFNC-causing mutation when expressed in Xenopus oocytes. The mutation 2513delG is a single basepair deletion located at amino acid (aa) position 838 (Fig. 3), just 7 aa before the regular stop codon. It induces a frame shift, change of the last 7 aa and prolongation by another 56 aa before the new stop codon. This mutation reduces the resulting K‡ current by 20-fold, as seen in the raw current traces in A and for the average of many such experiments in B. Coexpression of both wild-type (WT) and mutant channels did not result in signi®cantly less than 50% of WT current, suggesting that there was no dominant negative effect of the mutation [modi®ed after Lerche et al., 1999].

KCNQ4 mutations cause congenital than heart muscle ®bers, a fact that regions of the protein, in the P-loop deafness [Kubisch et al., 1999]. KCNQ5 apparently applies similarly to the Na‡ between segments S5 and S6 constitut- is the only channel in which disease- channel disorders in muscle and brain ing the pore region and in the long C- causing mutations have not been found (see below). terminus, which is speci®c for this family thus far [Lerche et al., 2000a; SchroÈder After the discovery of the neuron- of K‡ channels (Fig. 3). The pore muta- et al., 2000]. Functional expression of speci®c KCNQ2 and KCNQ3 channels, tions should reduce K‡ current by the known mutations revealed a con- it was shown that both interact with each affecting ionic conductance, whereas sistent reduction of the resulting potas- other, since the current size of KCNQ2 the C-terminus is most probably respon- sium current in KCNQ1±4 [Chouabe is enhanced by about 10-fold upon sible for assembly to heteromeric chan- et al., 1997; Wollnik et al., 1997; coexpression with KCNQ3, which nels. Although the stoichiometry of Biervert et al., 1998; SchroÈder et al., exhibits only very small currents when KCNQ channels has not been examined 1998; Kubisch et al., 1999; Lerche et al., expressed alone [Yanget al., 1998]. Both so far, it is well known from other 1999] (Fig. 4). This leads to an impair- channels most probably constitute the voltage-gated K‡ channels that they ment of membrane repolarization, so-called `M-current,' a neuronal K‡ assemble to form tetramers. A mutation explaining the occurrence of hyperex- current known for several decades to in the C-terminal part of KCNQ1 citability in the affected tissues. play an important role in the regulation causing Jervell and Lange-Nielson syn- However, the effects on current of the ®ring rate of neurons [Wang et al., drome disrupt assembly of KCNQ1 reduction were quite different for chan- 1998; Shapiro et al., 2000]. When the in channels [Schmitt et al., 2000] and nels expressed in heart muscle and outer vivo situation for dominant KCNQ2 experiments using chimeras between hair cells compared to those expressed and KCNQ3 mutations was mimicked KCNQ1, KCNQ2, and KCNQ3 chan- exclusively in brain. Whereas KCNQ1 in vitro by coexpressing, for example, nels show that the interaction of and KCNQ4 mutations exhibited strong WT and mutant KCNQ2 with WT KCNQ2 and KCNQ3 channels is dominant negative effects on WT chan- KCNQ3 channels in a 1:1:2 ratio in indeed mediated by this region [Lerche nels [Chouabe et al., 1997; Wollniket al., Xenopus oocytes, the reduction in cur- et al., 2000b; Maljevic et al., 2001]. 1997; Kubisch et al., 1999], KCNQ2 rent size was only 20±25% compared to Hence, C-terminal mutations probably and KCNQ3 mutations did not. The WT KCNQ2 combined with KCNQ3 reduce current size by inhibiting the latter cause a dominant disease by [SchroÈder et al., 1998]. Thus, as stated formation of functional heteromers haploinsuf®ciency [Biervert et al., above, relatively small changes of the M- inserting into the cell membrane. This 1998; SchroÈder et al., 1998; Lerche current seem to be suf®cient to cause hypothesis corresponds well to a reduced et al., 1999]. Hence, the brain seems to epileptic seizures. surface expression of a KCNQ2 mutant be more sensitive to changes in K‡ Disease-causing mutations in truncating the C-terminus. In con- conductance inducing hyperexcitability KCNQ channels are clustered in two trast, pore mutations in KCNQ2 and 152 AMERICAN JOURNAL OF MEDICAL GENETICS (SEMIN. MED. GENET.) ARTICLE

Figure 5. Proposed structure of the voltage-gated Na‡ channel containing mutations causing generalized epilepsy with febrile ‡ seizures plus (GEFS ) or severe myoclonic epilepsy of infancy (SMEI) in the a-subunit encoded by the gene SCN1A or the b1-subunit encoded by SCN1B. Na‡ channels are built similar to K‡ channels, as shown in Figure 3. They have four highly homologous repeats (I± IV) with six transmembrane segments each (S1±S6) forming a central pore (lower right). The a-subunit mutations that have been functionally characterized so far (Fig. 6) are located in the highly conserved voltage sensors in repeat II and IV, respectively (T875M, R1648H). The b1-subunit mutation disrupts a disul®de bridge between two cysteine residues in the extracellular loop that is essential for interaction with the a-subunit (see text).

KCNQ3 did not affect surface expres- attenuates KCNQ2 and KCNQ3 chan- life or in combination with afebrile sion [Schwake et al., 2000]. nels upon coexpression [Smith et al., generalized tonic-clonic seizures (called The question remains why the 2001] have been reported. However, it `FS‡'). The phenotypes FS and FS‡ reduced KCNQ2/KCNQ3 K‡ current remains unclear how these ®ndings were found in about two-thirds of results in seizures preferentially during contribute to the neonatal seizure phe- affected individuals. According to the the neonatal period. One possibility notype. additional seizure types occurring in the could be that the brain is generally more remaining third of the patients, pheno- likely to develop seizures in this pre- types such as `FS‡ with absences,' `FS‡ GENERALIZED EPILEPSY mature state than later in life [Swann with myoclonic seizures,' or `FS‡ with WITH FEBRILE SEIZURES et al., 1993]. Another explanation might atonic seizures' were described. The PLUS (GEFS‡) AND SEVERE be the differential expression of potas- MYOCLONIC EPILEPSY OF most severe phenotype was myoclonic sium channels during maturation, which INFANCY (SMEI) astatic epilepsy (MAE). Also, partial may attribute a dominant role to KCNQ epilepsies occurred in rare cases (`FS‡ channels in central neurons within the GEFS‡ was ®rst described in 1997 and with temporal lobe epilepsy'). The ®rst days to weeks of life. Either potas- 1999 by the group of Scheffer, Berkovic penetrance was about 60%. sium channels of the KCNQ family and colleagues [Scheffer and Berkovic, Severe myoclonic epilepsy of could be upregulated during this period 1997; Singh et al., 1999] as a childhood- infancy as ®rst described by Dravet or other voltage-gated potassium chan- onset syndrome featuring febrile con- [1978] is characterized by clonic and nels could still not have reached their full vulsions and a varietyof afebrile epileptic tonic-clonic seizures in the ®rst year of expression level. Differential expression seizure types within the same pedigree life that are often prolonged and asso- with reduced expression of KCNQ3 with autosomal dominant inheritance. ciated with fever. Later, patients have during the ®rst days of life [Tinel et al., Most common was the febrile convul- afebrile generalized seizures such as my- 1998] and expression of a shorter splice sion syndrome (FS), often with febrile oclonic, absence, or tonic-clonic, and variant of KCNQ2 in fetal brain that seizures persisting after the sixth year of also simple and complex partial seizures ARTICLE AMERICAN JOURNAL OF MEDICAL GENETICS (SEMIN. MED. GENET.) 153 occur. Developmental stagnation with dementia occurs in early childhood. In contrast to GEFS‡, the syndrome is usually resistant to pharmacotherapy. The ®rst genetic defect in GEFS‡ was found by Wallace et al. [1998]. The authors described linkage to chromo- some 19q13 and identi®ed a point mutation within the gene SCN1B encoding the b1-subunit of the vol- tage-gated Na‡ channel. The mutation predicts substitution of tryptophan for a cysteine residue at position 121 disrupt- ing a disul®de bridge and changing the secondary structure of the b1-subunit extracellular loop (Fig. 5). This leads to a loss of b-subunit function resulting electrophysiologically in a slight slowing of the inactivation time course of the resulting Na‡ current [Wallace et al.,

1998]. Although the b1-subunit is also expressed in skeletal muscle, interest- ingly, these patients were not reported to suffer from myotonia like others carrying mutations within the skeletal muscle Na‡ channel a-subunit gene SCN4A. Hence, the brain seems to be more sensitive to such changes of Figure 6. Functional consequences of two SCN1A GEFS‡-mutations in II/S4 excitability than skeletal muscle ®bers, and IV/S4 (T875M, R1648H; Fig. 5). The mutations were introduced in the highly homologous skeletal muscle a-subunit gene SCN4A (corresponding mutations or, alternatively, there are different T685M, R1460H) and studied in human embryonic kidney cells (tsA201) using the disease-causing mechanisms for both whole-cell patch-clamp technique. A: Families of raw current traces show only barely ‡ diseases, which is discussed in more measurable differences in the time course of inactivation and no persistent Na current at the end of the depolarizing test pulses for the mutations compared to wild type (WT) detail below. channels. B: A strong acceleration of the time course of recovery from inactivation was Subsequently, several groups found found for R1460H (shown at 100 mV). C: Another signi®cant difference was an acceleration of the time course of activation at potentials more negative than 20 mV, linkage to a cluster of genes encoding ‡ ‡ shown as a shortening of the rise time of the whole-cell Na current for both neuronal Na channel a-subunits on mutations. D,E: Steady-state fast (E) and slow (F) inactivation curves. For both chromosome 2q21-33 and the ®rst two mutations inactivation is enhanced resulting in a loss-of-function and decrease of point mutations were detected in membrane excitability in contrast to the gain-of-function mechanisms shown in B,C [modi®ed after Alekov et al., 2000, 2001]. SCN1A predicting amino acid changes within the voltage sensors (S4 segments) of domains II and IV [Escayg et al., 2000, 2001] (Fig. 6). The most obvious needed to elicit an action potential, 2000a] (Fig. 5). Recently, several more alteration of the IV/S4 mutant these alterations would increase mem- SCN1A mutations have been described (R1460H) was a threefold acceleration brane excitability. [Escayg et al., 2001; Wallace et al., of recovery from inactivation (Fig. 6B), However, the most obvious differ- 2001a] (Fig. 5) and there is evidence which was also reported in a preliminary ence in gating for the II/S4 mutation for further genetic as well as clinical study with expression of the mutation in (T685M) in comparison to the wild type heterogeneity [Lerche et al., 2001]. the SCN1A gene using Xenopus oocytes was an enhancement of both fast and Heterologous functional expression and two-microelectrode voltage clamp- slow inactivation of the channel. The of the ®rst two SCN1A mutations in ing [Escayg et al., 2000b]. In addition, steady-state fast and slow inactivation segments II/S4 and IV/S4 using the we found little acceleration of the curves were shifted by 10 or 20 mV, highly conserved gene SCN4A, human activation time course at potentials respectively, entry into slow inactivation embryonic kidney cells (tsA201), and more negative than 20 mV for both was accelerated and recovery from slow the whole-cell patch clamp technique mutations compared to wild-type chan- inactivation signi®cantly slowed. These revealed only subtle changes in sodium nels (Fig. 6C). By shortening the alterations were also found for the IV/S4 channel fast inactivation and activation refractory period after an action poten- mutation, although less pronounced and no persistent current [Alekov et al., tial and the time of depolarization (Fig. 6D,E) [Alekov et al., 2000, 2001]. 154 AMERICAN JOURNAL OF MEDICAL GENETICS (SEMIN. MED. GENET.) ARTICLE

Hence, the disease-causing mechanism tively to such alterations of excitability terall, 2000]. Therefore, SMEI is a loss- of sodium channel mutations found in than muscle ®bers, which seems to apply of-function sodium channel disorder GEFS‡ might be a loss-of-function by similarly for K‡ channel defects (see caused by haploinsuf®ciency and, from enhanced inactivation of the channel. above). a genetic point of view, a severe allelic Two recent advances in the genetics variant of GEFS‡.

of idiopathic epilepsies support the Finally, the loss of b1-subunit func- hypothesis that a decrease of excitability tion by the SCN1B mutation could also Hence, the disease-causing of inhibitory neurons is the most induce a loss-of-function of the sodium mechanism of sodium important disease-causing mechanism channel, since one of the major effects of ‡ channel mutations found for GEFS -causing sodium channel the b1-subunit upon coexpression with mutations. First, only recently mutations the a-subunit is to increase the current ‡ in GEFS might be a in two GEFS‡ families were found in the amplitude [Catterall, 2000]. Altogether, loss-of-function by g2-subunit of GABAA receptors. One of loss-of-function mechanisms (in inhibi- these families presented with a typical tory neurons) seem to predominate and enhanced inactivation GEFS‡ phenotype (FS and FS‡) [Baulac are common to all mutations causing of the channel. et al., 2001], the other with a frequent GEFS‡ or SMEI. combination of FS and absence seizures besides other syndromes described in ‡ EPISODIC ATAXIA TYPE 1 The increase of excitability due to GEFS [Wallace et al., 2001b]. The two WITH MYOKYMIA (AND acceleration of recovery from fast inac- mutations are located in different regions PARTIAL EPILEPSY) tivation or of the activation time of the channel, one in the benzodiaze- courseÐwhich would have to exert its pine binding domain in the N-terminal Another ion channel disorder with effect on excitatory neurons to explain extracellularloop(R43Q)[Wallaceetal., disturbed excitability of the CNS is the occurrence of epileptic seizuresÐ 2001b] and the other in the loop episodic ataxia type 1 with myokymia may be to small, in particular for the II/ connecting transmembrane segments (EA-1). S4 mutation. In contrast, enhancement M2 and M3 (K289M) [Baulac et al., of both fast and slow inactivation would 2001]. Functional expression of the decrease membrane excitability by redu- mutant receptor g2-subunits together Another ion channel disorder cing the number of available sodium with a1- and b2-subunits revealed two with disturbed excitability of channels. When acting on inhibitory distinct gating defects. Whereas muta- neurons, this effect could be responsible tion K289M reduced GABA-activated the CNS is episodic ataxia type for the occurrence of synchronous currents 10-fold, R43Q revealed nor- 1 with myokymia. activity in neuronal circuits causing mal GABA-activated currents, but abol- epileptic seizures. ished the sensitivity to benzodiazepines These ®ndings are in contrast to the such that activation by diazepam was no Dysfunction occurs predominantly in gain-of-function mechanism by a failure longer present. Thus, both mutations the cerebellum. Patients suffer from brief of inactivation that has been shown for lead to a loss-of-function of GABAA kinesiogenic attacks of gait and limb SCN4A mutations causing sodium receptors, although for R43Q it has to ataxia or cerebellar dysarthria. Interic- channel disorders of skeletal muscle, like be postulated that `endozepines'do exist tally they experience myokymia. In four myotonia and periodic paralysis [Leh- and can prevent the development of families, partial epileptic seizures were mann-Horn and Jurkat-Rott, 1999; epileptic seizures in vivo [Wallace et al., also reported, occurring in some family Cannon, 2000; Mitrovic and Lerche, 2001b]. For these mutations, undoubt- members affected by ataxia or myokymia 2000]. Nonetheless, such a gain-of- edly a decrease of excitability in inhibi- [van Dyke et al., 1975; Brunt and function has also been shown to induce tory neurons is the pathophysiological van Weerden, 1990; Zuberi et al., 1999; epileptic seizures in a transgenic mouse mechanism causing seizures, as it could Eunson et al., 2000]. Zuberi et al. [1999] model in which an SCN2A mutation similarly be explained by enhanced estimated a 10-fold increased risk to de- with slowing of the inactivation time inactivation of sodium channels. velop epilepsy when affected by EA-1. course and increased persistent current Second, novel mutations were id- Genetic analyses in EA-1 revealed was introduced. Only 25% of the animals enti®ed in SCN1A causing a more linkage to chromosome 12p13 and survived beyond 6 months of age; death severe phenotype than GEFS‡, that is mutations within the Shaker homolo- occurred due to severe status epilepticus SMEI [Claes et al., 2001]. Most of these gous gene KCNA1 encoding the K‡

[Kearneyet al., 2001]. The gating defects mutations predict an early stop codon channel Kv1.1 [Browne et al., 1994; Litt of inactivation were much less pro- and a truncated protein without func- et al., 1994] (Fig. 7). Functional expres- nounced than those found for SCN4A tion (Fig. 5), with regard to all we know sion in Xenopus oocytes or mammalian mutations causing myotonia, suggesting about structure±function relationships cells resulted in a reduction of the K‡ that the CNS reacts much more sensi- of voltage-gated sodium channels [Cat- currents either by diminished expression ARTICLE AMERICAN JOURNAL OF MEDICAL GENETICS (SEMIN. MED. GENET.) 155

trait of idiopathic generalized epilepsy (IGE) revealed loci on chromosomes 6p and 15q14 for juvenile myoclonic epi- lepsy [JME; 6p: Greenberg et al., 1988; Serratosa et al., 1996; Sander et al., 1997; 15q14: Elmslie et al., 1997] and on 8q24 for childhood absence epilepsy [CAE, Fong et al., 1998; Sugimoto et al., 2000]. In two linkage studies using a large number of smaller IGE families, the 8q24 locus was also found, while the 6p locus could not be veri®ed [Zara et al., 1995]; other potential loci were described on 2q36, 3q26, and 14q23; 15q14 was con®rmed [Sander et al., 2000]. Signi®cant linkage to the JME locus on chromosome 15 was recently also described for Rolando epilepsy [Neubauer et al., 1998]. Until now, mutations in genes at these locations have not been identi®ed but there are several ion channel or transporter encoding genes that are strong candi- dates: on chromosome 2q36 the chloride-bicarbonate exchanger gene SLC4A3, on 3q26 the voltage-gated K‡ channel b-subunit gene KCNA1B [Schultz et al., 1996], and the Cl channel gene CLCN2 [Cid et al., 1995], on 14q24 the Na‡/Ca2‡- exchanger gene SLC8A2 [Li et al.,

1994] and on 15q14 the a7-subunit gene of the neuronal nAChR CHRNA7 [Chini et al., 1994]. Several ion channel encoding genes were tested in association studies and mutation screenings if the play a role in the genetics of IGE. For KCNQ2 [Steinlein et al., 1999], KCNQ3 [Haug Figure 7. Proposed transmembrane structure of the voltage-gated potassium et al., 2000a], KCNJ3 and KCNJ6 ‡ channel Kv1.1, the human homolog of the Shaker K channel, encoded by the gene [Girk1 and Girk2: Hallmann et al., KCNA1. Mutations cause episodic ataxia type 1 with myokymia, two mutations are 2000], KCNN3 [hKCa3: Sander et al., associated with partial epilepsy, and one mutation causes isolated myokymia. 1999], CACNA1A [Sander et al., 1998], and SCN1B [Haug et al., 2000b], no or shifts in voltage-dependence. Ac- induce epileptic seizures, a knock-out association could be found. A possible cording to these studies, both dominant mouse model for Kv1.1 presented with association of a benign polymorphism negative effects on WT channels and an epileptic phenotype [Smart et al., in CHRNA4 with IGE [Steinlein et al., haploinsuf®ciency can cause EA-1 1998]. 1997b] could not be con®rmed in [Adelman et al., 1995; Zerr et al., another study [Chioza et al., 2000]. 1998; Bretschneider et al., 1999; Zuberi ASSOCIATION OF ION Recently, a mutation was discov- ered in a patient with juvenile myoclonic et al., 1999; Eunson et al., 2000]. A CHANNEL DEFECTS WITH speci®c defect for those mutations going epilepsy in the gene CACNB4, encod- COMMON FORMS OF along with an epilepsy phenotype could ing the b -subunit of the high voltage- IDIOPATHIC EPILEPSY 4 not be found [Zuberi et al., 1999; gated L-type Ca2‡ channel, and func- Eunson et al., 2000]. In support of the Genetic linkage studies in a few large tional studies revealed differences in hypothesis that KCNA1 mutations can families with a presumably monogenic channel gating for this mutation com- 156 AMERICAN JOURNAL OF MEDICAL GENETICS (SEMIN. MED. GENET.) ARTICLE

pared to the WT [Escayg et al., 2000c]. with KATP channel openers failed due to ataxia/myokymia syndrome is associated Naturally occurring mutations in differ- intolerable cardiovascular side effects, with point mutations in the human potas- sium channel gene, KCNA1. Nat Genet ent subunits of the same channel com- since KATP channels are expressed abun- 8:136±140. plex cause epilepsy with generalized dantly in heart, smooth muscle cells, and Brunt ER, van Weerden TW. 1990. Familial spike and wave discharges in the EEG other tissues [Lawson, 2000]. In con- paroxysmal kinesigenic ataxia and contin- uous myokymia. Brain 113:1361±1382. in several mouse models (Noebels, trast, KCNQ2 and KCNQ3 channels Cannon SC. 2000. Spectrum of sodium channel accompanying article). Mutations in are expressed speci®cally in neurons and, disturbances in the nondystrophic myoto- CACNA1A, encoding the a-subunit, therefore, side effects of retigabine nias and periodic paralyses. Kidney Int 57:772±779. cause episodic ataxia type II, familial should be diminished, since it has no Catterall WA. 2000. From ionic currents to hemiplegic migraine, or spinocerebellar effect on KCNQ1 channels expressed in molecular mechanisms: the structure and ataxia type 6 in man [Ophoff et al., 1996; the heart [Main et al., 2000b]. Clinical function of voltage-gated sodium channels. Neuron 26:13±25. Zhuchenko et al., 1997]. Interestingly, trials are currently under way. Chandy KG, Gutman GA. 1995. Voltage-gated the same mutation in CACNB4 causing K‡ channel genes. In: North RA, editor. JME caused EA-2 in a Canadian family Handbook of receptors and channels. REFERENCES Ligand- and voltage-gated ion channels. [Escayg et al., 2000c]. It remains to be Boca Raton, FL: CRC Press. p 1±71. proven in further studies if CACNB4 is Adelman JP, Bond CT, Pessia M, Maylie J. 1995. Charlier C, Singh NA, Ryan SG, Lewis TB, Reus an `epilepsy gene' involved in a larger Episodic ataxia results from voltage-depen- BE, Leach RJ, Leppert M. 1998. A pore dent potassium channels with altered func- mutation in a novel KQT-like potassium number of IGE families. tions. Neuron 15:1449±1454. channel gene in an idiopathic epilepsy Alekov AK, Rahman MM, Mitrovic N, Leh- family. Nat Genet 18:53±55. mann-Horn F, Lerche H. 2000. A sodium Chini B, Raimond E, Elgoyhen AB, Moralli D, IMPLICATIONS FOR channel mutation causing epilepsy in man Balzaretti M, Heinemann S. 1994. Mole- THERAPY exhibits subtle defects in fast inactivation and cular cloning and chromosomal localization activation in vitro. J Physiol 529:533±539. of the human alpha 7-nicotinic receptor The discovery of genetic defects and, in Alekov AK, Rahman MM, Mitrovic N, Leh- subunit gene. Genomics 19:379±381. particular, the electrophysiological char- mann-Horn F, Lerche H. 2001. Enhanced Chioza B, Goodwin H, Blower J, McCormick D, inactivation and acceleration of activation of Nashef L, Asherson P, Makoff AJ. 2000. acterization of mutant ion channels in the sodium channel associated with epilepsy Failure to replicate association between the hereditary forms of epilepsy elucidates in man. Eur J Neurosci 13:2171±2176. gene for the neuronal nicotinic acetylcho- pathophysiological concepts of hyper- Baulac S, Gour®nkel-An I, Picard F, Rosenberg- line receptor alpha 4 subunit (CHRNA4) Bourgin M, Prud'homme J-F, Baulac M, and IGE. Am J Med Genet 96:814±816. excitability in the CNS. This knowledge Brice A, LeGuern E. 1999. A second locus Chouabe C, Neyroud N, Guicheney P, Lazdunski enables new therapeutic strategies by for familial generalized epilepsy with febrile M, Romey G, Barhanin J. 1997. Properties antagonizing the epilepsy-causing seizures plus maps to chsomosome 2q21- of KvLQT1 K‡ channel mutations in q33. Am J Hum Genet 65:1078±1085. Romano-Ward and Jervell and Lange-Niel- mechanisms using the defective proteins Baulac S, Huberfeld G, Gour®nkel-An I, Mitro- sen inherited cardiac arrhythmias. EMBO J as pharmacological targets. In the case of poulou G, Beranger A, Prud'homme JF, 16:5472±5479. BFNC, a completely novel approach in Baulac M, Brice A, Bruzzone R, LeGuern Cid L, Montrose-Ra®zadeh C, Smith DI, Gug- E. 2001. First genetic evidence of GABA-A gino WB, Cutting GR. 1995. Cloning of a the treatment of epilepsies emerged from receptor dysfunction in epilepsy: a mutation putative human voltage-gated chloride identifying retigabine as an activator of in the g2-subunit gene. Nat Genet 28:46± channel (CLC-2) cDNA widely expressed M-currents conducted by KCNQ2 and 48. in human tissues. Hum Mol Genet 4:407± ‡ Bertrand D, Changeux JP. 1999. Nicotinic 413. KCNQ3 K channels. Retigabine shifts receptor: a prototype of allosteric ligand- Claes L, Del-Favero J, Ceulemans B, Lagae L, Van the voltage dependence of steady-state gated ion channels and its possible implica- Broeckhoven C, De Jonghe P. 2001. De activation of these channels by about 20 tions in epilepsy. Adv Neurol 79:171±188. novo mutations in the sodium-channel gene Bertrand S, Weiland S, Berkovic SF,Steinlein OK, scn1a cause severe myoclonic epilepsy of mV in the negative direction so that they Bertrand D. 1998. Properties of neuronal infancy. Am J Hum Genet 68:1327±1332. are active at the resting membrane nicotinic acetylcholine receptor mutants De Fusco M, Becchetti A, Patrignani A, Annesi potential. This stabilizes the cell mem- from human suffering from autosomal G, Gambardella A, Quattrone A, Ballabio A, dominant nocturnal frontal lobe epilepsy. Wanke E, Casari G. 2000. The nicotinic brane via hyperpolarization towards the Br J Pharmacol 125:751±760. receptor b2 subunit is mutant in nocturnal K‡ equilibrium potential [Rundtfeld Biervert C, Steinlein OK. 1999. Structural and frontal lobe epilepsy. Nat Genet 26:275± and Netzer, 2000; Main et al., 2000a; mutational analysis of KCNQ2, the major 276. gene locus for benign familial neonatal Dravet C. 1978. Les eÂplepsies graves de l'enfant. Wickenden et al., 2000]. convulsions. Hum Genet 104:234±240. Vie Med 8:543±548. It has been shown that for openers Biervert C, Schroeder BC, Kubisch C, Berkovic Elmslie FV, Rees M, Williamson MP, Kerr M, of ATP-dependent K‡ channels (K SF, Propping P, Jentsch TJ, Steinlein OK. Kjeldsen MJ, An Pang K, Sundqvist A, Friis ATP 1998. A potassium channel mutation in ML, Chadwick D, Richens A, Covanis A, channels) that they reduce hyperexcit- neonatal human epilepsy. Science 279:403± Santos M, Arzimanoglou A, Panayiotopou- ability and can reverse paralysis of 406. los CP, Curtis D, Whitehouse WP, Gardiner biopsied skeletal muscle ®bers from Bretschneider F, Wrisch A, Lehmann-Horn F, RM. 1997. Genetic mapping of a major Grissmer S. 1999. Electrophysiological char- susceptibility locus for juvenile myoclonic patients with myotonia or periodic acterization of two mutant Kv1.1 potassium epilepsy on chromosome 15q. Hum Mol paralysis in vitro by hyperpolarization channels causing episodic ataxia type 1 in Genet 6:1329±1334. of the cell membrane [Grafe et al., 1990; mammalian cells. Eur J Neurosci 11:2403± Escayg AP, MacDonald BT, Meisler MH, Baulac 2412. S, Huberfeld G, An-Gour®nkel I, Brice A, Quasthoff et al., 1990; Lerche et al., Browne DL, Gancher ST, Nutt JG, Brunt ERP, LeGuern E, Moulard B, Chaigne D, Buresi 1996]. Attempts to treat such patients Smith EA, Kramer P, Litt M. 1994. Episodic C, Malafosse A. 2000a. Mutations of ARTICLE AMERICAN JOURNAL OF MEDICAL GENETICS (SEMIN. MED. GENET.) 157

SCN1A, encoding a neuronal sodium Haug K, Sander T, Hallmann K, Rau B, Dullinger human skeletal muscle. Ann Neurol channel, in two families with GEFS‡2. JS, Elger CE, Propping P, Heils A. 2000b. 39:599±608. Nat Genet 24:343±345. The voltage-gated sodium channel beta2- Lerche H, Biervert C, Alekov AK, Schleithoff L, Escayg AP, MacDonald BT, Spmpanato J, Goldin subunit gene and idiopathic generalized Lindner M, Klingler W, Bretschneider F, AL, Meisler MH. 2000b. Coding and epilepsy. Neuroreport 11:2687±2689. Mitrovic N, Jurkat-Rott K, Bode H, noncoding variation in the neuronal sodium Hille B. 1992. Ionic channels of excitable Lehmann-Horn F, Steinlein OK. 1999. A channel SCN1A in patients with epilepsy. membranes. Sunderland: Sinauer. reduced K‡ current due to a novel mutation Soc Neurosci Abstr 26:222. Hirose S, Iwata H, Akiyoshi H, Kobayashi K, Ito in KCNQ2 causes neonatal convulsions. Escayg AP, De Waard M, Lee DD, Bichet D, Wolf M, Wada K, Kaneko S, Mitsudome A. 1999. Ann Neurol 46:305±312. P, Mayer T, Johnston J, Baloh R, Sander T, A novel mutation of CHRNA4 responsible Lerche C, Scherer CR, Seebohm G, Derst C, Wei Meisler MH. 2000c. Coding and noncoding for autosomal dominant nocturnal frontal AD, Busch AE, Steinmeyer K. 2000a. variation of the human calcium-channel b4- lobe epilepsy. Neurology 53:1749±1753. Molecular cloning and functional expression subunit gene CACNB4 in patients with Hirose S, Zenri F, Akiyoshi H, Fukuma G, Iwata of KCNQ5, a potassium channel subunit idiopathic generalized epilepsy and episodic H, Inoue T, Yonetani M, Tsutsumi M, that may contribute to neuronal M-current ataxia. Am J Hum Genet 66:1531±1539. Muranaka H, Kurokawa T, Hanai T, Wada diversity. J Biol Chem 275:22395±22400. Escayg A, Heils A, MacDonald BT, Haug K, K, Kaneko S, Mitsudome A. 2000. A novel Lerche C, Seebohm G, Schiebe M, Busch AE, Sander T, Meisler MH. 2001. A novel mutation of KCNQ3 (c.925T!C) in a Lerche H. 2000b. Evidence for assembly of SCN1A mutation associated with general- Japanese family with benign familial neona- KCNQ2 and KCNQ3 K‡ channels via the ized epilepsy with febrile seizures plus and tal convulsions. Ann Neurol 47:822± C-terminus. Soc Neurosci Abstr 26:1909. prevalence of variants in patients with 826. Lerche H, Weber YG, Baier H, Jurkat-Rott K, epilepsy. Am J Hum Genet 68:866±873. Jentsch TJ. 2000. Neuronal KCNQ potassium Kraus de Camargo O, Ludolph AC, Bode Eunson EL, Rea R, Zuberi SM, Youroukos S, channels: physiology and role in disease. Nat H, Lehmann-Horn F. 2001. Generalized Panayiotopoulos CP, Liguori R, Avoni P, Neurosci Rev 1:21±30. epilepsy with febrile seizures plus: further McWilliam RC, Stephenson JBP, Hanna Kandel ER, Siegelbaum SA. 2000. Synaptic heterogeneity in a large family. Neurology MG, Kullmann DM, Spauschus A. 2000. integration. In: Kandel ER, Schwartz JH, (in press). Clinical, genetic, and expression studies of Jessel MT, editors. Principles of neural Lewis TB, Leach RJ, Ward K, O'Connell P, Ryan mutations in the potassium channel gene science. New York: McGraw Hill. p 207± SG. 1993. Genetic heterogeneity in benign KCNA1 reveal new phenotypic variability. 228. familial neonatal convulsions: identi®cation Ann Neurol 48:647±656. Kearney JA, Plummer NW, Smith MR, Kapur J, of a new locus on chromosome 8q. Am J Figl A, Viseshakul N, Shafaee N, Forsayeth J, Cummins TR, Waxman SG, Goldin AL, Hum Genet 53:670±675. Cohen BN. 1998. Two mutations linked to Meisler MH. 2001. A gain-of-function Li Z, Matsuoka S, Hryshko LV, Nicoll DA, nocturnal frontal lobe epilepsy cause use- mutation in the sodium channel gene Scn2a Bersohn MM, Burke EP, Lifton RP, Phi- dependent potentiation of the nicotinic Ach results in seizures and behavioral abnormal- lipson KD. 1994. Cloning of the NCX2 response. J Physiol 513:655±670. ities. Neuroscience 102:307±317. isoform of the plasma membrane Na‡/Ca2‡ Fong GCY, Shah PU, Gee MN, Serratosa JM, Kubisch C, Schroeder BC, Friedrich T, LuÈtjo- exchanger. J Biol Chem 269:17434±17439. Castroviejo IP, Khan S, Ravat SH, Mani J, hann B, El-Amraoui A, Martin S, Petit C, Litt M, Kramer P, Browne D, Gancher S, Brunt Huang Y, Zhao HZ, Medina MT, Treiman Jentsch TJ. 1999. KCNQ4, a novel potas- ERP, Root D, Phromchotikul T, Dubay CJ, LJ, Pineda G, Delgado-Escueta AV. 1998. sium channel expressed in sensory outer hair Nutt J. 1994. A gene for episodic ataxia/ Childhood absence epilepsy with tonic- cells, is mutated in dominant deafness. Cell myokymia maps to chromosome 12p13. Am clonic seizures and electroencephalogram 96:437±446. J Hum Genet 55:702±709. 3-4-Hz spike and multispike-slow wave Kuryatov A, Gerzanich V, Nelson M, Olale F, Lopes-Cendes I, Scheffer IE, Berkovic SF, complexes: linkage to chromosome 8q24. Lindstrom J. 1997. Mutation causing auto- Rousseau M, Andermann E, Rouleau GA. Am J Hum Genet 63:1117±1129. somal dominant nocturnal frontal lobe 2000. A new locus for generalized epilepsy Goldin AL, Barchi RL, Caldwell JH, Hofmann F, epilepsy alters Ca2‡ permeability, conduc- with febrile seizures plus maps to chromo- Howe JR, Hunter JC, Kallen RG, Mandel tance, and gating of human alpha4beta2 some 2. Am J Hum Genet 66:698±701. G, Meisler MH, Netter YB, Noda M, nicotinic acetylcholine receptors. J Neurosci Main MJ, Cryan JE, Dupere JRB, Cox B, Clare JJ, Tamkun MM, Waxman SG, Wood JN, 17:9035±9047. Burbidge SA. 2000a. Modulation of Catterall WA. 2000. Nomenclature of Labarca C, Schwarz J, Deshpande P, Schwarz S, KCNQ2/3 potassium channels by the novel voltage-gated sodium channels. Neuron Nowak MW,Fonck C, Nashmi R, Kofuji P, anticonvulsant retigabine. Mol Pharm 28:368. Dang H, Shi W, Fidan M, Khakh BS, Chen 58:253±262. Grafe P, Quasthoff S, Strupp M, Lehmann-Horn Z, Bowers BJ, Boulter J, Wehner JM, Lester Main MJ, Tatulian L, Cryan JE, Selyanko A, F. 1990. Enhancement of K‡ conductance HA. 2001. Point mutant mice with hyper- Brown D, Clare JJ, Hayes A, Trezise DJ, improves in vitro the contraction force of sensitive alpha 4 nicotinic receptors show Burbidge SA. 2000b. Modulation of KCNQ skeletal muscle in hypokalemic periodic dopaminergic de®cits and increased anxiety. potassium channels by retigabine. Soc Neu- paralysis. Muscle Nerve 13:451±457. Proc Natl Acad Sci USA 98:2786±2791. rosci Abs 26:1908. Greenberg DA, Delgado-Escueta AV, Widelitz H, Lawson K. 2000. Potassium channel openers as Maljevic S, Lerche C, Seebohm G, Wuttke T, Sparkes RS, Treiman L, Maldonado HM, potential therapeutic weapons in ion chan- Alekov A, Busch AE, Lerche H. 2001. Park MS, Terasaki PI. 1988. Juvenile nel disease. Kidney Int 57:838±845. Evidence for assembly of KCNQ2 and myoclonic epilepsy (JME) may be linked Lehmann-Horn F, Jurkat-Rott K. 1999. Voltage- KCNQ3 K‡ channels via the C-terminus. to the BF and HLA loci on human gated ion channels and hereditary disease. P¯uÈgers Arch Eur J Physiol 441:R143. chromosome 6. Am J Med Genet 31:185± Physiol Rev 79:1317±1372. Mehta AK, Ticku MK. 1999. An update on 192. Lehmann-Horn F, Jurkat-Rott K. 2000. Chan- GABA-A receptors. Brain Res Rev Hallmann K, Durner M, Sander T, Steinlein OK. nelopathies: common mechanisms in 29:196±217. 2000. Mutation analysis of the inwardly aura, arrhythmia and alkalosis. Amsterdam: Miraglia del Giudice E, Coppola G, Scuccimarra rectifying K(‡) channels KCNJ6 (GIRK2) Elsevier. G, Cirillo G, Bellini G, Pascotto A. 2000. and KCNJ3 (GIRK1) in juvenile myoclonic Leppert M, Anderson VE, Quattlebaum T, Benign familial neonatal convulsions epilepsy. Am J Med Genet 96:8±11. Stauffer D, O'Connell P, Nakamura Y, (BFNC) resulting from mutation of the Haug K, Hallmann K, Horvath S, Sander T, Lalouel JM, White R. 1989. Benign familial KCNQ2 voltage sensor. Eur J Med Genet Kubisch C, Rau B, Dullinger J, Beyenburg neonatal convulsions linked to genetic 8:994±997. S, Elger CE, Propping P,Heils A. 2000a. No markers on chromosome 20. Nature 337: Mitrovic N, Lerche H. 2000. Sodium and calcium evidence for association between the 647±648. channelopathies of sarcolemma: periodic KCNQ3 gene and susceptibility to idio- Lerche H, Mitrovic N, Dubowitz V, Lehmann- paralyses, paramyotonia congenita and pathic generalized epilepsy. Epilepsy Res Horn F. 1996. Paramyotonia congenita: the potassium-aggravated myotonia. In: Leh- 42:57±62. R1448P sodium channel mutation in adult mann-Horn F, Jurkat-Rott K, editors. 158 AMERICAN JOURNAL OF MEDICAL GENETICS (SEMIN. MED. GENET.) ARTICLE

ChannelopathiesÐcommon mechanisms in human skeletal muscle in vitro. Eur J novel potassium channel broadly expressed aura, arrhythmia and alkalosis. Amsterdam: Pharmacol 186:125±128. in brain, mediates M-type currents. J Biol Elsevier-Science. p 3±32. Ronen GM, Rosales TO, Connolly M, Anderson Chem 275:24089±24095. Moulard B, Guipponi M, Chaigne D, Mouthon VE, Leppert M. 1993. Seizure character- Schultz D, Litt M, Smith L, Thayer M, McCor- D, Buresi C, Malafosse A. 1999. Identi®ca- istics in chromosome 20 benign familial mick K. 1996. Localization of two potas- tion of a new locus for generalized epilepsy neonatal convulsions. Neurology 43:1355± sium channel beta subunit genes, KCNA1B with febrile seizures plus (GEFS‡)on 1360. and KCNA2B. Genomics 31:389±391. chromosome 2q24-q33. Am J Hum Genet Ross SA, Wong JY, Clifford JJ, Kinsella A, Schwake M, Pusch M, Kharkovets T, Jentsch TJ. 65:1396±1400. Massalas JS, Horne MK, Scheffer IE, Kola 2000. Surface expression and single channel Neubauer BA, Fiedler B, Himmelein B, KaÈmpfer I, Waddington JL, Berkovic SF, Drago J. properties of KCNQ2/KCNQ3, M-type F, L aÈbker U, Schwabe G, Spanier I, Tams D, 2000. Phenotypic characterization of an K‡ channels involved in epilepsy. J Biol Bretscher O, Moldenhauer K, Kurlemann alpha 4 neuronal nicotinic acetylcholine Chem 275:13343±13348. G, Weise S, Tedroff K, Eeg-Olofsson O, receptor subunit knock-out mouse. J Neu- Serratosa JM, Delgado-Escueta AV, Medina MT, Wadelius C, Stephani U. 1998. Centrotem- rosci 20:6431±6441. Zhang Q, Iranmanesh R, Sparkes RS. 1996. poral spikes in families with rolandic Rundtfeld C, Netzer R. 2000. The novel antic- Clinical and genetic analysis of a large epilepsy. Linkage to chromosome 15q14. onvulsant retigabine activates M-currents in pedigree with juvenile myoclonic epilepsy. Neurology 51:1608±1612. Chinese hamster ovary-cells transfected with Ann Neurol 39:187±195. Ophoff RA, Terwindt GM, Vergouwe MN, van human KCNQ2/3 subunits. Neurosci Lett Shapiro MS, Roche JP, Kaftan EJ, Cruzblanca H, Eijk R, Oefner PJ, Hoffman SMG, Lamer- 282:73±76. Mackie K, Hille B. 2000. Reconstitution of din JE, Mohrenweiser HW, Bulman DE, Sander T, Bockenkamp B, Hildmann T, Blasczyk muscarinic modulation of the KCNQ2/ Ferrari M, Haan J, Lindhout D, van Ommen R, Kretz R, Wienker TF, Volz A, Schmitz KCNQ3 K‡ channels that underlie the GJB, Hofker MH, Ferrari MD, Frants RR. B, Beck-Mannagetta G, Rieb O, Epplen JT, neuronal M current. J Neurosci 20:1710± 1996. Familial hemiplegic migraine and Janz D, Ziegler A. 1997. Re®ned mapping 1721. episodic ataxia type-2 are caused by muta- of the epilepsy susceptibility locus EJM1 on Siegelbaum SA, Koester J. 2000. Ion channels. In: tions in the Ca2‡ channel gene chromosome 6. Neurology 49:842±847. Kandel ER, Schwartz JH, Jessel MT, editors. CACNL1A4. Cell 87:543±552. Sander T, Peters C, Janz D, Bianchi A, Bauer G, Principles of neural science. New York: Pfeiffer A, Thompson J, Charlier C, Otterud B, Wienker TF, Hildmann T, Epplen JT, Riess McGraw Hill. p 105±124. Varvil T, Pappas C, Barnitz C, Gruenthal K, O. 1998. The gene encoding the alpha1A- Sieghart W,Fuchs K, Tretter V,Ebert V,Jechlinger Kuhn R, Leppert M. 1999. A locus for voltage-dependent calcium channel M, Hoger H, Adamiker D. 1999. Structure febrile seizures (FEB3) maps to chromo- (CACN1A4) is not a candidate for causing and subunit composition of GABA(A) some 2q23-24. Ann Neurol 46:671±678. common subtypes of idiopathic generalized receptors. Neurochem Int 34:379±385. Phillips HA, Scheffer IE, Berkovic SF, Hollway epilepsy. Epilepsy Res 29:115±122. Singh NA, Charlier C, Stauffer D, DuPont BR, GE, Sutherland GR, Mulley JC. 1995. Sander T, Scholz L, Janz D, Epplen JT, Riess O. Leach RJ, Melis R, Ronen GM, Bjerre I, Localization of a gene for autosomal domi- 1999. Length variation of a polyglutamine Quattlebaum T, Murphy JV, McHarg ML, nant nocturnal frontal lobe epilepsy to array in the gene encoding a small-con- Gagnon D, Rosales TO, Peiffer A, Anderson chromosome 20q 13.2. Nat Genet 10: ductance, calcium-activated potassium E, Leppert M. 1998. A novel potassium 117±118. channel (hKCa3) and susceptibility to idio- channel gene, KCNQ2, is mutated in an Phillips HA, Marini C, Scheffer IE, Sutherland pathic generalized epilepsy. Epilepsy Res inherited epilepsy of newborns. Nat Genet GR, Mulley JC, Berkovic SF. 2000. A de 33:227±233. 18:25±29. novo mutation in sporadic nocturnal frontal Sander T, Schulz H, Saar K, Gennaro E, Riggio Singh R, Scheffer IE, Crossland K, Berkovic SF. lobe epilepsy. Ann Neurol 48:264±267. MC, Bianchi A, Zara F, Luna D, Bulteau C, 1999. Generalized epilepsy with febrile Phillips HA, Favre I, Kirkpatrick M, Zuberi SM, Kaminska A, Ville D, Cieuta C, Picard F, seizures plus: a common childhood-onset Goudie D, Heron SE, Scheffer IE, Suther- Prud'homme JF, Bate L, Sundquist A, genetic epilepsy syndrome. Ann Neurol land GR, Berkovic SF, Bertrand D, Mulley Gardiner RM, Janssen GA, Haan GJ, 45:75±81. JC. 2001. CHRNB2 is the second acetyl- Kasteleijn-Nolst-Trenite DG, Bader A, Smart SL, Lopantsev V, Zhang CL, Robbins CA, choline receptor subunit associated with Lindhout D, Riess O, Wienker TF, Janz D, Wang H, Chiu SY, Schwartzkroin PA, autosomal dominant nocturnal frontal Reis A. 2000. Genome search for suscept- Messing A, Tempel BL. 1998. Deletion of lobe epilepsy. Am J Hum Genet 68:225± ibility loci of common idiopathic general- the Kv1.1 potassium channel causes epilepsy 231. ised epilepsies. Hum Mol Genet 9:1465± in mice. Neuron 20:809±819. Picard F, Bertrand S, Steinlein OK, Bertrand D. 1472. Smith JS, Iannotti CA, Dargis P, Christian EP, 1999. Mutated nicotinic receptors respon- Scheffer IE, Berkovic SF. 1997. Generalized Aiyar J. 2001. Differential expression of sible for autosomal dominant nocturnal epilepsy with febrile seizures plus. A genetic KCNQ2 splice variants: implications to M frontal lobe epilepsy are more sensitive to disorder with heterogeneous clinical phe- current function during neuronal develop- carbamazepine. Epilepsia 40:1198±1209. notypes. Brain 120:479±490. ment. J Neurosci 21:1096±1103. Picard F, Baulac S, Kahane P, Hirsch E, Sebastia- Scheffer IE, Bhatia KP, Lopes-Cendes I, et al. Steinlein OK, Mulley JC, Propping P, Wallace nelli R, Thomas P, Vigevano F, Genton P, 1994. Autosomal dominant frontal epilepsy RH, Phillips HA, Sutherland GR, Scheffer Guerrini R, Gericke CA, An I, Rudolf G, misdiagnosed as sleep disorder. Lancet IE, Berkovic SF. 1995. A missense mutation Herman A, Brice A, Marescaux C, LeGuern 343:515±517. in the neuronal nicotinic acetylcholine E. 2000. Dominant partial epilepsies. A Scheffer IE, Bhatia KP, Lopes-Cendes I, et al. receptor a4 subunit is associated with clinical, electrophysiological and genetic 1995. Autosomal dominant nocturnal fron- autosomal dominant nocturnal frontal lobe study of 19 European families. Brain tal lobe epilepsy: a distinct clinical disorder. epilepsy. Nat Genet 11:201±203. 123:1247±1262. Brain 118:61±73. Steinlein OK, Magnusson A, Stoodt J, Bertrand S, Plouin P. 1994. Benign idiopathic neonatal Schmitt N, Schwarz M, Peretz A, Abitbol I, Attali Weiland S, Berkovic SF, Nakken KO, convulsions (familial and non-familial): B, Pongs O. 2000. A recessive C-terminal Propping P,Bertrand D. 1997a. An insertion open questions about these syndromes. In: Jervell and Lange-Nielsen mutation of the mutation of the CHRNA4 gene in a family Wolf P, editor. Epileptic seizures and KCNQ1 channel impairs subunit assembly. with autosomal dominant nocturnal frontal syndromes. London: John Libbey & Co. p EMBO J 19:332±340. lobe epilepsy. Hum Mol Genet 6:943± 193±201. Schroeder BC, Kubisch C, Stein V, Jentsch TJ. 947. Ptacek LJ. 1999. Ion channel diseases: episodic 1998. Moderate loss of function of cyclic- Steinlein OK, Sander T, Stoodt J, Kretz R, Janz D, disorders of the nervous system. Semin AMP-modulated KCNQ2/KCNQ3 K‡ Propping P. 1997b. Possible association of a Neurol 19:363±369. channels causes epilepsy. Nature 396:687± silent polymorphism in the neuronal nico- Quasthoff S, Spuler A, Spittelmeister W, Leh- 690. tinic acetylcholine receptor subunit alpha4 mann-Horn F, Grafe P. 1990. K‡ channel Schroeder BC, Hechenberger M, Weinreich F, with common idiopathic generalized epi- openers suppress myotonic activity of Kubisch C, Jentsch TJ. 2000. KCNQ5,a lepsies. Am J Med Genet 74:445±449. ARTICLE AMERICAN JOURNAL OF MEDICAL GENETICS (SEMIN. MED. GENET.) 159

Steinlein OK, Stoodt J, Biervert C, Janz D, Sander Gardner A, Sutherland GR, George AL Jr, Wollnik B, Schroeder BC, Kubisch C, Esperer T. 1999. The voltage gated potassium Mulley JC, Berkovic SF. 2001a. Neuronal HD, Wieacker P, Jentsch TJ. 1997. Patho- channel KCNQ2 and idiopathic generalized sodium-channel alpha1-subunit mutations physiological mechanisms of dominant and epilepsy. Neuroreport 10:1163±1166. in generalized epilepsy with febrile seizures recessive KVLQT1 K‡ channel mutations Sugimoto Y,Morita R, Amano K, Fong CY,Shah plus. Am J Hum Genet 68:859±865. found in inherited cardiac arrhythmias. PU, Castroviejo IP, Khan S, Delgado- Wallace RH, Marini C, Petrou S, Harkin LA, Hum Mol Gen 6:1943±1949. Escueta AV, Yamakawa K. 2000. Childhood Bowser DN, Panchal RG, Williams DA, Yang W-P, Levesque PC, Little WA, Conder ML, absence epilepsy in 8q24: re®nement of Sutherland GR, Mulley JC, Scheffer IE, Ramakrishnan P, Neubauer MG, Blanar candidate region and construction of phy- Berkovic SF. 2001b. Mutant GABA-A MA. 1998. Functional expression of two sical map. Genomics 68:264±272. receptor g2-subunit in childhood absence KvLQT1-related potassium channels re- Swann JW, Smith KL, Brady RJ, Pierson MG. epilepsy and febrile seizures. Nat Genet sponsible for an inherited idiopathic epi- 1993. Neurophysiological studies of altera- 28:49±52. lepsy. J Biol Chem 273:19419±19423. tions in seizure susceptibility during brain Wang Q, Curran ME, Splawski I, Burn TC, Zara F, Bianchi A, Avanzini G, Di Donato S, development. In: Schwartzkroin PA, editor. Millholland JM, VanRaay TJ, Shen J, Castellotti B, Patel PI, Pandolfo M. 1995. Epilepsy: models, mechanisms and concepts. Timothy KW, Vincent GM, de Jager T, Mapping of genes predisposing to idiopathic Cambridge: Cambridge University Press. p Schwartz PJ, Towbin JA, Moss AJ, Atkinson generalized epilepsy. Hum Mol Genet 209±243. DL, Landes GM, Connors TD, Keating MT. 4:1201±1207. Tinel N, Lauritzen I, Chouabe C, Lazdunski M, 1996. Positional cloning of a novel potas- Zerr P, Adelman JP, Maylie J. 1998. Characteriza- Borsotto M. 1998. The KCNQ2 potassium sium channel gene: KVLQT1 mutations tion of three episodic ataxia mutations in the channel: splice variants, functional and cause cardiac arrhythmias. Nat Genet human Kv1.1 potassium channel. FEBS Lett developmental expression. Brain localiza- 12:17±23. 431:461±464. tion and comparison with KCNQ3. FEBS Wang HS, Pan Z, Shi W,Brown BS, Wymore RS, Zhuchenko O, Bailey J, Bonnen P, Ashizawa T, Lett 438:171±176. Cohen IS, Dixon JE, McKinnon D. 1998. Stockton DW, Amos C, Dobyns WB, Van Dyke DH, Griggs RC, Murphy MJ, Gold- KCNQ2 and KCNQ3 potassium channel Subramony SH, Zoghbi HY, Lee CC. stein MN. 1975. Hereditary myokymia and subunits: molecular correlates of the M- 1997. Autosomal dominant cerebellar ataxia periodic ataxia. J Neurol 25:109±118. channel. Science 282:1890±1893. (SCA6) associated with small polyglutamine Wallace RH, Wang DW, Singh R, Scheffer IE, Weiland S, Witzemann V, Villarroel A, Propping expansions in the a1A-voltage-dependent George AL Jr, Phillips HA, Saar K, Reis A, P, Steinlein O. 1996. An amino acid calcium channel. Nat Genet 15:62± Johnson EW, Sutherland GR, Berkovic SF, exchange in the second transmembrane 69. Mulley JC. 1998. Febrile seizures and segment of a neuronal nicotinic receptor Zuberi SM, Eunson LH, Spauschus A, De Silva generalized epilepsy associated with a muta- causes partial epilepsy by altering ist desen- R, Tolmie J, Wood NW, McWilliam RC, ‡ tion in the Na channel b1 subunit gene sitization kinetics. FEBS Lett 398:91±96. Stephenson JPB, Kullmann DM, Hanna SCN1B. Nat Genet 19:366±370. Wickenden AD, Yu W, Zou A, Jegla T, Wagoner MG. 1999. A novel mutation in the human Wallace RH, Scheffer IE, Barnett S, Richards M, PK. 2000. Retigabine, a novel anti-con- voltage-gated potassium channel gene Dibbens L, Desai RR, Lerman-Sagie T, Lev vulsant, enhances activation of KCNQ2/3 (Kv1.1) associates with episodic ataxia type D, Mazarib A, Brand N, Ben-Zeev B, potassium channels. Mol Pharmacol 1 and sometimes with partial epilepsy. Brain Goikhman I, Singh R, Kremmidiotis G, 58:591±600. 122:817±825.