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

A Case of Early Infantile Epileptic Encephalopathy Due to KCNQ2 Gene Mutation Presenting With Episodes of Hiccups

Neonatal epilepsy syndromes are responsible for only 15% of the cases of neonatal seizure. An underlying genetic disorder can be detected in approximately 42% of this subgroup. KCNQ2 gene-associated epilepsies are very rare and more common presentations are self-limited familial neonatal epilepsy (SLFNE) and early infantile epileptic encephalopathies (EIEE). The most common initial seizure semiologies are tonic seizures with or without autonomic symptoms in EIEE resulting from KCNQ2 gene mutation. It is characterized by early neonatal onset seizures with suppression burst pattern on electroencephalogram and typically results in severe developmental delay. Therapeutic options for infants with KCNQ2-related EIEE are limited and there is no consensus about it in the literature. Herein, the neonate with EIEE with unexpected episodes of hiccups due to novel mutation of the KCNQ2 gene, which was reported second time, was presented and antiepileptic treatment strategies were discussed in the light of current literature.

Common pediatric epilepsy syndromes

Benign rolandic epilepsy (BRE), childhood idiopathic occipital epilepsy (CIOE), childhood absence epilepsy (CAE), and juvenile myoclonic epilepsy (JME) are some of the common epilepsy syndromes in the pediatric age group. Among the four, BRE is the most commonly encountered. BRE remits by age 16 years with many children requiring no treatment. Seizures in CAE also remit at the rate of approximately 80%; whereas, JME is considered a lifelong condition even with the use of antiepileptic drugs (AEDs). Neonates and infants may also present with seizures that are self-limited with no associated psychomotor disturbances. Benign familial neonatal convulsions caused by a channelopathy, and inherited in an autosomal dominant manner, have a favorable outcome with spontaneous resolution. Benign idiopathic neonatal seizures, also referred to as "fifth-day fits," are an example of another epilepsy syndrome in infants that carries a good prognosis. BRE, CIOE, benign familial neonatal convulsions, benign idiopathic neonatal seizures, and benign myoclonic epilepsy in infancy are characterized as "benign" idiopathic age-related epilepsies as they have favorable implications, no structural brain abnormality, are sensitive to AEDs, have a high remission rate, and have no associated psychomotor disturbances. However, sometimes selected patients may have associated comorbidities such as cognitive and language delay for which the term "benign" may not be appropriate.

The new KCNQ2 activator 4-Chlor-N-(6-chlor-pyridin-3-yl)-benzamid displays anticonvulsant potential

BACKGROUND AND PURPOSE

KCNQ2-5 channels are voltage-gated potassium channels that regulate neuronal excitability and represent suitable targets for the treatment of hyperexcitability disorders. The effect of Chlor-N-(6-chlor-pyridin-3-yl)-benzamid was tested on KCNQ subtypes for its ability to alter neuronal excitability and for its anticonvulsant potential.

EXPERIMENTAL APPROACH

The effect of 4-Chlor-N-(6-chlor-pyridin-3-yl)-benzamid was evaluated using whole-cell voltage-clamp recordings from CHO cells and Xenopus laevis oocytes expressing different types of KCNQ channels. Epileptiform afterdischarges were recorded in fully amygdala-kindled rats in vivo. Neuronal excitability was assessed using field potential and whole cell recording in rat hippocampus in vitro.

KEY RESULTS

4-Chlor-N-(6-chlor-pyridin-3-yl)-benzamid caused a hyperpolarizing shift of the activation curve and a pronounced slowing of deactivation in KCNQ2-mediated currents, whereas KCNQ3/5 heteromers remained unaffected. The effect was also apparent in the Retigabine-insensitive mutant KCNQ2-W236L. In fully amygdala-kindled rats, it elevated the threshold for induction of afterdischarges and reduced seizure severity and duration. In hippocampal CA1 cells, 4-Chlor-N-(6-chlor-pyridin-3-yl)-benzamid strongly damped neuronal excitability caused by a membrane hyperpolarization and a decrease in membrane resistance and induced an increase of the somatic resonance frequency on the single cell level, whereas synaptic transmission was unaffected. On the network level, 4-Chlor-N-(6-chlor-pyridin-3-yl)-benzamid caused a significant reduction of γ and θ oscillation peak power, with no significant change in oscillation frequency.

CONCLUSION AND IMPLICATIONS

Our data indicate that 4-Chlor-N-(6-chlor-pyridin-3-yl)-benzamid is a potent KCNQ activator with a selectivity for KCNQ2 containing channels. It strongly reduces neuronal excitability and displays anticonvulsant activity in vivo.

In vivo loss of slow potassium channel activity in individuals with benign familial neonatal epilepsy in remission

Benign familial neonatal epilepsy is a neuronal channelopathy most commonly caused by mutations in KCNQ2, which encodes the K(v)7.2 subunit of the slow K(+) channel. K(v)7.2 is expressed in both central and peripheral nervous systems. Seizures occur in the neonatal period, often in clusters within the first few days of life, and usually remit by 12 months of age. The mechanism of involvement of K(v)7.2 mutations in the process of seizure generation has not been established in vivo. In peripheral axons, K(v)7.2 contributes to the nodal slow K(+) current. The present study aimed to determine whether axonal excitability studies could detect changes in peripheral nerve function related to dysfunction or loss of slow potassium channel activity. Nerve excitability studies were performed on eight adults with KCNQ2 mutations and a history of benign familial neonatal epilepsy, now in remission. Studies detected distinctive changes in peripheral nerve, indicating a reduction in slow K(+) current. Specifically, accommodation to long-lasting depolarizing currents was reduced in mutation carriers by 24% compared with normal controls, and the threshold undershoot after 100 ms depolarizing currents was reduced by 22%. Additional changes in excitability included a reduction in the relative refractory period, an increase in superexcitability and a tendency towards reduced sub-excitability. Modelling of the nerve excitability changes suggested that peripheral nerve hyperexcitability may have been ameliorated by upregulation of other potassium channels. We conclude that subclinical dysfunction of K(v)7.2 in peripheral axons can be reliably detected non-invasively in adulthood. Related alterations in neuronal excitability may contribute to epilepsy associated with KCNQ2 mutations.

Novel KCNQ2 mutation in a large Emirati family with benign familial neonatal seizures

Mutations in voltage-gated potassium channel Kv7.2 are responsible for benign familial neonatal seizures type 1, a rare monogenic autosomal dominant inherited epilepsy syndrome. We describe a novel mutation (c.1126_1127delA) in exon 9 of KCNQ2, the gene encoding for the Kv7.2 channel, in a large Emirati family with benign familial neonatal seizures type 1. The mutation leads to a frameshift at amino acid position 376, triggering loss of function and haploinsufficiency. Patients with this mutation manifest repeated clonic seizures with normal interictal electroencephalograms and favorable prognoses. Signs occur within the first days of age, lingering well into puberty. KCNQ2 mutation screening, alongside genetic counseling, should be included in diagnostic evaluations of neonatal epileptic patients, potentially sparing the need for unnecessary investigations and treatment. To our knowledge, this report is the first of a KCNQ2 mutation in an Emirati family with benign familial neonatal seizures type 1.

Transcriptional control of KCNQ channel genes and the regulation of neuronal excitability

Regulation of the resting membrane potential and the repolarization of neurons are important in regulating neuronal excitability. The potassium channel subunits Kv7.2 and Kv7.3 play a key role in stabilizing neuronal activity. Mutations in KCNQ2 and KCNQ3, the genes encoding Kv7.2 and Kv7.3, cause a neonatal form of epilepsy, and activators of these channels have been identified as novel antiepileptics and analgesics. Despite the observations that regulation of these subunits has profound effects on neuronal function, almost nothing is known about the mechanisms responsible for controlling appropriate expression levels. Here we identify two mechanisms responsible for regulating KCNQ2 and KCNQ3 mRNA levels. We show that the transcription factor Sp1 activates expression of both KCNQ2 and KCNQ3, whereas the transcriptional repressor REST (repressor element 1-silencing transcription factor) represses expression of both of these genes. Furthermore, we show that transcriptional regulation of KCNQ genes is mirrored by the correlated changes in M-current density and excitability of native sensory neurons. We propose that these mechanisms are important in the control of excitability of neurons and may have implications in seizure activity and pain.

Novel mutation in KCNQ2 causing benign familial neonatal seizures

Potassium channel subunits encoded by several genes of the KCNQ family underlie the M-current. Specifically, KCNQ2 and KCNQ3 play a major role at most neuronal sites. Mutations in KCNQ2 or KCNQ3 that reduce the M-current are responsible for benign familial neonatal seizures, a rare autosomal dominant idiopathic epilepsy of the newborn. The aim of this study was to investigate a single family with benign familial neonatal seizures for mutations in KCNQ genes and to analyze the association of mutation type with disease prognosis. A family in which members in several generations had signs and symptoms compatible with a diagnosis of benign familial neonatal seizures had DNA testing with single-stranded conformation polymorphism analysis for various mutations known to cause benign familial neonatal seizures. A novel KCNQ2 mutation c.63-66delGGTG (p.K21fsX40), causing a framework shift and early chain termination, was identified in the affected family members. In all cases, there was complete remission of the seizures after the neonatal period. This KCNQ2 mutation has implications for diagnosis and prognosis of familial neonatal seizures. Its presence suggests a benign disease with good prognosis and its identification can spare patients and physicians the need for extensive investigations or prolonged therapy.

Linkage study of voltage-gated potassium channels in familial mesial temporal lobe epilepsy

Voltage-gated potassium channels (VGKCs) play a critical role in the regulation of neuronal excitability and have been implicated in some types of epilepsies. Recently, autoimmune limbic encephalitis (LE) was associated with antibodies against VGKC. In addition, patients with LE showed partial epilepsy and increased T2 signal abnormalities in limbic structures. We have reported familial mesial temporal lobe epilepsy (FMTLE) associated with hippocampal atrophy (HA) and other signs of mesial temporal sclerosis detected by magnetic resonance imaging (MRI). In order to investigate whether VGKC may be associated to HA present in FMTLE, we perform linkage study in these candidate genes. Seventy-three microsatellites markers were genotyped in different human autosomal chromosome. Two-point LOD scores did not show evidence for linkage with any of the microsatellite markers genotyped (Zmax ranging from 0.11to-9.53 at theta=0.00). In the present study, linkage data showed no evidence that VGKC are involved in the determination of HA in FMTLE.

Mutational scanning of potassium, sodium and chloride ion channels in malignant migrating partial seizures in infancy

The mutational analysis of potassium (KCNQ2, KCNQ3), sodium (SCN1A, SCN2A), and chloride (CLCN2) ion channels was performed in three children with typical features of the recently described syndrome of migrating partial seizures in infancy. Mutational analysis was performed by PCR and automatic sequencing. The coding regions, including the exon-intron boundaries, were amplified in the patients using appropriate primers sets. No mutations associated to migrating partial seizures have been found. Mutational screening of CLCN2 gene, revealed a homozygous mutation G2003C (exon 17), leading to a Ser/Thr substitution at the codon 668, in two of the three patients. The same variation has been found in 38 out of 100 control alleles. The identification of the genetic basis of this new epileptic encephalopathy requires further studies that might be enforced by familial cases.

Severe epilepsy resulting from genetic interaction between Scn2a and Kcnq2

A mutation in the voltage-gated sodium-channel Scn2a results in moderate epilepsy in transgenic Scn2a(Q54) mice maintained on a C57BL/6J strain background. The onset of progressive epilepsy begins in adults with short-duration partial seizures that originate in the hippocampus. The underlying abnormality is an increase in persistent sodium current in hippocampal neurons. The voltage-gated potassium channel Kcnq2 is responsible for generating M current (I(KM)) that is thought to control excitability and limit repetitive firing of hippocampal neurons. To determine whether impaired M current would exacerbate the seizure phenotype of Scn2a(Q54) mice, we carried out genetic crosses with two mutant alleles of Kcnq2. Szt1 mice carry a spontaneous deletion that removes the C-terminal domain of Kcnq2. A novel Kcnq2 missense mutation V182M was identified by screening the offspring of ENU-treated males for reduced threshold to electrically evoked minimal clonic seizures. Double mutant mice carrying the Scn2a(Q54) transgene together with either of the Kcnq2 mutations exhibited severe epilepsy with early onset, generalized tonic-clonic seizures and juvenile lethality by 3 weeks of age. This dramatic exacerbation of the sodium-channel mutant phenotype indicates that M current plays a critical role in preventing seizure initiation and spreading in this animal model. The genetic interaction between Scn2a and Kcnq2 demonstrates that combinations of mild alleles of monogenic epilepsy genes can result in severe disease and provides a model for complex inheritance of human epilepsy. The data suggest that interaction between these genes might contribute to the variable expressivity observed in human families with sodium-channel mutations. In a screen of 23 SMEI patients with missense mutations of SCN1A, no second-site mutations in KCNQ2 were identified.

Changes in KCNQ2 immunoreactivity in the amygdala in two rat models of temporal lobe epilepsy

Potassium channels containing the KCNQ2 subunit play an important role in the regulation of neuronal excitability and therefore have been implicated in epilepsy. This study describes the expression of KCNQ2 subunit immunoreactivity in the basolateral amygdala in two rat models of temporal lobe epilepsy, (1) amygdala kindling and (2) spontaneously epileptic rats after status epilepticus induced by hippocampal electrical stimulation. KCNQ2 subunit immunoreactivity was assessed with a commercial antibody raised against a C-terminal part of the KCNQ2 protein. We show that KCNQ2 subunit immunoreactivity is upregulated in the basolateral amygdala in both models and that generalized seizures are required to induce this upregulation. We hypothesize that the upregulation of potassium channels containing the KCNQ2 subunit might represent a mechanism to counteract seizures in experimental temporal lobe epilepsy.

The CHRNB2 mutation I312M is associated with epilepsy and distinct memory deficits

Mutations in nAChRs are found in a rare form of nocturnal frontal lobe epilepsy (ADNFLE). Previously, some nAChR mutations have been described that are associated with additional neurological features such as psychiatric disorders or cognitive defects. Here, we report a new CHRNB2 mutation located in transmembrane region 3 (M3), outside the known ADNFLE mutation cluster. The CHRNB2 mutation I312M, which occurred de novo in twins, markedly increases the receptor's sensitivity to acetylcholine. Phenotypically, the mutation is associated not only with typical ADNFLE, but also with distinct deficits in memory. The cognitive problems are most obvious in tasks requiring the organization and storage of verbal information.

A novel mutation in KCNQ2 gene causes benign familial neonatal convulsions in a Chinese family

Benign familial neonatal convulsions (BFNC) are a rare autosomal dominant inherited epilepsy syndrome. Two voltage-gated potassium channel genes, KCNQ2 on chromosome 20q13.3 and KCNQ3 on chromosome 8q24, have been identified as the genes responsible for benign familial neonatal convulsions. By linkage analysis and mutation analysis of KCNQ2 gene, we found a novel frameshift mutation of KCNQ2 gene, 1931delG, in a large Chinese family with benign familial neonatal convulsions. This mutation is located in the C-terminus of KCNQ2, in codon 644 predicting the replacement of the last 201 amino acids with a stretch of 257 amino acids showing a completely different sequence. An unusual clinical feature of this family is that the seizures of every patient did not remit until 12 to 18 months. This is the first report of KCNQ2 gene mutation in China.

The genetics of febrile seizures and related epilepsy syndromes

Febrile seizures (FS) may represent the most common seizure disorder in childhood and are known to be associated with putative genetic predispositions. Nevertheless, molecular genetic approaches toward understanding FS have been just initiated this decade. Recently, several genetic loci for FS have been mapped thereby assuring the genetic heterogeneity of FS. However, the exact molecular mechanisms of FS are yet to be elucidated. Genetic defects have been recently identified in autosomal dominant epilepsy with FS plus or generalized epilepsy with FS plus. The underlying mutations were found in genes encoding several Na+ channel subunits and the gamma2 subunit of gamma amino-butyric acid (GABA)A receptors in the brain. Furthermore, both channels are also associated with severe myoclonic epilepsy in infancy, where the seizure attacks often begin with prolonged FS and are precipitated by fever even afterwards. Na+ channels are associated with other temperature-sensitive disorders, and GABA(A) receptors are known to play an important role in the pathogenesis of FS. These lines of evidence suggest the involvement of various Na+ channels, GABA(A) receptors and additional auxiliary proteins in the pathogenesis of frequent FS and even in simple FS. This hypothesis may facilitate our understanding of the genetic background of FS.

Suggestive evidence for association of two potassium channel genes with different idiopathic generalised epilepsy syndromes

Several potassium channel genes have been implicated in epilepsy. We have investigated three such genes, KCNJ3, KCNJ6 and KCNQ2, by association studies using a broad sample of idiopathic generalised epilepsy (IGE) unselected by syndrome. One of the two single nucleotide polymorphisms (SNPs) examined in one of the inward rectifying potassium channel genes, KCNJ3, was associated with IGE by genotype (P=0.0097), while its association by allele was of borderline significance (P=0.051). Analysis of the different clinical subgroups within the IGE sample showed more significant association with the presence of absence seizures (P=0.0041) and which is still significant after correction for multiple testing. Neither SNP in the other rectifying potassium channel gene, KCNJ6, was associated with IGE or any subgroup. None of the three SNPs in the voltage-gated potassium channel gene, KCNQ2, was associated with IGE. However, one SNP was associated with epilepsy with generalised tonic clonic seizures only (P=0.016), as was an SNP approximately 56 kb distant in the closely linked nicotinic acetylcholine gene CHRNA4 (P=0.014). These two SNPs were not in linkage disequilibrium with each other, suggesting that if they are not true associations they have independently occurred by chance. Neither association remains significant after correcting for multiple testing.

Altered kinetics and benzodiazepine sensitivity of a GABAA receptor subunit mutation [gamma 2(R43Q)] found in human epilepsy

The gamma-aminobutyric acid type A (GABA(A)) receptor mediates fast inhibitory synaptic transmission in the CNS. Dysfunction of the GABA(A) receptor would be expected to cause neuronal hyperexcitability, a phenomenon linked with epileptogenesis. We have investigated the functional consequences of an arginine-to-glutamine mutation at position 43 within the GABA(A) gamma(2)-subunit found in a family with childhood absence epilepsy and febrile seizures. Rapid-application experiments performed on receptors expressed in HEK-293 cells demonstrated that the mutation slows GABA(A) receptor deactivation and increases the rate of desensitization, resulting in an accumulation of desensitized receptors during repeated, short applications. In Xenopus laevis oocytes, two-electrode voltage-clamp analysis of steady-state currents obtained from alpha(1)beta(2)gamma(2) or alpha(1)beta(2)gamma(2)(R43Q) receptors did not reveal any differences in GABA sensitivity. However, differences in the benzodiazepine pharmacology of mutant receptors were apparent. Mutant receptors expressed in oocytes displayed reduced sensitivity to diazepam and flunitrazepam but not the imidazopyridine zolpidem. These results provide evidence of impaired GABA(A) receptor function that could decrease the efficacy of transmission at inhibitory synapses, possibly generating a hyperexcitable neuronal state in thalamocortical networks of epileptic patients possessing the mutant subunit.

The interface of preclinical evaluation with clinical testing of antiepileptic drugs: role of pharmacogenomics and pharmacogenetics

Despite the release of eight antiepileptic drugs (AEDs) during the last decade, the incidence of pharmacoresistant epilepsy has changed relatively little. Predicting efficacy and safety of AEDs in people with epilepsy from acute seizure models in rodents is difficult and risky. It is becoming increasingly clear that genetic polymorphisms play an integral role in variability in both antiepileptic drug pharmacokinetics and pharmacodynamics. The publication of the human genome and increasing sophisticated and powerful genetic tools offers new methods for screening drugs and predicting deadly idiosyncratic side effects. In this review the use of pharmacogenomic and pharmacokinetic techniques in the development and monitoring of antiepileptic drug therapy is reviewed. Genetic techniques have the potential of identifying novel drug targets, predicting drug response, and identifying individuals at risk for serious idosyncratic reactions.

Genetic abnormalities underlying familial epilepsy syndromes

Genetic defects have been recently identified in certain inherited epilepsy syndromes in which the phenotypes are similar to common idiopathic epilepsies. Mutations in the neuronal nicotinic acetylcholine receptor 4 and 2 subunit genes have been detected in families with autosomal dominant nocturnal frontal lobe epilepsy. Both receptors are components of neuronal acetylcholine receptor, a ligand-gated ion channel in the brain. Furthermore, mutations of two K+-channel genes were also identified as the underlying genetic abnormalities of benign familial neonatal convulsions. Mutations in the voltage-gated Na+-channel 1, 2 and 1 and the gamma aminobutyric acid (GABAA) receptor 2 subunit genes were found as a cause of generalized epilepsy with febrile seizures plus, a clinical subset of febrile convulsions. Na+-channels, GABAA receptor and their auxiliaries may be involved in the pathogenesis of this subtype and even in simple febrile convulsions. Mutation of a voltage-gated K+-channel gene can cause partial seizures associated with periodic ataxia type 1 and some forms of juvenile myoclonic epilepsy and idiopathic generalized epilepsy can result from mutations of a Ca2+-channel. This line of evidence suggests the involvement of channels expressed in the brain in the pathogenesis of certain types of epilepsy. Our working hypothesis is to view certain idiopathic epilepsies as disorders of ion channels, i.e. 'channelopathies'. Such hypothesis should provide a new insight to our understanding of the genetic background of epilepsy.

Channelopathies can cause epilepsy in man

Idiopathic epilepsies, which account for up to 40% of all epilepsies, are mainly caused by genetic factors. Most idiopathic epilepsies are due to oligogenic or multifactorial rather than monogenetic inheritance. Nevertheless, most of what is known today about the molecular genetics of idiopathic epilepsies has been found by analysing large families with rare monogenetic forms of the disease. For the first time, gene defects can be linked to certain epilepsies. Mutations in the CHRNA4 or CHRNB subunits of the neuronal nicotinic acetylcholine receptor lead to familial nocturnal frontal lobe epilepsy, while defects in the voltage-gated potassium channels KCNQ2 and KCNQ3 have recently been found to cause benign familial neonatal convulsions. The voltage-gated sodium channel subunits SCN1B, SCN1A and SCN2A as well as the GABRG2 subunit of the GABA(A) receptor are involved in the pathology of the newly described syndrome generalized epilepsy with febrile seizures plus. These rare monogenetic epilepsies can serve as models for further genetic analysis of the common forms of idiopathic epilepsies.

Myokymia and neonatal epilepsy caused by a mutation in the voltage sensor of the KCNQ2 K+ channel

KCNQ2 and KCNQ3 are two homologous K(+) channel subunits that can combine to form heterotetrameric channels with properties of neuronal M channels. Loss-of-function mutations in either subunit can lead to benign familial neonatal convulsions (BFNC), a generalized, idiopathic epilepsy of the newborn. We now describe a syndrome in which BFNC is followed later in life by myokymia, involuntary contractions of skeletal muscles. All affected members of the myokymia/BFNC family carried a mutation (R207W) that neutralized a charged amino acid in the S4 voltage-sensor segment of KCNQ2. This substitution led to a shift of voltage-dependent activation of KCNQ2 and a dramatic slowing of activation upon depolarization. Myokymia is thought to result from hyperexcitability of the lower motoneuron, and indeed both KCNQ2 and KCNQ3 mRNAs were detected in the anterior horn of the spinal cord where the cells of the lower motoneurons arise. We propose that a difference in firing patterns between motoneurons and central neurons, combined with the drastically slowed voltage activation of the R207W mutant, explains why this particular KCNQ2 mutant causes myokymia in addition to BFNC.

Differential expression of kcnq2 splice variants: implications to m current function during neuronal development

The KCNQ family of K(+) channels has been implicated in several cardiac and neurological disease pathologies. KCNQ2 (Q2) is a brain-derived gene, which in association with KCNQ3 (Q3) has been shown to provide a molecular basis for the neuronal M current. We have cloned a long (Q2L) and a short (Q2S) splice variant of the human KCNQ2 gene; these variants differ in their C-terminal tail. Northern blot analysis reveals that Q2L is preferentially expressed in differentiated neurons, whereas the Q2S transcript is prominent in fetal brain, undifferentiated neuroblastoma cells, and brain tumors. Q2L, transfected into mammalian cells, produces a slowly activating, noninactivating voltage-gated K(+) current that is blocked potently by tetraethylammonium (TEA; IC(50), 0.14 mm). Q2S on the other hand produces no measurable potassium currents. Cotransfection of Q2S with either Q2L, Q3, or Q2L/Q3 heteromultimers results in attenuation of K(+) current, the suppression being most profound for Q3. Inclusion of Q2S in the heteromultimer also positively shifts the voltage dependence of current activation and alters affinity for the TEA block, suggesting that under these conditions, some Q2S subunits incorporate into functional channels on the plasma membrane. In view of the crucial role of M currents in modulating neuronal excitability, our findings provide important insight into the functional consequences of differential expression of KCNQ2 splice variants: dampened potassium conductances in the developing brain could shape firing repertoires to provide cues for proliferation rather than differentiation.

Alternative splicing of KCNQ2 potassium channel transcripts contributes to the functional diversity of M-currents

The region of alternative splicing in the KCNQ2 potassium channel gene was determined by RNase protection analysis of KCNQ2 mRNA transcripts. Systematic analysis of KCNQ2 alternative splice variant expression in rat superior cervical ganglia revealed multiple variant isoforms. One class of KCNQ2 splice variants, those that contained exon 15a, was found to have significantly different kinetics to those of the other isoforms. These transcripts encoded channel subunits that, when co-expressed with the KCNQ3 subunit, activated and deactivated approximately 2.5 times more slowly than other isoforms. Deletion of exon 15a in these isoforms produced a reversion to the faster kinetics. Comparison of the kinetic properties of the cloned channel splice variants with those of the native M-current suggests that alternative splicing of the KCNQ2 gene may contribute to the variation in M-current kinetics seen in vivo.

Are some idiopathic epilepsies disorders of ion channels?: A working hypothesis

Epilepsy is a common neurological disease and encompasses a variety of disorders with paroxysms. Although there is a genetic component in the pathogenesis of epilepsy, the molecular mechanisms of this syndrome remains poorly understood. Linkage analysis and positional cloning have not been sufficient tools for determining the pathogenic mechanisms of common idiopathic epilepsies, and hence, novel approaches, based on the etiology of epilepsy, are necessary. Recently, many paroxysmal disorders, including, epilepsy, have been considered to be due to ion channel abnormalities or channelopathies. Results of recent studies employing gene analysis in animal models of epilepsy and human familial epilepsies support the hypothesis that at least some of the so called idiopathic epilepsies, i.e. epilepsies currently, classified as idiopathic could be considered as a channelopathy. This hypothesis is consistent with the putative prerequisites for genes responsible for the majority of idiopathic epilepsies that can adequately explain the following characteristics of epilepsy. Neuronal hyperexcitability, dominant inheritance with various penetrance, pharmacological role of some conventional antiepileptic drugs, age dependency in the onset of epilepsy, and the involvement of genetic factors in the pathogenesis of post-traumatic epilepsy. Search for mutations in ion channels expressed in the central nervous system may help in finding defects underlying some of idiopathic epilepsies, thereby enhancing, our understanding of the molecular pathogenesis of epilepsy. A working hypothesis to view certain idiopathic epilepsies as disorders of ion channels should provide a new insight to our understanding of epilepsy and allow the design of novel therapies.

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

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