Benign familial neonatal convulsions: evidence for clinical and genetic heterogeneity

Familial neonatal seizures in 36 families: Clinical and genetic features correlate with outcome

OBJECTIVE

We evaluated seizure outcome in a large cohort of familial neonatal seizures (FNS), and examined phenotypic overlap with different molecular lesions.

METHODS

Detailed clinical data were collected from 36 families comprising two or more individuals with neonatal seizures. The seizure course and occurrence of seizures later in life were analyzed. Families were screened for KCNQ2, KCNQ3, SCN2A, and PRRT2 mutations, and linkage studies were performed in mutation-negative families to exclude known loci.

RESULTS

Thirty-three families fulfilled clinical criteria for benign familial neonatal epilepsy (BFNE); 27 of these families had KCNQ2 mutations, one had a KCNQ3 mutation, and two had SCN2A mutations. Seizures persisting after age 6 months were reported in 31% of individuals with KCNQ2 mutations; later seizures were associated with frequent neonatal seizures. Linkage mapping in two mutation-negative BFNE families excluded linkage to KCNQ2, KCNQ3, and SCN2A, but linkage to KCNQ2 could not be excluded in the third mutation-negative BFNE family. The three remaining families did not fulfill criteria of BFNE due to developmental delay or intellectual disability; a molecular lesion was identified in two; the other family remains unsolved.

SIGNIFICANCE

Most families in our cohort of familial neonatal seizures fulfill criteria for BFNE; the molecular cause was identified in 91%. Most had KCNQ2 mutations, but two families had SCN2A mutations, which are normally associated with a mixed picture of neonatal and infantile onset seizures. Seizures later in life are more common in BFNE than previously reported and are associated with a greater number of seizures in the neonatal period. Linkage studies in two families excluded known loci, suggesting a further gene is involved in BFNE.

The molecular biology of genetic-based epilepsies

Epilepsy is one of the most common neurological disorders characterized by abnormal electrical activity in the central nervous system. The clinical features of this disorder are recurrent seizures, difference in age onset, type, and frequency, leading to motor, sensory, cognitive, psychic, or autonomic disturbances. Since the discovery of the first monogenic gene mutation in 1995, it is proposed that genetic factor plays an important role in the mechanism of epilepsy. Genes discovered in idiopathic epilepsies encode for ion channel or neurotransmitter receptor proteins, whereas syndromes with epilepsy as a main feature are caused by genes that are involved in functions such as cortical development, mitochondrial function, and cell metabolism. The identification of these monogenic epilepsy-causing genes provides new insight into the pathogenesis of epilepsies. Although most of the identified gene mutations present a monogenic inheritance, most of idiopathic epilepsies are complex genetic diseases exhibiting a polygenic or oligogenic inheritance. This article reviews recent genetic and molecular progresses in exploring the pathogenesis of epilepsy, with special emphasis on monogenic epilepsy-causing genes, including voltage-gated channels (Na(+), K(+), Ca(2+), Cl(-), and HCN), ligand-gated channels (nicotinic acetylcholine and GABAA receptors), non-ion channel genes as well as the mitochondrial DNA genes. These progresses have improved our understanding of the complex neurological disorder.

Approach to neonatal seizures

Neonatal seizures can be difficult to recognize given the variation in their presentation. Hence, diagnosis and appropriate treatment may be delayed. Morphology of seizures in this age group is discussed, followed by common etiological entities. Special emphasis is laid on treatable disease states such as vitamin responsive seizure disorders and benign conditions that may not warrant aggressive treatment. Conditions that may have devastating neurological consequences are discussed in some detail so that the treating pediatrician can provide realistic information to the parents of such newborns. Imaging and laboratory workup is outlined, followed by guidelines for ordering and interpreting an electroencephalogram in this age group. Finally, treatment options in the form of antiepileptics are discussed with mention of new avenues for diagnosis and treatment that may become commonly employed in the future.

Genetics of epilepsy: an overview

Studies of the genetics of epilepsy have, until recently, involved epidemiologic or segregation analyses of phenotypic characteristics of a number of seizure disorders. Technical advances in molecular biology involving gene mapping and gene identification have made it possible to examine the heritability of various epilepsy syndromes. Using "reverse genetics" or positional cloning, it is possible to identify an abnormal protein through gene isolation and cloning. Genes are localized through analysis of linkage to phenotypic markers (proteins) or DNA markers such as restriction fragment length polymorphisms, variable number of tandem repeats, and dinucleotides. Methods used to obtain DNA of interest involve digestion of genomic DNA with specific restriction endonucleases or amplification of DNA by polymerase chain reaction technology. Gel electrophoresis is the basis for the separation of different sized DNA. Inherited disorders for which a gene has been cloned or localized have highly penetrant, well-defined clinical phenotypes with no remissions and abundant clinical material. Genetic epilepsies, however, are variably penetrant age-dependent disorders with heterogeneous clinical phenotypes. Despite these difficulties, three genetic epilepsies have been mapped to specific chromosomes: benign familial neonatal convulsions to 20q, juvenile myoclonic epilepsy to 6p, and Baltic progressive myoclonus epilepsy to 21q. Further progress in understanding genetic epilepsies will depend on better definition of syndrome phenotypes, isolation of the epilepsy gene(s), and identification of the abnormal protein(s).

The first Korean case of KCNQ2 mutation in a family with benign familial neonatal convulsions

Neonatal seizures represent a heterogeneous group of disorders with vastly different etiologies and outcomes. Benign familial neonatal convulsions (BFNC) are a distinctive epileptic syndrome of autosomal dominant inheritance with a favorable prognosis, characterized by the occurrence of unprovoked partial or generalized clonic seizures in the neonatal period or early infancy. Recently, mutations in two potassium channel genes, KCNQ2 and KCNQ3, have been described in this disorder. In this report, we describe a family with BFNC due to a KCNQ2 mutation, the first such family to be described in the Korean population. The diagnosis of BFNC can be made based on clinical suspicion and careful history taking with special emphasis on the familial nature of the disorder. KCNQ2 mutations may be associated with BFNC in a number of different races, as has been reported in other ethnic groups.

Mouse models of human KCNQ2 and KCNQ3 mutations for benign familial neonatal convulsions show seizures and neuronal plasticity without synaptic reorganization

The childhood epilepsy syndrome of benign familial neonatal convulsions (BFNC) exhibits the remarkable feature of clinical remission within a few weeks of onset and a favourable prognosis, sparing cognitive abilities despite persistent expression of the mutant KCNQ2 or KCNQ3 potassium channels throughout adulthood. To better understand such dynamic neuroprotective plasticity within the developing brain, we introduced missense mutations that underlie human BFNC into the orthologous murine Kcnq2 (Kv7.2) and Kcnq3 (Kv7.3) genes. Mutant mice were examined for altered thresholds to induced seizures, spontaneous seizure characteristics, hippocampal histology, and M-current properties of CA1 hippocampal pyramidal neurons. Adult Kcnq2(A306T/+) and Kcnq3(G311V/+) heterozygous knock-in mice exhibited reduced thresholds to electrically induced seizures compared to wild-type littermate mice. Both Kcnq2(A306T/A306T) and Kcnq3(G311V/G311V) homozygous mutant mice exhibited early onset spontaneous generalized tonic-clonic seizures concurrent with a significant reduction in amplitude and increased deactivation kinetics of the neuronal M-current. Mice had recurrent seizures into adulthood that triggered molecular plasticity including ectopic neuropeptide Y (NPY) expression in granule cells, but without hippocampal mossy fibre sprouting or neuronal loss. These novel knocking mice recapitulate proconvulsant features of the human disorder yet show that inherited M-current defects spare granule cells from reactive changes in adult hippocampal networks. The absence of seizure-induced pathology found in these epileptic mouse models parallels the benign neurodevelopmental cognitive profile exhibited by the majority of BFNC patients.

Germ-line mutation of KCNQ2, p.R213W, in a Japanese family with benign familial neonatal convulsion

BACKGROUND

Benign familial neonatal convulsion (BFNC) is an autosomal-dominantly inherited epilepsy of neonates. The KCNQ2 and KCNQ3 genes have been cloned as the responsible genes for BFNC. Detection of mutations in these genes is helpful for confirmation of BFNC or differential diagnosis of convulsive disorders in the neonatal period.

METHODS

A Japanese family with BFNC was investigated. Two siblings were clinically diagnosed as having BFNC. KCNQ2 and KCNQ3 were screened for mutations using a combination of polymerase chain reaction and denaturing high-performance liquid chromatography. Nucleotide substitutions were confirmed by direct sequencing.

RESULTS

In the affected siblings a C-to-T heterozygous substitution was detected at nucleotide 683 (c.683C>T) in KCNQ2, leading to substitution of arginine with tryptophan at amino acid position 213 (p.R213W) in the S4 voltage-sensing domain of the KCNQ2 protein. The detected mutation may disrupt this highly conserved region among potassium channel proteins. The c.683C>T substitution in KCNQ2 was not present in the parents. KCNQ3 was also analyzed and a single nucleotide polymorphism, c.1241A>G (National Center for Biotechnology Information (NCBI), SNP ID: rs2303995), was detected in the index family.

CONCLUSIONS

Two siblings with BFNC had a novel heterozygous missense mutation, p.R213W, in KCNQ2. This mutation may affect potassium gating, leading to neuronal excitability or convulsions in the patients. Furthermore, neither of the parents had the p.R213W mutation, indicating that it was a germ-line mutation. The possibility of recurrence of such a germ-line mutation in the next siblings should be explained during genetic counseling.

A novel mutation of KCNQ3 gene in a Chinese family with benign familial neonatal convulsions

Benign familial neonatal convulsions (BFNC, also named benign familial neonatal seizures, BFNS) is a rare autosomal dominant inherited epilepsy syndrome with clinical and genetic heterogeneity. Two voltage-gated potassium channel subunit genes, KCNQ2 and KCNQ3, have been identified to cause BFNC1 and BFNC2, respectively. To date, only three mutations of KCNQ3, all located within exon 5, have been reported. By limited linkage analysis and mutation analysis of KCNQ3 in a Chinese family with BFNC, we identified a novel missense mutation of KCNQ3, c.988C>T located within exon 6. c.988C>T led to the substitution Cys for Arg in amino acid position 330 (p.R330C) in KCNQ3 potassium channel, which possibly impaired the neuronal M-current and altered neuronal excitability. Seizures of all BFNC patients started from day 2 to 3 after birth and remitted during 1 month, and no recurrence was found. One family member who displayed fever-associated seizures for two times at age 5 years and was diagnosed as febrile seizures, however, did not carry this mutation, which suggests that febrile seizures and BFNC have different pathogenesis. To our knowledge, this is the first report of KCNQ3 mutation in Chinese family with BFNC.

The spectrum of benign infantile seizures

Benign epilepsies during infancy are a wide topic, which needs both clinical and nosological clarifications. Already in 1963 Fukuyama reported patients with seizures during infancy with a benign outcome. In the late 80s and early 90s, Watanabe reported series of infants with complex partial seizures or partial seizures with secondary generalization, with a normal development before onset and a benign outcome. In the same years Vigevano focused on familial cases: he described several families with seizures with onset around the 6-month of age, and autosomal dominant mode of inheritance. To define this condition, he coined the term "benign familial infantile seizures" (BFIS). Afterwards, studying families with this phenotype, loci on chromosomes 19, 16 and 2 responsible for BFIS were detected. Similar loci were found in families affected by BFIS and subsequent choreoathetosis, and BFIS associated with familial hemiplegic migraine. In most recent years a new form of benign epilepsy has been proposed, with an intermediate onset between the neonatal and infantile age, which was defined with the term benign familial neonatal-infantile seizures (BFNIS). This condition could have some clinical and genetic features overlapping with BFIS. Seizures with a benign outcome have been reported also in infants during episode of mild gastroenteritis (BIS with MG) frequently with positive Rotavirus antigen. Lastly, sleep EEG abnormalities have been reported in children with a peculiar form of epilepsy by Capovilla, who defined this condition as benign infantile focal epilepsy with midline spikes and waves during sleep (BIMSE). Some of these entities have been included in the last classification proposed by the ILAE and have been differentiated in familial and non-familial forms. The aim of this review is to describe these entities, discuss their nosological aspects, pointing out the similarities and differences with benign neonatal seizures and benign focal epilepsies appearing later in life such as early-onset benign occipital seizure susceptibility syndrome (EBOSS), or benign epilepsy of childhood with centro-temporal spikes (BECTS).

A novel splicing mutation in KCNQ2 in a multigenerational family with BFNC followed for 25 years

PURPOSE

A large multigenerational family with benign familial neonatal convulsions (BFNC) was revisited to identify the disease-causing mutation and to assess long-term outcome.

METHODS

We supplemented the original data with recent clinical and neurophysiologic data on patients and first-degree relatives, including information on seizure recurrence. We conducted linkage analysis at the EBN1 and EBN2 loci, followed by mutation analysis of KCNQ2. We evaluated the qualitative effect of the KCNQ2 mutation at the messenger RNA (mRNA) level by using reverse-transcribed total RNA isolated from leukocytes.

RESULTS

Thirteen relatives had a history of neonatal convulsions, 11 of whom showed remission within 2 months. One patient showed an atypical course of neonatal convulsions, developing photosensitive myoclonic epilepsy at age 13 years. We found suggestive linkage of the BFNC phenotype to the 20q13-EBN1 locus (lod score, 2.03) and an intronic mutation IVS14-6 C>A in KCNQ2 segregating with the trait in all affected members, but absent in 100 unrelated control subjects. This mutation creates a new, preferentially used, splice site. Alternative splicing adds 4 nt containing a premature stop codon to the transcript, resulting in a truncated protein after position R588.

CONCLUSIONS

We detected and characterized a novel splicing mutation in the brain-specific KCNQ2 gene by using easily accessible blood leukocytes. Aberrant splicing cosegregates with BFNC but not with photosensitivity.

Genetics of Idiopathic Generalized Epilepsies

The idiopathic generalized epilepsies (IGEs) are considered to be primarily genetic in origin. They encompass a number of rare mendelian or monogenic epilepsies and more common forms which are familial but manifest as complex, non-mendelian traits. Recent advances have demonstrated that many monogenic IGEs are ion channelopathies. These include benign familial neonatal convulsions due to mutations in KCNQ2 or KCNQ3, generalized epilepsy with febrile seizures plus due to mutations in SCN1A, SCN2A, SCN1B, and GABRG2, autosomal-dominant juvenile myoclonic epilepsy (JME) due to a mutation in GABRA1 and mutations in CLCN2 associated with several IGE sub-types. There has also been progress in understanding the non-mendelian IGEs. A haplotype in the Malic Enzyme 2 gene, ME2, increases the risk for IGE in the homozygous state. Five missense mutations have been identified in EFHC1 in 6 of 44 families with JME. Rare sequence variants have been identified in CACNA1H in sporadic patients with childhood absence epilepsy in the Chinese Han population. These advances should lead to new approaches to diagnosis and treatment.

Benign familial infantile seizures

In recent years, numerous publications have reported localization-related epilepsy with onset during early infancy, idiopathic etiology and favourable outcome. In 1963, Fukuyama reported cases occurring in the first 2 years of life characterized by partial seizures, absence of etiologic factors and benign outcome. Watanabe studied the localization and semiology of seizures. Later Vigevano and coworkers directed attention to the presence of cases with a family history of convulsions with benign outcome during infancy, with autosomal dominant inheritance, suggesting the term 'benign infantile familial convulsions' (BIFC). Similar cases have been described by several authors confirming that this is a new syndrome. In the last ILAE proposal of Classification of Epilepsy Syndromes this entity is called benign familial infantile seizures. Benign infantile seizures are divided now into familial and non-familial forms, although the two forms can overlap. Genetic studies led to the identification of a marker on chromosome 19. This was not confirmed by later studies, and genetic heterogeneity was hypothesized. Recently Malacarne studying eight Italian families with BIFC mapped a novel locus on chromosome 2. In 1997, Szepetowski described the association between BIFC and a later occurrence of paroxysmal choreoathetosis. Following the identification of a specific marker on chromosome 16, this entity constitutes a variant of the familial forms, called infantile convulsions and choreoathetosis. The age at onset, the semeiology of the seizures and the genetic data distinguish the benign familial infantile seizures from the benign familial neonatal seizures. Recent data suggested that this type of epilepsy would be due to a channellopathy.

Conditional transgenic suppression of M channels in mouse brain reveals functions in neuronal excitability, resonance and behavior

In humans, mutations in the KCNQ2 or KCNQ3 potassium-channel genes are associated with an inherited epilepsy syndrome. We have studied the contribution of KCNQ/M-channels to the control of neuronal excitability by using transgenic mice that conditionally express dominant-negative KCNQ2 subunits in brain. We show that suppression of the neuronal M current in mice is associated with spontaneous seizures, behavioral hyperactivity and morphological changes in the hippocampus. Restriction of transgene expression to defined developmental periods revealed that M-channel activity is critical to the development of normal hippocampal morphology during the first postnatal weeks. Suppression of the M current after this critical period resulted in mice with signs of increased neuronal excitability and deficits in hippocampus-dependent spatial memory. M-current-deficient hippocampal CA1 pyramidal neurons showed increased excitability, reduced spike-frequency adaptation, attenuated medium afterhyperpolarization and reduced intrinsic subthreshold theta resonance. M channels are thus critical determinants of cellular and neuronal network excitability, postnatal brain development and cognitive performance.

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.

Molecular basis of Mendelian idiopathic epilepsies

A genetic aetiology is estimated to be present in about 40% of patients with epilepsy. Significant progress has been made in understanding the molecular genetic basis of Mendelian epilepsies. Fourteen genes have been identified which underlie a group of rare, autosomal dominant Mendelian idiopathic epilepsies. All but two of these genes encode subunits of ion-channels, revealing that idiopathic Mendelian human epilepsies are predominantly channelopathies. The two non-ion-channel genes, LGl1 causing autosomal dominant lateral temporal lobe epilepsy and MASS1 causing febrile and afebrile seizures, both contain a novel repeat motif variously called the epilepsy-associated repeat (EAR) and epitempin (EPTP) repeat. This motif defines a subfamily of genes, some of which have also been implicated in epilepsy in mice and humans. Progress in dissecting the more common 'complex' genetic epilepsies remains slow, but ion channels represent the most biologically plausible candidates. Characterization of common population sequence variants for the entire cohort of ion channel genes and the development of high-throughput techniques should enable rapid advances in the understanding of the common idiopathic familial epilepsies.

Molecular genetics of human familial epilepsy syndromes

Genetic defects have been recently identified in certain inherited epilepsy syndromes in which the phenotypes are similar to those of common idiopathic epilepsies. Mutations in the neuronal nicotinic acetylcholine receptor alpha4 and beta2 subunit genes have been detected in families with autosomal dominant nocturnal frontal lobe epilepsy. Both receptors are components of neuronal acetylcholine receptor, a ligand-gated ion channel in the brain. Furthermore, mutations of two K+ channel genes also were identified as the underlying genetic abnormalities of benign familial neonatal convulsions. Mutations in the voltage-gated Na+-channel alpha1 and beta1 subunit genes were found as the cause of generalized epilepsy with febrile seizures plus, a clinical subset of 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 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 a hypothesis should provide a new insight into our understanding of the genetic background of epilepsy.

Search for alpha4 and alpha7 nicotinic acetylcholine receptor markers in a pedigree of benign familial infantile convulsions (BFIC)

In this study we investigate the possible involvement of the recently reported locus for benign familial infantile convulsions (BFIC) in human chromosome 19 and that of the neuronal acetylcholine receptor alpha4 (CHRNA4) and alpha7 (CHRNA7) subunits in a family with at least twelve clinically diagnosed cases of BFIC. Six polymorphic microsatellite markers covering the BFIC locus on chromosomal region 19q, one marker for CHRNA4 (chromosome 20) and two for CHRNA7 (chromosome 15) were used for the screening. The two-point lod score analysis showed no evidence of BFIC phenotype on chromosome 19. Similarly, when markers for chromosome 20 (CHRNA4 intron1, Amplimer: CHRNA4. PCR.1) and chromosome 15 (D15S165 and D15S1010) were used, score analysis showed no indication of linkage. The most likely interpretation of these results is that BFIC is a genetically heterogeneous form of epilepsy.

Genetics of epilepsy: current status and perspectives

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

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.

Neonatal seizures and neonatal epileptic syndromes

The neonatal period is defined as the first 28 days of life of a term infant; for premature infants the limit of this period is 44 completed weeks of the infant's conceptional age (CA)-defined as the chronological age plus gestational age (GA) at birth. The clinical and electroencephalographic (EEG) manifestations of seizures during this period are determined primarily by the development features of the immature brain at the time of seizure onset, but are also related to the type and diversity of etiologies and risk-factors for seizures neonates may face early in life. Neonatal seizures may be strikingly different from the clinical and electrical seizures of older children and adults. In addition, findings from basic science investigations suggest that immature animals are more likely to experience seizures in response to injury than more mature animals, although the developing brain is less susceptible to seizure-induced injury.

Neonatal seizures: early-onset seizure syndromes and their consequences for development

The determination of the developmental consequences of seizure syndromes in the neonate is based upon a number of factors which include: understanding of the clinical and electroencephalographic (EEG) features of neonatal seizures; current theories of the mechanisms by which neonatal seizures are generated; a current classification of neonatal seizures; potential etiologic and risk factors for seizures; and therapies. In addition, different seizure types, mechanisms of generation and etiologies of cerebral dysfunction may vary with conceptional age of the infant. There are a few distinct neonatal epileptic syndromes, which are rare, have been well described: benign neonatal convulsions; benign neonatal familial convulsions; early myoclonic encephalopathy and early infantile epileptic encephalopathy. The prognosis for the first two is relatively good while the outcome for the other two with encephalopathy is catastrophic. However, the majority of neonatal seizures occur as acute, reactive events in association with a wide range of etiologic factors. These etiologic factors, as well as those of the more traditionally defined syndromes, are the main determinants of eventual developmental outcome of neonates who experience seizures. Although experimental data suggests that some epileptic seizures eventually may have physiological, histological, metabolic, or behavioral consequences, there is yet direct evidence in humans to suggest that the occurrence of seizures themselves in the neonate is the main determinant of long-term outcome.

Cloning, localisation and functional expression of a novel human, cerebellum specific, two pore domain potassium channel

We have isolated, by degenerate PCR, a complementary DNA encoding a novel two pore domain potassium channel. This is the 7th functional member of the human tandem pore domain potassium channel family to be reported. It has an open reading frame of 1.125 kb and encodes a 374 amino acid protein which shows 62% identity to the human TASK-1 gene: identity to other human members of the family is 31-35% at the amino acid level. We believe this gene to be human TASK-3, the ortholog of the recently reported rat TASK-3 gene: amino acid identity between the two is 74%. 'Taqman' mRNA analysis demonstrated a very specific tissue distribution pattern, showing human TASK-3 mRNA to be localised largely in the cerebellum, in contrast rat TASK-3 was reported to be widely distributed. We have shown by radiation hybrid mapping that human TASK-3 can be assigned to chromosome 8q24.3. Human TASK-3 was demonstrated to endow Xenopus oocytes with a negative resting membrane potential through the presence of a large K(+) selective conductance. TASK-3 is inhibited by extracellular acidosis with a mid-point of inhibition around pH 6. 5, supporting the predictions from the sequence data that this is a third human TASK (TWIK-related acid sensitive K(+) channel) gene.

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.

Benign familial neonatal epilepsy with mutations in two potassium channel genes

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

Ion channels and the genetic contribution to epilepsy

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

Replication analysis of a putative susceptibility locus (EGI) for idiopathic generalized epilepsy on chromosome 8q24

PURPOSE

The present replication study was designed to test the validity of a previously mapped susceptibility locus (EGI) for common subtypes of idiopathic generalized epilepsy (IGE) in chromosomal region 8q24.

METHODS

Thirty-eight multiplex families of probands with common IGE syndromes were included in the present study. Parametric and nonparametric multipoint linkage analyses were conducted between the IGE trait (either "idiopathic" generalized seizure or generalized spike-wave EEG discharges) and three microsatellite polymorphisms (D8S256, D8S284, D8S1128) encompassing the putative EGI locus.

RESULTS

Parametric and nonparametric multipoint linkage analysis provided no evidence for linkage between the IGE trait and the markers encompassing the putative EGI locus. Moreover, we noted no indication favoring linkage to this chromosomal region in two distinct subsets of families subdivided by the absence (n = 18) or presence (n = 20) of family members with juvenile myoclonic epilepsy (JME).

CONCLUSIONS

We failed to replicate evidence of a major locus (EGI) for common familial IGE in chromosome region 8q24. On the contrary, our present parametric linkage results provide evidence against linkage across the region under a broad range of genetic models. If there is a susceptibility locus for IGE in this region, the effect size or the proportion of linked families is too small to detect linkage in these families. Taking into account the problems in replicating initial linkage claims in oligogenic traits, further linkage studies in additional family sets are necessary to evaluate the validity of the previous linkage finding.

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

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

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

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

Partial cosegregation of familial hemiplegic migraine and a benign familial infantile epileptic syndrome

PURPOSE

We studied a large Dutch-Canadian family, in which two very rare hereditary paroxysmal neurologic disorders, familial hemiplegic migraine (FHM) and a "benign familial infantile epileptic syndrome" concur and partially cosegregate. FHM is a dominantly inherited subtype of migraine with attacks of hemiparesis, linked to chromosome 19p13 in 50% of the families tested. Recently mutations in a brain-specific P/Q-type Ca2+ channel alpha1 subunit gene (CACNL1A4) were identified in families with chromosome 19-linked FHM. The infantile epileptic syndrome resembles to two other dominantly inherited benign epilepsies occurring in the first year of life, benign familial neonatal convulsions (BFNC), assigned to chromosomes 20q13.2 and 8q, and benign infantile familial convulsions (BIFC), as yet unlinked.

METHODS

Linkage analysis was performed for the known locations of FHM and BFNC. The question whether the two conditions in this family can be caused by a single gene defect was addressed by additional linkage analysis.

RESULTS

We excluded linkage of the infantile convulsions to markers on chromosome 20q13.2, 8q, or 19p13. This indicates the existence of a third locus for benign familial convulsions in the first year of life. Linkage of FHM to these markers was not formally excluded but seems very unlikely. Statistical analysis of whether, in this family, both conditions are caused by a single gene defect was inconclusive.

CONCLUSIONS

We describe a "benign familial infantile epileptic syndrome" with attacks of FHM at a later age. Further genetic studies in this family may help to unravel the genetic basis of epilepsy or migraine or both.

Possible association of a silent polymorphism in the neuronal nicotinic acetylcholine receptor subunit alpha4 with common idiopathic generalized epilepsies

The alpha4 subunit gene of the neuronal nicotinic acetylcholine receptor (CHRNA4) has recently been identified as the first gene underlying an idiopathic partial epilepsy syndrome in human, autosomal-dominant nocturnal frontal lobe epilepsy (ADNFLE). CHRNA4 is located in the candidate region for benign familial neonatal convulsions and low-voltage EEG on chromosome 20q. In the present study, we examined the possible role of CHRNA4 in common subtypes of idiopathic generalized epilepsy (IGE), comprising childhood and juvenile absence epilepsy and juvenile myoclonic epilepsy (JME), by systematically screening the coding region of the gene for sequence variants. We present here a population-based association study testing the hypothesis that variants of the CHRNA4 gene confer genetic susceptibility to common subtypes of IGE. The missense mutation (Ser248Phe), associated with ADNFLE, and four silent polymorphisms in the CHRNA4 gene were genotyped in 103 IGE patients and 92 controls by polymerase chain reaction and subsequent restriction analysis. Without correction for multiple testing, the frequency of the T-allele of the silent CfoI bp595 polymorphism was increased in the entire group of IGE patients (f(T) = 0.085) compared to that in the controls (f(T) = 0.027). The allelic association was not restricted to any subgroup of IGE with either JME or idiopathic absence epilepsies. This result suggests that variation of the CHRNA4 gene, or so-far-undetected sequence variants near the CHRNA4 locus, confer susceptibility to the common IGE syndromes.

Recent advances and some problems in the delineation of epileptic syndromes in children

The current International Classification of Epilepsies and Epileptic Syndromes has classified a number of age-related epileptic syndromes in children, but some of these entities have not been well delineated and many syndromes not included in the International Classification have been proposed. This article is intended to supplement some of the currently listed childhood epileptic syndromes, discuss certain problems with the classification system, and review some of the proposed pediatric syndromes.

Evidence of a third locus for benign familial convulsions

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

Common subtypes of idiopathic generalized epilepsies: lack of linkage to D20S19 close to candidate loci (EBN1, EEGV1) on chromosome 20

Hereditary factors play a major role in the etiology of idiopathic generalized epilepsies (IGEs). A trait locus (EBN1) for a rare subtype of IGEs, the benign neonatal familial convulsions, and a susceptibility gene (EEGV1) for the common human low-voltage electroencephalogram have been mapped close together with D20S19 to the chromosomal region 20q13.2. Both loci are potential candidates for the susceptibility to IGE spectra with age-related onset beyond the neonatal period. The present study tested the hypothesis that a putative susceptibility locus linked to D20S19 predisposes to spectra of IGEs with age-related onset from childhood to adolescence. Linkage analyses were conducted in 60 families ascertained through IGE patients with juvenile myoclonic epilepsy, juvenile absence epilepsy or childhood absence epilepsy. Our results provide evidence against linkage of a putative susceptibility gene for four hierarchically broadened IGE spectra with D20S19 assuming tentative single-locus genetic models. The extent of an "exclusion region" (lod scores below-2) varied from 0.5 cM up to 22 cM on either side of D20S19 depending on the trait assumed. These results are contrary to the expectation that a susceptibility gene in vicinity to D20S19 confers a common major gene effect to the expression of IGE spectra with age-related onset from childhood to adolescence.

Inherited epilepsies of childhood

Recent advances in neuroepidemiologic and molecular biological techniques have facilitated a growing understanding of the role that inherited factors play in epileptogenesis. During the last few years linkage analysis has mapped gene loci associated with the following epilepsy syndromes: benign familial neonatal convulsions, juvenile myoclonic epilepsy, Unverricht-Lundborg/Baltic/Mediterranean progressive myoclonic epilepsies, the juvenile form of ceroid lipofuscinosis, sialidosis I, and the myoclonus epilepsy with ragged red fibers (MERRF) syndrome. In addition, characterization of the inheritance patterns of other syndromes such as childhood epilepsy with occipital paroxysms and febrile convulsions has improved. It is apparent that a significant amount of clinical and genetic heterogeneity exists, which emphasizes the importance of accurate clinical classification. As genetic markers are found for well-defined groups of patients, traditional means of classification (seizure type, pathologic markers, progressive course, etc.) become less meaningful. It is proposed that the components of the phenotype of an epilepsy syndrome (eg, age of onset, seizure type, electroencephalographic pattern) may be controlled by multiple genes.

Evolution of neonatal seizures

The evolution of the diagnosis, etiology, management, and prognosis of neonatal seizures over the past two decades is reviewed. Seizures in the neonate are unique and require special classification. They result from acquired or congenital abnormalities of the central nervous system. Clustering of prognostic parameters, including seizure characteristics, perinatal factors, neurologic signs, cause, and neuroimaging and electroencephalographic abnormalities, allows neonatal seizures to be viewed as clinical syndromes with predictable outcomes.

Benign infantile familial convulsions are not an allelic form of the benign familial neonatal convulsions gene

Benign infantile familial convulsions (BIFC) and benign familial neonatal convulsions (BFNC) are two forms of familial convulsions having an age of onset within the first year of life. The gene responsible for BFNC has been mapped to chromosome 20q in the close vicinity of D20S19 and D20S20 markers. We performed linkage analysis between BIFC and D20S19-D20S20 in eight families in order to know whether the BFNC gene is also implicated in BIFC. Several apparent obligate crossovers between affected members were detected. The data here presented demonstrate that the BFNC gene is not responsible for BIFC.

Diagnosis of childhood seizure disorders

Convulsive epilepsies are generally unmistakable. Absence epilepsies, which are easily recognized by the provocation of an episode during hyperventilation and by the typical features of the EEG, can be overdiagnosed, especially in the child who daydreams in class and has scattered, asymptomatic, epileptogenic EEG changes. As in adults, complex partial seizures in children can be difficult to distinguish from certain behaviors. Several types of benign childhood epilepsies need to be distinguished from the more intractable and lesional childhood epilepsies. Two common examples, benign rolandic epilepsy and benign occipital epilepsy, can be recognized by their unique EEG changes and clinical features. Juvenile myoclonic epilepsy generally does not remit spontaneously but should be recognized because it appears to respond to valproate. Some recurrent nonepileptic events seen in children can be mistaken for seizures, including shuddering attacks, paroxysmal vertigo, breath-holding spells, cardiogenic syncope, night terrors, and movement disorders, such as paroxysmal kinesigenic choreoathetosis.

Benign infantile epilepsy with autosomal dominant inheritance

Benign cryptogenic infantile epilepsy occurred in 6 infants of 3 families, with similar characteristics suggesting a common physiopathology: onset between 3 and 12 months of age, clusters of brief generalized seizures easily controlled by anti-epileptic drugs, normal psychomotor development, usually normal EEG with, rarely, generalized interictal spike-waves, no recurrence after drug discontinuation, the treatment being no longer than 16 months in most cases. Identical histories were found in parents, uncles and aunts, suggesting an autosomal dominant mode of inheritance. This seems to correspond to an original form of early onset, benign infantile epilepsy.

Neonate with benign familial neonatal convulsions: recorded generalized and focal seizures

A neonate with benign familial neonatal convulsions is presented. Ictal electroencephalography demonstrated a seizure of right frontal onset with generalization and one of right frontal onset which remained confined to that hemisphere. Very few ictal recordings of this entity exist. In this patient a partial seizure and a seizure with generalization were recorded, emphasizing the difficulties with the present classification.

Progress in mapping human epilepsy genes

The chromosomal loci for seven epilepsy genes have been identified in chromosomes 1q, 6p, 8q, 16p, 20q, 21q, and 22q. In 1987, the first epilepsy locus was mapped in a common benign idiopathic generalized epilepsy syndrome, juvenile myoclonic epilepsy (JME). Properdin factor or Bf, human leukocyte antigen (HLA), and DNA markers in the HLA-DQ region were genetically linked to JME and the locus, named EJM1, was assigned to the short arm of chromosome 6. Our latest studies, as well as those by Whitehouse et al., show that not all families with JME have their genetic locus in chromosome 6p, and that childhood absence epilepsy does not map to the same EJM1 locus. Recent results, therefore, favor genetic heterogeneity for JME and for the common idiopathic generalized epilepsies. Heterogeneity also exists in benign familial neonatal convulsions, a rare form of idiopathic generalized epilepsy. Two loci are now recognized; one in chromosome 20q (EBN1) and another in chromosome 8q. Heterogeneity also exists for the broad group of debilitating and often fatal progressive myoclonus epilepsies (PME). The gene locus (EPM1) for both the Baltic and Mediterranean types of PME or Unverricht-Lundborg disease is the same and is located in the long arm of chromosome 21. Lafora type of PME does not map to the same EPM1 locus in chromosome 21. PME can be caused by the juvenile type of Gaucher's disease, which maps to chromosome 1q, by the juvenile type of neuronal ceroid lipofuscinoses (CLN3), which maps to chromosome 16p, and by the "cherry-red-spot-myoclonus" syndrome of Guazzi or sialidosis type I, which has been localized to chromosome 10. A point mutation in the mitochondrial tRNA(Lys) coding gene can also cause PME in children and adults (MERFF).

Electroclinical signs of benign neonatal familial convulsions

Benign neonatal familial convulsions comprise a distinct epileptic syndrome with an autosomal mode of transmission. The electroclinical signs of seizures in this syndrome are not yet well defined. In 3 children from two families presenting with benign neonatal familial convulsions, 14 seizures were recorded during electroencephalographic (EEG)-video sessions. All seizures occurred during sleep, after a short arousal reaction. Seizures started with bilateral, symmetrical flattening of the EEG for 5 to 19 seconds; simultaneously there was apnea and tonic motor activity. The EEG flattening was followed by a long (1-2-minute) bilateral discharge of spikes and sharp waves; simultaneously, there were vocalizations, chewing, and focal or generalized clonic activity. The prominence of EEG and motor abnormalities varied between the left and the right from one seizure to the next in any given child. The seizures stopped without EEG or clinical postictal depression. These electroclinical observations suggest that the convulsions of benign neonatal familial convulsions are a form of generalized tonic-clonic seizure whose expression may be asymmetrical, probably because of the immaturity of the corpus callosum or other structures ensuring seizure synchronization.

Searching for human epilepsy genes: a progress report

Application of new genetic techniques has brought remarkable discoveries in the study of genetic diseases. The potential benefits from applying such technology to idiopathic epilepsies include improved understanding of cellular mechanisms and potential new methods of prevention and treatment. The complex problems involved in studying the hereditary epilepsies include: defining of specific phenotypes; detecting genetic and non-genetic heterogeneity; and specifying the appropriate mode of inheritance and penetrance. The gene loci for three primary epilepsies have been localized to specific chromosomal regions, and serve to demonstrate the process used in generalized linkage studies of hereditary epilepsy syndromes. Benign familial neonatal convulsions (BFNC) and Unverricht-Lundborg progressive myoclonus epilepsy are rare single-gene disorders that are sufficiently localized to chromosomal regions that positional cloning studies are likely to succeed. Juvenile myoclonic epilepsy (JME), a common hereditary syndrome with an uncertain mode of inheritance, has been reported to be linked to chromosome 6p. JME presents a challenge for generalized linkage methodology that may be overcome by attending to potential problems reviewed here. The candidate-gene method, combined with studies using animal models, holds promise for understanding these as well as other hereditary epilepsies.

Infantile spasms in one member of a family with benign familial neonatal convulsions

Seven members of two generations experienced benign familial neonatal convulsions (BFNC) in the neonatal period and/or early infancy. All but 1 family member had a good prognosis. One family member with infantile spasms (IS) was delivered by cesarean section at 37 weeks gestation. Birth weight (2,562 g) was slightly lower than that of other family members. At age 20 days, adversive seizures started. At age 1 month, 10 days, she developed complex partial seizures (CPS) and IS. Interictal EEG showed hypsarrhythmia. Biochemical investigations and head magnetic resonance imaging (MRI) scan showed no abnormalities. Treatment with valproate (VPA) and carbamazepine (CBZ) stopped the seizures, and she had no seizures after age 3 months. Psychomotor development was moderately delayed at 8 months. This is the first reported case of a severe epilepsy, IS, in association with BFNC.