Benign Familial Neonatal Seizures Research Articles

The role of genetics and ethnicity in epilepsy management

Recent exciting developments in epilepsy genetics have led to significant insights into the mechanisms underlying seizure disorders. Success in epilepsy genetics research to date has resulted from identification of genes responsible for rare monogenic disorders, the majority encoding either voltage- or ligand-gated ion channels. For some conditions, such as benign familial neonatal seizures, an understanding of the underlying genetics is helpful in predicting prognosis. However, for other disorders, such as autosomal dominant nocturnal frontal lobe epilepsy, phenotypic severity is determined by factors other than the major dominant nicotinic subunit mutation found in some families. Further complexity arises when single-gene mutations give rise to heterogeneous phenotypes, as typically occur with generalized epilepsy with febrile seizures plus. Another area of increasing genetic endeavour, pharmacogenetics will allow tailoring of antiepileptic medication for each patient. Pharmacogenetics explores genetic polymorphisms in genes coding for drug-metabolizing enzymes, receptors and transporters. Polymorphisms have been identified that result in marked ethnic and interindividual differences in response to treatment. With further understanding of the impact of these differences, pharmacogenetic screening is likely to guide the management of epilepsy in the future.

Ion Channels and Epilepsy

The role of voltage-gated and ligand-gated ion channels in epileptogenesis of both genetic and acquired epilepsies, and as targets in the development of new antiepileptic drugs (AEDs) is reviewed. Voltage-gated Na+ channels are essential for action potentials, and their mutations are the substrate for generalised epilepsy with febrile seizures plus and benign familial neonatal infantile seizures; Na+ channel inhibition is the primary mechanism of carbamazepine, phenytoin and lamotrigine, and is a probable mechanism for many other classic and novel AEDs. Voltage-gated K+ channels are essential in the repolarisation and hyperpolarisation that follows paroxysmal depolarisation shifts (PDSs), and their mutations are the substrate for the benign neonatal epilepsy and episodic ataxia type 1; they are new targets for AEDs such as retigabine. Voltage-gated Ca2+ channels are involved in neurotransmitter release, in the sustained depolarisation-phase of PDSs, and in the generation of absence seizures; their mutations are a substrate for juvenile myoclonic epilepsy and the absence-like pattern seen in some mice; the antiabsence effect of ethosuximide is due to the inhibition of thalamic T-type Ca2+ channels. Voltage-gated Cl- channels are implicated in GABA(A) transmission, and mutations in these channels have been described in some families with juvenile myoclonic epilepsies, epilepsy with grand mal seizures on awakening or juvenile absence epilepsy. Hyperpolarisation-activated cation channels have been implicated in spike-wave seizures and in hippocampal epileptiform discharges. The Cl- ionophore of the GABA(A) receptor is responsible for the rapid post-PDS hyperpolarisation, it has been involved in epileptogenesis both in animals and humans, and mutations in these receptors have been found in families with juvenile myoclonic epilepsy or generalised epilepsy with febrile seizures plus; enhancement of GABA(A) inhibitory transmission is the primary mechanism of benzodiazepines and phenobarbital and is a mechanistic approach to the development of novel AEDs such as tiagabine or vigabatrin. Altered GABA(B)-receptor function is implicated in spike-wave seizures. Ionotropic glutamate receptors are implicated in the sustained depolarisation phase of PDS and in epileptogenesis both in animals and humans; felbamate, phenobarbital and topiramate block these receptors, and attenuation of glutamatergic excitatory transmission is another new mechanistic approach. Mutations in the nicotinic acetylcholine receptor are the substrates for the nocturnal frontal lobe epilepsy. The knowledge of the role of the ion channels in the epilepsies is allowing the design of new and more specific therapeutic strategies.

Ion channel alterations and epilepsy in mice and man

Physiological CNS function relies on a balance of impulses mediated by voltage and ligand gated ion channels. Various forms of epilepsy correlate with mutations of genes coding for voltage dependent sodium (e.g., benign familial neonatal seizures), potassium (e.g., benign familial neonatal convulsions), and calcium (e.g., juvenile myoclonic epilepsy) channels. Mice carrying mutations in the orthologous ion channel genes serve as models of corresponding human disorders. From these studies, it has become apparent that ion conducting as well as accessory subunits of these channel types may be affected. The mouse mutant entla develops an absence epilepsy characterized by 2 and 4Hz EEG patterns, in addition to ataxia and paroxysmal dyskinesia. The underlying mutation on mouse Chr 9 was identified as an allele of the Cacna2d2 gene. This gene encodes the accessory α2δ-2 subunit of P/Q type calcium channels, a target of the antiepileptic drug gabapentin. The entla allele harbors an exon duplication that interferes with posttranslational processing, i.e. the formation of a disulfide linkage. This results in a dramatic reduction of both, calcium currents and [3H]gabapentin binding. The entla mouse thus represents a model of genetically alteratered protein processing producing a complex absence epilepsy syndrome. At the postsynaptic face, the neurotransmitter signal is converted to excitatory or inhibitory currents by binding to ligand gated ion channels. Mutations of the nicotinic receptor α4 subunit gene (CHNRA4) cause autosomal dominant nocturnal frontal lobe epilepsy. In contrast, mutations of the glycine subunit genes α1 (GLRA1) and β (GLRB) result in hyperekplexia (startle disease, stiff baby syndrome), a non-epileptic neurological condition. Upon recombinant expression, mutant acetylcholine and glycine receptor subunits frequently display altered transmitter binding and ion conductance.

Genetics of Idiopathic Epilepsies

To search for clues to molecular genetics of common idiopathic epilepsy syndromes. Genetic defects have been identified recently in certain inherited epilepsy syndromes in which the phenotypes are similar to those of common idiopathic epilepsies. Mutations identified as the causes of inherited idiopathic epilepsies were reviewed. Mutations of the genes encoding two subunits of the neuronal nicotinic acetylcholine receptor were found in autosomal dominant nocturnal frontal lobe epilepsy. Mutations of two K(+)-channel genes were identified in benign familial neonatal convulsions. Mutations of the genes encoding several subunits of the voltage-gated Na(+)-channel and gamma-aminobutyric acid (GABA)(A) receptor also were identified as the underlying causes of various epilepsy syndromes, such as autosomal dominant epilepsy with febrile seizures plus, benign familial neonatal infantile seizures, and autosomal dominant juvenile myoclonic epilepsy. Mutations within the same gene may result in different epilepsy phenotypes. Thus, the Na(+) channel, GABA(A) receptor, and their auxiliaries may be involved in the pathogenesis of various types of epilepsy. Some forms of juvenile myoclonic epilepsy, idiopathic generalized epilepsy, and absence epilepsy may result from mutations of Ca(2+) channels. Mutations of the Cl(-) channel have been recently found to be associated with a certain type of epilepsy. The recent discovery that mutations of LGI1, a gene encoding a nonchannel molecule, are associated with autosomal partial epilepsy with auditory features may provide a new insight into our understanding of the genetics of idiopathic epilepsy. These findings suggest the involvement of brain channelopathies in the pathogenesis of certain types of idiopathic epilepsy.

KCNQ/M Currents in Sensory Neurons: Significance for Pain Therapy

Neuronal hyperexcitability is a feature of epilepsy and both inflammatory and neuropathic pain. M currents [IK(M)] play a key role in regulating neuronal excitability, and mutations in neuronal KCNQ2/3 subunits, the molecular correlates of IK(M), have previously been linked to benign familial neonatal epilepsy. Here, we demonstrate that KCNQ/M channels are also present in nociceptive sensory systems. IK(M) was identified, on the basis of biophysical and pharmacological properties, in cultured neurons isolated from dorsal root ganglia (DRGs) from 17-d-old rats. Currents were inhibited by the M-channel blockers linopirdine (IC50, 2.1 microm) and XE991 (IC50, 0.26 microm) and enhanced by retigabine (10 microm). The expression of neuronal KCNQ subunits in DRG neurons was confirmed using reverse transcription-PCR and single-cell PCR analysis and by immunofluorescence. Retigabine, applied to the dorsal spinal cord, inhibited C and Adelta fiber-mediated responses of dorsal horn neurons evoked by natural or electrical afferent stimulation and the progressive "windup" discharge with repetitive stimulation in normal rats and in rats subjected to spinal nerve ligation. Retigabine also inhibited responses to intrapaw application of carrageenan in a rat model of chronic pain; this was reversed by XE991. It is suggested that IK(M) plays a key role in controlling the excitability of nociceptors and may represent a novel analgesic target.

Channelopathies as a genetic cause of epilepsy

This review describes the significant number of new gene associations with epilepsy syndromes that have emerged during the past year, together with additional mutations and new electrophysiological data relating to previously known gene associations. Autosomal dominant juvenile myoclonic epilepsy was demonstrated to be a channelopathy associated with a GABA(A) receptor, alpha1 subunit mutation. Benign familial neonatal infantile seizures were delineated as another channelopathy of infancy, by molecular characterization of sodium channel, alpha2 subunit defects. A sodium channel, alpha2 subunit defect was previously found to be associated with generalized epilepsy with febrile seizures plus. Similarly, the clinical spectrum associated with potassium channel, KQT-like mutations was extended to include the channelopathy myokymia and neonatal epilepsy. Mutations in the non-ion channel genes, leucine-rich, glioma inactivated 1 gene and Aristaless related homeobox gene, have emerged as important causes of their specific syndromes, with mutations in the latter gene frequently underlying X-linked mental retardation with epilepsy. All but one of the idiopathic epilepsies with a known molecular basis are channelopathies. Where the ion channel defects have been identified, however, they generally account for a minority of families and sporadic cases with the syndrome in question. The data suggest that ion channel mutations of large effect are a common cause of rare monogenic idiopathic epilepsies, but are rare causes of common epilepsies. Additive effects of genetic variation, perhaps within the same ion channel gene families, are likely to underlie the common idiopathic generalized epilepsies with complex inheritance. The genetics of epilepsy is progressing rapidly toward a more detailed molecular dissection and definition of syndromes.

Sodium-channel defects in benign familial neonatal-infantile seizures

Ion-channel gene defects are associated with a range of paroxysmal disorders, including several monogenic epilepsy syndromes. Two autosomal dominant disorders present in the first year of life: benign familial neonatal seizures, which is associated with potassium-channel gene defects; and benign familial infantile seizures, for which no genes have been identified. Here, we describe a clinically intermediate variant, benign familial neonatal-infantile seizures, with mutations in the sodium-channel subunit gene SCN2A. This clinico-molecular correlation defines a new benign familial epilepsy syndrome beginning in early infancy, an age at which seizure disorders frequently have a sombre prognosis.

Síndromes epilépticos idiopáticos del recién nacido

Seizures during the neonatal period are the characteristic clinical expression of dysfunction of the nervous system. Not all the seizures seen during the neonatal period are due to epilepsy which only occurs in 10%. The aetiology of neonatal seizures is very varied and is mainly due to different types of aggression to the brain during this early stage of life. Epileptic syndromes are very rare during the neonatal period. In general the prognosis is very bad as occurs in infantile epileptic encephalopathy of early onset or myoclonic encephalopathy of early onset. However, the International League Against Epilepsy (ILAE) has identified and recognized some idiopathic epileptic syndromes of the neonatal period with a somewhat better prognosis. Two major groups have been established including the benign neonatal epilepsies (benign idiopathic neonatal epilepsies and benign familial neonatal seizures) and the group of status epilepticus (severe idiopathic status epilepticus). We analyze the different types of epilepsy of the newborn, form of onset, current knowledge of molecular biology, treatment and prognosis.

Neonatal Seizures

Neonatal Seizures

The neuronal nicotinic acetylcholine receptor in some hereditary epilepsies.

Recent advances in human genetics and in the neurobiology of neurotransmitter receptors and channels have led to the discovery of specific genes associated with hereditary epileptic phenotypes. All the genes identified to date code for ligand- and voltage-gated ion channels. Some clinically rare idiopathic epilepsies are associated with mutations in genes coding for different neuronal nicotinic acetylcholine receptor (AChR) subunits. Distinct alpha subunits are found in the brain and in the peripheral nervous system, and structural, non-alpha subunits like beta2 and beta4 confer different properties to neuronal receptors. Thus, the final properties of the oligomeric AChR depend on the different combinations of alpha and beta subunits. Most mutations found so far occur in the alpha4 chain, the most abundant subunit in the central nervous system. Specifically, the identification of mutations in the alpha4 subunit of neuronal AChR in human benign familial neonatal convulsions (BFNC) and autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) raise the possibility that the observed gene defects are linked (causatively) with these two diseases or, alternatively, that AChR alpha4 mutants increase the probability of epileptic discharges. We discuss testable hypotheses for unraveling the pathophysiology of these two disorders associated with AChR mutations.

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.

Susceptibility genes in human epilepsy.

Major advances in the identification of genetic loci and genes that predispose individuals to epilepsy have been made in the last several years. Two main themes for human, idiopathic epilepsies are emerging; genetic, or locus heterogeneity is not uncommon, and the discovery that epilepsy susceptibility genes are voltage-gated and ligand-gated ion channels. Knowledge that more than a single genetic locus is responsible for a single seizure type, along with a wide spectrum of disease mutations among families will complicate clinical, diagnostic issues. Disease gene identification, such as the two potassium ion channels (KCNQ2 and KCNQ3) for the two forms of benign familial neonatal seizures (BFNC) and the alpha4 subunit of the nicotinic receptor for autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE), however, should yield significant advances in drug discoveries. Understanding the primary defect in inherited epilepsies provides for specific protein and pathway targets for potential drug intervention.

An autosomal recessive form of benign familial neonatal seizures

We present a consanguineous sibship with benign familial neonatal seizures. The mode of transmission of the disorder in this family seems to be autosomal recessive, which is contrary to the usual autosomal dominant type. Linkage analysis failed to show tight linkage between the disease locus and the autosomal dominant locus assigned to chromosome 20q. We thus conclude that benign familial neonatal seizures is a genetically heterogeneous type of epilepsy.

Benign familial neonatal seizures: Clinical and electroencephalographic characteristics

Two families with benign familial neonatal seizures evident over three generations are presented with emphasis on clinical and electroencephalographic features. Benign familial neonatal seizures represent a disorder with the onset of frequent generalized seizures during the first weeks of life. Family history reveals autosomal dominant inheritance of this disorder. The neonates are neurologically normal and outcomes are usually benign. In our families, none of the patients had seizures after the first ten months of life, with long-term follow-up ranging from 10 months to 56 years. The use of antiepileptic medication seems to have little effect on course or eventual outcome. The electroencephalograms, both initially and subsequently, ranged from normal to epileptiform and did not correlate with risk for further seizures in infancy or subsequent epilepsy. Benign familial neonatal seizures are a distinct clinical entity whose key features and favorable prognosis should be appreciated by all those caring for sick neonates. This recognition would result in an early diagnosis and conservative treatment.