Epilepsy is a common neurological condition that reflects neuronal hyperexcitability arising from largely unknown cellular and molecular mechanisms. In generalized epilepsy with febrile seizures plus, an autosomal dominant epilepsy syndrome, mutations in three genes coding for voltage-gated sodium channel alpha or beta1 subunits (SCN1A, SCN2A, SCN1B) and one GABA receptor subunit gene (GABRG2) have been identified. Here, we characterize the functional effects of three mutations in the human neuronal sodium channel alpha subunit SCN1A by heterologous expression with its known accessory subunits, beta1 and beta2, in cultured mammalian cells. SCN1A mutations alter channel inactivation, resulting in persistent inward sodium current. This gain-of-function abnormality will likely enhance excitability of neuronal membranes by causing prolonged membrane depolarization, a plausible underlying biophysical mechanism responsible for this inherited human epilepsy.

Mutations in SCN1A, the gene encoding the brain voltage-gated sodium channel alpha1 subunit (NaV1.1), are associated with at least two forms of epilepsy, generalized epilepsy with febrile seizures plus (GEFS+) and severe myoclonic epilepsy of infancy (SMEI). We examined the functional properties of four GEFS+ alleles and one SMEI allele using whole-cell patch-clamp analysis of heterologously expressed recombinant human SCN1A. One previously reported GEFS+ mutation (I1656M) and an additional novel allele (R1657C), both affecting residues in a voltage-sensing S4 segment, exhibited a similar depolarizing shift in the voltage dependence of activation. Additionally, R1657C showed a 50% reduction in current density and accelerated recovery from slow inactivation. Unlike three other GEFS+ alleles that we recently characterized, neither R1657C nor I1656M gave rise to a persistent, noninactivating current. In contrast, two other GEFS+ mutations (A1685V and V1353L) and L986F, an SMEI-associated allele, exhibited complete loss of function. In conclusion, our data provide evidence for a wide spectrum of sodium channel dysfunction in familial epilepsy and demonstrate that both GEFS+ and SMEI can be associated with nonfunctional SCN1A alleles.

Mutations in SCN1A, the gene encoding the brain voltage-gated sodium channel ␣1 subunit (NaV1.1), are associated with at least two forms of epilepsy, generalized epilepsy with febrile seizures plus and severe myoclonic epilepsy of infancy (SMEI). We examined the functional properties of five SMEI mutations by using wholecell patch-clamp analysis of heterologously expressed recombinant human SCN1A. Two mutations (F902C and G1674R) rendered SCN1A channels nonfunctional, and a third allele (G1749E) exhibited minimal functional alterations. However, two mutations within or near the S4 segment of the fourth repeat domain (R1648C and F1661S) conferred significant impairments in fast inactivation, including persistent, noninactivating channel activity resembling the pattern of channel dysfunction observed for alleles associated with generalized epilepsy with febrile seizures plus. Our data provide evidence for a range of SCN1A functional abnormalities in SMEI, including gain-of-function defects that were not anticipated in this disorder. Our results further indicate that a complex relationship exists between phenotype and aberrant sodium channel function in these inherited epilepsies.seizure ͉ generalized epilepsy with febrile seizures plus ͉ SCN1A ͉ electrophysiology M utations in genes encoding neuronal voltage-gated sodium channels have been linked to inherited forms of epilepsy. Genetic defects in two pore-forming ␣ subunits (encoded by SCN1A and SCN2A) and the accessory ␤ 1 subunit (encoded by SCN1B) have been discovered in four distinguishable clinical syndromes with overlapping features (1-6). Generalized epilepsy with febrile seizures plus (GEFSϩ) is an autosomal dominant disorder characterized by childhood febrile seizures that persist beyond age 6 years, as well as afebrile generalized or partial seizures of various types. In 1998, Wallace et al.(1) described a single missense mutation in SCN1B, the gene encoding the voltage-gated sodium channel ␤ 1 subunit, in a large GEFSϩ pedigree. However, SCN1B mutations are rare causes of GEFSϩ (7,8). By contrast, mutations in SCN1A, the gene encoding the neuronal sodium channel ␣-subunit Na V 1.1, have been identified in several GEFSϩ families (2, 9-11).SCN1A mutations occur also in severe myoclonic epilepsy of infancy (SMEI), a rare convulsive disorder characterized by febrile seizures with onset during the first year of life, followed by intractable epilepsy, impaired psychomotor development, and ataxia (12, 13). Seizures in this disorder typically do not respond to standard anticonvulsant pharmacotherapy. More than 80 heterozygous, predominantly de novo, SCN1A mutations have been reported in this disorder (3,(14)(15)(16)(17). Because many of the SCN1A mutations discovered in SMEI probands are nonsense and frameshift alleles, loss of neuronal sodium channel function as the cause of this syndrome seems most plausible. This hypothesis is supported by the observation that certain missense mutations in this condition render SCN1A channels nonfunctional or severely impair...

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