Role of the human voltage-gated sodium channel, SCN1A, in familial epilepsy

Mutations in SCN1A encoding a neuronal voltage-gated sodium channel α₁ subunit (Na_v1.1) have been linked to inherited seizure disorders such as generalized epilepsy with febrile seizures plus, type 2 (GEFS+2), severe myoclonic epilepsy of infancy (SMEI), and intractable childhood epilepsy with frequent generalized tonic-clonic seizures (ICEGTC). We have cloned a full-length human SCN1A (SCN1A[full]) cDNA as well as two shorter splice variants (SCN1A[−33] and SCN1A[−84]) that arise from utilization of alternative 3′ ends of exon 11 resulting in lack of 11 or 28 amino acids in the D1-D2 linker. Interestingly, as determined by RNAse protection, SCN1A[−33] and SCN1A[−84] contribute 75% and 19%, respectively, to the brain's SCN1A population. Whole-cell patch clamp recordings of human tsA201 cells transiently transfected with wildtype SCN1A[full] and auxiliary human sodium channel subunits β₁ and β ₂ exhibited depolarization-induced rapidly activating and inactivating (τ_i ∼ 0.1 to 5 ms) inward sodium currents that were reversibly blocked by tetrodotoxin. Voltage-dependence of activation and inactivation was half-maximal at −26.4 and −67.5 mV, respectively. Cells expressing SCN1A[full] mutants associated with GEFS+2 (T875M, W1204R, R1648H, I1656M, R1657C) exhibited a wide spectrum of defects including reduced current density, persistent current (Lossin et al., Neuron 34:877–884, 2002), shifts in the voltage dependence of activation and steady-state inactivation, altered recovery from fast inactivation, as well as subtle changes in slower forms of inactivation. Mutant L986F (SMEI), as well as mutants V1353L and A1685V (GEFS+2) failed to exhibit voltage-sensitive current under the conditions employed. Introduction of the GEFS+2 mutation R1648H into SCN1A[−33] or SCN1A[−84] gave rise to isoform-specific defects and reduced the functional diversity seen among the three wildtype splice variants.; Our data are consistent with gain-of-function, as well as partial and complete loss-of-function in epilepsy-associated mutants of SCN1A. Despite the shared genetic origin of the disorders, a common disease-causing factor could not be detected. Post-transcriptional mRNA processing may play an important role in determining the function of the SCN1A gene product and epileptogenesis.