(What to Do) When Epilepsy Gene Mutations Stop Making Sense
Authors: Cooper Edward C. 
Published: January 2007

An Epilepsy Mutation in the Sodium Channel SCN1A That Decreases Channel Excitability. Barela AJ, Waddy SP, Lickfett JG, Hunter J, Anido A, Helmers SL, Goldin AL, Escayg A. J Neurosci 2006;26:2714–2723. Mutations in three voltage-gated sodium channel genes SCN1A, SCN2A, and SCN1B and two GABAA receptor subunit genes GABRG2 and GABRD have been identified in families with generalized epilepsy with febrile seizures plus (GEFS+). A novel mutation, R859C, in the Nav1.1 sodium channel was identified in a four-generation, 33-member white family with a clinical presentation consistent with GEFS++. The mutation neutralizes a positively charged arginine in the domain 2 S4 voltage sensor of the Nav1.1 channel subunit. This residue is conserved in mammalian sodium channels as well as in sodium channels from lower organisms. When the mutation was placed in the rat Nav1.1 channel and expressed in Xenopus oocytes, the mutant channel displayed a positive shift in the voltage dependence of sodium channel activation, slower recovery from slow inactivation, and lower levels of current compared with the wild-type channel. Computational analysis suggests that neurons expressing the mutant channel have higher thresholds for firing a single action potential and for firing multiple action potentials, along with decreased repetitive firing. Therefore, this mutation should lead to decreased neuronal excitability, in contrast to most previous GEFS+ sodium channel mutations, which have changes predicted to increase neuronal firing.

Sodium Channel Dysfunction in Intractable Childhood Epilepsy with Generalized Tonic–Clonic Seizures Rhodes TH, Vanoye CG, Ohmori I, Ogiwara I, Yamakawa K, George AL Jr. J Physiol 2005;569(Pt 2):433–445. Mutations in SCN1A, the gene encoding the brain voltage-gated sodium channel1 subunit (NaV1.1), are associated with genetic forms of epilepsy, including generalized epilepsy with febrile seizures plus (GEFS+ type 2), severe myoclonic epilepsy of infancy (SMEI), and related conditions. Several missense SCN1A mutations have been identified in probands affected by the syndrome of intractable childhood epilepsy with generalized tonic–clonic seizures (ICEGTC), which bears similarity to SMEI. To test whether ICEGTC arises from molecular mechanisms similar to those involved in SMEI, we characterized eight ICEGTC missense mutations by whole-cell patch clamp recording of recombinant human SCN1A heterologously expressed in cultured mammalian cells. Two mutations (G979R and T1709I) were nonfunctional. The remaining alleles (T808S, V983A, N1011I, V1611F, P1632S, and F1808L) exhibited measurable sodium current, but had heterogeneous biophysical phenotypes. Mutant channels exhibited lower (V983A, N1011I, and F1808L), greater (T808S), or similar (V1611F and P1632S) peak sodium current densities compared with wild-type (WT)-SCN1A. Three mutations (V1611F, P1632S, and F1808L) displayed hyperpolarized conductance–voltage relationships, while V983A exhibited a strong depolarizing shift in the voltage dependence of activation. All mutants except T808S had hyperpolarized shifts in the voltage dependence of steady-state channel availability. Three mutants (V1611F, P1632S, and F1808L) exhibited persistent sodium current ranging from 1–3% of peak current amplitude that was significantly greater than WT-SCN1A. Several mutants had impaired slow inactivation, with V983A showing the most prominent effect. Finally, all of the functional alleles exhibited reduced use-dependent channel inhibition. In summary, SCN1A mutations associated with ICEGTC result in a wide spectrum of biophysical defects, including mild-to-moderate gating impairments, shifted voltage dependence, and reduced use dependence. The constellation of biophysical abnormalities for some mutants is distinct from those previously observed for GEFS+ and SMEI, suggesting possible, but complex, genotype–phenotype correlations.

Single-Channel Properties of Human Nav1.1 and Mechanism of Channel Dysfunction in SCN1A-Associated Epilepsy. Vanoye CG, Lossin C, Rhodes TH, George AL Jr. J Gen Physiol 2006;127:1–14. Mutations in genes encoding neuronal voltage-gated sodium channel subunits have been linked to inherited forms of epilepsy. The majority of mutations (>100) associated with generalized epilepsy with febrile seizures plus (GEFS+) and severe myoclonic epilepsy of infancy (SMEI) occur in SCN1A encoding the Nav1.1 neuronal sodium channel subunit. Previous studies demonstrated functional heterogeneity among mutant SCN1A channels, revealing a complex relationship between clinical and biophysical phenotypes. To further understand the mechanisms responsible for mutant SCN1A behavior, we performed a comprehensive analysis of the single-channel properties of heterologously expressed recombinant WT- SCN1A channels. Based on these data, we then determined the mechanisms for dysfunction of two GEFS+-associated mutations (R1648H, R1657C) both affecting the S4 segment of domain 4. WT- SCN1A has a slope conductance (17 pS) similar to channels found in native mammalian neurons. The mean open time is 0.3 ms in the −30 to −10 mV range. The R1648H mutant, previously shown to display persistent sodium current in whole-cell recordings, exhibited similar slope conductance but had an increased probability of late reopening and a subfraction of channels with prolonged open times. We did not observe bursting behavior and found no evidence for a gating mode shift to explain the increased persistent current caused by R1648H. Cells expressing R1657C exhibited conductance, open probability, mean open time, and latency to first opening similar to WT channels but reduced whole-cell current density, suggesting decreased number of functional channels at the plasma membrane. In summary, our findings define single-channel properties for WT- SCN1A, detail the functional phenotypes for two human epilepsy-associated sodium channel mutants, and clarify the mechanism for increased persistent sodium current induced by the R1648H allele.