Several epilepsy syndromes, including severe myoclonic epilepsy in infancy (SMEI) and generalized epilepsy with febrile seizures plus (GEFS+), are caused by mutations in the voltage-gated sodium channels. Mutations in the sodium channel SCN1A are a major cause of GEFS+ and SMEI, even though three other sodium channels (SCN2A, SCN3A, and SCN8A) are expressed in the central nervous system (CNS). Mice with mutations in Scn1a and Scn2a exhibit lower seizure thresholds and spontaneous seizures. In marked contrast, we have shown that mutations in the mouse Scn8a gene lead to elevated seizure thresholds. Furthermore, altering Scn8a activity can restore normal seizure thresholds and life spans in mice with Scn1a mutations that serve as models of GEFS+ and SMEI. The goal of this application is to test the hypothesis that mutations in Scn8a can protect against seizures and to investigate the mechanism underlying such protection. This can be achieved in two specific aims. In the first aim we will determine the mechanism by which decreased Scn8a expression leads to higher seizure thresholds and protection against seizure induction. The first objective will establish whether the increase in seizure thresholds is due either to a direct effect of decreased Scn8a expression on neuronal excitability or an indirect effect of a compensatory increase in the expression of any of the other three CNS sodium channels, or possibly both. We will then determine whether reduced Scn8a expression leads to altered network excitability in hippocampal and cortical brain slices using electrophysiological recordings. The purpose of the second aim is to identify the neuronal cell types responsible for the elevation in seizure thresholds of Scn8a mutant mice. This will be accomplished by selectively deleting Scn8a from either pyramidal cells or interneurons in the cortex and hippocampus, and then evaluating the mice for elevated seizure thresholds and altered network excitability. Finally, we will use molecular genetic and electrophysiological approaches to investigate the mechanism by which altered Scn8a activity leads to the dramatic improvements seen in the seizure phenotype of an SMEI mouse model. These studies will provide important and clinically relevant insight into the mechanism by which altered Scn8a function leads to elevated seizure thresholds, enabling the pursuit of much-needed translational studies into the development of novel treatments for epilepsy.

Public Health Relevance

We have observed that mice with reduced activity of the sodium channel gene Scn8a are more seizure resistant. We will investigate the mechanism that underlies this observation. This study will provide important, clinically relevant information on the contribution of Scn8a to seizure resistance and will lay the foundation for further research on the feasibility of reducing the activity of the human SCN8A gene as a treatment for human epilepsy.

Agency
National Institute of Health (NIH)
Institute
National Institute of Neurological Disorders and Stroke (NINDS)
Type
Research Project (R01)
Project #
5R01NS065187-04
Application #
8241100
Study Section
Molecular Neurogenetics Study Section (MNG)
Program Officer
Fureman, Brandy E
Project Start
2009-04-01
Project End
2014-03-31
Budget Start
2012-04-01
Budget End
2014-03-31
Support Year
4
Fiscal Year
2012
Total Cost
$315,701
Indirect Cost
$58,953
Name
Emory University
Department
Genetics
Type
Schools of Medicine
DUNS #
066469933
City
Atlanta
State
GA
Country
United States
Zip Code
30322
Makinson, Christopher D; Tanaka, Brian S; Lamar, Tyra et al. (2014) Role of the hippocampus in Nav1.6 (Scn8a) mediated seizure resistance. Neurobiol Dis 68:16-25
Hawkins, Nicole A; Martin, Melinda S; Frankel, Wayne N et al. (2011) Neuronal voltage-gated ion channels are genetic modifiers of generalized epilepsy with febrile seizures plus. Neurobiol Dis 41:655-60
Papale, Ligia A; Paul, Ketema N; Sawyer, Nikki T et al. (2010) Dysfunction of the Scn8a voltage-gated sodium channel alters sleep architecture, reduces diurnal corticosterone levels, and enhances spatial memory. J Biol Chem 285:16553-61
Escayg, Andrew; Goldin, Alan L (2010) Sodium channel SCN1A and epilepsy: Mutations and mechanisms. Epilepsia :