Pathogenesis of epilepsy in a SCN8A human mutation mouse model

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De novo mutations of SCN8A, the gene that encodes for the sodium (Na) channel isoform Nav1.6, are known to cause early infantile epileptic encephalopathy 13 (EIEE13). To date, more than 150 SCN8A mutations have been identified. Patients experience a variety of seizure types and motor features that can lead to wheelchair dependence. Intellectual disability varies from mild to severe and becomes progressively worst with seizure onset. Sudden unexpected death in epilepsy (SUDEP) occurs in approximately 10% of patients and increases significantly if seizures are not controlled. Unfortunately, a majority of patients have drug refractory epilepsy or a mixed response to anti-epileptic drugs (AEDs). Very little is known about the pathogenesis of SCN8A epileptic encephalopathy or treatment options for patients. In this proposal we will use a highly novel and innovative knock-in mouse model, developed by the Meisler lab, carrying the human SCN8A encephalopathy mutation p.Asn1768Asp (N1768D). The mutation was identified in a child who presented with refractory epilepsy at the age of 6 months, intellectual disability, ataxia and SUDEP at 15 years of age. The mouse model exhibits many of the pathological phenotypes seen in patients, including spontaneous seizures and sudden death. In homozygous mutant mice (D/D), seizures begin at 3 weeks of age and progress to death within 24 hours. Heterozygous mutant mice (D/+) have later seizure onset starting at 8 weeks of age and progression to death within one to two months. The availability of this mouse model provides a unique opportunity to fully investigate the pathogenesis of this devastating human epileptic encephalopathy and to also test new and selective therapies. In this proposal we will investigate when alterations in Na channel physiology and membrane excitability begin to appear in our model of epileptic encephalopathy, testing both excitatory and inhibitory neurons within brain regions known to be involved in seizure activity. In Aim 1 we will determine the pathogenesis of these alterations at specific time points before and after seizure onset using mutant mice. In Aim 2, we will silence Nav1.6 using virally delivered, dox-inducible, Nav1.6 shRNA, targeting either excitatory or inhibitory neurons and determine the effects on Na channel activity, neuronal excitability and seizure activity in mutant mice. We will determine if we can delay the onset of seizures by targeting at a time point before the onset of seizures and also if we can modulate seizure activity in mice with spontaneous seizures. In Aim 3, we will test whether our Nav1.6 subtype selective compound (MV1505) can reduce seizure activity in heterozygous (D/+) mice and delay SUDEP. We will also evaluate two clinically used anticonvulsants (phenytoin and lacosamide). These studies will significantly impact our current understanding of the physiological consequences of increased Nav1.6 activity in SCN8A epileptic encephalopathy. They will also provide important insight into the selective targeting of Nav1.6 for therapy.