Abstracts

Rationale: De novo mutations of the voltage-gated sodium channel gene SCN8A have recently been recognized as a cause of epileptic encephalopathy, an early-onset disorder with refractory seizures, developmental delay and cognitive disabilities. We previously described the SCN8A missense mutation p.Asn1768Asp (N1768D) in a child with epileptic encephalopathy that included seizures, intellectual disability, ataxia, and SUDEP (sudden unexpected death in epilepsy) at 15 years of age (Veeramah et al. 2012). SCN8A^N1768D is a dominant, gain-of-function mutation that causes a large increase in persistent sodium current leading to hyperexcitability in transfected cells. We introduced the SCN8A^N1768D human mutation into the mouse genome to investigate the pathogenicity of the altered channel in vivo. Methods: We generated the Scn8a^N1768D knock-in mouse using TALEN technology with a targeting vector (Jones and Meisler 2014). We have studied homozygous, heterozygous, and functional hemizygous mice with one Scn8a^N1768D allele and one null Scn8a^medtg allele. We monitored seizure incidence and survival of mutant mice and performed video/EEG analysis. We also characterized motor coordination, learning and memory, and social interactions in heterozygous Scn8a^N1768D/+ mice. Results: Heterozygous Scn8a^N1768D/+ mice recapitulate the seizures and SUDEP of the heterozygous patient. Seizure onset occurs around three months of age with progression to SUDEP within a month of onset. Video/EEG analysis reveals ictal discharges that coincide with seizures. Prior to seizure onset, Scn8a^N1768D/+ heterozygotes were not defective in motor learning, fear conditioning or sociability, but did exhibit impairments in motor coordination and social discrimination. Homozygous Scn8a^{N1768D/N1768D} mice exhibit earlier seizure onset, at three weeks of age, and progression to SUDEP within 24 hours. The functionally hemizygous Scn8a^N1768D/- mice have an intermediate phenotype, indicating that severity is increased by a double dose of mutant protein and reduced by the presence of wildtype protein. Conclusions: Scn8a^N1768D mice provide a model of epileptic encephalopathy and SUDEP that will facilitate investigation of the cellular and molecular pathology underlying the disease as well as response to therapeutic interventions. This gain-of-function Scn8a mutant mouse is also a valuable tool for studying the in vivo effects of abnormal Na_v1.6 activity.