Abstracts

Rationale: SCN8A mutations have been detected in more than 100 patients with epileptic encephalopathy (EIEE13, OMIM #614558). These mutations arise de novo in the affected individuals. The epileptogenic mutations of SCN8A are missense mutations causing amino acid substitutions, rather than protein truncations. The functional consequences of these mutations are difficult to predict from the sequence change alone. Approximately one third of SCN8A mutations are recurrent at a small number of residues. In this study, we examined the functional effects of recurrent amino acid substitutions responsible for more than 10% of cases of EIEE13. SCN8A encodes Nav1.6, the abundant voltage-gated sodium channel that is localized at the axon initial segment of excitatory and inhibitory neurons, and at nodes of Ranvier in myelinated neurons. The clinical features associated with SCN8A mutations include seizure onset between 0-18 months of age, intellectual disability, and developmental delay. Nearly half of affected individuals have severe movement impairment resulting in wheelchair dependence. Sudden unexpected death in epilepsy (SUDEP) has been reported in approximately 10% of EIEE13 patients.Methods: SCN8A mutations were introduced into the Scn8a cDNA by site directed mutagenesis. Channel activity was characterized in transfected ND7/23 cells using patch clamp electrophysiology.Results: All three mutations resulted in impaired channel function, by differing molecular mechanisms consistent with their locations within the domains of the channel protein. Abnormalities included increased macroscopic Na channel current, depolarizing shifts in steady-state inactivation parameters, and elevated persistent current.Conclusions: Gain-of-function effects were characterized for three recurrent mutations of SCN8A. Gain of function mutations of Nav1.6 can lead to increased in neuronal excitability and generation of epileptic seizures. Distinct molecular mechanisms of gain-of-function mutations may be a factor in the variable responses of patients with EIEE13 to anti-epileptic drugs.