Abstracts

Rationale: Epilepsy is a severe neurological disease defined by recurrent seizures and associated with comorbid developmental delay and intellectual disability. The most severe type of epilepsy, the early infantile epileptic encephalopathies, are largely due to pathogenic variants in genes important for brain development and function. Mutations in genes encoding voltage-gated sodium (Na+) channel a subunits, which underlie action potential generation and propagation, are well-known causes of childhood epilepsy. We recently showed that heterozygous de novo pathogenic variants in SCN3A encoding the Na+ channel subunit Nav1.3 is a cause of very early onset epileptic encephalopathy; such variants produce increased slowly-inactivating/persistent current and a left-shift in the voltage dependence of activation to more hyperpolarized potentials. However, the effect of such variants on native neuronal function is not known. Methods: Here, we express wild-type and variant human Nav1.3 (Nav1.3-WT, Nav1.3-Ile875Thr, and Nav1.3-Val1769Ala) in cultured rat hippocampal pyramidal neurons via transient transfection, and in layer 2/3 pyramidal neurons in mouse neocortex using in utero electroporation. Results: Current clamp recordings revealed spontaneous bursting from resting membrane potential with transition to depolarization block in cells expressing mutant hNav1.3, but not in cells overexpressing hNav1.3-WT or in untransfected cultured neurons or in neighboring non-electroporated cells in brain slices. Conclusions: Such results support the pathogenic nature of the identified epilepsy-associated Nav1.3 variants yet indicate that the overall functional impact of such variants on neuronal excitability may be complex. Funding: NIH NINDS K08 NS097633 and a Burroughs Wellcome Fund Career Award for Medical Scientists to E.M.G.