Cellular and behavioral effects of altered Na(V)1.2 sodium channel ion permeability in Scn2a(K1422E) mice

Genetic variants in SCN2A, encoding the Na(V)1.2 voltage-gated sodium channel, are associated with a range of neurodevelopmental disorders with overlapping phenotypes. Some variants fit into a framework wherein gain-of-function missense variants that increase neuronal excitability lead to developmental and epileptic encephalopathy, while loss-of-function variants that reduce neuronal excitability lead to intellectual disability and/or autism spectrum disorder (ASD) with or without co-morbid seizures. One unique case less easily classified using this framework is the de novo missense variant SCN2A-p.K1422E, associated with infant-onset developmental delay, infantile spasms and features of ASD. Prior structure–function studies demonstrated that K1422E substitution alters ion selectivity of Na(V)1.2, conferring Ca(2+) permeability, lowering overall conductance and conferring resistance to tetrodotoxin (TTX). Based on heterologous expression of K1422E, we developed a compartmental neuron model incorporating variant channels that predicted reductions in peak action potential (AP) speed. We generated Scn2a(K1422E) mice and characterized effects on neurons and neurological/neurobehavioral phenotypes. Cultured cortical neurons from heterozygous Scn2a(K1422E/+) mice exhibited lower current density with a TTX-resistant component and reversal potential consistent with mixed ion permeation. Recordings from Scn2a(K1442E/+) cortical slices demonstrated impaired AP initiation and larger Ca(2+) transients at the axon initial segment during the rising phase of the AP, suggesting complex effects on channel function. Scn2a(K1422E/+) mice exhibited rare spontaneous seizures, interictal electroencephalogram abnormalities, altered induced seizure thresholds, reduced anxiety-like behavior and alterations in olfactory-guided social behavior. Overall, Scn2a(K1422E/+) mice present with phenotypes similar yet distinct from other Scn2a models, consistent with complex effects of K1422E on Na(V)1.2 channel function.