Cellular and behavioral effects of altered NaV1.2 sodium channel ion permeability in Scn2aK1422E mice
Genetic variants in SCN2A, encoding the NaV1.2 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 infantile epileptic encephalopathy, while loss-of-function variants that reduce neuronal excitability lead to developmental delay and/or autism spectrum disorder with or without co- morbid seizures. One unique case less easily classified using this binary paradigm is the de novo missense variant SCN2A p.K1422E, associated with infant-onset developmental delay, infantile spasms, and features of autism spectrum disorder. Prior structure-function studies demonstrated that K1422E substitution alters ion selectivity of NaV1.2, conferring Ca2+ permeability, lowering overall conductance, and conferring resistance to tetrodotoxin (TTX). Based on heterologous expression of K1422E, we developed a compartmental neuron model that predicted mixed effects on channel function and neuronal activity. We also generated Scn2aK1422E mice and characterized effects on neurons and neurological/neurobehavioral phenotypes. Dissociated neurons from heterozygous Scn2aK1422E/+ mice exhibited a novel TTX-resistant current with a reversal potential consistent with mixed ion permeation. Cortical slice recordings from Scn2aK1442E/+ tissue demonstrated impaired action potential initiation and larger Ca2+ transients at the axon initial segment during the rising phase of the action potential, suggesting mixed effects on channel function. Scn2aK1422E/+ mice exhibited rare spontaneous seizures, interictal EEG abnormalities, altered response to induced seizures, reduced anxiety-like behavior and alterations in olfactory-guided social behavior. Overall, Scn2aK1422E/+ mice present with phenotypes similar yet distinct from Scn2a knockout models, consistent with mixed effects of K1422E on NaV1.2 channel function. Significance StatementThe early-onset epilepsy variant SCN2A-p.K1422E displays unique biophysical properties in vitro. To model the impact of this rare variant, we generated Scn2aK1422E mice. Neurons from heterozygous Scn2aK1422E/+ mice showed functional deficits similar to the loss-of-function effects observed in the Scn2a haploinsufficiency model, as well as gain-of-function effects specific to the K1422E variant. There is also some overlap in neurobehavioral phenotypes between Scn2aK1422E/+ and Scn2a haploinsufficient mice. However, Scn2aK1422E/+ mice exhibited unique epilepsy-related phenotypes, including epileptiform events and seizures. Scn2aK1422E/+ mice serve as a useful platform to investigate phenotypic complexity of SCN2A-associated disorders.