Paradoxical hyperexcitability from Na V 1.2 sodium channel loss in neocortical pyramidal cells

Loss-of-function variants in the gene SCN2A , which encodes the sodium channel Na V 1.2, are strongly associated with autism spectrum disorder and intellectual disability. An estimated 20-30% of children with these variants are co-morbid for epilepsy, with altered neuronal activity originating in neocortex, a region where Na V 1.2 channels are expressed predominantly in excitatory pyramidal cells. This is paradoxical, as sodium channel loss in excitatory cells would be expected to dampen neocortical activity rather than promote seizure. Here, we examined pyramidal neurons lacking Na V 1.2 channels and found that they were intrinsically hyperexcitable, firing high-frequency bursts of action potentials (APs) despite decrements in AP size and speed. Compartmental modeling and dynamic clamp recordings revealed that Na V 1.2 loss prevented potassium channels from properly repolarizing neurons between APs, increasing overall excitability by allowing neurons to reach threshold for subsequent APs more rapidly. This cell-intrinsic mechanism may therefore account for why SCN2A loss-of-function can paradoxically promote seizure.