Berecki, G.; Tao, E.; Howell, K. B.; Coorg, R. K.; Kahlig, K.; Wolff, M.; Corry, B.; Petrou, S.

Mutations in the SCN2A gene encoding the Nav1.2 sodium channel can lead to neurodevelopmental disorders. We studied the N1662D variant associated with severe early-onset developmental and epileptic encephalopathy (DEE). The N1662D mutation almost completely prevented fast inactivation without affecting activation. The comparison of wild-type and N1662D channel structures suggested that the ambifunctional hydrogen bond formation between residues N1662 and Q1494 is essential for fast inactivation. Fast inactivation could also be prevented with engineered Q1494A or Q1494L Nav1.2 channel variants, whereas Q1494E or Q1494K variants resulted in incomplete inactivation and persistent current. Molecular dynamics simulations revealed a reduced affinity of the hydrophobic IFM-motif to its receptor site with N1662D and Q1494L variants relative to wild-type. These results demonstrate that the interactions between N1662 and Q1494 underpin the stability and the orientation of the inactivation gate and are essential for the development of fast inactivation. Six DEE-associated Nav1.2 variants, with mutations mapped to channel segments known to be implicated in fast inactivation were also evaluated. Remarkably, the L1657P variant also prevented fast inactivation and produced biophysical characteristics similar to N1662D, whereas the M1501V, M1501T, F1651C, P1658S, and A1659V variants resulted in biophysical properties that were consistent with gain-of-function and enhanced action potential firing of hybrid neurons in dynamic action potential clamp experiments. Paradoxically, low densities of N1662D or L1657P currents potentiated action potential firing, whereas increased densities resulted in sustained depolarization. The contribution of non-inactivating Nav1.2 channels to neuronal excitability may constitute a novel cellular mechanism in the pathogenesis of SCN2A-related DEE.