Functional and pharmacological characterization of the SCN2A variant p.C258R with mixed gain and loss of function...

Pathogenic variants in SCN2A cause dysfunction of the Na_V1.2 voltage-gated sodium channel and are associated with neurodevelopmental disorders with or without epilepsy. Treatment of epilepsy in these patients is challenging and depends on whether the variant results in a gain (GoF) or loss of function (LoF). We describe a patient with the de novo SCN2A variant p.C258R presenting with therapy-resistant developmental and epileptic encephalopathy type 11 (DEE type 11). We aimed to electrophysiologically analyze the functional consequences of this mutation and to examine the in vitro properties of phenytoin, lamotrigine, and topiramate, which resulted in clinical improvement. Depending on the specific functional readout after transient expression in HEK293T cells, mutation C258R of Na_V1.2 channels resulted in a mixed LoF and GoF phenotype. Na_V1.2-C258R channels exhibited slower inactivation kinetics and a left-shifted voltage dependence of steady-state inactivation. Current density was reduced when expressed in HEK293T cells although single-channel conductance and maximal channel open probability were only marginally affected. During repetitive stimulation, mutated channels showed stronger cumulative inactivation, consistent with LoF. Phenytoin and lamotrigine caused use-dependent block, but lamotrigine was less effective for C258R compared with the wild type. Topiramate diminished the current density in both variants with less use-dependence. The C258R mutation impairs Na_V1.2 function through both LoF and GoF mechanisms. Sodium channel blockers with a strong use-dependence, such as phenytoin, may be preferential for seizure control in such cases.