Abstracts

Rationale: Mutations in SCN2A, which encodes the NaV1.2 voltage-gated sodium channel, can result in a wide spectrum of epilepsy severity ranging from benign neonatal infantile seizures to early-onset epileptic encephalopathy. Only a small fraction of known SCN2A variants have been investigated for their functional consequences. Here we present a case of a patient with severe early-onset epileptic encephalopathy associated with a novel, de novo SCN2A variant predicting substitution of a highly conserved methionine residue within the distal carboxyl terminus to threonine (p.M1879T). We investigated the effect of this mutation using heterologously expressed NaV1.2 channels. Methods: The M1879T mutation was engineered in a recombinant human NaV1.2 plasmid. Plasmids encoding NaV1.2 wild-type or M1879T were co-transfected into HEK293T cells along with human β1 and β2 subunits. Whole-cell voltage clamp recording was performed using established protocols for assessing current-voltage and conductance-voltage relationships, kinetics and voltage-dependence of inactivation, and the kinetics of recovery from inactivation. Results: A 2-month old male presented with apneic events with cyanosis in the context of daily tonic seizures. The patient continued to experience daily seizures despite levetiracetam, topiramate, and pyridoxine. The child became seizure free after an intravenous loading dose of fosphenytoin. A novel de novo SCN2A variant of unknown significance was identified (c.5636T->C, p.M1879T). After discovery of the SCN2A variant, the patient’s treatment was modified to carbamazepine with a resulting decrease in seizure frequency. When expressed in HEK293T cells, the M1879T mutant channel exhibited a significant depolarizing shift in the voltage-dependence of inactivation compared to wild-type NaV1.2. The time course of inactivation was significantly slower in cells expressing M1879T compared to wild-type channel. There was no significant difference in the voltage-dependence of activation between M1879T and wild-type channels, and there was no difference in the kinetics of recovery from inactivation. Conclusions: A novel de novo SCN2A mutation (p.M1879T) associated with early-onset epileptic encephalopathy causes impaired kinetics and voltage-dependence of channel inactivation. We interpret these data as indicating a gain of channel function which is consistent with the pathogenesis of epilepsy and the observed clinical efficacy of the sodium channel blocking anticonvulsant drugs. Our findings further demonstrate the value of conducting functional analyses of SCN2A variants of unknown significance to establish pathogenicity and genotype-phenotype correlations. Funding: U54-NS108874, R25-NS070695