Differential effects of sodium channel blockers on SCN8A gain-of-function variants associated with drug-responsive or -resistant epilepsy

SCN8A

Gain-of-function variants in SCN8A (NaV1.6 channel): variants altering channel activation respond poorly to sodium channel blockers, unlike variants altering inactivation

The authors compared the clinical features of 173 individuals carrying 25 different SCN8A gain-of-function variants. Carriers of variants altering channel activation gating were more severely affected than those with variants altering inactivation: earlier age at onset (3 vs 5 months), higher prevalence of developmental and epileptic encephalopathy (75% vs 39%) and markedly poorer response to sodium channel blockers (20% vs 69% seizure free). Pharmacological studies on four representative variants showed that phenytoin failed to suppress neuronal firing for activation-related variants (F846S, M1760I) but remained effective for inactivation-related ones (G1475R, N1877S). PRAX-330, a blocker with faster binding kinetics, reduced neuronal firing in both groups by stabilising NaV1.6 in the inactivated state.

Synthesis written by Geno'X. For the full original abstract, please refer to the source publication.

This is a genotype-drug response correlation that operates below the gene level: what predicts response to sodium channel blockers is not SCN8A itself but the biophysical mechanism of the variant. The demonstration pairs a reasonably sized clinical series with electrophysiological validation of both categories, making it more robust than the purely descriptive correlations usual in this field. These variants are already captured by exome or genome sequencing: the challenge is to annotate the activation or inactivation mechanism at interpretation, information no routine database currently carries.

Analysis by Dr Thibaut Benquey