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

Gain-of-function variants in SCN8A, which encodes the Na_V1.6 sodium channel, lead to epilepsy syndromes ranging from drug-responsive self-limited and intermediate epilepsy to drug-resistant developmental and epileptic encephalopathy (DEE). It is currently unclear why individuals with SCN8A gain-of-function variants show variable responses to sodium channel blockers (SCBs). Here, we compared the clinical characteristics of 173 individuals with 25 different SCN8A gain-of-function variants. We found that individuals with variants altering channel activation gating were more severely affected than those with variants altering inactivation properties: They had an earlier age at onset (3 vs. 5 months, P < 0.0001), higher prevalence of DEE (75% vs. 39%; P < 0.0001), and poorer response to SCBs (20% vs. 69% seizure free; P < 0.0001). We performed pharmacological studies on representative and recurrent variants from each group: two variants (F846S and M1760I) causing hyperpolarizing shifts of the voltage-dependent activation curves, and two variants (G1475R and N1877S) causing depolarizing shifts of the voltage-dependent fast inactivation curves. Phenytoin failed to suppress neuronal firing in neurons expressing activation-related variants, but showed good suppressing effects in neurons expressing inactivation-related variants. In contrast, PRAX-330, a new SCB, which showed much faster binding rates than phenytoin, was effective for both groups of variants by markedly reducing neuronal firing through rapidly and persistently stabilizing Na_V1.6 in the inactivated state. Our findings provide new pharmacogenetic insights into the mechanism of drug-resistance in SCN8A related epilepsy and support PRAX-330 and compounds with similar pharmacological properties as promising candidates for targeted therapies.