SCN1A splice variants exhibit divergent sensitivity to commonly used antiepileptic drugs

A common genetic variant (rs3812718) in a splice donor consensus sequence within the neuronal sodium channel gene SCN1A (encoding Na(V) 1.1) modulates the proportion of transcripts incorporating either the canonical (5A) or alternative (5N) exon 5. A pharmacogenetic association has been reported whereby increased expression of exon 5N containing Na(V) 1.1 transcripts correlated with lower required doses of phenytoin in epileptics. We tested the hypothesis that SCN1A alternative splicing affects the pharmacology of Na(V) 1.1 channels.

To directly examine biophysical and pharmacologic differences between the exon 5 splice variants, we performed whole-cell patch clamp recording of tsA201 cells transiently coexpressing either Na(V) 1.1-5A or Na(V) 1.1-5N with the β1 and β2 accessory subunits. We examined tonic inhibition and use-dependent inhibition of Na(V) 1.1 splice isoforms by phenytoin, carbamazepine, and lamotrigine. We also examined the effects of phenytoin and lamotrigine on channel biophysical properties and determined concentration-response relationships for both splice variants.

We observed no significant differences in voltage dependence of activation, steady-state inactivation, and recovery from inactivation between splice variants. However, Na(V) 1.1-5N channels exhibited enhanced tonic block by phenytoin and lamotrigine compared to Na(V) 1.1-5A. In addition, Na(V) 1.1-5N exhibited enhanced use-dependent block by phenytoin and lamotrigine across a range of stimulation frequencies and concentrations. Phenytoin and lamotrigine induced shifts in steady-state inactivation and recovery from fast inactivation for both splice isoforms. No splice isoform differences were observed for channel inhibition by carbamazepine.

These results suggest Na(V) 1.1 channels containing exon 5N are more sensitive to the commonly used antiepileptic drugs phenytoin and lamotrigine.