Divergent Pharmacological Properties of SCN1A Splice Variants

Voltage-gated sodium channels undergo alternative mRNA splicing. In the human neuronal Nav1.1 channel encoded by SCN1A, a common genetic variant affecting an intron splice donor site alters the proportion of transcripts that incorporate the canonical exon 5 (exon 5A) or an alternative (exon 5N) encoding portions of the S3 and S4 segments of domain 1. Epileptic subjects with this genetic variant require lower doses of anticonvulsant drugs such as phenytoin compared with individuals lacking this variant. Because this genetic variant is associated with a larger proportion of exon 5N containing transcripts in brain, we hypothesized that differences in function and pharmacology of Nav1.1 channels containing either exon 5N or 5A account for the observed divergence in anticonvulsant dose requirements. To examine differences in drug efficacy of SCN1A splice variants, we performed whole-cell recording on tsA201 cells transiently co-transfected with either Nav1.1-5A or Nav1.1-5N and two accessory subunits (β1,β2). We examined voltage-dependence of activation, steady-state inactivation, and recovery from fast inactivation and observed no significant differences between splice variants. We also measured both steady-state block and use-dependent block (10Hz) by phenytoin, carbamazepine, and lamotrigine. Nav1.1-5N channels exhibited greater steady-state block by phenytoin(100μM) (16±5% vs. 2±6%) and lamotrigine(200μM) (25±4% vs. 14±2%) compared to Nav1.1-5A. Additionally, Nav1.1-5N exhibited greater use-dependent block by phenytoin (39±5% vs. 24±4%) and lamotrigine (29±6% vs. 18±2%). We tested cells stably transfected with either Nav1.1-5A or Nav1.1-5N and both β subunits using an automated planar patch clamp system (Patchliner, Nanion Inc.) to perform concentration-response curves to determine steady-state and inactivated state affinities for lamotrigine. Similar to conventional patch clamp experiments, lamotrigine exhibited greater steady-state and inactivated state affinity for Nav1.1-5N than Nav1.1-5A. These results suggest SCN1A transcripts containing the alternative exon 5N encode channels that are more sensitive to multiple anticonvulsant drugs.