Seizure phenotype and underlying cellular defects in drosophila knock-in models of DS (R1648C) and GEFS1 (R1648H) SCN1A epilepsy

Abstract

Mutations in the voltage-gated sodium channel gene SCN1A are associated with human epilepsy disorders, but how most of these mutations alter channel properties and result in seizures is unknown. This study fo-cuses on two different mutations occurring at one position within SCN1A. R1648C (R-C) is associated with the severe disorder Dravet syndrome, and R1648H (R-H), with the milder disorder GEFS1. To explore how these different mutations contribute to distinct seizure disorders, Drosophila lines with the R-C or R-H mutation, or R1648R (R-R) control substitution in the fly sodium channel gene para were generated by CRISPR-Cas9 gene editing. The R-C and R-H mutations are homozygous lethal. Animals heterozygous for R-C or R-H mutations displayed reduced life spans and spontaneous and temperature-induced seizures not observed in R-R con-trols. Electrophysiological recordings from adult GABAergic neurons in R-C and R-H mutants revealed the ap-pearance of sustained neuronal depolarizations and altered firing frequency that were exacerbated at elevated temperature. The only significant change observed in underlying sodium currents in both R-C and R-H mutants was a hyperpolarized deactivation threshold at room and elevated temperature compared with R-R con-trols. Since this change is constitutive, it is likely to interact with heat-induced changes in other cellular properties to result in the heat-induced increase in sustained depolarizations and seizure activity. Further, the similarity of the behavioral and cellular phenotypes in the R-C and R-H fly lines, suggests that disease symp-toms of different severity associated with these mutations in humans could be due in large part to differences in genetic background.