Abstracts

Rationale: Dravet syndrome (DS) is a severe developmental disorder that begins in infancy and occurs in ≈1:16000 births worldwide. DS is characterized by prolonged, intractable seizures, significant developmental delays, and increased risk for sudden unexpected death in epilepsy (SUDEP) and childhood mortality. Roughly 80% of patients with DS have a loss-of-function mutation in the SCN1A gene, leading to 50% reduced function of the NaV1.1 protein in GABAergic interneurons. Existing therapies for DS reduce seizures but do not address the underlying disease mechanism-SCN1A haploinsufficiency. Viral gene therapy can achieve this; however, current technologies have limited cell-type selectivity and other technical challenges that restrict their use.  Methods: To address these limitations, we have developed an adenoassociated viral (AAV) vector containing an engineered transcription factor for the upregulation of endogenous SCN1A controlled by a GABA-selective human regulatory element. Heterozygous SCN1A knockout mice (SCN1A+/−) or wild-type (WT) littermates were treated with our AAV construct or vehicle control on postnatal day 1 (P1) via bilateral intracerebroventricular injection. Mice were assessed for hyperthermic seizure sensitivity on P28 and by video-electroencephalogram (v-EEG) continuously for 9 days beginning on P22 or P23; mice were also assessed for survival up to P350. Cohorts of mice were independently assessed for hyperthermic seizures, electrographic seizures, and SUDEP. Results: Hyperthermic seizures were significantly reduced with our AAV vector versus vehicle control (P < 0.0001; Figure 1), with only 5.6% of the SCN1A+/− mice treated with AAV on P1 (n = 18) experiencing a seizure at 42.5°C (black vertical line) versus 53.3% of the control-treated mice (n = 15). Electrographic seizure frequency significantly decreased with AAV administration versus control (0.34 seizures/mouse/day [n = 9] vs 1.09 seizures/mouse/day [n = 9]; P = 0.0026); the average duration of electrographic seizures/mouse/day also significantly decreased (16.4 s vs 44.5 s; P = 0.0018). Finally, treatment with our AAV construct (n = 34) significantly reduced the risk for SUDEP by 89% compared with control (n = 53) in this model (HR, 0.11 [95% CI: 0.06-0.23]; P < 0.0001), with median survival not yet reached for AAV-treated mice by day 350 after administration (Figure 2). No difference in survival was observed between WT littermates +- AAV and SCN1A+/− mice + AAV.  Conclusions: Our GABA-selective AAV vector significantly reduced hyperthermic seizures, decreased the frequency and duration of electrographic seizures, and significantly reduced the risk for SUDEP by 89% when administered to SCN1A+/− mice. The reversal of key phenotypes in an established mouse model of DS supports further clinical development for the treatment of SCN1A-positive pediatric epilepsy. Funding: This work was supported by Encoded Therapeutics, Inc.