Cell-Selective Adeno-Associated Virus-Mediated <i>SCN1A</i> Gene Regulation Therapy Rescues Mortality and Seizure Phenotypes in a Dravet Syndrome Mouse Model and Is Well Tolerated in Nonhuman Primates

Abstract

Dravet syndrome (DS) is a developmental and epileptic encephalopathy caused by monoallelic loss-of-function variants in the <i>SCN1A</i> gene. <i>SCN1A</i> encodes for the alpha subunit of the voltage-gated type I sodium channel (Na_V1.1), the primary voltage-gated sodium channel responsible for generation of action potentials in GABAergic inhibitory interneurons. In these studies, we tested the efficacy of an adeno-associated virus serotype 9 (AAV9) <i>SCN1A</i> gene regulation therapy, AAV9-RE^GABA-eTF^<i>SCN1A</i>, designed to target transgene expression to GABAergic inhibitory neurons and reduce off-target expression within excitatory cells, in the <i>Scn1a</i>^+/- mouse model of DS. Biodistribution and preliminary safety were evaluated in nonhuman primates (NHPs). AAV9-RE^GABA-eTF^<i>SCN1A</i> was engineered to upregulate <i>SCN1A</i> expression levels within GABAergic inhibitory interneurons to correct the underlying haploinsufficiency and circuit dysfunction. A single bilateral intracerebroventricular (ICV) injection of AAV9-RE^GABA-eTF^<i>SCN1A</i> in <i>Scn1a</i>^+/- postnatal day 1 mice led to increased <i>SCN1A</i> mRNA transcripts, specifically within GABAergic inhibitory interneurons, and Na_V1.1 protein levels in the brain. This was associated with a significant decrease in the occurrence of spontaneous and hyperthermia-induced seizures, and prolonged survival for over a year. In NHPs, delivery of AAV9-RE^GABA-eTF^<i>SCN1A</i> by unilateral ICV injection led to widespread vector biodistribution and transgene expression throughout the brain, including key structures involved in epilepsy and cognitive behaviors, such as hippocampus and cortex. AAV9-RE^GABA-eTF^<i>SCN1A</i> was well tolerated, with no adverse events during administration, no detectable changes in clinical observations, no adverse findings in histopathology, and no dorsal root ganglion-related toxicity. Our results support the clinical development of AAV9-RE^GABA-eTF^<i>SCN1A</i> (ETX101) as an effective and targeted disease-modifying approach to SCN1A⁺ DS.