Cell-Selective Adeno-Associated Virus-Mediated SCN1A 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 SCN1A gene. SCN1A encodes for the alpha subunit of the voltage-gated type I sodium channel (Na V 1.1), 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) SCN1A gene regulation therapy, AAV9-RE GABA -eTF SCN1A , designed to target transgene expression to GABAergic inhibitory neurons and reduce off-target expression within excitatory cells, in the Scn1a +/− mouse model of DS. Biodistribution and preliminary safety were evaluated in nonhuman primates (NHPs). AAV9-RE GABA -eTF SCN1A was engineered to upregulate SCN1A 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 SCN1A in Scn1a +/− postnatal day 1 mice led to increased SCN1A mRNA transcripts, specifically within GABAergic inhibitory interneurons, and Na V 1.1 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 SCN1A 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 SCN1A 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 SCN1A (ETX101) as an effective and targeted disease-modifying approach to SCN1A + DS.

A hyperthermic seizure unleashes a surge of spreading depolarizations inScn1adeficient mice

“…In other words, the evolution of Scn1a deficient encephalopathy may reflect sustained proexcitatory plasticity mechanisms rather than being solely determined by a static developmental defect. This view is consistent with studies demonstrating that postnatal conditional Scn1a deletion can create an epileptic encephalopathy similar to the genomic model 56,57 , and also that the disease phenotype can be reversed or ameliorated when Scn1a expression is recovered at a juvenile age 43,58,59 .…”

“…This evidence suggests that disease progression by a febrile seizure can occur independent of developmental stages, and that the exacerbation of phenotype is not solely a developmental defect, but likely involves a proexcitatory plasticity mechanism. This is consistent with studies demonstrating that postnatal Scn1a deletion can create an epileptic encephalopathy similar to the genomic model 32,33 , and also that the disease phenotype can be reversed or ameliorated when Scn1a expression is recovered at a juvenile age [34][35][36] . These findings suggest a lifelong risk of exacerbation of disease to severe epilepsy encephalopathy, but also a potential for reversal of disease if the underlying mechanism can be appropriately targeted.…”

Heat-induced seizures, premature mortality, and hyperactivity in a novelScn1anonsense model for Dravet syndrome

“…Multiple studies have used DS mice models to examine the therapeutic potential of current and novel drug treatments (Hawkins et al, 2017; Han et al, 2020; Isom and Knupp, 2021; Pernici et al, 2021; Tanenhaus et al, 2022). Our characterization highlights several readouts that may be useful in future studies to examine the therapeutic benefit while using Scn1a WT/R613X mice on a mixed C57BL/6J: 129S1/SvImJ background.…”

Genetic therapeutic advancements for Dravet Syndrome

“…In vitro studies conducted on human-induced pluripotent stem cell-derived GABAergic interneurons led to increased SCN1A mRNA expression and Nav1.1 protein [79]. When administered to P1 Scn1a+/À DS mice via bilateral ICV, this resulted in increased survival, significant improvement in hyperthermic seizure threshold, and reduction in spontaneous seizures [80]. Furthermore, bio-distributional and safety studies were conducted in non-human primates (NHP).…”