In vivo adenine base editing ameliorates Dravet syndrome phenotypes in a mouse model

Dravet syndrome (DS) is a severe neurodevelopmental disorder characterized by drug-resistant epilepsy, temperature-sensitive seizures, cognitive impairment, and a high incidence of sudden unexpected death in epilepsy (SUDEP). DS is caused by loss-of-function variants in SCN1A, which encodes the α subunit of the voltage-gated sodium channel (Na_v1.1). Current approved treatments manage symptoms of DS but do not correct the root cause of the disease. Here, we describe the use of an adenine base editor (ABE) to directly correct SCN1A^R613X, a recurrent variant found in patients with DS. We identified ABE strategies to efficiently correct R613X in engineered homozygous SCN1A^R613X human embryonic kidney 293T and mouse Neuro-2a cells (72 and 92% correction efficiencies, respectively). We then used a dual-adeno-associated virus serotype 9 (AAV9) approach to deliver an optimized ABE system to Scn1a^R613X/+ mice, which recapitulate several key DS pathologies. AAV9-ABE treatment of Scn1a^R613X/+ neonates resulted in efficient DNA and mRNA editing (59 and 97%, respectively, in bulk neocortices), restoring parvalbumin-expressing inhibitory neuron excitability and sodium current to wild-type levels. This ameliorated both spontaneous and temperature-induced seizures and led to a 3.3-fold improvement in 45-day survival over vehicle-treated mice (ABE treated, 90%; and vehicle treated, 27%). Last, ABE treatment in 12-day-old mice resulted in a 3.0-fold improvement in 60-day survival over vehicle-treated mice (ABE treated, 82%; and vehicle treated, 27%). In conclusion, these data validate a strategy to correct SCN1A variants with ABE and highlight the potential of precision genome editing treatments for the treatment of DS and possibly other neurodevelopmental disorders.