Base Editing Rescue of Seizures and SUDEP in SCN8A Developmental Epileptic Encephalopathy

Abstract

SCN8A encodes the voltage-gated sodium channel Nav1.6 which plays a key role in facilitating neuronal excitability. Mutations in SCN8A, particularly gain-of-function missense variants, are associated with SCN8A developmental and epileptic encephalopathy (DEE), a severe epilepsy syndrome characterized by spontaneous seizures, movement disorders, cognitive dysfunction, and sudden unexpected death in epilepsy (SUDEP). The recurrent SCN8A variant R1872W destabilizes inactivation of the sodium channel, resulting in neuronal hyperexcitability and onset of seizures. Current treatments, including anti-seizure medications (ASMs) that broadly target sodium channels, are often ineffective in SCN8A DEE patients and are associated with significant side effects, highlighting the need for targeted therapies. In this study, we utilized base editing as a therapeutic strategy to correct the patient derived R1872W SCN8A variant. Using two engineered mammalian cell line screens, we identified several targeting constructs that successfully reverted the R1872W variant to the reference allele. Our most effective construct, a modified adenine base editor, along with a paired successful guide RNA, was selected and packaged within a dual PhP.eB-adeno-associated virus (AAV) delivery system. This dual AAV therapy, referred to as SCN8A-ABE, was administered to mice expressing the R1872W variant at P2. Treatment with SCN8A-ABE significantly increased survival of mice expressing R1872W and either significantly reduced or completely inhibited seizure occurrence. Assessment of editing efficiencies revealed approximately 30% conversion of the mutant tryptophan to wildtype arginine observable in RNA transcripts from hippocampal and cortex tissue. Electrophysiological recordings revealed a rescue of seizure-associated neuronal hyperexcitability and a suppression of the pathogenic sodium channel behavior in treated mice. Associated comorbidities, including movement disorders and anxiety-like behaviors, were also improved in treated mice. These findings demonstrate the profound potential of base editing as a targeted and effective therapeutic approach for SCN8A DEE, addressing the underlying genetic mutation driving the disease.