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RATIONALE: 

The clinical spectrum associated with SCN2A de novo mutations includes early and late seizure onset developmental and epileptic encephalopathy (DEE) as well as autism. Patients with early seizure-onset DEE are most frequently associated with SCN2A variants that show gain of function biophysical changes that would lead to enhanced neuronal excitability. We hypothesized that reduced expression of SCN2A in such gain of function cases would be clinically beneficial. To explore this concept a mouse model harboring a recurrent human early seizure onset DEE variant (human R1882Q) was generated and showed a strong seizure phenotype as early as P1 and severe mortality with survival rarely extending beyond P22. A gapmer ASO targeting mouse Scn2a was developed to assess efficacy in this mouse model.

METHODS:

Scn2a ASO was injected into the right intracerebral ventricles of mice heterozygous for the human R1882Q mutation. Efficacy was evaluated by survival, seizure number,electroencephalography (EEG), behavioral test batteries and whole cell recording in brain slices.

RESULTS:

Scn2a R1882Q “DEE” mice display seizures as early as postnatal day 1 and die before postnatal day 30. A single ICV injection of Scn2a ASO at postnatal day 1 significantly extended the lifespan. Survival was 29% on postnatal day 21 for untreated DEE mice compared to 95% inDEE mice treated with Scn2a ASO. The therapeutic effect of Scn2a ASO treatment was long lasting, with 69% of treated DEE mice surviving up to postnatal day 80. Scn2a ASO treatment also mitigated spontaneous seizures and restored the EEG activity of the DEE mice to that of wildtype. Scn2a ASO treated DEE mice performed similarly to wild type mice across a range of locomotor and social interaction tests. Efficacy of Scn2a ASO treatment was also evident at the single neuron level. Whole cell patch clamp recording showed that excitatory neurons expressing the Scn2a R1882Q mutation are considerably more excitable than wildtype neurons.Consistent with the observed in vivo efficacy, neurons from Scn2a ASO treated DEE mice had identical action potential input-output curves to wildtype neurons suggesting that intrinsic excitability could be restored.

CONCLUSIONS:

This study demonstrates the remarkable efficacy of Scn2a down regulation in rescuing the phenotype of a Scn2a gain-of-function DEE mouse model and has laid an important foundation for clinical development.

FUNDING: RogCon Biosciences