No abstract

Patients with SCN8A epileptic encephalopathy exhibit a range of clinical features, including multiple seizure types, movement disorders, and behavioral abnormalities, such as developmental delay, mild-to-severe intellectual disability, and autism. Recently, the de novo heterozygous SCN8A R1620L mutation was identified in an individual with autism, intellectual disability, and behavioral seizures without accompanying electrographic seizure activity. To date, the effects of SCN8A mutations that are primarily associated with behavioral abnormalities have not been studied in a mouse model. To better understand the phenotypic and functional consequences of the R1620L mutation, we used CRISPR/Cas9 technology to generate mice expressing the corresponding SCN8A amino acid substitution. Homozygous mutants exhibit tremors and a maximum lifespan of 22 days, while heterozygous mutants (RL/+) exhibit autistic-like behaviors, such as hyperactivity and learning and social deficits, increased seizure susceptibility, and spontaneous seizures. Current clamp analyses revealed a reduced threshold for firing action potentials in heterozygous CA3 pyramidal neurons and reduced firing frequency, suggesting that the R1620L mutation has both gain-and loss-of-function effects. In vivo calcium imaging using miniscopes in freely moving RL/+ mutants showed hyperexcitability of cortical excitatory neurons that is likely to increase seizure susceptibility. Finally, we found that oxcarbazepine and Huperzine A, a sodium channel blocker and reversible acetylcholinesterase inhibitor, respectively, were capable of conferring robust protection against induced seizures in RL/ + mutants. This mouse line will provide the opportunity to better understand the range of clinical phenotypes associated with SCN8A mutations and to develop new therapeutic approaches.

De novo loss‐of‐function mutations in SCN1A are the main cause of Dravet syndrome, a catastrophic encephalopathy characterized by recurrent early‐life febrile seizures, a number of other afebrile seizure types that are often refractory to treatment, and behavioral abnormalities including social deficits, motor dysfunction, and cognitive impairment. We previously demonstrated that the reversible acetylcholinesterase inhibitor, Huperzine A, increases seizure resistance in Scn1a mutants. In the present study, we evaluated the therapeutic potential of donepezil, a reversible acetylcholinesterase inhibitor approved by the Food and Drug Administration, in a mouse model of Dravet syndrome (Scn1a+/−). We found that donepezil conferred robust protection against induced seizures in Scn1a+/− mutants.

Mutations in the voltage‐gated sodium channel gene SCN8A cause a broad range of human diseases, including epilepsy, intellectual disability, and ataxia. Here we describe three mouse lines on the C57BL/6J background with novel, overlapping mutations in the Scn8a DIIS4 voltage sensor: an in‐frame 9 bp deletion (Δ9), an in‐frame 3 bp insertion (∇3) and a 35 bp deletion that results in a frameshift and the generation of a null allele (Δ35). Scn8a Δ9/+ and Scn8a ∇3/+ heterozygous mutants display subtle motor deficits, reduced acoustic startle response, and are resistant to induced seizures, suggesting that these mutations reduce activity of the Scn8a channel protein, Nav1.6. Heterozygous Scn8a Δ35/+ mutants show no alterations in motor function or acoustic startle response, but are resistant to induced seizures. Homozygous mutants from each line exhibit premature lethality and severe motor impairments, ranging from uncoordinated gait with tremor (Δ9 and ∇3) to loss of hindlimb control (Δ35). Scn8a Δ9/Δ9 and Scn8a ∇3/∇3 homozygous mutants also exhibit impaired nerve conduction velocity, while normal nerve conduction was observed in Scn8a Δ35/Δ35 homozygous mice. Our results suggest that hypomorphic mutations that reduce Nav1.6 activity will likely result in different clinical phenotypes compared to null alleles. These three mouse lines represent a valuable opportunity to examine the phenotypic impacts of hypomorphic and null Scn8a mutations without the confound of strain‐specific differences.