Abstracts

Rationale: Developmental and epileptic encephalopathies (DEEs) are rare genetic disorders characterized by refractory seizures and developmental delay. De novo mutations of SCN8A, encoding the voltage-gated sodium channel Na_v1.6, are responsible for >600 cases of DEE. The most common pathogenic mechanism of SCN8A-DEE is gain-of-function mutation causing premature channel opening or impaired channel inactivation. In a conditional animal model, intracerebroventricular administration of an antisense oligonucleotide (ASO) at postnatal day 2 resulted in down-regulation of Scn8a expression (Lenk et al, Ann Neurol 2020).  A single treatment delayed onset of seizures for 6 weeks, and multiple treatments extended the period of effectiveness.  We extended these studies to evaluate the administration of ASO after the onset of seizures, as a model of clinical application.  We also explored the efficacy of viral administration of an anti-Scn8a shRNA for extended down-regulation of Scn8a.  

Methods: Within a few days of the first observed seizure, Scn8a mutant mice expressing the DEE mutation N1768D were treated by ICV injection of Scn8a ASO. Seizure onset and length of survival were monitored in treated and control mice. An AAV10-Scn8a-shRNA virus that down-regulates Scn8a expression (Wong et al, Sci. Repts. 2018) was administered by ICV or by stereotactic injection into CA1, CA3 and dentate gyrus of the hippocampus.

Results: Survival of Scn8a^N1768D/+ mice was prolonged by administration of the ASO after seizure onset.  Administration of the AAV10-Scn8a-shRNA into a more severely affected Scn8a^R1872W/+ mouse line by ICV at P1 prevented seizures and extended survival beyond the time observed for ASO treatment. Scn8a-shRNA treatment at P1 also rescued survival of Dravet Syndrome mice with haploinsufficiency of Scn1a. Stereotactic administration of AAV10-Scn8a-shRNA to hippocampus of adult Scn8a^N1768D/+ mice resulted in 60% reduction of Scn8a transcript, demonstrating the potential for regional application of shRNA therapy.

Conclusions: These studies confirm the effectiveness of down-regulation of Scn8a to delay seizures and death in DEE-mice. The effectiveness of ASO administration after seizure onset indicates the absence of irreversible changes during prenatal and early postnatal development in Scn8a mutant mice.  Preliminary observations of administration of viral shRNA indicate that extended downregulation of Scn8a can be achieved. 

Funding: Supported by NIH R01 NS34509 and the Dravet Syndrome Foundation