Rationale:
Dravet syndrome (DS) is a severe and progressive developmental and epileptic encephalopathy characterized by high seizure frequency and severity, intellectual disability, and a high risk of sudden unexpected death in epilepsy (SUDEP). Approximately 85% of DS cases are caused by spontaneous, heterozygous loss of function mutations in the SCN1A gene which encodes the voltage-gated sodium channel α subunit NaV1.1. We developed a novel therapeutic approach to treat DS using antisense oligonucleotides (ASOs) to increase the endogenous expression of SCN1A mRNA and NaV1.1 protein. We are testing this approach using the F2:129S-Scn1a+/- x C57BL/6J DS mouse model that has been shown previously to recapitulate many patient phenotypes.
Method:
Targeted Augmentation of Nuclear Gene Output (TANGO), which modulates naturally occurring, non-productive splicing events to increase target protein expression, was used to screen and identify an ASO (STK-001) that can increase productive Scn1a mRNA and NaV1.1 protein in vivo. We administered 20µg TANGO ASO or PBS control to DS mice and wildtype (WT) littermates by single intracerebroventricular (ICV) injection at postnatal day (P)2. Mice were monitored by electroencephalography (EEG) pre (P13-19) and post (P20-40) weaning. DS mice were also crossed with WT mice hemizygous for a parvalbumin (PV)-tdTomato fluorescent reporter to produce DS mice expressing the tdTomato specifically in PV expressing interneurons. These mice received 20µg ASO or PBS by ICV administration at P2 and then electrophysiological recordings were taken from tdTomato expressing cells in the somatosensory cortex between P17 and 23.
Results:
No seizures were detected in any of the mice prior to day 16. Between P16-19, ASO-injected DS mice continued seizure free (0/5 animals) whereas 50% (4/8 animals) of PBS-injected control DS mice had seizures. When assessed after weaning (P20-40), 2/11 P2 ASO injected DS mice developed seizures and one animal died while 9/11 P2 PBS-injected control DS mice developed seizures and seven animals died. The firing frequency of PV interneurons in DS mice assessed between P17 to P23 was significantly impaired. DS PV interneurons fired at a significantly (P< 0.05) lower frequency than WT PV interneurons over a range of current injection steps. After ASO treatment at P2, the firing frequency of PV interneurons in DS mice was increased and was no longer significantly different from WT levels (Figure 1).
Conclusion:
These results provide evidence that increased Scn1a and NaV1.1 expression achieved using a TANGO ASO can greatly decrease seizures and death rates in a mouse model of SCN1A-linked DS. Further, the current data support the hypothesis that the improvement in DS phenotype is in part due to restoration of excitability of PV expressing interneurons.
Funding:
:Sponsored by Stoke Therapeutics
FIGURES
Figure 1