Antisense oligonucleotides restore excitability, GABA signalling and sodium current density in a Dravet syndrome model

Dravet syndrome is an intractable developmental and epileptic encephalopathy caused by de novo variants in SCN1A resulting in haploinsufficiency of the voltage-gated sodium channel Nav1.1. We showed previously that administration of the antisense oligonucleotide STK-001, also called ASO-22, generated using targeted augmentation of nuclear gene output technology to prevent inclusion of the nonsense-mediated decay, or poison, exon 20N in human SCN1A, increased productive Scn1a transcript and Nav1.1 expression and reduced the incidence of electrographic seizures and sudden unexpected death in epilepsy in a mouse model of Dravet syndrome. Here, we investigated the mechanism of action of ASO-84, a surrogate for ASO-22 that also targets splicing of SCN1A exon 20N, in Scn1a+/- Dravet syndrome mouse brain. Scn1a +/- Dravet syndrome and wild-type mice received a single intracerebroventricular injection of antisense oligonucleotide or vehicle at postnatal Day 2. We examined the electrophysiological properties of cortical pyramidal neurons and parvalbumin-positive fast-spiking interneurons in brain slices at postnatal Days 21-25 and measured sodium currents in parvalbumin-positive interneurons acutely dissociated from postnatal Day 21-25 brain slices. We show that, in untreated Dravet syndrome mice, intrinsic cortical pyramidal neuron excitability was unchanged while cortical parvalbumin-positive interneurons showed biphasic excitability with initial hyperexcitability followed by hypoexcitability and depolarization block. Dravet syndrome parvalbumin-positive interneuron sodium current density was decreased compared to wild-type. GABAergic signalling to cortical pyramidal neurons was reduced in Dravet syndrome mice, suggesting decreased GABA release from interneurons. ASO-84 treatment restored action potential firing, sodium current density and GABAergic signalling in Dravet syndrome parvalbumin-positive interneurons. Our work suggests that interneuron excitability is selectively affected by ASO-84. This new work provides critical insights into the mechanism of action of this antisense oligonucleotide and supports the potential of antisense oligonucleotide-mediated upregulation of Nav1.1 as a successful strategy to treat Dravet syndrome.

Dravet Syndrome: Novel Approaches for the Most Common Genetic Epilepsy

Dravet syndrome (DS) is a severe developmental and epileptic encephalopathy that is mainly associated with variants in SCN1A. While drug-resistant epilepsy is the most notable feature of this syndrome, numerous symptoms are present that have significant impact on patients' quality of life. In spite of novel, third-generation anti-seizure treatment options becoming available over the last several years, seizure freedom is often not attained and non-seizure symptoms remain. Precision medicine now offers realistic hope for seizure freedom in DS patients, with several approaches demonstrating preclinical success. Therapeutic approaches such as antisense oligonucleotides (ASO) and adeno-associated virus (AAV)-delivered gene modulation have expanded the potential treatment options for DS, with some of these approaches now transitioning to clinical trials. Several of these treatments may risk the exacerbation of gain-of-function variants and may not be reversible, therefore emphasizing the need for functional testing of new pathogenic variants. The current absence of treatments that address the overall disease, in addition to seizures, exposes the urgent need for reliable, valid measures of the entire complement of symptoms as outcome measures to truly know the impact of treatments on DS. Additionally, with so many treatment options on the horizon, there will be a need to understand how to select appropriate patients for each treatment, whether treatments are complementary or adverse to each other, and long-term risks of the treatment. Nevertheless, precision therapeutics hold tremendous potential to provide long-lasting seizure freedom and even complete cures for this devastating disease.