Just last month, a previous research review detailed how Stoke Therapeutics developed their targeted augmentation of nuclear gene output (TANGO), utilizing anti-sense oligonucleotides (ASOs) to upregulate expression of several genes, including SCN1A, in human cells and mouse brain. ASOs are single-stranded RNAs that bind to target RNA sequences and have the potential to alter the regulation of those target RNAs. The TANGO approach uses ASOs to decrease the amount of mRNA transcripts that include a “non-productive exon.” Non-productive exons, often referred to as “poison exons,” target the mRNA transcript to be destroyed, rather than utilized to produce a healthy protein. By decreasing the use of this “non-productive” exon in SCN1A using the TANGO approach with ASO-22, the amount of SCN1A mRNA that produces Nav1.1 is increased, and, in turn, the levels of the Nav1.1 sodium channel increase. Many patients with Dravet syndrome only have about half of the levels of Nav1.1 that are needed due to mutations in the SCN1A gene (called haploinsufficieny), making this an ideal disease modifying approach. The current study sought to show the pre-clinical efficacy of the TANGO approach to restore levels of SCN1A/Nav1.1 in a mouse model of Dravet syndrome.

Published in late August in the journal Science Translational Medicine, the study of TANGO in a mouse model of Dravet was a collaboration between Stoke Therapeutics and Lori Isom’s laboratory at the University of Michigan. The results are highlighted below and links to the paper can be found here.

ASO-22 decreased SUDEP in Dravet mice. When Dravet mice received ASO-22 injections into the brain at two-days old (before disease onset), it increased the survival of mice from only 23% in control-treated Dravet mice to 97% survival in ASO-22-treated mice when measured out to 90-days. When mice were injected with ASO-22 at 14-days old (closer to the onset of disease), 64% of Dravet mice that received a control injection survived to 90-days of age, while 85% of the ASO-22 treated Dravet mice survived.

ASO-22 increased SCN1A mRNA and Nav1.1 protein in mouse brain. Levels of SCN1A mRNA and the encoded sodium channel, Nav1.1, were significantly increased for up to 14 weeks following the single injection at two-days old of ASO-22 in both Dravet and control mice. The increase in SCN1A mRNA and the Nav1.1 sodium channel was detected at 90-days of age following the 14-day old injection of ASO-22, but Nav1.1 was not increased at an earlier timepoint (35 days). Researchers are not sure of why this difference was seen following the injection at 14-days, but postulate that it may be due to dosage, brain size, and efficacy of the ASO to integrate as efficiently at older ages in mice. While dosing at both 2- and 14-days of age increased Nav1.1 expression at the 90-day time-point, ASOs are not permanent and repeated dosing would be necessary to maintain efficacy as a long-term therapy.

ASO-22 modulated some seizure activity in Dravet mice. Seizure onset was delayed in mice injected with ASO-22 at two-days old. ASO-22 also reduced the frequency of behavioral and EEG-measured seizure activity in Dravet mice injected at both 2-days and 14-days old. A few mice that did experience seizure activity in the ASO-22 treatment group died subsequently due to SUDEP, suggesting that while the frequency of seizures was reduced, the severity of seizures was not affected by ASO-22. In addition, while behavioral seizure frequency was decreased, when seizure activity did occur, it could still be detected by EEG and looked similar to seizures in Dravet mice receiving the control-treatment.

ASO-22 treatment appears specific to SCN1A. Researchers did not detect any subsequent changes in other voltage-gated sodium channels in response to the increased SCN1A/Nav1.1 following ASO-22 treatment, indicating ASO-22 acts specifically on the SCN1A gene.

ASO-22 increased Nav1.1 in healthy mice without affecting survival or measured brain activity. Treatment of healthy control mice with ASO-22 did not affect survival or EEG activity despite increasing Nav1.1 above normal physiological levels. While still lacking a more in-depth analysis, these results suggest having “too much” healthy Nav1.1 may be tolerated.

The preclinical success of ASO-22 in reducing SUDEP and behavioral seizures in a mouse model of Dravet holds promise for this approach to be efficacious as a disease modifying therapy in human individuals diagnosed with Dravet syndrome that carry mutations in the SCN1A gene leading to haploinsufficiency. It is important to remember that while these mice display many features of Dravet syndrome (SUDEP, severe seizures) there are many differences including a much higher rate of SUDEP in mice (~50%) than is seen in human patients with Dravet syndrome (15-20%). The timing of disease in Dravet mice may also not translate directly to the progression of Dravet syndrome in humans, cautioning interpretations of the age-at-dose of ASO-22 to modify disease. Despite the limitations, these studies support the TANGO approach as a highly specific treatment for Dravet syndrome that has potential to truly modify the significant health outcomes for patients.

As of August 2020, at least one human patient with Dravet syndrome has been dosed with Stoke Therapeutics’ TANGO approach, STK-001. You can read more about the MONARCH trial for STK-001 at clincaltrials.gov  or at monarchstudy.com.

Original Article: Han Z, Chen C, Christiansen A, Ji S, Lin Q, Anumonwo C, Liu C, Leiser SC, Meena, Aznarez I, Lia G, and Isom LL. (2020) Antisense oligonucleotides increase Scn1a expression and reduce seizures and SUDEP incidence in a mouse model of Dravet Syndrome. Science Translational Medicine. 12:eaaz6100. doi: 10.1126/scitranslmed.aaz6100