NM_001165963.4(SCN1A):c.962C>A (p.Ser321Ter)

For SCN1A c.962C>A (p.Ser321Ter), a nonsense variant in the S5-S6 linker region of Nav1.1 domain I that creates a premature stop codon and complete loss-of-function from the mutant allele, the TANGO (Targeted Augmentation of Nuclear Gene Output) ASO zorevunersen represents the most clinically advanced precision therapy. Rather than correcting the mutation directly, zorevunersen targets the SCN1A non-productive alternatively spliced transcript (poison exon 20N) to increase productive mRNA output from the intact wild-type allele. This allele-independent upregulation strategy is ideal for haploinsufficiency variants like p.Ser321Ter, as it leverages the remaining functional allele to restore Nav1.1 sodium channel density in GABAergic inhibitory interneurons toward normal levels.

The MONARCH pivotal trial (Han et al., NEJM 2024; PMID:38587247) demonstrated that intrathecal zorevunersen significantly reduced convulsive seizure frequency in Dravet syndrome patients compared to sham procedure over a 16-week treatment period. The TANGO mechanism targets a naturally occurring non-productive splice variant of SCN1A containing poison exon 20N, which introduces a premature termination codon leading to NMD. By blocking inclusion of this poison exon with an ASO, zorevunersen shifts the splicing ratio toward productive SCN1A mRNA, effectively upregulating functional Nav1.1 protein from the wild-type allele. Preclinical studies in Scn1a+/- mice showed that this approach increases Nav1.1 protein levels and reduces seizure susceptibility (Lenk et al., Sci Transl Med 2020; PMID:32461334). For p.Ser321Ter specifically, this variant creates a stop codon in exon 7, within domain I of the Nav1.1 channel, truncating the protein before any of the four voltage-sensing or pore-forming domains are complete. The truncated product is non-functional and likely degraded by NMD, making this a clear haploinsufficiency allele. ClinVar classifies it as Pathogenic.

Zorevunersen requires repeated intrathecal administration (lumbar puncture every few months), imposing significant procedural burden on young children. The MONARCH trial, while positive, showed a 49% median reduction in convulsive seizures — meaningful but not seizure freedom for most patients. Whether TANGO-mediated SCN1A upregulation can fully compensate for 50% allele loss is unclear, as the degree of poison exon inclusion (and thus the therapeutic ceiling of this approach) varies between individuals. Long-term safety of chronic ASO administration in the pediatric CNS is being evaluated but not yet established over multi-year timeframes. Seizure frequency may not capture all relevant Dravet outcomes (cognitive development, SUDEP risk, quality of life). For p.Ser321Ter specifically, the therapeutic response should be equivalent to other loss-of-function variants, but individual variation in poison exon utilization could affect response magnitude. The approach does not address potential gain-of-function aspects of some SCN1A missense variants, though this is not relevant for a nonsense variant like p.Ser321Ter.