NM_001165963.4(SCN1A):c.3075T>A (p.Tyr1025Ter)

For SCN1A c.3075T>A (p.Tyr1025Ter), a pathogenic nonsense variant in the domain II-III cytoplasmic linker of Nav1.1, adenine base editing (ABE) offers the most direct correction strategy: reverting the premature TAA stop codon on the complementary strand (TTA→TAT→TAC at DNA level, effectively A→G on the target strand) to restore the original tyrosine codon. Unlike ASO-mediated upregulation approaches (which increase wild-type Nav1.1 from the normal allele), base editing would correct the disease allele itself, restoring full biallelic Nav1.1 expression in the corrected neurons.

Dravet syndrome is caused by heterozygous SCN1A loss-of-function, resulting in Nav1.1 haploinsufficiency predominantly in GABAergic inhibitory interneurons, leading to impaired inhibitory neurotransmission and seizures. The p.Tyr1025Ter variant creates a premature stop in the large intracellular loop between domains II and III. ABE8e and newer adenine base editors achieve A-to-G conversions with high efficiency (often >50-80%) in mammalian cells and have been demonstrated in vivo via AAV delivery to the mouse brain. For this specific variant, the antisense strand contains the targetable adenine within a TAA stop codon; ABE could convert it to restore a sense codon. Bhatt et al. have shown that readthrough-inducing ASOs can suppress premature stop codons in neuronal genes (PMID:39011883), providing a complementary RNA-level approach, but base editing offers permanent correction. CRISPR-based SCN1A activation (CRISPRa) using catalytically dead Cas9 fused to transcriptional activators has also shown efficacy in Dravet mouse models (Colasante et al., Nature 2020; PMID:31996851), establishing proof-of-concept for genetic rescue of Nav1.1 levels in vivo.

In vivo brain base editing faces major delivery challenges: the full ABE cassette (~5.3 kb for ABE8e + Cas9) fits AAV only as a split-intein dual vector, adding complexity. Neuron-specific transduction across the brain is required because SCN1A haploinsufficiency in even a subset of interneurons can cause seizures. Off-target A-to-G editing at bystander adenines in the editing window (positions 4-8 of the protospacer) could introduce unintended missense mutations in SCN1A, potentially creating a gain-of-function channel variant with different pathogenic consequences. The PAM site requirement (NGG for SpCas9, NRCH/NRN for relaxed-PAM variants) may constrain guide RNA design for this specific genomic position. No in vivo SCN1A base editing has been published in preclinical models. The approach is variant-specific — unlike ASO upregulation (which works for all haploinsufficiency variants), each nonsense variant requires a bespoke guide RNA, making regulatory and manufacturing scale-up challenging for an individually ultra-rare mutation.