Base Editing (BE4max) for PRNP c.385A>G (p.Met129Val) in Sporadic fatal insomnia

Base Editing (BE4max) via AAV9 delivery is a rationale-driven therapeutic strategy for Sporadic fatal insomnia targeting the PRNP c.385A>G (p.Met129Val) variant (Pathogenic, missense variant, 3 prime UTR variant). The editing system (BE4max (cytosine base editor)) converts the pathogenic C to T (or G to A on the target strand), restoring the wild-type codon. Target tissue: CNS. Therapeutic goal: Knock down or edit PRNP in CNS (thalamus-focused) to reduce PrP levels and prevent or slow prion propagation in sporadic fatal insomnia and related prion diseases. Risk profile: off-target Medium (bystander bases in editing window), delivery complexity Medium, immunogenicity High (AAV pre-existing immunity).

1. Molecular basis: PRNP NM_000311.5(PRNP):c.385A>G (p.Met129Val) is classified as Pathogenic (ClinVar variation ID 13399). Molecular consequence: missense variant, 3 prime UTR variant. Protein change: M129V, D178N. 2. Epidemiology: ~24 reported sFI cases worldwide, all PRNP 129MM, PrPSc type 2; ultra-rare sporadic prion disease with middle-age onset. 3. Standard of care: No approved disease-modifying therapy; management is supportive and palliative (symptom control, sleep/autonomic support). 4. Pipeline: No sFI-specific interventional trials; prion-wide programs include PRNP-targeting ASOs in phase I, preclinical AAV-based epigenetic editors and base editors, and historical immunotherapy attempts without clear efficacy. 5. CBE clinical validation: BE4max (Koblan et al. 2018) is the gold-standard cytosine base editor. Multiple CBE programs are in clinical development for liver and hematologic targets.

1. No published data specifically correcting PRNP c.385A>G (p.Met129Val) with Base Editing (BE4max); strategy is based on general principles and must be validated preclinically.
2. PAM availability and bystander base analysis for the specific genomic context have not been performed. If no canonical NGG PAM positions the target within the editing window, PAM-flexible variants (SpRY) may be needed.
3. Delivery to CNS tissue remains a major translational bottleneck. Current vectors have limited transduction efficiency in these compartments.
4. Long-term durability, off-target genome-wide effects, and immunogenicity in the target patient population require thorough preclinical and clinical evaluation.