ASO-mediated PrP lowering for PRNP c.305C>T (p.Pro102Leu): translating prion protein reduction to genetic prion disease

For PRNP c.305C>T (p.Pro102Leu), a pathogenic missense variant associated with Gerstmann-Sträussler-Scheinker syndrome (GSS) and the inherited CJD spectrum, antisense oligonucleotide (ASO)-mediated reduction of total PrP levels represents the most promising therapeutic strategy in development. The rationale is based on the principle that prion disease requires PrP substrate — reducing PrP expression below a critical threshold should delay or prevent pathogenic PrP conformational conversion. Ionis Pharmaceuticals has developed ION717, an ASO targeting PRNP mRNA, which has entered clinical trials for genetic prion disease. This approach is variant-agnostic for gain-of-function PRNP mutations, as it reduces total PrP rather than targeting the specific mutation.

Raymond et al. (Lancet Neurol 2019; PMID:31587955) and Bhatt et al. established that PRNP ASOs achieve dose-dependent PrP reduction in prion-infected mice and significantly extend survival. ION717 (Ionis) has entered clinical development for symptomatic and presymptomatic PRNP mutation carriers, based on CSF PrP as a pharmacodynamic biomarker. Minikel et al. (Sci Transl Med 2020; PMID:33268508) demonstrated that heterozygous PRNP loss-of-function is well-tolerated in humans, supporting the safety of partial PrP lowering — individuals with one truncating PRNP allele show no clinical phenotype, providing natural human genetic evidence for target safety. P102L is the most common GSS-associated variant, with autosomal dominant inheritance and variable penetrance. ClinVar classifies it as Pathogenic. The variant destabilizes the PrP globular domain and promotes misfolding into protease-resistant PrPSc conformers. CSF total PrP and RT-QuIC seed amplification serve as established biomarkers for monitoring disease activity and treatment response.

ION717 clinical trial data are preliminary, and no efficacy endpoints have been formally met. ASO delivery requires repeated intrathecal injections — the lifelong treatment burden and procedural risks are substantial. Whether PrP lowering can reverse established neurodegeneration or only slow/prevent further progression is unknown; presymptomatic treatment may be necessary for maximal benefit, but identifying the optimal intervention timepoint in presymptomatic carriers is challenging given variable penetrance and age of onset for P102L (typically 40s-60s but highly variable). Heterozygous PrP reduction may be insufficient if the mutant protein has a particularly high propensity for misfolding. Allele-specific approaches that selectively silence the mutant allele while preserving wild-type PrP could be theoretically superior but are more technically challenging. The extremely low prevalence of genetic prion disease creates challenges for trial enrollment and statistical power.