Prion protein (PrP) profiles in blood and CSF: insights into pre-symptomatic and symptomatic prion disease

The conversion of native prion protein (PrP) into its misfolded isoform, scrapie (PrP^Sc) and its intracellular accumulation represent central events in the pathogenesis of prion diseases. Reduction of native PrP in the central nervous system (CNS) has emerged as a promising strategy for treatment and prevention of prion diseases in humans. To facilitate translation into clinical practice, it is essential to identify at-risk individuals through biomarker development and to elucidate PrP behaviour across prion disease subtypes and biological fluids. Measurements of PrP in accessible biofluids, such as plasma and cerebrospinal fluid (CSF), may provide a pharmacodynamic readout and enable monitoring for PrP-targeted therapies. This study systematically quantifies PrP in plasma and CSF of individuals with sporadic and genetic prion diseases, healthy controls (HC), patients with non-neurodegenerative neurological conditions (ND) and Alzheimer's disease (AD). We analysed 136 plasma and 84 CSF samples, including HC, AD, sporadic Creutzfeldt-Jakob disease (sCJD), as well as symptomatic patients and asymptomatic carriers of the mutations D178N, E200K and P102L. Quantification of PrP was performed using a BetaPrion Human ELISA. Statistical analyses assessed differences between diagnostic groups, associations with demographic factors and PRNP codon 129 polymorphism, and diagnostic accuracy via ROC curves. Plasma PrP was significantly reduced in patients with sCJD (P = 0.043), in symptomatic patients with the E200K (P = 0.0078) and in both symptomatic and asymptomatic D178N carriers (P = 0.0002) compared to HC. Furthermore, symptomatic and asymptomatic D178N carriers had significantly lower plasma PrP levels than patients with AD (P = 0.0025 and P = 0.0041, respectively). In CSF, PrP concentrations were notably lower in D178N symptomatic patients (P = 0.0026) versus non-neurodegenerative (ND) controls. Plasma PrP levels showed no association with age, sex or disease onset and were lower in genetic prion disease patients with the methionine/valine (MV) genotype at PRNP codon 129. The diagnostic accuracy for PrP quantification in plasma as a biomarker discriminated D178N asymptomatic carriers [area under the curve (AUC) = 0.96] and D178N symptomatic patients (AUC = 0.90) from HC with excellent accuracy. In CSF, PrP quantification discriminated D178N symptomatic patients from ND with good accuracy (AUC = 0.64). Taken together, this study defines a characteristic profile of persistently low plasma and CSF PrP in D178N symptomatic and asymptomatic mutation carriers. Low plasma levels in sCJD and E200K, in contrast to P102L, are puzzling. Mutation-specific patterns of PrP need to be considered for monitoring purposes in clinical trials.