A diagnostic blood test for prion diseases

In this issue of Blood, Thomas et al¹ report on their comparative analysis of 3 highly specific blood assays to detect transmissible spongiform encephalopathy–associated prion protein (PrP^TSE) in the blood of sheep infected with bovine spongiform encephalopathy (BSE) as a model of human variant Creutzfeldt-Jakob disease (vCJD).

The search for a blood test for human vCJD for the diagnosis and screening of donated blood products and organs has seen few successes over the years and efforts have not led to a practical test to date. Among the many reasons for this situation are the technical challenges in converting a PrP^TSE assay developed in a research lab into a routine clinical laboratory test and the scarcity of human vCJD blood needed for assay validation. In their study, Thomas and colleagues address the latter limitation using blood from BSE-infected sheep. This group was first to develop and characterize the BSE sheep model and demonstrate its relevance as a model for human vCJD. vCJD is acquired mostly by dietary exposure to the agent that causes BSE in cattle,² and in a few cases by human-to-human transmission with blood transfusions.³ BSE and vCJD are part of a family of universally fatal neurodegenerative infections that affect both animals and humans, collectively called transmissible spongiform encephalopathies or prion diseases. The authors used their extensive and unique inventory of longitudinal sheep blood samples collected over many years and applied 3 in-house-developed assays to detect PrP^TSE. These assays are based on variations of protein misfolding cyclic amplification (PMCA) and real-time quaking-induced conversion (RT-QuIC) methods.^4-6 

The goal of the study was to evaluate the performance of the 3 assays, mb-PMCA, capture PMCA, and whole-blood iron oxide magnetic extraction RT-QuIC, in a side-by-side comparison. The RT-QuIC assay demonstrated lower analytical specificity compared with the 2 PMCA assays using serial dilutions of infected human and sheep brain homogenates. Importantly, all assays detected the human vCJD agent, suggesting their potential applicability in testing PrP^TSE in infected human blood. The investigation included a well-designed assay validation study using established algorithms to assign reactivity status to blinded panels of BSE-infected and uninfected sheep blood. The results are quite impressive: A clear pattern of early detection of PrP^TSE in blood emerged starting at 6 months postinfection and several months before disease onset. All 3 tests showed 100% specificity. RT-QuIC showed lower assay sensitivity when compared with the PMCA tests. The authors also demonstrate infectivity in sheep blood by infecting BSE-susceptible transgenic mice, confirming a positive association between PrP^TSE and infectivity. Notably, data from this work and from other large experimental animal models^7,8 coincide with data from the 4 human cases of transfusion-transmitted vCJD reported in the United Kingdom.³ The consistent feature is that infectivity and PrP^TSE were present in human and animal blood during the asymptomatic phase. This finding is encouraging for the prospect of using a preclinical human blood sample to identify infected individuals and prevent future iatrogenic vCJD transmissions. Such a diagnostic test would be useful even now when new cases of vCJD are very rare but the risk of vCJD is unquantifiable due to the uncertainty around the prevalence of vCJD in the United Kingdom and the fact that TSE disease can incubate undetected in infected persons for many decades.

Although the approach used by the authors is not novel, their work is highly relevant to TSE diagnostics and will accelerate the field toward the development of a human vCJD blood assay. The study focused on sheep blood, with no human blood being tested. This is reasonable because assay development must be conducted using samples with known disease status, such as those from experimentally infected sheep. The next step will be to confirm and validate the assays with human vCJD blood. Another highly desirable direction of future work would be to adapt the best assay to detect PrP^TSE in the blood of individuals infected with sporadic CJD agent—the most common human TSE. Such assays are urgently needed to facilitate early therapeutic intervention in genetic CJD cases and prevent or slow progression to clinical CJD.⁹ The same test could be used in clinical trials for all types of CJD as well as to screen living donors of human cellular and tissue-derived products to reduce the risk of transmission of iatrogenic CJD. The main challenge for such an assay is that the levels of PrP^TSE in sporadic CJD blood may be even lower than those in vCJD blood. This is a technical challenge that will likely be resolved by developing even more sensitive assays, and efforts toward this end are already underway.

Conflict-of-interest disclosure: L.G. declares no competing financial interests.