Quantification and diagnostic relevance of blood and heme-mediated inhibition of prion detection by RT-QuIC.

Prion diseases are characterized by misfolding of prion protein (PrP) from correctly folded PrPC to a disease-associated form, PrPD. Real-time quaking-induced conversion (RT-QuIC) detects prions by "seeding" reaction mixtures, which contain recombinant PrP, with samples suspected to contain prions, resulting in amplification of misfolded PrP. The assay is sensitive to inhibition by tissue constituents, including blood. Heme, a cofactor of hemoglobin (Hb), has been shown to bind PrP in an isoform-specific manner and to affect the stability of other pathogenic amyloids. Herein, tissue samples from scrapie-positive sheep were used to seed RT-QuIC reactions in the presence of heme-as free hemin, as a cofactor of Hb, and as present in whole blood. At equivalent heme concentrations, the inhibitory action of free heme was the least and that of blood the greatest, suggesting other components of Hb and whole blood have additional inhibitory actions. We also demonstrate that this inhibition of RT-QuIC acts through disruption of the recombinant PrP assay substrate, rather than destruction of PrPD seeds. Lastly, heme concentrations were measured in several ruminant tissues. Heme levels exceeded inhibitory thresholds in nearly all types of intact tissue but were reduced below inhibitory levels at a 1:1,000 dilution of most tissue types, with whole blood being one of a few notable exceptions. Our results suggest that detection of PrPD seeding activity is not precluded by exposure to heme in tissue samples, but that the final heme concentration introduced into the RT-QuIC assay mixture is the critical factor that impacts detection sensitivity. IMPORTANCE: Real-time quaking-induced conversion (RT-QuIC) is an ultrasensitive amplification assay for the detection of prions. The assay has shown exceptional performance in optimal laboratory conditions, on par with bioassay, and far surpassing current immunoassay diagnostics. However, efforts to apply RT-QuIC as a real-world diagnostic have been hampered by inconsistencies and unexpectedly low sensitivity in some field samples. This study aims to quantify and characterize the mechanism of inhibition from blood and its constituent parts, hemoglobin and heme-omnipresent components of most sample types. Such systematic evaluations of RT-QuIC inhibitory factors represent necessary steps toward the consistent and sensitive performance necessary for a field-applicable diagnostic assay.