Quantifying the Molecular Properties of the Elk Chronic Wasting Disease Agent with Mass Spectrometry

Chronic wasting disease (CWD) is a prion disease afflicting wild and farmed elk. CWD prions (PrP^Sc) are infectious protein conformations that replicate by inducing a natively expressed prion protein (PrP^C) to refold into the prion conformation. Mass spectrometry was used to study the prions resulting from a previously described experimental inoculation of MM132, ML132, and LL132 elk with a common CWD inoculum. Chymotryptic digestion times and instrument parameters were optimized to yield a set of six peptides, TNMK, MLGSAMSRPL, LLGSAMSRPL, ENMYR, MMER, and VVEQMCITQYQR. These peptides were used to quantify the amount, the M132 and L132 polymorphic composition, and the extent of methionine oxidation of elk PrP^Sc. The amount (ng/g brain tissue) of PrP^Sc present in each sample was determined to be: MM132 (5.4 × 10² ± 7 × 10¹), ML132 (3.3 × 10² ± 6 × 10¹ and 3.6 × 10² ± 3 × 10¹) and LL132 (0.7 × 10² ± 1 × 10¹, 0.2 × 10² ± 0.2 × 10¹, and 0.2 × 10² ± 0.5 × 10¹). The proportion of L132 polymorphism in ML132 (heterozygous) PrP^Sc from CWD-infected elk was determined to be 43% ± 2% or 36% ± 3%. Methionine oxidation was detected and quantified for the M132 and L132 polymorphisms in the samples. In this way, mass spectrometry can be used to characterize prion strains at a molecular level.