A traceback phenomenon can reveal the origin of prion infection.

The transmission of prions to animals with incongruent prion protein (PrP) gene (referred to as cross-sequence transmission) results in a relatively long incubation period and can generate a new prion strain with unique transmissibility designated as a traceback phenomenon. For example, cross-sequence transmission of bovine spongiform encephalopathy (BSE) prions to human generated variant Creutzfeldt-Jakob disease (vCJD) prions which retained the transmissibility to mice expressing bovine PrP. This finding suggests that traceback studies could enable us to identify the origin of prions. There are two distinct phenotypes in dura mater graft-associated Creutzfeldt-Jakob disease (dCJD), with the majority represented by a non-plaque-type of dCJD (np-dCJD) and the minority by a plaque-type of dCJD (p-dCJD). To identify the origin of p-dCJD, we performed a traceback study using mice expressing human PrP with methionine homozygosity (129M/M) or valine homozygosity (129V/V) at polymorphic codon 129. The characteristics of p-dCJD such as the accumulation of abnormal isoform of PrP (PrP(Sc)) intermediate in size between type 1 and type 2, and plaque-type PrP deposition in the brain were maintained after transmission to the 129M/M mice. Furthermore, the 129V/V mice were more susceptible to p-dCJD prions than the 129M/M mice and produced type 2 PrP(Sc) that were identical in size to those from the 129V/V mice inoculated with sporadic CJD prions from a patient with 129V/V and type 2 PrP(Sc) (sCJD-VV2). In addition, we performed intracerebral transmission of sCJD-VV2 prions to the 129M/M mice as an experimental model for p-dCJD. These 129M/M mice showed the accumulation of the intermediate type PrP(Sc) and plaque-type PrP deposition in the brain. These results suggest that p-dCJD could be caused by cross-sequence transmission of sCJD-VV2 prions to individuals with the 129M/M genotype.