Mixtures of prion substrains in natural scrapie cases revealed by ovinised murine models

Abstract

phenotypic variability in prion diseases, such as scrapie, is associated to the existence of prion strains, which are different pathogenic prion protein (PrP Sc) conformations with distinct pathobiological properties. to faithfully study scrapie strain variability in natural sheep isolates, transgenic mice expressing sheep cellular prion protein (PrP c) are used. in this study, we used two of such models to bioassay 20 scrapie isolates from the Spain-France-Andorra transboundary territory. Animals were intracerebrally inoculated and survival periods, proteinase K-resistant PrP (PrP res) banding patterns, lesion profiles and PrP Sc distribution were studied. inocula showed a remarkable homogeneity on banding patterns, all of them but one showing 19-kDa PrP res. However, a number of isolates caused accumulation of 21-kDa PrP res in TgShp XI. A different subgroup of isolates caused long survival periods and presence of 21-kDa PrP res in Tg338 mice. It seemed that one major 19-kDa prion isoform and two distinct 21-kDa variants coexisted in source inocula, and that they could be separated by bioassay in each transgenic model. The reason why each model favours a specific component of the mixture is unknown, although prp c expression level may play a role. our results indicate that coinfection with more than one substrain is more frequent than infection with a single component. Transmissible spongiform encephalopathies (TSEs), also known as prion diseases, are a group of rare, fatal and progressive neurodegenerative disorders that affect both animals and human beings. TSEs are caused by non-conventional etiologic agents called prions. According to the widely accepted prion hypothesis 1 , prions consist exclusively of a pathogenic protein conformer, termed PrP Sc , that derives from the physiological, host-encoded cellular prion protein (PrP C) 1-3 via a post-translational conformational change that is triggered by PrP Sc itself in a feedback manner 4. PrP Sc accumulates in nervous tissue 5 and through a yet unclarified mechanism gives rise to neuronal death, neuron loss and vacuolization 6,7 , together with astrogliosis, microgliosis 8-10 and other neuroinflammatory phenomena 11. This progressive degeneration of the central nervous system manifests as a set of neurological signs appearing after long incubation periods. Although, according to the protein-only model 1 , the agent responsible for these disorders lacks nucleic acids, the existence of several phenotypic variants has been proved for different TSEs. This was first described for scrapie when two different clinical syndromes were observed in scrapie-infected goats, which were reproducible by intracerebral inoculation 12. Prion variants associated to these different phenotypes were termed "strains", by analogy with other infectious agents. Further demonstration of the existence of scrapie strains was addressed by studies in wild type mice 13,14 , which also established a methodology to discriminate them according to survival periods and lesion profiles 15-17 .

Bovine adapted transmissible mink encephalopathy is similar to L-BSE after passage through sheep with the VRQ/VRQ genotype but not VRQ/ARQ

“…However, the effect of host adaptation on disease phenotype cannot be fully accounted for in the lesion pro les derived from rst passage mice. Subsequent repeated intraspecies passages is required to sift out strain variants and select for a new variant [15,17,[33][34][35][36][37][38]. This strain selection results in a decreased IP and stabilized neuropathology [39].…”

Bovine adapted transmissible mink encephalopathy is similar to L-BSE after passage through sheep with the VRQ/VRQ genotype but not VRQ/ARQ

“…However, the effect of host adaptation on disease phenotype cannot be fully accounted for in the lesion pro les derived from rst passage mice. Subsequent repeated intraspecies passages is required to sift out strain variants and select for a new variant [16,18,[37][38][39][40][41][42]. This strain selection results in a decreased IP and stabilized neuropathology [43].…”

Bovine adapted transmissible mink encephalopathy is similar to L-BSE after passage through sheep with the VRQ/VRQ genotype but not VRQ/ARQ

“…However, the effect of host adaptation on disease phenotype cannot be fully accounted for in the lesion profiles derived from first passage mice. Subsequent repeated intraspecies passages is required to sift out strain variants and select for a new variant [16,18,[37][38][39][40][41][42]. This strain selection results in a decreased IP and stabilized neuropathology [43].…”