Atypical Scrapie Prions from Sheep and Lack of Disease in Transgenic Mice Overexpressing Human Prion Protein

Jonathan D.F. Wadsworth

, Susan Joiner, Jacqueline M. Linehan, Anne Balkema-Buschmann, John Spiropoulos, Marion M. Simmons, Peter C. Griffiths, Martin H. Groschup, James Hope, Sebastian Brandner, Emmanuel A. Asante, and John Collinge

Author affiliations: University College London, London, UK (J.D.F Wadsworth, S. Joiner, J.M. Linehan, S. Brandner, E.A. Asante, J. Collinge); Federal Research Institute for Animal Health, Greifswald-Insel Riems, Germany (A. Anne Balkema-Buschmann, M.H. Groschup); Animal Health and Veterinary Laboratories Agency, Addlestone, UK (J. Spiropoulos, M.M. Simmons, P.C. Griffiths, J. Hope)

Abstract

Public and animal health controls to limit human exposure to animal prions are focused on bovine spongiform encephalopathy (BSE), but other prion strains in ruminants may also have zoonotic potential. One example is atypical/Nor98 scrapie, which evaded statutory diagnostic methods worldwide until the early 2000s. To investigate whether sheep infected with scrapie prions could be another source of infection, we inoculated transgenic mice that overexpressed human prion protein with brain tissue from sheep with natural field cases of classical and atypical scrapie, sheep with experimental BSE, and cattle with BSE. We found that these mice were susceptible to BSE prions, but disease did not develop after prolonged postinoculation periods when mice were inoculated with classical or atypical scrapie prions. These data are consistent with the conclusion that prion disease is less likely to develop in humans after exposure to naturally occurring prions of sheep than after exposure to epizootic BSE prions of ruminants.

Bovine spongiform encephalopathy (BSE) is the transmissible spongiform encephalopathy (TSE) or prion disease of domestic cattle. The BSE prion is an epizootic agent and causes variant Creutzfeldt-Jakob disease (vCJD) in humans after dietary exposure (1–4). Because the time lag between exposure and development of vCJD may be decades, uncertainty about the extent of the pathogenicity of BSE for humans continues (5), and subclinical forms of infection may exist (6,7). A recent immunohistochemical study that estimated prevalence of prion infection in the UK population by screening samples from surgically removed appendixes found 1 in 2,000 persons were positive for the disease-associated form of the prion protein (PrP) (8). Similar uncertainty exists in our understanding of scrapie, the TSE of small ruminants, which has been heightened in recent years by finding BSE in goats (9,10), the possibility of BSE in sheep (11), and the discovery of atypical scrapie (12,13), a form of small-ruminant TSE, which had evaded statutory diagnosis until the early 2000s.
Recent analysis of surveillance data of TSEs in small ruminants in Great Britain, collected over the past 10 years, has demonstrated a dramatic decrease (up to 90%) in number of confirmed cases of classical scrapie in the national flock. However, atypical scrapie continues to affect sheep bred for their relative resistance to the classical form of this prion disease, and the proportion of sheep with resistant genotypes in the national flock is likely to have increased over the past decade because of the National Scrapie Plan for Great Britain. This increase has rekindled speculation that atypical scrapie in small ruminants might be a source of human prion disease (11). Although atypical scrapie has been discovered retrospectively in 2 UK sheep culled in 1987 and 1989 (14,15), the level and duration of human exposure to atypical scrapie prions are unknown, and this lack of knowledge confounds a cause-and-effect investigation of epidemiologic links between this animal disease and some form of CJD (11).
Over the past 2 decades, surrogate methods have been developed to assess the relative pathogenicity of animal prions for humans. One approach involves the experimental transmission of disease by inoculating homogenized brain tissue from affected animals into transgenic mice that are overexpressing 1 of the 2 common polymorphic forms of the human PrP (either methionine or valine at residue 129) on a mouse PrP null background (16). Such transgenic mice are fully susceptible to infection with human prions (16) and, to a lesser extent, cattle and ovine BSE prions (2,4,17), but appear resistant to chronic wasting disease prions from cervids (18–20). In this study, we inoculated transgenic mice that overexpressed human PrP with brain tissue from field sheep with natural cases of classical and atypical scrapie, sheep with serially-passaged experimental BSE, and cattle with BSE to assess the pathogenicity of natural scrapie prions relative to that of the known epizootic TSE agent, the cattle BSE prion strain.