Volume 23, Number 9—September 2017
Chronic Wasting Disease Prion Strain Emergence and Host Range Expansion
Human and mouse prion proteins share a structural motif that regulates resistance to common chronic wasting disease (CWD) prion strains. Successful transmission of an emergent strain of CWD prion, H95+, into mice resulted in infection. Thus, emergent CWD prion strains may have higher zoonotic potential than common strains.
Chronic wasting disease (CWD) is a contagious prion disease of cervids that is spreading globally. CWD is enzootic in multiple cervid species, including deer and elk; the major foci of disease are Colorado/Wyoming (USA), Wisconsin/Illinois (USA), and Alberta/Saskatchewan (Canada). CWD is also present in captive cervids in South Korea and wild reindeer and moose in Norway (https://www.nwhc.usgs.gov/images/cwd/cwd_map.jpg). CWD results from the conformational transformation of the host-encoded cellular prion protein (PrPC) into protease-resistant, detergent-insoluble, β-sheet rich, amyloidogenic conformers, termed prions (PrPCWD). Within their conformation, prion strains encipher the information that directs the templated misfolding and aggregation of PrPC molecules into additional prions (1).
Although the sequence homology of PrP among mammals is high, the ability of particular prion strains to cause disease in different species is determined by the conformational compatibility between a given strain and the host PrPC (2). We previously identified 2 strains of CWD prion in white-tailed deer (3), Wisc-1 and H95+; these strains exhibit distinct biological properties in deer and transgenic cervidized mice. To ascertain the host range of different strains from cervids, we inoculated CWD prions isolated from experimentally infected deer with different PRNP genotypes (Q95G96 [wild type (wt)], S96/wt, H95/wt, and H95/S96) and from elk (CWD2 strain) into hamsters and mice. All isolates have been successfully transmitted into transgenic mice expressing wt cervid PrP and contain high titers of CWD prions (3).
Mice inoculated with H95+ CWD prions succumbed to clinical disease at 575 ± 47 or 692 ± 9 days, depending on the H95+ isolate (Table). Mice inoculated with Wisc-1 or elk CWD or uninfected deer homogenates were euthanized at day 708 after infection with no signs of prion disease. Clinical signs of H95+ CWD in C57Bl/6 mice included ataxia, lethargy, tail rigidity, and dermatitis. Protease-resistant PrPCWD was present in all mice infected with H95+ prions and was not detected in mice infected with Wisc-1 or CWD2 (Technical Appendix).
In contrast to mice, hamsters succumbed to clinical disease when inoculated with Wisc-1 CWD prions but were less susceptible to H95+ CWD prions (Table). Clinical signs of CWD in hamsters began with lethargy and, upon arousal, retrocollis; as the disease progressed, lethargy declined with increased dystonic movement including ataxia and tremors. Hyperesthesia was not observed. Subclinical disease (no clinical signs but PrP-res positive by Western blot) was observed in a subset of hamsters (Technical Appendix).
Successful interspecies prion transmission at the molecular level depends on the compatibility of the invading prion conformers and structural determinants imposed by host PrPC. One structural motif is the loop region between β sheet 2 and α helix 2 of PRPC at aa 170–174 (Technical Appendix). Host species containing PrPC molecules with a flexible β2-α2 loop (mice and humans) are hypothesized to be incompatible with prions derived from species containing a rigid loop (deer and elk) (4,5). Previous attempts to transmit CWD to mice have failed (6,7). Our data show that prions from a prototypic rigid-loop species (deer) can transmit to a flexible-loop species (mice). The transmission is strain dependent. H95+ overrides the conformational restriction imposed by the mouse PrP flexible loop that Wisc-1 and CWD2 cannot overcome, suggesting that the invading prion strain is a dominant contributor to the species/transmission barrier. How the N terminal amino acid polymorphism (Q95H) affects the conformation of PrP, altering the deer-to-mouse transmission barrier, is unknown. Further structural studies may clarify the effect of N terminal residues on β2-α2 loop rigidity.
Transmission of H95+ CWD prions to mice further confirms the value of specifying strain when defining species barriers. Experimental transmission of CWD prion into macaques and transgenic mice expressing human PrP suggests a considerable transmission barrier to CWD prions (although squirrel monkeys are susceptible), and human prion protein is converted inefficiently in vitro (8,9). Successful infection of a flexible-loop species (mice) with H95+ CWD raises concerns for the potential pathogenicity of H95+ prions to other flexible-loop species. Transmission studies with Wisc-1 and H95+ in transgenic humanized and bovinized mice are ongoing.
The increasing prevalence of CWD indicates selection for cervids with resistance alleles, such as S96 and H95. Genetic resistance to a given prion strain selects for the emergence of novel prion strains with altered properties such as H95+ and Nor98 (3,10). The iterative transmission of CWD prions to cervids with protective alleles of PrPC and the consequent emergence of new CWD prion strains highlights the dynamics of the CWD panzootic and the value of characterizing the host range of emergent CWD prion strains.
Dr. Herbst is a research associate and Dr. Duque Velásquez is a postdoctoral fellow at the University of Alberta. Their primary research interest is the mechanism(s) of pathogenicity underlying neurodegeneration, as exemplified by prion diseases in animals and humans.
We thank Catherine Graham for the elk CWD prions and Richard Rubenstein for the 3F4 monoclonal antibodies.
This work was supported by the Alberta Prion Research Institute, Natural Sciences and Engineering Research Council, the Alberta Livestock and Meat Agency, and the Genome Canada Large Scale Applied Research Program.
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1These authors contributed equally to this article.