Chronic wasting disease prion strain emergence and host range expansion.
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 [PrP.sup.C] (2). We previously identified 2 strains of CWD prion in white-tailed deer (3), Wisc-1 and [H95.sup.+]; 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.sup.+] CWD prions succumbed to clinical disease at 575 [+ or -] 47 or 692 [+ or -] 9 days, depending on the [H95.sup.+] 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.sup.+] CWD in C57B1/6 mice included ataxia, lethargy, tail rigidity, and dermatitis. Protease-resistant PrPCWD was present in all mice infected with [H95.sup.+] prions and was not detected in mice infected with Wisc-1 or CWD2 (online Technical Appendix, https://www.nc.cdc. gov/EID/article/23/9/16-1474-Techapp1.pdf).
In contrast to mice, hamsters succumbed to clinical disease when inoculated with Wise-1 CWD prions but were less susceptible to [H95.sup.+] 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 (online 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 [PrP.sup.c]. One structural motif is the loop region between [beta] sheet 2 and [alpha] helix 2 of [PRP.sup.C] at aa 170-174 (online Technical Appendix). Host species containing [PrP.sup.C] molecules with a flexible [beta]2-[alpha]2 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.sup.+] 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 [beta]2-[alpha]2 loop rigidity.
Transmission of [H95.sup.+] 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.sup.+] CWD raises concerns for the potential pathogenicity of [H95.sup.+] prions to other flexible-loop species. Transmission studies with Wisc-1 and [H95.sup.+] 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.sup.+] 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.
DOI: https://doi.org/10.3201 /eid2309.161474
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.
Dr. Herbst is a research associate and Dr. Duque Velasquez 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.
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Allen Herbst,  Camilo Duque Velasquez,  Elizabeth Triscott, Judd M. Aiken, Debbie McKenzie
Author affiliation: University of Alberta, Edmonton, Alberta, Canada
 These authors contributed equally to this article.
Address for correspondence: Debbie McKenzie, University of Alberta, 120 Brain and Aging Research Building, Edmonton, AB T6G 2R3, Canada; email: firstname.lastname@example.org
Table. Results of CWD prion inoculation into rodents * Recipient and PrP-res+ CWD inocula No. Clinical Subclinical Incubation period, d Mice wt/wt 6 0 0 NA S96/wt 6 0 0 NA H95/wt 7 5 2 669, 671, 706, 706, 706 H95/S96 7 7 0 306, 593, 593, 593, 593, 673, 675 Elk 4 0 0 NA Control 4 0 0 NA Hamsters wt/wt 8 3 5 652, 653, 653 S96/wt 8 1 4 634 H95/wt 8 1 6 652 H95/S96 8 0 1 NA Elk 8 2 2 673, 719 Control 8 0 0 NA * Mice infected with CWD prions were observed for up to 708 d; hamsters infected with white-tailed deer and elk CWD prions were observed for 659 and 726 d, respectively. Control mice and hamsters were inoculated with brain homogenates from CWD-negative wt/wt deer. CWD, chronic wasting disease; NA, not applicable; PrP-res+, positive for proteinase-K-resistant prion protein; wt, wild type.
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|Title Annotation:||RESEARCH LETTERS|
|Author:||Herbst, Allen; Velasquez, Camilo Duque; Triscott, Elizabeth; Aiken, Judd M.; McKenzie, Debbie|
|Publication:||Emerging Infectious Diseases|
|Article Type:||Letter to the editor|
|Date:||Sep 1, 2017|
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