Structural effects of PrP polymorphisms on intra- and interspecies prion transmission

Propagation of distinct CWD prion strains during peripheral and intracerebral challenges of gene-targeted mice

Since prion diseases result from infection and neurodegeneration of the central nervous system (CNS), experimental characterizations of prion strain properties customarily rely on the outcomes of intracerebral challenges. However, natural transmission of certain prions, including those causing chronic wasting disease (CWD) in elk and deer, depends on propagation in peripheral host compartments prior to CNS infection. Using gene-targeted GtE and GtQ mice, which accurately control cellular elk or deer PrP expression, we assessed the impact that peripheral or intracerebral exposures play on CWD prion strain propagation and resulting CNS abnormalities. Whereas oral and intraperitoneal transmissions produced identical neuropathological outcomes in GtE and GtQ mice and preserved the naturally convergent conformations of elk and deer CWD prions, intracerebral transmissions generated CNS prion strains with divergent biochemical properties in GtE and GtQ mice that were changed compared to their native counterparts. While CWD replication kinetics remained constant during iterative peripheral transmissions and brain titers reflected those found in native hosts, serial intracerebral transmissions produced 10-fold higher prion titers and accelerated incubation times. Our demonstration that peripherally and intracerebrally challenged Gt mice develop dissimilar CNS diseases which result from the propagation of distinct CWD prion strains points to the involvement of tissue-specific cofactors during strain selection in different host compartments. Since peripheral transmissions preserved the natural features of elk and deer prions, whereas intracerebral propagation produced divergent strains, our findings illustrate the importance of experimental characterizations using hosts that not only abrogate species barriers but also accurately recapitulate natural transmission routes of native strains.

Scrapie versus Chronic Wasting Disease in White-Tailed Deer

White-tailed deer are susceptible to scrapie (WTD scrapie) after oronasal inoculation with the classical scrapie agent from sheep. Deer affected by WTD scrapie are difficult to differentiate from deer infected with chronic wasting disease (CWD). To assess the transmissibility of the WTD scrapie agent and tissue phenotypes when further passaged in white-tailed deer, we oronasally inoculated wild-type white-tailed deer with WTD scrapie agent. We found that WTD scrapie and CWD agents were generally similar, although some differences were noted. The greatest differences were seen in bioassays of cervidized mice that exhibited significantly longer survival periods when inoculated with WTD scrapie agent than those inoculated with CWD agent. Our findings establish that white-tailed deer are susceptible to WTD scrapie and that the presence of WTD scrapie agent in the lymphoreticular system suggests the handling of suspected cases should be consistent with current CWD guidelines because environmental shedding may occur.

New developments in prion disease research using genetically modified mouse models

While much of what we know about the general principles of protein-based information transfer derives from studies of experimentally adapted rodent prions, these laboratory strains are limited in their ability to recapitulate features of human and animal prions and the diseases they produce. Here, we review how recent approaches using genetically modified mice have informed our understanding of naturally occurring prion diseases, their strain properties, and the factors controlling their transmission and evolution. In light of the increasing importance of chronic wasting disease, the application of mouse transgenesis to study this burgeoning and highly contagious prion disorder, in particular recent insights derived from gene-targeting approaches, will be a major focus of this review.

Mouse models of chronic wasting disease: A review

Animal models are essential tools for investigating and understanding complex prion diseases like chronic wasting disease (CWD), an infectious prion disease of cervids (elk, deer, moose, and reindeer). Over the past several decades, numerous mouse models have been generated to aid in the advancement of CWD knowledge and comprehension. These models have facilitated the investigation of pathogenesis, transmission, and potential therapies for CWD. Findings have impacted CWD management and disease outcomes, though much remains unknown, and a cure has yet to be discovered. Studying wildlife for CWD effects is singularly difficult due to the long incubation time, subtle clinical signs at early stages, lack of convenient in-the-field live testing methods, and lack of reproducibility of a controlled laboratory setting. Mouse models in many cases is the first step to understanding the mechanisms of disease in a shortened time frame. Here, we provide a comprehensive review of studies with mouse models in CWD research. We begin by reviewing studies that examined the use of mouse models for bioassays for tissues, bodily fluids, and excreta that spread disease, then address routes of infectivity and infectious load. Next, we delve into studies of genetic factors that influence protein structure. We then move on to immune factors, possible transmission through environmental contamination, and species barriers and differing prion strains. We conclude with studies that make use of cervidized mouse models in the search for therapies for CWD.

Novel Prion Strain as Cause of Chronic Wasting Disease in a Moose, Finland

Our previous studies using gene-targeted mouse models of chronic wasting disease (CWD) demonstrated that Norway and North America cervids are infected with distinct prion strains that respond differently to naturally occurring amino acid variation at residue 226 of the prion protein. Here we performed transmissions in gene-targeted mice to investigate the properties of prions causing newly emergent CWD in moose in Finland. Although CWD prions from Finland and Norway moose had comparable responses to primary structural differences at residue 226, other distinctive criteria, including transmission kinetics, patterns of neuronal degeneration, and conformational features of prions generated in the brains of diseased mice, demonstrated that the strain properties of Finland moose CWD prions are different from those previously characterized in Norway CWD. Our findings add to a growing body of evidence for a diverse portfolio of emergent strains in Nordic countries that are etiologically distinct from the comparatively consistent strain profile of North America CWD.

Extrapolating the susceptibility of Eld’s deer (Rucervus eldii thamin) to chronic wasting disease from prion protein gene (PRNP) polymorphisms

Chronic wasting disease (CWD) is a prion disease of North American cervids. The transmission of CWD to endangered cervid species is of concern for captive breeding programs. Trans-species transmission could occur via direct contact with infected wild deer, or via prion contaminated fomites. Variation in the prion protein gene, PRNP, is associated with differences in CWD susceptibility among cervids. We therefore sequenced PRNP in 36 endangered Eld’s deer (Rucervus eldii thamin), detecting five synonymous and two non-synonymous SNPs. Three haplotypes were inferred, suggesting that genetic management in captive breeding programs has been effective at maintaining PRNP diversity. The haplotypes encoded two PrP protein variants. The more common Eld’s deer PrP variant encodes methionine at codon 208 and glutamine at codon 226. Because this protein variant is identical to a common PrP variant in white-tailed deer and mule deer and is especially common in white-tailed deer positive for CWD, we recommend reducing the frequency of this variant in the breeding stock, while implementing strict management practices to avoid exposure to wild North American cervids. The frequency of the other PrP variant, which differs from variants present in these North American cervids, was low. It has the potential to reduce susceptibility to CWD and thus could be increased in frequency. While PRNP haplotype frequencies should be shifted, genetic diversity should be maintained. Ultimately protein diversity may be protective should CWD infect the species, and trans-species polymorphisms are suggestive of past balancing selection and a potential fitness advantage for PRNP diversity.

Gene-Edited Cell Models to Study Chronic Wasting Disease

Prion diseases are fatal infectious neurodegenerative disorders affecting both humans and animals. They are caused by the misfolded isoform of the cellular prion protein (PrPC), PrPSc, and currently no options exist to prevent or cure prion diseases. Chronic wasting disease (CWD) in deer, elk and other cervids is considered the most contagious prion disease, with extensive shedding of infectivity into the environment. Cell culture models provide a versatile platform for convenient quantification of prions, for studying the molecular and cellular biology of prions, and for performing high-throughput screening of potential therapeutic compounds. Unfortunately, only a very limited number of cell lines are available that facilitate robust and persistent propagation of CWD prions. Gene-editing using programmable nucleases (e.g., CRISPR-Cas9 (CC9)) has proven to be a valuable tool for high precision site-specific gene modification, including gene deletion, insertion, and replacement. CC9-based gene editing was used recently for replacing the PrP gene in mouse and cell culture models, as efficient prion propagation usually requires matching sequence homology between infecting prions and prion protein in the recipient host. As expected, such gene-editing proved to be useful for developing CWD models. Several transgenic mouse models were available that propagate CWD prions effectively, however, mostly fail to reproduce CWD pathogenesis as found in the cervid host, including CWD prion shedding. This is different for the few currently available knock-in mouse models that seem to do so. In this review, we discuss the available in vitro and in vivo models of CWD, and the impact of gene-editing strategies.

Review on PRNP genetics and susceptibility to chronic wasting disease of Cervidae

To date, chronic wasting disease (CWD) is the most infectious form of prion disease affecting several captive, free ranging and wild cervid species. Responsible for marked population declines in North America, its geographical spread is now becoming a major concern in Europe. Polymorphisms in the prion protein gene (PRNP) are an important factor influencing the susceptibility to prions and their rate of propagation. All reported cervid PRNP genotypes are affected by CWD. However, in each species, some polymorphisms are associated with lower attack rates and slower progression of the disease. This has potential consequences in terms of genetic selection, CWD diffusion and strain evolution. CWD also presents a zoonotic risk due to prions capacity to cross species barriers. This review summarizes our current understanding of CWD control, focusing on PRNP genetic, strain diversity and capacity to infect other animal species, including humans.

Chronic Wasting Disease Transmission Risk Assessment for Farmed Cervids in Minnesota and Wisconsin

CWD (chronic wasting disease) has emerged as one of the most important diseases of cervids and continues to adversely affect farmed and wild cervid populations, despite control and preventive measures. This study aims to use the current scientific understanding of CWD transmission and knowledge of farmed cervid operations to conduct a qualitative risk assessment for CWD transmission to cervid farms and, applying this risk assessment, systematically describe the CWD transmission risks experienced by CWD-positive farmed cervid operations in Minnesota and Wisconsin. A systematic review of literature related to CWD transmission informed our criteria to stratify CWD transmission risks to cervid operations into high-risk low uncertainty, moderate-risk high uncertainty, and negligible-risk low uncertainty categories. Case data from 34 CWD-positive farmed cervid operations in Minnesota and Wisconsin from 2002 to January 2019 were categorized by transmission risks exposure and evaluated for trends. The majority of case farms recorded high transmission risks (56%), which were likely sources of CWD, but many (44%) had only moderate or negligible transmission risks, including most of the herds (62%) detected since 2012. The presence of CWD-positive cervid farms with only moderate or low CWD transmission risks necessitates further investigation of these risks to inform effective control measures.

Chronic wasting disease in Norway-A survey of prion protein gene variation among cervids

Susceptibility of cervids to Chronic Wasting Disease (CWD), a prion disease, can be modulated by variations in the prion protein gene (PRNP), encoding the cellular prion protein (PrP^C ). In prion diseases, PrP^C is conformationally converted to pathogenic conformers (PrP^Sc ), aggregates of which comprise infectious prions. CWD has recently been observed in its contagious form in Norwegian reindeer (Rangifer tarandus) and in novel, potentially sporadic forms, here called 'atypical CWD', in moose (Alces alces) and red deer (Cervus elaphus). To estimate relative susceptibility of different Norwegian cervid species to CWD, their non-synonymous PRNP variants were analyzed. In reindeer, seven PRNP alleles were observed and in red deer and moose two alleles were present, whereas roe deer (Capreolus capreolus) PRNP was monomorphic. One 'archetypal' PRNP allele associated with susceptibility was common to all four cervid species. The distribution of PRNP alleles differed between wild and semi-domesticated reindeer, with alleles associated with a high susceptibility occurring, on average, above 55% in wild reindeer and below 20% in semi-domesticated reindeer. This difference may reflect the diverse origins of the populations and/or selection processes during domestication and breeding. Overall, PRNP genetic data indicate considerable susceptibility to CWD among Norwegian cervids and suggest that PRNP homozygosity may be a risk factor for the atypical CWD observed in moose. The CWD isolates found in the Norwegian cervid species differ from those previously found in Canada and USA. Our study provides an overview of the PRNP genetics in populations exposed to these emerging strains that will provide a basis for understanding these strains' dynamics in relation to PRNP variability.

Spatio-temporal analyses reveal infectious disease-driven selection in a free-ranging ungulate

Infectious diseases play an important role in wildlife population dynamics by altering individual fitness, but detecting disease-driven natural selection in free-ranging populations is difficult due to complex disease-host relationships. Chronic wasting disease (CWD) is a fatal infectious prion disease in cervids for which mutations in a single gene have been mechanistically linked to disease outcomes, providing a rare opportunity to study disease-driven selection in wildlife. In Wyoming, USA, CWD has gradually spread across mule deer (Odocoileus hemionus) populations, producing natural variation in disease history to evaluate selection pressure. We used spatial variation and a novel temporal comparison to investigate the relationship between CWD and a mutation at codon 225 of the mule deer prion protein gene that slows disease progression. We found that individuals with the 'slow' 225F allele were less likely to test positive for CWD, and the 225F allele was more common in herds exposed to CWD longer. We also found that in the past 2 decades, the 225F allele frequency increased more in herds with higher CWD prevalence. This study expanded on previous research by analysing spatio-temporal patterns of individual and herd-based disease data to present multiple lines of evidence for disease-driven selection in free-ranging wildlife.

New and distinct chronic wasting disease strains associated with cervid polymorphism at codon 116 of the Prnp gene

Chronic wasting disease (CWD) is a prion disease affecting cervids. Polymorphisms in the prion protein gene can result in extended survival of CWD-infected animals. However, the impact of polymorphisms on cellular prion protein (PrP^C) and prion properties is less understood. Previously, we characterized the effects of a polymorphism at codon 116 (A>G) of the white-tailed deer (WTD) prion protein and determined that it destabilizes PrP^C structure. Comparing CWD isolates from WTD expressing homozygous wild-type (116AA) or heterozygous (116AG) PrP, we found that 116AG-prions were conformationally less stable, more sensitive to proteases, with lower seeding activity in cell-free conversion and reduced infectivity. Here, we aimed to understand CWD strain emergence and adaptation. We show that the WTD-116AG isolate contains two different prion strains, distinguished by their host range, biochemical properties, and pathogenesis from WTD-116AA prions (Wisc-1). Serial passages of WTD-116AG prions in tg(CerPrP)1536^+/+ mice overexpressing wild-type deer-PrP^C revealed two populations of mice with short and long incubation periods, respectively, and remarkably prolonged clinical phase upon inoculation with WTD-116AG prions. Inoculation of serially diluted brain homogenates confirmed the presence of two strains in the 116AG isolate with distinct pathology in the brain. Interestingly, deglycosylation revealed proteinase K-resistant fragments with different electrophoretic mobility in both tg(CerPrP)1536^+/+ mice and Syrian golden hamsters infected with WTD-116AG. Infection of tg60 mice expressing deer S96-PrP with 116AG, but not Wisc-1 prions induced clinical disease. On the contrary, bank voles resisted 116AG prions, but not Wisc-1 infection. Our data indicate that two strains co-existed in the WTD-116AG isolate, expanding the variety of CWD prion strains. We argue that the 116AG isolate does not contain Wisc-1 prions, indicating that the presence of 116G-PrP^C diverted 116A-PrP^C from adopting a Wisc-1 structure. This can have important implications for their possible distinct capacities to cross species barriers into both cervids and non-cervids.

Chronic wasting disease belongs to the family of prion diseases. It is considered the most contagious prion disease and the only one that affects free ranging wildlife. The disease range is expanding in North America and Northern Europe. This work describes the emergence and characterization of new chronic wasting disease strains related to a polymorphism in the prion protein gene. It supports the concept that strains are a dynamic mixture of substrains that can influence and interfere with each other. Because transmission barriers are governed by the compatibility of a particular prion strain with the new host’s prion protein, it is critical to understand the emergence and variety of chronic wasting disease strains circulating in wild animals and their ability to infect new host species including humans.

Adaptive selection of a prion strain conformer corresponding to established North American CWD during propagation of novel emergent Norwegian strains in mice expressing elk or deer prion protein

Prions are infectious proteins causing fatal, transmissible neurodegenerative diseases of animals and humans. Replication involves template-directed refolding of host encoded prion protein, PrP^C, by its infectious conformation, PrP^Sc. Following its discovery in captive Colorado deer in 1967, uncontrollable contagious transmission of chronic wasting disease (CWD) led to an expanded geographic range in increasing numbers of free-ranging and captive North American (NA) cervids. Some five decades later, detection of PrP^Sc in free-ranging Norwegian (NO) reindeer and moose marked the first indication of CWD in Europe. To assess the properties of these emergent NO prions and compare them with NA CWD we used transgenic (Tg) and gene targeted (Gt) mice expressing PrP with glutamine (Q) or glutamate (E) at residue 226, a variation in wild type cervid PrP which influences prion strain selection in NA deer and elk. Transmissions of NO moose and reindeer prions to Tg and Gt mice recapitulated the characteristic features of CWD in natural hosts, revealing novel prion strains with disease kinetics, neuropathological profiles, and capacities to infect lymphoid tissues and cultured cells that were distinct from those causing NA CWD. In support of strain variation, PrP^Sc conformers comprising emergent NO moose and reindeer CWD were subject to selective effects imposed by variation at residue 226 that were different from those controlling established NA CWD. Transmission of particular NO moose CWD prions in mice expressing E at 226 resulted in selection of a kinetically optimized conformer, subsequent transmission of which revealed properties consistent with NA CWD. These findings illustrate the potential for adaptive selection of strain conformers with improved fitness during propagation of unstable NO prions. Their potential for contagious transmission has implications for risk analyses and management of emergent European CWD. Finally, we found that Gt mice expressing physiologically controlled PrP levels recapitulated the lymphotropic properties of naturally occurring CWD strains resulting in improved susceptibilities to emergent NO reindeer prions compared with over-expressing Tg counterparts. These findings underscore the refined advantages of Gt models for exploring the mechanisms and impacts of strain selection in peripheral compartments during natural prion transmission.

Prions cause fatal, transmissible neurodegenerative diseases in animals and humans. They are composed of an infectious, neurotoxic protein (PrP) which replicates by imposing pathogenic conformations on its normal, host-encoded counterpart. Chronic wasting disease (CWD) is a contagious prion disorder threatening increasing numbers of free-ranging and captive North American deer, elk, and moose. While CWD detection in Norwegian reindeer and moose in 2016 marked the advent of disease in Europe, its origins and relationship to North American CWD were initially unclear. Here we show, using mice engineered to express deer or elk PrP, that Norwegian reindeer and moose CWD are caused by novel prion strains with properties distinct from those of North American CWD. We found that selection and propagation of North American and Norwegian CWD strains was controlled by a key amino acid residue in host PrP. We also found that particular Norwegian isolates adapted during their propagation in mice to produce prions with characteristics of the North American strain. Our findings defining the transmission profiles of novel Norwegian prions and their unstable potential to produce adapted strains with improved fitness for contagious transmission have implications for risk analyses and management of emergent European CWD.

White-tailed deer S96 prion protein does not support stable in vitro propagation of most common CWD strains

PrP^C variation at residue 96 (G/S) plays an important role in the epidemiology of chronic wasting disease (CWD) in exposed white-tailed deer populations. In vivo studies have demonstrated the protective effect of serine at codon 96, which hinders the propagation of common CWD strains when expressed in homozygosis and increases the survival period of S96/wt heterozygous deer after challenge with CWD. Previous in vitro studies of the transmission barrier suggested that following a single amplification step, wt and S96 PrP^C were equally susceptible to misfolding when seeded with various CWD prions. When we performed serial prion amplification in vitro using S96-PrP^C, we observed a reduction in the efficiency of propagation with the Wisc-1 or CWD2 strains, suggesting these strains cannot stably template their conformations on this PrP^C once the primary sequence has changed after the first round of replication. Our data shows the S96-PrP^C polymorphism is detrimental to prion conversion of some CWD strains. These data suggests that deer homozygous for S96-PrP^C may not sustain prion transmission as compared to a deer expressing G96-PrP^C.

Characterization of the prion protein gene in axis deer (Axis axis) and implications for susceptibility to chronic wasting disease

Axis deer (Axis axis) occur both in captivity and free-ranging populations in portions of North America, but to-date, no data exist pertaining to the species’ susceptibility to CWD. We sequenced the prion protein gene (PRNP) from axis deer. We then compared axis deer PrP^C sequences and amino acid polymorphisms to those of CWD susceptible species. A single PRNP allele with no evidence of intraspecies variation was identified in axis deer that indicates axis deer PRNP is most similar to North American elk (Cervus canadensis) PRNP. Therefore, axis deer may be susceptible to CWD. We recommend proactively increasing CWD surveillance for axis deer, particularly where CWD has been detected and axis deer are sympatric with native North American CWD susceptible species.

Cervid Prion Protein Polymorphisms: Role in Chronic Wasting Disease Pathogenesis

Chronic wasting disease (CWD) is a prion disease found in both free-ranging and farmed cervids. Susceptibility of these animals to CWD is governed by various exogenous and endogenous factors. Past studies have demonstrated that polymorphisms within the prion protein (PrP) sequence itself affect an animal's susceptibility to CWD. PrP polymorphisms can modulate CWD pathogenesis in two ways: the ability of the endogenous prion protein (PrP^C) to convert into infectious prions (PrP^Sc) or it can give rise to novel prion strains. In vivo studies in susceptible cervids, complemented by studies in transgenic mice expressing the corresponding cervid PrP sequence, show that each polymorphism has distinct effects on both PrP^C and PrP^Sc. It is not entirely clear how these polymorphisms are responsible for these effects, but in vitro studies suggest they play a role in modifying PrP epitopes crucial for PrP^C to PrP^Sc conversion and determining PrP^C stability. PrP polymorphisms are unique to one or two cervid species and most confer a certain degree of reduced susceptibility to CWD. However, to date, there are no reports of polymorphic cervid PrP alleles providing absolute resistance to CWD. Studies on polymorphisms have focused on those found in CWD-endemic areas, with the hope that understanding the role of an animal's genetics in CWD can help to predict, contain, or prevent transmission of CWD.

Predicting the spread-risk potential of chronic wasting disease to sympatric ungulate species

Wildlife disease incidence is increasing, resulting in negative impacts on the economy, biodiversity, and potentially human health. Chronic wasting disease (CWD) is a fatal, transmissible spongiform encephalopathy of cervids (wild and captive) which continues to spread geographically resulting in exposure to potential new host species. The disease agent (PrPCWD) is a misfolded conformer of the cellular prion protein (PrPC). In Canada, the disease is endemic in Alberta and Saskatchewan, affecting mule and white-tail deer, with lesser impact on elk and moose. As the disease continues to expand, additional wild ungulate species including bison, bighorn sheep, mountain goat, and pronghorn antelope may be exposed. To better understand the species-barrier, we reviewed the current literature on taxa naturally or experimentally exposed to CWD to identify susceptible and resistant species. We created a phylogeny of these taxa using cytochrome B and found that CWD susceptibility followed the species phylogeny. Using this phylogeny we estimated the probability of CWD susceptibility for wild ungulate species. We then compared PrPC amino acid polymorphisms among these species to identify which sites segregated between susceptible and resistant species. We identified sites that were significantly associated with susceptibility, but they were not fully discriminating. Finally, we sequenced Prnp from 578 wild ungulates to further evaluate their potential susceptibility. Together, these data suggest the host-range for CWD will potentially include pronghorn, mountain goat and bighorn sheep, but bison are likely to be more resistant. These findings highlight the need for monitoring potentially susceptible species as CWD continues to expand.

Large-scale prion protein genotyping in Canadian caribou populations and potential impact on chronic wasting disease susceptibility

Polymorphisms within the prion protein gene (Prnp) are an intrinsic factor that can modulate chronic wasting disease (CWD) pathogenesis in cervids. Although wild European reindeer (Rangifer tarandus tarandus) were infected with CWD, as yet there have been no reports of the disease in North American caribou (R. tarandus spp.). Previous Prnp genotyping studies on approximately 200 caribou revealed single nucleotide polymorphisms (SNPs) at codons 2 (V/M), 129 (G/S), 138 (S/N), 146 (N/n) and 169 (V/M). The impact of these polymorphisms on CWD transmission is mostly unknown, except for codon 138. Reindeer carrying at least one allele encoding for asparagine (138NN or 138SN) are less susceptible to clinical CWD upon infection by natural routes, with the majority of prions limited to extraneural tissues. We sequenced the Prnp coding region of two caribou subspecies (n = 986) from British Columbia, Saskatchewan, Yukon, Nunavut and the Northwest Territories, to identify SNPs and their frequencies. Genotype frequencies at codon 138 differed significantly between barren-ground (R. t. groenlandicus) and woodland (R. t. caribou) caribou when we excluded the Chinchaga herd (p < .05). We also found new variants at codons 153 (Y/F) and 242 (P/L). Our findings show that the 138N allele is rare among caribou in areas with higher risk of contact with CWD-infected species. As both subspecies are classified as Threatened and play significant roles in North American Indigenous culture, history, food security and the economy, determining frequencies of Prnp genotypes associated with susceptibility to CWD is important for future wildlife management measures.

Identification of a homology-independent linchpin domain controlling mouse and bank vole prion protein conversion

Prions are unorthodox pathogens that cause fatal neurodegenerative diseases in humans and other mammals. Prion propagation occurs through the self-templating of the pathogenic conformer PrP^Sc, onto the cell-expressed conformer, PrP^C. Here we study the conversion of PrP^C to PrP^Sc using a recombinant mouse PrP^Sc conformer (mouse protein-only recPrP^Sc) as a unique tool that can convert bank vole but not mouse PrP^C substrates in vitro. Thus, its templating ability is not dependent on sequence homology with the substrate. In the present study, we used chimeric bank vole/mouse PrP^C substrates to systematically determine the domain that allows for conversion by Mo protein-only recPrP^Sc. Our results show that that either the presence of the bank vole amino acid residues E227 and S230 or the absence of the second N-linked glycan are sufficient to allow PrP^C substrates to be converted by Mo protein-only recPrP^Sc and several native infectious prion strains. We propose that residues 227 and 230 and the second glycan are part of a C-terminal domain that acts as a linchpin for bank vole and mouse prion conversion.

Prions are unconventional infectious agents that lack nucleic acids such as DNA and RNA, and the mechanism by which prions replicate is not fully understood. It has been established that a central feature of the replication mechanism involves the misfolding of a host protein (PrP^C) into an infectious shape termed PrP^Sc, but it is unclear how this misfolding occurs. Interestingly, it has been observed that a particular animal species, the European bank vole, is unusually susceptible to prion infection and that this near-universal susceptibility is caused by the specific PrP^C sequence of this protein. Here we use a powerful and unique biochemical system to determine the specific region of bank vole PrP^C that is primarily responsible for its propensity to misfold into PrP^Sc. This critical region, which is located at the extreme C-terminal end of the protein, appears to act as a linchpin domain that normally stabilizes the shape of PrP^C and thereby regulates its misfolding into PrP^Sc.

Incomplete glycosylation during prion infection unmasks a prion protein epitope that facilitates prion detection and strain discrimination

The causative factors underlying conformational conversion of cellular prion protein (PrP^C) into its infectious counterpart (PrP^Sc) during prion infection remain undetermined, in part because of a lack of monoclonal antibodies (mAbs) that can distinguish these conformational isoforms. Here we show that the anti-PrP mAb PRC7 recognizes an epitope that is shielded from detection when glycans are attached to Asn-196. We observed that whereas PrP^C is predisposed to full glycosylation and is therefore refractory to PRC7 detection, prion infection leads to diminished PrP^Sc glycosylation at Asn-196, resulting in an unshielded PRC7 epitope that is amenable to mAb recognition upon renaturation. Detection of PRC7-reactive PrP^Sc in experimental and natural infections with various mouse-adapted scrapie strains and with prions causing deer and elk chronic wasting disease and transmissible mink encephalopathy uncovered that incomplete PrP^Sc glycosylation is a consistent feature of prion pathogenesis. We also show that interrogating the conformational properties of the PRC7 epitope affords a direct means of distinguishing different prion strains. Because the specificity of our approach for prion detection and strain discrimination relies on the extent to which N-linked glycosylation shields or unshields PrP epitopes from antibody recognition, it dispenses with the requirement for additional standard manipulations to distinguish PrP^Sc from PrP^C, including evaluation of protease resistance. Our findings not only highlight an innovative and facile strategy for prion detection and strain differentiation, but are also consistent with a mechanism of prion replication in which structural instability of incompletely glycosylated PrP contributes to the conformational conversion of PrP^C to PrP^Sc.

Chronic wasting disease (CWD) prion strains evolve via adaptive diversification of conformers in hosts expressing prion protein polymorphisms

Chronic wasting disease (CWD) is caused by an unknown spectrum of prions and has become enzootic in populations of cervid species that express cellular prion protein (PrPC) molecules varying in amino acid composition. These PrPC polymorphisms can affect prion transmission, disease progression, neuropathology, and emergence of new prion strains, but the mechanistic steps in prion evolution are not understood. Here, using conformation-dependent immunoassay, conformation stability assay, and protein-misfolding cyclic amplification, we monitored the conformational and phenotypic characteristics of CWD prions passaged through deer and transgenic mice expressing different cervid PrPC polymorphisms. We observed that transmission through hosts with distinct PrPC sequences diversifies the PrPCWD conformations and causes a shift toward oligomers with defined structural organization, replication rate, and host range. When passaged in host environments that restrict prion replication, distinct co-existing PrPCWD conformers underwent competitive selection, stabilizing a new prion strain. Nonadaptive conformers exhibited unstable replication and accumulated only to low levels. These results suggest a continuously evolving diversity of CWD conformers and imply a critical interplay between CWD prion plasticity and PrPC polymorphisms during prion strain evolution.

Characterization of goat prions demonstrates geographical variation of scrapie strains in Europe and reveals the composite nature of prion strains

Bovine Spongiform Encephalopathy (BSE) is the only animal prion which has been recognized as a zoonotic agent so far. The identification of BSE in two goats raised the need to reliably identify BSE in small ruminants. However, our understanding of scrapie strain diversity in small ruminants remains ill-defined, thus limiting the accuracy of BSE surveillance and spreading fear that BSE might lurk unrecognized in goats. We investigated prion strain diversity in a large panel of European goats by a novel experimental approach that, instead of assessing the neuropathological profile after serial transmissions in a single animal model, was based on the direct interaction of prion isolates with several recipient rodent models expressing small ruminants or heterologous prion proteins. The findings show that the biological properties of scrapie isolates display different patterns of geographical distribution in Europe and suggest that goat BSE could be reliably discriminated from a wide range of biologically and geographically diverse goat prion isolates. Finally, most field prion isolates showed composite strain features, with discrete strain components or sub-strains being present in different proportions in individual goats or tissues. This has important implications for understanding the nature and evolution of scrapie strains and their transmissibility to other species, including humans.

In Vitro detection of Chronic Wasting Disease (CWD) prions in semen and reproductive tissues of white tailed deer bucks (Odocoileus virginianus)

Chronic Wasting Disease (CWD) is a prion disease affecting several cervid species. Among them, white-tailed deer (WTD) are of relevance due to their value in farming and game hunting. The exact events involved in CWD transmission in captive and wild animals are still unclear. An unexplored mechanism of CWD spread involves transmissions through germplasm, such as semen. Surprisingly, the presence and load of CWD prions in semen and male sexual tissues from WTD has not been explored. Here, we described the detection of CWD prions in semen and sexual tissues of WTD bucks utilizing the Protein Misfolding Cyclic Amplification (PMCA) technology. Samples were obtained post-mortem from farmed pre-clinical, CWD positive WTD bucks possessing polymorphisms at position 96 of the PRNP gene. Our results show that overall CWD detection in these samples had a sensitivity of 59.3%, with a specificity of 97.2%. The data indicate that the presence of CWD prions in male sexual organs and fluids is prevalent in late stage, pre-clinical, CWD-infected WTD (80%-100% of the animals depending on the sample type analyzed). Our findings reveal the presence of CWD prions in semen and sexual tissues of prion infected WTD bucks. Future studies will be necessary to determine whether sexual contact and/or artificial inseminations are plausible means of CWD transmission in susceptible animal species.

Chronic wasting disease associated with prion protein gene (PRNP) variation in Norwegian wild reindeer (Rangifer tarandus)

The emergence of CWD in Europe in 2016 and the first natural infection in wild reindeer warranted disease management. This led to the testing of 2424 hunted or culled reindeer during 2016–2018, from the infected subpopulation in the Nordfjella mountain range in Southern Norway. To identify any association between PRNP variation and CWD susceptibility, we characterized the open reading frame of the PRNP gene in 19 CWD positive reindeer and in 101 age category- and sex-matched CWD negative controls. Seven variant positions were identified: 6 single nucleotide variants (SNVs) and a 24 base pair (bp) deletion located between nucleotide position 238 and 272, encoding four instead of five octapeptide repeats. With a single exception, all variant positions but one were predicted to be non-synonymous. The synonymous SNV and the deletion are novel in reindeer. Various combinations of the non-synonymous variant positions resulted in the identification of five PRNP alleles (A-E) that structured into 14 genotypes. We identified an increased CWD risk in reindeer carrying two copies of the most common allele, A, coding for serine in position 225 (Ser225) and in those carrying allele A together with the 24 bp deletion.

Unique Structural Features of Mule Deer Prion Protein Provide Insights into Chronic Wasting Disease

Chronic wasting disease (CWD) is a highly infectious prion disease of cervids. Accumulation of prions, the disease-specific structural conformers of the cellular prion protein (PrP^C), in the central nervous system, is the key pathological event of the disorder. The analysis of cervid PrP^C sequences revealed the existence of polymorphism at position 226, in which deer PrP contains glutamine (Q), whereas elk PrP contains glutamate (E). The effects of this polymorphism on CWD are still unknown. We determined the high-resolution nuclear magnetic resonance structure of the mule deer prion protein that was compared to previously published PrP structures of elk and white-tailed deer. We found that the polymorphism Q226E could influence the long-range intramolecular interactions and packing of the β2-α2 loop and the C-terminus of the α3 helix of cervid PrP structures. This solvent-accessible epitope is believed to be involved in prion conversion. Additional differences were observed at the beginning of the well-defined C-terminus domain, in the α2-α3 region, and in its interactions with the α1 helix. Here, we highlight the importance of the PrP structure in prion susceptibility and how single amino acid differences might influence the overall protein folding.

Primary structural differences at residue 226 of deer and elk PrP dictate selection of distinct CWD prion strains in gene-targeted mice

Our gene-targeting strategy addresses several critical issues relating to chronic wasting disease (CWD), a contagious, lethal neurodegenerative prion disease affecting wild as well as captive cervids, which poses an uncertain risk to humans. First, we show that residue 226, the sole primary structural difference between deer and elk prion protein, dictates the selection and propagation of different CWD strains. Because the strain properties of prions affect their host-range potential, these findings suggest that CWD from elk and deer pose distinct risks to humans and other species. Second, we show that these gene-targeted mice offer an unprecedented means to address distinctive aspects of CWD peripheral pathogenesis and horizontal transmission that are not afforded by previously generated transgenic mouse models.

Although the unifying hallmark of prion diseases is CNS neurodegeneration caused by conformational corruption of host prion protein (PrP) to its infective counterpart, contagious transmission of chronic wasting disease (CWD) results from shedding of prions produced at high titers in the periphery of diseased cervids. While deer and elk PrP primary structures are equivalent except at residue 226, which is glutamate in elk and glutamine in deer, the effect of this difference on CWD pathogenesis is largely unknown. Using a gene-targeting approach where the mouse PrP coding sequence was replaced with elk or deer PrP, we show that the resulting GtE226 and GtQ226 mice had distinct kinetics of disease onset, prion conformations, and distributions of prions in the brains of diseased mice following intracerebral CWD challenge. These findings indicate that amino acid differences at PrP residue 226 dictate the selection and propagation of divergent strains in deer and elk with CWD. Because prion strain properties largely dictate host-range potential, our findings suggest that prion strains from elk and deer pose distinct risks to sympatric species or humans exposed to CWD. GtE226 and GtQ226 mice were also highly susceptible to CWD prions following intraperitoneal and oral exposures, a characteristic that stood in stark contrast to previously produced transgenic models. Remarkably, disease transmission was effective when infected mice were cohoused with naïve cagemates. Our findings indicate that gene-targeted mice provide unprecedented opportunities to accurately investigate CWD peripheral pathogenesis, CWD strains, and mechanisms of horizontal CWD transmission.

Prion protein polymorphisms associated with reduced CWD susceptibility limit peripheral PrPCWD deposition in orally infected white-tailed deer

Background

Chronic wasting disease (CWD) is a prion disease affecting members of the Cervidae family. PrP^C primary structures play a key role in CWD susceptibility resulting in extended incubation periods and regulating the propagation of CWD strains. We analyzed the distribution of abnormal prion protein (PrP^CWD) aggregates in brain and peripheral organs from orally inoculated white-tailed deer expressing four different PRNP genotypes: Q95G96/Q95G96 (wt/wt), S96/wt, H95/wt and H95/S96 to determine if there are substantial differences in the deposition pattern of PrP^CWD between different PRNP genotypes.

Results

Although we detected differences in certain brain areas, globally, the different genotypes showed similar PrP^CWD deposition patterns in the brain. However, we found that clinically affected deer expressing H95 PrP^C, despite having the longest survival periods, presented less PrP^CWD immunoreactivity in particular peripheral organs. In addition, no PrP^CWD was detected in skeletal muscle of any of the deer.

Conclusions

Our data suggest that expression of H95-PrP^C limits peripheral accumulation of PrP^CWD as detected by immunohistochemistry. Conversely, infected S96/wt and wt/wt deer presented with similar PrP^CWD peripheral distribution at terminal stage of disease, suggesting that the S96-PrP^C allele, although delaying CWD progression, does not completely limit the peripheral accumulation of the infectious agent.

Source genotype influence on cross species transmission of transmissible spongiform encephalopathies evaluated by RT-QuIC

Scrapie is a naturally occurring transmissible spongiform encephalopathy of sheep and goats. This fatal neurodegenerative disease is caused by misfolding of the cellular prion protein to pathogenic β-rich conformers (PrPSc) that accumulate in higher order structures of the brain and other tissues. This conversion has been used for in vitro assays including serial protein misfolding amplification and real-time quaking induced conversion (RT-QuIC). RT-QuIC can be used for the detection of prions and for strain discrimination in a variety of biological tissues from humans and animals. In this study, we evaluated how PrPSc isolated from sheep of different genotypes after inoculation with the scrapie agent influence the fibril formation in vitro using RT-QuIC. We found that reaction mixtures seeded with PrPSc from genotype VRQ/VRQ sheep brains have better conversion efficiency with 132M elk substrate compared to reactions seeded with PrPSc from the brains of sheep with the ARQ/ARQ genotype no matter which strain of scrapie was used to seed the reactions. We also inoculated transgenic mice expressing 132M elk PRNP (Tg12) with the scrapie agent from different genotypes of sheep to compare with our RT-QuIC results. The bioassays support the data showing a significantly shorter incubation period for inoculum from VRQ/VRQ sheep when compared to inoculum from ARQ/ARQ sheep. Thus, we conclude that the genotype of both source and recipient can strongly influence transmission.

Chronic wasting disease: an evolving prion disease of cervids

Chronic wasting disease (CWD) is a relatively new and burgeoning prion epidemic of deer, elk, reindeer, and moose, which are members of the cervid family. While the disease was first described in captive deer, its subsequent discovery in various species of free-ranging animals makes it the only currently recognized prion disorder of both wild and farmed animals. In addition to its expanding range of host species, CWD continues to spread from North America to new geographic areas, including South Korea, and most recently Norway, marking the first time this disease was detected in Europe. Its unparalleled efficiency of contagious transmission, combined with high densities of deer in certain areas, complicates strategies for controlling CWD, raising concerns about its potential for spread to new species. Because there is a high prevalence of CWD in deer and elk, which are commonly hunted and consumed by humans, and since prions from cattle with bovine spongiform encephalopathy have been transmitted to humans causing variant Creutzfeldt-Jakob disease, the possibility of zoonotic transmission of CWD is particularly concerning. Here we review the clinical and pathologic features of CWD and its disturbing epidemiology, and discuss features that affect its transmission, including genetic susceptibility, pathogenesis, and agent strain variability. Finally, we discuss evidence that speaks to the potential for zoonotic transmission of this emerging disease.

Modified Protein Misfolding Cyclic Amplification Overcomes Real-Time Quaking-Induced Conversion Assay Inhibitors in Deer Saliva To Detect Chronic Wasting Disease Prions

Chronic wasting disease (CWD), a fatal neurodegenerative prion disease of cervids, has spread across North America and has been detected in The Republic of Korea, Finland, and Norway. CWD appears to spread by horizontal transmission, and prions shed in saliva, feces, and urine are thought to contribute. However, studies investigating the rapid spread of CWD have been hampered by assay inhibitors and a lack of consistent and sensitive means to detect the relatively low levels of prions in these samples. Here we show that saliva frequently contains an inhibitor of the real-time quaking-induced conversion assay (RT-QuIC) and that the inhibitor is a member of the mucin family. To circumvent the inhibitor, we developed a modified protein misfolding cyclic amplification (PMCA) method to amplify CWD prions in saliva that were undetectable or ambiguous by RT-QuIC. Our results reinforce the impact of saliva in horizontal CWD transmission and highlight the importance of detection optimization.

Disulfide-crosslink scanning reveals prion-induced conformational changes and prion strain-specific structures of the pathological prion protein PrPSc

Prions are composed solely of the pathological isoform (PrP^Sc) of the normal cellular prion protein (PrP^C). Identification of different PrP^Sc structures is crucially important for understanding prion biology because the pathogenic properties of prions are hypothesized to be encoded in the structures of PrP^Sc However, these structures remain yet to be identified, because of the incompatibility of PrP^Sc with conventional high-resolution structural analysis methods. Previously, we reported that the region between the first and the second α-helix (H1∼H2) of PrP^C might cooperate with the more C-terminal side region for efficient interactions with PrP^Sc From this starting point, we created a series of PrP variants with two cysteine substitutions (C;C-PrP) forming a disulfide-crosslink between H1∼H2 and the distal region of the third helix (Ctrm). We then assessed the conversion capabilities of the C;C-PrP variants in N2a cells infected with mouse-adapted scrapie prions (22L-ScN2a). Specifically, Cys substitutions at residues 165, 166, or 168 in H1∼H2 were combined with cysteine scanning along Ctrm residues 220-229. We found that C;C-PrPs are expressed normally with glycosylation patterns and subcellular localization similar to WT PrP, albeit differing in expression levels. Interestingly, some C;C-PrPs converted to protease-resistant isoforms in the 22L-ScN2a cells, but not in Fukuoka1 prion-infected cells. Crosslink patterns of convertible C;C-PrPs indicated a positional change of H1∼H2 toward Ctrm in PrP^Sc-induced conformational conversion. Given the properties of the C;C-PrPs reported here, we propose that these PrP variants may be useful tools for investigating prion strain-specific structures and structure-phenotype relationships of PrP^Sc.

Molecular Mechanisms of Chronic Wasting Disease Prion Propagation

Prion disease epidemics, which have been unpredictable recurrences, are of significant concern for animal and human health. Examples include kuru, once the leading cause of death among the Fore people in Papua New Guinea and caused by mortuary feasting; bovine spongiform encephalopathy (BSE) and its subsequent transmission to humans in the form of variant Creutzfeldt-Jakob disease (vCJD), and repeated examples of large-scale prion disease epidemics in animals caused by contaminated vaccines. The etiology of chronic wasting disease (CWD), a relatively new and burgeoning prion epidemic in deer, elk, and moose (members of the cervid family), is more enigmatic. The disease was first described in captive and later in wild mule deer and subsequently in free-ranging as well as captive Rocky Mountain elk, white-tailed deer, and most recently moose. It is therefore the only recognized prion disorder of both wild and captive animals. In addition to its expanding range of hosts, CWD continues to spread to new geographical areas, including recent cases in Norway. The unparalleled efficiency of the contagious transmission of the disease combined with high densities of deer in certain areas of North America complicates strategies for controlling CWD and raises concerns about its potential spread to new species. Because there is a high prevalence of CWD in deer and elk, which are commonly hunted and consumed by humans, the possibility of zoonotic transmission is particularly concerning. Here, we review the current status of naturally occurring CWD and describe advances in our understanding of its molecular pathogenesis, as shown by studies of CWD prions in novel in vivo and in vitro systems.

Destabilizing polymorphism in cervid prion protein hydrophobic core determines prion conformation and conversion efficiency

Prion diseases are infectious neurodegenerative disorders of humans and animals caused by misfolded forms of the cellular prion protein PrP^C. Prions cause disease by converting PrP^C into aggregation-prone PrP^Sc. Chronic wasting disease (CWD) is the most contagious prion disease with substantial lateral transmission, affecting free-ranging and farmed cervids. Although the PrP primary structure is highly conserved among cervids, the disease phenotype can be modulated by species-specific polymorphisms in the prion protein gene. How the resulting amino-acid substitutions impact PrP^C and PrP^Sc structure and propagation is poorly understood. We investigated the effects of the cervid 116A>G substitution, located in the most conserved PrP domain, on PrP^C structure and conversion and on 116AG-prion conformation and infectivity. Molecular dynamics simulations revealed structural de-stabilization of 116G-PrP, which enhanced its in vitro conversion efficiency when used as recombinant PrP substrate in real-time quaking-induced conversion (RT-QuIC). We demonstrate that 116AG-prions are conformationally less stable, show lower activity as a seed in RT-QuIC and exhibit reduced infectivity in vitro and in vivo. Infectivity of 116AG-prions was significantly enhanced upon secondary passage in mice, yet conformational features were retained. These findings indicate that structurally de-stabilized PrP^C is readily convertible by cervid prions of different genetic background and results in a prion conformation adaptable to cervid wild-type PrP. Conformation is an important criterion when assessing transmission barrier, and conformational variants can target a different host range. Therefore, a thorough analysis of CWD isolates and re-assessment of species-barriers is important in order to fully exclude a zoonotic potential of CWD.

Chronic wasting disease (CWD) is a prion disease which affects wild and captive cervids. Prion diseases are infectious neurodegenerative disorders, and the causative agent consists of abnormally folded prion protein termed PrP^Sc. Prions replicate without genetic information, and their three-dimensional structure is thought to encode heritable information necessary to propagate using the cellular prion protein PrP^C as a substrate for conversion. In this study, we use in vitro and in vivo techniques to analyze the effect of a polymorphism at codon 116 (A>G) of the white-tailed deer prion protein on CWD prion conformation, propagation and pathogenesis. We observed differences in conformation, infectivity and seeding activity in vitro between CWD prions isolated from white-tailed deer encoding wild-type (116AA) PrP^C or 116AG-PrP^C. In mouse bioassays conformational differences are retained, however, 116AG CWD prions resulted in significantly shortened incubation times upon passages. Molecular dynamics simulations suggest that the structure of 116G-PrP^C is more flexible, which is supported by an improved convertibility in an in vitro conversion assay. Altogether these data indicate the importance of a variation in the most conserved PrP domain, and highlight the relationship between PrP^C structural flexibility, prion conformation and conversion, and pathogenesis of prion disease in vivo.

Proteopathic Strains and the Heterogeneity of Neurodegenerative Diseases

Most age-related neurodegenerative diseases are associated with the misfolding and aberrant accumulation of specific proteins in the nervous system. The proteins self-assemble and spread by a prion-like process of corruptive molecular templating, whereby abnormally folded proteins induce the misfolding and aggregation of like proteins into characteristic lesions. Despite the apparent simplicity of this process at the molecular level, diseases such as Alzheimer's, Parkinson's, Creutzfeldt-Jakob, and others display remarkable phenotypic heterogeneity, both clinically and pathologically. Evidence is growing that this variability is mediated, at least in part, by the acquisition of diverse molecular architectures by the misfolded proteins, variants referred to as proteopathic strains. The structural and functional diversity of the assemblies is influenced by genetic, epigenetic, and local contextual factors. Insights into proteopathic strains gleaned from the classical prion diseases can be profitably incorporated into research on other neurodegenerative diseases. Their potentially wide-ranging influence on disease phenotype also suggests that proteopathic strains should be considered in the design and interpretation of diagnostic and therapeutic approaches to these disorders.

Prion replication without host adaptation during interspecies transmissions

Adaptation of prions to new species is thought to reflect the capacity of the host-encoded cellular form of the prion protein (PrP^C) to selectively propagate optimized prion conformations from larger ensembles generated in the species of origin. Here we describe an alternate replicative process, termed nonadaptive prion amplification (NAPA), in which dominant conformers bypass this requirement during particular interspecies transmissions. To model susceptibility of horses to prions, we produced transgenic (Tg) mice expressing cognate PrP^C Although disease transmission to only a subset of infected TgEq indicated a significant barrier to EqPrP^C conversion, the resulting horse prions unexpectedly failed to cause disease upon further passage to TgEq. TgD expressing deer PrP^C was similarly refractory to deer prions from diseased TgD infected with mink prions. In both cases, the resulting prions transmitted to mice expressing PrP^C from the species of prion origin, demonstrating that transmission barrier eradication of the originating prions was ephemeral and adaptation superficial in TgEq and TgD. Horse prions produced in vitro by protein misfolding cyclic amplification of mouse prions using horse PrP^C also failed to infect TgEq but retained tropism for wild-type mice. Concordant patterns of neuropathology and prion deposition in susceptible mice infected with NAPA prions and the corresponding prion of origin confirmed preservation of strain properties. The comparable responses of both prion types to guanidine hydrochloride denaturation indicated this occurs because NAPA precludes selection of novel prion conformations. Our findings provide insights into mechanisms regulating interspecies prion transmission and a framework to reconcile puzzling epidemiological features of certain prion disorders.

Chronic wasting disease (CWD) in cervids

In April and May of 2016, Norway confirmed two cases of chronic wasting disease (CWD) in a wild reindeer and a wild moose, respectively. In the light of this emerging issue, the European Commission requested EFSA to recommend surveillance activities and, if necessary, additional animal health risk-based measures to prevent the introduction of the disease and the spread into/within the EU, specifically Estonia, Finland, Iceland, Latvia, Lithuania, Norway, Poland and Sweden, and considering seven wild, semidomesticated and farmed cervid species (Eurasian tundra reindeer, Finnish (Eurasian) forest reindeer, moose, roe deer, white-tailed deer, red deer and fallow deer). It was also asked to assess any new evidence on possible public health risks related to CWD. A 3-year surveillance system is proposed, differing for farmed and wild or semidomesticated cervids, with a two-stage sampling programme at the farm/geographically based population unit level (random sampling) and individual level (convenience sampling targeting high-risk animals). The current derogations of Commission Implementing Decision (EU) 2016/1918 present a risk of introduction of CWD into the EU. Measures to prevent the spread of CWD within the EU are dependent upon the assumption that the disease is already present; this is currently unknown. The measures listed are intended to contain (limit the geographic extent of a focus) and/or to control (actively stabilise/reduce infection rates in an affected herd or population) the disease where it occurs. With regard to the zoonotic potential, the human species barrier for CWD prions does not appear to be absolute. These prions are present in the skeletal muscle and other edible tissues, so humans may consume infected material in enzootic areas. Epidemiological investigations carried out to date make no association between the occurrence of sporadic Creutzfeldt-Jakob disease in humans and exposure to CWD prions.

Insights into Mechanisms of Transmission and Pathogenesis from Transgenic Mouse Models of Prion Diseases

Prions represent a new paradigm of protein-mediated information transfer. In the case of mammals, prions are the cause of fatal, transmissible neurodegenerative diseases, sometimes referred to as transmissible spongiform encephalopathies (TSEs), which frequently occur as epidemics. An increasing body of evidence indicates that the canonical mechanism of conformational corruption of cellular prion protein (PrP^C) by the pathogenic isoform (PrP^Sc) that is the basis of prion formation in TSEs is common to a spectrum of proteins associated with various additional human neurodegenerative disorders, including the more common Alzheimer's and Parkinson's diseases. The peerless infectious properties of TSE prions, and the unparalleled tools for their study, therefore enable elucidation of mechanisms of template-mediated conformational propagation that are generally applicable to these related disease states. Many unresolved issues remain including the exact molecular nature of the prion, the detailed cellular and molecular mechanisms of prion propagation, and the means by which prion diseases can be both genetic and infectious. In addition, we know little about the mechanism by which neurons degenerate during prion diseases. Tied to this, the physiological role of the normal form of the prion protein remains unclear and it is uncertain whether or not loss of this function contributes to prion pathogenesis. The factors governing the transmission of prions between species remain unclear, in particular the means by which prion strains and PrP primary structure interact to affect interspecies prion transmission. Despite all these unknowns, advances in our understanding of prions have occurred because of their transmissibility to experimental animals, and the development of transgenic (Tg) mouse models has done much to further our understanding about various aspects of prion biology. In this review, we will focus on advances in our understanding of prion biology that occurred in the past 8 years since our last review of this topic.

A brief history of prions

Proteins were described as distinct biological molecules and their significance in cellular processes was recognized as early as the 18th century. At the same time, Spanish shepherds observed a disease that compelled their Merino sheep to pathologically scrape against fences, a defining clinical sign that led to the disease being named scrapie. In the late 19th century, Robert Koch published his postulates for defining causative agents of disease. In the early 20th century, pathologists Creutzfeldt and Jakob described a neurodegenerative disease that would later be included with scrapie into a group of diseases known as transmissible spongiform encephalopathies (TSEs). Later that century, mounting evidence compelled a handful of scientists to betray the prevailing biological dogma governing pathogen replication that Watson and Crick so convincingly explained by cracking the genetic code just two decades earlier. Because TSEs seemed to defy these new rules, J.S. Griffith theorized mechanisms by which a pathogenic protein could encipher its own replication blueprint without a genetic code. Stanley Prusiner called this proteinaceous infectious pathogen a prion. Here we offer a concise account of the discovery of prions, the causative agent of TSEs, in the wider context of protein biochemistry and infectious disease. We highlight the discovery of prions in yeast and discuss the implication of prions as epigenomic carriers of biological and pathological information. We also consider expanding the prion hypothesis to include other proteins whose alternate isoforms confer new biological or pathological properties.

Deer Prion Proteins Modulate the Emergence and Adaptation of Chronic Wasting Disease Strains

Transmission of chronic wasting disease (CWD) between cervids is influenced by the primary structure of the host cellular prion protein (PrP^C). In white-tailed deer, PRNP alleles encode the polymorphisms Q95 G96 (wild type [wt]), Q95 S96 (referred to as the S96 allele), and H95 G96 (referred to as the H95 allele), which differentially impact CWD progression. We hypothesize that the transmission of CWD prions between deer expressing different allotypes of PrP^C modifies the contagious agent affecting disease spread. To evaluate the transmission properties of CWD prions derived experimentally from deer of four PRNP genotypes (wt/wt, S96/wt, H95/wt, or H95/S96), transgenic (tg) mice expressing the wt allele (tg33) or S96 allele (tg60) were challenged with these prion agents. Passage of deer CWD prions into tg33 mice resulted in 100% attack rates, with the CWD H95/S96 prions having significantly longer incubation periods. The disease signs and neuropathological and protease-resistant prion protein (PrP-res) profiles in infected tg33 mice were similar between groups, indicating that a prion strain (Wisc-1) common to all CWD inocula was amplified. In contrast, tg60 mice developed prion disease only when inoculated with the H95/wt and H95/S96 CWD allotypes. Serial passage in tg60 mice resulted in adaptation of a novel CWD strain (H95⁺) with distinct biological properties. Transmission of first-passage tg60CWD-H95⁺ isolates into tg33 mice, however, elicited two prion disease presentations consistent with a mixture of strains associated with different PrP-res glycotypes. Our data indicate that H95-PRNP heterozygous deer accumulated two CWD strains whose emergence was dictated by the PrP^C primary structure of the recipient host. These findings suggest that CWD transmission between cervids expressing distinct PrP^C molecules results in the generation of novel CWD strains.

IMPORTANCE CWD prions are contagious among wild and captive cervids in North America and in South Korea. We present data linking the amino acid variant Q95H in white-tailed deer cellular prion protein (PrP^C) to the emergence of a novel CWD strain (H95⁺). We show that, upon infection, deer expressing H95-PrP^C molecules accumulated a mixture of CWD strains that selectively propagated depending on the PRNP genotype of the host in which they were passaged. Our study also demonstrates that mice expressing the deer S96-PRNP allele, previously shown to be resistant to various cervid prions, are susceptible to H95⁺ CWD prions. The potential for the generation of novel strains raises the possibility of an expanded host range for CWD.