Chronic wasting disease and atypical forms of bovine spongiform encephalopathy and scrapie are not transmissible to mice expressing wild-type levels of human prion protein

Lack of Transmission of Chronic Wasting Disease Prions to Human Cerebral Organoids

Chronic wasting disease (CWD) is a cervid prion disease with unknown zoonotic potential that might pose a risk to humans who are exposed. To assess the potential of CWD to infect human neural tissue, we used human cerebral organoids with 2 different prion genotypes, 1 of which has previously been associated with susceptibility to zoonotic prion disease. We exposed organoids from both genotypes to high concentrations of CWD inocula from 3 different sources for 7 days, then screened for infection periodically for up to 180 days. No de novo CWD propagation or deposition of protease-resistant forms of human prions was evident in CWD-exposed organoids. Some persistence of the original inoculum was detected, which was equivalent in prion gene knockout organoids and thus not attributable to human prion propagation. Overall, the unsuccessful propagation of CWD in cerebral organoids supports a strong species barrier to transmission of CWD prions to humans.

Chronic Wasting Disease: State of the Science

Chronic wasting disease (CWD) is a prion disease affecting cervid species, both free-ranging and captive populations. As the geographic range continues to expand and disease prevalence continues to increase, CWD will have an impact on cervid populations, local economies, and ecosystem health. Mitigation of this "wicked" disease will require input from many different stakeholders including hunters, landowners, research biologists, wildlife managers, and others, working together. The NC1209 (North American interdisciplinary chronic wasting disease research consortium) is composed of scientists from different disciplines involved with investigating and managing CWD. Leveraging this broad breadth of expertise, the Consortium has created a state-of-the-science review of five key aspects of CWD, including current diagnostic capabilities for detecting prions, requirements for validating these diagnostics, the role of environmental transmission in CWD dynamics, and potential zoonotic risks associated with CWD. The goal of this review is to increase stakeholders', managers', and decision-makers' understanding of this disease informed by current scientific knowledge.

Oral vaccination as a potential strategy to manage chronic wasting disease in wild cervid populations

Prion diseases are a novel class of infectious disease based in the misfolding of the cellular prion protein (PrPC) into a pathological, self-propagating isoform (PrPSc). These fatal, untreatable neurodegenerative disorders affect a variety of species causing scrapie in sheep and goats, bovine spongiform encephalopathy (BSE) in cattle, chronic wasting disease (CWD) in cervids, and Creutzfeldt-Jacob disease (CJD) in humans. Of the animal prion diseases, CWD is currently regarded as the most significant threat due its ongoing geographical spread, environmental persistence, uptake into plants, unpredictable evolution, and emerging evidence of zoonotic potential. The extensive efforts to manage CWD have been largely ineffective, highlighting the need for new disease management tools, including vaccines. Development of an effective CWD vaccine is challenged by the unique biology of these diseases, including the necessity, and associated dangers, of overcoming immune tolerance, as well the logistical challenges of vaccinating wild animals. Despite these obstacles, there has been encouraging progress towards the identification of safe, protective antigens as well as effective strategies of formulation and delivery that would enable oral delivery to wild cervids. In this review we highlight recent strategies for antigen selection and optimization, as well as considerations of various platforms for oral delivery, that will enable researchers to accelerate the rate at which candidate CWD vaccines are developed and evaluated.

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.

The Zoonotic Potential of Chronic Wasting Disease-A Review

Prion diseases are transmissible neurodegenerative disorders that affect humans and ruminant species consumed by humans. Ruminant prion diseases include bovine spongiform encephalopathy (BSE) in cattle, scrapie in sheep and goats and chronic wasting disease (CWD) in cervids. In 1996, prions causing BSE were identified as the cause of a new prion disease in humans; variant Creutzfeldt-Jakob disease (vCJD). This sparked a food safety crisis and unprecedented protective measures to reduce human exposure to livestock prions. CWD continues to spread in North America, and now affects free-ranging and/or farmed cervids in 30 US states and four Canadian provinces. The recent discovery in Europe of previously unrecognized CWD strains has further heightened concerns about CWD as a food pathogen. The escalating CWD prevalence in enzootic areas and its appearance in a new species (reindeer) and new geographical locations, increase human exposure and the risk of CWD strain adaptation to humans. No cases of human prion disease caused by CWD have been recorded, and most experimental data suggest that the zoonotic risk of CWD is very low. However, the understanding of these diseases is still incomplete (e.g., origin, transmission properties and ecology), suggesting that precautionary measures should be implemented to minimize human exposure.

Second passage experiments of chronic wasting disease in transgenic mice overexpressing human prion protein

Chronic wasting disease (CWD) is a prion disease of cervids including deer, elk, reindeer, and moose. Human consumption of cervids is common, therefore assessing the risk potential of CWD transmission to humans is critical. In a previous study, we tested CWD transmission via intracerebral inoculation into transgenic mice (tg66 and tgRM) that over-expressed human prion protein. Mice screened by traditional prion detection assays were negative. However, in a group of 88 mice screened by the ultrasensitive RT-QuIC assay, we identified 4 tg66 mice that produced inconsistent positive RT-QuIC reactions. These data could be false positive reactions, residual input inoculum or indicative of subclinical infections suggestive of cross species transmission of CWD to humans. Additional experiments were required to understand the nature of the prion seeding activity in this model. In this manuscript, second passage experiments using brains from mice with weak prion seeding activity showed they were not infectious to additional recipient tg66 mice. Clearance experiments showed that input CWD prion seeding activity was eliminated by 180 days in tg66 mice and PrPKO mice, which are unable to replicate prion protein, indicating that the weak positive levels of seeding activity detected at later time points was not likely residual inoculum. The failure of CWD prions to cause disease in tg66 after two sequential passages suggested that a strong species barrier prevented CWD infection of mice expressing human prion protein.

Bona fide atypical scrapie faithfully reproduced for the first time in a rodent model

Atypical Scrapie, which is not linked to epidemics, is assumed to be an idiopathic spontaneous prion disease in small ruminants. Therefore, its occurrence is unlikely to be controlled through selective breeding or other strategies as it is done for classical scrapie outbreaks. Its spontaneous nature and its sporadic incidence worldwide is reminiscent of the incidence of idiopathic spontaneous prion diseases in humans, which account for more than 85% of the cases in humans. Hence, developing animal models that consistently reproduce this phenomenon of spontaneous PrP misfolding, is of importance to study the pathobiology of idiopathic spontaneous prion disorders. Transgenic mice overexpressing sheep PrP^C with I112 polymorphism (TgShI112, 1-2 × PrP levels compared to sheep brain) manifest clinical signs of a spongiform encephalopathy spontaneously as early as 380 days of age. The brains of these animals show the neuropathological hallmarks of prion disease and biochemical analyses of the misfolded prion protein show a ladder-like PrP^res pattern with a predominant 7-10 kDa band. Brain homogenates from spontaneously diseased transgenic mice were inoculated in several models to assess their transmissibility and characterize the prion strain generated: TgShI112 (ovine I112 ARQ PrP^C), Tg338 (ovine VRQ PrP^C), Tg501 (ovine ARQ PrP^C), Tg340 (human M129 PrP^C), Tg361 (human V129 PrP^C), TgVole (bank vole I109 PrP^C), bank vole (I109I PrP^C), and sheep (AHQ/ARR and AHQ/AHQ churra-tensina breeds). Our analysis of the results of these bioassays concludes that the strain generated in this model is indistinguishable to that causing atypical scrapie (Nor98). Thus, we present the first faithful model for a bona fide, transmissible, ovine, atypical scrapie prion disease.

Non-human primates in prion diseases

The fascinating history of prion diseases is intimately linked to the use of nonhuman primates as experimental models, which brought so fundamental and founding information about transmissibility, pathogenesis, and resistance of prions. These models are still of crucial need for risk assessment of human health and may contribute to pave a new way towards the moving field of prion-like entities which now includes the main human neurodegenerative diseases (especially Alzheimer's and Parkinson's diseases).

Chronic Wasting Disease (CWD) in Cervids and the Consequences of a Mutable Protein Conformation

Chronic wasting disease (CWD) is a prion disease of cervids (deer, elk, moose, etc.). It spreads readily from CWD-contaminated environments and among wild cervids. As of 2022, North American CWD has been found in 29 states, four Canadian provinces and South Korea. The Scandinavian form of CWD originated independently. Prions propagate their pathology by inducing a natively expressed prion protein (PrPC) to adopt the prion conformation (PrPSc). PrPC and PrPSc differ solely in their conformation. Like other prion diseases, transmissible CWD prions can arise spontaneously. The CWD prions can respond to selection pressures resulting in the emergence of new strain phenotypes. Annually, 11.5 million Americans hunt and harvest nearly 6 million deer, indicating that CWD is a potential threat to an important American food source. No tested CWD strain has been shown to be zoonotic. However, this may not be true for emerging strains. Should a zoonotic CWD strain emerge, it could adversely impact the hunting economy and game meat consumers.

Transmission of cervid prions to humanized mice demonstrates the zoonotic potential of CWD

Prions cause infectious and fatal neurodegenerative diseases in mammals. Chronic wasting disease (CWD), a prion disease of cervids, spreads efficiently among wild and farmed animals. Potential transmission to humans of CWD is a growing concern due to its increasing prevalence. Here, we provide evidence for a zoonotic potential of CWD prions, and its probable signature using mice expressing human prion protein (PrP) as an infection model. Inoculation of these mice with deer CWD isolates resulted in atypical clinical manifestation with prion seeding activity and efficient transmissible infectivity in the brain and, remarkably, in feces, but without classical neuropathological or Western blot appearances of prion diseases. Intriguingly, the protease-resistant PrP in the brain resembled that found in a familial human prion disease and was transmissible upon second passage. Our results suggest that CWD might infect humans, although the transmission barrier is likely higher compared to zoonotic transmission of cattle prions. Notably, our data suggest a different clinical presentation, prion signature, and tissue tropism, which causes challenges for detection by current diagnostic assays. Furthermore, the presence of infectious prions in feces is concerning because if this occurs in humans, it is a source for human-to-human transmission. These findings have strong implications for public health and CWD management.

Virus Infection, Genetic Mutations, and Prion Infection in Prion Protein Conversion

Conformational conversion of the cellular isoform of prion protein, PrP^C, into the abnormally folded, amyloidogenic isoform, PrP^Sc, is an underlying pathogenic mechanism in prion diseases. The diseases manifest as sporadic, hereditary, and acquired disorders. Etiological mechanisms driving the conversion of PrP^C into PrP^Sc are unknown in sporadic prion diseases, while prion infection and specific mutations in the PrP gene are known to cause the conversion of PrP^C into PrP^Sc in acquired and hereditary prion diseases, respectively. We recently reported that a neurotropic strain of influenza A virus (IAV) induced the conversion of PrP^C into PrP^Sc as well as formation of infectious prions in mouse neuroblastoma cells after infection, suggesting the causative role of the neuronal infection of IAV in sporadic prion diseases. Here, we discuss the conversion mechanism of PrP^C into PrP^Sc in different types of prion diseases, by presenting our findings of the IAV infection-induced conversion of PrP^C into PrP^Sc and by reviewing the so far reported transgenic animal models of hereditary prion diseases and the reverse genetic studies, which have revealed the structure-function relationship for PrP^C to convert into PrP^Sc after prion infection.

Prion Dissemination through the Environment and Medical Practices: Facts and Risks for Human Health

Prion diseases are a group of fatal, infectious neurodegenerative disorders affecting various species of mammals, including humans. The infectious agent in these diseases, termed prion, is composed exclusively of a misfolded protein that can spread and multiply in the absence of genetic materials. In this article, we provide an overview of the mechanisms of prion replication, interindividual transmission, and dissemination in communities. In particular, we review the potential role of the natural environment in prion transmission, including the mechanisms and pathways for prion entry and accumulation in the environment as well as its roles in prion mutation, adaptation, evolution, and transmission. We also discuss the transmission of prion diseases through medical practices, scientific research, and use of biological products. Detailed knowledge of these aspects is crucial to limit the spreading of existing prion diseases as well as to prevent the emergence of new diseases with possible catastrophic consequences for public health.

Prion Diseases: A Unique Transmissible Agent or a Model for Neurodegenerative Diseases?

The accumulation and propagation in the brain of misfolded proteins is a pathological hallmark shared by many neurodegenerative diseases such as Alzheimer's disease (Aβ and tau), Parkinson's disease (α-synuclein), and prion disease (prion protein). Currently, there is no epidemiological evidence to suggest that neurodegenerative disorders are infectious, apart from prion diseases. However, there is an increasing body of evidence from experimental models to suggest that other pathogenic proteins such as Aβ and tau can propagate in vivo and in vitro in a prion-like mechanism, inducing the formation of misfolded protein aggregates such as amyloid plaques and neurofibrillary tangles. Such similarities have raised concerns that misfolded proteins, other than the prion protein, could potentially transmit from person-to-person as rare events after lengthy incubation periods. Such concerns have been heightened following a number of recent reports of the possible inadvertent transmission of Aβ pathology via medical and surgical procedures. This review will provide a historical perspective on the unique transmissible nature of prion diseases, examining their impact on public health and the ongoing concerns raised by this rare group of disorders. Additionally, this review will provide an insight into current evidence supporting the potential transmissibility of other pathogenic proteins associated with more common neurodegenerative disorders and the potential implications for public health.

Met166 -Glu168 residues in human PrP β2-α2 loop account for evolutionary resistance to prion infection

Aims

The amino acid sequence of prion protein (PrP) is a key determinant in the transmissibility of prion diseases. While PrP sequence is highly conserved among mammalian species, minor changes in the PrP amino acid sequence may confer alterations in the transmissibility of prion diseases. Classical bovine spongiform encephalopathy (C‐BSE) is the only zoonotic prion strain reported to date causing variant Creutzfeldt‐Jacob disease (vCJD) in humans, although experimental transmission points to atypical L‐BSE and some classical scrapie isolates as also zoonotic. The precise molecular elements in the human PrP sequence that limit the transmissibility of prion strains such as sheep/goat scrapie or cervid chronic wasting disease (CWD) are not well known.

Methods

The transmissibility of a panel of diverse prions from different species was compared in transgenic mice expressing either wild‐type human PrP^C (MDE‐HuTg340) or a mutated human PrP^C harbouring Val₁₆₆‐Gln₁₆₈ amino acid changes (VDQ‐HuTg372) in the β2‐α2 loop instead of Met₁₆₆‐Glu₁₆₈ wild‐type variants.

Results

VDQ‐HuTg372 mice were more susceptible to prions than MDE‐HuTg340 mice in a strain‐dependent manner.

Conclusions

Met₁₆₆‐Glu₁₆₈ amino acid residues present in wild‐type human PrP^C are molecular determinants that limit the propagation of most prion strains assayed in the human PrP context.

Transgenic mouse models for the study of prion diseases

Prions are unique agents that challenge the molecular biology dogma by transmitting information on the protein level. They cause neurodegenerative diseases that lack of any cure or treatment called transmissible spongiform encephalopathies. The function of the normal form of the prion protein, the exact mechanism of prion propagation between species as well as at the cellular level and neuron degeneration remains elusive. However, great amount of information known for all these aspects has been achieved thanks to the use of animal models and more precisely to transgenic mouse models. In this chapter, the main contributions of these powerful research tools in the prion field are revised.

Radical Change in Zoonotic Abilities of Atypical BSE Prion Strains as Evidenced by Crossing of Sheep Species Barrier in Transgenic Mice

Classical bovine spongiform encephalopathy (BSE) is the only zoonotic prion disease described to date. Although the zoonotic potential of atypical BSE prions have been partially studied, an extensive analysis is still needed. We conducted a systematic study by inoculating atypical BSE isolates from different countries in Europe into transgenic mice overexpressing human prion protein (PrP): TgMet129, TgMet/Val129, and TgVal129. L-type BSE showed a higher zoonotic potential in TgMet129 mice than classical BSE, whereas Val129-PrP variant was a strong molecular protector against L-type BSE prions, even in heterozygosis. H-type BSE could not be transmitted to any of the mice. We also adapted 1 H- and 1 L-type BSE isolate to sheep-PrP transgenic mice and inoculated them into human-PrP transgenic mice. Atypical BSE prions showed a modification in their zoonotic ability after adaptation to sheep-PrP producing agents able to infect TgMet129 and TgVal129, bearing features that make them indistinguishable of sporadic Creutzfeldt-Jakob disease prions.

An Ex Vivo Brain Slice Culture Model of Chronic Wasting Disease: Implications for Disease Pathogenesis and Therapeutic Development

Chronic wasting disease (CWD) is a rapidly spreading prion disease of cervids, yet antemortem diagnosis, treatment, and control remain elusive. We recently developed an organotypic slice culture assay for sensitive detection of scrapie prions using ultrasensitive prion seeding. However, this model was not established for CWD prions due to their strong transmission barrier from deer (Odocoileus spp) to standard laboratory mice (Mus musculus). Therefore, we developed and characterized the ex vivo brain slice culture model for CWD, using a transgenic mouse model (Tg12) that expresses the elk (Cervus canadensis) prion protein gene (PRNP). We tested for CWD infectivity in cultured slices using sensitive seeding assays such as real-time quaking-induced conversion (RT-QuIC) and protein misfolding cyclic amplification (PMCA). Slice cultures from Tg12, but not from prnp^−/− mice, tested positive for CWD. Slice-generated CWD prions transmitted efficiently to Tg12 mice. Furthermore, we determined the activity of anti-prion compounds and optimized a screening protocol for the infectivity of biological samples in this CWD slice culture model. Our results demonstrate that this integrated brain slice model of CWD enables the study of pathogenic mechanisms with translational implications for controlling CWD.

Update on chronic wasting disease (CWD) III

The European Commission asked EFSA for a Scientific Opinion: to revise the state of knowledge about the differences between the chronic wasting disease (CWD) strains found in North America (NA) and Europe and within Europe; to review new scientific evidence on the zoonotic potential of CWD and to provide recommendations to address the potential risks and to identify risk factors for the spread of CWD in the European Union. Full characterisation of European isolates is being pursued, whereas most NA CWD isolates have not been characterised in this way. The differing surveillance programmes in these continents result in biases in the types of cases that can be detected. Preliminary data support the contention that the CWD strains identified in Europe and NA are different and suggest the presence of strain diversity in European cervids. Current data do not allow any conclusion on the implications of strain diversity on transmissibility, pathogenesis or prevalence. Available data do not allow any conclusion on the zoonotic potential of NA or European CWD isolates. The risk of CWD to humans through consumption of meat cannot be directly assessed. At individual level, consumers of meat, meat products and offal derived from CWD-infected cervids will be exposed to the CWD agent(s). Measures to reduce human dietary exposure could be applied, but exclusion from the food chain of whole carcasses of infected animals would be required to eliminate exposure. Based on NA experiences, all the risk factors identified for the spread of CWD may be associated with animals accumulating infectivity in both the peripheral tissues and the central nervous system. A subset of risk factors is relevant for infected animals without involvement of peripheral tissues. All the risk factors should be taken into account due to the potential co-localisation of animals presenting with different disease phenotypes.

Bovine Spongiform Encephalopathy - A Review from the Perspective of Food Safety

Bovine spongiform encephalopathy (BSE) is a fatal neurodegenerative disease that belongs to transmissible spongiform encephalopathy (TSE). Since the first case was identified in the UK in 1986, BSE spread to other countries including Japan. Its incidence peaked in 1992 in the UK and from 2001 to 2006 in many other countries, but a feed ban aimed at eliminating the recycling of the BSE agent and other control measures aimed at preventing food and feed contamination with the agent were highly effective at reducing the spread of BSE. In 2004, two types of atypical BSE, H-type BSE (H-BSE) and L-type BSE (L-BSE), which differ from classical BSE (C-BSE), were found in France and Italy. Atypical BSE, which is assumed to occur spontaneously, has also been detected among cattle in other countries including Japan. The BSE agent including atypical BSE agent is a unique food-safety hazard with different chemical and biological properties from the microbial pathogens and toxic chemicals that contaminate food. In this review, we summarize the reported findings on the tissue distribution of BSE prions in infected cattle and other aspects of BSE, as well as the control measures against the disease employed in Japan. Topics that require further studies are discussed based on the summarized findings from the perspective of food safety.

Susceptibility of Human Prion Protein to Conversion by Chronic Wasting Disease Prions

Chronic wasting disease (CWD) is a contagious and fatal neurodegenerative disease and a serious animal health issue for deer and elk in North America. The identification of the first cases of CWD among free-ranging reindeer and moose in Europe brings back into focus the unresolved issue of whether CWD can be zoonotic like bovine spongiform encephalopathy. We used a cell-free seeded protein misfolding assay to determine whether CWD prions from elk, white-tailed deer, and reindeer in North America can convert the human prion protein to the disease-associated form. We found that prions can convert, but the efficiency of conversion is affected by polymorphic variation in the cervid and human prion protein genes. In view of the similarity of reindeer, elk, and white-tailed deer in North America to reindeer, red deer, and roe deer, respectively, in Europe, a more comprehensive and thorough assessment of the zoonotic potential of CWD might be warranted.

Transmission studies of chronic wasting disease to transgenic mice overexpressing human prion protein using the RT-QuIC assay

Chronic wasting disease (CWD) is a fatal prion disease which infects deer, elk and moose. CWD was first described as a wasting syndrome in captive deer in Colorado and Wyoming wildlife facilities from 1967 to 1979. Currently, CWD has been reported in 26 states of the USA, three Canadian provinces, South Korea, Norway and Finland. Since human consumption of cervids is common, it is critical to determine if CWD can infect humans. Published research, including epidemiologic studies and transmission studies using animal models, including transgenic mice that express human prion protein, have suggested existence of a strong species barrier between cervid CWD and humans. In the current study, we tested CWD transmission into two additional strains of transgenic mice (tg66 and tgRM). These mice over-express human prion protein at high levels and are highly sensitive to infection by human-tropic prions. One hundred and eight mice were inoculated intracerebrally with three different sources of CWD. After long periods of observation, brain tissues from CWD-inoculated mice were screened for evidence of prion infection by RT-QuIC, immunohistochemistry (IHC) and immunoblot. No IHC or immunoblot evidence was found to suggest transmission had occurred, and most mice were negative by RT-QuIC assay. However, four mice with inconsistent positive RT-QuIC reactions were detected. The seeding activity detected in these mice may represent a low level of CWD agent, suggesting a possible transfer of CWD infection. Alternatively, these results might be due to false positive reactions or residual CWD inoculum.

Animal prion diseases: the risks to human health

Transmissible spongiform encephalopathies (TSEs) or prion diseases of animals notably include scrapie in small ruminants, chronic wasting disease (CWD) in cervids and classical bovine spongiform encephalopathy (C-BSE). As the transmission barrier phenomenon naturally limits the propagation of prions from one species to another, and the lack of epidemiological evidence for an association with human prion diseases, the zoonotic potential of these diseases was for a long time considered negligible. However, in 1996, C-BSE was recognized as the cause of a new human prion disease, variant Creutzfeldt-Jakob disease (vCJD), which triggered an unprecedented public health crisis in Europe. Large-scale epidemio-surveillance programs for scrapie and C-BSE that were implemented in the EU after the BSE crisis revealed that the distribution and prevalence of prion diseases in the ruminant population had previously been underestimated. They also led to the recognition of new forms of TSEs (named atypical) in cattle and small ruminants and to the recent identification of CWD in Europe. At this stage, the characterization of the strain diversity and zoonotic abilities associated with animal prion diseases remains largely incomplete. However, transmission experiments in nonhuman primates and transgenic mice expressing human PrP clearly indicate that classical scrapie, and certain forms of atypical BSE (L-BSE) or CWD may have the potential to infect humans. The remaining uncertainties about the origins and relationships between animal prion diseases emphasize the importance of the measures implemented to limit human exposure to these potentially zoonotic agents, and of continued surveillance for both animal and human prion diseases.

Insights into the Bidirectional Properties of the Sheep-Deer Prion Transmission Barrier

The large chronic wasting disease (CWD)-affected cervid population in the USA and Canada, and the risk of the disease being transmitted to humans through intermediate species, is a highly worrying issue that is still poorly understood. In this case, recombinant protein misfolding cyclic amplification was used to determine, in vitro, the relevance of each individual amino acid on cross-species prion transmission. Others and we have found that the β2-α2 loop is a key modulator of transmission barriers between species and markedly influences infection by sheep scrapie, bovine spongiform encephalopathy (BSE), or elk CWD. Amino acids that differentiate ovine and deer normal host prion protein (PrP^C) and associated with structural rigidity of the loop β2-α2 (S173N, N177T) appear to confer resistance to some prion diseases. However, addition of methionine at codon 208 together with the previously described rigid loop substitutions seems to hide a key in this species barrier, as it makes sheep recombinant prion protein highly susceptible to CWD-induced misfolding. These studies indicate that interspecies prion transmission is not only governed just by the β2-α2 loop amino acid sequence but also by its interactions with the α3-helix as shown by substitution I208M. Transmissible spongiform encephalopathies, characterized by long incubation periods and spongiform changes associated with neuronal loss in the brain, have been described in several mammalian species appearing either naturally (scrapie in sheep and goats, bovine spongiform encephalopathy in cattle, chronic wasting disease in cervids, Creutzfeldt-Jakob disease in humans) or by experimental transmission studies (scrapie in mice and hamsters). Much of the pathogenesis of the prion diseases has been determined in the last 40 years, such as the etiological agent or the fact that prions occur as different strains that show distinct biological and physicochemical properties. However, there are many unanswered questions regarding the strain phenomenon and interspecies transmissibility. To assess the risk of interspecies transmission between scrapie and chronic wasting disease, an in vitro prion propagation method has been used. This technique allows to predict the amino acids preventing the transmission between sheep and deer prion diseases.

Review: Update on Classical and Atypical Scrapie in Sheep and Goats

Scrapie is a naturally occurring transmissible spongiform encephalopathy (TSE) or prion disease of sheep and goats. Scrapie is a protein misfolding disease where the normal prion protein (PrP^C) misfolds into a pathogenic form (PrP^Sc) that is highly resistant to enzymatic breakdown within the cell and accumulates, eventually leading to neurodegeneration. The amino acid sequence of the prion protein and tissue distribution of PrP^Sc within affected hosts have a major role in determining susceptibility to and potential environmental contamination with the scrapie agent. Many countries have genotype-based eradication programs that emphasize using rams that express arginine at codon 171 in the prion protein, which is associated with resistance to the classical scrapie agent. In classical scrapie, accumulation of PrP^Sc within lymphoid and other tissues facilitates environmental contamination and spread of the disease within flocks. A major distinction can be made between classical scrapie strains that are readily spread within populations of susceptible sheep and goats and atypical (Nor-98) scrapie that has unique molecular and phenotype characteristics and is thought to occur spontaneously in older sheep or goats. This review provides an overview of classical and atypical scrapie with consideration of potential transmission of classical scrapie to other mammalian hosts.

Lack of Transmission of Chronic Wasting Disease to Cynomolgus Macaques

Chronic wasting disease (CWD) is a fatal prion disease that can infect deer, elk, and moose. CWD was first recognized in captive deer kept in wildlife facilities in Colorado from 1967 to 1979. CWD has now been detected in 25 U.S. states, 2 Canadian provinces, South Korea, Norway, and Finland. It is currently unknown if humans are susceptible to CWD infection. Understanding the health risk from consuming meat and/or products from CWD-infected cervids is a critical human health concern. Previous research using transgenic mouse models and in vitro conversion assays suggests that a significant species barrier exists between CWD and humans. To date, reported epidemiologic studies of humans consuming cervids in areas where CWD is endemic have found no evidence to confirm CWD transmission to humans. Previously, we reported data from ongoing cross-species CWD transmission studies using two species of nonhuman primates as models. Squirrel monkeys (SM) and cynomolgus macaques (CM) were inoculated by either the intracerebral or oral route with brain homogenates from CWD-infected deer and elk containing high levels of infectivity. SM were highly susceptible to CWD infection, while CM were not. In the present study, we present new data for seven CWD-inoculated CM euthanized 11 to 13 years after CWD inoculation and eight additional uninoculated control CM. New and archival CM tissues were screened for prion infection by using the ultrasensitive real-time quaking-induced conversion (RT-QuIC) assay, immunohistochemistry, and immunoblotting. In this study, there was no clinical, pathological, or biochemical evidence suggesting that CWD was transmitted from cervids to CM.IMPORTANCE Chronic wasting disease (CWD) is a fatal prion disease found in deer, elk, and moose. Since it was first discovered in the late 1960s, CWD has now spread to at least 25 U.S. states, 2 Canadian provinces, South Korea, Norway, and Finland. Eradication of CWD from areas of endemicity is very unlikely, and additional spread will occur. As the range and prevalence of CWD increase, so will the potential for human exposure to CWD prions. It is currently unknown if CWD poses a risk to human health. However, determining this risk is critical to preventing a scenario similar to that which occurred when mad cow disease was found to be transmissible to humans. In the present study, we used cynomolgus macaque monkeys as a surrogate model for CWD transmission to humans. After 13 years, no evidence for CWD transmission to macaques was detected clinically or by using highly sensitive prion disease-screening assays.

The zoonotic potential of animal prion diseases

Bovine spongiform encephalopathy (BSE) is the only animal prion disease that has been demonstrated to be zoonotic, causing variant Creutzfeldt-Jakob disease (vCJD) in humans. The link between BSE and vCJD was established by careful surveillance, epidemiologic investigations, and experimental studies using in vivo and in vitro models of cross-species transmission. Similar approaches have been used to assess the zoonotic potential of other animal prion diseases, including atypical forms identified through active surveillance. There is no epidemiologic evidence that classical or atypical scrapie, atypical forms of BSE, or chronic wasting disease (CWD) is associated with human prion disease, but the limitations of the epidemiologic data should be taken into account when interpreting these results. Transmission experiments in nonhuman primates and human PrP transgenic mice suggest that classic scrapie, L-type atypical BSE (L-BSE), and CWD may have zoonotic potential, which for L-BSE appears to be equal to or greater than that of classic BSE. The results of in vitro conversion assays to analyze the human transmission barrier correlate well with the in vivo data. However, it is still difficult to predict the likelihood that an animal prion disease will transmit to humans under conditions of field exposure from the results of in vivo or in vitro experiments. This emphasizes the importance of continuing systematic surveillance for both human and animal prion diseases in identifying zoonotic transmission of diseases other than classic BSE.

Experimental models of human prion diseases and prion strains

Prion strains occur in natural prion diseases, including prion diseases of humans. Prion strains can correspond with differences in the clinical signs and symptoms of disease and the distribution of prion infectivity in the host and are hypothesized to be encoded by strain-specific differences in the conformation of the disease-specific isoform of the host-encoded prion protein, PrP^TSE. Prion strains can differ in biochemical properties of PrP^TSE that can include the relative sensitivity to digestion with proteinase K and conformational stability in denaturants. These strain-specific biochemical properties of field isolates are maintained upon transmission to experimental animal models of prion disease. Experimental human models of prion disease include traditional and gene-targeted mice that express endogenous PrP^C. Transgenic mice that express different polymorphs of human PrP^C or mutations in human PrP^C that correspond with familial forms of human prion disease have been generated that can recapitulate the clinical, pathologic, and biochemical features of disease. These models aid in understanding disease pathogenesis, evaluating zoonotic potential of animal prion diseases, and assessing human-to-human transmission of disease. Models of sporadic or familial forms of disease offer an opportunity to define mechanisms of disease, identify key neurodegenerative pathways, and assess therapeutic interventions.

Evolution of Diagnostic Tests for Chronic Wasting Disease, a Naturally Occurring Prion Disease of Cervids

Since chronic wasting disease (CWD) was first identified nearly 50 years ago in a captive mule deer herd in the Rocky Mountains of the United States, it has slowly spread across North America through the natural and anthropogenic movement of cervids and their carcasses. As the endemic areas have expanded, so has the need for rapid, sensitive, and cost effective diagnostic tests—especially those which take advantage of samples collected antemortem. Over the past two decades, strategies have evolved from the recognition of microscopic spongiform pathology and associated immunohistochemical staining of the misfolded prion protein to enzyme-linked immunoassays capable of detecting the abnormal prion conformer in postmortem samples. In a history that parallels the diagnosis of more conventional infectious agents, both qualitative and real-time amplification assays have recently been developed to detect minute quantities of misfolded prions in a range of biological and environmental samples. With these more sensitive and semi-quantitative approaches has come a greater understanding of the pathogenesis and epidemiology of this disease in the native host. Because the molecular pathogenesis of prion protein misfolding is broadly analogous to the misfolding of other pathogenic proteins, including Aβ and α-synuclein, efforts are currently underway to apply these in vitro amplification techniques towards the diagnosis of Alzheimer’s disease, Parkinson’s disease, and other proteinopathies. Chronic wasting disease—once a rare disease of Colorado mule deer—now represents one of the most prevalent prion diseases, and should serve as a model for the continued development and implementation of novel diagnostic strategies for protein misfolding disorders in the natural host.

Gene Targeted Transgenic Mouse Models in Prion Research

The production of transgenic mice expressing different forms of the prion protein (PrP) or devoid of PrP has enabled researchers to study the role of PrP in the infectious process of a prion disease and its normal function in the healthy individual. A wide range of transgenic models have been produced ranging from PrP null mice, normal expression levels to overexpression models, models expressing different species of the Prnp gene and different mutations and polymorphisms within the gene. Using this range of transgenic models has allowed us to define the influence of PrP expression on disease susceptibility and transmission, assess zoonotic potential, define strains of human prion diseases, elucidate the function of PrP, and start to unravel the mechanisms involved in chronic neurodegeneration. This chapter focuses mainly on the use of the gene targeted transgenic models and summarizes the ways in which they have allowed us to study the role of PrP in prion disease and the insights they have provided into the mechanisms of neurodegenerative diseases.

Evaluation of rapid post-mortem test kits for bovine spongiform encephalopathy (BSE) screening in Japan: Their analytical sensitivity to atypical BSE prions

A classical type of bovine spongiform encephalopathy (C-BSE), recognized in 1987, had a large impact on public health due to its zoonotic link to variant Creutzfeldt-Jakob disease by the human consumption of dietary products contaminated with the C-BSE prion. Thus, a number of countries implemented BSE surveillance using rapid post-mortem test kits that were approved for detection of the C-BSE prion in the cattle brain. However, as atypical BSE (L- and H-BSE) cases emerged in subsequent years, the efficacy of the kits for the detection of atypical BSE prions became a matter of concern. In response to this, laboratories in the European Union and Canada evaluated the kits used in their countries. Here, we carried out an evaluation study of NippiBL®, a kit currently used for BSE screening in Japan. By applying the kit to cattle brains of field cases of C-BSE and L-BSE, and an experimental case of H-BSE, we showed its comparable sensitivities to C, L-, and H-BSE prions, and satisfactory performance required by the European Food Safety Authority. In addition to NippiBL®, two kits (TeSeE® and FRELISA®) formerly used in Japan were effective for detection of the L-BSE prion, although the two kits were unable to be tested for the H-BSE prion due to the discontinuation of domestic sales during this study. These results indicate that BSE screening in Japan is as effective as those in other countries, and it is unlikely that cases of atypical BSE have been overlooked.

Current evidence on the transmissibility of chronic wasting disease prions to humans-A systematic review

A number of prion diseases affect humans, including Creutzfeldt-Jakob disease; most of these are due to genetic mutations in the affected individual and occur sporadically, but some result from transmission of prion proteins from external sources. Of the known animal prion diseases, only bovine spongiform encephalopathy prions have been shown to be transmissible from animals to humans under non-experimental conditions. Chronic wasting disease (CWD) is a prion disease that affects cervids (e.g., deer and elk) in North America and isolated populations in Korea and Europe. Systematic review methodology was used to identify, select, critically appraise and analyse data from relevant research. Studies were evaluated for adherence to good conduct based on their study design following the Cochrane collaboration's approach to grading the quality of evidence and the strength of recommendations (GRADE). Twenty-three studies were included after screening 800 citations from the literature search and evaluating 78 full papers. Studies examined the transmissibility of CWD prions to humans using epidemiological study design, in vitro and in vivo experiments. Five epidemiological studies, two studies on macaques and seven studies on humanized transgenic mice provided no evidence to support the possibility of transmission of CWD prions to humans. Ongoing surveillance in the United States and Canada has not documented CWD transmission to humans. However, two studies on squirrel monkeys provided evidence that transmission of CWD prions resulting in prion disease is possible in these monkeys under experimental conditions and seven in vitro experiments provided evidence that CWD prions can convert human prion protein to a misfolded state. Therefore, future discovery of CWD transmission to humans cannot be entirely ruled out on the basis of current studies, particularly in the light of possible decades-long incubation periods for CWD prions in humans. It would be prudent to continue CWD research and epidemiologic surveillance, exercise caution when handling potentially contaminated material and explore CWD management opportunities.

Cross-species transmission of CWD prions

Prions cause fatal neurodegenerative diseases in humans and animals and can be transmitted zoonotically. Chronic wasting disease (CWD) is a highly transmissible prion disease of wild deer and elk that affects cervids over extensive regions of the United States and Canada. The risk of cross-species CWD transmission has been experimentally evaluated in a wide array of mammals, including non-human primates and mouse models expressing human cellular prion protein. Here we review the determinants of cross-species CWD transmission, and propose a model that may explain a structural barrier for CWD transmission to humans.

Evaluating the Species Barrier

A Transmissible Spongiform Encephalopathy (TSE) agent from one species generally transmits poorly to a new species, a phenomenon known as the species barrier. However once in the new species it generally but not always adapts and then more readily transmits within the new host. No single test is available to determine accurately the ability of a prion strain to transmit between species. Evaluating the species barrier for any prion strain has to take into consideration as much information as can be gathered for that strain from surveillance and research. The interactions of the agent with a particular host can be measured by in vivo and in vitro methods and assessing the species barrier needs to make full use of all the tools available. This review will identify the important considerations that need to be made when evaluating the species barrier.

Absence of Evidence for a Causal Link between Bovine Spongiform Encephalopathy Strain Variant L-BSE and Known Forms of Sporadic Creutzfeldt-Jakob Disease in Human PrP Transgenic Mice

Prions are proteinaceous pathogens responsible for subacute spongiform encephalopathies in animals and humans. The prions responsible for bovine spongiform encephalopathy (BSE) are zoonotic agents, causing variant Creutzfeldt-Jakob disease (CJD) in humans. The transfer of prions between species is limited by a species barrier, which is thought to reflect structural incompatibilities between the host cellular prion protein (PrP^C) and the infecting pathological PrP assemblies (PrP^Sc) constituting the prion. A BSE strain variant, designated L-BSE and responsible for atypical, supposedly spontaneous forms of prion diseases in aged cattle, demonstrates zoonotic potential, as evidenced by its capacity to propagate more easily than classical BSE in transgenic mice expressing human PrP^C and in nonhuman primates. In humanized mice, L-BSE propagates without any apparent species barrier and shares similar biochemical PrP^Sc signatures with the CJD subtype designated MM2-cortical, thus opening the possibility that certain CJD cases classified as sporadic may actually originate from L-type BSE cross-transmission. To address this issue, we compared the biological properties of L-BSE and those of a panel of CJD subtypes representative of the human prion strain diversity using standard strain-typing criteria in human PrP transgenic mice. We found no evidence that L-BSE causes a known form of sporadic CJD.

IMPORTANCE

Since the quasi-extinction of classical BSE, atypical BSE forms are the sole BSE variants circulating in cattle worldwide. They are observed in rare cases of old cattle, making them difficult to detect. Extrapolation of our results suggests that L-BSE may propagate in humans as an unrecognized form of CJD, and we urge both the continued utilization of precautionary measures to eliminate these agents from the human food chain and active surveillance for CJD phenotypes in the general population.

Assessment of the PrPc Amino-Terminal Domain in Prion Species Barriers

Chronic wasting disease (CWD) in cervids and bovine spongiform encephalopathy (BSE) in cattle are prion diseases that are caused by the same protein-misfolding mechanism, but they appear to pose different risks to humans. We are interested in understanding the differences between the species barriers of CWD and BSE. We used real-time, quaking-induced conversion (RT-QuIC) to model the central molecular event in prion disease, the templated misfolding of the normal prion protein, PrP^c, to a pathogenic, amyloid isoform, scrapie prion protein, PrP^Sc We examined the role of the PrP^c amino-terminal domain (N-terminal domain [NTD], amino acids [aa] 23 to 90) in cross-species conversion by comparing the conversion efficiency of various prion seeds in either full-length (aa 23 to 231) or truncated (aa 90 to 231) PrP^c We demonstrate that the presence of white-tailed deer and bovine NTDs hindered seeded conversion of PrP^c, but human and bank vole NTDs did the opposite. Additionally, full-length human and bank vole PrP^cs were more likely to be converted to amyloid by CWD prions than were their truncated forms. A chimera with replacement of the human NTD by the bovine NTD resembled human PrP^c The requirement for an NTD, but not for the specific human sequence, suggests that the NTD interacts with other regions of the human PrP^c to increase promiscuity. These data contribute to the evidence that, in addition to primary sequence, prion species barriers are controlled by interactions of the substrate NTD with the rest of the substrate PrP^c molecule.

IMPORTANCE

We demonstrate that the amino-terminal domain of the normal prion protein, PrP^c, hinders seeded conversion of bovine and white-tailed deer PrP^cs to the prion forms, but it facilitates conversion of the human and bank vole PrP^cs to the prion forms. Additionally, we demonstrate that the amino-terminal domain of human and bank vole PrP^cs requires interaction with the rest of the molecule to facilitate conversion by CWD prions. These data suggest that interactions of the amino-terminal domain with the rest of the PrP^c molecule play an important role in the susceptibility of humans to CWD prions.

Prion Diseases

PURPOSE OF REVIEW

This article presents an update on the clinical aspects of human prion disease, including the wide spectrum of their presentations.

RECENT FINDINGS

Prion diseases, a group of disorders caused by abnormally shaped proteins called prions, occur in sporadic (Jakob-Creutzfeldt disease), genetic (genetic Jakob-Creutzfeldt disease, Gerstmann-Sträussler-Scheinker syndrome, and fatal familial insomnia), and acquired (kuru, variant Jakob-Creutzfeldt disease, and iatrogenic Jakob-Creutzfeldt disease) forms. This article presents updated information on the clinical features and diagnostic methods for human prion diseases. New antemortem potential diagnostic tests based on amplifying prions in order to detect them are showing very high specificity. Understanding of the diversity of possible presentations of human prion diseases continues to evolve, with some genetic forms progressing slowly over decades, beginning with dysautonomia and neuropathy and progressing to a frontal-executive dementia with pathology of combined prionopathy and tauopathy. Unfortunately, to date, all human prion disease clinical trials have failed to show survival benefit. A very rare polymorphism in the prion protein gene recently has been identified that appears to protect against prion disease; this finding, in addition to providing greater understanding of the prionlike mechanisms of neurodegenerative disorders, might lead to potential treatments.

SUMMARY

Sporadic Jakob-Creutzfeldt disease is the most common form of human prion disease. Genetic prion diseases, resulting from mutations in the prion-related protein gene (PRNP), are classified based on the mutation, clinical phenotype, and neuropathologic features and can be difficult to diagnose because of their varied presentations. Perhaps most relevant to this Continuum issue on neuroinfectious diseases, acquired prion diseases are caused by accidental transmission to humans, but fortunately, they are the least common form and are becoming rarer as awareness of transmission risk has led to implementation of measures to prevent such occurrences.

The transmissible spongiform encephalopathies of livestock

Prion diseases or transmissible spongiform encephalopathies (TSEs) are fatal protein-misfolding neurodegenerative diseases. TSEs have been described in several species, including bovine spongiform encephalopathy (BSE) in cattle, scrapie in sheep and goats, chronic wasting disease (CWD) in cervids, transmissible mink encephalopathy (TME) in mink, and Kuru and Creutzfeldt-Jakob disease (CJD) in humans. These diseases are associated with the accumulation of a protease-resistant, disease-associated isoform of the prion protein (called PrP(Sc)) in the central nervous system and other tissues, depending on the host species. Typically, TSEs are acquired through exposure to infectious material, but inherited and spontaneous TSEs also occur. All TSEs share pathologic features and infectious mechanisms but have distinct differences in transmission and epidemiology due to host factors and strain differences encoded within the structure of the misfolded prion protein. The possibility that BSE can be transmitted to humans as the cause of variant Creutzfeldt-Jakob disease has brought attention to this family of diseases. This review is focused on the TSEs of livestock: bovine spongiform encephalopathy in cattle and scrapie in sheep and goats.

Expert elicitation on the uncertainties associated with chronic wasting disease

A high degree of uncertainty exists for chronic wasting disease (CWD) transmission factors in farmed and wild cervids. Evaluating the factors is important as it helps to inform future risk management strategies. Expert opinion is often used to assist decision making in a number of health, science, and technology domains where data may be sparse or missing. Using the "Classical Model" of elicitation, a group of experts was asked to estimate the most likely values for several risk factors affecting CWD transmission. The formalized expert elicitation helped structure the issues and hence provide a rational basis for estimating some transmission risk factors for which evidence is lacking. Considered judgments regarding environmental transmission, latency of CWD transmission, management, and species barrier were provided by the experts. Uncertainties for many items were determined to be large, highlighting areas requiring more research. The elicited values may be used as surrogate values until research evidence becomes available.

Insights into Chronic Wasting Disease and Bovine Spongiform Encephalopathy Species Barriers by Use of Real-Time Conversion

The propensity for transspecies prion transmission is related to the structural characteristics of the enciphering and new host PrP, although the exact mechanism remains incompletely understood. The effects of variability in prion protein on cross-species prion transmission have been studied with animal bioassays, but the influence of prion protein structure versus that of host cofactors (e.g., cellular constituents, trafficking, and innate immune interactions) remains difficult to dissect. To isolate the effects of protein-protein interactions on transspecies conversion, we used recombinant PrP(C) and real-time quaking-induced conversion (RT-QuIC) and compared chronic wasting disease (CWD) and classical bovine spongiform encephalopathy (cBSE) prions. To assess the impact of transmission to a new species, we studied feline CWD (fCWD) and feline BSE (i.e., feline spongiform encephalopathy [FSE]). We cross-seeded fCWD and FSE into each species' full-length, recombinant PrP(C) and measured the time required for conversion to the amyloid (PrP(Res)) form, which we describe here as the rate of amyloid conversion. These studies revealed the following: (i) CWD and BSE seeded their homologous species' PrP best; (ii) fCWD was a more efficient seed for feline rPrP than for white-tailed deer rPrP; (iii) conversely, FSE more efficiently converted bovine than feline rPrP; (iv) and CWD, fCWD, BSE, and FSE all converted human rPrP, although not as efficiently as homologous sCJD prions. These results suggest that (i) at the level of protein-protein interactions, CWD adapts to a new species more readily than does BSE and (ii) the barrier preventing transmission of CWD to humans may be less robust than estimated.

IMPORTANCE

We demonstrate that bovine spongiform encephalopathy prions maintain their transspecies conversion characteristics upon passage to cats but that chronic wasting disease prions adapt to the cat and are distinguishable from the original prion. Additionally, we showed that chronic wasting disease prions are effective at seeding the conversion of normal human prion protein to an amyloid conformation, perhaps the first step in crossing the species barrier.

Transmission of scrapie prions to primate after an extended silent incubation period

Classical bovine spongiform encephalopathy (c-BSE) is the only animal prion disease reputed to be zoonotic, causing variant Creutzfeldt-Jakob disease (vCJD) in humans and having guided protective measures for animal and human health against animal prion diseases. Recently, partial transmissions to humanized mice showed that the zoonotic potential of scrapie might be similar to c-BSE. We here report the direct transmission of a natural classical scrapie isolate to cynomolgus macaque, a highly relevant model for human prion diseases, after a 10-year silent incubation period, with features similar to those reported for human cases of sporadic CJD. Scrapie is thus actually transmissible to primates with incubation periods compatible with their life expectancy, although fourfold longer than BSE. Long-term experimental transmission studies are necessary to better assess the zoonotic potential of other prion diseases with high prevalence, notably Chronic Wasting Disease of deer and elk and atypical/Nor98 scrapie.

Correlation between infectivity and disease associated prion protein in the nervous system and selected edible tissues of naturally affected scrapie sheep

The transmissible spongiform encephalopathies (TSEs) or prion diseases are a group of fatal neurodegenerative disorders characterised by the accumulation of a pathological form of a host protein known as prion protein (PrP). The validation of abnormal PrP detection techniques is fundamental to allow the use of high-throughput laboratory based tests, avoiding the limitations of bioassays. We used scrapie, a prototype TSE, to examine the relationship between infectivity and laboratory based diagnostic tools. The data may help to optimise strategies to prevent exposure of humans to small ruminant TSE material via the food chain. Abnormal PrP distribution/accumulation was assessed by immunohistochemistry (IHC), Western blot (WB) and ELISA in samples from four animals. In addition, infectivity was detected using a sensitive bank vole bioassay with selected samples from two of the four sheep and protein misfolding cyclic amplification using bank vole brain as substrate (vPMCA) was also carried out in selected samples from one animal. Lymph nodes, oculomotor muscles, sciatic nerve and kidney were positive by IHC, WB and ELISA, although at levels 100-1000 fold lower than the brain, and contained detectable infectivity by bioassay. Tissues not infectious by bioassay were also negative by all laboratory tests including PMCA. Although discrepancies were observed in tissues with very low levels of abnormal PrP, there was an overall good correlation between IHC, WB, ELISA and bioassay results. Most importantly, there was a good correlation between the detection of abnormal PrP in tissues using laboratory tests and the levels of infectivity even when the titre was low. These findings provide useful information for risk modellers and represent a first step toward the validation of laboratory tests used to quantify prion infectivity, which would greatly aid TSE risk assessment policies.

Human prion protein sequence elements impede cross-species chronic wasting disease transmission

Chronic wasting disease (CWD) is a fatal prion disease of North American deer and elk and poses an unclear risk for transmission to humans. Human exposure to CWD occurs through hunting activities and consumption of venison from prion-infected animals. Although the amino acid residues of the prion protein (PrP) that prevent or permit human CWD infection are unknown, NMR-based structural studies suggest that the β2-α2 loop (residues 165-175) may impact species barriers. Here we sought to define PrP sequence determinants that affect CWD transmission to humans. We engineered transgenic mice that express human PrP with four amino acid substitutions that result in expression of PrP with a β2-α2 loop (residues 165-175) that exactly matches that of elk PrP. Compared with transgenic mice expressing unaltered human PrP, mice expressing the human-elk chimeric PrP were highly susceptible to elk and deer CWD prions but were concurrently less susceptible to human Creutzfeldt-Jakob disease prions. A systematic in vitro survey of amino acid differences between humans and cervids identified two additional residues that impacted CWD conversion of human PrP. This work identifies amino acids that constitute a substantial structural barrier for CWD transmission to humans and helps illuminate the molecular requirements for cross-species prion transmission.

Exploring the zoonotic potential of animal prion diseases: in vivo and in vitro approaches

Following the discovery of a causal link between bovine spongiform encephalopathy (BSE) in cattle and variant Creutzfeldt-Jakob disease (vCJD) in humans, several experimental approaches have been used to try to assess the potential risk of transmission of other animal transmissible spongiform encephalopathies (TSEs) to humans. Experimental challenge of non-human primates, humanised transgenic mice and cell-free conversion systems have all been used as models to explore the susceptibility of humans to animal TSEs. In this review we compare and contrast in vivo and in vitro evidence of the zoonotic risk to humans from sheep, cattle and deer prions, focusing primarily on chronic wasting disease and our own recent studies using protein misfolding cyclic amplification.

Evidence for zoonotic potential of ovine scrapie prions

Although Bovine Spongiform Encephalopathy (BSE) is the cause of variant Creutzfeldt Jakob disease (vCJD) in humans, the zoonotic potential of scrapie prions remains unknown. Mice genetically engineered to overexpress the human prion protein (tgHu) have emerged as highly relevant models for gauging the capacity of prions to transmit to humans. These models can propagate human prions without any apparent transmission barrier and have been used used to confirm the zoonotic ability of BSE. Here we show that a panel of sheep scrapie prions transmit to several tgHu mice models with an efficiency comparable to that of cattle BSE. The serial transmission of different scrapie isolates in these mice led to the propagation of prions that are phenotypically identical to those causing sporadic CJD (sCJD) in humans. These results demonstrate that scrapie prions have a zoonotic potential and raise new questions about the possible link between animal and human prions.

Chronic wasting disease agents in nonhuman primates

Chronic wasting disease is a prion disease of cervids. Assessment of its zoonotic potential is critical. To evaluate primate susceptibility, we tested monkeys from 2 genera. We found that 100% of intracerebrally inoculated and 92% of orally inoculated squirrel monkeys were susceptible, but cynomolgus macaques were not, suggesting possible low risk for humans.

Prion and prion-like diseases in animals

Transmissible spongiform encephalopaties (TSEs) are fatal neurodegenerative diseases characterized by the aggregation and accumulation of the misfolded prion protein in the brain. Other proteins such as β-amyloid, tau or Serum Amyloid-A (SAA) seem to share with prions some aspects of their pathogenic mechanism; causing a variety of so called prion-like diseases in humans and/or animals such as Alzheimer's, Parkinson's, Huntington's, Type II diabetes mellitus or amyloidosis. The question remains whether these misfolding proteins have the ability to self-propagate and transmit in a similar manner to prions. In this review, we describe the prion and prion-like diseases affecting animals as well as the recent findings suggesting the prion-like transmissibility of certain non-prion proteins.

Protein misfolding cyclic amplification (PMCA): Current status and future directions

Transmissible spongiform encephalopathies (TSEs) most commonly known as prion diseases are invariably fatal neurological disorders that affect humans and animals. These disorders differ from other neurodegenerative conformational diseases caused by the accumulation in the brain of misfolded proteins, sometimes with amyloid properties, in their ability to infect susceptible species by various routes. While the infectious properties of amyloidogenic proteins, other than misfolded prion protein (PrP(TSE)), are currently under scrutiny, their potential to transmit from cell to cell, one of the intrinsic properties of the prion, has been recently shown in vitro and in vivo. Over the decades, various cell culture and laboratory animal models have been developed to study TSEs. These assays have been widely used in a variety of applications but showed to be time consuming and entailed elevated costs. Novel economic and fast alternatives became available with the development of in vitro assays that are based on the property of conformationally abnormal PrP(TSE) to recruit normal cellular PrP(C) to misfold. These include the cell-free conversion assay, protein misfolding cyclic amplification (PMCA) and quaking induced conversion assay (QuIC), of which the PMCA has been the only technology shown to generate infectious prions. Moreover, it allows indefinite amplification of PrP(TSE) with strain-specific biochemical and biological properties of the original molecules and under certain conditions may give rise to new spontaneously generated prions. The method also allows addressing the species barrier phenomena and assessing possible risks of animal-to-animal and animal-to-human transmission. Additionally, its unprecedented sensitivity has made possible the detection of as little as one infectious dose of PrP(TSE) and the biochemical identification of this protein in different tissues and biological fluids, including blood, cerebral spinal fluid (CSF), semen, milk, urine and saliva during the pre-clinical and clinical phases of the disease. The mechanistic similarities between TSEs and other conformational disorders have resulted in the adaptation of the PMCA to the amplification and detection of various amyloidogenic proteins. Here we provide a compelling discussion of the different applications of this technology to the study of TSEs and other neurodegenerative diseases.

Variant CJD. 18 years of research and surveillance

It is now 18 years since the first identification of a case of vCJD in the UK. Since that time, there has been much speculation over how vCJD might impact human health. To date there have been 177 case reports in the UK and a further 51 cases worldwide in 11 different countries. Since establishing that BSE and vCJD are of the same strain of agent, we have also shown that there is broad similarity between UK and non-UK vCJD cases on first passage to mice. Transgenic mouse studies have indicated that all codon 129 genotypes are susceptible to vCJD and that genotype may influence whether disease appears in a clinical or asymptomatic form, supported by the appearance of the first case of potential asymptomatic vCJD infection in a PRNP 129MV patient. Following evidence of blood transfusion as a route of transmission, we have ascertained that all blood components and leucoreduced blood in a sheep model of vCJD have the ability to transmit disease. Importantly, we recently established that a PRNP 129MV patient blood recipient with an asymptomatic infection and limited PrP(Sc) deposition in the spleen could readily transmit disease into mice, demonstrating the potential for peripheral infection in the absence of clinical disease. This, along with the recent appendix survey which identified 16 positive appendices in a study of 32,441 cases, underlines the importance of continued CJD surveillance and maintaining control measures already in place to protect human health.

Molecular barriers to zoonotic transmission of prions

The risks posed to human health by individual animal prion diseases cannot be determined a priori and are difficult to address empirically. The fundamental event in prion disease pathogenesis is thought to be the seeded conversion of normal prion protein to its pathologic isoform. We used a rapid molecular conversion assay (protein misfolding cyclic amplification) to test whether brain homogenates from specimens of classical bovine spongiform encephalopathy (BSE), atypical BSE (H-type BSE and L-type BSE), classical scrapie, atypical scrapie, and chronic wasting disease can convert normal human prion protein to the abnormal disease-associated form. None of the tested prion isolates from diseased animals were as efficient as classical BSE in converting human prion protein. However, in the case of chronic wasting disease, there was no absolute barrier to conversion of the human prion protein.