Rapid bioassay of cervid prions in PrP transgenic Drosophila: addressing the threat to animal and human health from Chronic Wasting Disease

Prion diseases are fatal neurodegenerative disorders of humans and other animal species. These conditions are transmissible both within and between species. The pathogen that causes these transmissible brain diseases is an aggregated form of a normal protein found in neurons and is referred to as a prion. The only reliable method to detect prion infectivity is by bioassay in experimental animals. Animal prion diseases are a threat to human health because of their zoonotic potential, highlighted by the BSE epizootic in cattle and subsequent emergence of vCJD in humans.

Chronic wasting disease (CWD) is a highly contagious and increasingly prevalent fatal prion disease of deer, and related species. CWD is a potential zoonosis since infectious prions are present in cervid skeletal muscle, which is consumed by humans. This view is supported by studies that show non-human primates can be experimentally infected with CWD-infected cervid brain tissue. CWD was considered to be restricted to N. West America but has now been found in Europe with cases detected in Norway and Finland. In attempts to control the disease Norway initiated the large-scale cull of wild reindeer, while Finland banned the export of live deer, two acts that had a significant impact upon their respective cervid industries. Emergence of these new foci of CWD infection poses significant new unmet challenges to cervid and human health across Europe, including the UK. The BSE epizootic in the UK highlighted the critical need for early development of a tractable prion bioassay for preparedness in assessment of the risk to animal and human health from new animal prion diseases, or as in the case of CWD, new reservoirs of an existing prion disease.

It is important to develop a tractable prion bioassay that measures CWD prion infectivity in tissues and blood from affected cervids in order to formulate risk-based control measures that curb the spread of the disease amongst animals. In addition, it is necessary to carry out further CWD prion bioassays in experimental animals that are more ethically acceptable than primates in order to help predict the ability of this disease to infect humans. Both of these issues have been attempted using mice transgenic for cervid or human PrP, which provided murine models of deer and humans for CWD bioassays. These mouse prion bioassays involve injecting CWD-infected material into experimental mice and waiting to see if these animals develop prion disease. The mouse bioassay may take years to complete and is very expensive because of the cost of keeping large numbers of mice for a long period of time. Consequently, there is a need to develop a faster, cheaper, animal bioassay to assess CWD prion infectivity, that is as sensitive as the mouse prion bioassay.

In our lab, we model prion disease in the fruit fly Drosophila melanogaster because they are relatively easy to work with, enable more rapid data collection and are less expensive to keep compared to large experimental animals and mice. Here, we will use Drosophila to develop a sensitive bioassay to detect CWD prions. To do so, we have introduced the cervid or human PrP gene into the flies that allows them to produce the protein that aggregates in the brain of deer with CWD or humans with vCJD, respectively. We already know, from our previously published studies, that this mammalian protein will aggregate and cause prion disease in transgenic Drosophila when flies are fed infectious prions. We will determine the sensitivity of our fly-based CWD prion bioassay, including its ability to bioassay cervid blood, and investigate its ability to model human susceptibility to CWD prions. We estimate that PrP transgenic Drosophila can be used to bioassay prion inocula at 10% (or less) of the cost of the mouse prion bioassay. Furthermore, our fly-based prion bioassay is completed within 6 weeks, whereas the mouse bioassay may require one or more years to assess the same prion inocula.

Here we will demonstrate that cervid PrP transgenic Drosophila can bioassay CWD prion infectivity and that human PrP transgenic Drosophila can be used to model the zoonotic potential of cervid prions. Inocula from European and USA cervids of known CWD status will comprise brain material from animals at terminal prion disease, and blood from asymptomatic or clinical CWD-affected USA cervids. Prion-free cervid brain tissue and blood samples will serve as control material. Drosophila will be inoculated with prions at the larval stage while assessment of their locomotor ability, survival and detection of PrPSc, prion seeding activity and prion strain typing studies in mice will use adult flies. The sensitivity of our Drosophila bioassay will be compared with existing data for the bioassay of the USA-derived CWD inocula in the natural host or cervid PrP transgenic mice. Accordingly, we will: (1) inoculate cervid PrP transgenic Drosophila with CWD-infected cervid brain material, including a dilution series of these inocula, to determine the sensitivity of these flies to cervid prions by assessment of locomotor ability and survival. We will detect PrPSc and prion seeding activity in prion exposed flies to validate the specificity of our bioassay. We will strain type cervid prions passaged in Drosophila by fly-to-mouse transmission studies in cervid PrP transgenic mice; (2) inoculate cervid PrP transgenic Drosophila with whole blood or blood fractions from CWD-affected cervids at the asymptomatic or clinical phase of experimental or natural CWD in order to determine the ability of these flies to act as a confirmatory ante-mortem blood test for cervid prion disease; (3) inoculate human PrP transgenic Drosophila with CWD prions to model their zoonotic potential.

Highlight:

Chronic Wasting Disease (CWD) is a fatal transmissible prion disease of deer and related cervid species, previously considered restricted to N. America but now detected in Europe. The BSE epizootic in the UK highlighted the critical need for early development of a tractable prion bioassay for preparedness in assessment of the risk to animal and human health from new animal prion diseases, or as with CWD, new reservoirs of an existing prion disease. Our research proposed here will show that PrP transgenic Drosophila can rapidly and efficiently bioassay CWD prions, and can be used to model cervid prion permeation of the human species barrier. This will provide protection for animal and human health from the potentially zoonotic cervid prion disease CWD. While our research will primarily impact upon the CWD prion studies, our research will have significant translation approaches to other prion disease studies. This significant utility of our novel system will build confidence and encouragement for uptake of our Drosophila-based prion bioassay by other prion researchers both nationally and internationally.

Beneficiaries:

Industry: Should CWD arise in the UK it would have a significant impact upon industries that directly and in-directly rely upon cervids through a negative impact on the health and numbers of these animals. The rapidly developing UK venison industry is presently valued at approximately £120M annually, and the recreational and tourism industry centered upon deer hunting is valued at approximately £150M. Our research proposed here will show that PrP transgenic Drosophila can rapidly and efficiently bioassay CWD prions, including those in blood, in order to provide protection for animal health, and therefore, protect the UK cervid industry from CWD.

Government: The UK House of Commons Science & Technology Select Committee enquiry (2014) into the vCJD epidemic recommended development of a test for detection of prion-infected human blood. Development of a human PrP Drosophila-based prion bioassay to test human blood will directly impact on this requirement. Similarly, development of a human PrP Drosophila-based prion bioassay will address the Department of Health CJD Oversight Committee's need for an independent confirmatory bioassay for prion infectivity to support pre-clinical biochemical tests for vCJD.

Animal & Plant Health Agency (APHA) and Food Standards Agency (FSA): These agencies rely heavily on animal prion bioassays to monitor animal prion disease and human food safety. Our Drosophila-based prion bioassay is strongly supported by the APHA since it will contribute to the rapid detection of prion infectivity levels in tissues and fluids of prion diseased animals. The FSA is also very supportive of our study since it will allow us to establish a tractable system to help ensure the safety of animal products destined for human consumption.

Medical & Veterinary profession: Our outputs will impact upon human health care by development of a tractable and rapid diagnostic test to screen human organ donors for transplantation and as a confirmatory blood test for human prion infectivity. They will also impact upon the veterinary profession by development of new versatile diagnostic tests to screen blood for the prion status of farmed animals and improve the health of herds of cattle or flocks of sheep.

Experimental animals: The successful roll out of our Drosophila model will lead to reduction and replacement of the significant number of cervids and mice that currently undergo prion-induced neurodegeneration in bioassays that model human prion disease transmission and animal prion disease zoonosis. Animals other than cervids and mice will also benefit from the development of our invertebrate model because the ease of transgenesis in Drosophila allows the generation of species-specific PrP flies that detect autologous prions from any mammalian species.