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

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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.