The use of PrP transgenic Drosophila to replace and reduce mice in the bioassay of mammalian prions

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Prion diseases include BSE of cattle, chronic wasting disease of cervids, scrapie of sheep and CJD of humans. These conditions are infectious and can spread between individuals of the same or different species. Animal prion diseases can be transmitted to humans and are therefore a threat to public health, evidenced by the outbreak of classical BSE in cattle followed by the emergence of variant CJD (vCJD) in humans. The pathogen that causes these transmissible diseases is an aggregated rogue form of a normal protein found in neurons and is referred to as a prion. Transmission of BSE to humans is believed to have occurred by dietary intake of BSE-contaminated food. Strict controls now protect humans from BSE, including removal of cattle tissues most likely to contain infectious prions when the animal is slaughtered. These cattle tissues are called specified risk material (SRM) and do not enter the human food chain.The only reliable way to detect BSE prion infectivity is by bioassay in experimental animals, which have traditionally been rodents, such as mice. The mouse prion bioassay involves injecting suspected BSE-infected samples into experimental mice and waiting to see if these animals develop prion disease. These assays are slow and cumbersome since the incubation time for prion disease in mice may be 1-2 years before clinical signs become evident and a predetermined end-point is reached. Collectively, BSE prion infectivity studies have utilised large numbers of mice over a long time period, and have subjected a high proportion of these animals to experience terminal clinical signs of experimental neurological disease. Since the emergence of classical BSE, the increased surveillance for the condition has led to identification of novel forms of the disease, which are a new threat to the human food chain as we do not know which tissues in affected cattle contain these new forms of BSE prion infectivity. It is essential therefore, to verify if the current SRM control measures are sufficient to prevent the new forms of BSE prions from entering the human food chain. Many more mice will be used in proposed food safety research programmes in order to measure prion infectivity levels in an extensive range of samples from cattle infected with new forms of BSE. In their entirety, these mouse prion bioassays will use more than 100,000 mice. It is vital to apply the 3Rs principles to this intended research programme by reducing and replacing, where possible, this large number of experimental mice with a prion bioassay that uses a less sentient host.In our laboratory we model prion disease in the fruit fly Drosophila melanogaster because they are relatively easy and economical to work with, and are a widely accepted ethical alternative to higher organisms including mice. We have developed a Drosophila-based prion bioassay that can detect BSE prion infectivity in order to provide an alternative to mice for the bioassay of bovine prions, and reduce the use of mice, and other vertebrate animal species, to measure prion infectivity in general. To do so, we have introduced a gene into the flies that allows them to produce the protein that aggregates in the brain of animals with prion disease. We already know that this mammalian protein will aggregate and cause prion disease in transgenic Drosophila when flies are fed infectious prions. What is equally significant is that the response by the transgenic flies to prion-infected material is evident within a few weeks following exposure to prion material. We can now develop a faster, more versatile and more sensitive bioassay to detect BSE prion infectivity than currently exists. We aim to transfer this know-how to the APHA, a main user and internationally recognised reference laboratory of the mouse prion bioassay. Our Skills and Knowledge Transfer to the APHA will provide the best opportunity to ensure world-wide impact of our new Drosophila-based prion bioassay.