It is a fact that amino acid sequence variants in prion protein PrP determine whether a host is susceptible or resistant to prion infection. New research of Central Veterinary Institute (CVI), part of Wageningen UR, reveals however that this variation does not play a role in the type of prion that will emerge. This means that other structural factors are at work, e.g. refolding and aggregation of PrP, and perhaps the presence of one or more other components.
Knowledge of prion structure is essential for understanding the cause and development of prion diseases in animals, such as sheep with scrapie and cattle with BSE, or in humans who can acquire several types, particularly Creuzfeldt-Jacob disease (CJD), syndrome of Gerstmann-Sträussler-Scheinker (GSS), and fatal familial insomnia (FFI). However, up to now it proved to be hard to determine the structure of prions with the methods that are available right now. Much is expected from the use of cryo-electron microscopy that is developing rapidly. The availability of different prion materials from animals such as cattle and sheep and several prion production methods are indispensable for this kind of research.
The prion protein PrP plays a key role in prion diseases. This protein is considered the most important component of prions. These infectious diseases – first seen in sheep – cause a fatal irreversible degeneration of the nervous system.
The cause of prion diseases in humans remains in most cases unknown. Most forms are considered to have developed spontaneously. However, in the last century research in New Guinea concerning the disease kuru among Papuan tribes with cannibalistic rituals, showed that transmission (infection) could occur from person to person. This kind of research on this group of unique protein associated diseases was rewarded with three Nobel prizes to resp. D. Carlton Gajdusek (1976), Stanley B. Prusiner (1997) and Kurt Wüthrich (2002).
The usually normal prion protein (PrP_norm) is in the different types of prions converted into structurally different forms (PrP_dis). PrP_norm is present in the host as two copies. Through genetic variations these two PrP_norm copies can differ in only one of the approx. 250 amino acids of the protein. Amino acid variants in one or both PrP products in the host can determine whether the host is more susceptible or more resistant to prion infections. In humans and in sheep extraordinary resistant individuals were found, dependent on genetic variations of PrP. Since 1998 sheep are actively and successfully bred in The Netherlands for resistance in order to eradicate scrapie and also to minimize the chances of infection with BSE.
BSE and scrapie are different types of prions for both of which sheep are susceptible. The structure of the PrP_norm is largely elucidated. However, the structure of PrP_dis has up till now been inaccessible to current protein structure determination methods, such as röntgen diffraction or nucleair magnetic resonance (NMR). The question remains whether the variation in the amino acid sequence in PrP_dis in prions can cause the development of so many prion types. With this study we are getting closer to the answer by investigating BSE and scrapie prions in sheep. It is known that the structure of PrP_dis is different between these two prion types. It is assumed that both PrP_norm copies present in one animal can contribute to the emergence of prions. And it is known that both copies are produced in equal amounts, but it is not clear whether both become equally part of the prion as PrP_dis.
The study in sheep showed us - by using specific antibodies - that several genetic variants can be present in the prion material. Special animals were used that at the same time produce two special variants of PrP_norm, the ARR‑PrP and the VRQ‑PrP that cause the host to be resistant as well as extra susceptible). It was shown that in case of infection with scrapie over 90% of the total PrP_dis material existed of VRQ-PrP_dis. In case of infection with BSE the levels of ARR‑PrP_dis and VRQ‑PrP_dis were practically equal. This means that prions can vary in their variant PrP protein composition, while the prion type is not dependent on this because its BSE or scrapie character remains the same.
This study was made possible thanks to the availability of infectious material from Tours FR (INRA-Nouzilly), Edinburgh (Roslin institute University of Edinburgh) and CVI in Lelystad.