Introduction to the transmissible spongiform encephalopathies or prion diseases.

Sheep scrapie has been known for at least 200 years and was described as a transmissible disease over 100 years ago. Since then, three groups of transmissible spongiform encephalopathies or TSE diseases have been identified in humans including familial, infectious and sporadic types. The discovery of the prion protein (PrP) in the 1980s greatly accelerated knowledge of the biology and pathogenesis of TSE diseases as this protein was found to play a critical role in disease susceptibility and the TSE species-barrier and may also be a component of the infectious agent itself. Nevertheless, the nature of the TSE agents remains an enigma. Proof of the protein-only hypothesis may require generation of biologically active transmissible agent in a cell-free environment where a virus cannot replicate. Conversely, proof of a viral aetiology will require identification and isolation of a candidate virus. Further understanding of the structure of the disease-associated protease-resistant PrP should help elucidate the mechanism of PrP conversion from the normal to the abnormal form. Such information should open up new approaches to both diagnosis and therapy.