Prion diseases and emerging prion diseases.

Transmissible spongiform encephalopathies (TSEs), also called prion diseases, are fatal neurodegenerative disorders. An abnormal isoform of the prion protein (PrP(Sc)) generated by post-translational modification of the cellular prion protein (PrP(C)) is believed to be the main component of this infectious agent. PrP(Sc) is relatively resistant to proteinase K (PK) digestion. This characteristic has been widely accepted as the physicochemical basis for distinguishing between PrP(C) and PrP(Sc). PrP(C) is a glycoprotein that contains 2 Asn-linked glycosylation sites; it is present in the cells in 3 different glycoforms, including an unglycosylated form. Hence, for different prion strains, PrP(Sc) exhibits different glycoform patterns with different ratios of the 3 forms by western blot. Recently, phenotypes of TSEs have emerged that exhibit PrP(Sc) with different glycoform patterns and/or mild PK resistance in comparison with previously described typical cases. Regarding sheep scrapie, atypical scrapie cases that are represented by Nor98 have been reported among sheep previously presumed to be genetically scrapie-resistant. Moreover, atypical bovine spongiform encephalopathy (BSE) cases have been reported. These are classified into 2 phenotypes (H-type and L-type) based on the molecular weight of unglycosylated band of PK-digested PrP(Sc). The origin of these emerging prion diseases is obscure, conformational differences of PrP(Sc) may cause the different biological and biochemical characteristics of prion strains.