Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 6/18/2004
Publication Date: 6/21/2004
Citation: Stanker, L.H., Carter, J.M. 2004. Techniques and challenges in the diagnosis of prion diseases.. Meeting Abstract. Interpretive Summary:
Technical Abstract: Transmissible Spongiform Encephalopathies (TSEs) are fatal transmissible brain diseases that occur in animals and humans. The human diseases include Creutzfeldt-Jakob Disease (CJD), Gerstmann-Straüssler Scheinker syndrome, Fatal Familial Insomnia, kuru, iatrogenic CJD, and new variant CJD. Animal TSEs include scrapie in sheep, Chronic Wasting Disease (CWD) in cervids, Transmissible Mink Encephalopathy (TME), and Bovine Spongiform Encephalopathy (BSE), as well as TSEs in cats (Feline Spongiform Encephalopathy) and in exotic ungulates. The causative agents for these diseases are prions. Prions appear to be composed solely of protein. Although the product of a single gene, prion protein (PrP) may exist in at least two forms: a non-infectious conformer referred to as normal PrP (PrPc), and an infectious form referred to as PrPSc. The function of normal PrPc is unclear. It is a glycoprotein anchored in membranes by a GPI structure. Its tertiary structure contains three alpha-helical regions and a small beta-sheet region. While a crystallographic solution for the infectious form is not available, circular dichroism studies suggest that PrPSc has relatively less alpha-helical structure and more beta-sheet. PrPSc appears to aggregate and to have the ability to convert normal PrPc to the infectious PrPSc form. Transmission of BSE disease in bovines occurs via consumption of PrPSc contaminated feed, and transmission to humans (as vCJD) occurs via consumption of contaminated beef. No pre-clinical diagnostic tests (excluding a third eyelid test in sheep) are available for TSE diseases. Diagnosis is based on clinical observations and histochemical examination of brain. Typically, spongiform degeneration is observed surrounded by plaque-like aggregates of immunoreactive PrPSc at the rim of the vacuoles. Further complicating the production of effective diagnostics for TSEs has been the difficulty in obtaining antibodies that can distinguish between PrPc and PrPSc. In the EU and US, all tests currently approved for BSE screening are postmortem procedures that measure PrPSc in brain. The first generations of approved tests are based on the observation that PrPc is susceptible to digestion with proteases, whereas PrPSc is resistant. In these tests, brain samples are prepared and digested with proteinase K to remove PrPc. Then PrPSc that remains is detected using a nonspecific anti-PrP antibody. A second-generation test, the Conformation Dependent Immunoassay (CDI), does not rely on proteinase K digestion. Instead it is based on antibodies that bind an epitope exposed in PrPc but hidden in PrPSc. Improved diagnostics clearly are needed. To supplement existing clinical diagnostics and postmortem tests, live animal tests are essential for improved herd management. Furthermore, tests with greater sensitivity would further strengthen efforts to eliminate prion-contaminated material from feeds and food. Progress in development of sensitive non-immunochemical and immunochemical tests for PrPSc detection and characterization will be discussed.