Molecular discrimination of sheep bovine spongiform encephalopathy from scrapie

Sheep CH1641-like transmissible spongiform encephalopathy isolates have shown molecular similarities to bovine spongiform encephalopathy (BSE) isolates. We report that the prion protein PrPSc from sheep BSE is extremely resistant to denaturation. This feature, combined with the N-terminal PrPSc cleavage, allowed differentiation of classical scrapie, including CH1641-like, from natural goat BSE and experimental sheep BSE. Prion diseases, or transmissible spongiform encephalopathies (TSEs), are neurodegenerative disorders that include Creutzfeldt-Jakob disease (CJD) in humans, scrapie in sheep and goats, and bovine spongiform encephalopathy (BSE) in cattle. TSEs are characterized by accumulation of an abnormal isoform of the host-encoded prion protein (PrPC), termed PrPSc.

A novel human prion disease, variant CJD, was reported in 1995 and postulated to be caused by eating beef infected with BSE. Biologic and molecular analyses provided evidence that the same agent was involved in BSE and variant CJD Evidence of sheep and goat susceptibility to BSE and discovery of natural BSE infections in 2 goats prompted the European Commission to increase the search for BSE infections in small ruminants. Although the BSE agent can be recognized by biologic strain typing in conventional mice, large-scale testing of small ruminants required molecular tests able to discriminate BSE from the most common TSEs of small ruminants. Molecular criteria used to discriminate BSE from scrapie are based on the low molecular weight of proteinase K–treated PrPSc (PrPres), a high proportion of the diglycosylated PrPSc, and poor or absent binding with antibodies directed at N-terminal epitopes. This last characteristic was fundamental in developing the discriminatory methods currently approved for surveillance in Europe.

The experimental scrapie isolate CH1641 reportedly shares molecular features with experimental sheep BSE, although lack of transmissibility of CH1641 to conventional mice in comparison to successful transmission of BSE provided evidence that CH1641 and BSE are caused by distinct prion agents. A few natural isolates have been described in sheep, showing molecular and biologic similarities to CH1641, and were named CH1641-like. Subtle pathologic differences were exploited to distinguish these CH1641-like isolates from BSE by immunohistochemical and biochemical analyses by glycoform profiling. However, routine testing by using discriminatory Western blot (WB) methods does not easily distinguish CH1641 and CH1641-like isolates from BSE. We report 2 new CH1641-like isolates; analyze the conformational stability of CH1641-like isolates, BSE, and classical scrapie; and show that a reliable molecular differentiation of these 3 TSE sources is possible by an improved discriminatory WB method.

Emerging Infectious Diseases
April 19, 2011

Original web page at Emerging Infectious Diseases