Molecular typing of protease-resistant prion protein in transmissible spongiform encephalopathies of small ruminants, France, 2002–2009

The agent that causes bovine spongiform encephalopathy (BSE) may be infecting small ruminants, which could have serious implications for human health. To distinguish BSE from scrapie and to examine the molecular characteristics of the protease-resistant prion protein (PrPres), we used a specifically designed Western blot method to test isolates from 648 sheep and 53 goats. During 2002–2009, classical non-Nor98 transmissible spongiform encephalopathy had been confirmed among ≈1.7 million small ruminants in France. Five sheep and 2 goats that showed a PrPres pattern consistent with BSE, or with the CH1641 experimental scrapie source, were identified. Later, bioassays confirmed infection by the BSE agent in 1 of the 2 goats. Western blot testing of the 6 other isolates showed an additional C-terminally cleaved PrPres product, with an unglycosylated band at ≈14 kDa, similar to that found in the CH1641 experimental scrapie isolate and different from the BSE isolate. Transmissible spongiform encephalopathies (TSEs) are a group of fatal neurodegenerative diseases that include scrapie in sheep and goats, bovine spongiform encephalopathy (BSE) in cattle, and Creutzfeldt-Jakob disease (CJD) in humans. TSEs are characterized by accumulation in the brain of a disease-associated isoform (PrPd) of a host-encoded cellular prion protein (PrPc). PrPd, in comparison with the normal prion protein PrPc, clearly differs in secondary and tertiary structures and in biochemical characteristics. Proteinase K (PK) digestion destroys PrPc, but in PrPd it generates a protease-resistant fragment known as PrPres. Most TSE diagnostic methods (e.g., ELISA and Western blot tests) are based on detection of PrPres.

The transmissible agent involved in BSE in cattle is known to cause prion diseases in other species under natural conditions. BSE can also be experimentally transmitted to sheep and goats, including after oral challenge to test for transmission. Because BSE-contaminated meat and bone meal may have been fed to small ruminants, BSE may have been transmitted to sheep or goats. Also, the Scientific Steering Committee of the European Commission has hypothesized that the BSE agent might have originated from a scrapie agent in sheep or goats and that these animals may represent a reservoir. In view of these data, the European Commission defined a strategy to investigate the possible presence of BSE in sheep and goats under natural conditions.

The standard for strain typing TSE agents is based on analysis of the phenotypic characteristics of the disease after transmission in laboratory rodents. Biological characterization of the BSE agent in inbred wild-type mice appeared to be reliable, because it showed uniform features in mice. However, this approach is time-consuming and costly. The identification of uniform molecular features of PrPres by Western blot in human variant CJD paved the way to a similar approach for detecting possible BSE in small ruminants. The molecular criteria defined in these studies included electrophoretic mobilities, glycosylation characteristics, and immunolabeling with different monoclonal antibodies. The last criteria enabled mapping of the protease cleavage site of the PrP protein fragment obtained after PK digestion. More recently, the identification of additional C-terminal PrPres products may contribute to discrimination of the different types of CJD or of different scrapie and BSE sources. Discriminant molecular features of the prion protein can also be investigated by immunohistochemical analysis or ELISA. In all of these studies, it was assumed that the strain information was closely associated with the structural features of PrPd.

The Western blot method enabled discrimination of experimental BSE in sheep from most scrapie-affected animals. Nevertheless, discrimination was more difficult with the CH1641 experimental scrapie isolate, which otherwise clearly differs from BSE by its absence of transmissibility to wild-type mice. Similar molecular features to those of CH1641 have been described in a few natural scrapie cases in France and in the United Kingdom. We describe the molecular findings obtained for a large series of TSE infections in France identified in small ruminants by active surveillance during 2002–2009 and for CH1641-like isolates in sheep and in 1 goat.

Emerging Infectious Diseases
January 24, 2011

Original web page at Emerging Infectious Diseases