Report of outbreaks of classical scrapie in Dorper sheep and associated prion protein gene polymorphisms in affected flocks

Scrapie Resistance Gene Identification using Optimized Taqman Test qPCR Method in Sheep on the Territory of the Republic of Serbia

Scrapie is an infectious neurodegenerative disease affecting the central nervous system of sheep and goats that belongs to transmissible spongiform encephalopathies. The disease is caused by the accumulation of proteinase-resistant isoform of the prion protein. The sheep predisposition to scrapie is associated with polymorphisms of the PrP gene. Genetic susceptibility to scrapie is mainly related to codons 136, 154, and 171. ARR sheep are strongly scrapie resistant and VRQ genotype is the most susceptible. Many countries have scrapie eradication programs based on using rams with resistant genotype. The eradication program has not yet been implemented in the Republic of Serbia. To examine the genetic makeup of sheep in Serbia related to scrapie, we optimized TaqMan probes of real-time polymerase chain reaction (qPCR) technique for three codons. Blood samples from 100 sheep were analyzed by qPCR and the majority of the examined sheep were AA homozygous for the 136 codon. For codon 154 the most frequent genotype was RR and for codon 171 the most frequent genotype was QQ.

Classic and atypical scrapie - a genetic perspective

Scrapie was the first prion disease to be recognised and the study of this disease in sheep and goats has provided a wealth of information not only for scrapie but also for the other prion diseases. All prion diseases are under strong genetic control of the prion gene PRNP, independent of whether they are typical or atypical scrapie and which of the different prion strains is causing infection. Decades of studies using experimental disease challenges and field surveys have established disease association models, in which species-specific amino acid variations in the prion or PrP protein, encoded by the PRNP gene, can predict disease susceptibility or resistance. PRNP genetics represents an important and successful basis for implementing scrapie eradication strategies in sheep and goats. In general terms these studies have revealed that there appear to be many more amino acid changes in PrP leading to increased resistance than to higher susceptibility. Most changes are in the globular part of PrP protein and three regions appear to have major influence. This knowledge can be transferred into prion diseases of other species to facilitate genetic control strategies. However, an obstacle remains with the lack of fully understanding the underlying molecular mechanism, impeding our ability to deal with the difference in the genetic control between typical and atypical forms of scrapie or to predict association in newly infected species. This chapter will discuss the advances in both typical and atypical scrapie from a genetic perspective.

Scrapie, CWD, and Transmissible Mink Encephalopathy

Transmissible spongiform encephalopathies (TSEs), or prions, are neurodegenerative diseases that affect a variety of animal species, including humans. Cruetzfeldt-Jakob disease (CJD) in humans, sheep and goat scrapie, chronic wasting disease (CWD) of cervids, and transmissible mink encephalopathy (TME) of mink are classified as TSEs. According to the "protein-only" hypothesis (Prusiner, 1982),¹ prions are devoid of nucleic acids and consist of assemblies of misfolded host-encoded normal protein, the prion protein (PrP^C). Prion propagation is thought to occur by a templating mechanism during which PrP^C is recruited, converted to a disease-associated isoform (PrP^D), and assembled onto the growing amyloid fibril. This fibular assembly is infectious, with ability to initiate disease processes similar to other pathogenic agents. Evidence indicates that scrapie, CWD, and TME disease processes follow this rule.