The association between 171 K polymorphism and resistance against scrapie affection in Greek dairy sheep

Polymorphisms at codon 171 of the sheep prion protein gene (PRNP) are known to be genetically associated with scrapie (Hunter et al., 1993, Hunter et al., 1997). Glutamine (Q) and histidine (H) at codon 171 contribute to scrapie susceptibility, while arginine (R) is associated with resistance. In some breeds, lysine (K) occurs at codon 171 (171 K), but its effect on scrapie resistance has not been fully determined yet. To investigate the role of 171 K in dairy sheep in Greece, we initially analyzed blood samples from 2368 sheep with no clinical signs of the disease, from thirteen scrapie-affected flocks that were then monitored for 5 years for scrapie clinical signs. During this period 177 sheep resulted positive in classical scrapie, after obex testing. The 177 scrapie-affected brain samples PRNP polymorphisms were analyzed using a Real Time-PCR protocol while the 2368 sheep blood samples were analyzed by a micro-array method based on primer extension (Homogenous Mass Extend Assay - SEQUENOM, Genaissance Pharmaceuticals) in MALDI-TOF-MS. Single Nucleotide Polymorphisms (SNPs) present at PRNP codons 136A/V (Alanine/Valine), 154R/H and 171Q/R/H/K were detected. The ARQ haplotype as well as the ARQ/ARQ genotype predominated. In the studied population, ARK haplotype was found at the highest frequency reported so far (6.98%) whereas, ARK/-genotypes had a total frequency of 12.81%. Relative risk (RR) and the percent relative effect were calculated to assess the association between ARK/-genotypes and scrapie affection. The results showed that carriers of K allele at codon 171 compared to Q and H alleles had ca. 0.25 and 0.40 times the risk of scrapie affection, respectively, indicating a possible association of K allele at codon 171 with scrapie resistance.