Low frequency of the scrapie resistance-associated allele and presence of lysine-171 allele of the prion protein gene in Italian Biellese ovine breed.

Abstract

Frequencies of polymorphisms at codons 136, 154 and 171 of the prion protein (PrP) gene were studied in 1207 pure-bred and cross-bred Italian Biellese rams, a small ovine breed of about 65 000 head in Italy. Aside from the five most common alleles (VRQ, ARQ, ARR, AHQ and ARH), the rare ARK allele was also found, with the highest frequency reported so far in an ovine breed (2.5 %). ARK/--- genotypes had a total frequency of 4.9 %. The resistance-associated ARR allele was seen at a low frequency (8.3 %). Only 1.4 % of animals examined had a resistant ARR/ARR PrP genotype. Semi-resistant (ARR/ARQ, ARR/ARH and ARR/AHQ) PrP genotypes had a total frequency of 12.6 % and PrP genotypes that are associated with high scrapie susceptibility (e.g. VRQ/VRQ and ARQ/ARQ) had a total frequency of 81.1 %. Statistical analysis comparing PrP allele frequencies between pure-bred and cross-bred animals showed that the ARR allele occurred at a significantly lower frequency in pure-bred rams. Furthermore, comparison of PrP allele frequencies between pure-bred rams over 18 months of age and those below 18 months of age showed a significant decrease in the ARR allele in breeding rams over 18 months of age. Based on these results, breeding for scrapie resistance in the Biellese breed will have to take into account the low frequency of the ARR allele, which also seems to be subject to negative selection by farmers. Further investigation is required to understand whether the ARK allele is also associated with resistance to transmissible spongiform encephalopathies.