Prolonged incubation time in sheep with prion protein containing lysine at position 171

Sheep scrapie susceptibility or resistance is a function of genotype, with polymorphisms at codon 171 in the sheep prion gene playing a major role. Glutamine (Q) at codon 171 contributes to scrapie susceptibility, while arginine (R) is associated with resistance. In some breeds, lysine (K) occurs at codon 171, but its effect on scrapie resistance has not been determined. Charge and structural similarities between K and R suggest that they may contribute to prion disease susceptibility in a similar way, but studies have not been performed to confirm this. The purpose of the current study was to compare susceptibility and incubation times of AA(136)RR(154)QQ(171) (where the letter denotes the amino acid and the number the position) with AA(136)RR(154)QK(171) sheep after inoculation with scrapie. Barbado AA(136)RR(154)QQ(171) and AA(136)RR(154)QK(171) sheep were inoculated with scrapie intracerebrally to assess their susceptibility to scrapie. After inoculation, sheep were observed daily for clinical signs and were euthanized and necropsied after clinical signs were unequivocal. Tissues were collected at necropsy for immunohistochemistry and Western blot analyses. The QQ(171) sheep had clinical signs approximately 12 months after inoculation, whereas QK(171) animals had an average incubation time of 30 months to onset of clinical signs. The distribution of abnormal prion protein was similar in QQ(171) and QK(171) sheep. Results of the study indicate that sheep with a single K allele at codon 171 are susceptible to scrapie but with a prolonged incubation time. Work is currently underway to examine relative scrapie susceptibility or resistance of KK(171) sheep.