Ancient DNA polymorphisms analysis to investigate scrapie susceptibility in Sicilian sheep remains from medieval archaeological sites

Abstract

Encephalopathy in sheep was at first described in Ireland in 1732 and was called “scrapie“ because the animals tend to tear their hair. Historically it seems to be the result of an incestuous union as breeding practice in old farms. In Sicily the animal bones found in association with the human skeletal remains from the tombs or city-sites, comprised a broad range of domestic. Usually, species included in the collected bones are domestic animals commonly eaten as sheep or goats, cattle, pigs, chickens and a small partridge. In this contest, the assemblage and the species identification is often difficult. Based on DNA barcoding, all the investigated bones were confirmed as belonging to the Ovine specie. Most of the elements were not whitened or brittle, suggesting none exposure period to weathering and the soft tissues were decomposed slowly. The aim of this study was to investigate the SNP variants of the ovine PrP gene (PrnP) that is associated with the risk of scrapie disease. Sheep susceptibility to classic scrapie is associated with mutational points that leads to the recognized variants: codon 136 (Alanine/Valine), codon 141 (Leucine/Phenylalanine), codon 154 (Arginine/Histidine) and codon 171 (Arginine/Glutamine/Histidine). Ancient DNA was isolated directly from 200 animal skeletal remains and collected on four Sicilian medieval archaeological sites (10th – 12th century). aDNA extraction was performed from the epiphysis of the skeletal remains containing dry bone marrow cells and used as template to perform real time PCR and sequencing. The obtained data were analyzed for the assignment of the PrnP genotype at the 136, 141, 154 and 171 codons. Results showed that the 90% of the animals (n=180) had a resistant homozygote genotype at each codon (ALRR/ALRR), while only the 10% had a low sensitivity to scrapie disease (ALRR/ALRQ); no animals were found with the most well-known sensitive genotypes containing Q, F, V and H codons. The high incidence of animals resistant genotype in various independent districts, suggest a positive trend towards scrapie free herds for the animals bred in Sicily in old age. In conclusion, the genotype obtained for the medieval bone sheep differ strongly at the scrapie hot spot respect those of the modern ovine so that the polymorphisms that leads to susceptibility probably occurred in Sicily after the Middle Age.