PrP genotype frequencies and risk evaluation for scrapie in dairy sheep breeds from southern Italy

Novel Prion Protein Gene Polymorphisms in Awassi Sheep in Three Regions of the Fertile Crescent

Scrapie is a fatal, neurodegenerative disease that affects sheep and goats, and genetic susceptibility to scrapie in sheep is associated with polymorphisms in the prion protein (PRNP) gene. The aim of this study is to identify PRNP polymorphism in Awassi sheep from Türkiye, the Palestinian Authority, and Saudi Arabia. A total of 150 healthy sheep were genotyped for PRNP, using Sanger sequencing. There were seven alleles and eleven genotypes observed based on codons 136, 154, and 171 of PRNP. The ARQ allele was predominant in all populations. The most resistant allele to scrapie, ARR, was present in all three regions. The VRQ allele, associated with the highest susceptibility to scrapie, was detected only in Türkiye at a low frequency. In this study, twenty-seven amino acid substitutions were found. Eight of them (R40Q, G65E, H88L, S98T, A118P, S138T, V192F and L250I) have not been previously reported. These data indicate that sheep breeds close to the sheep domestication center have maintained high genetic diversity in the PRNP region. Our findings on PRNP will provide valuable insights for sheep breeding programs, aiding in the selection of genotypes resistant to scrapie in Türkiye, the Palestinian Authority, and Saudi Arabia.

Genetic variability following selection for scrapie resistance in six autochthonous sheep breeds in the province of Bolzano (northern Italy)

Scrapie is an ovine transmissible spongiform encephalopathy, and its susceptibility is associated with polymorphisms in the prion protein gene (PRNP). Genetic selection is currently the most effective mean for eradication of the susceptible VRQ allele in favour of resistant ARR allele. Maintenance of genetic diversity should be one of the major objectives in breeding programmes, especially in endangered breeds, and genetic information are an excellent alternative to pedigree data where these information are missing. The aim of our study was to determine changes of genetic variability in six native sheep breeds from autonomous province of Bolzano, northern Italy, following simulation of scrapie selection scenarios. A total of 684 rams were investigated for PRNP polymorphisms and for 10 microsatellite loci to estimate genetic variability. Across all loci, a total of 163 alleles were detected with a mean of 10.4 alleles per locus. Average observed (Ho) and unbiased expected (uHe) heterozygosity overall loci were 0.74 and 0.78, respectively, showing a statistically significant deviation from Hardy-Weinberg equilibrium (HWE) in all breeds. This heterozygosity deficit was confirmed by a positive fixation index (Fis), determining a moderate inbreeding in each breed. Simulating a soft selection, where only rams having at least a VRQ allele should be excluded from reproduction, Ho, uHe and Fis values remained almost unchanged, indicating that genetic variability should not be affected by the removal of these individuals. With a mild selection scenario, considering only rams with at least one ARR allele, we observed a decrease in the mean alleles per breed (8.9) and the maintenance of heterozygosity deficiency, except for two breeds, where it was any longer significant. These results showed that selection strategies allowing use of heterozygous as well homozygous ARR rams might be the right compromise to improve resistance to scrapie and to do not dramatically affect genetic variability of these breeds.

A decade of using small-to-medium throughput allele discrimination assay to determine prion protein gene ( Prnp) genotypes in sheep in Slovenia

Sheep with valine (V) at codon 136 and glutamine (Q) at codon 171 of the prion protein gene ( Prnp) are highly susceptible to classical scrapie, whereas phenylalanine (F) at codon 141 and histidine (H) at codon 154 play a major role in the susceptibility to atypical scrapie. A TaqMan real-time PCR assay was developed to determine Prnp alleles at codons 136, 141, 154, and 171 and used in classical scrapie eradication and breeding programs adopted in Slovenia. The frequency of the most resistant genotypes ARR/ARR and ARR/ARQ increased significantly in tested animals ( n = 35,138) from 6.7 and 27.1% of the tested sheep in 2006 to 12.1 and 32.4%, respectively, in 2015. Frequencies of more susceptible genotypes ARQ/ARQ and ARQ/VRQ decreased significantly from 36.4 and 3.5% in 2006 to 31.1 and 1.8%, respectively, in 2015. The most susceptible genotype VRQ/VRQ was detected in <0.5% of tested sheep. Frequencies of alleles AFRQ and AHQ affecting the susceptibility to atypical scrapie did not change significantly. The developed assay was suitable for genotyping on a small-to-medium throughput scale and was successfully used in classical scrapie eradication, as well as for the selection of classical scrapie-resistant sheep within breeding programs in Slovenia.

Evaluation of genetic variability within PrP genotyped sheep of endangered Italian Altamurana breed

In the last few decades, there has been increased awareness of preservation and exploitation of endangered breeds for the maintenance of biodiversity, as well as the concern for diseases in sheep breeding. This study was carried out in native endangered Altamurana dairy sheep breed from Southern Italy. The Altamurana breed was considered as two populations (Alt-Cav and Alt-Cra-Zoe), based on presumed cross-breed and remarkable differences in the PrP genotypes frequencies. The genetic diversity between the two Altamurana populations (Alt-Cav and Alt-Cra-Zoe) was evaluated in comparison to three Italian dairy breeds through fourteen microsatellite markers. Both measures of genetic distance and the population structure analysis highlighted that the Alt-Cav and Alt-Cra-Zoe sheep have a particular genetic background. The estimated fixation index (FST) and the genetic Nei's distances among the populations showed a higher level of genetic differentiation for Alt-Cav than Alt-Cra-Zoe. The Bayesian clustering analysis implemented in the STRUCTURE software showed clear and distinct clusters for the two Altamurana populations, confirming the hypothesis of Alt-Cav as a genetic group well differentiated from Alt-Cra-Zoe. Alt-Cav likely can be considered as belonging to the original strain of the Altamurana breed. This findings may be used to assist the programme for conservation and selection of scrapie resistance genotypes in endangered Altamurana sheep breed.