Ovine leukocyte profiles do not associate with variation in the prion gene, but are breed dependent.

Background: Lymphatic tissue is the accepted tissue type for live animal testing of scrapie, a prion disease in sheep, 1,2 for which genetic variation within the prion gene (PRNP) confers resistance. 3 Circulating lymphocytes infected with PrPSc can cause scrapie in sheep. 4,5 In cattle, a lymphocyte profile has been found to be associated with a PRNP genotype 6 and, if true in sheep, may have implications for live animal testing and disease progression. Therefore, our aim was to determine whether a sheep PRNP genotype was associated with leukocytes as measured by complete blood cell counts (CBC). Materials/Methods: Blood was collected from 589 ewes (Columbia, Polypay, Rambouillet, Suffolk) aged 2-6 years over 3 years (2011, 2013 and 2014) at the U.S. Sheep Experiment Station, Dubois, ID, USA, per USSES IACUC 10-07 and WSU permit 3171. DNA was extracted, and CBC counts were recorded. 7 The PRNP genotypes were identified with sequencing 8 (data may be accessed at: http://www.animalgenome.org/repository/pub/USDA2015.0917/). A reduced statistical model, which included breed, genotype and age as fixed effects with CBC sampling year and sire as random effects, was analyzed with the Mixed procedure of SAS 9.3 (SAS Institute). Genotypes with a lower than 1% frequency were removed from the analysis of that variant. Results/Conclusions: There were no associations (P > 0.10) of any PRNP genotype with any leukocyte count except for the variant at nucleotide position 335 (amino acid 101) with monocytes (P < 0.025) (Table 1). This difference is likely because only 10 heterozygotes were represented within these sheep and, if a Bonferroni correction is applied, there is no association. These results differ from cattle, 6 but we did not differentiate lymphocytes into B cells, CD4+ and CD8+ T cells. Breed of sheep impacted (P < 0.01) all leukocyte CBC measurements (Table S1). Generally, Rambouillet ewes had lower leukocyte counts than did ewes from other breeds and Suffolk had the greatest counts. Differences in leukocyte count due to sheep breed have been detected by others. 9 Younger sheep had greater (P < 0.01) WBC and lymphocyte counts than did older sheep (Table S2). Other counts did not differ (P > 0.16) due to ewe age. We did not find an association of leukocyte counts, as determined by CBC, with the 10 PRNP genotypic variations detected here.