Ovine Leukocyte Counts Do Not Associate with Variation in prnp, but are Breed and Age Dependent

Genetic variation within the prion gene (prnp) in sheep confers resistance to scrapie.  In cattle, lymphocyte profile was found to be associated with prnp genotype. Therefore, the aim of this study was to determine if sheep prnp genotypes were associated with any leukocyte population.   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.  DNA was extracted, complete blood cell counts (CBC) were recorded, and the prnp genotypes were identified by sequencing. Data was analyzed with a reduced mixed model where breed, genotype, and age were fixed effects with CBC sampling year and sire as random effects.  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).  Bonferroni correction eliminated this significance.  These results are contrary to a cattle study however, we did not differentiate the lymphocyte population into B cells, CD4+, and CD8+ T cells.  Breed of sheep impacted (P<0.01) all leukocyte measurements.  Generally, Rambouillet ewes had lower leukocyte counts compared with the other breeds and Suffolk had the greatest.  Younger sheep had greater (P<0.01) white blood cell and lymphocyte counts compared with older sheep.  Ewe age did not affect (P>0.16) monocyte, neutrophil, basophil or eosinophil counts.  We did not find an association of leukocyte counts, as determined by CBC, with the 10 prnp genotypic variations detected here.