Abstract

We obtained genomic DNA from members of 10 species and subspecies of wild North American ruminants and two Old World cervids and determined the DNA sequence of the prion protein (PrP) genes to investigate the degree of similarity between them. We also assessed genetic variability of this gene in the different species by ascertaining the number and location of any commonly recurring polymorphisms. The species sampled included the three natural hosts for chronic wasting disease (CWD), a transmissible spongiform encephalopathy seen in Cervus elaphus, Odocoileus hemionus, and Odocoileus virginianus. We also sampled Alces alces shirasi, Rangifer tarandus, Odocoileus hemionus columbianus, Odocoileus hemionus sitkensis, Antilocapra americana, Bison bison, Ovis canadensis, Dama dama, and Capreolus capreolus. We compared the locations of interspecific substitutions and of polymorphic loci relative to the other species and to the elements of secondary structure of the normal cellular protein.

Our findings are consistent with previous, large-scale comparisons of PrP protein sequences across a wide array of taxa that show conservation of amino acid sequence in the two beta-sheet and three alpha helix regions of mammalian prion proteins. The interspecific differences and 12 to 15 intraspecific polymorphisms of the mature polypeptides in the ruminant species examined fell in the carboxy terminal two-thirds of the protein. Most substitutions occurred outside the secondary structure features of cellular PrP, although in addition to the well-known substitution in beta-1 of leucine for methionine at codon 132, which occurs at low frequency in Cervus elaphus nelsoni, we noted an isoleucine-for-methionine substitution at 209 in Alces alces shirasi; this falls in alpha helix 3. In our samples (n=1 to >300), the number of non-rare, fixed polymorphic loci per species varies from zero to three, and none exhibits more than two alleles per locus. None of the polymorphisms are the same in any two species. About half of the 11 interspecific differences fall outside elements of secondary structure, while two each occur within the second and third alpha helices. In addition, bison, bovines and pronghorn carry a sixth copy of the eight-amino acid repeated region in the amino terminus of the protein. Overall, we found amino acid identities within these species to vary between 96% and 100% as compared to Cervus elaphus PrP.