The cervid PrP gene : patterns of variability and selection

The PrP prion protein (PrPC) is a glycoprotein believed to be present in all vertebrates. In mammals, a protease-resistant isoform of the prion protein (PrPSC), produced by posttranslational modifications of PrPC, is believed to be the agent or a component of the agent causing transmissible spongiform encephalopathies (TSEs). PrPC and PrPSC are encoded by the same gene (PRNP) and variation within this locus has been associated with disease development and modulation in a number of species. The selective forces acting on the PrP gene have now been investigated in a number of TSE affected species and although PRNP appears to have been under strong selective constraint throughout the mammalian evolutionary history, species-specific selective patterns seem to exist. The current lack of knowledge concerning the functional role of the PrP protein, and its exact role in disease, have made the interpretation of these different evolutionary scenarios very challenging; however, the potential for disease to shape the genetic variability of the PrP gene has been demonstrated in the human gene. Variation at codon 132 of the Cervus canadensis (wapiti) PRNP has been claimed to modulate Chronic Wasting Disease (CWD), a relatively new TSE affecting cervid species and currently the only TSE naturally affecting both captive and free-ranging populations. Codon 132 corresponds to the human codon 129 and variation at this position has been associated with TSE-related balancing selection in humans. This thesis investigated the genetic variability and selective patterns of coding and non-coding regions of PRNP in free-ranging populations of C. canadensis and C. elaphus (CWD-free species closely related to wapiti) to gain a better understanding of the possible functional or disease-related forces shaping PrP genetics. The study of codon 132 genotypic patterns in CWD +vi and CWD vE wapiti provided no evidence for genetic modulation of CWD susceptibility, challenging previously published data. Despite this, a modulatory role of this residue in CWD incubation time, as suggested by many, is still possible. The analysis of the variability patterns in the PrP gene of the two cervid species suggested the presence of purifying selection. This was also supported by analyses aimed at identifying positively selected sites, which showed that codon 100 was the only site under positive selection throughout mammalian evolution, while the rest of the protein was under strong purifying selection. These data provide further support for the hypothesis suggesting a key cellular role for the PrP protein. The adaptive pressures driving selection at codon 100 are unknown, although they are most likely to be related to PrP function. A role for variation at this position in the interactions of the PrP protein with cell membrane translocation factors is proposed. The study provided an insight into the possible forces shaping PrP genetics and revaluated the role of variation at codon 132 in the wapiti PRNP gene in relation to CWD susceptibility; as well as contributing to our knowledge of the level and pattern of DNA sequence variability in natural populations of mammals.