Evolution of transmissible spongiform encephalopathy and the prion protein gene (PRNP) in mammals

Abstract

24Wildlife managers are concerned with transmissible spongiform encephalopathies (TSEs) as they 25 are currently incurable, always fatal, and have the potential to cross species boundaries. 26 Although a wide range of mammals exhibit TSEs, it is currently unclear whether they are 27 evolutionarily clustered or if TSE+ species are randomly distributed phylogenetically. We tested 28 whether mammalian species with TSEs are phylogenetically underdispersed on a tree derived 29 from 102 PRNP sequences obtained from the Orthologous Mammalian Markers database. We 30 determined that the PRNP tree was topologically congruent with a species tree for these same 31 102 taxa constructed from 20 aligned gene sequences, excluding the PRNP sequence. Searches 32 in Google Scholar were done to determine whether a species is known to have expressed a TSE. 33TSEs were present in a variety of orders excluding Chiroptera, Eulipotyphyla, and Lagomorpha 34 and no marine mammals (Artiodactyla) were recorded to have a TSE. We calculated the 35 phylogenetic signal of binary traits (D-Value) to infer if the phylogenetic distribution of TSEs 36 are conserved or dispersed. The occurrence of TSEs in both trees is non-random (Species tree 37 D-value = 0.291; PRNP tree D-value = 0.273), and appears to have arisen independently in the 38 recent history of different mammalian groups. Our findings suggest that the evolution of TSEs 39 develops in groups of species irrespective of PRNP genotype. The evolution of TSEs merits 40 continued exploration at a more in-depth phylogenetic level, as well as the search for genetic 41 combinations that might underlie TSE diseases.42 43 44 3 45 Introduction 46 Wildlife managers are concerned with transmissible spongiform encephalopathies (TSEs) 47 as they are currently incurable, always fatal, and have the potential to cross species boundaries. 48 Known TSEs include chronic wasting disease (CWD) in cervids, scrapie in sheep, bovine 49 spongiform encephalopathy (BSE, also known as mad cow disease), transmissible mink 50 encephalopathy (TME), feline spongiform encephalopathy (FSE) and Creutzfeld-Jacob in 51 humans [1-3]. In response to health concerns of livestock and humans, research has focused on 52 learning how species contract TSEs, how they are spread, causes of immunity, and prevention or 53 cures [4]. 54Most researchers accept the hypothesis that resistance to TSEs in mammals results from 55 certain genotypes found at the highly conserved prion protein gene (PRNP) [5] . TSEs are 56 thought to be caused by the misfolding of the host's prion protein (PrP) whose primary 57 physiological function is not entirely clear. When correctly folded the prion protein has been 58 theorized to localize at synaptic membranes and be related to normal synaptic functioning, signal 59 transduction, and copper binding [6][7][8][9]. When misfolded the protein induces other prion proteins 60 to misfold as well, followed by ultimately fatal accumulation in the central nervous system 61 within the host [1, 5, 10]. Misfolded ...