Mutation and Selection of Prions

Citation: Weissmann C ( Mutation and Selection of Prions Charles Weissmann 0 1 Prion Diseases 0 1 Heather True-Krob, Washington University School of Medicine, United States of America 0 Funding: This study was supported by grants from the National Institutes of Health (1RO1NSO59543, 1R01NSO67214) and the Alafi Family Foundation. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript 1 Department of Infectology, Scripps Florida , Jupiter, Florida , United States of America Prion diseases, or transmissible spongiform encephalopathies (TSEs), occur naturally in several species, including humans, cattle, sheep, and deer, and can be transmitted experimentally to many others. Typically, incubation times are relatively long, extending to 40 years or more in humans; however, after appearance of clinical symptoms, death mostly ensues within less than a year, as a consequence of neurodegeneration accompanied by accumulation of abnormal conformers of the host protein PrP. Natural transmission usually occurs perorally, as exemplified by the kuru epidemic among the Fore people of Papua New Guinea, attributed to cannibalistic practices; the bovine spongiform encephalopathy (BSE) epizootic in the United Kingdom at the end of last century, caused by feeding of contaminated meat-andbone meal to cattle; or the current epizootic of chronic wasting disease afflicting cervids in 19 states of the United States. Transmission of BSE prions to young humans gave rise to a limited outbreak of a novel illness, variant Creutzfeldt-Jakob disease (vCJD), almost exclusively in the UK. Sporadic cases of prion disease occur at very low frequency in human populations (sCJD) and in cattle herds (atypical BSE), and are attributed to spontaneous generation of prions in the affected individuals. Finally, familial forms of human prion disease are linked to a variety of different, dominant mutations in the PRNP gene, and while afflicted families are rare, penetrance is very high. - Replication of Prions Prions consist mainly, if not solely, of PrPSc (scrapie prion protein), aggregated conformers of the GPI-linked host glycoprotein PrPC (cellular prion protein). PrPSc propagates by converting PrPC to a replica of itself (Figure 1A). PrPC may exist as an equilibrium mixture of conformers, some of which can accrete to PrPSc seeds at a critical rate [1,2]. This seeding model is supported by the protein misfolding cyclic amplification (PMCA) reaction, in which brain homogenate, as a source of PrPC, is spiked with a seed of infected brain homogenate and subjected to multiple cycles of sonication and incubation, ultimately yielding a vast excess of infectious prions [3]. Infectious prions arose spontaneously in PMCA-mediated, cell-free reactions from defined components [4], in particular from recombinant PrP, a phospholipid, and poly(A) or poly(dT) [5], definitively laying to rest the perennial proposal that the infectious agent is a virus-like entity [6]. Prion-like, seeded conversion into an aggregated state has been proposed for several mammalian proteins such as Abeta, a-synuclein, or serum amyloid, which underlie protein misfolding diseases, and for several fungal, in particular yeast, proteins. Prion populations may present as distinct strains: these differ in their phenotypic properties but are associated with PrPSc having the same amino acid sequence. Murine prion strains, originally characterized by the incubation time and the neuropathology they elicit, can be propagated indefinitely in mice homozygous for the PrP gene. Many classical strains currently propagated in mice and hamsters, such as 79A, 22L, and ME7, originated from scrapie-infected sheep or goats [7] and were cloned by endpoint dilution in mice. Strain-specific properties of the prion are believed to be enciphered in the conformation of the cognate PrPSc [8], and indeed, distinct strains are often associated with PrPSc species differing in physicochemical properties. Experiments with yeast prion strains have shown that specific conformations can be propagated in vitro by pure, unglycosylated proteins [9]. Nonetheless, in view of the vast multiplicity of mammalian prion strains and their tropism for particular cell lines, it is conceivable that post translational modifications of PrP, such as glycosylation or association with some cellular components, might favor certain PrP conformations and hence account for cell-specific preferential propagation of particular strains. The Species Barrier In general, there is a considerable barrier to transmission of prions between animal species, in that even massive intracerebral trans-species inoculation causes disease at only low frequency (low attack rate) and/or only after very long incubation times, if at all. This barrier was abolished in some instances by replacing the PrP gene of the recipient by its counterpart from the donor, but clearly factors other than mismatch of PrP sequences contribute to the incompatibility. Importantly, when prions are serially transmitted from the initial trans-species recipients to further animals of the same species, attack rates increase and incubation times decrease, reflecting adaptation to the new host [10]. Adaptation implies as a first step accretion of PrPC from the recipient host to the incoming PrPSc seed, which may be a very inefficient process if the amino acid sequence of the host PrP entrains a spectrum of conformations that are poorly compatible with that of the seed. Efficient propagation may only be enabled when the conformation of the seed changes, perhaps initially at the growing end [11], resulting in a mutation at the conformational level. Subsequently, prions may evolve to replicate more rapidly in the new host, accounting for the striking reduction of their incubation period as they are sequentially transferred within the new species. In some instances, transfer of a prion strain from one species to another, followed by several passages in the original host species, led to emergence of mutant strains. For example, when cloned murine 139A prions were passaged through hamster and subsequently passaged repeatedly in mouse a new strain, 139AH2M, was recovered; however, ME7 subjected to the same procedure remained apparently unchanged [12]. Evolution of Prions The finding that many murine prion strains replicated efficiently in selected murine cell lines created important new experimental opportunities. In particular, the slow, expensive, and imprecise mouse-based bioassay for murine prions could be replaced by a humane, rapid, and precise cell-based procedure, the standard scrapie cell assay (SSCA) [13]. The differential susceptibility of cell lines to various prion strains provided the basis of the cell panel assay (CPA), which rapidly differentiates between various prion strains on the basis of their cell tropism and their susceptibility to various drugs, such as swa (...truncated)