The interpretation of disease phenotypes to identify TSE strains following murine bioassay: characterisation of classical scrapie

Abstract

Mouse bioassay can be readily employed for strain typing of naturally occurring transmissible spongiform encephalopathy cases. Classical scrapie strains have been characterised historically based on the established methodology of assessing incubation period of disease and the distribution of disease-specific vacuolation across the brain following strain stabilisation in a given mouse line. More recent research has shown that additional methods could be used to characterise strains and thereby expand the definition of strain “phenotype”. Here we present the phenotypic characteristics of classical scrapie strains isolated from 24 UK ovine field cases through the wild-type mouse bioassay. PrPSc immunohistochemistry (IHC), paraffin embedded tissue blots (PET-blot) and Western blotting approaches were used to determine the neuroanatomical distribution and molecular profile of PrPSc associated with each strain, in conjunction with traditional methodologies. Results revealed three strains isolated through each mouse line, including a previously unidentified strain. Moreover IHC and PET-blot methodologies were effective in characterising the strain-associated types and neuroanatomical locations of PrPSc. The use of Western blotting as a parameter to define classical scrapie strains was limited. These data provide a comprehensive description of classical scrapie strain phenotypes on isolation through the mouse bioassay that can provide a reference for further scrapie strain identification.

Pathology of Animal Transmissible Spongiform Encephalopathies (TSEs)

“…By definition, the phenotype is host specific and includes a very large number of parameters which may evolve over time as new techniques are applied. For practical purposes in TSEs the parameters that are currently considered to define phenotype are incubation period, clinical signs, and quantitative aspects of vacuolation which give rise to lesion profiles, IHC deposition patterns and Western blot characteristics [31][32][33] . For the purposes of this review different phenotypes in the original host are considered as different TSE strains ie BSE, H-type BSE and L-type BSE in cattle, and classical and atypical scrapie in sheep.…”

“…However, after experimental transmission of a TSE to wild type or transgenic rodents a single or multiple phenotypes may emerge in a single mouse line, leading to the laboratory definition of a strain. According to this definition, classical scrapie includes several strains as various distinct phenotypes can be identified in mice [33][34][35] . In contrast every time a source of atypical scrapie has been bioassayed in mice a single phenotype has been detected suggesting that atypical scrapie is attributed to a single strain 36,37) .…”

“…In contrast every time a source of atypical scrapie has been bioassayed in mice a single phenotype has been detected suggesting that atypical scrapie is attributed to a single strain 36,37) . It must be emphasized that, particularly in rodents, the phenotypes are not simply host specific but mouse line specific 30,33,38) . This means that phenotypes between different mouse lines are not directly comparable even within wild type mice due to polymorphisms in the murine PrP and other genetic factors 34,[39][40][41] .…”

Scientific Opinion on a request for a review of a scientific publication concerning the zoonotic potential of ovine scrapie prions

“…Based on strain typing studies in mice it is well documented that more than one TSE strain is responsible for Classical scrapie in sheep Bruce, 2003;Thackray et al, 2011Thackray et al, , 2012Beck et al, 2012a;Cassard et al, 2014). However, the exact number of Classical scrapie strains is unknown despite characterisation of several scrapie isolates in both wild type and transgenic mouse models.…”

“…Therefore they retain little resemblance to the properties of the agent in the sheep, and cannot be considered to comprehensively represent the field situation. Since the BSE epidemic, further characterisation of naturally occurring Classical scrapie cases that were detected after 1985 was initiated to identify strain variation in natural Classical scrapie using both wild type and transgenic mouse models (Bruce, 2002;Bruce et al, 2002;Beck et al, 2012a;Thackray et al, 2011Thackray et al, , 2012Cassard et al, 2014). Remarkably, the studies conducted in wild type mice after 1985 revealed novel strains in addition to the 20 that were known historically, suggesting that there may be evolution of Classical scrapie strains (Bruce et al, 2002;Beck et al, 2012a).…”