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How do resistant ovine Prnp genotypes become susceptible genotypes? - SE1445

Description
Scrapie in sheep and goats belongs to the prion disease group, also known as transmissible spongiform encephalopathies (TSEs). The number of TSE affected animals in these livestock species is to a large extent controllable through breeding of specific prion (PrP) protein types (Prnp genotypes). The UK sheep National Scrapie Plan (NSP) during 2001-2009 had the effect of increasing numbers of apparently scrapie and BSE resistant ARR/ARR Prnp sheep. TSEs however exist in different forms (strains) and disease types which appear to attack different Prnp genotypes and also with the potential to change and evolve. This is exemplified by the occurrence of two quite distinct scrapie diseases named classical and atypical and also by the BSE agent which may have originally emerged from scrapie.
ARR/ARR resistance is not complete however and sheep with ARR/ARR genotypes may succumb to experimental BSE and can develop atypical scrapie by natural means. It is not understood what genetic factors are involved to allow this breach of resistance in these sheep, but in cattle and man it has already been shown that regulation of protein expression rather than protein sequence can be associated with susceptibility.
In a previous Defra funded TSE project (SE1432) sheep of various Prnp genotypes were challenged with BSE. Only 40% of ARR/ARR sheep were disease resistant and affected animals succumbed with a very wide incubation period range of 1000-2300 days. These and data from other studies and other genotypes suggests a modulation of disease phenotype independent of the prion protein sequence. The tissues from these experiments are in storage and are therefore both available and ideally suited to investigate novel aspects of genetic susceptibility in small ruminants.
The ARR allele can vary at codons other than 136/154/171 and in this current project more recently discovered Prnp gene alleles and other host genes known or suspected to be involved in TSE control will be tested by extensive sequencing to explain the incomplete TSE disease resistance of ARR/ARR sheep. Results will be verified on additional experimental sheep and goat material to search for new markers for predicting the outcome of infection in animals with a high but incomplete resistance to TSEs.
By improving our understanding of the genetics of susceptibility in sheep and goats it will become possible to breed animals for complete rather than partial resistance to TSEs. The results will also facilitate the optimization of control measures to combat emerging new strains which may be in the process of adapting to the change in genotype frequencies (of PrP and other genes) brought about by the effects of NSP. Stakeholders will be provided with novel resistance markers to be used in genetic selection which will enhance the ability to fight future TSE outbreaks. Having invested considerable time and resources into breeding sheep for specific Prnp genotypes, and thereby changing the genetics of the UK sheep population, it is crucial to understand the effectiveness of these measures over time and the potential of the TSE agent to adapt to the changed host genetics.
Objective
Genetics
(1) to establish the sequence of the regulatory regions of the Prnp (PrP) gene (~ 8kb) for BSE challenged and control animals and generate haplotype maps according to Saunder et al., 2009. (milestones 1-3)
(2) to genotype the Sprn and HSP90-AA1 genes in the BSE challenged and control animals according to Stewart et al, [15] and Marcos-Carcavilla et al, [8] respectively.
(3) to investigate ADAM10 sequence variation in regulatory gene region (promoter).
(4) to analyse all haplotypes and genotypes (single or combined) for association with susceptibility or incubation period length in BSE as well as for indications of links between these genes in healthy animals.
(5) Verification of new genetic marker association (objectives 1-4) in material from sheep and goats challenged with scrapie/BSE from other projects.
Molecular Biology
(6) to determine the transcript expression levels of Prnp, Sprn, HSP90 and ADAM10 in BSE challenged and control brain tissues.
(7) To analyse PrPC and Sho protein processing in TSE affected and healthy controls and in relation to the Prnp, Sprn or HSP90AA1 genotype or ADAM10 expression.
(8) to analyse whether a proposed inverse relationship between the PrPSc and Sho protein levels [16, 17] can be confirmed in these BSE cases.
(9) to analyse the transcript and protein levels of Prnp and Sprn, in a selection of peripheral tissues from ARR/ARR sheep in comparison to ARQ/ARQ sheep.
Time-Scale and Cost
From: 2012

To: 2015

Cost: £135,623
Contractor / Funded Organisations
University of Edinburgh
Keywords
Scrapie              
Fields of Study
Animal Health