Functional genomics approach to identifying peripheral markers for sheep scrapie
Scrapie is a transmissible spongiform encephalopathy (TSE) of sheep and goats, for which there is currently no ante-mortem diagnostic test. A rapid, ante-mortem diagnostic test for scrapie would also potentially be important for other TSEs such as bovine spongiform encephalopathy (BSE) and variant Creutzfeldt Jakob’s disease (vCJD). The hypothesis of this study was that there is differential gene expression in the blood and peripheral tissues of scrapie infected animals, and that a panel of differentially expressed genes could be identified and used as surrogate markers of infection. An expression screening approach, using real-time PCR and an EST microarray, was used to identify genes that were differentially expressed between SSBP/1 infected and mock-infected control sheep. The animals used in this study were New Zealand Cheviot sheep of three genotypes, the highly susceptible VRQ/VRQ (incubation time 193 ± 12 days), the intermediately susceptible VRQ/ARR (incubation time 325 ± 36 days) and the disease resistant ARR/ARR (no clinical signs of disease), experimentally infected with scrapie strain SSBP/1 and sacrificed at various time points post infection. No differentially expressed candidates were identified in blood. Other microarray experiments in our group had demonstrated evidence of differential expression in spleen fractions enriched for follicular dendritic cells (FDCs). These data were analysed and candidates were selected for quantitative real-time PCR validation, with a view to assessing the expression of validated candidates in blood as a more targeted approach to identifying markers of infection. The gene Early Growth Response 1 (EGR1) emerged as an interesting candidate as its expression was found to be significantly up-regulated in FDC-enriched spleen samples of VRQ/VRQ and ARR/ARR animals over a number of time points post infection. EGR1 expression was steady among all mock-infected controls. There was, however, no evidence of differential expression of EGR1 in blood. This is the first report of differential expression of EGR1 in preclinical spleen samples in sheep. EGR1 is an attractive candidate for a surrogate marker of preclinical infection, as its levels rise very early after infection and remain elevated for a sustained amount of time in the VRQ/VRQ sheep. Elevated expression is also detectable in VRQ/ARR and in ARR/ARR sheep. Further studies with larger sample numbers would be necessary to more accurately estimate the extent of differential expression and to assess its true worth as a diagnostic marker. Expression studies in samples from other TSEs and non-TSE neuropathological disease would also be necessary to establish whether differential expression of EGR1 is specific to TSE disease.