Genetic Markers to Identify the Resilient and Susceptible Phenotypes to Johne’s Disease in Red Deer (Cervus elaphus)

Johne’s disease, a chronic granulomatous enteritis caused by infection of the pathogen Mycobacterium avium subspecies paratuberculosis, can be a major cause of production loses in farmed ruminants. At present, there are no viable options for treatment or prevention of the disease. With prevalence of Johne’s disease suspected to be on the rise, a method of control is required. Red deer stags with resilient and susceptible phenotypes to Johne’s disease have been identified previously by the DRL, and these phenotypes appear to have a high paternal heritability. The present study is a contribution towards the identification of biomarkers for either phenotype to produce a diagnostic test to proactively identify whether a naïve animal is likely to be resilient or susceptible to the disease. This diagnostic assay would be useful for selection of resilient animals for breeding, producing a genetically resilient herd to reduce Johne’s disease associated production losses. As susceptibility to Johne’s disease was thought to be due to a dysregulation of the innate immune system, this project investigated differential expression of genetic markers from macrophages of resilient and susceptible animals. Monocyte-derived macrophages were cultured from cervine blood and stimulated with viable Mycobacterium paratuberculosis. Quantitative-PCR was used to analyse changes in expression of target genes. Of the investigated gene targets, ISG15 and ISG20 showed particular promise, where animals of the resilient phenotype upregulated expression to a greater degree than animals of the susceptible phenotype. Moreover, IL1A, IL12A, MAPK8, NOS2, PKLR, and STAT3 showed sufficient differential expression between the phenotypes to suggest their potential for further investigation. This study also investigated the feasibility of PBMC culture as a platform for a diagnostic assay. This assay presented favorably in terms of volume of blood required and culture duration compared to the MDM assay. However, no consistent differential gene expression was observed from the investigated genes, and more work is required to identify functional genetic markers. Taken together, these results demonstrate the potential for using differential gene expression for phenotypic diagnosis, where further investigations should be carried out to confirm robust and dependable genetic markers of resilience and susceptibility