Our research group aims to find DNA variations influencing resistance/susceptibility to paratuberculosis infection or Johne's disease in Friesian cattle. The focus is on genes related to innate immune system. Moreover, two years ago, we reported a genetic association between a microsatellite marker in 3'UTR region of bovine Nramp1 gene and the resistance to Johne's disease (8th International Colloquium on Paratuberculosis, Copenhagen, 2005).

In the present study the analysis is extended to a second gene, Card15, previously associated with Crohn disease, and new polymorphisms in Nramp1 gene are studied. A total of 69 SNPs distributed in those genes (17 from Card15 and 52 from Nramp1) are analysed: (1) 28 out of those 69 SNPs were newly discovered by comparative sequencing in 14 bovine breeds; (2) the distribution of 18 out of those 69 SNPs that showed variation are described in our Friesian cattle population. The MAF (minor allele frequency) was higher than 0.01 in all cases and the whole panel of SNPs was in Hardy-Weinberg equilibrium.

To evaluate the potential role of both candidate genes in determining the outcome of MAP infection, a case-control association study was performed in MAP naturally infected herds. Comparisons between cases (137 ELISA positive cattle) and controls (136 ELISA negative cattle) were performed for the 18 SNPs mentioned above. Three SNPs showed significant difference between the two groups: two ncSNP in Card15 gene (C12 p<0.01 and C25 p<0.002) and an intronic SNP in Nramp1 gene (N23 p<0.05).

These results suggest that bovine Card15 and Nramp1 genes might be associated with natural resistance to MAP infection in cattle, or at least, the results might reflect linkage disequilibrium with another resistance conferring polymorphism nearby in the chromosome. Further studies are needed to clarify this point.