Submitted to: Journal of Applied Genetics
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/4/2014
Publication Date: 4/3/2014
Publication URL: http://handle.nal.usda.gov/10113/59357
Citation: Kich, J.D., Uthe, J.J., Benavides, M.V., Cantao, M.E., Zanella, R., Tuggle, C.K., Bearson, S.M. 2014. TLR4 single nucleotide polymorphisms (SNPs) associated with Salmonella shedding in pigs. Journal of Applied Genetics. 55(2):267-271. Interpretive Summary: Salmonella has the ability to survive and thrive in a variety of different environments, thereby creating significant challenges for food-producing industries in controlling Salmonella in food chain products. Swine are an important reservoir of Salmonella because colonization and shedding of this human pathogen occurs in asymptomatic pigs, imposing elevated risks to public and animal health. Thus, diverse intervention strategies are needed to control the transmission of Salmonella from pigs to humans and to the environment. One potential control approach is through genetic improvement by identifying and selecting for pigs with decreased Salmonella colonization and shedding. In this study, we used two pig populations with extreme Salmonella shedding phenotypes (low shedding versus persistent shedding) to search for DNA sequence variations in an important activator of the pig’s immune response, Toll-like receptor 4 (TLR4). Genetic variations, called single nucleotide polymorphisms (SNPs), in the TLR4 gene have been associated with different infectious diseases in humans and with infection and disease in cattle, chicken and pigs. We identified eighteen SNPs in the TLR4 gene of the two pig populations, and a specific sequence was found more often in the low Salmonella shedding pigs than the persistent shedding pigs. Our results highlight the importance of linking genetic variations that may influence the function of a key immune regulator (TLR4) with a desirable trait - low Salmonella shedding in swine.
Technical Abstract: Toll-like receptor 4 (TLR4) is a key factor in the innate immune recognition of lipopolysaccharide (LPS) from Gram-negative bacteria. Previous studies from our group identified differences in the expression profile of TLR4 and genes affected by the TLR4 signaling pathway among pigs that shed varying levels of Salmonella, a Gram-negative bacterium. Therefore, genetic variation in this gene may be involved with the host’s immune response to bacterial infections. The current study screened for single nucleotide polymorphisms (SNPs) in the TLR4 gene and tested their association with Salmonella fecal shedding. Pigs (n=117) were intranasally challenged at 7 weeks of age with 1 x 109 CFU of S. Typhimurium chi4232 and were classified as low or persistent Salmonella shedders based on the levels of Salmonella being excreted in fecal material. Salmonella fecal shedding was determined by quantitative bacteriology on days 2, 7, 14, and 20/21 post exposure, and the cumulative levels of Salmonella were calculated to identify the low (n=20) and persistent (n=20) Salmonella shedder pigs. From those 40 animals, the TLR4 region was sequenced, and 18 single nucleotide polymorphisms (SNPs) in TLR4 were identified. Twelve SNPs have been previously described and six are novel SNPs of which five are in the 5’ untranslated region and one is in intron 2. Single marker association test identified 13 SNPs associated with the qualitative trait of Salmonella fecal shedding, and seven of those SNPs were also associated with a quantitative measurement of fecal shedding (P less than 0.05). Using a stepwise regression process, a haplotype composed of SNPs rs80787918 and rs80907449 (P less than or greater than 4.0 x 10-3) spanning a region of 4.9Kb was identified, thereby providing additional information of the influence of those SNPs on Salmonella fecal shedding in pigs.