Masters Theses

Date of Award

5-2004

Degree Type

Thesis

Degree Name

Master of Science

Major

Animal Science

Major Professor

Gina M. Pighetti

Committee Members

Arnold M. Saxton, Stephen P. Oliver

Abstract

Mastitis is the most economically devastating disease affecting the dairy industry. A genetic marker associated with inflammatory responses during mastitis could aid in selection for mastitis resistant dairy cattle. Objectives of this experiment were to identify single nucleotide polymorphisms (SNPs) and resulting haplotypes in the bovine CXCR2 gene for two breeds of dairy cattle, determine SNP and haplotype frequencies, and identify genotype associations with subclinical and clinical mastitis. Genomic DNA was isolated from whole blood collected from Jersey and Holstein cows. A 311 bp segment located within the coding region of exon 3 of the CXCR2 gene was amplified and sequenced. Five SNPs were expressed in both breeds of cattle. Strong linkage disequilibrium was exhibited for both breeds among all five SNPs (P<0.001). Four SNPs resulted in synonymous nucleotide changes, while one nonsynonymous nucleotide substitution (CXCR2+777 G→C) resulted in a GlnÆHis substitution at amino acid residue 245. Breed was shown to have a significant effect on frequencies for four of the five SNPs (P<0.001). Ten haplotypes were observed from the five polymorphisms. Six haplotypes were common between the two breeds, while Holsteins and Jerseys each uniquely expressed two haplotypes.

We then examined the association of CXCR2 SNPs and haplotype genotypes with subclinical and clinical mastitis. Subclinical mastitis was defined as the presence of the same pathogen in at least two out of three consecutive samples obtained during lactation. Clinical mastitis was defined as the presence of abnormal milk, and/or an abnormal udder, and/or systemic signs of intramammary infection that warranted treatment with intramammary therapies and/or use of anti-inflammatory agents. A significant difference was detected between CXCR2 SNP +777 genotypes and percentages of subclinical mastitis cases in Holstein cows. Holsteins with CXCR2 SNP +777 genotype GG had decreased percentages of subclinical mastitis (~22%), while genotype CC cows had increased percentages of subclinical mastitis (~37%). In summary, results from these experiments demonstrated that the CXCR2 gene is highly polymorphic in Jersey and Holstein cattle. CXCR2 SNP +777 genotypes were associated with subclinical mastitis in Holstein cows, indicating that the CXCR2 amino acid 245 may be involved in cellular signaling and function during mastitis. However, with observed levels of linkage disequilibrium, other genes near the CXCR2 locus may also be involved with cellular functions during mastitis. This research is promising for dairy producers as this approach may represent an effective means of marker assisted selection for mastitis resistance and other inflammatory diseases involving neutrophils.

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