Genome-wide Association Study of Entropion Eyelid in Multiple Breeds in Sheep

Entropion is an inversion of the eyelid margin causing lashes or external hairs to rub against the ocular surface. If uncorrected, discomfort, ocular damage, increased eye infection rates, and potential blindness can occur. Entropion affects many mammalian species, can be expressed in both upper and lower eyelids, and may be bilateral. Treatment includes antibiotic injection, physical reformation using Michel clips, and/or surgical eyelid reduction. These treatments increase production costs, raise potential animal welfare concerns, and are incentives to reduce the frequency of this undesirable trait. Entropion has been reported in 1% to 80% of sheep, depending on the breed composition, which supports a genetic basis in sheep. Discovering specific genes associated with entropion could facilitate development of genetic markers to select against entropion, and may improve understanding of developmental pathways. Thus, a genome-wide association scan was performed with 1,000 sheep genotyped using the Illumina OvineSNP50 marker set designed by the International Sheep Genomics Consortium. Entropion status was recorded within 24 hours of birth and overall prevalence was 5.65% in the 3 breeds of sheep (Columbia, Polypay, and Rambouillet) evaluated. The inclusion of substantial numbers of animals from multiple breeds improved the odds for detection of true positive associated genomic regions in multiple genetic backgrounds. Four genomic regions were found to be associated with entropion. Further evaluation of these regions is needed to identify underlying causal mutations, which would be useful as genetic markers for sheep producers.