Targeted SNP Testing of Horses Bearing Sarcoid Tumors

Recent research from our group using the equine SNP50 Chip identified two genetic loci associated with sarcoid tumors: the Major Histocompatibility Complex Class II region on horse chromosome 20 and the PROCR locus on chromosome 22. We then constructed Allelic Discrimination Assays (ADA) for these loci.  The assays were validated on small populations of known genotypes and then used to re-test the previously typed samples from 67 sarcoid-bearing horses and 112 controls. The ADAs confirmed the associations detected in our original study. Next we tested DNA isolated from formalin fixed tissues of skin biopsies submitted to the Cornell Veterinary Diagnostic Pathology Service. We tested 249 samples diagnosed as sarcoid and 89 skin samples diagnosed with other conditions. We used PCR and discriminating restriction enzyme digestion on the same samples. We amplified Bovine Papilloma Virus (the putative cause of sarcoid tumors) from 86% of the sarcoid tumors, but only 5% of the control (non-sarcoid) samples. The ADAs for the ECA-22 SNP showed no difference in allele frequency between test and control samples.  The ECA-20 (MHC class II region) SNP was increased in frequency in the sarcoid bearing horses compared to controls, but the result was less significant than in our original study, which used samples from sarcoid bearing horses presented to referral clinics.  We hypothesize that the genetic associations on ECA-20 and ECA-22 that we have identified may be restricted to horses bearing severe sarcoid, but not the mild forms of the tumors seen routinely in equine practice.