Polymelia in Holstein Cattle
Polymelia is a congenital condition where an animal has more than the normal number of limbs. Previous reports have suggested that polymelia is due to abnormal chromosomal breaks or alternatively due to a mutation found to segregate in Angus cattle. A male Holstein calf was presented to the WSU Veterinary hospital with scoliosis, a deviated tail, and two additional front legs originating from each scapula. The objective of this study was to determine if polymelia in this Holstein calf was due to the mutation identified in Angus cattle, a gross chromosomal abnormality or potentially another genetic cause. A 10 ml blood sample was taken via the jugular vein for genetic analyses. Five ml of blood was used to prepare a karyotype of the calf to identify chromosomal abnormalities. From the remaining 5 ml of blood, DNA was extracted for genotyping. Genotyping of this Holstein calf for the specific mutation present in Angus cattle was performed using a PCR-RFLP technique. Genotypes were also obtained from the Holstein calf using the Illumina bovine HD BeadChip. A genome wide association study (GWAS) was conducted with the polymelia calf compared with 2800 control Holstein calf samples. The statistical approach used for the GWAS was EMMAX (Efficient Mixed-Model Association eXpedited) within the SNP & Variation Suite 7 software package (Golden Helix, Bozeman, MT). GWAS data underwent quality control filtering for minor allele frequency (<1%), SNP call rate (<95%), and animal call rate (removal of animals with less than 95% of SNPs called). Population stratification was tested for (lambdaGC) prior to GWAS analysis. Loci associated with polymelia were found to be associated when P<1x10-50. The karyotype results showed no evidence of increased levels of chromosomal breaks in the Holstein calf. The PCR-RFLP genotype of the Holstein calf was consistent with a homozygous normal Angus animal. Furthermore, sequencing of the entire coding sequence of the gene mutated in Angus cattle, revealed no additional polymorphisms that might cause the polymelia phenotype. No population stratification was identified (lambdaGC=1.04) in the Holsteins genotyped by the Bovine HD SNP assay. The GWAS association analysis identified three loci associated with polymelia: one locus on BTA13 (P<1x10-281), BTA10 (P<2x10-110) and BTA20 (P<2x10-52). These preliminary results suggest that polymelia may be due to more than one locus and mutations that may cause polymelia are not shared across all breeds.
Keywords: polymelia genetics Holstein