Evidence That <i>GYPA</i> (Glycophorin A) Encodes the K Blood Group System in Horses

Although serological and genetic studies of equine blood group systems have been conducted for many years, the molecular basis of erythrocyte antigens' variability has remained largely unexplored. In this study, we aimed to elucidate the genetic basis of serological variation within equine blood group K. Using mRNA extracted from peripheral blood samples (n = 100) collected from horses with known serological blood types (Ka or K-), we performed a transcriptome-wide association study (TWAS), which revealed a significantly associated region on equine chromosome 2 (ECA2). A detailed analysis of this region identified GYPA (glycophorin A) as the most promising candidate gene. Resequencing its entire coding sequence revealed the presence of a dinucleotide missense variant in exon 3 (ENSECAT00000026370.3:c.145_146delinsAT; p.Asp49Ile), which is predicted to potentially alter the function of the GYPA protein. Genotyping this variant in a large, breed-diverse cohort, which included family-based samples, confirmed perfect cosegregation between the identified GYPA missense substitution and serological K blood group typing results. Our findings demonstrate that blood transcriptome-based approaches, despite certain limitations, can effectively reveal the molecular basis of equine erythrocyte antigen variability.