Identification of a novel A4GALT exon reveals the genetic basis of the P1/P2 histo-blood groups

Abstract

AbstractThe A4GALT locus encodes a glycosyltransferase that synthesizes the terminal Galα1-4Gal of the Pk (Gb3/CD77) glycosphingolipid, important in transfusion medicine, obstetrics, and pathogen susceptibility. Critical nucleotide changes in A4GALT not only abolish Pk formation but also another Galα1-4Gal–defined antigen, P1, which belongs to the only blood group system for which the responsible locus remains undefined. Since known A4GALT polymorphisms do not explain the P1−Pk+ phenotype, P2, we set out to elucidate the genetic basis of P1/P2. Despite marked differences (P1 > P2) in A4GALT transcript levels in blood, luciferase experiments showed no difference between P1/P2-related promoter sequences. Investigation of A4GALT mRNA in cultured human bone marrow cells revealed novel transcripts containing only the noncoding exon 1 and a sequence (here termed exon 2a) from intron 1. These 5′-capped transcripts include poly-A tails and 3 polymorphic sites, one of which was P1/P2-specific among > 200 donors and opens a short reading frame in P2 alleles. We exploited these data to devise the first genotyping assays to predict P1 status. P1/P2 genotypes correlated with both transcript levels and P1/Pk expression on red cells. Thus, P1 zygosity partially explains the well-known interindividual variation in P1 strength. Future investigations need to focus on regulatory mechanisms underlying P1 synthesis.