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

The A4GALT locus encodes a glycosyltransferase that synthesizes the terminal Galα1-4Gal of the P(k) (Gb3/CD77) glycosphingolipid, important in transfusion medicine, obstetrics, and pathogen susceptibility. Critical nucleotide changes in A4GALT not only abolish P(k) 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-P(k)+ phenotype, P(2), we set out to elucidate the genetic basis of P(1)/P(2). Despite marked differences (P(1) > P(2)) in A4GALT transcript levels in blood, luciferase experiments showed no difference between P(1)/P(2)-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 P(1)/P(2)-specific among > 200 donors and opens a short reading frame in P(2) alleles. We exploited these data to devise the first genotyping assays to predict P1 status. P(1)/P(2) genotypes correlated with both transcript levels and P1/P(k) expression on red cells. Thus, P(1) zygosity partially explains the well-known interindividual variation in P1 strength. Future investigations need to focus on regulatory mechanisms underlying P1 synthesis.