Identification of the molecular and genetic basis of PX2, a glycosphingolipid blood group antigen lacking on globoside-deficient erythrocytes

Abstract

The x 2 glycosphingolipid is expressed on erythrocytes from individuals of all common blood group phenotypes and elevated on cells of the rare P/P1/P k -negative p blood group phenotype. Globoside or P antigen is synthesized by UDP-N-acetylgalactosamine: globotriaosyl-ceramide 3-β-N-acetylgalactosaminyltransferase encoded by B3GALNT1. It is the most abundant non-acid glycosphingolipid on erythrocytes and displays the same terminal disaccharide, GalNAcβ3Gal, as x 2. We encountered a patient with mutations in B3GALNT1 causing the rare P-deficient P 1 k phenotype and whose pretransfusion plasma was unexpectedly incompatible with p erythrocytes. The same phenomenon was also noted in seven other unrelated P-deficient individuals. Thin-layer chromatography, mass spectrometry, and flow cytometry were used to show that the naturally occurring antibodies made by p individuals recognize x 2 and sialylated forms of x 2, whereas x 2 is lacking on P-deficient erythrocytes. Overexpression of B3GALNT1 resulted in synthesis of both P and x 2. Knockdown experiments with siRNA against B3GALNT1 diminished x 2 levels. We conclude that x 2 fulfills blood group criteria and is synthesized by UDP-N-acetylgalactosamine: globotriaosylceramide 3-β-N-acetylgalactosaminyltransferase. Based on this linkage, we proposed that x 2 joins P in the GLOB blood group system (ISBT 028) and is renamed PX2 (GLOB2). Thus, in the absence of a functional P synthase, neither P nor PX2 are formed. As a consequence, naturally occurring anti-P and anti-PX2 can be made. Until the clinical significance of anti-PX2 is known, we also recommend that rare P 1 k or P 2 k erythrocyte units are preferentially selected for transfusion to P k patients because p erythrocytes may pose a risk for hemolytic transfusion reactions due to their elevated PX2 levels.