DR(A-) POLYMORPHISM OF DECAY ACCELERATING FACTOR - BIOCHEMICAL, FUNCTIONAL, AND MOLECULAR CHARACTERIZATION AND PRODUCTION OF ALLELE-SPECIFIC TRANSFECTANTS

The Dr(a) antigen belongs to the Cromer-related blood group system, a series of antigens on decay accelerating factor (DAF), a glycosyl-phosphatidylinositol-anchored membrane protein that protects host cells from complement-mediated damage. We studied the rare inherited Dr(a-) phenotype to ascertain the associated biochemical and functional changes in DAF and to characterize the basis for this polymorphism. Radioimmunoassay and flow cytometric analysis of Dr(a-) erythrocytes demonstrated 40% of normal surface expression of DAF but normal levels of several other glycosyl-phosphatidylinositol-anchored proteins, distinguishing this phenotype from that of paroxysmal nocturnal hemoglobinuria. Western blots confirmed this reduced DAF expression and indicated a slightly faster mobility of the molecule on SDS-PAGE. Despite the reduced DAF expression, Dr(a-) erythrocytes functioned normally in the complement lysis sensitivity assay. Utilization of the polymerase chain reaction to amplify mononuclear cell genomic DNA from three unrelated Dr(a-) individuals demonstrated that a point mutation underlies the Dr(a-) phenotype: a C to T change in nucleotide 649 resulting in a serine165 to leucine change. This defines the Dr(b) allele of DAF, which can be distinguished from Dr(a) by a Taq I restriction fragment length polymorphism. We created transfected Chinese hamster ovary cell lines expressing either the Dr(a) or the Dr(b) allelic form of DAF. These allele-specific transfectants were tested by inhibition of hemagglutination or flow cytometry and confirmed the specificity of anti-Dr(a) alloantisera. The allele-specific transfectants could form the basis of a new serological approach to immunohematology.