THE HUMAN PLATELET ALLOANTIGENS BR(A) AND BR(B) ARE ASSOCIATED WITH A SINGLE AMINO-ACID POLYMORPHISM ON GLYCOPROTEIN-IA (INTEGRIN SUBUNIT ALPHA-2)

The human GPIa/IIa complex, also known as integrin alpha2beta1, serves as a major receptor for collagen in platelets and other cell types. In addition to its role in platelet adhesion to extracellular matrix, GPIa/IIa is also known to bear the clinically important Br(a) and Br(b) alloantigenic determinants, which can result in antibody-mediated platelet destruction. Immunochemical studies showed that the Br antigenic epitopes reside solely on the GP Ia subunit and do not depend on sialic acid residues. To define the polymorphism responsible for the Br alloantigen system platelet RNA PCR technique, was used to amplify GPIa mRNA transcripts. Nucleotide sequence analysis of the amplified platelet GPIa cDNA from Br(a/a) and Br(b/b) individuals revealed a single A <-- --> G polymorphism at base 1648. Mnl I RFLP analysis of cDNA from serologically determined individuals confirmed that this polymorphism segregates with Br phenotype. This single base change results in a substitution of Lys (AAG) in Br(a) to Glu (GAG) in Br(b) at amino acid residue 505. In spite of the reversal in charge at this position, however, we found no difference in the ability of Br(a) and Br(b) homozygous platelets to adhere to collagens types I, III, or V, nor did anti-Br(a) or anti-Br(b) alloantibodies interfere with platelet adhesion to any of these fibrillar collagens. The identification of the nucleotide substitution that defines the Br(a)/Br(b) alloantigen system will now permit both pre- and postnatal diagnosis for Br phenotype.