Single amino acid substitution in human platelet glycoprotein Ib beta is responsible for the formation of the platelet-specific alloantigen Iy(a)

We recently described a new low-frequency platelet alloantigen on the human platelet glycoprotein (GP) Ib-IX complex, termed Iy(a), Which was implicat ed in a severe case of neonatal alloimmune thrombocytopenia, Immunoprecipit ation studies with trypsin-treated platelets indicated that the Iy(a) alloa ntigenic determinants are formed by the membrane-associated remnant moiety of GP Ib alpha (GP Ib alpha(r)) together with GP Ib beta and GP IX. To eluc idate the molecular basis underlying the Iy(a) alloantigen, we amplified GP Ib alpha(r) , GPIb beta, and GPIX genes by polymerase chain reaction (PCR), Nucleotide-sequence analysis of these 3 genes showed a G to A transition a t position 141 on GPIb beta gene in a subject positive for Iy(a). This tran sition resulted in a Gly(15)Glu dimorphism on the N-terminal domain of GPIb beta, This finding was confirmed by genotyping analysis of 6 ly(a)-positiv e subjects by restriction fragment length polymorphism (RFLP) studies using NarI endonuclease. In 300 randomly selected healthy blood donors, one Iy(a )-positive individual was found. Phenotypes determined by monoclonal antibo dy-specific immobilization of platelet antigens assay and genotypes determi ned by RFLP were identical in this population. Analysis of Iy(a)-positive p latelets showed that the point mutation affected neither the degree of surf ace expression nor the function of the GP Ib alpha-GP Ib beta-IX complex on the platelet surface. Transient expression of the GP Ib-IX complex in CHO cells using wild-type GP Ib beta (Gly(15)) Or mutant GP Ib beta (Glu(15)) a llowed us to demonstrate that this single amino acid substitution is suffic ient to induce Iy(a)epitope(s). (C) 2000 by The American Society of Hematol ogy.