Single amino acid substitution in human platelet glycoprotein Ibβ 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 ly(a), which was implicated in a severe case of neonatal alloimmune thrombocytopenia. Immunoprecipitation studies with trypsin-treated platelets indicated that the ly(a) alloantigenic determinants are formed by the membrane-associated remnant moiety of GP Ibα (GP Ibα(r)) together with GP Ibβ and GP IX. To elucidate the molecular basis underlying the ly(a) alloantigen, we amplified GPIbαr, GPIbβ, and GPIX genes by polymerase chain reaction (PCR). Nucleotide-sequence analysis of these 3 genes showed a G to A transition at position 141 on GPIbβ gene in a subject positive for ly(a). This transition resulted in a Gly₁₅Glu dimorphism on the N-terminal domain of GPIbβ. This finding was confirmed by genotyping analysis of 6 ly(a)-positive subjects by restriction fragment length polymorphism (RFLP) studies using Narl endonuclease. In 300 randomly selected healthy blood donors, one ly(a)- positive individual was found. Phenotypes determined by monoclonal antibody- specific immobilization of platelet antigens assay and genotypes determined by RFLP were identical in this population. Analysis of ly(a)-positive platelets showed that the point mutation affected neither the degree of surface expression nor the function of the GP Ibα-GP Ibβ-IX complex on the platelet surface. Transient expression of the GP Ib-IX complex in CHO cells using wild-type GP Ibβ (Gly₁₅) or mutant GP Ibβ (Glu₁₅) allowed us to demonstrate that this single amino acid substitution is sufficient to induce ly(a) epitope(s). (C) 2000 by The American Society of Hematology.