A new Ser472Asn (Cab2a+) polymorphism localized within the αIIb “thigh” domain is involved in neonatal thrombocytopenia

Abstract

A new platelet antigen, Cab2(a+), was identified in a case of severe neonatal alloimmune thrombocytopenia (<8 × 10(9)/L) in twins. Coding sequences of αIIb and β3 genes from parents were amplified and sequenced. CHO cell lines expressing wild-type or mutated forms of the complex were established to study the role of the mutation in alloimmunization and in αIIbβ3 functions. The father and twins were heterozygous for a single αIIb c.1508G>A mutation leading to a Ser472Asn substitution. Immunologic assays with transfected CHO cells revealed the Asn472 form of αIIbβ3 responsible for the Cab2(a+) epitope but not an Ala472 form. Using these cells lines we demonstrated that both Ser472Asn and Ser472Ala substitutions produced limited structural alteration as revealed by the reactivity of a panel of anti-αIIbβ3 monoclonal antibodies (MoAbs). Activated Asn472 and Ala472 forms of αIIbβ3 supported 1) binding of soluble fibrinogen and of the ligand mimetic MoAb PAC-1, 2) ligand-induced binding site epitopes exposure (MoAbs AP-5 and D3GP3), and 3) cell aggregation. Adhesion onto adsorbed fibrinogen was conserved and was specifically inhibited by MoAb AP-2 or peptide RGDS. Finally outside-in signaling was not affected. We have characterized a new low-frequency alloantigen (<1%) resulting from the Ser472Asn substitution in αIIb and shown this polymorphism to have a limited effect, if any, on the αIIbβ3 complex functions.