A functional platelet fibrinogen receptor with a deletion in the cysteine-rich repeat region of the β3 integrin: the Oea alloantigen in neonatal alloimmune thrombocytopenia

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Abstract
This report describes a new low-frequency alloantigen, Oea, responsible for a case of neonatal alloimmune thrombocytopenia (NAIT). In a population study none of 600 unrelated blood donors was an Oea carrier. By immunochemical studies the Oea antigen could be assigned to platelet glycoprotein (GP) IIIa. Sequencing of GPIIIa complementary DNA from an Oea (+) individual showed deletion of a lysine residue at position 611 (ΔLys611). Analysis of 20 Oea(−) and 3 Oea (+) individuals showed that the ΔLys611 form of GPIIIa was related to the phenotype. Anti-Oea reacted with the ΔLys611, but not with the wild-type isoforms on stable transfectants expressing GPIIIa, indicating that ΔLys611 directly induces the expression of Oea epitopes. Under nonreducing conditions the Pro33ΔLys611 variant migrated with a slightly decreased molecular weight compared to the Pro33Lys611 isoform suggesting that ΔLys611 has an influence on the disulfide bonds of GPIIIa. The Pro33ΔLys611 GPIIIa could undergo conformational changes and bind to fibrinogen in a similar manner as the Pro33Lys611 isoform. No difference was found in the tyrosine phosphorylation of pp125FAK, suggesting that ΔLys611 has no effect on integrin function. In contrast to all other low-frequency antigens, the ΔLys611 isoform was associated with the HPA-1b, but not with the high frequency HPA-1a allele. Comparison with GPIIIa DNA from nonhuman primates indicated that the HPA-1a allele represents the ancestral form of GPIIIa. It can be assumed that the Oea form did arise as a result of a mutational event from an already mutated GPIIIa allele.