A point mutation in the EGF-4 domain of β₃ integrin is responsible for the formation of the Sec^a platelet alloantigen and affects receptor function

Summary

Neonatal alloimmune thrombocytopenia (NAIT) is caused by fetomaternal platelet incompatibility with maternal antibodies crossing the placenta and destroying fetal platelets. Antibodies against human platelet antigen-1a (HPA-1a) and HPA-5b are responsible for the majority of NAIT cases. We observed a suspected NAIT in a newborn with a platelet count of 25 G/l and petechial haemorrhages. Serological analysis of maternal serum revealed an immunisation against αIIbβ3 on paternal platelets only, indicating the presence of an antibody against a new rare alloantigen (Sec^a) residing on αIIbβ3. The location of Sec^a on αIIbβ3 was confirmed by immunoprecipitation. Nucleotide sequence analysis of paternal β3 revealed a single nucleotide exchange (G₁₈₁₈T) in exon 11 of the β3 gene (ITGB3), changing Lys₅₈₀ (wild-type) to Asn₅₈₀ (Sec^a). Two additional members of the family Sec were typed Sec^a positive, but none of 300 blood donors. Chinese hamster ovary cells expressing Asn₅₈₀, but not Lys₅₈₀ αIIbβ3, bound anti-Sec^a, which was corroborated by immunoprecipitation. Adhesion of transfected cells onto immobilised fibrinogen showed reduced binding of the Asn₅₈₀ variant compared to wild-type αIIbβ3. Analysis of transfected cells with anti-LIBS and PAC-1 antibody showed reduced binding when compared to the wild-type. No such effects were observed with Sec^a positive platelets, which, however, are heterozygous for the Lys₅₈₀Asn mutation. In this study, we describe a NAIT case caused by maternal alloimmunisation against a new antigen on αIIbβ3. Analysis with mutant transfected cells showed that the Lys₅₈₀Asn mutation responsible for the formation of the Sec^a antigenic determinant affects αIIbβ3 receptor function.

Keywords

NAIT - HPA - thrombocytopenia - GP IIb/IIIa

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