A point mutation in the EGF-4 domain of β(3) integrin is responsible for the formation of the Sec(a) platelet alloantigen and affects receptor function.

Neonatal alloimmune thrombocytopenia (WAIT) 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 WAIT cases. We observed a suspected WAIT in a newborn with a platelet count of 25 G/I and petechial haemorrhages. Serological analysis of maternal serum revealed an immunisation against alpha IIb beta 3 on paternal platelets only, indicating the presence of an antibody against a new rare alloantigen (Sec(a)) residing on alpha IIb beta 3. The location of Sec(a) on alpha IIb beta 3 was confirmed by immunoprecipitation. Nucleotide sequence analysis of paternal beta 3 revealed a single nucleotide exchange (G(1818)T) in exon 11 of the beta 3 gene (ITGB3), changing Lys(580) (wild-type) to Asn(580) (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 Asn(580), but not Lys(580) alpha IIb beta 3, bound anti-Sec(a), which was corroborated by immunoprecipitation. Adhesion of transfected cells onto immobilised fibrinogen showed reduced binding of the Asn(580) variant compared to wild-type alpha IIb beta 3. 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 Lys(580)Asn mutation. In this study, we describe a NAIT case caused by maternal alloimmunisation against a new antigen on alpha IIb beta 3. Analysis with mutant transfected cells showed that the Lys580Asn mutation responsible for the formation of the Sec(a) antigenic determinant affects alpha IIb beta 3 receptor function.