A V740L mutation in glycoprotein IIb defines a novel epitope (War) associated with fetomaternal alloimmune thrombocytopenia

Background Most recently described human platelet antigens ( HPAs ) have been detected in cases of fetomaternal alloimmune thrombocytopenia ( FMAIT ) where the mother has been immunized against a low‐frequency antigen that the fetus has inherited from the father. Low‐frequency antigens are not represented in normal panel platelets ( PLTs ) and antibody detection and identification in such cases requires incubation of maternal serum with paternal PLTs and definition of the causative mutation. Study Design and Methods A suspected case of FMAIT was investigated for PLT ‐specific antibodies using a panel of both HPA ‐typed and paternal PLTs . HPA typing was performed by polymerase chain reaction with sequence‐specific primers and further DNA analysis was performed using direct sequencing of the coding regions of the ITGA 2 B and ITGB 3 genes. Results Maternal antibodies reactive only with paternal PLTs were localized to glycoprotein ( GP ) IIb / IIIa using the monoclonal antibody immobilization of PLT antibody assay. A single‐nucleotide polymorphism was detected in E xon 23 of ITGA 2 B in the father and affected child, which predicted a V 740 L substitution in the mature protein. Recombinant V 740 L mutated GPIIb expressed in HEK 293 cells was specifically recognized by maternal antibodies. The polymorphism was not detected either in the mother or in a cohort of 100 donors. Conclusion The V 740 L polymorphism defines a new low‐frequency antigen implicated in two cases of FMAIT in a single family. Low‐frequency HPAs are clinically important and their elucidation requires both crossmatch studies and gene sequencing in cases where there is strong clinical evidence of FMAIT but initial laboratory investigations do not support the diagnosis.