MAX(A), A NEW LOW-FREQUENCY PLATELET-SPECIFIC ANTIGEN LOCALIZED ON GLYCOPROTEIN IIB, IS ASSOCIATED WITH NEONATAL ALLOIMMUNE THROMBOCYTOPENIA

We have identified a new platelet-specific alloantigen, Max(a), respon sible for a typical case of neonatal alloimmune thrombocytopenic purpu ra. The maternal serum reacted strongly with paternal platelets in the platelet immunofluorescence test, whereas platelet alloantigen typing showed that no known human platelet antigen (HPA)-system was involved . In the monoclonal antibody (MoAb)-specific immobilization of platele t antigens (MAIPA) assay, the new antigen was located on the platelet membrane glycoprotein (GP) IIb-IIIa complex, but immunoprecipitation a nd immunoblot experiments to further localize the antigen failed. Howe ver, in the MAIPA assay, the binding of the anti-Max(a) antibodies fro m the maternal serum was blocked by two anti-GPIIb MoAbs. Thus, the an tigen appeared to be located on GPIIb. Analysis of the family lead to the identification of six additional Max(a+) individuals. Three of the se six individuals and the father were tested in the platelet aggregat ion test and were found to be normal. In the analysis of normal donors , three of 500 were typed positive for the new platelet-specific antig en, indicating a phenotype frequency of 0.6% in the normal population. Platelet RNA was isolated from the newborn's Max(a+) father and from a healthy donor phenotyped as Max(a-), reverse-transcribed, and the en tire GPIIb coding region was amplified by polymerase chain reaction. S ubsequent nucleotide sequence analysis showed a single G --> A substit ution at position 2,603, predicting a valine --> methionine amino acid substitution at position 837 of the mature glycoprotein. This mutatio n abolished a BsiYI restriction site at the cDNA level and a BstNI res triction site at genomic DNA level, respectively. The genetic associat ion between the new antigen and this point mutation was confirmed by a llele-specific restriction analysis on cDNA and on genomic DNA, as wel l as by allele-specific primer amplification on genomic DNA. The new m utation is 19 bp upstream of the mutation underlying the HPA-3 system. Therefore, we also evaluated the association between Max and the HPA- 3 polymorphism. So far, all Max(a+) individuals were also found to be HPA-3b, whereas 50 HPA-3a individuals were all Max(a-). This may indic ate that Max(a) is a variant of the HPA-3b allele. (C) 1995 by The Ame rican Society of Hematology.