A third allele of the HPA-5 (Br) platelet alloantigen system identified in investigating a case of neonatal thrombocytopenia

摘要

HPA-5a/b (Brb/a) was identified in the late 1980s as an antigen system capable of causing maternal immunization leading to neonatal alloimmune thrombocytopenia (NAIT). Subsequent studies showed that the HPA-5a/b antigens result from a single-nucleotide polymorphism (G>A1600; "A" of the ATG start codon is here designated Nucleotide 1) in the gene encoding platelet (PLT) glycoprotein (GP) Ia (alpha2 integrin), leading to a Glu (HPA- 5a)>Lys (HPA-5b) amino acid substitution at Residue 505 in GPIa. Further studies showed that maternal-fetal incompatibility for HPA-5 antigens is second only to incompatibility for HPA-1a as a cause of NAIT. HPA-5a and -5b is thought to be a diallelic system with gene frequencies approximately 0.89 and 0.11, respectively. Here, we describe a third allele of HPA-5 identified in a Caucasian individual while studying paternal DNA in a suspected case of NAIT.