A naturally occurring Leu33Val mutation in β3-integrin impairs the HPA-1a epitope: the third allele of HPA-1

BACKGROUND: Single‐amino‐acid substitution Leu33Pro in the β3‐integrin is responsible for the formation of the human platelet antigen (HPA)‐1. Alloimmunization against HPA‐1a (β3‐Leu33) is the most frequent cause of neonatal alloimmune thrombocytopenia and posttransfusion purpura. STUDY DESIGN AND METHODS: While HPA‐1 genotyping a large cohort of patients with thromboembolic disease with a thermal cycler (LightCycler), one patient was identified with a unique HPA‐1a melting curve. RESULTS: Sequence analysis revealed a C‐to‐G transversion at nucleotide 175 in the β3‐integrin (ITGB3) gene that alters the Leu33 codon to Val33. Further genotyping of healthy blood donors (n = 2950) identified one nonrelated Pro33Val33‐positive individual. To examine whether the presence of Val33 affected the binding pattern of HPA‐1 alloantibodies, transfectants were generated expressing recombinant β3‐Leu33 or β3‐Val33. Interestingly, differences in the reactivity of anti‐HPA‐1a were observed, with some HPA‐1a alloantibodies showing diminished reactivity with β3‐Val33 compared to β3‐Leu33 and others reacting equally with both types. Similar findings were observed with recombinant human HPA‐1a antibodies, with one of the three not binding to β3‐Val33. CONCLUSIONS: Our results demonstrate that the naturally occurring Leu33Val mutation in the β3‐integrin can disrupt some HPA‐1a epitopes. These findings provide evidence for a heterogeneous humoral response against HPA‐1a that may have potential clinical implications for alloimmune thrombocytopenia disorders.