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 beta 3-integrin is responsible for the formation of the human platelet antigen (HPA)-1. Alloimmunization against HPA-1a (beta 3-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 beta 3-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 beta 3-Leu33 or beta 3-Val33. Interestingly, differences in the reactivity of anti-HPA-1a were observed, with some HPA-1a alloantibodies showing diminished reactivity with beta 3-Val33 compared to beta 3-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 beta 3-Val33. CONCLUSIONS: Our results demonstrate that the naturally occurring Leu33Val mutation in the beta 3-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.