Significance of valine/leucine²⁴⁷ polymorphism of β₂-glycoprotein I in antiphospholipid syndrome: Increased reactivity of anti-β₂-glycoprotein I autoantibodies to the valine²⁴⁷ β₂-glycoprotein I variant

Objective. To clarify the consequences of the valine/leucine polymorphism at position 247 of the β₂-glycoproiein I (β₂GPI) gene in patients with antiphospholipid syndrome (APS), by investigating the correlation between genotypes and the presence of anti-β₂GPI antibody. The reactivity of anti-β₂GPI antibodies was characterized using recombinant Val²⁴⁷ and Leu²⁴⁷ β₂GPI. Methods. Sixty-five Japanese patients with APS and/or systemic lupus erythematosus who were positive for antiphospholipid antibodies and 61 controls were analyzed for the presence of the Val/Leu²⁴⁷ polymorphism of β₂GPI. Polymorphism assignment was determined by polymerase chain reaction followed by restriction enzyme digestion. Recombinant Val²⁴⁷ and Leu²⁴⁷ β₂GPI were established to compare the reactivity of anti-β₂GPI antibodies to β₂GPI between these variants. The variants were prepared on polyoxygenated plates or cardiolipin-coated plates, and the reactivity of a series of anti-β₂GPI antibodies (immunized anti-human β₂GPI monoclonal antibodies [Cof-19-21] and autoimmune anti-β₂GPI monoclonal antibodies [EY1C8, EY2C9, and TM1G2]) and IgGs purified from patient sera was investigated. Results. A positive correlation between the Val²⁴⁷ allele and the presence of anti-β₂GPI antibodies was observed in the patient group. Human monoclonal/ polyclonal anti-β₂GPI autoantibodies showed higher binding to recombinant Val²⁴⁷ β₂GPI than to Leu ²⁴⁷ β₂GPI, although no difference in the reactivity of the immunized anti-β₂GPI between these variants was observed. Conformational optimization showed that the replacement of Leu²⁴⁷ by Val²⁴⁷ led to a significant alteration in the tertiary structure of domain V and/or the domain IV-V interaction. Conclusion. The Val²⁴⁷ β₂GPI allele was associated with both a high frequency of anti-β₂GPI antibodies and stronger reactivity with anti-β₂GPI antibodies compared with the Leu²⁴⁷ β₂GPI allele, suggesting that the Val²⁴⁷ β₂GPI allele may be one of the genetic risk factors for development of APS.