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Abstract

Objective

To clarify the consequences of the valine/leucine polymorphism at position 247 of the β2-glycoprotein 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 Leu247 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.