The prevalence of a non-phospholipid-binding form of beta2-glycoprotein I in human plasma--consequences for the development of anti-beta2-glycoprotein I antibodies.

The presence of antiphospholipid antibodies (aPL) is strongly correlated with venous and arterial thrombosis, fetal loss and thrombocytopenia. This relation is called the antiphospholipid syndrome (APS). It is well recognized that thrombosis related aPL are not directed against phospholipids alone, but to phospholipid bound plasma proteins like beta2-glycoprotein I (beta2GPI). aPL that need beta2GPI for the binding to negatively charged phospholipids are called anti-beta2GPI-antibodies. Recently, a mutation in the gene encoding beta2GPI has been described, which results in an amino acid substitution Trp316 into Ser316. This Ser316-beta2GPI did not bind to negatively charged phospholipids. Because only phospholipid bound beta2GPI is recognized by human anti-beta2GPI-antibodies, it might be argued that individuals carrying the Trp316Ser mutation are protected against the development of anti-beta2GPI-antibodies. To investigate this hypothesis, the prevalence of the Trp316Ser mutation was measured in 170 systemic lupus erythematosus (SLE) patients and in 18 patients with the primary antiphospholipid syndrome (PAPS) and the mutation was correlated with the presence of anti-beta2GPI-antibodies. In the total patient group 1 homozygous patient and 21 heterozygous patients were found. The allele frequency of the mutation in SLE patients with anti-beta2GPI-antibodies (0.063) was comparable to that found in SLE patients without anti-beta2GPI-antibodies (0.062). These results indicate that the heterozygous presence of Trp316Ser mutation does not prevent an individual from developing anti-beta2GPI-antibodies. We showed that this can be explained by the concentration of Trp316-beta2GPI in heterozygous patients, which is far above the minimal beta2GPI level necessary for optimal phospholipid binding. In our single patient homozygous for the Trp316Ser mutation no binding beta2GPI to the phospholipid surface was detected and no anti-beta2GPI-antibodies were present in the plasma of this patient. In conclusion, heterozygous Trp316Ser beta2GPI persons are not protected against the development of anti-beta2GPI-antibodies. To confirm that homozygotes do not develop anti-beta2GPI-antibodies a very large population is needed, due to the relatively low prevalence of the mutation.