Summary
Carriership of the factor V (FV) gene marked by the R2-haplotype, a series of linked
polymorphisms encoding several amino acid changes in FV, is associated with mild resistance
to activated protein C (APC) and with an increased risk of thrombosis. We compared
the functional properties of normal FV(a) and R2-FV(a) in model systems and in plasma.
FV and R2-FV were equally well activated by thrombin and expressed identical cofactor
activities in prothrombin activation. Rate constants of APC-catalyzed inactivation
of FVa and R2-FVa were similar both with and without protein S. However, significant
differences were observed between haemostatic parameters determined in plasma from
homozygous carriers of the R2-gene (n = 5) and age-matched non-carriers (n = 19).
Plasma from R2-carriers contained significantly lower FV levels and the ratio of the
two FV isoforms (FV1 and FV2) was shifted in favor of FV1. The FV2/FV1 ratio was 1.4 (95% CI = 1.3-1.5) in homozygous carriers of R2 and 2.8 (95% CI = 2.5-3.1)
in controls (p <0.00001). In an APC resistance test which quantifies the cofactor
activity of FV in APC-catalyzed FVIII(a) inactivation, homozygous R2-carriers had
significantly lower (p <0.00001) APC sensitivity ratios (APCsr = 1.54, 95% CI = 1.48-1.60)
than controls (APCsr = 2.17, 95% CI = 2.05-2.28). This indicates that R2-FV has reduced
cofactor activity in APC-catalyzed FVIII(a) inactivation. The changes of the relative
amounts of FV1 and FV2 in carriers of the R2-gene will result in increased thrombin formation in the presence
of APC and may provide a mechanistic explanation for the increased thrombotic risk
associated with the R2-haplotype.
Keywords
Factor V isoforms - R2-haplotype - APC-resistance