Identification and functional characterization of a novel F5 mutation (Ala512Val, FVBonn) associated with activated protein C resistance

Background Activated protein C (APC) resistance is a prevalent risk factor for venous thrombosis. This phenotype is most commonly associated with the factor V Arg506Gln mutation (FV Leiden), which impairs the APC-mediated inactivation of both activated FV (FVa) and activated FVIII (FVIIIa). Objectives Here, we report the identification and characterization of a novel FV mutation (Ala512Val, FVBonn) in six patients with APC resistance and venous thrombosis or recurrent abortions. Methods FVBonn was expressed in a recombinant system and compared with recombinant wild-type (WT) FV and FV Leiden in several functional assays. Results FVBonn conferred APC resistance to FV-depleted plasma, both in the activated partial thromboplastin time (APTT)-based test (APC sensitivity ratio [APCsr] of 1.98 for FVBonn versus 4.31 for WT FV and 1.59 for FV Leiden) and in the thrombin generation-based test (normalized APCsr of 5.41 for FVBonn versus 1.00 for WT FV and 8.99 for FV Leiden). The APC-mediated inactivation of FVa(Bonn) was slower than that of WT FVa (mainly because of delayed cleavage at Arg506), but was greatly stimulated by protein S. The APC cofactor activity of FVBonn in FVIIIa inactivation was similar to 24% lower than that of WT FV. In line with these findings, an in silico analysis showed that the Ala512Val mutation is located in the same loop as the Arg506 APC cleavage site and might hamper its interaction with APC. Moreover, FVBonn was more procoagulant than WT FV and FV Leiden in the absence of APC, because of an increased activation rate and, possibly, an enhanced interaction with activated FX. Conclusions FVBonn induces hypercoagulability via a combination of increased activation/procoagulant activity, decreased susceptibility to APC-mediated inactivation, and slightly reduced APC cofactor activity.