Pro- and anticoagulant properties of factor V in pathogenesis of thrombosis and bleeding disorders.

Factor V (FV) serves an important role in the regulation of blood coagulation, having both pro- and anticoagulant properties. The circulating high molecular weight single-chain FV molecule undergoes a series of proteolytic cleavages during both activation of coagulation and during anticoagulant regulation of coagulation by activated protein C (APC). It is noteworthy that mutations in the factor V gene (F5) either cause thrombosis or bleeding. New insights into the importance and complexity of FV functions have been generated from elucidation of the pathogenic mechanisms of two familial mutations in the F5 gene. The first mutation was identified as a result of the discovery of APC resistance as the most common risk factor for venous thrombosis. The mutation (FV Leiden) predicts the Arg(506) Gln replacement, which impairs the normal regulation of FVa by APC, as the Arg506 site is an important APC cleavage site. In addition, elucidation of APC resistance resulted in the discovery of the anticoagulant APC cofactor activity of FV. The second FV mutation (FV(A2440G) ), identified in a family with an autosomal dominant bleeding disorder, has led to the discovery of an alternative splicing generating a previously unidentified FV isoform (FV-Short), which inhibits coagulation via an unexpected and intriguing mechanism involving the coagulation inhibitor TFPI-α. These are naturally occurring mutations in the F5 gene that have generated new knowledge on the role of FV in regulation of coagulation and the importance of genetic risk factors for thrombosis and bleeding.