Resistance to activated protein C due to a factor V gene mutation The most common inherited risk factor of thrombosis.

The protein C anticoagulant pathway is of major importance in maintaining vascular patency. Resistance to the key enzyme of this system, activated protein C (APC), is a recently discovered congenital defect of the protein C system. This genetic defect is present in 20% to 60% of venous thrombosis patients, making it by far the most common known pathogenetic risk factor of thrombosis. APC resistance is due to a single point mutation in the factor V gene (G to A at nucleotide position 1691) that predicts the replacement of arginine(506) by glutamine. This is associated with the loss of one of three APC cleavage sites in factor Va, one of the substrates for APC, and hypercoagulability. The identification of APC resistance as an additional genetic risk factor in a large proportion of symptomatic protein C- and protein S-deficient families has provided evidence that thrombosis is a polygenetic disease. Thus, several genetic defects act in concert with environmental factors in the pathogenesis of venous thromboembolism.