Inherited resistance to activated protein C is corrected by anticoagulant cofactor activity found to be a property of factor V

Association of idiopathic venous thromboembolism with single point-mutation at Arg506 of factor V

Abnormal coagulation factor V may underlie the thrombotic events associated with resistance to activated protein C (APC). We analysed 27 consecutive patients with documented idiopathic (recurrent) thromboembolism for the occurrence of point mutations within the APC sensitive regions of blood coagulation factor V. In 10 patients we observed a single basepair mutation resulting in a substitution of Arg506 to Gln. This mutation was significantly linked to in-vitro resistance to APC in these subjects. This mutation at Arg506 of factor V may form the molecular basis for the thrombotic events associated with APC resistance.

Mutation in blood coagulation factor V associated with resistance to activated protein C

Activated protein C (APC) is a serine protease with potent anticoagulant properties, which is formed in blood on the endothelium from an inactive precursor. During normal haemostasis, APC limits clot formation by proteolytic inactivation of factors Va and VIIIa (ref. 2). To do this efficiently the enzyme needs a nonenzymatic cofactor, protein S (ref. 3). Recently it was found that the anticoagulant response to APC (APC resistance) was very weak in the plasma of 21% of unselected consecutive patients with thrombosis and about 50% of selected patients with a personal or family history of thrombosis; moreover, 5% of healthy individuals show APC resistance, which is associated with a sevenfold increase in the risk for deep vein thrombosis. Here we demonstrate that the phenotype of APC resistance is associated with heterozygosity or homozygosity for a single point mutation in the factor V gene (at nucleotide position 1,691, G-->A substitution) which predicts the synthesis of a factor V molecule (FV Q506, or FV Leiden) that is not properly inactivated by APC. The allelic frequency of the mutation in the Dutch population is approximately 2% and is at least tenfold higher than that of all other known genetic risk factors for thrombosis (protein C (ref. 8), protein S (ref. 9), antithrombin10 deficiency) together.

Resistance to activated protein C as a basis for venous thrombosis

BACKGROUND

In three families with various forms of venous thrombosis, we observed an apparently inherited poor response to the anticoagulant activated protein C (APC). The condition was due to a deficiency in a previously unrecognized anticoagulant factor that functioned as a cofactor to activated protein C.

METHODS

We conducted the present study to determine the prevalence of resistance to APC in patients with venous thrombosis. We compared 104 consecutive patients with venous thrombosis confirmed by objective tests with 130 controls. In addition, 211 members of 34 families of persons with resistance to APC were studied. The anticoagulant response to APC was measured with a modified version of the activated partial-thromboplastin time test; the results were expressed as APC ratios.

RESULTS

Forty-five percent of patients had a family history of thrombosis. A significant (P < 0.001) difference in APC ratios was observed between the controls and the patients with thrombosis. For 33 percent of patients, the APC ratio was below the 5th percentile of the control values, although the results of the family studies suggested that the prevalence of APC resistance may be even higher (approximately 40 percent) in the patients with thrombosis. The inherited nature of the defect was confirmed in a majority of cases, and the family studies suggested the mode of inheritance to be autosomal dominant. The thrombosis-free survival of APC-resistant family members was significantly less than that of non-APC-resistant family members.

CONCLUSIONS

There was a high prevalence of APC resistance among young persons with a history of venous thrombosis, and this resistance appeared to be inherited as an autosomal dominant trait.