Updated February 2, 2000 ACTIVATED PROTEIN C RESISTANCE (APC RESISTANCE) AND MUTATIONS IN FACTOR V MOLECULAR BASIS: When thrombin (IIa) is generated, it has both procoagulant activities and anticoagulant activities. Excess thrombin is washed downstream where it binds to thrombomodulin (TM) on endothelial cells of the vessel wall. Protein C from plasma binds to the IIa/TM complex and is cleaved to its active form, termed "activated Protein C" (APC). APC is one of the most physiologically important anticoagulants as it selectively degrades the coagulation cofactors, Va and VIIIa to limit thrombin generation, fibrin formation and blood clotting in vivo. It has been shown that APC resistance results from a mutant Factor Va molecule, termed Factor V Leiden. This molecule has a specific point mutation (Arg -> Gln ) which cannot be degraded by APC in more than 90% of all APC resistant patients2. The defective Va is able to clot as normal and clotting continues because of this resistance to inactivation by APC. Other APC cleavage sites in the Factor Va molecule are potential mutation sites. These sites include: Arg -> Gln and Arg -> Gln . The other 10% of APC resistance includes Acquired APC Resistance and these secondary sites. Recent studies have shown the occurrence of APC resistance to vary from 2 - 16% depending on the population studied. This data suggests that screening protocols for hereditary thrombotic disorders should include testing for APC resistance as an important genetic risk factor. Hypercoagulability has been explained by hereditary deficiencies of Protein C, Protein S and Antithrombin in only 9 to 21% of thrombotic cases in patients without the usual risk factors for thrombosis (i.e., cancer, recent surgery, lupus anticoagulant) | |
|