hematology and oncology
Factor V Resistance to Activated Protein C (APC)
Introduction
Mutations of factor V make it resistant to its normal cleavage and inactivation by activated protein C, and they also predispose to venous thrombosis. (See also Overview of Thrombotic Disorders .) Activated protein C (APC), in complex with protein S, degrades coagulation factors Va and VIIIa, thus inhibiting coagulation (see figure). Any of several mutations to factor V make it resistant to inactivation by APC, increasing the tendency for thrombosis. Factor V Leidenis the most common of these mutations. Homozygous mutations increase the risk of thrombosis more than do heterozygous mutations. Factor V Leiden as a single gene defect is present in about 5% of White Americans, but it rarely occurs in native Asian or African populations ( General references ). It is present in 20 to 60% (depending on patient selection) of patients with "spontaneous" venous thrombosis ( 