Summary
Vitamin K-dependent protein C is an important regulator of blood coagulation. After
its activation on the endothelial cell surface by thrombin bound to thrombomodulin,
it cleaves and inactivates procoagulant cofactors Va and Villa, protein S and intact
factor V working as cofactors. Until recently, genetic defects of protein C or protein
S were, together with antithrombin III deficiency, the established major causes of
familial venous thromboembolism, but they were found in fewer than 5-10% of patients
with thrombosis. In 1993, inherited resistance to activated protein C (APC) was described
as a major risk factor for venous thrombosis. It is found in up to 60% of patients
with venous thrombosis. In more than 90% of cases, the molecular background for the
APC resistance is a single point mutation in the factor V gene, which predicts substitution
of an arginine (R) at position 506 by a glutamine (Q). Mutated factor V (FV: Q506) is activated by thrombin or factor Xa in normal way, but impaired inactivation of
mutated factor Va by APC results in life-long hypercoagulability. The prevalence of
the FV:Q506 allele in the general population of Western countries varies between 2 and 15%, whereas
it is not found in several other populations with different ethnic backgrounds. Owing
to the high prevalence of FV:Q506 in Western populations, it occasionally occurs in patients with deficiency of protein
S, protein C, or antithrombin III. Individuals with combined defects suffer more severely
from thrombosis, and often at a younger age, than those with single defects, suggesting
severe thrombophilia to be a multigenetic disease.
Keywords
APC-resistance - factor V - protein C - protein S - thrombosis