Normal hemostasis requires a balance between procoagulant and anticoagulant systems which may be perturbed by either inherited or acquired defects. Acquired defects may be environmental or intrinsic. Inherited defects associated with bleeding disorders, such as hemophilia A or B and von Willebrand's disease, have been known and studied for many years. Inherited defects causing thrombosis and hypercoagulability are a more recently recognized phenomenon, and include factor V Leiden, antithrombin III deficiency, protein C and protein S deficiencies and a few others.
Venous thrombosis leads to 50,000 to 100,000 deaths and 500,000 hospitalizations annually in this country, and is a multifactorial problem with both heritable risk factors and environmental conditions implicated. The incidence of symptomatic venous thrombosis is 1:1,000 people per year. Risk factors for venous thrombosis include pregnancy, oral contraceptives, estrogen therapy, malignancy, stroke with paresis of extremities, trauma, surgery, and prolonged immobility. It also may occur in the absence of known risk factors. Genetic causes are implicated in 25% of unselected venous thrombosis cases, and approximately 63% of familial ones.
FVL is the most common genetic risk factor for venous thrombosis, and was discovered in 1993. Factor V is a cofactor in blood coagulation, and is regulated by the protein C system inhibitor complex. The FVL mutation is caused by a single point mutation in the gene for factor V. By destroying a cleavage site in the protein, factor V degradation by activated protein C (ACP), which inactivates factors V and VIII, is inhibited in those with the FVL mutation. This results in a state of hypercoagulability and "activated protein C resistance."
Individuals with FVL have a propensity to develop venous thromboembolic disease (deep venous thrombosis and pulmonary embolism) at a younger age than the normal population. Venous thrombosis is the third most common cardiovascular disease in the United States. The risk of venous thrombosis is increased seven-fold in heterozygotes and 80 fold in homozygotes. Coupling of a heterozygous mutation with oral contraception synergistically increases risk of venous thrombosis thirty-fold. Synergistic risk increases also occur when the mutation is coupled with elevated homocysteine levels or a factor II (prothrombin) 20210G→A mutation. FVL is also associated with arterial thrombosis (particularly in smokers), complications of pregnancy, and increased levels of factor VIII.
Twenty percent of venous thrombosis cases involve FVL, with 3 to 7% of the general Caucasian population and 1.2 % of the African-American population possessing the mutation. Up to 95% of cases of activated protein C resistance and subsequent defective anticoagulation are due to FVL. Populations considered at risk for this mutation include: (1) those with an initial venous thrombotic event at age 50 or younger; (2) relatives of those individuals; (3) young people with histories of recurrent venous thrombotic episodes or episodes in unusual sites (e.g. mesenteric venous thrombosis); (4) pregnant women with venous thrombotic events; (5) women with recurrent pregnancy loss, unexplained severe preeclampsia, placental abruption, intrauterine fetal growth retardation, or stillbirth; and (6) women placed on oral contraceptives or estrogen replacement at menopause who have had venous thrombotic events.
A new microtiter plate based Third Wave Invader assay that does not require PCR, restriction digestion, or gel electrophoresis has recently been added to the laboratory armamentarium. This method of FVL genotyping using invasive cleavage of oligonucleotide probes is a rapid and reliable alternative to genotyping by more traditional PCR-based methods. The new methodology showed complete concordance when 1,369 individuals were FVL genotyped by both the new assay and allele-specific PCR. Mutation analysis is not affected by heparin, oral anticoagulants, pregnancy, oral contraceptives, estrogen replacement therapy, or lupus anticoagulants.