Factor V Leiden Mutation Analysis

Details of how the blood coagulation system is regulated have become well understood in recent years. Many of the abnormalities that cause some patients to have an increased risk for thrombosis have been defined at the molecular level. The poor anticoagulant response to activated protein C (APC), or APC resistance, that is found in up to 50% of patients with a family history of thrombosis, and in 3% to 8% of apparently normal individuals, was shown to correlate with the inheritance of a single-point mutation in the factor V gene. This mutation has been reported in patients with deep vein thrombosis, pulmonary embolus, central retinal vein occlusion, cerebral sinus thrombosis, and hepatic vein thrombosis. Ten percent to 20% of people with a first-time venous clot have this mutation. Strikingly, 40% of people younger than 50 with a first-time venous clot, and 60% of pregnant women with venous thrombosis have this mutation as well. Beyond the risk for thrombophilia, this mutation has also been associated with an increased risk for recurrent pregnancy loss, severe preëclampsia, fetal growth retardation, stillbirth, and placental problems (infarction and abruption).

The mutation is characterized by a guanine to adenine substitution at nucleotide 1691 in exon 10 of the factor V gene that replaces an arginine at codon 506 with a glutamine. It is designated as FV R506Q (Leiden), and confers resistance to inactivation by activated protein C. As a result, factor V persists in the circulation, leading to a mild hypercoagulable state. The Leiden mutation accounts for 90% to 95% of APC resistance. Heterozygous carriers of this mutation have a four- to eightfold increased risk of thrombosis. Individuals homozygous for the mutation (ie, they have a copy of the mutation on each chromosome) carry an 80- to 100-fold risk of thrombosis. All offspring of a factor V Leiden homozygote will inherit at least a single copy of the mutation. Genetic counseling is recommended for these patients.

The risk of venous thrombosis increases exponentially in patients with more than one risk factor, including age, surgery, oral contraceptive use, pregnancy, elevated homocysteine levels, or malignancy. A 2005 study reports malignancy carries a sevenfold increased risk for thrombosis, and that this effect is most pronounced for hematological malignancies, for recently diagnosed cancers, and/or for patients with distant metastases. Malignancy and genetic predisposition together may compound risk, as carriers of the factor V Leiden mutation who have cancer are more likely to develop thrombosis than mutation carriers without cancer. Other risk factors to be considered in the work-up for venous thrombosis include the G20210A mutation in the factor II (prothrombin) gene, mutations in the MTHFR gene, protein S and C deficiency, and antithrombin deficiencies. Anticardiolipin antibody and lupus anticoagulant analysis may be appropriate for certain patients, as well as homocysteine levels.