Article: Factor V Leiden

Factor V Leiden (sometimes Factor V_Leiden) is the name given to a variant of human factor V that causes a hypercoagulability disorder. In this disorder the Leiden variant of factor V, cannot be inactivated by activated protein C. Factor V Leiden is the most common hereditary hypercoagulability disorder amongst Eurasians. It is named after the city Leiden (The Netherlands), where it was first identified in 1994 by Prof R. Bertina et al.

Pathophysiology

In the normal person, factor V functions as a cofactor to allow factor X to generate the active form of an enzyme called thrombin. Thrombin in turn cleaves fibrinogen to fibrin, which polymerizes to form the dense meshwork that makes up the majority of a clot. Activated protein C (aPC) is a natural anticoagulant that acts to limit the extent of clotting by cleaving and degrading factor V.

Factor V Leiden is an autosomal dominant condition in which the coagulation factor cannot be destroyed by aPC. Mutation of the gene encoding factor V—a single nucleotide substitution of adenine for guanine—changes the protein's 506th amino acid from arginine to glutamine . Since this amino acid is normally the cleavage site for aPC, the mutation prevents efficient inactivation of factor V. When factor V remains active, it facilitates overproduction of thrombin leading to excess fibrin generation and excess clotting.

The excessive clotting that occurs in this disorder is restricted to the veins, where the clotting may cause a deep vein thrombosis (DVT). If the venous clots break off, these clots can travel through the heart to the lung, where they block a pulmonary blood vessel and cause a pulmonary embolism. Women with the disorder have an increased risk of miscarriage and stillbirth. This disorder does not increase the formation of clots in arteries that can lead to stroke or heart attack, though a "mini-stroke" know as a transient ischemic attack may occur.

Epidemiology

Studies have found that about 5% of caucasians in North America have factor V Leiden. The disease is less common in Hispanics and African-Americans and is extremely rare in people of Asian descent.

Up to 30% of patients who present with deep vein thrombosis (DVT) or pulmonary embolism have this mutation. Factor V Leiden doubles the risk that a person will have a DVT during their life, but it is unclear whether these individuals are at increased risk for recurrent a venous thrombosis. While only 1% of people with factor V Leiden have two copies of the defective gene, these homozygous individuals have a more severe clinical condition. The presence of acquired risk factors for venous thrombosis -- including smoking, oral contraceptive use, and recent surgery -- further increase the chance that an individual with the factor V Leiden mutation.

Diagnosis

Suspicion of factor V Leiden being the cause for any thrombotic event should be considered in any white patient below the age of 45, or in any person with a family history of venous thrombosis.

This disease can be diagnosed by watching the aPTT (the time it takes for blood to clot) as activated protein C is added. With a normal patient, adding aPC increases the APTT. In patients with factor V Leiden, adding aPC will barely affect the time it takes for blood to clot.

There is also a simple genetic test that can be done for this disorder. The mutation (a 1691G→A substitution) removes a cleavage site of the restriction endonuclease MnlI, so simple PCR, treatment with MnlI, and then DNA electrophoresis will give a quick diagnosis.