Wednesday, 21 November 2007

Genetics and blood clotting disorders

Thrombosis is the presence or formation of a thrombus - a blood clot that forms within a vein or an artery. The condition arising from clots developing in the body’s deep veins (typically the leg and pelvic regions) is known as Deep Vein Thrombosis, or DVT. Sometimes the blood clot dissolves on its own, with no harm. But a clot that prohibits adequate amounts of blood to pass through a vein can result in necrosis, or tissue death. Even worse, the clot could break off, travel through the blood stream, and lodge itself in an artery in the lungs, causing a life-threatening condition known as pulmonary embolism (PE).

There are many reasons why these abnormal clots form. When a person’s ability to move about is limited, whether due to a long airplane flight (“Economy Class Syndrome”) or confinement to a bed after surgery, blood can pool and form clots. The use of oral contraceptives or estrogen therapy can also cause clotting.

Genetics can have a role in the development of thrombosis. There are several known mutations (heritable following an autosomal dominant pattern) that interfere with the body’s blood clotting process. The most common is the Factor V Leiden mutation, a resistance to Protein C, a protein that keeps the formation of blood clots in check. Present in 5 – 7 percent of the general population, the Factor V Leiden mutation is present in 20 percent of more of all people with thrombosis. Less frequently occurring mutations include Prothrombin II, Protein C deficiency; Protein S deficiency; Antithrombin III deficiency; Plasminogen deficiency; and Heparin cofactor II deficiency. It is possible for a person to have more than one of these mutations. When the Prothrombin II mutation is present with the Factor V Leiden mutation, the risk for DVT or PE is increased substantially.

The genetic test for Factor V Leiden and Prothrombin II is a DNA analysis taken from a sample of blood or cheek cells. There are also panel tests that screen for several of the mutations described above. Testing should be considered for: people who experience a thrombotic event—such as a DVT or PE— before age 50 or have multiple thrombotic events; those with a family history of thrombotic events, even if they themselves are not experiencing symptoms; and people with a first-degree relative who is known to have one of the thrombosis mutations.
A person who has had a thrombotic event and, after testing, is found to have the Factor V Leiden mutation or any of the other thrombilia-related mutations, is usually advised to take anticoagulant medication, such as Coumadin. The period of time in which the medication is taken will depend upon the specific circumstances surrounding the thrombotic event.
Additionally, affected persons as well as family members who have a mutation but no history or indication of clotting disorders would be advised to take preventive steps, such as not smoking, not taking birth control pills, wearing support hose and stretching during long airplane flights, and alerting their physicians if they are scheduled for surgery or are immobilized for a long period of time.

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