Factor V Leiden Thrombophilia Health Article

Definition

Factor V Leiden thrombophilia is a common genetic disorder that leads to a predisposition or increased chance to develop blood clots in the veins (venous thrombosis).

Description

Factor V Leiden thrombophilia is a disorder caused by an inherited change or mutation in the genetic instructions for making a substance called factor V. The factor V change leads to an increased chance to develop blood clots in blood vessels.

Blood clots form in two steps. In the first step, the body produces platelets that are "sticky" and can form initial plugs or clots when needed. However, the first platelets only form the first temporary plugs. To form a more lasting plug or clot the platelets release chemicals to attract more platelets and other substances called clotting factors (or clotting proteins). In the second step, the platelets come together with the clotting proteins and form fibers. The fibers weave together and make the clot stronger and longer lasting.

Individuals affected by factor V Leiden thrombophilia have a genetic mutation that makes a longer lasting, "stickier" form of the clotting factor or protein called factor V. This different form of factor V is called factor V Leiden. The factor V Leiden clotting protein lasts longer in the blood because a chemical produced by the body called Activated Protein C (or APC), which is supposed to help "break-down" the factor V clotting protein, cannot break down the factor V Leiden clotting protein as easily and quickly as it breaks down normal factor V. The factor V Leiden clotting protein breaks down ten times slower than an average clotting factor V and accordingly stays in the blood longer.

Since there is longer lasting, extra sticky Factor V Leiden in the blood, individuals affected by factor V Leiden thrombophilia have an increased chance to have free-floating blood clots (thrombosis) that can get stuck in the veins and other blood vessels. An alternative name used to describe this condition is Hereditary Resistance to Activated Protein C.

Genetic profile

Factor V Leiden thrombophilia occurs when a specific gene on the long arm of chromosome one is changed or mutated. This gene is called F5. Every person has approximately 30,000-35,000 genes that tell our bodies how to form and function. Each gene is present in pairs, since one is inherited from the mother, and one is inherited from the father. Depending on the inheritance of the changed or mutated F5 gene, factor V Leiden thrombophilia runs in families in a more severe and less severe form.

The less severe form of factor V Leiden thrombophilia is called "heterozygous" and occurs when an individual inherits only one copy of the altered or mutated gene that causes factor V Leiden. The more severe form of factor V is called "homozygous" and is caused by the inheritance of two non-working or mutated copies of the gene that causes factor V Leiden thrombophilia.

Heterozygous factor V Leiden is inherited in an autosomal dominant pattern. In an autosomal dominant condition, only one changed or mutated copy of the gene for a particular condition is necessary for a person to experience symptoms of the condition. If a parent has an autosomal dominant condition, there is a 50% chance for each child to have the same or similar condition. In heterozygous factor V Leiden thrombophilia, the chance of being affected by venous blood clots is four to eight times greater than the general population.

Homozygous factor V Leiden thrombophilia is inherited in an autosomal recessive pattern. An autosomal recessive condition is caused by the inheritance of two changed or mutated copies of a gene. Individuals who are affected by heterozygous factor V Leiden thrombophilia have only one copy of the altered gene. However, when two people with heterozygous factor V Leiden thrombophilia have children together, there is a 25% chance, with each pregnancy, for the child to inherit two copies, one from each parent. That child then has two altered copies of the gene and therefore, has homozygous factor V Leiden thrombophilia. When an individual inherits two non-working copies of the gene that lead to homozygous factor V Leiden thrombophilia, there is an up to 80 times increased risk to be affected by blood clots stuck in the veins (venous thrombosis). Additionally, most individuals affected by homozygous factor V Leiden thrombophilia develop blood clots at a younger age than individuals affected by heterozygous factor V Leiden thrombophilia.