The Prothrombin Gene
Prothrombin is a type of protein called a coagulation factor, and plays an important role in blood coagulation, or clotting. During the cascade of reactions that lead to blood clotting, prothrombin is cleaved by an enzyme into multiple parts which subsequently have their own roles to play in the clotting process.
Prothrombin is also known as Factor II, a designation which references the protein’s role in the clotting cascade. A mutation in the gene that codes for prothrombin is called a prothrombin gene mutation, PT gene mutation, F2 mutation, or Factor II mutation.
Prothrombin Gene Mutation
The designation of prothrombin as Factor II gives rise to the name of the gene that codes for prothrombin. The gene is called F2, with an official name of coagulation factor II (thrombin).
The F2 mutation that causes thrombophilia is located at gene position 20210, which designates the nucleotide position at which the mutation occurs. At this location, the normal guanine nucleotide is replaced by an adenine when the gene is mutated.
The effect of this mutation is to cause too much prothrombin to be produced. As a result, there is more prothrombin and more thrombin in the body, leading to an increased tendency for blood clots to form.
Prothrombin gene mutation is congenital, rather than spontaneous, meaning it is largely an inherited mutation. Someone with one copy of the mutated gene generally has a mild form of thrombophilia, while someone with two mutated copies has a more severe form.
Effects of Mutation
A mutation of the prothrombin gene can have serious effects on the health, due to the increased tendency of blood coagulation and clot formation.
Increased risk of venous thromboses, such as deep vein thrombosis, pulmonary embolism, or mesenteric vein thrombosis.
Increased risk of arterial blood clots leading to stroke or heart attack. The link between prothrombin gene mutation and stroke or heart attack is not proven, but is thought to be likely.
In women, an increased risk of miscarriage or stillbirth. In addition, pregnant women might have an increased risk of preeclampsia and eclampsia.
It is thought that up to 2% of the general population has one copy of the mutation; the prevalence is lower in some racial groups, including African-Americans and Asians (Rush). The risk of a blood clot is increased by two to five times, giving an approximate risk of 1 in 3700. For women taking certain oral contraceptives this increases to 1 in 800.
For someone with two mutated copies of the gene, the risk of developing a blood clot is very much higher. It is estimated that the risk is up to 50 times higher than normal in such cases.
Rush Hemophilia & Thrombophilia Center: Prothrombin Gene Mutation Educational Resource (PDF)
UNC Molecular Genetics Lab: Laboratory Test for Prothrombin Gene Mutation(G20210A) (PDF)
University of Iowa Healthcare: Thrombophilia Information for Patients
US National Library of Medicine Genetics Home Reference: Prothrombin Thrombophilia