Which genes are included in the thrombotic risk DNA test?
The thrombotic risk DNA panel identifies the presence or absence of the following mutations which are associated with an increased risk for thrombophilia:
- Leiden mutation: 1691G>A in theF5 gene encoding Factor V
- Prothrombin mutation: 20210G>A in theF2 gene encoding Prothrombin
- MTHFR mutations: 677C>T and 1298A>C in the MTHFR gene encoding methylenetetrahydrofolate reductase.
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Factor V Leiden Mutation (F5)
The F5 gene encodes a protein (Factor V) that promotes blood clotting. In the normal situation, the Factor V protein is inactivated by another protein called Protein C. This inactivation occurs by cleaving of the Factor V protein at three different arginine amino acids (at positions 306, 506 and 679). However, when the Factor V Leiden mutation occurs, the resulting protein resists inactivation and increases the risk of abnormal clotting. The Leiden mutation is a single base substitution (1691G>A) that changes an arginine at one of the cleavage points to a glutamine (Arg506Gln). The Leiden mutation can also be referred to as c.1601G>A or p.Arg534Gln.
The Factor V Leiden mutation is associated with an increased relative risk of thrombophilia and is also associated with a 2- to 11-fold increased relative risk for miscarriages. Individuals who are heterozygous for the Factor V Leiden mutation have between a 3- and 8-fold increased risk of thrombophilia. The risk is higher in individuals that have two copies of the Factor V Leiden mutation (homozygotes). Several different studies have predicted a wide range of relative risk values with an increased risk for homozygotes estimated to be anywhere between 10- and 80-fold.
Factor II Prothrombin Mutation (F2)
The F2 prothrombin gene encodes the prothrombin protein that promotes blood clotting. When the 20210G>A mutation (a.k.a. c.*97G>A) occurs in the prothrombin gene, more of the prothrombin protein is produced, hence an increased chance of blood clotting.
Individuals who are heterozygous for the 20210G>A prothrombin mutation have a 2- to 5-fold increased risk of thrombophilia. The risk is higher in individuals that have two copies of the 20210G>A mutation (homozygous).
The MTHFR gene encodes the methylenetetrahydrofolate reductase (MTHFR) enzyme that helps process Vitamin B9. Vitamin B9 is also known as folic acid or folate and is required in the process of converting homocysteine to methionine. When either the 677C>T or 1298A>C mutation occurs in the MTHFR gene, the enzyme is not as efficient at processing Vitamin B9/folate and an excess of homocysteine accumulates, leading to abnormal blood clotting.
The 677C>T mutation (a.k.a c.665C>T) is a single base substitution (C>T) that changes an alanine to a valine (p.Ala222Val) and reduces the MTHFR enzyme level to ~25% of normal. The 1298A>C mutation (a.k.a. c.1286A>C) is a single base substitution (A>C) that changes a glutamic acid to an alanine (p.Glu429Ala) and reduces the MTHFR enzyme level to ~60% of normal.
An increased risk of high homocysteine levels (and associated thrombophilia) only occurs when an individual has either two copies of the 677C>T mutation or one 677C>T and one 1298A>C allele. If an individual only has one 677C>T or one 1298A>C allele there is no increased risk of high homocysteine as the wildtype allele provides enough functional MTHFR to produce folate to prevent the build up of homocysteine. Interestingly, the presence of two 1298A>C alleles is also not associated with an increased risk.
High homocysteine levels due to MTHFR variants and/or other risk factors have also been linked to pregnancy complications including miscarriages, placental abruption (where the placenta pulls away from the wall of the uterus) and preeclampsia (high blood pressure and proteinuria).