What is being tested?
The methylenetetrahydrofolate reductase (MTHFR) gene contains the DNA code to produce the MTHFR enzyme.
The MTHFR enzyme metabolises one form of B vitamin, folate, into another. It is also part of the process that converts homocysteine into methionine, an important building block for many proteins.
The MTHFR gene test detects two of the most common variants in the gene, C677T and A1298C. These are known as single nucleotide polymorphisms (SNPs). Individuals can inherit one or both variants. There are also other rarer variants in MTHFR, but these are not detected by the common testing.
The C677T and A1298C variants are very common in the Australian population. In fact there are more people who carry one or both of these variants than those who don’t.
About 10 per cent of the Australian population is homozygous for C677T, meaning they have two copies of it one on each chromosome. The C677T variant results in a less active form of the MTHFR enzyme (~65% activity) and reduced ability to process folate and homocysteine. When a person has two copies of the MTHFR C677T gene mutation (homozygous) or one copy of MTHFR C677T and one copy of A1298C (compound heterozygous), a decrease in the activity of MTHFR enzyme slows down the homocysteine-to-methionine conversion process and this can lead to a build-up of homocysteine in the blood.
The increase in homocysteine is often mild to moderate but will vary from person to person depending on the amount of MTHFR enzyme activity and other factors such as dietary folate intake. Even if a person has two copies of the MTHFR mutation, many do not develop high homocysteine levels since adequate folate intake can counteract the effect of the MTHFR mutation. This is especially so in countries such as Australia where there is mandatory folate fortification of wheat flour used for making bread.
Results of some previous studies suggest that high levels of homocysteine in the blood may contribute to risk of CVD by damaging blood vessel walls and promoting formation of plaque (atherosclerosis) and inappropriate blood clots. However, a direct link between homocysteine levels and cardiovascular disease or thrombotic risk has not been found.
How is it used?
The methylenetetrahydrofolate reductase (MTHFR) mutation test is used to detect two relatively common mutations in the MTHFR gene that are associated with elevated levels of homocysteine in the blood. It is not routinely ordered.
This test is sometimes ordered as a follow-up to an elevated homocysteine test and may be occasionally ordered along with other cardiac risk tests if someone has a personal or family history of premature cardiovascular disease (CVD) or inappropriate blood clots (thrombosis) and no other cause can be found. However, its utility for assessing risk of CVD has not been established and most expert guidelines do not recommend it for thrombosis screening.
Although the MTHFR mutation test may be used to help determine the cause of elevated homocysteine, the value of measuring homocysteine levels is not clear. While evidence from some studies suggests that elevated homocysteine levels contribute to the risk of CVD and/or thrombosis, a direct link has not been established, and the overall risk is low. Routine testing for homocysteine levels as a cardiac risk marker is not recommended by the American Heart Association and an Australian study in 2004 came to the same conclusion. More than ten percent of the Australian population are homozygous (they have two copies) of the most common mutation, MTHFR C677T. However, its relationship with venous thrombosis remains controversial and it is not currently recommended as part of a screen for thrombophilia. The College of American Pathologists and the American College of Medical Genetics recommend against testing for the C677T variant, citing limited utility for patients with blood clots. The NSW Government Centre for Genetics Education information for GPs lists only a few indications for MTHFR testing. Also, the use of homocysteine levels for the purpose of determining risk of CVD, peripheral vascular disease and stroke is in doubt at this time given that several studies show no benefit or risk reduction in people who were treated with folic acid and vitamin B supplements that lowered their homocysteine level.
When is it requested?
The NSW Centre for Genetics Education states that there may be some evidence to consider MTHFR testing in the following limited circumstances:
The MTHFR test has become popular with many complementary and alternative medicine practitioners such as naturopaths who believe that it is involved in a wide variety of disease processes although the evidence on which these beliefs are based is extremely weak or non-existent.
What does the result mean?
Results typically are reported as negative or positive and, if positive, the report will name the mutation(s) present. Often, an interpretation of the results is also provided.
Only a small percentage of cases of elevated homocysteine are due to an inherited cause. Of these, MTHFR C677T and A1298C mutations are among the most common.
If someone has two copies (homozygous) of MTHFR C677T, or one copy of C677T and one of A1298C, it is likely that elevated homocysteine levels are due to these inherited mutations, or that the mutations are contributing to them.
Two copies of A1298C are not typically associated with increased homocysteine levels.
If the MTHFR mutation test is negative, the C677T and A1298C mutations were not detected and an elevated homocysteine level is likely to be due to another cause. Other, rarer MTHFR genetic mutations will not be detected with typical testing.
Those with MTHFR mutations and other clotting risk factors, such as Factor V mutation (Leiden) or PT 20210 mutations, may be at an increased risk of thrombosis.
Is there anything else I should know?
People who have elevated homocysteine levels may be at an increased risk of developing premature cardiovascular disease (CVD) and/or thrombosis, but many, including those with MTHFR mutations, will never develop CVD or thrombosis. The role of homocysteine in cardiac risk assessment is still in the process of being determined.
As well as MTHFR mutations there are other causes of elevated homocysteine levels including deficiency of vitamins B6, B12, and/or folate – vitamins required for homocysteine metabolism, kidney and other diseases, numerous drugs, and increasing age. Also, if a rarer mutation of MTHFR is causing elevated homocysteine levels, the C677T and A1298C tests will not detect these other mutations.
Some studies have shown links between MTHFR genetic mutations and an increased risk of neural tube defects, pre-eclampsia, and certain cancers, but the test is not used clinically with these conditions, because the evidence is weak and conflicting.
The MTHFR enzyme is involved in folate metabolism. Because of this, those who have MTHFR mutations and take drugs that affect folate metabolism, such as methotrexate, may be more likely to experience toxicity. An MTHFR mutation test may be performed for a person who is prescribed methotrexate in order to adjust dosages and reduce risk of toxicity although this is not routinely done.
Common questions
It is not offered in every laboratory. In most cases, your blood sample will be sent to a reference laboratory for testing.
No, you inherit a copy of the gene from each of your parents and they will not change over time.
Even when two people have the same MTHFR mutations, the results and their risks are often different. Many things can affect homocysteine levels, including MTHFR enzyme activity, folate levels and a person's health status.
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