Guide · DNA

MTHFR gene: what the C677T variant really means

One of the most tested genetic variants in the world, and what the large studies say about it.

In this article

1. What the MTHFR gene does
2. The C677T and A1298C variants
3. MTHFR and the heart: what the large studies showed
4. What guidelines say about MTHFR testing
5. I have the variant. What now?
6. Why MTHFR became a marketing topic
7. Frequently asked questions
8. Sources

What the MTHFR gene does

MTHFR stands for methylenetetrahydrofolate reductase, an enzyme that converts folate from food into its active form, 5-methyltetrahydrofolate. This active form donates a methyl group to convert homocysteine into methionine, an essential amino acid.

This pathway is part of the so-called methylation cycle, which depends on folate and vitamin B12 and takes part in making DNA and regulating genes. When folate or B12 is lacking, or the enzyme works more slowly, homocysteine tends to rise in the blood.

The link with B12 is direct: the two vitamins work at the same step. So high homocysteine can reflect a lack of B12 as much as variation in MTHFR.

The C677T and A1298C variants

The most studied variant of the gene is C677T (rs1801133). It changes one letter of DNA and produces an enzyme that is more heat-sensitive, with reduced activity. It was described in 1995 in a Nature Genetics paper, which found the variant in about 38% of chromosomes from unselected people and showed that those with two copies had higher homocysteine.

Since each person has two copies of the gene, a test result looks like this:

There is a second common variant, A1298C, which often appears alongside it on panels. The large studies cited in this text deal mainly with C677T, the most researched.

Frequency varies between populations. In a Brazilian study of women in Salvador, the C677T variant had a frequency of about 19%, both in women with a history of pregnancy loss and in the comparison group.

MTHFR and the heart: what the large studies showed

The MTHFR story is a good example of science correcting itself. In the 1990s the idea seemed solid: high homocysteine was associated with vascular disease, and the C677T variant raises homocysteine. So having the variant should raise risk.

The larger data told another story. A meta-analysis published in PLoS Medicine pooled 19 unpublished datasets, with 48,175 coronary disease cases and 67,961 controls. Comparing people with two copies of the variant (TT) with those with none (CC), the odds of coronary disease were practically the same: an odds ratio of 1.02. The same ratio appeared in populations with low folate and no supplementation.

In the published studies, by contrast, the association looked larger, at 1.15, and larger still in small studies. The authors concluded that this difference reflects publication bias, the tendency for studies with positive results to be published more, and that a moderate, lifelong rise in homocysteine has little or no effect on coronary disease.

The role of folate in the diet

A second analysis, published in The Lancet, pooled 237 genetic datasets and 13 clinical trials. The effect of the variant on homocysteine depended on the folate available in the population. In Asia, in regions with low folate, the difference between TT and CC was 3.12 µmol/L. In regions of the Americas, Australia and New Zealand with folic acid fortification of foods, it was only 0.13 µmol/L. Trials that lowered homocysteine showed no benefit for stroke in populations with good access to folate.

The practical reading: the environment, especially diet, shapes the effect of the variant. The gene does not act alone.

What guidelines say about MTHFR testing

The American College of Medical Genetics and Genomics (ACMG) published a guideline in 2013 with a direct title: lack of evidence for MTHFR polymorphism testing. The document explains that the test was often ordered when investigating thrombophilia, based on the hypothesis that mildly high homocysteine would increase the risk of venous thrombosis, coronary disease and recurrent pregnancy loss. Later meta-analyses ruled out the association between high homocysteine and coronary disease and between MTHFR genotype and venous thrombosis.

The ACMG's conclusion: MTHFR testing has minimal clinical utility and should not be ordered as part of a routine thrombophilia evaluation. In 2020 the college published an addendum to the same guideline.

In the Brazilian study of women with recurrent pregnancy loss, MTHFR variants and levels of homocysteine, B12 and folate were not associated with the losses. Investigations in this area are done with an obstetrician and, when indicated, a geneticist.

I have the variant. What now?

If a test showed you have the C677T variant, in one or two copies, a few points help put the result in perspective:

• It is very common. A large part of the population carries at least one copy.
• It is not a diagnosis. On its own, the variant does not define a disease or a relevant clinical risk, according to the guidelines.
• A blood test says more than the gene. If in doubt, your doctor can order homocysteine, folate and vitamin B12, which show how the cycle is working today.
• Medications and supplements are a medical decision. Do not start or change anything just because of your genotype.
• Diet remains the foundation. Dark green leafy vegetables, legumes and fruit are natural sources of folate, and animal foods provide B12.

If the result has worried you, genetic counselling can help you understand what it means for you and your family.

Why MTHFR became a marketing topic

MTHFR appears often in direct-to-consumer tests and in nutrition content online. Part of the appeal comes from simple, intuitive logic: "slower enzyme, so I need to compensate". The problem is that the intuitive logic did not hold up in large studies.

This does not mean nutrigenetics has no value. It can show tendencies and help explain why people respond differently to the same diet. The care lies in separating what has evidence from what is extrapolation.

Reading the variant is easy; interpreting it is the challenge

Technically, identifying C677T is simple: it is a single letter of DNA that can be read accurately from blood or saliva. The difficulty is the next step, giving the result clinical meaning. A report that turns a common variant into a cause for worry, without considering folate, B12, homocysteine and history, delivers more anxiety than information. A good genetic report says what the variant does, the size of the effect measured in large studies and whether it changes any course of action.

A good question for any genetic result is: does this change any decision? For MTHFR, according to the guidelines, the answer is usually no.

Frequently asked questions

Sources

1. Hickey SE et al. ACMG Practice Guideline: lack of evidence for MTHFR polymorphism testing. Genetics in Medicine, 2013. doi.org/10.1038/gim.2012.165 · Evidence A
2. Bashford MT et al. Addendum: ACMG Practice Guideline: lack of evidence for MTHFR polymorphism testing. Genetics in Medicine, 2020. doi.org/10.1038/s41436-020-0843-0 · Evidence A
3. Clarke R et al. Homocysteine and coronary heart disease: meta-analysis of MTHFR case-control studies, avoiding publication bias. PLoS Medicine, 2012. doi.org/10.1371/journal.pmed.1001177 · Evidence A
4. Holmes MV et al. Effect modification by population dietary folate on the association between MTHFR genotype, homocysteine, and stroke risk: a meta-analysis of genetic studies and randomised trials. Lancet, 2011. doi.org/10.1016/S0140-6736(11)60872-6 · Evidence A
5. Frosst P et al. A candidate genetic risk factor for vascular disease: a common mutation in methylenetetrahydrofolate reductase. Nature Genetics, 1995. doi.org/10.1038/ng0595-111 · Evidence C
6. Vilas Boas W et al. Metabolism and gene polymorphisms of the folate pathway in Brazilian women with history of recurrent abortion. Revista Brasileira de Ginecologia e Obstetrícia, 2015. doi.org/10.1590/SO100-720320140005223 · Evidence C