What Causes MTHFR Gene Mutations and How Are They Inherited?

MTHFR variants are not caused by anything you did or were exposed to. They are inherited DNA spelling differences, passed from parent to child in a straightforward pattern, present from the moment of conception. The two most studied variants, known as C677T and A1298C, are single-letter changes in the genetic code that reduce the activity of an enzyme involved in processing folate and regulating a compound called homocysteine. Because every person carries two copies of the MTHFR gene, one from each parent, the number of variant copies you inherit shapes how much enzyme activity you retain, and whether that reduction ever matters clinically.

What MTHFR Actually Does in the Body

The MTHFR gene provides instructions for building an enzyme called methylenetetrahydrofolate reductase. That enzyme sits at a key junction in your body’s folate-processing pathway, converting one form of folate into another form your cells need for two critical jobs: recycling homocysteine back into methionine (an amino acid) and supplying the chemical building blocks for DNA and protein synthesis.¹ When the enzyme works at full speed, homocysteine stays in balance and cells have enough of the active folate form (called 5-methyltetrahydrofolate, or 5-MTHF) to keep methylation running smoothly. When the enzyme is sluggish, homocysteine can accumulate and the supply of active folate drops.

The gene itself sits on chromosome 1, and the protein it encodes weighs about 70 kilodaltons. Researchers have catalogued more than a dozen rare mutations that cause severe enzyme deficiency, plus the two common polymorphisms that produce milder reductions in activity.² When people say they “have an MTHFR mutation,” they almost always mean one of those two common variants rather than the rare, clinically severe forms.

The Two Common Variants and What Makes Them Different

C677T and A1298C are both single-nucleotide changes. In C677T, a cytosine is swapped for a thymine at position 677 in the gene, which changes one amino acid in the enzyme (alanine becomes valine). In A1298C, an adenine becomes a cytosine at position 1298, swapping glutamic acid for alanine. Both substitutions reduce how well the enzyme works, though they do so to different degrees and affect different parts of the protein.³

C677T gets the most attention because its effect on enzyme activity is larger. People with two copies of the T version (homozygous TT) produce an enzyme that is thermolabile, meaning it loses function more readily at body temperature. Enzyme activity in TT individuals drops to roughly 30% of normal. People with one copy of each (CT, or heterozygous) retain more activity, around 65%. The A1298C variant causes a milder reduction, and its clinical significance on its own is less clear. Having one copy of each variant simultaneously, called compound heterozygosity, can also reduce enzyme function, though usually less dramatically than being homozygous for C677T.

How These Variants Are Inherited

MTHFR variants follow standard autosomal inheritance. The gene is on chromosome 1, not on a sex chromosome, so the pattern is the same regardless of sex. You get one copy from your mother and one from your father. If both parents carry the C677T variant (each being CT), each of their children has a one-in-four chance of inheriting two T copies, a one-in-two chance of inheriting one, and a one-in-four chance of inheriting none.

The situation gets more complex because the two variant positions are on the same gene. A person can carry a C677T variant on one copy and an A1298C variant on the other, producing compound heterozygosity. In rare cases, patients have been found to carry both common variants alongside rarer severe mutations on the same gene copies, which compounds the effect on enzyme function.⁴ The practical takeaway is that MTHFR status is not binary. It is a spectrum of enzyme activity shaped by which combination of variants, if any, you inherited from each parent.

Why These Variants Are So Common

A natural question is why evolution hasn’t weeded these variants out if they reduce enzyme function. The answer appears to be that the T allele at position 677 has actually been positively selected for in some populations. Research on genetic selection patterns has detected evidence that the T allele confers a survival advantage in early embryonic development, increasing fetal viability in certain contexts.⁵ This kind of balancing selection, where a variant has both costs and benefits, is a well-known pattern in human genetics. The variant persists because, under certain dietary and environmental conditions, it provided enough of an advantage to keep spreading through the population.

The result is that MTHFR variants are extremely prevalent worldwide, but their frequency varies dramatically by ethnicity and geography. In U.S. primary care populations, striking ethnogeographic differences have been documented: Hispanic and Caucasian populations carry elevated rates of C677T and A1298C respectively, while African Americans appear relatively protected from MTHFR deficiency.⁶ A study of women from different ethnic backgrounds found that about 18% of Mexican women had the homozygous TT genotype at C677T, compared with about 7% of white women, roughly 4% of Asian women, and 0% of the African American women sampled.⁷ Within China, the frequencies of both variants show geographical gradients, varying between regions and ethnic groups.⁸

These numbers matter because they mean many people walking around with MTHFR variants have no idea and experience no obvious health consequences. The variant is common enough that it cannot be treated as an automatic disease risk. Whether it matters for any given person depends heavily on other factors, especially diet.

The Homocysteine Connection

The most well-documented biochemical effect of reduced MTHFR activity is elevated homocysteine. Homocysteine is an amino acid that your body normally recycles back into methionine, a process that requires the active folate form produced by MTHFR. When the enzyme underperforms, this recycling slows down and homocysteine levels can rise.

But here is the part that changes everything: the effect of MTHFR variants on homocysteine is almost entirely dependent on your folate status. A landmark study found that among people with lower blood folate levels, those homozygous for the C677T variant had fasting homocysteine levels about 24% higher than people with no variant copies. But among people whose folate levels were adequate, this difference between genotypes disappeared entirely.⁹ More recent work has confirmed this pattern, showing that the difference in homocysteine between genotypes is eliminated once blood folate reaches a sufficient threshold.¹⁰

In other words, MTHFR variants load the gun, but low folate intake pulls the trigger. People with the TT genotype who eat plenty of folate-rich foods or supplement appropriately tend to maintain normal homocysteine levels. People with the same genotype on a folate-poor diet are much more likely to develop elevated homocysteine, which has downstream associations with cardiovascular and other health risks.

Folate, Methylation, and the Ripple Effects

Beyond homocysteine, reduced MTHFR activity constrains the supply of S-adenosylmethionine (SAM), the molecule your cells use as a methyl donor for hundreds of biochemical reactions, including the methylation of DNA. DNA methylation is one of the main ways your body controls which genes are turned on or off. When SAM supply drops because of impaired MTHFR function combined with low folate availability, this methylation machinery can become disrupted. Research has linked this to changes in gene expression patterns relevant to immune function and autoimmune conditions.¹¹

The MTHFR gene itself can be regulated by epigenetic modifications. The gene contains regions where methylation of the DNA can influence how actively it is read by the cell’s machinery, with transcription factors binding differently depending on whether those sites are methylated or not.¹² This creates a feedback loop: the gene’s variants reduce the enzyme that helps supply methyl groups, and the gene’s own activity can be further modified by the methylation environment it helped create.

Health Associations People Worry About

MTHFR variants have been linked in research to a range of health conditions, though the strength and clinical significance of these associations varies. The evidence is strongest in a few areas.

Neural Tube Defects and Pregnancy

The connection between MTHFR and neural tube defects like spina bifida was one of the earliest findings to draw clinical attention. Being homozygous for C677T (TT) has been associated with a modestly increased odds of spina bifida, with one study estimating an odds ratio of about 1.6.¹³ A meta-analysis looking at maternal MTHFR status and neural tube defects in offspring found a similar elevation in risk for the TT genotype.¹⁴ This is part of why folic acid supplementation before and during early pregnancy is universally recommended, and it is especially relevant for women with MTHFR variants. The interaction between MTHFR and other folate-pathway genes, like BHMT, may further modify the risk of neural tube defects when the T allele is transmitted to the child.¹⁵

Depression and Psychiatric Conditions

Multiple meta-analyses have found that the C677T variant is associated with an increased risk of major depression. A large meta-analysis reported that the TT genotype was associated with about a 36% higher odds of unipolar depression compared to the CC genotype, with no significant inconsistency between the studies, based on over 1,200 cases and more than 10,000 controls.¹⁶ A more recent meta-analysis found a somewhat larger effect, with the TT genotype associated with about 66% higher odds of major depression compared to CC.¹⁷ Associations with schizophrenia and bipolar disorder have also been reported, though these findings are more variable across studies. The hypothesized mechanism involves the downstream effects on methylation and neurotransmitter metabolism.

Cardiovascular Risk and Blood Clots

Elevated homocysteine has long been considered a risk factor for cardiovascular disease, and MTHFR variants that raise homocysteine levels, particularly in the context of low folate, have been implicated. In hypertensive patients, carriers of the T allele at C677T had modestly higher homocysteine levels than C allele carriers.¹⁸ A case series of neonates who developed blood clots found that every patient in the unit who developed a clot tested positive for MTHFR variants, though many had normal homocysteine levels, suggesting the relationship between MTHFR and clotting is not straightforward.¹⁹

It is worth noting that the cardiovascular field has grown more cautious about MTHFR over the past decade. Large trials of homocysteine-lowering therapy have generally not shown reductions in heart attacks or strokes, which has led many cardiologists to view elevated homocysteine as more of a biomarker than a direct cause. The MTHFR variant may contribute to cardiovascular risk through mechanisms beyond homocysteine alone, but the clinical evidence for routine screening based on heart risk is weak.

Folate Supplementation and the Form That Matters

For people with MTHFR variants, the practical question is usually about supplementation. Standard folic acid, the synthetic form found in most prenatal vitamins and fortified foods, must be converted through several enzymatic steps, including the one catalyzed by MTHFR, to become the active 5-MTHF form. If your MTHFR enzyme is underperforming, this conversion is slower and less efficient.

L-5-methyltetrahydrofolate (often sold as L-methylfolate) is the already-active form that bypasses the MTHFR step entirely. Pharmacokinetic studies have found that L-methylfolate and folic acid have comparable bioavailability and absorption at equivalent doses in the general population.²⁰ But for people with MTHFR variants, the case for methylfolate is different. Some clinicians have argued that standard folic acid should be replaced with methylfolate for carriers of MTHFR variants, particularly during pregnancy, because it sidesteps the enzymatic bottleneck and avoids the accumulation of unmetabolized folic acid, which has raised its own health concerns.²¹

Meanwhile, yeast studies have shown that multiple less-common MTHFR variants, beyond just C677T and A1298C, can be functionally rescued by increasing folate availability, suggesting that vitamin remediation of impaired enzymes may be a broadly applicable phenomenon.²² This supports the idea that adequate folate, in whatever form, is the single most important modifiable factor for people carrying MTHFR variants.

Drug Interactions Worth Knowing About

MTHFR status can influence how you respond to certain medications, particularly those that interact with folate metabolism. Methotrexate, widely used for cancer, autoimmune diseases, and ectopic pregnancy, works partly by disrupting folate pathways. While methotrexate does not directly inhibit the MTHFR enzyme, changes in MTHFR activity alter how folate is distributed inside cells, which can shift both the drug’s effectiveness and its side effect profile. In children with acute lymphoblastic leukemia treated with methotrexate, the C677T TT genotype was associated with a higher risk of hypokalemia, while the A1298C AA genotype was linked to increased risk of liver toxicity.²³

Fluoropyrimidine chemotherapy drugs, another class that interacts with the folate pathway, are also affected by MTHFR genotype. The same C677T and A1298C variants that alter folate distribution can change how these drugs are processed, potentially influencing both how well they work and how toxic they are.³ Pharmacogenomic testing for MTHFR is gaining traction in oncology precisely because knowing a patient’s genotype could help clinicians adjust dosing or anticipate adverse reactions.

Should You Get Tested

MTHFR testing has become widely available through direct-to-consumer genetic services and can be ordered by any physician. But the clinical utility of routine MTHFR testing is genuinely controversial. Several professional medical organizations have advised against blanket MTHFR testing for conditions like recurrent pregnancy loss or clotting disorders, arguing that the evidence does not support using the genotype alone to guide treatment decisions.²⁴

The reasoning is practical: the most important intervention for people with MTHFR variants, adequate folate intake, is already universally recommended. Testing homocysteine directly tells you whether there is an actual metabolic problem, regardless of genotype. And because MTHFR variants are so common, a positive test result can generate anxiety without changing what a clinician would recommend. Where MTHFR testing has clearer value is in pharmacogenomics, where knowing the genotype before starting drugs like methotrexate can help predict side effects, and in families with a history of severe MTHFR deficiency, the rare form that causes serious neurological symptoms in infancy.

If you do learn you have an MTHFR variant, the evidence consistently points toward the same actionable steps: ensure adequate folate intake (through diet or supplementation), consider methylfolate rather than synthetic folic acid if you are homozygous for C677T or compound heterozygous, and get your homocysteine checked periodically to see if there is a real biochemical issue rather than just a genetic predisposition. The variant is common, it is inherited in a completely predictable way, and for most carriers, it is manageable with something as simple as eating enough leafy greens or taking the right form of a vitamin.