MTHFR C677T β The Methylation Gatekeeper
MTHFR (methylenetetrahydrofolate reductase) is arguably the most talked-about gene
in nutritional genomics, and for good reason. It encodes the enzyme that converts
5,10-methylenetetrahydrofolate into 5-methyltetrahydrofolate | The active form of folate that enters the methylation cycle (methylfolate), the
biologically active form of folate that your body actually uses. Methylfolate is
essential for the methylation cycle | The methylation cycle adds methyl groups to DNA, proteins, and neurotransmitters β essential for hundreds of reactions, which affects DNA repair, neurotransmitter
production, detoxification, and hundreds of other biochemical reactions.
The Mechanism
The C677T variant (rs1801133) causes an alanine-to-valine substitution | Alanine-to-valine substitution at position 222 of the enzyme (p.Ala222Val) at position
222 of the MTHFR enzyme. This makes the enzyme thermolabile | Thermolabile: the enzyme loses stability and function at normal body temperature β it loses activity at
body temperature. The AA genotype | TT on the coding strand β 23andMe reports the complementary strand retains only about 30%
of normal enzyme activity, while the AG genotype | CT on the coding strand retains about 65%. This means
less dietary folate and supplemental folic acid gets converted to the methylfolate
your cells need.
The Evidence
The C677T variant is one of the most extensively studied genetic variants in human
biology. A meta-analysis of over 80 studies | Wen YY et al. Meta-analysis across 82 studies confirming the MTHFR-homocysteine link confirmed that the TT genotype is
associated with 25% higher homocysteine levels when folate intake is low. Elevated
homocysteine is an independent risk factor for cardiovascular disease | Mangoni AA & Jackson SHD. Homocysteine and cardiovascular disease. Am J Med, 2002, neural tube
defects, and possibly cognitive decline. However, the key finding is that adequate
folate intake essentially normalizes homocysteine in most TT individuals. A
large meta-analysis | Clarke R et al. Homocysteine and coronary heart disease meta-analysis, 2012 found a 15%
excess coronary heart disease risk in TT homozygotes compared to CC homozygotes.
The Folic Acid Question
Synthetic folic acid (found in fortified foods and cheap supplements) must be
converted by MTHFR to become active methylfolate. If your MTHFR is working at
only 30% capacity, this conversion is a bottleneck. Methylfolate supplements
bypass this step entirely, which is why they are often recommended for people with
the TT genotype. Riboflavin (vitamin B2) is an essential cofactor for MTHFR and
has been shown to lower blood pressure | McNulty H et al. showed riboflavin 1.6mg/day lowers blood pressure in MTHFR TT individuals by stabilizing the thermolabile enzyme
in TT individuals by stabilizing the thermolabile enzyme.
Practical Implications
The MTHFR C677T variant is extremely common β about 10-15% of Europeans are TT
and about 40% are CT. It is not a disease-causing mutation. With adequate folate
(especially as methylfolate), B12, B2, and B6 intake, most people with the TT
genotype function perfectly normally. The key is knowing your status so you can
optimize your B vitamin strategy.
Interactions
The C677T variant interacts importantly with the A1298C variant (rs1801131) β
compound heterozygosity (one copy of each) can reduce MTHFR activity to 40-50%.
It also interacts with SLC19A1 (rs1051266), which controls folate transport into
cells, and COMT (rs4680), which determines tolerance for methyl donors.
Methotrexate, an antifolate drug, has increased toxicity in C677T carriers.
All genotypes
Normal MTHFR enzyme activity
You have the common CC genotype at MTHFR C677T. Your enzyme activity is normal (100%), meaning you efficiently convert dietary folate and folic acid into methylfolate, the active form your body uses. About 45% of Europeans share this genotype.
One MTHFR variant β mildly reduced activity
You carry one copy of the C677T variant (CT genotype). Your MTHFR enzyme works at about 65% efficiency, which is usually adequate for most purposes. This is very common β about 40% of people have this genotype. Most people with CT function perfectly normally.
Two MTHFR variants β significantly reduced activity (~30%)
You have two copies of the C677T variant (TT genotype). Your MTHFR enzyme works at about 30% efficiency. This means you convert folate to methylfolate less efficiently than average. About 10-15% of Europeans have this genotype. It's associated with higher homocysteine levels if folate intake is inadequate.