MTHFR performs a specific folate-cycle reaction: it reduces 5,10-methylene-THF to 5-MTHF. This is the only enzyme reaction that produces 5-MTHF from 5,10-methylene-THF. The resulting 5-MTHF helps recycle homocysteine into methionine.
C677T and A1298C do not have the same effect. C677T makes the enzyme less stable, weakens FAD binding, and lowers activity, especially with two T alleles. A1298C is also common, but current public health guidance says there is not enough evidence that A1298C alone significantly changes folate processing.
A genotype cannot show current folate, vitamin B12, or homocysteine levels. It also cannot explain symptoms, clotting risk, pregnancy loss, or supplement need by itself. CDC states that people with common MTHFR variants can process folic acid and that folic acid intake affects blood folate more than genotype does.
What the MTHFR gene does
MTHFR is the gene symbol for methylenetetrahydrofolate reductase. The enzyme carries out the physiologically irreversible reduction of 5,10-methylene-THF to 5-MTHF, committing those folate-bound one-carbon units to the methionine cycle.
5-MTHF supplies a methyl group to methionine synthase, which also requires vitamin B12. That reaction converts homocysteine to methionine and releases THF back into the folate cycle. Reduced MTHFR activity can therefore limit both 5-MTHF supply and THF regeneration through this route, while more folate remains in non-methyl forms such as 5,10-methylene-THF.
This is a bottleneck and a redistribution of folate forms, not a complete chemical trap. 5,10-methylene-THF can still support thymidylate synthesis or interconvert with other folate forms. In a small red-blood-cell study, some people with 677TT had higher proportions of both THF and 5,10-methylene-THF, so the genotype does not automatically mean that total cellular THF is low.
Why C677T and A1298C get attention
C677T and A1298C are legacy cDNA names that remain common in reports and papers. On transcript NM_005957.5, their current HGVS names are c.665C>T and c.1286A>C. Coordinates depend on the transcript, so use the rsID, transcript accession, and HGVS name together when matching a result to a database record.
| Common name | dbSNP ID | HGVS on NM_005957.5 | Protein change |
|---|---|---|---|
| C677T | rs1801133 | NM_005957.5:c.665C>T | p.Ala222Val |
| A1298C | rs1801131 | NM_005957.5:c.1286A>C | p.Glu429Ala |
These are common polymorphisms. They are not the same as rare severe MTHFR deficiency, which involves pathogenic variants that can cause homocystinuria and very different clinical findings.
C677T in plain language
C677T changes alanine to valine at position 222 in the enzyme's catalytic domain, written p.Ala222Val. MTHFR normally forms a dimer and holds a FAD cofactor derived from riboflavin.
Human cell extracts from people with the TT genotype have been reported at about 30% of the activity measured in CC extracts. This is an assay-level group estimate, not a prediction that every person with TT has exactly 30% activity.
This is a real biochemical effect. It still should not be used by itself to diagnose a condition, predict a clot, explain pregnancy loss, or choose a supplement plan. CDC guidance notes that people with the 677TT genotype can still process folic acid, and that folic acid intake is more important than MTHFR genotype for blood folate levels.
A1298C in plain language
A1298C changes glutamate to alanine at position 429 in the enzyme's regulatory domain, written p.Glu429Ala. In purified recombinant enzyme, Yamada and colleagues found no measurable difference from the standard enzyme in catalytic activity, thermolability, or FAD release. Earlier cell-extract studies reported lower activity, so the isolated effect is less consistent rather than absent.
CDC concludes that evidence is insufficient to show that A1298C alone significantly changes folate processing. When both variants are heterozygous, an unphased DNA file usually cannot show whether the changed alleles are on the same chromosome or opposite chromosomes. Do not call the result compound heterozygous unless phase has been established.
What an MTHFR result cannot tell you
A raw DNA result cannot show current folate status, vitamin B12 status, homocysteine level, symptoms, pregnancy risk, medication response, or whether a variant is causing a health problem. It also cannot show whether every relevant MTHFR position was tested.
The ACMG Clinical Practice Resource says common MTHFR polymorphism testing has minimal clinical utility and should not be included in routine thrombophilia evaluation. This recommendation concerns the clinical use of the test. It does not mean that C677T has no biochemical effect.
How Gene Inspector Pro looks deeper
Gene Inspector Pro connects the legacy C677T or A1298C label to the exact rsID, transcript, HGVS name, protein change, and genotype. This prevents a common label from being mistaken for a complete description of the finding.
For sequencing files, GIP can surface read depth, genotype quality, coverage, and nearby variants when the file provides them. For SNP arrays, only selected positions were tested and read-level confirmation is unavailable. If both common variants are heterozygous, inheritance data may help establish phase.
A full review also checks rarer MTHFR variants and separates four questions: what the file measured, whether the call is credible, what the variant evidence shows, and whether current labs support a folate or homocysteine concern.
MTHFR review checklist
| Question | What to check | Why it matters |
|---|---|---|
| Which variant is it? | Common name, rsID, HGVS name, and genotype. | C677T, A1298C, and rare MTHFR variants should not be grouped together. |
| How was it measured? | SNP array, WGS, WES, or another source file. | The file type affects coverage, call quality, and what may be missing. |
| Is it common? | Population frequency in dbSNP or similar datasets. | Common variants are usually reviewed as context, not as single-gene diagnoses. |
| What does ClinVar say? | Classification, number of submissions, and review status. | Database labels need evidence level and agreement checks. |
| Is phase known? | Whether C677T and A1298C are on the same or opposite gene copies. | Unphased calls should not be labeled compound heterozygous. |
| What is the clinical question? | Folate status, homocysteine, pregnancy planning, medication review, or another reason. | The same genotype can matter differently depending on the question. |
Related review pages
For evidence context, read allele frequency and MAF in variant review, what a missense variant is, and how inheritance tracking helps establish phase. For file limits, compare SNP arrays with WGS and WES files.
Sources
- CDC, MTHFR Gene Variant and Folic Acid Facts, for folic acid guidance, C677T and A1298C plain-language descriptions, and evidence limits.
- MedlinePlus Genetics, MTHFR gene, for gene function, homocystinuria distinction, and common polymorphism context.
- NCBI Gene, MTHFR, for the official gene symbol, genomic context, and RefSeq summary.
- Frosst et al., Nature Genetics, 1995, for the original C677T report, reduced MTHFR activity, thermolability, and homocysteine association.
- Yamada et al., PNAS, 2001, for recombinant-enzyme data on Ala222Val, FAD loss, monomer dissociation, and Glu429Ala behavior.
- Huang et al., Rapid Communications in Mass Spectrometry, 2008, for measured red-blood-cell folate forms across MTHFR 677C>T genotypes.
- UniProtKB P42898, human MTHFR, for MTHFR function, FAD cofactor annotations, and the rs1801133 functional annotation.
- NCBI dbSNP, rs1801133, for the C677T reference SNP record.
- NCBI dbSNP, rs1801131, for the A1298C reference SNP record.
- Hickey, Curry, and Toriello, ACMG Practice Guideline: lack of evidence for MTHFR polymorphism testing, for clinical utility cautions around routine thrombophilia testing.
- ACMG addendum, 2020, for the reaffirmation of the MTHFR polymorphism-testing guideline.
- Brown et al., Genetics in Medicine, 2005, for the inability of routine unphased genotyping to distinguish cis from trans C677T and A1298C arrangements.