MTHFR C677T and A1298C Together: Why Both Genotypes Matter

By Sequencing Team, The team of bioinformaticians, Genetic Health Coaches, and writers at Sequencing.

Short answer: A combined MTHFR result uses the genotypes at two positions, rs1801133 (C677T) and rs1801131 (A1298C). Neither one gives the full picture alone. The activity categories below are population averages, not a diagnosis, and medical guidelines advise against routine MTHFR testing to assess clotting risk.

Genetic results are often presented one variant at a time: one rsID, one genotype, one annotation. That works when the meaning of a result depends on a single position.

Some familiar genetic results, however, are based on the combination of genotypes at two or more positions. Neither position provides the complete answer by itself.

APOE status and combined MTHFR activity estimates are two useful examples. In both cases, a system must find the genotype at each required position and interpret them together.

One genotype may provide part of the result. The combination provides the meaning.

One Position Is Only Part of the Answer

A genotype records the two bases observed at a particular genomic position. For example, a result might report CT at one rsID.

If a health or trait interpretation depends on two rsIDs, knowing only that CT genotype is incomplete. The genotype at the second position may place the result into a different category.

This means a combined result requires:

the correct genotype at the first position
the correct genotype at the second position
a lookup or rule that interprets the two genotypes together

MTHFR: Two Genotypes Inform the Activity Category

Two common MTHFR variants are also frequently considered together:

rs1801133, NM_005957.5:c.665C>T, historically called C677T
rs1801131, NM_005957.5:c.1286A>C, historically called A1298C

These variants can reduce average MTHFR enzyme activity. A combined result is more informative than either genotype alone because the expected activity category depends on both.

The list below uses genomic forward-strand genotypes. In this orientation, rs1801133 G>A corresponds to the historical C677T change, while rs1801131 T>G corresponds to A1298C. Each line gives the rs1801133 genotype, the rs1801131 genotype, the historical combined genotype, and the expected activity category.

rs1801133 AA and rs1801131 GT (677TT/1298AC): Strongly decreased; evidence is insufficient for a reliable percentage.
rs1801133 AG and rs1801131 GG (677CT/1298CC): Markedly decreased; evidence is insufficient for a reliable percentage.
rs1801133 AA and rs1801131 TT (677TT/1298AA): Strongly decreased; approximately 70–80% lower than typical.
rs1801133 AG and rs1801131 TT (677CT/1298AA): Moderately decreased; approximately 35–40% lower than typical.
rs1801133 GG and rs1801131 GG (677CC/1298CC): Mildly decreased; approximately 20% lower than typical.
rs1801133 GG and rs1801131 GT (677CC/1298AC): Slightly decreased; less than approximately 20% lower than typical.
rs1801133 AG and rs1801131 GT (677CT/1298AC): Markedly decreased; approximately 50% lower than typical.
rs1801133 GG and rs1801131 TT (677CC/1298AA): Typical activity.
rs1801133 AA and rs1801131 GG (677TT/1298CC): Likely incorrect or extremely rare; verify before interpretation.

Activity estimates are approximate population averages. They do not measure enzyme activity in an individual and do not diagnose a medical condition. Folate and vitamin B12 status, medications, health conditions, lifestyle, and other genetic factors can influence homocysteine and the practical effect of these common variants.

Why Both MTHFR Genotypes Matter

Consider a result of AG at rs1801133, corresponding to 677CT. The expected activity category depends on the genotype at rs1801131:

with TT at rs1801131, activity is moderately decreased
with GT at rs1801131, activity is markedly decreased
with GG at rs1801131, the combination is uncommon and should be interpreted cautiously

Again, the genotype at the first position is unchanged. The combined interpretation changes when the genotype at the second position changes.

Common MTHFR variants are not equivalent to the rare pathogenic variants that cause severe MTHFR deficiency. The CDC notes that people with a common MTHFR variant can process all types of folate, including folic acid. Read our MTHFR gene guide for what medical guidelines say about testing.

APOE: Two rsIDs Create One Status

The common APOE forms, ε2, ε3, and ε4, are determined using two positions: rs429358 and rs7412.

Neither rsID alone establishes a person's APOE status. The genotypes at both positions must be combined. Each line gives the rs429358 genotype, the rs7412 genotype, the APOE status, and the approximate Alzheimer's disease risk category.

rs429358 CC and rs7412 CC (ε4/ε4): High; approximately 30–55% lifetime risk.
rs429358 CT and rs7412 CC (ε3/ε4): Increased; approximately 20–25% lifetime risk.
rs429358 TT and rs7412 CC (ε3/ε3): Average; approximately 10–15% lifetime risk.
rs429358 TT and rs7412 CT (ε2/ε3): Average to reduced; below approximately 10–15%.
rs429358 CT and rs7412 CT (ε2/ε4): Increased; approximately 20–25% lifetime risk.
rs429358 TT and rs7412 TT (ε2/ε2): Reduced; below approximately 10–15%.
rs429358 CC and rs7412 TT (No standard status assigned): Verify the genotype calls and strand orientation.

These percentages are broad population estimates, not individualized predictions. APOE-associated risk varies with age, sex, ancestry, family history, lifestyle, other genetic factors, and the population used to calculate risk. APOE ε4 increases susceptibility; it does not diagnose Alzheimer's disease or make the disease inevitable.

Why both APOE genotypes matter

Consider a result of TT at rs429358. That result alone could contribute to several different APOE statuses:

with CC at rs7412, the result is ε3/ε3
with CT at rs7412, the result is ε2/ε3
with TT at rs7412, the result is ε2/ε2

The genotype at rs429358 has not changed. The combined APOE status changes because the genotype at rs7412 is different.

This is why neither rs429358 nor rs7412 should be described as an “APOE ε4 result” on its own. APOE status comes from the two-position lookup.

What a Reliable Combined Interpretation Requires

A confident call at every required position. A no-call is not the same as a reference genotype.
A consistent genome build and strand orientation. The allele letters must be interpreted in the correct coordinate system.
A validated lookup table. The system must map each supported genotype pair to the correct combined status.
The underlying genotypes alongside the derived result. Readers should be able to see how the combined status was calculated.
A verification path for unusual combinations. Unexpected genotype pairs should be reviewed rather than forced into a familiar category.

Key Takeaways

Some genetic interpretations require genotypes from more than one position.
Knowing one genotype does not establish the combined result.
APOE status requires both rs429358 and rs7412.
Combined MTHFR activity estimates use both rs1801133 and rs1801131.
Combined labels may not appear as individual records in Genome Explorer, NGDS, or a variants-only VCF.
Reliable reporting should show both component genotypes and the combined interpretation.

MTHFR Gene: Variants, Testing and What Your DNA Can Tell You: the guide this article belongs to.
Why MTHFR C677T Can Be Hard to Find in Genome Explorer: how the same variant appears under different names.
MTHFR Thermolabile Polymorphism (C677T): the common variant in more detail.
Decoding Elevated Homocysteine Levels: how homocysteine is measured and what raises it.

Get Tested With Whole Genome Sequencing

Whole genome sequencing reads 100% of your DNA, including over 30,000 genes, so both MTHFR positions and the APOE positions are part of your data. Sequencing's Kit Bundles include a Whole Genome Sequencing kit and Next Gen Disease Screening, which analyzes your genome for more than 15,000 conditions. Activity categories and risk estimates are averages, and guidelines advise against relying on a common MTHFR variant alone, so bring your results to a clinician.

Shop All Whole Genome Sequencing Kit Bundles

Frequently Asked Questions About Combined MTHFR Results

What does a combined MTHFR genotype mean?

A combined MTHFR result uses the genotypes at two positions, rs1801133 (C677T) and rs1801131 (A1298C). The expected activity category depends on both, so one genotype alone does not give the full result.

What does it mean to carry one copy of C677T and one copy of A1298C?

In the genomic forward-strand display, that is AG at rs1801133 and GT at rs1801131, written 677CT/1298AC in older articles. The expected activity category is markedly decreased, about 50% lower than typical on average.

Does a combined MTHFR result diagnose a condition?

No. The activity estimates are approximate population averages. They do not measure enzyme activity in an individual and do not diagnose a medical condition.

Do I need MTHFR testing?

Medical guidelines advise against routine MTHFR polymorphism testing. The ACMG guideline concluded it has minimal clinical use and should not be ordered as part of a routine evaluation for thrombophilia. Ask your clinician whether any testing fits your situation.

How do I find both MTHFR variants in Genome Explorer?

Search for rs1801133 and rs1801131. In the genomic display, C677T appears as G>A and A1298C appears as T>G. A combined label may not appear as its own record, so check both genotypes.

Does Sequencing test for MTHFR?

Sequencing's whole genome sequencing reads 100% of your DNA, which includes the MTHFR gene, so both variants appear in your data. Review any result with a doctor or genetic counselor.

References

Hickey SE, Curry CJ, Toriello HV. ACMG practice guideline: lack of evidence for MTHFR polymorphism testing. Genet Med. 2013;15(2):153-156. PubMed 23288205
Centers for Disease Control and Prevention. MTHFR gene variant and folic acid facts. cdc.gov
MedlinePlus Genetics. MTHFR gene. medlineplus.gov/genetics/gene/mthfr

This article is for education and is not a substitute for professional medical advice, diagnosis, or treatment. Talk to a qualified healthcare provider about your own health.