SHMT1 Genotype

Folate does not work the same way in every body. The enzyme called SHMT1 (serine hydroxymethyltransferase 1) helps your cells turn folate and B vitamins into the raw materials for DNA, methylation, and repair. Small inherited differences in the SHMT1 gene change how well that machinery runs, and those differences have been linked in research studies to risks for several cancers, cardiovascular disease, and developmental problems in pregnancy. The evidence is mostly from individual case-control and cohort studies, and SHMT1 genotyping is not currently part of any clinical guideline for cancer screening, cardiovascular risk, or prenatal planning.

This test reads the specific letters of DNA at the SHMT1 gene. The result is fixed for life, but its meaning plays out across decades, shaping how your body handles folate intake, how it pairs with other gene variants like MTHFR, and how it interacts with environmental exposures. Knowing your genotype gives you research-grade context for the decisions where folate biology may matter.

What This Test Reads

SHMT1 sits in a chemical assembly line that scientists call one-carbon metabolism. Its job is to convert serine and the active form of folate (tetrahydrofolate) into glycine and another folate compound (5,10-methylenetetrahydrofolate) your cells then use to make DNA building blocks (thymidylate and purines) and to make methionine, the molecule that drives DNA methylation. Methylation is how your cells decide which genes to turn on or off.

Three SHMT1 variants are most studied. The first, called C1420T or rs1979277 (also written as Leu474Phe), changes one letter of the gene, which changes one amino acid in the enzyme and shifts how active it is. The second, rs9909104, sits in a non-coding part of the gene but still tracks with disease risk in some studies. The third, rs4925166, behaves similarly. Different ancestry groups carry these variants at different rates, and the clinical meaning depends on which combination you carry.

Cancer Risk

SHMT1 variants have been linked to several cancers in individual studies, sometimes raising risk and sometimes lowering it. Importantly, larger meta-analyses pooling these studies have often found no significant overall association between SHMT1 C1420T and cancer risk, with the most reproducible signal being a modest protective effect in leukemia and in Asian populations. The findings below come from individual studies; treat them as suggestive, not settled.

Sources: Skibola et al. 2002; Hishida et al. 2003; Wang et al. 2007; Kelemen et al. 2008; Collin et al. 2009; Zhong et al. 2014; Wang Q et al. 2014; Kelemen et al. 2014; Yang et al. 2022; Meng et al. 2025.

What this means for you: a single SHMT1 result will not tell you whether you will get cancer, and no clinical guideline currently recommends using SHMT1 genotyping to guide cancer screening. It can be one piece of context to discuss with your doctor alongside your other risk factors and family history.

Reconciling the Opposite Directions

You may notice that the same C1420T T allele appears to lower leukemia and lymphoma risk in some studies while slightly raising prostate cancer risk in others. One theoretical framework, not yet a proven mechanism, is that SHMT1 shifts how folate is routed inside the cell: the same shift can protect cells in one tissue (where DNA synthesis is the rate-limiting step) and stress them in another (where methylation balance matters more). When reading any single result, focus on the disease categories where the evidence is strongest for your individual situation, not on the variant itself as universally good or bad.

Heart Disease Risk

SHMT1 does not work alone. In the Nurses' Health Study, the cardiovascular risk linked to the common MTHFR C677T variant was stronger in people who also carried the SHMT1 rs1979277 TT genotype. Carrying the SHMT1 variant by itself was not enough to raise heart risk in that study, but the combination of both variants pushed risk higher. The same interaction was not seen in the Health Professionals Follow-Up Study, so the finding is not yet consistent across cohorts. Separate work in men in the Normative Aging Study found a similar interaction between SHMT1 and MTHFR variants, with higher homocysteine in carriers of both.

What this means for you: if you carry both a known SHMT1 and a known MTHFR variant, your standard lipid panel may be only part of your cardiovascular picture. Homocysteine, folate, and vitamin B12 levels can be useful companion measurements, though SHMT1 genotyping is not part of routine cardiovascular risk stratification.

Pregnancy and Developmental Risk

Folate metabolism is the backbone of healthy fetal development, so SHMT1 variants show up in pregnancy outcomes. A Chilean case-control study of 417 participants found that the rs1979277 A allele was associated with lower risk of nonsyndromic cleft lip with or without cleft palate. A separate 2,824-participant study reported that maternal carriers of the rs9909104 GG or AG genotype had higher risk of congenital heart defects in offspring when SSRIs were used around conception (about 5.9-fold higher with GG and 2.4-fold higher with AG). This is a single observational study with a gene-environment interaction that has not been independently replicated.

What this means for you: if you are planning a pregnancy and you take an SSRI or carry one of these variants, this is information worth bringing to a prenatal genetic counselor before conception, not after. SHMT1 genotyping is not part of standard prenatal genetic screening.

Other Conditions

A genome-wide study of more than 15,000 people identified SHMT1 as one of several new susceptibility loci for multiple sclerosis, with replication in a Sardinian cohort. SHMT1 is not among the 233 established MS risk loci identified in the much larger International Multiple Sclerosis Genetics Consortium GWAS of more than 47,000 cases, so this finding remains unconfirmed in the largest dataset. A study of childhood ALL survival found that the rs9909104 TC genotype was linked to lower survival compared with TT. And in a study of 49 astronauts, the SHMT1 C1420 C allele (the wild-type, not the T allele) together with MTRR genotype and B-vitamin status helped predict vision changes during spaceflight; a follow-up bed rest study supported the association. These are association findings, not actionable diagnoses on their own.

How SHMT1 Interacts With Your Diet

SHMT1 variants do not act in isolation from what you eat. Carriers of certain SHMT1 genotypes have higher serum folate levels at the same intake, while the lung cancer risk associated with combined SHMT1 variants was more pronounced in people with low total folate intake. Ovarian cancer risk linked to SHMT1 was likewise modified by the availability of one-carbon units from diet. Knowing your genotype gives one input for thinking about how to track folate, vitamin B6, vitamin B9, and vitamin B12 status.

A One-Time Test

This is a once-in-a-lifetime test. Your SHMT1 genotype was set at conception and will be the same whether you test it today, next year, or thirty years from now. There is no benefit to retesting unless a confirmatory method (such as Sanger sequencing after a SNP chip call) is needed to verify an unexpected result.

The value comes from what you do with the result. The phenotypes downstream of SHMT1 (homocysteine, folate, vitamin B12, vitamin B6, and standard cardiovascular and cancer screening markers) are dynamic and do need ongoing tracking. A reasonable cadence is a baseline panel now, a follow-up in 3 to 6 months if you change folate or B-vitamin intake, then at least annually.

What an Unexpected Result Should Trigger

If your SHMT1 result identifies a variant linked to one of the conditions above, the next step is not panic and not retesting the gene. It is a conversation with your physician about the biology around it. Companion labs that some clinicians find useful in this context include homocysteine, vitamin B9 (folate), vitamin B12, and vitamin B6, the inputs your SHMT1 enzyme depends on. If you carry both a known SHMT1 and MTHFR variant, ApoB (apolipoprotein B) and a lipid panel can add to your cardiovascular picture.

If you are planning a pregnancy, bring the result to a prenatal genetic counselor, especially if you take SSRIs or have a family history of cleft lip or congenital heart defects. If you have a personal or family history of leukemia, lymphoma, lung cancer, ovarian cancer, prostate cancer, or thyroid cancer, share the result with the specialist who manages your screening, while keeping in mind that current screening guidelines are not driven by SHMT1 genotype. The SHMT1 result is a piece of context, not a diagnosis. It earns its value by changing the questions you ask elsewhere.

When Results Can Be Misleading

This is a genetic test, so the usual diet, time-of-day, and medication confounders that affect blood biomarkers do not apply. Different caveats matter:

• Variant panel coverage: this assay only detects the specific variants it is designed to detect. A result that does not flag a known risk variant does not rule out rare or untested variants in the same gene.
• Ancestry-specific allele frequency: SHMT1 variants are not distributed equally across populations. The risk attached to a given genotype depends on what is known in your ancestry group, and many studies are based on a single population.
• Direct-to-consumer vs clinical-grade testing: if you have seen a SHMT1 call from a consumer genomics report, the analytic accuracy may differ from a clinical-grade genotyping or sequencing assay. A clinically important result should be confirmed.
• Carrying a variant is not the same as developing the disease: most people carrying SHMT1 risk variants will never develop the associated conditions. The variant shifts probability, not destiny.
• Single-study findings vs pooled evidence: several SHMT1 cancer associations come from individual case-control studies that did not hold up in larger meta-analyses. Treat single-study effect sizes with caution.