rs1979277 — SHMT1 C1420T

Alters one-carbon metabolism and folate distribution; influences cancer risk and folate levels

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SHMT1 C1420T — A Folate Metabolism Variant with Complex Health Effects

The SHMT1 gene encodes serine hydroxymethyltransferase 1, a pyridoxal phosphate (vitamin B6)-dependent enzyme11 pyridoxal phosphate (vitamin B6)-dependent enzyme
SHMT1 requires vitamin B6 as a cofactor that sits at a critical junction in one-carbon metabolism22 one-carbon metabolism
the folate cycle that provides methyl groups for DNA synthesis, repair, and methylation. SHMT1 converts serine and tetrahydrofolate into glycine and 5,10-methylenetetrahydrofolate, supplying one-carbon units for thymidylate synthesis (DNA building blocks) and methylation reactions. The C1420T variant causes a leucine-to-phenylalanine substitution at position 474 of the protein, altering its cellular localization and affecting how efficiently the enzyme channels one-carbon units through different metabolic pathways.

The Mechanism

The Leu474Phe amino acid change affects the enzyme's cellular distribution33 affects the enzyme's cellular distribution
The T allele alters SHMT1 localization within cells, reducing availability of 5,10-methylenetetrahydrofolate rather than simply destroying its activity. SHMT1 normally shuttles between the cytoplasm and nucleus during DNA replication, providing one-carbon units directly where they're needed. The 1420T variant appears to disrupt this trafficking, causing the enzyme to favor certain metabolic routes over others. Specifically, it may reduce the supply of methylenetetrahydrofolate44 reduce the supply of methylenetetrahydrofolate
substrate required by the MTHFR enzyme that converts it to methylfolate for homocysteine remethylation. This creates a metabolic bottleneck: less substrate available for MTHFR, potentially leading to elevated homocysteine if folate intake is marginal.

The Evidence

The most robust data come from cancer association studies55 cancer association studies
Multiple meta-analyses with tens of thousands of participants. A meta-analysis of 7,309 lymphoma patients66 meta-analysis of 7,309 lymphoma patients
3,232 cases and 4,077 controls across eight studies found the TT genotype modestly increases non-Hodgkin lymphoma risk (OR = 1.18), with borderline significance. Mechanisms likely involve impaired DNA synthesis or methylation during rapid immune cell proliferation.

Conversely, a meta-analysis of 14,409 solid tumor cases77 meta-analysis of 14,409 solid tumor cases
14,409 cases and 16,996 controls from 23 studies revealed the TT genotype protects against breast cancer in Asian populations (OR = 0.79, p = 0.003), though not in Caucasians. For acute lymphoblastic leukemia88 acute lymphoblastic leukemia
Univariate analysis in adults, the TT genotype conferred a striking 3.3-fold risk reduction (OR = 0.31). These protective effects may arise from altered folate partitioning that reduces availability of nucleotides for rapidly dividing cancer cells.

Practical Implications

If you carry one or two T alleles, your folate metabolism is functional but may be less efficient at certain steps, particularly the conversion of folate intermediates into methylfolate for homocysteine metabolism. The clinical relevance depends heavily on context — your folate intake, the status of other folate-cycle genes (especially MTHFR), and which tissues are most metabolically active.

Prioritize methylfolate over synthetic folic acid, especially if you also carry MTHFR 677T or 1298C variants.

The cancer associations are complex and context-dependent. The T allele may increase risk for blood cancers involving rapid immune cell division, but appears protective against some solid tumors. This isn't a call to avoid or embrace the variant — it's fixed in your DNA — but rather a reminder that optimal folate status matters. Adequate B vitamin intake supports proper DNA synthesis and repair regardless of genotype.

Interactions

SHMT1 C1420T interacts most notably with MTHFR C677T (rs1801133) and A1298C (rs1801131). When SHMT1 TT reduces the supply of 5,10-methylenetetrahydrofolate, and MTHFR 677T reduces the enzyme's ability to convert that substrate, the combined effect significantly impairs methylfolate production and homocysteine remethylation. Individuals with both SHMT1 TT and MTHFR 677 CT or TT genotypes should prioritize methylfolate supplementation and monitor homocysteine levels.

SHMT1 also functions in the same pathway as SLC19A1 G80A (RFC1, rs1051266), the main folate transporter into cells, and MTRR A66G (rs1801394), which recycles methionine synthase. Variants in these genes can compound SHMT1-related inefficiencies by further limiting folate availability or homocysteine remethylation capacity.

Nutrient Interactions

folate altered_metabolism
vitamin B6 increased_need
riboflavin increased_need

Genotype Interpretations

What each possible genotype means for this variant.

CC Normal SHMT Activity Normal

Standard one-carbon metabolism with typical folate distribution

You have two copies of the common C allele, resulting in standard SHMT1 enzyme function. Your one-carbon metabolism operates at typical efficiency, channeling folate-derived methyl groups through DNA synthesis and methylation pathways without the altered cellular localization seen in T allele carriers. Interestingly, this genotype is associated with somewhat lower circulating folate levels, possibly due to more efficient utilization rather than storage. About 40-45% of people of European descent share this genotype.

GG Normal SHMT Activity Normal

Standard one-carbon metabolism with typical folate distribution

You have two copies of the common C allele, resulting in standard SHMT1 enzyme function. Your one-carbon metabolism operates at typical efficiency, channeling folate-derived methyl groups through DNA synthesis and methylation pathways without the altered cellular localization seen in T allele carriers. About 40-45% of people of European descent share this genotype.

CT Intermediate SHMT Function Intermediate

One altered copy moderately affects folate metabolism efficiency

You have one C and one T allele, giving you a mix of normal and altered SHMT1 enzyme. This typically produces an intermediate effect on one-carbon metabolism — not as efficient as CC in certain pathways, but not as significantly altered as TT. The T allele shifts how the enzyme distributes folate intermediates between cellular compartments and metabolic routes. About 45-48% of people of European descent carry this genotype.

AG Intermediate SHMT Function Intermediate

One altered copy moderately affects folate metabolism efficiency

You have one C and one T allele, giving you a mix of normal and altered SHMT1 enzyme. This typically produces an intermediate effect on one-carbon metabolism — not as efficient as CC in certain pathways, but not as significantly altered as TT. The T allele shifts how the enzyme distributes folate intermediates between cellular compartments and metabolic routes. About 45-48% of people of European descent carry this genotype.

TT Altered SHMT Function Reduced

Two altered copies significantly shift folate metabolism patterns

You have two copies of the T allele, causing a leucine-to-phenylalanine substitution that alters SHMT1 enzyme localization and activity. This changes how your cells distribute and utilize one-carbon units from folate, reducing the availability of methylenetetrahydrofolate for the MTHFR enzyme. Paradoxically, this genotype is associated with higher circulating folate levels (possibly reflecting reduced cellular uptake or altered compartmentalization), but may impair efficient conversion to methylfolate, especially when combined with MTHFR variants. About 10-15% of people of European descent carry this genotype. You may have reduced risk for some solid tumors (breast, rectal) but modestly increased risk for blood cancers.

AA Altered SHMT Function Reduced

Two altered copies significantly shift folate metabolism patterns

You have two copies of the T allele, causing a leucine-to-phenylalanine substitution that alters SHMT1 enzyme localization and activity. This changes how your cells distribute and utilize one-carbon units from folate, reducing the availability of methylenetetrahydrofolate for the MTHFR enzyme. This genotype may impair efficient conversion to methylfolate, especially when combined with MTHFR variants. About 10-15% of people of European descent carry this genotype. You may have reduced risk for some solid tumors (breast) but modestly increased risk for blood cancers.