rs202676 — FOLH1 T484A

Reduces intestinal folate hydrolase activity, impairing absorption of dietary polyglutamyl folates and lowering circulating folate available for methylation

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FOLH1 T484A — The Gut Gateway for Dietary Folate

Before folate from food can enter your bloodstream, it must be stripped of its glutamate chain. Dietary folates arrive from leafy greens, legumes, and liver as polyglutamylated forms 11 Polyglutamyl folates: folate molecules with 3–9 glutamate residues attached; found in natural foods but too large to cross the intestinal wall intact — long chains that cannot be absorbed as-is. FOLH1 (folate hydrolase 1, also called GCPII or glutamate carboxypeptidase II) is the enzyme anchored to the intestinal brush border that cleaves these chains down to the monoglutamyl form your cells can actually take up. Variants in FOLH1 that reduce its activity act as a bottleneck: even a folate-rich diet may not translate to adequate circulating folate levels if the gateway enzyme is underperforming.

The Mechanism

The T484A variant (rs202676) changes a tyrosine to a histidine at position 75 of the GCPII protein 22 p.Tyr75His using primary transcript NM_004476.3 (c.223T>C); the variant lies in exon 2 of FOLH1, which sits on the minus strand of chromosome 11. This substitution alters the surface charge and spatial structure of the enzyme's substrate-binding region, reducing its ability to bind and cleave folyl-polyglutamates. Carriers of the G allele (which encodes the histidine variant) show lower erythrocyte (red blood cell) folate levels despite equivalent dietary intake — the enzyme simply extracts less folate from food.

FOLH1 also serves a separate neurological function: in the brain, GCPII cleaves NAAG 33 N-acetylaspartylglutamate (NAAG): a neuropeptide that acts as a co-agonist at metabotropic glutamate receptors (mGluR3), dampening synaptic glutamate tone and supporting working memory, a neuropeptide that modulates glutamate signaling. The G allele was found to increase FOLH1 mRNA expression in the dorsolateral prefrontal cortex44 increase FOLH1 mRNA expression in the dorsolateral prefrontal cortex
Zink et al. 2020, American Journal of Psychiatry, resulting in more aggressive NAAG breakdown and lower synaptic NAAG availability. This dual role — gut folate absorption and brain NAAG metabolism — means the G allele may affect both methylation and cognitive function through distinct pathways.

The Evidence

The strongest evidence for the G allele's impact on folate status comes from a Chinese cohort study55 Chinese cohort study
Guo et al. 2013, Genes & Nutrition, 160 NTD cases and 320 controls showing that GG homozygotes had 18% lower plasma folate concentrations and 17% higher total homocysteine compared to non-carriers. The GG genotype was significantly over-represented in mothers of children with multiple neural tube defects (OR 2.16, p=0.030). A separate Eastern Indian study found a protective signal in a different direction, consistent with population-specific genetic backgrounds 66 Paul et al. 2018, Birth Defects Research, n=135 — illustrating that the variant's phenotypic effect may be modified by background folate intake and co-occurring folate pathway variants.

In the folate supplementation trial most directly relevant to actionable guidance, Roffman et al. 201377 Roffman et al. 2013
JAMA Psychiatry, randomized double-blind trial, n=140 schizophrenia patients, 16 weeks folate 2mg + B12 400mcg vs placebo showed that baseline RBC folate was inversely proportional to G allele load (p=0.03), and that only patients with the normal-functioning AA genotype showed meaningful clinical benefit from folate plus B12 supplementation — C allele (G allele plus-strand) carriers did not respond significantly through week 8, suggesting their folate absorption pathway was the rate-limiting step.

A large neuroimaging genetics study by Zink et al. 202088 Zink et al. 2020
American Journal of Psychiatry, postmortem brain samples + 7-Tesla MRS + fMRI in living subjects linked the G allele to increased FOLH1 brain expression, lower NAAG concentrations, and less efficient working-memory circuitry, with G allele carriers also scoring lower on IQ assessments.

Practical Actions

For G allele carriers, the critical insight from supplementation research is that folate from food and from standard folic acid supplements must still pass through FOLH1 before entering the circulation. Supplementing with methylfolate (5-MTHF)99 methylfolate (5-MTHF)
The monoglutamyl, already-activated form of folate; bypasses the FOLH1 bottleneck entirely bypasses the FOLH1 bottleneck entirely, since it is already in the absorbable monoglutamyl form. Monitoring homocysteine and erythrocyte (RBC) folate provides a direct readout of whether circulating folate is adequate.

Interactions

The G allele compounds with MTHFR C677T (rs1801133) and A1298C (rs1801131) along the same folate-methylation axis. An individual who both absorbs dietary folate poorly (FOLH1 G allele) and converts it inefficiently to methylfolate (MTHFR reduced function) faces a multiplicative deficit: less substrate entering the cycle and slower conversion at the MTHFR step. This interaction is clinically meaningful enough to warrant combined monitoring and preferential methylfolate supplementation. The SLC19A1/RFC1 A80G variant (rs1051266) affects intestinal folate transport and is another upstream modifier of folate availability that compounds with FOLH1 status.

Nutrient Interactions

folate reduced_absorption
vitamin B12 increased_need

Genotype Interpretations

What each possible genotype means for this variant.

AA Normal Folate Absorber Normal

Normal FOLH1 activity — dietary folate absorbed efficiently

The FOLH1 enzyme works at full capacity, meaning folate from leafy greens, legumes, and fortified foods is efficiently converted to the absorbable monoglutamyl form. This does not guarantee adequate folate status — dietary intake and cooking methods still matter — but the gateway step functions normally. Standard folic acid or folinic acid supplements are metabolized normally.

AG Reduced Folate Absorber Reduced

One copy of the reduced-activity variant — mildly impaired folate absorption from food

The Y75H substitution alters the charge and shape of the enzyme's substrate-binding region. With one functional copy and one reduced-activity copy, overall GCPII activity in the intestinal brush border is partially maintained. Under normal dietary conditions, this may result in only marginally lower circulating folate, but the deficit can become meaningful under conditions of low dietary folate intake, high physiological demand (pregnancy, rapid growth), or when co-occurring with MTHFR or SLC19A1 variants that further strain the folate pathway.

The G allele is also associated with increased FOLH1 expression in the brain, which may accelerate breakdown of the neuropeptide NAAG — potentially relevant to working-memory function, though the heterozygous effect is smaller than in GG homozygotes.

GG Poor Folate Absorber Poor

Significantly reduced FOLH1 activity — dietary polyglutamyl folates are poorly absorbed

With both copies of FOLH1 producing the histidine-75 variant, GCPII activity in the intestinal brush border is substantially impaired. Even a diet high in folate-containing foods may not translate to adequate circulating folate, because the enzyme that strips the glutamate chains needed for absorption is underperforming. This makes dietary folate sources inherently less bioavailable for GG carriers.

A 16-week randomized trial of folate (2 mg) plus B12 (400 mcg) supplementation in schizophrenia patients demonstrated that RBC folate levels were inversely correlated with the number of G (484C coding) alleles carried at baseline — and that clinical benefit from supplementation required the AA genotype. GG carriers' folate levels lagged behind through at least week 8, consistent with impaired absorption even from supplemental sources.

In the brain, the G allele is associated with increased FOLH1 mRNA expression in the dorsolateral prefrontal cortex, accelerating catabolism of NAAG. A 7-Tesla MRS neuroimaging study found GG-associated lower NAAG concentrations, less efficient working-memory circuitry by fMRI, and modestly lower IQ scores — a brain-specific consequence beyond the gut absorption deficit.