rs61886492 — FOLH1 H475Y

FOLH1 H475Y — The Folate Retention Variant

Before dietary folate from leafy greens, lentils, and liver can enter your bloodstream, it must first be stripped of its glutamate chain. In the intestinal brush border, FOLH1 (folate hydrolase 1, also called GCPII or glutamate carboxypeptidase II¹¹ GCPII or glutamate carboxypeptidase II
GCPII: a type II transmembrane metallopeptidase anchored to jejunal enterocytes; cleaves polyglutamated dietary folates to the absorbable monoglutamate form) performs this essential cleavage step. The H475Y variant at rs61886492 changes a histidine to a tyrosine at position 475 of the enzyme, reducing its catalytic activity by approximately 53% in laboratory models.

What makes this variant scientifically interesting — and clinically relevant — is the direction of its population-level effect: despite the measured reduction in enzyme activity, carriers consistently show higher blood folate and lower homocysteine than non-carriers in large population studies. Understanding why requires looking beyond gut absorption alone.

The Mechanism

The H475Y substitution ²² p.His475Tyr arises from the c.1561C>T change on the coding (minus) strand; on the plus strand of GRCh38 this appears as G>A at chromosome 11 position 49164722 (NC_000011.10:49164721:G:A) alters the spatial configuration of GCPII's substrate-binding region. In transfected COS-7 cells, membranes expressing the H475Y variant showed 53% less folylpolyglutamate carboxypeptidase activity³³ 53% less folylpolyglutamate carboxypeptidase activity
Devlin AM et al. Glutamate carboxypeptidase II: a polymorphism associated with lower levels of serum folate. Hum Mol Genet, 2000 than wild-type GCPII.

In the gut, GCPII cleaves dietary polyglutamated folates — but it also participates in folate catabolism. The net in-vivo effect may depend on which direction (absorption versus breakdown) dominates in a given tissue context. GCPII is also expressed in the brain, where it cleaves NAAG⁴⁴ NAAG
N-acetylaspartylglutamate: a neuropeptide acting as a mGluR3 co-agonist; its cleavage releases glutamate and modulates synaptic tone, a neuropeptide that modulates glutamate signaling. In the kidney, GCPII is expressed on proximal tubule cells and may influence folate reabsorption. The combination of these tissue-specific roles means the in-vivo folate phenotype of H475Y reflects a multi-tissue balance rather than gut absorption alone.

The Evidence

The largest and most directly informative study is a Dutch population cohort Lievers et al. 2002⁵⁵ Lievers et al. 2002
Lievers KJA et al. Influence of a GCPII polymorphism on plasma homocysteine, folate and vitamin B12. Atherosclerosis, 2002 examining 190 vascular disease patients and 601 healthy controls. The 1561C>T polymorphism (H475Y) was associated with significantly higher RBC folate and plasma folate concentrations (ANOVA p=0.013, linear trend p=0.03), and TT homozygotes (AA on the plus strand) tended toward lower fasting and post-load homocysteine.

A controlled folate bioavailability trial by Melse-Boonstra et al. 2004⁶⁶ Melse-Boonstra et al. 2004
Melse-Boonstra A et al. Bioavailability of polyglutamyl folic acid. Am J Clin Nutr, 2004 enrolled 180 healthy adults (50–75 years) and found CT carriers had significantly higher baseline erythrocyte and serum folate (p<0.05) than CC homozygotes; however, the bioavailability of supplemental polyglutamyl folic acid was similar across genotypes (64% vs. 70%). This suggests the folate-status advantage arises from expression-level differences rather than altered enzyme kinetics, consistent with the H475Y variant having tissue- and context-specific effects that go beyond simple gut absorption.

The large Norwegian Hordaland Homocysteine Study⁷⁷ Hordaland Homocysteine Study
Halsted CH et al. Relations of GCPII polymorphisms to folate and homocysteine. Am J Clin Nutr, 2007 confirmed that CT and TT carriers had higher plasma folate and lower total homocysteine concentrations than CC homozygotes. However, it also identified a cognitive nuance: TT homozygotes showed paradoxically lower Symbol Digit Modalities Test scores, particularly among non-drinking women. The authors speculated this could reflect altered NAAG metabolism in the brain — reduced GCPII cleavage preserving NAAG tone in most contexts, but interacting negatively with certain co-factors (alcohol metabolism, B12 status).

In 120 chronic dialysis patients, Fodinger et al. 2003⁸⁸ Fodinger et al. 2003
Fodinger M et al. Effect of GCPII and reduced folate carrier polymorphisms on folate and homocysteine in dialysis patients. J Am Soc Nephrol, 2003 found CT/TT genotypes independently predicted higher RBC folate (p=0.04), even in this high-homocysteine population where B vitamin metabolism is substantially altered.

Practical Actions

For people carrying the AA genotype (two copies of the H475Y T allele), circulating folate levels are typically higher than average. Standard dietary folate intake through whole foods is generally sufficient. The primary clinical consideration is ensuring that the folate advantage translates into adequate methylation capacity — which requires cofactors B12, B6, and riboflavin alongside folate. Monitoring homocysteine confirms that the methylation cycle is operating well.

For heterozygous AG carriers, the folate-raising effect is partial but still present. Supporting the methylation cycle with the full B vitamin complex remains the most practical approach.

The paradox from Halsted 2007 (lower cognitive scores in TT homozygotes) is worth noting but requires replication before strong clinical guidance is possible. Adequate B12 and avoidance of alcohol excess may be especially relevant for AA individuals given the suggested interaction.

Interactions

The H475Y variant sits at the gut absorption step that precedes the methylation cycle. It interacts with downstream methylation variants: MTHFR C677T (rs1801133) and A1298C (rs1801131) determine how efficiently absorbed folate is converted to methylfolate; COMT V158M (rs4680) affects how methyl groups are used. The FOLH1 T484A variant (rs202676) is a separate missense variant in the same gene with different population frequencies and a different direction of effect; the two variants may or may not be in linkage disequilibrium — this has not been formally established. SLC19A1 A80G (rs1051266) controls cellular folate uptake downstream of intestinal absorption.