rs1800909 — GGH c.16T>C (p.Cys6Arg)

Signal-peptide missense variant that alters GGH subcellular targeting; C allele carriers show modestly lower plasma homocysteine and altered methotrexate pharmacokinetics

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GGH c.16T>C — A Signal Peptide Variant That Shifts Folate Balance

Every folate molecule entering a cell faces the same fate: to be useful, it must be trapped. Cells accomplish this by adding chains of glutamate — called polyglutamate tails11 polyglutamate tails
Polyglutamylation: the enzymatic addition of 2–8 glutamate residues to folate, anchoring it inside the cell. Polyglutamylated folate is the metabolically active form used by folate-dependent enzymes; monoglutamylated folate can cross cell membranes and leave the cell — that anchor folate inside the cell. GGH (gamma-glutamyl hydrolase) is the enzyme that removes these tails, converting retained polyglutamates back to exportable monoglutamates. Adjusting GGH activity is therefore one of the primary levers controlling intracellular folate retention.

The rs1800909 variant (c.16T>C in coding notation) falls in a part of the GGH gene that most genetics tests would classify as "probably benign" — the signal peptide that directs newly synthesized GGH protein to the endoplasmic reticulum. Yet this variant shows up consistently in studies of folate metabolism and methotrexate pharmacokinetics, suggesting the signal peptide change has subtle but real functional consequences.

The Mechanism

GGH sits on chromosome 8 at position 63,038,752 on the plus strand (GRCh38), within the GGH gene which itself runs on the minus strand. The c.16T>C change (T at coding position 16, C in the variant) alters codon 6 of the precursor protein — or equivalently codon -19 relative to the start of the mature protein22 mature protein
Mature protein: GGH contains a signal peptide of about 25 amino acids that is cleaved during processing; the "mature" protein is what remains after signal peptide removal in the ER. Chave et al. (2003)33 Chave et al. (2003)
Chave KJ et al. Identification of single nucleotide polymorphisms in the human gamma-glutamyl hydrolase gene and characterization of promoter polymorphisms. Mutat Res, 2003 were the first to characterize this polymorphism, noting that it changes the signal peptide from cysteine to arginine (Cys6Arg). Since the signal peptide directs GGH to the ER for processing and eventual lysosomal localization, an amino acid change here could alter protein folding during transit, signal peptide cleavage efficiency, or the quantity of functional GGH that reaches its final destination.

Unlike the promoter variant rs11545076 (-124T>G), which straightforwardly increases GGH transcription, the mechanism of rs1800909 is more subtle. The bioinformatics prediction for this variant is "benign" per in-silico tools noted in the Singapore Chinese cohort study44 noted in the Singapore Chinese cohort study
Oppeneer SJ et al. Genetic variation in FPGS and GGH and plasma homocysteine levels in the Singapore Chinese Health Study. Mol Genet Metab, 2012, yet three independent human studies find meaningful associations with folate pathway outcomes — suggesting a real but modest functional effect that current computational prediction tools underestimate.

The Evidence

The most direct evidence for a folate-related effect comes from a cohort of 482 Singapore Chinese adults55 cohort of 482 Singapore Chinese adults
Oppeneer SJ et al. Genetic variation in FPGS and GGH and plasma homocysteine levels in the Singapore Chinese Health Study. Mol Genet Metab, 2012. Among nine GGH SNPs tested, rs1800909 was one of three associated with plasma homocysteine (p=0.005 multivariate-adjusted). The direction is counterintuitive: carriers of the C allele (G on the plus strand) had lower homocysteine than TT homozygotes — TT averaged 10.0 nmol/L versus 9.4 nmol/L in CT heterozygotes and 9.7 nmol/L in CC homozygotes. The R² was 0.022 (explaining ~2% of variance), and the authors noted the ~0.6 nmol/L difference "would not be considered clinically meaningful" on its own. Only the promoter variant rs11545076 survived multiple-comparison correction in this study.

The pharmacogenomics picture is more consistent. A study of 70 Japanese rheumatoid arthritis patients66 study of 70 Japanese rheumatoid arthritis patients
Hakamata J et al. Factors Predicting the Therapeutic Response to Methotrexate in Japanese Patients with Rheumatoid Arthritis. Biol Pharm Bull, 2018 found by multivariate logistic regression that patients carrying the C allele of GGH 16T>C had a better therapeutic response to methotrexate than TT homozygotes. A complementary study of 149 Chinese pediatric ALL patients77 study of 149 Chinese pediatric ALL patients
Li M et al. Frequency distribution of five SNPs in human GGH gene and their effects on clinical outcomes of Chinese pediatric patients with acute lymphoblastic leukemia. Pharmazie, 2020 found that a haplotype including rs1800909 was associated with improved event-free survival (89.2% vs 71.9%) and lower relapse rates versus wild-type haplotype carriers. A European RA study of 352 patients88 European RA study of 352 patients
van der Straaten RJHM et al. Exploratory analysis of four polymorphisms in human GGH and FPGS genes and their effect in methotrexate-treated rheumatoid arthritis patients. Pharmacogenomics, 2007 found no significant association with MTX efficacy or toxicity overall, though there was a trend in haplotype analysis. The GGH 16T>C variant was also identified among polymorphisms associated with chemotherapy toxicity in osteosarcoma99 chemotherapy toxicity in osteosarcoma
Hattinger CM et al. Candidate germline polymorphisms of genes belonging to the pathways of four drugs used in osteosarcoma standard chemotherapy. Oncotarget, 2016.

Practical Actions

The C allele (G on plus strand) of rs1800909 is associated with modestly lower plasma homocysteine and appears to improve methotrexate response in some populations. For everyday folate metabolism, the effect is subtle — standard dietary folate intake supports adequate homocysteine levels for most carriers. The more actionable consequence is pharmacogenomic: if you carry the G allele (C in coding notation) and take methotrexate for rheumatoid arthritis, psoriasis, or cancer, you may have altered MTX polyglutamate kinetics that your treating clinician should be aware of. Because GGH activity affects how long methotrexate is retained in cells as active polyglutamates, this variant may influence both MTX efficacy and side-effect burden.

The variant's context matters: rs1800909 never occurs in isolation in the GGH gene. Individuals who also carry the promoter variant rs11545076 (C allele) — which more strongly elevates GGH expression and depletes intracellular folate — face a compounded effect. Supporting folate status through methylfolate (5-MTHF) rather than synthetic folic acid makes sense for anyone carrying multiple GGH variants, since it bypasses the conversion step and provides the active form directly.

Interactions

The most clinically significant interaction is with rs11545076 (GGH -124T>G), the promoter variant that dramatically increases GGH transcription. Rs11545076 has a larger and better-replicated effect on folate retention and DNA uracil accumulation (up to 73% higher in CC homozygotes). Individuals carrying the risk allele at rs1800909 alongside the risk allele at rs11545076 face overlapping GGH pathway disruption through two independent mechanisms.

For methotrexate pharmacology, rs1800909 interacts with rs11545078 (GGH 452C>T, Thr127Ile), the coding variant with the strongest characterized functional effect on GGH catalytic activity (2.7-fold higher Km for long-chain substrates). Together, variants in the signal peptide (rs1800909) and catalytic domain (rs11545078) may produce compound effects on MTX polyglutamate retention in target tissues.

Rs1800909 is in partial linkage disequilibrium with rs3758149 (GGH -401C>T promoter variant) and other GGH polymorphisms — haplotype-level analysis often shows stronger associations than any individual SNP, as seen in the Li et al. 2020 leukemia study.

Drug Interactions

methotrexate altered_metabolism literature

Nutrient Interactions

folate altered_metabolism

Genotype Interpretations

What each possible genotype means for this variant.

AA Wild Type Normal

Common genotype — standard GGH signal peptide and typical homocysteine levels

You have the TT (wild-type) genotype at GGH 16T>C, appearing as AA in plus-strand notation. Your GGH signal peptide contains the ancestral cysteine at position 6, directing GGH to the endoplasmic reticulum through the standard pathway. Studies show TT homozygotes average approximately 10.0 nmol/L plasma homocysteine — within the normal reference range. About 58% of people globally share this genotype.

For methotrexate therapy, the TT genotype represents the reference pharmacokinetic profile used in standard dosing. Your folate metabolism via GGH follows the baseline pattern, without the signal-peptide modification associated with altered MTX response.

AG Heterozygous Carrier Intermediate

One copy — signal peptide variant associated with modestly lower homocysteine and altered MTX pharmacokinetics

The Cys6Arg change sits in the signal peptide that directs GGH from the ribosome to the endoplasmic reticulum. While bioinformatics tools rate this as likely benign, the consistent direction of effect across multiple human studies suggests a real functional consequence — possibly altered signal peptide cleavage kinetics, subtle changes in GGH protein folding, or differences in the rate of GGH degradation.

The paradoxical finding — C allele associated with lower homocysteine despite carrying a GGH variant — parallels a similar pattern seen with rs11545076 (the promoter variant). One hypothesis is that reduced GGH activity per molecule slightly increases intracellular folate polyglutamate retention, improving the methionine synthase reaction that remethylates homocysteine. This remains mechanistically unconfirmed in functional studies of rs1800909 specifically.

GG Homozygous Variant Decreased

Two copies — homozygous signal-peptide variant with consistent effect on folate metabolism and MTX response

The non-additive pattern seen in the homocysteine data (heterozygotes and homozygotes showing similar reductions versus wild-type) is common for missense variants with a dominant or semi-dominant mode of action. A single copy of the Arg6 signal peptide may be sufficient to produce most of the functional effect on GGH protein processing.

In methotrexate pharmacokinetics, the haplotype including rs1800909 was associated with higher MTX exposure metrics in the Li et al. 2020 pediatric ALL study (n=149). Whether this reflects reduced GGH degradation of MTX polyglutamates or an indirect effect through the broader GGH haplotype structure remains to be determined in functional studies. For CC homozygotes on long-term low-dose MTX (as in RA), the implications are similar to heterozygotes but warrant more attentive monitoring for cumulative hepatotoxicity.

Across cancer pharmacogenomics studies, rs1800909 appears in haplotype blocks alongside other GGH variants. If you also carry the promoter variant rs11545076 (CC genotype), you are combining a signal-peptide effect on GGH protein processing with a transcriptional effect on GGH expression — two independent routes to altered folate polyglutamate turnover.