rs3788200 — SLC19A1 SLC19A1 intron 2 variant

Intronic SLC19A1 tag variant in strong LD with the G80A folate-transporter coding variant — marks reduced folate carrier efficiency

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SLC19A1 Intronic Variant — Tagging the Folate Carrier Efficiency Haplotype

The SLC19A1 gene encodes the reduced folate carrier (RFC1), the primary mechanism by which folate moves from the bloodstream into your cells. Without efficient RFC1 function, intracellular folate levels fall — even when blood folate appears normal. The rs3788200 variant sits in intron 2 of SLC19A1 and has no direct effect on protein structure, but it serves as a reliable molecular tag for the same haplotype as rs1051266, the well-studied G80A coding variant that directly reduces folate transport kinetics.

The Mechanism

rs3788200 is an intronic variant that does not itself change amino acid sequence. Its biological relevance comes from its position in the genome: it is in very strong linkage disequilibrium11 very strong linkage disequilibrium
LD r²=0.98 — the two variants co-occur nearly perfectly across populations with rs1051266 (G80A, p.His27Arg), which directly alters transmembrane domain 1 of the RFC1 transporter protein and reduces folate uptake into cells. Carrying the G allele at rs3788200 therefore marks the same genetic background as carrying the T allele at rs1051266: reduced folate transport capacity.

The Evidence

A 2023 Italian cohort study Bugianesi et al. Biomedicines, 202322 Bugianesi et al. Biomedicines, 2023
Impaired Function of Solute Carrier Family 19 Leads to Low Folate Levels and Lipid Droplet Accumulation in Hepatocytes genotyped 756 individuals (452 NAFLD cases, 304 controls) and found rs3788200 significantly associated with non-alcoholic fatty liver disease (p=0.003). The G allele was associated with higher NAFLD risk, while the A allele was protective (0.6-fold decreased MAFLD risk). The LD with rs1051266 (r²=0.98) confirmed these two variants represent the same functional haplotype — impaired SLC19A1 transport reduces intracellular folate, dysregulates hepatic lipid metabolism genes, and promotes lipid droplet accumulation.

A family-based transmission disequilibrium test O'Byrne MR et al. Birth Defects Research, 201033 O'Byrne MR et al. Birth Defects Research, 2010
Association of SLC19A1 genes with meningomyelocele in 610 families found rs3788200 significantly associated with meningomyelocele risk (p=0.0195). Only 37 of 97 informative transmissions passed the G allele to affected offspring — fewer than expected — suggesting the G allele on its own shows a complex pattern, with the intronic variant's effect mediated through its LD relationship with the coding variant rs1051266.

A 2022 Chinese pharmacogenomics study Cen H et al. Pharmacogenomics and Personalized Medicine, 202244 Cen H et al. Pharmacogenomics and Personalized Medicine, 2022 of 104 rheumatoid arthritis patients on methotrexate found rs3788200 G-carrier status (AG+GG) associated with significantly better EULAR treatment response (RR=1.45, 95% CI=1.04–2.01, p=0.03). This seeming paradox — the G allele associated with reduced folate transport yet better methotrexate response — is mechanistically coherent: methotrexate enters cells via the same RFC1 transporter, so the G-tagged rs1051266-T haplotype may affect drug accumulation dynamics differently than folate in the context of pharmacological dosing.

Practical Implications

The clinical significance of rs3788200 runs parallel to rs1051266. If you carry the G allele, your folate transport efficiency is likely reduced, making adequate dietary and supplemental folate more important. Methylfolate (5-MTHF) is the preferred form — it is already in its active state and may be transported more efficiently than synthetic folic acid. Monitoring homocysteine provides a functional readout: elevated homocysteine signals that methylation is falling behind, potentially due to limited intracellular folate availability.

If you are prescribed methotrexate (for rheumatoid arthritis, psoriasis, or other conditions), your SLC19A1 genotype at this locus may influence how the drug accumulates in your cells. Inform your prescribing physician of this variant; dosing and monitoring protocols may benefit from adjustment.

Interactions

rs3788200 is in strong LD (r²=0.98) with rs1051266 and the two should be interpreted together — they report on the same underlying folate-transport haplotype. The combined effect is amplified when MTHFR variants (rs1801133, rs1801131) are also present: impaired methylfolate production (MTHFR) combined with reduced folate import (SLC19A1) creates a more significant bottleneck in the folate-methylation cycle than either variant alone. rs1888530, another SLC19A1 intronic variant (intron 5), showed even stronger meningomyelocele association in the O'Byrne 2010 study and may tag a partially distinct haplotype within the same gene.

Drug Interactions

methotrexate dose_adjustment literature

Nutrient Interactions

folate reduced_absorption

Genotype Interpretations

What each possible genotype means for this variant.

AA Full Transport Capacity Normal

Full folate carrier efficiency

You carry two copies of the A allele at this SLC19A1 intronic marker. This genotype tags the high-efficiency folate transport haplotype — your RFC1 carrier protein moves folate into cells with normal or above-average efficiency. About 22% of people globally share this genotype. Standard folate intake from diet and supplements is well-utilized.

AG Mildly Reduced Transport Intermediate

Mildly reduced folate transport capacity

You carry one G allele, which places you on the lower end of the folate transport capacity spectrum. The G allele tags the same haplotype as the rs1051266 T allele, which reduces RFC1 transporter efficiency. In heterozygotes this is typically a mild effect. About 50% of people globally share this genotype. The practical impact is greatest when dietary folate is marginal or when methylation demand is elevated (pregnancy, high homocysteine, combined MTHFR variants).

GG Reduced Transport Capacity Reduced

Reduced folate carrier efficiency

You carry two G alleles, placing you in the lowest folate transport capacity group at this locus. The GG genotype strongly tags the rs1051266-TT haplotype (LD r²=0.98), which is associated with meaningfully reduced RFC1 transporter function. About 29% of people globally share this genotype. This means that even with adequate blood folate, the amount actually reaching your cells may be suboptimal. A 2023 cohort study found this genotype associated with higher NAFLD risk, mechanistically linked to impaired intracellular folate and downstream lipid metabolism dysregulation.