rs2838956 — SLC19A1 RFC1 intronic variant

SLC19A1 RFC1 Intronic Variant — Folate Transport Efficiency and Methotrexate Response

Every cell in your body depends on a constant supply of folate to make DNA, synthesize amino acids, and run the methylation cycle. Yet folate cannot cross cell membranes on its own. The reduced folate carrier 1 (RFC1), encoded by the SLC19A1 gene on chromosome 21, is the primary gateway — a high-capacity, bidirectional transporter that ferries 5-methyltetrahydrofolate¹¹ 5-methyltetrahydrofolate
The predominant circulating form of folate; the active form used in the methylation cycle (5-MTHF) and other reduced folates into cells against concentration gradients. The rs2838956 variant lies within an intron of this gene and tags a haplotype block that influences how efficiently RFC1 does its job.

The Mechanism

rs2838956 (chr21:45525110, GRCh38) is an intronic A>G single-nucleotide variant in SLC19A1. Intronic variants are not silent bystanders — they can alter RNA splicing²² RNA splicing
The process that removes introns and joins exons to create messenger RNA; intronic variants can shift splice-site recognition, alter exon inclusion, or change expression levels or transcription factor binding, changing the amount or structure of the RFC1 protein produced. The mechanistic route for rs2838956 has not been directly resolved, but its consistent presence in haplotype blocks associated with altered RFC1 transport activity suggests it is a regulatory tag SNP rather than a purely neutral bystander. The G allele at this position defines the RFC1 haplotype associated with altered methotrexate uptake³³ altered methotrexate uptake
Methotrexate enters cells through the same RFC1 transporter as folate; reduced transport affects both therapeutic drug delivery and cellular folate homeostasis.

RFC1 operates as a secondary active transporter, exchanging intracellular organic anions (principally thiamine pyrophosphate) for extracellular folates. Its preferred substrates are reduced folates — 5-MTHF and 5-formylTHF — while oxidized folic acid has orders-of-magnitude lower affinity⁴⁴ orders-of-magnitude lower affinity for the carrier. This biochemical preference means RFC1 function matters most for how efficiently cells import the active folate forms from circulation, and less for folic acid absorbed directly from gut lumen (which uses separate transporters).

The Evidence

The clearest clinical signal for rs2838956 comes from pharmacogenomics — specifically, how this variant affects methotrexate (MTX) response in rheumatoid arthritis (RA) and childhood leukemia, because MTX enters cells through the same RFC1 transporter as folate.

A UK cohort study of 219 RA patients⁵⁵ UK cohort study of 219 RA patients
Daly AK et al. Genetic polymorphisms in key methotrexate pathway genes are associated with response to treatment in rheumatoid arthritis patients. Pharmacogenomics J, 2012 found the G allele of rs2838956 associated with MTX treatment failure (OR 1.45, 95% CI 1.00–2.10, p trend = 0.04). G allele carriers showed higher rates of inadequate disease response, suggesting reduced RFC1-mediated MTX uptake into inflamed synovial cells.

A Portuguese RA study⁶⁶ Portuguese RA study
Hider SL et al. SLC19A1, SLC46A1 and SLCO1B1 polymorphisms as predictors of methotrexate-related toxicity. Toxicol Sci, 2014 found the complementary pattern for gastrointestinal toxicity: A allele carriers had increased GI adverse effects (OR 3.21, p = 0.049). This apparent paradox — G allele linked to lower efficacy while A allele linked to higher toxicity — suggests the A allele may enhance RFC1 transport activity in some tissues, delivering more MTX to the GI epithelium (causing toxicity) while also enabling better drug delivery to target tissues (improving RA response). The GGAG haplotype incorporating rs2838956 was independently associated with GI toxicity (p = 0.029).

A pediatric ALL study⁷⁷ pediatric ALL study
Organista-Nava J et al. Folate transport gene polymorphisms in Mexican children with ALL. Front Pharmacol, 2016 of 73 cases and 133 controls found remarkably elevated ALL risk in AG heterozygotes (OR = 44.69, 95% CI 10.42–191.63, p = 0.0001). The extraordinarily large odds ratio in an over-dominant model suggests an interaction effect specific to heterozygous status in this admixed Mexican population; replication in larger independent cohorts is needed before this finding can be used clinically.

For the general population without autoimmune disease or chemotherapy exposure, the primary implication is folate transport efficiency — individuals carrying the G allele may have a modestly different RFC1 haplotype that alters how efficiently cells import dietary folates from the bloodstream.

Practical Actions

For most people, the rs2838956 variant has a modest effect that becomes clinically relevant mainly in two contexts: (1) if methotrexate is prescribed for RA or other autoimmune conditions, and (2) if other folate-pathway variants (particularly MTHFR C677T) compound the functional load on the folate transport system. Using 5-MTHF rather than folic acid is especially relevant for RFC1 variant carriers because RFC1 transports reduced folates far more efficiently than oxidized folic acid, meaning the form of folate matters independently of any downstream conversion issues.

Interactions

rs2838956 is most studied in the context of SLC19A1 haplotypes that include rs1051266 (H27R, the most-studied RFC1 coding variant), rs7499, and rs3788200. The haplotype context determines the net functional effect more reliably than any single SNP alone. For folate-pathway burden, rs2838956 compounds with MTHFR C677T (rs1801133) and A1298C (rs1801131) — individuals carrying both MTHFR impairment and an RFC1 transport variant face reduced folate at two steps: import into cells and conversion to the active methylfolate form. This two-step bottleneck scenario warrants more aggressive use of bioactive folate forms (5-MTHF) and monitoring of homocysteine levels. The interaction between MTHFR and SLC19A1 variants has been noted in the literature on one-carbon metabolism but has not been formally quantified in a compound-action study.