rs1050152 — SLC22A4 OCTN1 L503F

Missense variant in SLC22A4 encoding the organic cation/ergothioneine transporter OCTN1; the T allele (503F) increases ergothioneine transport efficiency and forms part of the IBD5 two-locus TC risk haplotype associated with Crohn's disease and other inflammatory conditions; strongly enriched in Europeans

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OCTN1 L503F — When a Transporter Variant Reshapes the IBD5 Risk Haplotype

OCTN111 OCTN1
Organic Cation Transporter Novel 1 — a membrane transport protein expressed in the intestine, kidney, liver, lung, and immune cells that shuttles organic cations and the dietary antioxidant ergothioneine into cells is the product of the SLC22A4 gene on chromosome 5q31. The region around SLC22A4 is one of the earliest and most replicated inflammatory bowel disease susceptibility loci in human genetics — a 250 kb haplotype block called the IBD5 locus22 IBD5 locus
A linkage-disequilibrium block on chromosome 5q31 first identified in a Canadian sib-pair study of Crohn's disease; the locus spans SLC22A4, SLC22A5, and several immune-regulatory genes including IRF1 and IL5. The L503F missense variant in OCTN1 is one of two functional variants proposed to underlie the IBD5 risk haplotype, the other being rs2631367 (-207G>C) in the SLC22A5 (OCTN2) promoter.

The Mechanism

OCTN1 uses a sodium gradient to transport ergothioneine33 ergothioneine
A naturally occurring amino acid synthesized only by fungi and some bacteria; humans obtain it entirely from diet, primarily through mushrooms, oats, and black beans; OCTN1 is the dedicated mammalian ergothioneine transporter into cells. The L503F substitution — a leucine-to-phenylalanine swap in the 11th transmembrane domain — meaningfully alters the transporter's kinetic profile.

Gründemann et al. showed44 Gründemann et al. showed
PNAS 2009 — the 503F variant has 3-fold higher substrate affinity (lower Km) and 2-fold lower maximal transport velocity (Vmax) for ergothioneine, yielding 50% higher net transport efficiency at the physiological ergothioneine concentrations found in food and portal blood. The result: 503F carriers (CT and TT) accumulate higher ergothioneine concentrations in OCTN1-expressing tissues — including intestinal epithelium, macrophages, and neutrophils — than 503L homozygotes (CC) eating the same diet. In parallel, the 503F variant shows reduced carnitine transport (2.7-fold lower uptake) and altered uptake of various organic cations.

The functional paradox is that 503F is an ergothioneine gain-of-function variant associated with increased inflammatory disease risk. Current evidence suggests the primary causal signal at IBD5 may reside in nearby variants near IRF1 (Interferon Regulatory Factor 1)55 Current evidence suggests the primary causal signal at IBD5 may reside in nearby variants near IRF1 (Interferon Regulatory Factor 1)
Festen et al., Nature Genetics 2011 — reanalysis of IBD5 suggested that 503F may be a recent European-enriched adaptation that swept to high frequency in linkage disequilibrium with the true causal signal, rather than being causal itself. Regardless of causality, the T allele remains a reliable risk tag for the IBD5 haplotype.

The Evidence

Peltekova et al.66 Peltekova et al.
Nature Genetics 2004 — first proposed the two-locus IBD5 TC haplotype (SLC22A4 L503F-T + SLC22A5 -207C) as functional Crohn's disease risk variants; TC heterozygotes showed OR 2.1–2.56; TC/TC homozygotes showed OR 3.43–5.14 in two independent Canadian cohorts. This seminal finding made the IBD5 TC haplotype one of the best-replicated Crohn's disease susceptibility signals in the pre-GWAS era.

Subsequent replication studies confirmed the association in European cohorts. Noble et al., Gastroenterology 200577 Noble et al., Gastroenterology 2005
Studied TC haplotype in 1,400 Crohn's disease patients and 725 controls; TC/TC homozygotes were more likely to require intestinal resection and had higher rates of stricturing or penetrating disease, establishing a genotype-severity relationship beyond susceptibility alone.

Population data are striking: the T allele reaches 42% in European-Americans but drops to approximately 2% in East Asians, consistent with the observation that the IBD5 association with Crohn's disease is not observed in Japanese populations. This European enrichment has been proposed to reflect a recent selective sweep in European ancestry, possibly related to dietary adaptation to mushroom-rich environments.

Practical Actions

For TT homozygotes — carrying two copies of the 503F allele — the OCTN1 transporter accumulates particularly high ergothioneine tissue concentrations. The clinical significance is primarily as a tag for the IBD5 risk haplotype: elevated Crohn's disease susceptibility, with evidence that disease course may be more severe. Monitoring for early gastrointestinal symptoms and optimizing the gut microbiome through diet are the most actionable responses.

Because OCTN1 also regulates carnitine transport (with 503F reducing carnitine uptake 2.7-fold in vitro), TT homozygotes may have modestly reduced intestinal carnitine absorption. Carnitine status can be supported through dietary sources (red meat, dairy) or supplementation.

Interactions

The T allele at rs1050152 is the primary coding component of the IBD5 TC haplotype. Its partner variant, rs2631367 (-207G>C in the SLC22A5/OCTN2 promoter), reduces OCTN2 expression and synergizes with L503F to confer full TC haplotype risk for Crohn's disease. TC/TC homozygotes — those carrying the T allele here AND the C allele at rs2631367 — show the highest disease risk (OR 3.43–5.14) in the original Peltekova data. Single-locus carriage at either site confers intermediate risk.

Downstream of IBD5, NOD2/CARD15 variants (rs2066844, rs2066845, rs2066847) act in a common pathogenic pathway with the OCTN haplotype; compound carriage of IBD5 TC haplotype plus NOD2 variants may substantially further elevate Crohn's disease risk.

Nutrient Interactions

ergothioneine altered_metabolism
carnitine altered_metabolism

Genotype Interpretations

What each possible genotype means for this variant.

CC OCTN1 Leu503 Homozygote Normal

Standard OCTN1 transporter function — not on the IBD5 risk haplotype

The wild-type OCTN1 Leu503 protein has standard kinetics for ergothioneine transport (the dietary antioxidant from mushrooms and grains). The L503F substitution present in CT and TT genotypes creates a transporter with higher ergothioneine affinity but lower carnitine uptake. CC carriers — with two Leu503 copies — have the baseline ergothioneine absorption profile. Absent the IBD5 T allele, CC individuals do not carry the coding component of the Crohn's disease risk haplotype at this locus, though the rs2631367 promoter variant in OCTN2 (the other IBD5 haplotype component) would still be relevant if present.

CT IBD5 TC Haplotype Carrier Intermediate

One copy of the OCTN1 L503F risk variant — heterozygous for the IBD5 haplotype

CT carriers express both the Leu503 (standard) and Phe503 (L503F) OCTN1 protein. The net ergothioneine transport capacity is intermediate — higher than CC but lower than TT. The clinical significance of CT genotype at this locus is primarily as a marker of partial IBD5 TC haplotype carriage. The full IBD5 haplotype confers the highest risk when both rs1050152-T and the rs2631367-C (OCTN2 promoter) are on the same chromosome. If you also carry the C allele at rs2631367, you are a TC/- carrier and your Crohn's disease susceptibility is intermediate between single haplotype carriage and TC/TC homozygosity. Whether the L503F coding variant itself contributes to disease or merely tags nearby causal variants (near IRF1) is debated — but the risk association is consistent across European populations.

TT IBD5 TC Haplotype Homozygote High Risk

Two copies of OCTN1 L503F — highest risk configuration for the IBD5 haplotype

TT homozygotes express only the Phe503 OCTN1 protein. Functional studies demonstrate this transporter has 3-fold higher ergothioneine affinity and 50% greater ergothioneine transport efficiency at physiological dietary concentrations compared to the Leu503 (CC) protein, resulting in measurably higher ergothioneine tissue concentrations in TT carriers eating the same diet. In parallel, carnitine uptake is reduced approximately 2.7-fold.

The IBD5 locus spans genes with broad immune-regulatory roles: IRF1 (interferon regulatory factor 1, a key antiviral and innate immune transcription factor), IL5, IL13, and several cytokine pathway elements. Current evidence suggests that causal Crohn's disease variants at IBD5 may primarily reside in or near IRF1, with L503F acting as a haplotype tag in Europeans. Whether L503F itself contributes mechanistically (through altered carnitine availability, altered ergothioneine signaling in gut macrophages, or altered xenobiotic transport affecting gut epithelium) remains an active research question.

Clinically, TT individuals in European populations should be aware of their substantially elevated Crohn's disease susceptibility, and should particularly note the additive risk of also carrying the C allele at rs2631367 (OCTN2 promoter variant). Disease severity data suggest TC/TC haplotype carriers are more likely to experience stricturing or penetrating disease courses requiring surgery.