rs2631367 — SLC22A5 OCTN2 -207C>G

SLC22A5 OCTN2 -207C>G — The Carnitine Transporter Variant at the Heart of IBD5

One of the most studied regions in Crohn's disease genetics sits on chromosome 5q31 — a 250-kilobase stretch called the IBD5 locus¹¹ IBD5 locus
IBD5 was the fifth inflammatory bowel disease susceptibility locus identified by genome-wide linkage analysis; multiple studies from 2001–2006 refined its boundaries and candidate genes. Within this locus, two functional variants in adjacent carnitine transporter genes form a two-allele haplotype that raises Crohn's disease risk by two- to fivefold. rs2631367 is one of those two variants: a single nucleotide change in the promoter of SLC22A5 (encoding OCTN2) that reduces how much carnitine transporter the intestine makes — with significant consequences for mucosal immune homeostasis.

The Mechanism

SLC22A5 encodes OCTN2, a high-affinity sodium-dependent carnitine transporter²² high-affinity sodium-dependent carnitine transporter
Carnitine is an amino acid derivative essential for shuttling long-chain fatty acids across the inner mitochondrial membrane for beta-oxidation; without adequate OCTN2 activity, cells cannot efficiently import carnitine from the circulation expressed prominently in intestinal epithelium, heart, liver, and skeletal muscle. At position -207 in the SLC22A5 promoter, the G allele creates a heat shock element³³ heat shock element
HSEs are short DNA sequences recognized by heat shock transcription factors (HSFs); HSF binding increases gene transcription, especially under stress conditions binding site for heat shock transcription factors. The C allele disrupts this binding site, reducing OCTN2 transcriptional activity. Luciferase reporter assays and lymphoblastoid cell line studies confirmed that haplotypes carrying the -207G allele produce higher OCTN2 mRNA than haplotypes carrying -207C.

In the intestine, this matters beyond fatty acid metabolism. OCTN2-deficient mice⁴⁴ OCTN2-deficient mice
A neonatal mouse model with homozygous OCTN2 deletion showing early intestinal inflammation develop villous atrophy, early lymphocytic and macrophage infiltration, spleen and thymus atrophy via apoptosis, and a pro-inflammatory cytokine profile — a gut and immune phenotype that shares features with Crohn's disease histology. During active inflammatory bowel disease, OCTN2 expression in the terminal ileum is decreased further, suggesting a vicious cycle where inflammation suppresses the transporter and reduced transporter activity fails to protect the epithelial barrier.

rs2631367 forms the promoter component of the IBD5 TC haplotype⁵⁵ TC haplotype
TC stands for the two risk alleles: T at position +1672 in SLC22A4 (OCTN1, rs1050152) and C at position -207 in SLC22A5 (OCTN2, rs2631367); the haplotype was named for the two alleles present simultaneously alongside rs1050152 (L503F in the related OCTN1 transporter). Individually each variant reduces transporter function; together they appear to act additively on intestinal carnitine handling and mucosal barrier integrity.

The Evidence

The landmark study identifying the TC haplotype was published by Peltekova et al. in Nature Genetics⁶⁶ Peltekova et al. in Nature Genetics
Peltekova VD et al., 2004, 36:471–475. They showed that individuals heterozygous for the TC haplotype had odds ratios of 2.1–2.56 for Crohn's disease, while those homozygous for TC/TC had odds ratios of 3.43–5.14 — among the strongest common variant effects reported for IBD at the time. The association was independent of CARD15 variants (the IBD16 locus), and combined TC homozygosity with CARD15 mutations further amplified risk.

Waller et al. (Gut, 2006)⁷⁷ Waller et al. (Gut, 2006)
Waller S et al., 55(6):809–814 confirmed in 1,104 IBD patients and 750 controls that rs2631367 CC genotype carried OR 1.69 (95% CI 1.29–2.22) versus GG, and that the association extended beyond Crohn's disease to ulcerative colitis with comparable effect sizes (P = 0.0001 for overall IBD). The C allele frequency was 52.5% in IBD cases versus 46.3% in controls.

The functional basis of rs2631367 was confirmed by Tahara et al. (J Pharmacol Exp Ther, 2009)⁸⁸ Tahara et al. (J Pharmacol Exp Ther, 2009)
Tahara H et al., 329(1):262–271, who demonstrated that the -207G allele increases OCTN2 transcriptional activity through a heat shock element, while the -207C allele lacks this enhancing element. The functional consequence of lower OCTN2 expression directly reduces intestinal carnitine uptake capacity.

Noble et al. (Gastroenterology, 2005)⁹⁹ Noble et al. (Gastroenterology, 2005)
Noble CL et al., 129(5):1516–1523 extended the analysis to show that OCTN TC haplotype carriers not only had increased Crohn's disease susceptibility but also greater disease severity — homozygous TC/TC carriers were more likely to require intestinal surgery than TC/non-TC patients, suggesting a gene-dosage effect on clinical course.

One important caveat: the entire IBD5 locus is in high linkage disequilibrium¹⁰¹⁰ linkage disequilibrium
LD describes the tendency for nearby variants on the same chromosome to be inherited together more often than random chance; high LD makes it difficult to determine which variant in a region is the true functional cause, and the question of whether the TC haplotype SNPs are causal or merely tagging unknown causal variants remains scientifically debated. The functional data for -207C>G directly affecting OCTN2 transcription strengthens the causal argument considerably.

Practical Implications

For CC homozygotes (highest risk): the evidence supports optimizing carnitine status through dietary sources and monitoring. Red meat and dairy are the richest dietary carnitine sources; individuals with lower OCTN2 expression may benefit from increased intake of carnitine-rich foods. Given the mucosal barrier implications, maintaining intestinal epithelial health through butyrate-producing dietary fiber is also specifically relevant to OCTN2-mediated intestinal dysfunction. Gastrointestinal symptoms — particularly right lower quadrant abdominal pain, changes in stool frequency, bloody stools, or unexplained weight loss — warrant prompt investigation in CC carriers.

For CG heterozygotes: the intermediate risk elevation (approximately 45% increased OR vs GG) is moderate but clinically notable. Awareness of Crohn's disease early symptoms and family history discussion is appropriate.

Interactions

rs2631367 functions as one half of the IBD5 TC haplotype. Its partner, rs1050152 (SLC22A4 L503F, T allele), is an independent OCTN1 missense variant that alters ergothioneine transport. The combined TC genotype at both loci produces the full IBD5 risk signal; carriers of the C allele at rs2631367 without the T allele at rs1050152 may have somewhat reduced risk compared to TC haplotype homozygotes.

The IBD5 locus interacts with the CARD15/NOD2 locus (chromosome 16): Peltekova et al. showed that TC haplotype homozygosity combined with CARD15 mutations substantially elevated Crohn's disease risk beyond either alone, consistent with independent pathway effects (bacterial sensing via NOD2 + intestinal barrier via OCTN2).