rs7517847 — IL23R

IL23R rs7517847 — The Original IBD Risk Signal and Its Protective Counterpart

When researchers published the first genome-wide association study to identify IL23R as an inflammatory bowel disease gene in 2006, rs7517847 was the single most significant marker in the entire locus¹¹ single most significant marker in the entire locus
P=3.36×10−13 in the combined analysis of both ileal CD case-control cohorts in Duerr et al. Science 2006 — the landmark study that opened the modern era of IBD genetics. That study found the less common G allele was dramatically less frequent in Crohn's disease patients than in healthy controls (33% vs. 44%), establishing the T allele as the risk variant and the G allele as protective. The variant sits in an intron of IL23R²² IL23R
the gene encoding the IL-23-specific subunit of the heterodimeric IL-23 receptor complex on chromosome 1p31.3, which together with IL12RB1/IL-12Rβ1 forms the complete receptor for the Th17-polarizing cytokine IL-23, the receptor for a cytokine that drives Th17 cell expansion and sustains the inflammatory cascades underlying Crohn's disease, ulcerative colitis, ankylosing spondylitis, and psoriasis.

Crucially, rs7517847 operates in a distinct linkage disequilibrium block³³ linkage disequilibrium block
LD blocks are chromosomal regions where nearby variants tend to be inherited together; low LD between two variants means they are statistically and biologically independent signals from the other shipped IL23R variant, rs2201841 (r²=0.03 between the two). They are independent signals capturing different aspects of IL23R genetic architecture within the same locus.

The Mechanism

IL23R encodes the IL-23-specific receptor subunit that, together with the shared IL-12Rβ1 subunit, forms the complete IL-23 receptor complex. Upon IL-23 binding, the receptor activates JAK2 and TYK2, which phosphorylate STAT3 and STAT4, driving expression of RORγt — the master transcription factor for Th17 cell differentiation. Sustained Th17 expansion underlies the epithelial damage in IBD, joint inflammation in ankylosing spondylitis, and keratinocyte hyperproliferation in psoriasis.

rs7517847 is an intronic variant that does not change the IL-23 receptor protein sequence. Its mechanism is regulatory⁴⁴ regulatory
intronic variants can influence splicing efficiency, mRNA stability, transcription factor binding within intronic enhancers, or alternative isoform ratios — any of which could subtly modulate receptor surface density or signaling output. The G allele is associated with reduced IL-23 pathway activation, parallel in direction (though independent in mechanism) to the well-characterized protective missense variant rs11209026 (R381Q), which directly reduces receptor surface expression. rs7517847 represents a second, distinct molecular entry point to damping the same IL-23/Th17 axis.

The T allele is the ancestral common form (~59% globally) and represents baseline or slightly elevated pathway activity. Homozygous T carriers do not have an overactive receptor — rather, GG carriers appear to have a slightly reduced set point for IL-23 signaling that confers population-level protection against Th17-driven inflammatory diseases, especially in people of European ancestry.

The Evidence

The foundational study by Duerr et al. published in Science in 2006⁵⁵ foundational study by Duerr et al. published in Science in 2006
A genome-wide association study identifies IL23R as an inflammatory bowel disease gene, Science 2006 genotyped 297 individuals with ileal Crohn's disease and 148 controls, then replicated in a second cohort, and found rs7517847 to be the most strongly associated IL23R marker (P=3.36×10−13). The G allele frequency was 0.443 in controls and only 0.331 in CD cases, giving an odds ratio of 0.62 for the protective G allele.

A meta-analysis of 25 studies (9,297 CD cases, 12,643 controls)⁶⁶ meta-analysis of 25 studies (9,297 CD cases, 12,643 controls)
Du et al. Scientific Reports 2015 confirmed robust protection: G allele OR=0.699 (95% CI 0.659–0.741, P<0.001) overall, with the Caucasian-specific effect even stronger (OR=0.669). No significant protective effect was seen in Asian or African populations, making this a Caucasian-predominant signal. A complementary meta-analysis of 11 Caucasian studies⁷⁷ meta-analysis of 11 Caucasian studies
Zhang et al. 2015 framing the T allele as the risk factor found TT vs GG homozygote comparison OR=1.890 (95% CI 1.465–2.437) and dominant model OR=1.652 (95% CI 1.277–2.137) for T risk-allele carriers.

For ankylosing spondylitis, a meta-analysis of 4 studies (1,006 AS cases, 1,190 controls)⁸⁸ ankylosing spondylitis, a meta-analysis of 4 studies (1,006 AS cases, 1,190 controls)
Xu et al. PeerJ 2015 found G allele protective OR=0.88 (95% CI 0.78–0.99, P=0.032) and GG vs TT OR=0.76 (P=0.038). The authors concluded that it was the rs7517847 polymorphism rather than rs2201841 that held the primary statistical association with AS. A separate meta-analysis of 12 UC studies (3,589 cases, 5,536 controls)⁹⁹ meta-analysis of 12 UC studies (3,589 cases, 5,536 controls)
Ye et al. 2017 showed G allele protective OR=0.818 (95% CI 0.768–0.871, P<0.001) in Caucasian populations.

The consistency across CD, UC, and AS — each independently replicated — establishes rs7517847 as one of the best-validated non-HLA susceptibility loci for the cluster of IL-23-driven inflammatory diseases.

Practical Implications

For GG carriers, the genetic data indicate a meaningful reduction in susceptibility to Crohn's disease, ulcerative colitis, and ankylosing spondylitis compared to TT carriers, particularly in people of European ancestry. This is a fortunate genotype from an inflammatory disease perspective — but it does not confer immunity, and other genetic and environmental factors still contribute substantially to disease risk.

For TT carriers — the most common genotype (~35% of Europeans) — the elevated risk is real but modest at the individual level. Most TT carriers will never develop Crohn's disease or AS. However, awareness of symptom patterns that suggest early IBD or spondyloarthritis is valuable, since early diagnosis and treatment dramatically improves long-term outcomes in both conditions. The IL-23/Th17 pathway is directly targeted by multiple approved biologics (ustekinumab, risankizumab, guselkumab for IBD and psoriatic disease; secukinumab, ixekizumab for AS), meaning that if inflammatory disease does develop, effective targeted therapies exist.

Note that this variant's protection is predominantly demonstrated in European populations. People of East Asian or African ancestry should interpret GG genotype with more caution, as the population-stratified meta-analyses showed no significant protective signal in these groups — possibly because the T allele frequency is already very high in some Asian populations, limiting the statistical power to detect differences.

Interactions

rs7517847 and rs2201841 are both intronic IL23R variants associated with overlapping disease spectra but tag independent LD blocks (r²=0.03). Carriers who are TT at rs7517847 (risk) and GG at rs2201841 (risk) face elevated susceptibility from both independent signals — the two variants capture distinct aspects of IL23R regulation within the same gene. Conversely, carrying the protective G allele at rs7517847 alongside the protective A allele at rs2201841 may provide additive dampening of IL-23 pathway activity.

The other major IL23R protective variant, rs11209026 (R381Q/Arg381Gln), acts through a different mechanism — directly reducing receptor surface expression — and is in very low LD with rs7517847 (r²=0.03). The three IL23R signals (rs7517847, rs2201841, rs11209026) each independently tag the same biological pathway through distinct molecular entry points.

For Crohn's disease specifically, documented gene-gene interactions between IL23R variants and NOD2/CARD15 variants (rs2066844, rs2066845) suggest that TT carriers who also carry NOD2 risk alleles face a substantially amplified IBD risk. The rs7517847 × NOD2 interaction has not been quantified directly but is biologically plausible given the established IL23R × NOD2 interaction reported for rs2201841.