rs1495965 — IL23R

IL23R-IL12RB2 rs1495965 — An Independent Inflammatory Axis Switch Linking Gut, Spine, and Eye

The region between the IL23R¹¹ IL23R
interleukin-23 receptor gene, encoding the cytokine-binding subunit of the receptor complex that captures interleukin-23 and initiates downstream Th17 cell activation and IL12RB2 genes on chromosome 1p31.3 has emerged as one of the most replicated non-HLA autoimmune risk loci across three distinct diseases — Behçet's disease, ankylosing spondylitis, and Crohn's disease. rs1495965 is an intergenic variant within this region that tags a functional haplotype independently associated with each of these IL-23-driven inflammatory conditions. It is not the same signal as the well-known IL23R R381Q protective variant rs11209026²² IL23R R381Q protective variant rs11209026
a missense change that directly reduces IL-23 receptor function and strongly protects against IBD, psoriasis, and AS; rs1495965 is a separate, independent association captured by a different LD block in the same region — it captures an independent source of IL-23 pathway variation at this locus.

The Mechanism

rs1495965 sits in the intergenic space between IL23R and IL12RB2, roughly 80 kb downstream of the IL23R coding sequence. As an intergenic/regulatory variant³³ intergenic/regulatory variant
a change in non-coding DNA that may alter transcription factor binding sites, enhancer elements, or regulatory regions controlling adjacent gene expression, it does not alter either receptor protein's amino acid sequence. The fine-mapping study⁴⁴ fine-mapping study
Cheon et al. 2017 Korean cohort — 453 BD cases, 2,283 controls — found rs1495965 in r²=0.99 linkage disequilibrium with two flanking variants (rs1495966, rs4655535), all localized to the intergenic region rather than the two flanking gene bodies establishes that the disease signal emanates from a regulatory element in the gap between the two receptor genes rather than from their coding sequences.

The likely mechanism involves altered expression or isoform balance of IL23R, IL12RB2, or both. Both encode receptor subunits for cytokines in the interleukin-12 family — IL23R pairs with IL12RB1 to form the IL-23 receptor, while IL12RB2 pairs with IL12RB1 for the IL-12 receptor. Regulatory variants in this region can shift the balance of Th17 versus Th1 immune responses by modulating which cytokine signal dominates T cell polarization. The net consequence of the risk haplotype is a subtly amplified IL-23/Th17 axis — the same axis that drives inflammation in the gut mucosa, spinal entheses, and uveal tissue of the eye.

The Evidence

The original association was identified in a genome-wide association study of Behçet's disease⁵⁵ genome-wide association study of Behçet's disease
Mizuki N et al., Nature Genetics, 2010 — initial Japanese cohort of 612 cases and 740 controls, confirmed in Turkish and Korean replication cohorts, which found rs1495965 at p=2×10⁻¹¹ with OR 1.35 (95% CI 1.24–1.47). Behçet's disease is a systemic vasculitis with inflammatory ulcers, joint involvement, and uveitis that is particularly prevalent in populations along the ancient Silk Road from the Middle East to East Asia. A 2018 clinical study of Behçet's uveitis⁶⁶ 2018 clinical study of Behçet's uveitis
Habot-Wilner et al. — Turkish and Middle Eastern Israeli patients found the risk allele in 77–79% of Behçet's uveitis patients versus only 27.8% of controls, underscoring how dominant this locus is in those ancestral backgrounds.

Korean fine-mapping⁷⁷ fine-mapping
Cheon et al. 2017 confirmed the signal with OR 1.5 (95% CI 1.3–1.7, p=2.5×10⁻⁷) and established that the three correlated variants (rs1495965, rs1495966, rs4655535) are effectively interchangeable tags for the same causal regulatory element.

For ankylosing spondylitis, a meta-analysis of 25 studies comprising 8,431 AS cases and 8,972 controls⁸⁸ meta-analysis of 25 studies comprising 8,431 AS cases and 8,972 controls
Zhong et al., Expert Review of Clinical Immunology, 2018 found the C allele at rs1495965 significantly enriched in AS cases across populations (p<0.001), identifying it as one of three IL23R-region variants with independently replicated AS susceptibility signals.

For Crohn's disease, a Korean pediatric cohort study⁹⁹ Korean pediatric cohort study
Kim et al. 2019 — 141 pediatric CD cases, 150 controls found risk allele OR 1.484 (p=0.018), with homozygous carriers at OR 2.256. Homozygous risk carriers were also more likely to develop penetrating or stenotic disease behavior — the more complicated CD phenotypes associated with worse long-term outcomes.

The evidence across three diseases is rated strong based on genome-wide significance in the original GWAS, independent replication across multiple ancestries, and a formal meta-analysis for AS. Effect sizes are modest (OR 1.35–1.5 per allele), consistent with the polygenic architecture of these conditions.

Practical Implications

The three conditions linked to rs1495965 — Behçet's disease, ankylosing spondylitis, and Crohn's disease — share the IL-23/Th17 inflammatory axis, and their genetic overlap at this locus reflects their shared pathobiology. Carrying one or two copies of the C risk allele elevates susceptibility to all three, and the diseases can co-occur in the same individual.

For Behçet's disease, the hallmark features are recurrent oral ulcers, genital ulcers, uveitis, and skin lesions. The condition is most prevalent in Middle Eastern, Turkish, and East Asian backgrounds. Unexplained recurrent oral ulcers — especially if accompanied by genital ulceration or eye inflammation — warrant rheumatology evaluation.

For ankylosing spondylitis, the key early symptom is inflammatory back pain: onset before age 45, worse at rest and at night, improving with movement and NSAIDs, with morning stiffness lasting more than 30 minutes. Early diagnosis with sacroiliac joint MRI enables treatment before structural fusion occurs.

For Crohn's disease, symptoms include persistent diarrhea, abdominal pain, blood in stool, and unintended weight loss. The pediatric data suggesting more penetrating disease behavior in homozygous risk carriers makes earlier endoscopic evaluation and proactive treatment escalation appropriate if CD is diagnosed.

All three conditions are treated with immunosuppressive therapies targeting the IL-23/Th17 axis, including IL-23 inhibitors¹⁰¹⁰ IL-23 inhibitors
risankizumab, guselkumab, tildrakizumab — approved for Crohn's disease, AS, and psoriasis/PsA; ustekinumab targets the shared IL-12/23 p40 subunit and IL-17 inhibitors.

Interactions

rs1495965 tags an independent signal from the other IL23R variants in the database. rs1004819 and rs2201841 each reside in different LD blocks within the IL23R locus — rs1004819 within an IL23R intron, rs2201841 in a separate intronic region, and rs1495965 in the downstream intergenic region. These three variants capture distinct regulatory effects on the same IL-23 pathway and may have additive susceptibility when risk alleles are co-inherited, though formal interaction analysis has not been published.

The protective missense variant rs11209026 (R381Q) is in the IL23R coding sequence and reduces receptor surface expression — it operates through a fundamentally different mechanism (protein loss-of-function) compared to the regulatory effects of rs1495965. Individuals carrying C risk alleles at rs1495965 who also carry the R381Q protective allele may have partially attenuated risk, as the coding change reduces receptor activity regardless of the regulatory signal from the intergenic region.