rs10045431 — IL12B

IL12B Upstream Haplotype Tag — Th1/Th17 Risk at the IL-12p40 Locus

The IL12B gene¹¹ IL12B gene
located at chromosome 5q33.3, encodes the 40-kDa p40 subunit shared by interleukin-12 and interleukin-23. This shared subunit means genetic variation at IL12B influences both cytokine axes simultaneously: IL-12 (p40 + p35) drives Th1 differentiation and IFN-γ production, while IL-23 (p40 + p19) sustains Th17 expansion and IL-17 production. Both arms converge on inflammatory skin disease, gut inflammation, and joint inflammation. rs10045431 lies approximately 57 kb upstream of IL12B and is an independent tagging SNP for the IL12B susceptibility haplotype, associated with increased risk for psoriasis (OR ~1.41), Crohn's disease, and other Th1/Th17-mediated autoimmune conditions²² increased risk for psoriasis (OR ~1.41), Crohn's disease, and other Th1/Th17-mediated autoimmune conditions
Capon et al. identified rs10045431 specifically as an independent signal from rs3212227 in the IL12B locus.

The Mechanism

rs10045431 resides within an antisense long non-coding RNA (IL12B-AS1) at the IL12B locus and is in linkage disequilibrium with the well-characterized IL12B risk haplotype defined by rs6887695 (approximately 60 kb further upstream) and rs3212227 (3′ UTR of IL12B). The C allele at rs10045431 (corresponding to G on the IL12B coding strand) tags this risk haplotype. Functional studies of the IL12B risk haplotype show that carriers of the C allele have elevated IL12B mRNA expression in monocytes and dendritic cells, translating to higher serum IL-12 and IFN-γ — with a paradoxical decrease in IL-23³³ elevated IL12B mRNA expression in monocytes and dendritic cells, translating to higher serum IL-12 and IFN-γ — with a paradoxical decrease in IL-23
The haplotype biases the IL-12/IL-23 balance toward Th1 rather than Th17, yet both pathways contribute to the psoriatic and IBD phenotypes. Mechanistically, the upstream regulatory region tagged by rs10045431 contains transcription factor binding sites that modulate IL12B transcription initiation, with the C risk allele altering binding affinity to enhance p40 production.

The Evidence

The initial identification of rs10045431 as an independent IL12B psoriasis-risk SNP came from Capon et al. 2007⁴⁴ Capon et al. 2007
Sequence variants in IL23R and IL12B confer protection against psoriasis; rs10045431 OR 1.41, p=0.0001 in North American + UK cohorts. This was replicated by Nair et al. 2008⁵⁵ Nair et al. 2008
Large-scale study of 2,178 cases and 1,789 controls confirming IL12B and identifying IL23R. In the Crohn's disease domain, Wang et al. 2009⁶⁶ Wang et al. 2009
Pathway-level GWAS meta-analysis identifying rs10045431 with combined p = 3.86 × 10⁻¹³ and OR 1.11 for CD embedded this SNP within the broader IL12/IL23 pathway story. A 2021 meta-analysis comprising 20 studies with over 10,700 CD patients, 10,900 UC patients, and 18,300 controls⁷⁷ 20 studies with over 10,700 CD patients, 10,900 UC patients, and 18,300 controls
Wang et al. 2021 confirmed that rs10045431 significantly modifies IBD risk across all genetic models. At the functional level, Capon et al. 2013⁸⁸ Capon et al. 2013
Susceptibility- associated genetic variation at IL12B enhances Th1 polarization through elevated monocyte p40 expression and increased serum IL-12 and IFN-γ linked the upstream haplotype tagged by rs10045431 to measurable immunological shifts in carriers.

The additive effect size at this locus is modest (OR ~1.11–1.41 per allele depending on condition and study), consistent with a polygenic susceptibility variant rather than a Mendelian cause. Over 80 loci contribute to psoriasis susceptibility; rs10045431 represents one component of the IL12B contribution, with independent signals from rs3212227 (3′ UTR) and rs6887695 (upstream regulatory).

Practical Actions

The most clinically actionable implication for CC allele carriers is pharmacogenomic. Ustekinumab (Stelara) is a monoclonal antibody that binds directly to the p40 subunit encoded by IL12B, blocking both IL-12 and IL-23 simultaneously. Galluzzo et al. 2016⁹⁹ Galluzzo et al. 2016
IL12B (p40) gene polymorphisms contribute to ustekinumab response prediction in HLA-Cw6+ psoriasis patients found that IL12B genotype helps predict ustekinumab response — the IL12B risk haplotype (of which rs10045431 is a tag) increases p40 production, potentially providing more antigen for antibody blockade. Risankizumab and guselkumab target only the p19 (IL-23-specific) subunit and behave differently in carriers with elevated p40 expression.

For carriers who have not yet developed disease, awareness of the IL12B locus's breadth of associations is valuable: this variant is part of the shared genetic architecture underlying psoriasis, psoriatic arthritis, Crohn's disease, ulcerative colitis, and ankylosing spondylitis — all of which share Th1/Th17 inflammatory pathophysiology and respond to the same class of biologics.

Interactions

rs10045431 is in linkage disequilibrium with rs3212227 (IL12B 3′ UTR) and rs6887695 (IL12B upstream). Together these form the IL12B risk haplotype; carriers of the full risk haplotype (rs6887695-C / rs3212227-T / rs10045431-C) have the strongest p40 overexpression phenotype. rs11209026 (IL23R R381Q) encodes a loss-of-function receptor variant strongly protective against psoriasis and IBD by dampening IL-23 signaling. Carrying the IL12B risk haplotype (elevated p40) alongside IL23R R381Q (reduced receptor sensitivity) represents a partially antagonistic combination. rs12188300 is another IL12B near-gene variant with partially overlapping psoriasis associations; individuals carrying risk alleles at both SNPs may have compounded susceptibility via independent regulatory mechanisms at the same locus.