rs104895431 — NOD2

Rare NOD2 missense variant (Ser431Leu) that reduces NF-κB activation in response to bacterial muramyl dipeptide, independently associated with Crohn's disease risk; often co-inherited on a haplotype with the V793M variant

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NOD2 S431L — A Rare Bacterial-Sensing Variant That Quietly Elevates Crohn's Risk

Your immune system never stops reading bacteria. In the lining of your small intestine, an intracellular sensor called NOD2 (nucleotide-binding oligomerization domain-containing protein 2)11 NOD2 (nucleotide-binding oligomerization domain-containing protein 2)
a pattern-recognition receptor expressed in intestinal epithelial cells, Paneth cells, and monocytes; detects muramyl dipeptide (MDP), a fragment of bacterial cell-wall peptidoglycan monitors every passing microbe and decides whether to mount a defensive response or stand down. NOD2 was the first gene linked to Crohn's disease22 first gene linked to Crohn's disease
identified in 2001 through positional cloning of the IBD1 locus on chromosome 16q12, and it remains the single strongest genetic risk factor for this condition.

rs104895431 encodes the S431L substitution — serine replaced by leucine at position 431. This is a rare variant, carried by roughly 0.16% of people of European descent, and it was identified as one of five rare NOD2 variants that reached genome-wide significance33 five rare NOD2 variants that reached genome-wide significance
Rivas et al. identified R311W, S431L, R703C, N852S, and M863V through deep resequencing of GWAS loci in 16,054 CD cases and 17,575 controls for Crohn's disease risk in a large resequencing study.

The Mechanism

Functional characterization of this variant shows borderline-to-significant reduction in both basal NF-κB activity and MDP-stimulated NF-κB activation44 borderline-to-significant reduction in both basal NF-κB activity and MDP-stimulated NF-κB activation
reported by Chamaillard et al.; compared to wild-type NOD2, S431L reduces the protein's ability to mount the downstream inflammatory cascade that normally clears invading bacteria compared to wild-type NOD2.

This reduced NOD2 function has two important downstream consequences. First, Paneth cells in the ileum55 Paneth cells in the ileum
specialized epithelial cells in the crypts of Lieberkühn that secrete antimicrobial peptides to regulate the small intestinal microbiome — which express high levels of NOD2 — produce fewer alpha-defensins (HD-5 and HD-6) when NOD2 signaling is impaired. Reduced alpha-defensin expression is significantly more pronounced in Crohn's disease patients who carry NOD2 mutations66 Reduced alpha-defensin expression is significantly more pronounced in Crohn's disease patients who carry NOD2 mutations
compared to both wild-type CD patients and healthy controls; the deficit allows overgrowth of bacterial species in the terminal ileum. Second, impaired NOD2 function blunts normal suppression of Toll-like receptor (TLR) signaling77 Toll-like receptor (TLR) signaling
NOD2 normally cross-regulates TLR2/4 to prevent excessive inflammatory responses to gut commensals; when NOD2 is weakened, TLR-driven inflammation runs less regulated, predisposing the mucosa to dysregulated inflammatory flares.

A notable feature of S431L is its very frequent co-inheritance with a second NOD2 variant, V793M. The two changes are in tight linkage disequilibrium88 tight linkage disequilibrium
they co-segregate on a shared ancestral chromosome so reliably they are treated as a single haplotype unit in most genetic analyses, and probands carrying S431L almost always carry V793M on the same chromosome (in cis). This haplotype pattern makes it difficult to assign independent functional effects to S431L alone.

The Evidence

The primary evidence comes from Rivas et al. 201199 Rivas et al. 2011
deep resequencing of GWAS loci across 56 genes in pooled sequencing of 350 cases/350 controls, with follow-up genotyping in nine independent series totaling 16,054 CD cases, 12,153 UC cases, and 17,575 controls, which identified S431L as reaching genome-wide significance (P < 5×10⁻⁸) for Crohn's disease risk independently of the three common NOD2 variants (R702W, G908R, L1007fs). This methodology — deep resequencing followed by large-scale genotyping — was designed specifically to detect rare variants missed by standard GWAS arrays.

The broader NOD2 risk architecture has been quantified in a meta-analysis of 75 case-control studies1010 meta-analysis of 75 case-control studies
18,727 CD cases and 17,102 controls: heterozygous NOD2 carriers show an OR of 2.4 for Crohn's disease; those with two variants (either compound heterozygous or homozygous) have ORs of 9.0 and 6.7 respectively. S431L falls within the rare variant category where individual odds ratios have not been established with precision, but the genome-wide significant signal confirms its independent contribution to risk.

The clinical relevance of NOD2 variants extends beyond diagnosis: a genomic biomarker study1111 genomic biomarker study
Ashton et al. 2023, testing cohort of 161 CD patients found that patients with NOD2 variant burden (high-risk group) had a 56.7% rate of stricturing disease versus 21.4% in the low-risk group (HR 2.09 overall; HR 4.89 in pediatric-onset disease combined with terminal ileal location). NOD2 carriers are specifically at risk for ileal and terminal ileal disease1212 ileal and terminal ileal disease
NOD2 is most highly expressed in the terminal ileum, making this segment the most vulnerable to impaired bacterial sensing and consequent inflammation.

A recessive inheritance study1313 recessive inheritance study
Horowitz et al. 2021; 1,183 pediatric IBD probands analyzed for biallelic NOD2 variants found that S431L and V793M were inherited together in cis from carrier parents in three complete trios.

Practical Implications

Carrying one copy of the S431L variant (CT genotype) means one of your two NOD2 copies has reduced MDP-sensing and NF-κB activation capacity. The other copy is fully functional, providing substantial backup. The primary concern is vigilance for Crohn's disease symptoms — particularly those implicating the terminal ileum — and early evaluation when symptoms arise.

Since NOD2 impairment specifically affects ileal Paneth cell function and microbiome composition1414 ileal Paneth cell function and microbiome composition
NOD2-deficient intestinal epithelium shows increased colonization by Enterobacteriaceae and decreased beneficial Firmicutes, particularly in the ileum, strategies that support gut microbiome diversity and mucosal barrier integrity are particularly relevant for NOD2 variant carriers.

Interactions

S431L almost always co-occurs with V793M on the same chromosome, forming a distinct NOD2 haplotype.

The downstream NF-κB pathway is shared with TNFAIP3 (A20) variants including rs2230926 and rs50299391515 TNFAIP3 (A20) variants including rs2230926 and rs5029939
A20 normally terminates NF-κB signals triggered by NOD2 activation; loss-of-function A20 variants could amplify the baseline inflammatory state in NOD2 risk carriers.

Genotype Interpretations

What each possible genotype means for this variant.

CC Normal NOD2 S431 Normal

Wild-type serine at position 431 — full NOD2 bacterial sensing function

You carry two copies of the common C allele at rs104895431, meaning both your NOD2 proteins have the normal serine amino acid at position 431. Your NOD2 pattern-recognition receptor is capable of fully sensing bacterial muramyl dipeptide and triggering appropriate NF-κB activation to defend the intestinal mucosa. About 99.7% of people carry this genotype. Your risk for Crohn's disease from this specific variant is at the population baseline.

CT S431L Carrier Intermediate

One NOD2 S431L variant — reduced bacterial sensing on one chromosome

Functional studies show this variant reduces both basal NF-κB activity and MDP-stimulated NF-κB activation relative to wild-type NOD2.

Since you still have one fully functional NOD2 copy, the clinical consequence is more subtle than for biallelic carriers. The primary concern is monitoring for Crohn's disease symptoms, particularly those involving the terminal ileum — the intestinal segment most dependent on NOD2-driven Paneth cell antimicrobial function. NOD2 risk variants as a class are associated with stricturing (scarring) disease patterns and ileal involvement if Crohn's disease does develop.

TT Homozygous S431L High Risk

Two NOD2 S431L variants — substantially impaired bacterial sensing on both chromosomes

Biallelic NOD2 variants account for approximately 8% of early-onset inflammatory bowel disease and are associated with a markedly severe phenotype: earlier disease onset, predominantly ileal location, high rates of stricturing behavior requiring surgical intervention, and a clinical course that more closely resembles a monogenic inflammatory disease than typical polygenic Crohn's disease.

With S431L on both chromosomes, both NOD2 copies show reduced MDP-sensing and NF-κB activation. Ileal Paneth cells — the front-line source of antimicrobial alpha-defensins in the small intestine — depend heavily on NOD2 function. Severely impaired NOD2 leads to reduced defensin output, bacterial dysbiosis in the terminal ileum (increased Enterobacteriaceae, decreased beneficial Firmicutes), and a mucosal environment that struggles to maintain homeostasis against commensal bacteria.

Proactive gastroenterology engagement is warranted even before symptoms develop.