rs2241879 — ATG16L1

Intronic ATG16L1 variant in tight LD with the T300A coding variant; the G risk allele tags a haplotype that sensitizes the ATG16L1 protein to caspase-3-mediated degradation, impairing autophagy and Paneth cell antimicrobial function, raising risk for Crohn's disease

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ATG16L1 rs2241879 — Autophagy, Paneth Cells, and Crohn's Disease Risk

ATG16L1 (Autophagy Related 16 Like 1)11 ATG16L1 (Autophagy Related 16 Like 1)
A core scaffold protein required for autophagosome formation — the membrane-bound compartment that engulfs and digests intracellular bacteria, damaged organelles, and protein aggregates is one of the strongest and most replicated genetic risk loci for Crohn's disease22 Crohn's disease
A form of inflammatory bowel disease (IBD) that causes transmural intestinal inflammation, most commonly affecting the ileum and colon, driven by a dysregulated immune response to gut bacteria. rs2241879 is an intronic variant within ATG16L1 on chromosome 2q37.1 that sits in tight linkage disequilibrium33 linkage disequilibrium
Two variants are in LD when they are inherited together far more often than expected by chance — they effectively act as proxies for each other with the coding variant rs2241880 (T300A). It tags the same disease-associated haplotype as T300A and reflects identical IBD risk in the European populations where both were originally characterised.

The Mechanism

ATG16L1 is indispensable for the formation of autophagosomes — the double-membrane vesicles that capture intracellular cargo and deliver it to lysosomes for degradation. In the gut, this xenophagy44 xenophagy
Selective autophagy specifically targeting intracellular pathogens rather than the cell's own components pathway is the primary mechanism by which intestinal epithelial cells clear bacteria such as Salmonella and Yersinia that breach the mucosal barrier. ATG16L1 cooperates with NOD255 NOD2
Nucleotide-binding oligomerisation domain 2, a bacterial pattern-recognition receptor that detects muramyl dipeptide (MDP) from bacterial cell walls and is itself a major Crohn's disease risk gene to initiate bacterial autophagy at the site of pathogen entry.

The critical functional consequence of the T300A haplotype (tagged by rs2241879) was revealed by Murthy et al. 201466 Murthy et al. 2014
A landmark Nature paper demonstrating that the threonine-to-alanine substitution at position 300 creates a hypersensitive caspase-3 cleavage motif within ATG16L1. Under conditions of metabolic stress, apoptotic signalling, or pathogen-induced cell death — all common events in an inflamed intestinal epithelium — caspase-3 becomes activated and cleaves ATG16L1. The T300A variant dramatically accelerates this cleavage, causing the protein to be destroyed faster than it can be replenished. The downstream consequence is an abrupt collapse of autophagy capacity precisely when the gut most needs to clear bacteria.

Paneth cells77 Paneth cells
Highly specialised secretory cells at the base of small intestinal crypts that release lysozyme, defensins, and other antimicrobial peptides into the intestinal lumen to maintain sterility of the stem cell niche are disproportionately affected. ATG16L1 is selectively critical for Paneth cell biology: loss-of-function in mice produces abnormal cytoplasmic granule morphology, reduced lysozyme secretion, and dysregulated cytokine production (including elevated leptin and IL-1β). Human Crohn's patients homozygous for the ATG16L1 risk allele display the same Paneth cell abnormalities, and this cellular phenotype is associated with dysbiosis of the ileal microbiome — the site where Crohn's most commonly begins.

The Evidence

The original German association study by Glas et al. 200888 Glas et al. 2008
768 CD patients, 507 UC patients, 1,615 healthy controls, 9 ATG16L1 variants genotyped; rs2241879 showed the joint-strongest CD association alongside rs2241880 found rs2241879 to carry the same effect size as the T300A coding variant (OR 0.74, 95% CI 0.65–0.84, p=3.6×10⁻⁶) — consistent with the two variants being in tight LD on the same haplotype. The A allele (non-risk) was protective across all nine ATG16L1 variants tested.

Replication came rapidly. Prescott et al. 200799 Prescott et al. 2007
Independent UK cohorts of 1,236 CD cases and 1,235 controls confirmed the T300A association with a 1.65-fold overall Crohn's risk and a 2.2-fold risk for ileal disease — the anatomical location where Paneth cells are most abundant. The ileal specificity is mechanistically coherent: Paneth cells exist only in the small intestine and are uniquely dependent on ATG16L1 for granule biology. The landmark Zhang et al. 2009 meta-analysis1010 Zhang et al. 2009 meta-analysis
24 studies, 13,022 CD cases, 17,532 controls confirmed OR 1.87 (95% CI 1.69–2.05) for GG vs AA genotypes in Caucasian populations, with no significant effect in Asian populations, consistent with the different allele frequency distributions across ancestries. The functional mechanism was completed by Murthy et al. 20141111 Murthy et al. 2014
Nature; knock-in mice with T300A showed defective Yersinia clearance and elevated inflammatory cytokines, rescued by caspase-3 deletion.

Evidence level is strong: multiple large European cohorts, a definitive meta-analysis, a clear molecular mechanism, and a causal animal model. The absence of association in Asian populations reflects the very different allele frequency spectrum (G allele ~72% in East Asians vs ~49% in Europeans), not a lack of biological plausibility.

Practical Actions

The rs2241879 G allele raises Crohn's disease risk through impaired bacterial clearance and Paneth cell dysfunction. Recommended actions focus on gut microbiome support, early recognition of IBD symptoms, and specific nutritional approaches that can compensate for impaired xenophagy.

For individuals carrying one or two G alleles, the most important modifiable factor is the gut microbiome: a diverse microbiome with high Lactobacillus and Bifidobacterium content reduces the challenge load that ATG16L1-dependent xenophagy must handle. Conversely, broad-spectrum antibiotic use and ultra-processed food diets that deplete microbial diversity increase the risk that uncleared bacteria trigger mucosal inflammation. Fermented foods that deliver live microorganisms directly to the ileum are particularly relevant given the ileal tropism of Paneth cell defects in this genotype.

Specific nutritional research on ATG16L1 variants suggests that reducing dietary patterns that promote intestinal permeability — particularly high intake of emulsifiers (carboxymethylcellulose, polysorbate 80) and refined sugars — may reduce luminal bacterial translocation and the demand for autophagy-mediated clearance. Vitamin D has a documented role in Paneth cell antimicrobial peptide production and ATG16L1 autophagy regulation, making adequate vitamin D status (serum 25(OH)D ≥50 nmol/L) especially relevant for carriers of this variant.

Interactions

rs2241879 (ATG16L1) and rs4958847 (IRGM) are partners in the same xenophagy pathway. IRGM encodes an immunity-related GTPase that acts upstream of ATG16L1, facilitating the selective recruitment of autophagy machinery to bacteria. Individuals carrying risk alleles at both loci face a compounded impairment: IRGM variants reduce autophagy initiation, and the ATG16L1 T300A haplotype accelerates degradation of the resulting ATG16L1 protein under stress. Multiple GWAS of IBD and CD have confirmed that risk allele burden across IRGM, ATG16L1, and NOD2 loci produces substantially greater CD risk than any single locus alone. The interaction is described in prose for compound action processing by the supervisor.

Genotype Interpretations

What each possible genotype means for this variant.

AA Protective Genotype Normal

No ATG16L1 Crohn's risk — full autophagy capacity

You carry two copies of the A allele at ATG16L1 rs2241879, the genotype associated with the lowest Crohn's disease risk at this locus. About 26% of people of European descent share this genotype. Your ATG16L1 protein is not sensitised to accelerated caspase-3 cleavage, meaning autophagy-mediated bacterial clearance and Paneth cell antimicrobial function remain intact at this locus. This genotype is more common in European populations (A allele ~51%) than in East Asian (A allele ~28%) or African (A allele ~31%) populations.

AG One Risk Allele Intermediate

One ATG16L1 risk allele — modestly reduced autophagy under gut stress

rs2241879 tags the ATG16L1 T300A haplotype (rs2241880, Thr300Ala). The threonine-to-alanine substitution at residue 300 creates a novel caspase-3 cleavage site in ATG16L1. Under cellular stress — including death-receptor activation (e.g. from TNF-α, abundant in inflamed gut), nutrient deprivation, and bacterial infection — caspase-3 is activated and cleaves ATG16L1 at this site. The T300A variant doubles or more the rate of cleavage relative to the wild-type protein, accelerating ATG16L1 degradation and collapsing xenophagy at the moment gut bacteria most need to be controlled.

In heterozygotes, one copy of the sensitised allele is partially buffered by the normal copy. The Glas et al. 2008 German cohort data (OR 0.74 for the A allele in dominant/additive models) and the Zhang et al. 2009 meta-analysis (GA vs AA: OR 1.39, 95% CI 1.27–1.51) are consistent with a meaningful but incomplete impairment in single-copy carriers. Ileal disease, specifically, carries higher risk because Paneth cells — the gut cell type most dependent on ATG16L1 — are concentrated in the ileum.

GG Two Risk Alleles High Risk

Two ATG16L1 risk alleles — substantially elevated Crohn's disease risk

Homozygous G carriers have both copies of ATG16L1 on the T300A haplotype. The Murthy et al. 2014 Nature paper demonstrated that the T300A variant creates a caspase-3 cleavage motif spanning amino acids 296–300 of ATG16L1. Under stress — TNF-α signalling, nutrient starvation, or bacterial infection — caspase-3 is activated in intestinal epithelial cells and cleaves ATG16L1 at this site. In GG homozygotes there is no wild-type copy to compensate, so the protein pool collapses under stress conditions precisely when xenophagy is most needed.

The Prescott et al. 2007 UK cohort documented that ATG16L1 risk homozygotes had 2.2-fold risk specifically for ileal Crohn's disease — anatomically matching the location of Paneth cells, which are the intestinal cell type with the highest ATG16L1 dependence. Human ileal biopsy studies show that GG carriers have structurally abnormal Paneth cell granules with reduced lysozyme content, impaired secretion, and elevated leptin expression — a pattern identical to mice with complete Atg16L1 deletion in the intestinal epithelium.

Crohn's risk compounds substantially with co-occurring NOD2 variants (rs2066844, rs2066845, rs2066847) and IRGM variants (rs4958847). The Prescott et al. 2007 data showed that combined homozygosity at ATG16L1, CARD15/NOD2, and IBD5 produced a 20.4-fold Crohn's risk relative to those carrying no risk alleles — demonstrating that these loci act in a shared autophagy and bacterial sensing pathway.