Do ATG16L1 rs2241880 and IL23R rs11209026 increase risk for IBD-related chronic diarrhea and malabsorption?

The claim that ATG16L1 rs2241880 and IL23R rs11209026 are inflammatory bowel disease (IBD) susceptibility variants that increase the risk for chronic inflammatory diarrhea and malabsorption is partially accurate regarding the first variant but fundamentally incorrect regarding the second.

Clinical and genetic evidence

The relationship between these variants and disease risk is divergent:

• ATG16L1 rs2241880: This is a well-validated susceptibility variant, particularly for Crohn’s disease (CD). Large-scale meta-analyses consistently show that the G allele (Thr300Ala) increases CD risk, with pooled odds ratios (OR) typically ranging from 1.23 to 1.35. While the association with ulcerative colitis is less pronounced, it remains statistically significant in various populations.
• IL23R rs11209026: In direct contrast to the claim, this variant is one of the most robust protective factors against IBD identified to date. The minor A allele (Gln381) is associated with a significantly reduced risk of Crohn's disease (OR ~0.62–0.66) and a moderate reduction in ulcerative colitis risk.

Mechanistic explanations

The biological pathways affected by these variants explain their opposing clinical roles:

• Autophagy and Barrier Integrity (ATG16L1): The rs2241880 variant makes the ATG16L1 protein highly sensitive to caspase-3 mediated cleavage, leading to its degradation. This causes autophagy dysfunction, which is particularly damaging to Paneth cells in the small intestine. These cells exhibit morphological defects and impaired secretion of antimicrobial α-defensins, leading to a compromised intestinal barrier and chronic inflammation.
• The IL-23/Th17 Axis (IL23R): The rs11209026 variant is a "loss-of-function" or hypomorphic mutation. It reduces the stability and surface expression of the IL-23 receptor, which dampens JAK2/STAT3 signaling. This decrease in signaling inhibits the differentiation and maintenance of pro-inflammatory Th17 cells. By weakening this primary inflammatory pathway, the variant protects against the gut inflammation that leads to diarrhea and malabsorption.

Clinical implications

While IBD itself is a primary cause of chronic inflammatory diarrhea and malabsorption due to mucosal damage and epithelial dysfunction:

• Carriers of the ATG16L1 rs2241880 variant are at an increased risk for these clinical manifestations because the variant promotes the underlying inflammatory process.
• Carriers of the IL23R rs11209026 variant are at a decreased risk for these symptoms because the variant acts as a molecular brake on the inflammatory response.

Bottom line

The claim is only half-supported. While ATG16L1 rs2241880 is a susceptibility variant that increases the risk of IBD-related diarrhea and malabsorption via impaired autophagy, IL23R rs11209026 is a protective variant that significantly reduces the risk of developing these conditions by dampening the IL-23/Th17 inflammatory pathway.