IL23R

Several variations (polymorphisms) in the IL23R gene have been found to influence the risk of ankylosing spondylitis. This condition is a form of painful, ongoing joint inflammation (chronic inflammatory arthritis) that primarily affects the spine. 

One of these IL23R gene polymorphisms appears to reduce the likelihood of developing this disorder. This genetic change alters a single protein building block (amino acid) in the IL-23 receptor, replacing the amino acid arginine with the amino acid glutamine at protein position 381 (written as Arg381Gln or R381Q). Other IL23R variations appear to increase the risk of developing ankylosing spondylitis. 

It is not fully known how these changes are related to a person's risk of developing this disorder, but studies suggest that the effects of IL23R variations are likely related to the IL-23 receptor's role in inflammation. Other genetic and environmental factors, many of which are unknown, also affect the chance of developing ankylosing spondylitis.

Several variants in or near the IL23R gene have been found to influence the risk of developing Crohn's disease. These associations have been reported primarily in people of northern European ancestry. For example, Arg381Gln, which is a protective factor for ankylosing spondylitis (described above), also appears to reduce the risk of developing Crohn's disease. Although it is unclear how this change protects against Crohn's disease, researchers believe that the receptor's role in triggering inflammation in the intestinal walls may underlie its connection with this disorder.

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Variants in the IL23R gene have also been associated with the risk of several other immune system-related conditions, including a skin disorder called psoriasis. In people with light skin, this chronic inflammatory condition causes patches of red, irritated skin that are often covered by flaky white scales, but these patches are darker looking in people with darker skin. Psoriasis likely results from a malfunction of the immune system in which the body's immune response turns against itself, attacking healthy skin cells by mistake.

Each of the known IL23R variations changes a single amino acid in the IL-23 receptor. One of these variations, Arg381Gln, appears to reduce the risk of developing psoriasis. (This variation has also been shown to protect against ankylosing spondylitis and Crohn's disease, described above.) Other IL23R variations may increase the risk of developing psoriasis. Researchers suggest that changes in the IL23R gene may contribute to general problems with regulation of the immune system, which may help explain why these variations are related to several different disorders characterized by immune system dysfunction.