What causes ankylosing spondylitis?
The cause of ankylosing spondylitis (AS) is unknown. It is most likely a combination between genes and the environment that play a role in AS. The main gene associated with a risk for getting ankylosing spondylitis is called HLA-B27. This marker is found in greater than 95% of Caucasians and over 90% of all cases of AS, but you don't necessarily have to be a carrier of the HLA-B27 gene variant to have AS. Recently, variants in two other genes, called IL23R and ERAP1, have also been shown to carry a increased risk for AS. Similarly, just because you have the gene, does not mean you will develop AS. Bacterial infections are also suspected to be involved in AS. It is thought that AS may begin when the defenses in the gut (intestines) break down and bacteria enters your bloodstream near the SI joints.