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Ankylosing Spondylitis (cont.)

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What causes ankylosing spondylitis?

The tendency to develop ankylosing spondylitis is believed to be genetically inherited, and a majority (nearly 90%) of people with ankylosing spondylitis are born with a gene known as the HLA-B27 gene. Blood tests have been developed to detect the HLA-B27 gene marker and have furthered our understanding of the relationship between HLA-B27 and ankylosing spondylitis. The HLA-B27 gene appears only to increase the tendency of developing ankylosing spondylitis, while some additional factor(s), perhaps environmental, are necessary for the disease to appear or become expressed. For example, while 7% of the United States population has the HLA-B27 gene, only 1% of the population actually has the disease ankylosing spondylitis. In northern Scandinavia (Lapland), 1.8% of the population has ankylosing spondylitis while 24% of the general population has the HLA-B27 gene. Even among HLA-B27-positive individuals, the risk of developing ankylosing spondylitis appears to be further related to heredity. In HLA-B27-positive individuals who have relatives with the disease, the risk of developing ankylosing spondylitis is 12% (six times greater than for those whose relatives do not have ankylosing spondylitis).

Recently, two more genes have been identified that are associated with ankylosing spondylitis. These genes are called ARTS1 and IL23R. These genes seem to play a role in influencing immune function. It is anticipated that by understanding the effects of each of these known genes researchers will make significant progress in discovering a cure for ankylosing spondylitis.

How inflammation occurs and persists in different organs and joints in ankylosing spondylitis is a subject of active research. Each individual tends to have their own unique pattern of presentation and activity of the illness. The initial inflammation may be a result of an activation of the body's immune system, perhaps by a preceding bacterial infection or a combination of infectious microbes. Once activated, the body's immune system becomes unable to turn itself off, even though the initial bacterial infection may have long subsided. Chronic tissue inflammation resulting from the continued activation of the body's own immune system in the absence of active infection is the hallmark of an inflammatory autoimmune disease.

Reviewed by Catherine Burt Driver, MD on 3/9/2012

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