Оригинал
There is no consensus on that one.
Citing from Mayo clinic website:
"Ankylosing spondylitis has no known specific cause, though genetic factors seem to be involved. In particular, people who have a gene called HLA-B27 are at greatly increased risk of developing ankylosing spondylitis. However, only some people with the gene develop the condition.
Risk factors
Your sex. Men are more likely to develop ankylosing spondylitis than are women.
Your age. Onset generally occurs in late adolescence or early adulthood.
Your heredity. Most people who have ankylosing spondylitis have the HLA-B27 gene. But many people who have this gene never develop ankylosing spondylitis."