What this means

HLA-B27 is one of the most strongly disease-associated immune system alleles. Around 90% of people with ankylosing spondylitis are HLA-B27 positive, but the inverse isn't true — only ~5% of HLA-B27 positive people develop ankylosing spondylitis. The variant is also associated with reactive arthritis, acute anterior uveitis, and psoriatic arthritis. rs4349859 is a strong tag SNP for the HLA-B27 allele.

HLA-B27 is a particular version of an immune system gene strongly linked to a kind of inflammatory back arthritis called ankylosing spondylitis. About 9 in 10 people who develop that disease carry HLA-B27 — but the reverse isn't true: only around 5 in 100 people who carry it ever go on to develop the disease. The same version is also linked to a few related conditions: reactive arthritis, a kind of eye inflammation called acute anterior uveitis, and psoriatic arthritis. Knowing you have it can help a doctor connect the dots if you ever get unexplained joint or eye inflammation.

Caveats

• Tag SNP correlation with the actual HLA allele is high but imperfect — clinical HLA typing confirms.
• Most HLA-B27 carriers never develop disease.
• Useful as one input to a clinical evaluation, not as a diagnosis by itself.

References

• Brown et al. — HLA class I associations of ankylosing spondylitis (Arthritis & Rheumatism, 1996)
• Reveille — The genetic basis of ankylosing spondylitis (Current Opinion in Rheumatology, 2006)