Genes Play Minor Role in Arthritis

Feb. 4, 2002 -- People with rheumatoid arthritis are often looking for reasons why they have the disabling joint pain and swelling. The latest study weighing in on this issue shows that it may have little to do with genes -- and much more to do with the environment.

rheumatoid arthritis, or RA, affects more than 2 million Americans. Both environmental and genetic reasons for the disease have been identified, but no single factor has emerged as significant enough to cause the disease, according to the researchers.

Anders J. Svendsen and colleagues used a study of more than 37,000 twins to compare environmental vs. genetic causes of RA. Twin studies are a simple but effective way of doing this.

Researchers compared the frequency of RA in identical twins -- who share 100% of the same genes -- to non-identical twins. Non-identical twins share about 50% of the same genes -- same as a non-twin brother or sister.

RA was seen in 13 identical twins and in 36 non-identical twins. There were no cases in which both identical twins had RA but two sets of non-identical twins had the disease.

These findings suggest that RA is no more common in identical twins than in non-identical twins. This means that the environment is likely to play a much bigger role in causing RA than genetics.

This study doesn't prove that there is no association between RA and genetics. It's not uncommon for people with RA to have another family member with the disease. In fact, in an editorial accompanying the study, Alan J. Silman says there is undoubtedly a genetic contribution to RA.

But these results do emphasize that the genetic effects are weak compared to the environmental factors, writes Silman.