A gene–environment interaction between smoking and shared epitope genes in HLA–DR provides a high risk of seropositive rheumatoid arthritis

The main genetic risk factor for rheumatoid arthritis (RA) is the shared epitope (SE) of HLA-DR, while smoking is an important environmental risk factor. We studied a potential gene-environment interaction between SE genes and smoking in the etiology of the 2 major subgroups of RA: rheumatoid factor (RF)-seropositive and RF-seronegative disease.

A population-based case-control study involving incident cases of RF-seropositive and RF-seronegative RA (858 cases and 1,048 controls) was performed in Sweden. Cases and controls were classified according to their cigarette smoking status and HLA-DRB1 genotypes. The relative risk of developing RA was calculated for different gene/smoking combinations and was compared with the relative risk in never smokers without SE genes.