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.
The relative risk of RF-seropositive RA was 2.8 (95% confidence interval [95% CI] 1.6-4.8) in never smokers with SE genes, 2.4 (95% CI 1.3-4.6) in current smokers without SE genes, and 7.5 (95% CI 4.2-13.1) in current smokers with SE genes. Smokers carrying double SE genes displayed a relative risk of RF-seropositive RA of 15.7 (95% CI 7.2-34.2). The interaction between smoking and SE genes was significant, as measured by the attributable proportion due to interaction, which was 0.4 (95% CI 0.2-0.7) for smoking and any SE, and 0.6 (95% CI 0.4-0.9) for smoking and a double SE. Neither smoking nor SE genes nor the combination of these factors increased the risk of developing RF-seronegative RA.
The disease risk of RF-seropositive RA associated with one of the classic genetic risk factors for immune-mediated diseases (the SE of HLA-DR) is strongly influenced by the presence of an environmental factor (smoking) in the population at risk.