To our knowledge, this is the first study to show an association of cigarette smoking with RA in African Americans. We found that this association is most striking in heavy smokers and those with HLA-DRB1 SE
containing alleles. Among African Americans with a cumulative smoking history exceeding 10 pack-years, the risk of RA is increased by more than two-fold, and this risk is increased to more than four-fold in the presence of SE
alleles. In contrast, the risk of RA among ever smokers with a cumulative exposure of less than 10 pack-years appears to be negligible. Assuming that the smoking behaviors reported by controls in this study (where 15% reported a smoking history of more than 10 pack-years) reflect those of African Americans nationally, the attributable risk of RA due to heavy smoking exposure in this population may be as high as 16%. Said in another way, our results suggest that approximately 1 in 6 new cases of RA occurring in African Americans could be prevented through smoking cessation or by limiting cumulative smoking exposure in this population to less than 10 pack-years. In light of reports suggesting that smoking is on the rise among African Americans (15
), our results suggest that RA incidence and disease burden may increase in this population over the next decades.
It is worth noting that some uncertainty remains regarding the optimal method to model gene
-environment interactions (31
). In contrast to prior studies that have examined smoking-SE
interactions in RA risk using only additive interaction (2
), we have examined measures of both additive and multiplicative interaction. Multiplicative interaction refers to the inclusion of a product term in regression analyses to generate an ‘optimal fit’ of the data in the statistical model. It is important to note that the absence of multiplicative interaction does not exclude the existence of important biologic or additive interactions, which in the case of this study show that at least one ‘pathway’ to RA development in African Americans requires the presence of two risk factors (i.e. heavy smoking and HLA-DRB1 SE
The results presented here are similar to a recent report from the Nurses’ Health Study (NHS), a nested case-control study of more than 100,000 women of European ancestry in the U.S. including 680 women with incident RA (11
). In the NHS study (11
), investigators found evidence of significant additive interaction between HLA-DRB1
and heavy smoking (> 10 pack-years) in overall RA risk with an AP of 0.39 (95% CI 0.08 to 0.69, p = 0.01). Although the NHS study did not yield evidence of multiplicative interaction in overall disease risk (p = 0.14), there was evidence of a significant multiplicative interaction between HLA-DRB1 SE
and heavy smoking in the development of seropositive RA (a phenotype based on RF status in some patients and anti-CCP antibody status in others) (11
). The absence of multiplicative interaction in our study of African Americans may relate to the smaller number of study participants and limited power relative to the larger NHS study. NHS investigators also found no evidence of additive interaction between HLA-DRB1 SE
and ever smoking status in disease risk (AP = 0.23; 95% CI −0.14 to 0.61, p = 0.23). Evidence of additive interaction in CLEAR between SE
and ever smoking was similarly attenuated compared to interaction between SE
and heavy smoking, although still statistically significant in CLEAR. Results from the NHS suggest that accounting for cumulative exposure is essential in assessing the role of cigarette smoking and gene-smoking interactions in RA. In light of our results, these conclusions can now be extended to African Americans. Failure to account for smoking “dose” could explain the lack of substantial SE
-smoking interaction found in other North American cohorts (28
Biologic interactions between HLA-DRB1 SE
alleles and smoking in RA risk have been shown in several epidemiological investigations, although ours is the first study to exclusively involve African Americans. Taken together, these studies suggest that smoking may trigger initial inflammatory events in RA that are HLA-DRB1
dependent. Previous studies, which involved populations of European ancestry, have shown that SE
-smoking interactions are most evident in the development of anti-CCP antibody positive disease. These results have been interpreted to mean that smoking either upregulates citrullination or enhances the immunogenicity of citrullinated peptides in the context of select HLA
alleles. However, our study of African Americans showed that the associations of heavy smoking with RA are similar for autoantibody positive and negative disease, although risk estimates were consistently higher for seropositive RA. This finding is in direct contrast to results from reports involving populations of European ancestry (2
) and one that will require replication in separate study populations. These results are consistent with a prior case-only analysis done in a subset of CLEAR-I patients showing no association of smoking with anti-CCP antibody positivity (32
). In the present study of African Americans, heavy smoking was associated with a significant and more than two-fold increased risk for both anti-CCP antibody negative RA and RF negative RA. By contrast, in a large national case-control study from Sweden, Klareskog and colleagues found no associations of smoking with the risk of anti-CCP antibody negative RA, regardless of HLA DRB1 SE
). Reasons for this apparent discrepancy are unknown, but it is possible that there are other genetic and/or environmental factors that could mediate the effect of smoking in autoantibody negative RA and the prevalence of these as of yet undefined factors could vary markedly in prevalence based on race/ethnicity. In addition to differences in the study populations and accounting for cumulative exposures, variations in study design, ascertainment of smoking status, and different methods of analysis could also serve to explain differences across published reports.
Limitations to this study are those inherent to its case-control design. These include possible recall bias and the possibility of a “healthy responder” bias among controls. This latter concern is mitigated by the recruitment and enrollment of healthy controls residing in the same census tracts as RA cases, individuals similar to cases in regards to sociodemographic characteristics and other “unmeasured confounders”. Similarities between cases and controls were further borne out at the genetic level, with examinations of AIMS showing very similar levels of European admixture in both groups.
The case-control design also prohibits conclusions regarding the ‘direction’ of the associations examined, although for all RA cases included in this study initial smoking exposure preceded disease onset, in most cases by many years. We also found no major differences in risk estimates corresponding to heavy smoking in analyses limited to CLEAR-I (RA cases with disease duration < 2 years) and analyses limited to CLEAR-II (RA cases with any disease duration), suggesting that recall bias and protopathic bias (disease onset leading to exposure) are less likely issues. If a protopathic bias had been operative in these findings, we would have expected to have observed much stronger associations of heavy smoking with RA in CLEAR-II, a cohort that included RA patients with established disease and a much longer time interval between disease onset and study enrollment. Despite these potential limitations, this effort represents the largest study to date examining the impact of smoking and gene-smoking interactions in RA in a well characterized African American population, a group that has been vastly understudied in RA epidemiology.
In summary, cigarette smoking is significantly associated with RA in African Americans, an association that is most pronounced with a cumulative smoking history exceeding 10 pack-years. Similar to reports involving populations of European ancestry, the risk attributed to smoking is highest in African Americans positive for HLA-DRB1 SE alleles with evidence of a significant biologic interaction between SE and heavy smoking in RA risk.