Smoking and risk for psoriasis: a population-based twin study

BackgroundSmoking is a potential risk factor for psoriasis. Both psoriasis and smoking habits are partly explained by genetic factors. However, twin studies investigating the association between these traits are limited. MethodsQuestionnaire-based data on smoking habits and psoriasis were collected for 34,781 twins, aged 20-71years, from the Danish Twin Registry. A co-twin control analysis was performed on 1700 twin pairs discordant for lifetime history of smoking. Genetic and environmental correlations between smoking and psoriasis were estimated using classical twin modeling. ResultsAfter multivariable adjustment, age group (50-71 vs. 20-49years) and childhood exposure to environmental tobacco smoke (ETS) were significantly associated with psoriasis in the whole population (odds ratio [OR] 1.15, 95% confidence interval [CI] 1.02-1.29 [P=0.021] and OR 1.28, 95% CI 1.10-1.49 [P=0.002], respectively). Risk for psoriasis increased substantially (OR 2.18, 95% CI 1.82-2.61; P<0.001) for smokers with a history of >5pack-years, even after adjusting for age, sex, and childhood ETS. Among twin pairs discordant for smoking, risk for psoriasis in the ever-smoking twin was lower among monozygotic twins (OR 1.23, 95% CI 0.59-2.56; P=0.578) than among same-sex dizygotic twins (OR 2.21, 95% CI 1.36-3.58; P=0.001). Genetic factors explained 20% (14-25%; P<0.001) of the correlation between psoriasis and smoking, whereas non-shared environmental factors explained 8% (0-22%; P=0.504). ConclusionsTobacco consumption and childhood ETS are significantly associated with psoriasis. Results indicate shared genetic factors for smoking and psoriasis.