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


METHODS
Questionnaire-based data on smoking habits and psoriasis were collected for 34,781 twins, aged 20-71 years, 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.


RESULTS
After multivariable adjustment, age group (50-71 vs. 20-49 years) 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 >5 pack-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).


CONCLUSIONS
Tobacco consumption and childhood ETS are significantly associated with psoriasis. Results indicate shared genetic factors for smoking and psoriasis.
Department of Dermato-Allergology, Gentofte Hospital, University of Copenhagen, Hellerup, Denmark.
Ann Sophie
AS
Lønnberg
Department of Dermato-Allergology, Gentofte Hospital, University of Copenhagen, Hellerup, Denmark.
Lone
L
Skov
Danish Twin Registry, University of Southern Denmark, Odense, Denmark.
Axel
A
Skytthe
Institute of Regional Health Services Research, University of Southern Denmark, Odense, Denmark.|Odense Patient Data Explorative Network (OPEN), Odense University Hospital, Odense, Denmark.
Kirsten Ohm
KO
Kyvik
Department of Clinical Immunology, Naestved Hospital, Naestved, Denmark.
Ole Birger
OB
Pedersen
Department of Dermato-Allergology, Gentofte Hospital, University of Copenhagen, Hellerup, Denmark.
Simon Francis
SF
Thomsen
D000328:Adult; D000367:Age Factors; D000368:Aged; D003718:Denmark / Q000453:epidemiology; D004781:Environmental Exposure / Q000706:statistics & numerical data*; D005260:Female; D059647:Gene-Environment Interaction; D006306:Health Surveys; D006801:Humans; D008297:Male; D008875:Middle Aged; D015995:Prevalence; D011565:Psoriasis / Q000453:epidemiology*; D012307:Risk Factors; D012907:Smoking / Q000453:epidemiology*; D014028:Tobacco Smoke Pollution / Q000706:statistics & numerical data*; D014429:Twins, Dizygotic / Q000706:statistics & numerical data; D014430:Twins, Monozygotic / Q000706:statistics & numerical data; D055815:Young Adult
D014028:Tobacco Smoke Pollution