Background: Psoriasis is a chronic T-cell-mediated immunological skin disease with a complex pathogenesis where both genetic and environmental factors are involved.
Objective: To study the conditional and relative risk of developing psoriasis in identical and fraternal twins whose co-twin has a positive history of the disease and to estimate the relative contribution of genetic and environmental factors on the liability for psoriasis in Norway.
Methods: Self-reported history of psoriasis in twins from the population-based Norwegian Twin Panel (N = 8045) was studied. Absolute and relative risks of developing psoriasis conditioned on the positive history of psoriasis in a co-twin were calculated by Kaplan-Meier survival analysis and Cox regression, respectively. Structural equation modelling was used to estimate genetic and environmental variance components.
Results: Altogether, 334 (4.2%) of the twins reported having psoriasis. No difference in prevalence of the disease across sexes and zygosity groups was found. Identical twins were more likely to develop psoriasis than fraternal twins if a co-twin reported having the disease. The best-fitting model showed that additive genetic effects could explain 66% of the variation in liability for psoriasis in this population, and the remaining 34% was due to non-shared environmental influences.
Conclusions: High heritability due to additive genetic effects together with considerable environmental contribution to the liability of psoriasis support the current opinion on the multifactorial aetiology of the disease. No sex-specific patterns of heritability of psoriasis were found.