Psoriasis is a chronic disorder characterized by cutaneous and systemic manifestations. Epidemiological studies have reported increased comorbidity of psoriasis with numerous complex diseases such as metabolic clinical measurements (Greb et al., 2016; Naito and Imafuku, 2016). However, interpretation of the comorbidity remains controversial to date, because causal inference between correlated phenotypes is difficult when depending solely on epidemiological studies. Identification of causal inference between correlated phenotypes has significant clinical impacts, because modification of the causal phenotypes could benefit treatment of the outcome phenotypes.
