The causal relationship between psoriasis, psoriatic arthritis, and inflammatory bowel diseases

Psoriasis is more common in patients with inflammatory bowel disease (IBD) than in the general population. Similarly, patients with psoriasis or psoriatic arthritis (PsA) have a higher incidence of IBD. However, whether this association is causal remains unknown. Therefore, we used a two-sample bidirectional Mendelian randomization (MR) analysis to identify this relationship. According to MR analysis, psoriasis and PsA causally increased the odds of developing Crohn’s disease (OR = 1.350 (1.066–1.709) P = 0.013; OR = 1.319 (1.166–1.492) P < 0.001). In contrast, MR estimates gave little support to a possible causal effect of psoriasis, PsA, on ulcerative colitis (OR = 1.101 (0.905–1.340) P = 0.335; OR = 1.007 (0.941–1.078) P = 0.831). Similarly, the reverse analysis suggested the Crohn’s disease causally increased the odds of psoriasis and PsA (OR = 1.425 (1.174–1.731) P < 0.001; OR = 1.448 (1.156–1.182) P = 0.001), whereas there are no causal association between ulcerative colitis and psoriasis, PsA (OR = 1.192 (0.921–1.542) P = 0.182; OR = 1.166 (0.818–1.664) P = 0.396). In summary, our MR analysis strengthens the evidence for the bidirectional dual causality between psoriasis (including PsA) and Crohn’s disease.