Is Malignancy in PsA an Extension of Autoimmune Dysfunction?

The link between autoimmune diseases and malignancy is frequently observed, but not entirely understood. Increased cancer signals have been reported in myositis, scleroderma, and rheumatoid arthritis. This may be due to an interplay between a misguided immune system, as well as the use of immunosuppressant medications. In myositis, one mechanism that has been proposed has been the theory of molecular mimicry, whereby tumour cells incite an immune response that can then be mounted against similar antigens in muscle.

What about other autoimmune diseases? Psoriatic arthritis (PsA) is a disease with almost a "double whammy" of immune dysfunction, which causes inflammation in skin as well as joints. Katelynn Winton and the group at the Mayo Clinic (poster abstract #1684) wanted to explore if the risk of malignancy is also increased in PsA patients. They retrospectively studied and analyzed a cohort of 217 patients with PsA, and compared this group to a cohort of 271 age- and sex-matched subjects without PsA for cancer occurrence. The cohorts consisted mostly of males and the average follow-up was ~15 years.

A total of 28 malignancies were detected in the PsA patients and 41 in the control population. There was no statistical difference in malignancy incidence between the two groups. In addition, the relative risk of most individual malignancies was not increased in the PsA group compared to controls. Interestingly, more cases of non-melanoma skin cancer were identified in the PsA group, although this difference was not statistically significant. In general, it was noted that skin cancers were increased to a greater extent in patients with psoriasis.

What is interesting is that, although PsA is an autoimmune disease, there does not seem to be an increase in other types of malignancies, as compared to some of its "cousin" diseases. This has also been reported by previous studies. Once again, this speaks to the complexity of the immune system as well as the multitude of immune pathways involved in autoimmune disease in general. If anything, it also reinforces the take home message to remember to warn our PsA patients to be vigilant about noting skin changes that may be akin to skin cancers, rather than assuming that all skin changes are more likely secondary to psoriasis itself.

More reports from ACR 2015