Long-term Remission of Pyoderma Gangrenosum, Acne, and Hidradenitis Suppurativa Syndrome

In further pursuit of PASH syndrome pathophysiology, many experts have sought to uncover the relationship between PASH syndrome and the previously described pyogenic arthritis, PG, and acne (PAPA) syndrome, another entity within the AIDs spectrum (Table). This condition was first recognized in 1997 in a 3-generation family with 10 affected members.¹ It is characterized by PG and acne, similar to PASH; however, PAPA syndrome includes PG arthritis and lacks HS. Pyogenic arthritis manifests as recurrent aseptic inflammation of the joints, mainly the elbows, knees, and ankles. Pyogenic arthritis commonly is the presenting symptom of PAPA syndrome, with onset in childhood.² As patients age, the arthritic symptoms decrease, and skin manifestations become more prominent.

PAPA syndrome has autosomal-dominant inheritance with mutations on chromosome 15 in the proline-serine-threonine phosphatase interacting protein 1 (PSTPIP1) gene.¹ This mutation induces hyperphosphorylation of PSTPIP1, allowing for increased binding affinity to pyrin. Both PSTPIP1 and pyrin are co-expressed as parts of the NLRP3 inflammasome in granulocytes and monocytes.¹ As a result, pyrin is more highly bound and loses its inhibitory effect on the NLRP3 inflammasome pathway. This lack of inhibition allows for uninhibited cleavage of pro–IL-1β to active IL-1β by the inflammasome.¹

Elevated concentrations of IL-1β in patients with PAPA syndrome result in a dysregulation of the innate immune system. IL-1β induces the release of proinflammatory cytokines, namely TNF-α; interferon γ; IL-8; and regulated on activation, normal T cell expressed and secreted (RANTES), all of which activate neutrophils and induce neutrophilic inflammation.² IL-1β not only initiates this entire cascade but also acts as an antiapoptotic signal for neutrophils.² When IL-1β reaches a critical threshold, it induces enough inflammation to cause severe tissue damage, thus causing joint and cutaneous disease in PAPA syndrome. IL-1 inhibitors (anakinra) or TNF-α inhibitors (etanercept, adalimumab, infliximab) have been used many times to successfully treat PAPA syndrome, with TNF-α inhibitors providing the most consistent results.

Another AIDs entity with similarities to both PAPA syndrome and PASH syndrome is pyogenic arthritis, PG, acne, and HS (PA-PASH) syndrome. First identified in 2012 by Bruzzese,⁹ genetic analyses revealed a p.E277D missense mutation in PSTPIP1 in PA-PASH syndrome. Research has suggested that the key molecular feature is neutrophil activation by T_H17 cells and the TNF-α axis.⁹ This syndrome has not been further characterized, and little is known regarding adequate treatment for PA-PASH syndrome.

Although it is similar in phenotype to aspects of PAPA and PA-PASH syndromes, PASH syndrome has distinct genotypic and immunologic abnormalities. Genetic analysis of this condition has shown an increased number of CCTG repeats in proximity to the PSTPIP1 promoter. It is hypothesized that these additional repeats predispose patients to neutrophilic inflammation in a similar manner to a condition described in France, termed aseptic abscess syndrome.^1,5 Other mutations have been identified, including those in IL-1N, PSMB8, MEFV, NOD2, NCSTN, and more.^2,7 However, it has been determined that the majority of these variants have already been filed in the Single Nucleotide Polymorphism Database or in the Registry of Hereditary Auto-inflammatory Disorders Mutations.² The question remains regarding the origin of inflammation seen in PASH syndrome; the potential role of biofilms; and the relationship between PASH, PAPA, and PA-PASH syndromes. Much work remains to be done in refining therapeutic options for PASH syndrome. Continued biochemical research is necessary, as well as collaboration among dermatologists worldwide who find success in treating this condition.

Conclusion

There are genotypic and phenotypic similarities between PASH, PAPA, and PA-PASH syndromes, with various mutations within or near the PSTPIP1 gene; however, their genetic discrepancies seem to play a major role in the pathophysiology of each syndrome. Much work remains to be done in PA-PASH syndrome, which has not yet been well described. Meanwhile, PAPA syndrome has been well characterized with mutations affecting proteins of the NLRP3 inflammasome, resulting in elevated IL-1β and excess neutrophilic inflammation. In PASH syndrome, the importance of increased repeats near the PSTPIP1 promoter is yet to be elucidated. It has been shown that these abnormalities predispose individuals to neutrophilic inflammation, but the mechanism by which they do so is unknown. In addition, consideration of biofilms and their predisposition to inflammation within the pathophysiology of PASH syndrome is a possibility that must be considered when discussing therapeutic options. Based on our case study and previous successes in treating PASH syndrome, it is clear that a multidrug approach is necessary for remission. It is likely that the etiology of PASH syndrome is multifaceted and involves hyperactivity in multiple arms of the innate immune system.

Patients with PASH syndrome have severely impaired quality of life and often experience social withdrawal due to the disfiguring sequelae and limited treatment options available. To improve patient outcomes, it is essential for physicians and scientists to report on successful treatment strategies and advances in immunologic understanding. Improved understanding of PASH syndrome calls for further genetic exploration into the role of additional genomic repeats and how these affect the PSTPIP1 gene and inflammasome activity. As medical advances improve understanding of the pathophysiology of this disease entity, it will likely become clear which mechanisms are most important in disease progression and how clinicians can best optimize treatment.