AP1S3 Mutations Cause Skin Autoinflammation by Disrupting Keratinocyte Autophagy and Up-Regulating IL-36 Production

Abstract

Prominent skin involvement is a defining characteristic of autoinflammatory disorders caused by abnormal IL-1 signaling. However, the pathways and cell types that drive cutaneous autoinflammatory features remain poorly understood. We sought to address this issue by investigating the pathogenesis of pustular psoriasis, a model of autoinflammatory disorders with predominant cutaneous manifestations. We specifically characterized the impact of mutations affecting AP1S3, a disease gene previously identified by our group and validated here in a newly ascertained patient resource. We first showed that AP1S3 expression is distinctively elevated in keratinocytes. Because AP1S3 encodes a protein implicated in autophagosome formation, we next investigated the effects of gene silencing on this pathway. We found that AP1S3 knockout disrupts keratinocyte autophagy, causing abnormal accumulation of p62, an adaptor protein mediating NF-κB activation. We showed that as a consequence, AP1S3-deficient cells up-regulate IL-1 signaling and overexpress IL-36α, a cytokine that is emerging as an important mediator of skin inflammation. These abnormal immune profiles were recapitulated by pharmacological inhibition of autophagy and verified in patient keratinocytes, where they were reversed by IL-36 blockade. These findings show that keratinocytes play a key role in skin autoinflammation and identify autophagy modulation of IL-36 signaling as a therapeutic target.

Regulation and function of interleukin‐36 cytokines

“…In addition, AP1S3 knockdown disrupts keratinocyte autophagy, causing abnormal accumulation of p62, an adaptor protein mediating NF‐κB activation. As a consequence, AP1S3 ‐deficient cells upregulated IL‐1 signaling and overexpress IL‐36α . All together, these results indicate that IL‐36 may play an important role in psoriasis as well as in other inflammatory skin pathologies (Figure ).…”

Autophagy and autoimmunity

“…Autophagy negatively regulates TLR2/6 mediated NF-κB activation, SQSTM1 expression, and cytokine secretion in human keratinocytes that are critical to skin inflammation as observed in psoriasis/psoriatic arthritis [54]. Indeed other studies have shown that mutation of psoriasis risk gene AP1S3 that leads to impaired autophagy, and accumulation of SQSTM1, results in up-regulation of IL-36 in keratinocytes and causes skin inflammation [84]. Furthermore, increased expression of autophagy-related protein ATG16L1 is observed in dendritic cells derived from psoriatic arthritis patients compared to healthy controls that suggests autophagy involvement in psoriatic arthritis pathogenesis [85].…”

The genetic basis for most patients with pustular skin disease remains elusive

“…Genetic variants in CARD14 have also been implicated as disease‐causing/disease‐contributing factors in patients with GPP and PPP; in vitro experiments of most of these variants suggest an activation of nuclear factor kappa B (NF‐κB) . Two missense variants in AP1S3 have further been described to contribute to ACH, GPP and PPP and in vitro analysis suggests a reduced protein function resulting in disruption of keratinocyte autophagy and an increased activation of NF‐κB leading to upregulation of IL‐1 signalling and overexpression of IL‐36α …”