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Haploinsufficiency of A20 impairs protein-protein interactome and leads into caspase-8-dependent enhancement of NLRP3 inflammasome activation

By Kristiina Rajamäki, Salla Keskitalo, Mikko Seppänen, Outi Kuismin, Paula Vähäsalo, Luca Trotta, Antti Väänänen, Virpi Glumoff, Paula Keskitalo, Riitta Kaarteenaho, Airi Jartti, Nina Hautala, Päivi Jackson, Dan C Nordström, Janna Saarela, Timo Hautala, Kari K Eklund, Markku Varjosalo

Posted 27 Aug 2018
bioRxiv DOI: 10.1101/401034 (published DOI: 10.1136/rmdopen-2018-000740)

Objectives: TNFAIP3 encodes A20 that negatively regulates nuclear factor kappa light chain enhancer of activated B cells (NF-κB), the major transcription factor coordinating inflammatory gene expression. TNFAIP3 polymorphisms have been linked with a spectrum of inflammatory and autoimmune diseases and recently, loss-of-function mutations in A20 were found to cause a novel inflammatory disease 'haploinsufficiency of A20' (HA20). Here we describe a family with HA20 caused by a novel TNFAIP3 loss-of-function mutation and elucidate the upstream molecular mechanisms linking HA20 to dysregulation of NF-κB and the related inflammasome pathway. Methods: NF-κB activation was studied in a mutation-expressing cell line using luciferase reporter assay. Physical and close-proximity protein–protein interactions of wild-type and TNFAIP3 p.(Lys91*) mutant A20 were analyzed using mass spectrometry. NF-κB-dependent transcription, cytokine secretion, and inflammasome activation were compared in immune cells of the HA20 patients and control subjects. Results: The protein–protein interactome of p.(Lys91*) mutant A20 was severely impaired, including interactions with proteins regulating NF-κB activation, DNA repair responses, and the NLR family pyrin domain containing 3 (NLRP3) inflammasome. The p.(Lys91*) mutant A20 failed to suppress NF-κB signaling, which led to increased NF-κB-dependent proinflammatory cytokine transcription. Functional experiments in the HA20 patients' immune cells uncovered a novel caspase-8-dependent mechanism of NLRP3 inflammasome hyperresponsiveness that mediated the excessive secretion of interleukin-1β and -18. Conclusions: The current findings significantly deepen our understanding of the molecular mechanisms underlying HA20 and other diseases associated with reduced A20 expression or function, paving the way for future therapeutic targeting of the pathway.

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