Background/Purpose:

We describe a new autoinflammatory syndrome caused by high penetrance heterozygous germline mutations in the NFκB regulatory protein TNFAIP3 (A20) in six unrelated families with early onset systemic inflammation. The syndrome resembles Behçet’s disease (BD), which is typically considered a polygenic disorder with onset in early adulthood. Previously, low-penetrance common variants of TNFAIP3 have been associated with a number of autoimmune and inflammatory diseases in multiple GWAS studies.  

Methods:

We performed whole-exome and candidate gene sequencing in the patients and their unaffected family members. Targeted sequencing of TNFAIP3 was performed in 384 Turkish and 384 Japanese patients with BD that were used for GWAS. We used overexpression experiments in 293T cells and Jurkat cells to establish the causality of candidate variants. Patient samples were analyzed using immunoblotting, immunohistochemistry, flow cytometry, real-time PCR, and cytokine profiling.  

Results:

We identified 5 high-penetrance dominantly inherited frameshift and nonsense TNFAIP3 mutations in 11 patients from 5 families who presented with early-onset systemic inflammation, arthralgia/arthritis, oral and genital ulcers, and ocular inflammation. Targeted sequencing of TNFAIP3 identified one patient from the GWAS cohort with a novel frameshift mutation. None of the mutations have been reported in any public database. Expression of wild type (WT) A20 was reduced in patients’ PBMCs and fibroblasts. A20 is a potent inhibitor of the NFκB signaling pathway. Over-expressed mutant proteins failed to suppress TNF-induced NFκB activity in comparison to WT A20, while co-transfection with the WT A20 normalized the NFκB activity.  A20 restricts cellular activation by cleaving K63-linked ubiquitin chains (Ub) from target substrates such as NEMO, RIP1, and TRAF6.  Cells transfected with A20 mutants showed a marked defect in the deubiquitination of each of these target molecules.  This defect was partially rescued by co-transfection with wild-type A20, mimicking the situation in the patients. These experiments suggest haploinsufficiency as a likely mechanism of disease.

Stimulated patients’ cells showed increased phosphorylation of IKKα/β, increased degradation of IκBα, and increased nuclear translocation of p65. Patients’ PBMCs displayed weak association of A20 with the TNFR complex and a defect in the deubiquitination of A20 target molecules. Patient cells had a sustained higher K63-ubiquitinaton level of NEMO and RIP1 and accumulated high-molecular weight Ub-aggregates. These results indicate that inefficient deubiquitination of A20 target proteins might explain a higher NFκB signaling activity in mutant cells.  Levels of IL-1β, TNF-α, IP-10, IL-17, and IL-9 were substantially increased in patients’ serum and in the supernatants of stimulated PBMCs relative to healthy controls. Patients’ cells showed constitutive activation of the NLRP3 inflammasome. Initial experience with agents targeting pro-inflammatory cytokines has been encouraging. 

Conclusion:

This study provides the first example of the effects of germline high-penetrance heterozygous truncating mutations in the TNFAIP3 gene.

« Back to 2015 ACR/ARHP Annual Meeting

ACR Meeting Abstracts - https://acrabstracts.org/abstract/ha20-a-novel-autoinflammatory-disease-caused-by-haploinsufficiency-of-a20-encoded-by-tnfaip3/