Background
The
TNFAIP3 gene that encodes the 90 kDa A20 protein was first described in 1990 as a TNF-α-induced primary response gene in endothelial cells [
1]. The protein A20 is a negative regulator of the TNF-nuclear factor-kB signaling pathway, which plays fundamental roles in various physiological and pathological processes, such as immunity, apoptosis, inflammation, and cancer [
2,
3]. Polymorphisms in
TNFAIP3 have been linked to the development of several autoimmune and autoinflammatory diseases in genome-wide association studies, such as systemic lupus erythematosus [
4‐
6], Sjögren’s Syndrome [
7], Crohn disease [
8], rheumatoid arthritis [
9,
10], type 1 diabetes [
11] and psoriasis [
12]. Recently, heterozygous germline mutations in the
TNFAIP3 gene have been found to cause the haploinsufficiency of A20 (HA20), which displays an early-onset autoinflammatory disease resembling Behçet’s disease [
13]. HA20, first described by Zhou et al. in 2016, is a new autoinflammatory disease [
13]. Zhou et al. found that the major phenotype of HA20 displays Behçet’s disease-like symptoms, including recurrent aphthous stomatitis, genital ulcers, and intestinal symptoms [
13]. However, a number of studies showed that some patients present with not only the features of autoinflammatory diseases but also several autoimmune-like features [
14‐
17]. Therefore, these preceding clinical reports suggested that there might be unexpected phenotypes in HA20. Cases with HA20 were not reported in China. In this study, we described four patients in two HA20 families, which were diagnosed by whole exons sequencing, to enrich the phenotype and genotype spectrum of HA20.
Discussion and conclusion
In this study, we describe the clinical manifestations of four Chinese patients with HA20 from two unrelated families. Four patients all had no signs of eye and skin problems, such as uveitis, rash, folliculitis and dermal abscess. Based on clinical features, P1 was initially diagnosed with SLE, liver fibrosis and hypothyroidism. She also had no oral, genital or perineal ulcers. P2 was initially diagnosed with Crohn’s disease and inflammatory bowel disease-related arthritis (IBD-RA). He had perianal abscess but no oral or genital ulcers. However, P3, the father of P1 and P2, only had mild oral ulcers, arthralgia, and archosyrinx. Therefore, three patients had different phenotypes in this family. Furthermore, signs of liver and thyroid problems were never reported before in HA20 patients in other studies.
P4 was initially diagnosed with polyarticular juvenile idiopathic arthritis (JIA) because of the chronic synovitis of her joints. Although ANA was positive, no other autoantibody was detected in her serum. She did not meet the classification criteria for SLE. However, she met the classification criteria of macrophage activation syndrome (MAS) in suspected systemic juvenile idiopathic arthritis (SJIA) due to fever, hyperferritinemia, hypertriglyceridemia and hypofibrinogenemia. She had a cough, and her lung CT scan showed ILD. ILD is one of the most common and clinically important manifestations of autoimmune diseases/connective tissue disease [
19]. To date, MAS and ILD in the HA20 patients from this study were the first cases reported. Thus, clinical features were significantly different between families with HA20 in our study.
HA20 was initially identified as a Behçet’s disease-like phenotype. The original description showed that all patients with HA20 had recurrent aphthous stomatitis and genital ulcers [
13]. However, not all patients included in our study and other studies developed some Behçet’s disease-like phenotypes [
14‐
17], even if some patients included in our study and other studies did not have a Behçet’s disease-like phenotype [
15‐
17]. Thus, HA20 is a very heterogeneous disease. Patients presenting with Behçet’s-like disease or other features of autoinflammatory and autoimmune disease starting in early childhood should be screened for the
TNFAIP3 mutation, especially if there is a family history of similar symptoms, as the clinical course and response to the treatment of this genetic disorder differs from common Behçet’s disease.
A20 functions as a negative regulator of the nuclear factor kB (NF-kB) pathway and modulates immune responses by preventing excessive activation of NF-kB in response to a variety of external stimuli [
20]. It is structurally divided into two types of domains – the N-terminal Ovarian tumor (OTU) domain and the C-terminal domain containing 7 zinc finger motifs (ZnF1–ZnF7) [
21]. It harbors a deubiquitination enzyme domain and activates multiple mechanisms to antagonize ubiquitination upstream of the NF-kB essential modulator (NEMO), a regulatory subunit of the IkB kinase (IKK) complex [
4]. The IKK complex has 2 catalytic subunits (IKKa and IKKb) and a subunit that regulates the canonical NF-kB pathway (NEMO/IKKg). The IKK regulates the noncanonical pathway by IKKa [
22]. The ubiquitin-induced recruitment of A20 to NEMO can downregulate IKK activation by blocking IKK phosphorylation. On activation, IKK phosphorylates the inhibitory IkBa protein, leading to its degradation and dissociation from NF-kB, after which it translocates to the nucleus [
23]. Hence, A20 deficiency leads to the activation of the NF-kB pathway. Both a mutation in the
TNFAIP3 gene in the N-terminal domain or the C-terminal domain can cause the loss of A20 function [
14‐
17]. Approximately 10 different disease-causing mutations of the
TNFAIP3 gene have been reported in various populations (
http://www.hgmd.cf.ac.uk/, last updated April, 2018). In our study, WES revealed that a heterozygous c.559C > T(p.Q187X) mutation in the
TNFAIP3 gene cosegregates with the disease in three patients of family 1. The mutation was confirmed by Sanger sequencing, but it was reported by other study [
16]. By sequence analysis in family 1, we found that P3 had this de novo mutation and transmitted it to his children. Interestingly, the three patients had the same genotype of the
TNFAIP3 gene but had different phenotypes. The heterozygous c.259C > T (p.R87X) mutation in the
TNFAIP3 gene was detected by WES and confirmed by Sanger sequencing in family 2. Sequence analysis in family 2 showed that the p.R87X mutation is a de novo mutation, where a nonsense mutation leads to the truncation of A20. The p.R87X mutation was first reported in our study. The p.Q187X and p.R87X mutations are both located in OUT domains, which controlled NF-κB signaling by deubiquitinating receptor-interacting protein (RIP) 1, RIP2 and TNF receptor-associated factor (TRAF) [
24].
Here, we provide four HA20 patients with novel features of considerable existing evidence implicating novel mutations in the TNFAIP3 gene. Our study expands the phenotype and genotype spectrum of HA20.
HA20 is a new autoinflammatory disease caused by heterozygous loss-of-function mutations in TNFAIP3. These mutations cause insufficient DUB activity of A20 and lead to an increased NF-κB signaling and phosphorylation of c-Jun N-terminal kinase and p38 mitogen-activated protein kinases (MAPKs). Clinical features were significantly different between families with HA20, even between numbers with the same TNFAIP3 mutation.
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