Research funded in part by the NIH’s National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) has revealed a new role for A20, a protein that regulates a key immune response pathway, in certain early-onset autoinflammatory diseases. The results suggest that targeting this pathway could be an effective strategy for treating these diseases, and possibly related conditions, as well. Two independent studies homed in on A20 in papers appearing in the journals Nature Genetics and the Proceedings of the National Academy of Sciences.
Autoinflammatory disorders are a diverse class of diseases caused by abnormal activation of the immune system. Each condition has distinct features, but they are all characterized by episodes of inflammation that result in symptoms such as fever, rash, or joint swelling.
In the past two decades, scientists have uncovered the genes that underlie several inherited forms of these diseases. Knowing which genes are involved has enabled physicians to target therapies to specific molecular pathways, an approach that has helped many patients. However, the genetic basis of many other cases has remained unknown.
In an effort to identify additional genes linked to autoinflammatory diseases, a team led by Ivona Aksentijevich, M.D., a senior staff scientist and medical geneticist at the NIH’s National Human Genome Research Institute, collaborated with Richard M. Siegel, M.D., Ph.D., who is the clinical director at NIAMS. They focused on a Canadian family of European descent with early-onset systemic inflammation. The affected family members, a mother and her two daughters, experienced signs of arthritis, oral and genital ulcers, and eye inflammation.
By analyzing all three affected family members’ protein-coding genes, the researchers discovered a defect in the A20 gene. They subsequently identified mutations in the A20 gene in five other unrelated families with similar disease symptoms. All the mutations resulted in shortened forms of the protein.
A20 is known to be a potent inhibitor of the NF-κB signaling pathway, a key part of the body’s immune defense and an important promoter of inflammation. In addition to autoinflammatory conditions, incorrect regulation of the NF-κB signaling pathway also has been linked to some common autoimmune disorders, such as systemic lupus erythematosus, rheumatoid arthritis, Crohn's disease, and psoriasis.
When the researchers examined cells from patients with A20 mutations, they saw more active NF-κB, compared to cells from healthy controls. In the patients’ blood samples, they also saw higher levels of cytokines, which are chemical messengers produced by activation of the NF-κB pathway. Additional experiments showed that the mutations hampered A20’s function, leaving it less able to restrain the NF-κB signaling pathway and the associated inflammation.
While these studies were ongoing, Eric P. Hanson, M.D., a pediatric rheumatologist and assistant clinical investigator at NIAMS, was studying families affected by mutations in the gene for NEMO, another regulator of the NF-κB signaling pathway. NEMO, which stimulates the pathway, is directly inhibited by A20.
People with NEMO mutations usually exhibit immune deficiency and are prone to infections. The affected individuals in a particular family, however, also had signs of systemic inflammation, including fevers, inflammatory bowel disease and arthritis.
When Dr. Hanson’s team investigated how this family’s NEMO mutation and other, related mutations cause excessive inflammation, they uncovered a defect in NEMO’s ability to bind to A20. In additional experiments, the researchers found that the NEMO mutations led to over-activation of the NF-κB signaling pathway and increased production of inflammatory cytokines.
These findings confirm the importance of A20 in dampening inflammatory responses, and show that failure of other proteins to interact with it can be as damaging as mutations that affect A20 itself. The results also suggest that raising A20 levels, or otherwise enhancing its function, could be an effective therapeutic strategy for certain autoinflammatory diseases.
"Our goal is to understand the genes and molecular mechanisms that cause autoinflammatory and autoimmune diseases so that we can design better ways to treat them," said Dr. Siegel. "In addition, what we learn from people with diseases caused by mutations in single genes, like A20 or NEMO, may be applicable to more common inflammatory conditions".
The Nature Genetics paper co-authored by Dr. Siegel was supported by the Intramural Research Programs at NIH’s NIAMS (Z01-AR041133-14); National Human Genome Research Institute; National Heart, Lung, and Blood Institute; National Institute of Allergy and Infectious Diseases; and Clinical Center. The Scientific and Technological Research Council of Turkey also contributed support.
Dr. Hanson’s study, published in the Proceedings of the National Academy of Sciences, was supported by the Intramural Research Program at NIAMS (Z01-AR041187).
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Zhou Q, Wang H, Schwartz DM, Stoffels M, Park YH, Zhang Y, Yang D, Demirkaya E, Takeuchi M, Tsai WL, Lyons JJ, Yu X, Ouyang C, Chen C, Chin DT, Zaal K, Chandrasekharappa SC, P Hanson E, Yu Z, Mullikin JC, Hasni SA, Wertz IE, Ombrello AK, Stone DL, Hoffmann P, Jones A, Barham BK, Leavis HL, van Royen-Kerkof A, Sibley C, Batu ED, Gül A, Siegel RM, Boehm M, Milner JD, Ozen S, Gadina M, Chae J, Laxer RM, Kastner DL, Aksentijevich I. Loss-of-function mutations in TNFAIP3 leading to A20 haploinsufficiency cause an early-onset autoinflammatory disease. Nat Genet. 2016 Jan;48(1):67-73. doi: 10.1038/ng.3459. Epub 2015 Dec 7. PMID: 26642243
Zilberman-Rudenko J, Shawver LM, Wessel AW, Luo Y, Pelletier M, Tsai WL, Lee Y, Vonortas S, Cheng L, Ashwell JD, Orange JS, Siegel RM, Hanson EP. Recruitment of A20 by the C-terminal domain of NEMO suppresses NF-κB activation and autoinflammatory disease. Proc Natl Acad Sci U S A. 2016 Feb 9;113(6):1612-7. doi: 10.1073/pnas.1518163113. Epub 2016 Jan 22. PMID: 26802121
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