Erratum: The uracil-DNA glycosylase UNG protects the fitness of normal and cancer B cells expressing AID.

[This corrects the article DOI: 10.1093/narcan/zcaa019.].

The uracil-DNA glycosylase UNG protects the fitness of normal and cancer B cells expressing AID.

In B lymphocytes, the uracil N-glycosylase (UNG) excises genomic uracils made by activation-induced deaminase (AID), thus underpinning antibody gene diversification and oncogenic chromosomal translocations, but also initiating faithful DNA repair. Ung-/- mice develop B-cell lymphoma (BCL). However, since UNG has anti- and pro-oncogenic activities, its tumor suppressor relevance is unclear. Moreover, how the constant DNA damage and repair caused by the AID and UNG interplay affects B-cell fitness and thereby the dynamics of cell populations in vivo is unknown. Here, we show that UNG specifically protects the fitness of germinal center B cells, which express AID, and not of any other B-cell subset, coincident with AID-induced telomere damage activating p53-dependent checkpoints. Consistent with AID expression being detrimental in UNG-deficient B cells, Ung-/- mice develop BCL originating from activated B cells but lose AID expression in the established tumor. Accordingly, we find that UNG is rarely lost in human BCL. The fitness preservation activity of UNG contingent to AID expression was confirmed in a B-cell leukemia model. Hence, UNG, typically considered a tumor suppressor, acquires tumor-enabling activity in cancer cell populations that express AID by protecting cell fitness.

UNG protects B cells from AID-induced telomere loss.

Activation-induced deaminase (AID) initiates antibody gene diversification by creating G:U mismatches in the immunoglobulin loci. However, AID also deaminates nonimmunoglobulin genes, and failure to faithfully repair these off-target lesions can cause B cell lymphoma. In this study, we identify a mechanism by which processing of G:U produced by AID at the telomeres can eliminate B cells at risk of genomic instability. We show that telomeres are off-target substrates of AID and that B cell proliferation depends on protective repair by uracil-DNA glycosylase (UNG). In contrast, in the absence of UNG activity, deleterious processing by mismatch repair leads to telomere loss and defective cell proliferation. Indeed, we show that UNG deficiency reduces B cell clonal expansion in the germinal center in mice and blocks the proliferation of tumor B cells expressing AID. We propose that AID-induced damage at telomeres acts as a fail-safe mechanism to limit the tumor promoting activity of AID when it overwhelms uracil excision repair.

HSP90 inhibitors decrease AID levels and activity in mice and in human cells.

Activation induced deaminase (AID) initiates somatic hypermutation and class switch recombination of the Ig genes in antigen-activated B cells, underpinning antibody affinity maturation and isotype switching. AID can also be pathogenic by contributing to autoimmune diseases and oncogenic mutations. Moreover, AID can exert noncanonical functions when aberrantly expressed in epithelial cells. The lack of specific inhibitors prevents therapeutic applications to modulate AID functions. Here, we have exploited our previous finding that the HSP90 molecular chaperoning pathway stabilizes AID in B cells, to test whether HSP90 inhibitors could target AID in vivo. We demonstrate that chronic administration of HSP90 inhibitors decreases AID protein levels and isotype switching in immunized mice. HSP90 inhibitors also reduce disease severity in a mouse model of acute B-cell lymphoblastic leukemia in which AID accelerates disease progression. We further show that human AID protein levels are sensitive to HSP90 inhibition in normal and leukemic B cells, and that HSP90 inhibition prevents AID-dependent epithelial to mesenchymal transition in a human breast cancer cell line in vitro. Thus, we provide proof-of-concept that HSP90 inhibitors indirectly target AID in vivo and that endogenous human AID is widely sensitive to them, which could have therapeutic applications.

Prognostic value of the high-mobility group box-1 in young patients with chest pain.

BACKGROUND: Atherosclerosis is accepted as an inflammatory disease. Evidence suggests that inflammation evoked by injury plays a pathogenic role in all stages of atherosclerosis. This study aimed to investigate whether the high-mobility group box-1 (HMGB1) a proinflammatory cytokine/nuclear protein, which is derived from both injured endothelium and activated macrophages/monocytes, could contribute to the progression of atherosclerosis and other cardiovascular diseases. METHODS: This study was designed as case-control. A total of 135 patients who referred to the hospital due to angina pectoris had the diagnosis of unstable angina and were candidates of angiography were recruited in this study. Forty patients who had coronary artery disease confirmed by angiography were considered as case group and control group consists of 40 persons who had no plaque, and 55 persons were excluded according to the exclusion criteria. At first, a questionnaire was filled for each patient including demographic factors and their medical history. Then a blood sample was taken to assess the level of HMGB1. Data were analyzed using SPSS, Student's independent t-test, and chi-square tests. RESULTS: The mean plasma level of HMGB1 in the case group was 27.1 ± 2.9 ng/ml, while it was 19.6 ± 1.9 ng/ml in control groups (P = 0.03). The odds ratio for coronary artery plaque associated with high (> 15.03 ng/ml) levels of HMGB1 was 2.50 (95% confidence interval, 1.02-6.17, P = 0.03). CONCLUSION: Increased plasma HMGB1 concentration may be associated with an increased risk of coronary atherosclerosis.

Separation of function between isotype switching and affinity maturation in vivo during acute immune responses and circulating autoantibodies in UNG-deficient mice.

Activation-induced deaminase converts deoxycytidine to deoxyuridine at the Ig loci. Complementary pathways, initiated by the uracil-DNA glycosylase (UNG) or the mismatch repair factor MSH2/MSH6, must process the deoxyuridine to initiate class-switch recombination (CSR) and somatic hypermutation. UNG deficiency most severely reduces CSR efficiency and only modestly affects the somatic hypermutation spectrum in vitro. This would predict isotype-switching deficiency but normal affinity maturation in Ung(-/-) mice in vivo, but this has not been tested. Moreover, puzzling differences in the amount of circulating Ig between UNG-deficient humans and mice make it unclear to what extent MSH2/MSH6 can complement for UNG in vivo. We find that Ab affinity maturation is indeed unaffected in Ung(-/-) mice, even allowing IgM responses with higher than normal affinity. Ung(-/-) mice display normal to only moderately reduced basal levels of most circulating Ig subclasses and gut-associated IgA, which are elicited in response to chronically available environmental Ag. In contrast, their ability to produce switched Ig in response to immunization or vesicular stomatitis virus infection is strongly impaired. Our results uncover a specific need for UNG in CSR for timely and efficient acute Ab responses in vivo. Furthermore, Ung(-/-) mice provide a novel model for separating isotype switching and affinity maturation during acute (but not chronic) Ab responses, which could be useful for dissecting their relative contribution to some infections. Interestingly, Ung(-/-) mice present with circulating autoantibodies, suggesting that UNG may impinge on tolerance.