Departments of Pathology, Environmental Health and Radiation Oncology, Geffen School of Medicine and School of Public Health, University of California, Los Angeles, CA 90095

* To whom correspondence should be addressed. E-mail: rschiestl@mednet.ucla.edu.

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Ataxia telangiectasia (AT), a human hereditary disorder resulting from mutations in the ATM gene, is characterized by a high incidence of lymphoid malignancies, neurodegeneration, immunodeficiency, premature aging, elevated radiosensitivity, and genomic instability. Evidence has been accumulating that ATM-deficient cells are in a continuous state of oxidative stress. A variety of markers of oxidative stress were detected in AT patients as well as Atm-deficient mice, used as an animal model of AT. Since then, it has been proposed that oxidative stress contributes to the clinical phenotype of AT, especially carcinogenesis and neurodegeneration, and several animal studies were conducted to determine whether exogenous antioxidants mitigate the symptoms of AT. Tempol, EUK-189, and N-acetyl cysteine have been tested as chemopreventive antioxidants in Atm-deficient mice. We review these findings, mainly focusing on the effect of N-acetyl cysteine, which is known as a safe and efficient drug and nutritional supplement.

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Ataxia telangiectasia (AT)4 is a severe, progressive, and terminal disorder resulting from homozygous mutations in the ATM (AT mutated) gene. It is a pleotropic disease characterized by early onset of neurodegeneration, a high incidence of cancer, immunodeficiency, oculocutaneous telangiectasias, infertility, premature aging, and hypersensitivity to ionizing radiation (1,2). About 40% of AT patients develop cancer, mostly in the lymphoid organs (3,4). No treatment is available to cure or even delay progression of the disease, and death typically occurs in the second or third decade.
ATM-deficient cells are characterized by genomic instability including chromosome breaks, chromosome gaps, translocations, and aneuploidy (5,6). The lack of functions of the ATM protein, in particular of its function in the cell cycle checkpoint control and repair responses to DNA double-stranded breaks, is believed to be responsible for the resulting genomic instability and high risk of cancer (5,6).

More recently, ATM deficiency was found to be associated with elevated oxidative stress. Elevated oxidative damage to lipids and DNA and reduced plasma antioxidant concentrations were measured in AT patients (7,8). More information on the presence of chronic oxidative stress in AT was obtained from Atm-deficient mice (9–12). Atm-deficient mice recapitulate most clinical and cellular features of AT and succumb to thymic lymphoma (13–15).

Because oxidative stress is linked to several human diseases, especially cancer and neurological disorders, it was postulated that oxidative stress is implicated in the pathogenesis of AT (16). Since then, several groups of researchers launched studies to investigate the effect of exogenous antioxidants in AT. Recently, chemopreventive properties of EUK-189, tempol, and N-acetyl cysteine were tested in Atm-deficient mice (17–19). Using different compounds and different routes of administration, all 3 studies provided evidence that antioxidant supplementation may act beneficially in Atm-deficient mice.

Chemoprevention studies in Atm-deficient mice

One of the tested compounds, EUK-189, belongs to a series of synthetic salen-manganese complexes termed EUK (Eukarion). EUKs were developed to mimic activities of the endogenous superoxide dismutase and catalase involved in neutralization of superoxide and hydrogen peroxide, respectively (20,21). Thus, the EUK complexes can potentially be useful against reactive oxygen species (ROS)-mediated diseases and were shown to be neuroprotective in animal models characterized by oxidative damage (20,21). Treatment of Atm-deficient mice with EUK-189 improved performance on a rotarod and showed a trend toward prolonged life span (P = 0.08) (17). After termination of the study at 5 mo, 31% of the vehicle-treated and 56% of the EUK-189–treated animals were still alive (17). Whether the effect was mediated by antioxidant activity was not examined. EUK-189 was delivered via an osmotic pump implanted subcutaneously, and treatment was started at age 40 d.

Tempol, a stable nitroxide free radical, is a murine radioprotector and superoxide dismutase mimetic (22). Tempol oxidizes redox-active trace metal ions, reduces quinone radicals, and is itself reduced by glutathione and ascorbic acid and thus has a potential to deplete thiols and ascorbic acid in biological systems (23–25). In the chemoprevention study in Atm-deficient mice, a tempol-supplemented diet was continuously given to Atm-deficient mice either from fertilization (Atm +/– mice were crossed with each other, and dams were given tempol-containing food) or from weaning (3-wk-old mice) throughout life (18). Tempol reduced oxidative stress, such as levels of ROS and heme-oxygenase-1. Tempol significantly increased the life span (mean survival 62 vs. 30 wk) when the diet was given from weaning, but no effect was found when the treatment was started from fertilization (18).

We examined the effect of dietary supplementation with the thiol-containing antioxidant, N-acetyl-L-cysteine (NAC) (19). NAC is a low-molecular-weight thiol-containing molecule that is rapidly absorbed by the gastrointestinal tract and appears in the plasma in less than 1 h following oral administration (26). NAC detoxifies reactive electrophiles and ROS and can enhance glutathione synthesis (27). Used for >40 y in clinical practice, NAC has found wide applications and was established as a safe drug, even when used at high doses and long term (26,28,29). NAC has been used for treating respiratory diseases (as a mucolytic agent) (30) and for treating acetaminophen overdose, preventing glutathione depletion in the liver (31). It has strong anticarcinogenic properties and is available as an over-the-counter dietary supplement (27). In human applications, NAC is most frequently administered orally.

We added NAC to drinking water of Atm-deficient mice from fertilization continuously throughout life. We started antioxidant administration as early as fertilization to protect against genome rearrangements that can occur during mouse development and lead to carcinogenesis later in life. NAC significantly increased the life span and reduced both the incidence and multiplicity of lymphoma (19). The mean survival of mice given NAC was 68 wk, whereas that of untreated mice was 50 wk. The incidence of lymphoma decreased by 2-fold (37.5% vs. 76.5%) (Fig. 1). Tumor tissue distribution suggested thymic origin, and we also found lymphomas in other organs. Tumor number was similar in the thymus, spleen, and liver, whereas in other organs, such as lymph nodes, lung, heart, kidney, pancreas, stomach, duodenum, and adrenal glands, there were fewer or no tumors in NAC-treated mice (Fig. 2). In total, the number of tumors per mouse decreased from 4.6 to 2.8.

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Figure 1 Lymphoma incidence in untreated and NAC-treated Atm-deficient mice. Reprinted with permission from Reliene and Schiestl (19). P = 0.02.

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Figure 2 Lymphoma tissue distribution in untreated and NAC-treated Atm-deficient mice. Only mice that had lymphoma are included in the calculation. Black bars depict untreated mice; gray bars show NAC-treated mice; P = 0.038. Lymph nodes affected were mesenteric and/or peripheral, thoracic, and perirenal. Lymphoma in the heart was seen in epicardium and/or pericardium. Adapted from Reliene and Schiestl (19).

Possible mechanism of lymphoma prevention by NAC
The results of the chemoprevention study clearly showed that NAC acts as a lymphoma-preventive agent in Atm-deficient mice. It is tempting to postulate a possible NAC action mechanism based on the available data. Current knowledge points to genome rearrangements, elevated oxidative stress, or both as causes of cancer in AT (5,6,16). Lymphoma in Atm-deficient mice arise from the rapidly proliferating CD4/CD8 double-positive T cells that display multiple chromosomal aberrations, frequently unbalanced rearrangements, with deletion or duplication of genetic material (32). We demonstrated that NAC suppressed both oxidative DNA damage and DNA deletions in Atm-deficient mice (Fig. 3) (12).

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Figure 3 The correlation between oxidative DNA damage and the frequency of DNA deletions. Oxidative DNA damage was determined as the number of oxidized guanine residues per 106 guanine residues (8-OHdG/106 dG) as determined by HPLC. The frequency of DNA deletions was determined as a number of eye-spots in the retinal pigment epithelium (RPE) of the eye. The eye-spots are derived from 70 kb DNA deletions at the pink-eyed unstable (pun) locus of the pink-eyed dilution (p) gene, which result in black pigment accumulation in the affected cells (37). Data for untreated mice are shown by a black triangle; results for NAC-treated mice are shown by a gray rectangle. Adapted from Reliene et al. (12).

Oxidative DNA damage may be an event preceding deletion formation, especially during DNA replication (33). Yan et al. (34) showed that NAC reduces spontaneously elevated DNA synthesis in T cells from Atm-deficient mice. By reducing increased oxidative DNA damage and elevated DNA synthesis, NAC may minimize the frequency of replication-mediated genome rearrangements. Unregulated DNA synthesis results in insufficient time for DNA repair; hence, the damaged template is used for DNA replication resulting in rearrangements. These findings provide sufficient evidence that NAC reduces genomic instability in Atm-deficient cells. In addition, NAC was shown to have antiinvasive, antimetastatic, and antiangiogenic activity in several systems. For example, NAC inhibits vascular endothelial growth factor production and growth of angiogenesis-driven Kaposi's sarcoma in nude mice (35). NAC promotes antiangiogenic factor angiostatin production and results in endothelial apoptosis and vascular collapse in an experimental breast cancer assay (36). Thus, NAC may suppress neovascularization and thereby reduce tumor growth and multiplicity.
Summary

The effect of EUK-189, tempol, and NAC was studied in Atm-deficient mice to give us insight into whether antioxidant therapy could find applications in human AT patients. All the reported compounds had some beneficial effect. Of the tested antioxidants, NAC offers the advantage of having a long history of safety and efficacy in clinical settings and thus has a potential to emerge as a dietary supplement aimed at tumor prevention in humans with cancer-prone syndromes, especially in those associated with oxidative stress. The antioxidant (including NAC) clinical trial currently being conducted in AT patients in A.I. duPont Hospital for Children in Wilmington, Delaware, and Thomas Jefferson University in Philadelphia, Pennsylvania, led by Dr. Gerard T. Berry (38) will reveal the role of antioxidants in human AT patients.

FOOTNOTES

1 Published in a supplement to The Journal of Nutrition. Presented as part of the International Research Conference on Food, Nutrition, and Cancer held in Washington, DC, July 13–14, 2006. This conference was organized by the American Institute for Cancer Research and the World Cancer Research Fund International and sponsored by (in alphabetical order) the California Walnut Commission; Campbell Soup Company; Cranberry Institute; Hormel Institute; IP-6 International, Inc.; Kyushu University, Japan Graduate School of Agriculture; National Fisheries Institute; and United Soybean Board. Guest editors for this symposium were Vay Liang W. Go, Susan Higginbotham, and Ivana Vucenik. Guest Editor Disclosure: V.L.W. Go, no relationships to disclose; S. Higginbotham and I. Vucenik are employed by the conference sponsor, the American Institute for Cancer Research.

2 Author Disclosure: No relationships to disclose.

3 This work is supported by grants from the National Institute of Environmental Health Sciences (NIH RO1 grant No. ES09519) the American Institute for Cancer Research (both to RHS) and a post-doctoral research fellowship of the Lymphoma Research Foundation Elizabeth Banks Jacobs & Byron Wade Strunk Memorial Fellowship (to RR).

4 Abbreviations used: AT, ataxia telangiectasia; NAC, N-acetyl-L-cysteine; ROS, reactive oxygen species.

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