PubMed · 15555187
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Michael Bigby, Sam Gibbs, Ian Harvey, Jane Sterling. 2003. Warts.. https://pubmed.ncbi.nlm.nih.gov/15555187/
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Bleomycins (BLMs) can utilize H2O2 to cleave DNA in the presence of ferric ions. DFT calculations were used to study the mechanism of O-O bond cleavage in the low-spin FeIII-hydroperoxo complex of BLM. The following alternative hypotheses were investigated using realistic structural models: (a) heterolytic cleavage of the O-O bond, generating a Compound I (Cpd I) like intermediate, formally BLM-FeV=O; (b) homolytic O-O cleavage, leading to a BLM-FeIV=O species and an OH* radical; and (c) a direct O-O cleavage/H-abstraction mechanism by ABLM. The calculations showed that (a) is a facile and viable mechanism; it involves acid-base proton reshuffle mediated by the side-chain linkers of BLM, causing thereby heterolytic cleavage of the O-O bond and generation of Cpd I. Formation of Cpd I is found to involve a barrier of 13.3 kcal/mol, which is lower than the barriers in the alternative mechanisms (b and c) that possess respective barriers of 31 and 17 kcal/mol. The so-formed Cpd I species with a radical on the side-chain linker, methylvalerate (V), adjacent to the BLM-FeIV=O complex, resembles the formation of the active species of cytochrome c peroxidase in the Poulos-Kraut proton-shuffle mechanism in heme peroxidases (Poulos, T. L.; Kraut, J. J. Biol. Chem. 1980, 255, 8199-8205). Experimental data are discussed and shown to be in accord with this proposal. It suggests that the high-valence Cpd I species of BLM participates in the DNA cleavage. This is an alternative mechanistic hypothesis to the exclusive reactivity scenario based on ABLM (FeIII-OOH).
PURPOSE: Early-stage head and neck cancer patients are at high risks for tumor recurrence and secondary primary tumor (SPT) development. We hypothesized that latent genetic instability and proliferation potential may be associated with elevated risks of SPT and recurrence. EXPERIMENTAL DESIGN: We conducted a nested case-control study within a randomized, placebo-controlled chemoprevention trial in patients with early-stage head and neck cancer. We compared prediagnostic bleomycin-induced chromatid breaks in peripheral blood lymphocyte cultures (as an indicator of latent genetic instability) between 298 cases (patients with SPT/recurrence) and 693 controls (patients without SPT/recurrence). We also determined the joint effects of latent genetic instability and elevated proliferation potential [indicated by serum insulin-like growth factor (IGF) level] in modulating the risk of SPT and recurrence. RESULTS: In the Cox proportional hazards model, patients with higher mutagen sensitivity (using a cutoff of > or =0.50 breaks per cell) exhibited a significantly increased risk of developing SPT/recurrence [hazard ratio, 1.38; 95% confidence interval (95% CI), 1.02-1.86]. Cases also exhibited significantly higher levels of IGF-I and IGF-binding protein-3 than controls (P = 0.022 and 0.042, respectively). Moreover, there were joint effects between mutagen sensitivity and IGFs in modulating SPT/recurrence risk. Using patients with low IGF-I level and low mutagen sensitivity profile as the reference group, the odds ratios of developing SPT/recurrence for patients with high IGF-I level alone, high mutagen sensitivity alone, and both high IGF-I level and high mutagen sensitivity were 2.85 (95% CI, 0.92-8.82), 3.92 (95% CI, 1.28-11.97), and 6.16 (95% CI, 2.03-18.71), respectively. A similar joint effect was observed for mutagen sensitivity and IGF-binding protein-3 level. CONCLUSIONS: This is the largest prospective study to evaluate mutagen sensitivity as a prognosis marker in head and neck cancer because mutagen sensitivity data were derived from baseline samples drawn before the development of SPT or tumor recurrence. The results also show for the first time that latent genetic instability and elevated proliferation potential jointly elevate the risk of second tumors in early-stage head and neck cancers.
The Artemis nuclease is required for V(D)J recombination and for repair of an as yet undefined subset of radiation-induced DNA double strand breaks. To assess the possibility that Artemis acts on oxidatively modified double strand break termini, its activity toward model DNA substrates, bearing either 3'-hydroxyl or 3'-phosphoglycolate moieties, was examined. A 3'-phosphoglycolate had little effect on Artemis-mediated trimming of long 3' overhangs (> or =9 nucleotides), which were efficiently trimmed to 4-5 nucleotides. However, 3'-phosphoglycolates on overhangs of 4-5 bases promoted Artemis-mediated removal of a single 3'-terminal nucleotide, while at least 2 nucleotides were trimmed from identical hydroxyl-terminated substrates. Artemis also efficiently removed a single nucleotide from a phosphoglycolate-terminated 3-base 3' overhang, while leaving an analogous hydroxyl-terminated overhang largely intact. Such removal was completely dependent on DNA-dependent protein kinase and ATP and was largely dependent on Ku, which markedly stimulated Artemis activity toward all 3' overhangs. Together, these data suggest that efficient Artemis-mediated cleavage of 3' overhangs requires a minimum of 2 nucleotides, or a nucleotide plus a phosphoglycolate, 3' to the cleavage site, as well as 2 unpaired nucleotides 5' to the cleavage site. Shorter 3'-phosphoglycolate-terminated overhangs and blunt ends were also processed by Artemis but much more slowly. Consistent with a role for Artemis in repair of terminally blocked double strand breaks in vivo, human cells lacking Artemis exhibited hypersensitivity to x-rays, bleomycin, and neocarzinostatin, which all induce 3'-phosphoglycolate-terminated double strand breaks.