Variations in the spectrum of lesions produced in the DNA of cells from mouse tissues after exposure to gamma-rays in air-breathing or in artificially anoxic animals

Repair pathways for radiation DNA damage under normoxic and hypoxic conditions: Assessment with a panel of repair-deficient human TK6 cells

Various types of DNA lesions are produced when cells are exposed to ionizing radiation (IR). The type and yield of IR-induced DNA damage is influenced by the oxygen concentration. Thus, different DNA repair mechanisms may be involved in the response of normoxic and hypoxic cells to irradiation with IR. However, differences between the repair mechanisms of IR-induced DNA damage under normoxic versus hypoxic conditions have not been clarified. Elucidating the relative contribution of individual repair factors to cell survival would give insight into the repair mechanisms operating in irradiated normoxic and hypoxic cells. In the present study, we used a panel of repair-deficient human TK6 cell lines that covered seven repair pathways. Cells were irradiated with X-rays under normoxic and hypoxic conditions, and the sensitivities of each mutant relative to the wild-type (i.e. relative sensitivity) were determined for normoxic and hypoxic conditions. The sensitivity of cells varied depending on the type of repair defects. However, for each repair mutant, the relative sensitivity under normoxic conditions was comparable to that under hypoxic conditions. This result indicates that the relative contribution of individual repair pathways to cell survival is comparable in normoxic and hypoxic cells, although the spectrum of IR-induced DNA damage in hypoxic cells differs from that of normoxic cells.

How to fix DNA-protein crosslinks

Proteins that act on DNA, or are in close proximity to it, can become inadvertently crosslinked to DNA and form highly toxic lesions, known as DNA-protein crosslinks (DPCs). DPCs are generated by different chemotherapeutics, environmental or endogenous sources of crosslinking agents, or by lesions on DNA that stall the catalytic cycle of certain DNA processing enzymes. These bulky adducts impair processes on DNA such as DNA replication or transcription, and therefore pose a serious threat to genome integrity. The large diversity of DPCs suggests that there is more than one canonical mechanism to repair them. Indeed, many different enzymes have been shown to act on DPCs by either processing the protein, the DNA or the crosslink itself. In addition, the cell cycle stage or cell type are likely to dictate pathway choice. In recent years, a detailed understanding of DPC repair during S phase has started to emerge. Here, we review the current knowledge on the mechanisms of replication-coupled DPC repair, and describe and also speculate on possible pathways that remove DPCs outside of S phase. Moreover, we highlight a recent paradigm shifting finding that indicates that DPCs are not always detrimental, but can also play a protective role, preserving the genome from more deleterious forms of DNA damage.

Radiation-induced DNA-protein cross-links: Mechanisms and biological significance

Ionizing radiation produces various DNA lesions such as base damage, DNA single-strand breaks (SSBs), DNA double-strand breaks (DSBs), and DNA-protein cross-links (DPCs). Of these, the biological significance of DPCs remains elusive. In this article, we focus on radiation-induced DPCs and review the current understanding of their induction, properties, repair, and biological consequences. When cells are irradiated, the formation of base damage, SSBs, and DSBs are promoted in the presence of oxygen. Conversely, that of DPCs is promoted in the absence of oxygen, suggesting their importance in hypoxic cells, such as those present in tumors. DNA and protein radicals generated by hydroxyl radicals (i.e., indirect effect) are responsible for DPC formation. In addition, DPCs can also be formed from guanine radical cations generated by the direct effect. Actin, histones, and other proteins have been identified as cross-linked proteins. Also, covalent linkages between DNA and protein constituents such as thymine-lysine and guanine-lysine have been identified and their structures are proposed. In irradiated cells and tissues, DPCs are repaired in a biphasic manner, consisting of fast and slow components. The half-time for the fast component is 20min-2h and that for the slow component is 2-70h. Notably, radiation-induced DPCs are repaired more slowly than DSBs. Homologous recombination plays a pivotal role in the repair of radiation-induced DPCs as well as DSBs. Recently, a novel mechanism of DPC repair mediated by a DPC protease was reported, wherein the resulting DNA-peptide cross-links were bypassed by translesion synthesis. The replication and transcription of DPC-bearing reporter plasmids are inhibited in cells, suggesting that DPCs are potentially lethal lesions. However, whether DPCs are mutagenic and induce gross chromosomal alterations remains to be determined.

Tumor hypoxia as a driving force in genetic instability

Sub-regions of hypoxia exist within all tumors and the presence of intratumoral hypoxia has an adverse impact on patient prognosis. Tumor hypoxia can increase metastatic capacity and lead to resistance to chemotherapy and radiotherapy. Hypoxia also leads to altered transcription and translation of a number of DNA damage response and repair genes. This can lead to inhibition of recombination-mediated repair of DNA double-strand breaks. Hypoxia can also increase the rate of mutation. Therefore, tumor cell adaptation to the hypoxic microenvironment can drive genetic instability and malignant progression. In this review, we focus on hypoxia-mediated genetic instability in the context of aberrant DNA damage signaling and DNA repair. Additionally, we discuss potential therapeutic approaches to specifically target repair-deficient hypoxic tumor cells.

Radiation-induced DNA damage in tumors and normal tissues. VI. Estimation of the hypoxic fraction of experimental tumors

For several years, we have concentrated our efforts on validating the use of radiation-induced DNA strand breaks and DNA-protein crosslinks to assess the oxygenation status of tumors and normal tissues. We have demonstrated that (1) the oxygen dependence of strand break formation is identical to that of radiation-induced cell killing; (2) the oxygen dependence of DNA-protein crosslink formation is the mirror image of that of radiation-induced cell killing; and (3) the formation of these radiation-induced DNA lesions is predominantly dependent on the oxygen concentration near the DNA and is independent of the cell type, metabolic status, proliferative status, pH of the surrounding environment, and composition or properties of the proteins tightly associated with the DNA. In the present study, the hypoxic fraction of three experimental tumors was estimated using our assay of radiation-induced DNA damage. The average hypoxic fraction of a large number of tumors estimated with this assay of radiation-induced DNA damage for (1) WiDR human colorectal carcinoma xenografts (40.8 +/- 4.2%), (2) 66 mouse mammary adenocarcinoma tumors (41.8 +/- 3.1%), and (3) subcutaneous tumors grown from 9L rat brain tumor cells (95% CI =-8.2-4.2%) was not statistically different from that of a large number of tumors measured for each of these tumor models by the paired survival curve method (38.3 +/- 6. 3%, 28.9 +/- 5.5%, 95% CI = 2.2-4.4%, respectively). When the hypoxic fraction measured by the alkaline elution method on one half of an individual tumor was compared to that measured by the paired survival curve method on the other half of the same tumor, no statistical correlation was found for either 66 or WiDR tumors. Although this assay of radiation-induced DNA damage can be used effectively in the laboratory to answer a number of important questions about the oxygenation status of animal tumors and normal tissues, failure to reliably estimate the hypoxic fraction of individual tumors and technical considerations make it unlikely that the assay can be used in the clinic to estimate the hypoxic fraction of human tumors.

ERCC1/ERCC4 5'-endonuclease activity as a determinant of hypoxic cell radiosensitivity

In this study, the relationships between cellular oxygen enhancement ratios (OER) and nucleotide excision repair capability were examined using the UV20 mutant cell line (which has a defective ERCC1 gene). Using a clonogenic survival assay, the OER for the killing of wild-type AA8 cells was 3.2 +/- 0.1, whereas that for UV20 cells was only 2.35 +/- 0.05; the decreased OER of UV20 cells was the result of their significantly greater radiosensitivity relative to wild-type cells under hypoxic conditions. In AA8 cells, hypoxia protected against DNA double-strand break (dsb) induction (determined by pulsed-field gel electrophoresis) by a factor 3.5 +/- 0.3; i.e. to a similar extent that it modulated cell killing. However, this correlation was not apparent in UV20 cells, where hypoxia protected against dsb induction to a similar extent as in wild-type cells (approximately 3.2-fold). Stably transfected UV20 cells over-expressing a full-length ERCC1 cDNA clone displayed a normal OER (3.5 +/- 0.1) in addition to wild-type resistance to UV light. Our data suggest that the hypoxic radiosensitivity of UV20 cells is a direct result of their ERCC1 deficiency and reflects their inability to process some type of DNA damage (not dsbs) that is induced preferentially in hypoxic cells.

Studies on mutagen sensitive strains of Drosophila melanogaster. XI. Survival (dominant lethality) after X-irradiation and relation to recessive lethals and translocations

Muller-5 males of Drosophila melanogaster were irradiated in N2 or O2 and mated to excision repair deficient, post-replication repair deficient (mei-9a, mei-41D5, mus101D1, mus201D1, mus302D1, mus306D1 and mus308D2) or repair proficient females. The surviving fraction (dominant lethality) was estimated in the F1 and used to reassess existing recessive lethal and translocation data. The surviving fraction was found to decrease if repair deficient females were used (maternal effect). The dose-effect curves are often biphasic with a steeper slope at low doses than at high (> or = 5 Gy) doses of X-rays. The high dose part of the curve is sensitive to oxygenation during irradiation and is affected significantly by the mutants with low fertility (mei-9, mus101 and mus302). The low dose component is not sensitive to oxygenation during irradiation and seems influenced by all seven repair deficient mutants. The sensitivity of the high dose part to oxygenation suggests that this part is related mainly to DNA break damage, while in the low dose part base damage seems more important. Existing recessive lethal and translocation data were plotted against the surviving fraction for a reassessment. In excision repair deficient mutants translocation induction is lower compared to repair proficient flies at the same level of survival (i.e., dominant lethality). Likewise in post-replication repair deficient mutants induction of recessive lethals is decreased. However the frequency of respectively induced recessive lethals and translocations obtained at the same level of X-rays was the same in repair deficient and proficient backgrounds. It is concluded that genetic damage recovered in a repair deficient background is likely to be qualitatively different even if the frequency of the damage induced by a given dose is not altered.