Tamulevicius, P., Wang, M. and Iliakis, G. Homology-Directed Repair is Required for the Development of Radioresistance during S Phase: Interplay between Double-Strand Break Repair and Checkpoint Response. Radiat. Res. 167, 1– 11 (2007).

The S-phase-dependent radioresistance to killing uniformly seen in eukaryotic cells is absent in radiosensitive mutants with defects in genes involved in the repair of DNA double-strand breaks (DSBs) by homologous recombination (homologous recombination repair: HRR). This implicates, for the first time, a concrete DNA repair process in the radiosensitivity of a specific cell cycle phase. The cell cycle-dependent fluctuations in radiosensitivity reflect a fundamental and well-documented radiobiological phenomenon that still awaits a detailed molecular characterization. The underlying mechanisms are likely to combine aspects of DNA repair and cell cycle regulation. Advances in both fields allow a first dissection in the cell cycle of the molecular interplay between DSB repair and DNA damage checkpoint response and its contribution to cell survival. Here we review the available literature on the topic, speculate on the ramifications of this information for our understanding of cellular responses to DNA damage, and discuss future directions in research. An effort is made to integrate relevant phenomena of radiation action, such as low-dose radiosensitivity and the G₂ assay in this scheme.

Suit, H., Goldberg, S., Niemierko, A., Ancukiewicz, M., Hall, E., Goitein, M., Wong, W. and Paganetti, H. Secondary Carcinogenesis in Patients Treated with Radiation: A Review of Data on Radiation-Induced Cancers in Human, Non-human Primate, Canine and Rodent Subjects. Radiat. Res. 167, 12–42 (2007).

Concern for risk of radiation-induced cancer is growing with the increasing number of cancer patients surviving long term. This study examined data on radiation transformation of mammalian cells in vitro and on the risk of an increased cancer incidence after irradiation of mice, dogs, monkeys, atomic bomb survivors, occupationally exposed persons, and patients treated with radiation. Transformation of cells lines in vitro increased linearly with dose from ∼1 to ∼4–5 Gy. At <0.1 Gy, transformation was not increased in all studies. Dose–response relationships for cancer incidence varied with mouse strain, gender and tissue/organ. Risk of cancer in Macaca mulatta was not raised at 0.25–2.8 Gy. From the atomic bomb survivor study, risk is accepted as increasing linearly to 2 Sv for establishing exposure standards. In irradiated patients, risk of cancer increased significantly from 1 to 45 Gy (a low to a high dose level) for stomach and pancreas, but not for bladder and rectum (1–60 Gy) or kidney (1–15 Gy). Risk for several organs/tissues increased substantially at doses far above 2 Gy. There is great heterogeneity in risk of radiation-associated cancer between species, strains of a species, and organs within a species. At present, the heterogeneity between and within patient populations of virtually every parameter considered in risk estimation results in substantial uncertainty in quantification of a general risk factor. An implication of this review is that reduced risks of secondary cancer should be achieved by any technique that achieved a dose reduction down to −0.1 Gy, i.e. dose to tissues distant from the target. The proportionate gain should be greatest for dose decrement to less than 2 Gy.

Benderitter, M., Isoir, M., Buard, V., Durand, V., Linard, C., Vozenin-Brotons, M. C., Steffanazi, J., Carsin, H. and Gourmelon, P. Collapse of Skin Antioxidant Status during the Subacute Period of Cutaneous Radiation Syndrome: A Case Report. Radiat. Res. 167, 43–50 (2007).

This case report describes a patient suffering from accidental cutaneous radiation syndrome. Clinical symptoms were characterized by the presence of moist epidermal denudation over approximately 8% of the body surface without signs of necrosis 88 days after radiation exposure. The skin transcriptional profile was obtained and provides a comprehensive overview of the changes in gene expression associated with skin wound healing after irradiation. In particular, our data show a specific set of genes, i.e. SOD1, GPX1, TDX1, TDX2 and HSP60, implicated in the redox control of normal skin repair after radiation exposure, whereas HOX1 and HOX2 were involved in the pathological skin repair. A reduction in the antioxidant capacity of the irradiated tissue concomitant with a progressive establishment of an uncontrolled inflammatory response was noted. Our data corroborate the hypothesis that ROS modulation is a key element of the healing response after cutaneous exposure to radiation and that the collapse of skin antioxidant status interferes directly with wound healing in skin after radiation exposure. Thus a better understanding of the molecular events through which oxidative stress modulates the healing response could result in a more rational therapeutic approach to the pathological process induced after exposure of skin to radiation.

Okazaki, R., Ootsuyama, A. and Norimura, T. TP53 and TP53-Related Genes Associated with Protection from Apoptosis in the Radioadaptive Response. Radiat. Res. 167, 51–57 (2007).

We investigated the effect of administering priming low-dose radiation prior to high-dose radiation on the level of apoptosis and on the expression of TP53 and TP53-related genes in mouse splenocytes. The percentage of apoptotic cells was significantly lower in TP53 ^/ mice receiving priming radiation 2 to 168 h before the high-dose irradiation, compared to TP53 ^/ mice exposed to 2 Gy alone. In contrast, TP53 ^/− mice exhibited a reduced level of apoptosis only when priming was performed for 2 or 4 h prior to the high-dose irradiation. In TP53 ^/ mice, primed mice had higher TP53 expression than mice exposed to 2 Gy. Phospho-TP53 (ser15/18) expression was the highest in mice exposed to 2 Gy and intermediate in primed mice. Expression of p21 (CDKN1A) was higher in primed mice compared with mice exposed to 2 Gy. MDM2 expression remained at a high level in all mice receiving 2 Gy. Elevated phospho-ATM expression was observed only in mice exposed to 2 Gy. We conclude that TP53 plays a critical role in the radioadaptive response and that TP53 and TP53-related genes might protect cells from apoptosis through activation of the intracellular repair system.

Alsbeih, G., Torres, M., Al-Harbi, N. and Al-Buhairi, M. Evidence that Individual Variations in TP53 and CDKN1A Protein Responsiveness are Related to Inherent Radiation Sensitivity. Radiat. Res. 167, 58–65 (2007).

We tested the hypothesis that individual variations in the induction of the TP53 tumor suppressor protein by radiation are related to inherent radiosensitivity. Thirty-two fibroblast cell strains were examined. Radiosensitivity was measured by a clonogenic survival assay. The induction of TP53 and its transcriptionally activated CDKN1A (p21) protein were studied by Western blotting 3 h after a single dose of 5 Gy. The relative cell culture age, as determined by the colony size distribution, was studied as a confounding factor. Survival curves showed wide range of radiosensitivity. The surviving fraction at 2 Gy (SF2) ranged between 0.02 and 0.49 (mean = 0.29, SD = 0.13). TP53 induction ranged between 1.28 and 2.34 (mean = 1.80, SD = 0.31). CDKN1A showed a wider induction (1.09–4.05, mean = 2.33, SD = 0.78). Positive correlations were observed between SF2 and TP53 induction (R² = 0.62, P < 0.001) and CDKN1A (R² = 0.64, P < 0.001). No correlation with the colony size distribution was observed. In conclusion, these results suggest that the individual variations in radiosensitivity and in the level of induction of TP53 (and consequently CDKN1A) are congruent, irrespective of the genetic background of these nontransformed fibroblasts. It is postulated that underlying mechanisms culminating in a stronger TP53 induction lead to higher survival, presumably due to more efficient repair of radiation-induced damage.

Ghosh, G., Li, G., Myung, K. and Hendrickson, E. A. The Lethality of Ku86 (XRCC5) Loss-of-Function Mutations in Human Cells is Independent of p53 (TP53). Radiat. Res. 167, 66–79 (2007).

Ku86 is one of the two regulatory subunits of the DNA-PK (DNA-dependent protein kinase) complex that is required for DNA double-strand break repair in mammalian cells. In a previous study, by means of somatic gene targeting, we generated human cell lines deficient in Ku86 (XRCC5). Heterozygous human Ku86 cells exhibited a wide array of haploinsufficient phenotypes, including sensitivity to ionizing radiation, defects in DNA-PK and DNA end-binding activities, elevated levels of p53 (TP53) and γ-H2AX foci, and a defect in cell proliferation with an increase in the frequency of aneuploid cells. Here we demonstrate that the overexpression of a human Ku86 cDNA complemented the deficiencies of these cells to wild-type levels. In contrast, Ku86 overexpression only partially rescued the telomere defects characteristic of Ku86 heterozygous cells and did not rescue their genetic instability. Additionally, in stark contrast to every other species described to date, we had shown earlier that homozygous human Ku86^−/− cells are inviable, because they undergo 8 to 10 rounds of cell division before succumbing to apoptosis. The tumor suppressor protein p53 regulates the DNA damage response in mammalian cells and triggers apoptosis in the face of excessive DNA damage. Correspondingly, ablation of p53 expression has repeatedly been shown to significantly ameliorate the pathological effects of loss-of-function mutations for a large number of DNA repair genes. Surprisingly, however, even in a p53-null genetic background, the absence of Ku86 proved lethal. Thus the gene encoding Ku86 (XRCC5) is an essential gene in human somatic cells, and its absence cannot be suppressed by the loss of p53 function. These results suggest that Ku86 performs an essential role in telomere maintenance in human cells.

Ansari, R., Gaber, M. W., Wang, B., Pattillo, C. B., Miyamoto, C. and Kiani, M. F. Anti-TNFA (TNF-α) Treatment Abrogates Radiation-Induced Changes in Vascular Density and Tissue Oxygenation. Radiat. Res. 167, 80–86 (2007).

Ionizing radiation significantly alters the structure and function of microvasculature, which regulates delivery of oxygen to brain tissue. Previous experimental and modeling studies have shown that tissue oxygenation patterns are significantly different in irradiated normal tissue compared to age-matched controls, and the differences are apparent as early as 3 days postirradiation. However, oxygen delivery to irradiated tissue recovers within 6 months postirradiation. Changes in perfusion and oxygenation were studied in a bilaterally (both cerebral hemispheres) and unilaterally (only one hemisphere) irradiated mouse brain model at 6 and 24 h as well as 3, 7, 30, 60 and 120 days postirradiation. The results indicate that significant changes in the number of perfused vessels (as measured by fluorescent DiOC₇ staining) and anatomical vessels (as indicated by CD31 immunohistochemical staining) and tissue oxygenation (by immunohistochemical detection of a fluorescently conjugated monoclonal antibody to EF5) are most pronounced at 3 days postirradiation, while a degree of recovery is observed at later times. However, in the unilaterally irradiated animals, both irradiated and unirradiated (out-of-field) cerebral hemispheres showed similarly significant changes in oxygenation and/or perfusion compared to unirradiated controls. Anti-TNFA treatment inhibited radiation-induced local as well as abscopal effects in the brain tissue.

Chauhan, V., Mariampillai, A., Kutzner, B., Wilkins, R. C., Ferrarotto, C., Bellier, P. V., Marro, L., Gajda, G. B., Lemay, E., Thansandote, A. and McNamee, J. P. Evaluating the Biological Effects of Intermittent 1.9 GHz Pulse-Modulated Radiofrequency Fields in a Series of Human-Derived Cell Lines. Radiat. Res. 167, 87–93 (2007).

Several recent studies have suggested that radiofrequency (RF) fields may cause changes in a variety of cellular functions that may eventually lead to potential long-term health effects. In the present study, we have assessed the ability of non-thermal RF-field exposure to affect a variety of biological processes (including apoptosis, cell cycle progression, viability and cytokine production) in a series of human-derived cell lines (TK6, HL60 and Mono-Mac-6). Exponentially growing cells were exposed to intermittent (5 min on, 10 min off) 1.9 GHz pulse-modulated RF fields for 6 h at mean specific absorption rates (SARs) of 0, 1 and 10 W/kg. Concurrent negative (incubator) and positive (heat shock for 1 h at 43°C) controls were included in each experiment. Immediately after the 6-h exposure period and 18 h after exposure, cell pellets were collected and analyzed for cell viability, the incidence of apoptosis, and alterations in cell cycle kinetics. The cell culture supernatants were assessed for the presence of a series of human inflammatory cytokines (TNFA, IL1B, IL6, IL8, IL10, IL12) using a cytometric bead array assay. No detectable changes in cell viability, cell cycle kinetics, incidence of apoptosis, or cytokine expression were observed in any of RF-field-exposed groups in any of the cell lines tested, relative to the sham controls. However, the positive (heat-shock) control samples displayed a significant decrease in cell viability, increase in apoptosis, and alteration in cell cycle kinetics (G₂/M block). Overall, we found no evidence that non-thermal RF-field exposure could elicit any detectable biological effect in three human-derived cell lines.

Yokota, Y., Yamada, S., Hase, Y., Shikazono, N., Narumi, I., Tanaka, A. and Inoue, M. Initial Yields of DNA Double-Strand Breaks and DNA Fragmentation Patterns Depend on Linear Energy Transfer in Tobacco BY-2 Protoplasts Irradiated with Helium, Carbon and Neon Ions. Radiat. Res. 167, 94–101 (2007).

The ability of ion beams to kill or mutate plant cells is known to depend on the linear energy transfer (LET) of the ions, although the mechanism of damage is poorly understood. In this study, DNA double-strand breaks (DSBs) were quantified by a DNA fragment-size analysis in tobacco protoplasts irradiated with high-LET ions. Tobacco BY-2 protoplasts, as a model of single plant cells, were irradiated with helium, carbon and neon ions having different LETs and with γ rays. After irradiation, DNA fragments were separated into sizes between 1600 and 6.6 kbp by pulsed-field gel electrophoresis. Information on DNA fragmentation was obtained by staining the gels with SYBR Green I. Initial DSB yields were found to depend on LET, and the highest relative biological effectiveness (about 1.6) was obtained at 124 and 241 keV/μm carbon ions. High-LET carbon and neon ions induced short DNA fragments more efficiently than γ rays. These results partially explain the large biological effects caused by high-LET ions in plants.

Yokoyama, K., Miyatake, S-I., Kajimoto, Y., Kawabata, S., Doi, A., Yoshida, T., Okabe, M., Kirihata, M., Ono, K. and Kuroiwa, T. Analysis of Boron Distribution In Vivo for Boron Neutron Capture Therapy using Two Different Boron Compounds by Secondary Ion Mass Spectrometry. Radiat. Res. 167, 102–109 (2007).

The efficiency of boron neutron capture therapy (BNCT) for malignant gliomas depends on the selective and absolute accumulation of ¹⁰B atoms in tumor tissues. Only two boron compounds, BPA and BSH, currently can be used clinically. However, the detailed distributions of these compounds have not been determined. Here we used secondary ion mass spectrometry (SIMS) to determine the histological distribution of ¹⁰B atoms derived from the boron compounds BSH and BPA. C6 tumor-bearing rats were given 500 mg/kg of BPA or 100 mg/kg of BSH intraperitoneally; 2.5 h later, their brains were sectioned and subjected to SIMS. In the main tumor mass, BPA accumulated heterogeneously, while BSH accumulated homogeneously. In the peritumoral area, both BPA and BSH accumulated measurably. Interestingly, in this area, BSH accumulated distinctively in a diffuse manner even 800 μm distant from the interface between the main tumor and normal brain. In the contralateral brain, BPA accumulated measurably, while BSH did not. In conclusion, both BPA and BSH each have advantages and disadvantages. These compounds are considered to be essential as boron delivery agents independently for clinical BNCT. There is some rationale for the simultaneous use of both compounds in clinical BNCT for malignant gliomas.

Emfietzoglou, D. and Nikjoo, H. Accurate Electron Inelastic Cross Sections and Stopping Powers for Liquid Water over the 0.1–10 keV Range Based on an Improved Dielectric Description of the Bethe Surface. Radiat. Res. 167, 110–120 (2007).

Electron inelastic cross sections and stopping powers for liquid water over the 0.1–10 keV range are presented based on a recently developed dielectric response model for liquid water (D. Emfietzoglou, F. Cucinotta and H. Nikjoo, Radiat. Res. 164, 202–211, 2005) that is consistent with the experimental data over the whole energy-momentum plane. Both exchange and second-order Born corrections are included in a material-specific way using the dielectric functions of liquid water. The numerical results are fitted by simple analytic functions to facilitate their further use. Compared to previous studies, differential cross sections are shifted toward smaller energy losses resulting in smaller inelastic and stopping cross sections with differences reaching, on average, the ∼20% and ∼50% level, respectively. Contrary to higher energies, it is shown that the dispersion model for the momentum dependence of the dielectric functions (Bethe ridge) is as important as the optical model used. Within the accuracy of the experimental data (a few percent) upon which our dielectric model is based, the calculations are “exact” to first order, while the uncertainty of the results beyond first order is estimated at the 5–10% level. The present work overcomes the limitations of Bethe's theory at low energies by a self-consistent account of inner-shell effects and may serve to extend the ICRU electron stopping power database for liquid water down to 100 eV with a level of uncertainty similar to that for the higher-energy values.