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Purpose: To ask whether dose-rate influences low-dose hyper- radiosensitivity and induced radioresistance (HRS/IRR) response in rat colon progressive (PRO) and regressive (REG) cells., Methods: Clonogenic survival was applied to tumorigenic PRO and non-tumorigenic REG cells irradiated with (60)Co γ-rays at 0.0025-500 mGy.min(-1). Both clonogenic survival and non-homologous end-joining (NHEJ) pathway involved in DNA double-strand breaks (DSB) repair assays were applied to PRO cells irradiated at 25 mGy.min(-1) with 75 kV X-rays only., Results: Irrespective of dose-rates, marked HRS/IRR responses were observed in PRO but not in REG cells. For PRO cells, the doses at which HRS and IRR responses are maximal were dependent on dose-rate; conversely exposure times during which HRS and IRR responses are maximal (t(HRSmax) and t(IRRmax)) were independent of dose-rate. The t(HRSmax) and t(IRRmax) values were 23 ± 5 s and 66 ± 7 s (mean ± standard error of the mean [SEM], n = 7), in agreement with literature data. Repair data show that t(HRSmax) may correspond to exposure time during which NHEJ is deficient while t(IRRmax) may correspond to exposure time during which NHEJ is complete., Conclusion: HRS response may be maximal if exposure times are shorter than t(HRSmax) irrespective of dose, dose-rate and cellular model. Potential application of HRS response in radiotherapy is discussed.

Immunofluorescence with antibodies against DNA damage repair and signaling protein is revolutionarising the estimation of the genotoxic risk. Indeed, a number of stress response proteins relocalize in nucleus as identifiable foci whose number, pattern and appearance/disappearance rate depend on several parameters such as the stress nature, dose, time and individual factor. Few authors proposed biomathematical tools to describe them in a unified formula that would be relevant for all the relocalizable proteins. Based on our two previous reports in this Journal (Foray et al., 2005; Gastaldo et al., 2008), we considered that foci response to stress is composed of a recognition and a repair phase, both described by an inverse power function provided from a Euler's Gamma distribution. The resulting unified formula called "Bodgi's function" is able to describe appearance/disappearance kinetics of nuclear foci after any condition of genotoxic stress. By applying the Bodgi's formula to DNA damage repair data from 45 patients treated with radiotherapy, we deduced a classification of human radiosensitivity based on objective molecular criteria, notably like the number of unrepaired DNA double-strand breaks and the radiation-induced nucleo-shuttling of the ATM kinase., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

What more natural than a stress response that would vary among individuals? The individual factor has been pointed out in numerous medical research areas, was alluded to in the antiquity under the term "idiosyncrasy" and used by Claude Bernard in the 19th century. The idea that each individual shows a specific response to radiation, as he does for any stress, is well-accepted. However, the history of radiobiology shows that this idea has been neglected. Today, it comes back in the debates of personalized medicine and in the evaluation of the risks of adverse reactions post-radiotherapy and of developing cancers linked to medical exposures., (© 2013 médecine/sciences – Inserm / SRMS.)

Purpose: By extending to other animal models the experiments on irradiated rabbit testes performed by the German Albers-Schönberg and Frieben and inspired by the preliminary observations of Regaud on spermatogenesis after irradiation, Bergonié and Tribondeau established a link between radiosensitivity and proliferation as a 'law' in 1906. Although it is still popular and taken as one of the founding laws of radiation oncology, it was early considered as a 'first approximation'., Conclusions: More than 100 years later, one tribute paid to these pioneers should be to at least avoid mixing notions like radiosensitivity, proliferation, differentiation, stem cells that they deeply contributed to define and to re-read their works in their original version in order to better understand what the technological and conceptual advances really are today.

Women with high risk of breast or ovarian cancers might be more susceptible to radiation-induced cancer because most of tumor suppressor genes are also implicated in the radio-induced DNA damage repair and signaling. Recent radiobiological advances may help to re-consider the potential cellular and molecular consequences of the standard two-view mammographic screening. A major radiobiological effect exacerbated in high family risk women caused by mammographic repeated doses was pointed out on relevant cellular model (untransformed and non tumoral human breast epithelial cells): the Low and Repeated Dose (LORD) effect. In parallel, while magnetic resonance imaging (MRI) is reported to be less sensitive than mammography for detection of ductal carcinoma in situ, a recent study highlighted the increased ability of MRI to detect them related to the experience both of radiologists and MRI centers. Hence, along with studies confirming improvement of the sensitivity of MRI to detect ductal carcinoma in situ, the supra-additivity effect induced by the two-view mammographic screening in high family risk patients suggests that mammographic exposures can be limited seriously. Consequently, a single view (oblique) per breast in association with annual MRI, with the sole aim to detect calcifications reflecting carcinoma in situ non detectable by MRI, might represent currently a compromise., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Poor local control and tumor escape are of major concern in head-and-neck cancers treated by conventional radiotherapy or hadrontherapy. Reduced glutathione (GSH) is suspected of playing an important role in mechanisms leading to radioresistance, and its depletion should enable oxidative stress insult, thereby modifying the nature of DNA lesions and the subsequent chromosomal changes that potentially lead to tumor escape.This study aimed to highlight the impact of a GSH-depletion strategy (dimethylfumarate, and L-buthionine sulfoximine association) combined with carbon ion or X-ray irradiation on types of DNA lesions (sparse or clustered) and the subsequent transmission of chromosomal changes to the progeny in a radioresistant cell line (SQ20B) expressing a high endogenous GSH content. Results are compared with those of a radiosensitive cell line (SCC61) displaying a low endogenous GSH level. DNA damage measurements (γH2AX/comet assay) demonstrated that a transient GSH depletion in resistant SQ20B cells potentiated the effects of irradiation by initially increasing sparse DNA breaks and oxidative lesions after X-ray irradiation, while carbon ion irradiation enhanced the complexity of clustered oxidative damage. Moreover, residual DNA double-strand breaks were measured whatever the radiation qualities. The nature of the initial DNA lesions and amount of residual DNA damage were similar to those observed in sensitive SCC61 cells after both types of irradiation. Misrepaired or unrepaired lesions may lead to chromosomal changes, estimated in cell progeny by the cytome assay. Both types of irradiation induced aberrations in nondepleted resistant SQ20B and sensitive SCC61 cells. The GSH-depletion strategy prevented the transmission of aberrations (complex rearrangements and chromosome break or loss) in radioresistant SQ20B only when associated with carbon ion irradiation. A GSH-depleting strategy combined with hadrontherapy may thus have considerable advantage in the care of patients, by minimizing genomic instability and improving the local control.

Purpose: To assess in vitro mammographic radiation-induced DNA damage in mammary epithelial cells from 30 patients with low (LR) or high (HR) family risk of breast cancer., Materials and Methods: Spontaneous and radiation-induced DNA double-strand breaks (DSB) were quantified by using immunofluorescence of the phosphorylated H2AX histone (γH2AX) in different conditions of mammography irradiation (2, 4, 2 + 2 mGy)., Results: HR patients showed significantly more spontaneous γH2AX foci than LR patients (p = 0.014). A significant dose-effect was observed, with an exacerbation in HR patients (p = 0.01). The dose repetition (2 + 2 mGy) provided more induced and more unrepaired DSB than 2 mGy and 4 mGy, and was exacerbated in HR (p = 0.006)., Conclusions: This study highlights the existence of DSB induced by mammography and revealed by γH2AX assay with two major radiobiological effects occurring: A low-dose effect, and a LOw and Repeated Dose (LORD) effect. All these effects were exacerbated in HR patients. These findings may lead us to re-evaluate the number of views performed in screening using a single view (oblique) in women whose mammographic benefit has not properly been proved such as HR patients.

At the beginning of the 21st century, radiation biology is at a major turning point in its history. It must meet the expectations of the radiation oncologists, radiologists and the general public, but its purpose remains the same: to understand the molecular, cellular and tissue levels of lethal and carcinogenic effects of ionizing radiation in order to better protect healthy tissues and to develop treatments more effective against tumours. Four major aspects of radiobiology that marked this decade will be discussed: technological developments, the importance of signalling and repair of radiation-induced deoxyribonucleic acid (DNA) damage, the impact of individual factor in the response to radiation and the contribution of radiobiology to better choose innovative therapies such as protontherapy or stereotactic body radiation therapy (SBRT). A translational radiobiology should emerge with the help of radiotherapists and radiation physicists and by facilitating access to the new radio and/or chemotherapy modalities., (Copyright © 2011. Published by Elsevier SAS.)

Synchrotrons are capable of producing intense low-energy X-rays that enable the photoactivation of high-Z elements. Photoactivation therapy (PAT) consists of loading tumors with photoactivatable drugs and thereafter irradiating them at an energy, generally close to the K-edge of the element, that enhances the photoelectric effect. To date, three major photoactivatable elements are used in PAT: platinum (cisplatin and carboplatin), iodine (iodinated contrast agents and iododeoxyuridine) and gadolinium (motexafin gadolinium). However, the molecular and cellular events specific to PAT and the radiobiological properties of these photoactivatable drugs are still misknown. Here, it is examined how standard and synchrotron X-rays combined with photoactivatable drugs impact on the cellular response of human endothelial cells. These findings suggest that the radiolysis products of the photoactivatable drugs may participate in the synergetic effects of PAT by increasing the severity of radiation-induced DNA double-strand breaks. Interestingly, subpopulation of highly damaged cells was found to be a cellular pattern specific to PAT. The data show that the efficiency of emerging anti-cancer modalities involving synchrotron photoactivation strongly depends on the choice of photoactivatable drugs, and important series of experiments are required to secure their clinical transfer before applying to humans.

To study the biological mechanisms of the repair of the radiation-induced DNA damage leads to two major medical applications: (1) the identification of the radiosensitive patients by using appropriate predictive assays in order to avoid toxicity due to radiation therapy and sometimes to chemotherapy; (2) the decrease of the radioresistance of tumour cells to obtain a better local control. To transpose fundamental biological knowledge from experimental in vitro clinic is delicate and sometimes too hasty, though necessary. In mechanistic terms, clinical features are once again a very rich and under-exploited approach to identify the molecular mechanisms of the DNA repair function. An exhaustive survey of the clinical cases of radiosensitivity with biological samples and with long course surveillance is an inverse approach, probably promising but still difficult to apply. Unlike classical reviews, this article attempts to identify the major genetic syndromes associated with radiosensitivity and cancer predisposition in order to deduce the different stages of major mechanisms of DNA double-strand breaks repair. Emphasis is placed on the importance of studying this repair at the functional level. Surprisingly, among the genetic syndromes associated to radiosensitivity there are some anomalies, not linked to DNA repair itself but to the intracellular trafficking. The repair function but therefore also the signalling are then logically therapeutic targets applicable in radiotherapy but with a very accurate ballistic sparing of the healthy tissues.

Individual response to ionizing radiation is an important information required to apply an efficient radiotherapy treatment against tumour and to avoid any adverse effects in normal tissues. In 1981, Fertil and Malaise have demonstrated that the post-irradiation local tumor control determined in vivo is correlated with clonogenic cell survival assessed in vitro. Furthermore, these authors have reminded the relevance of the concept of intrinsic radiosensitivity that is specific to each individual organ (Fertil and Malaise, 1981) [1]. To date, since clonogenicity assays are too time-consuming and do not provide any other molecular information, a plethora of research groups have attempted to determine the molecular bases of intrinsic radiosensitivity in order to propose reliable and faster predictive assays. To this aim, several approaches have been developed. Notably, the recent revolution in genomic and proteomic technologies is providing a considerable number of data but their link with radiosensitivity still remains to be elucidated. On another hand, the systematic screening of some candidate genes potentially involved in the radiation response is highlighting the complexity of the molecular and cellular mechanisms of DNA damage sensoring and signalling and shows that an abnormal radiation response is not necessarily due to the impairment of one single protein. Finally, more modest approaches consisting in focusing some specific functions of DNA repair seem to provide more reliable clues to predict over-acute reactions caused by radiotherapy. In this review, we endeavoured to analyse the contributions of these major approaches to predict human radiosensitivity., (Copyright © 2011 Académie des sciences. Published by Elsevier SAS. All rights reserved.)

Among post-translational modifications of chromatin proteins taking place in DNA double strand break (DSB) repair, acetylation plays a prominent role. This review lists several facts and hypotheses concerning this process. Lack of acetyltransferase TIP60 (HIV-Tat interacting protein of 60 kDa) activity results in cells with defective DSB repair. The enzyme is present in the nucleus in a multimeric protein complex. TIP60 dependent activation of ATM (ataxia telangiectasia mutated kinase) is an early event in the response to DNA breakage. Other important acetylations are those of histones H4 and γH2AX. Correct reconstruction of the damaged site is critical for survival and prevention of genetic and epigenetic changes in the cell that may affect the function of its daughter cells. Recently, two proteins with previously unsuspected functions in DSB repair have been identified as active in this process: Alzheimer β-amyloid precursor protein (APP) and its binding partner FE65, β-amyloid precursor binding protein. Their participation in DSB repair in both neuronal and non-neuronal cells is related to acetylation carried out by the acetyltransferase complex. The same function is ascribed to heterochromatin protein 1 (HP1). So far, the relations (if any) between TIP60 activation by HP1 and by the FE65 complex remain unidentified.

Exposure to radiation is one of the main concerns for space exploration by humans. By focusing deliberately on the works performed on human cells, we endeavored to review, decade by decade, the technological developments and conceptual advances of space radiation biology. Despite considerable efforts, the cancer and the toxicity risks remain to be quantified: 1) the nature and the frequency of secondary heavy ions need to be better characterized in order to estimate their contribution to the dose and to the final biological response; 2) the diversity of radiation history of each astronaut and the impact of individual susceptibility make very difficult any epidemiological analysis for estimating hazards specifically due to space radiation exposure. 3) Cytogenetic data undoubtedly revealed that space radiation exposure produce significant damage in cells. However, our knowledge of the basic mechanisms specific to low-dose, to repeated doses and to adaptive response is still poor. The application of new radiobiological techniques, like immunofluorescence, and the use of human tissue models different from blood, like skin fibroblasts, may help in clarifying all the above items.

Each technological development of radiotherapy is an example of interaction between physicians and physicists. In the past, it was the case for the first X-rays generators, betatrons and particle accelerators. To date, this is the case for Cyberknives and intensity modulation radiotherapy. In the future, this will be the case for proton- and hadron-therapy. However, in a general tendency of favouring higher radiation energies, leaving the 250kV orthovoltage irradiators and preferring accelerators delivering some tens MeV to reach the deepest tumours, how to consider the anti-cancer applications of synchrotron radiation that provides X-rays in the 10-100keV "only"? Since the first approaches developed in the USA in seventies until the last preclinical trials performed at the European Synchrotron Radiation Facility of Grenoble, the radiobiological features of the chemoradiotherapy involving synchrotron radiation will be described and analysed throughout a transversal view considering physicochemical bases, biomolecular and cellular mechanisms and results from the preclinical trials in order to provide a general outcome and some eventual transfer perspectives., (Copyright (c) 2010 Société française de radiothérapie oncologique (SFRO). Published by Elsevier SAS. All rights reserved.)

For the past 5 years, a radio-chemotherapy approach based on the photoactivation of platinum atoms (PAT-Plat) consisting of treating tumors with platinated compounds and irradiating them above the platinum K edge (78.4 keV) has been developed at the European Synchrotron Radiation Facility (Grenoble, France). Compared to other preclinical modalities, PAT-Plat provides the highest survivals of rats bearing the rodent F98 glioma. However, further investigations are required to optimize its efficiency and to allow its clinical application. Here we examined in vitro and in vivo whether monochromatic X rays are more efficient than high-energy photons in producing the PAT-Plat effect by measuring DNA double-strand breaks (DSBs) and survival of glioma-bearing rats and whether an increase in the platinum concentration in the tumor results in increased rat survival. DSBs were assessed by pulsed-field gel electrophoresis with different DNA fragment migration programs and with gamma-H2AX immunofluorescence. In vivo, F98 glioma cells were injected intracerebrally, treated with a single intracranial injection of cisplatin or carboplatin 13 days after tumor implantation, and irradiated the day after with 78.8 keV X rays or 6 MV photons. Our results indicate that 78.8 keV X rays are more efficient than high-energy photons at producing the PAT-Plat effect. At low concentrations, cisplatin is more efficient than carboplatin; this is likely due to more efficient DNA binding and DSB repair inhibition. High concentrations of carboplatin inside tumors do not necessarily lead to protracted survival of rats. The therapeutic benefit of anti-glioma synchrotron strategies appears to be correlated with the percentage of unrepaired DSBs but not with the number of DSBs induced.

Several human progerias, including Hutchinson-Gilford progeria syndrome (HGPS), are caused by the accumulation at the nuclear envelope of farnesylated forms of truncated prelamin A, a protein that is also altered during normal aging. Previous studies in cells from individuals with HGPS have shown that farnesyltransferase inhibitors (FTIs) improve nuclear abnormalities associated with prelamin A accumulation, suggesting that these compounds could represent a therapeutic approach for this devastating progeroid syndrome. We show herein that both prelamin A and its truncated form progerin/LADelta50 undergo alternative prenylation by geranylgeranyltransferase in the setting of farnesyltransferase inhibition, which could explain the low efficiency of FTIs in ameliorating the phenotypes of progeroid mouse models. We also show that a combination of statins and aminobisphosphonates efficiently inhibits both farnesylation and geranylgeranylation of progerin and prelamin A and markedly improves the aging-like phenotypes of mice deficient in the metalloproteinase Zmpste24, including growth retardation, loss of weight, lipodystrophy, hair loss and bone defects. Likewise, the longevity of these mice is substantially extended. These findings open a new therapeutic approach for human progeroid syndromes associated with nuclear-envelope abnormalities.

Purpose: To ask whether highly metastatic sublines show more marked low-dose hyper-radiosensitivity (HRS) response than poorly metastatic ones., Materials and Methods: The progressive (PRO) subline showing tumourigenicity and metastatic potential and the regressive (REG) subline showing neither tumourigenicity nor metastatic potential were both isolated from a parental rat colon tumour. Clonogenic survival, micronuclei and apoptosis, cell cycle distribution, DNA single- (SSB) and double-strand breaks (DSB) induction and repair were examined., Results: HRS phenomenon was demonstrated in PRO subline. Before irradiation, PRO cells show more spontaneous damage than REG cells. After 0.1 Gy, PRO cells displayed: (i) More DNA SSB 15 min post-irradiation, (ii) more unrepaired DNA DSB processed by the non-homologous end-joining (NHEJ) and by the RAD51-dependent recombination pathways, (iii) more micronuclei, than REG cells while neither apoptosis nor p53 phosphorylation nor cell cycle arrest was observed in both sublines., Conclusions: HRS response of PRO subline may be induced by impairments in NHEJ repair that targets G(1) cells and RAD51-dependent repair that targets S-G(2)/M cells. The cellular consequences of such impairments are a failure to arrest in cell cycle, the propagation of damage through cell cycle, mitotic death but not p53-dependent apoptosis. Tumourigenic cells with high metastatic potential may preferentially show HRS response.

Although cadmium still represents a public health problem and despite the fact that it has been classified as an IARC Group-I carcinogen, the molecular and cellular mechanisms responsible for the toxicity and the carcinogenicity of cadmium compounds are poorly known. Since unrepaired DNA double-strand breaks (DSBs) are considered to be key-lesions in cell lethality, and because misrepaired DSBs are a source of genomic instability leading to cancer proneness, the activity of the major DSB-repair pathways, i.e. non-homologous end-joining (NHEJ) and recombination, has been evaluated in human endothelial cells exposed to cadmium chloride and cadmium diacetate. Exposure to cadmium results in the production of DSBs a few hours after incubation. These breaks trigger the phosphorylation of H2AX proteins, which was used as an indirect measure of DSB in this study. The presence of cadmium in cells decreases the repair rate of X-ray-induced DSBs, suggesting an impact of cadmium upon the reparability of DSBs. Such an interpretation was consolidated by the finding that the DNA-PK kinase activity, essential for NHEJ, is affected by the presence of cadmium. These results suggest that the toxicity of cadmium compounds may be explained by the propagation of persistent DSBs. In parallel, the presence of cadmium was also associated with an over-activation of the MRE11-dependent repair pathway that may favour genomic instability. Altogether, our data provide a first example of the impact of cadmium upon DSB repair and signalling.

DNA is a key-target for genotoxic stress. Hence, the knowledge of induction and repair rate of DNA damage are crucial to describe and predict the impact of stress situations. Unfortunately, DNA damage induction and repair rates are generally assessed separately whereas they act either concomitantly or transiently in living organisms. Furthermore, the interplay of induction and repair raises the question whether DNA repair adapts to respond to different amounts of DNA damage. In a previous report, we proposed a stochastic interpretation of the repair rate of the major radiation-induced DNA damage. We provided evidence that the repair rate of individual DNA damage is time-independent whereas that of a population of DNA damage is time-dependent (Foray, N., Charvet, A.-M., Duchemin, D., Favaudon, V., Lavalette, D., 2005. The repair rate of radiation-induced DNA damage: a stochastic interpretation based on the gamma function. J. Theor. Biol. 236, 448-458). Here, to better describe situations in which DNA damage induction and repair occur together, our biostatistical model was modified by the introduction of a DNA damage induction parameter. Theoretical and experimental data were compared and discussed by taking concrete experimental situations: X-rays irradiation at different dose-rates, internal irradiation with radioactive compound, contamination with heavy metal and detection of DNA damage by immunofluorescence. By assuming that DNA repair rate is invariant whatever the amount of DNA damage, our model provides good prediction of experimental data suggesting its relevance for the description of complex situations of co-toxicities.

Purpose: Human diseases associated with acute radiation responses are rare genetic disorders with common clinical and biological features including radiosensitivity, genomic instability, chromosomal aberrations, and frequently immunodeficiency. To determine what molecular assays are predictive of cellular radiosensitivity whatever the genes mutations, the existence of a quantitative correlation between cellular radiosensitivity and unrepaired DNA double-strand breaks (DSB) repair defects was examined in a collection of 40 human fibroblasts representing 8 different syndromes., Materials and Methods: A number of techniques such as pulsed-field gel electrophoresis, plasmid assay and immunofluorescence with antibodies against MRE11, MDC1, 53BP1 and phosphorylated forms of H2AX, DNA-PK were applied systematically., Results and Conclusions: Survival fraction at 2 Gy was found to be inversely proportional to the amount of unrepaired DSB, whatever the genes mutations and the assay applied. However, no single assay discriminates the full range of human radiosensitivity. Particularly, nuclear foci formed by the phosphorylation of H2AX do not predict well moderate radiosensitivities. Our findings suggest the existence of an ATM-dependent interplay between the activation of DNA-PK and MRE11. A classification of diseases according their cellular radiosensitivity, their molecular response to radiation and the functional assays permitting their evaluation is proposed.

Purpose Anti-glioma strategies are generally based on trials involving rodent models whose choice remains based on proliferative capacity and availability. Recently, our group obtained the most protracted survival of rats bearing F98 gliomas by combining synchrotron X-rays and intracerebral cisplatin injection (Biston et al., Cancer Res, 64:2317-2323, 2004). The response to such treatment was suggested to be dependent on BRCA1, a tumour suppressor known to be involved in the response to radiation and cisplatin. In order to verify the impact of BRCA1 functionality upon success of anti-glioma trials, radiobiological features and BRCA1-dependent stress signalling were investigated in the most extensively used rodent glioma models. Methods Cell death pathways, cell cycle arrests, DNA repair and stress signalling were evaluated in response to radiation and cisplatin in C6, 9L and F98 models. Results Rodent glioma models showed a large spectrum of cellular radiation response. Surprisingly, BRCA1 was found to be functionally impaired in C6 and F98 favouring genomic instability, tumour heterogeneity and tolerance of unrepaired DNA damage. Significance Our findings strengthened the importance of the choice of the glioma model on genetic and radiobiological bases, inasmuch as all these rat glioma models are induced by nitrosourea-mediated mutagenesis that may favour specific gene mutations. Particularly, BRCA1 status may condition the response to anti-glioma treatments. Furthermore, since BRCA1 acts as a tumour suppressor in a number of malignancies, our findings raise also the question of the implication of BRCA1 in brain tumours formation.

Synchrotrons are opening new paths in innovative anti-cancer radiotherapy strategies. Indeed, the fluence of X-rays induced by synchrotrons is so high (10(6) times higher than standard medical irradiators) that it enables the production of X-ray beams tunable in energy (monochromatic beams) and in size (micrometric beams). Monochromatic synchrotron X-ray beams theoretically permit photoactivate high-Z elements to be introduced in or close to tumours in order to increase the yield of damage by enhanced energy photoabsorption. This is notably the case of attempts with iodinated contrast agents used in tumour imaging (the computed tomography therapy approach) and with platinated agents used in chemotherapy (the PAT-Plat approach). Micrometric synchrotron X-ray beams theoretically permit very high radiation doses to accumulate in tumours by using arrays of parallel microplanar beams that spare the surrounding tissues (the microbeam radiation therapy approach). These anti-cancer applications of synchrotron radiation have been developed at the European Synchrotron Radiation Facility to be applied to glioma, one of the tumour tissues most refractory to standard treatments. In the present paper the molecular and cellular mechanisms involved in these three approaches are reviewed, in the context of recent advances in radiobiology. Furthermore, by considering the unavoidable biases, an attempt to propose a comparison of the different results obtained in preclinical trials dealing with rats bearing tumours is given.

Despite a considerable amount of data, evaluation of the potential genotoxicity and cancer proneness of lead compounds remains unclear, probably due to the plethora of experimental procedures, biological endpoints and cellular models used. In parallel, the understanding in DNA damage formation, repair and signaling has considerably progressed all along these last years, notably for DNA double-strand breaks (DSBs). Here, were examined DNA damage formation and repair in human cells exposed to lead nitrate (Pb(NO(3))(2)) and their consequences upon the ATM-dependent stress signaling, cell cycle progression and cell death. As observed with anti-pH2AX immunofluorescence, exposure to Pb(NO(3))(2) results in formation of late DSBs, that would not originate from conversion of nucleotide damage but likely by a direct production of single-strand breaks. Lead contamination inhibits non-homologous end-joining repair process by preventing the DNA-PK kinase activity whereas the MRE11-dependent repair pathway is exacerbated. Lead contamination triggers successive synchronization of cells in G2/M phase in which the RAD51-dependent homologous recombination was found to be activated. Altogether, our findings support that lead contamination generates late unrepairable DSBs that impact upon the ATM-dependent stress signaling pathway by favoring propagation of errors. Such findings should help to consider more carefully the biological action of lead compounds in the frame of public and occupational exposures.

Radiobiologists have pointed out a novel radiobiological phenomenon observed in many tumor and normal cell lines: hyper-radiosensitivity to very low-dose (HRS) followed by induced radioresistance (IRR) after a threshold dose of 0.1-0.3 Gy that depends on the cell line. Radioresistance at high dose (i.e. higher than 0.5 Gy) and metastatic potential of tumor cells are likely major factors of failure in radiotherapy. A careful review of literature suggests that: 1) radiotherapy does not increase the metastatic potential of tumor cells; 2) radioresistance at high dose and metastatic potential are not related. However, inside a given tumor cell line, highly metastatic clones may elicit more cells showing HRS or are more radiosensitive at high dose than poorly metastatic ones. Recent data obtained from molecular techniques (comet and immunofluorescence assays) applied to single cells irradiated at very low radiation doses (1-100 mGy) suggest that DNA single-strand breaks (SSB) and double-strand breaks (DSB) may be the key-lesions responsible for the HRS phenomenon. These data suggest that the HRS phenomenon may find application in radiotherapy for micrometastasis. These early disseminated and probably unvascularised cells may escape the influence of high-dose chemotherapy after excision of the primary tumor. Considering the link between metastatic potential and HRS, we have previously proposed to apply very low-dose total body irradiation (TBI) at M(0) stage that may prevent the development of micrometastases. Literature data suggest that the smallest radiation dose that can produce HRS without increasing the risk of cancer may be in the milliGrays range.

Purpose: By allowing the visualization of the proteins inside cells, the immunofluorescence technique has revolutionized our view of events that follow radiation response. Particularly, the formation of nuclear foci, their kinetic of appearance and disappearance, and the association-dissociation of protein partners are useful endpoints to better understand the effects of ionizing radiation. Recently, the technique based on the phosphorylation of the histone 2A family, member X (H2AX) has generated a plethora of reports concerning the interaction between the major proteins involved in DNA repair and stress signaling pathways. However, some unavoidable overlaps of excitation and emission wavelength spectra (the so-called bleed-through phenomenon) of the available fluorescent markers are still generating discrepancies and misinterpretations in the choreography of DNA damage response. Biases are particularly strong with the fluorescein isothiocyanate (FITC)-rhodamine couple, tetramethyl rhodamine iso-thiocyanate (TRITC), the most extensively used markers., Method and Results: Here, two representative examples of biased co-immunofluorescence with pH2AX proteins that form radiation-induced nuclear foci or not are presented. A brief review of literature points out differences in kinetic of appearance and association-dissociation of radiation-induced pH2AX and MRE11 foci., Conclusion: Through this report, we would like authors to consider more carefully protein co-localizations by performing systematically, before any co-immunofluorescence, immunofluorescence of each protein separately to avoid bleed-through artifacts.

Among the large spectrum of DNA damage induced by radiation, DNA double-strand breaks (DSBs) are considered, to date, as the key-lesions responsible for the cell killing. However, although it was always intuitive to radiobiologists, such a conclusion has only been reached after technical developments and conceptual advances and remains consensual rather than demonstrated formally. In this article, we have reviewed the results that have lead to the conclusion that the assessment of successful DSB repair can be the basis of reliable assays predictive of the clinical response to radiotherapy and some chemotherapeutic treatments. We have discussed a number of technical artifacts, the biases due to the extrapolation of data obtained in yeast and rodent model systems to the human situation and the variety of phenotypes observed in human cells and in particular: 1) the most recent techniques developed, based on immunofluorescence, which have revolutionized our understanding of the molecular events occurring early after irradiation but have also raised the crucial questions about the choice of techniques to assess DSB repair and their specificity for different steps of the repair process; 2) While the homologous recombination repair pathway is predominant in yeasts, its importance in human cells appears less obvious, and raises the problem that the existence of randomized repair events may produce many more errors in human cells than in small genome organisms; 3) the impairment of DSB repair is observed in a plethora of genetic diseases, leading to radiosensitivity, immunodeficiency and sometimes cancer-proneness, but the low frequency and the pleiotropism of such diseases makes difficult the development of a single predictive assay. Therefore, although complete DSB repair appears to be crucial for cell survival, further research is still needed to provide innovative techniques fro measuring repair which can be successfully transferred to the clinic and used to ensure the avoidance of deleterious side-effects to cancer therapies.

There is a large body of evidence that stress-induced DNA damage may be responsible for cell lethality, cancer proneness and/or immune reaction. However, statistical features of their repair rate remain poorly documented. In order to interpret the shape of the radiation-induced DNA damage repair curves with a minimum of biological assumptions, we introduced the concept of repair probability, specific to any individual radiation-induced DNA damage, whatever its biochemical type. We strengthened the apparent paradox that the repair rate of a population of DNA damage is time-dependent even if the repair rate of the individual DNA damage is constant. Hence, the existing models, based on a dual approach of the DNA repair may be insufficient for describing the DNA repair rate over a large range of repair times. Since the repair probability of DNA damage cannot be assessed individually, the measurement of the DNA repair rate is assumed to consist in determining the instantaneous mean of all repair probabilities. The relevance of this model was examined with different endpoints: cell species, genotypes, radiation type and chromatin condensation. The Euler's Gamma function was shown to provide the distribution the most consistent with such hypotheses. Furthermore, formulas, deduced from the Gamma distribution, were found to be compatible with our previous model, empirically defined but based on a variable repair half-time.

Purpose: To date, iodinated contrast agents (ICA) are commonly used in medical imaging to improve tumor visualization by attenuating scanners X-rays. However, some adverse reactions to ICAs are still reported, and their molecular origin remains unclear. In 1983, it was proposed to visualize and treat ICA-loaded tumors by using scanners as therapy machines to enhance X-rays absorption at the iodine atoms. Theoretically, such physical conditions are optimized at 50 keV and can be easily obtained with synchrotrons., Methods and Materials: Here, we examined the molecular and cellular responses of mammalian endothelial cells to radiation in the presence of iomeprol, one of the most extensively used ICAs., Results: Irradiation with X-rays at 50 keV in the presence of iomeprol produced a strong radiosensitization effect. The same conclusion was reached with a standard medical irradiator but to a lesser extent. While such treatment did not produce additional DNA double-strand breaks, we observed a dose-dependent production of iodides due to the iomeprol radiolysis that inhibit double-strand break repair rate by decreasing DNA-PK kinase activity., Conclusions: Our data suggest that the concomitant use of ICA and radiation may be toxic when radiation-produced iodide concentrations and double-strand break yields are sufficient. The potential toxicity of ICAs during X-rays for diagnosis and therapy is discussed.

Background and Purpose: There is still confusion in the choice of the molecular assays to predict the radiation response of human cells. The case of tumours appears to be particularly complex, may be because of their instability and heterogeneity. The aim of this study was to investigate quantitatively the relationships between DNA double-strand breaks (DSB) repair, chromatin relaxation and cellular radiosensitivity. Nineteen human tumour cell lines, representing a large spectrum of radiation responses and tissues, were examined., Materials and Methods: Intrinsic radiosensitivity was quantified with surviving fraction at 2 Gy (SF2) as an endpoint. Standard and modified pulsed-field gel electrophoresis techniques were employed to assess DSB repair rate and chromatin relaxation. A cell-free assay was chosen to estimate DSB repair activity, independently of chromatin impairment., Results and Conclusions: Surviving fraction at 2 Gy (SF2) decreases linearly with the amount of unrepaired DSB and the extent of chromatin relaxation: one additional unrepaired DSB per cell or 1% chromatin decondensation produce a loss of about 1.5% surviving fraction. However, all the cell lines did not obey both correlations, suggesting that DSB repair and chromatin impairments contribute separately to increase the severity of DNA damage involved in cell lethality. Four cell lines groups showing different DSB repair and/or chromatin impairments were defined. Cell lines exhibiting both DSB repair defect and chromatin relaxation are the most radiosensitive.

High-grade gliomas are usually of poor prognosis, and conventional radiotherapy, even combined with chemotherapy, still fails to improve the survival of patients. Here, we propose an innovative therapeutic approach combining synchrotron radiation with cis-diamminedichloroplatinum (II) (CDDP). As suggested previously, monochromatic synchrotron irradiation of CDDP at 78.8 keV, just above the 78.4 keV platinum absorption K-edge, leads to an enhanced photoelectric effect and an increased local toxicity. To select a particular radiation energy that could provide supra-additive effect, we used pulsed-field gel electrophoresis to assess yields of DNA double-strand breaks induced in rat F98 glioma cells after CDDP treatment combined with synchrotron X-rays. Thereafter, intracerebral CDDP injection combined with synchrotron X-rays was applied to Fisher rats bearing F98 glioma. CDDP concentrations were mapped by synchrotron X-ray microfluorescence. An extra number of more slowly repaired double strand breaks were observed when irradiating CDDP-treated F98 cells at 78.8 keV. In vivo treatments were then performed with different radiation doses and CDDP concentrations. All cell inoculations in rat brain resulted in tumor development, and tumor presence was controlled by computed tomography. Among all of the conditions tested, the combination of 3 micro g of CDDP with 15 Gy resulted in the largest median survival time (206 days). After 1 year, about 34% of treated rats were still alive. This preclinical finding, validated by molecular analysis, represents the most protracted survival reported with this radioresistant glioma model and demonstrates the interest in powerful monochromatic X-ray sources as new tools for cancer treatments.

Combination of cis-platinum with ionizing radiation is one of the most promising anticancer treatments that appears to be more efficient than radiotherapy alone. Unlike conventional X-ray emitters, accelerators of high energy particles like synchrotrons display powerful and monochromatizable radiation that makes the induction of an Auger electron cascade in cis-platinum molecules [also called photoactivation of cis-platinum (PAT-Plat)] theoretically possible. Here, we examined the molecular consequences of one of the first attempts of synchrotron PAT-Plat, performed at the European Synchrotron Research Facility (Grenoble-France). PAT-Plat was found to result in an extra number of slowly repairable DNA double-strand breaks, inhibition of DNA-protein kinase activity, dramatic nuclear relocalization of RAD51, hyperphosphorylation of the BRCA1 protein, and activation of proto-oncogenic c-Abl tyrosine kinase.

BRCA1 plays an important role in mechanisms of response to double-strand breaks, participating in genome surveillance, DNA repair, and cell cycle checkpoint arrests. Here, we identify a constitutive BRCA1-c-Abl complex and provide evidence for a direct interaction between the PXXP motif in the C terminus of BRCA1 and the SH3 domain of c-Abl. Following exposure to ionizing radiation (IR), the BRCA1-c-Abl complex is disrupted in an ATM-dependent manner, which correlates temporally with ATM-dependent phosphorylation of BRCA1 and ATM-dependent enhancement of the tyrosine kinase activity of c-Abl. The BRCA1-c-Abl interaction is affected by radiation-induced modification to both BRCA1 and c-Abl. We show that the C terminus of BRCA1 is phosphorylated by c-Abl in vitro. In vivo, BRCA1 is phosphorylated at tyrosine residues in an ATM-dependent, radiation-dependent manner. Tyrosine phosphorylation of BRCA1, however, is not required for the disruption of the BRCA1-c-Abl complex. BRCA1-mutated cells exhibit constitutively high c-Abl kinase activity that is not further increased on exposure to IR. We suggest a model in which BRCA1 acts in concert with ATM to regulate c-Abl tyrosine kinase activity.

The loss of BRCA1 function appears as an essential step in breast and ovarian epithelial cells oncogenesis and is consistent with the concept that BRCA1 acts as a tumor suppressor gene. However, the mechanism underlying this activity is not understood. In 1996, a retroviral vector was used for BRCA1 delivery to demonstrate that the transfer of BRCA1 inhibits breast and ovarian cancer cell growth. Since this early observation, the tumor growth inhibitory activity of BRCA1 in vivo has not been further documented. Here we re-address this issue and report experiments designed to evaluate the potential of adenovirus-mediated BRCA1 delivery to suppress the growth of cells with various status of endogenous BRCA1 in comparison with p53 and p21. Delivery of wild-type BRCA1 by an adenovirus vector in breast and ovarian tumor cells, decreases in vitro proliferation and tumorigenicity. Similarly, in vivo administration of BRCA1 provokes tumor growth retardation or regression comparable to that obtained with p53 or p21. The antitumor effect of BRCA1 is not observed upon transfer of a mutant lacking the 542 C-terminal residues. The p53- or p21-mediated antiproliferative activities are likely to bear on their capacity to induce apoptosis and/or interfere with cell cycle checkpoint. By contrast, the data presented here show that neither of these mechanisms can account for the BRCA1-mediated antitumor activity and suggest the activation of an alternative route.

Cells derived from Nijmegen Breakage Syndrome (NBS) patients display radiosensitivity and cell cycle checkpoint defects. Here, we examine whether the radiosensitivity of NBS cells is the result of a repair defect or whether it can be attributed to impaired checkpoint arrest. We report a small increased fraction of unrejoined double strand breaks and, more significantly, increased chromosome breaks in noncycling NBS cells at 24 h after irradiation. One of the NBS lines examined (347BR) was atypical in showing a nearly normal checkpoint response. In contrast to the mild checkpoint defect, 347BR displays marked y-ray sensitivity similar to that shown by other NBS lines. Thus, the gamma-ray sensitivity correlates with the repair defect rather than impaired checkpoint control. Taken together, the results provide direct evidence for a repair defect in NBS cells and are inconsistent with the suggestion that the radiosensitivity is attributable only to impaired checkpoint arrest. 347BR also displays elevated spontaneous damage that cannot be attributed to impaired G2-M arrest, suggesting a function of Nbsl in decreasing or limiting the impact of spontaneously arising double strand breaks.

Ku86 has been shown to be involved in DNA double-strand break (DSB) repair and radiosensitivity in rodents, but its role in human cells is still under investigation. The purpose of this study was to evaluate the radiosensitivity and DSB repair after transfection of a Ku86-antisense in a human fibroblast cell line. Simian virus 40-transformed MRC5V1 human fibroblasts were transfected with a vector (pcDNA3) containing a Ku86-antisense cDNA. The main endpoints were Ku86 protein level, Ku DNA end-binding and DNA protein kinase activity, clonogenic survival, and DSB repair kinetics. After transfection of the Ku86-antisense, decreased Ku86 protein expression, Ku DNA end-binding activity, and DNA protein kinase activity were observed in the uncloned cellular population. The fibroblasts transfected with the Ku86-antisense showed also a radiosensitive phenotype, with a surviving fraction at 2 Gy of 0.29 compared with 0.75 for the control and 20% of unrepaired DSB observed at 24 hours after irradiation compared with 0% for the control. Several clones were also isolated with a decreased level of Ku86 protein, a surviving fraction at 2 Gy between 0.05 and 0.40, and 10-20% of unrepaired DSB at 24 hours. This study is the first to show the implication of Ku86 in DSB repair and in the radiosensitivity of human cells. This investigation strongly suggests that Ku86 could constitute an appealing target for combining gene therapy and radiation therapy.

There is now evidence to suggest that BRCA1 and BRCA2 are involved in the response of cells to DNA damage and cell cycle checkpoint control. This report examines the death pathways of human cells with various BRCA1 and BRCA2 genotypes after exposure to gamma-rays. A lack of functional BRCA1 and BRCA2 led to defective repair of DNA double-strand breaks in irradiated cells. This impairment resulted in a relaxation of cell cycle checkpoints, production of micronuclei, and a loss of proliferative capacity. Heterozygous BRCA1 and BRCA2 mutations also led to enhanced radiosensitivity, with an impaired proliferative capacity after irradiation. The existence of a phenotype related to radiosensitivity in BRCA1+/- and BRCA2+/- cells raises the question of the response of heterozygous women to radiation.

Purpose: To overcome the underestimation of the small residual damage when measuring DNA double-strand breaks (DSB) as fraction of activity released (FAR) by pulsed-field gel electrophoresis., Materials and Methods: The techniques used to assess DNA damage (e.g. pulsed-field gel electrophoresis, neutral elution, comet assay) do not directly measure the number of DSB. The Blöcher model can be used to express data as DSB after irradiation at 4 degrees C by calculating the distribution of all radiation-induced DNA fragments as a function of their size. We have used this model to measure the residual DSB (irradiation at 4 degrees C followed by incubation at 37 degrees C) in untransformed human fibroblasts., Results: The DSB induction rate after irradiation at 4 degrees C was 39.1+/-2.0 Gy(-1). The DSB repair rate obtained after doses of 10 to 80 Gy followed by repair times of 0 to 24 h was expressed as unrepaired DSB calculated from the Blöcher formula. All the damage appeared to be repaired at 24h when the data were expressed as FAR, whereas 15% of DSB remained unrepaired. The DSB repair rate and the chromosome break repair rate assessed by premature condensation chromosome (PCC) techniques were similar., Conclusion: The expression of repair data in terms of FAR dramatically underestimates the amount of unrepaired DNA damage. The Blöcher model that takes into account the size distribution of radiation-induced DNA fragments should therefore be used to avoid this bias. Applied to a normal human fibroblast cell line, this model shows that DSB repair is never complete.

Severe immunodeficiency characterized by lymphopenia was found in two siblings, one of whom was examined in detail. The calcium flux, pattern of tyrosine phosphorylation of proteins, and interleukin 2 (IL-2) production and proliferation in response to mitogens suggested that the peripheral blood T cells activated normally. The peripheral blood T cells were shown to have an activated phenotype with increased expression of CD45RO+ and CD95/Fas. Increased spontaneous apoptosis occurred in unstimulated lymphocyte cultures. The elevated apoptosis was not due to alterations in expression or to mutations in Bcl-2, Bcl-X(L), or Flip, nor could the spontaneous apoptosis be prevented by blocking Fas, suggesting that it was independent of Fas signaling. This is the first inherited combined immunodeficiency associated with impaired lymphocyte survival. Fibroblasts derived from the patient showed appreciable radiosensitivity in clonal assays, but apoptosis was not elevated. Our results show that the fibroblasts represent a new radiosensitive phenotype not associated with cell cycle checkpoint defects, V(D)J recombination defects, or elevated chromosome breakage. We suggest that the affected gene plays a role in an undetermined damage response mechanism that results in elevated spontaneous apoptosis in lymphoid cells and radiosensitivity in fibroblasts.

cis-Dichlorodiammineplatinum(II) (CDDP; cisplatin) is commonly used in combination with ionizing radiation (IR) in the treatment of various malignancies. In vitro, many observations suggest that acquisition of CDDP resistance in cell lines confers cross-resistance to IR, but the molecular mechanisms involved have not been well documented yet. We report here the selection and characterization of a murine CDDP-resistant L1210 cell line (L1210/3R) that exhibits cross-resistance to IR because of an increased capacity to repair double-strand breaks compared with parental cells (L1210/P). In resistant cells, electrophoretic mobility shift assays revealed an increased DNA-end binding activity that could be ascribed, by supershifting the retardation complexes with antibodies, to the autoantigen Ku. The heterodimeric Ku protein, composed of 86-kDa (Ku80) and 70-kDa (Ku70) subunits, is the DNA-targeting component of DNA-dependent protein kinase (DNA-PK), which plays a critical role in mammalian DNA double-strand breaks repair. The increased Ku-binding activity in resistant cells was associated with an overexpression affecting specifically the Ku80 subunit. These data strongly suggest that the increase in Ku activity is responsible for the phenotype of cross-resistance to IR. In addition, these observations, along with previous results from DNA-PK- mutant cells, provide evidence in favor of a role of Ku/DNA-PK in resistance to CDDP. These results suggest that Ku activity may be an important molecular target in cancer therapy at the crossroad between cellular responses to CDDP and IR.

Purpose: To propose a novel interpretation of DNA double-strand break (dsb) repair based on the distribution of energy micro-deposition., Materials and Methods: Double-strand break repair curves were studied either after irradiation at 4 degrees C or at 37 degrees C (low dose rate). Two human fibroblast cell lines were used: a control line, HF19, and an ataxia telangiectasia repair-deficient line, AT5BI. Irradiations were made with gamma-rays or alpha-particles (241Am). Repair data were fitted by the variable repair half-time (VRHT) model. Assuming that each dsb has its own inherent repair half-time (IRHT) and that the VRHT is the average of the IRHT at any time during repair, the distribution of the IRHT was calculated., Results: At the end of the irradiation, the distribution was a continuous asymmetric curve with a maximum of dsb having a short IRHT. After 1 h of repair, the curve became bell-shaped. There is a striking similarity between the distribution of dsb repair half-times and that of energy micro-deposition described by Goodhead et al. (1993)., Conclusion: This similarity suggests a possible causal relationship between the energy density deposition and the repair rate or the probability of dsb repair.

The 180BR cell line was derived from an acute lymphoblastic leukemia patient who overresponded to radiation therapy and died following radiation morbidity. 180BR cells are hypersensitive to the lethal effects of ionizing radiation and are defective in the repair of DNA double-strand breaks (DSBs). The levels and activity of the proteins of the DNA-dependent protein kinase complex are normal in 180BR cells. To facilitate a measurement of V(D)J recombination, we have characterized 180BRM, a SV40-transformed line derived from 180BR. 180BRM retains the radiosensitivity and defect in DSB repair characteristic of 180BR. The activities associated with DNA-dependent protein kinase are also normal in 180BRM cells. The ability to carry out V(D)J recombination is comparable in 180BRM and a reference control transformed human cell line, MRC5V1. These results show that 180BR and 180BRM differ from the rodent mutants belonging to ionizing radiation complementation groups 4, 5, 6, and 7 and, therefore, represent a new mutant phenotype, in which a defect in DNA DSB rejoining is not associated with defective V(D)J recombination. Furthermore, we have shown that 180BR can arrest at the G1-S and G2-M cell cycle checkpoints after irradiation. These results confirm that 180BR can be distinguished from ataxia telangiectasia.

A large number of reports suggest that DNA double-strand breaks (DSB) play a major role in the radiation-induced killing of mammalian cells. However, the arguments supporting the relationship between DSB and radiosensitivity are generally indirect. Furthermore, care must be taken to allow for the possible impact of the techniques and of the experimental protocols on the relationship between DSB and cell death. The recent data on DSB induction, repair and misrepair in human cell lines and their correlation with intrinsic radiosensitivity are reviewed.

We have studied the intrinsic radiosensitivity, repair of potentially lethal damage (PLD) and the repair rate of radiation-induced DNA double-strand breaks (DSB) in 11 non-transformed human fibroblast cell lines, four of which were homozygous for the A-T mutation and two that were heterozygous (A-TH). All the experiments were done on cells in plateau phase of growth (97-99% of cells in G0/G1). With a dose of 30 Gy delivered at 4 degrees C, the A-T cell lines had faster repair rates of up to 6 h, after which the repair curve crossed that of the control so that the residual damage at 24 h was higher in the A-T cells. Irradiation at 37 degrees C at low dose rate 1 cGy.min-1) produced even more marked differences between the A-T cells and controls: the residual DSB level was always higher in A-T cells than controls at doses of 5-40 Gy, due to defective repair of a small fraction of DSB in A-T cells. The two protocols showed DSB repair rates for the A-TH cell lines that were intermediate between those of the A-T and control cells. There was a quantitative relationship between the residual DSB after irradiation at 37 degrees C and the intrinsic radiosensitivity, and with the extent of PLD repair. There were very few apoptotic cells in the non-transformed control and A-T cell line, both before and after irradiation. In combination, these result support the contention that the defective repair of DSB is a mechanism of the hypersensitivity linked to the A-T mutation.

A review of reports dealing with fittings of the data for repair of DNA double-strand breaks (DSBs) and excess chromosome fragments (ECFs) shows that several models are used to fit the repair curves. Since DSBs and ECFs are correlated, it is worth developing a model describing both phenomena. The curve-fitting models used most extensively, the two repair half-times model for DSBs and the monoexponential plus residual model for ECFs, appear to be too inflexible to describe the repair curves for both DSBs and ECFs. We have therefore developed a new concept based on a variable repair half-time. According to this concept, the repair curve is continuously bending and dependent on time and probably reflects a continuous spectrum of damage repairability. The fits of the curves for DSB repair to the variable repair half-time and the variable repair half-time plus residual models were compared to those obtained with the two half-times plus residual and two half-times models. Similarly, the fits of the curves for ECF repair to the variable repair half-time and variable half-time plus residual models were compared to that obtained with the monoexponential plus residual model. The quality of fit and the dependence of adjustable parameters on the portion of the curve fitted were used as comparison criteria. We found that: (a) It is useful to postulate the existence of a residual term for unrepairable lesions, regardless of the model adopted. (b) With the two cell lines tested (a normal and a hypersensitive one), data for both DSBs and ECFs are best fitted to the variable repair half-time plus residual model, whatever the repair time range.