DNA Organization Affects Cellular Radiosensitivity and Detection of Initial DNA Strand Breaks

Direct and bystander radiation effects: a biophysical model and clinical perspectives

In planning treatment for each new patient, radiation oncologists pay attention to the aspects that they control. Thus their attention is usually focused on volume and dose. The dilemma for the physician is how to protract the treatment in a way that maximizes control of the tumor and minimizes normal tissue injury. The initial radiation-induced damage to DNA may be a biological indicator of the quantity of energy transferred to the DNA. However, until now the biophysical models proposed cannot explain either the early or the late adverse effects of radiation, and a more general theory appears to be required. The bystander component of tumor cell death after radiotherapy measured in many experimental works highlights the importance of confirming these observations in a clinical situation.

Evaluation of the in vivo genotoxic effects of gamma radiation on the peripheral blood leukocytes of head and neck cancer patients undergoing radiotherapy

The present study aimed to evaluate the genotoxic effects of ionizing radiation on non-target cells of Head and Neck Squamous Cell Carcinoma (HNSCC) patients exposed to various cumulative doses of gamma rays during radiotherapy. The ten patients (P1-P10) were treated with cobalt 60 gamma radiation (External Beam Radiotherapy) for a period of five to six weeks with a daily fraction of 2Gy for 5 days each week. The genotoxic effects of radiation (single strand breaks - SSBs) in these patients were analyzed using the alkaline single cell gel electrophoresis (SCGE) technique, with the Olive Tail Moment (OTM) as the critical parameter. A sample of each patient's peripheral blood before starting with radiotherapy (pre-therapy) served as the control, and blood collected at weekly time intervals during the course of the radiotherapy served as treated (10, 20, 30, 40, 50 and 60Gy) samples. In vivo radiosensitivity of these patients, as indicated by SSB's after the cumulative radiation doses at the various times, was assessed using Student's t-test. Significant DNA damage relative to the individual patient's pre-therapy baseline data was observed in all patients. Inter-individual variation of the genotoxic effects was analyzed using two-way ANOVA. The correlation between doses for the means of smoker and non-smoker patients was calculated using the Pearson test. The results of this study may indicate the need to reduce the daily radiotherapy dose further to prevent genotoxic effects on non-target cells, thus improving safety. Furthermore, these results may indicate that the estimation of DNA damage following exposure to a gamma radiation, as measured by the comet assay in whole blood leukocytes, can be used to screen human populations for radiation-induced genetic damage at the molecular level.

Comparative study of two evaluation methods for the genotoxic effects of environmental heavy metals on normal cells

Genetic modifications caused by chronic exposure to low levels of toxic metals may activate stress-signaling pathways, thus increasing cancer incidence among affected individuals. The aim of this study was to evaluate the relationship between exposure to heavy metals and the incidence of chromosomal aberrations and DNA lesions in a chronically exposed population by using specific biomarkers. The study included 156 subjects divided into two major groups: exposed individuals (in a heavy metal contaminated region, Maramures, Romania) and non-exposed population, as control group (Cluj, Romania). We compared the results of two cytogenetic methods for the detection and quantification of DNA lesions and chromosomal aberrations in normal human cells: Single Cell Gel Electrophoresis or Comet assay and Cytokinesis Block Micronucleus assay. The methods were performed on lymphocytes isolated from whole blood in density gradient. The basal DNA lesions and chromosomal aberrations were evaluated, as well as the repair capacity of the supplementary lesions induced by genotoxic agents such as ionizing radiations. Our results showed a great interindividual variability in the basal level of the DNA lesions and chromosomal aberrations, between and within the groups, the most affected being the heavy metals-exposed groups. Non-exposed subjects from rural area Cluj appeared to be more susceptible to the induction of supplementary DNA lesions and chromosomal aberrations by irradiation. The most efficient repair capacity of the radio-induced DNA lesions was observed in the non-exposed Cluj urban group. Both cytogenetic assays (as tools for detection of DNA lesions and chromosomal aberrations) may be used in human biomonitoring studies as indicators of early biological effects induced by exposure to heavy metals.

Chromatin‐ and temperature‐dependent modulation of radiation‐induced double‐strand breaks

PURPOSE

To investigate the influence of chromatin organization and scavenging capacity in relation to irradiation temperature on the induction of double-strand breaks (DSB) in structures derived from human diploid fibroblasts.

MATERIALS AND METHODS

Agarose plugs with different chromatin structures (intact cells+/-wortmannin, permeabilized cells with condensed chromatin, nucleoids and DNA) were prepared and irradiated with X-rays at 2 or 37 degrees C and lysed using two different lysis protocols (new ice-cold lysis or standard lysis at 37 degrees C). Induction of DSB was determined by constant-field gel electrophoresis.

RESULTS

The dose-modifying factor (DMF(temp)) for irradiation at 37 compared with 2 degrees C was 0.92 in intact cells (i.e. more DSB induced at 2 degrees C), but gradually increased to 1.5 in permeabilized cells, 2.2 in nucleoids and 2.6 in naked DNA, suggesting a role of chromatin organization for temperature modulation of DNA damage. In addition, DMF(temp) was influenced by the presence of 0.1 M DMSO or 30 mM glutathione, but not by post-irradiation temperature.

CONCLUSION

The protective effect of low temperature was correlated to the indirect effects of ionizing radiation and was not dependent on post-irradiation temperature. Reasons for a dose modifying factor <1 in intact cells are discussed.

Could a strong alkali deproteinization replace the standard lysis step in alkaline single cell gel electrophoresis (comet) assay (pH>13)?

The alkaline version of single cell gel electrophoresis (comet) assay is widely used for evaluating DNA damage at the individual cell level. The standard alkaline method of the comet assay involves deproteinization of cells embedded in agarose gel using a high salt-detergent lysis buffer, followed by denaturation of DNA and electrophoresis using a strong alkali at pH>13 [N.P. Singh, M.T. McCoy, R.R. Tice, E.L. Schneider, A simple technique for quantitation of low levels of DNA damage in individual cells, Exp. Cell. Res. 175 (1988) 184-191]. However, a recent report showed that a strong alkali treatment results in simultaneous deproteinization of cells and denaturation of genomic DNA [P. Sestili, C. Martinelli, V. Stocchi, The fast halo assay: an improved method to quantify genomic DNA strand breakage at the single cell-level, Mutat. Res. 607 (2006) 205-214]. This study was carried out to test whether the strong alkali deproteinization of cells could replace the high salt-detergent lysis step used in the standard method of the alkaline comet assay. Peripheral blood lymphocytes from 3 healthy individuals were irradiated with gamma rays at doses varying between 0 and 10 Gy. Following irradiation, the comet assay was performed according to the standard alkaline method (pH>13) and a modified method. In the modified method, agarose embedded cells were treated with a strong alkali (0.3M NaOH, 0.02 M Trizma and 1mM EDTA, pH>13) for 20 min to allow deproteinization of cells and denaturation of DNA. This was followed by electrophoresis using the same alkali solution to obtain comets. DNA damage expressed in terms of comet tail length, percentage of DNA in comet tail and tail moment obtained by the standard alkaline method and the modified method were compared. In both methods, DNA damage showed a good correlation with the dose of gamma ray. The results indicate a satisfactory sensitivity of the modified method in detecting radiation-induced DNA damage in human peripheral blood lymphocytes.

Enhanced sensitivity of the RET proto-oncogene to ionizing radiation in vitro

Exposure to ionizing radiation is a well-known risk factor for a number of human cancers, including leukemia and thyroid cancer. It has been known for a long time that exposure of cells to radiation results in extensive DNA damage; however, a small number of studies have tried to explain the mechanisms of radiation-induced carcinogenesis. The high prevalence of RET/PTC rearrangements in patients who have received external radiation, and the evidence of in vitro induction of RET rearrangements in human cells, suggest an enhanced sensitivity of the RET genomic region to damage by ionizing radiation. To assess whether RET is indeed more sensitive to radiations than other genomic regions, we used a COMET assay coupled with fluorescence in situ hybridization, which allows the measurement of DNA fragmentation in defined genomic regions of single cells. We compared the initial DNA damage of the genomic regions of RET, CXCL12/SDF1, ABL, MYC, PLA2G2A, p53, and JAK2 induced by ionizing radiation in both a lymphoblastoid and a fetal thyroid cell line. In both cell lines, RET fragmentation was significantly higher than in other genomic regions. Moreover, a differential distribution of signals within the COMET was associated with a higher percentage of RET fragments in the tail. RET was more susceptible to fragmentation in the thyroid-derived cells than in lymphoblasts. This enhanced susceptibility of RET to ionizing radiation suggests the possibility of using it as a radiation exposure marker.

Relationships between acute reactions to radiotherapy in head and neck cancer patients and parameters of radiation-induced DNA damage and repair in their lymphocytes

PURPOSE

To study the relationship between lymphocyte radiosensitivity measured in vitro and acute reactions to radiotherapy in patients with head and neck cancer.

MATERIALS AND METHODS

Acute reactions were measured in 34 patients using the Dische scale. Lymphocyte radiosensitivity was measured using the alkaline comet assay, the micronucleus assay, the nuclear division index and morphological assessment of apoptosis.

RESULTS

There was a weak, statistically significant correlation between in vitro radiosensitivity measured as the rate of DNA damage repair and the cumulative radiation dose exerting the maximum acute reaction scored (r = -0.366, p = 0.039, n = 34). Subgroup analyses showed that for patients with a low level of radiation-induced DNA damage there was a statistically significant relationship between lymphocyte radiosensitivity measured as inhibition of proliferation and acute toxicity (r = -0.621, p = 0.007, n = 18). For patients with a high level of residual DNA damage, there was a relationship between lymphocyte radiosensitivity measured using the micronucleus assay and acute toxicity (r = -0.597, p = 0.023, n = 14).

CONCLUSIONS

Combining two measures of radiosensitivity improves the ability to correlate in vitro lymphocyte radiosensitivity and acute radiotherapy toxicity data.

5-Bromodeoxyuridine radiosensitization: conformation-dependent DNA damage

DNA structure has recently emerged as one of the key factors governing the ability of 5-bromodeoxyuridine (BrdU) to radiosensitize DNA. Here, we report the dependence of the specific damage induced by BrdU for different DNA conformations. Strand breaks are specific for B-form DNA, whereas A-DNA only undergoes formation of piperidine-sensitive DNA lesions. Interstrand cross-links are only found in semi-complementary B-DNA. DNA conformation was altered by gradually rehydrating lyophilized DNA samples, which induces an A- to B-form transition. These results were also validated by irradiating DNA in solution, in the presence or absence of 80% ethanol to induce an A- or B-form, respectively. Alkali-labile DNA lesions were revealed using hot piperidine to transform both base and sugar lesions into strand breaks. We also analyzed the location of damage as a function of DNA structure: piperidine-sensitive lesions were observed exclusively at the site of BrdU substitution, whereas strand breaks were able to migrate along the DNA strand, with a clear preference for the adenine 5' of the BrdU. Thus, not only the hybridization state but also the DNA conformation affect the degree of sensitization by BrdU by influencing the amount and type of damage produced. Although clinical trials using BrdU as a radiosensitizer have been disappointing up to this point, these new findings point to several key features of BrdU radiosensitization that may alter future radiotherapeutic studies.

Cell proliferative activity estimated by histone H2B mRNA level correlates with cytogenetic damage induced by radiation in human glioblastoma cell lines

We studied the relationship between proliferative activity and radiation-induced DNA damage in human malignant gliomas in vitro. Nine human glioblastoma established cell lines were gamma-irradiated (60Co) over a dose range of 0-10 Gy. H2B and H4 histone mRNA level was assessed with quantitative RT-PCR technique (TaqMan) and histone labeling index (HLI) with in situ hybridization to define proliferation rate, while cytochalasin-block micronucleus assay was performed to measure cytogenetic damage. Micronucleus frequency correlated with H2B mRNA level (Spearman's R up to 0.82 at 8 Gy), HLI, nuclear division index (NDI) and percentage of binucleated cells (%BNC). There was a high correlation between H2B mRNA level and NDI (R = 0.80) as well as %BNC and HLI (R = 0.72). Histone H2B and H4 mRNA level (not significant), HLI, NDI, and %BNC (significant) were higher in cell lines sensitive to DNA damage. Proliferative activity correlates with radiation-induced DNA damage in human glioma cell lines. Histone H2B mRNA level and HLI may be a useful molecular predictor of the tumour response to radiation treatment in gliomas of the same histological grade, however the risk of potentially more rapid tumour-cell repopulation must be considered. Presumed protective activity of histones against radiation-induced DNA damage was not confirmed at the transcript level.

Chromatin organization contributes to non-randomly distributed double-strand breaks after exposure to high-LET radiation

The influence of higher-order chromatin structure on the non-random distribution of DNA double-strand breaks induced by high-LET radiation was investigated. Five different chromatin structures (intact cells, condensed and decondensed chromatin, nucleoids and naked genomic DNA) from GM5758 cells or K562 cells were irradiated with (137)Cs gamma-ray photons and 125 keV/microm nitrogen ions (16-25 MeV/nucleon). DNA was purified with a modified lysis procedure to avoid release of heat-labile sites, and fragment size distributions and double-strand break yields were analyzed by different pulsed-field gel electrophoresis protocols. Whereas double-strand breaks in photon-irradiated cells were randomly distributed, irradiation of intact K562 cells with high-LET nitrogen ions produced an excess of non-randomly distributed DNA fragments 10 kb-1 Mbp in size. Complete removal of proteins eliminated this non-random component. There was a gradual increase in the yield of double-strand breaks for each chromatin decondensation step, and compared to intact cells, the yields for naked DNA (in buffer without scavengers) increased 83 and 25 times after photon and nitrogen-ion irradiation, respectively. The corresponding relative biological effectiveness decreased from 1.6-1.8 for intact cells to 0.49 for the naked DNA. We conclude that the organization of DNA into chromatin fiber and higher-order structures is responsible for the majority of non-randomly distributed double-strand breaks induced by high-LET radiation. However, our data suggest a complex interaction between track structure and chromatin organization over several levels.

Molecular mechanisms of individual radiosensitivity studied in normal diploid human fibroblasts

The molecular mechanisms of individual radiosensitivity were studied in normal diploid human fibroblasts. For fibroblasts irradiated with X-rays in G1-phase the individual radiosensitivity was shown to be correlated with the extent of double-strand break (dsb) repair. The number of residual dsbs (including both non- and mis-rejoined dsbs) varied between 2 and 5% of the initial number induced and was low for resistant and high for sensitive strains. In the G1-phase dsbs are considered to be mostly repaired via the non-homologous end-joining pathway (NHEJ). However, so far none of the parameters tested for this pathway was found to be correlated with the number of residual dsbs. The parameters tested were mRNA expression, protein level and localisation and activity of the DNA-PK, which is the central complex of NHEJ. The dsb-repair capacity is also not regulated by the differentiation status, which varies substantially among fibroblast strains, whereas there is some indication that dsb repair might depend on the chromatin structure, with more efficient repair in cells with condensed DNA. Residual dsbs are converted into lethal chromosome aberrations finally leading to the loss of clonogenic activity, when cells pass through mitosis. Beside this so-called mitotic death, X-irradiated human fibroblasts are also inactivated via the TP53-dependent permanent G1-arrest, while apoptosis appears to be not important. On average, mitotic death and G1-arrest are equally effective, but there is a broad variation from one strain to the other, with a negative correlation between these two pathways. Fibroblast strains exhibiting only a moderate G1-arrest showed a high number of lethal aberrations and vice versa. This result points to a common regulator of both G1-arrest and dsb repair, which is presently under investigation.

Expression of phosphorylated histone H2AX in cultured cell lines following exposure to X-rays

PURPOSE

Exposure to ionizing radiation results in phosphorylation of histone H2AX (gammaH2AX) at sites of DNA double-strand breaks. To determine the relationship between gammaH2AX formation and radiosensitivity, the rate of formation and loss of gammaH2AX were examined in several cultured cell lines following exposure to 253 kV X-rays.

MATERIALS AND METHODS

Flow and image cytometry were both performed using a mouse monoclonal antibody against gammaH2AX. Immunoblotting was used to confirm cell line-dependent differences in antibody staining. Cell lines examined included V79 and CHO-K1 hamster cells, the human tumour cell lines SiHa, WiDr, DU145, WIL-2NS, HT144, HCC1937 and U87, and the normal cell strain HFL1. Radiosensitivity was measured using a standard clonogenic assay.

RESULTS

Using flow cytometry, gammaH2AX formation was detected 1 h after doses as low as 20 cGy. Peak levels of gammaH2AX were observed within 15-30 min after irradiation and both the rate of radiation-induced gammaH2AX formation and loss were cell type dependent. Maximum levels of gammaH2AX formation were lower for HT144 cells mutant for the ataxia telangiectasia gene. Half-times of loss after irradiation ranged from 1.6 to 7.2 h and were associated with a decrease in the total number of foci per cell. The half-time of loss of gammaH2AX was correlated with clonogenic survival for 10 cell lines (r2=0.66).

CONCLUSIONS

GammaH2AX can be detected with excellent sensitivity using both flow and image analysis. The rate of gammaH2AX loss may be an important factor in the response of cells to ionizing radiation, with more rapid loss and less retention associated with more radioresistant cell lines.

Single cell gel electrophoresis assay: methodology and applications

The single cell gel electrophoresis or Comet assay is a sensitive, reliable, and rapid method for DNA double- and single-strand breaks, alkali-labile sites and delayed repair site detection, in eukaryotic individual cells. Given its overall characteristics, this method has been widely used over the past few years in several different areas. In this paper we review the studies published to date about the principles, the basic methodology with currently used variations. We also explore the applications of this assay in: genotoxicology, clinical area, DNA repair studies, environmental biomonitoring and human monitoring.

Differences between a human bladder carcinoma cell line and its radiosensitive clone in the formation of radiation-induced DNA double-strand breaks in different chromatin substrates

It is well established that DNA-associated proteins, as well as soluble free-radical scavengers, can significantly influence the amount of damage inflicted in DNA by ionising radiation. It is not known, however, to what degree there is variation between cell lines in the effectiveness of these cellular components to protect DNA. In this study we have examined the level of strand break induction in a human bladder carcinoma cell line, MGH-U1, and its radiosensitive mutant, U1-S40b, when soluble scavengers and DNA-associated proteins were progressively removed. DNA double-strand breaks were measured using pulsed-field gel electrophoresis when cells were irradiated after lysis in solutions containing various salt concentrations. The two cell lines showed only a small, non-significant difference in damage induced in intact cells but isolated nuclei and chromatin devoid of non-histone proteins showed significantly more damage in the U1-S40b cells. Once the histone H1 was removed again there was no difference between the cell lines in the damage induced. We conclude that the different components of the cellular defences against free radical attack can have different influences in different cells. It is not clear whether this has an influence on the cellular sensitivity to the killing effects of radiation but it does suggest that artificial manipulation of the different components of the system may not affect overall damage induction to the same degree in all cells.

Cell fusion studies to examine the mechanism for etoposide resistance in Chinese hamster V79 spheroids

When exposed to etoposide, the outer cells from Chinese hamster V79 spheroids are about 10 times more resistant to DNA strand breaks and cell killing than V79 cells grown as monolayers. Previous results have shown that the outer cells of both spheroids and monolayers grow at the same rate and contain the same amount and activity of the target enzyme, topoisomerase II. In order to examine possible mechanisms for this resistance, cell fusion studies were conducted with fluorescent dye-tagged monolayer and spheroid cells. Fused cells were exposed for 30 min to 1.2 microg/ml etoposide and then separated using fluorescence-activated cell sorting into binucleate cells consisting of two monolayer cells, two spheroid cells, or a mixed doublet consisting of one cell of each type. Individual sorted cell doublets were examined for the presence of etoposide-induced DNA strand breaks using the alkaline comet assay. As expected, doublets of monolayer cells were sensitive to etoposide and doublets of spheroid cells were resistant. However, mixed doublets were as resistant to DNA damage by etoposide as spheroid doublets. In comparison, when etoposide- or adriamycin-resistant V79 monolayer cells were fused to the parent monolayer cells, the expected intermediate sensitivity to etoposide was observed for the mixed doublets. We conclude that etoposide resistance associated with the outer cells of spheroids can be "transferred" to produce resistance in monolayer cells. Rapid changes in phosphorylation that can affect topoisomerase II activity or localization, or that can alter chromatin structure, are suggested as possible mechanisms of resistance. In support of this hypothesis, topo IIalpha phosphorylation was at least 10 times greater in monolayers than in the outer cell layer of spheroids.

Radiation response of connexin43-transfected cells in relation to the "contact effect"

Some cell lines grown for only two cell doublings as multicell spheroids develop a form of resistance to killing by ionizing radiation that has been called the "contact" effect. While our previous results have implicated a role for higher order chromatin structure in the contact effect, another possible explanation is the presence of intercellular gap junctions that might facilitate communication between cells grown as spheroids and thereby enhance the ability of cells to resist or recover from radiation damage. To examine the role of gap junctions in the contact effect, rat glioma C6 and mouse EMT6 cell lines were transfected with a gene encoding the gap junctional protein connexin43. While C6 glioma cells are deficient in gap junctional communication, cells from spheroids were nonetheless more resistant than monolayers to killing by ionizing radiation, and the contact effect was present to a similar extent in the three transfected clones. For mouse EMT6 cells, radiosensitivity was similar whether cells were grown as monolayers or spheroids. Transfection of EMT6 cells with connexin43 increased gap junctional communication but did not promote development of a contact effect. Tumor volume doubling time in SCID mice increased significantly for one transfected clone; however, doubling time in vitro was also increased relative to the EMT6 parent. We conclude that extensive gap junctional communication is not a requirement for the increased radiation resistance observed when some cell lines are grown as spheroids.

In vitro evaluation of Taxol combined with radiations in human squamous cell carcinoma spheroids

The effect of Taxol on the radiation sensitivity of human squamous carcinoma of the head and neck region was determined in vitro, using clonogenic assays and multicellular tumor spheroids (MTS). Radiosensitivity parameters were determined by alpha and beta for clonogenic assays, and by the residual/control volume ratios at 2 Gy (RSV2) and the dose inducing 50% decrease in MTS number (SCD50) for spheroids. In HTB43 and CAL27 colonies, the combination was antagonist. In spheroids, Taxol induced a decrease of RSV2 and SCD50 in HTB43 and CAL27 MTS and their combinations with radiation were synergistic and additive, respectively. Therefore, the different results obtained by clonogenic assays and MTS may suggest higher drug incorporation through the multiple cell layers of the spheroids than in monolayers.

Chromatin structure and cellular radiosensitivity: a comparison of two human tumour cell lines

The role of variation in susceptibility to DNA damage induction was studied as a determinant for cellular radiosensitivity. Comparison of the radiosensitive HX142 and radioresistant RT112 cell lines previously revealed higher susceptibility to X-ray-induced DNA damage in the sensitive cell line using non-denaturing elution, but not when using alkaline unwinding. The present data also show that no difference in the amount of initial damage is seen when pulsed-field gel electrophoresis (PFGE) or comet analysis are used for DNA damage assessment. However, using the halo assay or a modified version of PFGE in which the higher DNA architecture remained partially intact, the radiosensitive cells showed steeper dose-response curves for initial DNA damage than the radioresistant cells. Analysis of the protein composition, of DNA-nucleoid structures revealed substantial differences when isolated from HX142 or RT112 cells. From our data, it is concluded that HX142 and RT112 differ in their structural organization of chromatin. As no differences in the kinetics of DNA damage rejoining were found, it is hypothesized that the same amount of lesions have a different impact in the two cell lines in that the 'presentation' of DNA damage alters the ratio of repairable to non-repairable DNA damage.

Comet assay study of DNA damage and repair of tumour cells following boron neutron capture irradiation with fast d(14) + Be neutrons

We compared the amount of radiation-induced DNA damage and the extent of DNA repair in human melanoma cells (MeWo) using the 'comet assay' after neutron, boron neutron capture and X-irradiation. Using a colony-forming assay it was shown earlier that lethal effects in tumour cells treated with fast neutrons may be increased by the neutron capture reaction 10B(n, alpha)7Li. The effectiveness of boron neutron capture in killing tumour cells depends on the number of 10B atoms delivered to the tumour, the subcellular distribution of 10B and the thermal neutron fluence at the side of the tumour. Using the 'comet assay' the DNA damage of fast neutrons (mean energy 5.8 MeV) was shown to be significantly greater than for the same absorbed dose of X-rays. The presence of 600 ppm 10B (boric acid H5 10BO3) in the cell medium during irradiation with d(14) + Be neutrons in a phantom enhances the DNA damage by 20% compared with neutron irradiation alone. After DNA damage induction by neutrons and neutron capture of boron, the DNA repair capacity of the MeWo cells is significantly reduced in comparison with X-irradiation resulting in proportionally more residual DNA damage after 180 min of repair time.

Radiosensitivity of human breast cancer cell lines of different hormonal responsiveness. Modulatory effects of oestradiol

Treatments which inhibit or retard progression of the cell through the cell cycle have been reported to reduce the effectiveness of ionizing radiation by increasing cellular radioresistance. We studied cellular radiosensitivity and radiation-induced DNA damage (double-strand break, dsb) in both hormone-sensitive and non-sensitive human breast cancer cell lines. After 72h of culture in an oestradiol-deprived medium, MCF-7 BUS and T47D B8 breast cancer cells showed a significant delay in growth, whereas no effect was seen in EVSA-T cell line. In oestradiol-free medium, MGF-7 BUS cells were arrested mainly in G(zero)/G1 phase (85-90% in G(zero)/G1, 5-7% in S, and 6-8% in G2/M). The growth-delayed MCF-7 BUS cells showed reduced radiosensitivity (survival fraction at 2 Gy, SF2 = 63%; initial DNA damage 1.00 dsb/Gy/DNA unit) in comparison with proliferating cells (SF2 = 33%, initial DNA damage 2.70 dsb/Gy/DNA unit). The radio-protective effect of oestrogen deprivation was abolished by rescuing MCF-7 cells with oestrogen-containing medium. At 24h after rescue, MCF-7 BUS cells reached a cell cycle distribution close to that found under standard culture conditions and their radiosensitivity was correspondingly increased (SF2 = 40%, DNA damage = 2.52 dsb/Gy/DNA unit). Our findings indicate that: (1) sensitivity to radiation and the proportion of proliferating cells are probably related, and (2) differences in radiosensitivity reflect differences in radiation-induced DNA damage.

DNA double-strand break rejoining rates, inherent radiation sensitivity and human tumour response to radiotherapy

The relationship between DNA double-strand break rejoining rates, inherent radiation sensitivity and tumour response to radiation therapy was determined for a group of 25 squamous cell carcinoma (SCC) and eight sarcoma (SAR) tumours. DNA double-strand break frequencies were measured by neutral filter elution in first passage following explant tumour samples after in vitro exposure to 100 Gy of 60Co gamma-rays. There was no significant difference between SCC and SAR tumour cells in their sensitivity to break induction, but in a 1 h time period SAR tumour cells rejoined significantly fewer breaks than SCC tumour cells, consistent with the greater sensitivity of SAR and suggesting that differences in rates of break rejoining account for the different distributions of radiosensitivities seen when different tumour types are compared. The percentage of breaks rejoined in 1 h in these tumour samples correlated well with D(o) and with the beta component of the survival curve, measured in vitro by clonogenic assay in tumour cell lines established from the tumour samples, but not with SF2 or the alpha component of the survival curve. The rates of DNA double-strand break rejoining therefore appear to influence the exponential portion of survival curves and probably the interactions between breaks. The percentage of breaks rejoined in 1 h was higher in SCC tumours that subsequently failed radiotherapy and, although the differences were not significant, they suggest that rates of break rejoining are an important component of tumour response to radiation therapy.

Biochemical changes associated with the adaptive response of human keratinocytes to N-methyl-N'-nitro-N-nitrosoguanidine

Exposure of cells to low doses of radiation or chemicals renders them more resistant to higher doses of these agents. This phenomenon, termed adaptive response, was studied in quiescent human keratinocytes exposed to the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). The cells were adapted with 2.5 nM MNNG for 60 min and challenged immediately thereafter with 2.5 microM MNNG for 30, 45 or 60 min. Clonogenic survival studies revealed that adapted cells were more resistant to the subsequent challenge treatment (up to 30% higher survival) than unadapted cells. In addition, formation of DNA strand breaks was lower in adapted cells. We monitored poly-ADP-ribosylation activity during expression of the adaptive response both at the substrate as well as the product level. NAD+ utilization in adapted and non-adapted cells exposed to the high dose of MNNG was similar, but recovery from NAD+ depletion was faster in low-dose pretreated cells. Induction of poly(ADP-ribose) formation was more than 2 times higher in low-dose adapted cells and this was associated with the formation of a distinct class of ADP-ribose polymers, i.e., branched polymers. These polymers exhibit a very high binding affinity for histones and can displace them from DNA. Elevated levels of poly(ADP-ribose) and, particularly, synthesis of branched polymers may play a critical role in low-dose adaptation.

Relationship between DNA damage, rejoining and cell killing by radiation in mammalian cells

The prevailing hypothesis on the mechanism of radiation-induced cell killing identifies the genetic material deoxyribonucleic acid (DNA) as the most important subcellular target at biologically relevant doses. In this review we present new data and summarize the role of the DNA double-strand breaks (dsb) induced by ionizing radiation and DNA dsb rejoining as determinants of cellular radiosensitivity. When cells were irradiated at high dose-rate, two molecular end-points were identified which often correlated with radiosensitivity: (1) the apparent number of DNA dsb induced per Gy per DNA unit and (2) the half-time of the fast component of the DNA dsb rejoining kinetics. These two molecular determinants, not mutually exclusive, may be linked through a common factor such as the conformation of DNA.

Adaptive response of the chicken embryo to low doses of x-irradiation

Chicken embryos were x-irradiated in ovo with 5-30 cGy (=priming dose) at the 13th-15th day of development. After 3-48 h, brain- and liver-cell suspensions were x-irradiated in vitro with (challenge) doses of 4-32 Gy. Significantly less radiation damage was observed when the radiation response was measured by scheduled DNA synthesis, nucleoid sedimentation and viscosity of alkaline cell lysates 12-36 h after the priming exposure. In vivo, pre-irradiation with 10 cGy enhanced regeneration as evidenced by the DNA content of chicken embryo brain and liver 24 h following a challenge dose of 4 Gy. From nucleoid sedimentation analyses in brain and liver cells immediately after irradiation with 16 Gy and after a 30-min repair period in the presence of aphidicolin, dideoxythymidine and 3-aminobenzamide or in the absence of these DNA repair inhibitors, it is concluded that a reduction of the initial radiation damage is the dominant mechanism of the "radio-adaptive" response of the chicken embryo. Sedimentation of nucleoids from ethidium bromide (EB) (0.75-400 micrograms/ml)-treated cells suggests a higher tendency of "radio-adapted" cells to undergo positive DNA supercoiling in the presence of high EB concentrations.

The comet assay: a comprehensive review

The comet assay is a sensitive and rapid method for DNA strand break detection in individual cells. Its use has increased significantly in the past few years. This paper is a review of the studies published to date that have made use of the comet assay. The principles of strand break detection using both the alkaline and neutral versions of the technique are discussed, and a basic methodology with currently used variations is presented. Applications in different fields are reviewed and possible future directions of the technique are briefly explored.

Radiation-induced DNA double-strand break rejoining in human tumour cells

Five established human breast cancer cell lines and one established human bladder cancer cell line of varying radiosensitivity have been used to determine whether the rejoining of DNA double-strand breaks (dsbs) shows a correlation with radiosensitivity. The kinetics of dsb rejoining was biphasic and both components proceeded exponentially with time. The half-time (t1/2) of rejoining ranged from 18.0 +/- 1.4 to 36.4 +/- 3.2 min (fast rejoining process) and from 1.5 +/- 0.2 to 5.1 +/- 0.2 h (slow rejoining process). We found a statistically significant relationship between the survival fraction at 2 Gy (SF2) and the t1/2 of the fast rejoining component (r = 0.949, P = 0.0039). Our results suggest that cell lines which show rapid rejoining are more radioresistant. These results support the view that, as well as the level of damage induction that we have reported previously, the repair process is a major determinant of cellular radiosensitivity. It is possible that the differences found in DNA dsb rejoining and the differences in DNA dsb induction are related by a common mechanism, e.g. conformation of chromatin in the cell.

Cellular radiosensitivity and DNA damage in primary human fibroblasts

PURPOSE

To evaluate the relationship between radiation-induced cell survival and DNA damage in primary human fibroblasts to decide whether the initial or residual DNA damage levels are the more predictive of normal tissue cellular radiosensitivity.

METHODS AND MATERIALS

Five primary human nonsyndromic and two primary ataxia telangiectasia fibroblast strains grown in monolayer were studied. Cell survival was assessed by clonogenic assay. Irradiation was given at high dose rate (HDR) 1-2 Gy/min. DNA damage was measured in stationary phase cells and expressed as fraction released from the well by pulsed-field gel electrophoresis (PFGE). For initial damage, cells were embedded in agarose and irradiated at HDR on ice. Residual DNA damage was measured in monolayer by allowing a 4-h repair period after HDR irradiation.

RESULTS

Following HDR irradiation, cell survival varied between SF2 0.025 to 0.23. Measurement of initial DNA damage demonstrated linear induction up to 30 Gy, with small differences in the slope of the dose-response curve between strains. No correlation between cell survival and initial damage was found. Residual damage increased linearly up to 80 Gy with a variation in slope by a factor of 3.2. Cell survival correlated with the slope of the dose-response curves for residual damage of the different strains (p = 0.003).

CONCLUSION

The relationship between radiation-induced cell survival and DNA damage in primary human fibroblasts of differing radiosensitivity is closest with the amount of DNA damage remaining after repair. If assays of DNA damage are to be used as predictors of normal tissue response to radiation, residual DNA damage provides the most likely correlation with cell survival.

Free radical modes of cytotoxicity of adriamycin and streptonigrin

Free radical modes of cytotoxicity of streptonigrin (STN) and Adriamycin (ADR) in Chinese hamster V79 cells under aerobic conditions were evaluated using 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TP), a low molecular weight stable nitroxide free radical with antioxidant properties and desferrioxamine (DF), a transition metal chelator. In addition, exogenous superoxide dismutase (SOD, EC 1.15.1.1) and catalase (CAT, EC 1.11.1.6), were tested for cytoprotective effects. EPR studies showed that TP reacts with the semiquinones of both ADR and STN and also with O2- radicals generated during aerobic redox cycling of the respective semiquinone radicals. Pulsed field gel electrophoresis studies confirmed that DNA double-strand breaks (dsb) induced by STN in V79 cells were inhibited completely by TP, whereas ADR-induced DNA dsb were not affected by TP. Clonogenic cell survival studies showed that STN-induced cytotoxicity could be inhibited completely by DF or TP. Both agents were ineffective in inhibiting ADR-induced cytotoxicity. SOD and CAT were ineffective in protecting against both STN and ADR cytotoxicity. Our results are consistent with a mechanism requiring the semiquinone radical intermediate of STN for cytotoxicity and minimal free radical involvement in ADR-induced V79 cell cytotoxicity.

The kinetics of cellular recovery in exponential and plateau growth phase human glioma cells following gamma-irradiation

PURPOSE

In this study the kinetics of recovery following irradiation was examined in a human glioma cell line. Specific objectives were: to determine whether recovery is mono- or biexponential in nature; to determine if recovery half-times are different in exponential and plateau growth phase cells; to compare recovery half-times as a function of dose or recovery levels; and finally, to compare the kinetics of sublethal damage recovery and potentially lethal damage recovery in plateau growth phase cells.

METHODS AND MATERIALS

U-87MG cells were irradiated in exponential and plateau growth phases and then subjected to incubation at 37 degrees C for various periods of time following or between doses prior to assaying for survival. Survival recovery curves were fit to a sum of exponential terms.

RESULTS

Potentially lethal damage recovery was monoexponential in both exponential and plateau growth phase cells and occurred at the same rate when isorecovery values were compared. Recovery half-times increased in an exponential manner within the observed dose range. Recovery between doses of radiation (sublethal damage recovery) proceeded at a slower rate than recovery following a single dose of radiation (potentially lethal damage recovery).

CONCLUSIONS

This study suggests that potentially lethal damage recovery is a saturated process and that the recovery half-time may increase in a linear-quadratic exponential function of dose similar to the absolute recovery level. In addition, if iso-recovery levels are compared, the recovery half-time is similar in rapidly and slowly proliferating cell populations.

Effect of diet and vitamin C on DNA strand breakage in freshly-isolated human white blood cells

We have measured DNA strand breaks induced by ionising radiation in nucleated cells from freshly isolated whole blood from normal human subjects. Samples were taken after subjects had fasted overnight and again 1 h after they had eaten breakfast in combination with approximately 35 mg/kg vitamin C. Damage was measured by single cell gel electrophoresis (the 'comet' assay), in which DNA single strand breaks generate a comet tail streaming from the nucleus. In repeat experiments on 6 subjects a reduction in DNA damage, as indicated by a highly significant decrease in overall comet length, was observed following vitamin C ingestion, both in the unirradiated control blood samples and in the dose response to ionising radiation damage. In addition, consistent differences in dose response between individual subjects were found. The peak effect was 4 h after intake of food and vitamin C. An effect was also seen with vitamin C alone and after breakfast without additional vitamin C. Protection against strand breakage was also seen in Ficoll-separated mononuclear cells but evidence was not obtained for protection of separated, mitogen stimulated T-lymphocytes either against ionising radiation cell killing in a clonal assay, or against clastogenicity assessed by micronucleus formation following one cell division. Exposure of separated lymphocytes in vitro to vitamin C, at doses greater than 200 microM, did not offer protection but induced strand breakage. Our results raise the possibility that variation in normal diet may not only affect susceptibility to endogenous oxidative damage, but may affect some responses of the individual to radiation.

Inhibition of potentially lethal damage recovery by altered pH, glucose utilization and proliferation in plateau growth phase human glioma cells

Recovery from potentially lethal damage (PLD) has been measured in plateau growth phase human glioblastoma cells (U-87MG) under four postirradiation medium conditions. Recovery was maximal in depleted medium at an acidic pH, conditions which inhibit cellular proliferation. Compared with this control, PLD recovery (PLDR) was increasingly inhibited by alkalization of the existing medium (to pH 7.4), exchanging the old medium with fresh medium-pH acidified (to pH 6.8), and exchanging the old medium with fresh medium-pH unaltered (pH 7.4), respectively. These three medium adjustments were made at the time of irradiation. Increased glucose utilization (glycolysis) was detected postirradiation in all three cases, while increased proliferation was detected only when fresh medium was exchanged for old medium. Thus inhibition of PLDR has been correlated with increased glycolysis and increased proliferation during the recovery period. When acting together, these two processes provided almost complete inhibition. This study was revealed that the degree of inhibition may be related to the amount of glycolysis and/or proliferation occurring during the recovery period. Examining, in vitro, the range of PLDR achieved by postirradiation manipulation of medium pH may provide some indication of the range in PLDR that may be expected in vivo. Our study demonstrates that the effect of pH on glycolysis and proliferation may be important when determining the ability of a particular cell type to recover from PLD.

Drug and radiation resistance in spheroids: cell contact and kinetics

Cells from multicellular spheroids are often more resistant than monolayers to drugs and radiation. While explanations for resistance can be based on differences in cell cycle distribution, inability of the drug to penetrate the spheroid, or the presence of hypoxic cells, these mechanisms do not adequately explain resistance to all agents. Small spheroids (containing about 25-50 cells) exposed to ionizing radiation, hyperthermia, photodynamic therapy, or topoisomerase II inhibitors, are more resistant to killing than monolayers; the close three-dimensional contact in spheroids has been implicated in this resistance. Proposed mechanisms for the 'contact effect' include gap junctional 'reciprocity', cell shape mediated changes in (repair-related) gene expression, and alterations in chromatin packaging which influence DNA repair. The consequences of the contact effect are especially important for multifraction exposures. Another form of resistance can be demonstrated during repetitive treatments; 'regrowth resistance' reflects the capacity of spheroid cells to proliferate more efficiently to compensate for cell killing.

Correlation between gamma-ray-induced DNA double-strand breakage and cell killing after biologically relevant doses: analysis by pulsed-field gel electrophoresis

We examined the degree of correlation between gamma-ray-induced lethality and DNA double-strand breaks (dsbs) after biologically relevant doses of radiation. Radiation lethality was modified by treating 14C-labelled Chinese hamster ovary cells with either of two aminothiols (WR-1065 or WR-25591) and the associated effect on dsb induction was determined by pulsed-field gel electrophoresis (PFGE). The use of phosphorimaging to analyse the distribution of 14C-activity in the gel greatly improved the low-dose resolution of the PFGE assay. Both WR-1065 and WR-25591 protected against dsb induction and lethality to a similar extent after low doses of radiation, although this correlation broke down when supralethal doses were used to induce dsbs. Thus, the level of dsbs induced in these cells as measured by PFGE after survival-curve doses of gamma-radiation is consistently predictive of the degree of lethality obtained, implying a case-effect relationship between these two parameters and confirming previous results obtained using the neutral filter elution assay for dsbs.

Initial radiation-induced DNA damage in human tumour cell lines: a correlation with intrinsic cellular radiosensitivity

The role of the initial DNA double-strand breaks (dsb) as a determinant of cellular radiosensitivity was studied in human breast and bladder cancer cell lines. Cell survival was measured by monolayer colony-forming assay as appropriate and differences in radiosensitivity were seen (alpha-values ranged from 0.12 to 0.54). After pulsed-field gel electrophoresis (PFGE) the initial slopes of dose-response curves were biphasic with a flattening of the curves above 30 Gy. When the frequency of DNA dsb induction was assessed using a mathematical model based on the DNA fragment size distribution into the gel lane, we found a statistically significant relationship between the number of DNA dsb induced and the corresponding alpha-values and fraction surviving after 2Gy (P = 0.0049 and P = 0.0031 respectively). These results support the view that initial damage is a major determinant of cell radiosensitivity.

Molecular determinants of radiosensitivity in mammalian cells

Knowledge of the biochemical and molecular basis of sensitivity to ionizing radiation will provide useful information regarding carcinogenesis, cancer proneness and patient responses to radiotherapy. Cellular endpoints following irradiation are primarily the product of the induction, processing and manifestation of DNA damage. There are therefore several points in the postirradiation sequelae that can be altered to modify the sensitivity of a cell. At the present time there is no consensus as to the single most important determinant of radiosensitivity, but maybe this is because it does not exist. There could be a basic cellular characteristic, such as DNA conformation, which can influence every aspect of the cellular response to radiation, but it is likely that the critical controlling steps differ in different cell systems.

Mechanisms of induction and repair of DNA double-strand breaks by ionizing radiation: some contradictions

The various aspects of formation and repair of radiation-induced double-strand breaks (DSB) are summarized. Concerning the structure of DSB found in irradiated cells, enzymatic and microdosimetric analysis hints at complex damage of the DNA structure at the position of a DSB. With increasing LET, the DSB damage may be more complex than that induced by low-LET irradiation. Most of the DSB are repaired in the irradiated cell; apparently the kinetics of DSB repair and the fraction of unrejoined DSB determine cell survival or cell death. We do not know the details of the complex machinery of DSB repair; certainly recombination processes are involved, but there are still contradictions between our current knowledge about the mechanisms of recombinational DSB repair and the observed kinetics.

DNA Supercoiling and Repair in Peripheral Lymphocytes as a Measure of Acute Radiation Response After Radiotherapy

DNA supercoiling density and incision kinetics during ultraviolet (UV) excision repair hav been measured in lymphocytes from 20 cancer patients and 17 healthy donors. Nucleoid sedimentation was used, which allows the sensitive detection of both DNA damage and alterations in chromatin structure. The release of DNA supercoiling after ethidium bromide intercalation and the kinetics of the incision step following UV irradiation were compared in lymphocytes derived from cancer patients and those from normal donors. The classification into lymphocytes with normal or reduced repair and normal or altered supercoiling, respectively, revealed that reduced repair as well as altered chromatin structure occurred more frequently in lymphocytes derived from patients (40% and 85%, respectively) than in those from healthy donors (35% and 23%, respectively). Even more striking was the simultaneous occurrence of both characteristics in tumor patients: in 34% of all cases reduced repair was associated with altered supercoiling density, whereas among healthy donors this association occurred in only 18% of all cases. Supercoiling density may be related to functional integrity of lymphocytes and repair capacity to recovery after radiation damage. Since both parameters are important for the radiation response of normal tissue, we consider these measurements a potential prognostic assay aimed at reducing acute reaction of the normal tissue.

Sensitivity of neutral filter elution but not PFGE can be modified by non-dsb chromatin damage

Hamster V79 fibroblast cells and human squamous carcinoma cells (Caski) were exposed to 60Co radiation and DNA double-strand break (dsb) induction was analysed by DNA elution at neutral pH from polycarbonate filter or out of an agarose matrix in pulsed-field electrophoresis (PFGE). While dsb yields were equal for the two cell lines (using 125-iodine calibration) a reduced responsiveness of filter elution was found for V79 versus Caski cells. This difference could be abolished when additional single-strand breaks (ssb) were introduced by an incubation at 10(-4) M H2O2 for up to 40 min that itself did not give a response in neutral elution. No such lack of specificity for the detection of dsb was seen in electrophoretic elution where also the influence of peroxide incubation was absent. The presumed potential of ssb to modify dsb detection was paralleled by the kinetics of dsb rejoining: a pronounced transient increase of DNA elution from filters was observed for V79 cells (less prominent with Caski cells) at 15-40 which is thought to reflect the occurrence of secondary ssb from incisions during base damage repair. Rejoining measured by PFGE did not show this behaviour. The results suggest that ssb may aid decondensation of the chromatin during lysis of cells required for an efficient release of dsb fragments when supported on filters, but which depends on cell type and is less critical in electrophoretic elution out of an agarose matrix. This involvement of ssb in the neutral filter elution assay appears to be contrary to published data obtained with different experimental systems. The finding of an increase of DNA elution from filters due to hyperthermia at 45 degrees C is also taken to indicate an involvement of non-dsb chromatin damage in the response of filter elution at neutral pH with V79 but not with Caski cells.