Differences in the DNA supercoiling response of irradiated cell lines from ataxia-telangiectasia versus unaffected individuals

Association of the ATM gene polymorphisms with papillary thyroid cancer

Papillary thyroid cancer (PTC) is the most common type of thyroid cancer, yet few genetic markers of PTC risk useful for screening exist. Our study aimed to evaluate the association between single nucleotide polymorphisms (SNPs) of the ataxia telangiectasia mutated (ATM) gene and PTC risk. 358 patients with PTC and 360 healthy controls were included in the case-control study. Four ATM SNPs (rs664677, rs373759, rs4988099, and rs189037) were genotyped by matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF-MS). The analysis of genetic data was performed using the SNPStats program. The allele frequencies and genotype distributions of the four ATM SNPs were not different between PTC patients and controls. We did not observe any tendency of increasing the frequency of the risk allele from controls, patients without metastasis to patients with metastasis (P(trend) > 0.05). Interestingly, the AG genotype of rs373759 was associated with PTC risk under an overdominant model of inheritance (adjusted OR = 1.38; 95 % CI, 1.03-1.87; P = 0.03). No haplotype was observed to be significantly associated with PTC risk. Our results suggest that heterozygosity for the ATM rs373759 polymorphism may be a potential risk factor for PTC.

Alterations in heat-induced radiosensitization accompanied by nuclear structure alterations in Chinese hamster cells

This paper examined heat-induced radiosensitization in two Chinese hamster heat-resistant cell lines, HR-1 and OC-14, that were isolated from the same wild-type HA-1 cell line. It found a reduction of the magnitude of heat-induced radiosensitization after exposure to 43 degrees C in both HR-1 and OC-14 cells and a similar reduction after exposure to 45 degrees C in HR-1 cells, but not in OC-14 cells. The effect of heat exposure on a class of ionizing radiation-induced DNA damage that inhibits the ability of nuclear DNA to undergo super-coiling changes was also studied using the fluorescent halo assay in these three cell lines. Wild type cells exposed to either 43 or 45 degrees C before irradiation had a DNA rewinding ability that was intermediate between control and unheated cells, a phenomenon previously described as a masking effect. This masking effect was significantly reduced in HR-1 cells exposed to either 43 or 45 degrees C or in OC-14 cells exposed to 43 degrees C under conditions that heat-induced radiosensitization was reduced. In contrast, the masking effect was not altered in OC-14 cells exposed to 45 degrees C, conditions under which heat-induced radiosensitization was similar to that observed in wild-type HA-1 cells. These results suggest that a reduction in the masking effect is associated with a reduction of the magnitude of heat-induced radiosensitization in the HR-1 and OC-14 heat-resistant cell lines. The reduction of the masking effect in the cell lines resistant to heat-induced radiosensitization was associated with neither a reduction in the magnitude of the heat-induced increase in total nuclear protein content nor major differences in the protein profiles of the nucleoids isolated from heated cells.

Quantified relationship between cellular radiosensitivity, DNA repair defects and chromatin relaxation: a study of 19 human tumour cell lines from different origin

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.

ATM: a mediator of multiple responses to genotoxic stress

The ATM protein kinase is the product of the gene responsible for the pleiotropic recessive disorder ataxia-telangiectasia. ATM-deficient cells show enhanced sensitivity and greatly reduced responses to genotoxic agents that generate DNA double strand breaks (DSBs), such as ionizing radiation and radiomimetic chemicals, but exhibit normal responses to DNA adducts and base modifications induced by other agents. Therefore, DSBs are most likely the predominant signal for the activation of ATM-mediated pathways. Identification of the ATM gene triggered extensive research aimed at elucidating the numerous functions of its large multifaceted protein product. While ATM has both nuclear and cytoplasmic functions, this review will focus on its roles in the nucleus where it plays a central role in the very early stages of damage detection and serves as a master controller of cellular responses to DSBs. By activating key regulators of multiple signal transduction pathways, ATM mediates the efficient induction of a signaling network responsible for repair of the damage, and for cellular recovery and survival.

Changes in DNA supercoiling in fibroblasts cultured in the presence of hydralazine

We have analyzed changes in the supercoiling of nucleoid DNA of murine fibroblasts cultured in the presence of hydralazine. The entire DNA attached to the nuclear matrix was extracted from the cells and sedimented in neutral sucrose density gradients containing ethidium bromide. Nucleoids from cells treated with hydralazine responded to increasing ethidium bromide concentrations in a different way than those from control cultures. That is, supercoiled loops of DNA unwound with lower concentrations of ethidium bromide sedimented less rapidly than those of control cells, indicating that hydralazine reduced the degree of DNA supercoiling. Also, nucleoids from the drug-treated cells resisted the transition from relaxed to positive supercoiling at higher concentrations of ethidium bromide. Changes in nucleoid DNA supercoiling correlated directly with the dose of hydralazine in the fibroblast culture.

Radiosensitivity and oxidative signalling in ataxia telangiectasia: an update

Radiosensitivity is a major hallmark of the human genetic disorder ataxia telangiectasia. This hypersensitivity to ionizing radiation has been demonstrated in vivo after exposure of patients to therapeutic doses of radiation and in cells in culture. Clearly an understanding of the nature of the molecular defect in ataxia telangiectasia will be of considerable assistance in delineating additional pathways that determine cellular radiosensitivity/radioresistance. Furthermore, since patients with this syndrome are also predisposed to developing a number of leukaemias and lymphomas, the possible connection between radiosensitivity and cancer predisposition is of interest. Now that the gene (ATM) responsible for this genetic disease has been cloned and identified, progress is being made in determining the role of the ATM protein in mediating the effects of cellular exposure to ionizing radiation and other forms of redox stress. Proteins such as the product of the tumour suppressor gene p53 and the proto-oncogene c-Abl (a protein tyrosine kinase) have been shown to interact with ATM. Since several intermediate steps in both the p53 and c-Abl pathways, activated by ionizing radiation, are known it will be possible to map the position of ATM in these pathways and describe its mechanism of action. What are the clinical implications of understanding the molecular basis of the defect in ataxia telangiectasia (A-T)? As outlined above, since radiosensitivity is a universal characteristic of A-T, understanding the mechanism of action of ATM will provide additional information on radiation signalling in human cells. With this information it may be possible to sensitize tumour cells to radiation and thus increase the therapeutic benefit of radiotherapy. This might involve the use of small molecules that would interfere with the normal ATM-controlled pathways and thus sensitize cells to radiation or alternatively it might involve the efficient introduction of ATM anti-sense cDNA constructs into tumours to achieve the same end-point.

Radiation-induced DNA double-strand breaks and the radiosensitivity of human cells: a closer look

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.

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.

Radiation-induced changes in nucleoid halo diameters of aerobic and hypoxic SF-126 human brain tumor cells

Nucleoid halo diameters were measured to assay changes in DNA supercoiling in human brain tumor cell line SF-126 after irradiation under aerobic or hypoxic conditions. In unirradiated aerobic cells, a typical propidium iodide titration curve showed that with increasing concentrations of propidium iodide, the halo diameter increased and then decreased with the unwinding and subsequent rewinding of DNA supercoils. In irradiated cells, the rewinding of DNA supercoils was inhibited, resulting in an increased halo diameter, in a radiation dose-dependent manner. To produce equal increases in halo diameter required about a threefold higher radiation dose in hypoxic cells than in aerobic cells. Quantitatively similar differences in the radiation sensitivities of hypoxic and aerobic cells were demonstrated by a colony-forming efficiency assay. These findings suggest that the nucleoid halo assay may be used as a rapid measure of the inherent radiation sensitivity of human tumors.

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.

The ionizing radiation-induced replication protein A phosphorylation response differs between ataxia telangiectasia and normal human cells

Replication protein A (RPA), the trimeric single-stranded DNA-binding protein complex of eukaryotic cells, is important to DNA replication and repair. Phosphorylation of the p34 subunit of RPA is modulated by the cell cycle, occurring during S and G2 but not during G1. The function of phosphorylated p34 remains unknown. We show that RPA p34 phosphorylation is significantly induced by ionizing radiation. The phosphorylated form, p36, is similar if not identical to the phosphorylated S/G2 form. gamma-Irradiation-induced phosphorylation occurs without new protein synthesis and in cells in G1. Mutation of cdc2-type protein kinase phosphorylation sites in p34 eliminates the ionizing radiation response. The gamma-irradiation-induced phosphorylation of RPA p34 is delayed in cells from ataxia telangiectasia, a human inherited disease conferring DNA repair defects and early-onset tumorigenesis. UV-induced phosphorylation of RPA p34 occurs less rapidly than gamma-irradiation-induced phosphorylation but is kinetically similar between ataxia telangiectasia and normal cells. This is the first time that modification of a repair protein, RPA, has been linked with a DNA damage response and suggests that phosphorylation may play a role in regulating DNA repair pathways.

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.

Intrinsic radiosensitivity of adult and cord blood lymphocytes as determined by the micronucleus assay

Predictive radiosensitivity testing necessitates rapid and reliable assays of radiosensitivity. We assessed the lymphocyte micronucleus assay as such an assay. We performed repeated experiments on lymphocytes from 10 healthy donors. Levels of radiation-induced micronuclei were measured following exposures of up to 4 Gy X-rays. When measuring the slope of the dose-response, we have found more variation between individuals than between repeated experiments on the same individual (F value 12.31, P < 0.001). There is also greater interindividual variation in the data following a single dose of X-rays of 2 Gy (F value 3.54, P < 0.01) and of 4 Gy (F value 7.55, P < 0.005). We performed the micronucleus assay on five different samples of cord blood lymphocytes (CBLs). Their radiosensitivities were compared with the mean radiosensitivity of the lymphocytes from the normal group of donors. Comparing the level of micronuclei induced by 2 Gy, only CBL1 (P < 0.01) and CBL2 (P < 0.02) were more radiosensitive than the mean of the adult lymphocytes. At 4 Gy, CBL1 (P < 0.001), CBL2 (P < 0.05), CBL3 (P < 0.01) and CBL5 (P < 0.01) were more radiosensitive than the mean radiosensitivity of the adult lymphocytes. This was also shown when the slope of the dose-response curves were measured. We conclude that the lymphocyte micronucleus assay shows more variability when applied to lymphocytes from different individuals than when repeatedly applied to lymphocytes from the same individual, a requirement for the determination of individual radiosensitivity.

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.

The ionizing radiation-induced replication protein A phosphorylation response differs between ataxia telangiectasia and normal human cells

Replication protein A (RPA), the trimeric single-stranded DNA-binding protein complex of eukaryotic cells, is important to DNA replication and repair. Phosphorylation of the p34 subunit of RPA is modulated by the cell cycle, occurring during S and G2 but not during G1. The function of phosphorylated p34 remains unknown. We show that RPA p34 phosphorylation is significantly induced by ionizing radiation. The phosphorylated form, p36, is similar if not identical to the phosphorylated S/G2 form. gamma-Irradiation-induced phosphorylation occurs without new protein synthesis and in cells in G1. Mutation of cdc2-type protein kinase phosphorylation sites in p34 eliminates the ionizing radiation response. The gamma-irradiation-induced phosphorylation of RPA p34 is delayed in cells from ataxia telangiectasia, a human inherited disease conferring DNA repair defects and early-onset tumorigenesis. UV-induced phosphorylation of RPA p34 occurs less rapidly than gamma-irradiation-induced phosphorylation but is kinetically similar between ataxia telangiectasia and normal cells. This is the first time that modification of a repair protein, RPA, has been linked with a DNA damage response and suggests that phosphorylation may play a role in regulating DNA repair pathways.

Differential post-irradiation caffeine response in normal diploid versus SV40-transformed human fibroblasts: potential role of nuclear organization and protein-composition

To test the hypothesis that the enhancement of cell killing by post-irradiation treatment with caffeine (CAF) is mediated by alterations in chromatin structure, several nuclear parameters were examined in both caffeine-responsive and non-responsive cell lines. Cell killing, as determined by clonogenic assay, was not enhanced by post-irradiation treatment with 5 mM caffeine in a human diploid fibroblast line (AG1522) but an effect was seen in a SV40 T-antigen transformed derivative (1522-a). CAF caused a complete reversal of the radiation-induced G2 + S phase cell-cycle delays in the transformed cell line but only a partial reversal was noted for the parental cell line. The nuclear endpoints examined, which may be indicative of chromatin conformational changes, included enzymatic accessibility, DNA loop structure, and nuclear protein composition. In assays of the ability of DNA to undergo supercoiling changes, it was found that nucleoids isolated from CAF-treated cells had a significantly reduced propidium-iodide relaxable DNA loop size. The constraints to DNA unwinding produced by CAF were also maintained even in the presence of large numbers of single strand breaks produced by a test dose of radiation (10 Gy). This effect did not correlate well with the ability of CAF to enhance radiation-induced cell killing. The two other nuclear endpoints did detect differences between the normal and transformed cell lines. CAF had no effect on the DNase I digestion kinetics of the normal fibroblasts. However, in the transformed cell line, CAF appeared to render an additional 10-15% of the genome accessible to DNase I digestion. Several radiation and CAF-induced changes in the polypeptide pattern of isolated nucleoids were detected after metabolic labelling with 35S-methionine or 32P-orthophosphoric acid. While the identities of these proteins remain to be established, many had relative molecular weights similar to the other reported radiation-altered proteins and human cell cycle control gene products. The present cell lines should provide a convenient system in which to identify a nuclear protein change specifically associated with the ability of CAF to enhance radiation-induced cell killing.

Cell killing and DNA damage by etoposide in Chinese hamster V79 monolayers and spheroids: influence of growth kinetics, growth environment and DNA packaging

Cells from V79 multicell spheroids must be exposed to approximately 50 times more etoposide than exponentially growing monolayers in order to produce the same amount of cell killing. A part of this difference in sensitivity is readily explained by the decrease in growth fraction of large spheroids, and by the protection afforded by nutrient deprivation which also reduces cellular ATP. However, cells composing the outer 10% of large (approximately 600 microns diameter) V79 spheroids, although actively cycling, were still ten times more resistant to etoposide than exponentially growing monolayers, regardless of whether cells were exposed in situ in spheroids or dispersed by trypsin immediately prior to exposure to the drug. Four cell doublings (48 h) as monolayers were required before the outer cells of spheroids regained drug sensitivity equivalent to that of exponentially growing monolayers. No differences in uptake/efflux of 3H-etoposide or in levels of p-glycoprotein were observed between monolayers and the outer cells of spheroids. In addition, topoisomerase II protein measured by immunoblotting and topoisomerase II activity measured by decatenation of kinetoplast DNA were not reduced in the outer cells of spheroids compared to monolayers. DNA strand breakage measured in individual cells using the DNA precipitation and comet assays correlated well with cell killing with one exception: DNA damage was not affected when cells were incubated with etoposide in phosphate-buffered saline, although the etoposide concentration required to produce a given amount of cell killing was increased approximately 7-fold compared to cells incubated with the drug in complete medium. These results indicate that etoposide toxicity towards V79 spheroids is influenced not only by proliferative status of the cells but also by factors which may include DNA packaging and the growth environment of the cell prior to and during treatment.

A correlation between DNA-nuclear matrix binding and relative radiosensitivity in two human squamous cell carcinoma cell lines

Three aspects of DNA topology were examined in two human squamous cell carcinoma lines of differing radiosensitivity (SQ-9G, D0 = 1.46 Gy; and SQ-20B, D0 = 2.36 Gy). High-salt-extracted nuclei (nucleoids) were taken from gamma-irradiated cells, stained with ethidium bromide and examined by flow cytometry. After 5 Gy, nucleoids from SQ-9G cells became 30% less efficient at adopting positive DNA supercoils than were unirradiated controls. In contrast, only a 4% difference was found with the radioresistant SQ-20B line. Both lines produced positive supercoils more efficiently after irradiation if first exposed to the topoisomerase II inhibitor VP16. Ethidium bromide titration of nucleoids was consistent with each containing similar numbers and sizes of DNA loops. In each line approximately 30-35% of DNA was accessible to trioxsalen, as shown by inter-strand crosslinking after UV photo-activation. Exhaustive digestion of nuclear DNA by DNase I removed more DNA from the radiosensitive than from the radioresistant cell line (12% vs 28% remaining). This difference was thought to be due to the increased accessibility of SQ-9G DNA in vitro. We suggest that a looser association of SQ-9G DNA with the nuclear matrix both promotes DNase I digestion and affects the ability of SQ-9G nucleoids to maintain positive DNA supercoils after irradiation. These data implicate the DNA matrix attachment region in the expression of radiation sensitivity in the cell lines studied.

Differential radioprotection of cultured human diploid fibroblasts and fibrosarcoma cells by WR1065

The present studies were performed to determine whether WR1065, the dephosphorylated, free-thiol active metabolite of WR2721, could provide differential radioprotection of normal and tumor cell lines in vitro and secondly to investigate potential mechanisms for the selective nature of the radioprotection at the cellular and molecular level. When 4 mM WR1065 was administered 30 min prior to and during irradiation, a protection factor of 1.9 was obtained in clonogenic assays performed with normal human diploid fibroblasts (AG1522) while no protection of fibrosarcoma cells (HT1080) was observed. Some radioprotection of fibrosarcoma cells was observed with higher drug concentrations (10-40 mM), but the increase in survival was considerably less than the plateau level reached with the diploid fibroblasts (3-fold vs 24-fold at 6 Gy). The observation of such a selective effect in vitro with WR1065 indicates that differences in tissue-specific variables such as blood flow, pH, pO2, and drug dephosphorylation cannot solely account for the selective nature of the radioprotection afforded by WR2721 in vivo. Incubation of nucleoids with increasing concentrations of the DNA intercalating dye propidium iodide was used to titrate the ability of DNA to undergo supercoiling changes. The relaxation and rewinding of supercoiled DNA loops in isolated nucleoids serves as an indicator of both the presence of DNA damage and inherent differences in DNA loop characteristics. Fibrosarcoma cells had a much larger propidium iodide-relaxable DNA loop size than fibroblasts. The rewinding phase of the DNA supercoiling response is impaired by the presence of radiation-induced DNA strand breaks. Four mM WR1065 resulted in a significant reduction in the amount of rewinding inhibition observed after a dose of 10 Gy in diploid fibroblasts (protection factor = 1.43) but did not alter the response of irradiated fibrosarcoma cells. These results, indicating that WR1065 had a preferential radioprotective effect in vitro on both survival and the manifestation of DNA damage at the nucleoid level, are consistent with the hypothesis that cell type differences in chromatin organization and DNA-drug associations could play a role in the selective radioprotection.

Molecular assays of radiation-induced DNA damage

There is a need for assays of DNA damage in many areas of laboratory research applied to radiation therapy, in order to understand the molecular processes involved in cell killing by ionising radiation and to predict in vivo response. Assays exist which measure many types of DNA damage following ionising radiation. From studies of the dose-response relationships for different types of damage, the double-strand break (dsb) has been shown to be the most significant lesion. Assays for DNA dsb have been of low sensitivity, such that supralethal doses of radiation had to be used in order to study dsb induction or repair. New assays, such as pulsed-field gel electrophoresis, are sensitive to dsb in a dose range relevant to cell survival. In addition, these assays can assess the distribution of dsb in different parts of the genome and determine heterogeneity of damage induction and repair. Assays which measure the effects of strand breaks on DNA complexed with nuclear matrix can reveal features of chromatin organisation and their influence on cellular radiosensitivity.