Lysis solution composition and non-linear dose-response to ionizing radiation in the non-denaturing DNA filter elution technique

The Role of Non-protein Sulphydryls in Determining the Chemical Repair Rates of Free Radical Precursors of DNA Damage and Cell Killing in Chinese Hamster V79 Cells

Chinese hamster V79 fibroblasts were irradiated in the gas explosion apparatus and the chemical repair rates of the oxygen-dependent free radical precursors of DNA double-strand breaks (dsb) and lethal lesions measured using filter elution (pH 9.6) and a clonogenic assay. Depletion of cellular GSH levels, from 4.16 fmol/cell to 0.05 fmol/cell, by treatment with buthionine sulphoximine (50 mumol dm-3; 18 h), led to sensitization as regards DNA dsb induction and cell killing. This was evident at all time settings but was particularly pronounced when the oxygen shot was given 1 ms after the irradiation pulse. A detailed analysis of the chemical repair kinetics showed that depletion of GSH led to a reduction in the first-order rate constant for dsb precursors from 385 s-1 to 144 s-1, and for lethal lesion precursors from 533 s-1 to 165 s-1. This is generally consistent with the role of GSH in the repair-fixation model of radiation damage at the critical DNA lesions. However, the reduction in chemical repair rate was not proportional to the severe thiol depletion (down to approximately 1% for GSH) and a residual repair capacity remained (approximately 30%). This was found not to be due to compartmentalization of residual GSH in the nucleus, as the repair rate for dsb precursors in isolated nuclei, washed virtually free of GSH, was identical to that found in GSH-depleted cells (144 s-1), also the OER remained substantially above unity. This suggests that other reducing agents may have a role to play in the chemical repair of oxygen-dependent damage. One possible candidate is the significant level of protein sulphydryls present in isolated nuclei.

Heavy Ion-induced DNA Double-strand Breaks in Yeast

DNA double-strand break (dsb) induction in diploid yeast was measured by neutral sucrose sedimentation after exposure to very heavy ions with values of linear energy transfer (LET) ranging from about 300 to 11500 ke V/microns. Linear fluence dependencies were found in all cases from which dsb production cross-sections (sigma dsb) could be calculated. Corresponding cross-sections for cell killing (sigma i) were derived from final slopes of survival curves measured in parallel and for the same fluence range. A close correlation was found between sigma i and sigma dsb. It is calculated that over the entire LET range, including 30 MeV electron irradiation, about 22 dsb are induced per lethal event when high exposures are considered.

Radiosensibilité intrinsèque et cassures double–brin de l'ADN dans les cellules humaines

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.

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.

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.

X-ray-induced DNA double-strand breaks as lethal lesions in diploid human fibroblasts compared to Chinese hamster ovary cells

Calibration of the non-denaturing (pH 9.6) filter elution technique by 125I decays has been used to examine the relationship between X-ray-induced DNA double-strand breaks (dsb) and cellular survival of CHO cells and human fibroblasts. DNA dsb induction varies markedly between the two cell lines, reflecting the different survival levels. The relationships between lethal lesions (-ln S) and DNA dsb are also significantly different for CHO cells and human fibroblasts, suggesting either different probabilities of dsb conversion into lethal lesions or different repair capacities.

The shape of DNA elution dose-response curves under non-denaturing conditions: the contribution of the degree of chromatin condensation

We have re-examined the effect of detergent type, pH and temperature of lysis on the shape of the DNA elution dose response under non-denaturing conditions using plateau-phase CHO cells. Results practically identical to those previously published (Okayasu and Iliakis, 1989) were obtained, with a 1 h incubation at 60 degrees C during lysis with sodium-N-laurylsarcosine (NLS) resulting in almost linear dose-response curves. We also examined chromatin decondensation as a contributing factor in the linearization observed in the elution dose-response curve under the above conditions. When nuclei with condensed chromatin were prepared from irradiated cells, applied on the filter and lysed with NLS at room temperature, a shoulder-type elution dose-response curve was obtained only slightly higher than that of cells lysed under the same conditions. However, when nuclei prepared from irradiated cells were applied on the filter after relaxation of chromatin by incubation in low ionic strength buffer and lysed with NLS at room temperature, an almost linear dose-response curve was obtained similar to that of cells lysed with NLS at 60 degrees C. Lysis with NLS at 60 degrees C of nuclei with relaxed chromatin did not further modify the DNA elution dose-response curve. Based on these results we propose that the linearization of the DNA elution dose-response curve observed after chromatin decondensation reflects a reduction in the degree of chromatin compactness in the nuclear complexes that leads to a relatively uniform distribution of the DNA on the filter and reduces trapping of elutable material in the compact nuclear structures otherwise present. Since high radiation doses dissolve compact nuclear structures, trapping of elutable material is expected to be highest at low doses of radiation, leading to the observed reduction in the fraction of DNA eluted per Gy at low versus high radiation doses and thus to the observed shoulder. Furthermore, we propose that the linearization of the DNA elution dose-response curves observed in cells lysed in NLS at 60 degrees C may also be due to a decondensation of the nuclear complexes on the filter as a result of the combined action of detergent and high temperature. The notion of a correlation between DNA elution dose response and cell radiosensitivity was examined in two human (SQ20B, SCC61) and two Chinese hamster (V-79, irs-2) cell lines with widely different radiosensitivities.(ABSTRACT TRUNCATED AT 400 WORDS)

Radiation-induced DNA double-strand break frequencies in human squamous cell carcinoma cell lines of different radiation sensitivities

DNA neutral (pH 9.6) filter elution was used to measure radiation-induced DNA double-strand break (dsb) frequencies in eight human squamous cell carcinoma cell lines with radiosensitivities (D0) ranging from 1.07 to 2.66 Gy and D values ranging from 1.46 to 4.08 Gy. The elution profiles of unirradiated samples from more radiosensitive cell lines were all steeper in slope than the profiles from resistant cells. The shapes of the dsb induction curves were curvilinear and there was some variability from cell line to cell line in the dose-response for the induction of DNA dsb after exposures to 5-100 Gy 60Co gamma-rays. There was no relation between the shapes of the survival curves and the shapes of the dose-responses for the induction of DNA dsb. At low doses (5-25 Gy), three out of four of the more sensitive cell lines (D less than 2.5 Gy) had larger initial break frequencies than the more resistant lines (D greater than 3.0 Gy). Although the low-dose (5-25 Gy) elution results were variable, they do suggest that DNA neutral elution will detect differences between sensitive and resistant tumour cells in initial DNA dsb frequencies.

Detection of DNA double-strand breaks in synchronous cultures of CHO cells by means of asymmetric field inversion gel electrophoresis

A pulsed field gel electrophoresis technique, asymmetric field inversion gel electrophoresis (AFIGE), was used to evaluate induction by X-rays of DNA damage in CHO cells. The fraction of DNA activity released from the plug (FAR) was used as a measure for the amount of radiation-induced DNA damage, predominantly DNA double-strand breaks (dsb) (Stamato and Denko 1990), and was determined at various stages of growth and phases of the cell cycle in a range of doses between zero and 70 Gy. The FAR per unit dose fluctuated throughout the cell cycle and was high for cells irradiated in G1; it decreased as cells entered S and reached a minimum in the middle of this phase. The FAR per unit dose increased again as cells progressed towards the end of S, and reached values in G2 similar to those measured in G1. When damage was introduced into DNA by means of 125I decay similar fluctuations in the FAR per decay were observed throughout the cell cycle, suggesting that the variations in the FAR per unit of radiation dose observed throughout the cell cycle do not derive from alterations in the induction of dsb. The fluctuations in the FAR per unit dose throughout the cell cycle were quantitatively similar to the fluctuations in the fraction of activity eluted in irradiated cells assayed by the non-unwinding filter elution assay throughout the cycle (Okayasu et al. 1988), and suggest that both techniques respond to similar DNA replication-associated alterations of the biophysical and/or biochemical properties of the DNA molecule. It is concluded that caution needs to be exercised before differences observed in the FAR between different cell lines or between various phases of the cell cycle after exposure to a given dose of radiation are interpreted as suggesting differences in the induction of DNA dsb.

Principles and practice of DNA filter elution

DNA filter elution assays have proved useful in studies of DNA strand breaks and crosslinks produced in mammalian cells or tissues by a wide variety of carcinogenic and cancer chemotherapeutic agents. The basic types of DNA lesions that can be measured include single and double-strand breaks, interstrand crosslinks and DNA-protein crosslinks. DNA filter elution has also been adapted to the assay of other lesions such as alkali-labile sites and the protein-associated strand breaks of topoisomerase DNA cleavage complexes. The essential concepts and theory of the technique are discussed and the applications of the technique to various types of studies are critically reviewed.

Significance and measurement of DNA double strand breaks in mammalian cells

Techniques for measuring DNA double strand breaks in mammalian cells are being used increasingly by researchers studying both physiological processes, such as recombination, replication, and apoptosis, as well as pathological processes, such as clastogenesis induced by ionizing radiation, chemotherapeutic drugs, and chemical toxicants. In this review we evaluate commonly used assays for measuring DNA double strand breaks, focusing on neutral filter elution and pulsed field gel electrophoresis, and explore the advantages and limitations of applying these techniques to problems of current interest in carcinogenesis and genetic toxicology.