An improved method for the treatment of data from DNA strand break measurements using filter elution techniques with an internal standard

Incorporating oxygenation levels in analytical DNA-damage models-quantifying the oxygen fixation mechanism

Purpose.To develop a framework to include oxygenation effects in radiation therapy treatment planning which is valid for all modalities, energy spectra and oxygen levels. The framework is based on predicting the difference in DNA-damage resulting from ionising radiation at variable oxygenation levels.Methods.Oxygen fixation is treated as a statistical process in a simplified model of complex and simple damage. We show that a linear transformation of the microscopic oxygen fixation process allows to extend this to all energies and modalities, resulting in a relatively simple rational polynomial expression. The model is expanded such that it can be applied for polyenergetic beams. The methodology is validated using Microdosimetric Monte Carlo Damage Simulation code (MCDS). This serves as a bootstrap to determine relevant parameters in the analytical expression, as MCDS is shown to be extensively verified with published empirical data. Double-strand break induction as calculated by this methodology is compared to published proton experiments. Finally, an example is worked out where the oxygen enhancement ratio (OER) is calculated at different positions in a clinically relevant spread out Bragg peak (SOBP) dose deposition in water. This dose deposition is obtained using a general Monte Carlo code (FLUKA) to determine dose deposition and locate fluence spectra.Results.For all modalities (electrons, protons), the damage categorised as complex could be parameterised to within 0.3% of the value calculated using microdosimetric Monte Carlo. The proton beam implementation showed some variation in OERs which differed slightly depending on where the assessment was made; before the SOBP, mid-SOBP or at the distal edge. Environment oxygenation was seen to be the more important variable.Conclusions.An analytic expression calculating complex damage depending on modality, energy spectrum, and oxygenation levels was shown to be effective and can be readily incorporated in treatment planning software, to take into account the impact of variable oxygenation, forming a first step to an optimised treatment based on biological factors.

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.

A Comparison of the Chemical Repair Rates of Free Radical Precursors of DNA Damage and Cell Killing in Chinese Hamster V79 Cells

One of the important temporal stages of radiation action in cellular systems is the chemical phase, where oxygen fixation reactions compete with chemical repair reactions involving reducing agents such as GSH. Using the gas explosion technique it is possible to follow the kinetics of these fast (greater than 1 ms) reactions in intact cells. We have compared the chemical repair kinetics of the oxygen-dependent free radical precursors leading to DNA single-strand and double-strand breaks, measured using filter elution techniques, with those leading to cell killing in V79 cells. The chemical repair rates for DNA dsb (670s-1 at pH 7.2 and 380s-1 at pH 9.6) and cell killing (530s-1) were similar. This is in agreement with the important role of DNA dsb in radiation induced cell lethality. The rate for DNA ssb precursors was significantly slower (210s-1). The difference in rate between DNA ssb and dsb precursors may be explained on the basis of a dsb free radical precursor consisting of a paired radical, one radical on each strand. The instantaneous probability of one or other of these radicals being chemically repaired and not proceeding to form a dsb will be twice that of ssb radical precursor. This agrees well with the concept of locally multiply damaged sites (LMDS) produced from clusters of ionizations in DNA (Ward 1985).

X-ray-induced DNA double-strand breaks in mouse l1210 cells: a new computational method for analyzing neutral filter elution data

The aim of this article is to present a method for studying the shape of the dose and repair responses for X-ray-induced double-strand breaks (DSBs) as measured by neutral filter elution (NFE). The approach is closely related to a method we developed for the use of specific molecular size markers and used for determination of the absolute number of randomly distributed radiation-induced DSBs by pulsed-field gel electrophoresis (PFGE). Mouse leukemia L1210 cells were X-irradiated with 0-50 Gy. Samples were then evaluated both with PFGE and with NFE. Assuming that with both migration (PFGE) and elution (NFE), a heterogeneous population of double-stranded DNA fragments will start with the smallest fragments and proceed with increasingly larger fragments, it is possible to match the migration behavior of fractions of fragments smaller than a certain size to the fraction eluted at a specific time. This assumption does not exclude the possibility of DNA being sheared in the NFE filter. The yield, as determined by the size markers in PFGE, was used to find the corresponding elution times in the NFE experiment. These experimentally used elution times could then reversely be interpreted as size markers which finally were used to calculate DSBs/Mbp as a function of X-ray dose. The resulting lines were almost straight. The data were also plotted as relative elution and showed that, as expected, the dose response then appears with a more pronounced sigmoid shape.

Direct measurement by pulsed-field gel electrophoresis of induction and rejoining of X-ray-induced double-strand breaks in cultured mouse cells

The induction and rejoining of X-ray-induced double-strand breaks (dsb) in chromosomal DNA has been difficult to measure. We have developed a pulsed-field gel electrophoresis (PFGE)-based system for directly estimating DNA sizes between 0.2 and 10 million base pairs. With this system we can estimate average DNA sizes from randomly broken chromosomes by measuring the approximate molecular weight of the maximum DNA concentration. In practice this is effective where the average is between 1 and 4 million bp allowing both shoulders of the distribution to be observed. This corresponds to a dose range of 20-80 Gy. Qualitative differences from non-irradiated DNA can be observed down to about 5 Gy. We have confirmed the dose response by utilizing methotrexate-resistant mouse cells containing circular double-minute (dm) chromosomes of 1, 1.5, and 3 million bp. The kinetics of dsb rejoining from doses of 50 and 5 Gy was investigated: 50 Gy reduced the chromosomal DNA to an average size of approximately 1 million bp, followed by a constant repair rate of 44 dsb per minute per cell for 3 h (assuming a total genome size of 10 million bp).

The irradiation of V79 mammalian cells by protons with energies below 2 MeV. Part II. Measurement of oxygen enhancement ratios and DNA damage

The effectiveness of low-energy (below 2 MeV) protons at inducing DNA damage in the form of single- and double-strand breaks has been determined. Protons with mean energies of 1.90 MeV, 1.15 MeV and 0.76 MeV corresponding to track average LETs of 17 keV/microns, 24 keV/microns and 32 keV/microns, respectively, have been used and compared with 250 kVp X-rays and 3.8 MeV 238Pu alpha-particles. Although there was variation in the RBE for DNA ssb induction with LET, the RBEs for dsb induction at all three proton energies and for 3.8 MeV alpha-particles were all around 1.0. This suggests that, if DNA dsb are important in radiation-induced cell lethality, the probability of an induced dsb leading to a lethal event increases with increasing LET of radiation. Oxygen enhancement ratios were measured for both cell survival and DNA dsb induction, and in both cases a decrease in OER with increasing LET was observed.

Non-linear dose-effect curve for DNA double-strand breaks by low LET radiation: the effect of eluting buffer composition on the measurement of breaks by the filter elution technique

Chinese hamster V79 cells were irradiated with 250 kVp X-rays and analysed for the presence of DNA double-strand breaks using filter elution. Changing the pH of the elution and lysis buffers from pH 9.6 to 7.2 leads to a decreased rate of elution of the DNA from the filters and a small difference in the dose-effect relationship obtained. In both cases the dose-effect curves were non-linear. When cells are irradiated with 238Pu alpha-particles a difference in the rate of elution of the DNA is also observed between the two pH values, although the dose-effect curves are linear. These findings suggest that there may be qualitative differences in the types of lesions being induced by low-LET radiation and being scored as dsb in the filter elution assay at pH 9.6. However, carrying out elution at pH 7.2 does not eliminate the nonlinear response measured after X-irradiation.