DNA fragmentation in V79 cells irradiated with light ions as measured by pulsed-field gel electrophoresis. I. Experimental results

PURPOSE

To compare the results on DNA fragmentation induced in Chinese hamster V79 cells by various doses of gamma-rays and low-energy protons and helium-4 ions.

MATERIALS AND METHODS

V79 cells were irradiated as monolayers with monoenergetic protons and helium-4 ions; gamma-rays were used as the reference radiation. DNA double-strand breaks were evaluated by calibrated pulsed-field gel electrophoresis using conditions covering the range 5.7 Mbp-23.1 kbp.

RESULTS

The fragment-counting method gave double-strand breaks yields and the relative biological effectiveness higher than those obtained by the fraction of activity released method. The frequency distribution of fragments showed that protons and helium ions induced more fragments below the Mbp region than did gamma-rays at the same dose. The distributions for both the irradiated and non-irradiated samples clearly appeared to be non-random.

CONCLUSION

Differences were observed in the yield and spatial correlation, at a molecular size scale characteristic of loop dimensions, of the double-strand breaks induced by gamma-rays and by light ions. These effects may have a role in the observed different cell response to these radiations.

Inactivation of human cells exposed to fractionated doses of low energy protons: relationship between cell sensitivity and recovery efficiency

Within the framework of radiation biophysics research in the hadrontherapy field, split-dose studies have been performed on four human cell lines with different radiation sensitivity (SCC25, HF19, H184B5 F5-1 M10, and SQ20B). Low energy protons of about 8 and 20 keV/micron LET and gamma-rays were used to study the relationship between the recovery ratio and the radiation quality. Each cell line was irradiated with two dose values corresponding to survival levels of about 5% and 1%. The same total dose was also delivered in two equal fractions separated by 1.5, 3, and 4.5 hours. A higher maximum recovery ratio was observed for radiosensitive cell lines as compared to radioresistant cells. The recovery potential after split doses was small for slow protons, compared to low-LET radiation. These data show that radiosensitivity may not be related to a deficient recovery, and suggest a possible involvement of inducible repair mechanisms.

DNA fragmentation in mammalian cells exposed to various light ions

Elucidation of how effects of densely ionizing radiation at cellular level are linked to DNA damage is fundamental for a better understanding of the mechanisms leading to genomic damage (especially chromosome aberrations) and developing biophysical models to predict space radiation effects. We have investigated the DNA fragmentation patterns induced in Chinese hamster V79 cells by 31 keV/micrometer protons, 123 keV/micrometer helium-4 ions and gamma rays in the size range 0.023-5.7 Mbp, using calibrated Pulsed Field Gel Electrophoresis (PFGE). The frequency distributions of fragments induced by the charged particles were shifted towards smaller sizes with respect to that induced by comparable doses of gamma rays. The DSB yields, evaluated from the fragments induced in the size range studied, were higher for protons and helium ions than for gamma rays by a factor of about 1.9 and 1.2, respectively. However, these ratios do not adequately reflect the RBE observed on the same cells for inactivation and mutation induced by these beams. This is a further indication for the lack of correlation between the effects exerted at cellular level and the initial yield of DSB. The dependence on radiation quality of the fragmentation pattern suggests that it may have a role in damage repairability. We have analyzed these patterns with a "random breakage" model generalized in order to consider the initial non-random distribution of the DNA molecules. Our results suggest that a random breakage mechanism can describe with a reasonable approximation the DNA fragmentation induced by gamma rays, while the approximation is not so good for light ions, likely due to the interplay between ion tracks and chromatin organization at the loop level.

DNA DSB induction and rejoining in V79 cells irradiated with light ions: a constant field gel electrophoresis study

PURPOSE

To study the induction and the time-course of rejoining of DNA double strand breaks (DSB) in V79 cells irradiated with light ions with different linear energy transfer (LET).

MATERIALS AND METHODS

V79 cells were irradiated in monolayer with monoenergetic proton, deuteron, helium-3 or helium-4 ion beams, each at two different energy values. Gamma rays were used as reference radiation. DSB have been measured by constant field gel electrophoresis (CFGE).

RESULTS

The initial yield depended little on the particle type and LET. The amount of DSB left unrejoined for up to 2 h incubation time could be roughly described by a decreasing exponential function with a final plateau, although more complex functions cannot be excluded. Radiation quality had little effect on the rejoining rate but affected the plateau. The amount of residual DSB after 2 h was higher for densely than for sparsely ionizing radiation, and for the same particle was dependent on LET. The corresponding RBE ranged from 1.8 to 6.0.

CONCLUSIONS

The results support the hypothesis that complex, less reparable DSB are induced in higher proportion by light ions with respect to gamma-rays and that, for the same ion, increasing LET leads to an increase in this proportion.

Inactivation of human normal and tumour cells irradiated with low energy protons

PURPOSE

To analyse the cell inactivation frequencies induced by low energy protons in human cells with different sensitivity to photon radiation.

MATERIALS AND METHODS

Four human cell lines with various sensitivities to photon irradiation were used: the SCC25 and SQ20B derived from human epithelium tumours of the tongue and larynx, respectively, and the normal lines M/10, derived from human mammary epithelium, and HF19 derived from a lung fibroblast. The cells were irradiated with y-rays and proton beams with linear energy transfer (LET) from 7 to 33 keV/microm. Clonogenic survival was assessed.

RESULTS

Survival curves are reported for each cell line following irradiation with gamma-rays and with various proton LETs. The surviving fraction after 2 Gy of gamma-rays was 0.72 for SQ20B cells, and 0.28-0.35 for the other cell lines. The maximum LET proton effectiveness was generally greater than that of gamma-rays. In particular there was a marked increase in beam effectiveness with increasing LET for the most resistant cells (SQ20B) whose 2 Gy-survival varied from 0.72 with gamma-radiation down to 0.37 with 30 keV/microm protons. The relative biological effectiveness (RBE(2 Gy gamma)) with the 30 keV/microm beam, evaluated as the ratio of 2 Gy to the proton dose producing the same inactivation level as that given by 2 Gy of gamma-rays, was 3.2, 1.8, 1.3 and 0.8 for SQ20B, M/10, SCC25, and HF19, respectively.

CONCLUSIONS

RBE for inactivation with high-LET protons increased with the cellular radioresistance to gamma-rays. The cell line with the greatest resistance to gamma-rays was the most responsive to the highest LET proton beam. A similar trend has also been found in studies reported in the literature with He, C, N ions with LET in the range 20-125 keV/microm on human tumour cell lines.

Micronuclei, CREST-positive micronuclei and cell inactivation induced in Chinese hamster cells by radiation with different quality

PURPOSE

To study the relative biological effectiveness-linear energy transfer (RBE-LET) relationship for micronuclei (MN) and cell inactivation, in Chinese hamster cells irradiated with low-energy protons (0.88 and 5.04 MeV, at the cell entrance surface). Chromosome loss was also investigated by means of antikinetochore CREST staining.

MATERIALS AND METHODS

Cl-1 cells were exposed to different doses of X-rays, gamma-rays, 7.7 keV/microm and 27.6 keV/microm protons. The induction of MN, the distribution of MN per cell and the frequency of CREST-positive MN were evaluated in cytokinesis-blocked binucleated cells (BN cells) in the dose range 0.125-3 Gy. In parallel, cell survival experiments were carried out in samples irradiated with 0.5 to 4 Gy.

RESULTS

MN yield and the frequency of BN cells carrying multiple MN (> or =2) were significantly higher after exposure to 27.6 keV/microm protons, compared with the other radiation types. In contrast, MN induction and MN distribution per BN cell were similar among 7.7 keV/microm protons, X- and gamma-rays up to 1 Gy. Cell survival experiments gave RBE values very close to those obtained with the MN assay. Both X-rays and 27.6 keV/microm protons yielded a significant proportion of CREST-positive MN at the highest doses investigated (0.75-3 Gy).

CONCLUSIONS

Good correlations between MN induction and cell inactivation were observed for both low- and high-LET radiation, indicating that the MN assay can be a useful tool to predict cell sensitivity to densely ionizing radiation with implications for tumour therapy with protons.

Reduction of the gamma-ray component from 252Cf fission neutron source--optimization for biological irradiations and comparison with MCNP code

Gamma-rays contribute 33% of the absorbed dose from an unfiltered 252Cf fission neutron source. To reduce this gamma-ray component and to enable radiobiological experiments at as high a dose rate as possible, Monte Carlo calculations for several filter materials (Al, Fe, Pb and concrete) have been made using MCNP neutron and photon transport code version 4a. A lead filter of thickness 4 cm was found to reduce the gamma-ray component to 6.7% of the total dose whilst reducing the neutron dose by only about 10%. Such a filter was installed at the MRC 252Cf neutron irradiation facility and dosimetric measurements were made using a TE-TE chamber and a 7LiF(Mg, Cu, P) TLD. Monte Carlo simulations agree with experimental measurements of neutron and gamma-ray doses within 6%. V79-4 Chinese hamster cells were irradiated with lead-filtered and unfiltered neutrons and also with 60Co gamma-rays at two dose rates. The survival fraction obtained for each radiation was consistent with the reduced gamma-ray dose. The relative biological effectiveness for neutrons alone, corrected for gamma-ray effects, was found to be 9.2 +/- 3.4 from the initial slopes and 3.1 +/- 0.5 at 10% survival, both relative to the acute gamma-rays.

RBE-LET relationships for cell inactivation and mutation induced by low energy protons in V79 cells: further results at the LNL facility

PURPOSE

RBE-LET relationships for cell inactivation and hprt mutation in V79 cells have been studied with mono-energetic low-energy proton beams at the radiobiological facility of the INFN-Laboratori Nazionali di Legnaro (LNL), Padova, Italy.

MATERIALS AND METHODS

V79 cells were irradiated in mono-layer on mylar coated stainless steel petri dishes, in air. Inactivation data were obtained at 7.7, 34.6 and 37.8 keV/microm and hprt mutation was studied at 7 7 and 37.8 keV/microm. Additional data were also collected for both the end points with the proton LET already considered in our previous publications, namely 11.0, 20.0 and 30.5 keV/microm.

RESULTS

A maximum in the RBE-LET relationship for cell inactivation was found at around 31 keV/microm, while the RBE for mutation induction increased continuously with LET.

CONCLUSIONS

The proton RBE-LET relationship for cell inactivation is shifted to lower LET values compared with that for heavier ions. For mutation induction, protons of LET equal to 7.7keV/microm gave an RBE value comparable with that obtained by helium ions of about 20 keV/microm. Mutagenicity and lethality caused by protons at low doses in the LET range 7.7-31 keV/microm were proportional, while the data at 37.8 keV/microm suggest that this may not hold at higher LET values.

Inactivation of C3H10T1/2 cells by low energy protons and deuterons

PURPOSE

To determine the RBE-LET relationship for C3H10T1/2 cell inactivation by protons in the LET range 11-33 keV/microm and to compare inactivation frequencies induced in C3H10T1/2 cells by protons and deuterons at two matching LET values in the range 11-20 keV/microm.

MATERIALS AND METHODS

C3H10T1/2 cells were irradiated with protons and deuterons at the radiobiological facility set up at the 7MV Van de Graaff accelerator at the LNL, Legnaro, Padova. Gamma rays from 60Co were used as reference radiation.

RESULTS

Proton RBE values (alpha/alphagamma) for inactivation of C3H10T1/2 cells are constant around a value of 2 between 11 and 20 keV/microm and then rise sharply to reach a value of 4.2+/-1.0 at 33 keV/microm. Deuteron RBE values are 1.7+/-0.4 and 2.2+/-0.6 at LET values of 13 and 18 keV/microm respectively.

CONCLUSIONS

Proton RBE values with C3H10T1/2 cells are significantly larger than unity at LET values as low as 11 keV/microm. No difference in effectiveness for inactivation of C3H10T1/2 has been found between protons and deuterons at two LET values in the range 10-20 keV/microm.

Production and dosimetry of copper L ultrasoft x-rays for biological and biochemical investigations

Ultrasoft x-rays provide a unique tool for investigating the intracellular mechanisms of radiation action. Secondary electrons are produced with a well defined energy and a range comparable with that of critical structures in the cell. Copper L characteristic x-rays of weighted average energy of 956 eV interact within the cell, mainly with the oxygen atom, typically producing a photoelectron with energy 424 eV (95%) followed by an Auger electron with an average energy of 505 eV, with a combined continuous slowing down approximation (csda) range of approximately 40 nm. The attenuation through the cell is similar to that of carbon K x-rays (277 eV, single electron), therefore a useful comparison can be made due to similar dose-averaging factors but different electron configurations (total range, and pairs versus singlets). The production, absorption, dosimetry and biological implications of Cu L x-rays using the Medical Research Council cold cathode source is described extending the number of energies available for study in the ultrasoft region. Design parameters were optimized to overcome the inherently low L-characteristic-to-bremsstrahlung yield ratio. Surface absorbed dose rates of 1 Gy min-1 have been obtained with a bremsstrahlung contamination of less than 0.5%. A confocal microscope was used to make thickness measurements on live cells to allow careful determination of the mean absorbed dose. Survival curves for V79-4 Chinese hamster cells were obtained, showing that Cu L x-rays are substantially more lethal per unit dose than are hard x-rays or gamma-rays, with a relative biological effectiveness (RBE) of 1.8. The data are consistent with the hypothesis that clustered damage at the DNA/chromatin level produced by low-energy electrons is biologically more effective.