Sublethal damage and DNA double strand breaks have similar RBE-LET relationships: evidence and implications

Decay of γ-H2AX foci correlates with potentially lethal damage repair in prostate cancer cells

To determine the relationship between ionizing radiation-induced levels of γ-H2AX foci and cell survival in cultured prostate cancer cell lines, three prostate cancer cell lines: LNCaP (wt TP53), DU145 (mut TP53) and PC3 (TP53 null), were studied. For γ-H2AX foci induction, cells were irradiated with a single dose of 2 Gy and foci levels were studied at 30 min and 24 h after irradiation. Cell survival was determined by clonogenic assay, directly and 24 h after irradiation with doses ranging from 0 to 8 Gy. Irradiation was performed with a Siemens Stabilipan 250 KeV X-ray machine at a dose rate of approximately 3 Gy/min. Survival curves were analyzed using the linear-quadratic model S(D)/S(0)=exp-(αD+βD2). LNCaP cells clearly demonstrated potentially lethal damage repair (PLDR) which was assessed as increased survival levels after delayed plating as compared to cells plated immediately after irradiation. DU145 cells demonstrated only a slight PLDR and PC3 cells did not show PLDR at all. Levels of γ-H2AX foci were significantly decreased in all cell lines at 24 h after irradiation, compared to levels after 30 min. The LNCaP cells which demonstrated a clear PLDR also showed the largest decay in the number of γ-H2AX foci. In addition, the PC cells which did not show PLDR had the lowest decay of γ-H2AX foci. A clear correlation was demonstrated between the degree of decay of γ-H2AX foci and PLDR.

Comparison of RBE values of high-LET α-particles for the induction of DNA-DSBs, chromosome aberrations and cell reproductive death

Background

Various types of radiation effects in mammalian cells have been studied with the aim to predict the radiosensitivity of tumours and normal tissues, e.g. DNA double strand breaks (DSB), chromosome aberrations and cell reproductive inactivation. However, variation in correlations with clinical results has reduced general application. An additional type of information is required for the increasing application of high-LET radiation in cancer therapy: the Relative Biological Effectiveness (RBE) for effects in tumours and normal tissues. Relevant information on RBE values might be derived from studies on cells in culture.

Methods

To evaluate relationships between DNA-DSB, chromosome aberrations and the clinically most relevant effect of cell reproductive death, for ionizing radiations of different LET, dose-effect relationships were determined for the induction of these effects in cultured SW-1573 cells irradiated with gamma-rays from a Cs-137 source or with α-particles from an Am-241 source. RBE values were derived for these effects. Ionizing radiation induced foci (IRIF) of DNA repair related proteins, indicative of DSB, were assessed by counting gamma-H2AX foci. Chromosome aberration frequencies were determined by scoring fragments and translocations using premature chromosome condensation. Cell survival was measured by colony formation assay. Analysis of dose-effect relations was based on the linear-quadratic model.

Results

Our results show that, although both investigated radiation types induce similar numbers of IRIF per absorbed dose, only a small fraction of the DSB induced by the low-LET gamma-rays result in chromosome rearrangements and cell reproductive death, while this fraction is considerably enhanced for the high-LET alpha-radiation. Calculated RBE values derived for the linear components of dose-effect relations for gamma-H2AX foci, cell reproductive death, chromosome fragments and colour junctions are 1.0 ± 0.3, 14.7 ± 5.1, 15.3 ± 5.9 and 13.3 ± 6.0 respectively.

Conclusions

These results indicate that RBE values for IRIF (DNA-DSB) induction provide little valid information on other biologically-relevant end points in cells exposed to high-LET radiations. Furthermore, the RBE values for the induction of the two types of chromosome aberrations are similar to those established for cell reproductive death. This suggests that assays of these aberrations might yield relevant information on the biological effectiveness in high-LET radiotherapy.

Induction of asymmetrical type of chromosomal aberrations in cultured human lymphocytes by ion beams of different energies at varying LET from HIMAC and RRC

Frequencies of asymmetrical type of chromosome aberration were scored in cultured human blood lymphocytes irradiated with carbon and neon beams. Blood cells were irradiated with various doses to establish dose response curves for chromosome aberration frequency vs. dose, and chromosome preparation was made by conventional method. Dose response curves for the per cell frequencies of the dicentrics and centric rings as well as the excess amount of acentric fragments were described for 7 different qualities (LET = 22.4, 40.0, 41.5, 69.9, 70.0, 100.0 and 150 KeV/micrometer) of carbon and neon beams with three different energies, 135, 290 and 400 MeV/u. From the analysis of those dose response curves, the maximum effect was found in the region of LET value at near 70 KeV/micrometer together with linear expression in the response from all endpoints examined. The 135 MeV/u of carbons (69.9 KeV/micrometer) and neons(70.0 KeV/micrometer) showed linear response. The 290 MeV/u of carbons (100 KeV/m) and neons (150 KeV/micrometer) showed medium effects with different shape of response, linear with a plateau and upward concavity. The 2 carbon beams (41.5 and 40 KeV/micrometer) from 2 different accelerators showed much discrepancy in the response. RBE-LET relationship was also described by comparing the coefficient alpha of the 7 different dose responses. The peak (near 70 KeV/m) was localized close to that (80 KeV/m) for the survivals of dsb repair deficient cells (Eguchi-Kasai et al. 1998), but in different position from that previously reported in many other studies (100-200 KeV/mm). Identification of the RBEmax in the present study has yet to be definitive.

Comparison of radiobiological effective depths in 65-MeV modulated proton beams

To assess the achievement of uniformity of radiobiological effectiveness at different depths in the proton spread-out Bragg peak (SOBP), Chinese hamster ovary (CHO) cells were exposed to 65-MeV modulated proton beams at the Research Center for Nuclear Physics (RCNP) of Osaka University. We selected four different irradiation positions: 2 mm depth, corresponding to the entrance, and 10, 18 and 23 mm depths, corresponding to different positions in the SOBP. Cell survival curves were generated with the in vitro colony formation method and fitted to the linear-quadratic model. With 137Cs gamma-rays as the reference irradiation, the relative biological effectiveness (RBE) values for a surviving fraction (SF) level of 0.1 are 1.05, 1.10, 1.12 and 1.19 for depths of 2, 10, 18 and 23 mm respectively. A significant difference was found between the survival curves at 10 and 23 mm (P < 0.05), but not between 18 and 10 mm or between 18 and 23 mm. There was a significant dependence of RBE on depths in modulated proton beams at the 0.1 surviving fraction level (P < 0.05). Moreover, the rise of RBEs significantly depended on increasing SF level or decreased approximately in correspondence with irradiation dose (P = 0.0001). To maintain uniformity of radiobiological effectiveness for the target volume, careful attention should be paid to the influence of depth of beam and irradiation dose.

Modelling of radiation damage by 125I on a nucleosome

Studies of early physical interactions of ionizing radiation in biological medium have evolved from water cylinders or spheres to structured volumes representing nucleosomal DNA, based on spatial co-ordinates for each individual atom. Regarding the physico-chemical and chemical stages, the models of DNA have evolved from inactive geometrical objects to active participation of DNA in the reactions with the radical species. In this paper data are presented on the modelling of the interaction of low energy electrons with nucleosomal DNA. The nucleosome core unit has been modelled as a 146-bp helical DNA, containing > 9000 atoms, wound around the core unit. The yields of strand breaks for low energy electrons has been obtained by placing the nucleosome target in a liquid water environment and introducing a number of assumptions for the induction of strand breaks. The calculated results for the induction of ssb and dsb by 125I decays agree with experimental data, confirming the plausibility of this nucleosome model as well as the characterization of the interaction of ionizing radiation in terms of the energy deposition and the assumptions made for the strand breaks.

RBE-LET relationships for different types of lethal radiation damage in mammalian cells: comparison with DNA dsb and an interpretation of differences in radiosensitivity

Relative biological effectiveness (RBE), as a function of linear energy transfer (LET), is evaluated for different types of damage contributing to mammalian cell reproductive death. Survival curves are analysed assuming a linear-quadratic dose dependence of lethal lesions. The linear term represents lethal damage due to single particle tracks, the quadratic term represents lethality due to interaction of lesions from independent tracks. RBE-LET relationships of single-track lethal damage, sublethal damage, potentially lethal damage and DNA double-strand breaks (dsb) are compared. Single-track lethal damage is shown to be composed of two components: damage that remains unrepaired in an interval between irradiation and assay, characterized by a very strong dependence on LET, with RBEs up to 20, and potentially lethal damage, which is weakly dependent on LET with RBEs < 3. Potentially lethal damage and sublethal damage depend similarly on LET as DNA dsb. The identification of these different components of damage leads to an interpretation of differences in radiosensitivity and in RBEs among various types of cells.

A nucleosome model for the simulation of DNA strand break experiments

Using a set of Monte Carlo simulation models, track structures of 125I Auger electrons generated in liquid water are superimposed on a nucleosome DNA model able to precisely localize energy deposition events on sub-molecular units of the DNA strands. After scoring direct hits taking place during the physical phase (at about 10(-15) s) the radiation chemistry of the whole system is simulated between 10(-12) and 10(-8) s, taking into account all reactions between water radio-chemical species, radicals, sub-molecular units of DNA (Ribose, Adenine, Thymine, Guanine, and Cytosine), and scavengers like Tris or Formate ions. The model's possibility to distinguish between direct and indirect hits has been utilized to introduce different assumptions for strand break induction by both hit modes. The number of SSB and DSB as well as their local distribution will be given and compared with experimental and theoretical results from the literature.

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.

Relative contributions of levels of initial damage and repair of double-strand breaks to the ionizing radiation-sensitive phenotype of the Chinese hamster cell mutant, XR-V15B. Part II. Neutrons

We have compared DNA double-strand break (dsb) induction and rejoining, using field-inversion gel electrophoresis, with survival in mutant (XR-V15B) and in wild-type parental (V79B) hamster cell lines after low dose neutron and X-irradiation. We found that neutrons do not appear to induce more dsbs than X-rays and deduce that increased sensitivity to neutrons is therefore not due to a higher initial yield of dsbs. Even with low doses of neutrons, there is a visible increase in the production of a smaller subset of DNA fragments which arise only after very high dose X-irradiation. In both cell lines, dsbs induced by neutrons are rejoined more slowly than those induced by X-rays. At long repair times (4 and 17 h) there are no significant differences in the fractions of unrejoined dsbs between neutrons and X-rays. We propose that neutron-induced dsbs have a higher probability of becoming lethal because they are more likely to be misrepaired during the slow stage of rejoining.