The indirect effect of radiation reduces the repair fidelity of NHEJ as verified in repair deficient CHO cell lines exposed to different radiation qualities and potassium bromate

DNA Damage by Radiopharmaceuticals and Mechanisms of Cellular Repair

DNA is an organic molecule that is highly vulnerable to chemical alterations and breaks caused by both internal and external factors. Cells possess complex and advanced mechanisms, including DNA repair, damage tolerance, cell cycle checkpoints, and cell death pathways, which together minimize the potentially harmful effects of DNA damage. However, in cancer cells, the normal DNA damage tolerance and response processes are disrupted or deregulated. This results in increased mutagenesis and genomic instability within the cancer cells, a known driver of cancer progression and therapeutic resistance. On the other hand, the inherent instability of the genome in rapidly dividing cancer cells can be exploited as a tool to kill by imposing DNA damage with radiopharmaceuticals. As the field of targeted radiopharmaceutical therapy (RPT) is rapidly growing in oncology, it is crucial to have a deep understanding of the impact of systemic radiation delivery by radiopharmaceuticals on the DNA of tumors and healthy tissues. The distribution and activation of DNA damage and repair pathways caused by RPT can be different based on the characteristics of the radioisotope and molecular target. Here we provide a comprehensive discussion of the biological effects of RPTs, with the main focus on the role of varying radioisotopes in inducing direct and indirect DNA damage and activating DNA repair pathways.

The Effects of Dimethylsulfoxide and Oxygen on DNA Damage Induction and Repair Outcomes for Cells Irradiated by 62 MeV Proton and 3.31 MeV Helium Ions

Reactive oxygen species (ROS) play an essential role in radiation-induced indirect actions. In terms of DNA damage, double strand breaks (DSBs) have the greatest effects on the repair of DNA damage, cell survival and transformation. This study evaluated the biological effects of the presence of ROS and oxygen on DSB induction and mutation frequency. The relative biological effectiveness (RBE) and oxygen enhancement ratio (OER) of 62 MeV therapeutic proton beams and 3.31 MeV helium ions were calculated using Monte Carlo damage simulation (MCDS) software. Monte Carlo excision repair (MCER) simulations were used to calculate the repair outcomes (mutation frequency). The RBE values of proton beams decreased to 0.75 in the presence of 0.4 M dimethylsulfoxide (DMSO) and then increases to 0.9 in the presence of 2 M DMSO while the RBE values of 3.31 MeV helium ions increased from 2.9 to 5.7 (0–2 M). The mutation frequency of proton beams also decreased from 0.008–0.065 to 0.004–0.034 per cell per Gy by the addition of 2 M DMSO, indicating that ROS affects both DSB induction and repair outcomes. These results show that the combined use of DMSO in normal tissues and an increased dose in tumor regions increases treatment efficiency.

Small is beautiful: low activity alpha and gamma sources for small-scale radiation protection research experiments

PURPOSE

Uncertainties regarding the magnitude of health effects following exposure to low doses of ionizing radiation remain a matter of concern both for professionals and for the public. There is consensus within the international radiation research community that more research is required on biological effects of radiation doses below 100 mGy applied at low dose rates. Moreover, there is a demand for increasing education and training of future radiation researchers and regulators. Research, education and training is primarily carried out at universities but university-based radiation research is often hampered by limited access to radiation sources. The aim of the present report is to describe small and cost-effective low activity gamma and alpha sources that can easily be installed and used in university laboratories.

METHODS AND RESULTS

A gamma radiation source was made from an euxenite-(Y) rock (Y,Ca,Ce,U,Th)(Nb,Ta,Ti)₂O₆) that was found in an abandoned mine in Sweden. It allows exposing cells grown in culture dishes to radiation at a dose rate of 50 µGy/h and lower. Three alpha sources were custom-made and yield a dose rate of 1 mGy/h each. The construction, dosimetry and cellular effects of the sources are described.

CONCLUSIONS

We hope that the report will stimulate research and training activities in the low dose field by facilitating access to radiation sources.

Effects of indirect actions and oxygen on relative biological effectiveness: estimate of DSB inductions and conversions induced by therapeutic proton beams

Purpose: This study evaluated the DNA double strand breaks (DSBs) induced by indirect actions and its misrepairs to estimate the relative biological effectiveness (RBE) of proton beams.Materials and methods: From experimental data, DSB induction was evaluated in cells irradiated by 62 MeV proton beams in the presence of dimethylsulphoxide (DMSO) and under hypoxic conditions. The DNA damage yields for calculating the RBE were estimated using Monte Carlo Damage Simulation (MCDS) software. The repair outcomes (correct repairs, mutations and DSB conversions) were estimated using Monte Carlo Excision Repair (MCER) simulations.Results: The values for RBE of 62 MeV protons (LET = 1.051 keV/μm) for DSB induction and enzymatic DSB under aerobic condition (21% O₂) was 1.02 and 0.94, respectively, as comparing to ⁶⁰Co γ-rays (LET = 2.4 keV/μm). DMSO mitigated the inference of indirect action and reduced DSB induction to a greater extent when damaged by protons rather than γ-rays, resulting in a decreased RBE of 0.86. DMSO also efficiently prevented enzymatic DSB yields triggered by proton irradiation and reduced the RBE to 0.83. However, hypoxia (2% O₂) produced a similar level of DSB induction with respect to the protons and γ-rays, with a comparable RBE of 1.02.Conclusions: The RBE values of proton beams estimated from DSB induction and enzymatic DSB decreased by 16% and 12%, respectively, in the presence of DMSO. Our findings indicate that the overall effects of DSB induction and enzymatic DSB could intensify the tumor killing, while alleviate normal tissue damage when indirect actions are effectively interrupted.

Impact of dimethyl sulfoxide on irradiation-related DNA double-strand-break induction, -repair and cell survival

Dimethyl sulfoxide (DMSO) is an effective radical scavenger and, when added to cells, reduces the initial number of radiation-induced DNA double-strand breaks (DSB). The aim of this study was to investigate modification by DMSO of both DSB induction and DSB repair by means of pulsed-field gel electrophoresis (PFGE) as well as gamma-H2AX immunofluorescence staining. WiDr cells (human colon carcinoma provided by DKFZ) were incubated with 2% DMSO for 2 h (or mock-treated) prior to irradiation with varying X-ray doses and subsequent incubation for repair. Sample processing for PFGE analysis or counting of γ-H2AX foci was performed according to standard protocols. Effects on apoptosis induction and cell survival were investigated additionally by standard protocols. DMSO reduced DSB yield after 20-80 Gy measured by PFGE. A qualitatively similar result was found after low-dose irradiation (1 Gy) using γ-H2AX immunofluorescence staining. During incubation for repair, both DNA fragment rejoining (PFGE) as well as γ-H2AX foci removal occurred at a reduced rate when cells had been pre-treated with DMSO. But this effect was clearly more pronounced for the PFGE-analyzed double-strand breakage, particularly at early repair times. WiDr cells treated with DMSO (2%) showed a significantly increased clonogenic survival after irradiation doses above 8 Gy. Apoptosis rates were not changed by DMSO. The radio-protective effect of DMSO, well known from other PFGE studies, could be confirmed for the formation of γ-H2AX foci. DSB generated in the presence of DMSO were less rapidly repaired. DMSO showed radio-protective effects on clonogenic survival but not on apoptosis.

Effects of indirect actions and oxygen on relative biological effectiveness: estimate of DSB induction and conversion induced by gamma rays and helium ions

Clustered DNA damage other than double-strand breaks (DSBs) can be detrimental to cells and can lead to mutagenesis or cell death. In addition to DSBs induced by ionizing radiation, misrepair of non-DSB clustered damage contributes extra DSBs converted from DNA misrepair via pathways for base excision repair and nucleotide excision repair. This study aimed to quantify the relative biological effectiveness (RBE) when DSB induction and conversion from non-DSB clustered damage misrepair were used as biological endpoints. The results showed that both linear energy transfer (LET) and indirect action had a strong impact on the yields for DSB induction and conversion. RBE values for DSB induction and maximum DSB conversion of helium ions (LET = 120 keV/μm) to (60)Co gamma rays were 3.0 and 3.2, respectively. These RBE values increased to 5.8 and 5.6 in the absence of interference of indirect action initiated by addition of 2-M dimethylsulfoxide. DSB conversion was ∼1-4% of the total non-DSB damage due to gamma rays, which was lower than the 10% estimate by experimental measurement. Five to twenty percent of total non-DSB damage due to helium ions was converted into DSBs. Hence, it may be possible to increase the yields of DSBs in cancerous cells through DNA repair pathways, ultimately enhancing cell killing.

Drug metabolism and homologous recombination repair in radiosensitization with gemcitabine

Gemcitabine (difluorodeoxycytidine; dFdCyd) is a potent radiosensitizer, noted for its ability to enhance cytotoxicity with radiation at noncytotoxic concentrations in vitro and subchemotherapeutic doses in patients. Radiosensitization in human tumor cells requires dFdCyd-mediated accumulation of cells in S phase with inhibition of ribonucleotide reductase, resulting in ≥80% deoxyadenosine triphosphate (dATP) depletion and errors of replication in DNA. Less is known of the role of specific DNA replication and repair pathways in the radiosensitization mechanism. Here the role of homologous recombination (HR) in relationship to the metabolic and cell cycle effects of dFdCyd was investigated using a matched pair of CHO cell lines that are either proficient (AA8 cells) or deficient (irs1SF cells) in HR based on expression of the HR protein XRCC3. The results demonstrated that the characteristics of radiosensitization in the rodent AA8 cells differed significantly from those in human tumor cells. In the AA8 cells, radiosensitization was achieved only under short (≤4 h) cytotoxic incubations, and S-phase accumulation did not appear to be required for radiosensitization. In contrast, human tumor cell lines were radiosensitized using noncytotoxic concentrations of dFdCyd and required early S-phase accumulation. Studies of the metabolic effects of dFdCyd demonstrated low dFdCyd concentrations did not deplete dATP by ≥80% in AA8 and irs1SF cells. However, at higher concentrations of dFdCyd, failure to radiosensitize the HR-deficient irs1SF cells could not be explained by a lack of dATP depletion or lack of S-phase accumulation. Thus, these parameters did not correspond to dFdCyd radiosensitization in the CHO cells. To evaluate directly the role of HR in radiosensitization, XRCC3 expression was suppressed in the AA8 cells with a lentiviral-delivered shRNA. Partial XRCC3 suppression significantly decreased radiosensitization [radiation enhancement ratio (RER) = 1.6 ± 0.15], compared to nontransduced (RER = 2.7 ± 0.27; P = 0.012), and a substantial decrease compared to nonspecific shRNA-transduced (RER = 2.5 ± 0.42; P = 0.056) AA8 cells. Although the results support a role for HR in radiosensitization with dFdCyd in CHO cells, the differences in the underlying metabolic and cell cycle characteristics suggest that dFdCyd radiosensitization in the nontumor-derived CHO cells is mechanistically distinct from that in human tumor cells.

DNA double strand breaks induced by the indirect effect of radiation are more efficiently repaired by non-homologous end joining compared to homologous recombination repair

The aim of this study was to investigate the relative involvement of three major DNA repair pathways, i.e., non-homologous end joining (NHEJ), homologous recombination (HRR) and base excision (BER) in repair of DNA lesions of different complexity induced by low- or high-LET radiation with emphasis on the contribution of the indirect effect of radiation for these radiation qualities. A panel of DNA repair-deficient CHO cell lines was irradiated by (137)Cs γ-rays or radon progeny α-particles. Irradiation was also performed in the presence of 2M DMSO to reduce the indirect effect of radiation and the complexity of the DNA damage formed. Clonogenic survival and micronucleus assays were used to estimate efficiencies of the different repair pathways for DNA damages produced by direct and indirect effects. Removal of the indirect effect of low-LET radiation by DMSO increased clonogenic survival and decreased MN formation for all cell lines investigated. A direct contribution of the indirect effect of radiation to DNA base damage was suggested by the significant protection by DMSO seen for the BER deficient cell line. Lesions formed by the indirect effect are more readily repaired by the NHEJ pathway than by HRR after irradiation with γ-rays or α-particles as evaluated by cell survival and the yields of MN. The results obtained with BER- and NHEJ-deficient cells suggest that the indirect effect of radiation contributes significantly to the formation of repair substrates for these pathways.