Programmed cellular response to ionizing radiation damage

Identifying patients at risk for late radiation-induced toxicity

The impact of curative radiotherapy depends mainly on the total dose delivered in the targeted volume. Nevertheless, the dose delivered to the surrounding healthy tissues may reduce the therapeutic ratio of many treatments. Two different side effects (acute and late) can occur during and after radiotherapy. Of particular interest are the radiation-induced late complications (LC) due to their irreversibility and the potential impact on quality of life. In one population treated with the same technique, it appears that individual radiosensitivity clearly exists. In the hypothesis that genetic is involved in this area of research, low CD4 and CD8 lymphocyte apoptosis were shown to be correlated with high grade of LC. In addition, recent data suggest that patients with severe radiation-induced LC possess 4 or more single nucleotide polymorphisms (SNPs) in candidate genes and low radiation-induced CD8 lymphocyte apoptosis in vitro. On-going studies are being analyzing the entire genome using a genome-wide association study (GWAS).

Concurrent or sequential adjuvant letrozole and radiotherapy after conservative surgery for early-stage breast cancer (CO-HO-RT): a phase 2 randomised trial

BACKGROUND

Letrozole radiosensitises breast cancer cells in vitro. In clinical settings, no data exist for the combination of letrozole and radiotherapy. We assessed concurrent and sequential radiotherapy and letrozole in the adjuvant setting.

METHODS

This phase 2 randomised trial was undertaken in two centres in France and one in Switzerland between Jan 12, 2005, and Feb 21, 2007. 150 postmenopausal women with early-stage breast cancer were randomly assigned after conserving surgery to either concurrent radiotherapy and letrozole (n=75) or sequential radiotherapy and letrozole (n=75). Randomisation was open label with a minimisation technique, stratified by investigational centres, chemotherapy (yes vs no), radiation boost (yes vs no), and value of radiation-induced lymphocyte apoptosis (< or = 16% vs >16%). Whole breast was irradiated to a total dose of 50 Gy in 25 fractions over 5 weeks. In the case of supraclavicular and internal mammary node irradiation, the dose was 44-50 Gy. Letrozole was administered orally once daily at a dose of 2.5 mg for 5 years (beginning 3 weeks pre-radiotherapy in the concomitant group, and 3 weeks post-radiotherapy in the sequential group). The primary endpoint was the occurrence of acute (during and within 6 weeks of radiotherapy) and late (within 2 years) radiation-induced grade 2 or worse toxic effects of the skin. Analyses were by intention to treat. This study is registered with ClinicalTrials.gov, number NCT00208273.

FINDINGS

All patients were analysed apart from one in the concurrent group who withdrew consent before any treatment. During radiotherapy and within the first 12 weeks after radiotherapy, 31 patients in the concurrent group and 31 in the sequential group had any grade 2 or worse skin-related toxicity. The most common skin-related adverse event was dermatitis: four patients in the concurrent group and six in the sequential group had grade 3 acute skin dermatitis during radiotherapy. At a median follow-up of 26 months (range 3-40), two patients in each group had grade 2 or worse late effects (both radiation-induced subcutaneous fibrosis).

INTERPRETATION

Letrozole can be safely delivered shortly after surgery and concomitantly with radiotherapy. Long-term follow-up is needed to investigate cardiac side-effects and cancer-specific outcomes.

FUNDING

Novartis Oncology France.

Novel Radiation Response Genes Identified in Gene-Trapped MCF10A Mammary Epithelial Cells

We have used a gene-trapping strategy to screen human mammary epithelial cells for radiation response genes. Relative mRNA expression levels of five candidate genes in MCF10A cells were analyzed, both with and without exposure to radiation. In all five cases, the trapped genes were significantly down-regulated after radiation treatment. Sequence analysis of the fusion transcripts identified the trapped genes: (1) the human androgen receptor, (2) the uncharacterized DREV1 gene, which has known homology to DNA methyltransferases, (3) the human creatine kinase gene, (4) the human eukaryotic translation elongation factor 1 beta 2, and (5) the human ribosomal protein L27. All five genes were down-regulated significantly after treatment with varying doses of ionizing radiation (0.10 to 4.0 Gy) and at varying times (2-30 h after treatment). The genes were also analyzed in human fibroblast and lymphoblastoid cell lines to determine whether the radiation response being observed was cell-type specific. The results verified that the observed radiation response was not a cell-type-specific phenomenon, suggesting that the genes play essential roles in the radiation damage control pathways. This study demonstrates the potential of the gene-trap approach for the identification and functional analysis of novel radiation response genes.

Role of cell cycle in mediating sensitivity to radiotherapy

Multiple pathways are involved in maintaining the genetic integrity of a cell after its exposure to ionizing radiation. Although repair mechanisms such as homologous recombination and nonhomologous end-joining are important mammalian responses to double-strand DNA damage, cell cycle regulation is perhaps the most important determinant of ionizing radiation sensitivity. A common cellular response to DNA-damaging agents is the activation of cell cycle checkpoints. The DNA damage induced by ionizing radiation initiates signals that can ultimately activate either temporary checkpoints that permit time for genetic repair or irreversible growth arrest that results in cell death (necrosis or apoptosis). Such checkpoint activation constitutes an integrated response that involves sensor (RAD, BRCA, NBS1), transducer (ATM, CHK), and effector (p53, p21, CDK) genes. One of the key proteins in the checkpoint pathways is the tumor suppressor gene p53, which coordinates DNA repair with cell cycle progression and apoptosis. Specifically, in addition to other mediators of the checkpoint response (CHK kinases, p21), p53 mediates the two major DNA damage-dependent cellular checkpoints, one at the G(1)-S transition and the other at the G(2)-M transition, although the influence on the former process is more direct and significant. The cell cycle phase also determines a cell's relative radiosensitivity, with cells being most radiosensitive in the G(2)-M phase, less sensitive in the G(1) phase, and least sensitive during the latter part of the S phase. This understanding has, therefore, led to the realization that one way in which chemotherapy and fractionated radiotherapy may work better is by partial synchronization of cells in the most radiosensitive phase of the cell cycle. We describe how cell cycle and DNA damage checkpoint control relates to exposure to ionizing radiation.

Increased radiation toxicity by enhanced apoptotic clearance of HL-60 cells in the presence of the pentapeptide thymopentin, which selectively binds to apoptotic cells

Radiotoxic insult to cells is associated with genetic instability and heritable damage [Mutat. Res. 517 (2002) 173]. A strengthened response to such insult by enhanced apoptotic clearance, which would be associated with anti-inflammatory [Nature 390 (1997) 350; Nature 407 (2000) 784] and anti-necrotic intercellular signaling [Nature 418 (2002) 191], has been previously reported. The pentapeptide thymopentin (TP5) improves immunological parameters in cancer patients following radiotherapy without clinically observable side effects. We assessed the effects of TP5 on human promyeloid leukemia HL-60 cells exposed to therapeutic (2Gy) doses of X-rays. We observed an increased accumulation of cells in the G2/M phase of the cell cycle after irradiation when treated with TP5. However, TP5 had no effect on the cell cycle distribution of non-irradiated HL-60 cells. Additionally, TP5 treatment of irradiated cells increased the number of cells undergoing apoptosis. Furthermore, TP5 was found to selectively bind to apoptotic cells. These findings represent a promising and novel approach employing TP5-mediated modulation of cellular radiation response to augment both clinical gain in radiation oncology and safety measures for radiation protection.

Flow cytometric detection of apoptotic bone marrow cells with fractional DNA content after application of WR-2721, cyclophosphamide, cisplatin, and exposure of mice to gamma rays

Little is known about the mechanisms of apoptosis triggered in normal cells of the haemopoietic system by the aminothiol WR-2721 (Amifostine), chemotherapeutic drugs, and ionizing radiation; thus, the present study was undertaken to evaluate the effects of WR-2721, cyclophosphamide (CP), cisplatin (CDDP), and 60Co gamma rays on induction of apoptotic DNA degradation in bone marrow cells. Adult male Swiss mice were treated with WR-2721 (400 mg/kg b.wt.), CP (200 mg/kg b.wt.), and CDDP (10 mg/kg b.wt.), and exposed to 6 Gy 60Co gamma rays. Alterations in the number of apoptotic cells with fractional DNA content and also the cell cycle position of the non-apoptotic cells were determined in the bone marrow at 7 and 24 hours after treatment of mice with these agents, using flow cytometric assay of the controlled extraction of low-MW DNA from apoptotic cells. The chemotherapeutic drugs CP and CDDP and 60Co gamma rays triggered apoptosis and affected the cell cycle position of the non-apoptotic cells in the mouse bone marrow. The pretreatment of mice with WR-2721 resulted in the modulatory action of the aminothiol on induction of apoptotic cell death and changes in the cell cycle distribution of the non-apoptotic cells caused by the DNA-damaging agents. The patterns of changes in the frequency of apoptotic cells and the cell cycle position of the non-apoptotic cells, observed in the bone marrow, were dependent on the agent(s) applied and the time interval after application of the drug(s) and exposure of mice to gamma rays. Understanding of the mechanisms responsible for triggering of apoptotic cell death and disturbing of the cell cycle by the DNA-damaging agents, and modulation of the apoptotic and cell cycle pathways by the aminothiol WR-2721, can lead to more effective therapy and chemo- and radio-protection of normal cells.

A single low dose of X-rays induces high frequencies of genetic instability (aneuploidy) and heritable damage (apoptosis), dependent on cell type and p53 status

We harvested and analyzed cells from four different non-transformed cell lines surviving a single X-ray exposure. Evidence of radiation-induced karyotype instability was observed in 100% of C3H 10T1/2 fibroblast clones and 11.3% of V79 fibroblast clones. Heritable damage: predisposition to apoptosis, but not karyotype instability, was induced in TK6 (p53(wt/wt)) and WTK1 (p53(mut/mut)) human B-lymphoblastoid cell clones. The studies indicate: (1) genetic instability and/or heritable damage are induced in cells exposed to radiation at a high frequency, and induction of genetic instability is not limited to morphologically transformed cells [Radiat. Res. 138 (1994) S105; Radiat. Environ. Biophys. 36 (1998) 255]; (2) sensitivity to genetic instability and heritable damage depend on cell type; (3) checkpoint stringency and p53 status significantly influence the frequency of radiation-induced genetic instability and heritable damage; (4) in some cell lines, damage induced by low doses of radiation (below 2 Gy) leads to heritable cytotoxic and genotoxic effects in 100% of cells exposed. The data suggest that mammalian cells misinterpret damage induced by ionizing radiation as if it were a physiological cell signal. This contrasts strongly with the response of mammalian cells to damage induced by other types of DNA-toxic agents where damage-specific repair mechanisms are activated.

Identification of radiation-responsive genes in vitro using a gene trap strategy predicts for modulation of expression by radiation in vivo

A large number of genes are known to be responsive to ionizing radiation, and there is strong evidence for the existence of inducible radiation resistance in mammalian cells. We have developed a gene trap insertional mutagenesis strategy to identify novel genes involved in responses to radiation. Using this approach, we have isolated four gene-trap integrations in embryonic stem cells. In three cases (9A, 3E and 9H) the trapped genes are radiation-inducible, and in one (7D) the gene is down-regulated. Sequence analysis of fusion transcripts from three of the integrations indicate one novel gene (3E), the mouse homologue (9A) of a known but uncharacterized human gene that encodes a protein with significant homology to several GTPase-activating proteins and a murine locus, Mym (9H). The embryonic stem cell clone with the 9A insertion was introduced into the mouse germline, and the in vivo expression pattern of 9A was studied in detail. A unique, spatially restricted pattern of expression in embryos and adult animals was observed. There is tissue-specific in vivo induction of the 9A gene in adult mice by radiation. This study demonstrates the potential of the gene trap approach for the identification and functional analysis of novel radiation-regulated genes. Similar strategies may facilitate the discovery and characterization of genes involved in other cellular stress responses.

Clonal variation in phenotype and life span of human embryonic fibroblasts (MRC-5) transduced with the catalytic component of telomerase (hTERT)

Expression of telomerase (hTERT) in certain cell types has been shown to extend cellular life span without malignant transformation. We studied the phenotype of 26 telomerase-transduced fibroblast clones (TTFC) generated from a mass culture of hTERT retrovirally transduced MRC-5 cells. About two-thirds of the transduced clones senesced at the expected time or shortly thereafter, despite high levels of expression of telomerase and telomere length maintenance. The remaining one-third of the clones were "immortalized" (followed for over 200 cumulative population doublings). All clones maintained a nontransformed phenotype: contact inhibition, anchorage dependency, lack of tumor formation in nude mice, dose dependency to serum and growth factors, low expression of a matrix metalloproteinase associated with metastatic invasion (MMP-9) and high expression of its inhibitor TIMP-1, and no cytogenetic abnormalities by G-banding. In addition, fibroblast-specific biological parameters, such as colony size, production of collagenase, and response to MMC and gamma radiation were tightly regulated at the clonal and subclonal levels.

Delayed re-endothelialization and T-cell infiltration following intracoronary radiation therapy in the porcine model

PURPOSE

To evaluate the late induction of apoptosis following intracoronary radiation (IR) and the effects of IR on inflammatory cells.

METHODS AND MATERIALS

Porcine coronaries were injured by balloon overstretch followed by either 0 or 15 Gy of 192Ir prescribed to 2 mm from the center of the source. Swine were euthanized at 3, 7, and 14 days posttreatment, and arteries were stained for markers of smooth muscle cells (SMCs alpha-actin), T cells (CD3), macrophages, endothelial cells, and apoptotic nuclei (terminal uridine nick end labeling, TUNEL). Intimal area (IA) and IA corrected for medial fracture length (IA/FL) were quantified by digital image analysis, which was also used to quantify the distribution of immunostain-positive cells in the adventitia, media, and neointima, respectively.

RESULTS

IA/FL was significantly reduced following treatment with 15 Gy, in association with decreased SMC density. Following injury and IR, TUNEL- and CD3-positive cell density increased significantly, and density of macrophages was increased in the adventitia and neointima. Staining for endothelial cells revealed a delay of re-endothelialization after radiation treatment.

CONCLUSION

Increased T-cell infiltration at the medial tear following IR, perhaps due to incomplete re-endothelialization, may indicate incomplete healing. The elevated apoptosis of these infiltrating T cells may indicate a mechanism for the resolution of inflammation.

Cell cycle and growth response of CHO cells to X-irradiation: threshold-free repair at low doses

PURPOSE

To test the hypothesis of a threshold for induced repair of DNA damage (IR) and, secondarily, of hyperradiosensitivity (HRS) to low-dose X-irradiation.

METHODS AND MATERIALS

Exponentially growing Chinese hamster ovary cells (CHO) were X-irradiated with doses from 0.2 to 8 Gy. Survival data were established by conventional colony-forming assay and flow-cytometric population counting. The early cell cycle response to radiation was studied based on DNA-profiles and bromodeoxyuridine pulse-labeling experiments.

RESULTS

Colony-forming data were consistent with HRS. However, these data were of low statistic significance. Population counting provided highly reproducible survival curves that were in perfect accord with the linear-quadratic (LQ) model. The dominant cell cycle reaction was a dose-dependent delay of G2 M and late S-phase.

CONCLUSION

There was no evidence for a threshold of IR and for low-dose HRS in X-irradiated CHO cells. It is suggested that DNA damage repair activity is constitutively expressed during S-phase and is additionally induced in a dose-dependent and threshold-free manner in late S-phase and G2. The resulting survival is precisely described by the LQ model.

Gamma ray induced DNA damage in human and mouse leucocytes measured by SCGE-Pro: a software developed for automated image analysis and data processing for Comet assay

The studies reported in this communication had two major objectives: first to validate the in-house developed SCGE-Pro: a software developed for automated image analysis and data processing for Comet assay using human peripheral blood leucocytes exposed to radiation doses, viz. 2, 4 and 8 Gy, which are known to produce DNA/chromosome damage using alkaline Comet assay. The second objective was to investigate the effect of gamma radiation on DNA damage in mouse peripheral blood leucocytes using identical doses and experimental conditions, e.g. lyses, electrophoretic conditions and duration of electrophoresis which are known to affect tail moment (TM) and tail length (TL) of comets. Human and mouse whole blood samples were irradiated with different doses of gamma rays, e.g. 2, 4 and 8 Gy at a dose rate of 0.668Gy/min between 0 and 4 degrees C in air. After lyses, cells were electrophorased under alkaline conditions at pH 13, washed and stained with propidium iodide. Images of the cells were acquired and analyzed using in-house developed imaging software, SCGE-Pro, for Comet assay. For each comet, total fluorescence, tail fluorescence and tail length were measured. Increase in TM and TL was considered as the criteria of DNA damage. Analysis of data revealed heterogeneity in the response of leucocytes to gamma ray induced DNA damage both in human as well as in mouse. A wide variation in TM and TL was observed in control and irradiated groups of all the three donors. Data were analyzed for statistical significance using one-way ANOVA. Though a small variation in basal level of TM and TL was observed amongst human and mouse controls, the differences were not statistically significant. A dose-dependent increase in TM (P<0.001) and TL (P<0.001) was obtained at all the radiation doses (2-8 Gy) both in human and mouse leucocytes. However, there was a difference in the nature of dose response curves for human and mouse leucocytes. In human leucocytes, a linear increase in TM and TL was observed up to the highest radiation dose of 8 Gy. However, in case of mouse leucocytes, a sharp increase in TM and TL was observed only up to 4 Gy, and there after saturation ensued. In human samples, the dose response of both TM and TL showed best fits with linear model (r(TM)=0.999 and r(TL)=0.999), where as in mouse, the best fit was obtained with Sigmoid (Boltzman) model. From the present data on leucocytes with increase in TM and TL as the criteria of DNA damage, it appears that mouse is relatively more sensitive to radiation damage than humans.

Vascular repair after balloon overstretch injury in porcine model effects of intracoronary radiation

OBJECTIVES

The purpose of this study was to evaluate the effect of IR on thrombus formation and dissection repair following overstretch balloon injury in porcine coronary arteries.

BACKGROUND

Exposure of blood to the injured arterial wall after percutaneous transluminal coronary angioplasty (PTCA) induces thrombus formation and inflammation in the dissection plane. Neointima formation is related to smooth muscle cell (SMC) proliferation and migration into the preformed thrombus. Intracoronary radiation (IR) with doses of 10 to 25 Gy using either beta or gamma emitters can prevent neointima accumulation by reducing SMC proliferation. However, there are some indications that IR may delay the process of dissection repair after PTCA. The purpose of this study was to evaluate the effect of IR on thrombus formation and dissection repair after overstretch balloon injury in porcine coronary arteries.

METHODS

Forty porcine coronaries were injured by balloon overstretch followed by either 0 or 18 Gy of 90Y prescribed to 1.2 mm from the balloon center. The animals were euthanized 14 days after treatment, and intimal area (IA) and IA corrected for medial fracture length (IA/FL) were quantified by digital image analysis. Dissections were quantified by tracing the length, thickness and area behind the dissection flap. The rate of dissections was calculated for each group. Thrombi were identified and designated as intraluminal thrombus or thrombus within dissection planes (mural thrombus), and area measurements were obtained.

RESULTS

The irradiated group showed a significant reduction of IA/FL (0.55 +/- 0.29 vs. 0.05 +/- 0.09; p < 0.001). No difference was observed in the rate of dissection between control and irradiated arteries (77% vs. 88%, respectively). The control group showed a smaller dissection area (0.19 +/- 0.28 mm2 vs. 0.32 +/- 0.29 mm2; p < 0.05) with smaller mural thrombi (0.03 +/-0.01 mm2 vs. 0.29 +/- 0.30 mm2; p < 0.001). A strong correlation between dissection area and neointima area was observed only in the control group (R2 = 0.474; p < 0.003; alpha0.05 = 0.862). A positive correlation between mural thrombus and dissection area was observed only in the irradiated group (R2 = 0.889; p < 0.001; alpha0.05 = 1.00).

CONCLUSIONS

These results suggest that the dissection area may be a useful parameter by which to quantify the extent of injury and repair after IR and may indicate an incomplete healing process after IR at this time point.

Assessment of DNA damage induced by high-LET ions in human lymphocytes using the comet assay

The alkaline single-cell gel electrophoresis assay (comet assay) was used to analyze DNA damage induced in human lymphocytes by irradiation with high linear energy transfer (LET) ions. Our aim was to measure DNA breaks and to demonstrate the heterogeneity of the damage levels in a lymphocyte population irradiated with ions of different energies and LETs. Four experiments with heavy ions (Ar, C and U), as well as gamma-ray exposure, were conducted to enable comparisons. We demonstrated that the comet assay is able to assess the variability in DNA damage induced at the single cell level. The amount of DNA damage and its heterogeneity increased with particle fluence and LET, but saturated at high LETs. However, when expressed in terms of the mean dose, gamma-rays were more efficient than most of the ions used. The comet assay also allowed the detection of highly damaged cells (HDC), which were previously described as cells in late apoptotic stages. The rapid emergence of HDC in this study suggests that they were generated following ion irradiation-induced creation of DNA break clusters induced by ion exposure. Another clue was that the proportion of HDC increased with LET and fluence. We hypothesized that the LET threshold observed and the higher efficiency of low-LET radiation might be linked to the impossibility of measuring small DNA fragments in HDC.

Altered apoptotic profiles in irradiated patients with increased toxicity

PURPOSE

A retrospective study of radiation-induced apoptosis in CD4 and CD8 T-lymphocytes, from 12 cancer patients who displayed enhanced toxicity to radiation therapy and 9 ataxia telangiectasia patients, was performed to test for altered response compared to healthy blood-donors and normal cancer patients.

METHODS AND MATERIALS

Three milliliters of heparinized blood from each donor was sent via express post to the Paul Scherrer Institute (PSI) for subsequent examination. The blood was diluted 1:10 in RPMI medium, irradiated with 0-, 2-, or 9-Gy X-rays, and incubated for 48 h. CD4 and CD8 T-lymphocytes were then labeled using FITC-conjugated antibodies, erythrocytes were lysed, and the DNA stained with propidium iodide. Subsequently, cells were analyzed using a Becton Dickinson FACScan flow cytometer. Radiation-induced apoptosis was recognized in leukocytes as reduced DNA content attributed to apoptosis-associated changes in chromatin structure. Apoptosis was confirmed by light microscopy, electron microscopy, and by the use of commercially available apoptosis detection kits (in situ nick translation and Annexin V). Data from hypersensitive individuals were compared to a standard database of 105 healthy blood-donors, and a database of 48 cancer patient blood donors who displayed normal toxicity to radiation therapy. To integrate radiosensitivity results from CD4 and CD8 T-lymphocytes after 2 and 9 Gy, z-score analyses were performed.

RESULTS

A cohort of 12 hypersensitive patients was evaluated; 8 showed enhanced early toxicity, 3 showed enhanced late toxicity, and 1 showed both. The cohort displayed less radiation-induced apoptosis (-1.8 sigma) than average age-matched donors. A cohort of 9 ataxia telangiectasia homozygotes displayed even less apoptosis (-3.6 sigma).

CONCLUSION

The leukocyte apoptosis assay appears to be a useful predictor of individuals likely to display increased toxicity to radiation therapy; however, validation of this requires a prospective study.

Modulation of protein expression and activity by radiation: relevance to intracoronary radiation for the prevention of restenosis

Restenosis is a common complication of percutaneous transluminal coronary angioplasty. Recent studies have demonstrated a striking reduction in the neointimal hyperplasia characteristic of restenosis following intracoronary radiation (IR), but the mechanisms by which radiation reduces neointima formation following balloon overstretch injury are not elucidated fully. In addition to direct antimitotic effects mediated via oxygen free radicals, ionizing radiation can induce the expression of numerous genes and thereby mediate indirect effects. Additionally, IR prevents restenosis at the cost of decreased healing and increased thrombosis, and we suggest that these adverse reactions can be modulated by adjunct pharmacology or gene-based strategies. This review discusses several genes and proteins modulated by radiation in the context of arterial injury, and their possible therapeutic relevance.