Relationship between DNA double-strand breaks, cell killing, and fibrosis studied in confluent skin fibroblasts derived from breast cancer patients

Bedrock radioactivity influences the rate and spectrum of mutation

All organisms on Earth are exposed to low doses of natural radioactivity but some habitats are more radioactive than others. Yet, documenting the influence of natural radioactivity on the evolution of biodiversity is challenging. Here, we addressed whether organisms living in naturally more radioactive habitats accumulate more mutations across generations using 14 species of waterlice living in subterranean habitats with contrasted levels of radioactivity. We found that the mitochondrial and nuclear mutation rates across a waterlouse species’ genome increased on average by 60% and 30%, respectively, when radioactivity increased by a factor of three. We also found a positive correlation between the level of radioactivity and the probability of G to T (and complementary C to A) mutations, a hallmark of oxidative stress. We conclude that even low doses of natural bedrock radioactivity influence the mutation rate possibly through the accumulation of oxidative damage, in particular in the mitochondrial genome.

AZD9291 Increases Sensitivity to Radiation in PC-9-IR Cells by Delaying DNA Damage Repair after Irradiation and Inducing Apoptosis

AZD9291 is a novel, irreversible epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor (TKI), which is administered orally. It has been proven effective in non-small cell lung cancer (NSCLC) patients, with both EGFR-sensitizing and EGFR T790M mutations in preclinical models. However, the potential therapeutic effects of AZD9291 combined with other modalities, including ionizing radiation, are not well understood. The presence of AZD9291 significantly increases the cell-killing effects of radiation in PC-9-IR cells with a secondary EGFR mutation (T790M), which was developed from NSCLC PC-9 cells (human lung adenocarcinoma cell with EGFR 19 exon 15 bp deletion) after chronic exposure to increasing doses of gefitinib, and in H1975 cells (human lung adenocarcinoma cell with EGFR exon 20 T790M mutation de novo), but not in PC-9 cells or in H460 cells (human lung adenocarcinoma cell with wild-type EGFR). In PC-9-IR cells, AZD9291 remarkably decreases phosphorylation levels of EGFR, extracellular regulated protein kinase (ERK), and protein kinase B (AKT). AZD9291 increases sensitivity to radiation in PC-9-IR cells by delaying deoxyribonucleic acid (DNA) damage repair after irradiation and inducing apoptosis, and enhances tumor growth inhibition when combined with radiation in PC-9-IR xenografts. Our findings suggest a potential therapeutic effect of AZD9291 as a radiation sensitizer in lung cancer cells with an acquired EGFR T790M mutation, providing a rationale for a clinical trial using the combination of AZD9291 and radiation in NSCLCs harboring acquired T790M mutation.

Radiation-Induced Fibrosis: Mechanisms and Opportunities to Mitigate. Report of an NCI Workshop, September 19, 2016

A workshop entitled "Radiation-Induced Fibrosis: Mechanisms and Opportunities to Mitigate" (held in Rockville, MD, September 19, 2016) was organized by the Radiation Research Program and Radiation Oncology Branch of the Center for Cancer Research (CCR) of the National Cancer Institute (NCI), to identify critical research areas and directions that will advance the understanding of radiation-induced fibrosis (RIF) and accelerate the development of strategies to mitigate or treat it. Experts in radiation biology, radiation oncology and related fields met to identify and prioritize the key areas for future research and clinical translation. The consensus was that several known and newly identified targets can prevent or mitigate RIF in pre-clinical models. Further, basic and translational research and focused clinical trials are needed to identify optimal agents and strategies for therapeutic use. It was felt that optimally designed preclinical models are needed to better study biomarkers that predict for development of RIF, as well as to understand when effective therapies need to be initiated in relationship to manifestation of injury. Integrating appropriate endpoints and defining efficacy in clinical trials testing treatment of RIF were felt to be critical to demonstrating efficacy. The objective of this meeting report is to (a) highlight the significance of RIF in a global context, (b) summarize recent advances in our understanding of mechanisms of RIF,

Polymorphisms in double strand break repair related genes influence radiosensitivity phenotype in lymphocytes from healthy individuals

BACKGROUND

A range of individual radiosensitivity observed in humans can influence individual's susceptibility toward cancer risk and radiotherapy outcome. Therefore, it is important to measure the variation in radiosensitivity and to identify the genetic factors influencing it.

METHODS

By adopting a pathway specific genotype-phenotype design, we established the variability in cellular radiosensitivity by performing γ-H2AX foci assay in healthy individuals. Further, we genotyped ten selected SNPs in candidate genes XRCC3 (rs861539), XRCC4 (rs1805377), XRCC5 (rs3835), XRCC6 (rs2267437), ATM (rs3218698, rs1800057), LIG4 (rs1805388), NBN (rs1805794), RAD51 (rs1801320) and PRKDC (rs7003908), and analysed their influence on observed variation in radiosensitivity.

RESULTS

The rs2267437 polymorphisms in XRCC6 was associated (P=0.0326) with increased DSB induction while rs1805388 in LIG4 (P=0.0240) was associated with increased radioresistance. Further, multiple risk alleles decreased the DSB repair capacity in an additive manner. Polymorphisms in candidate DSB repair genes can act individually or in combination to the efficacy of DSB repair process, resulting in variation of cellular radiosensitivity.

CONCLUSIONS

Current study suggests that γ-H2AX assay may fulfil the role of a rapid and sensitive biomarker that can be used for epidemiological studies to measure variations in radiosensitivity. DSB repair gene polymorphisms can impact the formation and repair of DSBs.

IMPACT

γ-H2AX foci analysis as well as DSBs repair gene polymorphisms can be used to assess cellular radiosensitivity, which will be useful in population risk assessment, disease prediction, individualization of radiotherapy and also in setting the radiation protection standards.

Afatinib increases sensitivity to radiation in non-small cell lung cancer cells with acquired EGFR T790M mutation

Afatinib is a second-generation of epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor and has shown a significant clinical benefit in non-small cell lung cancer (NSCLC) patients with EGFR-activating mutations. However, the potential therapeutic effects of afatinib combining with other modalities, including ionizing radiation (IR), are not well understood. In this study, we developed a gefitinib-resistant cell subline (PC-9-GR) with a secondary EGFR mutation (T790M) from NSCLC PC-9 cells after chronic exposures to increasing doses of gefitinib. The presence of afatinib significantly increases the cell killing effect of radiation in PC-9-GR cells harboring acquired T790M, but not in H1975 cells with de novo T790M or in H460 cells that express wild-type EGFR. In PC-9-GR cells, afatinib remarkable blocks baseline of EGFR and ERK phosphorylations, and causes delay of IR-induced AKT phosphorylation. Afatinib treatment also leads to increased apoptosis and suppressed DNA damage repair in irradiated PC-9-GR cells, and enhanced tumor growth inhibition when combined with IR in PC-9-GR xenografts. Our findings suggest a potential therapeutic impact of afatinib as a radiation sensitizer in lung cancer cells harboring acquired T790M mutation, providing a rationale for a clinical trial with combination of afatinib and radiation in NSCLCs with EGFR T790M mutation.

Analysis of Residual DSBs in Ataxia-Telangiectasia Lymphoblast Cells Initiating Apoptosis

In order to examine the relationship between accumulation of residual DNA double-strand breaks (DSBs) and cell death, we have used a control and an ATM (Ataxia-Telangiectasia Mutated) defective cell line, as Ataxia-Telangiectasia (AT) cells tend to accumulate residual DSBs at long times after damage infliction. After irradiation, AT cells showed checkpoint impairment and a fraction of cells displayed an abnormal centrosome number and tetraploid DNA content, and this fraction increased along with apoptosis rates. At all times analyzed, AT cells displayed a significantly higher rate of radiation-induced apoptosis than normal cells. Besides apoptosis, 70-85% of the AT viable cells (TUNEL-negative) carried ≥ 10 γH2AX foci/cell, while only 12-27% of normal cells did. The fraction of AT and normal cells undergoing early and late apoptosis were isolated by flow cytometry and residual DSBs were concretely scored in these populations. Half of the γH2AX-positive AT cells undergoing early apoptosis carried ≥ 10 γH2AX foci/cell and this fraction increased to 75% in late apoptosis. The results suggest that retention of DNA damage-induced γH2AX foci is an indicative of lethal DNA damage, as cells undergoing apoptosis are those accumulating more DSBs. Scoring of residual γH2AX foci might function as a predictive tool to assess radiation-induced apoptosis.

The impact of CDK9 on radiosensitivity, DNA damage repair and cell cycling of HNSCC cancer cells

Cyclin-dependent kinase 9 (CDK9), mainly involved in regulation of transcription, has recently been shown to impact on cell cycling and DNA repair. Despite the fact that CDK9 has been proposed as potential cancer target, it remains largely elusive whether CDK9 targeting alters tumor cell radiosensitivity. Five human head and neck squamous cell carcinoma (HNSCC) cell lines (SAS, FaDu, HSC4, Cal33, UTSCC5) as well as SAS cells stably transfected with CDK9-EGFP-N1 plasmid or empty vector controls were used. Upon either CDK9 small interfering RNA knockdown or treatment with a pan-CDK inhibitor (ZK304709), colony formation, DNA double strand breaks (DSBs), apoptosis, cell cycling, and expression and phosphorylation of major cell cycle and DNA damage repair proteins were examined. While CDK9 overexpression mediated radioprotection, CDK9 depletion clearly enhanced the radiosensitivity of HNSCC cells without an induction of apoptosis. While the cell cycle and cell cycle proteins were significantly modulated by CDK9 depletion, no further alterations in these parameters were observed after combined CDK9 knockdown with irradiation. ZK304709 showed concentration-dependent cytotoxicity but failed to radiosensitize HNSCC cells. Our findings suggest a potential role of CDK9 in the radiation response of HNSCC cells. Additional studies are warranted to clarify the usefulness to target CDK9 in the clinic.

The yield of DNA double strand breaks determined after exclusion of those forming from heat-labile lesions predicts tumor cell radiosensitivity to killing

BACKGROUND AND PURPOSE

The radiosensitivity to killing of tumor cells and in-field normal tissue are key determinants of radiotherapy response. In vitro radiosensitivity of tumor- and normal-tissue-derived cells often predicts radiation response, but high determination cost in time and resources compromise utility as routine response-predictor. Efforts to use induction or repair of DNA double-strand-breaks (DSBs) as surrogate-predictors of cell radiosensitivity to killing have met with limited success. Here, we re-visit this issue encouraged by our recent observations that ionizing radiation (IR) induces not only promptly-forming DSBs (prDSBs), but also DSBs developing after irradiation from the conversion to breaks of thermally-labile sugar-lesions (tlDSBs).

MATERIALS AND METHODS

We employ pulsed-field gel-electrophoresis and flow-cytometry protocols to measure total DSBs (tDSB=prDSB+tlDSBs) and prDSBs, as well as γH2AX and parameters of chromatin structure.

RESULTS

We report a fully unexpected and in many ways unprecedented correlation between yield of prDSBs and radiosensitivity to killing in a battery of ten tumor cell lines that is not matched by yields of tDSBs or γH2AX, and cannot be explained by simple parameters of chromatin structure.

CONCLUSIONS

We propose the introduction of prDSBs-yield as a novel and powerful surrogate-predictor of cell radiosensitivity to killing with potential for clinical application.

Misrepair of DNA double-strand breaks in patient with unidentified chromosomal fragility syndrome and family history of radiosensitivity

PURPOSE

To test the hypothesis that differences in DNA double-strand breaks (DSB) repair fidelity underlies differences in radiosensitivity.

MATERIALS AND METHODS

A primary fibroblast culture (C42) derived from a pediatric cancer patient treated with reduced radiation doses consequent to a family history of radiosensitivity reminiscent of chromosomal fragility syndrome, was compared to a normal control (C29). DNA DSB rejoining and repair fidelity were studied by Southern blotting and hybridization to specific fragments: Alu repetitive sequence representing the overall DSB rejoining capacity in the genome and a 3.2 Mbp NotI restriction fragment on chromosome 21 for DSB repair fidelity.

RESULTS

Although both assays showed statistically significant difference (p ≤ 0.05) between the two cell strains in residual misrepaired (un-or mis-rejoined) DSB (24 h after 30 or 80 Gy), the residual damage was lower in the Alu enriched genome assay compared to NotI assay (0.01-0.07 and 0.10-0.37, respectively).

CONCLUSIONS

These results suggest that, in comparison to classic DSB repair experiment, an assay of measuring DNA DSB repair fidelity can provide better resolution and a more accurate estimate of misrepair of radiation-induced DNA damage, which underlies genomic instability and increased radiosensitivity.

Suberoylanilide hydroxamic acid induces hypersensitivity to radiation therapy in acute myelogenous leukemia cells expressing constitutively active FLT3 mutants

Histone deacetylase inhibitors (HDIs) have shown promise as candidate radiosensitizer for many types of cancers. However, the mechanisms of action are not well understood, and whether they could have clinical impact on radiotherapy for leukemia is unclear. In this study, we demonstrate that suberoylanilide hydroxamic acid (SAHA) can increase radiosensitivity of acute myeloid leukemia (AML) cells through posttranslational modification of Rad51 protein responses and selective inhibition of the homology-directed repair (HDR) pathway. Our data also showed that AML cells with mutant, constitutively active FMS-like tyrosine kinase-3 (FLT3) were more radiation sensitive, caused by compromised non-homologous end joining (NHEJ) repair. Furthermore, SAHA-induced radiosensitization were enhanced in AML cells with expression of these FLT3 mutants. The results of this study suggest that SAHA, a recently approved HDI in clinical trials, may act as a candidate component for novel conditioning regimens to improve efficacy for AML patients undergoing radiotherapy and chemotherapy.

Effect of irradiation on the expression of DNA repair genes studied in human fibroblasts by real-time qPCR using three methods of reference gene validation

The aim of this study was to determine the effects of ionizing radiation on gene expression by using for a first time a qPCR platform specifically established for the detection of 94 DNA repair genes but also to test the robustness of these results by using three analytical methods (global pattern recognition, ΔΔCq/Normfinder and ΔΔCq/Genorm). Study was focused on these genes because DNA repair is known primarily to determine the radiation response. Six strains of normal human fibroblasts were exposed to 2 Gy, and changes in gene expression were analyzed 24 h thereafter. A significant change in gene expression was found for only few genes, but the genes detected were mostly different for the three analytical methods used. For GPR, a significant change was found for four genes, in contrast to the eight or nine genes when applying ΔΔCq/Genorm or ΔΔCq/Normfinder, respectively. When using all three methods, a significant change in expression was only seen for GADD45A and PCNA. These data demonstrate that (1) the genes identified to show an altered expression upon irradiation strongly depend on the analytical method applied, and that (2) overall GADD45A and PCNA appear to play a central role in this response, while no significant change is induced for any of the other DNA repair genes tested.

Defining molecular and cellular responses after low and high linear energy transfer radiations to develop biomarkers of carcinogenic risk or therapeutic outcome

The variability in radiosensitivity across the human population is governed in part by genetic factors. The ability to predict therapeutic response, identify individuals at greatest risk for adverse clinical responses after therapeutic radiation doses, or identify individuals at high risk for carcinogenesis from environmental or medical radiation exposures has a medical and economic impact on both the individual and society at large. As radiotherapy incorporates particles, particularly particles larger than protons, into therapy, the need for such discriminators, (i.e., biomarkers) will become ever more important. Cellular assays for survival, DNA repair, or chromatid/chromosomal analysis have been used to identify at-risk individuals, but they are not clinically applicable. Newer approaches, such as genome-wide analysis of gene expression or single nucleotide polymorphisms and small copy number variations within chromosomes, are examples of technologies being applied to the discovery process. Gene expression analysis of primary or immortalized human cells suggests that there are distinct gene expression patterns associated with radiation exposure to both low and high linear energy transfer radiations and that those most radiosensitive are discernible by their basal gene expression patterns. However, because the genetic alterations that drive radio response may be subtle and cumulative, the need for large sample sizes of specific cell or tissue types is required. A systems biology approach will ultimately be necessary. Potential biomarkers from cell lines or animal models will require validation in a human setting where possible and before being considered as a credible biomarker some understanding of the molecular mechanism is necessary.

Suberoylanilide hydroxamic acid as a radiosensitizer through modulation of RAD51 protein and inhibition of homology-directed repair in multiple myeloma

Histone deacetylase inhibitors (HDI) have shown promise as candidate radiosensitizers for many types of cancers. However, the mechanisms of action are not well understood, and whether they could sensitize multiple myeloma (MM) to radiation therapy is unclear. In this study, we show that suberoylanilide hydroxamic acid (SAHA) at low concentrations has minimal cytotoxic effects, yet can significantly increase radiosensitivity of MM cells. SAHA seems to block RAD51 protein response to ionizing radiation, consistent with an inhibitory effect on the formation of RAD51 focus in irradiated MM cells. These effects of SAHA on RAD51 focus are independent of cell-cycle distribution changes. Furthermore, we show that SAHA selectively inhibits the homology-directed repair (HDR) pathway. The results of this study suggest that SAHA, a recently approved HDI in clinical trials for malignancies, at lower concentrations may act as a radiosensitizer via disruption of the RAD51-dependent HDR pathway.

Combined low initial DNA damage and high radiation-induced apoptosis confers clinical resistance to long-term toxicity in breast cancer patients treated with high-dose radiotherapy

Background

Either higher levels of initial DNA damage or lower levels of radiation-induced apoptosis in peripheral blood lymphocytes have been associated to increased risk for develop late radiation-induced toxicity. It has been recently published that these two predictive tests are inversely related. The aim of the present study was to investigate the combined role of both tests in relation to clinical radiation-induced toxicity in a set of breast cancer patients treated with high dose hyperfractionated radical radiotherapy.

Methods

Peripheral blood lymphocytes were taken from 26 consecutive patients with locally advanced breast carcinoma treated with high-dose hyperfractioned radical radiotherapy. Acute and late cutaneous and subcutaneous toxicity was evaluated using the Radiation Therapy Oncology Group morbidity scoring schema. The mean follow-up of survivors (n = 13) was 197.23 months. Radiosensitivity of lymphocytes was quantified as the initial number of DNA double-strand breaks induced per Gy and per DNA unit (200 Mbp). Radiation-induced apoptosis (RIA) at 1, 2 and 8 Gy was measured by flow cytometry using annexin V/propidium iodide.

Results

Mean DSB/Gy/DNA unit obtained was 1.70 ± 0.83 (range 0.63-4.08; median, 1.46). Radiation-induced apoptosis increased with radiation dose (median 12.36, 17.79 and 24.83 for 1, 2, and 8 Gy respectively). We observed that those "expected resistant patients" (DSB values lower than 1.78 DSB/Gy per 200 Mbp and RIA values over 9.58, 14.40 or 24.83 for 1, 2 and 8 Gy respectively) were at low risk of suffer severe subcutaneous late toxicity (HR 0.223, 95%CI 0.073-0.678, P = 0.008; HR 0.206, 95%CI 0.063-0.677, P = 0.009; HR 0.239, 95%CI 0.062-0.929, P = 0.039, for RIA at 1, 2 and 8 Gy respectively) in multivariate analysis.

Conclusions

A radiation-resistant profile is proposed, where those patients who presented lower levels of initial DNA damage and higher levels of radiation induced apoptosis were at low risk of suffer severe subcutaneous late toxicity after clinical treatment at high radiation doses in our series. However, due to the small sample size, other prospective studies with higher number of patients are needed to validate these results.

Inter-individual and inter-cell type variation in residual DNA damage after in vivo irradiation of human skin

PURPOSE

The aim of this study was to compare inter-individual and inter-cell type variation in DNA double-strand break (DSB) repair following in vivo irradiation of human skin.

MATERIALS AND METHODS

Duplicate 4mm core biopsies of irradiated and unirradiated skin were collected from 35 patients 24h after 4Gy exposure using 6MeV electrons. Residual DSB were quantified by scoring 53BP1 foci in dermal fibroblasts, endothelial cells, superficial keratinocytes and basal epidermal cells.

RESULTS

Coefficients of inter-individual variation for levels of residual foci 24h after in vivo irradiation of skin were 39.9% in dermal fibroblasts, 44.3% in endothelial cells, 32.9% in superficial keratinocytes and 46.4% in basal epidermal cells (p<0.001, ANOVA). In contrast, the coefficient of inter-cell type variation for residual foci levels was only 11.3% in human skin between the different epidermal and dermal cells (p=0.034, ANOVA). Foci levels between the different skin cell types were correlated (Pearson's R=0.855-0.955, p<0.001).

CONCLUSIONS

Patient-specific factors appear to be more important than cell type-specific factors in determining residual foci levels following in vivo irradiation of human skin.

Stochastic simulation of DNA double-strand break repair by non-homologous end joining based on track structure calculations

A Monte Carlo simulation model for DNA repair via the non-homologous end-joining pathway has been developed. Initial DNA damage calculated by the Monte Carlo track structure code PARTRAC provides starting conditions concerning spatial distribution of double-strand breaks (DSBs) and characterization of lesion complexity. DNA termini undergo attachment and dissociation of repair enzymes described in stochastic first-order kinetics as well as step-by-step diffusive motion considering nuclear attachment sites. Pairs of DNA termini with attached DNA-PK enter synapsis under spatial proximity conditions. After synapsis, a single rate-limiting step is assumed for clean DNA ends, and step-by-step removal of nearby base lesions and strand breaks is considered for dirty DNA ends. Four simple model scenarios reflecting different hypotheses on the origin of the slow phase of DSB repair have been set up. Parameters for the presynaptic phase have been derived from experimental data for Ku70/Ku80 and DNA-PK association and dissociation kinetics. Time constants for the post-synaptic phase have been adapted to experimental DSB rejoining kinetics for human fibroblasts after (137)Cs gamma irradiation. In addition to DSB rejoining kinetics, the yields of residual DSBs, incorrectly rejoined DSBs, and chromosomal aberrations have been determined as a function of dose and compared with experimental data. Three of the model scenarios obviously overestimate residual DSBs after long-term repair after low-dose irradiation, whereas misrejoined DSBs and chromosomal aberrations are in surprisingly good agreement with measurements.

No correlation between radiosensitivity or double-strand break repair capacity of normal fibroblasts and acute normal tissue reaction after radiotherapy of breast cancer patients

The aim was to study the relationship between cellular radiosensitivity or double-strand break (dsb) repair capacity of skin fibroblasts and the extent of acute reaction after radiotherapy for breast cancer. The study was performed with 25 breast cancer patients submitted to the radiotherapy unit of the Egyptian National Cancer Institute after conserving surgery. Dermal fibroblasts, established from skin biopsies, were used to determine the cellular radiosensitivity via colony assay and the capacity of dsb repair by constant-field gel electrophoresis. Acute reactions were scored using the Radiation Therapy Oncology Group (RTOG) classification. The spectrum of acute reactions varied from grade 1 to 4, whereby most patients developed a grade 1 reaction after total doses ranging between 46 and 70 Gy. Skin fibroblasts showed a pronounced variation in both cellular radiosensitivity expressed as the mean inactivation dose (Dbar) (coefficient of variation, CV=25%) as well as in the number of residual dsb (CV=33%) with no significant correlation between these two endpoints (r2=0.20, p=0.14). Both parameters did not correlate with the extent of acute reaction of the respective patient. The data obtained indicate that the sensitivity of fibroblasts measured either by colony assay or by dsb repair capacity is not a major parameter determining the extent of acute reaction after radiotherapy of breast cancer patients.

NASA Radiation Biomarker Workshop, September 27-28, 2007

A summary is provided of presentations and discussions at the NASA Radiation Biomarker Workshop held September 27-28, 2007 at NASA Ames Research Center in Mountain View, CA. Invited speakers were distinguished scientists representing key sectors of the radiation research community. Speakers addressed recent developments in the biomarker and biotechnology fields that may provide new opportunities for health-related assessment of radiation-exposed individuals, including those exposed during long-duration space travel. Topics discussed included the space radiation environment, biomarkers of radiation sensitivity and individual susceptibility, molecular signatures of low-dose responses, multivariate analysis of gene expression, biomarkers in biodefense, biomarkers in radiation oncology, biomarkers and triage after large-scale radiological incidents, integrated and multiple biomarker approaches, advances in whole-genome tiling arrays, advances in mass spectrometry proteomics, radiation biodosimetry for estimation of cancer risk in a rat skin model, and confounding factors. A summary of conclusions is provided at the end of the report.

Exploring the possibility of unique molecular, biological, and tissue effects with hypofractionated radiotherapy

High-dose hypofractionated radiotherapy is feasible because of technological advances that allow for the precise delivery of radiation to a target volume that contains a limited amount of normal tissue. No longer constrained by the normal tissue response of large fields, very large doses of radiation can be delivered. However, with the application of high doses of radiation to very precise treatment volumes, we find ourselves wondering if the fundamental principles that govern conventional radiotherapy apply. The conventions of tumor hypoxia, reoxygenation, tumor repopulation, and intrinsic radiosensitivity are all likely to be unique or play new roles in tumor or normal tissue response. With that in mind, we highlight several facets of tumor or normal tissue response in which a better understanding of tumor biology and radiation biology could be beneficial to the application of high dose per fraction treatment modalities or in which caution should be exercised.

A defect in DNA double strand break processing in cells from unaffected parents of retinoblastoma patients and other apparently normal humans

Cells from unaffected parents of retinoblastoma (RB) patients were previously shown to be hypersensitive to radiation induced G(1) arrest and cell killing [1]. The hypersensitivity was similar to that reported for cells from ATM heterozygotes. The latter was consistent with a mild DNA DSB rejoining defect which we demonstrated using a gamma-H2AX focus assay after low dose-rate (LDR) irradiation of non-cycling G(0) cells [2,3]. Since neither parent carried the mutant RB allele of the RB heterozygous probands, these results suggested the possibility of an enhanced germline mutation rate, perhaps resulting from some mild defect in genome maintenance. We therefore examined levels of gamma-H2AX foci for cells from these RB parents in this G(0) LDR assay, which reflects the non-homologous end joining (NHEJ) capacity of cells and in a G(2)/M assay, which reflects additional contributions from other G(2)-related damage processing systems. For several of the cell strains parallel radiosensitivity comparisons were made for cell killing and for G(2) chromosomal radiosensitivities. G(0) cells from the RB parents were clearly hypersensitive both in the LDR gamma-H2AX assay, and for cell killing. In addition, cultured fibroblasts from 6 of 15 apparently normal individuals in this study (and one of six in a previous study) were also hypersensitive in the same assays. In the G(2)/M gamma-H2AX assay, the relative sensitivities were similar to those seen in the low dose-rate G(0) assay and tracked with chromosomal radiosensitivity, but some differences were observed.

Genetic determination of chromosomal radiosensitivities in G0- and G2-phase human lymphocytes

BACKGROUND AND PURPOSE

The radiosensitivity of human lymphocytes measured using a G0- or G2-assay has been linked with an individual's risk of developing normal tissue complications following radiotherapy. This study was performed to increase basic knowledge of the genetics of the human radiation response, and chromosomal aberration induction in particular.

MATERIALS AND METHODS

The study was carried out with blood samples taken from 15 monozygotic twin pairs. G0-assay was performed for cells irradiated with 6 Gy counting only deletions and G2-assay for cells irradiated with 0.5 Gy scoring only chromatid breaks.

RESULTS

The mean number of deletions measured at 6 Gy for all 30 samples using the G0-assay amounted to 2.96+/-0.37 (means+/-SD), which corresponds to a coefficient of variation (CV) of 13%. There is a highly significant intra-pair correlation for this number among twins (r(2)=0.911) demonstrating that this parameter is mostly determined by genetic factors. According to the mean number of deletions, a theoretical classification based on the definition < or = MV-SD as resistant, MV+/-SD as normal and > or = MV+SD as sensitive was made, identifying two pairs as sensitive or resistant, respectively, while nine were normal and two pairs are intermediate. For chromatid breaks measured at 0.5 Gy with the G2-assay the mean number was 1.35+/-0.42 (means+/-SD) corresponding to a CV of 31%. There was again a strong intra-pair correlation among twins with r(2)=0.837 showing that this sensitivity is also determined mostly by genetic factors. There was, however, no inter-assay correlation between the G0- and G2-sensitivity (r(2)=0.006) demonstrating that these two sensitivities depend on different genetic factors.

CONCLUSION

The chromosomal radiosensitivity of lymphocytes as defined by G0- or G2-assay is largely determined by different genetic factors, which may allow the use of genetic profiling as an indicator of the respective individual radiosensitivity.

Lymphoblastoid cell lines differing in p53 status show clear differences in basal gene expression with minor changes after irradiation

BACKGROUND AND PURPOSE

The genetic profile as determined by microarray is considered to be an ideal marker of the individual radiosensitivity. However, it is still an open question, whether this profile has to be determined prior to or only after irradiation, since the expression of some genes is affected by irradiation. These changes are induced mainly due to a p53-dependent transactivation.

MATERIALS AND METHODS

In this study gene expression profiles were measured for 3 lymphoblastoid cell lines differing in p53 status (p53 wt: TK6; p53null: TK6E6, p53mut: WTK1) measured either prior to or 3h after exposure to 2Gy. The gene expression profile was determined using the Affymetrix Human HG U133A GeneChip and for selective genes, variation in gene expression was validated by qRT-PCR. In addition, different assays were used to characterize the radioresponse of these three strains.

RESULTS

The three strains were found to be different in all aspects of radiosensitivity studied. Cells with p53wt showed more apoptosis, slightly stronger arrest in G1, but less lethal aberrations and a lower viability when compared to cells with mutated p53, whereas cells absent in p53 are characterized by an intermediate response. The gene expression profile measured prior to irradiation already revealed huge differences. Significance analysis of microarrays (SAM) identified 141 genes that changed expression twofold or more with a false discovery rate (FDR) of 5.4%. When compared to p53null cell line with p53wt showed a twofold difference in up- or down-regulation in 28 genes. A much higher variation was even found when p53mut cells were compared with p53null cells with a twofold difference in even 123 genes. The respective genes were found to be involved mainly in apoptosis, cell cycle regulation, metabolisms and signalling but with only one gene relevant for DNA repair. Radiation was found to affect this profile solely for cells with p53wt with a twofold significant up-regulation in only five genes. For selective genes (BCL2, CASP1, CCND2, DDB2, XPC, RAD51C, SESN1, FUCA1, CDKN1A, MDM2, XPC) array data were confirmed by qRT-PCR.

CONCLUSION

The result, that the gene expression profile of lymphoblastoid cells differing in p53 status already displayed clear differences when measured prior to irradiation with only few changes after irradiation, which are solely seen for p53wt cells, suggests, that the differences in radiosensitivity observed for these cells are primarily determined by the variation in expression profile present already prior to irradiation.

Inter-relation of apoptosis and DNA double-strand breaks in patients with multiple primary cancers

Since the development of multiple primary cancers in an individual is considered an unlikely event, it is suspected that a defect in DNA repair or apoptosis is the underlying cause for some of these patients. Therefore, this study was based on the hypothesis that such patients have increased remaining DNA double-strand breaks (DSBs) and reduced levels of apoptosis after in vitro irradiation. To investigate these mechanisms in cancer patients, 19 with multiple primary cancers were selected out of 25 121 cancer patients. For inclusion in this study, patients had to present with first malignancy at an early age, have a positive family history of cancer and no risk factors. The exclusion criteria were recurrence of cancer or metastasis, haematological tumours and tumours possibly connected to a patient risk factor such as smoking or drinking. Their peripheral blood lymphocytes were tested for proper repair of DNA DSBs and apoptosis after in vitro irradiation. DSBs were measured using constant field gel electrophoresis at 0, 8 and 24 h after irradiation. Apoptotic rates were determined at 24, 48 and 72 h after irradiation using the TUNEL assay. We found that patients' lymphocytes had significantly more initial DNA DSBs compared with controls, but there was no difference in the number of remaining DNA DSBs. Apoptotic rates of lymphocytes were only slightly lower in patients than in controls. These findings show that there are limited differences between patients with multiple cancers and healthy individuals. However, we found a trend towards an inverse correlation between remaining DNA DSBs and apoptotic rates in patients' lymphocytes. This is indicative of DNA DSBs persisting in patients' cells, presumably leading to a higher level of stable chromosomal aberrations that may contribute to tumour formation.

Wound healing morbidity in STS patients treated with preoperative radiotherapy in relation to in vitro skin fibroblast radiosensitivity, proliferative capacity and TGF-β activity

BACKGROUND AND PURPOSE

In a recent study, we demonstrated that the ability of dermal fibroblasts, obtained from soft tissue sarcoma (STS) patients, to undergo initial division in vitro following radiation exposure correlated with the development of wound healing morbidity in the patients following their treatment with preoperative radiotherapy. Transforming growth factor beta (TGF-beta) is thought to play an important role in fibroblast proliferation and radiosensitivity both of which may impact on wound healing. Thus, in this study we examined the interrelationship between TGF-beta activity, radiosensitivity and proliferation of cultured fibroblasts and the wound healing response of STS patients after preoperative radiotherapy to provide a validation cohort for our previous study and to investigate mechanisms.

PATIENTS AND METHODS

Skin fibroblasts were established from skin biopsies of 46 STS patients. The treatment group consisted of 28 patients who received preoperative radiotherapy. Eighteen patients constituted a control group who were either irradiated postoperatively or did not receive radiation treatment. Fibroblast cultures were subjected to the colony forming and cytokinesis-blocked binucleation assays (low dose rate: approximately 0.02 Gy/min) and TGF-beta assays (high dose-rate: approximately 1.06 Gy/min) following gamma-irradiation. Fibroblast radiosensitivity and initial proliferative ability were represented by the surviving fraction at 2.4 Gy (SF(2.4)) and binucleation index (BNI), respectively. Active and total TGF-beta levels in fibroblast cultures were determined using a biological assay. Wound healing complication (WHC), defined as the requirement for further surgery or prolonged deep wound packing, was the clinical endpoint examined.

RESULTS

Of the 28 patients treated with preoperative radiotherapy, 8 (29%) had wound healing difficulties. Fibroblasts from patients who developed WHC showed a trend to retain a significantly higher initial proliferative ability after irradiation compared with those from individuals in the treatment group with normal wound healing, consistent with the results of our previous study. No link was observed between fibroblast radiosensitivity and WHC. Neither active nor total TGF-beta levels in cultures were significantly affected by irradiation. Fibroblast proliferation in unirradiated and irradiated cultures, as well as radiosensitivity, was not influenced by TGF-beta content. TGF-beta expression in fibroblast cultures did not reflect wound healing morbidity.

CONCLUSIONS

These data are consistent with our previous study and combined the results suggest that in vitro fibroblast proliferation after irradiation may be a useful predictor of wound healing morbidity in STS patients treated with preoperative radiotherapy. TGF-beta levels in culture do not predict WHC, suggesting that the role of TGF-beta in wound healing is likely controlled by other in vivo factors.

Detecting Normal Cell Radiosensitivity via Assay of DNA Damage in Lymphocytes for Individualizing Radiotherapy in Head and Neck Cancer Patients

PURPOSE

The purpose of this study was to determine whether the distribution of radiosensitivities in normal tissues of head and neck cancer patients, measured using a DNA damage assay on lymphocytes, is likely to provide sufficient discrimination to enable reliable identification of patients with abnormal sensitivities.

MATERIAL AND METHODS

Radiosensitivity was assessed in 307 lymphocyte samples from unselected head and neck cancer patients and was quantified as the initial number of DNA double-strand breaks (dsb) induced per Gray and per DNA unit (200 Mbp).

RESULTS

The existence of an inter-individual variation in the radiosensitivity parameter is described by the range (0.41--9.38 dsb/Gy/DNA unit) of the values found. We detected 37 patients who developed severe skin reactions during radiotherapy treatment and we compared their radiosensitivity values with the remaining patients treated. Radiosensitivity values of >7.20 dsb/Gy/DNA unit should theoretically correspond to highly radiosensitive patients.

CONCLUSIONS

Our results suggest that initial DNA damage measured on lymphocytes offers an approach to predict the acute response of human normal tissues prior to radiotherapy. .

Radiation sensitivity, H2AX phosphorylation, and kinetics of repair of DNA strand breaks in irradiated cervical cancer cell lines

Six human cervical cancer cell lines [five human papillomavirus (HPV) positive, one HPV negative] for induction and rejoining of DNA strand breaks and for kinetics of formation and loss of serine 139 phosphorylated histone H2AX (gammaH2AX). X-rays induced the same level of DNA breakage for all cell lines. By 8 hours after 20 Gy, <2% of the initial single-strand breaks remained and no double-strand breaks could be detected. In contrast, 24 hours after irradiation, gammaH2AX representing up to 30% of the initial signal still present. SW756 cells showed almost four times higher background levels of gammaH2AX and no residual gammaH2AX compared with the most radiosensitive HPV-negative C33A cells that showed the lowest background and retained 30% of the maximum level of gammaH2AX. Radiation sensitivity, measured as clonogenic-surviving fraction after 2 Gy, was correlated with the fraction of gammaH2AX remaining 24 hours after irradiation. A substantial correlation with gammaH2AX loss half-time measured over the first 4 hours was seen only when cervical cell lines were included in a larger series of p53-deficient cell lines. Interestingly, p53 wild-type cell lines consistently showed faster gammaH2AX loss half-times than p53-deficient cell lines. We conclude that cell line-dependent differences in loss of gammaH2AX after irradiation are related in part to intrinsic radiosensitivity. The possibility that the presence of gammaH2AX foci may not always signify the presence of a physical break, notably in some tumor cell lines, is also supported by these results.

Clonogenic survival and cytokinesis-blocked binucleation of skin fibroblasts and normal tissue complications in soft tissue sarcoma patients treated with preoperative radiotherapy

BACKGROUND AND PURPOSE

To evaluate the clonogenic and cytokinesis-blocked assays in skin fibroblast cultures for their utility as tools for predicting normal tissue responses in soft tissue sarcoma (STS) patients treated with preoperative radiotherapy.

PATIENTS AND METHODS

Dermal fibroblast strains were established from skin biopsies of 26 STS patients who received preoperative radiotherapy. Cultures were subjected to the colony forming and cytokinesis-blocked assays after low (approximately 0.02 Gy/min) dose-rate 60Co -irradiation. Fibroblast radiosensitivity was expressed as the dose for 1% clonogenic survival, D0.01, based on colonies/clusters with >or=10 cells. Fibroblast proliferative capability was represented by binucleation index (BNI) and genomic damage was expressed in terms of micronucleus frequency. Wound healing complications (WHC) and subcutaneous fibrosis were the clinical endpoints examined. The ability of each in vitro parameter to detect patients at high risk of a given normal tissue complication was assessed using receiver operating characteristic (ROC) analysis.

RESULTS

While fibroblasts from patients without WHC were marginally more radiosensitive than fibroblasts from patients with WHC (P=0.08), the reduction in BNI following a dose of 2.4 Gy was significantly higher in strains from patients without WHC compared to those from patients with WHC (P=0.01). The area under the ROC curve (c-index) is indicative of the power of discrimination of D0.01 and BNI for WHC, and was found to be 0.68 and 0.79, respectively. Subcutaneous fibrosis was not associated with D0.01 (rs=0.09, P=0.66) and the percent reduction in BNI after 2.4 Gy (rs=-0.19, P=0.36). Micronucleus frequency did not reflect differences in normal tissue responses.

CONCLUSION

These data suggest that it is the ability of fibroblasts to undergo one-three divisions in vitro following radiation treatment that may reflect the development of wound healing morbidity or subcutaneous fibrosis in this population of patients.

Modeling and computer simulations of tumor growth and tumor response to radiotherapy

A model of tumor growth and tumor response to radiation is introduced in which each tumor cell is taken into account individually. Each cell is assigned a set of radiobiological parameters, and the status of each cell is checked in discrete intervals. Tumor proliferation is governed by the cell cycle times of tumor cells, the growth fraction, the apoptotic capacity of the tumor, and the degree of tumor angiogenesis. The response of tumor cells to radiation is determined by the radiosensitivities and the oxygenation status. Computer simulation is performed on a 3D rigid cubic lattice, starting out from a single tumor cell. Random processes are simulated by Monte Carlo methods. Short cell cycle time, high growth fraction, and tumor angiogenesis all increase tumor proliferation rates. Accelerated time-dose patterns result in lower total doses needed for tumor control, but the extent of dose reduction depends on the kinetics and the radiosensitivities of tumor cells. Tumor angiogenesis alters fully oxygenated and hypoxic fractions within the tumor and subsequently affects the radiation response. It is demonstrated for selected radiobiological parameters that the simulation tools are suitable to quantitatively assess the total doses needed for tumor control. Using the simulation tools, it is feasible to simulate time-dependent effects during fractionated radiotherapy and to compare different time-dose patterns in terms of their tumor control.

For X-irradiated normal human fibroblasts, only half of cell inactivation results from chromosomal damage

PURPOSE

To study the relationship between residual double-strand breaks (dsbs), chromosomal damage, and cell inactivation for X-irradiated normal human fibroblasts.

METHODS AND MATERIALS

The experiments were performed with 12 normal human fibroblast strains and, for comparison, a fibroblast line from a LiFraumeni patient (LFS2800), a squamous cell carcinoma line (FaDu), and CHO cells. Cells were irradiated in plateau phase, which was followed by immediate or delayed (14 h) plating. Chromosomal damage was measured by metaphase technique and loss of proliferative capacity by colony-forming assay. The data obtained were compared with residual double-strand breaks measured previously (Dikomey et al. IJROBP 2000;46:481-490).

RESULTS

For each fibroblast strain, the number of lethal chromosome aberrations (CAs) increased with dose, but with a substantial variation among the strains (coefficient of variation = 20%-26%). The number of lethal aberrations was significantly correlated with the number of residual dsbs measured for the same strain (r(2) = 0.71, p = 0.0006). The residual dsbs were assumed to represent both non- and also mis-rejoined dsbs. There was a significant correlation between lethal aberrations and cell survival, but only for delayed and not immediate plating (r(2) = 0.69, p < 0.0008 vs. r(2) = 0.19, p = 0.16). For delayed plating, the ratio between lethal events (LEs) and CAs amounted to LE:CA = 2.0 +/- 0.05:1, indicating that on average, only half of cell inactivation resulted from chromosomal damage. The other 50% was attributed to the p53-dependent permanent G1 arrest, because cells lacking in functional p53 (LFS2800, FaDu, CHO) showed a ratio of LE:CA = 1.01 +/- 0.02:1.

CONCLUSION

On average, up to 50% of the inactivation of X-irradiated normal human fibroblasts is a result of lethal chromosome aberrations, whereas the rest is due to a p53-dependent process, probably permanent G1 arrest.

Molecular mechanisms of individual radiosensitivity studied in normal diploid human fibroblasts

The molecular mechanisms of individual radiosensitivity were studied in normal diploid human fibroblasts. For fibroblasts irradiated with X-rays in G1-phase the individual radiosensitivity was shown to be correlated with the extent of double-strand break (dsb) repair. The number of residual dsbs (including both non- and mis-rejoined dsbs) varied between 2 and 5% of the initial number induced and was low for resistant and high for sensitive strains. In the G1-phase dsbs are considered to be mostly repaired via the non-homologous end-joining pathway (NHEJ). However, so far none of the parameters tested for this pathway was found to be correlated with the number of residual dsbs. The parameters tested were mRNA expression, protein level and localisation and activity of the DNA-PK, which is the central complex of NHEJ. The dsb-repair capacity is also not regulated by the differentiation status, which varies substantially among fibroblast strains, whereas there is some indication that dsb repair might depend on the chromatin structure, with more efficient repair in cells with condensed DNA. Residual dsbs are converted into lethal chromosome aberrations finally leading to the loss of clonogenic activity, when cells pass through mitosis. Beside this so-called mitotic death, X-irradiated human fibroblasts are also inactivated via the TP53-dependent permanent G1-arrest, while apoptosis appears to be not important. On average, mitotic death and G1-arrest are equally effective, but there is a broad variation from one strain to the other, with a negative correlation between these two pathways. Fibroblast strains exhibiting only a moderate G1-arrest showed a high number of lethal aberrations and vice versa. This result points to a common regulator of both G1-arrest and dsb repair, which is presently under investigation.

Prediction of normal tissue response and individualization of doses in radiotherapy

In this work we estimate the therapeutic gain that could be obtained using a radiotherapy programme in which doses were based on a radiosensitivity test that was able to predict the final response of normal tissues to radiation for each particular patient. To date, no good radiosensitivity assay has been demonstrated and by way of example we use an assay based on initial DNA damage. The individualized programme we propose is based on an increase in the dose delivered to patients showing a resistant behaviour to radiation and on the adoption of alternative programmes or a careful monitoring of those patients in whom an excessive reaction is expected. To quantify the results produced by the individualization programme, both analytical and Monte Carlo simulation methods are used. The increase in tumour control probability obtained by means of this individualization strongly depends on the dose-response curve for the particular tumour. In certain cases, this enhancement can be marked, and 40% of the patients considered in this work could attain more than 10% increase in tumour control probability. The quantitative estimations in this study indicate the need to seek a predictive assay of radiosensitivity, for both normal and tumour tissue response, in order to develop individualized treatment protocols.

Radiosensitivity of human tumour cells is correlated with the induction but not with the repair of DNA double-strand breaks

Nine human tumour cell lines (four mammary, one bladder, two prostate, one cervical, and one squamous cell carcinoma) were studied as to whether cellular radiosensitivity is related to the number of initial or residual double-strand breaks (dsb). Cellular sensitivity was measured by colony assay and dsb by means of constant- and graded-field gel electrophoresis (CFGE and GFGE, respectively). The nine tumour cell lines showed a broad variation in cellular sensitivity (SF2 0.17-0.63). The number of initial dsb as measured by GFGE ranged between 14 and 27 dsb/Gy/diploid DNA content. In contrast, normal fibroblasts raised from skin biopsies of seven individuals showed only a marginal variation with 18-20 dsb/Gy/diploid DNA content. For eight of the nine tumour cell lines, there was a significant correlation between the number of initial dsb and the cellular radiosensitivity. The tumour cells showed a broad variation in the amount of dsb measured 24 h after irradiation by CFGE, which, however, was not correlated with the cellular sensitivity. This residual damage was found to be influenced not only by the actual number of residual dsb, but also by apoptosis and cell cycle progression which had impact on CFGE measurements. Some cell line strains were able to proliferate even after exposure to 150 Gy while others were found to degrade their DNA. Our results suggest that for tumour cells, in contrast to normal cells, the variation in sensitivity is mainly determined by differences in the initial number of dsb induced.

The relationship between cellular radiosensitivity and radiation-induced DNA damage measured by the comet assay

The relationship between deoxyribonucleic acid (DNA) damage and the cell death induced by gamma-irradiation was examined in three kinds of cells, Chinese hamster ovary fibroblast CHO-K1, human melanoma HMV-II and mouse leukemia L5178Y. Cell survival was determined by a clonogenic assay. The induction and rejoining of DNA strand breaks induced by radiation were measured by the alkaline and neutral comet assays. L5178Y cells were the most radiosensitive, while CHO-K1 cells and HMV-II cells were radioresistant. There was an inverse relationship between the survival fraction at 2 Gy (SF2) and the yield of initial DNA strand breaks per unit dose under the alkaline condition for the comet assay, and also a relationship between SF2 and the residual DNA strand breaks (for 4 hr after irradiation) under the neutral condition for the comet assay, the latter being generally considered to be relative to cellular radiosensitivity. In the present analysis, it was considered that the alkaline condition for the comet assay was optimal for evaluating the initial DNA strand breaks, while the neutral condition was optimal for evaluating the residual DNA strand breaks. Since the comet assay is simpler and more rapid than other methods for detecting radiation-induced DNA damage, this assay appears to be a useful predictive assay for evaluating cellular clonogenic radiosensitivity of tumor cells.

Radiation-induced DNA damage and repair in lymphocytes from breast cancer patients and their correlation with acute skin reactions to radiotherapy

PURPOSE

Repair of radiation-induced DNA damage plays a critical role for both the susceptibility of patients to side effects after radiotherapy and their subsequent cancer risk. The study objective was to evaluate whether DNA repair data determined in vitro are correlated with the occurrence of acute side effects during radiotherapy.

METHODS AND MATERIALS

Breast cancer patients receiving radiation therapy after a breast-conserving surgery were recruited in a prospective epidemiologic study. As an indicator for clinical radiosensitivity, adverse reactions of the skin were recorded. Cryo-preserved lymphocytes from 113 study participants were gamma-irradiated with 5 Gy in vitro and analyzed using the alkaline comet assay. Reproducibility of the assay was determined by repeated analysis (n = 26) of cells from a healthy donor. A coefficient of variation of 0.3 was calculated.

RESULTS

The various parameters determined to characterize the individual DNA repair capacity showed large differences between patients. Eleven patients were identified with considerably enhanced DNA damage induction, and 7 patients exhibited severely reduced DNA repair capacity after 15 and 30 min. Six patients were considered as clinically radiosensitive, indicated by moist desquamation of the skin after a total radiation dose of about 50 Gy.

CONCLUSIONS

Using the alkaline comet assay as described here, breast cancer patients were identified showing abnormal cellular radiation effects, but this repair deficiency corresponded only at a very limited extent to the acute radiation sensitivity of the skin. Because impaired DNA repair could be involved in the development of late irradiation effects, individuals exhibiting severely reduced DNA repair capacity should be followed for the development of late clinical symptoms.

Indicators of late normal tissue response after radiotherapy for head and neck cancer: fibroblasts, lymphocytes, genetics, DNA repair, and chromosome aberrations

PURPOSE

To investigate the relationship between late tissue response after radiotherapy, cellular sensitivity and DNA repair capacity measured in dermal fibroblasts and chromosomal aberrations measured in lymphocytes. The study was in particular designed to compare cellular parameters of patients with maximum differences in late tissue reactions.

MATERIALS AND METHODS

The study was performed with 16 pair-wise matched head and neck cancer patients 2-7 years after curative therapy exhibiting maximum differences (grade 1 vs. grade 3) in late normal tissue reactions. Clinical endpoints were fibrosis, telangiectasia, mucositis and xerostomia using the radiation therapy oncology group score. Patients with grade 3 reactions were tested for mutations in ataxia telangiectasia (AT), Nijmegen Breakage Syndrome (NBS), MRE11, RAD50 and DNA ligase IV genes by means of polymerase chain reaction-single-strand conformation polymorphism and sequencing analysis. Skin fibroblasts obtained from biopsies were used to determine the cellular sensitivity by colony formation and the induction and repair of DNA double-strand breaks (dsb) using constant-field gel electrophoresis. Lymphocytes were taken to measure chromosomal damage either in metaphase using conventional chromosome analysis or in G(0) using premature chromosome condensation (PCC)-technique.

RESULTS

Patients with extreme late reactions (grade 3) showed no evidence for an AT, NBS, MRE11 or RAD50 mutation. Studies with fibroblasts revealed that extreme late reactions were associated neither with a pronounced cellular radiosensitivity nor with a difference in dsb repair capacity. In contrast, there was a significant difference in chromosomal damage measured in lymphocytes. After in vitro irradiation with 6Gy, lymphocytes taken from overreacting patients showed on average a significantly higher number of lethal aberrations than lymphocytes isolated from patients with mild reactions (7.2+/-0.8 vs. 5.0+/-0.3). Similar differences were found for PCC fragments.

CONCLUSION

This study suggests that lymphocytes are more promising than fibroblasts to predict patient's normal tissue response after radiotherapy.

Relationship between residual radiation-induced DNA double-strand breaks in cultured fibroblasts and late radiation reactions: a comparison of training and validation cohorts of breast cancer patients

BACKGROUND AND PURPOSE

Using pulsed field gel electrophoresis (PFGE) a significant correlation was demonstrated between residual DNA double-strand breaks (dsbs) and the development of late radiation fibrosis in a group of 39 breast cancer patients studied retrospectively. This group formed a training cohort generating a hypothesis that there is a relationship between residual radiation-induced DNA dsbs in cultured fibroblasts and late radiotherapy reactions in breast cancer patients. The aim of this study was to retest and validate the hypothesis.

MATERIALS AND METHODS

The study was retrospective. Skin biopsies were taken from a validation cohort of 50 breast cancer patients and PFGE was used to examine residual radiation-induced dsbs in cultured fibroblasts. Late morbidity was measured clinically as fibrosis and using the late effects on normal tissues scales that incorporate subjective, objective management and analytic data (LENT SOMA).

RESULTS

PFGE data were obtained for 49 biopsies. In the 49 patients there was no correlation between residual DNA damage and either fibrosis (r=-0.027, P=0.85) or LENT SOMA (r=-0.10, P=0.48) scores. There was no significant relationship between residual damage and fibrosis for the combined training and validation cohorts of 88 patients (r=0.20, P=0.063).

CONCLUSIONS

This study did not validate the hypothesis that there is a relationship between fibroblast residual DNA damage and late morbidity in breast cancer patients. The PFGE assay on fibroblasts is not a suitable test of the degree of late radiation-induced fibrosis in the breast.

Studies on radiosensitivity from an epidemiological point of view - overview of methods and results

BACKGROUND AND PURPOSE

The establishment of a predictive in vitro assay for radiosensitivity has been a goal in radiotherapy research. To date, no single assay has proven to be effective for this purpose. A review of the epidemiologic methods used in the studies has been undertaken to evaluate limitations associated with specific design options and to develop recommendations for future research.

MATERIALS AND METHODS

We focused on studies attempting to establish the usefulness of an assay in breast cancer patients undergoing radiotherapy using skin reactions as indicators for radiosensitivity. The 25 published studies included were evaluated with respect to criteria for good epidemiological studies: (a) study design, (b) study population, (c) assessment of radiation reaction, and (d) treatment of confounding factors.

RESULTS

Limitations in study design were often found among the studies reviewed. Possible sources of bias are, among others, misclassification due to non-standardized assessment of side effects, selection bias due to drawing convenience patient groups instead of representative patient groups, and confounding due to analysis not adjusted for important factors influencing the severity of side effects.

CONCLUSIONS

Further studies should make use of good epidemiological practice so that valid conclusions can be drawn with respect to the usefulness of an in vitro assay to distinguish between patients with different degrees of radiosensitivity in clinical practice.

Heat effects on DNA repair after ionising radiation: hyperthermia commonly increases the number of non-repaired double-strand breaks and structural rearrangements

After ionising radiation double-strand breaks (dsb) are lethal if not repaired or misrepaired. Cell killing is greatly enhanced by hyperthermia and it is questioned here whether heat not only affects dsb repair capacity but also fidelity in a chromosomal context. dsb repair experiments were designed so as to mainly score non-homologous end joining, while homologous recombination was largely precluded. Human male G(0) fibroblasts were either preheated (45 degrees C, 20 min) or not before X-irradiation. dsb induction and repair were measured by conventional gel electrophoresis and an assay combining restriction digestion using a rare cutting enzyme (NotI) and Southern hybridisation, which detects large chromosomal rearrangements (>100 kb). dsb induction rate in an X-chromosomal NotI fragment was 4.8 x 10(-3) dsb/Gy/MB: Similar values were found for the genome overall and also when cells were preheated. After 50 Gy, fibroblasts were competent to largely restore the original restriction fragment size. Five per cent of dsb remained non-rejoined and 14% were misrejoined. Correct restitution of restriction fragments occurred preferably during the first hour but continued at a slow rate for 12-16 h. In addition, dsb appeared to misrejoin throughout the entire repair period. After hyperthermia the fractions of non-rejoined and misrejoined dsb were similarly increased to 13 and 51%, respectively. It is suggested that heat increases the probability of dsb being incorrectly rejoined but it is not likely to interfere with one dsb repair pathway in particular.

[Radiation-induced superficial fibrosis and TGF-alpha 1]

Radiation-induced fibrosis is a late sequela of both therapeutic and accidental irradiations, which has been described in various tissues, including the lung, liver, kidney and skin. This review presents different aspects of superficial radiation-induced fibrosis, such as clinical observations, histological changes, cellular and molecular regulations, and medical management. Recent evidence on the critical role played by TGF-beta 1 in the initiation, development and persistence of fibrosis are discussed, as well as the possibility that this cytokine may constitute a specific target for antifibrotic agents.