Fibroblast radiosensitivity measured using the comet DNA-damage assay correlates with clonogenic survival parameters

Diagnosis of radiosensitive severe combined immunodeficiency disease (RS-SCID) by Comet Assay, management of bone marrow transplantation

BACKGROUND AND OBJECTIVE

Severe combined immunodeficiency disease (SCID) is a rare inherited severe immunodeficiency, in which functions of T cells and B cells are impaired. SCID is inherited either in X-linked recessive, or autosomal recessive forms, and is either radiosensitive or radioresistant. Artemis (DCLRE1C gene), DNA ligase IV, DNA-PKC, and Cernunnos/XLF proteins are regarded as NHEJ (Non-Homologous End-Joining) proteins that are involved in the repair process of double-strand DNA breaks and their mutations would lead to cellular radiosensitivity. Diagnostic radiosensitivity assays are important for the management of clinical BMT (Bone Marrow Transplantation) conditions, such as what conditioning agents and doses should be used.

MATERIALS AND METHODS

In this study, five SCID patients and healthy controls were examined. Skin fibroblasts were cultured. After X-irradiation, cells either underwent clonogenic assay or incubated to allow DNA repair and examined by the alkaline comet assay. Finally, DCLRE1C, RAG-1, and RAG-2 genes sequenced.

RESULTS

By clonogenic assay, three patients were detected as radiosensitive with possible mutations in NHEJ genes such as DCLRE1C gene. The percentage of DNA in the tail measured by comet assay, in all three patients, was significantly different from the two other patients and the control group (p-value < 0.05). By using Sanger sequencing, a mutation in DCLRE1C gene was detected in one of the radiosensitive patients and two mutations in RAG-1, and RAG-2 genes were detected in the two radioresistant patients.

CONCLUSION

Our findings suggest that comet assay is a fast technique for the diagnosis of the radiosensitive form of SCID and is very suitable for the timely diagnosis of RS-SCID before BMT.

Clinical and Functional Assays of Radiosensitivity and Radiation-Induced Second Cancer

Whilst the near instantaneous physical interaction of radiation energy with living cells leaves little opportunity for inter-individual variation in the initial yield of DNA damage, all the downstream processes in how damage is recognized, repaired or resolved and therefore the ultimate fate of cells can vary across the population. In the clinic, this variability is observed most readily as rare extreme sensitivity to radiotherapy with acute and late tissue toxic reactions. Though some radiosensitivity can be anticipated in individuals with known genetic predispositions manifest through recognizable phenotypes and clinical presentations, others exhibit unexpected radiosensitivity which nevertheless has an underlying genetic cause. Currently, functional assays for cellular radiosensitivity represent a strategy to identify patients with potential radiosensitivity before radiotherapy begins, without needing to discover or evaluate the impact of the precise genetic determinants. Yet, some of the genes responsible for extreme radiosensitivity would also be expected to confer susceptibility to radiation-induced cancer, which can be considered another late adverse event associated with radiotherapy. Here, the utility of functional assays of radiosensitivity for identifying individuals susceptible to radiotherapy-induced second cancer is discussed, considering both the common mechanisms and important differences between stochastic radiation carcinogenesis and the range of deterministic acute and late toxic effects of radiotherapy.

XAV939-mediated ARTD activity inhibition in human MB cell lines

Diphtheria toxin-like ADP-ribosyltransferases 1 and 5 (ARTD-1, ARTD-5) are poly ADP-ribose enzymes (PARP) involved in non-homologous end-joining (NHEJ), which is the major pathway of double-strand break (DSB) repair. In addition, ARTD-5, or Tankyrase (TNKS), is a positive regulator of the WNT signaling implicated in the development and biological behavior of many neoplasms, such as Medulloblastoma (MB), in which radiotherapy is an essential part of the treatment. The use of radiosensitizing agents may improve the therapeutic index in MB patients by increasing the efficacy of radiotherapy, while reducing toxicity to the neuroaxis. ARTD-5 seems to be a good molecular target for improving the current treatment of MB. In this study, we used the small molecule XAV939, a potent ARTD-5 inhibitor with a slight affinity for ARTD-1, in different human MB cell lines. XAV939 inhibited the WNT pathway and DNA-PKcs in our MB cells, with many biological consequences. The co-administration of XAV939 and ionizing radiations (IR) inhibited MB cells proliferation and clonogenic capacity, decreased their efficacy in repairing DNA damage, and increased IR-induced cell mortality. In conclusion, our in vitro data show that XAV939 could be a very promising small molecule in MB treatment, and these results lay the basis for further in vivo studies with the aim of improving the current therapy available for MB patients.

Agarose overlay selectively improves macrocolony formation and radiosensitivity assessment in primary fibroblasts

PURPOSE

Primary fibroblasts are not suitable for in vitro macrocolony assay due to their inability to form distinct colonies. Here we present a modification of agarose overlay that yielded extensive improvement in their colony formation and assessment of radiosensitivity.

MATERIALS AND METHODS

Macrocolony formation was assessed in primary human fibroblasts VH10 and HDFn with or without overlay using 0.5% agarose in growth medium at 24 h post-seeding. Malignant human cell lines (A549, U87) and transformed non-malignant fibroblasts (AA8 hamster, MRC5 human) were used for comparison.

RESULTS

Agarose overlay caused significant improvement marked by early appearance (one week) of distinct colonies with high cell density and multifold higher plating efficiency than conventional macrocolony assay in VH10 and HDFn human fibroblasts. Compared to conventional assay or feeder cell supplementation, agarose overlay resulted in broader cell morphology due to improved adherence, and yielded more compact colonies. Gamma-radiation dose-response survival curves could be successfully generated for both fibroblast cell lines using this method, which yielded no such effects in the transformed/malignant cell lines tested.

CONCLUSION

This easy and inexpensive 'agarose overlay technique' significantly and selectively improves the fibroblast plating efficiency, thus considerably reducing time and effort to greatly benefit the survival studies on primary fibroblasts.

Use of the comet-FISH assay to compare DNA damage and repair in p53 and hTERT genes following ionizing radiation

The alkaline single cell gel electrophoresis (comet) assay can be combined with fluorescent in situ hybridisation (FISH) methodology in order to investigate the localisation of specific gene domains within an individual cell. The number and position of the fluorescent signal(s) provides information about the relative damage and subsequent repair that is occurring in the targeted gene domain(s). In this study, we have optimised the comet-FISH assay to detect and compare DNA damage and repair in the p53 and hTERT gene regions of bladder cancer cell-lines RT4 and RT112, normal fibroblasts and Cockayne Syndrome (CS) fibroblasts following γ-radiation. Cells were exposed to 5Gy γ-radiation and repair followed for up to 60 minutes. At each repair time-point, the number and location of p53 and hTERT hybridisation spots was recorded in addition to standard comet measurements. In bladder cancer cell-lines and normal fibroblasts, the p53 gene region was found to be rapidly repaired relative to the hTERT gene region and the overall genome, a phenomenon that appeared to be independent of hTERT transcriptional activity. However, in the CS fibroblasts, which are defective in transcription coupled repair (TCR), this rapid repair of the p53 gene region was not observed when compared to both the hTERT gene region and the overall genome, proving the assay can detect variations in DNA repair in the same gene. In conclusion, we propose that the comet-FISH assay is a sensitive and rapid method for detecting differences in DNA damage and repair between different gene regions in individual cells in response to radiation. We suggest this increases its potential for measuring radiosensitivity in cells and may therefore have value in a clinical setting.

Radiosensitization of mammary carcinoma cells by telomere homolog oligonucleotide pretreatment

Introduction

Ionizing radiation (IR) is a widely used approach to cancer therapy, ranking second only to surgery in rate of utilization. Responses of cancer patients to radiotherapy depend in part on the intrinsic radiosensitivity of the tumor cells. Thus, promoting tumor cell sensitivity to IR could significantly enhance the treatment outcome and quality of life for patients.

Methods

Mammary tumor cells were treated by a 16-base phosphodiester-linked oligonucleotide homologous to the telomere G-rich sequence TTAGGG (T-oligo: GGTTAGGTGTAGGTTT) or a control-oligo (the partial complement, TAACCCTAACCCTAAC) followed by IR. The inhibition of tumor cell growth in vitro was assessed by cell counting and clonogenic cell survival assay. The tumorigenesis of tumor cells after various treatments was measured by tumor growth in mice. The mechanism underlying the radiosensitization by T-oligo was explored by immunofluorescent determination of phosphorylated histone H2AX (γH2AX) foci, β-galactosidase staining, comet and Terminal deoxynucleotidyl transferase dUTP Nick End Labeling (TUNEL) assays. The efficacy of the combined treatment was assessed in a spontaneous murine mammary tumor model.

Results

Pretreatment of tumor cells with T-oligo for 24 hours in vitro enhanced both senescence and apoptosis of irradiated tumor cells and reduced clonogenic potential. Radiosensitization by T-oligo was associated with increased formation and/or delayed resolution of γH2AX DNA damage foci and fragmented DNA. T-oligo also caused radiosensitization in two in vivo mammary tumor models. Indeed, combined T-oligo and IR-treatment in vivo led to a substantial reduction in tumor growth. Of further significance, treatment with T-oligo and IR led to synergistic inhibition of the growth of spontaneous mammary carcinomas. Despite these profound antitumor properties, T-oligo and IR caused no detectable side effects under our experimental conditions.

Conclusions

Pretreatment with T-oligo sensitizes mammary tumor cells to radiation in both in vitro and in vivo settings with minimal or no normal tissue side effects.

Calculated DNA Damage from Gadolinium Auger Electrons and Relation to Dose Distributions in a Head Phantom

PURPOSE

To calculate the number of 157Gadolinium (157Gd) neutron capture induced DNA double strand breaks (DSB) in tumor cells resulting from epithermal neutron irradiation of a human head when the peak tissue dose is 10 Gy. To assess the lethality of these Gd induced DSB. MATRIALS AND METHODS: DNA single and double strand breaks from Auger electrons emitted during 157Gd(n,gamma) events were calculated using an atomistic model of B-DNA with higher-order structure. When combined with gadolinium neutron capture reaction rates and neutron and photon physical dose rates calculated from the radiation transport through a model of the human head with explicit tumors, peak tissue dose can be related to the number of Auger electron induced DSB in tumor cell DNA. The lethality of these DNA DSB were assessed through a comparison with incorporated 125I decay cell survival curves and second comparison with the number of DSB resulting from neutron and photon interactions.

RESULTS

These calculations on a molecular scale (microscopic calculations) indicate that for incorporated 157Gd, each neutron capture reaction results in an average of 1.56 +/- 0.16 DNA single strand breaks (SSB) and 0.21 +/- 0.04 DBS in the immediate vicinity (approximately 40 nm) of the neutron capture. In an example case of Gd Neutron Capture Therapy (GdNCT), a 1 cm radius midline tumor, peak normal tissue dose of 10 Gy, and a tumor concentration of 1000 ppm Gd, result in a maximum of 140 +/- 27 DSBs per tumor cell.

CONCLUSIONS

The number of DSB from the background radiation components is one order of magnitude lower than the Gd Auger electron induced DSB. The cell survival of mammalian cell lines with a similar amount of complex DSB induced from incorporated 125I decay yield one to two magnitudes of cell killing. These two points indicate that gadolinium auger electrons could significantly contribute to cell killing in GdNCT.

Radiosensitization effect of zidovudine on human malignant glioma cells

Telomeres are shortened with each cell division and play an important role in maintaining chromosomal integrity and function. Telomerase, responsible for telomere synthesis, is activated in 90% of human tumor cells but seldom in normal somatic cells. Zidovudine (AZT) is a reverse transcriptase inhibitor. In this study, we have investigated the effects of gamma-radiation in combination with AZT on telomerase activity (TA), telomere length, DNA single-strand breaks (SSBs), DNA double-strand breaks (DSBs), and the changes in radiosensitivity of human malignant glioma cell line U251. The results showed that the TA was suppressed by AZT but enhanced by irradiation, resulting in a deceleration of restored rate of shortened telomere, decreased repair rate of DNA strand breaks, and increased radiosensitivity of U251 cells. Our results suggested that telomerase activity and telomere length may serve as markers for estimating the efficacy of cancer radiotherapy and reverse transcriptase inhibitors, such as AZT, may be used clinically as a new radiosensitizer in cancer radiotherapy.

Prediction of cellular radiosensitivity from DNA damage induced by gamma-rays and carbon ion irradiation in canine tumor cells

Diseases of companion animals are shifting from infectious diseases to neoplasms (cancer), and since radiation therapy is one of the effective choices available for cancer treatment, the application of radiotherapy in veterinary medicine is likely to increase. However tumor tissues have different radiosensitivities, and therefore it is important to determine the intrinsic radiosensitivity of tumors in individual patients in advance of radiotherapy. We have studied the relationship between the surviving cell fraction measured by a clonogenic assay and DNA double strand breaks detected by a comet assay under neutral conditions in three canine tumor cell lines, after gamma-ray and carbon ion irradiation. In all the cell lines, cell death assessed by the clonogenic assay was much higher following irradiation with carbon ions than with gamma-rays. The initial and residual (4 hr) DNA damage due to gamma-ray and carbon ion irradiation were higher in a radiosensitive cell line than in a radioresistant cell line. The surviving cell fraction at 2 Gy (SF2) showed a tendency for correlation with both the initial and residual DNA damage. In particular, the residual damage per Gy was significantly correlated with SF2, regardless of the type of radiation. This indicates that cellular radiosensitivity can be predicted by detection of radiation-induced residual DNA damage.

Comparison of clonogenic assay with premature chromosome condensation assay in prediction of human cell radiosensitivity

AIM: To determine whether the number of non-rejoining G2-chromatid breaks can predict the radiosensitivity of human cell lines.

METHODS: Cell lines of human ovary carcinoma cells (HO8910), human hepatoma cells (HepG2) and liver cells (L02) were irradiated with a range of doses and assessed both of cell survival and non-rejoining G2-chromatid breaks at 24 h after irradiation. Cell survival was documented by a colony assay. Non-rejoining G2-chromatid breaks were measured by counting the number of non-rejoining G2 chromatid breaks at 24 h after irradiation, detected by the prematurely chromosome condensed (PCC) technique.

RESULTS: A linear-quadratic survival curve was observed in three cell lines, and HepG2 was the most sensitive to γ-radiation. A dose-dependent linear increase was observed in radiation-induced non-rejoining G2-PCC breaks measured at 24 h after irradiation in all cell lines, and HepG2 was the most susceptible to induction of non-rejoining G2-PCC breaks. A close correlation was found between the clonogenic radiosensitivity and the radiation-induced non-rejoining G2-PCC breaks (r = 0.923). Furthermore, survival-aberration correlations for two or more than two doses lever were also significant.

CONCLUSION: The number of non-rejoining G2 PCC breaks holds considerable promise for predicting the radiosensitivity of normal and tumor cells when two or more than two doses lever is tested.

Measurements using the alkaline comet assay predict bladder cancer cell radiosensitivity

In the UK, the two main treatments of invasive bladder cancer are radiotherapy or cystectomy. However, ∼50% of patients undergoing radiotherapy fail to respond. If tumour radiosensitivity could be predicted in advance, it may be possible to improve control rates significantly by selecting for radiotherapy those patients whose tumours are radiosensitive. Additionally, patients who would benefit from surgery would be identified earlier. The alkaline comet assay (ACA) is a sensitive method for the detection of DNA strand break damage in cells. In the present study, using six bladder cancer cell lines of differing radiosensitivities, cell survival was compared to the manifestation of radiogenic DNA damage as assessed by ACA. For all the cell lines, the extent of comet formation strongly correlates with cell killing (R²>0.96), with a greater response being noted in radiosensitive cells. In repair studies, measures of residual damage correlate with survival fraction at 2 Gy (R²>0.96), but for only five of the cell lines. Finally, cells from human bladder tumour biopsies reveal a wide range of predicted radiosensitivies as determined by ACA. Overall, these studies demonstrate ACA to be a good predictive measure of bladder cancer cell radiosensitivity at low dose, with potential clinical application.

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.

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.

Collagen secretion after photodynamic therapy versus scar-inducing anti-cancer modalities: an in vitro study

Photodynamic therapy (PDT) has been associated anecdotally with good quality healing and an absence of scar formation. Our previous studies, examining the levels of the collagen specific molecular chaperone Hsp47, have noted differences in the response after photodynamic therapy and hyperthermia at both the transcriptional and translational levels. In the present study the levels of Hsp47 after exposure to two chemotherapeutic agents (bleomycin and mitomycin). ionising radiation, hyperthermia and haematoporphyrin ester (HpE) mediated PDT were compared in both mouse and human fibroblast cell lines. A rapid assay for soluble collagen has also been used to quantify soluble collagen levels at early time points after treatment. Peak Hsp47 levels were found to correlate well with peak collagen levels. The results show that the levels of collagen measured in vitro are elevated in modalities associated with scarring in vivo but not after HpE-PDT.

Does radiotherapy affect the outcome of the comet assay?

This study was designed to assess possible effects of fractionated radiotherapy (5 or 10 fractions at 2 Gy per fraction) on the DNA repair capacity of lymphocytes, as measured by the comet assay. 50 patients with various tumour types were chosen. They had received no chemotherapy during the 6 months prior to radiotherapy and did not receive cortisone. 10 ml of heparinized blood was collected before radiotherapy, after 5 fractions and after 10 fractions. Lymphocytes were isolated and analysed using the comet assay. On average, no effect on DNA repair capacity was observed that could be attributed to radiotherapy. On an individual basis, there were a few patients who showed a comparatively pronounced variability in their response to radiotherapy (three patients with a relative coefficient of variability of more than 30%). There was some indication of a weak correlation between poor repair capacity and severe side effects in normal tissue. We also found that alcohol in particular, and smoking to some extent, may impair repair capacity during radiotherapy. Age, gender, field size, medication and tumour entity showed no effect on repair capacity.

The use of DNA double-strand break quantification in radiotherapy

DNA double-strand breaks (DSB) are an important direct consequence of treating cells with ionizingradiation. A variety of evidence points toward DSBs being the key damage type linked to radiation-induced lethality. In particular, the link between DSB and chromosome breakage, which in turn closely correlates with cell death in some cell types, is strongly supportive of this concept. There has been much interest in the possibility of using measures of strand breaks as a pretreatment test of radiation response. This has largely been in the context of assessing inherent cellular sensitivity through damage induction or repair parameters. A number of studies have produced hopeful results, but overall there has been no parameter that can reliably predict radiosensitivity. This may be due to the inadequacies of the assays, but it is more likely to reflect the fact that the radiosensitivity of cells is dictated by a whole series of events; alterations in many of these can alter the overall response. In addition, it is now recognized that cell-signalling pathways form an essential part of the cellular response to damage, and these can be triggered by damage other than DSB. It is therefore possible that while DSBs are clearly important--and they may be the single most important lesion in some types--other damage types may be significant triggers of cell death pathways after ionizing radiation treatment.