The relationship between homologous recombination repair and the sensitivity of human epidermis to the size of daily doses over a 5-week course of breast radiotherapy

From 25 Fractions to Five: How Hypofractionation has Revolutionised Adjuvant Breast Radiotherapy

There is a sound empirical basis for hypofractionation in radiotherapy for breast cancer. This article reviews the radiobiological implications of hypofractionation in breast cancer derived from a series of clinical trials that began when 50 Gy in 25 fractions over 5 weeks was commonplace. These trials led first to 40 Gy in 15 fractions over 3 weeks and, subsequently, to 26 Gy in five fractions over 1 week being adopted as standards of care for many patients prescribed whole- or partial-breast radiotherapy after primary surgery.

How Low Can You Go? The Radiobiology of Hypofractionation

Hypofractionated radical radiotherapy is now an accepted standard of care for tumour sites such as prostate and breast cancer. Much research effort is being directed towards more profoundly hypofractionated (ultrahypofractionated) schedules, with some reaching UK standard of care (e.g. adjuvant breast). Hypofractionation exerts varying influences on each of the major clinical end points of radiotherapy studies: acute toxicity, late toxicity and local control. This review will discuss these effects from the viewpoint of the traditional 5 Rs of radiobiology, before considering non-canonical radiobiological effects that may be relevant to ultrahypofractionated radiotherapy. The principles outlined here may assist the reader in their interpretation of the wealth of clinical data presented in the tumour site-specific articles in this special issue.

TP53 modulates radiotherapy fraction size sensitivity in normal and malignant cells

Recent clinical trials in breast and prostate cancer have established that fewer, larger daily doses (fractions) of radiotherapy are safe and effective, but these do not represent personalised dosing on a patient-by-patient basis. Understanding cell and molecular mechanisms determining fraction size sensitivity is essential to fully exploit this therapeutic variable for patient benefit. The hypothesis under test in this study is that fraction size sensitivity is dependent on the presence of wild-type (WT) p53 and intact non-homologous end-joining (NHEJ). Using single or split-doses of radiation in a range of normal and malignant cells, split-dose recovery was determined using colony-survival assays. Both normal and tumour cells with WT p53 demonstrated significant split-dose recovery, whereas Li-Fraumeni fibroblasts and tumour cells with defective G1/S checkpoint had a large S/G2 component and lost the sparing effect of smaller fractions. There was lack of split-dose recovery in NHEJ-deficient cells and DNA-PKcs inhibitor increased sensitivity to split-doses in glioma cells. Furthermore, siRNA knockdown of p53 in fibroblasts reduced split-dose recovery. In summary, cells defective in p53 are less sensitive to radiotherapy fraction size and lack of split-dose recovery in DNA ligase IV and DNA-PKcs mutant cells suggests the dependence of fraction size sensitivity on intact NHEJ.

Epidermal Keratinocyte Depletion during Five Weeks of Radiotherapy is Associated with DNA Double-Strand Break Foci, Cell Growth Arrest and Apoptosis: Evidence of Increasing Radioresponsiveness and Lack of Repopulation; the Number of Melanocytes Remains Unchanged

During fractionated radiotherapy, epithelial cell populations are thought to decrease initially, followed by accelerated repopulation to compensate cell loss. However, previous findings in skin with daily 1.1 Gy dose fractions indicate continued and increasing cell depletion. Here we investigated epidermal keratinocyte response with daily 2 Gy fractions as well as accelerated and hypofractionation. Epidermal interfollicular melanocytes were also assessed. Skin-punch biopsies were collected from breast cancer patients before, during and after mastectomy radiotherapy to the thoracic wall with daily 2 Gy fractions for 5 weeks. In addition, 2.4 Gy radiotherapy four times per week and 4 Gy fractions twice per week for 5 weeks, and two times 2 Gy daily for 2.5 weeks, were used. Basal keratinocyte density of the interfollicular epidermis was determined and immunostainings of keratinocytes for DNA double-strand break (DSB) foci, growth arrest, apoptosis and mitosis were quantified. In addition, interfollicular melanocytes were counted. Initially minimal keratinocyte loss was observed followed by pronounced depletion during the second half of treatment and full recovery at 2 weeks post treatment. DSB foci per cell peaked towards the end of treatment. p21-stained cell counts increased during radiotherapy, especially the second half. Apoptotic frequency was low throughout radiotherapy but increased at treatment end. Mitotic cell count was significantly suppressed throughout radiotherapy and did not recover during weekend treatment gaps, but increased more than threefold compared to unexposed skin 2 weeks post-radiotherapy. The number of melanocytes remained constant over the study period. Germinal keratinocyte loss rate increased gradually during daily 2 Gy fractions for 5 weeks, and similarly for hypofractionation. DSB foci number after 2 Gy irradiation revealed an initial radioresistance followed by increasing radiosensitivity. Growth arrest mediated by p21 strongly suggests that cells within or recruited into the cell cycle during treatment are at high risk of loss and do not contribute significantly to repopulation. It is possible that quiescent (G₀) cells at treatment completion accounted for the accelerated post-treatment repopulation. Recent knowledge of epidermal tissue regeneration and cell cycle progression during genotoxic and mitogen stress allows for a credible explanation of the current finding. Melanocytes were radioresistant regarding cell depletion.

Simulation analysis for tumor radiotherapy based on three-component mathematical models

OBJECTIVE

To setup a three-component tumor growth mathematical model and discuss its basic application in tumor fractional radiotherapy with computer simulation.

METHOD

First, our three-component tumor growth model extended from the classical Gompertz tumor model was formulated and applied to a fractional radiotherapy with a series of proper parameters. With the computer simulation of our model, the impact of some parameters such as fractional dose, amount of quiescent tumor cells, and α/β value to the effect of radiotherapy was also analyzed, respectively.

RESULTS

With several optimal technologies, the model could run stably and output a series of convergent results. The simulation results showed that the fractional radiotherapy dose could impact the effect of radiotherapy significantly, while the amount of quiescent tumor cells and α/β value did that to a certain extent.

CONCLUSIONS

Supported with some proper parameters, our model can simulate and analyze the tumor radiotherapy program as well as give some theoretical instruction to radiotherapy personalized optimization.

Ki67 Is an Independent Predictor of Recurrence in the Largest Randomized Trial of 3 Radiation Fractionation Schedules in Localized Prostate Cancer

PURPOSE

To assess whether the cellular proliferation marker Ki67 provides prognostic information and predicts response to radiation therapy fractionation in patients with localized prostate tumors participating in a randomized trial of 3 radiation therapy fractionation schedules (74 Gy/37 fractions vs 60 Gy/20 fractions vs 57 Gy/19 fractions).

METHODS AND MATERIALS

A matched case-control study design was used; patients with biochemical/clinical failure >2 years after radiation therapy (BCR) were matched 1:1 to patients without recurrence using established prognostic factors (Gleason score, prostate-specific antigen, tumor stage) and fractionation schedule. Immunohistochemistry was used to stain diagnostic biopsy specimens for Ki67, which were scored using the unweighted global method. Conditional logistic regression models estimated the prognostic value of mean and maximum Ki67 scores on BCR risk. Biomarker-fractionation interaction terms determined whether Ki67 was predictive of BCR by fractionation.

RESULTS

Using 173 matched pairs, the median for mean and maximum Ki67 scores were 6.6% (interquartile range, 3.9%-9.8%) and 11.0% (interquartile range, 7.0%-15.0%) respectively. Both scores were significant predictors of BCR in models adjusted for established prognostic factors. Conditioning on matching variables and age, the odds of BCR were estimated to increase by 9% per 1% increase in mean Ki67 score (odds ratio 1.09; 95% confidence interval 1.04-1.15, P = .001). Interaction terms between Ki67 and fractionation schedules were not statistically significant.

CONCLUSIONS

Diagnostic Ki67 did not predict BCR according to fractionation schedule in CHHiP; however, it was a strong independent prognostic factor for BCR.

Low Rap1-interacting factor 1 and sirtuin 6 expression predict poor outcome in radiotherapy-treated Hodgkin lymphoma patients

Sirtuins (SIRTs) are a family of histone deacetylases, which widely regulate cellular metabolism and are also involved in DNA repair. Rap1-interacting factor 1 (Rif1) and O⁶-alkylguanine DNA alkyltransferase (MGMT) are DNA-repair enzymes, which may potentially be involved in resistance to treatment of classical Hodgkin lymphoma (HL). We assessed the expression levels of (previously unstudied) SIRT1, SIRT4, SIRT6, Rif1, and MGMT immunohistochemically in 85 patients with untreated classical HL. Aberrant distributions of SIRT1, SIRT4, and SIRT6 were detected in Hodgkin neoplastic Reed-Sternberg (RS) cells compared with reactive elements. Low-level expression of both Rif1 and SIRT6 predicted dismal relapse-free survival in radiotherapy-treated patients (multivariate analysis; HR 8.521; 95% CI 1.714-42.358; p = .0088). Expression levels of SIRT1, 4, and 6 were abnormally distributed in RS cells, suggesting a putative role of aberrant acetylation in classical HL carcinogenesis. Rif1 and SIRT6 may also have substantial prognostic and even predictive roles in classical HL.

DNA Repair

Cellular chromosomal DNA is the principal target through which ionising radiation exerts it diverse biological effects. This chapter summarises the relevant DNA damage signalling and repair pathways used by normal and tumour cells in response to irradiation. Strategies for tumour radiosensitisation are reviewed which exploit tumour-specific DNA repair deficiencies or signalling pathway addictions, with a special focus on growth factor signalling, PARP, cancer stem cells, cell cycle checkpoints and DNA replication. This chapter concludes with a discussion of DNA repair-related candidate biomarkers of tumour response which are of crucial importance for implementing precision medicine in radiation oncology.

Dynamic In Vivo Profiling of DNA Damage and Repair after Radiotherapy Using Canine Patients as a Model

Time resolved data of DNA damage and repair after radiotherapy elucidates the relation between damage, repair, and cell survival. While well characterized in vitro, little is known about the time-course of DNA damage response in tumors sampled from individual patients. Kinetics of DNA damage after radiotherapy was assessed in eight dogs using repeated in vivo samples of tumor and co-irradiated normal tissue analyzed with comet assay and phosphorylated H2AX (γH2AX) immunohistochemistry. In vivo results were then compared (in silico) with a dynamic mathematical model for DNA damage formation and repair. Maximum %DNA in tail was observed at 15–60 min after irradiation, with a rapid decrease. Time-courses of γH2AX-foci paralleled these findings with a small time delay and were not influenced by covariates. The evolutionary parameter search based on %DNA in tail revealed a good fit of the DNA repair model to in vivo data for pooled sarcoma time-courses, but fits for individual sarcoma time-courses suffer from the heterogeneous nature of the in vivo data. It was possible to follow dynamics of comet tail intensity and γH2AX-foci during a course of radiation using a minimally invasive approach. DNA repair can be quantitatively investigated as time-courses of individual patients by integrating this resulting data into a dynamic mathematical model.

Upregulation of RAD51 expression is associated with progression of thyroid carcinoma

AIMS

RAD51 participates in homologous recombination repair (HRR) of double-stranded DNA breaks (DSBs) which may cause genomic instability and cancer. The aim of this study was to investigate RAD51 gene expression at transcriptional and translational levels to measure mRNA and protein level and to correlate its relationship with proliferation marker, Ki67 in thyroid cancer patients. This study also explored correlation of these genes with different clinicopathological parameters of the study cohort by Spearman's rank correlation coefficient.

METHODS

Quantitative real time polymerase chain reaction (qRT-PCR) and immunohistochemistry were used to detect mRNA transcript levels and protein expression of RAD51 and Ki67 in 102 cases of thyroid cancer tissues and equal number of uninvolved healthy thyroid tissue controls.

RESULTS

Data showed that expression for both RAD51 and Ki67 was significantly increased in thyroid cancer (p<0.001). High RAD51 and Ki67 expression was associated with later stages, poor tissue differentiation, large tumor size, positive lymph node metastasis and distant metastasis. The correlation analysis demonstrated a strong positive correlation (r=0.461) between RAD51 and Ki67 on mRNA level and on protein level (r=0.866). Strong correlation was observed between clinicopathological characteristics and selected molecules.

CONCLUSION

The present study concluded that upregulation of RAD51 and overexpression of Ki67 may be associated with the progression of thyroid cancer.

Rosiglitazone enhances radiosensitivity by inhibiting repair of DNA damage in cervical cancer cells

Radiation therapy (RT) is one of the main treatment modalities for cervical cancer. Rosiglitazone (ROSI) has been reported to have antiproliferative effects against various types of cancer cells and also to induce antioxidant enzymes that can scavenge reactive oxygen species (ROS) and thereby modify radiosensitivity. Here, we explored the effect of ROSI on radiosensitivity and the underlying mechanisms in cervical cancer cells. Three cervical cancer cell lines (ME-180, HeLa, and SiHa) were used. The cells were pretreated with ROSI and then irradiated. Expression of proteins of interest was detected by western blot and immunofluorescence. Intracellular production of ROS was measured by H₂DCFDA. Radiosensitivity was assessed by monitoring clonogenic survival. Expression of antioxidant enzymes (catalase, superoxide dismutases) was increased by ROSI in HeLa and SiHa cells, but not in ME-180 cells. With ROSI pre-treatment, cell survival after irradiation remained unchanged in HeLa and SiHa cells, but decreased in ME-180 cells. Radiation-induced expression of γ-H2AX was increased and that of RAD51 was decreased by ROSI pre-treatment in ME-180 cells, but not in HeLa cells. ROSI increases radiosensitivity by inhibiting RAD51-mediated repair of DNA damage in some cervical cancer cell lines; therefore, ROSI is a potential inhibitor of RAD51 that can be used to enhance the effect of RT in the treatment of some cervical cancers.

Visualization of complex DNA double-strand breaks in a tumor treated with carbon ion radiotherapy

Carbon ion radiotherapy shows great potential as a cure for X-ray-resistant tumors. Basic research suggests that the strong cell-killing effect induced by carbon ions is based on their ability to cause complex DNA double-strand breaks (DSBs). However, evidence supporting the formation of complex DSBs in actual patients is lacking. Here, we used advanced high-resolution microscopy with deconvolution to show that complex DSBs are formed in a human tumor clinically treated with carbon ion radiotherapy, but not in a tumor treated with X-ray radiotherapy. Furthermore, analysis using a physics model suggested that the complexity of radiotherapy-induced DSBs is related to linear energy transfer, which is much higher for carbon ion beams than for X-rays. Visualization of complex DSBs in clinical specimens will help us to understand the anti-tumor effects of carbon ion radiotherapy.

Genomic and Histopathological Tissue Biomarkers That Predict Radiotherapy Response in Localised Prostate Cancer

Localised prostate cancer, in particular, intermediate risk disease, has varied survival outcomes that cannot be predicted accurately using current clinical risk factors. External beam radiotherapy (EBRT) is one of the standard curative treatment options for localised disease and its efficacy is related to wide ranging aspects of tumour biology. Histopathological techniques including immunohistochemistry and a variety of genomic assays have been used to identify biomarkers of tumour proliferation, cell cycle checkpoints, hypoxia, DNA repair, apoptosis, and androgen synthesis, which predict response to radiotherapy. Global measures of genomic instability also show exciting capacity to predict survival outcomes following EBRT. There is also an urgent clinical need for biomarkers to predict the radiotherapy fraction sensitivity of different prostate tumours and preclinical studies point to possible candidates. Finally, the increased resolution of next generation sequencing (NGS) is likely to enable yet more precise molecular predictions of radiotherapy response and fraction sensitivity.

Where Do We Look for Markers of Radiotherapy Fraction Size Sensitivity?

The response of human normal tissues to radiotherapy fraction size is often described in terms of cellular recovery, but the causal links between cellular and tissue responses to ionising radiation are not necessarily straightforward. This article reviews the evidence for a cellular basis to clinical fractionation sensitivity in normal tissues and discusses the significance of a long-established inverse association between fractionation sensitivity and proliferative indices. Molecular mechanisms of fractionation sensitivity involving DNA damage repair and cell cycle control are proposed that will probably require modification before being applicable to human cancer. The article concludes by discussing the kind of correlative research needed to test for and validate predictive biomarkers of tumour fractionation sensitivity.

DNA damage foci: Meaning and significance

The discovery of DNA damage response proteins such as γH2AX, ATM, 53BP1, RAD51, and the MRE11/RAD50/NBS1 complex, that accumulate and/or are modified in the vicinity of a chromosomal DNA double-strand break to form microscopically visible, subnuclear foci, has revolutionized the detection of these lesions and has enabled studies of the cellular machinery that contributes to their repair. Double-strand breaks are induced directly by a number of physical and chemical agents, including ionizing radiation and radiomimetic drugs, but can also arise as secondary lesions during replication and DNA repair following exposure to a wide range of genotoxins. Here we aim to review the biological meaning and significance of DNA damage foci, looking specifically at a range of different settings in which such markers of DNA damage and repair are being studied and interpreted.

DNA damage response and DNA repair - dog as a model?

Background

Companion animals like dogs frequently develop tumors with age and similarly to human malignancies, display interpatient tumoral heterogeneity. Tumors are frequently characterized with regard to their mutation spectra, changes in gene expression or protein levels. Among others, these changes affect proteins involved in the DNA damage response (DDR), which served as a basis for the development of numerous clinically relevant cancer therapies. Even though the effects of different DNA damaging agents, as well as DDR kinetics, have been well characterized in mammalian cells in vitro, very little is so far known about the kinetics of DDR in tumor and normal tissues in vivo.

Discussion

Due to (i) the similarities between human and canine genomes, (ii) the course of spontaneous tumor development, as well as (iii) common exposure to environmental agents, canine tumors are potentially an excellent model to study DDR in vivo. This is further supported by the fact that dogs show approximately the same rate of tumor development with age as humans. Though similarities between human and dog osteosarcoma, as well as mammary tumors have been well established, only few studies using canine tumor samples addressed the importance of affected DDR pathways in tumor progression, thus leaving many questions unanswered.

Summary

Studies in humans showed that misregulated DDR pathways play an important role during tumor development, as well as in treatment response. Since dogs are proposed to be a good tumor model in many aspects of cancer research, we herein critically investigate the current knowledge of canine DDR and discuss (i) its future potential for studies on the in vivo level, as well as (ii) its possible translation to veterinary and human medicine.

Homologous recombination mediates cellular resistance and fraction size sensitivity to radiation therapy

PURPOSE

Cellular sensitivity to radiotherapy total dose and fraction size is strongly influenced by DNA double strand break (DSB) repair. Here, we investigate response to radiotherapy fraction size using CHO cell lines deficient in specific DNA repair pathways in response to radiation induced DNA double strand breaks (DSB).

EXPERIMENTAL DESIGN

We irradiated CHO cell lines, AA8 (WT), irs1SF (XRCC3-), V3-3 (DNA-PKcs-) and EM9 (XRCC1-) with 16 Gy in 1 Gy daily fractions over 3 weeks or 16 Gy in 4 Gy daily fractions over 4 days, and studied clonogenic survival, DNA DSB repair kinetics (RAD51 and 53BP1 foci staining) and cell cycle profiles (flow cytometry).

RESULTS

In response to fractionated radiotherapy, wild-type and DNA repair defective cells accumulated in late S/G2 phase. In cells proficient in homologous recombination (HR), accumulation in S/G2 resulted in reduced sensitivity to fraction size and increased cellular resistance (clonogenic survival). Sensitivity to fraction size was also lost in NHEJ-defective V3-3 cells, which likely rely on functional HR. By contrast, HR-defective irs1SF cells, with functional NHEJ, remained equally sensitive to fractionation throughout the 3-week treatment.

CONCLUSIONS

The high fidelity of HR, which is independent of induced DNA damage level, is postulated to explain the low fractionation sensitivity and cellular resistance of cells in S/G2 phase. In conclusion, our results suggest that HR mediates resistance to fractionated radiotherapy, an observation that may help future efforts to improve radiotherapy outcome.

Deoxyribonucleic acid damage-associated biomarkers of ionising radiation: current status and future relevance for radiology and radiotherapy

Diagnostic and therapeutic radiation technology has developed dramatically in recent years, and its use has increased significantly, bringing clinical benefit. The use of diagnostic radiology has become widespread in modern society, particularly in paediatrics where the clinical benefit needs to be balanced with the risk of leukaemia and brain cancer increasing after exposure to low doses of radiation. With improving long-term survival rates of radiotherapy patients and the ever-increasing use of diagnostic and interventional radiology procedures, concern has risen over the long-term risks and side effects from such treatments. Biomarker development in radiology and radiotherapy has progressed significantly in recent years to investigate the effects of such use and optimise treatment. Recent biomarker development has focused on improving the limitations of established techniques by the use of automation, increasing sensitivity and developing novel biomarkers capable of quicker results. The effect of low-dose exposure (0-100 mGy) used in radiology, which is increasingly linked to cancer incidences, is being investigated, as some recent research challenges the linear-no-threshold model. Radiotherapy biomarkers are focused on identifying radiosensitive patients, determining the treatment-associated risk and allowing for a tailored and more successful treatment of cancer patients. For biomarkers in any of these areas to be successfully developed, stringent criteria must be applied in techniques and analysis of data to reduce variation among reports and allow data sets to be accurately compared. Newly developed biomarkers can then be used in combination with the established techniques to better understand and quantify the individual biological response to exposures associated with radiology tests and to personalise treatment plans for patients.

New paradigms and future challenges in radiation oncology: an update of biological targets and technology

Radiation oncology exploits the biological interaction of radiation within tissue to promote tumor death while minimizing damage to surrounding normal tissue. The clinical delivery of radiation relies on principles of radiation physics that define how radiation energy is deposited in the body, as well as technology that facilitates accurate tumor targeting. This review will summarize the current landscape of recent biological and technological advances in radiation oncology, describe the challenges that exist, and offer potential avenues for improvement.