Investigation of switch from ATM to ATR signaling at the sites of DNA damage induced by low and high LET radiation

YTHDC1 delays cellular senescence and pulmonary fibrosis by activating ATR in an m6A-independent manner

Accumulation of DNA damage in the lung induces cellular senescence and promotes age-related diseases such as idiopathic pulmonary fibrosis (IPF). Hence, understanding the mechanistic regulation of DNA damage repair is important for anti-aging therapies and disease control. Here, we identified an m6A-independent role of the RNA-binding protein YTHDC1 in counteracting stress-induced pulmonary senescence and fibrosis. YTHDC1 is primarily expressed in pulmonary alveolar epithelial type 2 (AECII) cells and its AECII expression is significantly decreased in AECIIs during fibrosis. Exogenous overexpression of YTHDC1 alleviates pulmonary senescence and fibrosis independent of its m6A-binding ability, while YTHDC1 deletion enhances disease progression in mice. Mechanistically, YTHDC1 promotes the interaction between TopBP1 and MRE11, thereby activating ATR and facilitating DNA damage repair. These findings reveal a noncanonical function of YTHDC1 in delaying cellular senescence, and suggest that enhancing YTHDC1 expression in the lung could be an effective treatment strategy for pulmonary fibrosis.

Multifaceted Influence of Histone Deacetylases on DNA Damage Repair: Implications for Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is one of the most commonly diagnosed cancers and a leading cause of cancer-related mortality worldwide, but its pathogenesis remains largely unknown. Nevertheless, genomic instability has been recognized as one of the facilitating characteristics of cancer hallmarks that expedites the acquisition of genetic diversity. Genomic instability is associated with a greater tendency to accumulate DNA damage and tumor-specific DNA repair defects, which gives rise to gene mutations and chromosomal damage and causes oncogenic transformation and tumor progression. Histone deacetylases (HDACs) have been shown to impair a variety of cellular processes of genome stability, including the regulation of DNA damage and repair, reactive oxygen species generation and elimination, and progression to mitosis. In this review, we provide an overview of the role of HDAC in the different aspects of DNA repair and genome instability in HCC as well as the current progress on the development of HDAC-specific inhibitors as new cancer therapies.

DNA Damage Response and Repair in Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) is an approach to the radiotherapy of solid tumors that was first outlined in the 1930s but has attracted considerable attention recently with the advent of a new generation of neutron sources. In BNCT, tumor cells accumulate 10B atoms that react with epithermal neutrons, producing energetic α particles and 7Li atoms that damage the cell's genome. The damage inflicted by BNCT appears not to be easily repairable and is thus lethal for the cell; however, the molecular events underlying the action of BNCT remain largely unaddressed. In this review, the chemistry of DNA damage during BNCT is outlined, the major mechanisms of DNA break sensing and repair are summarized, and the specifics of the repair of BNCT-induced DNA lesions are discussed.

Does particle radiation have superior radiobiological advantages for prostate cancer cells? A systematic review of in vitro studies

BACKGROUND

Charged particle beams from protons to carbon ions provide many significant physical benefits in radiation therapy. However, preclinical studies of charged particle therapy for prostate cancer are extremely limited. The aim of this study was to comprehensively investigate the biological effects of charged particles on prostate cancer from the perspective of in vitro studies.

METHODS

We conducted a systematic review by searching EMBASE (OVID), Medline (OVID), and Web of Science databases to identify the publications assessing the radiobiological effects of charged particle irradiation on prostate cancer cells. The data of relative biological effectiveness (RBE), surviving fraction (SF), standard enhancement ratio (SER) and oxygen enhancement ratio (OER) were extracted.

RESULTS

We found 12 studies met the eligible criteria. The relative biological effectiveness values of proton and carbon ion irradiation ranged from 0.94 to 1.52, and 1.67 to 3.7, respectively. Surviving fraction of 2 Gy were 0.17 ± 0.12, 0.55 ± 0.20 and 0.53 ± 0.16 in carbon ion, proton, and photon irradiation, respectively. PNKP inhibitor and gold nanoparticles were favorable sensitizing agents, while it was presented poorer performance in GANT61. The oxygen enhancement ratio values of photon and carbon ion irradiation were 2.32 ± 0.04, and 1.77 ± 0.13, respectively. Charged particle irradiation induced more G0-/G1- or G2-/M-phase arrest, more expression of γ-H2AX, more apoptosis, and lower motility and/or migration ability than photon irradiation.

CONCLUSIONS

Both carbon ion and proton irradiation have advantages over photon irradiation in radiobiological effects on prostate cancer cell lines. Carbon ion irradiation seems to have further advantages over proton irradiation.

Clinical Trial in a Dish for Space Radiation Countermeasure Discovery

NASA aims to return humans to the moon within the next five years and to land humans on Mars in a few decades. Space radiation exposure represents a major challenge to astronauts' health during long-duration missions, as it is linked to increased risks of cancer, cardiovascular dysfunctions, central nervous system (CNS) impairment, and other negative outcomes. Characterization of radiation health effects and developing corresponding countermeasures are high priorities for the preparation of long duration space travel. Due to limitations of animal and cell models, the development of novel physiologically relevant radiation models is needed to better predict these individual risks and bridge gaps between preclinical testing and clinical trials in drug development. "Clinical Trial in a Dish" (CTiD) is now possible with the use of human induced pluripotent stem cells (hiPSCs), offering a powerful tool for drug safety or efficacy testing using patient-specific cell models. Here we review the development and applications of CTiD for space radiation biology and countermeasure studies, focusing on progress made in the past decade.

Aberrantly reduced expression of miR-342-5p contributes to CCND1-associated chronic myeloid leukemia progression and imatinib resistance

Chronic myeloid leukemia (CML) is a myeloproliferative disorder associated with the Philadelphia chromosome, and the current standard of care is the use of tyrosine kinase inhibitors (TKI). However, some patients will not achieve a molecular response and may progress to blast crisis, and the underlying mechanisms remain to be clarified. In this study, next-generation sequencing was used to explore endogenous miRNAs in CML patients versus healthy volunteers, and miR-342-5p was identified as the primary target. We found that miR-342-5p was downregulated in CML patients and had a significant inhibitory effect on cell proliferation in CML. Through a luciferase reporter system, miR-342-5p was reported to target the 3'-UTR domain of CCND1 and downregulated its expression. Furthermore, overexpression of miR-342-5p enhanced imatinib-induced DNA double-strand breaks and apoptosis. Finally, by analyzing clinical databases, we further confirmed that miR-342-5p was associated with predicted molecular responses in CML patients. In conclusion, we found that both in vivo and in vitro experiments and database cohorts showed that miR-342-5p plays a key role in CML patients, indicating that miR-342-5p may be a potential target for future CML treatment or prognostic evaluation.

Phase I Study of Ceralasertib (AZD6738), a Novel DNA Damage Repair Agent, in Combination with Weekly Paclitaxel in Refractory Cancer

PURPOSE

Ceralasertib is a potent and selective oral inhibitor of the serine/threonine protein kinase ataxia telangiectasia and Rad3-related (ATR) protein.

PATIENTS AND METHODS

Eligible patients with solid tumors, enriched for melanoma, received ceralasertib in combination with a fixed dose of paclitaxel (80 mg/m² on D1, D8, D15) in 28-day cycles. The dose of ceralasertib was escalated to reach an MTD in a rolling 6 design. The starting dose of ceralasertib was 40 mg QD. Fifty-seven patients (33 patients with melanoma who failed prior PD1/L1 treatment) were enrolled in 7 dose cohorts ranging from 40 mg QD to 240 mg BD plus weekly paclitaxel.

RESULTS

The RP2D was established as ceralasertib 240 mg BD days 1-14 plus paclitaxel 80 mg/m² on D1, D8, D15 every 28 days. The most common toxicities were neutropenia (n = 39, 68%), anemia (n = 25, 44%), and thrombocytopenia (n = 21, 37%). In the full analysis set of 57 patients, the overall response rate (ORR) was 22.6% (95% CI, 12.5-35.3). In 33 patients with melanoma, resistant to prior anti-PD1 therapy, the ORR was 33.3% (95% CI, 18.0-51.8). In the melanoma subset, the mPFS was 3.6 months (95% CI, 2.0-5.8), the median duration of response was 9.9 months (95% CI, 3.7-23.2), and the mOS was 7.4 months (95% CI, 5.7-11.9).

CONCLUSIONS

Ceralasertib in combination with paclitaxel was well tolerated in patients with advanced malignancies and showed evidence of antitumor activity. Durable responses were observed in patients with advanced cutaneous, acral, and mucosal melanoma resistant to anti-PD1/L1 treatment.See related commentary by Ashworth, p. 4667.

The antiproliferative effects of ataxia-telangiectasia mutated and ATM- and Rad3-related inhibitions and their enhancements with the cytotoxicity of DNA damaging agents in cholangiocarcinoma cells

OBJECTIVE

To investigate whether the inhibitions of ataxia-telangiectasia mutated (ATM) and ATM- and Rad3-related (ATR) kinases by their specific inhibitors, KU-55933 and VE-821, respectively, are able to promote the cytotoxic activity of genotoxic agents including gemcitabine, 5-Fluorouracil, cisplatin and doxorubicin, in cholangiocarcinoma (CCA) and immortalized cholangiocyte cell lines.

METHODS

Cell viability of cells treated with DNA damaging agents, alone and in combination with KU-55933 and VE-821, was determined by MTT assay. The changes of cell cycle distribution were evaluated by flow cytometry analysis. Colony formation was conducted to assess the effects of KU-55933 and VE-821 on cell proliferation. The levels of protein expression and phosphorylation were examined by western blot analysis.

KEY FINDINGS

The cytotoxic effects of DNA damaging agents varied among CCA cell lines. Each DNA damaging drug induced different phases of the cell cycle in CCA cells. The combinations of both KU-55933 and VE-821 with DNA damaging agents promoted more cytotoxic activity than single inhibition in some CCA cell lines. ATM and ATR inhibitors decreased the effects of DNA damaging agent-induced ATM-Chk2 and ATR-Chk1 activations in CCA cells.

CONCLUSIONS

Inhibitions of ATM and ATR potentiated the cytotoxic effects of DNA damaging agents in CCA cells, especially p53 defective HuCCA1 and RMCC1 cell lines.

The Determinant of DNA Repair Pathway Choices in Ionising Radiation-Induced DNA Double-Strand Breaks

Ionising radiation- (IR-) induced DNA double-strand breaks (DSBs) are considered to be the deleterious DNA lesions that pose a serious threat to genomic stability. The major DNA repair pathways, including classical nonhomologous end joining, homologous recombination, single-strand annealing, and alternative end joining, play critical roles in countering and eliciting IR-induced DSBs to ensure genome integrity. If the IR-induced DNA DSBs are not repaired correctly, the residual or incorrectly repaired DSBs can result in genomic instability that is associated with certain human diseases. Although many efforts have been made in investigating the major mechanisms of IR-induced DNA DSB repair, it is still unclear what determines the choices of IR-induced DNA DSB repair pathways. In this review, we discuss how the mechanisms of IR-induced DSB repair pathway choices can operate in irradiated cells. We first briefly describe the main mechanisms of the major DNA DSB repair pathways and the related key repair proteins. Based on our understanding of the characteristics of IR-induced DNA DSBs and the regulatory mechanisms of DSB repair pathways in irradiated cells and recent advances in this field, We then highlight the main factors and associated challenges to determine the IR-induced DSB repair pathway choices. We conclude that the type and distribution of IR-induced DSBs, chromatin state, DNA-end structure, and DNA-end resection are the main determinants of the choice of the IR-induced DNA DSB repair pathway.

The Determinant of DNA Repair Pathway Choices in Ionising Radiation-Induced DNA Double-Strand Breaks

Ionising radiation- (IR-) induced DNA double-strand breaks (DSBs) are considered to be the deleterious DNA lesions that pose a serious threat to genomic stability. The major DNA repair pathways, including classical nonhomologous end joining, homologous recombination, single-strand annealing, and alternative end joining, play critical roles in countering and eliciting IR-induced DSBs to ensure genome integrity. If the IR-induced DNA DSBs are not repaired correctly, the residual or incorrectly repaired DSBs can result in genomic instability that is associated with certain human diseases. Although many efforts have been made in investigating the major mechanisms of IR-induced DNA DSB repair, it is still unclear what determines the choices of IR-induced DNA DSB repair pathways. In this review, we discuss how the mechanisms of IR-induced DSB repair pathway choices can operate in irradiated cells. We first briefly describe the main mechanisms of the major DNA DSB repair pathways and the related key repair proteins. Based on our understanding of the characteristics of IR-induced DNA DSBs and the regulatory mechanisms of DSB repair pathways in irradiated cells and recent advances in this field, We then highlight the main factors and associated challenges to determine the IR-induced DSB repair pathway choices. We conclude that the type and distribution of IR-induced DSBs, chromatin state, DNA-end structure, and DNA-end resection are the main determinants of the choice of the IR-induced DNA DSB repair pathway.

Nanoscopic analysis of 53BP1, BRCA1 and Rad51 reveals new insights in temporal progression of DNA-repair and pathway choice

The accumulation and spatial distribution of 53BP1, BRCA1 and Rad51, key proteins in DNA double-strand break (DSB) repair, was investigated with high temporal resolution over a time span of 24 h, using STED nanoscopy. DNA lesions were induced by irradiation with high-LET (linear energy transfer) α-particles. We show that 53BP1 IRIF formation occurs quickly in almost all cells and after about 6 h the fraction of 53BP1 IRIF positive cells slowly declines. Against the expectations BRCA1 and Rad51 IRIF formation is only shortly delayed but with the maximum of cells showing foci after 6 and 8 h after irradiation. At this stage, almost all IRIF in a given Rad51-positive cell show Rad51 accumulation, suggesting that repair via homologous recombination is attempted at almost all residual DSB sites. The frequency of BRCA1 IRIF positive cells increases much earlier and remains high after Rad51 positive cells start to decline, supporting models claiming that functional roles of BRCA1 change over time. Correlation analysis showed a high degree of correlation of Rad51 with BRCA1, while the exclusion of 53BP1 from the actual resection-zone is demonstrated by anti-correlation of Rad51 and 53BP1. Interestingly, these correlation and anti-correlation patterns exhibit complementary temporal variation.

Biological effects of space environmental factors: A possible interaction between space radiation and microgravity

In the mid-1980s, space experiments began to examine if microgravity could alter the biological effects of space radiation. In the late 1990s, repair of DNA strand breaks was reported to not be influenced by microgravity using the pre-irradiated cells, because the exposure doses of space radiation were few due to the short spaceflight. There were, however, conflicting reports depending on the biological endpoints used in various systems. While almost no attempts were made to assess the possibility that the microgravity effects could be altered by space radiation. This was probably due to the general understanding that microgravity plays a major role in space and works independently from space radiation. Recent ground-based simulation studies focusing on DNA oxidative damage and signal transduction suggested that combined effects of microgravity and space radiation might exist. These studies also implicated the importance of research focusing not only on chromosomal DNA but also on cytoplasm, especially mitochondria. Therefore, we propose a new model which accounts for the combined-effects through the window of cellular responses. In this model, the interactions between microgravity and space radiation might occur during the following cellular-responses; (A) damaging and signaling by ROS, (B) damage responses on DNA (repair, replication, transcription, etc.), and (C) expression of gene and protein (regulation by chromatin, epigenetic control, etc.).

Comparing Photon and Charged Particle Therapy Using DNA Damage Biomarkers

Treatment modalities for cancer radiation therapy have become increasingly diversified given the growing number of facilities providing proton and carbon-ion therapy in addition to the more historically accepted photon therapy. An understanding of high-LET radiobiology is critical for optimization of charged particle radiation therapy and potential DNA damage response. In this review, we present a comprehensive summary and comparison of these types of therapy monitored primarily by using DNA damage biomarkers. We focus on their relative profiles of dose distribution and mechanisms of action from the level of nucleic acid to tumor cell death.

Vive la radiorésistance!: converging research in radiobiology and biogerontology to enhance human radioresistance for deep space exploration and colonization

While many efforts have been made to pave the way toward human space colonization, little consideration has been given to the methods of protecting spacefarers against harsh cosmic and local radioactive environments and the high costs associated with protection from the deleterious physiological effects of exposure to high-Linear energy transfer (high-LET) radiation. Herein, we lay the foundations of a roadmap toward enhancing human radioresistance for the purposes of deep space colonization and exploration. We outline future research directions toward the goal of enhancing human radioresistance, including upregulation of endogenous repair and radioprotective mechanisms, possible leeways into gene therapy in order to enhance radioresistance via the translation of exogenous and engineered DNA repair and radioprotective mechanisms, the substitution of organic molecules with fortified isoforms, and methods of slowing metabolic activity while preserving cognitive function. We conclude by presenting the known associations between radioresistance and longevity, and articulating the position that enhancing human radioresistance is likely to extend the healthspan of human spacefarers as well.

Repair of exogenous DNA double-strand breaks promotes chromosome synapsis in SPO11-mutant mouse meiocytes, and is altered in the absence of HORMAD1

Repair of SPO11-dependent DNA double-strand breaks (DSBs) via homologous recombination (HR) is essential for stable homologous chromosome pairing and synapsis during meiotic prophase. Here, we induced radiation-induced DSBs to study meiotic recombination and homologous chromosome pairing in mouse meiocytes in the absence of SPO11 activity (Spo11^YF/YF model), and in the absence of both SPO11 and HORMAD1 (Spo11/Hormad1 dko). Within 30 min after 5 Gy irradiation of Spo11^YF/YF mice, 140-160 DSB repair foci were detected, which specifically localized to the synaptonemal complex axes. Repair of radiation-induced DSBs was incomplete in Spo11^YF/YF compared to Spo11^+/YF meiocytes. Still, repair of exogenous DSBs promoted partial recovery of chromosome pairing and synapsis in Spo11^YF/YF meiocytes. This indicates that at least part of the exogenous DSBs can be processed in an interhomolog recombination repair pathway. Interestingly, in a seperate experiment, using 3 Gy of irradiation, we observed that Spo11/Hormad1 dko spermatocytes contained fewer remaining DSB repair foci at 48 h after irradiation compared to irradiated Spo11 knockout spermatocytes. Together, these results show that recruitment of exogenous DSBs to the synaptonemal complex, in conjunction with repair of exogenous DSBs via the homologous chromosome, contributes to homology recognition. In addition, the data suggest a role for HORMAD1 in DNA repair pathway choice in mouse meiocytes.

Carbon Ion Radiotherapy: A Review of Clinical Experiences and Preclinical Research, with an Emphasis on DNA Damage/Repair

Compared to conventional photon-based external beam radiation (PhXRT), carbon ion radiotherapy (CIRT) has superior dose distribution, higher linear energy transfer (LET), and a higher relative biological effectiveness (RBE). This enhanced RBE is driven by a unique DNA damage signature characterized by clustered lesions that overwhelm the DNA repair capacity of malignant cells. These physical and radiobiological characteristics imbue heavy ions with potent tumoricidal capacity, while having the potential for simultaneously maximally sparing normal tissues. Thus, CIRT could potentially be used to treat some of the most difficult to treat tumors, including those that are hypoxic, radio-resistant, or deep-seated. Clinical data, mostly from Japan and Germany, are promising, with favorable oncologic outcomes and acceptable toxicity. In this manuscript, we review the physical and biological rationales for CIRT, with an emphasis on DNA damage and repair, as well as providing a comprehensive overview of the translational and clinical data using CIRT.

AZD6738, A Novel Oral Inhibitor of ATR, Induces Synthetic Lethality with ATM Deficiency in Gastric Cancer Cells

Ataxia telangiectasia and Rad3-related (ATR) can be considered an attractive target for cancer treatment due to its deleterious effect on cancer cells harboring a homologous recombination defect. The aim of this study was to investigate the potential use of the ATR inhibitor, AZD6738, to treat gastric cancer.In SNU-601 cells with dysfunctional ATM, AZD6738 treatment led to an accumulation of DNA damage due to dysfunctional RAD51 foci formation, S phase arrest, and caspase 3-dependent apoptosis. In contrast, SNU-484 cells with functional ATM were not sensitive to AZD6738. Inhibition of ATM in SNU-484 cells enhanced AZD6738 sensitivity to a level comparable with that observed in SNU-601 cells, showing that activation of the ATM-Chk2 signaling pathway attenuates AZD6738 sensitivity. In addition, decreased HDAC1 expression was found to be associated with ATM inactivation in SNU-601 cells, demonstrating the interaction between HDAC1 and ATM can affect sensitivity to AZD6738. Furthermore, in an in vivo tumor xenograft mouse model, AZD6738 significantly suppressed tumor growth and increased apoptosis.These findings suggest synthetic lethality between ATR inhibition and ATM deficiency in gastric cancer cells. Further clinical studies on the interaction between AZD 6738 and ATM deficiency are warranted to develop novel treatment strategies for gastric cancer. Mol Cancer Ther; 16(4); 566-77. ©2017 AACR.

Differences in the recruitment of DNA repair proteins at subtelomeric and interstitial I-SceI endonuclease-induced DNA double-strand breaks

Telomeres are nucleoprotein structures that are required to protect chromosome ends. Dysfunctional telomeres are recognized as DNA double-strand breaks (DSBs), and elicit the activation of a DNA damage response (DDR). We have previously reported that DSBs near telomeres are poorly repaired, resulting in a high frequency of large deletions and gross chromosome rearrangements (GCRs). Our previous genetic studies have demonstrated that this sensitivity of telomeric regions to DSBs is a result of excessive processing. In the current study, we have further investigated the sensitivity of telomeric regions to DSBs through the analysis of repair proteins associated with DSBs at interstitial and telomeric sites. Following the inducible expression of I-SceI endonuclease, chromatin immunoprecipitation (ChIP) and real-time quantitative PCR were used to compare the recruitment of repair proteins at I-SceI-induced DSBs at interstitial and subtelomeric sites. We observed that proteins that are specifically associated with processing of DSBs during homologous recombination repair, RAD51, BRCA1, and CtIP, are present at a much greater abundance at subtelomeric DSBs. In contrast, Ku70, which is specifically involved in classical nonhomologous end joining, showed no difference at interstitial and subtelomeric DSBs. Importantly, ATM was lower in abundance at subtelomeric DSBs, while ATR was in greater abundance at subtelomeric DSBs, consistent with the accumulation of processed DSBs near telomeres, since processing is accompanied by a transition from ATM to ATR binding. Combined, our results suggest that excessive processing is responsible for the increased frequency of large deletions and GCRs at DSBs near telomeres.

Novel Double-Hit Model of Radiation and Hyperoxia-Induced Oxidative Cell Damage Relevant to Space Travel

Spaceflight occasionally requires multiple extravehicular activities (EVA) that potentially subject astronauts to repeated changes in ambient oxygen superimposed on those of space radiation exposure. We thus developed a novel in vitro model system to test lung cell damage following repeated exposure to radiation and hyperoxia. Non-tumorigenic murine alveolar type II epithelial cells (C10) were exposed to >95% O₂ for 8 h only (O₂), 0.25 Gy ionizing γ-radiation (IR) only, or a double-hit combination of both challenges (O₂ + IR) followed by 16 h of normoxia (ambient air containing 21% O₂ and 5% CO₂) (1 cycle = 24 h, 2 cycles = 48 h). Cell survival, DNA damage, apoptosis, and indicators of oxidative stress were evaluated after 1 and 2 cycles of exposure. We observed a significant (p < 0.05) decrease in cell survival across all challenge conditions along with an increase in DNA damage, determined by Comet analysis and H2AX phosphorylation, and apoptosis, determined by Annexin-V staining, relative to cells unexposed to hyperoxia or radiation. DNA damage (GADD45α and cleaved-PARP), apoptotic (cleaved caspase-3 and BAX), and antioxidant (HO-1 and Nqo1) proteins were increased following radiation and hyperoxia exposure after 1 and 2 cycles of exposure. Importantly, exposure to combination challenge O₂ + IR exacerbated cell death and DNA damage compared to individual exposures O₂ or IR alone. Additionally levels of cell cycle proteins phospho-p53 and p21 were significantly increased, while levels of CDK1 and Cyclin B1 were decreased at both time points for all exposure groups. Similarly, proteins involved in cell cycle arrest was more profoundly changed with the combination challenges as compared to each stressor alone. These results correlate with a significant 4- to 6-fold increase in the ratio of cells in G2/G1 after 2 cycles of exposure to hyperoxic conditions. We have characterized a novel in vitro model of double-hit, low-level radiation and hyperoxia exposure that leads to oxidative lung cell injury, DNA damage, apoptosis, and cell cycle arrest.

Evaluating biomarkers to model cancer risk post cosmic ray exposure

Robust predictive models are essential to manage the risk of radiation-induced carcinogenesis. Chronic exposure to cosmic rays in the context of the complex deep space environment may place astronauts at high cancer risk. To estimate this risk, it is critical to understand how radiation-induced cellular stress impacts cell fate decisions and how this in turn alters the risk of carcinogenesis. Exposure to the heavy ion component of cosmic rays triggers a multitude of cellular changes, depending on the rate of exposure, the type of damage incurred and individual susceptibility. Heterogeneity in dose, dose rate, radiation quality, energy and particle flux contribute to the complexity of risk assessment. To unravel the impact of each of these factors, it is critical to identify sensitive biomarkers that can serve as inputs for robust modeling of individual risk of cancer or other long-term health consequences of exposure. Limitations in sensitivity of biomarkers to dose and dose rate, and the complexity of longitudinal monitoring, are some of the factors that increase uncertainties in the output from risk prediction models. Here, we critically evaluate candidate early and late biomarkers of radiation exposure and discuss their usefulness in predicting cell fate decisions. Some of the biomarkers we have reviewed include complex clustered DNA damage, persistent DNA repair foci, reactive oxygen species, chromosome aberrations and inflammation. Other biomarkers discussed, often assayed for at longer points post exposure, include mutations, chromosome aberrations, reactive oxygen species and telomere length changes. We discuss the relationship of biomarkers to different potential cell fates, including proliferation, apoptosis, senescence, and loss of stemness, which can propagate genomic instability and alter tissue composition and the underlying mRNA signatures that contribute to cell fate decisions. Our goal is to highlight factors that are important in choosing biomarkers and to evaluate the potential for biomarkers to inform models of post exposure cancer risk. Because cellular stress response pathways to space radiation and environmental carcinogens share common nodes, biomarker-driven risk models may be broadly applicable for estimating risks for other carcinogens.

Serum Interleukin-6 Levels and Pulmonary Function in Ataxia-Telangiectasia

OBJECTIVE

To evaluate the potential link between systemic inflammation and impaired lung function in people with ataxia-telangiectasia (A-T), we hypothesized that serum levels of interleukin (IL)-6, a proinflammatory cytokine, would correlate inversely with lung function in subjects with A-T.

STUDY DESIGN

Consecutive subjects with A-T were recruited from the Johns Hopkins Outpatient A-T Clinical Center. Serum levels of IL-6 and 8 were measured by enzyme-linked immunosorbent assay. Spirometry was performed in subjects ≥ 6 years of age on the same day that serum was obtained for measurements of cytokines.

RESULTS

Approximately 80% of subjects had elevated serum IL-6 levels (> 1.0 pg/mL). No association was found between elevated IL-6 and age. Elevated IL-8 levels were found in 23.6% of subjects, and all subjects with elevated IL-8 levels had elevated IL-6 levels. Subjects with elevated IL-6 levels (mean: 6.14 ± 7.47 pg/mL) had significantly lower mean percent forced vital capacity (FVC%, 50.5% ± 17.8%) compared with subjects with normal serum IL-6 levels (FVC% of 66.2 ± 16.1, P = .018). Greater IL-6 levels were associated with lower FVC% even after adjustment for receiving gamma globulin therapy (P = .024) and supplemental nutrition (P = .055).

CONCLUSIONS

An association was found between elevated serum IL-6 levels and lower lung function in subjects with A-T. In addition, subjects with both elevated IL-6 and IL-8 had the lowest mean lung function. These findings indicate that markers for systemic inflammation may be useful in identifying individuals with A-T at increased risk for lower lung function and may help in assessing response to therapy.

DNA Repair Processes and Checkpoint Pathways in Human Cells Exposed to Heavy Ion Beams

The DNA double-strand break (DSB) is the most deleterious of the ionizing radiation-induced DNA damages. Two major repair pathways for DSBs have been well studied, nonhomologous end-joining and homologous recombination. It is known that high linear energy transfer radiation, such as heavy ion beams, induces complex DSBs with clustered damages at the end and that, as a result, the efficiency of nonhomologous end-joining in repairing these DSBs is diminished. We have shown that more than 80% of complex DSBs in S/G2 human cells are subjected to DNA end resection, an early step in homologous recombination to generate single-strand DNA. Furthermore, recent work, including ours, revealed that a subpopulation of human G1 cells exhibit resection activity following ionizing radiation, which is dependent on CtIP, as in other cell cycle phases, and also dependent on the complexity of the DSB. Collectively, this recent progress indicates that the complexity of the DSB structure drastically enhances end resection, with CtIP being a significant factor required for complex DSB repair throughout the cell cycle. We further revealed that the ATR pathway, which is activated by end resection, plays a pivotal role in regulating early G2/M arrest in ATM-deficient cells exposed to high linear energy transfer ion beams. This suggests that the complexity of the DSB also influences the choice of the signaling pathway via the enhanced resection. Additionally, we discuss a possibility that CtIP has an additional function (or functions) after the initiation of resection. In conclusion, new findings and insight are pivotal to allow innovative progress in heavy ion-particle therapy by shedding light on the whole response at the molecular level in cells exposed to heavy ion beams.

DNA double-strand breaks cooperate with loss of Ink4 and Arf tumor suppressors to generate glioblastomas with frequent Met amplification

Glioblastomas (GBM) are highly radioresistant and lethal brain tumors. Ionizing radiation (IR)-induced DNA double-strand breaks (DSBs) are a risk factor for the development of GBM. In this study, we systematically examined the contribution of IR-induced DSBs to GBM development using transgenic mouse models harboring brain-targeted deletions of key tumor suppressors frequently lost in GBM, namely Ink4a, Ink4b, Arf and/or PTEN. Using low linear energy transfer (LET) X-rays to generate simple breaks or high LET HZE particles (Fe ions) to generate complex breaks, we found that DSBs induce high-grade gliomas in these mice which, otherwise, do not develop gliomas spontaneously. Loss of Ink4a and Arf was sufficient to trigger IR-induced glioma development but additional loss of Ink4b significantly increased tumor incidence. We analyzed IR-induced tumors for copy number alterations to identify oncogenic changes that were generated and selected for as a consequence of stochastic DSB events. We found Met amplification to be the most significant oncogenic event in these radiation-induced gliomas. Importantly, Met activation resulted in the expression of Sox2, a GBM cancer stem cell marker, and was obligatory for tumor formation. In sum, these results indicate that radiation-induced DSBs cooperate with loss of Ink4 and Arf tumor suppressors to generate high-grade gliomas that are commonly driven by Met amplification and activation.

Understanding cancer development processes after HZE-particle exposure: roles of ROS, DNA damage repair and inflammation

During space travel astronauts are exposed to a variety of radiations, including galactic cosmic rays composed of high-energy protons and high-energy charged (HZE) nuclei, and solar particle events containing low- to medium-energy protons. Risks from these exposures include carcinogenesis, central nervous system damage and degenerative tissue effects. Currently, career radiation limits are based on estimates of fatal cancer risks calculated using a model that incorporates human epidemiological data from exposed populations, estimates of relative biological effectiveness and dose-response data from relevant mammalian experimental models. A major goal of space radiation risk assessment is to link mechanistic data from biological studies at NASA Space Radiation Laboratory and other particle accelerators with risk models. Early phenotypes of HZE exposure, such as the induction of reactive oxygen species, DNA damage signaling and inflammation, are sensitive to HZE damage complexity. This review summarizes our current understanding of critical areas within the DNA damage and oxidative stress arena and provides insight into their mechanistic interdependence and their usefulness in accurately modeling cancer and other risks in astronauts exposed to space radiation. Our ultimate goals are to examine potential links and crosstalk between early response modules activated by charged particle exposure, to identify critical areas that require further research and to use these data to reduced uncertainties in modeling cancer risk for astronauts. A clearer understanding of the links between early mechanistic aspects of high-LET response and later surrogate cancer end points could reveal key nodes that can be therapeutically targeted to mitigate the health effects from charged particle exposures.

The complexity of DNA double strand break is a crucial factor for activating ATR signaling pathway for G2/M checkpoint regulation regardless of ATM function

DNA double strand break (DSB) repair pathway choice following ionizing radiation (IR) is currently an appealing research topic, which is still largely unclear. Our recent paper indicated that the complexity of DSBs is a critical factor that enhances DNA end resection. It has been well accepted that the RPA-coated single strand DNA produced by resection is a signaling structure for ATR activation. Therefore, taking advantage of high linear energy transfer (LET) radiation to effectively produce complex DSBs, we investigated how the complexity of DSB influences the function of ATR pathway on the G2/M checkpoint regulation. Human skin fibroblast cells with or without ATM were irradiated with X rays or heavy ion particles, and dual-parameter flow cytometry was used to quantitatively assess the mitotic entry at early period post radiation by detecting the cells positive for phosphor histone H3. In ATM-deficient cells, ATR pathway played a pivotal role and functioned in a dose- and LET-dependent way to regulate the early G2/M arrest even as low as 0.2Gy for heavy ion radiation, which indicated that ATR pathway could be rapidly activated and functioned in an ATM-independent, but DSB complexity-dependent manner following exposure to IR. Furthermore, ATR pathway also functioned more efficiently in ATM-proficient cells to block G2 to M transition at early period of particle radiation exposure. Accordingly, in contrast to ATM inhibitor, ATR inhibitor had a more effective radiosensitizing effect on survival fraction following heavy ion beams as compared with X ray radiation. Taken together, our results reveal that the complexity of DSBs is a crucial factor for the activation of ATR pathway for G2/M checkpoint regulation, and ATM-dependent end resection is not essential for the activation.

Biological characterization of low-energy ions with high-energy deposition on human cells

During space travel, astronauts are exposed to cosmic radiation that is comprised of high-energy nuclear particles. Cancer patients are also exposed to high-energy nuclear particles when treated with proton and carbon beams. Nuclear interactions from high-energy particles traversing shielding materials and tissue produce low-energy (<10 MeV/n) secondary particles of high-LET that contribute significantly to overall radiation exposures. Track structure theories suggest that high charge and energy (HZE) particles and low-energy secondary ions of similar LET will have distinct biological effects for cellular and tissue damage endpoints. We investigated the biological effects of low-energy ions of high LET utilizing the Tandem Van de Graaff accelerator at the Brookhaven National Laboratory (BNL), and compared these to experiments with HZE particles, that mimic the space environment produced at NASA Space Radiation Laboratory (NSRL) at BNL. Immunostaining for DNA damage response proteins was carried out after irradiation with 5.6 MeV/n boron (LET 205 keV/μm), 5.3 MeV/n silicon (LET 1241 keV/μm), 600 MeV/n Fe (LET 180 keV/μm) and 77 MeV/n oxygen (LET 58 keV/μm) particles. Low-energy ions caused more persistent DNA damage response (DDR) protein foci in irradiated human fibroblasts and esophageal epithelial cells compared to HZE particles. More detailed studies comparing boron ions to Fe particles, showed that boron-ion radiation resulted in a stronger G2 delay compared to Fe-particle exposure, and boron ions also showed an early recruitment of Rad51 at double-strand break (DSB) sites, which suggests a preference of homologous recombination for DSB repair in low-energy albeit high-LET particles. Our experiments suggest that the very high-energy radiation deposition by low-energy ions, representative of galactic cosmic radiation and solar particle event secondary radiation, generates massive but localized DNA damage leading to delayed DSB repair, and distinct cellular responses from HZE particles. Thus, low-energy heavy ions provide a valuable probe for studies of homologous recombination repair in radiation responses.

DNA end resection is needed for the repair of complex lesions in G1-phase human cells

Repair of DNA double strand breaks (DSBs) is influenced by the chemical complexity of the lesion. Clustered lesions (complex DSBs) are generally considered more difficult to repair and responsible for early and late cellular effects after exposure to genotoxic agents. Resection is commonly used by the cells as part of the homologous recombination (HR) pathway in S- and G2-phase. In contrast, DNA resection in G1-phase may lead to an error-prone microhomology-mediated end joining. We induced DNA lesions with a wide range of complexity by irradiation of mammalian cells with X-rays or accelerated ions of different velocity and mass. We found replication protein A (RPA) foci indicating DSB resection both in S/G2- and G1-cells, and the fraction of resection-positive cells correlates with the severity of lesion complexity throughout the cell cycle. Besides RPA, Ataxia telangiectasia and Rad3-related (ATR) was recruited to complex DSBs both in S/G2- and G1-cells. Resection of complex DSBs is driven by meiotic recombination 11 homolog A (MRE11), CTBP-interacting protein (CtIP), and exonuclease 1 (EXO1) but seems not controlled by the Ku heterodimer or by phosphorylation of H2AX. Reduced resection capacity by CtIP depletion increased cell killing and the fraction of unrepaired DSBs after exposure to densely ionizing heavy ions, but not to X-rays. We conclude that in mammalian cells resection is essential for repair of complex DSBs in all phases of the cell-cycle and targeting this process sensitizes mammalian cells to cytotoxic agents inducing clustered breaks, such as in heavy-ion cancer therapy.