Cell cycle phase dependent role of DNA polymerase beta in DNA repair and survival after ionizing radiation

TRIP12 governs DNA Polymerase β involvement in DNA damage response and repair

The multitude of DNA lesion types, and the nuclear dynamic context in which they occur, present a challenge for genome integrity maintenance as this requires the engagement of different DNA repair pathways. Specific 'repair controllers' that facilitate DNA repair pathway crosstalk between double strand break (DSB) repair and base excision repair (BER), and regulate BER protein trafficking at lesion sites, have yet to be identified. We find that DNA polymerase β (Polβ), crucial for BER, is ubiquitylated in a BER complex-dependent manner by TRIP12, an E3 ligase that partners with UBR5 and restrains DSB repair signaling. Here we find that, TRIP12, but not UBR5, controls cellular levels and chromatin loading of Polβ. Required for Polβ foci formation, TRIP12 regulates Polβ involvement after DNA damage. Notably, excessive TRIP12-mediated shuttling of Polβ affects DSB formation and radiation sensitivity, underscoring its precedence for BER. We conclude that the herein discovered trafficking function at the nexus of DNA repair signaling pathways, towards Polβ-directed BER, optimizes DNA repair pathway choice at complex lesion sites.

Regulation of base excision repair during adipogenesis and osteogenesis of bone marrow-derived mesenchymal stem cells

Bone marrow-derived human mesenchymal stem cells (hMSCs) can differentiate into various lineages, such as chondrocytes, adipocytes, osteoblasts, and neuronal lineages. It has been shown that the high-efficiency DNA-repair capacity of hMSCs is decreased during their differentiation. However, the underlying its mechanism during adipogenesis and osteogenesis is unknown. Herein, we investigated how alkyl-damage repair is modulated during adipogenic and osteogenic differentiation, especially focusing on the base excision repair (BER) pathway. Response to an alkylation agent was assessed via quantification of the double-strand break (DSB) foci and activities of BER-related enzymes during differentiation in hMSCs. Adipocytes showed high resistance against methyl methanesulfonate (MMS)-induced alkyl damage, whereas osteoblasts were more sensitive than hMSCs. During the differentiation, activities, and protein levels of uracil-DNA glycosylase were found to be regulated. In addition, ligation-related proteins, such as X-ray repair cross-complementing protein 1 (XRCC1) and DNA polymerase β, were upregulated in adipocytes, whereas their levels and recruitment declined during osteogenesis. These modulations of BER enzyme activity during differentiation influenced DNA repair efficiency and the accumulation of DSBs as repair intermediates in the nucleus. Taken together, we suggest that BER enzymatic activity is regulated in adipogenic and osteogenic differentiation and these alterations in the BER pathway led to different responses to alkyl damage from those in hMSCs.

High serum superoxide dismutase activity improves radiation-related quality of life in patients with esophageal squamous cell carcinoma

OBJECTIVES

Esophageal squamous cell carcinoma (ESCC) is one of the most common malignant tumors in China. Intensity-modulated radiation therapy and volume-modulated arc therapy have become the main treatments for esophageal carcinoma; however, side effects caused by radiotherapy greatly impact the quality of life in these patients. This study aimed to explore the impact of serum superoxide dismutase (SOD) levels on the prognosis of patients with ESCC undergoing radiotherapy.

METHODS

Patients aged between 18 and 80 years with lower-middle ESCC who underwent radiotherapy were eligible for this assessment. Adverse events, responses, treatment outcomes, and overall survival (OS) were assessed. Between 2012 and 2014, 195 patients were enrolled, of which 65 were assigned to the low- and high-SOD groups based on their serum SOD values.

RESULTS

The baseline characteristics were similar between the two groups, except for the T staging. Adverse events in the low-SOD group were significantly higher than those in the high-SOD group (radiation esophagitis, p=0.007; radiation pneumonitis, p=0.032; leukopenia, p=0.023; thrombocytopenia, p=0.037; anemia, p=0.041). There were no significant differences in response, treatment outcomes, or OS.

CONCLUSION

In conclusion, high serum SOD activity improved post-radiotherapy quality of life but did not impact the prognosis of patients with ESCC. To the best of our knowledge, this study is the first to report that serum SOD activity is associated with radiation-induced toxicity and moderately increased radiotherapeutic response in patients with ESCC undergoing radiotherapy.

In vitro studies of DNA damage and repair mechanisms induced by BNCT in a poorly differentiated thyroid carcinoma cell line

Boron neutron capture therapy (BNCT) for aggressive tumors is based on nuclear reaction [¹⁰B (n, α) ⁷Li]. Previously, we demonstrated that BNCT could be applied for the treatment of undifferentiated thyroid carcinoma. The aim of the present study was to describe the DNA damage pattern and the repair pathways that are activated by BNCT in thyroid cells. We analyzed γH2AX foci and the expression of Ku70, Rad51 and Rad54, main effector enzymes of non-homologous end joining (NHEJ) and homologous recombination repair (HRR) pathways, respectively, in thyroid follicular carcinoma cells. The studied groups were: (1) C [no irradiation], (2) gamma [⁶⁰Co source], (3) N [neutron beam alone], (4) BNCT [neutron beam plus 10 µg ¹⁰B/ml of boronphenylalanine (¹⁰BPA)]. The total absorbed dose was always 3 Gy. The results showed that the number of nuclear γH2AX foci was higher in the gamma group than in the N and BNCT groups (30 min-24 h) (p < 0.001). However, the focus size was significantly larger in BNCT compared to other groups (p < 0.01). The analysis of repair enzymes showed a significant increase in Rad51 and Rad54 mRNA at 4 and 6 h, respectively; in both N and BNCT groups and the expression of Ku70 did not show significant differences between groups. These findings are consistent with an activation of HRR mechanism in thyroid cells. A melanoma cell line showed different DNA damage pattern and activation of both repair pathways. These results will allow us to evaluate different blocking points, to potentiate the damage induced by BNCT.

HSP90 regulates DNA repair via the interaction between XRCC1 and DNA polymerase β

Cellular DNA repair processes are crucial to maintain genome stability and integrity. In DNA base excision repair, a tight heterodimer complex formed by DNA polymerase β (Polβ) and XRCC1 is thought to facilitate repair by recruiting Polβ to DNA damage sites. Here we show that disruption of the complex does not impact DNA damage response or DNA repair. Instead, the heterodimer formation is required to prevent ubiquitylation and degradation of Polβ. In contrast, the stability of the XRCC1 monomer is protected from CHIP-mediated ubiquitylation by interaction with the binding partner HSP90. In response to cellular proliferation and DNA damage, proteasome and HSP90-mediated regulation of Polβ and XRCC1 alters the DNA repair complex architecture. We propose that protein stability, mediated by DNA repair protein complex formation, functions as a regulatory mechanism for DNA repair pathway choice in the context of cell cycle progression and genome surveillance.

Critical dose and toxicity index of organs at risk in radiotherapy: analyzing the calculated effects of modified dose fractionation in non-small cell lung cancer

To increase the efficacy of radiotherapy for non-small cell lung cancer (NSCLC), many schemes of dose fractionation were assessed by a new "toxicity index" (I), which allows one to choose the fractionation schedules that produce less toxic treatments. Thirty-two patients affected by non resectable NSCLC were treated by standard 3-dimensional conformal radiotherapy (3DCRT) with a strategy of limited treated volume. Computed tomography datasets were employed to re plan by simultaneous integrated boost intensity-modulated radiotherapy (IMRT). The dose distributions from plans were used to test various schemes of dose fractionation, in 3DCRT as well as in IMRT, by transforming the dose-volume histogram (DVH) into a biological equivalent DVH (BDVH) and by varying the overall treatment time. The BDVHs were obtained through the toxicity index, which was defined for each of the organs at risk (OAR) by a linear quadratic model keeping an equivalent radiobiological effect on the target volume. The less toxic fractionation consisted in a severe/moderate hyper fractionation for the volume including the primary tumor and lymph nodes, followed by a hypofractionation for the reduced volume of the primary tumor. The 3DCRT and IMRT resulted, respectively, in 4.7% and 4.3% of dose sparing for the spinal cord, without significant changes for the combined-lungs toxicity (p < 0.001). Schedules with reduced overall treatment time (accelerated fractionations) led to a 12.5% dose sparing for the spinal cord (7.5% in IMRT), 8.3% dose sparing for V20 in the combined lungs (5.5% in IMRT), and also significant dose sparing for all the other OARs (p < 0.001). The toxicity index allows to choose fractionation schedules with reduced toxicity for all the OARs and equivalent radiobiological effect for the tumor in 3DCRT, as well as in IMRT, treatments of NSCLC.

DNA polymerase beta promoter mutations affect gene transcription, translation and the sensitivity of esophageal cancer cells to cisplatin treatment

The ability of a promoter to initiate transcription is important for the control of gene expression. Mutations in the DNA polymerase beta (po1β) promoter may affect the transcription of this gene; however, the relationship between these mutations and the upregulation of the expression of po1β remains unclear. Therefore, in the present study, three po1β promoter mutants (M1, -37 C→A; M2, -114 G→A, -37 C→A; M3, -194 T→C) were generated to examine the effect of promoter mutations on polβ gene expression and sensitivity to cisplatin. We found that the M1 and M2 mutant polβ promoter constructs showed higher RLA than the wild-type polβ promoter (P < 0.01), whereas the activity of the M3 polβ promoter did not differ significantly from that of the wild-type polβ promoter (P > 0.05). The expression levels of polβ mRNA and protein were significantly higher (P < 0.01) and the sensitivity to cisplatin was significantly lower (P < 0.05) in Eca9706(-/-)-M1 and Eca9706(-/-)-M2 cells than in Eca9706(-/-)-W. The expression levels of polβ mRNA and protein and the sensitivity to cisplatin were not significantly different between Eca9706(-/-)-M3 and Eca9706(-/-)-W cells (P > 0.05).These results revealed that specific mutations of the polymerase beta gene promoter significantly enhanced the gene's transcriptional activity. These mutations correspondingly increased the gene's mRNA and protein product, at the same time reduced the esophageal cancer cells' sensitivity to cisplatin.

Cyclophilin B expression is associated with in vitro radioresistance and clinical outcome after radiotherapy

The tools for predicting clinical outcome after radiotherapy are not yet optimal. To improve on this, we applied the COXEN informatics approach to in vitro radiation sensitivity data of transcriptionally profiled human cells and gene expression data from untreated head and neck squamous cell carcinoma (HNSCC) and bladder tumors to generate a multigene predictive model that is independent of histologic findings and reports on tumor radiosensitivity. The predictive ability of this 41-gene model was evaluated in patients with HNSCC and was found to stratify clinical outcome after radiotherapy. In contrast, this model was not useful in stratifying similar patients not treated with radiation. This led us to hypothesize that expression of some of the 41 genes contributes to tumor radioresistance and clinical recurrence. Hence, we evaluated the expression the 41 genes as a function of in vitro radioresistance in the NCI-60 cancer cell line panel and found cyclophilin B (PPIB), a peptidylprolyl isomerase and target of cyclosporine A (CsA), had the strongest direct correlation. Functional inhibition of PPIB by small interfering RNA depletion or CsA treatment leads to radiosensitization in cancer cells and reduced cellular DNA repair. Immunohistochemical evaluation of PPIB expression in patients with HNSCC was found to be associated with outcome after radiotherapy. This work demonstrates that a novel 41-gene expression model of radiation sensitivity developed in bladder cancer cell lines and human skin fibroblasts predicts clinical outcome after radiotherapy in head and neck cancer patients and identifies PPIB as a potential target for clinical radiosensitization.

Effects of formulation and route of administration on the systemic availability of Ex-RAD®, a new radioprotectant, in preclinical species

ON 01210.Na (Ex-RAD®) is a novel small molecule under development by Onconova Therapeutics, Inc. as a radiation protection agent. The purpose of this investigation was to evaluate the effect of various formulation approaches on the systemic exposure of ON 01210.Na. In vitro experiments were used to characterize the plasma binding and metabolic stability of ON 01210.Na using hepatocytes from several animal species (mouse, rat, rabbit, dog, monkey and human). In vivo studies were performed in rats, rabbits, dogs and monkeys, and involved several routes of administration (intravenous, subcutaneous, oral). Plasma protein binding was high across species (>83%), and the rate of ON 01210.Na metabolism was highest in rat and mouse hepatocytes. After intravenous administration, ON 01210.Na demonstrated biphasic elimination from the plasma. Systemic exposure parameters (Cmax, AUC) were dose-proportional up to 100 mg/kg. Following subcutaneous dosing, ON 01210.Na showed relatively low bioavailability upon administration of the suspension formulation. Developing a solution formulation significantly increased the bioavailability of the drug. This solution formulation demonstrated significant oral bioavailability in rabbit (70%) and monkey (30%). The findings from these preclinical studies provide an overview of the systemic disposition of ON 01210.Na, aiding in the development of optimal formulations and routes of administration for pivotal animal efficacy and clinical safety studies. A solution formulation of ON 01210.Na for s.c. administration is being developed, in addition to an oral dosage form for potential use of the compound as a radioprotectant and a radiation-mitigating agent in wider military and civilian populations.

DNA 3'-phosphatase activity is critical for rapid global rates of single-strand break repair following oxidative stress

Oxidative stress is a major source of chromosome single-strand breaks (SSBs), and the repair of these lesions is retarded in neurodegenerative disease. The rate of the repair of oxidative SSBs is accelerated by XRCC1, a scaffold protein that is essential for embryonic viability and that interacts with multiple DNA repair proteins. However, the relative importance of the interactions mediated by XRCC1 during oxidative stress in vivo is unknown. We show that mutations that disrupt the XRCC1 interaction with DNA polymerase beta or DNA ligase III fail to slow SSB repair in proliferating CHO cells following oxidative stress. In contrast, mutation of the domain that interacts with polynucleotide kinase/phosphatase (PNK) and Aprataxin retards repair, and truncated XRCC1 encoding this domain fully supports this process. Importantly, the impact of mutating the protein domain in XRCC1 that binds these end-processing factors is circumvented by the overexpression of wild-type PNK but not by the overexpression of PNK harboring a mutated DNA 3'-phosphatase domain. These data suggest that DNA 3'-phosphatase activity is critical for rapid rates of chromosomal SSB repair following oxidative stress, and that the XRCC1-PNK interaction ensures that this activity is not rate limiting in vivo.

Mechanism of cell killing after ionizing radiation by a dominant negative DNA polymerase beta

Several types of DNA lesion are induced after ionizing irradiation (IR) of which double strand breaks (DSBs) are expected to be the most lethal, although single strand breaks (SSBs) and DNA base damages are quantitatively in the majority. Proteins of the base excision repair (BER) pathway repair these numerous lesions. DNA polymerase beta has been identified as a crucial enzyme in BER and SSB repair (SSBR). We showed previously that inhibition of BER/SSBR by expressing a dominant negative DNA polymerase beta (polbetaDN) resulted in radiosensitization. We hypothesized increased kill to result from DSBs arising from unrepaired SSBs and BER intermediates. We find here higher numbers of IR-induced chromosome aberrations in polbetaDN expressing cells, confirming increased DSB formation. These aberrations did not result from changes in DSB induction or repair of the majority of lesions. SSB conversion to DSBs has been shown to occur during replication. We observed an increased induction of chromatid aberrations in polbetaDN expressing cells after IR, suggesting such a replication-dependence of secondary DSB formation. We also observed a pronounced increase of chromosomal deletions, the most likely cause of the increased kill. After H(2)O(2) treatment, polbetaDN expression only resulted in increased chromatid (not chromosome) aberrations. Together with the lack of sensitization to H(2)O(2), these data further suggest that the additional secondarily induced lethal DSBs resulted from repair attempts at complex clustered damage sites, unique to IR. Surprisingly, the polbetaDN induced increase in residual gammaH2AX foci number was unexpectedly low compared with the radiosensitization or induction of aberrations. Our data thus demonstrate the formation of secondary DSBs that are reflected by increased kill but not by residual gammaH2AX foci, indicating an escape from gammaH2AX-mediated DSB repair. In addition, we show that in the polbetaDN expressing cells secondary DSBs arise in a radiation-specific and partly replication-dependent manner.