Conchita Vens

Strategies to improve radiotherapy with targeted drugs

Radiotherapy is used to treat approximately 50% of all cancer patients, with varying success. The dose of ionizing radiation that can be given to the tumour is determined by the sensitivity of the surrounding normal tissues. Strategies to improve radiotherapy therefore aim to increase the effect on the tumour or to decrease the effects on normal tissues. These aims must be achieved without sensitizing the normal tissues in the first approach and without protecting the tumour in the second approach. Two factors have made such approaches feasible: namely, an improved understanding of the molecular response of cells and tissues to ionizing radiation and a new appreciation of the exploitable genetic alterations in tumours. These have led to the development of treatments combining pharmacological interventions with ionizing radiation that more specifically target either tumour or normal tissue, leading to improvements in efficacy.

Novel therapeutics in combination with radiotherapy to improve cancer treatment: rationale, mechanisms of action and clinical perspective.

Our increased understanding of the molecular processes underlying cellular sensitivity to ionizing radiation has led to the identification of novel targets for intervention. New agents have become available for combined use to overcome radioresistance and enhance the clinical efficacy of radiotherapy. This rational selection of potential radiosensitizers contrasts with the empirical approach that has dominated the field of chemo-radiotherapy over the last decades. It allows the identification of those patients who will benefit most from a specific combination by exploiting new predictive biomarkers of response. In this review we present several approaches of targeted radiosensitization and discuss the available in vitro and in vivo results that support their translation into clinical trials. We focus on EGFR-inhibiting, anti-angiogenic, apoptosis-modulating and PARP-interfering strategies.

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.

Extent of radiosensitization by the PARP inhibitor olaparib depends on its dose, the radiation dose and the integrity of the homologous recombination pathway of tumor cells

BACKGROUND AND PURPOSE The PARP inhibitor olaparib is currently tested in clinical phase 1 trials to define safe dose levels in combination with RT. However, certain clinically relevant insights are still lacking. Here we test, while comparing to single agent activity, the olaparib dose and genetic background dependence of olaparib-mediated radiosensitization. MATERIALS AND METHODS Long-term growth inhibition and clonogenic assays were used to assess radiosensitization in BRCA2-deficient and BRCA2-complemented cells and in a panel of human head and neck squamous cell carcinoma cell lines. RESULTS The extent of radiosensitization greatly depended on the olaparib dose, the radiation dose and the homologous recombination status of cells. Olaparib concentrations that resulted in radiosensitization prevented PAR induction by irradiation. Seven hours olaparib exposures were sufficient for radiosensitization. Importantly, the radiosensitizing effects can be observed at much lower olaparib doses than the single agent effects. CONCLUSION Extrapolation of these data to the clinic suggests that low olaparib doses are sufficient to cause radiosensitization, underlining the potential of the treatment. Here we show that drug doses achieving radiosensitization can greatly differ from those achieving single agent activities, an important consideration when developing combined radiotherapy strategies with novel targeted agents.

The role of DNA polymerase beta in determining sensitivity to ionizing radiation in human tumor cells.

Lethal lesions after ionizing radiation are thought to be mainly unrepaired or misrepaired DNA double-strand breaks, ultimately leading to lethal chromosome aberrations. However, studies with radioprotectors and repair inhibitors indicate that single-strand breaks, damaged nucleotides or abasic sites can also influence cell survival. This paper reports on studies to further define the role of base damage and base excision repair on the radiosensitivity of human cells. We retrovirally transduced human tumor cells with a dominant negative form of DNA polymerase beta, comprising the 14 kDa DNA-binding domain of DNA polymerase beta but lacking polymerase function. Radiosensitization of two human carcinoma cell lines, A549 and SQD9, was observed, achieving dose enhancement factors of 1.5-1.7. Sensitization was dependent on expression level of the dominant negative and was seen in both single cell clones and in unselected virally transduced populations. Sensitization was not due to changes in cell cycle distribution. Little or no sensitization was seen in G(1)-enriched populations, indicating cell cycle specificity for the observed sensitization. These results contrast with the lack of effect seen in DNA polymerase beta knockout cells, suggesting that polDN also inhibits the long patch, DNA polymerase beta-independent repair pathway. These data demonstrate an important role for BER in determining sensitivity to ionizing radiation and might help identify targets for radiosensitizing tumor cells.

Tight regulation of ubiquitin-mediated DNA damage response by USP3 preserves the functional integrity of hematopoietic stem cells.

In vivo deletion of USP3, a deubiquitinating enzyme involved in DNA damage repair, increases the incidence of spontaneous cancer and impairs the proliferation and repopulation ability of HSCs.

Targeting Base Excision Repair as a Sensitization Strategy in Radiotherapy

Cellular DNA repair determines survival after ionizing radiation. Human tumors commonly exhibit aberrant DNA repair since they drive mutagenesis and chromosomal instability. Recent reports have shown alterations in the base excision repair (BER) and single strand break repair (SSBR) pathways in human tumors. Here we review these reports with respect to radiation sensitivity and the attempts to target such tumor-specific BER/SSBR aberrations. These aberrations can alter cellular resistance to therapeutic agents, including radiation. Some strategies therefore aim to counteract the radioresistance mediated by such aberrant DNA repair. Other strategies aim to exploit the dependence of the tumor, but not the normal cells, on backup repair mechanisms after radiation, therefore increasing the therapeutic window. Such tumor-targeted radiosensitization holds promise for increasing the efficacy of radiotherapy.

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.

Ionizing Radiation Sensitivity of DNA Polymerase Lambda-Deficient Cells

Abstract Vermeulen, C., Bertocci, B., Begg, A. C. and Vens, C. Ionizing Radiation Sensitivity of DNA Polymerase Lambda-Deficient Cells. Radiat. Res. 168, 683–688 (2007). Ionizing radiation induces a diverse spectrum of DNA lesions, including strand breaks and oxidized bases. In mammalian cells, ionizing radiation-induced lesions are targets of non-homologous end joining, homologous recombination, and base excision repair. In vitro assays show a potential involvement of DNA polymerase lambda in non-homologous end joining and base excision repair. In this study, we investigated whether DNA polymerase lambda played a significant role in determining ionizing radiation sensitivity. Despite increased sensitivity to hydrogen peroxide, lambda-deficient mouse embryonic fibroblasts displayed equal survival after exposure to ionizing radiation compared to their wild-type counterparts. In addition, we found increased sensitivity to the topoisomerase inhibitors camptothecin and etoposide in the absence of polymerase lambda. These results do not reveal a major role for DNA polymerase lambda in determining radiosensitivity in vivo.

Tight regulation of ubiquitin-mediated DNA damage response by USP3 preserves the functional integrity of hematopoietic stem cells

Histone ubiquitination at DNA breaks is required for activation of the DNA damage response (DDR) and DNA repair. How the dynamic removal of this modification by deubiquitinating enzymes (DUBs) impacts genome maintenance in vivo is largely unknown. To address this question, we generated mice deficient for Ub-specific protease 3 (USP3; Usp3{delta}/{delta}), a histone H2A DUB which negatively regulates ubiquitin-dependent DDR signaling. Notably, USP3 deletion increased the levels of histone ubiquitination in adult tissues, reduced the hematopoietic stem cell (HSC) reserves over time, and shortened animal life span. Mechanistically, our data show that USP3 is important in HSC homeostasis, preserving HSC self-renewal, and repopulation potential in vivo and proliferation in vitro. A defective DDR and unresolved spontaneous DNA damage contribute to cell cycle restriction of Usp3{delta}/{delta} HSCs. Beyond the hematopoietic system, Usp3{delta}/{delta} animals spontaneously developed tumors, and primary Usp3{delta}/{delta} cells failed to preserve chromosomal integrity. These findings broadly support the regulation of chromatin ubiquitination as a key pathway in preserving tissue function through modulation of the response to genotoxic stress.