Norman Chan

Chronic Hypoxia Decreases Synthesis of Homologous Recombination Proteins to Offset Chemoresistance and Radioresistance

Hypoxic and/or anoxic tumor cells can have increased rates of mutagenesis and altered DNA repair protein expression. Yet very little is known regarding the functional consequences of any hypoxia-induced changes in the expression of proteins involved in DNA double-strand break repair. We have developed a unique hypoxic model system using H1299 cells expressing an integrated direct repeat green fluorescent protein (DR-GFP) homologous recombination (HR) reporter system to study HR under prolonged chronic hypoxia (up to 72 h under 0.2% O(2)) without bias from altered proliferation, cell cycle checkpoint activation, or severe cell toxicity. We observed decreased expression of HR proteins due to a novel mechanism involving decreased HR protein synthesis. Error-free HR was suppressed 3-fold under 0.2% O(2) as measured by the DR-GFP reporter system. This decrease in functional HR resulted in increased sensitivity to the DNA cross-linking agents mitomycin C and cisplatin but not to the microtubule-interfering agent, paclitaxel. Chronically hypoxic H1299 cells that had decreased functional HR were relatively radiosensitive [oxygen enhancement ratio (OER), 1.37] when compared with acutely hypoxic or anoxic cells (OER, 1.96-2.61). Using CAPAN1 cells isogenic for BRCA2 and siRNA to RAD51, we confirmed that the hypoxia-induced radiosensitivity was due to decreased HR capacity. Persistent down-regulation of HR function by the tumor microenvironment could result in low-fidelity DNA repair and have significant implications for response to therapy and genetic instability in human cancers.

A novel poly(ADP-ribose) polymerase inhibitor, ABT-888, radiosensitizes malignant human cell lines under hypoxia

The chemo- and radioresponse of tumor cells can be determined by genetic factors (e.g., those that modify cell cycle arrest, DNA damage repair or cell death) and microenvironmental factors, such as hypoxia. Poly(ADP-ribose) polymerase (PARP) is a nuclear enzyme that rapidly recognizes and binds to DNA breaks to facilitate DNA strand break repair. Pre-clinical data suggest that PARP inhibitors (PARPi) may potentiate the effects of radiotherapy and chemotherapy. However, it is unclear as to whether PARPi are effective against hypoxic cells. We therefore tested the role for a novel PARPi, ABT-888, as a radiosensitizing agent under hypoxic conditions. Using human prostate (DU-145, 22RV1) and non-small cell lung (H1299) cancer cell lines, we observed that ABT-888 inhibited both recombinant PARP activity and intracellular PARP activity (86% to 92% decrease in all 3 cells lines following 2.5 microM treatment). ABT-888 was toxic to both oxic and hypoxic cells. When ABT-888 was combined with ionizing radiation (IR), clonogenic radiation survival was decreased by 40-50% under oxic conditions. Under acute hypoxia, ABT-888 radiosensitized malignant cells to a level similar to oxic radiosensitivity. To our knowledge, this is the first study to demonstrate that inhibition of PARP activity can sensitize hypoxic cancer cells and the combination of IR-PARPi has the potential to improve the therapeutic ratio of radiotherapy.

Poly(ADP-Ribose) Polymerase Inhibition as a Model for Synthetic Lethality in Developing Radiation Oncology Targets

DNA double-strand breaks (DSBs) induced during clinical radiotherapy are potent inducers of cell death. Poly(ADP-ribose) polymerase (PARP)-1 is a 113-kD nuclear protein that binds to both single- and double-strand DNA breaks and is actively involved in DNA single-strand break repair and base excision repair. Recently, potent and specific chemical inhibitors of PARP activity have been developed that are effective tumor cell radiosensitizers in vitro and in vivo. Because of synthetic lethality, PARP inhibitors may be highly effective as a single agent in patients whose tumors have germline or somatic defects in DNA damage and repair genes (eg, ATM, BRCA1, BRCA2, and NBS1) or defects in genes involved in phosphatase and tensin homolog gene (PTEN) signaling. Defects in specific DNA repair pathways also appear to enhance the radiosensitizing effects of PARP inhibition. In addition to inherent genetics, tumor cells may also be preferentially sensitized to radiotherapy by diverse mechanisms, including proliferation-dependent radiosensitization, targeting of the endothelium and tumor vasculature, and increased sensitivity to PARP inhibitors within repair-deficient hypoxic cells. Because biologically active doses of PARP inhibitors caused minimal toxicity in phase I to II clinical trials, careful scheduling of these agents in combination with radiotherapy may maintain the therapeutic ratio and increase tumor radiocurability.

Rnf8 deficiency impairs class switch recombination, spermatogenesis, and genomic integrity and predisposes for cancer.

Signaling and repair of DNA double-strand breaks (DSBs) are critical for preventing immunodeficiency and cancer. These DNA breaks result from exogenous and endogenous DNA insults but are also programmed to occur during physiological processes such as meiosis and immunoglobulin heavy chain (IgH) class switch recombination (CSR). Recent studies reported that the E3 ligase RNF8 plays important roles in propagating DNA DSB signals and thereby facilitating the recruitment of various DNA damage response proteins, such as 53BP1 and BRCA1, to sites of damage. Using mouse models for Rnf8 mutation, we report that Rnf8 deficiency leads to impaired spermatogenesis and increased sensitivity to ionizing radiation both in vitro and in vivo. We also demonstrate the existence of alternative Rnf8-independent mechanisms that respond to irradiation and accounts for the partial recruitment of 53bp1 to sites of DNA damage in activated Rnf8−/− B cells. Remarkably, IgH CSR is impaired in a gene dose-dependent manner in Rnf8 mutant mice, revealing that these mice are immunodeficient. In addition, Rnf8−/− mice exhibit increased genomic instability and elevated risks for tumorigenesis indicating that Rnf8 is a novel tumor suppressor. These data unravel the in vivo pleiotropic effects of Rnf8.

PTEN Deletion in Prostate Cancer Cells Does Not Associate with Loss of RAD51 Function: Implications for Radiotherapy and Chemotherapy

Purpose: PTEN deletions in prostate cancer are associated with tumor aggression and poor outcome. Recent studies have implicated PTEN as a determinant of homologous recombination (HR) through defective RAD51 function. Similar to BRCA1/2-defective tumor cells, PTEN-null prostate and other cancer cells have been reported to be sensitive to PARP inhibitors (PARPi). To date, no direct comparison between PTEN and RAD51 expression in primary prostate tumors has been reported. Experimental Design: Prostate cancer cell lines and xenografts with known PTEN status (22RV1-PTEN+/+, DU145-PTEN+/−, PC3-PTEN−/−) and H1299 and HCT116 cancer cells were used to evaluate how PTEN loss affects RAD51 expression and PARPi sensitivity. Primary prostate cancers with known PTEN status were analyzed for RAD51 expression. Results: PTEN status is not associated with reduced RAD51 mRNA or protein expression in primary prostate cancers. Decreased PTEN expression did not reduce RAD51 expression or clonogenic survival following PARPi among prostate cancer cells that vary in TP53 and PTEN. PARPi sensitivity instead associated with a defect in MRE11 expression. PTEN-deficient cells had only mild PARPi sensitivity and no loss of HR or RAD51 recruitment. Clonogenic cell survival following a series of DNA damaging agents was variable: PTEN-deficient cells were sensitive to ionizing radiation, mitomycin-C, UV, H2O2, and methyl methanesulfonate but not to cisplatin, camptothecin, or paclitaxel. Conclusions: These data suggest that the relationship between PTEN status and survival following DNA damage is indirect and complex. It is unlikely that PTEN status will be a direct biomarker for HR status or PARPi response in prostate cancer clinical trials. Clin Cancer Res; 18(4); 1015–27. ©2011 AACR.

Tumor hypoxia, DNA repair and prostate cancer progression: new targets and new therapies

Increasingly, the tumor microenvironment and hypoxia are being studied as potential prognostic factors in prostate cancer given their effects on the hypoxia inducible factor-1alpha and vascular endothelial growth factor signaling pathways. Based on immunohistochemical studies using hypoxic cell markers and direct oxygen-electrode measurements, clinically relevant levels of hypoxia are detected in 30-90% of prostate cancers. Exciting new data suggest that hypoxia can alter cell-cycle checkpoints and DNA repair within the prostate epithelium, thereby driving genetic instability and tumor aggression. Novel therapies designed to target the hypoxic response and resulting defective DNA repair may therefore be effective as chemoprevention agents or as adjuncts to surgery, radiotherapy and chemotherapy to improve clinical outcome.

Contextual synthetic lethality and/or loss of heterozygosity: Tumor hypoxia and modification of DNA repair

Hypoxia exists in every solid tumor and is associated with poor prognosis because of both local and systemic therapeutic resistance. Recent studies have focused on the interaction between tumor cell genetics and the dynamic state of oxygenation and metabolism. Hypoxia generates aggressive tumor cell phenotypes in part owing to ongoing genetic instability and a “mutator” phenotype. The latter may be due to suppression of DNA mismatch repair (MMR), nucleotide excision repair (NER), and double-strand break (DSB) repair. We propose a theoretical model in which hypoxia-mediated defects in DNA repair can lead to “contextual loss of heterozygosity” and drive oncogenesis. Additionally, hypoxia-mediated repair defects can be specifically targeted by DNA damaging agents and/or “contextual synthetic lethality” to kill repair-deficient cells and preserve the therapeutic ratio. These proposed concepts support the interrogation of solid tumors to document repair defects in both oxic and hypoxic tumor subregions as a conduit to novel clinical trials within the context of personalized medicine. Clin Cancer Res; 16(18); 4553–60. ©2010 AACR.

Tumor Hypoxia as a Modifier of DNA Strand Break and Cross-Link Repair

Hypoxia is a common characteristic of many solid tumors and is associated with poor prognosis. Cells with low oxygen levels can have altered sensitivity to radiotherapy and chemotherapy secondary to changes in the incidence of DNA single- and double-strand breaks (DNA-ssb, DNA-dsb), DNA base damage, DNA-DNA cross-links and DNA-protein cross-links. Recent evidence also supports that cells exposed to chronic hypoxia have a decreased capacity of DNA-dsb repair. This review will examine the influence of short-term and prolonged hypoxia on the two major pathways of DNA-dsb repair: homologous recombination (HR) and non-homologous end-joining (NHEJ). Novel treatment strategies designed to exploit the hypoxic tumor microenvironment are also discussed. Modification of DNA damage sensing and repair due to fluctuating oxygen levels within a dynamic tumor microenvironment may have profound implications for tumor progression and treatment.

Efficacy of Combining GMX1777 with Radiation Therapy for Human Head and Neck Carcinoma

Purpose: Rapidly metabolizing tumor cells have elevated levels of nicotinamide phosphoribosyltransferase, an enzyme involved in NAD+ biosynthesis, which serves as an important substrate for proteins involved in DNA repair. GMX1777, which inhibits nicotinamide phosphoribosyltransferase, was evaluated in two human head and neck cancer models in combination with radiotherapy. Experimental Design: Effects of GMX1777-mediated radiosensitization were examined via metabolic and cytotoxicity assays in vitro; mechanism of action, in vivo antitumor efficacy, and radiosensitization were also investigated. Results: IC50 values of GMX1777 for FaDu and C666-1 cells were 10 and 5 nmol/L, respectively, which interacted synergistically with radiotherapy. GMX1777 induced a rapid decline in intracellular NAD+ followed by ATP reduction associated with significant cytotoxicity. These metabolic changes were slightly increased with the addition of radiotherapy, although poly(ADP-ribose) polymerase activity was significantly reduced when GMX1777 was combined with radiotherapy, thereby accounting for the synergistic cytotoxicity of these two modalities. Systemic GMX1777 administration with local tumor radiotherapy caused complete disappearance of FaDu and C666-1 tumors for 50 and 20 days, respectively. There was also significant reduction in tumor vascularity, particularly for the more sensitive FaDu model. [18F]FDG-positron emission tomography/computed tomography images showed reduction in [18F]FDG uptake after GMX1777 administration, showing decreased glucose metabolism in vivo. Conclusions: Our data represent the first report showing that GMX1777 plus radiotherapy is an effective therapeutic strategy for head and neck cancer, mediated via pleiotropic effects of inhibition of DNA repair and tumor angiogenesis, while sparing normal tissues. Therefore, GMX1777 combined with radiotherapy definitely warrants clinical evaluation in human head and neck cancer patients. Clin Cancer Res; 16(3); 898–911

Hypoxia Provokes Base Excision Repair Changes and a Repair-Deficient, Mutator Phenotype in Colorectal Cancer Cells

Regions of acute and chronic hypoxia exist within solid tumors and can lead to increased rates of mutagenesis and/or altered DNA damage and repair protein expression. Base excision repair (BER) is responsible for resolving small, non–helix-distorting lesions from the genome that potentially cause mutations by mispairing or promoting DNA breaks during replication. Germline and somatic mutations in BER genes, such as MutY Homolog (MUTYH/MYH) and DNA-directed polymerase (POLB), are associated with increased risk of colorectal cancer. However, very little is known about the expression and function of BER proteins under hypoxic stress. Using conditions of chronic hypoxia, decreased expression of BER proteins was observed because of a mechanism involving suppressed BER protein synthesis in multiple colorectal cancer cell lines. Functional BER was impaired as determined by MYH- and 8-oxoguanine (OGG1)–specific glycosylase assays. A formamidopyrimidine-DNA glycosylase (Fpg) Comet assay revealed elevated residual DNA base damage in hypoxic cells 24 hours after H2O2 treatment as compared with normoxic controls. Similarly, high-performance liquid chromatography analysis demonstrated that 8-oxo-2′-deoxyguanosine lesions were elevated in hypoxic cells 3 and 24 hours after potassium bromate (KBrO3) treatment when compared with aerobic cells. Correspondingly, decreased clonogenic survival was observed following exposure to the DNA base damaging agents H2O2 and MMS, but not to the microtubule interfering agent paclitaxel. Thus, a persistent downregulation of BER components by the microenvironment modifies and facilitates a mutator phenotype, driving genetic instability and cancer progression. Implications: Aberrant BER is a contributing factor for the observed genetic instability in hypoxic tumor cells. Mol Cancer Res; 12(10); 1407–15. ©2014 AACR.