Human REV1 modulates the cytotoxicity and mutagenicity of cisplatin in human ovarian carcinoma cells

Rev1 overexpression accelerates N-methyl-N-nitrosourea (MNU)-induced thymic lymphoma by increasing mutagenesis

Rev1 has two important functions in the translesion synthesis pathway, including dCMP transferase activity, and acts as a scaffolding protein for other polymerases involved in translesion synthesis. However, the role of Rev1 in mutagenesis and tumorigenesis in vivo remains unclear. We previously generated Rev1-overexpressing (Rev1-Tg) mice and reported that they exhibited a significantly increased incidence of intestinal adenoma and thymic lymphoma (TL) after N-methyl-N-nitrosourea (MNU) treatment. In this study, we investigated mutagenesis of MNU-induced TL tumorigenesis in wild-type (WT) and Rev1-Tg mice using diverse approaches, including whole-exome sequencing (WES). In Rev1-Tg TLs, the mutation frequency was higher than that in WT TL in most cases. However, no difference in the number of nonsynonymous mutations in the Catalogue of Somatic Mutations in Cancer (COSMIC) genes was observed, and mutations involved in Notch1 and MAPK signaling were similarly detected in both TLs. Mutational signature analysis of WT and Rev1-Tg TLs revealed cosine similarity with COSMIC mutational SBS5 (aging-related) and SBS11 (alkylation-related). Interestingly, the total number of mutations, but not the genotypes of WT and Rev1-Tg, was positively correlated with the relative contribution of SBS5 in individual TLs, suggesting that genetic instability could be accelerated in Rev1-Tg TLs. Finally, we demonstrated that preleukemic cells could be detected earlier in Rev1-Tg mice than in WT mice, following MNU treatment. In conclusion, Rev1 overexpression accelerates mutagenesis and increases the incidence of MNU-induced TL by shortening the latency period, which may be associated with more frequent DNA damage-induced genetic instability.

Leveraging the replication stress response to optimize cancer therapy

High-fidelity DNA replication is critical for the faithful transmission of genetic information to daughter cells. Following genotoxic stress, specialized DNA damage tolerance pathways are activated to ensure replication fork progression. These pathways include translesion DNA synthesis, template switching and repriming. In this Review, we describe how DNA damage tolerance pathways impact genome stability, their connection with tumorigenesis and their effects on cancer therapy response. We discuss recent findings that single-strand DNA gap accumulation impacts chemoresponse and explore a growing body of evidence that suggests that different DNA damage tolerance factors, including translesion synthesis polymerases, template switching proteins and enzymes affecting single-stranded DNA gaps, represent useful cancer targets. We further outline how the consequences of DNA damage tolerance mechanisms could inform the discovery of new biomarkers to refine cancer therapies.

REV1: A novel biomarker and potential therapeutic target for various cancers

Background: REV1 is a member of the translesion synthesis DNA polymerase Y family. It is an essential player in a variety of DNA replication activities, and perform major roles in the production of both spontaneous and DNA damage-induced mutations. This study aimed to explore the role of REV1 as a prognostic biomarker and its potential function regulating the sensitivity of anti-tumor drugs in various cancers.

Methods: We analyzed the impact of REV1 gene alterations on patient prognosis and the impact of different REV1 single nucleotide polymorphisms (SNP) on protein structure and function using multiple online prediction servers. REV1 expression was assessed using data from Oncomine, TCGA, and TIMER database. The correlation between REV1 expression and patient prognosis was performed using the PrognoScan and Kaplan-Meier plotter databases. The IC50 values of anti-cancer drugs were downloaded from the Genomics of Drug Sensitivity in Cancer database and the correlation analyses between REV1 expression and each drug pathway’s IC50 value in different tumor types were conducted.

Results: Progression free survival was longer in REV1 gene altered group comparing to unaltered group [Median progression free survival (PFS), 107.80 vs. 60.89 months, p value = 7.062e-3]. REV1 SNP rs183737771 (F427L) was predicted to be deleterious SNP. REV1 expression differs in different tumour types. Low REV1 expression is associated with better prognosis in colorectal disease specific survival (DSS), disease-free survival (DFS), gastric overall survival (OS), post progression survival (PPS) and ovarian (OS, PPS) cancer while high REV1 expression is associated with better prognosis in lung [OS, relapse free survival (RFS), first progession (FP), PPS] and breast (DSS, RFS) cancer. In colon adenocarcinoma and rectum adenocarcinoma and lung adenocarcinoma, low expression of REV1 may suggest resistance to drugs in certain pathways. Conversely, high expression of REV1 in acute myeloid leukemia, brain lower grade glioma, small cell lung cancer and thyroid carcinoma may indicate resistance to drugs in certain pathways.

Conclusion: REV1 plays different roles in different tumor types, drug susceptibility, and related biological events. REV1 expression is significantly correlated with different prognosis in colorectal, ovarian, lung, breast, and gastric cancer. REV1 expression can be used as predictive marker for various drugs of various pathways in different tumors.

REV1 promotes lung tumorigenesis by activating the Rad18/SERTAD2 axis

REV1 is the central member of the family of TLS polymerases, which participate in various DNA damage repair and tolerance pathways and play a significant role in maintaining genomic stability. However, the role of REV1 in tumors is rarely reported. In this study, we found that the expression of REV1 was significantly upregulated in lung cancer tissues compared with matched adjacent tissues and was associated with poor prognosis. Functional experiments demonstrated that REV1 silencing decreased the growth and proliferation capacity of lung cancer cells. Mechanistically, REV1 upregulated the expression of SERTAD2 in a Rad18-dependent manner, thereby promoting lung carcinogenesis. A novel REV1 inhibitor, JH-RE-06, suppressed lung tumorigenesis in vivo and in vitro and was shown to be safe and well tolerated. Our study confirmed that REV1 is a potential diagnostic marker and therapeutic target for lung cancer and that JH-RE-06 may be a safe and efficient therapeutic agent for NSCLC.

Targeting protein-protein interactions in the DNA damage response pathways for cancer chemotherapy

Cellular DNA damage response (DDR) is an extensive signaling network that orchestrates DNA damage recognition, repair and avoidance, cell cycle progression and cell death. DDR alteration is a hallmark of cancer, with the deficiency in one DDR capability often compensated by a dependency on alternative pathways endowing cancer cells with survival and growth advantage. Targeting these DDR pathways has provided multiple opportunities for the development of cancer therapies. Traditional drug discovery has mainly focused on catalytic inhibitors that block enzyme active sites, which limits the number of potential drug targets within the DDR pathways. This review article describes the emerging approach to the development of cancer therapeutics targeting essential protein-protein interactions (PPIs) in the DDR network. The overall strategy for the structure-based design of small molecule PPI inhibitors is discussed, followed by an overview of the major DNA damage sensing, DNA repair, and DNA damage tolerance pathways with a specific focus on PPI targets for anti-cancer drug design. The existing small molecule inhibitors of DDR PPIs are summarized that selectively kill cancer cells and/or sensitize cancers to front-line genotoxic therapies, and a range of new PPI targets are proposed that may lead to the development of novel chemotherapeutics.

Translesion DNA Synthesis and Reinitiation of DNA Synthesis in Chemotherapy Resistance

Many chemotherapy drugs block tumor cell division by damaging DNA. DNA polymerases eta (Pol η), iota (Pol ι), kappa (Pol κ), REV1 of the Y-family and zeta (Pol ζ) of the B-family efficiently incorporate nucleotides opposite a number of DNA lesions during translesion DNA synthesis. Primase-polymerase PrimPol and the Pol α-primase complex reinitiate DNA synthesis downstream of the damaged sites using their DNA primase activity. These enzymes can decrease the efficacy of chemotherapy drugs, contribute to the survival of tumor cells and to the progression of malignant diseases. DNA polymerases are promising targets for increasing the effectiveness of chemotherapy, and mutations and polymorphisms in some DNA polymerases can serve as additional prognostic markers in a number of oncological disorders.

Translesion synthesis inhibitors as a new class of cancer chemotherapeutics

Introduction: Translesion synthesis (TLS) is a DNA damage tolerance mechanism that replaces the replicative DNA polymerase with a specialized, low-fidelity TLS DNA polymerase that can copy past DNA lesions during active replication. Recent studies have demonstrated a primary role for TLS in replicating past DNA lesions induced by first-line genotoxic agents, resulting in decreased efficacy and acquired chemoresistance. With this in mind, targeting TLS as a combination strategy with first-line genotoxic agents has emerged as a promising approach to develop a new class of anti-cancer adjuvant agents. Areas covered: In this review, we provide a brief background on TLS and its role in cancer. We also discuss the identification and development of inhibitors that target various TLS DNA polymerases or key protein-protein interactions (PPIs) in the TLS machinery. Expert opinion: TLS inhibitors have demonstrated initial promise; however, their continued study is essential to more fully understand the clinical potential of this emerging class of anti-cancer chemotherapeutics. It will be important to determine whether a specific protein involved in TLS is an optimal target. In addition, an expanded understanding of what current genotoxic chemotherapies synergize with TLS inhibitors will guide the clinical strategies for devising combination therapies.

Post-replication repair: Rad5/HLTF regulation, activity on undamaged templates, and relationship to cancer

The eukaryotic post-replication repair (PRR) pathway allows completion of DNA replication when replication forks encounter lesions on the DNA template and are mediated by post-translational ubiquitination of the DNA sliding clamp proliferating cell nuclear antigen (PCNA). Monoubiquitinated PCNA recruits translesion synthesis (TLS) polymerases to replicate past DNA lesions in an error-prone manner while addition of K63-linked polyubiquitin chains signals for error-free template switching to the sister chromatid. Central to both branches is the E3 ubiquitin ligase and DNA helicase Rad5/helicase-like transcription factor (HLTF). Mutations in PRR pathway components lead to genomic rearrangements, cancer predisposition, and cancer progression. Recent studies have challenged the notion that the PRR pathway is involved only in DNA lesion tolerance and have shed new light on its roles in cancer progression. Molecular details of Rad5/HLTF recruitment and function at replication forks have emerged. Mounting evidence indicates that PRR is required during lesion-less replication stress, leading to TLS polymerase activity on undamaged templates. Analysis of PRR mutation status in human cancers and PRR function in cancer models indicates that down regulation of PRR activity is a viable strategy to inhibit cancer cell growth and reduce chemoresistance. Here, we review these findings, discuss how they change our views of current PRR models, and look forward to targeting the PRR pathway in the clinic.

DNA repair pathways and cisplatin resistance: an intimate relationship

The main goal of chemotherapeutic drugs is to induce massive cell death in tumors. Cisplatin is an antitumor drug widely used to treat several types of cancer. Despite its remarkable efficiency, most tumors show intrinsic or acquired drug resistance. The primary biological target of cisplatin is genomic DNA, and it causes a plethora of DNA lesions that block transcription and replication. These cisplatin-induced DNA lesions strongly induce cell death if they are not properly repaired or processed. To counteract cisplatin-induced DNA damage, cells use an intricate network of mechanisms, including DNA damage repair and translesion synthesis. In this review, we describe how cisplatin-induced DNA lesions are repaired or tolerated by cells and focus on the pivotal role of DNA repair and tolerance mechanisms in tumor resistance to cisplatin. In fact, several recent clinical findings have correlated the tumor cell status of DNA repair/translesion synthesis with patient response to cisplatin treatment. Furthermore, these mechanisms provide interesting targets for pharmacological modulation that can increase the efficiency of cisplatin chemotherapy.

Co-inhibition of Pol η and ATR sensitizes cisplatin-resistant non-small cell lung cancer cells to cisplatin by impeding DNA damage repair

For the majority of patients with advanced non-small cell lung cancer (NSCLC), the standard of care remains platinum-based chemotherapy. However, cisplatin resistance is a big obstacle to the treatment, and elucidation of its mechanism is warranted. In this study, we showed that there was no difference in intracellular uptake of cisplatin or the removal of platinum-DNA adducts between a cisplatin-resistant NSCLC cell line (A549/DR) and a cisplatin-sensitive NSCLC cell line (A549). However, the capacity to repair DNA interstrand crosslinks (ICLs) and double-strand breaks (DSBs) was significantly enhanced in the A549/DR cell line compared to 3 cisplatin-sensitive cell lines. We found that the protein and mRNA expression levels of Pol η, a Y-family translesion synthesis (TLS) polymerase, were markedly increased upon cisplatin exposure in A549/DR cells compared with A549 cells. Furthermore, intracellular co-localization of Pol η and proliferation cell nuclear antigen (PCNA) induced by cisplatin or cisplatin plus gemcitabine treatment was inhibited by depleting ataxia telangiectasia mutated and Rad-3-related (ATR). Pol η depletion by siRNA sensitized A549/DR cells to cisplatin; co-depletion of Pol η and ATR further increased A549/DR cell death induced by cisplatin or cisplatin plus gemcitabine compared to depletion of Pol η or ATR alone, concomitant with inhibition of DNA ICL and DSB repair and accumulation of DNA damage. No additional sensitization effect of co-depleting Pol η and ATR was observed in A549 cells. These results demonstrate that co-inhibition of Pol η and ATR reverses the drug resistance of cisplatin-resistant NSCLC cells by blocking the repair of DNA ICLs and DSBs induced by cisplatin or cisplatin plus gemcitabine.

Small-molecules that bind to the ubiquitin-binding motif of REV1 inhibit REV1 interaction with K164-monoubiquitinated PCNA and suppress DNA damage tolerance

REV1 protein is a mutagenic DNA damage tolerance (DDT) mediator and encodes two ubiquitin-binding motifs (i.e., UBM1 and UBM2) that are essential for the DDT function. REV1 interacts with K164-monoubiquitinated PCNA (UbPCNA) in cells upon DNA-damaging stress. By using AlphaScreen assays to detect inhibition of REV1 and UbPCNA protein interactions along with an NMR-based strategy, we identified small-molecule compounds that inhibit the REV1/UbPCNA interaction and that directly bind to REV1 UBM2. In cells, one of the compound prevented recruitment of REV1 to PCNA foci on chromatin upon cisplatin treatment, delayed removal of UV-induced cyclopyrimidine dimers from nuclei, prevented UV-induced mutation of HPRT gene, and diminished clonogenic survival of cells that were challenged by cyclophosphamide or cisplatin. This study demonstrates the potential utility of a small-molecule REV1 UBM2 inhibitor for preventing DDT.

A Small-Molecule Inhibitor of Human DNA Polymerase η Potentiates the Effects of Cisplatin in Tumor Cells

Translesion DNA synthesis (TLS) performed by human DNA polymerase eta (hpol η) allows tolerance of damage from cis-diamminedichloroplatinum(II) (CDDP or cisplatin). We have developed hpol η inhibitors derived from N-aryl-substituted indole barbituric acid (IBA), indole thiobarbituric acid (ITBA), and indole quinuclidine scaffolds and identified 5-((5-chloro-1-(naphthalen-2-ylmethyl)-1H-indol-3-yl)methylene)-2-thioxodihydropyrimidine-4,6(1H,5H)-dione (PNR-7-02), an ITBA derivative that inhibited hpol η activity with an IC₅₀ value of 8 μM and exhibited 5-10-fold specificity for hpol η over replicative pols. We conclude from kinetic analyses, chemical footprinting assays, and molecular docking that PNR-7-02 binds to a site on the little finger domain and interferes with the proper orientation of template DNA to inhibit hpol η. A synergistic increase in CDDP toxicity was observed in hpol η-proficient cells co-treated with PNR-7-02 (combination index values = 0.4-0.6). Increased γH2AX formation accompanied treatment of hpol η-proficient cells with CDDP and PNR-7-02. Importantly, PNR-7-02 did not impact the effect of CDDP on cell viability or γH2AX in hpol η-deficient cells. In summary, we observed hpol η-dependent effects on DNA damage/replication stress and sensitivity to CDDP in cells treated with PNR-7-02. The ability to employ a small-molecule inhibitor of hpol η to improve the cytotoxic effect of CDDP may aid in the development of more effective chemotherapeutic strategies.

Translesion DNA Synthesis in Cancer: Molecular Mechanisms and Therapeutic Opportunities

The genomic landscape of cancer is one marred by instability, but the mechanisms that underlie these alterations are multifaceted and remain a topic of intense research. Cellular responses to DNA damage and/or replication stress can affect genome stability in tumors and influence the response of patients to therapy. In addition to direct repair, DNA damage tolerance (DDT) is an element of genomic maintenance programs that contributes to the etiology of several types of cancer. DDT mechanisms primarily act to resolve replication stress, and this can influence the effectiveness of genotoxic drugs. Translesion DNA synthesis (TLS) is an important component of DDT that facilitates direct bypass of DNA adducts and other barriers to replication. The central role of TLS in the bypass of drug-induced DNA lesions, the promotion of tumor heterogeneity, and the involvement of these enzymes in the maintenance of the cancer stem cell niche presents an opportunity to leverage inhibition of TLS as a way of improving existing therapies. In the review that follows, we summarize mechanisms of DDT, misregulation of TLS in cancer, and discuss the potential for targeting these pathways as a means of improving cancer therapies.

Overexpression of Rev1 promotes the development of carcinogen-induced intestinal adenomas via accumulation of point mutation and suppression of apoptosis proportionally to the Rev1 expression level

Cancer development often involves mutagenic replication of damaged DNA by the error-prone translesion synthesis (TLS) pathway. Aberrant activation of this pathway plays a role in tumorigenesis by promoting genetic mutations. Rev1 controls the function of the TLS pathway, and Rev1 expression levels are associated with DNA damage induced cytotoxicity and mutagenicity. However, it remains unclear whether deregulated Rev1 expression triggers or promotes tumorigenesis in vivo. In this study, we generated a novel Rev1-overexpressing transgenic (Tg) mouse and characterized its susceptibility to tumorigenesis. Using a small intestinal tumor model induced by N-methyl-N-nitrosourea (MNU), we found that transgenic expression of Rev1 accelerated intestinal adenoma development in proportion to the Rev1 expression level; however, overexpression of Rev1 alone did not cause spontaneous development of intestinal adenomas. In Rev1 Tg mice, MNU-induced mutagenesis was elevated, whereas apoptosis was suppressed. The effects of hREV1 expression levels on the cytotoxicity and mutagenicity of MNU were confirmed in the human cancer cell line HT1080. These data indicate that dysregulation of cellular Rev1 levels leads to the accumulation of mutations and suppression of cell death, which accelerates the tumorigenic activities of DNA-damaging agents.

Identification of Small Molecule Translesion Synthesis Inhibitors That Target the Rev1-CT/RIR Protein-Protein Interaction

Translesion synthesis (TLS) is an important mechanism through which proliferating cells tolerate DNA damage during replication. The mutagenic Rev1/Polζ-dependent branch of TLS helps cancer cells survive first-line genotoxic chemotherapy and introduces mutations that can contribute to the acquired resistance so often observed with standard anticancer regimens. As such, inhibition of Rev1/Polζ-dependent TLS has recently emerged as a strategy to enhance the efficacy of first-line chemotherapy and reduce the acquisition of chemoresistance by decreasing tumor mutation rate. The TLS DNA polymerase Rev1 serves as an integral scaffolding protein that mediates the assembly of the active multiprotein TLS complexes. Protein-protein interactions (PPIs) between the C-terminal domain of Rev1 (Rev1-CT) and the Rev1-interacting region (RIR) of other TLS DNA polymerases play an essential role in regulating TLS activity. To probe whether disrupting the Rev1-CT/RIR PPI is a valid approach for developing a new class of targeted anticancer agents, we designed a fluorescence polarization-based assay that was utilized in a pilot screen for small molecule inhibitors of this PPI. Two small molecule scaffolds that disrupt this interaction were identified, and secondary validation assays confirmed that compound 5 binds to Rev1-CT at the RIR interface. Finally, survival and mutagenesis assays in mouse embryonic fibroblasts and human fibrosarcoma HT1080 cells treated with cisplatin and ultraviolet light indicate that these compounds inhibit mutagenic Rev1/Polζ-dependent TLS in cells, validating the Rev1-CT/RIR PPI for future anticancer drug discovery and identifying the first small molecule inhibitors of TLS that target Rev1-CT.

Targeting the Translesion Synthesis Pathway for the Development of Anti-Cancer Chemotherapeutics

Human cells possess tightly controlled mechanisms to rescue DNA replication following DNA damage caused by environmental and endogenous carcinogens using a set of low-fidelity translesion synthesis (TLS) DNA polymerases. These polymerases can copy over replication blocking DNA lesions while temporarily leaving them unrepaired, preventing cell death at the expense of increasing mutation rates and contributing to the onset and progression of cancer. In addition, TLS has been implicated as a major cellular mechanism promoting acquired resistance to genotoxic chemotherapy. Owing to its central role in mutagenesis and cell survival after DNA damage, inhibition of the TLS pathway has emerged as a potential target for the development of anticancer agents. This review will recap our current understanding of the structure and regulation of DNA polymerase complexes that mediate TLS and describe how this knowledge is beginning to translate into the development of small molecule TLS inhibitors.

Association of polymorphisms in translesion synthesis genes with prognosis of advanced non-small-cell lung cancer patients treated with platinum-based chemotherapy

BACKGROUND AND OBJECTIVES

Translesion synthesis (TLS) polymerases enable cells to bypass or overcome DNA damage during DNA replication and contributes to genomic instability and cancer. Inhibition of the expression of TLS genes enhances the sensitivity of cancer cells to cisplatin. This study aimed to investigate the relationship between single nucleotide polymorphisms (SNPs) in the TLS genes and clinical outcome of advanced non-small-cell lung cancer (NSCLC) patients treated with platinum-based chemotherapy.

METHODS

A total of 16 SNPs were genotyped and analyzed in 302 advanced NSCLC patients (discovery set), and the results were further validated in additional 428 NSCLC patients (validation set).

RESULTS

Analyses revealed significant associations of two SNPs, rs3213801 and rs3792136, with overall survival, with the lowest combined P values of 0.003 and 0.016, respectively. These effects also remained in stratification analyses by clinical variables. Furthermore, the number of risk genotypes of the two SNPs showed a cumulative effect on overall survival (P = 0.03).

CONCLUSIONS

Genetic polymorphisms in the TLS genes might serve as potential predictive biomarkers of prognosis of advanced NSCLC patients treated with platinum-based chemotherapy.

Translesion polymerase genes polymorphisms and haplotypes influence survival of osteosarcoma patients

Cytotoxic activity of most chemotherapeutic agents is based on their ability to induce DNA damage. Interstrand crosslinks are among the most detrimental forms of DNA damage as both DNA strands are affected. As translesion polymerases participate in their repair, they may be important for response to chemotherapeutic agents that induce such lesions, including commonly used cisplatin. Altered expression of translesion polymerase genes REV1 and REV3L may modify sensitivity to cisplatin. As osteosarcoma patients are commonly treated with cisplatin-based chemotherapy, our aim was to investigate if REV1 and REV3L polymorphisms influence survival of osteosarcoma patients treated with cisplatin-based chemotherapy. We determined the genotypes of common functional tag REV1 and REV3L polymorphisms in 66 osteosarcoma patients. Cox regression was used for survival analysis. Carriers of at least one polymorphic REV1 rs3087403 allele had significantly shorter EFS and overall survival (OS) (p = 0.004; HR = 3.79; 95%CI = 1.53-9.35 and p < 0.001; HR = 4.44; 95%CI = 1.92-10.27, respectively). Combination of REV1 rs3087403 and REV3L rs462779 polymorphisms was also significantly associated with shorter OS (ptrend<0.001) and shorter EFS (ptrend = 0.003). The results of this first study on polymorphisms in translesion polymerase genes in osteosarcoma suggest they could help predict outcome of cisplatin-based chemotherapy in osteosarcoma patients.

Enhanced expression of DNA polymerase eta contributes to cisplatin resistance of ovarian cancer stem cells

Cancer stem cells (CSCs) with enhanced tumorigenicity and chemoresistance are believed to be responsible for treatment failure and tumor relapse in ovarian cancer patients. However, it is still unclear how CSCs survive DNA-damaging agent treatment. Here, we report an elevated expression of DNA polymerase η (Pol η) in ovarian CSCs isolated from both ovarian cancer cell lines and primary tumors, indicating that CSCs may have intrinsically enhanced translesion DNA synthesis (TLS). Down-regulation of Pol η blocked cisplatin-induced CSC enrichment both in vitro and in vivo through the enhancement of cisplatin-induced apoptosis in CSCs, indicating that Pol η-mediated TLS contributes to the survival of CSCs upon cisplatin treatment. Furthermore, our data demonstrated a depletion of miR-93 in ovarian CSCs. Enforced expression of miR-93 in ovarian CSCs reduced Pol η expression and increased their sensitivity to cisplatin. Taken together, our data suggest that ovarian CSCs have intrinsically enhanced Pol η-mediated TLS, allowing CSCs to survive cisplatin treatment, leading to tumor relapse. Targeting Pol η, probably through enhancement of miR-93 expression, might be exploited as a strategy to increase the efficacy of cisplatin treatment.

Cancer therapy and replication stress: forks on the road to perdition

Deregulated DNA replication occurs in cancer where it contributes to genomic instability. This process is a target of cytotoxic therapies. Chemotherapies exploit high DNA replication in cancer cells by modifying the DNA template or by inhibiting vital enzymatic activities that lead to slowing or stalling replication fork progression. Stalled replication forks can be converted into toxic DNA double-strand breaks resulting in cell death, i.e., replication stress. While likely crucial for many cancer treatments, replication stress is poorly understood due to its complexity. While we still know relatively little about the role of replication stress in cancer therapy, technical advances in recent years have shed new light on the effect that cancer therapeutics have on replication forks and the molecular mechanisms that lead from obstructed fork progression to cell death. This chapter will give an overview of our current understanding of replication stress in the context of cancer therapy.

Can pharmacogenetics explain efficacy and safety of cisplatin pharmacotherapy?

Several recent pharmacogenetic studies have investigated the variability in both outcome and toxicity in cisplatin-based therapies. These studies have focused on the genetic variability of therapeutic targets that could affect cisplatin response and toxicity in diverse type of cancer including lung, gastric, ovarian, testicular, and esophageal cancer. In this review, we seek to update the reader in this area of investigation, focusing primarily on DNA reparation enzymes and cisplatin metabolism through Glutathione S-Transferases (GSTs). Current evidence indicates a potential application of pharmacogenetics in therapeutic schemes in which cisplatin is the cornerstone of these treatments. Therefore, a collaborative effort is required to study these molecular characteristics in order to generate a genetic panel with clinical utility.

Polymorphisms in translesion polymerase genes influence treatment outcome in malignant mesothelioma

Aim: We evaluated the influence of genetic variability in translesion polymerases REV1 and REV3L on the outcome of cisplatin treatment in malignant mesothelioma patients. Materials & methods: In total, 139 malignant mesothelioma patients were genotyped for seven tag SNPs in REV1 and REV3L. Logistic regression and Cox regression were used to assess the influence of SNPs on treatment outcome. Results: Polymorphic REV1 rs3087403 allele and REV1 TGT haplotype were associated with increased risk for leukopenia (p = 0.013 and p = 0.047, respectively) and neutropenia (p = 0.048 and p = 0.024, respectively). REV3L rs465646, rs462779 and REV3L CCGG haplotype were significantly associated with longer overall survival (p = 0.007, p = 0.022 and p = 0.013, respectively). Conclusion: Our results suggest for the first time that REV1 and REV3L SNPs might serve as potential predictive markers of outcome of cisplatin-based chemotherapy.

Original submitted 7 October 2013; Revision submitted 15 January 2014

REV1 and DNA polymerase zeta in DNA interstrand crosslink repair

DNA interstrand crosslinks (ICLs) are covalent linkages between two strands of DNA, and their presence interferes with essential metabolic processes such as transcription and replication. These lesions are extremely toxic, and their repair is essential for genome stability and cell survival. In this review, we will discuss how the removal of ICLs requires interplay between multiple genome maintenance pathways and can occur in the absence of replication (replication-independent ICL repair) or during S phase (replication-coupled ICL repair), the latter being the predominant pathway used in mammalian cells. It is now well recognized that translesion DNA synthesis (TLS), especially through the activities of REV1 and DNA polymerase zeta (Polζ), is necessary for both ICL repair pathways operating throughout the cell cycle. Recent studies suggest that the convergence of two replication forks upon an ICL initiates a cascade of events including unhooking of the lesion through the actions of structure-specific endonucleases, thereby creating a DNA double-stranded break (DSB). TLS across the unhooked lesion is necessary for restoring the sister chromatid before homologous recombination repair. Biochemical and genetic studies implicate REV1 and Polζ as being essential for performing lesion bypass across the unhooked crosslink, and this step appears to be important for subsequent events to repair the intermediate DSB. The potential role of Fanconi anemia pathway in the regulation of REV1 and Polζ-dependent TLS and the involvement of additional polymerases, including DNA polymerases kappa, nu, and theta, in the repair of ICLs is also discussed in this review.

The roles of DNA polymerase ζ and the Y family DNA polymerases in promoting or preventing genome instability

Cancer cells display numerous abnormal characteristics which are initiated and maintained by elevated mutation rates and genome instability. Chromosomal DNA is continuously surveyed for the presence of damage or blocked replication forks by the DNA Damage Response (DDR) network. The DDR is complex and includes activation of cell cycle checkpoints, DNA repair, gene transcription, and induction of apoptosis. Duplicating a damaged genome is associated with elevated risks to fork collapse and genome instability. Therefore, the DNA damage tolerance (DDT) pathway is also employed to enhance survival and involves the recruitment of translesion DNA synthesis (TLS) polymerases to sites of replication fork blockade or single stranded DNA gaps left after the completion of replication in order to restore DNA to its double stranded form before mitosis. TLS polymerases are specialized for inserting nucleotides opposite DNA adducts, abasic sites, or DNA crosslinks. By definition, the DDT pathway is not involved in the actual repair of damaged DNA, but provides a mechanism to tolerate DNA lesions during replication thereby increasing survival and lessening the chance for genome instability. However this may be associated with increased mutagenesis. In this review, we will describe the specialized functions of Y family polymerases (Rev1, Polη, Polι and Polκ) and DNA polymerase ζ in lesion bypass, mutagenesis, and prevention of genome instability, the latter due to newly appreciated roles in DNA repair. The recently described role of the Fanconi anemia pathway in regulating Rev1 and Polζ-dependent TLS is also discussed in terms of their involvement in TLS, interstrand crosslink repair, and homologous recombination.

Structural insights into the assembly of human translesion polymerase complexes

In addition to DNA repair pathways, cells utilize translesion DNA synthesis (TLS) to bypass DNA lesions during replication. During TLS, Y-family DNA polymerase (Polη, Polκ, Polı and Rev1) inserts specific nucleotide opposite preferred DNA lesions, and then Polζ consisting of two subunits, Rev3 and Rev7, carries out primer extension. Here, we report the complex structures of Rev3-Rev7-Rev1(CTD) and Rev3-Rev7-Rev1(CTD)-Polκ(RIR). These two structures demonstrate that Rev1(CTD) contains separate binding sites for Polκ and Rev7. Our BIAcore experiments provide additional support for the notion that the interaction between Rev3 and Rev7 increases the affinity of Rev7 and Rev1. We also verified through FRET experiment that Rev1, Rev3, Rev7 and Polκ form a stable quaternary complex in vivo, thereby suggesting an efficient switching mechanism where the "inserter" polymerase can be immediately replaced by an "extender" polymerase within the same quaternary complex.

Structural basis of Rev1-mediated assembly of a quaternary vertebrate translesion polymerase complex consisting of Rev1, heterodimeric polymerase (Pol) ζ, and Pol κ

DNA synthesis across lesions during genomic replication requires concerted actions of specialized DNA polymerases in a potentially mutagenic process known as translesion synthesis. Current models suggest that translesion synthesis in mammalian cells is achieved in two sequential steps, with a Y-family DNA polymerase (κ, η, ι, or Rev1) inserting a nucleotide opposite the lesion and with the heterodimeric B-family polymerase ζ, consisting of the catalytic Rev3 subunit and the accessory Rev7 subunit, replacing the insertion polymerase to carry out primer extension past the lesion. Effective translesion synthesis in vertebrates requires the scaffolding function of the C-terminal domain (CTD) of Rev1 that interacts with the Rev1-interacting region of polymerases κ, η, and ι and with the Rev7 subunit of polymerase ζ. We report the purification and structure determination of a quaternary translesion polymerase complex consisting of the Rev1 CTD, the heterodimeric Pol ζ complex, and the Pol κ Rev1-interacting region. Yeast two-hybrid assays were employed to identify important interface residues of the translesion polymerase complex. The structural elucidation of such a quaternary translesion polymerase complex encompassing both insertion and extension polymerases bridged by the Rev1 CTD provides the first molecular explanation of the essential scaffolding function of Rev1 and highlights the Rev1 CTD as a promising target for developing novel cancer therapeutics to suppress translesion synthesis. Our studies support the notion that vertebrate insertion and extension polymerases could structurally cooperate within a megatranslesion polymerase complex (translesionsome) nucleated by Rev1 to achieve efficient lesion bypass without incurring an additional switching mechanism.

Multifaceted recognition of vertebrate Rev1 by translesion polymerases ζ and κ

Translesion synthesis is a fundamental biological process that enables DNA replication across lesion sites to ensure timely duplication of genetic information at the cost of replication fidelity, and it is implicated in development of cancer drug resistance after chemotherapy. The eukaryotic Y-family polymerase Rev1 is an essential scaffolding protein in translesion synthesis. Its C-terminal domain (CTD), which interacts with translesion polymerase ζ through the Rev7 subunit and with polymerases κ, ι, and η in vertebrates through the Rev1-interacting region (RIR), is absolutely required for function. We report the first solution structures of the mouse Rev1 CTD and its complex with the Pol κ RIR, revealing an atypical four-helix bundle. Using yeast two-hybrid assays, we have identified a Rev7-binding surface centered at the α2-α3 loop and N-terminal half of α3 of the Rev1 CTD. Binding of the mouse Pol κ RIR to the Rev1 CTD induces folding of the disordered RIR peptide into a three-turn α-helix, with the helix stabilized by an N-terminal cap. RIR binding also induces folding of a disordered N-terminal loop of the Rev1 CTD into a β-hairpin that projects over the shallow α1-α2 surface and creates a deep hydrophobic cavity to interact with the essential FF residues juxtaposed on the same side of the RIR helix. Our combined structural and biochemical studies reveal two distinct surfaces of the Rev1 CTD that separately mediate the assembly of extension and insertion translesion polymerase complexes and provide a molecular framework for developing novel cancer therapeutics to inhibit translesion synthesis.

The Rev1 translesion synthesis polymerase has multiple distinct DNA binding modes

Rev1 is a eukaryotic DNA polymerase of the Y family involved in translesion synthesis (TLS), a major damage tolerance pathway that allows DNA replication at damaged templates. Uniquely amongst the Y family polymerases, the N-terminal part of Rev1, dubbed the BRCA1 C-terminal homology (BRCT) region, includes a BRCT domain. While most BRCT domains mediate protein-protein interactions, Rev1 contains a predicted α-helix N-terminal to the BRCT domain and in human Replication Factor C (RFC) such a BRCT region endows the protein with DNA binding capacity. Here, we studied the DNA binding properties of yeast and mouse Rev1. Our results show that the BRCT region of Rev1 specifically binds to a 5' phosphorylated, recessed, primer-template junction. This DNA binding depends on the extra α-helix, N-terminal to the BRCT domain. Surprisingly, a stretch of 20 amino acids N-terminal to the predicted α-helix is also critical for high-affinity DNA binding. In addition to 5' primer-template junction binding, Rev1 efficiently binds to a recessed 3' primer-template junction. These dual DNA binding characteristics are discussed in view of the proposed recruitment of Rev1 by 5' primer-template junctions, downstream of stalled replication forks.

Molecular chaperone Hsp90 regulates REV1-mediated mutagenesis

REV1 is a Y-family polymerase that plays a central role in mutagenic translesion DNA synthesis (TLS), contributing to tumor initiation and progression. In a current model, a monoubiquitinated form of the replication accessory protein, proliferating cell nuclear antigen (PCNA), serves as a platform to recruit REV1 to damaged sites on the DNA template. Emerging evidence indicates that posttranslational mechanisms regulate REV1 in yeast; however, the regulation of REV1 in higher eukaryotes is poorly understood. Here we show that the molecular chaperone Hsp90 is a critical regulator of REV1 in human cells. Hsp90 specifically binds REV1 in vivo and in vitro. Treatment with a specific inhibitor of Hsp90 reduces REV1 protein levels in several cell types through proteasomal degradation. This is associated with suppression of UV-induced mutagenesis. Furthermore, Hsp90 inhibition disrupts the interaction between REV1 and monoubiquitinated PCNA and suppresses UV-induced focus formation. These results indicate that Hsp90 promotes folding of REV1 into a stable and/or functional form(s) to bind to monoubiquitinated PCNA. The present findings reveal a novel role of Hsp90 in the regulation of TLS-mediated mutagenesis.

Proteasomal regulation of the mutagenic translesion DNA polymerase, Saccharomyces cerevisiae Rev1

Translesion DNA synthesis (TLS) functions as a tolerance mechanism for DNA damage at a potentially mutagenic cost. Three TLS polymerases (Pols) function to bypass DNA damage in Saccharomyces cerevisiae: Rev1, Pol ζ, a heterodimer of the Rev3 and Rev7 proteins, and Pol η (Rad30). Our lab has shown that S. cerevisiae Rev1 protein levels are under striking cell cycle regulation, being ∼50-fold higher during G2/M than during G1 and much of S phase (Waters and Walker, 2006). REV1 transcript levels only vary ∼3-fold in a similar cell cycle pattern, suggesting a posttranscriptional mechanism controls protein levels. Here, we show that the S. cerevisiae Rev1 protein is unstable during both the G1 and the G2/M phases of the cell cycle, however, the protein's half-life is shorter in G1 arrested cells than in G2/M arrested cells, indicating that the rate of proteolysis strongly contributes to Rev1's cell cycle regulation. In the presence of the proteasome inhibitor, MG132, the steady-state levels and half-life of Rev1 increase during G1 and G2/M. Through the use of a viable proteasome mutant, we confirm that the levels of Rev1 protein are dependent on proteasome-mediated degradation. The accumulation of higher migrating forms of Rev1 under certain conditions shows that the degradation of Rev1 is possibly directed through the addition of a polyubiquitination signal or another modification. These results support a model that proteasomal degradation acts as a regulatory system of mutagenic TLS mediated by Rev1.

Suppression of Rev3, the catalytic subunit of Pol{zeta}, sensitizes drug-resistant lung tumors to chemotherapy

Platinum-based chemotherapeutic drugs are front-line therapies for the treatment of non-small cell lung cancer. However, intrinsic drug resistance limits the clinical efficacy of these agents. Recent evidence suggests that loss of the translesion polymerase, Polζ, can sensitize tumor cell lines to cisplatin, although the relevance of these findings to the treatment of chemoresistant tumors in vivo has remained unclear. Here, we describe a tumor transplantation approach that enables the rapid introduction of defined genetic lesions into a preclinical model of lung adenocarcinoma. Using this approach, we examined the effect of impaired translesion DNA synthesis on cisplatin response in aggressive late-stage lung cancers. In the presence of reduced levels of Rev3, an essential component of Polζ, tumors exhibited pronounced sensitivity to cisplatin, leading to a significant extension in overall survival of treated recipient mice. Additionally, treated Rev3-deficient cells exhibited reduced cisplatin-induced mutation, a process that has been implicated in the induction of secondary malignancies following chemotherapy. Taken together, our data illustrate the potential of Rev3 inhibition as an adjuvant therapy for the treatment of chemoresistant malignancies, and highlight the utility of rapid transplantation methodologies for evaluating mechanisms of chemotherapeutic resistance in preclinical settings.

Unconventional ubiquitin recognition by the ubiquitin-binding motif within the Y family DNA polymerases iota and Rev1

Translesion synthesis is an essential cell survival strategy to promote replication after DNA damage. The accumulation of Y family polymerases (pol) iota and Rev1 at the stalled replication machinery is mediated by the ubiquitin-binding motifs (UBMs) of the polymerases and enhanced by PCNA monoubiquitination. We report the solution structures of the C-terminal UBM of human pol iota and its complex with ubiquitin. Distinct from other ubiquitin-binding domains, the UBM binds to the hydrophobic surface of ubiquitin centered at L8. Accordingly, mutation of L8A, but not I44A, of ubiquitin abolishes UBM binding. Human pol iota contains two functional UBMs, both contributing to replication foci formation. In contrast, only the second UBM of Saccharomyces cerevisiae Rev1 binds to ubiquitin and is essential for Rev1-dependent cell survival and mutagenesis. Point mutations disrupting the UBM-ubiquitin interaction also impair the accumulation of pol iota in replication foci and Rev1-mediated DNA damage tolerance in vivo.

REV3L confers chemoresistance to cisplatin in human gliomas: the potential of its RNAi for synergistic therapy

The REV3L gene, encoding the catalytic subunit of human polymerase zeta, plays a significant role in the cytotoxicity, mutagenicity, and chemoresistance of certain tumors. However, the role of REV3L in regulating the sensitivity of glioma cells to chemotherapy remains unknown. In this study, we investigated the expression of the REV3L gene in 10 normal brain specimens and 30 human glioma specimens and examined the value of REV3L as a potential modulator of cellular response to various DNA-damaging agents. Reverse transcriptase PCR/real-time PCR analysis revealed that REV3L was overexpressed in human gliomas compared with normal brain tissues. A glioma cell model with stable overexpression of REV3L was used to probe the role of REV3L in cisplatin treatment; upregulation of REV3L markedly attenuated cisplatin-induced apoptosis of the mitochondrial apoptotic pathway. We therefore assessed the REV3L-targeted treatment modality that combines suppression of REV3L expression using RNA interference (RNAi) with the cytotoxic effects of DNA-damaging agents. Downregulation of REV3L expression significantly enhanced the sensitivity of glioma cells to cisplatin, as evidenced by the increased apoptosis rate and marked alterations in the anti-apoptotic proteins B-cell lymphoma 2 (Bcl-2) and B-cell lymphoma-extra large (Bcl-xl) and proapoptotic Bcl-2-associated x protein (Bax) expression levels, and reduced mutation frequencies in surviving glioma cells. These results suggest that REV3L may potentially contribute to gliomagenesis and play a crucial role in regulating cellular response to the DNA cross-linking agent cisplatin. Our findings indicate that RNAi targeting REV3L combined with chemotherapy has synergistic therapeutic effects on glioma cells, which warrants further investigation as an effective novel therapeutic regimen for patients with this malignancy.

Eukaryotic translesion polymerases and their roles and regulation in DNA damage tolerance

DNA repair and DNA damage tolerance machineries are crucial to overcome the vast array of DNA damage that a cell encounters during its lifetime. In this review, we summarize the current state of knowledge about the eukaryotic DNA damage tolerance pathway translesion synthesis (TLS), a process in which specialized DNA polymerases replicate across from DNA lesions. TLS aids in resistance to DNA damage, presumably by restarting stalled replication forks or filling in gaps that remain in the genome due to the presence of DNA lesions. One consequence of this process is the potential risk of introducing mutations. Given the role of these translesion polymerases in mutagenesis, we discuss the significant regulatory mechanisms that control the five known eukaryotic translesion polymerases: Rev1, Pol zeta, Pol kappa, Pol eta, and Pol iota.

Mechanism of double-base lesion bypass catalyzed by a Y-family DNA polymerase

As a widely used anticancer drug, cis-diamminedichloroplatinum(II) (cisplatin) reacts with adjacent purine bases in DNA to form predominantly cis-[Pt(NH₃)₂{d(GpG)-N7(1),-N7(2)}] intrastrand cross-links. Drug resistance, one of the major limitations of cisplatin therapy, is partially due to the inherent ability of human Y-family DNA polymerases to perform translesion synthesis in the presence of DNA-distorting damage such as cisplatin–DNA adducts. To better understand the mechanistic basis of translesion synthesis contributing to cisplatin resistance, this study investigated the bypass of a single, site-specifically placed cisplatin-d(GpG) adduct by a model Y-family DNA polymerase, Sulfolobus solfataricus DNA polymerase IV (Dpo4). Dpo4 was able to bypass this double-base lesion, although, the incorporation efficiency of dCTP opposite the first and second cross-linked guanine bases was decreased by 72- and 860-fold, respectively. Moreover, the fidelity at the lesion decreased up to two orders of magnitude. The cisplatin-d(GpG) adduct affected six downstream nucleotide incorporations, but interestingly the fidelity was essentially unaltered. Biphasic kinetic analysis supported a universal kinetic mechanism for the bypass of DNA lesions catalyzed by various translesion DNA polymerases. In conclusion, if human Y-family DNA polymerases adhere to this bypass mechanism, then translesion synthesis by these error-prone enzymes is likely accountable for cisplatin resistance observed in cancer patients.

REV1 genetic variants associated with the risk of cervical carcinoma

PURPOSE

To explore the REV1 genetic variants effect the risk of cervical carcinoma.

METHODS

Total 543 cases, including 282 carcinoma and 261 CIN, and 480 normal controls were performed. Two single nucleotide polymorphisms (SNPs) (REV1 Phe257Ser and REV1 Asn373Ser) were genotyped by PCR-squencing, and analysis the correlation to clinical character including HPV infection.

RESULTS

Compared with the REV1 Phe257Ser, women carrying Ser257Ser and Phe257Ser genotypes had a significantly decreased the risk for cervical carcinoma or cervical squamous cell carcinoma. On contrary, homozygous Ser373Ser increased the risk for carcinoma. In addition, we found that the association of Phe257Ser and Asn373Ser with the risk for cervical carcinoma was specific to squamous cell carcinomas and not relevant for adenocarcinoma. Our results suggest that women carry Phe257Ser variant genotype decrease the risk for cervical carcinoma, more in women that have high-risk sexual reproductive histories, when women who carried Asn373Ser variant genotype and had high-risk sexual and reproductive histories had a significantly elevated risk for cervical carcinoma.

CONCLUSION

Our results support Phe257Ser and Ser257Ser genotypes are associated with a decreased risk for cervical carcinoma, while Asn373Ser and Ser373Ser genotypes increased the risk. In addition, the effects were more significant in the groups with high-risk sexual and reproductive histories.

DNA polymerase zeta (pol zeta) in higher eukaryotes

Most current knowledge about DNA polymerase zeta (pol zeta) comes from studies of the enzyme in the budding yeast Saccharomyces cerevisiae, where pol zeta consists of a complex of the catalytic subunit Rev3 with Rev7, which associates with Rev1. Most spontaneous and induced mutagenesis in yeast is dependent on these gene products, and yeast pol zeta can mediate translesion DNA synthesis past some adducts in DNA templates. Study of the homologous gene products in higher eukaryotes is in a relatively early stage, but additional functions for the eukaryotic proteins are already apparent. Suppression of vertebrate REV3L function not only reduces induced point mutagenesis but also causes larger-scale genome instability by raising the frequency of spontaneous chromosome translocations. Disruption of Rev3L function is tolerated in Drosophila, Arabidopsis, and in vertebrate cell lines under some conditions, but is incompatible with mouse embryonic development. Functions for REV3L and REV7(MAD2B) in higher eukaryotes have been suggested not only in translesion DNA synthesis but also in some forms of homologous recombination, repair of interstrand DNA crosslinks, somatic hypermutation of immunoglobulin genes and cell-cycle control. This review discusses recent developments in these areas.

Mechanisms of resistance to cisplatin and carboplatin

While cisplatin and carboplatin are active versus most common cancers, epithelial malignancies are incurable when metastatic. Even if an initial response occurs, acquired resistance due to mutations and epigenetic events limits efficacy. Resistance may be due to excess of a resistance factor, to saturation of factors required for tumor cell killing, or to mutation or alteration of a factor required for tumor cell killing. Platinum resistance could arise from decreased tumor blood flow, extracellular conditions, reduced platinum uptake, increased efflux, intracellular detoxification by glutathione, etc., decreased binding (e.g., due to high intracellular pH), DNA repair, decreased mismatch repair, defective apoptosis, antiapoptotic factors, effects of several signaling pathways, or presence of quiescent non-cycling cells. In lung cancer, flattening of dose-response curves at higher doses suggests that efficacy is limited by exhaustion of something required for cell killing, and several clinical observations suggest epigenetic events may play a major role in resistance.