Mutagenic and genotoxic effects of DNA adducts formed by the anticancer drug cis-diamminedichloroplatinum(II)

Diagnostic yield of colonoscopy surveillance in testicular cancer survivors treated with platinum-based chemotherapy: study protocol of a prospective cross-sectional cohort study

Background

Testicular cancer (TC) survivors have an increased risk of various second primary malignancies. A recent cohort study detected an increased risk of colorectal cancer (CRC) in TC survivors treated with platinum-based chemotherapy with a hazard ratio of 3.9. CRC risk increased with higher cisplatin-dose. We know that colonoscopy surveillance in high-risk populations results in reduced incidence and mortality of CRC. TC survivors treated with platinum-based chemotherapy can potentially benefit from colonoscopy surveillance; however, to which extent is unknown. Furthermore, the pathogenesis of these secondary CRCs is unknown, and better insights into the carcinogenesis may affect surveillance decisions.

Methods

This prospective multicenter study will be performed in four Dutch hospitals. TC survivors are eligible if treated with ≥ 3 cycles of cisplatin before age 50. Colonoscopy will be performed ≥ 8 years after initial treatment (minimum and maximum ages at colonoscopy, 35 and 75 years, respectively). The primary aim of the study is the diagnostic yield of advanced neoplasia detected during colonoscopy. As secondary aim, we will evaluate the molecular profile of advanced colorectal neoplasia and will assess current platinum levels in blood and urine and correlate blood-platinum levels with prevalence of colorectal lesions. Furthermore, we will investigate effectiveness of fecal immunochemical testing (FIT) and burden of colonoscopy by two questionnaires. Demographic data, previous history, results of colonoscopy, hemoglobin level of FIT and results of molecular and platinum levels will be obtained. Yield of colonoscopy will be determined by detection rate of adenoma and serrated lesions, advanced adenoma detection rate and CRC detection rate. The MISCAN model will be used for cost-effectiveness analyses of CRC surveillance. With 234 participants undergoing colonoscopy, we can detect an absolute difference of 6% of advanced neoplasia with 80% power.

Discussion

TC survivors treated with cisplatin-based chemotherapy can benefit from CRC surveillance. Evaluation of the diagnostic performance and patient acceptance of CRC surveillance is of importance to develop surveillance recommendations. Insight into the carcinogenesis of cisplatin-related advanced colorectal lesions will contribute to CRC prevention in the increasing number of TC survivors. The results may also be important for the many other cancer survivors treated with platinum-based chemotherapy.

Trial registration

Clinical Trials: NCT04180033, November 27, 2019, https://clinicaltrials.gov/ct2/show/NCT04180033.

Biological Evaluation of DNA Biomarkers in a Chemically Defined and Site-Specific Manner

As described elsewhere in this Special Issue on biomarkers, much progress has been made in the detection of modified DNA within organisms at endogenous and exogenous levels of exposure to chemical species, including putative carcinogens and chemotherapeutic agents. Advances in the detection of damaged or unnatural bases have been able to provide correlations to support or refute hypotheses between the level of exposure to oxidative, alkylative, and other stresses, and the resulting DNA damage (lesion formation). However, such stresses can form a plethora of modified nucleobases, and it is therefore difficult to determine the individual contribution of a particular modification to alter a cell's genetic fate, as measured in the form of toxicity by stalled replication past the damage, by subsequent mutation, and by lesion repair. Chemical incorporation of a modification at a specific site within a vector (site-specific mutagenesis) has been a useful tool to deconvolute what types of damage quantified in biologically relevant systems may lead to toxicity and/or mutagenicity, thereby allowing researchers to focus on the most relevant biomarkers that may impact human health. Here, we will review a sampling of the DNA modifications that have been studied by shuttle vector techniques.

Polymerase bypass of N7-guanine monoadducts of cisplatin, diepoxybutane, and epichlorohydrin

DNA oligonucleotides containing site-specific N7-guanine monoadducts of cisplatin, diepoxybutane, and epichlorohydrin were used as templates for DNA synthesis by two bacterial DNA polymerases and human polymerase β. These polymerases were able to bypass the lesions effectively, although the efficiency was decreased, with inhibition increasing with the size of the lesion. Fidelity of incorporation was essentially unaltered, suggesting that N7-guanine monoadducts do not significantly contribute to the mutational spectra of these agents.

MutY-glycosylase: an overview on mutagenesis and activities beyond the GO system

MutY is a glycosylase known for its role in DNA base excision repair (BER). It is critically important in the prevention of DNA mutations derived from 7,8-dihydro-8-oxoguanine (8-oxoG), which are the major lesions resulting from guanine oxidation. MutY has been described as a DNA repair enzyme in the GO system responsible for removing adenine residues misincorporated in 8-oxoG:A mispairs, avoiding G:C to T:A mutations. Further studies have shown that this enzyme binds to other mispairs, interacts with several enzymes, avoids different transversions/transitions in DNA, and is involved in different repair pathways. Additional activities have been reported for MutY, such as the repair of replication errors in newly synthesized DNA strands through its glycosylase activity. Moreover, MutY is a highly conserved enzyme present in several prokaryotic and eukaryotic organisms. MutY defects are associated with a hereditary colorectal cancer syndrome termed MUTYH-associated polyposis (MAP). Here, we have reviewed the roles of MutY in the repair of mispaired bases in DNA as well as its activities beyond the GO system.

Ruthenium dihydroxybipyridine complexes are tumor activated prodrugs due to low pH and blue light induced ligand release

Ruthenium drugs are potent anti-cancer agents, but inducing drug selectivity and enhancing their modest activity remain challenging. Slow Ru ligand loss limits the formation of free sites and subsequent binding to DNA base pairs. Herein, we designed a ligand that rapidly dissociates upon irradiation at low pH. Activation at low pH can lead to cancer selectivity, since many cancer cells have higher metabolism (and thus lower pH) than non-cancerous cells. We have used the pH sensitive ligand, 6,6'-dihydroxy-2,2'-bipyridine (66'bpy(OH)2), to generate [Ru(bpy)2(66'(bpy(OH)2)](2+), which contains two acidic hydroxyl groups with pKa1=5.26 and pKa2=7.27. Irradiation when protonated leads to photo-dissociation of the 66'bpy(OH)2 ligand. An in-depth study of the structural and electronic properties of the complex was carried out using X-ray crystallography, electrochemistry, UV/visible spectroscopy, and computational techniques. Notably, RuN bond lengths in the 66'bpy(OH)2 complex are longer (by ~0.3Å) than in polypyridyl complexes that lack 6 and 6' substitution. Thus, the longer bond length predisposes the complex for photo-dissociation and leads to the anti-cancer activity. When the complex is deprotonated, the 66'bpy(O(-))2 ligand molecular orbitals mix heavily with the ruthenium orbitals, making new mixed metal-ligand orbitals that lead to a higher bond order. We investigated the anti-cancer activities of [Ru(bpy)2(66'(bpy(OH)2)](2+), [Ru(bpy)2(44'(bpy(OH)2)](2+), and [Ru(bpy)3](2+) (44'(bpy(OH)2=4,4'-dihydroxy-2,2'-bipyridine) in HeLa cells, which have a relatively low pH. It is found that [Ru(bpy)2(66'(bpy(OH)2)](2+) is more cytotoxic than the other ruthenium complexes studied. Thus, we have identified a pH sensitive ruthenium scaffold that can be exploited for photo-induced anti-cancer activity.

Epistatic role of base excision repair and mismatch repair pathways in mediating cisplatin cytotoxicity

Base excision repair (BER) and mismatch repair (MMR) pathways play an important role in modulating cis-Diamminedichloroplatinum (II) (cisplatin) cytotoxicity. In this article, we identified a novel mechanistic role of both BER and MMR pathways in mediating cellular responses to cisplatin treatment. Cells defective in BER or MMR display a cisplatin-resistant phenotype. Targeting both BER and MMR pathways resulted in no additional resistance to cisplatin, suggesting that BER and MMR play epistatic roles in mediating cisplatin cytotoxicity. Using a DNA Polymerase β (Polβ) variant deficient in polymerase activity (D256A), we demonstrate that MMR acts downstream of BER and is dependent on the polymerase activity of Polβ in mediating cisplatin cytotoxicity. MSH2 preferentially binds a cisplatin interstrand cross-link (ICL) DNA substrate containing a mismatch compared with a cisplatin ICL substrate without a mismatch, suggesting a novel mutagenic role of Polβ in activating MMR in response to cisplatin. Collectively, these results provide the first mechanistic model for BER and MMR functioning within the same pathway to mediate cisplatin sensitivity via non-productive ICL processing. In this model, MMR participation in non-productive cisplatin ICL processing is downstream of BER processing and dependent on Polβ misincorporation at cisplatin ICL sites, which results in persistent cisplatin ICLs and sensitivity to cisplatin.

nfxB as a novel target for analysis of mutation spectra in Pseudomonas aeruginosa

nfxB encodes a negative regulator of the mexCD-oprJ genes for drug efflux in the opportunistic pathogen Pseudomonas aeruginosa. Inactivating mutations in this transcriptional regulator constitute one of the main mechanisms of resistance to ciprofloxacin (Cip^r). In this work, we evaluated the use of nfxB/Cip^r as a new test system to study mutation spectra in P. aeruginosa. The analysis of 240 mutations in nfxB occurring spontaneously in the wild-type and mutator backgrounds or induced by mutagens showed that nfxB/Cip^r offers several advantages compared with other mutation detection systems. Identification of nfxB mutations was easy since the entire open reading frame and its promoter region were sequenced from the chromosome using a single primer. Mutations detected in nfxB included all transitions and transversions, 1-bp deletions and insertions, >1-bp deletions and duplications. The broad mutation spectrum observed in nfxB relies on the selection of loss-of-function changes, as we confirmed by generating a structural model of the NfxB repressor and evaluating the significance of each detected mutation. The mutation spectra characterized in the mutS, mutT, mutY and mutM mutator backgrounds or induced by the mutagenic agents 2-aminopurine, cisplatin and hydrogen peroxide were in agreement with their predicted mutational specificities. Additionally, this system allowed the analysis of sequence context effects since point mutations occurred at 85 different sites distributed over the entire nfxB. Significant hotspots and preferred sequence contexts were observed for spontaneous and mutagen-induced mutation spectra. Finally, we demonstrated the utility of a luminescence-based reporter for identification of nfxB mutants previous to sequencing analysis. Thus, the nfxB/Cip^r system in combination with the luminescent reporter may be a valuable tool for studying mutational processes in Pseudomonas spp. wherein the genes encoding the NfxB repressor and the associated efflux pump are conserved.

A role for Myh1 in DNA repair after treatment with strand-breaking and crosslinking chemotherapeutic agents

The highly conserved DNA glycosylase MutY is implicated in repair of oxidative DNA damage, in particular in removing adenines misincorporated opposite 7,8-dihydro-8-oxoguanine (8-oxo-G). The MutY homologues (MutYH) physically associate with proteins implicated in replication, DNA repair, and checkpoint signaling, specifically with the DNA damage sensor complex 9-1-1 proteins. Here, we ask whether MutYH could have a broader function in sensing and repairing different types of DNA damage induced by conventional chemotherapeutics. Thus, we examined if deletion of the Schizosaccharomyces pombe MutY homologue, Myh1, alone or in combination with deletion of either component of the 9-1-1 sensor complex, influences survival after exposure to different classes of DNA damaging chemotherapeutics that do not act primarily by causing 8-oxoG lesions. We show that Myh1 contributes to survival on genotoxic stresses induced by the oxidizing, DNA double strand break-inducing, bleomycins, or the DNA crosslinking platinum compounds, particularly in a rad1 mutant background. Exposure of cells to cisplatin leads to a moderate overall accumulation of Myh1 protein. Interestingly, we found that DNA damage induced by phleomycin results in increased chromatin association of Myh1. Further, we demonstrate that Myh1 relocalizes to the nucleus after exposure to hydrogen peroxide or chemotherapeutics, most prominently seen after phleomycin treatment. These observations indicate a wider role of Myh1 in DNA repair and DNA damage-induced checkpoint activation than previously thought.

Thermodynamics of translesion synthesis across a major DNA adduct of antitumor oxaliplatin: differential scanning calorimetric study

Differential scanning calorimetry (DSC) was used to measure the thermodynamic changes associated with translesion synthesis across major lesion induced in DNA by antitumor oxaliplatin [1,2-d(GG) intrastrand cross-link]. Insertion of matched nucleotides dC at the primer terminus (across unique 3'- or 5'-dG in the unplatinated template) and subsequent extensions resulted in an incremental increase in thermodynamic parameters. In contrast, incorporation of dC opposite either platinated dG in the intrastrand cross-link formed in the template strand and subsequent extensions by one nucleotide resulted only in little changes in thermodynamics. A similar thermodynamic delay was observed for a control template primer containing a dG:dT mismatch across 3'- or 5'-dG in the template and subsequent Watson-Crick primer extensions. The thermodynamic scarcity generated by either the lesion or mismatches was not localized but extended to the 5'-downstream sites, which may be connected with the phenomenon termed "short-term memory" of replication errors retained by some DNA polymerases responding to DNA damages or mismatches. Interestingly, formation of the 1,2-d(GG) intrastrand cross-link of oxaliplatin altered the overall DSC profiles of the dG:dT mismatch template/primers only in a very small extent. While addition of matched nucleotide dC across either dG in the template strand was thermodynamically favored over the presence of a mismatched dT (ΔΔG(0)(310) was 7.6 or 6.8 kJ mol(-1), ΔΔH was 14 or 49 kJ mol(-1)), no such thermodynamic advantage was observed with the 1,2-d(GG) intrastrand cross-link of oxaliplatin at these positions (ΔΔG(0)(310) was 2.8 or -0.3 kJ mol(-1), ΔΔH was 4 or 9 kJ mol(-1)). The equilibrium thermodynamic data also provide insight into the processes associated with misincorporation of incorrect nucleotides during replication bypass across major cross-links of antitumor oxaliplatin. On the other hand, besides thermodynamic effects also kinetic factors play an important role in the processing of the cross-links of antitumor platinum drugs. The impact of the two effects in overall processing DNA adducts by a particular DNA polymerase will depend on its nature.

Differences in the cellular response and signaling pathways between cisplatin and monodentate organometallic Ru(II) antitumor complexes containing a terphenyl ligand

The new monofunctional Ru(II)-arene complex [(η⁶-arene)Ru(II)(en)Cl]+, where en = 1,2-diaminoethane and the arene is para-terphenyl (complex 1) exhibits promising cytotoxic effects in human tumor cells including those resistant to conventional cisplatin (J. Med. Chem.2008, 51, 5310). The present study is focused on the cellular pharmacology of 1 to elucidate more deeply the mechanisms underlying its antitumor effects. We have identified several cellular mechanisms induced by 1 in human ovarian carcinoma cells, including inhibition of DNA synthesis, overexpression and activation of p53, expression of proapoptotic proteins p21(WAF1) and Bax, G₀/G₁ arrest, and nuclear fragmentation as a result of apoptotic, and, to a much lower extent, also necrotic processes. Thus, 1 inhibits growth of the cancer cells through induction of apoptotic cell death and G₀/G₁ cell cycle arrest. Further investigations have shown that 1 induces apoptosis by regulating the expression of Bcl-2 family proteins. There were significant differences in cellular responses to the treatment with 1 and with conventional cisplatin, particularly in the kinetics and the extent of these responses. In addition, the distinct p53 activation profile of 1 compared with cisplatin provides an explanation for the activity of this ruthenium drug against cisplatin-resistant cells. Hence complex 1 may provide an alternative therapy in patients with acquired cisplatin resistance, particularly with respect to its very low mutagenicity and different mode of action compared to platinum antitumor drugs in clinical use.

Biological properties of single chemical-DNA adducts: a twenty year perspective

The genome and its nucleotide precursor pool are under sustained attack by radiation, reactive oxygen and nitrogen species, chemical carcinogens, hydrolytic reactions, and certain drugs. As a result, a large and heterogeneous population of damaged nucleotides forms in all cells. Some of the lesions are repaired, but for those that remain, there can be serious biological consequences. For example, lesions that form in DNA can lead to altered gene expression, mutation, and death. This perspective examines systems developed over the past 20 years to study the biological properties of single DNA lesions.

Genotoxicity (mitotic recombination) of the cancer chemotherapeutic agents cisplatin and cytosine arabinoside in Aspergillus nidulans

Cisplatin (cis-diamminedichloroplatinum, cis-DDP) and cytosine arabinoside (ara-C) are anticancer drugs used in the treatment of human cancer. The two chemotherapeutic drugs were tested in current research for their recombinogenic potential in diploid cells of Aspergillus nidulans. Non-cytotoxic concentrations of ara-C (0.4 and 0.8 microM) and cis-DDP (1.5, 3.0 and 6.0 microM) were strong recombinagens in A. nidulans UT448//A757 diploid strain, which induced homozygosis of recessive genetic markers, previously present in heterozygous condition. Drugs significantly increased homozygosity index (HI) values for five nutritional genetic markers when compared with those determined in the absence of anticancer drugs. Since mitotic recombination is a mechanism leading to malignant growth through loss of heterozygosity at tumor-suppressor loci, ara-C and cis-DDP may be characterized as secondary promoters of malignant neoplasia in diagnosed cancer patients, after chemotherapy treatment.

QSAR of genotoxic active benzazoles

Previously synthesized 2,5-disubstituted benzoxazole and benzimidazole derivatives, were tested for their genotoxic activity in the Bacillus subtilis rec- assay. The results revealed that 5-methyl-2-(p-aminobenzyl)benzoxazole exhibited the highest genotoxic response, which was comparable to 4-nitroquinoline 1-oxide (4-NQO), the reference agent of classical positive mutagen. Among the other tested compounds, four showed a genotoxic activity. A QSAR study revealed that structural parameters IY(C(2)H(4)) and IY(CH(2)O), indicating the bridge elements between the phenyl moiety and the fused ring system at position 2 and the quantum chemical parameter (DeltaE ), showing the difference between HOMO and LUMO energies, were found significant for enhancing the genotoxic activity in these compounds. In addition, the substituent effects on positions R and R(1) were found important for the activity as well as holding a substituent possessing a maximum length with a minimum width property on position R(1) like alkyl groups. On the other hand, substituting position R with an electron donating group instead of electron withdrawing group increased the genotoxic activity.

A model for triplet mutation formation based on error-prone translesional DNA synthesis opposite UV photolesions

A triplet mutation is defined as multiple base substitutions or frameshifts within a three-nucleotide sequence which includes a dipyrimidine sequence. Triplet mutations have recently been identified as a new type of UV-specific mutation, although the mechanism of their formation is unknown. A total of 163 triplet mutations were identified through an extensive search of previously published data on UV-induced mutations, including mutations from skin, skin cancer, and cultured mammalian cells. Seven common patterns of sequence changes were found: Type I, NTC-->TTT; Type IIa, NCC-->PyTT or PyCT (Py, pyrimidine); Type IIb, TCC-->PuTT or PuCT (Pu, purine); Type III, NCC-->NAT or NTA; Type IV, NTT-->AAT; Type Va, NCT-->NTX; and Type Vb, PuCT-->XTT (N and X, independent anonymous bases). Furthermore, it is suggested that the type of UV lesion responsible for each of these triplet mutation classes are (a) pyrimidine(6-4)pyrimidone photoproducts for Types I, IIb, III, IV and Vb, (b) cyclobutane pyrimidine dimers for Type Va, and (c) Dewar valence isomers for Types IIa and IIb. These estimations are based primarily on results from previous studies using photolyases specific for each type of UV lesion. A model is proposed to explain the formation of each type of triplet mutation, based on error-prone translesional DNA synthesis opposite UV-specific photolesions. The model is largely consistent with the 'A-rule', and predicts error-prone insertions not only opposite photolesions but also opposite the undamaged template base one-nucleotide downstream from the lesions.

G1 arrest induction represents a critical determinant for cisplatin cytotoxicity in G1 checkpoint-retaining human cancers

Cisplatin has been used effectively to treat various human cancer types; yet, the precise mechanism underlying its cytotoxicity remains unknown. In eukaryotes, progression through G1 is monitored by a checkpoint, which executes G1 arrest in the event of DNA damage to allow time for repair before initiating DNA replication. The retinoblastoma tumor suppressor gene is an integral component of the mammalian G1 checkpoint. The utility of the retinoblastoma gene as a therapeutic for human cancers has been investigated. Intriguingly, the cytotoxicity profile of the retinoblastoma gene therapy closely parallels the clinical targets of cisplatin. It prompted an investigation into the potential role of the checkpoint-induced G1 arrest in cisplatin cytotoxicity. Here, the evidence that G1 arrest induction represents a critical step in cisplatin-induced lytic path is presented. First, cisplatin-treated human cancer cells undergo a prolonged G1 arrest before dying. Second, triggering G1 arrest via infection with a recombinant adenovirus expressing the human retinoblastoma gene is sufficient to potentiate lethality in the absence of cisplatin. Third, the extent of the lethality induced correlates with the G1-arresting potential of the ectopically expressed human retinoblastoma polypeptide. Fourth, human cancer cells resistant to cisplatin do not undergo G1 arrest despite cisplatin treatment. The above mechanism may be exploited to develop therapeutics that preserve the efficacy of cisplatin yet bypass its mutagenicity associated with the formation of secondary tumors.

The 5'-GNC site for DNA interstrand cross-linking is conserved for diepoxybutane stereoisomers

The bifunctional alkylating agent 1,2,3,4-diepoxybutane forms interstrand DNA-DNA cross-links between the N7 positions of deoxyguanosine residues on opposite strands of the duplex. For racemic diepoxybutane, these cross-links predominate within 5'-GNC/3'CNG sequences, where N is any nucleotide. We used denaturing polyacrylamide gel electrophoresis (dPAGE) to examine the role of stereochemistry in the cross-linking reaction, subjecting a restriction fragment to cross-linking with S,S-DEB, R,R-DEB, or meso-DEB. DNA cross-links generated by each isomer were isolated by dPAGE, and the sites of cross-linking were identified by sequencing gel analysis of DNA fragments generated by hot piperidine cleavage. We found that the 5'-GNC consensus sequence of racemic DEB is conserved, but the efficiencies of cross-linking vary, with S,S- > R,R- > meso-DEB. These results help explain the observed differences between the biological activities of DEB stereoisomers.

DNA polymerase zeta accounts for the reduced cytotoxicity and enhanced mutagenicity of cisplatin in human colon carcinoma cells that have lost DNA mismatch repair

The mutagenicity of cis-diamminedichloroplatinum(II) (DDP; cisplatin) and the rate at which resistance develops with repeated exposure to DDP are dependent on mutagenic translesional replication across DDP DNA adducts, mediated in part by DNA polymerase zeta, and on the integrity of the DNA mismatch repair (MMR) system. The aim of this study was to determine whether disabling Pol zeta by suppressing expression of its hREV3 subunit in human cancer cells can reduce the mutagenicity of DDP and whether loss of MMR facilitates mutagenic Pol zeta-dependent translesional bypass. The HCT116+ch3 (MMR(+)/REV3(+)) and HCT116 (MMR(-)/REV3(+)) human colon carcinoma cell lines were engineered to suppress hREV3 mRNA by stable expression of a short hairpin interfering RNA targeted to hREV3. The effect of knocking down REV3 expression was to completely offset the DDP resistance mediated by loss of MMR. Knockdown of REV3 also reduced the mutagenicity of DDP and eliminated the enhanced mutagenicity of DDP observed in the MMR(-)/REV3(+) cells. Similar results were obtained when the ability of the cells to express luciferase from a platinated plasmid was measured. We conclude that Pol zeta plays a central role in the mutagenic bypass of DDP adducts and that the DDP resistance, enhanced mutagenicity, and the increased capacity of MMR(-)/REV3(+) cells to express a gene burdened by DDP adducts are all dependent on the Pol zeta pathway.

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

REV1 interacts with Y-type DNA polymerases (Pol) and Pol zeta to bypass many types of adducts that block the replicative DNA polymerases. This pathway accounts for many of the mutations induced by cisplatin (cis-diamminedichloroplatinium II, DDP). This study sought to determine how increasing human REV1 (hREV1) affects the cytotoxicity and mutagenicity of DDP. Human ovarian carcinoma 2008 cells were transfected with an hREV1 expression vector and 4 sublines developed in which the hREV1 mRNA level was increased by 6.3- to 23.4-fold and hREV1 protein by 2.7- to 6.2-fold. The sublines were 1.3- to 1.7-fold resistant to the cytotoxic effect of DDP and 2.3- to 5.1-fold hypersensitive to the mutagenic effect of DDP. The hREV1-transfected sublines were 1.5- to 1.8-fold better than the parental 2008 cells at managing DDP adducts as assessed by their ability to express Renilla reniformis luciferase from a vector that had been extensively loaded with DDP adducts before transfection. Increased hREV1 expression was associated with a 1.5-fold increase in the rate at which the whole population acquired resistance to DDP during sequential cycles of drug exposure. Increasing the abundance of hREV1 thus resulted in both resistance to DDP and a significant elevation in DDP-induced mutagenicity. This was accompanied by an enhanced capacity to synthesize a functional protein from a DDP-damaged gene and, most importantly, by more rapid development of resistance during sequential cycles of DDP exposure that mimic clinical schedules of DDP administration. We conclude that hREV1-dependent processes are important determinants of DDP-induced genomic instability and the development of resistance.

Evidence for Escherichia coli polymerase II mutagenic bypass of intrastrand DNA crosslinks

The mutagenic potentials of DNAs containing site- and stereospecific intrastrand DNA crosslinks were evaluated in Escherichia coli cells that contained a full complement of DNA polymerases or were deficient in either polymerases II, IV, or V. Crosslinks were made between adjacent N(6)-N(6) adenines and consisted of R,R- and S,S-butadiene crosslinks and unfunctionalized 2-, 3-, and 4-carbon tethers. Although replication of single-stranded DNAs containing the unfunctionalized 3- and 4-carbon tethers were non-mutagenic in all strains tested, replication past all the other intrastrand crosslinks was mutagenic in all E. coli strains, except the one deficient in polymerase II in which no mutations were ever detected. However, when mutagenesis was analyzed in cells induced for SOS, mutations were not detected, suggesting a possible change in the overall fidelity of polymerase II under SOS conditions. These data suggest that DNA polymerase II is responsible for the in vivo mutagenic bypass of these lesions in wild-type E. coli.

Comparative analysis of the mutagenic activity of oxaliplatin and cisplatin in the Hprt gene of CHO cells

Oxaliplatin is a platinum-derived antitumor drug that is active against cisplatin-resistant tumors and has lower overall toxicity than does cisplatin. DNA adduct formation is believed to mediate the cytotoxic activity of both compounds; however, the adducts may also be responsible for mutagenic and secondary tumorigenic activities. In this study, we have compared the mutagenicity of oxaliplatin and cisplatin in the Hprt gene of CHO-K1 cells. Both drugs produced dose-related increases in mutant frequency. For 1-hr treatments, oxaliplatin was less mutagenic than cisplatin at equimolar doses, while similar mutant frequencies were induced at equitoxic doses. Sequencing of mutant Hprt genes indicated that the mutation spectra of both oxaliplatin and cisplatin were significantly different from the spontaneous mutation spectrum (P = 0.014 and P = 0.008, respectively). A significant difference was also observed between the spectra of oxaliplatin- and cisplatin-induced mutations (P = 0.033). Although G:C-->T:A transversion was the most common mutation produced by both compounds, oxaliplatin produced higher frequencies of A:T-->T:A transversion than did cisplatin, most commonly at nucleotide 307, and higher frequencies of small deletions/insertions. Also, cisplatin induced tandem base-pair substitutions, mainly at positions 135/136, and a higher frequency of G:C-->A:T transition than did oxaliplatin. These results provide the first evidence that oxaliplatin is mutagenic and that the profiles of cisplatin- and oxaliplatin-induced mutations display not only similarities but also distinctive features relating to the type and sequence-context preference for mutation. Environ.

Suppression of hREV1 expression reduces the rate at which human ovarian carcinoma cells acquire resistance to cisplatin

Replicative bypass of many DNA adducts is dependent on the interaction of hREV1 with DNA polymerase zeta and potentially with members of the Y family of DNA polymerases. To examine the role of hREV1 in the development of cisplatin (DDP) resistance, a subline (2008-shREV1-3.3) of the ovarian carcinoma cell line 2008 was isolated in which stable expression of a short hairpin RNA suppressed hREV1 expression to 20% and reduced hREV1 protein level to 43% of that found in the parental cells. The 2008-shREV1-3.3 cells were 1.5-fold more sensitive to the cytotoxic effect of DDP but less sensitive to the mutagenic effect of DDP as evidenced by a 2.6- or 2.7-fold reduction in the ability to induce clones highly resistant to 6-thioguanine or DDP itself, respectively, in the surviving population. Reduction of hREV1 did not alter the initial rate of DDP adduct removal from DNA but did impair both spontaneous and DDP-induced extra-chromosomal homologous recombination, as measured by the recombination-sensitive reporter vector pBHRF. DDP induced an increase in hREV1 protein level. DDP resistance at the population level evolved 2.8-fold more slowly in the 2008-shREV1-3.3 cells than in the parental cells during repeated cycles of drug exposure. The results indicate that hREV1 functions to enhance both cell survival and the generation of drug-resistant variants in the surviving population. DDP up-regulates hREV1, suggesting that it may enhance its own mutagenicity. Most importantly, hREV1 controls the rate of emergence of resistance to DDP at the population level. Thus, hREV1 is an important contributor to DDP-induced genomic instability and the subsequent emergence of resistance.

DNA polymerase zeta regulates cisplatin cytotoxicity, mutagenicity, and the rate of development of cisplatin resistance

DNA polymerase zeta participates in translesional bypass replication. Here we show that reduced expression of the catalytic subunit hREV3 renders human fibroblasts more sensitive to the cytotoxic effect of cisplatin, reduces their sensitivity to the ability of cisplatin exposure to generate drug resistant variants in the surviving population, and reduces the rate of emergence of resistance to cisplatin at the population level. Reduction of REV3 mRNA did not alter the rate of cisplatin adduct removal but did impair both spontaneous and cisplatin-induced extrachromosomal homologous recombination and attenuated bypass replication as reflected by reduced ability to express luciferase from a platinated plasmid. Cisplatin induced a concentration- and time-dependent increase in hREV3 mRNA. The results indicate that, following formation of cisplatin adducts in DNA, REV3 mRNA levels increase, and polymerase zeta functions to promote both cell survival and the generation of drug-resistant variants in the surviving population. We conclude that when cisplatin adducts are present in the DNA, polymerase zeta is an important contributor to cisplatin-induced genomic instability and the subsequent emergence of resistance to this chemotherapeutic agent.

Induction of frameshift and base pair substitution mutations by the major DNA adduct of the endogenous carcinogen malondialdehyde

Instability of repetitive sequences is a hallmark of human cancer, and its enhancement has been linked to oxidative stress. Malondialdehyde is an endogenous product of oxidative stress that reacts with guanine to form the exocyclic adduct, pyrimido[1,2- alpha]purin-10(3H)-one (M1G). We used site-specifically modified single- and double-stranded vectors to investigate the mutagenic potential of M1G in bacteria and mammalian cells. M1G induced frameshift mutations (-1 and -2) when positioned in a reiterated (CpG)4 sequence but not when positioned in a nonreiterated sequence in Escherichia coli and in COS-7 cells. The frequency of frameshift mutations was highest when M1G was placed at the third G in the sequence. M1G induced base pair substitutions at comparable frequencies in both sequence contexts in COS-7 cells. These studies indicate that M1G, an endogenously generated product of oxidative stress, induces sequence-dependent frameshift mutations and base pair substitutions in bacteria and in mammalian cells. This finding suggests a potential role for the M1G lesion in the induction of mutations commonly associated with human diseases.

Binding discrimination of MutS to a set of lesions and compound lesions (base damage and mismatch) reveals its potential role as a cisplatin-damaged DNA sensing protein

The DNA mismatch repair (MMR) system plays a critical role in sensitizing both prokaryotic and eukaryotic cells to the clinically potent anticancer drug cisplatin. It is thought to mediate cytotoxicity through recognition of cisplatin DNA lesions. This drug generates a range of lesions that may also give rise to compound lesions resulting from the misincorporation of a base during translesion synthesis. Using gel mobility shift competition assays and surface plasmon resonance, we have analyzed the interaction of Escherichia coli MutS protein with site-specifically modified DNA oligonucleotides containing each of the four cisplatin cross-links or a set of compound lesions. The major 1,2-d(GpG) cisplatin intrastrand cross-link was recognized with only a 1.5-fold specificity, whereas a 47-fold specificity was found with a natural G/T containing DNA substrate. The rate of association, kon, for binding to the 1,2-d(GpG) adduct was 3.1 x 104 m-1 s-1 and the specificity of binding was essentially dependent on koff. DNA duplexes containing a single 1,2-d(ApG), 1,3-d(GpCpG) adduct, and an interstrand cross-link of cisplatin were not preferentially recognized. Among 12 DNA substrates, each containing a different cisplatin compound lesion derived from replicative misincorporation of one base opposite either of the 1,2-intrastrand adducts, 10 were specifically recognized including those that are more likely formed in vivo based on cisplatin mutation spectra. Moreover, among these lesions, two compound lesions formed when an adenine was misincorporated opposite a 1,2-d(GpG) adduct were not substrates for the MutY-dependent mismatch repair pathway. The ability of MutS to sense differentially various platinated DNA substrates suggests that cisplatin compound lesions formed during misincorporation of a base opposite either adducted base of both 1,2-intrastrand cross-links are more plausible critical lesions for MMR-mediated cisplatin cytotoxicity.

High-fidelity in vivo replication of DNA base shape mimics without Watson-Crick hydrogen bonds

We report studies testing the importance of Watson-Crick hydrogen bonding, base-pair geometry, and steric effects during DNA replication in living bacterial cells. Nonpolar DNA base shape mimics of thymine and adenine (abbreviated F and Q, respectively) were introduced into Escherichia coli by insertion into a phage genome followed by transfection of the vector into bacteria. Genetic assays showed that these two base mimics were bypassed with moderate to high efficiency in the cells and with very high efficiency under damage-response (SOS induction) conditions. Under both sets of conditions, the T-shape mimic (F) encoded genetic information in the bacteria as if it were thymine, directing incorporation of adenine opposite it with high fidelity. Similarly, the A mimic (Q) directed incorporation of thymine opposite itself with high fidelity. The data establish that Watson-Crick hydrogen bonding is not necessary for high-fidelity replication of a base pair in vivo. The results suggest that recognition of DNA base shape alone serves as the most powerful determinant of fidelity during transfer of genetic information in a living organism.

DNA modifications by antitumor platinum and ruthenium compounds: their recognition and repair

The development of metal-based antitumor drugs has been stimulated by the clinical success of cis-diamminedichloroplatinum(II) (cisplatin) and its analogs and by the clinical trials of other platinum and ruthenium complexes with activity against resistant tumors and reduced toxicity including orally available platinum drugs. Broadening the spectrum of antitumor drugs depends on understanding existing agents with a view toward developing new modes of attack. It is therefore of great interest to understand the details of molecular and biochemical mechanisms underlying the biological efficacy of platinum and other transition-metal compounds. There is a large body of experimental evidence that the success of platinum complexes in killing tumor cells results from their ability to form various types of covalent adducts on DNA; thus, the research of DNA interactions of metal-based antitumor drugs has predominated. The present review summarizes current knowledge on DNA modifications by platinum and ruthenium complexes, their recognition by specific proteins, and repair. It also provides strong support for the view that either platinum or ruthenium drugs, which bind to DNA in a fundamentally different manner from that of 'classical' cisplatin, have altered pharmacological properties. The present article also demonstrates that this concept has already led to the synthesis of several new unconventional platinum or ruthenium antitumor compounds that violate the original structure-activity relationships.

Nucleotide excision repair by extracts of human fetal hepatocytes

Human hepatocytes are particularly exposed to genotoxins, and nucleotide excision repair (NER) in these cells is essential for the maintenance of genome integrity. To characterize NER under conditions that closely resemble the pathway in vivo, we report the preparation and use of primary human fetal liver extracts to define the repair of a 1,3-intrastrand d(GpTpG)-cisplatin DNA lesion. Endonucleolytic cleavage at unique sites on either side of the adduct occurs at similar positions to the dominant NER incisions that have been reported for HeLa extracts. However, incisions effected by primary hepatocyte extracts are more precise as no secondary cleavage sites are detected 5' and 3' to the cisplatin lesion.

Mutagenic activity of cisplatin in the lacZ plasmid-based transgenic mouse model

Cis-diamminedichloroplatinum (II) (cisplatin) is a well characterized antitumor drug used for the treatment of a variety of human cancers. The cytotoxicity of cisplatin is mainly mediated through the formation of DNA adducts, which are also believed to be responsible for the secondary malignancies produced by the drug. The aim of this study was to determine the in vivo mutagenic activity of cisplatin in the lacZ plasmid-based transgenic mouse model. The mutant frequency (MF) and the spectrum of mutations induced by cisplatin in the mouse liver were analyzed and compared to controls. The mean MF in the lacZ gene was increased 2-fold in mice treated with a single 6 mg/kg body weight dose of cisplatin and sacrificed after 17 and 28 days (P = 0.001 and P < 0.0001). Restriction analysis and sequencing of mutant DNA showed that cisplatin was able to induce both large deletions and point mutations. A specific profile of base substitution and frameshift mutations was identified in treated mice, consisting primarily of G:C-->A:T transitions at GpG and ApG sites, the preferential DNA binding sites of cisplatin, and single basepair deletions/insertions. The present results provide the first evidence that cisplatin has mutagenic activity in vivo and induces a characteristic pattern of mutations in the mouse liver. This mutagenicity may be responsible for its tumorigenic activity.

Cisplatin adducts inhibit 1,N(6)-ethenoadenine repair by interacting with the human 3-methyladenine DNA glycosylase

The human 3-methyladenine DNA glycosylase (AAG) is a repair enzyme that removes a number of damaged bases from DNA, including adducts formed by some chemotherapeutic agents. Cisplatin is one of the most widely used anticancer drugs. Its success in killing tumor cells results from its ability to form DNA adducts and the cellular processes triggered by the presence of those adducts in DNA. Variations in tumor response to cisplatin may result from altered expression of cellular proteins that recognize cisplatin adducts. The present study focuses on the interaction between the cisplatin intrastrand cross-links and human AAG. Using site-specifically modified oligonucleotides containing each of the cisplatin intrastrand cross-links, we found that AAG readily recognized cisplatin adducts. The apparent dissociation constants for the 1, 2-d(GpG), the 1,2-d(ApG), and the 1,3-d(GpTpG) oligonucleotides were 115 nM, 71 nM, and 144 nM, respectively. For comparison, the apparent dissociation constant for an oligonucleotide containing a single 1,N(6)-ethenoadenine (epsilonA), which is repaired efficiently by AAG, was 26 nM. Despite the affinity of AAG for cisplatin adducts, AAG was not able to release any of these adducts from DNA. Furthermore, it was demonstrated that the presence of cisplatin adducts in the reactions inhibited the excision of epsilonA by AAG. These data suggest a previously unexplored dimension to the toxicological response of cells to cisplatin. We suggest that cisplatin adducts could titrate AAG away from its natural substrates, resulting in higher mutagenesis and/or cell death because of the persistence of AAG substrates in DNA.

Butadiene-induced intrastrand DNA cross-links: a possible role in deletion mutagenesis

To initiate studies designed to identify the mutagenic spectrum associated with butadiene diepoxide-induced N(2)-N(2) guanine intrastrand cross-links, site specifically adducted oligodeoxynucleotides were synthesized in which the adducted bases were centrally located within the context of the human ras 12 codon. The two stereospecifically modified DNAs and the corresponding unmodified DNA were ligated into a single-stranded M13mp7L2 vector and transfected into Escherichia coli. Both stereoisomeric forms (R, R and S,S) of the DNA cross-links resulted in very severely decreased plaque-forming ability, along with an increased mutagenic frequency for both single base substitutions and deletions compared with unadducted DNAs, with the S,S stereoisomer being the most mutagenic. Consistent with decreased plaque formation, in vitro replication of DNA templates containing the cross-links by the three major E. coli polymerases revealed replication blockage by both stereoisomeric forms of the cross-links. The same DNAs that were used for replication studies were also assembled into duplex DNAs and tested as substrates for the initiation of nucleotide excision repair by the E. coli UvrABC complex. UvrABC incised linear substrates containing these intrastrand cross-links with low efficiency, suggesting that these lesions may be inefficiently repaired by the nucleotide excision repair system.

The role of DNA mismatch repair in cisplatin mutagenicity

Cisplatin (DDP) is used with varying success for the treatment of a wide spectrum of human cancers. The most abundant lesions produced in DNA are intrastrand crosslinks, which are believed to account for not only the cytotoxic action but also the mutagenicity of the drug. The molecular basis for the mutagenicity of DDP adducts is believed to be related to bypass replication across the adducts by DNA polymerase. This results in misincorporation of non-complimentary bases by polymerase beta which, if left unpaired, will generate point or frameshift mutations. An important replication-associated correction function is provided by the post-replicative DNA mismatch repair (MMR) system. Loss of MMR activity is well documented to result in increased mutation rates and instability of genomic DNA. Inactivation of the MMR system also augments the intrinsic mutagenicity of DDP and enhances the risk of developing cells resistant to other drugs commonly used in combination with DDP. A future challenge will be to assess the clinical significance of the presence of MMR-deficient cells in tumors, and investigate new approaches to circumvent such multidrug resistance.

Effect of loss of DNA mismatch repair on development of topotecan-, gemcitabine-, and paclitaxel-resistant variants after exposure to cisplatin

Loss of DNA mismatch repair (MMR) causes genomic instability by markedly increasing the frequency of sporadic mutations in both coding and noncoding sequences. Little is known about how loss of MMR affects sensitivity to the mutagenic effect of chemotherapeutic agents. We wanted to determine how loss of MMR affects the ability of cisplatin, a known mutagen, to generate human tumor cell variants resistant to other drugs with which cisplatin is commonly combined in treatment regimens. We compared the ability of cisplatin to produce variants resistant to topotecan, gemcitabine, and paclitaxel in two pairs of MMR-proficient and -deficient cells that included sublines of the human colon carcinoma cell line HCT-116 and sublines of the human endometrial adenocarcinoma cell line HEC59. Cells were exposed to increasing concentrations of cisplatin for 1 h, and the surviving population was tested for the frequency of variants resistant to these single molecular target drugs 10 days later. The frequency of variants increased linearly with cisplatin concentration for all three drugs. Cisplatin was 2.6 +/- 0.3- (S.D.), 3.6 +/- 0.9-, and 2.3 +/- 0.1-fold more potent at producing topotecan-, gemcitabine-, and paclitaxel-resistant variants in the MMR-deficient than in the MMR-proficient HCT116 cells (P <.05 for all). Cisplatin was 1.4 +/- 0.3- and 1.4 +/- 0.4-fold more potent at generating topotecan- and gemcitabine-resistant variants in MMR-deficient HEC59 cells than in MMR-proficient HEC59+ch2 cells. Cisplatin was not more potent in generating paclitaxel-resistant variants in the MMR-deficient HEC59 cells. Spontaneous rates of generation of cells resistant to these three drugs were also measured in the HCT116 sublines. MMR-deficient HCT116 cells exhibited rates of generation of resistant variants that were 1.94- and 1.51-fold higher (P <.05) than those in the MMR-proficient cells for topotecan and gemcitabine, respectively; loss of MMR had no effect on the rate of generation of variants resistant to paclitaxel. We conclude that the loss of MMR increases the ability of cisplatin to generate variants resistant to topotecan, gemcitabine, and possibly paclitaxel and that MMR also plays a role in controlling the spontaneous rate of generation of variants resistant to topotecan and gemcitabine.

Pharmacodynamics of cisplatin in human head and neck cancer: correlation between platinum content, DNA adduct levels and drug sensitivity in vitro and in vivo

Total platinum contents and cisplatin-DNA adduct levels were determined in vivo in xenografted tumour tissues in mice and in vitro in cultured tumour cells of head and neck squamous cell carcinoma (HNSCC), and correlated with sensitivity to cisplatin. In vivo, a panel of five HNSCC tumour lines growing as xenografts in nude mice was used. In vitro, the panel consisted of five HNSCC cell lines, of which four had an in vivo equivalent. Sensitivity to cisplatin varied three- to sevenfold among cell lines and tumours respectively. However, the ranking of the sensitivities of the tumour lines (in vivo), also after reinjection of the cultured tumour cells, did not coincide with that of the corresponding cell lines, which showed that cell culture systems are not representative for the in vivo situation. Both in vitro and in vivo, however, significant correlations were found between total platinum levels, measured by atomic absorption spectrophotometry (AAS), and tumour response to cisplatin therapy at all time points tested. The levels of the two major cisplatin-DNA adduct types were determined by a recently developed and improved 32P post-labelling assay at various time points after cisplatin treatment. Evidence is presented that the platinum-AG adduct, in which platinum is bound to guanine and an adjacent adenine, may be the cytotoxic lesion because a significant correlation was found between the platinum-AG levels and the sensitivities in our panel of HNSCC, in vitro as well as in vivo. This correlation with the platinum-AG levels was established at 1 h (in vitro) and 3 h (in vivo) after the start of the cisplatin treatment, which emphasizes the importance of early sampling.

GG versus AG Platination: A Kinetic Study on Hairpin-Stabilized Duplex Oligonucleotides

The kinetics of the reactions between the diaqua form of the antitumor drug cisplatin, cis-[Pt(NH(3))(2)(H(2)O)(2)](2+), and two hairpin-stabilized duplex oligonucleotides, d(TATGGTATTTTTATACCATA) (I) and d(TATAGTATTTTTATACTATA) (II), were investigated. Oligonucleotides I and II were used as models for GG and AG sequences within duplex DNA, which are known as the major sites of platinum binding. The two GG guanines of I are shown to react with similar rates (k(5)(') = 18 +/- 2 and k(3)(') = 15 +/- 1 M(-)(1) s(-)(1)), roughly twice as fast as the AG guanine of II (k(3)(') = 9 +/- 1 M(-)(1) s(-)(1)). Platination of the AG adenine of II was also observed to a minor extent (k(5)(') = 1.5 +/- 0.3 M(-)(1) s(-)(1)), whereas no other adenine of I or II was platinated to a detectable extent. The overall platination rate of I is approximately three times larger than that of II. The 3'-monoadduct of I undergoes chelation to the GG intrastrand adduct with a rate 10.5 times larger than the 5'-monoadduct (k(3)(')(c) = (1.9 +/- 0.1) x 10(-)(3) s(-)(1) and k(5)(')(c) = (0.18 +/- 0.05) x 10(-)(3) s(-)(1)). For II, the chelation rate constants of the guanine- and adenine-bound monoadducts are k(5)(')(c) = 0.3 +/- 0.1 and k(3)(')(c) = 0.08 +/- 0.01 s(-)(1), respectively. These results are discussed in relation to the platination kinetics determined for other model systems.