DNA alkylation and interstrand cross-linking by treosulfan

The anti-tumour drug treosulfan (L-threitol 1,4-bismethanesulphonate, Ovastat) is a prodrug for epoxy compounds by converting non-enzymatically to L-diepoxybutane via the corresponding monoepoxide under physiological conditions. The present study supports the hypothesis that this conversion of treosulfan is required for cytotoxicity in vitro. DNA alkylation and interstrand cross-linking of plasmid DNA is observed after treosulfan treatment, but this is again produced via the epoxide species. Alkylation occurs at guanine bases with a sequence selectivity similar to other alkylating agents such as the nitrogen mustards. In treosulfan-treated K562 cells, cross-links form slowly, reaching a peak at approximately 24 h. Incubation of K562 cells with preformed epoxides shows faster and more efficient DNA cross-linking. © 1999 Cancer Research Campaign     

DNA-protein cross-linking and DNA interstrand cross-linking by haloethylnitrosoureas in L1210 cells.

Bifunctional alkylating agents are known to produce cross-links between DNA and protein and between paired DNA strands. The possibility of discriminating these two classes of cross-links in L1210 cells treated with haloethylnitrosoureas or nitrogen mustard was explored with the alkaline elution technique. Two classes of cross-links were demonstrated, based on sensitivity to proteinase K; the proteinase-sensitive cross-links appear to be DNA-protein cross-links, and the proteinase-resistant class may include interstrand cross-links. Proteinase-sensitive cross-links form more rapidly than do proteinase-resistant cross-links in cells treated with chloroethylnitrosoureas, perhaps because these agents can chloroethylate protein sulfhydryl or amino groups followed by rapid reaction of these chloroethylated groups with DNA. Although both types of cross-links produced by nitrogen mustard disappeared or were repaired after 24 hr, the removal of cross-links produced by chloroethylnitrosoureas either did not occur or was incomplete in 24 hr. In addition to cross-links, cells treated with haloethylnitrosoureas exhibited DNA strand breaks; a method is suggested for estimating the apparent frequencies of strand breaks and cross-links in the DNA.     

Synergistic cytotoxicity and DNA strand break formation by bromodeoxyuridine and bleomycin in human tumor cells

5-Bromo-2'-deoxyuridine (BrdUrd) is a thymidine analogue whose cellular effects are related to its incorporation into DNA. BrdUrd is a known radiosensitizing agent that could potentially enhance the activity of chemotherapeutic agents that interact directly with DNA. Therefore we studied the interaction of BrdUrd and bleomycin in a human head and neck squamous carcinoma cell line, SQ20B. Using a colony-forming assay and analyzing results by the median-effect method, we have shown that there is synergistic cytotoxicity between BrdUrd and bleomycin. Synergism is evident when BrdUrd is administered prior to bleomycin or when the two drugs are applied simultaneously and is evident at a variety of BrdUrd:bleomycin concentration ratios. Alkaline elution of DNA from cells exposed to BrdUrd and bleomycin demonstrated greater single strand break formation than expected from the individual single strand break frequencies induced by each drug alone. BrdUrd did not affect the rate of repair of bleomycin-induced single strand breaks or the formation of double strand breaks. Although the mechanism of this interaction at the molecular level is unclear, our studies suggest that a direct interaction of bleomycin with BrdUrd-substituted DNA may be the cause of the synergism of these two agents.     

Artemis‐dependent DNA double‐strand break formation at stalled replication forks

Stalled replication forks undergo DNA double‐strand breaks (DSBs) under certain conditions. However, the precise mechanism underlying DSB induction and the cellular response to persistent replication fork stalling are not fully understood. Here we show that, in response to hydroxyurea exposure, DSBs are generated in an Artemis nuclease‐dependent manner following prolonged stalling with subsequent activation of the ataxia–telangiectasia mutated (ATM) signaling pathway. The kinase activity of the catalytic subunit of the DNA‐dependent protein kinase, a prerequisite for stimulation of the endonuclease activity of Artemis, is also required for DSB generation and subsequent ATM activation. Our findings indicate a novel function of Artemis as a molecular switch that converts stalled replication forks harboring single‐stranded gap DNA lesions into DSBs, thereby activating the ATM signaling pathway following prolonged replication fork stalling.     

Quenching of DNA cross-link precursors of chloroethylnitrosoureas and attenuation of DNA interstrand cross-linking by glutathione

Interstrand DNA cross-linking is essential for the antitumor activity of chloroethylnitrosoureas (CENUs). The critical cross-links have been proposed to involve a rapid O6-guanine chloroethylation on one DNA strand, followed by a rearrangement of the O6-(2-chloroethyl)guanine and slow alkylation of the second DNA strand. In view of the relative intracellular abundance of glutathione (GSH) and nucleophilicity of its thiolate ion, the ability of GSH to react with and to inactivate 2-chlorethylated DNA and the possibility that this interaction decreases net DNA cross-linking by CENUs were investigated. Chloroethylated calf thymus DNA was reacted with GSH, the DNA was precipitated and redissolved, and subsequent DNA interstrand cross-linking was determined. The DNA cross-link index was compared for both GSH-treated and 2-chloroethylated untreated DNA. Simultaneously, Col E1 plasmid DNA was chloroethylated and reacted with GSH, and the extent of DNA interstrand cross-linking was determined by agarose gel electrophoresis and compared with controls. The results show both a time- and GSH concentration-dependent quenching of chloroethylated DNA, with a corresponding decrease in the DNA cross-link index. Using [methyl-3H]GSH, it was also demonstrated that 56% of the total GSH was bound to quenched 2-chloroethylated Col E1 DNA and 25% to quenched 2-chloroethylated calf thymus DNA. GSH binding to cross-linked DNA and native DNA was insignificant. It is concluded that, in addition to direct inactivation of reactive cytotoxic CENU species, GSH may also modulate cellular response to CENUs by quenching chloroethylated DNA, thereby decreasing the formation of potentially lethal DNA cross-links.     

Indoloquinone EO9: DNA interstrand cross-linking upon reduction by DT-diaphorase or xanthine oxidase.

We report DNA interstrand cross-linking caused by the anti-tumour indoloquinone EO9 following reductive activation with purified rat liver DT-diaphorase or xanthine oxidase. Reduction was a necessary event for cross-linking to occur. DNA cross-link formation by EO9 following DT-diaphorase reduction was completely inhibited by addition 10 microM dicoumarol, whereas only a minor effect of dicoumarol on xanthine oxidase-mediated DNA cross-linking by EO9 was observed. DNA cross-linking was pH dependent, with increasing cross-link formation from pH 5.5 to 7.0 for both DT-diaphorase and xanthine oxidase mediated reactions. Also, conversion of EO9 upon reduction was pH dependent. However, in contrast to DNA cross-linking, conversion rates of EO9 decreased at higher pH. EO9 was shown to be more efficient in DNA cross-linking than mitomycin C under identical conditions, using both DT-diaphorase and xanthine oxidase reductive activation at pH 5.5 and 7.0. This study indicates that the anti-tumour activity of EO9 may be at least partly mediated by interstrand DNA cross-link formation, and that various reducing enzymes may be important for activation of EO9 in vitro and in vivo.     

Ataxia-telangiectasia-like Chinese hamster V79 cell mutants with radioresistant DNA synthesis, chromosomal instability, and normal DNA strand break repair

We have isolated three radiosensitive mutants (V-C4, V-E5, and V-G8) of the Chinese hamster V79 cell line which also show increased sensitivities to killing by bleomycin (approximately 2-5-fold) and ethyl methanesulfonate (approximately 2-fold). Genetic complementation analysis indicates that all three mutants belong to one complementation group. The mutants show a radioresistant DNA synthesis following X-ray irradiation when compared to wild-type V79 cells. Both the level and the rate of repair of DNA single- and double-strand breaks measured by DNA elution were similar to those observed in wild-type V79 cells. The level of spontaneously occurring chromosome aberrations in two of these mutants differs severalfold from the level observed in wild-type V-79 cells and in V-G8, to approximately 2- and 6-fold increase in V-E5 and V-C4, respectively. X-irradiation of the mutants resulted in consistently 3-4-fold higher levels of chromatid gaps, breaks, and exchanges than observed in wild-type V79 cells. In addition, G1 irradiation of the mutant cells yielded both chromosome and chromatid types of aberrations. The level and pattern of chromosomal aberrations induced by X-rays in V-C4, V-E5, and V-G8 are similar to those observed in ataxia-telangiectasia cells. These results indicate that our mutants represent the first rodent cell mutants which show phenotypic characteristics strongly resembling those in cells from ataxia-telangiectasia patients.     

Comparison of DNA interstrand cross-linking and strand breakage by 1,3-bis(2-chloroethyl)-1-nitrosourea in polyamine-depleted and control human adenocarcinoma cells

Recent evidence from our laboratory and from others suggested that pretreatment with alpha-difluoromethylornithine (DFMO) sensitizes some human and rodent tumor cell lines to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). Many human tumor cells are resistant to chloroethylnitrosourea-induced DNA interstrand cross-linking and cell kill due to their high levels of the DNA repair protein O6-alkylguanine DNA alkyltransferase. We therefore investigated DFMO-mediated sensitization to BCNU in BCNU-sensitive and -resistant cells. Colony formation assays were used to compare BCNU cytotoxicity in DFMO-pretreated and control cultures of two colon tumor lines, HT-29 cells, which have high alkyltransferase levels and thus are BCNU-resistant, and BE cells, which are deficient in this repair capacity and thus are BCNU-sensitive. Polyamine depletion significantly enhanced BCNU cytotoxicity only for the repair-proficient HT-29 cell line. BE cells were 40-fold more sensitive to BCNU than were HT-29 cultures. However, in BE cells, no effect of polyamine depletion was found on cellular response to BCNU treatment at 72 h after DFMO treatment. Reverse-phase high-performance liquid chromatography assays of polyamine concentrations in cell extracts verified that DFMO produced comparable degrees of polyamine depletion for both cell lines. DNA alkaline elution analysis was used to monitor BCNU-induced formation of DNA single strand breaks, DNA interstrand cross-links, and DNA-protein cross-links. Equal concentrations of BCNU produced similar levels of strand breaks and DNA-protein cross-links in DFMO-pretreated and control cultures for both cell lines. These data suggest that DNA in polyamine-deficient HT-29 and BE cells is not more accessible to BCNU than is DNA in controls. No DNA interstrand cross-links were detected in either DFMO-pretreated or control HT-29 cells after BCNU treatment. Further, in BE cells which accumulate BCNU-induced DNA interstrand cross-links, no increase in the measureable levels of cross-links resulted from polyamine deficiency. Our observations suggest that mechanisms other than increased DNA interstrand cross-link formation may be mediating the enhanced efficacy of BCNU in polyamine-deficient HT-29 cell cultures. Our findings may also imply that cellular targets for BCNU other than DNA damage may be responsible for DFMO-induced chemosensitization in the repair-proficient cells.     

DNA damage-induced cell cycle checkpoints and DNA strand break repair in development and tumorigenesis

Several newly identified tumor suppressor genes including ATM, NBS1, BRCA1 and BRCA2 are involved in DNA double-strand break repair (DSBR) and DNA damage-induced checkpoint activation. Many of the gene products involved in checkpoint control and DSBR have been studied in great detail in yeast. In addition to evolutionarily conserved proteins such as Chk1 and Chk2, studies in mammalian cells have identified novel proteins such as p53 in executing checkpoint control. DSBR proteins including Mre11, Rad50, Rad51, Rad54, and Ku are present in yeast and in mammals. Many of the tumor suppressor gene products interact with these repair proteins as well as checkpoint regulators, thus providing a biochemical explanation for the pleiotropic phenotypes of mutant cells. This review focuses on the proteins mediating G1/S, S, and G2/M checkpoint control in mammalian cells. In addition, mammalian DSBR proteins and their activities are discussed. An intricate network among DNA damage signal transducers, cell cycle regulators and the DSBR pathways is illustrated. Mouse knockout models for genes involved in these processes have provided valuable insights into their function, establishing genomic instability as a major contributing factor in tumorigenesis.     

ＤＮＡ双链断裂修复通路与遗传性乳腺癌的关系

遗传性乳腺癌具有家族聚集、早发、双侧等特点,多为易感基因发生胚系突变所致。ＤＮＡ损伤修复是哺乳动物细胞保证遗传物质稳定性的重要机制。双链断裂是最严重的ＤＮＡ损伤之一,修复过程涉及同源重组和非同源末端连接通路。ＤＮＡ双链断裂修复或信号传导相关基因或蛋白功能缺陷可以诱导染色体不稳定而增加乳腺癌的易感性。与ＤＮＡ修复功能相关的链交联剂和ＰＡＲＰ-１抑制剂为ＢＲＣＡ相关遗传性乳腺癌的治疗提供了新的途径。本文就ＤＮＡ双链断裂修复通路相关基因的突变与遗传性乳腺癌发病的关系作一综述。     

Hypoxia down-regulates DNA double strand break repair gene expression in prostate cancer cells.

BACKGROUND AND PURPOSE: Intratumoral hypoxia has been correlated with poor clinical outcome in prostate cancer. Prostate cancer cells can be genetically unstable and have altered DNA repair. We, therefore, hypothesized that the expression of DNA double-strand break (DNA-dsb) repair genes in normal and malignant prostate cultures can be altered under hypoxic conditions. METHODS AND MATERIALS: The expression of homologous recombination (HR) and non-homologous recombination (NHEJ) genes following gas hypoxia (0.2%) or exposure to HIF1alpha-inducing agent, CoCl2 (100 microM), was determined for normal diploid fibroblasts (GM05757) and the pre-malignant and malignant prostate cell lines, BPH-1, 22RV-1, DU145 and PC3. RNA and protein levels were determined using RT-PCR and Western blotting. Additionally, p53 genotype and function, the level of hypoxia-induced apoptosis, and cell cycle distribution, were determined to correlate to changes in DNA-dsb gene expression. RESULTS: Induction of hypoxia was confirmed using HIF1alpha and VEGF expression in gas- and CoCl2-treated cultures. Hypoxia (48-72 h of 0.2% O2) decreased RNA expression of a number of HR-related genes (e.g. Rad51, Rad52, Rad54, BRCA1, BRCA2) in both normal and malignant cultures. Similar decreases in RNA pertaining to the NHEJ-related genes (e.g. Ku70, DNA-PKcs, DNA Ligase IV, Xrcc4) were observed. In selected cases, hypoxia-mediated decreases in RNA expression led to decreased DNA-dsb protein expression. CoCl2-treated cultures did not show decreased DNA-dsb protein expression. The ability of hypoxia to down-regulate Rad51 and other HR-associated genes under hypoxia was not correlated to c-Abl or c-Myc gene expression, p53 genotype or function, propensity for hypoxia-mediated apoptosis, or specific changes in cell cycle distribution. CONCLUSIONS: Hypoxia can down-regulate expression of DNA-dsb repair genes in both normal and cancer cells. If associated with a functional decrease in DNA-dsb repair, this observation could provide a potential basis for the observed genetic instability within tumor cells exposed to hypoxia.     

DNA interstrand crosslinking and strand break repair in human glioma cell lines of varying [1,3-bis(2-chloroethyl)-1-nitrosourea] resistance

The production of DNA interstrand crosslinks (ISC) by BCNU and other bifunctional alkylators and the effects of these drugs on the repair of radiation-induced DNA-single strand breaks (SSB) were studied in two human glioblastoma used to assess both DNA-ISCs and DNA-SSBs. BCNU-treated UWR2 and UWR3 cells showed a significant BCNU dose-dependent increase in radiation-induced DNA-SSBs at 6 hrs post-drug treatment, and at 100 microM BCNU DNA-ISC was completely masked in UWR2 cells. There was no enhancement of radiation-induced DNA-SSBs in both cell lines after treatment with cis-DDP, CHZ, or MNU. In the capillary clonogenic cell assay, UWR2 cells were 3.2 times more resistant than UWR3 cells; 0(6)-methylguanine-DNA methyltransferase activity was also 1.8 times higher in UWR2 than in UWR3. Our data suggest caution in the use of the standard alkaline elution technique (with 6 hrs between drug exposure and irradiation) to measure BCNU-induced DNA-ISC induction in highly BCNU-resistant cell lines. We provide evidence that the synergism between BCNU and radiation in the generation of DNA-SSBs is the result of low DNA-SSB repair capacity of the cells, and is further potentiated by the carbamoylating action of BCNU.     

Cytotoxic and antitumor activity of 1-nitroacridines as an aftereffect of their interstrand DNA cross-linking

To determine whether the toxic effects and changes in many cell functions caused by antitumor 1-nitroacridines are related to their enzymatically mediated covalent interstrand DNA cross-linking (J. Konopa, J. W. Pawlak, and K. Pawlak. Chem.-Biol. Interact., 43: 175-197, 1983), the cross-linking potency of the derivatives with structural modifications in position 9 of the acridine nucleus was estimated as their in vitro threshold concentrations (0.3 to 4.5 microM), beyond which the first interstrand DNA cross-links could be detected in DNA of cultured HeLa S3 cells with a polyethylene glycol 6000-Dextran T500 assay. Statistically significant (p less than 0.05) correlations exist between the cross-linking potency of 1-nitroacridines and their in vivo antitumor activity and toxicity against mice with Sarcoma 180 tumors in solid form (3 to 1065 mumol/kg of body weight), as well as their in vitro cytotoxicity against cultured HeLa or HeLa S3 cells (0.0005 to 7.2 microM), indicating that the interstrand DNA cross-linking potency might be one of primary determinants of in vivo and in vitro biological activity of 1-nitroacridine antineoplastic drugs. Susceptibility of the parent agents to reduction does not appear to be a rate-limiting factor of DNA cross-linking potency of 1-nitroacridines and their metabolic transformations (J. W. Pawlak, and J. Konopa. Biochem. Pharmacol., 28: 3391-3402, 1979), because no significant differences were observed among the agents with respect to their polarographic half-wave potentials estimated under anaerobic conditions.     

Structure-activity study with bioreductive benzoquinone alkylating agents: effects on DT-diaphorase-mediated DNA crosslink and strand break formation in relation to mechanisms of cytotoxicity

Purpose Structure-activity studies were carried out with the model bioreductive alkylating agent benzoquinone mustard (BM) and its structural analogs. The specific objectives were: (1) to investigate the effects of functional group substitutions to the benzoquinone ring on DNA crosslink and strand break formation subsequent to reduction of the analogs by DT-diaphorase (DTD) in vitro, (2) to correlate DNA crosslink and strand break formation by the analogs with anaerobic reduction of the BM analogs by DTD and their redox cycling in vitro, and (3) to correlate DNA crosslink and strand break formation by the BM analogs with their cytotoxic effects in cancer cells.Methods DNA interstrand crosslink and single-strand break formation were assessed using agarose gel assays. To determine DNA interstrand crosslinks or single-strand breaks, linearized or supercoiled plasmid DNA, respectively, were incubated with purified human DTD and increasing concentrations of each BM analog. Subsequently, DNA was electrophoresed on an agarose gel and DNA crosslink and strand break formation were quantified using densitometry. The rates of reduction of the BM analogs by purified human DTD were measured in vitro under hypoxic conditions, and the redox cycling potential was determined under aerobic conditions using HPLC analysis. The cytotoxic activities of these agents in human tumor cell lines were measured by the MTT assay, with and without the DTD inhibitor, dicoumarol.Results BM analogs with electron-donating groups (MeBM, MBM, m-MeBM), electron-withdrawing groups (CBM, FBM), sterically bulky groups (PBM, m-PBM, m-TBM) and positional isomers (MeBM, m-MeBM, PBM, m-PBM) were synthesized. After reduction by DTD, the BM analogs produced a concentration-dependent increase in DNA crosslink and DNA strand break formation. The E₁₀ (extent of DNA crosslink formation produced by 10 M BM analog) for DNA crosslink formation displayed the rank order MeBMMBM>m-MeBMPBMBM>CBM>FBM>m-PBMm-TBM. For DNA strand break formation, the E₁₀ values (extent of DNA strand break formation produced by 10 M BM analog) displayed the rank order MeBM>MBM>m-MeBM>PBM>BMCBM>FBM>m-PBMm-TBM. Importantly, the cytotoxic activity of the BM analogs in SK-Mel-28 human melanoma cells correlated positively with the E₁₀ values for DTD-mediated DNA crosslink formation (r _s=0.87, P<0.05) and DNA strand break formation (r _s=0.95, P<0.05). Similar correlations were observed in NCI-H661 human lung carcinoma cells. Furthermore, the D₁₀ values (concentration of BM analog that decreased the surviving cell fraction to 0.1) for cytotoxic activity of the BM analogs correlated with the maximum levels of DNA crosslinks formed with each BM analog, with r _s values of –0.85 (P<0.05) for the NCI-H661 cell line, and –0.81 (P<0.05) for the SK-MEL-28 cell line. The half-time of reduction (t_1/2) of the BM analogs by DTD did not correlate with DNA crosslink formation, DNA strand break formation, or cytotoxic potency of the analogs.Conclusions Functional groups on the benzoquinone ring affect the ability of BM to produce DNA crosslinks and strand breaks following reduction by DTD. Electron-donating groups increased DNA damage, whereas electron-withdrawing groups and sterically bulky groups at the C6 position had no effect or decreased the ability of the compounds to produce DNA damage compared to BM. Moreover, both DNA crosslink and strand break formation appear to have an important impact on the cytotoxicity of the BM analogs. These results may have significance for optimal use of BM-based antitumor agents and for rationalization of the development of novel therapeutic compounds that require bioactivation by DTD.Abbreviations AZQ 2,5-Diaziridinyl-3,6-bis(carboethoxyamino)-1,4-benzoquinone - BM 2-[Di(chloroethyl)amino]-1,4-benzoquinone - CBM 5-Chloro-2-[di(chloroethyl)amino]-1,4-benzoquinone - DCPIP 2,6-Di-chlorophenolindophenol - DIC Dicoumarol - DMF Dimethylformamide - DTD NAD(P)H:quinone oxidoreductase - DZQ 3,6-Diaziridinyl-1,4-benzoquinone - EO9 3-Hydroxymethyl-5-aziridinyl-1-methyl-2-(H-indole-4,7-dione)-prop-2-en-1-ol - FBM 5-Fluoro-2-[di(chloroethyl)amino]-1,4-benzoquinone - MBM 5-Methoxy-2-[di(chloroethyl)amino]-1,4-benzoquinone - MeBM 5-Methyl-2-[di(chloroethyl)amino]-1,4-benzoquinone - m-MeBM 6-Methyl-2-[di(chloroethyl)amino]-1,4-benzoquinone - MeDZQ 2,5-Diaziridinyl-3,6-dimethyl-1,4-benzoquinone - PBM 5-Phenyl-2-[di(chloroethyl)amino]-1,4-benzoquinone - m-PBM 6-Phenyl-2-[di(chloroethyl)amino]-1,4-benzoquinone - m-TBM 6-t-Butyl-2-[di(chloroethyl)amino]-1,4-benzoquinone - RH1 2,5-Diaziridinyl-3-hydroxymethyl-6-methyl-1,4-benzoquinone.     

DNA double strand break repair and chromosomal translocation: lessons from animal models

The maintenance of genomic stability is one of the most important defenses against neoplastic transformation. This objective must be accomplished despite a constant barrage of spontaneous DNA double strand breaks. These dangerous lesions are corrected by two primary pathways of double strand break repair; non homologous end joining and homologous recombination. Recent studies employing mouse models have shown that absence of either pathway leads to genomic instability, including potentially oncogenic translocations. Because translocations involve the union of different chromosomes, cellular machinery must exist that creates these structures in the context of unrepaired double strand breaks. Evidence is mounting that the pathways of double strand break repair that are so important for survival may themselves be the culprits that generate potentially fatal translocations. Evidence and models for the dual roles of double strand break repair in both preventing, and generating, oncogenic karyotypic changes are discussed.     

1-beta-D-arabinofuranosylcytosine and hydroxyurea production of cytotoxic synergy with cis-diamminedichloroplatinum(II) and modification of platinum-induced DNA interstrand cross-linking

1-beta-D-Arabinofuranosylcytosine (ara-C) and hydroxyurea (HU) were investigated as possible inhibitors for the repair of cis-diamminedichloroplatinum(II) (DDP)-induced DNA damage. HU and ara-C were chosen for their known ability to inhibit DNA excision repair following UV irradiation. Work by several groups has suggested that the repair of DDP-induced DNA damage may involve an excision-repair mechanism. The cytotoxic effects of dose, exposure duration, and sequence for the three drugs was studied in a human colon cancer cell line (HT-29) by colony formation assays. Significant synergistic cytotoxicity was seen whether HU + ara-C were given prior to, or following DDP exposure. Cytotoxic synergy was also seen between HU + ara-C themselves. The effect of the combined antimetabolites on the level and persistence of DDP-induced DNA interstrand cross-links was assessed by DNA alkaline elution. These were measured as an indicator of DDP-DNA adduct formation and removal. When HU + ara-C exposure preceded or followed DDP treatment, higher levels of interstrand cross-linking were found at late time points than were seen with DDP alone, suggesting repair inhibition. We conclude that the combination of HU, ara-C, and DDP shows synergistic cytotoxicity, and that this effect may be due in part to inhibition of DDP-induced DNA adduct repair. The concentrations of drugs used in vitro are achievable in humans. On the basis of these results, a Phase I/II clinical trial of the three agents in combination has been initiated.     

Cytotoxicity, DNA cross-linking, and single strand breaks induced by activated cyclophosphamide and acrolein in human leukemia cells

The in vitro cytotoxicity and mechanism of action of cyclophosphamide (CP) were studied in a dual cell culture system, using rat hepatocytes and K562 human chronic myeloid leukemia cells. Cytotoxicity and DNA damage were measurable in K562 cells using CP concentrations that are clinically attainable. Alkaline elution analysis of cellular DNA demonstrated the presence of concentration- and time-dependent DNA interstrand cross-links, DNA-protein cross-links, and DNA single strand breaks in K562 cells following a 1-h exposure to cyclophosphamide activated by hepatocytes. Hepatocyte-activated CP was 3 to 4 times more potent than phosphoramide mustard with regard to cytotoxicity and induction of DNA interstrand cross-links. Exposure to phosphoramide mustard did not produce single strand breaks, but exposure of K562 cells to acrolein resulted in substantial levels of single strand breaks. The demonstration of acrolein-induced single strand breaks following exposure to activated CP is a novel finding and suggests that acrolein may have a role in the cytotoxicity of CP.     

Gene-specific formation and repair of DNA monoadducts and interstrand cross-links after therapeutic exposure to nitrogen mustards.

PURPOSE: To investigate the possibility of measuring the gene-specific DNA damage after therapeutic exposure to nitrogen mustards and to examine its relationship with the clinical response. EXPERIMENTAL DESIGN: The kinetics of gene-specific monoadducts and interstrand cross-link formation/repair were measured in the p53 and N-ras genes. DNA extracted from human peripheral lymphocytes following in vitro exposure to melphalan or therapeutic exposure to melphalan or cyclophosphamide was used. RESULTS: When lymphocytes were treated in vitro with biologically relevant doses of melphalan, monoadducts accumulated rapidly in both p53 and N-ras genes, reaching maximal levels within 2 h, whereas the highest interstrand cross-link levels were found within 8 h. Thereafter, the adducts were repaired with half-lives of 14.5 +/- 0.3 h (p53) or 18.8 +/- 1.5 h (N-ras) for monoadducts and 12.4 +/- 0.8 h (p53) or 14.1 +/- 2.2 h (N-ras) for interstrand cross-links. Moreover, peak levels of monoadducts in both genes were observed 2 h after treatment in peripheral leukocytes from patients with multiple myeloma treated with high-dose i.v. melphalan, supported by autologous stem cell transplantation, whereas interstrand cross-links were maximal within 8 h. Of seven patients examined, the three who showed the least levels of DNA damage did not respond to the high-dose melphalan. CONCLUSIONS: This is the first report showing that it is feasible to measure gene-specific DNA damage in a readily accessible tissue of humans exposed to bifunctional alkylating drugs and to examine, at the level of the individual patient, the relationships between the induction/repair of cytotoxic DNA damage and clinical response or long-term complications.     

DNA interstrand crosslinking and sequence selectivity of dimethanesulphonates.

Members of the homologous series of alkanediol dimethanesulphonates of general formula H3C.SO2O.(CH2)n.O.SO2.CH3 have been tested for their ability to produce DNA interstrand crosslinking and DNA sequence selectivity of guanine-N7 alkylation. In a sensitive crosslinking gel assay the efficiency of DNA interstrand crosslink formation, dependent on the ability of the alkylating moiety to span critical nucleophilic distances within the DNA, was found at 6 h to be 1,6-hexanediol dimethanesulphonate (Hexa-DMS) (n = 6) greater than methylene dimethanesulphonate (MDMS) (n = 1) greater than 1,8-octanediol dimethanesulphonate (Octa-DMS) (n = 8) greater than Busulphan (n = 4). The DNA interstrand crosslinking produced by MDMS was not due to either of its hydrolysis products, formaldehyde or methanesulphonic acid (MSA). In contrast the extent of monoalkylation at guanine-N7 as determined by a modified DNA sequencing technique was found to be Busulphan much greater than Hexa-DMS = Octa-DMS, with a sequence selectivity somewhat less than that of other chemotherapeutic alkylating agents such as nitrogen mustards. MDMS at high levels induced a non-specific depurination as a result of the reduction in pH resulting from MSA release. More strikingly MDMS (and MSA) produced a single strong site of guanine reaction (depurination) in a guanine-rich 276 base pair fragment of pBR322 DNA in the sequence of 5''-ATGGTGG-3''. This was observed when non-specific depurination was negligible and was not seen with formic acid. Thus structurally similar alkylating agents can differ in their type and extent of DNA monoalkylation and interstrand crosslinking, and in some cases (e.g. MDMS/MSA) produce reactions with a high degree of selectivity.     

Oral cancer and genetic polymorphism of DNA double strand break gene Ku70 in Taiwan

The DNA repair gene Ku70, an important caretaker of the overall genome stability, is thought to play a major role in the DNA double strand break repair system. It is known that defects in double strand break repair capacity can lead to irreversible genomic instability. However, the polymorphic variants of Ku70 and their association with oral cancer susceptibility has never been reported on. In this hospital-based case-control study, the association of Ku70 promoter T-991C (rs5751129), promoter G-57C (rs2267437), promoter A-31G (rs132770), and intron3 (rs132774) polymorphisms with oral cancer risk in a Taiwanese population was investigated. In total, 318 patients with oral cancer and 318 age- and gender-matched healthy controls recruited from the China Medical Hospital in Taiwan were genotyped. The results showed that there were significant differences between the oral cancer and control groups in the distribution of their genotypes (P=0.0031) and allelic frequency (P=0.0009) in the Ku70 promoter T-991C polymorphism. Individuals who carried at least one C allele (T/C or C/C) had a 2.15-fold increased risk of developing oral cancer compared to those who carried the T/T wild-type genotype (95% CI: 1.37-3.36). In the other three polymorphisms, there was no difference between both groups in the distribution of either genotype or allelic frequency. In conclusion, the Ku70 promoter T-991C, but not the Ku70 promoter C-57G, promoter A-31G or intron3, is connected to oral cancer susceptibility. This polymorphism may be a novel useful marker for primary prevention and anticancer intervention.