A system in mouse liver for the repair of O6-methylguanine lesions in methylated DNA

Tamoxifen accelerates proteasomal degradation ofO6-methylguanine DNA methyltransferase in human cancer cells

Tamoxifen, a synthetic triphenyl-ethylene compound, is a member of a class of anticancer drugs known as selective estrogen receptor modulators. It may block tumor growth by mimicking estrogen and binding to the estrogen receptors, preventing cancerous growth. Clinical studies have demonstrated that a combination chemo/hormonal therapy regimen with tamoxifen and O(6)-alkylating drugs increased the tumor response rate in cancer patients. The mechanism of action of this combined regimen remains undefined. In this study, we demonstrated that treatment of human colorectal HT-29 carcinoma cells with tamoxifen decreased the repair activity and expression level of O(6)-methylguanine DNA methyltransferase (MGMT) protein in a concentration- and time-dependent manner. This inhibition was also shown in other malignant human cells, regardless of their estrogen receptor status. Furthermore, MGMT inactivation by tamoxifen was associated with a significantly increased susceptibility of cells to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). No alteration in MGMT mRNA levels was observed in tamoxifen-treated cells. The half-life of MGMT protein was markedly decreased in the presence of tamoxifen. Tamoxifen-induced MGMT degradation could be blocked by MG-132, a proteasome inhibitor. An increased level of ubiquitinated MGMT protein was found after tamoxifen treatment. We conclude that tamoxifen decreased the MGMT protein level by accelerating protein degradation through the ubiquitin-dependent proteasomal pathway. These findings provide a strong rationale for combined chemo/hormonal therapy with tamoxifen and BCNU in the treatment of human cancers.

Alkylation damage in DNA and RNA--repair mechanisms and medical significance

Alkylation lesions in DNA and RNA result from endogenous compounds, environmental agents and alkylating drugs. Simple methylating agents, e.g. methylnitrosourea, tobacco-specific nitrosamines and drugs like temozolomide or streptozotocin, form adducts at N- and O-atoms in DNA bases. These lesions are mainly repaired by direct base repair, base excision repair, and to some extent by nucleotide excision repair (NER). The identified carcinogenicity of O(6)-methylguanine (O(6)-meG) is largely caused by its miscoding properties. Mutations from this lesion are prevented by O(6)-alkylG-DNA alkyltransferase (MGMT or AGT) that repairs the base in one step. However, the genotoxicity and cytotoxicity of O(6)-meG is mainly due to recognition of O(6)-meG/T (or C) mispairs by the mismatch repair system (MMR) and induction of futile repair cycles, eventually resulting in cytotoxic double-strand breaks. Therefore, inactivation of the MMR system in an AGT-defective background causes resistance to the killing effects of O(6)-alkylating agents, but not to the mutagenic effect. Bifunctional alkylating agents, such as chlorambucil or carmustine (BCNU), are commonly used anti-cancer drugs. DNA lesions caused by these agents are complex and require complex repair mechanisms. Thus, primary chloroethyl adducts at O(6)-G are repaired by AGT, while the secondary highly cytotoxic interstrand cross-links (ICLs) require nucleotide excision repair factors (e.g. XPF-ERCC1) for incision and homologous recombination to complete repair. Recently, Escherichia coli protein AlkB and human homologues were shown to be oxidative demethylases that repair cytotoxic 1-methyladenine (1-meA) and 3-methylcytosine (3-meC) residues. Numerous AlkB homologues are found in viruses, bacteria and eukaryotes, including eight human homologues (hABH1-8). These have distinct locations in subcellular compartments and their functions are only starting to become understood. Surprisingly, AlkB and hABH3 also repair RNA. An evaluation of the biological effects of environmental mutagens, as well as understanding the mechanism of action and resistance to alkylating drugs require a detailed understanding of DNA repair processes.

Reversal of DNA alkylation damage by two human dioxygenases

The Escherichia coli AlkB protein protects against the cytotoxicity of methylating agents by repair of the DNA lesions 1-methyladenine and 3-methylcytosine, which are generated in single-stranded stretches of DNA. AlkB is an alpha-ketoglutarate- and Fe(II)-dependent dioxygenase that oxidizes the relevant methyl groups and releases them as formaldehyde. Here, we identify two human AlkB homologs, ABH2 and ABH3, by sequence and fold similarity, functional assays, and complementation of the E. coli alkB mutant phenotype. The levels of their mRNAs do not appear to correlate with cell proliferation but tissue distributions are different. Both enzymes remove 1-methyladenine and 3-methylcytosine from methylated polynucleotides in an alpha-ketoglutarate-dependent reaction, and act by direct damage reversal with the regeneration of the unsubstituted bases. AlkB, ABH2, and ABH3 can also repair 1-ethyladenine residues in DNA with the release of acetaldehyde.

Unmasking a killer: DNA O(6)-methylguanine and the cytotoxicity of methylating agents

Methylating agents are potent carcinogens that are mutagenic and cytotoxic towards bacteria and mammalian cells. Their effects can be ascribed to an ability to modify DNA covalently. Pioneering studies of the chemical reactivity of methylating agents towards DNA components and their effectiveness as animal carcinogens identified O(6)-methylguanine (O(6)meG) as a potentially important DNA lesion. Subsequent analysis of the effects of methylating carcinogens in bacteria and cultured mammalian cells - including the discovery of the inducible adaptive response to alkylating agents in Escherichia coli - have defined the contributions of O(6)meG and other methylated DNA bases to the biological effects of these chemicals. More recently, the role of O(6)meG in killing mammalian cells has been revealed by the lethal interaction between persistent DNA O(6)meG and the mismatch repair pathway. Here, we briefly review the results which led to the identification of the biological consequences of persistent DNA O(6)meG. We consider the possible consequences for a human cell of chronic exposure to low levels of a methylating agent. Such exposure may increase the probability that the cell's mismatch repair pathway becomes inactive. Loss of mismatch repair predisposes the cell to mutation induction, not only through uncorrected replication errors but also by methylating agents and other mutagens.

DNA repair and gene therapy: implications for translational uses

Gene therapy has been proposed to have implications in the treatment of cancer. By genetically manipulating the hematopoietic stem cell compartment with genes that confer resistance to chemotherapeutic agents, the dose escalation that is necessary to effectively treat the cancers could potentially be achieved. DNA repair genes are some of the potential candidates to confer increased resistance to chemotherapeutic agents. Although initial focus in this area has been on the direct reversal protein (MGMT), its protective ability is limited to those agents that produce O(6)-methylGuanine cross-links-agents that are not extensively used clinically (e.g., nitrosoureas). Furthermore, most alkylating agents attack more sites in DNA other than O(6)-methylGuanine, such that the protections afforded by MGMT may prevent the initial cytotoxicity, but at a price of increased mutational burden and potential secondary leukemias. Therefore, some of the genes that are being tested as candidates for gene transfer are base excision repair (BER) genes. We and others have found that overexpression of selective BER genes confers resistance to chemotherapeutic agents such as thiotepa, ionizing radiation, bleomycin, and other agents. As these "proof of concept" analyses mature, many more clinically relevant chemotherapeutic agents can be tested for BER protective ability.

Understanding and manipulating O6-methylguanine-DNA methyltransferase expression

O6-Methylguanine-DNA methyltransferase (MGMT) is a DNA repair protein that transfers methyl and alkyl lesions from the O6 position of guanine to a cysteine in its structure. The ability of MGMT to also remove precytotoxic O6-alkylguanine lesions induced by chemotherapeutic chloroethylnitrosoureas has made down-regulation of MGMT expression the key component in strategies designed to sensitize tumors to the cytotoxic potential of chloroethylnitrosoureas. The study of how to regulate MGMT expression at the gene, mRNA, and protein levels has contributed not only to the development of effective inhibitors of MGMT action, but also, in a broader sense, to a better understanding of gene regulation and protein structure/function.

The mus206 gene of Drosophila melanogaster is required in the excision repair of alkylation-induced DNA lesions

The mus206A1 mutation, previously identified in our laboratory on the basis of increased sensitivity to methyl methanesulfonate (MMS), has undergone further analysis. Genetic recombinational mapping data localize mus206 at 2-64.8. Sex-linked recessive lethal mutation tests indicate that mus206A1 exhibits significant alkylation-induced hypermutability, compared to the wild-type Oregon R progenitor strain, suggesting a defect in DNA repair function. Results of embryo viability tests show that mus206A1 and Oregon R embryos hatch to the first instar larvae at similar rates, indicating that the mus206A1 mutation does not confer embryonic lethality. Unscheduled DNA synthesis (UDS) studies with primary embryonic cell cultures subsequently demonstrated considerably less nucleotide incorporation following treatment with MMS, confirming that mus206A1 is deficient at or before the resynthesis step of alkylation-induced DNA excision repair. Previous genetic investigations have provided indirect support that at least 15 Drosophila genes which display MMS sensitivity are deficient in DNA repair functions. This study brings to 7 the number of mus genes displaying alkylation excision-repair deficiency.

Immunocytochemical evidence for a nuclear and a cytoplasmic O6-methylguanine repair mechanism in cultured rat hepatocytes

The localization of DNA and RNA adducts was studied at the ultrastructural level using antibodies directed against O6-metG and the protein A-gold technique. Primary rat hepatocyte cultures were exposed for 2-24 h to 5 mM N-nitrosodimethylamine (NDMA) or 0.1 mM N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). In both cases, the O6-metG immunoreactive sites were concentrated in the nucleus and in the rough endoplasmic reticulum (RER) rich cytoplasmic regions. The highest gold labeling density measured was observed at 2 h of NDMA or MNNG treatment. However, after a 24-h exposure, very little labeling was observed in both the nuclear and the cytoplasmic compartments. The rate of disappearance of immunoreactive sites was faster in the cytoplasm than in the nucleus, Untreated control preparations showed no specific immunogold labeling. Furthermore, when cells were exposed first to NDMA and MNNG for a few hours and then to culture medium containing no genotoxin, and subsequently were reexposed to NDMA or MNNG for a few hours, very little labeling of both the nuclear and cytoplasmic compartments was observed. Control preparations without a second genotoxin exposure showed a normal labeling pattern. Control preparations without genotoxin showed no gold labeling. Our results provide evidence for the existence of a cytoplasmic O6-metG repair mechanism that behaves like its nuclear counterpart.

Ellagic acid embryoprotection in vitro: distribution and effects on DNA adduct formation

Ellagic acid (EA) is a naturally occurring plant phenol that was recently demonstrated to protect cultured rat embryos from the embryotoxic effects of N-methyl-N-nitrosourea (MNU). The teratogenic mechanism of action of MNU, as well as other methylating agents, is debated: both cell death and mutation have been proposed. In some model systems, EA has been reported to selectively decrease the mutagenic DNA adduct O6-methylguanine (O6MG) when compared to the cytotoxic DNA adduct N7-methylguanine (N7MG). The present study was initiated to determine 1) the distribution of 14C-EA and 3H-MNU in the rat whole embryo culture model system and 2) the effects of EA on MNU-induced DNA adduct formation in this model system. 14C-EA (50 microM for 2 hr, known embryoprotective concentration; no MNU added) was used to demonstrate access of EA to the embryo within the 2 hr exposure period. The majority of EA (99.5%) remained in the media while tissue concentrations of 57.0 and 47.9 pmol/mg were attained in the yolk sacs and embryos, respectively. Similarly, MNU (75 microM for 1 hr, known effective concentration; no EA added) was distributed between the media, yolk sacs, and embryos at 99.7%, 73.7 and 112.9 pmol/mg, respectively. When non-radiolabeled EA (50 microM for 2 hr) was used to protect embryos prior to exposure to 3H-MNU (75 microM for 1 hr), the distribution of MNU in the model system was unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

Synergistic efficacy of O6-benzylguanine and 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) in a human colon cancer xenograft completely resistant to BCNU alone

The DNA repair protein O6-alkylguanine-DNA alkyltransferase (alkyltransferase) repairs cytotoxic DNA damage formed by 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). High levels of this repair protein cause tumor drug resistance to nitrosoureas. To investigate the ability of a direct alkyltransferase inhibitor, O6-benzylguanine, to reverse the nitrosourea resistance of human colon cancer cells, we studied the VACO 6 cell line which has high alkyltransferase and is completely resistant to BCNU at maximal tolerated doses in the xenograft model. O6-Benzylguanine at 0.5 microgram/mL for 1 hr inactivated VACO 6 alkyltransferase by > 98% and reduced the IC50 of BCNU by 3- to 4-fold. Further analysis indicated that these two agents act in a highly synergistic fashion. In xenograft bearing athymic mice, dose-dependent depletion of hepatic and tumor alkyltransferase was noted. To maintain alkyltransferase depletion in the xenograft for at least 24 hr, two doses of 60 mg/kg O6-benzylguanine were given 1 hr prior and 7 hr after BCNU. Under these conditions, VACO 6 xenografts became responsive to BCNU with significant reductions (P < 0.001) in the tumor growth rate. The combination increased toxicity to the host, reducing the maximum tolerated dose of BCNU by approximately 50%. This study provides definitive evidence that high alkyltransferase activity is responsible for BCNU resistance in human colon cancer xenografts and that with careful drug scheduling, O6-benzylguanine can sensitize a tumor which is completely unresponsive to BCNU alone. Further studies which optimize the therapeutic index of BCNU and O6-benzylguanine in vivo will define the schedule to be used in broader preclinical studies.

The prevention of thymic lymphomas in transgenic mice by human O6-alkylguanine-DNA alkyltransferase

Nitrosoureas form O6-alkylguanine-DNA adducts that are converted to G to A transitions, the mutation found in the activated ras oncogenes of nitrosourea-induced mouse lymphomas and rat mammary tumors. These adducts are removed by the DNA repair protein O6-alkylguanine-DNA alkyltransferase. Transgenic mice that express the human homolog of this protein in the thymus were found to be protected from developing thymic lymphomas after exposure to N-methyl-N-nitrosourea. Thus, transgenic expression of a single human DNA repair gene is sufficient to block chemical carcinogenesis. The transduction of DNA repair genes in vivo may unravel mechanisms of carcinogenesis and provide therapeutic protection from known carcinogens.

Characterization of cDNA encoding mouse DNA repair protein O6-methylguanine-DNA methyltransferase and high-level expression of the wild-type and mutant proteins in Escherichia coli

A mouse cDNA clone encoding O6-methylguanine-DNA methyltransferase (MGMT), responsible for repair of mutagenic O6-alkylguanine in DNA, was cloned from a lambda gt11 library. On the basis of an open reading frame in cDNA, the mouse protein contains 211 amino acids with a molecular mass of 22 kDa. The size and the predicted N-terminal sequence of the mouse protein were confirmed experimentally. The deduced amino acid sequence of the mouse MGMT is 70% homologous to that of the human MGMT. Cysteine-149 was shown to be the only alkyl acceptor residue in the mouse protein, in confirmation of the prediction based on conserved sequences of different MGMTs. Mouse MGMT protein is recognized by some monoclonal antibodies specific for human MGMT. Site-directed mutagenesis was utilized to reclone the mouse cDNA in a T7 promoter-based vector for overexpression of the native repair protein in Escherichia coli. The mouse protein has a tetrapeptide sequence, Pro-Glu-Gly-Val at positions 56-59, absent in the human protein. Neither deletion of this tetrapeptide nor substitution of valine-169 with alanine affected the activity of the mutant proteins.

Modulation of nitrosourea resistance in human colon cancer by O6-methylguanine

Human colon cancer is resistant to a variety of alkylating agents including the nitrosoureas. To specifically evaluate nitrosourea resistance, we studied the role of O6-alkylguanine-DNA alkyltransferase (alkyltransferase) which is known to repair nitrosourea-induced cytotoxic DNA damage. Alkyltransferase activity varied over a similar wide range in 25 colon cancer biopsies and 14 colon cancer cell lines but the activity was not correlated with differentiation status, Dukes' classification or in vitro growth characteristics. 1,3-Bis-(2-chloroethyl)-1-nitrosourea (BCNU) resistance and alkyltransferase activity were highly correlated (R2 = 0.929, P less than 0.001) in 7 different colon cancer cell lines, suggesting that the alkyltransferase is an important component of nitrosourea resistance in colon cancer cells. In the BCNU-resistant, high alkyltransferase VACO 6 cell line, inactivation of the alkyltransferase by O6-methylguanine caused a proportional decrease in the BCNU IC50, consistent with that predicted by the regression line. Enzyme inactivation was also associated with a marked increase in DNA cross-link formation. Because alkyltransferase correlates with BCNU resistance in colon cancer, and resistance can be reversed by inactivating the protein, the alkyltransferase may have an important role in nitrosourea resistance in human colon cancer cells. These data provide the rationale for clinical trials in colon cancer with biochemical modulators of the alkyltransferase to increase the therapeutic response to nitrosoureas.

Drosophila methyltransferase activity and the repair of alkylated DNA

The biochemical mechanism and developmental expression for the repair of alkylated DNA has been characterized from Drosophila. As in other organisms, the correction of O6-methylguanine in Drosophila was found to involve the transfer of a methyl group from DNA to a protein cysteine residue. Two methylated proteins with subunit molecular weights of 30 kDa and 19 kDa were identified following incubation with [3H]-methylated substrate DNA and denaturing polyacrylamide gel electrophoresis. Identical molecular weights were found for the unmethylated forms of protein through their reaction to an antibody prepared against the 19 kDa Escherichia coli methyltransferase. Both Drosophila proteins are serologically reactive in adult males and females and most of the other developmental stages tested, with embryos representing the possible exception. The Drosophila proteins do not appear to be induced by sublethal exposures to alkylating agent.

Role of Clara cells and type II cells in the development of pulmonary tumors in rats and mice following exposure to a tobacco-specific nitrosamine

The role of the Clara and type II cell in the development of pulmonary tumors in the A/J mouse and Fischer rat was investigated by determining the relationship of DNA methylation and repair in pulmonary cells to oncogene activation and by characterizing the morphology of pulmonary tumors induced by treatment with 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). Marked differences in the formation of the promutagenic adduct O6-methylguanine (O6MG) were observed in pulmonary cells following treatment of rats with NNK. Concentrations of this adduct in Clara cells greatly exceeded (3- to 30-fold) those detected in type II cells and whole lung with doses of NNK ranging from 0.1 to 50 mg/kg. In addition, very low rates of repair of this adduct were detected in Clara cells, whereas efficient adduct removal occurred in type II cells. The importance of this adduct and the role of cell specificity was suggested by the fact that a strong correlation was observed between the concentration of O6MG in Clara cells and tumor incidence in the Fischer rat with doses of NNK ranging from 0.03-50 mg/kg. In contrast, no differences in adduct concentration between type II and Clara cells from A/J mice were observed under conditions resulting in pulmonary tumor formation. Activation of the K-ras gene was detected in lung tumors from A/J mice. This gene was activated by a mutation in codon 12 involving a GC to AT transition (GGT to GAT) and is consistent with base mispairing produced by the formation of O6MG. Activation of this gene was not associated with lung tumor formation in the Fischer rat. DNA from rat lung tumors did induce tumors in the nude mouse carcinogenicity assay. In addition, rat repetitive sequences were detected in DNA isolated from these nude mouse tumors. In spite of the cell selectivity for DNA methylation in Clara cells from rat and the relationship between O6MG formation and tumorigenicity, early proliferative lesions observed in both mice and rats involved the alveolar areas. Ultrastructural examination of these lesions and adenomas revealed morphologic features characteristic of the type II cell. Thus the lack of agreement between biochemical and morphological findings makes it difficult to hypothesize a cell of origin for the pulmonary neoplasms induced by NNK. However, these studies indicate that the concentration of O6MG in Clara cells is an excellent indicator of the carcinogenic potency of NNK in the rat.(ABSTRACT TRUNCATED AT 400 WORDS)

Increased spontaneous mutation and alkylation sensitivity of Escherichia coli strains lacking the ogt O6-methylguanine DNA repair methyltransferase

Escherichia coli expresses two DNA repair methyltransferases (MTases) that repair the mutagenic O6-methylguanine (O6MeG) and O4-methylthymine (O4MeT) DNA lesions; one is the product of the inducible ada gene, and here we confirm that the other is the product of the constitutive ogt gene. We have generated various ogt disruption mutants. Double mutants (ada ogt) do not express any O6MeG/O4MeT DNA MTases, indicating that Ada and Ogt are probably the only two O6MeG/O4MeT DNA MTases in E. coli. ogt mutants were more sensitive to alkylation-induced mutation, and mutants arose linearly with dose, unlike ogt+ cells, which had a threshold dose below which no mutants accumulated; this ogt(+)-dependent threshold was seen in both ada+ and ada strains. ogt mutants were also more sensitive to alkylation-induced killing (in an ada background), and overexpression of the Ogt MTase from a plasmid provided ada, but not ada+, cells with increased resistance to killing by alkylating agents. The induction of the adaptive response was normal in ogt mutants. We infer from these results that the Ogt MTase prevents mutagenesis by low levels of alkylating agents and that, in ada cells, the Ogt MTase also protects cells from killing by alkylating agents. We also found that ada ogt E. coli had a higher rate of spontaneous mutation than wild-type, ada, and ogt cells and that this increased mutation occurred in nondividing cells. We infer that there is an endogenous source of O6MeG or O4MeT DNA damage in E. coli that is prevalent in nondividing cells.

Repair of O6-ethylguanine in DNA protects rat 208F cells from tumorigenic conversion by N-ethyl-N-nitrosourea

O6-Ethylguanine (O6-EtGua) is one of about a dozen different alkylation products formed in the DNA of cells exposed to the alkylating N-nitroso carcinogen N-ethyl-N-nitrosourea (EtNU). We have evaluated selectively the relative capacity of cells for the specific enzymatic repair of O6-EtGua as a determinant for the probability of malignant conversion. Eleven O6-EtGua-repair-proficient (R+) variant subclones were isolated from the O6-EtGua-repair-deficient (R-) clonal rat fibroblast line 208F by selection for resistance to 1,3-bis-(2-chloroethyl)-1-nitrosourea (frequency, approximately equal to 10(-5). Contrary to the 208F wild-type cells, all variants expressed O6-methylguanine-DNA methyltransferase activity, while both kinds of cells were deficient for repair of the DNA ethylation products O2- and O4-ethylthymine. After exposure to EtNU (less than or equal to 500 micrograms/ml; 20 min), cells were analyzed for the formation of piled-up foci in monolayer culture and of anchorage-independent colonies in semisolid agar medium. Depending on the EtNU concentration, the frequencies of piled-up foci and agar colonies, respectively, in the R+ variants were as low as 1/28th and 1/56th of those in the R- wild type. Contrasting with the cells from R+ variant-derived agar colonies, cells from 208F (R-) agar colonies gave rise to highly malignant tumors when implanted subcutaneously into syngeneic rats. No significant differences in the frequencies of piled-up foci were found between wild-type and variant cells after exposure to the major reactive metabolite of benzo[a]pyrene, (+)-7 beta, 8 alpha-dihydroxy-9,10 alpha-epoxy-7,8,9,10 alpha-tetrahydrobenzo[a] pyrene, for which stable binding to guanine O6 in cellular DNA has not been observed. The relative capacity of cells for repair of O6-alkylguanine is, therefore, a critical determinant for their risk of malignant conversion by N-nitroso carcinogens.

Purification to apparent homogeneity and partial amino acid sequence of rat liver O6-alkylguanine-DNA-alkyltransferase

O6-alkylguanine-DNA-alkyltransferase (ATase) activity was increased in rat liver from 80 to 320 fmoles/mg total protein 48 h after administration of 2-acetylaminofluorene at 60 mg/kg body weight. This tissue was used as a source of ATase which was purified by ammonium sulphate precipitation and DNA-cellulose, molecular exclusion and ion exchange chromatography (IEC). IEC purified material showed a major 24 kDa band after polyacrylamide gel electrophoresis (PAGE) with silver staining. Fluorography of purified ATase following incubation with [3H]-methylated substrate DNA and PAGE showed a single band at 24 kDa suggesting that, as with bacterial ATases, the protein itself accepts the alkyl group from O6-alkylguanine in substrate DNA during the repair reaction. Further purification of the protein using reverse phase HPLC resulted in a single peak representing approximately 125,000 fold purification. This was subjected to amino-terminal sequencing and it was found that the protein was blocked at the amino-terminal end: it was cleaved using trypsin or cyanogen bromide and the amino acid sequence of several reverse phase HPLC purified fragments was determined.

Combined effect of dimethylnitrosamine and a lysine-restricted diet on O6-methylguanine-DNA methyltransferase levels in mouse tissues

O6-Methylguanine is a lesion produced in DNA after exposure of animals to the procarcinogen dimethylnitrosamine. The lesion may lead to mutagenesis or carcinogenesis if not repaired. Repair is accomplished by the protein O6-methylguanine-DNA methyltransferase (MT). The methyl group is transferred to a cysteine residue of the protein, which is not regenerated. In mice, after exposure to alkylating agents, the synthesis of the protein is non-inducible. The inactivation of MT as a result of the transmethylation makes new synthesis of the protein molecules necessary for further dealkylation reactions. Protein synthesis activity correlates well with dietary protein quality. Nutritional conditions of amino acid restriction will limit the number of MT molecules synthesized. Continuous exposure of mice to dimethylnitrosamine will further diminish the pool of the protein. In this study, mice were fed a diet low in lysine and simultaneously given dimethylnitrosamine in the drinking water at concentrations resulting in dosages of zero, 0.4 mg or 1.2 mg/kg body weight/day. After 6 days MT was assayed in liver, kidney and lung. In liver and kidney, lysine restriction provoked a decrease in MT levels per mg of tissue DNA which was intensified by the presence of dimethylnitrosamine in the drinking water. Recovery from lysine restriction with respect to MT levels was achieved within 2 days. In lung, moderate effects on MT levels were observed when dietary lysine restriction was combined with the highest dosage of dimethylnitrosamine used (1.2 mg/kg body weight/day). The data strongly emphasize the importance of an adequate amino acid mixture in the diet, to support protein synthesis and to allow for high MT levels and repair of DNA lesions at the O-6 position of guanine during the exposure of the animals to alkylating agents.

Characterization of the major DNA repair methyltransferase activity in unadapted Escherichia coli and identification of a similar activity in Salmonella typhimurium

Escherichia coli has two DNA repair methyltransferases (MTases): the 39-kilodalton (kDa) Ada protein, which can undergo proteolysis to an active 19-kDa fragment, and the 19-kDa DNA MTase II. We characterized DNA MTase II in cell extracts of an ada deletion mutant and compared it with the purified 19-kDa Ada fragment. Like Ada, DNA MTase II repaired O6-methylguanine (O6MeG) lesions via transfer of the methyl group from DNA to a cysteine residue in the MTase. Substrate competition experiments indicated that DNA MTase II repaired O4-methylthymine lesions by transfer of the methyl group to the same active site within the DNA MTase II molecule. The repair kinetics of DNA MTase II were similar to those of Ada; both repaired O6MeG in double-stranded DNA much more efficiently than O6MeG in single-stranded DNA. Chronic pretreatment of ada deletion mutants with sublethal (adapting) levels of two alkylating agents resulted in the depletion of DNA MTase II. Thus, unlike Ada, DNA MTase II did not appear to be induced in response to chronic DNA alkylation at least in this ada deletion strain. DNA MTase II was much more heat labile than Ada. Heat lability studies indicated that more than 95% of the MTase in unadapted E. coli was DNA MTase II. We discuss the possible implications of these results for the mechanism of induction of the adaptive response. A similarly active 19-kDa O6MeG-O4-methylthymine DNA MTase was identified in Salmonella typhimurium.

Affinity purification and characterization of human O6-alkylguanine-DNA alkyltransferase complexed with BCNU-treated, synthetic oligonucleotide

Tumor cells resistant to chloroethylnitrosourea (CENU) therapy contain high levels of O6-alkylguanine DNA-alkyltransferase (GATase), a DNA repair enzyme that aborts DNA interstrand cross-linking by removing CENU-induced O6-alkylguanine adducts. Because the transferase binds covalently to CENU-treated oligonucleotides, we reacted partially purified GATase from cultured human lymphoblasts with a BCNU-treated, 35S-5'-end-labeled, synthetic oligonucleotide designed to have a polyadenylated 3' terminus. Immunoprobing Western blots of this reaction mixture with GATase-specific monoclonal antibody indicated that 25-30% of the transferase became complexed. We purified this complex by affinity chromatography with oligo(dT) cellulose, recovering homogenous material that appeared as a discrete 35-kDa Coomassie blue or silver-stained band after SDS-polyacrylamide gel electrophoresis. Autoradiography and Western immunoblotting confirmed that this band contained both the radiolabeled oligonucleotide and the GATase protein.

Purification and properties of O6-methylguanine-DNA-methyltransferase in human hepatic tissue

O6-Methylguanine-DNA-methyltransferase was partially purified from human liver. The transferase activity was purified by means of ammonium sulfate fractionation, DEAE-cellulose, Sepharose 6B, and double-strand DNA-cellulose chromatography. The native enzyme showed a molecular weight of about 44,000 as determined by gel filtration and a minimal molecular weight of 22,000 as obtained from SDS-PAGE. The native enzyme was unstable and underwent dissociation and decrease of activity in the presence of detergents.

Formation and persistence of ethylguanines in liver DNA of rainbow trout (Salmo gairdneri) treated with diethylnitrosamine by water exposure

Diethylnitrosamine exposure via the water resulted in the formation of 7-ethylguanine and O6-ethylguanine in rainbow trout liver DNA. The modified bases were quantitated by high-pressure liquid chromatography and fluorescence spectrophotometry. In vivo 7-ethylguanine and O6-ethylguanine levels were directly proportional to DEN concentrations between 62.5 and 250 ppm. 7-Ethylguanine and O6-ethylguanine levels were approximately directly proportional to duration of exposure to DEN between 0 and 6 hr under the conditions used, with less than proportionate increases thereafter. Removal of ethylguanines from liver DNA following a 24-hr exposure to 250 ppm DEN (a known carcinogenic regimen) was biphasic; 24% of the O6-ethylguanine and 32% of the 7-ethylguanine found immediately after exposure were removed in 12 hr but no significant decline was found over the period from 12 to 96 hr after exposure. Alkyl acceptor protein activity in trout liver was examined to assess the role of enzymatic repair in the observed loss of O6-ethylguanine in vivo. Although an O6-alkylguanine repair system similar to the alkyltransferase system reported in rodents was found in trout liver, only 4% of the O6-ethylguanine lost from DNA after exposure to 250 ppm DEN can be accounted for by activity of the alkyl acceptor protein. The high incidence of liver tumours observed in DEN-treated rainbow trout may be related to the rapid formation and substantial persistence of the promutagenic O6-ethylguanine in liver DNA.

Repair of alkylation damage in the fungus Aspergillus nidulans

The repair of alkylation damage in Aspergillus nidulans was investigated. We have assayed soluble protein fractions for enzymes known to be involved in the repair of this type of damage in DNA. The presence of a glycosylase activity that can remove 3-methyladenine from DNA was demonstrated, as well as a DNA methyltransferase activity that appears to act against O6-methylguanine. In addition to this approach, a series of mutants were isolated which display increased sensitivity to alkylating agents (sag mutants). 5 such mutants were further characterized, and at least 4 are shown to map to genes which have not previously been characterized. The behaviour of double mutant combinations demonstrates the existence of at least 2 pathways for the repair of alkylation damage. The majority of the sag mutants (sagA1, sagB2, sag4 and sagE5) exhibit an increased sensitivity to a range of alkylating agents, but not to UV light, while sagC3, when irradiated at the germling stage, also shows sensitivity to UV. None of the mutants isolated are defective in either the 3-methyladenine DNA glycosylase activity, or the DNA methyltransferase activity, and the nature of the defects in these strains remains to be determined.

The 1987 Walter Hubert lecture. Regulation and deficiencies in DNA repair

A number of rare human inherited syndromes are associated with apparent defects in DNA repair and a greatly increased frequency of cancer. Cell lines derived from such individuals phenotypically resemble certain bacterial mutant strains that have increased sensitivity to radiation or chemical agents and well characterised repair defects. This analogy provides leads for unravelling the molecular alterations in such cancer-prone human cells. The inducibility of DNA repair enzymes is also reviewed. Exposure of bacteria to alkylating agents, or oxygen radicals, causes the overproduction of several novel and interesting repair activities, and the induced bacteria provide an abundant source of these proteins for purification and biological characterisation. Enzymes with the same defined specificities are often present in human cells, presumably serving the same functions as in microorganisms, but these activities are only constitutively expressed at low levels.

6-Methylguanine and 6-methylguanosine inhibit colony-forming ability in a malignant xeroderma pigmentosum cell line but not in other xeroderma pigmentosum and normal human fibroblast strains after treatment with 1-(2-chloroethyl)-1-nitroso-3-(2-hydroxyethyl)-urea

The XP cell strain XP29MA, its malignant counterpart XP29MAmal and a normal human fibroblast strain were tested for colony-forming ability after treatment with HECNU in the presence of m6G, m6Gua, and he7G. In XP29MAmal, inhibition of post-HECNU colony-forming ability was 35% when 0.25 mM of either m6G or m6Gua were present, whereas in XP29MA and the normal fibroblast strain no inhibition was detected. The he7G caused a similar but smaller inhibitory effect in XP29MAmal, but failed to do so in XP29MA. HECNU predominantly exerts its killing effect by alkylating O-6 of DNA-bound guanine and causing DNA interstrand crosslinks. Alkylation of O-6 of guanine can be repaired by 6-methylguanine-DNA methyltransferase. From our experiments we conclude that in XP29MAmal this methyltransferase was inhibited in the presence of the 6-alkylguanines, thus leaving more 2-chloroethylated sites in DNA unrepaired. This results in sensitization in terms of decreased colony-forming ability observed only in the malignant cell line.

Delta-crystallin genes become hypomethylated in postmitotic lens cells during chicken development

Although it has been argued that the loss of 5-methylcytosine from specific sites in DNA plays an important role in activation of specific genes, the mechanism of hypomethylation is not well understood. One model links the process to DNA replication, proposing that it occurs by not remethylating cytosine on newly synthesized DNA. An alternative model argues that hypomethylation results from excision of part or all of the 5-methylcytosine. We were able to test whether hypomethylation can occur without replication by analysis of methylation changes in the delta-crystallin genes of the chicken lens. During embryonic development a large fraction of cells in the lens stops dividing as part of the differentiation process. Shortly after this stage, the delta-crystallin genes in samples of the whole lens become hypomethylated, suggesting the possibility that this process might be occurring in the subset of cells that is no longer dividing. We found that hypomethylation of these genes does occur in postmitotic lens cells, a result that implicates an excision mechanism in this tissue.

DNA damage and repair in mouse liver

The formation of DNA adducts in mouse liver has been demonstrated for numerous chemicals including members of most major classes of carcinogens. Considerably less is known about the persistence and repair of DNA adducts in mouse liver. Likewise, major gaps in present knowledge exist regarding the molecular dosimetry of DNA adducts and their potential for miscoding during continuous exposure to high versus low doses of carcinogens. A prime example of this is 2-acetylaminofluorene (2-AAF), the carcinogen used in the ED01 megamouse study. There are no molecular dosimetry studies on the DNA adducts of 2-AAF, even though such a unique data base exists for the dose-response relationship of mouse liver tumors. Reviewing the pertinent literature, identifying deficiencies, and conducting the required research will hopefully permit a better determination of the relevance of mouse liver tumors to man.

DNA-mediated transfer and expression of a human DNA repair gene that demethylates O6-methylguanine

Human liver DNA was transfected into CHO cells (mex-) along with pSV2gpt and colonies were selected first for resistance to mycophenolic acid and then to chloroethylnitrosourea. Transformants were obtained that contained approximately 10,000 molecules of O6-alkylguanine alkyltransferase (mex+) per cell. Their genome contained at least three copies of the human Alu sequence.

Effect of N-nitrosamines carcinogenic for oesophagus on O6-alkyl-guanine-DNA-methyl transferase in rat oesophagus and liver

Several O6-alkylGua adducts have been shown to be removed from DNA during its repair by transfer of the alkyl group to a cysteine residue in a specific AAP, with the formation of S-alkylcysteine. As the reaction is stoichiometric and irreversible, the AAP content of the cell can be reduced or depleted. In vivo depletion by a high dose of nitrosamine can be used to test for the formation of a repairable alkylation adduct at the O6-position of guanine. In addition, if the carcinogenic potency of a nitroso compound for a particular organ is related to the persistence of the adduct in DNA, potency would depend not on the level of alkylation attained after treatment, but on whether this was sufficient to deplete the AAP content of the organ concerned and so to slow down repair, i.e. depletion of AAP is a more relevant estimate of potency than is the initial extent of DNA alkylation. Dose-response studies on target and non-target organs showed that depletion of AAP correlated with organotropy for those nitrosamines known to methylate DNA, i.e. with NDMA for liver, and with NMBzA for oesophagus. With NDEA, the results supported the suggestion that other adducts in addition to O6-alkylGua may be involved. NMPhA, an oesophageal specific carcinogen, did not deplete AAP in oesophagus, and induced AAP in liver. This result adds to the evidence that NMPhA does not alkylate DNA.

Studies on the distribution of O6-methylguanine-DNA methyltransferase in rat

O6-Methylguanine-DNA methyltransferase, a DNA repair enzyme which transfers the methyl group of O6-methylguanine residue to a cysteinyl residue in the methyltransferase itself, was examined in rat organs by quantifying the S-methylcysteine formed in the methyl acceptor protein. Among the various organs examined, the spleen exhibited the highest enzyme specific activity followed by the thymus, liver, lung and testis. Brain had the lowest activity. The patterns of subcellular distribution of the methyltransferase in spleen and liver were different: while 75-80% of the activity was present in the nuclear fraction of the spleen, 54% of the activity in the liver was found in the nuclei and 35% in the cytosolic fraction. Forty-five and thirty-five percent of the total nuclear enzyme activity could be extracted with 1 M and 2 M NaCl solutions, respectively, indicating that the repair enzyme is not tightly bound to the nuclear matrix. When isolated nuclei were incubated with [methyl-3H]DNA substrate and subsequently fractionated into histone and non-histone protein fractions, over 90% of the radioactivity was coeluted on a Bio-Rex 70 column with the non-histone fraction and only a negligible amount of radioactivity was found to be associated with the histone fraction. The molecular mass of the [methyl-3H]methyltransferase in the non-histone fraction was determined to be 23,000, and its pI value was found to be 6.6 by two-dimensional polyacrylamide gel electrophoresis.

O6 alkylguanine-DNA alkyltransferase activity in human myeloid cells

The association between alkylating agent exposure and acute nonlymphocytic leukemia in humans indicates that myeloid cells may be particularly susceptible to mutagenic damage. Alkylating agent mutagenesis is frequently mediated through formation and persistence of a particular DNA base adduct, O6alkylguanine, which preferentially mispairs with thymine rather than cytosine, leading to point mutations. O6alkylguanine is repaired by O6alkylguanine-DNA alkyltransferase (alkyltransferase), a protein that removes the adduct, leaving an intact guanine base in DNA. We measured alkyltransferase activity in myeloid precursors and compared it with levels in other cells and tissues. In peripheral blood granulocytes, monocytes, T lymphocytes, and B lymphocytes, there was an eightfold range of activity between individuals but only a twofold range in the mean activity between cell types. Normal donors maintained stable levels of alkyltransferase activity over time. In bone marrow T lymphocytes and myeloid precursors, there was an eightfold range of alkyltransferase activity between donors. Alkyltransferase activity in the two cell types was closely correlated in individual donors, r = 0.69, P less than 0.005, but was significantly higher in the T lymphocytes than the myeloid precursors, P less than 0.05. Liver contained the highest levels of alkyltransferase of all tissues tested. By comparison, small intestine contained 34%, colon 14%, T lymphocytes 11%, brain 11%, and myeloid precursors 6.6% of the activity found in liver. Thus, human myeloid precursors have low levels of O6alkylguanine-DNA alkyltransferase compared with other tissues. Low levels of this DNA repair protein may increase the susceptibility of myeloid precursors to malignant transformation after exposure to certain alkylating agents.

DNA-mediated transfer and expression of a human DNA repair gene that demethylates O6-methylguanine

Human liver DNA was transfected into CHO cells (mex-) along with pSV2gpt and colonies were selected first for resistance to mycophenolic acid and then to chloroethylnitrosourea. Transformants were obtained that contained approximately 10,000 molecules of O6-alkylguanine alkyltransferase (mex+) per cell. Their genome contained at least three copies of the human Alu sequence.

Repair of O6-(2-chloroethyl)guanine mediates the biological effects of chloroethylnitrosoureas

Chloroethylnitrosoureas (CENUs) are alkylating and crosslinking agents used for the treatment of human cancer; they are both mutagenic and carcinogenic. We compared the levels of induction of sister chromatid exchanges (SCEs) and the cytotoxicity of nitrosoureas that alkylate only with CENUs. CENUs are 200-fold more cytotoxic and induce SCEs with 45-fold greater efficiency than agents that do not crosslink; therefore, crosslinking is probably the most important molecular event that leads to cell death and induction of SCEs. The biological and biochemical properties of both human and rat brain tumor cells that are sensitive or resistant to the cytotoxic effects of CENUs have been investigated. CENUs induce SCEs in both sensitive and resistant cells, but to induce similar levels of SCEs, resistant cells must be treated with a 5- to 14-fold higher concentration of CENUs than are used to treat sensitive cells. Resistant cells have a higher cellular level of O6-methylguanine-DNA methyl transferase, increased repair of O6-methylguanine, and 50% fewer DNA interstrand crosslinks formed than do sensitive cells treated with the same concentration of CENU. Based on these findings, we propose that cellular resistance to the cytotoxic effects of CENUs is mediated by O6-methylguanine-DNA methyltransferase and that DNA repair may also modify the mutagenic and carcinogenic properties of CENUs.

Repair of DNA alkylation adducts in mammalian cells

Carcinogenic alkylating agents, including nitrosamines, are able to alkylate DNA at various sites. This review presents evidence of the high degree of specificity in the type of DNA damage induced by various N-nitroso compounds and in the DNA repair processes among tissues or cells of different species. The O6-alkylguanine DNA alkyltransferase activity in various human and rodent tissues is discussed as well as the detection of O6-methylguanine in human DNA, using monoclonal antibodies and radioimmunoassay. The relevance of these findings to the mechanisms of cancer induction by nitrosamines is discussed.

Possible depletion of a DNA repair enzyme in human lymphoma cells by subversive repair

Mex+ human lymphoma cell lines contain O6-methylguanine-DNA methyltransferase, a DNA repair enzyme that undergoes suicide inactivation on interaction with its substrate. The cells are therefore competent to remove the alkylation lesion O6-methylguanine from their DNA. However, several repair-deficient lymphoma cell lines (Mex-) are also known. It is shown here that Mex+ cells can be converted temporarily to a Mex- phenotype by growth in nontoxic concentrations of free O6-methylguanine. The depletion of methyltransferase activity is not a result of O6-methylguanine incorporation into DNA and subsequent demethylation by the enzyme. It is proposed that O6-methylguanine is mistakenly incorporated into tRNA molecules by means of a post-transcriptional ribosyl transfer reaction. The demethylation of such bases in tRNA has been demonstrated by using bacterial and human DNA repair enzymes. The existence of such a subversive repair of a methylated base in tRNA raises the possibility of competition between DNA and RNA for cellular DNA repair enzymes. Furthermore, it is proposed that the known aberrant methylation of tRNA in certain transformed cells, together with subversive tRNA repair, could account for the Mex- phenotype.

Cloning of the E. coli O6-methylguanine and methylphosphotriester methyltransferase gene using a functional DNA repair assay

Alkylating agents react with various nitrogen and oxygen atoms in DNA and many of the products are substrates for repair processes. Oxygen atom derivatives such as O6-methylguanine (O6-meG) O4-methylthymine and methylphosphotriesters (MP) have been shown to undergo repair by methyl group removal. The proteins involved in the latter reaction can be considered to be methyltransferases (MT) because their action results in the transfer of the methyl group to a cysteine residue within a polypeptide. A rapid and sensitive assay for MT activity has been developed and used to screen extracts of bacteria harbouring an E. coli genomic DNA library carried in a plasmid vector. We report here the cloning of an E. coli gene coding for O6-meG and MP MT repair functions. These two activities reside on a 37Kd protein that can undergo a host-dependent cleavage to produce an 18Kd protein which contains only O6-meG MT and a 13Kd protein which contains only MP MT.

Induction and autoregulation of ada, a positively acting element regulating the response of Escherichia coli K-12 to methylating agents

The ada gene of Escherichia coli K-12, the regulatory locus for the adaptive response to methylating agents, coded for a 39,000-dalton protein. An adjacent gene coding for a 27,000-dalton protein was coregulated with ada. The Ada protein was strongly induced upon exposure of cells to methylating agents such as N-methyl-N'-nitro-N-nitrosoguanidine. An analysis of ada regulation with an ada-lacZ operon fusion showed that ada+ function was required for induction of ada transcription. Derivatives of the ada gene truncated from the 3' end produced proteins which were more potent in stimulating transcription than the product of the intact ada gene, indicating that the transcription-activating function of the Ada protein resided in its amino terminus. The sequence of the ada-regulatory region and the identification of the start site of ada transcription are also presented.