Fast repair of O6-ethylguanine, but not O6-methylguanine, in transcribed genes prevents mutation of H-ras in rat mammary tumorigenesis induced by ethylnitrosourea in place of methylnitrosourea

A Review of Stable Isotope Labeling and Mass Spectrometry Methods to Distinguish Exogenous from Endogenous DNA Adducts and Improve Dose-Response Assessments

Cancer remains the second most frequent cause of death in human populations worldwide, which has been reflected in the emphasis placed on management of risk from environmental chemicals considered to be potential human carcinogens. The formation of DNA adducts has been considered as one of the key events of cancer, and persistence and/or failure of repair of these adducts may lead to mutation, thus initiating cancer. Some chemical carcinogens can produce DNA adducts, and DNA adducts have been used as biomarkers of exposure. However, DNA adducts of various types are also produced endogenously in the course of normal metabolism. Since both endogenous physiological processes and exogenous exposure to xenobiotics can cause DNA adducts, the differentiation of the sources of DNA adducts can be highly informative for cancer risk assessment. This review summarizes a highly applicable methodology, termed stable isotope labeling and mass spectrometry (SILMS), that is superior to previous methods, as it not only provides absolute quantitation of DNA adducts but also differentiates the exogenous and endogenous origins of DNA adducts. SILMS uses stable isotope-labeled substances for exposure, followed by DNA adduct measurement with highly sensitive mass spectrometry. Herein, the utilities and advantage of SILMS have been demonstrated by the rich data sets generated over the last two decades in improving the risk assessment of chemicals with DNA adducts being induced by both endogenous and exogenous sources, such as formaldehyde, vinyl acetate, vinyl chloride, and ethylene oxide.

Aldehydes are the predominant forces inducing DNA damage and inhibiting DNA repair in tobacco smoke carcinogenesis

Tobacco smoke (TS) contains numerous cancer-causing agents, with polycyclic aromatic hydrocarbons (PAHs) and nitrosamines being most frequently cited as the major TS human cancer agents. Many lines of evidence seriously question this conclusion. To resolve this issue, we determined DNA adducts induced by the three major TS carcinogens: benzo(a)pyrene (BP), 4-(methylnitrosamine)-1-(3-pyridyl)-1-butanoe (NNK), and aldehydes in humans and mice. In mice, TS induces abundant aldehyde-induced γ-hydroxy-propano-deoxyguanosine (γ-OH-PdG) and α-methyl-γ-OH-PdG adducts in the lung and bladder, but not in the heart and liver. TS does not induce the BP- and NNK-DNA adducts in lung, heart, liver, and bladder. TS also reduces DNA repair activity and the abundance of repair proteins, XPC and OGG1/2, in lung tissues. These TS effects were greatly reduced by diet with polyphenols. We found that γ-OH-PdG and α-methyl-γ-OH-PdG are the major adducts formed in tobacco smokers' buccal cells as well as the normal lung tissues of tobacco-smoking lung cancer patients, but not in lung tissues of nonsmokers. However, the levels of BP- and NNK-DNA adducts are the same in lung tissues of smokers and nonsmokers. We found that while BP and NNK can induce BPDE-dG and O6-methyl-dG adducts in human lung and bladder epithelial cells, these inductions can be inhibited by acrolein. Acrolein also can reduce DNA repair activity and repair proteins. We propose a TS carcinogenesis paradigm. Aldehydes are major TS carcinogens exerting dominant effect: Aldehydes induce mutagenic PdG adducts, impair DNA repair functions, and inhibit many procarcinogens in TS from becoming DNA-damaging agents.

Combining molecular docking and 3-D pharmacophore generation to enclose the in vivo antigenotoxic activity of naturally occurring aromatic compounds: myricetin, quercetin, rutin, and rosmarinic acid

Considering the controversial results concerning the antimutagenicity of some phenolic compounds recorded in the literature, the antigenotoxic effects of four selected phenolic compounds, myricetin, quercetin, rutin, and rosmarinic acid, against DNA damage induced by alkylation with ethyl methanesulfonate (EMS), were evaluated in Drosophila melanogaster males using the sex-linked recessive lethal (SLRL) test. To assess the protective effects against DNA damage, D. melanogaster males were exposed to a monofunctional alkylating agent EMS in concentration of 0.75 ppm, 24 h prior to one of the selected phenolic compounds in the concentration of 100 ppm. The possible differences in mechanisms of protection by selected compounds were determined by molecular docking, after which structure-based 3-D pharmacophore models were generated. EMS induced considerable DNA damage as shown by significant increase in the frequency of germinative mutations. The frequency decreased with high significance (p<0.001***) after post-treatments with all selected phenolic compounds. Further, docking analysis revealed EMS pre-bond conformations against guanine and thymine as a necessary condition for alkylation, after which resulting O⁶-ethylguanine and O⁴-ethylthimine were docked into the active site of O⁶-alkylguanine-DNA alkyltransferase to confirm that particular lesions are going to be repaired. Finally, myricetin and quercetin protected dealkylated nucleotides from further EMS alkylation by forming the strong hydrogen bonds with O⁶-guanine and O⁴-thymine via B ring hydroxyl group (bond lengths lower than 2.5 Å). On the other side, rutin and rosmarinic acid encircled nucleotides and by fulfilling the EMS binding space they made an impermeable barrier for the EMS molecule and prevented further alkylation.

Simultaneous quantitative analysis of N3-ethyladenine and N7-ethylguanine in human leukocyte deoxyribonucleic acid by stable isotope dilution capillary liquid chromatography-nanospray ionization tandem mass spectrometry

Cigarette smoke contains ethylating agents which damage DNA producing ethylated DNA adducts, such as N(3)-ethyladenine (3-EtAde), N(7)-ethylguanine (7-EtGua), and regioisomers of ethylthymine. Among them, 3-EtAde and 7-EtGua are present in human urine and their levels are higher in smokers than in nonsmokers. The amount of ethylated DNA adducts in tissue DNA represents the steady-state levels of DNA adducts resulting from the ethylating agent after repair in vivo. In this study, we have developed a highly sensitive, accurate, and quantitative assay for simultaneous detection and quantification of 3-EtAde and 7-EtGua by stable isotope dilution capillary liquid chromatography-nanospray ionization tandem mass spectrometry (capLC-NSI/MS/MS). Under the highly selective reaction monitoring (H-SRM) mode, the detection limit of 3-EtAde and 7-EtGua injected on-column was 5.0 fg (31 amol) and 10 fg (56 amol), respectively. The quantification limit for the entire assay was 50 and 100 fg of 3-EtAde and 7-EtGua, corresponding to 4.7 and 8.6 adducts in 10(9) normal nucleotides, respectively, starting with 20 μg of DNA isolated from <1 mL of blood and injecting an equivalent of 4 μg of DNA on-column. The mean (±SD) levels of 3-EtAde and 7-EtGua in leukocyte DNA from 20 smokers were 16.0±7.8 and 9.7±8.3 in 10(8) normal nucleotides, respectively, which were statistically significantly higher than those of 5.4±2.6 3-EtAde and 0.3±0.8 7-EtGua in 10(8) normal nucleotides from 20 nonsmokers (p<0.0001). The levels of 3-EtAde and 7-EtGua in these 40 leukocyte DNA samples are positively correlated (γ=0.6970, p<0.0001). Furthermore, there are statistically significant associations between the number of cigarettes smoked per day, as well as the smoking index, and the levels of 3-EtAde and 7-EtGua. Levels of 3-EtAde and 7-EtGua are compared to those of ethylthymidine adducts. To our knowledge, this is the first assay for simultaneous quantification of 3-EtAde and 7-EtGua in the same DNA sample and is the first report of 3-EtAde in human DNA. This highly sensitive and specific stable isotope dilution capLC-NSI/MS/MS assay should be useful in measuring 3-EtAde and 7-EtGua in human leukocyte DNA as potential biomarkers for smoking-related cancers.

Drug resistance and DNA repair in leukaemia

Most cytotoxic agents exert their action via damage of DNA. Therefore, the repair of such lesions is of major importance for the sensitivity of malignant cells to chemotherapeutic agents. The underlying mechanisms of various DNA repair pathways have extensively been studied in yeast, bacteria and mammalian cells. Sensitive and drug resistant cancer cell lines have provided models for analysis of the contribution of DNA repair to chemosensitivity. However, the validity of results obtained by laboratory experiments with regard to the clinical situation is limited. In both acute and chronic leukaemias, the emergence of drug resistant cells is a major cause for treatment failure. Recently, assays have become available to measure cellular DNA repair capacity in clinical specimens at the single-cell level. Application of these assays to isolated lymphocytes from patients with chronic lymphatic leukaemia (CLL) revealed large interindividual differences in DNA repair rates. Accelerated O(6)-ethylguanine elimination from DNA and faster processing of repair-induced single-strand breaks were found in CLL lymphocytes from patients nonresponsive to chemotherapy with alkylating agents compared to untreated or treated sensitive patients. Moreover, modulators of DNA repair with different target mechanisms were identified which also influence the sensitivity of cancer cells to alkylating agents. In this article, we review the current knowledge about the contribution of DNA repair to drug resistance in human leukaemia.

Analysis of ethylated thymidine adducts in human leukocyte DNA by stable isotope dilution nanoflow liquid chromatography-nanospray ionization tandem mass spectrometry

Studies showed that levels of ethylated DNA adducts in certain tissues and urine are higher in smokers than in nonsmokers. Because cigarette smoking is a major risk factor of various cancers, DNA ethylation might play an important role in cigarette smoke-induced cancer formation. Among the ethylated DNA adducts, O(2)-ethylthymidine (O(2)-edT) and O(4)-ethylthymidine (O(4)-edT) are poorly repaired and are accumulated in the body. In addition, O(4)-edT possesses promutagenic properties. In this study, we have developed a highly sensitive, accurate, and quantitative assay for simultaneous detection and quantification of O(2)-edT, N(3)-edT (N(3)-ethylthymidine), and O(4)-edT adducts by isotope dilution nanoflow liquid chromatography-nanospray ionization tandem mass spectrometry (nanoLC-NSI/MS/MS). Under the highly selected reaction monitoring (H-SRM) mode, the detection limit of O(2)-edT, N(3)-edT, and O(4)-edT injected on-column was 5.0, 10, and 10 fg, respectively. The quantification limit for the entire assay was 50, 100, and 100 fg of O(2)-edT, N(3)-edT, and O(4)-edT, respectively, corresponding to 1.1, 2.3, and 2.3 adducts in 10(9) normal nucleotides, respectively, starting with 50 μg of DNA (from 1.5-2.0 mL of blood). Levels of O(2)-edT, N(3)-edT, and O(4)-edT in 20 smokers' leukocyte DNA were 44.8 ± 52.0, 41.1 ± 43.8, 48.3 ± 53.9 in 10(8) normal nucleotides, while those in 20 nonsmokers were 0.19 ± 0.87, 4.1 ± 13.3, and 1.0 ± 2.9, respectively. Levels of O(2)-edT, N(3)-edT, and O(4)-edT in human leukocyte DNA are all significantly higher in smokers than in nonsmokers, with pvalues of 0.0004, 0.0009, and 0.0004, respectively. Furthermore, levels of O(2)-edT show a statistically significant association (γ = 0.4789, p = 0.0327) with the smoking index in smokers. In the 40 leukocyte DNA samples, the extremely significant statistical correlations (p < 0.0001) are observed between levels of O(2)-edT and O(4)-edT (γ = 0.9896), between levels of O(2)-edT and N(3)-edT (γ = 0.9840), and between levels of N(3)-edT and O(4)-edT (γ = 0.9901). To our knowledge, this is the first mass spectrometry-based assay for ethylated thymidine adducts. Using this assay, the three ethylated thymidine adducts were detected and quantified for the first time. Therefore, this highly sensitive, specific, and accurate assay should be clinically feasible for simultaneous quantification of the three ethylated thymidine adducts as potential biomarkers for exposure to ethylating agents and for cancer risk assessment.

Repair of O4-alkylthymine by O6-alkylguanine-DNA alkyltransferases

O(6)-Alkylguanine-DNA alkyltransferase (AGT) plays a major role in repair of the cytotoxic and mutagenic lesion O(6)-methylguanine (m(6)G) in DNA. Unlike the Escherichia coli alkyltransferase Ogt that also repairs O(4)-methylthymine (m(4)T) efficiently, the human AGT (hAGT) acts poorly on m(4)T. Here we made several hAGT mutants in which residues near the cysteine acceptor site were replaced by corresponding residues from Ogt to investigate the basis for the inefficiency of hAGT in repair of m(4)T. Construct hAGT-03 (where hAGT sequence -V(149)CSSGAVGN(157)- was replaced with the corresponding Ogt -I(143)GRNGTMTG(151)-) exhibited enhanced m(4)T repair activity in vitro compared with hAGT. Three AGT proteins (hAGT, hAGT-03, and Ogt) exhibited similar protection from killing by N-methyl-N'-nitro-N-nitrosoguanidine and caused a reduction in m(6)G-induced G:C to A:T mutations in both nucleotide excision repair (NER)-proficient and -deficient Escherichia coli strains that lack endogenous AGTs. hAGT-03 resembled Ogt in totally reducing the m(4)T-induced T:A to C:G mutations in NER-proficient and -deficient strains. Surprisingly, wild type hAGT expression caused a significant but incomplete decrease in NER-deficient strains but a slight increase in T:A to C:G mutation frequency in NER-proficient strains. The T:A to C:G mutations due to O(4)-alkylthymine formed by ethylating and propylating agents were also efficiently reduced by either hAGT-03 or Ogt, whereas hAGT had little effect irrespective of NER status. These results show that specific alterations in the hAGT active site facilitate efficient recognition and repair of O(4)-alkylthymines and reveal damage-dependent interactions of base and nucleotide excision repair.

Chemical biology of mutagenesis and DNA repair: cellular responses to DNA alkylation

The reaction of DNA-damaging agents with the genome results in a plethora of lesions, commonly referred to as adducts. Adducts may cause DNA to mutate, they may represent the chemical precursors of lethal events and they can disrupt expression of genes. Determination of which adduct is responsible for each of these biological endpoints is difficult, but this task has been accomplished for some carcinogenic DNA-damaging agents. Here, we describe the respective contributions of specific DNA lesions to the biological effects of low molecular weight alkylating agents.

Differential effects of low- and high-dose X-rays on N-ethyl-N-nitrosourea-induced mutagenesis in thymocytes of B6C3F1 gpt-delta mice

Carcinogenesis in humans is thought to result from exposure to numerous environmental factors. Little is known, however, about how these different factors work in combination to cause cancer. Because thymic lymphoma is a good model of research for combined exposure, we examined the occurrence of mutations in thymic DNA following exposure of B6C3F1 gpt-delta mice to both ionizing radiation and N-ethyl-N-nitrosourea (ENU). Mice were exposed weekly to whole body X-irradiation (0.2 or 1.0 Gy), ENU (200 ppm) in the drinking water, or X-irradiation followed by ENU treatment. Thereafter, genomic DNA was prepared from the thymus and the number and types of mutations in the reporter transgene gpt was determined. ENU exposure alone increased mutant frequency by 10-fold compared to untreated controls and over 80% of mutants had expanded clonally. X-irradiation alone, at either low or high dose, unexpectedly, reduced mutant frequency. Combined exposure to 0.2 Gy X-rays with ENU dramatically decreased mutant frequency, specifically G:C to A:T and A:T to T:A mutations, compared to ENU treatment alone. In contrast, 1.0 Gy X-rays enhanced mutant frequency by about 30-fold and appeared to accelerate clonal expansion of mutated cells. In conclusion, repeated irradiation with 0.2 Gy X-rays not only reduced background mutation levels, but also suppressed ENU-induced mutations and clonal expansion. In contrast, 1.0 Gy irradiation in combination with ENU accelerated clonal expansion of mutated cells. These results indicate that the mode of the combined mutagenic effect is dose dependent.

Cytotoxicity and mutagenicity of endogenous DNA base lesions as potential cause of human aging

Endogenous factors constitute a substantial source of damage to the genomic DNA. The type of damage includes a number of different base lesions and single- and double-strand breaks. Unrepaired DNA damage can give rise to mutations and may cause cell death. A number of studies have demonstrated an association between aging and the accumulation of DNA damage. This may be attributed to reduced DNA repair with age, although this is apparently not a general feature for all types of damage and repair mechanisms. Therefore, detailed studies that improve our knowledge of DNA repair systems as well as mutagenic and toxic effects of DNA lesions will help us to gain a better insight into the mechanisms of aging. The aim of this review is to provide a brief description of cytotoxic and mutagenic endogenous DNA lesions that are mainly repaired by base excision repair and single-strand break repair pathways and to discuss the potential role of DNA lesions and DNA repair dysfunction in the onset of human aging.

Reduced methylation-induced mutagenesis in rat splenocytes in vivo by sub-chronic low dose exposure to N-metyl-N-nitrosourea

Estimates of genotoxic effects of mutagens at low and protracted doses are often based on linear extrapolation of data obtained at relatively high doses. To test the validity of such an approach, a comparison was made between the mutagenicity of N-methyl-N-nitrosourea (MNU) in T-lymphocytes of the rat following two treatment protocols, i.e. sub-chronic exposure to a low dose (15-45 repeated exposures to 1mg/kg of MNU) or acute exposure to a single high dose (15, 30 or 45 mg/kg of MNU). Mutation induction appeared dramatically lower following sub-chronic treatment compared to treatment with a single high exposure. Furthermore, DNA sequence analysis of the coding region of the hprt gene in MNU-induced mutants showed that acute high dose treatment causes mainly GC-->AT base pair changes, whereas sub-chronic treatment results in a significant contribution of AT base pair changes to mutation induction. We hypothesize that O(6)-methylguanine-DNA methyltransferase is saturated after acute treatments, while after sub-chronic treatment most O(6)-methylguanine is efficiently repaired. These data suggest (i) that risk estimations at low and protracted doses of MNU on the basis of linear extrapolation of effects measured at high dose are too high and (ii) that the protective effects of DNA repair processes are relatively strong at low sub-chronic exposure.

MGMT: key node in the battle against genotoxicity, carcinogenicity and apoptosis induced by alkylating agents

O(6)-methylguanine-DNA methyltransferase (MGMT) plays a crucial role in the defense against alkylating agents that generate, among other lesions, O(6)-alkylguanine in DNA (collectively termed O(6)-alkylating agents [O(6)AA]). The defense is highly important, since O(6)AA are common environmental carcinogens, are formed endogenously during normal cellular metabolism and possibly inflammation, and are being used in cancer therapy. O(6)AA induced DNA damage is subject to repair, which is executed by MGMT, AlkB homologous proteins (ABH) and base excision repair (BER). Although this review focuses on MGMT, the mechanism of repair by ABH and BER will also be discussed. Experimental systems, in which MGMT has been modulated, revealed that O(6)-methylguanine (O(6)MeG) and O(6)-chloroethylguanine are major mutagenic, carcinogenic, recombinogenic, clastogenic and killing lesions. O(6)MeG-induced clastogenicity and cell death require MutS alpha-dependent mismatch repair (MMR), whereas O(6)-chloroethylguanine-induced killing occurs independently of MMR. Extensive DNA replication is required for O(6)MeG to provoke cytotoxicity. In MGMT depleted cells, O(6)MeG induces apoptosis almost exclusively, barely any necrosis, which is presumably due to the remarkable ability of secondarily formed DNA double-strand breaks (DSBs) to trigger apoptosis via ATM/ATR, Chk1, Chk2, p53 and p73. Depending on the cellular background, O(6)MeG activates both the death receptor and the mitochondrial apoptotic pathway. The inter-individual expression of MGMT in human lymphocytes is highly variable. Given the key role of MGMT in cellular defense, determination of MGMT activity could be useful for assessing a patient's drug sensitivity. MGMT is expressed at highly variable amounts in human tumors. In gliomas, a correlation was found between MGMT activity, MGMT promoter methylation and response to O(6)AA. Although the human MGMT gene is inducible by glucocorticoids and genotoxins such as radiation and alkylating agents, the role of this induction in the protection against carcinogens and the development of chemotherapeutic alkylating drug resistance are still unclear. Modulation of MGMT expression in tumors and normal tissue is currently being investigated as a possible strategy for improving cancer therapy.

Proteomic analysis of human O6-methylguanine-DNA methyltransferase by affinity chromatography and tandem mass spectrometry

Recent evidence suggests that human O(6)-methylguanine-DNA methyltransferase (MGMT), a DNA repair protein that protects the genome against mutagens and accords tumor resistance to many anticancer alkylating agents, may have other roles besides repair. Therefore, we isolated MGMT-interacting proteins from extracts of HT29 human colon cancer cells using affinity chromatography on MGMT-Sepharose. Specific proteins bound to this column were identified by electrospray ionization tandem mass spectrometry and/or Western blotting. These procedures identified >60 MGMT-interacting proteins with diverse functions including those involved in DNA replication and repair (MCM2, PCNA, ORC1, DNA polymerase delta, MSH-2, and DNA-dependent protein kinase), cell cycle progression (CDK1, cyclin B, CDK2, CDC7, CDC10, 14-3-3 protein, and p21(waf1/cip1)), RNA processing and translation (poly(A)-binding protein, nucleolin, heterogeneous nuclear ribonucleoproteins, A2/B1, and elongation factor-1alpha), several histones (H4, H3.4, and H2A.1), and topoisomerase I. The heat shock proteins, HSP-90alpha and beta, also bound strongly with MGMT. The DNA repair activity of MGMT was greatly enhanced in the presence of interacting proteins or histones. These data, for the first time, suggest that human MGMT is likely to have additional functions, possibly, in sensing and integrating the DNA damage/repair-related signals with replication, cell cycle progression, and genomic stability.

Unifying concept of DNA repair: the polymerase scanning hypothesis

According to a series of experiments on untransformed mouse embryonic fibroblasts, quiescent mouse cells lack global genomic repair (GGR) of premutagenic DNA damage. The gene used to assess mutation and premutagenic DNA damage was the lacI transgene incorporated permanently in the DNA in a lambda shuttle vector. The transgene lacks mammalian transcription signals and thus is unexpressed in the cells. Although the cells conducted transcription-coupled repair (TCR) of UV damage, the transgene was not repaired over a 4-day interval. These cells are not terminally differentiated and can readily be induced to resume cellular division. In this article, we discuss the interpretation of these results and suggest a new hypothesis for DNA scanning, the mechanism by which cells discover DNA damage and initiate DNA repair. Our hypothesis, which we call the polymerase scanning hypothesis, is that GGR is initiated in very much the same way as TCR, by a polymerase complex encountering the damage. We call the two together polymerase-coupled repair (PC repair). In the case of GGR, it would be the DNA replication complex during the S-phase. This is, we suggest, the dominant mechanism of repair of DNA at low doses for untranscribed genes. Evidence contrary to this hypothesis exists, which we discuss, but it should be noted that existing hypotheses about DNA scanning and DNA repair cannot account for the results that we have obtained.

Intra- and intercellular variations in the repair efficiency of O6-methylguanine, and their contribution to kinetic complexity

Following administration to rats of various doses of N-nitrosodimethylamine (NDMA), O(6)-methylguanine (O(6)-meG) was lost from the DNA of four tissues (liver, white blood cells, lymph nodes, bone marrow) over two, sharply demarcated phases with substantially differing repair rates. Repair during each phase followed approximately first-order kinetics in O(6)-meG, even after a high dose of NDMA which caused substantial depletion of O(6)-alkylguanine-DNA alkyltransferase (AGT), a suicide repair protein. This is compatible with rate-determining adduct repair being brought about by a distinct, minor pool of AGT molecules which is rapidly replenished by de novo AGT synthesis. Similar biphasic repair kinetics were also observed in HepG2 cells treated in vitro with NDMA. In this case, the first phase of repair was inhibited by alpha-amanitin, an inhibitor of RNA polymerase II-mediated transcription. However, no dependence on transcriptional activity was found when O(6)-meG repair in specific gene sequences with different transcriptional status in rat liver was examined, suggesting that the effects of alpha-amanitin in HepG2 cells did not reflect inhibition of preferential repair of transcribed sequences. Repair was also examined in rat liver hepatocytes and non-parenchymal cells separately after administration of NDMA at non-AGT depleting doses. Within each cell-population, the repair followed single phase, first-order kinetics, with adduct loss from AGT-rich hepatocytes being significantly faster than from the relatively AGT-deficient non-parenchymal cells. In conclusion, differences in the AGT content of different cell subpopulations in the liver (and probably in other tissues), as well as additional cellular factors affecting repair efficiency, appear to determine the observed variation in the kinetics of repair of O(6)-meG. The additional cellular factors involved appear not to be related to the transcriptional state of the sequences being repaired, but may reflect different states of chromatin condensation.

Generating mutations but providing chemosensitivity: the role of O6-methylguanine DNA methyltransferase in human cancer

O(6)-methylguanine DNA methyltransferase (MGMT) is a key enzyme in the DNA repair network. MGMT removes mutagenic and cytotoxic adducts from O(6)-guanine in DNA, the preferred point of attack of many carcinogens (i.e. methylnitrosourea) and alkylating chemotherapeutic agents (i.e. BCNU, temozolamide, etc.). Hypermethylation of the CpG island located in the promoter region of MGMT is primarily responsible for the loss of MGMT function in many tumor types. The methylation-mediated silencing of MGMT has two consequences for cancer. First, tumors with MGMT methylation have a new mutator phenotype characterized by the generation of transition point mutations in genes involved in cancer etiology, such as the tumor suppressor p53 and the oncogene K-ras. Second, MGMT hypermethylation demonstrates the possibility of pharmacoepigenomics: methylated tumors are more sensitive to the killing effects of alkylating drugs used in chemotherapy. These recent results underscore the importance of MGMT in basic and translational cancer research.

Synthesis and preliminary biological evaluation of radiolabeled O6-benzylguanine derivatives, new potential PET imaging agents for the DNA repair protein O6-alkylguanine-DNA alkyltransferase in breast cancer

Novel radiolabeled O(6)-benzylguanine (O(6)-BG) derivatives, 2-amino-6-O-[(11)C]-[(methoxymethyl)benzyloxy]-9-methyl purines ([(11)C]p-O(6)-AMMP, 1a; [(11)C]m-O(6)-AMMP, 1b; [(11)C]o-O(6)-AMMP, 1c), 2-amino-6-O-benzyloxy-9-[(11)C]-[(methoxycarbonyl)methyl]purine ([(11)C]ABMMP, 2), and 2-amino-6-O-benzyloxy-9-[(11)C]-[(4'-methoxycarbonyl)benzyl]purine ([(11)C]ABMBP, 3), have been synthesized for evaluation as new potential positron emission tomography (PET) imaging agents for the DNA repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT) in breast cancer. The appropriate precursors for radiolabeling were obtained in two to three steps from starting material 2-amino-6-chloropurine with moderate to excellent chemical yields. Tracers were prepared by O-[(11)C]methylation of hydroxymethyl or acid precursors using [(11)C]methyl triflate. Pure target compounds were isolated by solid-phase extraction (SPE) purification procedure in 45-65% radiochemical yields (decay corrected to end of bombardment), and a synthesis time of 20-25 min. The activity of unlabeled standard samples of 1-3 was evaluated via an in vitro AGT oligonucleotide assay. Preliminary findings from biological assay indicate the synthesized analogs have similar strong inhibitory effectiveness on AGT in comparison with the parent compound O(6)-BG. The results warrant further evaluation of these radiotracers as new potential PET imaging agents for the DNA repair protein AGT in breast cancer in vivo.

Assessing cancer risks from short-term exposures in children

For the vast majority of chemicals that have cancer potency estimates on IRIS, the underlying database is deficient with respect to early-life exposures. This data gap has prevented derivation of cancer potency factors that are relevant to this time period, and so assessments may not fully address children's risks. This article provides a review of juvenile animal bioassay data in comparison to adult animal data for a broad array of carcinogens. This comparison indicates that short-term exposures in early life are likely to yield a greater tumor response than short-term exposures in adults, but similar tumor response when compared to long-term exposures in adults. This evidence is brought into a risk assessment context by proposing an approach that: (1) does not prorate children's exposures over the entire life span or mix them with exposures that occur at other ages; (2) applies the cancer slope factor from adult animal or human epidemiology studies to the children's exposure dose to calculate the cancer risk associated with the early-life period; and (3) adds the cancer risk for young children to that for older children/adults to yield a total lifetime cancer risk. The proposed approach allows for the unique exposure and pharmacokinetic factors associated with young children to be fully weighted in the cancer risk assessment. It is very similar to the approach currently used by U.S. EPA for vinyl chloride. The current analysis finds that the database of early life and adult cancer bioassays supports extension of this approach from vinyl chloride to other carcinogens of diverse mode of action. This approach should be enhanced by early-life data specific to the particular carcinogen under analysis whenever possible.

Coffee and its chemopreventive components Kahweol and Cafestol increase the activity of O6-methylguanine-DNA methyltransferase in rat liver--comparison with phase II xenobiotic metabolism

A lower rate of colon cancer was observed in consumers of coffee with a high content of the diterpenes Kahweol and Cafestol (K/C). In animal models, K/C have been found to protect against the mutagenic/carcinogenic effects of compounds such as 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), aflatoxin B1, and 7,12-dimethylbenz[a]anthracene. Thus far, such chemoprotection by K/C has been attributed to modifications of xenobiotic metabolism, e.g. enhanced detoxification by UDP-glucuronosyltransferase (UDPGT) and/or glutathione transferase (GST). In the present study, we investigated the potential of several coffee-related treatments (K/C [1:1], Cafestol-alone, Turkish coffee) to modify the expression level of the DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT) which is involved in the reversal of the precarcinogenic DNA damage O(6)-alkylguanine induced by alkylating agents. The results show that, in the male F344 rat, K/C and Cafestol increase hepatic MGMT in a dose-dependent manner up to a maximum of 2.6-fold at 0.122% K/C in the feed. Turkish coffee led to enhancements of up to 16%, the more moderate increase being associated with the lower estimated K/C intake through the beverage. In the livers of the rats receiving Turkish coffee, we also found 10-30% increases in several GST-related parameters (overall GST, GST-pi, glutathione, gamma-glutamylcysteine-synthetase) and a two-fold increase in UDPGT activity. Dose-response studies with K/C revealed that MGMT increased in parallel with three of the four GST-related parameters whereas the dose-response curves of UDPGT and of GST-pi activity displayed a steeper slope. Increased expression level of MGMT may extend the antimutagenic/anticarcinogenic potential of coffee components to protection against DNA alkylating agents.

O6-alkylguanine-DNA alkyltransferase: role in carcinogenesis and chemotherapy

The DNA in human cells is continuously undergoing damage as consequences of both endogenous processes and exposure to exogenous agents. The resulting structural changes can be repaired by a number of systems that function to preserve genome integrity. Most pathways are multicomponent, involving incision in the damaged DNA strand and resynthesis using the undamaged strand as a template. In contrast, O(6)-alkylguanine-DNA alkyltransferase is able to act as a single protein that reverses specific types of alkylation damage simply by removing the offending alkyl group, which becomes covalently attached to the protein and inactivates it. The types of damage that ATase repairs are potentially toxic, mutagenic, recombinogenic and clastogenic. They are generated by certain classes of carcinogenic and chemotherapeutic alkylating agents. There is consequently a great deal of interest in this repair system in relation to both carcinogenesis and cancer chemotherapy.

Modulatory effects of neonatal exposure to TCDD, or a mixture of PCBs, p,p'-DDT, and p-p'-DDE, on methylnitrosourea-induced mammary tumor development in the rat

The role of organochlorine (OC) exposure in the etiology of breast cancer remains controversial. Thus, our objective was to determine whether the most abundant and toxic OCs found in human milk could, when ingested during the neonatal period, modulate the development of mammary tumors in the rat. We prepared a mixture composed of p,p'-dichlorodiphenyltrichloroethane (DDT), its major metabolite, p,p'-dichlorodiphenyldichloroethene (DDE), and 19 polychlorinated biphenyls (PCB) based on their concentrations found in the milk of Canadian women. Neonate rats at 1, 5, 10, 15, and 20 days of age were gavaged with this mixture, at 10, 100, and 1,000 times the amount that a human baby would consume. An additional group received 2.5 microg 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)/kg body weight (bw) by gavage at 18 days of age, instead of the mixture. On day 21, all treatment groups, except for a control group and a 1,000-mix group, received a single intraperitoneal injection of methylnitrosourea (MNU, 30 mg/kg bw), the initiator of the carcinogenic process. The average number of rats per treatment group was 33. Rats were sacrificed when their tumors reached 1 cm in size, or at 308 days of age. We prepared mammary tumors and mammary gland whole mounts for histologic analysis. There were no significant effects when only the malignant or only the benign tumors were considered. After all benign and malignant lesions were pooled, the number of mammary tumors differed among all MNU-treated groups (p = 0.02) with more lesions developing in the MNU-1,000[times] (median = 4.5; p = 0.05) and MNU-TCDD (median = 5.5; p = 0.07) compared to the MNU-0 rats (median = 2). Compared to the MNU-0 group, the percentage of rats that developed palpable tumors (benign plus malignant) was slightly higher (p = 0.06) in the MNU-TCDD group, but not in the MNU-1,000[times] group. The percentage of palpable tumors that were malignant was higher (p = 0.02) in the MNU-100[times] group (15/16, 94%) than in the MNU-0 group (10/18, 56%). The highest dose of the mixture delayed (p = 0.03) the development of tumors, but this was not observed with the MNU-TCDD treatment. These results suggest that neonatal exposure to high doses of organochlorines could favor the development of MNU-induced mammary lesions, but also delays the development of palpable tumors in the rat.

Epigenetic lesions causing genetic lesions in human cancer: promoter hypermethylation of DNA repair genes

The existence of genetic alterations affecting genes involved in cellular proliferation and death, such as TP53 and K-ras, is one of the most common features of tumour cells. Recently, gene inactivation by promoter hypermethylation has been demonstrated. Methylation is the main epigenetic modification in mammals and abnormal methylation of the CpG islands located in the promoter region of the genes leads to transcriptional silencing. Examples include the p16INK4a, p15INK4B, p14ARF, Von Hippel-Lindau (VHL), the oestrogen and progesterone receptors, E-cadherin, death associated protein (DAP) kinase and the first tumour suppressor gene described, retinoblastoma (Rb) gene. In most cases, methylation involves loss of expression, absence of a coding mutation and restoration of transcription by the use of demethylating agents. However, is there a linkage between genetic and epigenetic alterations? Our results show one side of this puzzle demonstrating that epigenetic lesions drive genetic lesions in cancer. Four specific epigenetic lesions, promoter hypermethylation of the DNA mismatch repair gene hMLH1, the DNA alkyl-repair gene O(6)-methylguanine-DNA methyltransferase (MGMT), the detoxifier glutathione S-transferase P1 (GSTP1) and the familial breast cancer gene BRCA1 may lead to four specific genetic lesions, microsatellite instability, G to A transitions, steroid-related adducts and double-strand breaks in DNA. This is probably only the beginning of an extensive list of epigenetic events that change and make the genetic environment of the transformed cell unstable.

Coexposure to ethanol with N-nitrosodimethylamine or 4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone during lactation of rats: marked increase in O(6)-methylguanine-DNA adducts in maternal mammary gland and in suckling lung and kidney

Use of alcoholic beverages increases risk of cancer at several target sites, including the breast. Of several possible mechanisms for this effect, competitive inhibition by ethanol of hepatic clearance of nitrosamines, resulting in increased dose delivery to posthepatic tissues, gives the quantitatively most pronounced enhancement. We investigated whether this effect would pertain to the mammary gland, and to ethanol and nitrosamines delivered translactationally to sucklings. Ethanol (1.6 g/kg) was administered by gavage to nursing Sprague-Dawley rats 10 min before 5 mg/kg N-nitrosodimethylamine (NDMA) or 50 mg/kg 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK); treatment was on postnatal days 1, 7, or 14. Tissues taken 4 h later for analysis of O(6)-methylguanine in DNA were liver, blood, and mammary glands from the mothers, and liver, lung, kidney, and blood from the sucklings. Ethanol cotreatment resulted in a marked, 10-fold increase in O(6)-methylguanine adducts from NDMA in mammary gland, as well as smaller but significant increases in this tissue from NNK and in maternal blood cells from both chemicals; adducts in maternal liver decreased slightly. In the sucklings, ethanol cotreatment also lowered adducts in liver after NDMA or NNK treatment. After NDMA, adducts were also detected in suckling lung and kidney and were increased five- to 10-fold after ethanol coexposure. Adducts from either chemical, with or without ethanol, decreased markedly in all suckling tissues with development from postnatal day 1 to day 14. Thus ethanol coexposure with nitrosamines increases O(6)-methylguanine DNA adducts in mammary gland and strongly influences adduct formation in suckling tissues after translactational delivery.

The importance of being K-Ras

The ras genes give rise to a family of related proteins that have strong transforming potential. Typical in vitro studies fail to discriminate between the transforming activity of the Ras proteins. Although activating mutations in ras genes are commonly found in human disease, they are not evenly distributed between the different ras members. Instead, they are concentrated in k-ras. With the absence of evidence to suggest that k-ras DNA is more prone to mutation than h-ras DNA, this imbalance in mutational frequency suggests a special biological role for the K-Ras protein in vivo.

Role of DNA repair in carcinogen-induced ras mutation

In this contribution we discuss the gene- and cell type-specific repair of miscoding DNA alkylation products as a risk parameter in both mutation induction and malignant transformation by N-nitroso carcinogens. Upon exposure to N-nitroso compounds such as N-methyl-N-nitrosourea (MeNU) or N-ethyl-N-nitrosourea (EtNU), about a dozen different alkylation products are formed in cellular DNA. Among these are O(6)-methylguanine (O(6)-MeGua) and O(6)-ethylguanine (O(6)-EtGua), respectively, which differ only by one CH(2) group in their alkyl residue and, when unrepaired, cause G:C-->A:T transition mutations by anomalous base pairing during DNA replication. We have analyzed the global and gene-specific repair of O(6)-MeGua and O(6)-EtGua in target cell DNA, ras gene mutation frequencies, and tumor incidence, in the model of mammary carcinogenesis induced in 50-day-old female Sprague-Dawley rats by a single application of MeNU or EtNU. Both carcinogens induce histologically indistinguishable mammary adenocarcinomas at high yield. In the target mammary epithelia, O(6)-MeGua is repaired at similar slow rates in both transcriptionally active genes (Ha-ras, beta-actin), silent genes (lgE heavy chain), and in bulk DNA, by the one-step repair protein O(6)-alkylguanine-DNA alkyltransferase (MGMT; low level of expression in the target cells). The slow repair of O(6)-MeGua translates into a high frequency of mutations at the central position of Ha-ras codon 12 (GGA) in MeNU-induced tumors. O(6)-EtGua, however, is removed approximately 20 times faster than O(6)-MeGua selectively from transcribed genes via an MGMT independent, as yet uncharacterized excision mechanism. Accordingly, no Ha-ras codon 12 mutations are found in the EtNU-induced mammary tumors. Neither MeNU- nor EtNU-induced tumors exhibit mutations at codons 13 and 61 of Ha-ras or at codons 12, 13 and 61 of Ki-ras. While a moderate surplus MGMT activity of the target cells - contributed by a bacterial MGMT transgene (ada) - significantly counteracts mammary tumorigenesis in MeNU-exposed rats, this is not the case in the EtNU-treated animals. Differential repair of structurally distinct DNA lesions in transcribed or (temporarily) silent genes thus determines the probability of mutation and, together with cell type-specific and interindividual differences in DNA repair capacity, influences carcinogenic risk.

Repair of O(6)-alkylguanine by alkyltransferases

The predominant pathway for the repair of O(6)-methylguanine in DNA is via the activity of an alkyltransferase protein that transfers the methyl group to a cysteine acceptor site on the protein itself. This review article describes recent studies on this alkyltransferase. The protein repairs not only methyl groups but also 2-chloroethyl-, benzyl- and pyridyloxobutyl-adducts. It acts on double-stranded DNA by flipping the O(6)-guanine adduct out of the DNA helix and into a binding pocket. The free base, O(6)-benzylguanine, is able to bind in this pocket and react with the cysteine, rendering it an effective inactivator of mammalian alkyltransferases. The alkylated form of the protein is rapidly degraded by the ubiquitin/proteasomal system. Some tumor cells do not express alkyltransferase despite having an intact gene. Methylation of key sites in CpG-rich islands in the promoter region are involved in this silencing and a change in the nuclear localization of an enhancer binding protein may also contribute. The alkyltransferase promoter contains Sp1, GRE and AP-1 sites and is slightly inducible by glucocorticoids and protein kinase C activators. There is a complex relationship between p53 and alkyltransferase expression with p53 mediating a rise in alkyltransferase in response to ionizing radiation but having no clear effect on basal levels. DNA adducts at the O(6)-position of guanine are a major factor in the carcinogenic, mutagenic, apoptopic and clastogenic actions of methylating agents and chloroethylating agents. Studies with transgenic mice in which alkyltransferase levels are increased or decreased confirm the importance of this repair pathway in protecting against carcinogenesis. Alkyltransferase activity in tumors protects them from therapeutic agents such as temozolomide and BCNU. This resistance is abolished by O(6)-benzylguanine and this drug is currently in clinical trials to enhance cancer chemotherapy by these agents. Studies are in progress to reduce the toxicity of such therapy towards the bone marrow by gene therapy to express alkyltransferases with mutations imparting resistance to O(6)-benzylguanine at high levels in marrow stem cells. Several polymorphisms in the human alkyltransferase gene have been identified but the significance of these in terms of alkyltransferase action is currently unknown.

DNA repair: counteragent in mutagenesis and carcinogenesis-- accomplice in cancer therapy resistance

DNA-reactive carcinogens and anticancer drugs induce many structurally distinct mutagenic and cytotoxic DNA lesions. The varying capability of normal and malignant cells to recognize and repair specific DNA lesions influences both cancer risk and the relative sensitivity or resistance of cancer cells towards cytotoxic agents. Using monoclonal antibody-based immunoanalytical assays, very low amounts of defined carcinogen-DNA adducts can be quantified in bulk genomic DNA, in individual genes, and in the nuclear DNA of single cells. DNA repair kinetics can, thus, be measured in a lesion-, gene-, and cell type-specific manner, and the DNA repair profiles of malignant cells can be monitored in individual patients. Even structurally very similar DNA lesions may be repaired with strongly differing efficiency. The miscoding DNA alkylation products O(6)-methylguanine and O(6)-ethylguanine, for example, differ only by one CH(2) group. These lesions are formed in DNA upon exposure to N-methyl-N-nitrosourea or N-ethyl-N-nitrosourea, both of which induce mammary adenocarcinomas in female rats at high yield. Unrepaired O(6)-alkylguanines in DNA cause G:C-->A:T transition mutations via mispairing during DNA replication. O(6)-methylguanines are repaired at a similar slow rate in both transcriptionally active (H-ras, beta-actin) and inactive genes (IgE heavy chain; bulk DNA) of the target mammary epithelia (which express the repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT) at a very low level). In contrast, O(6)-ethylguanines are repaired approximately 20 times faster than O(6)-methylguanines in both DNA strands of the transcribed genes selectively via an AGT-independent, as yet unclarified excision mechanism. Accordingly, G:C-->A:T transitions resulting from the misreplication of an O(6)-methylated guanine at the second position of codon 12 (GGA) of H-ras represent a frequent "signature" mutation in rat mammary adenocarcinomas that develop after exposure to N-methyl-N-nitrosourea. However, this mutation is not observed when these tumors are induced by N-ethyl-N-nitrosourea, due to the fast repair of O(6)-ethylguanines in the H-ras gene. The key importance of "conventional" and "conditional" gene knockout technology for resolving the intricacies of the complex network of DNA repair pathways is briefly discussed.

PCR assay of DNA damage and repair at the gene level in brain and spleen of gamma-irradiated young and old rats

The PCR amplification of fragments of transcribed (beta-actin, p53) and nontranscribed (IgE, heavy chain) genes in brain and spleen DNA from gamma-irradiated and unirradiated 2- and 28-month-old rats was studied. The amplification levels of fragments of these genes in DNA from old rats were substantially lower than those from young rats, which suggested that these gene fragments in old-rat DNA contained lesions blocking thermostable polymerase in PCR. The beta-actin and IgE gene fragments of spleen DNA from old rats exhibited a significantly higher level of lesions inhibiting Tth polymerase compared to analogous fragments of brain DNA from the same animals. DNA from the tissues of gamma-irradiated rats showed the amount of damage inhibiting amplification to be dependent on animal age and the postirradiation time before DNA isolation. As judged from the changes in the amplification level of gene fragments, there was no preferential fast repair of lesions in the actively transcribed gene beta-actin compared to the nontranscribed gene IgE (heavy chain) in the brain and spleen of gamma-irradiated young and old rats. The amplification results suggest that equal amounts of DNA lesions were repaired in the brain of both old and young rats during the first 0.5 h of the postirradiation time (fast-repair phase), whereas in the subsequent postirradiation period over 5 h (slow-repair phase), the efficiency of damage elimination in the brain DNA of old rats was markedly lower. As for the spleen tissue, the elimination of lesions blocking Tth polymerase was much lower in old gamma-irradiated animals for both of the repair phases.

Mutagenicities of N-nitrosodimethylamine and N-nitrosodiethylamine in Drosophila and their relationship to the levels of O-alkyl adducts in DNA

N-Nitrosodialkylamines are potent carcinogens in experimental animals. Previously, we reported that the mutagenicity of N-nitrosodimethylamine (NDMA) was 10 times higher than that of N-nitrosodiethylamine (NDEA) in the Drosophila wing spot test. To find out how to explain this difference, we have measured the levels of O-alkylated bases in the DNA of exposed Drosophila larvae. Third instar larvae were fed for 3 or 6 h with NDMA or NDEA. Part of the treated larvae were grown to adult flies to score their wings for the presence of mutant spots. From the remaining larvae, DNA was isolated and digested to deoxyribonucleosides, and the digest fractionated by high-performance liquid chromatography (HPLC). The amounts of specific alkyldeoxyribonucleosides present in the fractions were quantified by a radioimmunoassay (RIA) using monoclonal antibodies. Dose-dependent O6-methylguanine, O6-ethylguanine and O4-ethylthymine formations were found to be correlated with the induction frequencies of mutant wing spots. At the same exposure dose, the values of O6-alkylde- oxyguanosine/106 deoxyguanosine were similar for NDMA and NDEA: on feeding 20 micromol/1.5 ml feeding solution, the values for NDMA were 4.0 with 3 h and 18.5 with 6 h of exposure; with 20 micromol NDEA, the corresponding values were 5.4 with 3 h and 14.6 with 6 h of exposure. The wing spot frequencies were very different; however, with NDMA, the total numbers of spots/wing were 3.5 (3 h) and 15 (6 h), and with NDEA 0.8 (3 h) and 0.9 (6 h). Similar discrepancies exist as well between the mutagenicities and the alkylation rates observed for O4-alkylthymidines. These results suggest that the difference between the mutagenic potencies of NDMA and NDEA cannot be explained by the amounts of O-alkyl adducts formed. Different mechanisms are considered by which NDMA and NDEA may produce the genetic effects observed.

Relevance of DNA repair to carcinogenesis and cancer therapy

DNA-reactive carcinogens and anticancer drugs induce many structurally distinct cytotoxic and potentially mutagenic DNA lesions. The capability of normal and malignant cells to recognize and repair different DNA lesions is an important variable influencing the risk of mutation and cancer as well as therapy resistance. Using monoclonal antibody-based immunoanalytical assays, very low amounts of defined carcinogen-DNA adducts can be quantified in bulk genomic DNA, individual genes, and in the nuclear DNA of single cells. The kinetics of DNA repair can thus be measured in a lesion-, gene-, and cell type-specific manner, and the DNA repair profiles of malignant cells can be monitored in individual patients. Even structurally very similar DNa lesions may be repaired with extremely different efficiency. The miscoding DNA alkylation products O6-methylguanine (O6-MeGua) and O6-ethylguanine (O6-EtGua), for example, differ only by one CH2 group. These lesions are formed in DNA upon exposure to N-methyl-N-nitrosourea (MeNU) or N-ethyl-N-nitrosourea (EtNU), both of which induce mammary adenocarcinomas in female rats at high yield. Unrepaired O6-alkylguanines cause transition mutations via mispairing during DNA replication. O6-MeGua is repaired at a similar slow rate in transcribed (H-ras, beta-actin) and inactive genes (IgE heavy chain; bulk DNA) of the target mammary epithelia (which express the repair protein O6-alkylguanine-DNA alkyltransferase at a very low level). O6-EtGua, however, via an alkyltransferase-independent mechanism, is excised approximately 20 times faster than O6-MeGua from the transcribed genes selectively. Correspondingly, G:C-->A:T transitions arising from unrepaired O6-MeGua at the second nucleotide of codon 12 (GGA) of the H-ras gene are frequently found in MeNU-induced mammary tumors, but are absent in their EtNU-induced counterparts.

O6-Alkylguanine-DNA alkyltransferase: influence on susceptibility to the genetic effects of alkylating agents

O6-Alkylguanine-DNA alkyltransferase (AGT) repairs DNA containing O6-alkylguanine by a suicide mechanism involving transfer of the alkyl group to its active site. In this way AGT protects cells from the mutagenic and cytotoxic effects of O6-alkylguanine-type lesions such as O6-methylguanine (O6-meG), an observation which has during recent years been confirmed by studies in transgenic animals either over-expressing or completely lacking this activity. While the levels of expression of AGT have been shown to affect strongly the repair of O6-meG after high doses of methylating agents inducing complete and prolonged depletion of the cellular AGT pool, other data suggest that within smaller variations of AGT levels (such as the interindividual variations observed in man or as observed after low or moderate exposures to alkylating agents) the dependence of O6-meG repair is limited. This phenomenon may reflect the intracellular distribution of the repair protein and must be taken into account when assessing the role of AGT in determining susceptibility to alkylating agents of environmental or clinical significance.

Links between chromatin structure, DNA repair and chromosome fragility

This paper is a brief overview of the studies we have recently conducted to unravel how chromatin structure and DNA repair modulate the fragility of diverse chromosomes and chromosomal regions. We have employed a combination of molecular cytogenetic techniques, including interphase and metaphase multicolour FISH, reverse FISH with CpG-rich probes or repaired DNA fractions, and several combinations of FISH and immunocytogenetics with antibodies against acetylated histones. The targets of our investigation were human constitutive and facultative heterochromatin, chromosomes with high and low gene density and human and hamster fragile sites. The role of DNA repair was investigated by using DNA repair deficient mutants and DNA repair inhibitors. We found that intragenomic heterogeneity in DNA repair and chromatin structure may explain a substantial part of the differential fragility of diverse chromosomes and chromosomal regions.