Chromatin structure, transcriptional activity and DNA repair efficiency affect the outcome of chemotherapy in multiple myeloma

Background:

Melphalan is one of the most active chemotherapeutic agents in the treatment of multiple myeloma (MM). However, the mechanism underlying differential patient responses to melphalan therapy is unknown.

Methods:

Chromatin structure, transcriptional activity and DNA damage response signals were examined following ex vivo treatment with melphalan of both malignant bone marrow plasma cells (BMPCs) and peripheral blood mononuclear cells (PBMCs) of MM patients, responders (n=57) or non-responders (n=28) to melphalan therapy. PBMCs from healthy controls (n=25) were also included in the study.

Results:

In both BMPCs and PBMCs, the local chromatin looseness, transcriptional activity and repair efficiency of the transcribed strand (TS) were significantly higher in non-responders than in responders and lowest in healthy controls (all P<0.05). Moreover, we found that melphalan-induced apoptosis inversely correlated with the repair efficiency of the TS, with the duration of the inhibition of mRNA synthesis, phosphorylation of p53 at serine 15 and apoptosis rates being higher in responders than in non-responders (all P<0.001).

Conclusions:

Our findings provide a mechanistic basis for the link between DNA repair efficiency and response to melphalan therapy. Interestingly, the observation of these phenomena in PBMCs provides a novel approach for the prediction of response to anti-myeloma therapy.

Prediagnostic transcriptomic markers of Chronic lymphocytic leukemia reveal perturbations 10 years before diagnosis

BACKGROUND

B-cell lymphomas are a diverse group of hematological neoplasms with differential etiology and clinical trajectories. Increased insights in the etiology and the discovery of prediagnostic markers have the potential to improve the clinical course of these neoplasms.

METHODS

We investigated in a prospective study global gene expression in peripheral blood mononuclear cells of 263 incident B-cell lymphoma cases, diagnosed between 1 and 17 years after blood sample collection, and 439 controls, nested within two European cohorts.

RESULTS

Our analyses identified only transcriptomic markers for specific lymphoma subtypes; few markers of multiple myeloma (N = 3), and 745 differentially expressed genes in relation to future risk of chronic lymphocytic leukemia (CLL). The strongest of these associations were consistently found in both cohorts and were related to (B-) cell signaling networks and immune system regulation pathways. CLL markers exhibited very high predictive abilities of disease onset even in cases diagnosed more than 10 years after blood collection.

CONCLUSIONS

This is the first investigation on blood cell global gene expression and future risk of B-cell lymphomas. We mainly identified genes in relation to future risk of CLL that are involved in biological pathways, which appear to be mechanistically involved in CLL pathogenesis. Many but not all of the top hits we identified have been reported previously in studies based on tumor tissues, therefore suggesting that a mixture of preclinical and early disease markers can be detected several years before CLL clinical diagnosis.

Investigation of cell death induced by N-methyl-N-nitrosourea in cell lines of human origin and implication of RNA binding protein alterations

Methylating agents, a widely used class of anticancer drugs, induce DNA methylation adducts, the most biologically significant being O(6)-methylguanine. The efficacy of these drugs depends on the interplay of three DNA repair systems: base excision repair (BER), methyl-directed mismatch repair (MMR) and direct damage reversal by O(6)-methylguanine-DNA methyltransferase (MGMT). An MGMT-inducible, MMR- and BER-proficient HeLa cell line was treated with different concentrations of N-methyl-N-nitrosourea (MNU), a model S(N)1 methylating agent, analogous to widely used methylating cancer chemotherapeutic drugs, under different expression levels of the repair enzyme (MGMT). MNU induced MGMT-dependent apoptotic cell death. In this particular cellular context, the induction of apoptosis was accompanied by modifications of the RNA binding protein poly(A)polymerase and significant down-regulation of the heterogeneous nuclear ribonucleoprotein (hnRNP) C1/C2. These results implicate alterations of the above mentioned RNA binding proteins in S(N)1 methylating agent-induced cell death and apoptosis, providing a possible perspective regarding their use as biomarkers of tumor resistance/sensitivity to chemotherapy.

Benzo[a]pyrene-enhanced mutagenesis by man-made mineral fibres in the lung of lamda-lacI transgenic rats

In an attempt to examine the interaction of man-made mineral fibres with benzo[a]pyrene (B[a]P), homozygous X-lacI transgenic F344 rats were intratracheally treated with rock (stone) wool RWI and glass wool MMVF 10 fibres together with B[a]P. To analyze the induction of gene mutations by fibres and B[a]P in lung, single doses of 1 and 2 mg fibres/animal or multiple doses of 2 mg fibres/animal were administered weekly on 4 consecutive weeks (total dose 8 mg/animal). B[a]P (10 mg/animal) was administered either simultaneously with fibres (for single dose treatment with fibres) or together with the last fiber treatment (for multiple dose treatment with fibres). Animals were scarified 4 weeks after the last treatment. Benzo[a]pyrene administered simultaneously with RW1 fibres exhibited a strong synergistic effect on mutagenicity, the observed mutant frequency (MF) being more than three-fold higher than the net sum of the MF induced after separate administration of both agents. Our data suggest that DNA adducts induced by simultaneous B[a]P and fiber treatment lead to a strong increase in mutatant frequencies.

Mutagenesis by man-made mineral fibres in the lung of rats

The potential of two asbestos substitute mineral fibres--rock (stone) wool RW1 and glass wool MMVF10--to induce gene mutations, DNA strand breaks, inflammation and oxidative stress has been studied in rats. Male homozygous lamda-lacI transgenic F344 rats were intratracheally instilled with single doses of 1 and 2 mg/animal of fibres or with multiple doses of 2 mg/animal administered weekly on four consecutive weeks (8 mg in total). Exposure to RW1 fibres for 16 weeks significantly increased mutant frequency (MF) in the lung in a dose-dependent manner, while MMVF10 fibres did not exhibit any increase of MF at any dose. RW1 fibres gave a significant increase of MF at a dose of 1 mg. Four weeks after instillation, neither the single nor the multiple doses significantly increased MF for both fibre types. To investigate mechanisms for induction of mutations, other genotoxicity markers and parameters of inflammatory and oxidative damage were determined in relation to MF. A weak correlation of mutagenicity data with other genotoxicity parameters studied was observed. DNA strand breaks as measured by comet assay were increased in alveolar macrophages and lung epithelial cells of RW1 and MMVF10 treated rats. RWl fibres caused more extensive lung inflammation as measured by release of neutrophils into broncho-alveolar lavage fluid than MMVF10 fibres. The effects were observed 16 weeks post-exposure, indicating a persistence of the pathogenic process during the exposure period. Only minor differences in the extent of inflammatory processes were observed between the doses of 2 mg and 4 x 2 mg, suggesting that any threshold for inflammation lies below the dose of 2 mg. With the exception of the highest dose of MMVF10 fibres after 16 weeks of exposure, no significant increase of oxidative damage as measured by levels of malondialdehyde in lung tissue was observed. MMVF10 fibres caused weaker inflammation in the lung of rats and did not exhibit any mutagenic effect. We conclude that a weak but chronic inflammation (more likely than acute inflammation or direct oxidative damage) in the lung tissue of fibre treated rats characterized by moderate influx of inflammatory cells into BAL is probably responsible for the observed mutagenic effect of RW1 fibres.

Mutagenesis by asbestos in the lung of lambda-lacI transgenic rats

In order to get more insight into the mechanism of asbestos-related lung cancer, the mutagenic potential of asbestos was examined in vivo in rat lung. Groups of five transgenic lambda-lacI (Big Blue) rats were intratracheally instilled with single doses of 1 or 2mg, or with four weekly doses of 2mg, per animal of the amosite asbestos. Sixteen weeks after instillation, the mutation frequency was found to be increased in lung DNA by 2-fold at doses of 2 mg (P = 0.035) and of 4 x 2 mg (P = 0.007) amosite. No significant changes were observed after 4 weeks of exposure. In separate experiments, wild-type F344 rats were treated by the same regimen as described above and markers of inflammation, genotoxicity, cell proliferation and lung tissue damage were analysed. Our results indicate a weak but persistent inflammation and cell proliferation which possibly plays a major role in the observed mutagenic effect.

Benzo[a]pyrene-enhanced mutagenesis by asbestos in the lung of lambda-lacI transgenic rats

To study the suspected mechanism of the interaction between tobacco smoking and asbestos exposure in the modulation of cancer risk, the mutagenic potential of asbestos in combination with the tobacco smoke carcinogen benzo[a]pyrene (B[a]P) was examined in vivo in the rat lung. B[a]P was administered intratracheally in one set of experiments, or by two daily intraperitoneal injections in another set of experiments, to lambdalacI transgenic rats, together with 1, 2 or 4 x 2 mg amosite in one experiment. In the first experiment, the combined action of amosite and B[a]P caused a synergistic (superadditive) increase of mutation frequency in the lung, as compared to groups treated only with asbestos or B[a]P. In the second experiment, i.p. treatment with B[a]P did not significantly alter the mutation frequency induced by amosite, neither after 4 nor after 16 weeks of exposure. The B[a]P-DNA adduct levels were unaffected by amosite co-treatment in both experiments. We assume that the synergistic increase of mutation frequency after intratracheal treatment was due to the mitogenic activities of B[a]P and of amosite. In conclusion, our findings indicate that a weak and delayed mutagenic effect of amosite in rat lung observed in another study was strongly enhanced by the concomitant action of B[a]P. The striking enhancement effect of B[a]P may provide a basis for understanding the suspected synergism of smoking on asbestos carcinogenesis.

Impact of phase I or phase II enzyme polymorphisms on lymphocyte DNA adducts in subjects exposed to urban air pollution and environmental tobacco smoke

Little is known about the impact of genetic variation on the genetic damage induced by urban air pollution or environmental tobacco smoke (ETS) in exposed populations. The levels of bulky DNA adducts ( 32P-postlabelling, nuclease P1 enrichment) and chromosomal aberrations were measured in lymphocytes of 194 non-smoking students living in the city of Athens, and the rural region of Halkida, Greece. In these individuals personal exposure to PAH was also measured. Furthermore, genetic polymorphisms were examined in cytochromes P450 1A1, 1B1, in the GSTM1, GSTP1 and GSTT1 as well as in microsomal epoxy hydrolase (EPHX) genes. Subjects with the CYP1*2A mutant genotype also suffering significant ETS exposure tended to exhibit higher adduct levels and % aberrant cells. In contrast, CYP1B1 polymorphisms seemed to have an impact on the DNA adduct levels only among individual with negligible ETS exposure. Subjects carrying both the CYP1*2A mutant genotype and the GSTM1 null genotype tended to have higher DNA adduct levels. A similar effect was also observed with the combined CYP1A1*2A/GSTP1 (Ile/Val) and the CYP1A1*2A/mEH "slow" polymorphisms. In both cases, the effect was more pronounced among subjects with higher levels of ETS exposure. Stepwise restriction of the observations to subjects characterised by (a) GSTP1 mutant, (b) GSTM1 null, (c) mEH "slow" (His139His) genotypes and (d) ETS exposure resulted in a significant trend of increasing DNA adduct levels only among individuals with at least one CYP1A1*2A mutated allele, illustrating the importance and complexity of gene-exposure and gene-gene interactions in determining the level of genotoxic damage on an individual levels.

Some lung cellular parameters reflecting inflammation after combined inhalation of amosite dust with cigarette smoke by rats

Cellular changes were followed in lung cell suspensions after 175 day inhalation by rats of concentrations 30 mg/m3 or 60 mg/m3 of amosite asbestos every second day combined with daily exposure to cigarette smoke at 30 mg of total particulate matter (TPM)/m3 air. Concomitantly, lung inflammation was assessed by changes in the bronchoalveolar lavage fluid (BALF). A dose-dependent rise in the BALF inflammatory parameters was found. The rise of the proportion of binucleate (BNC) and multinucleate cells (MNC) in lung cell suspensions was also dose-dependent. It is concluded that, in the experimental assessment of effects of fibrogenic dusts, the number of BNC and of MNC in lung cell suspensions may serve as a useful semiquantitative biomarker of the inflammation.

Evaluation of bronchoalveolar lavage fluid cytotoxic parameters after inhalation exposure to amosite and wollastonite fibrous dusts combined with cigarette smoke

The aim of this work was to compare the influence of amosite-asbestos and wollastonite fibrous dusts combined with cigarette smoke on chosen cytotoxic parameters of bronchoalveolar lavage fluid (BALF) in rats. Fisher 344 rats inhaled wollastonite or amosite fibrous dusts (60 or 30 mg x m(-3) air) one hour every two days combined with daily breathing of diluted mainstream tobacco smoke (30 mg of TPM x m(-3) air). The experiment lasted 6 months. After sacrifying the animals bronchoalveolar lavage (BAL) was performed and the viability and phagocytic activity of alveolar macrophages (AM), lactate dehydrogenase (LDH) and alkaline phosphatase activity (in the cell-free BALF), acid phosphatase (ACP) and cathepsin D activity (in cell-free BALF and BAL cell suspension) were examined. Exposure to amosite without tobacco smoke significantly decreased the viability of AM and increased the cathepsin D activity in BAL cells. Exposure to wollastonite significantly increased only the cathepsin D activity in BAL cells. Smoking significantly depressed the phagocytic activity of AM and amplified the amosite-induced increase of lysosomal enzyme activities--especially the activity of cathepsin D in BAL cells.

Monitoring for DNA damage of humans occupationally exposed to methyl bromide

BACKGROUND

Methyl bromide (MeBr) is a methylating agent, weak mutagen and possible animal carcinogen. A molecular epidemiological study to examine human exposure to, and consequent DNA damage by MeBr was conducted in an area where this agent is used extensively for soil sterilisation in greenhouses.

MATERIALS AND METHODS

During the first part of the study, blood samples were collected from 21 persons within 24 hours after use of MeBr for greenhouse sterilisation, as well as from 19 non-exposed subjects. Personal air sampling was also carried out, indicating mean air concentrations for different subjects in the range 11-78 mg/m3. In the second part of the study, an attempt was made to examine professional applicators of MeBr who suffered particularly high exposures (mean exposures, based on personal monitoring 23-165 mg/m3). The levels of N7-methylguanine and O6-methylguanine, two DNA adducts known to be induced by MeBr, were assessed in blood leukocyte DNA.

RESULTS

Concerning the first part, two subjects (one exposed and one control) were found to be positive for N7-methylguanine, while none of the blood samples analysed had detectable levels of O6-methylguanine. Among 6 such persons examined during the second part, 2 were found positive for N7-methylguanine while none was positive for O6-methylguanine.

CONCLUSION

Within the detection power of this limited study, no significant evidence of induction of DNA damage in blood leukocyte DNA by MeBr was found.

Biomarkers of genotoxicity of urban air pollution. Overview and descriptive data from a molecular epidemiology study on populations exposed to moderate-to-low levels of polycyclic aromatic hydrocarbons: the AULIS project

Epidemiologic studies indicate that prolonged exposure to high pollution levels is associated with increased risk of cancer, especially lung cancer. However, under conditions of moderate or low air pollution, epidemiologic evidence does not permit reliable conclusions. Biomarker-based population studies may serve as complementary tools providing a better understanding of the relative contribution of ambient atmospheric pollution to the overall genotoxic burden suffered by city dwellers. However, past efforts to apply biomarkers to studies of low levels exposure to urban air pollution have given inconclusive results, partly because of the absence of adequate data on personal exposure, covering a time-window which is appropriate for the biomarkers being examined, as well as a battery of biomarkers reflecting different stages of the carcinogenic process. In the present paper, the potential of biomarker-based population studies to aid the assessment of the genotoxic and carcinogenic effects of urban air pollution is reviewed by reference to the achievements and limitations of earlier reported studies. The design and methodology adopted in a recently completed large-scale population study, carried out in the context of the European Union Environment and Climate Programme, known by the short name of AULIS project, is discussed and descriptive statistics of the main findings of the project are presented. These findings indicate that for cohorts suffering moderate-to-low exposures to airborne particulate-bound polycyclic aromatic hydrocarbons (PAHs), no simple correlation with biomarkers of genotoxicity existed and suggest that additional factors made a significant contribution to the overall genotoxic burden.

Biomarkers of genotoxicity of air pollution (the AULIS project): bulky DNA adducts in subjects with moderate to low exposures to airborne polycyclic aromatic hydrocarbons and their relationship to environmental tobacco smoke and other parameters

The levels of bulky DNA adducts were measured by (32)P-post-labelling in lymphocytes of 194 non-smoking students living in the city of Athens and the region of Halkida, Greece, once in the winter and again in the following summer. Personal exposures to particulate-bound polycyclic aromatic hydrocarbons (PAH) were significantly higher in Athens subjects during both seasons. There was hardly any diagonal radioactive zone in the pattern of DNA adducts observed. Highest adduct levels were observed in a sub-group of subjects living in or near the Halkida Institute campus, which was located in rural surroundings with a minimal burden of urban air pollution. The remaining Halkida subjects had intermediate levels, while Athens subjects showed the lowest levels. This trend, which was observed over both monitoring seasons, consistently paralleled the variation in three markers of exposure to environmental tobacco smoke (ETS), namely (i) declared times of exposure to ETS during the 24 h prior to blood donation, (ii) plasma cotinine levels and (iii) chrysene/benzo[g,h,i]perylene ratios in the profile of personal PAH exposure. Furthermore, among the Halkida campus area subjects (but not the remaining subjects) positive correlations were observed between DNA adducts and (i) measured personal exposures to chrysene or benzo[a]pyrene, (ii) time of declared ETS exposure and (iii) chrysene/benzo[g,h,i] perylene ratios. These correlations suggest that, for a group suffering minimal exposure to urban air pollution, exposure to ETS was a significant determinant of the observed DNA damage. Gender had a consistent and significant effect on adduct levels (males having higher levels), which remained significant even after multiple regression analysis. Habitual consumption of roasted meat was significantly associated with an enhancement of adduct levels and the effect was strengthened when only individuals unexposed to ETS were taken into consideration. No significant effects were observed for other dietary parameters or factors reflecting exposure to air pollution.

Personal exposures to PM(2.5) and polycyclic aromatic hydrocarbons and their relationship to environmental tobacco smoke at two locations in Greece

In the context of a large-scale molecular epidemiology study of biomarkers of genotoxicity of air pollution, 24-h mean personal exposures to airborne PM(2.5) (particulate matter <2.5 microm) and associated polycyclic aromatic hydrocarbon (PAHs) were measured in 194 non-smoking technical institute students living in the city of Athens, Greece (an area with moderately high levels of air pollution) and the nearby small town of Halkida anticipated to have lower pollution levels. Extensive information relevant to the assessment of long-term and recent exposure to PAH was obtained from questionnaires as well as a time-location-activity diary (TLAD) which was kept by all subjects during a 4-day observation period. During the last 24 h of this period, subjects underwent personal exposure monitoring for PM(2.5) and PAH, while a sample of blood was donated at the end of this period. All subjects were monitored in this way twice; once during a winter season (October-February) and once during the following summer season (June-September). Nine subjects with plasma cotinine levels above 20 ng/ml were considered as unreported smokers and excluded from the study. Winter PM(2.5) exposures were lower in Athens (geometric mean 39.7 microg/m(3)) than Halkida (geometric mean 56.2 microg/m(3)) (P<0.001), while there was no significant location difference during the summer (Athens: geometric mean 32.3 microg/m(3), Halkida: geometric mean 32.9 microg/m(3); P=0.79). On the other hand, PAH exposures (sum of the eight carcinogenic PAHs) were significantly higher in Athens than in Halkida during the winter (Athens: geometric mean 8.26 ng/m(3), Halkida: geometric mean 5.80 ng/m(3); P<0.001) as well as during the summer (Athens: geometric mean 4.44 ng/m(3), Halkida: geometric mean 1.48 ng/m(3); P<0.001). There was a significant difference in the profile of the PAH exposures at the two locations, the proportion of lighter PAH (benzo[a]anthracene, chrysene [CHRYS], benzo[k]fluoranthene, and benzo[b]fluoranthene) being higher, and that of heavier PAH (benzo[ghi]perylene [BPer] and indeno[1,2,3,cd]pyrene) lower, in Halkida than in Athens, regardless of season. This difference appeared to be related to individual exposure to environmental tobacco smoke (ETS), as indicated by (a) the correlation at the individual level between the CHRYS/BPer ratio and declared time of recent exposure to ETS as well as plasma cotinine levels, especially during the winter; (b) the parallel variation of the mean levels of all three markers (declared ETS exposure, cotinine levels, CHRYS/BPer ratio) among three subgroups of subjects (Athens subjects who had lowest levels of all three markers; Halkida subjects other than those living in the institute campus area; and Halkida subjects living in the institute campus area who had the highest levels of all three markers). This demonstrates that ETS can have a distinctive effect on the PAH exposure profile of subjects exposed to relatively low levels of urban air pollution.

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.

Ubiquitous presence of O6-methylguanine in human peripheral and cord blood DNA

O6-Methylguanine (O6-meG) is a powerful premutagenic lesion that can arise from exposure to methylating agents. Although it has been reported to occur in human DNA, no systematic epidemiological analysis of its occurrence in populations suffering general environmental exposure is available. We report here results from a study of the presence of O6-meG in maternal and cord blood leukocyte DNA of women not knowingly exposed to methylating agents. Using a modification of an already existing method capable of detecting the lesion at levels as low as 16 nmol/molG, the adduct was detected in 31 of 36 maternal and 30 of 36 cord samples, at levels ranging up to 192 nmol/molG. Adduct levels in maternal blood DNA were significantly higher than those in cord blood DNA (P < 0.05), and there was a strong correlation between adduct levels in the two tissues (P < 0.001). In bivariate analysis, no significant association of adduct levels in either tissue and residence air pollution, active and passive smoking status, or eating habits was found. However, intake of fruits/vegetables and of vitamin supplements showed nonstatistically significant trends toward being associated with lower adduct levels in both maternal and cord blood DNA. The same trend was observed after multivariate analysis where all the above variables were controlled for. These findings indicate that premutagenic methylation DNA damage is commonplace in individuals not known to have suffered excessive exposure to environmental methylating agents or their precursors and are compatible with an endogenous origin of this damage, possibly associated with endogenous nitrosation processes.

Toxicity, mutation frequency and mutation spectrum induced by dacarbazine in CHO cells expressing different levels of O(6)-methylguanine-DNA methyltransferase

The toxicity and mutagenicity (including the mutation spectrum induced) of dacarbazine, a methylating cytostatic drug, was examined in CHO cells expressing different levels of the repair enzyme O(6)-methylguanine-DNA methyltransferase (MGMT). Expression of low or high levels of a transfected human MGMT gene under the control of the metallothionein promoter protected the cells against dacarbazine-induced toxicity and mutagenesis. In the absence of MGMT expression, the mutation spectrum in the HPRT locus was dominated by GC-->AT transitions (mostly found at 5'Pu-G sequences), while there were also a few AT-->GC transitions. Expression MGMT was associated with a substantial decrease of GC-->AT mutations, suggesting that these mutations arose primarily via O(6)-methylguanine. These data illustrate the important role of the latter lesion in the drug's mutagenic and cytotoxic activity.

Induction of somatic mutations but not methylated DNA adducts in lambdalacZ transgenic mice by dichlorvos

In order to examine the in vivo genotoxic activity of dichlorvos, lambdalacZ transgenic mice (Muta Mouse) were treated i.p. with single (4.4 or 11 mg/kg) or multiple (5 x 11 mg/kg) doses of this agent and sacrificed 4 h or 14 days post-treatment for DNA adduct measurement or mutant frequency analysis, respectively. Neither methylated DNA adducts nor an increase in mutant frequency were detected in the bone marrow, white blood cells, liver, spleen, lung, brain and sperm cells after the single doses. However, following multiple dosing a statistically significant 3-fold increase in mutant frequency was observed in the liver, while a non-statistically significant increase was observed in the bone marrow. In contrast, dimethylsulphate, a model methylating agent, gave rise to detectable DNA adducts but no increase in mutant frequency following i.p. administration of single (30 mg/kg) or multiple (10 x 6 mg/kg) doses.

Molecular epidemiological approaches to the study of the genotoxic effects of urban air pollution

Direct epidemiological observations suggest that exposure to high levels of urban air pollution may result in increased risk of lung cancer, sufficient to account for a few (approximately 1-3) percent of total lung cancer incidence. Extrapolation from occupational exposure and risk data suggests that among potential carcinogens present in polluted urban air, polycyclic aromatic hydrocarbons (PAHs) may make a major contribution to air pollution-associated lung cancer risks. The use of biomarkers of genotoxocity in large-scale population studies may help to reduce the uncertainty involved in the assessment of such risks, especially those associated with relatively low pollution levels such as nowadays found in many Western cities. Increases in biomarkers of exposure to urban air PAHs as well as biomarkers of early effects have been detected in situations of relatively high levels of air pollution (e. g., ambient PAH concentrations of the order of a few tens of micrograms per cubic meter). Evidence has also been found about the modulation genetic damage accumulation in different individuals by polymorphisms in genes involved in the activation or detoxification of PAHs, especially of polymorphisms GSTM1 and CYP1A1 genes. However, the inconsistencies in the currently reported effects of genetic polymorphisms suggest that additional factors may also be important in the modulation of individual susceptibility to the accumulation of PAH-derived genetic damage. Biomarkers studies in populations exposed to relatively low ambient PAH concentrations (below 20 microg/m(3)) have not demonstrated clear dose-related effects (e.g., on DNA adduct levels), possibly because of the existence of multiple sources and routes of human exposure to PAHs in addition to inhalation of urban air (including, for example, home heating, environmental tobacco smoke and diet), and the consequent difficulty of adequately and specifically assessing atmospheric air-related exposure. This makes it imperative that molecular epidemiology studies be designed in such a way as to allow adequate assessment of exposure to urban air PAHs at the individual level and over short-, medium- and long-term time periods which correspond to the expression times of different biomarkers.

Methyl bromide causes DNA methylation in rats and mice but fails to induce somatic mutations in lambda lacZ transgenic mice

Following single or multiple oral treatments of rats or lambda lacZ transgenic mice with methyl bromide, methylated DNA adducts (N7- and/or O6-methylguanine) were found at comparable levels in various tissues, including among others the glandular stomach, the forestomach and the liver. Multiple rat treatment resulted in substantial decreases in the repair enzyme O6-alkylguanine-DNA alkyltransferase which were probably due in part to direct interaction of the enzyme with methyl bromide. However, no induction of mutagenesis in the lacZ transgene could be detected in any tissue 14 days after single treatments of up to 50 mg/kg or after multiple treatments of as many as 10 daily treatments of 25 mg/kg MeBr.

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.

DNA adducts in humans after exposure to methylating agents

Human exposure to methylating agents appears to be widespread, as indicated by the frequent occurrence of methylated DNA adducts in human DNA. The high incidence of methylated DNA adducts even in humans thought not to have suffered extensive exposure to environmental methylating agents implies that chemicals of endogenous origin, probably N-nitroso compounds such as the strongly carcinogenic N-nitrosodimethylamine (NDMA), may be primarily responsible for their formation and raises the question of the carcinogenic risks associated with such exposure. In addition to accumulation of DNA damage, other factors (such as induced cell proliferation) appear to be important in determining the probability of induction of mutation or cancer by NDMA, implying that high to low dose risk extrapolations should not be based on the assumption of dose- or even adduct-linearity. Comparative studies of the accumulation and repair of methylated adducts in humans and animals treated with methylating cytostatic drugs do not reveal significant species differences. Based on this and the dosimetry of adduct accumulation in rats chronically exposed to very low doses of NDMA, it is suggested that the exposure needed to account for the levels of adducts found in human DNA may be of the order of hundreds of micrograms NDMA (or equivalent) per day, a level of exposure which may well represent a significant carcinogenic hazard for man.

DNA adducts, mutant frequencies and mutation spectra in lambda lacZ transgenic mice treated with N-nitrosodimethylamine

Groups of lambda lacZ transgenic mice were treated i.p. with N-nitrosodimethylamine (NDMA) as single doses of 5 mg/kg or 10 mg/kg or as 10 daily doses of 1 mg/kg and changes in DNA N7- or O6-methylguanine or the repair enzyme O6-alkylguanine-DNA alkyltransferase (AGT) were followed for up to 14 days in various tissues. Adduct induction in the liver exceeded by at least one order of magnitude than observed in the next nearest target tissue (lung), and was approximately linearly related to dose, except for O6-methylguanine after the first dose of 1 mg/kg which was lower than expected. Substantial induction of lambda lacZ mutagenesis was observed only in the liver, where the mutant frequency was already maximal within 7 days after 5 mg/kg NDMA and remained unchanged thereafter up to 49 days. Small but marginally significant increases in mutant frequency were consistently observed in the spleen after all three modes of treatment. A lack of proportionality between mutation induction and the administered dose or the corresponding adduct levels was observed, probably reflecting the importance of toxicity-related cell proliferation caused by NDMA at higher doses. Twenty eight days after a dose of 10 mg/kg (causing a 3.6-fold increase in mutant frequency), NDMA was found to increase the frequency of GC-->AT mutations (with a concomitant shift of their preferential location from CpG sites to GpG sites), which made up approximately 60% of the induced mutations. Surprisingly, NDMA also caused a significant increase in deletions of a few (up to 11) base-pairs (22%).

DNA damage and mutagenesis induced by procarbazine in lambda lacZ transgenic mice: evidence that bone marrow mutations do not arise primarily through miscoding by O6-methylguanine

The DNA damaging and mutagenic activities of procarbazine, a methylating drug employed in cancer chemotherapy and suspected of causing therapy-related leukaemia, were investigated in the liver and bone marrow of lambda lacZ transgenic mice (MutaMouse). The drug was administered using two different protocols, a 'high-dose' one involving 5 daily doses of 200 mg/kg, expected to cause depletion of the repair enzyme O6-alkylguanine-DNA alkyltransferase (AGT) and thus favour the selective accumulation of the premutagenic lesion O6-methylguanine (O6-meG) relative to other adducts, and a 'low-dose' one involving 10 daily doses of 20 mg/kg procarbazine. Substantial accumulation of O6-meG was observed in both tissues examined 6 h after the end of the 'high-dose' treatment, with the liver accumulating somewhat higher levels than the bone marrow (28.0 +/- 1.8 fmol/microg DNA and 18.5 +/- 1.1 fmol/microg DNA respectively). However, significant increases in mutant frequency 10 days after the end of treatment were observed only in the bone marrow, reaching a 16-fold increase over background following the 5 x 200 mg/kg treatment. Sequence analysis of the mutations induced after this treatment revealed a mixed spectrum, in which G:C-->A:T transitions (characteristic of O6-meG miscoding) were only a secondary feature: Among 20 mutants analysed, only six such mutations were found, including three at CpG sites, which might have arisen from deamination of 5-methylcytosine. The other mutations observed included 1 A:T-->G:C transition, five transversions (one G:C-->T:A, one double G:C-->C:G, two A:T-->T:A, one A:T-->C:G), five deletions and three insertions. The mechanistic and clinical significance of these findings is discussed.

A phase II study evaluating the effect of tamoxifen on DNA repair in melanoma patients treated with dacarbazine

The addition of tamoxifen to dacarbazine containing chemotherapy regimens used in the treatment of melanoma, has been shown to increase response rates, but the mechanism of any interaction is uncertain. The object of this study was to determine whether the addition of tamoxifen to dacarbazine, would modify DNA repair in-vivo and cause an increase in O6-meG adducts in peripheral blood leucocytes. This would provide some insight into the nature of the interaction between these two drugs. Twenty three patients with metastatic malignant melanoma received dacarbazine (DTIC) 1 g/m2 every three weeks for a maximum of six cycles. Tamoxifen 20 mg daily, was started after the first cycle of chemotherapy and then taken continuously during the treatment. Adduct levels after the second cycle of treatment were significantly higher than those after the first cycle (p = 0.0001). A similar rise however, was also produced when a cohort of patients were given dacarbazine without tamoxifen during the second cycle of treatment. This study did not show an additional increase of O6-meG adducts when tamoxifen was administered and therefore this mechanism does not support a postulated interaction between tamoxifen and dacarbazine. This is in agreement with the recent randomised study which did not show any significant increase in response rate with the addition of tamoxifen.

DNA adducts and the mechanism of carcinogenesis and cytotoxicity of methylating agents of environmental and clinical significance

DNA adducts are covalent complexes formed between genotoxic carcinogens and DNA bases, and constitute a critical early intermediate on the pathway of chemical carcinogenesis. Their accumulation in different tissues reflects the amount of activated carcinogen reaching DNA, and can therefore serve as an index of the biologically relevant dose reaching the target tissues or cells. Methylating agents are of interest in view of their occurrence in the environment and their use as cytotoxic drugs in cancer chemotherapy. Current evidence indicates that O6-methylguanine plays a particularly important role in the mutagenic, carcinogenic, and cytotoxic activities of methylating agents. O6-Methylguanine is repaired efficiently by the enzyme O6-alkylguanine-DNA alkyltransferase (AGT). Lack of this enzyme results in excessive accumulation of O6-methylguanine and recent evidence suggests that significant quantitative effects on adduct accumulation may be linked to conditions of very low AGT levels. This would be important from the point of view of clinical practice, since modulation of AGT is under investigation as a means of enhancing the therapeutic efficacy of clinical agents acting via the production of O6-methylguanine and related adducts, such as, for example, procarbazine, dacarbazine, and some nitrosoureas. The measurement of O6-methylguanine in human DNA has been employed as a tool to investigate the role of environmental methylating agents in human carcinogenesis. While the nature and origin of the methylating agents responsible for these adducts is currently unknown, recent studies in patas monkeys have shown that N-nitrosodimethylamine, a methylating carcinogen to which human exposure is well documented, is capable of efficiently generating O6-methylguanine in most tissues, including fetal tissues. Furthermore, it has been found that this damage is substantially enhanced by the coadministration of ethyl alcohol which acts by inhibiting the liver first-pass metabolism of the carcinogen, an observation which supports the hypothesis that alcohol consumption may act as a risk factor in human carcinogenesis by augmenting the action of nitrosamines.

Biomonitoring human exposure to environmental carcinogenic chemicals

A coordinated study was carried out on the development, evaluation and application of biomonitoring procedures for populations exposed to environmental genotoxic pollutants. The procedures used involved both direct measurement of DNA or protein damage (adducts) and assessment of second biological effects (mutation and cytogenetic damage). Adduct detection at the level of DNA or protein (haemoglobin) was carried out by 32P-postlabelling, immunochemical, HPLC or mass spectrometric methods. Urinary excretion products resulting from DNA damage were also estimated (immunochemical assay, mass spectrometry). The measurement of adducts was focused on those from genotoxicants that result from petrochemical combustion or processing, e.g. low-molecular-weight alkylating agents, PAHs and compounds that cause oxidative DNA damage. Cytogenetic analysis of lymphocytes was undertaken (micronuclei, chromosome aberrations and sister chromatid exchanges) and mutation frequency was estimated at a number of loci including the hprt gene and genes involving in cancer development. Blood and urine samples from individuals exposed to urban pollution were collected. Populations exposed through occupational or medical sources to larger amounts of some of the genotoxic compounds present in the environmental samples were used as positive controls for the environmentally exposed population. Samples from rural areas were used as negative controls. The project has led to new, more sensitive and more selective approaches for detecting carcinogen-induced damage to DNA and proteins, and subsequent biological effects. These methods were validated with the occupational exposures, which showed evidence of DNA and/or protein and/or chromosome damage in workers in a coke oven plant, garage workers exposed to diesel exhaust and workers exposed to ethylene oxide in a sterilization plant. Dose reponse and adduct repair were studied for methylated adducts in patients treated with methylating cytostatic drugs. The biomonitoring methods have also demonstrated their potential for detecting environmental exposure to genotoxic compounds in nine groups of non-smoking individuals, 32P-postlabelling of DNA adducts being shown to have the greatest sensitivity.

Liver tumorigenesis by Helicobacter hepaticus: considerations of mechanism

A new animal model for the causation of liver tumors via a bacterial infection presented itself fortuitously in the form of a new species, Helicobacter hepaticus. This species of Helicobacter colonizes the hepatic bile canaliculi in susceptible strains of mice, resulting in hepatitis and hepatocellular and hepatocholangiolar adenomas and carcinomas. The mechanism by which this infection leads to cancer is unknown. Tests with Helicobacter hepaticus have revealed thus far that the bacteria do not secrete a mutagen which is capable of detection by the Ames Assay. Measurement of oxidatively damaged bases in the liver DNA of hepaticus infected mice have shown accumulation of 8-oxodeoxyguanosine with disease progression. Other promutagenic DNA lesions, 7-methylguanine and O6-methylguanine, indicative of nitrosation of endogenous amines by nitric oxide, were not detected. Analysis of carcinomas and adenomas taken from H. hepaticus infected A/JCr mice revealed no mutations in ras oncogenes or in exons 5-8 of the p53 gene. These preliminary results indicate that a non-genotoxic tumor promotion mechanism, possibly implemented by reactive oxygen species from the immune response, is more likely than a genotoxic mechanism.

Nitrosamines, alcohol, and gastrointestinal tract cancer: recent epidemiology and experimentation

Recent epidemiological and experimental data continues to implicate nitrosamines in causation of gastrointestinal cancers. The evidence is strong for pharynx, esophagus, and stomach, and more problematic for liver, pancreas, and colorectum. Substantial levels of the promutagenic DNA adduct, Ob-methylguanine, in DNA from these organs in patas monkeys after a low dose of N-nitrosodimethylamine confirms the capacity for activation of environmental nitrosamines in these primate tissues. Alcohol is both an independent and a tobacco-interactive risk factor, influencing cancer incidence for oropharynx and esophagus strongly, and for stomach, colorectum, and liver more moderately. In a tabulation of experimental effects of ethanol potentially related to cancer-enhancing effects, toxicokinetic inhibition of hepatic first-pass clearance of nitrosamines is quantitatively greatest, and may be a major part of the mechanism of alcohol's effect on cancer risk for oropharnx, esophagus, and colon. Other operative mechanisms supported by experimental data are induction of activating enzymes, inhibition of DNA repair, and tumor promotion.

Comparative study of the formation and repair of O6-methylguanine in humans and rodents treated with dacarbazine

The mutagenic, carcinogenic and cytotoxic activity of dacarbazine, a drug employed in cancer chemotherapy, may be related to the induction in DNA of O6-methylguanine (O6-meG), a quantitatively minor but biologically important lesion. In the present study the kinetics of O6-meG formation and repair in blood leukocyte DNA were examined in 20 Hodgkins lymphoma patients treated i.v. with 180 +/- 13 (mean +/- SD) mg/m2 dacarbazine and compared with those observed in various tissues of rodents treated with different doses of the drug. In Hodgkin's lymphoma patients adduct levels reached a value of 0.27 +/- 0.14 fmol/microgram DNA 2 h after dacarbazine administration, while the rate of subsequent loss suggested an adduct half-life of < or = 30 h. Measurement of adduct levels in the same individuals after successive courses of treatment spaced 3 weeks apart (up to 10 treatment courses) demonstrated a consistent individual response and statistical analysis of variance confirmed that intra-individual variation in adduct accumulation after a given dose of dacarbazine accounted for only 5% of the total variance observed. In contrast, inter-individual variation accounted for 70% of the observed variance, with adduct levels 2 h after drug treatment varying approximately 7.5-fold among adduct-positive individuals. No significant depletion of lymphocyte O6-alkylguanine-DNA alkyltransferase (AGT) occurred after patient treatment with dacarbazine. No significant relationship between adduct levels and clinical response to treatment was observed. In rats treated with single or multiple doses of dacarbazine causing varying degrees of AGT depletion the highest levels of O6-meG were seen in the liver, followed by the lymph nodes, bone marrow and blood leukocytes, which showed up to approximately 2-fold lower levels. A similar tissue distribution was also observed in mice and in a single rabbit. These observations suggest that O6-meG levels assayed in blood leukocytes of therapeutically treated humans reflect those present in the -lymph nodes (target tissue for chemotherapy) and the bone marrow (target tissue for leukaemogenesis) and may be utilized as a measure of the drug dose reaching these tissues. The quantitative data reported in this study show that under conditions of no depletion of AGT O6-meG accumulates in blood leukocyte DNA of humans at a rate similar to that observed in rats, suggesting that human susceptibility to any O6-meG-mediated genotoxic effects of dacarbazine may be comparable with that of the rat.

N-nitrosodimethylamine-derived O(6)-methylguanine in DNA of monkey gastrointestinal and urogenital organs and enhancement by ethanol

N-nitrosodimethylamine (NDMA) is a human cancer initiator suspect. Ethanol, a cancer risk factor, may synergize with nitrosamines by suppressing hepatic clearance, to increase internal exposure. A limitation to these hypotheses is lack of activation of NDMA by many rodent tissues. However, systemtic primate studies are lacking. Patas monkeys were utilized to investigate NDMA activation by primate tissues in vivo, generating the promutagenic DNA lesion 0(6)-methylguanine (0(6)-meG). Adult monkeys received 0. 1 mg/kg NDMA by gavage, in some cases preceded by ethanol. Four hours after NDMA only, 0(6)-meG was detected in DNA from all tissues. Levels were highest in gastric mucosa and liver and were only about 50% lower in DNA from white blood cells, esophagus, ovary, pancreas, urinary bladder and uterus. With ethanol co-exposure, amounts of 0(6)-meG increased at least 2-fold in all tissues except liver. The largest effect was in esophagus (17-fold increase), followed by ovary, large intestine, urinary bladder, spleen and cerebellum (9- to 13-fold increases), and uterus, cerebrum and brain stem (7- to 8-fold increases). Alkylguanine alkyltransferase activities varied over a 30-fold range and were highest in liver and stomach. Thus primate tissues, especially those of the gastrointestinal and urogenital organs, are sensitive targets for DNA adduct damage due to NDMA, and ethanol co-exposure leads to striking increases in adducts. Our data support epidemiology implicating nitrosamines in causation of cancers of stomach and other organs, and alcohol as enhancing internal exposure to nitrosamines.

Methyl DNA adducts, DNA repair, and hypoxanthine-guanine phosphoribosyl transferase mutations in peripheral white blood cells from patients with malignant melanoma treated with dacarbazine and hydroxyurea

Dacarbazine (DTIC) is a DNA-methylating drug used in the treatment of malignant melanoma. Among the DNA dducts induced by DTIC are N7-methylguanine (N7-meG) and O6-methylguanine (O6-meG). The latter adduct, in particular, may be important in the mutagenic as well as the cytotoxic activity of DTIC. Repair of O6-meG is carried out by the enzyme O6-alkylguanine-DNA-alkyltransferase (AGT) by a process which results in its autoinactivation. N7-meG is lost from DNA partly spontaneously and partly by enzymatic depurination followed by excision repair of the resulting apurinic site. The purpose of this study was to determine the in vivo kinetics of formation and repair of O6-meG and N7-meG and the changes in AGT in peripheral WBCs with repeated doses of DTIC, and to determine the effects on these processes of concomitant administration of hydroxyurea. In addition, we examined the induction of mutations at the HPRT gene locus. Thirty-four patients with malignant melanoma received 1.0 g/m2 DTIC i.v. every 3 weeks. Hydroxyurea was added to the second and subsequent doses of DTIC in 19 patients. The concentrations of O6-meG, N7-meG, and AGT in peripheral blood lymphocytes were determined up to 24 h after each of the first two doses of DTIC. Mutations at the HPRT gene locus were determined using the T-cell clonal assay. Peak O6-meG levels were detected 1 and 4 h after the first and second dose of DTIC, respectively. AGT concentrations declined to 56.7% (range, 40.3-76.9%) and 55.0% (range, 45.4-58.9%) of pretreatment levels 24 h after the first and second doses of DTIC, respectively, and were still approximately 25%below their initial levels just prior to administration of the second dose of DTIC. An increase in formation of O6-meG was observed at all time points after the second dose of DTIC (P = 0.0001), which was not affected by cotreatment with hydroxyurea (P > 0.5). There was a negative correlation between pretreatment AGT levels and the O6-meG concentration at 24 h after therapy (r = -0.554, P = 0.014). N7-meG levels peaked at 6 h after DTIC therapy and were not significantly influenced by the cycle number. Cotreatment with hydroxyurea tended to be associated with lower levels of N7-meG (P = 0.08). There was no correlation between either O6-meG or N7-meG levels and the grade of neutropenia. On the basis of a limited series of blood samples analyzed, there was no firm evidence that chemotherapy with DTIC resulted in induction of HPRT mutations in lymphocytes. In conclusion, repeated administrations of DTIC resulted in higher concentrations of O6-meG, probably due to reduction in cellular AGT. Hydroxyurea did not significantly influence the kinetics of O6-meG, and N7-meG adduct formation. There was no significant induction of HPRT gene mutations with DTIC. This study suggests that sequencing of DTIC doses should be evaluated using the time course of cellular AGT depletion and DNA adduct formation to achieve higher cytotoxic efficiency.

O6-methylguanine DNA adduct formation and modulation by ethanol in placenta and fetal tissues after exposure of pregnant patas monkeys to N-nitrosodimethylamine

Perinatal nitrosamine exposures may contribute to childhood cancer risk. To test primate fetal susceptibility to formation of cancer initiation-related DNA adducts from nitrosamines, pregnant patas monkeys were given 1.0 or 0.1 mg/kg N-nitrosodimethylamine. Appreciable levels of the promutagenic O6-methylguanine adduct occurred in placental and fetal liver DNA after both doses and were lower but detectable in other fetal tissues after the higher dose. Coadministered ethanol (1.6 g/kg) reduced adducts in placenta and fetal liver by one-half and increased levels in other fetal tissues to the same degree. Thus, primate placenta and fetal tissues have a significant, ethanol-modulated capacity to activate N-nitrosodimethylamine, supporting implication of nitrosamines in human perinatal carcinogenesis and of alcohol as a modulating factor.

Variability in DNA repair and individual susceptibility to genotoxins

DNA repair is an important mechanism of cellular protection from the effects of genotoxic chemicals. Although extensive evidence from studies in experimental systems indicates that variation in DNA repair can significantly influence susceptibility to genotoxins, corresponding studies in human populations are so far limited, mainly because of methodological difficulties. One system, using observations of the accumulation and repair of DNA damage in cancer patients treated with alkylating cytostatic drugs, has provided useful information for assessing the effects of interindividual variation in DNA repair activity on the induction of genotoxic effects in humans. The most detailed studies of this kind have been carried out on patients with cancer (i.e., Hodgkin disease, malignant melanoma) treated with the methylating cytostatic drugs procarbazine or dacarbazine; these studies have provided detailed information on dose-response relationships. They have also demonstrated the protective role of the repair enzyme O6-alkylguanine-DNA alkyltransferase against the accumulation of the premutagenic methylated DNA lesion O6-methylguanine in patients' DNA. Given the strong evidence that exposure of the general population to environmental methylating agents may be extensive, as indicated by the frequent discovery of methylated DNA adducts in human DNA, data on DNA damage and repair in alkylating drug-treated patients and their modulation by host factors may prove useful in efforts to assess the possible carcinogenic risks posed by exposure to environmental methylating agents.

Variability in DNA repair and individual susceptibility to genotoxins

DNA repair is an important mechanism of cellular protection from the effects of genotoxic chemicals. Although extensive evidence from studies in experimental systems indicates that variation in DNA repair can significantly influence susceptibility to genotoxins, corresponding studies in human populations are so far limited, mainly because of methodological difficulties. One system, using observations of the accumulation and repair of DNA damage in cancer patients treated with alkylating cytostatic drugs, has provided useful information for assessing the effects of interindividual variation in DNA repair activity on the induction of genotoxic effects in humans. The most detailed studies of this kind have been carried out on patients with cancer (i.e., Hodgkin disease, malignant melanoma) treated with the methylating cytostatic drugs procarbazine or dacarbazine; these studies have provided detailed information on dose-response relationships. They have also demonstrated the protective role of the repair enzyme O6-alkylguanine-DNA alkyltransferase against the accumulation of the premutagenic methylated DNA lesion O6-methylguanine in patients' DNA. Given the strong evidence that exposure of the general population to environmental methylating agents may be extensive, as indicated by the frequent discovery of methylated DNA adducts in human DNA, data on DNA damage and repair in alkylating drug-treated patients and their modulation by host factors may prove useful in efforts to assess the possible carcinogenic risks posed by exposure to environmental methylating agents.

Dosimetry of O6-methylguanine in rat DNA after low-dose, chronic exposure to N-nitrosodimethylamine (NDMA). Implications for the mechanism of NDMA hepatocarcinogenesis

Groups of female Wistar Furth/NCr rats, aged 6 weeks or 7 months at the start of the experiment, were administered drinking water containing N-nitrosodimethylamine (NDMA) for up to 28 days at concentrations in the range 0.2-2.64 p.p.m., resulting in daily intakes in the range 28-372 micrograms/kg/day at age 10 weeks. The levels of the premutagenic DNA adduct O6-methylguanine (O6-meG) in liver and blood leukocyte DNA were measured at different times during this exposure as well as on the days immediately following cessation of exposure. The adduct was found to accumulate rapidly in both tissues, reaching within 2-7 days steady states in the range 0.08-0.45 mumol/molG, similar in young and adult animals. Accumulation of O6-meG in blood leukocytes was approximately 30% lower than in the liver. Following cessation of NDMA treatment, adducts were lost rapidly from the DNA of both tissues, with an apparent t1/2 of approximately 19-23 h for the liver and 30-35 h for blood leukocytes. No change in liver O6-alkylguanine-DNA alkyltransferase (AGT) took place throughout this treatment. The steady-state adduct levels were approximately linearly related to NDMA dose-rate, except that a clear break (corresponding to a 2.6-fold lower slope) was observed at dose rates > 0.4 p.p.m. (approximately 56 micrograms/kg/day). This dose-response relationship is in contrast to the sharp increase in the liver tumour induction in rats chronically treated with similar concentrations of NDMA reported by Peto et al. (Cancer Res., 51, 6415-6451) and suggests that accumulation of O6-meG cannot by itself account for the hepatocarcinogenic efficacy of NDMA in the rat. Following i.g. administration of single doses of NDMA in a range approximately corresponding to the daily intake during the above mentioned chronic exposure study (25, 50 or 100 micrograms/kg), repair of O6-meG followed sharply biphasic kinetics in both liver and blood leukocytes. In the liver, an initial rapid phase (t1/2 approximately 1.5-1.7 h) was followed by much slower repair (t1/2 approximately 20.7-24.9 h) despite the absence of any change in AGT. Extrapolation of the two segments of the repair kinetics plots back to 0 time suggests that approximately 52-61% of the adducts originally formed in the liver and 33-35% of those formed in blood leukocytes belonged to the rapidly repaired category.(ABSTRACT TRUNCATED AT 400 WORDS)

Alcohol-related cancer risk: a toxicokinetic hypothesis

Consumption of alcoholic beverages is an accepted social custom world-wide. This makes its involvement in events contributing to human cancer risk very important. Although it is neither tumorigenic nor genotoxic in animals, ethanol can potentiate the carcinogenic risk associated with certain environmentally present agents. The reasons for such a synergistic action are speculative, but among theories postulated may be ethanol's ability to modify the toxicokinetics/dynamics of carcinogen metabolism. Experiments conducted with rodents and primates support this hypothesis, demonstrating increased exposure of posthepatic organs to nitrosamines when given in combination with ethanol, followed by enhancement of DNA adduct formation and, at least in rodents, of tumor development. In addition, ethanol may induce enzymes responsible for carcinogen activation, including hepatic cytochrome P450 2E1 in rodents and humans, and in lung, kidney, and brain in rodents. Studies have also shown that these effects can extend to the next generation via maternal and in utero fetal exposure. What impact such ethanol-induced modulations have on tumorigenesis during childhood and later stages of life needs to be investigated further.

Comparative study of mutagenesis by O6-methylguanine in the human Ha-ras oncogene in E. coli and in vitro

Single residues of O6-methylguanine (O6-meG) were introduced into the first or second position of codon 12 (GGC; positions 12G1 or 12G2, respectively) or the first position of codon 13 (GGT; position 13G1) of the human Ha-ras oncogene in phage M13-based vectors. After transformation of E.coli, higher mutant plaque frequencies (MPF) were observed at 12G1 and 13G1 than at 12G2 if O6-alkylguanine-DNA alkyltransferase (AGT) had been depleted, while similar MPF were observed at all three positions in the presence of active AGT. Taken together, these observations suggest reduced AGT repair at 12G2. Kinetic analysis of in vitro DNA replication in the same sequences using E. coli DNA polymerase I (Klenow fragment) indicated that variation in polymerase fidelity may contribute to the overall sequence specificity of mutagenesis. By constructing vectors which direct methyl-directed mismatch repair to the (+) or the (-) strand and comparing the MPF values in bacteria proficient or deficient in mismatch repair and/or AGT, it was concluded that, while mutS-mediated mismatch repair did not remove O6-meG from O6-meG:C pairs, this repair mechanism can affect O6-meG mutagenesis by repairing G:T pairs generated through AGT-induced demethylation of O6-meG:T replication intermediates.

Differential effects of procarbazine and methylnitrosourea on the accumulation of O6-methylguanine and the depletion and recovery of O6-alkylguanine-DNA alkyltransferase in rat tissues

The kinetics of accumulation of the premutagenic DNA adduct O6-methylguanine (O6-meG) in the liver, blood leukocytes, lymph nodes and bone marrow of rats was examined and compared after single or multiple doses of procarbazine, a methylating cytostatic drug employed in the treatment of Hodgkin's lymphoma patients, and methylnitrosourea (MNU), an experimental methylating agent and carcinogen. Maximal O6-meG levels occurred 1-2 h after administration of single doses of procarbazine (10 mg/kg) or MNU (1 mg/kg), thereafter decreasing with half-lives of approximately 20-45 h, depending on the tissue. A relatively uniform tissue distribution was observed with both agents, with the liver generally showing highest adduct levels, followed by the lymph nodes, bone marrow and blood leukocytes which contained broadly similar amounts of O6-meG. During daily, oral administration to rats of procarbazine for 10 days at dose rates of 2.5, 5, 10 or 20 mg/kg/day (treatment analogous to that of the MOOP chemotherapy protocol for Hodgkin's lymphoma) followed by animal death on different days (in each case 24 h after the last treatment), a biphasic mode of O6-meG induction was observed: an initially steep build-up during the first 3-4 days was followed by a transient decline in the rate of accumulation, in turn followed by a second wave of accumulation and then a further slow-down. During the same treatment, liver O6-methylguanine-DNA alkyltransferase (AGT) declined in a dose-related manner. AGT recovery after the end of treatment was slow, taking nearly 20 days after the end of the high-dose treatment to return to control levels, despite the fact that all detectable adducts had been lost from DNA within 3 days after the end of treatment. A similar depletion and slow recovery of AGT in the liver, blood lymphocytes, bone marrow and lymph nodes was observed after treatment with a single dose of 100 mg/kg procarbazine. In contrast to these observations, O6-meG accumulated smoothly during a 10 day administration of MNU (1 or 10 mg/kg/day) to reach a steady-state within 5-6 days, while liver AGT was partially depleted after the high dose and recovered fully within 72 h of cessation of treatment. Similarly, a single dose of MNU (35 mg/kg) resulted in AGT depletion followed by rapid recovery in all four tissues examined. It is concluded that procarbazine (but not MNU) causes a decrease in cellular AGT concentrations by a mechanism additional to suicide repair of O6-meG.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparative dosimetry of O6-methylguanine in humans and rodents treated with procarbazine

The accumulation of O6-methylguanine (O6-meG) in the DNA of blood leukocytes of 21 Hodgkin's lymphoma patients (followed for up to 12 cycles of treatment) treated in the context of MOPP combination chemotherapy with 150 mg procarbazine daily for 10 days was examined and compared to that observed in rats treated with different doses of procarbazine as a single agent once per day for 10 days. In humans, the adduct accumulated in a dose-related fashion and appeared to approach a steady-state after 7-8 days of treatment. Adduct levels on day 10 of the treatment cycle averaged 0.25 +/- 0.09 (mean +/- SD) mumol/molG and, for different individuals, covered a 3-fold range. Intra-individual variability between different treatment cycles was much more limited than inter-individual variability, the two parameters accounting for 8.9% and 84.5% respectively of adduct variance at a constant cumulative dose. Comparison of the dose-response relationships for humans and rats indicates that, under conditions of no depletion of O6-alkylguanine-DNA alkyltransferase (AGT), O6-meG accumulates in blood leukocyte DNA of humans at a rate which is only approximately 2-fold lower than in rats, implying that, to the extent to which O6-meG contributes to the genotoxic activity of procarbazine, human susceptibility to it is likely to be comparable to that of the rat. This is likely to be true also of the bone marrow (the tissue of interest as a target tissue for leukaemogenesis), since the tissue distribution of O6-meG induced by low doses of procarbazine in rats, mice and rabbits indicated that blood leukocyte levels of this adduct closely reflect those in the bone marrow. Based on these results, it is estimated that by the end of a MOPP chemotherapy cycle O6-meG reaches levels of the order of 0.2-0.3 fmol/microgram DNA (0.3-0.5 mumol/molG) in human bone marrow (the target tissue of leukaemogenesis observed after such treatment).

Accumulation of O6-methylguanine in human DNA after therapeutic exposure to methylating agents and its relationship with biological effects

O6-Methylguanine has been measured in peripheral blood leukocytes of 14 patients during one or more cycles of treatment with procarbazine (daily treatment for 10 days) and in 12 patients during one or more cycles of treatment with dacarbazine (single dose per cycle). Adduct formation at levels up to about 0.4 fmole/microgram DNA was detected in all procarbazine- and all but one dacarbazine-treated patients at some point after treatment. O6-Methylguanine accumulated during procarbazine treatment in a dose-related manner (mean rate of accumulation 2.8 x 10(-4) fmole/microgram DNA per mg/m2 dose) and appeared to approach a plateau by the end of the cycle (above 600 mg/m2 cumulative dose). The average rate of O6-methylguanine formation 2 hr after dacarbazine treatment was 11 +/- 8 x 10(-4) fmole/microgram DNA per mg/m2 dose. Individuals examined on more than one treatment cycle with either drug showed broadly similar methylation responses. The rate of adduct accumulation showed a nonsignificant, negative correlation with the pretreatment lymphocyte levels of the repair enzyme O6-alkylguanine-DNA alkyltransferase (AGT) in the case of procarbazine and no correlation in the case of dacarbazine. No consistent lymphocyte AGT depletion was noted as a result of treatment with either drug. No correlation between O6-methylguanine formation and hematological toxicity was observed. In eight patients showing full remission after treatment with dacarbazine, the value of O6-methylguanine (averaged over all the cycles) was 0.252 +/- 0.120 fmole/microgram DNA while in four patients showing partial or no response it was 0.087 +/- 0.110 fmole/microgram DNA (p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Mutagenesis by O6 meG residues within codon 12 of the human Ha-ras proto-oncogene in monkey cells

The first or/and the second guanines of the human Ha-ras codon 12 (normally GGC) were substituted by O6 meG residues and the modified sequence was subsequently introduced into an SV40-based shuttle vector able to replicate in both simian cells and bacteria. After replication in simian COS7 cells (proficient in O6-alkyl-guanine transferase), plasmid DNA was extracted and mutations were screened in E. coli DH5 alpha cells. The vast majority of the mutations induced by O6 meG were G----A transitions. The mutation frequency observed at the second guanine of codon 12 (12G2 position: 3.75% +/- 0.4) was higher than the one observed at the first guanine (12G1 position: 1.09% +/- 0.6). This difference was confirmed by the results obtained when two adjacent O6 meG residues were positioned within codon 12. The higher mutation frequency observed for the 12G2 position could be attributed to differential repair or/and variation in polymerase fidelity. These results are in agreement with animal experiments where alkylating agents gave rise to mutation on G2 position of codon 12.

Studies in gastric carcinogenesis. V. The effects of ascorbic acid on N-nitroso compound formation in human gastric juice in vivo and in vitro

The concentrations of nitrite, thermo- and acetic acid-labile TEA-responsive compounds (TACs) and N-nitroso compounds (NOCs) as a group were measured in human gastric juice collected just before and 1, 2 and 4 h after oral ingestion of 1 g ascorbic acid (AA) or 200 mg sodium nitrate, separately or in combination. Individual responses of gastric [nitrite] following ingestion of AA alone varied widely, with both decreases and increases being observed, and showed no correlation with gastric pH. While a mixed response was also noted for [NOC] and [TAC], substantial decreases were observed in 5/6 individuals with initial [NOC] greater than 0.2 microM and 3/3 individuals with initial [TAC] greater than 0.2 microM, implying that (i) AA effectively inhibited gastric nitrosation and (ii) a basal amount of NOCs and TACs was present in gastric juice which could not be lowered by AA ingestion. Statistical analysis indicated that global mean values of gastric [NOC] were significantly reduced (P less than 0.02) 1-4 h after ingestion of AA. Ingestion of 200 mg sodium nitrate alone resulted in increases in gastric [NOC], which in some cases were very substantial. While nitrosation appeared lower following ingestion of the same dose of nitrate in combination with 1 g AA, the difference from the effects of nitrate alone was not statistically significant. In aqueous buffer, pH 2.5, and in the presence of 1 mM AA, 50 microM nitrite was consumed with a t1/2 of 50 min only if molecular oxygen had first been removed from the system. In the presence of oxygen, no consumption of nitrite could be detected in 50 min, reflecting nitrite recycling (oxidation of nitric oxide to higher oxides of nitrogen and hydrolysis back to nitrite). It is likely that nitrite recycling occurring after collection of gastric juice accounted for the inconsistent responses of gastric nitrite following ingestion of AA. Incubation of human gastric juice, pH 2.5, in vitro in the presence of 50 microM sodium nitrite for 60 min resulted in an increase of [NOC] and [TAC] from 0.10 to 0.70 and 1.10 microM respectively. Nitrosation was efficiently inhibited by AA, 2.27 mM AA resulting in 87 and 100% inhibition respectively. Removal of oxygen from the reaction mixture did not have any significant effect on the extent of nitrosation in the presence or absence of AA.

In vivo formation and repair of O6-methylguanine in human leukocyte DNA after intravenous exposure to dacarbazine

Blood leukocyte DNA obtained from 11 Hodgkin's disease patients undergoing ABVD chemotherapy was analysed for the presence of the precarcinogenic adduct O6-methylguanine (O6-meG) at various times (1-2 h up to 49 h) after i.v. treatment with the methylating drug dacarbazine. Adduct formation was detected in all but one of the patients examined at levels ranging up to 0.45 fmol/micrograms DNA (7.2 x 10(-7) mol/mol guanine). The levels of the adduct decreased by approximately 30% over the 24 h following exposure and were usually not detectable 49 h after exposure. In five out of seven individuals examined after more than one treatment, consistent methylation responses were noted, while in the remaining two cases the responses were mixed. No correlation between the extent of adduct formation and lymphocyte levels of the repair enzyme O6-alkylguanine-DNA alkyltransferase was observed. The average extent of O6-meG formation 1 h after dacarbazine treatment was (4.3 +/- 3.1) x 10(-2) fmol/micrograms DNA per mg/kg dose [( 1.2 +/- 0.8) x 10(-3) fmol/micrograms DNA per mg/m2 dose)]. Following exposure of rats to similar doses of dacarbazine, the corresponding levels of adduct in blood leukocyte DNA were 1.1 x 10(-2) fmol/micrograms DNA per mg/kg dose (2.6 x 10(-3) fmol/micrograms DNA per mg/m2 dose).

Accumulation of O6-methylguanine in human blood leukocyte DNA during exposure to procarbazine and its relationships with dose and repair

O6-Methylguanine was measured by a competitive repair assay in blood leukocyte DNA of seven patients with Hodgkin's or non-Hodgkin's lymphoma during therapeutic exposure to procarbazine involving three daily p.o. doses (50 mg each) for 10 days (corresponding to 2.1 mg/kg/day for a 70-kg human). Adduct accumulation was observed in all seven cases, reaching levels up to 0.28 fmol/microgram of DNA (0.45 mumol/mol of guanine). In one individual, maximal levels of adduct were reached after 7 days of exposure, followed by a steady decline, whereas in all other individuals continuous accumulation was observed throughout the exposure period. In four individuals for which data were available for Day 11 (12 to 16 h after the final intake of procarbazine), decreased amounts of O6-methylguanine were observed relative to the last previous measurements. The accumulation of O6-methylguanine was linearly correlated (P less than 0.01) with the cumulative dose of procarbazine, with a slope of 0.011 fmol of O6-methylguanine/microgram of DNA per mg/kg of body weight or 2.68 x 10(-4) fmol of O6 methylguanine DNA per mg/m2. (Two h after the administration of single p.o. doses of 1 to 10 mg/kg of procarbazine to rats, O6-methylguanine formation in leukocyte DNA was just under half that in liver DNA and showed a linear relationship with dose with a slope of 0.017 fmol/microgram of DNA per mg/kg of body weight or 5.67 x 10(-4) fmol of O6-methylguanine/microgram of DNA per mg/m2. A negative correlation (P less than 0.05) between the rate of accumulation of O6-methylguanine in different individuals and lymphocyte O6-alkylguanine-DNA alkyltransferase (AGT) was observed, demonstrating a probable protective effect of AGT against the accumulation of O6-methylguanine during exposure to methylating agents. This observation supports the suggestion of a possible role of procarbazine-induced O6-methylguanine in the pathogenesis of acute nonlymphocytic leukemia appearing after treatment with chemotherapeutic protocols which include procarbazine, based on the finding of low lymphocyte AGT levels in patients with such therapy-related neoplastic disease (Sagher et al., Cancer Res., 48: 3084-3089, 1988). Lymphocyte AGT levels were mainly in the range of 5 to 10 fmol/micrograms of DNA and showed no consistent variation during procarbazine exposure.