Mapping polycyclic aromatic hydrocarbon and aromatic amine-induced DNA damage in cancer-related genes at the sequence level

Reactions of deoxyribonucleotide bases with sulfooxymethyl or halomethyl polycyclic aromatic hydrocarbons induce unwinding of DNA supercoils

Torsional stress in double-stranded DNA enables and regulates facets of chromosomal metabolism, replication, and transcription and requires regulatory enzymatic systems including topoisomerases and histone methyltransferases. As such, this machinery may be subject to deleterious effects from reactive mutagens, including ones from carcinogenic polycyclic aromatic hydrocarbon (PAH) adduct formation with DNA. Supercoiled plasmid DNA was investigated for its torsional responses to adducts formed in vitro from PAH benzylic carbocation reactive intermediates created spontaneously by release of leaving groups. PAH sulfate esters were found to (1) unwind DNA in a concentration dependent manner, and (2) provide maximum unwinding in a pattern consistent with known carcinogenicities of the parent PAHs, that is, 6-methylbenzo[a]pyrene > 7,12-methylbenz[a]anthracene > 3-methylcholanthrene > 9-methylanthracene > 7-methylbenz[a]anthracene > 1-methylpyrene. Supercoil unwinding was demonstrated to be dependent on the presence of sulfate or chloride leaving groups such that reactive carbocations were generated in situ by hydrolysis. In silico modeling of intercalative complex topology showed PAH benzylic carbocation reactive functional groups in alignment with target nucleophiles on guanine bases in a 5'-dCdG-3' pocket in agreement with known formation of nucleotide adducts. Inhibitory or modulatory effects on PAH-induced supercoil unwinding were seen with ascorbic acid and an experimental antineoplastic agent Antineoplaston A10 in agreement with their known anticarcinogenic properties. In summary, the reactive PAH intermediates studied here undoubtedly participate in well-known mutational mechanisms such as frameshifts and apurinic site generation. However, they are also capable of random disruption of chromosomal supercoiling in a manner consistent with the known carcinogenicities of the parent compounds, and this mechanism may represent an additional detrimental motif worthy of further study for a more complete understanding of chemical carcinogenicity.

DNA damage, DNA repair and carcinogenicity: Tobacco smoke versus electronic cigarette aerosol

The allure of tobacco smoking is linked to the instant gratification provided by inhaled nicotine. Unfortunately, tobacco curing and burning generates many mutagens including more than 70 carcinogens. There are two types of mutagens and carcinogens in tobacco smoke (TS): direct DNA damaging carcinogens and procarcinogens, which require metabolic activation to become DNA damaging. Recent studies provide three new insights on TS-induced DNA damage. First, two major types of TS DNA damage are induced by direct carcinogen aldehydes, cyclic-1,N2-hydroxy-deoxyguanosine (γ-OH-PdG) and α-methyl-1, N2-γ-OH-PdG, rather than by the procarcinogens, polycyclic aromatic hydrocarbons and aromatic amines. Second, TS reduces DNA repair proteins and activity levels. TS aldehydes also prevent procarcinogen activation. Based on these findings, we propose that aldehydes are major sources of TS induce DNA damage and a driving force for carcinogenesis. E-cigarettes (E-cigs) are designed to deliver nicotine in an aerosol state, without burning tobacco. E-cigarette aerosols (ECAs) contain nicotine, propylene glycol and vegetable glycerin. ECAs induce O6-methyl-deoxyguanosines (O6-medG) and cyclic γ-hydroxy-1,N2--propano-dG (γ-OH-PdG) in mouse lung, heart and bladder tissues and causes a reduction of DNA repair proteins and activity in lungs. Nicotine and nicotine-derived nitrosamine ketone (NNK) induce the same types of DNA adducts and cause DNA repair inhibition in human cells. After long-term exposure, ECAs induce lung adenocarcinoma and bladder urothelial hyperplasia in mice. We propose that E-cig nicotine can be nitrosated in mouse and human cells becoming nitrosamines, thereby causing two carcinogenic effects, induction of DNA damage and inhibition of DNA repair, and that ECA is carcinogenic in mice. Thus, this article reviews the newest literature on DNA adducts and DNA repair inhibition induced by nicotine and ECAs in mice and cultured human cells, and provides insights into ECA carcinogenicity in mice.

Evaluation of impairment of DNA integrity in marine gastropods (Cronia contracta) as a biomarker of genotoxic contaminants in coastal water around Goa, West coast of India

The measurement of the impairment of DNA in marine gastropod (Cronia contracta) provides an insight into the genotoxic effects of contaminants on marine organisms along the Goa coast. The impact of genotoxic contaminants on Goan coastal environment was evaluated in terms of the loss of DNA integrity (expressed as the value of 'I') in marine snails with respect to those from the reference site (Palolem) over a period from April 2004 to May 2005 using the technique of alkaline unwinding assay. The DNA integrity in marine snails was found to be significantly damaged at Dona Paula (58%), Vasco (73.5%), and Velsao (48.5%) during the monsoon period (July-August 2004). Similar trend in the loss of DNA integrity in marine gastropods was also detected during the post-monsoon (November-December 2004) and the pre-monsoon (April-May 2005) periods. The low integrities of DNA in marine gastropods at these sites can be attributed to exposure to genotoxic contaminants especially polycyclic aromatic hydrocarbons (PAHs) and toxic heavy metals (Pb, Cd, Cu, Fe, and Mn) prevalent in the marine environment as evident by their accumulation in the tissues of the marine snails inhabiting different sites along the Goa coast. The contaminant-induced DNA strand breaks in marine snails increased significantly at Dona Paula, Vasco, and Velsao clearly indicating the levels of contamination of the sites by genotoxic compounds in those regions. The genotoxic effects of contaminants were further substantiated by detection of the impairment (39%) of DNA integrity in marine snails in a field experiment in which the same species of marine snails (C. contracta) collected from the reference site, Palolem, were deployed at Dona Paula and caged for 25 days for exposure to ambient marine pollutants. The impairment of DNA integrity in marine gastropods along the Goa coast can thus act as a biomarker for marine pollution monitoring of genotoxic contaminants.

Exposure to environmental polycyclic aromatic hydrocarbons: influences on cellular susceptibility to DNA damage (sampling Kosice and Sofia)

The aim of this study was to investigate a possible influence of occupational exposure to carcinogenic environmental polycyclic aromatic hydrocarbons (c-PAHs) on cellular susceptibility to the induction of the DNA damage. Monitoring was performed and blood samples were collected from two groups of male subjects: occupationally exposed and matched controls. The group exposed to c-PAHs (average age of 35.1 years) consisted of 52 policemen from Kosice and 26 policemen and 25 bus drivers (51 altogether) from Sofia. The control group (average age of 36.4 years) consisted of 54 unexposed subjects from Kosice and 24 from Sofia. In the investigated groups 52.5% of exposed subjects and 45.3% of control were current smokers. A challenging dose of X-rays (3Gy) and an alkaline version of the single cell gel electrophoresis (SCGE) assay, known as Comet assay, were used to evaluate levels of induced DNA damage and repair kinetics in isolated human blood lymphocytes. DNA damage detected in lymphocytes prior to or after irradiation did not differ significantly between exposed and unexposed subjects. A significant decrease in repair efficiency due to exposure to PAHs was observed in the exposed individuals from Kosice and Sofia, when analysed separately or together. A negative influence of tobacco smoking on the efficiency of DNA repair was observed. Statistically significant differences were found between subgroups stratified according to education level in Sofia: the half times for DNA repair declined with the increasing level of education. These results confirm that environmental exposure to c-PAHs can alter the ability of blood lymphocytes to repair DNA damage and, as a result could potentially lead to effects that are hazardous to human health.

Influence of environmental exposure to PAHs on the susceptibility of lymphocytes to DNA-damage induction and on their repair capacity

The influence of occupational exposure to environmental carcinogenic polycyclic aromatic hydrocarbons (c-PAHs) on DNA damage detected in lymphocytes of exposed people (city policemen) was studied. The cellular susceptibility to the induction of the DNA damage and the repair capacity of exposed donors are presented in comparison with matched controls. Monitoring was performed and blood samples (164 donors) were collected in Prague, Czech Republic, during the winter and summer seasons. The single-cell gel electrophoresis (SCGE) assay with an internal standard was applied to evaluate the DNA damage. A challenging dose of 2Gy of X-rays was used to study cellular capacities. In the results of studies of the DNA damage induced in vivo or as an immediate response to the challenging treatment no significant difference was found between exposed and unexposed subgroups. The percentage of non-repaired X-ray-induced DNA damage (residual damage, RD) overall in both seasons was significantly higher in lymphocytes of policemen exposed to c-PAHs than in matched controls (RD(T-DNA), %DNA in the comet tail: winter 36.4+/-22.1 versus 22.7+/-10.8, p < 0.001; summer 47.7+/-22.9 versus 34.7+/-15.2, p < 0.001). The results suggest that occupational exposure to environmental c-PAHs significantly reduces the cellular capacity to repair the DNA damage induced by a challenging treatment. A significant decrease of repair efficiency in donors occupationally exposed to environmental c-PAHs was also observed when subgroups were stratified according to smoking history. In conclusion, our results suggest that environmental exposure to c-PAHs affects the cellular repair processes and can lead to harmful effects hazardous to human health.

Sequence specificity of Cr(III)-DNA adduct formation in the p53 gene: NGG sequences are preferential adduct-forming sites

Hexavalent chromium [Cr(VI)] is a known etiological factor in human lung cancer. Cr(VI) exposure-related lung cancer has a high mutation incidence in the p53 gene. Upon intake in human cells Cr(VI) is reduced to Cr(III), which is able to conjugate with amino acids and consequently form either binary Cr(III)-DNA or ternary Cr(III)-amino acid-DNA adducts. Both binary and ternary Cr(III)-DNA adducts are mutagenic. We have found that the Escherichia coli nucleotide excision enzyme UvrABC nuclease is able to incise Cr(III)- and Cr(III)-histidine-modified plasmid DNA and the extent of incision is proportional to the amount of Cr(III)-DNA adducts in the plasmid. In order to determine the role of Cr(III)-DNA adducts in the mutagenesis of the p53 gene in human cancer using the UvrABC nuclease incision method, we have mapped the Cr(III)-DNA distribution in PCR DNA fragments amplified from exons 5, 7 and 8 of the p53 gene. We have found that the sequence specificities of Cr(III)-DNA and Cr(III)-histidine-DNA adducts in the p53 gene sequence are identical and that both types of adducts are preferentially formed at -NGG- sequences, including codons 245, 248 and 249, the mutational hotspots in human lung cancer. It has been found that Cr(III)-DNA adducts induce mainly G to T mutations. Therefore, these results suggest that Cr(III)-DNA adduct formation contributes to the p53 gene mutations in lung carcinogenesis.

Superfund research program--accomplishments and future opportunities

This special issue of the International Journal of Hygiene and Environmental Health provides extensive background on the Superfund programs, findings of specific research studies and future directions. Three federal agencies are leading the effort in this program. They are the National Institute of Environmental Health Sciences, U.S. Environmental Protection Agency and the Agency for Toxic Substances and Disease Registry. As a result of their collaboration, a comprehensive program has been developed that ranges from basic to applied research with the aim of improving public health services and protection. This paper highlights the research within areas of toxicological investigation, exposure assessment, risk evaluation and engaging communities. Each of the agencies has developed strategies and initiatives to enhance the effectiveness of the Superfund research program. The continuation of research will contribute significantly towards achieving the Healthy People 2010 goals that have been set for the United States.