Preferred sites of strand scission in DNA modified by andi-diol epoxide of benzo[a]pyrene

Bioinformatic analysis of benzo-α-pyrene-induced damage to the human placental insulin-like growth factor-1 gene

INTRODUCTION

Intrauterine growth restriction (IUGR) has been associated with exposure to polyaromatic hydrocarbons (PAHs) which are released in the combustion of oil, fuel, gas, garbage, and tobacco. Pregnant women exposed to PAHs are at risk of the effects of these environmental toxins; for example, benzo-α-pyrene (BαP) is able to enter the blood stream and could contribute to IUGR or other developmental abnormalities via effects on the placental cells. Since IUGR has been associated with decreased cord blood concentrations of immunoreactive insulin-like growth factor 1 (ir-IGF-1) and IUGR has been associated with disordered development and fetal programming, we tested the effects of BαP on human placental trophoblast cells in culture.

EXPERIMENTAL

IGF-1 expression and activation was studied using an immortalized human placental trophoblast cell line (HTR-8). The cells were treated with vehicle control or 1 µmol/L BαP, or 5 µmol/L BαP for 12 hours. RNA was extracted and the exons of IGF-1 were amplified using reverse transcriptase-polymerase chain reaction (RT-PCR). The ir-IGF-1 expression levels were compared using gel electrophoresis. The PCR products were sequenced, and levels of mutation were measured with comparative sequence analysis. A computational protein analysis (computer simulation) was performed in order to assess the potential impact of BαP-associated mutation on IGF-1 protein function.

RESULTS

The IGF-1 expression decreased considerably in BαP-treated cells relative to untreated controls (P < .05), also in a dose-dependent manner. Comparative sequence analysis indicated that the level of BαP exposure correlated with the percentage of base pair mutations in IGF-1 nucleotide sequences for both treatment groups (P < .05). Shifts were observed in the open reading frame, indicating a possible change in the IGF-1 start codon. Protein folding simulation analysis indicated that the base pair changes induced by BαP weakened IGF-1-IGF binding protein (IGFBP) interaction.

CONCLUSIONS

In concordance with the previous findings, exposure of human placental trophoblast cells to BαP exposure results in reduction of IGF-1 expression and base pair mutations. The direct action of BαP on the placenta indicates that it may not be necessary for BαP to access other maternal tissues in order for gene abnormalities to occur. Given that PAHs are known to work through aryl hydrocarbon hydrolase (AHH), these results are likely due to the presence of AHH in HTR cells. Computational modeling of BαP actions on IGF1, substrate-ligand binding, supports the biological premise of this work and underlines the need to determine actual biological effects rather than equating immune to bioactivity of IGF1.

DNA–carcinogen interaction: covalent DNA-adducts of benzo(a)pyrene 7, 8-dihydrodiol 9, 10-epoxides studied by biochemical and biophysical techniques

Exposure to various chemicals, either due to occupation or lifestyle, is considered to be a major contributing factor to tumour formation in man (Higginson, 1969; Doll and Peto, 1981). An important and prevalent class of potent carcinogenic compounds present in he environment is polycyclic aromatic hydrocarbons (PAHs), which are found in various petroleum and combustion products derived from heat and power generation and motor vehicle exhausts (Baum, 1978). Furthermore, since PAHs are generally formed by pyrolysis of organic matters such as tobacco smoking and certain procedures of food preparation, the PAH exposure to humans is extensive.

DNA–carcinogen interaction: covalent DNA-adducts of benzo(a)pyrene 7, 8-dihydrodiol 9, 10-epoxides studied by biochemical and biophysical techniques

Exposure to various chemicals, either due to occupation or lifestyle, is considered to be a major contributing factor to tumour formation in man (Higginson, 1969; Doll & Peto, 1981). An important and prevalent class of potent carcinogenic compounds present in the environment is polycyclic aromatic hydrocarbons (PAHs), which are found in various petroleum and combustion products derived from heat and power generation and motor vehicle exhausts (Baum, 1978). Furthermore, since PAHs are generally formed by pyrolysis of organic matters such as tobacco smoking and certain procedures of food preparation, the PAH exposure to humans is extensive

DNA–carcinogen interaction: covalent DNA-adducts of benzo(a)pyrene 7, 8-dihydrodiol 9, 10-epoxides studied by biochemical and biophysical techniques

Exposure to various chemicals, either due to occupation or lifestyle, is considered to be a major contributing factor to tumour formation in man (Higginson, 1969; Doll & Peto, 1981). An important and prevalent class of potent carcinogenic compounds present in the environment is polycyclic aromatic hydrocarbons (PAHs), which are found in various petroleum and combustion products derived from heat and power generation and motor vehicle exhausts (Baum, 1978). Furthermore, since PAHs are generally formed by pyrolysis of organic matters such as tobacco smoking and certain procedures of food preparation, the PAH exposure to humans is extensive.

DNA–carcinogen interaction: covalent DNA-adducts of benzo(a)pyrene 7, 8-dihydrodiol 9, 10-epoxides studied by biochemical and biophysical techniques

Exposure to various chemicals, either due to occupation or lifestyle, is considered to be a major contributing factor to tumour formation in man (Higginson, 1969; Doll and Peto, 1981). An important and prevalent class of potent carcinogeniccompounds present in the environment is polycyclic aromatic hydrocarbons(PAHs), which are found in various petroleum and combustion products derived from heat and power generation and motor vehicle exhausts (Baum, 1978). Furthermore, since PAHs are generally formed by pyrolysis of organic matters such as tobacco smoking and certain procedures of food preparation, the PAH exposure to humans is extensive.

A survey of the sequence-specific interaction of damaging agents with DNA: emphasis on antitumor agents

This article reviews the literature concerning the sequence specificity of DNA-damaging agents. DNA-damaging agents are widely used in cancer chemotherapy. It is important to understand fully the determinants of DNA sequence specificity so that more effective DNA-damaging agents can be developed as antitumor drugs. There are five main methods of DNA sequence specificity analysis: cleavage of end-labeled fragments, linear amplification with Taq DNA polymerase, ligation-mediated polymerase chain reaction (PCR), single-strand ligation PCR, and footprinting. The DNA sequence specificity in purified DNA and in intact mammalian cells is reviewed for several classes of DNA-damaging agent. These include agents that form covalent adducts with DNA, free radical generators, topoisomerase inhibitors, intercalators and minor groove binders, enzymes, and electromagnetic radiation. The main sites of adduct formation are at the N-7 of guanine in the major groove of DNA and the N-3 of adenine in the minor groove, whereas free radical generators abstract hydrogen from the deoxyribose sugar and topoisomerase inhibitors cause enzyme-DNA cross-links to form. Several issues involved in the determination of the DNA sequence specificity are discussed. The future directions of the field, with respect to cancer chemotherapy, are also examined.

Preferential promutagenic lesions at exons 7-8 of human p53 genomic DNA induced by the direct-acting hepatocarcinogens N-nitroso-2-acetylaminofluorene and N-acetoxy-2-acetylaminofluorene

DNA polymerase fingerprint analysis (DPFA) was employed for identifying DNA-carcinogen adduct formation in the human p53 and lac gene sequence. Two 'hot regions' at codons 223-250 and 257-283 of the p53 gene were easily attacked by nitroso-2-acetylaminofluorene or acetoxy-2-acetylaminofluorene. However, the promutational lesions in lac gene were rather randomly distributed. The chemical treated plasmid (pUC 19) which contains lac gene were transfected into Escherichia coli JM109 cells and the induced lac gene mutants were selected with X-Gal plate as indicated by the appearance of white colonies. No mutational hot regions were found in the lac gene.

p53 gene mutational spectra in hepatocellular carcinomas induced by 2-acetylaminofluorene and N-nitroso-2-acetylaminofluorene in rats

In this study, the mutation frequencies of the p53 gene in rat hepatocellular carcinomas (HCCs) induced by N-nitroso-2-acetylaminofluorene (NO-AAF) and 2-acetylaminofluorene (AAF) were 19.23% (20 of 104) and 31.1% (33 of 106), respectively. Four noteworthy features of the mutation spectrums of the p53 gene in HCCs induced by both NO-AAF and AAF were observed: (i) There was preferential clustering of mutations at exons 5-8 in both the NO-AAF or AAF groups. (ii) Nearly all the mutations (98%) induced by NO-AAF and AAF were point mutations. (iii) A high frequency of the p53 mutations were transition mutations, and the ratios of transition to transversion in the NO-AAF and the AAF group were 13:6 and 21:12, respectively. Almost all the mutations were G-->A transitions and guanosine was the major target base. (iv) The frequency and base location of p53 mutations were significantly associated with cancer cell differentiation. In poorly differentiated HCCs (58 individual tumor samples), mutations were detected in 24 of 58 samples (41.1%) and clustered mostly in exons 7 and 8 (19 of 24 samples), whereas in well-differentiated HCCs (105 individual tumor samples), the incidence of mutations was low (one of 10 in the AAF group, 17 of 95 in the NO-AAF group), and the mutations were located in exon 5 (11 of 18). The biological significance of these different mutational spectra among p53 genes deserves further investigation.

Benzo[a]pyrene-induced mutagenesis of p53 hot-spot codons 248 and 249 in human hepatocytes

Human tobacco-related cancers show a high frequency of G-to-T transversions in several mutation hot-spot regions of the p53 tumor suppressor gene, probably the result of specific mutagens in tobacco smoke, most notably benzo[a]pyrene. To gain insight into the mechanism of formation of these G-to-T transversions in tobacco-associated carcinogenesis, we studied the mutagenesis of p53 codons 247-250 by benzo[a]pyrene in human hepatocellular carcinoma cells by restriction fragment length polymorphism-polymerase chain reaction genotypic analysis. Benzo[a]pyrene preferentially induced G-to-T transversion in the second and third positions of codon 248 and C-to-A transversion in the first position of codon 248. However, benzo[a]pyrene did not induce base-pair changes in codon 249, which is a mutational hot-spot in aflatoxin-related hepatocarcinogenesis, in which predominantly G-to-T transversion in the third position of codon 249 is observed. The benzo[a]pyrene-induced G-to-T transversion in the middle position of codon 248, in which arginine is changed into leucine, is frequently observed in tumors of the lung. The other two benzo[a]pyrene-induced base-pair changes in codon 248, namely the C-to-A transversion in the first position and G-to-T transversion in the third position, do not lead to a change in the amino-acid composition of the p53 protein. These mutations are silent and therefore are not selected in tumors. It follows that benzo[a]pyrene-induced mutability on the DNA level in p53 codons 247-250 correlates well with the type of mutation found in tumors of the lung. Therefore, our results support the hypothesis that benzo[a]pyrene is the etiological agent in tobacco-related cancers.

Distribution of DNA cleavages induced by bleomycin and neocarzinostatin in a defined sequence of rat glioma cells

We have demonstrated the usefulness of a highly reiterated sequence of rat DNA as a probe sequence for evaluating the effect of bleomycin (BLM) and neocarzinostatin (NCS) at the level of individual nucleotides. The 370 base pairs (bp) DNA fragment, purified from rat glioma C6 cells after Hind III digestion, was labeled with 32P at either the 3'- or the 5'-ends and then divided into 167 bp and 203 bp by Hae III. These end-labeled DNA fragments were reacted in vitro with BLM or NCS, and electrophoresed on the denaturating 8% polyacrylamide gels according to Maxam and Gilbert's sequencing protocol. BLM created DNA strand breaks at the guanine-cytosine and guanine-thymine (5'----3') sequences, and NCS cleaved DNA at the position of thymines and adenines. The highly reiterated sequence of rat brain tumor DNA therefore provides adequate knowledge of DNA damages induced by BLM and NCS.

Low absolute mutagenic efficiency but high cytotoxicity of a non-bay region diol epoxide derived from benzo[a]pyrene

Insights into the mechanisms of chemical carcinogenesis can sometimes be gained by comparing the effects of closely related chemicals which differ in carcinogenic potency. We have treated Chinese hamster ovary (CHO) cells with a non-carcinogenic metabolite of benzo[a]pyrene, 9r,10t-dihydroxy-7c,8c-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE-III), and measured the formation and persistence of DNA adducts. We have correlated this binding data with cytotoxicity and mutagenicity in a DNA-repair-proficient CHO cell line (AT3-2) and in two derived lines, UVL-1 and UVL-10, which are unable to repair bulky DNA adducts. These data are compared with similar studies of the effects of the carcinogenic metabolite, 7r,8t-dihydroxy-9t,10t-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE-I). Synchronous fluorescence spectroscopy was used to measure the levels of BPDE-III-DNA adducts in treated cells. Adduct levels increased linearly with dose, but the absolute binding levels were about 30-fold lower than in comparable incubations with BPDE-I. Measurements of the removal of adducts derived from these two diol epoxides indicated no significant difference in the rate of repair measured 24 h post-treatment. When cells were treated with increasing doses of BPDE-III, survival curves were obtained which exhibited a shoulder region at low doses and an exponential decrease in plating efficiency at higher doses. By comparison of the D0's, the DNA-repair-deficient cell lines were found to be 4-5-fold more sensitive to the killing effects of BPDE-III than were the repair-proficient AT3-2 cells.

DNA cleavage by phenylhydroquinone: the major metabolite of a fungicide o-phenylphenol

The interaction of o-phenylphenol (OPP) and its metabolites with DNA was examined. As a model system, the reactivities of OPP and its metabolites with DNA were studied by using pUC18 DNA. The major metabolite formed in vitro from OPP by mixed function oxidase was phenylhydroquinone (PHQ). This result corresponds to the findings that PHQ in the form of glucuronide conjugate was the main product detected in bladder of OPP fed rats in vivo. When supercoiled pUC18 DNA (form I) was incubated with PHQ at concentrations from 1 X 10(-6) M to 2 X 10(-1) M, DNA strand scission by PHQ was observed at a concentration as low as 1 X 10(-5) M and the amount of linear form (form III) increased with increasing PHQ concentration. PHQ causes DNA strand scission. The DNA cleavage by OPP and phenyl-p-benzoquinone (PBQ) was barely detectable. The DNA cleavage by PHQ was inhibited by superoxide dismutase (SOD), catalase and several oxygen radical scavengers such as polyethylene glycol, tert-butanol, dimethyl sulfoxide, sodium azide, sodium benzoate, bovine serum albumin and methionine. The production of superoxide radical from PHQ was confirmed by cytochrome c reduction assay. These results indicate that the oxygen radicals such as superoxide, hydroxyl radicals and some others generated in the process of oxidation of PHQ in aqueous solution are responsible for the DNA cleavage. In order to identify the sites of cleavage of DNA by PHQ, a 5'-end 32P-labeled 206 base-pair EcoRI-BglI fragment of pUC18 DNA was incubated with PHQ. The DNA was then analyzed by sequencing gel electrophoresis followed by autoradiography. When the DNA was incubated with PHQ and further treated with piperidine, cleavage was detected relatively more frequently at guanine residues. The attack seemed to occur at guanine residues in general, but was not restricted to guanines with specific residues in the vicinity.

Linear dichroism properties and orientations of different ultraviolet transition moments of benzo[a]pyrene derivatives bound noncovalently and covalently to DNA

Linear dichroism and absorption methods are used to study the orientations of transition moments of absorption bands of polycyclic aromatic epoxide derivatives which overlap with those of the DNA band in the 240-300 nm region. Both the short and long axes of the pyrene residues of 1-oxiranylpyrene (1-OP) and the (+) and (-) enantiomers of trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) noncovalently bound to double-stranded native DNA are oriented approximately perpendicular to the axis of the DNA helix, consistent with intercalative modes of binding. The covalent binding of these three epoxide derivatives to DNA is accompanied by reorientations of both the short and long axes of the pyrene residues. Covalent adducts derived from the highly mutagenic (+)-anti-BPDE are characterized by tilts of the short axis within 35 degrees or less, and of the long axis by more than 60-80 degrees, with respect to the planes of the DNA bases. In the adducts derived from the binding of the less mutagenic (-)-anti-BPDE and 1-OP epoxide derivatives to DNA, the long axes of the pyrenyl rings are predominantly oriented within 25 degrees of the planes of the DNA bases; however, in the case of the (-) enantiomer of BPDE, there is significant heterogeneity of conformations. In the case of the 1-OP covalent DNA adducts, the short axis of the pyrene ring system is tilted away from the planes of the DNA bases, and the pyrene ring system is not intercalated between DNA base-pairs as in the noncovalent complexes. The stereochemical properties of the saturated 7,8,9,10-ring in BPDE, or the lack of the 7 and 8 carbon atoms in 1-OP, do not seem to affect noncovalent intercalative complex formation which, most likely, is influenced mainly by the flat pyrenyl residues. These structural features, however, strongly influence the conformations of the covalent adducts, which in turn may be responsible for the differences in the mutagenic activities of these molecules.

Origins of stereospecificity in DNA damage by anti-benzo[a]pyrene diol-epoxides. A molecular modelling study

A general computational procedure for the modelling of intercalated DNA-ligand complexes has been developed, and is used here to model intercalated complexes of the (+)-anti and (-)-anti enantiomers of benzo[a]pyrene diol-epoxide (BPDE) with cytosine-3',5'-guanosine double-stranded DNA sequences (dCpG). Results are presented indicating differences between the behaviours of the two enantiomers which have implications for the understanding of the stereospecificity of DNA strand breakage by benzo[a]pyrene diol-epoxides.

Detection of DNA damage in human cells and tissue using sequencing techniques

Methods have been developed that permit both identification and location of sites of alterations is defined DNA sequences. These methods can be extended to human tissues using the alphoid segment, which comprises 1% of the human genome. This segment can be isolated in ample quantities from human cells and tissues. Once purified and end labeled, this defined segment can be used to detect sites of altered DNA moieties by combining Maxam-Gilbert sequencing protocols with appropriate enzymatic probes and chemical techniques. These studies can be performed in cultured cells or in tissues obtained by surgical excision or autopsy.

T4 DNA polymerase (3'-5') exonuclease, an enzyme for the detection and quantitation of stable DNA lesions: the ultraviolet light example

Ultraviolet light irradiation of DNA results in the formation of two major types of photoproducts, cyclobutane dimers and 6-4' [pyrimidin-2'-one] -pyrimidine photoproducts. The enzyme T4 DNA polymerase possesses a 3' to 5' exonuclease activity and hydrolyzes both single and double stranded DNA in the absence of deoxynucleotide triphosphate substrates. Here we describe the use of T4 DNA polymerase associated exonuclease for the detection and quantitation of UV light-induced damage on both single and double stranded DNA. Hydrolysis of UV-irradiated single or double stranded DNA by the DNA polymerase associated exonuclease is quantitatively blocked by both cyclobutane dimers and (6-4) photoproducts. The enzyme terminates digestion of UV-irradiated DNA at the 3' pyrimidine of both cyclobutane dimers and (6-4) photoproducts. For a given photoproduct site, the induction of cyclobutane dimers was the same for both single and double stranded DNA. A similar relationship was also found for the induction of (6-4) photoproducts. These results suggest that the T4 DNA polymerase proofreading activity alone cannot remove these UV photoproducts present on DNA templates, but instead must function together with enzymes such as the T4 pyrimidine dimer-specific endonuclease in the repair of DNA photoproducts. The T4 DNA polymerase associated exonuclease should be useful for the analysis of a wide variety of bulky, stable DNA adducts.

Depurination of benzo[a]pyrene-diolepoxide treated DNA

Rat liver DNA was treated in vitro with benzo[a]pyrene-diolepoxide (BPDE), the ultimate carcinogenic metabolite derived from the polycyclic hydrocarbon benzo[a]pyrene. On incubation of the reacted DNA, apurinic sites developed which gave rise to strand breakage in alkaline solution. The reduction in molecular weight produced by these breaks was measured by analytical ultracentrifugation. In the case of anti-BPDE this depurination was shown to occur in two stages. The first was mainly due to attack at the 7-position of guanine, to yield an adduct which was lost from the DNA within a few hours. The second stage was due to much slower loss of the major N2-guanine adduct. The separated enantiomers, (+)- and (-)-anti-BPDE, and syn-BPDE all caused depurination to various extents. It is argued that although these processes are important in a study of the action of BPDE on DNA in vitro, their contribution to the biological activity of BPDE is probably negligible.

DNA modification by chemical carcinogens

The chemistry and molecular biology of DNA adducts is only one part of the carcinogenic process. Many other factors will determine whether a particular chemical will exert a carcinogenic effect. For example, the size of particles upon which a carcinogenic may be adsorbed will influence whether or not, and if so where, deposition within the lung will occur. The simultaneous exposure to several different agents may enhance or inhibit the metabolism of a chemical to its ultimate carcinogenic form (Rice et al., 1984; Smolarek and Baird, 1984). The ultimate carcinogenic metabolites may be influenced in their ability to react with DNA by a number of factors such as internal levels of detoxifying enzymes, the presence of other metabolic intermediates such as glutathione with which they could react either enzymatically or non-enzymatically, and the state of DNA which is probably most heavily influenced by whether or not the cell is undergoing replication or particular sequences being expressed. Replicating forks have been shown to be more extensively modified than other areas of DNA. Another critical factor which can influence the final outcome of the DNA damage is whether or not the modifications can be repaired. If this occurs with high fidelity and the cell has not previously undergone replication then the effect of the damage by the carcinogen is likely to be minimal. The major area in which progress is needed is an understanding of what this damage really does to the cell such that after an additional period of time, which may be as long as twenty or more years, these prior events are expressed and cell proliferation occurs. Clearly additional stimulatory factors, for example tumor promoting agents such as the phorbol esters or phenobarbital, are often needed. After such prolonged periods it seems likely that the DNA adducts would no longer be present. However, the way in which their earlier presence is remembered is not clear. Simple mutations do not explain all the characteristics of tumor progression and, when it occurs, regression. Even if a specific site mutation does occur then its expression must be under other types of control. Any explanation of the action of DNA modification at the molecular level also requires that account be taken of the diverse nature of the DNA adducts from simple modifications such as methylation to bulkier adducts such as benzo[a]pyrene, aflatoxin or aromatic amines.(ABSTRACT TRUNCATED AT 400 WORDS)

Site-specific carcinogen binding to DNA

Benzo[alpha]pyrene diol epoxide (BPDE) is a well-studied environmental carcinogen that binds covalently to DNA. Here we describe a photochemical technique that allows us to map BPDE-binding sites within cloned gene sequences. The technique is based upon our observation that, when irradiated with laser light at 355 nm, one single-strand DNA cut is produced at each BPDE binding site. In initial experiments we have studied the distribution of such cuts in cloned DNA from the chicken adult beta-globin gene. We find that BPDE binding in this gene sequence is distinctly nonrandom. While several prominent BPDE-binding sites are evident, a 300-base-pair sequence immediately 5' to the RNA cap site is most strongly attacked by the carcinogen. This region is believed to contain important transcriptional control sequences. We discuss the possibility that sequence-specific binding to such regulatory elements may be an important feature of the mechanism of the carcinogen.

Interaction of (+/-)-7r,8t-dihydroxy-9t,10t-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene with relaxed circular pBR322 DNA

The interaction of (+/-)-BPDE (1) with DNA at neutral pH was investigated by the application of relaxed circular pBR322 DNA. (+/-)-BPDE causes a rapid positive supercoiling of this DNA followed by a slower spontaneous relaxation. The results indicate that there are two clearly discernible types of chemical interactions between 1 and DNA, a rapid intercalative covalent binding and a slower strand breakage. The implications of these findings are discussed.

Base sequence selectivity in binding of aromatic hydrocarbons with synthetic polynucleotides

The interactions between DOBP (3) and calf thymus DNA as well as four synthetic polynucleotides, poly(dA-dT), polydA:polydT, poly(dG-dC), and polydG:polydC, were investigated by spectroscopic techniques. It was found that the binding of 3 with poly(dA-dT) is favored appreciably over other synthetic polynucleotides and DNA. The results suggest that the initial association of carcinogenic BPDE (2) with DNA may take place preferentially at certain specific base sequences in DNA.

Sequence specificity of drug-DNA interactions

Methods for determining sequence specificities of anticancer drugs, carcinogens, and mutagens which interact with natural DNA's are presented. For drugs which nick or covalently bind to DNA and thus leave a permanent record of their residence position on the helix, the sequences important in drug action can be readily determined. For agents which interact with DNA in an equilibrium fashion, "footprinting" analysis, a technique used to investigate protein-DNA binding, has proved to be useful in studying drug-DNA interactions. The sequence specificities of a number of small ligands which interact with natural DNA's are also presented.

The purification of human DNA fragments containing benzpyrene adducts

DNA was isolated from human diploid lung epithelial cells treated in culture with [3H]benzpyrene. The DNA contained one covalently bound benzpyrene group per 38 kb and it was digested with a series of restriction endonucleases giving decreasing fragment sizes, and also with DNAase I to 96% acid solubility. The digests were applied to octyl-Sepharose columns under conditions which promote hydrophobic interaction of the benzpyrene groups on the DNA with the octyl groups in the column matrix. Separation of fragments without and with benzpyrene groups was achieved with successive high salt and ethanediol washes. As DNA fragment size is decreased, more DNA-associated benzpyrene is eluted with ethanediol. Under these conditions, DNA from untreated cells is totally removed in the high salt wash and free benzpyrene metabolites are retained on the column. The separation of DNA fragments with covalently-bound hydrophobic benzpyrene groups, from less modified or unmodified DNA will facilitate examination of the distribution of benzpyrene adducts in defined regions of the human genome.

Use of exonuclease III to determine the site of stable lesions in defined sequences of DNA: the cyclobutane pyrimidine dimer and cis and trans dichlorodiammine platinum II examples

A method to detect chemically stable lesions in DNA has been developed using Exonuclease III, a double strand specific nuclease, to digest 5'-end labeled DNA. The products, when analyzed on high resolution DNA sequencing gels, reveal the sites of DNA modification. Cyclobutane pyrimidine dimers induced by UV irradiation can be localized by comparison of the fragments produced by Exonuclease III digestion with fragments obtained after digestion of the DNA with UV specific endonuclease. The experiments demonstrate the Exonuclease III stops one base away from the cyclobutane pyrimidine dimers. Similar experiments with cis- and trans-dichlorodiammine-platinum (II) showed that modification of DNA by these agents also impede Exonuclease III digestion. In general the same stop sites were found for cis-and trans-platinum adducts. They occur at sites of guanine bases. Additional stop sites were found for cis-platinum at sites of adjacent guanine bases. These results are in agreement with the model that cis-platinum forms intrastrand guanine-guanine dimers, whereas trans-platinum does not.