Discrimination by antibodies between local defects in DNA induced by 2-aminofluorene derivatives

Synthesis of 1-phenyl-3-biphenyl-5-(N-ethylcarbazole-3-yl)-2-pyrazoline and its use as DNA probe

A novel pyrazoline derivative, named 1-phenyl-3-biphenyl-5-(N-ethylcarbazole-3-yl)-2-pyrazoline, was synthesized, and its structure was confirmed by means of IR, (1)H NMR, and elementary analysis. The compound emits strong yellow fluorescence. Decrease of fluorescence intensity of the pyrazoline derivative in the presence of calf thymus DNA (ct DNA) is observed, and the quenching obey Stern-Volmer equation. There is a single quenching mechanism for the complex, which belongs to static quenching. KI quenching study shows that the magnitude of K(SV) of the bound pyrazoline is lower than that of the free one. It is also found that ionic strength could affect the interaction. Binding constants for pyrazoline with ct DNA and salmon sperm DNA (ss DNA) are in the same order of 10(4) molL(-1), and binding site size are about 1 per base pairs. Experimental results indicate that the new compound might insert into DNA base pairs by intercalative binding mode.

Observing Translesion Synthesis of an Aromatic Amine DNA Adduct by a High-fidelity DNA Polymerase

Aromatic amines have been studied for more than a half-century as model carcinogens representing a class of chemicals that form bulky adducts to the C8 position of guanine in DNA. Among these guanine adducts, the N-(2'-deoxyguanosin-8-yl)-aminofluorene (G-AF) and N-2-(2'-deoxyguanosin-8-yl)-acetylaminofluorene (G-AAF) derivatives are the best studied. Although G-AF and G-AAF differ by only an acetyl group, they exert different effects on DNA replication by replicative and high-fidelity DNA polymerases. Translesion synthesis of G-AF is achieved with high-fidelity polymerases, whereas replication of G-AAF requires specialized bypass polymerases. Here we have presented structures of G-AF as it undergoes one round of accurate replication by a high-fidelity DNA polymerase. Nucleotide incorporation opposite G-AF is achieved in solution and in the crystal, revealing how the polymerase accommodates and replicates past G-AF, but not G-AAF. Like an unmodified guanine, G-AF adopts a conformation that allows it to form Watson-Crick hydrogen bonds with an opposing cytosine that results in protrusion of the bulky fluorene moiety into the major groove. Although incorporation opposite G-AF is observed, the C:G-AF base pair induces distortions to the polymerase active site that slow translesion synthesis.

Genetic toxicity of 2-acetylaminofluorene, 2-aminofluorene and some of their metabolites and model metabolites

2-Acetylaminofluorene and 2-aminofluorene are among the most intensively studied of all chemical mutagens and carcinogens. Fundamental research findings concerning the metabolism of 2-acetylaminofluorene to electrophilic derivatives, the interaction of these derivatives with DNA, and the carcinogenic and mutagenic responses that are associated with the resulting DNA damage have formed the foundation upon which much of genetic toxicity testing is based. The parent compounds and their proximate and ultimate mutagenic and carcinogenic derivatives have been evaluated in a variety of prokaryotic and eukaryotic assays for mutagenesis and DNA damage. The reactive derivatives are active in virtually all systems, while 2-acetylaminofluorene and 2-aminofluorene are active in most systems that provide adequate metabolic activation. Knowledge of the structures of the DNA adducts formed by 2-acetylaminofluorene and 2-aminofluorene, the effects of the adducts on DNA conformation and synthesis, adduct distribution in tissues, cells and DNA, and adduct repair have been used to develop hypotheses to understand the genotoxic and carcinogenic effects of these compounds. Molecular analysis of mutations produced in cell-free, bacterial, in vitro mammalian, and intact animal systems have recently been used to extend these hypotheses.

2-Aminofluorene modified DNA duplex exists in two interchangeable conformations

One- and two-dimensional NMR shows that the carcinogen 2-aminofluorene exists in two unique, interchangeable conformations when covalently bound to a model human c-H-ras1 proto-oncogene codon 61 oligomer duplex. In one conformation the 2-aminofluorene moiety protrudes out of the major groove leaving the Watson-Crick base pairing of the cytosine and 2-aminofluorene-guanine bases intact, consistent with the ability of replicating enzymes to bypass the lesion and correctly incorporate cytosine. The second form of the modified oligomer duplex may be representative of a pre-mutagenic conformation in that the 2-aminofluorene moiety is stacked within the DNA helix, disrupting base pairing between the 2-aminofluorene-modified guanine and its complementary cytosine.

Use of single-turnover kinetics to study bulky adduct bypass by T7 DNA polymerase

The mechanism by which T7 DNA polymerase (exo-) bypasses N-2-acetylaminofluorene (AAF) and N-2-aminofluorene (AF) adducts was studied by single-turnover kinetics. These adducts are known to be mutagenic in several cell types, and their bypass was studied in the framework of understanding how they promote mutations. Synthetic primer/templates were made from a template sequence containing a single guanine, to which the adducts were covalently attached, and one of three primers whose 3' ends were various distances from the adduct in the annealed substrates. Upon approaching the site of either adduct, the polymerase was found to add nucleotides as rapidly as to unmodified primer/templates, until just opposite the lesion. The incorporation rate of dCTP (at 100 microM) opposite AF-dG or AAF-dG was approximately 5 x 10(4)- and 4 x 10(6)-fold slower, respectively, than incorporation at the same position into an unmodified primer/template. The polymerase dissociated from the sites of the adducts at approximately the same rate that it dissociated from unmodified DNA. Correct nucleotide incorporation was favored both opposite and immediately after AF-dG. However, at both positions, dATP was the most rapidly misincorporated nucleotide. Misincorporation of dATP was more rapid than correct nucleotide incorporation both opposite and immediately after AAF-dG. These results are discussed in terms of the effects of AF and AAF adducts in vivo.

DNA sequence analysis of mutations induced by N-2-acetylamino-7-iodofluorene in plasmid pBR322 in Escherichia coli

The spectrum of mutations induced by N-2-acetylamino-7-iodofluorene (AAIF) was analyzed in a forward mutation system based on mutagenesis directed to a small restriction fragment in the tetracycline resistance gene of plasmid pBR322. AAIF was found to induce frameshift mutations and base-pair substitutions at approximately equal frequencies. The frameshift mutations were mostly deletions of single base-pairs, but -2 frameshifts and +1 frameshifts were also detected. With one exception, the substitutions were transversions initiated at a G.C base-pair. Both frameshift mutations and transversions occurred preferentially at sites of repetitive guanine residues. Although AAIF and the related aromatic amines N-2-acetylaminofluorene (AAF) and N-2-aminofluorene (AF) all bind to the C-8 position of guanine, they have different effects on DNA conformation, and these differences are reflected in their mutation spectra. Previous studies have provided evidence that AAF adducts can trigger a B to Z conformational change in alternating GC sequences or displacement of the guanine by the fluorene ring in other sequences; the principal result is two classes of frameshift mutations. AF, whose DNA interaction involves outside binding rather than insertion and denaturation, primarily induces base-pair substitutions. AAIF adducts are chemically similar to AAF adducts, but the iodo group apparently hinders insertion of the fluorene ring into DNA. Consistent with this model, the mutation spectrum of AAIF combines properties of the mutation spectra of both AAF and AF.

The DNA bending by acetylaminofluorene residues and by apurinic sites

We have studied the distortions induced in double-stranded oligonucleotides by covalently bound acetylaminofluorene residues and by apurinic sites. Within the acetylaminofluorene-modified oligonucleotide three base-pairs are unpaired as detected by the chemical probes chloroacetaldehyde and osmium tetroxide. These two probes reveal that the bases adjacent to the apurinic site are paired. In both the modified double-stranded oligonucleotides, the backbone on the 5' side of the modification is more reactive with 1,10-phenanthroline copper than the backbone on the 3' side. On polyacrylamide gels, the ligated multimers of acetylaminofluorene or apurinic site-modified oligonucleotides migrate slower than the multimers of the unmodified oligonucleotides. It is suggested that the acetylaminofluorene-modified guanine residues and the apurinic sites behave more as hinge joints than as the centres of directed bends.

Detection of DNA damage in individual cells by flow cytometric analysis using anti-DNA monoclonal antibody

A new method for the measurement of DNA damage in individual cells treated with alkylating agents is described. The method is based on the binding of anti-DNA monoclonal antibody to DNA in situ. Monoclonal antibody F7-26 was obtained by fusion of mouse myeloma cells with spleen cells isolated from a mouse immunized with DNA treated by nitrogen mustard (HN2). Binding of antibody was evaluated by flow cytometry with indirect immunofluorescence. No binding of antibody to DNA in non-treated HeLa S3 cells was detected. Treatment of cells with HN2 or L-phenylalanine mustard induced binding of antibody to DNA in situ. Binding of antibody was observed after treating cells with doses of drugs which reduced the surviving fraction below 20%. Intensity of binding increased in proportion to the drug dose. Two-parameter analysis for the antibody binding and DNA content showed no binding of antibody to replicating DNA in control cells. In HN2-treated cells a cell subset with the lowest antibody binding was observed among cells in G1 phase. Binding of antibody to DNA in HN2-treated cells was eliminated by single-strand (ss) specific S1 nuclease. In competition assay, antibody was inhibited by thermally denatured DNA, but not by native double-stranded (ds) DNA, RNA, nucleosides and deoxyribohomopolymers. Binding of monoclonal antibody specific for the determinants expressed on ssDNA to the cells treated with alkylating agents may be attributed to local DNA denaturation. Potentiation of L-phenylalanine mustard cytotoxicity by buthionine sulfoximine or hyperthermia was accompanied by increased antibody binding to cellular DNA. Immunoreactivity of cells with the monoclonal antibody F7-26 may be a useful probe for the assessment of cell damage induced by alkylating agents, especially in heterogeneous cell populations.

N-hydroxyaminofluorene: a chemical probe for DNA conformation

The importance of the polymorphism of DNA in the reaction with the chemical carcinogen N-hydroxyaminofluorene is studied by means of a supercoiled plasmid containing an insert of (dC-dG). Immunochemical titration and the determination of the binding spectrum of -aminofluorene adducts show that the carcinogen reacts with B-DNA but not with Z-DNA and that conformational changes of the B-DNA-Z-DNA junctions occur as a function of the superhelical density.

DNA binding and mutation spectra of the carcinogen N-2-aminofluorene in Escherichia coli. A correlation between the conformation of the premutagenic lesion and the mutation specificity

When the chemical carcinogen N-2-acetylaminofluorene binds to DNA in vivo, two major adducts are formed, both at position C-8 of the guanine residue. One of these (the acetylaminofluorene adduct) retains the acetyl group, while the other (the aminofluorene adduct) is the corresponding deacetylated form. Unlike -AAF adducts, which trigger important structural changes of the DNA secondary structure (either the insertion-denaturation model or the induction of a Z-DNA structure, depending upon the local nucleotide sequence), -AF adducts bind to the C-8 of guanine residues without causing any major conformational change of the B-DNA structure. Well-defined adducts (either -AF or -AAF) can be formed in vitro by reacting DNA with either N-hydroxy-N-2-aminofluorene or N-acetoxy-N-2-acetylaminofluorene. Specific cleavage of the phosphodiester backbone at -AF adducts can be achieved by treating -AF-modified DNA in 1 M-piperidine at 90 degrees C. This observation led us to construct the spectrum for -AF binding to a defined DNA restriction fragment. It is found that only guanine residues react to form alkali-labile lesions and that the reactivity among the different guanines is similar. In a forward mutation assay, namely the inactivation of the tetracycline resistance gene, we found previously that more than 90% of mutations induced by -AAF adducts are frameshift mutations. Using the same assay, we show here that -AF adducts induce primarily base substitution mutations (85%), mainly of the G to T transversion type. There is therefore a strong correlation between the nature of the carcinogen-induced conformational change of the DNA structure and the corresponding mutation specificity. The -AF-induced base substitution mutations depend upon the umuC gene function(s). The data obtained in our forward mutation assay are compared to the data previously obtained in the histidine reversion assay (Ames test).

Immunological detection of lesions in DNA

The purpose of this paper is to show that the antibodies to nucleic acids, to nucleosides or to DNA damaged by a physical or a chemical agent, are useful tools in the study of DNA damage and repair. The results obtained with antibodies to nucleosides, antibodies to nucleosides and DNA modified by chemical carcinogens emphasize the potential of immunological methods in three main areas, a) the sensitive detection and quantitation of adducts; b) the visualization of adducts in tissues, individual cells, and along the DNA double helix; c) the study of conformational changes of DNA induced by adducts.

An internal fluorene model for iodo-N-2-acetylaminofluorene modified DNA

Minimized conformational potential energy calculations have been performed for the 7-iodo (AAIF) and 7-fluoro (AAFF) derivatives of N-2-acetylaminofluorene (AAF), linked covalently to guanine C-8 in dCpdG. Both the iodo and the fluoro derivatives are carcinogenic and mutagenic. The lowest energy forms on the dinucleoside monophosphate level have syn guanine and fluorene-cytidine stacking. However, the iodo adduct cannot adopt this conformation in larger polymers, according to earlier experimental studies (Fuchs et al., Biochemistry, 15 (1976) 3347) and model building, because of iodine's large Van der Waal's radius. Therefore, a model consistent with all the experimental evidence, incorporating the second lowest energy conformation in B form duplex (dCdG)3 was constructed. In this model the modified guanine is syn, yet still stacked with the adjacent cytidine in one direction, the fluorene is located primarily at the helix interior between the base pairing sites, rupturing two base pairs, and the iodine atom and its adjoining ring protrude to the helix exterior.

Z-DNA conformation of N-2-acetylaminofluorene modified poly(dG-dC).poly(dG-dC) determined by reactivity with anti cytidine antibodies and minimized potential energy calculations

The conformation of poly(dG-dC).poly(dG-dC), poly(dG).poly(dC), and calf thymus DNA modified with N-acetoxy-N-2-acetylaminofluorene (N-acetoxy-AAF) was examined by extent of reaction with anti cytidine antibodies. In contrast to modified poly(dG).poly(dC0 and DNA, modified poly(dG-dC).poly (dG-dC) failed to react with the antibodies indicating that the base pairing in this polymer is intact. This in consistent with induction of the Z-DNA conformation in AAF modified poly(dG-dC).poly(dG-dC). Using minimized potential energy calculations on the dCpdG-AAF dimer as a model for the modified polymer, it is shown that the proposed Z-DNA conformation is energetically stable. A model is proposed for an AAF modified tetramer, dGpdCpdGpdC, in which the AAF is external to the Z-DNA duplex.

Alternative conformations of DNA modified by N-2-acetylaminofluorene

Modification of DNA by the carcinogen N-acetoxy-N-2-acetylaminofluorene gives two adducts, a major one at the C-8 position of guanine and a minor one at the N-2 position with differing conformations. Binding at the C-8 position results in a large distortion of the DNA helix referred to as the "base displacement model" with the carcinogen inserted into the DNA helix and the guanosine displaced to the outside. The result is increased susceptibility to nuclease S1 digestion due to the presence of large, single-stranded regions in the modified DNA. In contrast, the N-2 adduct results in much less distortion of the helix and is less susceptible to nuclease S1 digestion. A third and predominant adduct is formed in vivo, the deacetylated C-8 guanine adduct. The conformation of this adduct has been investigated using the dimer dApdG as a model for DNA. The attachment of aminofluorene (AF) residues introduced smaller changes in the circular dichroism (CD) spectra of dApdG than binding of acetylaminofluorene (AAF) residues. Similarly, binding of AF residues caused lower upfield shifts for the H-2 and H-8 protons of adenine than the AAF residues. These results suggest that AF residues are less stacked with neighboring bases than AAF and induce less distortion in conformation of the modified regions than AAF. An alternative conformation of AAF-modified deoxyguanosine has been suggested based on studies of poly(dG-dC).(dG-dC). Modification of his copolymer with AAF to an extent of 28% showed a CD spectrum that had the characteristics of the left-handed Z conformation seen in unmodified poly(dG-dC).poly(dG-dC) at high ethanol or salt concentrations. Poly(dG-dC).poly(dG-dC) which does not undergo the B to Z transition at high ethanol concentrations, did not show this type of conformational change with high AAF modifications. Differences in conformation were suggested by single-strand specific nuclease S1 digestion and reactivity with anticytidine antibodies. Highly modified poly(GS-dC).poly(DG-dC) was almost completely resistant to nuclease S1 hydrolysis, while, modified DNa and poly(dG).poly(dC) are highly susceptible to digestion. Two possible conformations for deoxyguanosine modified at the C-8 position by AAF are compared depending on whether its position is in alternating purine-pyrimidine sequences or random sequence DNA.

'Non-specific' binding of lac repressor of E. coli to a carcinogen-modified DNA

The 'non-specific' binding of lac repressor to a N-hydroxyaminofluorene (N-OH-AF) modified DNA was studied using circular dichroism. The binding process was not altered by the modification of the DNA. A small increase of the binding constant was observed. These results are discussed with respect to the role of carcinogen modification in the protein-nucleic acid recognition phenomena.