An efficient route to N6 deoxyadenosine adducts of diol epoxides of carcinogenic polycyclic aromatic hydrocarbons

Hypothesis-based weight-of-evidence evaluation and risk assessment for naphthalene carcinogenesis

Inhalation of naphthalene causes olfactory epithelial nasal tumors in rats (but not in mice) and benign lung adenomas in mice (but not in rats). The limited available human data have not identified an association between naphthalene exposure and increased respiratory cancer risk. Assessing naphthalene's carcinogenicity in humans, therefore, depends entirely on experimental evidence from rodents. We evaluated the respiratory carcinogenicity of naphthalene in rodents, and its potential relevance to humans, using our Hypothesis-Based Weight-of-Evidence (HBWoE) approach. We systematically and comparatively reviewed data relevant to key elements in the hypothesized modes of action (MoA) to determine which is best supported by the available data, allowing all of the data from each realm of investigation to inform interpretation of one another. Our analysis supports a mechanism that involves initial metabolism of naphthalene to the epoxide, followed by GSH depletion, cytotoxicity, chronic inflammation, regenerative hyperplasia, and tumor formation, with possible weak genotoxicity from downstream metabolites occurring only at high cytotoxic doses, strongly supporting a non-mutagenic threshold MoA in the rat nose. We also conducted a dose-response analysis, based on the likely MoA, which suggests that the rat nasal MoA is not relevant in human respiratory tissues at typical environmental exposures. Our analysis illustrates how a thorough WoE evaluation can be used to support a MoA, even when a mechanism of action cannot be fully elucidated. A non-mutagenic threshold MoA for naphthalene-induced rat nasal tumors should be considered as a basis to determine human relevance and to guide regulatory and risk-management decisions.

Sequence context modulation of polycyclic aromatic hydrocarbon-induced mutagenesis

DNA structural perturbations that are induced by site specifically and stereospecifically defined benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide (BPDE) adducts are directly correlated with mutagenesis, leading to cellular transformation. Although previous investigations had established that replication of DNAs containing N(6) -BPDE dA adducts at the second position in the N-ras codon 61(CAA) (61(2) ) resulted exclusively in A to G transitions, NMR analyses not only established the structural basis for this transition mutation but also predicted that if the adduct were positioned at the third position in the same codon, an expanded spectra of mutations was possible. To test this prediction, replication of DNAs containing C10 S-BPDE and C10 R-BPDE lesions linked through the N(6) position of adenine in the sequence context N-ras codon 61, position 3 (C10 S-BPDE and C10 R-BPDE at 61(3) ) was carried out in Escherichia coli, and these data revealed a wide mutation spectrum. In addition to A to G transitions produced by replication of both lesions, replication of the C10 S-BPDE and C10 R-BPDE adducts also yielded A to C and A to T transversions, respectively. Analyses of single nucleotide incorporation using Sequenase 2.0 and exonuclease-deficient E. coli Klenow fragment and pol II not only revealed high fidelity synthesis but also demonstrated the same hierarchy of preference opposite a particular lesion, independent of the sequence context. Primer extension assays with the two lesions at N-ras 61(3) resulted in truncated products, with the C10 S-BPDE adducts being more blocking than C10 R-BPDE lesions, and termination of synthesis was more pronounced at position 61(3) than at 61(2) for each of the lesions.

Inhibition of HIV-1 reverse transcriptase-catalyzed synthesis by intercalated DNA Benzo[a]Pyrene 7,8-Dihydrodiol-9,10-Epoxide adducts

To aid in the characterization of the relationship of structure and function for human immunodeficiency virus type-1 reverse transcriptase (HIV-1 RT), this investigation utilized DNAs containing benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide (BPDE)-modified primers and templates as a probe of the architecture of this complex. BPDE lesions that differed in their stereochemistry around the C₁₀ position were covalently linked to N⁶-adenine and positioned in either the primer or template strand of a duplex template-primer. HIV-1 RT exhibited a stereoisomer-specific and strand-specific difference in replication when the BPDE-lesion was placed in the template versus the primer strand. When the C₁₀R-BPDE adduct was positioned in the primer strand in duplex DNA, 5 nucleotides from the 3΄ end of the primer terminus, HIV-1 RT could not fully replicate the template, producing truncated products; this block to further synthesis did not affect rates of dissociation or DNA binding affinity. Additionally, when the adducts were in the same relative position, but located in the template strand, similar truncated products were observed with both the C₁₀R and C₁₀S BPDE adducts. These data suggest that the presence of covalently-linked intercalative DNA adducts distant from the active site can lead to termination of DNA synthesis catalyzed by HIV-1 RT.

Synthesis of N6 ,N6-Dialkyl Adenine Nucleosides With In Situ Formed Hexaalkylphosphorus Triamides

Reactions between secondary amines and phosphorus trichloride (PCl₃) leads to the formation of the corresponding tris(dialkylamino)phosphines or hexaalkylphosphorus triamides [HAPT: (R₂N)₃P]. Reaction of silyl-protected 2'-deoxyinosine and acetyl-protected inosine with the in situ formed HAPT and iodine (I₂) leads to the formation of N⁶,N⁶-dialkyl adenosine and 2'-deoxyadenosine. In some cases the stoichiometry of the amine is important as also the use of a tertiary amine base. The effect of amine stoichiometry on the reaction of HAPT with I₂ has been studied by ³¹P{¹H} NMR.

Biological properties of single chemical-DNA adducts: a twenty year perspective

The genome and its nucleotide precursor pool are under sustained attack by radiation, reactive oxygen and nitrogen species, chemical carcinogens, hydrolytic reactions, and certain drugs. As a result, a large and heterogeneous population of damaged nucleotides forms in all cells. Some of the lesions are repaired, but for those that remain, there can be serious biological consequences. For example, lesions that form in DNA can lead to altered gene expression, mutation, and death. This perspective examines systems developed over the past 20 years to study the biological properties of single DNA lesions.

Synthesis of the minor acrolein adducts of 2(')-deoxyguanosine and their generation in oligomeric DNA

Acrolein, a known mutagen, undergoes reaction in vitro under physiological conditions with both 2(')-deoxyguanosine and native DNA to give rise to exocyclic adducts of the 5,6,7,8-tetrahydropyrimido[1,2-a]purine-10(3H)-one class having an hydroxy group at either the 6 or the 8 position. Previously we have shown that the 8-hydroxy derivative in a bacterial system has very low mutagenicity probably because in double-stranded DNA this residue exists in the open-chain aldehydic form [N(2)-(3-oxopropyl)-2(')-deoxyguanosine] (3). To continue our investigation in this area, we needed ample supplies of the 6-hydroxy isomers. This current paper describes high-yield simple methods for the synthesis in bulk of the 6-hydroxy adduct 1 and its incorporation into DNA oligomers. The basic methods for the synthesis of the adduct 1, involve 1-substitution of dG derivatives with a 3-butenyl group, dihydroxylation of the olefin with osmium tetroxide and N-methylmorpholine N-oxide, then diol cleavage with periodate ion after incorporation of the 1-(3,4-diacetoxybutyl)-2(')-deoxyguanosine into oligomeric DNA.

Utilization of tetrabutylammonium triphenyldifluorosilicate (TBAT) in the synthesis of 6-fluoropurine nucleosides

Tetrabutylammonium triphenydifluorosilicate (TBAT) has been found to be a useful reagent for the conversion of 6-chloropurine nucleosides to 6-fluoropurine derivatives. The 6-chloropurine nucleosides were reacted with trimethylamine to form quaternary trimethylammonium salts which were treated in situ with TBAT in DMF to effect conversion to the 6-fluoro derivatives in yields of 59-72%.

Highly mutagenic bypass synthesis by T7 RNA polymerase of site-specific benzo[a]pyrene diol epoxide-adducted template DNA

We have previously developed an in vitro system that allows quantitative evaluation of the fidelity of transcription during synthesis on a natural template in the presence of all four nucleotides. Here, we have employed this system using a TAA ochre codon reversion assay to examine the fidelity of transcription by T7 RNA polymerase past an adenine residue adducted at the N6-position with (-)-anti-trans- or (+)-anti-trans-benzo[a]pyrene diol epoxide (BPDE). T7 RNAP was capable of transcribing past either BPDE isomer to generate full-length run-off transcripts. The extent of bypass was found to be 32% for the (-)-anti-trans-isomer and 18% for the (+)-anti-trans-isomer. Transcription past both adducts was highly mutagenic. The reversion frequency of bypass synthesis of the (-)-anti-trans-isomer was elevated 11,000-fold and that of the (+)-anti-trans-isomer 6000-fold, relative to the reversion frequency of transcription on unadducted template. Adenine was misinserted preferentially, followed by guanine, opposite the adenine adducted with either BPDE isomer. Although base substitution errors were by far the most frequent mutation on the adducted template, three- and six-base deletions were also observed. These results suggest that transcriptional errors, particularly with regard to damage bypass, may contribute to the mutational burden of the cell.

Lack of correlation between in vitro and in vivo replication of precisely defined benz-a-anthracene adducted DNAs

Like other polycyclic aromatic hydrocarbons, certain metabolites of benz[a]anthracene have been implicated as potent carcinogens. These effects are thought to be caused by the covalent binding of these species to nucleophilic groups on the bases of DNA. To address the molecular mechanisms by which these molecules induce mutations, this study employed oligonucleotides containing four site-specific N6 adenine-benz[a]anthracene diol epoxide adducts. Using a prokaryotic in vivo replication system, we have shown that both non-bay region anti-trans-benz[a]anthracene adducts are essentially nonmutagenic. In contrast, the bay region anti-trans-benz[a]anthracene lesions do induce point mutations at the adduct site. The mutagenic frequency of these bay region lesions is dependent on the stereochemistry about the adduct-forming bond, as well as the strain of Escherichia coli in which they are replicated. The ability of the bacterial replication machinery to bypass the lesions does not correlate with the differences observed in their mutagenesis. While both non-bay region adducts are readily bypassed in vivo, the bay region adducts are both blocking to approximately the same degree. In vitro studies of the interactions of E. coli DNA polymerase III with these adducts have also been undertaken to further dissect the relationship between adduct structure and biological activity.