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Nitrosamine-induced cancer: O4-alkylthymine produces sites of DNA hyperflexibility
P Georgiadis1, Y Z Xu, P F Swann
1Department of Biochemistry and Molecular Biology, University College London, U.K.
Abstract:
The carcinogenic properties of N-nitroso compounds are associated with their ability to alkylate DNA, in particular to form O6-alkylguanine and O4-alkylthymine. DNA duplexes containing either O6-alkylguanine or O4-alkylthymine were synthesized, and each duplex was ligated to form a set of DNAs of increasing length with the alkylated base out of phase (16 base-pairs apart) or in phase (21 base-pairs apart) with the helical repeat of the DNA. The DNA contained the sequence 5' CAA 3', which is the 61st codon of the K-ras gene, because this codon is a preferred site of mutation for a number of carcinogens including the methylating carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1- butanone (NNK). O4-Methylthymine or O4-ethylthymine replaced thymine in either of the two A.T base-pairs of this codon (normally CAA), and O6-methylguanine replaced the guanine in the G.C pair. All the sequences containing O4-alkylthymine exhibited anomalous, slow, gel migration and ligated to form circles of unusually small diameter. In general, the effect was seen when the alkylated base-pair was out of phase with the helical repeat as well as when it was in phase, suggesting that the alkylated base-pair confers flexibility which is largely isotropic, i.e., has no preferred direction, rather than anisotropic flexibility or bending. However, at pH 8.3 the 21-base-pair set containing O4-alkylT.A had significantly greater anomalous migration than the 16-base-pair set, suggesting that the flexibility produced by this base-pair has a significant anisotropic component and thus resembles true bending.(ABSTRACT TRUNCATED AT 250 WORDS)
Insights
Carcinogenic N-nitroso compounds alkylate DNA, forming O6-alkylguanine and O4-alkylthymine. Alkylated DNA duplexes exhibit altered flexibility, influencing DNA structure and potentially mutation sites like the K-ras gene.
Area of Science:
- Molecular Biology
- Chemical Carcinogenesis
- DNA Structure and Dynamics
Background:
- N-nitroso compounds are known carcinogens that exert their effects by alkylating DNA.
- Specific DNA adducts, such as O6-alkylguanine and O4-alkylthymine, are critical intermediates in chemical carcinogenesis.
- The K-ras gene's 61st codon (CAA) is a frequent mutation hotspot for various carcinogens, including NNK.
Purpose of the Study:
- To investigate the structural and dynamic consequences of specific DNA alkylation relevant to carcinogenesis.
- To synthesize and analyze DNA duplexes containing O4-alkylthymine and O6-alkylguanine adducts within the K-ras 61st codon.
- To determine how these adducts affect DNA conformation and flexibility, particularly in relation to helical repeat and potential bending.
Main Methods:
- Synthesis of DNA duplexes containing O4-methylthymine, O4-ethylthymine, or O6-methylguanine at specific positions within the K-ras 61st codon.
- Ligation of these duplexes into linear and circular forms of varying lengths (16 and 21 base-pairs).
- Analysis of DNA migration in gel electrophoresis to assess structural anomalies and flexibility.
- pH-dependent analysis to differentiate between isotropic and anisotropic flexibility.
Main Results:
- DNA duplexes containing O4-alkylthymine exhibited anomalous, slow gel migration and formed unusually small circles.
- This flexibility effect was observed regardless of whether the alkylated base-pair was in or out of phase with the DNA helical repeat, suggesting isotropic flexibility.
- At pH 8.3, a significant anisotropic component (bending) was detected in the O4-alkylT.A containing sequences, particularly in the 21-base-pair set.
Conclusions:
- O4-alkylthymine adducts confer significant flexibility to DNA, which is largely isotropic but can exhibit anisotropic characteristics (bending) under specific conditions (pH 8.3).
- This altered DNA flexibility may play a role in the mutagenic potential of N-nitroso compounds at critical sites like the K-ras gene.
- Understanding these structural changes is crucial for elucidating the mechanisms of chemical carcinogenesis.