Topoisomerase I-mediated cytotoxicity of N-methyl-N'-nitro-N-nitrosoguanidine: Trapping of topoisomerase I by the O6-methylguanine

Philippe Pourquier, Jessica L. Waltman, Yoshimasa Urasaki, Natalia A. Loktionova, Anthony E. Pegg, John L. Nitiss, Yves Pommier

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90 Scopus citations


Alkylating agents such as N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) are known to covalently link alkyl groups at the position 6 of guanines (O6MG) in DNA. O6-alkylguanine-DNA alkyltransferase (AGT) specifically removes the methyl group of the O6MG. Using purified human topoisomerase I (Top1), we found an 8-10-fold enhancement of Top1 cleavage complexes when O6MG is incorporated in oligonucleotides at the +1 position relative to a unique Top1 cleavage site. Top1 poisoning by O6MG is attributable to a decrease of the Top1-mediated DNA religation as well as an increase in the enzyme cleavage step. Increased cleavage is probably linked to a change in the hydrogen bonding pattern, such as in the case of the 8-oxoguanine, whereas inhibition of religation could be attributed to altered base pairing, such as a basic sites or base mismatches, because incorporation of a 6-thioguanine did not affect Top1 activity. Top1-DNA covalent complexes are also induced in MNNG-treated CHO cells constitutively lacking the AGT enzyme. Conversely, no increase could be detected in CHO cells transfected with the wild-type human AGT. Moreover, we show that yeasts overexpressing the human Top1 are more sensitive to MNNG, whereas knock-out Top1 strain cells display some resistance to the drug. Altogether, these results suggest a role for Top1 poisoning by alkylated bases in the antiproliferative activity of alkylating agents as well as in the DNA lesions resulting from endogenous and carcinogenic DNA modifications.

Original languageEnglish (US)
Pages (from-to)53-58
Number of pages6
JournalCancer Research
Issue number1
StatePublished - Jan 1 2001

All Science Journal Classification (ASJC) codes

  • Oncology
  • Cancer Research


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