Published online 5 May 2006
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Genetic effects of oxidative DNA damages: comparative mutagenesis of the imidazole ring-opened formamidopyrimidines (Fapy lesions) and 8-oxo-purines in simian kidney cells
Department of Chemistry, University of Connecticut Storrs, CT 06269, USA 1 Department of Chemistry, Johns Hopkins University Baltimore, MD 21218, USA 2 Biology Department, Boston University Boston, MA 02118, USA 3 Department of Pharmacological Sciences, State University of New York Stony Brook, NY 11794, USA
*To whom correspondence should be addressed. Tel: +1 860 486 3965; Fax: +1 860 486 2981; Email: ashis.basu{at}uconn.edu
Received December 1, 2005. Revised January 9, 2006. Accepted March 7, 2006.
Fapy·dG and 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dG) are formed in DNA by hydroxyl radical damage. In order to study replication past these lesions in cells, we constructed a single-stranded shuttle vector containing the lesion in 5'-TGT and 5'-TGA sequence contexts. Replication of the modified vector in simian kidney (COS-7) cells showed that Fapy·dG is mutagenic inducing primarily targeted Fapy·G
T transversions. In the 5'-TGT sequence mutational frequency of Fapy·dG was 
30%, whereas in the 5'-TGA sequence it was 8%. In parallel studies 8-oxo-dG was found to be slightly less mutagenic than Fapy·dG, though it also exhibited a similar context effect: 4-fold GT transversions (24% versus 6%) occurred in the 5'-TGT sequence relative to 5'-TGA. To investigate a possible structural basis for the higher GT mutations induced by both lesions when their 3' neighbor was T, we carried out a molecular modeling investigation in the active site of DNA polymerase ß, which is known to incorporate both dCTP (no mutation) and dATP (GT substitution) opposite 8-oxo-G. In pol ß, the syn-8-oxo-G:dATP pair showed greater stacking with the 3'-T:A base pair in the 5'-TGT sequence compared with the 3'-A:T in the 5'-TGA sequence, whereas stacking for the anti-8-oxo-G:dCTP pair was similar in both 5'-TGT and 5'-TGA sequences. Similarly, syn-Fapy·G:dATP pairing showed greater stacking in the 5'-TGT sequence compared with the 5'-TGA sequence, while stacking for anti-Fapy·G:dCTP pairs was similar in the two sequences. Thus, for both lesions less efficient base stacking between the lesion:dATP pair and the 3'-A:T base pair in the 5'-TGA sequence might cause lower GT mutational frequencies in the 5'-TGA sequence compared to 5'-TGT. The corresponding lesions derived from 2'-deoxyadenosine, Fapy·dA and 8-oxo-dA, were not detectably mutagenic in the 5'-TAT sequence, and were only weakly mutagenic (<1%) in the 5'-TAA sequence context, where both lesions induced targeted AC transversions. To our knowledge this is the first investigation using extrachromosomal probes containing a Fapy·dG or Fapy·dA site-specifically incorporated, which showed unequivocally that in simian kidney cells Fapy·GT substitutions occur at a higher frequency than 8-oxo-GT and that Fapy·dA is very weakly mutagenic, as is 8-oxo-dA.
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