DNA and Double-Stranded Oligonucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 379

-radia-
tion (Yokoya et al. 2002). Here, additional DSBs are at least twice that of prompt
DSBs. In contrast, enzymatic treatment of DNA films by

Clustered lesions are also apparent when DNA films are subjected to γ

-radiation does not
raise the number of DSBs, and it has been suggested that in this case the com-
plexity of the clustered damage may be greater than that formed upon

α

-radia-
tion and that the enzymes can no longer adequately work on such sites (Yokoya
et al. 2003).
It is beyond the scope of this topic to present and discuss details of the de-
tection and repair of clustered lesions, but some papers that deal with these
questions are mentioned here. For example, the action of various enzymes on
clustered lesions have been investigated in some detail, such as E. coli EndoIII
(Chaudry and Weinfeld 1995), yeast and human Ogg1 proteins (David-Cordon-
nier et al. 2001a,b) as well as XRS5 nuclear extract and E. coli Nth and Fpg pro-
teins (David-Cordonnier et al. 2000; Gulston et al. 2002, 2004; Sutherland et al.
2000). The artificial clustered lesion, 8-oxo-G positioned on one strand and Ura
on the other at 1

γ

6 bp distance causes mutations when incorporated into wild
type E. coli (Pearson et al. 2004). Mutation frequencies were enhanced in strains
lacking the DNA glycosylases Fpg and MutY. The mutation rate decreases with
an increase in the distance between the two lesions. For another clustered lesion
involving 8-oxo-G and a SSB, the repair rate of the ligase III/XRCC1 complex was
markedly reduced (Lomax et al. 2004). An even more complex lesion consisting
of an 8-oxo-G and an 8-oxo-A on one strand and 5OHUra, 5FoUra and a one
nucleotide gap on the other strand was also constructed and its repair studied
(Eot-Houlier et al. 2005). Bistranded APs in close proximity are clustered lesions
that could turn into a DSB upon the action of repair enzymes. For this reason
the action of human Ape and E. coli Exo III on a variety of such lesions have been
investigated in some detail (Chaudry and Weinfeld 1997).

−

12 . 5.1
Tandem Lesions

The tandem lesion, such as two adjacent damaged bases at the same strand, is a
special case of a clustered lesion. Mechanistically they are very interesting, since
they are induced by a single radical, that is, they are due to a damage amplifica-
tion reaction. Mechanistic aspects are discussed in Chapters 10 and 11.2. Since
clustered lesions constitute a severer damage than a single lesion, it is conceiv-
able that 'repair' of DNA radicals by thiols (Sect. 12.12) that reduces the severity
of damage, especially in the presence of O 2 , also interferes with the formation
of tandem lesions. To our knowledge, this has not yet been investigated. The
longevity of DNA peroxyl radicals allow intramolecular reactions to occur that
would be very much less pronounced on the nucleotide level. This may be con-
cluded from the observation that • OH-induced O 2 -uptake is noticeably higher in
DNA than in an equivalent mixture of nucleotides (Isildar et al. 1982).
One of these tandem lesions in DNA consist of a Fo (precursor: T or C) next
to an 8-oxo-G lesion (Box et al. 2000; Maccubbin et al. 2000; Bourdat et al. 2000;
Cadet et al. 2002). Within DNA, two types of this lesion are expected, the 8-
oxo-G/Fo and the Fo/8-oxo-G lesion. They are formed in considerably different