DNA and Double-Stranded Oligonucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 345

12 .1
General Remarks
There are various agents that may induce DNA damage via free radicals
(Chap. 2). The effects of ionizing radiation have been most widely studied. Here,
DNA damage is induced by • OH (H • and e aq − ). These reactions dominate when
DNA is irradiated in dilute aqueous solution. However in cells or with DNA in
aqueous solution that contain high • OH-scavenger concentration, a second phe-
nomenon plays a role, the direct effect (ionization of DNA; see below). Ionizing
radiation is deposited in small packages (spurs, tracks; Chap. 2.2) containing
several radicals. This causes the formation of clustered lesions. Yet, some so-
called radiomimetic drugs are also capable of producing at least two radicals in
close proximity. Moreover, there are damage amplification reactions that result
in more than one damaged site resulting from the attack by just one • OH radical.
Fenton-type reactions (H 2 O 2 + low-valent transition ions) may produce • OH site
specifically, when the transition-metal ion is bound to DNA, and with this free-
radical source one has to take into account that H 2 O 2 and the transition-metal
ions can modify the • OH-induced damage (Sect. 12.8).
The main types of damage that can be formed in DNA (base damage, apy-
rimidnic/apurinic (AP) site, single-strand break (SSB), double-strand break
(DSB), tandem lesions and various clustered lesions) are shown schematically in
Fig. 12.1. There are, however, further lesions such as DNA/DNA and DNA/pro-
tein cross-links.
As to the severity of the above lesions, it is important to note that single base
damages, SSBs and even DSBs do not necessarily constitute a lethal damage (Ta-
ble 12.1).
Also, in the treatment with BLM, 150 SSBs and 30 DSBs are formed at the LD 37
level of survival (Ward 1988). It has hence be concluded that the cellular repair
enzymes can cope very efficiently with these simple lesions, and much more com-
plex lesions (clustered lesions, above the level of a DSB, see below) are required to
cause a lethal event. This is supported by the observation that densely ionizing
radiations show a higher RBE then low-LET radiation (Goodhead 1994).
Various tools have been used to study quite specifically DNA damage, e.g.,
substituting Thy by a halouracil such as 5BrUra or 5IUra. Some of the radio-
mimetic drugs also react with DNA very specifically and so do some inorganic
radicals. Charge transfer through DNA is another aspect that leads to the prefer-
ential formation of damage at certain sites.
In cells, DNA is always surrounded by proteins (cf. Fig. 12.2). The attach-
ment of proteins modifies the DNA damage by various effects: hampering an
easy access of free-radicals, protection by compaction and repair of some of the
damage by electron/hydrogen-donation. On the other hand, it may even induce
some DNA damage via reactive intermediates formed upon the reaction of free
radicals with the proteins. For this reason, the structure of DNA in solution and
in the cellular environment will also have to be addressed.
Some of the aspects that are dealt with in this chapter have been reviewed
in much more detail than it will be possible here, and reference is thus made to
these reviews: the state of the art in the 1970s and 1980s (Blok and Loman 1973;