Active oxygen species and carcinogenesis

It is now clear that certain initiators (e.g. radiation and some chemical carcinogens) produce, or bring about, the production of radicals [123-125], Moreover, active oxygen species such as superoxide radicals and hydrogen peroxide have been associated with the induction of cancer [126,127]. 

A carcinogen that can generate free radicals can, in principle, bring about the formation of modified bases such as thymine glycols in cellular DNA [128], but radicals can also cause single-strand and double-strand DNA breaks [129,130]. Critically however many radical-induced DNA damaging events can be abolished by free radical scavengers [131]. 

Whilst cells are normally equipped with extensive DNA repair systems these can sometimes become overwhelmed, or they may be defective for genetic reasons. Patients with genetic defects in their DNA repair systems (e.g. Xeroderma pigmentosum, Fanconi s syndrome, Bloom s syndrome) are predisposed to the development of cancer [132]. 

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