**8. Conclusion**

256 Selected Topics in DNA Repair

and purine biosynthesis and induces epigenetic changes leading to global DNA hypomethylation and/or changes in gene-specific methylation and protooncogene activation (Duthie and McMillan, 1997; Duthie, 2010; Duthie *et al.*, 2000; Melnyk et al., 1999). In vivo, folate deficiency induced DNA repair, such as increase of OGG1 and MGMT (Duthie *et al.*, 2010). However, remains unclear if increasing folate intake improve DNA repair. Generaly, the protective role of folate against carcinogenesis is not completely

**7. Effects of phytochemicals through DNA repair modulation and their** 

Some DNA-damaging agents (specially alkylating agents) are used in cancer therapy due to their ability to induce DNA damage and subsequently apoptosis of tumor cells (Maynard et al., 2009). The efficacy of DNA damage-based cancer therapy is modulated by DNA repair pathways. Therefore these pathways may, attenuate the therapeutic effects of alkylating agents. These drugs are usually classified in two classes: monofunctional (e.g., N-methyl-N\_-nitro-N-nitrosoguanidine [MNNG], temozolomide [TMZ] and dacarbazine) and bifunctional alkylating agents (e.g., carmustine [BCNU], cyclophosphamide, lomustine

BCNU, induces several kinds of DNA damage such as cross-linking, strand breaks and modified bases (Kondo et al., 2010). Between alkylation damage, N7 alkyl-guanine is the most abundant (around 90% of the total alkylation events) and O6 alkyl-guanine is the less frequent. O6 alkyl-guanine, if not repaired before cell division, can form base pairs with T, generating GCAT transitions mutations by action of MMR pathway. O6 alkyl-guanine is the lesion mainly responsible for the cytotoxic and mutagenic effects of these alkylating drugs. However O6 alkyl-guanine can be repaired by MGMT. In human colorectal adenoma, reduced MGMT activity has been found. Therefore, more mutations occur when cells are treated with alkylating agent (Lees *et al.*, 2004; Lees *et al.*, 2002). However, high levels of MGMT where found in some other tumors (Baer et al., 1993). In the context of therapy with alkylating agents, inhibition of MGMT activity in tumor cells is desirable. The coadjuvant drug O6-benzylguanine (O6-BG) inactivates MGMT, acting as a pseudosubstrate. Effect of O6-BG has been investigated when in combination with an alkylating drug to increase its

N7-alkylG is the most frequent alkylation damage however is not considered to be as mutagenic as O6-alkylG because it is efficiently repaired by BER pathway. N-methylpurine-DNAglycoslase (MPG), the only glycosylase that recognizes alkylation lesions in animal cells, removes N7-alkylG by hydrolysis of the N-glycosylic bond creating an AP site that is repaired by the other enzymes of BER pathway (Drablos et al., 2004). An overexpression of MPG increases the production of AP sites. If the levels of the other enzymes of BER pathway remain unaltered, repair of AP sites is low and accumulation of these lesions becomes cytotoxic and mutagenic. Therefore, an overexpression of MPG or a decrease in other enzymes involved in the subsequent steps in BER pathway, increases cytotoxicity of alkylating drugs. This cytoxicity becomes more relevant when cells are resistant to

MGMT and MMR have contrasting effects on DNA O6-alkylG. While MGMT is an efficient mechanism of repair, MMR in contrast, does not remove the alkylated base but introduce more lesions like strand break in an attempt to repair the mismatch. Accumulation of strand

**interaction with alkylating agents used as chemotherapeutic** 

understood.

[CCNU] and fotemustine).

efficacy (Liu *et al.*, 2002).

cytotoxicity from O6-alkylation.

Prevention of DNA damage and/or enhanced DNA repair activity by dietary agents constitute an important strategy to prevent mutations and consequently inhibit the carcinogenic process. Diet supplementation with phytochemicals, with beneficial effects, increases their concentrations in the organism. However, effects of supra-physiological concentrations need to be evaluated in each case since a safe phytochemical at physiological concentrations can be toxic at higher concentrations.

The comet assay is a powerful tool to assess the effects of diet on DNA. The immense literature (more than 5500 papers in Pubmed since 1995) that use comet assay (standard and modified versions) demonstrates the real potential of this assay. Some dietary agents have

DNA Damage Protection and Induction of Repair

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base excision repair and nucleotide excision repair in humans are influenced by

reduce oxidative DNA damage and lipid peroxidation and quench free radicals.

shown ability to prevent DNA damage (oxidative and/or alkylanting) in several experimental models from in vitro to human intervention studies. Less is known about the effect of diet on DNA repair modulation. However, the modifications of comet assay (e.g. use specific-enzymes) gives an opportunity to enhance the knowledge in this field. In spite of the large number of publications much remains to be done. An emergent field is the effect of combinations between diet and drugs used in chemotherapy. In our view, comet assay can be a useful approach also to understand the role of dietary constituents on chemotherapy.
