**3.2 Nucleotide Excision Repair**

NER is another important repair pathway involved in DNA adduct repair (e.g. thymidine dimers and 6–4 photoproducts) that are induced by ultraviolet radiation, chemicals or ROS (Benhamou and Sarasin, 2000). These adducts change the normal structure of the DNA helix, breaking transcription and replication processes. Two distinct NER sub-pathways, transcription coupled repair (TCR) and global genomic repair (GGR), have been described. Briefly, TCR repair transcription-blocking lesions present in transcribed DNA strands; and GGR pathway repairs lesions over the bulk genome including non-transcribed strands of active genes (Knudsen et al., 2009). Lesions like thymidine dimmers are usually repaired by

DNA Damage Protection and Induction of Repair

aberrant crypt foci (ACF) (Bugni *et al.*, 2009; Wali *et al.*, 1999).

free process (Dudas and Chovanec, 2004; Helleday *et al.*, 2007).

**4. Chemopreventive activities of dietary phytochemicals** 

and alkylating agents will be covered by this review.

and colorectum (WCRF, 2007).

**3.5 Homologous recombination and Non-homologous end-joining repair** 

by Dietary Phytochemicals and Cancer Prevention: What Do We Know? 243

resistance of tumors to alkylating agents toxicity (Eker *et al.*, 2009; Nystrom and Mutanen, 2009). Removal of the methyl group from O6MeG by MGMT is dependent on the rate of MGMT syntheses which is induced in response to DNA damage (Doak et al., 2008). Depletion of MGMT by reducing MGMT activity or decreasing gene expression can occur using a specific inhibitor O6-benzylguanine (BG) or through epigenetic silencing, (Eker et al., 2009). Inhibition of MGMT with BG in rats increases azoxymethane (AOM)-induced colon tumors, and transgenic expression of MGMT in mice protects against AOM-induced

In mammalian cells double strand breaks (DSBs), one of the most deleterious damage, can be repaired by two different types of mechanism: 1) non-homologous end-joining (NHEJ) that rejoins the two broken ends in a template independent way with concomitant loss of sequence information. After overlapping of the two DNA ends, the ligase IV complex start the ligation process of the two broken ends; and 2) homologous recombination repair (HR) that uses a homologue undamaged DNA sequence (sister-chromatid or homologous chromosome) to repair the missing sequence between the two DNA ends. HR is an error-

If not properly repaired DSB can cause loss of chromosomes and consequently generate mutations with or without induction of cell death (Dudas and Chovanec, 2004). Singlestrand breaks repair (SSBR) is a DNA repair pathway extremely important to avoid the deleterious effects of single-strand breaks (for more detail see the review Caldecott, 2007). Since DNA damage is recognized as the initial step in chemical carcinogenesis, inhibition of DNA damage and/or induction of repair would be the first line of defense against cancer caused by carcinogens. Chemoprevention by diet and dietary constituints against oxidative

Diet and lifestyle play crucial roles in cancer aetiology. Nowadays, the idea that prevention of any disease is preferable over treatment is accepted by all. In this context, in the last decades, several studies suggest that regular consumption of fruits, vegetables and spices have health benefits including risk reduction of developing a cancer, namely, colon cancer (Terry et al., 2001). Much of the protective effects have been attributed to phytochemicals such as polyphenols, terpenes and alkaloids, present in low levels in plants (Barth et al., 2005). For instance, flavonoids (polyphenolic compounds) have been reported to possess potential on prevention of several cancers specially cancers of gastrointestinal tract, like oral cavity and colon cancer (Kuo, 1996). The World Cancer Research Fund (WCRF) in its report about diet and prevention of cancer in 2007, mentioned that fruits and vegetables in general probably protect against cancers of the mouth, pharynx, larynx, oesophagus, lung, and stomach and there are limited evidences that suggest protective effects of fruits against cancers of the nasopharynx, pancreas, liver,

The use of plants for the prevention of diseases is an ancient practice. However, it was in the last decades that scientific community started to show interest in the medicinal properties of plants. The first scientific evidences showing that vegetables and fruits might be protective against some cancers emerged in the 1990s. Nevertheless, twenty year on no consensus

TCR, while other lesions as 6–4 photoproducts are efficiently repaired by GGR (for review see Hanawalt, 2002; Nouspikel, 2009).

#### **3.3 Mismatch Repair**

MMR is a post-replicative DNA repair mechanism that mainly corrects base–base mismatches which are caused by errors of DNA polymerases and insertion/deletion loops (IDLs). Two complexes are responsible for the repair initiation, MutSα (MSH2/MSH6) and MutSβ (MSH2/MSH3) complexes. MutSα recognize base–base mismatches and small IDLs (with one or two extrahelical nucleotides) while MutSβ recognize the larger IDLs. Repair of the new synthesized strand give the DNA polymerase the chance to generate an error-free copy of the template sequence, protecting cells from point mutations and possibly cancer development (Jiricny, 2006; Knudsen *et al.*, 2009). Loss of MMR function prevents the correction of replicative errors, leading to instability of the genome (Davis and Milner, 2007). As more details about NER and MMR pathways are known, relation between deficiencies on these pathways and cancer become stronger (Hegde et al., 2008).

DNA repair pathways have an important role during all carcinogenic process and in its treatment. Defects in DNA-repair pathways, like MMR, BER and NER, lead to an accumulation of mutations and consequently to carcinogenesis (Jiricny and Marra, 2003). Some of these pathways are inactivated due to mutation or epigenetic modifications in some human cancer, for instance, loss of MMR is observed in 15% of sporadic colorectal cancers (Casorelli *et al.*, 2008).

#### **3.4 Direct damage reversal repair**

Human cells have several DNA repair mechanisms that are capable of correcting specific types of alkylating damage. O6MeG lesions are repaired by direct damage reversal repair carried out by MGMT also referred to as alkylguanine transferase (AGT). Cells with deficient repair of O6MeG by MGMT are hypersensitive to chromosome aberration induced by alkylating agents (Armstrong and Galloway, 1997). MGMT is a key suicide enzyme that efficiently repairs O6MeG before replication, through direct transfer of the adducted methyl group from the oxygen in the guanine to a cysteine residue in the catalytic site of MGMT. O6MeG is highly mutagenic and carcinogenic because it possess a high potential to mispair with thymine during replication. This lesion is read as adenine by DNA polymerases causing GC-AT transitions (Eker *et al.*, 2009). The toxicity of the O6MeG lesion is attributed to the recognition of O6MeG:T mispairs by the MMR pathway that remove the new thymine. In the next round of replication another thymine mispairs with O6MeG that will be removed by MMR. Recognition by MMR creates a gap in DNA by incision on the new replicated strand. If O6MeG remains in one of the template strands the repair process will be repeated, creating a "futile repair loop". This loop will eventually result in toxic double-strand breaks leading to chromosomal aberrations, cell-cycle arrest or apoptosis (Bugni *et al.*, 2009; Kaina *et al.*, 2007; Kaina *et al.*, 2010; Kondo *et al.*, 2010). When both of these systems fail to repair, O6MeG results in point mutations that can possibly initiate the carcinogenic process. Beyond the ability to remove methyl adducts, MGMT can also remove larger adducts such as, ethyl, propyl and butyl adducts, however at a lower efficiency (Doak et al., 2008).

Some authors mention the important role of MGMT in protection against sporadic human colorectal cancer, once that epigenetic silencing of MGMT gene was observed in 50% of these cancers (Lind et al., 2004). MGMT expression in tumor cells have been related with the

TCR, while other lesions as 6–4 photoproducts are efficiently repaired by GGR (for review

MMR is a post-replicative DNA repair mechanism that mainly corrects base–base mismatches which are caused by errors of DNA polymerases and insertion/deletion loops (IDLs). Two complexes are responsible for the repair initiation, MutSα (MSH2/MSH6) and MutSβ (MSH2/MSH3) complexes. MutSα recognize base–base mismatches and small IDLs (with one or two extrahelical nucleotides) while MutSβ recognize the larger IDLs. Repair of the new synthesized strand give the DNA polymerase the chance to generate an error-free copy of the template sequence, protecting cells from point mutations and possibly cancer development (Jiricny, 2006; Knudsen *et al.*, 2009). Loss of MMR function prevents the correction of replicative errors, leading to instability of the genome (Davis and Milner, 2007). As more details about NER and MMR pathways are known, relation between deficiencies

DNA repair pathways have an important role during all carcinogenic process and in its treatment. Defects in DNA-repair pathways, like MMR, BER and NER, lead to an accumulation of mutations and consequently to carcinogenesis (Jiricny and Marra, 2003). Some of these pathways are inactivated due to mutation or epigenetic modifications in some human cancer, for instance, loss of MMR is observed in 15% of sporadic colorectal cancers

Human cells have several DNA repair mechanisms that are capable of correcting specific types of alkylating damage. O6MeG lesions are repaired by direct damage reversal repair carried out by MGMT also referred to as alkylguanine transferase (AGT). Cells with deficient repair of O6MeG by MGMT are hypersensitive to chromosome aberration induced by alkylating agents (Armstrong and Galloway, 1997). MGMT is a key suicide enzyme that efficiently repairs O6MeG before replication, through direct transfer of the adducted methyl group from the oxygen in the guanine to a cysteine residue in the catalytic site of MGMT. O6MeG is highly mutagenic and carcinogenic because it possess a high potential to mispair with thymine during replication. This lesion is read as adenine by DNA polymerases causing GC-AT transitions (Eker *et al.*, 2009). The toxicity of the O6MeG lesion is attributed to the recognition of O6MeG:T mispairs by the MMR pathway that remove the new thymine. In the next round of replication another thymine mispairs with O6MeG that will be removed by MMR. Recognition by MMR creates a gap in DNA by incision on the new replicated strand. If O6MeG remains in one of the template strands the repair process will be repeated, creating a "futile repair loop". This loop will eventually result in toxic double-strand breaks leading to chromosomal aberrations, cell-cycle arrest or apoptosis (Bugni *et al.*, 2009; Kaina *et al.*, 2007; Kaina *et al.*, 2010; Kondo *et al.*, 2010). When both of these systems fail to repair, O6MeG results in point mutations that can possibly initiate the carcinogenic process. Beyond the ability to remove methyl adducts, MGMT can also remove larger adducts such as, ethyl,

propyl and butyl adducts, however at a lower efficiency (Doak et al., 2008).

Some authors mention the important role of MGMT in protection against sporadic human colorectal cancer, once that epigenetic silencing of MGMT gene was observed in 50% of these cancers (Lind et al., 2004). MGMT expression in tumor cells have been related with the

on these pathways and cancer become stronger (Hegde et al., 2008).

see Hanawalt, 2002; Nouspikel, 2009).

**3.3 Mismatch Repair** 

(Casorelli *et al.*, 2008).

**3.4 Direct damage reversal repair** 

resistance of tumors to alkylating agents toxicity (Eker *et al.*, 2009; Nystrom and Mutanen, 2009). Removal of the methyl group from O6MeG by MGMT is dependent on the rate of MGMT syntheses which is induced in response to DNA damage (Doak et al., 2008). Depletion of MGMT by reducing MGMT activity or decreasing gene expression can occur using a specific inhibitor O6-benzylguanine (BG) or through epigenetic silencing, (Eker et al., 2009). Inhibition of MGMT with BG in rats increases azoxymethane (AOM)-induced colon tumors, and transgenic expression of MGMT in mice protects against AOM-induced aberrant crypt foci (ACF) (Bugni *et al.*, 2009; Wali *et al.*, 1999).
