**6. Conclusions**

136 Selected Topics in DNA Repair

The MMR system is responsible for the post-replicative repair of mismatches and small single stranded DNA loops, and is essential to the prevention of recombination between homologous DNA sequences. Although MMR repair in eukaryotic cells is not fully understood, studies of this mechanism in bacteria have contributed considerably to our understanding of this pathway. MMR is dependent on three highly conserved proteins: MutS, MutL, and MutH. In eukaryotes, homologous proteins as PCNA, RPA, HMGB1, RFC, and ligase I participate and coordinate the MMR process. Eukaryotic cells that are deficient in MMR have a 10-1,000-fold increase in mutation rates (Kunkel & Erie, 2005). In another study, MMR deficient mice showed an increase in PAH-induced lymphoma compared to controls (Zienolddiny, et al., 2006). MMR corrects strand-specific base mispairs and small loop structures. Furthermore, this mechanism recognizes smaller base analoges as mispairs that are participating in oxidative DNA damage repair. Cells deficient in the hMLH1 protein (essential for MMR initiation) have a reduced repair activity when exposed to anthracene (Desler, et al., 2009). This result indicates that MMR has a probable role in DNA repair

Further studies are necessary to confirm two other possible mechanisms involved in DNA repair following PAH exposure: non-homologous end joining (NHEJ) and transcription coupled repair (TCR). NHEJ appears to participate in DNA repair after PAH exposure. A recent report described that the V3-3 CHO cell line (NHEJ-deficient), showed a statistically significant dose-dependent enhancement during induced sister chromatid exchanges under PAH exposure. This result indicates that this process is important in DNA repair (Meschini,

Some adducts perform alterations to the transcription process. The current model for TCR is dependent on the stalling of RNA pol II. Some PAHs such as BPDE and B[c]PhDE produce adducts repaired by TCR, while other studies indicate that PAHs such as B[g]Ch-DE and DB[a,l]P-DE also produce adducts. The efficiency and rate for repairing DNA depends of intrinsic adduct properties (Zhong, et al., 2010). For a complete review of this process we

DNA damage associated with PAH exposure is mainly assessed by measuring the number of adducts formed. Several methods, such as 32P-postlabelling or immunochemical analysis with specific antibodies, have been applied extensively. However, the arrival of new molecular techniques and innovations in previously well-established biochemical methods, have increased the possibility of the early detection of DNA damage. Polymorphisms and gene expression analysis are helping to determine susceptibility of individuals and populations. Use of mass spectrometry and high performance liquid chromatography (HPLC) with increased sensitivity is another common tool to evaluate and quantify adduct formation. A brief description of the available methods to detect DNA damage is listed in

On the other hand, some studies have proposed using molecules included in natural compounds to reduce DNA damage induced by PAHs (Chan, et al., 2003). Green tea consumption may reduce the risk of lung cancer by several hypothesized mechanisms

**4.5 Mismatch repair (MMR)** 

response resulting from PAH exposure.

suggest consulting Scicchitano (2005).

**5. Methods for detection of DNA damage** 

**4.6 Other repair mechanisms** 

et al., 2010).

Table 2.

Worldwide, the population is, to some extent, exposed to PAHs. Of primary concern are the carcinogenic, teratogenic and mutagenic properties exhibited by some PAHs. The

DNA Damage Caused by Polycyclic Aromatic Hydrocarbons: Mechanisms and Markers 139

Bhattacharyya, N. P., Maher, V. M.& McCormick, J. J. (1989). Ability of structurally related

Binkova, B., Chvatalova, I., Lnenickova, Z., Milcova, A., Tulupova, E., Farmer, P. B.& Sram,

*and Molecular Mechanisms of Mutagenesis*, Vol. 620, No.1-2, pp. 49-61, Boffetta, P., Jourenkova, N.& Gustavsson, P. (1997). Cancer risk from occupational and

582, No.1-2, (Apr 4), pp. 53-60, 0027-5107 (Print) 0027-5107 (Linking) Boysen, G.& Hecht, S. S. (2003). Analysis of DNA and protein adducts of benzo[a]pyrene in

Braithwaite, E., Wu, X.& Wang, Z. (1998). Repair of DNA lesions induced by polycyclic

Briede, J. J., Godschalk, R. W., Emans, M. T., De Kok, T. M., Van Agen, E., Van Maanen, J.,

Buterin, T., Hess, M. T., Luneva, N., Geacintov, N. E., Amin, S., Kroth, H., Seidel, A.&

Castorena-Torres, F., Bermudez de Leon, M., Cisneros, B., Zapata-Perez, O., Salinas, J. E.&

Chan, H. Y., Wang, H., Tsang, D. S., Chen, Z. Y.& Leung, L. K. (2003). Screening of

*Mutation Research*, Vol. 543, No.1, pp. 17-30,

1002, 1071-5762 (Print)1029-2470 (Linking)

No.2, (Mar), pp. 411-421, 0887-2333 (Print)

100, 0163-5581 (Print) 0163-5581 (Linking)

1), pp. 1849-1856, 0008-5472 (Print) 0008-5472 (Linking)

3334 (Print) 0143-3334 (Linking)

205-214,

polycyclic aromatic carcinogens to induce homologous recombination between duplicated chromosomal sequences in mouse L cells. *Mutation Research/Fundamental and Molecular Mechanisms of Mutagenesis*, Vol. 211, No.2, pp.

R. J. (2007). PAH-DNA adducts in environmentally exposed population in relation to metabolic and DNA repair gene polymorphisms. *Mutation Research/Fundamental* 

environmental exposure to polycyclic aromatic hydrocarbons. *Cancer Causes Control*, Vol. 8, No.3, (May), pp. 444-472, 0957-5243 (Print) 0957-5243 (Linking) Bolognesi, C., Parrini, M., Aiello, C.& Rossi, L. (1991). DNA damage induced by 7,12-

dimethylbenz[a]anthracene in the liver and the mammary gland of rats exposed to polycyclic aromatic hydrocarbon enzyme inducers during perinatal life. *Mutagenesis*, Vol. 6, No.2, (Mar), pp. 113-116, 0267-8357 (Print) 0267-8357 (Linking) Bonner, M. R., Rothman, N., Mumford, J. L., He, X., Shen, M., Welch, R., Yeager, M.,

Chanock, S., Caporaso, N.& Lan, Q. (2005). Green tea consumption, genetic susceptibility, PAH-rich smoky coal, and the risk of lung cancer. Mutat Res, Vol.

human tissues using structure-specific methods. *Mutation Research/Reviews in* 

aromatic hydrocarbons in human cell-free extracts: involvement of two excision repair mechanisms in vitro. *Carcinogenesis*, Vol. 19, No.7, (Jul), pp. 1239-1246, 0143-

Van Schooten, F. J.& Kleinjans, J. C. (2004). In vitro and in vivo studies on oxygen free radical and DNA adduct formation in rat lung and liver during benzo[a]pyrene metabolism*. Free Radicals Research*, Vol. 38, No.9, (Sep), pp. 995-

Naegeli, H. (2000). Unrepaired fjord region polycyclic aromatic hydrocarbon-DNA adducts in ras codon 61 mutational hot spots. *Cancer Research*, Vol. 60, No.7, (Apr

Albores, A. (2008). Changes in gene expression induced by polycyclic aromatic hydrocarbons in the human cell lines HepG2 and A549. *Toxicology In Vitro*, Vol. 22,

chemopreventive tea polyphenols against PAH genotoxicity in breast cancer cells by a XRE-luciferase reporter construct. *Nutrition and Cancer*, Vol. 46, No.1, pp. 93-

occurrence of cancer and other diseases associated with PAH exposure has increased in recent decades. As we discussed in this chapter, the major mechanisms of carcinogenesis induced by these compounds are the interaction of PAHs with DNA to form adducts and the generation of reactive oxygen species. Both activities result in DNA damage and mutagenesis in important sites of the genome. Maintenance of genome integrity is critically dependent on efficient repair of DNA lesions by specific DNA repair mechanisms. In addition, the metabolism of PAHs is related with the ability of cells to prevent damage. Polymorphisms in DNA repair or xenobiotic metabolism-related genes are therefore associated with PAH-induced carcinogenesis. For this reason, understanding the metabolic pathways, biochemical transformations and interactions of PAHs with DNA will help to develop better strategies for risk analysis in exposed individuals. Some studies have shown that several natural compounds could help to reduce DNA damage caused by PAH exposure.

These findings offer the possibility for the development of novel drugs that help in the treatment of diseases related to PAH exposure. The analysis of specific polymorphisms of DNA repair genes will help to determine susceptibility in defined populations and the development of new biomarkers and diagnostic tools. Moreover, the study of DNA damage induced by these compounds has included other environmentally important species, such as fish, shrimp and worms. These studies are important in the development of biomarkers for biomonitoring and environmental assessment.
