Oxidative stress, oxidative DNA damage and resulting mutations play a role in colorectal carcinogenesis. Impaired equilibrium between DNA damage formation, antioxidant status, and DNA repair capacity is responsible for the accumulation of genetic mutations and genomic instability. The lesion-specific DNA glycosylases, e.g., hOGG1 and MUTYH, initiate the repair of oxidative DNA damage. Hereditary syndromes (MUTYH-associated polyposis, NTHL1-associated tumor syndrome) with germline mutations causing a loss-of-function in base excision repair glycosylases, serve as straight forward evidence on the role of oxidative DNA damage and its repair. Altered or inhibited function of above glycosylases result in an accumulation of oxidative DNA damage and contribute to the adenoma-adenocarcinoma transition. Oxidative DNA damage, unless repaired, often gives rise G:C > T:A mutations in tumor suppressor genes and proto-oncogenes with subsequent occurrence of chromosomal copy-neutral loss of heterozygosity. For instance, G>T transversions in position c.34 of a KRAS gene serves as a pre-screening tool for MUTYH-associated polyposis diagnosis. Since sporadic colorectal cancer represents more complex and heterogenous disease, the situation is more complicated. In the present study we focused on the roles of base excision repair glycosylases (hOGG1, MUTYH) in colorectal cancer patients by investigating tumor and adjacent mucosa tissues. Although we found downregulation of both glycosylases and significantly lower expression of hOGG1 in tumor tissues, accompanied with G>T mutations in KRAS gene, oxidative DNA damage and its repair cannot solely explain the onset of sporadic colorectal cancer. In this respect, other factors (especially microenvironment) per se or in combination with oxidative DNA damage warrant further attention. Base excision repair characteristics determined in colorectal cancer tissues and their association with disease prognosis have been discussed as well.

Biomedical Centre Faculty of Medicine in Pilsen Charles University Alej Svobody 1655 323 00 Pilsen Czech Republic

Department of Biology Faculty of Education J Selye University Bratislavska 3322 945 01 Komarno Slovakia

Department of Molecular Biology of Cancer Institute of Experimental Medicine of the Czech Academy of Sciences Videnska 1083 142 00 Prague Czech Republic

Department of Oncology 1st Faculty of Medicine Charles University and Thomayer Hospital 140 59 Prague Czech Republic

Department of Surgery General University Hospital Prague 1st Medical Faculty Charles University Katerinska 1660 128 00 Prague Czech Republic

Institute of Biology and Medical Genetics 1st Faculty of Medicine Charles University Albertov 4 128 00 Prague Czech Republic

Laboratory of Cellular and Molecular Immunology Institute of Microbiology of the Czech Academy of Sciences Videnska 1083 142 20 Prague Czech Republic