3-n(4)-ethanocytosine and Colonic-Neoplasms

More than 50% of colon cancer-associated mutations in the p53 tumor suppressor gene are C-->T transitions. The majority of them locate in CpG dinucleotides and are thought to have arisen through spontaneous hydrolytic deamination of 5-methylcytosine. This deamination process gives rise to G.T mispairs that need to be repaired to G.C in order to avoid C-->T mutation. Similarly, deamination of cytosine generates G.U mispairs that also produce C-->T transitions if not repaired. Restoration of both G.T and G.U mismatches was shown to be mediated by a short-patch excision repair pathway, and one principal player implicated in this process may be thymine DNA glycosylase (TDG). Human TDG was discovered as an enzyme that has the potential to specifically remove thymine and uracil bases mispaired with guanine through hydrolysis of their N-glycosidic bond, thereby generating abasic sites in DNA and initiating a base excision repair reaction. The same protein was later found to interact physically and functionally with the retinoid receptors RAR and RXR, and this implicated an unexpected function of TDG in nuclear receptor-mediated transcriptional activation of gene expression. The objective of this chapter is to put together the results of different lines of experimentation that have explored the thymine DNA glycosylase since its discovery and to critically evaluate their implications for possible physiological roles of this enzyme.

Topics: Amino Acid Sequence; Animals; Bacterial Proteins; Base Pair Mismatch; Base Sequence; Cell Transformation, Neoplastic; Colonic Neoplasms; Cytosine; Deamination; Deoxyribonuclease (Pyrimidine Dimer); DNA Damage; DNA Repair; DNA, Neoplasm; Endodeoxyribonucleases; Evolution, Molecular; Guanine; Humans; Mice; Models, Genetic; Molecular Sequence Data; Receptors, Retinoic Acid; Sequence Alignment; Sequence Homology, Amino Acid; Structure-Activity Relationship; Substrate Specificity; Thymine; Transcription, Genetic; Transfection; Uracil

Oxidative DNA damage may be a risk factor for development of various pathologies, including malignancy. We studied inflammation triggered modulation of repair activity in the intestines of three weeks old rats injected i.p. with E.coli or S. typhimurium lipopolysaccharides (LPS) at doses of 1, 5 or 10 mg/kg. Subsequent formation in these animals of colonic preneoplastic lesions, aberrant crypt foci (ACF) was also investigated. Five days after LPS administration no differences were observed in repair rate of 1,N(6)-ethenoadenine (εA), 3,N(4)-ethenocytosine (εC) and 8-oxoguanine (8-oxoG) in intestines of these rats, as measured by the nicking assay. However a significant increase in all three repair activities was found within one and two months after S. typhimurium LPS treatment. E. coli LPS significantly increased only the 8-oxoG repair. S. typhimurium LPS stimulated mRNA transcription of pro-inflammatory proteins, lipooxygenase-12 and cyclooxygenase-2, as well as some DNA repair enzymes like AP-endonuclease (Ape1) and εC-glycosylase (Tdg). mRNA level of DNA glycosylases excising εA (MPG) and 8-oxoG (OGG1) was also increased by LPS treatment, but only at the highest dose. Transcription of all enzymes increased for up to 30 days after LPS, and subsequently decreased to the level observed before treatment, with the exception of APE1, which remained elevated even two months after LPS administration. Thus, the repair efficiency of εA, εC and 8-oxoG depends on the availability of APE1, which increases OGG1 and TDG turnover on damaged DNA, and presumably stimulates MPG. One and two months after administration of E. coli or S. typhimurium LPS, the number of aberrant crypt foci in rat colons increased in a dose and time dependent manner. Thus, inflammation stimulates the repair capacity for εA, εC and 8-oxoG, but simultaneously triggers the appearance of preneoplastic changes in the colons. This may be due to increased oxidative stress and imbalance in DNA repair.

Topics: Adenine; Animals; Animals, Newborn; Arachidonate 12-Lipoxygenase; Colon; Colonic Neoplasms; Cyclooxygenase 2; Cytosine; DNA Damage; DNA Repair; Escherichia coli; Guanine; Inflammation; Lipopolysaccharides; Oxidative Stress; Precancerous Conditions; Rats, Wistar; Salmonella typhimurium