DESCRIPTION: A growing body of evidence suggests that endogenous damage to DNA may contribute significantly to genomic instability, human disease and aging. An understanding of the formation and persistence of endogenous DNA damage is also required in order to assess the impact of DNA damage induced by exogenous mutagens and carcinogens.The purpose of the work described in this proposal is to investigate several components of endogenous DNA damage. Endogenous damage to DNA may result from either hydrolysis or oxidation of DNA and its components. Among the purines, hydrolysis resulting in the abasic sites and oxidation to form 8-oxopurines have dominated the literature. Purines may also be damaged by imidazole ring-opening generating formamidopyrimidine (FAPY) derivatives via both hydrolysis and oxidation. Hydrolytic formation of FAPY derivatives has received minimal attention in the literature. The kinetics of purine hydrolysis must be understood in order to assess accurately the significance of FAPY derivatives.The preliminary results from Dr. Sowers' laboratory indicate that current and often reported measurements of FAPY derivatives may yield extreme underestimates of the actual levels. Similarly, hydrolysis of the oxopurines may be biologically significant, and can lead to erroneous quantitation of oxidation damaged products. Oxidation of thymidine generating 5- hydroxymethyl-2'-deoxyuridine (HMdU) is a frequent DNA lesion. HMdU is controversial in terms of both amount formed and biological significance. Dr. Sowers' laboratory has identified chemical characteristics of HMdU which may explain the divergent quantitative reports. The investigators propose a series of experiments which should shed light on the potential biological impact of HMdU formation in DNA. In parallel with the oxidation of the thymine methyl group, they present preliminary data showing that the methyl group of 5-methylcytosine is also a target for oxidative damage. The oxidative damage to 5- methylcytosine is relatively unexplored. Because of the pivotal role of cytosine methylation in gene control, oxidation of 5mC may be part of an unrecognized mechanism for inappropriate activation of oncogenes and latent viral genes.