UVB and UVA induced formation of photoproducts within cellular DNA

Table of contents

Abstract
1.1 Introduction 3
1.2 Distribution of UVB radiation-induced dimeric
pyrimidime photoproducts within cellular DNA 5
1.3 UVC and UVB-induced formation of monomeric base photoproducts 7
1.4 Damage induced by UVA radiation to cellular DNA 8
1.5 Conclusions 10
References 10

Abstract

Emphasis is placed in this short survey on recent aspects of the photochemistry of cellular DNA that involve both the effects of UVB and UVA radiations. Direct excitation of the purine and pyrimidine bases of DNA is known to mostly generate dimeric pyrimidine photoproducts in oxygen independent photoreactions. Interestingly, the twelve possible dimeric photoproducts at the four main bipyrimidine sites can now be singled out as dinucleoside monophosphates. This is achieved using a specific and sensitive assay that associates high performance liquid chromatography to tandem mass spectrometry (HPLC-MS/MS) operating in the electrospray ionization (ESI) mode. Thus, it was found that UVB irradiation of human monocyte cells gives rise predominantly to cis-syn cyclobutadithymine, thymine-cytosine pyrimidine(6-4) pyrimidone adduct and related cyclobutyl dimer. In contrast the dimeric photoproducts at (di)cytosine sites are generated in very low yields although characteristic tandem mutations of UV-B irradiation are observed at the latter CC sequences. Further, cytosine photohydrate and Dewar valence isomers of the (6-4) photoproducts are at the best minor UV-B photoproducts. Relevant information on UVA-sensitized oxidative damage to cellular DNA was gained from measurements using chromatographic methods and the modified comet assay. Thus, it was shown that 8-oxo-7,8-dihydro-2'-deoxyguanosine is the predominant DNA oxidation product, as mostly, the result of singlet oxygen oxidation. In addition, oxidized pyrimidine bases and DNA strand breaks whose formation involves •OH radical, are produced in much lower yields. Work is in progress to assess the UVA-induced formation of other markers of oxidative stress. These include on the one hand DNA-protein crosslinks and on the other hand DNA adducts with reactive aldehydes that arise from the breakdown of initially generated lipid peroxides.

1.1 Introduction

Solar UV radiation appears to be the main etiological factor responsible for the induction of skin cancers in human population. It is well established that UVB and UVA radiations act mostly on cellular DNA via direct and photosensitized reactions respectively [for earlier reviews, see 1,2]. Precise assessment of the final products of these photoreactions has been hampered for years by the lack of accurate and quantitative methods of measurement. This particularly concerns the individual determination of dimeric pyrimidine photoproducts including cis-syn cyclobutadi-pyrimidines (CPDs), pyrimidine (6-4) pyrimidone photoadducts (6-4PPs) and related valence Dewar isomers (DewarPPs) for which only limited information was available until recently. However, relevant data on the distribution and repair of the three latter classes of photoproducts within the DNA of plasmids, isolated cells and tissues were gained mostly from serological approaches. These include ELISA, RIA and immuno-dot-blot measurements together with immunostaining detection through the availability of monoclonal and polyclonal antibodies. We may also mention the recent development of an immunological method aimed at measuring CPDs and 6-4PPs in the DNA of isolated cells in association with the comet assay. Another suitable method that is receiving major attention is the ligation-mediated polymerase chain reaction (LM-PCR). This allows the mapping within DNA at the nucleotide resolution of dimeric pyrimidine photoproducts and particularly of CPDs, the latter lesions being revealed through the nicking activity of T4 endonuclease V. Thus, it was found that methylation of cytosine residues in 5'-CCG and 5'-TCG sequences leads to a 10-fold increase in the UVB formation of CPDs. Interestingly the latter lesions were found to constitute major p53 mutation hot spots in mouse skin tumors. It was also shown that accumulation of dimeric DNA photoproducts takes place at the same locations of the p53 gene in...