39] DNA damage induced by ultraviolet and visible light and its wavelength dependence

Abstract

DNA damage induced by solar radiation in mammalian cells consists largely of two types of modification: pyrimidine dimers and oxidative modifications. Pyrimidine dimmers that can be subdivided into cyclobutane pyrimidine dimmers, (CPDs) and (6-4) photoproducts are the characteristic and most abundant modifications after direct excitation of DNA, although they can also be formed indirectly by energy transfer from other excited molecules such as carbonyl compounds. Oxidative DNA damage, which includes various pyrimidine and purine modifications, sites of base loss (AP sites), and strand breaks, is generated in only low yield after direct excitation of DNA (except at very short wavelengths, e.g., 193 nm 3 ). It is, however, the most frequent type of DNA damage generated by so-called “indirect mechanisms,” which emanate from the excitation of cellular chromophores other than DNA. These chromophores (endogenous photosensitizers) can react directly with DNA (type I reaction) or give rise to the formation of reactive oxygen species (ROS) such as singlet oxygen ( 1 O 2 ) or superoxide (O 2 - ). This chapter describes the application of this approach to determine the wavelength dependencies (action spectra) of the generation of both cyclobutane pyrimidine dimers and oxidative DNA modifications in the range between 290 and 500 nm in cultured mammalian cells by means of the alkaline elution technique. In addition, DNA damage profiles (ratios of various types of DNA modification) induced by various wavelength ranges and by natural sunlight are described. Scope and limitations of the technique are discussed.