Photocarcinogenesis is the result of mutation formation at sites of UVR-induced DNA damage. Types of DNA damage induced by UVR are pyrimidine dimers (cyclobutane pyrimidine dimers and 6-4 photoproducts), oxidative base modifications (mostly of guanine), and single-strand breaks. Although UVR-induced oxidative DNA damage is mutagenic, mutation spectra of melanoma and nonmelanoma skin cancers point to pyrimidine dimers as the most important UVR-induced type of DNA damage in photocarcinogenesis. Formation of pyrimidine dimers can either be mediated by direct photoexcitation of DNA itself or, as recently described, by a photosensitized reaction via photoexcitation of melanin, in particular pheomelanin.
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