DNA repair of UV photoproducts and mutagenesis in human mitochondrial DNA

The induction and repair of DNA photolesions and mutations in the mitochondrial (mt) DNA of human cells in culture were analysed after cell exposure to UV-C light. The level of induction of cyclobutane pyrimidine dimers (CPD) in mitochondrial and nuclear DNA was comparable, while a higher frequency of pyrimidine (6–4) pyrimidone photoproducts (6–4 PP) was detected in mitochondrial than in nuclear DNA. Besides the known defect in CPD removal, mitochondria were shown to be deficient also in the excision of 6–4 PP. The effects of repair-defective conditions for the two major UV photolesions on mutagenesis was assessed by analysing the frequency and spectrum of spontaneous and UV-induced mutations by restriction site mutation (RSM) method in a restriction endonuclease site, NciI (5′CCCGG3′) located within the coding sequence of the mitochondrial gene for tRNA Leu. The spontaneous mutation frequency and spectrum at the NciI site of mitochondrial DNA was very similar to the RSM background mutation frequency (approximately 10 −5) and type (predominantly GC > AT transitions at G 1 of the NciI site). Conversely, an approximately tenfold increase over background mutation frequency was recorded after cell exposure to 20 J/m 2. In this case, the majority of mutations were C > T transitions preferentially located on the non-transcribed DNA strand at C 1 and C 2 of the NciI site. This mutation spectrum is expected by UV mutagenesis. This is the first evidence of induction of mutations in mitochondrial DNA by treatment of human cells with a carcinogen.