DNA-REPAIR OF UV PHOTOPRODUCTS AND MUTAGENESIS IN HUMAN MITOCHONDRIAL-DNA

The induction and repair of DNA photolesions and mutations in the mito chondrial (mt) DNA of human cells in culture were analysed after cell exposure to UV-C. light. The level of induction of cyclobutane pyrimid ine dimers (CPD) in mitochondrial and nuclear DNA was comparable, whil e a higher frequency of pyrimidine (6-4) pyrimidone photoproducts (6-4 PP) was detected in mitochondrial than in nuclear DNA. Besides the kn own defect in CPD removal, mitochondria were shown to be deficient als o in the excision of 6-4 PP. The effects of repair-defective condition s for the two major UV photolesions on mutagenesis was assessed by ana lysing the frequency and spectrum of spontaneous and UV-induced mutati ons by restriction site mutation (RSM) method in a restriction endonuc lease 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 and type (pre dominantly GC > AT transitions at G(1) of the NciI site). Conversely, an approximately tenfold increase over background mutation frequency w as recorded after cell exposure to 20J/m(2). In this case, the majorit y of mutations were C > T transitions preferentially located on the no n-transcribed DNA strand at C-1 and C-2 of the NciI site. This mutatio n spectrum is expected by UV mutagenesis. This is the first evidence o f induction of mutations in mitochondrial DNA by treatment of human ce lls with a carcinogen. (C) 1997 Academic Press Limited.