Spontaneous mutation spectra in supF: comparative analysis of mammalian cell line base substitution spectra

The last decade has seen a dramatic accumulation of mutation data from repo rter genes utilized in mutagenesis experiments involving DNA reactive agent s allowing comparisons for the mutagenic potential between many different m utagens. When analysing chemically induced mutation spectra it is important to establish the potential spontaneous background before drawing conclusio ns concerning specific chemically induced hotspots. A major mutation report er system gene used in mammalian cells is the supF suppressor tRNA gene. Th e Mammalian Gene Mutation Database (MGMD) contains a considerable number of supF spontaneous mutations permitting a thorough analysis of spontaneous m utations in mammalian cell lines from different species and tissues. Analys es of spontaneous mutation spectra were performed using a range of statisti cal techniques. Spontaneous mutations were observed at 82.4% of the nucleot ides in the supF suppressor tRNA sequence although the pattern of significa nt hotspots differed between cell lines. Our analyses of spontaneous mutati on spectra show considerable variation both within and between cell lines f or the distributions of spontaneous mutations occurring with no clear tissu e or species-specific patterns emerging. In addition, spectra derived from supF recovered from liver and skin of transgenic mice, were similar to each other, but showed significant differences from many in vitro spectra. The most common base substitutions were G:C>TA transversions and G:C>A:T transi tions, although levels of each type differed between cell lines. There was also variation between cell lines for the most mutable dinucleotides, howev er, significant hotspots were frequently observed at CpG sites and sequence s containing GG/CC. We conclude that the number of varying distributions an d potential hotspots for spontaneous mutations should thus be considered wh en comparing chemically induced mutation spectra in supF. The spectra prese nted here will be a useful reference for analysis and re-analysis of chemic ally induced spectra as well as for use in comparison with the spontaneous spectra of other gene systems.