Studies on direct and indirect effects of DNA damage on mutagenesis in monkey cells using an SV40-based shuttle vector

Mutation spectra in supF: approaches to elucidating sequence context effects

Shuttle vectors carrying the supF suppressor tRNA gene were originally developed for mutagenesis experiments in primate and human cells. Since then, the supF gene has been used as a mutation reporter in other mammalian cells, yeast, Escherichia coli, and transgenic mice. The widespread use of the vector for studies of many DNA reactive agents has produced a large database of mutation spectra. These provide primary information on the kinds and distribution of mutations provoked by many agents and, in many instances, allow comparisons between related agents or the same agent in different cell backgrounds. In this review we will discuss some of these data with a primary focus on the interpretation of UV mutation spectra. We will also describe our development and application of custom supF marker genes as an approach to studying the effect of sequence context on mutation hotspots and cold spots. Our studies suggest that C-C photoproducts are not mutagenic in certain sequence contexts in which T-C photoproducts are mutation hotspots. In addition, we have found several examples of sequence context effects acting as much as 80 bases away from the site of mutation. We will consider some of the problems raised by these studies and the possible resolution of some of them offered by the newly discovered family of damage bypass DNA polymerases.

Induction of mutagenic DNA damage by chromium (VI) and glutathione

Certain chromium (Cr) compounds are known to be carcinogenic in humans and mutagenic in cell culture. However, the mechanism of Cr mutagenesis is not well understood. It appears that intracellular reduction of Cr by agents such as glutathione plays a role in the induction of DNA damage. We have used a simian virus 40-based shuttle vector to investigate the relationship between chromium-induced DNA damage and Cr mutagenicity. The treatment of the plasmid pZ189 with Cr(VI) plus glutathione (GSH) induced DNA strand breaks and reduced the plasmid biological activity, whereas Cr(III) treatment with or without GSH did not give rise to such DNA damage. When Cr(VI)/GSH- or Cr(III)/GSH-treated pZ189 was replicated in mammalian cells, a dose-dependent increase in mutant frequency was observed with Cr(VI)/GSH-treated pZ189, but not with Cr(III)/GSH-treated plasmid. About 43% of the mutants from Cr(VI)/GSH-treated pZ189 were deletion mutants. The remainder were base substitution mutants, mostly GC-->AT transitions and GC-->TA transversions. This pattern of mutagenesis is similar to that observed with other agents that cause oxidative DNA damage such as ionizing radiation and H2O2. These results support the hypothesis that Cr mutagenesis can be induced by the generation of reactive oxygen intermediates during the reduction of Cr(VI) by glutathione.

Analysis of complex mutations induced in cells by herpes simplex virus type-1

The shuttle vector plasmid pZ189 was used as a target for mutagenesis in COS-1 cells. Complex mutations were analyzed in 5 plasmids that were recovered from noninfected cells and 15 plasmids that were recovered from cells that were infected with herpes simplex virus type-1 (HSV-1). Complex mutations in noninfected cells consisted of duplications and rearrangements of plasmid DNA, while those from cells that were infected with HSV-1 included 8 that were enlarged due to insertion of other DNA sequences. Plasmids that contained inserted DNA showed an increase in size of from 118 bp up to around 4500 bp. Maps were constructed based on restriction enzyme digestion, and some or all of each plasmid was examined by DNA sequencing. The inserted DNA was not derived from HSV-1 in any case, since it did not hybridize to DNA from HSV-1 and showed no sequence similarities to the virus. Instead, inserted DNA was found to hybridize to HindIII-digested cellular DNA as a single or double band in 5 plasmids and contained multiple repeat sequences such as alpha satellite, Alu or Kpn repeats in 4 plasmids. In four enlarged plasmids the identity of the inserted sequences could not be determined. The junctions between the shuttle vector and the inserted DNA did not show features of transposable elements and no homology was detected between inserted sequences and the sequence at the insertion site. No preferred site for recombination was detected. Although no similarities were found among the inserted sequences, it is possible that the cellular sequences represent cellular targets for virus-mediated rearrangement. It appears that HSV-1 stimulates nonhomologous recombination between DNA sequences in virus-infected cells.

DNA sequence of mutations induced in cells by herpes simplex virus type-1

The shuttle vector plasmid pZ189 was used to find the kinds of mutations that are induced in cells by herpes simplex virus type-1 (HSV-1). A significant increase in mutation frequency was detected as early as 2 hr after infection, and reached a peak of two- to sevenfold over background at 4 hr after infection. Several differences were detected between spontaneous mutants and those induced by HSV-1 when they were analyzed by gel electrophoresis and DNA sequencing. Point mutations accounted for 63% of spontaneous mutants but for only 44% of HSV-1-induced mutants (P less than 0.05). In each case the predominant type of point mutation was the G:C to A:T transition, which comprised 51% of point mutations induced by HSV-1, and 32% of spontaneous point mutations. Deletions of DNA were seen in HSV-1-induced mutants at a frequency of 44%, compared with only 29% in spontaneous mutants. HSV-1-induced deletions were less than half the length of spontaneous deletions, and 3 contained short filler sequences. An increase in size was seen in 13% of HSV-1-induced mutants and was due either to duplication of plasmid DNA, or, in 8 instances, to insertion of sequences derived from cellular DNA. Among spontaneous mutants, only 8% were increased in size and none of them had inserted cellular DNA. The proportion of complex mutants increased as infection by the virus progressed and they accounted for 79% of mutants at 24 hr after infection. The observed mutations have implications for understanding the "hit and run" mechanism of malignant transformation of cells by HSV-1.