UV Light-Induced Cyclobutane Pyrimidine Dimers Are Mutagenic in Mammalian Cells

Abstract

We used a simian virus 40-based shuttle vector plasmid, pZ189, to determine the role of pyrimidine cyclobutane dimers in UV light-induced mutagenesis in monkey cells. The vector DNA was UV irradiated and then introduced into monkey cells by transfection. After replication, vector DNA was recovered from the cells and tested for mutations in its supF suppressor tRNA marker gene by transformation of Escherichia coli carrying a nonsense mutation in the beta-galactosidase gene. When the irradiated vector was treated with E. coli photolyase prior to transfection, pyrimidine cyclobutane dimers were removed selectively. Removal of approximately 90% of the pyrimidine cyclobutane dimers increased the biological activity of the vector by 75% and reduced its mutation frequency by 80%. Sequence analysis of 72 mutants recovered indicated that there were significantly fewer tandem double-base changes and G X C----A X T transitions (particularly at CC sites) after photoreactivation of the DNA. UV-induced photoproducts remained (although at greatly reduced levels) at all pyr-pyr sites after photoreactivation, but there was a relative increase in photoproducts at CC and TC sites and a relative decrease at TT and CT sites, presumably due to a persistence of (6-4) photoproducts at some CC and TC sites. These observations are consistent with the fact that mutations were found after photoreactivation at many sites at which only cyclobutane dimers would be expected to occur. From these results we conclude that UV-induced pyrimidine cyclobutane dimers are mutagenic in DNA replicated in monkey cells.

Similar Mutagenicity of Photoactivated Porphyrins and Ultraviolet A Radiation in Mouse Embryonic Fibroblasts: Involvement of Oxidative DNA Lesions in Mutagenesis

“…Furthermore, the observed mutation spectrum corroborates the nondetectability of the photolesions mentioned earlier. Accordingly, the characteristic mutation signature of dimeric photolesions, i.e., C → T and CC → TT transitions at “dipyrimidine sites”, was not discernible in the induced mutation spectrum ( − ) (Figure B). In fact, the relative frequency of C → T transitions at “dipyrimidine sites” was not different between the induced and spontaneous mutation spectra ( p = 0.12) nor was there any occurrence of CC → TT transitions in either spectrum of mutations.…”

In vivo evidence that UV-induced C .fwdarw. T mutations at dipyrimidine sites could result from the replicative bypass of cis-syn cyclobutane dimers or their deamination products

“…That a large percentage of this tandem mutation must result from the replicative bypass of the CC dimer or its deamination products comes from comparison of mutation spectra of photodamaged DNA that had, or had not, been photoreversed prior to transfection. It was found that 100% of 254 nminduced tandem CC -*• TT mutations in the shuttle vector pZ189 were removed by enzymatic photoreversal when propagated in an monkey cell line (Protic-Sabljic et al, 1986) whereas only 50% were removed when propagated in an XP cell line (Brash et al, 1987).…”

Photopharmacology: Beyond Proof of Principle

“…Due to the steepness of the sigmoidal-shaped dose−response curve, it is in principle possible to find conditions (d) where the concentration of the active state (right sphere) is beyond the saturation point (dashed orange line), and complete on−off control can be achieved. The toxicity of UV light has been studied extensively, and it has been shown that UV light is carcinogenic, 61 can cause mutations, 62 and can induce cell apoptosis. 63,64 Therefore, UV light should be avoided in therapies, and UV-switchable compounds cannot be used in the clinic for applications that involve the direct irradiation of a living tissue.…”

“…The toxicity of UV light has been studied extensively, and it has been shown that UV light is carcinogenic, can cause mutations, and can induce cell apoptosis. , Therefore, UV light should be avoided in therapies, and UV-switchable compounds cannot be used in the clinic for applications that involve the direct irradiation of a living tissue.…”