Misincorporation rate and type on the leading and lagging strands of UV-damaged DNA

Development and validation of a new transgenic hairless albino mouse as a mutational model for potential assessment of photocarcinogenicity

Short-term phototoxicity testing is useful in selecting test agents for the longer and more expensive photocarcinogenesis safety tests; however, no validated short-term tests have been proven reliable in predicting the outcome of a photocarcinogenesis safety test. A transgenic, hairless, albino (THA) mouse model was developed that carries the gpt and red/gam [Spi(-)] genes from the gpt delta mouse background and the phenotypes from the SKH-1 mouse background to use as a short-term test in lieu of photocarcinogenesis safety tests. Validation of the THA mouse model was confirmed by exposing groups of male mice to sub-erythemal doses of ultraviolet B (UVB) irradiation for three consecutive days emitted from calibrated overhead, Kodacel-filtered fluorescent lamps and measuring the mutant frequencies (MFs) in the gpt and red/gam (Spi(-)) genes and types of mutations in the gpt gene. The doses or irradiation were monitored with broad-spectrum dosimeters that were calibrated to a NIST-traceable standard and cumulative CIE-weighted doses were 20.55 and 41.0mJ/cm(2) (effective). Mice were sacrificed 14 days after the final UVB exposure and MFs in both the gpt and red/gam genes were evaluated in the epidermis. The exposure of mice to UVB induced significant ten- to twelve-fold increases in the gpt MF and three- to five-fold increases in the Spi(-) MF over their respective background MF, 26±3×10(-6) and 9±1×10(-6). The gpt mutation spectra were significantly different between that of the UVB-irradiated and that of non-irradiated mice although the mutation spectra of both groups were dominated by C→T transitions (84% and 66%). In mice exposed to UVB, the C→T transitions occurred almost exclusively at dipyrimidine sites (92%), whereas in non-irradiated control mice, the C→T transitions occurred at CpG sites (86%). These results suggest that the newly developed THA mice are a useful and reliable model for testing UVB-induced mutagenicity in skin tissue. The application of this model for short-term prediction of solar-induced skin carcinogenicity is presently under investigation.

The robustness of naturally and artificially selected nucleic acid secondary structures

Thermodynamic stability and mutational robustness of secondary structure are critical to the function and evolutionary longevity of RNA molecules. We hypothesize that natural and artificial selection for functional molecules favors the formation of structures that are stable to both thermal and mutational perturbation. There is little direct evidence, however, that functional RNA molecules have been selected for their stability. Here we use thermodynamic secondary structure prediction algorithms to compare the thermal and mutational robustness of over 1000 naturally and artificially evolved molecules. Although we find evidence for the evolution of both types of stability in both sets of molecules, the naturally evolved functional RNA molecules were significantly more stable than those selected in vitro, and artificially evolved catalysts (ribozymes) were more stable than artificially evolved binding species (aptamers). The thermostability of RNA molecules bred in the laboratory is probably not constrained by a lack of suitable variation in the sequence pool but, rather, by intrinsic biases in the selection process.

Asymmetry of DNA replication and translesion synthesis of UV-induced thymine dimers

In vitro replication assays for detection and quantification of bypass of UV-induced DNA photoproducts were used to compare the capacity of extracts prepared from different human cell lines to replicate past the cis,syn cyclobutane thymine dimer ([c,s]TT). The results demonstrated that neither nucleotide excision repair (NER) nor mismatch repair (MMR) activities in the intact cells interfered with measurements of bypass replication efficiencies in vitro. Extracts prepared from HeLa (NER- and MMR-proficient), xeroderma pigmentosum group A (NER-deficient), and HCT116 (MMR-deficient) cells displayed similar capacity for translesion synthesis, when the substrate carried the site-specific [c,s]TT on the template for the leading or the lagging strand of nascent DNA. Extracts from xeroderma pigmentosum variant cells, which lack DNA polymerase eta, were devoid of bypass activity. Bypass-proficient extracts as a group (n=16 for 3 extracts) displayed higher efficiency (P=0.005) for replication past the [c,s]TT during leading strand synthesis (84+/-22%) than during lagging strand synthesis (64+/-13%). These findings are compared to previous results concerning the bypass of the (6-4) photoproduct [Biochemistry 40 (2001) 15215] and analyzed in the context of the reported characteristics of bypass DNA polymerases implicated in translesion synthesis of UV-induced DNA lesions. Models to explain how these enzymes might interact with the DNA replication machinery are considered. An alternative pathway of bypass replication, which avoids translesion synthesis, and the mutagenic potential of post-replication repair mechanisms that contribute to the duplication of the human genome damaged by UV are discussed.

Recent advances in the protocols of transgenic mouse mutation assays

Transgenic mutation assays were developed to detect gene mutations in multiple organs of mice or rats. The assays permit (1) quantitative measurements of mutation frequencies in all tissues/organs including germ cells and (2) molecular analysis of induced and spontaneous mutations by DNA sequencing analysis. The protocols of recently developed selections in the lambda phage-based transgenic mutation assays, i.e. cII, Spi(-) and 6-thioguanine selections, are described, and a data set of transgenic mutation assays, including those using Big Blue and Muta Mouse, is presented.

The (6-4) photoproduct of thymine-thymine induces targeted substitution mutations in mammalian cells

Two major ultraviolet-induced photolesions of TpT, a (6-4) photoproduct [T(6-4)T] and a cis-syn cyclobutane TT dimer (T=T), were incorporated into a predetermined site of one of the leading and lagging template strands of a double-stranded vector, and the modified DNAs were transfected into simian COS-7 cells. The DNAs replicated in the cells were recovered and were transfected again into Escherichia coli. The DNA replication efficiencies of plasmids containing T(6-4)T and T=T in the template strand for lagging strand synthesis were 93 and 79%, respectively, as compared with the unmodified DNA. Similar inhibitory effects were observed in the cases of the photoproducts in the template strand for leading strand synthesis (71 and 58%, respectively). These results indicated that T(6-4)T blocked DNA replication more weakly than T=T during leading and lagging strand syntheses in mammalian cells. The mutation frequencies of T(6-4)T were 2.3 and 4.7% in the leading and lagging template strands, respectively. The T=T lesion was less mutagenic and induced mutations with 0.2-0.7% frequencies. The T(6-4)T lesion primarily elicited 3'-T-->C substitutions, and T=T induced various types of mutations. These results indicate that T(6-4)T is more mutagenic than T=T during leading and lagging strand syntheses in simian cells. Moreover, this is the first evidence that shows T(6-4)T mainly elicits targeted substitutions at its 3'-T site in mammalian cells.

Mutation spectra resulting from carcinogenic exposure: from model systems to cancer-related genes

The events leading to cancer are complex and interactive. Alteration of cancer genes, such as the tumor suppressor gene p53, plays a central role in this process. Analysis of the frequency, type and site of mutations in important cancer-related genes may provide clues to the identification of etiological factors and sources of exposure. In this chapter we have selected a few examples of environmental human carcinogens and have attempted to use the knowledge of their mechanisms of mutagenesis, as derived from in vitro cell systems, as a key to understanding the complexity of p53 mutation spectra in tumors arising at the putative target organ. The analysis will focus on environmental exposure to UV radiation. The examples of tobacco smoke, dietary aflatoxin and vinyl chloride will be also briefly discussed.

Persistent DNA damage inhibits S-phase and G2 progression, and results in apoptosis

We used genetically related Chinese hamster ovary cell lines proficient or deficient in DNA repair to determine the direct role of UV-induced DNA photoproducts in inhibition of DNA replication and in induction of G2 arrest and apoptosis. UV irradiation of S-phase-synchronized cells causes delays in completion of the S-phase sometimes followed by an extended G2 arrest and apoptosis. The effects of UV irradiation during the S-phase on subsequent cell cycle progression are magnified in repair-deficient cells, indicating that these effects are initiated by persistent DNA damage and not by direct UV activation of signal transduction pathways. Moreover, among the lesions introduced by UV irradiation, persistence of (6-4) photoproducts inhibits DNA synthesis much more than persistence of cyclobutane pyrimidine dimers (which appear to be efficiently bypassed by the DNA replication apparatus). Apoptosis begins approximately 24 h after UV irradiation of S-phase-synchronized cells, occurs to a greater extent in repair-deficient cells, and correlates well with the inability to escape from an extended late S-phase-G2 arrest. We also find that nucleotide excision repair activity (including its coupling to transcription) is similar in the S-phase to what we have previously measured in G1 and G2.

Formation and processing of UV photoproducts: effects of DNA sequence and chromatin environment

Cyclobutane pyrimidine dimers and (6-4) photoproducts are the two major classes of lesions produced in DNA by UVB and UVC irradiation. Their distribution along genes is nucleotide sequence-dependent. In vivo, the frequency of these lesions at specific sites is modulated by nucleosomes and other DNA binding proteins. Repair of UV photoproducts is dependent on the transcriptional status of the sequences to be repaired and on the chromatin environment. The formation of DNA photolesions by UV light is responsible for the induction of mutations and the development of skin cancer. To understand the mechanisms of UV mutagenesis, it is important to know how these lesions are formed, by which cellular pathways they are repaired and how they are dealt with by DNA polymerases.