Molecular nature of intrachromosomal deletions and base substitutions induced by environmental mutagens

Abstract

Cellular DNA is exposed to a variety of exogenous and endogenous mutagens. A complete understanding of the importance of different types of DNA damage requires knowledge of the specific molecular alterations induced by different types of agents in specific target tissues in vivo. The gpt delta transgenic mouse model provides the opportunity to characterize tissue-specific DNA alterations because small and large deletions as well as base substitutions can be analyzed. Here, we summarize the characteristics of intrachromosomal deletions and base substitutions induced by ionizing radiation in liver and spleen, ultraviolet B (UVB) radiation in epidermis, mitomycin C (MMC) in bone marrow, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in colon, and aminophenylnorharman (APNH) in liver of gpt delta mice. Carbon-ion radiation, UVB, and MMC induced large deletions of more than 1 kb. About half of the large deletions occurred between short direct-repeat sequences and the remainder had flush ends, suggesting the involvement of nonhomologous end joining of double-stranded breaks (DSBs) in DNA. UV photoproducts and interstrand crosslinks by MMC may block DNA replication, thereby inducing DSBs. In contrast, PhIP and APNH mainly generated 1 bp deletions in runs of guanine bases. As for base substitutions, UVB and MMC induced G:C-->A:T transitions at dipyrimidine sites and tandem base substitutions at GG sites, respectively. PhIP and APNH induced G:C-->T:A transversions. Translesion DNA synthesis across the lesions, i.e., UV photoproducts, intrastrand crosslinks by MMC, and guanine adducts by the heterocyclic amines, may be involved in the induction of base substitutions. These results indicate the importance of sequence information to elucidate the mechanisms underlying deletions and base substitutions induced in vivo by environmental mutagens.

Novel DNA Polymerases and Novel Genotoxicity Assays

“…These one bp deletions are probably induced by TLS across DNA lesions induced by PhIP and APNH. Most of spontaneous Spi -mutations are one bp deletions in runs of A:T bps (14). The most prominent one is one base deletions from A6 to A5.…”

“…The recovered phage particles are introduced to two diŠerent E. coli strains for selection of the gpt and Spi -mutants. We have analyzed in vivo mutations in the gpt gene in various organs of mice treated with diŠerent mutagens (14). UVB irradiation induces G:C to A:T transitions at dipyrimidine sites such as 5?-TC-3?…”

“…2). The second topic is bacterial and rodent genotoxicity assays that are useful to evaluate the genotoxic risk of environmental chemicals (13,14). Finally, I discuss future directions of research on environmental mutagenesis and genotoxicology.…”

Spontaneous Mutagenesis in Rodents: Spontaneous Gene Mutations Identified by Neutral Reporter Genes in gpt Delta Transgenic Mice and Rats

“…The methodology and the characteristics of the chemical-and radiation-induced Spi − mutations have been described in detail. 1,14) Spi − selection takes advantage of the restricted growth of wild type lambda phage in P2 lysogens. 44) This phenotype is called Spi (sensitive to P2 interference).…”

“…The gpt and Spi − assay systems have been validated primarily in mice with many chemical mutagens/carcinogens, UV and ionizing radiation, for which mutagenicity, organ specificity and mutation spectrum have been thoroughly characterized. 1,[10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] Recently, the outbred gpt delta SD rat was backcrossed with Fisher 344 (F344) rat, to establish an inbred gpt delta rat (F344). In this review, we focus on the spontaneous mutations detected by the gpt and Spi − assays in gpt delta mice and rats and discuss possible mechanisms underlying these in vivo mutations.…”