Human beings are exposed to numerous natural and man-made agents that are potentially carcinogenic. Therefore, cancer risk by ionizing radiation (IR) should be assessed as a result of combined exposures with other agents. These agents include genotoxic and non-genotoxic chemical carcinogens such as, tobacco, hormones, viruses, metals etc. Carcinogenesis is a multi-step process that accumulates several genetic and epigenetic changes of oncogenes and tumor suppressor genes. For agents having similar biological function and affecting the same step of carcinogenesis, additivity is generally expected, while for agents acting at different rate-limiting step, combined exposure is expected to be deviated from additivity. Conceptually, carcinogens are classified as initiator and promoter. IR could function at several steps as initiator, promoter or both. In order to predict the mode of combined action of IR with other agents, the sequence and time interval of the exposures, the dose, and the type of exposure (acute or chronic) are the critical factors. In this review, we focus on the combined effect of IR and alkylating agents. The data in the literatures and in our laboratory on mouse thymic lymphomas indicate that combined effect of these two genotoxic agents is synergistic, additive or antagonistic, depending on the dose and the sequence. Mechanistic approach determining frequency and spectrum of cancer-related genes and loss of heterozygosity (LOH) shows that role of IR differs in combined exposures depending on the dose. At low dose range, in general, the combined effect may not deviate from additivity. More information on the mode and the mechanism of low-level exposures, which occasionally encountered in environmental and occupational situation, are required for reaching a unifying concept.

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A perceived shortcoming of transgenic rodent mutation assays is the relatively high spontaneous mutant frequencies (MFs) of the bacterial transgenes compared to endogenous genes. This background is dominated by G:C→A:T mutation, frequently in a CpG context, mammalian sites of cytosine methylation. The single A:T target site of the ΦX174 transgenic mouse reversion assay might avoid this background, yet in vivo mutagenic sensitivity at this site was poor because of background mutation in the recovery bacteria. In order to determine the actual spontaneous MF of the ΦX174 transgene in the mouse cell, several research tools have been developed: 1) single burst analysis, for distinguishing mouse from bacterial mutation; 2) a transgenic mouse embryonic cell line, PX-2; and 3) a forward mutational assay, which has few CpG sites among its target sites. In this study, single burst analysis was applied to the transgenic cell line for the first time, evaluating the response to UVB irradiation for potential phototoxicity studies. Under appropriate plating conditions, single burst analysis lowered the spontaneous MF10-fold from the original report to 0.17×10^-5. The MF 72 h after 70 J/m² UVB irradiation was 8.3×10^-5. The characteristic UVB-induced mutant spectrum included >80% G:C→A:T at dipyrimidine sites (primarily TpC dinucleotides) with 13% of these at a single CpG site, and 20% as multiple mutants, tandem and non-tandem. The spontaneous MF per nucleotide, 3×10^-8, was comparable to that of human disease genes in the germline. When normalized for target number and dose, single burst analysis of the ΦX174 forward mutational assay produced a UVB-induced MF that was equivalent to that of the cII transgene in mouse cell culture, but a spontaneous MF an order of magnitude lower. The results suggest this cell line is highly sensitive to UVB irradiation.

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9-(4′-Aminophenyl)-9H-pyrido[3,4-b]indole (aminophenylnorharman, APNH), a novel endogenous mutagenic/ carcinogenic heterocyclic amine, is known to be a reaction product of 9H-pyrido[3,4-b]indole (norharman) and aniline. The major APNH-DNA adduct has been reported to be 2′-deoxyguanosin-8-yl-aminophenylnorharman (dGuo-C8-APNH). However, RNA adducts may also be important. We here demonstrated formation of APNH-RNA adducts and conducted a structural analysis using various spectrometric approaches. When a reaction mixture of an ultimate mutagenic form of APNH, N-acetoxy-APNH, and guanosine (Guo) was subjected to LC-ESI/MS analysis, one peak, with a similar UV spectrum to dGuo-C8-APNH, exhibited a molecular ion peak at m/z 541 along with a fragment peak at m/z 409, consistent with loss of a ribose moiety. From ¹H-NMR analysis, its chemical structure was concluded to be N⁴-(guanosin-8-yl)-9-(4′-aminophenyl)-9H-pyrido[3,4-b]indole (Guo-C8-APNH). The same adduct was yielded in yeast tRNA incubated with N-acetoxy-APNH. Digestion of tRNA treated with N-acetoxy-APNH resulted in the appearance of one adduct spot visualized by the ³²P-postlabeling method, corresponding to Guo-C8-APNH. No spot was seen with tRNA alone. Additional analysis of in vivo adduct formation in the livers of rats administered APNH at a concentration of 100 mg/kg revealed that several adduct spots, including one corresponded to Guo-C8-APNH, were observed. The total adduct levels of APNH-RNA were 28±13.3 (mean±SD) adducts per 10⁶ nucleotides. Comparisons demonstrated six times higher levels of total APNH-RNA than total APNH-DNA adducts in the same rat liver samples. These results indicate that APNH-RNA might be a useful biomarker for exposure to APNH.

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The Escherichia coli Orf135 (NudG) protein, a MutT-type enzyme, catalyzes the hydrolysis of 2-hydroxy-dATP and 8-hydroxy-dGTP, and its deficiency causes an increase in the mutation frequency. In this study, Orf135 proteins with substitutions at the Gly-36, Gly-37, Lys-38, Glu-43, Arg-51, Glu-52, Leu-53, Glu-55, and Glu-56 residues, which are conserved in three MutT-type proteins (Orf135, MutT, and MTH1), were each expressed in the orf135^- strain, and the rpoB mutant frequency upon H₂O₂ treatment was examined. The in vivo mutation suppression abilities and the in vitro enzymatic activities obtained in a previous study were compared. The expression of the enzymatically active Orf135 mutants in the orf135^- strain tended to reduce the rpoB mutant frequency induced by H₂O₂. This result suggests the importance of the phosphohydrolase activity in the suppression of mutations by the Orf135 protein.

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Mutagenicity was monitored in the surface water from the river Thames in 1997, 2002, 2003 and 2005. All samples from the Thames taken at sites in London (a place close to Tower bridge and a site near Teddington lock) and in Windsor show significant mutagenicity in the Ames test. This suggests mutagenic pollution in the Thames was not improved from 1997 to 2005. Water samples from the Serpentine in Hyde Park, London and a sample from the river Dee in Chester also show mutagenic activities toward S. typhimurium YG1024. The mutagenicity (500-2500 revertants per liter) toward S. typhimurium YG1024 in the presence of metabolic activation found in the water from the Thames in London was comparable with that found in the water from the river Asahi in Okayama and the river Katsura in Kyoto, Japan in 2003 and 2005.
A comparison of the degree of mutagenicity was made between water samples from the Thames, Asahi and the Katsura rivers. Higher mutagenicity was observed with the O-acetyltransferase-overproducing strains, S. typhimurium YG1024 and YG1029, than with the parental strains S. typhimurium TA98 and TA100. This suggests the presence of amino- or nitro-groups in the structure of molecules polluting the water from all three rivers. Water samples taken from the Thames in 2002 and 2005 exerted mutagenicity in S. typhimurium YG1029, YG1024 and TA98 in the presence of metabolic activation but not in the absence of S9 mix, whereas samples from the Asahi and the Katsura showed mutagenicity in S. typhimurium YG1029, YG1024 and TA98 both in the presence and in the absence of S9 mix. This suggests that mutagenic pollution in the Thames and the Japanese rivers took place with different mutagenic contaminants.

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The publisher would like to draw the reader's attention to errors in the pagination of the Volume 29 Number 1, February 2007 and Short communication by Katsuhito Kino et al..
The publisher apologizes for these errors and any confusion they may have caused. Wrong:Vol. 29 (2007) , No. 1 p.23-28
Right:Vol. 29 (2007) , No. 1 p.23-27

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