Cytotoxic and mutagenic response of mismatch repair-defective human cancer cells exposed to a food-associated heterocyclic amine

Environmental factors and colorectal tumor risk in individuals with hereditary nonpolyposis colorectal cancer

BACKGROUND & AIMS

Individuals with hereditary nonpolyposis colorectal cancer (HNPCC) are at increased risk for colorectal cancer. Environmental factors might play a role in HNPCC-associated carcinogenesis. The aim of this study was to gain insight into the effects of environmental factors on colorectal tumor risk in individuals with HNPCC.

METHODS

We examined associations between dietary factors, cigarette smoking, and HNPCC-associated colorectal tumors in a Dutch case-control study (145 cases, 103 tumor-free controls; all study participants were known or suspected carriers of a germline mutation in one of the DNA mismatch repair genes). We also assessed associations between the various environmental factors and occurrence of adenomatous polyposis coli (APC) mutations in HNPCC-associated polyps in a subset of the study population.

RESULTS

Fruit consumption was inversely associated with ever developing HNPCC-associated colorectal tumors (odds ratio [95% confidence interval] for highest vs lowest tertile, 0.4 [0.2-0.9]; P(trend) = .03); a borderline significant inverse association was observed for dietary fiber intake (0.5 [0.2-1.0]; P(trend) = .06). Cigarette smoking seemed to increase the risk of HNPCC-associated colorectal tumors. Truncating APC mutations were detected in 30 (37.5%) of the 80 available HNPCC-associated polyps; frameshift mutations were most common (73.3%). None of the evaluated environmental factors was distinctively associated with a specific APC status of the polyps.

CONCLUSIONS

Our data suggest that fruit consumption and dietary fiber intake might decrease the risk of colorectal tumors in individuals with HNPCC, whereas cigarette smoking might increase the risk of HNPCC-associated colorectal tumors. The observed associations support the hypothesis that HNPCC-associated outcomes might be modified by environmental factors.

Antimutagenicity of cinnamaldehyde and vanillin in human cells: Global gene expression and possible role of DNA damage and repair

Vanillin (VAN) and cinnamaldehyde (CIN) are dietary flavorings that exhibit antimutagenic activity against mutagen-induced and spontaneous mutations in bacteria. Although these compounds were antimutagenic against chromosomal mutations in mammalian cells, they have not been studied for antimutagenesis against spontaneous gene mutations in mammalian cells. Thus, we initiated studies with VAN and CIN in human mismatch repair-deficient (hMLH1(-)) HCT116 colon cancer cells, which exhibit high spontaneous mutation rates (mutations/cell/generation) at the HPRT locus, permitting analysis of antimutagenic effects of agents against spontaneous mutation. Long-term (1-3 weeks) treatment of HCT116 cells with VAN at minimally toxic concentrations (0.5-2.5mM) reduced the spontaneous HPRT mutant fraction (MF, mutants/10(6) survivors) in a concentration-related manner by 19-73%. A similar treatment with CIN at 2.5-7.5microM yielded a 13-56% reduction of the spontaneous MF. Short-term (4-h) treatments also reduced the spontaneous MF by 64% (VAN) and 31% (CIN). To investigate the mechanisms of antimutagenesis, we evaluated the ability of VAN and CIN to induce DNA damage (comet assay) and to alter global gene expression (Affymetrix GeneChip) after 4-h treatments. Both VAN and CIN induced DNA damage in both mismatch repair-proficient (HCT116+chr3) and deficient (HCT116) cells at concentrations that were antimutagenic in HCT116 cells. There were 64 genes whose expression was changed similarly by both VAN and CIN; these included genes related to DNA damage, stress responses, oxidative damage, apoptosis, and cell growth. RT-PCR results paralleled the Affymetrix results for four selected genes (HMOX1, DDIT4, GCLM, and CLK4). Our results show for the first time that VAN and CIN are antimutagenic against spontaneous mutations in mammalian (human) cells. These and other data lead us to propose that VAN and CIN may induce DNA damage that elicits recombinational DNA repair, which reduces spontaneous mutations.

Human sensitivity to PhIP: a novel marker for prostate cancer risk

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) has been implicated in the development of colon, prostate and mammary gland tumors in rats. In this study, we developed a modified in vitro mutagen sensitivity assay, with activated PhIP (N-OH-PhIP) as the challenge mutagen and chromosome aberrations as the endpoint, and applied it in a pilot prostate cancer case-control study of 81 cases and 84 age and ethnicity-matched controls. Our results showed significantly higher baseline breaks among the cases, mean+/-S.E.=1.86+/-0.23 versus 0.96+/-0.14 in controls; P=0.006. Individuals with high baseline breaks (dichotomized at the control median) had a 36% increased risk for PC (OR=1.36; 95% CI=1.08-1.72). In stratified analysis, high baseline breaks was associated in younger participants (< or = 60 years) with an OR of 1.71 (1.14-2.57) and in those with a positive family history of PC, an OR of 1.43 (0.97-2.11). PhIP treatment induced significantly higher breaks in cases, mean+/-S.E.=5.07+/-0.39 versus 3.83+/-0.24 in controls; P=0.05. Higher PhIP-induced breaks was associated with an overall 17% increased risk for PC (OR=1.17; 95% CI=1.03-1.33), a significantly increased risks (OR=1.19; 95% CI=1.00-1.41) among younger participants, non-smokers (OR=1.39, 1.03-1.88) and 1.20 (1.00-1.45) among those with no family history of PC. Results from this pilot study demonstrate differential sensitivity to PhIP among subgroups and therefore, this assay have potential as a susceptibility marker for prostate cancer risk.

DNA mismatch repair mediates protection from mutagenesis induced by short-wave ultraviolet light

To investigate involvement of DNA mismatch repair in the response to short-wave ultraviolet (UVC) light, we compared UVC-induced mutant frequencies and mutational spectra at the Hprt gene between wild type and mismatch-repair-deficient mouse embryonic stem (ES) cells. Whereas mismatch repair gene status did not significantly affect survival of these cells after UVC irradiation, UVC induced substantially more mutations in ES cells that lack the MutSalpha mismatch-recognizing heterodimer than in wild type ES cells. The global UVC-induced mutational spectra at Hprt and the distribution of most spectral mutational hotspots were found to be similar in mismatch-repair-deficient and wild type cells. However, at one predominant spectral hot spot for mutagenesis in wild type cells, the UVC-induced mutation frequency was not affected by the mismatch repair status. Together these data reveal a major role of mismatch repair in controlling mutagenesis induced by UVC light and may suggest the sequence context-dependent direct mismatch repair of misincorporations opposite UVC-induced pyrimidine dimers.

Induction of aberrant crypt foci in DNA mismatch repair-deficient mice by the food-borne carcinogen 2-amino-1-methyl-6-phenylimidazo [4,5-b] pyridine (PhIP)

Disruption of the DNA mismatch repair (MMR) pathway results in elevated mutation rates, inappropriate survival of cells bearing DNA damage, and increased cancer risk. Relatively little is known about the impact of environmentally relevant carcinogens on cancer risk in individuals with MMR-deficiency. We evaluated the effect of MMR status (Mlh1(+/+) versus Mlh1(-/-)) on the carcinogenic potential of the cooked-meat mutagen, 2-amino-1-methyl-6-phenylimidazo [4,5-b] pyridine (PhIP) in mice. PhIP exposure did not obviously increase lymphoma or small intestinal tumorigenesis in either Mlh1-deficient or -proficient mice. In contrast, the frequency of aberrant crypt foci (ACF), a preneoplastic biomarker for colon tumorigenesis, was increased by PhIP, and the increase due to PhIP was significantly greater in Mlh1(-/-) versus wild-type littermates. This apparent heightened susceptibility to induction of ACF parallels the previously reported hypermutability of Mlh1-deficient mice to PhIP and is consistent with the hypothesis that MMR-deficiency would increase the likelihood of PhIP-induced carcinogenic mutations. Further evaluation of the risk that consumption of heterocyclic amines may impart to MMR-deficient individuals therefore is warranted.

DNA mismatch repair status may influence anti-neoplastic effects of butyrate

HNPCC (hereditary non-polyposis colon cancer) is an autosomal-dominant disorder characterized by early-onset CRC (colorectal cancer). HNPCC is most often associated with mutations in the MMR (mismatch repair) genes hMLH1, hMSH2, hMSH6 or hPMS2. The mutator phenotype of a defective MMR system is MSI (microsatellite instability), which also occurs in approx. 15-25% of sporadic CRC cases, where it is associated with the hypermethylation of the promoter region of hMLH1. Dietary factors, including excessive alcohol consumption, ingestion of red meat and low folate intake, may increase the risk of MSI high tumour development. In contrast, aspirin may suppress MSI in MMR-deficient CRC cell lines. Butyrate, a short-chain-fatty-acid end product of carbohydrate fermentation in the colon, shares a number of anti-neoplastic properties with aspirin, including inhibiting proliferation and inducing apoptosis of CRC cells. Recent in vitro studies suggest that physiological concentrations of butyrate (0.5-2 mM) may have more potent anti-neoplastic effects in CRC cell lines deficient in MMR, but mechanisms for such a differential response remain to be established.

Mlh1-dependent suppression of specific mutations induced in vivo by the food-borne carcinogen 2-amino-1-methyl-6-phenylimidazo [4,5-b] pyridine (PhIP)

Disruption of the DNA mismatch repair (MMR) pathway results in elevated mutation rates, inappropriate survival of cells bearing DNA damage, and increased cancer risk. Relatively little is known about the potential impact of environmentally relevant carcinogens on cancer risk in individuals with MMR-deficiency. We determined the effect of MMR status (Mlh1+/+ versus Mlh1-/-) on mutagenesis induced by the cooked-meat mutagen, 2-amino-1-methyl-6-phenylimidazo [4,5-b] pyridine (PhIP) within cII and supFG1 transgene reporters. Despite being a lymphomagen in mice, PhIP was not mutagenic in thymus. In colon, PhIP exposure induced 3-fold more mutations in Mlh1-deficient mice compared to their Mlh1+/+ littermates. Similar induction was seen in Mlh1-/- small intestine. Analysis of mutational spectra revealed that G/C to T/A transversions, the "signature PhIP mutation", were induced to similar levels regardless of Mlh1 status. In contrast, Mlh1-/- mice exhibited hypermutability to frameshifts, G/C to A/T transitions, and G/C to C/G transversions. Thus, both the level and types of mutation induced by PhIP are influenced by the activity of the MMR system. MMR may suppress PhIP-induced mutation through recognition and processing of specific mispairs (PhIP-G/T, PhIP-G/G, and PhIP-G/loop mispairs). In contrast, the PhIP-G/A mispair is unlikely to be a MMR substrate. In addition, the similar induction of both transversions and transitions in Mlh1-/- mice suggests that mutagenic bypass of PhIP-G is similarly efficient with dATP, dTTP, and dGTP, in contrast to previously published conclusions. Our data suggests that MMR-deficiency would increase the likelihood of PhIP-induced carcinogenic mutations. Further evaluation of the risk that consumption of heterocyclic amines may impart to MMR-deficient individuals therefore is warranted.

In vitro toxicity of 7H-dibenzo[c,g]carbazole in human liver cell lines

7H-Dibenzo[c,g]carbazole (DBC) is a model N-heterocyclic aromatic compound (NHA) which is both a hepatotoxin and hepatocarcinogen in rodents. The focus of this investigation was to determine whether human liver cell lines display differential sensitivities to DBC-induced toxicity. Treatment of cell lines with increasing DBC concentrations produced apoptosis only in HepG2 cells. Although DBC inhibited the clonogenic growth of all cell lines at high concentrations, only the survival of HepG2 cells was reduced at lower concentrations. DBC inhibited DNA synthesis in two (HepG2, HLF) of the three cell lines at lower concentrations and was effective only at a high concentration in Mahlavu cells. Differences in DBC uptake were not observed in any of the cell lines, suggesting that bioavailability was not a limiting factor. DBC-DNA adducts were not detected in HLF or Mahlavu cells at either low or high concentrations of DBC. Consistent with the DNA adduct data, RP-HPLC analysis indicated that DBC was metabolized to a lesser degree in the HLF and Mahlavu cells. These results suggest that human liver cell lines differ markedly in the ability to metabolize DBC to toxic species and that DBC-induced apoptosis is only observed in cells that produce detectable metabolites and DBC-DNA adducts.

DNA mismatch repair defects: role in colorectal carcinogenesis

The inactivation of the DNA mismah repair (MMR) system, which is associated with the predisposition to the hereditary non-polyposis colorectal cancer (HNPCC), has also been documented in nearly 20% of the sporadic colorectal cancers. These tumors are characterized by a high frequency of microsatellite instability (MSI(+) phenotype), resulting from the accumulation of small insertions or deletions that frequently arise during replication of these short repeated sequences. A germline mutation of one of the two major MMR genes (hMSH2 or hMLH1) is found in half to two-thirds of the patients with HNPCC, whereas in sporadic cases hypermethylation of the hMLH1 promoter is the major cause of the MMR defect. Germline mutations in hMSH6 are rare and rather confer predisposition to late-onset familial colorectal cancer, and frequent extracolonic tumors. Yet, the genetic background of a number of HNPCC patients remains unexplained, indicating that other genes participate in MMR and play important roles in cancer susceptibility. The tumor-suppressor genes that are potential targets for the MSI-driven mutations because they contain hypermutable repeated sequences are likely to contribute to the etiology and tissue specificity of the MSI-associated carcinogenesis. Because the prognosis and the chemosensitivity of the MSI(+) colorectal tumors differ from those without instability, the determination of the MSI phenotype is expected to improve the clinical management of patients. This review gives an overview of various aspects of the biochemistry and genetics of the DNA mismah repair system, with particular emphasis in its role in colorectal carcinogenesis.

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-induced mutagenesis in cultured Big Blue rat mammary epithelial and fibroblast cells

Epithelial cells are the primary site of carcinogenesis in most tissues, including the mammary gland. As an alternative to the study of mutation induction in whole tissues in vivo, we have established Big Blue transgenic rat cell lines from the mammary epithelium (BBR/ME) and the mammary stroma (BBR/MFib), to permit a comparison of their mutagenic responses to carcinogens. We previously demonstrated their responsiveness to the alkylating agent N-ethyl-N-nitrosourea (ENU) (McDiarmid H et al. [2001]: Mutat Res 497:39-47). Here, we examined the responses of cultured epithelial and stromal cells to the protein pyrolysis product and mammary carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). Rat hepatic S9 was used as a source of bioactivation enzymes. Mutant induction (cII locus) and clonogenic survival were measured as a function of PhIP concentration. PhIP mutagenicity was observed in the fibroblast cells, but the greater toxicity of PhIP to the epithelial cells prevented a definitive evaluation of mutagenicity. Since PhIP may be detoxified by conjugation with glutathione, we measured glutathione levels and glutathione-S-transferase expression and activities in both cell lines. The epithelial cells had higher glutathione-S-transferase enzyme activity and protein expression than did the fibroblast cell line. Because the epithelial cells were more sensitive to toxicity, glutathione conjugation evidently plays only a minor role in PhIP toxicity and mutagenicity in our cell lines.

Mechanisms of tolerance to DNA damaging therapeutic drugs

The cytotoxic effect of many anticancer drugs relies on their ability to damage DNA. Drug resistance can be associated with the ability to remove potentially lethal DNA lesions. DNA damage tolerance offers an alternative route to resistance. In a drug-tolerant cell, persistent DNA damage has become uncoupled from cell death. Tolerance to some DNA damaging drugs is accompanied by inactivation of the cell's DNA mismatch repair pathway. This is widely acknowledged as the mechanism underlying resistance to methylating agents and to 6-thioguanine which produce structurally similar types of DNA damage. Defects in mismatch repair are also associated with resistance to numerous drugs that produce a wide variety of structurally diverse DNA lesions. Here I consider possible mechanisms by which mismatch repair might influence drug resistance and the extent to which loss of mismatch repair might be considered to confer a multidrug resistance phenotype.

Msh2 DNA mismatch repair gene deficiency and the food-borne mutagen 2-amino-1-methy1-6-phenolimidazo [4,5-b] pyridine (PhIP) synergistically affect mutagenesis in mouse colon

Msh2 deficiency and food-borne carcinogen PhIP have been implicated as genetic and environmental factors, respectively, in human colon carcinogenesis. It is not clear whether loss of one or both alleles of Msh2 gene increases the mutational sensitivity in colon when exposed to environmental carcinogens. In the current study, Msh2(+/-)/lacI and Msh2(-/-)/lacI double transgenic mice were treated with PhIP and mutations in the lacI gene were studied in the colon. The spontaneous mutation frequency (MF) is approximately eightfold higher in Msh2(-/-) mice than in Msh2(+/+) mice, while Msh2(+/-) mice display similar levels of spontaneous mutation as the Msh2 wild type mice. PhIP induced a significant increase in MF in all genotypes of mice. However, induced MF is much higher in Msh2(-/-) mice compared to Msh2(+/+) and Msh2(+/-) mice. Msh2(+/-) mice displayed an increased level of G:C>T:A transversions and -1 frameshifts upon PhIP treatment. In contrast, loss of both Msh2 alleles mainly results in increased frequency of G:C>A:T transitions when exposed to PhIP. These results suggest that a defect in mismatch repair may result in an enhanced sensitivity from exposure to a dietary carcinogen. It also provides insight into interaction between genetic and environmental factors in human carcinogenesis.

Heterocyclic amine induced apoptotic response in the human lymphoblastoid cell line TK6 is linked to mismatch repair status

The human lymphoblastoid cell, TK6, exhibited a dose-dependent cytotoxic and apoptotic response following treatment with the food borne heterocyclic amine, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). Augmentation of the p53 protein and increases in p21-WAF1 levels were also observed. Comparison of the survival by clonogenic assays and the percentage of apoptotic cells (cells containing subG1 DNA or condensed nuclei) revealed that only 10-20% of the PhIP-induced cell death could be attributed to apoptosis that occurred in the first 24h after treatment. MT1, a derivative of TK6 that contains mutations in both alleles of its hMSH6 gene and is mismatch repair deficient, showed a decreased apoptotic response. A significant increase (P<0.05) in apoptosis was observed in TK6 and not in MT1 following treatment with 2.5microg/ml PhIP. A five- to six-fold increase and less than a two-fold increase in the fraction of apoptotic cells were observed in TK6 and MT1, respectively. Treatment with 5microg/ml PhIP resulted in significant increases in apoptosis (P<0.05) in TK6 and MT1. The percentages of apoptotic cells were, however, two- to three-fold higher in TK6 than in MT1. HCT116, a hMLH1 defective mismatch repair deficient colorectal carcinoma cell line, also exhibited lower PhIP-induced apoptosis than its mismatch repair proficient chromosome transfer cell line (HCT116+chr3) following PhIP treatment. These results show that PhIP-induced apoptosis is mediated through a mismatch repair dependent pathway. Accumulation of p53 in TK6 and MT1 were evident in samples taken 24h after PhIP treatment. Increases in p21-WAF1 were also observed in both cell lines confirming that the p53 was functional. The lower apoptotic response of MT1 but similar p53 accumulation in TK6 and MT1 suggest that the mismatch repair protein(s) are involved downstream of p53 or that PhIP-induced apoptosis is p53-independent.

Comparison of the mutagenic responses of mismatch repair-proficient (TK6) and mismatch repair-deficient (MT1) human lymphoblast cells to the food-borne carcinogen PhIP

Heterocyclic amines are ubiquitously present in cooked meats and fish. They represent an important class of food-borne carcinogens. We describe the cytotoxic, apoptotic, and mutagenic responses of mismatch repair-proficient (TK6) and mismatch repair-deficient (MT1) human lymphoblastoid cells to PhIP, the most abundant heterocyclic amine. Dose-dependent increases in cytotoxicity, in apoptosis, and in mutant fractions at the hprt locus were observed following PhIP treatment. We present a statistical method that is useful for comparing two populations. With this method, we show that the data fitted a model that assumes that the PhIP-induced mutation rate is dependent on the cell line. Estimated rates of increase of 22.8 x 10(-6) and 2.2 x 10(-6) mutation per cell per microg PhIP were found in MT1 and TK6, respectively, showing that MT1 is hypermutable to PhIP. MT1 also exhibited lower PhIP-induced apoptosis. We conclude from these results that mismatch repair-deficient cells are hypermutable to the food-borne carcinogen PhIP and that the PhIP-DNA adducts, when not eliminated by apoptosis, can be transformed into mutations.

Cytotoxicity and keratinocyte microsome-mediated mutagenic activation of carcinogens in cultured epidermal cells

Four model carcinogens (aflatoxin B(1), 6-nitrochrysene, 3-amino-1-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-1), 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2)) were examined for their ability to inhibit the growth of cultured human and rat epidermal cells. To find a basis for observed differences in growth inhibition, aflatoxin B(1), Trp-P-1 and Trp-P-2 were tested for activation by microsomes isolated from these cells in a bacterial mutagenesis assay. Treated rat cultures exhibited sensitivity to Trp-P-1 and Trp-P-2 and especially aflatoxin toxicity (growth inhibition) despite their microsomes being unable to induce bacterial mutagenicity. In treated human cultures, the toxicities of Trp-P-1, Trp-P-2 and AFB(1) were stimulated by 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD), consistent with their dependence on the biotransformation reactions this agent induces; however, the toxicity correlated poorly with observed bacterial mutagenicity mediated by their isolated microsomes. 6-Nitrochrysene, a known direct-acting mutagen in bacteria, was highly toxic to the rat but not to the human cells. Since toxic effects can modify carcinogenic outcomes, these findings are compatible with a complex relationship between toxicity, mutagenicity and carcinogenicity and indicate the utility of keratinocytes for clarifying this relationship.

Unmasking a killer: DNA O(6)-methylguanine and the cytotoxicity of methylating agents

Methylating agents are potent carcinogens that are mutagenic and cytotoxic towards bacteria and mammalian cells. Their effects can be ascribed to an ability to modify DNA covalently. Pioneering studies of the chemical reactivity of methylating agents towards DNA components and their effectiveness as animal carcinogens identified O(6)-methylguanine (O(6)meG) as a potentially important DNA lesion. Subsequent analysis of the effects of methylating carcinogens in bacteria and cultured mammalian cells - including the discovery of the inducible adaptive response to alkylating agents in Escherichia coli - have defined the contributions of O(6)meG and other methylated DNA bases to the biological effects of these chemicals. More recently, the role of O(6)meG in killing mammalian cells has been revealed by the lethal interaction between persistent DNA O(6)meG and the mismatch repair pathway. Here, we briefly review the results which led to the identification of the biological consequences of persistent DNA O(6)meG. We consider the possible consequences for a human cell of chronic exposure to low levels of a methylating agent. Such exposure may increase the probability that the cell's mismatch repair pathway becomes inactive. Loss of mismatch repair predisposes the cell to mutation induction, not only through uncorrected replication errors but also by methylating agents and other mutagens.

Mammalian DNA mismatch repair

DNA mismatch repair (MMR) is one of multiple replication, repair, and recombination processes that are required to maintain genomic stability in prokaryotes and eukaryotes. In the wake of the discoveries that hereditary nonpolyposis colorectal cancer (HNPCC) and other human cancers are associated with mutations in MMR genes, intensive efforts are under way to elucidate the biochemical functions of mammalian MutS and MutL homologs, and the consequences of defects in these genes. Genetic studies in cultured mammalian cells and mice are proving to be instrumental in defining the relationship between the functions of MMR in mutation and tumor avoidance. Furthermore, these approaches have raised awareness that MMR homologs contribute to DNA damage surveillance, transcription-coupled repair, and recombinogenic and meiotic processes.

Cytotoxicity and mutagenicity of frameshift-inducing agent ICR191 in mismatch repair-deficient colon cancer cells

BACKGROUND

Deficiency of DNA mismatch repair is a common feature of cancers exhibiting instability of microsatellite DNA sequences. Cancers with microsatellite instability are recognizable by their high rate of spontaneous frameshift mutations within microsatellite sequences, their resistance to killing by cytotoxic agents, and their localization to specific tissues, e.g., the proximal colon and stomach. We hypothesized that the mismatch repair deficiency of these cancers would make them vulnerable to environmental or chemical frameshift-inducing agents. This study was undertaken to test whether exogenous frameshift-inducing agents selectively induce mutations in mismatch repair-deficient cells of mutagen-exposed tissues like the colon and whether cytotoxic doses of these agents would preferentially kill those cells.

METHODS

Cytotoxicity of the acridine mutagen 6-chloro-9-[3-(2-chloroethylamino)propylamino]-2-methoxy-acridine (ICR191), a DNA frameshift inducer, was determined in the mismatch repair-deficient human colon carcinoma cell line HCT116 versus the repair-reconstituted derivative HCT116+C3. Vulnerability to the mutagenic effects of ICR191 was determined by transfection of HCT116 or HCT116+C3 cells with a frameshift reporter vector, followed by treatment of the cells with ICR191. Alternatively, the reporter vector was reacted ex vivo with ICR191, and the derivatized vector was then transfected into HCT116 or HCT116+C3 cells.

RESULTS

ICR191 proved to be fivefold to 10-fold more potent in inducing mutations in mismatch repair-deficient HCT116 cells than in mismatch repair-proficient HCT116+C3 cells. Moreover, at cytotoxic doses of ICR191, repair-deficient HCT116 cells proved to be fivefold more vulnerable to killing than did HCT116+C3 cells.

CONCLUSIONS

Frameshift-inducing mutagens can selectively induce mutations in mismatch repair-deficient cells versus mismatch repair-proficient cells. Environmental exposures may, therefore, favor development of cancers with microsatellite instability in tissues like the gut. Frameshift-inducing agents can, however, also preferentially kill mismatch repair-deficient cancer cells and, thus, may be promising as model therapeutic compounds.