Characterization of p53 mutations in methylene chloride-induced lung tumors from B6C3F1 mice

BRG1 and BRM loss selectively impacts RB and P53, respectively: BRG1 and BRM have differential functions in vivo

The SWI/SNF complex is an important regulator of gene expression that functions by interacting with a diverse array of cellular proteins. The catalytic subunits of SWI/SNF, BRG1 and BRM, are frequently lost alone or concomitantly in a range of different cancer types. This loss abrogates SWI/SNF complex function as well as the functions of proteins that are required for SWI/SNF function, such as RB1 and TP53. Yet while both proteins are known to be dependent on SWI/SNF, we found that BRG1, but not BRM, is functionally linked to RB1, such that loss of BRG1 can directly or indirectly inactivate the RB1 pathway. This newly discovered dependence of RB1 on BRG1 is important because it explains why BRG1 loss can blunt the growth-inhibitory effect of tyrosine kinase inhibitors (TKIs). We also observed that selection for Trp53 mutations occurred in Brm-positive tumors but did not occur in Brm-negative tumors. Hence, these data indicate that, during cancer development, Trp53 is functionally dependent on Brm but not Brg1. Our findings show for the first time the key differences in Brm- and Brg1-specific SWI/SNF complexes and help explain why concomitant loss of Brg1 and Brm frequently occurs in cancer, as well as how their loss impacts cancer development.

Silica-induced chronic inflammation promotes lung carcinogenesis in the context of an immunosuppressive microenvironment

The association between inflammation and lung tumor development has been clearly demonstrated. However, little is known concerning the molecular events preceding the development of lung cancer. In this study, we characterize a chemically induced lung cancer mouse model in which lung cancer developed in the presence of silicotic chronic inflammation. Silica-induced lung inflammation increased the incidence and multiplicity of lung cancer in mice treated with N-nitrosodimethylamine, a carcinogen found in tobacco smoke. Histologic and molecular analysis revealed that concomitant chronic inflammation contributed to lung tumorigenesis through induction of preneoplastic changes in lung epithelial cells. In addition, silica-mediated inflammation generated an immunosuppressive microenvironment in which we observed increased expression of programmed cell death protein 1 (PD-1), transforming growth factor-β1, monocyte chemotactic protein 1 (MCP-1), lymphocyte-activation gene 3 (LAG3), and forkhead box P3 (FOXP3), as well as the presence of regulatory T cells. Finally, the K-RAS mutational profile of the tumors changed from Q61R to G12D mutations in the inflammatory milieu. In summary, we describe some of the early molecular changes associated to lung carcinogenesis in a chronic inflammatory microenvironment and provide novel information concerning the mechanisms underlying the formation and the fate of preneoplastic lesions in the silicotic lung.

Genetic alterations in K-ras and p53 cancer genes in lung neoplasms from B6C3F1 mice exposed to cumene

The incidences of alveolar/bronchiolar adenomas and carcinomas in cumene-treated B6C3F1 mice were significantly greater than those of the control animals. We evaluated these lung neoplasms for point mutations in the K-ras and p53 genes that are often mutated in humans. K-ras and p53 mutations were detected by cycle sequencing of PCR-amplified DNA isolated from paraffin-embedded neoplasms. K-ras mutations were detected in 87% of cumene-induced lung neoplasms, and the predominant mutations were exon 1 codon 12 G to T transversions and exon 2 codon 61 A to G transitions. P53 protein expression was detected by immunohistochemistry in 56% of cumene-induced neoplasms, and mutations were detected in 52% of neoplasms. The predominant mutations were exon 5, codon 155 G to A transitions, and codon 133 C to T transitions. No p53 mutations and one of seven (14%) K-ras mutations were detected in spontaneous neoplasms. Cumene-induced lung carcinomas showed loss of heterozygosity (LOH) on chromosome 4 near the p16 gene (13%) and on chromosome 6 near the K-ras gene (12%). No LOH was observed in spontaneous carcinomas or normal lung tissues examined. The pattern of mutations identified in the lung tumors suggests that DNA damage and genomic instability may be contributing factors to the mutation profile and development of lung cancer in mice exposed to cumene.

Molecular analysis of a multistep lung cancer model induced by chronic inflammation reveals epigenetic regulation of p16 and activation of the DNA damage response pathway

The molecular hallmarks of inflammation-mediated lung carcinogenesis have not been fully clarified, mainly due to the scarcity of appropriate animal models. We have used a silica-induced multistep lung carcinogenesis model driven by chronic inflammation to study the evolution of molecular markers and genetic alterations. We analyzed markers of DNA damage response (DDR), proliferative stress, and telomeric stress: gamma-H2AX, p16, p53, and TERT. Lung cancer-related epigenetic and genetic alterations, including promoter hypermethylation status of p16(CDKN2A), APC, CDH13, Rassf1, and Nore1A, as well as mutations of Tp53, epidermal growth factor receptor, K-ras, N-ras, and c-H-ras, have been also studied. Our results showed DDR pathway activation in preneoplastic lesions, in association with inducible nitric oxide synthase and p53 induction. p16 was also induced in early tumorigenic progression and was inactivated in bronchiolar dysplasias and tumors. Remarkably, lack of mutations of Ras and epidermal growth factor receptor, and a very low frequency of Tp53 mutations suggest that they are not required for tumorigenesis in this model. In contrast, epigenetic alterations in p16(CDKN2A), CDH13, and APC, but not in Rassf1 and Nore1A, were clearly observed. These data suggest the existence of a specific molecular signature of inflammation-driven lung carcinogenesis that shares some, but not all, of the molecular landmarks of chemically induced lung cancer.

Analysis of DNA adducts formed in vivo in rats and mice from 1,2-dibromoethane, 1,2-dichloroethane, dibromomethane, and dichloromethane using HPLC/accelerator mass spectrometry and relevance to risk estimates

Dihaloalkanes are of toxicological interest because of their high-volume use in industry and their abilities to cause tumors in rodents, particularly dichloromethane and 1,2-dichloroethane. The brominated analogues are not used as extensively but are known to produce more toxicity in some systems. Rats and mice were treated i.p. with (14)C-dichloromethane, -dibromomethane, -1,2-dichloroethane, or -1,2-dibromoethane [5 mg (kg body weight)(-1)], and livers and kidneys were collected to rapidly isolate DNA. The DNA was digested using a procedure designed to minimize processing time, because some of the potential dihalomethane-derived DNA-glutathione (GSH) adducts are known to be unstable, and the HPLC fractions corresponding to major adduct standards were separated and analyzed for (14)C using accelerator mass spectrometry. The level of liver or kidney S-[2-(N(7)-guanyl)ethyl]GSH in rats treated with 1,2-dibromoethane was approximately 1 adduct/10(5) DNA bases; in male or female mice, the level was approximately one-half of this. The levels of 1,2-dichloroethane adducts were 10-50-fold lower. None of four known (in vitro) GSH-DNA adducts was detected at a level of >2/10(8) DNA bases from dibromomethane or dichloromethane. These results provide parameters for risk assessment of these compounds: DNA binding occurs with 1,2-dichloroethane but is considerably less than from 1,2-dibromoethane in vivo, and low exposure to dihalomethanes does not produce appreciable DNA adduct levels in rat or mouse liver and kidney of the doses used. The results may be used to address issues in human risk assessment.

Genetic alterations in K-ras and p53 cancer genes in lung neoplasms from Swiss (CD-1) male mice exposed transplacentally to AZT

A transplacental carcinogenicity study was conducted by exposing pregnant Swiss (CD-1) mice to 0, 50, 100, 200, or 300 mg of 3'-azido-3'-deoxythymidine (AZT)/kg bw/day, through a 18 to 19-day gestation [National Toxicology Program, NIH Pub. No. 04-4458, 2004]. The incidences of alveolar/bronchiolar adenomas and carcinomas, in the 200 and 300 mg/kg male treatment groups, were significantly greater than that of the controls. In the present study, we evaluated the benign and malignant lung neoplasms from this bioassay for point mutations, in the K-ras and p53 cancer genes that are often mutated in human lung tumors. K-ras and p53 mutations were detected by cycle sequencing of polymerase chain reaction-amplified DNA, isolated from formalin-fixed, paraffin-embedded neoplasms. K-ras mutations were detected in 25 of 38 (66%) of the AZT-induced lung tumors, and the predominant mutations were codon 12 G-->T transversions. p53 mutations were detected in 32 of 38 (84%) of the AZT-induced lung tumors, with the predominant mutations being exon 8, codon 285 A-->T transversions, and exon 6, codon 198 T-->A transversions. No K-ras or p53 mutations were detected in five tumors, examined from control mice. The patterns of mutations identified in the lung tumors suggest that incorporation of AZT or its metabolites into DNA, oxidative stress, and genomic instability may be the contributing factors to the mutation profile and development of lung cancer in these mice.

Overview of the molecular carcinogenesis of mouse lung tumor models of human lung cancer

Lung cancer is the leading cause of cancer death worldwide, and the need to develop better diagnostic techniques and therapies is urgent. Mouse models have been utilized for studying carcinogenesis of human lung cancers, and many of the major genetic alterations detected in human lung cancers have also been identified in mouse lung tumors. The importance of mouse models for understanding human lung carcinogenic processes and in developing early diagnostic techniques, preventive measures and therapies cannot be overstated. In this report, the major known molecular alterations in lung tumorigenesis of mice are reviewed and compared to those in humans.

No complementation between TP53 or RB-1 and v-src in astrocytomas of GFAP-v-src transgenic mice

Human low-grade astrocytomas frequently recur and progress to states of higher malignancy. During tumor progression TP53 alterations are among the first genetic changes, while derangement of the p16/p14ARF/RB-1 system occurs later. To probe the pathogenetic significance of TP53 and RB-1 alterations, we introduced a v-src transgene driven by glial fibrillary acidic protein (GFAP) regulatory elements (which causes preneoplastic astrocytic lesions and stochastically astrocytomas of varying degrees of malignancy) into TP53+/- or RB-1+/- mice. Hemizygosity for TP53 or RB-1 did not increase the incidence or shorten the latency of astrocytic tumors in GFAP-v-src mice over a period of up to 76 weeks. Single strand conformation analysis of exons 5 to 8 of non-ablated TP53 alleles revealed altered migration patterns in only 3/16 tumors analyzed. Wild-type RB-1 alleles were retained in all RB-1+/-GFAP-v-src mice-derived astrocytic tumors analyzed, and pRb immunostaining revealed protein expression in all tumors. Conversely, the GFAP-v-src transgene did not influence the development of extraneural tumors related to TP53 or RB-1 hemizygosity. Therefore, the present study indicates that neither loss of RB-1 nor of TP53 confer a growth advantage in vivo to preneoplastic astrocytes expressing v-src, and suggests that RB-1 and TP53 belong to one single complementation group along with v-src in this transgenic model of astrocytoma development. The stochastic development of astrocytic tumors in GFAP-v-src, TP53+/- GFAP-v-src, and RB-1+/- GFAP-v-src transgenic mice indicates that additional hitherto unknown genetic lesions of astrocytes contribute to tumorigenesis, whose elucidation may prove important for our understanding of astrocytoma initiation and progression.

Genetic alterations in cancer knowledge system: analysis of gene mutations in mouse and human liver and lung tumors

Mutational incidence and spectra for genes examined in both human and mouse lung and liver tumors were analyzed using the National Institute of Environmental Health Sciences (NIEHS) Genetic Alterations in Cancer (GAC) knowledge system. GAC is a publicly available, web-based system for evaluating data obtained from peer-reviewed studies of genetic changes in tumors associated with exposure to chemical, physical, or biological agents, as well as spontaneous tumors. In mice, mutations in Kras2 and Hras-1 were the most common events reported for lung and liver tumors, respectively, whether chemically induced or spontaneous. There was a significant difference in Kras2 mutation incidence for spontaneous versus induced mouse lung tumors and in Hras-1 mutation incidence and spectrum for spontaneous versus induced mouse liver tumors. The major gene changes reported for human lung and liver tumors were in KRAS2 (lung only) and TP53. The KRAS2 mutation incidence was similar for spontaneous and asbestos-induced human lung tumors, while the TP53 mutation incidence differed significantly. Aflatoxin B1, hepatitis B virus, hepatitis C virus, and vinyl chloride all caused TP53 mutations in human liver tumors, but the mutation spectrum for each agent differed. The incidence of KRAS2 mutations in human compared to mouse lung tumors differed significantly, as did the incidence of Hras and p53 gene mutations in human compared to mouse liver tumors. Differences observed in the mutation spectra for agent-induced compared to spontaneous tumors and similarities in spectra for structurally similar agents support the concept that mutation spectra can serve as a "fingerprint" of exposure based on chemical structure.

Organ-specific susceptibility of p53 knockout mice to N-bis(2-hydroxypropyl)nitrosamine carcinogenesis

To elucidate which is the major determinant of susceptibility of p53 deficient mice, the carcinogen or the target organ, N-bis(2-hydroxypropyl)nitrosamine was administered to induce tumors in multi-organs. In a 15-week experiment, the incidences of both lung and hepatic vascular tumors were found to be significantly higher in p53 nullizygous (-/-) than in heterozygous (+/-) and wild-type (+/+) mice, indicating universal susceptibility of p53 (-/-) mice. In a 40-week experiment, p53 (+/-) mice showed increased susceptibility only with regard to vascular tumors, coinciding with significantly more frequent (60%) p53 gene mutations, in comparison with lung tumors with their low mutation rate (10.8%) (P<0.005). These results indicate that the target organ may be a more important factor than the carcinogen in determining susceptibility of p53 (+/-) mice.

Relevance of animal carcinogenesis findings to human cancer predictions and prevention

Use of laboratory animals to identify carcinogenic potential of chemicals, mixtures, and other agents has a modern history of greater than 40 years from which much useful scientific and public health information can be derived. While laboratory animals differ from humans in some respects that may affect responses to hazardous exposures, use of such models is based on experimental evidence indicating that there are more genetic, genomic, physiological, biochemical, and metabolic similarities than differences among mammalian species. Issues of concordance of responses between rodent species and between rodents and humans as well as repeatability and site-specificity are important considerations in evaluating laboratory animal carcinogenicity results. Variables in experimental design such as animal strain, diet, route of exposure, and study, duration as well as single-site versus multisite carcinogenic responses all influence interpretation and intelligent use of study data. Similarities and differences in site-specific laboratory animal and corresponding human cancers should also be considered in study evaluation. Recent attempts to explore genetically engineered mice and to humanize the mouse for more relevant identification of carcinogen hazard identification have yielded mixed results. In the end we are confronted by the realization that virtually all animal cancer models are useful but imperfect surrogates for humans. Assuming the percentage of chemicals currently in commerce that are estimated to be potent animal or human carcinogens is quite low, the task of identifying agents with significant carcinogenic potential is daunting and important. The biological conundrum of scientific debate regarding the relevance of carcinogenicity studies in laboratory animals is likely to continue. Nonetheless public health considerations must take precedence when deciding human safety issues.

Aberrant promoter hypermethylation of the death-associated protein kinase gene is early and frequent in murine lung tumors induced by cigarette smoke and tobacco carcinogens

Loss of expression of the death-associated protein (DAP)-kinase gene by aberrant promoter methylation may play an important role in cancer development and progression. The purpose of this investigation was to determine the commonality for inactivation of the DAP-kinase gene in adenocarcinomas induced in mice by chronic exposure to mainstream cigarette smoke, the tobacco carcinogens 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and vinyl carbamate, and the occupational carcinogen methylene chloride. The timing for inactivation was also determined in alveolar hyperplasias that arise in lung cancer induced in the A/J mouse by NNK. The DAP-kinase gene was not expressed in three of five NNK-induced lung tumor-derived cell lines or in a spontaneously arising lung tumor-derived cell line. Treatment with 5-aza-2'-deoxycytidine restored expression; dense methylation throughout the DAP-kinase CpG island detected by bisulfite sequencing supported methylation as the inactivating event in these cell lines. Methylation-specific PCR detected inactivation of the DAP-kinase gene in 43% of tumors associated with cigarette smoke, a frequency similar to those reported in human non-small cell lung cancer. In addition, DAP-kinase methylation was detected in 52%, 60%, and 50% of tumors associated with NNK, vinyl carbamate, and methylene chloride, respectively. Methylation was observed at similar prevalence in both NNK-induced hyperplasias and adenocarcinomas (46% versus 52%), suggesting that inactivation of this gene is one pathway for tumor development in the mouse lung. Bisulfite sequencing of both premalignant and malignant lesions revealed dense methylation, substantiating that this gene is functionally inactivated at the earliest histological stages of adenocarcinoma development. This study is the first to use a murine model of cigarette smoke-induced lung cancer and demonstrate commonality for inactivation by promoter hypermethylation of a gene implicated in the development of this disease in humans.

Transplacental lung carcinogenesis: molecular mechanisms and pathogenesis

A wide variety of studies in both animal models and human populations have demonstrated age-related differences in the susceptibility of the developing organism to environmentally prevalent toxicants. While this differential susceptibility has been clearly established, the mechanistic basis for these age-related differences is still poorly understood. The developing fetus utilizes many of the same metabolic and signaling pathways as adult organisms in responding to environmental agents. However, it is becoming increasingly evident that the fetus is not a "little adult" and exhibits unique biochemical responses and gene expression profiles to chemical and physical agents. Because of the rapid growth and developmental changes that occur during gestation, the fetus represents a particularly challenging research subject as a result of the dynamic alterations that occur in gene expression pathways as gene systems are activated or repressed during specific stages of development. Thus, an understanding of the mechanism(s) that render the developing organism more or less susceptible to specific carcinogenic agents is crucial for both regulatory decisions regarding the determination of safe levels of toxic chemicals released into the environment and also for determining the effects of therapeutic compounds in younger age groups and pregnant women. Concentrating on studies from the author's laboratory, this review will highlight recent research on the molecular pathogenesis of transplacentally induced tumors. While focusing on the lung, other animal models and recent human epidemiological studies will also be discussed to contrast similarities and differences in the developing and adult organisms in terms of responses to toxic chemicals, including metabolism of environmentally prevalent toxicants and alterations in gene systems at the molecular level.

Comparative study on Tp53 gene mutations in lung tumors from rats exposed to 239Pu, 237Np and 222Rn

The tumor suppressor gene Tp53 was analyzed by polymerase chain reaction-amplification of genomic DNA extracted from paraffin-embedded tissue sections of rat lung tumors to compare mutations that occurred after inhalation exposures to plutonium dioxide, neptunium dioxide, or radon and radon progenies. Exons 5 to 8 of the gene were amplified in 16 plutonium-, 23 neptunium- and 15 radon-induced lung tumors, and their polymerase chain reaction products were examined for mutations by single strand conformational polymorphism analysis and direct sequencing method. Two point mutations were detected in the plutonium-induced tumors, i.e., a guanine to adenine transition at codon 219 of exon 6 and a cytosine to thymine transition at codon 266 of exon 8. Although only one point mutation was found at codon 175 of exon 5 (cytosine to thymine transition) from neptunium-induced tumors, no mutations were detectable from radon-induced tumors. These results indicate that the abnormalities of the Tp53 gene might not be so critical for the pulmonary carcinogenesis after the inhalation of different alpha emitters, even though the presence and frequencies of the Tp53 gene mutations were different.

Chemical-specific alterations in ras, p53, and beta-catenin genes in hemangiosarcomas from B6C3F1 mice exposed to o-nitrotoluene or riddelliine for 2 years

The most prominent neoplastic lesions in mice in the 2-year studies of o-nitrotoluene and riddelliine were hemangiosarcomas. Fifteen o-nitrotoluene-induced hemangiosarcomas of the skeletal muscle, subcutaneous tissue, and mesentery; 12 riddelliine-induced hemangiosarcomas of the liver; and 15 spontaneous subcutaneous hemangiosarcomas were examined for genetic alterations in ras, p53, and beta-catenin genes. Mutations in at least one of these genes were identified in 13 of 15 (87%) of the o-nitrotoluene-induced hemangiosarcomas with missense mutations in p53 exons 5-8 detected in 11 of 15 (73%) of these neoplasms. Seven of 15 (47%) hemangiosarcomas from mice exposed to o-nitrotoluene had deletions at exon 2 splice sites or smaller deletions in the beta-catenin gene. K-ras mutation was detected in only 1 of the 15 (7%) o-nitrotoluene-induced hemangiosarcomas. In contrast to the o-nitrotoluene study, 7/12 (58%) riddelliine-induced hemangiosarcomas had K-ras codon 12 GTT mutations and, when screened by immunohistochemistry, 9/12 (75%) had strong staining for the p53 protein in malignant endothelial cells, the cells of origin of hemangiosarcomas. Riddelliine-induced hemangiosarcomas were negative for the beta-catenin protein. Spontaneous hemangiosarcomas from control mice lacked both p53 and beta-catenin protein expression and ras mutations. Our data indicated that p53 and beta-catenin mutations in the o-nitrotoluene-induced hemangiosarcomas and K-ras mutations and p53 protein expression in riddelliine-induced hemangiosarcomas most likely occurred as a result of the genotoxic effects of these chemicals. It also suggests that these mutations play a role in the pathogenesis of the respective hemangiosarcomas in B6C3F1(1) mice.

Inhibition of Rel/Nuclear Factor-kappaB signaling in skin results in defective DNA damage-induced cell cycle arrest and Ha-ras- and p53-independent tumor development

In recent years a growth inhibitory role in skin for the Rel/NF-kappaB transcription factors has been established, and the block of Rel/NF-kappaB signaling results in rapid development of spontaneous skin cancer. The molecular mechanism underlying tumor development is however unknown. In the present study, we show that inhibition of NF-kappaB signaling in mouse skin by targeted expression of degradation resistant IkappaB-alpha generates transgenic keratinocytes unable to arrest the cell cycle in response to DNA damage induced by gamma-radiation. The results indicate that transgenic keratinocytes have a defect at the G1-S checkpoint whereas the G2-M checkpoint response was found to be intact. However, transgenic keratinocytes still respond by induction of the cyclin dependent kinase inhibitor p21(Cip1/Waf) after exposure to gamma-radiation. In the spontaneous skin tumors that develop in transgenic mice no mutations were found in the Ha-ras or p53 gene, suggesting that inhibition of NF-kappaB signaling in skin can induce cancer development independently of initiating mutations in the Ha-ras gene or additional mutations in the p53 gene. These findings demonstrate an involvement of NF-kappaB signaling in the DNA damage response and cell cycle checkpoint control in the skin.

Enhancement of natural killer cytotoxicity delayed murine carcinogenesis by a probiotic microorganism

Regulation of innate immunity may be an effective means of cancer control. Delaying cancer onset is regarded as an important mode of action in cancer prevention. We have been investigating the chemopreventive mechanisms of Lactobacillus casei Shirota (LcS), a probiotic strain. In this study, we evaluated the effect of LcS on tumor onset and the involvement of natural killer (NK) cells using a 3-methylcholanthrene-induced carcinogenesis model. C3H/HeN mice were divided into three groups, according to treatment: vehicle-treated, treated with vehicle only; control, 3-methylcholanthrene treated; LcS, 3-methylcholanthrene and LcS treated. 3-Methylcholanthrene was injected intradermally at 7 weeks of age. LcS was mixed into the diet (0.05%, w/w), which the mice were fed from the day of 3-methylcholanthrene injection onward. Tumor incidence was observed weekly. Profiles of splenic NK cells, in vitro cytotoxicity and the proportion, in the early stage of carcinogenesis were analyzed at 5 weeks after the injection. The tumor suppressive effect of LcS was also evaluated in a beige mouse model that is genetically deficient in NK cells. LcS delayed tumor onset and reduced tumor incidence in the results with C3H/HeN mice (P< 0.05). More specifically, tumor incidence in the control group was 33% at 6 weeks after the injection and 83% at 11 weeks as opposed to 0 and 42%, respectively, in the LcS group. NK cell cytotoxicity was significantly higher than in the control group, and the number of NK cells also increased in the LcS group of C3H/HeN mice. However, LcS failed to suppress tumorigenesis in the beige mouse. These findings suggest that enhancement of the cytotoxicity of NK cells by LcS delays tumor onset.

The potential use of mutation spectra in cancer related genes in genetic toxicology: a statement of a GUM working group

In recent years, there has been widespread interest in the relationship between carcinogenic exposure and mutation spectra in cancer-related genes. To evaluate potential benefits and/or limitations in the use of mutation spectra in genetic toxicology, a GUM working group has been established to discuss this subject. Based on methodological possibilities and limitations, the impact of mutation spectra in the interpretation of animal experiments and in the identification of etiological agents in human cancer has been considered. With respect to experimental animals, the analyses of mutation spectra within long-term rodent carcinogenicity studies may provide some additional information on the mode of action of the respective carcinogen, however, the interpretation of results should be done carefully and only in context with other toxicological data available. Regarding human exposure, the analysis of mutation spectra in p53 or ras genes supplies information on the genotoxic properties of the respective agent. Nevertheless, on the individual level, the presence or absence of defined mutations in cancer-related genes in human tumors does not permit a definite conclusion about the causative agent.

Mutations of ras protooncogenes and p53 tumor suppressor gene in cardiac hemangiosarcomas from B6C3F1 mice exposed to 1,3-butadiene for 2 years

1,3-Butadiene is a multisite carcinogen in rodents. Incidences of cardiac hemangiosarcomas were significantly increased in male and female B6C3F1 mice that inhaled 1,3-butadiene (BD) for 2 years. Eleven BD-induced cardiac hemangiosarcomas were examined for genetic alterations in ras protooncogenes and in the p53 tumor suppressor gene. Nine of 11 (82%) BD-induced hemangiosarcomas had K-ras mutations and 5 of 11 (46%) had H-ras mutations. All of the K-ras mutations were G-->C transversions (GGC-->CGC) at codon 13; this pattern is consistent with reported results in BD-induced lung neoplasms and lymphomas. Both K-ras codon 13 CGC mutations and H-ras codon 61 CGA mutations were detected in 5 of 9 (56%) hemangiosarcomas. The 11 hemangiosarcomas stained positive for p53 protein by immunohistochemistry and were analyzed for p53 mutations using cycle sequencing of polymerase chain reaction (PCR) amplified DNA isolated from paraffin-embedded sections. Mutations in exons 5 to 8 of the p53 gene were identified in 5 of 11 (46%) hemangiosarcomas, and all of these were from the 200- or 625-ppm exposure groups that also had K-ras codon 13 CGC mutations. Our data indicate that K-ras, H-ras, and p53 mutations in these hemangiosarcomas most likely occurred as a result of the genotoxic effects of BD and that these mutations may play a role in the pathogenesis of BD-induced cardiac hemangiosarcomas in the B6C3F1 mouse.

Use of transgenic animals for carcinogenicity testing: considerations and implications for risk assessment

Advances in genetic engineering have created opportunities for improved understanding of the molecular basis of carcinogenesis. Through selective introduction, activation, and inactivation of specific genes, investigators can produce mice of unique genotypes and phenotypes that afford insights into the events and mechanisms responsible for tumor formation. It has been suggested that such animals might be used for routine testing of chemicals to determine their carcinogenic potential because the animals may be mechanistically relevant for understanding and predicting the human response to exposure to the chemical being tested. Before transgenic and knockout mice can be used as an adjunct or alternative to the conventional 2-year rodent bioassay, information related to the animal line to be used, study design, and data analysis and interpretation must be carefully considered. Here, we identify and review such information relative to Tg.AC and rasH2 transgenic mice and p53+/- and XPA-/- knockout mice, all of which have been proposed for use in chemical carcinogenicity testing. In addition, the implications of findings of tumors in transgenic and knockout animals when exposed to chemicals is discussed in the context of human health risk assessment.

Induction of basal cell carcinomas and trichoepitheliomas in mice overexpressing GLI-1

Basal cell carcinoma is the most prevalent cancer in the western world, showing a rapid increase in incidence. Activation of the Sonic hedgehog/Patched (PTCH) signaling pathway because of PTCH1 inactivation is a key event in sporadic and familial basal cell carcinoma development in humans and is associated with transcriptional activation of specific target genes, including PTCH1 itself. These changes are analogous to the situation in Drosophila where hedgehog activates the zinc-finger transcription factor Cubitus interruptus, leading to increased transcription of target genes. In the present study, we show that mice ectopically expressing the human Cubitus interruptus homolog GLI-1 in the skin develop tumors closely resembling human BCCs as well as other hair follicle-derived neoplasias, such as trichoepitheliomas, cylindromas, and trichoblastomas. Furthermore, examination of the tumors revealed wild-type p53 and Ha ras genes. These findings firmly establish that increased GLI-1 expression is central and probably sufficient for tumor development and suggest that GLI-1-induced tumor development does not depend on additional p53 or Ha ras mutations.

Induction of basal cell carcinomas and trichoepitheliomas in mice overexpressing GLI-1

Basal cell carcinoma is the most prevalent cancer in the western world, showing a rapid increase in incidence. Activation of the Sonic hedgehog/Patched (PTCH) signaling pathway because of PTCH1 inactivation is a key event in sporadic and familial basal cell carcinoma development in humans and is associated with transcriptional activation of specific target genes, including PTCH1 itself. These changes are analogous to the situation in Drosophila where hedgehog activates the zinc-finger transcription factor Cubitus interruptus, leading to increased transcription of target genes. In the present study, we show that mice ectopically expressing the human Cubitus interruptus homolog GLI-1 in the skin develop tumors closely resembling human BCCs as well as other hair follicle-derived neoplasias, such as trichoepitheliomas, cylindromas, and trichoblastomas. Furthermore, examination of the tumors revealed wild-type p53 and Ha ras genes. These findings firmly establish that increased GLI-1 expression is central and probably sufficient for tumor development and suggest that GLI-1-induced tumor development does not depend on additional p53 or Ha ras mutations.

New approaches to understanding p53 gene tumor mutation spectra

The first p53 gene mutation arising in a human tumor was described a decade ago by Baker et al. [S.J. Baker, E.R. Fearon, J.M. Nigro, S.R. Hamilton, A.C. Preisinger, J.M. Jessup, P. van Tuinen, D.H. Ledbetter, D.F. Barker, Y. Nakamura, R. White, B. Vogelstein, Chromosome 17 deletions and p53 gene mutations in colorectal carcinomas, Science 244 (1989) 217-221]. There are now over 10,000 mutations extracted from the published literature in the IARC database of human p53 tumor mutations [P. Hainaut, T. Hernandez, A. Robinson, P. Rodriguez-Tome, T. Flores, M. Hollstein, C.C. Harris, R. Montesano, IARC database of p53 gene mutations in human tumors and cell lines: updated compilation, revised formats and new visualization tools, Nucleic Acids Res. 26 (1998) 205-213; Version R3, January 1999]. A large and diverse collection of tumor mutations in cancer patients provides important information on the nature of environmental factors or biological processes that are important causes of human gene mutation, since xenobiotic mutagens as well as endogenous mechanisms of genetic change produce characteristic types of patterns in target DNA [J.H. Miller, Mutational specificity in bacteria, Annu. Rev. Genet. 17 (1983) 215-238; T. Lindahl, Instability and decay of the primary structure of DNA, Nature 362 (1993) 709-715; S.P. Hussain, C.C. Harris, Molecular epidemiology of human cancer: contribution of mutation spectra studies of tumor suppressor genes, Cancer Res. 58 (1998) 4023-4037; P. Hainaut, M. Hollstein, p53 and human cancer: the first ten thousand mutations, Adv. Cancer Res. 2000]. P53 gene mutations in cancers can be compared to point mutation spectra at the HPRT locus of human lymphocytes from patients or healthy individuals with known exposure histories, and accumulated data indicate that mutation patterns at the two loci share certain general features. Hypotheses regarding specific cancer risk factors can be tested by comparing p53 tumor mutations typical of a defined patient group against mutations generated experimentally in rodents or in prokaryotic and eukaryotic cells in vitro. Refinements of this approach to hypothesis testing are being explored that employ human p53 sequences introduced artificially into experimental organisms used in laboratory mutagenesis assays. P53-specific laboratory models, combined with DNA microchips designed for high through-put mutation screening promise to unmask information currently hidden in the compilation of human tumor p53 mutations.

Molecular pathogenesis of transplacentally induced mouse lung tumors

Previous studies from this and other laboratories have shown that treatment of pregnant mice with 3-methylcholanthrene (MC) caused lung tumors in the offspring, the incidence of which correlated with fetal inducibility of Cyp1a1. Analysis of paraffin-embedded lung tissue for Ki-ras-2 mutations indicated that 79% of the lesions examined contained point mutations in codons 12 and 13 of the Ki-ras-2 gene locus, the majority of which (84%) were G-->T transversions. The mutational spectrum was dependent on the tumor stage, as both the incidence of mutation and type of mutation produced correlated with malignant progression of the tumor. Mutations occurred in 60% of the hyperplasias, 80% of the adenomas, and 100% of the adenocarcinomas. In the tumors with mutations, GLY12-->CYS12 transversions occurred in 100% of the hyperplasias, 42% of the adenomas, and 14% of the adenocarcinomas. GLY12-->VAL12 transversions were not observed in hyperplasias and occurred in 42% of the adenomas and 57% of the adenocarcinomas. The remaining ASP12 and ARG13 mutations occurred only in adenomas (17%) and adenocarcinomas (29%). The tumors were also analyzed for alterations in the structure or function of the tumor suppressor genes Rb, p53, and Cdkn2a. No mutations were observed in exons 5-8 of the p53 gene. SSCP analysis demonstrated that 2 of 15 lung tumors contained shifted bands at the Cdkn2a gene locus. Sequence analysis had identified these as mutations in exon 2, with a CAC-->TAC transition at base 301 (HIS74-->TYR74) in tumor 23-1 and GGG-->GAG transition at base 350 (GLY90-->GLU90) in tumor 36-1. Northern blot analysis of the larger tumors revealed that 14 of 14 of these large lung tumors exhibited markedly decreased expression of Rb gene transcripts. These results were confirmed by immunohistochemistry. The larger tumors, which exhibited features of adenocarcinomas, showed a marked reduction or almost complete absence of nuclear pRb staining compared with smaller adenomas and normal lung tissue. The results suggest that Ki-ras-2 mutations are an early and frequent event in lung tumorigenesis, and that the type of mutation produced by environmental chemicals can influence the carcinogenic potential of the tumor. The results obtained with the Cdkn2a and Rb genes suggest that alterations in the Rb regulatory axis may play a key role in the pathogenesis of the pulmonary tumors and appear to occur later in the neoplastic process. It appears from these experiments that the combination of mutated Ki-ras-2 and alterations in the Rb regulatory gene locus, which are frequent alterations in human lung tumors, may be the preferred pathway for lung tumor pathogenesis in mice exposed transplacentally to environmental carcinogens.

Carcinogenic responses of transgenic heterozygous p53 knockout mice to inhaled 239PuO2 or metallic beryllium

The transgenic heterozygous p53+/- knockout mouse has been a model for assessing the tumorigenicity of selected carcinogens administered by noninhalation routes of exposure. The sensitivity of the model for predicting cancer by inhaled chemicals has not been examined. This study addresses this issue by acutely exposing p53+/- mice of both sexes by nose-only inhalation to either air (controls), or to 1 of 2 levels of 239PuO2 (500 or 100 Bq 239Pu) or beryllium (Be) metal (60 or 15 micrograms). Additional wild-type p53+/+ mice were exposed by inhalation to either 500 Bq of 239PuO2 or 60 micrograms of Be metal. These carcinogens were selected because they operate by differing mechanisms and because of their use in other pulmonary carcinogenesis studies in our laboratory. Four or 5 of the 15 mice per sex from each group were sacrificed 6 mo after exposure, and only 2 pulmonary neoplasms were observed. The remainder of the mice were held for life-span observation and euthanasia as they became moribund. Survival of the p53+/- knockout mice was reduced compared to the p53+/+ wild-type mice. No lung neoplasms were observed in p53+/- mice exposed to air alone. Eleven of the p53+/- mice inhaling 239PuO2 developed pulmonary neoplasms. Seven p53+/+ mice exposed to 239PuO2 also developed pulmonary neoplasms, but the latency period for pulmonary neoplasia was significantly shorter in the p53+/ mice. Four pulmonary neoplasms were observed in p53+/- mice exposed to the higher dose of Be, whereas none were observed in the wild-type mice or in the heterozygous mice exposed to the lower dose of Be. Thus, both p53+/- and p53+/+ mice were susceptible to 239Pu-induced carcinogenesis, whereas the p53+/- but not the p53+/+ mice were susceptible to Be-induced carcinogenesis. However, only 2 pulmonary neoplasms (1 in each of the 239PuO2 exposure groups) were observed in the 59 p53+/ mice that were sacrificed or euthanatized within 9 mo after exposure, indicating that the p53+/- knockout mouse might not be appropriate for a 6-mo model of carcinogenesis for these inhaled carcinogens.

Predominant p53 G-->A transition mutation and enhanced cell proliferation in uterine sarcomas of CBA mice treated with 1,2-dimethylhydrazine

Mouse uterine tumors were examined for genetic alterations in the ras proto-oncogene and p53 tumor suppressor gene and for other biologically relevant immunohistochemical markers that may increase our understanding of the events that occur in uterine cancer. Fourteen dimethylhydrazine (DMH)-induced uterine sarcomas, including 3 primary malignant fibrous histiocytomas (MFH), 7 transplanted MFH, 3 stromal sarcomas, and 1 undifferentiated sarcoma, were first screened by immunohistochemistry for p53 missense mutations, followed by single strand conformation polymorphism analysis and DNA sequencing for the identification of point mutations. There was 100% correlation between p53 protein immunopositivity and subsequent detection of p53 mutations in DMH-induced malignant fibrous histiocytomas. All MFH had a characteristic p53 G:C-->A:T transition mutation, consistent with O6-methylguanine mispairing with thymine, the most common DNA lesion caused by alkylating agents. DMH-induced uterine MFH with p53 mutations also had a higher proliferative rate (qualitatively evaluated by immunohistochemical detection of proliferating cell nuclear antigen) when compared with other DMH-induced sarcomas. Uterine sarcomas were further evaluated for biological end points, such as estrogen receptor and desmin. Neoplastic cells from stromal sarcomas (SS), undifferentiated sarcomas (US), and MFH did not stain for desmin. The estrogen receptor was detected in normal uteri and a small portion of MFH, SS, and US. Our data suggest that DMH-induced uterine sarcomas are not consistent with smooth muscle cell origin and that a subset of these tumors, specifically DMH-induced malignant fibrous histiocytomas, have unique p53 G:C-->A:T transitions and a high proliferative rate.

Ki-ras gene mutations and absence of p53 gene mutations in spontaneous and urethane-induced early lung lesions in CBA/J mice

Ki-ras and p53 genes are involved in human lung carcinogenesis; however, the role of these genes in experimental lung tumors is not well known. In our study, the CBA/J mouse strain was used to investigate the presence of Ki-ras and p53 alterations in lung carcinogenesis of spontaneous tumors and tumors induced with high and low doses of urethane (ethyl carbamate). To study the presence of these alterations in the early stages of lung carcinogenesis and in very small lung tumors, restriction fragment length polymorphism and single-strand conformation polymorphism analyses were performed on polymerase chain reaction-amplified DNA from microdissected tumoral and normal lung samples. Ki-ras gene mutations in codons 12 and 61 were detected in all types of lung lesions, even in small and preneoplastic lesions, and their incidence increased with progression from lung hyperplasias (18%) to adenomas (75%) and to carcinomas (80%). Urethane exposure, in both high and low doses, increased the incidence of Ki-ras mutations in lung tumors, especially in adenomas. The presence of Ki-ras gene mutations in very small urethane-induced lung tumors and the absence of hyperplasias among the treated-group lesions may indicate that urethane accelerates tumoral progression. No p53 mutations were detected in exons 5-8 in any of the epithelium-derived lung tumors. Only one p53 mutation in exon 5 was found in a spontaneous lymphoma. Therefore, p53 mutations do not seem to cooperate with Ki-ras gene mutations or represent an alternative molecular pathway in murine carcinogenesis.

Role of tumor suppressor genes in transplacental lung carcinogenesis

Most human cancers involve multiple genetic changes, including activation of oncogenes such as Ki-ras-2 (Kras2) and inactivation of any one of a number of tumor suppressor genes such as p53 and members of the retinoblastoma (Rb) regulatory axis. As part of an ongoing project to determine how in utero exposure to chemical carcinogens affects the molecular pathogenesis of murine lung tumors, the p53 and p16Cdkn2a genes were analyzed by using paraffin-embedded lung tissues from mice treated transplacentally with 3-methylcholanthrene. Single-strand conformation polymorphism analysis of exons 5-8 of the p53 gene, as well as their flanking introns, demonstrated an absence of mutations at this gene locus. However, a genetic polymorphism was identified at nt 708 in intron 4 of the DBA/2 strain of mice 5 bp downstream of a 3' branching-point splice signal. Analysis of exons 1 and 2 of the Cdkn2a gene by single-strand conformation polymorphism and sequence analyses revealed mutations in exon 2 in 7% of the tumors examined. Tumor 23-1 exhibited a CAC-->TAC transition at nt 301 (His74-->Tyr74), and tumor 36-1 exhibited a GGG-->GAG transition at nucleotide 350 (Gly90-->Glu90). Northern blot analysis of 14 of the larger tumors showed a marked decrease in the levels of Rb RNA expression. Immunohistochemical analysis revealed a spectrum of pRb expression, with the smaller adenomas showing moderate numbers of nuclei with heterogeneous staining for pRb in contrast with a highly reduced or near-complete absence of expression in the nuclei of larger tumors with features of adenocarcinomas. The low incidence of mutations at tumor suppressor loci suggested that inactivation of tumor suppressor genes was a late event in murine lung tumor pathogenesis. The identification of both mutations at the Cdkn2a gene locus and reduced levels of Rb expression combined with previous studies demonstrating a high incidence of mutated Kras2 alleles in these tumors implies that alterations of the Rb regulatory axis, in combination with mutation of Kras2, may be the preferred pathway for the pathogenesis of pulmonary tumors in transplacentally exposed mice.

p53 gene mutation and loss of heterozygosity of chromosome 11 in methylcholanthrene-induced mouse sarcomas

Mutations of the p53 tumor suppressor gene are the most prevalent genetic alteration observed in a wide variety of human cancers. In this study we examined 63 methylcholanthrene (MCA)-induced sarcomas from C57BL/6N x C3H/HeN F1 (BCF1) or C3H/HeN x C57BL/6N F1 (CBF1) mice for p53 gene mutations and loss of heterozygosity (LOH) of chromosome 11. Mutation analysis was done on exons 5 to 8 of the p53 gene by polymerase chain reaction-single strand conformation polymorphism analysis. This identified 53 potential mutations in 45 sarcomas. Mutations were further confirmed by direct sequencing of the region. Forty-nine of the 53 cases (94%) were missense mutations, while the rest included two nonsense mutations, one silent mutation and one insertional mutation. Spectra of base substitutions were: 25 cases (47%) of G:C-->T:A transversion, 13 cases (25%) of G:C-->A:T transition (CpG site 15%), 13 cases (24%) of G:C-->C:G transversion, a case (2%) of A:T-->T:A transversion and a case (2%) of insertion. In addition, analysis of 5 polymorphic markers of mouse chromosome 11 revealed LOH in ten cases (22%) among those carrying p53 mutations. In nine of these 10 cases, the loss involved all 5 markers. In addition, the loss was biased toward the C57BL allele (9 cases). The present study establishes the pattern of mutation of the p53 gene in MCA-induced mouse sarcomas.

Phenolphthalein induces thymic lymphomas accompanied by loss of the p53 wild type allele in heterozygous p53-deficient (+/-) mice

Epidemiology studies have indicated that many human cancers are influenced by environmental factors. Genetically altered mouse model systems offer us the opportunity to study the interaction of chemicals with genetic predisposition to cancer. Using the heterozygous p53-deficient (+/-) mouse, an animal model carrying one wild type p53 gene and one p53 null allele, we studied the effects of phenolphthalein on tumor induction and p53 gene alterations. Earlier studies showed that phenolphthalein caused carcinogenic effects in Fisher 344 rats and B6C3F1 mice after a 2-yr dosing period (Dunnick and Hailey, Cancer Res. 56: 4922-4926, 1996). The p53 (+/-) mice received phenolphthalein in the feed at concentrations of 200, 375, 750, 3,000, or 12,000 ppm (approximately 43, 84, 174, 689, or 2,375 mg/kg body weight/day or 129, 252, 522, 2,867, or 7,128 mg/m2 body surface area/day) for up to 6 mo. A target organ cancer site that accumulated p53 protein in the B6C3F1 mouse (i.e., thymic lymphoma) was also a target site for cancer in the p53 (+/-) mouse. In the p53 (+/-) mouse, treatment-related atypical hyperplasia and malignant lymphoma of thymic origin were seen in the control and dosed groups at a combined incidence of 0, 5, 5, 25, 100, and 95%, respectively. Twenty-one of the thymic lymphomas were examined for p53 gene changes, and all showed loss of the p53 wild type allele. Chemical-induced ovarian tumors in the B6C3F1 mouse showed no evidence for p53 protein accumulation and did not occur in the p53 (+/-) mouse. The p53-deficient (+/-) mouse model responded to phenolphthalein treatment with a carcinogenic response in the thymus after only 4 mo of dosing. This carcinogenic response took 2 yr to develop in the conventional B6C3F1 mouse bioassay. The p53-deficient (+/-) mouse is an important model for identifying a carcinogenic response after short-term (< 6 mo) exposures. Our studies show that exposure to phenolphthalein combined with a genetic predisposition to cancer can potentiate the carcinogenic process and cause p53 gene alterations, a gene alteration found in many human cancers.

Genome wide search for genetic damage in p53 transgenic mouse lung tumours reveals consistent loss of chromosome 4

The tumour which develops most frequently in mice carrying a p53 Val135 transgene is adenocarcinoma of the lung. We established 10 cell lines from these tumours and investigated their karyotypes by detailed cytogenetic analysis using a complete set of mouse chromosome-specific paints. Consistent loss of chromosome 4 material was noted in 9 out of 10 cell lines; this loss was detected in tetraploid but not diploid cells of the same cell line, suggesting that mouse chromosome 4 plays a critical role in the progression of lung adenocarcinomas. Other frequently observed chromosome aberrations involved chromosomes 7, 5 and 8. Atypical bronchial epithelium was observed together with lung tumours and in tumour-free, apparently normal lungs indicating that mouse lung tumours induced due to the presence of a mutant p53 transgene may develop via pre-invasive lesions and thus may be effective models for the study of lung tumour progression.

The mouse lymphoma L5178Y Tk+/- cell line is heterozygous for a codon 170 mutation in the p53 tumor suppressor gene

The p53 tumor suppressor protein plays an important role in regulating the cellular response to DNA damage, including cell cycle arrest and apoptosis induction. Normal p53 function is critical for the maintenance of genomic stability. The mouse lymphoma L5178Y/TK(+/-)-3.7.2C cell line is widely used in genetic toxicology for mutagenesis and clastogenesis testing. A related line L5178Y-R, has previously been shown to react with antibodies specific for mutant as well as wild-type p53 protein and to exhibit delayed cell death after radiation. For this reason, as well as the mouse lymphoma assay's reputation for high sensitivity of detection for genotoxic agents but low specificity, we examined several clones of L5178Y cells for mutations in the conserved core domain (exons 5-8) of the p53 gene. Using single-strand conformational polymorphism analysis, we found evidence for the same mutation in exon 5 of p53 in L5178Y-R, L5178Y-S and L5178Y/TK(+/+)-3.7.2C cells. The mutation was identified by sequencing of exon 5 as a TGC (Cys) to CGC (Arg) transition in codon 170 (= codon 176 in humans). Sequencing showed approximately equivalent signals for the mutant and normal alleles for all 3 lines. The mutation in codon 170 is adjacent to a mutation hotspot of the human p53 gene (codon 175) and eliminates a critical zinc-coordinating cysteine residue such that the mutant protein is likely to be denatured and have a dominant negative effect on normal p53 function. Western blots showed approximately 100-fold higher levels of p53 protein in unirradiated L5178Y cells as compared to induced levels of p53 in normal mouse splenocytes 4 h after 5 Gy of gamma radiation. The high levels of p53 protein in L5178Y cells were not further inducible by radiation, whereas an 11-fold induction was seen in the irradiated splenocytes. These results indicate that p53 protein in L5178Y cells is dysfunctional and suggest that this line may therefore be abnormally susceptible to the induction of genetic alterations.

Ki-ras and p53 mutations are early and late events, respectively, in urethane-induced pulmonary carcinogenesis in A/J mice

In the A/J strain of mice, urethane (ethyl carbamate) induces lung hyperplasia, adenoma, and adenocarcinoma in a time-dependent manner. These distinct morphological stages may correlate with sequential molecular genetic changes in this mouse model. To test this hypothesis, we investigated the presence of mutations involving Ki-ras and p53 in urethane-induced lung lesions in A/J mice at early and late stages of tumorigenesis. We precisely microdissected 40 lung lesions from paraffin-embedded sections. Ki-ras mutations around codon 61 and p53 mutations in exons 5-8 were identified by polymerase chain reaction-single-strand conformation polymorphism and DNA sequencing techniques. In 29 early-stage lung lesions classified as hyperplasias (seven) or adenomas (22), we observed 19 Ki-ras mutations (66%), including three silent mutations and one double mutation at different codons, and one silent p53 mutation (3.5%). In 11 late-stage adenomas, we identified nine activating Ki-ras mutations (82%) and four missense p53 mutations (36%). These results indicate that Ki-ras mutations arise early, whereas p53 mutations occur relatively late during the benign stages of urethane-induced lung carcinogenesis in A/J mice.

Somatic genetic events linked to the Apc locus in intestinal adenomas of the Min mouse

We have found previously that all spontaneous intestinal adenomas from Apc+/ApcMin mice lose the wild type Apc marker on two genetic backgrounds. On the (AKR x B6)F1 background, this event involves loss of the entire homolog of mouse chromosome 18 carrying Apc+. This chromosome carries both the Mcc and Dcc genes, which are homologs of genes that have been implicated in human colorectal cancer. To determine whether the loss of alleles of Mcc and/or Dcc is necessary for the formation of intestinal adenomas, subchromosomal somatic events were induced by gamma-irradiation. The observed spectrum of intrachromosomal somatic genetic losses rules out a requirement for loss of heterozygosity at either locus during adenoma formation. Subchromosomal allelic losses linked to Apc+ occur spontaneously on other genetic backgrounds. In the majority of these events, the Apc+ allele itself was somatically lost, as judged by the wild type marker at the Min site. However, on the [M. musculus castaneous (CAST) x B6-Min]F1 and (129/Sv x B6-Min)F1 backgrounds, spontaneous adenomas were observed in which the wild type marker at the Min site was retained. Further analysis will be required to determine whether these exceptions involve intra-Apc mutations. If not, then these events would illustrate routes to intestinal neoplasia that do not require complete inactivation of wild type Apc function.

Studies of oncogene activation and tumor suppressor gene inactivation in normal and neoplastic rodent tissue

Emerging short-term bioassays for chemically-induced carcinogenesis are dependent for their relevance to human risk assessment on the degree of coincidence of human and rodent tumor pathways. Since these pathways do not always converge, these new tests may have a number of unanticipated pitfalls. Models of liver and renal tumors are described. The results from Rb and p53 tumor suppressor gene transgenic animals are compared to human tumor syndromes. The question of mutagenic and epigenetic fingerprints of chemicals versus the cell-specific selection of spontaneous mutations is debated. Examples of specific pitfalls, such as the recently discovered Helicobacter hepaticus promoted liver tumors in mice are presented. The rat pseudogenes for p53 and the rare role of p53 in most important rodent tumor models other than epithelial tumors present experimental quandaries. The differential effects of carcinogens during various stages of rodent perinatal and adult development are also discussed. It is concluded that the pathways of both animal models and their human counterparts should be better identified so that realistic endpoint markers can be chosen for human carcinogenic risk assessment.

PBPK modeling/Monte Carlo simulation of methylene chloride kinetic changes in mice in relation to age and acute, subchronic, and chronic inhalation exposure

During a 2-year chronic inhalation study on methylene chloride (2000 or 0 ppm; 6 hr/day, 5 days/week), gas-uptake pharmacokinetic studies and tissue partition coefficient determinations were conducted on female B6C3F1, mice after 1 day, 1 month, 1 year, and 2 years of exposure. Using physiologically based pharmacokinetic (PBPK) modeling coupled with Monte Carlo simulation and bootstrap resampling for data analyses, a significant induction in the mixed function oxidase (MFO) rate constant (Vmaxc) was observed at the 1-day and 1-month exposure points when compared to concurrent control mice while decreases in glutathione S-transferase (GST) rate constant (Kfc) were observed in the 1-day and 1-month exposed mice. Within exposure groups, the apparent Vmaxc maintained significant increases in the 1-month and 2-year control groups. Although the same initial increase exists in the exposed group, the 2-year Vmaxc is significantly smaller than the 1-month group (p < 0.001). Within group differences in median Kfc values show a significant decrease in both 1-month and 2-year groups among control and exposed mice (p < 0.001). Although no changes in methylene chloride solubility as a result of prior exposure were observed in blood, muscle, liver, or lung, a marginal decrease in the fat:air partition coefficient was found in the exposed mice at p = 0.053. Age related solubility differences were found in muscle:air, liver:air, lung:air, and fat:air partition coefficients at p < 0.001, while the solubility of methylene chloride in blood was not affected by age (p = 0.461). As a result of this study, we conclude that age and prior exposure to methylene chloride can produce notable changes in disposition and metabolism and may represent important factors in the interpretation for toxicologic data and its application to risk assessment.

Loss of heterozygosity on chromosomes 1, 11, 12, and 14 in hybrid mouse lung adenocarcinomas

An allelotype analysis of lung tumors in mouse hybrids was conducted to identify common regions of allelic loss. By using 50 informative genetic markers, the autosomes of 36 (A/J x C3H/HeJ) F1 adenocarcinomas were examined. Additional adenocarcinomas from as many as 72 (C3H/HeJ x A/J) F1 and 15 (BALB/cJ x DBA/2J) F1 hybrids also were analyzed for DNA loss at some of the loci. Loss of heterozygosity (LOH) was observed at multiple loci and occurred with the most regularity at markers on chromosomes 12 (28%), 14 (28%), 11 (21%), and 1 (20%). The frequency of LOH was not greater than 11% on any of the other chromosomes. Chromosomes 11 and 14 often displayed allelic loss at markers located near the p53 and retinoblastoma tumor suppressor loci, respectively. LOH at markers on chromosomes 12 and 14 was associated with tumors having overall frequencies of allelic loss that exceeded the median value. Losses on chromosomes 1, 11, 12, and 14 also showed a significant association with the adenocarcinoma stage of mouse lung tumorigenesis, suggesting that the inactivation of tumor suppressor loci on these chromosomes may participate in the progression of these tumors.

Hepatic and pulmonary carcinogenicity of methylene chloride in mice: a search for mechanisms

An inhalation study utilizing over 1400 female B6C3F1 mice was undertaken to study mechanistic factors associated with liver and lung tumor induction following exposure to 2000 ppm of methylene chloride. Mice were exposed to methylene chloride (treated) or chamber air (controls) 6 h per day, for varying durations up to 104 weeks. Several interim sacrifices and 'stop exposures' were included. Exposure to 2000 ppm methylene chloride caused an increase in liver and lung neoplasia in the absence of overt cytotoxicity. Measurement of replicative DNA synthesis done after 13, 26, 52 and 78 weeks of exposure showed a significant decrease in the hepatocyte labeling index at 13 weeks. Replicative DNA synthesis in pulmonary airways after 1, 2, 3, 4, 13 and 26 weeks of exposure to methylene chloride was significantly lower than in air-exposed controls. Likewise, the increase in tumor induction in treated mice was not associated with increased replicative DNA synthesis in liver foci or in alveolar parenchyma. The frequency and pattern of H-ras gene activation were similar in control and methylene chloride-induced liver neoplasms. Similarly, the frequency and pattern of K-ras activation in lung neoplasms were not altered by exposure to methylene chloride. Early exposure to methylene chloride for only 26 weeks was sufficient to cause an increase in lung tumors by 2 years, suggesting that methylene chloride may cause early and persistent loss of growth control in lung cells. This implies that risk management strategies should be aimed at minimizing or eliminating exposure to methylene chloride. Liver neoplasms continued to increase in incidence and multiplicity as exposure continued, suggesting that methylene chloride-induced hepatocarcinogenesis is facilitated by continuing exposure to methylene chloride. Since methylene chloride is a more potent inducer of lung than liver neoplasia, it is recommended that health risk assessment be based on the lung data. While no novel molecular lesions have been found to explain the induction of lung and liver neoplasia in mice, ongoing studies may identify other molecular changes that are important in the genesis of these neoplasms. Hence, it may be necessary to revise risk assessment and management strategies in light of future research findings.

Analysis of loss of heterozygosity in murine hepatocellular tumors

Because allelotype analysis of tumors has been important in the identification of new tumor suppressor genes, here we studied loss of heterozygosity (LOH) in a well-defined animal experimental system. We analyzed spontaneous liver tumors from C3HHc x C57BL/6J (B6C3F1) mice and urethane-induced hepatocellular tumors from (C3H/He x Mus spretus) x C57BL/6JBy (HSB) interspecific mice. A total of 95 different genetic markers were tested: 13 in 24 B6C3F1 tumors, 76 in 58 HSB tumors, and six in both groups. Minisatellite finger-printing analysis detected one case of LOH and less than 1% genomic rearrangements in polymorphic and nonpolymorphic bands, respectively. There were no changes at hepatocellular susceptibility loci or at markers homologous to loci frequently lost in human hepatocellular carcinomas. Therefore, our results suggest that LOH and genomic rearrangements are uncommon in mouse hepatocellular tumors.

Molecular aspects of chemical carcinogenesis: the roles of oncogenes and tumour suppressor genes

The observation that oncogenes are frequently activated in human tumours raises the question of whether these genes are involved in chemical carcinogenesis. H-ras activation is probably an initiating event in mouse skin and rat mammary gland systems. The H-ras oncogene is also important in mouse liver tumours; in mouse lung the K-ras gene is commonly activated. In both, the mutations observed are usually those predicted from the adduct-forming properties of the carcinogen. Among non-ras oncogenes, only raf and neu have been detected in experimental tumours. Tumour suppressor genes are frequently inactivated in human tumours. Searches for such phenomena in animal tumours have generally had disappointing results. p53 and Rb gene alterations are rarely observed in chemically-induced tumours. The reason may be that unknown tumour suppressor genes are involved in animal tumour development. Several novel genes have been identified using animal tumour susceptibility models. Thus, ras genes are important in chemical carcinogenesis, but as the methodology for studying other genes improves, their roles will be seen in perspective.

Transformation of type 1 astrocytes with N-ethyl-N-nitrosourea: establishment of an in vitro system and the role of the p53 gene

N-ethyl-N-nitrosourea (ENU)-induced gliomas, animal models of human gliomas, are most frequently oligodendrocytic, while human gliomas tend to be astrocytic. To facilitate a detailed study of human glial carcinogenesis, we developed an in vitro system using type 1 astrocyte transformation with ENU. Type 1 astrocytes from fetal Wistar rat brain were treated by a single dose of ENU. Transformed colonies appeared 50 days after exposure to single doses of ENU greater than 150 micrograms/mL. Cloned cells from these colonies retained the immunohistochemical characteristics of type 1 astrocytes. They showed rapid growth and high saturation densities, colony formation in low (2%) serum medium and gave rise to tumors when injected into nude mice. When p53 expression was studied at each passage, a single cell positive for mutant p53 protein emerged 40 days after ENU treatment. In the next 1-3 passages, the mutant p53 positive cell formed piled-up colonies and exhibited dominant growth. Northern blot analysis showed markedly increased accumulations of p53 mRNA in transformed cells. This in vitro transformation system of type 1 astrocytes provides a valuable tool for further investigations of astrocyte carcinogenesis.