Genetics of murine lung tumors

Drug Sensitivity and Allele Specificity of First-Line Osimertinib Resistance EGFR Mutations

Osimertinib, a mutant-specific third-generation EGFR tyrosine kinase inhibitor, is emerging as the preferred first-line therapy for EGFR-mutant lung cancer, yet resistance inevitably develops in patients. We modeled acquired resistance to osimertinib in transgenic mouse models of EGFR^L858R -induced lung adenocarcinoma and found that it is mediated largely through secondary mutations in EGFR-either C797S or L718V/Q. Analysis of circulating free DNA data from patients revealed that L718Q/V mutations almost always occur in the context of an L858R driver mutation. Therapeutic testing in mice revealed that both erlotinib and afatinib caused regression of osimertinib-resistant C797S-containing tumors, whereas only afatinib was effective on L718Q mutant tumors. Combination first-line osimertinib plus erlotinib treatment prevented the emergence of secondary mutations in EGFR. These findings highlight how knowledge of the specific characteristics of resistance mutations is important for determining potential subsequent treatment approaches and suggest strategies to overcome or prevent osimertinib resistance in vivo. SIGNIFICANCE: This study provides insight into the biological and molecular properties of osimertinib resistance EGFR mutations and evaluates therapeutic strategies to overcome resistance. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/80/10/2017/F1.large.jpg.

Germline control of somatic Kras mutations in mouse lung tumors

Somatic KRAS mutations are common in human lung adenocarcinomas and are associated with worse prognosis. In mice, Kras is frequently mutated in both spontaneous and experimentally induced lung tumors, although the pattern of mutation varies among strains, suggesting that such mutations are not random events. We tested if the occurrence of Kras mutations is under genetic control in two mouse intercrosses. Codon 61 mutations were prevalent, but the patterns of nucleotide changes differed between the intercrosses. Whole genome analysis with SNPs in (A/J x C57BL/6)F4 mice revealed a significant linkage between a locus on chromosome 19 and 2 particular codon 61 variants (CTA and CGA). In (AIRmax × AIRmin) F2 mice, there was a significant linkage between SNPs located on distal chromosome 6 (around 135 Mbp) and the frequency of codon 61 mutation. These results reveal the presence of two loci, on chromosomes 6 and 19, that modulate Kras mutation frequency in different mouse intercrosses. These findings indicate that somatic mutation frequency and type are not simple random events, but are under genetic control.

Complex genetic control of lung tumorigenesis in resistant mice strains

The SM/J mouse strain is resistant to chemically‐induced lung tumorigenesis despite having a haplotype, in the pulmonary adenoma susceptibility locus (Pas1) locus, that confers tumor susceptibility in other strains. To clarify this inconsistent genotype‐phenotype correlation, we crossed SM/J mice with another resistant strain and conducted genome‐wide linkage analysis in the (C57BL/6J × SM/J)F2 progeny exposed to urethane to induce lung tumors. Overall, >80% of F2 mice of both sexes developed from 1 to 20 lung tumors. Genotyping of 372 F2 mice for 744 informative non‐redundant SNPs dispersed over all autosomal chromosomes revealed four quantitative trait loci (QTLs) affecting lung tumor multiplicity, on chromosomes 3 (near rs13477379), 15 (rs6285067), 17 (rs33373629) and 18 (rs3706601), all with logarithm of the odds (LOD) scores >5. Four QTLs modulated total lung tumor volume, on chromosome 3 (rs13477379), 10 (rs13480702), 15 (rs6285067) and 17 (rs3682923), all with LOD scores >4. No QTL modulating lung tumor multiplicity or total volume was detected in Pas1 on chromosome 6. The present study demonstrates that the SM/J strain carries, at the Pas1 locus, the resistance allele: a finding that will facilitate identification of the Pas1 causal element. More generally, it demonstrates that lung tumorigenesis is under complex polygenic control even in a pedigree with low susceptibility to this neoplasia, suggesting that the genetics of lung tumorigenesis is much more complex than evidenced by the pulmonary adenoma susceptibility and resistance loci that have, so far, been mapped in a small number of crosses between a few inbred strains.

Afatinib plus Cetuximab Delays Resistance Compared to Single-Agent Erlotinib or Afatinib in Mouse Models of TKI-Naïve EGFR L858R-Induced Lung Adenocarcinoma

PURPOSE

The EGFR tyrosine kinase inhibitors (TKIs), erlotinib and afatinib, have transformed the treatment of advanced EGFR-mutant lung adenocarcinoma. However, almost all patients who respond develop acquired resistance on average approximately 1 year after starting therapy. Resistance is commonly due to a secondary mutation in EGFR (EGFR(T790M)). We previously found that the combination of the EGFR TKI afatinib and the EGFR antibody cetuximab could overcome EGFR(T790M)-mediated resistance in preclinical models. This combination has shown a 29% response rate in a clinical trial in patients with acquired resistance to first-generation TKIs. An outstanding question is whether this regimen is beneficial when used as first-line therapy.

EXPERIMENTAL DESIGN

Using mouse models of EGFR-mutant lung cancer, we tested whether the combination of afatinib plus cetuximab delivered upfront to mice with TKI-naïve EGFR(L858R)-induced lung adenocarcinomas delayed tumor relapse and drug-resistance compared with single-agent TKIs.

RESULTS

Afatinib plus cetuximab markedly delayed the time to relapse and incidence of drug-resistant tumors, which occurred in only 63.6% of the mice, in contrast to erlotinib or afatinib treatment where 100% of mice developed resistance. Mechanisms of tumor escape observed in afatinib plus cetuximab resistant tumors include the EGFR(T790M) mutation and Kras mutations. Experiments in cell lines and xenografts confirmed that the afatinib plus cetuximab combination does not suppress the emergence of EGFR(T790M).

CONCLUSIONS

These results highlight the potential of afatinib plus cetuximab as an effective treatment strategy for patients with TKI-naïve EGFR-mutant lung cancer and indicate that clinical trial development in this area is warranted.

Animal Models of Chemical Carcinogenesis: Driving Breakthroughs in Cancer Research for 100 Years

The identification of carcinogens in the workplace, diet, and environment through chemical carcinogenesis studies in animals has directly contributed to a reduction of cancer burden in the human population. Reduced exposure to these carcinogens through lifestyle changes, government regulation, or change in industry practices has reduced cancer incidence in exposed populations. In addition to providing the first experimental evidence for cancer's relationship to chemical and radiation exposure, animal models of environmentally induced cancer have and will continue to provide important insight into the causes, mechanisms, and conceptual frameworks of cancer. More recently, combining chemical carcinogens with genetically engineered mouse models has emerged as an invaluable approach to study the complex interaction between genotype and environment that contributes to cancer development. In the future, animal models of environmentally induced cancer are likely to provide insight into areas such as the epigenetic basis of cancer, genetic modifiers of cancer susceptibility, the systems biology of cancer, inflammation and cancer, and cancer prevention.

Pharmacokinetic and Genomic Effects of Arsenite in Drinking Water on Mouse Lung in a 30-Day Exposure

The 2 objectives of this subchronic study were to determine the arsenite drinking water exposure dependent increases in female C3H mouse liver and lung tissue arsenicals and to characterize the dose response (to 0, 0.05, 0.25, 1, 10, and 85 ppm arsenite in drinking water for 30 days and a purified AIN-93M diet) for genomic mouse lung expression patterns. Mouse lungs were analyzed for inorganic arsenic, monomethylated, and dimethylated arsenicals by hydride generation atomic absorption spectroscopy. The total lung mean arsenical levels were 1.4, 22.5, 30.1, 50.9, 105.3, and 316.4 ng/g lung tissue after 0, 0.05, 0.25, 1, 10, and 85 ppm, respectively. At 85 ppm, the total mean lung arsenical levels increased 14-fold and 131-fold when compared to either the lowest noncontrol dose (0.05 ppm) or the control dose, respectively. We found that arsenic exposure elicited minimal numbers of differentially expressed genes (DEGs; 77, 38, 90, 87, and 87 DEGs) after 0.05, 0.25, 1, 10, and 85 ppm, respectively, which were associated with cardiovascular disease, development, differentiation, apoptosis, proliferation, and stress response. After 30 days of arsenite exposure, this study showed monotonic increases in mouse lung arsenical (total arsenic and dimethylarsinic acid) concentrations but no clear dose-related increases in DEG numbers.

Nrf2 prevents initiation but accelerates progression through the Kras signaling pathway during lung carcinogenesis

Nrf2 (Nfe2l2) governs cellular defenses against oxidative and electrophilic stresses and protects against chemical carcinogenesis. However, many cancers have been found to accumulate NRF2 protein, raising questions of precisely how Nrf2 contributes to carcinogenesis. In this report, we explored such questions in an established urethane-induced multistep model of lung carcinogenesis. Consistent with earlier observations, Nrf2-deficient (Nrf2(-/-)) mice exhibited a relative increase in tumor foci by 8 weeks after urethane administration. However, after 16 weeks, we observed a relative reduction in the number of tumors with more malignant characteristics in Nrf2(-/-) mice. Furthermore, all Nrf2(+/+) tumors harbored activated mutations in Kras, whereas Nrf2(-/-) tumors were rarely associated with similar Kras mutations. Overall, our results established that Nrf2 has two roles during carcinogenesis, one of which is preventive during tumor initiation and the second that promotes malignant progression. These findings establish Nrf2 inhibitors as rational tools to prevent malignant progression in lung cancer, whereas Nrf2 activators are more suited for lung cancer prevention.

Erlotinib resistance in mouse models of epidermal growth factor receptor-induced lung adenocarcinoma

Seventy-five percent of lung adenocarcinomas with epidermal growth factor receptor (EGFR) mutations respond to treatment with the tyrosine kinase inhibitors (TKIs) gefitinib and erlotinib; however, drug-resistant tumors eventually emerge. In 60% of cases, resistant tumors carry a secondary mutation in EGFR (T790M), amplification of MET, or both. Here, we describe the establishment of erlotinib resistance in lung tumors, which were induced by mutant EGFR, in transgenic mice after multiple cycles of drug treatment; we detect the T790M mutation in five out of 24 tumors or Met amplification in one out of 11 tumors in these mice. This preclinical mouse model, therefore, recapitulates the molecular changes responsible for resistance to TKIs in human tumors and holds promise for the discovery of additional mechanisms of drug resistance in lung cancer.

Mechanisms Involved in A/J Mouse Lung Tumorigenesis Induced by Inhalation of an Environmental Tobacco Smoke Surrogate

Lung tumors have been reproducibly induced in A/J mice exposed to a surrogate for experimental environmental tobacco smoke (ETSS) in a 5-mo inhalation period followed by 4 mo without further exposure. In order to increase our mechanistic understanding of this model, male mice were whole-body exposed for 6 h/d, 5 d/wk to ETSS with a particulate matter concentration of 100 mg/m(3). Food restriction regimens were included to model or exceed the ETSS-related impairment of body weight development. Half of the mice were pretreated with a single ip injection of urethane to study the effect of the above treatments on lung tumor development induced by this substance. At 5 mo, the tumor response was statistically the same for all groups of non-pretreated mice; however, the expected urethane-induced lung tumorigenesis was significantly inhibited by approximately 25% by ETSS and food restriction. This inhibition was accompanied by a threefold increase in blood corticosterone as a common stress marker for both ETSS and food restriction. At 9 mo, in mice not pretreated, the lung tumor incidence and multiplicity were significantly increased by twofold in the ETSS group; in the urethane-treated groups, the same high tumor multiplicity was reached regardless of previous treatment. The predominant tumor type in all groups was bronchiolo-alveolar adenoma. There was no induction of a specific K-ras mutation pattern by ETSS exposure. These data suggest a stress-induced inhibition of lung tumorigenesis in this model, explaining the need for the posttreatment period.

Cis-acting genomic elements of the Pas1 locus control Kras mutability in lung tumors

The Pas1 locus is the major tumor modifier of lung tumorigenesis in mouse inbred strains. Of six genes contained in a conserved haplotype, three (Casc1, Kras and Ifltd1) have been proposed as Pas1 candidates, but mechanistic evidence is sparse. Herein, we examined urethane-induced lung tumorigenesis in a new mouse model developed by replacing the Kras gene with an Hras gene in the susceptible A/J-type Pas1 locus and crossing these mice with either C57BL/6J or A/J mice. Heterozygous mice carrying the Hras-replacement gene were more susceptible than wild-type mice to lung carcinogenesis, indicating that Hras replacement not only compensates for Kras functions, but is more active. Indeed, most lung tumors carried a Gln61Leu mutation in the Hras-replacement gene, whereas no mutations were observed in the endogenous Hras gene. Thus, the context of the Kras locus determined mutability of ras genes. In mice carrying the Hras-replacement gene, the mutation frequency affecting the wild-type Kras gene was much higher when this gene was located in the A/J type than in the C57BL/6J-type Pas1 locus (12 versus 0%, -log P=5.0). These findings identify cis-acting elements in the Pas1 locus as the functional components controlling genetic susceptibility to lung tumorigenesis by modulating mutability of the Kras gene.

Strain-specific induction of murine lung tumors following in utero exposure to 3-methylcholanthrene

Fetal mice are more sensitive to chemical carcinogens than are adults. We previously demonstrated that resistant offspring of a DBA/2 x (C57BL/6 x DBA2) backcross exhibited a high incidence of lung tumors 12-13 mo after transplacental exposure to 3-methylcholanthrene (MC). We compared the effects of in utero treatment with MC on lung tumor incidence in the offspring of intermediately susceptible BALB/c (C), resistant C57BL/6 (B6), and reciprocal crosses between these strains. Pregnant mice were treated with 45 mg/kg of MC on day 17 of gestation and tumor incidence, multiplicity, and the Ki-ras mutational spectrum determined in the offspring 12-18 mo after birth. Tumor incidences in C mice and reciprocal crosses were 86% and 100%, respectively, while B6 mice demonstrated resistance to tumorigenesis, with a tumor incidence of 11%. Tumor multiplicities in C, B6C, CB6, and B6 mice were 3.3 +/- 3.2, 5.8 +/- 3.2, 5.0 +/- 2.7, and <0.1, respectively. Ki-ras mutations, which occurred chiefly in the K(s) allele (96%), were found in 79-81% of reciprocally crossed F1 mice, 64% of C mice, and 50% of B6 mice, with the Val(12), Asp(12), and Arg(13) mutations associated with more aggressive tumors. A subset of these mice was used to demonstrate the utility of computer tomography (CT) for the visualization and measurement of lung tumors in the submillimeter range in vivo. Based on known genetic differences in murine strains for lung cancer, our results suggest the presence of a previously unidentified genetic factor(s) which appears to specifically influence lung tumorigenesis following exposure to carcinogens during fetal development.

A V141L polymorphism of the human LRMP gene is associated with survival of lung cancer patients

Mouse Lrmp and Casc1 genes are candidates for the pulmonary adenoma susceptibility 1 (Pas1) locus, the major determinant of strain variation in lung tumor susceptibility. These genes contain coding and non-coding single nucleotide polymorphisms (SNPs) strongly associated with lung tumor risk in mice. Analysis of LRMP and CASC1 gene SNPs in 361 lung adenocarcinoma (ADCA) patients and 327 healthy controls revealed common SNPs in LRMP (V141L and S197C) and CASC1 (R33S and three intronic variations), and none showed a significant association with lung ADCA risk. However, in the time-dependent Cox regression model, after adjustment for age, gender, smoking history and clinical stage, the carrier status of the Leu variation (V141L) of the LRMP gene was associated with higher mortality in patients with age at tumor onset < or = 65 years [hazard ratio (HR) 2.3; 95% CI 1.4-3.7; P = 0.001]. These findings suggest that the LRMP V141L polymorphism can predict survival in lung ADCA and that the role of LRMP and CASC1 in human lung cancer risk may differ from that in mice.

Induction of Cyp1a1 and Cyp1b1 and formation of DNA adducts in C57BL/6, Balb/c, and F1 mice following in utero exposure to 3-methylcholanthrene

Fetal mice are more sensitive to chemical carcinogens than are adults. Previous studies from our laboratory demonstrated differences in the mutational spectrum induced in the Ki-ras gene from lung tumors isolated from [D2 x B6D2F1]F2 mice and Balb/c mice treated in utero with 3-methylcholanthrene (MC). We thus determined if differences in metabolism, adduct formation, or adduct repair influence strain-specific responses to transplacental MC exposure in C57BL/6 (B6), Balb/c (BC), and reciprocal F1 crosses between these two strains of mice. The induction of Cyp1a1 and Cyp1b1 in fetal lung and liver tissue was determined by quantitative fluorescent real-time PCR. MC treatment caused maximal induction of Cyp1a1 and Cyp1b1 RNA 2-8 h after injection in both organs. RNA levels for both genes then declined in both fetal organs, but a small biphasic, secondary increase in Cyp1a1 was observed specifically in the fetal lung 24-48 h after MC exposure in all four strains. Cyp1a1 induction by MC at 4 h was 2-5 times greater in fetal liver (7000- to 16,000-fold) than fetal lung (2000- to 6000-fold). Cyp1b1 induction in both fetal lung and liver was similar and much lower than that observed for Cyp1a1, with induction ratios of 8- to 18-fold in fetal lung and 10- to 20-fold in fetal liver. The overall kinetics and patterns of induction were thus very similar across the four strains of mice. The only significant strain-specific effect appeared to be the relatively poor induction of Cyp1b1 in the parental strain of B6 mice, especially in fetal lung tissue. We also measured the levels of MC adducts and their disappearance from lung tissue by the P(32) post-labeling assay on gestation days 18 and 19 and postnatal days 1, 4, 11, and 18. Few differences were seen between the different strains of mice; the parental strain of B6 mice had nominally higher levels of DNA adducts 2 (gestation day 19) and 4 (postnatal day 1) days after injection, although this was not statistically significant. These results indicate that differences in Phase I metabolism of MC and formation of MC-DNA adducts are unlikely to account for the marked differences observed in the Ki-ras mutational spectrum seen in previous studies. Further, the results suggest that other genetic factors may interact with chemical carcinogens in determining individual susceptibility to these agents during development.

The Mouse Tumor Biology Database: integrated access to mouse cancer biology data

Mice have long been used as models for the study of human cancer. The National Cancer Institute has included among its research areas of extraordinary opportunity the development of new mouse genetic models of human cancer and the exploration of cancer imaging as a research tool. Because of the volume and interconnectedness of relevant data, the creation and maintenance of bioinformatics resources of mouse tumor biology is necessary to facilitate current and future cancer research. The Mouse Tumor Biology (MTB) Database provides electronic access to data generated through the study of spontaneous and induced tumors in genetically defined mice (inbred, hybrid, spontaneous and induced mutant, and genetically engineered strains of mice).

Genetic linkage between Pol iota deficiency and increased susceptibility to lung tumors in mice

Pol iota is a member of the Y-family DNA polymerases, characterized by their capacity for translesion DNA synthesis and low fidelity base incorporation, and has therefore been assumed to play important roles in mutagenesis and carcinogenesis. In fact, the mouse Pol iota gene is located within the Par2 (pulmonary adenoma resistance 2) locus on distal chromosome 18, which we have identified as a major susceptibility locus regarding urethane induction of pulmonary adenomas. Indeed, Pol iota has been suggested to be a candidate for Par2 from both the genetic and biological standpoints. Taking advantage of 129X1/SvJ mice naturally deficient in Pol iota due to a nonsense mutation within the coding region of the gene, we here analyzed urethane-treated (A/J x 129X1/SvJ)F(1) x A/J backcross and (A/J x 129X1/SvJ)F(2) intercross mice and observed the defective 129X1/SvJ Pol iota allele to be genetically linked with an increased susceptibility to lung tumors relative to the A/J allele. Thus, among the already known mouse Pol iota alleles, the defective 129X1/SvJ allele is associated exclusively with the highest susceptibility to lung tumors. The result indicates a possibility that the Pol iota gene may participate in error-free repair of damaged DNA and prevention of lung tumor development.

Genetic selection for high acute inflammatory response confers resistance to lung carcinogenesis in the mouse

Mice selected for a high acute inflammatory response (AIRmax) are resistant to chemically induced lung tumorigenesis, whereas the low responders (AIRmin) are susceptible. In urethane-treated mice, anti-inflammatory drugs increased the tumor incidence in AIRmax but not AIRmin mice, and an inverse correlation (P<.001) between the degree of acute inflammatory response (AIR) and lung tumorigenesis was found in an F2 (AIRmax x AIRmin) intercross population. The results provide evidence for the involvement of lung tumor modifier loci in AIR regulation and implicate AIR quantitative trait loci in the inherited predisposition to lung cancer.

Immunity against mouse thymus-leukemia antigen (TL) protects against development of lymphomas induced by a chemical carcinogen, N-butyl-N-nitrosourea

Mouse thymus-leukemia antigens (TL) are aberrantly expressed on T lymphomas in C57BL/6 (B6) and C3H/He (C3H) mice, while they are not expressed on normal T lymphocytes in these strains. When N-butyl-N-nitrosourea (NBU), a chemical carcinogen, was administered orally to B6 and C3H strains, lymphoma development was slower than in T3(b)-TL gene-transduced counterpart strains expressing TL ubiquitously as self-antigens, suggesting that anti-TL immunity may play a protective role. In addition, the development of lymphomas was slightly slower in C3H than in B6, which seems to be in accordance with the results of skin graft experiments indicating that both cellular and humoral immunities against TL were stronger in C3H than B6 mice. The interesting finding that B lymphomas derived from a T3(b)-TL transgenic strain (C3H background) expressing a very high level of TL were rejected in C3H, but not in H-2K(b) transgenic mice (C3H background), raises the possibility that TL-specific effector T cell populations are eliminated and/or energized to a certain extent by interacting with H-2K(b) molecules.

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.

Molecular profiling of mouse lung tumors: association with tumor progression, lung development, and human lung adenocarcinomas

We have performed oligonucleotide array analysis on various murine lung tissues [normal lungs, lung adenomas, and lung adenocarcinomas (ACs)] using Affymetrix U74Av2 GeneChips to examine the complex genetic changes occurring during lung carcinogenesis. Analysis yielded 20 novel genes differentially expressed in both lung adenomas and ACs versus normal lungs, including the tumor suppressor APC2 and the oncogene Ros 1. In addition, 50 genes were found to be differentially expressed in lung adenomas versus lung ACs, including the differentiation factor Hox C6, the oncogene Ets 2, and the Ras nuclear transport factor, nuclear transport factor 2. To understand the potential relationship between genes expressed in murine lung tumors and its relationship to altered gene expression observed during embryogenesis and postnatal development, tissues from embryonic lungs and from lungs of mice up to 4 weeks following birth were examined using Affymetrix U74Av2 GeneChips. From this analysis, approximately 1300 genes were determined to exhibit differential expression in fetal lung versus postnatal lung. When we compared lung adenomas, lung ACs, and normal lung parenchyma, 24 developmentally regulated genes were found aberrantly expressed in lung tumors; these included the cell cycle control factor CDC5, the cellular differentiation factor TEA domain 4, and the proapoptotic factor BNIP 2. Finally, we compared the murine lung tumor gene expression data to the expression of genes in human lung cancer, in order to assess the relevance of murine lung cancer models in the study of human AC formation. When the 17 human lung ACs and six human lung large cell carcinomas were examined, it was found that 13 of the 17 human lung ACs clustered tightly together in a pattern that was different from the remaining four human lung ACs and six large cell carcinomas, which exhibited a different pattern. Interestingly, the mouse lung adenomas appeared similar to 13 clustered ACs, while mouse lung ACs appeared more similar in pattern to the group consisting of four ACs and six large-cell carcinomas (LCCs). Nevertheless, when compared with the combined human ACs, 39 genes with similar expression changes in murine lung tumors and human ACs/LCCs were identified, such as the oncogene-related BCL7B, the cell cycle regulator CDK4, and the proapoptotic Endophilin B1. Overall, we have determined, for the first time, the expression profiles during murine lung tumor progression and have established, at the molecular level, an association between murine lung tumorigenesis and lung development. We have also attempted to compare the expression profiles found in mouse lung cancers and those in human lung ACs.

Genetic susceptibility to thymic lymphomas and K-ras gene mutation in mice after exposure to X-rays and N-ethyl-N-nitrosourea

PURPOSE

Ras activation is one of the major mechanisms for the development of murine thymic lymphomas by radiation and chemical carcinogens. To gain insight into the relationship between genetic susceptibility and ras gene mutation, the frequency and spectrum of ras gene mutation was examined in thymic lymphomas from susceptible and resistant mice.

MATERIALS AND METHODS

K- and N-ras mutations in thymic lymphomas that arose in X-ray-irradiated and N-ethyl-N-nitrosourea (ENU)-treated mice of susceptible C57BL/6, rather resistant C3H and their hybrid B6C3F1 were analysed by polymerase chain reaction-single-strand conformation polymorphism and subsequent DNA sequencing.

RESULTS

C57BL/6 exhibited a higher incidence of thymic lymphomas after exposure to X-rays and ENU than C3H, with B6C3F1 being intermediate. K-ras gene mutations occurred frequently in the pathogenesis of ENU-induced thymic lymphomas in susceptible C57BL/6 as opposed to resistant C3H. The ras mutations were more frequent in ENU-induced thymic lymphomas than X-ray-induced thymic lymphomas, and with the latter, there was no clear evidence for strain differences, suggesting that the genetic susceptibility to X-rays was independent of ras activation. The mutations of K-ras in thymic lymphomas from C57BL/6 were predominantly GGT to GAT in codon 12, whereas this mutation type was never found in those from C3H. No strain difference was observed in the nucleotide sequence or expression levels of O(6)-alkylguanine alkyltransferase, indicating that this enzyme did not account for the genetic susceptibility to ras activation.

CONCLUSIONS

The results indicate that there is a clear strain and carcinogen dependency of K-ras mutation and that the frequency of ras mutation might determine the genetic susceptibility to ENU-induced lymphomagenesis, whereas pathways independent of ras activation might determine the susceptibility to X-ray-induced lymphomagenesis.

Analysis of lung tumorigenesis in chimeric mice indicates the Pulmonary adenoma resistance 2 (Par2) locus to operate in the tumor-initiation stage in a cell-autonomous manner: detection of polymorphisms in the Poli gene as a candidate for Par2

The Pulmonary adenoma resistance 2 (Par2) locus of the BALB/cByJ mouse, located within 0.5 cM of chromosome 18, is responsible for reducing the mean multiplicity of urethane-induced lung tumors relative to those in C57BL/6J, A/J and C3H/HeJ mice. Thus, BALB/B6-Par2 congenic strain genetically identical to BALB/cByJ except carrying C57BL/6J Par2 alleles develops seven times more tumors than BALB/cByJ. To gain clues for identification of Par2 candidate genes, we analysed lung tumorigenesis in BALB/cByJ<-->BALB.B6-Par2 chimeric animals. Of 100 tumors induced by urethane in 16 chimeras, 82 originated from BALB.B6-Par2 cells, indicating the Par2 phenotype to be cell-autonomous. In addition, the BALB.B6-Par2- and BALB/cByJ-derived tumors were similar in mean size, implying that the phenotype is primarily expressed during initiation rather than in the promotion stage of carcinogenesis. Given these results, we surveyed a comprehensive mouse genome database and physically mapped Par2 within a 2.3 Mbp segment containing three known genes, Poli, Mbd2 and Dcc. Among those, the Poli seemed to be the most reasonable Par2 candidate, since it encodes an extremely error-prone DNA polymerase preferentially incorporating G or T opposite template T in vitro, reminiscent of the Kras2 activation because of an A to G or T point mutation within codon 61 with which most urethane-induced lung tumors are initiated. Indeed, our sequencing of Poli cDNAs from BALB/cByJ, C57BL/6J, A/J and C3H/HeJ lungs revealed 21 BALB/cByJ-specific single-nucleotide polymorphisms in the coding region accompanied by seven amino-acid substitutions and an elevated frequency of alternative splicing, while no polymorphisms associated with tumor susceptibility were found for either Mbd2 or Dcc. Notably, we obtained evidence that BALB/cByJ Par2 alleles may selectively decrease the frequency of Kras2-mutated tumors compared with C57BL/6J alleles. Consequently, the Poli is an intriguing Par2 candidate clearly deserving further evaluation.

Hypermethylation of the p16 (Ink4a) promoter in B6C3F1 mouse primary lung adenocarcinomas and mouse lung cell lines

Primary lung tumors from B6C3F1 mice and mouse lung cell lines were examined to investigate the role of transcriptional silencing of the p16 (Ink4a) tumor suppressor gene by DNA hypermethylation during mouse lung carcinogenesis. Hypermethylation (>/=50% methylation at two or more of the CpG sites examined) of the p16 (Ink4a) promoter region was detected in DNA from 12 of 17 (70%) of the B6C3F1 primary mouse lung adenocarcinomas examined, whereas hypermethylation was not detected in normal B6C3F1, C57BL/6 and C3H/He mouse lung tissues. Immunohistochemistry performed on the B6C3F1 lung adenocarcinomas revealed heterogeneous expression of the p16 protein within and among the tumors. Laser capture microdissection was employed to collect cells from immunostained sections of four tumors displaying areas of relatively high and low p16 expression. The methylation status of the microdissected samples was assessed by sodium bisulfite genomic sequencing. The pattern of p16 expression correlated inversely with the DNA methylation pattern at promoter CpG sites in nine of 11 (82%) of the microdissected areas displaying variable p16 expression. To provide further evidence that hypermethylation is involved in the loss of p16 (Ink4a) gene expression, three mouse lung tumor cell lines (C10, sp6c and CMT64) displaying complete methylation at seven promoter CpG sites and no p16 (Ink4a) expression were treated with the demethylating agent, 5-aza-2'-deoxycytidine. Re-expression of p16 (Ink4a) and partial demethylation of the p16 (Ink4a) promoter were observed in two cell lines (C10 and sp6c) following treatment. These are the first reported studies to provide strong evidence that DNA methylation is a mechanism for p16 inactivation in mouse lung tumors.

Etheno-adduct-forming chemicals: from mutagenicity testing to tumor mutation spectra

During the past 25 years, ethenobases have emerged as a new class of DNA lesions with promutagenic potential. Ethenobases were first investigated as DNA reaction products of vinyl chloride, an occupational carcinogen causing angiosarcoma of the liver (ASL). They were subsequently shown to be formed by several carcinogenic agents, including urethane (ethyl carbamate), and more recently, to occur in various tissues of unexposed humans and rodents. The endogenous source of ethenobases in DNA is thought to be a lipid peroxidation (LPO) product. Initial studies on metabolic activation, mutagenicity and carcinogenicity moved to the analyses of the formation of ethenobases in vivo and to the determination of their promutagenic properties. Quantification of etheno adducts in vivo became possible with the development of ultrasensitive techniques of analysis. To study the miscoding properties of ethenobases, the initial assays on the fidelity of replication or of transcription were replaced by site-directed mutagenesis assays in vivo. Ethenobases generate mainly base pair substitution mutations. With the advent of new techniques of molecular biology, mutations were investigated in the ras and p53 genes of tumors induced by vinyl chloride and urethane. In liver tumors induced by vinyl chloride, specific mutational patterns were found in the Ki-ras gene in human ASL, in the Ha-ras gene in hepatocellular carcinoma (HCC) in rats, and in the p53 gene in human and rat ASL. In tumors induced by urethane in mice, codon 61 of the Ha-ras gene (liver, skin) and of the Ki-ras gene (lung) seems to be a characteristic target. These tumor mutation spectra are compatible with the promutagenic properties of etheno adducts and with their formation in target tissues, suggesting that ethenobases can be initiating lesions in carcinogenesis. Another recent focus has been given to the repair of etheno adducts, and DNA glycosylases able to excise these adducts in vitro have been identified. The last two decades have brought ethenobases to light as potentially important DNA lesions in carcinogenesis. More research is needed to better understand the environmental and genetic factors that affect the formation and persistence of ethenobases in vivo.

Predisposition to lung tumorigenesis

Mouse inbred strains with inherited predisposition and resistance to lung cancer provide an essential tool for the dissection of the genetics of this complex disease. We have previously mapped a major locus (Pulmonary adenoma susceptibility 1, Pas1) affecting inherited predisposition to lung cancer in mice on chromosome 6, near Kras2. Appropriate crosses that include susceptible mice (Pas1(s)) provide a model system for identifying loci that can modify the lung cancer predisposition phenotype caused by Pas1. Using this approach we have mapped the Pulmonary adenoma resistance 1 (Par1) locus that behaves like a modifier locus of Pas1. More recently, we mapped additional lung tumor resistance loci (Par2, and Par4), and a locus specifically involved with lung tumor progression (Papg1). The mapping of Pas1 in mice stimulated us to test the possible association of genetic markers located in the homologous human region (12p12) with risk and prognosis of lung adenocarcinomas in man. In the Italian population, we carried out an association study by genotyping lung adenocarcinoma patients and healthy controls for genetic markers located in the putative region of interest. Homozygosity of the A2 allele at a Kras2/RsaI polymorphism, and allele 2 at a VNTR polymorphism in the PTHLH gene showed borderline statistically significant associations with lung cancer risk. Furthermore, the same alleles were significantly associated with tumor prognosis. Studies on association were then performed in the Japanese and in European populations. In the Japanese population, the KRAS2/RsaI marker was significantly associated with prognosis of lung adenocarcinoma, whereas the European study did not confirm this association. Our results may provide evidence for the existence of the human PAS1 locus, suggesting that the mouse model of inherited predisposition to lung tumorigenesis is predictive of a human genetic mechanism of susceptibility to lung cancer.

Carcinogenesis in mouse and human cells: parallels and paradoxes

It has been known since the last century that genetic changes are important in carcinogenesis [Boveri,T. (1914) Zur Frage der Erstehung Maligner Tumoren. Gustav Fischer, Jena]. Observations of tumor cells growing in tissue culture led to the prediction, even before the true nature of the genetic material was known, that alterations at the chromosomal level were critically involved in the process of neoplastic development. The past 20 years have seen the transition of carcinogenesis studies from the purely observational to the molecular genetic level. Although much more needs to be done, it is nevertheless gratifying to be able to piece together the sequence of events from carcinogen exposure, metabolism of the carcinogen to the activated form, formation of specific carcinogen-DNA adducts, misrepair leading to the fixation of mutations in particular target genes, and the resulting selective outgrowth of neoplastic cells. The nature of many of these steps has been clarified only in the relatively recent past, and only for a small number of specific target genes, but the fact that we can say with confidence that such processes occur and are causal changes in tumorigenesis represents a tremendous advance over the situation pertaining 20 years ago. The purpose of this review is to summarize the advances over this time period in our understanding of some of the genetic alterations that contribute to neoplasia, with particular emphasis on chemical carcinogenesis in rodents and the parallels with transformation of human cells.

Lung cancer: occurrence and new possibilities for detection

Lung cancer is the leading cause of death worldwide. Physical and chemical agents such as tobacco smoke are the leading cause of various lung cancers. The intrinsic heterogeneity of normal lung tissue may be affected in different ways, giving rise to different types of lung cancers classified as either small‐cell lung cancer (SCLC) or non‐small cell lung cancer (NSCLC). Adenocarcinoma, a NSCLC, accounts for 40 percent of all lung cancer cases and the incidence is increasing worldwide, especially among women. The survival rate and prognosis is poorest for adenocarcinoma. Therefore, diagnosis at the earliest stage (Stage I, localized) is critical for increasing survival rates of those suffering from lung cancer. However, many factors affect early diagnosis including the variable natural growth of tumors plus technological and human factors associated with manipulation of tissue samples and interpretation of results. This article reviews potential problems associated with diagnosing lung cancer and considers future directions of diagnostic technology.

Genetic mapping of cancer susceptibility/resistance loci in the mouse

Genetic linkage experiments using crosses between mouse inbred strains with an inherited predisposition and resistance to lung cancer make it possible to investigate the genetics of the complex inheritance of susceptibility and resistance to lung cancer. We have previously mapped a major locus (pulmonary adenoma susceptibility 1, Pas1) affecting inherited predisposition to lung cancer in mice onto chromosome 6, near Kras2. Appropriate crosses that include Pas1/+ mice provide a model system for identifying loci that can modify the lung cancer predisposition phenotype caused by Pas1. Using this approach, we mapped the pulmonary adenoma resistance 1 (Par1) locus on to mouse chromosome 11; this locus selectively inhibits lung tumor development in Pas1/+ animals and therefore behaves like a modulator gene of Pas1. More recently, we have mapped lung tumor modifier loci specifically affecting the initiation and progression of lung cancer. Thus experimental models provide an essential tool for the mapping of lung cancer susceptibility/resistance genes and for the subsequent cloning of candidate genes.

Intratracheal injection of adenovirus containing the human MnSOD transgene protects athymic nude mice from irradiation-induced organizing alveolitis

PURPOSE

A dose and volume limiting factor in radiation treatment of thoracic cancer is the development of fibrosis in normal lung. The goal of the present study was to determine whether expression prior to irradiation of a transgene for human manganese superoxide dismutase (MnSOD) or human copper/zinc superoxide dismutase (Cu/ZnSOD) protects against irradiation-induced lung damage in mice.

METHODS AND MATERIALS

Athymic Nude (Nu/J) mice were intratracheally injected with 10(9) plaque-forming units (PFU) of a replication-incompetent mutant adenovirus construct containing the gene for either human MnSOD, human copper/zinc superoxide dismutase (Cu/ZnSOD) or LacZ. Four days later the mice were irradiated to the pulmonary cavity to doses of 850, 900, or 950 cGy. To demonstrate adenoviral infection, nested reverse transcriptase-polymerase chain reaction (RT-PCR) was carried out with primers specific for either human MnSOD or Cu/ZnSOD transgene on freshly explanted lung, trachea, or alveolar type II cells, and immunohistochemistry was used to measure LacZ expression. RNA was extracted on day 0, 1, 4, or 7 after 850 cGy of irradiation from lungs of mice that had previously received adenovirus or had no treatment. Slot blot analysis was performed to quantitate RNA expression for IL-1, tumor necrosis factor (TNF)-alpha, TGF-beta, MnSOD, or Cu/ZnSOD. Lung tissue was explanted and tested for biochemical activity of MnSOD or Cu/ZnSOD after adenovirus injection. Other mice were sacrificed 132 days after irradiation, lungs excised, frozen in OCT, (polyvinyl alcohol, polyethylene glycol mixture) sectioned, H&E stained, and evaluated for percent of the lung demonstrating organizing alveolitis.

RESULTS

Mice injected intratracheally with adenovirus containing the gene for human MnSOD had significantly reduced chronic lung irradiation damage following 950 cGy, compared to control mice or mice injected with adenovirus containing the gene for human Cu/ZnSOD or LacZ. Immunohistochemistry for LacZ protein in adenovirus LacZ (Ad-LacZ)-injected mice demonstrated expression of LacZ in both the upper and lower airway. Nested RT-PCR showed lung expression of MnSOD and Cu/ZnSOD for at least 11 days following infection with each respective adenovirus construct. Nested RT-PCR using primers specific for human MnSOD demonstrated increased expression of the human MnSOD transgene in the trachea and alveolar type II cells 4 days after virus injection on the day of irradiation. At this time point, increased biochemical activity of MnSOD and Cu/ZnSOD respectively, was detected in lungs from these two adenovirus groups, compared to each other or to control or adenovirus LacZ mice. Slot blot analysis of RNA from lungs of mice in each group following 850 cGy irradiation demonstrated decreased expression of mRNA for interleukin-I (IL-1), TNF-alpha, and transforming growth factor-beta (TGF-beta) in the MnSOD adenovirus-injected mice, compared to irradiated control, LacZ, or Cu/ZnSOD adenovirus-injected, irradiated mice. Mice receiving adenovirus MnSOD showed decreased organizing alveolitis at 132 days in all three dose groups, compared to irradiated control or Ad-LacZ, or Ad-Cu/ZnSOD mice.

CONCLUSIONS

Overexpression of MnSOD in the lungs of mice prior to irradiation prevents irradiation-induced acute and chronic damage quantitated as decreased levels of mRNA for IL-1, TNF-alpha, and TGF-beta in the days immediately following irradiation, and decrease in the percent of lung demonstrating fibrosis or organizing alveolitis at 132 days. These data provide a rational basis for development of gene therapy as a method of protection of the normal lung from acute and chronic sequelae of ionizing irradiation.

Progress in understanding the molecular pathogenesis of human lung cancer

We review the molecular pathogenesis of lung cancer including alterations in dominant oncogenes, recessive oncogenes/tumor suppressor genes, alterations in growth regulatory signaling pathways, abnormalities in other pathways, such as apoptosis, autocrine and paracrine growth stimulatory loops, angiogenesis, and host immune responses, other mechanisms of genetic changes, such as microsatellite and methylation alterations, and the potential for inherited predisposition to lung cancer. These changes are related to multistage carcinogenesis involving preneoplastic lesions, and lung development and differentiation. The translational applications of these findings for developing new ways of early detection, prevention, treatment, and prognosis of lung cancer are discussed.

A review of chronic inhalation studies with mainstream cigarette smoke in rats and mice

In this paper, I review the results of a representative selection of chronic inhalation studies with rats and mice exposed to mainstream cigarette smoke and describe the inhalation exposures and the histopathological changes reported by various authors. Many of the studies used nose-only exposure systems, whereas others simply used large whole-body chambers. Smoke-induced epithelial hypertrophy, hyperplasia, and squamous metaplasia were reported in the conducting airways in most of the studies, along with increased numbers of intra-alveolar macrophages that were occasionally associated with alveolar metaplasia. Lung adenomas and adenocarcinomas were reported in only a few of the studies. No statistically significant increase in the incidence of malignant lung tumors was seen in either species as a result of smoke exposure, a finding that does not agree with the results of epidemiological studies in humans. Possible reasons for this lack of correlation are given.

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.

Alterations in DNA methylation: a fundamental aspect of neoplasia

Neoplastic cells simultaneously harbor widespread genomic hypomethylation, more regional areas of hypermethylation, and increased DNA-methyltransferase (DNA-MTase) activity. Each component of this "methylation imbalance" may fundamentally contribute to tumor progression. The precise role of the hypomethylation is unclear, but this change may well be involved in the widespread chromosomal alterations in tumor cells. A main target of the regional hypermethylation are normally unmethylated CpG islands located in gene promoter regions. This hypermethylation correlates with transcriptional repression that can serve as an alternative to coding region mutations for inactivation of tumor suppressor genes, including p16, p15, VHL, and E-cad. Each gene can be partially reactivated by demethylation, and the selective advantage for loss of gene function is identical to that seen for loss by classic mutations. How abnormal methylation, in general, and hypermethylation, in particular, evolve during tumorigenesis are just beginning to be defined. Normally, unmethylated CpG islands appear protected from dense methylation affecting immediate flanking regions. In neoplastic cells, this protection is lost, possibly by chronic exposure to increased DNA-MTase activity and/or disruption of local protective mechanisms. Hypermethylation of some genes appears to occur only after onset of neoplastic evolution, whereas others, including the estrogen receptor, become hypermethylated in normal cells during aging. This latter change may predispose to neoplasia because tumors frequently are hypermethylated for these same genes. A model is proposed wherein tumor progression results from episodic clonal expansion of heterogeneous cell populations driven by continuous interaction between these methylation abnormalities and classic genetic changes.

Pas1 is a common lung cancer susceptibility locus in three mouse strains

Inherited predisposition to lung cancer is a phenotypic trait shared by different mouse inbred strains that show either a high or an intermediate predisposition. Other strains are instead genetically resistant. The Pas1 locus is the major determinant of lung cancer predisposition in the A/J strain (Gariboldi et al. 1993). To define the determinants of susceptibility to lung tumorigenesis in the highly susceptible SWR/J and in the intermediately susceptible BALB/c mice, we analyzed (BALB/c x SWR/J)F2 and (BALB/c x C3H/He)F2 crosses by genetic linkage experiments. The present results provide unequivocal evidence that the same Pas1/+ allele that leads to lung cancer predisposition is shared by A/J, SWR/J, and BALB/c strains. The intermediate susceptibility of the BALB/c strain would result by interaction of Pas1 locus with lung cancer resistance loci.

Tissue-specific expression of the K-ras allele from the A/J parent in (A/J x TSG-p53) F1 mice

Tissue-specific expression of parental K-ras allele(s) was investigated by single-strand conformation polymorphism analysis of the 3' untranslated region of the K-ras gene in normal lung, spleen, liver and kidney from (A/J x TSG-p53) F1 mice. The expression of A/J K-ras allele was equal to that of C57BL/6J allele in normal spleen, liver and kidney. However, transcripts from A/J K-ras allele were found to be 2-12-times greater than those from C57BL/6J allele in lung tissues harvested over a 20-week period. Similar to our previous observation with dimethylnitrosamine- and benzo[a] pyrene-induced lung tumors, K-ras mRNA transcribed from A/J allele was 10-40-times more abundant than those from C57BL/6J allele in all of 40 (A/J x TSG-p53) F1 mouse lung tumors induced by 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone. In addition, K-ras mutations (G to A transitions at the second base of codon 12) were detected in 38 of 40 (95%) lung tumors and all of the mutations were found on the allele inherited from the A/J parent. These data demonstrate tissue-specific allele-specific transcription of the K-ras gene and provide further support to the thesis that K-ras allele itself is a primary mouse lung tumor susceptibility gene.

Induction of aberrant crypt foci in C57BL/6N mice by 2-amino-9H-pyrido[2,3-b]indole (A alphaC) and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx)

The inductions of aberrant crypt foci (ACF) by two carcinogenic heterocyclic amines (HCAs), 2-amino-9H-pyrido[2,3-b]indole (A alphaC) and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), were studied in the large intestine of C57BL/6N mice. Seven-week-old mice were fed a diet supplemented with 500 or 800 ppm of A alphaC, or 400 or 600 ppm of MeIQx for 7 weeks, followed by a basal diet for another 7 weeks. A alphaC at 800 ppm induced 8.0 +/- 1.9 and 7.8 +/- 2.5 ACF in female and male mice, respectively. MeIQx at 600 ppm induced 2.8 +/- 1.8 and 1.6 +/- 0.8 ACF in females and males, respectively. At lower concentrations of A alphaC and MeIQx, many fewer ACF were induced. No ACF were induced in the control group. The size of ACF (number of aberrant crypts/ACF) in all experimental groups was between 1.0 and 1.5. More than half the A alphaC-induced ACF were located in the region about 20-40% of the distance from the ileocecal portion to the anus. Although both of these HCAs were reported to induce tumors in the liver and other organs, but not in the large intestine, of CDF1 mice, these findings suggest that both these HCAs, and especially A alphaC, induce large intestinal tumors in C57BL/6N mice.

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.

The role of individual susceptibility in cancer burden related to environmental exposure

Individual susceptibility to cancer may result from host factors including differences n metabolism, DNA repair, altered expression of protooncogenes and tumor suppressor genes, and nutritional status. Since most carcinogens require metabolic activation before binding to DNA, variations in an individual's metabolic phenotype that have detected in enzymes involved in activation and detoxification should play an essential role in the development of environmental cancer. This phenotypic metabolic variation has now been related to genetic polymorphisms, and many genes encoding carcinogen-metabolizing enzymes have been identified and cloned. Consequently, allelic variants or genetic defects that give rise to the observed variation and new polymorphisms have been recognized. Development of simple polymerase chain reaction (PCR)-based assays has enabled identification of an individual's genotype for a variety of metabolic polymorphisms. Thus, recent knowledge of the genetic basis for individual metabolic variation has opened new possibilities of studies focusing on increased individual susceptibility to environmentally induced cancer, which are reviewed with special reference to smoking-induced lung cancer. Cancer susceptibility due to chemical exposure is likely to be determined by an individual's phenotype for a number of enzymes (both activating and detoxifying) relevant to that of a single carcinogen or mixtures of carcinogens. Given the number and variability in expression of carcinogen-metabolizing enzymes and the complexity of chemical exposures, assessment of a single polymorphic enzyme (genotype) may not be sufficient. Mutations in the p53 gene are among the most common genetic changes in human cancer. The frequency and type p53 mutations can act as a fingerprint of carcinogen exposure and may therefore provide information about external etiological agents, intensity of exposure, and host factors affecting the tumorigenesis process. In human lung cancer, p53 mutations (both the mutation pattern and frequency) have been linked with tobacco smoking; the type of mutation most frequently observed is G:C to T:A transversion, a mutation preferentially induced by benzo[a]pyrene diol epoxide. An association between the presence of this transversion and the genotype deficient in glutathione S-transferase M1-mediated detoxification has been observed in lung cancer. Taken together, these findings suggest that determination of metabolic at risk genotypes in combination with levels of DNA adducts in target (surrogate) tissues and the p53 mutation pattern should allow the identification of susceptible individuals and subgroups in carcinogen-exposed populations.

Genetic mapping of a pulmonary adenoma resistance (Par1) in mouse

Lung cancer, a major cause of death in the Western world, has a poor prognosis. So far, therapeutic strategies have had only a limited effect. Lung cancer risk is strongly associated with cigarette smoking and lung cancer pedigrees are rare. However, a possible polygenic nature of inherited predisposition to this cancer has been envisaged. Mouse inbred strains with inherited predisposition and resistance to lung cancer provide an important tool for the dissection of the genetics of this complex disease. The A/J strain carries the pulmonary adenoma susceptibility 1 (Pas1) locus and develops many lung tumours. We have mapped the M. spretus-derived locus that strongly resists the lung tumorigenesis in Pas1/+ mice. This locus, pulmonary adenoma resistance 1 (Par1) maps to mouse chromosome 11, near the Rara locus, with a lod score of 5.3. In Pas1/+ mice Par1 accounts for 23% of the phenotypic variance and 10 fold reduction in total tumour volume. These results provide evidence for a major resistance locus affecting the expression of an inherited predisposition to lung cancer.