Role of tumor suppressor genes in transplacental lung carcinogenesis

Genetic and epigenetic alterations in lung tumors from bitransgenic Ki-rasG12C expressing mice

Mutations in Ki-ras occur in approximately 30-50% of patients with adenocarcinoma (AC) of the lung. We previously reported the development of a bitransgenic mouse model that expressed the human Ki-ras(G12C) allele in a lung-specific, tetracycline-inducible manner and gave rise to benign lung tumors. In the current study, these benign tumors, which represent relatively early lesions in neoplastic progression, were analyzed for molecular alterations secondary to mutant Ki-ras expression to determine the gene(s) that contribute to adenoma (AD) development. Tumors were removed following doxycycline (DOX) treatment for 9 and 12 mo and examined for alterations in cell-cycle regulatory genes. Quantification of mRNA expression for cyclin D1, retinoblastoma, p16(Ink4a), p19(Arf), and survivin was carried out by real-time PCR. All of the tumors examined exhibited a mean reduction of approximately fivefold for the retinoblastoma gene (P < 0.02). Increased expression of both p19(Arf) and survivin were detected in a majority of the tumors examined (P < 0.01 and 0.001, respectively), but no change in cyclin D1 RNA expression was observed. A subset of the lung tumors (8/28) displayed reduced levels of p16(Ink4a) expression (P = 0.02). Immunohistochemical analysis confirmed the upregulation of p19(Arf) and survivin in all 10 of the lung tumors examined. However, increased staining for cyclin D1 was observed in the tumor tissue. In addition, increased levels of activated p53 were found in lung tumor tissues stained with an anti-phospho-p53 antibody, while an absence of staining was observed with an anti-phospho-pRb antibody in both normal control and tumor tissue. Analysis of the methylation status of p16(Ink4a) by methylation-specific PCR (MSP) demonstrated that seven of eight tumors exhibiting decreased expression of p16(Ink4a) had at least partial methylation of the promoter region. Single stranded conformational polymorphism (SSCP) analysis demonstrated that neither exons 1 or 2 of p16(Ink4a) nor exons 5-8 of p53 exhibited mutations. These data thus identify alterations in specific genes and pathways that combine with the mutation in Ki-ras to promote the formation of benign lung tumors and suggest potential targets for the development of novel chemotherapeutic and chemopreventive agents during the early stages of lung tumor progression.

Loss of heterozygosity of Kras2 gene on 12p12-13 in Chinese colon carcinoma patients

AIM: To study the loss of heterozygosity (LOH) on 12p12-13 in Chinese colon carcinoma patients.

METHODS: DNA was extracted from 10 specimens of cancer tissue, 10 specimens of adjacent tissue and 10 specimens of normal tissue, respectively. LOH of Kras2 gene was analyzed by polymerase chain reaction (PCR) and denaturing polyacrylamide gel electrophoresis using 11 microsatellite markers on 12p-12-13.

RESULTS: LOH of Kras gene was detected at least on one marker of 12p-12-13 in 30% (3/10) of adjacent tissue specimens. The highest frequency of LOH was identified on D12S1034 in 28.57% (2/7) of adjacent tissue specimens. LOH was detected at least on one marker of 12p12-13 in 60% (6/10) of carcinoma tissue specimens, the most frequent LOH was found on D12S1034 and D12S1591 in 42.86% (3/7) of carcinoma tissue specimens. LOH was detected in 30% (3/10) of carcinoma tissue specimens, 30% (3/10) of adjacent tissue specimens, and no signal in 1% (1/0) carcinoma tissue specimen. The occurrence of LOH did not correlate with sex, age, tumor size and lymph node metastasis.

CONCLUSION: Genomic instability may occur on 12p-12-13 of Kras2 gene in the development and progression of colon carcinoma. The high LOH of Kras2 gene may directly influence the transcription and translation of wild type Kras2 gene.

In utero Exposure of Mice to Dibenzo[a,l]Pyrene Produces Lymphoma in the Offspring: Role of the Aryl Hydrocarbon Receptor

Lymphoma and leukemia are the most common cancers in children and young adults; in utero carcinogen exposure may contribute to the etiology of these cancers. A polycyclic aromatic hydrocarbon (PAH), dibenzo[a,l]pyrene (DBP), was given to pregnant mice (15 mg/kg body weight, gavage) on gestation day 17. Significant mortalities in offspring, beginning at 12 weeks of age, were observed due to an aggressive T-cell lymphoblastic lymphoma. Lymphocytes invaded numerous tissues. All mice surviving 10 months, exposed in utero to DBP, exhibited lung tumors; some mice also had liver tumors. To assess the role of the aryl hydrocarbon receptor (AHR) in DBP transplacental cancer, B6129SF1/J (AHR(b-1/d), responsive) mice were crossed with strain 129S1/SvIm (AHR(d/d), nonresponsive) to determine the effect of maternal and fetal AHR status on carcinogenesis. Offspring born to nonresponsive mothers had greater susceptibility to lymphoma, irrespective of offspring phenotype. However, when the mother was responsive, an AHR-responsive phenotype in offspring increased mortality by 2-fold. In DBP-induced lymphomas, no evidence was found for TP53, beta-catenin, or Ki-ras mutations but lung adenomas of mice surviving to 10 months of age had mutations in Ki-ras codons 12 and 13. Lung adenomas exhibited a 50% decrease and a 35-fold increase in expression of Rb and p19/ARF mRNA, respectively. This is the first demonstration that transplacental exposure to an environmental PAH can induce a highly aggressive lymphoma in mice and raises the possibility that PAH exposures to pregnant women could contribute to similar cancers in children and young adults.

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.

Alterations at the Ink4a locus in transplacentally induced murine lung tumors

The malignant phenotype results from multiple genetic alterations, including the activation of oncogenes and inactivation of tumor suppressor genes. Activation of the Ki-ras oncogene has been implicated as an early event in the pathogenesis of lung adenocarcinomas in humans and experimental animal models. Previous studies from this laboratory have shown that, following treatment of pregnant [D2 x B6D2F(1)]F(2) or Balb/c mice with the polycyclic aromatic hydrocarbon, 3-methylcholanthrene (MC), lung tumors from the transplacentally exposed offspring exhibited a high incidence of mutations in the Ki-ras gene. The role of genetic alterations at other oncogenic or tumor suppressor loci that can mediate lung tumor initiation and/or progression have not been well characterized in either human or murine models. Using the transplacental carcinogenesis model, which results in the induction of both lung and liver tumors following in utero exposure to MC, the results of this and our previous studies show that alterations in the Ink4a locus occur in only 15 and 27% of the lung and liver tumors, respectively. Preliminary data also suggests that the type of mutation induced in the Ki-ras gene following the initial exposure to MC may influence lung tumor progression. These results imply that damage to the Ink4a gene is not a frequent pathway to malignant progression in mouse lung and liver tumors following in utero exposure to environmental carcinogens.

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.

Absence of p16, p21 and p53 gene alterations in hepatocellular carcinomas induced by N-nitrosodiethylamine or a choline-deficient L-amino acid-defined diet in rats

To clarify the involvement of tumor suppressor genes in exogenous and endogenous liver carcinogenesis, alterations of p16, p21 and p53 in hepatocellular carcinomas (HCCs) induced by N-nitrosodiethylamine (DEN) and a choline deficient L-amino acid-defined (CDAA) diet in rats were investigated. Male Fischer 344 rats received DEN at 6-week of age followed by partial hepatectomy (PH), with colchicine to induce cell cycle disturbance, and a selection pressure regimen. Sacrifice was after 42 weeks. Other animals continuously received a CDAA diet for 75 weeks and were then killed. Eleven and 15 HCCs were obtained, respectively. Total RNA was extracted from and cDNA was synthesized with reverse transcriptase to allow investigation of mutations in p16, p21 and p53 by polymerase chain reaction single strand conformation polymorphism (PCR-SSCP) analysis. Expression of p16 and p21 mRNA was also analyzed by reverse transcription (RT)-PCR. The results showed no mutations or deletions of p16, p21 and p53 in any of the HCCs induced by DEN or CDAA. Loss or decrease of p16 and p21 expression were also not found, suggesting that p16, p21 and p53 alteration may not be necessary for either exogenous or endogenous liver carcinogenesis in rats.

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.

Strain-dependent lung tumor formation in mice transplacentally exposed to 3-methylcholanthrene and post-natally exposed to butylated hydroxytoluene

The carcinogenic effects of in utero exposure to 3-methylcholanthrene (MC) have been demonstrated in the tumor-resistant C57BL/6 (B6) and DBA (D2) strains of mice. In this study, we determined the effects of in utero exposure to MC in BALB/c mice, a strain which demonstrates greater susceptibility to lung tumor induction, and compared our findings with those previously found in [D2xB6D2F(1)]F(2) mice. In addition, we assessed the molecular pathogenesis of the chemically induced tumors and examined the effects of the putative lung tumor promoter butylated hydroxytoluene (BHT) in BALB/c mice. BALB/c mice were treated on day 17 of gestation with 5, 15 or 45 mg/kg MC and 6 weeks after birth with BHT for 6 consecutive weeks. Mice were killed at 6 months of age. Ki-ras, p16Ink4a and p19ARF gene loci were amplified from paraffin-embedded lung tumor tissue and screened for the presence of point mutations via allele-specific oligonucleotide hybridization and single strand conformation polymorphism (SSCP) analyses. Ki-ras point mutations were found in 56% (20/36) of BALB/c lung tumors, with 33% (2/6) of the hyperplasias, 58% (10/19) of the adenomas and 73% (8/11) of the carcinomas exhibiting point mutations at this gene locus. Similar incidences of Ki-ras mutations were previously found following transplacental exposure of [D2xB6D2F(1)]F(2) mice to MC and treatment of adult A/J mice with urethane. Interestingly, a strain-dependent difference was observed in the mutational spectrum. Sixty-two and 38% of the lung lesions in BALB/c mice exhibited G-->C and G-->T transversions, respectively, in contrast to the 13 and 84% incidences previously observed in [D2xB6D2F(1)]F(2) mice. SSCP analysis of the tumor suppressor gene p16Ink4a showed a 6% incidence of point mutations, consistent with that found in [D2xB6D2F(1)]F(2) mice. No mutations were found in exon 1beta of the p19ARF gene of either strain. BHT, a lung tumor promoter in adult mice, had no statistically significant effects on either tumor incidence, tumor multiplicity or the mutational spectrum produced in the Ki-ras gene by in utero MC treatment. However, though not significant, there was an observable trend in increased tumor multiplicity in mice co-treated with BHT. These data demonstrate the transplacental carcinogenic effect of MC in BALB/c mice and show that mutagenic damage to Ki-ras is a critical early event mediating murine lung tumorigenesis in both the tumor-sensitive and tumor-resistant strains. Unlike what occurs when adult BALB/c mice are treated with MC, BHT does not appear to significantly promote the formation of lung tumors following transplacental exposure to MC, possibly due to the rapid growth and cell proliferation in the developing organism. Strain-dependent differences in the Ki-ras mutational spectrum may be associated with their differential susceptibility to lung tumor initiation.

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.