Thai-Vu T. Ton

K-ras Mutations in Lung Tumors and Tumors from Other Organs are Consistent with a Common Mechanism of Ethylene Oxide Tumorigenesis in the B6C3F1 Mouse

Ethylene oxide is a multisite carcinogen in rodents and classified as a human carcinogen by the National Toxicology Program. In 2-year mouse studies, ethylene oxide (EO) induced lung, Harderian gland (HG), and uterine neoplasms. We evaluated representative EO-induced and equivalent spontaneous neoplasms for K-ras mutations in codons 12, 13, and 61. K-ras mutations were identified in 100% (23/23) of the EO-induced lung neoplasms and 25% (27/108) of the spontaneous lung neoplasms. Codon 12 G to T transversions were common in EO-induced lung neoplasms (21/23) but infrequent in spontaneous lung neoplasms (1/108). K-ras mutations were found in 86% (18/21) of the EO-induced HG neoplasms and 7% (2/27) of the spontaneous HG neoplasms. Codon 13 G to C and codon 12 G to T transversions were predominant in the EO-induced HG neoplasms but absent in spontaneous HG neoplasms (0/27). K-ras mutations occurred in 83% (5/6) of the EO-induced uterine carcinomas and all were codon 13 C to T transitions. These data show a strong predilection for development of K-ras mutations in EO-induced lung, Harderian gland, and uterine neoplasms. This suggests that EO specifically targets the K-ras gene in multiple tissue types and that this event is a critical component of EO-induced tumorigenesis.

Frequent p53 and H-ras mutations in benzene- and ethylene oxide-induced mammary gland carcinomas from B6C3F1 mice.

Benzene and ethylene oxide are multisite carcinogens in rodents and classified as human carcinogens by the National Toxicology Program. In 2-year mouse studies, both chemicals induced mammary carcinomas. We examined spontaneous, benzene-, and ethylene oxide-induced mouse mammary carcinomas for p53 protein expression, using immunohistochemistry, and p53 (exons 5–8) and H-ras (codon 61) mutations using cycle sequencing techniques. p53 protein expression was detected in 42% (8/19) of spontaneous, 43% (6/14) of benzene-, and 67% (8/12) of ethylene oxide-induced carcinomas. However, semiquantitative evaluation of p53 protein expression revealed that benzene- and ethylene oxide-induced carcinomas exhibited expression levels five- to six-fold higher than spontaneous carcinomas. p53 mutations were found in 58% (7/12) of spontaneous, 57% (8/14) of benzene-, and 67% (8/12) of ethylene oxide-induced carcinomas. H-ras mutations were identified in 26% (5/19) of spontaneous, 50% (7/14) of benzene-, and 33% (4/12) of ethylene oxide-induced carcinomas. When H-ras mutations were present, concurrent p53 mutations were identified in 40% (2/5) of spontaneous, 71% (5/7) of benzene-, and 75% (3/4) of ethylene oxide-induced carcinomas. Our results demonstrate that p53 and H-ras mutations are relatively common in control and chemically induced mouse mammary carcinomas although both chemicals can alter the mutational spectra and more commonly induce concurrent mutations.

High Frequency of Ras Mutations in Forestomach and Lung Tumors of B6C3F1 Mice Exposed to 1-Amino-2,4-dibromoanthraquinone for 2 Years

1-Amino-2,4-dibromoanthraquinone (ADBAQ) is an anthraquinone-derived vat dye, and a potent carcinogen in laboratory animals. In a 2-year study with dietary exposure to 10,000 or 20,000 ppm ADBAQ, increased incidence of forestomach and lung tumors were observed in B6C3F1 mice. The present study indentified genetic alterations in H-ras and K-ras proto-oncogenes in ADBAQ-induced tumors. Point mutations in ras proto-oncogenes were identified by restriction fragment length polymorphism, single-stranded conformational polymorphis m analysis and cycle sequencing of polymerase chain reaction-amplified DNA isolated from paraffin-embedded squamous cell papillomas and carcinomas in the forestomach, and alveolar/bronchiolar adenomas and carcinomas in the lung. A higher frequency of ras mutations was identified in ADBAQ-induced forestomach (23/32, 72%) and lung tumors (16/23, 70%) than in spontaneous forestomach (4/11, 36%) and lung tumors (26/86, 30%). H-ras codon 61 CTA mutations were detected in (4/8, 50%) ADBAQ-induced forestomach squamous cell papillomas and (10/24, 42%) squamous cell carcinomas, but not in the spontaneous forestomach tumors examined. H-ras codon 61 CGA mutation (6/24, 25%) was also detected in ADBAQ-induced forestomach squamous cell carcinomas. K-ras codon 61 A to T transversions and A to G transitions were prominent in ADBAQ-induced lung alveolar/bronchiolar adenomas and alveolar/bronchiolar carcinomas. The major finding of A to T transversions or A to G transitions in forestomach and lung tumors suggests that ADBAQ or its metabolites target adenine bases in the ras proto-oncogene s and that these mutations play a dominant role in multi-organ carcinogenesi s in the B6C3F1 mouse.

Gene Expression and Mutation Assessment Provide Clues of Genetic and Epigenetic Mechanisms in Liver Tumors of Oxazepam-Exposed Mice

Liver tumors from a previous National Toxicology Program study were examined using global gene expression and mutation analysis to define the mechanisms of carcinogenesis in mice exposed to oxazepam. Five hepatocellular adenomas and 5 hepatocellular carcinomas from male B6C3F1 mice exposed to 5000 ppm oxazepam and 6 histologically normal liver samples from control animals were examined. One of the major findings in the study was upregulation of the Wnt/β-catenin signaling pathway. Genes that activate β-catenin, such as Sox4, were upregulated, whereas genes that inhibit Wnt signaling, such as APC and Crebbp, were downregulated. In addition, liver tumors from oxazepam-exposed mice displayed β-catenin mutations and increased protein expression of glutamine synthetase, a downstream target in the Wnt signaling pathway. Another important finding in this study was the altered expression of oxidative stress–related genes, specifically increased expression of cytochrome p450 genes, including Cyp1a2 and Cyp2b10, and decreased expression of genes that protect against oxidative stress, such as Sod2 and Cat. Increased oxidative stress was confirmed by measuring isoprostane expression using mass spectrometry. Furthermore, global gene expression identified altered expression of genes that are associated with epigenetic mechanisms of cancer. There was decreased expression of genes that are hypermethylated in human liver cancer, including tumor suppressors APC and Pten. Oxazepam-induced tumors also exhibited decreased expression of genes involved in DNA methylation (Crebbp, Dnmt3b) and histone modification (Sirt1). These data suggest that formation of hepatocellular adenomas and carcinomas in oxazepam-exposed mice involves alteration of the Wnt signaling pathway, oxidative stress, and potential epigenetic alterations.

Gene Expression Studies Reveal That DNA Damage, Vascular Perturbation, and Inflammation Contribute to the Pathogenesis of Carbonyl Sulfide Neurotoxicity

Carbonyl sulfide (COS) is an odorless gas that produces highly reproducible lesions in the central nervous system. In the present study, the time course for the development of the neurotoxicological lesions was defined and the gene expression changes occurring in the posterior colliculus upon exposure to COS were characterized. Fischer 344 rats were exposed to 0 or 500 ppm COS for one, two, three, four, five, eight, or ten days, six hours per day. On days 1 and 2, no morphological changes were detected; on day 3, 10/10 (100%) rats had necrosis in the posterior colliculi; and on day 4 and later, necrosis was observed in numerous areas of the brain. Important gene expression changes occurring in the posterior colliculi after one or two days of COS exposure that were predictive of the subsequent morphological findings included up-regulation of genes associated with DNA damage and G1/S checkpoint regulation (KLF4, BTG2, GADD45g), apoptosis (TGM2, GADD45g, RIPK3), and vascular mediators (ADAMTS, CTGF, CYR61, VEGFC). Proinflammatory mediators (CCL2, CEBPD) were up-regulated prior to increases in expression of the astrocytic marker GFAP and macrophage marker CSF2rb1. These gene expression findings were predictive of later CNS lesions caused by COS exposure and serve as a model for future investigations into the mechanisms of disease in the central nervous system.

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.

A PCR-RFLP method for the detection of Helicobacter hepaticus in frozen or fixed liver from B6C3F1 mice

Establishing the diagnosis of Helicobacter hepaticus infection in mouse liver has recently become important for the interpretation of rodent carcinogenicity bioassays. A seminested primer polymerase chain reaction (PCR) amplification of the bacterial 16S ribosomal RNA gene in combination with a restriction fragment length polymorphism (RFLP) assay was designed to identify and distinguish H. hepaticus from H. muridarum and H. bilis in mouse liver. The PCR-RFLP assay was applied to formalin-fixed, paraffin-embedded and, when available, corresponding frozen liver tissues from male and female B6C3F, mice with or without histologic evidence of infection from various National Toxicology Program 2-yr bioassay studies. PCR products consistent with H. hepaticus were detected in 10-80% of livers from mice in studies with other evidence of infection that were frozen or fixed for less than 24 hr but not in liver fixed for several weeks. The sensitivity of the PCR-RFLP assay for H. hepaticus on formalin-fixed, paraffin-embedded mouse liver varied between studies from markedly decreased when compared to the results from frozen liver or histologic evaluation to nearly equivalent or more sensitive than histologic evaluation. The PCR-RFLP results appeared dependent on the duration of fixation and bacterial load but not on the presence of hepatitis, sampling from neoplastic or nonneoplastic liver, or sex of the mouse.

Gene expression in obliterative bronchiolitis-like lesions in 2,3-pentanedione-exposed rats.

Obliterative bronchiolitis (OB) is an irreversible lung disease characterized by progressive fibrosis in the small airways with eventual occlusion of the airway lumens. OB is most commonly associated with lung transplant rejection; however, OB has also been diagnosed in workers exposed to artificial butter flavoring (ABF) vapors. Research has been limited by the lack of an adequate animal model of OB, and as a result the mechanism(s) is unclear and there are no effective treatments for this condition. Exposure of rats to the ABF component, 2,3-pentanedione (PD) results in airway lesions that are histopathologically similar to those in human OB. We used this animal model to evaluate changes in gene expression in the distal bronchi of rats with PD-induced OB. Male Wistar Han rats were exposed to 200 ppm PD or air 6 h/d, 5 d/wk for 2-wks. Bronchial tissues were laser microdissected from serial sections of frozen lung. In exposed lungs, both fibrotic and non-fibrotic airways were collected. Following RNA extraction and microarray analysis, differential gene expression was evaluated. In non-fibrotic bronchi of exposed rats, 4683 genes were significantly altered relative to air-exposed controls with notable down-regulation of many inflammatory cytokines and chemokines. In contrast, in fibrotic bronchi, 3807 genes were significantly altered with a majority of genes being up-regulated in affected pathways. Tgf-β2 and downstream genes implicated in fibrosis were significantly up-regulated in fibrotic lesions. Genes for collagens and extracellular matrix proteins were highly up-regulated. In addition, expression of genes for peptidases and peptidase inhibitors were significantly altered, indicative of the tissue remodeling that occurs during airway fibrosis. Our data provide new insights into the molecular mechanisms of OB. This new information is of potential significance with regard to future therapeutic targets for treatment.

Analysis of p53 Tumor Suppressor Gene, H-ras Protooncogene and Proliferating Cell Nuclear Antigen (PCNA) in Squamous Cell Carcinomas of HRA/Skh Mice Following Exposure to 8-Methoxypsoralen (8-MOP) and UVA Radiation (PUVA Therapy)

Treatment with 8-methoxypsoralen (8-MOP) and ultraviolet radiation (primarily UVA), called PUVA therapy, has been used to treat different chronic skin diseases but led to a significant increased risk for skin cancer. The National Toxicology Program (NTP) performed a study in mice treated with PUVA that showed a significant increase in squamous cell carcinomas of the skin. In the present study, we evaluated the protein expression of p53 and PCNA and DNA mutations of p53 and H-ras genes in both hyperplastic and neoplastic squamous cell lesions from the NTP study. By immunohistochemical staining, protein expression of both p53 and PCNA was detected in 3/16 (19%) of hyperplastic lesions and 14/17 (82%) of SCCs in groups treated with both 8-MOP and UVA. The mutation frequency of p53 in SCCs from mice administered 8-MOP plus UVA was 15/17 (88%) with a predominant distribution of mutations in exon 6 (14/15 – 93%). No H-ras mutations were detected in the hyperplastic lesions/tumors. The mutagenic effect of PUVA on the p53 tumor suppressor gene may lead to a conformational modification and inactivation of the p53 protein, which are considered critical steps in PUVA-induced skin carcinogenesis. The p53 mutational frequency and patterns from our study were different from those reported in human PUVA-type tumors.

Predominant K-ras Codon 12 G → A Transition in Chemically Induced Lung Neoplasms in B6C3F1 Mice

Based on long-term toxicity and carcinogencity studies in B6C3F1 mice conducted by the National Toxicology Program, 2,2-Bis(bromomethyl)-1,3-propanediol (BMP) and tetranitromethane (TNM) have been identified as carcinogens. Following 2 yr of exposure to 312, 625, or 1,250 ppm BMP in feed, or exposure to 0.5 or 2 ppm TNM by inhalation, increased incidences of lung neoplasms were observed in B6C3F1 mice at all exposure concentrations compared to unexposed mice. The present study characterizes genetic alterations in the K-ras protooncogene in BMP- and TNM-induced lung neoplasms, respectively, and compares the findings to spontaneous lung neoplasms from corresponding control mice. The frequencies of the K-ras mutations were 57% (29/51) in BMP-induced lung neoplasms compared to 15% (3/20) in lung neoplasms from dosed feed control mice, and 54% (14/26) in TNM-induced lung neoplasms compared to 60% (3/5) in lung neoplasms from inhalation control mice. G → A transitions at the second base of the K-ras codon 12 (GGT → GAT) were the most frequent pattern of K-ras mutations identified in BMP-induced (20/29) and TNM-induced lung neoplasms (13/14), which differed from the mutational patterns identified in the lung neoplasms from unexposed control mice. These results indicate that mutations in the K-ras gene are involved in B6C3F1 lung carcinogenesis following BMP- and TNM-exposure, and the high frequency and specificity of the ras mutation profile in lung neoplasms (G → A transition) may be due to in vivo genotoxicity by the parent compounds or their metabolites.