An evaluation of the hemizygous transgenic Tg.AC mouse for carcinogenicity testing of pharmaceuticals. II. A genotypic marker that predicts tumorigenic responsiveness

26-Week Dermal Oncogenicity Study Evaluating Pure trans-Capsaicin in Tg.AC Hemizygous Mice (FBV/N)

The objective of this study was to assess the oncogenic potential of trans-capsaicin when administered weekly via topical application to the dorsal skin of Tg.AC mice for 26 weeks. Male and female Tg.AC mice (25 mice/sex/group) received dose formulations containing trans-capsaicin dissolved in diethylene glycol monoethyl ether (DGME). The positive control was tetradecanoylphorbol-13-acetate (TPA) dissolved in DGME. Appropriate controls, including a topical lidocaine local anesthetic pretreatment (4% w/ w), were maintained. All groups were dosed once weekly, except for the TPA group, which was dosed twice per week. Analysis of the macroscopic observations after the final sacrifice revealed no noteworthy treatment-related findings, with the exception of dermal masses that were randomly dispersed throughout all treatment groups for both males and females. The frequency of dermal masses in the capsaicin-treated groups (at a dose level of up to 102 mg/kg and an application rate of 25.6 mg/cm2/kg/week) was not elevated in comparison to either concurrent vehicle or untreated controls. In contrast, a notable increase in the frequency of dermal masses was observed in the TPA-treated mice compared to both the concurrent vehicle and untreated controls. Dermal application of capsaicin resulted in no increased incidence of preneoplastic or neoplastic skin lesions. In contrast, over half the male and female mice exposed to TPA had multiple skin papillomas; the majority of the TPA-treated animals either died early or was humanely euthanized due to tumor load. Spontaneously occurring neoplasms were not appreciably increased in capsaicin-treated animals. Capsaicin-related non-neoplastic microscopic findings were seen sporadically in both genders and included acanthosis, hyperkeratosis/parakeratosis (primarily females), epidermal crusts, subepidermal fibrosis, epidermal ulcerations/erosions, and chronic-active inflammation. There was no evidence of a dose response in either the incidence or severity of these findings. The lidocaine- (at a dose level of 162 mg/kg and at an application rate of 40.5 mg/cm2/kg/week) and DGME-treated (at a dose level of 4.0 g/kg and at an application rate of 1 g/cm2/kg/week) control groups also did not display any evidence of increase in dermal masses. Based on these results, trans-capsaicin, lidocaine, and DGME should be considered nononcogenic in the Tg.AC mouse dermal model.

The use of genetically modified mice in cancer risk assessment: challenges and limitations

The use of genetically modified (GM) mice to assess carcinogenicity is playing an increasingly important role in the safety evaluation of chemicals. While progress has been made in developing and evaluating mouse models such as the Trp53⁺/⁻, Tg.AC and the rasH2, the suitability of these models as replacements for the conventional rodent cancer bioassay and for assessing human health risks remains uncertain. The objective of this research was to evaluate the use of accelerated cancer bioassays with GM mice for assessing the potential health risks associated with exposure to carcinogenic agents. We compared the published results from the GM bioassays to those obtained in the National Toxicology Program's conventional chronic mouse bioassay for their potential use in risk assessment. Our analysis indicates that the GM models are less efficient in detecting carcinogenic agents but more consistent in identifying non-carcinogenic agents. We identified several issues of concern related to the design of the accelerated bioassays (e.g., sample size, study duration, genetic stability and reproducibility) as well as pathway-dependency of effects, and different carcinogenic mechanisms operable in GM and non-GM mice. The use of the GM models for dose-response assessment is particularly problematic as these models are, at times, much more or less sensitive than the conventional rodent cancer bioassays. Thus, the existing GM mouse models may be useful for hazard identification, but will be of limited use for dose-response assessment. Hence, caution should be exercised when using GM mouse models to assess the carcinogenic risks of chemicals.

The transgenic mouse assay as an alternative test method for regulatory carcinogenicity studies--implications for REACH

REACH, an EU regulation that requires the submission of safety data in support of the protection of human and environmental health, mandates that registration should be achieved with the minimum amount of animal testing possible. Under REACH, a two-year carcinogenicity assay may be required for certain chemicals produced at >1000 metric tonnes per year. In addition, some chemicals that are found to be genotoxic will also require testing. Alternative methods have been explored in an attempt to improve the predictivity of this bioassay as well as to reduce the number of animals used for such testing. This research has focused on the use of transgenic/knockout mouse models. Study results from selected models indicate that they are useful in hazard identification, even if they are not entirely suitable for risk assessment on their own. Carcinogenic hazard assessment can be greatly enhanced and animal use reduced if the traditional two-year rat bioassay is combined with a well conducted transgenic mouse assay. Importantly, the use of transgenic animals to supplement a traditional two-year carcinogenicity study may help reduce the number of false negatives, one of the unstated goals of REACH via the precautionary principle.

Biological, cellular, and molecular characteristics of an inducible transgenic skin tumor model: a review

The genetically initiated Tg.AC transgenic mouse carries a transgene consisting of an oncogenic v-Ha-ras coding region flanked 5' by a mouse zeta-globin promoter and 3' by an SV-40 polyadenylation sequence. Located on chromosome 11, the transgene is transcriptionally silent until activated by chemical carcinogens, UV light, or full-thickness wounding. Expression of the transgene is an early event that drives cellular proliferation resulting in clonal expansion and tumor formation, the unique characteristics now associated with the Tg.AC mouse. This ras-dependent phenotype has resulted in the widespread interest and use of the Tg.AC mouse in experimental skin carcinogenesis and as an alternative carcinogenesis assay. This review examines the general biology of the tumorigenic responses observed in Tg.AC mice, the genetic interactions of the ras transgene, and explores the cellular and molecular regulation of zeta-globin promoted transgene expression. As a prototype alternative model to the current long-term rodent bioassays, the Tg.AC has generated a healthy discussion on the future of transgenic bioassays, and opened the doors for subsequent models for toxicity testing. The further exploration and elucidation of the molecular controls of transgene expression will enhance the usefulness of this mouse and enable a better understanding of the Tg.AC's discriminate response to chemical carcinogens.

Strong synergy between mutant ras and HPV16 E6/E7 in the development of primary tumors

E6/E7 oncogenes of high-risk human papilloma virus (HPV) subtypes are essential for the development of certain types of cancers. However, these oncogenes are insufficient to transform normal cells into an immortalized or malignant state. Mutant Ha-ras cooperates with E6/E7 of HPV subtype 16 in transformation of cells in vitro and may contribute to some HPV-associated cancers in humans. This study investigates whether HPV16 E6/E7 and v-Ha-ras synergize in vivo. FVB/n mice transgenic for v-Ha-ras gene (R+) were crossed with transgenic C57BL/6 mice that harbor E6/E7 of HPV16 (E+). Beginning at about 3 months of age, the bitransgenic E(+)R(+)(C57BL/6 x FVB/n) F1 mice developed mouth, eye and ear tumors. By 6 months, the prevalence of these types of mouth, eye and ear tumors was 100, 71 and 79% respectively in the E(+)R+ mice. Most tumors grew progressively until the mice had to be killed. The median times for the appearance of the first mouth, eye and ear tumor were 3.6, 4.3 and 4.2 months, respectively. For the two singly transgenic groups of mice, the prevalence of mouth, eye and ear tumors was 0, 0 and 6% (E(-)R+) and 0, 0 and 0% (E(+)R-), respectively, and the median time to first tumor was greater than 12 months for singly transgenic mice (E(-)R+, E(+)R-). Thus, a remarkable synergy occurred between the v-Ha-ras and HPV16 E6/E7 oncogenes in the development of primary tumors in mice.

Transgenic tumor models for carcinogen identification: the heterozygous Trp53-deficient and RasH2 mouse lines

Genetically altered mouse models (GAMM) for human cancers have been critical to the investigation and characterization of oncogene and tumor suppressor gene expression and function and the associated cancer phenotype. Similarly, several of the mouse models with defined genetic alterations have shown promise for identification of potential human carcinogens and investigation of mechanisms of carcinogen-gene interactions and tumorigenesis. In particular, both the B6.129N5-Trp53 mouse, heterozygous for a p53 null allele, and the CB6F1-RasH2 mouse, hemizygous for the human H-ras transgene, have been extensively investigated. Using 26-week exposure protocols at or approaching the maximum tolerated dose, the summary results to date indicate the potential for GAMM to identify and, possibly, classify chemicals of potential risk to humans using short-term carcinogenicity experiments. This IWGT session focused on: (1) the development of recommendations for genetic/molecular characterization required in animals, tissues, and tumors before and after treatment for identification of presumptive human carcinogens based on the current state of knowledge, (2) identification of data gaps in our current state of knowledge, and (3) development of recommendations for research strategies for further development of our knowledge base of these particular models. By optimization of protocols and identification of significant outcomes and responses to chemical exposure in appropriate short-term mechanism-based genetically altered rodent models, strategies for prevention and intervention may be developed and employed to the benefit of public health.

Evaluation of the Tg.AC assay: specificity testing with three noncarcinogenic pharmaceuticals that induce selected stress gene promoters in vitro and the inhibitory effects of solvent components

Understanding the strengths and limitations of alternative models, such as the Tg.AC assay, for evaluation of the potential carcinogenicity of pharmaceuticals requires assessment of assay specificity through studies that specifically target biologically active compounds that are known to not be carcinogens in rodents. To identify drugs that might provoke a false positive response in the Tg.AC assay, we screened pharmaceuticals for in vitro induction of the gadd153 promoter and the zeta-globin promoter. We have previously found a high correlation between induction of the gadd153 promoter in HepG2 cells and activity in the Tg.AC assay. The three drugs selected through screening 99 noncarcinogenic pharmaceuticals were amiloride, dipyridamole, and pyrimethamine. A 26-week skin paint study was conducted in hemizygous Tg.AC mice with the three drugs at two doses selected by a 4-week dose range finding study. Evidence of systemic toxicity was observed in animals dosed chronically with pyrimethamine or amiloride, but no skin papillomas were observed in mice treated with amiloride, dipyridamole, or pyrimethamine for 26 weeks. All male mice and 80% of female mice treated with 12-O-tetradecanoylphorbol-13-acetate (TPA) in acetone developed a maximal tumor burden. However, mice treated with TPA in a vehicle containing 2.4% DMSO had greatly reduced incidences of papillomas. In summary, the correct negative response was shown in the Tg.AC assay for three noncarcinogenic pharmaceuticals, which adds further favorable evidence of appropriate specificity of this model system. However, vehicle composition must be carefully selected because the outcome of this assay can be confounded by certain commonly used solvents.

Selection of drugs to test the specificity of the Tg.AC assay by screening for induction of the gadd153 promoter in vitro

Short-term assays for carcinogenicity testing of chemicals that use transgenic mice designed to have altered expression of genes mechanistically relevant to carcinogenesis are attractive alternatives to two-year dosing studies in rodents. The models that have been the received the greatest level of performance evaluation include p53(+/-), rasH2, Xpa/p53(+/-), and Tg.AC mice. For use of these models in a regulatory setting to evaluate the carcinogenic potential of pharmaceuticals, it is important to establish an assurance of assay specificity and positive predictivity based on studies using drugs with a wide spectrum of pharmacologic activity. For this purpose, 99 noncarcinogenic drugs were prioritized based on their activity in an in vitro induction assay correlative with a positive response in the Tg.AC assay (induction of the gadd153 promoter in HepG2 cells). Activities in two assays less predictive of Tg.AC activity (induction of c-fos and zeta-globin gene promoters) were also measured. Nine percent of the screened drugs induced the gadd153 promoter by at least fourfold. Several criteria were used to select candidates for subsequent in vivo testing in the Tg.AC assay: (1) sufficient drug solubility in appropriate skin paint vehicles to elicit systemic toxicity, (2) the level of induction of the gadd153 promoter by the drug, (3) the in vitro potency of the drug, and (4) the cost of the drug required for a 6-month study. Based on these criteria, amiloride, dipyridamole, and pyrimethamine were selected from 99 rodent noncarcinogens in a drug database for testing the specificity of the Tg.AC assay.

p27(Kip1) (Cdkn1b)-deficient mice are susceptible to chemical carcinogenesis and may be a useful model for carcinogen screening

The two-year rodent bioassay is one of several tests that are widely used by governmental regulatory agencies as well as pharmaceutical and chemical companies to determine the carcinogenic potential of chemicals or environmental agents where human exposure is anticipated. That this assay has remained relatively unchanged for the last 25 years is a testament to the power of this approach to identify carcinogens and thus to minimize human exposure. However, there has long been controversy over the specificity and relevance of the rodent bioassay as well as its high cost in terms of time, expense, and numbers of mice. Much discussion has been generated in recent years over how to improve the 2-year rodent bioassay for more accurate and faster detection of potential human carcinogens. Here, we argue that the use of p27(Kip1) (Cdkn1b) knockout mouse for carcinogen screening may solve several shortcomings inherent in the conventional bioassay while preserving its best quality, that is, protecting public health by providing reliable in vivo information on the potential of chemicals to cause cancer.

Glucocorticoid receptor functions as a potent suppressor of mouse skin carcinogenesis

Glucocorticoids are effective inhibitors of epidermal proliferation and skin tumorigenesis. Glucocorticoids affect cellular functions via glucocorticoid receptor (GR), a well-known transcription factor. Recently, we generated skin-targeted transgenic mice overexpressing GR under control of the keratin5 promoter (K5-GR mice). To test the hypothesis that GR plays a role as a tumor suppressor in skin, we bred K5-GR transgenic mice with Tg.AC transgenic mice, which express v-Ha-ras oncogene in the skin, and compared the susceptibility of F1 offspring to TPA-induced skin carcinogenesis. GR overexpression in the epidermis dramatically inhibited skin tumor development. In K5-GR/ras+ double transgenic mice papillomas developed later and the average number of tumors per animal was 15% (in males) and 40% (in females) of the number seen in wild type (w.t./ras+) littermates. In addition, the papillomas in w.t./ras+ animals were eight to nine times larger. GR overexpression resulted in a decrease in keratinocyte proliferation combined with a modest increase in apoptosis and differentiation of keratinocytes in K5-GR/ras+ papillomas. Our data clearly indicate that interference of GR transgenic protein with nuclear factor kappa B (NF-kappaB) transcription factor had resulted in NF-kappaB blockage in K5-GR/ras+ tumors. We discuss the role of NF-kappaB blockage in tumor-suppressor effect of GR.

Genetic interaction between the unstable v-Ha-RAS transgene (Tg.AC) and the murine Werner syndrome gene: transgene instability and tumorigenesis

Tg.AC transgenic mice provide a sensitive assay for oncogenic agents and a convenient alternative to the two-stage initiation/promoter model of skin tumorigenesis. Although extensively used, this model has remained in part an enigma since mice that carry the Tg.AC transgene (consisting of v-Ha-Ras driven by an embryonic zeta-globin promoter) would not ordinarily be expected to develop skin and other adult tumors. Cloning and characterizing the inserted transgene has provided an insight into the Tg.AC phenotype. We find that the transgene is inserted into a Line-1 element in such a way as to create extended inverted repeats consisting of both transgene and Line-1 sequences. Such structures would be expected to contribute to the instability of the Tg.AC locus and we suggest that this instability is critical to the Tg.AC phenotype. Further, we strengthen this notion by introducing an inactivating mutation in the murine Wrn gene (a gene important in maintenance of genome stability) and showing that bigenic Tg.AC/Wrn(Deltahel/Deltahel) mice experience an eightfold increase in inactivating germline mutations at the Tg.AC locus. Similarly, Tg.AC/Wrn(Deltahel/Deltahel) mice that retain an intact and thus active Tg.AC locus experience a sharp increase in papillomas as compared to Tg.AC/Wrn(+/+) mice. This work demonstrates a genetic interaction between the instability of the multicopy transgene and the Werner Syndrome gene. From this, we conclude that genetic instability remains a key element in this tumor promoter model.

Transgene stability and features of rasH2 mice as an animal model for short-term carcinogenicity testing

The transgenic mouse rasH2 line, in which the mouse carries the human c-Ha-ras gene under the control of its own enhancer and promoter, has been proposed as one of the alternative short-term models for carcinogenicity testing. To apply this purpose, we have produced a genetically homogeneous population as C57BL/6JJic-TgN(RASH2) (Tg-rasH2) by continuous backcrossing. In this study, we examined the transgene stability between different generations and the detailed transgene architecture of the integrated human c-Ha-ras gene. Fluorescence in situ hybridization analysis showed that the integrated human c-Ha-ras gene was stably located on chromosome 15E3 in Tg-rasH2 mice at generation number (N) 15 and 20. Southern and Northern blot analysis did not show any differences in the hybridized band pattern in each generation. Southern blot analyses showed that the Tg-rasH2 mouse contained three copies of the human c-Ha-ras gene arrayed in a head-to-tail configuration. We also determined the nucleotide sequence of the transgene in the Tg-rasH2 mouse at N20 and confirmed that the sequence of the coding region was perfectly matched with human c-Ha-ras cDNA. Cloning and sequencing of genome/transgene junctions revealed that integration of the microinjected human c-Ha-ras gene into mouse host genome resulted in a 1820-bp deletion in the rasH2 line. The deleted sequence did not have any sequence homologies with known functional genes. We assumed that either the deletion or the transgene insertion, or both, would not cause insertional mutation. In short-term carcinogenicity testing with a genetically engineered mouse model, confirmation of the transgene or modified gene stability at each generation is one of the important factors that affect the sensitivity to carcinogenic compounds in the same way as the genetic background, age and route of administration.

The Tg.AC (v-Ha-ras) transgenic mouse: nature of the model

The Tg.AC (v-Ha-ras) transgenic mouse model provides a reporter phenotype of skin papillomas in response to either genotoxic or nongenotoxic carcinogens. In common with the conventional bioassay, the Tg.AC model responds to known human carcinogens and does not respond to noncarcinogens. It also does not respond to most chemicals that are positive in conventional bioassays principally at sites of high spontaneous tumor incidence. The mechanism of response of the Tg.AC model is related to the structure and genomic position of the transgene and the induction of transgene expression through specific mediated interactions between the chemicals and target cells in the skin.

Tg.AC genetically altered mouse: assay working group overview of available data

In a Government/Industry/Academic partnership to evaluate alternative approaches to carcinogenicity testing, 21 pharmaceutical agents representing a variety of chemical and pharmacological classes and possessing known human and or rodent carcinogenic potential were selected for study in several rodent models. The studies from this partnership project, coordinated by the International Life Sciences Institute, provide additional data to better understand the models' limitations and sensitivity in identifying carcinogens. The results of these alternative model studies were reviewed by members of Assay Working Groups (AWG) composed of scientists from government and industry with expertise in toxicology, genetics, statistics, and pathology. The Tg.AC genetically manipulated mouse was one of the models selected for this project based on previous studies indicating that Tg.AC mice seem to respond to topical application of either mutagenic or nonmutagenic carcinogens with papilloma formation at the site of application. This communication describes the results and AWG interpretations of studies conducted on 14 chemicals administered by the topical and oral (gavage and/or diet) routes to Tg.AC genetically manipulated mice. Cyclosporin A, an immunosuppresant human carcinogen, ethinyl estradiol and diethylstilbestrol (human hormone carcinogens) and clofibrate, an hepatocarcinogenic peroxisome proliferator in rodents, were considered clearly positive in the topical studies. In the oral studies, ethinyl estradiol and diethylstilbestrol were negative, cyclosporin was considered equivocal, and results were not available for the clofibrate study. Of the 3 genotoxic human carcinogens (phenacetin, melphalan, and cyclophosphamide), phenacetin was negative by both the topical and oral routes. Melphalan and cyclophosphamide are, respectively, direct and indirect DNA alkylating agents and topical administration of both caused equivocal responses. With the exception of clofibrate, Tg.AC mice did not exhibit tumor responses to the rodent carcinogens that were putative human noncarcinogens, (di(2-ethylhexyl) phthalate, methapyraline HCl, phenobarbital Na, reserpine, sulfamethoxazole or WY-14643, or the nongenotoxic, noncarcinogen, sulfisoxazole) regardless of route of administration. Based on the observed responses in these studies, it was concluded by the AWG that the Tg.AC model was not overly sensitive and possesses utility as an adjunct to the battery of toxicity studies used to establish human carcinogenic risk.

How many food additives are rodent carcinogens?

One generally assumes that chemical agents added to foods are reasonably free of risks to human health, and practically everyone consumes some additives in his or her food daily throughout life. In the United States, the 1958 Food Additives Amendment to the Federal Food, Drug and Cosmetic Act of 1938 requires food manufacturers to demonstrate the safety of food additives to the Food and Drug Administration (FDA). The Amendment contains a provision that prohibits approval of an additive if it is found to cause cancer in humans or animals. In the present study, data from the National Toxicology Program rodent bioassay (NTPRB) were used to identify a sample of approximately 50 rodent-tested additives and other chemicals added to food that had been evaluated independently of the FDA/food industry. Surprisingly, the sample shows more than 40% of these food chemicals to be carcinogenic in one or more rodent groups. If this percentage is extrapolated to all substances added to food in the United States, it would imply that more than 1000 of such substances are potential rodent carcinogens. The NTP and FDA test guidelines use similar, though not necessarily identical, rodent test procedures, including near lifetime exposures to the maximum tolerated dose. The FDA specifies that test chemicals should be administered by the oral route. However, the oral route includes three methods of delivering chemicals, that is, mixed in the food or water or delivered by stomach tube (gavage). The NTP data show only 1 of 18 food chemicals mixed in the food are rodent carcinogens, but 16 of 23 gavage-administered food chemicals are carcinogenic to rodents. The distribution suggests that among orally delivered chemicals, those administered in the feed will more likely prove to be noncarcinogens than chemicals given by gavage. The rodent data also reveal that effects may vary according to dose and genotype, as well as by route of administration, to further complicate extrapolation to humans. Human experience with known carcinogens such as tobacco, asbestos, and benzidine convinces us that environmental carcinogens constitute a real threat to human health, although predicting human carcinogens from rodent tests involves a number of uncertainties. These uncertainties do not mean that we should simply ignore the presence of carcinogens. Rather, in the interests of public safety, a serious effort should be made to resolve the questions surrounding the presence of chemicals identified as rodent carcinogens in our food. Environ. Mol. Mutagen. 39:69-80, 2002 Published 2002 Wiley-Liss, Inc.

Use of genetically engineered mice in drug discovery and development: wielding Occam's razor to prune the product portfolio

Genetically engineered mice (GEMs) that either overexpress (transgenic) or lack (gene-targeted, or "knock-out") genes are used increasingly in industry to investigate molecular mechanisms of disease, to evaluate innovative therapeutic targets, and to screen agents for efficacy and/or toxicity. High throughput GEM construction in drug discovery and development (DDD) serves two main purposes: to test whether a given gene participates in a disease condition, or to determine the function(s) of a protein that is encoded by an expressed sequence tag (EST, an mRNA fragment for a previously uncharacterized protein). In some instances, phenotypes induced by such novel GEMs also may yield clues regarding potential target organs and toxic effects of potential therapeutic molecules. The battery of tests used in phenotypic analysis of GEMs varies between companies, but the goal is to define one or more easily measured endpoints that can be used to monitor the disease course--especially during in vivo treatment with novel drug candidates. In many DDD projects, overt phenotypes are subtle or absent even in GEMs in which high-level expression or total ablation of an engineered gene can be confirmed. This outcome presents a major quandary for biotechnology and pharmaceutical firms: given the significant expense and labor required to generate GEMs, what should be done with "negative" constructs? The 14th century philosophical principle known as Occam's razor-that the simplest explanation for a phenomenon is likely the truth-provides a reasonable basis for pruning potential therapeutic molecules and targets. In the context of DDD, Occam's razor may be construed to mean that correctly engineered GEMs lacking obvious functional or structural phenotypes have none because the affected gene is not uniquely essential to normal homeostasis or disease progression. Thus, a "negative" GEM construct suggests that the gene under investigation encodes a ligand or target molecule without significant therapeutic potential. This interpretation indicates that, at least in a market-driven industrial setting, such "negative" projects should be pruned aggressively so that resources may be redirected to more promising DDD ventures.

Evaluation of the Tg.AC transgenic mouse assay for testing the human carcinogenic potential of pharmaceuticals--practical pointers, mechanistic clues, and new questions

Transgenic mouse strains with genetic alterations known to play a role in the multistage process of carcinogenesis are being used increasingly as models for evaluating the human carcinogenic potential of chemicals and pharmaceuticals. The Tg.AC transgenic mouse is one of the strains currently being used in such alternative short-term carcinogenicity testing protocols. This review is focused on recent data from studies designed to evaluate this model's ability to discriminate carcinogens from noncarcinogens. Details relating to protocol design that can significantly impact study outcome are described. Data relating to mechanisms of chemical tumor induction in the Tg.AC model are reviewed, and questions have been formulated to encourage research to further guide appropriate future applications of this model.

Loss of critical palindromic transgene promoter sequence in chemically induced Tg.AC mouse skin papillomas expressing transgene-derived mRNA

The Tg.AC transgenic mouse carries a v-Ha-ras transgene. Skin papillomas develop in Tg.AC mice upon repeated dermal application of tumor promoters and carcinogens. The transgene is inserted at a single site on chromosome 11 in a multiple-copy array. Although most of the >or= 40 copies are arranged in a direct-repeat orientation, two copies of the transgene are inserted in a palindromic, inverted-repeat orientation. Deletion of the palindromic transgene promoter sequence is associated strongly with and diagnostic of loss of phenotypic responsiveness to Tg.AC papillomagens, such as 12-O-tetradecanoylphorbol-13-acetate (TPA). Unexpectedly, a loss of palindromic transgene sequence, in the absence of an observable reduction in copy number of the direct-repeat-oriented transgene sequence, is seen in DNA from papillomas when compared to genomic DNA from tail clips or skin samples away from the application site. Transgene-derived transcripts were detectable in all Tg.AC papillomas sampled. The transgene locus was hypomethylated in papillomas but not in samples from tail clips from the same animal or from skin samples away from the application site in responder Tg.AC mice, as shown by loss of resistance to digestion by HpaII. A cell line derived from a Tg.AC squamous cell carcinoma showed complete loss of the palindromic transgene sequence, hypomethylation of the transgene locus, and strong expression of v-Ha-ras mRNA. These data indicate that the palindromic transgene sequence, which appears to be necessary for initial responsiveness to tumorigens, may be susceptible to deletion during rapid cellular proliferation and is not required for transgene expression in later phases of papilloma growth.

Dermal carcinogenicity in transgenic mice: effect of vehicle on responsiveness of hemizygous Tg.AC mice to phorbol 12-myristate 13-acetate (TPA)

The Tg.AC mouse is being evaluated for use in short-term carcinogenicity bioassays. Because the dermal test protocol necessitates dissolving test agents we determined the effects of several solvents on responsiveness of hemizygous mice to dermal applications of the classical skin tumor promoter. phorbol 12-myristate 13-acetate (TPA). Mice of both sexes received dermal applications of either acetone (negative control) or TPA in various vehicles [acetone, 100% methanol, 70% and 100% ethanol, DMSO and mixtures of acetone and ethanol (1:1), acetone and DMSO (4:1 and 1: 1). and acetone and olive oil (4:1)]. Negative control animals did not exhibit papillomas. When administered in acetone. ethanolic or methanolic vehicles TPA caused prompt and robust papillomatous responses. TPA was also tumorigenic in all nonalcoholic vehicles, except the acetone-olive oil mixture. Papilloma responses were generally delayed when TPA was applied in the nonalcoholic solvents but the distinction between TPA-dosed and negative control groups was unequivocal. These results show that choice of vehicle may affect the quantitative and qualitative nature of the response of Tg.AC mice to TPA, but 8 of 9 vehicles proved satisfactory for delivery of TPA.

Radial transformation-associated recombination cloning from the mouse genome: isolation of Tg.AC transgene with flanking DNAs

Transformation-associated recombination (TAR) cloning allows entire genes and large chromosomal regions to be specifically, accurately, and quickly isolated from total genomic DNA. We report the first example of radial TAR cloning from the mouse genome. Tg.AC mice carry a zeta-globin promoter/v-Ha-ras transgene. Fluorescence in situ hybridization localized the transgene integrant as a single site proximal to the centromere of chromosome 11. Radial TAR cloning in yeast was utilized to create orientation-specific yeast artificial chromosomes (YACs) to explore the possibility that cis-flanking regions were involved in transgene expression. YACs containing variable lengths of 5' or 3' flanking chromosome 11 DNA and the Tg.AC transgene were specifically chosen, converted to bacterial artificial chromosomes (BACs), and assayed for their ability to promote transcription of the transgene following transfection into an FVB/N carcinoma cell line. A transgene-specific reverse transcription-polymerase chain reaction assay was utilized to examine RNA transcripts from stably transfected clones. All Tg.AC BACs expressed the transgene in this in vitro system. This report describes the cloning of the v-Ha-ras transgene and suggests that transcriptional activity may not require cis elements flanking the transgene's integration site.

Enhanced growth of primary tumors in cancer-prone mice after immunization against the mutant region of an inherited oncoprotein

One major objective of tumor immunologists is to prevent cancer development in individuals at high risk. (TG.AC x C57BL/6)F1 mice serve as a model for testing the feasibility of this objective. The mice carry in the germline a mutant ras oncogene that has an arginine at codon 12 instead of glycine present in the wild-type, and after physical (wounding) or chemical promotion, these mice have a high probability for developing papillomas that progress to cancer. Furthermore, F1 mice immunized with Arg(12) mutant ras peptide in complete Freund's adjuvant (CFA) develop T cells within 10 d that proliferate in vitro on stimulation with the Arg(12) mutant ras peptide. Within 14 d, these mice have delayed-type hypersensitivity to the peptide. Immunization with CFA alone or with a different Arg(12) mutant ras peptide in CFA induced neither response. To determine the effect of immunization on development of tumors, mice immunized 3 wk earlier were painted on the back with phorbol 12-myristate 13-acetate every 3 d for 8 wk. The time of appearance and the number of papillomas were about the same in immunized and control mice, but the tumors grew faster and became much larger in the mice immunized with the Arg(12) mutant ras peptide. Thus, the immunization failed to protect against growth of papillomas. The peptide-induced CD4(+) T cells preferentially recognized the peptide but not the native mutant ras protein. On the other hand, mice immunized with Arg(12) mutant ras peptide and bearing papillomas had serum antibodies that did bind native mutant ras protein. Together, these studies indicate that active immunization of cancer-prone individuals may result in immune responses that fail to eradicate mutant oncogene-expressing tumor cells, but rather induce a remarkable enhancement of tumor growth.

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

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

Current status and use of short/medium term models for carcinogenicity testing of pharmaceuticals--scientific perspective

Short- and medium-term rodent bioassays have been proposed under ICH guidelines for use in testing for the carcinogenic potential of pharmaceuticals. Further evaluation of these models is needed urgently and coordinated efforts are in progress worldwide to expand the available database. Models currently being investigated include transgenic mice (Tg-rasH2, Tg.AC, p53(+/-), XPA(-/-)) and neonatal mice. As more data become available on the performance of these assays, regulatory and industry scientists will be faced with the difficult challenge of determining how the performance (accuracy) of each assay will be measured and deciding which assays have value in the risk assessment process.

Dermal carcinogenicity in transgenic mice: relative responsiveness of male and female hemizygous and homozygous Tg.AC mice to 12-O-tetradecanoylphorbol 13-acetate (TPA) and benzene

Assessment of the carcinogenic potential of chemical agents continues to rely primarily upon the chronic rodent bioassay, a resource-intensive exercise. Recent advances in transgenic technology offer a potential resource conserving approach to carcinogen detection. Incorporation of oncogenes with known roles in the development of neoplasms into the genomes of laboratory rodents may provide new models with the potential of quickly and accurately separating carcinogenic from noncarcinogenic chemicals. The insertion of the v-Ha-ras oncogene into the genome of FVB/N mice imparts the qualities of genetically initiated skin in the transgenic mouse line designated as Tg.AC. The skin of either hemizygous (animals carrying the transgene on 1 allele) or homozygous (transgene copies on both alleles) Tg.AC mice promptly responds to the application of nongenotoxic carcinogens, such as the classical tumor promoting phorbol esters, with the development of squamous papillomas. Tumor production generally begins after 8-10 applications of 2.5 micrograms/mouse (3 times/wk) of 12-O-tetradecanoylphorbol 13-acetate (TPA). Maximal tumor response is usually in evidence within 20 wk. If this transgenic mouse line is to be useful in the identification of carcinogenic chemicals, experimental protocols must be systematically optimized. Experiments were conducted to compare the relative responsiveness of male and female hemizygous and homozygous Tg.AC mice to the dermal application of TPA and the known human leukemogen, benzene. Results revealed shipment-related variabilities in the relative responsiveness of hemizygous male and female mice to the application of the proliferative agent. Homozygous mice of both sexes were more reliable and uniform in responsiveness to both TPA and benzene. Therefore, our standard protocol for the conduct of bioassays with the Tg.AC mouse line specifies the use of homozygous males and/or females.

An evaluation of the hemizygous transgenic Tg.AC mouse for carcinogenicity testing of pharmaceuticals. I. Evidence for a confounding nonresponder phenotype

We have completed 2 26-wk studies to evaluate the hemizygous transgenic Tg.AC mouse, which has been proposed as an alternative short term model for testing carcinogenicity. We attempted to evaluate the response to the known rodent carcinogens cyclophosphamide, phenolphthalein, and tamoxifen and to the noncarcinogen chlorpheniramine following topical application. In the first study, a weak response (2/17 animals) was observed to the positive control 12-O-tetradecanoylphorbol 13-acetate (TPA in ethanol, 1.25 micrograms), and no response was observed to cyclophosphamide, phenolphthalein, or chlorpheniramine, despite evidence for skin penetration. The second study compared 1.25 micrograms and 6.25 micrograms of TPA in ethanol and acetone solutions. Tamoxifen was also evaluated in both solvents and orally. No significant response was observed to tamoxifen by skin paint or oral routes. Over 60% of the high dose TPA-treated animals showed no (0 or 1) papilloma response, and 30% of the animals each developed more than 32 papillomas. The heterogenous response to high dose TPA may be related to variability in the responsiveness of hemizygous animals. In light of these findings, further Tg.AC studies should employ homozygous animals, and the underlying cause for heterogeneity in the tumorigenic response of Tg.AC mice should be identified and eliminated.