Transgenic Tumor Models Research Articles

Targeted Liposomal Delivery of TLR9 Ligands Activates Spontaneous Antitumor Immunity in an Autochthonous Cancer Model

Accessibility of tumors for highly effective local treatment represents a major challenge for anticancer therapy. Immunostimulatory oligodeoxynucleotides (ODN) with CpG motifs are ligands of TLR9, which prime spontaneous antitumor immunity, but are less effective when applied systemically. We therefore developed a liposome-based agent for selective delivery of CpG-ODN into the tumor environment. A peptide that specifically targets angiogenic endothelial cells in a transgenic tumor model for islet cell carcinogenesis was engrafted into CpG-ODN containing liposomes. Intravenous injection of these liposomes resulted in specific accumulation around tumor vessels, increased uptake by tumor-resident macrophages, and retention over time. In contrast, nontargeted liposomes did not localize to the tumor vasculature. Consequently, only vascular targeting of CpG-ODN liposomes provoked a marked inflammatory response at vessel walls with enhanced CD8(+) and CD4(+) T cell infiltration and, importantly, activation of spontaneous, tumor-specific cytotoxicity. In a therapeutic setting, 40% of tumor-bearing, transgenic mice survived beyond week 45 after systemic administration of vascular-directed CpG-ODN liposomes. In contrast, control mice survived up to 30 wk. Therapeutic efficacy was further improved by increasing the frequency of tumor-specific effector cells through adoptive transfers. NK cells and CD8(+) T cells were major effectors which induced tumor cell death and acted in conjunction with antivascular effects. Thus, tumor homing with CpG-ODN-loaded liposomes is as potent as direct injection of free CpG-ODN and has the potential to overcome some major limitations of conventional CpG-ODN monotherapy.

Mammary Tumorigenesis through LPA Receptor Signaling

Lysophosphatidic acid (LPA) is a lipid growth factor that is produced by an extracellular phospholipase, termed autotaxin (ATX), and acts via G protein-coupled receptors. In this issue of Cancer Cell, Liu et al. show that transgenic overexpression of ATX or LPA receptors leads to invasive and metastatic mammary cancer.

A systems biology approach to defining metastatic biomarkers and signaling pathways

Metastasis is the final stage of cancer and the primary cause of mortality for most solid malignancies. This terminal phase of cancer progression has been investigated using a variety of high-throughput technologies (i.e., gene expression arrays, array comparative genomic hybridization (aCGH), and proteomics) to identify prognostic expression profiles and better characterize the metastatic process. For decades, the predominant model for the metastatic process has been the 'progression model', yet recent microarray results tend to support an inherent metastatic capability within primary tumors. Moreover, studies using a highly metastatic transgenic mammary tumor model suggest that germline polymorphisms are significant determinants of metastatic efficiency. Likewise, a strong concordance of survival has been observed between family members with cancer, further supporting the link between genetic inheritance and survival. In addition, chromosomal aberrations and signaling pathways related to metastatic capacity have been identified by array comparative genomic hybridization (aCGH) and proteomic studies, respectively. Lastly, carcinoma enzyme activity profiles using activity-based proteomics (ABPP), may be more clinically useful than expression-based proteomics for certain cancers. Most importantly, the application of these high-throughput techniques should expedite the search for additional biomarkers, germline polymorphisms, and expression signatures with greater prognostic value.

Ag-specific type 1 CD8 effector cells enhance methotrexate-mediated antitumor responses by modulating differentiated T cell localization, activation and chemokine production in established breast cancer

The chemotherapeutic agent methotrexate is widely used in the treatment of breast cancer. Although its mechanism-of-action has been defined, less is known about its interaction with T cell-mediated antitumor responses. Type 1 CD8 T cell-mediated immune responses (Tc1) are cytolytic, produce IFN-gamma and are associated with effective antitumor responses. Using a murine transgenic TCR tumor model, we show that single-dose treatment with methotrexate enhanced CD8-mediated type 1 antitumor responses when administered 3 days prior to Tc1 effector cell transfer. Co-treatment with methotrexate not only enhanced donor Tc1 cell accumulation and persistence at sites of primary tumor growth, but also promoted elevated levels of activated donor TIL cells. This markedly enhanced the appearance of endogenous differentiated (CD44 High) CD8 tumor-infiltrating cells when compared to that of corresponding groups receiving either MTX or Tc1 cell transfer alone. Such cells were acutely activated as defined by co-expression of surface markers associated with TCR engagement (CD69) and T cell activation (CD25) at both early (days 1–8) and late (days 12–20) stages following treatment. Conversely, such animals showed an early decrease in CD4 +/CD44 High/CD25 +/CD69 + T cells that correlated with delays in tumor growth in vivo. Moreover, cellular response kinetics appeared to further correlate with the up-regulation of endogenous T cells producing the chemokine IP-10 in vivo. This suggested that Tc1 cell transfer, in combination with chemotherapy, can enhance antitumor responses by modulating immunoregulatory T cells involved in homeostasis and immune tolerance within the tumor environment. These studies offer insight into mechanisms that enhance T cell-based immunotherapy in cancer.

Combined Inactivation of MYC and K-Ras Oncogenes Reverses Tumorigenesis in Lung Adenocarcinomas and Lymphomas

BackgroundConditional transgenic models have established that tumors require sustained oncogene activation for tumor maintenance, exhibiting the phenomenon known as “oncogene-addiction.” However, most cancers are caused by multiple genetic events making it difficult to determine which oncogenes or combination of oncogenes will be the most effective targets for their treatment.Methodology/Principal FindingsTo examine how the MYC and K-rasG12D oncogenes cooperate for the initiation and maintenance of tumorigenesis, we generated double conditional transgenic tumor models of lung adenocarcinoma and lymphoma. The ability of MYC and K-rasG12D to cooperate for tumorigenesis and the ability of the inactivation of these oncogenes to result in tumor regression depended upon the specific tissue context. MYC-, K-rasG12D- or MYC/K-rasG12D-induced lymphomas exhibited sustained regression upon the inactivation of either or both oncogenes. However, in marked contrast, MYC-induced lung tumors failed to regress completely upon oncogene inactivation; whereas K-rasG12D-induced lung tumors regressed completely. Importantly, the combined inactivation of both MYC and K-rasG12D resulted more frequently in complete lung tumor regression. To account for the different roles of MYC and K-rasG12D in maintenance of lung tumors, we found that the down-stream mediators of K-rasG12D signaling, Stat3 and Stat5, are dephosphorylated following conditional K-rasG12D but not MYC inactivation. In contrast, Stat3 becomes dephosphorylated in lymphoma cells upon inactivation of MYC and/or K-rasG12D. Interestingly, MYC-induced lung tumors that failed to regress upon MYC inactivation were found to have persistent Stat3 and Stat5 phosphorylation.Conclusions/SignificanceTaken together, our findings point to the importance of the K-Ras and associated down-stream Stat effector pathways in the initiation and maintenance of lymphomas and lung tumors. We suggest that combined targeting of oncogenic pathways is more likely to be effective in the treatment of lung cancers and lymphomas.

Tumor Models for Efficacy Determination

The first in vivo tumor models were developed in the mid-1960s. These models were mouse leukemia models grown as ascites. The growth pattern was like that of bacteria in vivo and therefore it was possible to apply similar mathematics of growth and response to these tumors as had been worked out for bacteria. Since the development of the murine leukemia models, investigators have devoted a large effort to modeling solid tumors in mice. There are now a variety of models including syngeneic mouse tumors and human tumor xenografts grown as s.c. nodules, syngeneic mouse tumors and human tumor xenografts grown in orthotopic sites, models of disseminated disease, labeled tumor models that can be visualized using varied technologies, and transgenic tumor models. Each of these types of models has advantages and disadvantages to the drug hunter searching for improved treatments.

Non-invasive MRI tumor imaging and synergistic anticancer effect of HSP90 inhibitor and glycolysis inhibitor in RIP1-Tag2 transgenic pancreatic tumor model

To utilize non-invasive MRI imaging for real-time testing the synergistic effects of HSP90 inhibitor and glycolysis inhibitor for pancreatic cancer therapy in spontaneous pancreatic cancer mouse model. Transgenic RIP1-Tag2 spontaneous pancreatic cancer mice were treated with geldanamycin (GA, 5 mg/kg) and /or 3-Bromo-pyruvate (3-BrPA, 5 mg/kg) from 8 to 12 weeks of age. Non-invasive MRI imaging measured and calculated the total tumor mass and volumes in real-time and compared to ex vivo tumors size. Serum VEGF levels were measured by ELISA. HSP 90 client protein levels (AKT and VEGF) were measured by western blots. RIP-Tag2 transgenic mice developed pancreatic tumors from 8 to 12 weeks of age. Non-invasive MRI imaging detected primary tumors in pancreas and metastasis in intestine and mesenterium with minimal resolution of 20 mm(3). VEGF, AKT, hexokinase II, and Hsp90 were expressed in the pancreatic cancer tissues from RIP1-Tag2 transgenic mice. Combination of GA and 3-BrPA decreased serum VEGF levels by 70% compared to control group. Non-invasive MRI imaging showed that combination of GA and 3-BrPA inhibited pancreatic tumor and metastasis by more than 90% and significantly prolonged life span of RIP1-Tag2 transgenic pancreatic cancer mice. The synergistic effect of geldanamycin and 3-BrPA is through inhibition of two different pathways on HSP90 for its client protein degradation and on HK II for energy metabolism. Non-invasive MRI imaging revealed synergistic effects of Hsp90 inhibitors and glycolysis inhibitors, which may provide a new therapeutic option for pancreatic cancer therapy.

Ag-Specific Type 1 CD8 Effector Cells Enhance Methotrexate-Mediated Antitumor Responses by Modulating Endogenous CD49b-Expressing CD4 and CD8 T Effector Cell Subpopulations Producing IL-10

The chemotherapeutic agent methotrexate is widely used in the treatment of breast cancer. Although its mechanism-of-action has been defined, less is known about its interaction with Ag-specific T cell-mediated antitumor responses. Type 1 CD8 T cell-mediated immune responses (Tc1) are cytolytic, produce IFN-gamma and are associated with effective antitumor responses. Using a murine transgenic TCR tumor model, we show that single-dose-treatment with methotrexate enhanced CD8-mediated type 1 antitumor responses when administered three days prior to Tc1 effector cell transfer. Co-treatment with methotrexate not only enhanced donor Tc1 cell accumulation and persistence at sites of primary tumor growth, but also promoted elevated levels of activated CD25+ expressing donor TIL cells. This correlated with a marked decrease in the appearance of endogenous differentiated (CD44High) CD3/CD8/CD49b and CD3/CD4/CD49b tumor-infiltrating effector T cells at both early (Days 1–8) and late (Days 12–20) stages following treatment when compared to that of corresponding groups receiving either MTX or Tc1 cell transfer alone. Moreover, such cellular response kinetics appeared to further correlate with the down-regulation of endogenous CD4/CD44High/CD49b effector T cells producing IL-10 and delays in tumor growth in vivo. This suggested that Ag-specific Tc1 cell transfer, in combination with chemotherapy, can enhance antitumor responses by modulating select CD49b-expressing T effector/memory cell subpopulations involved in homeostasis and immune tolerance within the tumor environment. These studies offer insight into mechanisms that enhance T cell-based immunotherapy in cancer. Supplementary materials are available for this article. Go to the publisher's online edition of Immunological Investigations for the following free supplemental resource(s): Addendum 1.

PHA-739358, a potent inhibitor of Aurora kinases with a selective target inhibition profile relevant to cancer

PHA-739358 is a small-molecule 3-aminopyrazole derivative with strong activity against Aurora kinases and cross-reactivities with some receptor tyrosine kinases relevant for cancer. PHA-739358 inhibits all Aurora kinase family members and shows a dominant Aurora B kinase inhibition-related cellular phenotype and mechanism of action in cells in vitro and in vivo. p53 status-dependent endoreduplication is observed upon treatment of cells with PHA-739358, and phosphorylation of histone H3 in Ser(10) is inhibited. The compound has significant antitumor activity in different xenografts and spontaneous and transgenic animal tumor models and shows a favorable pharmacokinetic and safety profile. In vivo target modulation is observed as assessed by the inhibition of the phosphorylation of histone H3, which has been validated preclinically as a candidate biomarker for the clinical phase. Pharmacokinetics/pharmacodynamics modeling was used to define drug potency and to support the prediction of active clinical doses and schedules. We conclude that PHA-739358, which is currently tested in clinical trials, has great therapeutic potential in anticancer therapy in a wide range of cancers.

A novel Flk1-TVA transgenic mouse model for gene delivery to angiogenic vasculature

The genes that regulate the formation of blood vessels in adult tissues represent promising therapeutic targets because angiogenesis plays a role in many diseases, including cancer. We wished to develop a mouse model allowing characterization of gene function in adult angiogenic vasculature while minimizing effects on embryonic vasculature or adult quiescent vasculature. Here we describe a transgenic mouse model that allows expression of proteins in the endothelial cells of newly forming blood vessels in the adult using a selective retroviral gene delivery system. We generated transgenic mouse lines that express the TVA receptor for the RCAS avian-specific retrovirus from Flk1 gene regulatory elements that drive expression in proliferating endothelial cells. Several of these Flk1-TVA lines expressed TVA mRNA in the embryonic vasculature and TVA protein in new blood vessels growing into subcutaneous extracellular matrix implants in adult mice. In a Flk1-TVA line that was crossed with the MMTV-PyMT transgenic mammary tumor model, tumor endothelial cells also expressed the TVA protein. Furthermore, endothelial cells in extracellular matrix implants and the tumors of Flk1-TVA mice were susceptible to RCAS infection, as determined by expression of green fluorescent protein encoded by the virus. The Flk1-TVA mouse model in conjunction with the RCAS gene delivery system will be useful to study molecular mechanisms underlying adult forms of angiogenesis.

Efficacy and Long Term Survival of Genetically Modified T Cells Targeting CD19 in a Syngeneic Immune Competent Transgenic Murine Tumor Model Is Dependent on Prior Lymphodepleting Chemotherapy.

T cells may be genetically modified ex vivo to target specific antigens by retroviral transduction of genes encoding chimeric antigen receptors (CARs). We have previously constructed a CAR, termed 19z1, specific for the CD19 antigen expressed on most B cell malignancies. Human T cells modified to express the 19z1 CAR specifically eradicate systemic human CD19+ tumors in SCID-Beige mice. However, these models are limited by the xenogeneic nature of the human T cells and tumor cells and the immune compromised state of the host. Here, we studied the biology of adoptively transferred 19z1+ T cells in a syngeneic immune competent murine model designed to better mimic the clinical setting of patients with B cell malignancies. We utilized transgenic C57BL6 mice which lack expression of mouse CD19 (mCD19−/−) and have a single copy of the human CD19 (hCD19+/−) gene (C57BL6(mCD19−/− hCD19+/−)) kindly provided by Dr. T. Tedder, Duke University. These mice are functionally immune-competent with hCD19 expression restricted to the B cell population. To assess whether syngeneic 19z1+ T cells were capable of eradicating normal hCD19+ B cells, we infused C57BL6(mCD19−/− hCD19+/−) mice with either 19z1+ or control prostate specific membrane antigen-targeted (Pz1+) T cells. As assessed by flow cytometric analysis of peripheral blood, we neither found evidence of hCD19+ B cell aplasias in 19z1+ T cell treated mice nor were able to demonstrate the persistence of infused CAR+ T cells. To investigate whether the lack of 19z1+ T cell efficacy and persistence was due to an absence of homeostatic drive, we next lymphodepleted C57BL6(mCD19−/− hCD19+/−) mice with cyclophosphamide prior to T cell infusion. Mice lymphodepleted prior to 19z1+ T cell infusion demonstrated marked and sustained B cell aplasias when compared to lymphodepleted Pz1+ T cell and non-lymphodepleted T cell treated controls. Furthermore, while no CAR+ T cells were identifiable in the Pz1 and non-lymphodepleted control groups, 19z1+ T cells were consistently present in the peripheral blood of the cyclophosphamide pre-treated, 19z1+ T cell treated mice (3–5% of white blood cells). To assess the anti-tumor efficacy of the 19z1+ T cells, we next established a systemic tumor model utilizing mouse EL4 thymoma cells retrovirally modified to express hCD19 (EL4(hCD19)). C57BL6(mCD19−/− hCD19+/−) mice pre-treated with cyclophosphamide, subsequently infused systemically with EL4(hCD19) tumor, followed by systemic 19z1+ T cell infusion, had a significant survival advantage (80% survival at >120 days) over untreated controls or controls treated with Pz1+ T cells or 19z1+ T cells in the absence of lymphodepletion (0% survival). In conclusion, we have developed a syngeneic immune competent tumor model of hCD19 disease that is highly relevant to the clinical setting. Using this model, we demonstrate the significance of lymphodepletion on the prolonged in vivo persistence and anti-tumor efficacy of 19z1+ T cells. Data derived from this model will be correlated to findings obtained from a recently initiated clinical trial for patients with chronic lymphocytic leukemia, and will significantly impact the design of subsequent trials in the future.

Chemoprevention by cyclooxygenase-2 inhibition reduces immature myeloid suppressor cell expansion

Selective inhibitors of cyclooxygenase-2 (COX-2) enzyme activity have shown chemopreventive activity in carcinogen-induced and transgenic rodent tumor models and clinically for colon cancer. However, the mechanism(s) by which COX-2 inhibitors reduce carcinogenesis remains controversial. We report herein that administration of the selective COX-2 inhibitor, celecoxib, significantly reduces the number of Gr1 +CD11b + immature myeloid suppressor cells (IMSCs) during chemoprevention of 1,2-dimethylhydrazine diHCl-(1,2-DMH-) induction of large intestinal tumors in Swiss mice. Celecoxib administration also increased splenic lymphatic number and tumor infiltration by lymphocytes. The 1,2-DMH induction of large intestinal tumors was associated with a four-fold increase in IMSCs, and a decrease in splenic T cell number and function. Concordant with the changes in the IMSC frequency, messenger ribonucleic acid (mRNA) levels of inducible nitric oxide synthase (NOS-2) and arginase (Arg) were increased in the spleen of the tumor-bearing mice and normalized by celecoxib administration. In addition to delaying tumor induction, reducing tumor number, and increasing lymphocyte infiltration of tumors, celecoxib therapy reversed CD4 + T cell loss, decreased IMSC numbers and increased mRNA levels of NOS-2 and Arg in the spleen. In summary, our results suggest that celecoxib chemoprevention of autochthonous intestinal tumors can regulate IMSCs and CD4 + T cell numbers.

Tumor models for efficacy determination

The first in vivo tumor models were developed in the mid-1960s. These models were mouse leukemia models grown as ascites. The growth pattern was like that of bacteria in vivo and therefore it was possible to apply similar mathematics of growth and response to these tumors as had been worked out for bacteria. Since the development of the murine leukemia models, investigators have devoted a large effort to modeling solid tumors in mice. There are now a variety of models including syngeneic mouse tumors and human tumor xenografts grown as s.c. nodules, syngeneic mouse tumors and human tumor xenografts grown in orthotopic sites, models of disseminated disease, "labeled" tumor models that can be visualized using varied technologies, and transgenic tumor models. Each of these types of models has advantages and disadvantages to the "drug hunter" searching for improved treatments.

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.

In Vivo Synergy of IL-21 Plus IL-2 in A Murine Transgenic pmel-1 Therapeutic Melanoma Tumor Model Results in Significant Anti-Tumor Immunity and Durable Long-Term Curative Effect

s: Abstracts for the 20th Annual Scientific Meeting of the International Society for Biological Therapy of Cancer (Primary Authors are Italicized): CYTOKINES: BIOLOGY AND THERAPEUTICS

Of mice and Myc: c-Myc and mammary tumorigenesis.

The proto-oncogene c- myc is involved in regulating proliferation and apoptosis, and its deregulation via genomic and postgenomic mechanisms, contributes to the development and progression of multiple human cancers, including those of the breast. Deregulated expression of c-Myc also contributes to neoplastic transformation by altering cellular differentiation pathways and by facilitating mutagenesis through induction of genomic instability. Transgenic and gene-knockout mice are frequently utilized to resolve the mechanisms through which specific genes influence the development and progression of malignancies. In this review, we discuss how research findings obtained from various c- myc transgenic mammary tumor models help to improve our resolution of c-Myc's role both in tumorigenesis of the murine mammary gland and cancer of the human breast.

Signal transducer and activator of transcription 6 (Stat6) and CD1: inhibitors of immunosurveillance against primary tumors and metastatic disease.

Many tumor immunologists favor the hypothesis that optimal anti-tumor activity is mediated by type 1 CD4(+) and CD8(+) T cells, and that the production of type 2 CD4(+) T cells may be counterproductive for effective anti-tumor immunity. Since Stat6-deficient or "knockout" mice lack the signal transducer and activator of transcription-6 protein and are unable to transmit signals initiated by the type 2 cytokines, IL-4 and IL-13, they have been studied to confirm the T(h)1 vs T(h)2 paradigm. Using transplantable tumor cells that cause primary solid tumors and metastatic disease, as well as a spontaneous transgenic tumor model, multiple studies have demonstrated that Stat6(-/-) mice are able to reject or delay primary tumor growth, prevent recurrence of primary tumors, and/or reject established, spontaneous metastatic disease. Deletion of the Stat6 gene, therefore, provides significantly enhanced immunosurveillance. Comparable experiments with CD1-deficient mice, which lack NKT cells and hence are deficient for IL-13, give similar results and suggest that removal of NKT cells also enhances immunosurveillance. Because immunity is enhanced in the absence of Stat6 or CD1, it has been hypothesized that these deletions result in the removal of an inhibitor that blocks constitutive immunosurveillance. Several mechanisms have been tested as potential inhibitors, including CD4(+)CD25(+) T regulatory cells, IL-13, a T(h)2 shift, and myeloid suppressor cells. Although the first three mechanisms do not appear to be relevant, regression of myeloid suppressor cells in Stat6-deficient and CD1-deficient mice may be responsible for enhanced immunosurveillance. Although additional studies are clearly needed to clarify the mechanism(s) underlying improved anti-tumor immunity in Stat6(-/-) and CD1(-/-) mice, deletion of these genes results in a potent anti-tumor immunity and may be a basis for an immunotherapy strategy.

Role of animal models in oncology drug discovery

Tumor xenografts have been used for over three decades in oncology drug development with little predictive value. A case has been made for use of orthotopic xenograft models as well as transgenic tumor models as a better reflection of tumor biology. However, their use in drug development has been limited, thus far, due to technical challenges of monitoring tumor growth. Recent advances in small animal imaging are addressing some of those challenges; however, the predictive ability of these models with regards to clinical response is still questionable. This presentation will focus on the utility of animal models in oncology drug development and a paradigm shift in their value as tools to evaluate biological effects of new agents as well as understanding the pharmacokinetic/pharmacodynamic relationship to aid in better clinical development.

Isolation and characterization of functional mammary gland stem cells.

Significant advances in the stem-cell biology of several tissues, including the mammary gland, have occurred over the past several years. Recent progress on stem-cell fate determination, molecular markers, signalling pathways and niche interactions in haematopoietic, neuronal and muscle tissue may provide parallel insight into the biology of mammary epithelial stem cells. Taking advantage of approaches similar to those employed to isolate and characterize haematopoietic and epidermal stem cells, we have identified a mammary epithelial cell population with several stem/progenitor cell qualities. In this article, we review some recent data on mammary epithelial stem/progenitor cells in genetically engineered mouse models. We also discuss several potential molecular markers, including stem-cell antigen-1 (Sca-1), which may be useful for both the isolation of functional mammary epithelial stem/progenitor cells and the analysis of tumour aetiology and phenotype in genetically engineered mouse models. In different transgenic mammary tumour models, Sca-1 expression levels, as well as several other putative markers of progenitors including keratin-6, possess dramatically altered expression profiles. These data suggest that the heterogeneity of mouse models of breast cancer may partially reflect the selection or expansion of different progenitors.

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 CB6F 1- 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.