Andrea Rosner

Pathway Pathology: Histological Differences Between ErbB/Ras and Wnt Pathway Transgenic Mammary Tumors

To study phenotype-genotype correlations, ErbB/Ras pathway tumors (transgenic for ErbB2, c-Neu, mutants of c-Neu, polyomavirus middle T antigene (PyV-mT), Ras, and bi-transgenic for ErbB2/Neu with ErbB3 and with progesterone receptor) from four different institutions were histopathologically compared with Wnt pathway tumors [transgenes Wnt1, Wnt10b, dominant-negative glycogen synthase kinase 3-beta, beta-Catenin, and spontaneous mutants of adenomatous polyposis coli gene (Apc)]. ErbB/Ras pathway tumors tend to form solid nodules consisting of poorly differentiated cells with abundant cytoplasm. ErbB/Ras pathway tumors also have scanty stroma and lack myoepithelial or squamous differentiation. In contrast, Wnt pathway tumors exhibit myoepithelial, acinar, or glandular differentiation, and, frequently, combinations of these. Squamous metaplasia is frequent and may include transdifferentiation to epidermal and pilar structures. Most Wnt pathway tumors form caricatures of elongated, branched ductules, and have well-developed stroma, inflammatory infiltrates, and pushing margins. Tumors transgenic for interacting genes such as protein kinase CK2alpha (casein kinase IIalpha), and the fibroblast growth factors (Fgf) Int2/Fgf3 or keratinocyte growth factor (Kgf/Fgf7) also have the Wnt pathway phenotype. Because the tumors from the ErbB/Ras and the Wnt pathway are so distinct and can be readily identified using routine hematoxylin and eosin sections, we suggest that pathway pathology is applicable in both basic and clinical cancer research.

AML1 deletion in adult mice causes splenomegaly and lymphomas

AML1 (RUNX1) encodes a DNA-binding subunit of the CBF transcription factor family and is required for the establishment of definitive hematopoiesis. AML1 is one of the most frequently mutated genes associated with human acute leukemia, suggesting that genetic alterations of the gene contribute to leukemogenesis. Here, we report the analysis of mice carrying conditional AML1 knockout alleles that were inactivated using the Cre/loxP system. AML1 was deleted in adult mice by inducing Cre activity to replicate AML1 deletions found in human MDS, familial platelet disorder and rare de novo human AML. At a latency of 2 months after induction, the thymus was reduced in size and frequently populated by immature double negative thymocytes, indicating defective T-lymphocyte maturation, resulting in lymphatic diseases with 50% penetrance, including atypical hyperplasia and thymic lymphoma. Metastatic lymphomas to the liver and the meninges were observed. Mice also developed splenomegaly with an expansion of the myeloid compartment. Increased Howell-Jolly body counts indicated splenic hypofunction. Thrombocytopenia occurred due to immaturity of mini-megakaryocytes in the bone marrow. Together with mild lymphocytopenia in the peripheral blood and increased fractions of immature cells in the bone marrow, AML1 deficient mice display features of a myelodysplastic syndrome, suggesting a preleukemic state.

Activation of different Wnt/β-catenin signaling components in mammary epithelium induces transdifferentiation and the formation of pilar tumors

The Wnt/β-catenin signaling pathway controls cell fate and neoplastic transformation. Expression of an endogenous stabilized β-catenin (ΔE3 β-catenin) in mammary epithelium leads to the transdifferentiation into epidermis- and pilar-like structures. Signaling molecules in the canonical Wnt pathway upstream from β-catenin induce glandular tumors but it is not clear whether they also cause squamous transdifferentiation. To address this question we have now investigated mammary epithelium from transgenic mice that express activating molecules of the Wnt pathway: Wnt10b, Int2/Fgf3, CK2α, ΔE3 β-catenin, Cyclin D1, and dominant negative (dn) GSK3β. Cytokeratin 5 (CK5), which is expressed in both mammary myoepithelium and epidermis, and the epidermis-specific CK1 and CK6 were used as differentiation markers. Extensive squamous metaplasias and widespread expression of CK1 and CK6 were observed in ΔE3 β-catenin transgenic mammary tissue. Wnt10b and Int2 transgenes also induced squamous metaplasias, but expression of CK1 and CK6 was sporadic. While CK5 expression in Wnt10b transgenic tissue was still confined to the lining cell layer, its expression in Int2 transgenic tissue was completely disorganized. In contrast, cytokeratin expression in CK2α, dnGSK3β and Cyclin D1 transgenic mammary tissues was similar to that in ΔE3 β-catenin tissue. In support of transdifferentiation, expression of hard keratins specific for hair and nails was observed in pilar tumors. These results demonstrate that the activation of Wnt signaling components in mammary epithelium induces not only glandular tumors but also squamous differentiation, possibly by activating LEF-1, which is expressed in normal mammary epithelium.

Kinase-Inactive Glycogen Synthase Kinase 3B Promotes Wnt Signaling and Mammary Tumorigenesis

Recent studies have implicated ectopic activation of the Wnt pathway in many human cancers, including breast cancer. beta-catenin is a critical coactivator in this signaling pathway and is regulated in a complex fashion by phosphorylation, degradation, and nuclear translocation. Glycogen synthase kinase 3beta (GSK3beta) phosphorylation of the NH2-terminal domain of beta-catenin targets it for ubiquitination and proteosomal degradation. We hypothesized that expression of kinase-inactive GSK3beta (KI-GSK3beta) in mammary glands would function in a dominant-negative fashion by antagonizing the endogenous activity of GSK3beta and promoting breast cancer development. Consistent with this, we find that KI-GSK3beta stabilizes beta-catenin expression, catalyzes its localization to the nucleus, and up-regulates the downstream target gene, cyclin D1, in vitro. In vivo, transgenic mice overexpressing the KI-GSK3beta under the control of the mouse mammary tumor virus-long terminal repeat develop mammary tumors with overexpression of beta-catenin and cyclin D1. Thus, antagonism of GSK3beta activity is oncogenic in the mammary epithelium; mutation or pharmacologic down-regulation of GSK3beta could promote mammary tumors.

Molecular analysis of metastasis in a polyomavirus middle T mouse model: the role of osteopontin

IntroductionIn order to study metastatic disease, we employed the use of two related polyomavirus middle T transgenic mouse tumor transplant models of mammary carcinoma (termed Met and Db) that display significant differences in metastatic potential.MethodsThrough suppression subtractive hybridization coupled to the microarray, we found osteopontin (OPN) to be a highly expressed gene in the tumors of the metastatic mouse model, and a lowly expressed gene in the tumors of the lowly metastatic mouse model. We further analyzed the role of OPN in this model by examining sense and antisense constructs using in vitro and in vivo methods.ResultsWith in vivo metastasis assays, the antisense Met cells showed no metastatic tumor formation to the lungs of recipient mice, while wild-type Met cells, with higher levels of OPN, showed significant amounts of metastasis. The Db cells showed a significantly reduced metastasis rate in the in vivo metastasis assay as compared with the Met cells. Db cells with enforced overexpression of OPN showed elevated levels of OPN but did not demonstrate an increase in the rate of metastasis compared with the wild-type Db cells.ConclusionsWe conclude that OPN is an essential regulator of the metastatic phenotype seen in polyomavirus middle T-induced mammary tumors. Yet OPN expression alone is not sufficient to cause metastasis. These data suggest a link between metastasis and phosphatidylinositol-3-kinase-mediated transcriptional upregulation of OPN, but additional phosphatidylinositol-3-kinase-regulated genes may be essential in precipitating the metastasis phenotype in the polyomavirus middle T model.

Validation: The New Challenge for Pathology

Modern pathologists have been challenged to “validate” mouse models of human cancer. Validation requires matching of morphological attributes of the model to human disease. Computers can assist in the validation process. However, adequate controlled, computer-readable vocabularies that can match terms do not currently exist in mouse pathology. Further, current standard diagnostic terminologies do not include the new concepts discussed here such as pathway pathology and mammary intraepithelial neoplasia. The terminologies must be revised and improved to meet the challenge. Human medicine has traditionally used “guilt-by-association” to validate interpretations of disease. Experimental pathology uses experimental verification exemplified by “test-by-transplantation.” Genetically Engineered Mice (GEM) develop unique tumor phenotypes bringing new structural-functional insights and reevaluation of concepts. Novel GEM-related tumors appear in all organ systems but mouse models of human breast cancer are prototypes. For example, mammary tumors induced by Mouse Mammary Tumor Virus (MMTV), chemical, radiation or other carcinogenic stimuli have limited phenotypes. These “spontaneous” or induced mammary tumors have never resembled human breast cancers. GEM tumors created with genes associated with human cancer are strikingly different. GEM tumors have unique histological phenotypes. Depending on the genes, the tumors may: 1) resemble MMTV-induced tumors, 2) display “signature” phenotypes, and 3) mimic human breast cancers. The phenotypes can be placed into structural and functional clusters with shared characteristics leading to the concepts of Pathway Pathology: tumor phenotype reflects the genotype.

Acceleration of Mouse Mammary Tumor Virus-Induced Murine Mammary Tumorigenesis by a p53172H Transgene: Influence of FVB Background on Tumor Latency and Identification of Novel Sites of Proviral Insertion

We previously showed that a mammary-specific dominant-negative p53 transgene (WAP-p53(172H)) could accelerate ErbB2-induced mammary tumorigenesis in mice, but was not tumorigenic on its own. To identify other genes that cooperate with WAP-p53(172H) in tumorigenesis, we performed mouse mammary tumor virus (MMTV) proviral mutagenesis. We derived F1, N2, and N4/N5 mice from p53(172H) transgenic FVB mice backcrossed onto MMTV+ C3H/He mice. Results show the latency of MMTV tumorigenesis is correlated with FVB contribution. F1 tumors had the shortest latency (217 days), had a higher rate of metastasis, and were less differentiated than the N2 and N4/N5 tumors. The latency was 269 days in N2 mice, and lengthened to 346 days in N4/N5 mice. p53(172H) significantly accelerated MMTV tumorigenesis only in N2 mice, indicating cooperativity between p53(172H) and MMTV in this cohort. To identify genes that may be causally involved in MMTV-induced mammary tumorigenesis, we identified 60 sites of proviral insertion in the N2 tumors. Among the insertions in p53(172H) transgenic tumors were 10 genes not previously found as sites of MMTV insertion including genes involved in signaling (Pdgfra, Pde1b, Cnk1), cell adhesion (Cd44), angiogenesis (Galgt1), and transcriptional regulation (Olig1, Olig2, and Uncx4.1). These may represent cellular functions that are likely not deregulated by mutation in p53.