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The role of Snail transcription factor in mammary tumorigenesis
Background: The p53 tumor suppressor gene is mutated in ~ 30% of breast cancers. To understand how loss of p53 promotes breast tumorigenesis, we inactivated the p53 gene specifically in mammary epithelial cells using conditional mutagenesis. Mammary tumors developing in these females were composed of solid irregular nests of malignant E-cadherin positive epithelial cells. In about 80% of the cases spindle cell transformation (sarcomatous metaplasia) featuring long fascicles were noted, suggestive of carcinomas undergoing epithelial to mesenchymal transition (EMT). Consistently, these cells did not express epithelial markers like E-cadherin, ß-catenin and cytokeratins 6 and 14. Instead, the tumors stained strongly positive for mesenchymal markers (i.e vimentin). Moreover, the gene-expression analysis of these tumors identified upregulation of several genes that are involved in the EMT process, including the zinc-finger transcription factor, snail. Snail is a transcriptional repressor of E-cadherin, and has been shown to be up-regulated in invasive human breast cancers. The main objective is to elucidate the role of snail in mammary tumor development and progression in vivo. Therefore, our specific aims are: (1) To inactivate conditional snail and p53 alleles concomitantly in mammary epithelial cells to test the hypothesis that ablation of snail in p53-deficient cells may suppress (or delay) the development of p53-deficient tumors. (2) To elucidate the role of snail overexpression in normal mammary gland development and in malignancy, we will induce overexpression of snail specifically in mammary epithelial cells in either wild type or conditional p53 mutant mice. Study design: (1) Using targeted mutagenesis in mouse embryonic stem cells we will generate conditional snail mutant mice to specifically ablate snail function in p53-deficient mammary epithelial cells and monitor these animals for tumor latency and progression. (2) Employing the tetracycline inducible (tet/ON) system, we will overexpress snail in wild-type and p53-deficient mammary epithelial cells in vivo to assess its role in mammary development and tumorigenesis. A comprehensive analysis of primary tumor and tumor cell lines should identify transcriptional targets of snail besides E-cadherin, highlight signaling pathways that are critical for tumor invasion and/or metastasis and potentially uncover diagnostic markers and targets for effective therapy.
Background: ~25% of breast cancer patients will develop invasive tumors that spread (‘metastasis’) to other organs like lungs and bone. Metastases are responsible for most of the breast cancer fatalities because they are notoriously refractory to most treatments unlike primary tumors. Unfortunately, at present we know very little about the changes that take place to convert a primary tumor cell into a deadly metastasis. 90% of breast cancers arise from uncontrolled proliferation of ‘epithelial’ cells that line the milk ducts and their finer branches. Epithelial cells have distinct top (‘apical’) and bottom (‘basal’) surfaces, are tightly held to one another by the molecular glue E-cadherin and are immobile. However, when breast tumors become metastatic, they lose these epithelial features. This process is called epithelial-to-mesenchymal transition (‘EMT’ for short). Loss of E-cadherin (the molecular glue) from primary tumors is the hallmark of EMT. There are many molecular switches present within an epithelial cell to precisely control E-cadherin production. One can imagine that abnormal production of a molecular switch which normally turns-off E-cadherin production could allow primary tumors metastasize. Recently, the Snail protein was discovered to be a molecular switch that turns off E-cadherin production in an epithelial cell to jump-start the EMT process. It was also found that invasive human breast cancers produced high levels of Snail protein, but very low or none of E-cadherin. These discoveries suggest that abnormal production of Snail may be connected with breast cancer development and metastasis. Hence the main objective of this proposal is to develop animal models of breast cancer to study the role of the Snail protein in breast cancer development and metastasis. To this purpose, we will generate two strains of mice: 1) those that lack Snail protein in the mammary gland (the equivalent organ to human breast) and 2) others that over-produce Snail protein in the mammary gland. These animals will be monitored for tumor development and metastasis. Tumors that develop in these animals will be used to identify additional genes involved in tumor development and metastasis. These genes could be potential targets for developing more effective therapies against breast cancer metastasis, as well as could be used diagnostic markers for metastasis, thus helping to reduce breast cancer mortalities.