Colorado PROFILES, The Colorado Clinical and Translational Sciences Institute (CCTSI)
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Targeting Oncogene Expression to Skin in Transgenic Mice

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Greater than 1 million new cases of non-melanoma skin cancer (NMSC) occur annually in the United States with a lifetime risk nearly equal to that of all other cancers combined. The costs associated with treatment of NMSC per year in 2004 was 1.4 billion dollars and certainly much higher in 2010. The majority of NMSC are basal cell carcinomas (BCC), however most deaths (~3000 annually) result from SCCs. The incidence of SCCs (~300,000 new cases/yr) has increased at an alarming rate, especially in areas of high UV exposure such as Colorado. In addition, NMSC patients have a high risk of acquiring additional cancers that are aggressive and resistant to traditional therapies such as radiation and chemotherapy. They are characterized by multiple recurrences, a large size, deep invasion and a high incidence of metastases. Most SCCs occur on the face and hands and complex reconstructive surgery may leave patients disfigured and functionally impaired. To deal with the dramatic increase in referrals of patients with advanced stages of skin SCCs, our clinical collaborators have recently established a new multidisciplinary Skin SCC Medical Oncology Program (SSMOP). The establishment of the SSMOP, together with our existing High Risk Skin Cancer Specialty Clinic (HRSCSC), which was established to treat organ transplant recipients (OTRs) who exhibit an increased risk to develop NMSC (especially SCCs, in some cases up to 250-fold), illustrates how serious this growing public health problem is for residents in the greater Rocky Mountain Region. The purpose of this proposal is to dissect the molecular mechanisms that govern the formation of highly malignant and metastatic skin tumors by using mouse models that have been genetically engineered to express genetic alterations frequently found in human skin cancers. In order to recapitulate events that have been shown to occur in human skin cancers, my laboratory has developed an inducible system that allows the focal induction of genetic alterations in epidermal stem cells that constantly regenerate the skin. Mutations in p53 are very frequent in human squamous cell carcinomas (SCCs) and can be divided into gain-of-function (GOF) or loss-of-function (LOF) p53 mutations. Moreover, SCCs also show frequent activation of the ras signaling pathway, deregulation of the mitotic kinase Aurora-A and amplification of Myc. During the last phase of this grant, we generated three inducible mouse models of malignant and metastatic SCCs by the overexpression of either Aurora-A or Myc, or by the induction of both a mutant Kras allele and a gain-of-function (GOF) p53 mutant allele. This approach allowed us to generate mice with genetic changes that closely mimic the sporadic focal accumulation of somatic mutations in epidermal stem cells in human skin tumors. We analyzed tumors that developed in these models with a combination of comparative genomic hybridization arrays and gene expression profiling. Our analyses revealed that these models are characterized by a high level of genomic instability and we have shown that oncogenic Aurora-A targets wild type p53 signaling, while SCCs that form in GOF p53 mice are defined by both Myc gene amplification and by deregulation of Aurora-A. GOF p53 tumors are far more aggressive compared to tumors arising from loss of p53, and in this regard, mimic the worst, most malignant human tumors, e.g. similar to those observed in organ transplant recipient (OTR) patients, 25-50% of which harbor MYC amplification. These studies have led to the novel hypothesis that the mutational status of p53 determines the molecular pathways governing skin SCC malignancy. We also postulate that cross regulation of signaling pathways mediated by Myc, Aurora-A and mutant p53 mediate genomic instability and the Rho signaling pathway mediates the invasive metastatic properties of SCCs. The pathways governed by Aurora-A, Myc and effectors of mutant p53 converge on cell cycle regulation and signaling from extracelluar matrix components, integrins and Rho effectors, that we believe promote aggressively growing and invasive tumors. To establish the clinical relevance of our GOF p53 mouse model with human skin SCCs, we have initiated collaborations with our clinical colleagues to establish a direct patient tumor xenograft model (DPTXM) in which individual patient tumors are transplanted into immune compromised mice and then passaged through several generations. These tumors retain the original tumor epithelia and stroma, and are an ideal platform both to expand human skin SCCs and to test therapeutic approaches. This model represents a key clinical link as any therapies developed in this application can be rapidly translated into clinical trials and personalized therapeutic approaches as our clinical colleagues provide clinical care to the same patients whose cancer samples are used to generate the DPTXM. This application proposes to test our novel hypothesis and use both our mouse models and DPTXM for human skin SCCs to test new therapeutic approaches for highly metastatic and aggressive skin SCCs.
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