Date of Award

January 2012

Document Type

Open Access Thesis

Degree Name

Medical Doctor (MD)

Department

Medicine

First Advisor

Marcus Bosenberg

Subject Area(s)

Medicine, Cellular biology, Oncology

Abstract

Melanoma is the most lethal form of skin cancer and accounts for the majority of skin cancer related mortality. Early metastases are characteristic of melanoma and are an ominous sign, as current therapeutic interventions have little effect on overall survival. The lack of accurate prognostic indicators and effective therapies emphasize the need for a better understanding of the genetic and phenotypic changes in melanoma formation and progression. One such change is the loss of E-cadherin, which normally plays a role in cell-cell adhesion and is thought to be a central feature of epithelial-mesenchymal transition (EMT). Maintenance of E-cadherin in melanoma may function to restrict melanoma invasion, and loss of E-cadherin expression is associated with mesenchymal features and melanoma metastasis. While several studies have suggested that loss of E-cadherin is associated with cancer progression, functional evaluation of the effects of E-cadherin loss on metastasis has not been well established.

In order to evaluate these important issues on the role of E-cadherin in melanoma formation and metastasis, we have utilized a well-characterized conditional mouse model combining the activating Braf V600E mutation with Pten tumor suppressor loss (Braf/Pten tumor model). This model allows us to alter these genes in mouse melanocytes in a spatially and temporally restricted manner, producing melanoma specifically and reproducibly. We have combined this model with conditional E-cadherin inactivation in order to evaluate the role of E-cadherin loss on melanoma formation, progression and metastasis as well as changes in cell morphology and signaling. Loss of E-cadherin significantly decreases survival of mice in the Braf/Pten tumor model (p=0.024) and alters the expression of markers of EMT as demonstrated by RNA microarray analysis. Surprisingly, no difference in metastases to either the lymph nodes (p=0.28) or lungs (p=0.39) are seen with loss of E-cadherin.

Additionally, Braf activation in combination with E-cadherin loss (Braf/Ecad model) leads to melanoma in 100% of mice with tumor death occurring after an average of 302 days (range: 199 - 395 days) or approximately 10 months. This novel model is in striking contrast to the Braf/Pten tumor model, which has an average lifespan of 40 days. No tumors develop in mice with Braf activation alone or E-cadherin loss alone nor in mice with both loss of E-cadherin and loss of Pten. The Braf/Ecad tumor model shows activation of mammalian target of rapamycin (mTOR) signaling, a commonly dysregulated protein in human cancers, as well as activity in the P13K/Akt pathway which regulates cellular proliferation and survival. Some markers of EMT are also upregulated in the Braf/Ecad tumor model.

These findings functionally delineate the role of E-cadherin loss in melanoma formation, metastasis and EMT. Additional evidence is garnered which supports the role of E-cadherin as a tumor suppressor gene and that its loss is functionally important in altering key pathways of cellular growth and survival. With better understanding of melanoma, new treatments will be developed to help manage this devastating cancer.

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