Abstract 4291: Hras versus Braf activation determines follicular versus papillary thyroid cancer development.

Proceedings: AACR 104th Annual Meeting 2013; Apr 6-10, 2013; Washington, DC Thyroid cancer is the most common endocrine malignancy, and incidences are rising. MAP kinase (MAPK) signaling has been implicated in playing a critical role in the initiation and maintenance of thyroid cancer, as evidenced by the high incidence of non-overlapping mutations of the genes encoding RET and TRK, as well as of NRAS, HRAS, KRAS and BRAF. Follicular thyroid cancer (FTC) is commonly associated with mutations of the RAS family of oncoproteins, while papillary thyroid cancer (PTC) is often associated with mutations in BRAF. To determine whether mode of MAPK activation played a role in thyroid cancer development in mice, we generated HrasG12V and BrafV600E knock-in models. We report that a thyroid specific knock-in of HrasG12V and Pten inactivation leads to the development of FTCs and poorly differentiated thyroid cancer (PDTCs) by 47-52 weeks of age in HrasG12V/Ptenhom/TPO-cre mice. In stark contrast, mice with a thyroid specific knock-in of BrafV600E and loss of Pten (BrafV600E/Ptenhom/TPO-cre) rapidly develop PDTCs and anaplastic thyroid cancer with complete lethality by weaning, suggesting that Braf and Hras, in cooperation with PI3K signaling, likely play distinct roles in the development and progression of disease. We next sought to dissect the role of Hras versus Braf activation in FTC versus PTC development. We derived stable cell lines from HrasG12V/Ptenhom/TPO-cre and BrafV600E/Ptenhom/TPO-cre tumors in order to identify cellular and molecular consequences of Hras versus Braf activation and their potential impact on tumor phenotype. We have found significant differences in phosphorylation of kinases in response to Hras versus Braf activation. Hras activation leads to increased phosphorylation of mTOR effectors, while Braf activation increases CREB activation. Studies are ongoing to identify the molecular mechanism of these observed differences. We have also shown a significant decrease in the growth of HrasG12V/Ptenhom (p=0.0009) and BrafV600E/Ptenhom (p=0.0001) cells treated with the MEK1/2 inhibitor PD0325901, suggesting that activated Hras or Braf-even in the context of Pten loss-partially relies on MAPK signaling to reach its full oncogenic potential. We have recently employed a PCR array approach to identify differences in the expression of genes related to the MAPK pathway in HrasG12V/Ptenhom and BrafV600E/Ptenhom cell lines. We noted an up-regulation of Myc, Cdkn1a, and Sfn in BrafV600E/Ptenhom cells in comparison to HrasG12V/Ptenhom cells, which may contribute to the different pathophysiology of HrasG12V/Ptenhom and BrafV600E/Ptenhom tumor sub-types. Because FTCs and PTCs have different responses to therapy, we seek to identify the cellular and physiological consequences of Ras versus Raf activation which will hopefully aid in the discovery of novel therapeutic and prevention strategies for thyroid cancer. Citation Format: Lee Ann King, Jeffrey Knauf, Ronald Ghossein, James Fagin, Aime T. Franco. Hras versus Braf activation determines follicular versus papillary thyroid cancer development. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4291. doi:10.1158/1538-7445.AM2013-4291