| Summary: | Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Biology, 2010. === In title on title page, doubled underscored "Arf" appears as superscript. Vita. Cataloged from PDF version of thesis. === Includes bibliographical references. === Tumor development is a multi-step process driven by the collective action of gain-of-function mutations in oncogenes and loss-of-function alterations in tumor suppressor genes. The particular spectrum of mutations in a given cancer is rarely the result of random chance but instead derives from the intimate connections between proliferative networks and those suppressing growth and transformation. Specifically, hyper-active oncogenes directly engage tumor suppressor programs, such that cells harboring oncogenic lesions frequently must acquire secondary mutations that disable these anti-proliferative responses before progressing to overt transformation. This tight coupling represents a critical checkpoint protecting against tumor formation. Whether different cell types exhibit variability in the extent and/or timing of this oncogene-induced tumor suppression is largely unknown. The ability of oncogenic Ras to induce the tumor suppressive p1 9 Arf-p5.3 pathway and cause irreversible cell cycle arrest typifies this phenomenon. Using this-well established interaction as model, we investigated the cell-type specificity of oncogene-induced tumor suppression. By combining K-rasL mice with a reporter for p19Arf expression (Ar FP), we identify a tissue-specific, onocogenic K-ras-dependent expression pattern of 19Arfin lung tumors and sarcomas that correlates with each tissue's genetic requirements for tumorigenesis. Lung tumors, which can arise in the presence of p19Arf and show modest increases in tumor progression in its absence, exhibit very minimal p19 Arf induction. Conversely, sarcomas, which depend on p19 f-p53 mutation for tumor formation, display robust p 1 9 Af up-regulation. While previous studies proposed oncogene levels as the main determinant of p19A induction, we find equivalent signaling levels and instead highlight tissue-specific differences in the epigenetic regulation of Ink4a/Arf Using in vivo RNAi, we implicate Polycomb group (PcG) proteinmediated repression in lung tumors and SWI/SNF-dependent activation in sarcomas as being critically important for each tissue's unique expression pattern of p1 9 Arf During normal tumor progression, mutations in oncogenes and tumor suppressors occur in a sequential fashion, although whether unique orders of mutations dictate distinct phenotypes is unknown. The requirement for complete p53 pathway abrogation during oncogenic K-rasdependent sarcomagenesis suggested that tumor development in the muscle critically depends on early p53 mutation. To test this we generated a Flp-inducible allele of K-rasG12D (K-rasFSF-G12D) that when combined with established reagents for Cre-dependent p53 deletion permits the separate regulation of K-ras activation and p53 loss. Strikingly, although simultaneous mutation results in robust tumor formation, delaying p53 deletion relative to oncogenic K-ras expression === (cont.) significantly diminishes tumor penetrance. This indicates that the tumorigenic capacity of KrasG12D mutant muscle cells is rapidly and severely compromised by a strong p53-dependent response, which is entirely different from the mode of action of p53 during lung tumorigenesis. Further genetic analysis implicates the p53 target gene p21 in this suppression, implying that p53 irreversibly constrains sarcoma development through cell cycle arrest mechanisms. Together, these results highlight tissue-specific variability in the relationship of oncogenic K-ras and the p53 pathway. Robust pathway up-regulation, as seen in muscle cells, affords potent inhibition of tumor initiation, while modest induction, such as in lung cells, permits tumor development and only hinders more advanced stages of progression. These differences might help explain the spectrum of tumors associated with K-Ras mutations as well as the overall frequency of difference cancer types. === by Nathan Price Young. === Ph.D.
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