Temporal dissection ofp53 function in vivo and in vitro

The tumor suppressor protein p53 is a sequence specific transcription factor that plays a key role in suppressing the emergence of neoplasia. Notably, p53 serves no significant role in the control of normal cell proliferation but, rather, acts as an integrator of responses to abnormal and pathological stimuli. Importantly, the pathways responsible for p53-mediated tumor suppression remain unclear, largely because so many different upstream signals and insults that arise in tumor cells have been shown to activate p53.; A novel type of targeted mutant mouse model was created, in which the endogenous p53 gene was replaced by one encoding p53ERTAM, a p53 fusion protein whose function is dependent upon ectopic provision of the synthetic steroid 4-hydroxytamoxifen. In this thesis, the p53ER TAM knock-in mouse model was validated and used to investigate the temporal relationship between p53 status, DNA damage, oncogene activation and tumor suppression. The kinetics, persistence and reversibility of p53-mediated responses to DNA damage in tissues in vivo, and to the activation of the Ras oncoprotein in cells in vitro, are defined for the first time. In vivo studies of radiation-induced tumorigenesis show that the function of p53 in the response to acute DNA damage is distinct from its function as a tumor suppressor. Rather, it is shown that the tumor suppressive function of p53 is dependent on the presence of p19ARF , a tumor suppressor that is activated by deregulated oncogenes. This suggests that the p53-activating signal in incipient tumors is the activation of mitogenic oncogenes as opposed to DNA damage. Furthermore, this study shows that the presence of p53 in tissues at the time of acute genotoxic stress is disadvantageous for long-term tumor suppression. Preliminary evidence suggests that this is due to selective pressure for inactivation of p53.; The field of cancer biology is currently exploring pharmacological strategies for restoring p53 in tumors and early pre-cancerous lesions. The findings in this thesis suggest that it should be possible to restore p53 function effectively, without resorting to DNA damage as a stimulus for p53 activity, thereby avoiding the associated pathologies and secondary malignancies caused by current cancer therapies.