Temporal dissection of p53 function in vitro and in vivo

To investigate the functions of the p53 tumor suppressor, we created a new knock- in gene replacement mouse model in which the endogenous Trp53 gene is substituted by one encoding p53ER(TAM), a p53 fusion protein whose function is completely dependent on ectopic provision of 4- hydroxytamoxifen. We show here that both tissues in vivo and cells in vitro derived from such mice can be rapidly toggled between wild- type and p53 knockout states. Using this rapid perturbation model, we define the kinetics, dependence, persistence and reversibility of p53- mediated responses to DNA damage in tissues in vivo and to activation of the Ras oncoprotein and stress in vitro. This is the first example to our knowledge of a new class of genetic model that allows the specific, rapid and reversible perturbation of the function of a single endogenous gene in vivo.