Abstract

Mouse p53 is phosphorylated at Ser18 and Ser23 after DNA damage. To determine whether these two phosphorylation events have synergistic functions in activating p53 responses, we simultaneously introduced Ser18/23 to Ala mutations into the endogenous p53 locus in mice. While partial defects in apoptosis are observed in p53^S18A and p53^S23A thymocytes exposed to IR, p53-dependent apoptosis is essentially abolished in p53^S18/23A thymocytes, indicating that these two events have critical and synergistic roles in activating p53-dependent apoptosis. In addition, p53^S18/23A, but not p53^S18A, could completely rescue embryonic lethality of Xrcc4^-/- mice that is caused by massive p53-dependent neuronal apoptosis. However, certain p53-dependent functions, including G₁/S checkpoint and cellular senescence, are partially retained in p53^S18/23A cells. While p53^S18A mice are not cancer prone, p53^S18/23A mice developed a spectrum of malignancies distinct from p53^S23A and p53^-/- mice. Interestingly, Xrcc4^-/-p53^S18/23A mice fail to develop tumors like the pro-B cell lymphomas uniformly developed in Xrcc4^-/- p53^-/- animals, but exhibit developmental defects typical of accelerated ageing. Therefore, Ser18 and Ser23 phosphorylation is important for p53-dependent suppression of tumorigenesis in certain physiological context.