The p53 tumor suppressor acts as a guardian of the genome by preventing the propagation of DNA damage-induced breaks and mutations to subsequent generations of cells. We have previously shown that phosphorylation of the Mdm2 oncoprotein at Ser394 by the ATM kinase is required for robust p53 stabilization and activation in cells treated with ionizing radiation, and that loss of Mdm2 Ser394 phosphorylation leads to spontaneous tumorigenesis and radioresistance in Mdm2S394A mice. Previous in vitro data indicate that the c-Abl kinase phosphorylates Mdm2 at the neighboring residue (Tyr393) in response to DNA damage to regulate p53-dependent apoptosis. In this present study, we have generated an Mdm2 mutant mouse (Mdm2Y393F) to determine whether c-Abl phosphorylation of Mdm2 regulates the p53-mediated DNA damage response or p53 tumor suppression in vivo. The Mdm2Y393F mice develop accelerated spontaneous and oncogene-induced tumors, yet display no defects in p53 stabilization and activity following acute genotoxic stress. Although apoptosis is unaltered in these mice, they recover more rapidly from radiation-induced bone marrow ablation and are more resistant to whole-body radiation-induced lethality. These data reveal an in vivo role for c-Abl phosphorylation of Mdm2 in regulation of p53 tumor suppression and bone marrow failure. However, c-Abl phosphorylation of Mdm2 Tyr393 appears to play a lesser role in governing Mdm2-p53 signaling than ATM phosphorylation of Mdm2 Ser394. Furthermore, the effects of these phosphorylation events on p53 regulation are not additive, as Mdm2Y393F/S394A mice and Mdm2S394A mice display similar phenotypes.

ATM phosphorylation of Mdm2-S394 is required for robust p53 stabilization and activation in DNA-damaged cells. We have now utilized Mdm2(S394A) knockin mice to determine that phosphorylation of Mdm2-S394 regulates p53 activity and the DNA damage response in lymphatic tissues in vivo by modulating Mdm2 stability. Mdm2-S394 phosphorylation delays lymphomagenesis in Emu-myc transgenic mice, and preventing Mdm2-S394 phosphorylation obviates the need for p53 mutation in Myc-driven tumorigenesis. However, irradiated Mdm2(S394A) mice also have increased hematopoietic stem and progenitor cell functions, and we observed decreased lymphomagenesis in sub-lethally irradiated Mdm2(S394A) mice. These findings document contrasting effects of ATM-Mdm2 signaling on p53 tumor suppression and reveal that destabilizing Mdm2 by promoting its phosphorylation by ATM would be effective in treating oncogene-induced malignancies, while inhibiting Mdm2-S394 phosphorylation during radiation exposure or chemotherapy would ameliorate bone marrow failure and prevent the development of secondary hematological malignancies.

The p53 tumor suppressor protein is upregulated in response to DNA damage and other stress signals. The upregulation of p53 involves freeing it from negative regulation imposed by Mdm2 and MdmX (Mdm4). Accumulating evidence indicates that phosphorylation of Mdm proteins by different stress-activated kinases such as ATM or c-Abl significantly impacts p53 functions. We have previously shown that ATM phosphorylation of Mdm2 Ser394 is required for robust p53 stabilization and activation following DNA damage. This dissertation describes in vivo examination of the mechanism by which Mdm2 Ser394 phosphorylation impacts p53 activities and its contribution to suppression of oncogene and DNA damage-induced tumors. We determine that phosphorylation of Mdm2 Ser394 regulates p53 activity by modulating Mdm2 stability and paradoxically delays Myc-driven lymphomagenesis while increasing lymphomagenesis in sub-lethally irradiated mice. c-Abl phosphorylates the residue neighboring Mdm2 Ser394, Mdm2 Tyr393. This dissertation describes the generation of a novel Mdm2Y393F mutant mouse to determine if c-Abl phosphorylation of Mdm2 regulates p53-mediated DNA damage responses or tumor suppression in vivo. Mdm2Y393F mice develop accelerated spontaneous and oncogene-induced tumors, yet display no defects in p53 stabilization and activity following acute genotoxic stress. Furthermore, the effects of these phosphorylation events on p53 regulation are not additive, as Mdm2Y393F/S394A mice and Mdm2S394A mice display similar phenotypes. The studies presented herein further our understanding of the mechanisms by which DNA damage-associated kinases stabilize and activate p53, and influence p53-dependent responses and tumor suppression. A better understanding of the in vivo effects of Mdm2 phosphorylation may facilitate the development of novel therapeutics capable of stimulating p53 anti-tumor activity or alleviating p53- dependent toxicities in non-malignant tissues.