The Role of MDM2 in Mouse Development and its Implication in the Pathogenesis of Cancer and Developmental Diseases

The tumor suppressor protein p53, encoded by Tp53 gene, is a transcription factor that regulates cell cycle arrest and apoptosis following cellular stresses that compromise DNA integrity and normal cellular function. Tp53 is mutated in approximately 50% of human cancers, thereby allowing cancer cells to replicate uncontrollably. In cancers in which Tp53 is not mutated, p53 is frequently functionally inactivated through other mechanisms. For example, Mdm2, a proximal negative regulator p53 is often overexpressed in cancers in which p53 is wild-type. Mdm2 is E3 ubiquitin ligase that binds to and targets p53 for proteasomal degradation and as well as inhibits p53 transcriptional activity. Pharmacological disruption of the Mdm2-p53 interaction in cancer cells with wild-type p53 is currently being explored as a strategy to enhance p53-mediated cell death in response to conventional chemotherapeutics. Nutlin-3, an Mdm2 inhibitor, promotes cell death in cultured cells from human medulloblastoma (MB), a common cerebellar pediatric cancer, suggesting that Mdm2 is a promising target to treat this tumor type. Consistent with this idea, studies in a mouse model of MB have shown that loss of Mdm2 limits the development of preneoplastic lesion in the cerebellum. The developing nature of the cerebellum in the youngest of MB patients is a major contributing factor to the side-effects resulting from current MB therapies. Studies in adult rodents suggest that nutlin-3 is non-genotoxic in normal homeostatic tissues; however the effects of nutlin-3 have not been evaluated in developing tissues. To gain insight into the potential side effects of p53 activation on the developing cerebellum, the pharmacological effects of Mdm2 inhibition in Granule Neuron Precursor cells (GNPs) was mimicked genetically using a mouse model in which Mdm2 could be selectively deleted in postnatal GNPs. My studies revealed that deletion of Mdm2 in GNPs led to a reduction in cerebellum size but did not negatively impact gross motor coordination. These results suggest that Mdm2 inhibitors may promote the killing of MB tumor cells of pediatric patients without minimal side effects on normal cerebellum development

In addition to cancer, p53 has an important role guarding proliferating cells during development. Activation of p53 has been implicated in the pathology of several human congenital syndromes, and mice lacking Mdm2 die in utero due to p53-mediated apoptosis. These studies highlight the need for p53 function to be tightly regulated as even modest decreases or increases in p53 function can promote cancer or disrupt normal development, respectively. During the course of my studies on Mdm2 inhibition in MB, it was serendipitously discovered that in the absence of a wild-type level of Mdm2, the phenotypic consequences of p53 activation on the developing mouse embryo were strongly influenced by the genetic background. On a 129S6/B6 F1 hybrid genetic background, mice expressing ~30% the wild-type level of Mdm2 were viable, while mice on an inbred C57BL/6 genetic background died at birth and exhibited an array of craniofacial abnormalities including coloboma, exencephaly, and cleft palate. This is the first demonstration of a role for Mdm2 in craniofacial development. The genotype-dependence, further, indicates the presence of additional genes affecting craniofacial dysmorphology. In human pleiotropic malformation syndromes, there is often clinical variability amongst individuals with an identical underlying mutation at the major effect locus. Currently, the modifier genes that influence craniofacial dysmorphology are unknown. The allelic variants encoded by the divergent genetic backgrounds that increase the penetrance and expressivity of craniofacial malformations in the Mdm2 hypomorphic mice identify the gene and protein networks governing craniofacial development. In the future, it will be important to determine the genes that are differentially expressed between mice that express low levels of Mdm2 in C57BL/6 and 129S6/B6 F1 genetic backgrounds. The results from this comparison are predicted to lead to the identification of candidate genes that influence craniofacial development through the modulation of p53 function.