Abstract 5213: MDM2 at the crossroads of cell fate decisions in cerebellar development and medulloblastoma

During post-natal cerebellar development, Granule Neuron Precursors (GNPs) undergo a proliferative expansion in response to Sonic hedgehog (Shh) signaling, prior to differentiation and migration to their final niche. Aberrations in this tightly controlled process can tip the balance toward persistent proliferation leading to Medulloblastoma (MB), or toward apoptosis or premature differentiation leading to neurodegeneratiion. Here, we report that MDM2, the principal inhibitor of the p53 tumor suppressor protein, is required for cerebellum development as well as for MB tumorigenesis. Our studies employed a novel hypomorphic allele of Mdm2 (Mdm2puro) in combination with the Mdm2del7-9 null allele. Mdm2puro/del7-9 mice express ∼30% of the wild-type level of MDM2. Mdm2puro/del7-9 cerebella exhibit decreased foliation, reduction in GNP numbers, and laminar disorganization. These phenotypes are attributed, in part, to high levels of p53-dependent apoptosis. In addition, we have recently determined through fate mapping studies that loss of MDM2 also promotes the premature migration of GNPs. Global transcriptome analyses of the Mdm2puro/del7-9 cerebellum supports dysregulation of the transcriptional programs that regulate GNP differentiation. Further, Western blot analyses reveal an increase in the pro-differentiation protein, Numb, and the neuronal maturation marker, βIII-Tubulin, in GNPs. A concomitant decrease in Gli1 and Gli2, two effectors of Shh signaling known to be degraded by Numb, suggests an increase in Numb activity. These experiments suggest that loss of MDM2 and subsequent increase in p53 activity accelerates GNPs migration and possibly favors GNP differentiation over proliferation. In complementary experiments, we employed Shh-responsive primary GNP cultures from wild-type mice. Shh stimulation of GNPs results in an increase in the steady state level of MDM2 and an increase in MDM2 phosphorylation at Ser166, a modification known to enhance the anti-p53 function of MDM2. These findings suggest that Shh signaling may upregulate MDM2 to prevent p53 activation during GNP proliferation. In keeping with the concept that GNP proliferation and MB tumor formation are governed by similar mechanisms, we also observed increased levels of p-MDM2Ser166 in Shh-induced preneoplastic lesions (PNLs) and MB tumors in a Ptch1+/- mice, a model of human MB. Notably, a 70% reduction in the level of MDM2 in these mice abrogates the formation of PNLs, supporting a key role for MDM2 in the initiation of some types of MB. Overall, our results suggest that MDM2 mediated inhibition of p53 plays in important role in survival and migration of GNPs cerebellum development. This role of MDM2 may be high-jacked by deregulated Shh signaling in certain MB tumors thereby limiting the tumor suppressive function of p53 and obviating a need for p53 gene mutations. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5213. doi:1538-7445.AM2012-5213