The role of Mdm2 in cerebellum development and medulloblastoma tumorigenesis

Reem Malek, Purdue University

Abstract

During postnatal 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, or toward arrest, apoptosis or premature differentiation leading to cerebellar malformations. Here, we report that Mdm2, the principal inhibitor of the p53 tumor suppressor protein, is required for the development of the cerebellum as well as for the initiation of medulloblastoma tumors. We employed a novel hypomorphic allele Mdm2puro that expresses reduced levels of Mdm2. Mdm2puro/Δ7-9 mice expressing ~30% of the wild-type Mdm2 show reduced cerebellar size, foliation and reduction in GNPs. This phenotype can be attributed in part to high levels of p53-dependent apoptosis. Additionally, through fate mapping studies we have shown that loss of Mdm2 also promotes the premature migration of GNPs. Global transcriptome analyses of the Mdm2 puro/Δ7-9 cerebellum supports dysregulation of the transcriptional programs that regulate GNP differentiation. A decrease in Shh target gene expression and decreased levels Gli1 and Gli2 proteins, the transducers of Shh signaling, suggest a role for p53 in attenuation of Shh signaling. These studies reveal that Mdm2 is required to limit p53 activation during GNP development and precocious p53 activation can lead to massive apoptosis and acceleration migration. Using Shh-responsive primary GNP cultures from wild-type mice we demonstrate that Shh stimulation of GNPs results in an increase in the steady state level of Mdm2 and an increase in Mdm2 phosphorylation at Ser 166, 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 parallel mechanisms governing GNP proliferation and medulloblastoma tumor formation, we also observed increased levels of p-Mdm2SER166 in Shh-induced MB tumors and preneoplastic lesions in a mouse model of MB (Ptch1+/-). Notably, 70% decrease in Mdm abrogates the formation of PNLs while 50% reduction in Mdm2 (Mdm2+/Δ7-9) decreases PNLs two-fold. Sustained Shh signaling in Ptch1+/- mice promotes proliferation and survival of GNPs in the EGL from which PNLs arise. However, loss of Mdm2 decreases proliferation and increases apoptosis in GNPs, restoring them back to wild-type levels. We further demonstrate restoration of p53 levels and it's activated form p53SER15 in EGL of Ptch1+/- Mdm2+/Δ7-9 as compared to Ptch1+/- mice. Overall, our results suggest that Mdm2 plays in important role in cerebellum development specifically in survival and migration of GNPs. This mechanism maybe hijacked by deregulated Shh signaling in certain medulloblastoma tumors resulting in Mdm2 dependent p53 inactivation.

Degree

Ph.D.

Advisors

Mendrysa, Purdue University.

Subject Area

Cellular biology|Developmental biology|Oncology

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