Functional Cooperation Between Ptch1 And Pten Tumor Suppressor Genes During The Tumorigenesis Of Medulloblastoma

Medulloblastoma is atumor of the cerebellum and the most common malignant pediatric brain tumor. Medulloblastoma occurs most frequently in children between the ages of 5 and 10 but may occur in adults as well. Although 5-year in overall survival rates have reached 60–80%, the prognosis for many medulloblastoma patients remains Bleak.Chromosome 10 q deletion was identified frequently in human medulloblastoma, and was shown to be highly significant with survival probability.Chromosome 10 q loss was primarily limited to SHH and group C tumors. Adult Shh-medulloblastomas harboring 10 q loss exhibit particularly poor progression-free survival and overall survival probabilities. Tumor suppressor gene of phosphatase and tensin homolog, Pten are located on the chromosomal region 10q23.31. We established a mouse model to present an important role for cross talk between Pten loss and sonic hedgehog signaling pathways in the pathogenesis of medulloblastoma.We inactivated Pten during brain development in neuronal and glial cell populations in a precisely defined area of the cerebellum. This was achieved by using Nes-cre transgenic mice, in which cre was active in external granular layer, the Bergmann glia and white matter of cerebellum. Ptenfl/fl;Nes-Cre mice developed seizures and ataxia early in life and died prematurely similarly to other people’s research. Although PTEN is frequently inactivated in malignant human brain tumors, inactivation of Pten in the mouse brain does not lead to tumor development, which may be due to the fact that additional genetic alterations are necessary. We used the Ptch1+/- mouse model of medulloblastoma to study the effect of Pten loss on medulloblastoma tumorigenesis. We found that heterozygosity for Pten, in the context of heterozygosity for Ptch1, altered tumor histology from classic to desmoplastic histology and accelerated medulloblastoma tumorigenesis in Ptenfl/+;Nes-cre;Ptch1+/-mice. Although spontaneous medulloblastoma that harbor only monoallelic mutations of Pten possess at least one functional wildtype Pten allele, yet they further or completely lose Pten protein immunoreactivity. As a result, loss of Pten expression lead to activation of PI3 K signaling: increased expression of activated Akt, phosphorylated on serine 473, in Ptenfl/+;Nes-cre;Ptch1+/-medulloblastomas.Of interest is our finding that the expression of several genes, including Gli1 and Mycn in the Shh signaling pathway was down regulated in Ptenfl/+;Nes-cre;Ptch1+/-medulloblastoma. And the RNA or protein level of Pten is downregulated by Shh signaling.Here we examine the role of mi R-183~96~182 in the context of PI3 K signaling activated medulloblastoma. The miR-183~96~182 cluster is upregulated across Ptenfl/+;Nes-cre;Ptch1+/-medulloblastoma samples relative to Ptch1+/-medulloblastoma. miR-183~96~182 has been implicated in retinal development and stem-cell maintenance. Because expression of Nes-cre and recombination occur in scattered EGL cells before neoplastic transformation, we conclude that in Ptenfl/+;Nes-cre;Ptch1+/-mouse model medulloblastomas arise from cerebellar granule cell precursors retained on the cerebellar surface. GCP cells taken from the P6~7 cerebellum have a greater proliferative capacity than those harvested later when neuronal progenitors have already begun to exit the cell division cycle and migrate into the IGL. When explanted into culture in the absence of Shh, GCP cells rapidly exit the cell cycle and differentiate, but Shh addition extends their proliferative potential.GCP cells harvested from C57BL/6 mice and engineered to ectopically express miR-183~96~182 by retroviral transfer demonstrated a greater proliferative advantage than their Retro-GFP counterparts, in the presence of Shh. However, its role in promoting GCP cells proliferation is dependent on hedgehog signaling activation. Our data conclude that miR-183~96~182 alone was unable to maintain cell proliferation in the absence of Shh. miR-183~96~182 could amplify the effects of Shh signaling, perhaps by inhibiting a transcriptional repressor of a subset of Shh target genes.In conclusion, our study provides both in vitro and in vivo evidence for an important role of miR-183~96~182 cluster in Pten loss associated medulloblastomas. Antigomirs to the miR-183~96~182 cluster might provide a potential new therapeutic strategy for patients with medulloblastomas harboring a constitutively activated PI3 K signaling pathway.