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PTEN heterozygosity promotes tumorigenesis in a Smoothened active mouse model of medulloblastoma

ABSTRACT: Recent publications have described an important role for cross talk between PI-3 kinase and sonic hedgehog signaling pathways in the pathogenesis of medulloblastoma. Our primary objective was to determine if loss of Pten promotes medulloblastoma tumor formation. When mice expressing SmoA1 were crossed with mice heterozygous for Pten, SmoA1 +; Pten +/- mice exhibited decreased survival, an increased incidence, and a shorter time to onset of symptoms, compared to SmoA1 +; Pten +/+ mice. Tumors from SmoA1 +; Pten +/+ mice exhibited classic histology. In contrast, tumors from SmoA1 +; Pten +/- mice exhibited predominantly nodular/desmoplastic histology and evidence of neuronal differentiation. Analysis by gene expression microarray revealed a clear separation of gene signatures, with upregulation of genes such as Pik3cb, Mapk8, Frap1, and multiple genes involved in angiogenesis in SmoA1+; Pten +/- tumors. Downregulated genes included Pten, Trp53, and Rb1. Western blotting and immunohistochemical analysis revealed decreased expression of Pten and increased activation of Akt in the SmoA1+; Pten +/- tumors. Consistent with these findings, we detected strong expression of PCNA throughout the Pten +/+ tumors and focal areas of intense staining for PCNA in tumors from SmoA1 +/-; Pten +/- mice. Conversely, staining for Cleaved Caspase 3 revealed multiple punctuate foci in SmoA1 +; Pten +/+ tumors and almost no staining in SmoA1 +/-; Pten +/- tumors. These results suggest that increased signaling through PI-3 kinase pathways promotes MB tumorigenesis by driving proliferation and inhibiting apoptosis of granule neuron precursor cells in the developing cerebellum.

ORGANISM(S): Mus musculus

PROVIDER: GSE19059 | GEO | 2009/11/21

SECONDARY ACCESSION(S): PRJNA120563

REPOSITORIES: GEO

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Project description:Deregulated developmental processes in the cerebellum cause medulloblastoma, the most common malignant tumor of the central nervous system. About 20-30% of cases are caused by mutations increasing the activity of the Sonic hedgehog (Shh) pathway, a critical mitogen in cerebellar development. The proto-oncogene Smoothened is a key transducer of the Shh pathway. Activating mutations in Smoothened that lead to constitutive activity of the Shh pathway have been identified in human medulloblastoma. To understand the molecular and cellular effects of Smoothened variants in normal development and medulloblastoma genesis, we generated the SmoA2 transgenic mouse model which expresses the transgene exclusively in granule neuron precursors. In this study, we demonstrate how two point mutations in a single molecule can produce starkly different phenotypes as seen in comparison to our previous model, ND2:SmoA1. The SmoA2 mice have severe aberrations in cerebellar development whereas the SmoA1 mice are largely normal during development. Medulloblastomas in the SmoA2 mice develop in the dysplastic cerebellar milieu. Intriguingly, despite disruptions in the stereotypic organization of the cerebellum, the SmoA2 mice do not exhibit any overt abnormalities in motor coordination. The differences in the global transcriptional profiles downstream of SmoA1 and SmoA2 further demonstrate the distinctiveness of the two oncogenic Smoothened mutations. The SmoA2 model serves as a unique spatiotemporal tool to investigate the functional significance of the reiterative cerebellar circuitry as well as to further understand Shh pathway mechanics in cerebellar development and oncogenesis. We previously generated a SmoA1 transgenic mouse medulloblastoma model, which expresses the SmoA1 transgene driven by the GNP-specific fragment of the promoter of ND2 transcription factor leading to constitutively active Shh signaling exclusively in the cerebellum. In this study, we characterize the ND2:SmoA2 transgenic mouse model with a similarly designed transgene expressing the SmoA2 mutation. To assess transcriptional changes downstream of SmoA1 and SmoA2, we evaluated global gene expression profiles of P5 SmoA1, SmoA2 and Wt age-matched cerebella. We chose this specific developmental stage because (1) the phenotypes of SmoA1 and SmoA2 are robust and distinct at P5; (2) at P5 GNPs undergo proliferation, migration and differentiation and therefore expression profiling could capture key differences in multiple processes.

Project description:Previous studies have evaluated the role of miRNAs in the initiation and progression of cancer. MiR-34a was found to be downregulated in several tumors, including medulloblastoma. We here analysed the function of miR-34a in vivo by targeted transgenesis to generate mice with constitutive deletion of the miR-34a gene, which resulted in the absence of mir-34a in all analysed tissues. Nevertheless, these mice were viable and fertile. A comprehensive standardized phenotypic analysis including more than 300 single parameters performed by the German Mouse Clinic revealed no apparent phenotype. Analysis of miR-34a expression in human medulloblastomas and medulloblastoma cell lines revealed significant downregulation as compared to human cerebellum. Re-expression of mir-34a in human medulloblastoma cells in vitro reduced cell viability, cell proliferation and induced apoptosis. Among the targets downregulated by miR-34a in human medulloblastoma cells were NMYC and SIRT1. Activation of the Shh pathway by targeted overexpression of SmoA1 causes medulloblastoma in mice, which is dependent on the presence and upregulation of NMYC. Analysis of miR-34a in ND2:SmoA1-derived medulloblastomas revealed significant suppression of miR-34a compared to normal cerebellum. Crossbreeding these mice with miR-34a knockout mice significantly accelerated medulloblastoma growth in mice deficient for miR-34a. Interestingly, NMYC and SIRT1 were highly expressed in medulloblastomas derived from these mice. We here demonstrate that miR-34a is dispensable for normal development, but that its loss accelerates medulloblastoma. Strategies aiming to re-express miR-34a in tumors could therefore represent an efficient therapy option. For genome-wide expression analysis total RNA from brain and thymus of three or four male miR34a and four control mice was isolated using RNeasy Midi kit (Qiagen, Hilden, Germany). The cDNA microarrays were generated, hybridized and analysed as described (Horsch et al 2009). Two chip hybridizations were performed with total RNA for each individual mutant mouse against a reference RNA pool of the same organ.

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PTEN heterozygosity promotes tumorigenesis in a Smoothened active mouse model of medulloblastoma

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