Novel MYC-driven medulloblastoma models from multiple embryonic cerebellar cells.

Group3 medulloblastoma (MBG3) that predominantly occur in young children are usually associated with MYC amplification and/or overexpression, frequent metastasis and a dismal prognosis. Physiologically relevant MBG3 models are currently lacking, making inferences related to their cellular origin thus far limited. Using in utero electroporation, we here report that MBG3 mouse models can be developed in situ from different multipotent embryonic cerebellar progenitor cells via conditional expression of Myc and loss of Trp53 function in several Cre driver mouse lines. The Blbp-Cre driver that targets embryonic neural progenitors induced tumors exhibiting a large-cell/anaplastic histopathology adjacent to the fourth ventricle, recapitulating human MBG3. Enforced co-expression of luciferase together with Myc and a dominant-negative form of Trp53 revealed that GABAergic neuronal progenitors as well as cerebellar granule cells give rise to MBG3 with their distinct growth kinetics. Cross-species gene expression analysis revealed that these novel MBG3 models shared molecular characteristics with human MBG3, irrespective of their cellular origin. We here developed MBG3 mouse models in their physiological environment and we show that oncogenic insults drive this MB subgroup in different cerebellar lineages rather than in a specific cell of origin.

The WIP1 oncogene promotes progression and invasion of aggressive medulloblastoma variants.

Recent studies suggest that medulloblastoma, the most common malignant brain tumor of childhood, is comprised of four disease variants. The WIP1 oncogene is overexpressed in Group 3 and 4 tumors, which contain medulloblastomas with the most aggressive clinical behavior. Our data demonstrate increased WIP1 expression in metastatic medulloblastomas, and inferior progression-free and overall survival of patients with WIP1 high-expressing medulloblastoma. Microarray analysis identified upregulation of genes involved in tumor metastasis, including the G protein-coupled receptor CXCR4, in medulloblastoma cells with high WIP1 expression. Stimulation with the CXCR4 ligand SDF1α activated PI-3 kinase signaling, and promoted growth and invasion of WIP1 high-expressing medulloblastoma cells in a p53-dependent manner. When xenografted into the cerebellum of immunodeficient mice, medulloblastoma cells with stable or endogenous high WIP1 expression exhibited strong expression of CXCR4 and activated AKT in primary and invasive tumor cells. WIP1 or CXCR4 knockdown inhibited medulloblastoma growth and invasion. WIP1 knockdown also improved the survival of mice xenografted with WIP1 high-expressing medulloblastoma cells. WIP1 knockdown inhibited cell surface localization of CXCR4 by suppressing expression of the G protein receptor kinase 5, GRK5. Restoration of wild-type GRK5 promoted Ser339 phosphorylation of CXCR4 and inhibited the growth of WIP1-stable medulloblastoma cells. Conversely, GRK5 knockdown inhibited Ser339 phosphorylation of CXCR4, increased cell surface localization of CXCR4 and promoted the growth of medulloblastoma cells with low WIP1 expression. These results demonstrate crosstalk among WIP1, CXCR4 and GRK5, which may be important for the aggressive phenotype of a subclass of medulloblastomas in children.

Epigenomic alterations define lethal CIMP-positive ependymomas of infancy.

Ependymomas are common childhood brain tumours that occur throughout the nervous system, but are most common in the paediatric hindbrain. Current standard therapy comprises surgery and radiation, but not cytotoxic chemotherapy as it does not further increase survival. Whole-genome and whole-exome sequencing of 47 hindbrain ependymomas reveals an extremely low mutation rate, and zero significant recurrent somatic single nucleotide variants. Although devoid of recurrent single nucleotide variants and focal copy number aberrations, poor-prognosis hindbrain ependymomas exhibit a CpG island methylator phenotype. Transcriptional silencing driven by CpG methylation converges exclusively on targets of the Polycomb repressive complex 2 which represses expression of differentiation genes through trimethylation of H3K27. CpG island methylator phenotype-positive hindbrain ependymomas are responsive to clinical drugs that target either DNA or H3K27 methylation both in vitro and in vivo. We conclude that epigenetic modifiers are the first rational therapeutic candidates for this deadly malignancy, which is epigenetically deregulated but genetically bland.

Transitioning from genotypes to epigenotypes: why the time has come for medulloblastoma epigenomics.

Recent advances in genomic technologies have allowed for tremendous progress in our understanding of the biology underlying medulloblastoma, a malignant childhood brain tumor. Consensus molecular subgroups have been put forth by the pediatric neuro-oncology community and next-generation genomic studies have led to an improved description of driver genes and pathways somatically altered in these subgroups. In contrast to the impressive pace at which advances have been made at the level of the medulloblastoma genome, comparable studies of the epigenome have lagged behind. Complementary data yielded from genomic sequencing and copy number profiling have verified frequent targeting of chromatin modifiers in medulloblastoma, highly suggestive of prominent epigenetic deregulation in the disease. Past studies of DNA methylation-dependent gene silencing and microRNA expression analyses further support the concept of medulloblastoma as an epigenetic disease. In this Review, we aim to summarize the key findings of past reports pertaining to medulloblastoma epigenetics as well as recent and ongoing genomic efforts linking somatic alterations of the genome with inferred deregulation of the epigenome. In addition, we predict what is on the horizon for medulloblastoma epigenetics and how aberrant changes in the medulloblastoma epigenome might serve as an attractive target for future therapies.

The SFRP family of WNT inhibitors function as novel tumor suppressor genes epigenetically silenced in medulloblastoma.

Medulloblastoma (MB) is the most common malignant pediatric brain tumor. Dysregulation of WNT signaling occurs in up to 20% of cases. Using a genome-wide approach, we identified the secreted frizzled-related protein 1, 2 and 3 (SFRP1, SFRP2 and SFRP3) family of WNT inhibitors as putative tumor suppressor genes silenced by promoter region methylation in MB. SFRP1, SFRP2 and SFRP3 expression increased after 5-aza-2'-deoxycytidine treatment. SFRP1, SFRP2 and SFRP3 methylation was identified in 23.5, 3.9 and 15.7% of primary MB specimens, respectively, by methylation-specific PCR. Stable SFRP1, SFRP2 and SFRP3 expression reduced phospho-DVL2 levels and hindered MB cell proliferation and colony formation in soft agar in vitro. In 60% of primary tumors, SFRP1 was expressed at levels twofold lower than that in normal cerebellum. SFRP1 expression impaired tumor formation in vivo in flank and orthotopic intracerebellar xenograft models and conferred a significant survival advantage (P<0.0001). We identify for the first time tumor suppressor gene function of SFRP genes in MB, and suggest that loss of WNT pathway inhibition due to SFRP gene silencing is an additional mechanism that may contribute to excessive WNT signaling in this disease.