Glioblastoma (GBM) is a highly lethal brain tumor presenting as one of two subtypes with distinct clinical histories and molecular profiles. The primary GBM subtype presents acutely as high-grade disease that typically harbors EGFR, Pten and Ink4a/Arf mutations, and the secondary GBM subtype evolves from the slow progression of low-grade disease that classically possesses PDGF and p53 events1–3. Here, we show that concomitant CNS-specific deletion of p53 and Pten in the mouse CNS generates a penetrant acute-onset high-grade malignant glioma phenotype with striking clinical, pathological and molecular resemblance to primary GBM in humans. This genetic observation prompted p53 and Pten mutational analysis in human primary GBM, demonstrating unexpectedly frequent inactivating mutations of p53 as well the expected Pten mutations. Integrated transcriptomic profiling, in silico promoter analysis and functional studies of murine neural stem cells (NSCs) established that dual, but not singular, inactivation of p53 and Pten promotes an undifferentiated state with high renewal potential and drives elevated c-Myc levels and its associated signature. Functional studies validated increased c-Myc activity as a potent contributor to the impaired differentiation and enhanced renewal of p53-Pten null NSCs as well as tumor neurospheres (TNSs) derived from this model. c-Myc also serves to maintain robust tumorigenic potential of p53-Pten null TNSs. These murine modeling studies, together with confirmatory transcriptomic/promoter studies in human primary GBM, validate a pathogenetic role of a common tumor suppressor mutation profile in human primary GBM and establish c-Myc as a key target for cooperative actions of p53 and Pten in the regulation of normal and malignant stem/progenitor cell differentiation, self-renewal and tumorigenic potential.

Stimulation of the Ras-mitogen-activated protein kinase (MAPK) signal transduction pathway results in a multitude of events including expression of the immediate-early genes, c-fos and c-myc. Downstream targets of this stimulated pathway are the mitogen- and stress-activated protein kinases (MSK) 1 and 2, which are histone H3 kinases. In chromatin immunoprecipitation assays, it has been shown that the mitogen-induced phosphorylated H3 is associated with the immediate-early genes and that MSK1/2 activity and H3 phosphorylation have roles in chromatin remodeling and transcription of these genes. In oncogene-transformed fibroblasts in which the Ras-MAPK pathway is constitutively active, histone H1 and H3 phosphorylation is increased and the chromatin of these cells has a more relaxed structure than the parental cells. In this review we explore the deregulation of the Ras-MAPK pathway in cancer, with an emphasis on breast cancer. We discuss the features of MSK1 and 2 and the impact of a constitutively activated Ras-MAPK pathway on chromatin remodeling and gene expression.

Glioblastoma (GBM) is a highly lethal primary brain cancer with hallmark features of diffuse invasion, intense apoptosis resistance and florid necrosis, robust angiogenesis, and an immature profile with developmental plasticity. In the course of assessing the developmental consequences of central nervous system (CNS)-specific deletion of p53 and Pten, we observed a penetrant acute-onset malignant glioma phenotype with striking clinical, pathological, and molecular resemblance to primary GBM in humans. This primary, as opposed to secondary, GBM presentation in the mouse prompted genetic analysis of human primary GBM samples that revealed combined p53 and Pten mutations as the most common tumor suppressor defects in primary GBM. On the mechanistic level, the "multiforme" histopathological presentation and immature differentiation marker profile of the murine tumors motivated transcriptomic promoter-binding element and functional studies of neural stem cells (NSCs), which revealed that dual, but not singular, inactivation of p53 and Pten promotes cellular c-Myc activation. This increased c-Myc activity is associated not only with impaired differentiation, enhanced self-renewal capacity of NSCs, and tumor-initiating cells (TICs), but also with maintenance of TIC tumorigenic potential. Together, these murine studies have provided a highly faithful model of primary GBM, revealed a common tumor suppressor mutational pattern in human disease, and established c-Myc as a key component of p53 and Pten cooperative actions in the regulation of normal and malignant stem/progenitor cell differentiation, self-renewal, and tumorigenic potential. ment of cancer because it would imply that complete eradication of the TIC compartment would not lead to durable cures, as is widely assumed, and that such eradication would need to be complemented with differentiationinducing agents. In this light, it stands to reason that an improved understanding of the pathways governing processes of differentiation along the NSC-glial axis and, by extension, the TIC-progeny axis, may prove instrumental in guiding more effective drug development efforts. Cold Spring Harbor Symposia onDuring the past 20 years, molecular and human genetic studies have revealed obligate pathways involved in GBM pathogenesis, and principal among these are the RBp16/p18-CDK4/6, p53-MDM2-ARF, and PI3K/PTEN/ AKT pathways (Zhu and Parada 2002;Furnari et al. 2007). A wealth of studies has established a central role for the phosphoinositol-3 kinase (PI3K) pathway in the pathogenesis of glioma on the genomic, genetic, and epigenetic levels (Furnari et al. 2007). Historically, p53 inactivation has been considered to be a more classical genetic lesion in low-grade astrocytomas that invariably evolve into secondary GBM, whereas primary GBMs are thought to more commonly sustain loss of the CDKN2A locus, which contains both cyclin-dependent kinase inhibitor 2A (Ink4a) and an alternate open reading frame (ARF), a negative regulator of the p53 pathway (Watanabe et al. 1996;Kleihues an...

Leveraging TCGA’s multi-dimensional data in glioblastoma (GBM), we inferred the putative regulatory network between microRNA and mRNA using the Context-Likelihood-Relatedness (1) modeling algorithm. Interrogation of the network in context of defined molecular subtypes identified 8 microRNAs with a strong discriminatory potential between proneural and mesenchymal subtypes. Integrative in silico analyses, functional genetic screen and experimental validation identified miR-34a as a tumor suppressor in proneural subtype GBM. Mechanistically, in addition to its direct regulation of PDGFRA, promoter enrichment analysis of CLR-inferred mRNA nodes established miR-34a as a novel regulator of a Smad4 transcriptional network. Clinically, miR-34a expression level is shown to be prognostic, where miR-34a low-expressing GBMs exhibited better overall survival. This work illustrates the potential of comprehensive multi-dimensional cancer genomic data combined with computational and experimental models in enabling mechanistic exploration of relationships among different genetic elements across the genome space in cancer.