BMI1-BMP Connection in Medulloblastoma Pathogenesis

PhDMedulloblastoma (MB) is the commonest intracranial childhood malignancy and despite recent advances, current therapeutic approaches are still associated with high morbidity and mortality. A novel molecular classification has recently been proposed for these tumours – WNT Group (best prognosis), SHH Group (intermediate prognosis), Group 3 (worst prognosis) and Group 4 (intermediate prognosis). BMI1, a transcriptional repressor of the Polycomb group genes, is overexpressed in MB, most significantly in those of Group 4 MBs. Bone Morphogenetic Proteins (BMPs) are morphogens belonging to TGF-β superfamily of growth factors, and are known to inhibit MB cell proliferation and induce apoptosis in vitro, and to inhibit tumour growth in vivo. Our team have recently demonstrated that Bmi1 regulates cell adhesion properties during cerebellar development through repression of the BMP pathway. The aim of this project is to assess whether BMI1 overexpression may contribute to MB pathogenesis through repression of the BMP pathway. Here we demonstrate that BMI1 knock down derepresses BMP pathway, and using a novel xenograft model of human MB of Group 4, we show that BMI1 controls tumour volume and intraparenchymal invasion. In in vitro assays on MB cell lines we show that cell adhesion and motility is controlled by BMI1 in a BMP dependent manner and that deregulation of extracellular matrix proteins are key mediators of this effect. Furthermore, we demonstrate that BMP treatment to BMI1 overexpressing MB cells reduces cell proliferation and invasion, suggesting BMI1 as a possible biomarker for those tumours that could benefit from treatment with BMP agonist small molecules.National Institute of Health Research (NIHR), Academic Clinical Fellowship (ACF)Medical Research Council (MRC), Clinical Research Training Fellowship (CRTF Ali’s Dream and Charlie’s Challenge Charitie

Kruppel-Like Factor 4 Regulates Granule Cell Pax6 Expression and Cell Proliferation in Early Cerebellar Development.

Kruppel-like factor 4 (Klf4) is a transcription factor that regulates many important cellular processes in stem cell biology, cancer, and development. We used histological and molecular methods to study the expression of Klf4 in embryonic development of the normal and Klf4 knockout cerebellum. We find that Klf4 is expressed strongly in early granule cell progenitor development but tails-off considerably by the end of embryonic development. Klf4 is also co-expressed with Pax6 in these cells. In the Klf4-null mouse, which is perinatal lethal, Klf4 positively regulates Pax6 expression and regulates the proliferation of neuronal progenitors in the rhombic lip, external granular layer and the neuroepithelium. This paper is the first to describe a role for Klf4 in the cerebellum and provides insight into this gene's function in neuronal development

Kruppel-Like Factor 4 Regulates Granule Cell Pax6 Expression and Cell Proliferation in Early Cerebellar Development

<div><p>Kruppel-like factor 4 (Klf4) is a transcription factor that regulates many important cellular processes in stem cell biology, cancer, and development. We used histological and molecular methods to study the expression of Klf4 in embryonic development of the normal and Klf4 knockout cerebellum. We find that Klf4 is expressed strongly in early granule cell progenitor development but tails-off considerably by the end of embryonic development. Klf4 is also co-expressed with Pax6 in these cells. In the Klf4-null mouse, which is perinatal lethal, Klf4 positively regulates Pax6 expression and regulates the proliferation of neuronal progenitors in the rhombic lip, external granular layer and the neuroepithelium. This paper is the first to describe a role for Klf4 in the cerebellum and provides insight into this gene’s function in neuronal development.</p></div

Concerted microRNA control of Hedgehog signalling in cerebellar neuronal progenitor and tumour cells

MicroRNAs (miRNA) are crucial post-transcriptional regulators of gene expression and control cell differentiation and proliferation. However, little is known about their targeting of specific developmental pathways. Hedgehog (Hh) signalling controls cerebellar granule cell progenitor development and a subversion of this pathway leads to neoplastic transformation into medulloblastoma (MB). Using a miRNA high-throughput profile screening, we identify here a downregulated miRNA signature in human MBs with high Hh signalling. Specifically, we identify miR-125b and miR-326 as suppressors of the pathway activator Smoothened together with miR-324-5p, which also targets the downstream transcription factor Gli1. Downregulation of these miRNAs allows high levels of Hh-dependent gene expression leading to tumour cell proliferation. Interestingly, the downregulation of miR-324-5p is genetically determined by MB-associated deletion of chromosome 17p. We also report that whereas miRNA expression is downregulated in cerebellar neuronal progenitors, it increases alongside differentiation, thereby allowing cell maturation and growth inhibition. These findings identify a novel regulatory circuitry of the Hh signalling and suggest that misregulation of specific miRNAs, leading to its aberrant activation, sustain cancer development

Basal Suppression of the Sonic Hedgehog Pathway by the G-Protein-Coupled Receptor Gpr161 Restricts Medulloblastoma Pathogenesis

Summary: Sonic hedgehog (Shh) determines cerebellar granule cell (GC) progenitor proliferation and medulloblastoma pathogenesis. However, the pathways regulating GC progenitors during embryogenesis before Shh production by Purkinje neurons and their roles in tumorigenesis remain unclear. The cilium-localized G-protein-coupled receptor Gpr161 suppresses Shh-mediated signaling in the neural tube. Here, by deleting Gpr161 in mouse neural stem cells or GC progenitors, we establish Gpr161 as a tumor suppressor in Shh subtype medulloblastoma. Irrespective of Shh production in the cerebellum, Gpr161 deletion increased downstream activity of the Shh pathway by restricting Gli3-mediated repression, causing more extensive generation and proliferation of GC progenitors. Moreover, earlier deletion of Gpr161 during embryogenesis increased tumor incidence and severity. GC progenitor overproduction during embryogenesis from Gpr161 deletion was cilium dependent, unlike normal development. Low GPR161 expression correlated with poor survival of SHH subtype medulloblastoma patients. Gpr161 restricts GC progenitor production by preventing premature and Shh-dependent pathway activity, highlighting the importance of basal pathway suppression in tumorigenesis. : Shimada et al. identify the ciliary G-protein-coupled receptor Gpr161 as a tumor suppressor in Shh subtype medulloblastoma. The authors suggest that Gpr161 restricts premature Shh pathway activity during granule cell progenitor development, implying that cilium-mediated pathway suppression preceding Shh signaling prevents tumorigenesis. Keywords: cerebellum, granule cell, medulloblastoma, sonic hedgehog, primary cilia, G-protein-coupled receptor, Gpr161, tumor suppresso