PTPS-03EPIGENETIC SILENCING OF β-ARRESTIN1 AND ITS INTRAGENIC miR-326 CONTROLS MEDULLOBLASTOMA GROWTH

Abstract

Medulloblastoma (MB) is the most common malignant brain tumor. It is classified into four subgroups characterized by distinct mutations, deregulation of specific signaling pathways and different clinical outcomes. A series of pathways and epigenetic mechanisms are deregulated across multiple subtypes, thus representing a window of opportunity for the identification of common targets. A common feature is the presence of stem-like cells (SLCs), a fraction of the tumor bulk which retains the ability to sustain tumor growth and may represent the progenitor giving rise to MB. Recent studies have highlighted the crucial role of microRNAs tumor signaling pathway deregulation. We have previously identified microRNAs deregulated MB and showed that miR-326 is strongly downregulated and represses Hedgehog/Gli signaling. Human and murine MB SLCs were derived and cultured as oncospheres. We investigated the expression levels of miR-326 and its host gene β-arrestin1 in MB specimens and in SLC, the role of the two molecules in MB and the regulation of miR-326/ β-arrestin1 transcriptional unit in SLC. A pharmacological approach was utilized to modulate the expression of miR-326/ β-arrestin1 in MB both in vitro and in vivo. We found that miR-326 cooperates with its host gene β-arrestin1 as tumor suppressor, and is lost in both MB and SLC. Such unit suppresses Hedgehog signaling at multiple levels: β-arrestin1 modulates Gli1-K518 acetylation while miR-326 controls Gli2 and Smo, activatory molecules of the pathway. The analysis of the potential mechanisms involved in the downregulation of this transcriptional unit showed that β-arrestin1/miR-326 are silenced through epigenetic mechanisms at histone levels. Indeed, epigenetic drugs are able to reactivate the miR-326/β- arrestin1 expression and suppress MB and SLC growth in vitro and in vivo. Our study reveals a new microRNA/host gene network in MB and proposes miR-326/β-arrestin1 as tumor suppressors for medulloblastoma patients, susceptible to be re-expressed by epigenetic treatments.