Downregulation of miR-326 and its host gene β-arrestin1 induces pro-survival activity of E2F1 and promotes medulloblastoma growth.

Evelina Miele,Lucia Di Marcotullio,Franco Locatelli,Marcel Kool,Natalia Pediconi,Zein Mersini Besharat,Andrea Carai,Elisabetta Ferretti,Enrico De Smaele,Felice Giangaspero,Alessandra Vacca,Claudia Sabato,Massimo Levrero,Gianluca Canettieri,Stefan M Pfister,Giuseppina Catanzaro,Antonello Mai,Angela Mastronuzzi,Luana Abballe,Agnese Pò

doi:10.1002/1878-0261.12800

Abstract

Persistent mortality rates of medulloblastoma (MB) and severe side effects of the current therapies require the definition of the molecular mechanisms that contribute to tumor progression. Using cultured MB cancer stem cells and xenograft tumors generated in mice, we show that low expression of miR-326 and its host gene β-arrestin1 (ARRB1) promotes tumor growth enhancing the E2F1 pro-survival function. Our models revealed that miR-326 and ARRB1 are controlled by a bivalent domain, since the H3K27me3 repressive mark is found at their regulatory region together with the activation-associated H3K4me3 mark. High levels of EZH2, a feature of MB, are responsible for the presence of H3K27me3. Ectopic expression of miR-326 and ARRB1 provides hints into how their low levels regulate E2F1 activity. MiR-326 targets E2F1 mRNA, thereby reducing its protein levels; ARRB1, triggering E2F1 acetylation, reverses its function into pro-apoptotic activity. Similar to miR-326 and ARRB1 overexpression, we also show that EZH2 inhibition restores miR-326/ARRB1 expression, limiting E2F1 pro-proliferative activity. Our results reveal a new regulatory molecular axis critical for MB progression.

Highlights

Brain tumors are an important cause of cancer-related morbidity and mortality in children, and medulloblastoma (MB) is the most common pediatric malignant brain tumor
Transfer of cancer stem cells (CSCs) to differentiation medium (DFM) was accompanied by significantly upregulated expression of miR-326 and ARRB1 at both the transcriptional and protein levels (Fig. 1D). These findings indicate that miR-326 and ARRB1 show the same expression pattern and that their markedly downregulated expression is a feature of MB cells, including their CSC subset
We identified a previously undescribed mechanism that promotes MB growth and maintains the CSC subpopulation in an undifferentiated state characterized by proliferation, enhanced self-renewal and resistance to apoptosis

Summary

Introduction

Brain tumors are an important cause of cancer-related morbidity and mortality in children, and medulloblastoma (MB) is the most common pediatric malignant brain tumor. Therapeutic approaches consist mainly of maximally safe surgical resection, high-dose cytotoxic chemotherapy and, for patients over the age of three, craniospinal irradiation. These methods have substantially improved survival, they are frequently associated with severe long-term adverse effects, and approximately, onethird of patients still die from the disease [6,7]. We previously identified a subset of microRNAs with remarkably low expression levels in MBs [12,13] These microRNAs were expressed at low levels in cerebellar granule cell progenitors (GCPs), the proliferating and undifferentiated cells of the developing cerebellum, described as cell of origin of certain MBs [14]. MiR-326 was shown to have an onco-suppressive role [16,17,18,19,20] and low levels of miR-326 have been reported in brain tumors of glial origin [21,22,23]

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Discussion

Conclusion