Epigenetic regulators Rbbp4 and Hdac1 are overexpressed in a zebrafish model of RB1 embryonal brain tumor, and are required for neural progenitor survival and proliferation.

Laura E Schultz,Heather R Shive,Wesley A Wierson,Jordan A Helmer,Elizabeth J Sandquist,Maura Mcgrail,Trevor J Weiss,Jeffrey A Haltom,Staci L Solin,Maira P Almeida

doi:10.1242/dmm.034124

Abstract

ABSTRACTIn this study, we used comparative genomics and developmental genetics to identify epigenetic regulators driving oncogenesis in a zebrafish retinoblastoma 1 (rb1) somatic-targeting model of RB1 mutant embryonal brain tumors. Zebrafish rb1 brain tumors caused by TALEN or CRISPR targeting are histologically similar to human central nervous system primitive neuroectodermal tumors (CNS-PNETs). Like the human oligoneural OLIG2+/SOX10+ CNS-PNET subtype, zebrafish rb1 tumors show elevated expression of neural progenitor transcription factors olig2, sox10, sox8b and the receptor tyrosine kinase erbb3a oncogene. Comparison of rb1 tumor and rb1/rb1 germline mutant larval transcriptomes shows that the altered oligoneural precursor signature is specific to tumor tissue. More than 170 chromatin regulators were differentially expressed in rb1 tumors, including overexpression of chromatin remodeler components histone deacetylase 1 (hdac1) and retinoblastoma binding protein 4 (rbbp4). Germline mutant analysis confirms that zebrafish rb1, rbbp4 and hdac1 are required during brain development. rb1 is necessary for neural precursor cell cycle exit and terminal differentiation, rbbp4 is required for survival of postmitotic precursors, and hdac1 maintains proliferation of the neural stem cell/progenitor pool. We present an in vivo assay using somatic CRISPR targeting plus live imaging of histone-H2A.F/Z-GFP fusion protein in developing larval brain to rapidly test the role of chromatin remodelers in neural stem and progenitor cells. Our somatic assay recapitulates germline mutant phenotypes and reveals a dynamic view of their roles in neural cell populations. Our study provides new insight into the epigenetic processes that might drive pathogenesis in RB1 brain tumors, and identifies Rbbp4 and its associated chromatin remodeling complexes as potential target pathways to induce apoptosis in RB1 mutant brain cancer cells.This article has an associated First Person interview with the first author of the paper.

Highlights

The retinoblastoma tumor suppressor Retinoblastoma 1 (RB1) plays distinct roles in regulating proliferation and differentiation in stem, progenitor and lineage-restricted cell populations (Fong and Slack, 2017; Julian and Blais, 2015; Sage, 2012)
Germline mutant analysis confirms that zebrafish rb1, rbbp4 and hdac1 are required during brain development. rb1 is necessary for neural precursor cell cycle exit and terminal differentiation, rbbp4 is required for survival of postmitotic precursors, and hdac1 maintains proliferation of the neural stem cell/progenitor pool
The absence of apoptosis combined with an overall reduced size suggests that hdac1 is required for persistent proliferation of stem cells to generate the neural progenitor pool and new neurons. These results show a distinct requirement for rbbp4 in neural progenitor or precursor survival, whereas hdac1 is necessary for maintaining proliferation of the neural stem and/or progenitor pool

Summary

Introduction

The retinoblastoma tumor suppressor Retinoblastoma 1 (RB1) plays distinct roles in regulating proliferation and differentiation in stem, progenitor and lineage-restricted cell populations (Fong and Slack, 2017; Julian and Blais, 2015; Sage, 2012). The canonical tumor suppressor role of RB1 is to regulate proliferation by transcriptional repression of E2F targets driving cell cycle entry (Dyson, 2016). RB1 transcriptional control is mediated, in part, through association with chromatin remodelers that modify post-translational marks on histones to activate or repress gene expression (Beshiri et al, 2012; Brehm et al, 1998; Lin et al, 2011). Like RB1, many chromatin remodelers are mutated in human cancers (Vogelstein et al, 2013), implicating epigenetic control of gene expression as a significant contributor to oncogenesis (Conway et al, 2015; Suva et al, 2013). Understanding how RB1 loss affects epigenetic control in neural stem and progenitor cells during development and in brain cancer is important for identifying new pathways that contribute to carcinogenesis

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