PRC1 Prevents Replication Stress during Chondrogenic Transit Amplification

Frank Spaapen,Yoshihiro Takihara,Hendrik Marks,Peggy Prickaerts,Vivian Dahlmans,Jan Voncken,Bradly Wouters,Juliette Salvaing,Frans Kruitz,Michiel Adriaens,Lars Eijssen,Donald Surtel,Tim Welting,Roel Kuijer,Hendrik Stunnenberg,Miguel Vidal

doi:10.3390/epigenomes1030022

Abstract

Transit amplification (TA), a state of combined, rapid proliferative expansion and differentiation of stem cell-descendants, remains poorly defined at the molecular level. The Polycomb Repressive Complex 1 (PRC1) protein BMI1 has been localized to TA compartments, yet its exact role in TA is unclear. PRC1 proteins control gene expression, cell proliferation and DNA-damage repair. Coordination of such DNA-templated activities during TA is predicted to be crucial to support DNA replication and differentiation-associated transcriptional programming. We here examined whether chondrogenesis provides a relevant biological context for synchronized coordination of these chromatin-based tasks by BMI1. Taking advantage of a prominently featuring TA-phase during chondrogenesis in vitro and in vivo, we here report that TA is completely dependent on intact PRC1 function. BMI1-depleted chondrogenic progenitors rapidly accumulate double strand DNA breaks during DNA replication, present massive non-H3K27me3-directed transcriptional deregulation and fail to undergo chondrogenic TA. Genome-wide accumulation of Topoisomerase 2α and Geminin suggests a model in which PRC1 synchronizes replication and transcription during rapid chondrogenic progenitor expansion. Our combined data reveals for the first time a vital cell-autonomous role for PRC1 during chondrogenesis. We provide evidence that chondrocyte hyper-replication and hypertrophy represent a unique example of programmed senescence in vivo. These findings provide new perspectives on PRC1 function in development and disease.

Highlights

Throughout development, the cooperative action of Polycomb Repressive Complexes (PRCs) and their counterparts the Trithorax group proteins (TrxG) is thought to constitute part of an epigenetic transcriptional memory, vital for maintenance of cell identity [1,2,3,4,5]
We define a crucial role for Polycomb Repressive Complex 1 (PRC1) during differentiation-associated Transit amplification (TA) and provide a detailed analysis of the cellular and molecular consequences of PRC1 functional deficiency in TA chondrogenic progenitors
GMNN accumulation correlates with hampered chondrogenic transcriptional programming, as evidenced by coinciding, apparently non-H3K27me3-directed, transcriptional deregulation, hyperphosphorylation and enhanced degradation of RPOL2A

Summary

Introduction

Throughout development, the cooperative action of Polycomb Repressive Complexes (PRCs) and their counterparts the Trithorax group proteins (TrxG) is thought to constitute part of an epigenetic transcriptional memory, vital for maintenance of cell identity [1,2,3,4,5]. Many PRC1-mutants display antero-posterior (A–P) patterning defects in insect and rodent models as a result of failing Hox gene expression-boundary maintenance. In mice, such defects are manifested as skeletal deformations, which are exacerbated by compound mutation [6]. PRC1-mutations are known to cause defects in DNA replication in Drosophila melanogaster syncytial larvae through as yet unknown mechanisms [7,8]. PRCs function in numerous DNA-templated processes, each with obvious importance for development, differentiation and tissue homeostasis

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