SPT6-driven error-free DNA repair safeguards genomic stability of glioblastoma cancer stem-like cells

Elisabeth Anne Adanma Obara,Jane Skjoth-Rasmussen,Kristoffer Vitting-Serup,Diana Aguilar-Morante,Petra Hamerlik,Robert Strauss,Kamilla Ellermann Jensen,Alex Frias,Kirstine Juul Elbæk,Jiri Bartek,Lucie Tuckova,Jannick Brennum,Yi Chieh Lim,Lina Vardouli,Henriette Pedersen,Rikke Darling Rasmussen,Christoffel Dinant

doi:10.1038/s41467-020-18549-8

Abstract

Glioblastoma cancer-stem like cells (GSCs) display marked resistance to ionizing radiation (IR), a standard of care for glioblastoma patients. Mechanisms underpinning radio-resistance of GSCs remain largely unknown. Chromatin state and the accessibility of DNA lesions to DNA repair machineries are crucial for the maintenance of genomic stability. Understanding the functional impact of chromatin remodeling on DNA repair in GSCs may lay the foundation for advancing the efficacy of radio-sensitizing therapies. Here, we present the results of a high-content siRNA microscopy screen, revealing the transcriptional elongation factor SPT6 to be critical for the genomic stability and self-renewal of GSCs. Mechanistically, SPT6 transcriptionally up-regulates BRCA1 and thereby drives an error-free DNA repair in GSCs. SPT6 loss impairs the self-renewal, genomic stability and tumor initiating capacity of GSCs. Collectively, our results provide mechanistic insights into how SPT6 regulates DNA repair and identify SPT6 as a putative therapeutic target in glioblastoma.

Highlights

Glioblastoma cancer-stem like cells (GSCs) display marked resistance to ionizing radiation (IR), a standard of care for glioblastoma patients
The GSCs accumulate a significantly lower amount of double-strand breaks (DSBs) (Fig. 1a) compared to differentiated GBM cells (DGCs). This observation together with previously reported dependency of GSCs on chromatin remodeling factors[19,22] prompted us to interrogate the impact of the chromatin state on DNA repair efficiency of matched GSC and DGC sub-populations
The largest induction of γH2AX mean intensity (MI) was seen upon downregulation of the Suppressor of Ty 6 homolog (SPT6), a histone chaperon that binds the C-terminal repeat domain (CTD) of RNA polymerase II (RNAP II) via its tandem SH2 domain[23,24,25,26], and is essential for transcription elongation of RNAP II-transcribed genes[27]

Summary

Introduction

Glioblastoma cancer-stem like cells (GSCs) display marked resistance to ionizing radiation (IR), a standard of care for glioblastoma patients. Supporting the clinical relevance of chromatin regulation, several chromatin remodeling factors were identified as essential for GSC maintenance, opening a novel avenue for therapeutic intervention in GBM19,20 Based on this background, we hypothesized that GSCs and the differentiated GBM cells (DGCs) differ in their dependency on chromatin remodeling genes to enable efficient DNA repair, which confers the superior capacity of GSCs to evade DNA damaging therapies. Using matched patient-derived GSCs and DGCs, we carried out a high-content siRNA screen, which identified the histone chaperone and transcription elongation factor SPT6 (Suppressor of Ty 6 homolog) as a key regulator of error-free DNA repair in GSCs. SPT6 drives a stem cell transcriptional program to promote GSC maintenance and tumorigenicity in vivo, supporting SPT6 as a novel therapeutic target in GBM

Methods

Results

Conclusion