Efficient DNA Repair Mitigates Replication Stress Resulting in Less Immunogenic Cytosolic DNA in Radioresistant Breast Cancer Stem Cells

Felix Meyer,Tim Wagner,Cordula Petersen,Kai Rothkamm,Kerstin Borgmann,Claudia Peitzsch,Anna Maria Engel,Anna Dubrovska,Ann Kristin Krause,Lena Poole

doi:10.3389/fimmu.2022.765284

Abstract

Cancer stem cells (CSCs) are a major cause of tumor therapy failure. This is mainly attributed to increased DNA repair capacity and immune escape. Recent studies have shown that functional DNA repair via homologous recombination (HR) prevents radiation-induced accumulation of DNA in the cytoplasm, thereby inhibiting the intracellular immune response. However, it is unclear whether CSCs can suppress radiation-induced cytoplasmic dsDNA formation. Here, we show that the increased radioresistance of ALDH1-positive breast cancer stem cells (BCSCs) in S phase is mediated by both enhanced DNA double-strand break repair and improved replication fork protection due to HR. Both HR-mediated processes lead to suppression of radiation-induced replication stress and consequently reduction of cytoplasmic dsDNA. The amount of cytoplasmic dsDNA correlated significantly with BCSC content (p=0.0002). This clearly indicates that HR-dependent avoidance of radiation-induced replication stress mediates radioresistance and contributes to its immune evasion. Consistent with this, enhancement of replication stress by inhibition of ataxia telangiectasia and RAD3 related (ATR) resulted in significant radiosensitization (SER37 increase 1.7-2.8 Gy, p<0.0001). Therefore, disruption of HR-mediated processes, particularly in replication, opens a CSC-specific radiosensitization option by enhancing their intracellular immune response.

Highlights

Accumulation of DNA in the cytoplasm in the cell activates the innate immune response through cyclic GMP-AMP synthase and binding to the activator protein stimulator of interferon genes (STING)
Cytosolic DNA Correlates With Breast Cancer Cell Proportion (BCSC)
Cells with a DNA repair defect in homologous recombination (HR) experience increased activation of the cyclic GMP-AMP synthase (cGAS)/STING pathway and subsequent activation of the intracellular immune response due to elevated cytosolic dsDNA [4]. It has not been investigated whether Cancer stem cells (CSCs) can suppress the induction of cytosolic dsDNA and an intracellular immune response, through efficient DNA repair mechanisms

Summary

Introduction

Accumulation of DNA in the cytoplasm in the cell activates the innate immune response through cyclic GMP-AMP synthase (cGAS) and binding to the activator protein stimulator of interferon genes (STING). STING induces phosphorylation and translocation of the transcription factor interferon regulatory factor 3 (IRF3) and initiates the expression of type-1 interferon (type-1 IFN) This intracellular immune response primarily serves to defend against foreign DNA but cannot distinguish it from its own cytosolic DNA. Increased accumulation of cytosolic DNA and activation of the cGAS/STING pathway have been observed in RAD51-, BRCA1-, or BRCA2-deficient carcinoma cell lines. Factors of HR, such as RAD51, BRCA1 and BRCA2, stabilize DNA at active replication forks and protect it from degradation by nucleases such as MRE11 [6, 7] This mediates repair and restart of replication forks, prevents formation of single-ended replication-associated DSBs, and avoids DNA replication stress. The S-phase DNA damage response and DNA repair by HR to avoid DSB and replication stress are critical factors for the activation of the intracellular immune response

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Conclusion