R-loops (RNA–DNA hybrids with displaced single-stranded DNA) have emerged as a potent source of DNA damage and genomic instability. The termination of defective RNA polymerase II (RNAPII) is one of the major sources of R-loop formation. 5′-3′-exoribonuclease 2 (XRN2) promotes genome-wide efficient RNAPII termination, and XRN2-deficient cells exhibit increased DNA damage emanating from elevated R-loops. Recently, we showed that DNA damage instigated by XRN2 depletion in human fibroblast cells resulted in enhanced poly(ADP-ribose) polymerase 1 (PARP1) activity. Additionally, we established a synthetic lethal relationship between XRN2 and PARP1. However, the underlying cellular stress response promoting this synthetic lethality remains elusive. Here, we delineate the molecular consequences leading to the synthetic lethality of XRN2-deficient cancer cells induced by PARP inhibition. We found that XRN2-deficient lung and breast cancer cells display sensitivity to two clinically relevant PARP inhibitors, Rucaparib and Olaparib. At a mechanistic level, PARP inhibition combined with XRN2 deficiency exacerbates R-loop and DNA double-strand break formation in cancer cells. Consistent with our previous findings using several different siRNAs, we also show that XRN2 deficiency in cancer cells hyperactivates PARP1. Furthermore, we observed enhanced replication stress in XRN2-deficient cancer cells treated with PARP inhibitors. Finally, the enhanced stress response instigated by compromised PARP1 catalytic function in XRN2-deficient cells activates caspase-3 to initiate cell death. Collectively, these findings provide mechanistic insights into the sensitivity of XRN2-deficient cancer cells to PARP inhibition and strengthen the underlying translational implications for targeted therapy.
R环（RNA-DNA杂交体伴随一条被置换的单链DNA）已成为DNA损伤和基因组不稳定的重要来源。缺陷性RNA聚合酶II（RNAPII）的终止是R环形成的主要来源之一。5′-3′外切核糖核酸酶2（XRN2）促进全基因组范围内高效的RNAPII终止，而XRN2缺陷细胞会因R环水平升高而表现出DNA损伤增加。最近，我们发现人成纤维细胞中由XRN2缺失引发的DNA损伤导致聚腺苷二磷酸核糖聚合酶1（PARP1）活性增强。此外，我们证实了XRN2与PARP1之间存在合成致死关系。然而，促进这种合成致死性的潜在细胞应激反应机制尚不明确。本文系统阐述了PARP抑制诱导XRN2缺陷癌细胞发生合成致死效应的分子机制。我们发现XRN2缺陷的肺癌和乳腺癌细胞对两种临床相关PARP抑制剂Rucaparib和Olaparib表现出敏感性。在机制层面，PARP抑制与XRN2缺陷共同加剧了癌细胞中R环和DNA双链断裂的形成。与我们先前使用多种不同siRNA的研究结果一致，本研究也证实癌细胞中XRN2缺陷会导致PARP1过度激活。此外，我们观察到经PARP抑制剂处理的XRN2缺陷癌细胞复制应激显著增强。最终，XRN2缺陷细胞中PARP1催化功能受损引发的强化应激反应激活了caspase-3，从而启动细胞死亡程序。这些发现从机制层面揭示了XRN2缺陷癌细胞对PARP抑制敏感性的内在原理，并为靶向治疗的临床转化提供了理论依据。
Molecular Basis of XRN2-Deficient Cancer Cell Sensitivity to Poly(ADP-ribose) Polymerase Inhibition
肿瘤(癌症)患者之家