Molecular Basis of XRN2-Deficient Cancer Cell Sensitivity to Poly(ADP-ribose) Polymerase Inhibition.

Simple Summary: Many cancers exhibit compromised 5′-3′-exoribonuclease 2 (XRN2) expression. XRN2 is a major regulator of RNA polymerase II (RNAPII) at the transcription termination sites of protein-coding genes. Deregulated transcription termination facilitates the formation of triple-stranded nucleic acid structures known as R-loops (RNA–DNA hybrids with displaced single-strand DNA). Elevated levels of unscheduled R-loops promote genomic instability. In the absence of XRN2, R-loop levels increase and promote DNA damage that activates DNA damage surveillance protein poly(ADP-ribose) polymerase 1 (PARP1). Previously, we discovered that the simultaneous absence of XRN2 and PARP1 compromises the survival of non-cancer and cancer cells; however, the underlying cellular stress response remained unknown. Here, we aimed to uncover the molecular consequences of concurrent XRN2 depletion and PARP1 inhibition. Our findings provide a mechanistic understanding of why cancer cells rely on PARP1 when XRN2 is absent and strengthen the translational aspect of targeting XRN2 cancer vulnerabilities using PARP inhibitors. 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. [ABSTRACT FROM AUTHOR]