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R-loops are RNA–DNA-hybrid-containing nucleic acids with important cellular roles. Deregulation of R-loop dynamics can lead to DNA damage and genome instability 1 , which has been linked to the action of endonucleases such as XPG 2 – 4 . However, the mechanisms and cellular consequences of such processing have remained unclear. Here we identify a new population of RNA–DNA hybrids in the cytoplasm that are R-loop-processing products. When nuclear R-loops were perturbed by depleting the RNA–DNA helicase senataxin ( SETX ) or the breast cancer gene BRCA1  (refs. 5 – 7 ), we observed XPG- and XPF-dependent cytoplasmic hybrid formation. We identify their source as a subset of stable, overlapping nuclear hybrids with a specific nucleotide signature. Cytoplasmic hybrids bind to the pattern recognition receptors cGAS and TLR3 (ref.  8 ), activating IRF3 and inducing apoptosis. Excised hybrids and an R-loop-induced innate immune response were also observed in SETX -mutated cells from patients with ataxia oculomotor apraxia type 2 (ref.  9 ) and in BRCA1 -mutated cancer cells 10 . These findings establish RNA–DNA hybrids as immunogenic species that aberrantly accumulate in the cytoplasm after R-loop processing, linking R-loop accumulation to cell death through the innate immune response. Aberrant R-loop processing and subsequent innate immune activation may contribute to many diseases, such as neurodegeneration and cancer. RNA–DNA hybrids are immunogenic species that can aberrantly accumulate in the cytoplasm after R-loop processing, linking R-loop accumulation to cell death through the innate immune response.

R-loops have both positive and negative impacts on chromosome functions. To identify toxic R-loops in the human genome, here, we map RNA:DNA hybrids, replication stress markers and DNA double-strand breaks (DSBs) in cells depleted for Topoisomerase I (Top1), an enzyme that relaxes DNA supercoiling and prevents R-loop formation. RNA:DNA hybrids are found at both promoters (TSS) and terminators (TTS) of highly expressed genes. In contrast, the phosphorylation of RPA by ATR is only detected at TTS, which are preferentially replicated in a head-on orientation relative to the direction of transcription. In Top1-depleted cells, DSBs also accumulate at TTS, leading to persistent checkpoint activation, spreading of γ-H2AX on chromatin and global replication fork slowdown. These data indicate that fork pausing at the TTS of highly expressed genes containing R-loops prevents head-on conflicts between replication and transcription and maintains genome integrity in a Top1-dependent manner.

Transcription poses a threat to genomic stability through the formation of R-loops that can obstruct progression of replication forks. R-loops are three-stranded nucleic acid structures formed by an RNA–DNA hybrid with a displaced non-template DNA strand. We developed RNA–DNA Proximity Proteomics to map the R-loop proximal proteome of human cells using quantitative mass spectrometry. We implicate different cellular proteins in R-loop regulation and identify a role of the tumor suppressor DDX41 in opposing R-loop and double strand DNA break accumulation in promoters. DDX41 is enriched in promoter regions in vivo, and can unwind RNA–DNA hybrids in vitro. R-loop accumulation upon loss of DDX41 is accompanied with replication stress, an increase in the formation of double strand DNA breaks and transcriptome changes associated with the inflammatory response. Germline loss-of-function mutations in DDX41 lead to predisposition to acute myeloid leukemia in adulthood. We propose that R-loop accumulation and genomic instability-associated inflammatory response may contribute to the development of familial AML with mutated DDX41. Transcription can pose a threat to genomic instability through the formation of R-loops, which are RNA–DNA hybrids with a displaced non-template DNA strand. Here the authors mapped the R-loop proximal proteome in human cells and identified a role of the tumor suppressor DDX41 in opposing R-loop and double strand DNA break accumulation in gene promoters.

Homologous recombination (HR) repairs DNA double-strand breaks (DSBs) in the S and G2 phases of the cell cycle 1 – 3 . Several HR proteins are preferentially recruited to DSBs at transcriptionally active loci 4 – 10 , but how transcription promotes HR is poorly understood. Here we develop an assay to assess the effect of local transcription on HR. Using this assay, we find that transcription stimulates HR to a substantial extent. Tethering RNA transcripts to the vicinity of DSBs recapitulates the effects of local transcription, which suggests that transcription enhances HR through RNA transcripts. Tethered RNA transcripts stimulate HR in a sequence- and orientation-dependent manner, indicating that they function by forming DNA–RNA hybrids. In contrast to most HR proteins, RAD51-associated protein 1 (RAD51AP1) only promotes HR when local transcription is active. RAD51AP1 drives the formation of R-loops in vitro and is required for tethered RNAs to stimulate HR in cells. Notably, RAD51AP1 is necessary for the DSB-induced formation of DNA–RNA hybrids in donor DNA, linking R-loops to D-loops. In vitro, RAD51AP1-generated R-loops enhance the RAD51-mediated formation of D-loops locally and give rise to intermediates that we term ‘DR-loops’, which contain both DNA–DNA and DNA–RNA hybrids and favour RAD51 function. Thus, at DSBs in transcribed regions, RAD51AP1 promotes the invasion of RNA transcripts into donor DNA, and stimulates HR through the formation of DR-loops. RNA transcripts stimulate homologous recombination through the formation of DR-loops, intermediate structures that contain both DNA–DNA and DNA–RNA hybrids.

R‐loops are three‐stranded nucleic acid structures comprising an RNA/DNA hybrid and a displaced single‐stranded DNA. While transient R‐loop formation is essential for various physiological processes, their persistent accumulation leads to genomic instability. Cancer cells exhibit elevated R‐loop levels due to hypertranscription, replication stress, and impaired DNA repair pathways. In this review, we provide a comprehensive overview of the molecular machinery that resolves R‐loops, including chromatin remodelers, transcriptional regulators, nucleases, and helicases. We also highlight the emerging roles of long noncoding RNAs (lncRNAs) in modulating R‐loop dynamics and explore how these RNA‐based mechanisms cooperate with canonical resolution pathways. Finally, we explore the potential of targeting R‐loop regulatory networks as a novel therapeutic strategy in cancer treatment. R‐loops are three‐stranded nucleic acid structures whose dysregulation leads to genomic instability and cancer progression. This review summarizes the protein and lncRNA machineries that regulate R‐loop dynamics and discusses the therapeutic potential of targeting these pathways in cancer.

R-loops are three-stranded nucleic acid structures that are abundant and widespread across the genome and that have important physiological roles in many nuclear processes. Their accumulation is observed in cancers and neurodegenerative disorders. Recent studies have implicated a function for R-loops and G-quadruplex (G4) structures, which can form on the displaced single strand of R-loops, in three-dimensional genome organization in both physiological and pathological contexts. Here we discuss the interconnected functions of DNA:RNA hybrids and G4s within R-loops, their impact on DNA repair and gene regulatory networks, and their emerging roles in genome organization during development and disease. R-loops and G-quadruplexes are non-canonical nucleic acid structures with known roles in genome organization. Here, Wulfridge and Sarma highlight emerging roles in DNA repair and transcriptional and epigenetic gene regulation.

THO/TREX is a conserved nuclear complex that functions in mRNP biogenesis and prevents transcription‐associated recombination. Whether or not it has a ubiquitous role in the genome is unknown. Chromatin immunoprecipitation (ChIP)‐chip studies reveal that the Hpr1 component of THO and the Sub2 RNA‐dependent ATPase have genome‐wide distributions at active ORFs in yeast. In contrast to RNA polymerase II, evenly distributed from promoter to termination regions, THO and Sub2 are absent at promoters and distributed in a gradual 5′ → 3′ gradient. This is accompanied by a genome‐wide impact of THO–Sub2 deletions on expression of highly expressed, long and high G+C‐content genes. Importantly, ChIP‐chips reveal an over‐recruitment of Rrm3 in active genes in THO mutants that is reduced by RNaseH1 overexpression. Our work establishes a genome‐wide function for THO–Sub2 in transcription elongation and mRNP biogenesis that function to prevent the accumulation of transcription‐mediated replication obstacles, including R‐loops. THO/TREX is a conserved nuclear complex that functions in mRNP biogenesis and prevents transcription‐associated recombination. Here, genome‐wide binding analysis of THO/TREX describes a role for the complex in preventing R‐loop formation during transcription, which would then impair replication fork progression.

Tumour hypoxia is associated with poor patient prognosis and therapy resistance. A unique transcriptional response is initiated by hypoxia which includes the rapid activation of numerous transcription factors in a background of reduced global transcription. Here, we show that the biological response to hypoxia includes the accumulation of R-loops and the induction of the RNA/DNA helicase SETX. In the absence of hypoxia-induced SETX, R-loop levels increase, DNA damage accumulates, and DNA replication rates decrease. Therefore, suggesting that, SETX plays a role in protecting cells from DNA damage induced during transcription in hypoxia. Importantly, we propose that the mechanism of SETX induction in hypoxia is reliant on the PERK/ATF4 arm of the unfolded protein response. These data not only highlight the unique cellular response to hypoxia, which includes both a replication stress-dependent DNA damage response and an unfolded protein response but uncover a novel link between these two distinct pathways. Hypoxia induces a change in transcriptional response in mammalian cells. Here the authors reveal a role for the RNA/DNA helicase Senataxin in protecting cells from DNA damage induced during transcription in hypoxia.

Proper chromatin function and maintenance of genomic stability depends on spatiotemporal coordination between the transcription and replication machinery. Loss of this coordination can lead to DNA damage from increased transcription-replication collision events. We report that deregulated transcription following BRD4 loss in cancer cells leads to the accumulation of RNA:DNA hybrids (R-loops) and collisions with the replication machinery causing replication stress and DNA damage. Whole genome BRD4 and γH2AX ChIP-Seq with R-loop IP qPCR reveals that BRD4 inhibition leads to accumulation of R-loops and DNA damage at a subset of known BDR4, JMJD6, and CHD4 co-regulated genes. Interference with BRD4 function causes transcriptional downregulation of the DNA damage response protein TopBP1, resulting in failure to activate the ATR-Chk1 pathway despite increased replication stress, leading to apoptotic cell death in S-phase and mitotic catastrophe. These findings demonstrate that inhibition of BRD4 induces transcription-replication conflicts, DNA damage, and cell death in oncogenic cells. In order to avoid transcription-replication conflicts (TRCs) on shared DNA templates, cell must maintain strict spatiotemporal co-ordination of transcription with replication. Here the authors uncover a role for BRD4 in preventing TRCs and DNA damage checkpoint signaling in oncogenic cells.

RNase H2 is a specialized enzyme that degrades RNA in RNA/DNA hybrids and deficiency of this enzyme causes a severe neuroinflammatory disease, Aicardi Goutières syndrome (AGS). However, the molecular mechanism underlying AGS is still unclear. Here, we show that RNase H2 is associated with a subset of genes, in a transcription-dependent manner where it interacts with RNA Polymerase II. RNase H2 depletion impairs transcription leading to accumulation of R-loops, structures that comprise RNA/DNA hybrids and a displaced DNA strand, mainly associated with short and intronless genes. Importantly, accumulated R-loops are processed by XPG and XPF endonucleases which leads to DNA damage and activation of the immune response, features associated with AGS. Consequently, we uncover a key role for RNase H2 in the transcription of human genes by maintaining R-loop homeostasis. Our results provide insight into the mechanistic contribution of R-loops to AGS pathogenesis. RnaseH2 is mutated in severe neuro-inflammatory disorder Aicardi‐Goutières syndrome. Here the authors reveal that RNase H2 controls cellular R-loop homeostasis to promote transcription, genome integrity and prevent R-loop-associated inflammation.