Sources, resolution and physiological relevance of R-loops and RNA-DNA hybrids

Biological function molecular pathways and druggability of DNMT2/TRDMT1

5-methylcytosine (m5C) is among the most common epigenetic modification in DNA and RNA molecules, and plays an important role in the animal development and disease pathogenesis. Interestingly, unlike other m5C DNA methyltransferases (DNMTs), DNMT2/TRDMT1 has the double-substrate specificity and adopts a DNMT-similar catalytic mechanism to methylate RNA. Moreover, it is widely involved in a variety of physiological regulatory processes, such as the gene expression, precise protein synthesis, immune response, and disease occurrence. Thus, comprehending the epigenetic mechanism and function of DNMT2/TRDMT1 will probably provide new strategies to treat some refractory diseases. Here, we discuss recent studies on the spatiotemporal expression pattern and post-translational modifications of DNMT2/TRDMT1, and summarize the research advances in substrate characteristics, catalytic recognition mechanism, DNMT2/TRDMT1-related genes or proteins, pharmacological application, and inhibitor development. This review will shed light on the pharmacological design by targeting DNMT2/TRDMT1 to treat parasitic, viral and oncologic diseases.

Molecular insights into regulatory RNAs in the cellular machinery

It is apparent that various functional units within the cellular machinery are derived from RNAs. The evolution of sequencing techniques has resulted in significant insights into approaches for transcriptome studies. Organisms utilize RNA to govern cellular systems, and a heterogeneous class of RNAs is involved in regulatory functions. In particular, regulatory RNAs are increasingly recognized to participate in intricately functioning machinery across almost all levels of biological systems. These systems include those mediating chromatin arrangement, transcription, suborganelle stabilization, and posttranscriptional modifications. Any class of RNA exhibiting regulatory activity can be termed a class of regulatory RNA and is typically represented by noncoding RNAs, which constitute a substantial portion of the genome. These RNAs function based on the principle of structural changes through cis and/or trans regulation to facilitate mutual RNA‒RNA, RNA‒DNA, and RNA‒protein interactions. It has not been clearly elucidated whether regulatory RNAs identified through deep sequencing actually function in the anticipated mechanisms. This review addresses the dominant properties of regulatory RNAs at various layers of the cellular machinery and covers regulatory activities, structural dynamics, modifications, associated molecules, and further challenges related to therapeutics and deep learning.

CircR-loop: a novel RNA:DNA interaction on genome instability

CircR-loop, a recently unearthed regulatory mechanism situated at the crossroads of circular RNA and DNA interactions, constitute a subset of R-loop. This circR-loop have emerged as a crucial player in pivotal regulatory functions within both animal and plant systems. The journey into the realm of circR-loop commenced with their discovery within the human mitochondrial genome, where they serve as critical directors of mitochondrial DNA replication. In the plant kingdom, circR-loop wield influence over processes such as alternative splicing and centromere organization, impacting the intricacies of floral development and genome stability, respectively. Their significance extends to the animal domain, where circR-loop has captured attention for their roles in cancer-related phenomena, exerting control over transcription, chromatin architecture, and orchestrating responses to DNA damage. Moreover, their involvement in nuclear export anomalies further underscores their prominence in cellular regulation. This article summarizes the important regulatory mechanisms and physiological roles of circR-loop in plants and animals, and offers a comprehensive exploration of the methodologies employed for the identification, characterization, and functional analysis of circR-loop, underscoring the pressing need for innovative approaches that can effectively distinguish them from their linear RNA counterparts while elucidating their precise functions. Lastly, the article sheds light on the challenges and opportunities that lie ahead in the field of circR-loop research, emphasizing the vital importance of continued investigations to uncover their regulatory roles and potential applications in the realm of biology. In summary, circR-loop represents a captivating and novel regulatory mechanism with broad-reaching implications spanning the realms of genetics, epigenetics, and disease biology. Their exploration opens new avenues for comprehending gene regulation and holds significant promise for future therapeutic interventions.

LncRNA DYNLRB2-AS1 promotes gemcitabine resistance of nasopharyngeal carcinoma by inhibiting the ubiquitination degradation of DHX9 protein

Gemcitabine (GEM) based induction chemotherapy is a standard treatment for locoregionally advanced nasopharyngeal carcinoma (NPC). However, approximately 15 % of patients are still resistant to GEM-containing chemotherapy, which leads to treatment failure. Nevertheless, the underlying mechanisms of GEM resistance remain poorly understood. Herein, based on a microarray analysis, we identified 221 dysregulated lncRNAs, of which, DYNLRB2-AS1 was one of the most upregulated lncRNAs in GEM-resistance NPC cell lines. DYNLRB2-AS1 was shown to function as contain an oncogenic lncRNA that promoted NPC GEM resistance, cell proliferation, but inhibited cell apoptosis. Mechanistically, DYNLRB2-AS1 could directly bind to the DHX9 protein and prevent its interaction with the E3 ubiquitin ligase PRPF19, and thus blocking PRPF19-mediated DHX9 degradation, which ultimately facilitated the repair of DNA damage in the presence of GEM. Clinically, higher DYNLRB2-AS1 expression indicated an unfavourable overall survival of NPC patients who received induction chemotherapy. Overall, this study identified the oncogenic lncRNA DYNLRB2-AS1 as an independent prognostic biomarker for patients with locally advanced NPC and as a potential therapeutic target for overcoming GEM chemoresistance in NPC.

Unprocessed genomic uracil as a source of DNA replication stress in cancer cells

Alterations of bases in DNA constitute a major source of genomic instability. It is believed that base alterations trigger base excision repair (BER), generating DNA repair intermediates interfering with DNA replication. Here, we show that genomic uracil, a common type of base alteration, induces DNA replication stress (RS) without being processed by BER. In the absence of uracil DNA glycosylase (UNG), genomic uracil accumulates to high levels, DNA replication forks slow down, and PrimPol-mediated repriming is enhanced, generating single-stranded gaps in nascent DNA. ATR inhibition in UNG-deficient cells blocks the repair of uracil-induced gaps, increasing replication fork collapse and cell death. Notably, a subset of cancer cells upregulates UNG2 to suppress genomic uracil and limit RS, and these cancer cells are hypersensitive to co-treatment with ATR inhibitors and drugs increasing genomic uracil. These results reveal unprocessed genomic uracil as an unexpected source of RS and a targetable vulnerability of cancer cells.

The loss of DNA polymerase epsilon accessory subunits POLE3-POLE4 leads to BRCA1-independent PARP inhibitor sensitivity

The clinical success of PARP1/2 inhibitors (PARPi) prompts the expansion of their applicability beyond homologous recombination deficiency. Here, we demonstrate that the loss of the accessory subunits of DNA polymerase epsilon, POLE3 and POLE4, sensitizes cells to PARPi. We show that the sensitivity of POLE4 knockouts is not due to compromised response to DNA damage or homologous recombination deficiency. Instead, POLE4 loss affects replication speed leading to the accumulation of single-stranded DNA gaps behind replication forks upon PARPi treatment, due to impaired post-replicative repair. POLE4 knockouts elicit elevated replication stress signaling involving ATR and DNA-PK. We find POLE4 to act parallel to BRCA1 in inducing sensitivity to PARPi and counteracts acquired resistance associated with restoration of homologous recombination. Altogether, our findings establish POLE4 as a promising target to improve PARPi driven therapies and hamper acquired PARPi resistance.

Dormant origin firing promotes head-on transcription-replication conflicts at transcription termination sites in response to BRCA2 deficiency

BRCA2 is a tumor suppressor protein responsible for safeguarding the cellular genome from replication stress and genotoxicity, but the specific mechanism(s) by which this is achieved to prevent early oncogenesis remains unclear. Here, we provide evidence that BRCA2 acts as a critical suppressor of head-on transcription-replication conflicts (HO-TRCs). Using Okazaki-fragment sequencing (Ok-seq) and computational analysis, we identified origins (dormant origins) that are activated near the transcription termination sites (TTS) of highly expressed, long genes in response to replication stress. Dormant origins are a source for HO-TRCs, and drug treatments that inhibit dormant origin firing led to a reduction in HO-TRCs, R-loop formation, and DNA damage. Using super-resolution microscopy, we showed that HO-TRC events track with elongating RNA polymerase II, but not with transcription initiation. Importantly, RNase H2 is recruited to sites of HO-TRCs in a BRCA2-dependent manner to help alleviate toxic R-loops associated with HO-TRCs. Collectively, our results provide a mechanistic basis for how BRCA2 shields against genomic instability by preventing HO-TRCs through both direct and indirect means occurring at predetermined genomic sites based on the pre-cancer transcriptome.

RNA-DNA triplexes: molecular mechanisms and functional relevance

Interactions of RNA with DNA are principles of gene expression control that have recently gained considerable attention. Among RNA-DNA interactions are R-loops and RNA-DNA hybrid G-quadruplexes, as well as RNA-DNA triplexes. It is proposed that RNA-DNA triplexes guide RNA-associated regulatory proteins to specific genomic locations, influencing transcription and epigenetic decision making. Although triplex formation initially was considered solely an in vitro event, recent progress in computational, biochemical, and biophysical methods support in vivo functionality with relevance for gene expression control. Here, we review the central methodology and biology of triplexes, outline paradigms required for triplex function, and provide examples of physiologically important triplex-forming long non-coding RNAs.

Maintenance of R-loop structures by phosphorylated hTERT preserves genome integrity

As aberrant accumulation of RNA-DNA hybrids (R-loops) causes DNA damage and genome instability, cells express regulators of R-loop structures. Here we report that RNA-dependent RNA polymerase (RdRP) activity of human telomerase reverse transcriptase (hTERT) regulates R-loop formation. We found that the phosphorylated form of hTERT (p-hTERT) exhibits RdRP activity in nuclear speckles both in telomerase-positive cells and telomerase-negative cells with alternative lengthening of telomeres (ALT) activity. The p-hTERT did not associate with telomerase RNA component in nuclear speckles but, instead, with TERRA RNAs to resolve R-loops. Targeting of the TERT gene in ALT cells ablated RdRP activity and impaired tumour growth. Using a genome-scale CRISPR loss-of-function screen, we identified Fanconi anaemia/BRCA genes as synthetic lethal partners of hTERT RdRP. Inactivation of RdRP and Fanconi anaemia/BRCA genes caused accumulation of R-loop structures and DNA damage. These findings indicate that RdRP activity of p-hTERT guards against genome instability by removing R-loop structures.

RNA helicase DEAD-box-5 is involved in R-loop dynamics of preimplantation embryos

OBJECTIVE

R-loops are DNA:RNA triplex hybrids, and their metabolism is tightly regulated by transcriptional regulation, DNA damage response, and chromatin structure dynamics. R-loop homeostasis is dynamically regulated and closely associated with gene transcription in mouse zygotes. However, the factors responsible for regulating these dynamic changes in the R-loops of fertilized mouse eggs have not yet been investigated. This study examined the functions of candidate factors that interact with R-loops during zygotic gene activation.

METHODS

In this study, we used publicly available next-generation sequencing datasets, including low-input ribosome profiling analysis and polymerase II chromatin immunoprecipitation-sequencing (ChIP-seq), to identify potential regulators of R-loop dynamics in zygotes. These datasets were downloaded, reanalyzed, and compared with mass spectrometry data to identify candidate factors involved in regulating R-loop dynamics. To validate the functions of these candidate factors, we treated mouse zygotes with chemical inhibitors using in vitro fertilization. Immunofluorescence with an anti-R-loop antibody was then performed to quantify changes in R-loop metabolism.

RESULTS

We identified DEAD-box-5 (DDX5) and histone deacetylase-2 (HDAC2) as candidates that potentially regulate R-loop metabolism in oocytes, zygotes and two-cell embryos based on change of their gene translation. Our analysis revealed that the DDX5 inhibition of activity led to decreased R-loop accumulation in pronuclei, indicating its involvement in regulating R-loop dynamics. However, the inhibition of histone deacetylase-2 activity did not significantly affect R-loop levels in pronuclei.

CONCLUSION

These findings suggest that dynamic changes in R-loops during mouse zygote development are likely regulated by RNA helicases, particularly DDX5, in conjunction with transcriptional processes. Our study provides compelling evidence for the involvement of these factors in regulating R-loop dynamics during early embryonic development.

The GATAD2B-NuRD complex drives DNA:RNA hybrid-dependent chromatin boundary formation upon DNA damage

Double-strand breaks (DSBs) are the most lethal form of DNA damage. Transcriptional activity at DSBs, as well as transcriptional repression around DSBs, are both required for efficient DNA repair. The chromatin landscape defines and coordinates these two opposing events. However, how the open and condensed chromatin architecture is regulated remains unclear. Here, we show that the GATAD2B-NuRD complex associates with DSBs in a transcription- and DNA:RNA hybrid-dependent manner, to promote histone deacetylation and chromatin condensation. This activity establishes a spatio-temporal boundary between open and closed chromatin, which is necessary for the correct termination of DNA end resection. The lack of the GATAD2B-NuRD complex leads to chromatin hyperrelaxation and extended DNA end resection, resulting in homologous recombination (HR) repair failure. Our results suggest that the GATAD2B-NuRD complex is a key coordinator of the dynamic interplay between transcription and the chromatin landscape, underscoring its biological significance in the RNA-dependent DNA damage response.

Transcription-coupled DNA repair protects genome stability upon oxidative stress-derived DNA strand breaks

Elevated oxidative stress, which threatens genome stability, has been detected in almost all types of cancers. Cells employ various DNA repair pathways to cope with DNA damage induced by oxidative stress. Recently, a lot of studies have provided insights into DNA damage response upon oxidative stress, specifically in the context of transcriptionally active genomes. Here, we summarize recent studies to help understand how the transcription is regulated upon DNA double strand breaks (DSB) and how DNA repair pathways are selectively activated at the damage sites coupling with transcription. The role of RNA molecules, especially R-loops and RNA modifications during the DNA repair process, is critical for protecting genome stability. This review provides an update on how cells protect transcribed genome loci via transcription-coupled repair pathways.

DNA damage response defects in hematologic malignancies: mechanistic insights and therapeutic strategies

ABSTRACT

The DNA damage response (DDR) encompasses the detection and repair of DNA lesions and is fundamental to the maintenance of genome integrity. Germ line DDR alterations underlie hereditary chromosome instability syndromes by promoting the acquisition of pathogenic structural variants in hematopoietic cells, resulting in increased predisposition to hematologic malignancies. Also frequent in hematologic malignancies are somatic mutations of DDR genes, typically arising from replication stress triggered by oncogene activation or deregulated tumor proliferation that provides a selective pressure for DDR loss. These defects impair homology-directed DNA repair or replication stress response, leading to an excessive reliance on error-prone DNA repair mechanisms that results in genomic instability and tumor progression. In hematologic malignancies, loss-of-function DDR alterations confer clonal growth advantage and adverse prognostic impact but may also provide therapeutic opportunities. Selective targeting of functional dependencies arising from these defects could achieve synthetic lethality, a therapeutic concept exemplified by inhibition of poly-(adenosine 5'-diphosphate ribose) polymerase or the ataxia telangiectasia and Rad 3 related-CHK1-WEE1 axis in malignancies harboring the BRCAness phenotype or genetic defects that increase replication stress. Furthermore, the role of DDR defects as a source of tumor immunogenicity, as well as their impact on the cross talk between DDR, inflammation, and tumor immunity are increasingly recognized, thus providing rationale for combining DDR modulation with immune modulation. The nature of the DDR-immune interface and the cellular vulnerabilities conferred by DDR defects may nonetheless be disease-specific and remain incompletely understood in many hematologic malignancies. Their comprehensive elucidation will be critical for optimizing therapeutic strategies to target DDR defects in these diseases.

AAV-mediated genome editing is influenced by the formation of R-loops

Recombinant adeno-associated viral vectors (rAAV) hold an intrinsic ability to stimulate homologous recombination (AAV-HR) and are the most used in clinical settings for in vivo gene therapy. However, rAAVs also integrate throughout the genome. Here, we describe DNA-RNA immunoprecipitation sequencing (DRIP-seq) in murine HEPA1-6 hepatoma cells and whole murine liver to establish the similarities and differences in genomic R-loop formation in a transformed cell line and intact tissue. We show enhanced AAV-HR in mice upon genetic and pharmacological upregulation of R-loops. Selecting the highly expressed Albumin gene as a model locus for genome editing in both in vitro and in vivo experiments showed that the R-loop prone, 3' end of Albumin was efficiently edited by AAV-HR, whereas the upstream R-loop-deficient region did not result in detectable vector integration. In addition, we found a positive correlation between previously reported off-target rAAV integration sites and R-loop enriched genomic regions. Thus, we conclude that high levels of R-loops, present in highly transcribed genes, promote rAAV vector genome integration. These findings may shed light on potential mechanisms for improving the safety and efficacy of genome editing by modulating R-loops and may enhance our ability to predict regions most susceptible to off-target insertional mutagenesis by rAAV vectors.

Super enhancer-associated circRNA-circLrch3 regulates hypoxia-induced pulmonary arterial smooth muscle cells pyroptosis by formation of R-loop with host gene

BACKGROUND

Pulmonary hypertension (PH) is a complex vascular disorder, characterized by pulmonary vessel remodeling and perivascular inflammation. Pulmonary arterial smooth muscle cells (PASMCs) pyroptosis is a novel pathological mechanism implicated of pulmonary vessel remodeling. However, the involvement of circRNAs in the process of pyroptosis and the underlying regulatory mechanisms remain inadequately understood.

METHODS

Western blotting, PI staining and LDH release were used to explore the role of circLrch3 in PASMCs pyroptosis. Moreover, S9.6 dot blot and DRIP-PCR were used to assess the formation of R-loop between circLrch3 and its host gene Lrch3. Chip-qPCR were used to evaluate the mechanism of super enhancer-associated circLrh3, which is transcriptionally activated by the transcription factor Tbx2.

RESULTS

CircLrch3 was markedly upregulated in hypoxic PASMCs. CircLrch3 knockdown inhibited hypoxia induced PASMCs pyroptosis in vivo and in vitro. Mechanistically, circLrch3 can form R-loop with host gene to upregulate the protein and mRNA expression of Lrch3. Furthermore, super enhancer interacted with the Tbx2 at the Lrch3 promoter locus, mediating the augmented transcription of circLrch3.

CONCLUSION

Our findings clarify the role of a super enhancer-associated circLrch3 in the formation of R-loop with the host gene Lrch3 to modulate pyroptosis in PASMCs, ultimately promoting the development of PH.

Linear interaction between replication and transcription shapes DNA break dynamics at recurrent DNA break Clusters

Recurrent DNA break clusters (RDCs) are replication-transcription collision hotspots; many are unique to neural progenitor cells. Through high-resolution replication sequencing and a capture-ligation assay in mouse neural progenitor cells experiencing replication stress, we unravel the replication features dictating RDC location and orientation. Most RDCs occur at the replication forks traversing timing transition regions (TTRs), where sparse replication origins connect unidirectional forks. Leftward-moving forks generate telomere-connected DNA double-strand breaks (DSBs), while rightward-moving forks lead to centromere-connected DSBs. Strand-specific mapping for DNA-bound RNA reveals co-transcriptional dual-strand DNA:RNA hybrids present at a higher density in RDC than in other actively transcribed long genes. In addition, mapping RNA polymerase activity uncovers that head-to-head interactions between replication and transcription machinery result in 60% DSB contribution to the head-on compared to 40% for co-directional. Taken together we reveal TTR as a fragile class and show how the linear interaction between transcription and replication impacts genome stability.

R-loop and diseases: the cell cycle matters

The cell cycle is a crucial biological process that is involved in cell growth, development, and reproduction. It can be divided into G1, S, G2, and M phases, and each period is closely regulated to ensure the production of two similar daughter cells with the same genetic material. However, many obstacles influence the cell cycle, including the R-loop that is formed throughout this process. R-loop is a triple-stranded structure, composed of an RNA: DNA hybrid and a single DNA strand, which is ubiquitous in organisms from bacteria to mammals. The existence of the R-loop has important significance for the regulation of various physiological processes. However, aberrant accumulation of R-loop due to its limited resolving ability will be detrimental for cells. For example, DNA damage and genomic instability, caused by the R-loop, can activate checkpoints in the cell cycle, which in turn induce cell cycle arrest and cell death. At present, a growing number of factors have been proven to prevent or eliminate the accumulation of R-loop thereby avoiding DNA damage and mutations. Therefore, we need to gain detailed insight into the R-loop resolution factors at different stages of the cell cycle. In this review, we review the current knowledge of factors that play a role in resolving the R-loop at different stages of the cell cycle, as well as how mutations of these factors lead to the onset and progression of diseases.

Chromatin dynamics and RNA metabolism are double-edged swords for the maintenance of plant genome integrity

Maintenance of genome integrity is an essential process in all organisms. Mechanisms avoiding the formation of DNA lesions or mutations are well described in animals because of their relevance to human health and cancer. In plants, they are of growing interest because DNA damage accumulation is increasingly recognized as one of the consequences of stress. Although the cellular response to DNA damage is mostly studied in response to genotoxic treatments, the main source of DNA lesions is cellular activity itself. This can occur through the production of reactive oxygen species as well as DNA processing mechanisms such as DNA replication or transcription and chromatin dynamics. In addition, how lesions are formed and repaired is greatly influenced by chromatin features and dynamics and by DNA and RNA metabolism. Notably, actively transcribed regions or replicating DNA, because they are less condensed and are sites of DNA processing, are more exposed to DNA damage. However, at the same time, a wealth of cellular mechanisms cooperate to favour DNA repair at these genomic loci. These intricate relationships that shape the distribution of mutations along the genome have been studied extensively in animals but much less in plants. In this Review, we summarize how chromatin dynamics influence lesion formation and DNA repair in plants, providing a comprehensive view of current knowledge and highlighting open questions with regard to what is known in other organisms.

Rat1 promotes premature transcription termination at R-loops

Certain DNA sequences can adopt a non-B form in the genome that interfere with DNA-templated processes, including transcription. Among the sequences that are intrinsically difficult to transcribe are those that tend to form R-loops, three-stranded nucleic acid structures formed by a DNA-RNA hybrid and the displaced ssDNA. Here we compared the transcription of an endogenous gene with and without an R-loop-forming sequence inserted. We show that, in agreement with previous in vivo and in vitro analyses, transcription elongation is delayed by R-loops in yeast. Importantly, we demonstrate that the Rat1 transcription terminator factor facilitates transcription throughout such structures by inducing premature termination of arrested RNAPIIs. We propose that RNase H degrades the RNA moiety of the hybrid, providing an entry site for Rat1. Thus, we have uncovered an unanticipated function of Rat1 as a transcription restoring factor opening up the possibility that it may also promote transcription through other genomic DNA structures intrinsically difficult to transcribe. If R-loop-mediated transcriptional stress is not relieved by Rat1, it will cause genomic instability, probably through the increase of transcription-replication conflicts, a deleterious situation that could lead to cancer.

A rewiring of DNA replication mediated by MRE11 exonuclease underlies primed-to-naive cell de-differentiation

Mouse embryonic stem cells (mESCs) in the primed pluripotency state, which resembles the post-implantation epiblast, can be de-differentiated in culture to a naive state that resembles the pre-implantation inner cell mass. We report that primed-to-naive mESC transition entails a significant slowdown of DNA replication forks and the compensatory activation of dormant origins. Using isolation of proteins on nascent DNA coupled to mass spectrometry, we identify key changes in replisome composition that are responsible for these effects. Naive mESC forks are enriched in MRE11 nuclease and other DNA repair proteins. MRE11 is recruited to newly synthesized DNA in response to transcription-replication conflicts, and its inhibition or genetic downregulation in naive mESCs is sufficient to restore the fork rate of primed cells. Transcriptomic analyses indicate that MRE11 exonuclease activity is required for the complete primed-to-naive mESC transition, demonstrating a direct link between DNA replication dynamics and the mESC de-differentiation process.

The MCM2-7 Complex: Roles beyond DNA Unwinding

The MCM2-7 complex is a hexameric protein complex that serves as a DNA helicase. It unwinds the DNA double helix during DNA replication, thereby providing the single-stranded replication template. In recent years, it has become clear that the MCM2-7 complex has additional functions that extend well beyond its role in DNA replication. Through physical and functional interactions with different pathways, it impacts other nuclear events and activities, including folding of the genome, histone inheritance, chromosome segregation, DNA damage sensing and repair, and gene transcription. Collectively, the diverse roles of the MCM2-7 complex suggest it plays a critical role in maintaining genome integrity by integrating the regulation of DNA replication with other pathways in the nucleus.

DHX9 maintains epithelial homeostasis by restraining R-loop-mediated genomic instability in intestinal stem cells

Epithelial barrier dysfunction and crypt destruction are hallmarks of inflammatory bowel disease (IBD). Intestinal stem cells (ISCs) residing in the crypts play a crucial role in the continuous self-renewal and rapid recovery of intestinal epithelial cells (IECs). However, how ISCs are dysregulated in IBD remains poorly understood. Here, we observe reduced DHX9 protein levels in IBD patients, and mice with conditional DHX9 depletion in the intestinal epithelium (Dhx9ΔIEC) exhibit an increased susceptibility to experimental colitis. Notably, Dhx9ΔIEC mice display a significant reduction in the numbers of ISCs and Paneth cells. Further investigation using ISC-specific or Paneth cell-specific Dhx9-deficient mice demonstrates the involvement of ISC-expressed DHX9 in maintaining epithelial homeostasis. Mechanistically, DHX9 deficiency leads to abnormal R-loop accumulation, resulting in genomic instability and the cGAS-STING-mediated inflammatory response, which together impair ISC function and contribute to the pathogenesis of IBD. Collectively, our findings highlight R-loop-mediated genomic instability in ISCs as a risk factor in IBD.

Activation of ERβ hijacks the splicing machinery to trigger R-loop formation in triple-negative breast cancer

Triple-negative breast cancer (TNBC) is a subtype of breast cancer with aggressive behavior and poor prognosis. Current therapeutic options available for TNBC patients are primarily chemotherapy. With our evolving understanding of this disease, novel targeted therapies, including poly ADP-ribose polymerase (PARP) inhibitors, antibody-drug conjugates, and immune-checkpoint inhibitors, have been developed for clinical use. Previous reports have demonstrated the essential role of estrogen receptor β (ERβ) in TNBC, but the detailed molecular mechanisms downstream ERβ activation in TNBC are still far from elucidated. In this study, we demonstrated that a specific ERβ agonist, LY500307, potently induces R-loop formation and DNA damage in TNBC cells. Subsequent interactome experiments indicated that the residues 151 to 165 of U2 small nuclear RNA auxiliary factor 1 (U2AF1) and the Trp439 and Lys443 of ERβ were critical for the binding between U2AF1 and ERβ. Combined RNA sequencing and ribosome sequencing analysis demonstrated that U2AF1-regulated downstream RNA splicing of 5-oxoprolinase (OPLAH) could affect its enzymatic activity and is essential for ERβ-induced R-loop formation and DNA damage. In clinical samples including 115 patients from The Cancer Genome Atlas (TCGA) and 32 patients from an in-house cohort, we found a close correlation in the expression of ESR2 and U2AF1 in TNBC patients. Collectively, our study has unraveled the molecular mechanisms that explain the therapeutic effects of ERβ activation in TNBC, which provides rationale for ERβ activation-based single or combined therapy for patients with TNBC.

Coarse-grained modeling of DNA-RNA hybrids

We introduce oxNA, a new model for the simulation of DNA-RNA hybrids that is based on two previously developed coarse-grained models-oxDNA and oxRNA. The model naturally reproduces the physical properties of hybrid duplexes, including their structure, persistence length, and force-extension characteristics. By parameterizing the DNA-RNA hydrogen bonding interaction, we fit the model's thermodynamic properties to experimental data using both average-sequence and sequence-dependent parameters. To demonstrate the model's applicability, we provide three examples of its use-calculating the free energy profiles of hybrid strand displacement reactions, studying the resolution of a short R-loop, and simulating RNA-scaffolded wireframe origami.

Preferential formation of Z-RNA over intercalated motifs in long noncoding RNA

Secondary structure is a principal determinant of lncRNA function, predominantly regarding scaffold formation and interfaces with target molecules. Noncanonical secondary structures that form in nucleic acids have known roles in regulating gene expression and include G-quadruplexes (G4s), intercalated motifs (iMs), and R-loops (RLs). In this paper, we used the computational tools G4-iM Grinder and QmRLFS-finder to predict the formation of each of these structures throughout the lncRNA transcriptome in comparison to protein-coding transcripts. The importance of the predicted structures in lncRNAs in biological contexts was assessed by combining our results with publicly available lncRNA tissue expression data followed by pathway analysis. The formation of predicted G4 (pG4) and iM (piM) structures in select lncRNA sequences was confirmed in vitro using biophysical experiments under near-physiological conditions. We find that the majority of the tested pG4s form highly stable G4 structures, and identify many previously unreported G4s in biologically important lncRNAs. In contrast, none of the piM sequences are able to form iM structures, consistent with the idea that RNA is unable to form stable iMs. Unexpectedly, these C-rich sequences instead form Z-RNA structures, which have not been previously observed in regions containing cytosine repeats and represent an interesting and underexplored target for protein-RNA interactions. Our results highlight the prevalence and potential structure-associated functions of noncanonical secondary structures in lncRNAs, and show G4 and Z-RNA structure formation in many lncRNA sequences for the first time, furthering the understanding of the structure-function relationship in lncRNAs.

End resection and telomere healing of DNA double-strand breaks during nematode programmed DNA elimination

Most DNA double-strand breaks (DSBs) are harmful to genome integrity. However, some forms of DSBs are essential to biological processes, such as meiotic recombination and V(D)J recombination. DSBs are also required for programmed DNA elimination (PDE) in ciliates and nematodes. In nematodes, the DSBs are healed with telomere addition. While telomere addition sites have been well-characterized, little is known regarding the DSBs that fragment nematode chromosomes. Here, we used embryos from the nematode Ascaris to study the timing of PDE breaks and examine the DSBs and their end processing. Using END-seq, we characterize the DSB ends and demonstrate that DNA breaks are introduced before mitosis, followed by extensive end resection. The resection profile is unique for each break site, and the resection generates 3' overhangs before the addition of telomeres. Interestingly, telomere healing occurs much more frequently on retained DSB ends than on eliminated ends. This biased repair of the DSB ends in Ascaris may be due to the sequestration of the eliminated DNA into micronuclei, preventing their ends from telomere healing. Additional DNA breaks occur within the eliminated DNA in both Ascaris and Parascaris, ensuring chromosomal breakage and providing a fail-safe mechanism for nematode PDE.

AKT1 interacts with DHX9 to Mitigate R Loop-Induced Replication Stress in Ovarian Cancer

PARP inhibitor (PARPi)-resistant BRCA-mutant (BRCAm) high-grade serous ovarian cancer (HGSOC) represents a new clinical challenge with unmet therapeutic needs. Here, we performed a quantitative high-throughput drug combination screen that identified the combination of an ATR inhibitor (ATRi) and an AKT inhibitor (AKTi) as an effective treatment strategy for both PARPi-sensitive and PARPi-resistant BRCAm HGSOC. The ATRi and AKTi combination induced DNA damage and R loop-mediated replication stress (RS). Mechanistically, the kinase domain of AKT1 directly interacted with DHX9 and facilitated recruitment of DHX9 to R loops. AKTi increased ATRi-induced R loop-mediated RS by mitigating recruitment of DHX9 to R loops. Moreover, DHX9 was upregulated in tumors from patients with PARPi-resistant BRCAm HGSOC, and high coexpression of DHX9 and AKT1 correlated with worse survival. Together, this study reveals an interaction between AKT1 and DHX9 that facilitates R loop resolution and identifies combining ATRi and AKTi as a rational treatment strategy for BRCAm HGSOC irrespective of PARPi resistance status.

SIGNIFICANCE

Inhibition of the AKT and ATR pathways cooperatively induces R loop-associated replication stress in high-grade serous ovarian cancer, providing rationale to support the clinical development of AKT and ATR inhibitor combinations. See related commentary by Ramanarayanan and Oberdoerffer, p. 793.

TOP1 and R-loops facilitate transcriptional DSBs at hypertranscribed cancer driver genes

DNA double-stranded breaks (DSBs) pose a significant threat to genomic integrity, and their generation during essential cellular processes like transcription remains poorly understood. In this study, we employ several techniques to map DSBs, R-loops, and topoisomerase 1 cleavage complex (TOP1cc) to comprehensively investigate the interplay between transcription, DSBs, topoisomerase 1 (TOP1), and R-loops. Our findings reveal the presence of DSBs at highly expressed genes enriched with TOP1 and R-loops. Remarkably, transcription-associated DSBs at these loci are significantly reduced upon depletion of R-loops and TOP1, uncovering the pivotal roles of TOP1 and R-loops in transcriptional DSB formation. By elucidating the intricate interplay between TOP1cc trapping, R-loops, and DSBs, our study provides insights into the mechanisms underlying transcription-associated genomic instability. Moreover, we establish a link between transcriptional DSBs and early molecular changes driving cancer development, highlighting the distinct etiology and molecular characteristics of driver mutations compared to passenger mutations.

RPA and Rad27 limit templated and inverted insertions at DNA breaks

Formation of templated insertions at DNA double-strand breaks (DSBs) is very common in cancer cells. The mechanisms and enzymes regulating these events are largely unknown. Here, we investigated templated insertions in yeast at DSBs using amplicon sequencing across a repaired locus. We document very short (most ∼5-34 bp), templated inverted duplications at DSBs. They are generated through a foldback mechanism that utilizes microhomologies adjacent to the DSB. Enzymatic requirements suggest a hybrid mechanism wherein one end requires Polδ-mediated synthesis while the other end is captured by nonhomologous end joining (NHEJ). This process is exacerbated in mutants with low levels or mutated RPA ( rtt105 Δ; rfa1 -t33) or extensive resection mutant ( sgs1 Δ exo1 Δ). Templated insertions from various distant genomic locations also increase in these mutants as well as in rad27 Δ and originate from fragile regions of the genome. Among complex insertions, common events are insertions of two sequences, originating from the same locus and with inverted orientation. We propose that these inversions are also formed by microhomology-mediated template switching. Taken together, we propose that a shortage of RPA typical in cancer cells is one possible factor stimulating the formation of templated insertions.

Targeting DHX9 Triggers Tumor-Intrinsic Interferon Response and Replication Stress in Small Cell Lung Cancer

Activating innate immunity in cancer cells through cytoplasmic nucleic acid sensing pathways, a phenomenon known as "viral mimicry," has emerged as an effective strategy to convert immunologically "cold" tumors into "hot." Through a curated CRISPR-based screen of RNA helicases, we identified DExD/H-box helicase 9 (DHX9) as a potent repressor of double-stranded RNA (dsRNA) in small cell lung cancers (SCLC). Depletion of DHX9 induced accumulation of cytoplasmic dsRNA and triggered tumor-intrinsic innate immunity. Intriguingly, ablating DHX9 also induced aberrant accumulation of R-loops, which resulted in an increase of DNA damage-derived cytoplasmic DNA and replication stress in SCLCs. In vivo, DHX9 deletion promoted a decrease in tumor growth while inducing a more immunogenic tumor microenvironment, invigorating responsiveness to immune-checkpoint blockade. These findings suggest that DHX9 is a crucial repressor of tumor-intrinsic innate immunity and replication stress, representing a promising target for SCLC and other "cold" tumors in which genomic instability contributes to pathology.

SIGNIFICANCE

One promising strategy to trigger an immune response within tumors and enhance immunotherapy efficacy is by inducing endogenous "virus-mimetic" nucleic acid accumulation. Here, we identify DHX9 as a viral-mimicry-inducing factor involved in the suppression of double-stranded RNAs and R-loops and propose DHX9 as a novel target to enhance antitumor immunity. See related commentary by Chiappinelli, p. 389. This article is featured in Selected Articles from This Issue, p. 384.

Microglial inflammation in genome instability: A neurodegenerative perspective

The maintenance of genome stability is crucial for cell homeostasis and tissue integrity. Numerous human neuropathologies display chronic inflammation in the central nervous system, set against a backdrop of genome instability, implying a close interplay between the DNA damage and immune responses in the context of neurological disease. Dissecting the molecular mechanisms of this crosstalk is essential for holistic understanding of neuroinflammatory pathways in genome instability disorders. Non-neuronal cell types, specifically microglia, are major drivers of neuroinflammation in the central nervous system with neuro-protective and -toxic capabilities. Here, we discuss how persistent DNA damage affects microglial homeostasis, zooming in on the cytosolic DNA sensing cGAS-STING pathway and the downstream inflammatory response, which can drive neurotoxic outcomes in the context of genome instability.

Non-coding RNAs in disease: from mechanisms to therapeutics

Non-coding RNAs (ncRNAs) are a heterogeneous group of transcripts that, by definition, are not translated into proteins. Since their discovery, ncRNAs have emerged as important regulators of multiple biological functions across a range of cell types and tissues, and their dysregulation has been implicated in disease. Notably, much research has focused on the link between microRNAs (miRNAs) and human cancers, although other ncRNAs, such as long non-coding RNAs (lncRNAs) and circular RNAs (circRNAs), are also emerging as relevant contributors to human disease. In this Review, we summarize our current understanding of the roles of miRNAs, lncRNAs and circRNAs in cancer and other major human diseases, notably cardiovascular, neurological and infectious diseases. Further, we discuss the potential use of ncRNAs as biomarkers of disease and as therapeutic targets.

The ARID1A-METTL3-m6A axis ensures effective RNase H1-mediated resolution of R-loops and genome stability

R-loops are three-stranded structures that can pose threats to genome stability. RNase H1 precisely recognizes R-loops to drive their resolution within the genome, but the underlying mechanism is unclear. Here, we report that ARID1A recognizes R-loops with high affinity in an ATM-dependent manner. ARID1A recruits METTL3 and METTL14 to the R-loop, leading to the m6A methylation of R-loop RNA. This m6A modification facilitates the recruitment of RNase H1 to the R-loop, driving its resolution and promoting DNA end resection at DSBs, thereby ensuring genome stability. Depletion of ARID1A, METTL3, or METTL14 leads to R-loop accumulation and reduced cell survival upon exposure to cytotoxic agents. Therefore, ARID1A, METTL3, and METTL14 function in a coordinated, temporal order at DSB sites to recruit RNase H1 and to ensure efficient R-loop resolution. Given the association of high ARID1A levels with resistance to genotoxic therapies in patients, these findings open avenues for exploring potential therapeutic strategies for cancers with ARID1A abnormalities.

Linear Interaction Between Replication and Transcription Shapes DNA Break Dynamics at Recurrent DNA Break Clusters

Recurrent DNA break clusters (RDCs) are replication-transcription collision hotspots; many are unique to neural progenitor cells. Through high-resolution replication sequencing and a capture-ligation assay in mouse neural progenitor cells experiencing replication stress, we unraveled the replication features dictating RDC location and orientation. Most RDCs occur at the replication forks traversing timing transition regions (TTRs), where sparse replication origins connect unidirectional forks. Leftward-moving forks generate telomere-connected DNA double-strand breaks (DSBs), while rightward-moving forks lead to centromere-connected DSBs. Strand-specific mapping for DNA-bound RNA revealed co-transcriptional dual-strand DNA:RNA hybrids present at a higher density in RDC than in other actively transcribed long genes. In addition, mapping RNA polymerase activity revealed that head-to-head interactions between replication and transcription machinery resulted in 60% DSB contribution to the head-on compared to 40% for co-directional. Our findings revealed TTR as a novel fragile class and highlighted how the linear interaction between transcription and replication impacts genome stability.

R-Loop Functions in Brca1 -Associated Mammary Tumorigenesis

Excessive R-loops, a DNA-RNA hybrid structure, are associated with genome instability and BRCA1 mutation-related breast cancer. Yet the causality of R-loops in tumorigenesis remains unclear. Here we show that R-loop removal by Rnaseh1 overexpression (Rh1-OE) in Brca1 -knockout (BKO) mouse mammary epithelium exacerbates DNA replication stress without affecting homology-directed DNA repair. R-loop removal also diminishes luminal progenitors, the cell of origin for estrogen receptor α (ERα)-negative BKO tumors. However, R-loop reduction does not dampen spontaneous BKO tumor incidence. Rather, it gives rise to a significant percentage of ERα-expressing BKO tumors. Thus, R-loops reshape mammary tumor subtype rather than promoting tumorigenesis.

Human PKD1 sequences form R-loop structures in vitro

Autosomal dominant polycystic kidney disease results from the loss of the PKD1 gene product, polycystin 1. Regulatory mechanisms are unresolved, but an apparent G/C sequence bias in the gene is consistent with co-transcriptional R-loop formation. R-loops regulate gene expression and stability, and they form when newly synthesized RNA extensively pairs with the template DNA to displace the non-template strand. In this study, we tested two human PKD1 sequences for co-transcriptional R-loop formation in vitro. We observed RNase H-sensitive R-loop formation in intron 1 and 22 sequences, but only in one transcriptional orientation. Therefore, R-loops may participate in PKD1 expression or stability.

Telomeres cooperate with the nuclear envelope to maintain genome stability

Mammalian telomeres have evolved safeguards to prevent their recognition as DNA double-stranded breaks by suppressing the activation of various DNA sensing and repair proteins. We have shown that the telomere-binding proteins TRF2 and RAP1 cooperate to prevent telomeres from undergoing aberrant homology-directed recombination by mediating t-loop protection. Our recent findings also suggest that mammalian telomere-binding proteins interact with the nuclear envelope to maintain chromosome stability. RAP1 interacts with nuclear lamins through KU70/KU80, and disruption of RAP1 and TRF2 function result in nuclear envelope rupture, promoting telomere-telomere recombination to form structures termed ultrabright telomeres. In this review, we discuss the importance of the interactions between shelterin components and the nuclear envelope to maintain telomere homeostasis and genome stability.

Maintaining transcriptional homeostasis during cell cycle

The preservation of gene expression patterns that define cellular identity throughout the cell division cycle is essential to perpetuate cellular lineages. However, the progression of cells through different phases of the cell cycle severely disrupts chromatin accessibility, epigenetic marks, and the recruitment of transcriptional regulators. Notably, chromatin is transiently disassembled during S-phase and undergoes drastic condensation during mitosis, which is a significant challenge to the preservation of gene expression patterns between cell generations. This article delves into the specific gene expression and chromatin regulatory mechanisms that facilitate the preservation of transcriptional identity during replication and mitosis. Furthermore, we emphasize our recent findings revealing the unconventional role of yeast centromeres and mitotic chromosomes in maintaining transcriptional fidelity beyond mitosis.

PARP1, DIDO3, and DHX9 Proteins Mutually Interact in Mouse Fibroblasts, with Effects on DNA Replication Dynamics, Senescence, and Oncogenic Transformation

The regulated formation and resolution of R-loops is a natural process in physiological gene expression. Defects in R-loop metabolism can lead to DNA replication stress, which is associated with a variety of diseases and, ultimately, with cancer. The proteins PARP1, DIDO3, and DHX9 are important players in R-loop regulation. We previously described the interaction between DIDO3 and DHX9. Here, we show that, in mouse embryonic fibroblasts, the three proteins are physically linked and dependent on PARP1 activity. The C-terminal truncation of DIDO3 leads to the impairment of this interaction; concomitantly, the cells show increased replication stress and senescence. DIDO3 truncation also renders the cells partially resistant to in vitro oncogenic transformation, an effect that can be reversed by immortalization. We propose that PARP1, DIDO3, and DHX9 proteins form a ternary complex that regulates R-loop metabolism, preventing DNA replication stress and subsequent senescence.

Aberrant R-loop-mediated immune evasion, cellular communication, and metabolic reprogramming affect cancer progression: a single-cell analysis

Dysregulation of R-loop homeostasis is closely related to various human diseases, including cancer. However, the causality of aberrant R-loops in tumor progression remains unclear. In this study, using single-cell RNA-sequencing datasets from lung adenocarcinoma (LUAD), we constructed an R-loop scoring model to characterize the R-loop state according to the identified R-loop regulators related to EGFR mutations, tissue origins, and TNM stage. We then evaluated the relationships of the R-loop score with the tumor microenvironment (TME) and treatment response. Furthermore, the potential roles of FANCI-mediated R-loops in LUAD were explored using a series of in vitro experiments. Results showed that malignant cells with low R-loop scores displayed glycolysis and epithelial-mesenchymal transition pathway activation and immune escape promotion, thereby hampering the antitumor therapeutic effects. Cell communication analysis suggested that low R-loop scores contributed to T cell exhaustion. We subsequently validated the prognostic value of R-loop scores by using bulk transcriptome datasets across 33 tumor types. The R-loop scoring model well predicted patients' therapeutic response to targeted therapy, chemotherapy, or immunotherapy in 32 independent cohorts. Remarkably, changes in R-loop distribution mediated by FANCI deficiency blocked the activity of Ras signaling pathway, suppressing tumor-cell proliferation and dissemination. In conclusion, this study reveals the underlying molecular mechanism of metabolic reprogramming and T cell exhaustion under R-loop score patterns, and the changes in R-loops mediated by R-loop regulators resulting in tumor progression. Therefore, incorporating anticancer methods based on R-loop or R-loop regulators into the treatment schemes of precision medicine may be beneficial.

PSIP1/LEDGF reduces R-loops at transcription sites to maintain genome integrity

R-loops that accumulate at transcription sites pose a persistent threat to genome integrity. PSIP1 is a chromatin protein associated with transcriptional elongation complex, possesses histone chaperone activity, and is implicated in recruiting RNA processing and DNA repair factors to transcription sites. Here, we show that PSIP1 interacts with R-loops and other proteins involved in R-loop homeostasis, including PARP1. Genome-wide mapping of PSIP1, R-loops and γ-H2AX in PSIP1-depleted human and mouse cell lines revealed an accumulation of R-loops and DNA damage at gene promoters in the absence of PSIP1. R-loop accumulation causes local transcriptional arrest and transcription-replication conflict, leading to DNA damage. PSIP1 depletion increases 53BP1 foci and reduces RAD51 foci, suggesting altered DNA repair choice. Furthermore, PSIP1 depletion increases the sensitivity of cancer cells to PARP1 inhibitors and DNA-damaging agents that induce R-loop-induced DNA damage. These findings provide insights into the mechanism through which PSIP1 maintains genome integrity at the site of transcription.

DNA damage and transcription stress

Genome damage and transcription are intimately linked. Tens to hundreds of thousands of DNA lesions arise in each cell each day, many of which can directly or indirectly impede transcription. Conversely, the process of gene expression is itself a source of endogenous DNA lesions as a result of the susceptibility of single-stranded DNA to damage, conflicts with the DNA replication machinery, and engagement by cells of topoisomerases and base excision repair enzymes to regulate the initiation and progression of gene transcription. Although such processes are tightly regulated and normally accurate, on occasion, they can become abortive and leave behind DNA breaks that can drive genome rearrangements, instability, or cell death.

Mechanisms of immune checkpoint inhibitors: insights into the regulation of circular RNAS involved in cancer hallmarks

Current treatment strategies for cancer, especially advanced cancer, are limited and unsatisfactory. One of the most substantial advances in cancer therapy, in the last decades, was the discovery of a new layer of immunotherapy approach, immune checkpoint inhibitors (ICIs), which can specifically activate immune cells by targeting immune checkpoints. Immune checkpoints are a type of immunosuppressive molecules expressed on immune cells, which can regulate the degree of immune activation and avoid autoimmune responses. ICIs, such as anti-PD-1/PD-L1 drugs, has shown inspiring efficacy and broad applicability across various cancers. Unfortunately, not all cancer patients benefit remarkably from ICIs, and the overall response rates to ICIs remain relatively low for most cancer types. Moreover, the primary and acquired resistance to ICIs pose serious challenges to the clinical application of cancer immunotherapy. Thus, a deeper understanding of the molecular biological properties and regulatory mechanisms of immune checkpoints is urgently needed to improve clinical options for current therapies. Recently, circular RNAs (circRNAs) have attracted increasing attention, not only due to their involvement in various aspects of cancer hallmarks, but also for their impact on immune checkpoints in shaping the tumor immune microenvironment. In this review, we systematically summarize the current status of immune checkpoints in cancer and the existing regulatory roles of circRNAs on immune checkpoints. Meanwhile, we also aim to settle the issue in an evidence-oriented manner that circRNAs involved in cancer hallmarks regulate the effects and resistance of ICIs by targeting immune checkpoints.

Causes and consequences of DNA single-strand breaks

DNA single-strand breaks (SSBs) are among the most common lesions arising in human cells, with tens to hundreds of thousands arising in each cell, each day. Cells have efficient mechanisms for the sensing and repair of these ubiquitous DNA lesions, but the failure of these processes to rapidly remove SSBs can lead to a variety of pathogenic outcomes. The threat posed by unrepaired SSBs is illustrated by the existence of at least six genetic diseases in which SSB repair (SSBR) is defective, all of which are characterised by neurodevelopmental and/or neurodegenerative pathology. Here, I review current understanding of how SSBs arise and impact on critical molecular processes, such as DNA replication and gene transcription, and their links to human disease.

Looping out of control: R-loops in transcription-replication conflict

Transcription-replication conflict is a major cause of replication stress that arises when replication forks collide with the transcription machinery. Replication fork stalling at sites of transcription compromises chromosome replication fidelity and can induce DNA damage with potentially deleterious consequences for genome stability and organismal health. The block to DNA replication by the transcription machinery is complex and can involve stalled or elongating RNA polymerases, promoter-bound transcription factor complexes, or DNA topology constraints. In addition, studies over the past two decades have identified co-transcriptional R-loops as a major source for impairment of DNA replication forks at active genes. However, how R-loops impede DNA replication at the molecular level is incompletely understood. Current evidence suggests that RNA:DNA hybrids, DNA secondary structures, stalled RNA polymerases, and condensed chromatin states associated with R-loops contribute to the of fork progression. Moreover, since both R-loops and replication forks are intrinsically asymmetric structures, the outcome of R-loop-replisome collisions is influenced by collision orientation. Collectively, the data suggest that the impact of R-loops on DNA replication is highly dependent on their specific structural composition. Here, we will summarize our current understanding of the molecular basis for R-loop-induced replication fork progression defects.

Genome maintenance meets mechanobiology

Genome stability is key for healthy cells in healthy organisms, and deregulated maintenance of genome integrity is a hallmark of aging and of age-associated diseases including cancer and neurodegeneration. To maintain a stable genome, genome surveillance and repair pathways are closely intertwined with cell cycle regulation and with DNA transactions that occur during transcription and DNA replication. Coordination of these processes across different time and length scales involves dynamic changes of chromatin topology, clustering of fragile genomic regions and repair factors into nuclear repair centers, mobilization of the nuclear cytoskeleton, and activation of cell cycle checkpoints. Here, we provide a general overview of cell cycle regulation and of the processes involved in genome duplication in human cells, followed by an introduction to replication stress and to the cellular responses elicited by perturbed DNA synthesis. We discuss fragile genomic regions that experience high levels of replication stress, with a particular focus on telomere fragility caused by replication stress at the ends of linear chromosomes. Using alternative lengthening of telomeres (ALT) in cancer cells and ALT-associated PML bodies (APBs) as examples of replication stress-associated clustered DNA damage, we discuss compartmentalization of DNA repair reactions and the role of protein properties implicated in phase separation. Finally, we highlight emerging connections between DNA repair and mechanobiology and discuss how biomolecular condensates, components of the nuclear cytoskeleton, and interfaces between membrane-bound organelles and membraneless macromolecular condensates may cooperate to coordinate genome maintenance in space and time.

Multifaceted Role of PARP1 in Maintaining Genome Stability Through Its Binding to Alternative DNA Structures

Alternative DNA structures that differ from the canonical B-form of DNA can arise from repetitive sequences and play beneficial roles in many cellular processes such as gene regulation and chromatin organization. However, they also threaten genomic stability in several ways including mutagenesis and collisions with replication and/or transcription machinery, which lead to genomic instability that is associated with human disease. Thus, the careful regulation of non-B-DNA structure formation and resolution is crucial for the maintenance of genome integrity. Several protein factors have been demonstrated to associate with alternative DNA structures to facilitate their removal, one of which is the ADP-ribose transferase (ART) PARP1 (also called ADP-ribosyltransferase diphtheria toxin-like 1 or ARTD1), a multifaceted DNA repair enzyme that recognizes single- and double-stranded DNA breaks and synthesizes chains of poly (ADP-ribose) (PAR) to recruit DNA repair proteins. It is now well appreciated that PARP1 recognizes several nucleic acid structures beyond DNA lesions, including stalled replication forks, DNA hairpins and cruciforms, R-loops, and DNA G-quadruplexes (G4 DNA). In this review, we summarize the current evidence of a direct association of PARP1 with each of these aforementioned alternative DNA structures, as well as discuss the role of PARP1 in the prevention of non-B-DNA structure-induced genetic instability. We will focus on the mechanisms of the recognition and binding by PARP1 to each alternative structure and the structure-based stimulation of PARP1 catalytic activity upon binding. Finally, we will discuss some of the outstanding gaps in the literature and offer speculative insight for questions that remain to be experimentally addressed.

MBD1 protects replication fork stability by recruiting PARP1 and controlling transcription-replication conflicts

The replication-stress response is essential to ensure the faithful transmission of genetic information to daughter cells. Although several stress-resolution pathways have been identified to deal with replication stress, the precise regulatory mechanisms for replication fork stability are not fully understood. Our study identified Methyl-CpG Binding Domain 1 (MBD1) as essential for the maintaining genomic stability and protecting stalled replication forks in mammalian cells. Depletion of MBD1 increases DNA lesions and sensitivity to replication stress. Mechanistically, we found that loss of MBD1 leads to the dissociation of Poly(ADP-ribose) polymerase 1 (PARP1) from the replication fork, potentially accelerating fork progression and resulting in higher levels of transcription-replication conflicts (T-R conflicts). Using a proximity ligation assay combined with 5-ethynyl-2'-deoxyuridine, we revealed that the MBD1 and PARP1 proteins were recruited to stalled forks under hydroxyurea (HU) treatment. In addition, our study showed that the level of R-loops also increased in MBD1-delated cells. Without MBD1, stalled replication forks resulting from T-R conflicts were primarily degraded by the DNA2 nuclease. Our findings shed light on a new aspect of MBD1 in maintaining genome stability and providing insights into the mechanisms underlying replication stress response.