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R-loops are cellular structures composed of an RNA/DNA hybrid, which is formed when the RNA hybridises to a complementary DNA strand and a displaced single-stranded DNA. R-loops have been detected in various organisms from bacteria to mammals and play crucial roles in regulating gene expression, DNA and histone modifications, immunoglobulin class switch recombination, DNA replication, and genome stability. Recent evidence suggests that R-loops are also involved in molecular mechanisms of neurological diseases and cancer. In addition, mutations in factors implicated in R-loop biology, such as RNase H and SETX (senataxin), lead to devastating human neurodegenerative disorders, highlighting the importance of correctly regulating the level of R-loops in human cells. In this review we summarise current advances in this field, with a particular focus on diseases associated with dysregulation of R-loop structures. We also discuss potential therapeutic approaches for such diseases and highlight future research directions.

Cancer is a disease associated with genomic instability that often results from oncogene activation. This in turn leads to hyperproliferation and replication stress. However, the molecular mechanisms that underlie oncogene-induced replication stress are still poorly understood. Oncogenes such as HRAS V12 promote proliferation by upregulating general transcription factors to stimulate RNA synthesis. Here we investigate whether this increase in transcription underlies oncogene-induced replication stress. We show that in cells overexpressing HRAS V12 , elevated expression of the general transcription factor TATA-box binding protein (TBP) leads to increased RNA synthesis, which together with R-loop accumulation results in replication fork slowing and DNA damage. Furthermore, overexpression of TBP alone causes the hallmarks of oncogene-induced replication stress, including replication fork slowing, DNA damage and senescence. Consequently, we reveal that increased transcription can be a mechanism of oncogene-induced DNA damage, providing a molecular link between upregulation of the transcription machinery and genomic instability in cancer. Cancer cells proliferate at high rates and incur replication stress. Here, the authors show that this can be the consequence of oncogene-induced higher transcriptional activity, which, through increased RNA synthesis and R-loop accumulation, results in replication fork slowing and DNA damage.

Friedreich ataxia (FRDA) and Fragile X syndrome (FXS) are among 40 diseases associated with expansion of repeated sequences (TREDs). Although their molecular pathology is not well understood, formation of repressive chromatin and unusual DNA structures over repeat regions were proposed to play a role. Our study now shows that RNA/DNA hybrids (R-loops) form in patient cells on expanded repeats of endogenous FXN and FMR1 genes, associated with FRDA and FXS. These transcription-dependent R-loops are stable, co-localise with repressive H3K9me2 chromatin mark and impede RNA Polymerase II transcription in patient cells. We investigated the interplay between repressive chromatin marks and R-loops on the FXN gene. We show that decrease in repressive H3K9me2 chromatin mark has no effect on R-loop levels. Importantly, increasing R-loop levels by treatment with DNA topoisomerase inhibitor camptothecin leads to up-regulation of repressive chromatin marks, resulting in FXN transcriptional silencing. This provides a direct molecular link between R-loops and the pathology of TREDs, suggesting that R-loops act as an initial trigger to promote FXN and FMR1 silencing. Thus R-loops represent a common feature of nucleotide expansion disorders and provide a new target for therapeutic interventions.

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.

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.

R-loops are nucleic acid structures formed by an RNA:DNA hybrid and unpaired single-stranded DNA that represent a source of genomic instability in mammalian cells 1 – 4 . Here we show that N 6 -methyladenosine (m 6 A) modification, contributing to different aspects of messenger RNA metabolism 5 , 6 , is detectable on the majority of RNA:DNA hybrids in human pluripotent stem cells. We demonstrate that m 6 A-containing R-loops accumulate during G 2 /M and are depleted at G 0 /G 1 phases of the cell cycle, and that the m 6 A reader promoting mRNA degradation, YTHDF2 (ref. 7 ), interacts with R-loop-enriched loci in dividing cells. Consequently, YTHDF2 knockout leads to increased R-loop levels, cell growth retardation and accumulation of γH2AX, a marker for DNA double-strand breaks, in mammalian cells. Our results suggest that m 6 A regulates accumulation of R-loops, implying a role for this modification in safeguarding genomic stability. N 6 -methyladenosine (m 6 A) is prevalent at RNA:DNA hybrids in human pluripotent stem cells. The m 6 A reader YTHDF2 interacts with R-loop-enriched loci in dividing cells, and YTHDF2 loss leads to increased R-loop levels and accumulation of γH2AX.

The single-stranded DNA cytosine-to-uracil deaminase APOBEC3B is an antiviral protein implicated in cancer. However, its substrates in cells are not fully delineated. Here APOBEC3B proteomics reveal interactions with a surprising number of R-loop factors. Biochemical experiments show APOBEC3B binding to R-loops in cells and in vitro. Genetic experiments demonstrate R-loop increases in cells lacking APOBEC3B and decreases in cells overexpressing APOBEC3B. Genome-wide analyses show major changes in the overall landscape of physiological and stimulus-induced R-loops with thousands of differentially altered regions, as well as binding of APOBEC3B to many of these sites. APOBEC3 mutagenesis impacts genes overexpressed in tumors and splice factor mutant tumors preferentially, and APOBEC3-attributed kataegis are enriched in RTCW motifs consistent with APOBEC3B deamination. Taken together with the fact that APOBEC3B binds single-stranded DNA and RNA and preferentially deaminates DNA, these results support a mechanism in which APOBEC3B regulates R-loops and contributes to R-loop mutagenesis in cancer. APOBEC3B interacts with R-loops and helps mediate their resolution in a deamination-dependent way. This association also renders R-loops susceptible to enhanced APOBEC3B-dependent mutagenesis.

MicroRNAs are processed by Drosha, and previous data had suggested that this occurred soon after transcription. Following Drosha recruitment and using nuclear run-on technology, data now indicate that miRNAs both within and outside of introns are processed co-transcriptionally and that exonucleases enter the fray to aid processing before splicing, implying that Drosha cleavage influences the maturation of the pre-mRNAs in which miRNAs reside. microRNAs (miRNAs) are generated from long primary (pri-) RNA polymerase II (Pol II)–derived transcripts by two RNase III processing reactions: Drosha cleavage of nuclear pri-miRNAs and Dicer cleavage of cytoplasmic pre-miRNAs. Here we show that Drosha cleavage occurs during transcription acting on both independently transcribed and intron-encoded miRNAs. We also show that both 5′-3′ and 3′-5′ exonucleases associate with the sites where co-transcriptional Drosha cleavage occurs, promoting intron degradation before splicing. We finally demonstrate that miRNAs can also derive from 3` flanking transcripts of Pol II genes. Our results demonstrate that multiple miRNA-containing transcripts are co-transcriptionally cleaved during their synthesis and suggest that exonucleolytic degradation from Drosha cleavage sites in pre-mRNAs may influence the splicing and maturation of numerous mRNAs.

Eukaryotic protein-encoding genes possess poly(A) signals that define the end of the messenger RNA and mediate downstream transcriptional termination by RNA polymerase II (Pol II) 1 . Termination could occur through an ‘anti-termination’ mechanism whereby elongation factors dissociate when the poly(A) signal is encountered, producing termination-competent Pol II 2 , 3 . An alternative ‘torpedo’ model postulated that poly(A) site cleavage provides an unprotected RNA 5′ end that is degraded by 5′ → 3′ exonuclease activities (torpedoes) and so induces dissociation of Pol II from the DNA template 1 , 4 . This model has been questioned because unprocessed transcripts read all the way to the site of transcriptional termination before upstream polyadenylation 5 , 6 , 7 . However, nascent transcripts located 1 kilobase downstream of the human β-globin gene poly(A) signal are associated with a co-transcriptional cleavage (CoTC) activity 8 that acts with the poly(A) signal to elicit efficient transcriptional termination. The CoTC sequence is an autocatalytic RNA structure that undergoes rapid self-cleavage 9 . Here we show that CoTC acts as a precursor to termination by presenting a free RNA 5′ end that is recognized by the human 5′ → 3′ exonuclease Xrn2. Degradation of the downstream cleavage product by Xrn2 results in transcriptional termination, as envisaged in the torpedo model.

R-loops are cellular structures composed of an RNA/DNA hybrid, which is formed when the RNA hybridises to a complementary DNA strand and a displaced single-stranded DNA. R-loops have been detected in various organisms from bacteria to mammals and play crucial roles in regulating gene expression, DNA and histone modifications, immunoglobulin class switch recombination, DNA replication, and genome stability. Recent evidence suggests that R-loops are also involved in molecular mechanisms of neurological diseases and cancer. In addition, mutations in factors implicated in R-loop biology, such as RNase H and SETX (senataxin), lead to devastating human neurodegenerative disorders, highlighting the importance of correctly regulating the level of R-loops in human cells. In this review we summarise current advances in this field, with a particular focus on diseases associated with dysregulation of R-loop structures. We also discuss potential therapeutic approaches for such diseases and highlight future research directions.