The chromatin network helps prevent cancer-associated mutagenesis at transcription-replication conflicts.

Genome instability is a feature of cancer cells, transcription being an important source of DNA damage. This is in large part associated with R-loops, which hamper replication, especially at head-on transcription-replication conflicts (TRCs). Here we show that TRCs trigger a DNA Damage Response (DDR) involving the chromatin network to prevent genome instability. Depletion of the key chromatin factors INO80, SMARCA5 and MTA2 results in TRCs, fork stalling and R-loop-mediated DNA damage which mostly accumulates at S/G2, while histone H3 Ser10 phosphorylation, a mark of chromatin compaction, is enriched at TRCs. Strikingly, TRC regions show increased mutagenesis in cancer cells with signatures of homologous recombination deficiency, transcription-coupled nucleotide excision repair (TC-NER) and of the AID/APOBEC cytidine deaminases, being predominant at head-on collisions. Thus, our results support that the chromatin network prevents R-loops and TRCs from genomic instability and mutagenic signatures frequently associated with cancer.

Centriolar subdistal appendages promote double-strand break repair through homologous recombination.

The centrosome is a cytoplasmic organelle with roles in microtubule organization that has also been proposed to act as a hub for cellular signaling. Some centrosomal components are required for full activation of the DNA damage response. However, whether the centrosome regulates specific DNA repair pathways is not known. Here, we show that centrosome presence is required to fully activate recombination, specifically to completely license its initial step, the so-called DNA end resection. Furthermore, we identify a centriolar structure, the subdistal appendages, and a specific factor, CEP170, as the critical centrosomal component involved in the regulation of recombination and resection. Cells lacking centrosomes or depleted for CEP170 are, consequently, hypersensitive to DNA damaging agents. Moreover, low levels of CEP170 in multiple cancer types correlate with an increase of the mutation burden associated with specific mutational signatures and a better prognosis, suggesting that changes in CEP170 can act as a mutation driver but could also be targeted to improve current oncological treatments.

The role of chromatin at transcription-replication conflicts as a genome safeguard.

DNA replication ensures the correct copying of the genome and the faithful transfer of the genetic information to the offspring. However, obstacles to replication fork (RF) progression cause RF stalling and compromise efficient genome duplication. Since replication uses the same DNA template as transcription, both transcription and replication must be coordinated to prevent Transcription-Replication Conflicts (TRCs) that could stall RF progression. Several factors contribute to limit the occurrence of such conflicts and their harmful impact on genome integrity. Increasing evidence indicates that chromatin homeostasis plays a key role in the cellular response to TRCs as well as in the preservation of genome integrity. Indeed, chromatin regulating enzymes are frequently mutated in cancer cells, a common characteristic of which is genome instability. Therefore, understanding the role of chromatin in TRC occurrence and resolution may help identify the molecular mechanism by which chromatin protects genome integrity, and the causes and physiological relevance of the high mutation rates of chromatin regulating factors in cancer. Here we review the current knowledge in the field, as well as the perspectives and future applications.

The SWI/SNF complex, transcription-replication conflicts and cancer: a connection with high therapeutic potential.

Genome instability is a hallmark of cancer. ATP-dependent chromatin remodelers are frequently altered in cancer. We have recently reported that the SWItch/Sucrose Non-Fermentable (SWI/SNF) complex protects the genome by limiting R-loop-mediated genome instability, mainly that caused by transcription-replication conflicts. Here we discuss the significance and biomedical applications of this finding.

The SWI/SNF chromatin remodeling complex helps resolve R-loop-mediated transcription-replication conflicts.

ATP-dependent chromatin remodelers are commonly mutated in human cancer. Mammalian SWI/SNF complexes comprise three conserved multisubunit chromatin remodelers (cBAF, ncBAF and PBAF) that share the BRG1 (also known as SMARCA4) subunit responsible for the main ATPase activity. BRG1 is the most frequently mutated Snf2-like ATPase in cancer. In the present study, we have investigated the role of SWI/SNF in genome instability, a hallmark of cancer cells, given its role in transcription, DNA replication and DNA-damage repair. We show that depletion of BRG1 increases R-loops and R-loop-dependent DNA breaks, as well as transcription-replication (T-R) conflicts. BRG1 colocalizes with R-loops and replication fork blocks, as determined by FANCD2 foci, with BRG1 depletion being epistatic to FANCD2 silencing. Our study, extended to other components of SWI/SNF, uncovers a key role of the SWI/SNF complex, in particular cBAF, in helping resolve R-loop-mediated T-R conflicts, thus, unveiling a new mechanism by which chromatin remodeling protects genome integrity.

TDP-43 mutations link Amyotrophic Lateral Sclerosis with R-loop homeostasis and R loop-mediated DNA damage.

TDP-43 is a DNA and RNA binding protein involved in RNA processing and with structural resemblance to heterogeneous ribonucleoproteins (hnRNPs), whose depletion sensitizes neurons to double strand DNA breaks (DSBs). Amyotrophic Lateral Sclerosis (ALS) is a neurodegenerative disorder, in which 97% of patients are familial and sporadic cases associated with TDP-43 proteinopathies and conditions clearing TDP-43 from the nucleus, but we know little about the molecular basis of the disease. After showing with the non-neuronal model of HeLa cells that TDP-43 depletion increases R loops and associated genome instability, we prove that mislocalization of mutated TDP-43 (A382T) in transfected neuronal SH-SY5Y and lymphoblastoid cell lines (LCLs) from an ALS patient cause R-loop accumulation, R loop-dependent increased DSBs and Fanconi Anemia repair centers. These results uncover a new role of TDP-43 in the control of co-transcriptional R loops and the maintenance of genome integrity by preventing harmful R-loop accumulation. Our findings thus link TDP-43 pathology to increased R loops and R loop-mediated DNA damage opening the possibility that R-loop modulation in TDP-43-defective cells might help develop ALS therapies.

UAP56/DDX39B is a major cotranscriptional RNA-DNA helicase that unwinds harmful R loops genome-wide.

Nonscheduled R loops represent a major source of DNA damage and replication stress. Cells have different ways to prevent R-loop accumulation. One mechanism relies on the conserved THO complex in association with cotranscriptional RNA processing factors including the RNA-dependent ATPase UAP56/DDX39B and histone modifiers such as the SIN3 deacetylase in humans. We investigated the function of UAP56/DDX39B in R-loop removal. We show that UAP56 depletion causes R-loop accumulation, R-loop-mediated genome instability, and replication fork stalling. We demonstrate an RNA-DNA helicase activity in UAP56 and show that its overexpression suppresses R loops and genome instability induced by depleting five different unrelated factors. UAP56/DDX39B localizes to active chromatin and prevents the accumulation of RNA-DNA hybrids over the entire genome. We propose that, in addition to its RNA processing role, UAP56/DDX39B is a key helicase required to eliminate harmful cotranscriptional RNA structures that otherwise would block transcription and replication.

Human THO-Sin3A interaction reveals new mechanisms to prevent R-loops that cause genome instability.

R-loops, formed by co-transcriptional DNA-RNA hybrids and a displaced DNA single strand (ssDNA), fulfill certain positive regulatory roles but are also a source of genomic instability. One key cellular mechanism to prevent R-loop accumulation centers on the conserved THO/TREX complex, an RNA-binding factor involved in transcription elongation and RNA export that contributes to messenger ribonucleoprotein (mRNP) assembly, but whose precise function is still unclear. To understand how THO restrains harmful R-loops, we searched for new THO-interacting factors. We found that human THO interacts with the Sin3A histone deacetylase complex to suppress co-transcriptional R-loops, DNA damage, and replication impairment. Functional analyses show that histone hypo-acetylation prevents accumulation of harmful R-loops and RNA-mediated genomic instability. Diminished histone deacetylase activity in THO- and Sin3A-depleted cell lines correlates with increased R-loop formation, genomic instability, and replication fork stalling. Our study thus uncovers physical and functional crosstalk between RNA-binding factors and chromatin modifiers with a major role in preventing R-loop formation and RNA-mediated genome instability.

Linker histone H1 prevents R-loop accumulation and genome instability in heterochromatin.

Linker histone H1 is an important structural component of chromatin that stabilizes the nucleosome and compacts the nucleofilament into higher-order structures. The biology of histone H1 remains, however, poorly understood. Here we show that Drosophila histone H1 (dH1) prevents genome instability as indicated by the increased γH2Av (H2AvS137P) content and the high incidence of DNA breaks and sister-chromatid exchanges observed in dH1-depleted cells. Increased γH2Av occurs preferentially at heterochromatic elements, which are upregulated upon dH1 depletion, and is due to the abnormal accumulation of DNA:RNA hybrids (R-loops). R-loops accumulation is readily detectable in G1-phase, whereas γH2Av increases mainly during DNA replication. These defects induce JNK-mediated apoptosis and are specific of dH1 depletion since they are not observed when heterochromatin silencing is relieved by HP1a depletion. Altogether, our results suggest that histone H1 prevents R-loops-induced DNA damage in heterochromatin and unveil its essential contribution to maintenance of genome stability.While structural importance of linker histone H1 in packaging eukaryotic genome into chromatin is well known, its biological function remains poorly understood. Here the authors reveal that Drosophila linker histone H1 prevents DNA:RNA hybrids accumulation and genome instability in heterochromatin.

Histone H1: Lessons from Drosophila.

Eukaryotic genomes are structured in the form of chromatin with the help of a set of five small basic proteins, the histones. Four of them are highly conserved through evolution, form the basic unit of the chromatin, the nucleosome, and have been intensively studied and are well characterized. The fifth histone, histone H1, adds to this basic structure through its interaction at the entry/exit site of DNA in the nucleosome and makes an essential contribution to the higher order folding of the chromatin fiber. Histone H1 is the less conserved histone and the less known of them. Though for long time considered as a general repressor of gene expression, recent studies in Drosophila have rejected this view and have contributed to uncover important functions on genome stability and development. Here we present some of the most recent data obtained in the Drosophila model system and discuss how the lessons learnt in these studies compare and could be applied to all other eukaryotes.