Human DNA topoisomerase I poisoning causes R loop-mediated genome instability attenuated by transcription factor IIS.

DNA topoisomerase I can contribute to cancer genome instability. During catalytic activity, topoisomerase I forms a transient intermediate, topoisomerase I-DNA cleavage complex (Top1cc) to allow strand rotation and duplex relaxation, which can lead to elevated levels of DNA-RNA hybrids and micronuclei. To comprehend the underlying mechanisms, we have integrated genomic data of Top1cc-triggered hybrids and DNA double-strand breaks (DSBs) shortly after Top1cc induction, revealing that Top1ccs increase hybrid levels with different mechanisms. DSBs are at highly transcribed genes in early replicating initiation zones and overlap with hybrids downstream of accumulated RNA polymerase II (RNAPII) at gene 5'-ends. A transcription factor IIS mutant impairing transcription elongation further increased RNAPII accumulation likely due to backtracking. Moreover, Top1ccs can trigger micronuclei when occurring during late G1 or early/mid S, but not during late S. As micronuclei and transcription-replication conflicts are attenuated by transcription factor IIS, our results support a role of RNAPII arrest in Top1cc-induced transcription-replication conflicts leading to DSBs and micronuclei.

TDP1 mutation causing SCAN1 neurodegenerative syndrome hampers the repair of transcriptional DNA double-strand breaks.

TDP1 removes transcription-blocking topoisomerase I cleavage complexes (TOP1ccs), and its inactivating H493R mutation causes the neurodegenerative syndrome SCAN1. However, the molecular mechanism underlying the SCAN1 phenotype is unclear. Here, we generate human SCAN1 cell models using CRISPR-Cas9 and show that they accumulate TOP1ccs along with changes in gene expression and genomic distribution of R-loops. SCAN1 cells also accumulate transcriptional DNA double-strand breaks (DSBs) specifically in the G1 cell population due to increased DSB formation and lack of repair, both resulting from abortive removal of transcription-blocking TOP1ccs. Deficient TDP1 activity causes increased DSB production, and the presence of mutated TDP1 protein hampers DSB repair by a TDP2-dependent backup pathway. This study provides powerful models to study TDP1 functions under physiological and pathological conditions and unravels that a gain of function of the mutated TDP1 protein, which prevents DSB repair, rather than a loss of TDP1 activity itself, could contribute to SCAN1 pathogenesis.

Immunofluorescence microscopy of G-quadruplexes and R-loops.

A large variety of non-B secondary structures can be formed between DNA and RNA. In this chapter, we focus on G-quadruplexes (G4) and R-loops, which can have a close structural interplay. In recent years, increasing evidence pointed to the fact that they can strongly influence each other in vivo, both having physiological and pathological roles in normal and cancer cells. Here, we detail specific and accurate methods for purification of BG4 and S9.6 antibodies, and their subsequent use in immunofluorescence microscopy, enabling single-cell analysis of extent and localization of G4s and R-loops.

Non-B DNA structures as a booster of genome instability.

Non-canonical secondary structures (NCSs) are alternative nucleic acid structures that differ from the canonical B-DNA conformation. NCSs often occur in repetitive DNA sequences and can adopt different conformations depending on the sequence. The majority of these structures form in the context of physiological processes, such as transcription-associated R-loops, G4s, as well as hairpins and slipped-strand DNA, whose formation can be dependent on DNA replication. It is therefore not surprising that NCSs play important roles in the regulation of key biological processes. In the last years, increasing published data have supported their biological role thanks to genome-wide studies and the development of bioinformatic prediction tools. Data have also highlighted the pathological role of these secondary structures. Indeed, the alteration or stabilization of NCSs can cause the impairment of transcription and DNA replication, modification in chromatin structure and DNA damage. These events lead to a wide range of recombination events, deletions, mutations and chromosomal aberrations, well-known hallmarks of genome instability which are strongly associated with human diseases. In this review, we summarize molecular processes through which NCSs trigger genome instability, with a focus on G-quadruplex, i-motif, R-loop, Z-DNA, hairpin, cruciform and multi-stranded structures known as triplexes.

MYCN Amplification, along with Wild-Type RB1 Expression, Enhances CDK4/6 Inhibitors' Efficacy in Neuroblastoma Cells.

Neuroblastoma (NB) is one of the primary causes of death for pediatric malignancies. Given the high heterogeneity in NB's mutation landscape, optimizing individualized therapies is still challenging. In the context of genomic alterations, MYCN amplification is the most correlated event with poor outcomes. MYCN is involved in the regulation of several cellular mechanisms, including cell cycle. Thus, studying the influence of MYCN overexpression in the G1/S transition checkpoint of the cell cycle may unveil novel druggable targets for the development of personalized therapeutical approaches. Here, we show that high expression of E2F3 and MYCN correlate with poor prognosis in NB despite the RB1 mRNA levels. Moreover, we demonstrate through luciferase reporter assays that MYCN bypasses RB function by incrementing E2F3-responsive promoter activity. We showed that MYCN overexpression leads to RB inactivation by inducing RB hyperphosphorylation during the G1 phase through cell cycle synchronization experiments. Moreover, we generated two MYCN-amplified NB cell lines conditionally knockdown (cKD) for the RB1 gene through a CRISPRi approach. Indeed, RB KD did not affect cell proliferation, whereas cell proliferation was strongly influenced when a non-phosphorylatable RB mutant was expressed. This finding revealed the dispensable role of RB in regulating MYCN-amplified NB's cell cycle. The described genetic interaction between MYCN and RB1 provides the rationale for using cyclin/CDK complexes inhibitors in NBs carrying MYCN amplification and relatively high levels of RB1 expression.

Expression of down-regulated ERV LTR elements associates with immune activation in human small-cell lung cancers.

Small-cell lung cancer (SCLC) is an aggressive cancer characterized by immunosuppressive features leading to poor responses to current immunotherapies. Activation of transposable elements (TE) can trigger an innate immune response, which can synergize with immunotherapeutic protocols in patients. However, TE activity in relation to immune gene response is not fully known in human SCLC. Here, we compared TE expression in 104 human SCLC and 24 normal tissues and established their involvement in innate immune responses. We observed that different intergenic TEs, mainly endogenous retroviral (ERV) families, are deregulated in SCLC. Similarly to other cancers, we detected a subset of LTRs that correlate with innate immune gene signatures and cytosolic RNA sensors, such as RIG-I. These LTRs are downregulated in SCLC tumors vs. normal tissues, and are mainly located at transcriptional repressed regions, marked with H3K4me2 in different cell lines. Analyses of different genomic datasets show that chromatin repression is likely due to de-methylase LSD1 activity. Moreover, high expression levels of ERV LTRs predict a better survival upon chemotherapy of SCLC patients. The findings reveal a specific pattern of TE-mediated activation of innate immune genes in SCLC, which can be exploited to establish more effective immunotherapeutic combinations.

Ligands stimulating antitumour immunity as the next G-quadruplex challenge.

G-quadruplex (G4) binders have been investigated to discover new anticancer drugs worldwide in past decades. As these ligands are generally not highly cytotoxic, the discovery rational was mainly based on increasing the cell-killing potency. Nevertheless, no G4 binder has been shown yet to be effective in cancer patients. Here, G4 binder activity at low dosages will be discussed as a critical feature to discover ligands with therapeutic effects in cancer patients. Specific effects of G4 binders al low doses have been reported to occur in cancer and normal cells. Among them, genome instability and the stimulation of cytoplasmic processes related to autophagy and innate immune response open to the use of G4 binders as immune-stimulating agents. Thus, we propose a new rational of drug discovery, which is not based on cytotoxic potency but rather on immune gene activation at non-cytotoxic dosage.

Balancing Affinity, Selectivity, and Cytotoxicity of Hydrazone-Based G-Quadruplex Ligands for Activation of Interferon ß Genes in Cancer Cells.

G-quadruplex (G4) ligands are investigated to discover new anticancer drugs with increased cell-killing potency. These ligands can induce genome instability and activate innate immune genes at non-cytotoxic doses, opening the discovery of cytostatic immune-stimulating ligands. However, the interplay of G4 affinity/selectivity with cytotoxicity and immune gene activation is not well-understood. We investigated a series of closely related hydrazone derivatives to define the molecular bases of immune-stimulation activity. Although they are closely related to each other, such derivatives differ in G4 affinity, cytotoxicity, genome instability, and immune gene activation. Our findings show that G4 affinity of ligands is a critical feature for immune gene activation, whereas a high cytotoxic potency interferes with it. The balance of G4 stabilization versus cytotoxicity can determine the level of immune gene activation in cancer cells. Thus, we propose a new rationale based on low cell-killing potency and high immune stimulation to discover effective anticancer G4 ligands.

Topoisomerase I poison-triggered immune gene activation is markedly reduced in human small-cell lung cancers by impairment of the cGAS/STING pathway.

BACKGROUND: Current immunotherapy strategies have contrasting clinical results in human lung cancer patients as small-cell lung cancers (SCLC) often show features of immunological cold tumours. Topoisomerase 1 (TOP1) poisons are effective antitumor drugs with good efficacy against lung cancers. METHODS: We used molecular, genetic and bioinformatic approaches to determine the mechanism of micronuclei formation induced by two TOP1 poisons in different human cancer cells, including SCLC cell lines. RESULTS: TOP1 poisons stimulate similar levels of micronuclei in all tested cell lines but downstream effects can vary markedly. TOP1 poisons increase micronuclei levels with a mechanism involving R-loops as overexpression of RNaseH1 markedly reduces or abolishes both H2AX phosphorylation and micronuclei formation. TOP1 poison-induced micronuclei activate the cGAS/STING pathway leading to increased expression of immune genes in HeLa cells, but not in human SCLC cell lines, mainly due to lack of STING and/or cGAS expression. Moreover, the expression of STING and antigen-presenting machinery genes is generally downregulated in patient tumours of human lung cancer datasets. CONCLUSIONS: Altogether, our data reveal an immune signalling mechanism activated by TOP1 poisons, which is often impaired in human SCLC tumours.

Highly Purified Top1-Bound DNA Fragments.

Topoisomerase 1-DNA cleavage complexes (Top1ccs) form at DNA sites of Top1 activity and are increased by a highly specific anticancer drug (camptothecin, CPT) in living cells. Various methods are available to detect Top1ccs in cultured cells, including protocols based on the use of specific antibodies. Here, we describe a protocol to isolate Top1ccs at high purity, which does not depend on antibodies.

G-quadruplex binders as cytostatic modulators of innate immune genes in cancer cells.

G-quadruplexes (G4s) are non-canonical nucleic acid structures involved in fundamental biological processes. As G4s are promising anticancer targets, in past decades the search for effective anticancer G4 binders aimed at the discovery of more cytotoxic ligands interfering with specific G4 structures at oncogenes or telomeres. Here, we have instead observed a significant activation of innate immune genes by two unrelated ligands at non-cytotoxic concentrations. The studied G4 binders (pyridostatin and PhenDC3) can induce an increase of micronuclei triggering the activation of the cytoplasmic STING (stimulator of interferon response cGAMP interactor 1) signaling pathway in human and murine cancer cells. Ligand activity can then lead to type I interferon production and innate immune gene activation. Moreover, specific gene expression patterns mediated by a G4 binder in cancer cells correlate with immunological hot features and better survival in human TCGA (The Cancer Genome Atlas) breast tumors. The findings open to the development of cytostatic G4 binders as effective immunomodulators for combination immunotherapies in unresponsive tumors.

G-quadruplex-R-loop interactions and the mechanism of anticancer G-quadruplex binders.

Genomic DNA and cellular RNAs can form a variety of non-B secondary structures, including G-quadruplex (G4) and R-loops. G4s are constituted by stacked guanine tetrads held together by Hoogsteen hydrogen bonds and can form at key regulatory sites of eukaryote genomes and transcripts, including gene promoters, untranslated exon regions and telomeres. R-loops are 3-stranded structures wherein the two strands of a DNA duplex are melted and one of them is annealed to an RNA. Specific G4 binders are intensively investigated to discover new effective anticancer drugs based on a common rationale, i.e.: the selective inhibition of oncogene expression or specific impairment of telomere maintenance. However, despite the high number of known G4 binders, such a selective molecular activity has not been fully established and several published data point to a different mode of action. We will review published data that address the close structural interplay between G4s and R-loops in vitro and in vivo, and how these interactions can have functional consequences in relation to G4 binder activity. We propose that R-loops can play a previously-underestimated role in G4 binder action, in relation to DNA damage induction, telomere maintenance, genome and epigenome instability and alterations of gene expression programs.

Monohydrazone Based G-Quadruplex Selective Ligands Induce DNA Damage and Genome Instability in Human Cancer Cells.

Targeting G-quadruplex structures is currently viewed as a promising anticancer strategy. Searching for potent and selective G-quadruplex binders, here we describe a small series of new monohydrazone derivatives designed as analogues of a lead which was proved to stabilize G-quadruplex structures and increase R loop levels in human cancer cells. To investigate the G-quadruplex binding properties of the new molecules, in vitro biophysical studies were performed employing both telomeric and oncogene promoter G-quadruplex-forming sequences. The obtained results allowed the identification of a highly selective G-quadruplex ligand that, when studied in human cancer cells, proved to be able to stabilize both G-quadruplexes and R loops and showed a potent cell killing activity associated with the formation of micronuclei, a clear sign of genome instability.

Dual Processing of R-Loops and Topoisomerase I Induces Transcription-Dependent DNA Double-Strand Breaks.

Although accumulation of DNA damage and genomic instability in resting cells can cause neurodegenerative disorders, our understanding of how transcription produces DNA double-strand breaks (DSBs) is limited. Transcription-blocking topoisomerase I cleavage complexes (TOP1ccs) are frequent events that prime DSB production in non-replicating cells. Here, we report a mechanism of their formation by showing that they arise from two nearby single-strand breaks (SSBs) on opposing DNA strands: one SSB from the removal of transcription-blocking TOP1ccs by the TDP1 pathway and the other from the cleavage of R-loops by endonucleases, including XPF, XPG, and FEN1. Genetic defects in TOP1cc removal (TDP1, PNKP, and XRCC1) or in the resolution of R-loops (SETX) enhance DSB formation and prevent their repair. Such deficiencies cause neurological disorders. Owing to the high frequency of TOP1cc trapping and the widespread distribution of R-loops, these persistent transcriptional DSBs could accumulate over time in neuronal cells, contributing to the neurodegenerative diseases.

DROPA: DRIP-seq optimized peak annotator.

BACKGROUND: R-loops are three-stranded nucleic acid structures that usually form during transcription and that may lead to gene regulation or genome instability. DRIP (DNA:RNA Immunoprecipitation)-seq techniques are widely used to map R-loops genome-wide providing insights into R-loop biology. However, annotation of DRIP-seq peaks to genes can be a tricky step, due to the lack of strand information when using the common basic DRIP technique. RESULTS: Here, we introduce DRIP-seq Optimized Peak Annotator (DROPA), a new tool for gene annotation of R-loop peaks based on gene expression information. DROPA allows a full customization of annotation options, ranging from the choice of reference datasets to gene feature definitions. DROPA allows to assign R-loop peaks to the DNA template strand in gene body with a false positive rate of less than 7%. A comparison of DROPA performance with three widely used annotation tools show that it identifies less false positive annotations than the others. CONCLUSIONS: DROPA is a fully customizable peak-annotation tool optimized for co-transcriptional DRIP-seq peaks, which allows a finest gene annotation based on gene expression information. Its output can easily be integrated into pipelines to perform downstream analyses, while useful and informative summary plots and statistical enrichment tests can be produced.

DNA damage and genome instability by G-quadruplex ligands are mediated by R loops in human cancer cells.

G quadruplexes (G4s) and R loops are noncanonical DNA structures that can regulate basic nuclear processes and trigger DNA damage, genome instability, and cell killing. By different technical approaches, we here establish that specific G4 ligands stabilize G4s and simultaneously increase R-loop levels within minutes in human cancer cells. Genome-wide mapping of R loops showed that the studied G4 ligands likely cause the spreading of R loops to adjacent regions containing G4 structures, preferentially at 3'-end regions of expressed genes, which are partially ligand-specific. Overexpression of an exogenous human RNaseH1 rescued DNA damage induced by G4 ligands in BRCA2-proficient and BRCA2-silenced cancer cells. Moreover, even if the studied G4 ligands increased noncanonical DNA structures at similar levels in nuclear chromatin, their cellular effects were different in relation to cell-killing activity and stimulation of micronuclei, a hallmark of genome instability. Our findings therefore establish that G4 ligands can induce DNA damage by an R loop-dependent mechanism that can eventually lead to different cellular consequences depending on the chemical nature of the ligands.

Anthracyclines as Topoisomerase II Poisons: From Early Studies to New Perspectives.

Mammalian DNA topoisomerases II are targets of anticancer anthracyclines that act by stabilizing enzyme-DNA complexes wherein DNA strands are cut and covalently linked to the protein. This molecular mechanism is the molecular basis of anthracycline anticancer activity as well as the toxic effects such as cardiomyopathy and induction of secondary cancers. Even though anthracyclines have been used in the clinic for more than 50 years for solid and blood cancers, the search of breakthrough analogs has substantially failed. The recent developments of personalized medicine, availability of individual genomic information, and immune therapy are expected to change significantly human cancer therapy. Here, we discuss the knowledge of anthracyclines as Topoisomerase II poisons, their molecular and cellular effects and toxicity along with current efforts to improve the therapeutic index. Then, we discuss the contribution of the immune system in the anticancer activity of anthracyclines, and the need to increase our knowledge of molecular mechanisms connecting the drug targets to the immune stimulatory pathways in cancer cells. We propose that the complete definition of the molecular interaction of anthracyclines with the immune system may open up more effective and safer ways to treat patients with these drugs.