Mechanisms of DNA Replication and Repair: Insights from the Study of G-Quadruplexes

Chronic RNA G-quadruplex accumulation in aging and Alzheimer's disease

As the world population ages, new molecular targets in aging and Alzheimer's disease (AD) are needed to combat the expected influx of new AD cases. Until now, the role of RNA structure in aging and neurodegeneration has largely remained unexplored. In this study, we examined human hippocampal postmortem tissue for the formation of RNA G-quadruplexes (rG4s) in aging and AD. We found that rG4 immunostaining strongly increased in the hippocampus with both age and with AD severity. We further found that neurons with the accumulation of phospho-tau immunostaining contained rG4s, rG4 structure can drive tau aggregation, and rG4 staining density depended on APOE genotype in the human tissue examined. Combined with previous studies showing the dependence of rG4 structure on stress and the extreme power of rG4s at oligomerizing proteins, we propose a model of neurodegeneration in which chronic rG4 formation is linked to proteostasis collapse. These morphological findings suggest that further investigation of RNA structure in neurodegeneration is a critical avenue for future treatments and diagnoses.

Exploring the Electronic Interactions of Adenine, Cytosine, and Guanine with Graphene: A DFT Study

This study has provided new insights into the interaction between graphene and DNA nucleobases (adenine, cytosine, and guanine). It compares how each nucleobase interacts with graphene, examining their selectivity and binding energy. The research also explores how these interactions impact the electronic properties of graphene, showing potential applications in graphene-based biosensors and DNA sequencing technologies. Additionally, the findings suggest potential uses in DNA sensing and the functionalization of graphene for various biomedical applications. This study employs density functional theory (DFT) methods, utilizing the B3LYP functional with the 6-311G basis set, to explore the electronic interactions between DNA nucleobases (adenine, cytosine, and guanine) with pure graphene (Gr). We investigate various properties, including adsorption energy, HOMO-LUMO energy levels, charge transfer mechanisms, dipole moments, energy gaps, and density of states (DOS). Our findings indicate that cytosine interacts most favorably with graphene through its oxygen site (Gr-Cyt-O), exhibiting the strongest adsorption. Additionally, adenine's interaction significantly enhances its electronegativity and chemical potential, particularly at the nitrogen position, while decreasing its electrophilicity. Guanine, characterized by the smallest energy gap, demonstrates the highest conductivity among the nucleobases. These results suggest that graphene possesses advantageous properties as an adsorbent for guanine, highlighting its potential applications in biosensor technology.

Oncogenic and telomeric G-quadruplexes: Targets for porphyrin-triphenylphosphonium conjugates

DNA chains with sequential guanine (G) repeats can lead to the formation of G-quadruplexes (G4), which are found in functional DNA and RNA regions like telomeres and oncogene promoters. The development of molecules with adequate structural features to selectively stabilize G4 structures can counteract cell immortality, highly described for cancer cells, and also downregulate transcription events underlying cell apoptosis and/or senescence processes. We describe here, the efficiency of four highly charged porphyrins-phosphonium conjugates to act as G4 stabilizing agents. The spectrophotometric results allowed to select the conjugates P2-PPh3 and P3-PPh3 as the most promising ones to stabilize selectively G4 structures. Molecular dynamics simulation experiments were performed and support the preferential binding of P2-PPh3 namely to MYC and of P3-PPh3 to KRAS. The ability of both ligands to block the activity of Taq polymerase was confirmed and also their higher cytotoxicity against the two melanoma cell lines A375 and SK-MEL-28 than to immortalized skin keratinocytes. Both ligands present efficient cellular uptake, nuclear co-localization and high ability to generate 1O2 namely when interacting with G4 structure. The obtained data points the synthesized porphyrins as promising ligands to be used in a dual approach that can combine G4 stabilization and Photodynamic therapy (PDT).

G-quadruplex as an essential structural element in cytomegalovirus replication origin

G-quadruplex (G4) structures are found in eukaryotic cell replication origins, but their role in origin function remains unclear. In this study G4 motifs are found in the lytic DNA replication origin (oriLyt) of human cytomegalovirus (HCMV) and recombinant viruses show that a G4 motif in oriLyt essential region I (ER-I) is necessary for viral growth. Replication assays of oriLyt-containing plasmids and biochemical/biophysical analyses show that G4 formation in ER-I is crucial for viral DNA replication. G4 pull-down analysis identifies viral DNA replication factors, such as IE2, UL84, and UL44, as G4-binding proteins. In enzyme-linked immunosorbent assays, specific G4-binding ligands inhibit G4 binding by the viral proteins. The Epstein-Barr virus oriLyt core element also forms a stable G4 that could substitute for the oriLyt ER-I G4 in HCMV. These results demonstrate that viral G4s in replication origins represent an essential structural element in recruiting replication factors and might be a therapeutic target against viral infections.

Enhanced recognition of G-quadruplex DNA oxidative damage based on DNA-mediated charge transfer

G-quadruplex (G4) DNA is present in human telomere oligonucleotide sequences. Oxidative damage to telomeric DNA accelerates telomere shortening, which is strongly associated with aging and cancer. Most of the current analyses on oxidative DNA damage are based on ds-DNA. Here, we developed a electrochemiluminescence (ECL) probe for enhanced recognition of oxidative damage in G4-DNA based on DNA-mediated charge transfer (CT), which could specifically recognize damaged sites depending on the position of 8-oxoguanine (8-oxoG). First, a uniform G4-DNA monolayer interface was fabricated; the G4-DNA mediated CT properties were examined using an iridium(III) complex [Ir(ppy)2(pip)]PF6 stacked with G4-DNA as an indicator. The results showed that G4-DNA with 8-oxoG attenuated DNA CT. The topological effects of oxidative damage at different sites of G4-DNA and their effects on DNA CT were revealed. The sensing platform was also used for the sensitive and quantitative detection of 8-oxoG in G4-DNA, with a detection limit of 28.9 fmol. Overall, these findings present a sensitive platform to study G4-DNA structural and stability changes caused by oxidative damage as well as the specific and quantitative detection of oxidation sites. The different damage sites in the G-quadruplex could provide detailed clues for understanding the function of G4-associated telomere functional enzymes.

Poly (ADP-ribose) polymerase inhibitor therapy and mechanisms of resistance in epithelial ovarian cancer

Advanced epithelial ovarian cancer is the commonest cause of gynaecological cancer deaths. First-line treatment for advanced disease includes a combination of platinum-taxane chemotherapy (post-operatively or peri-operatively) and maximal debulking surgery whenever feasible. Initial response rate to chemotherapy is high (up to 80%) but most patients will develop recurrence (approximately 70-90%) and succumb to the disease. Recently, poly-ADP-ribose polymerase (PARP) inhibition (by drugs such as Olaparib, Niraparib or Rucaparib) directed synthetic lethality approach in BRCA germline mutant or platinum sensitive disease has generated real hope for patients. PARP inhibitor (PARPi) maintenance therapy can prolong survival but therapeutic response is not sustained due to intrinsic or acquired secondary resistance to PARPi therapy. Reversion of BRCA1/2 mutation can lead to clinical PARPi resistance in BRCA-germline mutated ovarian cancer. However, in the more common platinum sensitive sporadic HGSOC, the clinical mechanisms of development of PARPi resistance remains to be defined. Here we provide a comprehensive review of the current status of PARPi and the mechanisms of resistance to therapy.

KDM6A-SND1 interaction maintains genomic stability by protecting the nascent DNA and contributes to cancer chemoresistance

Genomic instability is one of the hallmarks of cancer. While loss of histone demethylase KDM6A increases the risk of tumorigenesis, its specific role in maintaining genomic stability remains poorly understood. Here, we propose a mechanism in which KDM6A maintains genomic stability independently on its demethylase activity. This occurs through its interaction with SND1, resulting in the establishment of a protective chromatin state that prevents replication fork collapse by recruiting of RPA and Ku70 to nascent DNA strand. Notably, KDM6A-SND1 interaction is up-regulated by KDM6A SUMOylation, while KDM6AK90A mutation almost abolish the interaction. Loss of KDM6A or SND1 leads to increased enrichment of H3K9ac and H4K8ac but attenuates the enrichment of Ku70 and H3K4me3 at nascent DNA strand. This subsequently results in enhanced cellular sensitivity to genotoxins and genomic instability. Consistent with these findings, knockdown of KDM6A and SND1 in esophageal squamous cell carcinoma (ESCC) cells increases genotoxin sensitivity. Intriguingly, KDM6A H101D & P110S, N1156T and D1216N mutations identified in ESCC patients promote genotoxin resistance via increased SND1 association. Our finding provides novel insights into the pivotal role of KDM6A-SND1 in genomic stability and chemoresistance, implying that targeting KDM6A and/or its interaction with SND1 may be a promising strategy to overcome the chemoresistance.

G-Quadruplex Forming DNA Sequence Context Is Enriched around Points of Somatic Mutations in a Subset of Multiple Myeloma Patients

Multiple myeloma (MM) is the second most common hematological malignancy, which remains incurable despite recent advances in treatment strategies. Like other forms of cancer, MM is characterized by genomic instability, caused by defects in DNA repair. Along with mutations in DNA repair genes and genotoxic drugs used to treat MM, non-canonical secondary DNA structures (four-stranded G-quadruplex structures) can affect accumulation of somatic mutations and chromosomal abnormalities in the tumor cells of MM patients. Here, we tested the hypothesis that G-quadruplex structures may influence the distribution of somatic mutations in the tumor cells of MM patients. We sequenced exomes of normal and tumor cells of 11 MM patients and analyzed the data for the presence of G4 context around points of somatic mutations. To identify molecular mechanisms that could affect mutational profile of tumors, we also analyzed mutational signatures in tumor cells as well as germline mutations for the presence of specific SNPs in DNA repair genes or in genes regulating G-quadruplex unwinding. In several patients, we found that sites of somatic mutations are frequently located in regions with G4 context. This pattern correlated with specific germline variants found in these patients. We discuss the possible implications of these variants for mutation accumulation and specificity in MM and propose that the extent of G4 context enrichment around somatic mutation sites may be a novel metric characterizing mutational processes in tumors.

The translocon protein FtsHi1 is an ATP-dependent DNA/RNA helicase that prevents R-loop accumulation in chloroplasts

R-loops, three-stranded nucleic acid structures consisting of a DNA: RNA hybrid and displaced single-stranded DNA, play critical roles in gene expression and genome stability. How R-loop homeostasis is integrated into chloroplast gene expression remains largely unknown. We found an unexpected function of FtsHi1, an inner envelope membrane-bound AAA-ATPase in chloroplast R-loop homeostasis of Arabidopsis thaliana. Previously, this protein was shown to function as a component of the import motor complex for nuclear-encoded chloroplast proteins. However, this study provides evidence that FtsHi1 is an ATP-dependent helicase that efficiently unwinds both DNA-DNA and DNA-RNA duplexes, thereby preventing R-loop accumulation. Over-accumulation of R-loops could impair chloroplast transcription but not necessarily genome integrity. The dual function of FtsHi1 in both protein import and chloroplast gene expression may be important to coordinate the biogenesis of nuclear- and chloroplast-encoded subunits of multi-protein photosynthetic complexes. This study suggests a mechanical link between protein import and R-loop homeostasis in chloroplasts of higher plants.

Guardians of the Genome: How the Single-Stranded DNA-Binding Proteins RPA and CST Facilitate Telomere Replication

Telomeres act as the protective caps of eukaryotic linear chromosomes; thus, proper telomere maintenance is crucial for genome stability. Successful telomere replication is a cornerstone of telomere length regulation, but this process can be fraught due to the many intrinsic challenges telomeres pose to the replication machinery. In addition to the famous "end replication" problem due to the discontinuous nature of lagging strand synthesis, telomeres require various telomere-specific steps for maintaining the proper 3' overhang length. Bulk telomere replication also encounters its own difficulties as telomeres are prone to various forms of replication roadblocks. These roadblocks can result in an increase in replication stress that can cause replication forks to slow, stall, or become reversed. Ultimately, this leads to excess single-stranded DNA (ssDNA) that needs to be managed and protected for replication to continue and to prevent DNA damage and genome instability. RPA and CST are single-stranded DNA-binding protein complexes that play key roles in performing this task and help stabilize stalled forks for continued replication. The interplay between RPA and CST, their functions at telomeres during replication, and their specialized features for helping overcome replication stress at telomeres are the focus of this review.

Detection of G-Quadruplex DNA Structures in Macrophages

In addition to the canonical B-DNA conformation, DNA can fold into different secondary structures. Among them are G-quadruplex structures (G4s). G4 structures are very stable and can fold in specific guanine-rich regions in DNA and RNA. Different in silico, in vitro, and in cellulo experiments have shown that G4 structures form so far in all tested organisms. There are over 700,000 predicted G4s in higher eukaryotes, but it is so far assumed that not all will form at the same time. Their formation is dynamically regulated by proteins and is cell type-specific and even changes during the cell cycle or during different exogenous or endogenous stimuli (e.g., infection or developmental stages) can alter the G4 level. G4s have been shown to accumulate in cancer cells where they contribute to gene expression changes and the mutagenic burden of the tumor. Specific targeting of G4 structures to impact the expression of oncogenes is currently discussed as an anti-cancer treatment. In a tumor microenvironment, not only the tumor cells will be targeted by G4 stabilization but also immune cells such as macrophages. Although G4s were detected in multiple organisms and different cell types, only little is known about their role in immune cells. Here, we provide a detailed protocol to detect G4 formation in the nucleus of macrophages of vertebrates and invertebrates by microscopic imaging.

G-Quadruplexes as Sensors of Intracellular Na+/K+ Ratio: Potential Role in Regulation of Transcription and Translation

Data on the structure of G-quadruplexes, noncanonical nucleic acid forms, supporting an idea of their potential participation in regulation of gene expression in response to the change in intracellular Na+i/K+i ratio are considered in the review. Structural variety of G-quadruplexes, role of monovalent cations in formation of this structure, and thermodynamic stability of G-quadruplexes are described. Data on the methods of their identification in the cells and biological functions of these structures are presented. Analysis of information about specific interactions of G-quadruplexes with some proteins was conducted, and their potential participation in the development of some pathological conditions, in particular, cancer and neurodegenerative diseases, is considered. Special attention is given to the plausible role of G-quadruplexes as sensors of intracellular Na+i/K+i ratio, because alteration of this parameter affects folding of G-quadruplexes changing their stability and, thereby, organization of the regulatory elements of nucleic acids. The data presented in the conclusion section demonstrate significant change in the expression of some early response genes under certain physiological conditions of cells and tissues depending on the intracellular Na+i/K+i ratio.

Hydrophobic Interaction-Induced Topology-Independent Destabilization of G-Quadruplex

Since the inception of the G-quadruplex (G4), enormous attention has been devoted to designing small molecules which can stabilize the G-quadruplex. In contrast, the knowledge about the molecules and mechanisms involved in the destabilization of G4 is sparse, although it is well recognized that destabilization of G4 is important in neurobiology and age-related genetic issues. In this study, it has been shown that amphiphilic molecules having a long hydrocarbon chain can destabilize G4, regardless of its topology, using various biophysical and molecular dynamics simulation methods. It has been observed that the hydrophobic interaction induced by the long hydrocarbon chain of amphiphilic molecules is the main contributor in triggering the destabilization of G4, although hydrogen bonding by the polar part of the molecules also cooperates in the destabilization process. The experiment and simulation studies suggest that a long hydrocarbon chain containing amphiphilic molecules gets aggregated, and their hydrocarbon chain as well as the polar group intrude in the quartet region from the 5' side and interact with guanine bases as well as nearby loops through hydrophobic and electrostatic interactions, which trigger the destabilization of G4.

Harnessing G-quadruplex ligands for lung cancer treatment: A comprehensive overview

Lung cancer (LC) remains a leading cause of mortality worldwide, and new therapeutic strategies are urgently needed. One such approach revolves around the utilization of four-stranded nucleic acid secondary structures, known as G-quadruplexes (G4), which are formed by G-rich sequences. Ligands that bind selectively to G4 structures present a promising strategy for regulating crucial cellular processes involved in the progression of LC, rendering them potent agents for lung cancer treatment. In this review, we offer a summary of recent advancements in the development of G4 ligands capable of targeting specific genes associated with the development and progression of lung cancer.

G-quadruplexes associated with R-loops promote CTCF binding

CTCF is a critical regulator of genome architecture and gene expression that binds thousands of sites on chromatin. CTCF genomic localization is controlled by the recognition of a DNA sequence motif and regulated by DNA modifications. However, CTCF does not bind to all its potential sites in all cell types, raising the question of whether the underlying chromatin structure can regulate CTCF occupancy. Here, we report that R-loops facilitate CTCF binding through the formation of associated G-quadruplex (G4) structures. R-loops and G4s co-localize with CTCF at many genomic regions in mouse embryonic stem cells and promote CTCF binding to its cognate DNA motif in vitro. R-loop attenuation reduces CTCF binding in vivo. Deletion of a specific G4-forming motif in a gene reduces CTCF binding and alters gene expression. Conversely, chemical stabilization of G4s results in CTCF gains and accompanying alterations in chromatin organization, suggesting a pivotal role for G4 structures in reinforcing long-range genome interactions through CTCF.

Significant destabilization of human telomeric G-quadruplex upon peptide binding: dramatic effect of flanking bases

Human telomere is composed of highly repeated hexanucleotide sequence TTAGGG and a 3' single-stranded DNA tail. Many telomere G4 topologies characterized at atomic level by X-ray crystallography and NMR studies. Until now, various small ligands developed to interact with G-quadruplex mainly to stabilize the structure and least is known for its destabilization. In this study, we provide the first evidence of human telomeric G4 destabilization upon peptide binding in dilute and cell-mimicking molecular crowing conditions due to the changes in flanking bases of human telomeric sequences. Hence, our findings will open the new ways to target diseases related with increasing the efficiency of DNA replication, transcription or duplex reannealing.Communicated by Ramaswamy H. Sarma.

A local ATR-dependent checkpoint pathway is activated by a site-specific replication fork block in human cells

When replication forks encounter DNA lesions that cause polymerase stalling, a checkpoint pathway is activated. The ATR-dependent intra-S checkpoint pathway mediates detection and processing of sites of replication fork stalling to maintain genomic integrity. Several factors involved in the global checkpoint pathway have been identified, but the response to a single replication fork barrier (RFB) is poorly understood. We utilized the Escherichia coli-based Tus-Ter system in human MCF7 cells and showed that the Tus protein binding to TerB sequences creates an efficient site-specific RFB. The single fork RFB was sufficient to activate a local, but not global, ATR-dependent checkpoint response that leads to phosphorylation and accumulation of DNA damage sensor protein γH2AX, confined locally to within a kilobase of the site of stalling. These data support a model of local management of fork stalling, which allows global replication at sites other than the RFB to continue to progress without delay.

Application of G-quadruplex targets in gastrointestinal cancers: Advancements, challenges and prospects

Genomic instability and inflammation are considered to be two enabling characteristics that support cancer development and progression. G-quadruplex structure is a key element that contributes to genomic instability and inflammation. G-quadruplexes were once regarded as simply an obstacle that can block the transcription of oncogenes. A ligand targeting G-quadruplexes was found to have anticancer activity, making G-quadruplexes potential anticancer targets. However, further investigation has revealed that G-quadruplexes are widely distributed throughout the human genome and have many functions, such as regulating DNA replication, DNA repair, transcription, translation, epigenetics, and inflammatory response. G-quadruplexes play double regulatory roles in transcription and translation. In this review, we focus on G-quadruplexes as novel targets for the treatment of gastrointestinal cancers. We summarize the application basis of G-quadruplexes in gastrointestinal cancers, including their distribution sites, structural characteristics, and physiological functions. We describe the current status of applications for the treatment of esophageal cancer, pancreatic cancer, hepatocellular carcinoma, gastric cancer, colorectal cancer, and gastrointestinal stromal tumors, as well as the associated challenges. Finally, we review the prospective clinical applications of G-quadruplex targets, providing references for targeted treatment strategies in gastrointestinal cancers.

G-quadruplexes and associated proteins in aging and Alzheimer's disease

Aging is a prominent risk factor for many neurodegenerative disorders, such as Alzheimer's disease (AD). Alzheimer's disease is characterized by progressive cognitive decline, memory loss, and neuropsychiatric and behavioral symptoms, accounting for most of the reported dementia cases. This disease is now becoming a major challenge and burden on modern society, especially with the aging population. Over the last few decades, a significant understanding of the pathophysiology of AD has been gained by studying amyloid deposition, hyperphosphorylated tau, synaptic dysfunction, oxidative stress, calcium dysregulation, and neuroinflammation. This review focuses on the role of non-canonical secondary structures of DNA/RNA G-quadruplexes (G4s, G4-DNA, and G4-RNA), G4-binding proteins (G4BPs), and helicases, and their roles in aging and AD. Being critically important for cellular function, G4s are involved in the regulation of DNA and RNA processes, such as replication, transcription, translation, RNA localization, and degradation. Recent studies have also highlighted G4-DNA's roles in inducing DNA double-strand breaks that cause genomic instability and G4-RNA's participation in regulating stress granule formation. This review emphasizes the significance of G4s in aging processes and how their homeostatic imbalance may contribute to the pathophysiology of AD.

A local ATR-dependent checkpoint pathway is activated by a site-specific replication fork block in human cells

When replication forks encounter DNA lesions that cause polymerase stalling a checkpoint pathway is activated. The ATR-dependent intra-S checkpoint pathway mediates detection and processing of sites of replication fork stalling to maintain genomic integrity. Several factors involved in the global checkpoint pathway have been identified, but the response to a single replication fork barrier (RFB) is poorly understood. We utilized the E.coli -based Tus- Ter system in human MCF7 cells and showed that the Tus protein binding to TerB sequences creates an efficient site-specific RFB. The single fork RFB was sufficient to activate a local, but not global, ATR-dependent checkpoint response that leads to phosphorylation and accumulation of DNA damage sensor protein γH2AX, confined locally to within a kilobase of the site of stalling. These data support a model of local management of fork stalling, which allows global replication at sites other than the RFB to continue to progress without delay.

Current Status of Oligonucleotide-Based Protein Degraders

Transcription factors (TFs) and RNA-binding proteins (RBPs) have long been considered undruggable, mainly because they lack ligand-binding sites and are equipped with flat and narrow protein surfaces. Protein-specific oligonucleotides have been harnessed to target these proteins with some satisfactory preclinical results. The emerging proteolysis-targeting chimera (PROTAC) technology is no exception, utilizing protein-specific oligonucleotides as warheads to target TFs and RBPs. In addition, proteolysis by proteases is another type of protein degradation. In this review article, we discuss the current status of oligonucleotide-based protein degraders that are dependent either on the ubiquitin–proteasome system or a protease, providing a reference for the future development of degraders.

Antigenic diversity in malaria parasites is maintained on extrachromosomal DNA

Sequence variation among antigenic var genes enables Plasmodium falciparum malaria parasites to evade host immunity. Using long sequence reads from haploid clones from a mutation accumulation experiment, we detect var diversity inconsistent with simple chromosomal inheritance. We discover putatively circular DNA that is strongly enriched for var genes, which exist in multiple alleles per locus separated by recombination and indel events. Extrachromosomal DNA likely contributes to rapid antigenic diversification in P. falciparum.

Roles of G4-DNA and G4-RNA in Class Switch Recombination and Additional Regulations in B-Lymphocytes

Mature B cells notably diversify immunoglobulin (Ig) production through class switch recombination (CSR), allowing the junction of distant "switch" (S) regions. CSR is initiated by activation-induced deaminase (AID), which targets cytosines adequately exposed within single-stranded DNA of transcribed targeted S regions, with a specific affinity for WRCY motifs. In mammals, G-rich sequences are additionally present in S regions, forming canonical G-quadruplexes (G4s) DNA structures, which favor CSR. Small molecules interacting with G4-DNA (G4 ligands), proved able to regulate CSR in B lymphocytes, either positively (such as for nucleoside diphosphate kinase isoforms) or negatively (such as for RHPS4). G4-DNA is also implicated in the control of transcription, and due to their impact on both CSR and transcriptional regulation, G4-rich sequences likely play a role in the natural history of B cell malignancies. Since G4-DNA stands at multiple locations in the genome, notably within oncogene promoters, it remains to be clarified how it can more specifically promote legitimate CSR in physiology, rather than pathogenic translocation. The specific regulatory role of G4 structures in transcribed DNA and/or in corresponding transcripts and recombination hereby appears as a major issue for understanding immune responses and lymphomagenesis.

The Intertwined Role of 8-oxodG and G4 in Transcription Regulation

The guanine base in nucleic acids is, among the other bases, the most susceptible to being converted into 8-Oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) when exposed to reactive oxygen species. In double-helix DNA, 8-oxodG can pair with adenine; hence, it may cause a G > T (C > A) mutation; it is frequently referred to as a form of DNA damage and promptly corrected by DNA repair mechanisms. Moreover, 8-oxodG has recently been redefined as an epigenetic factor that impacts transcriptional regulatory elements and other epigenetic modifications. It has been proposed that 8-oxodG exerts epigenetic control through interplay with the G-quadruplex (G4), a non-canonical DNA structure, in transcription regulatory regions. In this review, we focused on the epigenetic roles of 8-oxodG and the G4 and explored their interplay at the genomic level.

New DNA Plasmid Model for Studying DNA Mismatch Repair Response to the G4 Structure

G-quadruplexes (G4s), the most widely studied alternative DNA structures, are implicated in the regulation of the key cellular processes. In recent years, their involvement in DNA repair machinery has become the subject of intense research. Here, we evaluated the effect of G4 on the prokaryotic DNA mismatch repair (MMR) pathway from two bacterial sources with different mismatch repair mechanisms. The G4 folding, which competes with the maintenance of double-stranded DNA, is known to be controlled by numerous opposing factors. To overcome the kinetic barrier of G4 formation, we stabilized a parallel G4 formed by the d(GGGT)₄ sequence in a DNA plasmid lacking a fragment complementary to the G4 motif. Unlike commonly used isolated G4 structures, our plasmid with an embedded stable G4 structure contained elements, such as a MutH cleavage site, required to initiate the repair process. G4 formation in the designed construct was confirmed by Taq polymerase stop assay and dimethyl sulfate probing. The G4-carrying plasmid, together with control ones (lacking a looped area or containing unstructured d(GT)₈ insert instead of the G4 motif), were used as new type models to answer the question of whether G4 formation interferes with DNA cleavage as a basic function of MMR.

Characterization of DNA Polymerase Genes in Amazonian Amerindian Populations

Due to their continuing geographic isolation, the Amerindian populations of the Brazilian Amazon present a different genetic profile when compared to other continental populations. Few studies have investigated genetic variants present in these populations, especially in the context of next-generation sequencing. Knowledge of the molecular profile of a population is one of the bases for inferences about human evolutionary history, in addition, it has the ability to assist in the validation of molecular biomarkers of susceptibility to complex and rare diseases, and in the improvement of specific precision medicine protocols applied to these populations and to populations with high Amerindian ancestry, such as Brazilians. DNA polymerases play essential roles in DNA replication, repair, recombination, or damage repair, and their influence on various clinical phenotypes has been demonstrated in the specialized literature. Thus, the aim of this study is to characterize the molecular profile of POLA1, POLE, POLG, POLQ, and REV3L genes in Amerindian populations from the Brazilian Amazon, comparing these findings with genomic data from five continental populations described in the gnomAD database, and with data from the Brazilian population described in ABraOM. We performed the whole exome sequencing (WES) of 63 Indigenous individuals. Our study described for the first time the allele frequency of 45 variants already described in the other continental populations, but never before described in the investigated Amerindian populations. Our results also describe eight unique variants of the investigated Amerindians populations, with predictions of moderate, modifier and high clinical impact. Our findings demonstrate the unique genetic profile of the Indigenous population of the Brazilian Amazon, reinforcing the need for further studies on these populations, and may contribute to the creation of public policies that optimize not only the quality of life of this population, but also of the Brazilian population.

Telomere length analysis in amyotrophic lateral sclerosis using large-scale whole genome sequence data

Background

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by the loss of upper and lower motor neurons, leading to progressive weakness of voluntary muscles, with death following from neuromuscular respiratory failure, typically within 3 to 5 years. There is a strong genetic contribution to ALS risk. In 10% or more, a family history of ALS or frontotemporal dementia is obtained, and the Mendelian genes responsible for ALS in such families have now been identified in about 50% of cases. Only about 14% of apparently sporadic ALS is explained by known genetic variation, suggesting that other forms of genetic variation are important. Telomeres maintain DNA integrity during cellular replication, differ between sexes, and shorten naturally with age. Sex and age are risk factors for ALS and we therefore investigated telomere length in ALS.

Methods

Samples were from Project MinE, an international ALS whole genome sequencing consortium that includes phenotype data. For validation we used donated brain samples from motor cortex from people with ALS and controls. Ancestry and relatedness were evaluated by principal components analysis and relationship matrices of DNA microarray data. Whole genome sequence data were from Illumina HiSeq platforms and aligned using the Isaac pipeline. TelSeq was used to quantify telomere length using whole genome sequence data. We tested the association of telomere length with ALS and ALS survival using Cox regression.

Results

There were 6,580 whole genome sequences, reducing to 6,195 samples (4,315 from people with ALS and 1,880 controls) after quality control, and 159 brain samples (106 ALS, 53 controls). Accounting for age and sex, there was a 20% (95% CI 14%, 25%) increase of telomere length in people with ALS compared to controls (p = 1.1 × 10-12), validated in the brain samples (p = 0.03). Those with shorter telomeres had a 10% increase in median survival (p = 5.0×10-7). Although there was no difference in telomere length between sporadic ALS and familial ALS (p=0.64), telomere length in 334 people with ALS due to expanded C9orf72 repeats was shorter than in those without expanded C9orf72 repeats (p = 5.0×10-4).

Discussion

Although telomeres shorten with age, longer telomeres are a risk factor for ALS and worsen prognosis. Longer telomeres are associated with ALS.

Dual Targeting Topoisomerase/G-Quadruplex Agents in Cancer Therapy-An Overview

Topoisomerase (Topo) inhibitors have long been known as clinically effective drugs, while G-quadruplex (G4)-targeting compounds are emerging as a promising new strategy to target tumor cells and could support personalized treatment approaches in the near future. G-quadruplex (G4) is a secondary four-stranded DNA helical structure constituted of guanine-rich nucleic acids, and its stabilization impairs telomere replication, triggering the activation of several protein factors at telomere levels, including Topos. Thus, the pharmacological intervention through the simultaneous G4 stabilization and Topos inhibition offers a new opportunity to achieve greater antiproliferative activity and circumvent cellular insensitivity and resistance. In this line, dual ligands targeting both Topos and G4 emerge as innovative, efficient agents in cancer therapy. Although the research in this field is still limited, to date, some chemotypes have been identified, showing this dual activity and an interesting pharmacological profile. This paper reviews the available literature on dual Topo inhibitors/G4 stabilizing agents, with particular attention to the structure-activity relationship studies correlating the dual activity with the cytotoxic activity.

Strand switching mechanism of Pif1 helicase induced by its collision with a G-quadruplex embedded in dsDNA

G-rich sequences found at multiple sites throughout all genomes may form secondary structures called G-quadruplexes (G4), which act as roadblocks for molecular motors. Among the enzymes thought to process these structures, the Pif1 DNA helicase is considered as an archetypical G4-resolvase and its absence has been linked to G4-related genomic instabilities in yeast. Here we developed a single-molecule assay to observe Pif1 opening a DNA duplex and resolving the G4 in real time. In support of former enzymological studies, we show that the helicase reduces the lifetime of G4 from hours to seconds. However, we observe that in the presence of a G4, Pif1 exhibits a strong strand switching behavior, which can lead to Pif1 escaping G4 resolution, depending on the structural context surrounding the substrate. This behavior is also detected in the presence of other roadblocks (LNA or RNA). We propose that the efficiency of Pif1 to remove a roadblock (G4 or other) is affected by its strand switching behavior and depends on the context surrounding the obstacle. We discuss how this switching behavior may explain several aspects of Pif1 substrate preference and affect its activity as a G4 resolvase in vivo.

A novel DNA tweezers-based nanobiosensor for multiple detections of circulating exosomal microRNAs in breast cancer

Breast cancer is the prevalent disease in women, and diagnosis of it in early stage and takes preventive measures is very critical. Recently, circulating microRNAs have emerged as promising early biomarkers of cancer. MiR-21 and miR-155 are two significant biomarkers that act as oncomir in breast cancer. In this study, to detect both microRNAs in one test simultaneously, a novel colorimetric nanobiosensor was developed upon the peroxidation property of a specific G-quadruplex nanostructure. The nanostructure forms a DNA Nano-Tweezers after self-assembly of three DNA oligonucleotides with target sequences, and TMB (2, 2'-azino-bis (3-ethylbenzothiazo-line-6-sulfonic acid)) is used as a reporter to produce color. The high sensitivity of the nanobiosensor was determined (in buffer and blood) using different concentrations of target sequences with a linear response range from 0 to 10 nM, and detection limit of 0.38 nM (R² = 0.98). The method precisely detected target sequences from non-target sequences in both buffer and blood media. These findings demonstrate, the nanobiosensor is superior to most previous published works due to its simultaneous dual detection, simplicity, low response time, and cost. The analytical data is convenient for accurately use for clinical purposes to detect breast cancer in early stage, more significantly.

Multimeric G-quadruplexes: A review on their biological roles and targeting

In human cells, nucleic acids adopt several non-canonical structures that regulate key cellular processes. Among them, G-quadruplexes (G4s) are stable structures that form in guanine-rich regions in vitro and in cells. G4 folded/unfolded state shapes numerous cellular processes, including genome replication, transcription, and translation. Moreover, G4 folding is involved in genomic instability. G4s have been described to multimerize, forming high-order structures in both DNA and/or RNA strands. Multimeric G4s can be formed by adjacent intramolecular G4s joined by stacking interactions or connected by short loops. Multimeric G4s can also originate from the assembly of guanines embedded on independent DNA or RNA strands. Notably, crucial regions of the human genome, such as the 3'-terminal overhang of the telomeric DNA as well as the open reading frame of genes involved in the preservation of neuron viability in the human central and peripheral nervous system are prone to form multimeric G4s. The biological importance of such structures has been recently described, with multimeric G4s playing potentially protective or deleterious effects in the pathogenic cascade of various diseases. Here, we portray the multifaceted scenario of multimeric G4s, in terms of structural properties, biological roles, and targeting strategies.

G-Quadruplex Matters in Tissue-Specific Tumorigenesis by BRCA1 Deficiency

How and why distinct genetic alterations, such as BRCA1 mutation, promote tumorigenesis in certain tissues, but not others, remain an important issue in cancer research. The underlying mechanisms may reveal tissue-specific therapeutic vulnerabilities. Although the roles of BRCA1, such as DNA damage repair and stalled fork stabilization, obviously contribute to tumor suppression, these ubiquitously important functions cannot explain tissue-specific tumorigenesis by BRCA1 mutations. Recent advances in our understanding of the cancer genome and fundamental cellular processes on DNA, such as transcription and DNA replication, have provided new insights regarding BRCA1-associated tumorigenesis, suggesting that G-quadruplex (G4) plays a critical role. In this review, we summarize the importance of G4 structures in mutagenesis of the cancer genome and cell type-specific gene regulation, and discuss a recently revealed molecular mechanism of G4/base excision repair (BER)-mediated transcriptional activation. The latter adequately explains the correlation between the accumulation of unresolved transcriptional regulatory G4s and multi-level genomic alterations observed in BRCA1-associated tumors. In summary, tissue-specific tumorigenesis by BRCA1 deficiency can be explained by cell type-specific levels of transcriptional regulatory G4s and the role of BRCA1 in resolving it. This mechanism would provide an integrated understanding of the initiation and development of BRCA1-associated tumors.

G4-PROTAC: targeted degradation of a G-quadruplex binding protein

G-quadruplex (G4) binding proteins regulate important biological processes, but their interaction networks are poorly understood. We report the first use of G4 as a warhead of a proteolysis-targeting chimera (G4-PROTAC) for targeted degradation of a G4-binding protein (RHAU/DHX36). G4-PROTAC provides a new way to explore G4-protein networks and to develop potential therapeutics.

The Relevance of G-Quadruplexes for DNA Repair

DNA molecules can adopt a variety of alternative structures. Among these structures are G-quadruplex DNA structures (G4s), which support cellular function by affecting transcription, translation, and telomere maintenance. These structures can also induce genome instability by stalling replication, increasing DNA damage, and recombination events. G-quadruplex-driven genome instability is connected to tumorigenesis and other genetic disorders. In recent years, the connection between genome stability, DNA repair and G4 formation was further underlined by the identification of multiple DNA repair proteins and ligands which bind and stabilize said G4 structures to block specific DNA repair pathways. The relevance of G4s for different DNA repair pathways is complex and depends on the repair pathway itself. G4 structures can induce DNA damage and block efficient DNA repair, but they can also support the activity and function of certain repair pathways. In this review, we highlight the roles and consequences of G4 DNA structures for DNA repair initiation, processing, and the efficiency of various DNA repair pathways.

G-Quadruplexes Formation at the Upstream Region of Replication Origin (OriL) of the Pseudorabies Virus: Implications for Antiviral Targets

Pseudorabies virus (PRV) is the causative agent of Aujeszky's disease, which still causes large economic losses for the swine industry. Therefore, it is urgent to find a new strategy to prevent and control PRV infection. Previous studies have proven that guanine (G)-rich DNA or RNA sequences in some other viruses' genomes have the potential to form G-quadruplex (G4), which serve as promising antivirus targets. In this study, we identified two novel G4-forming sequences, OriL-A and OriL-S, which are located at the upstream origin of replication (OriL) in the PRV genome and conserved across 32 PRV strains. Circular dichroism (CD) spectroscopy and a gel electrophoresis assay showed that the two G-rich sequences can fold into parallel G4 structures in vitro. Moreover, fluorescence resonance energy transfer (FRET) melting and a Taq polymerase stop assay indicated that the G4 ligand PhenDC3 has the capacity to bind and stabilize the G4. Notably, the treatment of PRV-infected cells with G4-stabilizer PhenDC3 significantly inhibited PRV DNA replication in host cells but did not affect PRV's attachment and entry. These results not only expand our knowledge about the G4 characteristics in the PRV genome but also suggest that G4 may serve as an innovative therapeutic target against PRV.

Folding dynamics of polymorphic G-quadruplex structures

G-quadruplexes (G4), found in numerous places within the human genome, are involved in essential processes of cell regulation. Chromosomal DNA G4s are involved for example, in replication and transcription as first steps of gene expression. Hence, they influence a plethora of downstream processes. G4s possess an intricate structure that differs from canonical B-form DNA. Identical DNA G4 sequences can adopt multiple long-lived conformations, a phenomenon known as G4 polymorphism. A detailed understanding of the molecular mechanisms that drive G4 folding is essential to understand their ambivalent regulatory roles. Disentangling the inherent dynamic and polymorphic nature of G4 structures thus is key to unravel their biological functions and make them amenable as molecular targets in novel therapeutic approaches. We here review recent experimental approaches to monitor G4 folding and discuss structural aspects for possible folding pathways. Substantial progress in the understanding of G4 folding within the recent years now allows drawing comprehensive models of the complex folding energy landscape of G4s that we herein evaluate based on computational and experimental evidence.

Action and function of helicases on RNA G-quadruplexes

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Methodological progresses and piling evidence prove the rG4 biology in vivo.

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rG4s step in virtually every aspect of RNA biology.

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Helicases unwinding of rG4s is a fine regulatory layer to the downstream processes and general cell homeostasis.

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The current knowledge is however limited to a few cell lines.

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The regulation of helicases themselves is delineating as a important question.

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Non-helicase rG4-processing proteins likely play a role.

The nucleic acid structure called G-quadruplex (G4) is currently discussed to function in nucleic acid-based mechanisms that influence several cellular processes. They can modulate the cellular machinery either positively or negatively, both at the DNA and RNA level. The majority of what we know about G4 biology comes from DNA G4 (dG4) research. RNA G4s (rG4), on the other hand, are gaining interest as researchers become more aware of their role in several aspects of cellular homeostasis. In either case, the correct regulation of G4 structures within cells is essential and demands specialized proteins able to resolve them. Small changes in the formation and unfolding of G4 structures can have severe consequences for the cells that could even stimulate genome instability, apoptosis or proliferation. Helicases are the most relevant negative G4 regulators, which prevent and unfold G4 formation within cells during different pathways. Yet, and despite their importance only a handful of rG4 unwinding helicases have been identified and characterized thus far. This review addresses the current knowledge on rG4s-processing helicases with a focus on methodological approaches. An example of a non-helicase rG4s-unwinding protein is also briefly described.

Volumetric Properties of Four-Stranded DNA Structures

Four-stranded non-canonical DNA structures including G-quadruplexes and i-motifs have been found in the genome and are thought to be involved in regulation of biological function. These structures have been implicated in telomere biology, genomic instability, and regulation of transcription and translation events. To gain an understanding of the molecular determinants underlying the biological role of four-stranded DNA structures, their biophysical properties have been extensively studied. The limited libraries on volume, expansibility, and compressibility accumulated to date have begun to provide insights into the molecular origins of helix-to-coil and helix-to-helix conformational transitions involving four-stranded DNA structures. In this article, we review the recent progress in volumetric investigations of G-quadruplexes and i-motifs, emphasizing how such data can be used to characterize intra-and intermolecular interactions, including solvation. We describe how volumetric data can be interpreted at the molecular level to yield a better understanding of the role that solute-solvent interactions play in modulating the stability and recognition events of nucleic acids. Taken together, volumetric studies facilitate unveiling the molecular determinants of biological events involving biopolymers, including G-quadruplexes and i-motifs, by providing one more piece to the thermodynamic puzzle describing the energetics of cellular processes in vitro and, by extension, in vivo.

Identifying G-Quadruplex-DNA-Disrupting Small Molecules

The quest for small molecules that strongly bind to G-quadruplex-DNA (G4), so-called G4 ligands, has invigorated the G4 research field from its very inception. Massive efforts have been invested to discover or rationally design G4 ligands, evaluate their G4-interacting properties in vitro through a series of now widely accepted and routinely implemented assays, and use them as innovative chemical biology tools to interrogate cellular networks that might involve G4s. In sharp contrast, only uncoordinated efforts aimed at developing small molecules that destabilize G4s have been invested to date, even though it is now recognized that such molecular tools would have tremendous application in neurobiology as many genetic and age-related diseases are caused by an overrepresentation of G4s. Herein, we report on our efforts to develop in vitro assays to reliably identify molecules able to destabilize G4s. This workflow comprises the newly designed G4-unfold assay, adapted from the G4-helicase assay implemented with Pif1, as well as a series of biophysical and biochemical techniques classically used to study G4/ligand interactions (CD, UV-vis, PAGE, and FRET-melting), and a qPCR stop assay, adapted from a Taq-based protocol recently used to identify G4s in the genomic DNA of Schizosaccharomyces pombe. This unique, multipronged approach leads to the characterization of a phenylpyrrolocytosine (PhpC)-based G-clamp analog as a prototype of G4-disrupting small molecule whose properties are validated through many different and complementary in vitro evaluations.

GGGCTA repeats can fold into hairpins poorly unfolded by replication protein A: a possible origin of the length-dependent instability of GGGCTA variant repeats in human telomeres

Human telomeres are composed of GGGTTA repeats and interspersed with variant repeats. The GGGCTA variant motif was identified in the proximal regions of human telomeres about 10 years ago and was shown to display a length-dependent instability. In parallel, a structural study showed that four GGGCTA repeats folded into a non-canonical G-quadruplex (G4) comprising a Watson-Crick GCGC tetrad. It was proposed that this non-canonical G4 might be an additional obstacle for telomere replication. In the present study, we demonstrate that longer GGGCTA arrays fold into G4 and into hairpins. We also demonstrate that replication protein A (RPA) efficiently binds to GGGCTA repeats structured into G4 but poorly binds to GGGCTA repeats structured into hairpins. Our results (along with results obtained with a more stable variant motif) suggest that GGGCTA hairpins are at the origin of GGGCTA length-dependent instability. They also suggest, as working hypothesis, that failure of efficient binding of RPA to GGGCTA structured into hairpins might be involved in the mechanism of GGGCTA array instability. On the basis of our present and past studies about telomeric G4 and their interaction with RPA, we propose an original point of view about telomeric G4 and the evolution of telomeric motifs.

Repriming DNA synthesis: an intrinsic restart pathway that maintains efficient genome replication

To bypass a diverse range of fork stalling impediments encountered during genome replication, cells possess a variety of DNA damage tolerance (DDT) mechanisms including translesion synthesis, template switching, and fork reversal. These pathways function to bypass obstacles and allow efficient DNA synthesis to be maintained. In addition, lagging strand obstacles can also be circumvented by downstream priming during Okazaki fragment generation, leaving gaps to be filled post-replication. Whether repriming occurs on the leading strand has been intensely debated over the past half-century. Early studies indicated that both DNA strands were synthesised discontinuously. Although later studies suggested that leading strand synthesis was continuous, leading to the preferred semi-discontinuous replication model. However, more recently it has been established that replicative primases can perform leading strand repriming in prokaryotes. An analogous fork restart mechanism has also been identified in most eukaryotes, which possess a specialist primase called PrimPol that conducts repriming downstream of stalling lesions and structures. PrimPol also plays a more general role in maintaining efficient fork progression. Here, we review and discuss the historical evidence and recent discoveries that substantiate repriming as an intrinsic replication restart pathway for maintaining efficient genome duplication across all domains of life.

Folding and persistence times of intramolecular G-quadruplexes transiently embedded in a DNA duplex

G-quadruplex (G4) DNA structures have emerged as important regulatory elements during DNA metabolic transactions. While many in vitro studies have focused on the kinetics of G4 formation within DNA single-strands, G4 are found in vivo in double-stranded DNA regions, where their formation is challenged by the complementary strand. Since the energy of hybridization of Watson-Crick structures dominates the energy of G4 folding, this competition should play a critical role on G4 persistence. To address this, we designed a single-molecule assay allowing to measure G4 folding and persistence times in the presence of the complementary strand. We quantified both folding and unfolding rates of biologically relevant G4 sequences, such as the cMYC and cKIT oncogene promoters, human telomeres and an avian replication origin. We confirmed that G4s are found much more stable in tested replication origin and promoters than in human telomere repeats. In addition, we characterized how G4 dynamics was affected by G4 ligands and showed that both folding rate and persistence time increased. Our assay opens new perspectives for the measurement of G4 dynamics in double-stranded DNA mimicking a replication fork, which is important to understand their role in DNA replication and gene regulation at a mechanistic level.

Exploiting synthetic lethality to target BRCA1/2-deficient tumors: where we stand

The principle of synthetic lethality, which refers to the loss of viability resulting from the disruption of two genes, which, individually, do not cause lethality, has become an attractive target approach due to the development and clinical success of Poly (ADP-ribose) polymerase (PARP) inhibitors (PARPi). In this review, we present the most recent findings on the use of PARPi in the clinic, which are currently approved for second-line therapy for advanced ovarian and breast cancer associated with mutations of BRCA1 or BRCA2 (BRCA1/2) genes. PARPi efficacy, however, appears to be limited by acquired and inherent resistance, highlighting the need for alternative and synergistic targets to eliminate these tumors. Here, we explore other identified synthetic lethal interactors of BRCA1/2, including DNA polymerase theta (POLQ), Fanconi anemia complementation group D2 (FANDC2), radiation sensitive 52 (RAD52), Flap structure-specific endonuclease 1 (FEN1), and apurinic/apyrimidinic endodeoxyribonuclease 2 (APE2), as well as other protein and nonprotein targets, for BRCA1/2-mutated cancers and their implications for future therapies. A wealth of information now exists for phenotypic and functional characterization of these novel synthetic lethal interactors of BRCA1/2, and leveraging these findings can pave the way for the development of new targeted therapies for patients suffering from these cancers.

Discovery of MTR-106 as a highly potent G-quadruplex stabilizer for treating BRCA-deficient cancers

G-quadruplexes (G4s) are DNA or RNA structures formed by guanine-rich repeating sequences. Recently, G4s have become a highly attractive therapeutic target for BRCA-deficient cancers. Here, we show that a substituted quinolone amide compound, MTR-106, stabilizes DNA G-quadruplexes in vitro. MTR-106 displayed significant antiproliferative activity in homologous recombination repair (HR)-deficient and PARP inhibitor (PARPi)-resistant cancer cells. Moreover, MTR-106 increased DNA damage and promoted cell cycle arrest and apoptosis to inhibit cell growth. Importantly, its oral and i.v. administration significantly impaired tumor growth in BRCA-deficient xenograft mouse models. However, MTR-106 showed modest activity against talazoparib-resistant xenograft models. In rats, the drug rapidly distributes to tissues within 5 min, and its average concentrations were 12-fold higher in the tissues than in the plasma. Overall, we identified MTR-106 as a novel G-quadruplex stabilizer with high tissue distribution, and it may serve as a potential anticancer agent.

G-quadruplexes: a promising target for cancer therapy

DNA and RNA can fold into a variety of alternative conformations. In recent years, a particular nucleic acid structure was discussed to play a role in malignant transformation and cancer development. This structure is called a G-quadruplex (G4). G4 structure formation can drive genome instability by creating mutations, deletions and stimulating recombination events. The importance of G4 structures in the characterization of malignant cells was currently demonstrated in breast cancer samples. In this analysis a correlation between G4 structure formation and an increased intratumor heterogeneity was identified. This suggests that G4 structures might allow breast cancer stratification and supports the identification of new personalized treatment options. Because of the stability of G4 structures and their presence within most human oncogenic promoters and at telomeres, G4 structures are currently tested as a therapeutic target to downregulate transcription or to block telomere elongation in cancer cells. To date, different chemical molecules (G4 ligands) have been developed that aim to target G4 structures. In this review we discuss and compare G4 function and relevance for therapeutic approaches and their impact on cancer development for three cancer entities, which differ significantly in their amount and type of mutations: pancreatic cancer, leukemia and malignant melanoma. G4 structures might present a promising new strategy to individually target tumor cells and could support personalized treatment approaches in the future.

How to untie G-quadruplex knots and why?

For over two decades, the prime objective of the chemical biology community studying G-quadruplexes (G4s) has been to use chemicals to interact with and stabilize G4s in cells to obtain mechanistic interpretations. This strategy has been undoubtedly successful, as demonstrated by recent advances. However, these insights have also led to a fundamental rethinking of G4-targeting strategies: due to the prevalence of G4s in the human genome, transcriptome, and ncRNAome (collectively referred to as the G4ome), and their involvement in human diseases, should we continue developing G4-stabilizing ligands or should we invest in designing molecular tools to unfold G4s? Here, we first focus on how, when, and where G4s fold in cells; then, we describe the enzymatic systems that have evolved to counteract G4 folding and how they have been used as tools to manipulate G4s in cells; finally, we present strategies currently being implemented to devise new molecular G4 unwinding agents.

DNA folds threaten genetic stability and can be leveraged for chemotherapy

Damaging DNA is a current and efficient strategy to fight against cancer cell proliferation. Numerous mechanisms exist to counteract DNA damage, collectively referred to as the DNA damage response (DDR) and which are commonly dysregulated in cancer cells. Precise knowledge of these mechanisms is necessary to optimise chemotherapeutic DNA targeting. New research on DDR has uncovered a series of promising therapeutic targets, proteins and nucleic acids, with application notably via an approach referred to as combination therapy or combinatorial synthetic lethality. In this review, we summarise the cornerstone discoveries which gave way to the DNA being considered as an anticancer target, and the manipulation of DDR pathways as a valuable anticancer strategy. We describe in detail the DDR signalling and repair pathways activated in response to DNA damage. We then summarise the current understanding of non-B DNA folds, such as G-quadruplexes and DNA junctions, when they are formed and why they can offer a more specific therapeutic target compared to that of canonical B-DNA. Finally, we merge these subjects to depict the new and highly promising chemotherapeutic strategy which combines enhanced-specificity DNA damaging and DDR targeting agents. This review thus highlights how chemical biology has given rise to significant scientific advances thanks to resolutely multidisciplinary research efforts combining molecular and cell biology, chemistry and biophysics. We aim to provide the non-specialist reader a gateway into this exciting field and the specialist reader with a new perspective on the latest results achieved and strategies devised.

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.

Mgs1 function at G-quadruplex structures during DNA replication

The coordinated action of DNA polymerases and DNA helicases is essential at genomic sites that are hard to replicate. Among these are sites that harbour G-quadruplex DNA structures (G4). G4s are stable alternative DNA structures, which have been implicated to be involved in important cellular processes like the regulation of gene expression or telomere maintenance. G4 structures were shown to hinder replication fork progression and cause genomic deletions, mutations and recombination events. Many helicases unwind G4 structures and preserve genome stability, but a detailed understanding of G4 replication and the re-start of stalled replication forks around formed G4 structures is not clear, yet. In our recent study, we identified that Mgs1 preferentially binds to G4 DNA structures in vitro and is associated with putative G4-forming chromosomal regions in vivo. Mgs1 binding to G4 motifs in vivo is partially dependent on the helicase Pif1. Pif1 is the major G4-unwinding helicase in S. cerevisiae. In the absence of Mgs1, we determined elevated gross chromosomal rearrangement (GCR) rates in yeast, similar to Pif1 deletion. Here, we highlight the recent findings and set these into context with a new mechanistic model. We propose that Mgs1's functions support DNA replication at G4-forming regions.