FANCJ promotes DNA synthesis through G‐quadruplex structures

Our genome contains many G-rich sequences, which have the propensity to fold into stable secondary DNA structures called G4 or G-quadruplex structures. These structures have been implicated in cellular processes such as gene regulation and telomere maintenance. However, G4 sequences are prone to mutations particularly upon replication stress or in the absence of specific helicases. To investigate how G-quadruplex structures are resolved during DNA replication, we developed a model system using ssDNA templates and Xenopus egg extracts that recapitulates eukaryotic G4 replication. Here, we show that G-quadruplex structures form a barrier for DNA replication. Nascent strand synthesis is blocked at one or two nucleotides from the G4. After transient stalling, G-quadruplexes are efficiently unwound and replicated. In contrast, depletion of the FANCJ/BRIP1 helicase causes persistent replication stalling at G-quadruplex structures, demonstrating a vital role for this helicase in resolving these structures. FANCJ performs this function independently of the classical Fanconi anemia pathway. These data provide evidence that the G4 sequence instability in FANCJ^−/− cells and Fancj/dog1 deficient C. elegans is caused by replication stalling at G-quadruplexes.     

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.     

DNA replication through G-quadruplex motifs is promoted by the Saccharomyces cerevisiae Pif1 DNA helicase

G-quadruplex (G4) DNA structures are extremely stable four-stranded secondary structures held together by noncanonical G-G base pairs. Genome-wide chromatin immunoprecipitation was used to determine the in?vivo binding sites of the multifunctional Saccharomyces cerevisiae Pif1 DNA helicase, a potent unwinder of G4 structures in?vitro. G4 motifs were a significant subset of the high-confidence Pif1-binding sites. Replication slowed in the vicinity of these motifs, and they were prone to breakage in Pif1-deficient cells, whereas non-G4 Pif1-binding sites did not show this behavior. Introducing many copies of G4 motifs caused slow growth in replication-stressed Pif1-deficient cells, which was relieved by spontaneous mutations that eliminated their ability to form G4 structures, bind Pif1, slow DNA replication, and stimulate DNA breakage. These data suggest that G4 structures form in?vivo and that they are resolved by Pif1 to prevent replication fork stalling and DNA breakage.     

CDK4/6 inhibitors induce replication stress to cause long‐term cell cycle withdrawal

CDK4/6 inhibitors arrest the cell cycle in G1‐phase. They are approved to treat breast cancer and are also undergoing clinical trials against a range of other tumour types. To facilitate these efforts, it is important to understand why a cytostatic arrest in G1 causes long‐lasting effects on tumour growth. Here, we demonstrate that a prolonged G1 arrest following CDK4/6 inhibition downregulates replisome components and impairs origin licencing. Upon release from that arrest, many cells fail to complete DNA replication and exit the cell cycle in a p53‐dependent manner. If cells fail to withdraw from the cell cycle following DNA replication problems, they enter mitosis and missegregate chromosomes causing excessive DNA damage, which further limits their proliferative potential. These effects are observed in a range of tumour types, including breast cancer, implying that genotoxic stress is a common outcome of CDK4/6 inhibition. This unanticipated ability of CDK4/6 inhibitors to induce DNA damage now provides a rationale to better predict responsive tumour types and effective combination therapies, as demonstrated by the fact that CDK4/6 inhibition induces sensitivity to chemotherapeutics that also cause replication stress.     

The Yeast Pif1 Helicase Prevents Genomic Instability Caused by G-Quadruplex-Forming CEB1 Sequences In Vivo

In budding yeast, the Pif1 DNA helicase is involved in the maintenance of both nuclear and mitochondrial genomes, but its role in these processes is still poorly understood. Here, we provide evidence for a new Pif1 function by demonstrating that its absence promotes genetic instability of alleles of the G-rich human minisatellite CEB1 inserted in the Saccharomyces cerevisiae genome, but not of other tandem repeats. Inactivation of other DNA helicases, including Sgs1, had no effect on CEB1 stability. In vitro, we show that CEB1 repeats formed stable G-quadruplex (G4) secondary structures and the Pif1 protein unwinds these structures more efficiently than regular B-DNA. Finally, synthetic CEB1 arrays in which we mutated the potential G4-forming sequences were no longer destabilized in pif1Δ cells. Hence, we conclude that CEB1 instability in pif1Δ cells depends on the potential to form G-quadruplex structures, suggesting that Pif1 could play a role in the metabolism of G4-forming sequences.     

G-quadruplexes Significantly Stimulate Pif1 Helicase-catalyzed Duplex DNA Unwinding

The evolutionarily conserved G-quadruplexes (G4s) are faithfully inherited and serve a variety of cellular functions such as telomere maintenance, gene regulation, DNA replication initiation, and epigenetic regulation. Different from the Watson-Crick base-pairing found in duplex DNA, G4s are formed via Hoogsteen base pairing and are very stable and compact DNA structures. Failure of untangling them in the cell impedes DNA-based transactions and leads to genome instability. Cells have evolved highly specific helicases to resolve G4 structures. We used a recombinant nuclear form of Saccharomyces cerevisiae Pif1 to characterize Pif1-mediated DNA unwinding with a substrate mimicking an ongoing lagging strand synthesis stalled by G4s, which resembles a replication origin and a G4-structured flap in Okazaki fragment maturation. We find that the presence of G4 may greatly stimulate the Pif1 helicase to unwind duplex DNA. Further studies reveal that this stimulation results from G4-enhanced Pif1 dimerization, which is required for duplex DNA unwinding. This finding provides new insights into the properties and functions of G4s. We discuss the observed activation phenomenon in relation to the possible regulatory role of G4s in the rapid rescue of the stalled lagging strand synthesis by helping the replicator recognize and activate the replication origin as well as by quickly removing the G4-structured flap during Okazaki fragment maturation.     

G-quadruplexes and their regulatory roles in biology

‘If G-quadruplexes form so readily in vitro, Nature will have found a way of using them in vivo’ (Statement by Aaron Klug over 30 years ago).During the last decade, four-stranded helical structures called G-quadruplex (or G4) have emerged from being a structural curiosity observed in vitro, to being recognized as a possible nucleic acid based mechanism for regulating multiple biological processes in vivo. The sequencing of many genomes has revealed that they are rich in sequence motifs that have the potential to form G-quadruplexes and that their location is non-random, correlating with functionally important genomic regions. In this short review, we summarize recent evidence for the in vivo presence and function of DNA and RNA G-quadruplexes in various cellular pathways including DNA replication, gene expression and telomere maintenance. We also highlight remaining open questions that will have to be addressed in the future.     

G-quadruplexes in viruses: function and potential therapeutic applications

G-rich nucleic acids can form non-canonical G-quadruplex structures (G4s) in which four guanines fold in a planar arrangement through Hoogsteen hydrogen bonds. Although many biochemical and structural studies have focused on DNA sequences containing successive, adjacent guanines that spontaneously fold into G4s, evidence for their in vivo relevance has recently begun to accumulate. Complete sequencing of the human genome highlighted the presence of ∼300 000 sequences that can potentially form G4s. Likewise, the presence of putative G4-sequences has been reported in various viruses genomes [e.g., Human immunodeficiency virus (HIV-1), Epstein–Barr virus (EBV), papillomavirus (HPV)]. Many studies have focused on telomeric G4s and how their dynamics are regulated to enable telomere synthesis. Moreover, a role for G4s has been proposed in cellular and viral replication, recombination and gene expression control. In parallel, DNA aptamers that form G4s have been described as inhibitors and diagnostic tools to detect viruses [e.g., hepatitis A virus (HAV), EBV, cauliflower mosaic virus (CaMV), severe acute respiratory syndrome virus (SARS), simian virus 40 (SV40)]. Here, special emphasis will be given to the possible role of these structures in a virus life cycle as well as the use of G4-forming oligonucleotides as potential antiviral agents and innovative tools.     

Werner syndrome protein suppresses the formation of large deletions during the replication of human telomeric sequences

Werner syndrome (WS) is a disorder characterized by features of premature aging and increased cancer that is caused by loss of the RecQ helicase WRN. Telomeres consisting of duplex TTAGGG repeats in humans protect chromosome ends and sustain cellular proliferation. WRN prevents the loss of telomeres replicated from the G-rich strand, which can form secondary G-quadruplex (G4) structures. Here, we dissected WRN roles in the replication of telomeric sequences by examining factors inherent to telomeric repeats, such as G4 DNA, independently from other factors at chromosome ends that can also impede replication. For this we used the supF shuttle vector (SV) mutagenesis assay. We demonstrate that SVs with [TTAGGG]₆ sequences are stably replicated in human cells, and that the repeats suppress the frequency of large deletions despite G4 folding potential. WRN depletion increased the supF mutant frequency for both the telomeric and non-telomeric SVs, compared with the control cells, but this increase was much greater (27-fold) for telomeric SVs. The higher SV mutant frequencies in WRN-deficient cells were primarily due to an increase in large sequence deletions and rearrangements. However, WRN depletion caused a more dramatic increase in deletions and rearrangements arising within the telomeric SV (70-fold), compared with non-telomeric SV (8-fold). Our results indicate that WRN prevents large deletions and rearrangements during replication, and that this role is particularly important in templates with telomeric sequence. This provides a possible explanation for increased telomere loss in WS cells.     

Short LNA-modified oligonucleotide probes as efficient disruptors of DNA G-quadruplexes

G-quadruplexes (G4s) are well known non-canonical DNA secondary structures that can form in human cells. Most of the tools available to investigate G4-biology rely on small molecule ligands that stabilise these structures. However, the development of probes that disrupt G4s is equally important to study their biology. In this study, we investigated the disruption of G4s using Locked Nucleic Acids (LNA) as invader probes. We demonstrated that strategic positioning of LNA-modifications within short oligonucleotides (10 nts.) can significantly accelerate the rate of G4-disruption. Single-molecule experiments revealed that short LNA-probes can promote disruption of G4s with mechanical stability sufficient to stall polymerases. We corroborated this using a single-step extension assay, revealing that short LNA-probes can relieve replication dependent polymerase-stalling at G4 sites. We further demonstrated the potential of such LNA-based probes to study G4-biology in cells. By using a dual-luciferase assay, we found that short LNA probes can enhance the expression of c-KIT to levels similar to those observed when the c-KIT promoter is mutated to prevent the formation of the c-KIT1 G4. Collectively, our data suggest a potential use of rationally designed LNA-modified oligonucleotides as an accessible chemical-biology tool for disrupting individual G4s and interrogating their biological functions in cells.     

Re-evaluation of G-quadruplex propensity with G4Hunter

Critical evidence for the biological relevance of G-quadruplexes (G4) has recently been obtained in seminal studies performed in a variety of organisms. Four-stranded G-quadruplex DNA structures are promising drug targets as these non-canonical structures appear to be involved in a number of key biological processes. Given the growing interest for G4, accurate tools to predict G-quadruplex propensity of a given DNA or RNA sequence are needed. Several algorithms such as Quadparser predict quadruplex forming propensity. However, a number of studies have established that sequences that are not detected by these tools do form G4 structures (false negatives) and that other sequences predicted to form G4 structures do not (false positives). Here we report development and testing of a radically different algorithm, G4Hunter that takes into account G-richness and G-skewness of a given sequence and gives a quadruplex propensity score as output. To validate this model, we tested it on a large dataset of 392 published sequences and experimentally evaluated quadruplex forming potential of 209 sequences using a combination of biophysical methods to assess quadruplex formation in vitro. We experimentally validated the G4Hunter algorithm on a short complete genome, that of the human mitochondria (16.6 kb), because of its relatively high GC content and GC skewness as well as the biological relevance of these quadruplexes near instability hotspots. We then applied the algorithm to genomes of a number of species, including humans, allowing us to conclude that the number of sequences capable of forming stable quadruplexes (at least in vitro) in the human genome is significantly higher, by a factor of 2–10, than previously thought.     

Detection of G-quadruplex DNA in mammalian cells

It has been proposed that guanine-rich DNA forms four-stranded structures in vivo called G-quadruplexes or G4 DNA. G4 DNA has been implicated in several biological processes, but tools to study G4 DNA structures in cells are limited. Here we report the development of novel murine monoclonal antibodies specific for different G4 DNA structures. We show that one of these antibodies designated 1H6 exhibits strong nuclear staining in most human and murine cells. Staining intensity increased on treatment of cells with agents that stabilize G4 DNA and, strikingly, cells deficient in FANCJ, a G4 DNA-specific helicase, showed stronger nuclear staining than controls. Our data strongly support the existence of G4 DNA structures in mammalian cells and indicate that the abundance of such structures is increased in the absence of FANCJ. We conclude that monoclonal antibody 1H6 is a valuable tool for further studies on the role of G4 DNA in cell and molecular biology.     

Structural Basis for Human DNA Polymerase Kappa to Bypass Cisplatin Intrastrand Cross-Link (Pt-GG) Lesion as an Efficient and Accurate Extender

Cisplatin (cis-diamminedichloroplatinum) is a common chemotherapeutic drug that reacts with the N7 atoms of adjacent guanines in DNA to form the Pt-1,2-d(GpG) intrastrand cross-link (Pt-GG), a major product to block DNA replication. Translesion DNA synthesis has been implicated in chemoresistance during cisplatin treatment of cancer due to Pt-GG lesion bypass. Gene knockdown studies in human cells have indicated a role for polκ during translesion synthesis of the Pt-GG lesion. However, the bypass activity of polκ with cisplatin lesions has not been well characterized. In this study, we investigated polκ's ability to bypass Pt-GG lesion in vitro and determined two crystal structures of polκ in complex with Pt-GG DNA. The ternary complex structures represent two consecutive stages of lesion bypass: nucleotide insertion opposite the 5′G (Pt-GG2) and primer extension immediately after the lesion (Pt-GG3). Our biochemical data showed that polκ is very efficient and accurate in extending DNA primers after the first G of the Pt-GG lesion. The structures demonstrate that the efficiency and accuracy is achieved by stably accommodating the bases with the cisplatin adduct in the active site for proper Watson–Crick base pairing with the incoming nucleotide in both the second insertion and post-insertion complexes. Our studies suggest that polκ works as an extender for efficient replication of the Pt-GG lesion in cells. This work holds promise for considering polκ, along with polη, as potential targets for drug design, which together could improve the efficacy of cisplatin treatment for cancer therapy.