Differential Gene Expression following DHX36/G4R1 Knockout Is Associated with G-Quadruplex Content and Cancer

G-quadruplexes (G4s) are secondary DNA and RNA structures stabilized by positive cations in a central channel formed by stacked tetrads of Hoogsteen base-paired guanines. G4s form from G-rich sequences across the genome, whose biased distribution in regulatory regions points towards a gene-regulatory role. G4s can themselves be regulated by helicases, such as DHX36 (aliases: G4R1 and RHAU), which possess the necessary activity to resolve these stable structures. G4s have been shown to both positively and negatively regulate gene expression when stabilized by ligands, or through the loss of helicase activity. Using DHX36 knockout Jurkat cell lines, we identified widespread, although often subtle, effects on gene expression that are associated with the presence or number of observed G-quadruplexes in promoters or gene regions. Genes that significantly change their expression, particularly those that show a significant increase in RNA abundance under DHX36 knockout, are associated with a range of cellular functions and processes, including numerous transcription factors and oncogenes, and are linked to several cancers. Our work highlights the direct and indirect role of DHX36 in the transcriptome of T-lymphocyte leukemia cells and the potential for DHX36 dysregulation in cancer.

Dynamic alternative DNA structures in biology and disease

Repetitive elements in the human genome, once considered 'junk DNA', are now known to adopt more than a dozen alternative (that is, non-B) DNA structures, such as self-annealed hairpins, left-handed Z-DNA, three-stranded triplexes (H-DNA) or four-stranded guanine quadruplex structures (G4 DNA). These dynamic conformations can act as functional genomic elements involved in DNA replication and transcription, chromatin organization and genome stability. In addition, recent studies have revealed a role for these alternative structures in triggering error-generating DNA repair processes, thereby actively enabling genome plasticity. As a driving force for genetic variation, non-B DNA structures thus contribute to both disease aetiology and evolution.

G-Quadruplex DNA and Other Non-Canonical B-Form DNA Motifs Influence Productive and Latent HIV-1 Integration and Reactivation Potential

The integration of the HIV-1 genome into the host genome is an essential step in the life cycle of the virus and it plays a critical role in the expression, long-term persistence, and reactivation of HIV expression. To better understand the local genomic environment surrounding HIV-1 proviruses, we assessed the influence of non-canonical B-form DNA (non-B DNA) on the HIV-1 integration site selection. We showed that productively and latently infected cells exhibit different integration site biases towards non-B DNA motifs. We identified a correlation between the integration sites of the latent proviruses and non-B DNA features known to potently influence gene expression (e.g., cruciform, guanine-quadruplex (G4), triplex, and Z-DNA). The reactivation potential of latent proviruses with latency reversal agents also correlated with their proximity to specific non-B DNA motifs. The perturbation of G4 structures in vitro using G4 structure-destabilizing or -stabilizing ligands resulted in a significant reduction in integration within 100 base pairs of G4 motifs. The stabilization of G4 structures increased the integration within 300-500 base pairs from G4 motifs, increased integration near transcription start sites, and increased the proportion of latently infected cells. Moreover, we showed that host lens epithelium-derived growth factor (LEDGF)/p75 and cleavage and polyadenylation specificity factor 6 (CPSF6) influenced the distribution of integration sites near several non-B DNA motifs, especially G4 DNA. Our findings identify non-B DNA motifs as important factors that influence productive and latent HIV-1 integration and the reactivation potential of latent proviruses.

Creation and resolution of non-B-DNA structural impediments during replication

During replication, folding of the DNA template into non-B-form secondary structures provides one of the most abundant impediments to the smooth progression of the replisome. The core replisome collaborates with multiple accessory factors to ensure timely and accurate duplication of the genome and epigenome. Here, we discuss the forces that drive non-B structure formation and the evidence that secondary structures are a significant and frequent source of replication stress that must be actively countered. Taking advantage of recent advances in the molecular and structural biology of the yeast and human replisomes, we examine how structures form and how they may be sensed and resolved during replication.

High-throughput techniques enable advances in the roles of DNA and RNA secondary structures in transcriptional and post-transcriptional gene regulation

The most stable structure of DNA is the canonical right-handed double helix termed B DNA. However, certain environments and sequence motifs favor alternative conformations, termed non-canonical secondary structures. The roles of DNA and RNA secondary structures in transcriptional regulation remain incompletely understood. However, advances in high-throughput assays have enabled genome wide characterization of some secondary structures. Here, we describe their regulatory functions in promoters and 3’UTRs, providing insights into key mechanisms through which they regulate gene expression. We discuss their implication in human disease, and how advances in molecular technologies and emerging high-throughput experimental methods could provide additional insights.

G-quadruplexes in helminth parasites

Parasitic helminths infecting humans are highly prevalent infecting ∼2 billion people worldwide, causing inflammatory responses, malnutrition and anemia that are the primary cause of morbidity. In addition, helminth infections of cattle have a significant economic impact on livestock production, milk yield and fertility. The etiological agents of helminth infections are mainly Nematodes (roundworms) and Platyhelminths (flatworms). G-quadruplexes (G4) are unusual nucleic acid structures formed by G-rich sequences that can be recognized by specific G4 ligands. Here we used the G4Hunter Web Tool to identify and compare potential G4 sequences (PQS) in the nuclear and mitochondrial genomes of various helminths to identify G4 ligand targets. PQS are nonrandomly distributed in these genomes and often located in the proximity of genes. Unexpectedly, a Nematode, Ascaris lumbricoides, was found to be highly enriched in stable PQS. This species can tolerate high-stability G4 structures, which are not counter selected at all, in stark contrast to most other species. We experimentally confirmed G4 formation for sequences found in four different parasitic helminths. Small molecules able to selectively recognize G4 were found to bind to Schistosoma mansoni G4 motifs. Two of these ligands demonstrated potent activity both against larval and adult stages of this parasite.

Modulating gene expression in breast cancer via DNA secondary structure and the CRISPR toolbox

Breast cancer is the most commonly diagnosed malignancy in women, and while the survival prognosis of patients with early-stage, non-metastatic disease is ∼75%, recurrence poses a significant risk and advanced and/or metastatic breast cancer is incurable. A distinctive feature of advanced breast cancer is an unstable genome and altered gene expression patterns that result in disease heterogeneity. Transcription factors represent a unique therapeutic opportunity in breast cancer, since they are known regulators of gene expression, including gene expression involved in differentiation and cell death, which are themselves often mutated or dysregulated in cancer. While transcription factors have traditionally been viewed as 'undruggable', progress has been made in the development of small-molecule therapeutics to target relevant protein-protein, protein-DNA and enzymatic active sites, with varying levels of success. However, non-traditional approaches such as epigenetic editing, transcriptional control via CRISPR/dCas9 systems, and gene regulation through non-canonical nucleic acid secondary structures represent new directions yet to be fully explored. Here, we discuss these new approaches and current limitations in light of new therapeutic opportunities for breast cancers.

Cell Biology of Giant Cell Tumour of Bone: Crosstalk between m/wt Nucleosome H3.3, Telomeres and Osteoclastogenesis

Giant cell tumour of bone (GCTB) is a rare and intriguing primary bone neoplasm. Worrisome clinical features are its local destructive behaviour, its high tendency to recur after surgical therapy and its ability to create so-called benign lung metastases (lung 'plugs'). GCTB displays a complex and difficult-to-understand cell biological behaviour because of its heterogenous morphology. Recently, a driver mutation in histone H3.3 was found. This mutation is highly conserved in GCTB but can also be detected in glioblastoma. Denosumab was recently introduced as an extra option of medical treatment next to traditional surgical and in rare cases, radiotherapy. Despite these new insights, many 'old' questions about the key features of GCTB remain unanswered, such as the presence of telomeric associations (TAs), the reactivation of hTERT, and its slight genomic instability. This review summarises the recent relevant literature of histone H3.3 in relation to the GCTB-specific G34W mutation and pays specific attention to the G34W mutation in relation to the development of TAs, genomic instability, and the characteristic morphology of GCTB. As pieces of an etiogenetic puzzle, this review tries fitting all these molecular features and the unique H3.3 G34W mutation together in GCTB.

Guanine quadruplexes and their roles in molecular processes

The role of guanine quadruplexes (G4) in fundamental biological processes like DNA replication, transcription, translation and telomere maintenance is recognized. G4 structure dynamics is regulated by G4 structure binding proteins and is thought to be crucial for the maintenance of genome integrity in both prokaryotic and eukaryotic cells. Growing research over the last decade has expanded the existing knowledge of the functional diversity of G4 (DNA and RNA) structures across the working models. The control of G4 structure dynamics using G4 binding drugs has been suggested as the putative targets in the control of cancer and bacterial pathogenesis. This review has brought forth the collections of recent information that indicate G4 (mostly G4 DNA) roles in microbial pathogenesis, DNA damaging stress response in bacteria and mammalian cells. Studies in mitochondrial gene function regulation by G4s have also been underscored. Finally, the interdependence of G4s and epigenetic modifications and their speculated medical implications through G4 interacting proteins has been discussed.

Fundamentals of G-quadruplex biology

Several decades elapsed between the first descriptions of G-quadruplex nucleic acid structures (G4s) assembled in vitro and the emergence of experimental findings indicating that such structures can form and function in living systems. A large body of evidence now supports roles for G4s in many aspects of nucleic acid biology, spanning processes from transcription and chromatin structure, mRNA processing, protein translation, DNA replication and genome stability, and telomere and mitochondrial function. Nonetheless, it must be acknowledged that some of this evidence is tentative, which is not surprising given the technical challenges associated with demonstrating G4s in biology. Here I provide an overview of evidence for G4 biology, focusing particularly on the many potential pitfalls that can be encountered in its investigation, and briefly discuss some of broader biological processes that may be impacted by G4s including cancer.

Epigenetic Modulation of Chromatin States and Gene Expression by G-Quadruplex Structures

G-quadruplexes are four-stranded helical nucleic acid structures formed by guanine-rich sequences. A considerable number of studies have revealed that these noncanonical structural motifs are widespread throughout the genome and transcriptome of numerous organisms, including humans. In particular, G-quadruplexes occupy strategic locations in genomic DNA and both coding and noncoding RNA molecules, being involved in many essential cellular and organismal functions. In this review, we first outline the fundamental structural features of G-quadruplexes and then focus on the concept that these DNA and RNA structures convey a distinctive layer of epigenetic information that is critical for the complex regulation, either positive or negative, of biological activities in different contexts. In this framework, we summarize and discuss the proposed mechanisms underlying the functions of G-quadruplexes and their interacting factors. Furthermore, we give special emphasis to the interplay between G-quadruplex formation/disruption and other epigenetic marks, including biochemical modifications of DNA bases and histones, nucleosome positioning, and three-dimensional organization of chromatin. Finally, epigenetic roles of RNA G-quadruplexes in post-transcriptional regulation of gene expression are also discussed. Undoubtedly, the issues addressed in this review take on particular importance in the field of comparative epigenetics, as well as in translational research.

The Interplay between G-quadruplex and Transcription

G4 DNA is a non-canonical DNA structure consisting of a stacked array of Gquartets held together by base pairing between guanine bases. The formation of G4 DNA requires a cluster of guanine-runs within a strand of DNA. Even though the chemistry of this remarkable DNA structure has been under investigation for decades, evidence supporting the biological relevance of G4 DNA has only begun to emerge and point to very important and conserved biological functions. This review will specifically focus on the interplay between transcription and G4 DNA and discuss two alternative but interconnected perspectives. The first part of the review will describe the evidence substantiating the intriguing idea that a shift in DNA structural conformation could be another layer of non-genetic or epigenetic regulator of gene expression and thereby an important determinant of cell fate. The second part will describe the recent genetic studies showing that those genomic loci containing G4 DNA-forming guanine-rich sequences are potential hotspots of genome instability and that the level and orientation of transcription is critical in the materialization of genome instability associated with these sequences.

DNA G-quadruplex stability, position and chromatin accessibility are associated with CpG island methylation

CpG islands (CGI) are genomic regions associated with gene promoters and involved in gene expression regulation. Despite their high CpG content and unlike bulk genomic DNA methylation pattern, these regions are usually hypomethylated. So far, the mechanisms controlling the CGI methylation patterning remain unclear. G-quadruplex (G4) structures can inhibit DNA methyltransferases 1 enzymatic activity, leading to CGI hypomethylation. Our aim was to analyse the association of G4 forming sequences (G4FS) and CGI methylation as well as to determine the intrinsic and extrinsic characteristics of G4FS that may modulate this phenomenon. Using methylation data from human embryonic stem cells (hESCs) and three hESC-derived populations, we showed that hypomethylated CpGs located inside CGI (CGI/CpG) tend to be associated with highly stable G4FS (Minimum free energy ≤ -30 kcal·mol^-1 ). The association of highly stable G4FS and hypomethylation tend to be stronger when these structures are located at shorter distances (~ < 150 bp) from GCI/CpGs, when G4FS and CpGs are located within open chromatin and G4FS are inside CGI. Moreover, this association is not strongly influenced by the CpG content of CGI. Conversely, highly methylated CGI/CpG tend to be associated with low stability G4FS. Although CpGs inside CGIs without a G4FS tend to be more methylated, high stability G4FS within CGI neighbourhood were associated with decreased methylation. In summary, our data indicate that G4FS may act as protective cis elements against CGI methylation, and this effect seems to be influenced by the G4FS folding potential, its presence within CGI, CpG distance from G4FS and chromatin accessibility.

Replication of G Quadruplex DNA

A cursory look at any textbook image of DNA replication might suggest that the complex machine that is the replisome runs smoothly along the chromosomal DNA. However, many DNA sequences can adopt non-B form secondary structures and these have the potential to impede progression of the replisome. A picture is emerging in which the maintenance of processive DNA replication requires the action of a significant number of additional proteins beyond the core replisome to resolve secondary structures in the DNA template. By ensuring that DNA synthesis remains closely coupled to DNA unwinding by the replicative helicase, these factors prevent impediments to the replisome from causing genetic and epigenetic instability. This review considers the circumstances in which DNA forms secondary structures, the potential responses of the eukaryotic replisome to these impediments in the light of recent advances in our understanding of its structure and operation and the mechanisms cells deploy to remove secondary structure from the DNA. To illustrate the principles involved, we focus on one of the best understood DNA secondary structures, G quadruplexes (G4s), and on the helicases that promote their resolution.

Non-duplex G-Quadruplex Structures Emerge as Mediators of Epigenetic Modifications

The role of non-duplex DNA, the guanine-quadruplex structure in particular, is becoming widely appreciated. Increasing evidence in the last decade implicates quadruplexes in important processes such as transcription and replication. Interestingly, more recent work suggests roles for quadruplexes, in association with quadruplex-interacting proteins, in epigenetics through both DNA and histone modifications. Here, we review the effect of the quadruplex structure on post-replication epigenetic memory and quadruplex-induced promoter DNA/histone modifications. Furthermore, we highlight the epigenetic state of the telomerase promoter where quadruplexes could play a key regulatory role. Finally, we discuss the possibility that DNA structures such as quadruplexes, within a largely duplex DNA background, could act as molecular anchors for locally induced epigenetic modifications.

G Quadruplex in Plants: A Ubiquitous Regulatory Element and Its Biological Relevance

G quadruplexes (G4) are higher-order DNA and RNA secondary structures formed by G-rich sequences that are built around tetrads of hydrogen-bonded guanine bases. Potential G4 quadruplex sequences have been identified in G-rich eukaryotic non-telomeric and telomeric genomic regions. Upon function, G4 formation is known to involve in chromatin remodeling, gene regulation and has been associated with genomic instability, genetic diseases and cancer progression. The natural role and biological validation of G4 structures is starting to be explored, and is of particular interest for the therapeutic interventions for human diseases. However, the existence and physiological role of G4 DNA and G4 RNA in plants species have not been much investigated yet and therefore, is of great interest for the development of improved crop varieties for sustainable agriculture. In this context, several recent studies suggests that these highly diverse G4 structures in plants can be employed to regulate expression of genes involved in several pathophysiological conditions including stress response to biotic and abiotic stresses as well as DNA damage. In the current review, we summarize the recent findings regarding the emerging functional significance of G4 structures in plants and discuss their potential value in the development of improved crop varieties.

G-Quadruplex surveillance in BCL-2 gene: a promising therapeutic intervention in cancer treatment

Recently, therapeutic implications of BCL-2 quadruplex invigorated the field of clinical oncology. This Keynote review discusses how a BCL-2 quadruplex-selective approach circumvents the limitations of existing therapeutics; and which improvisations might ameliorate the recent trends of quadruplex-based treatment.

Mechanisms and regulation of DNA replication initiation in eukaryotes

Cellular DNA replication is initiated through the action of multiprotein complexes that recognize replication start sites in the chromosome (termed origins) and facilitate duplex DNA melting within these regions. In a typical cell cycle, initiation occurs only once per origin and each round of replication is tightly coupled to cell division. To avoid aberrant origin firing and re-replication, eukaryotes tightly regulate two events in the initiation process: loading of the replicative helicase, MCM2-7, onto chromatin by the origin recognition complex (ORC), and subsequent activation of the helicase by its incorporation into a complex known as the CMG. Recent work has begun to reveal the details of an orchestrated and sequential exchange of initiation factors on DNA that give rise to a replication-competent complex, the replisome. Here, we review the molecular mechanisms that underpin eukaryotic DNA replication initiation - from selecting replication start sites to replicative helicase loading and activation - and describe how these events are often distinctly regulated across different eukaryotic model organisms.

Interaction of a Cationic Porphyrin and Its Metal Derivatives with G-Quadruplex DNA

G-quadruplex (GQ) structures formed from guanine-rich sequences are found throughout the genome and are overrepresented in the promoter regions of some oncogenes, at the telomeric ends of eukaryotic chromosomes, and at the 5'-untranslated regions of mRNA. Interaction of small molecule ligands with GQ DNA is an area of great research interest to develop novel anticancer therapeutics and GQ sensors. In this paper we examine the interactions of TMPyP4, its isomer TMPyP2 (containing N-methyl-2-pyridyl substituents, N-Me-2Py) as well as two metal derivatives ZnTMPyP4 and CuTMPyP4 with GQs formed by dT₄G₄ and dT₄G₄T in 100 mM K⁺ or Na⁺ conditions. The DNA sequences were chosen to elucidate the effect of the 3'-T on the stabilization effect of porphyrins, binding modes, affinities, and stoichiometries determined via circular dichroism melting studies, UV-vis titrations, continuous variation analysis, and fluorescence studies. Our findings demonstrate that the stabilizing abilities of porphyrins are stronger toward (dT₄G₄)₄ as compared to (dT₄G₄T)₄ (ΔT_m is 4.4 vs -6.4 for TMPyP4; 12.7 vs 5.7 for TMPyP2; 16.4 vs 12.1 for ZnTMPyP4; and 1.9 vs -8.4 °C for CuTMPyP4) suggesting that the 3'G-tetrad presents at least one of the binding sites. The binding affinity was determined to be moderate (K_a ∼ 10⁶-10⁷ μM^-1) with a typical binding stoichiometry of 1:1 or 2:1 porphyrin-to-GQ. In all studies, ZnTMPyP4 emerged as a ligand superior to TMPyP4. Overall, our work contributes to clearer understanding of interactions between porphyrins and GQ DNA.

Transposable elements and G-quadruplexes

A significant part of eukaryotic genomes is formed by transposable elements (TEs) containing not only genes but also regulatory sequences. Some of the regulatory sequences located within TEs can form secondary structures like hairpins or three-stranded (triplex DNA) and four-stranded (quadruplex DNA) conformations. This review focuses on recent evidence showing that G-quadruplex-forming sequences in particular are often present in specific parts of TEs in plants and humans. We discuss the potential role of these structures in the TE life cycle as well as the impact of G-quadruplexes on replication, transcription, translation, chromatin status, and recombination. The aim of this review is to emphasize that TEs may serve as vehicles for the genomic spread of G-quadruplexes. These non-canonical DNA structures and their conformational switches may constitute another regulatory system that, together with small and long non-coding RNA molecules and proteins, contribute to the complex cellular network resulting in the large diversity of eukaryotes.

QuadBase2: web server for multiplexed guanine quadruplex mining and visualization

DNA guanine quadruplexes or G4s are non-canonical DNA secondary structures which affect genomic processes like replication, transcription and recombination. G4s are computationally identified by specific nucleotide motifs which are also called putative G4 (PG4) motifs. Despite the general relevance of these structures, there is currently no tool available that can allow batch queries and genome-wide analysis of these motifs in a user-friendly interface. QuadBase2 (quadbase.igib.res.in) presents a completely reinvented web server version of previously published QuadBase database. QuadBase2 enables users to mine PG4 motifs in up to 178 eukaryotes through the EuQuad module. This module interfaces with Ensembl Compara database, to allow users mine PG4 motifs in the orthologues of genes of interest across eukaryotes. PG4 motifs can be mined across genes and their promoter sequences in 1719 prokaryotes through ProQuad module. This module includes a feature that allows genome-wide mining of PG4 motifs and their visualization as circular histograms. TetraplexFinder, the module for mining PG4 motifs in user-provided sequences is now capable of handling up to 20 MB of data. QuadBase2 is a comprehensive PG4 motif mining tool that further expands the configurations and algorithms for mining PG4 motifs in a user-friendly way.

The chromatin environment shapes DNA replication origin organization and defines origin classes

To unveil the still-elusive nature of metazoan replication origins, we identified them genome-wide and at unprecedented high-resolution in mouse ES cells. This allowed initiation sites (IS) and initiation zones (IZ) to be differentiated. We then characterized their genetic signatures and organization and integrated these data with 43 chromatin marks and factors. Our results reveal that replication origins can be grouped into three main classes with distinct organization, chromatin environment, and sequence motifs. Class 1 contains relatively isolated, low-efficiency origins that are poor in epigenetic marks and are enriched in an asymmetric AC repeat at the initiation site. Late origins are mainly found in this class. Class 2 origins are particularly rich in enhancer elements. Class 3 origins are the most efficient and are associated with open chromatin and polycomb protein-enriched regions. The presence of Origin G-rich Repeated elements (OGRE) potentially forming G-quadruplexes (G4) was confirmed at most origins. These coincide with nucleosome-depleted regions located upstream of the initiation sites, which are associated with a labile nucleosome containing H3K64ac. These data demonstrate that specific chromatin landscapes and combinations of specific signatures regulate origin localization. They explain the frequently observed links between DNA replication and transcription. They also emphasize the plasticity of metazoan replication origins and suggest that in multicellular eukaryotes, the combination of distinct genetic features and chromatin configurations act in synergy to define and adapt the origin profile.

Histone variant H3.3 provides the heterochromatic H3 lysine 9 tri-methylation mark at telomeres

In addition to being a hallmark at active genes, histone variant H3.3 is deposited by ATRX at repressive chromatin regions, including the telomeres. It is unclear how H3.3 promotes heterochromatin assembly. We show that H3.3 is targeted for K9 trimethylation to establish a heterochromatic state enriched in trimethylated H3.3K9 at telomeres. In H3f3a^−/− and H3f3b^−/− mouse embryonic stem cells (ESCs), H3.3 deficiency results in reduced levels of H3K9me3, H4K20me3 and ATRX at telomeres. The H3f3b^−/− cells show increased levels of telomeric damage and sister chromatid exchange (t-SCE) activity when telomeres are compromised by treatment with a G-quadruplex (G4) DNA binding ligand or by ASF1 depletion. Overexpression of wild-type H3.3 (but not a H3.3K9 mutant) in H3f3b^−/− cells increases H3K9 trimethylation level at telomeres and represses t-SCE activity induced by a G4 ligand. This study demonstrates the importance of H3.3K9 trimethylation in heterochromatin formation at telomeres. It provides insights into H3.3 function in maintaining integrity of mammalian constitutive heterochromatin, adding to its role in mediating transcription memory in the genome.

R-loops and initiation of DNA replication in human cells: a missing link?

The unanticipated widespread occurrence of stable hybrid DNA/RNA structures (R-loops) in human cells and the increasing evidence of their involvement in several human malignancies have invigorated the research on R-loop biology in recent years. Here we propose that physiological R-loop formation at CpG island promoters can contribute to DNA replication origin specification at these regions, the most efficient replication initiation sites in mammalian cells. Quite likely, this occurs by the strand-displacement reaction activating the formation of G-quadruplex structures that target the origin recognition complex (ORC) in the single-stranded conformation. In agreement with this, we found that R-loops co-localize with the ORC within the same CpG island region in a significant fraction of these efficient replication origins, precisely at the position displaying the highest density of G4 motifs. This scenario builds on the connection between transcription and replication in human cells and suggests that R-loop dysregulation at CpG island promoter-origins might contribute to the phenotype of DNA replication abnormalities and loss of genome integrity detected in cancer cells.

Characterizing and controlling intrinsic biases of lambda exonuclease in nascent strand sequencing reveals phasing between nucleosomes and G-quadruplex motifs around a subset of human replication origins

Nascent strand sequencing (NS-seq) is used to discover DNA replication origins genome-wide, allowing identification of features for their specification. NS-seq depends on the ability of lambda exonuclease (λ-exo) to efficiently digest parental DNA while leaving RNA-primer protected nascent strands intact. We used genomics and biochemical approaches to determine if λ-exo digests all parental DNA sequences equally. We report that λ-exo does not efficiently digest G-quadruplex (G4) structures in a plasmid. Moreover, λ-exo digestion of nonreplicating genomic DNA (LexoG0) enriches GC-rich DNA and G4 motifs genome-wide. We used LexoG0 data to control for nascent strand–independent λ-exo biases in NS-seq and validated this approach at the rDNA locus. The λ-exo–controlled NS-seq peaks are not GC-rich, and only 35.5% overlap with 6.8% of all G4s, suggesting that G4s are not general determinants for origin specification but may play a role for a subset. Interestingly, we observed a periodic spacing of G4 motifs and nucleosomes around the peak summits, suggesting that G4s may position nucleosomes at this subset of origins. Finally, we demonstrate that use of Na⁺ instead of K⁺ in the λ-exo digestion buffer reduced the effect of G4s on λ-exo digestion and discuss ways to increase both the sensitivity and specificity of NS-seq.

Guanine quadruplexes are formed by specific regions of human transposable elements

Background

Transposable elements form a significant proportion of eukaryotic genomes. Recently, Lexa et al. (Nucleic Acids Res 42:968-978, 2014) reported that plant long terminal repeat (LTR) retrotransposons often contain potential quadruplex sequences (PQSs) in their LTRs and experimentally confirmed their ability to adopt four-stranded DNA conformations.

Results

Here, we searched for PQSs in human retrotransposons and found that PQSs are specifically localized in the 3’-UTR of LINE-1 elements, in LTRs of HERV elements and are strongly accumulated in specific regions of SVA elements. Circular dichroism spectroscopy confirmed that most PQSs had adopted monomolecular or bimolecular guanine quadruplex structures. Evolutionarily young SVA elements contained more PQSs than older elements and their propensity to form quadruplex DNA was higher. Full-length L1 elements contained more PQSs than truncated elements; the highest proportion of PQSs was found inside transpositionally active L1 elements (PA2 and HS families).

Conclusions

Conservation of quadruplexes at specific positions of transposable elements implies their importance in their life cycle. The increasing quadruplex presence in evolutionarily young LINE-1 and SVA families makes these elements important contributors toward present genome-wide quadruplex distribution.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1032) contains supplementary material, which is available to authorized users.

RAN translation and frameshifting as translational challenges at simple repeats of human neurodegenerative disorders

Repeat-associated disorders caused by expansions of short sequences have been classified as coding and noncoding and are thought to be caused by protein gain-of-function and RNA gain-of-function mechanisms, respectively. The boundary between such classifications has recently been blurred by the discovery of repeat-associated non-AUG (RAN) translation reported in spinocerebellar ataxia type 8, myotonic dystrophy type 1, fragile X tremor/ataxia syndrome and C9ORF72 amyotrophic lateral sclerosis and frontotemporal dementia. This noncanonical translation requires no AUG start codon and can initiate in multiple frames of CAG, CGG and GGGGCC repeats of the sense and antisense strands of disease-relevant transcripts. RNA structures formed by the repeats have been suggested as possible triggers; however, the precise mechanism of the translation initiation remains elusive. Templates containing expansions of microsatellites have also been shown to challenge translation elongation, as frameshifting has been recognized across CAG repeats in spinocerebellar ataxia type 3 and Huntington's disease. Determining the critical requirements for RAN translation and frameshifting is essential to decipher the mechanisms that govern these processes. The contribution of unusual translation products to pathogenesis needs to be better understood. In this review, we present current knowledge regarding RAN translation and frameshifting and discuss the proposed mechanisms of translational challenges imposed by simple repeat expansions.

Impact of alternative DNA structures on DNA damage, DNA repair, and genetic instability

Repetitive genomic sequences can adopt a number of alternative DNA structures that differ from the canonical B-form duplex (i.e. non-B DNA). These non-B DNA-forming sequences have been shown to have many important biological functions related to DNA metabolic processes; for example, they may have regulatory roles in DNA transcription and replication. In addition to these regulatory functions, non-B DNA can stimulate genetic instability in the presence or absence of DNA damage, via replication-dependent and/or replication-independent pathways. This review focuses on the interactions of non-B DNA conformations with DNA repair proteins and how these interactions impact genetic instability.

Insight into the molecular recognition of spermine by DNA quadruplexes from an NMR study of the association of spermine with the thrombin-binding aptamer

The preferred residence sites and the conformation of DNA-bound polyamines are central to understanding the regulatory roles of polyamines. To this end, we have used a series of selective (13)C-edited and selective total correlation spectroscopy-edited one-dimensional (1D) nuclear Overhauser effect spectroscopy NMR experiments to determine a number of intramolecular (1)H nuclear Overhauser effect (NOE) connectivities in (13)C-labelled spermine bound to the thrombin-binding aptamer. The results provide evidence that the aptamer-bound spermine adopts a conformation that optimizes electrostatic and hydrogen bond contacts with the aptamer backbone. The distance between the nitrogen atoms of the central aminobutyl is reduced by an increase in the population of gauche conformers at the C6-C7 bonds, which results in either a curved or S-shaped spermine conformation. Molecular modelling contributes insight toward the mode of spermine binding of these spermine structures within the narrow grooves of DNA quadruplexes. In each case, the N5 ammonium group makes hydrogen bonds with two nearby phosphates across the narrow groove. Our results have implications for the understanding of chromatin structure and the rational design of quadruplex-binding drugs.

DNA thermodynamics shape chromosome organization and topology

How much information is encoded in the DNA sequence of an organism? We argue that the informational, mechanical and topological properties of DNA are interdependent and act together to specify the primary characteristics of genetic organization and chromatin structures. Superhelicity generated in vivo, in part by the action of DNA translocases, can be transmitted to topologically sensitive regions encoded by less stable DNA sequences.

The G4 genome

Recent experiments provide fascinating examples of how G4 DNA and G4 RNA structures—aka quadruplexes—may contribute to normal biology and to genomic pathologies. Quadruplexes are transient and therefore difficult to identify directly in living cells, which initially caused skepticism regarding not only their biological relevance but even their existence. There is now compelling evidence for functions of some G4 motifs and the corresponding quadruplexes in essential processes, including initiation of DNA replication, telomere maintenance, regulated recombination in immune evasion and the immune response, control of gene expression, and genetic and epigenetic instability. Recognition and resolution of quadruplex structures is therefore an essential component of genome biology. We propose that G4 motifs and structures that participate in key processes compose the G4 genome, analogous to the transcriptome, proteome, or metabolome. This is a new view of the genome, which sees DNA as not only a simple alphabet but also a more complex geography. The challenge for the future is to systematically identify the G4 motifs that form quadruplexes in living cells and the features that confer on specific G4 motifs the ability to function as structural elements.

The yin and yang of repair mechanisms in DNA structure-induced genetic instability

DNA can adopt a variety of secondary structures that deviate from the canonical Watson-Crick B-DNA form. More than 10 types of non-canonical or non-B DNA secondary structures have been characterized, and the sequences that have the capacity to adopt such structures are very abundant in the human genome. Non-B DNA structures have been implicated in many important biological processes and can serve as sources of genetic instability, implicating them in disease and evolution. Non-B DNA conformations interact with a wide variety of proteins involved in replication, transcription, DNA repair, and chromatin architectural regulation. In this review, we will focus on the interactions of DNA repair proteins with non-B DNA and their roles in genetic instability, as the proteins and DNA involved in such interactions may represent plausible targets for selective therapeutic intervention.

Genome-wide chromatin remodeling identified at GC-rich long nucleosome-free regions

To gain deeper insights into principles of cell biology, it is essential to understand how cells reorganize their genomes by chromatin remodeling. We analyzed chromatin remodeling on next generation sequencing data from resting and activated T cells to determine a whole-genome chromatin remodeling landscape. We consider chromatin remodeling in terms of nucleosome repositioning which can be observed most robustly in long nucleosome-free regions (LNFRs) that are occupied by nucleosomes in another cell state. We found that LNFR sequences are either AT-rich or GC-rich, where nucleosome repositioning was observed much more prominently in GC-rich LNFRs - a considerable proportion of them outside promoter regions. Using support vector machines with string kernels, we identified a GC-rich DNA sequence pattern indicating loci of nucleosome repositioning in resting T cells. This pattern appears to be also typical for CpG islands. We found out that nucleosome repositioning in GC-rich LNFRs is indeed associated with CpG islands and with binding sites of the CpG-island-binding ZF-CXXC proteins KDM2A and CFP1. That this association occurs prominently inside and also prominently outside of promoter regions hints at a mechanism governing nucleosome repositioning that acts on a whole-genome scale.

Bisquinolinium compounds induce quadruplex-specific transcriptome changes in HeLa S3 cell lines

Background

Guanosine rich sequences capable of forming G-quadruplex (G4) motifs are enriched near the gene transcription start site (TSS) in the human genome. When probed at the single gene level, G-quadruplex motifs residing in promoter regions show substantial effects on gene transcription. Moreover, further changes in transcription levels are noticed when G4-motifs are targeted with G-quadruplex-specific small molecules.

Results

Global studies concerning general changes of the transcriptome via targeting promoter-based G-quadruplex motifs are very limited and have so far only been carried out with compounds displaying weak selectivity for quadruplex sequences. Here we utilize two G-quadruplex-specific bisquinolinium derivatives PhenDC3 and 360A and investigate their effects on the expression of the HeLa S3 transcriptome. Our results show wide-spread changes in the transcriptome with specificity for genes with G-quadruplex motifs near their transcription start sites (TSS). Using real-time PCR we further confirmed the specificity of PhenDC3 and 360A as potent molecules to target G-quadruplex-regulated genes.

Conclusions

Specific effects on quadruplex-containing genes have been observed utilizing whole-transcriptome analysis upon treatment of cultured cells with quadruplex-selective bisquinolinium compounds.

Reporter assays for studying quadruplex nucleic acids

DNA and RNA G-quadruplexes have gained increasing attention due to their potential role in a wide range of biological functions. The majority of functional studies characterize the influence of quadruplexes in gene expression including transcription and translation. Many of these studies have used reporter assays to elucidate the effect of quadruplexes at certain positions in promoters and untranslated mRNA regions (UTRs). Reporter assays are the preferred method to ascertain the biological function of DNA or RNA G-quadruplexes intracellularly due to their ready availability, fast cloning and experimental setup and reproducibility. Moreover, these reporter assays are also helpful to compare or screen for selectivity and efficacy of small molecules that target DNA and RNA G-quadruplexes in the cellular context. Here we briefly discuss various aspects of reporter assays followed by a review of available studies using reporter assays to understand the role and functions of DNA and RNA quadruplexes in gene expression.

Quadruplex-single nucleotide polymorphisms (Quad-SNP) influence gene expression difference among individuals

Non-canonical guanine quadruplex structures are not only predominant but also conserved among bacterial and mammalian promoters. Moreover recent findings directly implicate quadruplex structures in transcription. These argue for an intrinsic role of the structural motif and thereby posit that single nucleotide polymorphisms (SNP) that compromise the quadruplex architecture could influence function. To test this, we analysed SNPs within quadruplex motifs (Quad-SNP) and gene expression in 270 individuals across four populations (HapMap) representing more than 14,500 genotypes. Findings reveal significant association between quadruplex-SNPs and expression of the corresponding gene in individuals (P < 0.0001). Furthermore, analysis of Quad-SNPs obtained from population-scale sequencing of 1000 human genomes showed relative selection bias against alteration of the structural motif. To directly test the quadruplex-SNP-transcription connection, we constructed a reporter system using the RPS3 promoter-remarkable difference in promoter activity in the 'quadruplex-destabilized' versus 'quadruplex-intact' promoter was noticed. As a further test, we incorporated a quadruplex motif or its disrupted counterpart within a synthetic promoter reporter construct. The quadruplex motif, and not the disrupted-motif, enhanced transcription in human cell lines of different origin. Together, these findings build direct support for quadruplex-mediated transcription and suggest quadruplex-SNPs may play significant role in mechanistically understanding variations in gene expression among individuals.

Zinc-finger transcription factors are associated with guanine quadruplex motifs in human, chimpanzee, mouse and rat promoters genome-wide

Function of non-B DNA structures are poorly understood though several bioinformatics studies predict role of the G-quadruplex DNA structure in transcription. Earlier, using transcriptome profiling we found evidence of widespread G-quadruplex-mediated gene regulation. Herein, we asked whether potential G-quadruplex (PG4) motifs associate with transcription factors (TF). This was analyzed using 220 position weight matrices [designated as transcription factor binding sites (TFBS)], representing 187 unique TF, in >75 000 genes in human, chimpanzee, mouse and rat. Results show binding sites of nine TFs, including that of AP-2, SP1, MAZ and VDR, occurred significantly within 100 bases of the PG4 motif (P < 1.24E-10). PG4–TFBS combinations were conserved in ‘orthologously’ related promoters across all four organisms and were associated with >850 genes in each genome. Remarkably, seven of the nine TFs were zinc-finger binding proteins indicating a novel characteristic of PG4 motifs. To test these findings, transcriptome profiles from human cell lines treated with G-quadruplex-specific molecules were used; 66 genes were significantly differentially expressed across both cell-types, which also harbored conserved PG4 motifs along with one/more of the nine TFBS. In addition, genes regulated by PG4–TFBS combinations were found to be co-regulated in human tissues, further emphasizing the regulatory significance of the associations.

DNA secondary structures and epigenetic determinants of cancer genome evolution

An unstable genome is a hallmark of many cancers. It is unclear, however, whether some mutagenic features driving somatic alterations in cancer are encoded in the genome sequence and whether they can operate in a tissue-specific manner. We performed a genome-wide analysis of 663,446 DNA breakpoints associated with somatic copy-number alterations (SCNAs) from 2,792 cancer samples classified into 26 cancer types. Many SCNA breakpoints are spatially clustered in cancer genomes. We observed a significant enrichment for G-quadruplex sequences (G4s) in the vicinity of SCNA breakpoints and established that SCNAs show a strand bias consistent with G4-mediated structural alterations. Notably, abnormal hypomethylation near G4s-rich regions is a common signature for many SCNA breakpoint hotspots. We propose a mechanistic hypothesis that abnormal hypomethylation in genomic regions enriched for G4s acts as a mutagenic factor driving tissue-specific mutational landscapes in cancer.

Targeting G-quadruplexes in gene promoters: a novel anticancer strategy?

G-quadruplexes are four-stranded DNA structures that are over-represented in gene promoter regions and are viewed as emerging therapeutic targets in oncology, as transcriptional repression of oncogenes through stabilization of these structures could be a novel anticancer strategy. Many gene promoter G-quadruplexes have physicochemical properties and structural characteristics that might make them druggable, and their structural diversity suggests that a high degree of selectivity might be possible. Here, we describe the evidence for G-quadruplexes in gene promoters and discuss their potential as therapeutic targets, as well as progress in the development of strategies to harness this potential through intervention with small-molecule ligands.

RNA quadruplexes

Opposed to DNA quadruplex sequences, RNA quadruplexes are still less well characterized. On the other hand, RNA quadruplexes are found to be at least as stable as their DNA counterparts. They show the same dependence on metal ions but seem to be much more restricted with respect to the adopted conformations. Other than DNA, which is mostly found to be double-stranded inside cells, RNAs are produced during transcription without its complementary sequence. The absence of a second strand that is able to hybridize and form a duplex makes the folding of RNA quadruplexes a likely event of intramolecular structure formation. Consequently, the formation of RNA quadruplexes in cellular RNAs has recently been suggested and the study of their influence and potential roles in cellular processes has just started. Here we give an overview of the RNA quadruplex field, summarizing issues such as structures, stabilities, and anticipated roles of these interesting four-stranded, guanosine-rich sequences.

Self-organization of G-quadruplex structures in the hTERT core promoter stabilized by polyaminic side chain perylene derivatives

hTERT core promoter regulates telomerase transcription in human cells, thus its structural features are of large interest. We have found that the G-rich hTERT core promoter region, corresponding to the major DNase I hypersensitive site in chromatin organization, contains nine putative G-quadruplex forming sequences (PQS) and is unfavorable for nucleosome formation. Here we show that four PQS are effectively able to form stable parallel intramolecular G-quadruplexes, using PAGE and CD spectroscopy analysis. The PQS-region, as a whole, appears to be organized in three self-interacting G-quadruplexes, probably giving rise to a helicoidal superstructure, as shown by CD and polymerase stop assay. POL-HPDI drugs, that we previously found useful in selectively stabilizing telomeric G-quadruplex, are able to stabilize both the single intramolecular G-quadruplex and the PQS-region superstructure. The features of their induced CD spectra suggest that POL-HPDIs bind to single G-quadruplexes and to whole PQS-region superstructure, mainly by end-stacking interactions.

Dual sensing of hairpin and quadruplex DNA structures using multicolored peptide nucleic acid fluorescent probes

Synthesis of water-soluble 5-mer peptide nucleic acids (PNAs) functionalized at their 5'- and 3'-ends with two original precursors of pentamethine cyanine dye synthesis is reported. The successful use of these PNA probes for sensing DNA hairpin structures in vitro was also demonstrated where specific hairpin formation was associated with the appearance of a characteristic fluorescence signal at 660 nm. A comparative study between three different strategies where PNAs were targeting either the stem or the loop of the hairpin was carried out. Best sensitivity was obtained using PNA sequences complementary to the loop sequence and directing both functional moieties toward the base of loop. Unprecedented proof-of-concept for the simultaneous sensing of hairpin and quadruplex DNAs with a nonoverlapping two-color system (C3 and C5) is also demonstrated.

Guanine quadruplex DNA structure restricts methylation of CpG dinucleotides genome-wide

Cytosine methylation in mammals is important for epigenetic control of the transcriptome. Although altered methylation is frequently encountered in disease situations, particularly cancer, the relationship between genome-wide methylation and DNA structure is poorly understood. It is now evident that alternative DNA forms are functionally relevant in replication, recombination and transcription. Herein, we researched the role of alternative DNA structure in cytosine methylation using quadruplex DNA as a case study. Our findings from analysis of 2.1 million CpGs in humans, across 12 tissues from the Human Epigenome Project (HEP), revealed a striking correlation within each tissue: CpGs with low methylation were enriched (P = 5.24E(-20)) whereas CpGs with high methylation were relatively depleted (P = 9.28E(-15)), within quadruplex-forming regions. This was further substantiated on considering 1.07E(8) methylcytosines from genome-wide sequencing within embryonic stem cells and differentiated fibroblasts. To further test the predictions we experimentally determined methylation in >600,000 CpGs across 18 individuals using bisulfite mapping and found significantly low methylation of CpGs within quadruplex-forming regions (P = 1.36E(-08)). Together, these suggest the role of guanine-quadruplexes in CpG methylation and directly impact our understanding of the inter-relationship between DNA conformation and global cytosine methylation.

Structure, location and interactions of G-quadruplexes

Four-stranded G-rich DNA structures called G-quadruplexes have been the subject of increasing interest recently. Experimental and computational techniques have been used to implicate them in important biological processes such as transcription and translation. In this minireview, I discuss how they form, what structures they adopt and with what stability. I then discuss the computational approaches used to predict them on a genomic scale and how the information derived can be combined with experiments to understand their biological functions. Other minireviews in this series deal with G-quadruplex nucleic acids and human disease [Wu Y & Brosh RM Jr (2010) FEBS J] and making sense of G-quadruplex and i-motif function in oncogene promoters [Brooks TA et al. (2010) FEBS J].

A toolbox for predicting g-quadruplex formation and stability

G-quadruplexes are four stranded nucleic acid structures formed around a core of guanines, arranged in squares with mutual hydrogen bonding. Many of these structures are highly thermally stable, especially in the presence of monovalent cations, such as those found under physiological conditions. Understanding of their physiological roles is expanding rapidly, and they have been implicated in regulating gene transcription and translation among other functions. We have built a community-focused website to act as a repository for the information that is now being developed. At its core, this site has a detailed database (QuadDB) of predicted G-quadruplexes in the human and other genomes, together with the predictive algorithm used to identify them. We also provide a QuadPredict server, which predicts thermal stability and acts as a repository for experimental data from all researchers. There are also a number of other data sources with computational predictions. We anticipate that the wide availability of this information will be of use both to researchers already active in this exciting field and to those who wish to investigate a particular gene hypothesis.

Genetic instability triggered by G-quadruplex interacting Phen-DC compounds in Saccharomyces cerevisiae

G-quadruplexes are nucleic acid secondary structures for which many biological roles have been proposed but whose existence in vivo has remained elusive. To assess their formation, highly specific G-quadruplex ligands are needed. Here, we tested Phen-DC₃ and Phen-DC₆, two recently released ligands of the bisquinolinium class. In vitro, both compounds exhibit high affinity for the G4 formed by the human minisatellite CEB1 and inhibit efficiently their unwinding by the yeast Pif1 helicase. In vivo, both compounds rapidly induced recombination-dependent rearrangements of CEB1 inserted in the Saccharomyces cerevisiae genome, but did not affect the stability of other tandem repeats lacking G-quadruplex forming sequences. The rearrangements yielded simple-deletion, double-deletion or complex reshuffling of the polymorphic motif units, mimicking the phenotype of the Pif1 inactivation. Treatment of Pif1-deficient cells with the Phen-DC compounds further increased CEB1 instability, revealing additional G4 formation per cell. In sharp contrast, the commonly used N-methyl-mesoporphyrin IX G-quadruplex ligand did not affect CEB1 stability. Altogether, these results demonstrate that the Phen-DC bisquinolinium compounds are potent molecular tools for probing the formation of G-quadruplexes in vivo, interfere with their processing and elucidate their biological roles.

Altered gene expression in the Werner and Bloom syndromes is associated with sequences having G-quadruplex forming potential

The human Werner and Bloom syndromes (WS and BS) are caused by deficiencies in the WRN and BLM RecQ helicases, respectively. WRN, BLM and their Saccharomyces cerevisiae homologue Sgs1, are particularly active in vitro in unwinding G-quadruplex DNA (G4-DNA), a family of non-canonical nucleic acid structures formed by certain G-rich sequences. Recently, mRNA levels from loci containing potential G-quadruplex-forming sequences (PQS) were found to be preferentially altered in sgs1Δ mutants, suggesting that G4-DNA targeting by Sgs1 directly affects gene expression. Here, we extend these findings to human cells. Using microarrays to measure mRNAs obtained from human fibroblasts deficient for various RecQ family helicases, we observe significant associations between loci that are upregulated in WS or BS cells and loci that have PQS. No such PQS associations were observed for control expression datasets, however. Furthermore, upregulated genes in WS and BS showed no or dramatically reduced associations with sequences similar to PQS but that have considerably reduced potential to form intramolecular G4-DNA. These findings indicate that, like Sgs1, WRN and BLM can regulate transcription globally by targeting G4-DNA.

Predictable suppression of gene expression by 5'-UTR-based RNA quadruplexes

Four-stranded DNA and RNA quadruplexes or G4 motifs are non-B DNA conformations that are presumed to form in vivo, although only few explicit evidence has been reported. Using bioinformatics the presence of putative DNA G-quadruplexes within critical promoter regions has been demonstrated and a regulatory role in transcription has been suspected. However, in genomic DNA the presence of the complementary strand interferes with the potential to form a quadruplex motif. Contrarily RNA G4 motifs have no such limitation and consequently strong interference with gene expression is suspected. Nevertheless, experimental evidence is scarce. Here we show a well-defined structure–function relationship of synthetic quadruplex sequences in 5′-UTRs in multiple mammalian cell-lines. We establish a universal ‘translational suppressor’ effect of these motifs on gene expression at the translational level and show for the first time that specific features such as loop-length and the number of ‘GGG’-repeats further determine the suppressive impact. Moreover, a consistent and predictable repression of gene expression is observed for naturally occurring RNA G4 motifs, augmenting the functional relevance of these unusual nucleic acid structures.