Oxidative Modification of Guanine in a Potential Z-DNA-Forming Sequence of a Gene Promoter Impacts Gene Expression

Global identification and functional characterization of Z-DNA in rice

Z-DNA is a left-handed double helix form of DNA that is believed to be involved in various DNA transactions. However, comprehensive investigations aimed at global profiling of Z-DNA landscapes are still missing in both humans and plants. We here report the development of two techniques: anti-Z-DNA antibody-based immunoprecipitation followed by sequencing (ZIP-seq), and cleavage under targets and tagmentation (CUT&TAG) for characterizing Z-DNA in nipponbare rice (Oryza sativa L., Japonica). We found that Z-DNA-IP+ (Z-DNA recognized by the antibody) exhibits distinct genomic features as compared to Z-DNA-IP- (Z-DNA not recognized by the antibody). The concomitant presence of G-quadruplexes (G4s) and i-motifs (iMs) may promote Z-DNA formation. DNA modifications such as DNA-6mA/-4acC generally disfavours Z-DNA formation, while modifications like DNA-5mC (CHH) and 8-oxodG promote it, highlighting the distinct roles of DNA base modifications in modulating Z-DNA formation. Importantly, Z-DNA located at transcription start sites (TSSs) enhances gene expression, whereas Z-DNA in genic regions represses it, underscoring its dual roles in regulating the expression of genes involved in fundamental biological functions and responses to salt stress. Furthermore, Z-DNA may play a role in transcriptional initiation and termination rather than in transcriptional elongation. Finally, the presence of Z-DNA in promoters is correlated with the coevolution of overlapping genes, thereby regulating gene domestication. Consequently, our study represents as a pivotal point and a solid foundation for reliably launching genome-wide investigations of Z-DNA, thereby advancing the understanding of Z-DNA biology in both plants and non-plant systems.

Z-DNA at the crossroads: untangling its role in genome dynamics

DNA can fold into noncanonical left-handed Z-DNA conformation beyond the right-handed B-DNA. While its crystal structure was discovered nearly four decades ago, it was predominantly considered a structural curiosity. Recent evidence suggests that Z-DNA formation occurs in nuclear and mitochondrial DNA (mtDNA), with significant biological implications. However, our understanding of its roles remains in its infancy, primarily due to a lack of study tools. In this review we summarize the structure and function of Z-DNA within the genome while addressing the difficulties associated with identifying and investigating its role(s). We then critically evaluate several intracellular factors that can modulate and regulate Z-DNA. Additionally, we discuss the recent technological and methodological advances that may overcome the challenges and enhance our understanding of Z-DNA.

Oxidative damage within alternative DNA structures results in aberrant mutagenic processing

Genetic instability is a hallmark of cancer, and mutation hotspots in human cancer genomes co-localize with alternative DNA structure-forming sequences (e.g. H-DNA), implicating them in cancer etiology. H-DNA has been shown to stimulate genetic instability in mammals. Here, we demonstrate a new paradigm of genetic instability, where a cancer-associated H-DNA-forming sequence accumulates more oxidative lesions than B-DNA under conditions of oxidative stress (OS), often found in tumor microenvironments. We show that OS results in destabilization of the H-DNA structure and attenuates the fold increase in H-DNA-induced mutations over control B-DNA in mammalian cells. Furthermore, the mutation spectra revealed that the damaged H-DNA-containing region was processed differently compared to H-DNA in the absence of oxidative damage in mammalian cells. The oxidatively modified H-DNA elicits differential recruitment of DNA repair proteins from both the base excision repair and nucleotide excision repair mechanisms. Altogether, these results suggest a new model of genetic instability whereby H-DNA-forming regions are hotspots for DNA damage in oxidative microenvironments, resulting in its altered mutagenic processing. Our findings provide valuable insights into the role of OS in DNA structure-induced genetic instability and may establish H-DNA-forming sequences as promising genomic biomarkers and potential therapeutic targets for genetic diseases.

Why the ROS matters: One-electron oxidants focus DNA damage and repair on G-quadruplexes for gene regulation

Hydrogen peroxide is a precursor to reactive oxygen species (ROS) in cells because of its high reactivity with iron(II) carbonate complexes formed in the labile iron pool due to a high concentration of intracellular bicarbonate (25-100 mM). This chemistry leads to the formation of carbonate radical anion rather than hydroxyl radical, and unlike the latter ROS, CO3•- is a milder one-electron oxidant with high specificity for guanine oxidation in DNA and RNA. In addition to metabolism, another major source of DNA oxidation is inflammation which generates peroxynitrite, another precursor to CO3•- via reaction with dissolved CO2. The identity of the ROS is important because not all radicals react with DNA in the same way. Whereas hydroxyl radical forms adducts at all four bases and reacts with multiple positions on ribose leading to base loss and strand breaks, carbonate radical anion is focused on guanosine oxidation to yield 8-oxo-7,8-dihydroguanosine in nucleic acids and the nucleotide pool, a modification that can function epigenetically in the context of a G-quadruplex. DNA sequences of multiple adjacent guanines, as found in G-quadruplex-forming sequences of gene promoters, are particularly susceptible to oxidative damage, and the focusing of CO3•- chemistry on these sites can lead to a transcriptional response during base excision repair. In this pathway, AP-endonuclease 1 plays a key role in accelerating G-quadruplex folding as well as recruiting activating transcription factors to impact gene expression.

A Compendium of G-Flipon Biological Functions That Have Experimental Validation

As with all new fields of discovery, work on the biological role of G-quadruplexes (GQs) has produced a number of results that at first glance are quite baffling, sometimes because they do not fit well together, but mostly because they are different from commonly held expectations. Like other classes of flipons, those that form G-quadruplexes have a repeat sequence motif that enables the fold. The canonical DNA motif (G3N1-7)3G3, where N is any nucleotide and G is guanine, is a feature that is under active selection in avian and mammalian genomes. The involvement of G-flipons in genome maintenance traces back to the invertebrate Caenorhabditis elegans and to ancient DNA repair pathways. The role of GQs in transcription is supported by the observation that yeast Rap1 protein binds both B-DNA, in a sequence-specific manner, and GQs, in a structure-specific manner, through the same helix. Other sequence-specific transcription factors (TFs) also engage both conformations to actuate cellular transactions. Noncoding RNAs can also modulate GQ formation in a sequence-specific manner and engage the same cellular machinery as localized by TFs, linking the ancient RNA world with the modern protein world. The coevolution of noncoding RNAs and sequence-specific proteins is supported by studies of early embryonic development, where the transient formation of G-quadruplexes coordinates the epigenetic specification of cell fate.

Quantum mechanical electronic and geometric parameters for DNA k-mers as features for machine learning

We are witnessing a steep increase in model development initiatives in genomics that employ high-end machine learning methodologies. Of particular interest are models that predict certain genomic characteristics based solely on DNA sequence. These models, however, treat the DNA as a mere collection of four, A, T, G and C, letters, dismissing the past advancements in science that can enable the use of more intricate information from nucleic acid sequences. Here, we provide a comprehensive database of quantum mechanical (QM) and geometric features for all the permutations of 7-meric DNA in their representative B, A and Z conformations. The database is generated by employing the applicable high-cost and time-consuming QM methodologies. This can thus make it seamless to associate a wealth of novel molecular features to any DNA sequence, by scanning it with a matching k-meric window and pulling the pre-computed values from our database for further use in modelling. We demonstrate the usefulness of our deposited features through their exclusive use in developing a model for A->C mutation rates.

Methodologies for the detection and sequencing of the epigenetic-like oxidative DNA modification, 8-oxo-7,8-dihydroguanine

The human genome is constantly threatened by endogenous and environmental DNA damaging agents that can induce a variety of chemically modified DNA lesions including 8-oxo-7,8-dihydroguanine (OG). Increasing evidence has indicated that OG is not only a biomarker for oxidative DNA damage but also a novel epigenetic-like modification involved in regulation of gene expression in mammalian cells. Here we summarize the recent progress in OG research focusing on the following points: (i) the mechanism of OG production in organisms and its biological consequences in cells, (ii) the accurate identification of OG in low-abundance genomes and complex biological backgrounds, (iii) the development of OG sequencing methods. These studies will be helpful for further understanding of the molecular mechanisms of OG-induced mutagenesis and its potential roles in human development and diseases such as cancer.

The role of DNA in the pathogenesis of SLE: DNA as a molecular chameleon

Systemic lupus erythematosus (SLE) is a prototypic autoimmune disease characterised by antibodies to DNA (anti-DNA) and other nuclear macromolecules. Anti-DNA antibodies are markers for classification and disease activity and promote pathogenesis by forming immune complexes that deposit in the tissue or stimulate cytokine production. Studies on the antibody response to DNA have focused primarily on a conformation of DNA known as B-DNA, the classic right-handed double helix. Among other conformations of DNA, Z-DNA is a left-handed helix with a zig-zag backbone; hence, the term Z-DNA. Z-DNA formation is favoured by certain base sequences, with the energetically unfavourable flip from B-DNA to Z-DNA dependent on conditions. Z-DNA differs from B-DNA in its immunogenicity in animal models. Furthermore, anti-Z-DNA antibodies, but not anti-B-DNA antibodies, can be present in otherwise healthy individuals. In SLE, antibodies to Z-DNA can occur in association with antibodies to B-DNA as a cross-reactive response, rising and falling together. While formed transiently in chromosomal DNA, Z-DNA is stably present in bacterial biofilms; biofilms can provide protection against antibiotics and other challenges including elements of host defence. The high GC content of certain bacterial DNA also favours Z-DNA formation as do DNA-binding proteins of bacterial or host origin. Together, these findings suggest that sources of Z-DNA can enhance the immunogenicity of DNA and, in SLE, stimulate the production of cross-reactive antibodies that bind both B-DNA and Z-DNA. As such, DNA can act as a molecular chameleon that, when stabilised in the Z-DNA conformation, can drive autoimmunity.

Pharmacological OGG1 inhibition decreases murine allergic airway inflammation

Background and aim: Allergic asthma is a complex inflammatory disease involving type 2 innate lymphoid cells, type 2 T helper cells, macrophages, and eosinophils. The disease is characterized by wheezing, dyspnea, coughing, chest tightness and variable airflow limitation for which there is no cure and is symptomatically treated with inhaled corticosteroids and β2-agonists. Molecular mechanisms underlying its complex pathogenesis are not fully understood. However, 8-oxoguanine DNA glycosylase-1 (OGG1), a DNA repair protein may play a central role, as OGG1 deficiency decreases both innate and allergic inflammation.

Methods: Using a murine ovalbumin (OVA) model of allergic airway inflammation we assessed the utility of an inhibitor of OGG1 (TH5487) in this disease context. Cytokines and chemokines, promoting immune cell recruitment were measured using a 23-multiplex assay and Western blotting. Additionally, immune cell recruitment to bronchi was measured using flow cytometry. Histological analyses and immunofluorescent staining were used to confirm immune cell influx and goblet cell hyperplasia of the airways. A PCR array was used to assess asthma-related genes in murine lung tissue following TH5487 treatment. Finally, airway hyperresponsiveness was determined using in vivo lung function measurement.

Results: In this study, administration of TH5487 to mice with OVA-induced allergic airway inflammation significantly decreased goblet cell hyperplasia and mucus production. TH5487 treatment also decreased levels of activated NF-κB and expression of proinflammatory cytokines and chemokines resulting in significantly lower recruitment of eosinophils and other immune cells to the lungs. Gene expression profiling of asthma and allergy-related proteins after TH5487 treatment revealed differences in several important regulators, including down regulation of Tnfrsf4, Arg1, Ccl12 and Ccl11, and upregulation of the negative regulator of type 2 inflammation, Bcl6. Furthermore, the gene Clca1 was upregulated following TH5487 treatment, which should be explored further due to its ambiguous role in allergic asthma. In addition, the OVA-induced airway hyperresponsiveness was significantly reduced by TH5487 treatment.

Conclusion: Taken together, the data presented in this study suggest OGG1 as a clinically relevant pharmacological target for the treatment of allergic inflammation.

8-oxodG accumulation within super-enhancers marks fragile CTCF-mediated chromatin loops

8-Oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG), a major product of the DNA oxidization process, has been proposed to have an epigenetic function in gene regulation and has been associated with genome instability. NGS-based methodologies are contributing to the characterization of the 8-oxodG function in the genome. However, the 8-oxodG epigenetic role at a genomic level and the mechanisms controlling the genomic 8-oxodG accumulation/maintenance have not yet been fully characterized. In this study, we report the identification and characterization of a set of enhancer regions accumulating 8-oxodG in human epithelial cells. We found that these oxidized enhancers are mainly super-enhancers and are associated with bidirectional-transcribed enhancer RNAs and DNA Damage Response activation. Moreover, using ChIA-PET and HiC data, we identified specific CTCF-mediated chromatin loops in which the oxidized enhancer and promoter regions physically associate. Oxidized enhancers and their associated chromatin loops accumulate endogenous double-strand breaks which are in turn repaired by NHEJ pathway through a transcription-dependent mechanism. Our work suggests that 8-oxodG accumulation in enhancers-promoters pairs occurs in a transcription-dependent manner and provides novel mechanistic insights on the intrinsic fragility of chromatin loops containing oxidized enhancers-promoters interactions.

DNA damage and repair and epigenetic modification in the role of oxoguanine glycosylase 1 (OGG1) in brain development

Oxoguanine glycosylase 1 (OGG1) repairs the predominant reactive oxygen species (ROS)-initiated DNA lesion 8-oxoguanine (8-oxoG). Human OGG1 polymorphisms resulting in reduced DNA repair associate with an increased risk for disorders like cancer and diabetes, but the role of OGG1 in brain development is unclear. Herein, we show that Ogg1 knockout mice at 2-3 months of age exhibit enhanced gene- and sex-dependent DNA damage (strand breaks) and decreased epigenetic DNA methylation marks (5-methylcytosine, 5-hydroxymethylcytosine), both of which were associated with increased cerebellar calbindin levels, reduced hippocampal postsynaptic function, altered body weight with age and disorders of brain function reflected in behavioural tests for goal-directed repetitive behaviour, anxiety and fear, object recognition and spatial memory, motor coordination and startle response. These results suggest that OGG1 plays an important role in normal brain development, possibly via both its DNA repair activity and its role as an epigenetic modifier, with OGG1 deficiencies potentially contributing to neurodevelopmental disorders.

Searching for New Z-DNA/Z-RNA Binding Proteins Based on Structural Similarity to Experimentally Validated Zα Domain

Z-DNA and Z-RNA are functionally important left-handed structures of nucleic acids, which play a significant role in several molecular and biological processes including DNA replication, gene expression regulation and viral nucleic acid sensing. Most proteins that have been proven to interact with Z-DNA/Z-RNA contain the so-called Zα domain, which is structurally well conserved. To date, only eight proteins with Zα domain have been described within a few organisms (including human, mouse, Danio rerio, Trypanosoma brucei and some viruses). Therefore, this paper aimed to search for new Z-DNA/Z-RNA binding proteins in the complete PDB structures database and from the AlphaFold2 protein models. A structure-based similarity search found 14 proteins with highly similar Zα domain structure in experimentally-defined proteins and 185 proteins with a putative Zα domain using the AlphaFold2 models. Structure-based alignment and molecular docking confirmed high functional conservation of amino acids involved in Z-DNA/Z-RNA, suggesting that Z-DNA/Z-RNA recognition may play an important role in a variety of cellular processes.

Remodeling Chromatin Induces Z-DNA Conformation Detected through Fourier Transform Infrared Spectroscopy

The SWI/SNF complex is a highly conserved chromatin remodeling complex and can hydrolyze ATP by its catalytic subunit BRG1 or BRM to reconstruct the chromatin. To investigate whether this ATP-dependent chromatin remodeling could affect the DNA conformation, we therefore regulated (knocked down or overexpressed) BRG1/BRM in the cells and applied Fourier transform infrared (FTIR) spectroscopy to probe DNA conformational changes. As a result, we found that BRG1/BRM was indeed associated with the DNA conformational changes, in which knockdown of BRG1/BRM reduced Z-DNA conformation, while overexpression of BRG1/BRM enhanced Z-DNA conformation. This Z-DNA conformational transformation was also verified using the Z-DNA-binding proteins. Therefore, this work has provided a direct analytical tool to probe Z-DNA transformation upon ATP-dependent chromatin remodeling.

Cruciform DNA Sequences in Gene Promoters Can Impact Transcription upon Oxidative Modification of 2'-Deoxyguanosine

Sequences of DNA typically adopt B-form duplexes in genomes, although noncanonical structures such as G-quadruplexes, i-motifs, Z-DNA, and cruciform structures can occur. A challenge is to determine the functions of these various structures in cellular processes. We and others have hypothesized that G-rich G-quadruplex-forming sequences in human genome promoters serve to sense oxidative damage generated during oxidative stress impacting gene regulation. Herein, chemical tools and a cell-based assay were used to study the oxidation of guanine to 8-oxo-7,8-dihydroguanine (OG) in the context of a cruciform-forming sequence in a gene promoter to determine the impact on transcription. We found that OG in the nontemplate strand in the loop of a cruciform-forming sequence could induce gene expression; conversely when OG was in the same sequence on the template strand, gene expression was inhibited. A model for the transcriptional changes observed is proposed in which OG focuses the DNA repair process on the promoter to impact expression. Our cellular and biophysical studies and literature sources support the idea that removal of OG from duplex DNA by OGG1 yields an abasic site (AP) that triggers a structural shift to the cruciform fold. The AP-bearing cruciform structure is presented to APE1, which functions as a conduit between DNA repair and gene regulation. The significance is enhanced by a bioinformatic study of all human gene promoters and transcription termination sites for inverted repeats (IRs). Comparison of the two regions showed that promoters have stable and G-rich IRs at a low frequency and termination sites have many AT-rich IRs with low stability.

Interplay of Guanine Oxidation and G-Quadruplex Folding in Gene Promoters

Living in an oxygen atmosphere demands an ability to thrive in the presence of reactive oxygen species (ROS). Aerobic organisms have successfully found solutions to the oxidative threats imposed by ROS by evolving an elaborate detoxification system, upregulating ROS during inflammation, and utilizing ROS as messenger molecules. In this Perspective, recent studies are discussed that demonstrate ROS as signaling molecules for gene regulation by combining two emergent properties of the guanine (G) heterocycle in DNA, namely, oxidation sensitivity and a propensity for G-quadruplex (G4) folding, both of which depend upon sequence context. In human gene promoters, this results from an elevated 5'-GG-3' dinucleotide frequency and GC enrichment near transcription start sites. Oxidation of DNA by ROS drives conversion of G to 8-oxo-7,8-dihydroguanine (OG) to mark target promoters for base excision repair initiated by OG-glycosylase I (OGG1). Sequence-dependent mechanisms for gene activation are available to OGG1 to induce transcription. Either OGG1 releases OG to yield an abasic site driving formation of a non-canonical fold, such as a G4, to be displayed to apurinic/apyrimidinic 1 (APE1) and stalling on the fold to recruit activating factors, or OGG1 binds OG and facilitates activator protein recruitment. The mechanisms described drive induction of stress response, DNA repair, or estrogen-induced genes, and these pathways are novel potential anticancer targets for therapeutic intervention. Chemical concepts provide a framework to discuss the regulatory or possible epigenetic potential of the OG modification in DNA, in which DNA "damage" and non-canonical folds collaborate to turn on or off gene expression. The next steps for scientific discovery in this growing field are discussed.

The genomics of oxidative DNA damage, repair, and resulting mutagenesis

Reactive oxygen species are a constant threat to DNA as they modify bases with the risk of disrupting genome function, inducing genome instability and mutation. Such risks are due to primary oxidative DNA damage and also mediated by the repair process. This leads to a delicate decision process for the cell as to whether to repair a damaged base at a specific genomic location or better leave it unrepaired. Persistent DNA damage can disrupt genome function, but on the other hand it can also contribute to gene regulation by serving as an epigenetic mark. When such processes are out of balance, pathophysiological conditions could get accelerated, because oxidative DNA damage and resulting mutagenic processes are tightly linked to ageing, inflammation, and the development of multiple age-related diseases, such as cancer and neurodegenerative disorders. Recent technological advancements and novel data analysis strategies have revealed that oxidative DNA damage, its repair, and related mutations distribute heterogeneously over the genome at multiple levels of resolution. The involved mechanisms act in the context of genome sequence, in interaction with genome function and chromatin. This review addresses what we currently know about the genome distribution of oxidative DNA damage, repair intermediates, and mutations. It will specifically focus on the various methodologies to measure oxidative DNA damage distribution and discuss the mechanistic conclusions derived from the different approaches. It will also address the consequences of oxidative DNA damage, specifically how it gives rise to mutations, genome instability, and how it can act as an epigenetic mark.

Roles of OGG1 in transcriptional regulation and maintenance of metabolic homeostasis

Cellular damage produced by conditions generating oxidative stress have far-reaching implications in human disease that encompass, but are not restricted to aging, cardiovascular disease, type 2 diabetes, airway inflammation/asthma, cancer, and metabolic syndrome including visceral obesity, insulin resistance, fatty liver disease, and dyslipidemia. Although there are numerous sources and cellular targets of oxidative stress, this review will highlight literature that has investigated downstream consequences of oxidatively-induced DNA damage in both nuclear and mitochondrial genomes. The presence of such damage can in turn, directly and indirectly modulate cellular transcriptional and repair responses to such stressors. As such, the persistence of base damage can serve as a key regulator in coordinated gene-response cascades. Conversely, repair of these DNA lesions serves as both a suppressor of mutagenesis and by inference carcinogenesis, and as a signal for the cessation of ongoing oxidative stress. A key enzyme in all these processes is 8-oxoguanine DNA glycosylase (OGG1), which, via non-catalytic binding to oxidatively-induced DNA damage in promoter regions, serves as a nucleation site around which changes in large-scale regulation of inflammation-associated gene expression can occur. Further, the catalytic function of OGG1 can alter the three-dimensional structure of specialized DNA sequences, leading to changes in transcriptional profiles. This review will concentrate on adverse deleterious health effects that are associated with both the diminution of OGG1 activity via population-specific polymorphic variants and the complete loss of OGG1 in murine models. This mouse model displays diet- and age-related induction of metabolic syndrome, highlighting a key role for OGG1 in protecting against these phenotypes. Conversely, recent investigations using murine models having enhanced global expression of a mitochondrial-targeted OGG1 demonstrate that they are highly resistant to diet-induced disease. These data suggest strategies through which therapeutic interventions could be designed for reducing or limiting adverse human health consequences to these ubiquitous stressors.