Abasic and oxidized abasic site reactivity in DNA: enzyme inhibition, cross-linking, and nucleosome catalyzed reactions

Ultrasensitive mass spectrometric quantitation of apurinic/apyrimidinic sites in genomic DNA of mammalian cell lines exposed to genotoxic reagents

The apurinic/apyrimidinic (AP) site is an important intermediate in the DNA base excision repair (BER) pathway, having the potential of being a biomarker for DNA damage. AP sites could lead to the stalling of polymerases, the misincorporation of bases and DNA strand breaks, which might affect physiological function of cells. However, the abundance of AP sites in genomic DNA is very low (less than 2 AP sites/106 nts), which requires a sensitive and accurate method to meet its detection requirements. Here, we described an ultrasensitive quantification method based on a hydrazine-s-triazine reagent (i-Pr2N) labeling for AP sites combining with high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). The limit of detection reached an ultralow level (40 amol), realizing the most sensitive MS-based quantification for the AP site. To guarantee the accuracy of the quantitative results, the labeling reaction was carried out directly on DNA strands instead of labeling after DNA enzymatic digestion to reduce artifacts that might be produced during the enzymatic process of DNA strands. And selective detection was realized by MS to avoid introducing the false-positive signals from other aldehyde species, which could also react with i-Pr2N. Genomic DNA samples from different mammalian cell lines were successfully analyzed using this method. There were 0.4-0.8 AP sites per 106 nucleotides, and the values would increase 16.1 and 2.75 times when cells were treated with genotoxic substances methyl methanesulfonate and 5-fluorouracil, respectively. This method has good potential in the analysis of a small number of cell samples and clinical samples, is expected to be useful for evaluating the damage level of DNA bases, the genotoxicity of compounds and the drug resistance of cancer cells, and provides a new tool for cell function research and clinical precise treatment.

DNA abasic sites act as rational therapeutic targets to synergize temozolomide response in both MMR-proficient and deficient cancer

Temozolomide (TMZ) is widely used in cancer treatment, yet resistance to this agent limits its therapeutic effectiveness, particularly in mismatch-repair (MMR) deficient cancer. Concurrently, the Base Excision Repair (BER) pathway exerts a mitigating role. Our results demonstrated that the increasing TMZ concentrations correlate with an elevated accumulation of DNA abasic sites via the BER pathway in both MMR-proficient and deficient cancer cells, implicating abasic sites as promising targets to enhance the TMZ response. Amino-quinoxaline small molecules (RA-1) have been developed, whose hydrophobic core facilitates selective binding to apurinic/apyrimidinic (AP) sites, particularly adenine as the complementary nucleobase opposite to the AP-sites via base stacking. RA-1 effectively cleaves TMZ-induced DNA abasic sites in-vitro at minimal concentrations through Schiff-base formation. Remarkably, the combination of TMZ and RA-1 exerts a notable synergistic effect on both types of cells. The underlying mechanism of this synergy is rooted in the cleavage of TMZ-induced DNA abasic sites, which impairs the BER pathway, leading to the formation of DNA double-strand breaks. Consequently, the ATM-Chk2/ATR-Chk1 signalling pathways are activated, prompting S-phase arrest and ultimately driving apoptosis. These findings provide a compelling rationale for targeting DNA abasic sites to synergistically augment TMZ responses in both MMR-proficient and deficient cancer cells.

Selective anti-tumor activity of glutathione-responsive abasic site trapping agent in anaplastic thyroid carcinoma

Anaplastic thyroid carcinoma (ATC) is a rare but highly aggressive thyroid cancer with poor prognosis. Killing cancer cells by inducing DNA damage or blockage of DNA repair is a promising strategy for chemotherapy. It is reported that aldehyde-reactive alkoxyamines can capture the AP sites, one of the most common DNA lesions, and inhibit apurinic/apyrimidinic endonuclease 1(APE1)-mediated base excision repair (BER), leading to cell death. Whether this strategy can be employed for ATC treatment is rarely investigated. The aim of this study is to exploit GSH-responsive AP site capture reagent (AP probe-net), which responses to the elevated glutathione (GSH) levels in the tumor micro-environment (TME), releasing reactive alkoxyamine to trap AP sites and block the APE1-mediated BER for targeted anti-tumor activity against ATC. In vitro experiments, including MTT andγ-H2AX assays, demonstrate their selective cytotoxicity towards ATC cells over normal thyroid cells. Flow cytometry analysis suggests that AP probe-net arrests the cell cycle in the G2/M phase and induces apoptosis. Western blotting (WB) results show that the expression of apoptotic protein increased with the increased concentration of AP probe-net. Further in vivo experiments reveal that the AP probe-net has a good therapeutic effect on subcutaneous tumors of the ATC cells. In conclusion, taking advantage of the elevated GSH in TME, our study affords a new strategy for targeted chemotherapy of ATC with high selectivity and reduced adverse effects.

The Effects of Supplementation of the Freezing Extender with Silymarin on the Quality Parameters of Frozen-Thawed Arabian Stallion Sperm: A Preliminary Evaluation

This study evaluated the effects of supplementation of the freezing extender with different concentrations of silymarin on the quality of frozen-thawed Arabian stallion spermatozoa. Semen samples from three stallions (1, 2, and 3) were suspended in the freezing extender without or with silymarin (0, 25 μg/mL, 50 μg/mL, 75 μg/mL, and 100 μg/mL) and cryopreserved in 0.5 mL straws. After 1 month of storage, the frozen semen samples in straws were thawed and evaluated in terms of viability, mitochondrial membrane potential, kinematic parameters, total and progressive motility, plasma membrane integrity, lipid peroxidation, and DNA fragmentation. The findings indicated that 25-100 μg/mL of silymarin significantly improved viability and mitochondrial membrane potential while reducing the stallion sperm lipid peroxidation, DNA fragmentation, and apoptosis compared with the control group (p < 0.05). Silymarin concentrations of 75 μg/mL and 100 μg/mL significantly increased progressive motility and plasma membrane integrity (p < 0.05). Based on our findings, it can be inferred that silymarin exhibited a dose-dependent enhancement in the frozen-thawed Arabian stallion sperm quality. The most favorable outcomes were observed when 100 μg/mL silymarin was used.

Formation and Properties of DNA Adducts Generated by Reactions of Abasic Sites with 1,2-Aminothiols Including Cysteamine, Cysteine Methyl Ester, and Peptides Containing N-Terminal Cysteine Residues

The reaction of 1,2-aminothiol groups with aldehyde residues in aqueous solution generates thiazolidine products, and this process has been developed as a catalyst-free click reaction for bioconjugation. The work reported here characterized reactions of the biologically relevant 1,2-aminothiols including cysteamine, cysteine methyl ester, and peptides containing N-terminal cysteine residues with the aldehyde residue of apurinic/apyrimidinic (AP) sites in DNA oligomers. These 1,2-aminothiol-containing compounds rapidly generated adducts with AP sites in single-stranded and double-stranded DNA. NMR and MALDI-TOF-MS analyses provided evidence that the reaction generated a thiazolidine product. Conversion of an AP site to a thiazolidine-AP adduct protected against the rapid cleavage normally induced at AP sites by the endonuclease action of the enzyme APE1 and the AP-lyase activity of the biogenic amine spermine. In the presence of excess 1,2-aminothiols, the thiazolidine-AP adducts underwent slow strand cleavage via a β-lyase reaction that generated products with 1,2-aminothiol-modified sugar residues on the 3'-end of the strand break. In the absence of excess 1,2-aminothiols, the thiazolidine-AP adducts dissociated to release the parent AP-containing oligonucleotide. The properties of the thiazolidine-AP adducts described here mirror critical properties of SRAP proteins HMCES and YedK that capture AP sites in single-stranded regions of cellular DNA and protect them from cleavage.

Enzymatic Processing of DNA-Protein Crosslinks

DNA-protein crosslinks (DPCs) represent a unique and complex form of DNA damage formed by covalent attachment of proteins to DNA. DPCs are formed through a variety of mechanisms and can significantly impede essential cellular processes such as transcription and replication. For this reason, anti-cancer drugs that form DPCs have proven effective in cancer therapy. While cells rely on numerous different processes to remove DPCs, the molecular mechanisms responsible for orchestrating these processes remain obscure. Having this insight could potentially be harnessed therapeutically to improve clinical outcomes in the battle against cancer. In this review, we describe the ways cells enzymatically process DPCs. These processing events include direct reversal of the DPC via hydrolysis, nuclease digestion of the DNA backbone to delete the DPC and surrounding DNA, proteolytic processing of the crosslinked protein, as well as covalent modification of the DNA-crosslinked proteins with ubiquitin, SUMO, and Poly(ADP) Ribose (PAR).

Sustained pigmentation causes DNA damage and invokes translesion polymerase Polκ for repair in melanocytes

Melanin protects skin cells from ultraviolet radiation-induced DNA damage. However, intermediates of eumelanin are highly reactive quinones that are potentially genotoxic. In this study, we systematically investigate the effect of sustained elevation of melanogenesis and map the consequent cellular repair response of melanocytes. Pigmentation increases γH2AX foci, DNA abasic sites, causes replication stress and invokes translesion polymerase Polκ in primary human melanocytes, as well as mouse melanoma cells. Confirming the causal link, CRISPR-based genetic ablation of tyrosinase results in depigmented cells with low Polκ levels. During pigmentation, Polκ activates replication stress response and keeps a check on uncontrolled proliferation of cells harboring melanin-damaged DNA. The mutational landscape observed in human melanoma could in part explain the error-prone bypass of DNA lesions by Polκ, whose absence would lead to genome instability. Thereby, translesion polymerase Polκ is a critical response of pigmenting melanocytes to combat melanin-induced DNA alterations. Our study illuminates the dark side of melanin and identifies (eu)melanogenesis as a key missing link between tanning response and mutagenesis, mediated via the necessary evil translesion polymerase, Polκ.

Dual chemical labeling enables nucleotide-resolution mapping of DNA abasic sites and common alkylation damage in human mitochondrial DNA

Mitochondrial DNA (mtDNA) modifications play an emerging role in innate immunity and inflammatory diseases. Nonetheless, relatively little is known regarding the locations of mtDNA modifications. Such information is critically important for deciphering their roles in mtDNA instability, mtDNA-mediated immune and inflammatory responses, and mitochondrial disorders. The affinity probe-based enrichment of lesion-containing DNA represents a key strategy for sequencing DNA modifications. Existing methods are limited in the enrichment specificity of abasic (AP) sites, a prevalent DNA modification and repair intermediate. Herein, we devise a novel approach, termed dual chemical labeling-assisted sequencing (DCL-seq), for mapping AP sites. DCL-seq features two designer compounds for enriching and mapping AP sites specifically at single-nucleotide resolution. For proof of principle, we mapped AP sites in mtDNA from HeLa cells under different biological conditions. The resulting AP site maps coincide with mtDNA regions with low TFAM (mitochondrial transcription factor A) coverage and with potential G-quadruplex-forming sequences. In addition, we demonstrated the broader applicability of the method in sequencing other DNA modifications in mtDNA, such as N7-methyl-2'-deoxyguanosine and N3-methyl-2'-deoxyadenosine, when coupled with a lesion-specific repair enzyme. Together, DCL-seq holds the promise to sequence multiple DNA modifications in various biological samples.

Abasic site-peptide cross-links are blocking lesions repaired by AP endonucleases

Apurinic/apyrimidinic (AP) sites are abundant DNA lesions arising from spontaneous hydrolysis of the N-glycosidic bond and as base excision repair (BER) intermediates. AP sites and their derivatives readily trap DNA-bound proteins, resulting in DNA-protein cross-links. Those are subject to proteolysis but the fate of the resulting AP-peptide cross-links (APPXLs) is unclear. Here, we report two in vitro models of APPXLs synthesized by cross-linking of DNA glycosylases Fpg and OGG1 to DNA followed by trypsinolysis. The reaction with Fpg produces a 10-mer peptide cross-linked through its N-terminus, while OGG1 yields a 23-mer peptide attached through an internal lysine. Both adducts strongly blocked Klenow fragment, phage RB69 polymerase, Saccharolobus solfataricus Dpo4, and African swine fever virus PolX. In the residual lesion bypass, mostly dAMP and dGMP were incorporated by Klenow and RB69 polymerases, while Dpo4 and PolX used primer/template misalignment. Of AP endonucleases involved in BER, Escherichia coli endonuclease IV and its yeast homolog Apn1p efficiently hydrolyzed both adducts. In contrast, E. coli exonuclease III and human APE1 showed little activity on APPXL substrates. Our data suggest that APPXLs produced by proteolysis of AP site-trapped proteins may be removed by the BER pathway, at least in bacterial and yeast cells.

Bypass of Abasic Site-Peptide Cross-Links by Human Repair and Translesion DNA Polymerases

DNA-protein cross-links remain the least-studied type of DNA damage. Recently, their repair was shown to involve proteolysis; however, the fate of the peptide remnant attached to DNA is unclear. Particularly, peptide cross-links could interfere with DNA polymerases. Apurinuic/apyrimidinic (AP) sites, abundant and spontaneously arising DNA lesions, readily form cross-links with proteins. Their degradation products (AP site-peptide cross-links, APPXLs) are non-instructive and should be even more problematic for polymerases. Here, we address the ability of human DNA polymerases involved in DNA repair and translesion synthesis (POLβ, POLλ, POLη, POLκ and PrimPOL) to carry out synthesis on templates containing AP sites cross-linked to the N-terminus of a 10-mer peptide (APPXL-I) or to an internal lysine of a 23-mer peptide (APPXL-Y). Generally, APPXLs strongly blocked processive DNA synthesis. The blocking properties of APPXL-I were comparable with those of an AP site, while APPXL-Y constituted a much stronger obstruction. POLη and POLκ demonstrated the highest bypass ability. DNA polymerases mostly used dNTP-stabilized template misalignment to incorporate nucleotides when encountering an APPXL. We conclude that APPXLs are likely highly cytotoxic and mutagenic intermediates of AP site-protein cross-link repair and must be quickly eliminated before replication.

Genotoxic effects of the major alkylation damage N7-methylguanine and methyl formamidopyrimidine

Various alkylating agents are known to preferentially modify guanine in DNA, resulting in the formation of N7-alkylguanine (N7-alkylG) and the imidazole ring opened alkyl-formamidopyrimidine (alkyl-FapyG) lesions. Evaluating the mutagenic effects of N7-alkylG has been challenging due to the instability of the positively charged N7-alkylG. To address this issue, we developed a 2'-fluorine-mediated transition-state destabilization approach, which stabilizes N7-alkylG and prevents spontaneous depurination. We also developed a postsynthetic conversion of 2'-F-N7-alkylG DNA into 2'-F-alkyl-FapyG DNA. Using these methods, we incorporated site-specific N7-methylG and methyl-FapyG into pSP189 plasmid and determined their mutagenic properties in bacterial cells using the supF-based colony screening assay. The mutation frequency of N7-methylG was found to be less than 0.5%. Our crystal structure analysis revealed that N7-methylation did not significantly alter base pairing properties, as evidenced by a correct base pairing between 2'-F-N7-methylG and dCTP in Dpo4 polymerase catalytic site. In contrast, the mutation frequency of methyl-FapyG was 6.3%, highlighting the mutagenic nature of this secondary lesion. Interestingly, all mutations arising from methyl-FapyG in the 5'-GGT(methyl-FapyG)G-3' context were single nucleotide deletions at the 5'-G of the lesion. Overall, our results demonstrate that 2'-fluorination technology is a useful tool for studying the chemically labile N7-alkylG and alkyl-FapyG lesions.

A homogeneous fluorescence assay for rapid and sensitive quantification of the global level of abasic sites in genomic DNA

The apurinic/apyrimidinic (AP) sites are frequent DNA lesions in genomic DNA (gDNA). Here we report a facile approach for rapid quantification of the AP sites in gDNA with high selectivity and sensitivity. With the assistance of T4 pyrimidine dimer glycosylase, we covalently labeled the AP sites with 5'-hydroxylamine-modified oligonucleotide strand with high chemical selectivity against to naturally occurring formylated-bases, such as 5-formylcytosine and 5-formyluracil. Next, we sequentially removed the excessive labeling strands and triggered a signal amplification reaction with the labeled strands in a homogeneous system by flexible variation of the 3' or 5' terminal bases of an assistant strand and a fluorescent probe in the presence of a versatile exonuclease (lambda exonuclease). The detection of AP sites in gDNA was realized with an input of gDNA less than 500 ng and a limit of detection down to 0.2 fmol. The method enabled quantification of AP sites in gDNA from both normal cells and cells exposed to external damaging agents, showing the variation of AP sites level along with damaging and repair processes. The work has also provided a useful strategy for the rapid detection of other targeted sites in gDNA in a homogeneous system.

Mechanistic Investigation of the Androgen Receptor DNA-Binding Domain and Modulation via Direct Interactions with DNA Abasic Sites: Understanding the Mechanisms Involved in Castration-Resistant Prostate Cancer

The androgen receptor (AR) is an important drug target in prostate cancer and a driver of castration-resistant prostate cancer (CRPC). A significant challenge in designing effective drugs lies in targeting constitutively active AR variants and, most importantly, nearly all AR variants lacking the ligand-binding domain (LBD). Recent findings show that an AR's constitutive activity may occur in the presence of somatic DNA mutations within non-coding regions, but the role of these mutations remains elusive. The discovery of new drugs targeting CRPC is hampered by the limited molecular understanding of how AR binds mutated DNA sequences, frequently observed in prostate cancer, and how mutations within the protein and DNA regulate AR-DNA interactions. Using atomistic molecular dynamics (MD) simulations and quantum mechanical calculations, we focused our efforts on (i) rationalising the role of several activating DBD mutations linked to prostate cancer, and (ii) DBD interactions in the presence of abasic DNA lesions, which frequently occur in CRPC. Our results elucidate the role of mutations within DBD through their modulation of the intrinsic dynamics of the DBD-DNA ternary complex. Furthermore, our results indicate that the DNA apurinic lesions occurring in the androgen-responsive element (ARE) enhance direct AR-DNA interactions and stabilise the DBD homodimerisation interface. Moreover, our results strongly suggest that those abasic lesions may form reversible covalent crosslinks between DNA and lysine residues of an AR via a Schiff base. In addition to providing an atomistic model explaining how protein mutations within the AR DNA-binding domain affect AR dimerisation and AR-DNA interactions, our findings provide insight into how somatic mutations occurring in DNA non-coding regions may activate ARs. These mutations are frequently observed in prostate cancer and may contribute to disease progression by enhancing direct AR-DNA interactions.

Ligands for Abasic Site-containing DNA and their Use as Fluorescent Probes

Apurinic and apyrimidinic sites, also referred to as abasic or AP sites, are residues of duplex DNA in which one DNA base is removed from a Watson-Crick base pair. They are formed during the enzymatic repair of DNA and offer binding sites for a variety of guest molecules. Specifically, the AP site may bind an appropriate ligand as a substitute for the missing nucleic base, thus stabilizing the abasic site-containing DNA (AP-DNA). Notably, ligands that bind selectively to abasic sites may be employed for analytical and therapeutical purposes. As a result, there is a search for structural features that establish a strong and selective association of a given ligand with the abasic position in DNA. Against this background, this review provides an overview of the different classes of ligands for abasic site-containing DNA (AP-DNA). This review covers covalently binding substrates, namely amine and oxyamine derivatives, as well as ligands that bind to AP-DNA by noncovalent association, as represented by small heterocyclic aromatic compounds, metal-organic complexes, macrocyclic cyclophanes, and intercalator-nucleobase conjugates. As the systematic development of fluorescent probes for AP-DNA has been somewhat neglected so far, this review article contains a survey of the available reports on the fluorimetric response of the ligand upon binding to the AP-DNA. Based on these data, this compilation shall present a perspective for future developments of fluorescent probes for AP-DNA.

Bulky Adducts in Clustered DNA Lesions: Causes of Resistance to the NER System

The nucleotide excision repair (NER) system removes a wide range of bulky DNA lesions that cause significant distortions of the regular double helix structure. These lesions, mainly bulky covalent DNA adducts, are induced by ultraviolet and ionizing radiation or the interaction between exogenous/endogenous chemically active substances and nitrogenous DNA bases. As the number of DNA lesions increases, e.g., due to intensive chemotherapy and combination therapy of various diseases or DNA repair impairment, clustered lesions containing bulky adducts may occur. Clustered lesions are two or more lesions located within one or two turns of the DNA helix. Despite the fact that repair of single DNA lesions by the NER system in eukaryotic cells has been studied quite thoroughly, the repair mechanism of these lesions in clusters remains obscure. Identification of the structural features of the DNA regions containing irreparable clustered lesions is of considerable interest, in particular due to a relationship between the efficiency of some antitumor drugs and the activity of cellular repair systems. In this review, we analyzed data on the induction of clustered lesions containing bulky adducts, the potential biological significance of these lesions, and methods for quantification of DNA lesions and considered the causes for the inhibition of NER-catalyzed excision of clustered bulky lesions.

Selective Antitumor Activity and Photocytotoxicity of Glutathione-Activated Abasic Site Trapping Agents

Abasic (AP) sites are one of the most common DNA lesions in cells. Aldehyde-reactive alkoxyamines capture AP sites and block the activity of APE1, the enzyme responsible for initiating their repair. Blocking the APE1 repair of AP sites leads to cell death, and it is an actively investigated approach for treating cancer. However, unselective AP site capture in different cells produces side effects and limits the application of alkoxyamines in chemotherapy. Herein we take advantage of the higher glutathione (GSH) concentration in cancer cells over normal cells to develop GSH-inducible agents that selectively kill cancer cells. 2,4-Dinitrobenzenesulfonamide caged coumarin-based alkoxyamines 1 and 2 are selectively revealed by GSH to release SO₂ and fluorescent coumarin-based alkoxyamines 3 and 4 that trap AP sites in cells. GSH-directed AP site trapping and SO₂ release result in selective cytotoxicity (defined as IC₅₀WI38/IC₅₀H1299) against H1299 lung cancer cells over normal WI38 lung cells, ranging from 1.8 to 2.8 for 1 and 2. The alkylating agent methylmethanesulfonate (MMS) promotes the formation of AP sites in cells and enhances the cytotoxicity of agent 1 in a dose-dependent way. Moreover, the comet assay and γH2AX assay suggest that AP adducts form a highly toxic DNA interstrand cross-link (ICL) upon photolysis, leading to further cell death. DNA flow cytometric analysis showed that 1 promoted cell apoptosis in the early stage and induced G2/M phase cell-cycle arrest. The 2,4-dinitrobenzenesulfonamide-caged alkoxyamines exhibited selective antitumor activity and photocytotoxicity in cancer cells, illuminating their potential as GSH-directed chemotherapeutic agents.

Single nucleus multi-omics identifies human cortical cell regulatory genome diversity

Single-cell technologies measure unique cellular signatures but are typically limited to a single modality. Computational approaches allow the fusion of diverse single-cell data types, but their efficacy is difficult to validate in the absence of authentic multi-omic measurements. To comprehensively assess the molecular phenotypes of single cells, we devised single-nucleus methylcytosine, chromatin accessibility, and transcriptome sequencing (snmCAT-seq) and applied it to postmortem human frontal cortex tissue. We developed a cross-validation approach using multi-modal information to validate fine-grained cell types and assessed the effectiveness of computational data fusion methods. Correlation analysis in individual cells revealed distinct relations between methylation and gene expression. Our integrative approach enabled joint analyses of the methylome, transcriptome, chromatin accessibility, and conformation for 63 human cortical cell types. We reconstructed regulatory lineages for cortical cell populations and found specific enrichment of genetic risk for neuropsychiatric traits, enabling the prediction of cell types that are associated with diseases.

Effects of N7-Alkylguanine Conformation and Metal Cofactors on the Translesion Synthesis by Human DNA Polymerase η

Non-enzymatic alkylation on DNA often generates N7-alkyl-2'-deoxyguanosine (N7alkylG) adducts as major lesions. N7alkylG adducts significantly block replicative DNA polymerases and can be bypassed by translesion synthesis (TLS) polymerases such as polymerase η (polη). To gain insights into the bypass of N7alkylG by TLS polymerases, we conducted kinetic and structural studies of polη catalyzing across N7BnG, a genotoxic lesion generated by the carcinogenic N-nitrosobenzylmethylamine. The presence of templating N7BnG in the polη catalytic site decreased the replication fidelity by ∼9-fold, highlighting the promutagenicity of N7BnG. The catalytic efficiency for dCTP incorporation opposite N7BnG decreased ∼22-fold and ∼7-fold compared to the incorporation opposite undamaged guanine in the presence of Mg²⁺ and Mn²⁺, respectively. A crystal structure of the complexes grown with polη, templating N7BnG, incoming dCTP, and Mg²⁺ ions showed the lack of the incoming nucleotide and metal cofactors in the polη catalytic site. Interestingly, the templating N7BnG adopted a syn conformation, which has not been observed in the published N7alkylG structures. The preferential formation of syn-N7BnG conformation at the templating site may deter the binding of an incoming dCTP, causing the inefficient bypass by polη. In contrast, the use of Mn²⁺ in place of Mg²⁺ in co-crystallization yielded a ternary complex displaying an anti-N7BnG:dCTP base pair and catalytic metal ions, which would be a close mimic of a catalytically competent state. We conclude that certain bulky N7-alkylG lesions can slow TLS polymerase-mediated bypass by adopting a catalytically unfavorable syn conformation in the replicating base pair site.

AOP report: Development of an adverse outcome pathway for oxidative DNA damage leading to mutations and chromosomal aberrations

The Genetic Toxicology Technical Committee (GTTC) of the Health and Environmental Sciences Institute (HESI) is developing adverse outcome pathways (AOPs) that describe modes of action leading to potentially heritable genomic damage. The goal was to enhance the use of mechanistic information in genotoxicity assessment by building empirical support for the relationships between relevant molecular initiating events (MIEs) and regulatory endpoints in genetic toxicology. Herein, we present an AOP network that links oxidative DNA damage to two adverse outcomes (AOs): mutations and chromosomal aberrations. We collected empirical evidence from the literature to evaluate the key event relationships between the MIE and the AOs, and assessed the weight of evidence using the modified Bradford-Hill criteria for causality. Oxidative DNA damage is constantly induced and repaired in cells given the ubiquitous presence of reactive oxygen species and free radicals. However, xenobiotic exposures may increase damage above baseline levels through a variety of mechanisms and overwhelm DNA repair and endogenous antioxidant capacity. Unrepaired oxidative DNA base damage can lead to base substitutions during replication and, along with repair intermediates, can also cause DNA strand breaks that can lead to mutations and chromosomal aberrations if not repaired adequately. This AOP network identifies knowledge gaps that could be filled by targeted studies designed to better define the quantitative relationships between key events, which could be leveraged for quantitative chemical safety assessment. We anticipate that this AOP network will provide the building blocks for additional genotoxicity-associated AOPs and aid in designing novel integrated testing approaches for genotoxicity.

Genotoxic C8-Arylamino-2'-deoxyadenosines Act as Latent Alkylating Agents to Induce DNA Interstrand Cross-Links

DNA interstrand cross-links (ICLs) are extremely deleterious and structurally diverse, driving the evolution of ICL repair pathways. Discovering ICL-inducing agents is, thus, crucial for the characterization of ICL repair pathways and Fanconi anemia, a genetic disease caused by mutations in ICL repair genes. Although several studies point to oxidative stress as a cause of ICLs, oxidative stress-induced cross-linking events remain poorly characterized. Also, polycyclic aromatic amines, potent environmental carcinogens, have been implicated in producing ICLs, but their identities and sequences are unknown. To close this knowledge gap, we tested whether ICLs arise by the oxidation of 8-arylamino-2'-deoxyadenosine (ArNHdA) lesions, adducts produced by arylamino carcinogens. Herein, we report that ArNHdA acts as a latent cross-linking agent to generate ICLs under oxidative conditions. The formation of an ICL from 8-aminoadenine, but not from 8-aminoguanine, highlights the specificity of 8-aminopurine-mediated ICL production. Under the influence of the reactive oxygen species (ROS) nitrosoperoxycarbonate, ArNHdA (Ar = biphenyl, fluorenyl) lesions were selectively oxidized to generate ICLs. The cross-linking reaction may occur between the C2-ArNHdA and N2-dG, presumably via oxidation of ArNHdA into a reactive diiminoadenine intermediate followed by the nucleophilic attack of the N2-dG on the diiminoadenine. Overall, ArNHdA-mediated ICLs represent rare examples of ROS-induced ICLs and polycyclic aromatic amine-mediated ICLs. These results reveal novel cross-linking chemistry and the genotoxic effects of arylamino carcinogens and support the hypothesis that C8-modified adenines with low redox potential can cause ICLs in oxidative stress.

High-Resolution Mapping of Amino Acid Residues in DNA-Protein Cross-Links Enabled by Ribonucleotide-Containing DNA

DNA-protein cross-links have broad applications in mapping DNA-protein interactions and provide structural insights into macromolecular structures. However, high-resolution mapping of DNA-interacting amino acid residues with tandem mass spectrometry remains challenging due to difficulties in sample preparation and data analysis. Herein, we developed a method for identifying cross-linking amino residues in DNA-protein cross-links at single amino acid resolution. We leveraged the alkaline lability of ribonucleotides and designed ribonucleotide-containing DNA to produce structurally defined nucleic acid-peptide cross-links under our optimized ribonucleotide cleavage conditions. The structurally defined oligonucleotide-peptide heteroconjugates improved ionization, reduced the database search space, and facilitated the identification of cross-linking residues in peptides. We applied the workflow to identifying abasic (AP) site-interacting residues in human mitochondrial transcription factor A (TFAM)-DNA cross-links. With sub-nmol sample input, we obtained high-quality fragmentation spectra for nucleic acid-peptide cross-links and identified 14 cross-linked lysine residues with the home-built AP_CrosslinkFinder program. Semi-quantification based on integrated peak areas revealed that K186 of TFAM is the major cross-linking residue, consistent with K186 being the closest (to the AP modification) lysine residue in solved TFAM:DNA crystal structures. Additional cross-linking lysine residues (K69, K76, K136, K154) support the dynamic characteristics of TFAM:DNA complexes. Overall, our combined workflow using ribonucleotide as a chemically cleavable DNA modification together with optimized sample preparation and data analysis offers a simple yet powerful approach for mapping cross-linking sites in DNA-protein cross-links. The method is amendable to other chemical or photo-cross-linking systems and can be extended to complex biological samples.

Mild phenotype of knockouts of the major apurinic/apyrimidinic endonuclease APEX1 in a non-cancer human cell line

The major human apurinic/apyrimidinic (AP) site endonuclease, APEX1, is a central player in the base excision DNA repair (BER) pathway and has a role in the regulation of DNA binding by transcription factors. In vertebrates, APEX1 knockouts are embryonic lethal, and only a handful of knockout cell lines are known. To facilitate studies of multiple functions of this protein in human cells, we have used the CRISPR/Cas9 system to knock out the APEX1 gene in a widely used non-cancer hypotriploid HEK 293FT cell line. Two stable knockout lines were obtained, one carrying two single-base deletion alleles and one single-base insertion allele in exon 3, another homozygous in the single-base insertion allele. Both mutations cause a frameshift that leads to premature translation termination before the start of the protein's catalytic domain. Both cell lines totally lacked the APEX1 protein and AP site-cleaving activity, and showed significantly lower levels of the APEX1 transcript. The APEX1-null cells were unable to support BER on uracil- or AP site-containing substrates. Phenotypically, they showed a moderately increased sensitivity to methyl methanesulfonate (MMS; ~2-fold lower EC50 compared with wild-type cells), and their background level of natural AP sites detected by the aldehyde-reactive probe was elevated ~1.5-2-fold. However, the knockout lines retained a nearly wild-type sensitivity to oxidizing agents hydrogen peroxide and potassium bromate. Interestingly, despite the increased MMS cytotoxicity, we observed no additional increase in AP sites in knockout cells upon MMS treatment, which could indicate their conversion into more toxic products in the absence of repair. Overall, the relatively mild cell phenotype in the absence of APEX1-dependent BER suggests that mammalian cells possess mechanisms of tolerance or alternative repair of AP sites. The knockout derivatives of the extensively characterized HEK 293FT cell line may provide a valuable tool for studies of APEX1 in DNA repair and beyond.

Reactivity and DNA Damage by Independently Generated 2'-Deoxycytidin-N4-yl Radical

Oxidative stress produces a variety of radicals in DNA, including pyrimidine nucleobase radicals. The nitrogen-centered DNA radical 2'-deoxycytidin-N4-yl radical (dC·) plays a role in DNA damage mediated by one electron oxidants, such as HOCl and ionizing radiation. However, the reactivity of dC· is not well understood. To reduce this knowledge gap, we photochemically generated dC· from a nitrophenyl oxime nucleoside and within chemically synthesized oligonucleotides from the same precursor. dC· formation is confirmed by transient UV-absorption spectroscopy in laser flash photolysis (LFP) experiments. LFP and duplex DNA cleavage experiments indicate that dC· oxidizes dG. Transient formation of the dG radical cation (dG^+•) is observed in LFP experiments. Oxidation of the opposing dG in DNA results in hole transfer when the opposing dG is part of a dGGG sequence. The sequence dependence is attributed to a competition between rapid proton transfer from dG^+• to the opposing dC anion formed and hole transfer. Enhanced hole transfer when less acidic O6-methyl-2'-deoxyguanosine is opposite dC· supports this proposal. dC· produces tandem lesions in sequences containing thymidine at the 5'-position by abstracting a hydrogen atom from the thymine methyl group. The corresponding thymidine peroxyl radical completes tandem lesion formation by reacting with the 5'-adjacent nucleotide. As dC· is reduced to dC, its role in the process is traceless and is only detectable because of the ability to independently generate it from a stable precursor. These experiments reveal that dC· oxidizes neighboring nucleotides, resulting in deleterious tandem lesions and hole transfer in appropriate sequences.

Recognition and removal of clustered DNA lesions via nucleotide excision repair

Clustered damage of DNA consists of two or more lesions located within one or two turns of the DNA helix. Clusters consisting of lesions of various structures can arise under the influence of strong damaging factors, especially if the cells have a compromised repair status. In this work, we analyzed how the presence of an analog of the apurinic/apyrimidinic site - a non-nucleoside residue consisting of diethylene glycol phosphodiester (DEG) - affects the recognition and removal of a bulky lesion (a non-nucleoside site of the modified DNA strand containing a fluorescein residue, nFlu) from DNA by a mammalian nucleotide excision repair system. Here we demonstrated that the efficiency of nFlu removal decreases in the presence of DEG in the complementary strand and is completely suppressed when the DEG is located opposite the nFlu. By contrast, protein factor XPC-RAD23B, which initiates global genomic nucleotide excision repair, has higher affinity for DNA containing clustered damage as compared to DNA containing a single bulky lesion; the affinity of XPC strengthens as the positions of DEG and nFlu become closer. The changes in the double-stranded DNA's geometry caused by the presence of clustered damage were also assessed. The obtained experimental data together with the results of molecular dynamics simulations make it possible to get insight into the structural features of DNA containing clustered lesions that determine the efficiency of repair. Speaking more broadly, this study should help to understand the probable fate of bulky adduct-containing clusters of various topologies in the mammalian cell.

Effect of N7-methylation on base pairing patterns of guanine: a DFT study

In this paper, we aim to determine whether the N7-methylation can influence the base pairing properties of guanine by promoting the formation of guanine enol-tautomers. The keto- to -enol-tautomerization of N7-methylguanine (N7mG) and its base pairing patterns with all the canonical DNA bases have been investigated at the M06-2X/6-311+G(d,p) level of density functional theory. The barrier free energy calculations reveal that N7-methylation does not promote the keto- to enol- tautomerization of guanine. The Watson-Crick-like enol-N7mG:T1 or enol-N7mG:T2 base pair similar to what is observed experimentally is found to be energetically more stable than the keto-N7mG:T base pairs. However, the keto-N7mG:C1 which is structurally similar to the canonical G:C base pair is the most stable base pair among all the base pairs studied here. Thus, our calculations predict that N7mG would pair preferably with cytosine during DNA replication but there is also a probability that it can cause mutation through mispairing with thymine, in agreement with experimental observations.

A base-repair based electrochemiluminescent genotoxicity sensor that detects abasic sites in double-stranded DNA films

A novel genotoxicity sensor was developed based on the base repair process associated with the electrochemiluminescence (ECL) detection of abasic sites in a double-stranded DNA monolayer. This is the first time that an ECL sensor with the ability to identify the removed nucleobases in a DNA duplex has been studied. The successful detection of abasic sites created by DNA glycosylase indicates its further applications for examining some other specific types of DNA damage.

Hg(II) Binding to Thymine Bases in DNA

The compounds of mercury can be highly toxic and can interfere with a range of biological processes, although many aspects of the mechanism of toxicity are still obscure or unknown. One especially intriguing property of Hg(II) is its ability to bind DNA directly, making interstrand cross-links between thymine nucleobases in AT-rich sequences. We have used a combination of small molecule X-ray diffraction, X-ray spectroscopies, and computational chemistry to study the interactions of Hg(II) with thymine. We find that the energetically preferred mode of thymine binding in DNA is to the N3 and predict only minor distortions of the DNA structure on binding one Hg(II) to two cross-adjacent thymine nucleotides. The preferred geometry is predicted to be twisted away from coplanar through a torsion angle of between 32 and 43°. Using 1-methylthymine as a model, the bis-thymine coordination of Hg(II) is found to give a highly characteristic X-ray spectroscopic signature that is quite distinct from other previously described biological modes of binding of Hg(II). This work enlarges and deepens our view of significant biological targets of Hg(II) and demonstrates tools that can provide a characteristic signature for the binding of Hg(II) to DNA in more complex matrices including intact cells and tissues, laying the foundation for future studies of mechanisms of mercury toxicity.

Crosslinker-modified nucleic acid probes for improved target identification and biomarker detection

Understanding the intricate interaction pattern of nucleic acids with other molecules is essential to gain further insight in biological processes and disease mechanisms. To this end, a multitude of hybridization-based assays have been designed that rely on the non-covalent recognition between complementary nucleic acid sequences. However, the ephemeral nature of these interactions complicates straightforward analysis as low efficiency and specificity are rule rather than exception. By covalently locking nucleic acid interactions by means of a crosslinking agent, the overall efficiency, specificity and selectivity of hybridization-based assays could be increased. In this mini-review we highlight methodologies that exploit the use of crosslinker-modified nucleic acid probes for interstrand nucleic acid crosslinking with the objective to study, detect and identify important targets as well as nucleic acid sequences that can be considered relevant biomarkers. We emphasize on the usefulness and advantages of crosslinking agents and elaborate on the chemistry behind the crosslinking reactions they induce.

Nucleosomal embedding reshapes the dynamics of abasic sites

Apurinic/apyrimidinic (AP) sites are the most common DNA lesions, which benefit from a most efficient repair by the base excision pathway. The impact of losing a nucleobase on the conformation and dynamics of B-DNA is well characterized. Yet AP sites seem to present an entirely different chemistry in nucleosomal DNA, with lifetimes reduced up to 100-fold, and the much increased formation of covalent DNA-protein cross-links leading to strand breaks, refractory to repair. We report microsecond range, all-atom molecular dynamics simulations that capture the conformational dynamics of AP sites and their tetrahydrofuran analogs at two symmetrical positions within a nucleosome core particle, starting from a recent crystal structure. Different behaviours between the deoxyribo-based and tetrahydrofuran-type abasic sites are evidenced. The two solvent-exposed lesion sites present contrasted extrahelicities, revealing the crucial role of the position of a defect around the histone core. Our all-atom simulations also identify and quantify the frequency of several spontaneous, non-covalent interactions between AP and positively-charged residues from the histones H2A and H2B tails that prefigure DNA-protein cross-links. Such an in silico mapping of DNA-protein cross-links gives important insights for further experimental studies involving mutagenesis and truncation of histone tails to unravel mechanisms of DPCs formation.

Premature Termination Codon-Bearing mRNA Mediates Genetic Compensation Response

The genetic compensation response (GCR), triggered by deleterious mutations but not by gene knockdown, has been proposed to explain many phenotypic discrepancies between gene-knockout and gene-knockdown models. GCRs have been observed in many model organisms from mice to Arabidopsis. Although the GCR is beneficial for organism survival, it impedes the exploration of gene function as many knockout mutants do not display discernible phenotypes due to the GCR. Uncovering how the mechanism of GCR operates is not only a fundamental goal in biology but also may provide a key solution in the unmasking of phenotypes in mutants displaying GCRs. Using zebrafish as the model, two recent studies have provided a molecular basis to explain this genetic paradox by demonstrating that the nonsense-mediated mRNA decay pathway is essential for nonsense mRNA to upregulate the expression of its homologous genes through an enhancement of histone H3 Lys4 trimethylation (H3K4me3) at the transcription start site regions of the compensatory genes. Here, we summarize the progress on the molecular mechanism of the GCR and make suggestions on how to overcome GCRs in the generation of genetic mutants.

Molecular and structural characterization of disease-associated APE1 polymorphisms

Under conditions of oxidative stress, reactive oxygen species (ROS) continuously assault the structure of DNA resulting in oxidation and fragmentation of the nucleobases. When the nucleobase structure is altered, its base-pairing properties may also be altered, promoting mutations. Consequently, oxidative DNA damage is a major source of the mutation load that gives rise to numerous human maladies, including cancer. Base excision repair (BER) is the primary pathway tasked with removing and replacing mutagenic DNA base damage. Apurinic/apyrimidinic endonuclease 1 (APE1) is a central enzyme with AP-endonuclease and 3' to 5' exonuclease functions during BER, and therefore is key to maintenance of genome stability. Polymorphisms, or SNPs, in the gene encoding APE1 (APEX1) have been identified among specific human populations and result in variants of APE1 with modified function. These defects in APE1 potentially result in impaired DNA repair capabilities and consequently an increased risk of disease for individuals within these populations. In the present study, we determined the X-ray crystal structures of three prevalent disease-associated APE1 SNPs (D148E, L104R, and R237C). Each APE1 SNP results in unique localized changes in protein structure, including protein dynamics and DNA binding contacts. Combined with comprehensive biochemical characterization, including pre-steady-state kinetic and DNA binding analyses, variant APE1:DNA complex structures with both AP-endonuclease and exonuclease substrates were analyzed to elucidate how these SNPs might perturb the two major repair functions employed by APE1 during BER.

Single-stranded DNA damage: Protecting the single-stranded DNA from chemical attack

Cellular processes, such as DNA replication, recombination and transcription, require DNA strands separation and single-stranded DNA is formation. The single-stranded DNA is promptly wrapped by human single-stranded DNA binding proteins, replication protein A (RPA) complex. RPA binding not only prevent nuclease degradation and annealing, but it also coordinates cell-cycle checkpoint activation and DNA repair. However, RPA binding offers little protection against the chemical modification of DNA bases. This review focuses on the type of DNA base damage that occurs in single-stranded DNA and how the damage is rectified in human cells. The discovery of DNA repair proteins, such as ALKBH3, AGT, UNG2, NEIL3, being able to repair the damaged base in the single-stranded DNA, renewed the interest to study single-stranded DNA repair. These mechanistically different proteins work independently from each other with the overarching goal of increasing fidelity of recombination and promoting error-free replication.

Nucleotide excision repair of abasic DNA lesions

Apurinic/apyrimidinic (AP) sites are a class of highly mutagenic and toxic DNA lesions arising in the genome from a number of exogenous and endogenous sources. Repair of AP lesions takes place predominantly by the base excision pathway (BER). However, among chemically heterogeneous AP lesions formed in DNA, some are resistant to the endonuclease APE1 and thus refractory to BER. Here, we employed two types of reporter constructs accommodating synthetic APE1-resistant AP lesions to investigate the auxiliary repair mechanisms in human cells. By combined analyses of recovery of the transcription rate and suppression of transcriptional mutagenesis at specifically positioned AP lesions, we demonstrate that nucleotide excision repair pathway (NER) efficiently removes BER-resistant AP lesions and significantly enhances the repair of APE1-sensitive ones. Our results further indicate that core NER components XPA and XPF are equally required and that both global genome (GG-NER) and transcription coupled (TC-NER) subpathways contribute to the repair.

Specificity and Efficiency of the Uracil DNA Glycosylase-Mediated Strand Cleavage Surveyed on Large Sequence Libraries

Uracil-DNA glycosylase (UDG) is a critical DNA repair enzyme that is well conserved and ubiquitous in nearly all life forms. UDG protects genomic information integrity by catalyzing the excision from DNA of uracil nucleobases resulting from misincorporation or spontaneous cytosine deamination. UDG-mediated strand cleavage is also an important tool in molecular biotechnology, allowing for controlled and location-specific cleavage of single- and double DNA chemically or enzymatically synthesized with single or multiple incorporations of deoxyuridine. Although the cleavage mechanism is well-understood, detailed knowledge of efficiency and sequence specificity, in both single and double-stranded DNA contexts, has so far remained incomplete. Here we use an experimental approach based on the large-scale photolithographic synthesis of uracil-containing DNA oligonucleotides to comprehensively probe the context-dependent uracil excision efficiency of UDG.

Redox Regulation and Oxidative Stress: The Particular Case of the Stallion Spermatozoa

Redox regulation and oxidative stress have become areas of major interest in spermatology. Alteration of redox homeostasis is recognized as a significant cause of male factor infertility and is behind the damage that spermatozoa experience after freezing and thawing or conservation in a liquid state. While for a long time, oxidative stress was just considered an overproduction of reactive oxygen species, nowadays it is considered as a consequence of redox deregulation. Many essential aspects of spermatozoa functionality are redox regulated, with reversible oxidation of thiols in cysteine residues of key proteins acting as an “on–off” switch controlling sperm function. However, if deregulation occurs, these residues may experience irreversible oxidation and oxidative stress, leading to malfunction and ultimately death of the spermatozoa. Stallion spermatozoa are “professional producers” of reactive oxygen species due to their intense mitochondrial activity, and thus sophisticated systems to control redox homeostasis are also characteristic of the spermatozoa in the horse. As a result, and combined with the fact that embryos can easily be collected in this species, horses are a good model for the study of redox biology in the spermatozoa and its impact on the embryo.

Bypass of the Major Alkylative DNA Lesion by Human DNA Polymerase η

A wide range of endogenous and exogenous alkylating agents attack DNA to generate various alkylation adducts. N7-methyl-2-deoxyguanosine (Fm7dG) is the most abundant alkylative DNA lesion. If not repaired, Fm7dG can undergo spontaneous depurination, imidazole ring-opening, or bypass by translesion synthesis DNA polymerases. Human DNA polymerase η (polη) efficiently catalyzes across Fm7dG in vitro, but its structural basis is unknown. Herein, we report a crystal structure of polη in complex with templating Fm7dG and an incoming nonhydrolyzable dCTP analog, where a 2'-fluorine-mediated transition destabilization approach was used to prevent the spontaneous depurination of Fm7dG. The structure showed that polη readily accommodated the Fm7dG:dCTP base pair with little conformational change of protein and DNA. In the catalytic site, Fm7dG and dCTP formed three hydrogen bonds with a Watson-Crick geometry, indicating that the major keto tautomer of Fm7dG is involved in base pairing. The polη-Fm7dG:dCTP structure was essentially identical to the corresponding undamaged structure, which explained the efficient bypass of the major methylated lesion. Overall, the first structure of translesion synthesis DNA polymerase bypassing Fm7dG suggests that in the catalytic site of Y-family DNA polymerases, small N7-alkylguanine adducts may be well tolerated and form the canonical Watson-Crick base pair with dCTP through their keto tautomers.

Preparation and Purification of Oligodeoxynucleotide Duplexes Containing a Site-Specific, Reduced, Chemically Stable Covalent Interstrand Cross-Link Between a Guanine Residue and an Abasic Site

Methods for the preparation of DNA duplexes containing interstrand covalent cross-links may facilitate research in the fields of biochemistry, molecular biology, nanotechnology, and materials science. Here we report methods for the synthesis and isolation of DNA duplexes containing a site-specific, chemically stable, reduced covalent interstrand cross-link between a guanine residue and an abasic site. The method uses experimental techniques and equipment that are common in most biochemical laboratories and inexpensive, commercially available oligonucleotides and reagents.

Mitochondrial transcription factor A promotes DNA strand cleavage at abasic sites

In higher eukaryotic cells, mitochondria are essential subcellular organelles for energy production, cell signaling, and the biosynthesis of biomolecules. The mitochondrial DNA (mtDNA) genome is indispensable for mitochondrial function because it encodes protein subunits of the electron transport chain and a full set of transfer and ribosomal RNAs. MtDNA degradation has emerged as an essential quality control measure to maintain mtDNA and to cope with mtDNA damage resulting from endogenous and environmental factors. Among all types of DNA damage known, abasic (AP) sites, sourced from base excision repair and spontaneous base loss, are the most abundant endogenous DNA lesions in cells. In mitochondria, AP sites trigger rapid DNA loss; however, the mechanism and molecular factors involved in the process remain elusive. Herein, we demonstrate that the stability of AP sites is reduced dramatically upon binding to a major mtDNA packaging protein, mitochondrial transcription factor A (TFAM). The half-life of AP lesions within TFAM-DNA complexes is 2 to 3 orders of magnitude shorter than that in free DNA, depending on their position. The TFAM-catalyzed AP-DNA destabilization occurs with nonspecific DNA or mitochondrial light-strand promoter sequence, yielding DNA single-strand breaks and DNA-TFAM cross-links. TFAM-DNA cross-link intermediates prior to the strand scission were also observed upon treating AP-DNA with mitochondrial extracts of human cells. In situ trapping of the reaction intermediates (DNA-TFAM cross-links) revealed that the reaction proceeds via Schiff base chemistry facilitated by lysine residues. Collectively, our data suggest a novel role of TFAM in facilitating the turnover of abasic DNA.

Mitochondrial DNA degradation: A quality control measure for mitochondrial genome maintenance and stress response

Mitochondria play a central role in bioenergetics, and fulfill a plethora of functions in cell signaling, programmed cell death, and biosynthesis of key protein cofactors. Mitochondria harbor their own genomic DNA, which encodes protein subunits of the electron transport chain and a full set of transfer and ribosomal RNAs. Mitochondrial DNA (mtDNA) is essential for cellular and organismal functions, and defects in mitochondrial genome maintenance have been implicated in common human diseases and mitochondrial disorders. mtDNA repair and degradation are known pathways to cope with mtDNA damage; however, molecular factors involved in this process have remained unclear. Such knowledge is fundamental to the understanding of mitochondrial genomic maintenance and pathology, because mtDNA degradation may contribute to the etiology of mtDNA depletion syndromes and to the activation of the innate immune response by fragmented mtDNA. This article reviews the current literature regarding the importance of mitochondrial DNA degradation in mtDNA maintenance and stress response, and the recent progress in uncovering molecular factors involved in mtDNA degradation. These factors include key components of the mtDNA replication machinery, such as DNA polymerase γ, helicase Twinkle, and exonuclease MGME1, as well as a major DNA-packaging protein, mitochondrial transcription factor A (TFAM).

Supramolecularly Multicolor DNA Decoding Using an Indicator Competition Assay

Relative to the individual intensity-dependent strategy, the multicolor fluorescence sensor has promise to achieve a high signaling contrast. In this work, we develop a cucurbituril-based supramolecular and multicolor DNA recognition rationale via indicator competition assay (ICA). Alkaloids of coptisine (COP) and palmatine (PAL) are identified as the proof-of-principle indicators with a lighting-up fluorescence upon supramolecular complexation to cucurbit[7]uril (CB[7]). With an introduced abasic site (AP site) as the contestant, DNAs having pyrimidines opposite this site can compete for COP with CB[7] to bring an emission color change from green to yellow brown, while those having purines opposite the AP site do not compete for COP and still have the green emission, indicative of a high selectivity for the multicolor nucleotide transversion recognition. However, because of the relatively weaker binding of PAL with CB[7], the AP site-containing DNA can take away PAL from its CB[7] complex and resultantly bring a blue-to-green emission color change independent of the AP site-opposite nucleotide identity, dissimilar to the remaining blue color for the fully matched DNA without the AP site, suggesting a preferable strategy for the AP site biomarker detection. Our method demonstrates a new way to develop an ICA-based multicolor DNA sensor with the supramolecular cucurbituril complexation to ensure a highly selective performance.

Incomplete base excision repair contributes to cell death from antibiotics and other stresses

Numerous lethal stresses in bacteria including antibiotics, thymineless death, and MalE-LacZ expression trigger an increase in the production of reactive oxygen species. This results in the oxidation of the nucleotide pool by radicals produced by Fenton chemistry. Following the incorporation of these oxidized nucleotides into the genome, the cell's unsuccessful attempt to repair these lesions through base excision repair (BER) contributes causally to the lethality of these stresses. We review the evidence for this phenomenon of incomplete BER-mediated cell death and discuss how better understanding this pathway could contribute to the development of new antibiotics.

Structural and Kinetic Studies of the Effect of Guanine N7 Alkylation and Metal Cofactors on DNA Replication

A wide variety of endogenous and exogenous alkylating agents attack DNA to preferentially generate N7-alkylguanine (N7-alkylG) adducts. Studies of the effect of N7-alkylG lesions on biological processes have been difficult in part because of complications arising from the chemical lability of the positively charged N7-alkylG, which can readily produce secondary lesions. To assess the effect of bulky N7-alkylG on DNA replication, we prepared chemically stable N7-benzylguanine (N7bnG)-containing DNA and evaluated nucleotide incorporation opposite the lesion by human DNA polymerase β (polβ), a model enzyme for high-fidelity DNA polymerases. Kinetic studies showed that the N7-benzyl-G lesion greatly inhibited dCTP incorporation by polβ. The crystal structure of polβ incorporating dCTP opposite N7bnG showed a Watson-Crick N7bnG:dCTP structure. The polβ-N7bnG:dCTP structure showed an open protein conformation, a relatively disordered dCTP, and a lack of catalytic metal, which explained the inefficient nucleotide incorporation opposite N7bnG. This indicates that polβ is sensitive to major groove adducts in the templating base side and deters nucleotide incorporation opposite bulky N7-alkylG adducts by adopting a catalytically incompetent conformation. Substituting Mg²⁺ for Mn²⁺ induced an open-to-closed conformational change due to the presence of catalytic metal and stably bound dCTP and increased the catalytic efficiency by ∼10-fold, highlighting the effect of binding of the incoming nucleotide and catalytic metal on protein conformation and nucleotidyl transfer reaction. Overall, these results suggest that, although bulky alkyl groups at guanine-N7 may not alter base pairing properties of guanine, the major groove-positioned lesions in the template could impede nucleotidyl transfer by some DNA polymerases.

Mechanistic Insight through Irreversible Inhibition: DNA Polymerase θ Uses a Common Active Site for Polymerase and Lyase Activities

DNA polymerase θ (Pol θ) is a multifunctional enzyme. It is nonessential in normal cells, but its upregulation in cancer cells correlates with cellular resistance to oxidative damage and poor prognosis. Pol θ possesses polymerase activity and poorly characterized lyase activity. We examined the Pol θ lyase activity on various abasic sites and determined that the enzyme is inactivated upon attempted removal of the oxidized abasic site commonly associated with C4'-oxidation (pC4-AP). Covalent modification of Pol θ by the DNA lesion enabled determination of the primary nucleophile (Lys2383) responsible for Schiff base formation in the lyase reaction. Unlike some other base excision repair polymerases, Pol θ uses a single active site for polymerase and lyase activity. Mutation of Lys2383 significantly reduces both enzyme activities but not DNA binding. Demonstration that Lys2383 is required for polymerase and lyase activities indicates that this residue is an Achilles heel for Pol θ and suggests a path forward for designing inhibitors of this attractive anticancer target.