N-methylpurine DNA glycosylase and OGG1 DNA repair activities: opposite associations with lung cancer risk

Construction of single-molecule counting-based biosensors for DNA-modifying enzymes: A review

DNA-modifying enzymes act as critical regulators in a wide range of genetic functions (e.g., DNA damage & repair, DNA replication), and their aberrant expression may interfere with regular genetic functions and induce various malignant diseases including cancers. DNA-modifying enzymes have emerged as the potential biomarkers in early diagnosis of diseases and new therapeutic targets in genomic research. Consequently, the development of highly specific and sensitive biosensors for the detection of DNA-modifying enzymes is of great importance for basic biomedical research, disease diagnosis, and drug discovery. Single-molecule fluorescence detection has been widely implemented in the field of molecular diagnosis due to its simplicity, high sensitivity, visualization capability, and low sample consumption. In this paper, we summarize the recent advances in single-molecule counting-based biosensors for DNA-modifying enzyme (i.e, alkaline phosphatase, DNA methyltransferase, DNA glycosylase, flap endonuclease 1, and telomerase) assays in the past four years (2019 - 2023). We highlight the principles and applications of these biosensors, and give new insight into the future challenges and perspectives in the development of single-molecule counting-based biosensors.

Activatable Self-Dissociation of Watson-Crick Structures with Fluorescent Nucleotides for Sensing Multiple Human Glycosylases at Single-Cell Level

Nucleobase oxidation and alkylation can destroy Watson-Crick base-pairing to challenge the genomic integrity. Human 8-oxoguanine glycosylase 1 (hOGG1) and alkyladenine glycosylase (hAAG) are evolved to counter these two cytotoxic lesions through base-excision repair, and their deregulations are implicated with multifactorial diseases and cancers. Herein, we demonstrate activatable self-dissociation of Watson-Crick structures with fluorescent nucleotides for sensing multiple human glycosylases at single-cell level. The presence of hOGG1 and hAAG catalyzes 8-oxoG and deoxyinosine removal in functional probe 1 to release two trigger probes (1 and 2). Then, trigger probes hybridize with functional probe 2 to activate the autocatalytic degradation of functional probes 2 (Cycle I) and 3 (Cycle II), replicating abundant trigger probes (1-4) and releasing two fluorophores (2-aminopurine (2-AP) and pyrrolo-dC (P-dC)). New trigger probes (1, 2) and (3, 4), in turn, hybridize with free functional probes 2 and 3, repeating Cycles I and II turnovers. Through multicycle self-dissociation of Watson-Crick structures, 2-AP and P-dC are exponentially accumulated for the simultaneous quantification of hOGG1 and hAAG. This nanodevice exhibits high sensitivity with a detection limit of 2.9 × 10^-3 U/mL for hOOG1 and 1.5 × 10^-3 U/mL for hAAG, and it can measure enzymatic kinetics, identify potential inhibitors, discriminate glycosylases between cancer and normal cell lines, and even quantify glycosylase activities in a single HeLa cell. Moreover, this assay may be rapidly and isothermally performed in one tube with only one tool enzyme in a quencher-free manner, promising a simple and powerful platform for multiple human glycosylase detection.

Generation of 3'-OH terminal-triggered encoding of multicolor fluorescence for simultaneous detection of different DNA glycosylases

Uracil DNA glycosylase (UDG) and human alkyladenine DNA glycosylase (hAAG) are the important DNA glycosylases for initiating the repair of DNA damage, and the aberrant expression of DNA glycosylases is closely associated with various diseases, such as Parkinson's disease, several cancers, and human immunodeficiency. The simultaneous detection of UDG and hAAG is helpful for the study of early clinical diagnosis. However, the reported methods for multiple DNA glycosylase assay suffer from the application of an expensive single-molecule instrument, labor-tedious magnetic separation, and complicated design. Herein, we develop a simple fluorescence method with only three necessary DNA strands for the selective and sensitive detection of multiple DNA glycosylase activity based on the generation of 3'-OH terminal-triggered encoding of multicolor fluorescence. The method can achieve the detection limits of 5.5 × 10^-5 U/mL for UDG and 3.3 × 10^-3 U/mL for hAAG, which are lower than those of the reported fluorescence methods. Moreover, it can be further used to detect multiple DNA glycosylases in the human cervical carcinoma cell line (HeLa cells), normal human renal epithelial cells (293 T cells), and biological fluid and measure the enzyme kinetic parameters of UDG and hAAG.

DNA Deformation Exerted by Regulatory DNA-Binding Motifs in Human Alkyladenine DNA Glycosylase Promotes Base Flipping

Human alkyladenine DNA glycosylase (AAG) is a key enzyme that corrects a broad range of alkylated and deaminated nucleobases to maintain genomic integrity. When encountering the lesions, AAG adopts a base-flipping strategy to extrude the target base from the DNA duplex to its active site, thereby cleaving the glycosidic bond. Despite its functional importance, the detailed mechanism of such base extrusion and how AAG distinguishes the lesions from an excess of normal bases both remain elusive. Here, through the Markov state model constructed on extensive all-atom molecular dynamics simulations, we find that the alkylated nucleobase (N3-methyladenine, 3MeA) everts through the DNA major groove. Two key AAG motifs, the intercalation and E131-N146 motifs, play active roles in bending/pressing the DNA backbone and widening the DNA minor groove during 3MeA eversion. In particular, the intercalated residue Y162 is involved in buckling the target site at the early stage of 3MeA eversion. Our traveling-salesman based automated path searching algorithm further revealed that a non-target normal adenine tends to be trapped in an exo site near the active site, which however barely exists for a target base 3MeA. Collectively, these results suggest that the Markov state model combined with traveling-salesman based automated path searching acts as a promising approach for studying complex conformational changes of biomolecules and dissecting the elaborate mechanism of target recognition by this unique enzyme.

Construction of a dephosphorylation-mediated chemiluminescent biosensor for multiplexed detection of DNA glycosylases in cancer cells

DNA glycosylases are engaged in the base excision repair process and play a vital role in maintaining genomic integrity. It remains a challenge for multiplexed detection of DNA glycosylases in cancer cells. Herein, we demonstrate the construction of a dephosphorylation-mediated chemiluminescent biosensor for multiplexed detection of human alkyladenine DNA glycosylase (hAAG) and uracil DNA glycosylase (UDG) in cancer cells. In this biosensor, the generation of chemiluminescence signals relies on the dephosphorylation of 3-(2'-spiroadamantyl)-4-methoxy-4-(3''-phosphoryloxyphenyl)-1,2-dioxetane (AMPPD) catalyzed by alkaline phosphatase (ALP). We design a bifunctional double-stranded DNA (dsDNA) substrate, a biotin-labelled poly-(T) probe, and two capture probes for the hAAG and UDG assay. This assay involves four steps including (1) the cleavage of the bifunctional dsDNA substrate induced by DNA glycosylases, (2) the recognition of the 3'-OH terminus of the primer by TdT and the subsequent TdT-mediated polymerization reaction, (3) the construction of the AuNPs-dsDNA-ALP nanostructures, and (4) the streptavidin-alkaline phosphatase (SA-ALP)-initiated dephosphorylation of AMPPD for the generation of an enhanced chemiluminescence signal. By taking advantage of the unique features of TdT-mediated polymerization and the intrinsic superiority of the ALP-AMPPD-based chemiluminescence system, this biosensor exhibits good specificity and high sensitivity with a detection limit of 1.53 × 10^-6 U mL^-1 for hAAG and 1.77 × 10^-6 U mL^-1 for UDG, and it can even quantify multiple DNA glycosylases at the single-cell level. Moreover, this biosensor can be applied for the measurement of kinetic parameters and the screening of DNA glycosylase inhibitors, holding great potential in DNA damage-related biomedical research and disease diagnostics.

Development of a CRISPR-Cas-Based Biosensor for Rapid and Sensitive Detection of 8-Oxoguanine DNA Glycosylase

8-Oxoguanine DNA glycosylase is essential for maintaining genomic integrity and stability, while its abnormal activity may lead to the disturbance in the normal DNA damage repair and the occurrence of carcinogenicity and teratogenicity. Herein, we construct a CRISPR-Cas-based biosensor for rapid and sensitive measurement of 8-oxoguanine DNA glycosylases. This biosensor involves a hairpin probe and integrates quadratic strand displacement amplification (SDA) with a CRISPR/Cas12a effector with the characteristics of rapidity (within 40 min) and isothermal assay. The presence of 8-oxoguanine DNA glycosylase can initiate the quadratic SDA to produce large amounts of activators with the assistance of polynucleotide kinase (PNK). Subsequently, the activators can bind with crRNA to activate Cas12a, cleaving signal probes and recovering Cy5 fluorescence, which can be accurately quantified by single-molecule imaging. Notably, the designed hairpin probes can effectively block the hybridization of the generated activators with free hairpin probes, endowing this biosensor with high sensitivity. In addition, the utilization of PNK instead of apurinic/apyrimidinic endonuclease (APE1) greatly simplifies the experimental procedure to only a one-step reaction. The introduction of a single-molecule detection further reduces the sample consumption and improves the sensitivity. This biosensor displays a detection limit of 4.24 × 10^-9 U μL^-1, and it can accurately quantify cellular human 8-oxoguanine DNA glycosylase at a single-cell level. Furthermore, this biosensor can be applied for the screening of inhibitors, the analysis of kinetic parameters, and the discrimination of cancer cells from normal cells, with potential applications in molecular diagnostic and point-of-care testing.

Combination of bidirectional strand displacement amplification with single-molecule detection for multiplexed DNA glycosylases assay

DNA glycosylases can initiate base excision repair pathway to repair endogenous DNA base damages for the maintenance of genome stability. Multiple DNA glycosylases exhibit abnormal in various diseases, and the simultaneous measurement of different DNA glycosylases is critical to clinical diagnosis and drug discovery. Herein, we take advantage of single-molecule detection and bidirectional strand displacement amplification (SDA) to simultaneously detect uracil DNA glycolase (UDG) and human alkyladenine DNA glycosylase (hAAG). We design a partial double-stranded DNA (dsDNA) substrate modified with specific recognition sites of UDG and hAAG. The dsDNA substrate is labeled with BHQ1 and BHQ2 at the 5'-ends and then hybridizes with the Cy3/Cy5-labeled reporter probes to obtain the BHQ1/Cy3 and BHQ2/Cy5 base pairs, resulting in the quenching of Cy3/Cy5 fluorescence by BHQ1/BHQ2 via fluorescence resonance energy transfer (FRET). When UDG and hAAG are present, they can induce the base excision repair reaction and subsequently initiate the bidirectional SDA amplification process, releasing the Cy5/Cy3-labeled reporter probes from the dsDNA substrate and consequently the recovery of Cy5 and Cy3 fluorescence, which can be measured by single-molecule detection, with Cy5 indicating UDG and Cy3 indicating hAAG. This method possesses high sensitivity and good selectivity with the capability of quantifying multiple DNA glycosylases at the single-cell level. Furthermore, it can be used to simultaneously screen DNA glycosylase inhibitors and determine enzyme kinetic parameters, with the potential of sensing various DNA/RNA enzymes by simple changing the recognition sites of DNA substrates.

CometChip enables parallel analysis of multiple DNA repair activities

DNA damage can be cytotoxic and mutagenic, and it is directly linked to aging, cancer, and other diseases. To counteract the deleterious effects of DNA damage, cells have evolved highly conserved DNA repair pathways. Many commonly used DNA repair assays are relatively low throughput and are limited to analysis of one protein or one pathway. Here, we have explored the capacity of the CometChip platform for parallel analysis of multiple DNA repair activities. Taking advantage of the versatility of the traditional comet assay and leveraging micropatterning techniques, the CometChip platform offers increased throughput and sensitivity compared to the traditional comet assay. By exposing cells to DNA damaging agents that create substrates of Base Excision Repair, Nucleotide Excision Repair, and Non-Homologous End Joining, we show that the CometChip is an effective method for assessing repair deficiencies in all three pathways. With these applications of the CometChip platform, we expand the utility of the comet assay for precise, high-throughput, parallel analysis of multiple DNA repair activities.

Excision of mutagenic replication-blocking lesions suppresses cancer but promotes cytotoxicity and lethality in nitrosamine-exposed mice

N-Nitrosodimethylamine (NDMA) is a DNA-methylating agent that has been discovered to contaminate water, food, and drugs. The alkyladenine DNA glycosylase (AAG) removes methylated bases to initiate the base excision repair (BER) pathway. To understand how gene-environment interactions impact disease susceptibility, we study Aag-knockout (Aag-/-) and Aag-overexpressing mice that harbor increased levels of either replication-blocking lesions (3-methyladenine [3MeA]) or strand breaks (BER intermediates), respectively. Remarkably, the disease outcome switches from cancer to lethality simply by changing AAG levels. To understand the underlying basis for this observation, we integrate a suite of molecular, cellular, and physiological analyses. We find that unrepaired 3MeA is somewhat toxic, but highly mutagenic (promoting cancer), whereas excess strand breaks are poorly mutagenic and highly toxic (suppressing cancer and promoting lethality). We demonstrate that the levels of a single DNA repair protein tip the balance between blocks and breaks and thus dictate the disease consequences of DNA damage.

Role of Base Excision Repair Pathway in the Processing of Complex DNA Damage Generated by Oxidative Stress and Anticancer Drugs

Chemical alterations in DNA induced by genotoxic factors can have a complex nature such as bulky DNA adducts, interstrand DNA cross-links (ICLs), and clustered DNA lesions (including double-strand breaks, DSB). Complex DNA damage (CDD) has a complex character/structure as compared to singular lesions like randomly distributed abasic sites, deaminated, alkylated, and oxidized DNA bases. CDD is thought to be critical since they are more challenging to repair than singular lesions. Although CDD naturally constitutes a relatively minor fraction of the overall DNA damage induced by free radicals, DNA cross-linking agents, and ionizing radiation, if left unrepaired, these lesions cause a number of serious consequences, such as gross chromosomal rearrangements and genome instability. If not tightly controlled, the repair of ICLs and clustered bi-stranded oxidized bases via DNA excision repair will either inhibit initial steps of repair or produce persistent chromosomal breaks and consequently be lethal for the cells. Biochemical and genetic evidences indicate that the removal of CDD requires concurrent involvement of a number of distinct DNA repair pathways including poly(ADP-ribose) polymerase (PARP)-mediated DNA strand break repair, base excision repair (BER), nucleotide incision repair (NIR), global genome and transcription coupled nucleotide excision repair (GG-NER and TC-NER, respectively), mismatch repair (MMR), homologous recombination (HR), non-homologous end joining (NHEJ), and translesion DNA synthesis (TLS) pathways. In this review, we describe the role of DNA glycosylase-mediated BER pathway in the removal of complex DNA lesions.

Sensitive homogeneous fluorescent detection of DNA glycosylase by target-triggering ligation-dependent tricyclic cascade amplification

Abnormal DNA glycosylases are concerned with the aging process as well as numerous pathologies in humans. Herein, a sensitive fluorescence method utilizing target-induced ligation-dependent tricyclic cascade amplification reaction was developed for the detecting DNA glycosylase activity. The presence of DNA glycosylase triggered the cleavage of damaged base in hairpin substrate, successively activating ligation-dependent strand displacement amplification (SDA) and exponential amplification reaction (EXPAR) for the generation of large amount of reporter probes. The resultant reporter probes bound with the signal probes to form stable dsDNA duplexes. And then the signal probes could be digested circularly in the dsDNA duplexes by T7 exonuclease, leading to the generation of an enhanced fluorescence signal. Due to the high efficiency of tricyclic cascade amplification and the low background signal deriving from the inhibition of nonspecific amplification, this method exhibited a detection limit of 0.14 U/mL and a dynamic range from 0.16 to 8.0 U/mL. Moreover, it could be applied for detecting DNA glycosylase activity in human serum with good selectivity and high sensitivity, and even quantifying other types of enzyme with 5'-PO₄ residue cleavage product by rationally designing the corresponding substrate. Importantly, this method could be performed in homogenous solution without any complicated separation steps, providing a new strategy for DNA glycosylase-related biomedical research.

An optimized comet-based in vitro DNA repair assay to assess base and nucleotide excision repair activity

This optimized protocol (including links to instruction videos) describes a comet-based in vitro DNA repair assay that is relatively simple, versatile, and inexpensive, enabling the detection of base and nucleotide excision repair activity. Protein extracts from samples are incubated with agarose-embedded substrate nucleoids ('naked' supercoiled DNA) containing specifically induced DNA lesions (e.g., resulting from oxidation, UVC radiation or benzo[a]pyrene-diol epoxide treatment). DNA incisions produced during the incubation reaction are quantified as strand breaks after electrophoresis, reflecting the extract's incision activity. The method has been applied in cell culture model systems, human biomonitoring and clinical investigations, and animal studies, using isolated blood cells and various solid tissues. Once extracts and substrates are prepared, the assay can be completed within 2 d.

Rolling circle amplification-driven encoding of different fluorescent molecules for simultaneous detection of multiple DNA repair enzymes at the single-molecule level

Integration of single-molecule detection with rolling circle amplification-driven encoding of different fluorescent molecules enables simultaneous detection of multiple DNA repair enzymes.

DNA repair enzymes (e.g., DNA glycosylases) play a critical role in the repair of DNA lesions, and their aberrant levels are associated with various diseases. Herein, we develop a sensitive method for simultaneous detection of multiple DNA repair enzymes based on the integration of single-molecule detection with rolling circle amplification (RCA)-driven encoding of different fluorescent molecules. We use human alkyladenine DNA glycosylase (hAAG) and uracil DNA glycosylase (UDG) as the target analytes. We design a bifunctional double-stranded DNA (dsDNA) substrate with a hypoxanthine base (I) in one strand for hAAG recognition and an uracil (U) base in the other strand for UDG recognition, whose cleavage by APE1 generates two corresponding primers. The resultant two primers can hybridize with their respective circular templates to initiate RCA, resulting in the incorporation of multiple Cy3-dCTP and Cy5-dGTP nucleotides into the amplified products. After magnetic separation and exonuclease cleavage, the Cy3 and Cy5 fluorescent molecules in the amplified products are released into the solution and subsequently quantified by total internal reflection fluorescence (TIRF)-based single-molecule detection, with Cy3 indicating the presence of hAAG and Cy5 indicating the presence of UDG. This strategy greatly increases the number of fluorescent molecules per concatemer through the introduction of RCA-driven encoding of different fluorescent molecules, without the requirement of any specially labeled detection probes for simultaneous detection. Due to the high amplification efficiency of RCA and the high signal-to-ratio of single-molecule detection, this method can achieve a detection limit of 6.10 × 10^–9 U mL^–1 for hAAG and 1.54 × 10^–9 U mL^–1 for UDG. It can be further applied for simultaneous detection of multiple DNA glycosylases in cancer cells at the single-cell level and the screening of DNA glycosylase inhibitors, holding great potential in early clinical diagnosis and drug discovery.

Oxidative Damage in Sporadic Colorectal Cancer: Molecular Mapping of Base Excision Repair Glycosylases in Colorectal Cancer Patients

Oxidative stress with subsequent premutagenic oxidative DNA damage has been implicated in colorectal carcinogenesis. The repair of oxidative DNA damage is initiated by lesion-specific DNA glycosylases (hOGG1, NTH1, MUTYH). The direct evidence of the role of oxidative DNA damage and its repair is proven by hereditary syndromes (MUTYH-associated polyposis, NTHL1-associated tumor syndrome), where germline mutations cause loss-of-function in glycosylases of base excision repair, thus enabling the accumulation of oxidative DNA damage and leading to the adenoma-colorectal cancer transition. Unrepaired oxidative DNA damage often results in G:C>T:A mutations in tumor suppressor genes and proto-oncogenes and widespread occurrence of chromosomal copy-neutral loss of heterozygosity. However, the situation is more complicated in complex and heterogeneous disease, such as sporadic colorectal cancer. Here we summarized our current knowledge of the role of oxidative DNA damage and its repair on the onset, prognosis and treatment of sporadic colorectal cancer. Molecular and histological tumor heterogeneity was considered. Our study has also suggested an additional important source of oxidative DNA damage due to intestinal dysbiosis. The roles of base excision repair glycosylases (hOGG1, MUTYH) in tumor and adjacent mucosa tissues of colorectal cancer patients, particularly in the interplay with other factors (especially microenvironment), deserve further attention. Base excision repair characteristics determined in colorectal cancer tissues reflect, rather, a disease prognosis. Finally, we discuss the role of DNA repair in the treatment of colon cancer, since acquired or inherited defects in DNA repair pathways can be effectively used in therapy.

hOGG1 promoter methylation, hOGG1 genetic variants and their interactions for risk of coal-borne arsenicosis: A case-control study

To identify the effect of hOGG1 methylation, Ser326Cys polymorphism and their interactions on the risk of coal-borne arsenicosis, 113 coal-borne arsenicosis subjects and 55 reference subjects were recruited. Urinary arsenic contents were analyzed with ICP-MS. hOGG1 methylation and Ser326Cys polymorphism was measured by mehtylation-specific PCR and restriction fragment length polymorphism PCR in PBLCs, respectively. The results showed that the prevalence of methylated hOGG1 and variation genotype (326 ^Ser/Cys & 326 ^Cys/Cys) were increased with raised levels of urinary arsenic in arsenicosis subjects. Increased prevalence of methylated hOGG1 and variation genotype were associated with raised risk of arsenicosis. Moreover, the results revealed that variant genotype might increase the susceptibility to hOGG1 methylation. The interactions of methylated hOGG1 and variation genotype were also found to contribute to increased risk of arsenicosis. Taken together, hOGG1 hypermethylation, hOGG1 variants and their interactions might be potential biomarkers for evaluating risk of coal-borne arsenicosis.

DNA Repair Biomarker for Lung Cancer Risk and its Correlation With Airway Cells Gene Expression

BACKGROUND

Improving lung cancer risk assessment is required because current early-detection screening criteria miss most cases. We therefore examined the utility for lung cancer risk assessment of a DNA Repair score obtained from OGG1, MPG, and APE1 blood tests. In addition, we examined the relationship between the level of DNA repair and global gene expression.

METHODS

We conducted a blinded case-control study with 150 non-small cell lung cancer case patients and 143 control individuals. DNA Repair activity was measured in peripheral blood mononuclear cells, and the transcriptome of nasal and bronchial cells was determined by RNA sequencing. A combined DNA Repair score was formed using logistic regression, and its correlation with disease was assessed using cross-validation; correlation of expression to DNA Repair was analyzed using Gene Ontology enrichment.

RESULTS

DNA Repair score was lower in case patients than in control individuals, regardless of the case's disease stage. Individuals at the lowest tertile of DNA Repair score had an increased risk of lung cancer compared to individuals at the highest tertile, with an odds ratio (OR) of 7.2 (95% confidence interval [CI] = 3.0 to 17.5; P < .001), and independent of smoking. Receiver operating characteristic analysis yielded an area under the curve  of 0.89 (95% CI = 0.82 to 0.93). Remarkably, low DNA Repair score correlated with a broad upregulation of gene expression of immune pathways in patients but not in control individuals.

CONCLUSIONS

The DNA Repair score, previously shown to be a lung cancer risk factor in the Israeli population, was validated in this independent study as a mechanism-based cancer risk biomarker and can substantially improve current lung cancer risk prediction, assisting prevention and early detection by computed tomography scanning.

A panel of colorimetric assays to measure enzymatic activity in the base excision DNA repair pathway

DNA repair is essential for the maintenance of genomic integrity, and evidence suggest that inter-individual variation in DNA repair efficiency may contribute to disease risk. However, robust assays suitable for quantitative determination of DNA repair capacity in large cohort and clinical trials are needed to evaluate these apparent associations fully. We describe here a set of microplate-based oligonucleotide assays for high-throughput, non-radioactive and quantitative determination of repair enzyme activity at individual steps and over multiple steps of the DNA base excision repair pathway. The assays are highly sensitive: using HepG2 nuclear extract, enzyme activities were quantifiable at concentrations of 0.0002 to 0.181 μg per reaction, depending on the enzyme being measured. Assay coefficients of variation are comparable with other microplate-based assays. The assay format requires no specialist equipment and has the potential to be extended for analysis of a wide range of DNA repair enzyme activities. As such, these assays hold considerable promise for gaining new mechanistic insights into how DNA repair is related to individual genetics, disease status or progression and other environmental factors and investigating whether DNA repair activities can be used a biomarker of disease risk.

A single quantum dot-based nanosensor with multilayer of multiple acceptors for ultrasensitive detection of human alkyladenine DNA glycosylase

Base excision repair (BER) is an important DNA repair pathway involved in the maintenance of genome stability. As the initiator of BER, DNA glycosylase can remove a damaged base from DNA through cleaving the N-glycosidic bond between the sugar moiety and the damaged base. Accurate quantification of DNA glycosylase is essential for the early diagnosis of various human diseases. However, conventional methods for DNA glycosylase assay usually suffer from poor sensitivity and complex probe design. Herein, we develop a single quantum dot-based nanosensor with multilayer of multiple acceptors for ultrasensitive detection of human alkyladenine DNA glycosylase (hAAG) using apurinic/apyrimidinic endonuclease 1 (APE1)-assisted cyclic cleavage-mediated signal amplification in combination with the DNA polymerase-assisted multiple cyanine 5 (Cy5)-mediated fluorescence resonance energy transfer (FRET). The presence of hAAG induces the cleavage of the hairpin substrate, generating a trigger. The resultant trigger can hybridize with a probe modified with an AP site, initiating the APE1-mediated cyclic cleavage to produce a large number of primers. The primers can subsequently initiate the polymerase-mediated signal amplification with a biotin-modified capture probe as the template, generating the biotin-/multiple Cy5-labeled double-stranded DNAs (dsDNAs). The resultant dsDNAs can assemble onto the QD surface to form the QD-dsDNA-Cy5 nanostructure, leading to efficient FRET from the QD to Cy5 under the excitation of 405 nm. In contrast to the typical QD-based FRET approaches, the assembly of multilayer of multiple Cy5 molecules onto a single QD significantly amplifies the FRET signal. We further verify the FRET model with one donor and multilayered acceptors theoretically and experimentally. This single QD-based nanosensor can sensitively detect hAAG with a detection limit of as low as 4.42 × 10^-12 U μL^-1. Moreover, it can detect hAAG even in a single cancer cell, and distinguish the cancer cells from the normal cells. Importantly, this single QD-based nanosensor can be used for the kinetic study and inhibition assay, and it may become a universal platform for the detection of other DNA repair enzymes by designing appropriate DNA substrates.

Inflammation-induced DNA damage, mutations and cancer

The relationships between inflammation and cancer are varied and complex. An important connection linking inflammation to cancer development is DNA damage. During inflammation reactive oxygen and nitrogen species (RONS) are created to combat pathogens and to stimulate tissue repair and regeneration, but these chemicals can also damage DNA, which in turn can promote mutations that initiate and promote cancer. DNA repair pathways are essential for preventing DNA damage from causing mutations and cytotoxicity, but RONS can interfere with repair mechanisms, reducing their efficacy. Further, cellular responses to DNA damage, such as damage signaling and cytotoxicity, can promote inflammation, creating a positive feedback loop. Despite coordination of DNA repair and oxidative stress responses, there are nevertheless examples whereby inflammation has been shown to promote mutagenesis, tissue damage, and ultimately carcinogenesis. Here, we discuss the DNA damage-mediated associations between inflammation, mutagenesis and cancer.

Controllable Autocatalytic Cleavage-Mediated Fluorescence Recovery for Homogeneous Sensing of Alkyladenine DNA Glycosylase from Human Cancer Cells

DNA alkylation and oxidation are two most common forms of cytotoxic damage with the characteristics of mutagenic and carcinogenic. Human alkyladenine DNA glycosylase (hAAG) is the only glycosylase known to repair a wide variety of alkylative and oxidative DNA lesions. However, few approaches are capable of real-time monitoring hAAG activity.

Methods: Herein, we develop a facile fluorescent strategy for homogeneous and sensitive sensing of hAAG activity based on the controllable autocatalytic cleavage-mediated fluorescence recovery. The presence of hAAG enables the cleavage of hairpin probe 1 (HP1) at the damaged 2′-deoxyinosine site by AP endonuclease 1 (APE1), forming a DNA duplex. The trigger 1 built in the resultant DNA duplex may hybridize with hairpin probe 2 (HP2) to induce the T7 exonuclease (T7 exo)-catalyzed recycling cleavage of HP2 (Cycle I) to release trigger 2. The trigger 2 can further hybridize with the signal probe (a fluorophore (FAM) and a quencher (BHQ1) modified at its 5′ and 3′ ends) to induce the subsequent recycling cleavage of signal probes (Cycle II) to liberate FAM molecules. Through two-recycling autocatalytic cleavage processes, large amounts of fluorophore molecules (i.e., FAM) are liberated from the FAM-BHQ1 fluorescence resonance energy transfer (FRET) pair, leading to the amplified fluorescence recovery.

Results: Taking advantage of the high accuracy of in vivo DNA repair mechanism, the high specificity of T7 exo-catalyzed mononucleotides hydrolysis, and the high efficiency of autocatalytic recycling amplification, this strategy exhibits high sensitivity with a detection limit of 4.9 × 10^-6 U/μL and a large dynamic range of 4 orders of magnitude from 1 × 10^-5 to 0.1 U/μL, and it can further accurately evaluate the enzyme kinetic parameters, screen the potential inhibitors, and even quantify the hAAG activity from 1 cancer cell.

Conclusion: The proposed strategy can provide a facile and universal platform for the monitoring of DNA damage-related repair enzymes, holding great potential for DNA repair-related biochemical research, clinical diagnosis, drug discovery, and cancer therapy.

Base excision repair mediated cascading triple-signal amplification for the sensitive detection of human alkyladenine DNA glycosylase

DNA glycosylase (DG) plays a significant role in repairing DNA lesions, and the dysregulation of DG activity is associated with a variety of human pathologies. Thus, the detection of DG activity is essential for biomedical research and clinical diagnosis. Herein, we develop a facile fluorometric method based on the base excision repair (BER) mediated cascading triple-signal amplification for the sensitive detection of DG. The presence of human alkyladenine DNA glycosylase (hAAG) can initiate the cleavage of the substrate at the mismatched deoxyinosine site by endonuclease IV (Endo IV), resulting in the breaking of the DNA substrate. The cleaved DNA substrate functions as both a primer and a template to initiate strand displacement amplification (SDA) to release primers. The released primers can further bind to a circular template to induce an exponential primer generation rolling circle amplification (PG-RCA) reaction, producing a large number of primers. The primers that resulted from the SDA and PG-RCA reaction can induce the subsequent recycling cleavage of signal probes, leading to the generation of a fluorescence signal. Taking advantage of the high amplification efficiency of triple-signal amplification and the low background signal resulting from single uracil repair-mediated inhibition of nonspecific amplification, this method exhibits a low detection limit of 0.026 U mL^-1 and a large dynamic range of 4 orders of magnitude for hAAG. Moreover, this method has distinct advantages of simplicity and low cost, and it can further quantify the hAAG activity from HeLa cell extracts, holding great potential in clinical diagnosis and biomedical research.

DNA repair as a human biomonitoring tool: Comet assay approaches

The comet assay offers the opportunity to measure both DNA damage and repair. Various comet assay based methods are available to measure DNA repair activity, but some requirements should be met for their effective use in human biomonitoring studies. These conditions include i) robustness of the assay, ii) sources of inter- and intra-individual variability must be known, iii) DNA repair kinetics should be assessed to optimize sampling timing; and iv) DNA repair in accessible surrogate tissues should reflect repair activity in target tissues prone to carcinogenic effects. DNA repair phenotyping can be performed on frozen and fresh samples, and is a more direct measurement than genomic or transcriptomic approaches. There are mixed reports concerning the regulation of DNA repair by environmental and dietary factors. In general, exposure to genotoxic agents did not change base excision repair (BER) activity, whereas some studies reported that dietary interventions affected BER activity. On the other hand, in vitro and in vivo studies indicated that nucleotide excision repair (NER) can be altered by exposure to genotoxic agents, but studies on other life style related factors, such as diet, are rare. Thus, crucial questions concerning the factors regulating DNA repair and inter-individual variation remain unanswered. Intra-individual variation over a period of days to weeks seems limited, which is favourable for DNA repair phenotyping in biomonitoring studies. Despite this reported low intra-individual variation, timing of sampling remains an issue that needs further investigation. A correlation was reported between the repair activity in easily accessible peripheral blood mononuclear cells (PBMCs) and internal organs for both NER and BER. However, no correlation was found between tumour tissue and blood cells. In conclusion, although comet assay based approaches to measure BER/NER phenotypes are feasible and promising, more work is needed to further optimize their application in human biomonitoring and intervention studies.

DNA repair as an emerging target for COPD-lung cancer overlap

Cigarette smoking is the leading cause of lung cancer and chronic obstructive pulmonary disease (COPD). Many of the detrimental effects of cigarette smoke have been attributed to the development of DNA damage, either directly from chemicals contained in cigarette smoke or as a product of cigarette smoke-induced inflammation and oxidative stress. In this review, we discuss the environmental, epidemiological, and physiological links between COPD and lung cancer and the likely role of DNA damage and repair in COPD and lung cancer development. We explore alterations in DNA damage repair by DNA repair proteins and pathways. We discuss emerging data supporting a key role for the DNA repair protein, xeroderma pigmentosum group C (XPC), in cigarette smoke-induced COPD and early lung cancer development. Understanding the interplay between cigarette smoke, DNA damage repair, COPD, and lung cancer may lead to prognostic tools and new, potentially targetable, pathways for lung cancer prevention and treatment.

Target-mediated hyperbranched amplification for sensitive detection of human alkyladenine DNA glycosylase from HeLa cells

Human alkyladenine DNA glycosylase (hAAG) is an important protein enzyme which can specifically recognize and initiate the repair of a variety of alkylated purines and hypoxanthine, and the dysregulation of hAAG activity is associated with various human diseases. Although there are several methods focusing on hAAG detection, they share common defects such as time-consuming protocols, laborious operation or requirement of expensive analytical instruments. Herein, taking advantage of the high amplification efficiency of hyperbranched signal amplification and the low background signals by modifying NH₂ at 3' terminus of hairpin substrate and signal probe to prevent the terminal deoxynucleotidyl transferase (TdT)-activated nonspecific amplification, a fluoresence method for sensitive detection of hAAG was established using TdT-activated Endonuclease IV (Endo IV)-assisted hyperbranched signal amplification. This method exhibits high sensitivity with a limit of detection of 0.090 U/mL for pure hAAG and shows a large dynamic range of 3 orders of magnitude from 0.1 to 50 U/mL, and it can be applied for accurate detection of hAAG in complicated HeLa nuclear extract. Moreover, the method can be used for discrimination of hAAG from other DNA glycosylases, holding great potential in hAAG-related biomedical research and clinical diagnosis.

Roles of DNA repair enzyme OGG1 in innate immunity and its significance for lung cancer

Cytokines are pivotal mediators of the immune response, and their coordinated expression protects host tissue from excessive damage and oxidant stress. Nevertheless, the development of lung pathology, including asthma, chronic obstructive pulmonary disease, and ozone-induced lung injury, is associated with oxidant stress; as evidence, there is a significant increase in levels of the modified guanine base 7,8-dihydro-8-oxoguanine (8-oxoG) in the genome. 8-OxoG is primarily recognized by 8-oxoguanine glycosylase 1 (OGG1), which catalyzes the first step in the DNA base excision repair pathway. However, oxidant stress in the cell transiently halts enzymatic activity of substrate-bound OGG1. The stalled OGG1 facilitates DNA binding of transactivators, including NF-κB, to their cognate sites to enable expression of cytokines and chemokines, with ensuing recruitments of inflammatory cells. Hence, defective OGG1 will modulate the coordination between innate and adaptive immunity through excessive oxidant stress and cytokine dysregulation. Both oxidant stress and cytokine dysregulation constitute key elements of oncogenesis by KRAS, which is mechanistically coupled to OGG1. Thus, analysis of the mechanism by which OGG1 modulates gene expression helps discern between beneficial and detrimental effects of oxidant stress, exposes a missing functional link as a marker, and yields a novel target for lung cancer.

Simultaneous sensitive detection of multiple DNA glycosylases from lung cancer cells at the single-molecule level

We demonstrate the simultaneous detection of human 8-oxoguanine DNA glycosylase 1 and human alkyladenine DNA glycosylase at the single-molecule level.

DNA glycosylases are involved in the base excision repair pathway, and all mammals express multiple DNA glycosylases to maintain genome stability. However, the simultaneous detection of multiple DNA glycosylase still remains a great challenge. Here, we develop a single-molecule detection method for the simultaneous detection of human 8-oxoguanine DNA glycosylase 1 (hOGG1) and human alkyladenine DNA glycosylase (hAAG) on the basis of DNA glycosylase-mediated cleavage of molecular beacons. We designed a Cy3-labeled molecular beacon modified with 8-oxoguanine (8-oxoG) for a hOGG1 assay and a Cy5-labeled molecular beacon modified with deoxyinosine for a hAAG assay. hOGG1 may catalyze the removal of 8-oxoG from 8-oxoG/C base pairs to generate an apurinic/apyrimidinic (AP) site, and hAAG may catalyze the removal of deoxyinosine from deoxyinosine/T base pairs to generate an AP site. With the assistance of apurinic/apyrimidinic endonuclease (APE1), the cleavage of AP sites results in the cleavage of molecular beacons, with Cy3 indicating the presence of hOGG1 and Cy5 indicating the presence of hAAG. Both of the Cy3 and Cy5 signals can be simply quantified by total internal reflection fluorescence-based single-molecule detection. This method can simultaneously detect multiple DNA glycosylases with a detection limit of 2.23 × 10^–6 U μL^–1 for hOGG1 and 8.69 × 10^–7 U μL^–1 for hAAG without the involvement of any target amplification. Moreover, this method can be used for the screening of enzyme inhibitors and the simultaneous detection of hOGG1 and hAAG from lung cancer cells, having great potential for further application in early clinical diagnosis.

Methylated +322-327 CpG site decreases hOGG1 mRNA expression in non-small cell lung cancer

hOGG1 plays a role in several disease pathways, including various cancers. Despite such functional importance, how hOGG1 is regulated at the transcriptional level in human non-small cell lung cancer (NSCLC) remains unknown, particularly via DNA methylation changes. We obtained NSCLC tissues and adjacent non-cancerous tissues and examined hOGG1 mRNA expression levels. NSCLC cells were treated with 5-Aza to test whether DNA methylation can influence the expression of hOGG1. The MassARRAY EpiTYPER and luciferase reporter gene assays were used to define the functional region of the hOGG1 gene (including CpG sites). Finally, ChIP assay was utilized to verify transcription factor binding to the hOGG1 5'-UTR region. Our previous studies supported the idea that the methylation of the hOGG1 gene promoter region occurs frequently in NSCLC. Treatment with 5-Aza, a demethylating agent, led to a significant restoration of hOGG1 expression in NSCLC cell lines. Quantitative PCR and MassARRAY EpiTYPER assays demonstrated that methylation of the +322-327 CpG site in the 5'-UTR region of hOGG1 was higher in NSCLC tissues compared with adjacent non-cancerous tissues. Notably, the methylation level of +322-327 site (T/N) was inversely correlated with that of hOGG1 mRNA level (T/N) in 25 NSCLC tissues. ChIP assay and in silico prediction showed an association between the +322-327 CpG site and Sp1, which has been reported to be an activator of transcription. Importantly, luciferase reporter gene and ChIP assays showed that +322-327 CpG site methylation particularly reduced the recruitment of Sp1 to the 5'-UTR sequence in hOGG1 and reduced transcriptional activity ~50%. In summary, we have demonstrated that hOGG1 mRNA is downregulated in NSCLC tissues. Moreover, we identified that the methylated +322-327 CpG site in the hOGG1 5'-UTR is associated with reduced expression of hOGG1 by decreasing the recruitment of Sp1 to the 5'-UTR of hOGG1.

Aberrant base excision repair pathway of oxidatively damaged DNA: Implications for degenerative diseases

In cellular organisms composition of DNA is constrained to only four nucleobases A, G, T and C, except for minor DNA base modifications such as methylation which serves for defence against foreign DNA or gene expression regulation. Interestingly, this severe evolutionary constraint among other things demands DNA repair systems to discriminate between regular and modified bases. DNA glycosylases specifically recognize and excise damaged bases among vast majority of regular bases in the base excision repair (BER) pathway. However, the mismatched base pairs in DNA can occur from a spontaneous conversion of 5-methylcytosine to thymine and DNA polymerase errors during replication. To counteract these mutagenic threats to genome stability, cells evolved special DNA repair systems that target the non-damaged DNA strand in a duplex to remove mismatched regular DNA bases. Mismatch-specific adenine- and thymine-DNA glycosylases (MutY/MUTYH and TDG/MBD4, respectively) initiated BER and mismatch repair (MMR) pathways can recognize and remove normal DNA bases in mismatched DNA duplexes. Importantly, in DNA repair deficient cells bacterial MutY, human TDG and mammalian MMR can act in the aberrant manner: MutY and TDG removes adenine and thymine opposite misincorporated 8-oxoguanine and damaged adenine, respectively, whereas MMR removes thymine opposite to O⁶-methylguanine. These unusual activities lead either to mutations or futile DNA repair, thus indicating that the DNA repair pathways which target non-damaged DNA strand can act in aberrant manner and introduce genome instability in the presence of unrepaired DNA lesions. Evidences accumulated showing that in addition to the accumulation of oxidatively damaged DNA in cells, the aberrant DNA repair can also contribute to cancer, brain disorders and premature senescence. For example, the aberrant BER and MMR pathways for oxidized guanine residues can lead to trinucleotide expansion that underlies Huntington's disease, a severe hereditary neurodegenerative syndrome. This review summarises the present knowledge about the aberrant DNA repair pathways for oxidized base modifications and their possible role in age-related diseases.

DNA Base-Excision Repair Genes OGG1 and NTH1 in Brazilian Lung Cancer Patients

INTRODUCTION

Lung cancer is the leading global cause of cancer-related mortality and is associated with poor prognosis. To improve survival rates of lung cancer patients, better understanding of tumorigenic mechanisms is necessary, which may lead to development of new therapeutic strategies. The hOGG1 and NTH1 genes act in the DNA BER repair pathway and their involvement in lung cancer pathogenesis has been analyzed in several populations.

METHODS

We analyzed targeted regions of the hOGG1 and NTH1 genes in 96 Brazilian patients with non-small-cell lung cancer (NSCLC) and 89 cancer-free, ethnically matched controls.

RESULTS

The NTH1 c.98G>T polymorphism rs2302172 (p = 0.02 and p = 0.02 for allele and genotype frequency between cases and controls, respectively) and the 140-17C> T variant (rs2233518) (p = 0.02 and p = 0.02 for allele and genotype frequency between cases and controls, respectively) were detected in four lung cancer cases (4 %) while the NTH1 Q131K (C391A) polymorphism was found in seven lung cancer cases (7 %) (p = 0.001 and p = 0.008, for allele and genotype frequency between cases and controls, respectively). None of these sequence variants were detected in controls. The Ser326Cys (C1245G, rs1052133) polymorphism in the OGG1 gene was detected in 42 % of analyzed NSCLC patients and in 34 % of the controls (p = 0.11 and p = 0.25 for allele and genotype frequency between cases and controls, respectively).

CONCLUSIONS

Our study provides preliminary evidence that polymorphisms in OGG1 do not contribute to development of NSCLC in Brazilian patients and that NTH1 polymorphisms may be associated with NSCLC pathogenesis.

Development of APE1 enzymatic DNA repair assays: low APE1 activity is associated with increase lung cancer risk

We developed radioactivity-based and fluorescence-based assays for the DNA repair enzyme APE1 and showed that its decreased activity is associated with increased lung cancer risk. This suggests that ‘bad DNA repair’ rather than ‘bad luck’ is involved in cancer etiology.

The key role of DNA repair in removing DNA damage and minimizing mutations makes it an attractive target for cancer risk assessment and prevention. Here we describe the development of a robust assay for apurinic/apyrimidinic (AP) endonuclease 1 (APE1; APEX1), an essential enzyme involved in the repair of oxidative DNA damage. APE1 DNA repair enzymatic activity was measured in peripheral blood mononuclear cell protein extracts using a radioactivity-based assay, and its association with lung cancer was determined using conditional logistic regression with specimens from a population-based case–control study with 96 lung cancer cases and 96 matched control subjects. The mean APE1 enzyme activity in case patients was 691 [95% confidence interval (CI) = 655–727] units/ng protein, significantly lower than in control subjects (mean = 793, 95% CI = 751–834 units/ng protein, P = 0.0006). The adjusted odds ratio for lung cancer associated with 1 SD (211 units) decrease in APE1 activity was 2.0 (95% CI = 1.3–3.1; P = 0.002). Comparison of radioactivity- and fluorescence-based assays showed that the two are equivalent, indicating no interference by the fluorescent tag. The APE1Asp148Glu SNP was associated neither with APE1 enzyme activity nor with lung cancer risk. Taken together, our results indicate that low APE1 activity is associated with lung cancer risk, consistent with the hypothesis that ‘bad DNA repair’, rather than ‘bad luck’, is involved in cancer etiology. Such assays may be useful, along with additional DNA repair biomarkers, for risk assessment of lung cancer and perhaps other cancers, and for selecting individuals to undergo early detection techniques such as low-dose CT.

Germ line variants of human N-methylpurine DNA glycosylase show impaired DNA repair activity and facilitate 1,N6-ethenoadenine-induced mutations

Human N-methylpurine DNA glycosylase (hMPG) initiates base excision repair of a number of structurally diverse purine bases including 1,N(6)-ethenoadenine, hypoxanthine, and alkylation adducts in DNA. Genetic studies discovered at least eight validated non-synonymous single nucleotide polymorphisms (nsSNPs) of the hMPG gene in human populations that result in specific single amino acid substitutions. In this study, we tested the functional consequences of these nsSNPs of hMPG. Our results showed that two specific arginine residues, Arg-141 and Arg-120, are important for the activity of hMPG as the germ line variants R120C and R141Q had reduced enzymatic activity in vitro as well as in mammalian cells. Expression of these two variants in mammalian cells lacking endogenous MPG also showed an increase in mutations and sensitivity to an alkylating agent compared with the WT hMPG. Real time binding experiments by surface plasmon resonance spectroscopy suggested that these variants have substantial reduction in the equilibrium dissociation constant of binding (KD) of hMPG toward 1,N(6)-ethenoadenine-containing oligonucleotide (ϵA-DNA). Pre-steady-state kinetic studies showed that the substitutions at arginine residues affected the turnover of the enzyme significantly under multiple turnover condition. Surface plasmon resonance spectroscopy further showed that both variants had significantly decreased nonspecific (undamaged) DNA binding. Molecular modeling suggested that R141Q substitution may have resulted in a direct loss of the salt bridge between ϵA-DNA and hMPG, whereas R120C substitution redistributed, at a distance, the interactions among residues in the catalytic pocket. Together our results suggest that individuals carrying R120C and R141Q MPG variants may be at risk for genomic instability and associated diseases as a consequence.

Enzymatic MPG DNA repair assays for two different oxidative DNA lesions reveal associations with increased lung cancer risk

DNA repair is a major mechanism for minimizing mutations and reducing cancer risk. Here, we present the development of reproducible and specific enzymatic assays for methylpurine DNA glycosylase (MPG) repairing the oxidative lesions 1,N6-ethenoadenine (εA) and hypoxanthine (Hx) in peripheral blood mononuclear cells protein extracts. Association of these DNA repair activities with lung cancer was determined using conditional logistic regression with specimens from a population-based case-control study with 96 lung cancer cases and 96 matched control subjects. The mean MPG-εA in case patients was 15.8 units/μg protein (95% CI 15.3-16.3), significantly higher than in control subjects-15.1 (14.6-15.5), *P = 0.011. The adjusted odds ratio for lung cancer associated with a one SD increase in MPG-εA activity (2.48 units) was significantly bigger than 1 (OR = 1.6, 95% CI = 1.1-2.4; *P = 0.013). When activity of OGG1, a different DNA repair enzyme for oxidative damage, was included in the model, the estimated odds ratio/SD for a combined MPG-εA-OGG1 score was 2.6 (95% CI 1.6-4.2) *P = 0.0001, higher than the odds ratio for each single assay. The MPG enzyme activity assays described provide robust functional risk biomarkers, with increased MPG-εA activity being associated with increased lung cancer risk, similar to the behavior of MPG-Hx. This underscores the notion that imbalances in DNA repair, including high DNA repair, usually perceived as beneficial, can cause cancer risk. Such DNA repair risk biomarkers may be useful for risk assessment of lung cancer and perhaps other cancer types, and for early detection techniques such as low-dose CT.

DNA glycosylase activity and cell proliferation are key factors in modulating homologous recombination in vivo

Cancer susceptibility varies between people, affected by genotoxic exposures, genetic makeup and physiological state. Yet, how these factors interact among each other to define cancer risk is largely unknown. Here, we uncover the interactive effects of genetical, environmental and physiological factors on genome rearrangements driven by homologous recombination (HR). Using FYDR mice to quantify HR-driven rearrangements in pancreas tissue, we show that DNA methylation damage (induced by methylnitrosourea) and cell proliferation (induced by thyroid hormone) each induce HR and together act synergistically to induce HR-driven rearrangements in vivo. These results imply that developmental or regenerative proliferation as well as mitogenic exposures may sensitize tissues to DNA damaging exposures. We exploited mice genetically deficient in alkyl-adenine DNA glycosylase (Aag) to analyse the relative contributions of unrepaired DNA base lesions versus intermediates formed during base excision repair (BER). Remarkably, results show that, in the pancreas, Aag is a major driver of spontaneous HR, indicating that BER intermediates (including abasic sites and single strand breaks) are more recombinogenic than the spontaneous base lesions removed by Aag. Given that mammals have about a dozen DNA glycosylases, these results point to BER as a major source of pressure on the HR pathway in vivo. Taken together, methylation damage, cell proliferation and Aag interact to define the risk of HR-driven sequence rearrangements in vivo. These data identify important sources of sequence changes in a cancer-relevant organ, and advance the effort to identify populations at high-risk for cancer.

Lighting up individual DNA damage sites by in vitro repair synthesis

DNA damage and repair are linked to fundamental biological processes such as metabolism, disease, and aging. Single-strand lesions are the most abundant form of DNA damage; however, methods for characterizing these damage lesions are lacking. To avoid double-strand breaks and genomic instability, DNA damage is constantly repaired by efficient enzymatic machinery. We take advantage of this natural process and harness the repair capacity of a bacterial enzymatic cocktail to repair damaged DNA in vitro and incorporate fluorescent nucleotides into damage sites as part of the repair process. We use single-molecule imaging to detect individual damage sites in genomic DNA samples. When the labeled DNA is extended on a microscope slide, damage sites are visualized as fluorescent spots along the DNA contour, and the extent of damage is easily quantified. We demonstrate the ability to quantitatively follow the damage dose response to different damaging agents as well as repair dynamics in response to UV irradiation in several cell types. Finally, we show the modularity of this single-molecule approach by labeling DNA damage in conjunction with 5-hydroxymethylcytosine in genomic DNA extracted from mouse brain tissue.

Inter-individual variation in DNA repair capacity: a need for multi-pathway functional assays to promote translational DNA repair research

Why does a constant barrage of DNA damage lead to disease in some individuals, while others remain healthy? This article surveys current work addressing the implications of inter-individual variation in DNA repair capacity for human health, and discusses the status of DNA repair assays as potential clinical tools for personalized prevention or treatment of disease. In particular, we highlight research showing that there are significant inter-individual variations in DNA repair capacity (DRC), and that measuring these differences provides important biological insight regarding disease susceptibility and cancer treatment efficacy. We emphasize work showing that it is important to measure repair capacity in multiple pathways, and that functional assays are required to fill a gap left by genome wide association studies, global gene expression and proteomics. Finally, we discuss research that will be needed to overcome barriers that currently limit the use of DNA repair assays in the clinic.

Multiplexed DNA repair assays for multiple lesions and multiple doses via transcription inhibition and transcriptional mutagenesis

The capacity to repair different types of DNA damage varies among individuals, making them more or less susceptible to the detrimental health consequences of damage exposures. Current methods for measuring DNA repair capacity (DRC) are relatively labor intensive, often indirect, and usually limited to a single repair pathway. Here, we describe a fluorescence-based multiplex flow-cytometric host cell reactivation assay (FM-HCR) that measures the ability of human cells to repair plasmid reporters, each bearing a different type of DNA damage or different doses of the same type of DNA damage. FM-HCR simultaneously measures repair capacity in any four of the following pathways: nucleotide excision repair, mismatch repair, base excision repair, nonhomologous end joining, homologous recombination, and methylguanine methyltransferase. We show that FM-HCR can measure interindividual DRC differences in a panel of 24 cell lines derived from genetically diverse, apparently healthy individuals, and we show that FM-HCR may be used to identify inhibitors or enhancers of DRC. We further develop a next-generation sequencing-based HCR assay (HCR-Seq) that detects rare transcriptional mutagenesis events due to lesion bypass by RNA polymerase, providing an added dimension to DRC measurements. FM-HCR and HCR-Seq provide powerful tools for exploring relationships among global DRC, disease susceptibility, and optimal treatment.

Low integrated DNA repair score and lung cancer risk

DNA repair is a prime mechanism for preventing DNA damage, mutation, and cancers. Adopting a functional approach, we examined the association with lung cancer risk of an integrated DNA repair score, measured by a panel of three enzymatic DNA repair activities in peripheral blood mononuclear cells. The panel included assays for AP endonuclease 1 (APE1), 8-oxoguanine DNA glycosylase (OGG1), and methylpurine DNA glycosylase (MPG), all of which repair oxidative DNA damage as part of the base excision repair pathways. A blinded population-based case-control study was conducted with 96 patients with lung cancer and 96 control subjects matched by gender, age (±1 year), place of residence, and ethnic group (Jews/non-Jews). The three DNA repair activities were measured, and an integrated DNA repair OMA (OGG1, MPG, and APE1) score was calculated for each individual. Conditional logistic regression analysis revealed that individuals in the lowest tertile of the integrated DNA repair OMA score had an increased risk of lung cancer compared with the highest tertile, with OR = 9.7; 95% confidence interval (CI), 3.1-29.8; P < 0.001, or OR = 5.6; 95% CI, 2.1-15.1; P < 0.001 after cross-validation. These results suggest that pending validation, this DNA repair panel of risk factors may be useful for lung cancer risk assessment, assisting prevention and referral to early detection by technologies such as low-dose computed tomography scanning.

Topographical study of O(6)-alkylguanine DNA alkyltransferase repair activity and N7-methylguanine levels in resected lung tissue

BACKGROUND

Tobacco specific nitrosamines such as 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) are genotoxic alkylating agents found within cigarette smoke that induce lung adenocarcinomas in animal models. In humans, adenocarcinomas originate most frequently in the lung periphery. The aim of this study was to determine whether peripheral lung has increased susceptibility to the genotoxic effects of alkylating agents by comparing DNA alkylation damage (N7-methylguanine: N7-meG) and repair (O(6)-alkylguanine DNA alkyltransferase: MGMT) in peripheral relative to central lung tissue.

METHODS

Macroscopically normal lung tissue, resected from patients undergoing surgery for lung cancer, was sampled at equidistant points from central to peripheral lung along a bronchus. N7-meG levels were determined using an immunoslotblot technique and MGMT activity with a [32P]-labelled oligodeoxynucleotide cleavage assay.

RESULTS

A total of 20 subjects were recruited, 12 males and 8 females with a mean age of 68.7±5.8years. There were 14 former and 6 current smokers with a mean smoking exposure of 34.0±18.3packyears. N7-meG (mean 0.75±0.57/10(6)dG, n=65 samples from 14 patients) and MGMT repair (geometric mean 9.57±1.62fmol/μg DNA, n=79 samples from 16 patients) were detected in all samples assayed. MGMT activity increased towards the lung periphery (r=0.28, p=0.023; n=16) with a highly significant association in current (r=0.53, p=0.008; n=6) but not former smokers (r=0.13; p=0.41; n=10). No correlation was seen with N7-meG levels and lung position (r=-0.18; p=0.21; n=14). N7-meG levels were higher in current compared to former smokers reaching significance in two lung positions including peripheral lung (p=0.047).

CONCLUSIONS

The findings in this study do not support the hypothesis that peripheral tissue is more susceptible to the genotoxic effects of alkylating agents than central lung tissue. In addition exposure to cigarette smoke reduced the level of MGMT in central bronchial tissue possibly through increased alkylating agent exposure.

Identification of UHRF1/2 as new N-methylpurine DNA glycosylase-interacting proteins

N-methylpurine DNA glycosylase (MPG), a DNA repair enzyme, functions in the DNA base excision repair (BER) pathway. Aberrant over-expression of MPG in various cancers suggests an important role of MPG in carcinogenesis. Identification of MPG-interacting proteins will help to dissect the molecular link between MPG and cancer development. In the present study, using immunoprecipitation coupled with mass spectrometry (IP/MS), we screened ubiquitin-like, containing PHD and RING finger domains 1 (UHRF1), an essential protein required for the maintenance of DNA methylation, as a MPG-interacting protein. Endogenous co-immunoprecipitation assay in cancer cells confirmed that UHRF1 interacted with MPG in a p53 status-independent manner. Confocal microscopy showed that endogenous MPG and UHRF1 were co-localized in the nucleoplasm. Furthermore, co-immunoprecipitation assay indicated that UHRF2, the homolog of UHRF1, could also interact with MPG. These results show that MPG and the UHRF family of proteins interact, thus providing a functional linkage between MPG and UHRF1/2.

Aag DNA glycosylase promotes alkylation-induced tissue damage mediated by Parp1

Alkylating agents comprise a major class of front-line cancer chemotherapeutic compounds, and while these agents effectively kill tumor cells, they also damage healthy tissues. Although base excision repair (BER) is essential in repairing DNA alkylation damage, under certain conditions, initiation of BER can be detrimental. Here we illustrate that the alkyladenine DNA glycosylase (AAG) mediates alkylation-induced tissue damage and whole-animal lethality following exposure to alkylating agents. Aag-dependent tissue damage, as observed in cerebellar granule cells, splenocytes, thymocytes, bone marrow cells, pancreatic β-cells, and retinal photoreceptor cells, was detected in wild-type mice, exacerbated in Aag transgenic mice, and completely suppressed in Aag^−/− mice. Additional genetic experiments dissected the effects of modulating both BER and Parp1 on alkylation sensitivity in mice and determined that Aag acts upstream of Parp1 in alkylation-induced tissue damage; in fact, cytotoxicity in WT and Aag transgenic mice was abrogated in the absence of Parp1. These results provide in vivo evidence that Aag-initiated BER may play a critical role in determining the side-effects of alkylating agent chemotherapies and that Parp1 plays a crucial role in Aag-mediated tissue damage.

Alkylating agents are genotoxic chemicals that induce both toxic and mutagenic DNA damage through addition of an alkyl group to DNA. Alkylating agents are routinely and successfully used as chemotherapeutic therapies for cancer patients, with one major disadvantage being the significant toxicity induced in non-tumor tissues. Accordingly, identifying factors that modify susceptibility to alkylation-induced toxicity will provide valuable information in designing cancer therapeutic regimens. This study used mouse genetic experiments to investigate whether proteins important in the base excision repair pathway modulate susceptibility to alkylating agents. In addition to whole-animal toxicity at high doses, treatment of mice with alkylating agents resulted in severe damage to numerous tissues including the cerebellum, retina, bone marrow, spleen, thymus, and the pancreas. We illustrate that the DNA glycosylase Aag can actually confer, rather than prevent, alkylation sensitivity at both the whole-animal and tissue level; i.e., Aag transgenic animals are more susceptible than wild type, whereas Aag-deficient animals are less susceptible than wild type to alkylation-induced toxicity. Further genetic experiments show that the Aag-mediated alkylation sensitivity is dependent on Parp1. Given that we observe a wide range of human AAG expression among healthy individuals, this and other base excision repair proteins may be important factors modulating alkylation susceptibility.