Mammalian Base Excision Repair: Functional Partnership between PARP-1 and APE1 in AP-Site Repair

AP endonuclease 1: Biological updates and advances in activity analysis

Apurinic/apyrimidinic endodeoxyribonuclease 1 (APE1, APEX1, REF1, HAP1) is an abasic site-specific endonuclease holding critical roles in numerous biological functions including base excision repair, the DNA damage response, redox regulation of transcription factors, RNA processing, and gene regulation. Pathologically, APE1 expression and function is linked with numerous human diseases including cancer, highlighting the importance of sensitive and quantitative assays to measure APE1 activity. Here, we summarize biochemical and biological roles for APE1 and expand on the discovery of APE1 inhibitors. Finally, we highlight the development of assays to monitor APE1 activity, detailing a recently improved and stabilized DNA Repair Molecular Beacon assay to analyze APE1 activity. The assay is amenable to analysis of purified protein, to measure changes in APE1 activity in cell lysates, to monitor human patient samples for defects in APE1 function, or the cellular and biochemical response to APE1 inhibitors.

Role of condensates in modulating DNA repair pathways and its implication for chemoresistance

For cells, it is important to repair DNA damage, such as double strand and single strand DNA breaks, because unrepaired DNA can compromise genetic integrity, potentially leading to cell death or cancer. Cells have multiple DNA damage repair pathways that have been the subject of detailed genetic, biochemical, and structural studies. Recently, the scientific community has started to gain evidence that the repair of DNA double strand breaks may occur within biomolecular condensates and that condensates may also contribute to DNA damage through concentrating genotoxic agents used to treat various cancers. Here, we summarize key features of biomolecular condensates and note where they have been implicated in the repair of DNA double strand breaks. We also describe evidence suggesting that condensates may be involved in the repair of other types of DNA damage, including single strand DNA breaks, nucleotide modifications (e.g., mismatch and oxidized bases) and bulky lesions, among others. Finally, we discuss old and new mysteries that could now be addressed considering the properties of condensates, including chemoresistance mechanisms.

In Vitro Reconstitutive Base Excision Repair (BER) Assay

The mammalian cell genome is continuously exposed to endogenous and exogenous insults that modify its DNA. These modifications can be single-base lesions, bulky DNA adducts, base dimers, base alkylation, cytosine deamination, nitrosation, or other types of base alteration which interfere with DNA replication. Mammalian cells have evolved with a robust defense mechanism to repair these base modifications (damages) to preserve genomic stability. Base excision repair (BER) is the major defense mechanism for cells to remove these oxidative or alkylated single-base modifications. The base excision repair process involves replacement of a single-nucleotide residue by two sub-pathways, the single-nucleotide (SN) and the multi-nucleotide or long-patch (LP) base excision repair pathways. These reactions have been reproduced in vitro using cell free extracts or purified recombinant proteins involved in the base excision repair pathway. In the present chapter, we describe the detailed methodology to reconstitute base excision repair assay systems. These reconstitutive BER assay systems use artificially synthesized and modified DNA. These reconstitutive assay system will be a true representation of biologically occurring damages and their repair.

PARP inhibitor BMN-673 induced apoptosis by trapping PARP-1 and inhibiting base excision repair via modulation of pol-β in chromatin of breast cancer cells

PARP inhibitors emerged as clinically effective anti-tumor agents in combination with DNA damaging agents but the toxicity of DNA damaging agents and their off-target effects caused serious problems in cancer therapy. They confer cytotoxicity in cancer cells both by catalytic inhibition and trapping of PARP-1 at the DNA damage site. There is a lack of direct evidence to quantitatively determine the trapped PARP-1 in cellular DNA. Here, we have precisely evaluated the mechanism of PARP trapping mediated anti-cancer action of Quinacrine (QC), BMN-673, and their combination (QC + BMN-673) in breast cancer cells. We introduced a strategy to measure the cellular PARP trapping potentiality of BMN-673 in QC pretreated cells using a fluorescence-based assay system. It was found that QC+ BMN-673 induced apoptosis by triggering DNA damage in breast cancer cells. Treatment with QC + BMN-673 stimulated the expression of PARP-1 in the chromatin compared to that of PARP-2 and PARP-3. QC + BMN-673 treatment also caused a dose-dependent and time-dependent accumulation of PARP-1 and inhibition of PARylation in the chromatin. Upregulation of BER components (pol-β and FEN-1), an unchanged HR and NHEJ pathway proteins, and reduction of luciferase activity of the cells transfected with R-p21-P (LP-BER) were noted in combined drug-treated cells. Interestingly, silencing of pol-β resulted in unchanged PARP-1 trapping and PAR activity in the chromatin with increasing time after QC + BMN-673 treatment without altering APC and FEN-1 expression. Thus, our data suggested that the QC + BMN-673 augmented breast cancer cell death by pol-β mediated repair inhibition primarily through trapping of PARP-1 besides PARP-1 catalytic inhibition.

Role of YB-1 in Regulation of Poly(ADP-Ribosylation) Catalyzed by Poly(ADP-Ribose) Polymerases

Poly(ADP-ribosyl)ation is a post-translational modification of proteins that performs an essential regulatory function in the cellular response to DNA damage. The key enzyme synthesizing poly(ADP-ribose) (PAR) in the cells is poly(ADP-ribose) polymerase 1 (PARP1). Understanding the mechanisms of the PARP1 activity regulation within the cells is necessary for development of the PARP1-targeted antitumor therapy. This review is devoted to the studies of the role of the RNA-binding protein YB-1 in the PARP1-catalyzed PARylation. The mechanisms of PARP1 activity stimulation by YB-1 protein can possibly be extended to other RNA-binding proteins involved in the maintenance of the genome stability.

Apurinic/Apyrimidinic Endonuclease 2 (APE2): An ancillary enzyme for contextual base excision repair mechanisms to preserve genome stability

The genome of living organisms frequently undergoes various types of modifications which are recognized and repaired by the relevant repair mechanisms. These repair pathways are increasingly being deciphered to understand the mechanisms. Base excision repair (BER) is indispensable to maintain genome stability. One of the enigmatic repair proteins of BER, Apurinic/Apyrimidinic Endonuclease 2 (APE2), like APE1, is truly multifunctional and demonstrates the independent and non-redundant function in maintaining the genome integrity. APE2 is involved in ATR-Chk1 mediated DNA damage response. It also resolves topoisomerase1 mediated cleavage complex intermediate which is formed while repairing misincorporated ribonucleotides in the absence of functional RNase H2 mediated excision repair pathway. BER participates in the demethylation pathway and the role of Arabidopsis thaliana APE2 is demonstrated in this process. Moreover, APE2 is synthetically lethal to BRCA1, BRCA2, and RNase H2, and its homolog, APE1 fails to complement the function. Hence, the role of APE2 is not just an alternate to the repair mechanisms but has implications in diverse functional pathways related to the maintenance of genome integrity. This review analyses genomic features of APE2 and delineates its enzyme function as error-prone as well as efficient and accurate repair protein based on the studies on mammalian or its homolog proteins from model systems such as Arabidopsis thaliana, Schizosaccharomyces pombe, Trypanosoma curzi, Xenopus laevis, Danio rerio, Mus musculus, and Homo sapiens.

Increased DNA repair capacity augments resistance of glioblastoma cells to photodynamic therapy

Photodynamic therapy (PDT) is a clinically approved cancer therapy of low invasiveness. The therapeutic procedure involves administering a photosensitizing drug (PS), which is then activated with monochromatic light of a specific wavelength. The photochemical reaction produces highly toxic oxygen species. The development of resistance to PDT in some cancer cells is its main limitation. Several mechanisms are known to be involved in the development of cellular defense against cytotoxic effects of PDT, including activation of antioxidant enzymes, drug efflux pumps, degradation of PS, and overexpression of protein chaperons. Another putative factor that plays an important role in the development of resistance of cancer cells to PDT seems to be DNA repair; however, it has not been well studied so far. To explore the role of DNA repair and other potential novel mechanisms associated with the resistance to PDT in the glioblastoma cells, cells stably resistant to PDT were isolated from PDT sensitive cells following repetitive PDT cycles. Duly characterization of isolated PDT-resistant glioblastoma revealed that the resistance to PDT might be a consequence of several mechanisms, including higher repair efficiency of oxidative DNA damage and repair of DNA breaks. Higher activity of APE1 endonuclease and increased expression and activation of DNA damage kinase ATM was demonstrated in the U-87 MGR cell line, suggesting and proving that they are good targets for sensitization of resistant cells to PDT.

Differences in PARP Inhibitors for the Treatment of Ovarian Cancer: Mechanisms of Action, Pharmacology, Safety, and Efficacy

Poly(ADP-ribose) polymerases (PARP) are proteins responsible for DNA damage detection and signal transduction. PARP inhibitors (PARPi) are able to interact with the binding site for PARP cofactor (NAD+) and trapping PARP on the DNA. In this way, they inhibit single-strand DNA damage repair. These drugs have been approved in recent years for the treatment of ovarian cancer. Although they share some similarities, from the point of view of the chemical structure and pharmacodynamic, pharmacokinetic properties, these drugs also have some substantial differences. These differences may underlie the different safety profiles and activity of PARPi.

PARPs' impact on base excision DNA repair

Poly(ADP-ribosyl)ation is one of immediate cellular responses to DNA damage and is catalyzed by poly(ADP-ribose) polymerases (PARPs). PARP1 is a well-known regulator of DNA repair. Another member of this family, PARP2, was discovered later. The study of PARP1 and PARP2 functions started a long time ago, and special attention has been given to the role of these enzymes in base excision repair. This review summarizes my lab's data on the functions of PARP1 and PARP2 in base excision repair as well as the results obtained in the course of our collaboration with Dr. Samuel H. Wilson.

New perspectives in cancer biology from a study of canonical and non-canonical functions of base excision repair proteins with a focus on early steps

Alterations of DNA repair enzymes and consequential triggering of aberrant DNA damage response (DDR) pathways are thought to play a pivotal role in genomic instabilities associated with cancer development, and are further thought to be important predictive biomarkers for therapy using the synthetic lethality paradigm. However, novel unpredicted perspectives are emerging from the identification of several non-canonical roles of DNA repair enzymes, particularly in gene expression regulation, by different molecular mechanisms, such as (i) non-coding RNA regulation of tumour suppressors, (ii) epigenetic and transcriptional regulation of genes involved in genotoxic responses and (iii) paracrine effects of secreted DNA repair enzymes triggering the cell senescence phenotype. The base excision repair (BER) pathway, canonically involved in the repair of non-distorting DNA lesions generated by oxidative stress, ionising radiation, alkylation damage and spontaneous or enzymatic deamination of nucleotide bases, represents a paradigm for the multifaceted roles of complex DDR in human cells. This review will focus on what is known about the canonical and non-canonical functions of BER enzymes related to cancer development, highlighting novel opportunities to understand the biology of cancer and representing future perspectives for designing new anticancer strategies. We will specifically focus on APE1 as an example of a pleiotropic and multifunctional BER protein.

Mass spectrometry-based protein‒protein interaction techniques and their applications in studies of DNA damage repair

Proteins are major functional units that are tightly connected to form complex and dynamic networks. These networks enable cells and organisms to operate properly and respond efficiently to environmental cues. Over the past decades, many biochemical methods have been developed to search for protein-binding partners in order to understand how protein networks are constructed and connected. At the same time, rapid development in proteomics and mass spectrometry (MS) techniques makes it possible to identify interacting proteins and build comprehensive protein‒protein interaction networks. The resulting interactomes and networks have proven informative in the investigation of biological functions, such as in the field of DNA damage repair. In recent years, a number of proteins involved in DNA damage response and DNA repair pathways have been uncovered with MS-based protein‒protein interaction studies. As the technologies for enriching associated proteins and MS become more sophisticated, the studies of protein‒protein interactions are entering a new era. In this review, we summarize the strategies and recent developments for exploring protein‒protein interaction. In addition, we discuss the application of these tools in the investigation of protein‒protein interaction networks involved in DNA damage response and DNA repair.

The Interaction of Aging and Cellular Stress Contributes to Pathogenesis in Mouse and Human Huntington Disease Neurons

Huntington disease (HD) is a fatal, inherited neurodegenerative disorder caused by a mutation in the huntingtin (HTT) gene. While mutant HTT is present ubiquitously throughout life, HD onset typically occurs in mid-life. Oxidative damage accumulates in the aging brain and is a feature of HD. We sought to interrogate the roles and interaction of age and oxidative stress in HD using primary Hu97/18 mouse neurons, neurons differentiated from HD patient induced pluripotent stem cells (iPSCs), and the brains of HD mice. We find that primary neurons must be matured in culture for canonical stress responses to occur. Furthermore, when aging is accelerated in mature HD neurons, mutant HTT accumulates and sensitivity to oxidative stress is selectively enhanced. Furthermore, we observe HD-specific phenotypes in neurons and mouse brains that have undergone accelerated aging, including a selective increase in DNA damage. These findings suggest a role for aging in HD pathogenesis and an interaction between the biological age of HD neurons and sensitivity to exogenous stress.

WITHDRAWN: Requirements for PARP-1 covalent crosslinking to DNA (PARP-1 DPC)

The Publisher regrets that this article is an accidental duplication of an article that has already been published in DNA Repair, 90C (2020) 102850, https://doi.org/10.1016/j.dnarep.2020.102850. The duplicate article has therefore been withdrawn. The full Elsevier Policy on Article Withdrawal can be found at https://www.elsevier.com/about/our-business/policies/article-withdrawal.

Functional Role of N-Terminal Extension of Human AP Endonuclease 1 In Coordination of Base Excision DNA Repair via Protein-Protein Interactions

Human apurinic/apyrimidinic endonuclease 1 (APE1) has multiple functions in base excision DNA repair (BER) and other cellular processes. Its eukaryote-specific N-terminal extension plays diverse regulatory roles in interaction with different partners. Here, we explored its involvement in interaction with canonical BER proteins. Using fluorescence based-techniques, we compared binding affinities of the full-length and N-terminally truncated forms of APE1 (APE1NΔ35 and APE1NΔ61) for functionally and structurally different DNA polymerase β (Polβ), X-ray repair cross-complementing protein 1 (XRCC1), and poly(adenosine diphosphate (ADP)-ribose) polymerase 1 (PARP1), in the absence and presence of model DNA intermediates. Influence of the N-terminal truncation on binding the AP site-containing DNA was additionally explored. These data suggest that the interaction domain for proteins is basically formed by the conserved catalytic core of APE1. The N-terminal extension being capable of dynamically interacting with the protein and DNA partners is mostly responsible for DNA-dependent modulation of protein–protein interactions. Polβ, XRCC1, and PARP1 were shown to more efficiently regulate the endonuclease activity of the full-length protein than that of APE1NΔ61, further suggesting contribution of the N-terminal extension to BER coordination. Our results advance the understanding of functional roles of eukaryote-specific protein extensions in highly coordinated BER processes.

MacroH2A1 Regulation of Poly(ADP-Ribose) Synthesis and Stability Prevents Necrosis and Promotes DNA Repair

Through its ability to bind the ends of poly(ADP-ribose) (PAR) chains, the function of the histone variant macroH2A1.1, including its ability to regulate transcription, is coupled to PAR polymerases (PARPs). PARP1 also has a major role in DNA damage response (DDR) signaling, and our results show that macroH2A1 alters the kinetics of PAR accumulation following acute DNA damage by both suppressing PARP activity and simultaneously protecting PAR chains from degradation.

Through its ability to bind the ends of poly(ADP-ribose) (PAR) chains, the function of the histone variant macroH2A1.1, including its ability to regulate transcription, is coupled to PAR polymerases (PARPs). PARP1 also has a major role in DNA damage response (DDR) signaling, and our results show that macroH2A1 alters the kinetics of PAR accumulation following acute DNA damage by both suppressing PARP activity and simultaneously protecting PAR chains from degradation. In this way, we demonstrate that macroH2A1 prevents cellular NAD⁺ depletion, subsequently preventing necrotic cell death that would otherwise occur due to PARP overactivation. We also show that macroH2A1-dependent PAR stabilization promotes efficient repair of oxidative DNA damage. While the role of PAR in recruiting and regulating macrodomain-containing proteins has been established, our results demonstrate that, conversely, macrodomain-containing proteins, and specifically those containing macroH2A1, can regulate PARP1 function through a novel mechanism that promotes both survival and efficient repair during DNA damage response.

Shining light on the response to repair intermediates in DNA of living cells

Fluorescently-tagged repair proteins have been widely used to probe recruitment to micro-irradiation-induced nuclear DNA damage in living cells. Here, we quantify APE1 dynamics after micro-irradiation. Markers of DNA damage are characterized and UV-A laser micro-irradiation energy conditions are selected for formation of oxidatively-induced DNA base damage and single strand breaks, but without detectable double strand breaks. Increased energy of laser micro-irradiation, compared with that used previously in our work, enables study of APE1 dynamics at the lesion site. APE1 shows rapid transient kinetics, with recruitment half-time of less than 1 s and dissociation half-time of less than 15 s. In cells co-transfected with APE1 and PARP1, the recruitment half-time of PARP1 was slower than that of APE1, indicating APE1 is a rapid responder to the damage site. While recruitment of APE1 is unchanged in the presence of co-transfected PARP1, APE1 dissociation is 3-fold slower, revealing PARP1 involvement in APE1 dynamics. Further, we find that APE1 dissociation kinetics are strongly modified in the absence of DNA polymerase β (pol β). After unchanged recruitment to the damage site, dissociation of APE1 became undetectable. This indicates a necessary role for pol β in APE1 release after its recruitment to the damage site. These observations represent an advance in our understanding of in vivo dynamics of base excision repair factors APE1, PARP1 and pol β.

Tyrosyl-DNA Phosphodiesterase I N-Terminal Domain Modifications and Interactions Regulate Cellular Function

The conserved eukaryotic DNA repair enzyme Tyrosyl-DNA phosphodiesterase I (Tdp1) removes a diverse array of adducts from the end of DNA strand breaks. Tdp1 specifically catalyzes the hydrolysis of phosphodiester linked DNA-adducts. These DNA lesions range from damaged nucleotides to peptide-DNA adducts to protein-DNA covalent complexes and are products of endogenously or exogenously induced insults or simply failed reaction products. These adducts include DNA inserted ribonucleotides and non-conventional nucleotides, as well as covalent reaction intermediates of DNA topoisomerases with DNA and a Tdp1-DNA adduct in trans. This implies that Tdp1 plays a role in maintaining genome stability and cellular homeostasis. Dysregulation of Tdp1 protein levels or catalysis shifts the equilibrium to genome instability and is associated with driving human pathologies such as cancer and neurodegeneration. In this review, we highlight the function of the N-terminal domain of Tdp1. This domain is understudied, structurally unresolved, and the least conserved in amino acid sequence and length compared to the rest of the enzyme. However, over time it emerged that the N-terminal domain was post-translationally modified by, among others, phosphorylation, SUMOylation, and Ubiquitinoylation, which regulate Tdp1 protein interactions with other DNA repair associated proteins, cellular localization, and Tdp1 protein stability.

Human apurinic/apyrimidinic endonuclease 1 is modified in vitro by poly(ADP-ribose) polymerase 1 under control of the structure of damaged DNA

Apurinic/apyrimidinic endonuclease 1 (APE1) is an essential multifunctional protein in mammals involved in base excision DNA repair (BER), regulation of gene expression and RNA metabolism. Its major enzymatic function is incision of AP sites. Poly(ADP-ribose) polymerase 1 (PARP1) modifies itself and target proteins with poly(ADP-ribose) (PAR), contributing to regulation of many processes. To understand molecular basis of functional cooperation between APE1 and PARP1 in BER, we examined PAR-binding activity and ADP-ribosylation of human APE1 in comparison with known targets of PARP1, using the full-length, N-terminally truncated and catalytically inactive forms of APE1. The protein binds preferentially large ADP-ribose polymers, being very similar to DNA polymerase β (Polβ) but contrasting with the scaffold XRCC1 protein. The interaction with PAR involves the universally conserved catalytic portion and the eukaryote-specific extension of APE1. The ADP-ribosylation of APE1 depends on the structure of PARP1-activating DNA, contrasting APE1 with Polβ and XRCC1. Relative levels of APE1 modification in the presence of different DNA substrates were found to correlate with affinities of the DNAs for APE1 and substrate activities in the enzymatic incision, suggesting the ADP-ribosylation to occur within the DNA-mediated ternary complex. This conclusion was confirmed by importance of the length of DNA region 3' to the AP site for the modification. Deletion of the N-terminal extension of APE1 produced no significant influence on both the ADP-ribosylation efficiency and hydrolytic stability of the modified protein, suggesting localization of target amino acids in the conserved catalytic portion. The most efficient ADP-ribosylation of the catalytically inactive APE1 mutant was shown to reduce the level of PARP1 automodification, suggesting possible role of APE1 in modulating PARP1 activity. Our data on primary role of DNA in controlling the PARP-catalysed modification provide new insights into mechanisms of protein targeting for ADP-ribosylation.

Damage sensor role of UV-DDB during base excision repair

UV-DDB, a key protein in human global nucleotide excision repair (NER), binds avidly to abasic sites and 8-oxo-guanine (8-oxoG), suggesting a noncanonical role in base excision repair (BER). We investigated whether UV-DDB can stimulate BER for these two common forms of DNA damage, 8-oxoG and abasic sites, which are repaired by 8-oxoguanine glycosylase (OGG1) and apurinic/apyrimidinic endonuclease (APE1), respectively. UV-DDB increased both OGG1 and APE1 strand cleavage and stimulated subsequent DNA polymerase β-gap filling activity by 30-fold. Single-molecule real-time imaging revealed that UV-DDB forms transient complexes with OGG1 or APE1, facilitating their dissociation from DNA. Furthermore, UV-DDB moves to sites of 8-oxoG repair in cells, and UV-DDB depletion sensitizes cells to oxidative DNA damage. We propose that UV-DDB is a general sensor of DNA damage in both NER and BER pathways, facilitating damage recognition in the context of chromatin.

When you're strange: Unusual features of the MUTYH glycosylase and implications in cancer

MUTYH is a base-excision repair glycosylase that removes adenine opposite 8-oxoguanine (OG). Variants of MUTYH defective in functional activity lead to MUTYH-associated polyposis (MAP), which progresses to cancer with very high penetrance. Whole genome and whole exome sequencing studies have found MUTYH deficiencies in an increasing number of cancer types. While the canonical OG:A repair activity of MUTYH is well characterized and similar to bacterial MutY, here we review more recent evidence that MUTYH has activities independent of OG:A repair and appear centered on the interdomain connector (IDC) region of MUTYH. We summarize evidence that MUTYH is involved in rapid DNA damage response (DDR) signaling, including PARP activation, 9-1-1 and ATR signaling, and SIRT6 activity. MUTYH alters survival and DDR to a wide variety of DNA damaging agents in a time course that is not consistent with the formation of OG:A mispairs. Studies that suggest MUTYH inhibits the repair of alkyl-DNA damage and cyclopyrimidine dimers (CPDs) is reviewed, and evidence of a synthetic lethal interaction with mismatch repair (MMR) is summarized. Based on these studies we suggest that MUTYH has evolved from an OG:A mispair glycosylase to a multifunctional scaffold for DNA damage response signaling.

DNA repair after oxidative stress: current challenges

Reactive oxygen and nitrogen species damage cellular macromolecules including DNA. Cells have a robust base excision repair pathway to deal with this damage in both nuclear and mitochondrial genomes. However, mitochondria lack nucleotide excision repair. Evidence suggests that chronic oxidative stress can induce protective pathways lowering genotoxicity. Understanding oxidant injury to DNA and its repair is critical for our understanding the pathophysiology of a wide range of human disorders.

PARP1 changes from three-dimensional DNA damage searching to one-dimensional diffusion after auto-PARylation or in the presence of APE1

PARP1-dependent poly-ADP-ribosylation (PARylation) participates in the repair of many forms of DNA damage. Here, we used atomic force microscopy (AFM) and single molecule fluorescence microscopy to examine the interactions of PARP1 with common DNA repair intermediates. AFM volume analysis indicates that PARP1 binds to DNA at nicks, abasic (AP) sites, and ends as a monomer. Single molecule DNA tightrope assays were used to follow the real-time dynamic behavior of PARP1 in the absence and presence of AP endonuclease (APE1) on AP DNA damage arrays. These experiments revealed that PARP1 conducted damage search mostly through 3D diffusion. Co-localization of APE1 with PARP1 on DNA was found capable of inducing 1D diffusion of otherwise nonmotile PARP1, while excess APE1 also facilitated the dissociation of DNA-bound PARP1. Moreover, auto-PARylation of PARP1 allowed the protein to switch its damage search strategy by causing a 3-fold increase in linear diffusion. Finally, we demonstrated that PARP inhibitor olaparib did not significantly alter the rate of PARP1 dissociation from DNA, but instead resulted in more motility of DNA-bound PARP1 molecules.

Clair DK, Kyprianou N. Profiles of Radioresistance Mechanisms in Prostate Cancer

Radiation therapy (RT) is commonly used for the treatment of localized prostate cancer (PCa). However, cancer cells often develop resistance to radiation through unknown mechanisms and pose an intractable challenge. Radiation resistance is highly unpredictable, rendering the treatment less effective in many patients and frequently causing metastasis and cancer recurrence. Understanding the molecular events that cause radioresistance in PCa will enable us to develop adjuvant treatments for enhancing the efficacy of RT. Radioresistant PCa depends on the elevated DNA repair system and the intracellular levels of reactive oxygen species (ROS) to proliferate, self-renew, and scavenge anti-cancer regimens, whereas the elevated heat shock protein 90 (HSP90) and the epithelial-mesenchymal transition (EMT) enable radioresistant PCa cells to metastasize after exposure to radiation. The up-regulation of the DNA repairing system, ROS, HSP90, and EMT effectors has been studied extensively, but not targeted by adjuvant therapy of radioresistant PCa. Here, we emphasize the effects of ionizing radiation and the mechanisms driving the emergence of radioresistant PCa. We also address the markers of radioresistance, the gene signatures for the predictive response to radiotherapy, and novel therapeutic platforms for targeting radioresistant PCa. This review provides significant insights into enhancing the current knowledge and the understanding toward optimization of these markers for the treatment of radioresistant PCa.

Alkylation induced cerebellar degeneration dependent on Aag and Parp1 does not occur via previously established cell death mechanisms

Alkylating agents are ubiquitous in our internal and external environments, causing DNA damage that contributes to mutations and cell death that can result in aging, tissue degeneration and cancer. Repair of methylated DNA bases occurs primarily through the base excision repair (BER) pathway, a multi-enzyme pathway initiated by the alkyladenine DNA glycosylase (Aag, also known as Mpg). Previous work demonstrated that mice treated with the alkylating agent methyl methanesulfonate (MMS) undergo cerebellar degeneration in an Aag-dependent manner, whereby increased BER initiation by Aag causes increased tissue damage that is dependent on activation of poly (ADP-ribose) polymerase 1 (Parp1). Here, we dissect the molecular mechanism of cerebellar granule neuron (CGN) sensitivity to MMS using primary ex vivo neuronal cultures. We first established a high-throughput fluorescent imaging method to assess primary neuron sensitivity to treatment with DNA damaging agents. Next, we verified that the alkylation sensitivity of CGNs is an intrinsic phenotype that accurately recapitulates the in vivo dependency of alkylation-induced CGN cell death on Aag and Parp1 activity. Finally, we show that MMS-induced CGN toxicity is independent of all the cellular events that have previously been associated with Parp-mediated toxicity, including mitochondrial depolarization, AIF translocation, calcium fluxes, and NAD+ consumption. We therefore believe that further investigation is needed to adequately describe all varieties of Parp-mediated cell death.

Pre-Replicative Repair of Oxidized Bases Maintains Fidelity in Mammalian Genomes: The Cowcatcher Role of NEIL1 DNA Glycosylase

Genomic fidelity in the humans is continuously challenged by genotoxic reactive oxygen species (ROS) generated both endogenously during metabolic processes, and by exogenous agents. Mispairing of most ROS-induced oxidized base lesions during DNA replication induces mutations. Although bulky base adducts induced by ultraviolet light and other environmental mutagens block replicative DNA polymerases, most oxidized base lesions do not block DNA synthesis. In 8-oxo-G:A mispairs generated by the incorporation of A opposite unrepaired 8-oxo-G, A is removed by MutYH (MYH) for post-replicative repair, and other oxidized base lesions must be repaired prior to replication in order to prevent mutation fixation. Our earlier studies documented S phase-specific overexpression of endonuclease VIII-like 1 (NEIL1) DNA glycosylase (DG), one of five oxidized base excision repair (BER)-initiating enzymes in mammalian cells, and its high affinity for replication fork-mimicking single-stranded (ss)DNA substrates. We recently provided experimental evidence for the role of NEIL1 in replicating-strand repair, and proposed the "cowcatcher" model of pre-replicative BER, where NEIL1's nonproductive binding to the lesion base in ssDNA template blocks DNA chain elongation, causing fork regression. Repair of the lesion in the then re-annealed duplex is carried out by NEIL1 in association with the DNA replication proteins. In this commentary, we highlight the critical role of pre-replicative BER in preventing mutagenesis, and discuss the distinction between pre-replicative vs. post-replicative BER.

Coordination of DNA single strand break repair

The genetic material of all organisms is susceptible to modification. In some instances, these changes are programmed, such as the formation of DNA double strand breaks during meiotic recombination to generate gamete variety or class switch recombination to create antibody diversity. However, in most cases, genomic damage is potentially harmful to the health of the organism, contributing to disease and aging by promoting deleterious cellular outcomes. A proportion of DNA modifications are caused by exogenous agents, both physical (namely ultraviolet sunlight and ionizing radiation) and chemical (such as benzopyrene, alkylating agents, platinum compounds and psoralens), which can produce numerous forms of DNA damage, including a range of "simple" and helix-distorting base lesions, abasic sites, crosslinks and various types of phosphodiester strand breaks. More significant in terms of frequency are endogenous mechanisms of modification, which include hydrolytic disintegration of DNA chemical bonds, attack by reactive oxygen species and other byproducts of normal cellular metabolism, or incomplete or necessary enzymatic reactions (such as topoisomerases or repair nucleases). Both exogenous and endogenous mechanisms are associated with a high risk of single strand breakage, either produced directly or generated as intermediates of DNA repair. This review will focus upon the creation, consequences and resolution of single strand breaks, with a particular focus on two major coordinating repair proteins: poly(ADP-ribose) polymerase 1 (PARP1) and X-ray repair cross-complementing protein 1 (XRCC1).

Base excision repair of oxidative DNA damage: from mechanism to disease

Reactive oxygen species continuously assault the structure of DNA resulting in oxidation and fragmentation of the nucleobases. Both oxidative DNA damage itself and its repair mediate the progression of many prevalent human maladies. The major pathway tasked with removal of oxidative DNA damage, and hence maintaining genomic integrity, is base excision repair (BER). The aphorism that structure often dictates function has proven true, as numerous recent structural biology studies have aided in clarifying the molecular mechanisms used by key BER enzymes during the repair of damaged DNA. This review focuses on the mechanistic details of the individual BER enzymes and the association of these enzymes during the development and progression of human diseases, including cancer and neurological diseases. Expanding on these structural and biochemical studies to further clarify still elusive BER mechanisms, and focusing our efforts toward gaining an improved appreciation of how these enzymes form co-complexes to facilitate DNA repair is a crucial next step toward understanding how BER contributes to human maladies and how it can be manipulated to alter patient outcomes.

Repair of oxidatively induced DNA damage by DNA glycosylases: Mechanisms of action, substrate specificities and excision kinetics

Endogenous and exogenous reactive species cause oxidatively induced DNA damage in living organisms by a variety of mechanisms. As a result, a plethora of mutagenic and/or cytotoxic products are formed in cellular DNA. This type of DNA damage is repaired by base excision repair, although nucleotide excision repair also plays a limited role. DNA glycosylases remove modified DNA bases from DNA by hydrolyzing the glycosidic bond leaving behind an apurinic/apyrimidinic (AP) site. Some of them also possess an accompanying AP-lyase activity that cleaves the sugar-phosphate chain of DNA. Since the first discovery of a DNA glycosylase, many studies have elucidated the mechanisms of action, substrate specificities and excision kinetics of these enzymes present in all living organisms. For this purpose, most studies used single- or double-stranded oligodeoxynucleotides with a single DNA lesion embedded at a defined position. High-molecular weight DNA with multiple base lesions has been used in other studies with the advantage of the simultaneous investigation of many DNA base lesions as substrates. Differences between the substrate specificities and excision kinetics of DNA glycosylases have been found when these two different substrates were used. Some DNA glycosylases possess varying substrate specificities for either purine-derived lesions or pyrimidine-derived lesions, whereas others exhibit cross-activity for both types of lesions. Laboratory animals with knockouts of the genes of DNA glycosylases have also been used to provide unequivocal evidence for the substrates, which had previously been found in in vitro studies, to be the actual substrates in vivo as well. On the basis of the knowledge gained from the past studies, efforts are being made to discover small molecule inhibitors of DNA glycosylases that may be used as potential drugs in cancer therapy.

DNA maintenance following bleomycin-induced strand breaks does not require poly(ADP-ribosyl)ation activation in Drosophila S2 cells

BACKGROUND

Poly-ADP ribosylation (PARylation) is a post translational modification, catalyzed by Poly(ADP-ribose)polymerase (PARP) family. In Drosophila, PARP-I (human PARP-1 ortholog) is considered to be the only enzymatically active isoform. PARylation is involved in various cellular processes such as DNA repair in case of base excision and strand-breaks.

OBSERVATIONS

Strand-breaks (SSB and DSB) are detrimental to cell viability and, in Drosophila, that has a unique PARP family organization, little is known on PARP involvement in the control of strand-breaks repair process. In our study, strands-breaks (SSB and DSB) are chemically induced in S2 Drosophila cells using bleomycin. These breaks are efficiently repaired in S2 cells. During the bleomycin treatment, changes in PARylation levels are only detectable in a few cells, and an increase in PARP-I and PARP-II mRNAs is only observed during the recovery period. These results differ strongly from those obtained with Human cells, where PARylation is strongly activating when DNA breaks are generated. Finally, in PARP knock-down cells, DNA stability is altered but no change in strand-breaks repair can be observed.

CONCLUSIONS

PARP responses in DNA strands-breaks context are functional in Drosophila model as demonstrated by PARP-I and PARP-II mRNA increases. However, no modification of the global PARylation profile is observed during strand-breaks generation, only changes at cellular levels are detectable. Taking together, these results demonstrate that PARylation process in Drosophila is tightly regulated in the context of strands-breaks repair and that PARP is essential during the maintenance of DNA integrity but dispensable in the DNA repair process.

Pathways of cardiac toxicity: comparison between chemotherapeutic drugs doxorubicin and mitoxantrone

Anthracyclines, e.g., doxorubicin (DOX), and anthracenediones, e.g., mitoxantrone (MTX), are drugs used in the chemotherapy of several cancer types, including solid and non-solid malignancies such as breast cancer, leukemia, lymphomas, and sarcomas. Although they are effective in tumor therapy, treatment with these two drugs may lead to side effects such as arrhythmia and heart failure. At the same clinically equivalent dose, MTX causes slightly reduced cardiotoxicity compared with DOX. These drugs interact with iron to generate reactive oxygen species (ROS), target topoisomerase 2 (Top2), and impair mitochondria. These are some of the mechanisms through which these drugs induce late cardiomyopathy. In this review, we compare the cardiotoxicities of these two chemotherapeutic drugs, DOX and MTX. As described here, even though they share similarities in their modes of toxicant action, DOX and MTX seem to differ in a key aspect. DOX is a more redox-interfering drug, while MTX induces energy imbalance. In addition, DOX toxicity can be explained by underlying mechanisms that include targeting of Top2 beta, mitochondrial impairment, and increases in ROS generation. These modes of action have not yet been demonstrated for MTX, and this knowledge gap needs to be filled.