PARP-1 and its associated nucleases in DNA damage response

Abstract

Poly(ADP-ribose) (PAR) polymerase-1 (PARP-1) acts as a DNA damage sensor. It recognizes DNA damage and facilitates DNA repair by recruiting DNA repair machinery to damage sites. Recent studies reported that PARP-1 also plays an important role in DNA replication by recognizing the unligated Okazaki fragments and controlling the speed of fork elongation. On the other hand, emerging evidence reveals that excessive activation of PARP-1 causes chromatin DNA fragmentation and triggers an intrinsic PARP-1-dependent cell death program designated parthanatos, which can be blocked by genetic deletion or pharmacological inhibition of PARP-1. Therefore, PARP-1 plays an essential role in maintaining genomic stability by either facilitating DNA repair/replication or triggering DNA fragmentation to kill cells. A group of structure-specific nucleases is crucial for executing DNA incision and fragmentation following PARP-1 activation. In this review, we will discuss how PARP-1 coordinates with its associated nucleases to maintain genomic integrity and control the decision of cell life and death.

1. Introduction

Poly(ADP-ribose) (PAR) polymerase-1 (PARP-1) is the most extensively studied nuclear enzyme of the PARP superfamily [1–3]. It functions as a DNA damage sensor [1–3]. Upon DNA damage, PARP-1 utilizes NAD+ as the substrate and catalyzes the addition of mono-ADP-ribose or PAR to different acceptor proteins, including PARP-1 itself, which is the earliest event in response to DNA damage [1–3]. As a consequence, this event leads to the recruitment of DNA repair proteins and nucleases to damage sites, thereby facilitating DNA damage repair [4–10]. Recently, it was also shown that PARP-1 recognizes the unligated Okazaki fragments and promotes repair during DNA replication [11], indicating its important role in DNA replication. Therefore, inhibition of PARP-1 may sensitize cancer cells to death by interfering with DNA repair and replication. Indeed, PARP inhibitors have been approved by the FDA for the treatment of patients with homologous recombination (HR)-deficient ovarian and breast cancers [12–14].

On the other hand, excessive activation of PARP-1 causes large chromatin DNA fragmentation and triggers an intrinsic PARP-1-dependent cell death program designated parthanatos, which occurs in a variety of organ systems and is widely involved in different neurologic and non-neurologic diseases [15–17]. Pharmacological inhibition or genetic deletion of PARP-1 is profoundly protective against such cellular injury in models of human diseases [15–19]. Therefore, PARP-1 plays an essential role in maintaining genomic stability by either facilitating DNA repair/replication or triggering DNA fragmentation to kill cells (Figure 1). A group of structure-specific nucleases is required for the incision of DNA in multiple PARP-1-evoked DNA repair /damage pathways including single-stranded break (SSB) repair, double-stranded break (DSB) repair, and DNA replication process as well as parthanatos in response to DNA damage [4–10, 15]. In this review, we will summarize the progresses how PARP-1 coordinates with its associated nucleases to maintain genomic integrity and control cell fate.

Figure 1. PARP-1 cooperates with nucleases and controls genomic stability.

Upon DNA damage induced by oxidative stress or alkylating agents abundant in the environment, PARP-1 is activated to synthesize PAR. As a consequence, specific nucleases are recruited to DNA damage sites to either facilitate DNA damage repair/replication, or trigger DNA degradation leading to cell death. Me, methylation; OH, hydroxylation.

2. Role of PARP-1 in DNA replication and DNA damage repair

DNA replication is an essential process for maintaining genomic integrity. Emerging evidence shows that PARP-1 plays a role in DNA replication and genomic maintenance. Previously, it has been shown that the synthesis of PAR, the bioproduct of PARP-1 activation, is detected at sites of DNA replication fork stalling following DNA damage induced by replication inhibition and recruits meiotic recombination 11 homolog 1 (MRE11) to resect the fork and promote DNA replication restart [5]. Recently, it was found that endogenous PAR is detected at sites of DNA replication during S phase, which is mainly caused by unligated Okazaki fragments [11]. Suppression of Okazaki fragment formation decreases PARP-1 activity. Therefore, PARP-1 serves as a sensor of unligated Okazaki fragments to facilitate a XRCC1-dependent repair during DNA replication [11]. In line with this, another recent study showed that PARP-1 activation acts as a sensor of replication stress and controls the velocity of replication forks [20]. Inhibition of PARP increases the speed of fork elongation without causing fork stalling, which might reduce the fidelity of DNA polymerases and increase DNA damage and genomic instability.

PARP-1 has been well characterized for its role in base excision repair (BER), following DNA base modifications by exogenous genotoxic agents, such as alkylating agents, which are the most common damage to form SSBs [21]. PARP-1 rapidly recognizes these breaks and mediates the recruitment of a scaffold protein XRCC1 and subsequent assembly with other core factors at break sites to process the repair [22]. Besides BER, PAPR-1 has also been suggested to play a role in nucleotide excision repair (NER) by poly(ADP-ribos)ylating (PARylating) NER proteins xeroderma pigmentosum complementation group A (XPA) and XPC [23–25]. Inhibition of PARP-1 may impair SSB repair and cause DSBs.

DNA DSB is often caused by either exogenous damaging agents or endogenous collapse of DNA replication forks. HR and non-homologous end joining (NHEJ) are two major pathways responsible for DSB repair. Pharmacological inhibition of PARP-1 has shown a promising therapeutic effect on BRCA1/2-deficient ovarian cancer [9, 10]. PARP inhibitors have also been approved to treat BRCA mutated metastatic breast and prostate cancers [12–14]. BRCA1/2 are essential proteins required for HR repair. Although increasing evidences have linked PARP-1 to DSB repair, how PARP-1 contributes to DSB repair remains obscure. Nevertheless, two types of models about PARP-1 involvement in DSB repair have been reported. The first model is that PARP-1 directly regulates DSB repair. Upon PARP-1 activation, PAR is synthesized and controls ataxia telangiectasia mutated (ATM) activation and the recruitment of HR proteins to DNA damage sites [1, 26]. PARP-1 deficiency or inhibition interferes with the recruitment of the MRN complex proteins MRE11 and NBS1 to DNA lesions [6]. The 5’-3’ exonuclease activity of MRE11 is essential to generate 3’ single-stranded DNA (ssDNA) overhangs during HR initiation [27]. PARP-1 also contributes to the recruitment of another important HR component BRCA1 to DNA damage sites [28]. BRCA1 forms a heterodimer with BRAD1 and the BRCT domain of BRAD 1 can mediate the recmitment of BRCA1 to DNA lesions through binding with PAR [28]. Poly(ADP-ribos)ylation (PARylation) of BRCA1 stabilizes the BRCA1-RAP80 complex and PARP-1 has been proposed to limit HR process [1, 29]. Besides HR, PARP-1 has been shown to controls NHEJ by PARylating an important NHEJ factor DNA-dependent protein kinase catalytic subunit (DNA-PKcs) [30]. In the absence of Ku70 and Ku80, PARP-1 binds at DSB sites and recruits MRE11 to resect DNA. Then the resulting DNA ends join together by their microhomologue sequences and the gap is further sealed by polymerase Θ and ligase III, thereby promoting an error-prone alternative NHEJ (aNHEJ) repair for cell survival [1]. The second model is that PARP-1 indirectly impacts on DSB repair by controlling the SSB repair pathway, especially the BER. Inhibition of PARP-1 blocks the BER, and turns the SSBs into DSBs, thereby increasing HR stress in BRCA1/2-deficient cells, which at least partially explains the molecular mechanism underlying synthetic lethality of the PARP inhibitor in BRCA1/2-deficient cells [31].

Collectively, these findings suggest that PARP-1 plays a critical role in DNA replication and DNA damage repair to maintain genomic stability.

3. PARP-1 hyperactivation mediates a unique cell death program-parthanatos

As the other side of coin, excessive activation of PARP-1 leads to a unique cell death program designated parthanatos, which is a new type of programmed necrosis different from apoptosis and necrosis (Figure 2) [15–17]. Parthanatos has several key features, including nuclear shrinkage, chromatin condensation, and large DNA fragmentations ranged from 15 kb to 50 kb. Parthanatos occurs in many organ systems and is widely involved in the pathological processes of different neurologic and non-neurologic diseases, including ischemia-reperfusion injury after stroke and myocardial infarction, glutamate excitotoxicity, neurodegenerative diseases, inflammatory injury, reactive oxygen species (ROS)–induced injury [15–19]. Parthanatos specifically depends on PARP-1 hyperactivation and PAR accumulation. PARP-1 inhibitors or genetic deletion of PARP-1 completely block parthanatos, whereas caspase inhibitors fail to do so. PAR polymer has a very short half-life and is rapidly degraded by PAR glycohydrolase (PARG) [16]. It functions as a cell death signal and mediates parthanatos [32]. Following oxidative stress or brain injury, PARP-1 evokes PAR-mediated deadly crosstalk between the nucleus and mitochondria by triggering apoptosis-inducing factor (AIF) release from mitochondria (Figure 2). Then AIF recruits microphage migration inhibitory factor (MIF), a newly identified 3’ exonuclease, to the nucleus, where MIF cleaves the genomic DNA into large DNA fragments leading to cell death [15]. Genetic deletion of MIF, abolition of MIF nuclease activity, or disruption of MIF/AIF interaction prevents DNA degradation and cell death. These findings uncover a critical role of PARP-1 in cell death and may ultimately lead to new ways to protect against brain injury.

Figure 2. MIF is the executor for PARP-1 dependent cell death (parthanatos).

Stresses induce DNA damage, PARP1 activation, and subsequently PAR formation. PAR facilitates the release of AIF from mitochondria. AIF recruits MIF to the nucleus, where MIF cleaves genomic DNA into large DNA fragments and causes cell death. Interference with this cascade by blocking either the formation of the AIF-MIF complex or MIF nuclease activity prevents DNA fragmentation and promotes cell survival.

Besides its role in brain injury, the role of PARP-1 in alkylating agents-induced DNA damage and cell death has also been documented [33, 34]. N-Methyl-N′-nitro-N-nitrosoguanidine (MNNG), which belongs to the mono- (SN1-) subtype alkylating agent, was first discovered in 1959 as an anti-cancer drug through a national cooperative cancer drug screen program [35]. It acts by adding alkyl groups to the O⁶ of guanine and O⁴ of thymine, thereby causing PARP-1 hyperactivation and eventually leading to cell death [33, 34]. PARP-1 inhibition blocks PAR synthesis and prevents MNNG-induced cell death [15]. MNNG is structurally similar to toxic compounds found in common chemotherapy drugs [33]. Its derivatives Lomustine and Carmustine are the currently most commonly used alkylating chemotherapy drugs [36]. Similarly, SN2-subtype alkylating agent methyl methanesulfonate (MMS) also activates PARP-1 to produce PAR and causes cell death, which can be robustly blocked by PARP-1 deletion both in vitro and in vivo [33, 34]. Although alkylating agents are the most commonly used chemotherapy drugs to kill cancer cells [33, 36–38], the importance of PARP-1 in alkylating agents-induced cancer cell death has not brought much attention yet. The precise mechanisms by which PARP-1 hyperactivation following alkylating agent treatment induces DNA degradation and cancer cell death still remain to be explored.

4. Nucleases and genomic stability

PARP-1 is widely involved in different DNA repair pathways, DNA replication and cell death, in response to DNA damage. The structure-specific nucleases are often required for DNA incision during these PARP-1-evoked processes to regulate genomic stability either by facilitating DNA repair or triggering DNA fragmentation. Multiple nucleases, including Flap endonuclease 1 (FEN1), DNA replication helicase/nuclease 2 (DNA2), MUS81-EME1, Apurinic/apyrimidinic endonuclease (APE1), 3’ repair exonuclease (TREX), MRE11, and exonuclease 1 (EXO1), have been previously identified for their critical role in the process of DNA replication, SSB repair, and DSB repair (Table 1). Recently, we discovered that MIF is a novel PARP-1 activity associated nuclease (PAAN) involved in DNA degradation during parthanatos [15]. The functions of these nucleases in PARP-1-mediated genomic integrity are summarized below.

Table 1.

Summary of PARP-1-associated nucleases in DNA repair, replication and cell death pathways.

Nucleases	Polarity	Nuclease Activity	Functional Pathways	References
APE1	3’-5’	AP site endonuclease; 3’ exonuclease	BER; DNA replication	[54, 55, 58]
FEN1	5’-3’	5’ flap endonuclease; 5’ exonuclease	BER; Okazaki fragment processing	[11, 39–41, 51]
DNA2	5’-3’	5’ flap endo/exonuclease; Resolve the long 5’ flap;	Long-patch BER; Maintenance of telomere integrity; Okazaki fragment processing; Stalled replication fork resolution	[42–49, 80]
Mus81	3’-5’	3’ flap endonuclease	Stalled replication fork resolution	[50, 81]
TREX1/2	3’-5’	Aberrant ssDNA removal; 3’ exonuclease	DNA damage repair; Immune response; DNA replication proofreader	[59–62]
MRE11	3’-5’	Blunt or recessed 3’ exonuclease	Stalled replication fork restart; telomere length maintenance; DSB repair	[5, 6, 27, 63]
EXO1	5’-3’	5’ exonuclease and 5’ flap endonuclease	DNA replication; MMR; BER; Translesion synthesis (TLS); HR	[64–67]
MIF	3’-5’	3’ exonuclease; DNA degradation;	Parthanatos	[15]

4.1. FEN1

FEN1 is a structure-specific nuclease with 5’ flap endonuclease and 5’ exonuclease activity. It plays an essential role in DNA replication as well as BER. During DNA replication, FEN1 is required for the removal of RNA-DNA primers in lagging-strand and the resulting nick is sealed by DNA ligase I, leading to the maturation of Okazaki fragments [39]. Recently, it was shown that perturbation of FEN1 increases the unligated Okazaki fragments, which causes PARP-1 activation and PAR accumulation in S phase and further activates XRCC-1-dependent repair during DNA replication [11]. The function of FEN1 to remove the 5’-flap structure has also been widely explored in BER. The long-patch BER includes more than one nucleotide incorporation through strand-displacement reaction, which activates PARP-1- and XRCC1-dependent repair pathway and requires FEN1 to resolve the 5’-flap intermediate [40]. The nuclease-deficient FEN1 mutant causes BER deficiency and genomic instability, and makes mice more susceptible to DNA alkylating agent methylnitrosourea-induced lung adenocarcinoma [41].

4.2. DNA 2

DNA2 possesses 5′-3′ helicase and endonuclease activities and plays an essential role in DNA replication and repair in both nucleus and mitochondrion [42]. It resolves the long 5’ flap during the process of Okazaki fragment maturation and long-patch BER [43]. When the 5’ flap length is more than 27 nt, the replication protein A (RPA) complex recruits DNA2 to incise the long flap into a short one suitable for FEN1 cleavage [44]. DNA replication stress often results in stalled or reversed replication fork. DNA2 can degrade the reversed forks and coordinate with the Werner syndrome ATP-dependent helicase (WRN) as well as the Bloom syndrome protein (BLM) to restart the arrested replication forks [45]. DNA2 also plays a role in maintenance of telomere integrity [46]. It cleaves G-quadruplex DNA in vitro. DNA2 deficiency displays a variety of genome instabilities and impairs telomere replication [46]. The small-molecule inhibitor of DNA2 and PARP inhibitor have a synergistic effect on sensitization of human cancer cells to camptothecin chemotherapy [47]. Thus, combined inhibition of PARP-1 and DNA2 has been considered as a potential strategy for inducing synthetic lethality [43, 47–49].

4.3. MUS81-EME1

MUS81-EME1 is another structure-dependent endonuclease involving in restart of the stalled DNA replication fork. It prefers to remove the branched DNA structure and provokes the DSBs under conditions of replication inhibition [50]. Lack of functional MUS81 makes cells more sensitive to DNA damaging agents and impairs DNA replication fork progression [51, 52]. Furthermore, MUS81 is also essential for maintaining normal DNA replication by reducing the frequency of the initiation events [53].

4.4. APE1

APE1, a primary apurinic/apyrimidinic endonuclease, plays an essential role in the BER pathway. DNA base modification and damage often cause SSBs, which can be rapidly detected by PARP-1. The latter recruits the scaffold protein XRCC1 and APE1 to damage sites to process the repair [22]. APE1 cleaves the phosphodiester backbone immediately adjacent to the 5’ to an AP site to generate 3’-hydroxyl and 5’-deoxyribose phosphate terminuses [54]. In the short-patch BER, the DNA nick incised by APE1 is sealed by DNA polymerase β and DNA ligase 3 [55]. In the long-patch BER, following APE1 incision, PCNA and DNA polymerase δ or β are recruited, which displaces 2 to 8 nucleotides from the damage sites and further elongate the nucleotides to seal the gap with the assistant of FEN1 to cleave the displaced nucleotides and DNA ligase I to incorporate de novo nucleotides [56, 57]. In vitro reconstituted BER system implies that APE1 contributes the rejoining efficiency and increases the fidelity of BER [54]. Besides its endonuclease activity, APE1 also functions as a 3’ to 5’ exonuclease to remove the 3’-mispaired termini, such as phosphoglycolate (3’-PG) caused by ROS [58].

4.5. TREX

TREX1 and TREX2 are the major 3’ exonucleases that cleave the mismatched DNA end. They serve as proofreaders to maintain the accuracy and fidelity of DNA replication. TREX1 predominantly locates in the cytoplasm and digests cytosolic ssDNA to prevent cytosolic DNA-induced immune response [59]. Upon DNA damage induced by genotoxic stress such as UV and γ irradiation, TREX1 translocates to the nucleus where it directly interacts with PARP-1 and contributes to DNA damage responses [60]. TREX1 deficiency causes persistent ssDNA accumulation in endoplasmic reticulum, impairs Gl/S transition during the cell cycle, and provokes ATM-dependent DNA damage checkpoint activation [61]. TREX2 is reported to interact with DNA polymerase δ and increases its accuracy to prevent mutagenesis in the cell [59, 62].

4.6. MRE11

MRE11 possesses endonuclease and 5’ exonuclease activities and is widely involved in DNA replication, telomere length maintenance, and DSB repair [5, 6, 27, 63]. Its 5’ exonuclease activity is essential to generate 3’ ssDNA overhangs during HR initiation [27]. MRE11 is one of the first nucleases recruited to DNA lesions, where it forms a MRN (Mre11-Rad50-Nbs1) complex with Nbs1 and Rad50 [6]. MRE11 physically interacts with PARP-1 via its PAR biding motif and PAPR-1 is required for the recruitment of MRE11 to DNA damage lesions. In addition, PARP-1 can be activated by stalled replication forks and further recruits MRE11 to resect the fork for initiation restart [5]. Interestingly, PARP-1 could protect the excessive degradation of nascent DNA caused by MRE11 in BRCA2-deficient cells. Thus, BRCA2-deficient cells exhibit more stalled forks degradation upon PAPR-1 inhibition. This phenomenon could partially explain the synthetic lethal effect of BRCA2 deficiency combined with PARP-1 inhibitors [63].

4.7. EXO1

EXO1 is a multifunctional nuclease with 5’ exonuclease activity and 5’ flap endonuclease activity and is involved in DNA replication, DNA mismatch repair, and DSB repair in the cell [64–67]. It has been shown that PARP-1 interacts with EXO1 and promotes its early recruitment to sites of DNA damage in cells upon laser micro irradiation. This recruitment process depends on PARP-1-induced PARylation at the N-terminal PIN domain of EXO1. Zhang F et al. also showed that BRCA2 is PARylated by PARP-1 and mediates the early recruitment of EXO1 for DNA end resection [68]. However, PAR binding inhibits EXO1’s nuclease activity. Deletion of EXO1 prevents PARP inhibitor-induced Rad51 accumulation in chromatin and switches HR repair to NHEJ repair pathway [69].

4.8. MIF/PAAN

MIF is a pleiotropic cytokine-like protein, which was originally identified by Bloom and Bennett as a protein secreted by T cells and inhibiting migration of macrophage [70, 71]. To date, it becomes clear that MIF is widely expressed in multiple cell types, including neurons, monocytes, macrophages, vascular smooth muscle cells, cardiomyocytes, and cancer cells, and plays an important role in inflammation, immune response, and tumor growth. Hypoxia-inducible factor induces the expression of MIF in hypoxic cells [72]. Increased expression of MIF has been observed in cancer patients and is correlated with poor clinical outcome in these patients [73]. Besides its known function as a non-classically secreted cytokines, MIF also possesses oxido-reductase and tautomerase activities in vitro, although its biological substrates and physiological relevance remain unknown. MIF tautomerase-null knock-in mice reveals that MIF-interacting protein rather than its tautomerase activity mediates MIF-dependent growth regulation [74]. Interestingly, MIF knockout mice have a longer life span than control mice [75].

We recently identified MIF as a novel Mg²⁺- and Ca²⁺-dependent nuclease [15]. It possesses 3’ exonuclease activity and has relatively higher (about 10-fold) binding affinity to the ssDNA with a stem-loop like structure than the double-stranded DNA in vitro. By screening DNA substrates either with different structures or different sequences using both in vitro nuclease assay and electrophoretic mobility shift assay, we discovered that MIF binds to the 5’ free unpaired region of ssDNA but cleaves it at the 3’ unpaired region (Figure 3), indicating that MIF recognizes the ssDNA substrate in a structure-dependent rather than a sequence-dependent manner. MIF binds to chromatin DNA and cleaves it into large DNA fragments in neurons following N-methyl-D-aspartate (NMDA) neurotoxicity or in cancer cells following a strong alkylating agent MNNG treatment, eventually leading to cell death [15]. MIF knockout or nuclease-deficient MIF mutant prevents large DNA fragmentations and protects from NMDA or MNNG-induced cell death (Figure 3). Since MIF is widely involved in inflammation, immune response, and cancer biology, further studies are required to study if MIF’s nuclease activity contributes to its effects on inflammation, immune response and tumor growth.

Figure 3. MIF is a novel Mg²⁺/Ca²⁺-dependent 3’ exonuclease.

MIF trimer specifically recognizes the ssDNA with a stem loop structure as the substrate. It binds to the 5’ unpaired region of stem loop structured ssDNA, but selectively cleaves at its 3’ unpaired ssDNA.

5. Perspective

PARP-1 controls cell life and death in a context-dependent manner (Figure 4). In response to the mild DNA damage, PARP-1 activation facilitates DNA repair leading to cell survival [3, 4, 7–10, 76]. In this context, PARP inhibition may sensitize cancer cells to death by interfering with DNA repair. Indeed, the discovery of synthetic lethality of PARP inhibitors in BRCA1/2-deficient tumors has brought much attention and enthusiasm into the field of DNA repair and cancer therapy [9, 10]. Currently, four PARP inhibitors olaparib, rucaparib, niraparib and talazoparib have been approved by the FDA [12–14] and many others are being extensively tested in clinical trials either as a single agent or in combination with chemotherapy or radiotherapy approaches to treat different types of solid tumors. On the other hand, we and other groups have shown that PARP-1 mediates an intrinsic caspase-independent cell death program designated parthanatos in response to severe DNA damage [15–19, 77, 78]. In this context, inhibition of PARP-1 may prevent alkylating agent-induced cytotoxicity and increase chemoresistance. The precise mechanism by which PARP-1 regulates these antagonistic functions: DNA repair/cell survival versus DNA fragments/cell death, remains unclear. Interestingly, Tulin group recently showed that the interaction of N-terminal DNA binding domain of PARP-1 with DNA leads to a “hit and run activation of PARP-1”, which may initiate the DNA repair pathway [79]. In contrast, the interaction of the C-terminal domain of PARP-1 with histone H4 in the nucleosome leads to the long-term activation of PARP-1, which causes chromatin loosening. These studies indicate that different types of interaction with PARP-1 may contribute to distinct functions of PARP-1. Further studies are needed to explore PARP-1 interactors at its N- and C-terminal domains, which may help understand the regulation of the precise functions of PARP-1 under different DNA damage contexts.

Figure 4. The model of context-dependent functions of PARP-1 and nucleases in DNA repair and cell death.

The role of PARP-1 is highly context-dependent in response to different types of DNA damage in different types of cells and tissues. In BRCA1/2-deficient cancer cells, PARP-1 plays a critical role in DNA damage repair. PARP-1 inhibitors combined with DNA damage chemotherapy drugs kill cancer cells. However, in BRCA1/2-proficient cells, DNA alkylating agents or oxidative stress may cause PARP-1 hyperactivation, PAR accumulation and DNA degradation leading to cell death. Inhibition of PARP-1 prevents cell death. A group of structure-specific nucleases is recruited by activated PARP-1 in response to DNA damage. Future studies are required to understand if these PARP-1-associated nucleases execute different functions under the different contexts.

Although it is clear that a group of structure-specific nucleases associates with PARP-1 activation and is crucial for maintaining genome integrity and controlling cell fate, several outstanding questions remain to be answered. For example, how does PARP-1 differentially coordinate with these nucleases under different DNA damage contexts? The polymerases ε and δ have been known to have the proofreading functions with its 3’ exonuclease activity. However, it is still unclear if PARP-1 recruits other structural-specific Ύ exonucleases to monitor the accuracy of DNA replication in real time and removes the misincorporated nucleotide while the polymerases copy DNA at a high speed. It is also important to know if the roles of these PARP-1-associated nucleases are context-dependent.

Overall, a comprehensive understanding of the crucial roles of PARP-1 and its associated nucleases in DNA repair and cell death is of paramount importance for the development of successful therapeutic regiments for the treatment of different cancers as well as prevention of normal tissue injury.

Abbreviations:

AIF:

Apoptosis-inducing factor

APE1:

Apurinic/apyrimidinic endonuclease

ATM:

Ataxia Telangiectasia Mutated

BER:

Base excision repair

DSB:

Double-stranded break

EXO1:

Exonuclease 1

FEN1:

Flap endonuclease 1

HR:

Homologous recombination

MIF:

Microphage migration inhibitory factor

MRE11:

Meiotic Recombination 11 Homolog 1

NHEJ:

Non-homologous end joining

MNNG:

N-Methyl-N′-nitro-N-nitrosoguanidine

NMDA:

N-methyl-D-aspartate

PAAN:

PARP-1 activity associated nuclease

PARP-1:

Poly (ADP-ribose) (PAR) polymerase-1

PAR:

Poly (ADP-ribose)

PARylation:

Poly (ADP-ribos)ylation

Parthanatos:

PARP-1 dependent cell death

ROS:

Reactive Oxygen Species

SSB:

Single-stranded break

TREX:

3’Repair Exonuclease