Polynucleotide kinase as a potential target for enhancing cytotoxicity by ionizing radiation and topoisomerase I inhibitors

DNA double-strand break genetic variants in patients with premature ovarian insufficiency

Premature ovarian insufficiency (POI) is a clinically heterogeneous disease that may seriously affect the physical and mental health of women of reproductive age. POI primarily manifests as ovarian function decline and endocrine disorders in women prior to age 40 and is an established cause of female infertility. It is crucial to elucidate the causative factors of POI, not only to expand the understanding of ovarian physiology, but also to provide genetic counselling and fertility guidance to affected patients. Factors leading to POI are multifaceted with genetic factors accounting for 7% to 30%. In recent years, an increasing number of DNA damage-repair-related genes have been linked with the occurrence of POI. Among them, DNA double-strand breaks (DSBs), one of the most damaging to DNA, and its main repair methods including homologous recombination (HR) and non-homologous end joining (NHEJ) are of particular interest. Numerous genes are known to be involved in the regulation of programmed DSB formation and damage repair. The abnormal expression of several genes have been shown to trigger defects in the overall repair pathway and induce POI and other diseases. This review summarises the DSB-related genes that may contribute to the development of POI and their potential regulatory mechanisms, which will help to further establish role of DSB in the pathogenesis of POI and provide theoretical guidance for the study of the pathogenesis and clinical treatment of this disease.

Nano-Delivery of a Novel Inhibitor of Polynucleotide Kinase/Phosphatase (PNKP) for Targeted Sensitization of Colorectal Cancer to Radiation-Induced DNA Damage

Inhibition of the DNA repair enzyme polynucleotide kinase/phosphatase (PNKP) increases the sensitivity of cancer cells to DNA damage by ionizing radiation (IR). We have developed a novel inhibitor of PNKP, i.e., A83B4C63, as a potential radio-sensitizer for the treatment of solid tumors. Systemic delivery of A83B4C63, however, may sensitize both cancer and normal cells to DNA damaging therapeutics. Preferential delivery of A83B4C63 to solid tumors by nanoparticles (NP) was proposed to reduce potential side effects of this PNKP inhibitor to normal tissue, particularly when combined with DNA damaging therapies. Here, we investigated the radio-sensitizing activity of A83B4C63 encapsulated in NPs (NP/A83) based on methoxy poly(ethylene oxide)-b-poly(α-benzyl carboxylate-ε-caprolactone) (mPEO-b-PBCL) or solubilized with the aid of Cremophor EL: Ethanol (CE/A83) in human HCT116 colorectal cancer (CRC) models. Levels of γ-H2AX were measured and the biodistribution of CE/A83 and NP/A83 administered intravenously was determined in subcutaneous HCT116 CRC xenografts. The radio-sensitization effect of A83B4C63 was measured following fractionated tumor irradiation using an image-guided Small Animal Radiation Research Platform (SARRP), with 24 h pre-administration of CE/A83 and NP/A83 to Luc⁺/HCT116 bearing mice. Therapeutic effects were analyzed by monitoring tumor growth and functional imaging using Positron Emission Tomography (PET) and [¹⁸F]-fluoro-3’-deoxy-3’-L:-fluorothymidine ([¹⁸F]FLT) as a radiotracer for cell proliferation. The results showed an increased persistence of DNA damage in cells treated with a combination of CE/A83 or NP/A83 and IR compared to those only exposed to IR. Significantly higher tumor growth delay in mice treated with a combination of IR and NP/A83 than those treated with IR plus CE/A83 was observed. [¹⁸F]FLT PET displayed significant functional changes for tumor proliferation for the drug-loaded NP. This observation was attributed to the higher A83B4C63 levels in the tumors for NP/A83-treated mice compared to those treated with CE/A83. Overall, the results demonstrated a potential for A83B4C63-loaded NP as a novel radio-sensitizer for the treatment of CRC.

Patent highlights October-November 2018

A snapshot of noteworthy recent developments in the patent literature of relevance to pharmaceutical and medical research and development.

Domain analysis of PNKP-XRCC1 interactions: Influence of genetic variants of XRCC1

Polynucleotide kinase/phosphatase (PNKP) and X-ray repair cross-complementing 1 (XRCC1) are key proteins in the single-strand DNA break repair pathway. Phosphorylated XRCC1 stimulates PNKP by binding to its forkhead-associated (FHA) domain, whereas nonphosphorylated XRCC1 stimulates PNKP by interacting with the PNKP catalytic domain. Here, we have further studied the interactions between these two proteins, including two variants of XRCC1 (R194W and R280H) arising from single-nucleotide polymorphisms (SNPs) that have been associated with elevated cancer risk in some reports. We observed that interaction of the PNKP FHA domain with phosphorylated XRCC1 extends beyond the immediate, well-characterized phosphorylated region of XRCC1 (residues 515-526). We also found that an XRCC1 fragment, comprising residues 166-436, binds tightly to PNKP and DNA and efficiently activates PNKP's kinase activity. However, interaction of either of the SNP-derived variants of this fragment with PNKP was considerably weaker, and their stimulation of PNKP was severely reduced, although they still could bind DNA effectively. Laser microirradiation revealed reduced recruitment of PNKP to damaged DNA in cells expressing either XRCC1 variant compared with PNKP recruitment in cells expressing WT XRCC1 even though WT and variant XRCC1s were equally efficient at localizing to the damaged DNA. These findings suggest that the elevated risk of cancer associated with these XRCC1 SNPs reported in some studies may be due in part to the reduced ability of these XRCC1 variants to recruit PNKP to damaged DNA.

Persistent 3'-phosphate termini and increased cytotoxicity of radiomimetic DNA double-strand breaks in cells lacking polynucleotide kinase/phosphatase despite presence of an alternative 3'-phosphatase

Polynucleotide kinase/phosphatase (PNKP) has been implicated in non-homologous end joining (NHEJ) of DNA double-strand breaks (DSBs). To assess the consequences of PNKP deficiency for NHEJ of 3'-phosphate-ended DSBs, PNKP-deficient derivatives of HCT116 and of HeLa cells were generated using CRISPR/CAS9. For both cell lines, PNKP deficiency conferred sensitivity to ionizing radiation as well as to neocarzinostatin (NCS), which specifically induces DSBs bearing protruding 3'-phosphate termini. Moreover, NCS-induced DSBs, detected as 53BP1 foci, were more persistent in PNKP ^-/- HCT116 cells compared to their wild-type (WT) counterparts. Surprisingly, PNKP-deficient whole-cell and nuclear extracts were biochemically competent in removing both protruding and recessed 3'-phosphates from synthetic DSB substrates, albeit much less efficiently than WT extracts, suggesting an alternative 3'-phosphatase. Measurements by ligation-mediated PCR showed that PNKP-deficient HeLa cells contained significantly more 3'-phosphate-terminated and fewer 3'-hydroxyl-terminated DSBs than parental cells 5-15 min after NCS treatment, but this difference disappeared by 1 h. These results suggest that, despite presence of an alternative 3'-phosphatase, loss of PNKP significantly sensitizes cells to 3'-phosphate-terminated DSBs, due to a 3'-dephosphorylation defect.

The effector AvrRxo1 phosphorylates NAD in planta

Gram-negative bacterial pathogens of plants and animals employ type III secreted effectors to suppress innate immunity. Most characterized effectors work through modification of host proteins or transcriptional regulators, although a few are known to modify small molecule targets. The Xanthomonas type III secreted avirulence factor AvrRxo1 is a structural homolog of the zeta toxin family of sugar-nucleotide kinases that suppresses bacterial growth. AvrRxo1 was recently reported to phosphorylate the central metabolite and signaling molecule NAD in vitro, suggesting that the effector might enhance bacterial virulence on plants through manipulation of primary metabolic pathways. In this study, we determine that AvrRxo1 phosphorylates NAD in planta, and that its kinase catalytic sites are necessary for its toxic and resistance-triggering phenotypes. A global metabolomics approach was used to independently identify 3'-NADP as the sole detectable product of AvrRxo1 expression in yeast and bacteria, and NAD kinase activity was confirmed in vitro. 3'-NADP accumulated upon transient expression of AvrRxo1 in Nicotiana benthamiana and in rice leaves infected with avrRxo1-expressing strains of X. oryzae. Mutation of the catalytic aspartic acid residue D193 abolished AvrRxo1 kinase activity and several phenotypes of AvrRxo1, including toxicity in yeast, bacteria, and plants, suppression of the flg22-triggered ROS burst, and ability to trigger an R gene-mediated hypersensitive response. A mutation in the Walker A ATP-binding motif abolished the toxicity of AvrRxo1, but did not abolish the 3'-NADP production, virulence enhancement, ROS suppression, or HR-triggering phenotypes of AvrRxo1. These results demonstrate that a type III effector targets the central metabolite and redox carrier NAD in planta, and that this catalytic activity is required for toxicity and suppression of the ROS burst.

Characterization of DNA Substrate Binding to the Phosphatase Domain of the DNA Repair Enzyme Polynucleotide Kinase/Phosphatase

Polynucleotide kinase/phosphatase (PNKP) is a DNA strand break repair enzyme that uses separate 5' kinase and 3' phosphatase active sites to convert damaged 5'-hydroxyl/3'-phosphate strand termini to ligatable 5'-phosphate/3'-hydroxyl ends. The phosphatase active site has received particular attention as a target of inhibition in cancer therapy development. The phosphatase domain dephosphorylates a range of single- and double-stranded substrates; however, structural studies have shown that the phosphatase catalytic cleft can bind only single-stranded substrates. Here we use a catalytically inactive but structurally intact phosphatase mutant to probe interactions between PNKP and a variety of single- and double-stranded DNA substrates using an electrophoretic mobility shift assay. This work indicates that the phosphatase domain binds 3'-phosphorylated single-stranded DNAs in a manner that is highly dependent on the presence of the 3'-phosphate. Double-stranded substrate binding, in contrast, is not as dependent on the 3'-phosphate. Experiments comparing blunt-end, 3'-overhanging, and frayed-end substrates indicate that the predicted loss of energy due to base pair disruption upon binding of the phosphatase active site is likely balanced by favorable interactions between the liberated complementary strand and PNKP. Comparison of the effects on substrate binding of mutations within the phosphatase active site cleft with mutations in surrounding positively charged surfaces suggests that the surrounding surfaces are important for binding to double-stranded substrates. We further show that while fluorescence polarization methods can detect specific binding of single-stranded DNAs with the phosphatase domain, this method does not detect specific interactions between the PNKP phosphatase and double-stranded substrates.

A label-free cyclic assembly of G-quadruplex nanowires for cascade amplification detection of T4 polynucleotide kinase activity and inhibition

Several fluorescence methods have been developed for sensitive detection of PNK activity based on signal amplification techniques, but they need fluorescently labeled DNA probes and superabundant assistant enzymes. We have addressed these limitations and report here a label-free and enzyme-free amplification strategy for sensitively and specifically studying PNK activity and inhibition via hybridization chain reaction (HCR). First, the phosphorylation of hairpin DNA H1 by T4 PNK makes it be specifically digested by lambda exonuclease (λ exo) from 5' to 3' direction to generate a single-stranded initiator which can successively open hairpins H2 and H3 to trigger an autonomous assembly of long DNA nanowires. Meanwhile, an intermolecular G-quadruplex is formed between H2 and H3, thereby providing fluorescence enhancement of N-methyl mesoporphyrin IX (NMM) which is a highly quadruplex-selective fluorophore. So, the PNK activity can be facilely and sensitively detected by using NMM as a signal probe which provides a low background signal to improve the overall sensitivity, resulting in the detection limit of 3.37 × 10(-4) U mL(-1). More importantly, its successful application for detecting PNK activity in a complex biological matrix and studying the inhibition effects of PNK inhibitors demonstrated that it provides a promising platform for screening PNK inhibitors as well as detecting PNK activity. Therefore, it is a highly sensitive, specific, reliable and cost-effective strategy which shows great potential for biological process research, drug discovery, and clinical diagnostics.

Gold(III) macrocycles: nucleotide-specific unconventional catalytic inhibitors of human topoisomerase I

Topoisomerase IB (Top1) is a key eukaryotic nuclear enzyme that regulates the topology of DNA during replication and gene transcription. Anticancer drugs that block Top1 are either well-characterized interfacial poisons or lesser-known catalytic inhibitor compounds. Here we describe a new class of cytotoxic redox-stable cationic Au³⁺ macrocycles which, through hierarchical cluster analysis of cytotoxicity data for the lead compound, 3, were identified as either poisons or inhibitors of Top1. Two pivotal enzyme inhibition assays prove that the compounds are true catalytic inhibitors of Top1. Inhibition of human topoisomerase IIα (Top2α) by 3 was 2 orders of magnitude weaker than its inhibition of Top1, confirming that 3 is a type I-specific catalytic inhibitor. Importantly, Au³⁺ is essential for both DNA intercalation and enzyme inhibition. Macromolecular simulations show that 3 intercalates directly at the 5′-TA-3′ dinucleotide sequence targeted by Top1 via crucial electrostatic interactions, which include π–π stacking and an Au···O contact involving a thymine carbonyl group, resolving the ambiguity of conventional (drug binds protein) vs unconventional (drug binds substrate) catalytic inhibition of the enzyme. Surface plasmon resonance studies confirm the molecular mechanism of action elucidated by the simulations.

Structural basis for the phosphatase activity of polynucleotide kinase/phosphatase on single- and double-stranded DNA substrates

Polynucleotide kinase/phosphatase (PNKP) is a critical mammalian DNA repair enzyme that generates 5'-phosphate and 3'-hydroxyl groups at damaged DNA termini that are required for subsequent processing by DNA ligases and polymerases. The PNKP phosphatase domain recognizes 3'-phosphate termini within DNA nicks, gaps, or at double- or single-strand breaks. Here we present a mechanistic rationale for the recognition of damaged DNA termini by the PNKP phosphatase domain. The crystal structures of PNKP bound to single-stranded DNA substrates reveals a narrow active site cleft that accommodates a single-stranded substrate in a sequence-independent manner. Biochemical studies suggest that the terminal base pairs of double-stranded substrates near the 3'-phosphate are destabilized by PNKP to allow substrate access to the active site. A positively charged surface distinct from the active site specifically facilitates interactions with double-stranded substrates, providing a complex DNA binding surface that enables the recognition of diverse substrates.

Phosphorylation of polynucleotide kinase/ phosphatase by DNA-dependent protein kinase and ataxia-telangiectasia mutated regulates its association with sites of DNA damage

Human polynucleotide kinase/phosphatase (PNKP) is a dual specificity 5'-DNA kinase/3'-DNA phosphatase, with roles in base excision repair, DNA single-strand break repair and non-homologous end joining (NHEJ); yet precisely how PNKP functions in the repair of DNA double strand breaks (DSBs) remains unclear. We demonstrate that PNKP is phosphorylated by the DNA-dependent protein kinase (DNA-PK) and ataxia-telangiectasia mutated (ATM) in vitro. The major phosphorylation site for both kinases was serine 114, with serine 126 being a minor site. Ionizing radiation (IR)-induced phosphorylation of cellular PNKP on S114 was ATM dependent, whereas phosphorylation of PNKP on S126 required both ATM and DNA-PK. Inactivation of DNA-PK and/or ATM led to reduced PNKP at DNA damage sites in vivo. Cells expressing PNKP with alanine or aspartic acid at serines 114 and 126 were modestly radiosensitive and IR enhanced the association of PNKP with XRCC4 and DNA ligase IV; however, this interaction was not affected by mutation of PNKP phosphorylation sites. Purified PNKP protein with mutation of serines 114 and 126 had decreased DNA kinase and DNA phosphatase activities and reduced affinity for DNA in vitro. Together, our results reveal that IR-induced phosphorylation of PNKP by ATM and DNA-PK regulates PNKP function at DSBs.

Tidying up loose ends: the role of polynucleotide kinase/phosphatase in DNA strand break repair

The termini of DNA strand breaks induced by internal and external factors often require processing before missing nucleotides can be replaced by DNA polymerases and the strands rejoined by DNA ligases. Polynucleotide kinase/phosphatase (PNKP) serves a crucial role in the repair of DNA strand breaks by catalyzing the restoration of 5'-phosphate and 3'-hydroxyl termini. It participates in several DNA repair pathways through interactions with other DNA repair proteins, notably XRCC1 and XRCC4. Recent studies have highlighted the physiological importance of PNKP in maintaining the genomic stability of normal tissues, particularly developing neural cells, as well as enhancing the resistance of cancer cells to genotoxic therapeutic agents.

DNA end-processing enzyme polynucleotide kinase as a potential target in the treatment of cancer

Pharmacological inhibition of DNA-repair pathways as an approach for the potentiation of chemo- and radio-therapeutic cancer treatments has attracted increasing levels of interest in recent years. Inhibitors of several enzymes involved in the repair of DNA strand breaks are currently at various stages of the drug development process. Polynucleotide kinase (PNK), a bifunctional DNA-repair enzyme that possesses both 3'-phosphatase and 5'-kinase activities, plays an important role in the repair of both single strand and double strand breaks and as a result, RNAi-mediated knockdown of PNK sensitizes cells to a range of DNA-damaging agents. Recently, a small molecule inhibitor of PNK has been developed that is able to sensitize cells to ionizing radiation and the topoisomerase I poison, camptothecin. Although still in the early stages of development, PNK inhibition represents a promising means of enhancing the efficacy of existing cancer treatments.

Dual modes of interaction between XRCC4 and polynucleotide kinase/phosphatase: implications for nonhomologous end joining

XRCC4 plays a crucial role in the nonhomologous end joining (NHEJ) pathway of DNA double-strand break repair acting as a scaffold protein that recruits other NHEJ proteins to double-strand breaks. Phosphorylation of XRCC4 by protein kinase CK2 promotes a high affinity interaction with the forkhead-associated domain of the end-processing enzyme polynucleotide kinase/phosphatase (PNKP). Here we reveal that unphosphorylated XRCC4 also interacts with PNKP through a lower affinity interaction site within the catalytic domain and that this interaction stimulates the turnover of PNKP. Unexpectedly, CK2-phosphorylated XRCC4 inhibited PNKP activity. Moreover, the XRCC4·DNA ligase IV complex also stimulated PNKP enzyme turnover, and this effect was independent of the phosphorylation of XRCC4 at threonine 233. Our results reveal that CK2-mediated phosphorylation of XRCC4 can have different effects on PNKP activity, with implications for the roles of XRCC4 and PNKP in NHEJ.

The mechanism of double-strand DNA break repair by the nonhomologous DNA end-joining pathway

Double-strand DNA breaks are common events in eukaryotic cells, and there are two major pathways for repairing them: homologous recombination (HR) and nonhomologous DNA end joining (NHEJ). The various causes of double-strand breaks (DSBs) result in a diverse chemistry of DNA ends that must be repaired. Across NHEJ evolution, the enzymes of the NHEJ pathway exhibit a remarkable degree of structural tolerance in the range of DNA end substrate configurations upon which they can act. In vertebrate cells, the nuclease, DNA polymerases, and ligase of NHEJ are the most mechanistically flexible and multifunctional enzymes in each of their classes. Unlike repair pathways for more defined lesions, NHEJ repair enzymes act iteratively, act in any order, and can function independently of one another at each of the two DNA ends being joined. NHEJ is critical not only for the repair of pathologic DSBs as in chromosomal translocations, but also for the repair of physiologic DSBs created during variable (diversity) joining [V(D)J] recombination and class switch recombination (CSR). Therefore, patients lacking normal NHEJ are not only sensitive to ionizing radiation (IR), but also severely immunodeficient.

Beyond Drosophila: RNAi in vivo and functional genomics in insects

The increasing availability of insect genomes has revealed a large number of genes with unknown functions and the resulting problem of how to discover these functions. The RNA interference (RNAi) technique, which generates loss-of-function phenotypes by depletion of a chosen transcript, can help to overcome this challenge. RNAi can unveil the functions of new genes, lead to the discovery of new functions for old genes, and find the genes for old functions. Moreover, the possibility of studying the functions of homologous genes in different species can allow comparisons of the genetic networks regulating a given function in different insect groups, thereby facilitating an evolutionary insight into developmental processes. RNAi also has drawbacks and obscure points, however, such as those related to differences in species sensitivity. Disentangling these differences is one of the main challenges in the RNAi field.

Mechanism of action of an imidopiperidine inhibitor of human polynucleotide kinase/phosphatase

The small molecule, 2-(1-hydroxyundecyl)-1-(4-nitrophenylamino)-6-phenyl-6,7a-dihydro-1H-pyrrolo[3,4-b]pyridine-5,7(2H,4aH)-dione (A12B4C3), is a potent inhibitor of the phosphatase activity of human polynucleotide kinase/phosphatase (PNKP) in vitro. Kinetic analysis revealed that A12B4C3 acts as a noncompetitive inhibitor, and this was confirmed by fluorescence quenching, which showed that the inhibitor can form a ternary complex with PNKP and a DNA substrate, i.e. A12B4C3 does not prevent DNA from binding to the phosphatase DNA binding site. Conformational analysis using circular dichroism, UV difference spectroscopy, and fluorescence resonance energy transfer all indicate that A12B4C3 disrupts the secondary structure of PNKP. Investigation of the potential site of binding of A12B4C3 to PNKP using site-directed mutagenesis pointed to interaction between Trp(402) of PNKP and the inhibitor. Cellular studies revealed that A12B4C3 sensitizes A549 human lung cancer cells to the topoisomerase I poison, camptothecin, but not the topoisomerase II poison, etoposide, in a manner similar to small interfering RNA against PNKP. A12B4C3 also inhibits the repair of DNA single and double strand breaks following exposure of cells to ionizing radiation, but does not inhibit two other key strand-break repair enzymes, DNA polymerase beta or DNA ligase III, providing additional evidence that PNKP is the cellular target of the inhibitor.

Identification of a small molecule inhibitor of the human DNA repair enzyme polynucleotide kinase/phosphatase

Human polynucleotide kinase/phosphatase (hPNKP) is a 57.1-kDa enzyme that phosphorylates DNA 5'-termini and dephosphorylates DNA 3'-termini. hPNKP is involved in both single- and double-strand break repair, and cells depleted of hPNKP show a marked sensitivity to ionizing radiation. Therefore, small molecule inhibitors of hPNKP should potentially increase the sensitivity of human tumors to gamma-radiation. To identify small molecule inhibitors of hPNKP, we modified a novel fluorescence-based assay to measure the phosphatase activity of the protein, and screened a diverse library of over 200 polysubstituted piperidines. We identified five compounds that significantly inhibited hPNKP phosphatase activity. Further analysis revealed that one of these compounds, 2-(1-hydroxyundecyl)-1-(4-nitrophenylamino)-6-phenyl-6,7a-dihydro-1H-pyrrolo[3,4-b]pyridine-5,7(2H,4aH)-dione (A12B4C3), was the most effective, with an IC50 of 0.06 micromol/L. When tested for its specificity, A12B4C3 displayed no inhibition of two well-known eukaryotic protein phosphatases, calcineurin and protein phosphatase-1, or APTX, another human DNA 3'-phosphatase, and only limited inhibition of the related PNKP from Schizosaccharomyces pombe. At a nontoxic dose (1 micromol/L), A12B4C3 enhanced the radiosensitivity of human A549 lung carcinoma and MDA-MB-231 breast adenocarcinoma cells by a factor of two, which was almost identical to the increased sensitivity resulting from shRNA-mediated depletion of hPNKP. Importantly, A12B4C3 failed to increase the radiosensitivity of the hPNKP-depleted cells, implicating hPNKP as the principal cellular target of A12B4C3 responsible for increasing the response to radiation. A12B4C3 is thus a useful reagent for probing hPNKP cellular function and will serve as the lead compound for further development of PNKP-targeting drugs.

Hitting the bull's eye: novel directed cancer therapy through helicase-targeted synthetic lethality

Designing strategies for anti-cancer therapy have posed a significant challenge. One approach has been to inhibit specific DNA repair proteins and their respective pathways to enhance chemotherapy and radiation therapy used to treat cancer patients. Synthetic lethality represents an approach that exploits pre-existing DNA repair deficiencies in certain tumors to develop inhibitors of DNA repair pathways that compensate for the tumor-associated repair deficiency. Since helicases play critical roles in the DNA damage response and DNA repair, particularly in actively dividing and replicating cells, it is proposed that the identification and characterization of synthetic lethal relationships of DNA helicases will be of value in developing improved anti-cancer treatment strategies. In this review, we discuss this hypothesis and current evidence for synthetic lethal interactions of eukaryotic DNA helicases in model systems.

Repair of ionizing radiation-induced DNA double-strand breaks by non-homologous end-joining

DNA DSBs (double-strand breaks) are considered the most cytotoxic type of DNA lesion. They can be introduced by external sources such as IR (ionizing radiation), by chemotherapeutic drugs such as topoisomerase poisons and by normal biological processes such as V(D)J recombination. If left unrepaired, DSBs can cause cell death. If misrepaired, DSBs may lead to chromosomal translocations and genomic instability. One of the major pathways for the repair of IR-induced DSBs in mammalian cells is NHEJ (non-homologous end-joining). The main proteins required for NHEJ in mammalian cells are the Ku heterodimer (Ku70/80 heterodimer), DNA-PKcs [the catalytic subunit of DNA-PK (DNA-dependent protein kinase)], Artemis, XRCC4 (X-ray-complementing Chinese hamster gene 4), DNA ligase IV and XLF (XRCC4-like factor; also called Cernunnos). Additional proteins, including DNA polymerases mu and lambda, PNK (polynucleotide kinase) and WRN (Werner's Syndrome helicase), may also play a role. In the present review, we will discuss our current understanding of the mechanism of NHEJ in mammalian cells and discuss the roles of DNA-PKcs and DNA-PK-mediated phosphorylation in NHEJ.