Nucleolar localization and dynamic roles of flap endonuclease 1 in ribosomal DNA replication and damage repair

FEN1 Inhibition as a Potential Novel Targeted Therapy against Breast Cancer and the Prognostic Relevance of FEN1

To improve breast cancer treatment and to enable new strategies for therapeutic resistance, therapeutic targets are constantly being studied. Potential targets are proteins of DNA repair and replication and genomic integrity, such as Flap Endonuclease 1 (FEN1). This study investigated the effects of FEN1 inhibitor FEN1-IN-4 in combination with ionizing radiation on cell death, clonogenic survival, the cell cycle, senescence, doubling time, DNA double-strand breaks and micronuclei in breast cancer cells, breast cells and healthy skin fibroblasts. Furthermore, the variation in the baseline FEN1 level and its influence on treatment prognosis was investigated. The cell lines show specific response patterns in the aspects studied and have heterogeneous baseline FEN1 levels. FEN1-IN-4 has cytotoxic, cytostatic and radiosensitizing effects, expressed through increasing cell death by apoptosis and necrosis, G2M share, senescence, double-strand breaks and a reduced survival fraction. Nevertheless, some cells are less affected by the cytotoxicity and fibroblasts show a rather limited response. In vivo, high FEN1 mRNA expression worsens the prognosis of breast cancer patients. Due to the increased expression in breast cancer tissue, FEN1 could represent a new tumor and prognosis marker and FEN1-IN-4 may serve as a new potent agent in personalized medicine and targeted breast cancer therapy.

Identification of VPS34-PI(3)P-FEN1-mediated DNA repair pathway as a potential drug target to overcome chemoresistance

Intrinsic or acquired chemoresistance represents a major obstacle in cancer treatment. Multiple mechanisms can contribute to cancer cells' resistance to chemotherapy. Among them, an aberrantly strengthened DNA repair mechanism is responsible for a large proportion of drug resistance to alkylating agents and radiation therapy. In cancer cells, damping overactivated DNA repair system can overcome survival advantages conferred by chromosomal translocations or mutations and lead to cytostatic effects or cytotoxic. Therefore, selectively targeting DNA repair system in cancer cells holds promise for overcoming chemoresistance. In this study, we revealed that the endonuclease Flap Endonuclease 1 (FEN1), essential for DNA replication and repair, directly interacts with phosphatidylinositol 3-phosphate [PI(3)P], and FEN1-R378 is the primary PI(3)P-binding site. PI(3)P-binding deficient FEN1 mutant (FEN1-R378A) cells exhibited abnormal chromosomal structures and were hypersensitized to DNA damage. The PI(3)P-mediated FEN1 functionality was essential for repairing DNA damages caused by multiple mechanisms. Furthermore, VPS34, the major PI(3)P synthesizing enzyme, was negatively associated with patients' survival in various cancer types, and VPS34 inhibitors significantly sensitized chemoresistant cancer cells to genotoxic agents. These findings open up an avenue for counteracting chemoresistance by targeting VPS34-PI(3)P-mediated DNA repair pathway, and call for assessing the efficacy of this strategy in patients suffering from chemoresistance-mediated cancer recurrence in clinical trials.

BmNPV Orf 65 (Bm65) Is Identified as an Endonuclease Directly Facilitating UV-Induced DNA Damage Repair

Baculoviruses have been used as biopesticides for the control of Lepidoptera larvae. However, solar UV radiation reduces the activity of baculovirus. In this study, an UV endonuclease, Bm65, was found encoded in the genome of Bombyx mori nuclear polyhedrosis virus (BmNPV). Bm65 (the ortholog of AcMNPV orf79) was guided by a key nuclear localization signal to enter the nucleus and accumulated at UV-induced DNA damage sites. Subsequent results further showed that Bm65-mediated DNA damage repair was not the only UV damage repair pathway of BmNPV. BmNPV also used host DNA repair proteins to repair UV-induced DNA damage. In summary, these results revealed that Bm65 was very important in UV-induced DNA damage repair of BmNPV, and BmNPV repaired UV-damaged DNA through a variety of ways. IMPORTANCE Baculovirus biopesticides are environmentally friendly insecticides and specifically infect invertebrates. UV radiation from the sunlight greatly reduces the activity of baculovirus biopesticides. However, the molecular mechanisms of most baculoviruses to repair UV-induced DNA damage remain unclear. Nucleotide excision repair (NER) is a major DNA repair pathway that removes UV-induced DNA lesions. At present, there are few reports about the nucleotide excision repair pathway in viruses. Here, we showed for the first time that the baculovirus Bm65 endonuclease actually cleaved UV-damaged DNA. Meanwhile, we found that BmNPV used both viral-encoded enzymes and host DNA damage repair proteins to reverse UV-induced DNA damage. These results will provide a reference for the research of UV damage repair of other viruses.

Interaction between 038R and 125R of Cherax quadricarinatus iridovirus (CQIV) and their effects on virus replication

Iridovirids are a group icosahedral viruses containing linear double-stranded DNA, and mainly infect invertebrates and poikilothermic vertebrates. Cherax quadricarinatus iridovirus (CQIV) is a new species of the family Iridoviridae and can cause high mortality in shrimps. In CQIV genome, there are 25 conserved genes and the putative products are involved in several viral processes. In this study, three core protein including CQIV-032R, CQIV-125R and CQIV-160L were identified to interact with CQIV-038R by yeast two-hybrid (Y2H), and the interaction between CQIV-038R and CQIV-125R was further confirmed by co-immunoprecipitation (Co-IP) assays. In the expression system, EGFP-038R and mCherry-125R were colocalized in the cytoplasm when co-expressed in Sf9 cells. Moreover, silencing the expression of 038R, 125R or both of these two proteins respectively in C. quadricarinatus cells by small interfering RNAs showed significantly inhibit CQIV replication. Collectively, we identified the interaction between 038R and 125R, and demonstrated they are essential for CQIV replication.

The prognostic significance of Flap Endonuclease 1 (FEN1) in breast ductal carcinoma in situ

Background

Impaired DNA repair mechanism is one of the cancer hallmarks. Flap Endonuclease 1 (FEN1) is essential for genomic integrity. FEN1 has key roles during base excision repair (BER) and replication. We hypothesised a role for FEN1 in breast cancer pathogenesis. This study aims to assess the role of FEN1 in breast ductal carcinoma in situ (DCIS).

Methods

Expression of FEN1 protein was evaluated in a large (n = 1015) well-characterised cohort of DCIS, comprising pure (n = 776) and mixed (DCIS coexists with invasive breast cancer (IBC); n = 239) using immunohistochemistry (IHC).

Results

FEN1 high expression in DCIS was associated with aggressive and high-risk features including higher nuclear grade, larger tumour size, comedo type necrosis, hormonal receptors negativity, higher proliferation index and triple-negative phenotype. DCIS coexisting with invasive BC showed higher FEN1 nuclear expression compared to normal breast tissue and pure DCIS but revealed significantly lower expression when compared to the invasive component. However, FEN1 protein expression in DCIS was not an independent predictor of local recurrence-free interval.

Conclusion

High FEN1 expression is linked to features of aggressive tumour behaviour and may play a role in the direct progression of DCIS to invasive disease. Further studies are warranted to evaluate its mechanistic roles in DCIS progression and prognosis.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10549-021-06271-y.

Biochemical characterization of a unique DNA polymerase A from the extreme radioresistant organism Deinococcus radiodurans

Deinococcus radiodurans survives extraordinary doses of ionizing radiation and desiccation that cause numerous DNA strand breaks. D. radiodurans DNA polymerase A (DrPolA) is essential for reassembling the shattered genome, while its biochemical property has not been fully demonstrated. In this study, we systematically examined the enzymatic activities of DrPolA and characterized its unique features. DrPolA contains an N-terminal nuclease domain (DrPolA-NTD) and a C-terminal Klenow fragment (KlenDr). Compared with the Klenow fragment of E. coli Pol I, KlenDr shows higher fidelity despite the lacking of 3'-5' exonuclease proofreading activity and prefers double-strand DNA rather than Primer-Template substrates. Apart from the well-annotated 5'-3' exonuclease and flap endonuclease activities, DrPolA-NTD displays approximately 140-fold higher gap endonuclease activity than its homolog in E. coli and Human FEN1. Its 5'-3' exonuclease activity on ssDNA, gap endonuclease, and Holliday junction cleavage activities are greatly enhanced by Mn²⁺. The DrPolA-NTD deficient strain shows increased sensitivity to UV and gamma-ray radiation. Collectively, our results reveal distinct biochemical characteristics of DrPolA during DNA degradation and re-synthesis, which provide new insight into the outstanding DNA repair capacity of D. radiodurans.

Functional regulation of the structure-specific endonuclease FEN1 by the human cytomegalovirus protein IE1 suggests a role for the re-initiation of stalled viral replication forks

Flap endonuclease 1 (FEN1) is a member of the family of structure-specific endonucleases implicated in regulation of DNA damage response and DNA replication. So far, knowledge on the role of FEN1 during viral infections is limited. Previous publications indicated that poxviruses encode a conserved protein that acts in a manner similar to FEN1 to stimulate homologous recombination, double-strand break (DSB) repair and full-size genome formation. Only recently, cellular FEN1 has been identified as a key component for hepatitis B virus cccDNA formation. Here, we report on a novel functional interaction between Flap endonuclease 1 (FEN1) and the human cytomegalovirus (HCMV) immediate early protein 1 (IE1). Our results provide evidence that IE1 manipulates FEN1 in an unprecedented manner: we observed that direct IE1 binding does not only enhance FEN1 protein stability but also phosphorylation at serine 187. This correlates with nucleolar exclusion of FEN1 stimulating its DSB-generating gap endonuclease activity. Depletion of FEN1 and inhibition of its enzymatic activity during HCMV infection significantly reduced nascent viral DNA synthesis demonstrating a supportive role for efficient HCMV DNA replication. Furthermore, our results indicate that FEN1 is required for the formation of DSBs during HCMV infection suggesting that IE1 acts as viral activator of FEN1 in order to re-initiate stalled replication forks. In summary, we propose a novel mechanism of viral FEN1 activation to overcome replication fork barriers at difficult-to-replicate sites in viral genomes.

Fluorescence imaging of intracellular nucleases-A review

Nucleases play crucial roles in maintaining genomic integrity. Visualization of intracellular distribution and translocation of nucleases are of great importance for understanding the in-vivo physiological functions of these enzymes and their roles in DNA repair and other cellular signaling pathways. Here we review the recently developed approaches for fluorescence imaging of nucleases in various eukaryotic cells. We mainly focused on the immunofluorescence techniques, the genetically encoded fluorescent probes and the chemically synthesized fluorescent DNA-substrate probes that enabled in-situ visualization of the subcellular localization of nucleases and their interactions with other protein/DNA molecules within cells. The targeted nucleases included important endonucleases, 3' exonucleases and 5' exonucleases that were involved in the DNA damage repair pathways and the intracellular DNA degradation. The advantages and limitations of the available tools were summarized and discussed.

Transcriptome Analysis of FEN1 Knockdown HEK293T Cell Strain Reveals Alteration in Nucleic Acid Metabolism, Virus Infection, Cell Morphogenesis and Cancer Development

AIM AND OBJECTIVE

Flap endonuclease-1 (FEN1) plays a central role in DNA replication and DNA damage repair process. In mammals, FEN1 functional sites variation is related to cancer and chronic inflammation, and supports the role of FEN1 as a tumor suppressor. However, FEN1 is overexpressed in multiple types of cancer cells and is associated with drug resistance, supporting its role as an oncogene. Hence, it is vital to explore the multi-functions of FEN1 in normal cell metabolic process. This study was undertaken to examine how the gene expression profile changes when FEN1 is downregulated in 293T cells.

MATERIALS AND METHODS

Using the RNA sequencing and real-time PCR approaches, the transcript expression profile of FEN1 knockdown HEK293T cells have been detected for the next step evaluation, analyzation, and validation.

RESULTS

Our results confirmed that FEN1 is important for cell viability. We showed that when FEN1 downregulation led to the interruption of nucleic acids related metabolisms, cell cycle related metabolisms are significantly interrupted. FEN1 may also participate in non-coding RNA processing, ribosome RNA processing, transfer RNA processing, ribosome biogenesis, virus infection and cell morphogenesis.

CONCLUSION

These findings provide insight into how FEN1 nuclease might regulate a wide variety of biological processes, and laid the foundation for understanding the role of other RAD2 family nucleases in cell growth and metabolism.

PRMT1 is critical to FEN1 expression and drug resistance in lung cancer cells

The up-regulation of PRMT1 is critical to the cell growth and cancer progression of lung cancer cells. In our research, we found that PRMT1 is important to the DNA repair ability and drug resistance of lung cancer cells. To demonstrate the functions of PRMT1, we identified Flap endonuclease 1 (FEN1) as a post-translationally modified downstream target protein of PRMT1. As a major component of Base Excision Repair pathway, FEN1 plays an important role in DNA replication and DNA damage repair. However, the detailed mechanism of FEN1 up-regulation in lung cancer cells remains unclear. In our study, we identified PRMT1 as a key factor that maintains the high expression levels of FEN1, which is critical to the DNA repair ability and the chemotherapeutic drug resistance of lung cancer cells.

TRAF6 mediates human DNA2 polyubiquitination and nuclear localization to maintain nuclear genome integrity

The multifunctional human DNA2 (hDNA2) nuclease/helicase is required to process DNA ends for homology-directed recombination repair (HDR) and to counteract replication stress. To participate in these processes, hDNA2 must localize to the nucleus and be recruited to the replication or repair sites. However, because hDNA2 lacks the nuclear localization signal that is found in its yeast homolog, it is unclear how its migration into the nucleus is regulated during replication or in response to DNA damage. Here, we report that the E3 ligase TRAF6 binds to and mediates the K63-linked polyubiquitination of hDNA2, increasing the stability of hDNA2 and promoting its nuclear localization. Inhibiting TRAF6-mediated polyubiquitination abolishes the nuclear localization of hDNA2, consequently impairing DNA end resection and HDR. Thus, the current study reveals a mechanism for the regulation of hDNA2 localization and establishes that TRAF6-mediated hDNA2 ubiquitination activates DNA repair pathways to maintain nuclear genome integrity.

SUMO-1 modification of FEN1 facilitates its interaction with Rad9-Rad1-Hus1 to counteract DNA replication stress

Human flap endonuclease 1 (FEN1) is a structure-specific, multi-functional endonuclease essential for DNA replication and repair. We and others have shown that during DNA replication, FEN1 processes Okazaki fragments via its interaction with the proliferating cell nuclear antigen (PCNA). Alternatively, in response to DNA damage, FEN1 interacts with the PCNA-like Rad9–Rad1–Hus1 complex instead of PCNA to engage in DNA repair activities, such as homology-directed repair of stalled DNA replication forks. However, it is unclear how FEN1 is able to switch between these interactions and its roles in DNA replication and DNA repair. Here, we report that FEN1 undergoes SUMOylation by SUMO-1 in response to DNA replication fork-stalling agents, such as UV irradiation, hydroxyurea, and mitomycin C. This DNA damage-induced SUMO-1 modification promotes the interaction of FEN1 with the Rad9–Rad1–Hus1 complex. Furthermore, we found that FEN1 mutations that prevent its SUMO-1 modification also impair its ability to interact with HUS1 and to rescue stalled replication forks. These impairments lead to the accumulation of DNA damage and heightened sensitivity to fork-stalling agents. Altogether, our findings suggest an important role of the SUMO-1 modification of FEN1 in regulating its roles in DNA replication and repair.

BmNPV infection correlates with the enhancement of the resistance of Bombyx mori cells to UV radiation

At present, the effect of ultraviolet (UV) radiation on the interaction between Bombyx mori nucleopolyhedrovirus (BmNPV) and host remains unclear. In the current study, UV treatment significantly reduced the activity of BmNPV budded viruses (BVs), and UV-damaged BmN cells were not conducive to BmNPV proliferation. BmNPV infection significantly reduced the viability of host cells, but increased the viability of high-dose UV-treated host cells. Furthermore, the quantitative reverse-transcription PCR (qPCR) results suggested that BmNPV and Bombyx mori might mutually use the same DNA repair proteins for repairing UV-induced damage and BmNPV infection promote the ability of host cells to repair UV-induced damage.

Structural basis of 5' flap recognition and protein-protein interactions of human flap endonuclease 1

Human flap endonuclease 1 (hFEN1) is a structure-specific nuclease essential for DNA replication and repair processes. hFEN1 has 5′ flap removal activity, as well as gap endonuclease activity that is critical for restarting stalled replication forks. Here, we report the crystal structures of wild-type and mutant hFEN1 proteins in complex with DNA substrates, followed by mutagenesis studies that provide mechanistic insight into the protein–protein interactions of hFEN1. We found that in an α-helix forming the helical gateway of hFEN1 recognizes the 5′ flap prior to its threading into the active site for cleavage. We also found that the β-pin region is rigidified into a short helix in R192F hFEN1–DNA structures, suppressing its gap endonuclease activity and cycle-dependent kinase interactions. Our findings suggest that a single mutation at the primary methylation site can alter the function of hFEN1 and provide insight into the role of the β-pin region in hFEN1 protein interactions that are essential for DNA replication and repair.

Regulation of Structure-Specific Endonucleases in Replication Stress

Replication stress results in various forms of aberrant replication intermediates that need to be resolved for faithful chromosome segregation. Structure-specific endonucleases (SSEs) recognize DNA secondary structures rather than primary sequences and play key roles during DNA repair and replication stress. Holliday junction resolvase MUS81 (methyl methane sulfonate (MMS), and UV-sensitive protein 81) and XPF (xeroderma pigmentosum group F-complementing protein) are a subset of SSEs that resolve aberrant replication structures. To ensure genome stability and prevent unnecessary DNA breakage, these SSEs are tightly regulated by the cell cycle and replication checkpoints. We discuss the regulatory network that control activities of MUS81 and XPF and briefly mention other SSEs involved in the resolution of replication intermediates.

Iron-Requiring Enzymes in the Spotlight of Oxygen

Iron (Fe) is a cofactor required for a variety of essential redox reactions in plant metabolism. Thus, plants have developed a complex network of interacting pathways to withstand Fe deficiency, including metabolic reprogramming. This opinion aims at revisiting such reprogramming by focusing on: (i) the functional relationships of Fe-requiring enzymes (FeREs) with respect to oxygen; and (ii) the progression of FeREs engagement, occurring under Fe deficiency stress. In particular, we considered such progression of FeREs engagement as strain responses of increasing severity during the stress phases of alarm, resistance, and exhaustion. This approach can contribute to reconcile the variety of experimental results obtained so far from different plant species and/or different Fe supplies.

Control of structure-specific endonucleases to maintain genome stability

Structure-specific endonucleases (SSEs) have key roles in DNA replication, recombination and repair, and emerging roles in transcription. These enzymes have specificity for DNA secondary structure rather than for sequence, and therefore their activity must be precisely controlled to ensure genome stability. In this Review, we discuss how SSEs are controlled as part of genome maintenance pathways in eukaryotes, with an emphasis on the elaborate mechanisms that regulate the members of the major SSE families - including the xeroderma pigmentosum group F-complementing protein (XPF) and MMS and UV-sensitive protein 81 (MUS81)-dependent nucleases, and the flap endonuclease 1 (FEN1), XPG and XPG-like endonuclease 1 (GEN1) enzymes - during processes such as DNA adduct repair, Holliday junction processing and replication stress. We also discuss newly characterized connections between SSEs and other classes of DNA-remodelling enzymes and cell cycle control machineries, which reveal the importance of SSE scaffolds such as the synthetic lethal of unknown function 4 (SLX4) tumour suppressor for the maintenance of genome stability.

Targeting DNA Flap Endonuclease 1 to Impede Breast Cancer Progression

DNA flap endonuclease 1 (FEN1) plays critical roles in maintaining genome stability and integrity by participating in both DNA replication and repair. Suppression of FEN1 in cells leads to the retardation of DNA replication and accumulation of unrepaired DNA intermediates, resulting in DNA double strand breaks (DSBs) and apoptosis. Therefore, targeting FEN1 could serve as a potent strategy for cancer therapy. In this study, we demonstrated that FEN1 is overexpressed in breast cancers and is essential for rapid proliferation of cancer cells. We showed that manipulating FEN1 levels in cells alters the response of cancer cells to chemotherapeutic drugs. Furthermore, we identified a small molecular compound, SC13 that specifically inhibits FEN1 activity, thereby interfering with DNA replication and repair in vitro and in cells. SC13 suppresses cancer cell proliferation and induces chromosome instability and cytotoxicity in cells. Importantly, SC13 sensitizes cancer cells to DNA damage-inducing therapeutic modalities and impedes cancer progression in a mouse model. These findings could establish a paradigm for the treatment of breast cancer and other cancers as well.

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FEN1 is overexpressed in cancer cells and essential for cancer cell growth;

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Down regulation of FEN1 leads to retarded cell growth and sensitizes cancer cells to chemotherapeutic agents;

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SC13, a FEN1 specific inhibitor, inhibits cancer growth in vitro and in xenograft tumor mice.

Most anticancer agents used in clinic today kill cells by interfering DNA replication or inducing DNA damage, which in turn lead to cell apoptosis. However, cancer cells have evolved a compilation of highly effective DNA replication and repair systems to meet up the requirement of rapidly dividing of cancer cells and protect DNA against both endogenous and exogenous DNA damage. FEN1 has been shown to be an important factor in both DNA replication and repair pathways, making FEN1 a logical target for developing anticancer drugs as stand-alone agents for treating cancers that rely on its activity and as a therapy in combination with chemotherapeutic agents that cause DNA damage.

DNA damage-induced inflammation and nuclear architecture

Nuclear architecture and the chromatin state affect most-if not all- DNA-dependent transactions, including the ability of cells to sense DNA lesions and restore damaged DNA back to its native form. Recent evidence points to functional links between DNA damage sensors, DNA repair mechanisms and the innate immune responses. The latter raises the question of how such seemingly disparate processes operate within the intrinsically complex nuclear landscape and the chromatin environment. Here, we discuss how DNA damage-induced immune responses operate within chromatin and the distinct sub-nuclear compartments highlighting their relevance to chronic inflammation.

Role of FEN1 S187 phosphorylation in counteracting oxygen-induced stress and regulating postnatal heart development

Flap endonuclease 1 (FEN1) phosphorylation is proposed to regulate the action of FEN1 in DNA repair as well as Okazaki fragment maturation. However, the biologic significance of FEN1 phosphorylation in response to DNA damage remains unknown. Here, we report an in vivo role for FEN1 phosphorylation, using a mouse line carrying S187A FEN1, which abolishes FEN1 phosphorylation. Although S187A mouse embryonic fibroblast cells showed normal proliferation under low oxygen levels (2%), the mutant cells accumulated oxidative DNA damage, activated DNA damage checkpoints, and showed G₁-phase arrest at atmospheric oxygen levels (21%). This suggests an essential role for FEN1 phosphorylation in repairing oxygen-induced DNA damage and maintaining proper cell cycle progression. Consistently, the mutant cardiomyocytes showed G₁-phase arrest due to activation of the p53-mediated DNA damage response at the neonatal stage, which reduces the proliferation potential of the cardiomyocytes and impairs heart development. Nearly 50% of newborns with the S187A mutant died in the first week due to failure to undergo the peroxisome proliferator-activated receptor signaling-dependent switch from glycolysis to fatty acid oxidation. The adult mutant mice developed dilated hearts and showed significantly shorter life spans. Altogether, our results reveal an important role of FEN1 phosphorylation to counteract oxygen-induced stress in the heart during the fetal-to-neonatal transition.-Zhou, L., Dai, H., Wu, J., Zhou, M., Yuan, H., Du, J., Yang, L., Wu, X., Xu, H., Hua, Y., Xu, J., Zheng, L., Shen, B. Role of FEN1 S187 phosphorylation in counteracting oxygen-induced stress and regulating postnatal heart development.

Requirement for flap endonuclease 1 (FEN1) to maintain genomic stability and transcriptional gene silencing in Arabidopsis

As a central component in the maturation of Okazaki fragments, flap endonuclease 1 (FEN1) removes the 5'-flap and maintains genomic stability. Here, FEN1 was cloned as a suppressor of transcriptional gene silencing (TGS) from a forward genetic screen. FEN1 is abundant in the root and shoot apical meristems and FEN1-GFP shows a nucleolus-localized signal in tobacco cells. The Arabidopsis fen1-1 mutant is hypersensitive to methyl methanesulfonate and shows reduced telomere length. Interestingly, genome-wide chromatin immunoprecipitation and RNA sequencing results demonstrate that FEN1 mutation leads to a decrease in the level of H3K27me3 and an increase in the expression of a subset of genes marked with H3K27me3. Overall, these results uncover a role for FEN1 in mediating TGS as well as maintaining genome stability in Arabidopsis.

The FEN1 L209P mutation interferes with long-patch base excision repair and induces cellular transformation

Flap endonuclease-1 (FEN1) is a multifunctional, structure-specific nuclease that has a critical role in maintaining human genome stability. FEN1 mutations have been detected in human cancer specimens and have been suggested to cause genomic instability and cancer predisposition. However, the exact relationship between FEN1 deficiency and cancer susceptibility remains unclear. In the current work, we report a novel colorectal cancer-associated FEN1 mutation, L209P. This mutant protein lacks the FEN, exonuclease (EXO) and gap endonuclease (GEN) activities of FEN1 but retains DNA-binding affinity. The L209P FEN1 variant interferes with the function of the wild-type FEN1 enzyme in a dominant-negative manner and impairs long-patch base excision repair in vitro and in vivo. Expression of L209P FEN1 sensitizes cells to DNA damage, resulting in endogenous genomic instability and cellular transformation, as well as tumor growth in a mouse xenograft model. These data indicate that human cancer-associated genetic alterations in the FEN1 gene can contribute substantially to cancer development.

Base Excision Repair, a Pathway Regulated by Posttranslational Modifications

Base excision repair (BER) is an essential DNA repair pathway involved in the maintenance of genome stability and thus in the prevention of human diseases, such as premature aging, neurodegenerative diseases, and cancer. Protein posttranslational modifications (PTMs), including acetylation, methylation, phosphorylation, SUMOylation, and ubiquitylation, have emerged as important contributors in controlling cellular BER protein levels, enzymatic activities, protein-protein interactions, and protein cellular localization. These PTMs therefore play key roles in regulating the BER pathway and are consequently crucial for coordinating an efficient cellular DNA damage response. In this review, we summarize the presently available data on characterized PTMs of key BER proteins, the functional consequences of these modifications at the protein level, and also the impact on BER in vitro and in vivo.

Shade avoidance 6 encodes an Arabidopsis flap endonuclease required for maintenance of genome integrity and development

Flap endonuclease-1 (FEN1) belongs to the Rad2 family of structure-specific nucleases. It is required for several DNA metabolic pathways, including DNA replication and DNA damage repair. Here, we have identified a shade avoidance mutant, sav6, which reduces the mRNA splicing efficiency of SAV6. We have demonstrated that SAV6 is an FEN1 homologue that shows double-flap endonuclease and gap-dependent endonuclease activity, but lacks exonuclease activity. sav6 mutants are hypersensitive to DNA damage induced by ultraviolet (UV)-C radiation and reagents that induce double-stranded DNA breaks, but exhibit normal responses to chemicals that block DNA replication. Signalling components that respond to DNA damage are constitutively activated in sav6 mutants. These data indicate that SAV6 is required for DNA damage repair and the maintenance of genome integrity. Mutant sav6 plants also show reduced root apical meristem (RAM) size and defective quiescent centre (QC) development. The expression of SMR7, a cell cycle regulatory gene, and ERF115 and PSK5, regulators of QC division, is increased in sav6 mutants. Their constitutive induction is likely due to the elevated DNA damage responses in sav6 and may lead to defects in the development of the RAM and QC. Therefore, SAV6 assures proper root development through maintenance of genome integrity.

Nucleophosmin modulates stability, activity, and nucleolar accumulation of base excision repair proteins

Nucleophosmin (NPM1) is a multifunctional protein that controls cell growth and genome stability via a mechanism that involves nucleolar-cytoplasmic shuttling. It is clear that NPM1 also contributes to the DNA damage response, yet its exact function is poorly understood. We recently linked NPM1 expression to the functional activation of the major abasic endonuclease in mammalian base excision repair (BER), apurinic/apyrimidinic endonuclease 1 (APE1). Here we unveil a novel role for NPM1 as a modulator of the whole BER pathway by 1) controlling BER protein levels, 2) regulating total BER capacity, and 3) modulating the nucleolar localization of several BER enzymes. We find that cell treatment with the genotoxin cisplatin leads to concurrent relocalization of NPM1 and BER components from nucleoli to the nucleoplasm, and cellular experiments targeting APE1 suggest a role for the redistribution of nucleolar BER factors in determining cisplatin toxicity. Finally, based on the use of APE1 as a representative protein of the BER pathway, our data suggest a function for BER proteins in the regulation of ribogenesis.

Dual control of Yen1 nuclease activity and cellular localization by Cdk and Cdc14 prevents genome instability

The careful orchestration of cellular events such as DNA replication, repair, and segregation is essential for equal distribution of the duplicated genome into two daughter cells. To ensure that persistent recombination intermediates are resolved prior to cell division, the Yen1 Holliday junction resolvase is activated at anaphase. Here, we show that the master cell-cycle regulators, cyclin-dependent kinase (Cdk) and Cdc14 phosphatase, control the actions of Yen1. During S phase, Cdk-mediated phosphorylation of Yen1 promotes its nuclear exclusion and inhibits catalytic activity by reducing the efficiency of DNA binding. Later in the cell cycle, at anaphase, Cdc14 drives Yen1 dephosphorylation, leading to its nuclear relocalization and enzymatic activation. Using a constitutively activated form of Yen1, we show that uncontrolled Yen1 activity is detrimental to the cell: spatial and temporal restriction of Yen1 protects against genotoxic stress and, by avoiding competition with the noncrossover-promoting repair pathways, prevents loss of heterozygosity.

Regulatory mechanisms of RNA function: emerging roles of DNA repair enzymes

The acquisition of an appropriate set of chemical modifications is required in order to establish correct structure of RNA molecules, and essential for their function. Modification of RNA bases affects RNA maturation, RNA processing, RNA quality control, and protein translation. Some RNA modifications are directly involved in the regulation of these processes. RNA epigenetics is emerging as a mechanism to achieve dynamic regulation of RNA function. Other modifications may prevent or be a signal for degradation. All types of RNA species are subject to processing or degradation, and numerous cellular mechanisms are involved. Unexpectedly, several studies during the last decade have established a connection between DNA and RNA surveillance mechanisms in eukaryotes. Several proteins that respond to DNA damage, either to process or to signal the presence of damaged DNA, have been shown to participate in RNA quality control, turnover or processing. Some enzymes that repair DNA damage may also process modified RNA substrates. In this review, we give an overview of the DNA repair proteins that function in RNA metabolism. We also discuss the roles of two base excision repair enzymes, SMUG1 and APE1, in RNA quality control.

Emerging roles of the nucleolus in regulating the DNA damage response: the noncanonical DNA repair enzyme APE1/Ref-1 as a paradigmatical example

SIGNIFICANCE

An emerging concept in DNA repair mechanisms is the evidence that some key enzymes, besides their role in the maintenance of genome stability, display also unexpected noncanonical functions associated with RNA metabolism in specific subcellular districts (e.g., nucleoli). During the evolution of these key enzymes, the acquisition of unfolded domains significantly amplified the possibility to interact with different partners and substrates, possibly explaining their phylogenetic gain of functions.

RECENT ADVANCES

After nucleolar stress or DNA damage, many DNA repair proteins can freely relocalize from nucleoli to the nucleoplasm. This process may represent a surveillance mechanism to monitor the synthesis and correct assembly of ribosomal units affecting cell cycle progression or inducing p53-mediated apoptosis or senescence.

CRITICAL ISSUES

A paradigm for this kind of regulation is represented by some enzymes of the DNA base excision repair (BER) pathway, such as apurinic/apyrimidinic endonuclease 1 (APE1). In this review, the role of the nucleolus and the noncanonical functions of the APE1 protein are discussed in light of their possible implications in human pathologies.

FUTURE DIRECTIONS

A productive cross-talk between DNA repair enzymes and proteins involved in RNA metabolism seems reasonable as the nucleolus is emerging as a dynamic functional hub that coordinates cell growth arrest and DNA repair mechanisms. These findings will drive further analyses on other BER proteins and might imply that nucleic acid processing enzymes are more versatile than originally thought having evolved DNA-targeted functions after a previous life in the early RNA world.

Functional implications of K63-linked ubiquitination in the iron deficiency response of Arabidopsis roots

Iron is an essential micronutrient that plays important roles as a redox cofactor in a variety of processes, many of which are related to DNA metabolism. The E2 ubiquitin conjugase UBC13, the only E2 protein that is capable of catalyzing the formation of non-canonical K63-linked ubiquitin chains, has been associated with the DNA damage tolerance pathway in eukaryotes, critical for maintenance of genome stability and integrity. We previously showed that UBC13 and an interacting E3 ubiquitin ligase, RGLG, affect the differentiation of root epidermal cells in Arabidopsis. When grown on iron-free media, Arabidopsis plants develops root hairs that are branched at their base, a response that has been interpreted as an adaption to reduced iron availability. Mutations in UBC13A abolished the branched root hair phenotype. Unexpectedly, mutations in RGLG genes caused constitutive root hair branching. Based on recent results that link endocytotic turnover of plasma membrane-bound PIN transporters to K63-linked ubiquitination, we reinterpreted our results in a context that classifies the root hair phenotype of iron-deficient plants as a consequence of altered auxin distribution. We show here that UBC13A/B and RGLG1/2 are involved in DNA damage repair and hypothesize that UBC13 protein becomes limited under iron-deficient conditions to prioritize DNA metabolism. The data suggest that genes involved in combating detrimental effects on genome stability may represent essential components in the plant's stress response.

A cryptic targeting signal creates a mitochondrial FEN1 isoform with tailed R-Loop binding properties

A growing number of DNA transacting proteins is found in the nucleus and in mitochondria, including the DNA repair and replication protein Flap endonuclease 1, FEN1. Here we show a truncated FEN1 isoform is generated by alternative translation initiation, exposing a mitochondrial targeting signal. The shortened form of FEN1, which we term FENMIT, localizes to mitochondria, based on import into isolated organelles, immunocytochemistry and subcellular fractionation. In vitro FENMIT binds to flap structures containing a 5' RNA flap, and prefers such substrates to single-stranded RNA. FENMIT can also bind to R-loops, and to a lesser extent to D-loops. Exposing human cells to ethidium bromide results in the generation of RNA/DNA hybrids near the origin of mitochondrial DNA replication. FENMIT is recruited to the DNA under these conditions, and is released by RNase treatment. Moreover, high levels of recombinant FENMIT expression inhibit mtDNA replication, following ethidium bromide treatment. These findings suggest FENMIT interacts with RNA/DNA hybrids in mitochondrial DNA, such as those found at the origin of replication.

Flap endonuclease 1

First discovered as a structure-specific endonuclease that evolved to cut at the base of single-stranded flaps, flap endonuclease (FEN1) is now recognized as a central component of cellular DNA metabolism. Substrate specificity allows FEN1 to process intermediates of Okazaki fragment maturation, long-patch base excision repair, telomere maintenance, and stalled replication fork rescue. For Okazaki fragments, the RNA primer is displaced into a 5' flap and then cleaved off. FEN1 binds to the flap base and then threads the 5' end of the flap through its helical arch and active site to create a configuration for cleavage. The threading requirement prevents this active nuclease from cutting the single-stranded template between Okazaki fragments. FEN1 efficiency and specificity are critical to the maintenance of genome fidelity. Overall, recent advances in our knowledge of FEN1 suggest that it was an ancient protein that has been fine-tuned over eons to coordinate many essential DNA transactions.

Kinetics of endogenous mouse FEN1 in base excision repair

The structure specific flap endonuclease 1 (FEN1) plays an essential role in long-patch base excision repair (BER) and in DNA replication. We have generated a fluorescently tagged FEN1 expressing mouse which allows monitoring the localization and kinetics of FEN1 in response to DNA damage in living cells and tissues. The expression of FEN1, which is tagged at its C-terminal end with enhanced yellow fluorescent protein (FEN1-YFP), is under control of the endogenous Fen1 transcriptional regulatory elements. In line with its role in processing of Okazaki fragments during DNA replication, we found that FEN1-YFP expression is mainly observed in highly proliferating tissue. Moreover, the FEN1-YFP fusion protein allowed us to investigate repair kinetics in cells challenged with local and global DNA damage. In vivo multi-photon fluorescence microscopy demonstrates rapid localization of FEN1 to local laser-induced DNA damage sites in nuclei, providing evidence of a highly mobile protein that accumulates fast at DNA lesion sites with high turnover rate. Inhibition of poly (ADP-ribose) polymerase 1 (PARP1) disrupts FEN1 accumulation at sites of DNA damage, indicating that PARP1 is required for FEN1 recruitment to DNA repair intermediates in BER.

Sequential posttranslational modifications program FEN1 degradation during cell-cycle progression

We propose that cell-cycle-dependent timing of FEN1 nuclease activity is essential for cell-cycle progression and the maintenance of genome stability. After DNA replication is complete at the exit point of the S phase, removal of excess FEN1 may be crucial. Here, we report a mechanism that controls the programmed degradation of FEN1 via a sequential cascade of posttranslational modifications. We found that FEN1 phosphorylation stimulated its SUMOylation, which in turn stimulated its ubiquitination and ultimately led to its degradation via the proteasome pathway. Mutations or inhibitors that blocked the modification at any step in this pathway suppressed FEN1 degradation. Critically, the presence of SUMOylation- or ubiquitination-defective, nondegradable FEN1 mutant protein caused accumulation of Cyclin B, delays in the G1 and G2/M phases, and polyploidy. These findings may represent a newly identified regulatory mechanism used by cells to ensure precise cell-cycle progression and to prevent transformation.

Rad9B responds to nucleolar stress through ATR and JNK signalling, and delays the G1-S transition

The complex formed by Rad9, Rad1 and Hus1 (9-1-1) protects against genomic instability by activating DNA damage checkpoint and DNA damage repair pathways, mainly in response to replication fork collapse and UV lesions. Here we compare the role of Rad9A (also known as Rad9) with the human paralogue Rad9B. Unlike Rad9A, overexpression of Rad9B delays cells in G1 phase. Moreover, Rad9B migrates to nucleoli after nucleolar stress in an ATR- and JNK-dependent manner, in a newly described nucleolar domain structure containing p21. Analysis of chimeras of Rad9A and Rad9B demonstrate that localisation to nucleoli and the block in G1 phase upon overexpression crucially depend on the Rad9B C-terminal tail. Taken together, data presented here show a relationship between Rad9B and pathways for checkpoints, stress response and nucleolar function.

Functional analyses of human DNA repair proteins important for aging and genomic stability using yeast genetics

Model systems have been extremely useful for studying various theories of aging. Studies of yeast have been particularly helpful to explore the molecular mechanisms and pathways that affect aging at the cellular level in the simple eukaryote. Although genetic analysis has been useful to interrogate the aging process, there has been both interest and debate over how functionally conserved the mechanisms of aging are between yeast and higher eukaryotes, especially mammalian cells. One area of interest has been the importance of genomic stability for age-related processes, and the potential conservation of proteins and pathways between yeast and human. Translational genetics have been employed to examine the functional roles of mammalian proteins using yeast as a pliable model system. In the current review recent advancements made in this area are discussed, highlighting work which shows that the cellular functions of human proteins in DNA repair and maintenance of genomic stability can be elucidated by genetic rescue experiments performed in yeast.

Regulation of base excision repair in eukaryotes by dynamic localization strategies

This chapter discusses base excision repair (BER) and the known mechanisms defined thus far regulating BER in eukaryotes. Unlike the situation with nucleotide excision repair and double-strand break repair, little is known about how BER is regulated to allow for efficient and accurate repair of many types of DNA base damage in both nuclear and mitochondrial genomes. Regulation of BER has been proposed to occur at multiple, different levels including transcription, posttranslational modification, protein-protein interactions, and protein localization; however, none of these regulatory mechanisms characterized thus far affect a large spectrum of BER proteins. This chapter discusses a recently discovered mode of BER regulation defined in budding yeast cells that involves mobilization of DNA repair proteins to DNA-containing organelles in response to genotoxic stress.

The wonders of flap endonucleases: structure, function, mechanism and regulation

Processing of Okazaki fragments to complete lagging strand DNA synthesis requires coordination among several proteins. RNA primers and DNA synthesised by DNA polymerase α are displaced by DNA polymerase δ to create bifurcated nucleic acid structures known as 5'-flaps. These 5'-flaps are removed by Flap Endonuclease 1 (FEN), a structure-specific nuclease whose divalent metal ion-dependent phosphodiesterase activity cleaves 5'-flaps with exquisite specificity. FENs are paradigms for the 5' nuclease superfamily, whose members perform a wide variety of roles in nucleic acid metabolism using a similar nuclease core domain that displays common biochemical properties and structural features. A detailed review of FEN structure is undertaken to show how DNA substrate recognition occurs and how FEN achieves cleavage at a single phosphate diester. A proposed double nucleotide unpairing trap (DoNUT) is discussed with regards to FEN and has relevance to the wider 5' nuclease superfamily. The homotrimeric proliferating cell nuclear antigen protein (PCNA) coordinates the actions of DNA polymerase, FEN and DNA ligase by facilitating the hand-off intermediates between each protein during Okazaki fragment maturation to maximise through-put and minimise consequences of intermediates being released into the wider cellular environment. FEN has numerous partner proteins that modulate and control its action during DNA replication and is also controlled by several post-translational modification events, all acting in concert to maintain precise and appropriate cleavage of Okazaki fragment intermediates during DNA replication.

The DNA damage effector Chk1 kinase regulates Cdc14B nucleolar shuttling during cell cycle progression

Chk1 is a critical effector of DNA damage checkpoints necessary for the maintenance of chromosome integrity during cell cycle progression. Here we report, that Chk1 co-localized with the nucleolar marker, fibrillarin in response to radiation-induced DNA damage in human cells. Interestingly, in vitro studies using GST pull down assays identified the dual-specificity serine/threonine nucleolar phosphatase Cdc14B as a Chk1 substrate. Furthermore, Chk1, but not a kinase-dead Chk1 control, was shown to phosphorylate Cdc14B using an in vitro kinase assay. Co-immunoprecipitation experiments using exogenous Cdc14B transfected into human cells confirmed the interaction of Cdc14B and Chk1 during cell cycle. In addition, reduction of Chk1 levels via siRNA or UCN-01 treatment demonstrated that Chk1 activation following DNA damage was required for Cdc14B export from the nucleolus. These studies have revealed a novel interplay between Chk1 kinase and Cdc14B phosphatase involving radiation-induced nucleolar shuttling to facilitate error-free cell cycle progression and prevent genomic instability.

Oxygen as a friend and enemy: How to combat the mutational potential of 8-oxo-guanine

The maintenance of genetic stability is of crucial importance for any form of life. Prior to cell division in each mammalian cell, the process of DNA replication must faithfully duplicate the three billion bases with an absolute minimum of mistakes. Various environmental and endogenous agents, such as reactive oxygen species (ROS), can modify the structural properties of DNA bases and thus damage the DNA. Upon exposure of cells to oxidative stress, an often generated and highly mutagenic DNA damage is 7,8-dihydro-8-oxo-guanine (8-oxo-G). The estimated steady-state level of 8-oxo-G lesions is about 10(3) per cell/per day in normal tissues and up to 10(5) lesions per cell/per day in cancer tissues. The presence of 8-oxo-G on the replicating strand leads to frequent (10-75%) misincorporations of adenine opposite the lesion (formation of A:8-oxo-G mispairs), subsequently resulting in C:G to A:T transversion mutations. These mutations are among the most predominant somatic mutations in lung, breast, ovarian, gastric and colorectal cancers. Thus, in order to reduce the mutational burden of ROS, human cells have evolved base excision repair (BER) pathways ensuring (i) the correct and efficient repair of A:8-oxo-G mispairs and (ii) the removal of 8-oxo-G lesions from the genome. Very recently it was shown that MutY glycosylase homologue (MUTYH) and DNA polymerase lambda play a crucial role in the accurate repair of A:8-oxo-G mispairs. Here we review the importance of accurate BER of 8-oxo-G damage and its regulation in prevention of cancer.

Functional regulation of FEN1 nuclease and its link to cancer

Flap endonuclease-1 (FEN1) is a member of the Rad2 structure-specific nuclease family. FEN1 possesses FEN, 5′-exonuclease and gap-endonuclease activities. The multiple nuclease activities of FEN1 allow it to participate in numerous DNA metabolic pathways, including Okazaki fragment maturation, stalled replication fork rescue, telomere maintenance, long-patch base excision repair and apoptotic DNA fragmentation. Here, we summarize the distinct roles of the different nuclease activities of FEN1 in these pathways. Recent biochemical and genetic studies indicate that FEN1 interacts with more than 30 proteins and undergoes post-translational modifications. We discuss how FEN1 is regulated via these mechanisms. Moreover, FEN1 interacts with five distinct groups of DNA metabolic proteins, allowing the nuclease to be recruited to a specific DNA metabolic complex, such as the DNA replication machinery for RNA primer removal or the DNA degradosome for apoptotic DNA fragmentation. Some FEN1 interaction partners also stimulate FEN1 nuclease activities to further ensure efficient action in processing of different DNA structures. Post-translational modifications, on the other hand, may be critical to regulate protein–protein interactions and cellular localizations of FEN1. Lastly, we also review the biological significance of FEN1 as a tumor suppressor, with an emphasis on studies of human mutations and mouse models.

Methylation of FEN1 suppresses nearby phosphorylation and facilitates PCNA binding

Flap endonuclease 1 (FEN1), a structure-specific endo- and exonuclease, has multiple functions that determine essential biological processes, such as cell proliferation and cell death. As such, the enzyme must be precisely regulated to execute each of its functions with the right timing and in a specific subcellular location. Here we report that FEN1 is methylated at arginine residues, primarily at Arg192. The methylation suppresses FEN1 phosphorylation at Ser187. The methylated form, but not the phosphorylated form, of FEN1 strongly interacts with proliferating cell nuclear antigen (PCNA), ensuring the 'on' and 'off' timing of its reaction. Mutations of FEN1 disrupting arginine methylation and PCNA interaction result in unscheduled phosphorylation and a failure to localize to DNA replication or repair foci. This consequently leads to a defect in Okazaki fragment maturation, a delay in cell cycle progression, impairment of DNA repair and a high frequency of genome-wide mutations.

BRCA1 protein and nucleolin colocalize in breast carcinoma tissue and cancer cell lines

The breast and ovarian cancer susceptibility gene BRCA1 encodes a tumor suppressor. BRCA1 protein, which is involved in DNA damage response, has been thought to be found primarily in cell nuclei. In the present investigation, immunohistological studies of BRCA1 protein in frozen breast cancer tissue and MCF7 and HeLa cell lines revealed BRCA1 expression in both nucleoli and nucleoplasmic foci. Immunoelectron microscopic studies of estrogen-stimulated MCF7 cells demonstrated BRCA1 protein localization in the granular components of the nucleolus. Moreover, immunofluorescence of BRCA1 and nucleolin double-labeling showed colocalization in both nucleoli and nucleoplasmic foci in breast tumor cells and asynchronously growing MCF7 and HeLa cells. Multiparameter analysis of BRCA1 and nucleolin in relation to cell cycle position (DNA content) showed expression during G1-S and persistence of BRCA1 during G2/M. After gamma-irradiation of MCF7 cells, BRCA1 protein dispersed from nucleoli and nucleoplasmic foci to other nucleoplasmic sites, which did not colocalize with nucleolin. Small interfering RNA-mediated knockdown of BRCA1 protein resulted in decreased immunofluorescence staining, which was confirmed by Western blotting. The observed colocalization of BRCA1 and nucleolin raises new possibilities for the nucleoplasm-nucleolus pathways of these proteins and their functional significance.

Distinct activities of exonuclease 1 and flap endonuclease 1 at telomeric g4 DNA

Background

Exonuclease 1 (EXO1) and Flap endonuclease 1 (FEN1) are members of the RAD2 family of structure-specific nucleases. Genetic analysis has identified roles for EXO1 and FEN1 in replication, recombination, DNA repair and maintenance of telomeres. Telomeres are composed of G-rich repeats that readily form G4 DNA. We recently showed that human EXO1 and FEN1 exhibit distinct activities on G4 DNA substrates representative of intermediates in immunoglobulin class switch recombination.

Methodology/Principal Findings

We have now compared activities of these enzymes on telomeric substrates bearing G4 DNA, identifying non-overlapping functions that provide mechanistic insight into the distinct telomeric phenotypes caused by their deficiencies. We show that hFEN1 but not hEXO1 cleaves substrates bearing telomeric G4 DNA 5′-flaps, consistent with the requirement for FEN1 in telomeric lagging strand replication. Both hEXO1 and hFEN1 are active on substrates bearing telomeric G4 DNA tails, resembling uncapped telomeres. Notably, hEXO1 but not hFEN1 is active on transcribed telomeric G-loops.

Conclusion/Significance

Our results suggest that EXO1 may act at transcription-induced telomeric structures to promote telomere recombination while FEN1 has a dominant role in lagging strand replication at telomeres. Both enzymes can create ssDNA at uncapped telomere ends thereby contributing to recombination.