Telomere proteins POT1, TRF1 and TRF2 augment long-patch base excision repair in vitro

Human CST Stimulates Base Excision Repair to Prevent the Accumulation of Oxidative DNA Damage

CTC1-STN1-TEN1 (CST) is a single-stranded DNA binding protein vital for telomere length maintenance with additional genome-wide roles in DNA replication and repair. While CST was previously shown to function in double-strand break repair and promote replication restart, it is currently unclear whether it has specialized roles in other DNA repair pathways. Proper and efficient repair of DNA is critical to protecting genome integrity. Telomeres and other G-rich regions are strongly predisposed to oxidative DNA damage in the form of 8-oxoguanines, which are typically repaired by the base-excision repair (BER) pathway. Moreover, recent studies suggest that CST functions in the repair of oxidative DNA lesions. Therefore, we tested whether CST interacts with and regulates BER protein activity. Here, we show that CST robustly stimulates proteins involved in BER, including OGG1, Pol β, APE1, and LIGI, on both telomeric and non-telomeric DNA substrates. Biochemical reconstitution of the pathway indicates that CST stimulates BER. Finally, knockout of STN1 or CTC1 leads to increased levels of 8-oxoguanine, suggesting defective BER in the absence of CST. Combined, our results define an undiscovered function of CST in BER, where it acts as a stimulatory factor to promote efficient genome-wide oxidative repair.

Freezing Does Not Alter Sperm Telomere Length despite Increasing DNA Oxidation and Fragmentation

Correlations were reported between sperm telomere length (STL) and male fertility, sperm DNA fragmentation, and oxidation. Sperm freezing is widely used for assisted reproductive techniques, fertility preservation, and sperm donation. However, its impact on STL remains unknown. For this study, semen surplus from patients who underwent routine semen analysis were used. The impact of slow freezing on STL was analyzed by performing qPCR before and after freezing. Sperm populations with different STL were evaluated using Q-FISH. The relationship between sperm DNA oxidation, DNA fragmentation, and STL was assessed in fresh and frozen sperm samples. No significant impact of slow freezing on STL was observed, neither measured by qPCR nor Q-FISH. However, Q-FISH allowed for the distinguishing of sperm populations with different STLs within individual sperm samples. Slow freezing induced different STL distributions for some of the analyzed sperm samples, but no correlation was found between STL and sperm DNA fragmentation or oxidation. Slow freezing does not alter STL despite increasing sperm DNA oxidation and fragmentation. As STL alterations could be transmitted to offspring, the lack of impact of the slow freezing method on STL ensures the safety of this procedure.

Defective DNA polymerase beta invoke a cytosolic DNA mediated inflammatory response

Base excision repair (BER) has evolved to maintain the genomic integrity of DNA following endogenous and exogenous agent induced DNA base damage. In contrast, aberrant BER induces genomic instability, promotes malignant transformation and can even trigger cancer development. Previously, we have shown that deoxyribo-5'-phosphate (dRP) lyase deficient DNA polymerase beta (POLB) causes replication associated genomic instability and sensitivity to both endogenous and exogenous DNA damaging agents. Specifically, it has been established that this loss of dRP lyase function promotes inflammation associated gastric cancer. However, the way that aberrant POLB impacts the immune signaling and inflammatory responses is still unknown. Here we show that a dRP lyase deficient variant of POLB (Leu22Pro, or L22P) increases mitotic dysfunction associated genomic instability, which eventually leads to a cytosolic DNA mediated inflammatory response. Furthermore, poly(ADP-ribose) polymerase 1 inhibition exacerbates chromosomal instability and enhances the cytosolic DNA mediated inflammatory response. Our results suggest that POLB plays a significant role in modulating inflammatory signaling, and they provide a mechanistic basis for future potential cancer immunotherapies.

Plant telomere biology: The green solution to the end-replication problem

Telomere maintenance is a fundamental cellular process conserved across all eukaryotic lineages. Although plants and animals diverged over 1.5 billion years ago, lessons learned from plants continue to push the boundaries of science, revealing detailed molecular mechanisms in telomere biology with broad implications for human health, aging biology, and stress responses. Recent studies of plant telomeres have unveiled unexpected divergence in telomere sequence and architecture, and the proteins that engage telomeric DNA and telomerase. The discovery of telomerase RNA components in the plant kingdom and some algae groups revealed new insight into the divergent evolution and the universal core of telomerase across major eukaryotic kingdoms. In addition, resources cataloging the abundant natural variation in Arabidopsis thaliana, maize (Zea mays), and other plants are providing unparalleled opportunities to understand the genetic networks that govern telomere length polymorphism and, as a result, are uncovering unanticipated crosstalk between telomeres, environmental factors, organismal fitness, and plant physiology. Here we recap current advances in plant telomere biology and put this field in perspective relative to telomere and telomerase research in other eukaryotic lineages.

Telomere Attrition in Induced Pluripotent Stem Cell-Derived Neurons From ALS/FTD-Related C9ORF72 Repeat Expansion Carriers

The GGGGCC (G4C2) repeat expansion in C9ORF72 is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Dysregulated DNA damage response and the generation of reactive oxygen species (ROS) have been postulated as major drivers of toxicity in C9ORF72 pathogenesis. Telomeres are tandem-repeated nucleotide sequences that are located at the end of chromosomes and protect them from degradation. Interestingly, it has been established that telomeres are sensitive to ROS. Here, we analyzed telomere length in neurons and neural progenitor cells from several induced pluripotent stem cell (iPSC) lines from control subjects and C9ORF72 repeat expansion carriers. We found an age-dependent decrease in telomere length in two-month-old iPSC-derived motor neurons from C9ORF72 carriers as compared to control subjects and a dysregulation in the protein levels of shelterin complex members TRF2 and POT1.

APE1/Ref-1 Role in Inflammation and Immune Response

Apurinic/apyrimidinic endonuclease 1/redox effector factor 1 (APE1/Ref-1) is a multifunctional enzyme that is essential for maintaining cellular homeostasis. APE1 is the major apurinic/apyrimidinic endonuclease in the base excision repair pathway and acts as a redox-dependent regulator of several transcription factors, including NF-κB, AP-1, HIF-1α, and STAT3. These functions render APE1 vital to regulating cell signaling, senescence, and inflammatory pathways. In addition to regulating cytokine and chemokine expression through activation of redox sensitive transcription factors, APE1 participates in other critical processes in the immune response, including production of reactive oxygen species and class switch recombination. Furthermore, through participation in active chromatin demethylation, the repair function of APE1 also regulates transcription of some genes, including cytokines such as TNFα. The multiple functions of APE1 make it an essential regulator of the pathogenesis of several diseases, including cancer and neurological disorders. Therefore, APE1 inhibitors have therapeutic potential. APE1 is highly expressed in the central nervous system (CNS) and participates in tissue homeostasis, and its roles in neurodegenerative and neuroinflammatory diseases have been elucidated. This review discusses known roles of APE1 in innate and adaptive immunity, especially in the CNS, recent evidence of a role in the extracellular environment, and the therapeutic potential of APE1 inhibitors in infectious/immune diseases.

The Intra- and Extra-Telomeric Role of TRF2 in the DNA Damage Response

Telomere repeat binding factor 2 (TRF2) has a well-known function at the telomeres, which acts to protect the telomere end from being recognized as a DNA break or from unwanted recombination. This protection mechanism prevents DNA instability from mutation and subsequent severe diseases caused by the changes in DNA, such as cancer. Since TRF2 actively inhibits the DNA damage response factors from recognizing the telomere end as a DNA break, many more studies have also shown its interactions outside of the telomeres. However, very little has been discovered on the mechanisms involved in these interactions. This review aims to discuss the known function of TRF2 and its interaction with the DNA damage response (DDR) factors at both telomeric and non-telomeric regions. In this review, we will summarize recent progress and findings on the interactions between TRF2 and DDR factors at telomeres and outside of telomeres.

Significance of base excision repair to human health

Oxidative and alkylating DNA damage occurs under normal physiological conditions and exogenous exposure to DNA damaging agents. To counteract DNA base damage, cells have evolved several defense mechanisms that act at different levels to prevent or repair DNA base damage. Cells combat genomic lesions like these including base modifications, abasic sites, as well as single-strand breaks, via the base excision repair (BER) pathway. In general, the core BER process involves well-coordinated five-step reactions to correct DNA base damage. In this review, we will uncover the current understanding of BER mechanisms to maintain genomic stability and the biological consequences of its failure due to repair gene mutations. The malfunction of BER can often lead to BER intermediate accumulation, which is genotoxic and can lead to different types of human disease. Finally, we will address the use of BER intermediates for targeted cancer therapy.

Telomere Length and Oxidative Stress and Its Relation with Metabolic Syndrome Components in the Aging

A great amount of scientific evidence supports that Oxidative Stress (OxS) can contribute to telomeric attrition and also plays an important role in the development of certain age-related diseases, among them the metabolic syndrome (MetS), which is characterised by clinical and biochemical alterations such as obesity, dyslipidaemia, arterial hypertension, hyperglycaemia, and insulin resistance, all of which are considered as risk factors for type 2 diabetes mellitus (T2DM) and cardiovascular diseases, which are associated in turn with an increase of OxS. In this sense, we review scientific evidence that supports the association between OxS with telomere length (TL) dynamics and the relationship with MetS components in aging. It was analysed whether each MetS component affects the telomere length separately or if they all affect it together. Likewise, this review provides a summary of the structure and function of telomeres and telomerase, the mechanisms of telomeric DNA repair, how telomere length may influence the fate of cells or be linked to inflammation and the development of age-related diseases, and finally, how the lifestyles can affect telomere length.

Genome (in)stability at tandem repeats

Repeat sequences account for over half of the human genome and represent a significant source of variation that underlies physiological and pathological states. Yet, their study has been hindered due to limitations in short-reads sequencing technology and difficulties in assembly. A important category of repetitive DNA in the human genome is comprised of tandem repeats (TRs), where repetitive units are arranged in a head-to-tail pattern. Compared to other regions of the genome, TRs carry between 10 and 10,000 fold higher mutation rate. There are several mutagenic mechanisms that can give rise to this propensity toward instability, but their precise contribution remains speculative. Given the high degree of homology between these sequences and their arrangement in tandem, once damaged, TRs have an intrinsic propensity to undergo aberrant recombination with non-allelic exchange and generate harmful rearrangements that may undermine the stability of the entire genome. The dynamic mutagenesis at TRs has been found to underlie individual polymorphism associated with neurodegenerative and neuromuscular disorders, as well as complex genetic diseases like cancer and diabetes. Here, we review our current understanding of the surveillance and repair mechanisms operating within these regions, and we describe how alterations in these protective processes can readily trigger mutational signatures found at TRs, ultimately resulting in the pathological correlation between TRs instability and human diseases. Finally, we provide a viewpoint to counter the detrimental effects that TRs pose in light of their selection and conservation, as important drivers of human evolution.

An ordered assembly of MYH glycosylase, SIRT6 protein deacetylase, and Rad9-Rad1-Hus1 checkpoint clamp at oxidatively damaged telomeres

In the base excision repair pathway, MYH/MUTYH DNA glycosylase prevents mutations by removing adenine mispaired with 8-oxoG, a frequent oxidative lesion. MYH glycosylase activity is enhanced by Rad9-Rad1-Hus1 (9-1-1) checkpoint clamp and SIRT6 histone/protein deacetylase. Here, we show that MYH, SIRT6, and 9-1-1 are recruited to confined oxidatively damaged regions on telomeres in mammalian cells. Using different knockout cells, we show that SIRT6 responds to damaged telomeres very early, and then recruits MYH and Hus1 following oxidative stress. However, the recruitment of Hus1 to damaged telomeres is partially dependent on SIRT6. The catalytic activities of SIRT6 are not important for SIRT6 response but are essential for MYH recruitment to damaged telomeres. Compared to wild-type MYH, the recruitment of hMYH^V315A mutant (defective in both SIRT6 and Hus1 interactions), but not hMYH^Q324H mutant (defective in Hus1 interaction only), to damaged telomeres is severely reduced. The formation of MYH/SIRT6/9-1-1 complex is of biological significance as interrupting their interactions can increase cell's sensitivity to H₂O₂ and/or elevate cellular 8-oxoG levels after H₂O₂ treatment. Our results establish that SIRT6 acts as an early sensor of BER enzymes and both SIRT6 and 9-1-1 serve critical roles in DNA repair to maintain telomere integrity.

A Dual Face of APE1 in the Maintenance of Genetic Stability in Monocytes: An Overview of the Current Status and Future Perspectives

Monocytes, which play a crucial role in the immune system, are characterized by an enormous sensitivity to oxidative stress. As they lack four key proteins responsible for DNA damage response (DDR) pathways, they are especially prone to reactive oxygen species (ROS) exposure leading to oxidative DNA lesions and, consequently, ROS-driven apoptosis. Although such a phenomenon is of important biological significance in the regulation of monocyte/macrophage/dendritic cells’ balance, it also a challenge for monocytic mechanisms that have to provide and maintain genetic stability of its own DNA. Interestingly, apurinic/apyrimidinic endonuclease 1 (APE1), which is one of the key proteins in two DDR mechanisms, base excision repair (BER) and non-homologous end joining (NHEJ) pathways, operates in monocytic cells, although both BER and NHEJ are impaired in these cells. Thus, on the one hand, APE1 endonucleolytic activity leads to enhanced levels of both single- and double-strand DNA breaks (SSDs and DSBs, respectively) in monocytic DNA that remain unrepaired because of the impaired BER and NHEJ. On the other hand, there is some experimental evidence suggesting that APE1 is a crucial player in monocytic genome maintenance and stability through different molecular mechanisms, including induction of cytoprotective and antioxidant genes. Here, the dual face of APE1 is discussed.

Back to the future: The intimate and evolving connection between telomere-related factors and genotoxic stress

The conversion of circular genomes to linear chromosomes during molecular evolution required the invention of telomeres. This entailed the acquisition of factors necessary to fulfill two new requirements: the need to fully replicate terminal DNA sequences and the ability to distinguish chromosome ends from damaged DNA. Here we consider the multifaceted functions of factors recruited to perpetuate and stabilize telomeres. We discuss recent theories for how telomere factors evolved from existing cellular machineries and examine their engagement in nontelomeric functions such as DNA repair, replication, and transcriptional regulation. We highlight the remarkable versatility of protection of telomeres 1 (POT1) proteins that was fueled by gene duplication and divergence events that occurred independently across several eukaryotic lineages. Finally, we consider the relationship between oxidative stress and telomeres and the enigmatic role of telomere-associated proteins in mitochondria. These findings point to an evolving and intimate connection between telomeres and cellular physiology and the strong drive to maintain chromosome integrity.

APE1 deficiency promotes cellular senescence and premature aging features

Base excision repair (BER) handles many forms of endogenous DNA damage, and apurinic/apyrimidinic endonuclease 1 (APE1) is central to this process. Deletion of both alleles of APE1 (a.k.a. Apex1) in mice leads to embryonic lethality, and deficiency in cells can promote cell death. Unlike most other BER proteins, APE1 expression is inversely correlated with cellular senescence in primary human fibroblasts. Depletion of APE1 via shRNA induced senescence in normal human BJ fibroblasts, a phenotype that was not seen in counterpart cells expressing telomerase. APE1 knock-down in primary fibroblasts resulted in global DNA damage accumulation, and the induction of p16^INK4a and p21^WAF1 stress response pathways; the DNA damage response, as assessed by γ-H2AX, was particularly pronounced at telomeres. Conditional knock-out of Apex1 in mice at post-natal day 7/12 resulted in impaired growth, reduced organ size, and increased cellular senescence. The effect of Apex1 deletion at post-natal week 6 was less obvious, other than cellular senescence, until ∼8-months of age, when premature aging characteristics, such as hair loss and impaired wound healing, were seen. Low APE1 expression in patient cancer tissue also correlated with increased senescence. Our results point to a key role for APE1 in regulating cellular senescence and aging features, with telomere status apparently affecting the outcome.

Telomere Length: A Potential Biomarker for the Risk and Prognosis of Stroke

Stroke is one of the leading causes of death and disability worldwide. Age is associated with increased risk of stroke, while telomere length shortening plays a pivotal role in the process of aging. Moreover, telomere length shortening is associated with many risk factors of stroke in addition to age. Accumulated evidence shows that short leukocyte telomere length is not only associated with stroke occurrence but also associated with post-stroke recovery in the elderly population. In this review, we aimed to summarize the association between leukocyte telomere length and stroke, and discuss that telomere length might serve as a potential biomarker to predict the risk and prognosis of stroke.

The impact of oxidative DNA damage and stress on telomere homeostasis

Telomeres are dynamic nucleoprotein-DNA structures that cap and protect linear chromosome ends. Because telomeres shorten progressively with each replication, they impose a functional limit on the number of times a cell can divide. Critically short telomeres trigger cellular senescence in normal cells, or genomic instability in pre-malignant cells, which contribute to numerous degenerative and aging-related diseases including cancer. Therefore, a detailed understanding of the mechanisms of telomere loss and preservation is important for human health. Numerous studies have shown that oxidative stress is associated with accelerated telomere shortening and dysfunction. Oxidative stress caused by inflammation, intrinsic cell factors or environmental exposures, contributes to the pathogenesis of many degenerative diseases and cancer. Here we review the studies demonstrating associations between oxidative stress and accelerated telomere attrition in human tissue, mice and cell culture, and discuss possible mechanisms and cellular pathways that protect telomeres from oxidative damage.

Human AP-endonuclease (Ape1) activity on telomeric G4 structures is modulated by acetylatable lysine residues in the N-terminal sequence

Loss of telomeres stability is a hallmark of cancer cells. Exposed telomeres are prone to aberrant end-joining reactions leading to chromosomal fusions and translocations. Human telomeres contain repeated TTAGGG elements, in which the 3' exposed strand may adopt a G-quadruplex (G4) structure. The guanine-rich regions of telomeres are hotspots for oxidation forming 8-oxoguanine, a lesion that is handled by the base excision repair (BER) pathway. One key player of this pathway is Ape1, the main human endonuclease processing abasic sites. Recent evidences showed an important role for Ape1 in telomeric physiology, but the molecular details regulating Ape1 enzymatic activities on G4-telomeric sequences are lacking. Through a combination of in vitro assays, we demonstrate that Ape1 can bind and process different G4 structures and that this interaction involves specific acetylatable lysine residues (i.e. K^27/31/32/35) present in the unstructured N-terminal sequence of the protein. The cleavage of an abasic site located in a G4 structure by Ape1 depends on the DNA conformation or the position of the lesion and on electrostatic interactions between the protein and the nucleic acids. Moreover, Ape1 mutants mimicking the acetylated protein display increased cleavage activity for abasic sites. We found that nucleophosmin (NPM1), which binds the N-terminal sequence of Ape1, plays a role in modulating telomere length and Ape1 activity at abasic G4 structures. Thus, the Ape1 N-terminal sequence is an important relay site for regulating the enzyme's activity on G4-telomeric sequences, and specific acetylatable lysine residues constitute key regulatory sites of Ape1 enzymatic activity dynamics at telomeres.

NEIL3 Repairs Telomere Damage during S Phase to Secure Chromosome Segregation at Mitosis

Oxidative damage to telomere DNA compromises telomere integrity. We recently reported that the DNA glycosylase NEIL3 preferentially repairs oxidative lesions in telomere sequences in vitro. Here, we show that loss of NEIL3 causes anaphase DNA bridging because of telomere dysfunction. NEIL3 expression increases during S phase and reaches maximal levels in late S/G2. NEIL3 co-localizes with TRF2 and associates with telomeres during S phase, and this association increases upon oxidative stress. Mechanistic studies reveal that NEIL3 binds to single-stranded DNA via its intrinsically disordered C terminus in a telomere-sequence-independent manner. Moreover, NEIL3 is recruited to telomeres through its interaction with TRF1, and this interaction enhances the enzymatic activity of purified NEIL3. Finally, we show that NEIL3 interacts with AP Endonuclease 1 (APE1) and the long-patch base excision repair proteins PCNA and FEN1. Taken together, we propose that NEIL3 protects genome stability through targeted repair of oxidative damage in telomeres during S/G2 phase.

Decreased expression of TERT correlated with postnatal cochlear development and proliferation reduction of cochlear progenitor cells

Cochlear progenitor cells are considered as one of the best candidates for hair cell regeneration, thus, the regulation of cochlear progenitor cell proliferation has become a focus in this field. Several genes expressed in the inner ear during postnatal development have been demonstrated to be involved in maintaining the proliferative potential of progenitor cells, but the mechanism for regulating the proliferation and differentiation of cochlear progenitor cells remains poorly understood. Telomerase reverse transcriptase (TERT) has rate limiting telomerase activity and the overexpression of TERT has been shown to promote cell proliferation in series of cell lines. The aim of the present study was to evaluate the expression of TERT in the postnatal development of the cochlea and progenitor cells. The results demonstrated that TERT was expressed in the basilar membranes during the first postnatal week. In vitro, TERT expression in progenitor cells reached a maximum at day 4 after culture and decreased as the culture time prolonged or the cell passage number increased. These results led us to hypothesize that TERT may be involved in the development of the cochlea and in maintaining the proliferation ability of progenitor cells.

Clusters of Multiple Mutations: Incidence and Molecular Mechanisms

It has been long understood that mutation distribution is not completely random across genomic space and in time. Indeed, recent surprising discoveries identified multiple simultaneous mutations occurring in tiny regions within chromosomes while the rest of the genome remains relatively mutation-free. Mechanistic elucidation of these phenomena, called mutation showers, mutation clusters, or kataegis, in parallel with findings of abundant clustered mutagenesis in cancer genomes, is ongoing. So far, the combination of factors most important for clustered mutagenesis is the induction of DNA lesions within unusually long and persistent single-strand DNA intermediates. In addition to being a fascinating phenomenon, clustered mutagenesis also became an indispensable tool for identifying a previously unrecognized major source of mutation in cancer, APOBEC cytidine deaminases. Future research on clustered mutagenesis may shed light onto important mechanistic details of genome maintenance, with potentially profound implications for human health.

Dynamic Interactions of Arabidopsis TEN1: Stabilizing Telomeres in Response to Heat Stress

Telomeres are the essential nucleoprotein structures that provide a physical cap for the ends of linear chromosomes. The highly conserved CST (CTC1/STN1/TEN1) protein complex facilitates telomeric DNA replication and promotes telomere stability. Here we report three unexpected properties of Arabidopsis thaliana TEN1 that indicate it possesses functions distinct from other previously characterized telomere proteins. First, we show that telomeres in ten1 mutants are highly sensitive to thermal stress. Heat shock causes abrupt and dramatic loss of telomeric DNA in ten1 plants, likely via deletional recombination. Second, we show that AtTEN1 has the properties of a heat-shock induced molecular chaperone. At elevated temperature, AtTEN1 rapidly assembles into high molecular weight homo-oligomeric complexes that efficiently suppress heat-induced aggregation of model protein substrates in vitro. Finally, we report that AtTEN1 specifically protects CTC1 from heat-induced aggregation in vitro, and from heat-induced protein degradation and loss of telomere association in vivo. Collectively, these observations define Arabidopsis TEN1 as a highly dynamic protein that works in concert with CTC1 to preserve telomere integrity in response to environmental stress.

DNA damage processing at telomeres: The ends justify the means

Telomeres at chromosome ends are nucleoprotein structures consisting of tandem TTAGGG repeats and a complex of proteins termed shelterin. DNA damage and repair at telomeres is uniquely influenced by the ability of telomeric DNA to form alternate structures including loops and G-quadruplexes, coupled with the ability of shelterin proteins to interact with and regulate enzymes in every known DNA repair pathway. The role of shelterin proteins in preventing telomeric ends from being falsely recognized and processed as DNA double strand breaks is well established. Here we focus instead on recent developments in understanding the roles of shelterin proteins and telomeric DNA sequence and structure in processing genuine damage at telomeres induced by endogenous and exogenous DNA damage agents. We will highlight advances in double strand break repair, base excision repair and nucleotide excision repair at telomeres, and will discuss important questions remaining in the field.

DNA excision repair at telomeres

DNA damage is caused by either endogenous cellular metabolic processes such as hydrolysis, oxidation, alkylation, and DNA base mismatches, or exogenous sources including ultraviolet (UV) light, ionizing radiation, and chemical agents. Damaged DNA that is not properly repaired can lead to genomic instability, driving tumorigenesis. To protect genomic stability, mammalian cells have evolved highly conserved DNA repair mechanisms to remove and repair DNA lesions. Telomeres are composed of long tandem TTAGGG repeats located at the ends of chromosomes. Maintenance of functional telomeres is critical for preventing genome instability. The telomeric sequence possesses unique features that predispose telomeres to a variety of DNA damage induced by environmental genotoxins. This review briefly describes the relevance of excision repair pathways in telomere maintenance, with the focus on base excision repair (BER), nucleotide excision repair (NER), and mismatch repair (MMR). By summarizing current knowledge on excision repair of telomere damage and outlining many unanswered questions, it is our hope to stimulate further interest in a better understanding of excision repair processes at telomeres and in how these processes contribute to telomere maintenance.

BRG1, the ATPase subunit of SWI/SNF chromatin remodeling complex, interacts with HDAC2 to modulate telomerase expression in human cancer cells

Telomerase is often upregulated during initiation and/or progression of human tumors, suggesting that repression of telomerase might inhibit cancer growth or progression. Here, we report that BRG1, the ATPase subunit of the SWI/SNF chromatin remodeling complex, is a general suppressor of hTERT transcription in human cancer cells. While overexpression of BRG1 inhibits hTERT transcription, depletion of BRG1 stimulates transcription of hTERT, leading to higher telomerase activity and longer telomeres. Chromatin-immunoprecipitation assays revealed that BRG1 binds to the transcription start site (TSS) of the hTERT promoter and forms a ternary complex with histone deacetylase 2 (HDAC2). BRG1 remodels chromatin structure to facilitate the action of HDAC2, leading to deacetylation of H3K9ac and H4ac at the TSS and suppression of hTERT transcription. On the other hand, β-catenin binds to the TSS and stimulates hTERT transcription. Thus, BRG1/HDAC2 and β-catenin constitute a manipulative apparatus at the TSS to play opposite but complementary roles in regulating hTERT expression. These results uncover a yin-yang mechanism in modulating hTERT transcription and provide explanation for limited transcription of hTERT in human cancer cells. BRG1/HDAC2 may have a potential as an anti-cancer therapeutic and/or for reactivating cellular proliferative capacity in the context of in vitro tissue engineering.

The basic N-terminal domain of TRF2 limits recombination endonuclease action at human telomeres

The stability of mammalian telomeres depends upon TRF2, which prevents inappropriate repair and checkpoint activation. By using a plasmid integration assay in yeasts carrying humanized telomeres, we demonstrated that TRF2 possesses the intrinsic property to both stimulate initial homologous recombination events and to prevent their resolution via its basic N-terminal domain. In human cells, we further showed that this TRF2 domain prevents telomere shortening mediated by the resolvase-associated protein SLX4 as well as GEN1 and MUS81, 2 different types of endonucleases with resolvase activities. We propose that various types of resolvase activities are kept in check by the basic N-terminal domain of TRF2 in order to favor an accurate repair of the stalled forks that occur during telomere replication.

Targeted DNA damage at individual telomeres disrupts their integrity and triggers cell death

Cellular DNA is organized into chromosomes and capped by a unique nucleoprotein structure, the telomere. Both oxidative stress and telomere shortening/dysfunction cause aging-related degenerative pathologies and increase cancer risk. However, a direct connection between oxidative damage to telomeric DNA, comprising <1% of the genome, and telomere dysfunction has not been established. By fusing the KillerRed chromophore with the telomere repeat binding factor 1, TRF1, we developed a novel approach to generate localized damage to telomere DNA and to monitor the real time damage response at the single telomere level. We found that DNA damage at long telomeres in U2OS cells is not repaired efficiently compared to DNA damage in non-telomeric regions of the same length in heterochromatin. Telomeric DNA damage shortens the average length of telomeres and leads to cell senescence in HeLa cells and cell death in HeLa, U2OS and IMR90 cells, when DNA damage at non-telomeric regions is undetectable. Telomere-specific damage induces chromosomal aberrations, including chromatid telomere loss and telomere associations, distinct from the damage induced by ionizing irradiation. Taken together, our results demonstrate that oxidative damage induces telomere dysfunction and underline the importance of maintaining telomere integrity upon oxidative damage.

Base excision repair: a critical player in many games

This perspective reviews the many dimensions of base excision repair from a 10,000 foot vantage point and provides one person's view on where the field is headed. Enzyme function is considered under the lens of X-ray diffraction and single molecule studies. Base excision repair in chromatin and telomeres, regulation of expression and the role of posttranslational modifications are also discussed in the context of enzyme activities, cellular localization and interacting partners. The specialized roles that base excision repair play in transcriptional activation by active demethylation and targeted oxidation as well as how base excision repair functions in the immune processes of somatic hypermutation and class switch recombination and its possible involvement in retroviral infection are also discussed. Finally the complexities of oxidative damage and its repair and its link to neurodegenerative disorders, as well as the role of base excision repair as a tumor suppressor are examined in the context of damage, repair and aging. By outlining the many base excision repair-related mysteries that have yet to be unraveled, hopefully this perspective will stimulate further interest in the field.

APE1 incision activity at abasic sites in tandem repeat sequences

Repetitive DNA sequences, such as those present in microsatellites and minisatellites, telomeres, and trinucleotide repeats (linked to fragile X syndrome, Huntington disease, etc.), account for nearly 30% of the human genome. These domains exhibit enhanced susceptibility to oxidative attack to yield base modifications, strand breaks, and abasic sites; have a propensity to adopt non-canonical DNA forms modulated by the positions of the lesions; and, when not properly processed, can contribute to genome instability that underlies aging and disease development. Knowledge on the repair efficiencies of DNA damage within such repetitive sequences is therefore crucial for understanding the impact of such domains on genomic integrity. In the present study, using strategically designed oligonucleotide substrates, we determined the ability of human apurinic/apyrimidinic endonuclease 1 (APE1) to cleave at apurinic/apyrimidinic (AP) sites in a collection of tandem DNA repeat landscapes involving telomeric and CAG/CTG repeat sequences. Our studies reveal the differential influence of domain sequence, conformation, and AP site location/relative positioning on the efficiency of APE1 binding and strand incision. Intriguingly, our data demonstrate that APE1 endonuclease efficiency correlates with the thermodynamic stability of the DNA substrate. We discuss how these results have both predictive and mechanistic consequences for understanding the success and failure of repair protein activity associated with such oxidatively sensitive, conformationally plastic/dynamic repetitive DNA domains.

Essential role for mammalian apurinic/apyrimidinic (AP) endonuclease Ape1/Ref-1 in telomere maintenance

The major mammalian apurinic/apyrimidinic endonuclease Ape1 is a multifunctional protein operating in protection of cells from oxidative stress via its DNA repair, redox, and transcription regulatory activities. The importance of Ape1 has been marked by previous work demonstrating its requirement for viability in mammalian cells. However, beyond a requirement for Ape1-dependent DNA repair activity, deeper molecular mechanisms of the fundamental role of Ape1 in cell survival have not been defined. Here, we report that Ape1 is an essential factor stabilizing telomeric DNA, and its deficiency is associated with telomere dysfunction and segregation defects in immortalized cells maintaining telomeres by either the alternative lengthening of telomeres pathway (U2OS) or telomerase expression (BJ-hTERT), or in normal human fibroblasts (IMR90). Through the expression of Ape1 derivatives with site-specific changes, we found that the DNA repair and N-terminal acetylation domains are required for the Ape1 function at telomeres. Ape1 associates with telomere proteins in U2OS cells, and Ape1 depletion causes dissociation of TRF2 protein from telomeres. Consistent with this effect, we also observed that Ape1 depletion caused telomere shortening in both BJ-hTERT and in HeLa cells. Thus, our study describes a unique and unpredicted role for Ape1 in telomere protection, providing a direct link between base excision DNA repair activities and telomere metabolism.

Recruitment of TRF2 to laser-induced DNA damage sites

Several lines of evidence suggest that the telomere-associated protein TRF2 plays critical roles in the DNA damage response. TRF2 is rapidly and transiently phosphorylated by an ATM-dependent pathway in response to DNA damage and this DNA damage-induced phosphoryation is essential for the DNA-PK-dependent pathway of DNA double-strand break repair (DSB). However, the type of DNA damage that induces TRF2 localization to the damage sites, the requirement for DNA damage-induced phosphorylation of TRF2 for its recruitment, as well as the detailed kinetics of TRF2 accumulation at DNA damage sites have not been fully investigated. In order to address these questions, we used an ultrafast femtosecond multiphoton laser and a continuous wave 405-nm single photon laser to induce DNA damage at defined nuclear locations. Our results showed that DNA damage produced by a femtosecond multiphoton laser was sufficient for localization of TRF2 to these DNA damage sites. We also demonstrate that ectopically expressed TRF2 was recruited to DNA lesions created by a 405-nm laser. Our data suggest that ATM and DNA-PKcs kinases are not required for TRF2 localization to DNA damage sites. Furthermore, we found that phosphorylation of TRF2 at residue T188 was not essential for its recruitment to laser-induced DNA damage sites. Thus, we provide further evidence that a protein known to function in telomere maintenance, TRF2, is recruited to sites of DNA damage and plays critical roles in the DNA damage response.