DNA damage response at functional and dysfunctional telomeres

The relationship between neuropsychiatric disorders and aging: A review on telomere length, oxidative stress, and inflammation

Aging is the group of time-independent changes that occur in an organism and that ultimately end in death. The relationship between aging and neuropsychiatric disorders is complex. Not only does the incidence of several neuropsychiatric disorders rise with age, but also these disorders are linked with premature mortality and are even thought to be syndromes of accelerated biological aging. Oxidative stress, inflammation and telomere length are factors commonly used to assess biological aging. The purpose of this review is to sum up the existing information about the state of those factors in schizophrenia, depression, bipolar disorder and anxiety disorders, and to summarize the effects of treatment on telomere length in patients with those neuropsychiatric disorders. The main focus, however, is on telomere length seeing the highly controversial study results on this biomarker in neuropsychiatric disorders. There is no scientific consensus on the state of those factors in the mentioned neuropsychiatric disorders or on the effects of treatment on telomere length, thus further research is needed where confounding variables are controlled. Regarding telomere length, it is highly important to explore whether short telomeres lead to the development of neuropsychiatric disorders or vice versa, as it carries huge clinical potential.

The crosstalk between telomeres and DNA repair mechanisms: an overview to mammalian somatic cells, germ cells, and preimplantation embryos

Telomeres are located at the ends of linear chromosomes and play a critical role in maintaining genomic stability by preventing premature activation of DNA repair mechanisms. Because of exposure to various genotoxic agents, telomeres can undergo shortening and genetic changes. In mammalian cells, the basic DNA repair mechanisms, including base excision repair, nucleotide excision repair, double-strand break repair, and mismatch repair, function in repairing potential damages in telomeres. If these damages are not repaired correctly in time, the unfavorable results such as apoptosis, cell cycle arrest, and cancerous transition may occur. During lifespan, mammalian somatic cells, male and female germ cells, and preimplantation embryos experience a number of telomeric damages. Herein, we comprehensively reviewed the crosstalk between telomeres and the DNA repair mechanisms in the somatic cells, germ cells, and embryos. Infertility development resulting from possible defects in this crosstalk is also discussed in the light of existing studies.

Leukocyte telomere length mediates the association between cadmium exposure and cognitive function in US older adults

BACKGROUND

Long-term exposure to cadmium-polluted environments may lead to shortened leukocyte telomere length and cognitive decline. This study aims to investigate (1) the associations among blood cadmium levels, leukocyte telomere length, and cognitive function, and (2) the mediating role of leukocyte telomere length between blood cadmium levels and cognitive function among older adults in the United States.

METHODS

Using data from the National Health and Nutrition Examination Survey (NHANES) 1999-2002. Cadmium exposure level was assessed by measuring cadmium levels in blood samples. Leukocyte telomere length was measured by quantitative polymerase chain reaction, and cognitive function was measured by the digit symbol substitution test (DSST).

RESULTS

A total of 2185 older adults aged over 60 were included in this study, comprising 1109 (49.65%) males. Elevated blood cadmium levels were significantly associated with the risk of a decline in cognitive function (β = - 2.842, p = 0.018). Shorter leukocyte telomere lengths were significantly associated with a higher risk of a decline in cognitive function (β = 4.144, p = 0.020). The total indirect effect on the blood cadmium level and cognitive function via leukocyte telomere length was - 0.218 (p = 0.012). The mediation effect was estimated to be 0.218/2.084 × 100% = 10.46%.

CONCLUSION

The findings suggest that cadmium exposure may increase the risk of cognitive impairment by causing shortened leukocyte telomere length.

Mechanical stress abnormalities promote chondrocyte senescence - The pathogenesis of knee osteoarthritis

Knee osteoarthritis (KOA) is a common chronic disease in orthopedics, which brings great pain to patients' life and spirit. Therefore, it is necessary to elucidate the pathogenesis of KOA. The pathophysiology of KOA has been linked to numerous factors, including oxidative stress, apoptosis, cellular senescence, mitochondrial dysfunction, and inflammatory factors. Cellular senescence has grown in importance as a topic of study for age-related illnesses recently. KOA has also been discovered to be closely related to human aging, a process in which chondrocyte senescence may be crucial. Numerous researches have looked at the pathogenesis of KOA from the perspectives of mechanical stress abnormalities, oxidative stress, inflammatory overexpression, and mitochondrial dysfunction. Many studies have discovered that the primary pathogenesis of KOA is inflammatory overexpression and chondrocyte death brought on by an imbalance in the joint microenvironment. And abnormal mechanical stress is the initiating cause of oxidative stress, inflammation, and mitochondrial disorders. However, few findings have been reported in the literature on the relationship between these factors, especially for mechanical stress abnormalities, and chondrocyte senescence. This time, in order to better understand the pathogenesis of KOA and identify potential connections between chondrocyte senescence and these microenvironments in KOA, as well as oxidative stress, inflammatory overexpression, and mitochondrial dysfunction microenvironmental dysfunctions, we will use chondrocyte senescence as a starting point. This will allow us to develop new therapeutic approaches for KOA.

Potential effects of assisted reproductive technology on telomere length and telomerase activity in human oocytes and early embryos

Telomeres are repetitive DNA sequences at eukaryotic chromosome ends and function in maintaining genome integrity and stability. These unique structures undergo shortening due to various factors including biological aging, consecutive DNA replication, oxidative stress, and genotoxic agents. Shortened telomeres can be lengthened by the enzyme telomerase and alternative lengthening of telomeres in germ cells, early embryos, stem cells, and activated lymphocytes. If telomeres reach to critical length, it may lead to genomic instability, chromosome segregation defects, aneuploidy, and apoptosis. These phenotypes also occur in the oocytes and early embryos, produced using assisted reproductive technologies (ARTs). Thus, a number of studies have examined the potential effects of ART applications such as ovarian stimulation, culture conditions, and cryopreservation procedures on telomeres. Herein, we comprehensively reviewed impacts of these applications on telomere length and telomerase activity in ART-derived oocytes and embryos. Further, we discussed use of these parameters in ART centers as a biomarker in determining oocyte and embryo quality.

Early Environment and Telomeres: a Long-Term Toxic Relationship

PURPOSE OF REVIEW

Telomere length (TL) shortening is a hallmark of biological aging. While studies have extensively focused on the impact of environmental exposures on TL in older populations, consistent evidence indicates that prenatal environmental exposures to air pollutants, polycyclic aromatic hydrocarbons, metals, and endocrine-disrupting chemicals influence TL shortening. Here, we summarize evidence linking prenatal environmental exposures with children's TL and discuss potential long-term effects.

RECENT FINDINGS

Current evidence shows that prenatal environmental exposures alter TL and identify pregnancy as a critical window of susceptibility for telomere damage in children. However, results vary across studies, possibly depending on the source, exposure time window, and stage evaluated. Additional research is needed to investigate whether early TL alterations mediate long-term health effects of offspring. Prenatal environmental exposures induce early childhood changes in TL. Based on known links between TL and biological aging, these alterations may have long-term impact on individuals' health throughout life.

Close Ties between the Nuclear Envelope and Mammalian Telomeres: Give Me Shelter

The nuclear envelope (NE) in eukaryotic cells is essential to provide a protective compartment for the genome. Beside its role in connecting the nucleus with the cytoplasm, the NE has numerous important functions including chromatin organization, DNA replication and repair. NE alterations have been linked to different human diseases, such as laminopathies, and are a hallmark of cancer cells. Telomeres, the ends of eukaryotic chromosomes, are crucial for preserving genome stability. Their maintenance involves specific telomeric proteins, repair proteins and several additional factors, including NE proteins. Links between telomere maintenance and the NE have been well established in yeast, in which telomere tethering to the NE is critical for their preservation and beyond. For a long time, in mammalian cells, except during meiosis, telomeres were thought to be randomly localized throughout the nucleus, but recent advances have uncovered close ties between mammalian telomeres and the NE that play important roles for maintaining genome integrity. In this review, we will summarize these connections, with a special focus on telomere dynamics and the nuclear lamina, one of the main NE components, and discuss the evolutionary conservation of these mechanisms.

Understanding the Impact of Obesity on Ageing in the Radiance of DNA Metabolism

Ageing is a multi-factorial phenomenon which is considered as a major risk factor for the development of neurodegeneration, osteoporosis, cardiovascular disease, dementia, cancer, and other chronic diseases. Phenotypically, ageing is related with a combination of molecular, cellular, and physiological levels like genomic and epi-genomic alterations, loss of proteostasis, deregulation of cellular and subcellular function and mitochondrial dysfunction. Though, no single molecular mechanism accounts for the functional decline of different organ systems in older humans but accumulation of DNA damage or mutations is a dominant theory which contributes largely to the development of ageing and age-related diseases. However, mechanistic, and hierarchical order of these features of ageing has not been clarified yet. Scientific community now focus on the effect of obesity on accelerated ageing process. Obesity is a complex chronic disease that affects multiple organs and tissues. It can not only lead to various health conditions such as diabetes, cancer, and cardiovascular disease but also can decrease life expectancy which shows similar phenotype of ageing. Higher loads of DNA damage were also observed in the genome of obese people. Thus, inability of DNA damage repair may contribute to both ageing and obesity apart from cancer predisposition. The present review emphasizes on the involvement of molecular phenomenon of DNA metabolism in development of obesity and how it accelerates ageing in mammals.

Telomeric chromatin structure

Eukaryotic DNA is packaged into nucleosomes, which further condenses into chromosomes. The telomeres, which form the protective end-capping of chromosomes, play a pivotal role in ageing and cancer. Recently, significant advances have been made in understanding the nucleosomal and telomeric chromatin structure at the molecular level. In addition, recent studies shed light on the nucleosomal organisation at telomeres revealing its ultrastructural organisation, the atomic structure at the nucleosome level, its dynamic properties, and higher-order packaging of telomeric chromatin. Considerable advances have furthermore been made in understanding the structure, function and organisation of shelterin, telomerase and CST complexes. Here we discuss these recent advances in the organisation of telomeric nucleosomes and chromatin and highlight progress in the structural understanding of shelterin, telomerase and CST complexes.

Artificial chromosome technology and its potential application in plants

Plant genetic engineering and transgenic technology are powerful ways to study the function of genes and improve crop yield and quality in the past few years. However, only a few genes could be transformed by most available genetic engineering and transgenic technologies, so changes still need to be made to meet the demands for high throughput studies, such as investigating the whole genetic pathway of crop traits and avoiding undesirable genes simultaneously in the next generation. Plant artificial chromosome (PAC) technology provides a carrier which allows us to assemble multiple and specific genes to produce a variety of products by minichromosome. However, PAC technology also have limitations that may hinder its further development and application. In this review, we will introduce the current state of PACs technology from PACs formation, factors on PACs formation, problems and potential solutions of PACs and exogenous gene(s) integration.

Columnar structure of human telomeric chromatin

Telomeres, the ends of eukaryotic chromosomes, play pivotal parts in ageing and cancer and are targets of DNA damage and the DNA damage response^1-5. Little is known about the structure of telomeric chromatin at the molecular level. Here we used negative stain electron microscopy and single-molecule magnetic tweezers to characterize 3-kbp-long telomeric chromatin fibres. We also obtained the cryogenic electron microscopy structure of the condensed telomeric tetranucleosome and its dinucleosome unit. The structure displayed close stacking of nucleosomes with a columnar arrangement, and an unusually short nucleosome repeat  length that comprised about 132 bp DNA wound in a continuous superhelix around histone octamers. This columnar structure is primarily stabilized by the H2A carboxy-terminal and histone amino-terminal tails in a synergistic manner. The columnar conformation results in exposure of the DNA helix, which may make it susceptible to both DNA damage and the DNA damage response. The conformation also exists in an alternative open state, in which one nucleosome is unstacked and flipped out, which exposes the acidic patch of the histone surface. The structural features revealed in this work suggest mechanisms by which protein factors involved in telomere maintenance can access telomeric chromatin in its compact form.

Telomere attrition and inflammation: the chicken and the egg story

The challenge to improve human life span has progressed with the advent of health care services and technologies. This improvement poses a new challenge of an associated wave of diseases and pathologies that have not been observed or experienced. This has led to rise in geriatric population who are currently facing health challenges that needs to be addressed by the research community. This review focuses primarily on two mechanisms that have contributed to aging and associated pathologies: telomere attrition and inflammatory insults. A strong interplay appears to exist between telomere attrition and inflammation, and this could be the basis of many pathologies associated with increasing age. This creates a scientific dilemma as to what comes first: telomere attrition or inflammation. This review will enthuse the reader to the underlying molecules and mechanisms associated with telomere attrition and inflammation and their contribution to aging.

Telomeric Repeat-Containing RNA (TERRA): A Review of the Literature and First Assessment in Cutaneous T-Cell Lymphomas

Telomeric Repeat-containing RNA (TERRA) are long non-coding RNAs transcribed from telomeric DNA sequences from multiple chromosome ends. Major research efforts have been made to understand TERRA roles and functions in several physiological and pathological processes. We summarize herein available data regarding TERRA’s roles in human cells and we report the first investigation in cutaneous T-cells lymphomas (CTCL) using real-time PCR. Among the TERRA analysed, our data suggest a particular role for TERRA 16p downregulation and TERRA 11q upregulation in CTCL lymphomagenesis.

Cancer treatment-induced NAD+ depletion in premature senescence and late cardiovascular complications

Numerous studies have revealed the critical role of premature senescence induced by various cancer treatment modalities in the pathogenesis of aging-related diseases. Senescence-associated secretory phenotype (SASP) can be induced by telomere dysfunction. Telomeric DNA damage response induced by some cancer treatments can persist for months, possibly accounting for long-term sequelae of cancer treatments. Telomeric DNA damage-induced mitochondrial dysfunction and increased reactive oxygen species production are hallmarks of premature senescence. Recently, we reported that the nucleus-mitochondria positive feedback loop formed by p90 ribosomal S6 kinase (p90RSK) and phosphorylation of S496 on ERK5 (a unique member of the mitogen-activated protein kinase family that is not only a kinase but also a transcriptional co-activator) were vital signaling events that played crucial roles in linking mitochondrial dysfunction, nuclear telomere dysfunction, persistent SASP induction, and atherosclerosis. In this review, we will discuss the role of NAD⁺ depletion in instigating SASP and its downstream signaling and regulatory mechanisms that lead to the premature onset of atherosclerotic cardiovascular diseases in cancer survivors.

Mammalian MutY Homolog (MYH or MUTYH) is Critical for Telomere Integrity under Oxidative Stress

Telomeres consist of special features and proteins to protect the ends of each chromosome from deterioration and fusion. The telomeric DNA repeats are highly susceptible to oxidative damage that can accelerate telomere shortening and affect telomere integrity. Several DNA repair factors including MYH/MUTYH DNA glycosylase, its interacting partners Rad9/Rad1/Hus1 checkpoint clamp, and SIRT6 aging regulator, are associated with the telomeres. MYH prevents C:G to A:T mutation by removing adenine mispaired with a frequent oxidative DNA lesion, 8-oxoguanine. Here, we show that hMYH knockout (KO) human HEK-293T cells are more sensitive to H₂O₂ treatment, have higher levels of DNA strand breaks and shorter telomeres than the control hMYH ^+/+ cells. SIRT6 foci increase at both the global genome and at telomeric regions in H₂O₂-treated hMYH ^+/+ cells. However, in untreated hMYH KO HEK-293T cells, SIRT6 foci only increase at the global genome, but not at the telomeric regions. In addition, the hMYH KO HEK-293T cells have increased extra-chromosomal and intra-chromosomal telomeres compared to the control cells, even in the absence of H₂O₂ treatment. After H₂O₂ treatment, the frequency of extra-chromosomal telomeres increased in control HEK-293T cells. Remarkably, in H₂O₂-treated hMYH KO cells, the frequencies of extra-chromosomal telomeres, intra-chromosomal telomeres, and telomere fusions are further increased. We further found that the sensitivity to H₂O₂ and shortened telomeres of hMYH KO cells, are restored by expressing wild-type hMYH, and partially rescued by expressing hMYH^Q324H mutant (defective in Hus1 interaction only), but not by expressing hMYH^V315A mutant (defective in both SIRT6 and Hus1 interactions). Thus, MYH interactions with SIRT6 and Hus1 are critical for maintaining cell viability and telomeric stability. Therefore, the failure to coordinate 8-oxoG repair is detrimental to telomere integrity.

Impact of Chromatin Dynamics and DNA Repair on Genomic Stability and Treatment Resistance in Pediatric High-Grade Gliomas

Simple Summary

Pediatric high-grade gliomas (pHGGs) are the leading cause of mortality in pediatric neuro-oncology, due in great part to treatment resistance driven by complex DNA repair mechanisms. pHGGs have recently been divided into molecular subtypes based on mutations affecting the N-terminal tail of the histone variant H3.3 and the ATRX/DAXX histone chaperone that deposits H3.3 at repetitive heterochromatin loci that are of paramount importance to the stability of our genome. This review addresses the functions of H3.3 and ATRX/DAXX in chromatin dynamics and DNA repair, as well as the impact of mutations affecting H3.3/ATRX/DAXX on treatment resistance and how the vulnerabilities they expose could foster novel therapeutic strategies.

Abstract

Despite their low incidence, pediatric high-grade gliomas (pHGGs), including diffuse intrinsic pontine gliomas (DIPGs), are the leading cause of mortality in pediatric neuro-oncology. Recurrent, mutually exclusive mutations affecting K27 (K27M) and G34 (G34R/V) in the N-terminal tail of histones H3.3 and H3.1 act as key biological drivers of pHGGs. Notably, mutations in H3.3 are frequently associated with mutations affecting ATRX and DAXX, which encode a chaperone complex that deposits H3.3 into heterochromatic regions, including telomeres. The K27M and G34R/V mutations lead to distinct epigenetic reprogramming, telomere maintenance mechanisms, and oncogenesis scenarios, resulting in distinct subgroups of patients characterized by differences in tumor localization, clinical outcome, as well as concurrent epigenetic and genetic alterations. Contrasting with our understanding of the molecular biology of pHGGs, there has been little improvement in the treatment of pHGGs, with the current mainstays of therapy—genotoxic chemotherapy and ionizing radiation (IR)—facing the development of tumor resistance driven by complex DNA repair pathways. Chromatin and nucleosome dynamics constitute important modulators of the DNA damage response (DDR). Here, we summarize the major DNA repair pathways that contribute to resistance to current DNA damaging agent-based therapeutic strategies and describe the telomere maintenance mechanisms encountered in pHGGs. We then review the functions of H3.3 and its chaperones in chromatin dynamics and DNA repair, as well as examining the impact of their mutation/alteration on these processes. Finally, we discuss potential strategies targeting DNA repair and epigenetic mechanisms as well as telomere maintenance mechanisms, to improve the treatment of pHGGs.

Increase in lamin B1 promotes telomere instability by disrupting the shelterin complex in human cells

Telomere maintenance is essential to preserve genomic stability and involves telomere-specific proteins, DNA replication and repair proteins. Lamins are key components of the nuclear envelope and play numerous roles, including maintenance of the nuclear integrity, regulation of transcription, and DNA replication. Elevated levels of lamin B1, one of the major lamins, have been observed in some human pathologies and several cancers. Yet, the effect of lamin B1 dysregulation on telomere maintenance remains unknown. Here, we unveil that lamin B1 overexpression drives telomere instability through the disruption of the shelterin complex. Indeed, lamin B1 dysregulation leads to an increase in telomere dysfunction-induced foci, telomeric fusions and telomere losses in human cells. Telomere aberrations were preceded by mislocalizations of TRF2 and its binding partner RAP1. Interestingly, we identified new interactions between lamin B1 and these shelterin proteins, which are strongly enhanced at the nuclear periphery upon lamin B1 overexpression. Importantly, chromosomal fusions induced by lamin B1 in excess were rescued by TRF2 overexpression. These data indicated that lamin B1 overexpression triggers telomere instability through a mislocalization of TRF2. Altogether our results point to lamin B1 as a new interacting partner of TRF2, that is involved in telomere stability.

Crosstalk among DNA Damage, Mitochondrial Dysfunction, Impaired Mitophagy, Stem Cell Attrition, and Senescence in the Accelerated Ageing Disorder Werner Syndrome

Werner syndrome (WS) is an accelerated ageing disease caused by multiple mutations in the gene encoding the Werner DNA helicase (WRN). The major clinical features of WS include wrinkles, grey hair, osteoporosis, and metabolic phenomena such as atherosclerosis, diabetes, and fatty liver, and resemble those seen in normal ageing, but occur earlier, in middle age. Defective DNA repair resulting from mutations in WRN explain the majority of the clinical features of WS, but the underlying mechanisms driving the larger metabolic dysfunction remain elusive. Recent studies in animal models of WS and in WS patient cells and blood samples suggest the involvement of impaired mitophagy, NAD+ depletion, and accumulation of damaged mitochondria in metabolic dysfunction. This mini-review summarizes recent progress in the understanding of the molecular mechanisms of metabolic dysfunction in WS, with the involvement of DNA damage, mitochondrial dysfunction, mitophagy reduction, stem cell impairment, and senescence. Future studies on NAD+ and mitophagy may shed light on potential therapeutic strategies for the WS patients.

Functional characterization of miR-708 microRNA in telomerase positive and negative human cancer cells

Activation of a telomere length maintenance mechanism (TMM), including telomerase and alternative lengthening of telomeres (ALT), is essential for replicative immortality of tumor cells, although its regulatory mechanisms are incompletely understood. We conducted a microRNA (miRNA) microarray analysis on isogenic telomerase positive (TEP) and ALT cancer cell lines. Amongst nine miRNAs that showed difference in their expression in TEP and ALT cancer cells in array analysis, miR-708 was selected for further analysis since it was consistently highly expressed in a large panel of ALT cells. miR-708 in TEP and ALT cancer cells was not correlated with C-circle levels, an established feature of ALT cells. Its overexpression induced suppression of cell migration, invasion, and angiogenesis in both TEP and ALT cells, although cell proliferation was inhibited only in TEP cells suggesting that ALT cells may have acquired the ability to escape inhibition of cell proliferation by sustained miR-708 overexpression. Further, cell proliferation regulation in TEP cells by miR708 appears to be through the CARF-p53 pathway. We demonstrate here that miR-708 (i) is the first miRNA shown to be differentially regulated in TEP and ALT cancer cells, (ii) possesses tumor suppressor function, and (iii) deregulates CARF and p21^WAF1-mediated signaling to limit proliferation in TEP cells.

Telomere associated gene expression as well as TERT protein level and telomerase activity are altered in the ovarian follicles of aged mice

Telomeres cap the ends of eukaryotic chromosomes to maintain genomic stability and integrity during an organism’s lifespan. The length of telomeres inevitably shortens due to DNA replication, genotoxic agents, and biological aging. A limited number of cell types, e.g., stem cells, germline cells, and early embryos can elongate shortened telomeres via the enzymatic action of telomerase, which is composed of telomerase reverse transcriptase (TERT) and telomerase RNA component (Terc). Additionally, telomere-associated proteins including telomeric repeat binding factor 1 (TRF1) and 2 (TRF2), as well as protection of telomeres 1a (POT1a), bind to telomeres to maintain their structural integrity and length. During ovarian aging in mammals, telomeres progressively shorten, accompanied by fertility loss; however, the molecular mechanism underlying this attrition during follicle development remains unclear. In this study, the primary, secondary, preantral, and antral follicles were obtained either from 6-week-old adult (n = 19) or 52-week-old aged (n = 12) mice. We revealed that the Tert, Terc, Trf1, Trf2, and Pot1a gene expression (P < 0.001) and TERT protein (P < 0.01) levels significantly decreased in certain ovarian follicles of the aged group when compared to those of the adult group. Also, telomerase activity exhibited remarkable changes in the follicles of both groups. Consequently, altered telomere-associated gene expression and reduced TERT protein levels in the follicles of aged mice may be a determinant of telomere shortening during ovarian aging, and infertility appearing in the later decades of reproductive lifespan. Further investigations are required to determine the molecular mechanisms underlying these alterations in the follicles during ovarian aging.

Structural genomics approach to investigate deleterious impact of nsSNPs in conserved telomere maintenance component 1

Conserved telomere maintenance component 1 (CTC1) is an important component of the CST (CTC1-STN1-TEN1) complex, involved in maintaining the stability of telomeric DNA. Several non-synonymous single-nucleotide polymorphisms (nsSNPs) in CTC1 have been reported to cause Coats plus syndrome and Dyskeratosis congenital diseases. Here, we have performed sequence and structure analyses of nsSNPs of CTC1 using state-of-the-art computational methods. The structure-based study focuses on the C-terminal OB-fold region of CTC1. There are 11 pathogenic mutations identified, and detailed structural analyses were performed. These mutations cause a significant disruption of noncovalent interactions, which may be a possible reason for CTC1 instability and consequent diseases. To see the impact of such mutations on the protein conformation, all-atom molecular dynamics (MD) simulations of CTC1-wild-type (WT) and two of the selected mutations, R806C and R806L for 200 ns, were carried out. A significant conformational change in the structure of the R806C mutant was observed. This study provides a valuable direction to understand the molecular basis of CTC1 dysfunction in disease progression, including Coats plus syndrome.

Continuous reference intervals for leukocyte telomere length in children: the method matters

OBJECTIVES

Children with very short telomeres commonly develop bone marrow failure and other severe diseases. Identifying the individuals with short telomeres can improve outcome of bone marrow transplantation, with accurate diagnosis requiring the use of age-matched reference intervals (RIs). This study aimed to establish RIs for telomere length (TL) in children using three commonly used methods for TL measurement.

METHODS

Healthy children aged 30 days to 18 years were recruited for assessment using age as a continuous variable. Venous blood samples were collected and leukocyte TL was measured using terminal restriction fragment (TRF) analysis, quantitative PCR (QPCR) and flow cytometry with fluorescence in situ hybridization (Flow-FISH). Fractional polynomial model and quantile regression were performed to generate continuous RIs. Factors that might contribute to variation in TL, such as gender, were also examined.

RESULTS

A total of 212 samples were analyzed. Continuous RIs are presented as functions of age. TRF analysis and QPCR showed significant negative correlation between TL and age (r=-0.28 and r=-0.38, p<0.001). In contrast, Flow-FISH showed no change in TL with age (r=-0.08, p=0.23). Gender did not have significant influence on TL in children.

CONCLUSIONS

This study provides three options to assess TL in children by establishing method-specific continuous RIs. Choosing which method to use will depend on several factors such as amount and type of sample available and required sensitivity to age-related change.

The MRN complex promotes DNA repair by homologous recombination and restrains antigenic variation in African trypanosomes

Homologous recombination dominates as the major form of DNA repair in Trypanosoma brucei, and is especially important for recombination of the subtelomeric variant surface glycoprotein during antigenic variation. RAD50, a component of the MRN complex (MRE11, RAD50, NBS1), is central to homologous recombination through facilitating resection and governing the DNA damage response. The function of RAD50 in trypanosomes is untested. Here we report that RAD50 and MRE11 are required for RAD51-dependent homologous recombination and phosphorylation of histone H2A following a DNA double strand break (DSB), but neither MRE11 nor RAD50 substantially influence DSB resection at a chromosome-internal locus. In addition, we reveal intrinsic separation-of-function between T. brucei RAD50 and MRE11, with only RAD50 suppressing DSB repair using donors with short stretches of homology at a subtelomeric locus, and only MRE11 directing DSB resection at the same locus. Finally, we show that loss of either MRE11 or RAD50 causes a greater diversity of expressed VSG variants following DSB repair. We conclude that MRN promotes stringent homologous recombination at subtelomeric loci and restrains antigenic variation.

Small molecule inhibitor of OGG1 blocks oxidative DNA damage repair at telomeres and potentiates methotrexate anticancer effects

The most common oxidative DNA lesion is 8-oxoguanine which is mainly recognized and excised by the 8-oxoG DNA glycosylase 1 (OGG1), initiating the base excision repair (BER) pathway. Telomeres are particularly sensitive to oxidative stress (OS) which disrupts telomere homeostasis triggering genome instability. In the present study, we have investigated the effects of inactivating BER in OS conditions, by using a specific inhibitor of OGG1 (TH5487). We have found that in OS conditions, TH5487 blocks BER initiation at telomeres causing an accumulation of oxidized bases, that is correlated with telomere losses, micronuclei formation and mild proliferation defects. Moreover, the antimetabolite methotrexate synergizes with TH5487 through induction of intracellular reactive oxygen species (ROS) formation, which potentiates TH5487-mediated telomere and genome instability. Our findings demonstrate that OGG1 is required to protect telomeres from OS and present OGG1 inhibitors as a tool to induce oxidative DNA damage at telomeres, with the potential for developing new combination therapies for cancer treatment.

DNA Damage and the Aging Epigenome

In mammals, genome instability and aging are intimately linked as illustrated by the growing list of patients with progeroid and animal models with inborn DNA repair defects. Until recently, DNA damage was thought to drive aging by compromising transcription or DNA replication, thereby leading to age-related cellular malfunction and somatic mutations triggering cancer. However, recent evidence suggests that DNA lesions also elicit widespread epigenetic alterations that threaten cell homeostasis as a function of age. In this review, we discuss the functional links of persistent DNA damage with the epigenome in the context of aging and age-related diseases.

Photosensitizers Based on G-Quadruplex Ligand for Cancer Photodynamic Therapy

G-quadruplex (G4) is the non-canonical secondary structure of DNA and RNA formed by guanine-rich sequences. G4-forming sequences are abundantly located in telomeric regions and in the promoter and untranslated regions (UTR) of cancer-related genes, such as RAS and MYC. Extensive research has suggested that G4 is a potential molecular target for cancer therapy. Here, we reviewed G4 ligands as photosensitizers for cancer photodynamic therapy (PDT), which is a minimally invasive therapeutic approach. The photosensitizers, such as porphyrins, were found to be highly toxic against cancer cells via the generation of reactive oxidative species (ROS) upon photo-irradiation. Several porphyrin derivatives and analogs, such as phthalocyanines, which can generate ROS upon photo-irradiation, have been reported to act as G4 ligands. Therefore, they have been implicated as promising photosensitizers that can selectively break down cancer-related DNA and RNA forming G4. In this review, we majorly focused on the potential application of G4 ligands as photosensitizers, which would provide a novel strategy for PDT, especially molecularly targeted PDT (mtPDT).

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.

Human Telomerase RNA: Telomerase Component or More?

Telomerase is a ribonucleoprotein complex that maintains the lengths of telomeres. Most studies of telomerase function have focused on the involvement of telomerase activation in the immortalization of cancer cells and cellular rejuvenation. However, some studies demonstrated that the results do not meet expectations for telomerase action in telomere maintenance. Recent results give reason to think that major telomerase components-the reverse transcriptase protein subunit and telomerase RNA-may participate in many cellular processes, including the regulation of apoptosis and autophagy, cell survival, pro-proliferative effects, regulation of gene expression, and protection against oxidative stress. However, the difficulties faced by scientist when researching telomerase component functions often reduce confidence in the minor effects observed in experiments. In this review, we focus on the analysis of the functions of telomerase components (paying more attention to the telomerase RNA component), both as a complex and as independent components, providing effects that are not associated with telomerase activity and telomere length maintenance. Despite the fact that the data on alternative roles of telomerase components look illusory, it would be wrong to completely reject the possibility of their involvement in other biological processes excluded from research/discussion. Investigations to improve the understanding of every aspect of the functioning of telomerase components will provide the basis for a more precise development of approaches to regulate cellular homeostasis, which is important for carcinogenesis and aging.

Cellular Stress and General Pathological Processes

From the viewpoint of the general pathology, most of the human diseases are associated with a limited number of pathogenic processes such as inflammation, tumor growth, thrombosis, necrosis, fibrosis, atrophy, pathological hypertrophy, dysplasia and metaplasia. The phenomenon of chronic low-grade inflammation could be attributed to non-classical forms of inflammation, which include many neurodegenerative processes, pathological variants of insulin resistance, atherosclerosis, and other manifestations of the endothelial dysfunction. Individual and universal manifestations of cellular stress could be considered as a basic element of all these pathologies, which has both physiological and pathophysiological significance. The review examines the causes, main phenomena, developmental directions and outcomes of cellular stress using a phylogenetically conservative set of genes and their activation pathways, as well as tissue stress and its role in inflammatory and para-inflammatory processes. The main ways towards the realization of cellular stress and its functional blocks were outlined. The main stages of tissue stress and the classification of its typical manifestations, as well as its participation in the development of the classical and non-classical variants of the inflammatory process, were also described. The mechanisms of cellular and tissue stress are structured into the complex systems, which include networks that enable the exchange of information with multidirectional signaling pathways which together make these systems internally contradictory, and the result of their effects is often unpredictable. However, the possible solutions require new theoretical and methodological approaches, one of which includes the transition to integral criteria, which plausibly reflect the holistic image of these processes.

KEOPS complex promotes homologous recombination via DNA resection

KEOPS complex is one of the most conserved protein complexes in eukaryotes. It plays important roles in both telomere uncapping and tRNA N⁶-threonylcarbamoyladenosine (t⁶A) modification in budding yeast. But whether KEOPS complex plays any roles in DNA repair remains unknown. Here, we show that KEOPS complex plays positive roles in both DNA damage response and homologous recombination-mediated DNA repair independently of its t⁶A synthesis function. Additionally, KEOPS displays DNA binding activity in vitro, and is recruited to the chromatin at DNA breaks in vivo, suggesting a direct role of KEOPS in DSB repair. Mechanistically, KEOPS complex appears to promote DNA end resection through facilitating the association of Exo1 and Dna2 with DNA breaks. Interestingly, inactivation of both KEOPS and Mre11/Rad50/Xrs2 (MRX) complexes results in synergistic defect in DNA resection, revealing that KEOPS and MRX have some redundant functions in DNA resection. Thus we uncover a t⁶A-independent role of KEOPS complex in DNA resection, and propose that KEOPS might be a DSB sensor to assist cells in maintaining chromosome stability.

CSL controls telomere maintenance and genome stability in human dermal fibroblasts

Genomic instability is a hallmark of cancer. Whether it also occurs in Cancer Associated Fibroblasts (CAFs) remains to be carefully investigated. Loss of CSL/RBP-Jκ, the effector of canonical NOTCH signaling with intrinsic transcription repressive function, causes conversion of dermal fibroblasts into CAFs. Here, we find that CSL down-modulation triggers DNA damage, telomere loss and chromosome end fusions that also occur in skin Squamous Cell Carcinoma (SCC)-associated CAFs, in which CSL is decreased. Separately from its role in transcription, we show that CSL is part of a multiprotein telomere protective complex, binding directly and with high affinity to telomeric DNA as well as to UPF1 and Ku70/Ku80 proteins and being required for their telomere association. Taken together, the findings point to a central role of CSL in telomere homeostasis with important implications for genomic instability of cancer stromal cells and beyond.

Conversion of dermal fibroblasts into Cancer Associated Fibroblasts (CAFs) can play an important role in keratinocyte tumour development. Here the authors reveal that CSL plays a role in maintenance of telomeres and genomic integrity in both dermal fibroblasts and CAFs.

Tpz1TPP1 prevents telomerase activation and protects telomeres by modulating the Stn1-Ten1 complex in fission yeast

In both mammalian and fission yeast cells, conserved shelterin and CST (CTC1-STN1-TEN1) complexes play critical roles in protection of telomeres and regulation of telomerase, an enzyme required to overcome the end replication problem. However, molecular details that govern proper coordination among shelterin, CST, and telomerase have not yet been fully understood. Here, we establish a conserved SWSSS motif, located adjacent to the Lys242 SUMOylation site in the fission yeast shelterin subunit Tpz1, as a new functional regulatory element for telomere protection and telomere length homeostasis. The SWSSS motif works redundantly with Lys242 SUMOylation to promote binding of Stn1-Ten1 at telomere and sub-telomere regions to protect against single-strand annealing (SSA)-dependent telomere fusions, and to prevent telomerase accumulation at telomeres. In addition, we provide evidence that the SWSSS motif defines an unanticipated role of Tpz1 in limiting telomerase activation at telomeres to prevent uncontrolled telomere elongation.

Structural and functional impact of non-synonymous SNPs in the CST complex subunit TEN1: structural genomics approach

TEN1 protein is a key component of CST complex, implicated in maintaining the telomere homeostasis, and provides stability to the eukaryotic genome. Mutations in TEN1 gene have higher chances of deleterious impact; thus, interpreting the number of mutations and their consequential impact on the structure, stability, and function is essentially important. Here, we have investigated the structural and functional consequences of nsSNPs in the TEN1 gene. A wide array of sequence- and structure-based computational prediction tools were employed to identify the effects of 78 nsSNPs on the structure and function of TEN1 protein and to identify the deleterious nsSNPs. These deleterious or destabilizing nsSNPs are scattered throughout the structure of TEN1. However, major mutations were observed in the α1-helix (12–16 residues) and β5-strand (88–96 residues). We further observed that mutations at the C-terminal region were having higher tendency to form aggregate. In-depth structural analysis of these mutations reveals that the pathogenicity of these mutations are driven mainly through larger structural changes because of alterations in non-covalent interactions. This work provides a blueprint to pinpoint the possible consequences of pathogenic mutations in the CST complex subunit TEN1.

Telomere Biology and Human Phenotype

Telomeres are nucleoprotein structures that cap the end of each chromosome arm and function to maintain genome stability. The length of telomeres is known to shorten with each cell division and it is well-established that telomere attrition is related to replicative capacity in vitro. Moreover, telomere loss is also correlated with the process of aging in vivo. In this review, we discuss the mechanisms that lead to telomere shortening and summarise telomere homeostasis in humans throughout a lifetime. In addition, we discuss the available evidence that shows that telomere shortening is related to human aging and the onset of age-related disease.

hnRNPA2 mediated acetylation reduces telomere length in response to mitochondrial dysfunction

Telomeres protect against chromosomal damage. Accelerated telomere loss has been associated with premature aging syndromes such as Werner's syndrome and Dyskeratosis Congenita, while, progressive telomere loss activates a DNA damage response leading to chromosomal instability, typically observed in cancer cells and senescent cells. Therefore, identifying mechanisms of telomere length maintenance is critical for understanding human pathologies. In this paper we demonstrate that mitochondrial dysfunction plays a causal role in telomere shortening. Furthermore, hnRNPA2, a mitochondrial stress responsive lysine acetyltransferase (KAT) acetylates telomere histone H4at lysine 8 of (H4K8) and this acetylation is associated with telomere attrition. Cells containing dysfunctional mitochondria have higher telomere H4K8 acetylation and shorter telomeres independent of cell proliferation rates. Ectopic expression of KAT mutant hnRNPA2 rescued telomere length possibly due to impaired H4K8 acetylation coupled with inability to activate telomerase expression. The phenotypic outcome of telomere shortening in immortalized cells included chromosomal instability (end-fusions) and telomerase activation, typical of an oncogenic transformation; while in non-telomerase expressing fibroblasts, mitochondrial dysfunction induced-telomere attrition resulted in senescence. Our findings provide a mechanistic association between dysfunctional mitochondria and telomere loss and therefore describe a novel epigenetic signal for telomere length maintenance.

DDM1 guards against telomere truncation in Arabidopsis

Prolonged hypomethylation of DNA leads to telomere truncation correlated with increased telomere recombination, transposon mobilization and stem cell death. Epigenetic pathways, including DNA methylation, are crucial for telomere maintenance. Deficient in DNA Methylation 1 (DDM1) encodes a nucleosome remodeling protein, required to maintain DNA methylation in Arabidopsis thaliana. Plants lacking DDM1 can be self-propagated, but in the sixth generation (G6) hypomethylation leads to rampant transposon activation and infertility. Here we examine the role of DDM1 in telomere length homeostasis through a longitudinal study of successive generations of ddm1-2 mutants. We report that bulk telomere length remains within the wild-type range for the first five generations (G1-G5), and then precipitously drops in G6. While telomerase activity becomes more variable in later generation ddm1-2 mutants, there is no correlation between enzyme activity and telomere length. Plants lacking DDM1 also exhibit no dysregulation of several known telomere-associated transcripts, including TERRA. Instead, telomere shortening coincides with increased G-overhangs and extra-chromosomal circles, consistent with deletional recombination. Telomere shortening also correlates with transcriptional activation of retrotransposons, and a hypersensitive DNA damage response in root apical meristems. Since abiotic stresses, including DNA damage, stimulate homologous recombination, we hypothesize that telomere deletion in G6 ddm1-2 mutants is a by-product of elevated genome-wide recombination in response to transposon mobilization. Further, we speculate that telomere truncation may be beneficial in adverse environmental conditions by accelerating the elimination of stem cells with aberrant genomes.

LARP7-like protein Pof8 regulates telomerase assembly and poly(A)+TERRA expression in fission yeast

Telomerase is a reverse transcriptase complex that ensures stable maintenance of linear eukaryotic chromosome ends by overcoming the end replication problem, posed by the inability of replicative DNA polymerases to fully replicate linear DNA. The catalytic subunit TERT must be assembled properly with its telomerase RNA for telomerase to function, and studies in Tetrahymena have established that p65, a La-related protein 7 (LARP7) family protein, utilizes its C-terminal xRRM domain to promote assembly of the telomerase ribonucleoprotein (RNP) complex. However, LARP7-dependent telomerase complex assembly has been considered as unique to ciliates that utilize RNA polymerase III to transcribe telomerase RNA. Here we show evidence that fission yeast Schizosaccharomyces pombe utilizes the p65-related protein Pof8 and its xRRM domain to promote assembly of RNA polymerase II-encoded telomerase RNA with TERT. Furthermore, we show that Pof8 contributes to repression of the transcription of noncoding RNAs at telomeres.

A functional telomerase complex requires that the catalytic TERT subunit be assembled with the template RNA TER1. Here the authors show that Pof8, a possible LARP7 family protein, is required for assembly of the telomerase complex, and repression of lncRNA transcripts at telomeres in S. pombe.

Telomere targeting with a novel G-quadruplex-interactive ligand BRACO-19 induces T-loop disassembly and telomerase displacement in human glioblastoma cells

Interference with telomerase and telomere maintenance is emerging as an attractive target for anticancer therapies. Ligand-induced stabilization of G-quadruplex formation by the telomeric DNA 3'-overhang inhibits telomerase from catalyzing telomeric DNA synthesis and from capping telomeric ends, making these ligands good candidates for chemotherapeutic purposes. BRACO-19 is one of the most effective and specific ligand for telomeric G4. It is shown here that BRACO-19 suppresses proliferation and reduces telomerase activity in human glioblastoma cells, paralleled by the displacement of telomerase from nuclear to cytoplasm. Meanwhile, BRACO-19 triggers extensive DNA damage response at telomere, which may result from uncapping and disassembly of telomeric T-loop structure, characterized by the formation of anaphase bridge and telomere fusion, as well as the release of telomere-binding protein from telomere. The resulting dysfunctional telomere ultimately provokes p53 and p21-mediated cell cycle arrest, apoptosis and senescence. Notably, normal primary astrocytes do not respond to the treatment of BRACO-19, suggesting the agent's good selectivity for cancer cells. These results reinforce the notion that G-quadruplex binding compounds can act as broad inhibitors of telomere-related processes and have potential as selective antineoplastic drugs for various tumors including malignant gliomas.

The shortening telomere length of T lymphocytes maybe associated with hyper‑function in servere aplastic anemia

Severe aplastic anemia (SAA) is a primary disorder of severe bone marrow failure characterizing with extreme pancytopenia and a profound diminution of bone marrow progenitor cells, which is associated with T cell hyper‑function. Abnormal telomere shortening of bone marrow mononuclear cell has been reported in AA, which may lead to genomic instability, and result in cell senescence or apoptosis. Notably, certain studies identfieid that lymphocytes of shortening telomere length have undergone apoptosis escape in autoimmune diseases. In order to investigate the association between telomere lengths and function of T lymphocytes in SAA, the relative telomere lengths (RTLs) of different subtypes of T lymphocytes were investigated by flow‑fluorescent in situ hybridization in 30 patients with SAA and 25 healthy controls. Then the levels of expression of cluster of differentiation 28 (CD28), CD158 and CD70 were measured, which represent the function of T lymphocytes. The apoptosis rate and the cell cycle progression of CD8+T lymphocytes, and the level of secretion interferon‑γ and tumor necrosis factor‑α were also measured. Finally, the correlation between telomere length and these functional events of CD8+T lymphocytes was analyzed in patients with SAA. The results showed that RTLs of CD8+T lymphocytes in SAA were significantly shorter compared with those in controls. Furthermore, in patients with SAA, CD8+T lymphocytes are associated with T cell hyper‑function, which is related to the RTL. Thus, the shorter RTLs of CD8+T lymphocytes in SAA may be associated with hyper‑function of these cells, which contribute to the pathogenesis of SAA.

Homeostatic nuclear RAGE-ATM interaction is essential for efficient DNA repair

The integrity of genome is a prerequisite for healthy life. Indeed, defects in DNA repair have been associated with several human diseases, including tissue-fibrosis, neurodegeneration and cancer. Despite decades of extensive research, the spatio-mechanical processes of double-strand break (DSB)-repair, especially the auxiliary factor(s) that can stimulate accurate and timely repair, have remained elusive. Here, we report an ATM-kinase dependent, unforeseen function of the nuclear isoform of the Receptor for Advanced Glycation End-products (nRAGE) in DSB-repair. RAGE is phosphorylated at Serine³⁷⁶ and Serine³⁸⁹ by the ATM kinase and is recruited to the site of DNA-DSBs via an early DNA damage response. nRAGE preferentially co-localized with the MRE11 nuclease subunit of the MRN complex and orchestrates its nucleolytic activity to the ATR kinase signaling. This promotes efficient RPA2^S4-S8 and CHK1^S345 phosphorylation and thereby prevents cellular senescence, IPF and carcinoma formation. Accordingly, loss of RAGE causatively linked to perpetual DSBs signaling, cellular senescence and fibrosis. Importantly, in a mouse model of idiopathic pulmonary fibrosis (RAGE^−/−), reconstitution of RAGE efficiently restored DSB-repair and reversed pathological anomalies. Collectively, this study identifies nRAGE as a master regulator of DSB-repair, the absence of which orchestrates persistent DSB signaling to senescence, tissue-fibrosis and oncogenesis.

Replication Protein A-1 Has a Preference for the Telomeric G-rich Sequence in Trypanosoma cruzi

Replication protein A (RPA), the major eukaryotic single-stranded binding protein, is a heterotrimeric complex formed by RPA-1, RPA-2, and RPA-3. RPA is a fundamental player in replication, repair, recombination, and checkpoint signaling. In addition, increasing evidences have been adding functions to RPA in telomere maintenance, such as interaction with telomerase to facilitate its activity and also involvement in telomere capping in some conditions. Trypanosoma cruzi, the etiological agent of Chagas disease is a protozoa parasite that appears early in the evolution of eukaryotes. Recently, we have showed that T. cruziRPA presents canonical functions being involved with DNA replication and DNA damage response. Here, we found by FISH/IF assays that T. cruziRPA localizes at telomeres even outside replication (S) phase. In vitro analysis showed that one telomeric repeat is sufficient to bind RPA-1. Telomeric DNA induces different secondary structural modifications on RPA-1 in comparison with other types of DNA. In addition, RPA-1 presents a higher affinity for telomeric sequence compared to randomic sequence, suggesting that RPA may play specific roles in T. cruzi telomeric region.

Tankyrase1-mediated poly(ADP-ribosyl)ation of TRF1 maintains cell survival after telomeric DNA damage

Oxidative DNA damage triggers telomere erosion and cellular senescence. However, how repair is initiated at telomeres is largely unknown. Here, we found unlike PARP1-mediated Poly-ADP-Ribosylation (PARylation) at genomic damage sites, PARylation at telomeres is mainly dependent on tankyrase1 (TNKS1). TNKS1 is recruited to damaged telomeres via its interaction with TRF1, which subsequently facilitates the PARylation of TRF1 after damage. TNKS inhibition abolishes the recruitment of the repair proteins XRCC1 and polymerase β at damaged telomeres, while the PARP1/2 inhibitor only has such an effect at non-telomeric damage sites. The ANK domain of TNKS1 is essential for the telomeric damage response and TRF1 interaction. Mutation of the tankyrase-binding motif (TBM) on TRF1 (13R/18G to AA) disrupts its interaction with TNKS1 concomitant recruitment of TNKS1 and repair proteins after damage. Either TNKS1 inhibition or TBM mutated TRF1 expression markedly sensitizes cells to telomere oxidative damage as well as XRCC1 inhibition. Together, our data reveal a novel role of TNKS1 in facilitating SSBR at damaged telomeres through PARylation of TRF1, thereby protecting genome stability and cell viability.

Human MLH1 suppresses the insertion of telomeric sequences at intra-chromosomal sites in telomerase-expressing cells

Aberrant formation of interstitial telomeric sequences (ITSs) promotes genome instabilities. However, it is unclear how aberrant ITS formation is suppressed in human cells. Here, we report that MLH1, a key protein involved in mismatch repair (MMR), suppresses telomeric sequence insertion (TSI) at intra-chromosomal regions. The frequency of TSI can be elevated by double-strand break (DSB) inducer and abolished by ATM/ATR inhibition. Suppression of TSI requires MLH1 recruitment to DSBs, indicating that MLH1's role in DSB response/repair is important for suppressing TSI. Moreover, TSI requires telomerase activity but is independent of the functional status of p53 and Rb. Lastly, we show that TSI is associated with chromosome instabilities including chromosome loss, micronuclei formation and chromosome breakage that are further elevated by replication stress. Our studies uncover a novel link between MLH1, telomerase, telomere and genome stability.

Cigarette smoking and telomere length: A systematic review of 84 studies and meta-analysis

BACKGROUND

Cigarette smoking is a risk factor for ageing-related disease, but its association with biological ageing, indicated by telomere length, is unclear.

METHODS

We systematically reviewed evidence evaluating association between smoking status and telomere length. Searches were performed in MEDLINE (Ovid) and EMBASE (Ovid) databases, combining variation of keywords "smoking" and "telomere". Data was extracted for study characteristics and estimates for association between smoking and telomere length. Quality of studies was assessed with a risk of bias score, and publication bias was assessed with a funnel plot. I2 test was used to observe heterogeneity. Meta-analysis was carried out to compare mean difference in telomere length by smoking status, and a dose-response approach was carried out for pack-years of smoking and telomere length. A sensitivity analysis was carried out to examine sources of heterogeneity.

RESULTS

A total of 84 studies were included in the review, and 30 among them were included in our meta-analysis. Potential bias was addressed in half of included studies, and there was little evidence of small study bias. Telomere length was shorter among ever smokers compared to never smokers (summary standard mean difference [SMD]: -0.11 (95% CI -0.16 to -0.07)). Similarly, shorter telomere length was found among smokers compared to non-smokers, and among current smokers compared to never or former smokers. Dose-response meta-analysis suggested an inverse trend between pack-years of smoking and telomere length. However, heterogeneity among some analyses was observed.

CONCLUSION

Shorter telomeres among ever smokers compared to those who never smoked may imply mechanisms linking tobacco smoke exposure to ageing-related disease.

Epigenetic suppression of human telomerase (hTERT) is mediated by the metastasis suppressor NME2 in a G-quadruplex-dependent fashion

Transcriptional activation of the human telomerase reverse transcriptase (hTERT) gene, which remains repressed in adult somatic cells, is critical during tumorigenesis. Several transcription factors and the epigenetic state of the hTERT promoter are known to be important for tight control of hTERT in normal tissues, but the molecular mechanisms leading to hTERT reactivation in cancer are not well-understood. Surprisingly, here we found occupancy of the metastasis suppressor non-metastatic 2 (NME2) within the hTERT core promoter in HT1080 fibrosarcoma cells and HCT116 colon cancer cells and NME2-mediated transcriptional repression of hTERT in these cells. We also report that loss of NME2 results in up-regulated hTERT expression. Mechanistically, additional results indicated that the RE1-silencing transcription factor (REST)–lysine-specific histone demethylase 1 (LSD1) co-repressor complex associates with the hTERT promoter in an NME2-dependent way and that this assembly is required for maintaining repressive chromatin at the hTERT promoter. Interestingly, a G-quadruplex motif at the hTERT promoter was essential for occupancy of NME2 and the REST repressor complex on the hTERT promoter. In light of this mechanistic insight, we studied the effects of G-quadruplex–binding ligands on hTERT expression and observed that several of these ligands repressed hTERT expression. Together, our results support a mechanism of hTERT epigenetic control involving a G-quadruplex promoter motif, which potentially can be targeted by tailored small molecules.

The Telomeric Complex and Metabolic Disease

The attrition of telomeres is believed to be a key event not only in mammalian aging, but also in disturbed nutrient sensing, which could lead to numerous metabolic dysfunctions. The current debate focuses mainly on the question whether telomere shortening, e.g., as a heritable trait, may act as a cause or rather represents a consequence of such chronic diseases. This review discusses the damaging events that ultimately may lead or contribute to telomere shortening and can be associated with metabolic diseases.

The genome-wide transcription response to telomerase deficiency in the thermotolerant yeast Hansenula polymorpha DL-1

BACKGROUND

In the course of replication of eukaryotic chromosomes, the telomere length is maintained due to activity of telomerase, the ribonucleoprotein reverse transcriptase. Abolishing telomerase function causes progressive shortening of telomeres and, ultimately, cell cycle arrest and replicative senescence. To better understand the cellular response to telomerase deficiency, we performed a transcriptomic study for the thermotolerant methylotrophic yeast Hansenula polymorpha DL-1 lacking telomerase activity.

RESULTS

Mutant strain of H. polymorpha carrying a disrupted telomerase RNA gene was produced, grown to senescence and analyzed by RNA-seq along with wild type strain. Telomere shortening induced a transcriptional response involving genes relevant to telomere structure and maintenance, DNA damage response, information processing, and some metabolic pathways. Genes involved in DNA replication and repair, response to environmental stresses and intracellular traffic were up-regulated in senescent H. polymorpha cells, while strong down-regulation was observed for genes involved in transcription and translation, as well as core histones.

CONCLUSIONS

Comparison of the telomerase deletion transcription responses by Saccharomyces cerevisiae and H. polymorpha demonstrates that senescence makes different impact on the main metabolic pathways of these yeast species but induces similar changes in processes related to nucleic acids metabolism and protein synthesis. Up-regulation of a subunit of the TORC1 complex is clearly relevant for both types of yeast.