The end protection problem-an unexpected twist in the tail

α-Terpineol Induces Shelterin Components TRF1 and TRF2 to Mitigate Senescence and Telomere Integrity Loss via A Telomerase-Independent Pathway

Cellular senescence is a hallmark of aging characterized by irreversible growth arrest and functional decline. Progressive telomeric DNA shortening in dividing somatic cells, programmed during development, leads to critically short telomeres that trigger replicative senescence and thereby contribute to aging. Therefore, protecting telomeres from DNA damage is essential in order to avoid entry into senescence and organismal aging. In several organisms, including mammals, telomeres are protected by a protein complex named shelterin that prevents DNA damage at the chromosome ends through the specific function of its subunits. Here, we reveal that the nuclear protein levels of shelterin components TRF1 and TRF2 decline in fibroblasts reaching senescence. Notably, we identify α-terpineol as an activator that effectively enhances TRF1 and TRF2 levels in a telomerase-independent manner, counteracting the senescence-associated decline in these crucial proteins. Moreover, α-terpineol ameliorates the cells' response to oxidative DNA damage, particularly at the telomeric regions, thus preserving telomere length and delaying senescence. More importantly, our findings reveal the significance of the PI3K/AKT pathway in the regulation of shelterin components responsible for preserving telomere integrity. In conclusion, this study deepens our understanding of the molecular pathways involved in senescence-associated telomere dysfunction and highlights the potential of shelterin components to serve as targets of therapeutic interventions, aimed at promoting healthy aging and combating age-related diseases.

Investigating the Relationship between Telomere-Related Gene Variants and Leukocyte Telomere Length in Optic Neuritis Patients

Optic neuritis (ON) is a condition marked by optic nerve inflammation due to various potential triggers. Research indicates a link between telomeres and inflammation, as studies demonstrate that inflammation can lead to increased telomere shortening. Aim: We aimed to determine the associations of telomere-related telomeric repeat binding factor 1 (TERF1) rs1545827, rs10107605, and telomeric repeat binding factor 2 (TERF2) rs251796 polymorphisms and relative leukocyte telomere length (LTL) with the occurrence of ON. Methods: In this research, a total of 73 individuals diagnosed with optic neuritis (ON) were studied and the control group included 170 individuals without any health issues. The DNA samples were obtained from peripheral blood leukocytes, which were purified using the DNA salting-out technique. Real-time polymerase chain reaction (RT-PCR) assessed single-nucleotide polymorphisms (SNPs) and relative leukocyte telomere lengths (LTL). The data obtained were processed and analyzed using the "IBM SPSS Statistics 29.0" program. Results: Our study revealed the following results: in the male group, TERF2 rs251796 (AA, AG, and TT) statistically significantly differed between the long and short telomere group, with frequencies of 65.7%, 22.9%, and 2.0% in long telomeres, compared to 35.1%, 56.8%, and 8.1% in the short telomere group (p = 0.013). The TERF2 rs251796 CT genotype, compared to CC, under the codominant genetic model, was associated with 4.7-fold decreased odds of telomere shortening (p = 0.005). Meanwhile, CT+TT genotypes, compared to CC under the dominant genetic model, were associated with 3.5-fold decreased odds of telomere shortening (p = 0.011). Also, the CT genotype, compared to CC+TT, under the overdominant genetic model, was associated with 4.4-fold decreased odds of telomere shortening (p = 0.004). Conclusions: The current evidence may suggest a protective role of TERF2 rs251796 in the occurrence of ON in men.

The Influence of Telomere-Related Gene Variants, Serum Levels, and Relative Leukocyte Telomere Length in Pituitary Adenoma Occurrence and Recurrence

In this study, we examined 130 patients with pituitary adenomas (PAs) and 320 healthy subjects, using DNA samples from peripheral blood leukocytes purified through the DNA salting-out method. Real-time polymerase chain reaction (RT-PCR) was used to assess single nucleotide polymorphisms (SNPs) and relative leukocyte telomere lengths (RLTLs), while enzyme-linked immunosorbent assay (ELISA) was used to determine the levels of TERF1, TERF2, TNKS2, CTC1, and ZNF676 in blood serum. Our findings reveal several significant associations. Genetic associations with pituitary adenoma occurrence: the TERF1 rs1545827 CT + TT genotypes were linked to 2.9-fold decreased odds of PA occurrence. Conversely, the TNKS2 rs10509637 GG genotype showed 6.5-fold increased odds of PA occurrence. Gender-specific genetic associations with PA occurrence: in females, the TERF1 rs1545827 CC + TT genotypes indicated 3.1-fold decreased odds of PA occurrence, while the TNKS2 rs10509637 AA genotype was associated with 4.6-fold increased odds. In males, the presence of the TERF1 rs1545827 T allele was associated with 2.2-fold decreased odds of PA occurrence, while the TNKS2 rs10509637 AA genotype was linked to a substantial 10.6-fold increase in odds. Associations with pituitary adenoma recurrence: the TNKS2 rs10509637 AA genotype was associated with 4.2-fold increased odds of PA recurrence. On the other hand, the TERF1 rs1545827 CT + TT genotypes were linked to 3.5-fold decreased odds of PA without recurrence, while the TNKS2 rs10509637 AA genotype was associated with 6.4-fold increased odds of PA without recurrence. Serum TERF2 and TERF1 levels: patients with PA exhibited elevated serum TERF2 levels compared to the reference group. Conversely, patients with PA had decreased TERF1 serum levels compared to the reference group. Relative leukocyte telomere length (RLTL): a significant difference in RLTL between the PA group and the reference group was observed, with PA patients having longer telomeres. Genetic associations with telomere shortening: the TERF1 rs1545827 T allele was associated with 1.4-fold decreased odds of telomere shortening. In contrast, the CTC1 rs3027234 TT genotype was linked to 4.8-fold increased odds of telomere shortening. These findings suggest a complex interplay between genetic factors, telomere length, and pituitary adenoma occurrence and recurrence, with potential gender-specific effects. Furthermore, variations in TERF1 and TNKS2 genes may play crucial roles in telomere length regulation and disease susceptibility.

The C-terminal extension of dyskerin is a dyskeratosis congenita mutational hotspot that modulates interaction with telomerase RNA and subcellular localization

Dyskerin is a component of the human telomerase complex and is involved in stabilizing the human telomerase RNA (hTR). Many mutations in the DKC1 gene encoding dyskerin are found in X-linked dyskeratosis congenita (X-DC), a premature aging disorder and other related diseases. The C-terminal extension (CTE) of dyskerin contributes to its interaction with the molecular chaperone SHQ1 during the early stage of telomerase biogenesis. Disease mutations in this region were proposed to disrupt dyskerin-SHQ1 interaction and destabilize dyskerin, reducing hTR levels indirectly. However, biochemical evidence supporting this hypothesis is still lacking. In addition, the effects of many CTE disease mutations on hTR have not been examined. In this study, we tested eight dyskerin CTE variants and showed that they failed to maintain hTR levels. These mutants showed slightly reduced but not abolished interaction with SHQ1, and caused defective binding to hTR. Deletion of the CTE further reduced binding to hTR, and perturbed localization of dyskerin to the Cajal bodies and the nucleolus, and the interaction with TCAB1 as well as GAR1. Our findings suggest impaired dyskerin-hTR interaction in cells as a previously overlooked mechanism through which dyskerin CTE mutations cause X-DC and related telomere syndromes.

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.

Unwrap RAP1's Mystery at Kinetoplastid Telomeres

Although located at the chromosome end, telomeres are an essential chromosome component that helps maintain genome integrity and chromosome stability from protozoa to mammals. The role of telomere proteins in chromosome end protection is conserved, where they suppress various DNA damage response machineries and block nucleolytic degradation of the natural chromosome ends, although the detailed underlying mechanisms are not identical. In addition, the specialized telomere structure exerts a repressive epigenetic effect on expression of genes located at subtelomeres in a number of eukaryotic organisms. This so-called telomeric silencing also affects virulence of a number of microbial pathogens that undergo antigenic variation/phenotypic switching. Telomere proteins, particularly the RAP1 homologs, have been shown to be a key player for telomeric silencing. RAP1 homologs also suppress the expression of Telomere Repeat-containing RNA (TERRA), which is linked to their roles in telomere stability maintenance. The functions of RAP1s in suppressing telomere recombination are largely conserved from kinetoplastids to mammals. However, the underlying mechanisms of RAP1-mediated telomeric silencing have many species-specific features. In this review, I will focus on Trypanosoma brucei RAP1's functions in suppressing telomeric/subtelomeric DNA recombination and in the regulation of monoallelic expression of subtelomere-located major surface antigen genes. Common and unique mechanisms will be compared among RAP1 homologs, and their implications will be discussed.

Telomere maintenance in African trypanosomes

Telomere maintenance is essential for genome integrity and chromosome stability in eukaryotic cells harboring linear chromosomes, as telomere forms a specialized structure to mask the natural chromosome ends from DNA damage repair machineries and to prevent nucleolytic degradation of the telomeric DNA. In Trypanosoma brucei and several other microbial pathogens, virulence genes involved in antigenic variation, a key pathogenesis mechanism essential for host immune evasion and long-term infections, are located at subtelomeres, and expression and switching of these major surface antigens are regulated by telomere proteins and the telomere structure. Therefore, understanding telomere maintenance mechanisms and how these pathogens achieve a balance between stability and plasticity at telomere/subtelomere will help develop better means to eradicate human diseases caused by these pathogens. Telomere replication faces several challenges, and the "end replication problem" is a key obstacle that can cause progressive telomere shortening in proliferating cells. To overcome this challenge, most eukaryotes use telomerase to extend the G-rich telomere strand. In addition, a number of telomere proteins use sophisticated mechanisms to coordinate the telomerase-mediated de novo telomere G-strand synthesis and the telomere C-strand fill-in, which has been extensively studied in mammalian cells. However, we recently discovered that trypanosomes lack many telomere proteins identified in its mammalian host that are critical for telomere end processing. Rather, T. brucei uses a unique DNA polymerase, PolIE that belongs to the DNA polymerase A family (E. coli DNA PolI family), to coordinate the telomere G- and C-strand syntheses. In this review, I will first briefly summarize current understanding of telomere end processing in mammals. Subsequently, I will describe PolIE-mediated coordination of telomere G- and C-strand synthesis in T. brucei and implication of this recent discovery.

Drug repositioning strategy for the identification of novel telomere-damaging agents: A role for NAMPT inhibitors

Drug repositioning strategy represents a valid tool to accelerate the pharmacological development through the identification of new applications for already existing compounds. In this view, we aimed at discovering molecules able to trigger telomere-localized DNA damage and tumor cell death. By applying an automated high-content spinning-disk microscopy, we performed a screening aimed at identifying, on a library of 527 drugs, molecules able to negatively affect the expression of TRF2, a key protein in telomere maintenance. FK866, resulting from the screening as the best candidate hit, was then validated at biochemical and molecular levels and the mechanism underlying its activity in telomere deprotection was elucidated both in vitro and in vivo. The results of this study allow us to discover a novel role of FK866 in promoting, through the production of reactive oxygen species, telomere loss and deprotection, two events leading to an accumulation of DNA damage and tumor cell death. The ability of FK866 to induce telomere damage and apoptosis was also demonstrated in advanced preclinical models evidencing the antitumoral activity of FK866 in triple-negative breast cancer-a particularly aggressive breast cancer subtype still orphan of targeted therapies and characterized by high expression levels of both NAMPT and TRF2. Overall, our findings pave the way to the development of novel anticancer strategies to counteract triple-negative breast cancer, based on the use of telomere deprotecting agents, including NAMPT inhibitors, that would rapidly progress from bench to bedside.

Dietary nucleotides can prevent glucocorticoid-induced telomere attrition in a fast-growing wild vertebrate

Telomeres are chromosome protectors that shorten during eukaryotic cell replication and in stressful conditions. Developing individuals are susceptible to telomere erosion when their growth is fast and resources are limited. This is critical because the rate of telomere attrition in early life is linked to health and life span of adults. The metabolic telomere attrition hypothesis (MeTA) suggests that telomere dynamics can respond to biochemical signals conveying information about the organism's energetic state. Among these signals are glucocorticoids, hormones that promote catabolic processes, potentially impairing costly telomere maintenance, and nucleotides, which activate anabolic pathways through the cellular enzyme target of rapamycin (TOR), thus preventing telomere attrition. During the energetically demanding growth phase, the regulation of telomeres in response to two contrasting signals - one promoting telomere maintenance and the other attrition - provides an ideal experimental setting to test the MeTA. We studied nestlings of a rapidly developing free-living passerine, the great tit (Parus major), that either received glucocorticoids (Cort-chicks), nucleotides (Nuc-chicks) or a combination of both (NucCort-chicks), comparing these with controls (Cnt-chicks). As expected, Cort-chicks showed telomere attrition, while NucCort- and Nuc-chicks did not. NucCort-chicks was the only group showing increased expression of a proxy for TOR activation (the gene TELO2), of mitochondrial enzymes linked to ATP production (cytochrome oxidase and ATP-synthase) and a higher efficiency in aerobically producing ATP. NucCort-chicks had also a higher expression of telomere maintenance genes (shelterin protein TERF2 and telomerase TERT) and of enzymatic antioxidant genes (glutathione peroxidase and superoxide dismutase). The findings show that nucleotide availability is crucial for preventing telomere erosion during fast growth in stressful environments.

Associations between telomere attrition, genetic variants in telomere maintenance genes, and non-small cell lung cancer risk in the Jammu and Kashmir population of North India

BACKGROUND

Telomeres are repetitive DNA sequences located at the ends of chromosomes, playing a vital role in maintaining chromosomal integrity and stability. Dysregulation of telomeres has been implicated in the development of various cancers, including non-small cell lung cancer (NSCLC), which is the most common type of lung cancer. Genetic variations within telomere maintenance genes may influence the risk of developing NSCLC. The present study aimed to evaluate the genetic associations of select variants within telomere maintenance genes in a population from Jammu and Kashmir, North India, and to investigate the relationship between telomere length and NSCLC risk.

METHODS

We employed the cost-effective and high-throughput MassARRAY MALDI-TOF platform to assess the genetic associations of select variants within telomere maintenance genes in a population from Jammu and Kashmir, North India. Additionally, we used TaqMan genotyping to validate our results. Furthermore, we investigated telomere length variation and its relation to NSCLC risk in the same population using dual-labeled fluorescence-based qPCR.

RESULTS

Our findings revealed significant associations of TERT rs10069690 and POT1 rs10228682 with NSCLC risk (adjusted p-values = 0.019 and 0.002, respectively), while TERF2 rs251796 and rs2975843 showed no significant associations. The TaqMan genotyping validation further substantiated the associations of TERT rs10069690 and rs2242652 with NSCLC risk (adjusted p-values = 0.02 and 0.003, respectively). Our results also demonstrated significantly shorter telomere lengths in NSCLC patients compared to controls (p = 0.0004).

CONCLUSION

This study highlights the crucial interplay between genetic variation in telomere maintenance genes, telomere attrition, and NSCLC risk in the Jammu and Kashmir population of North India. Our findings suggest that TERT and POT1 gene variants, along with telomere length, may serve as potential biomarkers and therapeutic targets for NSCLC in this population. Further research is warranted to elucidate the underlying mechanisms and to explore the potential clinical applications of these findings.

Homology directed telomere clustering, ultrabright telomere formation and nuclear envelope rupture in cells lacking TRF2B and RAP1

Double-strand breaks (DSBs) due to genotoxic stress represent potential threats to genome stability. Dysfunctional telomeres are recognized as DSBs and are repaired by distinct DNA repair mechanisms. RAP1 and TRF2 are telomere binding proteins essential to protect telomeres from engaging in homology directed repair (HDR), but how this occurs remains unclear. In this study, we examined how the basic domain of TRF2 (TRF2B) and RAP1 cooperate to repress HDR at telomeres. Telomeres lacking TRF2B and RAP1 cluster into structures termed ultrabright telomeres (UTs). HDR factors localize to UTs, and UT formation is abolished by RNaseH1, DDX21 and ADAR1p110, suggesting that they contain DNA-RNA hybrids. Interaction between the BRCT domain of RAP1 and KU70/KU80 is also required to repress UT formation. Expressing TRF2∆B in Rap1-/- cells resulted in aberrant lamin A localization in the nuclear envelope and dramatically increased UT formation. Expressing lamin A phosphomimetic mutants induced nuclear envelope rupturing and aberrant HDR-mediated UT formation. Our results highlight the importance of shelterin and proteins in the nuclear envelope in repressing aberrant telomere-telomere recombination to maintain telomere homeostasis.

Genomic Patterns of Malignant Peripheral Nerve Sheath Tumor (MPNST) Evolution Correlate with Clinical Outcome and Are Detectable in Cell-Free DNA

Malignant peripheral nerve sheath tumor (MPNST), an aggressive soft-tissue sarcoma, occurs in people with neurofibromatosis type 1 (NF1) and sporadically. Whole-genome and multiregional exome sequencing, transcriptomic, and methylation profiling of 95 tumor samples revealed the order of genomic events in tumor evolution. Following biallelic inactivation of NF1, loss of CDKN2A or TP53 with or without inactivation of polycomb repressive complex 2 (PRC2) leads to extensive somatic copy-number aberrations (SCNA). Distinct pathways of tumor evolution are associated with inactivation of PRC2 genes and H3K27 trimethylation (H3K27me3) status. Tumors with H3K27me3 loss evolve through extensive chromosomal losses followed by whole-genome doubling and chromosome 8 amplification, and show lower levels of immune cell infiltration. Retention of H3K27me3 leads to extensive genomic instability, but an immune cell-rich phenotype. Specific SCNAs detected in both tumor samples and cell-free DNA (cfDNA) act as a surrogate for H3K27me3 loss and immune infiltration, and predict prognosis.

SIGNIFICANCE

MPNST is the most common cause of death and morbidity for individuals with NF1, a relatively common tumor predisposition syndrome. Our results suggest that somatic copy-number and methylation profiling of tumor or cfDNA could serve as a biomarker for early diagnosis and to stratify patients into prognostic and treatment-related subgroups. This article is highlighted in the In This Issue feature, p. 517.

Telomere Fragility and MiDAS: Managing the Gaps at the End of the Road

Telomeres present inherent difficulties to the DNA replication machinery due to their repetitive sequence content, formation of non-B DNA secondary structures, and the presence of the nucleo-protein t-loop. Especially in cancer cells, telomeres are hot spots for replication stress, which can result in a visible phenotype in metaphase cells termed "telomere fragility". A mechanism cells employ to mitigate replication stress, including at telomeres, is DNA synthesis in mitosis (MiDAS). While these phenomena are both observed in mitotic cells, the relationship between them is poorly understood; however, a common link is DNA replication stress. In this review, we will summarize what is known to regulate telomere fragility and telomere MiDAS, paying special attention to the proteins which play a role in these telomere phenotypes.

Nonhomologous Chromosome Interactions in Prophase I: Dynamics of Bizarre Meiotic Contacts in the Alay Mole Vole Ellobius alaicus (Mammalia, Rodentia)

Nonhomologous chromosome interactions take place in both somatic and meiotic cells. Prior to this study, we had discovered special contacts through the SYCP3 (synaptonemal complex protein 3) filament between the short arms of nonhomologous acrocentrics at the pachytene stage in the Alay mole vole, and these contacts demonstrate several patterns from proximity to the complete fusion stage. Here, we investigated the nonhomologous chromosome contacts in meiotic prophase I. It turned out that such contacts do not introduce changes into the classic distribution of DNA double-strand breaks. It is noteworthy that not all meiotic contacts were localized in the H3k9me3-positive heterochromatic environment. Both in the mid zygotene and in the early-mid diplotene, three types of contacts (proximity, touching, and anchoring/tethering) were observed, whereas fusion seems to be characteristic only for pachytene. The number of contacts in the mid pachytene is significantly higher than that in the zygotene, and the distance between centromeres in nonhomologous contacts is also the smallest in mid pachytene for all types of contacts. Thus, this work provides a new insight into the behavior of meiotic contacts during prophase I and points to avenues of further research.

Connecting telomere maintenance and regulation to the developmental origin and differentiation states of neuroblastoma tumor cells

A cardinal feature that distinguishes clinically high-risk neuroblastoma from low-risk tumors is telomere maintenance. Specifically, neuroblastoma tumors with either active telomerase or alternative lengthening of telomeres exhibit aggressive growth characteristics that lead to poor outcomes, whereas tumors without telomere maintenance can be managed with observation or minimal treatment. Even though the need for cancer cells to maintain telomere DNA-in order to sustain cell proliferation-is well established, recent studies suggest that the neural crest origin of neuroblastoma may enforce unique relationships between telomeres and tumor malignancy. Specifically in neuroblastoma, telomere structure and telomerase activity are correlated with the adrenergic/mesenchymal differentiation states, and manipulating telomerase activity can trigger tumor cell differentiation. Both findings may reflect features of normal neural crest development. This review summarizes recent advances in the characterization of telomere structure and telomere maintenance mechanisms in neuroblastoma and discusses the findings in the context of relevant literature on telomeres during embryonic and neural development. Understanding the canonical and non-canonical roles of telomere maintenance in neuroblastoma could reveal vulnerabilities for telomere-directed therapies with potential applications to other pediatric malignancies.

Telomeric 8-oxo-guanine drives rapid premature senescence in the absence of telomere shortening

Oxidative stress is a primary cause of cellular senescence and contributes to the etiology of numerous human diseases. Oxidative damage to telomeric DNA has been proposed to cause premature senescence by accelerating telomere shortening. Here, we tested this model directly using a precision chemoptogenetic tool to produce the common lesion 8-oxo-guanine (8oxoG) exclusively at telomeres in human fibroblasts and epithelial cells. A single induction of telomeric 8oxoG is sufficient to trigger multiple hallmarks of p53-dependent senescence. Telomeric 8oxoG activates ATM and ATR signaling, and enriches for markers of telomere dysfunction in replicating, but not quiescent cells. Acute 8oxoG production fails to shorten telomeres, but rather generates fragile sites and mitotic DNA synthesis at telomeres, indicative of impaired replication. Based on our results, we propose that oxidative stress promotes rapid senescence by producing oxidative base lesions that drive replication-dependent telomere fragility and dysfunction in the absence of shortening and shelterin loss.

This study uncovers a new mechanism linking oxidative stress to telomere-driven senescence. A common oxidative lesion at telomeres causes rapid premature cellular aging by inducing telomere fragility, rather than telomere shortening.

GSK3 inhibition rescues growth and telomere dysfunction in dyskeratosis congenita iPSC-derived type II alveolar epithelial cells

Dyskeratosis congenita (DC) is a rare genetic disorder characterized by deficiencies in telomere maintenance leading to very short telomeres and the premature onset of certain age-related diseases, including pulmonary fibrosis (PF). PF is thought to derive from epithelial failure, particularly that of type II alveolar epithelial (AT2) cells, which are highly dependent on Wnt signaling during development and adult regeneration. We use human induced pluripotent stem cell-derived AT2 (iAT2) cells to model how short telomeres affect AT2 cells. Cultured DC mutant iAT2 cells accumulate shortened, uncapped telomeres and manifest defects in the growth of alveolospheres, hallmarks of senescence, and apparent defects in Wnt signaling. The GSK3 inhibitor, CHIR99021, which mimics the output of canonical Wnt signaling, enhances telomerase activity and rescues the defects. These findings support further investigation of Wnt agonists as potential therapies for DC-related pathologies.

Telomere Targeting Approaches in Cancer: Beyond Length Maintenance

Telomeres are crucial structures that preserve genome stability. Their progressive erosion over numerous DNA duplications determines the senescence of cells and organisms. As telomere length homeostasis is critical for cancer development, nowadays, telomere maintenance mechanisms are established targets in cancer treatment. Besides telomere elongation, telomere dysfunction impinges on intracellular signaling pathways, in particular DNA damage signaling and repair, affecting cancer cell survival and proliferation. This review summarizes and discusses recent findings in anticancer drug development targeting different “telosome” components.

Functions of ADP-ribose transferases in the maintenance of telomere integrity

The ADP-ribose transferase (ART) family comprises 17 enzymes that catalyze mono- or poly-ADP-ribosylation, a post-translational modification of proteins. Present in all subcellular compartments, ARTs are implicated in a growing number of biological processes including DNA repair, replication, transcription regulation, intra- and extra-cellular signaling, viral infection and cell death. Five members of the family, PARP1, PARP2, PARP3, tankyrase 1 and tankyrase 2 are mainly described for their crucial functions in the maintenance of genome stability. It is well established that the most describedrole of PARP1, 2 and 3 is the repair of DNA lesions while tankyrases 1 and 2 are crucial for maintaining the integrity of telomeres. Telomeres, nucleoprotein complexes located at the ends of eukaryotic chromosomes, utilize their unique structure and associated set of proteins to orchestrate the mechanisms necessary for their own protection and replication. While the functions of tankyrases 1 and 2 at telomeres are well known, several studies have also brought PARP1, 2 and 3 to the forefront of telomere protection. The singular quality of the telomeric environment has highlighted protein interactions and molecular pathways distinct from those described throughout the genome. The aim of this review is to provide an overview of the current knowledge on the multiple roles of PARP1, PARP2, PARP3, tankyrase 1 and tankyrase 2 in the maintenance and preservation of telomere integrity.

Telomeric C-circles localize at nuclear pore complexes in Saccharomyces cerevisiae

As in human cells, yeast telomeres can be maintained in cells lacking telomerase activity by recombination-based mechanisms known as ALT (Alternative Lengthening of Telomeres). A hallmark of ALT human cancer cells are extrachromosomal telomeric DNA elements called C-circles, whose origin and function have remained unclear. Here, we show that extrachromosomal telomeric C-circles in yeast can be detected shortly after senescence crisis and concomitantly with the production of survivors arising from "type II" recombination events. We uncover that C-circles bind to the nuclear pore complex (NPC) and to the SAGA-TREX2 complex, similar to other non-centromeric episomal DNA. Disrupting the integrity of the SAGA/TREX2 complex affects both C-circle binding to NPCs and type II telomere recombination, suggesting that NPC tethering of C-circles facilitates formation and/or propagation of the long telomere repeats characteristic of type II survivors. Furthermore, we find that disruption of the nuclear diffusion barrier impairs type II recombination. These results support a model in which concentration of C-circles at NPCs benefits type II telomere recombination, highlighting the importance of spatial coordination in ALT-type mechanisms of telomere maintenance.

Mechanisms and Regulation of Cellular Senescence

Cellular senescence entails a state of an essentially irreversible proliferative arrest in which cells remain metabolically active and secrete a range of pro-inflammatory and proteolytic factors as part of the senescence-associated secretory phenotype. There are different types of senescent cells, and senescence can be induced in response to many DNA damage signals. Senescent cells accumulate in different tissues and organs where they have distinct physiological and pathological functions. Despite this diversity, all senescent cells must be able to survive in a nondividing state while protecting themselves from positive feedback loops linked to the constant activation of the DNA damage response. This capacity requires changes in core cellular programs. Understanding how different cell types can undergo extensive changes in their transcriptional programs, metabolism, heterochromatin patterns, and cellular structures to induce a common cellular state is crucial to preventing cancer development/progression and to improving health during aging. In this review, we discuss how senescent cells continuously evolve after their initial proliferative arrest and highlight the unifying features that define the senescent state.

A hypomorphic allele of telomerase uncovers the minimal functional length of telomeres in Arabidopsis

Despite the essential requirement of telomeric DNA for genome stability, the length of telomere tracts between species substantially differs, raising the question of the minimal length of telomeric DNA necessary for proper function. Here, we address this question using a hypomorphic allele of the telomerase catalytic subunit, TERT. We show that although this construct partially restored telomerase activity to a tert mutant, telomeres continued to shorten over several generations, ultimately stabilizing at a bimodal size distribution. Telomeres on two chromosome arms were maintained at a length of 1 kb, while the remaining telomeres were maintained at 400 bp. The longest telomeres identified in this background were also significantly longer in wild-type populations, suggesting cis-acting elements on these arms either promote telomerase processivity or recruitment. Genetically disrupting telomerase processivity in this background resulted in immediate lethality. Thus, telomeres of 400 bp are both necessary and sufficient for Arabidopsis viability. As this length is the estimated minimal length for t-loop formation, our data suggest that telomeres long enough to form a t-loop constitute the minimal functional length.

Occurrence, functionality and abundance of the TERT promoter mutations

Telomere shortening at chromosomal ends due to the constraints of the DNA replication process acts as a tumor suppressor by restricting the replicative potential in primary cells. Cancers evade that limitation primarily through the reactivation of telomerase via different mechanisms. Mutations within the promoter of the telomerase reverse transcriptase (TERT) gene represent a definite mechanism for the ribonucleic enzyme regeneration predominantly in cancers that arise from tissues with low rates of self-renewal. The promoter mutations cause a moderate increase in TERT transcription and consequent telomerase upregulation to the levels sufficient to delay replicative senescence but not prevent bulk telomere shortening and genomic instability. Since the discovery, a staggering number of studies have resolved the discrete aspects, effects and clinical relevance of the TERT promoter mutations. The promoter mutations link transcription of TERT with oncogenic pathways, associate with markers of poor outcome and define patients with reduced survivals in several cancers. In this review, we discuss the occurrence and impact of the promoter mutations and highlight the mechanism of TERT activation. We further deliberate on the foundational question of the abundance of the TERT promoter mutations and a general dearth of functional mutations within noncoding sequences, as evident from pan-cancer analysis of the whole-genomes. We posit that the favorable genomic constellation within the TERT promoter may be less than a common occurrence in other noncoding functional elements. Besides, the evolutionary constraints limit the functional fraction within the human genome, hence the lack of abundant mutations outside the coding sequences.

Mechanism of MRX inhibition by Rif2 at telomeres

Specific proteins present at telomeres ensure chromosome end stability, in large part through unknown mechanisms. In this work, we address how the Saccharomyces cerevisiae ORC-related Rif2 protein protects telomere. We show that the small N-terminal Rif2 BAT motif (Blocks Addition of Telomeres) previously known to limit telomere elongation and Tel1 activity is also sufficient to block NHEJ and 5' end resection. The BAT motif inhibits the ability of the Mre11-Rad50-Xrs2 complex (MRX) to capture DNA ends. It acts through a direct contact with Rad50 ATP-binding Head domains. Through genetic approaches guided by structural predictions, we identify residues at the surface of Rad50 that are essential for the interaction with Rif2 and its inhibition. Finally, a docking model predicts how BAT binding could specifically destabilise the DNA-bound state of the MRX complex. From these results, we propose that when an MRX complex approaches a telomere, the Rif2 BAT motif binds MRX Head in its ATP-bound resting state. This antagonises MRX transition to its DNA-bound state, and favours a rapid return to the ATP-bound state. Unable to stably capture the telomere end, the MRX complex cannot proceed with the subsequent steps of NHEJ, Tel1-activation and 5' resection.

Sequence, Chromatin and Evolution of Satellite DNA

Satellite DNA consists of abundant tandem repeats that play important roles in cellular processes, including chromosome segregation, genome organization and chromosome end protection. Most satellite DNA repeat units are either of nucleosomal length or 5–10 bp long and occupy centromeric, pericentromeric or telomeric regions. Due to high repetitiveness, satellite DNA sequences have largely been absent from genome assemblies. Although few conserved satellite-specific sequence motifs have been identified, DNA curvature, dyad symmetries and inverted repeats are features of various satellite DNAs in several organisms. Satellite DNA sequences are either embedded in highly compact gene-poor heterochromatin or specialized chromatin that is distinct from euchromatin. Nevertheless, some satellite DNAs are transcribed into non-coding RNAs that may play important roles in satellite DNA function. Intriguingly, satellite DNAs are among the most rapidly evolving genomic elements, such that a large fraction is species-specific in most organisms. Here we describe the different classes of satellite DNA sequences, their satellite-specific chromatin features, and how these features may contribute to satellite DNA biology and evolution. We also discuss how the evolution of functional satellite DNA classes may contribute to speciation in plants and animals.