TPP1 Blocks an ATR-Mediated Resection Mechanism at Telomeres

Molecular Studies on Plant Telomeres: Expanding Horizons in Plant Biology

The integrity of plant genomes is intricately safeguarded by telomeres, the protective caps located at the ends of the chromosome. This review provides a comprehensive analysis of the molecular mechanisms governing the structure, maintenance, and dynamics of plant telomeres, highlighting their genetic and epigenetic regulation and their pivotal roles in plant development, longevity, stress adaptation, and disease resistance. Recent advancements, such as next-generation sequencing and single-molecule imaging, have revolutionized our understanding of telomere biology, unveiling new insights into telomerase activity and telomere-associated genetic variants. Additionally, the review also discusses the challenges and future directions of telomere research, including the potential applications of telomere biology in plant breeding and genetic engineering.

Mechanisms and regulation of DNA end resection in the maintenance of genome stability

DNA end resection is a crucial early step in most DNA double-strand break (DSB) repair pathways. Resection involves the nucleolytic degradation of 5' ends at DSB sites to generate 3' single-stranded DNA overhangs. The first, short-range resection step is catalysed by the nuclease MRE11, acting as part of the MRE11-RAD50-NBS1 complex. Subsequent long-range resection is catalysed by the nucleases EXO1 and/or DNA2. Resected DNA is necessary for homology search and the priming of DNA synthesis in homologous recombination. DNA overhangs may also mediate DNA annealing in the microhomology-mediated end-joining and single-strand annealing pathways, and activate the DNA damage response. By contrast, DNA end resection inhibits DSB repair by non-homologous end-joining. In this Review, we discuss the importance of DNA end resection in various DSB repair pathways, the molecular mechanisms of end resection and its regulation, focusing on phosphorylation and other post-translational modifications that control resection throughout the cell cycle and in response to DNA damage.

Canonical and non-canonical functions of the non-coding RNA component (TERC) of telomerase complex

The telomerase complex consists of a protein component (TERT), which has reverse transcriptase activity, and an RNA component (TERC), which serves as a template for telomere synthesis. Evidence is rapidly accumulating regarding the non-canonical functions of these components in both normal or diseased cells. An oligonucleotide-based drug, the first telomerase inhibitor, secured FDA approval in June 2024. We recently summarized the non-canonical functions of TERT in viral infections and cancer. In this review, we expand on these non-canonical functions of TERC beyond telomere maintenance. Specifically, we explore TERC's roles in cellular aging and senescence, immune regulation, genetic diseases, human cancer, as well as involvement in viral infections and host interactions. Finally, we discuss a transcription product of telomere repeats, TERRA, and explore strategies for targeting TERC as a therapeutic approach.

Targeting Ataxia Telangiectasia-Mutated and Rad3-Related for Anaplastic Thyroid Cancer

BACKGROUND

Anaplastic thyroid cancer (ATC) is one of the most aggressive human malignancies and has a poor prognosis. Ataxia telangiectasia mutated and Rad3 related (ATR) is a key regulator for the DNA damage response and a potential target to treat cancer.

METHODS

We assessed the efficacy of BAY 1895344, an ATR inhibitor, in three ATC cell lines.

RESULTS

BAY 1895344 caused dose-response cytotoxicity in three ATC cell lines. BAY 1895344 induced S-phase and G2-phase arrest, activated caspase-3 activity and induced apoptosis in ATC cells. BAY 1895344 meaningfully retarded the tumor growth of an ATC xenograft model. BAY 1895344 therapy, combined with dabrafenib and trametinib, had synergism in vitro and revealed robust tumor growth suppression in vivo in two xenograft models of ATC harboring mutant BRAFV600E. Furthermore, the combination of BAY 1895344 with lenvatinib was more effective than either agent alone in a xenograft model of ATC.

CONCLUSIONS

These results reveal that BAY 1895344 has potential in treating ATC.

How to write an ending: Telomere replication as a multistep process

Telomeres are protective nucleoprotein caps found at the natural ends of eukaryotic chromosomes and are crucial for the preservation of stable chromosomal structure. In cycling cells, telomeres are maintained by a multi-step process called telomere replication, which involves the eukaryotic replisome navigating a complex repetitive template tightly bound by specific proteins, before terminating at the chromosome end prior to a 5' resection step that generates a protective 3' overhang. In this review, we examine mechanistic aspects of the telomere replication process and consider how individual parts of this multistep event are integrated and coordinated with one-another.

Telomeres and SIRT1 as Biomarkers of Gamete Oxidative Stress, Fertility, and Potential IVF Outcome

The number of infertile couples undergoing in vitro fertilisation (IVF) has increased significantly. The efficacy of this procedure is contingent upon a multitude of factors, including gamete quality. One factor influencing gamete quality is oxidative stress, which leads to telomere damage and accelerates cellular ageing. Identifying new biomarkers that can predict the success of assisted reproduction techniques is a current relevant area of research. In this review, we discuss the potential role of SIRT1, a protein known to protect against oxidative stress and telomeres, which are responsible for genome stability, as biomarkers of gamete quality and assisted reproduction technique outcomes.

Telomere length as a predictor of therapy response and survival in patients diagnosed with ovarian carcinoma

Impaired telomere length (TL) maintenance in ovarian tissue may play a pivotal role in the onset of epithelial ovarian cancer (OvC). TL in either target or surrogate tissue (blood) is currently being investigated for use as a predictor in anti-OvC therapy or as a biomarker of the disease progression, respectively. There is currently an urgent need for an appropriate approach to chemotherapy response prediction. We performed a monochrome multiplex qPCR measurement of TL in peripheral blood leukocytes (PBL) and tumor tissues of 209 OvC patients. The methylation status and gene expression of the shelterin complex and telomerase catalytic subunit (hTERT) were determined within tumor tissues by High-Throughput DNA methylation profiling and RNA sequencing (RNA-Seq) analysis, respectively. The patients sensitive to cancer treatment (n = 46) had shorter telomeres in PBL compared to treatment-resistant patients (n = 93; P = 0.037). In the patients with a different therapy response, transcriptomic analysis showed alterations in the peroxisome proliferator-activated receptor (PPAR) signaling pathway (q = 0.001). Moreover, tumor TL shorter than the median corresponded to better overall survival (OS) (P = 0.006). TPP1 gene expression was positively associated with TL in tumor tissue (P = 0.026). TL measured in PBL could serve as a marker of platinum therapy response in OvC patients. Additionally, TL determined in tumor tissue provides information on OvC patients' OS.

Modified Peptide Molecules As Potential Modulators of Shelterin Protein Functions; TRF1

In this work, we present studies on relatively new and still not well-explored potential anticancer targets which are shelterin proteins, in particular the TRF1 protein can be blocked by in silico designed "peptidomimetic" molecules. TRF1 interacts directly with the TIN2 protein, and this protein-protein interaction is crucial for the proper functioning of telomere, which could be blocked by our novel modified peptide molecules. Our chemotherapeutic approach is based on assumption that modulation of TRF1-TIN2 interaction may be more harmful for cancer cells as cancer telomeres are more fragile than in normal cells. We have shown in vitro within SPR experiments that our modified peptide PEP1 molecule interacts with TRF1, presumably at the site originally occupied by the TIN2 protein. Disturbance of the shelterin complex by studied molecule may not in short term lead to cytotoxic effects, however blocking TRF1-TIN2 resulted in cellular senescence in cellular breast cancer lines used as a cancer model. Thus, our compounds appeared useful as starting model compounds for precise blockage of TRF proteins.

DNA-PK and the TRF2 iDDR inhibit MRN-initiated resection at leading-end telomeres

Telomeres replicated by leading-strand synthesis lack the 3' overhang required for telomere protection. Surprisingly, resection of these blunt telomeres is initiated by the telomere-specific 5' exonuclease Apollo rather than the Mre11-Rad50-Nbs1 (MRN) complex, the nuclease that acts at DNA breaks. Without Apollo, leading-end telomeres undergo fusion, which, as demonstrated here, is mediated by alternative end joining. Here, we show that DNA-PK and TRF2 coordinate the repression of MRN at blunt mouse telomeres. DNA-PK represses an MRN-dependent long-range resection, while the endonuclease activity of MRN-CtIP, which could cleave DNA-PK off of blunt telomere ends, is inhibited in vitro and in vivo by the iDDR of TRF2. AlphaFold-Multimer predicts a conserved association of the iDDR with Rad50, potentially interfering with CtIP binding and MRN endonuclease activation. We propose that repression of MRN-mediated resection is a conserved aspect of telomere maintenance and represents an ancient feature of DNA-PK and the iDDR.

Human POT1 protects the telomeric ds-ss DNA junction by capping the 5' end of the chromosome

Protection of telomeres 1 (POT1) is the 3' single-stranded overhang-binding telomeric protein that prevents an ataxia telangiectasia and Rad3-related (ATR) DNA damage response (DDR) at chromosome ends. What precludes the DDR machinery from accessing the telomeric double-stranded-single-stranded junction is unknown. We demonstrate that human POT1 binds this junction by recognizing the phosphorylated 5' end of the chromosome. High-resolution crystallographic structures reveal that the junction is capped by POT1 through a "POT-hole" surface, the mutation of which compromises junction protection in vitro and telomeric 5'-end definition and DDR suppression in human cells. Whereas both mouse POT1 paralogs bind the single-stranded overhang, POT1a, not POT1b, contains a POT-hole and binds the junction, which explains POT1a's sufficiency for end protection. Our study shifts the paradigm for DDR suppression at telomeres by highlighting the importance of protecting the double-stranded-single-stranded junction.

A novel signature predicts prognosis and immunotherapy in lung adenocarcinoma based on cancer-associated fibroblasts

BACKGROUND

Extensive research has established the significant correlations between cancer-associated fibroblasts (CAFs) and various stages of cancer development, including initiation, angiogenesis, progression, and resistance to therapy. In this study, we aimed to investigate the characteristics of CAFs in lung adenocarcinoma (LUAD) and develop a risk signature to predict the prognosis of patients with LUAD.

METHODS

We obtained single-cell RNA sequencing (scRNA-seq) and bulk RNA-seq data from the public database. The Seurat R package was used to process the scRNA-seq data and identify CAF clusters based on several biomarkers. CAF-related prognostic genes were further identified using univariate Cox regression analysis. To reduce the number of genes, Lasso regression was performed, and a risk signature was established. A novel nomogram that incorporated the risk signature and clinicopathological features was developed to predict the clinical applicability of the model. Additionally, we conducted immune landscape and immunotherapy responsiveness analyses. Finally, we performed in vitro experiments to verify the functions of EXO1 in LUAD.

RESULTS

We identified 5 CAF clusters in LUAD using scRNA-seq data, of which 3 clusters were significantly associated with prognosis in LUAD. A total of 492 genes were found to be significantly linked to CAF clusters from 1731 DEGs and were used to construct a risk signature. Moreover, our immune landscape exploration revealed that the risk signature was significantly related to immune scores, and its ability to predict responsiveness to immunotherapy was confirmed. Furthermore, a novel nomogram incorporating the risk signature and clinicopathological features showed excellent clinical applicability. Finally, we verified the functions of EXP1 in LUAD through in vitro experiments.

CONCLUSIONS

The risk signature has proven to be an excellent predictor of LUAD prognosis, stratifying patients more appropriately and precisely predicting immunotherapy responsiveness. The comprehensive characterization of LUAD based on the CAF signature can predict the response of LUAD to immunotherapy, thus offering fresh perspectives into the management of LUAD patients. Our study ultimately confirms the role of EXP1 in facilitating the invasion and growth of tumor cells in LUAD. Nevertheless, further validation can be achieved by conducting in vivo experiments.

CST/Polα/primase-mediated fill-in synthesis at DSBs

DNA double-strand breaks (DSBs) pose a major threat to the genome, so the efficient repair of such breaks is essential. DSB processing and repair is affected by 53BP1, which has been proposed to determine repair pathway choice and/or promote repair fidelity. 53BP1 and its downstream effectors, RIF1 and shieldin, control 3' overhang length, and the mechanism has been a topic of intensive research. Here, we highlight recent evidence that 3' overhang control by 53BP1 occurs through fill-in synthesis of resected DSBs by CST/Polα/primase. We focus on the crucial role of fill-in synthesis in BRCA1-deficient cells treated with PARPi and discuss the notion of fill-in synthesis in other specialized settings and in the repair of random DSBs. We argue that - in addition to other determinants - repair pathway choice may be influenced by the DNA sequence at the break which can impact CST binding and therefore the deployment of Polα/primase fill-in.

Targeting the DNA Damage Response for Cancer Therapy by Inhibiting the Kinase Wee1

Cancer cells typically heavily rely on the G2/M checkpoint to survive endogenous and exogenous DNA damage, such as genotoxic stress due to genome instability or radiation and chemotherapy. The key regulator of the G2/M checkpoint, the cyclin-dependent kinase 1 (CDK1), is tightly controlled, including by its phosphorylation state. This posttranslational modification, which is determined by the opposing activities of the phosphatase cdc25 and the kinase Wee1, allows for a more rapid response to cellular stress than via the synthesis or degradation of modulatory interacting proteins, such as p21 or cyclin B. Reducing Wee1 activity results in ectopic activation of CDK1 activity and drives premature entry into mitosis with unrepaired or under-replicated DNA and causing mitotic catastrophe. Here, we review efforts to use small molecule inhibitors of Wee1 for therapeutic purposes, including strategies to combine Wee1 inhibition with genotoxic agents, such as radiation therapy or drugs inducing replication stress, or inhibitors of pathways that show synthetic lethality with Wee1. Furthermore, it become increasingly clear that Wee1 inhibition can also modulate therapeutic immune responses. We will discuss the mechanisms underlying combination treatments identifying both cell intrinsic and systemic anti-tumor activities.

Regulation of Gene Expression by Telomere Position Effect

Many diseases that involve malignant tumors in the elderly affect the quality of human life; therefore, the relationship between aging and pathogenesis in geriatric diseases must be under-stood to develop appropriate treatments for these diseases. Recent reports have shown that epigenetic regulation caused by changes in the local chromatin structure plays an essential role in aging. This review provides an overview of the roles of telomere shortening on genomic structural changes during an age-dependent shift in gene expression. Telomere shortening is one of the most prominent events that is involved in cellular aging and it affects global gene expression through genome rearrangement. This review provides novel insights into the roles of telomere shortening in disease-affected cells during pathogenesis and suggests novel therapeutic approaches.

Keeping Balance Between Genetic Stability and Plasticity at the Telomere and Subtelomere of Trypanosoma brucei

Telomeres, the nucleoprotein complexes at chromosome ends, are well-known for their essential roles in genome integrity and chromosome stability. Yet, telomeres and subtelomeres are frequently less stable than chromosome internal regions. Many subtelomeric genes are important for responding to environmental cues, and subtelomeric instability can facilitate organismal adaptation to extracellular changes, which is a common theme in a number of microbial pathogens. In this review, I will focus on the delicate and important balance between stability and plasticity at telomeres and subtelomeres of a kinetoplastid parasite, Trypanosoma brucei, which causes human African trypanosomiasis and undergoes antigenic variation to evade the host immune response. I will summarize the current understanding about T. brucei telomere protein complex, the telomeric transcript, and telomeric R-loops, focusing on their roles in maintaining telomere and subtelomere stability and integrity. The similarities and differences in functions and underlying mechanisms of T. brucei telomere factors will be compared with those in human and yeast cells.

Shelterin complex gene: Prognosis and therapeutic vulnerability in cancer

Telomere encompasses a (TTAGGG)n tandem repeats, and its dysfunction has emerged as the epicenter of driving carcinogenesis by promoting genetic instability. Indeed, they play an essential role in stabilizing chromosomes and therefore protecting them from end-to-end fusion and DNA degradation. Telomere length homeostasis is regulated by several key players including shelterin complex genes, telomerase, and various other regulators. Targeting these regulatory players can be a good approach to combat cancer as telomere length is increasingly correlated with cancer initiation and progression. In this review, we have aimed to describe the telomere length regulator's role in prognostic significance and important drug targets in breast cancer. Moreover, we also assessed alteration in telomeric function by various telomere length regulators and compares this to the regulatory mechanisms that can be associated with clinical biomarkers in cancer. Using publicly available software we summarized mutational and CpG island prediction analysis of the TERT gene breast cancer patient database. Studies have reported that the TERT gene has prognostic significance in breast cancer progression however mechanistic approaches are not defined yet. Interestingly, we reported using the UCSC Xena web-based tool, we confirmed a positive correlation of shelterin complex genes TERF1 and TERF2 in recurrent free survival, indicating the critical role of these genes in breast cancer prognosis. Moreover, the epigenetic landscape of DNA damage repair genes in different breast cancer subtypes also being analyzed using the UCSC Xena database. Together, these datasets provide a comprehensive resource for shelterin complex gene profiles and define epigenetic landscapes of DNA damage repair genes which reveals the key role of shelterin complex genes in breast cancer with the potential to identify novel and actionable targets for treatment.

Identification and characterization of novel ACD variants: modulation of TPP1 protein level offsets the impact of germline loss-of-function variants on telomere length

Telomere biology disorders, largely characterized by telomere lengths below the first centile for age, are caused by variants in genes associated with telomere replication, structure, or function. One of these genes, ACD, which encodes the shelterin protein TPP1, is associated with both autosomal dominantly and autosomal recessively inherited telomere biology disorders. TPP1 recruits telomerase to telomeres and stimulates telomerase processivity. Several studies probing the effect of various synthetic or patient-derived variants have mapped specific residues and regions of TPP1 that are important for interaction with TERT, the catalytic component of telomerase. However, these studies have come to differing conclusions regarding ACD haploinsufficiency. Here, we report a proband with compound heterozygous novel variants in ACD (NM_001082486.1)-c.505_507delGAG, p.(Glu169del); and c.619delG, p.(Asp207Thrfs*22)-and a second proband with a heterozygous chromosomal deletion encompassing ACD: arr[hg19] 16q22.1(67,628,846-67,813,408)x1. Clinical data, including symptoms and telomere length within the pedigrees, suggested that loss of one ACD allele was insufficient to induce telomere shortening or confer clinical features. Further analyses of lymphoblastoid cell lines showed decreased nascent ACD RNA and steady-state mRNA, but normal TPP1 protein levels, in cells containing heterozygous ACD c.619delG, p.(Asp207Thrfs*22), or the ACD-encompassing chromosomal deletion compared to controls. Based on our results, we conclude that cells are able to compensate for loss of one ACD allele by activating a mechanism to maintain TPP1 protein levels, thus maintaining normal telomere length.

Characterization of t-loop formation by TRF2

T-loops are thought to hide telomeres from DNA damage signaling and DSB repair pathways. T-loop formation requires the shelterin component TRF2, which represses ATM signaling and NHEJ. Here we establish that TRF2 alone, in the absence of other shelterin proteins can form t-loops. Mouse and human cells contain two isoforms of TRF2, one of which is uncharacterized. We show that both isoforms protect telomeres and form t-loops. The isoforms are not cell cycle regulated and t-loops are present in G1, S, and G2.  Using the DNA wrapping deficient TRF2 Topless mutant, we confirm its inability to form t-loops and repress ATM. However, since the mutant is also defective in repression of NHEJ and telomeric localization, the role of topological changes in telomere protection remains unclear.  Finally, we show that Rad51 does not affect t-loop frequencies or telomere protection. Therefore, alternative models for how TRF2 forms t-loops should be explored.

TPP1 Enhances the Therapeutic Effects of Transplanted Aged Mesenchymal Stem Cells in Infarcted Hearts via the MRE11/AKT Pathway

Background

Poor cell survival after transplantation restricts the therapeutic potential of mesenchymal stem cell (MSC) transplantation into infarcted hearts, particularly in older individuals. TPP1, a component of the shelterin complex that is involved in telomere protection, is highly expressed in young MSCs but declines in aged ones. Here, we explore whether TPP1 overexpression in aged mouse MSCs improves cell viability in vivo and in vitro.

Methods

Aged mouse MSCs overexpressing TPP1 were injected into the peri-infarct area of the mouse heart after left anterior descending coronary artery ligation. In parallel, to evaluate cellular-level effects, H₂O₂ was applied to MSCs in vitro to mimic the microenvironment of myocardial injury.

Results

In vivo, the transplantation of aged MSCs overexpressing TPP1 resulted in improved cell survival, enhanced cardiac function, and reduced fibrosis compared to unmodified aged MSCs. In vitro, TPP1 overexpression protected aged MSCs from H₂O₂-induced apoptosis and enhanced DNA double-strand break (DSB) repair. In addition, the phosphorylation of AKT and the key DSB repair protein MRE11 were both significantly upregulated in aged MSCs that overexpressed TPP1.

Conclusions

Our results reveal that TPP1 can enhance DNA repair through the AKT/MRE11 pathway, thereby improving the therapeutic effects of aged MSC transplantation and offering significant potential for the clinical application of autologous transplantation in aged patients.

Identification of Germline Mutations in Melanoma Patients with Early Onset, Double Primary Tumors, or Family Cancer History by NGS Analysis of 217 Genes

Cutaneous melanoma is the deadliest skin malignity with a rising prevalence worldwide. Patients carrying germline mutations in melanoma-susceptibility genes face an increased risk of melanoma and other cancers. To assess the spectrum of germline variants, we analyzed 264 Czech melanoma patients indicated for testing due to early melanoma (at <25 years) or the presence of multiple primary melanoma/melanoma and other cancer in their personal and/or family history. All patients were analyzed by panel next-generation sequencing targeting 217 genes in four groups: high-to-moderate melanoma risk genes, low melanoma risk genes, cancer syndrome genes, and other genes with an uncertain melanoma risk. Population frequencies were assessed in 1479 population-matched controls. Selected POT1 and CHEK2 variants were characterized by functional assays. Mutations in clinically relevant genes were significantly more frequent in melanoma patients than in controls (31/264; 11.7% vs. 58/1479; 3.9%; p = 2.0 × 10⁻⁶). A total of 9 patients (3.4%) carried mutations in high-to-moderate melanoma risk genes (CDKN2A, POT1, ACD) and 22 (8.3%) patients in other cancer syndrome genes (NBN, BRCA1/2, CHEK2, ATM, WRN, RB1). Mutations in high-to-moderate melanoma risk genes (OR = 52.2; 95%CI 6.6–413.1; p = 3.2 × 10⁻⁷) and in other cancer syndrome genes (OR = 2.3; 95%CI 1.4–3.8; p = 0.003) were significantly associated with melanoma risk. We found an increased potential to carry these mutations (OR = 2.9; 95%CI 1.2–6.8) in patients with double primary melanoma, melanoma and other primary cancer, but not in patients with early age at onset. The analysis revealed affected genes in Czech melanoma patients and identified individuals who may benefit from genetic testing and future surveillance management of mutation carriers.

Shelterin Complex at Telomeres: Implications in Ageing

Different factors influence the development‎ and control of ageing. It is well known that progressive telomere shorting is one of the molecular mechanisms underlying ageing. The shelterin complex consists of six telomere-specific proteins which are involved in the protection of chromosome ends. More particularly, this vital complex protects the telomeres from degradation, prevents from activation of unwanted repair systems, regulates the activity of telomerase, and has a crucial role in cellular senescent and ageing-related pathologies. This review explores the organization and function of telomeric DNA along with the mechanism of telomeres during ageing, followed by a discussion of the critical role of shelterin components and their changes during ageing.

The Replisome Mediates A-NHEJ Repair of Telomeres Lacking POT1-TPP1 Independently of MRN Function

Telomeres use shelterin to protect chromosome ends from activating the DNA damage sensor MRE11-RAD50-NBS1 (MRN), repressing ataxia-telangiectasia, mutated (ATM) and ATM and Rad3-related (ATR) dependent DNA damage checkpoint responses. The MRE11 nuclease is thought to be essential for the resection of the 5' C-strand to generate the microhomologies necessary for alternative non-homologous end joining (A-NHEJ) repair. In the present study, we uncover DNA damage signaling and repair pathways engaged by components of the replisome complex to repair dysfunctional telomeres. In cells lacking MRN, single-stranded telomeric overhangs devoid of POT1-TPP1 do not recruit replication protein A (RPA), ATR-interacting protein (ATRIP), and RAD 51. Rather, components of the replisome complex, including Claspin, Proliferating cell nuclear antigen (PCNA), and Downstream neighbor of SON (DONSON), initiate DNA-PKcs-mediated p-CHK1 activation and A-NHEJ repair. In addition, Claspin directly interacts with TRF2 and recruits EXO1 to newly replicated telomeres to promote 5' end resection. Our data indicate that MRN is dispensable for the repair of dysfunctional telomeres lacking POT1-TPP1 and highlight the contributions of the replisome in telomere repair.

Moving Mountains-The BRCA1 Promotion of DNA Resection

DNA double-strand breaks (DSBs) occur in our cells in the context of chromatin. This type of lesion is toxic, entirely preventing genome continuity and causing cell death or terminal arrest. Several repair mechanisms can act on DNA surrounding a DSB, only some of which carry a low risk of mutation, so that which repair process is utilized is critical to the stability of genetic material of cells. A key component of repair outcome is the degree of DNA resection directed to either side of the break site. This in turn determines the subsequent forms of repair in which DNA homology plays a part. Here we will focus on chromatin and chromatin-bound complexes which constitute the "mountains" that block resection, with a particular focus on how the breast and ovarian cancer predisposition protein-1 (BRCA1) contributes to repair outcomes through overcoming these blocks.

The N Terminus of the OB Domain of Telomere Protein TPP1 Is Critical for Telomerase Action

Telomerase recruitment to telomeres and enzymatic processivity are mediated by TPP1, an essential component of telomere integrity and telomerase function. A surface on the OB domain of TPP1 called the TEL patch is critical for TPP1's telomerase-associated functions. Here, we identify a separate region in the N terminus of the OB domain (termed NOB) of TPP1 that, like the TEL patch, is essential for telomerase repeat addition processivity in vitro as well as telomerase recruitment to telomeres and telomere lengthening in cells. Although well-conserved among most mammalian TPP1 homologs, the NOB region in mice is distinct. Swapping the sequence of human NOB into mouse TPP1 allows it to stimulate human telomerase, qualifying NOB as an important determinant of species specificity for TPP1-telomerase interaction. Our studies show that TPP1 NOB is critical for telomerase function and demonstrate that the telomerase interaction surface on TPP1 is more elaborate than previously appreciated.

Telomerase and Telomeres in Endometrial Cancer

Telomeres at the termini of human chromosomes are shortened with each round of cell division due to the “end replication problem” as well as oxidative stress. During carcinogenesis, cells acquire or retain mechanisms to maintain telomeres to avoid initiation of cellular senescence or apoptosis and halting cell division by critically short telomeres. The unique reverse transcriptase enzyme complex, telomerase, catalyzes the maintenance of telomeres but most human somatic cells do not have sufficient telomerase activity to prevent telomere shortening. Tissues with high and prolonged replicative potential demonstrate adequate cellular telomerase activity to prevent telomere erosion, and high telomerase activity appears to be a critical feature of most (80–90%) epithelial cancers, including endometrial cancer. Endometrial cancers regress in response to progesterone which is frequently used to treat advanced endometrial cancer. Endometrial telomerase is inhibited by progestogens and deciphering telomere and telomerase biology in endometrial cancer is therefore important, as targeting telomerase (a downstream target of progestogens) in endometrial cancer may provide novel and more effective therapeutic avenues. This review aims to examine the available evidence for the role and importance of telomere and telomerase biology in endometrial cancer.

Shieldin - the protector of DNA ends

DNA double-strand breaks are a threat to genome integrity and cell viability. The nucleolytic processing of broken DNA ends plays a central role in dictating the repair processes that will mend these lesions. Usually, DNA end resection promotes repair by homologous recombination, whereas minimally processed ends are repaired by non-homologous end joining. Important in this process is the chromatin-binding protein 53BP1, which inhibits DNA end resection. How 53BP1 shields DNA ends from nucleases has been an enduring mystery. The recent discovery of shieldin, a four-subunit protein complex with single-stranded DNA-binding activity, illuminated a strong candidate for the ultimate effector of 53BP1-dependent end protection. Shieldin consists of REV7, a known 53BP1-pathway component, and three hitherto uncharacterized proteins: C20orf196 (SHLD1), FAM35A (SHLD2), and CTC-534A2.2 (SHLD3). Shieldin promotes many 53BP1-associated activities, such as the protection of DNA ends, non-homologous end joining, and immunoglobulin class switching. This review summarizes the identification of shieldin and the various models of shieldin action and highlights some outstanding questions requiring answers to gain a full molecular understanding of shieldin function.

An N-terminal Flag-tag impairs TPP1 regulation of telomerase function

The shelterin protein complex protects natural chromosome ends from being recognized as DNA damage sites and also regulates the synthesis of telomeric repeats by telomerase. TPP1, a shelterin subunit that is essential for telomerase extension of telomeres, has been studied intensively in recent years. Many such studies utilize epitope tagged TPP1, but it is unclear how the tags may affect the multiple cellular functions of TPP1. Here we analyzed the effect of adding a 3x Flag epitope tag to the N- or C-terminus of TPP1. While the position of the tag did not affect TPP1's interaction within the shelterin complex or its localization to telomeres, the N-terminal Flag tag on TPP1 impaired telomerase function, resulting in reduced telomerase processivity in vitro and a failure to stimulate telomere elongation in vivo. The C-terminally Flag-tagged TPP1, in contrast, behaved similarly to untagged TPP1 in all functional aspects examined. These findings suggest that caution is required when utilizing epitope tagged TPP1 to study its regulation of telomerase function.

Inhibiting Wee1 and ATR kinases produces tumor-selective synthetic lethality and suppresses metastasis

We used the cancer-intrinsic property of oncogene-induced DNA damage as the base for a conditional synthetic lethality approach. To target mechanisms important for cancer cell adaptation to genotoxic stress and thereby to achieve cancer cell-specific killing, we combined inhibition of the kinases ATR and Wee1. Wee1 regulates cell cycle progression, whereas ATR is an apical kinase in the DNA-damage response. In an orthotopic breast cancer model, tumor-selective synthetic lethality of the combination of bioavailable ATR and Wee1 inhibitors led to tumor remission and inhibited metastasis with minimal side effects. ATR and Wee1 inhibition had a higher synergistic effect in cancer stem cells than in bulk cancer cells, compensating for the lower sensitivity of cancer stem cells to the individual drugs. Mechanistically, the combination treatment caused cells with unrepaired or under-replicated DNA to enter mitosis leading to mitotic catastrophe. As these inhibitors of ATR and Wee1 are already in phase I/II clinical trials, this knowledge could soon be translated into the clinic, especially as we showed that the combination treatment targets a wide range of tumor cells. Particularly, the antimetastatic effect of combined Wee1/ATR inhibition and the low toxicity of ATR inhibitors compared with Chk1 inhibitors have great clinical potential.

Telomeres in Plants and Humans: Not So Different, Not So Similar

Parallel research on multiple model organisms shows that while some principles of telomere biology are conserved among all eukaryotic kingdoms, we also find some deviations that reflect different evolutionary paths and life strategies, which may have diversified after the establishment of telomerase as a primary mechanism for telomere maintenance. Much more than animals, plants have to cope with environmental stressors, including genotoxic factors, due to their sessile lifestyle. This is, in principle, made possible by an increased capacity and efficiency of the molecular systems ensuring maintenance of genome stability, as well as a higher tolerance to genome instability. Furthermore, plant ontogenesis differs from that of animals in which tissue differentiation and telomerase silencing occur during early embryonic development, and the “telomere clock” in somatic cells may act as a preventive measure against carcinogenesis. This does not happen in plants, where growth and ontogenesis occur through the serial division of apical meristems consisting of a small group of stem cells that generate a linear series of cells, which differentiate into an array of cell types that make a shoot and root. Flowers, as generative plant organs, initiate from the shoot apical meristem in mature plants which is incompatible with the human-like developmental telomere shortening. In this review, we discuss differences between human and plant telomere biology and the implications for aging, genome stability, and cell and organism survival. In particular, we provide a comprehensive comparative overview of telomere proteins acting in humans and in Arabidopsis thaliana model plant, and discuss distinct epigenetic features of telomeric chromatin in these species.

Roles of Telomere Biology in Cell Senescence, Replicative and Chronological Ageing

Telomeres with G-rich repetitive DNA and particular proteins as special heterochromatin structures at the termini of eukaryotic chromosomes are tightly maintained to safeguard genetic integrity and functionality. Telomerase as a specialized reverse transcriptase uses its intrinsic RNA template to lengthen telomeric G-rich strand in yeast and human cells. Cells sense telomere length shortening and respond with cell cycle arrest at a certain size of telomeres referring to the "Hayflick limit." In addition to regulating the cell replicative senescence, telomere biology plays a fundamental role in regulating the chronological post-mitotic cell ageing. In this review, we summarize the current understandings of telomere regulation of cell replicative and chronological ageing in the pioneer model system Saccharomyces cerevisiae and provide an overview on telomere regulation of animal lifespans. We focus on the mechanisms of survivals by telomere elongation, DNA damage response and environmental factors in the absence of telomerase maintenance of telomeres in the yeast and mammals.

Telomere and its role in the aging pathways: telomere shortening, cell senescence and mitochondria dysfunction

Aging is a biological process characterized by a progressive functional decline in tissues and organs, which eventually leads to mortality. Telomeres, the repetitive DNA repeat sequences at the end of linear eukaryotic chromosomes protecting chromosome ends from degradation and illegitimate recombination, play a crucial role in cell fate and aging. Due to the mechanism of replication, telomeres shorten as cells proliferate, which consequently contributes to cellular senescence and mitochondrial dysfunction. Cells are the basic unit of organismal structure and function, and mitochondria are the powerhouse and metabolic center of cells. Therefore, cellular senescence and mitochondrial dysfunction would result in tissue or organ degeneration and dysfunction followed by somatic aging through multiple pathways. In this review, we summarized the main mechanisms of cellular senescence, mitochondrial malfunction and aging triggered by telomere attrition. Understanding the molecular mechanisms involved in the aging process may elicit new strategies for improving health and extending lifespan.

Shelterin-Mediated Telomere Protection

For more than a decade, it has been known that mammalian cells use shelterin to protect chromosome ends. Much progress has been made on the mechanism by which shelterin prevents telomeres from inadvertently activating DNA damage signaling and double-strand break (DSB) repair pathways. Shelterin averts activation of three DNA damage response enzymes [the ataxia-telangiectasia-mutated (ATM) and ataxia telangiectasia and Rad3-related (ATR) kinases and poly(ADP-ribose) polymerase 1 (PARP1)], blocks three DSB repair pathways [classical nonhomologous end joining (c-NHEJ), alternative (alt)-NHEJ, and homology-directed repair (HDR)], and prevents hyper-resection at telomeres. For several of these functions, mechanistic insights have emerged. In addition, much has been learned about how shelterin maintains the telomeric 3' overhang, forms and protects the t-loop structure, and promotes replication through telomeres. These studies revealed that shelterin is compartmentalized, with individual subunits dedicated to distinct aspects of the end-protection problem. This review focuses on the current knowledge of shelterin-mediated telomere protection, highlights differences between human and mouse shelterin, and discusses some of the questions that remain.

53BP1-RIF1-shieldin counteracts DSB resection through CST- and Polα-dependent fill-in

Resection of double-strand breaks (DSBs) dictates the choice between Homology-Directed Repair (HDR), which requires a 3′ overhang, and classical Non-Homologous End Joining (c-NHEJ), which can join unresected ends^1,2. BRCA1 mutant cancers show minimal DSB resection, rendering them HDR deficient and sensitive to PARP1 inhibitors (PARPi)^3–8. When BRCA1 is absent, DSB resection is thought to be prevented by 53BP1, Rif1, and the Rev7/Shld1/Shld2/Shld3 (Shieldin) complex and loss of these factors diminishes PARPi sensitivity^4,6–9. Here we address the mechanism by which 53BP1/Rif1/Shieldin regulate the generation of recombinogenic 3′ overhangs. We report that CST (Ctc1, Stn1, Ten1¹⁰), an RPA-like complex that functions as a Polymeraseα/primase accessory factor¹¹ is a downstream effector in the 53BP1 pathway. CST interacts with Shieldin and localizes with Polα to sites of DNA damage in a 53BP1- and Shieldin-dependent manner. Like loss of 53BP1/Rif1/Shieldin, CST depletion leads to increased resection. Furthermore, in BRCA1-deficient cells, CST blocks Rad51 loading and promotes PARPi efficacy. Finally, Polα inhibition diminishes the effect of PARPi in BRCA1-deficient cells. These data suggest that CST/Polα-mediated fill-in contributes to the control of DSB repair by 53BP1, Rif1, and Shieldin.

Inflammatory factor receptor Toll-like receptor 4 controls telomeres through heterochromatin protein 1 isoforms in liver cancer stem cell

Toll-like receptor 4 (TLR4) which acts as a receptor for lipopolysaccharide (LPS) has been reported to be involved in carcinogenesis. However, the regulatory mechanism of it has not been elucidated. Herein, we demonstrate that TLR4 promotes the malignant growth of liver cancer stem cells. Mechanistically, TLR4 promotes the expression of histone-lysine N-methyltransferase (SUV39 h2) and increases the formation of trimethyl histone H3 lysine 9-heterochromatin protein 1-telomere repeat binding factor 2 (H3K9me3-HP1-TRF2) complex at the telomeric locus under mediation by long non coding RNA urothelial cancer-associated 1 (CUDR). At the telomeric locus, this complex promotes binding of POT1, pPOT1, Exo1, pExo1, SNM1B and pSNM1B but prevents binding of CST/AAF to telomere, thus controlling telomere and maintaining telomere length. Furthermore, TLR4 enhances interaction between HP1α and DNA methyltransferase (DNMT3b), which limits RNA polymerase II deposition on the telomeric repeat-containing RNA (TERRA) promoter region and its elongation, thus inhibiting transcription of TERRA. Ultimately, TLR4 enhances the telomerase activity by reducing the interplay between telomerase reverse transcriptase catalytic subunit (TERT) and TERRA. More importantly, our results reveal that tri-complexes of HP1 isoforms (α, β and γ) are required for the oncogenic action of TLR4. This study elucidates a novel protection mechanism of TLR4 in liver cancer stem cells and suggests that TLR4 can be used as a novel therapeutic target for liver cancer.

Transient Telomerase Inhibition with Imetelstat Impacts DNA Damage Signals and Cell-Cycle Kinetics

Telomerase is the ribonucleoprotein reverse transcriptase that catalyzes the synthesis of telomeres at the ends of linear chromosomes and contributes to proper telomere-loop (T-loop) formation. Formation of the T-loop, an obligate step before cell division can proceed, requires the generation of a 3'-overhang on the G-rich strand of telomeric DNA via telomerase or C-strand specific nucleases. Here, it is discovered that telomerase activity is critical for efficient cell-cycle progression using transient chemical inhibition by the telomerase inhibitor, imetelstat. Telomerase inhibition changed cell cycle kinetics and increased the proportion of cells in G₂-phase, suggesting delayed clearance through this checkpoint. Investigating the possible contribution of unstructured telomere ends to these cell-cycle distribution changes, it was observed that imetelstat treatment induced γH2AX DNA damage foci in a subset of telomerase-positive cells but not telomerase-negative primary human fibroblasts. Chromatin-immunoprecipitation with γH2AX antibodies demonstrated imetelstat treatment-dependent enrichment of this DNA damage marker at telomeres. Notably, the effects of telomerase inhibition on cell cycle profile alterations were abrogated by pharmacological inhibition of the DNA-damage-repair transducer, ATM. Also, imetelstat potentiation of etoposide, a DNA-damaging drug that acts preferentially during S-G₂ phases of the cell cycle, depends on functional ATM signaling. Thus, telomerase inhibition delays the removal of ATM-dependent DNA damage signals from telomeres in telomerase-positive cancer cells and interferes with cell cycle progression through G₂Implications: This study demonstrates that telomerase activity directly facilitates the progression of the cell cycle through modulation of transient telomere dysfunction signals. Mol Cancer Res; 16(8); 1215-25. ©2018 AACR.

Inflammatory related gene IKKα, IKKβ, IKKγ cooperates to determine liver cancer stem cells progression by altering telomere via heterochromatin protein 1-HOTAIR axis

Cancer stem cells are associated with tumor recurrence. IKK is a protein kinase that is composed of IKKα, IKKβ, IKKγ. Herein, we demonstrate that IKKα plus IKKβ promoted and IKKγ inhibited liver cancer stem cell growth in vitro and in vivo. Mechanistically, IKKα plus IKKβ enhanced and IKKγ inhibited the interplay among HP1α, HP1β and HP1γ that competes for the interaction among HP1α, SUZ12, HEZ2. Therefore, IKKα plus IKKβ inhibited and IKKγ enhanced the activity of H3K27 methyltransferase SUZ12 and EZH2, which methylates H3K27 immediately sites on HOTAIR promoter region. Therefore, IKKα plus IKKβ increased and IKKγ decreased the HOTAIR expression. Strikingly, IKKα plus IKKβ decreases and IKKγ increases the HP1α interplays with DNA methyltransferase DNMT3b, which increases or decreases TERRA promoter DNA methylation. Thus IKKα plus IKKβ reduces and IKKγ increases to recruit TRF1 and RNA polymerase II deposition and elongation on the TERRA promoter locus, which increases or decreases TERRA expression. Furthermore, IKKα plus IKKβ decreases/increases and IKKγ increases/decreases the interplay between TERT and TRRRA/between TERT and TREC. Ultimately, IKKα plus IKKβ increases and IKKγ decreases the telomerase activity. On the other hand, at the telomere locus, IKKα plus IKKβ increases/drcreases and IKKγ decreases/increases TRF2, POT1, pPOT1, Exo1, pExo1, SNM1B, pSNM1B/CST-AAF binding, which keep active telomere regulatory genes and poised for telomere length. Strikingly, HOTAIR is required for IKKα plus IKKβ and IKKγ to control telomerase activity and telomere length. These observations suggest that HOTAIR operates the action of IKKα, IKKβ, IKKγ in liver cancer stem cells. This study provides a novel basis to elucidate the oncogenic action of IKKα, IKKβ, IKKγ and prompts that IKKα, IKKβ, IKKγ cooperate to HOTAR to be used as a novel therapeutic targets for liver cancer.

Telomeres: Implications for Cancer Development

Telomeres facilitate the protection of natural ends of chromosomes from constitutive exposure to the DNA damage response (DDR). This is most likely achieved by a lariat structure that hides the linear telomeric DNA through protein-protein and protein-DNA interactions. The telomere shortening associated with DNA replication in the absence of a compensatory mechanism culminates in unmasked telomeres. Then, the subsequent activation of the DDR will define the fate of cells according to the functionality of cell cycle checkpoints. Dysfunctional telomeres can suppress cancer development by engaging replicative senescence or apoptotic pathways, but they can also promote tumour initiation. Studies in telomere dynamics and karyotype analysis underpin telomere crisis as a key event driving genomic instability. Significant attainment of telomerase or alternative lengthening of telomeres (ALT)-pathway to maintain telomere length may be permissive and required for clonal evolution of genomically-unstable cells during progression to malignancy. We summarise current knowledge of the role of telomeres in the maintenance of chromosomal stability and carcinogenesis.

Structural and functional analyses of the mammalian TIN2-TPP1-TRF2 telomeric complex

Telomeres are nucleoprotein complexes that play essential roles in protecting chromosome ends. Mammalian telomeres consist of repetitive DNA sequences bound by the shelterin complex. In this complex, the POT1-TPP1 heterodimer binds to single-stranded telomeric DNAs, while TRF1 and TRF2-RAP1 interact with double-stranded telomeric DNAs. TIN2, the linchpin of this complex, simultaneously interacts with TRF1, TRF2, and TPP1 to mediate the stable assembly of the shelterin complex. However, the molecular mechanism by which TIN2 interacts with these proteins to orchestrate telomere protection remains poorly understood. Here, we report the crystal structure of the N-terminal domain of TIN2 in complex with TIN2-binding motifs from TPP1 and TRF2, revealing how TIN2 interacts cooperatively with TPP1 and TRF2. Unexpectedly, TIN2 contains a telomeric repeat factor homology (TRFH)-like domain that functions as a protein-protein interaction platform. Structure-based mutagenesis analyses suggest that TIN2 plays an important role in maintaining the stable shelterin complex required for proper telomere end protection.

Telomerase in hematologic malignancies

PURPOSE OF REVIEW

The activation of telomere maintenance pathways has long been regarded as a key hallmark of cancer and this has propelled the development of novel inhibitors of telomerase. In this review, we detail the background biology on telomere maintenance in health and disease, then concentrate on the recent preclinical and clinical development behind targeting telomerase in blood cancers.

RECENT FINDINGS

Preclinical and clinical studies have shown that imetelstat, a competitive inhibitor of telomerase, has activity in certain hematologic malignancies, in particular the myeloproliferative neoplasms and acute myeloid leukemia.

SUMMARY

Telomerase inhibition has shown remarkable efficacy in myeloid malignancies, and current and future preclinical and clinical studies are necessary to comprehensively investigate its underlying mechanism of action. Future work should identify the potential genetic susceptibilities to telomerase inhibition therapy, and evaluate rational combinations of telomerase inhibitors with chemotherapy and other novel agents. Robust preclinical evaluation is essential to best translate these new agents successfully into our clinical treatment algorithm for myeloid and other blood cancers.

Abnormal mRNA Expression Levels of Telomere-Binding Proteins Represent Biomarkers in Myelodysplastic Syndromes: A Case-Control Study

Objective:

As evidence was shown that abnormal shortening of telomeres begins to accumulate in myelodysplastic syndrome (MDS) patients, this study was conducted to determine the relationship between the mRNA expression levels of telomere-binding proteins (TRF1/TRF2/TIN2/TPP1/POT1/RAP1) and the risk level in MDS.

Materials and Methods:

There were 40 patients with MDS and 40 normal controls in this study. Methods including telomere content assays and quantitative reverse transcription-polymerase chain reaction were used to examine the mRNA levels of TRF1/TRF2/TIN2/TPP1/POT1/RAP1 in patients with MDS.

Results:

Compared to the normal group used as a control, the mRNA expression levels of RAP1/POT1/TPP1 of the patients with MDS were decreased, whereas their levels of TRF1/TRF2 and TIN2 were increased. A positive correlation was found between the TRF1, TRF2, and TIN2 mRNA expression levels and the risk level of the International Prognostic Scoring System (IPSS) and the World Health Organization Prognostic Scoring System (WPSS) criteria; however, a negative correlation was found between RAP1/POT1/TPP1 mRNA expression levels and the risk levels of IPSS and WPSS criteria.

Conclusion:

Because the reduction of TRF1/TRF2/TIN2 mRNA expression and the increase of RAP1/POT1/TPP1 mRNA expression are closely related to the risk levels of the IPSS and WPSS criteria in MDS, it is thought that these telomere-binding proteins could lead to abnormal telomere length and function, which cause chromosomal abnormalities in MDS. With this evidence, we suggest that those proteins’ mRNA expressions could be used as biomarkers for the assessment of the risk degree of MDS patients.

FANCM, BRCA1, and BLM cooperatively resolve the replication stress at the ALT telomeres

In the mammalian genome, certain genomic loci/regions pose greater challenges to the DNA replication machinery (i.e., the replisome) than others. Such known genomic loci/regions include centromeres, common fragile sites, subtelomeres, and telomeres. However, the detailed mechanism of how mammalian cells cope with the replication stress at these loci/regions is largely unknown. Here we show that depletion of FANCM, or of one of its obligatory binding partners, FAAP24, MHF1, and MHF2, induces replication stress primarily at the telomeres of cells that use the alternative lengthening of telomeres (ALT) pathway as their telomere maintenance mechanism. Using the telomere-specific single-molecule analysis of replicated DNA technique, we found that depletion of FANCM dramatically reduces the replication efficiency at ALT telomeres. We further show that FANCM, BRCA1, and BLM are actively recruited to the ALT telomeres that are experiencing replication stress and that the recruitment of BRCA1 and BLM to these damaged telomeres is interdependent and is regulated by both ATR and Chk1. Mechanistically, we demonstrated that, in FANCM-depleted ALT cells, BRCA1 and BLM help to resolve the telomeric replication stress by stimulating DNA end resection and homologous recombination (HR). Consistent with their roles in resolving the replication stress induced by FANCM deficiency, simultaneous depletion of BLM and FANCM, or of BRCA1 and FANCM, leads to increased micronuclei formation and synthetic lethality in ALT cells. We propose that these synthetic lethal interactions can be explored for targeting the ALT cancers.

DNA Replication Origins and Fork Progression at Mammalian Telomeres

Telomeres are essential chromosomal regions that prevent critical shortening of linear chromosomes and genomic instability in eukaryotic cells. The bulk of telomeric DNA is replicated by semi-conservative DNA replication in the same way as the rest of the genome. However, recent findings revealed that replication of telomeric repeats is a potential cause of chromosomal instability, because DNA replication through telomeres is challenged by the repetitive telomeric sequences and specific structures that hamper the replication fork. In this review, we summarize current understanding of the mechanisms by which telomeres are faithfully and safely replicated in mammalian cells. Various telomere-associated proteins ensure efficient telomere replication at different steps, such as licensing of replication origins, passage of replication forks, proper fork restart after replication stress, and dissolution of post-replicative structures. In particular, shelterin proteins have central roles in the control of telomere replication. Through physical interactions, accessory proteins are recruited to maintain telomere integrity during DNA replication. Dormant replication origins and/or homology-directed repair may rescue inappropriate fork stalling or collapse that can cause defects in telomere structure and functions.

PHF11 promotes DSB resection, ATR signaling, and HR

Resection of double-strand breaks (DSBs) plays a critical role in their detection and appropriate repair. The 3' ssDNA protrusion formed through resection activates the ATR-dependent DNA damage response (DDR) and is required for DSB repair by homologous recombination (HR). Here we report that PHF11 (plant homeodomain finger 11) encodes a previously unknown DDR factor involved in 5' end resection, ATR signaling, and HR. PHF11 was identified based on its association with deprotected telomeres and localized to sites of DNA damage in S phase. Depletion of PHF11 diminished the ATR signaling response to telomere dysfunction and genome-wide DNA damage, reduced end resection at sites of DNA damage, resulted in compromised HR and misrejoining of S-phase DSBs, and increased the sensitivity to DNA-damaging agents. PHF11 interacted with the ssDNA-binding protein RPA and was found in a complex with several nucleases, including the 5' dsDNA exonuclease EXO1. Biochemical experiments demonstrated that PHF11 stimulates EXO1 by overcoming its inhibition by RPA, suggesting that PHF11 acts (in part) by promoting 5' end resection at RPA-bound sites of DNA damage. These findings reveal a role for PHF11 in DSB resection, DNA damage signaling, and DSB repair.

Structural and functional consequences of a disease mutation in the telomere protein TPP1

Telomerase replicates chromosome ends to facilitate continued cell division. Mutations that compromise telomerase function result in stem cell failure diseases, such as dyskeratosis congenita (DC). One such mutation (K170Δ), residing in the telomerase-recruitment factor TPP1, provides an excellent opportunity to structurally, biochemically, and genetically dissect the mechanism of such diseases. We show through site-directed mutagenesis and X-ray crystallography that this TPP1 disease mutation deforms the conformation of two critical amino acids of the TEL [TPP1's glutamate (E) and leucine-rich (L)] patch, the surface of TPP1 that binds telomerase. Using CRISPR-Cas9 technology, we demonstrate that introduction of this mutation in a heterozygous manner is sufficient to shorten telomeres in human cells. Our findings rule out dominant-negative effects of the mutation. Instead, these findings implicate reduced TEL patch dosage in causing telomere shortening. Our studies provide mechanistic insight into telomerase-deficiency diseases and encourage the development of gene therapies to counter such diseases.

Dynamic peptides of human TPP1 fulfill diverse functions in telomere maintenance

Telomeres are specialized nucleoprotein complexes that comprise the ends of linear chromosomes. Human telomeres end in a short, single-stranded DNA (ssDNA) overhang that is recognized and bound by two telomere proteins, POT1 and TPP1. Whereas POT1 binds directly to telomere ssDNA, its interaction with TPP1 is essential for localization of POT1 to the telomere. TPP1 also provides enhanced binding and sequence discrimination that regulates POT1-TPP1 interactions exclusively with telomere ssDNA. Finally, TPP1 recruits telomerase, the enzyme responsible for synthesis of telomere DNA, to the telomere. While the oligosaccharide-oligonucleotide-binding (OB)-fold domain of TPP1 has been solved by X-ray crystallography, the molecular interactions within the POT1-TPP1-ssDNA ternary complex and the conformational changes that contribute to its diverse functions remain ambiguous. We employed hydrogen/deuterium exchange combined with mass spectrometry to identify three peptides, all residing within the OB-fold of TPP1, that exhibit altered exchange rates upon complex formation or ssDNA binding. Mutation of these regions combined with functional assays revealed the diverse contributions of each moiety in protein-protein interactions, regulating telomerase activity or DNA-binding. Together, these functional data combined with biophysical analyses and homology modeling provide a molecular understanding of the diverse contributions of TPP1 in telomere maintenance.