Human RTEL1 stabilizes long G-overhangs allowing telomerase-dependent over-extension.

Telomere maintenance protects the cell against genome instability and senescence. Accelerated telomere attrition is a characteristic of premature aging syndromes including Dyskeratosis congenita (DC). Mutations in hRTEL1 are associated with a severe form of DC called Hoyeraal-Hreidarsson syndrome (HHS). HHS patients carry short telomeres and HHS cells display telomere damage. Here we investigated how hRTEL1 contributes to telomere maintenance in human primary as well as tumor cells. Transient depletion of hRTEL1 resulted in rapid telomere shortening only in the context of telomerase-positive cells with very long telomeres and high levels of telomerase. The effect of hRTEL1 on telomere length is telomerase dependent without impacting telomerase biogenesis or targeting of the enzyme to telomeres. Instead, RTEL1 depletion led to a decrease in both G-overhang content and POT1 association with telomeres with limited telomere uncapping. Strikingly, overexpression of POT1 restored telomere length but not the overhang, demonstrating that G-overhang loss is the primary defect caused by RTEL1 depletion. We propose that hRTEL1 contributes to the maintenance of long telomeres by preserving long G-overhangs, thereby facilitating POT1 binding and elongation by telomerase.

Quantitative assays for measuring human telomerase activity and DNA binding properties.

Telomerase is the ribonucleoprotein enzyme that catalyzes the processive addition of the telomeric DNA repeat 5'-TTAGGG-3' onto chromosome ends. In addition to its fascinating biochemical and enzymatic properties, clinical interest in telomerase stems from its dysregulated expression in ∼90% of human cancers, representing a broad spectrum of diseases. Exploiting telomerase as a therapeutic target and hence identifying and/or evaluating potential inhibitors requires quantitative measurement of its activity. This article presents procedures for measuring multiple aspects of telomerase enzymology that are relevant to both fundamental biochemistry and drug discovery: direct activity assays, DNA binding affinity, DNA dissociation, and cell-based over-expression of the active enzyme complex.

Two-step mechanism involving active-site conformational changes regulates human telomerase DNA binding.

The ribonucleoprotein enzyme telomerase maintains telomeres and is essential for cellular immortality in most cancers. Insight into the telomerase mechanism can be gained from syndromes such as dyskeratosis congenita, in which mutation of telomerase components manifests in telomere dysfunction. We carried out detailed kinetic and thermodynamic analyses of wild-type telomerase and two disease-associated mutations in the reverse transcriptase domain. Differences in dissociation rates between primers with different 3' ends were independent of DNA affinities, revealing that initial binding of telomerase to telomeric DNA occurs through a previously undescribed two-step mechanism involving enzyme conformational changes. Both mutations affected DNA binding, but through different mechanisms: P704S specifically affected protein conformational changes during DNA binding, whereas R865H showed defects in binding to the 3' region of the DNA. To gain further insight at the structural level, we generated the first homology model of the human telomerase reverse transcriptase domain; the positions of P704S and R865H corroborate their observed mechanistic defects, providing validation for the structural model. Our data reveal the importance of protein interactions with the 3' end of telomeric DNA and the role of protein conformational change in telomerase DNA binding, and highlight naturally occurring disease mutations as a rich source of mechanistic insight.

Quantitative Assessment of Right Ventricular Volumes and Ejection Fraction in Patients with Left Ventricular Systolic Dysfunction by Real Time Three-Dimensional Echocardiography versus Cardiac Magnetic Resonance Imaging.

AIMS: The aim of this study was to assess the accuracy and reproducibility of real time three-dimensional echocardiographic (RT3DE) for the determination of right ventricular (RV) volumes and function in patients with left ventricular (LV) systolic dysfunction. METHODS AND RESULTS: Dedicated RT3DE was prospectively performed to assess RV volumes and EF in patients with LV systolic function identified on routine clinical cardiac magnetic resonance (CMR) imaging. RV end-diastolic volume (RV EDV), RV end-systolic volume (RV ESV), and RV EF were obtained using an offline analysis software (TomTec) by two observers blinded to CMR results. In this population of 27 patients with LV systolic dysfunction with a mean LV EF of 36 ± 12%, RV RT3DE dataset could be assessed in 27 of 30 patients (90%). High correlation was noted between RT3DE and CMR for RV EDV, ESV, and EF (r = 0.90, 0.89, and 0.77, respectively). RV EDV was lower by RT3DE as compared to CMR (129 ± 52 vs. 142 ± 53 mL, P = 0.005) while there was no significant difference in RV ESV and RV EF (71 ± 37 vs. 77 ± 45 mL, P = 0.146; 45 ± 11 vs. 48 ± 13%, P = 0.134, respectively). The intraclass correlation coefficient ranged from 0.94 to 0.94 between measurements and from 0.84 to 0.96 between observers. CONCLUSION: Overall, RV volumes and EF assessed by RT3DE correlate well with CMR measurements in patients with LV dysfunction. RT3DE may be used as a more widely available and versatile alternative to CMR for the quantitative assessment of RV size and function in patients with LV dysfunction.

Direct involvement of the TEN domain at the active site of human telomerase.

Telomerase is a ribonucleoprotein that adds DNA to the ends of chromosomes. The catalytic protein subunit of telomerase (TERT) contains an N-terminal domain (TEN) that is important for activity and processivity. Here we describe a mutation in the TEN domain of human TERT that results in a greatly increased primer K(d), supporting a role for the TEN domain in DNA affinity. Measurement of enzyme kinetic parameters has revealed that this mutant enzyme is also defective in dNTP polymerization, particularly while copying position 51 of the RNA template. The catalytic defect is independent of the presence of binding interactions at the 5'-region of the DNA primer, and is not a defect in translocation rate. These data suggest that the TEN domain is involved in conformational changes required to position the 3'-end of the primer in the active site during nucleotide addition, a function which is distinct from the role of the TEN domain in providing DNA binding affinity.

TCAB1 prevents nucleolar accumulation of the telomerase RNA to facilitate telomerase assembly.

Localization of a variety of RNAs to non-membrane-bound cellular compartments such as nucleoli and Cajal bodies is critical for their stability and function. The molecular mechanisms that underly the recruitment and exclusion of RNAs from these phase-separated organelles is incompletely understood. Telomerase is a ribonucleoprotein composed of the reverse transcriptase protein telomerase reverse transcriptase (TERT), the telomerase RNA (TR), and several auxiliary proteins, including TCAB1. Here we show that in the absence of TCAB1, a large fraction of TR is tightly bound to the nucleolus, while TERT is largely excluded from the nucleolus, reducing telomerase assembly. This suggests that nuclear compartmentalization by the non-membrane-bound nucleolus counteracts telomerase assembly, and TCAB1 is required to retain TR in the nucleoplasm. Our work provides insight into the mechanism and functional consequences of RNA recruitment to organelles formed by phase separation and demonstrates that TCAB1 plays an important role in telomerase assembly.

Preprocedural Anxiety in Adults With Congenital Heart Disease: The PANIC Study.

Background: Preprocedural anxiety may have detrimental effects both cognitively and physiologically. Objectives: The objective of this study was to determine the association between state (situational) and trait (persistent in everyday life) anxiety and differences between the adult congenital heart disease (ACHD) and acquired heart disease populations. Methods: The State-Trait Anxiety Inventory and financial stress scale were administered to adults with acquired and CHD at 4 tertiary referral centers in the United States prior to cardiac catheterization. Student's t-test and least absolute shrinkage and selection operator regression analyses were used to assess differences in anxiety between groups and identify the optimal model of predictors of anxiety. Results: Of the 291 patients enrolled, those with CHD (n = 91) were younger (age 41.3 ± 16.3 years vs 64.7 ± 11.3 years, P < 0.001), underwent more cardiac surgeries (P < 0.001), and had higher levels of trait anxiety (t[171] = 2.62, P = 0.001, d = 0.33). There was no difference in state anxiety between groups (t[158.65] = 1.37, P = 0.17, d = 0.18). State anxiety was singularly associated with trait anxiety. Trait anxiety was negatively associated with age and positively associated with state anxiety and financial stress. Patients with CHD of great complexity were more trait (F[2,88] = 4.21, P = 0.02) and state anxious (F[2,87] = 4.59, P = 0.01), though with relatively small effect size. Conclusions: Trait anxiety levels are higher in the ACHD population and directly associated with state anxiety. Specialists caring for ACHD patients should not only recognize the frequency of trait anxiety but also high-risk subgroups that may benefit from psychological or social interventions to reduce preprocedural anxiety.

Targeting telomerase reverse transcriptase with the covalent inhibitor NU-1 confers immunogenic radiation sensitization.

Beyond synthesizing telomere repeats, the telomerase reverse transcriptase (TERT) also serves multiple other roles supporting cancer growth. Blocking telomerase to drive telomere erosion appears impractical, but TERT's non-canonical activities have yet to be fully explored as cancer targets. Here, we used an irreversible TERT inhibitor, NU-1, to examine impacts on resistance to conventional cancer therapies. In vitro, inhibiting TERT sensitized cells to chemotherapy and radiation. NU-1 delayed repair of double-strand breaks, resulting in persistent DNA damage signaling and cellular senescence. Although NU-1 alone did not impact growth of syngeneic CT26 tumors in BALB/c mice, it dramatically enhanced the effects of radiation, leading to immune-dependent tumor elimination. Tumors displayed persistent DNA damage, suppressed proliferation, and increased activated immune infiltrate. Our studies confirm TERT's role in limiting genotoxic effects of conventional therapy but also implicate TERT as a determinant of immune evasion and therapy resistance.

Functional interaction between compound heterozygous TERT mutations causes severe telomere biology disorder.

Telomere biology disorders (TBDs) are a spectrum of multisystem inherited disorders characterized by bone marrow failure, resulting from mutations in the genes encoding telomerase or other proteins involved in maintaining telomere length and integrity. Pathogenicity of variants in these genes can be hard to evaluate, because TBD mutations show highly variable penetrance and genetic anticipation related to inheritance of shorter telomeres with each generation. Thus, detailed functional analysis of newly identified variants is often essential. Herein, we describe a patient with compound heterozygous variants in the TERT gene, which encodes the catalytic subunit of telomerase, hTERT. This patient had the extremely severe Hoyeraal-Hreidarsson form of TBD, although his heterozygous parents were clinically unaffected. Molecular dynamic modeling and detailed biochemical analyses demonstrate that one allele (L557P) affects association of hTERT with its cognate RNA component hTR, whereas the other (K1050E) affects the binding of telomerase to its DNA substrate and enzyme processivity. Unexpectedly, the data demonstrate a functional interaction between the proteins encoded by the two alleles, with wild-type hTERT rescuing the effect of K1050E on processivity, whereas L557P hTERT does not. These data contribute to the mechanistic understanding of telomerase, indicating that RNA binding in one hTERT molecule affects the processivity of telomere addition by the other molecule. This work emphasizes the importance of functional characterization of TERT variants to reach a definitive molecular diagnosis for patients with TBD, and, in particular, it illustrates the importance of analyzing the effects of compound heterozygous variants in combination, to reveal interallelic effects.

A sensitive direct human telomerase activity assay.

Expression of telomerase, the specialized reverse transcriptase that adds 5'-TTAGGG-3' repeats to the ends of human chromosomes, is upregulated in > or =85% of human cancers and tumor cell lines. We describe a direct primer-extension activity assay for human telomerase that displays sensitivity to approximately 10(6) telomerase-positive cells, making the method suitable for use with standard cell culture-based research (Fig. 1). Telomerase is first immunoaffinity purified from cell lysate using an antibody to telomerase and captured using protein G-agarose beads (Steps 14-17). Then telomerase is dissociated from the beads using excess peptide antigen (Step 19). A second affinity purification exploits the stable binding interaction between human telomerase and the telomeric DNA substrate 5'-(TTAGGG)3-3' (dissociation half-life > or = 10 h at 23 degrees C). Modifying neutravidin beads with 5'-biotin-CTAGACCTGTCATCA(TTAGGG)3-3' (Step 5) generates an affinity reagent that captures >90% of immunopurified telomerase (Step 22), providing highly enriched telomerase bound to its DNA substrate in a volume of 20 mul of beads. Addition of assay buffer results in extension of the bead-immobilized DNA substrate (Step 24). Telomerase extension products are released by heating in denaturing formamide buffer to disrupt the avidin-biotin interaction, and then separated and visualized by standard techniques.

Targeted Covalent Inhibition of Telomerase.

Telomerase is a ribonuceloprotein complex responsible for maintaining telomeres and protecting chromosomal integrity. The human telomerase reverse transcriptase (hTERT) is expressed in ∼90% of cancer cells where it confers the capacity for limitless proliferation. Along with its established role in telomere lengthening, telomerase also serves noncanonical extra-telomeric roles in oncogenic signaling, resistance to apoptosis, and enhanced DNA damage response. We report a new class of natural-product-inspired covalent inhibitors of telomerase that target the catalytic active site.

Dephosphorylation of YB-1 is Required for Nuclear Localisation During G2 Phase of the Cell Cycle.

Elevated levels of nuclear Y-box binding protein 1 (YB-1) are linked to poor prognosis in cancer. It has been proposed that entry into the nucleus requires specific proteasomal cleavage. However, evidence for cleavage is contradictory and high YB-1 levels are prognostic regardless of cellular location. Here, using confocal microscopy and mass spectrometry, we find no evidence of specific proteolytic cleavage. Doxorubicin treatment, and the resultant G2 arrest, leads to a significant increase in the number of cells where YB-1 is not found in the cytoplasm, suggesting that its cellular localisation is variable during the cell cycle. Live cell imaging reveals that the location of YB1 is linked to progression through the cell cycle. Primarily perinuclear during G1 and S phases, YB-1 enters the nucleus as cells transition through late G2/M and exits at the completion of mitosis. Atomistic modelling and molecular dynamics simulations show that dephosphorylation of YB1 at serine residues 102, 165 and 176 increases the accessibility of the nuclear localisation signal (NLS). We propose that this conformational change facilitates nuclear entry during late G2/M. Thus, the phosphorylation status of YB1 determines its cellular location.

A mechanism for the extension and unfolding of parallel telomeric G-quadruplexes by human telomerase at single-molecule resolution.

Telomeric G-quadruplexes (G4) were long believed to form a protective structure at telomeres, preventing their extension by the ribonucleoprotein telomerase. Contrary to this belief, we have previously demonstrated that parallel-stranded conformations of telomeric G4 can be extended by human and ciliate telomerase. However, a mechanistic understanding of the interaction of telomerase with structured DNA remained elusive. Here, we use single-molecule fluorescence resonance energy transfer (smFRET) microscopy and bulk-phase enzymology to propose a mechanism for the resolution and extension of parallel G4 by telomerase. Binding is initiated by the RNA template of telomerase interacting with the G-quadruplex; nucleotide addition then proceeds to the end of the RNA template. It is only through the large conformational change of translocation following synthesis that the G-quadruplex structure is completely unfolded to a linear product. Surprisingly, parallel G4 stabilization with either small molecule ligands or by chemical modification does not always inhibit G4 unfolding and extension by telomerase. These data reveal that telomerase is a parallel G-quadruplex resolvase.

Critical Role for Cold Shock Protein YB-1 in Cytokinesis.

High levels of the cold shock protein Y-box-binding protein-1, YB-1, are tightly correlated with increased cell proliferation and progression. However, the precise mechanism by which YB-1 regulates proliferation is unknown. Here, we found that YB-1 depletion in several cancer cell lines and in immortalized fibroblasts resulted in cytokinesis failure and consequent multinucleation. Rescue experiments indicated that YB-1 was required for completion of cytokinesis. Using confocal imaging we found that YB-1 was essential for orchestrating the spatio-temporal distribution of the microtubules, ß-actin and the chromosome passenger complex (CPC) to define the cleavage plane. We show that phosphorylation at six serine residues was essential for cytokinesis, of which novel sites were identified using mass spectrometry. Using atomistic modelling we show how phosphorylation at multiple sites alters YB-1 conformation, allowing it to interact with protein partners. Our results establish phosphorylated YB-1 as a critical regulator of cytokinesis, defining precisely how YB-1 regulates cell division.

ACC/AHA/ASE/HRS/ISACHD/SCAI/SCCT/SCMR/SOPE 2020 Appropriate Use Criteria for Multimodality Imaging During the Follow-Up Care of Patients With Congenital Heart Disease: A Report of the American College of Cardiology Solution Set Oversight Committee and Appropriate Use Criteria Task Force, American Heart Association, American Society of Echocardiography, Heart Rhythm Society, International Society for Adult Congenital Heart Disease, Society for Cardiovascular Angiography and Interventions, Society of Cardiovascular Computed Tomography, Society for Cardiovascular Magnetic Resonance, and Society of Pediatric Echocardiography.

The American College of Cardiology (ACC) collaborated with the American Heart Association, American Society of Echocardiography, Heart Rhythm Society, International Society for Adult Congenital Heart Disease, Society for Cardiovascular Angiography and Interventions, Society of Cardiovascular Computed Tomography, Society for Cardiovascular Magnetic Resonance, and the Society of Pediatric Echocardiography to develop Appropriate Use Criteria (AUC) for multimodality imaging during the follow-up care of patients with congenital heart disease (CHD). This is the first AUC to address cardiac imaging in adult and pediatric patients with established CHD. A number of common patient scenarios (also termed "indications") and associated assumptions and definitions were developed using guidelines, clinical trial data, and expert opinion in the field of CHD.1 The indications relate primarily to evaluation before and after cardiac surgery or catheter-based intervention, and they address routine surveillance as well as evaluation of new-onset signs or symptoms. The writing group developed 324 clinical indications, which they separated into 19 tables according to the type of cardiac lesion. Noninvasive cardiac imaging modalities that could potentially be used for these indications were incorporated into the tables, resulting in a total of 1,035 unique scenarios. These scenarios were presented to a separate, independent panel for rating, with each being scored on a scale of 1 to 9, with 1 to 3 categorized as "Rarely Appropriate," 4 to 6 as "May Be Appropriate," and 7 to 9 as "Appropriate." Forty-four percent of the scenarios were rated as Appropriate, 39% as May Be Appropriate, and 17% as Rarely Appropriate. This AUC document will provide guidance to clinicians in the care of patients with established CHD by identifying the reasonable imaging modality options available for evaluation and surveillance of such patients. It will also serve as an educational and quality improvement tool to identify patterns of care and reduce the number of Rarely Appropriate tests in clinical practice.

Modular ssDNA binding and inhibition of telomerase activity by designer PPR proteins.

DNA is typically found as a double helix, however it must be separated into single strands during all phases of DNA metabolism; including transcription, replication, recombination and repair. Although recent breakthroughs have enabled the design of modular RNA- and double-stranded DNA-binding proteins, there are currently no tools available to manipulate single-stranded DNA (ssDNA). Here we show that artificial pentatricopeptide repeat (PPR) proteins can be programmed for sequence-specific ssDNA binding. Interactions occur using the same code and specificity as for RNA binding. We solve the structures of DNA-bound and apo proteins revealing the basis for ssDNA binding and how hydrogen bond rearrangements enable the PPR structure to envelope its ssDNA target. Finally, we show that engineered PPRs can be designed to bind telomeric ssDNA and can block telomerase activity. The modular mode of ssDNA binding by PPR proteins provides tools to target ssDNA and to understand its importance in cells.

Forced vital capacity predicts morbidity and mortality in adults with repaired tetralogy of Fallot.

OBJECTIVE: Abnormal lung function characterized by a reduced forced vital capacity (FVC) is common in adults with repaired tetralogy of Fallot (TOF) and is associated with previous thoracotomies and sternotomies. The impact of abnormal lung function on clinical outcomes in adult patients with repaired TOF is unclear. The aim of this study was to determine the impact of abnormal lung function on the outcome of hospitalization and death in adults with repaired TOF when analyzed with other traditional cardiac risk factors. DESIGN: Retrospective study of adults with repaired TOF, who underwent spirometry between 2000 and 2014. FVC < 60% of predicted was categorized as moderate-to-severely reduced lung function. Primary outcome measure was the combined clinical endpoint of death, cardiac transplantation, or nonelective hospitalization for primary cardiac or respiratory indication. RESULTS: A total of 122 patients were included. Average age at spirometry testing was 31 ± 10.1 years. FVC was < 60% predicted in 23 (19%) patients. During a mean follow-up period of 3.97 ± 2.65 years, 23 (19%) patients reached the combined clinical outcome of nonelective hospitalization and/or death. FVC < 60% predicted was independently associated with the risk for the combined clinical outcome (RR 6.68 (95% CI 2.49-17.94), P < .001). CONCLUSIONS: Abnormal pulmonary function characterized by reduced FVC is common in adults with repaired TOF. Patients with FVC < 60% predicted had a 6 times higher rate of hospitalization and/or death compared to those with FVC ≥ 60%.

Takeuchi repair of anomalous left coronary artery from the pulmonary artery.

Anomalous left coronary artery from the pulmonary artery (ALCAPA) is a rare form of congenital heart disease that has been successfully palliated for decades. Prior to coronary reimplantation, the Takeuchi repair was the most common operative palliation. The Takeuchi repair is still seen today at less experienced congenital centers or when reimplantation is not possible. Patients who have had the Takeuchi repair are at risk of having subsequent complications related to this repair. Unfortunately, due to the surgical rarity, the post surgical anatomy is often poorly understood by cardiologists leading to inadequate risk factor assessment and compromised patient care. Coronary computed tomography angiography is a useful imaging modality to follow patients who have had the Takeuchi repair.

The Impact of Obesity on Postoperative Outcomes in Adults with Congenital Heart Disease Undergoing Pulmonary Valve Replacement.

OBJECTIVE: The impact of obesity on surgical morbidity in adults with congenital heart disease is currently unknown. The aim of our study was to investigate the impact of obesity on postoperative outcomes in adults with congenital heart disease undergoing reoperation for pulmonary valve replacement. METHODS: A retrospective analysis was performed assessing the influence of obesity on surgical outcomes. Obesity was defined as a body mass index ≥30 kg/m2. RESULTS: The mean body mass index of the cohort was 25.9 ± 6.9 kg/m2 . The cohort included 71 patients with 17 patients (24%) being obese. There was no postoperative mortality. Obese patients had a longer hospital length of stay (6.6 vs. 4.7 days; P < .001) and increased incidence of postoperative arrhythmias (29% vs. 5.6%; P = .003) compared with nonobese patients. Multivariable analysis performed using logistic regression with backwards elimination demonstrated obesity was independently associated with hospital length of stay >5 days (odds ratio [OR] = 5.2; 95% confidence interval [CI]: 1.5-18.2, P = .01) and with increased postoperative arrhythmias (OR = 4.2; 95% CI: 1.7-40, P < .01). CONCLUSIONS: Obesity is associated with increased morbidity in adults with congenital heart disease undergoing pulmonary valve replacement, including longer hospitalization and higher risk for postoperative arrhythmias.

Telomeric G-quadruplexes are a substrate and site of localization for human telomerase.

It has been hypothesized that G-quadruplexes can sequester the 3' end of the telomere and prevent it from being extended by telomerase. Here we purify and characterize stable, conformationally homogenous human telomeric G-quadruplexes, and demonstrate that human telomerase is able to extend parallel, intermolecular conformations in vitro. These G-quadruplexes align correctly with the RNA template of telomerase, demonstrating that at least partial G-quadruplex resolution is required. A highly purified preparation of human telomerase retains this extension ability, establishing that the core telomerase enzyme complex is sufficient for partial G-quadruplex resolution and extension. The parallel-specific G-quadruplex ligand N-methyl mesoporphyrin IX (NMM) causes an increase in telomeric G-quadruplexes, and we show that telomerase colocalizes with a subset of telomeric G-quadruplexes in vivo. The ability of telomerase to partially unwind, extend and localize to these structures implies that parallel telomeric G-quadruplexes may play an important biological role.