Telomere length regulation: coupling DNA end processing to feedback regulation of telomerase

Prenatal and early life exposure to fine particulate matter and telomere length in early childhood

BACKGROUND

Telomere length is a biomarker of molecular aging that may be impacted by air pollution exposure starting in utero. We aimed to examine the association between prenatal and early life exposure to fine particulate matter (PM2.5) and leukocyte telomere length (LTL) in children and explore sex differences.

METHODS

Analyses included 384 mother-child pairs enrolled in the Programming Research in Obesity, Growth, and Environmental Stressors (PROGRESS) birth cohort in Mexico City. Exposure to PM2.5 was estimated at the residential level using a satellite based spatio-temporally resolved prediction model. Average relative LTL was measured in DNA isolated from blood collected at age 4-6 years using quantitative real-time polymerase chain reaction. Linear regression models were used to examine the association between average PM2.5 across pregnancy, individual trimesters, first postnatal year, and LTL. Models were adjusted for maternal age and education at enrollment, prenatal environmental tobacco smoke exposure, child sex, age, and body mass index z-score at LTL measurement. Effect modification by sex was investigated with interaction terms and stratification.

RESULTS

In trimester specific models, we found an association between 2nd trimester PM2.5 and elongated LTL (β: 4.34, 95%CI [0.42, 8.42], per 5 μg/m3 increase). There was suggestive effect modification by sex on average 2nd trimester PM2.5 with stronger associations seen in females compared to males (β: 7.12, [95%CI: 0.98, 13.6] and β: 1.43 [95%CI: -3.46, 6.57]) per 5 μg/m3 increase respectively.

CONCLUSION

Second trimester PM2.5 levels were associated with changes in LTL in early childhood. Understanding temporal and sex differences in PM2.5 exposure may provide insights into telomere dynamics over early life.

Dysfunction of Telomeric Cdc13-Stn1-Ten1 Simultaneously Activates DNA Damage and Spindle Checkpoints

Telomeres, the ends of eukaryotic linear chromosomes, are composed of repeated DNA sequences and specialized proteins, with the conserved telomeric Cdc13/CTC1-Stn1-Ten1 (CST) complex providing chromosome stability via telomere end protection and the regulation of telomerase accessibility. In this study, SIZ1, coding for a SUMO E3 ligase, and TOP2 (a SUMO target for Siz1 and Siz2) were isolated as extragenic suppressors of Saccharomyces cerevisiae CST temperature-sensitive mutants. ten1-sz, stn1-sz and cdc13-sz mutants were isolated next due to being sensitive to intracellular Siz1 dosage. In parallel, strong negative genetic interactions between mutants of CST and septins were identified, with septins being noticeably sumoylated through the action of Siz1. The temperature-sensitive arrest in these new mutants of CST was dependent on the G2/M Mad2-mediated and Bub2-mediated spindle checkpoints as well as on the G2/M Mec1-mediated DNA damage checkpoint. Our data suggest the existence of yet unknown functions of the telomeric Cdc13-Stn1-Ten1 complex associated with mitotic spindle positioning and/or assembly that could be further elucidated by studying these new ten1-sz, stn1-sz and cdc13-sz mutants.

Human telomere length is chromosome end-specific and conserved across individuals

Short telomeres cause age-related disease, and long telomeres contribute to cancer; however, the mechanisms regulating telomere length are unclear. We developed a nanopore-based method, which we call Telomere Profiling, to determine telomere length at nearly single-nucleotide resolution. Mapping telomere reads to chromosome ends showed chromosome end-specific length distributions that could differ by more than six kilobases. Examination of telomere lengths in 147 individuals revealed that certain chromosome ends were consistently longer or shorter. The same rank order was found in newborn cord blood, suggesting that telomere length is determined at birth and that chromosome end-specific telomere length differences are maintained as telomeres shorten with age. Telomere Profiling makes precision investigation of telomere length widely accessible for laboratory, clinical, and drug discovery efforts and will allow deeper insights into telomere biology.

Telomere length is an epigenetic trait - Implications for the use of telomerase-deficient organisms to model human disease

Telomere length, unlike most genetic traits, is epigenetic, in the sense that it is not fully coded by the genome. Telomeres vary in length and randomly assort to the progeny leaving some individuals with longer and others with shorter telomeres. Telomerase activity counteracts this by extending telomeres in the germline and during embryogenesis but sizeable variances remain in telomere length. This effect is exacerbated by the absence of fully active telomerase. Telomerase heterozygous animals (tert+/-) have reduced telomerase activity and their telomeres fail to be elongated to wild-type average length, meaning that - with every generation - they decrease. After a given number of successive generations of telomerase-insufficient crosses, telomeres become critically short and cause organismal defects that, in humans, are known as telomere biology disorders. Importantly, these defects also occur in wild-type (tert+/+) animals derived from such tert+/- incrosses. Despite these tert+/+ animals being proficient for telomerase, they have shorter than average telomere length and, although milder, develop phenotypes that are similar to those of telomerase mutants. Here, we discuss the impact of this phenomenon on human pathologies associated with telomere length, provide a brief overview of telomere biology across species and propose specific measures for working with telomerase-deficient zebrafish.

Human telomere length is chromosome specific and conserved across individuals

Short telomeres cause age-related disease and long telomeres predispose to cancer; however, the mechanisms regulating telomere length are unclear. To probe these mechanisms, we developed a nanopore sequencing method, Telomere Profiling, that is easy to implement, precise, and cost effective with broad applications in research and the clinic. We sequenced telomeres from individuals with short telomere syndromes and found similar telomere lengths to the clinical FlowFISH assay. We mapped telomere reads to specific chromosome end and identified both chromosome end-specific and haplotype-specific telomere length distributions. In the T2T HG002 genome, where the average telomere length is 5kb, we found a remarkable 6kb difference in lengths between some telomeres. Further, we found that specific chromosome ends were consistently shorter or longer than the average length across 147 individuals. The presence of conserved chromosome end-specific telomere lengths suggests there are new paradigms in telomere biology that are yet to be explored. Understanding the mechanisms regulating length will allow deeper insights into telomere biology that can lead to new approaches to disease.

Pot1b -/- tumors activate G-quadruplex-induced DNA damage to promote telomere hyper-elongation

Malignant cancers must activate telomere maintenance mechanisms to achieve replicative immortality. Mutations in the human Protection of Telomeres 1 (POT1) gene are frequently detected in cancers with abnormally long telomeres, suggesting that the loss of POT1 function disrupts the regulation of telomere length homeostasis to promote telomere elongation. However, our understanding of the mechanisms leading to elongated telomeres remains incomplete. The mouse genome encodes two POT1 proteins, POT1a and POT1b possessing separation of hPOT1 functions. We performed serial transplantation of Pot1b-/- sarcomas to better understand the role of POT1b in regulating telomere length maintenance. While early-generation Pot1b-/- sarcomas initially possessed shortened telomeres, late-generation Pot1b-/- cells display markedly hyper-elongated telomeres that were recognized as damaged DNA by the Replication Protein A (RPA) complex. The RPA-ATR-dependent DNA damage response at telomeres promotes telomerase recruitment to facilitate telomere hyper-elongation. POT1b, but not POT1a, was able to unfold G-quadruplex present in hyper-elongated telomeres to repress the DNA damage response. Our findings demonstrate that the repression of the RPA-ATR DDR is conserved between POT1b and human POT1, suggesting that similar mechanisms may underly the phenotypes observed in human cancers harboring human POT1 mutations.

Different Approaches for the Profiling of Cancer Pathway-Related Genes in Glioblastoma Cells

Deregulation of signalling pathways that regulate cell growth, survival, metabolism, and migration can frequently lead to the progression of cancer. Brain tumours are a large group of malignancies characterised by inter- and intratumoral heterogeneity, with glioblastoma (GBM) being the most aggressive and fatal. The present study aimed to characterise the expression of cancer pathway-related genes (n = 84) in glial tumour cell lines (A172, SW1088, and T98G). The transcriptomic data obtained by the qRT-PCR method were compared to different control groups, and the most appropriate control for subsequent interpretation of the obtained results was chosen. We analysed three widely used control groups (non-glioma cells) in glioblastoma research: Human Dermal Fibroblasts (HDFa), Normal Human Astrocytes (NHA), and commercially available mRNAs extracted from healthy human brain tissues (hRNA). The gene expression profiles of individual glioblastoma cell lines may vary due to the selection of a different control group to correlate with. Moreover, we present the original multicriterial decision making (MCDM) for the possible characterization of gene expression profiles. We observed deregulation of 75 genes out of 78 tested in the A172 cell line, while T98G and SW1088 cells exhibited changes in 72 genes. By comparing the delta cycle threshold value of the tumour groups to the mean value of the three controls, only changes in the expression of 26 genes belonging to the following pathways were identified: angiogenesis FGF2; apoptosis APAF1, CFLAR, XIAP; cellular senescence BM1, ETS2, IGFBP5, IGFBP7, SOD1, TBX2; DNA damage and repair ERCC5, PPP1R15A; epithelial to mesenchymal transition SNAI3, SOX10; hypoxia ADM, ARNT, LDHA; metabolism ATP5A1, COX5A, CPT2, PFKL, UQCRFS1; telomeres and telomerase PINX1, TINF2, TNKS, and TNKS2. We identified a human astrocyte cell line and normal human brain tissue as the appropriate control group for an in vitro model, despite the small sample size. A different method of assessing gene expression levels produced the same disparities, highlighting the need for caution when interpreting the accuracy of tumorigenesis markers.

Ambient ozone exposure during pregnancy and telomere length in newborns: a prospective investigation in Wuhan, China

Recent studies suggest that environmental exposures, including air pollution, may influence initial (newborn) telomere length (TL), which has important implications for lifetime health. However, the effect of prenatal ozone exposure on newborn TL is unclear. This study aimed to examine the association of ozone exposure during pregnancy with newborn TL. We used data from a birth cohort study of 762 mother-newborn pairs performed in Wuhan, China, during 2013-2015. Land-use regression models were used to assess prenatal ozone exposure. Newborn TL was quantified in cord blood by qPCR assay. We applied multiple informant model to explore the relationship of prenatal ozone exposure with newborn TL. After adjustment for potential confounders, an interquartile range (IQR) increase in ozone exposure during the 2nd trimester, 3rd trimester, and whole pregnancy were associated with 6.00% (95% confidence interval [CI]: 1.59%, 10.62%), 12.64% (95% CI: 7.52%, 18.00%), and 7.10% (95% CI: 4.09%, 10.20%) longer cord blood TL, respectively. In contrast, an IQR increase in ozone exposure during the 1st trimester was associated with a 8.39% (95% CI: - 12.90%, - 3.65%) shorter cord blood TL. In multipollutant models, consistent associations were observed between ozone exposures during the 2nd trimester and whole pregnancy and cord blood TL, but not significant for the 1st and 3rd trimesters. In conclusion, our findings suggest positive associations of ozone exposure during the 2nd trimester, 3rd trimester, and whole pregnancy with newborn TL and a negative association during the 1st trimester. This study provides new evidence in humans for a potential "programming" mechanism linking maternal ozone exposure to the initial (newborn) setting of offspring's telomere biology.

The telomeric 5' end nucleotide is regulated in the budding yeast Naumovozyma castellii

The junction between the double-stranded and single-stranded telomeric DNA (ds-ss junction) is fundamental in the maintenance of the telomeric chromatin, as it directs the assembly of the telomere binding proteins. In budding yeast, multiple Rap1 proteins bind the telomeric dsDNA, while ssDNA repeats are bound by the Cdc13 protein. Here, we aimed to determine, for the first time, the telomeric 5' end nucleotide in a budding yeast. To this end, we developed a permutation-specific PCR-based method directed towards the regular 8-mer telomeric repeats in Naumovozyma castellii. We find that, in logarithmically growing cells, the 320 ± 30 bp long telomeres mainly terminate in either of two specific 5' end permutations of the repeat, both corresponding to a terminal adenine nucleotide. Strikingly, two permutations are completely absent at the 5' end, indicating that not all ds-ss junction structures would allow the establishment of the protective telomere chromatin cap structure. Using in vitro DNA end protection assays, we determined that binding of Rap1 and Cdc13 around the most abundant ds-ss junction ensures the protection of both 5' ends and 3' overhangs from exonucleolytic degradation. Our results provide mechanistic insights into telomere protection, and reveal that Rap1 and Cdc13 have complementary roles.

Establishment of an Efficient Immortalization Strategy Using HMEJ-Based bTERT Insertion for Bovine Cells

Immortalized cell lines have been used in a wide range of applications in research on immune disorders and cellular metabolic regulation due to the stability and uniformity of their cellular characteristics. At present, the investigation into molecular functions and signaling pathways within bovine cells remains largely limited by the lack of immortalized model cells. Current methods for immortalizing bovine cells are mainly restricted to the ectopic expression of human telomerase reverse transcriptase (hTERT) through transient transfection or virus-mediated delivery, which have defects in efficiency and reliability. In this study, we identified bovine TERT (bTERT) as a novel potent biofactor for immortalizing bovine cells with great advantages over hTERT, and established an efficient and easily manipulated strategy for the immortalization of bovine primary cells. Through the homology-mediated end-joining-based insertion of bTERT at the ROSA26 locus, we successfully generated immortalized bovine fetal fibroblast cell lines with stable characteristics. The observed limitation of this strategy in immortalizing bovine bone marrow-derived macrophages was attributed to the post-translational modification of bTERT, causing inhibited nuclear localization and depressed activity of bTERT in this terminally differentiated cell. In summary, we constructed an innovative method to achieve the high-quality immortalization of bovine primary cells, thereby expanding the prospects for the future application of immortalized bovine model cell lines.

Telomere dynamics across the early life course: Findings from a longitudinal study in children

Telomeres are protective caps on chromosome ends that shorten with each cell division. Telomere length (TL) predicts the onset of cellular senescence and correlates with longevity and age-related disease risk. Previous research suggests that adults display fixed ranking and tracking of TL by age 20 years, supporting the importance of TL at birth and attrition during childhood. However, longitudinal research examining telomere dynamics during early life is sparse. Here, we used monochrome multiplex quantitative polymerase chain reaction to measure relative TL in leukocytes isolated from cord blood and child blood collected at ages 3, 5, 7, and 9 years among 224 minority children enrolled in a New York City-based birth cohort. We also measured maternal TL at delivery in a subset of 197 participants with a biobanked blood sample. TL decreased most rapidly in the first years of life (birth to 3 years), followed by a period of maintenance into the pre-puberty period. Mothers with longer telomeres gave birth to newborns with longer telomeres that remained longer across childhood, suggesting that the fixed ranking and tracking of TL observed among adults may extend to early childhood or even the prenatal period with a potential transgenerational basis. We did not find significant sex differences in the pattern of child TL change across development. These findings emphasize the need to understand factors and mechanisms that determine TL during early childhood.

A unified alternative telomere-lengthening pathway in yeast survivor cells

Alternative lengthening of telomeres (ALT) is a recombination process that maintains telomeres in the absence of telomerase and helps cancer cells to survive. Yeast has been used as a robust model of ALT; however, the inability to determine the frequency and structure of ALT survivors hinders understanding of the ALT mechanism. Here, using population and molecular genetics approaches, we overcome these problems and demonstrate that contrary to the current view, both RAD51-dependent and RAD51-independent mechanisms are required for a unified ALT survivor pathway. This conclusion is based on the calculation of ALT frequencies, as well as on ultra-long sequencing of ALT products that revealed hybrid sequences containing features attributed to both recombination pathways. Sequencing of ALT intermediates demonstrates that recombination begins with Rad51-mediated strand invasion to form DNA substrates that are matured by a Rad51-independent ssDNA annealing pathway. A similar unified ALT pathway may operate in other organisms, including humans.

Crossroads of telomere biology and anticancer drug discovery

The telomere is the specialized nucleoprotein complex at the end of the chromosome. Its highly conserved 5'-TTAGGG-3' repeats and shelterin protein complexes form a protective loop structure to maintain the integrity and stability of linear chromosomes. Although human somatic cells gradually shorten telomeres to undergo senescence or crisis, cancer cells activate telomerase, or the recombination-based mechanism to maintain telomeres and exhibit immortality. As the most frequent non-coding mutations in cancer, gain-of-function mutations in the promoter region of the telomerase catalytic subunit, TERT, trigger telomerase activation. Promoter methylation and copy number gain are also associated with the enhanced TERT expression. Although telomerase inhibitors were pioneered from telomere-directed therapeutics, their efficacies are limited to cancer with short telomeres and some hematological malignancies. Other therapeutic approaches include a nucleoside analog incorporated to telomeres and TERT promoter-driven oncolytic adenoviruses. Tankyrase poly(ADP-ribose) polymerase, a positive regulator of telomerase, has been rediscovered as a target for Wnt-driven cancer. Meanwhile, telomeric nucleic acids form a higher-order structure called a G-quadruplex (G4). G4s are formed genome-wide and their dynamics affect various events, including replication, transcription, and translation. G4-stabilizing compounds (G4 ligands) exert anticancer effects and are in clinical investigations. Collectively, telomere biology has provided clues for deeper understanding of cancer, which expands opportunities to discover innovative anticancer drugs.

Telomeres and telomerase in oncogenesis

Telomeres are located at the ends of chromosomes and protect them from degradation. Suppressing the activity of telomerase, a telomere-synthesizing enzyme, and maintaining short telomeres is a protective mechanism against cancer in humans. In most human somatic cells, the expression of telomerase reverse transcriptase (TERT) is repressed and telomerase activity is inhibited. This leads to the progressive shortening of telomeres and inhibition of cell growth in a process called replicative senescence. Most types of primary cancer exhibit telomerase activation, which allows uncontrolled cell proliferation. Previous research indicates that TERT activation also affects cancer development through activities other than the canonical function of mediating telomere elongation. Recent studies have improved the understanding of the structure and function of telomeres and telomerase as well as key mechanisms underlying the activation of TERT and its role in oncogenesis. These advances led to a search for drugs that inhibit telomerase as a target for cancer therapy. The present review article summarizes the organization and function of telomeres, their role in carcinogenesis, and advances in telomerase-targeted therapy.

TERRA, a Multifaceted Regulator of Telomerase Activity at Telomeres

In eukaryotes, telomeres are repetitive sequences at the end of chromosomes, which are maintained in a constitutive heterochromatin state. It is now known that telomeres can be actively transcribed, leading to the production of a telomeric repeat-containing noncoding RNA called TERRA. Due to its sequence complementarity to the telomerase template, it was suggested early on that TERRA could be an inhibitor of telomerase. Since then, TERRA has been shown to be involved in heterochromatin formation at telomeres, to invade telomeric dsDNA and form R-loops, and even to promote telomerase recruitment at short telomeres. All these functions depend on the diverse capacities of this lncRNA to bind various cofactors, act as a scaffold, and promote higher-order complexes in cells. In this review, it will be highlighted as to how these properties of TERRA work together to regulate telomerase activity at telomeres.

Asymmetric Processing of DNA Ends at a Double-Strand Break Leads to Unconstrained Dynamics and Ectopic Translocation

Multiple pathways regulate the repair of double-strand breaks (DSBs) to suppress potentially dangerous ectopic recombination. Both sequence and chromatin context are thought to influence pathway choice between non-homologous end-joining (NHEJ) and homology-driven recombination. To test the effect of repetitive sequences on break processing, we have inserted TG-rich repeats on one side of an inducible DSB at the budding yeast MAT locus on chromosome III. Five clustered Rap1 sites within a break-proximal TG repeat are sufficient to block Mre11-Rad50-Xrs2 recruitment, impair resection, and favor elongation by telomerase. The two sides of the break lose end-to-end tethering and show enhanced, uncoordinated movement. Only the TG-free side is resected and shifts to the nuclear periphery. In contrast to persistent DSBs without TG repeats that are repaired by imprecise NHEJ, nearly all survivors of repeat-proximal DSBs repair the break by a homology-driven, non-reciprocal translocation from ChrIII-R to ChrVII-L. This suppression of imprecise NHEJ at TG-repeat-flanked DSBs requires the Uls1 translocase activity.

RNA-DNA Hybrids Support Recombination-Based Telomere Maintenance in Fission Yeast

A subset of cancers rely on telomerase-independent mechanisms to maintain their chromosome ends. The predominant "alternative lengthening of telomeres" pathway appears dependent on homology-directed repair (HDR) to maintain telomeric DNA. However, the molecular changes needed for cells to productively engage in telomeric HDR are poorly understood. To gain new insights into this transition, we monitored the state of telomeres during serial culture of fission yeast (Schizosaccharomyces pombe) lacking the telomerase recruitment factor Ccq1. Rad52 is loaded onto critically short telomeres shortly after germination despite continued telomere erosion, suggesting that recruitment of recombination factors is not sufficient to maintain telomeres in the absence of telomerase function. Instead, survivor formation coincides with the derepression of telomeric repeat-containing RNA (TERRA). In this context, degradation of TERRA associated with the telomere in the form of R-loops drives a severe growth crisis, ultimately leading to a novel type of survivor with linear chromosomes and altered cytological telomere characteristics, including the loss of the shelterin component Rap1 (but not the TRF1/TRF2 ortholog, Taz1) from the telomere. We demonstrate that deletion of Rap1 is protective in this context, preventing the growth crisis that is otherwise triggered by degradation of telomeric R-loops in survivors with linear chromosomes. These findings suggest that upregulation of telomere-engaged TERRA, or altered recruitment of shelterin components, can support telomerase-independent telomere maintenance.

Breast Tissue Biology Expands the Possibilities for Prevention of Age-Related Breast Cancers

Preventing breast cancer before it is able to form is an ideal way to stop breast cancer. However, there are limited existing options for prevention of breast cancer. Changes in the breast tissue resulting from the aging process contribute to breast cancer susceptibility and progression and may therefore provide promising targets for prevention. Here, we describe new potential targets, immortalization and inflammaging, that may be useful for prevention of age-related breast cancers. We also summarize existing studies of warfarin and metformin, current drugs used for non-cancerous diseases, that also may be repurposed for breast cancer prevention.

Prenatal Exposure to Phthalates and Newborn Telomere Length: A Birth Cohort Study in Wuhan, China

BACKGROUND

Telomere length (TL) is a marker of biological aging and is inversely related to aging-related diseases. The setting of TL at birth may have important implications for lifelong telomere dynamics; however, its determinants remain poorly understood.

OBJECTIVES

The purpose of our study was to explore the relationships between prenatal exposure to phthalates and umbilical cord blood TL.

METHODS

A total of 762 mother–newborn pairs were recruited from a birth cohort study performed between November 2013 and March 2015 in Wuhan, China. Relative cord blood TL was measured using quantitative real-time polymerase chain reaction. Six phthalate metabolites were measured in urine samples acquired from pregnant women during the three trimesters. Multiple informant models were applied to estimate the associations between prenatal exposure to phthalates and cord blood TL and to evaluate potential windows of vulnerability.

RESULTS

Exposure to mono-ethyl phthalate (MEP), mono(2-ethyl-5-carboxypentyl) phthalate (MECPP), mono(2-ethyl-5-oxohexyl) phthalate (MEOHP), mono(2-ethyl-5-hydroxyhexyl) phthalate (MEHHP), mono-butyl phthalate (MBP), and di(2-ethylhexyl) phthalate ([Formula: see text]) during the first trimester were inversely related to cord blood TL. In addition, we observed a female-specific association between maternal exposure to MEP during the first trimester and cord blood TL ([Formula: see text]). The associations between maternal exposure to MECPP, MEHHP, MEOHP, and [Formula: see text] during the first trimester and cord blood TL were consistent between males and females (all [Formula: see text]).

CONCLUSION

This prospective study demonstrated that prenatal exposure to some phthalate metabolites were associated with shorter cord blood TL. Our results, if confirmed in other populations, may provide more evidence of adverse health outcomes of phthalate exposure and support the hypothesis that the intrauterine environment may be one of the major determinants for newborn TL. https://doi.org/10.1289/EHP4492.

Catalysis-dependent inactivation of human telomerase and its reactivation by intracellular telomerase-activating factors (iTAFs)

Human telomerase maintains genome stability by adding telomeric repeats to the ends of linear chromosomes. Although previous studies have revealed profound insights into telomerase functions, the low cellular abundance of functional telomerase and the difficulties in quantifying its activity leave its thermodynamic and kinetic properties only partially characterized. Employing a stable cell line overexpressing both the human telomerase RNA component and the N-terminally biotinylated human telomerase reverse transcriptase and using a newly developed method to count individual extension products, we demonstrate here that human telomerase holoenzymes contain fast- and slow-acting catalytic sites. Surprisingly, both active sites became inactive after two consecutive rounds of catalysis, named single-run catalysis. The fast active sites turned off ∼40-fold quicker than the slow ones and exhibited higher affinities to DNA substrates. In a dimeric enzyme, the two active sites work in tandem, with the faster site functioning before the slower one, and in the monomeric enzyme, the active sites also perform single-run catalysis. Interestingly, inactive enzymes could be reactivated by intracellular telomerase-activating factors (iTAFs) from multiple cell types. We conclude that the single-run catalysis and the iTAF-triggered reactivation serve as an unprecedented control circuit for dynamic regulation of telomerase. They endow native telomerase holoenzymes with the ability to match their total number of active sites to the number of telomeres they extend. We propose that the exquisite kinetic control of telomerase activity may play important roles in both cell division and cell aging.

Association between prenatal particulate air pollution exposure and telomere length in cord blood: Effect modification by fetal sex

INTRODUCTION

In utero particulate matter exposure produces oxidative stress that impacts cellular processes that include telomere biology. Newborn telomere length is likely critical to an individual's telomere biology; reduction in this initial telomere setting may signal increased susceptibility to adverse outcomes later in life. We examined associations between prenatal particulate matter with diameter ≤2.5 µm (PM_2.5) and relative leukocyte telomere length (LTL) measured in cord blood using a data-driven approach to characterize sensitive windows of prenatal PM_2.5 effects and explore sex differences.

METHODS

Women who were residents of Mexico City and affiliated with the Mexican Social Security System were recruited during pregnancy (n = 423 for analyses). Mothers' prenatal exposure to PM_2.5 was estimated based on residence during pregnancy using a validated satellite-based spatio-temporally resolved prediction model. Leukocyte DNA was extracted from cord blood obtained at delivery. Duplex quantitative polymerase chain reaction was used to compare the relative amplification of the telomere repeat copy number to single gene (albumin) copy number. A distributed lag model incorporating weekly averages for PM_2.5 over gestation was used in order to explore sensitive windows. Sex-specific associations were examined using Bayesian distributed lag interaction models.

RESULTS

In models that included child's sex, mother's age at delivery, prenatal environmental tobacco smoke exposure, pre-pregnancy BMI, gestational age, birth season and assay batch, we found significant associations between higher PM_2.5 exposure during early pregnancy (4-9 weeks) and shorter LTL in cord blood. We also identified two more windows at 14-19 and 34-36 weeks in which increased PM_2.5 exposure was associated with longer LTL. In stratified analyses, the mean and cumulative associations between PM_2.5 and shortened LTL were stronger in girls when compared to boys.

CONCLUSIONS

Increased PM_2.5 during specific prenatal windows was associated with shorter LTL and longer LTL. PM_2.5 was more strongly associated with shortened LTL in girls when compared to boys. Understanding sex and temporal differences in response to air pollution may provide unique insight into mechanisms.

Biological Actions of the Hsp90-binding Immunophilins FKBP51 and FKBP52

Immunophilins are a family of proteins whose signature domain is the peptidylprolyl-isomerase domain. High molecular weight immunophilins are characterized by the additional presence of tetratricopeptide-repeats (TPR) through which they bind to the 90-kDa heat-shock protein (Hsp90), and via this chaperone, immunophilins contribute to the regulation of the biological functions of several client-proteins. Among these Hsp90-binding immunophilins, there are two highly homologous members named FKBP51 and FKBP52 (FK506-binding protein of 51-kDa and 52-kDa, respectively) that were first characterized as components of the Hsp90-based heterocomplex associated to steroid receptors. Afterwards, they emerged as likely contributors to a variety of other hormone-dependent diseases, stress-related pathologies, psychiatric disorders, cancer, and other syndromes characterized by misfolded proteins. The differential biological actions of these immunophilins have been assigned to the structurally similar, but functionally divergent enzymatic domain. Nonetheless, they also require the complementary input of the TPR domain, most likely due to their dependence with the association to Hsp90 as a functional unit. FKBP51 and FKBP52 regulate a variety of biological processes such as steroid receptor action, transcriptional activity, protein conformation, protein trafficking, cell differentiation, apoptosis, cancer progression, telomerase activity, cytoskeleton architecture, etc. In this article we discuss the biology of these events and some mechanistic aspects.

Revisiting Telomere Shortening in Cancer

Telomeres, the protective structures of chromosome ends are gradually shortened by each cell division, eventually leading to senescence or apoptosis. Cancer cells maintain the telomere length for unlimited growth by telomerase reactivation or a recombination-based mechanism. Recent genome-wide analyses have unveiled genetic and epigenetic alterations of the telomere maintenance machinery in cancer. While telomerase inhibition reveals that longer telomeres are more advantageous for cell survival, cancer cells often have paradoxically shorter telomeres compared with those found in the normal tissues. In this review, we summarize the latest knowledge about telomere length alterations in cancer and revisit its rationality. Finally, we discuss the potential utility of telomere length as a prognostic biomarker.

Characterization in humans of in vitro leucocyte maximal telomerase activity capacity and association with stress

The goal of this study was to develop and validate a measure of maximal telomerase activity capacity (mTAC) for use in human studies of telomere biology, and to determine its association with measures of stress and stress responsivity. The study was conducted in a population of 28 healthy young women and men who were assessed serially across two separate days, at multiple time points, and in response to a standardized laboratory stressor. Venous blood was collected at each of these multiple assessments, and an in vitro mitogen challenge (phytohaemagglutinin supplemented with interleukin-2) was used to stimulate telomerase activity in leucocytes. After first establishing the optimal post-stimulation time course to characterize mTAC, we determined the within-subject stability and the between-subject variability of mTAC. The major findings of our study are as follows: (i) the optimal time point to quantify human leucocyte mTAC appears to be at 72 h after mitogen stimulation; (ii) mTAC exhibits substantial within-subject stability (correlations were in the range of r 0.68-0.82) and between-subject variability, with a high intra-class coefficient (0.70), indicating greater between-subject relative to within-subject variability; (iii) mTAC is not influenced by situational factors including time of day, cortisol, acute stress exposure and immune cell distribution in the pre-stimulation blood sample; and (iv) a significant proportion of the between-subject variability in mTAC is associated with measures of stress and stress responsivity (mTAC is lower in subjects reporting higher levels of perceived (chronic) stress and exhibiting higher psychophysiological stress reactivity). Based collectively on these findings, it appears that mTAC, as proposed and operationalized, empirically meets the key criteria to represent a potentially useful individual difference measure of telomerase activity capacity of human leucocytes.This article is part of the theme issue 'Understanding diversity in telomere dynamics'.

Fundamental mechanisms of telomerase action in yeasts and mammals: understanding telomeres and telomerase in cancer cells

Aberrant activation of telomerase occurs in 85–90% of all cancers and underpins the ability of cancer cells to bypass their proliferative limit, rendering them immortal. The activity of telomerase is tightly controlled at multiple levels, from transcriptional regulation of the telomerase components to holoenzyme biogenesis and recruitment to the telomere, and finally activation and processivity. However, studies using cancer cell lines and other model systems have begun to reveal features of telomeres and telomerase that are unique to cancer. This review summarizes our current knowledge on the mechanisms of telomerase recruitment and activation using insights from studies in mammals and budding and fission yeasts. Finally, we discuss the differences in telomere homeostasis between normal cells and cancer cells, which may provide a foundation for telomere/telomerase targeted cancer treatments.

Telomere length is reduced in 9- to 16-year-old girls exposed to gestational diabetes in utero

AIMS/HYPOTHESIS

Shortened telomere length is a marker of cell damage and is associated with oxidative stress, chronic inflammation and metabolic disease. We hypothesised that the offspring of women with gestational diabetes mellitus (GDM) with increased risk of cardiovascular and metabolic diseases might exhibit shorter telomere length.

METHODS

We investigated telomere length in 439 GDM and 469 control group offspring, aged between 9 and 16 years, recruited from the Danish National Birth Cohort. Relative telomere length was measured in peripheral blood DNA (n = 908) using a quantitative PCR approach. Multivariate regression analysis was used to investigate the association between mothers' GDM status and telomere length in the offspring.

RESULTS

Female offspring had longer telomeres than males. Offspring of mothers with GDM had significantly shorter telomere length than control offspring, but this difference was observed only in girls. There was a negative association between telomere length and GDM exposure among the female offspring (14% shorter telomeres, p = 0.003) following adjustment for the age of the offspring. Telomere length in female offspring was negatively associated with fasting insulin levels and HOMA-IR (p = 0.03). Maternal age, smoking, gestational age, birthweight and the offspring's anthropometric characteristics were not associated with telomere length (p ≥ 0.1).

CONCLUSIONS/INTERPRETATION

The 9- to 16-year-old girls of mothers with GDM had shorter telomeres than those from the control population. Further studies are needed to understand the extent to which shortened telomere length predicts and/or contributes to the increased risk of disease later in life among the offspring of women with GDM.

The fetal programming of telomere biology hypothesis: an update

Research on mechanisms underlying fetal programming of health and disease risk has focused primarily on processes that are specific to cell types, organs or phenotypes of interest. However, the observation that developmental conditions concomitantly influence a diverse set of phenotypes, the majority of which are implicated in age-related disorders, raises the possibility that such developmental conditions may additionally exert effects via a common underlying mechanism that involves cellular/molecular ageing-related processes. In this context, we submit that telomere biology represents a process of particular interest in humans because, firstly, this system represents among the most salient antecedent cellular phenotypes for common age-related disorders; secondly, its initial (newborn) setting appears to be particularly important for its long-term effects; and thirdly, its initial setting appears to be plastic and under developmental regulation. We propose that the effects of suboptimal intrauterine conditions on the initial setting of telomere length and telomerase expression/activity capacity may be mediated by the programming actions of stress-related maternal-placental-fetal oxidative, immune, endocrine and metabolic pathways in a manner that may ultimately accelerate cellular dysfunction, ageing and disease susceptibility over the lifespan. This perspectives paper provides an overview of each of the elements underlying this hypothesis, with an emphasis on recent developments, findings and future directions.This article is part of the theme issue 'Understanding diversity in telomere dynamics'.

Molecular chemotherapeutic potential of butein: A concise review

Butein is a biologically active flavonoid isolated from the bark of Rhus verniciflua Stokes, which is known to have therapeutic potential against various cancers. Notably, butein inhibits cancer cell growth by inducing G₂/M phase arrest and apoptosis. Butein-induced G₂/M phase arrest is associated with increased phosphorylation of ataxia telangiectasia mutated (ATM) and Chk1/2, and consequently, with reduced cdc25C levels. In addition, butein-induced apoptosis is mediated through the activation of caspase-3, which is associated with changes in the expression of Bcl-2 and Bax proteins. Intriguingly, butein sensitizes cells to tumor necrosis factor-related apoptosis-inducing ligand-induced apoptosis via ERK-mediated Sp1 activation, which promotes the transcription of specific death receptor 5. Butein also inhibits the migration and invasion of human cancer cells by suppressing nuclear factor-κB- and extracellular signal-regulated kinases 1/2-mediated expression of matrix metalloproteinase-9 and vascular endothelial growth factor. Additionally, butein downregulates the expression of human telomerase reverse transcriptase and causes a concomitant decrease in telomerase activity. These findings provide the basis for the pharmaceutical development of butein. The aim of this review is to provide an update on the mechanisms underlying the anticancer activity of butein, with a special focus on its effects on different cellular signaling cascades.

Telomere shortening triggers a feedback loop to enhance end protection

Telomere homeostasis is controlled by both telomerase machinery and end protection. Telomere shortening induces DNA damage sensing kinases ATM/ATR for telomerase recruitment. Yet, whether telomere shortening also governs end protection is poorly understood. Here we discover that yeast ATM/ATR controls end protection. Rap1 is phosphorylated by Tel1 and Mec1 kinases at serine 731, and this regulation is stimulated by DNA damage and telomere shortening. Compromised Rap1 phosphorylation hampers the interaction between Rap1 and its interacting partner Rif1, which thereby disturbs the end protection. As expected, reduction of Rap1–Rif1 association impairs telomere length regulation and increases telomere–telomere recombination. These results indicate that ATM/ATR DNA damage checkpoint signal contributes to telomere protection by strengthening the Rap1–Rif1 interaction at short telomeres, and the checkpoint signal oversees both telomerase recruitment and end capping pathways to maintain telomere homeostasis.

Mec1ATR is needed for extensive telomere elongation in response to ethanol in yeast

Telomere length homeostasis is essential for cell survival. In humans, telomeres shorten as a function of age. Short telomeres are known determinants of cell senescence and longevity. The yeast Saccharomyces cerevisiae expresses telomerase and maintains a strict telomere length homeostasis during vegetative growth. We have previously reported that different environmental signals promote changes in telomere length in S. cerevisiae. In particular, exposure to ethanol induces an extensive telomere elongation response due to a reduction in RAP1 mRNA and protein levels. Here we show that the reduction in Rap1 protein levels disrupts the physical interaction between Rap1 and Rif1, which in turn reduces the recruitment of these two proteins to telomeres during G2-phase. Although elongation of the shortest telomeres has been shown to depend on the Rif2 telomeric protein and on the Tel1(ATM) protein kinase, we show here that the extensive telomere elongation in response to ethanol exposure is Rif1 and Mec1 (ATR)-dependent. Our results fit a model in which Rif1 and Rap1 form a complex that is loaded onto telomeres at the end of S-phase. Reduced levels of the Rap1-Rif1 complex in ethanol lead to continuous telomere elongation in a Mec1-dependent process.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Regulating telomere length from the inside out: the replication fork model

In this Hypothesis, Greider describes a new model for telomere length regulation, which links DNA replication and telomere elongation.

Telomere length is regulated around an equilibrium set point. Telomeres shorten during replication and are lengthened by telomerase. Disruption of the length equilibrium leads to disease; thus, it is important to understand the mechanisms that regulate length at the molecular level. The prevailing protein-counting model for regulating telomerase access to elongate the telomere does not explain accumulating evidence of a role of DNA replication in telomere length regulation. Here I present an alternative model: the replication fork model that can explain how passage of a replication fork and regulation of origin firing affect telomere length.

Nucleophosmin Interacts with PIN2/TERF1-interacting Telomerase Inhibitor 1 (PinX1) and Attenuates the PinX1 Inhibition on Telomerase Activity

Telomerase activation and telomere maintenance are critical for cellular immortalization and transformation. PIN2/TERF1-interacting telomerase inhibitor 1 (PinX1) is a telomerase regulator and the aberrant expression of PinX1 causes telomere shortening. Identifying PinX1-interacting proteins is important for understanding telomere maintenance. We found that PinX1 directly interacts with nucleophosmin (NPM), a protein that has been shown to positively correlate with telomerase activity. We further showed that PinX1 acts as a linker in the association between NPM and hTERT, the catalytic subunit of telomerase. Additionally, the recruitment of NPM by PinX1 to the telomerase complex could partially attenuate the PinX1-mediated inhibition on telomerase activity. Taken together, our data reveal a novel mechanism that regulates telomerase activation through the interaction between NPM, PinX1 and the telomerase complex.

Life and Death of Yeast Telomerase RNA

Telomerase reverse transcriptase elongates telomeres to overcome their natural attrition and allow unlimited cellular proliferation, a characteristic shared by stem cells and the majority of malignant cancerous cells. The telomerase holoenzyme comprises a core RNA molecule, a catalytic protein subunit, and other accessory proteins. Malfunction of certain telomerase components can cause serious genetic disorders including dyskeratosis congenita and aplastic anaemia. A hierarchy of tightly regulated steps constitutes the process of telomerase biogenesis, which, if interrupted or misregulated, can impede the production of a functional enzyme and severely affect telomere maintenance. Here, we take a closer look at the budding yeast telomerase RNA component, TLC1, in its long lifetime journey around the cell. We review the extensive knowledge on TLC1 transcription and processing. We focus on exciting recent studies on telomerase assembly, trafficking, and nuclear dynamics, which for the first time unveil striking similarities between the yeast and human telomerase ribonucleoproteins. Finally, we identify questions yet to be answered and new directions to be followed, which, in the future, might improve our knowledge of telomerase biology and trigger the development of new therapies against cancer and other telomerase-related diseases.

Regulation of Elg1 activity by phosphorylation

ELG1 is a conserved gene with important roles in the maintenance of genome stability. Elg1's activity prevents gross chromosomal rearrangements, maintains proper telomere length regulation, helps repairing DNA damage created by a number of genotoxins and participates in sister chromatid cohesion. Elg1 is evolutionarily conserved, and its Fanconi Anemia-related mammalian ortholog (also known as ATAD5) is embryonic lethal when lost in mice and acts as a tumor suppressor in mice and humans. Elg1 encodes a protein that forms an RFC-like complex that unloads the replicative clamp, PCNA, from DNA, mainly in its SUMOylated form. We have identified 2 different regions in yeast Elg1 that undergo phosphorylation. Phosphorylation of one of them, S112, is dependent on the ATR yeast ortholog, Mec1, and probably is a direct target of this kinase. We show that phosphorylation of Elg1 is important for its role at telomeres. Mutants unable to undergo phosphorylation suppress the DNA damage sensitivity of Δrad5 mutants, defective for an error-free post-replicational bypass pathway. This indicates a role of phosphorylation in the regulation of DNA repair. Our results open the way to investigate the mechanisms by which the activity of Elg1 is regulated during DNA replication and in response to DNA damage.

Early Loss of Telomerase Action in Yeast Creates a Dependence on the DNA Damage Response Adaptor Proteins

Telomeres cap the ends of chromosomes, protecting them from degradation and inappropriate DNA repair processes that can lead to genomic instability. A short telomere elicits increased telomerase action on itself that replenishes telomere length, thereby stabilizing the telomere. In the prolonged absence of telomerase activity in dividing cells, telomeres eventually become critically short, inducing a permanent cell cycle arrest (senescence). We recently showed that even early after telomerase inactivation (ETI), yeast cells have accelerated mother cell aging and mildly perturbed cell cycles. Here, we show that the complete disruption of DNA damage response (DDR) adaptor proteins in ETI cells causes severe growth defects. This synthetic-lethality phenotype was as pronounced as that caused by extensive DNA damage in wild-type cells but showed genetic dependencies distinct from such damage and was completely alleviated by SML1 deletion, which increases deoxynucleoside triphosphate (dNTP) pools. Our results indicated that these deleterious effects in ETI cells cannot be accounted for solely by the slow erosion of telomeres due to incomplete replication that leads to senescence. We propose that normally occurring telomeric DNA replication stress is resolved by telomerase activity and the DDR in two parallel pathways and that deletion of Sml1 prevents this stress.

Telomerase inhibitors: a patent review (2010-2015)

INTRODUCTION

Telomerase is a ribonucleoprotein that catalyses the addition of telomeric repeat sequences (having the sequence 5'-TTAGGG-3' in humans) to the ends of chromosomes. Telomerase activity is detected in most types of human tumours, but it is almost undetectable in normal somatic cells. Therefore, telomerase is a promising therapeutic target. To date, the known inhibitors of telomerase include nucleoside analogues, oligonucleotides and G-quadruplex stabilizers. This review highlights recent advances in our understanding of telomerase inhibitors, the relationships between telomerase inhibitors, cancer, and fields such as inflammation.

AREAS COVERED

This review summarizes new patents published on telomerase inhibitors from 2010 to 2015.

EXPERT OPINION

The review provides a brief account of the background, development, and on-going issues involving telomerase inhibitors. In particular, this review emphasizes imetelstat (GRN163L) and some typical G-quadruplex stabilizers that participate in telomerase inhibition. Overall, the research scope of antineoplastic is becoming broader and telomerase inhibitors have been shown to be a promising therapeutic target. Therefore, novel antineoplastic agents with greater activity and higher specificity must be developed.

Epidemiology

More than 250,000 new cases of primary malignant brain tumors are diagnosed annually worldwide, 77% of which are gliomas. A small proportion of gliomas are caused by the inheritance of rare high-penetrance genetic variants or high-dose radiation. Since 2009, inherited genetic variants in 10 regions near eight different genes have been consistently associated with glioma risk via genome-wide association studies. Most of these variants increase glioma risk by 20-40%, but two have higher relative risks. One on chromosome 8 increases risk of IDH-mutated gliomas sixfold and another that affects TP53 function confers a 2.5-fold increased risk of glioma. Functions of some of the other risk variants are known or suspected, but future research will determine functions of other risk loci. Recent progress also has been made in defining subgroups of glioma based on acquired alterations within tumors. Allergy history has been consistently associated with reduced glioma risk, though the mechanisms have not yet been clarified. Future studies will need to be large enough so that environmental and constitutive genetic risk factors can be examined within molecularly defined, etiologically homogeneous subgroups.

Prenatal stress, development, health and disease risk: A psychobiological perspective-2015 Curt Richter Award Paper

The long-term consequences of exposure to excess stress, particularly during sensitive developmental windows, on the initiation and progression of many complex, common physical and mental disorders that confer a major global burden of disease are well established. The period of intrauterine life represents among the most sensitive of these windows, at which time the effects of stress may be transmitted inter-generationally from a mother to her as-yet-unborn child. As explicated by the concept of fetal or developmental programming of health and disease susceptibility, a growing body of evidence supports the notion that health and disease susceptibility is determined by the dynamic interplay between genetic makeup and environment, particularly during intrauterine and early postnatal life. Except in extreme cases, an adverse intrauterine exposure may not, per se, 'cause' disease, but, instead, may determine propensity for disease(s) in later life (by shaping phenotypic responsivity to endogenous and exogenous disease-related risk conditions). Accumulating evidence suggests that maternal psychological and social stress during pregnancy represents one such condition that may adversely affect the developing child, with important implications for a diverse range of physical and mental health outcomes. In this paper we review primarily our own contributions to the field of maternal stress during pregnancy and child mental and physical health-related outcomes. We present findings on stress-related maternal-placental-fetal endocrine and immune/inflammatory processes that may mediate the effects of various adverse conditions during pregnancy on the developing human embryo and fetus. We enunciate conceptual and methodological issues related to the assessment of stress during pregnancy and discuss potential mechanisms of intergenerational transmission of the effects of stress. Lastly, we describe on-going research and some future directions of our program.

Early-Life Telomere Dynamics Differ between the Sexes and Predict Growth in the Barn Swallow (Hirundo rustica)

Telomeres are conserved DNA-protein structures at the termini of eukaryotic chromosomes which contribute to maintenance of genome integrity, and their shortening leads to cell senescence, with negative consequences for organismal functions. Because telomere erosion is influenced by extrinsic and endogenous factors, telomere dynamics may provide a mechanistic basis for evolutionary and physiological trade-offs. Yet, knowledge of fundamental aspects of telomere biology under natural selection regimes, including sex- and context-dependent variation in early-life, and the covariation between telomere dynamics and growth, is scant. In this study of barn swallows (Hirundo rustica) we investigated the sex-dependent telomere erosion during nestling period, and the covariation between relative telomere length and body and plumage growth. Finally, we tested whether any covariation between growth traits and relative telomere length depends on the social environment, as influenced by sibling sex ratio. Relative telomere length declined on average over the period of nestling maximal growth rate (between 7 and 16 days of age) and differently covaried with initial relative telomere length in either sex. The frequency distribution of changes in relative telomere length was bimodal, with most nestlings decreasing and some increasing relative telomere length, but none of the offspring traits predicted the a posteriori identified group to which individual nestlings belonged. Tail and wing length increased with relative telomere length, but more steeply in males than females, and this relationship held both at the within- and among-broods levels. Moreover, the increase in plumage phenotypic values was steeper when the sex ratio of an individual’s siblings was female-biased. Our study provides evidence for telomere shortening during early life according to subtly different dynamics in either sex. Furthermore, it shows that the positive covariation between growth and relative telomere length depends on sex as well as social environment, in terms of sibling sex ratio.

Cell cycle control (and more) by programmed -1 ribosomal frameshifting: implications for disease and therapeutics

Like most basic molecular mechanisms, programmed –1 ribosomal frameshifting (−1 PRF) was first identified in viruses. Early observations that global dysregulation of −1 PRF had deleterious effects on yeast cell growth suggested that −1 PRF may be used to control cellular gene expression, and the cell cycle in particular. Collection of sufficient numbers of viral −1 PRF signals coupled with advances in computer sciences enabled 2 complementary computational approaches to identify −1 PRF signals in free living organisms. The unexpected observation that almost all −1 PRF events on eukaryotic mRNAs direct ribosomes to premature termination codons engendered the hypothesis that −1 PRF signals post-transcriptionally regulate gene expression by functioning as mRNA destabilizing elements. Emerging research suggests that some human diseases are associated with global defects in −1 PRF. The recent discovery of −1 PRF signal-specific trans-acting regulators may provide insight into novel therapeutic strategies aimed at treating diseases caused by changes in gene expression patterns.

Regulation of Telomere Length Requires a Conserved N-Terminal Domain of Rif2 in Saccharomyces cerevisiae

The regulation of telomere length equilibrium is essential for cell growth and survival since critically short telomeres signal DNA damage and cell cycle arrest. While the broad principles of length regulation are well established, the molecular mechanism of how these steps occur is not fully understood. We mutagenized the RIF2 gene in Saccharomyces cerevisiae to understand how this protein blocks excess telomere elongation. We identified an N-terminal domain in Rif2 that is essential for length regulation, which we have termed BAT domain for Blocks Addition of Telomeres. Tethering this BAT domain to Rap1 blocked telomere elongation not only in rif2Δ mutants but also in rif1Δ and rap1C-terminal deletion mutants. Mutation of a single amino acid in the BAT domain, phenylalanine at position 8 to alanine, recapitulated the rif2Δ mutant phenotype. Substitution of F8 with tryptophan mimicked the wild-type phenylalanine, suggesting the aromatic amino acid represents a protein interaction site that is essential for telomere length regulation.

Binding of Multiple Rap1 Proteins Stimulates Chromosome Breakage Induction during DNA Replication

Telomeres, the ends of linear eukaryotic chromosomes, have a specialized chromatin structure that provides a stable chromosomal terminus. In budding yeast Rap1 protein binds to telomeric TG repeat and negatively regulates telomere length. Here we show that binding of multiple Rap1 proteins stimulates DNA double-stranded break (DSB) induction at both telomeric and non-telomeric regions. Consistent with the role of DSB induction, Rap1 stimulates nearby recombination events in a dosage-dependent manner. Rap1 recruits Rif1 and Rif2 to telomeres, but neither Rif1 nor Rif2 is required for DSB induction. Rap1-mediated DSB induction involves replication fork progression but inactivation of checkpoint kinase Mec1 does not affect DSB induction. Rap1 tethering shortens artificially elongated telomeres in parallel with telomerase inhibition, and this telomere shortening does not require homologous recombination. These results suggest that Rap1 contributes to telomere homeostasis by promoting chromosome breakage.

Telomere length is maintained primarily through equilibrium between telomerase-mediated lengthening and the loss of telomeric sequence through the end-replication problem. In budding yeast Rap1 protein binds to telomeric TG repeat and negatively regulates telomerase recruitment in a dosage-dependent manner. In this paper we provide evidence suggesting an alternative Rap1-dependent telomere shortening mechanism in which binding of multiple Rap1 proteins mediates DNA break induction during DNA replication. This process does not involve recombination events; therefore, it is distinct from loop-mediated telomere trimming.

Telomere maintenance and the etiology of adult glioma

A growing body of epidemiologic and tumor genomic research has identified an important role for telomere maintenance in glioma susceptibility, initiation, and prognosis. Telomere length has long been investigated in relation to cancer, but whether longer or shorter telomere length might be associated with glioma risk has remained elusive. Recent data address this question and are reviewed here. Common inherited variants near the telomerase-component genes TERC and TERT are associated both with longer telomere length and increased risk of glioma. Exome sequencing of glioma patients from families with multiple affected members has identified rare inherited mutations in POT1 (protection of telomeres protein 1) as high-penetrance glioma risk factors. These heritable POT1 mutations are also associated with increased telomere length in leukocytes. Tumor sequencing studies further indicate that acquired somatic mutations of TERT and ATRX are among the most frequent alterations found in adult gliomas. These mutations facilitate telomere lengthening, thus bypassing a critical mechanism of apoptosis. Although future research is needed, mounting evidence suggests that glioma is, at least in part, a disease of telomere dysregulation. Specifically, several inherited and acquired variants underlying gliomagenesis affect telomere pathways and are also associated with increased telomere length.

Unique C. elegans telomeric overhang structures reveal the evolutionarily conserved properties of telomeric DNA

There are two basic mechanisms that are associated with the maintenance of the telomere length, which endows cancer cells with unlimited proliferative potential. One mechanism, referred to as alternative lengthening of telomeres (ALT), accounts for approximately 10–15% of all human cancers. Tumours engaged in the ALT pathway are characterised by the presence of the single stranded 5′-C-rich telomeric overhang (C-overhang). This recently identified hallmark of ALT cancers distinguishes them from healthy tissues and renders the C-overhang as a clear target for anticancer therapy. We analysed structures of the 5′-C-rich and 3′-G-rich telomeric overhangs from human and Caenorhabditis elegans, the recently established multicellular in vivo model of ALT tumours. We show that the telomeric DNA from C. elegans and humans forms fundamentally different secondary structures. The unique structural characteristics of C. elegans telomeric DNA that are distinct not only from those of humans but also from those of other multicellular eukaryotes allowed us to identify evolutionarily conserved properties of telomeric DNA. Differences in structural organisation of the telomeric DNA between the C. elegans and human impose limitations on the use of the C. elegans as an ALT tumour model.

Elg1, a central player in genome stability

ELG1 is a conserved gene uncovered in a number of genetic screens in yeast aimed at identifying factors important in the maintenance of genome stability. Elg1's activity prevents gross chromosomal rearrangements, maintains proper telomere length regulation, helps repairing DNA damage created by a number of genotoxins and participates in sister chromatid cohesion. Elg1 is evolutionarily conserved, and its mammalian ortholog (also known as ATAD5) is embryonic lethal when lost in mice, acts as a tumor suppressor in mice and humans, exhibits physical interactions with components of the human Fanconi Anemia pathway and may be responsible for some of the phenotypes associated with neurofibromatosis. In this review, we summarize the information available on Elg1-related activities in yeast and mammals, and present models to explain how the different phenotypes observed in the absence of Elg1 activity are related.

Tpz1TPP1 SUMOylation reveals evolutionary conservation of SUMO-dependent Stn1 telomere association

Elongation of the telomeric overhang by telomerase is counteracted by synthesis of the complementary strand by the CST complex, CTC1(Cdc13)/Stn1/Ten1. Interaction of budding yeast Stn1 with overhang-binding Cdc13 is increased by Cdc13 SUMOylation. Human and fission yeast CST instead interact with overhang-binding TPP1/POT1. We show that the fission yeast TPP1 ortholog, Tpz1, is SUMOylated. Tpz1 SUMOylation restricts telomere elongation and promotes Stn1/Ten1 telomere association, and a SUMO-Tpz1 fusion protein has increased affinity for Stn1. Our data suggest that SUMO inhibits telomerase through stimulation of Stn1/Ten1 action by Tpz1, highlighting the evolutionary conservation of the regulation of CST function by SUMOylation.

Telomere length kinetics assay (TELKA) sorts the telomere length maintenance (tlm) mutants into functional groups

Genome-wide systematic screens in yeast have uncovered a large gene network (the telomere length maintenance network or TLM), encompassing more than 400 genes, which acts coordinatively to maintain telomere length. Identifying the genes was an important first stage; the next challenge is to decipher their mechanism of action and to organize then into functional groups or pathways. Here we present a new telomere-length measuring program, TelQuant, and a novel assay, telomere length kinetics assay, and use them to organize tlm mutants into functional classes. Our results show that a mutant defective for the relatively unknown MET7 gene has the same telomeric kinetics as mutants defective for the ribonucleotide reductase subunit Rnr1, in charge of the limiting step in dNTP synthesis, or for the Ku heterodimer, a well-established telomere complex. We confirm the epistatic relationship between the mutants and show that physical interactions exist between Rnr1 and Met7. We also show that Met7 and the Ku heterodimer affect dNTP formation, and play a role in non-homologous end joining. Thus, our telomere kinetics assay uncovers new functional groups, as well as complex genetic interactions between tlm mutants.