High incidence of rapid telomere loss in telomerase-deficient Caenorhabditis elegans

Genomic Instabilities, Cellular Senescence, and Aging: In Vitro, In Vivo and Aging-Like Human Syndromes

As average life span and elderly people prevalence in the western world population is gradually increasing, the incidence of age-related diseases such as cancer, heart diseases, diabetes, and dementia is increasing, bearing social and economic consequences worldwide. Understanding the molecular basis of aging-related processes can help extend the organism’s health span, i.e., the life period in which the organism is free of chronic diseases or decrease in basic body functions. During the last few decades, immense progress was made in the understanding of major components of aging and healthy aging biology, including genomic instability, telomere attrition, epigenetic changes, proteostasis, nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and intracellular communications. This progress has been made by three spear-headed strategies: in vitro (cell and tissue culture from various sources), in vivo (includes diverse model and non-model organisms), both can be manipulated and translated to human biology, and the study of aging-like human syndromes and human populations. Herein, we will focus on current repository of genomic “senescence” stage of aging, which includes health decline, structural changes of the genome, faulty DNA damage response and DNA damage, telomere shortening, and epigenetic alterations. Although aging is a complex process, many of the “hallmarks” of aging are directly related to DNA structure and function. This review will illustrate the variety of these studies, done in in vitro, in vivo and human levels, and highlight the unique potential and contribution of each research level and eventually the link between them.

Chromosome ends as adaptive beginnings: the potential role of dysfunctional telomeres in subtelomeric evolvability

Telomeres serve as protective caps that help the cell differentiate between the naturally occurring ends of chromosomes and double-stranded breaks. When telomere capping function becomes compromised, chromosome ends are subjected to elevated rates of chromosome alterations. These effects can be particularly dramatic in the telomere-adjacent subtelomeric region. While the catastrophic impact of severe telomere dysfunction on genome stability has been well documented, the adaptive telomere failure hypothesis considers an alternative role telomere dysfunction may play in adaptive evolution. This hypothesis suggests that low levels of telomere failure, induced by certain environmental stresses, can lead to elevated subtelomeric recombination. Mutational loss, duplication, or modification of subtelomeric contingency genes could ultimately facilitate adaptation by generating novel mutants better able to survive environmental stress. In this perspective, we discuss recent work that examined mild telomere dysfunction and its role in altering the adaptive potential of subtelomeric genes.

Maintenance of Genome Integrity by Mi2 Homologs CHD-3 and LET-418 in Caenorhabditis elegans

Meiotic recombination depends upon the tightly coordinated regulation of chromosome dynamics and is essential for the production of haploid gametes. Central to this process is the formation and repair of meiotic double-stranded breaks (DSBs), which must take place within the constraints of a specialized chromatin architecture. Here, we demonstrate a role for the nucleosome remodeling and deacetylase (NuRD) complex in orchestrating meiotic chromosome dynamics in Caenorhabditis elegans Our data reveal that the conserved Mi2 homologs Chromodomain helicase DNA-binding protein (CHD-3) and its paralog LET-418 facilitate meiotic progression by ensuring faithful repair of DSBs through homologous recombination. We discovered that loss of either CHD-3 or LET-418 results in elevated p53-dependent germ line apoptosis, which relies on the activation of the conserved checkpoint kinase CHK-1 Consistent with these findings, chd-3 and let-418 mutants produce a reduced number of offspring, indicating a role for Mi2 in forming viable gametes. When Mi2 function is compromised, persisting recombination intermediates are detected in late pachytene nuclei, indicating a failure in the timely repair of DSBs. Intriguingly, our data indicate that in Mi2 mutant germ lines, a subset of DSBs are repaired by nonhomologous end joining, which manifests as chromosomal fusions. We find that meiotic defects are exacerbated in Mi2 mutants lacking CKU-80, as evidenced by increased recombination intermediates, corpses, and defects in chromosomal integrity. Taken together, our findings support a model wherein the C. elegans Mi2 complex maintains genomic integrity through reinforcement of a chromatin landscape suitable for homology-driven repair mechanisms.

Synthesis and antitumor mechanism of a new iron(iii) complex with 5,7-dichloro-2-methyl-8-quinolinol as ligands

A new iron(iii) complex with 5,7-dichloro-2-methyl-8-quinolinol (HClMQ) as ligands, i.e., [Fe(ClMQ)2Cl] (1), was synthesized and evaluated for its anticancer activity. Compared to the HClMQ ligand, complex 1 showed a higher cytotoxicity towards a series of tumor cell lines, including Hep-G2, BEL-7404, NCI-H460, A549, and T-24, with IC50 values in the range of 5.04-14.35 μM. Notably, the Hep-G2 cell line was the most sensitive to complex 1. Mechanistic studies indicated that complex 1 is a telomerase inhibitor targeting c-myc G-quadruplex DNA and can trigger cell apoptosis via inducing cell cycle arrest and DNA damage.

The Genetic Basis of Natural Variation in Caenorhabditis elegans Telomere Length

Telomeres are involved in the maintenance of chromosomes and the prevention of genome instability. Despite this central importance, significant variation in telomere length has been observed in a variety of organisms. The genetic determinants of telomere-length variation and their effects on organismal fitness are largely unexplored. Here, we describe natural variation in telomere length across the Caenorhabditis elegans species. We identify a large-effect variant that contributes to differences in telomere length. The variant alters the conserved oligonucleotide/oligosaccharide-binding fold of protection of telomeres 2 (POT-2), a homolog of a human telomere-capping shelterin complex subunit. Mutations within this domain likely reduce the ability of POT-2 to bind telomeric DNA, thereby increasing telomere length. We find that telomere-length variation does not correlate with offspring production or longevity in C. elegans wild isolates, suggesting that naturally long telomeres play a limited role in modifying fitness phenotypes in C. elegans.

Premature aging in telomerase-deficient zebrafish

The study of telomere biology is crucial to the understanding of aging and cancer. In the pursuit of greater knowledge in the field of human telomere biology, the mouse has been used extensively as a model. However, there are fundamental differences between mouse and human cells. Therefore, additional models are required. In light of this, we have characterized telomerase-deficient zebrafish (Danio rerio) as the second vertebrate model for human telomerase-driven diseases. We found that telomerase-deficient zebrafish show p53-dependent premature aging and reduced lifespan in the first generation, as occurs in humans but not in mice, probably reflecting the similar telomere length in fish and humans. Among these aging symptoms, spinal curvature, liver and retina degeneration, and infertility were the most remarkable. Although the second-generation embryos died in early developmental stages, restoration of telomerase activity rescued telomere length and survival, indicating that telomerase dosage is crucial. Importantly, this model also reproduces the disease anticipation observed in humans with dyskeratosis congenita (DC). Thus, telomerase haploinsufficiency leads to anticipation phenomenon in longevity, which is related to telomere shortening and, specifically, with the proportion of short telomeres. Furthermore, p53 was induced by telomere attrition, leading to growth arrest and apoptosis. Importantly, genetic inhibition of p53 rescued the adverse effects of telomere loss, indicating that the molecular mechanisms induced by telomere shortening are conserved from fish to mammals. The partial rescue of telomere length and longevity by restoration of telomerase activity, together with the feasibility of the zebrafish for high-throughput chemical screening, both point to the usefulness of this model for the discovery of new drugs able to reactivate telomerase in individuals with DC.

Extreme telomere length dimorphism in the Tasmanian devil and related marsupials suggests parental control of telomere length

Telomeres, specialised structures that protect chromosome ends, play a critical role in preserving chromosome integrity. Telomere dynamics in the Tasmanian devil (Sarcophilus harrisii) are of particular interest in light of the emergence of devil facial tumour disease (DFTD), a transmissible malignancy that causes rapid mortality and threatens the species with extinction. We used fluorescent in situ hybridisation to investigate telomere length in DFTD cells, in healthy Tasmanian devils and in four closely related marsupial species. Here we report that animals in the Order Dasyuromorphia have chromosomes characterised by striking telomere length dimorphism between homologues. Findings in sex chromosomes suggest that telomere length dimorphism may be regulated by events in the parental germlines. Long telomeres on the Y chromosome imply that telomere lengthening occurs during spermatogenesis, whereas telomere diminution occurs during oogenesis. Although found in several somatic cell tissue types, telomere length dimorphism was not found in DFTD cancer cells, which are characterised by uniformly short telomeres. This is, to our knowledge, the first report of naturally occurring telomere length dimorphism in any species and suggests a novel strategy of telomere length control. Comparative studies in five distantly related marsupials and a monotreme indicate that telomere dimorphism evolved at least 50 million years ago.

DNA synthesis generates terminal duplications that seal end-to-end chromosome fusions

End-to-end chromosome fusions that occur in the context of telomerase deficiency can trigger genomic duplications. For more than 70 years, these duplications have been attributed solely to breakage-fusion-bridge cycles. To test this hypothesis, we examined end-to-end fusions isolated from Caenorhabditis elegans telomere replication mutants. Genome-level rearrangements revealed fused chromosome ends having interrupted terminal duplications accompanied by template-switching events. These features are very similar to disease-associated duplications of interstitial segments of the human genome. A model termed Fork Stalling and Template Switching has been proposed previously to explain such duplications, where promiscuous replication of large, noncontiguous segments of the genome occurs. Thus, a DNA synthesis-based process may create duplications that seal end-to-end fusions, in the absence of breakage-fusion-bridge cycles.

Recombination can cause telomere elongations as well as truncations deep within telomeres in wild-type Kluyveromyces lactis cells

In this study, we examined the role of recombination at the telomeres of the yeast Kluyveromyces lactis. We demonstrated that an abnormally long and mutationally tagged telomere was subject to high rates of telomere rapid deletion (TRD) that preferentially truncated the telomere to near-wild-type size. Unlike the case in Saccharomyces cerevisiae, however, there was not a great increase in TRD in meiosis. About half of mitotic TRD events were associated with deep turnover of telomeric repeats, suggesting that telomeres were often cleaved to well below normal length prior to being reextended by telomerase. Despite its high rate of TRD, the long telomere showed no increase in the rate of subtelomeric gene conversion, a highly sensitive test of telomere dysfunction. We also showed that the long telomere was subject to appreciable rates of becoming elongated substantially further through a recombinational mechanism that added additional tagged repeats. Finally, we showed that the deep turnover that occurs within normal-length telomeres was diminished in the absence of RAD52. Taken together, our results suggest that homologous recombination is a significant process acting on both abnormally long and normally sized telomeres in K. lactis.

Comparing effects of mTR and mTERT deletion on gene expression and DNA damage response: a critical examination of telomere length maintenance-independent roles of telomerase

Telomerase, the essential enzyme that maintains telomere length, contains two core components, TERT and TR. Early studies in yeast and mouse showed that loss of telomerase leads to phenotypes only after several generations, due to telomere shortening. However, recent studies have suggested additional roles for telomerase components in transcription and the response to DNA damage. To examine these potential telomere length maintenance-independent roles of telomerase components, we examined first generation mTR(-/-) and mTERT(-/-) mice with long telomeres. We used gene expression profiling and found no genes that were differentially expressed in mTR(-/-) G1 mice and mTERT(-/-) G1 mice compared with wild-type mice. We also compared the response to DNA damage in mTR(-/-)G1 and mTERT(-/-) G1 mouse embryonic fibroblasts, and found no increase in the response to DNA damage in the absence of either telomerase component compared to wild-type. We conclude that, under physiologic conditions, neither mTR nor mTERT acts as a transcription factor or plays a role in the DNA damage response.

Telomeres and disease

The telomeres of most eukaryotes are characterized by guanine-rich repeats synthesized by the reverse transcriptase telomerase. Complete loss of telomerase is tolerated for several generations in most species, but modestly reduced telomerase levels in human beings are implicated in bone marrow failure, pulmonary fibrosis and a spectrum of other diseases including cancer. Differences in telomerase deficiency phenotypes between species most likely reflect a tumour suppressor function of telomeres in long-lived mammals that does not exist as such in short-lived organisms. Another puzzle provided by current observations is that family members with the same genetic defect, haplo-insufficiency for one of the telomerase genes, can present with widely different diseases. Here, the crucial role of telomeres and telomerase in human (stem cell) biology is discussed from a Darwinian perspective. It is proposed that the variable phenotype and penetrance of heritable human telomerase deficiencies result from additional environmental, genetic and stochastic factors or combinations thereof.

End joining at Caenorhabditis elegans telomeres

Critically shortened telomeres can be subjected to DNA repair events that generate end-to-end chromosome fusions. The resulting dicentric chromosomes can enter breakage-fusion-bridge cycles, thereby impeding elucidation of the structures of the initial fusion events and a mechanistic understanding of their genesis. Current models for the molecular basis of fusion of critically shortened, uncapped telomeres rely on PCR assays that typically capture fusion breakpoints created by direct ligation of chromosome ends. Here we use independent approaches that rely on distinctive features of Caenorhabditis elegans to study the frequency of direct end-to-end chromosome fusion in telomerase mutants: (1) holocentric chromosomes that allow for genetic isolation of stable end-to-end fusions and (2) unique subtelomeric sequences that allow for thorough PCR analysis of samples of genomic DNA harboring multiple end-to-end fusions. Surprisingly, only a minority of end-to-end fusion events resulted from direct end joining with no additional genome rearrangements. We also demonstrate that deficiency for the C. elegans Ku DNA repair heterodimer does not affect telomere length or cause synthetic effects in the absence of telomerase.

Actions of human telomerase beyond telomeres

Telomerase has fundamental roles in bypassing cellular aging and in cancer progression by maintaining telomere homeostasis and integrity. However, recent studies have led some investigators to suggest novel biochemical properties of telomerase in several essential cell signaling pathways without apparent involvement of its well established function in telomere maintenance. These observations may further enhance our understanding of the molecular actions of telomerase in aging and cancer. This review will provide an update on the extracurricular activities of telomerase in apoptosis, DNA repair, stem cell function, and in the regulation of gene expression.

Telomeres and aging

Telomeres play a central role in cell fate and aging by adjusting the cellular response to stress and growth stimulation on the basis of previous cell divisions and DNA damage. At least a few hundred nucleotides of telomere repeats must "cap" each chromosome end to avoid activation of DNA repair pathways. Repair of critically short or "uncapped" telomeres by telomerase or recombination is limited in most somatic cells and apoptosis or cellular senescence is triggered when too many "uncapped" telomeres accumulate. The chance of the latter increases as the average telomere length decreases. The average telomere length is set and maintained in cells of the germline which typically express high levels of telomerase. In somatic cells, telomere length is very heterogeneous but typically declines with age, posing a barrier to tumor growth but also contributing to loss of cells with age. Loss of (stem) cells via telomere attrition provides strong selection for abnormal and malignant cells, a process facilitated by the genome instability and aneuploidy triggered by dysfunctional telomeres. The crucial role of telomeres in cell turnover and aging is highlighted by patients with 50% of normal telomerase levels resulting from a mutation in one of the telomerase genes. Short telomeres in such patients are implicated in a variety of disorders including dyskeratosis congenita, aplastic anemia, pulmonary fibrosis, and cancer. Here the role of telomeres and telomerase in human aging and aging-associated diseases is reviewed.

Genome integrity is regulated by the Caenorhabditis elegans Rad51D homolog rfs-1

Multiple mechanisms ensure genome maintenance through DNA damage repair, suppression of transposition, and telomere length regulation. The mortal germline (Mrt) mutants in Caenorhabditis elegans are defective in maintaining genome integrity, resulting in a progressive loss of fertility over many generations. Here I show that the high incidence of males (him)-15 locus, defined by the deficiency eDf25, is allelic to rfs-1, the sole rad-51 paralog group member in C. elegans. The rfs-1/eDf25 mutant displays a Mrt phenotype and mutant animals manifest features of chromosome fusions prior to the onset of sterility. Unlike other Mrt genes, rfs-1 manifests fluctuations in telomere lengths and functions independently of telomerase. These data suggest that rfs-1 is a novel regulator of genome stability.

Protection against chromosome degradation at the telomeres

Telomeres, the ends of linear chromosomes, contain repeated TG-rich sequences which, in dividing cells, must be constantly replenished in order to avoid chromosome erosion and, hence, genomic instability. Moreover, unprotected telomeres are prone to end-to-end fusions. Telomerase, a specialized reverse transcriptase with a built-in RNA template, or, in the absence of telomerase, alternative pathways of telomere maintenance are required for continuous cell proliferation in actively dividing cells as well as in cancerous cells emerging in deregulated somatic tissues. The challenge is to keep these free DNA ends masked from the nucleolytic attacks that will readily operate on any DNA double-strand break in the cell, while also allowing the recruitment of telomerase at intervals. Specialized telomeric proteins, as well as DNA repair and checkpoint proteins with a dual role in telomere maintenance and DNA damage signaling/repair, protect the telomere ends from degradation and some of them also function in telomerase recruitment or other aspects of telomere length homeostasis. Phosphorylation of some telomeric proteins by checkpoint protein kinases appears to represent a mode of regulation of telomeric mechanisms. Finally, recent studies have allowed starting to understand the coupling between progression of the replication forks through telomeric regions and the subsequent telomere replication by telomerase, as well as retroaction of telomerase in cis on the firing of nearby replication origins.

Telomere length inheritance and aging

Telomere shortening accompanies human aging, and premature aging syndromes are often associated with short telomeres. These two observations are central to the hypothesis that telomere length directly influences longevity. If true, genetically determined mechanisms of telomere length homeostasis should significantly contribute to variations of longevity in the human population. On the other hand, telomere shortening is also observed in the course of many aging-associated disorders but determining whether it is a cause or a consequence is not an easy task. Here, we review the most relevant experimental and descriptive data relating telomere length, as a quantitative trait, to aging and longevity.

MBD2 is required for correct spatial gene expression in the gut

Gene expression in the gut is segmentally regulated, but little is known of the molecular origin of patterning. Analysis of gene expression in colons from mice lacking the methyl-CpG binding repressor MBD2 revealed frequent activation of genes that are normally only expressed in the exocrine pancreas and duodenum. Reduced DNA methylation activated the same gene set in the colon. No significant differences in DNA methylation between the colon and duodenum were detected, but MBD2 was significantly more abundant in the colon. The relevance of MBD2 concentration was tested in a human colon cancer cell line. Depletion of MBD2 was again found to activate exocrine pancreatic genes. Gene activation in this cell culture model was accompanied by loss of promoter-bound MBD2 and increased histone acetylation. The results suggest that modulation of MBD2 during gut development establishes a region-specific gene expression pattern that is essential for establishing correct segmental character.

The role of the nonhomologous end-joining DNA double-strand break repair pathway in telomere biology

Double-strand breaks are a cataclysmic threat to genome integrity. In higher eukaryotes the predominant recourse is the nonhomologous end-joining (NHEJ) double-strand break repair pathway. NHEJ is a versatile mechanism employing the Ku heterodimer, ligase IV/XRCC4 and a host of other proteins that juxtapose two free DNA ends for ligation. A critical function of telomeres is their ability to distinguish the ends of linear chromosomes from double-strand breaks, and avoid NHEJ. Telomeres accomplish this feat by forming a unique higher order nucleoprotein structure. Paradoxically, key components of NHEJ associate with normal telomeres and are required for proper length regulation and end protection. Here we review the biochemical mechanism of NHEJ in double-strand break repair, and in the response to dysfunctional telomeres. We discuss the ways in which NHEJ proteins contribute to telomere biology, and highlight how the NHEJ machinery and the telomere complex are evolving to maintain genome stability.

Telomere rapid deletion regulates telomere length in Arabidopsis thaliana

Telomere length is maintained in species-specific equilibrium primarily through a competition between telomerase-mediated elongation and the loss of terminal DNA through the end-replication problem. Recombinational activities are also capable of both lengthening and shortening telomeres. Here we demonstrate that elongated telomeres in Arabidopsis Ku70 mutants reach a new length set point after three generations. Restoration of wild-type Ku70 in these mutants leads to discrete telomere-shortening events consistent with telomere rapid deletion (TRD). These findings imply that the longer telomere length set point is achieved through competition between overactive telomerase and TRD. Surprisingly, in the absence of telomerase, a subset of elongated telomeres was further lengthened, suggesting that in this background a mechanism of telomerase-independent lengthening of telomeres operates. Unexpectedly, we also found that plants possessing wild-type-length telomeres exhibit TRD when telomerase is inactivated. TRD is stochastic, and all chromosome ends appear to be equally susceptible. The frequency of TRD decreases as telomeres shorten; telomeres less than 2 kb in length are rarely subject to TRD. We conclude that TRD functions as a potent force to regulate telomere length in Arabidopsis.

Recent aging research in Caenorhabditis elegans

Evidence gathered over the past 15 years shows that the nematode Caenorhabditis elegans is excellently suited as a model to study aging processes in the entire organism. Genetic approaches have been used to identify and elucidate multiple mechanisms and their corresponding genes that limit the life span of C. elegans. These highly conserved pathways include the well-studied insulin/IGF-1 receptor-like signaling pathway, which is thought to be a central determinant of life span, since several other mechanisms depend or converge on the insulin/IGF-1 pathway transcription factor DAF-16/FoxO. In this review we focus on new insights into the molecular mechanisms of aging in C. elegans, including new genes acting in the insulin/IGF-1 pathway and germline signaling. In addition, stress response pathways and mitochondrial mechanisms, dietary restriction, SIR2 deacetylase activity, TOR and TUBBY signaling, as well as telomere length contribution are discussed in relation to recent developments in C. elegans aging research.