Those dam-aged telomeres!

Telomere length, ZNF208 genetic variants and risk of chronic obstructive pulmonary disease in the Hainan Li population

BACKGROUND

Chronic obstructive pulmonary disease (COPD) is a disease characterized by airflow limitation. It is not completely reversible and shows progressive development. ZNF208 rs8105767 affects telomere length, although the impact of telomere on COPD is still controversial. In the present study, we aimed to explore the impact of the ZNF208 gene polymorphism on telomere length and also that of telomere length on COPD in the Hainan Li population.

METHODS

In total, 270 COPD patients and 288 controls were recruited. Telomere length was measured by a quantitative real-time polymerase chain reaction. Five single nucleotide polymorphisms in ZNF208 were selected and genotyping was performed using MassARRAY software (Agena Bioscience Co. Ltd, San Diego, CA, USA). Differences in telomere length among the subjects with three genotypes of related genes were assessed using analysis of variance. Unconditional logistic regression was used to calculate odds ratios (OR) as the indicator of association between telomere length and COPD risk.

RESULTS

Relative telomere length in the COPD group and control group was 0.66 ± 0.47 and 1.44 ± 0.89, respectively. We grouped according to a median of 0.8284 for telomere length and observed that the risk of COPD for individuals with a telomere length less than 0.8284 is 2.92 times that for individuals with a telomere length longer than 0.8284 (OR = 2.92, 95% confidence interval = 2.01-4.25, p = 1.91 × 10^-8 ). Subjects carrying the G allele of rs2188972 had a longer telomere length. Subjects carrying the carrying the CA genotype of rs8103163 and AC genotype of rs7248488 had a longer telomere length compared to wild-type individuals.

CONCLUSIONS

Shorter telomeres increase COPD risk and the ZNF208 polymorphism affects telomere length in COPD patients.

Function, replication and structure of the mammalian telomere

Telomeres are specialized structures at the ends of linear chromosomes that were originally defined functionally based on observations first by Muller (1938) and subsequently by McClintock (1941) that naturally occurring chromosome ends do not behave as double-stranded DNA breaks, in spite of the fact that they are the physical end of a linear, duplex DNA molecule. Double-stranded DNA breaks are highly unstable entities, being susceptible to nucleolytic attack and giving rise to chromosome rearrangements through end-to-end fusions and recombination events. In contrast, telomeres confer stability upon chromosome termini, as evidenced by the fact that chromosomes are extraordinarily stable through multiple cell divisions and even across evolutionary time. This protective function of telomeres is due to the formation of a nucleoprotein complex that sequesters the end of the DNA molecule, rendering it inaccessible to nucleases and recombinases as well as preventing the telomere from activating the DNA damage checkpoint pathways. The capacity of a functional end-protective complex to form is dependent upon maintenance of sufficient telomeric DNA. We have learned a great deal about telomere structure and how this specialized nucleoprotein complex confers stability on chromosome ends since the original observations that defined telomeres were made. This review summarizes our current understanding of mammalian telomere replication, structure and function.

ERCC1/XPF protects short telomeres from homologous recombination in Arabidopsis thaliana

Many repair and recombination proteins play essential roles in telomere function and chromosome stability, notwithstanding the role of telomeres in "hiding" chromosome ends from DNA repair and recombination. Among these are XPF and ERCC1, which form a structure-specific endonuclease known for its essential role in nucleotide excision repair and is the subject of considerable interest in studies of recombination. In contrast to observations in mammalian cells, we observe no enhancement of chromosomal instability in Arabidopsis plants mutated for either XPF (AtRAD1) or ERCC1 (AtERCC1) orthologs, which develop normally and show wild-type telomere length. However, in the absence of telomerase, mutation of either of these two genes induces a significantly earlier onset of chromosomal instability. This early appearance of telomere instability is not due to a general acceleration of telomeric repeat loss, but is associated with the presence of dicentric chromosome bridges and cytologically visible extrachromosomal DNA fragments in mitotic anaphase. Such extrachromosomal fragments are not observed in later-generation single-telomerase mutant plants presenting similar frequencies of anaphase bridges. Extensive FISH analyses show that these DNAs are broken chromosomes and correspond to two specific chromosome arms. Analysis of the Arabidopsis genome sequence identified two extensive blocks of degenerate telomeric repeats, which lie at the bases of these two arms. Our data thus indicate a protective role of ERCC1/XPF against 3' G-strand overhang invasion of interstitial telomeric repeats. The fact that the Atercc1 (and Atrad1) mutants dramatically potentiate levels of chromosome instability in Attert mutants, and the absence of such events in the presence of telomerase, have important implications for models of the roles of recombination at telomeres and is a striking illustration of the impact of genome structure on the outcomes of equivalent recombination processes in different organisms.

No attenuation of the ATM-dependent DNA damage response in murine telomerase-deficient cells

Inactivation of mammalian telomerase leads to telomere attrition, eventually culminating in uncapped telomeres, which elicit a DNA damage response and cell cycle arrest or death. In some instances, telomerase modulation evokes a response not obviously attributable to changes in telomere length. One such example is the suppression of the DNA damage response (DDR) and changes in histone modification that occur upon repression of the telomerase reverse transcriptase, TERT, in human primary cells [K. Masutomi, R. Possemato, J.M. Wong, J.L. Currier, Z. Tothova, J.B. Manola, S. Ganesan, P.M. Lansdorp, K. Collins and W.C. Hahn, The telomerase reverse transcriptase regulates chromatin state and DNA damage responses, Proc. Natl. Acad. Sci. U.S.A. 102 (2005) 8222-8227]. Here, we evaluate the contribution of TERT to the DDR in murine Tert(-/-) cells without critically shortened telomeres. We treated mTert(-/-) embryonic stem (ES) cells and murine embryonic fibroblasts (MEFs) with etoposide and irradiation, and assessed the status of p53(pS15), 53BP1, ATM(pS1981), SMC1(pS957), and gammaH2AX by indirect immunofluorescence or western blotting. In four independently derived mTert(-/-) ES cell lines, there was no significant difference in the induction of gammaH2AX, 53BP1 foci, or the phosphorylation of ATM targets (ATM, SMC1, p53) between wildtype and mTert(-/-) ES cells and MEFs. A slight difference in post-translational modification of histones H3 and H4 was observed in a subset of mTert(-/-) ES cells, however this difference was reflected in the cellular levels of H3 and H4. Thus, in contrast to previous studies in human cells, the absence of Tert does not overtly affect the ATM-dependent response to DNA damage in murine cells.

Telomere shortening in chronic obstructive pulmonary disease

Chronic oxidative stress and systemic inflammation contribute to the pathology of several chronic diseases, one among which is chronic obstructive pulmonary disease (COPD). In addition, increased oxidative stress and inflammation have been observed to be negatively associated with telomere length (TL). Our aim was to investigate the TL in COPD patients in relation to pulmonary and extrapulmonary disease severity. Furthermore, based on experimental evidence suggesting the effects of oxidative stress on telomere shortening, we studied the association of TL with the antioxidant enzyme superoxide dismutase (SOD). One hundred and two COPD patients with moderate to severe COPD were studied and compared with 19 healthy age-matched controls. Patients were characterized by elevated levels of inflammatory markers (CRP, sTNF-receptors) and lower SOD-activity than the controls (p<0.001), irrespective of the SOD genotype. TL was negatively associated with age (p<0.01) and was significantly shorter in COPD patients than controls (p<0.05). Within the patient group age-adjusted TL variability could not be explained by lung function and smoking history but a modest association was found with the percentage of fat mass (p<0.05). These data provide evidence for a relationship between a disturbed oxidant/antioxidant balance and telomere shortening and indicate that preservation of fat mass may be protective in delaying telomere shortening in COPD patients.

Telomere length assessment: biomarker of chronic oxidative stress?

Telomeres are nucleoprotein structures, located at the ends of chromosomes and are subject to shortening at each cycle of cell division. They prevent chromosomal ends from being recognized as double strand breaks and protect them from end to end fusion and degradation. Telomeres consist of stretches of repetitive DNA with a high G-C content and are reported to be highly sensitive to damage induced by oxidative stress. The resulting DNA strand breaks can be formed either directly or as an intermediate step during the repair of oxidative bases. In contrast to the majority of genomic DNA, there is evidence that telomeric DNA is deficient in the repair of single strand breaks. Since chronic oxidative stress plays a major role in the pathophysiology of several chronic inflammatory diseases, it is hypothesized that telomere length is reducing at a faster rate during oxidative stress. Therefore, assessment of telomere length might be a useful biomarker of disease progression. In this review several features of telomere length regulation, their relation with oxidative stress, and the potential application of measurement of telomere length as biomarker of chronic oxidative stress, will be discussed.

Circular dichroism of quadruplex DNAs: applications to structure, cation effects and ligand binding

Circular dichroism, CD, spectra can be used to gain information about quadruplex structures of DNAs as well as the effects of sequence, cations, chemical modification and ligand binding on quadruplex structure. There is not yet a validated approach to calculate a CD spectrum from a quadruplex structure nor is their one to go from a CD spectrum to a structure. However, it is possible to empirically correlate CD spectra features with quadruplex structural type in many cases. In this article four case studies are presented to indicate the strengths and limitations of CD in investigations of the properties of quadruplex structures formed by telomere repeat sequences. The case studies include determination of the quadruplex structural type present as a function of potassium concentration, the effect of sequence on the equilibrium between quadruplex structural types as a function of potassium concentration, the effect of ligand binding on quadruplex structure and the effect of 5' phosphorylation on quadruplex structural type.

Nucleus pulposus cellular longevity by telomerase gene therapy

STUDY DESIGN

Nonviral transfection of nucleus pulposus cells with a telomerase expression construct to assess the effects on cellular lifespan, function, karyotypic stability, and transformation properties.

OBJECTIVES

To investigate whether telomerase gene therapy can extend the cellular lifespan while retaining functionality of nucleus pulposus cells in a safe manner.

SUMMARY OF BACKGROUND DATA

Degeneration of the intervertebral disc is an age-related condition in which cells responsible for the maintenance and health of the disc deteriorate with age. Telomerase can extend the cellular lifespan and function of other musculoskeletal tissues, such as the heart, bones, and connective tissues. Therefore, extension of the cellular lifespan and matrix production of intervertebral disc cells may have the potential to delay the degeneration process.

METHODS

Ovine nucleus pulposus cells were lipofectamine transfected in vitro with a human telomerase reverse transcriptase (hTERT) expression construct. Cellular lifespan and matrix transcript levels were determined by cumulative population doublings and real-time RT-PCR, respectively. G1-cell cycle checkpoint, p53 functionality, growth of transfected cells in anchorage-independent or serum starvation conditions, and karyotypic analysis were performed.

RESULTS

Transfection was achieved successfully with 340% +/- 7% (mean +/- SD) relative telomerase activity in hTERT-transfected cells. hTERT transfection enabled a 50% extension in mean cellular lifespan and prolonged matrix production of collagen 1 and 2 for more than 282 days. Karyotypic instability was detected but G1-cell cycle checkpoint and p53 was functionally comparable to parental cells with no growth in serum starvation or anchorage-independent conditions.

CONCLUSIONS

Telomerase can extend the cellular lifespan of nucleus pulposus cells and prolong the production of extracellular matrix. Safety is still unresolved, as karyotypic instability was detected but no loss of contact inhibition, mitogen dependency, or G1-cell cycle checkpoint control was evident.

Themes in ssDNA recognition by telomere-end protection proteins

The ends of eukaryotic linear chromosomes are unique structures that require special management by the cell. If left unattended, the ends are inappropriately processed, leading to genomic instability and problems with proliferation. Telomeres are specialized nucleoprotein structures that restore chromosome stability by protecting and maintaining chromosome ends. Proper telomere function is facilitated, in part, by the telomere-end protection (TEP) family of proteins, which targets the 3' single-stranded (ss) overhang region of the telomere via a specialized ssDNA-binding domain (DBD). With the recent availability of the structures of these DBDs, the ssDNA-binding characteristics of TEP proteins can be compared and the common underlying mechanisms of ssDNA recognition identified, thus providing insights into telomere function.

Identification of the determinants for the specific recognition of single-strand telomeric DNA by Cdc13

The single-strand overhang present at telomeres plays a critical role in mediating both the capping and telomerase regulation functions of telomeres. The telomere end-binding proteins, Cdc13 in Saccharomyces cerevisiae, Pot1 in higher eukaryotes, and TEBP in the ciliated protozoan Oxytricha nova, exhibit sequence-specific binding to their respective single-strand overhangs. S. cerevisiae telomeres are composed of a heterogeneous mixture of GT-rich telomeric sequence, unlike in higher eukaryotes which have a simple repeat that is maintained with high fidelity. In yeast, the telomeric overhang is recognized by the essential protein Cdc13, which coordinates end-capping and telomerase activities at the telomere. The Cdc13 DNA-binding domain (Cdc13-DBD) binds these telomere sequences with high affinity (3 pM) and sequence specificity. To better understand the basis for this remarkable recognition, we have investigated the binding of the Cdc13-DBD to a series of altered DNA substrates. Although an 11-mer of GT-rich sequence is required for full binding affinity, only three of these 11 bases are recognized with high specificity. This specificity differs from that observed in the other known telomere end-binding proteins, but is well suited to the specific role of Cdc13 at yeast telomeres. These studies expand our understanding of telomere recognition by the Cdc13-DBD and of the unique molecular recognition properties of ssDNA binding.

Cell cycle-regulated trafficking of human telomerase to telomeres

Telomerase synthesizes telomeres at the ends of human chromosomes during S phase. The results presented here suggest that telomerase activity may be regulated by intranuclear trafficking of the key components of the enzyme in human cells. We examined the subcellular localization of endogenous human telomerase RNA (hTR) and telomerase reverse transcriptase (hTERT) in HeLa cervical carcinoma cells. Throughout most of the cell cycle, we found that the two essential components of telomerase accumulate at intranuclear sites separate from telomeres. However, during S phase, both hTR and hTERT are specifically recruited to subsets of telomeres. The localization of telomerase to telomeres is dynamic, peaking at mid-S phase. We also found complex associations of both hTR and hTERT with nucleoli and Cajal bodies during S phase, implicating both structures in the biogenesis and trafficking of telomerase. Our results mark the first observation of human telomerase at telomeres and provide a mechanism for the cell cycle-dependent regulation of telomere synthesis in human cells.

DNA repair and recombination functions in Arabidopsis telomere maintenance

In this review, we discuss recent advances in the knowledge of plant telomere maintenance, focusing on the model plant Arabidopsis thaliana and, in particular, on the roles of proteins involved in DNA repair and recombination. The question of the interrelationships between DNA repair and recombination pathways and proteins with telomere function and maintenance is of increasing interest and has been the subject of a number of recent reviews (Cech 2004, d'Adda di Fagagna et al. 2004, Hande 2004, Harrington 2004, Maser and DePinho 2004). Understanding of telomere biology, DNA repair and recombination in plants has rapidly progressed over the last decade, substantially due to genetic approaches in Arabidopsis, and we feel that this is an appropriate time to review current knowledge in this field. A number of recent reviews have dealt more generally with the subject of plant telomere structure and evolution (Riha et al. 2001, McKnight et al. 2002, Riha and Shippen 2003b, McKnight and Shippen 2004, Fajkus et al. 2005) and we thus focus specifically on plant telomere biology in the context of DNA repair and recombination in Arabidopsis.

Thermodynamic and electrostatic properties of ternary Oxytricha nova TEBP-DNA complex

Telomeres constitute the nucleoprotein ends of eukaryotic chromosomes which are essential for their proper function. Telomere end binding protein (TEBP) from Oxytricha nova was among the first telomeric proteins, which were well characterized biologically. TEBP consists of two protein subunits (alpha, beta) and forms a ternary complex with single stranded telomeric DNA containing tandem repeats TTTTGGGG. This work presents the characterization of the thermodynamic and electrostatic properties of this complex by computational chemistry methods (continuum Poisson-Boltzmann and solvent accessible surface calculations). Our calculations give a new insight into molecular properties of studied system. Based on the thermodynamic analysis we provide a rationale for the experimental observation that alpha and ssDNA forms a binary complex and the beta subunit joins alpha:ssDNA complex only after the latter is formed. Calculations of distribution of the molecular electrostatic potential for protein subunits alone and for all possible binary complexes revealed the important role of the "guiding funnel" potential generated by alpha:ssDNA complex. This potential may help the beta subunit to dock to the already formed alpha:DNA intermediate in highly steric and electrostatic favorable manner. Our pK(a) calculations of TEBP are able to explain the experimental mobility shifts of the complex in electrophoretic non-denaturating gels.

Dyskeratosis congenita: telomerase, telomeres and anticipation

Dyskeratosis congenita (DC) is a rare bone marrow failure syndrome that displays marked clinical and genetic heterogeneity. The identification of dyskeratosis congenita gene 1 (DKC1) mutations in X-linked recessive patients initially suggested that DC is a defective pseudouridylation disorder. The subsequent identification of mutations in the telomerase RNA component (TERC) of autosomal dominant DC patients together with the discovery that both TERC and the DKC1-encoded protein, dyskerin, are closely associated in the telomerase complex have suggested that the pathophysiology of DC predominantly relates to defective telomere maintenance. Recent discoveries have shown that autosomal dominant DC exhibits disease anticipation and that this is associated with progressive telomere shortening owing to the haplo-insufficiency of TERC.

Telomere shortening is associated with malformation in p53-deficient mice after irradiation during specific stages of development

The natural ends of linear chromosomes, the telomeres, recruit specific proteins in the formation of protective caps that preserve the integrity of the genome. Unprotected chromosomes induce DNA damage checkpoint cascades and ultimately lead to senescence both in mouse and man in a p53 dependent manner and initial telomere length setting therefore determines the proliferative capacity of each cell. Yet, only little information is available on telomere biology during embryonic development. We have previously shown that the p53 gene plays a crucial role in the development of malformations (exencephaly, gastroschisis, polydactyly, cleft palate and dwarfism) in control and irradiated mouse embryos. Here, we investigated telomere biology and the outcome of radiation exposure in wild type (p53+/+) and p53-mutant (p53+/-- and--/--) C57BL mouse foetuses irradiated at three different developmental stages. We show that telomeres are significantly shorter in malformed foetuses as compared to normal counterparts. In addition, our results indicate that the observed telomere attrition is primarily associated with p53-deficiency but is also modulated by irradiation, more specifically during the gastrulation and organogenesis stages. In conclusion, we formulate a hypothesis in which telomere shortening is linked to the absence of p53 in mouse foetuses and that when, in the presence of shorter telomeres, these foetuses are irradiated, the chance for the occurrence of developmental defects increases substantially.

Chromosome instability in neoplasia: chaotic roots to continuous growth

Multiple rearrangements of chromosome number and structure are common manifestations of genomic instability encountered in mammalian tumors. In neoplasia, in continuous immortalized growth in vitro, and in animal models, the accumulation of various defects on DNA repair and telomere maintenance machineries, mitotic spindle abnormalities, and breakage-fusion-bridge cycles, deteriorate the precise mitotic distribution of the genomic content, thus producing various types of chromosomal anomalies. These lesions generate tremendous genomic imbalances, which are evolutionary selected, since they force the function of the whole genome towards continuous growth. For more than a century chromosomal rearrangements and aneuploidy in neoplasia have been discussed and a vast number of genes and pathways, directly or indirectly implicated, have been described. In this review, we focus on the biological mechanisms that generate numerical or structural deviations of the normal diploid chromosomal constitution in epithelial neoplasia. There is growing evidence that chromosomal instability is both an epiphenomenon and a leading cause of cancer. We will discuss the roles of genes, chromosome structure, and telomere dysfunction in the initiation of chromosomal instability. We will explore research strategies that can be applied to identify rates of chromosomal instability in a specimen, and the putative biological consequences of karyotypic heterogeneity. Finally, we will re-examine the longstanding hypothesis of the generation of aneuploidy in the context of telomere dysfunction and restoration.

Irreversible cellular senescence induced by prolonged exposure to H2O2 involves DNA-damage-and-repair genes and telomere shortening

H2O2 has been the most commonly used inducer for stress-induced premature senescence (SIPS), which shares features of replicative senescence. However, there is still uncertainty whether SIPS and replicative senescence differ or utilize different pathways. 'Young' human diploid fibroblasts (HDFs), treated with prolonged low doses of hydrogen peroxide, led to irreversible cellular senescence. Cells exhibited senescent-morphological features, irreversible G1 cell cycle arrest and irreversible senescence-associated beta-galactosidase positivity. The appearance of these cellular senescence markers was accompanied by significant increases of p21, gadd45 expression and p53 binding activity, as well as a significant decline in DNA repair capability and accelerated telomere shortening. Our results suggest that multiple pathways might be involved in oxidative SIPS, including genes related to DNA-damage-and-repair and telomere shortening, and that SIPS shares the same mechanisms with replicative senescence in vivo. Our findings indicate that several aging theories can be merged together by a common mechanism of oxidative damage, and that the level of oxidative DNA-damage-and-repair capacity may be exploited as reliable markers of cell senescence.

Exo1 and Rad24 differentially regulate generation of ssDNA at telomeres of Saccharomyces cerevisiae cdc13-1 mutants

Cell cycle arrest in response to DNA damage depends upon coordinated interactions between DNA repair and checkpoint pathways. Here we examine the role of DNA repair and checkpoint genes in responding to unprotected telomeres in budding yeast cdc13-1 mutants. We show that Exo1 is unique among the repair genes tested because like Rad9 and Rad24 checkpoint proteins, Exo1 inhibits the growth of cdc13-1 mutants at the semipermissive temperatures. In contrast Mre11, Rad50, Xrs2, and Rad27 contribute to the vitality of cdc13-1 strains grown at permissive temperatures, while Din7, Msh2, Nuc1, Rad2, Rad52, and Yen1 show no effect. Exo1 is not required for cell cycle arrest of cdc13-1 mutants at 36 degrees but is required to maintain arrest. Exo1 affects but is not essential for the production of ssDNA in subtelomeric Y' repeats of cdc13-1 mutants. However, Exo1 is critical for generating ssDNA in subtelomeric X repeats and internal single-copy sequences. Surprisingly, and in contrast to Rad24, Exo1 is not essential to generate ssDNA in X or single-copy sequences in cdc13-1 rad9Delta mutants. We conclude that Rad24 and Exo1 regulate nucleases with different properties at uncapped telomeres and propose a model to explain our findings.

Vertebrate telomere repeat DNAs favor external loop propeller quadruplex structures in the presence of high concentrations of potassium

The circular dichroism, CD, spectra of the telomere repeats of vertebrates, d(TTAGGG), indicate that parallel type quadruplex structures or disordered single-stranded structures are formed in low salt. Anti-parallel quadruplex structures are favored in the presence of high concentrations, 140 mM, of sodium. External loop, also known as propeller, parallel type structures are favored in the presence of high concentrations, 100 mM, of potassium in the presence of either 5 or 140 mM sodium. The cation dependence of the CD spectra of the vertebrate telomere repeat DNAs is distinctly different from that of the telomere repeats of Tetrahymena and Oxytricha as well as that of the thrombin binding aptamer. These results indicate that the external loop structures may be present in vertebrate telomeres under the conditions of high potassium and low sodium concentration found in nuclei.

Telomerase- and recombination-independent immortalization of budding yeast

It is generally assumed that there are only two ways to maintain the ends of chromosomes in yeast and mammalian nuclei: telomerase and recombination. Without telomerase and recombination, cells enter senescence, a state of permanent growth arrest. We found that the decisive role in preventing senescent budding yeast cells from dividing is played by the Exo1 nuclease. In the absence of Exo1, telomerase- and recombination-defective yeast can resume cell cycle progression, despite degradation of telomeric regions from many chromosomes. As degradation progresses toward internal chromosomal regions, a progressive decrease in viability would be expected, caused by loss of essential genes. However, this was not the case. We demonstrate that extensive degradation and loss of essential genes can be efficiently prevented through a little-studied mechanism of DNA double-strand-break repair, in which short DNA palindromes induce formation of large DNA palindromes. For the first time, we show that large palindromes form as a natural consequence of postsenescence growth and that they become essential for immortalization in the absence of telomerase activity.

Senescence and immortalization: role of telomeres and telomerase

Telomere dynamics are a critical component of both aging and cancer. Telomeres progressively shorten in almost all dividing cells and most human cells do not express or maintain sufficient telomerase activity to fully maintain telomeres. There is accumulating evidence that when only a few telomeres are short, they form end-associations, leading to a DNA damage signal resulting in replicative senescence (a cellular growth arrest, also called the M1 stage). In the absence of cell-cycle checkpoint pathways (e.g. p53 and or p16/Rb), cells bypass M1 senescence and telomeres continue to shorten eventually resulting in crisis (also called the M2 stage). M2 is characterized by many 'uncapped' chromosome ends, end-fusions, chromosome breakage fusion-bridge cycles, mitotic catastrophe and a high fraction of apoptotic cells. In a rare M2 cell, telomerase (a cellular reverse transcriptase) can be reactivated or up-regulated, resulting in indefinite cell proliferation. This cellular immortalization is a potentially rate-limiting step in carcinogenesis that is important for the continuing evolution of most advanced cancers. In this perspective we will present our views on the evidence for telomere dysfunction in aging and in cancer progression. We will argue that telomere shortening in the absence of other alterations may be a potent tumor suppressor mechanism and we will discuss the evidence for and against the major molecular mechanisms proposed to initiate replicative senescence.

Biochemical properties of Trypanosoma cruzi telomerase

Trypanosomatid parasite infections have a devastating impact on human health. Little is known about the requirements for parasite growth during any stage of their complex, multi-host life cycle. In most eukaryotic organisms, sustained cell proliferation requires telomerase-dependent telomere length maintenance. Here we investigate the regulation and biochemical features of telomerase from Trypanosoma cruzi, the causative agent of Chagas disease. We found that T.cruzi telomerase is active in extracts from multiple developmental stages of the parasite life cycle. Detailed characterization of the enzymatic properties of telomerase using epimatigote-stage extract revealed a unique combination of substrate specificities, consistent with the evolutionary divergence of trypanosomes from previously established model systems for telomerase biochemical characterization. Results from partial purification of T.cruzi telomerase suggest that the catalytically active enzyme is a large ribonucleoprotein complex and that the internal RNA template has an atypical, cytosine-rich permutation. These results expand our understanding of telomerase enzymology and should encourage the development of parasite-specific telomerase inhibitors as a method for disease therapy.

The End in Sight

The HP1 heterochromatic protein has become an archetypical example of a chromatin protein recruited by binding a specific histone modification. Surprisingly, recent work in Drosophila reveals that the essential function of HP1 is accomplished by another mode of binding.