New ways not to make ends meet: telomerase, DNA damage proteins and heterochromatin

Telomeres and cancer: a tale with many endings

Telomerase activity is necessary to maintain the integrity of telomeres, which in turn prevent chromosome ends from being processed and signaled as damaged DNA. That cancer cells rely on telomerase to maintain functional telomeres and to divide indefinitely has highlighted the potential for developing novel therapeutic approaches that target telomerase.

Induction of a p95/Nbs1-mediated S phase checkpoint by telomere 3' overhang specific DNA

Telomere shortening induces a nonproliferative senescent phenotype, believed to reduce cancer risk, and telomeres are involved in a poorly understood manner in responses to DNA damage. Although telomere disruption induces p53 and triggers apoptosis or cell cycle arrest, the features of the disrupted telomere that trigger this response and the precise mechanism involved are poorly understood. Using human cells, we show that DNA oligonucleotides homologous to the telomere 3' overhang sequence specifically induce and activate p53 and activate an S phase checkpoint by modifying the Nijmegen breakage syndrome protein, known to mediate the S phase checkpoint after DNA damage. These responses are mediated, at least in part, by the ATM kinase and are not attributable to disruption of cellular telomeres. Based on these and earlier data, we propose that these oligonucleotides mimic a physiological signal, exposure of the telomere 3' overhang due to opening of the normal telomere loop structure, and hence evoke these protective antiproliferative responses in the absence of DNA damage or telomere disruption.

hTERT associates with human telomeres and enhances genomic stability and DNA repair

Ectopic expression of telomerase in telomerase-silent cells is sufficient to overcome senescence and to extend cellular lifespan. We show here that the catalytic subunit of human telomerase (hTERT) crosslinks telomeres. This interaction is blocked by the telomere repeat binding factor 1, but not by a dominant negative form of this protein. It is also abolished by destruction of the RNA component of telomerase as well as by mutations in the hTERT protein. Ectopic expression of hTERT leads to transcriptional alterations of a subset of genes and changes in the interaction of the telomeres with the nuclear matrix. This is associated with reduction of spontaneous chromosome damage in G(1) cells, enhancement of the kinetics of DNA repair and an increase in NTP levels. The effect on DNA repair is likely indirect as TERT does not directly affect DNA end rejoining in vitro or meiotic recombination in vivo. The observed effects of hTERT occurred rapidly before any significant lengthening of telomeres was observed. Our findings establish an intimate relationship between hTERT-telomere interactions and alteration in transcription of a subset of genes that may lead to increased genomic stability and enhanced repair of genetic damage. These novel functions of telomerase are distinct from its known effect on telomere length and have potentially important biological consequences.

Ablation of telomerase and telomere loss leads to cardiac dilatation and heart failure associated with p53 upregulation

Cardiac failure is a frequent cause of death in the aging human population. Telomere attrition occurs with age, and is proposed to be causal for the aging process. To determine whether telomere shortening leads to a cardiac phenotype, we studied heart function in the telomerase knockout mouse, Terc-/-. We studied Terc-/- mice at the second, G2, and fifth, G5, generation. Telomere shortening in G2 and G5 Terc-/- mice was coupled with attenuation in cardiac myocyte proliferation, increased apoptosis and cardiac myocyte hypertrophy. On a single-cell basis, telomere shortening was coincidental with increased expression of p53, indicating the presence of dysfunctional telomeres in cardiac myocytes from G5 Terc-/- mice. The impairment in cell division, the enhanced cardiac myocyte death and cellular hypertrophy, are concomitant with ventricular dilation, thinning of the wall and cardiac dysfunction. Thus, inhibition of cardiac myocyte replication provoked by telomere shortening, results in de-compensated eccentric hypertrophy and heart failure in mice. Telomere shortening with age could also contribute to cardiac failure in humans, opening the possibility for new therapies.

RecQ helicases and cellular responses to DNA damage

The faithful replication of the genome is essential for the survival of all organisms. It is not surprising therefore that numerous mechanisms have evolved to ensure that duplication of the genome occurs with only minimal risk of mutation induction. One mechanism of genome destabilization is replication fork demise, which can occur when a translocating fork meets a lesion or adduct in the template. Indeed, the collapse of replication forks has been suggested to occur in every replicative cell cycle making this a potentially significant problem for all proliferating cells. The RecQ helicases, which are essential for the maintenance of genome stability, are thought to function during DNA replication. In particular, RecQ helicase mutants display replication defects and have phenotypes consistent with an inability to efficiently reinitiate replication following replication fork demise. Here, we review some current models for how replication fork repair might be effected, and discuss potential roles for RecQ helicases in this process.

Functional interaction between DNA-PKcs and telomerase in telomere length maintenance

DNA-PKcs is the catalytic subunit of the DNA-dependent protein kinase (DNA-PK) complex that functions in the non-homologous end-joining of double-strand breaks, and it has been shown previously to have a role in telomere capping. In particular, DNA-PKcs deficiency leads to chromosome fusions involving telomeres produced by leading-strand synthesis. Here, by generating mice doubly deficient in DNA-PKcs and telomerase (Terc(-/-)/DNA-PKcs(-/-)), we demonstrate that DNA-PKcs also has a fundamental role in telomere length maintenance. In particular, Terc(-/-)/DNA-PKcs(-/-) mice displayed an accelerated rate of telomere shortening when compared with Terc(-/-) controls, suggesting a functional interaction between both activities in maintaining telomere length. In addition, we also provide direct demonstration that DNA-PKcs is essential for both end-to-end fusions and apoptosis triggered by critically short telomeres. Our data predict that, in telomerase-deficient cells, i.e. human somatic cells, DNA-PKcs abrogation may lead to a faster rate of telomere degradation and cell cycle arrest in the absence of increased apoptosis and/or fusion of telomere-exhausted chromosomes. These results suggest a critical role of DNA-PKcs in both cancer and aging.

Mouse models to study the role of telomeres in cancer, aging and DNA repair

The chromosome ends have protective structures that distinguish them from broken chromosomes, known as telomeres. The function of telomeres, and that of the cellular activity that synthesises them, telomerase, are proposed to be biological determinants in the processes of cancer and aging. In this review, we will focus on mammalian telomeres and, in particular, on the analysis of different mouse models for proteins that are important for telomere function, such as telomerase and various telomere-binding proteins. These mouse models have allowed the relevance of telomeres and telomerase in tumour development and the aging of the organism to be directly tested.

A role for the Rb family of proteins in controlling telomere length

The molecular mechanisms of cellular mortality have recently begun to be unraveled. In particular, it has been discovered that cells that lack telomerase are subject to telomere attrition with each round of replication, eventually leading to loss of telomere capping function at chromosome ends. Critically short telomeres and telomeres lacking telomere-binding proteins lose their functionality and are metabolized as DNA breaks, thus generating chromosomal fusions. Telomerase activity is sufficient to rescue short telomeres and confers an unlimited proliferative capacity. In addition, the tumor-suppressor pathway Cdkn2a/Rb1 has also been implicated as a barrier to immortalization. Here, we report a connection between the members of the retinoblastoma family of proteins, Rb1 (retinoblastoma 1), Rbl1 (retinoblastoma-like 1) and Rbl2 (retinoblastoma-like 2), and the mechanisms that regulate telomere length. In particular, mouse embryonic fibroblasts doubly deficient in Rbl1 and Rbl2 or triply deficient in Rbl1, Rbl2 and Rb1 have markedly elongated telomeres compared with those of wildtype or Rb1-deficient cells. This deregulation of telomere length is not associated with increased telomerase activity. Notably, the abnormally elongated telomeres in doubly or triply deficient cells retain their end-capping function, as shown by the normal frequency of chromosomal fusions. These findings demonstrate a connection between the Rb1 family and the control of telomere length in mammalian cells.

Cooperation between p53 mutation and high telomerase transgenic expression in spontaneous cancer development

Telomerase reintroduction in adult somatic tissues is envisioned as a way to extend their proliferative capacity. It is still a question, however, whether constitutive telomerase expression in adult tissues impacts the normal aging and spontaneous cancer incidence of an organism. Here, we studied the aging and spontaneous cancer incidence of mice with transgenic telomerase expression in a wide range of adult tissues, K5-Tert mice. For this, we maintained large colonies of K5-Tert mice for more than 2 years. K5-Tert mice showed a decreased life span compared to wild-type cohorts associated with a higher incidence of preneoplastic and neoplastic lesions in various tissue types. Neoplasias in K5-Tert mice were coincident with transgene expression in the affected tissues. These observations suggest that high telomerase activity may cooperate with genetic alterations that occur with age to promote tumorigenesis. Indeed, we demonstrate here that increased cancer incidence and the reduced viability of K5-Tert mice are aggravated in a p53(+/-) genetic background, indicating that telomerase cooperates with loss of p53 function in inducing tumorigenesis. Altogether, these results demonstrate that constitutive high levels of telomerase activity result in a decreased life span associated with an increased incidence of neoplasias as the organism ages.

Reversible manipulation of telomerase expression and telomere length. Implications for the ionizing radiation response and replicative senescence of human cells

Most human cells do not express telomerase and irreversibly arrest proliferation after a finite number of divisions (replicative senescence). Several lines of evidence suggest that replicative senescence is caused by short dysfunctional telomeres, which arise when DNA is replicated in the absence of adequate telomerase activity. We describe a method to reversibly bypass replicative senescence and generate mass cultures that have different average telomere lengths. A retrovirus carrying hTERT flanked by excision sites for Cre recombinase rendered normal human fibroblasts telomerase-positive and replicatively immortal. Superinfection with retroviruses carrying wild-type or mutant forms of TIN2, a negative regulator of telomere length, created telomerase-positive, immortal populations with varying average telomere lengths. Subsequent infection with a Cre-expressing retrovirus abolished telomerase activity, creating mortal cells with varying telomere lengths. Using these cell populations, we show that, after hTERT excision, cells senesce with shorter telomeres than parental cells. Moreover, long telomeres, but not telomerase, protected cells from the loss of division potential caused by ionizing radiation. Finally, although telomerase-negative cells with short telomeres senesced after fewer doublings than those with long telomeres, telomere length per se did not correlate with senescence. Our results support a role for telomere structure, rather than length, in replicative senescence.

Telomerase beyond telomeres

The role of telomerase in actively proliferating cells is assumed to be restricted to maintaining of telomeres above a minimum-length threshold, thereby preventing telomere exhaustion and chromosomal instability. However, forced telomerase expression in cells and mice with normal-length telomeres has shown that telomerase promotes growth and survival in a manner that is uncoupled from net telomere lengthening. These findings imply that telomerase might have a fundamental role in tumour growth and survival, even at stages when telomeres are sufficiently long.

Replicative senescence revisited

Forty years after its discovery, replicative senescence remains a rich source of information about cell-cycle regulation and the progression from a normal to a transformed phenotype. Effectors of this growth-arrested state are being discovered at a great pace. This review discusses the latest findings on the players responsible for establishing replicative senescence, as well as the associated telomere shortening.

Are Drosophila telomeres an exception or the rule?

At the ends of eukaryotic chromosomes are telomeres, specialized structures with unusual properties. Specific efforts to compare sequences and properties of telomeres across species can reveal the generalities of telomere properties.