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Cardiovascular Research Advance Access originally published online on December 1, 2008
Cardiovascular Research 2009 81(2):244-252; doi:10.1093/cvr/cvn337
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Published on behalf of the European Society of Cardiology. All rights reserved. © The Author 2008. For permissions please email: journals.permissions@oxfordjournals.org

Telomere biology in cardiovascular disease: the TERC–/– mouse as a model for heart failure and ageing

Liza S.M. Wong, Hisko Oeseburg, Rudolf A. de Boer, Wiek H. van Gilst, Dirk J. van Veldhuisen and Pim van der Harst*

Department of Cardiology, University Medical Center Groningen, University of Groningen, Hanzeplein 1, 9700RB Groningen, The Netherlands

* Corresponding author. Tel: +31 50 3612355; fax: +31 50 3614391. E-mail address: P.van.der.Harst{at}thorax.umcg.nl

Received 23 August 2008; revised 8 November 2008; accepted 27 November 2008

Time for primary review: 34 days


   Abstract
Top
 Abstract
1. Introduction
 2. Telomere biology
 3. Telomere biology and...
 4. The telomerase-deficient mice
 5. Counteracting the effect...
 6. Conclusions and future...
 Funding
 References
 
Atherosclerosis and heart failure are major causes of morbidity and mortality in Western countries. Recent studies are suggesting involvement of telomere biology in the development and progression of age-associated conditions, including hypertension, atherosclerosis, and heart failure. Whether any of these reported associations are based on causal relationships remains to be elucidated. The construction of telomerase-deficient (telomerase RNA component, TERC–/–) mice might provide a potential instrumental model to study the involvement of telomere biology in cardiovascular disease. Here, we review the current available information from all studies performed in TERC–/– mice providing information on the cardiovascular phenotypic characteristics. Although this mouse model has proven its value in the understanding of the role of telomere biology in cancer, stem cell, and basic telomere research, only few studies were specifically designed to answer cardiovascular-related questions. The TERC–/– mice provide exciting opportunities to expand our knowledge of telomere biology in cardiovascular disease and the potential identification of novel targets of treatment.

KEYWORDS Telomeres; Telomerase; Animal model; Cardiovascular phenotype


   1. Introduction
Top
 Abstract
 1. Introduction
2. Telomere biology
 3. Telomere biology and...
 4. The telomerase-deficient mice
 5. Counteracting the effect...
 6. Conclusions and future...
 Funding
 References
 
Cardiovascular disease is the leading cause of morbidity and mortality in Western countries.1 Although experimental and epidemiological studies have identified many factors involved in pathogenesis of atherosclerosis and heart failure, our understanding is still incomplete. Recently, telomere biology has entered the cardiovascular research field as a potential factor involved in the initiation and progression of atherosclerosis and heart failure.2–5 Decreased telomere length has been associated with heart failure and atherosclerosis in human cross-sectional studies. However, there is no convincing human evidence implicating telomeres as a cause of atherosclerosis or heart failure. The construction of genetically modified mice with short telomeres provides an opportunity to increase our knowledge on the nature of the relationship between changes in telomere biology and cardiovascular disease. This mouse model has advanced our understanding of the role of telomeres in cancer, but there are only a few specific studies focussed on cardiovascular disease. Here, we will review all available information on cardiovascular phenotypic characteristics of the telomerase-deficient mice. We expect that future experiments in these mice might help to fill in important deficiencies in our knowledge on the role of telomeres in cardiovascular disease.


   2. Telomere biology
Top
 Abstract
 1. Introduction
 2. Telomere biology
3. Telomere biology and...
 4. The telomerase-deficient mice
 5. Counteracting the effect...
 6. Conclusions and future...
 Funding
 References
 
2.1 Structure and function of telomeres
Telomeres are the distal ends of chromosomes—present in all eukaryotes—and are made up of tandem repeats of specific DNA sequences (TTAGGG in vertebrates). The length of telomeres varies among and within species, but take up to ~15 kb in humans and 40–80 kb in mice.6 Telomeric DNA terminates in a 3'-single-stranded overhang, which is well-protected in a three-dimensional structure, the so-called T-loop.5 Little is known about the dynamics of the T-loop formation, but it is clear that telomeric proteins are involved in this process. Telomeres associate with several proteins to form a telomere–protein complex called the shelterin complex.7,8 Thus far, six telomeric proteins have been identified—the best-known proteins probably being telomere binding factor 1 and 2 (TRF1 and TRF2). Other telomeric proteins are protection of telomeres 1 (POT1), Ras-associated protein 1 (Rap1), tripeptidylpeptidase 1, and TRF1- and TRF2-interacting nuclear protein 2.8,9 As far as known today, these proteins do not have any function elsewhere in the cellular machinery.8

2.2 Cellular signalling in response to telomere dysfunction
The essential function of telomeres is protecting the chromosomal ends from being recognized as double-stranded DNA breaks. The protective mechanisms of telomeres are generally attributed to the unique features of the T-loop, as it hides the final open end of the DNA strand. If telomeres become dysfunctional or critically short, they will lose their protective properties and several DNA damage signalling mechanisms will be activated.

In mammalian cells, DNA breaks will be marked by DNA damage foci following phosphorylation of the histone H2AX sites by the protein kinases ATM (ataxia-telangiectasia mutated) and ATR (ataxia-telangiectasia- and Rad3-related).10 ATM and ATR will also activate the DNA checkpoint kinases Chk1 and Chk2 (which have an important role in cell cycle control). Chk1 and Chk2, in turn, will activate the anti-apoptotic protein p53,11 leading to the expression of p21, a cyclin-dependent kinase inhibitor. P21 expression is linked to cellular senescence (Figure 1). Activation of the checkpoint kinases can also lead to senescence, through inhibition of Cdc25 (cell division cycle 25). Next to the induction of senescence through p21, p53 can also initiate the apoptosis pathway.7,8 A far less well-understood mechanism through which telomere dysfunction causes senescence is the induction of p16. In senescent human and mouse cardiomyocytes, telomere reduction was related to upregulation of p16.12–14 p16 blocks the cyclin-dependent kinases CDK4 and CDK6. As a consequence, the protein RB (retinoblastoma protein) remains in its active, hypophosphorylated form. Active RB inhibits cell cycle progression and induces senescence (Figure 1).7,15


Figure 1
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  		Figure 1 Schematic overview of some cellular signalling pathways in response to telomere dysfunction. POT1, protection of telomeres 1; ATR, ataxia-telangiectasia mutated; TRF2, telomere binding factor 2; ATR, ataxia-telangiectasia- and Rad3-related; Cdc25, cell division cycle 25; Cdk4/6, cyclin-dependent kinases 4 and 6; RB, retinoblastoma protein. For details see text.7–11,15

 
In yeasts, the exonuclease 1 (EXO1) is shown to be involved in telomere-associated cellular damage responses when telomeres become critically short.16,17 Recently, a role for EXO1 in these damage responses has been confirmed in mice.18 The role of EXO1 is discussed in more detail below.

Cellular damage response initiated by telomere signalling can also involve DNA break repair systems. If telomeres are critically short or the shelterin complex is disturbed, the telomeric end will be recognized as DNA break in need of repair. In response, two DNA repair mechanisms will come into action; (i) non-homologous end joining (NHEJ) and (ii) homologous recombination (HR). NHEJ of telomeres results in chromosomal end-to-end fusion and causes further telomere damage and dysfunction. Both NHEJ and HR eventually will lead to cellular dysfunction, genomic instability and apoptosis (Figure 1). More details on NHEJ and HR at telomeric ends are extensively reviewed elsewhere.8

2.3 Acquired telomere erosion
After every cell cycle, telomeres lose a number of telomeric base pairs. This phenomenon is also known as the ‘end-replication problem’. As a consequence, telomeres mark replicative history and are therefore considered a marker of chronological ageing.12,19,20 Telomere length of human peripheral white blood cells has indeed consistently been associated with age.21–24 Besides replicative stress, several external stressors have also been associated with telomere shortening. Most evidence exists for the telomere eroding effects of UV radiation and oxidative stress.25,26 Telomere length at any time is a resultant of length provided at birth, replicative, and environmental stresses. Telomere attrition is variable and might be different during life time, but has been estimated to be on average ~30 bp/year.27

2.4 Structure and function of telomerase
Telomeres can be elongated by the ribonucleoprotein enzyme telomerase, which adds TTAGGG repeats to the 3'-end of DNA strands (Figure 2). Telomerase consists of two core compounds, telomerase reverse transcriptase (TERT) and telomerase RNA component (TERC), which serves as a template for addition of telomeric repeats to DNA strands.28 Under physiological circumstances, telomerase expression is undetectable in most human cells, with the major exceptions being embryonic stem cells, germline cells, and certain epithelial and lymphoid progenitor cells.29,30


Figure 2
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  		Figure 2 Elongation of a telomere by telomerase. Telomerase can elongate telomeres. Telomerase carries an RNA template that is used as a template to add telomeric nucleotide repeats, in human TTAGGG, to the 3'-ends of telomeres.

 

   3. Telomere biology and the cardiovascular phenotype in humans
Top
 Abstract
 1. Introduction
 2. Telomere biology
 3. Telomere biology and...
4. The telomerase-deficient mice
 5. Counteracting the effect...
 6. Conclusions and future...
 Funding
 References
 
Telomere length has repetitively been linked to cardiovascular disease states. One possible explanation for the association between telomere dysfunction and cardiovascular disease is that short telomeres lead to cellular dysfunction (e.g. diminished proliferative capacity) and increased senescence and apoptosis. Implications of diminished proliferative capacity for the cardiovascular system might include the exhaustion of the progenitor pool with repair capacity.31 As a result, neovascularization in ischaemic heart disease and vascular regeneration in atherosclerotic disease could be limited. In addition, cellular stability is essential for maintenance of organ function, especially in scarcely regenerating tissue, such as the myocardium. Increased cellular vulnerability or actual increased senescence and apoptosis due to telomere dysfunction can promote heart failure. Alternatively, the association between telomere dysfunction and cardiovascular disease may be that the disease itself causes the telomere to dysfunction or shorten. So although the precise mechanisms connecting telomere biology to the different cardiovascular phenotypes remain to be defined, convincing lines of evidence are supporting a role for telomeres in cardiovascular (patho)physiology, and below we briefly summarize these data.

3.1 Telomeres and cardiovascular risk factors
Reduced telomere length is associated with the presence of cardiovascular risk factors.4 For smoking, the evidence is most convincing.32–34 Reduced telomere length is also associated with insulin resistance and obesity,24,27,32 type 1 and type 2 diabetes,35,36 hypertension,37 activation of the renin–angiotensin–aldosterone system,38 and renal failure.39

3.2 Telomeres and atherosclerosis
Short telomeres have been implicated in vascular senescence and atherosclerosis.40 Endothelial cells derived from atherosclerotic plaques have shorter telomeres compared with endothelial cells derived from non-atherosclerotic areas of the same individual.41,42 Patients with atherosclerosis also have shorter telomeres in their leukocytes compared with healthy, age-matched controls.41,43,44

3.3 Telomeres and heart failure
Evidence is accumulating involving telomere biology in the development of heart failure. In endomyocardial biopsies from patients with dilated cardiomyopathies, more cells were senescent and telomeres were shorter compared with age-matched controls.13,45 Interfering with either function or expression of TRF2, one of the proteins that associate with telomeres, triggers telomere erosion, and apoptosis in cardiomyocytes.45 As myocardial tissue is difficult to obtain, human studies have evaluated telomere length in circulating leukocytes. Telomere length of leukocytes is also shorter in patients with heart failure compared with healthy controls.43 Moreover, shorter telomeres have recently been found in bone marrow cells of patients with atherosclerotic disease.46 Shorter telomere length in the bone marrow led to the speculation that it might affect the function of endothelial progenitor or other repair cells.31

The suggestion of the existence of cardiac progenitor cells (CPCs) as a repair source of the adult heart has raised the possibility that telomeres are important for the function of these cells.47–49 Telomere shortening is indeed observed and associated with aged and senescent CPCs.49

Unfortunately, despite significant clinical, in vivo and in vitro associations between cardiovascular disease phenotypes and telomere biology, definite proof for a causal role of telomeres in the development of atherosclerosis or heart failure has not been generated.


   4. The telomerase-deficient mice
Top
 Abstract
 1. Introduction
 2. Telomere biology
 3. Telomere biology and...
 4. The telomerase-deficient mice
5. Counteracting the effect...
 6. Conclusions and future...
 Funding
 References
 
4.1 Telomerase-deficient mouse models
The notion that mouse and human tumours express elevated levels of telomerase activity, while normal adjacent tissue lacks measurable activity, led to the development of models to study telomere biology. In 1997, Dr Maria A. Blasco reported the successful generation of telomerase-deficient mice by knocking out the RNA template of telomerase (TERC–/– mice).50 This mouse model was established to serve as a tool to study tumour formation and cell viability in the absence of telomerase. Besides the RNA template, another essential component of telomerase is the TERT. Not surprisingly, TERT–/– mice have also been constructed.

Although originally generated to serve research related to oncology, the telomerase-deficient mice have provided valuable information for age-associated diseases, including those related to cardiovascular disease.

4.2 General phenotype of the telomerase-deficient mouse
The telomeres of the TERC–/– mice shorten at a rate of ~5 kb in every subsequent generation.50 Therefore, experiments using TERC–/– mice commonly involve several generations (G) of these mouse compared with wild types. Telomere length in G3 TERC–/– mice decreases as much as 50% during ageing from 1 to 16 months.51 TERC–/– mice of later generations show increasingly severe structural abnormalities on cellular level, including undetectable short telomeres, aneuploidy, and chromosomal end-to-end fusions.50 In general, animals with phenotypical features of ageing have shorter telomeres than age-matched controls from the same generation lacking a clear ageing phenotype. Later generations TERC–/– mice show a decreasing life span (Table 1). Autopsy of late generation spontaneously diseased animals could not identify a clear cause of death, comparable to natural death in humans.51 Later generation and aged TERC–/– mice show many more defects than earlier generation and younger TERC–/– mice. These defects include reduced body size and weight, hair graying and loss, infertility and testicular atrophy, spleen atrophy, signs of immunosenescence, and bone marrow proliferative defects.51,52 Wound healing is also impaired in aged animals TERC–/–.51 Reduced angiogenic potential of these mice has been suggested as an explanation for this observation. Below we will focus in more detail on the knowledge of the angiogenic potential in these mice.


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  		Table 1 Characteristics of different generations of TERC–/– mice

 
Although some interesting studies have been performed on TERT–/– mice, these mice are substantially less well-studied compared with TERC–/– mice. The construction of the TERT–/– mice has first been reported in 1999.53 Both the TERT–/– and the TERC–/– mice do not show significant phenotypic abnormalities at early age in the first generation. Unfortunately, information on the general phenotype of the TERT–/– mice of later generations is sparse and data on fertility of these mice are reported differently by different groups, possibly due to different backgrounds of the mice. Unchanged litter size in G1 and G2 TERT–/– mice (progeny from a chimeric and a wild-type C57Bl/6 mouse) has been reported by some,53 while others experienced dramatically reduced litter size in G2 TERT–/– mice on a pure C57Bl/6 background.54 Indisputable is the fact that later generation TERT–/– and TERC–/– mice both have considerably shorter telomeres than their wild-type litter mates.54–56

In general, homozygous TERT and TERC deficient mice display a similar phenotype. However, one remarkable difference between heterozygous TERT and TERC deficient mice has been observed. In contrast to TERC+/– mice, TERT+/– mice do not have detectable defects in telomere maintenance and elongation. In both heterozygous mice, the mRNA levels of their knocked out gene are approximately 30–50% of that of wild-type mice. The differences are therefore unlikely to be attributable to differences in target gene expression. Presumably, both TERC and TERT are essential for telomere maintenance and elongation, but in contrast to TERT, gene copy number and transcriptional regulation of TERC are limiting for telomerase activity.56,57

4.3 Hypertension
Several cross-sectional human studies associate blood pressure parameters with telomere length of circulating leukocytes.37,58 Only one study is available evaluating blood pressure in telomerase-deficient mice.59 In conscious mice, tail sphygmomanometry revealed that TERC–/– mice from G1 showed higher systolic blood pressures compared with wild type. In G3 mice, both systolic and diastolic blood pressures were increased compared with wild-type and G1 mice. Studying anaesthesized G3 and wild-type mice with invasive haemodynamic studies did not show functional modifications in the nitric oxide system or the responsiveness to angiotensin II. Thus, the differences in blood pressure do not seem to be attributable to these two systems. Interestingly, the response to endothelin—a powerful vasoconstrictor—was diminished in G3. In concordance, treatment with bosentan, an endothelin receptor antagonist, resulted in a more pronounced drop of blood pressure in G3 mice compared with wild type. Furthermore, plasma and urine endothelin levels were gradually and significantly increased in G1 and G3 mice. To further explore the role of endothelin in the observed differences in blood pressure between TERC–/– and wild type mice, mRNA expression of endothelin converting enzyme (ECE) was measured. TERC–/– mice showed higher mRNA expression of ECE-1 and specific inhibition of ECE only reduced blood pressure in TERC–/– mice. In vitro experiments with transfection of deletion mutants of the ECE-1 promoter suggested that the AP-1 binding sequence of the ECE-1 promoter is involved, so that transcriptional control of ECE-1 may be disrupted in TERC–/– mice.59 Whether long-term treatment with endothelin receptor blockers is beneficial in TERC–/– mice remains to be determined. In addition, this functional data obtained in the TERC–/– mice have not been translated back to human studies, meaning we do not know yet whether ECE activity is also involved in the association between hypertension and telomere length in humans.

4.4 Atherosclerosis
The association between reduced telomere length in leukocytes with the development and presence of atherosclerotic manifestations in humans is demonstrated by several independent groups.41,44,60,61 As mice in general are resistant to the development of atherosclerosis,62 many groups are using the apolipoprotein-E (ApoE)-deficient mice, which do develop atherosclerosis when exposed to a high fat diet.63 Interestingly, ApoE–/– mice were inter-crossed with TERC–/– mice to study the effects of telomeres on the development of atherosclerosis.64 Fourth generations of TERC–/–ApoE–/– and TERC+/+ApoE–/– mice were challenged with a high cholesterol high fat diet. Remarkably, generation four (G4) TERC–/–ApoE–/– mice developed less atherosclerotic lesions compared with G4 TERC+/+ApoE–/– mice (Table 2). In addition, the atherosclerotic plaques of G4 TERC–/–ApoE–/– mice were morphologically in a less advanced stage of atherosclerosis, compared with G4 TERC+/+ApoE–/–. This suggests that the absence of telomerase activity is protective for atherosclerotic disease. These observations could not be attributed to differences in serum cholesterol levels.64 It was also observed that the proliferative capacity of macrophages and lymphocytes was decreased in G4 TERC–/–ApoE–/– mice compared with G4 TERC+/+ApoE–/–, suggesting reduced inflammatory capacity. This might explain the differences in atherosclerotic manifestations in this experimental model, since progression of atherosclerosis is partly dependent of functional immunocompetent cells.64


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  		Table 2 Mouse models of telomerase manipulation and the cardiovascular phenotype

 
Experiments in these mice providing more definite proof, e.g. by a bone-marrow switch with immune-competent cells or endothelial-specific TERC–/–, have not been reported up to date.

4.5 Angiogenesis
Evidence for involvement of telomerase in angiogenesis has been provided by adenovirus-mediated transfer of TERT in the rat hind limb ischaemia, which enhances capillary density in the ischaemic tissue.65 Also, TERC–/– mice have been studied using both matrigel implants and murine melanoma grafts.66 In an in vivo matrigel assay, late generation TERC–/– mice showed diminished angiogenic potential compared with wild-type mice. Early generation TERC–/– mice, who have normal telomere length, did not have impairment of angiogenesis as assessed by the in vivo Matrigel assay.66 This suggests that short telomere length limits the angiogenic potential, and not the absence of functional telomerase itself. Also an in vivo angiogenesis model using murine melanoma cells showed decreased tumour formation efficiency and growth rate in later generations TERC–/– mice. Microvessel density in tumour cryosections was stained with an anti-CD31 antibody—an endothelial cell marker—and it was shown that the microvessel density of G5 TERC–/– tumours was only half of the wild-type and G2 TERC–/– tumours.66

4.6 Cardiac myocytes and ventricular failure
The effects of telomerase deficiency on cardiac myocyte size, number, proliferative potential, and myocyte apoptosis has been studied in combination with cardiac function in G2 and G5 TERC–/– mice of the original mixed background and compared with wild-type mice. The progressive decrease of telomere length in cardiomyocytes of successive generations of TERC–/– mice was associated with an increase of p53 expression.67 G5 TERC–/– mice suffer from severe left ventricular failure, characterized by increased end diastolic left ventricular pressure, decreased maximally developed left ventricular pressure and disturbed contractility and relaxation of the left ventricle. These mice also showed anatomical changes of the heart, similar to dilated cardiomyopathy in human, together with decreased total number of myocytes and increase of myocyte hypertrophy. In addition, apoptosis of myocytes was an approximate 40% greater in G5 TERC–/– mice, compared with wild-type and G2 TERC–/– mice.67 In G2 TERC–/– mice, only a slight decrease in left ventricular pressure compared with wild-type mice was observed.67 These data suggest that late generation of TERC–/– mice spontaneously develop pathological cardiac remodelling and severe ventricular dysfunction. Another study showed that exercise increased TRF2 expression and prevented doxorubicin-induced cardiac apoptosis in wild-type mice, but not in TERT–/– mice. This suggests that, in absence of telomerase, upregulation of telomere-stabilizing proteins is challenged and cardiac apoptosis is more severe.68

In conclusion, telomerase-deficient mice provide a model to study the efficacy of telomerase-based therapies for heart failure. However, it should be taken into account that the majority of patients who develop heart failure have coronary artery disease.

4.7 Stem cell biology and tissue regeneration
A key process in tissue and organ homeostasis is the mobilization of stem cells for maintenance and repair. Evidence is supporting a role for bone marrow derived cells in the maintenance and regeneration the endothelium.69 More controversial is whether the heart is also harbouring progenitor cells in adult life.48,49 The TERC–/– mice have provided us with more insights in the role of telomerase and telomere length in several well-characterized stem-cell subtypes, including haematopoietic, epidermal, and neural stem cells.

Haematopoietic progenitor cells from G1 TERC–/– mice have a normal capacity to grow and differentiate in vitro. Mature haematopoietic organ structure and function seem to be well compensated in TERC–/– deficient mice, as no changes in peripheral blood count and profile were observed through successive generations and mature immunocytes show normal responses to mitogenic or infectious stimuli.70 However, in vitro haematopoietic colony-forming unit (CFU) assays revealed that later generation TERC–/– mice have a significant decrease in total number of CFU-granulocyte–monocyte, CFU-granulocyte, -erythrocyte, -monocyte, -megakaryocyte, and decreased high-proliferative-potential colony forming cell colonies.70 In addition, serial and competitive transplantations of TERC–/– bone marrow stem cells showed reduced long- term repopulating capacity compared with wild-type cells.71,72 This indicates that long-term renewal of haematopoietic stem cells is compromised upon telomere loss.

In different generations of TERC–/– inbred mouse, the epidermal stem cell number has been compared.73 In G1 and even more pronounced in G3 TERC–/– mice, greater numbers of epidermal stem cells were present in the bulge area of the hair follicle. Interestingly, the epidermal stem cells in TERC–/– mice showed a defect in their mobilization. Coincidentally, the proliferation index in different compartments was lower than that of wild-type follicles. In addition, in vitro culture of keratinocytes from G1 and G3 TERC–/– mice formed fewer and smaller colonies than those of wild type.73 Thus, epidermal stem cells in TERC–/– mice are less functional than in wild type, and the increased numbers of epidermal stem cells in the TERC–/– mice are possibly due to accumulation in the follicles, as the epidermal stem cells have impaired capacity to mobilize.

Although bone marrow and stem cells are considered important in the pathogenesis and possible treatment of cardiovascular disease, including atherosclerosis and heart failure, studies focussed on the role of telomere biology are lacking. In this regard also, the TERC–/– model provides a good model to study the efficacy of stem cell-based therapies for heart failure.


   5. Counteracting the effect of telomere dysfunction
Top
 Abstract
 1. Introduction
 2. Telomere biology
 3. Telomere biology and...
 4. The telomerase-deficient mice
 5. Counteracting the effect...
6. Conclusions and future...
 Funding
 References
 
As telomere biology is linked to cardiovascular pathology, targeting it provides new and promising therapeutic avenues to be explored. Here, we will discuss data investigating several possibilities of counteracting the deteriorating effects of dysfunctional telomeres.

5.1 Exonuclease-1 deletion
One of the proteins that mediate response to telomere dysfunction is EXO1.18 It has been shown that EXO1 deletion prolongs the lifespan of G3 TERC–/–EXO1–/– mice, compared with G3 TERC–/–EXO1+/+ mice. EXO1 deletion preserves organ function of the intestinal tract and the haematopoietic system,18 both organ systems that are normally affected in the TERC–/– mice.50 Remarkably, this beneficial effect is present in 12–15, but not 24 months old G3 TERC–/–EXO1–/– mice, suggesting that in old mice EXO1-independent mechanisms are responsible for the disturbed organ homeostasis. The beneficial effect of EXO1-deletion is not caused by preservation of telomere length. Telomere length in the G3 TERC–/–EXO1–/– mice is comparable to G3 TERC–/– EXO1+/+ mice. The precise mechanisms of the beneficial effects of EXO1-deletion on mice with dysfunctional telomeres remain to be elucidated.

5.2 Telomerase upregulation
Under physiological circumstances, telomerase activity is absent or undetectable in the adult myocardium. Overexpression of TERT rescues telomerase activity and preserves telomere length in the adult mouse myocardium and induces cardiomyocyte hypertrophy, without fibrosis or impaired cardiac function. In addition, infarct size after experimental myocardial infarction was substantially reduced in TERT transgenic mice compared with wild-type mice.74


   6. Conclusions and future perspectives
Top
 Abstract
 1. Introduction
 2. Telomere biology
 3. Telomere biology and...
 4. The telomerase-deficient mice
 5. Counteracting the effect...
 6. Conclusions and future...
Funding
 References
 
Evidence in humans for an association between telomere length and cardiovascular-related phenotypes, including atherosclerosis and heart failure, is rapidly accumulating.2–4,43 Understanding of the nature of these associations might be paramount in our understanding of the aetiology and pathogenesis of these diseases. Telomere biology already offers therapeutic targets in the treatment of cancer, but whether it can also provide novel therapeutic targets in coronary artery disease, heart failure, or optimalization of stem cells treatment remains to be discovered. For a rapid increment of our knowledge on the involvement of telomere biology in cardiovascular disease, the TERC–/– mice might turn out to be instrumental. Although this mouse model has proven its value in oncology, stem cell, and basic telomere research, up to date it is only sparsely used in the cardiovascular field. Studies with the TERC–/– mice might teach us valuable lessons on the involvement of telomere biology in age-associated hypertension, atherosclerosis, angiogenesis, and cardiac remodelling after myocardial infarction, as well as efficacy of telomerase and stem-cell- based therapies.

Conflict of interest: none declared.


   Funding
Top
 Abstract
 1. Introduction
 2. Telomere biology
 3. Telomere biology and...
 4. The telomerase-deficient mice
 5. Counteracting the effect...
 6. Conclusions and future...
 Funding
References
 
This work was supported by the Innovational Research Incentives Scheme program of the Netherlands Organisation for Scientific Research (NWO VENI, grant 916.76.170 to P.H.). P.H. is a research fellow of the Interuniversitair Cardiologisch Instituut Nederland (ICIN). The Netherlands Heart Foundation supports L.S.M.W. (grant 2008T028), R.A.B. (grants 2004T004 and 2007T046), D.J.V. (grant D97-017), and P.H. (grant 2006T003).


   References
Top
 Abstract
 1. Introduction
 2. Telomere biology
 3. Telomere biology and...
 4. The telomerase-deficient mice
 5. Counteracting the effect...
 6. Conclusions and future...
 Funding
 References
 

  1. van Jaarsveld CH, Ranchor AV, Kempen GI, Coyne JC, van Veldhuisen DJ, Sanderman R. Epidemiology of heart failure in a community-based study of subjects aged > or =57 years: incidence and long-term survival. Eur J Heart Fail (2006) 8:23–30.[CrossRef][Web of Science][Medline]
  2. Samani NJ, van der Harst P. Biological ageing and cardiovascular disease. Heart (2008) 94:537–539.[Free Full Text]
  3. Fuster JJ, Andres V. Telomere biology and cardiovascular disease. Circ Res (2006) 99:1167–1180.[Abstract/Free Full Text]
  4. Edo MD, Andres V. Aging, telomeres, and atherosclerosis. Cardiovasc Res (2005) 66:213–221.[Abstract/Free Full Text]
  5. Wong LS, de Boer RA, Samani NJ, van Veldhuisen DJ, van der Harst P. Telomere biology in heart failure. Eur J Heart Fail (2008) 10:1049–1056.[Abstract/Free Full Text]
  6. Chang S. Modeling aging and cancer in the telomerase knockout mouse. Mutat Res (2005) 576:39–53.[Web of Science][Medline]
  7. Deng Y, Chan SS, Chang S. Telomere dysfunction and tumour suppression: the senescence connection. Nat Rev Cancer (2008) 8:450–458.[CrossRef][Web of Science][Medline]
  8. Palm W, de Lange T. How shelterin protects mammalian telomeres. Annu Rev Genet (2008).
  9. Blasco MA. Telomeres and human disease: ageing, cancer and beyond. Nat Rev Genet (2005) 6:611–622.[CrossRef][Web of Science][Medline]
  10. Shiloh Y. ATM and related protein kinases: safeguarding genome integrity. Nat Rev Cancer (2003) 3:155–168.[CrossRef][Web of Science][Medline]
  11. Gire V, Roux P, Wynford-Thomas D, Brondello JM, Dulic V. DNA damage checkpoint kinase Chk2 triggers replicative senescence. EMBO J (2004) 23:2554–2563.[CrossRef][Web of Science][Medline]
  12. Kajstura J, Pertoldi B, Leri A, Beltrami CA, Deptala A, Darzynkiewicz Z, et al. Telomere shortening is an in vivo marker of myocyte replication and aging. Am J Pathol (2000) 156:813–819.[Abstract/Free Full Text]
  13. Chimenti C, Kajstura J, Torella D, Urbanek K, Heleniak H, Colussi C, et al. Senescence and death of primitive cells and myocytes lead to premature cardiac aging and heart failure. Circ Res (2003) 93:604–613.[Abstract/Free Full Text]
  14. Torella D, Rota M, Nurzynska D, Musso E, Monsen A, Shiraishi I, et al. Cardiac stem cell and myocyte aging, heart failure, and insulin-like growth factor-1 overexpression. Circ Res (2004) 94:514–524.[Abstract/Free Full Text]
  15. Jeyapalan JC, Sedivy JM. Cellular senescence and organismal aging. Mech Ageing Dev (2008) 129:467–474.[CrossRef][Web of Science][Medline]
  16. Maringele L, Lydall D. EXO1-dependent single-stranded DNA at telomeres activates subsets of DNA damage and spindle checkpoint pathways in budding yeast yku70Delta mutants. Genes Dev (2002) 16:1919–1933.[Abstract/Free Full Text]
  17. Hackett JA, Greider CW. End resection initiates genomic instability in the absence of telomerase. Mol Cell Biol (2003) 23:8450–8461.[Abstract/Free Full Text]
  18. Schaetzlein S, Kodandaramireddy NR, Ju Z, Lechel A, Stepczynska A, Lilli DR, et al. Exonuclease-1 deletion impairs DNA damage signaling and prolongs lifespan of telomere-dysfunctional mice. Cell (2007) 130:863–877.[CrossRef][Web of Science][Medline]
  19. Cherif H, Tarry JL, Ozanne SE, Hales CN. Ageing and telomeres: a study into organ- and gender-specific telomere shortening. Nucleic Acids Res (2003) 31:1576–1583.[Abstract/Free Full Text]
  20. Hastings R, Li NC, Lacy PS, Patel H, Herbert KE, Stanley AG, et al. Rapid telomere attrition in cardiac tissue of the ageing Wistar rat. Exp Gerontol (2004) 39:855–857.[CrossRef][Web of Science][Medline]
  21. Slagboom PE, Droog S, Boomsma DI. Genetic determination of telomere size in humans: a twin study of three age groups. Am J Hum Genet (1994) 55:876–882.[Web of Science][Medline]
  22. Brouilette SW, Moore JS, McMahon AD, Thompson JR, Ford I, Shepherd J, et al. Telomere length, risk of coronary heart disease, and statin treatment in the West of Scotland Primary Prevention Study: a nested case-control study. Lancet (2007) 369:107–114.[CrossRef][Web of Science][Medline]
  23. Nawrot TS, Staessen JA, Gardner JP, Aviv A. Telomere length and possible link to X chromosome. Lancet (2004) 363:507–510.[CrossRef][Web of Science][Medline]
  24. Fitzpatrick AL, Kronmal RA, Gardner JP, Psaty BM, Jenny NS, Tracy RP, et al. Leukocyte telomere length and cardiovascular disease in the cardiovascular health study. Am J Epidemiol (2007) 165:14–21.[Abstract/Free Full Text]
  25. Oikawa S, Tada-Oikawa S, Kawanishi S. Site-specific DNA damage at the GGG sequence by UVA involves acceleration of telomere shortening. Biochemistry (2001) 40:4763–4768.[CrossRef][Web of Science][Medline]
  26. von Zglinicki T. Oxidative stress shortens telomeres. Trends Biochem Sci (2002) 27:339–344.[CrossRef][Web of Science][Medline]
  27. Gardner JP, Li S, Srinivasan SR, Chen W, Kimura M, Lu X, et al. Rise in insulin resistance is associated with escalated telomere attrition. Circulation (2005) 111:2171–2177.[Abstract/Free Full Text]
  28. Cohen SB, Graham ME, Lovrecz GO, Bache N, Robinson PJ, Reddel RR. Protein composition of catalytically active human telomerase from immortal cells. Science (2007) 315:1850–1853.[Abstract/Free Full Text]
  29. Hiyama E, Hiyama K. Telomere and telomerase in stem cells. Br J Cancer (2007) 96:1020–1024.[CrossRef][Web of Science][Medline]
  30. Collins K. The biogenesis and regulation of telomerase holoenzymes. Nat Rev Mol Cell Biol (2006) 7:484–494.[CrossRef][Web of Science][Medline]
  31. van der Harst P, van Veldhuisen DJ, Samani NJ. Expanding the concept of telomere dysfunction in cardiovascular disease. Arterioscler Thromb Vasc Biol (2008) 28:807–808.[Free Full Text]
  32. Valdes AM, Andrew T, Gardner JP, Kimura M, Oelsner E, Cherkas LF, et al. Obesity, cigarette smoking, and telomere length in women. Lancet (2005) 366:662–664.[CrossRef][Web of Science][Medline]
  33. Morla M, Busquets X, Pons J, Sauleda J, MacNee W, Agusti AG. Telomere shortening in smokers with and without COPD. Eur Respir J (2006) 27:525–528.[Abstract/Free Full Text]
  34. McGrath M, Wong JY, Michaud D, Hunter DJ, De V I. Telomere length, cigarette smoking, and bladder cancer risk in men and women. Cancer Epidemiol Biomarkers Prev (2007) 16:815–819.[Abstract/Free Full Text]
  35. Jeanclos E, Krolewski A, Skurnick J, Kimura M, Aviv H, Warram JH, et al. Shortened telomere length in white blood cells of patients with IDDM. Diabetes (1998) 47:482–486.[Abstract]
  36. Adaikalakoteswari A, Balasubramanyam M, Mohan V. Telomere shortening occurs in Asian Indian Type 2 diabetic patients. Diabet Med (2005) 22:1151–1156.[CrossRef][Web of Science][Medline]
  37. Demissie S, Levy D, Benjamin EJ, Cupples LA, Gardner JP, Herbert A, et al. Insulin resistance, oxidative stress, hypertension, and leukocyte telomere length in men from the Framingham Heart Study. Aging Cell (2006) 5:325–330.[CrossRef][Web of Science][Medline]
  38. Vasan RS, Demissie S, Kimura M, Cupples LA, Rifai N, White C, et al. Association of leukocyte telomere length with circulating biomarkers of the renin-angiotensin-aldosterone system. The Framingham Heart Study. Circulation (2008) 117:1138–1144.[Abstract/Free Full Text]
  39. van der Harst P, Wong LS, de Boer RA, Brouilette SW, van der SG, Voors AA, et al. Possible association between telomere length and renal dysfunction in patients with chronic heart failure. Am J Cardiol (2008) 102:207–210.[CrossRef][Web of Science][Medline]
  40. Gorenne I, Kavurma M, Scott S, Bennett M. Vascular smooth muscle cell senescence in atherosclerosis. Cardiovasc Res (2006) 72:9–17.[Abstract/Free Full Text]
  41. Ogami M, Ikura Y, Ohsawa M, Matsuo T, Kayo S, Yoshimi N, et al. Telomere shortening in human coronary artery diseases. Arterioscler Thromb Vasc Biol (2004) 24:546–550.[Abstract/Free Full Text]
  42. Matthews C, Gorenne I, Scott S, Figg N, Kirkpatrick P, Ritchie A, et al. Vascular smooth muscle cells undergo telomere-based senescence in human atherosclerosis: effects of telomerase and oxidative stress. Circ Res (2006) 99:156–164.[Abstract/Free Full Text]
  43. van der Harst P, van der Steege G, de Boer RA, Voors AA, Hall AS, Mulder MJ, et al. Telomere length of circulating leukocytes is decreased in patients with chronic heart failure. J Am Coll Cardiol (2007) 49:1459–1464.[Abstract/Free Full Text]
  44. Samani NJ, Boultby R, Butler R, Thompson JR, Goodall AH. Telomere shortening in atherosclerosis. Lancet (2001) 358:472–473.[CrossRef][Web of Science][Medline]
  45. Oh H, Wang SC, Prahash A, Sano M, Moravec CS, Taffet GE, et al. Telomere attrition and Chk2 activation in human heart failure. Proc Natl Acad Sci USA (2003) 100:5378–5383.[Abstract/Free Full Text]
  46. Spyridopoulos I, Erben Y, Brummendorf TH, Haendeler J, Dietz K, Seeger F, et al. Telomere gap between granulocytes and lymphocytes is a determinant for haematopoetic progenitor cell impairment in patients with previous myocardial infarction. Arterioscler Thromb Vasc Biol (2008) 28:968–974.[Abstract/Free Full Text]
  47. Zimmermann WH, Eschenhagen T. Questioning the relevance of circulating cardiac progenitor cells in cardiac regeneration. Cardiovasc Res (2005) 68:344–346.[Free Full Text]
  48. Gonzalez A, Rota M, Nurzynska D, Misao Y, Tillmanns J, Ojaimi C, et al. Activation of cardiac progenitor cells reverses the failing heart senescent phenotype and prolongs lifespan. Circ Res (2008) 102:597–606.[Abstract/Free Full Text]
  49. Urbanek K, Torella D, Sheikh F, De Angelis A, Nurzynska D, Silvestri F, et al. Myocardial regeneration by activation of multipotent cardiac stem cells in ischemic heart failure. Proc Natl Acad Sci USA (2005) 102:8692–8697.[Abstract/Free Full Text]
  50. Blasco MA, Lee HW, Hande MP, Samper E, Lansdorp PM, DePinho RA, et al. Telomere shortening and tumor formation by mouse cells lacking telomerase RNA. Cell (1997) 91:25–34.[CrossRef][Web of Science][Medline]
  51. Rudolph KL, Chang S, Lee HW, Blasco M, Gottlieb GJ, Greider C, et al. Longevity, stress response, and cancer in aging telomerase-deficient mice. Cell (1999) 96:701–712.[CrossRef][Web of Science][Medline]
  52. Herrera E, Samper E, Martin-Caballero J, Flores JM, Lee HW, Blasco MA. Disease states associated with telomerase deficiency appear earlier in mice with short telomeres. EMBO J (1999) 18:2950–2960.[CrossRef][Web of Science][Medline]
  53. Yuan X, Ishibashi S, Hatakeyama S, Saito M, Nakayama J, Nikaido R, et al. Presence of telomeric G-strand tails in the telomerase catalytic subunit TERT knockout mice. Genes Cells (1999) 4:563–572.[Abstract]
  54. Erdmann N, Liu Y, Harrington L. Distinct dosage requirements for the maintenance of long and short telomeres in mTert heterozygous mice. Proc Natl Acad Sci USA (2004) 101:6080–6085.[Abstract/Free Full Text]
  55. Liu Y, Snow BE, Hande MP, Yeung D, Erdmann NJ, Wakeham A, et al. The telomerase reverse transcriptase is limiting and necessary for telomerase function in vivo. Curr Biol (2000) 10:1459–1462.[CrossRef][Web of Science][Medline]
  56. Chiang YJ, Hemann MT, Hathcock KS, Tessarollo L, Feigenbaum L, Hahn WC, et al. Expression of telomerase RNA template, but not telomerase reverse transcriptase, is limiting for telomere length maintenance in vivo. Mol Cell Biol (2004) 24:7024–7031.[Abstract/Free Full Text]
  57. Hathcock KS, Hemann MT, Opperman KK, Strong MA, Greider CW, Hodes RJ. Haploinsufficiency of mTR results in defects in telomere elongation. Proc Natl Acad Sci USA (2002) 99:3591–3596.[Abstract/Free Full Text]
  58. Jeanclos E, Schork NJ, Kyvik KO, Kimura M, Skurnick JH, Aviv A. Telomere length inversely correlates with pulse pressure and is highly familial. Hypertension (2000) 36:195–200.[Abstract/Free Full Text]
  59. Perez-Rivero G, Ruiz-Torres MP, Rivas-Elena JV, Jerkic M, Diez-Marques ML, Lopez-Novoa JM, et al. Mice deficient in telomerase activity develop hypertension because of an excess of endothelin production. Circulation (2006) 114:309–317.[Abstract/Free Full Text]
  60. Benetos A, Gardner JP, Zureik M, Labat C, Xiaobin L, Adamopoulos C, et al. Short telomeres are associated with increased carotid atherosclerosis in hypertensive subjects. Hypertension (2004) 43:182–185.[Abstract/Free Full Text]
  61. Obana N, Takagi S, Kinouchi Y, Tokita Y, Sekikawa A, Takahashi S, et al. Telomere shortening of peripheral blood mononuclear cells in coronary disease patients with metabolic disorders. Intern Med (2003) 42:150–153.[Web of Science][Medline]
  62. Breslow JL. Mouse models of atherosclerosis. Science (1996) 272:685–688.[Abstract]
  63. Meir KS, Leitersdorf E. Atherosclerosis in the apolipoprotein-E-deficient mouse: a decade of progress. Arterioscler Thromb Vasc Biol (2004) 24:1006–1014.[Abstract/Free Full Text]
  64. Poch E, Carbonell P, Franco S, Diez-Juan A, Blasco MA, Andres V. Short telomeres protect from diet-induced atherosclerosis in apolipoprotein E-null mice. FASEB J (2004) 18:418–420.[Abstract/Free Full Text]
  65. Zaccagnini G, Gaetano C, Della PL, Nanni S, Grasselli A, Mangoni A, et al. Telomerase mediates vascular endothelial growth factor-dependent responsiveness in a rat model of hind limb ischemia. J Biol Chem (2005) 280:14790–14798.[Abstract/Free Full Text]
  66. Franco S, Segura I, Riese HH, Blasco MA. Decreased B16F10 melanoma growth and impaired vascularization in telomerase-deficient mice with critically short telomeres. Cancer Res (2002) 62:552–559.[Abstract/Free Full Text]
  67. Leri A, Franco S, Zacheo A, Barlucchi L, Chimenti S, Limana F, et al. Ablation of telomerase and telomere loss leads to cardiac dilatation and heart failure associated with p53 upregulation. EMBO J (2003) 22:131–139.[CrossRef][Web of Science][Medline]
  68. Werner C, Hanhoun M, Widmann T, Kazakov A, Semenov A, Poss J, et al. Effects of physical exercise on myocardial telomere-regulating proteins, survival pathways, and apoptosis. J Am Coll Cardiol (2008) 52:470–482.[Abstract/Free Full Text]
  69. Suzuki T, Nishida M, Futami S, Fukino K, Amaki T, Aizawa K, et al. Neoendothelialization after peripheral blood stem cell transplantation in humans: a case report of a Tokaimura nuclear accident victim. Cardiovasc Res (2003) 58:487–492.[Abstract/Free Full Text]
  70. Lee HW, Blasco MA, Gottlieb GJ, Horner JW, Greider CW, DePinho RA. Essential role of mouse telomerase in highly proliferative organs. Nature (1998) 392:569–574.[CrossRef][Medline]
  71. Samper E, Fernandez P, Eguia R, Martin-Rivera L, Bernad A, Blasco MA, et al. Long-term repopulating ability of telomerase-deficient murine hematopoietic stem cells. Blood (2002) 99:2767–2775.[Abstract/Free Full Text]
  72. Allsopp RC, Morin GB, DePinho R, Harley CB, Weissman IL. Telomerase is required to slow telomere shortening and extend replicative lifespan of HSCs during serial transplantation. Blood (2003) 102:517–520.[Abstract/Free Full Text]
  73. Flores I, Cayuela ML, Blasco MA. Effects of telomerase and telomere length on epidermal stem cell behavior. Science (2005) 309:1253–1256.[Abstract/Free Full Text]
  74. Oh H, Taffet GE, Youker KA, Entman ML, Overbeek PA, Michael LH, et al. Telomerase reverse transcriptase promotes cardiac muscle cell proliferation, hypertrophy, and survival. Proc Natl Acad Sci USA (2001) 98:10308–10313.[Abstract/Free Full Text]
  75. Wong KK, Maser RS, Bachoo RM, Menon J, Carrasco DR, Gu Y, et al. Telomere dysfunction and Atm deficiency compromises organ homeostasis and accelerates ageing. Nature (2003) 421:643–648.[CrossRef][Medline]
  76. Chang S, Multani AS, Cabrera NG, Naylor ML, Laud P, Lombard D, et al. Essential role of limiting telomeres in the pathogenesis of Werner syndrome. Nat Genet (2004) 36:877–882.[CrossRef][Web of Science][Medline]

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P. van der Harst, R. A. de Boer, and D. J. van Veldhuisen
The Nobel Prize for medicine for telomere biology and relevance to heart failure research
Eur J Heart Fail, December 1, 2009; 11(12): 1113 - 1115.
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