Telomere shortening in human diseases

Authors

  • Chiou Mee Kong,

    1. Department of Biochemistry, Yong Loo Lin School of Medicine, National University of Singapore, Singapore
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  • Xiao Wen Lee,

    1. Department of Biochemistry, Yong Loo Lin School of Medicine, National University of Singapore, Singapore
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  • Xueying Wang

    Corresponding author
    1. Department of Biochemistry, Yong Loo Lin School of Medicine, National University of Singapore, Singapore
    • Correspondence

      X. Wang, Department of Biochemistry, Yong Loo Lin School of Medicine, National University of Singapore, Centre for Translational Medicine, 14 Medical Drive, #10-01, Singapore 117599, Singapore

      Fax: +65 6779 1453

      Tel.: +65 6601 2360

      E-mail: bchwxy@nus.edu.sg

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  • Chiou Mee Kong and Xiao Wen Lee contributed equally to this work

Abstract

The discovery of telomeres dates back to the early 20th century. In humans, telomeres are heterochromatic structures with tandem DNA repeats of 5′-TTAGGG-3′ at the chromosomal ends. Telomere length varies greatly among species and ranges from 10 to 15 kb in humans. With each cell division, telomeres shorten progressively because of the ‘end-replication problem’. Short or dysfunctional telomeres are often recognized as DNA DSBs, triggering cell-cycle arrest and result in cellular senescence or apoptotic cell death. Therefore, telomere shortening serves as an important tumor-suppressive mechanism by limiting cellular proliferative capacity by regulating senescence checkpoint activation. Although telomeres serve as a mitotic clock to cells, they also confer capping on chromosomes, with help from telomere-associated proteins. Over the past decades, many studies of telomere biology have demonstrated that telomeres and telomere-associated proteins are implicated in human genetic diseases. In addition, it has become more apparent that accelerated telomere erosion is associated with a myriad of metabolic and inflammatory diseases. Moreover, critically short or unprotected telomeres are likely to form telomeric fusions, leading to genomic instability, the cornerstone for carcinogenesis. In light of these, this minireview summarizes studies on telomeres and telomere-associated proteins in human diseases. Elucidating the roles of telomeres involved in the mechanisms underlying pathogenesis of these diseases may open up new possibilities for novel molecular targets as well as provide important diagnostic and therapeutic implications.

Abbreviations
AA

aplastic anemia

ALT

alternate lengthening of telomeres

A–T

ataxia–telangiectasia

ATM

ataxia–telangiectasia mutated

BS

Bloom syndrome

DC

dyskeratosis congenita

D-loop

displacement loop

DSBs

double-strand breaks

FA

Fanconi anemia

IPF

idiopathic pulmonary fibrosis

MRN

MRE11-RAD50-NBS1

NBS

Nijmegen breakage syndrome

NHEJ

nonhomologous end joining

TER

telomerase RNA

TERT

telomerase reverse transcriptase

T-loop

telomere loops

WS

Werner syndrome

Introduction

The ends of linear eukaryotic chromosomes mirror DNA double-strand breaks (DSBs) and are potential sites for DNA repair proteins, which mediate homologous recombination or nonhomologous end joining (NHEJ) [1], resulting in detrimental chromosomal fusions. It was not until the late 1970s that a major breakthrough by E. Blackburn and J. Szostak led to the discovery of how linear eukaryotic chromosomes are protected from deleterious degradation by telomeres [2]. Telomeres are heterochromatic structures with highly conserved long stretches of tandem repeats of the hexamer 5′-TTAGGG-3′ at the extremities of chromosomes; they confer protection on chromosomal ends and ensure proper chromosome replication. Telomere consists of a G-rich 3′-overhang which bends back on itself, leading to the invasion and displacement of upstream 3′ telomeric DNA, thus creating a displacement loop (D-loop) via base pairing. This D-loop is essential for the stabilization of large telomere loops (T-loop). Formation of such loops masks the telomere ends from access by other proteins and prevents telomeric end fusions; collectively serving to protect the chromosomes [3, 4]. Moreover, formation of T-loop structures is mediated by shelterin, a multiprotein complex that is made up of six core telomere-specific binding proteins: TRF1, TRF2, TIN2, RAP1, TPP1 and POT1 [5]. These proteins bind to telomeric DNA sequences and confer specificity in the recognition of telomere ends. In addition, the shelterin complex protects telomeric DNA by maintaining telomere length as well as regulating the synthesis of telomeric DNA [5].

In humans, the average telomere length typically ranges from 10 to 15 kb [6]. Telomeric DNA inevitably shortens upon each cell replication, at a rate of 50–200 bp [7] from the loss of DNA at the termini as a result of the ‘end-replication problem’, as well as degradation by nucleases. This shortening continues until the telomere reaches a critical length [8], which in turn triggers cell-cycle arrest, leading to senescence or apoptotic cell death. Such a unique characteristic allows telomeres to act as a mitotic clock by restricting cells capacity for division. Germ cells and stem cells, nonetheless, counteract progressive telomere erosion with the presence of telomerase, a RNA-dependent reverse transcriptase, which can synthesize telomeric DNA de novo. Telomerase comprises the telomerase reverse transcriptase (TERT) for the synthesis of telomeric repeats, and telomerase RNA (TER) which serves as the template for the elongation of telomeric DNA. The catalytic unit of telomerase consists of two molecules each of TERT and TER as well as dyskerin. Dyskerin is a member of the H/ACA box small nucleolar complex involved in 3′-end processing of TER and its assembly into active telomerase [9]. Alternatively, telomere elongation can occur via alternate lengthening of telomeres (ALT), a telomerase-independent mechanism, which involves homologous recombination of telomeres.

Over the past decades, emerging evidence has shown that telomeres are essential regulators in cellular aging and are causally linked to a myriad of human diseases, for example, dyskeratosis congenita (DC), Werner syndrome (WS), Bloom syndrome (BS), ataxia–telangiectasia (A–T), Nijmegen breakage syndrome (NBS) and Fanconi anemia (FA) (Table 1). A common hallmark of these diseases is that patients have critically short telomeres compared with healthy individuals of the same sex and age. In view of this, a summary of the roles of telomeres and telomere-associated proteins in human diseases is discussed below. These findings have significant and far-reaching implications for improving our understanding of the diseases as well as the development of future approaches designed to improve diagnostic and therapeutic strategies. In addition, understanding the deleterious consequences of telomere dysfunction at the adult stem cell level could provide insights for utilizing stem cell transplantation as potential therapy for the premature aging syndromes.

Table 1. A summary of human diseases with telomere shortening
DiseaseKey componentsChromosome locationPathway/functionTelomeric function
Werner syndromeWRN (RECQ3)8p12DNA helicasesTelomere end processing; telomere structure maintenance; associated with TRF1, TRF2 and POT1
Bloom syndromeBLM (RECQ2)15q26DNA helicasesTelomere end processing; telomere structure maintenance; associated with TRF1 and TRF2
Ataxia–telangiectasiaATM11q22DNA damage responseAssociated with TRF1 and TRF2
Nijmegen breakage syndromeNBS18q21DNA damage responseAssociated with TRF1 and TRF2
Ataxia–telangiectasia like syndromeMRE11/RAD50/NBS111q21, 5q31, 8q21DNA damage responseAssociated with TRF2; possible role in telomerase activation
Fanconi anemiaFANCA16q24FA pathway via DNA cross-linkingTelomeric breakage suppression
FANCBXp22  
FANCC9q22  
FANCD1 (BRCA2)13q12  
FANCD23p26  
FANCE6p22-p21  
FANCF11p15  
FANCG9p13  
FANCI15q26  
FANCJ (BRIP1,BACH1)17q22  
FANCL2p16  
FANCM14q21  
FANCN (PALB2)16p12  
FANCP (SLX4)16p13  
Aplastic anaemiaTERT5p15TelomeraseChromosomal lengthening at 3′-end
TERC3q26  
TRF18q21Shelterin complexTelomere-capping function
TRF216q22  
TIN214q12  
SBDS7q11 Possible role in telomerase activity
Pulmonary fibrosisTERT5p15TelomeraseChromosomal lengthening at 3′-end
TERC3q26  
Dyskeratosis congenitaTERT5p15TelomeraseChromosomal lengthening at 3′-end
TERC3q26  
DKC1Xq28  
TIN214q12Shelterin complexTelomere-capping function
NOP10 (NOLA3)15q14 – q15 Confer telomerase stability.
NHP2 (NOLA2)5q35 Confer telomerase stability.
WRAP5317p13 Direct telomerase to Cajal bodies for elongation.

Dyskeratosis congenita

DC is a disease associated with bone marrow failure and cancer predisposition, which has been attributed to defective telomere biology. Remarkable phenotypes of this disease include nail dystrophy, reticular skin pigmentation and oral leukoplakia. DC is genetically heterogeneous and mutations in at least seven telomere- and telomerase-associated genes have been linked to DC. Among these, DKC1 was the first DC gene to be cloned [10], suggesting a link between the disease and telomeres. Dyskerin, encoded by DKC1, binds to the H/ACA box of TER and plays vital roles in the stabilization of the TER template, TER and the assembly of telomerase. Moreover, subsequent studies have demonstrated the involvement of telomerase components, i.e. TER [11] and TERT [12] genes in DC. In agreement with this, studies on effect of telomerase deficiency using telomerase knockout mice (TER−/−) have revealed a myriad of abnormalities in DC, which started to appear in successive generations of mice, probably explainable by the gradual decrease in telomere length [13]. Mutations in one of the three genes of the telomerase components constitute only ~ 50% of incidences of DC; the remaining DC cases still lack identified mutations, suggesting possible involvement of other telomere-associated genes. In support of this notion, DC patients with shelterin component TIN2 gene mutation have been identified [14], suggesting that TIN2 may be involved in telomere elongation, in addition to its telomere-capping function [15]. Furthermore, mutations in other telomere-associated genes, for example, NOP10 [16], NHP2 [17] and WRAP53 [18] have been identified in DC. Figure 1 illustrates a general model of telomere dysfunction in the pathobiology of human diseases. Mutations in telomere- and telomerase-associated genes cause telomere erosion and accelerated cell turnover, which eventually leads to premature cellular senescence, as well as stem cell pool exhaustion, and is eventually manifest as hematological and nonhematological clinical features i.e. oral leukoplakia and bone marrow failure in DC patients.

Figure 1.

Schematic model illustrating possible underlying telomere dysfunction in the pathogenesis of human diseases. Defects in telomere- and telomerase-associated proteins lead to progressive telomere shortening. This in turn promotes premature senescence of mammalian cells and functional tissue loss. In addition, telomere attrition coupled with deficiency in telomerase activity culminates in replicative senescence in adult stem cells, leading to a depletion in the stem cell reserve. Collectively, these eventually manifest as hematological (i.e. aplastic anemia, pancytopenia, bone marrow failure) or nonhematological (i.e. skin abnormalities, pulmonary diseases, liver diseases) clinical features. Moreover, short dysfunctional telomeres inevitably result in telomeric fusions, leading to genome instability, the cornerstone for carcinogenesis.

Aplastic anemia and idiopathic pulmonary fibrosis

Apart from DC, telomerase deficiencies are genetic risk factors for AA and IPF. AA is characterized by peripheral blood pancytopenia due to insufficient hematopoiesis and hypocellular bone marrow with excessive amounts of fat cells. Defects in telomerase components, i.e. TERT [19] and TER [20] mutations, are common in these patients. Patients with mutated genes are found to possess significantly shorter telomeres than age-matched healthy individuals. In addition, an increased frequency of chromosomal breaks and chromosomal aberrations, particularly monosomy 7, is commonly found in AA patients [21]. Mutations in telomerase components result in low telomerase activity and accelerate telomere loss, thus leading to a dramatic reduction in the stem cell reserve (primarily hematopoietic stem cells), the molecular basis behind AA.

IPF is a chronic progressive lethal disease characterized by dyspnea, lung scarring and damage, particularly in the alveoli. Genetically, mutations in essential telomerase genes [22], i.e. TERT and TER, account for up to one-sixth of IPF cases and < 3% of sporadic cases [23], suggesting a role for telomerase in the pathogenesis of IPF. Heterozygous carriers that exhibit haploinsufficiency in TERT and TER genes demonstrated accelerated telomere attrition when compared with age-matched controls. Progressive telomere shortening causes accelerated cell turnover and premature cellular senescence, especially alveolar epithelial cells, which underlies the pathobiology of IPF [24].

Fanconi anemia

FA is an autosomal recessive disease characterized by bone marrow failure and a predisposition towards acute leukemia. In addition, patients suffering from FA are distinguished by their hypersensitivity to DNA cross-linking chemicals. To date, at least 15 genes have been identified for their involvement in the development of FA. These FA proteins constitute the FA pathway, and are involved in DNA damage signaling in response to DNA cross-linkers as well as the maintenance of genome integrity. Following exposure to DNA cross-linkers, defects in FA pathway induce prolonged cell-cycle arrest in the G2/M phase [25], which is associated with significant chromatid anomalies. Accumulating studies have provided a strong correlation of FA with telomere defects. Telomeres in FA patients were found to be shorter [26-30] and breakage-prone [31], with a noticeable increase in end fusions [28]. Consequently, accelerated telomere attrition observed in FA patients leads to an inability of their hematopoietic progenitor cells to replenish cell lineages, thus leading to progressive pancytopenia, a typical symptom of bone marrow failure [26]. This is consistent with other studies which showed that the degree of telomere attrition was correlated with the severity of the hematopoietic status (pancytopenia, bone marrow failure, aplastic anemia) in FA patients [26, 32]. Despite FA and its manifestations being well correlated with telomere dysfunction, the direct link between FA pathway and telomere dysfunction is largely unknown. Studies using mouse or human cells lacking FANCC [31] or FANCG [33] (FA nuclear core complex proteins) revealed unaltered telomere integrity (telomere length, telomere-capping and telomerase activity), suggesting that telomere attrition may be secondary to FA-deficiency diseases and that the FA pathway has no direct role in telomere maintenance in telomerase-positive cells. In agreement with this, the FA pathway was found to be involved in telomere maintenance via an ALT mechanism [34]. Fan and colleagues demonstrated that FANCD2 colocalized with telomeres in ALT cells, but not in non-ALT cells. Furthermore, transient knockdown of FANCD2 or FANCA resulted in telomere loss and a decrease in homologous recombination between telomeres, emphasizing a role for the FA pathway in ALT telomere maintenance [34].

Werner syndrome and Bloom syndrome

WS is an autosomal recessive disorder characterized by an early onset of aging symptoms such as osteoporosis, atherosclerosis, cataracts and a strong predisposition to cancer, primarily sarcomas, mesenchymal tumors and other rare types [35]. WRN, encoding the Werner protein, is the genetic causative factor of WS. It is a member of RecQ family of DNA helicases that binds to DNA and serves to unwind double-stranded DNA for replication or DNA damage repair. In addition to well-characterized functions in DNA metabolic pathways (DNA replication and cell-cycle progression, DNA repair and transcription), mounting evidence has shown that WRN is directly implicated in telomere maintenance. The helicase and exonuclease activities of WRN are implicated in processing telomere ends and regulating the telomeric structures in vitro [36-38], therefore facilitating telomere DNA replication. Crabbe et al. [39] showed that WS fibroblasts lacking helicase-deficient WRN displayed significant loss of telomeres from single sister chromatids and the catastrophic loss of telomeres could be rescued by telomerase expression, suggesting its significance in telomere DNA replication. Moreover, WRN inhibits the formation of large deletions and rearrangements, thus preventing telomere loss during replication [40]. In addition, WRN physically interacts with the shelterin complex components: TRF1, TRF2 [36, 41-43] and POT1 [44, 45]. TRF2 further enhances the helicase and exonuclease activities of WRN [41, 42], facilitating resolution of the D-loop [41]. Similarly, POT1 recruits WRN and stimulates its helicase activity, promoting efficient displacement of telomere duplexes, D-loop structures and G-quadruplexes [44, 46]. Failure to resolve telomeric structures may explain the loss of telomeres and premature senescence observed in WS cells [47] and such a phenotype could be rescued by expression of exogenous telomerase [48]. Furthermore, fibroblasts deficient in WRN displayed a high frequency of chromosomal fusions caused by telomere loss, and telomerase expression in these cells might reduce the frequency of chromosomal aberrations. This underscores the importance of telomere dysfunction in genomic instability and premature cellular senescence in WS cells [49-51]. Studies using WRN mutant mice further support the hypothesis that WRN is involved in telomere maintenance and provide a direct relation between telomere dysfunction and WS phenotypes. Late-generation mice lacking WRN and telomerase RNA template (TER−/−WRN−/−) recapitulates telomere shortening as well as typical clinical characteristics of WS premature symptoms, but not in age-matched late-generation TER single mutant mice (TER−/−WRN+/+) [52, 53]. Moreover, double-knockout mice showed WS-associated cellular phenotypes such as replicative senescence, increased chromosomal instability and tumor formation [51, 53]. Collectively, these studies suggest that WS-associated premature aging pathology may be linked to defects in telomere maintenance, emphasizing its role in telomere maintenance.

Mutation in another member of the RecQ family, the BLM gene, gives rise to BS. BS patients are characterized by male infertility, hypo-/hyperpigmented skin and an elevated risk of many types of cancer, including sarcomas. Functionally, BLM prevents homologous recombination between sister chromatids in the S and G2 phases [54]. Consistent with this, karyotype analysis of BS patients often revealed a high frequency of chromosomal breakage, coupled with an elevated level of recombination and sister chromatid exchange [50, 55, 56]. Similar to WRN, BLM is also implicated in telomeric end processing and the resolution of telomere structures, thereby maintaining telomere integrity. BLM interacts with TRF1 [57], TRF2 [41, 57, 58] and POT1 [38, 44], stimulating the unwinding of telomeric and/or nontelomeric structures. In addition, BLM associates with topoisomerase III alpha [59, 60], BLAP75/RMI1 [61] and BLAP18/RMI2 [62] to form a multiprotein (BTB) complex, which in turn promotes Holliday junction branch migration and D-loop resolution [63-66]. In addition to telomere replication, BLM, as well as WRN, was found to colocalize in foci containing telomeric DNA particularly in telomerase-negative cells which utilize ALT to lengthen the telomeres [67, 68], and overexpression of BLM was found to rescue telomere shortening in ALT cells [58]. Collectively, this proposes another role for BLM and WRN in telomere maintenance via recombination mechanisms [69]. Studies using BLM mutant mice have revealed that BLM gene mutation accentuates phenotypes in late-generation mice deficient of telomerase RNA template (TER−/−), such as infertility, impaired wound healing, and end-to-end chromosome fusions [53]. Taken together, these demonstrate the role of BLM in maintaining telomere integrity.

Ataxia–telangiectasia and Nijmegen breakage syndrome

A–T is a premature aging disease characterized by hypersensitivity to ionizing radiation, neurological deterioration and a predisposition to lymphoreticular malignancies. A–T is caused by a defective ataxia–telangiectasia mutated (ATM) gene. ATM is the key regulator molecule in several signaling cascades such as cell-cycle checkpoint regulation, DNA damage repair and other stress responses that respond to DSBs. It belongs to a superfamily of protein kinases that have sequence homology to phosphatidylinositol 3-kinase. ATM deficiency results in a defective DNA damage-response pathway and cell-cycle control. In addition, studies have suggested that ATM plays a vital role in telomere maintenance. A–T cells were found to possess considerable telomere attrition and increased chromosomal end fusions compared with normal age-matched controls [70, 71] and ectopic expression of catalytic unit of telomerase (TERT) in primary A–T fibroblasts was able to reverse the premature senescence phenotypes [72]. This is corroborated by findings from animal studies in which double ATM and telomerase knockout mice (ATM−/−TER−/−) exhibited accelerated telomere attrition and genomic instability, suggesting that A–T pathophysiology is linked to the functional state of telomeres [73, 74]. Consistent with this finding, ATM and telomerase null mice exhibited an increased germ cell death and chromosomal fusions compared with single knockout mice [75], suggesting that deficiency in ATM accentuates telomere dysfunction. In addition, ATM was found to interact with the shelterin component TRF1, regulating telomere length and maintaining genome stability [76]. In addition, TRF2 binds to ATM and obviates ATM activation at the telomere ends, which subsequently prevents short telomeres from triggering DNA damage checkpoints [77]. Studies of telomere–nuclear matrix interactions in A–T cells have revealed that the majority of telomeres in A–T cells were found to be associated with the nuclear matrix and such interactions were attributed to ATM mutation [78, 79]. Ectopic expression of ATM was able to reverse the altered telomere–nuclear matrix interactions.

Like A–T, NBS is a rare autosomal recessive disorder due to mutation of the NBS1 gene. It is characterized by pleiotropic clinical phenotypes, microcephaly, increased sensitivity to ionizing radiation and a strong predisposition to cancer. Hypomorphic Nbs1 mutant (Nbs1ΔB/ΔB) mice possess phenotypes resembling NBS patients such as hypersensitivity to radiation, defective cell-cycle checkpoints and chromosomal anomalies [80]. Like ATM, NBS1 is also a primary regulator in the ATM-dependent DNA damage signaling pathways, as well as cell-cycle checkpoint regulation in response to DSBs. Defect in the NBS1 gene results in disruption of the NBS1 protein, and thus renders ATM phosphorylation incomplete. This results in a delay in cell-cycle arrest, ineffective apoptosis activation and an accumulation of chromosomal mutations. Consequently, cells isolated from NBS patients display cellular and molecular phenotypes including defective cell-cycle checkpoints, accelerated telomere attrition and frequent chromosomal mutations. In addition to its functions in DNA damage signaling pathways, NBS1 has also been implicated in telomere maintenance. NBS1 was found to interact with TRF2 [81], specifically during the S phase (telomere replication), as well as TRF1 [82]. Telomeres in cells from NBS patients were shorter compared with controls, and such defects could be restored by exogenous introduction of telomerase [83]. In addition to telomere loss, NBS1-deficient cells were also shown to have a higher frequency of chromosomal instability, resulting from telomeric fusions [84]. Moreover, NBS1 is also implicated in telomere maintenance via the ALT mechanism and NBS1 was found to play a role in the assembly of ALT-associated promyelocytic leukemia bodies [85]. Promyelocytic leukemia body, a telomere-associated nuclear body, is proposed to be involved in ALT through recombination [86]. Furthermore, depletion of NBS1 in ALT cells resulted in the formation of extrachromosomal telomeric circles [87, 88], suggesting the role of NBS1 in telomere maintenance via ALT. Taken together, these studies demonstrate the existence of an intricate relationship between DNA damage-response pathways and telomere maintenance machinery in safeguarding genomic integrity.

Ataxia–telangiectasia-like disorder

NBS1, along with MRE11 and RAD50, constitutes the MRE11–RAD50–NBS1 (MRN) complex, a multifunctional and evolutionarily conserved protein complex, which acts as a core player in DNA damage signaling (serves as DSBs sensor), and DSB repair machinery mechanisms of homologous recombination and NHEJ [89]. Hypomorphic mutation in MRE11 gene has been linked to the development of ataxia–telangiectasia-like disorder [90], which resembles A–T and NBS. Ataxia–telangiectasia-like disorder patients are characterized by ataxia, dysarthria, abnormal eye movements and neurodegeneration. In animals, null mutations in all components of the MRN complex are lethal [91-94]. Apart from its well-known functions in DNA damage signaling and repair, the MRN complex has been implicated in telomere integrity maintenance. MRN mediates telomere length extension by engaging ATM at telomeres, leading to subsequent TRF1 phosphorylation and allowing increased access for telomerase to the telomere ends [95]. In addition, MRN complex was found to colocalize at telomeres and interact with TRF2 in a cell-cycle-dependent association, therefore promoting the formation of T-loops [81]. Moreover, reducing MRN expression shortens the 3′ telomeric overhang in telomerase-positive cells, but not in telomerase-negative cells, pointing to a possible role of the MRN complex in the recruitment or action of telomerase at telomeres [96]. In addition, in vitro and in vivo studies of dysfunctional telomeres have revealed a role for MRE11 in sensing telomere dysfunction as well as promoting telomere fusion by NHEJ [91, 97-99]. Deng et al. [98] demonstrated that, in response to dysfunctional telomeres, MRE11 nuclease activity was indispensable in triggering ATM phosphorylation, thus initiating the DNA damage-response. Moreover, the presence of MRE11 nuclease activity eliminated the 3′-overhang at telomeric ends and such elimination promoted efficient NHEJ of dysfunctional telomeres, resulting in increased chromosomal fusions [98]. Similarly, Attwooll and colleagues showed that the frequency of telomere fusions was reduced in hypomorphic Mre11 mutant (MRE11ATLD1/ATLD1 TERΔ/Δ) cells compared with telomerase-deficient cells [97], supporting the notion that the MRE11 complex plays a role in sensing telomere dysfunction and promoting the fusion of dysfunctional telomeres. In addition, studies have also demonstrated that MRN complex promotes telomerase action [96, 100], nonetheless, the exact mechanism is yet to be elucidated. Collectively, these studies suggest a role of MRN complex in telomere maintenance.

Metabolic diseases and infections

Accelerated telomere shortening has been implicated in numerous metabolic and inflammatory diseases, such as cardiovascular disease, diabetes mellitus, ulcerative colitis, liver cirrhosis and systemic lupus erythematous (Table 2). Such metabolic and inflammatory diseases are heavily modulated by external factors, i.e. inflammation and oxidative stress, which promote telomere attrition. Inflammation triggers cellular proliferation and accelerates cell turnover, thus facilitating telomere attrition due to the ‘end-replication problem’. Oxidative stress, however, promotes DSBs, particularly at telomeric regions, resulting in telomere shortening with each cell division [101]. Consequently, this may lead to premature senescence of mammalian cells and result in the loss of functional cells. As such, accelerated telomere shortening may serve as a surrogate for oxidative stress or inflammation. Moreover, accelerated telomere loss has also been observed in patients with infections (Table 2) and mortality in the elderly [102].

Table 2. Telomere attrition has been linked with metabolic/inflammatory and vascular diseases, infections and cancers
DiseasesReference
Cardiovascular disease [111]
Heart failure [112]
Atherosclerosis [113]
Myocardial infarction [114]
Ischemic heart disease [114]
Aortic dissection [115]
Coronary heart disease [116]
Diabetes mellitus [117]
Ulcerative colitis [118]
Liver cirrhosis [119]
Chronic liver disease [120]
Chronic obstructive pulmonary disease[[121],[122]]
Systemic lupus erythematous[[123],[124]]
Chronic gastroesophageal reflux disease [125]
Celiac disease [126]
Plasma cell disorder [127]
Dementia [128]
Alzheimer's dementia[[129], [130]]
Parkinson's disease [131]
AIDS [132]
Cytomegalovirus [133]
Hepatitis C virus [134]
Epstein-Barr virus [135]
Pancreatic invasive neoplasia [136]
Gastric carcinoma [137]
Breast cancer [138]
Hepatocellular carcinoma [139]
Epithelial cancer [140]

Cancer

Telomere shortening has been proposed to have a dual role in carcinogenesis: it promotes genome instability thus leading to cancer initiation; and suppresses tumor formation. Critically, short telomeres have been reported to be common and prevalent early genetic alterations in cancer initiation [103]. Short telomeres are potentially recognized as DSBs, leading to induction of the DNA damage response machinery and activation of DNA damage repair through the NHEJ pathway, resulting in end-to-end chromosomal fusions. When cells with fused chromosomes enter subsequent mitotic cycles, these chromosomal fusions are likely to break and result in chromosomal aberrations. Repeated breakage–fusion–bridge cycles cause accumulation of chromosomal instability that serves to fuel malignant transformation. Studies using telomerase-deficient mice (mTER−/−) have revealed that the loss of telomeres is correlated with an elevated frequency of cytogenetic abnormalities (end-to-end chromosomal fusions) [13, 104]. This is further supported by other studies using telomerase and p53 double-knockout mice in which increased genomic instability, characterized by the accumulation of dicentric chromosomes and an inclination toward oncogenic transformation, is observed [105]. Taken together, these findings suggest that with the loss of genomic checkpoint functions, telomere attrition might accelerate genomic instability and thus promote carcinogenesis (Fig. 1). Paradoxically, critically shortened telomeres may constrain the replicative capacity of tumor cells by triggering cell-cycle arrest, cellular senescence or apoptosis. This unique feature is envisioned to be an important tumor-suppressive mechanism. In support of this notion, studies using double-knockout telomerase and INK4a (TER−/− INK4a−/−) mice demonstrated a reduction in tumor formation in successive generations of mice with short telomeres when they were exposed to two-step DMBA/UVB carcinogenesis protocols [106]. Similar findings were observed in vitro where cellular growth and colony formation were greatly reduced in cells lacking telomerase and INK4a with short telomeres, suggesting oncogenic resistance in cells with short telomeres [106]. In addition, expression of mutant human TERT in cancer cells resulted in telomere shortening as well as apoptosis [107, 108], further corroborating the notion that telomere shortening inhibits tumorigenesis. Although there have been studies showing the role of telomere shortening in the suppression of tumor formation [106-109], other studies have demonstrated a progressive increase in the spontaneous tumor formation rate in successive generation of TER−/− mice with shorter telomeres [13, 104], suggesting conflicting and inconclusive roles of telomere shortening in suppressing tumorigenesis.

Concluding remarks

Telomere shortening is envisioned to be an important tumor-suppressive mechanism by limiting cellular proliferative capacity via regulating senescence checkpoint activation. Defective telomeres are causally linked with human diseases and carcinogenesis, emphasizing the importance of telomere maintenance. In light of this, telomere length assessment might serve as an important prognostic and diagnostic tool for clinical monitoring and management. Further investigation, however, is warranted in determining the cause–effect relationships between telomere length and disease progression. Another fundamental area that is worthy of further study is designing therapies directed to slow the process of telomere shortening, which may have a profound impact for early prevention of these aforementioned pathologies.

Although there have been studies showing that shorter telomere lengths are associated with metabolic and inflammatory diseases, these diseases are complex, polygenic and multifactorial in nature, with interplay between genetic and environment factors. Such mechanistic insights into how telomere defects play a role in the disease pathogenesis are still lacking. Additional studies are therefore needed to unravel the underlying molecular mechanisms of telomeres loss in the pathobiology of these diseases and findings from these studies might aid in revealing novel molecular targets for these diseases. Furthermore, large-scale prospective epidemiological studies are warranted to complement findings from mechanistic studies and provide evidence for the causal relationship between telomere attrition and metabolic diseases.

In addition, many of the proteins involved in the DNA repair machinery are implicated in telomere maintenance. Telomere attrition causes accelerated cell turnover, thus leading to premature aging symptoms and predisposition to cancer. Similarly, defects in DNA repair machinery are often associated with premature aging and an elevated risk of cancer. It is yet to be elucidated whether telomere-associated genes complement DNA damage signaling pathways. Although an ‘integrative’ model of telomere maintenance in DNA damage responses mechanism has been suggested [110], it remains a formidable challenge to uncover the intricate relationship between telomeres maintenance and DNA repair machinery and thus, in-depth future works are warranted.

Acknowledgements

This work was supported by the Ministry of Education Academic Research Fund Tier 1 grants, R-183-000-295-112 and R-183-000-320-112 to Xueying Wang, National University Health System (NUHS), National University of Singapore (NUS), Singapore. Chiou Mee Kong is a recipient of research scholarships from Yong Loo Lin School of Medicine, NUHS, NUS, Singapore. The authors sincerely thank Aloysius Loo Jia Wei, Grishma Rane and Kanchi Madhumathi for proofreading the manuscript.

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