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4 Homologous Recombination Based: Alternative Lengthening of Telomeres (ALT)

4 Homologous Recombination Based: Alternative Lengthening of Telomeres (ALT)

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Telomeres and Chromosome Stability


DNA, telosome components, and factors implicated in DNA repair, replication and

recombination (O’Sullivan and Almouzni 2014). A recent study revealed a role for

PML bodies in telomeric chromatin compaction and telomeric repeat clustering,

leading to formation of APBs and depletion of TRF2 from the telomeres (Osterwald

et al. 2015). This subsequently promotes ATM autophosphorylation in the APBs

and activation of the DDR, whereby telomeres are replicated via the ALT pathway

(Osterwald et al. 2015).

In normal cells, three variants of the telomeric sequence (TGAGGG, TCAGGG,

TTGGGG) are detected at human telomeres and restricted to the telomeric proximal

end (Conomos et al. 2012). In ALT cells however, variant sequences are detectable

throughout the telomeres and are most likely the consequence of HR-based telomere replication (Conomos et al. 2012). The presence of non-cognate telomeric

sequences was proposed to decrease the sequence-specific binding of the telosome

and other telomere-associated proteins, and sequence-specifically recruit different

proteins such as nuclear receptors, potentially altering the architecture of ALT

telomeres to promote recombination (Conomos et al. 2012). For example, in ALT

cells, the ratio of TRF2/total amount of telomeric DNA is reduced compared to

ALT-negative cells (Conomos et al. 2013). Since TRF2 typically represses HR,

telomeres in ALT cells are therefore prone to HR (Conomos et al. 2013). A decrease

in the binding of the telosome in ALT cells could also explain the high levels of

genomic instability at ALT telomeres (TIFs) but nevertheless, ALT cells are able to

progress through the cell cycle, suggesting that the telomere uncapping is only a

transient state and that dysfunctional telomeres are a requirement for ALT

(Conomos et al. 2013).


Factors Regulating ALT

Numerous proteins associated with APBs, telomeres and HR are potentially implicated in the ALT pathway but only a few proteins or protein complexes are required

for ALT-mediated telomere length maintenance (Bhattacharyya et al. 2010; Cesare

and Reddel 2010). These include the MRN and SMC5/6 ‘structural maintenance of

chromosome’ recombination complexes, consistent with ALT being

HR-dependent, the MUS81 and FEN1 endonucleases, and the FANCD2 and

FANCA Fanconi anemia proteins.

No ALT inhibitors have been developed to date primarily because the molecular

mechanisms governing the ALT pathway are incompletely characterized. A better

understanding of the ALT mechanism will greatly contribute to the development of

therapeutic approaches to specifically target ALT. One major challenge in the

development of ALT specific inhibitors is that proteins with potential roles in

telomeric recombination and the ALT pathway also regulate DNA transactions,

including DDR proteins, recombination proteins, helicases and nucleases and are

present in normal cells (Cesare and Reddel 2010). Nonetheless, depletion of

MUS81, a DNA structure-specific recombination endonuclease induces decreased


T.W. Chu and C. Autexier

telomeric sister chromatid exchanges and proliferation arrest in ALT cells, but not

in telomerase-positive cells (Zeng et al. 2009). Interestingly, RNaseHI was recently

found to associate specifically with ALT telomeres to regulate TERRA-telomeric

hybrids and RPA at telomeres, and altering cellular RNaseH1 levels distinctively

perturbed telomere homeostasis in ALT but not telomerase-positive cells (Arora

et al. 2014). Indeed, ALT cancer cells were recently shown to display specific

hypersensitivity to inhibition of ATR, a protein recruited by RPA and which

localizes to and maintains APBs (Flynn et al. 2015). These results suggest that it

may be possible to specifically target telomeres and viability of ALT cancer cells.


ALT in Human Cancer

In human cancer, 20–60 % of certain tumor types with a high unmet medical need,

such as osteosarcomas, pancreatic neuroendocrine cancers (PanNETs), glioblastomas, and other tumors of the central nervous system (CNS), maintain telomere

length by the ALT pathway (O’Sullivan and Almouzni 2014).

ATRX is part of the transcription/chromatin remodelling complex and functions

to maintain a closed heterochromatin structure at pericentric heterochromatin,

telomeres and several transcription factor binding sites by depositing the

replication-independent histone 3 variant H3.3 (Schwartzentruber et al. 2012).

Recently, mutation and loss of ATRX were found to correlate with features of

ALT in pediatric glioblastomas, tumors of the central nervous system and

PanNETs, consistent with ATRX being a repressor of the ALT pathway (Heaphy

et al. 2011; Schwartzentruber et al. 2012; O’Sullivan and Almouzni 2014; Clynes

et al. 2015). Loss of ATRX expression in PanNET patients is associated with

increased genomic instability and shorter time of survival (Marinoni et al. 2014).

The ALT phenotype is also associated with p53 deficiency (95 % of ALT cell lines,

and ~78 % of ALT tumours have non-functional p53) (Cesare and Reddel 2008;

Gocha et al. 2012). p53 mutations at codons 248 or 273 have been reported as

hotspots in human tumours, including ALT tumours (Gocha et al. 2012). Many

ALT tumors with these p53 mutations also harbor ATRX mutations (Gocha

et al. 2012).

The mechanisms by which ATRX regulates telomeric recombination and the

ALT pathway are unknown, but mutations or loss of ATRX could promote a more

open chromatin conformation and favor telomeric recombination. Recent studies

show that chromatin compaction is reduced at ALT telomeres and that ALT

phenotypes are induced by depletion of the histone chaperone ASF1, supporting

the hypothesis that ALT is a consequence of histone management dysfunction

(O’Sullivan et al. 2014; Episkopou et al. 2014).

Telomeres and Chromosome Stability


5 Telomeres and Epigenetics

Unlike the rest of the genome, mammalian telomeres do not contain genes and the

subtelomeres are gene poor. The telomeric chromatin structure has the ability to

reversibly silence subtelomeric genes, a repressive effect termed “telomere position

effect” (TPE) (Gottschling et al. 1990). TPE was first discovered in Drosophila

melanogaster and later extensively studied in yeast, and was shown to be dependent

on the high binding affinity of the chromatin compaction protein heterochromatin

protein 1 (HP1) to trimethylated H3K9me3 (Schoeftner and Blasco 2010). This

association triggers a switch from the open euchromatin conformation into the

closed heterochromatin state. In mammals, the existence of TPE remained controversial for many years. Studies using insertion of transgenes near human telomeres

demonstrated that the NAD-dependent protein deacetylase SIRT6 acts as a key

player in the maintenance of TPE by keeping the telomeres in a repressive

hypoacetylated state (Tennen et al. 2011). Furthermore, telomere length determines

the extent of TPE, with long telomeres exhibiting a stronger silencing effect due to a

higher density of repressive histone marks and enrichment of chromatin compaction proteins (Baur et al. 2001).

The assumption that telomeres merely serve as a protective cap and are consistently in a heterochromatin state persisted for many years. It was only with the

discovery of TERRA, transcribed by RNA polymerase II (RNAPII), that this notion

was disproved (Azzalin et al. 2007; Schoeftner and Blasco 2008). TERRA is

transcribed from subtelomeric promoters toward the telomeric regions (Fig. 1b).

They are comprised of RNA sequences ranging in size from 100 bases to approximately 9 kb complementary to both subtelomeric and telomeric sequences

(Schoeftner and Blasco 2010). In 2012, Decottignies’s group assessed the physiological existence of TPE at native human chromosomal ends for the first time by

monitoring the transcription of the subtelomeric regions into TERRA (Arnoult

et al. 2012). Mammalian subtelomeric regions can adopt a more open conformation

characterized by enrichment of H2BK5me1 and H3K4me3. Consistent with earlier

yeast studies, Decottignies’s group showed that long human telomeres negatively

impact the level of TERRA transcription, with a corresponding enrichment of

H3K9me3 and HP1α at the telomeres. Longer telomeres also result in a concomitant increase in TERRA length despite the lower number of TERRA molecules

(Arnoult et al. 2012). Due to the complementary nature of the TERRA sequence

(UUAGGG)N to the telomeric C-strand (AATCCC)N sequences, these RNAs can

transiently bind to subsets of telomeres and, in turn, recruit heterochromatinassociated proteins such as HP1α, resulting in a negative feedback that shifts the

telomeres from the open euchromatin conformation back to the heterochromatin

state (Arnoult et al. 2012). Furthermore, a new study in telomerase-negative yeast

cells (Type II survivors in yeast equivalent to ALT in mammalian cells) showed

that the association of TERRA with telomeric DNA promotes telomeric recombination and senescence bypass (Yu et al. 2014).


T.W. Chu and C. Autexier

TERRAs are strictly nuclear and are cell cycle regulated, with levels peaking in

G1 and early S phase (Maicher et al. 2014). Furthermore, the shelterin protein TRF1

can interact with RNAPII and changes in TRF1 or RNAPII expression impact

TERRA levels (Schoeftner and Blasco 2008). Another mechanism regulating

TERRA was found by Azzalin’s group, when they discovered that factors of the

nonsense-mediated RNA decay pathway can physically interact with the telomeric

chromatin and trigger TERRA degradation or displacement from the telomeres

(Azzalin et al. 2007). A recent study demonstrated that the chromatin organizing

factor CTCF and the multiprotein complex cohesin are fundamental components of

the subtelomeres in human cells (Deng et al. 2012). CTCF and cohesin recruit

RNAPII to telomeres, promote TERRA expression, while their depletion reduces

TERRA expression and induces TIF formation, suggesting a potential role for

TERRA in telomere end-protection (Deng et al. 2012).

Nucleosomes at mammalian telomeric and subtelomeric regions are spaced

distinctly compared to their organization in the rest of the genome (Schoeftner

and Blasco 2010). More recently, a study showed that in vitro, the shelterin

component TRF1 can change the nucleosomal spacing, thereby allowing chromatin

remodelling at telomeres (Galati et al. 2015). Increasing evidence supports a role

for epigenetics in telomere length regulation. Several studies have shown that

defective epigenetic control leads to compromised telomere length maintenance

and integrity. SUV39H1/H2 are suppressors of variegation histone

methyltransferases responsible for the trimethylation of H3K9 (Peters

et al. 2001). Deficiency in SUV39H1/H2 results in the decrease of H3K9me3,

altered telomeric heterochromatin state and aberrantly elongated telomeres

(Garcia-Cao et al. 2004). Mammalian telomeres are not methylated due to the

absence of CpG sequences, which are the substrates of DNA methyltransferases

(DNMTs) (Gonzalo et al. 2006). Instead, association of DNMT1, 3a and 3b has

been reported at the subtelomeric regions enriched in CpG sequences (Gonzalo

et al. 2006). Subtelomeric DNA methylation enforces TPE and restricts the access

of HR proteins to telomeres, thus suppressing ALT-mediated TMM (Gonzalo

et al. 2006).

Emerging evidence suggests a link between the epigenetic status of human

telomeric and subtelomeric chromatin and disease states. Such a connection has

already been demonstrated by the altered telomeric methylation status in

Alzheimer’s disease (AD) (Guan et al. 2013). Also, TERRA downregulation has

been observed in advanced stages of human larynx, colon and lymph node cancer,

compared to normal tissue (Schoeftner and Blasco 2008). An interesting cohort

study on astrocytic tumors revealed that promoter methylation-mediated epigenetic

silencing leads to TERRA downregulation (Sampl et al. 2012). Elevated TERRA

level inversely correlates with tumor grade and is associated with better patient

survival (Sampl et al. 2012). Increasing efforts are currently directed towards

studies that investigate the use of telomere epigenetic marks as diagnostic or

prognostic tools in human disease.

Telomeres and Chromosome Stability


6 Nuclear Organization and Telomeres

Initial studies in yeast showed that telomeres are distributed in a non-random

fashion and are localized to specific subnuclear domains in a cell cycle-dependent

manner. This specific telomere distribution implicating Sir3, Sir4 and Rap1 was

first described in yeast, through the observation that telomeric clustering forms at

the nuclear rim (Gotta et al. 1996). In human interphase cells, telomeres are also

attached to the nuclear matrix (de Lange 1992). Based on live cell imaging data,

telomeres are highly dynamic structures with a mobility that increases with telomere attrition and uncapping (Dimitrova et al. 2008; Wang et al. 2008).

Type A lamin, the main component of the lamina layer, is a member of the class

V intermediate filaments generally involved in the maintenance of nuclear structural integrity and the regulation of transcription. Depletion of lamin A, which

interacts with telomeres, causes telomere redistribution while mutation in this

filament protein leads to lamina layer distortion and the formation of telomere

aggregates (Novo and Londono-Vallejo 2013). Members of the shelterin complex

are also implicated in telomere positioning, serving as intermediates that bridge the

telomeres to the nuclear matrix. Time-lapse confocal microscopy showed that

similarly to yeast, human Rap1 is required for the association of telomeres with

the nuclear architecture (Crabbe et al. 2012). In Caenorhabditis elegans, POT1 is

essential for nuclear peripheral tethering of the telomeres (Ferreira et al. 2013).

Furthermore, TIN2L, an isoform of TIN2, binds strongly to the nuclear matrix and

is required for telomere tethering (Kaminker et al. 2009).

To decipher the 3D telomere positioning in the nucleus, Mai’s group developed a

technique using 3D nuclear imaging in combination with TeloView (Klonisch

et al. 2010). This technique led to the discovery that the nuclear positioning of

telomeres differs between normal and cancer cells. In normal cells, telomeres are

confined within an ellipsoid area and are positioned at the periphery of a central

telomeric disk formed in G2 (Klonisch et al. 2010). In cancer cells however,

telomeric fusions generate telomeric aggregates (TAs), hallmarks of tumor cells,

in the interphase nucleus (Chuang et al. 2004). TAs promote genomic instability

and nuclear architecture remodelling distinct from that in normal cells (Novo and

Londono-Vallejo 2013). In recent years, Mai’s group conducted studies profiling

the telomere architecture of multiple myeloma, myelodysplastic syndrome and

acute myeloid leukemia patients by measuring the telomere number, TAs, telomere

signal intensity, nuclear volume as well as telomeric distribution (Gadji et al. 2012;

Klewes et al. 2013). This 3D imaging approach allows the assessment of these

various parameters with a potential application in disease stratification and the

development of patient-specific treatments (Gadji et al. 2012; Klewes et al. 2013).


T.W. Chu and C. Autexier

7 Telomere and Disease

The existence of a link between telomere length and longevity in humans remains

an open question. However, convincing evidence suggests that deprotected telomeres contribute to the development of cancer and various premature aging diseases. This was first demonstrated using telomerase knockout mice characterized by

critically short, uncapped telomeres. They also displayed increased incidence of

tumorigenesis with several characteristics of premature aging (Artandi et al. 2000;

Blasco et al. 1997; Sahin and Depinho 2010).


Telomeres and Aging

Telomere shortening results from the natural replication dependent-aging process.

However, environmental factors, including pollution, smoking, diet, infection,

inflammation, and DNA damaging agents such as UV light, enhance the rate of

telomere loss (Calado and Dumitriu 2013). The heritability of telomere length has

also been demonstrated in human leukocytes, where the paternal telomere length

determines the ones in the offsprings (Eisenberg et al. 2012). Heritability of

telomere length is particularly important in individuals with a defective telomere

maintenance mechanism in their stem and germ cell compartments. Persons with

degenerative premature aging disorders caused by defective telomere maintenance,

termed “telomeropathies”, exhibit disease anticipation; that is, the onset of the

disease occurs at a younger age and symptoms are more severe with each subsequent generation (Holohan et al. 2014). These diseases arise from defects in the

shelterin complex, telomerase or its accessory proteins such as dyskerin, in addition

to other factors required for telomere replication, for example, the RTEL1 helicase.

The first disease identified to be directly associated with defective telomere

biology is dyskeratosis congenita (DC) (Mitchell et al. 1999). DC patients are

generally diagnosed by a triad of nail dystrophy, oral leukoplakia and reticulated

hyperpigmentation of the skin (Armanios and Blackburn 2012). The lengths of

germ line telomeres are very short in these individuals due to the inability to sustain

telomere maintenance and, as such, they often suffer from aplastic anemia, bone

marrow failure, pulmonary fibrosis and/or liver cirrhosis. Furthermore, high levels

of genomic instability resulting from short telomeres predispose patients to cancer

development (acute myeloid leukemia and myelodysplastic syndrome) (Armanios

and Blackburn 2012). The average lifespan of DC patient is 30 years as a result of

bone marrow failure (Holohan et al. 2014). DC is an extremely rare inherited

genetic disorder that can be classified based on the pattern of inheritance: autosomal

dominant, recessive or X-linked. To date, germline mutations causing DC and

Hoyeraal-Hreidarsson syndrome (HHS), a more severe form of DC, have been

identified in genes involved in telomere maintenance including hTERT, hTERC,

DKC1, NOP10, NHP2, TCAB1, TINF2 (codes for TIN2), CTC1, RTEL1 and TPP1

Telomeres and Chromosome Stability


(Holohan et al. 2014; Kocak et al. 2014; Guo et al. 2014). The manifestation of DC

and HHS is highly heterogeneous due to differences in the level of penetrance

(Holohan et al. 2014). Detrimental compound heterozygous RTEL1 mutations have

also been identified in hereditary HHS (Walne et al. 2013; Holohan et al. 2014).

RTEL1 was extensively studied in mice and evidence support its role in the

resolution of G-quadruplex (G4)-DNA (see below) and D-loop HR intermediates

during telomere replication (Vannier et al. 2012). More recently, an HHS mutation

within the TRF2-interacting site of RTEL1, the C4C4 domain, was shown to disrupt

its interaction with TRF2, thus preventing the recruitment of RTEL1 to the telomeres for T-loop unwinding during S-phase (Sarek et al. 2015). Loss of RTEL1 in

human cells also results in hyper-recombination, suggesting a role for RTEL1 in the

suppression of HR at telomeres (Holohan et al. 2014).

Other severe premature aging diseases resulting from aberrant telomere maintenance includes the Revesz syndrome (TINF2), and the Coats Plus syndrome

(CTC1), which also presents features that overlap with DC. Idiophathic pulmonary

fibrosis (IPF) is typically sporadic and is associated with mutations in hTERT and

hTR, possibly as a result of haploinsufficiency (Armanios and Blackburn 2012).

IPF is a progressive disease that usually develops around the age of 50 and patients

usually do not survive more than 3 years post-diagnosis (Armanios and Blackburn


Tremendous efforts continue to be invested into deciphering the role of the

different components to uncover new targets for regenerative medicine. The only

option available at this time is tissue or organ transplant but the wait for a

compatible donor and potential relapses limit the chances of survival. Research

focusing on the development of small molecules that can specifically activate

telomerase without triggering tumorigenesis is currently ongoing. Using a heterozygous TERT murine model of IPF, a novel molecule activator of telomerase,

GRN510, showed encouraging results in suppressing lung damages (Le Saux

et al. 2013). Telomerase gene therapy based on telomerase overexpression, tested

in both adult and old mice, has been shown to delay aging and increase longevity

without increasing cancer incidence (Bernardes de Jesus et al. 2012).


Telomeres and Cancer

The core components of the telomere maintenance machinery are potentially

specific targets for the development of anti-cancer therapies. Telomerase is active

in ~85 % of tumors, but only weakly active in primary cells, thus it is an attractive

target for cancer cell-specific therapy. The telomerase inhibitor GRN163L

(Imetelstat) is an hTR antagonist that has been tested in human cancer cell lines

including pancreatic, esophageal, and leukemic cancer cells and in xenograft

models. Results initially showed promising inhibitory effects on cell growth and

increased sensitization of cells to DNA damaging agents (Harley 2008). The

efficacy of GRN163L was investigated in various clinical trials for different types


T.W. Chu and C. Autexier

of cancer, but to date it has had limited efficacy in the clinic, with beneficial effects

noted in hematological cancers, in tumors with short telomeres and in combination

therapy (Buseman et al. 2012; Williams 2013).

One drawback of anti-telomerase based therapy is the lag period between the

time of treatment and the observable proliferative arrest resulting from telomere

erosion, which requires several rounds of cell division and thus, would likely only

be effective in cancer cells with short telomeres. Furthermore, 15 % of cancer cells

employ the ALT pathway for telomere maintenance and are insensitive to telomerase inhibition (Shay et al. 2012). An ingenious method to overcome such limitations involves directly triggering telomere uncapping, the activation of the DNA

damage checkpoint and ultimately cellular growth arrest (McEachern et al. 2000;

Guiducci et al. 2001). In recent years, G4-DNA-stabilizing ligands and mutant hTR

gene therapy-based approaches demonstrated their efficacy in triggering telomere

uncapping (Harley 2008; Cerone et al. 2006; Brault and Autexier 2011; Tauchi

et al. 2006; Goldkorn and Blackburn 2006). Recently, a small molecule telomerase

substrate was reported to induce telomere uncapping in a telomerase-dependent

fashion in cell and xenograft models (Mender et al. 2015).

Formation of the telomere cap is dependent on the telomere sequence-specific

binding of the TRF1, TRF2 and POT1 shelterin proteins. Alteration in the telomeric

sequences by mutant hTR can trigger telomere uncapping resulting from impaired

shelterin-telomere associations. In eukaryotes, the single-stranded guanine-rich

DNA sequences at the telomeres favor the formation of higher order intramolecular

structures known as G4 (Harley 2008) (Fig. 1a). The observation that certain G4

ligands and mutant hTR expression lead to specific anti-proliferative effects in

cancer cells implies that perhaps in these cells, the telomere cap is distinct from that

in normal cells (Fakhoury et al. 2007; Riou 2004; Mahalingam et al. 2011). Importantly, though G4 structures are typically viewed as preventing the access of

telomerase to the telomeres, a recent study demonstrated that in vivo, telomerase

can localize to parallel G4 formed at telomeres and subsequently allow DNA

extension (Moye et al. 2015). Their data suggest that specific G4 conformations

and location have important biological implications.

The altered expression profile of telosome components has been reported in

some human tumors (Oh et al. 2005; Poncet et al. 2008). TRF2 expression was

shown to contribute to multidrug resistance in gastric cancer (Ning et al. 2006)

while other studies found an increased expression of POT1 and TPP1 in multiple

myeloma (Panero et al. 2014; Ferrandon et al. 2013). POT1 was also recently

identified as the first shelterin protein mutated in human cancer (Ramsay

et al. 2013). Altogether, these results support the hypothesis that the telomere cap

in cancer cells is distinct from that in normal cells and highlight the potential of

telomere uncapping strategies.

Nonetheless, the development of telomerase and telomere-based anti-cancer and

regenerative therapies remains challenging due to the potential risk to induce

tumorigenesis upon telomerase activation or the development of resistance to

anti-cancer therapies. Both ALT and telomerase-based telomere maintenance can

co-exist in human immortalized and cancer cells (Queisser et al. 2013).

Telomeres and Chromosome Stability


Consequently, telomerase inhibition in these cells may elicit a switch from telomerase to ALT-dependent telomere maintenance, allowing the cells to resist antitelomerase-based strategies. Indeed, telomerase inhibition or deletion of telomerase

components in human cells or in mice leads to telomeric recombination, ALT or

ALT-like activation and cell survival (Bechter et al. 2004; Chang et al. 2003; Hande

et al. 1999; Morrish and Greider 2009; Niida et al. 2000; Laud et al. 2005; Hu

et al. 2012; Queisser et al. 2013). Telomere uncapping strategies in yeast and mice

also cause an increase in telomeric recombination and cell survival (Bechard

et al. 2009; Grandin et al. 2001; Iyer et al. 2005; Celli et al. 2006; He et al. 2006;

Wu et al. 2006). Recently, ALT activity was detected in normal mammalian

somatic cells (Neumann et al. 2013) and telomere uncapping in telomerase-positive

human cells activates an ALT-like phenotype characterized by telomeric HR

(Brault and Autexier 2011; Conomos et al. 2012). These results expose a possible

resistance mechanism against telomere uncapping-mediated anti-cancer therapies.

There is growing interest in combining anti-telomerase strategies with conventional therapies to maximize the efficacy and specificity of these methods. A study

on Barrett’s adenocarcinoma (BAC) demonstrated that targeting telomerase using

GRN163L in combination with an HR inhibitor such as nilotinib or an shRNA

against RAD51 decreases telomere length and increases BAC cell apoptosis

(Lu et al. 2014). A phase II trial using the telomerase vaccine GV1001 (15 amino

acid peptide) post-chemoradiotherapy treatment showed an elevated immune

response rate of 80 % with low level of toxicity in non-small cell lung cancer

(NSCLC) patients (Brunsvig et al. 2011). The increased survival observed in

responders suggests that GV1001, in combination with chemoradiotherapy, may

have a beneficial effect in NSCLC patients (Brunsvig et al. 2011).

8 Concluding Remarks

Despite the tremendous amount of knowledge we currently possess about telomere

biology and regulation, there are still many pending questions regarding the underlying mechanisms of telomere homeostasis. Furthermore, an increasing number of

studies aim to develop telomere-based strategies to target cancer cells more specifically and efficiently. Importantly, regenerative therapies are also under investigation, such as the generation of human induced pluripotent stem cells and telomere

length resetting, in the hope to find a cure for individuals suffering from premature

aging syndromes.

Acknowledgements The authors wish to thank Johanna Mancini and Deanna MacNeil for

comments on the manuscript. Work in the lab of C. Autexier is supported by the Canadian

Institutes of Health, the Canadian Cancer Society, the Cancer Research Society and the Natural

Sciences and Engineering Research Council of Canada. T.W. Chu is a recipient of Le Fonds de

recherche du Que´bec – Sante´ Doctoral Award.


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