Oncogene (2004) 23, 7283–7289. doi:10.1038/sj.onc.1207948

Does the reservoir for self-renewal stem from the ends?

Lea Harrington1

1Department of Medical Biophysics, Ontario Cancer Institute, University of Toronto, 620 University Avenue, Toronto, Ontario, Canada M5G 2C1

Correspondence: L Harrington, Department of Medical Biophysics, Ontario Cancer Institute, University of Toronto, 620 University Avenue, Room 932, Toronto, Ontario, Canada M5G 2C1. E-mail:



Stem cell research is a burgeoning field with an alluring potential for therapeutic intervention, and thus begs a critical understanding of the long-term consequences of stem cell replacement. Operationally, a stem cell may be defined as a rarely dividing cell with the capacity for self-renewal throughout the lifetime of the organism, and an ability to reconstitute its appropriate lineages via proliferation and differentiation. In many differentiated normal and cancer cell types, the maintenance of telomeres plays a pivotal role in their continued division potential. Taken together with the presence of the enzymatic activity responsible for telomere addition, telomerase, in several progenitor cell lineages, it is presumed that telomere maintenance will be critical for the replenishment of stem cells or their successors. The purpose of this review is to discuss the role of telomere length maintenance in self-renewal, and the consequent challenges and potential pitfalls to the manipulation of normal and cancer-derived stem cells.


stem/progenitor cell, self-renewal, telomeres, telomerase



In 1978, Blackburn and Gall molecularly defined the first telomere in the ciliate Tetrahymena thermophila, a single-celled protozoan with an indefinite capacity for self-renewal and an unusual property of chromosome fragmentation that leads to an abundance of newly formed telomeres (Blackburn and Gall, 1978). With a few interesting exceptions, these simple repetitive G-rich terminal sequences – consisting of a few hundred base pairs of TTGGGG in Tetrahymena, and several kilobase pairs of TTAGGG in humans – are shared among most eukaryotes and play a crucial role in the protection of linear chromosome ends against degradation, recombination, and fusion (Blackburn, 2001).

The Blackburn laboratory posited that the ability of self-renewing organisms such as ciliates and yeast to maintain telomeres (in fact, in some instances, to lengthen them) depended on active replenishment to compensate for the shortcomings of the DNA replication machinery, and in 1985 discovered such an activity, termed telomere terminal transferase or telomerase (Greider and Blackburn, 1985, 1987). The catalytic components of telomerase were first cloned in ciliates and budding yeast, and comprise an RNA component (TER) that carries a short template complementary to telomere DNA, and a reverse-transcriptase protein component (TERT) that copies the RNA template, one nucleotide at a time, onto the 3'-end of linear DNA (Greider and Blackburn, 1989; Shippen-Lentz and Blackburn, 1990; Romero and Blackburn, 1991; Singer and Gottschling, 1994; Lingner et al., 1997). The mammalian homologues of these components were identified soon thereafter (Blasco et al., 1995; Feng et al., 1995; Harrington et al., 1997; Kilian et al., 1997; Meyerson et al., 1997; Nakamura et al., 1997). A number of telomerase-associated proteins have been identified over the last decade in yeasts, ciliates, and mammals; except for a few, their precise role in the telomerase complex remains enigmatic (Blackburn, 2001; Cong et al., 2002).

Unicellular organisms usually express both essential subunits of telomerase, and thus are able to maintain their average telomere length (Blackburn, 2001). Net telomere length is also dictated by telomere-binding factors that regulate telomerase access and help mask the telomere from detection as a DNA break (Blackburn, 2001). Genetic evidence that telomere homeostasis is essential for indefinite propagation was first obtained in yeasts and ciliates; disruption of either the telomerase RNA or the telomerase reverse transcriptase leads to progressive telomere erosion, with an eventual increase in chromosome instability and infertility until the population becomes 'senescent' (Singer and Gottschling, 1994; McEachern and Blackburn, 1995; Lendvay et al., 1996; Miller and Collins, 2000). Human cell populations without a means to maintain a minimal amount of telomeric DNA also show limited replicative potential in vitro (Harley, 1997). In rare instances, cells without telomerase activity are able to survive via homologous recombination between telomeric DNA tracts, termed 'survivors' or ALT (for alternate telomere maintenance) in yeast and human cells, respectively (Reddel et al., 2001).

In multicellular organisms, the requirement for telomere length maintenance is more difficult to ascertain since telomerase is often not universally expressed. In some species, such as mammals, telomerase activity is developmentally and anatomically restricted to cell types that are undifferentiated or maintain a high proliferative index, such as germ tissues and progenitor cells (i.e. multipotent, but not necessarily stem cells) (Greider, 1998a). In tissues without detectable telomerase activity, the telomerase RNA transcript is usually present, but the TERT mRNA is repressed or produced as an inactive spliced variant (Ulaner et al., 1998, 2001).

Despite the complicated regulation of telomerase activity during development and differentiation, genetic experiments in mice and Arabidopsis have recapitulated the essential role of telomere length maintenance for long-term cellular proliferation (Riha et al., 2001; Hackett and Greider, 2002). While not immediately detrimental, deletion of either the telomerase RNA or TERT in mice leads to a loss of telomerase activity and progressive telomere shortening in all tissues (Blasco et al., 1997; Yuan et al., 1999; Liu et al., 2000; Erdmann et al., 2004). Eventually, detectable telomere DNA is lost from chromosome ends, leading to myriad consequences including genetic instability, infertility and increased apoptosis in germ cells, immune deficits due to reduced proliferation of activated T and B cells, premature loss of hair, and decreased wound healing (Hackett and Greider, 2002; Erdmann et al., 2004). In some murine genetic backgrounds, loss of telomere function predisposes to certain types of age-associated malignancies, while in other genetic contexts, short telomeres hinder tumor incidence and progression (Hackett and Greider, 2002). Similarly, human cells that lack robust telomerase activity usually demonstrate limited replicative capacity in vitro; restoration of ectopic telomerase activity can allow indefinite lifespan extension in some cell types (Greider, 1998b). Finally, the notion that telomere length maintenance is critical for mammalian cell proliferation is also supported by telomerase inhibition studies, in which loss of telomerase activity in normal and cancerous human cell lines leads to telomere erosion and premature replicative senescence or apoptosis (Harrington and Robinson, 2002).


Telomerase activity in stem cells

Telomerase enzymatic activity is usually detected via the incorporation of radiolabeled nucleotides onto a telomeric single-stranded DNA substrate (Greider and Blackburn, 1985; Morin, 1989; Prowse et al., 1993), and the minimum detection threshold can be improved to as little as 1–10 cells via the PCR amplification of the elongation products (Kim et al., 1994). Using this biochemical assay, telomerase activity has now been detected in numerous progenitor cell types within humans and mice, and in populations recognized as stem cells (Table 1). By way of comparison, plant stem cells such as the Arabidopsis meristem often contain detectable telomerase activity (Riha et al., 2001). However, not all human stem cell populations contain active telomerase; for example, mesenchymal stem cells retain the ability to regenerate multiple lineages including stromal cells and chondrocytes, and yet telomerase activity cannot be detected above background levels in some instances (Table 1) (Banfi et al., 2002; Gronthos et al., 2003; Zimmermann et al., 2003; Parsch et al., 2004). In these populations, it is not yet completely clear whether telomeres undergo the usual attrition in the absence of telomerase activity (Banfi et al., 2002; Gronthos et al., 2003; Zimmermann et al., 2003; Parsch et al., 2004).

It is possible that some stem cell types do not possess telomerase activity because limited telomere attrition can be tolerated. If we consider that telomere loss occurs as a natural consequence of DNA replication during cell division, and a stem cell only undergoes a limited number of doublings during the lifetime of the organism, then telomere replenishment may be dispensable (Figure 1). Stem cell successors, on the other hand, often represent rapidly dividing populations, and thus telomerase activity may be induced in progenitors arising from the stem cell population. Since some stem cell types might not undergo as many divisions as other types, some stem cells might need only possess the potential to induce telomerase activity in their successors, such as undifferentiated progenitor cells (Figure 1).

Figure 1.
Figure 1 - Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, please contact or the author

Telomerase status during cell lineage commitment. Color designations are indicated at lower right. (a) Representative examples of telomerase expression in stem and progenitor cells. (b) Hypothetical scenario for the potential for telomerase inhibition to force a cancerous stem cell into senescence/apoptosis (top) or for enforced telomerase expression to restore replicative capacity to an arrested cell (bottom). The blue arrows indicate potential pitfalls to the manipulation of telomerase status, that is, continuous telomerase expression might be permissive for accumulation of genetic abnormalities (arrow, bottom to top), or that telomerase inhibition might lead to premature replicative exhaustion of certain cell lineages (arrow, top to bottom). Note that the presence of telomerase activity is not always synonymous with telomere length maintenance. See text for details and references

Full figure and legend (146K)


Telomere length maintenance in stem cells

The role of telomere length maintenance in stem cell renewal is less well understood, but is an equally important parameter. In Arabidopsis and murine genetic models, loss of telomerase function leads to infertility, and an eventual depletion of stem cells such as germ cells and meristem (Lee et al., 1998; Hemann et al., 2001; Riha and Shippen, 2003). Other phenotypes associated with loss of telomere function suggest, but do not prove, that stem cell function for certain lineages have been compromised, for example, alopecia, and decreased immune function (Lee et al., 1998; Rudolph et al., 1999). These observations argue that stem cell function can be compromised when telomere erosion exceeds a critical limit. In some progenitor cell types, such as T-lymphocytes, low levels of telomerase activity are present, and yet telomere attrition is observed during growth in culture (Vaziri et al., 1994; Buchkovich and Greider, 1996; Chiu et al., 1996; Rufer et al., 1998). Inhibition of telomerase activity in T-lymphocytes, via overexpression of catalytically inactive hTERT, leads to premature senescence, suggesting that the low levels of telomerase activity play an important role in telomere integrity despite an inability to maintain longer telomere lengths (Roth et al., 2003b).

While compelling, the above observations do not necessarily prove that telomere maintenance is an essential requisite for stem cell function in mammals (Allsopp and Weissman, 2002). Two recent studies serve to underscore the potential pitfalls in asserting that telomere erosion is a physiological and causal determinant of self-renewal. In mice, wild-type donor hematopoietic stem cells (HSC, as operationally defined in this study) are capable of serial reconstitution of irradiated recipients up to four times. Interestingly, during serial transplantation, loss of the average telomere length in the reconstituted population is observed (Allsopp et al., 2003a). Mice deficient in the telomerase RNA or TERT, on the other hand, show accelerated telomere erosion (as expected, since telomerase activity is absent); however, HSC were unable to reconstitute irradiated recipients beyond two serial transplantations (Allsopp et al., 2003a). Based on this experiment, one might justifiably conclude that telomere erosion was a limiting determinant of serial HSC transplantation (Allsopp and Weissman, 2002; Samper et al., 2002; Allsopp et al., 2003a). However, in mice that overexpress mTert, telomere erosion was not observed during serial HSC transplantation, and the potential for serial transplantation did not exceed that of wild-type HSC (Allsopp et al., 2003b). This finding does not support a direct and causal link between telomere length and replicative potential. One interpretation of these experiments is that telomere erosion can lead to premature failure of HSC reconstitution, but it is unlikely to be the limiting factor for the reconstitution of wild-type HSC in mice.

In humans, the role of telomere length maintenance in HSC function may be more prominent (Lansdorp, 1998; Greenwood and Lansdorp, 2003; Bessler et al., 2004). In the past few years, autosomal dominant forms of dyskeratosis congenita, a disorder characterized by early bone marrow failure, have been linked to mutations in the telomerase RNA (Vulliamy et al., 2001a, 2001b; Fogarty et al., 2003; Mason, 2003). Most of these telomerase RNA mutations appear to be inactivating and patients show telomere shortening in all tissues (Bessler et al., 2004). At present, it is not clear whether the inactivating mutation represents a true haploinsufficiency or a dominant interfering mutation (Vulliamy et al., 2001a). Since average telomere lengths are shorter in humans than in inbred mouse strains, it is possible that telomere attrition may contribute some of the manifestations of this disorder. However, in X-linked DKC, the disorder is linked to a defect in ribosome function (Ruggero et al., 2003). Although telomere attrition has been observed in both humans and mice bearing mutations in dyskerin, it is thought that the defects in rRNA modification may be responsible for the disease (Ruggero et al., 2003; Bessler et al., 2004). Bone marrow transplantation of telomerase-transduced HSCs would be one possibility to test whether some of the phenotypes associated with autosomal dominant or X-linked DKC may be ameliorated.


Consequences of enforced telomerase expression in stem cells

Overexpression of TERT can, in some instances, extend the capacity to expand certain progenitor and stem cell populations in vitro (Figure 1). In other studies where cell proliferation has been altered upon TERT overexpression without an obvious effect on average telomere length, it has been argued that TERT may play a role in cell proliferation unrelated to the maintenance of telomere DNA (Gonzalez-Suarez et al., 2001; Artandi et al., 2002). Before reaching such a conclusion, however, it is important to examine telomere DNA at each chromosome end (i.e. via hybridization of a telomeric fluorescent probe in metaphase spreads) in addition to examining the average telomere length of a population. This technique, termed quantitative FISH or Q-FISH, was first developed by Lansdorp and co-workers, and has proven instrumental in discerning subtle alterations in telomere length dynamics that are not visible using other techniques (Blasco et al., 1997; Poon et al., 1999; Baerlocher et al., 2002). To illustrate just one example, two wild-type copies of mTert are sufficient to maintain long average telomere lengths, whereas mice heterozygous for mTert appear competent only for the maintenance of a minimal amount of telomeric DNA at each chromosome end (Liu et al., 2002). Mice that possess only one functional allele of mTert thus possess similar 'average' telomere lengths to mice that lack mTert altogether, and yet the former do not suffer from the same telomere instability and infertility as mTert null animals (Erdmann et al., 2004). Without measuring the telomeric DNA signal at each chromosome end, it may have been incorrectly inferred that telomere status in these two genotypes is equivalent, and that mTert must possess another cellular role separate from telomere length maintenance.


Future challenges and pitfalls in manipulating telomere homeostasis

The ability of TERT expression to expand indefinitely the replicative capacity of some cell types in vitro has generated considerable interest in using a similar approach to extend the proliferative potential of stem cells, in vivo and ex vivo. Before undertaking such an approach, it must be determined whether TERT overexpression has any other phenotypic ramifications for progenitor or stem cell populations. The long-term consequences of TERT expression in some normal human diploid fibroblasts or epithelial cells does not appear to impede cellular responses to DNA damage, and at gross level chromosomal aberrations do not appear enriched (Morales et al., 1999, 2003; Vaziri et al., 1999). However, it remains a possibility that TERT overexpression may generate a permissive environment for the continued propagation of progenitor cells with chromosome abnormalities. In support of this notion, mTert overexpression in murine skin and mammary gland leads to an increase in transient hyperplasia or an increased incidence of mammary epithelial cancer in older mice (Gonzalez-Suarez et al., 2001; Artandi et al., 2002). One strategy to circumvent this potential problem is to transiently overproduce telomerase activity, whether by transient infection or by introduction and subsequent excision of TERT. However, if the culprit is lengthened telomeres rather than telomerase status, the proliferative advantage may linger long after the removal of TERT. Using our mTert murine model, we wish to determine whether low levels of telomerase activity serve to protect mice from the same progenitor cell depletion as telomerase-deficient mice with a comparably short average telomere length. For example, it will be interesting to determine whether it is possible to protect mice from the adverse effects of telomere DNA loss without a concomitant perturbation in progenitor subtypes or an increase in age-associated neoplasias.

A second interesting question that remains to be addressed is whether strategies to inhibit telomerase activity would have adverse phenotypic consequences for normal progenitor and stem cell function. In cell culture, numerous studies have established that telomerase inhibition does lead to cell death or cell arrest in both normal and cancerous cells (Harrington and Robinson, 2002; Masutomi et al., 2003). Telomerase inhibition in some cancers thought to arise from immature or progenitor populations, such as AML, also leads to cell death (Roth et al., 2003a). Thus, it is not yet clear whether the inhibition of human telomerase might lead to defects in some cell types, either immediately or perhaps several years after treatment. Inbred mouse strains have very long telomeres compared with most human tissues, and thus make it difficult to model the consequences of telomerase inhibition on normal tissues. Wild-derived mouse strains, telomerase deficient strains, or mTert heterozygous mice have much shorter telomeres, and thus might provide a more appropriate model in which to examine this question. As yet, small molecule inhibitors of telomerase have so far only been developed in human cells (Damm et al., 2001; Pascolo et al., 2002).

As Charles Darwin said, 'Ignorance more frequently begets confidence than does knowledge' (Darwin, 1898). While we have much to be excited about with regard to the potential role of telomere homeostasis in modulating stem and progenitor cell function, we would be overly confident to assert that we do not have as much yet, or more, to learn.



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Where possible, several other relevant reviews on telomere homeostasis and stem cells have been referenced throughout. Unfortunately, space limitations have curtailed the inclusion of all primary literature. I thank Norman Iscove, John Dick, Guy Sauvageau, Rich Allsopp and Irv Weissman, and Jane Roskams for attempting to educate me about stem cells. The NIH (AG16629-04) provided financial support for the generation and analysis of mice deficient in mTert.