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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
Cell. Author manuscript; available in PMC 2010 August 7.
Published in final edited form as:
PMCID: PMC2726829

Telomere extension occurs at most chromosome ends and is uncoupled from fill-in in human cancer cells


Telomeres are thought to be maintained by the preferential recruitment of telomerase to the shortest telomeres. The extension of the G-rich telomeric strand by telomerase is also believed to be coordinated with the complementary synthesis of the C-strand by the conventional replication machinery. However, we show that under telomere length-maintenance conditions in cancer cells, human telomerase extends most chromosome ends during each S phase and is not preferentially recruited to the shortest telomeres. Telomerase rapidly extends the G-rich strand following telomere replication but fill-in of the C-strand is delayed into late S phase. This late C-strand fill-in is not executed by conventional Okazaki fragment synthesis but by a mechanism using a series of small incremental steps. These findings highlight differences between telomerase actions during steady state versus non-equilibrium conditions and reveal steps in the human telomere maintenance pathway that may provide additional targets for the development of anti-telomerase therapeutics.

Keywords: telomerase, DNA replication, replicative aging


The ends of linear eukaryotic chromosomes are capped by telomeres that protect chromosome termini from degradation, end-to-end fusion and recombination (Blackburn, 2001; de Lange, 2002). In human cells, the DNA component of telomeres is composed of ~5–15kb of double-stranded 5’-TTAGGG-3’ repeats terminating in a 3’ single-stranded overhang of ~12–300 bases (Makarov et al., 1997; Wright et al., 1997; Zhao et al., 2008). Because of the unidirectional nature of DNA polymerase and processing events, human telomeres lose ~50–200 base pairs during each cell division. Eventually, critically short telomeres trigger replicative senescence or apoptosis (Smogorzewska and De Lange, 2004). The majority of tumor cells and germ line cells overcome this proliferative limit by the action of telomerase, a ribonucleoprotein with reverse transcriptase activity that adds telomeric DNA repeats to the 3’ overhang of telomeres (Collins and Mitchell, 2002; Greider and Blackburn, 1985; Kim et al., 1994). An understanding of telomerase biology thus has important implications for both cancer and aging.

A central question in telomere biology concerns how telomerase maintains telomere length. Many reports indicate that telomerase is preferentially recruited to the shortest telomeres (Bianchi and Shore, 2008; Hemann et al., 2001; Marcand et al., 1997; Ouellette et al., 2000; Samper et al., 2001; Steinert et al., 2000; Zhu et al., 1998). In these studies the data supporting this conclusion were obtained under conditions of changing telomere length, and it is unknown whether significant preference for the shortest telomeres exists when overall telomere length is being maintained. Six essential proteins form a complex called shelterin that binds telomeric DNA, and all are believed to play important roles in the recruitment and regulation of telomerase at the G-strand (Bianchi and Shore, 2008; Palm and de Lange, 2008). Several studies indicate that disruption of C-strand DNA synthesis can also impair telomere length control (Price, 1997).

In the ciliate Euplotes, inhibition of DNA polymerase α and β during de novo telomere synthesis alters telomeric C-strand length and also causes an increase in G-strand length (Fan and Price, 1997). Similarly, propagation of S. cerevisiae strains defective for components of the lagging strand replication machinery causes a telomerase-dependent increase in telomere length (Adams and Holm, 1996; Adams Martin et al., 2000). Mouse cells harboring a temperature-sensitive allele of DNA polymerase α undergo sequential G-rich overhang elongation (that is not dependent on telomerase) followed by a marked increase in overall telomere length (that is telomerase dependent) (Nakamura et al., 2005).

Following extension of the 3' end of the G-rich strand by telomerase, fill in synthesis of the complementary C-strand completes replication of the double-stranded telomeric DNA. Increasing evidence shows that C-strand fill-in can be tightly regulated. In Euplotes, the newly synthesized G-strands are heterogeneous in length with the majority 95–100 nucleotides long. In contrast, most C-strands are exactly 84 nucleotides long, suggesting that the length of the C-strand is the more precisely regulated step (Price et al., 1994). End-processing of the C-strand is also more specific than that of the G-strand. Up to 80% of the C-rich strands terminate in CCAATC-5’ in human cells, whereas the G-terminal nucleotide is less precise (Sfeir et al., 2005).

S. cerevisiae telomeres replicate late in S phase due to late firing of subtelomeric origins of DNA replication (McCarroll and Fangman, 1988). Elongation of an artificially shortened telomere coincides with semiconservative telomere replication, which occurs very late in S phase (Marcand et al., 2000), suggesting coupling between the two processes. De novo addition of a telomere to a newly created double strand (ds) break adjacent to a telomere seeding sequence also suggests coupling between the replication machinery and telomerase. Using an inducible HO endonuclease to control the timing, Diede and Gottshling (Diede and Gottschling, 1999) showed that telomerase addition could not occur during G1 but that it could in M-phase arrested cells. This allowed them to explore the role of essential DNA polymerase components after a normal S-phase was completed. Telomere addition of even the G-rich strand was blocked in the absence of Pol α, Pol δ and DNA primase (Diede and Gottschling, 1999), reinforcing the concept that telomerase addition and C-strand fill-in are coupled to the replication complex.

Human telomeres replicate throughout S phase (Ten Hagen et al., 1990; Wright et al., 1999), providing the opportunity to distinguish whether telomerase extension/processing is a late S/G2/M process or is directly coupled to semiconservative replication. Recent in situ analysis indicates an S-phase-specific co-localization of telomerase with small numbers of telomeres at any one time (Jady et al., 2006; Tomlinson et al., 2006), consistent with telomerase extension occurring during the heterogeneous replication of telomeres throughout S phase. However, direct proof that telomerase action is restricted to this part of the cell cycle is lacking. We developed several independent assays to monitor G-strand extension and C-strand fill-in. We demonstrate that telomere elongation by telomerase occurs within 30 minutes of telomere replication throughout S phase, and that 70%-100% of the ends in Hela and H1299 cells are extended during each cell cycle. Telomerase is thus not preferentially recruited to the shortest telomeres under steady state conditions of telomere length maintenance. We also observed the unanticipated result that C-strand fill-in is delayed until late S phase. The demonstration that fill-in is uncoupled from telomerase extension defines a two-step model for telomere maintenance in cancer cells, implies an unexpected regulatory step in telomere extension, and provides a target for telomerase-inhibition therapies.


C-strand fill-in is delayed after telomerase elongation

Single Telomere Length Analysis (STELA) (Baird et al., 2003) can determine the length of individual human telomeres, and thus their elongation by telomerase during a single cell cycle. For this purpose, the approximate timing of replication of the target telomere needs to be established, telomeres need to be short and uniform enough so that small changes can be detected, and telomeres need to be elongating so that a change in length can be detected. We first determined the timing of replication of the Xp/Yp telomere in human fibroblasts. ReDFISH (Bailey et al., 2004; Zou et al., 2004) (Fig. 1A) produces a specific pattern of hybridization of telomeres that have replicated during a pulse of 5-bromodeoxyuridine/bromodoxycytidine (BrdU/BrdC). The removal of the BrdU/BrdC-containing strands allows hybridization of fluorescent C-rich and G-rich probes to only one of the two daughter chromatids at each end. The fractions of ends exhibiting this pattern are then determined by hourly pulses during S phase. 70% of the Xp/Yp telomeres replicate between 2–4 hours following release from a hydroxyurea block (Fig. 1B). This timing, occurring relatively early, provided a good window for distinguishing events coupled to replication from those that occur late in S-phase.

Figure 1
Timing of replication of the Xp/Yp telomere

Human fibroblasts have telomeres that are sufficiently long so that measuring small changes during a single S-phase would be difficult. Therefore, a clone of A549 lung cancer cells was picked that had very short telomeres (~3.5kb). This clone was treated with the telomerase inhibitor GRN163L (Herbert et al., 2005) in order to induce further telomere shortening. Finally, it was transduced with a retrovirus overexpressing the human TERT catalytic subunit in order to cause rapid enough telomere elongation to measure in a single cell cycle. Telomere length immediately following infection was 2.5kb, which elongated at about 500 bp/division during the first few divisions (Fig. 2A). Cells were synchronized at G1/S using aphidicolin immediately after selection for antibiotic resistance, and then samples were harvested every two hours for FACS (Fig. 2B) and STELA (Fig. 2C–E). Approximately 100 individual telomeric bands were analyzed at each time point. The size of the Xp telomere increased by approximately 250 bp between 4 and 6 hours after release into S phase. FACS analysis indicated that ~50% of the cells entered S phase following release (Fig. 2B, time points 6 and 8 hours), so the measured rate of telomere elongation by STELA (~250 nt) corresponds closely with the value predicted from telomere restriction fragment gels (~500 bp ÷2) (Fig. 2A).

Figure 2
Dissociation of telomerase extension and C-strand fill-in in human cancer cells (A) Telomere restriction fragments (TRF) analysis of hTERT retrovirus infected A549 cells. Telomere length increased at ~500bp/PD. (B) FACS analysis of DNA content in BJ fibroblasts ...

Comparison of the FACS profile of BJ fibroblasts synchronized with hydroxyurea and A549 cells synchronized with aphidicolin show similar rates of progression through S phase, so most of the Xp/Yp telomeres in A549 cells should have replicated by 4 hours after release (Fig. 2B). ReDFISH analysis of A549 cells confirmed that most Xp/Yp replication had occurred by 4 hours after release (data not shown). However, STELA products only elongated after 6 hours (Fig. 2D–E). This raised the possibility that telomerase action might be delayed following replication. However, the STELA procedure used above measures the size of the C-rich telomeric strand, and telomerase might have elongated the G-rich strand at an earlier time. The samples were thus reanalyzed using a modification in which a long C-rich oligonucleotide is first annealed to the telomeric overhangs (Sfeir et al., 2005). This "platform" oligonucleotide creates a single-stranded C-rich overhang to which an appropriately modified G-telorette oligonucleotide can be annealed and ligated to the G-strand (Fig. 2F). Amplification then occurs based upon the length of the G- rather than the C-strand. G-strand STELA products exhibited an ~250 nt elongation between 2 and 4 hours after release into S phase, consistent with the timing of replication of the Xp/Yp telomere (Fig. 2G–H). This established that there is a significant delay between the time at which telomerase elongates the G-strand (2–4 hours) and when the extended overhang is filled in (starting at 6 hours). Control experiments using asynchronously dividing cells showed only the expected ~100 nt difference between G- and C-strands due to the 3’overhangs (see Figure S1 in the Supplemental Data).

Overhangs become longer during S phase

A delay between the extension of the G-strand by telomerase and the fill-in of the C-strand on the Xp/Yp telomere predicts that overhang length on other telomeres should become longer during S phase than in either G1 or G2. Global overhang length was analyzed using DSN (duplex specific nuclease), which digests total double-stranded DNA to <10 bp while leaving the single-stranded telomeric overhang intact (Zhao et al., 2008). Because of the anticipated large size of these overhangs, the DNA was run on agarose rather than the polyacrylamide gels usually used for this analysis. Fig. 3A shows that total overhangs become hundreds of nt longer during S phase, confirming the STELA results that very long G-overhangs accumulate on many different chromosome ends in these cells and that C-strand fill-in is delayed until the end of S phase.

Figure 3
Overhang elongation during S phase

The A549 experiments used cells overexpressing telomerase, which might have altered normal replication and processing. We thus examined overhang length in synchronized Hela cells expressing only endogenous telomerase. Because of their heterogeneity, the overhangs exhibit a wide distribution of sizes on polyacrylamide gels (Fig. 3B). After adjusting the signal intensity for size (since longer overhangs hybridize to more probe), the average overhang size was compared at different times after release into S-phase. Total overhang length increased from ~65 nt to ~105 nt by 6 hours after release, dropping back to 65 nt by 10 hours. The results from this DSN analysis were compared to that from native gel hybridization, in which the signal hybridizing to the overhang in double-stranded (native) telomeres is compared to the total telomere signal following denaturation. This only gives a relative value for the size of the overhang rather than a quantitative measure, but it confirmed the progressive elongation of the overhangs during S phase (Fig.3C). Repetitive analysis of logarithmically growing Hela cells did not show a change in relative overhang length (see Figure S2 in the Supplemental Data).

In order to increase the resolution for how quickly telomerase acted following replication, we looked at overhang lengths in telomeres labeled during a very short BrdU exposure. Hela cells were released into S phase and then either continuously exposed to BrdU or pulsed for 30 minutes in mid-S and harvested immediately. DNA that had replicated during the 30 minute pulse was separated from unreplicated and previously replicated DNA on CsCl gradients (Fig. 3D). The increased amount of BrdU incorporated into the G-rich telomeric sequence allows the separation of daughter strands synthesized by leading and lagging strand synthesis (Chai et al., 2006). Figure. 3E shows that the average size of both leading and lagging strand overhangs is elongated within 30 minutes of replication. Figure 3F quantitates the differences shown in 3E as a function of size, and shows that at 30 min twice as many overhangs are longer than 50 nt (60–70% vs 30%) than at 8 hours for both leading and lagging daughter telomeres.

Direct evidence of elongation by telomerase

S. cerevesia telomeres exhibit a transient increase in overhang length at the end of S-phase (Wellinger et al., 1996; Wellinger et al., 1993). Because this transient increase is also observed in the absence of telomerase, it is presumed to represent a resection of the C-rich strand. Although we did not observe a statistically significant decrease in C-strand STELA Xp/Yp products during the first four hours after release into S phase, the standard deviations are large and we do not believe we could have detected a 50 nt decrease in length. It thus remains formally possible that the increase in overhang length observed in Fig. 3 is due to C-strand resection rather than telomerase elongation. In order to provide direct evidence for elongation by telomerase, we examined the BrdU content of lagging strand overhangs. The parental G-strand provides the template for lagging strand synthesis, and following replication initially contains only thymidine in synchronized cells released into S-phase in the presence of BrdU (Fig. 4A). However, if telomerase acts it will incorporate BrdU into the newly synthesized GGTTAG repeats. If C-strand fill-in is delayed, these overhangs will then contain a mixture of thymidine and BrdU containing repeats and should have an intermediate density on CsCl gradients. Depending on the amount of elongation, C-strand fill-in might then convert the thymidine-containing portion of the overhang to ds DNA, leaving a fully BrdU substituted overhang. Since the G-rich strand is the newly synthesized strand during leading strand synthesis, its overhangs will be fully BrdU substituted regardless of whether or not telomerase has acted.

Figure 4
Direct detection of telomerase addition during S phase

Despite their small size and thus high rates of diffusion, we were able to separate thymidine containing and fully BrdU-substituted overhangs on CsCl gradients (Fig. 4B). The signals were abolished by treatment with the 3' to 5' nuclease Exo I prior to DSN digestion, showing that the signals on slot blots used to analyze the gradient were not due to small undigested dsDNA (data not shown). Synchronized Hela cells were released for 3 hours in the presence of BrdU. Total DNA was digested with DSN and the surviving single-stranded G-rich overhangs were separated on CsCl gradients. The intermediate density peak half-way between the thymidine and fully substituted overhangs directly demonstrates that telomerase elongation of the lagging overhangs had occurred (Fig. 4C). This intermediate density peak was absent in similar experiments performed on normal human fibroblasts (telomerase-negative) and Hela cells in which telomerase had been inhibited with the hTR-template antagonist GRN163L, demonstrating that it required active telomerase to be produced. Probing the slot blot with a G-rich probe to identify the presence of any C-strand DNA surviving the DSN digestion gave only background signals (Fig. 4D), further confirming that the intermediate peak represented G-rich single-stranded overhangs.

Delay of C-strand fill-in until late S phase

The time course of telomerase extension and C-strand fill-in was examined at different intervals after release from a G1/S block. Figure 5B shows that increasing amounts of intermediate density overhangs accumulated until 4–6 hours after release. By 8 hours, the intermediate density peak disappeared, and the relative intensity of the thymidine and BrdU overhang peaks matched the FACS profile of cells still in G1 versus those in late S/G2 (Fig. 5A). The early appearance of intermediate density overhangs and their late conversion into fully substituted overhangs confirmed that telomerase action and C-strand fill-in are uncoupled, and extension products do not get filled-in until late S phase.

Figure 5
Telomerase extension versus C-Strand fill-in in Hela cells. (A) FACS analysis of synchronized Hela cells released into S phase for different times in the presence of BrdU. (B) Time course of overhang density distribution during S phase. The intermediate ...

The data in Figure 5B was obtained in the continuous presence of BrdU. In order to follow a single cohort of replicated telomeres, cells were pulsed with BrdU for the first 2.5 hours after release from a G1/S block and then chased for different times (Fig. 5C). A distinct peak of intermediate density overhangs appeared during the first 2.5 hour pulse, which persisted at the 5 hour time point before disappearing in DNA isolated 8 hours after release (Fig. 5D). Thus, even the telomeres that had replicated at the earliest time point had their overhangs extended by telomerase early, and delayed C-strand fill-in until late S.

Most lagging strand telomeres are extended by telomerase each cell cycle

It is currently unknown whether telomerase maintains telomeres by adding small amounts to most of the telomeres during each cell division or by adding much more to a small fraction of the ends. The intensity of the intermediate density peak in Fig. 4Fig. 5 suggested that a large fraction of the ends were being extended, and the intermediate density suggested that the amount added was approximately equal to the size of the newly replicated lagging daughter overhang. In order to directly determine the fraction of extended lagging ends, we labeled Hela cells with BrdU for the first four hours following replication, and then purified lagging daughter telomeres on CsCl gradients (Fig. 6A). These telomeres were then digested with DSN and their overhangs reanalyzed on a second CsCl gradient. Telomeres that had not been elongated by telomerase should contain only thymidine (Fig. 4A). Most of the replicated telomeres exhibited overhangs with an intermediate density (Fig. 6B), indicating that in Hela cells telomerase is maintaining telomere length by elongating most of the ends during every cell division. Fitting the observed distribution to overlapping Gaussian distributions indicated that ~18% of the signal was in the thymidine-only peak (Fig. 6C). The position of the intermediate-density peak half-way between the thymidine only and fully BrdU substituted peaks implies that the telomerase-extended overhangs are twice as long and have roughly equal numbers of thymidine and BrdU-containing repeats. They will thus give twice the signal per overhang when probed. Normalizing the signals for this difference indicated that ~70% of the telomeres were elongated by telomerase during every cell cycle (Fig. 6D). The fraction of ends elongated by telomerase was also examined in a second tumor cell line, H1299 lung adenocarcinoma cells. In these cells, 100% of the lagging daughter overhangs exhibited an intermediate density and were thus elongated by telomerase during a single cell cycle (Fig. 6 E&F). Analysis of the unreplicated and leading fractions yielded overhangs containing only thymidine or BrdU respectively (Fig. 6 G&H).

Figure 6
Fraction of lagging daughter telomeres extended by telomerase in one cell cycle. (A) DNA from G1/S synchronized Hela cells released in BrdU for 4 hours was separated on CsCl gradients. (B) Four fractions from the lagging strand peak in (A) were pooled ...


The investigation of telomerase action in cells has been challenging. Telomere length measurement on gels by TRF permits monitoring the long-term consequences of telomerase activity but only provides an average of many telomeres over many divisions. The STELA based method allows the length of chromosome-specific telomeres to be measured during a single cell cycle under conditions of changing telomere length. The BrdU/DSN method is able to monitor groups of replicating telomeres, and has the advantage that it provides a direct measurement of telomerase activity on lagging strand daughter telomeres in cells expressing only their endogenous telomerase. Both STELA and DSN demonstrate that telomerase extension occurs soon after replication. This is consistent with yeast observations (Teixeira et al., 2004) and in situ cytological evidence showing the S-phase-specific association of telomerase with telomeres (Jady et al., 2006; Tomlinson et al., 2006). However, the demonstration that fill-in is uncoupled from extension and delayed until late S phase represents an entirely unanticipated result that indicates an additional regulatory step in telomere replication. Blocking this step could potentially increase the efficacy of telomerase-inhibition of cancer cells.

Cell cycle-dependent regulation of telomerase extension

In yeast, passage of the replication fork is required before telomerase can act (Dionne and Wellinger, 1998; Marcand et al., 2000), and adding a de novo telomere during M-phase requires pol α, pol β and primase components of the replication complex (Diede and Gottschling, 1999). This suggests a coupling of telomerase action and telomere replication. The asynchronous timing of human telomere replication and the in situ demonstration that hTERT and hTR (hTERC) only associate with a few telomeres during any specific time during S-phase (Jady et al., 2006; Tomlinson et al., 2006) is consistent with telomerase being recruited to act rapidly following replication. Our results provide direct evidence that telomerase elongates human telomeres within 30 minutes following replication.

C-strand fill-in at the end of S phase

ChIP analysis has revealed the association of many DNA repair factors at telomeres at the end of S-phase (Verdun et al., 2005). In addition to ATM and the MRN complex, the localization of factors involved in the repair of stalled replication fork such as ATR, Rad17, Fen1 and DNA polymerase β led to the hypothesis that telomere replication might be stalled during S-phase ("phase I"), and that restart, completion of synthesis and G-overhang generation only occurred in late S/G2 ("phase II") (Verdun and Karlseder, 2006). Several lines of evidence argue against this interpretation. First, STELA of the Xp/Yp established that telomerase extends the G-strand 2–4 hours after release into S-phase (Fig. 2). In order to be compatible with stalled forks at telomeres, this would require telomerase to act before replication of the 3' end of the telomere was complete. This has been considered as a formal possibility (Chakhparonian and Wellinger, 2003). This could produce the increased overhang size observed as S phase progresses (Fig. 3). However, in the absence of completion of telomere replication all of the overhangs would be composed of unreplicated thymidine-containing overhangs that had not been extended by telomerase or intermediate density overhangs that had been extended, and there should be no fully-substituted overhangs. Approximately equal amounts of intermediate and heavy overhangs were observed, even at the earliest time points measured (Fig. 5B). Furthermore, when the overhangs from unreplicated DNA were examined, no heavy or hybrid density overhangs were found (Fig. 6G). This thus eliminates the possibility that telomerase might act on stalled replication forks prior to the completion of both leading and lagging daughter telomere replication. Second, if telomerase acted prior to replication, one would expect normal lagging strand synthesis to copy much of the extended overhang, preventing the appearance of the observed intermediate density overhangs in the DSN analysis and producing an early elongation of the C-strand by STELA. This was not observed. Finally, stalled replication forks would yield a complex pattern and not the clean separation of leading versus lagging telomeres on CsCl gradients within 30 minutes of replication that was observed (Fig. 3D).

Because a free G-rich overhang is required at both elongation and fill-in steps, it is difficult to imagine a mechanism by which t-loops could be maintained both during replication and during C-strand fill-in. It is unknown at present what fraction of ends are in t-loops, whether or not t-loops re-fold immediately following replication or whether telomeres remain as linear molecules with "free" 3'-overhangs until after C-strand fill-in occurs. The recognition of telomeres as damaged DNA at the end of S-phase (Verdun et al., 2005) is consistent with a signal for C-strand fill-in, t-loop re-folding or both. It is also possible that t-loops refold after replication and then are unfolded again at the end of S-phase in order to permit the final stages of C-strand replication to occur.

Incremental C-strand fill-in

An obvious mechanism for C-strand fill-in would use the conventional lagging-strand machinery, in which pol α/primase produces an RNA primer that is extended and added to the end of the C-strand. If the primer was synthesized ~50–100 nt from the end of the extended G-strand, the newly synthesized "Okazaki-like fragment" would rapidly convert the thymidine-containing portion of the single-stranded lagging overhang to double-stranded DNA, so that the remaining overhang would be fully substituted with BrdU. This would cause each molecule in the intermediate density pool of extended overhangs to rapidly transit from an intermediate to a heavy density, so that the intermediate peak should decrease and the heavy peak should increase with time. This is not what was observed. The six hour time point in Fig. 5B clearly shows that the intermediate density overhang peak has shifted to higher but not fully substituted densities, implying an incremental fill-in of the C-strand. The mechanism for this incremental process remains to be determined.

Fraction of extended ends

Approximately 70 % of Hela and 100% of H1299 lagging strand telomeres are elongated during one cell cycle under telomere steady state maintenance conditions, in which total telomere length is neither elongating nor shortening. Telomerase activity has been shown to be sporadically absent in Hela cells. Approximately 35% of Hela clones lacked telomerase activity immediately after cloning but became positive with further culture (Bryan et al., 1998). This suggests that almost all of the telomeres are being extended in the ~65% of Hela cells that are telomerase positive at any point in time, and that the fraction of extended ends is close to 100% in both Hela and H1299 cells. Preliminary data suggests that the majority of ends in human ES cells are also extended by telomerase during every cell cycle (data not shown). This is very different from the pattern observed in yeast, where only a small fraction (~7%) of ends are extended (Teixeira et al., 2004). Yeast telomeres only shorten at a rate of ~3 nt/division in the absence of telomerase (Marcand et al., 1999). Since yeast telomerase adds an average of 44 nucleotides when it acts on wt telomeres >200 in length (Teixeira et al., 2004), it could not extend a large fraction of ends without increasing overall telomere length. In contrast, human telomeres generally shorten at a rate of 50–100 nt/division in cell culture. In order to maintain a constant telomere length, human telomerase would thus either have to add kilobases of repeats to a small fraction of ends or 50–100 nt to most of the ends. The size of the intermediate density lagging daughter overhangs extended by telomerase is about 100 nucleotides (Fig. 3E), and its density implies an equal number of thymidine and fully-substituted repeats. This implies that the lagging-strand overhang prior to telomerase elongation is about 50 nt and that telomerase adds about 50 nt. Hela telomeres shorten by ~45 bp/cell division when telomerase activity is inhibited (Fig. S3 Supplemental data). The increase in size of ~50 nt to 70% of the ends during each cell cycle is roughly consistent with the amount that would need to be added to counteract telomere shortening. The amount of elongation produced following telomerase action is thus similar in S. cerevisiae and humans (44 vs ~50 nt), while the fraction of ends extended is an order of magnitude larger in humans (7% vs 70%-100%).

In the absence of changes in the rate of shortening, any model of telomere homeostasis requires that telomerase acts more when telomeres are too short and less when telomeres get too long. The shelterin complex has been implicated as part of the mechanism by which length is measured, since reducing shelterin components causes telomere elongation and overexpression of shelterin components leads to telomere shortening (reviewed in (Bianchi and Shore, 2008; Palm and de Lange, 2008)). There is currently a general consensus that telomerase is preferentially recruited to the shortest telomeres (Bianchi and Shore, 2008; Hemann et al., 2001; Marcand et al., 1997; Ouellette et al., 2000; Samper et al., 2001; Zhu et al., 1998). However, all of the above studies examined non-equilibrium conditions in which telomere length was changing. Although telomerase is preferentially recruited to the shortest telomere when it is restored to yeast mutants with artificially shortened telomeres, under length maintenance conditions in wt yeast telomerase randomly extends ends independent of length (Teixeira et al., 2004). Siince 70% (Hela) and 100% (H1299) of telomere ends are extended in every cell cycle in cancer cells maintaining a stable telomere length is inconsistent with a significant preference between telomeres. Under length maintenance conditions, although occasional telomeres may become too long or too short and be preferentially ignored or elongated, the vast majority of ends are treated equally. This suggests that the sensitivity for determining telomere length is sufficiently coarse that under maintenance conditions telomerase does not distinguish between slightly longer and slightly shorter telomeres, while it can be preferentially recruited to the shortest telomeres when the departure from the appropriate maintenance length is large. Regulation of the recruitment/action of telomerase on telomeres thus varies with context and is likely to depend on multiple different factors.

Extension of leading ends

The increased length of leading daughter telomeric overhangs during S-phase (Fig. 3E) is consistent with the hypothesis that telomerase is elongating both leading and lagging daughter chromatids. In contrast to telomerase-negative cells, in which lagging overhangs are ~3 times longer than leading overhangs (Zhao et al., 2008), fully processed leading and lagging overhangs are approximately the same size in telomerase-expressing cells (Fig. 3E & Fig. 3F and (Chai et al., 2006)). However, the initially extended overhangs of leading telomeric daughter strands are smaller than that of lagging daughters (70 vs 100 nt, Fig. 3E). This is consistent with the concept that immediately following replication the presumably blunt leading daughter is processed to yield a shorter overhang than is present on lagging daughters. However, it is currently unknown how much C-strand processing occurs before telomerase acts. It is possible that, as in yeast, a transient long G-rich overhang occurs that is later filled in, so that the simple measurement of an increased length does not rigorously distinguish between C-strand resection and telomerase elongation. However, the ~50nt added by telomerase to 70% of the lagging ends is unlikely to be sufficient to maintain telomere length unless telomerase is also adding ~50 nt to 70% of the leading ends. If this is the case, the size of the 30% non-extended leading overhangs would be ~35nt (the same size as leading overhangs in telomerase-negative fibroblasts (Zhao et al., 2008)) and that of the 70% extended overhangs would be 85nt prior to C-strand fill-in (yielding the observed average of 70 nt). Our data thus supports a working model that shortly following replication leading overhangs in Hela cells are ~35nt, lagging overhangs are ~50nt, and that telomerase adds ~50nt to 70% of both leading and lagging ends.

The processivity of telomerase on model substrates increases from one to about four in the presence of both Pot1 and TPP1(Wang et al., 2007). Four repeats accounts for only half of the observed ~50nt telomerase extension. Either other factors are contributing to telomerase processivity in vivo, or telomerase is acting more than once on each telomere per cell cycle.

The end-replication problem does not produce shortening of the lagging daughter telomere, so telomerase extension of most of these ends should produce continuous telomere elongation. Significant resection of the elongated G-strand (so that net elongation of lagging daughter telomeres would be small) would result in the intermediate density overhangs becoming lighter rather than heavier as G-strand processing and C-strand fill-in took place, rather than progressing to the fully substituted density at the end of S phase (Fig. 5B). Stochastic events (such as oxidative damage or failure to re-establish stalled replication forks) that can produce telomere shortening in mammalian telomeres are likely to balance the elongation of both leading and lagging daughters so that a steady state maintenance length is achieved.

Figure 7 summarizes these observations. If all telomeres are packaged into t-loops, then they must unfold during the passage of the replication fork, and following replication some initial processing should occur in order for telomerase to be able to elongate the (presumably) initially blunt leading strand daughter. Initial processing could also occur on lagging daughter telomeres. Between 70–100% of lagging (and leading) daughter telomeric ends are then rapidly (within 30 minutes) elongated by telomerase. C-strand fill-in that is not coordinated with replication/telomerase action but is delayed by many hours until late in S phase. This implies that telomeres must be in an unpackaged configuration (not in t-loops) at two very different times: at specific times during S-phase (to permit chromosome-specific end-replication) and again at the end of S-phase (to permit C-strand fill-in). It is currently unknown whether telomeres refold into t-loops following replication or whether they remain linear until after C-strand fill-in at late S/G2. Fill-in is incremental rather than rapid, as would be expected if it were performed by the conventional lagging-strand replication machinery. An additional step of end-processing must occur following fill-in that leaves the final C in the sequence 3'-CCAATC-5' as the terminal C-strand nucleotide (Sfeir et al., 2005). Finally, the telomeres would be re-folded into t-loops. This model raises multiple new steps of potential regulatory control that may be exploited for therapeutic applications. The ability to assay telomerase during a single cell cycle provides a powerful technique to address many of the issues raised by these observations.

Figure 7
Model for the replication of mammalian telomeres. See text for details.

Experimental Procedures

Cell culture and Treatments

Hela cervical carcinoma, H1299 lung adenocarcinoma and A549 lung adenocarcinoma cells were cultured at 37°C in 5% CO2 in medium containing 10% cosmic calf serum (HyClone, Logan, UT). A clone of A549 cells with short telomeres (TRF=3.5kb) was treated with lµM GRN163L every three days for one month before infection with retroviral pBabepurohTERT and analysis by STELA. Telomerase activity in Hela was inhibited by the addition of 2µM GRN163L (Geron Corp., Menlo Park, CA) for one week with two treatments at three day intervals before DSN analysis.

Cell cycle synchronization

Exponentially growing HeLa and H1299 cells synchronized with thymidine (2 mM) for 19 hr, washed with pre-warmed PBS (3x), then released into fresh medium for 9 hr. Thymine (2 mM) was then added for 17 hr followed by washing with pre-warmed PBS (3x) before release into fresh medium containing 5-bromo-2′-deoxyuridine (BrdU, 100µM) for 0–24 hr.

BJ and A549 cells were grown in low serum medium (DMEM with 0.1% calf serum and 20mM HEPES) for 2 days, then treated with 4µg/ml aphidicolin (overhang analysis) or hydroxyurea (timing of Xp/Yp replication) in fresh medium containing 10% serum for 24h. Cells were the washed 3x with pre-warmed PBS and released into fresh medium for 0–10h.

Strand-specific in situ Hybridization: Replicative Detargeting FISH (ReDFISH)

ReDFISH was performed as described (Zou et al., 2004). Briefly, synchronized BJ cells were released and 10 µM BrdU / 3.3 µM 5’-bromo-2’-deoxycytidine (BrdC) was added for 1h pulses at increasing times. Colcemid was added after 10h, metaphase spreads were prepared 1h later, and sequentially hybridized to cy3 and cy5 labeled telomeric probes after nicking the DNA with UV and digesting with ExoIII. The fraction of Xp/Yp ends exhibiting differential hybridization to the two chromatid arms was then determined.

C- and G-STELA

Procedures were performed as described (Sfeir et al., 2005) with minor modifications. Briefly, 320ng EcoRI digested genomic DNA was incubated in 20µl (1× ligase buffer, 50 U T4 ligase from NEB, 1 µM of six C-telorettes [0.16µM for each]) at RT for 12h. Multiple PCR reactions (28 cycles of 95°C for 15 s, 62°C for 20 s, and 68°C for 10 min) used Hi-Fidelity PCR Master Mix (2x provided by Abgene) in 25µl containing 2ng ligased DNA and 0.5µM primers (XpYpE2 forward primer and C-Teltail reverse primer). Amplification products transferred onto positively charged nylon membranes (Hybond N+, GE healthcare life sciences) were fixed by baking at 85° for 2h and hybridized with a subtelomeric probe (generated by PCR using XpYpE2 and XpYpB2 and labeled by random priming). The membrane was exposed to a PhosphorImager screen and scanned.

For G-STELA, EcoRI digested genomic DNA was incubated with 1nM platform oligonucleotide (AATCCC)10 at RT for 3h. The subsequent ligation and PCR amplification followed the same conditions as above except that six G-telorettes were used for ligation and a G-Teltail reverse primer was used for PCR.

Genomic DNA purification

Genomic DNA was isolated using the DNAeasy kit (<5M cells) or Blood&Cell culture Midi kit (Qiagen, Valencia CA). Eluted DNA was re-precipitated by adding two volumes 100% ethanol, washed twice with 70%ethanol and suspended in distilled water at <1 µg/µl. Overnight incubation at 37°C ensured that all the DNA was completely solubilized.

DSN Overhang assay and CsCl gradient separation of telomeric overhangs

The DSN assay was performed as described (Zhao et al., 2008). For the density analysis of overhangs, typically 50µg of purified genomic DNA from cells grown in 100 µM BrdU was digested with 2 U of DSN (Duplex Specific Nuclease, Evrogen, Russia) at 37°C for 2h. Digestion was stopped by adding EDTA to 20mM and the sample was mixed into CsCl with an initial density of 1.78 g/ml containing 5 mM Tris pH 8.0 and 1 mM EDTA. Samples were centrifuged at 60,000 rpm for 20 h at 25°C in a VTI80 vertical rotor (Beckman). Fractions were collected and analyzed for density (refractive index). Telomeric overhangs in each fraction were detected by slot blot and hybridization to a high specific activity C-rich telomeric probe under non-denaturing conditions. Double-stranded leading and lagging daughter telomeres were isolated and purified from CsCl as previously described (Zhao et al., 2008).

TRF analysis

Mean telomere length was evaluated by TRF analysis. Isolated DNAs were digested with Hinf I and Rsa I and resolved on a 0.7% agarose gel. The denatured and dried gel was hybridized with 32P-labeled oligonucleotides [(TTAGGG)4] and exposed to a PhosphorImager screen. The weighted mean telomere length was calculated as described previously (Ouellette et al., 2000).

Supplementary Material



This work was supported by grants AG01228 and P50 CA70907 from the National Institutes of Health, fellowship support from the American Federation for Aging Research (AFAR) to Y.Zhao, and the Department of Defense Breast Cancer Program to CB and TC. We thank Angela Diehl for help in preparing Fig. 1.


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