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Jan n&v final.indd
Miguel Godinho Ferreira Each round of DNA replication results in the erosion of telomeres, the ends of linear chromosomes. Telomerase counteracts telomere shortening by synthesizing new DNA sequences in a time period restricted to late S and G2 cell-cycle phases, even though the enzyme is active throughout the cell cycle. A recent study directly implicates cyclin dependent kinase 1 (Cdk1) in the regulation of telomere elongation during the cell cycle.
In recent years, the telomere field has been fac-ing an interesting riddle: although telomerase
purified from cell extracts is active throughout the cell cycle, telomeres are only extended in late S and G2 phases. Carol Greider and col-leagues now provide evidence for the direct involvement of the cell-cycle master regulator Cdk1 (ref. 1).
Telomeres have unique properties that dis-
tinguish them from damage-induced DNA double-strand breaks (DSBs). Alterations in these properties have been shown to promote
genomic instability associated with cancer, as well as to ultimately limit the proliferation of
both normal and cancer cells. Telomeres are
composed of small tandem arrays of duplex
repeats with a G-rich 3ʹ strand protruding
beyond the double-stranded region to form
an overhang. Both double- and single-stranded
telomeric regions must be maintained, and protective proteins bind specifically to each Figure 1 Telomere elongation follows the cycles of activation of Cdk1. Cdk1 regulation allows for two structure. These proteins fulfill two main func-
stages in the cell cycle: in early G1 (low Cdk1 activity), telomeres, together with DSBs, are refractive
tions: first, they protect chromosome-ends to 5ʹ-end degradation. Consequently, unresected ends at DSBs favour NHEJ and a refractory status
for telomeres. During S, G2 and M phases (high Cdk1 activity), an MRN-dependent nuclease activity
from illegitimate DNA repair, which can cause
degrades 5ʹ-ends thus generating extended ssDNA both at telomeres and DSBs. These extended
fusions or extensive degradation; second, they
3ʹ-overhangs allow for the engagement of telomerase at telomeres and homologous recombination-
promote the complete replication of chromo-
some-ends by engaging telomerase — a reverse transcriptase that uses an integral RNA compo-
Similarly to other organisms, yeast telomer-
The MRX complex is involved in the formation
nent as a template to lengthen the G-rich strand
ase activity can be reconstituted from extracts
of the telomeric 3ʹ-overhang in S-phase cells,
derived from different phases of the cell cycle4.
thus allowing the binding of Cdc13 and the
During telomere DNA replication, leading-
However, in synchronized cells, telomere elon-
subsequent recruitment of telomerase5,6.
strand synthesis generates a blunt-ended mol-
gation is observed only in late S phase3. Frank et
ecule, whereas lagging-strand synthesis leaves
al. used a telomere-healing assay that relies on
3ʹ-overhangs at DSBs was recently shown to be
behind a small G-rich overhang. To elongate
the formation of a new telomere ‘seed’ (gener-
under the control of Cdk1 (ref. 2). This finding
these ends, telomerase requires an extended 3ʹ-
ated in vivo by HO endonuclease) that is subse-
begged the question of whether Cdk1 regula-
overhang that is provided by a nuclease activ-
quently elongated by telomerase1. This method
tion of MRX was also involved in generating
ity under the control of Mre11–Rad50–Nbs1
was previously used to show that a telomere the telomeric 3ʹ-overhang and, consequently,
(MRN), a protein complex recently shown to
seed resulting from a DSB could be extended
telomere elongation. Using a drug-regulatable
be involved in relaying the cell-cycle status to
by telomerase in G2/M phases, but not in G1-
Cdk1 mutant7, Frank et al. showed that tel-
arrested cells. It also provided a means to test
omere elongation requires Cdk1 activity, pos-
Telomere elongation has been best studied
the genetic requirements for telomerase activ-
sibly through the MRX-dependent generation
in budding yeast using two different assays3,4.
ity at telomeres. Apart from requiring telom-
of the 3ʹ-overhang both at telomere seeds and
erase, its regulators (Est1-3, Tlc1 and Cdc13)
at native telomeres1. This result is reminiscent
Miguel Godinho Ferreira is in the Telomere and
and DNA polymerases α and δ4, elongation of the current model for cell-cycle regulation of
Genome Stability Laboratory, Instituto Gulbenkian de
of this broken-end also needs the MRX com-
DSB repair, in which low Cdk1 activity favours
Ciência, Apartado 14, 2781-901 Oeiras, Portugal. e-mail: [email protected]
plex5 (the budding yeast version of MRN). non-homologous end joining (NHEJ) DNA
NATURE CELL BIOLOGY VOLUME 9 | NUMBER 1 | JANUARY 2007
repair in G1 because of inefficient resection of
response, similar to DSB repair, so that ATM
origins of replication), homologous recombina-
DSBs2. Conversely, high Cdk1 activity favours
and Mre11 are specifically recruited to telom-
tion repair takes over as the major DSB repair
homologous recombination repair in the eres in G2 phase12. Thus, passage of a repli-
pathway and telomeres become available for
remainder of the cell cycle, due to the activation
cation fork could be a prerequisite for MRN telomerase. If the extrapolation from DNA rep-
of MRN-dependent resection of DSBs, making
binding and processing of telomere ends. This
lication holds true, we would expect that multi-
them better substrates for homologous recom-
hypothesis would also explain the seemingly
ple mechanisms linked to Cdk1 are involved in
bination repair. Thus, an attractive unifying conflicting result from the two telomere-elon-
controlling the elongation of telomeres. These
theme seems to be emerging (Fig. 1): telomeres
gation assays3,4: in one assay, telomeres could
may include cell cycle-dependent protein deg-
and damage-induced DSBs are viewed by the
be elongated from broken telomere seeds in radation (for example, an essential component
cell in a similar manner8. When Cdk1 activity
G2/M blocked cells, but this was not the case
of the telomerase complex, Est1, is degraded by
is low in G1, MRN is unable to promote resec-
in an independent assay, where telomeres the proteosome in G1; ref. 14), activating and
tion at terminal DNA ends, leading to NHEJ
were shortened by removing internal telomere
inhibitory phosphorylation events dependent
at DSBs and exclusion of telomerase from tel-
repeats by the flp recombinase. In the latter on Cdk1 and physical exclusion mechanisms
omeres. Later in the cell cycle, Cdk1 becomes
assay, although telomere length increases in (such as nuclear exclusion of regulatory com-
active, permitting MRN activity both at DSBs
S phase, they fail to elongate further in G2/
ponents). What is missing in this puzzle is the
and at telomeres; this engages the homologous
M-blocked cells3, suggesting that telomeres discovery of critical Cdk1 substrate(s) in tel-
recombination (HR)-repair machinery and tel-
would be refractory to telomerase later in the
omerase at their respective substrates.
cell cycle. However, normal telomere length is
However, like in every good riddle, several
recovered in subsequent cell cycles, as replica-
1. Frank, C. J., Hyde, M. & Greider, C. W. Mol. Cell24,
new questions arise. Perhaps the most pressing
tion forks traverse the telomere and allow MRN
et al.Nature 431, 1011–1017 (2004).
one is the role of DNA replication in the resec-
3. Marcand, S., Brevet, V., Mann, C. & Gilson, E. Cell
cycle restriction of telomere elongation. Curr. Biol.10,
tion of telomere-ends. Resection of telomere
Studies from cell-cycle regulation of DNA
4. Diede, S. J. & Gottschling, D. E. Cell 99, 723–733
ends and consequent telomere lengthening replication have provided the groundwork for
was found to require the passage of a replica-
most of our current understanding of ‘once-per-
5. Diede, S. J. & Gottschling, D. E. Curr. Biol.11, 1336–
tion fork3,9. One possible explanation could be
cell cycle’ events. Similar to the establishment of
6. Larrivee, M., LeBel, C. & Wellinger, R. J. Genes Dev.18,
that, although Cdk1 activation of MRN can competent origins of replication, cycles of Cdk1
et al.Nature 407, 395–401 (2000).
occur at newly formed DSBs, MRN can only
activation work as a binary switch between two
8. Ferreira, M. G. & Cooper, J. P. Genes Dev.18, 2249–
access telomeres after they have been opened
periods in the cell cycle13. In one stage Cdk1
9. Dionne, I. & Wellinger, R. J. Nucleic Acids Res.26,
by the passage of a replication fork. Consistent
activity is lacking (early G1), cells establish
with this hypothesis are the observations that
competent origins of replication, possess higher
10. Aylon, Y., Liefshitz, B. & Kupiec, M. EMBO J.23,
MRX-dependent resection of DSBs can occur
levels of NHEJ and telomeres are refractive to
11. Vodenicharov, M. D. & Wellinger, R. J. Mol. Cell24,
in late G1 (in a cdc7ts block) but telomeres telomerase. In the second stage, which com-
12. Verdun, R. E., Crabbe, L., Haggblom, C. & Karlseder, J.
are left unresected in an early S-phase block prises late G1 until M phase, Cdk1 is active,
Mol. Cell20, 551–561 (2005).
(hydroxyurea arrest)10,11. In human cells, rep-
DNA replication can be initiated (in addition
13. Diffley, J. F. Curr. Biol.14, R778–R786 (2004). 14. Osterhage, J. L., Talley, J. M. & Friedman, K. L. Nature
lication of telomeres triggers a DNA-damage
to the consequent inhibition of new competent
Struct. Mol .Biol.13, 720–728 (2006). NATURE CELL BIOLOGY VOLUME 9 | NUMBER 1 | JANUARY 2007
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