REVIEWS
APOPTOSIS AND GENOMIC
INSTABILITY
Boris Zhivotovsky* and Guido Kroemer ‡
Abstract | Genomic instability is intrinsically linked to significant alterations in apoptosis control.
Chromosomal and microsatellite instability can cause the inactivation of pro-apoptotic pathways.
In addition, the inhibition of apoptosis itself can be permissive for the survival and ongoing
division of cells that have failed to repair DNA double-strand breaks, experience telomere
dysfunction or are in an abnormal polyploid state. Furthermore, DNA-repair proteins can regulate
apoptosis. So, genomic instability and apoptosis are intimately linked phenomena, with
important implications for the pathophysiology of cancer.
GENOMIC INSTABILITY
The failure to transmit an
accurate copy of the entire
genome from one cell to its two
daughter cells. Note that this
term does not describe a state
but, rather, a process.
CENTROSOME
A specialized organelle that
duplicates during interphase
and that constitutes the centre of
the mitotic spindle.
*Institute of Environmental
Medicine, Karolinska
Institutet, Box 210,
Nobels väg 13,
SE-171 77 Stockholm,
Sweden.
‡
Centre National de la
Recherche ScientifiqueUMR8125,
Institut Gustave Roussy,
38 rue Camille Desmoulins,
F-94805 Villejuif, France.
e-mails: [email protected];
[email protected]
doi:10.1038/nrm1443
752
Genome integrity and cell proliferation and survival are
regulated by an intricate network of pathways that
include cell-cycle checkpoints, DNA repair and recombination, and programmed cell death. Permanent or
transient GENOMIC INSTABILITY (BOX 1), which represents
one of the fundamental characteristics of cancer, might
be ascribed to deficiencies in numerous cellular
processes including mitotic-checkpoint regulation, and
DNA-damage signalling and repair, as well as telomere
maintenance and CENTROSOME function. This review will
focus on the complex interplay between genomic
(in)stability and apoptosis regulation (BOX 2) that participates in carcinogenesis.
The relationship between genomic integrity and
cell-death regulation can follow at least three different
non-exclusive patterns, all of which might be important
for the development of cancer. First, genomic instability
can lead to the mutation, or altered expression levels, of
cell-death regulators. Second, disabled apoptosis can
favour genomic instability. Indeed, numerous cellular
mechanisms enforce the rule ‘better dead than wrong’,
which means that cells that have a damaged genome or
are afflicted by many disorders will be aborted by apoptosis — thereby avoiding the propagation of potentially
oncogenic mutations. DNA double-strand breaks
(DSBs), telomere dysfunction, illicit POLYPLOIDY or
abnormal mitoses can directly trigger apoptosis
through a default pathway. However, if apoptosis is
inhibited for some reason, this increases the risk of
| SEPTEMBER 2004 | VOLUME 5
CHROMOSOMAL INSTABILITY (CIN) at several levels, and cells
that are sufficiently fit to survive can be at a growth
advantage, which can lead to cancer. Third, a single protein or process might be involved in the control of both
apoptosis and genomic instability, which allows
‘crosstalk’ between the processes.
Here, we will discuss these different possibilities,
their molecular mechanisms and their possible impact
on carcinogenesis.
Genomic instability: disabling apoptosis
Genomic instability is a hallmark of cancer, the
pathogenesis of which is also characterized by specific
genetic and epigenetic changes that can result in
defective apoptosis1. It is tempting to assume that
genetic instability, after selection, will result in the
expansion of a cell population that is relatively resistant to apoptosis induction. Because disabling apoptosis, in itself, might favour genetic instability (see
below), it becomes plausible that both mechanisms
might cooperate to increase the oncogenic and
metastatic potential of transformed cells. During the
initial stages of oncogenesis, a series of random alterations in the unstable genome can lead to a collection
of nonrandom genetic alterations that affect a
restricted set of oncogenes (for example, oncogenes
that encode apoptosis inhibitors) and tumour-suppressor genes (which might encode apoptosis facilitators). These genetic alterations would be nonrandom,
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Box 1 | Distinct types of genetic instability
Mutation of caretakers and
gatekeepers, enhanced
proliferation and telomere attrition
Tetra- or polyploidization
due to fusion, endomitosis
or failed division
Inappropriate
centrosome
duplication
End-to-end fusions resulting in
dicentric chromosomes
Loss of polyploidy/
tetraploidy checkpoint
Defective spindle
checkpoint
Asymmetric division with
unequal chromosome
distribution
Non-reciprocal translocations due
to breakage-fusion-bridge crisis
Selection and re-expression
of telomerase
Apoptosis
Selection and adaptation
Structural chromosomal
instability
Aneuploid
cancer cells
Numeric chromosomal
instability
Genetic instability can be subdivided into chromosomal instability (CIN) and microsatellite
instability (MSI), the latter being due to mutations in, or silencing of, DNA-mismatch-repair
genes (including MLH1, MSH2, PMS1, PMS2 and MLH6). CIN can be classified broadly
into numeric CIN (aneuploidy with monosomies, trisomies or higher-order polysomies)
and structural CIN (see figure). Structural CIN leads to chromosomal deletions,
translocations, homogeneously staining regions (HSRs) or double minutes (acentric
chromosomes that lack functional centromeres). Structural instability can result from the
defective activation of the DNA-damage checkpoint (the ‘gate keeper’, which involves ATM,
ATR, CHK1, CHK2, p53 and so on). In addition, structural CIN is favoured by defective
DNA double-strand break (DSB) repair by the mutation of elements of the two repair
mechanisms (‘caretakers’). These are non-homologous end joining (NHEJ), which involves
DNA-PK, Ku70, Ku80, DNA ligase-4 and XRCC4, or homologous recombination, which
involves MRE11/RAD50/NBS1. This results in the generation of broken chromosomes,
including the fusion of two centromere-containing chromosomes to form a dicentric
chromosome. Such dicentric chromosomes break during mitosis, after which they fuse
again, thereby entering the highly unstable breakage-fusion-bridge (BFB) cycle. Similarly,
telomere attrition with consequent end-to-end fusions of uncapped chromosomes can lead
to the formation of dicentric chromosomes and subsequent BFB cycles.
Numeric CIN can result from various events. These include aberrant polyploidization,
which occurs, for example, as a result of cell fusion, endomitosis or failed
division. The latter occurs, for example, as a consequence of a deficient cytokinesis
checkpoint that could be due to mutations in the anaphase-promoting complex.
Alternatively, numeric CIN can arise from inappropriate centrosome duplication with
multipolar mitosis, which occurs, for example, as a result of cyclin-E overexpression,
BRCA1 and BRCA2 mutations, Aurora-A amplification or p53 inactivation. Similarly, it
can be caused by defects in the spindle checkpoint, which can occur due to mutations in
BUB1, BUBR1 (a homologue of yeast MAD3), MAD1 and MAD2.
The main processes that lead to structural CIN or numeric CIN tend to induce
apoptosis as a default pathway that aborts cells during the crisis that is associated with
replicative senescence — for example, after DNA damage, after polyploidization or after
asymmetric cell division. Disabled apoptosis therefore increases the probability that cells
which are sufficiently fit to survive can be selected for and generate cancers.
POLYPLOIDY
A situation in which a cell has
more than two complete sets of
chromosomes in G1 or more
than four sets in G2/M: so,
triploid cells carry 3N in G1 and
tetraploid cells have 4N. Note
that polyploidy (not to be
confused with hyperploidy, a
special case of aneuploidy with
too many chromosomes) refers
to simple multiples of the
normal chromosome number.
because essential and HAPLOINSUFFICIENT genes would
have to be maintained in at least one and two copies,
respectively. Simultaneously, an increase in the copies of
genes that have adverse effects on the metabolism of
the cell or on signal transduction would have to be
avoided. These constraints apply not only to single
genes, but also to adjacent genes and functionally
interrelated genes, to maintain LINKAGE DISEQUILIBRIA2.
So, the genetic ‘signature’ of a given cancer type can
result from the selection of genetic variants of unstable
genomes.
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One well-characterized example of how genetic
instability can disable apoptosis is provided by the
HCT116 colon carcinoma cell line, which is affected
by MICROSATELLITE INSTABILITY (MSI) due to mutation of
the DNA-mismatch-repair gene MSH2 (mutS homologue-2). MSI often leads to an inactivating
frameshift mutation of BAX, a pro-apoptotic member
of the BCL2 FAMILY, which results in the increased resistance to apoptosis. This process is involved in tumour
progression in vivo, for example when HCT116 cells
are injected into immunodeficient mice3, or in vitro,
for example after selection for resistance to oxaliplatin
(Eloxatin; Sanofi-Synthelabo Inc.)4. Furthermore,
inactivating mutations of BAX seem to be under
strong selective pressure in colon carcinoma cells
with MSI, where the mutation of BAX indicates a
poor prognosis for the patient3.
CIN can also favour the deletion of master genes that
are involved in apoptosis regulation. One example is the
deletion of the short arm of chromosome 17 (17p) that
contains the p53 gene, and which can lead to the inactivation of p53, provided that the other allele is mutated
or deleted. The allelic loss of 17p, in turn, can predispose
cells to polyploidy and ANEUPLOIDY in a variety of different
cancers5,6, because the inactivation of p53 abolishes a
series of checkpoints that maintain genetic stability.
These examples point to a complex, dual relationship
between apoptosis inhibition and genomic instability.
Apoptosis represses CIN after DNA damage
The genome scrambling that is typical of cancer is most
likely catalysed by inappropriately repaired DSBs or by
eroded telomere ends (see below) that are sensed and
processed as DSBs. This is exemplified by the high incidence of cancer in DSB-repair-deficiency syndromes
and by the experimental knockout of genes that are
involved in DNA repair by homologous recombination
and NON-HOMOLOGOUS END JOINING (NHEJ). DSBs that are
repaired by the joining of heterologous ends can generate DICENTRIC or ring chromosomes that initiate BREAKAGE-FUSION-BRIDGE (BFB) CYCLES (see below), thereby generating complex non-reciprocal translocations, which
are characteristic of human carcinomas. Indeed, such
translocations can be oncogenic — because they
might carry chimeric or deregulated oncogenes at
their breakpoints, or because they can increase the
number of copies of an oncogene or delete a tumoursuppressor gene.
Under normal conditions, cells that experience a
degree of DNA damage that is beyond repair either
undergo apoptosis or enter a senescent state (BOX 3). So,
when apoptosis is inhibited, cells with altered and potentially unstable genomes, which therefore should have
been eliminated, can survive. So, it is important to
understand how DSBs can trigger apoptosis.
The DNA-damage response is sensed by proteins that
belong to the family of phosphatidylinositol 3-kinaselike kinases7,8. This family includes ATM (ataxia telangiectasia mutated), ATR (ATM and RAD3-related) and
DNA-dependent protein kinase (DNA-PK). These
kinases operate at, or close to, the sites of primary DNA
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REVIEWS
Box 2 | Morphological and biochemical definition of apoptosis
Apoptosis is morphologically defined by cellular and nuclear shrinkage (pyknosis),
chromatin condensation, blebbing, nuclear fragmentation (karyorrhexis) and the
formation of apoptotic bodies81. At the biochemical level, apoptosis of mammalian cells
is characterized by mitochondrial-membrane permeabilization (MMP) and/or massive
caspase activation82–84. Two main pathways can lead to apoptosis.
The intrinsic (or stress) pathway
MMP is the rate-limiting event in this pathway. MMP causes bioenergetic failure as well
as the release of potentially lethal proteins from the mitochondrial intermembrane
space. Such lethal proteins include caspase activators (for example, cytochrome c, which
activates the APOPTOSOME caspase-activation complex) and caspase-independent death
effectors (such as the apoptosis-inducing factor (AIF), which translocates to the
nucleus). MMP is regulated, at least in part, by proteins of the BCL2 family 85.
The extrinsic (or death-receptor) pathway
This pathway involves the activation of the plasma-membrane receptor of the TNFreceptor superfamily (for example, CD95 (also known as Apo1 or Fas), DR3 and DR4),
which leads to the receptor-proximal recruitment of a caspase-activation complex 86. The
resulting activation of caspase-8 is either sufficient to trigger the proteolytic activation of
other caspases (and to set off a powerful chain reaction) or requires the proteolytic
activation of pro-apoptotic proteins of the BCL2 family (in particular BID and BIM),
which causes MMP and therefore triggers the ‘mitochondrial amplification loop’.
p53 is an upstream inducer of apoptosis that can stimulate the expression of death
receptors (for example, DR5 and CD95) and transactivates genes that encode MMPinducing proteins, either from the BCL2 family (for example, BAX, BID, NOXA and
PUMA) or other apoptotic regulators (for example, mtCLIC, p53AIP1, ferredoxin
reductase, proline oxidase and the BAX adaptor protein ASC)63. So, p53 stimulates the
extrinsic and/or the intrinsic pathway, depending on the cellular context. p53 also transrepresses the transcription of MMP inhibitors such as BCL2 and survivin. In addition,
p53 can exert transcription-independent pro-MMP effects, either by activating
pre-existing BAX (which is pro-apoptotic)21 or by neutralizing BCL2 or BCL-XL (which
are anti-apoptotic)20.
CHROMOSOMAL INSTABILITY
(CIN). Genetic changes that are
manifested at the level of
chromatin maintenance and
segregation.
HAPLOINSUFFICIENT
A gene that requires bi-allelic
expression. Suppression of one
allele would reduce the gene
dosage below the critical level.
LINKAGE DISEQUILIBRIUM
Non-independent assortment of
genes during cell division; for
example, because they are
situated on the same
chromosome.
MICROSATELLITE INSTABILITY
(MSI). Alterations in the length
of short repetitive sequences
(microsatellites), which can be
detected by the PCR
amplification of tumour DNA.
After PCR, new bands that were
not present in PCR products of
the corresponding normal DNA
will appear if MSI has occurred.
754
damage: ATM responds preferentially to DSBs, and
ATR to single-stranded breaks that are coated with
replication protein A. In human cells, the interaction
of RAD17 protein with the 9-1-1 complex (which
comprises RAD9, RAD1 and HUS1) is responsible
for the recognition of damaged DNA9. This results in
the activating phosphorylation of downstream
kinases (such as the checkpoint kinases (CHK)1 and 2)
by ATM/ATR10. CHK2, in turn, can phosphorylate p53,
thereby increasing the DNA-binding activity of p53 as
well as its stability. The relative contribution of these
various phosphorylation events to physiological DNAdamage-induced apoptosis versus DNA repair have
been difficult to assess. But experiments have shown
that the expression of a dominant-negative mutant
form of CHK2 inhibited p53-mediated apoptosis11.
Moreover, CHK2-deficient mice show significantly
reduced radiation-induced apoptosis in neurons,
thymocytes and splenocytes12, which indicates that
CHK2 mediates cell-cycle arrest, thereby facilitating
DNA repair while inhibiting apoptosis.
Another target for ATM and ATR is H2AX, a minor
histone-H2A variant, which undergoes specific rapid
phosphorylation on Ser139 following DNA damage.
Phosphorylation of H2AX at DSBs is essential for the
recruitment of several proteins that are involved in
the regulation and/or the execution of DNA repair,
| SEPTEMBER 2004 | VOLUME 5
namely 53BP1 (p53-binding protein-1), BRCA1
(breast-cancer-associated protein-1), MDC1 (mediator
of DNA-damage checkpoint-1) and the MRE11 (meiotic recombination-11)–RAD50–NBS1 (Nijmegen
breakage syndrome-1) complex to the site of DNA
damage10. H2AX-deficient mouse embryonic fibroblasts manifest defects in the localization of DNA-repair
factors to stable foci at DSBs and develop CIN after
DNA damage13. Conversely, the phosphorylation of
H2AX is an acute consequence of DNA damage and a
predictor of DNA-damage-induced cell death14.
Surprisingly, DSBs that are induced by γ-irradiation do
not only cause acute effects but can also lead to
delayed cell death, as late as 30–35 generations after the
initial DNA lesion. This radiation-induced delayed cell
death is accompanied by the appearance of foci of the
phosphorylated histone H2AX (which indicates the
presence of DNA lesions), as well as by the activating
phosphorylation of CHK2 and p53 and the consequent reactivation of p53-inducible genes15. It is still a
matter of speculation whether genomic instability that
results, for example, from BFB cycles can account for
this delayed cell death.
The tumour-suppressor protein p53 mediates part
of the response of mammalian cells to DNA damage,
either by stimulating DNA repair or — beyond a certain
threshold of DNA damage — by initiating apoptosis
(FIG. 1). p53, which is a transcription factor, transactivates a series of pro-apoptotic proteins from the BCL2
family (in particular BAX, BID, PUMA and NOXA)16,
which induce MITOCHONDRIAL-MEMBRANE PERMEABILIZATION
(MMP) and therefore release apoptogenic factors from
the mitochondrial intermembrane space. p53 upregulates the adaptor protein ASK (activator of S-phase
kinase; which promotes the activation of BAX and its
interaction with mitochondria), as well as several proteins that locate to mitochondria. These proteins favour
MMP through oxidative reactions, such as those that
are catalysed by ferredoxin reductase and proline oxidase, or through unknown mechanisms, such as those
that are mediated by p53AIP1 (p53-regulated apoptosisinducing protein-1) and mtCLIC (mitochondrial chloride intracellular channel-4). p53 also represses the
anti-apoptotic protein BCL2, which works on mitochondria to prevent membrane permeabilization. In
addition, p53 can initiate apoptosis through proteins
that localize to the endoplasmic reticulum (ER; for
example, Scotin) or the plasma membrane (such as
CD95, DR5 and PERP). Finally, in response to DSBs,
p53 can somehow stimulate the nuclear release of histone H1.2, which then works on mitochondria to stimulate MMP17. So, p53 can engage several, in part celltype-specific, pro-apoptotic pathways and promotes cell
death by transactivating a wide array of apoptosisinducing genes16.
p53 can also induce apoptosis in a transcription-independent manner18, although to what extent this is important for DNA-damage-induced apoptosis is controversial.
p53 has been reported to bind to the outer mitochondrial
membrane and antagonize the anti-apoptotic function
of BCL2 and BCL-XL19,20. Importantly, some p53 mutants
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Box 3 | Senescence and apoptosis
In the 1960s, L. Hayflick observed that non-transformed cells undergo only a limited
number of passages, and then enter a state that he called ‘cellular senescence’87. Although
proliferation of these cells was arrested, senescent cells were metabolically active and
could be maintained in culture for several years. The so-called ‘Hayflick limit’ was linked
to the progressive shortening of chromosomal ends during cell division88. The senescence
phenotype is associated with the upregulation of several marker proteins, such as INK4A,
ARF, p53, PML (promyelocytic leukaemia), clusterin and plasminogen-activator
inhibitor. Biochemical and morphological features of replicative senescence (that is, the
situation when telomere shortening has reached a critical length) are similar to those
observed during ‘premature’ cellular senescence, which is induced by DNA-damaging
agents and other types of anti-cancer therapy. Senescence of fibroblasts is associated with
resistance to radiation-induced apoptosis89. Moreover, some senescent fibroblasts are
unable to undergo p53-dependent apoptosis and, on DNA damage, become necrotic90.
However, resistance to apoptosis is not a general feature of senescent cells, as they can also
be apoptosis prone: it is cell-type and stimuli dependent.
Senescence has been proposed as a mechanism to block immortalization and
tumorigenesis. Moreover, in addition to apoptosis, premature or inducible senescence
was identified as an effective response to chemotherapy91. Interestingly, the inactivation
of ARF, INK4a or p53 is sufficient to disable both senescence and apoptosis. Consistently,
activation of the ARF–p53 pathway is important for the effective removal of cancer cells.
Recently, it was proposed that the simultaneous stimulation of a mitogen-activated
pathway and the downstream inhibition of cyclin-dependent kinases might lead to cell
senescence92. This model can distinguish between two types of growth arrest: first, exit to
G0 phase, which can be the result of mitogen withdrawal and might lead to apoptosis;
and second, so-called hypermitogenic arrest, which might be stimulated by mitogens and
lead to senescence. Additional research is required to test this hypothesis.
BCL2 FAMILY
A family of proteins that all
contain at least one BCL2
homology (BH) region. The
family is divided into antiapoptotic multidomain proteins
(such as BCL2 and BCL-XL),
which contain four BH domains
(BH1, BH2, BH3, BH4),
pro-apoptotic multidomain
proteins (for example, BAX and
BAK), which contain BH1, BH2
and BH3, and the pro-apoptotic
BH3-only protein family (for
example, BID, BIM and PUMA).
ANEUPLOIDY
The ploidy of a cell refers to the
number of chromosome sets
that it contains. Aneuploid
karyotypes are chromosome
complements that are not a
simple multiple of the haploid
set.
NON-HOMOLOGOUS END
JOINING
(NHEJ). The main pathway that
is used throughout the cell cycle
to repair chromosomal doublestrand DNA breaks in somatic
cells. In contrast to homologousrecombination repair, NHEJ is
error-prone because it leads to
the joining of heterologous ends.
simultaneously lose the capacity to bind to DNA and to
BCL2 and BCL-XL (REF. 20). Alternatively, p53 might activate the pore-forming, MMP-inducing function of BAX21
or BAK22. So, p53 triggers numerous transcriptiondependent and transcription-independent pathways that
link DNA damage to apoptosis. This seems important in
light of the fact that the genetic or functional inactivation
of p53 can lead to genomic instability (see below).
The pro-apoptotic and cell-cycle-arresting functions
of p53 have been attributed to distinct transcriptional profiles (for example, increased transcription of
BAX for apoptosis induction versus increased transcription of p21 for cell-cycle arrest). These profiles
correlate, to some extent, with the phosphorylation of
p53 on Ser46 (which augments its pro-apoptotic
potential)16. Whether such phosphorylation events
also affect the non-transcriptional effects of p53 is
currently unknown. There are growing suspicions that
p53-independent events might link DNA damage to
apoptosis induction. Such links could involve p73,
which functions as a p53-related transcription factor23,
and NUR77 (also known as NGF1β or TR3), which is
an orphan steroid receptor that can translocate to
mitochondria and specifically interact with BCL2,
thereby inducing MMP24. There might also be a link
through CASPASE-2 — which can be activated in the
nucleus by the ‘PIDDosome’, a molecular complex that
contains the protein PIDD (a p53-inducible, deathdomain-containing protein) and the protein
RAIDD/CRAIDD (an adaptor protein containing a
death domain). Activated caspase-2 can then trigger
the mitochondrial apoptotic pathway25. However, it is
NATURE REVIEWS | MOLECUL AR CELL BIOLOGY
unclear at present to what extent, if at all, the inhibition of p73, NUR77 or caspase-2 might contribute to
genomic instability.
Telomeric instability and apoptosis
Telomeres cap the ends of eukaryotic chromosomes,
thereby preventing them from being recognized as DSBs
that can be processed by DNA-repair mechanisms. They
comprise tandem repeats of the TTAGGG sequence and
the T-loop (which is formed by the invasion of the single-stranded 3′ overhang into an upstream doublestranded region of the telomere), as well as an array of
protective telomere-binding proteins (including telomeric-repeat-binding factor (TRF)1, TRF2, Ku86 and the
DNA-PK catalytic subunit (DNA-PKcs)). The loss of
TTAGGG repeats increases with ongoing cell division,
unless the telomerase (which comprises an RNA molecule known as TERC and a catalytic protein subunit
known as TERT) is activated.
Telomere attrition or mutation of telomere-binding
proteins causes telomere uncapping with the subsequent
activation of the telomere checkpoint responses (that is,
replicative SENESCENCE and apoptosis) or, alternatively,
chromosome end-to-end fusions, which results in karyotypic disarray. Mechanistically, telomere attrition ultimately prevents the formation of the T-loop, which leads
to the formation of an unprotected chromosome end
that is detected as damaged DNA and processed by the
DNA-repair machinery (in particular the NHEJ machinery)26. So, telomere dysfunction and the associated formation of dicentric chromosomes can set BFB cycles
in motion, which produce extensive and rapid changes in
gene dosage as well as complex cytogenetic alterations
(structural CIN)27. Moreover, telomere dysfunction can
cause cytokinesis to fail, which results in tetraploidization
and the accumulation of supernumerary centrosomes,
thereby causing numeric CIN (BOX 1)28.
Inactivation of the telomere checkpoint, coupled with
growth beyond the Hayflick limit (the lifespan of normal
fibroblasts in vitro; see BOX 3), leads to a ‘crisis’ — a period
of severe telomere dysfunction that is accompanied
by rampant genomic instability and massive cell
death 29 (FIG. 2). The absence of the retinoblastoma
(Rb)/INK4a/p53-dependent senescence checkpoint (BOX
3) prevents some of the phenomena that are elicited by
short or dysfunctional telomeres, such as apoptosis or
cell-cycle arrest, but it has no effect on end-to-end fusions
in vitro. In vivo, the knockout of the p53 gene (and to
some extent also that of the p53-activating kinase that is
encoded by the Atm gene) reduces apoptosis in the gastrointestinal crypts of mice that lack the telomerase RNA
component Terc. But this does not suppress the generation of dicentric chromosomes30. So, p53 inactivation
enhances the survival of cells with short, dysfunctional
telomeres and therefore generates wholesale genomic
changes that might drive the neoplastic process31.
Importantly, it appears that only after reactivation of
the telomerase, which quells severe CIN, does a subset
of post-crisis cells emerge with a genetic profile that is
permissive for malignant progression. So, the role of
telomerase in carcinogenesis is complex, in that the
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p53-dependent
p53-independent
DNA damage
Caspase-2
Mitochondrion
P
BAX
H1.2
DICENTRIC CHROMOSOME
A chromosome that carries two
centromeres, which arise from
the aberrant fusion of ‘naked’
telomeres or interstitial doublestrand breaks.
H2AX
RAD17
NOXA
ATM/ATR
NUR77
BCL-XL
Dicentric chromosomes —
which are generated by the
fusion of two centrosomecontaining chromosome
fragments or two ‘naked’
telomeres — break apart in
mitosis, after which they fuse
again, forming a mutated
dicentric chromosome.
APOPTOSOME
A complex that forms when
cytochrome c is released from
mitochondria and interacts with
the cytosolic protein APAF1,
which, in turn, recruits procaspase-9. In the presence of
dATP, this interaction results in
the allosteric activation of
caspase-9 and in the formation
of a caspase-3 activation
complex.
MITOCHONDRIAL-MEMBRANE
PERMEABILIZATION
(MMP). A pro-apoptotic process
whereby mitochondrial
membranes undergo transient
and stable permeabilization and
become structurally
reorganized. As a result, proteins
that are normally retained in the
intermembrane space are
released through the outer
membrane and the bioenergetic
function of mitochondria is
compromised.
CASPASE
A family of cysteine proteases
that cleave after asparagine
residues. Initiator caspases are
typically activated in response to
particular stimuli (for example,
caspase-8 after death-receptor
ligation, caspase-9 after
apoptosome activation, caspase-2
after DNA damage and PIDD
activation), whereas effector
caspases (mainly caspase-3, -6
and -7) are particularly
important for the ordered
dismantling of vital cellular
structures.
756
?
9-1-1
PUMA
BCL2
BREAKAGE-FUSION-BRIDGE
(BFB) CYCLE
BAX
p73
PUMA
BAX
BAK
CHK2
p53
P
Cytochrome c
Cytochrome c
Nucleus
Cytoplasm
Pro-caspases
Figure 1 | p53-dependent and p53-independent processes that link double-strand DNA breaks to the apoptotic default
pathway. p53-mediated apoptosis might require the transcriptional activation of several genes that function at the level of the
plasma membrane (including CD95; not shown in the figure) or the mitochondria (NOXA or PUMA) to induce apoptosis. Alternatively,
extranuclear p53 protein can function in a transcription-independent manner, by physical interaction with multidomain proteins from
the BCL2 family, which leads to the neutralization of anti-apoptotic proteins such as BCL2 or BCL-XL and/or to the activation of
pro-apoptotic proteins BAX and BAK. These effects are, in part, mediated by transcription-independent mechanisms and direct
physical interactions between p53 and members of the BCL2 and the BAX family (dashed arrows). In addition, damaged nuclei can
release histone H1.2 in a p53-dependent fashion, and histone H1.2 then works on mitochondria. Alternatively, DNA damage can
trigger apoptosis by p53-independent routes that might involve caspase-2, NUR77 and p73. Whether the pathway that links DNA
damage to apoptosis is p53-dependent or p53-independent, the activation of the mitochondria-mediated pathway is essential for
programmed cell death. In the centre of the diagram, a series of proteins that are involved in the formation of DNA-damage foci (ATM
(ataxia telangiectasia mutated)/ATR (ATM- and Rad3-related), RAD17, histone H2AX, 9-1-1 and CHK2 (checkpoint kinase-2)) are
linked with both DNA-repair and apoptotic machineries.
absence of telomerase activity must be followed by the
reacquisition of telomerase function to guarantee effective immortalization after genome scrambling27.
Importantly, it is possible that the telomerase itself has an
anti-apoptotic function, which works at the pre-mitochondrial level32. This anti-apoptotic function operates
in vivo, as transgene-enforced expression of the catalytic
subunit of telomerase protects against brain injury that is
caused by ischaemia33. In theory, pharmacological inhibition of telomerase could therefore have a direct apoptosis-facilitating effect on cancer cells.
Ploidy control and apoptosis
Polyploidy can be generated by cell fusion (BOX 4) or, in
a cell-autonomous fashion, by the multiplication of
chromosomes without accompanying cellular division34. This could involve ENDOREPLICATION, as seen in
megakaryocytes, which undergo successive rounds of
DNA replication accompanied by incomplete mitoses
(which proceed through anaphase A but omit anaphase B
and cytokinesis), resulting in mononuclear polyploidy35. Alternatively, it can involve an abortive cell
cycle in which, for example, nuclear division is not
| SEPTEMBER 2004 | VOLUME 5
accompanied by cytokinesis, which produces binucleate cells36. In addition, tetraploidization can be
induced by microtubule-stabilizing agents such as taxol
or nocodazole37.
Following spindle damage, cells become transiently
arrested at the metaphase–anaphase transition point and
then escape from the block, a process known as ‘mitotic
slippage’, and exit mitosis without proper segregation of
sister chromatids and cytokinesis (FIG. 3). Cells that have a
damaged spindle then arrest permanently in a tetraploid
G1 state. This final arrest is mediated through p53,
whereas the transient arrest at the metaphase–anaphase
transition and mitotic slippage are probably not influenced by p53. p53-deficient cells that are in the tetraploid
G1 state are not prevented from re-entering the cell cycle
to reduplicate their DNA unchecked, which leads to
polyploidy and subsequent CIN.
In the presence of p53, tetraploidy (or polyploidy in
general) causes the activation of p21 and an irreversible
arrest in the cell cycle, or causes cell death, thereby preventing the propagation of errors of late mitosis and
the generation of aneuploidy38,39. The absence of p21
also relaxes the polyploidy checkpoint and causes
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Cell death
p53
Senescence
and apoptosis
End-to-end joining,
BFB cycles
Telomere attrition
or dysfunction
Genomic
instability
Re-expression
of telomerase
Cancer
Figure 2 | Telomeric instability. When telomeres become dysfunctional, ‘naked’ telomeres are
recognized as double-strand DNA breaks and trigger the p53-dependent apoptotic default
pathway. In the absence of functional p53, genomic instability results from futile end-to-end
joining and breakage-fusion-bridge (BFB) cycles, which normally result in cell death. Only upon
re-expression of telomerase can immortal cancer cells be formed.
SENESCENCE
A nearly irreversible stage of
permanent G1 cell-cycle arrest,
which is linked to morphological
changes (flattening of the cells),
metabolic changes and changes
in gene expression (for example,
β-galactosidase). The induction
of senescence depends on p53
and cell-cycle inhibitors such as
p21 and p16.
ENDOREPLICATION
The replication of DNA during
S phase of the cell cycle without
the subsequent completion of
mitosis.
PASSENGER PROTEIN
A protein that shares a
characteristic pattern of
association with chromatin in
prophase, centromeres in
metaphase and early anaphase,
and then the midzone and
midbody in late anaphase and
telophase, respectively.
ANISOCYTOSIS
Abnormal heterogeneity in cell
size.
ANISOKARYOSIS
Abnormal heterogeneity in
nuclear size and/or in the
cytoplasm:nucleus ratio.
DNA-STRUCTURE CHECKPOINT
A checkpoint that arrests cellcycle advancement until DNA
mutations such as double-strand
breaks are repaired, or until the
replication of complementary
strands has been completed.
haematopoietic MO7e cells to manifest centrosome
overduplication with polyploidy and multilobular
nuclei in response to nocodazole37. This implicates p21
as one of the main p53 target genes in this context.
Importantly, the overexpression of BCL2 allows polyploid p53-deficient cells that are generated by nocodazole treatment to survive more efficiently40. So, p53
abrogation and apoptosis inhibition can cooperate to
induce rapid and progressive polyploidization following
mitotic spindle damage. It has also been reported that
p53-dependent apoptosis occurs in cells that undergo
mitotic slippage and in aneuploid cells that are the result
of aberrant multipolar mitoses, and that this process
involves the increased expression of the death receptor
CD95 and the cell-cycle regulators p21 and B-cell
translocation gene-2 (BTG2)41.
The absence of functional p53 is permissive for the
survival of polyploid cells that can be formed in several
ways. These include the downregulation of the PASSENGER
42
PROTEIN survivin by siRNA , treatment with spindle poisons such as nocodazole43, cell fusion with polyethylene
glycol44, or endopolyploidization due to the expression
of the mitotic kinases Aurora-A, Aurora-B and pololike kinases45,46 or K-cyclin (the Kaposi’s sarcoma-associated herpesvirus cyclin-D homologue)47. Similarly,
p53-knockout mice manifest the accumulation of multinuclear cells in various organs48. The inactivation of
p53 (and Rb) by the SV40 large T-cell antigen can lead
to tetraploidy in the mouse pancreas49 as well as to cytological abnormalities (polyploidy, ANISOCYTOSIS and
ANISOKARYIOSIS) in the mouse liver, which precede hepatic
carcinogenesis50. This latter effect is prevented by liverspecific overexpression of a p53 transgene50. The link
between p53 abrogation and aneuploidy has also been
established in a series of human cancers. For example,
allelic losses of 17p (which contains the p53 gene) occur
in diploid cells of patients with Barrett’s oesophagus
before aneuploidy becomes manifest51. So, p53-dependent apoptosis could be an important mechanism that
leads to the removal of polyploid cells, which constitute
bona fide precursors of aneuploid cells.
Mitotic cell death and genomic instability
Mitotic catastrophe. Cell death that occurs during
metaphase (a phenomenon that is known as mitotic
catastrophe) can be induced by DNA damage or by
the fusion of asynchronous cells, provided that the
DNA-STRUCTURE CHECKPOINT is inactivated (for example, by
the inhibition of CHK2; REF. 52; FIG. 4). This type of cell
death is independent of p53. Although morphologically
distinct from apoptosis53, mitotic catastrophe does
involve the activation of the apoptotic machinery. This
includes signs of MMP, such as the loss of the mitochondrial transmembrane potential (∆Ψm), the mitochondrial release of CYTOCHROME C and APOPTOSIS-INDUCING FACTOR
(AIF), caspase activation and DNA fragmentation54. In
one model of mitotic catastrophe — the metaphaseassociated death of CHK2-inhibited HeLa SYNCYTIA —
the order of lethal events has been established. Here,
caspase-2, which is activated by DNA damage (see
above), functions as an initiator caspase upstream of
MMP, which, in turn, is required for the activation of
the effector caspase, caspase-3 (REF. 55).
Box 4 | Cell fusion as a source of tumour-cell diversity
Syncytium formation is a physiological process that is coupled to irreversible end-stage differentiation in the generation
of the syncytiotrophoblast, muscle fibres, osteoclasts or foreign body giant cells (cells that are formed during local
inflammation, in response to alien particles). Such syncytia are characterized by cytogamy (cytoplasmic fusion) without
karyogamy (nuclear fusion) and therefore remain polynuclear (with all nuclei in the G0/G1 phase).
Heterotropic, presumably non-physiological, cell fusion has been extensively documented in vivo, by experiments in
which stem cells that bear certain genetic markers (such as Y chromosomes, major histocompatability complex (MHC)
alleles or antibiotic-resistance genes) are transfused into recipient mice93–95. As the fusion products are mononuclear,
they must result from a combination of cytogamy and karyogamy. Initially, this phenomenon has been misinterpreted
as ‘transdifferentiation’ (the conversion of one cell type to another one). Similarly, tumour cells of human origin can fuse
with host cells when inoculated into hamsters or mice96. And, in chimeric mice, carcinogens can result in the formation
of tumours that bear markers from both parental strains97. It has been suggested that tumour cells are intrinsically more
fusogenic than their normal counterparts98.
Cell fusion might lead to the acquisition of chemotherapy resistance from both parental cells99, as well as to a
temporary resistance to apoptosis98,100. Surviving hybrid cells carry chromosomal aberrations including chromosome
dysfunctions, mitotic recombinations, deletions, insertions and inversions98. Moreover, cell fusion (for example, between
melanoma cells and macrophages) can result in tumour cells with an enhanced metastatic potential101.
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2N
2N
Cell fusion
Endoreplication
2N
p53
Apoptosis
or
Defective cytoor karyokinesis
BCL2
Diploidy
4N
Polyploidy
2N (+/– nC)
Aneuploidy
Figure 3 | Relationship between polyploidy and apoptosis. Polyploidy can result from a
variety of alterations; for example, as a result of cell fusion, endomitosis or failed cell division
(BOX 1). Polyploid cells normally undergo apoptosis following the activation of a p53-dependent
checkpoint. Inhibition of apoptosis, for example by overexpressed BCL2, favours multipolar and
asymmetric divisions and therefore results in aneuploidy. nC, number of chromosomes.
CYTOCHROME C
A haem protein that is normally
confined to the mitochondrial
intermembrane space. Following
induction of apoptosis,
cytochrome c is released from
mitochondria and triggers the
formation of the apoptosome, a
caspase-activation complex.
APOPTOSIS-INDUCING FACTOR
(AIF). A flavoprotein that is
normally present in the
mitochondrial intermembrane
space. Following the induction
of apoptosis, AIF translocates to
the nucleus where it activates a
molecular complex that causes
large-scale DNA fragmentation,
presumably in a caspaseindependent fashion.
SYNCYTIUM
A cytoplasm that contains
several nuclei within the same
plasma membrane and without
internal cell boundaries.
Syncytia are mostly generated by
cell fusion.
KARYOKINESIS
The physical separation of the
daughter nuclei at the end of
mitosis.
SPINDLE-ASSEMBLY
CHECKPOINT
A checkpoint that monitors the
correct attachment of
chromosomes to spindles in the
metaphase–anaphase transition.
Activation of this checkpoint
causes cell-cycle arrest as a result
of the inhibition of anaphasepromoting complex/cyclosome
(APC/C) and is mediated by a
cytoplasmic activity known as
cytostatic factor (CSF).
758
Importantly, the inhibition of apoptosis by the
suppression of caspases or MMP prevents mitotic catastrophe55. In conditions of disabled apoptosis, cells
can continue mitosis beyond the metaphase and complete both KARYOKINESIS and cytokinesis. Fluorescent
videomicroscopy of syncytia — in which chromosomes have been labelled with a histone H2A–GFP
(green fluorescent protein) fusion protein — shows
that, in conditions of apoptosis suppression, cells can
simultaneously divide into three or more heterogeneous daughter cells. This cellular heterogeneity is
manifest in terms of both cell size (anisocytosis) and
DNA content (anisokaryosis). Most of the cells that are
generated by asymmetric division are aneuploid55.
Similar data have been reported for a model of DNA
damage in which the simultaneous abolition of the
DNA-structure checkpoint and inhibition of apoptosis
leads to rapid aneuploidization55. So, mitotic catastrophe emerges as a special case of metaphase-associated
apoptosis, the suppression of which leads to asymmetric division and aneuploidy.
The spindle-assembly checkpoint. The SPINDLE-ASSEMBLY
CHECKPOINT delays mitotic progression until all chromosomes have achieved a proper bipolar orientation on
the mitotic spindle. The checkpoint is triggered by
KINETOCHORES that lack attached microtubules and/or
by a lack of tension in the spindle46. A defective spindle
checkpoint or imprecise sister-chromatid separation
are associated with numeric CIN. So, mutations in
BUB1, MAD1, MAD2, BUBR1 (a homologue of yeast
Mad3) or securin, which are all involved in these
processes, can participate in the induction of cancerassociated CIN56. The expression of BUBR1, a mitotic
checkpoint kinase, is significantly reduced in polyploid
cells that were created by sustained damage to the
mitotic spindle (for example, by culturing cells in the
presence of nocodazole), as well as in a significant fraction of colon carcinomas. The reduction of BUBR1
expression might be explained by increased ubiquitindependent proteolysis in polyploid cells that arises during prolonged mitotic arrest57. Re-introduction of
BUBR1 triggered the apoptosis of polyploid cells that
| SEPTEMBER 2004 | VOLUME 5
are formed by aberrant exit from mitosis, and inhibited
the growth of human adenocarcinomas that were
transplanted into nude mice57.
The mitotic kinase Aurora-B and its associated passenger proteins INCENP and survivin participate in the
tension-activated arm of the spindle-assembly checkpoint. Knockdown or pharmacological inhibition of
either of these proteins can cause defects in chromosome
segregation and cytokinesis46,58. Moreover, knockdown
of survivin expression can cause several centrosomal
defects, aberrant multipolar spindle formation and
chromatin missegregation, and these phenotypes are
exacerbated in p53 –/– and p21 –/– cells42. The knockdown
of survivin expression also causes p53-independent
MMP and apoptosis42, a finding that has been confirmed by the overexpression of a dominant-negative
survivin mutant, which triggers MMP and the rapid
release of AIF and cytochrome c 59. This points to an
important degree of networking between the spindle
checkpoint and apoptosis regulation. It remains to be
determined how survivin suppresses apoptosis —
whether it is by a direct inhibitory effect on the mitochondrial caspase activator SMAC/DIABLO60 or via a ternary
interaction with two other proteins, HBXPIP (hepatitis-B
X-interacting protein) and pro-caspase-9, which would
also result in caspase inhibition61.
Apoptosis versus cell-cycle regulation by p53
As mentioned above, p53 is frequently required for the
suppression of DSBs, ‘naked’ telomeres, polyploid
genomes or defective mitoses. Accordingly, p53-null cell
lines or tumours frequently become aneuploid. It has
been a matter of intense debate as to whether the
absence of p53 induces aneuploidy through a failure of
the cells to arrest the cell cycle or through deficient
apoptosis. Myc-induced lymphomas that arise in EµMyc-transgenic mice develop in an accelerated fashion,
both in the context of p53 deficiency or in the presence
of an additional BCL2 transgene. However, p53-null
lymphomas, which showed checkpoint defects and were
highly aneuploid, differed from BCL2-expressing lymphomas (that have intact p53), which retained intact
checkpoints and were largely diploid. This has been
used as an argument to say that the loss of p53 in
murine lymphomas causes genomic instability through
mechanisms that are not related to apoptosis62,63. In
accordance with this particular interpretation, a knockin mutation of p53, which maintained the p53-mediated G1-phase arrest (through its capacity to induce
p21) but abolished the pro-apoptotic function of p53,
reduced the frequency of thymic lymphomas as well as
the degree of aneuploidy found in such tumours, compared with p53-knockout mice64.
On the basis of these latter results, it has been postulated that cell-cycle arrest, rather than apoptosis induction, might mediate the genome-stabilizing action of
p53 (REF. 64). However, this debate is ongoing. First, p21
is now thought to have a second, apoptosis-inhibitory
function65, thereby raising doubts about the clear distinction between cell-cycle regulation and apoptosis
induction by p53. Second, some data indicate that, in
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p53
Apoptosis
CHK2
Cell-cycle
arrest at G2/M
CDK1
Metaphase
CDK1
Asynchronous
cell fusion
Mitotic
prophase
Apoptosis
(mitotic
catastrophe)
Asymmetric cell division
Figure 4 | Mitotic catastrophe as a default pathway to avoid asymmetric division. Fusion
of asynchronous cells results in a CHK2-dependent prophase arrest, which eventually activates
p53 and apoptosis, as a default pathway. The inactivation of CHK2 allows entry into metaphase,
at which point cells undergo mitotic catastrophe unless the apoptotic machinery is disabled, for
example, due to the inhibition of caspases or expression of BCL2.
KINETOCHORE
A specialized condensed
chromosomal region in which
the chromatids are held together
to form an X shape.
SMAC/DIABLO
A mitochondrial
intermembrane protein that, on
apoptotic release, can interact
with inhibitor of apoptosis
(IAP) proteins such as XIAP,
thereby inhibiting their function
and facilitating caspase
activation.
BASE-EXCISION REPAIR
(BER). The main pathway that is
responsible for the repair of
apurinic and apyrimidinic (AP)
sites in DNA. BER is catalyzed in
four consecutive steps by a DNA
glycosylase, which removes the
damaged base; an AP
endonuclease, which processes
the abasic site; a DNA
polymerase, which inserts the
new nucleotide(s); and DNA
ligase, which rejoins the
DNA strand.
NUCLEOTIDE-EXCISION REPAIR
(NER). A process in which a
small region of the DNA strand
that surrounds DNA damage is
removed from the DNA helix as
an oligonucleotide.
model organisms such as Drosophila melanogaster, the
pro-death role of p53 prevails over its anti-proliferative
action in assuring genomic instability 66. The elimination
of the D. melanogaster p53 homologue (Dmp53) causes
genomic instability, radiation hypersensitivity, apoptosis
defects (and the reduced expression of the apoptosis
effectors Reaper and Sickle in response to DNA damage), yet has no effects on DNA-damage-induced cellcycle arrest66. Finally, the genome-stabilizing and
tumour-suppressive effects of p53 might be regulated
differentially. In MYC-induced lymphomas, which
develop under the influence of a BCL2 transgene, there
is no selective pressure for losing p53 expression. This
indicates that the principal anti-tumour role of p53 is in
inducing apoptosis rather than preventing genomic
instability 62. This is at odds with the knock-in study 64,
according to which the cell-cycle-inhibitory function of
p53 (which also represses genomic instability) would
suffice for tumour suppression. Future studies will have
to address the relationship between p53, genomic instability, tumour development and cancer treatment in
defined (rather than several distinct) experimental systems, so that the contribution of p53-mediated cellcycle control versus apoptosis induction can be weighed.
Crosstalk between apoptosis and DNA repair
There are three scenarios for how apoptosis might relate
to DNA repair. First, proteins that detect DNA damage
can directly relay to the apoptotic machinery (see
above). Second, apoptosis regulators such as BCL2 can
participate in the regulation of DNA repair. Third,
oncogenic kinases can simultaneously inhibit apoptosis
and DNA repair.
There are numerous examples of proteins that participate in the detection of DNA damage and in DNA
repair and that also can stimulate apoptosis, as a default
pathway. The poly(ADP-ribose) polymerase-1 and -2
(PARP-1, PARP-2) can participate in BASE-EXCISION REPAIR
(BER), as well as in signalling pathways that lead to
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apoptosis. When highly activated, PARP causes the
depletion of nicotine adenine dinucleotide (NAD)
and/or the accumulation of poly(ADP-ribose), which in
turn stimulates MMP and the release of AIF, thereby
triggering caspase-independent apoptosis67,68. Many
proteins in the BRCA1-associated genome surveillance
complex (BASC) — which contains the breast-cancerassociated protein BRCA1, the Bloom syndrome DNA
helicase BLM, mismatch-repair proteins (MSH2,
MSH6, MLH1), replication factor C (RFC) and ATM —
also participate both in DNA repair and in signalling
DNA damage to p53. Consistent with this idea, the stimulation of p53 phosphorylation, which is induced by
DNA-methylation damage, is dependent on functional
MSH and MLH mismatch-repair proteins that are capable of recognizing O6-methylguanine69. The apoptotic
response that is elicited by O6-methylguanine is mainly
mediated by the intrinsic, mitochondrial pathway of
apoptosis70 (BOX 2).
The BLM and the Werner syndrome (WRN) DNA
helicases are involved in the unwinding of intermediates
of recombination, thereby preventing uncontrolled
recombination events. Their associated deficiencies give
rise to elevated levels of recombination (the hyperrecombination phenotype), which result in chromosomal aberrations, including the loss of heterozygosity.
Importantly, BLM and WRN can bind to p53, and BLM
and WRN deficiencies attenuate p53-mediated apoptosis following DNA damage71,72.
High levels of chromosome aberrations after treatment with ionizing radiation have been reported in
human lymphoblast cell lines that express the antiapoptotic regulator BCL2 (REF. 73). Similarly, BCL2 overexpression can increase the rate of mutagenesis that is
induced by genotoxic stress, such as the oxidative stress
that is caused by benzene metabolites. This correlates
with a reduced removal of damaged bases (8-hydroxydeoxyguanosine and thymol glycol) from the DNA of
the cell74. BCL2 overexpression (and that of the related
BCL2-family member BCL-XL) can reduce the expression of functional RAD51 protein, thereby reducing
RAD51-dependent DNA repair75. As this effect has also
been found for the BCL2 mutant in which Gly145 is
mutated to Ala, and which has lost its anti-apoptotic
potential, it has been argued that anti-apoptotic proteins
of the BCL2 family favour a mutator phenotype that is
due to the reduction of error-free DNA repair, and that
this effect is independent of its anti-apoptotic activity75.
Oncogenic tyrosine kinases can combine anti-apoptotic and anti-repair effects. The constitutively active
BCR–ABL kinase (which arises from the fusion of the
kinase domain of ABL with the BCR protein) inhibits
apoptosis through the transcriptional activation of
BCL-XL and inactivates the pro-apoptotic regulator
BAD, in part by its direct phosphorylation.
Overexpression of the p210 BCR–ABL isoform modulates NUCLEOTIDE-EXCISION REPAIR (NER) in a lineage-specific fashion; it enhances NER in myeloid cells and
reduces NER in lymphoid cells76. Moreover, BCR–ABL
might stimulate DSB repair by homologous recombination, presumably due to the signal transducer and
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REVIEWS
activator of transcription (STAT)5-mediated induction
of RAD51 (REF. 77). By contrast, BCR–ABL reportedly
downregulates DNA-PKcs, which is involved in NHEJ 78.
These phenomena might participate in the resistance of
chronic myeloid leukaemia cells to genotoxic therapies.
It has been speculated that the reduced NHEJ and the
enhanced homologous-recombination-repair activities
might be responsible for intra-chromosomal or interchromosomal deletions and chromosomal translocations that are observed in BCR–ABL-positive
leukaemias79. Another example is provided by the
oncogenic tyrosine kinase LCK, which inhibits DSB
repair as well as the DNA-damage-induced BCL-XL
deamidation, a post-transcriptional modification that
inactivates BCL-XL (REF. 80). This process might well be
involved in T-cell transformation (in the case of LCK),
as well as in transformation that is mediated by other
oncogenic tyrosine kinases.
Altogether, these findings point to an intricate interplay between regulators and effectors of the DNA-repair
and apoptosis-execution machineries. It seems that deficient DNA repair can be coupled to a failure to elicit an
apoptotic response, and this particular association could
participate in oncogenesis.
Conclusions and perspectives
As discussed here, the machinery that controls and
executes cell death and the mechanisms that stabilize or
endanger the accurate replication of the genome engage
in an intricate interplay, the detailed comprehension of
which is still in its infancy. However, a few rules have
emerged that control the relationship between apoptosis
and genome maintenance. Inhibition of apoptosis can
favour CIN at several levels. So, DSBs cause structural
CIN when the default pathway that leads to senescence
and apoptosis is blocked. Similarly, telomere dysfunction
entails rampant structural CIN only when the p53dependent senescence or apoptosis pathway is disabled.
Inactivation of p53 is also permissive for the survival of
polyploid cells. Disabling the DNA-structure checkpoint
can favour metaphase-associated death. But suppression
of this mitotic catastrophe by caspase inhibitors or BCL2
overexpression results in asymmetric division and aneuploidy. Furthermore, the spindle-assembly checkpoint is
functionally linked to apoptosis regulation by the proapoptotic BUBR1 and anti-apoptotic survivin proteins.
Oncogenic kinases can simultaneously inhibit DNA
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Acknowledgements
The authors’ own work is supported by the European Commission.
Competing interests statement
The authors declare no competing financial interests.
Online links
DATABASES
The following terms in this article are linked online to:
Entrez: http://www.ncbi.nlm.nih.gov/entrez/query.fcgi
MSH2 | p53 | TERC
Swiss-Prot: http://us.expasy.org/sprot/
AIF | ATM | Aurora-A | Aurora-B | BAK | BCL2 | BCL-XL | BLM |
BRCA1 | caspase-2 | CD95 | CHK1 | CHK2 | cytochrome c |
H2AX | histone H1.2 | NUR77 | p21 | p73 | RAD1 | survivin | TERT |
WRN
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