Atlas of Genetics and Cytogenetics

in Oncology and Haematology

OPEN ACCESS JOURNAL INIST-CNRS

Deep Insight Section

Cell cycle, checkpoints and cancer

Laura Carrassa
Laboratory of Molecular Pharmacology Department of Oncology, IRCCS - Istituto di Ricerche
Farmacologiche Mario Negri Via La Masa 19, 20156 Milan, Italy (LC)

Published in Atlas Database: July 2013

Online updated version : http://AtlasGeneticsOncology.org/Deep/CellCycleandCancerID20123.html
DOI: 10.4267/2042/52080
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 2.0 France Licence.
© 2014 Atlas of Genetics and Cytogenetics in Oncology and Haematology

Abstract: Deep insight on cell cycle, checkpoints and cáncer.

Introduction S and G2 phases represent the interphase of a
proliferating cell and constitute the time lapse between
Maintenance of genomic integrity is a pre-requisite for
two consecutive mitoses. The differentiated cells that
a safe and long lasting life and prevents development of
do not proliferate enter in the so called G0 phase which
diseases associated with genomic instability such as
is a steady state phase or resting phase (Vermeulen et
cancer. DNA is constantly subjected and damaged by a
al., 2003).
large variety of chemical and physical agents, thus cells
The progression of a cell through the cell cycle is
had to set up a number of surveillance mechanisms that
strictly regulated by key regulatory proteins called
constantly monitor the DNA integrity and the cell cycle
CDK (cyclin dependent kinase) which avoid the
progression and in the presence of any type of DNA
initiation of a cell cycle phase before the completion of
damage activate pathways that lead to cell cycle
the preceding one. The cdks are a family of
checkpoints, DNA repair, apoptosis and transcription.
serine/threonine protein kinases that are activated at
In recent years checkpoint pathways have been
specific points of the cell cycle consisting of a catalytic
elucidated as an integral part of the DNA damage
subunit with a low intrinsic enzymatic activity and of a
response and in fact dysfunctions or mutations of these
fundamental positive regulatory subunit called cyclin
pathways are important in the pathogenesis of
(Pavletich, 1999). Cyclin protein levels rise and fall
malignant tumors. Understanding the molecular
during the cell cycle, activating the corresponding cdk,
mechanisms regulating the cell cycle progression and
whereas the cdk protein levels are kept constant
checkpoints and how these processes are altered in
throughout the cell cycle. Once the complex cdk-cyclin
malignant cells may be crucial to better define the
is formed, it gets activated by the protein CAK (cdk
events behind such a complex and devastating desease
activating protein) which phosphorylates the complex
like cancer (Poehlmann and Roessner, 2010;
ensuring the subsequent phosphorylation of target gene
Vermeulen et al., 2003; Aarts et al., 2013; Kastan and
products required for the progression of the cell
Bartek, 2004).
through the cell cycle (Morgan, 1995). When quiescent
Cell cycle regulation cells are stimulated by mitogen signals, CDK4 and
The cell cycle is a succession of very well organized CDK6 are activated by association with D type cyclins.
molecular events that give the ability to the cell to These above cited cdk-cyclin complexes are important
produce the exact itself's copy. The DNA replication for the progression through the G1 phase and the
and the segregation of replicated chromosomes are the restriction point preparing the cell to the replicative
main events of the cell cycle. The DNA replication phase by phosphorylating the oncosuppressor protein
occurs during the so called S phase (synthetic phase) pRb which causes the activation of the E2F family
which is preceded by the DNA synthesis preparatory transcription factors. The activation of CDK4 and
phase (Gap1 or G1 phase), whereas the nuclear division CDK6 is followed by the subsequent activation of
occurs in mitosis (M phase) and is preceded by the CDK2 by cyclin E and cyclin A, which in turn initiates
mitotic preparatory phase (gap 2 or G2 phase). The G1, DNA replication. As the DNA replication process

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finishes, the Cdk1/cyclin B complex is activated regulatory phosphorylation of Cdk1 prevents premature
leading to mitosis (Vermeulen et al., 2003; Sherr and phosphorylation of mitotic targets and the entry in
Roberts, 1999). Until the end of G2 phase, CDK1 is mitosis (Yang et al., 1998). Other examples are the
phos-phorylated at Thr14 and Tyr15 by the kinases CDK inactivating kinases Wee1 and Myt1 located
WEE1 and MYT1, resulting in inhibition of cyclin B- respectively in the nucleus and Golgi complex
CDK1 activity. Mitotic entry is ultimately initiated by protecting the cells from premature mitosis and the 14-
depho-sphorylation of these residues by the CDC25 3-3 group of proteins that regulate the intracellular
family of phosphatases, initiating a positive feedback trafficking of different proteins such as the phosphatase
loop that stimulates cyclin B-CDK1 activity and entry Cdc25C (Peng et al., 1997). The above mentioned
into mitosis (Lindqvist et al., 2009). The activation events are very well monitored by signaling pathways
status of the cdk-cyclin complexes is also monitored by called checkpoints which constantly make sure that
negative regulation of the ATP binding site by upstream events are successfully completed before the
phosphorylation in specific residues and subsequent initiation of the next phase. It's in fact important that
reactivation by specific phosphatases which alterations in duplication of the DNA during S phase do
dephosphorylate the same residues. Inhibitory proteins not occur, to avoid the segregation of aberrant genetic
also contribute to negatively regulate the cdks by material to the daughter cells hence ensuring accurate
forming either binary complexes with cdks or ternary genetic information's transmission throughout cellular
complexes with cyclin cdk dimers (figure 1). Three generations. Lack of fidelity in cell cycle processes
distinct families of these so called cyclin dependent creates a situation of genetic instability which
kinase inhibitors (CKI) can be distinguished. The first contributes to the development of cancer desease. In
one is called INK family and is composed by four cancer, the genetic control of cell division is altered
members: p15, p16, p18 and p19. They mainly regulate resulting in a massive cell proliferation. Mutations
the G1-S transition of the cell cycle targeting to CDK4 mainly occur in two classes of genes: proto-oncogenes
and CDK6 by binding the cdk subunit and causing a and tumor suppressor genes.
conformational change of the kinases which become In normal cells the proto oncogenes products act at
inactive precluding the cyclin binding. The second different levels in pathways that stimulate proper cell
family of inhibitors is the Cip/Kip family and consists proliferation while the mutated proto-oncogenes or
of three members: p21cip1, p27kip1 and p57kip2. The oncogenes can promote tumor growth due to
components of this group negatively regulate the uncontrolled cell proliferation. Tumor-suppressor genes
cdk2/cyclinA and cdk2/cyclinE complexes whereas normally keep cell numbers down, either by halting the
they positively regulate the cdk4/6 cyclinD complexes cell cycle and thereby preventing cellular division or by
by facilitating and stabilizing the association of cyclin promoting programmed cell death. When these genes
and CDKs. The final class of inhibitors is the pRb are rendered non-functional through mutation, the cell
protein family which consists of two members: p107 becomes malignant. Defective proto-oncogenes and
and p130. These proteins, better known as tumor-suppressor genes act similarly at a physiologic
transcriptional inhibitors, act as potent cyclin E/A-cdk2 level: they promote the inception of cancer by
inhibitors by binding both to cyclin and to cdk sites increasing tumor cell number through the stimulation
(Vermeulen et al., 2003; Cobrinik, 2005). of cell division or the inhibition of cell death or cell
An additional level of cdk regulation is the control of cycle arrest. Uncontrolled cell proliferation which
nuclear import/export which can be easily exemplified evolves in cancer can occur through mutation of
by the cyclinB1-Cdk1 complex that is kept out of the proteins important at different levels of the cell cycle
nucleus through an active nuclear export until late G2, such as CDK, cyclins, CKI and CDK substrates.
when the nuclear exporting signals are inactivated by Defects in cell cycle checkpoints can also result in gene
phosphorylation ensuring nuclear accumulation. The mutations, chromosome damages and aneuploidy all of
regulation of the Cdk1-cyclinB1 complex via which can contribute to tumorigenesis.
cytoplasmic sequestration together with the negative

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Figure 1. Schematic summary of the levels of regulation of the cyclin dependent kinases (Cdk). 1 and 2. Synthesis and
degradation of cyclins at specific stages of the cell cycle. 3. Association of cdks to cyclins in order to be active. 4. Activation of the
cdk/cyclin complexes by CAK. 5. Inactivation of cdk/cyclin complexes by phosphorylation at thr14 and tyr15 (5a) and reactivation by
phosphatases acting on these sites (5b). 6. Cdk inhibitor proteins (CKI) preventing either the assembly of cdk/cyclin complexes (6a) or
the activation of the cdk in the complex (6b). The activated cdk/cyclin complexes can phosphorylate substrates necessary for transition to
the next cell cycle phase.

Targeting cell cycle regulators in mutation, silencing by methylation or homozygous

deletion of CDKN2A (encoding p14ARF and
cancer p16INK4A) (Pinyol et al., 1997). Elevated levels of
Cyclins and their associated cyclin-dependent kinases phosphorylated RB and relatively low levels of
(CDKs) are the key drivers of the cell cycle and p16INK4A may provide biomarkers of CDK4/6
specific transitions in the cell cycle are controlled dependence (Konecny et al., 2011). Mouse double
solely by specific CDKs. When this specificity is knockout studies of CDK4 and CDK6 suggest that the
maintained in tumour cells, selective inhibition of these CDK4/6 kinases are only essential in specific tissue
kinases presents a potential attractive strategy to compartments (Malumbres et al., 2004), presenting a
tumour therapy, suggesting that a therapeutic window therapeutic window where tumour cells are more
could be achieved. In normal cells, commitment for the reliant on CDK4/6 than many proliferating normal
progression through the cell cycle and beginning of tissues. CDK4/6 inhibition has great promise for the
replication process is controlled by cyclin D-CDK4/6 at treatment of multiple cancer types, and multiple clinical
the restriction point (Musgrove et al., 2011). CDK4 and studies are ongoing.
CDK6 initiate the phosphorylation of the Cyclin B-CDK1 activity, as mentioned before, governs
retinoblastoma (RB) protein family, resulting in mitotic entry and is tightly controlled by an intricate
dissociation and thereby activation of E2F transcription network of feedback loops (Lindqvist et al., 2009). A
factors which initiate the S phase gene expression number of potential issues make CDK1 a less attractive
program, including the expression of both cyclin E and target than CDK4/6. CDK1 is essential for mitosis in
CDK2, resulting in further RB phosphorylation and most normal cells, which may limit the ability to dose
ultimately S phase entry (Malumbres and Barbacid, CDK1 inhibitors in the clinic. If CDK1 inhibition
2009). Deregulation of the restriction point is a causes a reversible G2 arrest in cancer cells, it is
common event in cancer, yet CDK4/6 is a potential unclear whether a CDK1 inhibitor could be dosed
therapeutic target in only a subset of cancers. Many sufficiently to achieve tumour control and studies are
oncogenes overcome the restriction point by promoting undergoing. Polo-like kinase 1 (PLK1) and Aurora
CDK4/6 activity (Huillard et al., 2012). CDK4 can be kinase A (AURKA), promote progression through
activated more directly by point mutation/amplification mitosis. Inhibition of these kinases presents a potential
or via amplification of CCND1 (cyclin D1) (Curtis et therapeutic opportunity through inhibiting appropriate
al., 2012; Kim and Diehl, 2009), or indirectly via progression through mitosis. PLK1 is a serine/threonine

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kinase involved in centrosome maturation, spindle strand crosslinks. Inhibitors of DNA topoisomerase can
formation, chromosome segregation and cytokinesis cause DNA lesions leading to enhanced single or
(Strebhardt, 2010). Besides its mitotic functions, PLK1 double strand
is essential for inactivating or removing key break depending on which topisomerase is inhibited
components of the DNA damage response, such as and on the phase of the cell cycle. Different
CHK1 (via Claspin), WEE1 and 53BP1, to inactivate mechanisms are required to repair the damage to the
checkpoint signalling and promote cell cycle DNA backbone or to the DNA bases and the repairing
resumption (Strebhardt, 2010). Inhibition of PLK1 mechanisms may also vary depending on the different
causes cells to arrest in mitosis with a monopolar or phases of the cell cycle.
disorganised spindle followed by mitotic cell death The DNA damage checkpoint activation pathway is the
(Lens et al., 2010). The Aurora kinase family members response to a variety of internal factors (e.g. incomplete
(A, B and C) each coordinate distinct processes during DNA replication due to stalled replication forks,
cell division. AURKA is critical for centrosome reactive oxygen species-ROS) and external sources
maturation and proper formation of the mitotic spindle. (e.g. UV light, ionizing radiation-IR, DNA-damaging
Selective inhibition of AURKA leads to abnormal chemotherapeutic agents).
mitotic spindles and a temporary mitotic arrest The checkpoint activation is part of the signaling
followed by chromosome segregation errors as cells network (the DNA damage response) that involves
exit mitosis. The amplification and overexpression of multiple pathways including checkpoints, DNA repair,
AURKA has been reported in many human tumours, transcriptional regulation and apoptosis (Bartek and
including breast, colon, neuroblastoma, pancreatic and Lukas, 2007; Branzei and Foiani, 2008).
ovarian cancers, with high AURKA expression levels When DNA damage occurs, a signal transduction
being associated with poor prognosis and genomic pathway cascade is activated in which sensor proteins
instability (Lens et al., 2010). This makes AURKA an recognize the damage and transmit signals that are
attractive anti-mitotic drug target and as in fact, amplified and propagated by adaptors/mediators to the
AURKA inhibitors are currently being evaluated pre- downstream effectors that connect the checkpoint with
clinically and in clinical trials. Clinical data with the cell cycle machinery and final cell fate.
mitotic kinase inhibitors have not yet been really Generally the cell cycle progression is hampered at the
promising. The AURKA-selective inhibitor MLN8237 stage in the cell cycle where the cell was at the time of
(alisertib) had low levels of activity in a phase II study injury: before entry in S phase (G1/S phase
in unselected ovarian cancer (Matulonis et al., 2012), checkpoint), during S phase progression (intra S phase
and only modest activity was seen in initial clinical or S phase checkpoint), before mitotic entry (G2/M
trials of PLK1 inhibitors (Olmos et al., 2011). phase checkpoint) or during mitosis (mitotic spindle
However, none of these studies have yet selected for checkpoint).
potentially sensitive tumours, so further insights in The cell cycle arrest gives cell time to fix the damage
determining the most responsive tumors are required in by activating a series of DNA repair pathways. If the
future trials. damage exceeds the capacity for repair, pathways
leading to cell death are activated mostly by apoptosis
DNA damage checkpoint (by p-53 dependent and independent pathways) (Zhou
A faithful transmission of genetic informations from and Elledge, 2000).
one cell to its daughters requires the ability of a cell to Chk1 protein kinase is one of the main component of
survive to spontaneous and induced DNA damage to DNA damage checkpoints pathways and represent a
minimize the number of heritable mutations. To vital link between the upstream sensors of the
achieve this fidelity, cells have evolved surveillance checkpoints (i.e. ATM and ATR) and the cell cycle
mechanisms composed by an intricate network of engine (i.e. cdk/cyclins) (Zhou and Elledge, 2000;
checkpoint proteins that tells the cell to stop or delay Stracker et al., 2009).
the cell cycle progression providing enough time for A brief description of its network is herein summarized
DNA repair. When the damage could not be repaired to show just an example of how in general checkpoints
cells undergo apoptosis. Many different lesions can proteins are strictly interconnected and inter-related
occur in the cells which are coupled to different repair each others.
mechanisms. First, normal metabolic processes or Chk1 regulates the checkpoints by targeting the Cdc25
exposure to external ionizing radiations generate free family of dual specificity phosphatases, Cdc25A at the
oxygen radicals and can break the phospho diester G1/S and S phase checkpoints and Cdc25A and
bonds in the backbone of the DNA helix (single strand Cdc25C at the G2/M checkpoint.(Peng et al., 1997;
break). When two of these breaks are close to each Mailand et al., 2000)
other but on opposite DNA strands, a double strand Phosphorylation of Cdc25A by Chk1 at multiple sites
break (DSB) is present. Second, alkylating agents can increases proteosomal degradation of the phosphatase
modify purine bases and can cause intra strand or inter and inability of Cdc25A to interact with its cyclin/cdks
substrates.

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Chk1 phosphorylates Cdc25C at ser216, leading to normal cells also depend on a functional G1 checkpoint
formation af a complex with 14-3-3 proteins and (Dai and Grant, 2010; Ma et al., 2011). Experimental
cytoplasmic sequestration of the phosphatase (Peng et evidence showed that inhibiting the S and G2
al., 1997; Mailand et al., 2000; Zhao et al., 2002), thus checkpoints by inactivation of ATR or CHK1
avoiding activation of the cyclinB1-CDK1 complex abrogated DNA damage-induced G2 checkpoint arrest
which regulates the entry in mitosis. Chk1 is activated and sensitized cancer cells to a variety of DNA-
after DNA damage, which ultimately causes single damaging chemotherapeutic agents (Carrassa et al.,
strand (ss) DNA breaks, by ATM- and ATR-dependent 2004; Ganzinelli et al., 2008; Massagué, 2004).
phosphorylation of C-terminal residues (ser317 and Furthermore, oncogenic replicative stress may render
ser345). In particular, after formation of ssDNA breaks cancer cells sensitive to inhibitors that prevent the S
(induced for example by UV, replication stresses, DNA and G2 checkpoints as single agents. As mentioned
damaging agents), replication protein A (RPA) binds to previously, CHK1 is a key signalling kinase involved
ssDNA and recruits Rad17/9-1-1 and ATR/ATRIP in the intra-S phase and G2/M checkpoints (Kastan and
complexes, leading to Chk1 phosphorylation. Chk1 Bartek, 2004). In response to replication stress or
activation by ATR also requires mediators such as genotoxic insults, CHK1 is activated via ATR-
claspin, BRCA1, TOBP1. Indirectly, as ssDNA breaks dependent phosphorylation. During unperturbed S
also serve as an intermediate of double strand DNA phase, CHK1 controls replication fork speed and
(dsDNA) breaks, ATM too is involved in Chk1 suppresses excess origin firing (Petermann et al., 2010),
activation. ATM is recruited at the level of DSBs prevents premature activation of cyclin B-CDK1 and
(induced by IR for example) by the MRN complex may be involved in spindle checkpoint signalling
leading to Chk2 activation. ATM and MRN mediate (Zachos et al., 2007; Chilà et al., 2013, Carrassa and
DSB resection leading to ssDNA formation as an Damia, 2011). Oncogene driven replication is abnormal
intermediate structure of DNA repair, leading to Chk1 and results in high levels of replication stress, and
activation through RPA/ATR-ATRIP recruitment inhibition of CHK1 may increase the replication stress
(Bartek and Lukas, 2007; Gottifredi and Prives, 2005; to sufficiently high levels to be lethal as a single agent
Jazayeri et al., 2006). in certain contexts (Jazayeri et al., 2006; Syljuåsen et
Chk1 also plays a role in the mitotic spindle checkpoint al., 2005). The tyrosine kinase Wee1, together with
which ensures the fidelity of mitotic segregation during Chk1, has also to be considered a crucial checkpoint
mitosis, preventing chromosomal instability and protein controlling S and G2 checkpoint (Figure 2).
aneuploidy (Carrassa et al., 2009; Zachos et al., 2007; The WEE1 kinase prevents mitotic entry via inhibitory
Suijkerbuijk and Kops, 2008; Chilà et al., 2013). phosphorylation of CDK1 at Tyr15 (Lindqvist et al.,
2009). Recently, it is becoming clear that WEE1 is also
Targeting cell cycle checkpoints as required for the maintenance of genome integrity
therapeutic strategy in cancer during DNA replication (Sørensen and Syljuåsen,
The DNA damage response requires the integration of 2012; Beck et al., 2012). WEE1 controls CDK1 and
cell cycle control via checkpoint signalling to allow CDK2 activity during S phase, thereby suppressing
time for repair to prevent DNA damage before DNA excessive firing of replication origins, promoting
replication and mitosis take place. The importance of homologous recombination, and preventing excessive
checkpoints pathways in the cellular response to DNA resection of stalled replication forks (Beck et al., 2012;
damage (both endogenous and exogenous) is at the Krajewska et al., 2013).
basis of the use of checkpoint inhibitors to increase the Thus both Chk1 and Wee1 are required during normal
efficacy of cancer radio- and chemo-therapy. Chemo- S phase to avoid deleterious DNA breakage, and
and radio-therapy are strong inducers of the DNA thereby prevent loss of genome integrity in the absence
damage response pathways being able to cause of exogenous DNA damage (Sørensen and Syljuåsen,
different types of DNA damage and variably able to 2012). Several Chk1 and Wee1 inhibitors have now
activate checkpoints, and the abrogation of these been developed and tested in combination with DNA
checkpoints can potentiate the cytotoxic activity of damaging agents to increase their efficacy, especially in
various anticancer agents (Poehlmann and Roessner, tumors with a defective G1/S checkpoint (e.g. p53
2010). Targeting the S and G2 checkpoints has been defects) (Carrassa and Damia, 2011; Stathis and Oza,
considering attractive for cancer therapy because loss 2010)
of G1 checkpoint control is a common feature of cancer WEE1 inhibitors have been developed, and some have
cells (due to mutation of tumor suppressor protein p53), entered into clinical trials but clinical data are still
making them more reliant on the S and G2 checkpoints limited.
to prevent DNA damage triggering cell death, while

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Figure 2. Schematic representation of the role of Chk1 and Wee1 in regulation of the CDK-cyclin complexes involved in S phase and M
phase entry.

The pyeazolo-pyrimidine derivative MK-1775 is the lethal, causing cells to die, if they occur
most potent and highly selective inhibitor of Wee1, and simultaneously. Synthetic lethal interactions have been
has recently reached phase I (in combination with widely reported for loss and gain of function mutations.
gemcitabine, cisplatin, or carboplatin) and II studies (in The synthetic lethality-driven approach offers the ideal
combination with paclitaxel and carboplatin in ovarian cancer therapy as it allows indirect targeting of non-
cancer) (Stathis and Oza, 2010; De Witt Hamer et al., druggable cancer-promoting lesions with
2011). Most research has focused on the development pharmacological inhibition of the druggable synthetic
of CHK1 inhibitors, which have entered clinical lethal interactor and as it should be exclusively
studies. UCN 01 was the first of this type of inhibitor to selective for cancer cells, and well tolerated by healthy
enter clinical trials, but after Phase II trials it was normal cells, that lack the cancer-specific mutation,
discontinued owing to dose-limiting toxicities and a with a wide therapeutic window (Kaelin Jr, 2005;
lack of convincing efficacy that was probably due to Canaani, 2009). This concept is at the basis of the
poor specificity and pharmacokinetics. The newer, efficacy in preclinical systems of PARP inhibitors in
more specific inhibitors of CHK1 have generally been homologous recombination defective cells, due to
combined with gemcitabine in Phase I studies, in which mutation of genes such as BRCA1/BRCA2 and it has
myelosuppression was the major toxicity that led to the already undergone proof-of-principle in the clinical
termination of the trials, and no efficacy data have yet setting. Substantial durable antitumor activity was
been presented (Carrassa and Damia, 2011; Blasina et observed after treatment with PARP inhibitors in
al., 2008). Recently, a selective orally available patients with BRCA1/2-mutated cancers, including
inhibitor developed from a high-throughput screening ovarian, breast and prostate cancers (Bryant et al.,
hit, GNE-900, gave promising pre-clinical studies and 2005; Fong et al., 2009). Chk1 inhibition has been
is now undergoing Phase I clinical trials (Blackwood et proposed as a strategy for targeting FA (Fanconi
al., 2013). Anemia) pathway deficient tumors. In fact, tumor cells
deficient in the FA pathway are hypersensitive to Chk1
Synthetic lethality approach in inhibition, suggesting a possible use of these inhibitors
cancer therapy in FA deficient tumors (Chen et al., 2009). The FA
The most promising prospect for the future of cancer pathway is a DNA repair pathway required for the
treatment seems to be the exploitation of dysregulated cellular response to different DNA damaging agents,
DNA Damage Response, by the synthetic lethality including cross-linking agents (e.g. cis-platinum) in
approach. The synthetic lethality concept states that cooperation with the homologous recombination
mutations of two different genes are not lethal in the pathway.
cells when they occur at once, but are synthetically

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Figure 3. Schematic representation of the effects of Chk1 and Wee1 inhibition on CDK-CYCLIN complex regulation, that gets more
activated being unphosphorylated.

A range of sporadic tumors with genetic and epigenetic recently corroborated by other groups, suggesting that
disruption of the FA genes have been reported. at least in solid tumors this drug combination could be
Hyperactive growth factor signalling and oncogene- a very new promising anticancer strategy deserving
induced replicative stress increase DNA breakage that clinical investigation (Russell et al., 2013; Guertin et
activates the ATR-CHK1 pathway, and some examples al., 2012). Many other successful synthetic lethality
of the synthetic lethality of checkpoint or DNA repair combinations exist and many more probably need to be
inhibitors in cells harbouring activated oncogenes have explored and they will provide in the near future new
been identified. ATR knockdown was synthetically potential effective tools for cancer therapy (Reinhardt
lethal in cells transformed with mutant KRAS (Gilad et et al., 2013; Curtin, 2012).
al., 2010), and inhibition of CHK1 and CHK2
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