Cancer cell-specific synthetic lethal interactions entail promising therapeutic possibilities. In this issue of Cancer Cell, Pfister et al. describe a synthetic lethal interaction where cancer cells deficient in H3K36me3 owing to SETD2 loss-of-function mutation are strongly sensitized to inhibition of WEE1, a cell cycle controlling kinase

Shoaib, Muhammad

Sørensen, Claus Storgaard

Sørensen, Claus Storgaard

Copenhagen University Research Information System

Epigenetic Deficiencies and Replicative Stress:Driving Cancer Cells to an Early Grave

Elsevier - Publisher Connector 

Cancer Cell
PreviewsEpigenetic Deficiencies and Replicative
Stress: Driving Cancer Cells to an Early GraveMuhammad Shoaib1 and Claus Storgaard Sørensen1,*
1Biotech Research and Innovation Centre (BRIC), University of Copenhagen, Ole Maaløes Vej 5, 2200 Copenhagen N, Denmark
*Correspondence: claus.storgaard@bric.ku.dk
http://dx.doi.org/10.1016/j.ccell.2015.10.009
Cancer cell-specific synthetic lethal interactions entail promising therapeutic possibilities. In this issue of
Cancer Cell, Pfister et al. describe a synthetic lethal interaction where cancer cells deficient in H3K36me3
owing to SETD2 loss-of-function mutation are strongly sensitized to inhibition of WEE1, a cell cycle control-
ling kinase.Epigenetic alterations are important
emerging players in cancer causation
and progression (Morgan and Shilatifard,
2015). In the last decade, high throughput
sequencing and proteomic studies have
highlighted epigenetic abnormalities as
an important hallmark of cancer cells
(Shen and Laird, 2013). The field of cancer
epigenetics was initially largely centered
upon studying abnormalities in patterns
of DNA methylation and developing tar-
geted therapies directed toward DNA
methyltransferases. However, in the past
decade, this focus has merged with a
burst of data linking covalent histone
modifications to gene expression pat-
terns and cancer (Morgan and Shilatifard,
2015).
The epigenetic information in the form
of histone modifications is dynamically
regulated by chromatin modifying en-
zymes and helps create global and local
chromatin states, regulating DNA-tem-
plated processes. Consequently, any
disturbance in transcriptional pattern or
mutations in chromatin modifiers critically
affects these processes and can lead to
initiation and progression of various can-
cers. In this regards, recent years have
seen great progress in targeting specific
chromatin modifying enzymes, and, in
particular, those that regulate histone
acetylation and methylation (Campbell
and Tummino, 2014). Inhibitors targeting
histone methyltransferases EZH2 and
DOT1L and histone deacetylase inhibitors
are currently in clinical trials (Campbell
and Tummino, 2014). Despite these dis-
coveries and advances, scientific chal-
lenges remain, in particular, due to the
fact that there are few mutations, translo-
cations, or synthetic lethal relationships
known in cancers that directly implicatechromatin modifications and the enzymes
that create them.
Synthetic lethality (SL) describes the
relationship between two factors whereby
loss or inhibition of either is compatible
with cell viability, but loss or inhibition of
both results in cell death. SL-based
screening has shown great promise in
identifying targets with high therapeutic
efficacy, low toxicity, and high selectivity
against tumor cells (Fece de la Cruz
et al., 2015). Because, chromatin modi-
fiers are critically involved in DNA-based
cellular processes, their inactivating mu-
tations in a variety of cancer types could
potentially be exploited in combination
with drugs targeting other DNA-tem-
plated processes. In this issue of Cancer
Cell, Pfister et al. (2015) describe a syn-
thetic lethal interaction where SETD2
loss-of-function mutation renders cancer
cells extremely sensitive to inhibition of
WEE1, a cell cycle controlling kinase.
SETD2 is the sole histone H3K36me3
methyltransferase and has recently been
highlighted as a tumor suppressor in a
variety of cancer types. Furthermore, its
role in transcription, DNA repair, and
chromatin structure modulation has also
been established, highlighting SETD2 in-
activating mutations as potential targets
for SL-based cancer therapy. Interest-
ingly, by identifying a critical SL interac-
tion between H3K36me3 deficiency and
WEE1 inhibition, Pfister et al. (2015)
targeted replicative stress, a common
hallmark of all cancer cells. Although
replicative stress is one of the drivers of
malignant transformation, further height-
ening replicative stress in a catastrophic
manner may make it the Achilles’ heel of
tumor cells and could be therapeutically
targeted to drive them to cell death (Dob-Cancer Cell 28,belstein and Sørensen, 2015). Replicative
stress is the direct consequence of both
endogenous and exogenous obstacles
to DNA replication. Furthermore, an in-
sufficient supply of key DNA building
blocks such as deoxyribonucleoside tri-
phosphates (dNTPs) or dNTP precursors
decrease the overall activity of DNA poly-
merases, leading to replicative stress
(Dobbelstein and Sørensen, 2015).
The SL described by Pfister et al. (2015)
involves a critical within-pathway interac-
tion between SETD2 loss and WEE1 inhi-
bition, resulting in extremely low levels of
RRM2 protein and enhanced replicative
stress. In addition to this within-pathway
interaction, WEE1 inhibition allows firing
of inactive DNA replication origins, which
further heightens the replicative stress to
a critical level that eventually leads to
cell cycle arrest and cell death (Figure 1).
Through their research in the genetic
model organism Schizosaccharomyces
pombe, the authors first observed SL be-
tween combined loss of Set2 (SETD2 or-
tholog) and Wee1 (a WEE1 ortholog).
Later on, they extended this finding to hu-
man cancer cell lines selectively deficient
in H3K36me3, which were found to be
extremely sensitive to AZD1775, an inhib-
itor of WEE1.
WEE1 has a very well-established role
in guarding timely entry into mitosis
through control of Cyclin B1-CDK1 activ-
ity; however, more recent findings have
identified a role for WEE1 in suppression
of replication stress (Beck et al., 2010).
This is mediated by WEE1 suppression
of CDK activity in S phase, which secures
an orderly replication program without
premature initiation of DNA replication
at multiple sites. Notably, theWEE1 inhib-
itor AZD1775 blocked DNA replication,November 9, 2015 ª2015 Elsevier Inc. 545
dNTPs
RRM2
Pol SETD2
H3K36me3
Transcription
RRM2 gene
WEE1
No dNTPs
RRM2
Pol SETD2
No Transcription
RRM2 gene
WEE1
NORMAL REPLICATION
AZD1775
CELL CYCLE ARREST
CELL DEATH
ENHANCED
REPLICATIVE STRESS
REPLICATION
FORK COLLAPSE &
GENOME INSTABILITY
A
B
Pol
Po
l
INACTIVE Origin
Origin Firing
Po
l
Pol
Po
l
Pol
ACTIVE Origin
Po
l
Pol
Po
l
Pol
CDK1/2
Origin Firing
CDK1/2
Figure 1. Synthetic Lethal Interaction between Epigenetic Deficiency and Replicative Stress
(A) In normally proliferating cells, RRM2 levels are regulated at transcriptional level by SETD2-dependent H3K36me3 and at protein level by WEE1 dependent
suppression of CDK1/2 activity. Furthermore, WEE1, by inhibiting CDK1/2, also controls firing of inactive replication origins. Both SETD2 and WEE1, therefore,
control the rate of DNA replication by a regular supply of dNTPs and keeping replicative stress to a minimal level.
(B) In cancer cells carrying a loss-of-functionmutation in SETD2 leading to loss of H3K36me3, the transcription ofRRM2 is severely affected, leading to low levels
of dNTPs. In this background, treatment of cells with AZD1775, a selective WEE1 inhibitor, caused RRM2 to be degraded via a CDK1/2-dependent pathway,
causing critically low levels of dNTPs. Lack of DNA building blocks severely hampers DNA replication and causes replicative stress. In addition, inhibition ofWEE1
leads to activation of previously inactive replication origins, which further heightens the replicative stress due to dNTP starvation. Subsequently, the replication
fork collapses, leading to genome instability and cell cycle arrest and drives cancer cells to an early grave.
Cancer Cell
Previews
546 Cancer Cell 28, November 9, 2015 ª2015 Elsevier Inc.
Cancer Cell
Previewsparticularly in SETD2-deficient cells. This
was not linked with obvious DNA repair
deficiencies or the p53 response, but
rather, RRM2 levels were severely
reduced by SETD2 and WEE1 ablation.
RRM2 is a subunit of ribonucleotide
reductase (RNR) that catalyzes the for-
mation of dNTPs, the key building blocks
for DNA duplication. In the absence
of SETD2-mediated H3K36me3, RRM2
mRNA levels were more than 2-fold
reduced. Reduced RRM2 transcription
was not simply a generalized transcrip-
tional effect resulting from H3K36me3
loss, because previous RNA-seq studies
found limited expression changes in
SETD2-deficient cells. The underlying
mechanism for the RRM2 specificity re-
mains to be determined.
Next, the authors investigated how
WEE1 inhibition contributes to lethality.
Here, two modes of action were identi-
fied. First, WEE1 contributes via previ-
ously discovered control of replication
initiation (Beck et al., 2012). Second,
WEE1 promotes RRM2 stability. This is
a so-called within-pathway interaction
whereby WEE1 secures RNR activity
(Figure 1A). In the absence of WEE1,
elevated CDK1/2 activity phosphorylates
RRM2, promoting untimely degradation
via the Cyclin F ubiquitin ligase. This re-
sults in RRM2 degradation at times
when dNTPs are still needed for DNA
replication. Notably, the RRM2 inhibitors
hydroxyurea (HU) and gemcitabine (GM)
may not replace AZD1775 in targeting
H3K36me3-deficient cells. This is likely
because the within-pathway effect isinsufficient to promote cell death and the
second effect of WEE1 inhibition, aber-
rant origin firing (Beck et al., 2012), is
required to enhance replicative stress
levels, leading to SL (Figure 1B). Finally,
the authors confirmed their findings
in vivo, because AZD1775 regressed
SETD2-deficient xenograft tumors.
SL provides a conceptual framework
for discovering drugs that selectively kill
cancer cells while sparing normal tissues.
The close interplay between chromatin
modifiers and pathways regulating DNA
replication could be further exploited in
the context of discovering new SL links
between chromatin modifiers and in-
ducers of replicative stress. WEE1 inhibi-
tion in the context of H3K36me3 defi-
ciency is one such approach that
resulted in increased replicative stress
and has been demonstrated by Pfister
et al. (2015) to have the potential to treat
cancers deficient in H3K36me3. This
study paves the way for further research
into discovering links between cancers
deficient in chromatin modifications and
agents enhancing replication stress.
Furthermore, it significantly underscores
the idea of exploiting heightened replica-
tive stress in cancer cells as a source of
intoxication, which would turn their selec-
tive growth advantage into a lethal disad-
vantage. It will be highly relevant in this
regard to direct future screening efforts
toward developing SL-based therapies
by inducing replicative stress using inhib-
itors for WEE1, ATR, or CHK1 for treating
cancers with aberrations in mitogenic
pathways, DNA damage response fac-Cancer Cell 28,tors, anti-apoptotic pathways, and immu-
nomodulators. We hope that the concept
of mechanism-based target identification
will uncover new vulnerabilities in cancer
cells that can be leveraged to develop
new treatment modalities. The therapeu-
tic targeting of ‘‘Hallmarks of Cancer’’
(Hanahan and Weinberg, 2011) in combi-
nation with enhancing replicative stress
holds exciting potential to develop preci-
sion cancer therapy and improve clinical
outcome in cancer patients.REFERENCES
Beck, H., Na¨hse, V., Larsen, M.S., Groth, P.,
Clancy, T., Lees, M., Jørgensen, M., Helleday, T.,
Syljua˚sen, R.G., and Sørensen, C.S. (2010).
J. Cell Biol. 188, 629–638.
Beck, H., Na¨hse-Kumpf, V., Larsen, M.S., O’Han-
lon, K.A., Patzke, S., Holmberg, C., Mejlvang, J.,
Groth, A., Nielsen, O., Syljua˚sen, R.G., and Søren-
sen, C.S. (2012). Mol. Cell. Biol. 32, 4226–4236.
Campbell, R.M., and Tummino, P.J. (2014). J. Clin.
Invest. 124, 64–69.
Dobbelstein, M., and Sørensen, C.S. (2015). Nat.
Rev. Drug Discov. 14, 405–423.
Fece de la Cruz, F., Gapp, B.V., and Nijman, S.M.
(2015). Annu. Rev. Pharmacol. Toxicol. 55,
513–531.
Hanahan, D., and Weinberg, R.A. (2011). Cell 144,
646–674.
Morgan, M.A., and Shilatifard, A. (2015). Genes
Dev. 29, 238–249.
Pfister, S.X., Markkanen, E., Jiang, Y., Sarkar, S.,
Woodcock, M., Orlando, G., Mavrommati, I., Pai,
C.C., Zalmas, L.P., Drobnitzky, N., et al. (2015).
Cancer Cell 28, this issue, 557–568.
Shen, H., and Laird, P.W. (2013). Cell 153, 38–55.November 9, 2015 ª2015 Elsevier Inc. 547


Epigenetic Deficiencies and Replicative Stress: Driving Cancer Cells to an Early Grave 

file:///data/core-remote/dit/data/elsevier/pdf/398/aHR0cDovL2FwaS5lbHNldmllci5jb20vY29udGVudC9hcnRpY2xlL3BpaS9zMTUzNTYxMDgxNTAwMzg2NA==.pdf

Epigenetic Deficiencies and Replicative Stress: Driving Cancer Cells to an Early Grave

Abstract

Similar works

Full text

Available Versions

Copenhagen University Research Information System

Elsevier - Publisher Connector