Biology Reference
In-Depth Information
of FANCD2, as well as foci formation of 53BP1, phospho-
ATM, NBS1, BRCA1, FANCD2, and RAD51.
332
In addi-
tional studies, chemical inhibition of the proteasome
suppressed HRR in a repair substrate as well as MDC1
degradation and subsequent BRCA recruitment.
333,334
However, proteasome inhibition affects multiple cellular
pathways and to which degree disrupted HRR accounts
for cell kill has not been established. Clinical trials
combining a proteasome inhibitor such as bortezomib
with DNA damaging therapeutics in malignancies such
as multiple myeloma and ovarian cancer were successful
and there are now a multitude of clinical trials involving
proteasome inhibitors.
335
However, howmuch of this effect is due to possibly sup-
pressed HRR or other mechanisms is unknown. In addi-
tion, the combination of EGFR kinase inhibitors with
platinum-based regimens in non-small cell lung cancer
has failed to produce a clinical benefit. Additional
preclinical studies are needed to determine how EGFR
might promote HRR processes.
c-Met Inhibition
c-Met is a receptor tyrosine kinase, similar to EGFR,
and implicated in cancer cell growth and survival.
Recent evidence has also implicated this receptor in
the promotion of HRR activities. Mutational activation
of c-Met was found to stimulate c-Abl and RAD51,
and this could be reversed by pharmacological inhibi-
tion of c-Met with SU11274.
343
In another preclinical
study on eight glioblastoma cell lines
in vitro
and
in
vivo
, the c-Met inhibitor MP470 (SuperGen) was found
to suppress RAD51 levels, increase DSB levels, and
cause radiosensitization.
344
However, this inhibitor has
multiple targets in addition to c-Met and it remains
thus unknown whether the effect on HRR is mediated
via c-Met or another mechanism.
Fanconi Anemia Pathway Inhibition
The FA pathway regulates aspects of HRR through its
downstream effectors FANCD2, FANCI, BRCA2/
FANCD1, and FANCN/PALB2 (see “HRR at Stalled or
Collapsed DNA Replication Forks” above). Disruption
of mono-ubiquitination of FANCD2 causes hypersensi-
tivity to crosslinking agents and likely impairs HRR,
though the requirement for mono-ubiquitination for
replication associated HRR is not entirely clear.
163
In
a cell-based screen, D'Andrea and colleagues identified
four inhibitors of the FA pathway, including curcumin,
a natural compound derived from the rhizome of Cur-
cuma Longa, an East Indian plant and regarded as safe
in humans.
336
Curcumin blocked mono-ubiquitination
of FANCD2 through an unknown mechanism and sensi-
tized breast and ovarian cancer cells to cisplatin by
causing apoptosis. Several clinical trials using curcumin
as anticancer agent are in progress, though we are not
aware that any of these trials has incorporated correla-
tive science to address the status of FANCD2 in tumors.
HDAC Inhibition
Histone deacetylase (HDAC) activity counteracts acet-
ylation of histones, causing compaction of the DNA/
histone complex and blocking gene transcription and
cell differentiation.
345
HDAC inhibitors cause growth
arrest, differentiation, apoptosis, and interfere with
DNA repair. These compounds have entered clinical
trials in monotherapy or combination therapy, including
combination with radiation and DNA damaging chemo-
therapeutics. Interestingly, some of these inhibitor effects
may bemediatedby a disruption of HRR.
346,347
In a recent
study by Parvin and colleagues, HDAC9 and 10 were
found to promote HRR, and treatment of HeLa cells
with HDAC inhibitors or depletion of HDAC9/10 by
RNA interference caused mitomycin C sensitivity. The
emerging picture is that HDACs regulate DNA repair
on chromatin and different HDACs regulate different
DSB repair processes. Specific inhibitors of HDAC9/10-
dependent HRR have not yet been developed.
MRN Inhibition
The MRN complex has multiple functions in DSB
signaling and repair including promotion of HRR.
Recently, in a forward chemical genetic screen, an inhib-
itor of this complex, mirin, was identified.
337
Mirin pre-
vented MRN-dependent activation of ATM, inhibited
Mre11 exonuclease activity, abolished the damage
induced G2/M checkpoint, and impaired HRR.
However, mirin does not only affect HRR but also DSB
repair by NHEJ.
338
We are not aware that this compound
has yet entered clinical testing.
Targeting ATR-Chk1 and ATM-Chk2
Pathways in Cancer
The ATR-Chk1 and ATM-Chk2 DNA damage
signaling pathways present attractive therapeutic targets,
not only because of their role in promoting HRR, as
detailed above, but also because the cell-cycle function
of these pathways is likely required by HRR-deficient
cells to tolerate and survive the high levels of DSB that
accumulate in the absence of fully functional HRR.
EGFR Inhibition
The epidermal growth factor receptor (EGFR) regu-
lates cell survival and growth pathways.
339
While there
exists no established link to the HRR machinery, small
molecule tyrosine kinase inhibitors have been shown to
reduce RAD51 expression in preclinical cell line
models.
340,341
EGFR inhibitors are known to sensitize
cells to chemotherapeutic agents and radiation
in vitro.
342
