Biology Reference
In-Depth Information
ubiquitously expressed
in vivo
, it is possible that the
therapeutic window for activity in normal cells versus
tumor cells will limit their use as radiation sensitizers.
Nonetheless, several preclinical studies have indicated
the potential utility of these agents. For example,
NU7441 was shown to sensitize 42 of 49 B-cell lympho-
cytic leukemia cell
suppresses its nuclear import, which in turn suppresses
DNA-PK activity.
278
Synthetic Lethality and PARP Inhibitors
Perhaps one of the most significant new findings in
the DNA repair field in recent years has been that
BRCA-deficient breast cancer cell lines are extremely
sensitive to small molecule inhibitors of the DNA
SSB sensing enzyme, poly(ADP-ribose) polymerase
(PARP).
279,280
These findings have been rapidly trans-
lated into the clinic and the PARP inhibitor olaparib
has shown a great deal of promise in phase 1 and phase
2 clinical trials against BRCA deficient breast and
ovarian cancers.
281
e
283
Inhibition of PARP is thought to
lead to the accumulation of DNA SSBs that require
repair by BRCA-dependent HR repair. PARP inhibition
is therefore non-toxic to normal cells but toxic to cells
defective in BRCA1 or BRCA2.
143,284
This concept has
been referred to as “synthetic lethality,” a term bor-
rowed from molecular genetics, in which two genes
are synthetic lethal if mutation of either alone is compat-
ible with viability but mutation of both leads to cell
death.
285
Although first described for BRCA1 and 2,
experimental studies have suggested that the concept
can be applied to other genes in the DNA damage
response pathway, including ATM.
286
Indeed, PARP
inhibitors have been shown to target ATM deficient
lymphoid malignancies, including mantle cell
lymphoma
287
and B-cell chronic lymphocytic
leukaemia
288
and it is possible that this approach may
be applicable to human tumors characterized by defi-
ciencies in other DNA repair genes as well.
lines to the topo II
inhibitor,
mitoxantrone.
272
Small Molecule Inhibitors of DNA Ligases
The ultimate purpose of NHEJ is to facilitate DNA
ligation, therefore DNA ligases are also considered
attractive therapeutic targets. An
in silico
screen for
small molecules that bound the catalytic domain of
DNA ligase I yielded a number of potential DNA ligase
inhibitors, one of which, L189, had activity against
NHEJ-specific DNA ligase IV,
273
suggesting that struc-
ture based strategies may also prove useful in designing
inhibitors to DNA repair enzymes.
Small Molecule Inhibitors of PNKP
An alternative target is the strand-break processing
enzyme PNKP. Given its role in removing 3
0
-phosphate
and 3
0
-phosphoglycolate groups at DSB termini gener-
ated by IR and other agents, it is not surprising that
downregulation of PNKP by shRNA reduces the rate
of DSB repair and sensitizes cells to IR and the topoiso-
merase I poison, camptothecin, but not to the topoiso-
merase II poison, etoposide.
274
A screen for small
molecule inhibitors of the PNKP phosphatase activity
led to the identification of an imidopiperidine
derivative A12B4C3 (2-(1-hydroxyundecyl)-1-(4-nitro-
phenylamino)-6-phenyl-6,7a-dihydro-1
H
-pyrrolo[3,4-
b
]
pyridine-5,7(2
H
,4a
H
)-dione) that similarly sensitizes
cells to IR and camptothecin, but not to etoposide.
275,276
A12B4C3 hinders the repair of radiation-induced strand
breaks by altering PNKP conformation and is thus
a non-competitive inhibitor of PNKP.
276
CONCLUDING REMARKS
In this chapter we have described the main cellular
pathways for the detection and repair of DNA DSBs
and illustrated how knowledge about these important
pathways has the potential for clinical applications.
Yet, despite many advances in our understanding of
NHEJ and DSB repair over the past decade clearly
much more work remains to be done particularly in
understanding the mechanism of NHEJ, and how it
interfaces with chromatin as well as other repair path-
ways, in particular HR and ATM-dependent signaling.
We envisage that as our understanding of these impor-
tant pathways increases so too will the opportunities
for translating these findings into the clinic.
Alternative Approaches to NHEJ Inhibition
The discussion above focused on small molecule
inhibitors specifically targeting NHEJ proteins.
However, alternative approaches have also been
explored. An interesting example is the use of an adeno-
viral vector system to overexpress a truncated form of
XRCC4 that inhibited NHEJ by interfering with DNA
ligase IV. This dominant negative approach, which
makes use of a mutated protein that can bind to its
partner proteins but lacks or prevents protein function,
was able to sensitize MDA-MB-231 breast cancer cells
to IR.
277
Another potentially useful approach is the use
of inhibitors of proteins that directly or indirectly modu-
late NHEJ. One example is the monoclonal antibody,
Cetuximab, which binds to the ligand-binding domain
of the epidermal growth factor receptor (EGFR) and
References
1. Langlois RG, Bigbee WL, Jensen RH, German J. (1989) Evidence
for increased in vivo mutation and somatic recombination in
Bloom's syndrome.
Proc Natl Acad Sci USA
1989;
86
:670
e
4.
