Biology Reference
In-Depth Information
ERCC1/XPF
ERCC1/XPF is a heterodimeric protein complex
required for NER.
15,37
It also has roles in recombination,
DSB repair, interstrand crosslink (ICL) repair and telo-
mere maintenance.
38-44
ERCC1/XPF is a structure-
specific endonuclease that cleaves at the 5' side of
a ssDNA-dsDNA junction.
45,46
The endonuclease active
site resides within the XPF protein, but activity requires
ERCC1.
47
Structural analysis reveals that ERCC1 DNA
binding activity helps position the XPF protein for
DNA cleavage.
47
e
49
During NER, ERCC1/XPF is
recruited to the damaged DNA site and interacts with
XPA and RPA, respectively, which helps position the
nuclease to cleave the damaged DNA strand.
37,50,51
In
ICL repair via HR, ERCC1/XPF plays a role in the later
stages of the pathway and helps resolve the DNA struc-
tural intermediates.
38
cancer cells to chemotherapeutic agents like cisplatin.
Using this approach, cancer cells gain 2
8-fold sensi-
tivity to chemotherapy when ERCC1 and XPF protein
levels are reduced.
62
e
66
In addition to targeting the enzymatic activity of
ERCC1/XPF, transcriptional regulation of ERCC1/XPF
has also been shown to be effective in the treatment of
cancers. The use of cetuximab, a chimeric IgG1 mono-
clonal antibody targeting the epidermal growth factor
receptor, has been shown to reduce the expression of
XPF and ERCC1.
67,68
Cetuximab inhibition of the
EGFR signaling pathway and ultimately a reduction in
MAPK and AKT activation, results in down-regulation
of ERCC1 and XPF protein.
68,69
The combination of
cetuximab with platinum-based chemotherapy has
been shown to have better efficacy than monotherapy
alone in both cell culture and xenograft models.
67,68
These
data suggest that small molecules that target the path-
ways that regulate the expression of DNA repair genes
can enhance chemotherapeutic efficacy.
e
ERCC1/XPF Protein Levels Mediate
the Response to Chemotherapy
Testicular cancer is curable when treated with
cisplatin, bleomycin, and etoposide.
52
All three drugs
target DNA but affect different DNA repair pathways.
It has been shown that testicular cancer cells repair the
damaged DNA at a slower rate and have lower levels
of DNA repair proteins.
28
Recent evidence suggests
that the hypersensitivity to cisplatin, specifically, is
a consequence of deficiencies in ICL repair and low
ERCC1/XPF levels.
31
In these studies, over-expression
of ERCC1/XPF in testicular cancer cells increased
cisplatin ICL repair and resulted in increased cisplatin
resistance.
31
These data highlight the importance of
low ERCC1/XPF protein levels in cisplatin hypersensi-
tivity in testicular cancers. In addition to findings in
testicular cancer, recent studies have implicated
ERCC1 as a prognostic indicator in bladder and non-
small cell lung cancers (NSCLC).
53
e
55
Low levels of
ERCC1 or a polymorphism in the ERCC1 gene in patient
populations correlate with better clinical outcomes
following cisplatin treatment in both cancers.
55
e
57
ERCC1/XPF has been shown to be over-expressed in
cisplatin resistant ovarian cancer cells.
32,58
e
60
The higher
ERCC1/XPF levels correlate with faster DNA repair of
the cisplatin DNA adducts. In addition to findings in
ovarian cancer cells, recent evidence suggests that high
levels of XPF in some NSCLC patients results in resis-
tance to chemotherapy treatment.
61
XPG
XPG is a structure-specific endonuclease required for
NER and incises 3' of the damaged DNA site.
70
XPG is
recruited to the damaged DNA site by TFIIH and inter-
acts with the N-terminus of RPA for correct polarity and
positioning.
45,71
XPG needs to be present to promote 5'
incision by the ERCC1/XPF endonuclease.
72
Recent
evidence suggests that ERCC1/XPF generates the 5'
incision prior to 3' incision by XPG.
11
Due to the require-
ment of XPG for 5' incision by ERCC1/XPF, targeting
the XPG protein may be a relevant small molecule target
to enhance chemotherapeutic agents.
XPG Levels Mediate the Response
to Platinum Therapy
Loss of heterozygosity (LOH) of the XPG (ERCC5)
gene, polymorphisms, and XPG gene expression have
been shown to be prognostic indicators to platinum
based chemotherapy in ovarian, lung, and colon can-
cer.
73
e
75
These data fit well with the general model
that downregulation or targeting the NER endonucle-
ases will enhance chemotherapeutic efficacy.
Targeting XPG to Enhance Chemotherapy
A recent study using RNAi to downregulate the XPG
protein shows that this endonuclease can influence cell
sensitivity to platinum-based chemotherapies.
65
As
with the ERCC1/XPF complex, a high level (
90%) of
protein knockdown was required to elicit the hypersen-
sitivity to platinum-based agents.
62,65
These data
suggest that with a small molecule targeting approach,
a high affinity and potent effect on the endonuclease
activity and/or DNA binding activity will be required
to enhance chemotherapeutic efficacy.
>
Targeting ERCC1/XPF to Enhance Chemotherapy
Targeting ERCC1 and XPF by RNAi has shown
good promise for enhancing chemotherapy in ovarian,
lung, prostate, and breast cancer cells. This approach
yields sufficient protein knockdown, which results in
reduced DNA repair capacity and sensitization of
