Biology Reference
In-Depth Information
defect. The main defect in TTD, however, appears to be
in TFIIH-mediated transactivation of a subset of nuclear
receptor-responsive genes.
regarding XP and NER contributed to the identification
of DNA as the therapeutic target of cisplatin and
provided the initial recognition of the potential impor-
tance of NER in modulating the repair of cisplatin-DNA
adducts. This potential was realized in the analysis of
many model systems where the sensitivity to cisplatin
was dramatically impacted by the NERpathway. Possibly
nowhere was this more interesting than in the case of
testicular cancer where initially the extreme sensitivity
of this cancer to cisplatin was attributed to an abysmal
capacity to repair cisplatin-DNA lesions. 27 e 29 Subsequent
analyses revealed decreased expression of a number of
XP proteins. 29 e 31 This presented the possibility that
reducing NER capacity in a cancer holds the potential to
sensitize the cancer to cisplatin. Around the same time,
analysis of the cellular response to cisplatin revealed
that increased DNA repair capacity was a common func-
tion in cancers that were inherently resistant to cisplatin
or that acquired resistance following treatment. 32,33
THE RESPONSE TO DNA DAMAGING
CHEMOTHERAPY IS MODULATED BY
NER
Platinum Agents
Many common chemotherapeutics used in the treat-
ment of cancer induce DNA damage. Probably the
most relevant are the platinum-based agents, cisplatin,
carboplatin, and oxaliplatin ( Figure 6.2 ). Each of these
agents evokes their therapeutic effect by forming DNA
adducts and altering DNA metabolism including DNA
replication and transcription. 23 These agents commonly
crosslink adjacent purine bases via the N7 to form intra-
strand Pt-DNA adducts. The most prevalent adduct
formed is the 1,2-d(GpG) Pt lesion which has been exten-
sively studied structurally and biochemically. 24 The
cisplatin dGpG lesion induces a modest degree of distor-
tion though the adducted guanines remain within base
pairing distance from the complementary cytosines. 25
Interestingly, a cisplatin 1,3 d(GpXpG) adduct is more
distorting and creates a localized single-stranded char-
acter. Consistent with the role of DDB in recognition of
DNA adducts, repair of the 1,2 lesion is relatively ineffi-
cient and is stimulated by DDB while the repair of the
1,3 lesion is more efficient and does not require DDB. 15
Early analysis of XP cell lines revealed significant
hypersensitivity to cisplatin 26
TARGETING CATALYTIC ACTIVITIES
IN NER
Structure-Specific Endonucleases
Nucleases are an abundant class of enzymes that cata-
lyze the scission of a phosphodiester bond between two
adjacent bases in a nucleic acid substrate. Endonucleases
cleave an internal phosphodiester while exonucleases
cleave terminal bonds releasing mononucleotide prod-
ucts. Endonucleases range from the highly sequence
specific to the relatively non-specific, while structure
specific nucleases, as the name implies recognize and
cleave specific structures. Nucleases are often metal-
dependent enzymes, which coordinate most commonly
Mg þþ or Mn þþ in their active sites to facilitate catalysis.
In the context of NER, ERCC1/XPF and XPG are
members of the Rad1 and Rad2 family of structure-
specific nucleases, respectively. 34 e 36 These are relatively
small families of nucleases that participate in specialized
DNAmetabolic functions including base excision repair,
homologous recombination, and interstrand crosslink
(ICL) repair, which supports the contention that specific
inhibitors may be identified to target a specific nuclease.
In addition, there are a number of in vitro assays for
structure-specific nuclease activity that could be devel-
oped for high throughput screening. These assays
coupled with the currently available, highly efficient
expression systems for a number of these nucleases
puts the identification of inhibitors via in vitro screening
well within reach. In addition, structural information is
available for a number of these mammalian nucleases
including FEN-1, ERCC1/XPF, and XPG, which will be
very useful in any drug discovery effort.
and this
information
FIGURE 6.2 Platinum-based chemotherapeutics. Cisplatin, car-
boplatin and oxaliplatin are approved for the treatment of a variety of
malignancies and impact their activity via the formation of DNA
adducts.
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