Chemistry Reference
In-Depth Information
with the 1,2-GG and 1,3-GXG intrastrand crosslinks with similar rates. The more
rapid rate of dissociation of XPC-HR23B from the 1,2-GG compared to the 1,3-
GXG intrastrand crosslink is likely a result of the lack of localized melting of the
duplex DNA containing the 1,2-GG intrastrand adduct.
RPA is one of the proteins that belongs to the initial damage-sensing factors
initiating NER (see above). The relative binding affi nities of RPA to the different
cisplatin adducts correlate with the repair of the adducts observed in an
in vitro
NER assay;
61
RPA has higher affi nity for the 1,3-GTG intrastrand than for the 1,2-
GG intrastrand crosslink of cisplatin, and the 1,3-GTG adduct is more effi ciently
repaired by the human exonuclease than the 1,2-GG adduct.
53
Thus, binding of RPA
to damaged DNA is a sensitive indicator to predict whether mammalian NER could
be effective in the removal of damaged nucleotides. BBR3464 is the novel phase II
antitumour polynuclear platinum drug ([(
trans
- PtCl(NH
3
)
2
)
2
( m -
trans
- Pt - (NH
3
)
2
(NH
2
(CH
2
)
6
NH
2
)
2
)]
4+
), which forms on DNA so-called long-range intra- and interstrand
crosslinks. The effect of 1,4-GG intrastrand and interstrand crosslinks of BBR3464
on RPA binding was examined
62
(the platinated bases in 1,4-crosslinks are separated
by two intervening base pairs). Interestingly, RPA did not bind to DNA containing
interstrand crosslinks of BBR3464. Thus, these results predict very low effi ciency of
the mammalian NER pathway to remove the interstrand crosslinks of BBR3464,
subsequently proved by
in vitro
experiments.
62,63
TC-NER is initiated by stalling RNA polymerase II. Therefore, cisplatin DNA
adduct processing by RNA polymerase II has been investigated.
64 - 67
It was found
that RNA polymerase II stalls in front of a cisplatin intrastrand crosslink because
it does not pass a translocation barrier that impairs delivery of the bulky crosslink
into the active site.
In vitro
assays indicate that a considerable fraction of stalled
RNA polymerase II proteins remains strongly associated with DNA damaged by
cisplatin following RNA polymerase II arrest.
66
RNA polymerase II stalled in front
of a cisplatin adduct (1,3-GTG intrastrand crosslink) serves as a decoy to sequen-
tially recruit TFIIH, XPA, RPA, XPG and XPF repair factors in an ATP-dependent
manner. This RNA polymerase II/repair complex allows the ATP-dependent removal
of the cisplatin lesion.
65
In conclusion, the anticancer drug cisplatin forms DNA
intrastrand crosslinks that stall RNA polymerase II, and in this way cisplatin triggers
TC - NER.
67
In addition, recent observations demonstrate a role for poly-(ADP-
ribose)polymerase - 1 (PARP - 1) in TC - NER
68
and PARP - 1 binding to 1,2 - GG intras-
trand crosslinks in nuclear cell extracts.
69
Thus, the latter observations indicate that
poly(ADP-ribosylation) activity may be crucial for cellular NER of platinated
DNA.
6.3.3 Mismatch Repair System
Similarly to the case of NER, the enhanced affi nity to DNA damaged by cisplatin
is exhibited mainly by the MMR proteins involved in damage recognition, i.e. by one
of two heterodimers of MutS homologues, hMutSa (heterodimer of hMSH2 and
hMSH6) or hMutLa (hMLH1 and hPMS2 heterodimer). The purifi ed hMSH2 protein
binds to DNA containing a single 1,2-GG intrastrand crosslink.
70
Moreover, hMSH2
