Biology Reference
In-Depth Information
CHAPTER
9
Defective DNA Mismatch Repair-dependent
c-Abl-p73-GADD45a Expression Confers
Can
cer Chemoresista
nce
Long Shan Li, Mark Wagner, Mark Meyers, David A. Boothman
University of Texas Southwestern Medical Center at Dallas, Dallas, TX
OVERVIEW OF DNA MISMATCH REPAIR:
MECHANISMS OF DNA LESION
DETECTION/REPAIR
repair, excising and re-synthesizing a repair patch of
up to ~1000 base pairs. Next, bacterial MutH allows
the MMR complex to discriminate between new and
old DNA strands by the lack of methylation on the
new strand and incises the DNA at unique hemi-meth-
ylated GATC sites from up to 1,000 base pairs 5
0
- from
the initial DNA lesion. The DNA break caused by
MutH is further recognized by the MutU helicase, which
is essential for the long-patch repair capacity of DNA
polymerase. MMR in E. coli has been successfully recon-
stituted in vitro and requires MutS, MutL, MutH, and
UvrD (helicase II) proteins along with the DNA poly-
merase III holoenzyme, DNA ligase, single-stranded
DNA (ssDNA) binding protein and one of four ssDNA
exonucleases (Exos) (Exo I, Exo VII, Rec J or Exo
X).
7
e
13
Several models for MMR processing have been
offered, but evidence appears to strongly support the
“Molecular Switch Model” simulated by Berardini
et al.,
14
and recently reviewed in Li et al.
6
MMR is highly conserved in nature. Genes encoding
homologs of bacterial MutS and MutL have been identi-
fied in a variety of eukaryotic model systems, including
yeast, plants, insects, nematodes, and mammals.
3,15
e
21
No eukaryotic homologue of MutH has been identified
to date. The complete human MMR reaction has been
reconstituted using cellular extracts
22
e
24
and purified
proteins.
25,26
Available data demonstrated that human
MutS and MutL homologues function as heterodimers
in the recognition and processing of different types of
DNA base mismatches.
27,28
Human homologs of MutS
formed specific MSH2-MSH6 and/or MSH2-MSH3 het-
erodimers. Single-nucleotide loops and DNA
mismatches derived from DNA damage or incorporated
base lesions (e.g., 5-fluorouracil (5-FU) incorporated into
Conserved MMR Genes and Functions
DNA mismatch repair (MMR) is a highly conserved
repair pathway that plays an important role in the detec-
tion and correction of errors created during/after DNA
replication (i.e., post-replication repair) and/or after
natural genetic recombination events.
1
Disruption of
MMR results in
10- to ~100-fold increases in the muta-
tion rates of affected cells, leading to a mutator pheno-
type.
2,3
The mutator phenotype of cells defective in
MMR was first characterized in Escherichia coli.
4,5
In
E. coli, initial studies showed that MMR corrects poly-
merase mis-incorporation errors through a “long-patch”
repair system, whereby DNA excision reactions were
genetically dependent on MutS, MutL, MutH, and
MutU (Uvrd) gene products, reviewed recently by Li
et al.
6
The MutS MMR complex exists as a mixture of
dimers and ultimately tetramers (with MLH1 dimeric
family members, MutL) that scan and sense initial
DNA lesions, providing genome-stabilizing, proof-
reading capacity to the cell. This complex also distin-
guishes between template and newly replicated DNA
strands that importantly allows specific detection of
post-replication repair lesions, such as mismatched
DNA base pairs or short DNA loops caused by DNA
polymerase errors or slippage, respectively. In brief,
DNA lesions are first detected by the MutS complex,
which in turn allows recruitment of the MutL to form
a heteroduplex in an ATP-dependent process. The
MutS:MutL heteroduplex then initiates
>
long-patch
