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From the Imperial Cancer Research Fund, Clare Hall
Laboratories, South Mimms, Herts EN6 3LD, United Kingdom
A versatile strategy for repairing damaged DNA,
termed nucleotide excision repair
(NER),1 is found throughout
the natural world in organisms ranging from mycoplasma to mammals. In
humans, NER is a major defense against the carcinogenic effects of
ultraviolet light from the sun. This repair pathway acts with varying
efficiencies on a wide variety of DNA alterations and is especially
important for bulky, helix-distorting lesions. The key event in NER is
incision of the damaged strand on each side of a lesion in DNA,
releasing the damage in a fragment that is ~24-32 nucleotides (nt)
long in eukaryotes. Nucleotide excision repair defects of various types
are found in individuals with the inherited syndromes xeroderma
pigmentosum (XP), Cockayne syndrome (CS), and trichothiodystrophy
(1). Individuals with XP are sun-sensitive and generally show a greatly
increased incidence of UV-induced skin cancers. The disorder has seven
genetic complementation groups, designated XP-A through XP-G, and a
variant form, XP-V. The recommended nomenclature uses a hyphen to
refer to cells of a given complementation group (for example XP-A),
distinguishing this from the affected gene XPA and the
normal protein product XPA (2). This minireview focuses on the
biochemistry of NER in mammalian cells, with an emphasis on recent
developments.
Mechanism of Core NER Reaction in Eukaryotes Many details of the NER reaction mechanism are being elucidated
with purified proteins and reconstituted systems. At a minimum, the
dual incision reaction of NER requires the factors XPA, RPA, XPC,
TFIIH, XPG, and ERCC1-XPF in mammalian cells or the homologous components in yeast (3, 4). These are summarized in Table I. XPA and the single-stranded binding
protein RPA associate with one another and are able to preferentially
bind to damaged DNA. TFIIH includes the XPB and XPD DNA helicases among
its subunits (5) and is involved in local opening of DNA around a site
of damage. XPC, usually bound to a protein partner hHR23B, may also be
involved in damage recognition and opening. The XPG and ERCC1-XPF factors are structure-specific DNA nucleases and are responsible for
cleaving on the 3 Table I.
Identified proteins participating in the dual incision reaction of
mammalian NER
The cDNA encoding XPF was recently isolated, completing the
list of cloned XP core factors (7, 8). XPF forms a tight complex with
the ERCC1 protein, directly homologous to the similar Rad1-Rad10
complex in the budding yeast Saccharomyces cerevisiae. Both
the Rad1-Rad10 and ERCC1-XPF complexes have a structure-specific nuclease activity that cleaves near the border between single-stranded and duplex DNA when the single strand has a polarity 5 The structure-specific cleavage properties of the NER nucleases have
strongly suggested that they act on an opened, "bubble" intermediate during repair (Fig. 1D). Evidence for the
existence of such an intermediate has been provided by probing for
potassium permanganate sensitivity around a lesion during repair
in vitro. ATP-dependent opening of a region of
approximately 25 base pairs occurs around the damaged site before
dual incision (9). Opening does not require the nuclease activity of
XPG (9) but is dependent on the DNA helicase activity of
TFIIH.2
XPG and Rad2 are members of a family of enzymes that includes the DNase
IV/FEN1 group of structure-specific nucleases which function in DNA
replication (10). Two conserved domains in all of these nucleases are
related to sequences in a group of prokaryotic exonucleases,
exemplified by the 5 Although the 3 XPC-hHR23B Complex and Transcription-coupled Repair The XPC protein plays a role in NER that is just beginning to be
revealed. This 106-kDa human protein copurifies with a tightly bound
43-kDa partner, hHR23B. XPC binds DNA with a preference for
single-stranded (16, 17) or damaged DNA (16). In some reconstituted
repair systems, recombinant XPC protein alone is sufficient, and adding
hHR23B does not increase repair (16). In other fractionated and
purified systems, only weak repair occurs with XPC alone, and repair is
significantly stimulated by adding hHR23B (17). Intriguingly, the XPC
subunit is dispensable for the repair of two different types of DNA
structures in vitro, an observation that has implications
for the mechanism of transcription-coupled repair in human cells.
Transcription-coupled nucleotide excision repair is a specialized mode
of NER that removes DNA adducts significantly faster from the
transcribed strand of genes than from the non-transcribed strand or the
bulk of DNA. In mammalian cells, transcription-coupled repair of
pyrimidine dimers can take place in the absence of XPC (while requiring
the other NER subunits including XPA and TFIIH) and is believed to be
initiated when RNA polymerase is stalled at a lesion in DNA (18). A
particular cholesterol moiety incorporated synthetically into a DNA
backbone is thought to cause an unusual degree of helix distortion, and
NER of this moiety does not require XPC (14). Similarly, a model
substrate consisting of a pyrimidine dimer flanked on the 3 Accessory Factors and Cellular Interactions An XP group that remains to be fully explained at the molecular
level is XP-E. XP-E patients are generally mildly affected and show
50% or more residual repair capacity. Cells from some but not all XP-E
individuals are missing an activity that binds damaged DNA, as measured
in electrophoretic mobility shift experiments. This activity is
associated with two polypeptides of 127 and 48 kDa. Sequence changes in
the 48-kDa subunit are present in cell lines that are missing the DNA
damage-binding activity (20). This XP-E-associated complex is not
required for the core NER system but may well play an accessory
role.
Further accessory proteins that modulate NER activity are being
uncovered. A factor designated IF7 was found to be needed for
appreciable repair of UV-irradiated plasmid DNA (21) and stimulates
incision at a cisplatin lesion in DNA by about
6-fold,3 but a gene encoding
this stimulatory protein component has not been identified. The p53
protein or some of the downstream target genes controlled by p53 may
also be accessory factors for NER. A study with human fibroblasts
showed that cells homozygous for p53 mutations have about 50-60% of
the normal rate of excision of pyrimidine dimers from the overall
genome (22). Although p53 can bind to some components of the NER
apparatus, including TFIIH (reviewed in Ref. 23), this binding
may not have a direct effect on repair efficiency. Instead, protein
products of genes that are transcriptionally regulated by p53, such as
Gadd45 and p21cip1/waf1, may enhance NER. There is good
evidence that in human cells, less repair of UV damage to DNA occurs in
the absence of Gadd45 or p21cip1/waf1 induction (24-26).
Post-translational modifications are also likely to regulate the
activity of core factors. For example, the proper phosphorylation state
of one or more components is necessary to observe significant NER in a
cell-free system (27).
NER in eukaryotes might work either by sequential assembly of
individual factors or by the action of a preformed "repairosome" or
by the interaction of intermediate subassemblies. Many interactions that would be consistent with any of these possibilities have been
documented between NER proteins. An extreme view is that all the
required repair factors can preassemble in the cell, and evidence has
been provided for such assemblies in yeast (28) and mammalian cells
(29). An intermediate hypothesis is that several subassemblies (for
example Rad1-Rad10-Rad14 and TFIIH-Rad2 in yeast) exist in the cell,
which are then sequentially recruited to a repair site (30). The
details of observed interactions vary and depend on the exact
purification and analysis procedures. In particular, moderate salt
conditions disrupt some interactions that may be significant. A line of
study that could help clarify this subject is a systematic and
quantitative measurement of the binding constants between different NER
proteins.
The densely packed chromosome structure in the nucleus introduces an
additional level of complexity, in that chromatin disassembly and
reassembly need to be coupled with DNA repair. A recent study analyzed
nucleotide excision repair of DNA in a cell-free system capable of
chromatin assembly (31). Chromatin formation occurred concomitantly
with repair DNA synthesis and required the chromatin assembly factor
CAF-1. Yeast lacking CAF-1 are viable but UV-sensitive (32). Like DNA
replication and transcription, NER might be preferentially localized at
a limited number of foci in the nucleus. Using immunofluorescence techniques, the XPG protein was observed to change position in the
nucleus after UV irradiation of cells, suggesting that NER may occur at
specific sites (33).
Cockayne Syndrome and Transcription-coupled Repair Cockayne syndrome is an NER-related disorder of particular current
interest. Like XP patients, individuals with CS are also sun-sensitive
but show a distinctive array of severe developmental and neurological
abnormalities. Classical CS is caused by mutations in the
CSA or CSB genes. The disease also occurs
simultaneously with XP in rare patients belonging to XP groups B, D,
and G. Remarkably, features of CS can therefore be caused by particular
mutations in at least five genes (1).
Cells in the CS-A and CS-B groups do not preferentially remove DNA
damage such as UV-induced pyrimidine dimers from the transcribed strand
of active RNA polymerase II-transcribed genes (34, 35). However, the
clinical features of CS are not simply explained by a defect in
transcription-coupled repair of pyrimidine dimers, since individuals
with complete defects in NER (e.g. most XP-A and some XP-G
patients) do not have CS symptoms. One possibility is that like XPB and
XPD, CSA and CSB are somehow involved in basal transcription and that
particular mutant forms of these proteins lead to subtle transcription
defects that can account for the developmental abnormalities of CS (34,
35). The CS proteins have properties consistent with this view; the
44-kDa CSA contains WD repeat motifs, and the 168-kDa CSB has motifs common with the Swi/Snf family (34, 35). Such an explanation still
leaves the dilemma of the occurrence of CS in some XP-G patients, even
though XPG has no obvious role in transcription. There are indications,
however, that XPG may interact with some subunits of TFIIH (36).
Moreover, with one purification protocol, active yeast TFIIH copurified
with overexpressed Rad2, the S. cerevisiae homolog of XPG
(30). It is possible that lack of a proper XPG-TFIIH association can
adversely affect transcription, giving rise to the CS symptoms in some
XP-G patients. Interestingly, it has been found that CS patients from
XP group G have mutations that would produce severely truncated XPG
protein, whereas XP-G patients without CS produce full-length XPG
protein with mutations that reduce or eliminate the nuclease function
(37). The absence of full-length XPG might affect transcription of some
genes.
An alternative hypothesis is that CS is instead due to a deficiency in
transcription-coupled repair of some types of endogenous oxidative
damage (37, 38). Some lesions induced in DNA by active oxygen, such as
thymine glycols, are subject to preferential removal from the
transcribed strand of active genes. Unlike pyrimidine dimer removal,
this strand-specific repair of oxidative damage is apparently
independent of XPA, XPF, and the nuclease function of XPG but depends
on CSA and CSB. The repair is, however, disrupted in those XP-G cells
with severely truncating mutations (38). This suggests that XPG may
have a second function in addition to its role in NER, aiding the
transcription-coupled repair of some forms of oxidative damage. It
might, for example, interact with a transcription complex blocked at a
thymine glycol and accelerate thymine glycol-DNA glycosylase-mediated
base excision repair at such a site.
It has been further suggested that some lesions in DNA block
transcription and are not well removed by normal NER but may only be
efficiently dealt with by a CSA- and CSB-dependent repair mechanism. The CSA and CSB proteins might also help RNA polymerase to
occasionally bypass damaged sites on a template without repair or play
a role in releasing transcription complexes blocked at damage (35, 39).
If blocking lesions (for example, certain types of oxidative damage)
accumulated with age, these adducts could trap transcription components
on the DNA and gradually cripple transcription in CS cells (35). This
might help account for the severe neurological symptoms and short
lifespan of human CS patients.
How do the CSA and CSB proteins function in transcription-coupled
repair? One possibility is that they are "coupling factors," mediating an interaction between a stalled RNA polymerase and the
repair proteins and thereby helping to attract DNA repair to a damaged
site on a transcribed strand. Such a mechanism appears to operate
during transcription-coupled repair of pyrimidine dimers in E. coli, where the Mfd protein acts as a coupling factor that promotes the interaction of RNA polymerase with the NER factor UvrA
(40). An indication that the CS proteins actually do interact directly
or indirectly with RNA polymerase II is the observation that UV light
induces modification of the catalytic subunit of human RNA polymerase
II by ubiquitination and that this modification is defective in CS-A
and CS-B cells (41). It is not yet known if the ubiquitination affects
transcription activity or the ability to repair damaged templates.
Connections between Mismatch Repair and Nucleotide Excision
Repair? Single-base mismatches and short unpaired loops can arise during
DNA replication. To prevent mutations, human cells use a repertoire of
homologues of the E. coli mismatch repair proteins MutS and
MutL to recognize and initiate correction of mismatches (42).
Unexpectedly, disruptions of the DNA mismatch repair genes mutS and mutL were found to reduce
transcription-coupled NER of the lactose operon in E. coli
(43), and human cells with mutations in particular mismatch repair
genes were likewise found to have a deficiency in transcription-coupled
repair of UV-induced pyrimidine dimers (44). S. cerevisiae
mutants defective in the homologous mismatch repair genes are not,
however, defective in transcription-coupled repair of UV-induced DNA
damage (45).
The mechanism of any influence of mismatch repair proteins on NER is
unknown, but it has been observed that the mismatch binding protein
hMSH2 and a functional complex denoted hMutS I thank the members of my laboratory and
Deborah Barnes, Stephanie Kong, and Alain van Gool for helpful
comments. Charlie Haden and Robert Plant provided useful assistance in
preparing the manuscript.
REFERENCES4
Volume 272, Number 38,
Issue of September 19, 1997
pp. 23465-23468
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.
MINIREVIEW:

INTRODUCTION
Mechanism of Core NER Reaction in Eukaryotes
NER Nucleases
XPC-hHR23B Complex and Transcription-coupled Repair
Accessory Factors and Cellular Interactions
Cockayne Syndrome and Transcription-coupled Repair
Connections between Mismatch Repair and Nucleotide Excision
Repair?
FOOTNOTES
ACKNOWLEDGEMENTS
REFERENCES4
- and 5
-sides of a lesion, respectively (Fig.
1). The excised fragment is replaced in a
DNA repair synthesis reaction mediated by DNA polymerase
or
holoenzyme (6), and the process is completed by sealing the repair
patch of about 30 nt with a DNA ligase.
Human protein
Nearest S. cerevisiae
homolog
Comments
XPA
Rad14
Preferentially
binds damaged and single-stranded DNA
RPA
Rpa
3-subunit
single-stranded DNA binding protein; binds to XPA
XPC
Rad4
Preferentially binds damaged and
single-stranded DNA; not needed for transcription-coupled repair or
some DNA lesions
hHR23B
Rad23
Binds to XPC and stimulates
activity
TFIIH
XPB
Ssl2 (Rad25)
DNA helicase
(3
to 5
); also known as ERCC3
XPD
Rad3
DNA helicase (5
to 3
); also known as ERCC2
p62
Tfb1
TFIIH subunit; also
known as GTF2H1
p52
Tfb2
TFIIH subunit; formerly known as
p41
p44
Ssl1
TFIIH subunit
p34
Tfb4
TFIIH
subunit
XPG
Rad2
DNA endonuclease for 3
side of
damage; also known as ERCC5
ERCC1
Rad10
Subunit of
DNA endonuclease for 5
side of damage
XPF
Rad1
Subunit of
DNA endonuclease for 5
side of damage; also known as ERCC4
Fig. 1.
Model for the core nucleotide excision repair
reaction in mammalian cells. A, a site of damage such as a
UV-induced pyrimidine dimer (represented by the square black
rectangle) causes some local distortion of the helix.
B, the damage is recognized by a complex of NER proteins,
thought to include XPA, RPA, and in most cases the XPC-hHR23B complex.
Limited opening of the DNA may occur at this stage. XPC-hHR23B is
dispensable for NER in special instances, such as strand-specific
transcription-coupled repair. C, a larger region around the
damage is opened up in an ATP-dependent process that
depends on TFIIH. D, the damaged strand is cleaved on the
3
-side of the lesion by XPG nuclease and on the 5
-side by ERCC1-XPF
nuclease, releasing the damage in an oligonucleotide 24-32 residues
long. E, repair synthesis then takes place, mediated by a
proliferating cell nuclear antigen (PCNA)-dependent DNA polymerase
or
holoenzyme, and the repair patch of about 30 nucleotides is ligated to
finish the process. In yeast, similar events occur with homologous gene
products (see Table I).
[View Larger Version of this Image (22K GIF file)]
to 3
moving
away from the border. Junctions between duplex and single-stranded DNA
are cleaved with the opposite polarity by the XPG protein and its
S. cerevisiae homolog Rad2.
-3
exonuclease domain of Escherichia coli DNA polymerase I. The solved structures of eubacterial
enzymes in this family, such as bacteriophage T5
5
-3
-exo/endonuclease (11), show that the conserved domains are
folded together in the predicted active site and coordinate the binding
of two Mg2+ ions. The T5 enzyme has a helical arch with a
size and ionic environment appropriate for single-stranded DNA to
thread through, starting at a free end. It will be interesting to learn
how DNA is bound during repair by XPG or Rad2, which must load onto
sites in DNA where there is no free 5
-end. The intervening region
between the conserved domains in the various nucleases in this family probably mediates specific protein-protein interactions for replication and repair. The XPG and ERCC1-XPF nucleases require
Mg2+ or Mn2+ but not ATP and on some model
substrates do not require other protein factors for their
structure-specific cleavage activity (7, 9). With other substrates, RPA
protein can dramatically stimulate nuclease activity (12).
- and 5
-incisions are nearly simultaneous, the
3
-incision mediated by XPG is normally first (13, 14). For some
lesions, the 3
-incision can be carried out by purified core incision
factors in the absence of ERCC1-XPF (14), although repair of a
1,3-intrastrand d(GpTpG)-cisplatin cross-link in DNA required all
components to be present to form either incision (15). After the
5
-incision, mediated by ERCC1-XPF nuclease (7, 14), the damaged
oligonucleotide is excised and seems to be bound to one or more
components of the repair complex (14). In repair-proficient cell
extracts some uncoupled 3
- or 5
-incisions can occur, indicating that
the order of incisions is not always strict, although uncoupled
3
-incisions are more frequent than uncoupled 5
-incisions (7, 14). The
precise positions of incisions for several different lesions have been
mapped in the human system. Depending on the adduct, incisions are
introduced 2-9 phosphodiester bonds away from the 3
-side of a lesion
and 16-25 phosphodiester bonds away from the 5
-side. A unifying
feature is that the modal length of incision products is 26-27 nt
in each case (15).
-side by a
10-nt unpaired region is repaired independently of XPC, while still
requiring the other NER subunits (19). Perhaps XPC is involved in
forming or stabilizing the damage recognition-incision complex at the
3
-side of a lesion and is not needed when the DNA structure is already
sufficiently altered on this side by a DNA adduct or by a stalled RNA
polymerase (Fig. 1B).
(a heterodimer of hMSH2
and hMSH6) can bind to DNA base damage to some extent. This has been
demonstrated for pyrimidine dimers and for a 1,2-intrastrand d(GpG)-cisplatin cross-link (46, 47). The binding of hMutS
can be
considerably enhanced when the adducts are paired to noncomplementary bases (48, 49). However, in vitro studies have yet to reveal any appreciable effect of such binding on NER. Nucleotide excision repair is in fact 3-15-fold more efficient when an adduct has noncomplementary bases in the opposite strand, but this phenomenon appears unrelated to mismatch repair because NER of such mispaired adducts occurs equally well with extracts from cells either proficient or deficient in mismatch correction (48, 50). Similarly, addition of
purified hMutS
to in vitro NER reactions does not
suppress or enhance NER (48). Further investigation of possible
transcription-coupled repair defects in mismatch repair-deficient cells
is required to establish a functional connection in mammalian cells.
Studies of physical interactions between mismatch repair proteins and NER proteins should be informative in this regard.
*
This minireview will be reprinted
in the 1997 Minireview Compendium, which
will be available in December, 1997. This is the first article of four in the
"Eukaryotic DNA Repair Minireview Series."
To whom correspondence should be addressed.
1
The abbreviations used are: NER, nucleotide
excision repair; XP, xeroderma pigmentosum; CS, Cockayne syndrome; nt,
nucleotide(s).
2
E. Evans and R. D. Wood, unpublished
results.
3
J. G. Moggs and R. D. Wood,
unpublished results.
4
Because of space limitations, a number of the
citations here are to other specialized review articles.
, M., Hübscher, U., Egly, J.-M., and Wood, R. D.
(1995)
Cell
80,
859-868
[CrossRef][Medline]
[Order article via Infotrieve]
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.
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J.-S. You, M. Wang, and S.-H. Lee Biochemical Analysis of the Damage Recognition Process in Nucleotide Excision Repair J. Biol. Chem., February 21, 2003; 278(9): 7476 - 7485. [Abstract] [Full Text] [PDF] |
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R. H. Elder and G. L. Dianov Repair of Dihydrouracil Supported by Base Excision Repair in mNTH1 Knock-out Cell Extracts J. Biol. Chem., December 20, 2002; 277(52): 50487 - 50490. [Abstract] [Full Text] [PDF] |
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