As a major defense against environmental damage to cells DNA repair is present in all organisms examined including bacteria, yeast, drosophila, fish, amphibians, rodents and humans. DNA repair is involved in processes that minimize cell killling, mutations, replication errors, persistence of DNA damage and genomic instability. Abnormalities in these processes have been implicated in cancer and aging (Figure 1).
There are several different repair pathways in mammalian cells (Figure 2):
An example of the single step reaction is the direct reversal that can be
accomplished by the bacterial photolyase enzyme: a cyclobutane pyrimidine
dimer is converted into two adjacent pyrimidines, and thereby the lesion is
Simple base modifications such as monofunctional alkylations can be removed
by the base excision repair system whereas more complex, bulky lesions are
dealt with by the nucleotide excision repair pathways.
Figure 3 - Nucleotide Excision Repair scheme
The most important DNA repair pathway is nucleotide excision repair
(figure 3) that fixes the majority of bulky lesions in DNA. These lesions
include UV induced photoproducts, and bulky adducts such as those derived
from cisplatin and 4-nitroquinoline oxide. Understanding of the enzymology
was previously based on knowledge from work done in E.coli, but now the
molecular events are being characterized in human cells. Nucleotide
excision repair involves recognition, incision, degradation,
polymerization, and finally, ligation. The recognition steps involve the
ERCC1, XPA and XPF gene products followed by the interaction with the TFIIH
transcription factor. This factor contains the repair genes XPB and XPD
and thus represents a direct molecular link between DNA repair and
transcription. A dual incision event is accomplished by the ERCC1 and XPG
products, and this is followed by excinuclease activity, polymerization and
ligation. There are a number of recent reviews that discuss this pathway
in detail and compare the pathways in bacteria and mammalian cells .
Nucleotide excision repair pathways differ in different parts of the
mammalian genome: separate pathways operate for the repair of active or
essential genomic regions versus regions that are non coding. The in vitro
cell free extract assays, that have been used with considerable success to
determine aspects of the DNA repair enzymology, are all limited to studying
inactive DNA, and as yet there is no assay for in vitro repair of active
genes in mammalian cells. Several laboratories are working on this
problem, and that approach is necessary for optimal biochemical analysis of
the biochemistry of gene specific DNA repair. There are distinct DNA
repair pathways for the bulk genome (inactive genomic regions) and for
gene specific repair of active genes. Some genes may have preferential
repair and even a strand bias of the repair process, and these need to be
understood. Further, there can be variations within genes as well: certain
codons are repaired better than others.
Patients with Cockayne syndrome have sun sensitivity, short stature, and
progressive neurologic degeneration. Unlike XP, Cockayne syndrome is not
associated with cancer. Cultured cells from Cockayne syndrome patients are
hypersensitive to killing by UV and have defective DNA repair of actively
transcribing genes. There are 2 complementatoin groups in Cockayne
syndrome. The genes that are defective in Cockayne syndrome are also
involved in both nucleotide excision repair and transcription however,
their precise function is not yet known.
Bohr, V.A., Wassermann, K., and Kraemer, K.H. DNA Repair Mechanisms, Alfred
Benzon Symposium No. 35, Copenhagen:Munksgaard, 1993.pp. 1-428.
Friedberg, E.C., Walker, G.C., and Siede, W. DNA repair and mutagenesis,
Washington, D.C.ASM Press, 1995. pp 1-698.
Trends in Biochemical Sciences (TIBS) vol 20 No 10: October 1995 (237).
This issue is entirely devoted to review articles on DNA repair. pp.