Recognition and Repair of Compound DNA Lesions (Base Damage and Mismatch) by Human Mismatch Repair and Excision Repair Systems

Abstract

Nucleotide excision repair and the long-patch mismatch repair systems correct abnormal DNA structures arising from DNA damage and replication errors, respectively. DNA synthesis past a damaged base (translesion replication) often causes misincorporation at the lesion site. In addition, mismatches are hot spots for DNA damage because of increased susceptibility of unpaired bases to chemical modification. We call such a DNA lesion, that is, a base damage superimposed on a mismatch, a compound lesion. To learn about the processing of compound lesions by human cells, synthetic compound lesions containing UV photoproducts or cisplatin 1,2-d(GpG) intrastrand cross-link and mismatch were tested for binding to the human mismatch recognition complex hMutS␣ and for excision by the human excision nuclease. No functional overlap between excision repair and mismatch repair was observed. The presence of a thymine dimer or a cisplatin diadduct in the context of a G-T mismatch reduced the affinity of hMutS␣ for the mismatch. In contrast, the damaged bases in these compound lesions were excised three-to fourfold faster than simple lesions by the human excision nuclease, regardless of the presence of hMutS␣ in the reaction. These results provide a new perspective on how excision repair, a cellular defense system for maintaining genomic integrity, can fix mutations under certain circumstances.Mismatches in DNA resulting from replication errors, and base damage caused by physical (UV light) and chemical (polyaromatic hydrocarbons) agents, are responsible for the majority of human cancers (18). Although certain mismatches and base lesions can be eliminated from DNA by the base excision repair pathway initiated by glycosylases with narrow substrate ranges, in human cells there exists a general mismatch repair system and a general damage repair system of wide substrate range. The mismatch repair system removes the mismatched base as a nucleotide (44), and the excision repair system excises the damaged base(s) in an oligonucleotide (52, 67).The general mismatch repair system (long-patch mismatch repair) corrects all eight single-base mismatches as well as small insertion sequence loops with comparable efficiencies (31, 44). The general nucleotide excision repair (excision repair) system (8) not only is the sole repair pathway for bulky lesions such as thymine dimers (TϽϾT) and cisplatin-guanine adducts but also repairs a wide variety of nonbulky lesions such as O 6 -methylguanine (O 6 -meG) at physiologically relevant rates (52). Thus, it appears that both repair systems recognize many dissimilar non-B DNA forms rather than a specific lesion structure.Given the wide substrate ranges of both systems, it is not unreasonable to expect overlaps between the two substrate spectra, or that one repair system may facilitate the function of the other. Indeed, it has been shown that the excision repair system recognizes mismatches and removes the mismatched base in a manner identical to the removal of damaged bases (27). Since a DNA lesion, by de...

Loss of DNA mismatch repair facilitates reactivation of a reporter plasmid damaged by cisplatin

“…Such a mechanism has been proposed to explain the ability of another group of cisplatin adduct-binding proteins, the HMG proteins, to interfere with adduct repair (Huang et al, 1994). However, in a recent study, (Mu et al, 1997) reported that addition of the hMSH2/hMSH6 heterodimer to a cell-free excision repair system did not impair the ability of the nucleotide excision repair system to remove cisplatin adducts from DNA. The assay system utilized by these investigators measured only the excision nuclease activity in the absence of transcription, and the possibility of a negative interaction between the MMR and nucleotide excision repair systems in assays including transcription needs further investigation.…”

Msh2 deficiency does not contribute to cisplatin resistance in mouse embryonic stem cells

“…This low-level resistance was sufficient for enrichment of an MMR-deficient cell population in vitro (Fink et al, , 1998) and a reduced response of tumor cells in an in vivo model . Furthermore, purified MSH2 protein, either alone or in complex with its dimerization partner MSH6, can bind to oligonucleotides containing a cisplatin adduct (Duckett et al, 1996;Mello et al, 1996;Mu et al, 1997). Recently, the MMR proteins have been suggested to have a function as general sensors of DNA damage with the ability to signal directly to cell cycle checkpoints and apoptosis.…”

Frequent loss of heterozygosity at the DNA mismatch-repair loci hMLH1 and hMSH3 in sporadic breast cancer

“…Therefore, a gene dosage effect affecting the stoichiometry and the activity of the heteromolecular mismatch-repair complex may be sufficient to promote cancer by impairing functions other than the correction of replication errors. Mismatchrepair proteins are involved in a variety of vital cellular processes, including the homologous recombination (Jones et al, 1987;deWind et al, 1995), the mediation of the G2 checkpoint (Hawn et al, 1995), transcription-coupled nucleotide excision repair (Mellon et al, 1996) and in the recognition of DNA damage and/or in the signalling pathway contributing to the generation of apoptotic cells (Kat et al, 1993;Mu et al, 1997). Interestingly, the influence of environmental factors on the genome stability in cells defective in nucleotide excision repair and mismatch-repair could be substantial.…”