Recognition of DNA alterations by the mismatch repair system

Abstract

Misincorporation of non-complementary bases by DNA polymerases is a major source of the occurrence of promutagenic base-pairing errors during DNA replication or repair. Base-base mismatches or loops of extra bases can arise which, if left unrepaired, will generate point or frameshift mutations respectively. To counteract this mutagenic potential, organisms have developed a number of elaborate surveillance and repair strategies which co-operate to maintain the integrity of their genomes. An important replication-associated correction function is provided by the post-replicative mismatch repair system. This system is highly conserved among species and appears to be the major pathway for strand-specific elimination of base-base mispairs and short insertion/deletion loops (IDLs), not only during DNA replication, but also in intermediates of homologous recombination. The efficiency of repair of different base-pairing errors in the DNA varies, and appears to depend on multiple factors, such as the physical structure of the mismatch and sequence context effects. These structural aspects of mismatch repair are poorly understood. In contrast, remarkable progress in understanding the biochemical role of error-recognition proteins has been made in the recent past. In eukaryotes, two heterodimers consisting of MutS-homologous proteins have been shown to share the function of mismatch recognition in vivo and in vitro. A first MutS homologue, MSH2, is present in both heterodimers, and the specificity for mismatch recognition is dictated by its association with either of two other MutS homologues: MSH6 for recognition of base-base mismatches and small IDLs, or MSH3 for recognition of IDLs only. Mismatch repair deficiency in cells can arise through mutation, transcriptional silencing or as a result of imbalanced expression of these genes.

Elevated mutant frequencies and increased C : G→T : A transitions in Mlh1−/− versus Pms2−/− murine small intestinal epithelial cells

“…Interestingly, and consistent with our ®ndings, the spectra of mutations in hMlh1 and hPms2 de®cient human tumor cell lines using hypoxanthine-guanine phosphoribosyltransferase (hprt) as the reporter, demonstrated a reduction in C : G?T : A transitions in cells lacking hPMS2, relative to the hMLH1 de®cient cells (Kato et al, 1998). The major causes of C : G?T : A transitions in mammals include the generation of G : T (or C : A) mismatches during DNA replication, recombination events, or deamination of 5-methylC at CpG sites (reviewed by Marra and Schar, 1999). Non-methylated cytosines can also undergo deamination to uracil, which can potentially lead to C : G?T : A changes.…”

“…In bacteria, initial recognition of mismatches arising during DNA replication is carried out by the MutS protein, which in turn recruits MutL and then MutH, the latter nicking the DNA at the nearest hemimethylated GATC site. The nascent strand is excised by exonuclease activity which is followed by patch repair (reviewed in Kolodner, 1995;Marra and Schar, 1999). Eukaryotes have a system similar to that of bacteria, and a number of MutS and MutL orthologs have been identi®ed in both yeast and mammals.…”

“…The orthologs of MutS, MSH2, MSH6, and MSH3 participate in primary recognition, with MSH2 functioning as an obligate partner in two heterodimers: MutSa, composed of MSH2 and MSH6, primarily binds to single base loop and base/base mispairs; and MutSb, made up of MSH2 and MSH3, recognizes insertion/deletion loops (IDLs) (reviewed in Jiricny, 1998;Kolodner and Marsischky, 1999;Marra and Schar, 1999). Yeast MutLa, which appears to be involved in the repair of both mismatches and IDLs, is a MLH1 and PMS1 heterodimer, whereas mammalian MutLa is composed of MLH1 and PMS2 (PMS2 being the closest mammalian ortholog to yeast PMS1) (Guerrette et al, 1999;Jiricny, 1998;Kolodner and Marsischky, 1999;Marra and Schar, 1999;Prolla et al, 1994). In yeast a second complex containing MLH1 and MLH3 (MLH3 being the closest ortholog of mammalian PMS1), MutLb, also participates in the repair of a subset of frameshift errors (Flores-Rozas and Kolodner, 1998).…”

Binding Discrimination of MutS to a Set of Lesions and Compound Lesions (Base Damage and Mismatch) Reveals Its Potential Role as a Cisplatin-damaged DNA Sensing Protein

“…A quantitative study reported that hMutS␣ displayed weak affinity for 1,2-d(GpG) cisplatin intrastrand cross-links (34) but in contrast a qualitative study reported that this protein exhibits a strong preference for cisplatin compound lesions formed when a thymine is misincorporated opposite the major cisplatin adduct (35). This protein was also found to have a reduced affinity for a G/T mismatch in the context of a 1,2-d(GpG) adduct (13) suggesting that compound lesions were not involved in MMR activity (36). hMSH2 alone also displayed weak affinity for the major intrastrand cross-link (37).…”

Elevated mutant frequencies and predominance of G:C to A:T transition mutations in Msh6−/− small intestinal epithelium

“…The major causes of G:C to A:T transitions in mammals include: the generation of G:T (or C:A) mispairs during semi-conservative DNA replication, recombination events, deamination of 5-methylC at CpG sites, deamination of non-methylated cytosines to uracil, and methylation of the O 6 position of guanine which can lead to thymine misincorporation by DNA polymerases (Marra and Schar, 1999). With respect to the relative importance of CpG sites in determining the mutational spectrum, we found that of all G:C to A:T transitions, 42% were at non-CpG sites.…”