exo1-Dependent mutator mutations: model system for studying functional interactions in mismatch repair

Human exonuclease I is required for 5' and 3' mismatch repair

We have partially purified a human activity that restores mismatch-dependent, bi-directional excision to a human nuclear extract fraction depleted for one or more mismatch repair excision activities. Human EXOI co-purifies with the excision activity, and the purified activity can be replaced by near homogeneous recombinant hEXOI. Despite the reported 5' to 3' hydrolytic polarity of this activity, hEXOI participates in mismatch-provoked excision directed by a strand break located either 5' or 3' to the mispair. When the strand break that directs repair is located 3' to the mispair, hEXOI- and mismatch-dependent gap formation in excision-depleted extracts requires both hMutSalpha and hMutLalpha. However, excision directed by a 5' strand break requires hMutSalpha but can occur in absence of hMutLalpha. In systems comprised of pure components, the 5' to 3' hydrolytic activity of hEXOI is activated by hMutSalpha in a mismatch-dependent manner. These observations indicate a hydrolytic function for hEXOI in 5'-heteroduplex correction. The involvement of hEXOI in 3'-heteroduplex repair suggests that it has a regulatory/structural role in assembly of the 3'-excision complex or that the protein possesses a cryptic 3' to 5' hydrolytic activity.

DNA mismatch repair and mutation avoidance pathways

Unpaired and mispaired bases in DNA can arise by replication errors, spontaneous or induced base modifications, and during recombination. The major pathway for correction of mismatches arising during replication is the MutHLS pathway of Escherichia coli and related pathways in other organisms. MutS initiates repair by binding to the mismatch, and activates together with MutL the MutH endonuclease, which incises at hemimethylated dam sites and thereby mediates strand discrimination. Multiple MutS and MutL homologues exist in eukaryotes, which play different roles in the mismatch repair (MMR) pathway or in recombination. No MutH homologues have been identified in eukaryotes, suggesting that strand discrimination is different to E. coli. Repair can be initiated by the heterodimers MSH2-MSH6 (MutSalpha) and MSH2-MSH3 (MutSbeta). Interestingly, MSH3 (and thus MutSbeta) is missing in some genomes, as for example in Drosophila, or is present as in Schizosaccharomyces pombe but appears to play no role in MMR. MLH1-PMS1 (MutLalpha) is the major MutL homologous heterodimer. Again some, but not all, eukaryotes have additional MutL homologues, which all form a heterodimer with MLH1 and which play a minor role in MMR. Additional factors with a possible function in eukaryotic MMR are PCNA, EXO1, and the DNA polymerases delta and epsilon. MMR-independent pathways or factors that can process some types of mismatches in DNA are nucleotide-excision repair (NER), some base excision repair (BER) glycosylases, and the flap endonuclease FEN-1. A pathway has been identified in Saccharomyces cerevisiae and human that corrects loops with about 16 to several hundreds of unpaired nucleotides. Such large loops cannot be processed by MMR.

Pol32, a subunit of Saccharomyces cerevisiae DNA polymerase delta, suppresses genomic deletions and is involved in the mutagenic bypass pathway

The Pol32 subunit of S. cerevisiae DNA polymerase (Pol) delta plays an important role in replication and mutagenesis. Here, by measuring the CAN1 forward mutation rate, we found that either POL32 or REV3 (which encodes the Pol zeta catalytic subunit) inactivation produces overlapping antimutator effects against rad mutators belonging to three epistasis groups. In contrast, the msh2Delta pol32Delta double mutant exhibits a synergistic mutator phenotype. Can(r) mutation spectrum analysis of pol32Delta strains revealed a substantial increase in the frequency of deletions and duplications (primarily deletions) of sequences flanked by short direct repeats, which appears to be RAD52 and RAD10 independent. To better understand the pol32Delta and rev3Delta antimutator effects in rad backgrounds and the pol32Delta mutator effect in a msh2Delta background, we determined Can(r) mutation spectra for rad5Delta, rad5Delta pol32Delta, rad5Delta rev3Delta, msh2Delta, msh2Delta pol32Delta, and msh2Delta rev3Delta strains. Both rad5Delta pol32Delta and rad5Delta rev3Delta mutants exhibit a reduction in frameshifts and base substitutions, attributable to antimutator effects conferred by the pol32Delta and rev3Delta mutations. In contrast, an increase in these two types of alterations is attributable to a synergistic mutator effect between the pol32Delta and msh2Delta mutations. Taken together, these observations indicate that Pol32 is important in ensuring genome stability and in mutagenesis.

Partial reconstitution of human DNA mismatch repair in vitro: characterization of the role of human replication protein A

DNA mismatch repair (MMR) is a critical genome-stabilization system. However, the molecular mechanism of MMR in human cells remains obscure because many of the components have not yet been identified. Using a functional in vitro reconstitution system, this study identified three HeLa cell fractions essential for in vitro MMR. These fractions divide human MMR into two distinct stages: mismatch-provoked excision and repair synthesis. In vitro dissection of the MMR reaction and crucial intermediates elucidated biochemical functions of individual fractions in human MMR and identified hitherto unknown functions of human replication protein A (hRPA) in MMR. Thus, one fraction carries out nick-directed and mismatch-dependent excision; the second carries out DNA repair synthesis and DNA ligation; and the third provides hRPA, which plays multiple roles in human MMR by protecting the template DNA strand from degradation, enhancing repair excision, and facilitating repair synthesis. It is anticipated that further analysis of these fractions will identify additional MMR components and enable the complete reconstitution of the human MMR pathway with purified proteins.

Analysis of conditional mutations in the Saccharomyces cerevisiae MLH1 gene in mismatch repair and in meiotic crossing over

In mismatch repair (MMR), members of the MLH gene family have been proposed to act as key molecular matchmakers to coordinate mismatch recognition with downstream repair functions that result in mispair excision. Two members of this gene family, MLH1 and MLH3, have also been implicated in meiotic crossing over. These diverse roles suggest that a mutational analysis of MLH genes could provide reagents required to identify interactions between gene products and to test whether the different roles ascribed to a subset of these genes can be separated. In this report we show that in Saccharomyces cerevisiae the mlh1Delta mutation confers inviability in pol3-01 strain backgrounds that are defective in the Poldelta proofreading exonuclease activity. This phenotype was exploited to identify four mlh1 alleles that each confer a temperature-sensitive phenotype for viability in pol3-01 strains. In three different mutator assays, strains bearing conditional mlh1 alleles displayed wild-type or nearly wild-type mutation rates at 26 degrees. At 35 degrees, these strains exhibited mutation rates that approached those observed in mlh1Delta mutants. The mutator phenotype exhibited in mlh1-I296S strains was partially suppressed at 35 degrees by EXO1 overexpression. The mlh1-F228S and -I296S mutations conferred a separation-of-function phenotype in meiosis; both mlh1-F228S and -I296S strains displayed strong defects in meiotic mismatch repair but showed nearly wild-type levels of crossing over, suggesting that the conditional mutations differentially affected MLH1 functions. These genetic studies suggest that the conditional mlh1 mutations can be used to separate the MMR and meiotic crossing-over functions of MLH1 and to identify interactions between MLH1 and downstream repair components.

DNA mismatch repair defects: role in colorectal carcinogenesis

The inactivation of the DNA mismah repair (MMR) system, which is associated with the predisposition to the hereditary non-polyposis colorectal cancer (HNPCC), has also been documented in nearly 20% of the sporadic colorectal cancers. These tumors are characterized by a high frequency of microsatellite instability (MSI(+) phenotype), resulting from the accumulation of small insertions or deletions that frequently arise during replication of these short repeated sequences. A germline mutation of one of the two major MMR genes (hMSH2 or hMLH1) is found in half to two-thirds of the patients with HNPCC, whereas in sporadic cases hypermethylation of the hMLH1 promoter is the major cause of the MMR defect. Germline mutations in hMSH6 are rare and rather confer predisposition to late-onset familial colorectal cancer, and frequent extracolonic tumors. Yet, the genetic background of a number of HNPCC patients remains unexplained, indicating that other genes participate in MMR and play important roles in cancer susceptibility. The tumor-suppressor genes that are potential targets for the MSI-driven mutations because they contain hypermutable repeated sequences are likely to contribute to the etiology and tissue specificity of the MSI-associated carcinogenesis. Because the prognosis and the chemosensitivity of the MSI(+) colorectal tumors differ from those without instability, the determination of the MSI phenotype is expected to improve the clinical management of patients. This review gives an overview of various aspects of the biochemistry and genetics of the DNA mismah repair system, with particular emphasis in its role in colorectal carcinogenesis.

Interactions of Exo1p with components of MutLalpha in Saccharomyces cerevisiae

Previously, we reported evidence suggesting that Saccharomyces cerevisiae MutLalpha, composed of Mlh1p and Pms1p, was a functional member of the gyrase b/Hsp90/MutL (GHL) dimeric ATPase superfamily characterized by highly conserved ATPase domains. Similar to other GHL ATPases, these putative ATPase domains of MutLalpha may be important for the recruitment and/or activation of downstream effectors. One downstream effector candidate is Exo1p, a 5'-3' double stranded DNA exonuclease that has previously been implicated in DNA mismatch repair (MMR). Here we report yeast two-hybrid results suggesting that Exo1p can interact physically with MutLalpha through the Mlh1p subunit. We also report epistasis analysis involving MutLalpha ATPase mutations combined with exo1Delta. One interpretation of our genetic results is that MutLalpha ATPase domains function to direct Exo1p and other functionally redundant exonucleases during MMR. Finally, our results show that much of the increase in spontaneous mutation observed in an exo1Delta strain is REV3-dependent, in turn suggesting that Exo1p is also involved in one or more MMR-independent mutation avoidance pathways.