Functional overlap in mismatch repair by human MSH3 and MSH6

Molecular mechanisms of human carcinogenesis

Intensive research efforts during the last several decades have increased our understanding of carcinogenesis, and have identified a genetic basis for the multi-step process of cancer development. Tumors grow through a process of clonal expansion driven by mutation. Several forms of molecular alteration have been described in human cancers, and these can be generally classified as chromosomal abnormalities and nucleotide sequence abnormalities. Most cancer cells display a phenotype characterized by genomic hypermutability, suggesting that genomic instability may precede the acquisition of transforming mutations in critical target genes. Reduced to its essence, cancer is a disease of abnormal gene expression, and these genetic abnormalities contribute to cancer pathogenesis through inactivation of negative mediators of cell proliferation (including tumor suppressor genes) and activation of positive mediators of cell proliferation (including proto-oncogenes). In several human tumor systems, specific genetic alterations have been shown to correlate with well-defined histopathological stages of tumor development and progression. Although the significance of mutations to the etiological mechanisms of tumor development has been debated, a causal role for such genetic lesions is now commonly accepted for most human cancers. Thus, genetic lesions represent an integral part of the processes of neoplastic transformation, tumorigenesis, and tumor progression, and as such represent potentially valuable markers for cancer detection and staging.

Contributions by MutL homologues Mlh3 and Pms2 to DNA mismatch repair and tumor suppression in the mouse

Germ line DNA mismatch repair mutations in MLH1 and MSH2 underlie the vast majority of hereditary non-polyposis colon cancer. Four mammalian homologues of Escherichia coli MutL heterodimerize to form three distinct complexes: MLH1/PMS2, MLH1/MLH3, and MLH1/PMS1. Although MLH1/PMS2 is generally thought to have the major MutL activity, the precise contributions of each MutL heterodimer to mismatch repair functions are poorly understood. Here, we show that Mlh3 contributes to mechanisms of tumor suppression in the mouse. Mlh3 deficiency alone causes microsatellite instability, impaired DNA-damage response, and increased gastrointestinal tumor susceptibility. Furthermore, Mlh3;Pms2 double-deficient mice have tumor susceptibility, shorter life span, microsatellite instability, and DNA-damage response phenotypes that are indistinguishable from Mlh1-deficient mice. Our data support previous results from budding yeast that show partial functional redundancy between MLH3 and PMS2 orthologues for mutation avoidance and show a role for Mlh3 in gastrointestinal and extragastrointestinal tumor suppression. The data also suggest a mechanistic basis for the more severe mismatch repair-related phenotypes and cancer susceptibility in Mlh1- versus Mlh3- or Pms2-deficient mice. Contributions by both MLH1/MLH3 and MLH1/PMS2 complexes to mechanisms of mismatch repair-mediated tumor suppression, therefore, provide an explanation why, among MutL homologues, only germ line mutations in MLH1 are common in hereditary non-polyposis colon cancer.

Somatic hypermutation and mismatch repair in non-B cells

Mismatch repair contributes to hypermutation in B lymphocytes, both by increasing the frequency of mutations and by changing the mutational patterns. In this paper, we investigated whether or not mismatch repair influences activation-induced cytidine deaminase (AID)-mediated hypermutation in a non-B lymphocyte line. We did so by regulating expression of MutL homologue MLH 1, which is essential in mismatch repair, in a kidney cell line that had been transduced by an AID-containing vector. Whether or not MLH1 was expressed, we found no difference in the mutation rates of an indicator gene. We conclude that in order to contribute to hypermutation, mismatch repair needs additional factors that are present in activated B lymphocytes, but absent in the cell line investigated.

Microsatellite Instability and DNA Mismatch Repair Deficiency Testing in Hereditary and Sporadic Gastrointestinal Cancers

The reference cancers associated with DNA mismatch repair (MMR)deficiency are the adenocarcinomas of patients with hereditary nonpolyposis colorectal cancer, also known as Lynch syndrome. Sporadic gastrointestinal (GI) carcinomas, most commonly colorectal and gastric carcinomas, may also be associated with deficiencies of DNA mismatch repair. Deficiency in cellular MMR leads to wide-spread mutagenesis and neoplastic development and progression. An important diagnostic feature of MMR-deficient tumors is the high rate of mutations that accumulate in repetitive nucleotide regions, and these mutations are known as microsatellite instability(MSI). A standard panel of markers to test for MSI in tumors has been recommended and efficiently separates tumors into those with high, low, or no microsatellite instability (MSI-H, MSI-L, or MSS). Tumors characterized by MSI-H characteristically show loss of one of the main DNA MMR proteins, mLH1 or MSH2, and rarely MSH6 and PMS2, detected by immunohistochemistry (IHC). The combination of MSI testing and IHC for MMR proteins in tumors tissues is used to identify underlying DNA MMR deficiency andis clinically relevant screen patients who might have hereditary non-polyposis colorectal cancer for DNA repair gene germline testing. Increasing evidence demonstrates that tumors with a positive MSI status have lower lymph node metastases burden, and these patients have an overall improved survival, suggesting that the MSI and MMR status may contribute to decision making regarding treatment approaches. Updated guidelines for MSI and IHC for DNAMMR testing, and the biological and potential clinical implications of MMR deficiency and microsatellite instability in GI polyps and cancers are reviewed.

Cadmium inhibits the functions of eukaryotic MutS complexes

Exposure of yeast cells to low concentrations of cadmium results in elevated mutation rates due to loss of mismatch repair (MMR), and cadmium inhibits MMR activity in extracts of human cells. Here we show that cadmium inhibits both Msh2-Msh6- and Msh2-Msh3-dependent human MMR activity in vitro. This inhibition, which occurs at a step or steps preceding repair DNA synthesis, is observed for repair directed by either a 3' or a 5' nick. In an attempt to identify the protein target(s) of cadmium inhibition, we show that cadmium inhibition of MMR is not reversed by addition of zinc to the repair reaction, suggesting that the target is not a zinc metalloprotein. We then show that cadmium inhibits ATP hydrolysis by yeast Msh2-Msh6 but has no effect on ATPase hydrolysis by yeast Mlh1-Pms1. Steady state kinetic analysis with wild type Msh2-Msh6, and with heterodimers containing subunit-specific Glu to Ala replacements inferred to inactivate the ATPase activity of either Msh2 or Msh6, suggest that cadmium inhibits ATP hydrolysis by Msh6 but not Msh2. Cadmium also reduces DNA binding by Msh2-Msh6 and more so for mismatched than matched duplexes. These data indicate that eukaryotic Msh2-Msh3 and Msh2-Msh6 complexes are targets for inhibition of MMR by cadmium, a human lung carcinogen that is ubiquitous in the environment.

Involvement of the Arabidopsis thaliana AtPMS1 gene in somatic repeat instability

Mismatch repair (MMR) genes participate in the maintenance of genome stability in all organisms. Based on its high degree of sequence conservation, it seems likely that the AtPMS1 gene of Arabidopsis thaliana is part of the MMR system in this model plant. To test this hypothesis, we aimed to disrupt AtPMS1 function by over-expressing mutated alleles expected to result in a dominant negative effect. To create one mutant allele we substituted two amino acids in the MutL-box, and for the other mutant allele we deleted 87 amino acids comprising the whole MutL-box. Contrary to published reports in some eukaryotes, transgenic plants expressing these alleles did not exhibit a decrease in fertility nor any other visible phenotype. To examine the impact of these mutations on microsatellite instability, the phenotype most often observed in organisms defective in MMR, reporter lines containing a uidA (GUS) gene inactivated by the insertion of a synthetic microsatellite (G7 or G16) were used. GUS gene function in these lines can be restored following the loss of one base or the gain of two bases in the repetitive tract. This results in the observation of blue sectors on a white background following histochemical staining. In a subset of the transformants, a significant increase (2- to 28-fold) in microsatellite instability was observed relative to wild-type. This report shows that MMR function can be disrupted via a dominant negative approach, and it is the first report to describe the phenotypic consequence of disrupting the function of a MutL homolog in plants.

Loss of DNA mismatch repair function and cancer predisposition in the mouse: animal models for human hereditary nonpolyposis colorectal cancer

Germline mutations in DNA mismatch repair genes underlie one of the most common hereditary cancer predisposition syndromes known in humans, hereditary nonpolyposis colorectal cancer (HNPCC). Defects of the DNA mismatch repair system are also prevalent in sporadic colorectal cancers. The generation of mice with targeted inactivating mutations in the mismatch repair genes has facilitated the in vivo study of how these genes function and how their individual loss contributes to tumorigenesis. Although there are notable limitations when using murine models to study the molecular basis of human cancer, there is remarkable similarity between the two species with respect to the contribution of individual members of the mismatch repair system to cancer susceptibility, and mouse mutants have greatly enhanced our understanding of the normal role of these genes in mutation avoidance and suppression of tumorigenesis.

Dominant effects of an Msh6 missense mutation on DNA repair and cancer susceptibility

Mutations in DNA mismatch repair (MMR) genes cause hereditary nonpolyposis colorectal cancer (HNPCC), and MMR defects are associated with a significant proportion of sporadic cancers. MMR maintains genome stability and suppresses tumor formation by preventing the accumulation of mutations and by mediating an apoptotic response to DNA damage. We describe the analysis of a dominant MSH6 missense mutation in yeast and mice that causes loss of DNA repair function while having no effect on the apoptotic response to DNA damaging agents. Our results demonstrate that MSH6 missense mutations can effectively separate the two functions, and that increased mutation rates associated with the loss of DNA repair are sufficient to drive tumorigenesis in MMR-defective tumors.

Apoptotic signaling in response to a single type of DNA lesion, O(6)-methylguanine

Until now, it has been difficult to establish exactly how a specific DNA lesion signals apoptosis because each DNA damaging agent produces a collection of distinct DNA lesions and produces damage in RNA, protein, and lipids. We have developed a system in human cells that focuses on the response to a single type of DNA lesion, namely O(6)-methylguanine (O(6)MeG). We dissect the signaling pathways involved in O(6)MeG-induced apoptosis, a response dependent on the MutSalpha heterodimer that is normally involved in DNA mismatch repair. O(6)MeG triggers robust activation of caspases associated with both death receptor- and mitochondrial-mediated apoptosis. Despite this, O(6)MeG/MutSalpha-triggered apoptosis is only partly dependent on caspase activation; moreover, it is mediated solely by mitochondrial signaling and not at all by death receptor signaling. Finally, while Bcl-2 and Bcl-x(L), negative regulators of mitochondrial-regulated apoptosis, could effectively block O(6)MeG/MutSalpha-dependent apoptosis, they were unable to prevent the cells from ultimately dying.

Differential specificities and simultaneous occupancy of human MutSalpha nucleotide binding sites

We have examined the permissible nucleotide occupancy states of human MutSalpha. The MSH2.MSH6 heterodimer binds 1 mol of ADP and 1 mol of adenosine 5'-O-(thiotriphosphate) (ATPgammaS), with a K(d) for each nucleotide of about 1 microm. Anisotropy measurements using BODIPY TR and BODIPY FL fluorescent derivatives of ADP and 5'-adenylyl-beta,gamma-imidodiphosphate (AMPPNP) also indicate an interaction stoichiometry of 1 mol of ADP and 1 mol of triphosphate analogue per MutSalpha heterodimer. Di- and triphosphate sites can be simultaneously occupied as judged by sequential filling of the two binding site classes with differentially radiolabeled ADP and ATPgammaS and by fluorescence resonance energy transfer between BODIPY TR- and BODIPY FL-labeled ADP and AMPPNP. ATP hydrolysis by MutSalpha is accompanied by a pre-steady-state burst of ADP formation, and analysis of MutSalpha-bound nucleotide during the first turnover has demonstrated the presence of both ADP and ATP. Simultaneous presence of ADP and a nonhydrolyzable ATP analogue modulates MutSalpha.heteroduplex interaction in a manner that is distinct from that observed in the presence of ADP or nonhydrolyzable triphosphate alone, and it is unlikely that this effect is due to the presence of a mixed population of binary complexes between MutSalpha and ADP or a triphosphate analogue. These findings imply that MutSalpha has two nucleotide binding sites with differential specificities for ADP and ATP and suggest that the ADP.MutSalpha.ATP ternary complex has an important role in mismatch repair.

An Msh2 point mutation uncouples DNA mismatch repair and apoptosis

Mutations in the human DNA mismatch repair gene MSH2 are associated with hereditary nonpolyposis colorectal cancer as well as a significant proportion of sporadic colorectal cancer. The inactivation of MSH2 results in the accumulation of somatic mutations in the genome of tumor cells and resistance to the genotoxic effects of a variety of chemotherapeutic agents. Here we show that the DNA repair and DNA damage-induced apoptosis functions of Msh2 can be uncoupled using mice that carry the G674A missense mutation in the conserved ATPase domain. As a consequence, although Msh2(G674A) homozygous mutant mice are highly tumor prone, the onset of tumorigenesis is delayed as compared with Msh2-null mice. In addition, tumors that carry the mutant allele remain responsive to treatment with a chemotherapeutic agent. Our results indicate that Msh2-mediated apoptosis is an important component of tumor suppression and that certain MSH2 missense mutations can cause mismatch repair deficiency while retaining the signaling functions that confer sensitivity to chemotherapeutic agents.

DNA mismatch repair: molecular mechanisms and biological function

DNA mismatch repair (MMR) guards the integrity of the genome in virtually all cells. It contributes about 1000-fold to the overall fidelity of replication and targets mispaired bases that arise through replication errors, during homologous recombination, and as a result of DNA damage. Cells deficient in MMR have a mutator phenotype in which the rate of spontaneous mutation is greatly elevated, and they frequently exhibit microsatellite instability at mono- and dinucleotide repeats. The importance of MMR in mutation avoidance is highlighted by the finding that defects in MMR predispose individuals to hereditary nonpolyposis colorectal cancer. In addition to its role in postreplication repair, the MMR machinery serves to police homologous recombination events and acts as a barrier to genetic exchange between species.

Regulation of the human MSH6 gene by the Sp1 transcription factor and alteration of promoter activity and expression by polymorphisms

Defects in human DNA mismatch repair have been reported to underlie a variety of hereditary and sporadic cancer cases. We characterized the structure of the MSH6 promoter region to examine the mechanisms of transcriptional regulation of the MSH6 gene. The 5'-flanking region of the MSH6 gene was found to contain seven functional Sp1 transcription factor binding sites that each bind Sp1 and Sp3 and contribute to promoter activity. Transcription did not appear to require a TATA box and resulted in multiple start sites, including two major start sites and at least nine minor start sites. Three common polymorphisms were identified in the promoter region (-557 T-->G, -448 G-->A, and -159 C-->T): the latter two were always associated, and each of these functionally inactivated a different Sp1 site. The polymorphic allele -448 A -159 T was demonstrated to be a common Caucasian polymorphism found in 16% of Caucasians and resulted in a five-Sp1-site promoter that had 50% less promoter activity and was more sensitive to inactivation by DNA methylation than the more common seven Sp1 site promoter allele, which was only partially inactivated by DNA methylation. In cell lines, this five-Sp1-site polymorphism resulted in reduced MSH6 expression at both the mRNA and protein level. An additional 2% of Caucasians contained another polymorphism, -210 C-->T, which inactivated a single Sp1 site that also contributes to promoter activity.

Inactivation of Exonuclease 1 in mice results in DNA mismatch repair defects, increased cancer susceptibility, and male and female sterility

Exonuclease 1 (Exo1) is a 5'-3' exonuclease that interacts with MutS and MutL homologs and has been implicated in the excision step of DNA mismatch repair. To investigate the role of Exo1 in mammalian mismatch repair and assess its importance for tumorigenesis and meiosis, we generated an Exo1 mutant mouse line. Analysis of Exo1(-/-) cells for mismatch repair activity in vitro showed that Exo1 is required for the repair of base:base and single-base insertion/deletion mismatches in both 5' and 3' nick-directed repair. The repair defect in Exo1(-/-) cells also caused elevated microsatellite instability at a mononucleotide repeat marker and a significant increase in mutation rate at the Hprt locus. Exo1(-/-) animals displayed reduced survival and increased susceptibility to the development of lymphomas. In addition, Exo1(-/-) male and female mice were sterile because of a meiotic defect. Meiosis in Exo1(-/-) animals proceeded through prophase I; however, the chromosomes exhibited dynamic loss of chiasmata during metaphase I, resulting in meiotic failure and apoptosis. Our results show that mammalian Exo1 functions in mutation avoidance and is essential for male and female meiosis.

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

The DNA mismatch repair (MMR) system is primarily responsible for purging newly synthesized DNA of errors incurred during semi-conservative replication. Lesion recognition is initially carried out by one of two heterodimeric protein complexes, MutS(alpha) or MutS(beta). While the former, comprised of MSH2 and MSH6, recognizes mispairs as well as short (1-2 nucleotide) insertions/deletions (IDLs), the latter, made up of MSH2 and MSH3, is primarily responsible for recognizing 2-6 nucleotide IDLs. As most of the functional information on these heterodimers is derived from in vitro studies, it was of interest to study the in vivo consequences of a lack of MutS(alpha). To this end, Big Blue( trade mark ) mice, that carry a lacI(+) transgenic lambda shuttle-phage mutational reporter, were crossed with Msh6(-/-) mice to evaluate the specific contribution of MutS(alpha) to genome integrity. Consistent with the importance of MutS(alpha) in lesion surveillance, small intestine epithelial cell DNA derived from lacI(+) Msh6(-/-) mice exhibited striking increases (average of 41-fold) in spontaneous mutant frequencies. Furthermore, the lacI gene mutation spectrum was dominated by G:C to A:T transitions, highlighting the critical importance of the MutS(alpha) complex in suppressing this frequently observed type of spontaneous mutation.

Helicobacter pylori impairs DNA mismatch repair in gastric epithelial cells

BACKGROUND & AIMS

Helicobacter pylori infection is a major gastric cancer risk factor. H. pylori gastritis occurs more frequently in individuals with microsatellite instability-positive than those with microsatellite instability-negative gastric cancers, raising the possibility that H. pylori infection affects DNA mismatch repair (MMR). The aim of this study was to determine the effect of H. pylori on the expression of DNA MMR proteins and RNA in gastric epithelial cells.

METHODS

Gastric cancer cell lines were cocultured with H. pylori, bacterial extracts, and Campylobacter jejuni or Escherichia coli. MutS (hMSH2 and hMSH6) and MutL (hMLH1, hPMS2, and hPMS1) DNA MMR protein and RNA levels were determined.

RESULTS

All cell lines examined showed decreased levels of MutS and MutL DNA MMR proteins in a dose-dependent manner after coculture with H. pylori strains. The reduction in DNA MMR protein levels was caused by heat-sensitive H. pylori products. The levels of DNA MMR proteins were affected by C. jejuni but not by E. coli. RNA levels of hMSH2 and hMSH6 were also reduced after exposure to H. pylori.

CONCLUSIONS

H. pylori infection of gastric epithelial cells leads to a decrease in DNA MMR proteins that is at least in part related to an H. pylori-induced decrease in messenger RNA levels of repair genes. These data suggest that H. pylori infection might lead to a deficiency of DNA MMR in gastric epithelial cells that may increase the risk of mutation accumulation in gastric mucosa cells and the risk of gastric cancer during chronic H. pylori infection.

Frequent germline mutations and somatic repeat instability in DNA mismatch-repair-deficient Caenorhabditis elegans

Mismatch-repair-deficient mutants were initially recognized as mutation-prone derivatives of bacteria, and later mismatch repair deficiency was found to predispose humans to colon cancers (HNPCC). We generated mismatch-repair-deficient Caenorhabditis elegans by deleting the msh-6 gene and analyzed the fidelity of transmission of genetic information to subsequent generations. msh-6-defective animals show an elevated level of spontaneous mutants in both the male and female germline; also repeated DNA tracts are unstable. To monitor DNA repeat instability in somatic tissue, we developed a sensitive system, making use of heat-shock promoter-driven lacZ transgenes, but with a repeat that puts this reporter gene out of frame. In genetic msh-6-deficient animals lacZ+ patches are observed as a result of somatic repeat instability. RNA interference by feeding wild-type animals dsRNA homologous to msh-2 or msh-6 also resulted in somatic DNA instability, as well as in germline mutagenesis, indicating that one can use C. elegans as a model system to discover genes involved in maintaining DNA stability by large-scale RNAi screens.

Methylation tolerance in mismatch repair proficient cells with low MSH2 protein level

Loss of DNA mismatch repair has been found in tumors associated with the familial cancer predisposition syndrome HNPCC (hereditary non-polyposis colorectal cancer) and a subset of sporadic cancers. MSH2 deficiency abolishes the action of the mismatch repair system, resulting in a phenotype which is characterized by an increased accumulation of base substitutions and frameshifts, enhanced recombination between homologous but non-identical DNA sequences, and tolerance to the cytotoxic effects of methylating agents. In this study we describe an embryonic stem cell line in which the level of MSH2 protein is 10-fold reduced compared to that in wild-type cells. Remarkably, these MSH2-low cells were as resistant to killing by methylating agents as cells completely lacking MSH2, while they had retained almost maximal mismatch repair capacity as judged from their anti-mutagenic and anti-recombinogenic capacity and the absence of microsatellite instability. In contrast, MSH2-low cells were highly sensitive to methylation-damage induced mutagenesis. Thus, 10-fold reduced MSH2 protein levels render cells resistant to the toxic and highly sensitive to the mutagenic effects of methylating agents. This condition is not manifested by microsatellite instability and may have implications for both the etiology and treatment of cancer.

Involvement of hMSH6 in the development of hereditary and sporadic colorectal cancer revealed by immunostaining is based on germline mutations, but rarely on somatic inactivation

Germline mutations in human mismatch repair (MMR) genes yield a predisposition for the hereditary nonpolyposis colon cancer (HNPCC) syndrome. In contrast to hMLH1 and hMSH2, little is known about the overall involvement of hMSH6 in colorectal cancer. We investigated 82 tumors from patients who fulfilled the Bethesda guidelines for HNPCC as well as 146 sporadic tumors, analyzing microsatellite instability and expression of the 4 MMR proteins hMSH6, hMSH2, hMLH1 and hPMS2. Four tumors with lost expression and 1 tumor with cytoplasmic expression of hMSH6 were identified. Sequence analysis revealed germline mutations in 4 of the 5 patients, including 1 patient with sporadic disease. The lost or reduced expression of hMSH2 and hMLH1 was always identical to its heterodimerization partners, hMSH6 and hPMS2, respectively. Furthermore, hMSH2 expression was reduced upon hMSH6 deficiency. Abnormal expression of 1 or more of the 4 proteins was always associated with a high level of microsatellite instability (MSI-H). Conversely, all but 1 of the 44 MSI-H tumors had abnormal expression of 1 or more of the proteins, basically excluding additional genes associated with the MSI-H phenotype. We conclude that the involvement of somatic or epigenetic hMSH6 inactivation in colorectal cancer is rare.

Functional interactions and signaling properties of mammalian DNA mismatch repair proteins

The mismatch repair (MMR) system promotes genomic fidelity by repairing base-base mismatches, insertion-deletion loops and heterologies generated during DNA replication and recombination. This function is critically dependent on the assembling of multimeric complexes involved in mismatch recognition and signal transduction to downstream repair events. In addition, MMR proteins coordinate a complex network of physical and functional interactions that mediate other DNA transactions, such as transcription-coupled repair, base excision repair and recombination. MMR proteins are also involved in activation of cell cycle checkpoint and induction of apoptosis when DNA damage overwhelms a critical threshold. For this reason, they play a role in cell death by alkylating agents and other chemotherapeutic drugs, including cisplatin. Inactivation of MMR genes in hereditary and sporadic cancer is associated with a mutator phenotype and inhibition of apoptosis. In the future, a deeper understanding of the molecular mechanisms and functional interactions of MMR proteins will lead to the development of more effective cancer prevention and treatment strategies.

Mismatch repair in correction of replication errors and processing of DNA damage

The primary role of mismatch repair (MMR) is to maintain genomic stability by removing replication errors from DNA. This repair pathway was originally implicated in human cancer through an association between microsatellite instability in colorectal tumors in hereditary nonpolyposis colon cancer (HNPCC) kindreds. Microsatellites are short repetitive sequences which are often copied incorrectly by DNA polymerases because the template and daughter strands in these regions are particularly prone to misalignment. These replication-dependent events create loops of extrahelical bases which would produce frameshift mutations unless reversed by MMR. One consequence of MMR loss is a widespread expansion and contraction of these repeated sequences that affects the whole genome. Defective MMR is therefore associated with a mutator phenotype. Since the same pathway is also responsible for repairing base:base mismatches, defective cells also experience large increases in the frequency of spontaneous transition and transversion mutations. Three different approaches have been used to investigate the function of individual components of the MMR pathway. The first is based on the biochemical characterization of the purified protein complexes using synthetic DNA substrates containing loops or single mismatches. In the second, the biological consequences of MMR loss are inferred from the phenotype of cell lines established from repair-deficient human tumors, from tolerant cells or from mice defective in single MMR genes. In particular, molecular analysis of the mutations in endogenous or reporter genes helped to identify the DNA substrates for MMR. Finally, mice bearing single inactive MMR genes have helped to define the involvement of MMR in cancer prevention.

DNA mismatch repair and genetic instability

Mismatch repair (MMR) systems play a central role in promoting genetic stability by repairing DNA replication errors, inhibiting recombination between non-identical DNA sequences and participating in responses to DNA damage. The discovery of a link between human cancer and MMR defects has led to an explosion of research on eukaryotic MMR. The key proteins in MMR are highly conserved from bacteria to mammals, and this conservation has been critical for defining the components of eukaryotic MMR systems. In eukaryotes, there are multiple homologs of the key bacterial MutS and MutL MMR proteins, and these homologs form heterodimers that have discrete roles in MMR-related processes. This review describes the genetic and biochemical approaches used to study MMR, and summarizes the diverse roles that MMR proteins play in maintaining genetic stability.

Fidelity of replication of repetitive DNA in mutS and repair proficient Escherichia coli

Replication fidelity is not constant among strains within a species or at all genetic loci within a genome. Altered fidelity of replication may affect patterns of pathogenesis and the evolution of these strains. We have been studying replication fidelity in Escherichia coli, both in laboratory attenuated strains and in food-borne pathogens. To understand the altered patterns of mutagenesis at the molecular level, we used a shuttle vector plasmid with a tRNA mutational marker gene which had been altered to include homopolymeric runs of five, seven and nine [G:C] pairs, as well as non-repetitive DNA. Replication of the plasmid in mutS strains resulted in a 20-fold increase in mutant progeny plasmids. The mutations were almost all (>90%) frameshift mutations, while base substitution mutations were rare. Most mutations were insertions or deletions of one or two [G:C] pairs in the longest homopolymeric runs. Larger deletions (5 to >70bp), also targeted to the repetitive sequence, were likewise common. Mutations increased exponentially with the length of the homopolymeric run. These patterns of mutation, including unexpectedly high levels in repair proficient strains, led to an examination of the E. coli K-12 genome for homopolymeric DNA. This sequence motif was found to be rare, particularly in genes and open reading frames. Amino acid homotrimers were found to avoid usage of homopolymeric codons, even when they are preferred among synonymous codons in E. coli. There appears to be active selection against tandem direct nucleotide repeats in the E. coli genome, correlated with the inability of the organism to accurately replicate such sequence.

In vivo and in vitro studies of immunoglobulin gene somatic hypermutation

Following antigen encounter, two distinct processes modify immunoglobulin genes. The variable region is diversified by somatic hypermutation while the constant region may be changed by class-switch recombination. Although both genetic events can occur concurrently within germinal centre B cells, there are examples of each occurring independently of the other. Here we compare the contributions of class-switch recombination and somatic hypermutation to the diversification of the serum immunoglobulin repertoire and review evidence that suggests that, despite clear differences, the two processes may share some aspects of their mechanism in common.

Tumor-associated Apc mutations in Mlh1-/- Apc1638N mice reveal a mutational signature of Mlh1 deficiency

Apc1638N mice, which are heterozygous for a germline mutation in Apc, typically develop three to five spontaneous intestinal tumors per animal. In most cases this is associated with allelic loss of wildtype Apc. We have previously reported that the multiplicity of intestinal tumors is increased dramatically by crossing Apc1638N with an Mlh1-deficient mouse strain that represents an animal model of hereditary non-polyposis colorectal cancer (HNPCC). The increased tumor multiplicity in these mice was associated with somatic mutations in the Apc tumor suppressor gene. Here, we have examined the nature and distribution of 91 Apc mutations implicated in the development of intestinal tumors in Mlh1-/- Apc1638N animals. Protein truncation mutations were detected in a majority of tumor samples, indicating that the prevailing mechanism of Apc mutation in tumors is altered from allelic loss to intragenic mutation as a result of Mlh1 deficiency. The observed mutations were a mixture of base substitutions (27%) and frameshifts (73%). Most frameshifts were detected within dinucleotide repeats and there were prominent mutational hotspots within sequences of this sort at codons 927-929, 1209-1211 and 1461-1464. The observed Apc mutations caused protein truncation upstream of the third 20 amino acid beta-catenin binding domain and the first Axin-binding SAMP repeat, yielding Apc proteins that are predicted to be deficient in destabilizing beta-catenin. Our results reveal a characteristic mutational signature in Apc that is attributable to Mlh1 deficiency. This demonstrates a direct effect of Mlh1 deficiency in the mutation of Apc in these tumors, and provides data that clarify the role of Mlh1 in mammalian DNA mismatch repair.

Germline and somatic mutation analysis of MLH3 in MSI-positive colorectal cancer

Microsatellite instability (MSI) is characteristic of hereditary nonpolyposis colorectal cancer, and occurs in a subset (10 to 15%) of unselected colorectal cancer cases. In hereditary nonpolyposis colorectal cancer, MSI is caused by defects in five mismatch repair genes, and in sporadic cases the main cause seems to be somatic MLH1 promoter methylation. Most likely additional hereditary nonpolyposis colorectal cancer genes remain to be discovered. Genes with simple repeats in their coding region are often targets for deletions in MSI-positive tumors. Several genes (TGFbeta RII, IGFIIR, MSH3, MSH6, BAX, MBD4) with significance in tumorigenesis harbor repeats in their coding regions and are often somatically inactivated because of deletions causing frameshifts. Recently, a novel human mismatch repair gene, MLH3, was cloned and shown to be involved in mammalian mismatch repair. To evaluate the possible role of MLH3 in hereditary cancer, we performed germline single-strand conformation polymorphism-analysis for 52 patients displaying features of inherited colorectal cancer. Forty-six of these had been diagnosed with MSI-positive tumors. No germline mutations were found. Similar to MSH3 and MSH6, MLH3 harbors mononucleotide repeats, ie, (A(6))-(A(9)), in its coding region, which makes it a putative target for somatic mutations in MSI-positive tumors. To evaluate its somatic inactivation we performed a deletion search focusing on eight exonic MLH3 mononucleotide repeats in a series of 93 MSI-positive tumors. Somatic deletions were found in 8.6% of the samples, a frequency similar to one detected in neutral noncoding mononucleotide repeats. No evidence of involvement of MLH3 in MSI tumorigenesis was obtained.

Mismatch repair proteins and mitotic genome stability

Mismatch repair (MMR) proteins play a critical role in maintaining the mitotic stability of eukaryotic genomes. MMR proteins repair errors made during DNA replication and in their absence, mutations accumulate at elevated rates. In addition, MMR proteins inhibit recombination between non-identical DNA sequences, and hence prevent genome rearrangements resulting from interactions between repetitive elements. This review provides an overview of the anti-mutator and anti-recombination functions of MMR proteins in the yeast Saccharomyces cerevisiae.

Steady-state regulation of the human DNA mismatch repair system

Steady-state levels of human DNA mismatch repair (MMR) transcripts and proteins were measured in MMR-proficient and -deficient cell lines by the newly developed competitive quantitative reverse transcription- polymerase chain reaction and Western analysis normalized with purified proteins. In MMR-proficient cells, hMSH2 is the most abundant MMR protein and is expressed 3 to 5 times more than hMLH1. The hMLH1 protein was expressed 1.5 to 2.5 times more than hPMS2. Steady-state levels of mRNA expression correlated well with protein expression. hMSH2-mutated LoVo cells did not express detectable hMSH3 or hMSH6 proteins. Similarly, hMLH1-mutated HCT116 cells did not express detectable hMLH1 or hPMS2 protein, whereas in hMLH1-restored HCT116+ch3 cells, hPMS2 protein was re-expressed. In hMSH6-mutated HCT15 cells, both hMSH3 protein and mRNA were increased. In SV40-transformed lung fibroblasts, all MMR mRNAs and proteins examined were expressed at levels 1.5-5-fold higher than in their nontransformed counterpart. The steady-state levels of MMR proteins indicate that substantially more hMutS proteins, which are involved in DNA mismatch recognition, are present in comparison with the hMutL proteins. Stability of hMSH3 and hMSH6 proteins appears to depend upon the presence of the hMSH2 protein, and, similarly, the stability of the hPMS2 protein depends upon hMLH1. When the hMSH6 is mutationally inactivated, hMSH3 increases by both transcriptional up-regulation and enhanced protein stability. A balanced up-regulation of all of the components was seen after viral transformation in a fibroblast model. Quantitative changes of the MMR components are a potential mechanism to modify the DNA MMR capabilities of a cell.

Arabidopsis MutS homologs-AtMSH2, AtMSH3, AtMSH6, and a novel AtMSH7-form three distinct protein heterodimers with different specificities for mismatched DNA

Arabidopsis mismatch repair genes predict MutS-like proteins remarkably similar to eukaryotic MutS homologs-MSH2, MSH3, and MSH6. A novel feature in Arabidopsis is the presence of two MSH6-like proteins, designated AtMSH6 and AtMSH7. Combinations of Arabidopsis AtMSH2 with AtMSH3, AtMSH6, or AtMSH7 proteins-products of in vitro transcription and translation-were analyzed for interactions by analytical gel filtration chromatography. The AtMSH2 protein formed heterodimers with AtMSH3, AtMSH6, and AtMSH7, but no single proteins formed homodimers. The abilities of the various heterodimers to bind to mismatched 51-mer duplexes were measured by electrophoretic mobility-shift assays. Similar to the behavior of the corresponding human proteins, AtMSH2*AtMSH3 heterodimers bound "insertion-deletion" DNA with three nucleotides (+AAG) or one nucleotide (+T) looped out much better than they bound DNA with a base/base mispair (T/G), whereas AtMSH2*AtMSH6 bound the (+T) substrate strongly, (T/G) well, and (+AAG) no better than it did a (T/A) homoduplex. However, AtMSH2*AtMSH7 showed a different specificity: moderate affinity for a (T/G) substrate and weak binding of (+T). Thus, AtMSH2*AtMSH7 may be specialized for lesions/base mispairs not tested or for (T/G) mispairs in special contexts.

Identification of a second MutL DNA mismatch repair complex (hPMS1 and hMLH1) in human epithelial cells

Deficiencies of MutL DNA mismatch repair-complex proteins (hMLH1, hPMS2, and hPMS1) typically result in microsatellite instability in human cancers. We examined the association patterns of MutL proteins in human epithelial cancer cell lines with (HCT-116, N87, SNU-1, and SNU-638) and without microsatellite instability (HeLa, AGS, KATO-III, and SNU-16). The analysis of hMLH1, hPMS2, and hPMS1 was performed using Northern blot, Western blot, and co-immunoprecipitation studies. Our data provide evidence that MutL proteins form two different complexes, MutL-alpha (hPMS2 and hMLH1) and MutL-beta (hPMS1 and hMLH1). Gastric and colorectal cancer cells lines with microsatellite instability lacked detectable hMLH1. Decreased levels of hMLH1 protein were associated with markedly reduced levels of hPMS2 and hPMS1 proteins, but the RNA levels of hPMS1 and hPMS2 were normal. In this study, we describe the association of hPMS1 with hMLH1 as a heterodimer, in human cells. Furthermore, normal levels of hMLH1 protein appear to be important in maintaining normal levels of hPMS1 and hPMS2 proteins.

MutS homolog 4 localization to meiotic chromosomes is required for chromosome pairing during meiosis in male and female mice

Msh4 (MutS homolog 4) is a member of the mammalian mismatch repair gene family whose members are involved in postreplicative DNA mismatch repair as well as in the control of meiotic recombination. In this report we show that MSH4 has an essential role in the control of male and female meiosis. We demonstrate that MSH4 is present in the nuclei of spermatocytes early in prophase I and that it forms discrete foci along meiotic chromosomes during the zygotene and pachytene stages of meiosis. Disruption of the Msh4 gene in mice results in male and female sterility due to meiotic failure. Although meiosis is initiated in Msh4 mutant male and female mice, as indicated by the chromosomal localization of RAD51 and COR1 during leptonema/zygonema, the chromosomes fail to undergo normal pairing. Our results show that MSH4 localization on chromosomes during the early stages of meiosis is essential for normal chromosome synapsis in prophase I and that it acts in the same pathway as MSH5.

MutS homolog 4 localization to meiotic chromosomes is required for chromosome pairing during meiosis in male and female mice

Msh4 (MutS homolog 4) is a member of the mammalian mismatch repair gene family whose members are involved in postreplicative DNA mismatch repair as well as in the control of meiotic recombination. In this report we show that MSH4 has an essential role in the control of male and female meiosis. We demonstrate that MSH4 is present in the nuclei of spermatocytes early in prophase I and that it forms discrete foci along meiotic chromosomes during the zygotene and pachytene stages of meiosis. Disruption of the Msh4 gene in mice results in male and female sterility due to meiotic failure. Although meiosis is initiated in Msh4 mutant male and female mice, as indicated by the chromosomal localization of RAD51 and COR1 during leptonema/zygonema, the chromosomes fail to undergo normal pairing. Our results show that MSH4 localization on chromosomes during the early stages of meiosis is essential for normal chromosome synapsis in prophase I and that it acts in the same pathway as MSH5.

Mammalian DNA mismatch repair

DNA mismatch repair (MMR) is one of multiple replication, repair, and recombination processes that are required to maintain genomic stability in prokaryotes and eukaryotes. In the wake of the discoveries that hereditary nonpolyposis colorectal cancer (HNPCC) and other human cancers are associated with mutations in MMR genes, intensive efforts are under way to elucidate the biochemical functions of mammalian MutS and MutL homologs, and the consequences of defects in these genes. Genetic studies in cultured mammalian cells and mice are proving to be instrumental in defining the relationship between the functions of MMR in mutation and tumor avoidance. Furthermore, these approaches have raised awareness that MMR homologs contribute to DNA damage surveillance, transcription-coupled repair, and recombinogenic and meiotic processes.

Mutation spectrum of MSH3-deficient HHUA/chr.2 cells reflects in vivo activity of the MSH3 gene product in mismatch repair

The endometrial tumor cell line HHUA carries mutations in two mismatch repair (MMR) genes MSH3 and MSH6. We have established an MSH3-deficient HHUA/chr.2 cell line by introducing human chromosome 2, which carries wild-type MSH6 and MSH2 genes, to HHUA cells. Introduction of chromosome 2 to HHUA cells partially restored G:G MMR activity to the cell extract and reduced the frequency of mutation at the hypoxanthine-guanine phosphoribosyltransferase (hprt*) locus to about 3% that of the parental HHUA cells, which is five-fold the frequency in MMR-proficient cells, indicating that the residual mutator activity in HHUA/chr.2 is due to an MSH3-deficiency in these cells. The spectrum of mutations occurring at the HPRT locus of HHUA/chr.2 was determined with 71 spontaneous 6TG(r) clones. Base substitutions and +/-1 bp frameshifts were the major mutational events constituting, respectively, 54% and 42% of the total mutations, and more than 70% of them occurred at A:T sites. A possible explanation for the apparent bias of mutations to A:T sites in HHUA/chr.2 is haploinsufficiency of the MSH6 gene on the transferred chromosome 2. Comparison of the mutation spectra of HHUA/chr.2 with that of the MSH6-deficient HCT-15 cell line [S. Ohzeki, A. Tachibana, K. Tatsumi, T. Kato, Carcinogenesis 18 (1997) 1127-1133.] suggests that in vivo the MutSalpha (MSH2:MSH6) efficiently repairs both mismatch and unpaired extrahelical bases, whereas MutSbeta (MSH2:MSH3) efficiently repairs extrahelical bases and repairs mismatch bases to a limited extent.

Regulation of mitotic homeologous recombination in yeast. Functions of mismatch repair and nucleotide excision repair genes

The Saccharomyces cerevisiae homologs of the bacterial mismatch repair proteins MutS and MutL correct replication errors and prevent recombination between homeologous (nonidentical) sequences. Previously, we demonstrated that Msh2p, Msh3p, and Pms1p regulate recombination between 91% identical inverted repeats, and here use the same substrates to show that Mlh1p and Msh6p have important antirecombination roles. In addition, substrates containing defined types of mismatches (base-base mismatches; 1-, 4-, or 12-nt insertion/deletion loops; or 18-nt palindromes) were used to examine recognition of these mismatches in mitotic recombination intermediates. Msh2p was required for recognition of all types of mismatches, whereas Msh6p recognized only base-base mismatches and 1-nt insertion/deletion loops. Msh3p was involved in recognition of the palindrome and all loops, but also had an unexpected antirecombination role when the potential heteroduplex contained only base-base mismatches. In contrast to their similar antimutator roles, Pms1p consistently inhibited recombination to a lesser degree than did Msh2p. In addition to the yeast MutS and MutL homologs, the exonuclease Exo1p and the nucleotide excision repair proteins Rad1p and Rad10p were found to have roles in inhibiting recombination between mismatched substrates.

Genetic analysis of mouse embryonic stem cells bearing Msh3 and Msh2 single and compound mutations

We have previously described the use of homologous recombination and CRE-loxP-mediated marker recycling to generate mouse embryonic stem (ES) cell lines homozygous for mutations at the Msh3, Msh2, and both Msh3 and Msh2 loci (2). In this study, we describe the analysis of these ES cells with respect to processes known to be affected by DNA mismatch repair. ES cells homozygous for the Msh2 mutation displayed increased resistance to killing by the cytotoxic drug 6-thioguanine (6TG), indicating that the 6TG cytotoxic mechanism is mediated by Msh2. The mutation rate of the herpes simplex virus thymidine kinase 1 (HSV-tk1) gene was unchanged in Msh3-deficient ES cell lines but markedly elevated in Msh2-deficient and Msh3 Msh2 double-mutant cells. Notably, the HSV-tk1 mutation rate was 11-fold higher, on average, than that of the hypoxanthine-guanine phosphoribosyl transferase (Hprt) locus in Msh2-deficient cells. Sequence analysis of HSV-tk1 mutants from these cells indicated the presence of a frameshift hotspot within the HSV-tk1 coding region. Msh3-deficient cells displayed a modest (16-fold) elevation in the instability of a dinucleotide repeat, whereas Msh2-deficient and Msh2 Msh3 double-mutant cells displayed markedly increased levels of repeat instability. Targeting frequencies of nonisogenic vectors were elevated in Msh2-deficient ES cell lines, confirming the role of Msh2 in blocking recombination between diverged sequences (homeologous recombination) in mammalian cells. These results are consistent with accumulating data from other laboratories and support the current model of DNA mismatch repair in mammalian cells.

HNPCC-like cancer predisposition in mice through simultaneous loss of Msh3 and Msh6 mismatch-repair protein functions

Cancer predisposition in hereditary non-polyposis colon cancer (HNPCC) is caused by defects in DNA mismatch repair (MMR). Mismatch recognition is attributed to two heterodimeric protein complexes: MutSalpha (refs 2, 3, 4, 5), a dimer of MutS homologues MSH2 and MSH6; and MutSbeta (refs 2,7), a dimer of MSH2 and MSH3. These complexes have specific and redundant mismatch recognition capacity. Whereas MSH2 deficiency ablates the activity of both dimers, causing strong cancer predisposition in mice and men, loss of MSH3 or MSH6 (also known as GTBP) function causes a partial MMR defect. This may explain the rarity of MSH6 and absence of MSH3 germline mutations in HNPCC families. To test this, we have inactivated the mouse genes Msh3 (formerly Rep3 ) and Msh6 (formerly Gtmbp). Msh6-deficient mice were prone to cancer; most animals developed lymphomas or epithelial tumours originating from the skin and uterus but only rarely from the intestine. Msh3 deficiency did not cause cancer predisposition, but in an Msh6 -deficient background, loss of Msh3 accelerated intestinal tumorigenesis. Lymphomagenesis was not affected. Furthermore, mismatch-directed anti-recombination and sensitivity to methylating agents required Msh2 and Msh6, but not Msh3. Thus, loss of MMR functions specific to Msh2/Msh6 is sufficient for lymphoma development in mice, whereas predisposition to intestinal cancer requires loss of function of both Msh2/Msh6 and Msh2/Msh3.

Mismatch repair and drug responses in cancer

Defects in mismatch repair contribute to development of approximately 15% of colon cancers and to origination of endometrial, gastric and other cancers. Tumors with defects in mismatch repair exhibit marked resistance to alkylators and a variety of anticancer agents that modify DNA to create substrates for the mismatch repair system. These altered drug responses appear to derive from requirements for mismatch repair proteins in signalling apoptosis, altered cell cycle checkpoint behaviour and/or loss of mismatch repair dependent toxicity arising from futile repair cycling. Altered repair mechanisms for mismatched substrates in mismatch repair defective tumors provide both challenges for development of tumor-phenotype-screening methodologies to assure appropriate therapy is administered for these cancers and foci for development of new therapy approaches that capitalize on modified drug responses in mismatch repair- defective cells. Copyright 1999 Harcourt Publishers Ltd.

Biochemical characterization of the interaction between the Saccharomyces cerevisiae MSH2-MSH6 complex and mispaired bases in DNA

The interaction of the Saccharomyces cerevisiae MSH2-MSH6 complex with mispaired bases was analyzed using gel mobility shift assays and surface plasmon resonance methods. Under equilibrium binding conditions, MSH2-MSH6 bound to homoduplex DNA with a K(d) of 3.9 nM and bound oligonucleotide duplexes containing T:G, +1, +2, +4, and +10 insertion/deletion loop (IDL) mispairs with K(d) values of 0.20, 0.25, 11, 3.2, and 0.55 nM, respectively. Competition binding experiments using 65 different substrates revealed a 10-fold range in mispair discrimination. In general, base-base mispairs and a +1 insertion/deletion mispair were recognized better than intermediate sized insertion/deletion mispairs of 2-8 bases. Larger IDL mispairs (>8 bases) were recognized almost as well as the +1 IDL mispair. Recognition of mispairs by MSH2-MSH6 was influenced by sequence context, with the 6-nucleotide region surrounding the mispair being primarily responsible for influencing mispair recognition. Effects of sequences as far away as 15 nucleotides were also observed. Differential effects of ATP on the stability of MSH2-MSH6-mispair complexes suggested that base-base mispairs and the smaller IDL mispairs were recognized by a different binding mode than larger IDL mispairs, consistent with genetic experiments indicating that MSH2-MSH6 functions primarily in the repair of base-base and small IDL mispairs.

Role of DNA mismatch repair and p53 in signaling induction of apoptosis by alkylating agents

All cells are unavoidably exposed to chemicals that can alkylate DNA to form genotoxic damage. Among the various DNA lesions formed, O(6)-alkylguanine lesions can be highly cytotoxic, and we recently demonstrated that O(6)-methylguanine (O(6)MeG) and O(6)-chloroethylguanine (O(6)CEG) specifically initiate apoptosis in hamster cells. Here we show, in both hamster and human cells, that the MutSalpha branch of the DNA mismatch repair pathway (but not the MutSbeta branch) is absolutely required for signaling the initiation of apoptosis in response to O(6)MeGs and is partially required for signaling apoptosis in response to O(6)CEGs. Further, O(6)MeG lesions signal the stabilization of the p53 tumor suppressor, and such signaling is also MutSalpha-dependent. Despite this, MutSalpha-dependent apoptosis can be executed in a p53-independent manner. DNA mismatch repair status did not influence the response of cells to other inducers of p53 and apoptosis. Thus, it appears that mismatch repair status, rather than p53 status, is a strong indicator of the susceptibility of cells to alkylation-induced apoptosis. This experimental system will allow dissection of the signal transduction events that couple a specific type of DNA base lesion with the final outcome of apoptotic cell death.

Genomic amplification of the human DHFR/MSH3 locus remodels mismatch recognition and repair activities

Mismatch recognition in human cells is mediated by two heterodimers, MutS alpha and MutS beta. MutS alpha appears to shoulder primary responsibility for mismatch correction during replication, based on its relative abundance and ability to recognize a broad spectrum of base-base and base-insertion mismatches. Because MutS alpha and MutS beta share a common component, MSH2, conditions that influence the expression or degradation of MSH3 or MSH6 can redistribute the profile of mismatch recognition and repair. MSH3 is linked by a shared promoter with DHFR, connecting two pathways with key roles in DNA metabolism. In a classic example of gene amplification, the DHFR (and MSH3) locus can become amplified to several hundred copies in the presence of methotrexate. Under these conditions, MutS beta forms at the expense of MutS alpha, and the mutation rate in these tumor cells rises more than 100-fold. The implications for cancer chemotherapy include a potential increase in mutability when tumors are treated with methotrexate, which could increase the frequency of subsequent mutations that influence the tumor's drug sensitivity or aggressiveness. Because processing certain types of DNA damage by the mismatch repair pathway has also been implicated in tumor sensitivity to agents such as cisplatin, changes in expression at the DHFR/MSH3 locus may have further relevance to the outcome of multi-drug treatment regimens.

Mononucleotide microsatellite instability and germline MSH6 mutation analysis in early onset colorectal cancer

Germline mutations in the MSH2 and MLH1 mismatch repair genes account for most cases of hereditary non-polyposis colon cancer syndrome (HNPCC). In addition, germline MSH2 and MLH1 mutations have been detected in patients with non-HNPCC early onset colorectal cancer. Germline MSH6 mutations appear to be rare in classical HNPCC families, but their frequency in young colorectal cancer cases has not been studied previously. In a population based study of early onset colorectal cancer (<50 years) investigated for tumour microsatellite instability (MSI), we identified a subgroup of tumours with MSI for mono- but not dinucleotide repeat markers (m-MSI+ group). In contrast to tumours with classical MSI for dinucleotide markers (d-MSI+), the m-MSI+ group cancers were mainly left sided (6/7). As MSH6 mutations in yeast and human cell lines are associated with weak (and preferential mononucleotide) MSI, the complete MSH6 gene coding region was sequenced in blood DNA from the five m-MSI+ cases available for analysis. A germline nonsense mutation was identified in an isolated case of early onset colorectal cancer (age 43 years). These results support previous findings that germline MSH6 mutations may not be associated with classical MSI and suggest a role for germline MSH6 mutations in isolated early onset colorectal cancer.

Correction of the UDP-glucuronosyltransferase gene defect in the gunn rat model of crigler-najjar syndrome type I with a chimeric oligonucleotide

Crigler-Najjar syndrome type I is characterized by unconjugated hyperbilirubinemia resulting from an autosomal recessive inherited deficiency of hepatic UDP-glucuronosyltransferase (UGT) 1A1 activity. The enzyme is essential for glucuronidation and biliary excretion of bilirubin, and its absence can be fatal. The Gunn rat is an excellent animal model of this disease, exhibiting a single guanosine (G) base deletion within the UGT1A1 gene. The defect results in a frameshift and a premature stop codon, absence of enzyme activity, and hyperbilirubinemia. Here, we show permanent correction of the UGT1A1 genetic defect in Gunn rat liver with site-specific replacement of the absent G residue at nucleotide 1206 by using an RNA/DNA oligonucleotide designed to promote endogenous repair of genomic DNA. The chimeric oligonucleotide was either complexed with polyethylenimine or encapsulated in anionic liposomes, administered i.v., and targeted to the hepatocyte via the asialoglycoprotein receptor. G insertion was determined by PCR amplification, colony lift hybridizations, restriction endonuclease digestion, and DNA sequencing, and confirmed by genomic Southern blot analysis. DNA repair was specific, efficient, stable throughout the 6-month observation period, and associated with reduction of serum bilirubin levels. Our results indicate that correction of the UGT1A1 genetic lesion in the Gunn rat restores enzyme expression and bilirubin conjugating activity, with consequent improvement in the metabolic abnormality.

Dissociation of mismatch recognition and ATPase activity by hMSH2-hMSH3

MSH2-MSH3 directs the repair of insertion/deletion loops of up to 13 nucleotides in vivo and in vitro. To examine the biochemical basis of this repair specificity, we characterized the mispair binding and ATPase activity of hMSH2-hMSH3. The ATPase was found to be regulated by a mismatch-stimulated ADP --> ATP exchange, which induces a conformational transition by the protein complex. We demonstrated strong binding of hMSH2-hMSH3 to an insertion/deletion loop containing 24 nucleotides that is incapable of provoking ADP --> ATP exchange, suggesting that mismatch recognition appears to be necessary but not sufficient to induce the intrinsic ATPase. These studies support the idea that hMSH2-hMSH3 functions as an adenosine nucleotide-regulated molecular switch that must be activated by mismatched nucleotides for classical mismatch repair to occur.

Specificity of mutations induced by methyl methanesulfonate in mismatch repair-deficient human cancer cell lines

Recently, we showed that the cytotoxic and mutagenic response in human cells to the model SN2 alkylating agent methyl methanesulfonate (MMS) can be modulated by the mismatch repair (MMR) pathway. That is, human cancer cell lines defective in MMR are more resistant to the cytotoxic effects of MMS exposure and suffer more induced mutations at the HPRT locus than MMR-proficient cell lines. Since MMS produces little O6-methylguanine (O6-meG), the observed hypermutability and resistance to cytotoxicity in MMR-defective cells likely results from lesions other than O6-meG. MMS produces a high yield of N7-methylguanine (N7-meG) and N3-methyladenine (N3-meA), which can lead to the formation of promutagenic abasic sites, and these lesions may be responsible for the observed cytotoxic and/or mutagenic effects of MMS. To further investigate the mechanism of MMS mutagenesis, two MMR-defective human cancer cell lines were treated with MMS and the frequency and the types of mutations produced at the HPRT locus were determined. MMS treatment (1.5 mM) produced a 1.6- and a 2.2-fold increase in mutations above spontaneous levels in HCT116 and DLD-1 cell lines, respectively. An average 3.7-fold increase in transversion mutations was observed, which accounted for greater than one-third of all induced mutations in both cell lines. In contrast, an average 1.6-fold increase was seen among transition mutations (the class expected from O-alkylation products). Since transversion mutations are not produced by O6-meG, these findings suggest that abasic sites may be the lesion responsible for a large proportion of MMS mutagenicity in MMR-defective cells. Furthermore, these data suggest the MMS-induced damage, either abasic site-inducing base alterations (i.e., N7-meG and N3-meA) or the resulting abasic sites themselves, may be substrates for recognition and/or repair by MMR proteins.

Specific binding of human MSH2.MSH6 mismatch-repair protein heterodimers to DNA incorporating thymine- or uracil-containing UV light photoproducts opposite mismatched bases

Previous studies have demonstrated recognition of DNA-containing UV light photoproducts by bacterial (Feng, W.-Y., Lee, E., and Hays, J. B. (1991) Genetics 129, 1007-1020) and human (Mu, D., Tursun, M., Duckett, D. R., Drummond, J. T., Modrich, P., and Sancar, A. (1997) Mol. Cell. Biol. 17, 760-769) long-patch mismatch-repair systems. Mismatch repair directed specifically against incorrect bases inserted during semi-conservative DNA replication might efficiently antagonize UV mutagenesis. To test this hypothesis, DNA 51-mers containing site-specific T-T cis-syn-cyclobutane pyrimidine-dimers or T-T pyrimidine-(6-4')pyrimidinone photoproducts, with all four possible bases opposite the respective 3'-thymines in the photoproducts, were analyzed for the ability to compete with radiolabeled (T/G)-mismatched DNA for binding by highly purified human MSH2.MSH6 heterodimer protein (hMutSalpha). Both (cyclobutane-dimer)/AG and ((6-4)photoproduct)/AG mismatches competed about as well as non-photoproduct T/T mismatches. The two respective pairs of photoproduct/(A(T or C)) mismatches also showed higher hMutSalpha affinity than photoproduct/AA "matches"; the apparent affinity of hMutSalpha for the ((6-4)photoproduct)/AA-"matched" substrate was actually less than that for TT/AA homoduplexes. Surprisingly, although hMutSalpha affinities for both non-photoproduct UU/GG double mismatches and for (uracil-cyclobutane-dimer)/AG single mismatches were high, affinity for the (uracil-cyclobutane-dimer)/GG mismatch was quite low. Equilibrium binding of hMutSalpha to DNA containing (photoproduct/base) mismatches and to (T/G)-mismatched DNA was reduced similarly by ATP (in the absence of magnesium).

Cytotoxic and mutagenic response of mismatch repair-defective human cancer cells exposed to a food-associated heterocyclic amine

The cytotoxic and mutagenic effects of 2-amino-1-methyl-6-phenylimidazo-[4,5-b]-pyridine (PhIP), a food-associated heterocyclic amine, were measured in three human cancer cell lines possessing different mismatch repair (MMR) defects and in matched cell lines corrected for the MMR deficiencies by specific chromosome transfer. Cells deficient in MMR were more resistant to PhIP-induced cytotoxicity and displayed approximately 3-fold more induced mutations at the hypoxanthine-guanine phosphoribosyl transferase locus. These results suggest that defects in MMR carried by patients with hereditary nonpolyposis colorectal cancer syndrome may result in enhanced sensitivity to certain dietary and environmental carcinogens such as PhIP.

Functional analysis of human MutSalpha and MutSbeta complexes in yeast

Mismatch repair (MMR) is initiated when a heterodimer of hMSH2*hMSH6 or hMSH2*hMSH3 binds to mismatches. Here we perform functional analyses of these human protein complexes in yeast. We use a sensitive genetic system wherein the rate of single-base deletions in a homopolymeric run in the LYS2 gene is 10 000-fold higher in an msh2 mutant than in a wild-type strain. Expression of the human proteins alone or in combination does not reduce the mutation rate of the msh2 strain, and expression of the individual human proteins does not increase the low mutation rate of a wild-type strain. However, co-expression of hMSH2 and hMSH6 in wild-type yeast increases the mutation rate 4000-fold, while co-expression of hMSH2 and hMSH3 elevates the rate 5-fold. Analysis of cell extracts indicates that the proteins are expressed and bind to mismatched DNA. The results suggest that hMutSalpha and hMutSbeta complexes form, bind to and prevent correction of replication slippage errors in yeast. Expression of hMSH6 with hMSH2 containing a proline substituted for a conserved Arg524 eliminates the mutator effect and reduces mismatch binding. The analogous mutation in humans is associated with microsatellite instability, defective MMR and cancer, illustrating the utility of the yeast system for studying human disease alleles.

Eukaryotic DNA mismatch repair

Eukaryotic mismatch repair (MMR) has been shown to require two different heterodimeric complexes of MutS-related proteins: MSH2-MSH3 and MSH2-MSH6. These two complexes have different mispair recognition properties and different abilities to support MMR. Alternative models have been proposed for how these MSH complexes function in MMR. Two different heterodimeric complexes of MutL-related proteins, MLH1-PMS1 (human PMS2) and MLH1-MLH3 (human PMS1) also function in MMR and appear to interact with other MMR proteins including the MSH complexes and replication factors. A number of other proteins have been implicated in MMR, including DNA polymerase delta, RPA (replication protein A), PCNA (proliferating cell nuclear antigen), RFC (replication factor C), Exonuclease 1, FEN1 (RAD27) and the DNA polymerase delta and epsilon associated exonucleases. MMR proteins have also been shown to function in other types of repair and recombination that appear distinct from MMR. MMR proteins function in these processes in conjunction with components of nucleotide excision repair (NER) and, possibly, recombination.