Post-replicative DNA Mismatch Repair Research Articles

Functional Analysis of Cancer-Associated DNA Polymerase ε Variants in Saccharomyces cerevisiae.

DNA replication fidelity relies on base selectivity of the replicative DNA polymerases, exonucleolytic proofreading, and postreplicative DNA mismatch repair (MMR). Ultramutated human cancers without MMR defects carry alterations in the exonuclease domain of DNA polymerase ε (Polε). They have been hypothesized to result from defective proofreading. However, modeling of the most common variant, Polε-P286R, in yeast produced an unexpectedly strong mutator effect that exceeded the effect of proofreading deficiency by two orders of magnitude and indicated the involvement of other infidelity factors. The in vivo consequences of many additional Polε mutations reported in cancers remain poorly understood. Here, we genetically characterized 13 cancer-associated Polε variants in the yeast system. Only variants directly altering the DNA binding cleft in the exonuclease domain elevated the mutation rate. Among these, frequently recurring variants were stronger mutators than rare variants, in agreement with the idea that mutator phenotype has a causative role in tumorigenesis. In nearly all cases, the mutator effects exceeded those of an exonuclease-null allele, suggesting that mechanisms distinct from loss of proofreading may drive the genome instability in most ultramutated tumors. All mutator alleles were semidominant, supporting the view that heterozygosity for the polymerase mutations is sufficient for tumor development. In contrast to the DNA binding cleft alterations, peripherally located variants, including a highly recurrent V411L, did not significantly elevate mutagenesis. Finally, the analysis of Polε variants found in MMR-deficient tumors suggested that the majority cause no mutator phenotype alone but some can synergize with MMR deficiency to increase the mutation rate.

DNA replication and mismatch repair safeguard against metabolic imbalances.

During DNA replication, the major DNA replicative polymerases Polδ and Pole introduce noncomplementary nucleotides approximately once in every 100,000 polymerization events. Proofreading properties of these polymerases improve their fidelity by 10- to 100-fold, and postreplicative DNA mismatch repair (MMR), which acts as a spell checker to remove mismatches that escape polymerase proofreading, improves this fidelity even further, resulting in mutation rates as low as 2 × 10−10 substitutions per base per cell division (1). Mutations in genes encoding MMR proteins and DNA polymerase proofreading activities are linked to elevated mutation rates and diseases such as cancer, and these rates are synergistically increased in double mutants (2, 3). In PNAS, Schmidt et al. (4) describe work in which they screened for defects in genome stability in genetic backgrounds where processes that limit mutagenesis were compromised. Their work provides new insights for how cellular metabolism and nucleotide pool homeostasis interact to avoid mutation and maintain genome stability. They also provide evidence that DNA polymerase fidelity and MMR functions are sufficiently robust to compensate at least partially for defects in cellular metabolism (Fig. 1). Fig. 1. Removing the buffering capabilities of DNA replication and DNA MMR uncovers new factors that contribute to nucleotide pool homeostasis. Multiple mechanisms contribute to the overall fidelity of DNA replication. For example, each of the four mechanisms outlined in this figure can compensate for defects in the others. ( i ) A homeostasis mechanism responds to cellular metabolism to regulate the biosynthesis of dNTP and dNTP precursors to maintain a balanced concentration of dNTPs. ( ii ) DNA polymerase proofreading activities: During eukaryotic DNA replication, the major replicative polymerases Pole and Polδ both contain proofreading activities that enable them to excise misincorporation errors. However, Polα, which initiates DNA synthesis at origins and at Okazaki fragments, does not contain such a function. … [↵][1]1To whom correspondence should be addressed. Email: eea3{at}cornell.edu. [1]: #xref-corresp-1-1

Epigenetic Enhancement of the Post-replicative DNA Mismatch Repair of Mammalian Genomes by a Hemi-mCpG-Np95-Dnmt1 Axis.

DNA methylation at C of CpG dyads (mCpG) in vertebrate genomes is essential for gene regulation, genome stability and development. We show in this study that proper functioning of post-replicative DNA mismatch repair (MMR) in mammalian cells relies on the presence of genomic mCpG, as well as on the maintenance DNA methyltransferase Dnmt1 independently of its catalytic activity. More importantly, high efficiency of mammalian MMR surveillance is achieved through a hemi-mCpG-Np95(Uhrf1)-Dnmt1 axis, in which the MMR surveillance complex(es) is recruited to post-replicative DNA by Dnmt1, requiring its interactions with MutSα, as well as with Np95 bound at the hemi-methylated CpG sites. Thus, efficiency of MMR surveillance over the mammalian genome in vivo is enhanced at the epigenetic level. This synergy endows vertebrate CpG methylation with a new biological significance and, consequently, an additional mechanism for the maintenance of vertebrate genome stability.

The Rate and Spectrum of Spontaneous Mutations in Mycobacterium smegmatis, a Bacterium Naturally Devoid of the Postreplicative Mismatch Repair Pathway.

Mycobacterium smegmatis is a bacterium that is naturally devoid of known postreplicative DNA mismatch repair (MMR) homologs, mutS and mutL, providing an opportunity to investigate how the mutation rate and spectrum has evolved in the absence of a highly conserved primary repair pathway. Mutation accumulation experiments of M. smegmatis yielded a base-substitution mutation rate of 5.27 × 10−10 per site per generation, or 0.0036 per genome per generation, which is surprisingly similar to the mutation rate in MMR-functional unicellular organisms. Transitions were found more frequently than transversions, with the A:T→G:C transition rate significantly higher than the G:C→A:T transition rate, opposite to what is observed in most studied bacteria. We also found that the transition-mutation rate of M. smegmatis is significantly lower than that of other naturally MMR-devoid or MMR-knockout organisms. Two possible candidates that could be responsible for maintaining high DNA fidelity in this MMR-deficient organism are the ancestral-like DNA polymerase DnaE1, which contains a highly efficient DNA proofreading histidinol phosphatase (PHP) domain, and/or the existence of a uracil-DNA glycosylase B (UdgB) homolog that might protect the GC-rich M. smegmatis genome against DNA damage arising from oxidation or deamination. Our results suggest that M. smegmatis has a noncanonical Dam (DNA adenine methylase) methylation system, with target motifs differing from those previously reported. The mutation features of M. smegmatis provide further evidence that genomes harbor alternative routes for improving replication fidelity, even in the absence of major repair pathways.

Abstract 2426: Exacerbation of oxidative DNA damage as a synthetic lethal approach in mismatch repair deficient tumors

Abstract The post-replicative DNA mismatch repair (MMR) pathway plays a key role in the maintenance of genomic stability. MMR corrects mismatches within the DNA helix that arise due to errors in nucleotide incorporation during either semi-conservative replication or synthesis associated with DNA repair. The importance of this system is testified by the fact that mutations affecting components of this repair pathway cause a “mutator” phenotype and predisposition to a wide variety of malignancies including colorectal, gastric, endometrial, and urothelial cancers. For example, the hereditable disorder Lynch syndrome (hereditary nonpolyposis colon cancer) is associated with germline loss of function mutations in the MMR tumor suppressor genes MLH1, MSH2, PMS2, and MSH6. In addition to this familial syndrome, MMR deficiency is present in 15% to 17% of all colorectal cancer cases. Synthetic lethal relationships have been identified that involve specific MMR genes, suggesting that synthetic lethal treatments could be applied to MMR deficient tumors. Previously, efforts to functionally identify genetic DNA mismatch repair synthetic lethalities and potential therapeutic targets have focused on the analysis of either candidate genes or relatively small gene subsets such as protein kinases. In order to define additional SL targets in MMR-deficient genetic backgrounds, we describe here the application of an shRNA screen in matched isogenic dMMR/pMMR (MMR-deficient/MMR-proficient) model cell lines. Using this screening approach, we identify new synthetic lethal interactions between MMR deficiency and components of the base excision repair and of the nucleotide sanitization systems, which restrict the extent of oxidative DNA damage. We propose that inhibition of the processes that minimize the oxidized DNA load could constitute a promising therapeutic approach in mismatch repair deficient cancers. Citation Format: Yari Fontebasso, Madeleine Hewish, Chris J. Lord, Alan Ashworth. Exacerbation of oxidative DNA damage as a synthetic lethal approach in mismatch repair deficient tumors. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2426. doi:10.1158/1538-7445.AM2014-2426

Hypermutability and error catastrophe due to defects in ribonucleotide reductase

The enzyme ribonucleotide reductase (RNR) plays a critical role in the production of deoxynucleoside-5'-triphosphates (dNTPs), the building blocks for DNA synthesis and replication. The levels of the cellular dNTPs are tightly controlled, in large part through allosteric control of RNR. One important reason for controlling the dNTPs relates to their ability to affect the fidelity of DNA replication and, hence, the cellular mutation rate. We have previously isolated a set of mutants of Escherichia coli RNR that are characterized by altered dNTP pools and increased mutation rates (mutator mutants). Here, we show that one particular set of RNR mutants, carrying alterations at the enzyme's allosteric specificity site, is characterized by relatively modest dNTP pool deviations but exceptionally strong mutator phenotypes, when measured in a mutational forward assay (>1,000-fold increases). We provide evidence indicating that this high mutability is due to a saturation of the DNA mismatch repair system, leading to hypermutability and error catastrophe. The results indicate that, surprisingly, even modest deviations of the cellular dNTP pools, particularly when the pool deviations promote particular types of replication errors, can have dramatic consequences for mutation rates.

Abstract A09: Induced aggravation of oxidative DNA damage as a targeted synthetic lethal approach in mismatch repair-deficient tumors

Abstract The post-replicative DNA mismatch repair (MMR) pathway plays a key role in the maintenance of genomic stability. MMR corrects mismatches within the DNA helix that arise due to errors in nucleotide incorporation during either semi-conservative replication or synthesis associated with DNA repair. The importance of this system is testified by the fact that mutations affecting components of this repair pathway cause a “mutator” phenotype and predisposition to a wide variety of malignancies including colorectal, gastric, endometrial, and urothelial cancers. For example, the hereditable disorder Lynch syndrome (hereditary nonpolyposis colon cancer) is associated with germline loss of function mutations in the MMR tumor suppressor genes MLH1, MSH2, PMS2 and MSH6. In addition to this familial syndrome, MMR deficiency is present in 15% to 17% of all colorectal cancer cases and a common cause of nonfamilial MMR deficiency is biallelic hypermethylation of the MLH1 promoter that results in a reduction of MLH1 expression. Synthetic lethal relationships have been identified that involve specific MMR genes, suggesting that the synthetic lethal paradigm could be applied to MMR deficiency. Previously, efforts to functionally identify genetic DNA mismatch repair synthetic lethalities and potential therapeutic targets have focused on the analysis of either candidate genes or relatively small gene subsets such as protein kinases. In order to define additional SL targets in MMR-deficient genetic backgrounds, we describe here the application of an shRNA screen in matched isogenic dMMR/pMMR (MMR-deficient/MMR-proficient) model cell lines. Using this screening approach, we identify new synthetic lethal interactions between MMR deficiency and components of the base excision repair and of the nucleotide sanitization systems. We propose that selective aggravation of oxidative damage constitutes a promising therapeutic approach in mismatch repair - deficient cancer cells. Citation Format: Yari Fontebasso, Christopher J. Lord, Alan Ashworth. Induced aggravation of oxidative DNA damage as a targeted synthetic lethal approach in mismatch repair-deficient tumors. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Synthetic Lethal Approaches to Cancer Vulnerabilities; May 17-20, 2013; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(5 Suppl):Abstract nr A09.

The effect of acute dose charge particle radiation on expression of DNA repair genes in mice

The space radiation environment consists of trapped particle radiation, solar particle radiation, and galactic cosmic radiation (GCR), in which protons are the most abundant particle type. During missions to the moon or to Mars, the constant exposure to GCR and occasional exposure to particles emitted from solar particle events (SPE) are major health concerns for astronauts. Therefore, in order to determine health risks during space missions, an understanding of cellular responses to proton exposure is of primary importance. The expression of DNA repair genes in response to ionizing radiation (X-rays and gamma rays) has been studied, but data on DNA repair in response to protons is lacking. Using qPCR analysis, we investigated changes in gene expression induced by positively charged particles (protons) in four categories (0, 0.1, 1.0, and 2.0 Gy) in nine different DNA repair genes isolated from the testes of irradiated mice. DNA repair genes were selected on the basis of their known functions. These genes include ERCC1 (5' incision subunit, DNA strand break repair), ERCC2/NER (opening DNA around the damage, Nucleotide Excision Repair), XRCC1 (5' incision subunit, DNA strand break repair), XRCC3 (DNA break and cross-link repair), XPA (binds damaged DNA in preincision complex), XPC (damage recognition), ATA or ATM (activates checkpoint signaling upon double strand breaks), MLH1 (post-replicative DNA mismatch repair), and PARP1 (base excision repair). Our results demonstrate that ERCC1, PARP1, and XPA genes showed no change at 0.1 Gy radiation, up-regulation at 1.0 Gy radiation (1.09 fold, 7.32 fold, 0.75 fold, respectively), and a remarkable increase in gene expression at 2.0 Gy radiation (4.83 fold, 57.58 fold and 87.58 fold, respectively). Expression of other genes, including ATM and XRCC3, was unchanged at 0.1 and 1.0 Gy radiation but showed up-regulation at 2.0 Gy radiation (2.64 fold and 2.86 fold, respectively). We were unable to detect gene expression for the remaining four genes (XPC, ERCC2, XRCC1, and MLH1) in either the experimental or control animals.

Unusual presentation of Lynch Syndrome

Lynch Syndrome/HNPCC is a syndrome of cancer predisposition linked to inherited mutations of genes participating in post-replicative DNA mismatch repair (MMR). The spectrum of cancer associated with Lynch Syndrome includes tumours of the colorectum, endometrium, ovary, upper gastrointestinal tract and the urothelium although other cancers are rarely described. We describe a family of Lynch Syndrome with an hMLH1 mutation, that harbours an unusual tumour spectrum and its diagnostic and management challenges.

Conformational Change in MSH2-MSH6 upon Binding DNA Coupled to ATPase Activity

Postreplication DNA mismatch repair is initiated by the eukaryotic protein MSH2-MSH6 or the prokaryotic protein MutS, both showing overall conserved structure and functionality. Crystal structures of MSH2-MSH6 and MutS bound to the mismatch DNA reveal a closed architecture of the clamp and the lever domains exhibiting strong contacts with the bent DNA backbone. Long molecular dynamics simulations of the human MSH2-MSH6 protein in the absence of a DNA show an altered conformation of the protein that reflects the protein's state before binding to DNA. The clamp and the lever domains of both MSH6 and MSH2 open in an asymmetric and dramatic fashion. The opening of the clamp and the lever domains in the absence of DNA is coupled to changes in the ATPase domains, which explains the experimentally observed diminished ATPase activity in DNA-free MSH2-MSH6 and illustrates the allosteric coupling between DNA binding and ATPase activity.

Deciphering the Mismatch Recognition Cycle in MutS and MSH2-MSH6 Using Normal-Mode Analysis

Postreplication DNA mismatch repair is essential for maintaining the integrity of genomic information in prokaryotes and eukaryotes. The first step in mismatch repair is the recognition of base-base mismatches and insertions/deletions by bacterial MutS or eukaryotic MSH2-MSH6. Crystal structures of both proteins bound to mismatch DNA reveal a similar molecular architecture but provide limited insight into the detailed molecular mechanism of long-range allostery involved in mismatch recognition and repair initiation. This study describes normal-mode calculations of MutS and MSH2-MSH6 with and without DNA. The results reveal similar protein flexibilities and suggest common dynamic and functional characteristics. A strongly correlated motion is present between the lever domain and ATPase domains, which suggests a pathway for long-range allostery from the N-terminal DNA binding domain to the C-terminal ATPase domains, as indicated by experimental studies. A detailed analysis of individual low-frequency modes of both MutS and MSH2-MSH6 shows changes in the DNA-binding domains coupled to the ATPase sites, which are interpreted in the context of experimental data to arrive at a complete molecular-level mismatch recognition cycle. Distinct conformational states are proposed for DNA scanning, mismatch recognition, repair initiation, and sliding along DNA after mismatch recognition. Hypotheses based on the results presented here form the basis for further experimental and computational studies.

Mismatch Recognition Cycle in MutS and MSH2-MSH6 from Normal Mode Analysis and Simulations

Post-replication DNA mismatch repair (MMR) is crucial in ensuring genetic fidelity in prokaryots and eukaryots. The initial step of MMR is recognition of defective DNA by MutS or its eukaryotic homologs. Binding of MutS to mismatched DNA, the subsequent initiation of repair, and eventual recovery to a mismatch scanning mode is coupled to ATPase activity in MutS. Crystal structures of MutS and the eukaryotic MSH2:MSH6 system place the ATPase domain far away from the DNA binding domains, implicating a complex allosteric mechanism.Normal mode calculations and molecular dynamics simulations of MutS and MSH2:MSH6 structures were carried out to explore the coupling between DNA binding and ATPase activity. The mode analysis reveals conserved dynamics between the bacterial and eukaryotic complexes. Individual modes correlate ATPase activity with the probing of DNA kinking that is characteristic of mismatched DNA. Furthermore, differential ATPase activity between the MutS dimer moieties as observed experimentally is coupled to release of MutS from the mismatch during repair. Based on the calculations and consistent with available experimental data, a detailed mechanistic model of the allosteric conformational changes during DNA mismatch recognition by MutS is proposed.View Large Image | View Hi-Res Image | Download PowerPoint Slide

Genomic Instability and Carcinogenesis: An Update

Cancers arise as a result of stepwise accumulation of mutations which may occur at the nucleotide level and/or the gross chromosomal level. Many cancers particularly those of the colon display a form of genomic instability which may facilitate and speed up tumor initiation and development. In few instances, a “mutator mutation” has been clearly implicated in driving the accumulation of other carcinogenic mutations. For example, the post-replicative DNA mismatch repair deficiency results in dramatic increase in insertion/deletion mutations giving rise to the microsatellite instability (MSI) phenotype and may predispose to a spectrum of tumours when it occurs in the germline. Although many sporadic cancers show multiple mutations suggesting unstable genome, the role of this instability in carcinogenesis, as opposed to the power of natural selection, has been a matter of controversy. This review gives an update of the latest data on these issues particularly recent data from genome-wide, high throughput techniques as well as mathematical modelling. Throughout this review, reference will be made to the relevance of genomic instability to the pathogenesis of colorectal carcinoma particularly its hereditary and familial subsets.

Assessement of the pathogenic role of MSH2 missense variants in HNPCC syndrome

22154 Background: Hereditary Non-Polyposis Colorectal Cancer (HNPCC) is associated with inherited defects in genes involved in postreplicative DNA MisMatch Repair (MMR), mainly MLH1 and MSH2. A significant proportion (20–25%) of the reported variants consists of missense mutations: a single amino acid change with uncertain pathogenetic significance. Up to date a validated test, able to clarify the role of MSH2 missense mutations, is not available. This arouses uncertainty regarding clinical surveillance for patients and families carrying MSH2 missense variants. We analysed 3 non-truncating MSH2 mutations (G162R D167H R359S), found in clinically suspected HNPCC families, in order to assess in a human expression system their ability to correctly localize into the nucleus. Methods: Clinical data were correlated with nature of AA change, MSI (Micro Satellite Instability) and IHC (ImmunoHystoChemistry). Localization assay: the 3 missense variants were cloned into the pEGFP-N1 eucariotic expression vector in frame with the EGFP (Enhanced Green Fluorescent Protein) encoding gene. Wild type and a pathogenetic variant were used as controls. Subcellular localization of controls and mutated MSH2-EGFP fusion proteins was analysed in the human 293 MMR proficient cell line by direct fluorescence microscopy. Results: See Table. Conclusions: Localization analysis seems to be of aid in order to establish a pathogenetic role of MSH2 missense mutations and could represent a useful tool in clinical practice and in genetic counselling. Age/sex Family history AA change Nature AA change MSI IHC MSH2 IHC MLH1 Localization Pathogenetic 55/F Bethesda G162R nonpolar→ Basic 60% − + Altered (Nucleus+ cytoplasm) Likely 42/M Bethesda D167H Acidic→ Basic ND + − Normal (Nucleus) Unlikely 59/F Amsterdam R359S Basic→ Uncharged polar 80% − + Altered (Cytoplasm) Likely No significant financial relationships to disclose.

The DNA-Mismatch Repair Enzyme hMSH2 Modulates UV-B-Induced Cell Cycle Arrest and Apoptosis in Melanoma Cells

The mechanisms by which the post-replicative DNA mismatch repair (MMR) enzyme MSH2 is involved in the complex response mechanisms to UV damage are yet to be clarified. Here, we show increased levels of MSH2 mRNA in malignant melanoma, metastases of melanoma, and melanoma cell (MeWo) lines as compared with melanocytic nevi or primary cultured benign melanocytes. UV-B treatment modulated MSH2 expression and silencing of MSH2 gene expression using small interfering RNA technology regulated UV-B-induced cell cycle arrest and apoptosis in human MeWo. We show that MSH2-deficient non-malignant mouse fibroblasts (MEF-/-) are partially resistant against UV-B-induced apoptosis and show reduced S-Phase accumulation. In addition, we show that an Msh2 point mutation (MEFGA) that affects MMR does not affect UV-B-induced apoptosis. In conclusion, we demonstrate that MSH2 modulates in human melanocytes both UV-B-induced cell cycle regulation and apoptosis, most likely via independent, uncoupled mechanisms.

The C-terminal region of Escherichia coli MutS and protein oligomerization

Escherichia coli MutS, an 853 amino acids oligomeric protein, is involved in the postreplicative DNA mismatch repair and avoidance of homeologous recombination. By constructing MutS mutated versions of the C-terminal region, we determined that deletion of the last 7 C-terminal amino acids is enough to abolish tetramer formation and that the K850A substitution destabilize the tetramer structure. It is proposed that the C-terminal extreme alpha helix (residues 839–850) of the protein may play an important role in protein oligomerization. We also show that the C-terminal region or the C-terminal plus the HTH domain of MutS, fused to the monomeric Maltose Binding Protein promote oligomerization of the chimeric protein. However, chemical cross-linking experiments indicate that the HTH domain improves the oligomerization properties of the fused protein. Escherichia coli cells expressing the fused proteins become hypermutator suggesting that the C-terminal region of MutS plays an important role in vivo.

Interaction analysis of hMLH1 missense variants in patients selected for HNPCC syndrome

9690 Background: Hereditary Non-Polyposis Colorectal Cancer (HNPCC) is linked to inherited mutations in genes involved in postreplicative DNA MisMatch Repair (MMR). It is estimated that about 30% of hMLH1 mutations results in a single amino acid change with uncertain pathological significance (missense mutations). Here we describe four hMLH1 missense mutations which were found in five patients with suspected HNPCC. The ability of these variants to interact with hPMS2 was assessed in vitro. In addition we correlated these genetic variants with the clinical and pathologic features of the patients. Methods: Five patients harbouring 4 hMLH1 missense mutations which were, either novel mutations or mutations for which a link to HNPCC was previously suggested, were identified among those who were referred to our Department for genetic counselling. Genetic studies included mutation analysis of hMLH1 and hMSH2 by direct sequencing, assessment of MSI (MSI: MicroSatellite Instability as determined in a panel of 13 loci) and immunohistochemistry (IHC) of hMLH1 and hMSH2. Using a PCR-based site-direct mutagenesis method we generated the 4 hMLH1 missense mutations (Y646C, P648S, L559R, K618A) detected in our patients and we studied their interaction with normal hPMS2. Results: (see table) Conclusions: Our observations confirm: a) the possibility to perform a test assessing the biological consequences of missense mutations; b) the possible role of missense mutation in the pathogenesis of HNPCC. Author Disclosure Results Family Nucleotidechange AA change Nature AA change Conserved residue MSI IHC hMLH1 Interaction hHMLH1/hPMS2 Fam 1 TAT-TGT exon 17 Y646C Polar to non polar yes 23% - - Fam 2 CCC-TCC exon 17 P648S Non polar to polar yes 23% + - Fam 3 CTG-CGG exon 15 L559R Non polar to positive charged yes 15% - - Fam 4 AAG-GCG exon 16 K618A Positive charged to non polar yes 15% - +/- Fam 5 TAT-TGT exon 17 Y646C Polar to non polar yes 8% - - No significant financial relationships to disclose.

Mutation Rates, Spectra and Hotspots in Mismatch Repair-Deficient Caenorhabditis elegansSequence data from this article have been deposited with the EMBL/GenBank Data Libraries under accession nos. AY863110, AY863111, AY863112, AY863113, AY863114, AY863115, AY863116, AY863117, AY863118, AY863119, AY863120, AY863121, AY863122, AY863123, AY863124, AY863125, AY863126, AY863127, AY863128, AY863129, AY863130, AY863131, AY863132, AY863133, AY863134, AY863135, AY863136, AY863137, AY863138, AY863139, AY863140, AY863141, AY863142, AY863143.

Although it is clear that postreplicative DNA mismatch repair (MMR) plays a critical role in maintaining genomic stability in nearly all forms of life surveyed, much remains to be understood about the genome-wide impact of MMR on spontaneous mutation processes and the extent to which MMR-deficient mutation patterns vary among species. We analyzed spontaneous mutation processes across multiple genomic regions using two sets of mismatch repair-deficient (msh-2 and msh-6) Caenorhabditis elegans mutation-accumulation (MA) lines and compared our observations to mutation spectra in a set of wild-type (WT), repair-proficient C. elegans MA lines. Across most sequences surveyed in the MMR-deficient MA lines, mutation rates were approximately 100-fold higher than rates in the WT MA lines, although homopolymeric nucleotide-run (HP) loci composed of A:T base pairs mutated at an approximately 500-fold greater rate. In contrast to yeast and humans where mutation spectra vary substantially with respect to different specific MMR-deficient genotypes, mutation rates and patterns were overall highly similar between the msh-2 and msh-6 C. elegans MA lines. This, along with the apparent absence of a Saccharomyces cerevisiae MSH3 ortholog in the C. elegans genome, suggests that C. elegans MMR surveillance is carried out by a single Msh-2/Msh-6 heterodimer.

Inactivation of deoxyadenosine methyltransferase (dam) attenuates Haemophilus influenzae virulence.

Mutants in deoxyadenosine methyltransferase (dam) from many Gram-negative pathogens suggest multiple roles for Dam methylase: directing post-replicative DNA mismatch repair to the correct strand, guiding the temporal control of DNA replication and regulating the expression of multiple genes (including virulence factors) by differential promoter methylation. Dam methylase (HI0209) in strain Rd KW20 was inactivated in Haemophilus influenzae strains Rd KW20, Strain 12 and INT-1; restriction with Dam methylation-sensitive enzymes DpnI and DpnII confirmed the absence of Dam methylation, which was restored by complementation with a single copy of dam ectopically expressed in cis. Despite the lack of increased mutation frequency, the dam mutants had a 2-aminopurine-susceptible phenotype that could be suppressed by secondary mutations in mutS, suggesting a role for Dam in H. influenzae DNA mismatch repair. Invasion of human brain microvascular endothelial cells (HBMECs) and human respiratory epithelial cells (NCI-H292) by the dam mutants was significantly attenuated in all strains, suggesting the absence of a Dam-regulated event necessary for uptake or invasion of host cells. Intracellular replication was inhibited only in the Strain 12 dam mutant, whereas in the infant rat model of infection, the INT-1 dam mutant was less virulent. Dam activity appears to be necessary for both in vitro and in vivo virulence in a strain-dependent fashion and may function as a regulator of gene expression including virulence factors.

Homologs of MutS and MutL during mammalian meiosis

In eukaryotes, homologs of the Escherichia coli MutS and MutL proteins are crucial for both meiotic recombination and post-replicative DNA mismatch repair. Both pathways require the formation of a MutS homolog complex which interacts with a second heterodimer, composed of two MutL homologs. During mammalian meiosis, it is likely that chromosome synapsis requires the presence of a MSH4-MSH5 heterodimer. PMS2, a MutL homolog, seems to play an important role in this process. A MSH4-MSH5 heterodimer is also likely present later with other MutL homologs (MLH1 and MLH3) and is involved in the crossing-over process. The phenotype of msh4-/- mutant mice and MSH4 immunolocalization on meiotic chromosomes suggest that MSH4 has an early function in mammalian meiotic recombination. Both MSH4 and PMS2 directly interact with the RAD51 DNA strand exchange protein. In addition, MSH4 and RAD51 proteins co-localize on mouse meiotic chromosome cores. These results suggest that MSH4 and its partners could act, just after strand exchange promoted by RAD51, to check the homology of DNA heteroduplexes.