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

Acetaldehyde and defective mismatch repair increase colonic tumours in a Lynch syndrome model with Aldh1b1 inactivation

ALDH1B1 expressed in the intestinal epithelium metabolises acetaldehyde to acetate, protecting against acetaldehyde-induced DNA damage. MSH2 is a key component of the DNA mismatch repair (MMR) pathway involved in Lynch syndrome (LS)-associated colorectal cancers. Here, we show that defective MMR (dMMR) interacts with acetaldehyde, in a gene/environment interaction, enhancing dMMR-driven colonic tumour formation in a LS murine model of Msh2 conditional inactivation (Lgr5-CreER; Msh2flox/-, or Msh2-LS) combined with Aldh1b1 inactivation. Conditional (Aldh1b1flox/flox) or constitutive (Aldh1b1-/-) Aldh1b1 knockout alleles combined with the conditional Msh2flox/- intestinal knockout mouse model of LS (Msh2-LS) received either ethanol, which is metabolised to acetaldehyde, or water. We demonstrated that 41.7% of ethanol-treated Aldh1b1flox/flox Msh2-LS mice and 66.7% of Aldh1b1-/- Msh2-LS mice developed colonic epithelial hyperproliferation and adenoma formation, in 4.5 and 6 months, respectively, significantly greater than 0% in water-treated control mice. Significantly higher numbers of dMMR colonic crypt foci precursors and increased plasma acetaldehyde levels were observed in ethanol-treated Aldh1b1flox/flox Msh2-LS and Aldh1b1-/- Msh2-LS mice compared with those in water-treated controls. Hence, ALDH1B1 loss increases acetaldehyde levels and DNA damage that interacts with dMMR to accelerate colonic, but not small intestinal, tumour formation.

Resolution of sequence divergence for repeat-mediated deletions shows a polarity that is mediated by MLH1

Repeat-mediated deletions (RMDs) are a type of chromosomal rearrangement between two homologous sequences that causes loss of the sequence between the repeats, along with one of the repeats. Sequence divergence between repeats suppresses RMDs; the mechanisms of such suppression and of resolution of the sequence divergence remains poorly understood. We identified RMD regulators using a set of reporter assays in mouse cells that test two key parameters: repeat sequence divergence and the distances between one repeat and the initiating chromosomal break. We found that the mismatch repair factor MLH1 suppresses RMDs with sequence divergence in the same pathway as MSH2 and MSH6, and which is dependent on residues in MLH1 and its binding partner PMS2 that are important for nuclease activity. Additionally, we found that the resolution of sequence divergence in the RMD product has a specific polarity, where divergent bases that are proximal to the chromosomal break end are preferentially removed. Moreover, we found that the domain of MLH1 that forms part of the MLH1-PMS2 endonuclease is important for polarity of resolution of sequence divergence. We also identified distinctions between MLH1 versus TOP3α in regulation of RMDs. We suggest that MLH1 suppresses RMDs with sequence divergence, while also promoting directional resolution of sequence divergence in the RMD product.

Ethanol-induced formation of colorectal tumours and precursors in a mouse model of Lynch syndrome

Lynch syndrome (LS) confers inherited cancer predisposition due to germline mutations in a DNA mismatch repair (MMR) gene, e.g. MSH2. MMR is a repair pathway for removal of base mismatches and insertion/deletion loops caused by endogenous and exogenous factors. Loss of MMR through somatic alteration of the wild-type allele in LS results in defective MMR (dMMR). Lifestyle/environmental factors can modify colorectal cancer risk in sporadic and LS patients. Ethanol and its metabolite acetaldehyde are classified as group one carcinogens, and acetaldehyde causes a range of DNA lesions. However, DNA repair pathways responsible for correcting most of such DNA lesions remain uncharacterised. We hypothesised that MMR plays a role in protecting colorectal epithelium from ethanol/acetaldehyde-induced DNA damage. Here, an LS mouse model (intestinal epithelial conditional-knockout for Msh2) was used to determine if there is a gene-environment interaction between dMMR and ethanol/acetaldehyde that accelerates colorectal tumourigenesis in LS. Mice underwent either long-term ethanol treatment or water treatment. Most ethanol-treated mice demonstrated colonic hyperproliferation and adenoma formation (with some invasive adenocarcinomas) within 6 months (15/23, 65%), compared with one colonic tumour after 15 months in water-treated mice (1/23, 4%) (p < 0.0001, Fisher's exact test). A significantly greater number of dMMR colonic crypt foci precursors were observed in ethanol-treated compared with water-treated mice (p = 0.0029, Student's t-test). Moreover, increased plasma acetaldehyde levels were detected in ethanol-treated compared with water-treated mice (p = 0.0019, Mann-Whitney U-test), along with significantly increased DNA damage response in the colonic epithelium. Long-term ethanol treatment was associated with significantly increased colonic epithelial proliferation and markedly reduced apoptosis in dMMR adenomas, consistent with enhanced survival of aberrant dMMR relative to MMR-proficient colonic epithelium. In conclusion, there is strong evidence for a gene-environment interaction between dMMR and acetaldehyde, causing acceleration of dMMR-driven colonic tumour formation in this LS model, indicating that advice to limit alcohol consumption should be considered for LS patients. © 2021 The Authors. The Journal of Pathology published by John Wiley & Sons, Ltd. on behalf of The Pathological Society of Great Britain and Ireland.

BLM has Contrary Effects on Repeat-Mediated Deletions, based on the Distance of DNA DSBs to a Repeat and Repeat Divergence

Repeat-mediated deletions (RMDs) often involve repetitive elements (e.g., short interspersed elements) with sequence divergence that is separated by several kilobase pairs (kbps). We have examined RMDs induced by DNA double-strand breaks (DSBs) under varying conditions of repeat sequence divergence (identical versus 1% and 3% divergent) and DSB/repeat distance (16 bp–28.4 kbp). We find that the BLM helicase promotes RMDs with long DSB/repeat distances (e.g., 28.4 kbp), which is consistent with a role in extensive DSB end resection, because the resection nucleases EXO1 and DNA2 affect RMDs similarly to BLM. In contrast, BLM suppresses RMDs with sequence divergence and intermediate (e.g., 3.3 kbp) DSB/repeat distances, which supports a role in heteroduplex rejection. The role of BLM in heteroduplex rejection is not epistatic with MSH2 and is independent of the annealing factor RAD52. Accordingly, the role of BLM on RMDs is substantially affected by DSB/repeat distance and repeat sequence divergence.

Mendez-Dorantes et al. identify the BLM helicase as a key regulator of repeat-mediated deletions (RMDs). BLM, EXO1, and DNA2 mediate RMDs with remarkably long DNA break/repeat distances. BLM suppresses RMDs with sequence divergence that is optimal with a long non-homologous tail and is independent of MSH2 and RAD52.

Phytochemicals: Current strategy to sensitize cancer cells to cisplatin

Cisplatin-based chemotherapeutic regimens are the most frequently used adjuvant treatments for many types of cancer. However, the development of chemoresistance to cisplatin results in treatment failure. Despite the significant developments in understanding the mechanisms of cisplatin resistance, effective strategies to enhance the chemosensitivity of cisplatin are lacking. Phytochemicals are naturally occurring plant-based compounds that can augment the anti-cancer activity of cisplatin, with minimal side effects. Notably, some novel phytochemicals, such as curcumin, not only increase the efficacy of cisplatin but also decrease toxicity induced by cisplatin. However, the exact mechanisms underlying this process remain unclear. In this review, we discussed the progress made in utilizing phytochemicals to enhance the anti-cancer efficacy of cisplatin. We also presented some ideal phytochemicals as novel agents for counteracting cisplatin-induced organ damage.

Repeat-mediated deletions can be induced by a chromosomal break far from a repeat, but multiple pathways suppress such rearrangements

Here, Mendez-Dorantes et al. investigated how far a chromosomal double-strand break (DSB) can be positioned from a repeat sequence to induce repeat-mediated rearrangements in mammalian cells. Using a novel reporter assay in mouse embryonic stem cells, they found that a DSB separated from the 3′ repeat by 28.4 kb can still substantially induce RMDs, indicating that a DSB is sufficient to induce RMDs at a relatively far distance.

Chromosomal deletion rearrangements mediated by repetitive elements often involve repeats separated by several kilobases and sequences that are divergent. While such rearrangements are likely induced by DNA double-strand breaks (DSBs), it has been unclear how the proximity of DSBs relative to repeat sequences affects the frequency of such events. We generated a reporter assay in mouse cells for a deletion rearrangement involving repeats separated by 0.4 Mb. We induced this repeat-mediated deletion (RMD) rearrangement with two DSBs: the 5′ DSB that is just downstream from the first repeat and the 3′ DSB that is varying distances upstream of the second repeat. Strikingly, we found that increasing the 3′ DSB/repeat distance from 3.3 kb to 28.4 kb causes only a modest decrease in rearrangement frequency. We also found that RMDs are suppressed by KU70 and RAD51 and promoted by RAD52, CtIP, and BRCA1. In addition, we found that 1%–3% sequence divergence substantially suppresses these rearrangements in a manner dependent on the mismatch repair factor MSH2, which is dominant over the suppressive role of KU70. We suggest that a DSB far from a repeat can stimulate repeat-mediated rearrangements, but multiple pathways suppress these events.

DNA mismatch repair and the DNA damage response

This review discusses the role of DNA mismatch repair (MMR) in the DNA damage response (DDR) that triggers cell cycle arrest and, in some cases, apoptosis. Although the focus is on findings from mammalian cells, much has been learned from studies in other organisms including bacteria and yeast [1,2]. MMR promotes a DDR mediated by a key signaling kinase, ATM and Rad3-related (ATR), in response to various types of DNA damage including some encountered in widely used chemotherapy regimes. An introduction to the DDR mediated by ATR reveals its immense complexity and highlights the many biological and mechanistic questions that remain. Recent findings and future directions are highlighted.

Temozolomide increases the number of mismatch repair-deficient intestinal crypts and accelerates tumorigenesis in a mouse model of Lynch syndrome

BACKGROUND & AIMS

Lynch syndrome, a nonpolyposis form of hereditary colorectal cancer, is caused by inherited defects in DNA mismatch repair (MMR) genes. Most patients carry a germline mutation in 1 allele of the MMR genes MSH2 or MLH1. With spontaneous loss of the wild-type allele, cells with defects in MMR exist among MMR-proficient cells, as observed in healthy intestinal tissues from patients with Lynch syndrome. We aimed to create a mouse model of this situation to aid in identification of environmental factors that affect MMR-defective cells and their propensity for oncogenic transformation.

METHODS

We created mice in which the MMR gene Msh2 can be inactivated in a defined fraction of crypt base columnar stem cells to generate MSH2-deficient intestinal crypts among an excess of wild-type crypts (Lgr5-CreERT2;Msh2(flox/-) mice). Intestinal tissues were collected; immunohistochemical analyses were performed for MSH2, along with allele-specific PCR assays. We traced the fate of MSH2-deficient crypts under the influence of different external factors.

RESULTS

Lgr5-CreERT2;Msh2(flox/-) mice developed more adenomas and adenocarcinomas than control mice; all tumors were MSH2 deficient. Exposure of Lgr5-CreERT2;Msh2(flox/-) mice to the methylating agent temozolomide caused MSH2-deficient intestinal stem cells to proliferate more rapidly than wild-type stem cells. The MSH2-deficient intestinal stem cells were able to colonize the intestinal epithelium and many underwent oncogenic transformation, forming intestinal neoplasias.

CONCLUSIONS

We developed a mouse model of Lynch syndrome (Lgr5-CreERT2;Msh2(flox/-) mice) and found that environmental factors can modify the number and mutability of the MMR-deficient stem cells. These findings provide evidence that environmental factors can promote development of neoplasias and tumors in patients with Lynch syndrome.

Functional analysis of MSH2 unclassified variants found in suspected Lynch syndrome patients reveals pathogenicity due to attenuated mismatch repair

BACKGROUND

Lynch syndrome, an autosomal-dominant disorder characterised by high colorectal and endometrial cancer risks, is caused by inherited mutations in DNA mismatch repair (MMR) genes. Mutations fully abrogating gene function are unambiguously disease causing. However, missense mutations often have unknown functional implications, hampering genetic counselling. We have applied a novel approach to study three MSH2 unclassified variants (UVs) found in Dutch families with suspected Lynch syndrome.

METHODS

The three mutations were recreated in the endogenous Msh2 gene in mouse embryonic stem cells by oligonucleotide-directed gene modification. The effect of the UVs on MMR activity was then tested using a set of functional assays interrogating the main MMR functions.

RESULTS

We recreated and functionally tested three MSH2 UVs: MSH2-Y165D (c.493T>G), MSH2-Q690E (c.2068C>G) and MSH2-M813V (c.2437A>G). We observed reduced levels of MSH2-Y165D and MSH2-Q690E but not MSH2-M813V proteins. MSH2-M813V was able to support all MMR functions similar to wild-type MSH2, whereas MSH2-Y165D and MSH2-Q690E showed partial defects.

CONCLUSIONS

Based on the results from our functional assays, we conclude that the MSH2-M813V variant is not disease causing. The MSH2-Y165D and MSH2-Q690E variants affect MMR function and are therefore likely the underlying cause of familial cancer predisposition. Since the MMR defect is partial, these variants may represent low penetrance alleles.

Embryonic lethality after combined inactivation of Fancd2 and Mlh1 in mice

DNA repair defects are frequently encountered in human cancers. These defects are utilized by traditional therapeutics but also offer novel cancer treatment strategies based on synthetic lethality. To determine the consequences of combined Fanconi anemia (FA) and mismatch repair pathway inactivation, defects in Fancd2 and Mlh1 were combined in one mouse model. Fancd2/Mlh1 double-mutant embryos displayed growth retardation resulting in embryonic lethality and significant underrepresentation among progeny. Additional inactivation of Trp53 failed to improve the survival of Fancd2/Mlh1-deficient embryos. Mouse fibroblasts were obtained and challenged with cross-linking agents. Fancd2-deficient cells displayed the FA-characteristic growth inhibition after mitomycin C (MMC) exposure. In primary fibroblasts, the absence of Mlh1 did not greatly affect the MMC sensitivity of Fancd2-deficient and Fancd2-proficient cells. However, in Trp53 mutant immortalized fibroblasts, Mlh1 deficiency reduced the growth-inhibiting effect of MMC in Fancd2 mutant and complemented cells. Similar data were obtained using psoralen/UVA, signifying that MLH1 influences the cellular sensitivity to DNA interstrand cross-links. Next, the effect of MLH1 deficiency on the formation of chromosomal aberrations in response to cross-linking agents was determined. Surprisingly, Mlh1 mutant fibroblasts displayed a modest but noticeable decrease in induced chromosomal breakage and interchange frequencies, suggesting that MLH1 promotes interstrand cross-link repair catastrophe. In conclusion, the combined inactivation of Fancd2 and Mlh1 did not result in synthetic lethality at the cellular level. Although the absence of Fancd2 sensitized Mlh1/Trp53 mutant fibroblasts to MMC, the differential survival of primary and immortalized fibroblasts advocates against systemic inactivation of FANCD2 to enhance treatment of MLH1-deficient tumors.

Limiting the persistence of a chromosome break diminishes its mutagenic potential

To characterize the repair pathways of chromosome double-strand breaks (DSBs), one approach involves monitoring the repair of site-specific DSBs generated by rare-cutting endonucleases, such as I-SceI. Using this method, we first describe the roles of Ercc1, Msh2, Nbs1, Xrcc4, and Brca1 in a set of distinct repair events. Subsequently, we considered that the outcome of such assays could be influenced by the persistent nature of I-SceI-induced DSBs, in that end-joining (EJ) products that restore the I-SceI site are prone to repeated cutting. To address this aspect of repair, we modified I-SceI-induced DSBs by co-expressing I-SceI with a non-processive 3′ exonuclease, Trex2, which we predicted would cause partial degradation of I-SceI 3′ overhangs. We find that Trex2 expression facilitates the formation of I-SceI-resistant EJ products, which reduces the potential for repeated cutting by I-SceI and, hence, limits the persistence of I-SceI-induced DSBs. Using this approach, we find that Trex2 expression causes a significant reduction in the frequency of repair pathways that result in substantial deletion mutations: EJ between distal ends of two tandem DSBs, single-strand annealing, and alternative-NHEJ. In contrast, Trex2 expression does not inhibit homology-directed repair. These results indicate that limiting the persistence of a DSB causes a reduction in the frequency of repair pathways that lead to significant genetic loss. Furthermore, we find that individual genetic factors play distinct roles during repair of non-cohesive DSB ends that are generated via co-expression of I-SceI with Trex2.

A deleterious lesion in DNA is a break of both strands, or a chromosome double-strand break (DSB). DSBs can arise during normal cellular metabolism, but are also a consequence of many forms of cancer therapy. If DSBs are not repaired prior to cell division, entire segments of a chromosome can be lost. Several pathways ensure that DSBs are repaired, though some pathways are prone to causing mutations and/or chromosomal rearrangements, each of which can contribute to cancer development. In the first part of this study, we describe the roles of individual genetic factors in distinct repair pathways of DSBs generated by the I-SceI endonuclease. From these studies, we find that some factors can function in multiple repair pathways. In the second part of this study, we present a method for partially degrading the cohesive DSB overhangs that are generated by I-SceI, which we find facilitates repair products that are not prone to being re-cut by the endonuclease. As a consequence, we have limited the persistence of such breaks, which we find causes a reduction in repair pathways that lead to significant genetic loss. As well, we use this method to characterize the role of individual genetic factors during the repair of non-cohesive DSB ends.

Novel DNA mismatch-repair activity involving YB-1 in human mitochondria

Maintenance of the mitochondrial genome (mtDNA) is essential for proper cellular function. The accumulation of damage and mutations in the mtDNA leads to diseases, cancer, and aging. Mammalian mitochondria have proficient base excision repair, but the existence of other DNA repair pathways is still unclear. Deficiencies in DNA mismatch repair (MMR), which corrects base mismatches and small loops, are associated with DNA microsatellite instability, accumulation of mutations, and cancer. MMR proteins have been identified in yeast and coral mitochondria; however, MMR proteins and function have not yet been detected in human mitochondria. Here we show that human mitochondria have a robust mismatch-repair activity, which is distinct from nuclear MMR. Key nuclear MMR factors were not detected in mitochondria, and similar mismatch-binding activity was observed in mitochondrial extracts from cells lacking MSH2, suggesting distinctive pathways for nuclear and mitochondrial MMR. We identified the repair factor YB-1 as a key candidate for a mitochondrial mismatch-binding protein. This protein localizes to mitochondria in human cells, and contributes significantly to the mismatch-binding and mismatch-repair activity detected in HeLa mitochondrial extracts, which are significantly decreased when the intracellular levels of YB-1 are diminished. Moreover, YB-1 depletion in cells increases mitochondrial DNA mutagenesis. Our results show that human mitochondria contain a functional MMR repair pathway in which YB-1 participates, likely in the mismatch-binding and recognition steps.

DNA mismatch repair-induced double-strand breaks

Escherichia coli dam mutants are sensitized to the cytotoxic action of base analogs, cisplatin and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), while their mismatch repair (MMR)-deficient derivatives are tolerant to these agents. We showed previously, using pulse field gel electrophoresis (PFGE), that MMR-mediated double-strand breaks (DSBs) are produced by cisplatin in dam recB(Ts) cells at the non-permissive temperature. We demonstrate here that the majority of these DSBs require DNA replication for their formation, consistent with a model in which replication forks collapse at nicks or gaps formed during MMR. DSBs were also detected in dam recB(Ts) ada ogt cells exposed to MNNG in a dose- and MMR-dependent manner. In contrast to cisplatin, the formation of these DSBs was not affected by DNA replication and it is proposed that two separate mechanisms result in DSB formation. Replication-independent DSBs arise from overlapping base excision and MMR repair tracts on complementary strands and constitute the majority of detectable DSBs in dam recB(Ts) ada ogt cells exposed to MNNG. Replication-dependent DSBs result from replication fork collapse at O(6)-methylguanine (O(6)-meG) base pairs undergoing MMR futile cycling and are more likely to contribute to cytotoxicity. This model is consistent with the observation that fast-growing dam recB(Ts) ada ogt cells, which have more chromosome replication origins, are more sensitive to the cytotoxic effect of MNNG than the same cells growing slowly.

Mouse embryonic stem cells are hypersensitive to apoptosis triggered by the DNA damage O(6)-methylguanine due to high E2F1 regulated mismatch repair

Exposure of stem cells to genotoxins may lead to embryonic lethality or teratogenic effects. This can be prevented by efficient DNA repair or by eliminating genetically damaged cells. Using undifferentiated mouse embryonic stem (ES) cells as a pluripotent model system, we compared ES cells with differentiated cells, with regard to apoptosis induction by alkylating agents forming the highly mutagenic and killing DNA adduct O(6)-methylguanine. Upon treatment with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), ES cells undergo apoptosis at much higher frequency than differentiated cells, although they express a high level of the repair protein O(6)-methylguanine-DNA methyltransferase (MGMT). Apoptosis induced by MNNG is due to O(6)-methylguanine DNA adducts, since inhibition of MGMT sensitized ES cells. The high sensitivity of ES cells to O(6)-methylating agents is due to high expression of the mismatch repair proteins MSH2 and MSH6 (MutSalpha), which declines during differentiation. High MutSalpha expression in ES cells was related to a high hyperphosphorylated retinoblastoma (ppRb) level and E2F1 activity that upregulates MSH2, causing, in turn, stabilization of MSH6. Non-repaired O(6)-methylguanine adducts were shown to cause DNA double-stranded breaks, stabilization of p53 and upregulation of Fas/CD95/Apo-1 at significantly higher level in ES cells than in fibroblasts. The high apoptotic response of ES cells to O(6)-methylguanine adducts may contribute to reduction of the mutational load in the progenitor population.

Mismatch repair deficiencies transforming stem cells into cancer stem cells and therapeutic implications

For the exceptional self-renewal capacity, regulated cell proliferation and differential potential to a wide variety of cell types, the stem cells must maintain the intact genome. The cells under continuous exogenous and endogenous genotoxic stress accumulate DNA errors, drive proliferative expansion and transform into cancer stem cells with a heterogeneous population of tumor cells. These cells are a common phenomenon for the hematological malignancies and solid tumors. In response to DNA damage, the complex cellular mechanisms including cell cycle arrest, transcription induction and DNA repair are activated. The cells when exposed to cytotoxic agents, the apoptosis lead to cell death. However, the absence of repair machinery makes the cells resistant to tumor sensitizing agents and result in malignant transformation. Mismatch repair gene defects are recently identified in hematopoietic malignancies, leukemia and lymphoma cell lines. This review emphasizes the importance of MMR systems in maintaining the stem cell functioning and its therapeutic implications in the eradication of cancer stem cells and differentiated tumor cells as well. The understanding of the biological functions of mismatch repair in the stem cells and its malignant counterparts could help in developing an effective novel therapies leaving residual non-tumorigenic population of cells resulting in potential cancer cures.

ATR kinase activation mediated by MutSalpha and MutLalpha in response to cytotoxic O6-methylguanine adducts

S(N)1-type alkylating agents that produce cytotoxic O(6)-methyl-G (O(6)-meG) DNA adducts induce cell cycle arrest and apoptosis in a manner requiring the DNA mismatch repair (MMR) proteins MutSalpha and MutLalpha. Here, we show that checkpoint signaling in response to DNA methylation occurs during S phase and requires DNA replication that gives rise to O(6)-meG/T mispairs. DNA binding studies reveal that MutSalpha specifically recognizes O(6)-meG/T mispairs, but not O(6)-meG/C. In an in vitro assay, ATR-ATRIP, but not RPA, is preferentially recruited to O(6)-meG/T mismatches in a MutSalpha- and MutLalpha-dependent manner. Furthermore, ATR kinase is activated to phosphorylate Chk1 in the presence of O(6)-meG/T mispairs and MMR proteins. These results suggest that MMR proteins can act as direct sensors of methylation damage and help recruit ATR-ATRIP to sites of cytotoxic O(6)-meG adducts to initiate ATR checkpoint signaling.

MSH2 is essential for the preservation of genome integrity and prevents homeologous recombination in the moss Physcomitrella patens

MSH2 is a central component of the mismatch repair pathway that targets mismatches arising during DNA replication, homologous recombination (HR) and in response to genotoxic stresses. Here, we describe the function of MSH2 in the moss Physcomitrella patens, as deciphered by the analysis of loss of function mutants. Ppmsh2 mutants display pleiotropic growth and developmental defects, which reflect genomic instability. Based on loss of function of the APT gene, we estimated this mutator phenotype to be at least 130 times higher in the mutants than in wild type. We also found that MSH2 is involved in some but not all the moss responses to genotoxic stresses we tested. Indeed, the Ppmsh2 mutants were more tolerant to cisplatin and show higher sensitivity to UV-B radiations. PpMSH2 gene involvement in HR was studied by assessing gene targeting (GT) efficiency with homologous and homeologous sequences. GT efficiency with homologous sequences was slightly decreased in the Ppmsh2 mutant compared with wild type. Strikingly GT efficiency with homeologous sequences decreased proportionally to sequence divergence in the wild type whereas it remained unaffected in the mutants. Those results demonstrate the role of PpMSH2 in the maintenance of genome integrity and in homologous and homeologous recombination.

Msh2 deficiency increases susceptibility to benzo[a]pyrene-induced lymphomagenesis

DNA mismatch repair (MMR) is essential for repair of single-base mismatches and insertion/deletion loops. MMR proteins also participate in cellular response to DNA damaging agents such as various alkylating agents. Mice deficient in the MMR gene Msh2 develop tumors earlier after exposure to alkylating agents when compared to unexposed mice. The interaction between the MMR system and polycyclic aromatic hydrocarbons such as benzo[a]pyrene (B[a]P) has not been investigated in vivo. Here, we show that treatment of Msh2-deficient mice with B[a]P enhances susceptibility to lymphomagenesis. Carrying at least one intact copy of the Msh2 gene had a protective effect. B[a]P treatment only induced lymphomas in 3 of the 40 (7.5%) mice with at least one intact copy of the Msh2 gene as compared to 13 of the 17 (76.5%) Msh2-deficient mice and occurs only after a much longer time period. The B[a]P-DNA adduct levels measured in lung, liver, spleen and forestomach of B[a]P-treated Msh2-/- mice were not significantly different from B[a]P-treated Msh2+/+ mice. In summary, the results suggest that B[a]P accelerates lymphomagenesis in Msh2-deficient mice. Furthermore, Msh2 deficiency does not have any significant effect on B[a]P-DNA adduct levels.

Signalling cell cycle arrest and cell death through the MMR System

Loss of DNA mismatch repair (MMR) in mammalian cells, as well as having a causative role in cancer, has been linked to resistance to certain DNA damaging agents including clinically important cytotoxic chemotherapeutics. MMR-deficient cells exhibit defects in G2/M cell cycle arrest and cell killing when treated with these agents. MMR-dependent cell cycle arrest occurs, at least for low doses of alkylating agents, only after the second S-phase following DNA alkylation, suggesting that two rounds of DNA replication are required to generate a checkpoint signal. These results point to an indirect role for MMR proteins in damage signalling where aberrant processing of mismatches leads to the generation of DNA structures (single-strand gaps and/or double-strand breaks) that provoke checkpoint activation and cell killing. Significantly, recent studies have revealed that the role of MMR proteins in mismatch repair can be uncoupled from the MMR-dependent damage responses. Thus, there is a threshold of expression of MSH2 or MLH1 required for proper checkpoint and cell-death signalling, even though sub-threshold levels are sufficient for fully functional MMR repair activity. Segregation is also revealed through the identification of mutations in MLH1 or MSH2 that provide alleles functional in MMR but not in DNA damage responses and mutations in MSH6 that compromise MMR but not in apoptotic responses to DNA damaging agents. These studies suggest a direct role for MMR proteins in recognizing and signalling DNA damage responses that is independent of the MMR catalytic repair process. How MMR-dependent G2 arrest may link to cell death remains elusive and we speculate that it is perhaps the resolution of the MMR-dependent G2 cell cycle arrest following DNA damage that is important in terms of cell survival.

Differential effects of cisplatin and MNNG on dna mutants of Escherichia coli

DNA mismatch repair (MMR) in mammalian cells or Escherichia coli dam mutants increases the cytotoxic effects of cisplatin and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). We found that, unlike wildtype, the dnaE486 (alpha catalytic subunit of DNA polymerase III holoenzyme) mutant, and a DnaX (clamp loader subunits) over-producer, are sensitive to cisplatin but resistant to MNNG at the permissive temperature for growth. Survival of dam-13 dnaN159 (beta sliding clamp) bacteria to cisplatin was significantly less than dam cells, suggesting decreased MMR, which may be due to reduced MutS-beta clamp interaction. We also found an elevated spontaneous mutant frequency to rifampicin resistance in dnaE486 (10-fold), dnaN159 (35-fold) and dnaX36 (10-fold) strains. The mutation spectrum in the dnaN159 strain was consistent with increased SOS induction and not indicative of MMR deficiency.

MutS inhibits RecA-mediated strand transfer with methylated DNA substrates

DNA mismatch repair (MMR) sensitizes human and Escherichia coli dam cells to the cytotoxic action of N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) while abrogation of such repair results in drug resistance. In DNA methylated by MNNG, MMR action is the result of MutS recognition of O⁶-methylguanine base pairs. MutS and Ada methyltransferase compete for the MNNG-induced O⁶-methylguanine residues, and MMR-induced cytotoxicity is abrogated when Ada is present at higher concentrations than normal. To test the hypothesis that MMR sensitization is due to decreased recombinational repair, we used a RecA-mediated strand exchange assay between homologous phiX174 substrate molecules, one of which was methylated with MNNG. MutS inhibited strand transfer on such substrates in a concentration-dependent manner and its inhibitory effect was enhanced by MutL. There was no effect of these proteins on RecA activity with unmethylated substrates. We quantified the number of O⁶-methylguanine residues in methylated DNA by HPLC-MS/MS and 5–10 of these residues in phiX174 DNA (5386 bp) were sufficient to block the RecA reaction in the presence of MutS and MutL. These results are consistent with a model in which methylated DNA is perceived by the cell as homeologous and prevented from recombining with homologous DNA by the MMR system.

Cellular processing of platinum anticancer drugs

Cisplatin, carboplatin and oxaliplatin are platinum-based drugs that are widely used in cancer chemotherapy. Platinum-DNA adducts, which are formed following uptake of the drug into the nucleus of cells, activate several cellular processes that mediate the cytotoxicity of these platinum drugs. This review focuses on recently discovered cellular pathways that are activated in response to cisplatin, including those involved in regulating drug uptake, the signalling of DNA damage, cell-cycle checkpoints and arrest, DNA repair and cell death. Such knowledge of the cellular processing of cisplatin adducts with DNA provides valuable clues for the rational design of more efficient platinum-based drugs as well as the development of new therapeutic strategies.

Separation of mutation avoidance and antirecombination functions in an Escherichia coli mutS mutant

DNA mismatch repair in Escherichia coli has been shown to be involved in two distinct processes: mutation avoidance, which removes potential mutations arising as replication errors, and antirecombination which prevents recombination between related, but not identical (homeologous), DNA sequences. We show that cells with the mutSΔ800 mutation (which removes the C-terminal 53 amino acids of MutS) on a multicopy plasmid are proficient for mutation avoidance. In interspecies genetic crosses, however, recipients with the mutSΔ800 mutation show increased recombination by up to 280-fold relative to mutS⁺. The MutSΔ800 protein binds to O⁶-methylguanine mismatches but not to intrastrand platinated GG cross-links, explaining why dam bacteria with the mutSΔ800 mutation are resistant to cisplatin, but not MNNG, toxicity. The results indicate that the C-terminal end of MutS is necessary for antirecombination and cisplatin sensitization, but less significant for mutation avoidance. The inability of MutSΔ800 to form tetramers may indicate that these are the active form of MutS.

Role of DNA mismatch repair in apoptotic responses to therapeutic agents

Deficiencies in DNA mismatch repair (MMR) have been found in both hereditary cancer (i.e., hereditary nonpolyposis colorectal cancer) and sporadic cancers of various tissues. In addition to its primary roles in the correction of DNA replication errors and suppression of recombination, research in the last 10 years has shown that MMR is involved in many other processes, such as interaction with other DNA repair pathways, cell cycle checkpoint regulation, and apoptosis. Indeed, a cell's MMR status can influence its response to a wide variety of chemotherapeutic agents, such as temozolomide (and many other methylating agents), 6-thioguanine, cisplatin, ionizing radiation, etoposide, and 5-fluorouracil. For this reason, identification of a tumor's MMR deficiency (as indicated by the presence of microsatellite instability) is being utilized more and more as a prognostic indicator in the clinic. Here, we describe the basic mechanisms of MMR and apoptosis and investigate the literature examining the influence of MMR status on the apoptotic response following treatment with various therapeutic agents. Furthermore, using isogenic MMR-deficient (HCT116) and MMR-proficient (HCT116 3-6) cells, we demonstrate that there is no enhanced apoptosis in MMR-proficient cells following treatment with 5-fluoro-2'-deoxyuridine. In fact, apoptosis accounts for only a small portion of the induced cell death response.