High Doses of SN1 Type Methylating Agents Activate DNA Damage Signaling Cascades that are Largely Independent of Mismatch Repair

FANCD2-Associated Nuclease 1 Partially Compensates for the Lack of Exonuclease 1 in Mismatch Repair

Germline mutations in the mismatch repair (MMR) genes MSH2, MSH6, MLH1, and PMS2 are linked to cancer of the colon and other organs, characterized by microsatellite instability and a large increase in mutation frequency. Unexpectedly, mutations in EXO1, encoding the only exonuclease genetically implicated in MMR, are not linked to familial cancer and cause a substantially weaker mutator phenotype. This difference could be explained if eukaryotic cells possessed additional exonucleases redundant with EXO1. Analysis of the MLH1 interactome identified FANCD2-associated nuclease 1 (FAN1), a novel enzyme with biochemical properties resembling EXO1. We now show that FAN1 efficiently substitutes for EXO1 in MMR assays and that this functional complementation is modulated by its interaction with MLH1. FAN1 also contributes to MMR in vivo; cells lacking both EXO1 and FAN1 have an MMR defect and display resistance to N-methyl-N-nitrosourea (MNU) and 6-thioguanine (TG). Moreover, FAN1 loss amplifies the mutational profile of EXO1-deficient cells, suggesting that the two nucleases act redundantly in the same antimutagenic pathway. However, the increased drug resistance and mutator phenotype of FAN1/EXO1-deficient cells are less prominent than those seen in cells lacking MSH6 or MLH1. Eukaryotic cells thus apparently possess additional mechanisms that compensate for the loss of EXO1.

Biological Basis for Threshold Responses to Methylating Agents

The cellular outcomes of chemical exposure are as much about the cellular response to the chemical as it is an effect of the chemical. We are growing in our understanding of the genotoxic interaction between chemistry and biology. For example, recent data has revealed the biological basis for mutation induction curves for a methylating chemical, which has been shown to be dependent on the repair capacity of the cells. However, this is just one end point in the toxicity pathway from chemical exposure to cell death. Much remains to be known in order for us to predict how cells will respond to a certain dose. Methylating agents, a subset of alkylating agents, are of particular interest, because of the variety of adverse genetic end points that can result, not only at increasing doses, but also over time. For instance, methylating agents are mutagenic, their potency, for this end point, is determined by the cellular repair capacity of an enzyme called methylguanine DNA-methyltransferase (MGMT) and its ability to repair the induceed methyl adducts. However, methyl adducts can become clastogenic. Erroneous biological processing will convert mutagenic adducts to clastogenic events in the form of double strand breaks (DSBs). How the cell responds to DSBs is via a cascade of protein kinases, which is called the DNA damage response (DDR), which will determine if the damage is repaired effectively, via homologous recombination, or with errors, via nonhomologous end joining, or whether the cell dies via apoptosis or enters senescence. The fate of cells may be determined by the extent of damage and the resulting strength of DDR signaling. Therefore, thresholds of damage may exist that determine cell fate. Such thresholds would be dependent on each of the repair and response mechanisms that these methyl adducts stimulate. The molecular mechanism of how methyl adducts kill cells is still to be fully resolved. If we are able to quantify each of these thresholds of damage for a given cell, then we can ascertain, of the many adducts that are induced, what proportion of them are mutagenic, what proportion are clastogenic, and how many of these clastogenic events are toxic. This review examines the possibility of dose and damage thresholds for methylating agents, from the perspective of the underlying evolutionary mechanisms that may be accountable.

The DNA repair enzyme MUTYH potentiates cytotoxicity of the alkylating agent MNNG by interacting with abasic sites

Higher expression of the human DNA repair enzyme MUTYH has previously been shown to be strongly associated with reduced survival in a panel of 24 human lymphoblastoid cell lines exposed to the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). The molecular mechanism of MUTYH-enhanced MNNG cytotoxicity is unclear, because MUTYH has a well-established role in the repair of oxidative DNA lesions. Here, we show in mouse embryonic fibroblasts (MEFs) that this MNNG-dependent phenotype does not involve oxidative DNA damage and occurs independently of both O⁶-methyl guanine adduct cytotoxicity and MUTYH-dependent glycosylase activity. We found that blocking of abasic (AP) sites abolishes higher survival of Mutyh-deficient (Mutyh ^-/-) MEFs, but this blockade had no additive cytotoxicity in WT MEFs, suggesting the cytotoxicity is due to MUTYH interactions with MNNG-induced AP sites. We found that recombinant mouse MUTYH tightly binds AP sites opposite all four canonical undamaged bases and stimulated apurinic/apyrimidinic endonuclease 1 (APE1)-mediated DNA incision. Consistent with these observations, we found that stable expression of WT, but not catalytically-inactive MUTYH, enhances MNNG cytotoxicity in Mutyh ^-/- MEFs and that MUTYH expression enhances MNNG-induced genomic strand breaks. Taken together, these results suggest that MUTYH enhances the rapid accumulation of AP-site intermediates by interacting with APE1, implicating MUTYH as a factor that modulates the delicate process of base-excision repair independently of its glycosylase activity.

Impact of DNA mismatch repair system alterations on human fertility and related treatments

DNA mismatch repair (MMR) is one of the biological pathways, which plays a critical role in DNA homeostasis, primarily by repairing base-pair mismatches and insertion/deletion loops that occur during DNA replication. MMR also takes part in other metabolic pathways and regulates cell cycle arrest. Defects in MMR are associated with genomic instability, predisposition to certain types of cancers and resistance to certain therapeutic drugs. Moreover, genetic and epigenetic alterations in the MMR system demonstrate a significant relationship with human fertility and related treatments, which helps us to understand the etiology and susceptibility of human infertility. Alterations in the MMR system may also influence the health of offspring conceived by assisted reproductive technology in humans. However, further studies are needed to explore the specific mechanisms by which the MMR system may affect human infertility. This review addresses the physiological mechanisms of the MMR system and associations between alterations of the MMR system and human fertility and related treatments, and potential effects on the next generation.

Cellular response to alkylating agent MNNG is impaired in STAT1-deficients cells

The SN 1 alkylating agents activate the mismatch repair system leading to delayed G2 /M cell cycle arrest and DNA repair with subsequent survival or cell death. STAT1, an anti-proliferative and pro-apoptotic transcription factor is known to potentiate p53 and to affect DNA-damage cellular response. We studied whether STAT1 may modulate cell fate following activation of the mismatch repair system upon exposure to the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Using STAT1-proficient or -deficient cell lines, we found that STAT1 is required for: (i) reduction in the extent of DNA lesions, (ii) rapid phosphorylation of T68-CHK2 and of S15-p53, (iii) progression through the G2 /M checkpoint and (iv) long-term survival following treatment with MNNG. Presence of STAT1 is critical for the formation of a p53-DNA complex comprising: STAT1, c-Abl and MLH1 following exposure to MNNG. Importantly, presence of STAT1 allows recruitment of c-Abl to p53-DNA complex and links c-Abl tyrosine kinase activity to MNNG-toxicity. Thus, our data highlight the important modulatory role of STAT1 in the signalling pathway activated by the mismatch repair system. This ability of STAT1 to favour resistance to MNNG indicates the targeting of STAT1 pathway as a therapeutic option for enhancing the efficacy of SN1 alkylating agent-based chemotherapy.

Chromosome Missegregation Associated with RUVBL1 Deficiency

RUVBL1 (RuvB-like1) and RUVBL2 (RuvB-like 2) are integral components of multisubunit protein complexes involved in processes ranging from cellular metabolism, transcription and chromatin remodeling to DNA repair. Here, we show that although RUVBL1 and RUVBL2 are known to form heterodimeric complexes in which they stabilize each other, the subunits separate during cytokinesis. In anaphase-to-telophase transition, RUVBL1 localizes to structures of the mitotic spindle apparatus, where it partially co-localizes with polo-like kinase 1 (PLK1). The ability of PLK1 to phosphorylate RUVBL1-but not RUVBL2-in vitro and their physical association in vivo suggest that this kinase differentially regulates the function of the RuvB-like proteins during mitosis. We further show that siRNA-mediated knock-down of RuvB-like proteins causes severe defects in chromosome alignment and segregation. In addition, we show that the ATPase activity of RUVBL1 is indispensable for cell proliferation. Our data thus demonstrate that RUVBL1 is essential for efficient mitosis and proliferation.

Theoretical considerations for thresholds in chemical carcinogenesis

There is increasing evidence for non-linear relationships for gene mutations, chromosomal aberrations and even tumor incidences in response to low doses of genotoxic carcinogens. To attain the biological relevance of such non-linear responses, there is a need to identify the underlying defense mechanisms that allow tolerance to low doses of genotoxicants. This communication discusses presumptive cancer prevention mechanisms that may contribute to thresholds, i.e. points of departure, for each endpoint, from initial DNA lesion to tumor formation. We discuss a sequential order of genome protection during carcinogenesis where genotoxicant scavenging, cellular efflux, DNA repair, elimination of damaged cells by apoptosis, autophagy, silencing by DNA damage-triggered replicative senescence, and finally, elimination of transformed (premalignant) cells by the immune system are thought to be responsible for a threshold in tumor formation. We highlight DNA repair, for which experimental evidence has been recently provided to dictate a role in PoDs. In conclusion, from a theoretical perspective it is reasonable to posit that tolerance to low dose levels exists for each requisite step of tumor formation and these tolerance mechanisms are critical in determining thresholds in chemical carcinogenesis.

NPM-ALK mediates phosphorylation of MSH2 at tyrosine 238, creating a functional deficiency in MSH2 and the loss of mismatch repair

The vast majority of anaplastic lymphoma kinase-positive anaplastic large cell lymphoma (ALK+ALCL) tumors express the characteristic oncogenic fusion protein NPM-ALK, which mediates tumorigenesis by exerting its constitutive tyrosine kinase activity on various substrates. We recently identified MSH2, a protein central to DNA mismatch repair (MMR), as a novel binding partner and phosphorylation substrate of NPM-ALK. Here, using liquid chromatography–mass spectrometry, we report for the first time that MSH2 is phosphorylated by NPM-ALK at a specific residue, tyrosine 238. Using GP293 cells transfected with NPM-ALK, we confirmed that the MSH2^Y238F mutant is not tyrosine phosphorylated. Furthermore, transfection of MSH2^Y238F into these cells substantially decreased the tyrosine phosphorylation of endogenous MSH2. Importantly, gene transfection of MSH2^Y238F abrogated the binding of NPM-ALK with endogenous MSH2, re-established the dimerization of MSH2:MSH6 and restored the sensitivity to DNA mismatch-inducing drugs, indicative of MMR return. Parallel findings were observed in two ALK+ALCL cell lines, Karpas 299 and SUP-M2. In addition, we found that enforced expression of MSH2^Y238F into ALK+ALCL cells alone was sufficient to induce spontaneous apoptosis. In conclusion, our findings have identified NPM-ALK-induced phosphorylation of MSH2 at Y238 as a crucial event in suppressing MMR. Our studies have provided novel insights into the mechanism by which oncogenic tyrosine kinases disrupt MMR.

Role of AKT signaling in DNA repair and clinical response to cancer therapy

Effective cancer treatment has been limited by the emergence of resistant cancer cells. The results of many studies indicate that AKT activation plays an important role in the acquisition of resistance to anticancer therapy. AKT is a critical effector serine/threonine kinase in the receptor tyrosine kinase/phosphatase and tensin homolog/phospho-inositide 3-kinase pathway and controls a myriad of cellular functions. Activation of AKT not only supports tumor growth and progression but also contributes to tumor-cell evasion of the cytotoxic effects of cancer therapy through many avenues including the promotion of anti-apoptosis, proliferation, and migration and regulation of the cell cycle. Accumulating evidence has implicated AKT as a direct participant in the DNA damage response and repair induced by commonly used genotoxic agents. In this review, we discuss the molecular mechanisms by which genotoxic agents activate AKT and therefore contribute to resistance to cancer therapeutics, with particular emphasis on DNA repair.

N-nitroso-N-ethylurea activates DNA damage surveillance pathways and induces transformation in mammalian cells

Background

The DNA damage checkpoint signalling cascade sense damaged DNA and coordinates cell cycle arrest, DNA repair, and/or apoptosis. However, it is still not well understood how the signalling system differentiates between different kinds of DNA damage. N-nitroso-N-ethylurea (NEU), a DNA ethylating agent induces both transversions and transition mutations.

Methods

Immunoblot and comet assays were performed to detect DNA breaks and activation of the canonical checkpoint signalling kinases following NEU damage upto 2 hours. To investigate whether mismatch repair played a role in checkpoint activation, knock-down studies were performed while flow cytometry analysis was done to understand whether the activation of the checkpoint kinases was cell cycle phase specific. Finally, breast epithelial cells were grown as 3-dimensional spheroid cultures to study whether NEU can induce upregulation of vimentin as well as disrupt cell polarity of the breast acini, thus causing transformation of epithelial cells in culture.

Results

We report a novel finding that NEU causes activation of major checkpoint signalling kinases, Chk1 and Chk2. This activation is temporally controlled with Chk2 activation preceding Chk1 phosphorylation, and absence of cross talk between the two parallel signalling pathways, ATM and ATR. Damage caused by NEU leads to the temporal formation of both double strand and single strand breaks. Activation of checkpoints following NEU damage is cell cycle phase dependent wherein Chk2 is primarily activated during G2-M phase whilst in S phase, there is immediate Chk1 phosphorylation and delayed Chk2 response. Surprisingly, the mismatch repair system does not play a role in checkpoint activation, at doses and duration of NEU used in the experiments. Interestingly, NEU caused disruption of the well-formed polarised spheroid archithecture and upregulation of vimentin in three-dimensional breast acini cultures of non-malignant breast epithelial cells upon NEU treatment indicating NEU to have the potential to cause early transformation in the cells.

Conclusion

NEU causes damage in mammalian cells in the form of double strand and single strand breaks that temporally activate the major checkpoint signalling kinases without the occurrence of cross-talk between the pathways. NEU also appear to cause transformation in three-dimensional spheroid cultures.

Early Chk1 phosphorylation is driven by temozolomide-induced, DNA double strand break- and mismatch repair-independent DNA damage

Temozolomide (TMZ) is a DNA methylating agent used to treat brain cancer. TMZ-induced O6-methylguanine adducts, in the absence of repair by O6-methylguanine DNA methyltransferase (MGMT), mispair during DNA replication and trigger cycles of futile mismatch repair (MMR). Futile MMR in turn leads to the formation of DNA single and double strand breaks, Chk1 and Chk2 phosphorylation/activation, cell cycle arrest, and ultimately cell death. Although both pChk1 and pChk2 are considered to be biomarkers of TMZ-induced DNA damage, cell-cycle arrest, and TMZ induced cytotoxicity, we found that levels of pChk1 (ser345), its downstream target pCdc25C (ser216), and the activity of its upstream activator ATR, were elevated within 3 hours of TMZ exposure, long before the onset of TMZ-induced DNA double strand breaks, Chk2 phosphorylation/activation, and cell cycle arrest. Furthermore, TMZ-induced early phosphorylation of Chk1 was noted in glioma cells regardless of whether they were MGMT-proficient or MGMT-deficient, and regardless of their MMR status. Early Chk1 phosphorylation was not associated with TMZ-induced reactive oxygen species, but was temporally associated with TMZ-induced alkalai-labile DNA damage produced by the non-O6-methylguanine DNA adducts and which, like Chk1 phosphorylation, was transient in MGMT-proficient cells but persistent in MGMT-deficient cells. These results re-define the TMZ-induced DNA damage response, and show that Chk1 phosphorylation is driven by TMZ-induced mismatch repair-independent DNA damage independently of DNA double strand breaks, Chk2 activation, and cell cycle arrest, and as such is a suboptimal biomarker of TMZ-induced drug action.

Decoupling of DNA damage response signaling from DNA damages underlies temozolomide resistance in glioblastoma cells

Glioblastoma multiforme (GBM) is the most aggressive primary brain tumor in adults. Current therapy includes surgery, radiation and chemotherapy with temozolomide (TMZ). Major determinants of clinical response to TMZ include methylation status of the O6-methylguanine-DNA methyltransferase (MGMT) promoter and mismatch repair (MMR) status. Though the MGMT promoter is methylated in 45% of cases, for the first nine months of follow-up, TMZ does not change survival outcome. Furthermore, MMR deficiency makes little contribution to clinical resistance, suggesting that there exist unrecognized mechanisms of resistance. We generated paired GBM cell lines whose resistance was attributed to neither MGMT nor MMR. We show that, responding to TMZ, these cells exhibit a decoupling of DNA damage response (DDR) from ongoing DNA damages. They display methylation-resistant synthesis in which ongoing DNA synthesis is not inhibited. They are also defective in the activation of the S and G2 phase checkpoint. DDR proteins ATM, Chk2, MDC1, NBS1 and gammaH2AX also fail to form discrete foci. These results demonstrate that failure of DDR may play an active role in chemoresistance to TMZ. DNA damages by TMZ are repaired by MMR proteins in a futile, reiterative process, which activates DDR signaling network that ultimately leads to the onset of cell death. GBM cells may survive genetic insults in the absence of DDR. We anticipate that our findings will lead to more studies that seek to further define the role of DDR in ultimately determining the fate of a tumor cell in response to TMZ and other DNA methylators.

Exonuclease 1 (Exo1) is required for activating response to S(N)1 DNA methylating agents

S(N)1 DNA methylating agents are genotoxic agents that methylate numerous nucleophilic centers within DNA including the O(6) position of guanine (O(6)meG). Methylation of this extracyclic oxygen forces mispairing with thymine during DNA replication. The mismatch repair (MMR) system recognizes these O(6)meG:T mispairs and is required to activate DNA damage response (DDR). Exonuclease I (EXO1) is a key component of MMR by resecting the damaged strand; however, whether EXO1 is required to activate MMR-dependent DDR remains unknown. Here we show that knockdown of the mouse ortholog (mExo1) in mouse embryonic fibroblasts (MEFs) results in decreased G2/M checkpoint response, limited effects on cell proliferation, and increased cell viability following exposure to the S(N)1 methylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), establishing a phenotype paralleling MMR deficiency. MNNG treatment induced formation of γ-H2AX foci with which EXO1 co-localized in MEFs, but mExo1-depleted MEFs displayed a significant diminishment of γ-H2AX foci formation. mExo1 depletion also reduced MSH2 association with DNA duplexes containing G:T mismatches in vitro, decreased MSH2 association with alkylated chromatin in vivo, and abrogated MNNG-induced MSH2/CHK1 interaction. To determine if nuclease activity is required to activate DDR we stably overexpressed a nuclease defective form of human EXO1 (hEXO1) in mExo1-depleted MEFs. These experiments indicated that expression of wildtype and catalytically null hEXO1 was able to restore normal response to MNNG. This study indicates that EXO1 is required to activate MMR-dependent DDR in response to S(N)1 methylating agents; however, this function of EXO1 is independent of its nucleolytic activity.

DNA damage-induced cell death: from specific DNA lesions to the DNA damage response and apoptosis

DNA damaging agents are potent inducers of cell death triggered by apoptosis. Since these agents induce a plethora of different DNA lesions, it is firstly important to identify the specific lesions responsible for initiating apoptosis before the apoptotic executing pathways can be elucidated. Here, we describe specific DNA lesions that have been identified as apoptosis triggers, their repair and the signaling provoked by them. We discuss methylating agents such as temozolomide, ionizing radiation and cisplatin, all of them are important in cancer therapy. We show that the potentially lethal events for the cell are O(6)-methylguanine adducts that are converted by mismatch repair into DNA double-strand breaks (DSBs), non-repaired N-methylpurines and abasic sites as well as bulky adducts that block DNA replication leading to DSBs that are also directly induced following ionizing radiation. Transcriptional inhibition may also contribute to apoptosis. Cells are equipped with sensors that detect DNA damage and relay the signal via kinases to executors, who on their turn evoke a process that inhibits cell cycle progression and provokes DNA repair or, if this fails, activate the receptor and/or mitochondrial apoptotic cascade. The main DNA damage recognition factors MRN and the PI3 kinases ATM, ATR and DNA-PK, which phosphorylate a multitude of proteins and thus induce the DNA damage response (DDR), will be discussed as well as the downstream players p53, NF-κB, Akt and survivin. We review data and models describing the signaling from DNA damage to the apoptosis executing machinery and discuss the complex interplay between cell survival and death.

Loss of DNA mismatch repair imparts a selective advantage in planarian adult stem cells

Lynch syndrome (LS) leads to an increased risk of early-onset colorectal and other types of cancer and is caused by germline mutations in DNA mismatch repair (MMR) genes. Loss of MMR function results in a mutator phenotype that likely underlies its role in tumorigenesis. However, loss of MMR also results in the elimination of a DNA damage-induced checkpoint/apoptosis activation barrier that may allow damaged cells to grow unchecked. A fundamental question is whether loss of MMR provides pre-cancerous stem cells an immediate selective advantage in addition to establishing a mutator phenotype. To test this hypothesis in an in vivo system, we utilized the planarian Schmidtea mediterranea which contains a significant population of identifiable adult stem cells. We identified a planarian homolog of human MSH2, a MMR gene which is mutated in 38% of LS cases. The planarian Smed-msh2 is expressed in stem cells and some progeny. We depleted Smed-msh2 mRNA levels by RNA-interference and found a striking survival advantage in these animals treated with a cytotoxic DNA alkylating agent compared to control animals. We demonstrated that this tolerance to DNA damage is due to the survival of mitotically active, MMR-deficient stem cells. Our results suggest that loss of MMR provides an in vivo survival advantage to the stem cell population in the presence of DNA damage that may have implications for tumorigenesis.

Conservation of functional asymmetry in the mammalian MutLα ATPase

The DNA mismatch repair (MMR) protein dimer MutLα is comprised of the MutL homologues MLH1 and PMS2, which each belong to the family of GHL ATPases. These ATPases undergo functionally important conformational changes, including dimerization of the NH₂-termini associated with ATP binding and hydrolysis. Previous studies in yeast and biochemical studies with the mammalian proteins established the importance of the MutLα ATPase for overall MMR function. Additionally, the studies in yeast demonstrated a functional asymmetry between the contributions of the Mlh1 and Pms1 ATPase domains to MMR that was not reflected in the biochemical studies. We investigated the effect of mutating the highly conserved ATP hydrolysis and Mg²(+) binding residues of MLH1 and PMS2 in mammalian cell lines. Amino acid substitutions in MLH1 intended to impact either ATP binding or hydrolysis disabled MMR, as measured by instability at microsatellite sequences, to an extent similar to MLH1-null mutation. Furthermore, cells expressing these MLH1 mutations exhibited resistance to the MMR-dependent cytotoxic effect of 6-thioguanine (6-TG). In contrast, ATP hydrolysis and binding mutants of PMS2 displayed no measurable increase in microsatellite instability or resistance to 6-TG. Our findings suggest that, in vivo, the integrity of the MLH1 ATPase domain is more critical than the PMS2 ATPase domain for normal MMR functions. These in vivo results are in contrast to results obtained previously in vitro that showed no functional asymmetry within the MutLα ATPase, highlighting the differences between in vivo and in vitro systems.

Interaction between human mismatch repair recognition proteins and checkpoint sensor Rad9-Rad1-Hus1

In eukaryotic cells, the cell cycle checkpoint proteins Rad9, Rad1, and Hus1 form the 9-1-1 complex which is structurally similar to the proliferating cell nuclear antigen (PCNA) sliding clamp. hMSH2/hMSH6 (hMutS alpha) and hMSH2/hMSH3 (hMutS beta) are the mismatch recognition factors of the mismatch repair pathway. hMutS alpha has been shown to physically and functionally interact with PCNA. Moreover, DNA methylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) treatment induces the G2/M cell cycle arrest that is dependent on the presence of hMutS alpha and hMutL alpha. In this study, we show that each subunit of the human 9-1-1 complex physically interacts with hMSH2, hMSH3, and hMSH6. The 9-1-1 complex from both humans and Schizosaccharomyces pombe can stimulate hMutS alpha binding with G/T-containing DNA. Rad9, Rad1, and Hus1 individual subunits can also stimulate the DNA binding activity of hMutS alpha. Human Rad9 and hMSH6 colocalize to nuclear foci of HeLa cells after exposure to MNNG. However, Rad9 does not form foci in MSH6 defective cells following MNNG treatment. In Rad9 knockdown untreated cells, the majority of the MSH6 is in cytoplasm. Following MNNG treatment, Rad9 knockdown cells has abnormal nuclear morphology and MSH6 is distributed around nuclear envelop. Our findings suggest that the 9-1-1 complex is a component of the mismatch repair involved in MNNG-induced damage response.

The mismatch repair system modulates curcumin sensitivity through induction of DNA strand breaks and activation of G2-M checkpoint

The highly conserved mismatch (MMR) repair system corrects postreplicative errors and modulates cellular responses to genotoxic agents. Here, we show that the MMR system strongly influences cellular sensitivity to curcumin. Compared with MMR-proficient cells, isogenically matched MMR-deficient cells displayed enhanced sensitivity to curcumin. Similarly, cells suppressed for MLH1 or MSH2 expression by RNA interference displayed increased curcumin sensitivity. Curcumin treatment generated comparable levels of reactive oxygen species and the mutagenic adduct 8-oxo-guanine in MMR-proficient and MMR-deficient cells; however, accumulation of gammaH2AX foci, a marker for DNA double-strand breaks (DSB), occurred only in MMR-positive cells in response to curcumin treatment. Additionally, MMR-positive cells showed activation of Chk1 and induction of G(2)-M cell cycle checkpoint following curcumin treatment and inhibition of Chk1 by UCN-01 abrogated Chk1 activation and heightened apoptosis in MMR-proficient cells. These results indicate that curcumin triggers the accumulation of DNA DSB and induction of a checkpoint response through a MMR-dependent mechanism. Conversely, in MMR-compromised cells, curcumin-induced DSB is significantly blunted, and as a result, cells fail to undergo cell cycle arrest, enter mitosis, and die through mitotic catastrophe. The results have potential therapeutic value, especially in the treatment of tumors with compromised MMR function.

Nuclear reorganization of DNA mismatch repair proteins in response to DNA damage

The DNA mismatch repair (MMR) system is highly conserved and vital for preserving genomic integrity. Current mechanistic models for MMR are mainly derived from in vitro assays including reconstitution of strand-specific MMR and DNA binding assays using short oligonucleotides. However, fundamental questions regarding the mechanism and regulation in the context of cellular DNA replication remain. Using synchronized populations of HeLa cells we demonstrated that hMSH2, hMLH1 and PCNA localize to the chromatin during S-phase, and accumulate to a greater extent in cells treated with a DNA alkylating agent. In addition, using small interfering RNA to deplete hMSH2, we demonstrated that hMLH1 localization to the chromatin is hMSH2-dependent. hMSH2/hMLH1/PCNA proteins, when associated with the chromatin, form a complex that is greatly enhanced by DNA damage. The DNA damage caused by high doses of alkylating agents leads to a G(2) arrest after only one round of replication. In these G(2)-arrested cells, an hMSH2/hMLH1 complex persists on chromatin, however, PCNA is no longer in the complex. Cells treated with a lower dose of alkylating agent require two rounds of replication before cells arrest in G(2). In the first S-phase, the MMR proteins form a complex with PCNA, however, during the second S-phase PCNA is missing from that complex. The distinction between these complexes may suggest separate functions for the MMR proteins in damage repair and signaling. Additionally, using confocal immunofluorescence, we observed a population of hMSH6 that localized to the nucleolus. This population is significantly reduced after DNA damage suggesting that the protein is shuttled out of the nucleolus in response to damage. In contrast, hMLH1 is excluded from the nucleolus at all times. Thus, the nucleolus may act to segregate a population of hMSH2-hMSH6 from hMLH1-hPMS2 such that, in the absence of DNA damage, an inappropriate response is not invoked.

Residual gammaH2AX foci as an indication of lethal DNA lesions

Background

Evidence suggests that tumor cells exposed to some DNA damaging agents are more likely to die if they retain microscopically visible γH2AX foci that are known to mark sites of double-strand breaks. This appears to be true even after exposure to the alkylating agent MNNG that does not cause direct double-strand breaks but does produce γH2AX foci when damaged DNA undergoes replication.

Methods

To examine this predictive ability further, SiHa human cervical carcinoma cells were exposed to 8 DNA damaging drugs (camptothecin, cisplatin, doxorubicin, etoposide, hydrogen peroxide, MNNG, temozolomide, and tirapazamine) and the fraction of cells that retained γH2AX foci 24 hours after a 30 or 60 min treatment was compared with the fraction of cells that lost clonogenicity. To determine if cells with residual repair foci are the cells that die, SiHa cervical cancer cells were stably transfected with a RAD51-GFP construct and live cell analysis was used to follow the fate of irradiated cells with RAD51-GFP foci.

Results

For all drugs regardless of their mechanism of interaction with DNA, close to a 1:1 correlation was observed between clonogenic surviving fraction and the fraction of cells that retained γH2AX foci 24 hours after treatment. Initial studies established that the fraction of cells that retained RAD51 foci after irradiation was similar to the fraction of cells that retained γH2AX foci and subsequently lost clonogenicity. Tracking individual irradiated live cells confirmed that SiHa cells with RAD51-GFP foci 24 hours after irradiation were more likely to die.

Conclusion

Retention of DNA damage-induced γH2AX foci appears to be indicative of lethal DNA damage so that it may be possible to predict tumor cell killing by a wide variety of DNA damaging agents simply by scoring the fraction of cells that retain γH2AX foci.

Cell death induced by N-methyl-N-nitrosourea, a model S(N)1 methylating agent, in two lung cancer cell lines of human origin

New therapeutic approaches are needed for lung cancer, the leading cause of cancer death. Methylating agents constitute a widely used class of anticancer drugs, the effect of which on human non small cell lung cancer (NSCLC) has not been adequately studied. N-methyl-N-nitrosourea (MNU), a model S(N)1 methylating agent, induced cell death through a distinct mechanism in two human NSCLC cell lines studied, A549(p53(wt)) and H157(p53(null)). In A549(p53(wt)), MNU induced G2/M arrest, accompanied by cdc25A degradation, hnRNP B1 induction, hnRNP C1/C2 downregulation. Non-apoptotic cell death was confirmed by the lack of increase in the sub-G1 DNA content, Poly (ADP-ribose) polymerase cleavage and caspase-3, -7 activation. In H157(p53(null)), MNU induced apoptotic cell death, confirmed by cytofluorometry of DNA content and immunodetection of apoptotic markers, accompanied by overexpression of hnRNP B1 and C1/C2. Thus, the mechanism of the cell death induced by S(N)1 methylating agents is cell type-dependent and must be assessed prior treatment.

Bifunctional DNA alkylator 1,3-bis(2-chloroethyl)-1-nitrosourea activates the ATR-Chk1 pathway independently of the mismatch repair pathway

The presence of DNA damage initiates signaling through the ataxia-telangiectasia mutated kinase (ATM) and the ATM- and the Rad3-related kinase (ATR), which phosphorylate, thus activating, the checkpoint kinases (Chk) 1 and 2, which leads to cell cycle arrest. The bifunctional DNA alkylator 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) is cytotoxic primarily by inducing DNA monoadducts and ultimately, interstrand cross-links, which block DNA replication. In this study, we investigated the activation of the ATR-Chk1 pathway in response to BCNU treatment and the dependence of this response on the DNA mismatch repair (MMR) capacity. Medulloblastoma cells were exposed to low and moderate doses of BCNU, and the effects on this DNA damage signaling pathway were examined. In response to BCNU, Chk1 was found to be phosphorylated at serine 345 and exhibited increased kinase activity. Caffeine and wortmannin, which are broad-spectrum inhibitors of ATM and ATR, reduced this phosphorylation. Cell cycle analysis further revealed an accumulation of cells in the S phase in response to BCNU, an effect that was attenuated by caffeine. Small interfering RNA knockdown of ATR also reduced Chk1 phosphorylation after exposure to BCNU. However, knockdown of ATM had no effect on the observed Chk1 phosphorylation, suggesting that ATR was primarily responsible for Chk1 activation. Analysis of Chk1 activation in cells deficient in MMR proteins MutLalpha or MutSalpha indicated that the DNA damage response induced by BCNU was independent of the MMR apparatus. This MMR-independent activation seems to be the result of DNA interstrand cross-link formation.

Interplay of DNA repair pathways controls methylation damage toxicity in Saccharomyces cerevisiae

Methylating agents of S(N)1 type are widely used in cancer chemotherapy, but their mode of action is poorly understood. In particular, it is unclear how the primary cytotoxic lesion, O(6)-methylguanine ((Me)G), causes cell death. One hypothesis stipulates that binding of mismatch repair (MMR) proteins to (Me)G/T mispairs arising during DNA replication triggers cell-cycle arrest and cell death. An alternative hypothesis posits that (Me)G cytotoxicity is linked to futile processing of (Me)G-containing base pairs by the MMR system. In this study, we provide compelling genetic evidence in support of the latter hypothesis. Treatment of 4644 deletion mutants of Saccharomyces cerevisiae with the prototypic S(N)1-type methylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) identified MMR as the only pathway that sensitizes cells to MNNG. In contrast, homologous recombination (HR), postreplicative repair, DNA helicases, and chromatin maintenance factors protect yeast cells against the cytotoxicity of this chemical. Notably, DNA damage signaling proteins played a protective rather than sensitizing role in the MNNG response. Taken together, this evidence demonstrates that (Me)G-containing lesions in yeast must be processed to be cytotoxic.

Validation of real-time methylation-specific PCR to determine O6-methylguanine-DNA methyltransferase gene promoter methylation in glioma

Epigenetic silencing of the DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT) by promoter methylation predicts successful alkylating agent therapy, such as with temozolomide, in glioblastoma patients. Stratified therapy assignment of patients in prospective clinical trials according to tumor MGMT status requires a standardized diagnostic test, suitable for high-throughput analysis of small amounts of formalin-fixed, paraffin-embedded tumor tissue. A direct, real-time methylation-specific PCR (MSP) assay was developed to determine methylation status of the MGMT gene promoter. Assay specificity was obtained by selective amplification of methylated DNA sequences of sodium bisulfite-modified DNA. The copy number of the methylated MGMT promoter, normalized to the beta-actin gene, provides a quantitative test result. We analyzed 134 clinical glioma samples, comparing the new test with the previously validated nested gel-based MSP assay, which yields a binary readout. A cut-off value for the MGMT methylation status was suggested by fitting a bimodal normal mixture model to the real-time results, supporting the hypothesis that there are two distinct populations within the test samples. Comparison of the tests showed high concordance of the results (82/91 [90%]; Cohen's kappa = 0.80; 95% confidence interval, 0.82-0.95). The direct, real-time MSP assay was highly reproducible (Pearson correlation 0.996) and showed valid test results for 93% (125/134) of samples compared with 75% (94/125) for the nested, gel-based MSP assay. This high-throughput test provides an important pharmacogenomic tool for individualized management of alkylating agent chemotherapy.

Inhibition of the 5' to 3' exonuclease activity of hEXO1 by 8-oxoguanine

The mismatch repair pathway is responsible for maintaining genomic stability by correcting base-base mismatches and insertion/deletion loops that arise mainly via replication errors. Additionally, the mismatch repair pathway performs a central role in the cellular response to both alkylation and reactive oxygen species induced DNA damage. An important step in mismatch processing is the recruitment of hEXO1, a 5' to 3' exonuclease, by hMSH2-hMSH6 to remove the nascent DNA strand. However, very little is currently known about the capacity of hEXO1 to exonucleolytically process damaged DNA bases. Therefore, we examined whether hEXO1 can degrade double-stranded DNA substrates containing alkylated or oxidized nucleotides. Our results demonstrated that hEXO1 is capable of degrading duplex DNA containing an O6-methylguanine (O6-meG) adduct paired with either a C or a T. Additionally, the hMSH2-hMSH6 complex stimulated hEXO1 exonuclease activity on the O6-meG/T and O6-meG/C DNA substrates. In contrast, hEXO1 exonuclease activity was significantly blocked by the presence of an 8-oxoguanine adduct in both single and double stranded DNA substrates. Further, hMSH2-hMSH6 was not able to alleviate the nucleolytic block caused by the 8-oxoguanine adduct in heteroduplex DNA.

Thiopurines in current medical practice: molecular mechanisms and contributions to therapy-related cancer

Thiopurines have diverse clinical applications and their long-term use as anti-rejection drugs in transplant patients has been associated with a significantly increased risk of various types of cancer. Although they are slowly being replaced by a new generation of non-thiopurine immunosuppressants, it is anticipated that their use in the management of inflammatory and autoimmune diseases will continue to increase. Therapy-related cancer will remain a potential consequence of prolonged treatment for these generally non-life-threatening conditions. Understanding how thiopurines contribute to the development of cancer will facilitate clinical decisions about the potential risks to patients of long-term treatment for chronic inflammatory disorders.

Reduced methylation-induced mutagenesis in rat splenocytes in vivo by sub-chronic low dose exposure to N-metyl-N-nitrosourea

Estimates of genotoxic effects of mutagens at low and protracted doses are often based on linear extrapolation of data obtained at relatively high doses. To test the validity of such an approach, a comparison was made between the mutagenicity of N-methyl-N-nitrosourea (MNU) in T-lymphocytes of the rat following two treatment protocols, i.e. sub-chronic exposure to a low dose (15-45 repeated exposures to 1mg/kg of MNU) or acute exposure to a single high dose (15, 30 or 45 mg/kg of MNU). Mutation induction appeared dramatically lower following sub-chronic treatment compared to treatment with a single high exposure. Furthermore, DNA sequence analysis of the coding region of the hprt gene in MNU-induced mutants showed that acute high dose treatment causes mainly GC-->AT base pair changes, whereas sub-chronic treatment results in a significant contribution of AT base pair changes to mutation induction. We hypothesize that O(6)-methylguanine-DNA methyltransferase is saturated after acute treatments, while after sub-chronic treatment most O(6)-methylguanine is efficiently repaired. These data suggest (i) that risk estimations at low and protracted doses of MNU on the basis of linear extrapolation of effects measured at high dose are too high and (ii) that the protective effects of DNA repair processes are relatively strong at low sub-chronic exposure.

N-methyl-N'-nitro-N-nitrosoguanidine activates multiple cell death mechanisms in human fibroblasts

Response to genotoxic stress may trigger the activation of distinct mechanisms that serve to promote cell death, including apoptosis and necrosis. In this study we examined the response of human fibroblasts, either proficient or deficient for the damage-activated protein kinase ataxia telangiectasia-mutated (ATM), to the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Analysis of both long- and short-term viability shows that both ATM-proficient YZ-5 and ATM-deficient EBS-7 fibroblasts display a cytotoxic response to MNNG. Consistent with activation of apoptosis in response to MNNG, we observed increased caspase-3 cleavage and activity, appearance of fragmented nuclei, and increased staining with annexin V in both ATM-proficient and -deficient fibroblasts. Flow cytometry demonstrated that these cell lines also display a nonapoptotic cell death in response to MNNG. This form of cell death is associated with activation of poly-ADP ribose polymerase (PARP), and analysis of PARP activity indicated increased protein poly(ADP-ribosylation) in YZ-5 when compared to EBS-7. This PARP activity was accompanied by apoptosis-inducing factor release and translocation from the mitochondria to the nucleus. Finally, the PARP inhibitor 3,4-dihydro-5-[4-(1-piperidinyl)butoxy]-1(2H)-isoquinolinone (DPQ) or the caspase-3 inhibitor benzyloxycarbonyl-VAD-fluoromethyl ketone dramatically diminished the cytotoxic response to MNNG, reinforcing the roles for apoptotic and nonapoptotic cell death in human fibroblasts treated with MNNG. From these findings, we conclude that MNNG induces a heterogeneous death response in human fibroblasts.

MLH1- and ATM-dependent MAPK signaling is activated through c-Abl in response to the alkylator N-methyl-N'-nitro-N'-nitrosoguanidine

N-Methyl-N'-nitro-N'-nitrosoguanidine (MNNG) is a DNA-methylating agent, and deficiency in mismatch repair (MMR) results in lack of sensitivity to this genotoxin (termed alkylation tolerance). A number of DNA damage response pathways are activated in a MMR-dependent manner following MNNG, and several also require ATM kinase activity. Here we show that activation of the transcription factor c-Jun is dependent upon both the MMR component MLH1 and ATM, but not ATR, in response to MNNG. In addition to c-Jun, the upstream MAPKs JNK and MKK4 are also activated in a MLH1- and ATM-dependent manner. We document that c-Jun activation is dependent on the MAPK kinase kinase MEKK1. Additionally, the tyrosine kinase c-Abl is required to activate this signaling cascade and forms a complex with MEKK1 and MLH1. This study indicates that an arm of DNA damage-activated MAPK signaling is activated in an MLH1- and ATM-dependent manner in response to MNNG and perhaps suggests that dysregulation of this signaling is responsible, in part, for alkylation tolerance.

Ataxia telangiectasia mutated (ATM) signaling network is modulated by a novel poly(ADP-ribose)-dependent pathway in the early response to DNA-damaging agents

Poly(ADP-ribosyl)ation is a post-translational modification that is instantly stimulated by DNA strand breaks creating a unique signal for the modulation of protein functions in DNA repair and cell cycle checkpoint pathways. Here we report that lack of poly(ADP-ribose) synthesis leads to a compromised response to DNA damage. Deficiency in poly(ADP-ribosyl)ation metabolism induces profound cellular sensitivity to DNA-damaging agents, particularly in cells deficient for the protein kinase ataxia telangiectasia mutated (ATM). At the biochemical level, we examined the significance of poly(ADP-ribose) synthesis on the regulation of early DNA damage-induced signaling cascade initiated by ATM. Using potent PARP inhibitors and PARP-1 knock-out cells, we demonstrate a functional interplay between ATM and poly(ADP-ribose) that is important for the phosphorylation of p53, SMC1, and H2AX. For the first time, we demonstrate a functional and physical interaction between the major DSB signaling kinase, ATM and poly(ADP-ribosyl)ation by PARP-1, a key enzyme of chromatin remodeling. This study suggests that poly(ADP-ribose) might serve as a DNA damage sensory molecule that is critical for early DNA damage signaling.

Methylating agents and DNA repair responses: Methylated bases and sources of strand breaks

The chemical methylating agents methylmethane sulfonate (MMS) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) have been used for decades as classical DNA damaging agents. These agents have been utilized to uncover and explore pathways of DNA repair, DNA damage response, and mutagenesis. MMS and MNNG modify DNA by adding methyl groups to a number of nucleophilic sites on the DNA bases, although MNNG produces a greater percentage of O-methyl adducts. There has been substantial progress elucidating direct reversal proteins that remove methyl groups and base excision repair (BER), which removes and replaces methylated bases. Direct reversal proteins and BER, thus, counteract the toxic, mutagenic, and clastogenic effects of methylating agents. Despite recent progress, the complexity of DNA damage responses to methylating agents is still being discovered. In particular, there is growing understanding of pathways such as homologous recombination, lesion bypass, and mismatch repair that react when the response of direct reversal proteins and BER is insufficient. Furthermore, the importance of proper balance within the steps in BER has been uncovered with the knowledge that DNA structural intermediates during BER are deleterious. A number of issues complicate the elucidation of the downstream responses when direct reversal is insufficient or BER is imbalanced. These include inter-species differences, cell-type-specific differences within mammals and between cancer cell lines, and the type of methyl damage or BER intermediate encountered. MMS also carries a misleading reputation of being a radiomimetic, that is, capable of directly producing strand breaks. This review focuses on the DNA methyl damage caused by MMS and MNNG for each site of potential methylation to summarize what is known about the repair of such damage and the downstream responses and consequences if the damage is not repaired.

Apoptotic effect of ethyl-4-isothiocyanatobutanoate is associated with DNA damage, proteasomal activity and induction of p53 and p21cip1/waf1

The effect of synthetic isothiocyanate ethyl-4-isothiocyanatobutanoate (E-4IB) on survival of mismatch repair-proficient TK6 and -deficient MT1 cell lines as well as the influence of proteasomal inhibitor MG132, caspase inhibitor Z-VAD-fmk, and ATM inhibitor caffeine on E-4IB modulation of cell cycle and apoptosis was evaluated. Flow cytometric analyses of DNA double strand breaks (gamma-H2AX), mitotic fraction (phospho-histone H3), cell cycle modulation, apoptosis induction (sub-G(0) fraction and fluorescein diacetate staining), and dissipation of transmembrane mitochondrial potential (JC-1 staining) were performed. Western blotting was used for the evaluation of ERK activation, expression of p53, p21(cip1/waf1) and GADD45alpha proteins, as well as PARP fragmentation. Analysis of mitotic nuclei was performed for chromosomal aberrations assessment. MT1 cells were more resistant to E-4IB treatment then TK6 cells (IC(50) 8 muM vs. 4 muM). In both cell lines E-4IB treatment induced phosphorylation of H2AX, increase of p53 protein level, phospho-histone H3 staining, and G(2)/M arrest. The sub-G(0) fragmentation was accompanied by PARP degradation, decreased mitochondrial transmembrane potential, and diminished p21(cip1/waf1) protein expression in TK6 cells. Caspase inhibitor Z-VAD-fmk decreased E-4IB induced sub-G(0) fragmentation and extent of apoptosis in TK6 cells, while proteasome inhibitor MG132 increased number of apoptotic cells in both cell lines tested. A number of aberrant metaphases and clastogenic effect of high E-4IB concentration was observed. The synthetic isothiocyanate E-4IB induced DNA strand breaks, increased mitotic fraction and apoptosis potentiated by MG132 inhibitor in both mismatch repair-proficient and -deficient cell lines.

Temozolomide-Mediated Radiation Enhancement in Glioblastoma: A Report on Underlying Mechanisms

PURPOSE

In this study, we investigated the mechanisms by which temozolomide enhances radiation response in glioblastoma cells.

EXPERIMENTAL DESIGN

Using a panel of four primary human glioblastoma cell lines with heterogeneous O(6)-methylguanine-DNA methyltransferase (MGMT) protein expression, normal human astrocytes, and U87 xenografts, we investigated (a) the relationship of MGMT status with efficacy of temozolomide-based chemoradiation using a panel of in vitro and in vivo assays; (b) underlying mechanisms by which temozolomide enhances radiation effect in glioblastoma cells; and (c) strategies to overcome resistance to radiation + temozolomide.

RESULTS

Temozolomide enhances radiation response most effectively in glioblastomas without detectable MGMT expression. On concurrent radiation + temozolomide administration in MGMT-negative glioblastomas, there seems to be decreased double-strand DNA (dsDNA) repair capacity and enhanced dsDNA damage compared either with radiation alone or with sequentially administered temozolomide. Our data suggest that O(6)-benzylguanine can enhance the antitumor effects of concurrent radiation + temozolomide in MGMT-positive cells by enhancing apoptosis and the degree of dsDNA damage. O(6)-Benzylguanine was most effective when administered concurrently with radiation + temozolomide and had less of an effect when administered with temozolomide in the absence of radiation or when administered sequentially with radiation. Our in vivo data using U87 xenografts confirmed our in vitro findings.

CONCLUSIONS

The present study shows that temozolomide enhances radiation response most effectively in MGMT-negative glioblastomas by increasing the degree of radiation-induced double-strand DNA damage. In MGMT-positive glioblastomas, depletion of MGMT by the addition of O(6)-benzylguanine significantly enhances the antitumor effect of concurrent radiation + temozolomide. These are among the first data showing mechanisms of synergy between radiation and temozolomide and the effect of MGMT.

Homologous recombination rescues mismatch-repair-dependent cytotoxicity of S(N)1-type methylating agents in S. cerevisiae

Resistance of mammalian cells to S(N)1-type methylating agents such as N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) generally arises through increased expression of methylguanine methyltransferase (MGMT), which reverts the cytotoxic O(6)-methylguanine ((Me)G) to guanine, or through inactivation of the mismatch repair (MMR) system, which triggers cell death through aberrant processing of (Me)G/T mispairs generated during DNA replication when MGMT capacity is exceeded. Given that MMR and (Me)G-detoxifying proteins are functionally conserved through evolution, and that MMR-deficient Escherichia coli dam(-) strains are also resistant to MNNG, the finding that MMR status did not affect the sensitivity of Saccharomyces cerevisiae to MNNG was unexpected. Because (Me)G residues in DNA trigger homologous recombination (HR), we wondered whether the efficient HR in S. cerevisiae might alleviate the cytotoxic effects of (Me)G processing. We now show that HR inactivation sensitizes S. cerevisiae to MNNG and that, as in human cells, defects in the MMR genes MLH1 and MSH2 rescue this sensitivity. Inactivation of the EXO1 gene, which encodes the only exonuclease implicated in MMR to date, failed to rescue the hypersensitivity, which implies that scExo1 is not involved in the processing of (Me)G residues by the S. cerevisiae MMR system.

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.

N-Methyl-N′-nitro-N-nitrosoguanidine Activates Cell-Cycle Arrest through Distinct Mechanisms Activated in a Dose-Dependent Manner

S(N)1-alkylating agents, such as the mutagenic and cytotoxic drug N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), robustly activate the DNA damage-responsive G(2) checkpoint. Establishment of this checkpoint is dependent on a functional mismatch repair (MMR) system; however, exposure to high doses of MNNG overrides the requirement for MMR to trigger G(2) arrest. In addition, unlike moderate-dose exposure, in which the G(2) checkpoint is attenuated in ataxia-telangiectasia, mutated (ATM)-deficient cells, high-dose MNNG treatment activates G(2) arrest through an ATM-independent mechanism. We document that this arrest is sensitive to the pharmacological agents caffeine and 7-hydroxystaurosporine (UCN-01) that inhibit the checkpoint kinases ATM/ATM and Rad-3-related (ATR) and Chk1/Chk2, respectively. Furthermore, these agents block inactivation of the cell-cycle regulatory molecules Cdc25C and Cdc2, establishing the downstream mechanism through which high-dose MNNG establishes G(2) arrest. Activation of both Chk2 and Chk1 was independent of ATM and MMR in response to high-dose MNNG, unlike the response to moderate doses of this drug. Chk2 was found to be dispensable for cell-cycle arrest in response to high-dose MNNG treatment; however, ATR deficiency and decreased Chk1 expression forced by RNA interference resulted in diminished checkpoint response. These results indicate that MNNG activates the G(2) checkpoint through different mechanisms activated in a dose-dependent fashion.

MSH2 missense mutations alter cisplatin cytotoxicity and promote cisplatin-induced genome instability

Defects in the mismatch repair protein MSH2 cause tolerance to DNA damage. We report how cancer-derived and polymorphic MSH2 missense mutations affect cisplatin cytotoxicity. The chemotolerance phenotype was compared with the mutator phenotype in a yeast model system. MSH2 missense mutations display a strikingly different effect on cell death and genome instability. A mutator phenotype does not predict chemotolerance or vice versa. MSH2 mutations that were identified in tumors (Y109C) or as genetic variations (L402F) promote tolerance to cisplatin, but leave the initial mutation rate of cells unaltered. A secondary increase in the mutation rate is observed after cisplatin exposure in these strains. The mutation spectrum of cisplatin-resistant mutators identifies persistent cisplatin adduction as the cause for this acquired genome instability. Our results demonstrate that MSH2 missense mutations that were identified in tumors or as polymorphic variations can cause increased cisplatin tolerance independent of an initial mutator phenotype. Cisplatin exposure promotes drug-induced genome instability. From a mechanistical standpoint, these data demonstrate functional separation between MSH2-dependent cisplatin cytotoxicity and repair. From a clinical standpoint, these data provide valuable information on the consequences of point mutations for the success of chemotherapy and the risk for secondary carcinogenesis.