Signaling mismatch repair in cancer

Genetic hits and mutation rate in colorectal tumorigenesis: versatility of Knudson's theory and implications for cancer prevention

The multistep model of carcinogenesis is now widely accepted for colorectal cancer and other common epithelial tumors of the adult. The challenges ahead are to define the number of genetic hits necessary for conversion of a normal cell into a cancer cell and to determine whether the observed increase in the mutation rate (mutator phenotype) is required. The beauty of Knudson's two-hit theory is its ability to explain diverse experimental situations and guide specific predictions, including some directly relevant to cancer prevention.

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.

Mismatch repair and response to DNA-damaging antitumour therapies

Most antitumour therapies damage tumour cell DNA either directly or indirectly. DNA damage responses, and particularly DNA repair, influence the outcome of therapy. Because DNA repair normally excises lethal DNA lesions, it is intuitive that efficient repair will contribute to intrinsic drug resistance. Indeed, in certain circumstances reduced levels of DNA nucleotide excision repair are associated with a good therapeutic outlook (Curr Biol 9 (1999) 273). A paradoxical relationship between DNA mismatch repair (MMR) and drug sensitivity has been revealed by model studies in cell lines. This suggests that connections between MMR and tumour therapy might be more complex. Here, we briefly review how MMR deficiency can affect drug resistance and the extent to which loss of MMR is a prognostic factor in certain cancer therapies. We also consider how the inverse relationship between MMR activity and drug resistance might influence the development of treatment-related malignancies which are increasingly linked to MMR defects.

Natural Selection and the Evolution of Genome Imprinting

Sexual reproduction results from the fusion of gametes in which the chromatin configuration of maternal and paternal chromosomes is distinct at fertilization. Although many of the differences are erased during successive cellular divisions and chromatin modifications, some are retained in both somatic and germline cells. These epigenetic modifications can confer different characteristics on maternal and paternal chromosomes and such differences can be selected during any process that has the ability to distinguish between homologues. The end result of these selective forces are parental origin effects, writ large. The range of effects observed, including transcriptional imprinting and effects on chromosome segregation and heterochromatization, reflects the diversity of selective forces in operation. However, a closer look at these effects suggests that parental origin-dependent differences in chromatin structure might be subject to some common forces and that these forces may explain many of the "nontranscriptional" parental origin effects observed in mammals.

A role for DNA mismatch repair in sensing and responding to fluoropyrimidine damage

The phenomenon of damage tolerance, whereby cells incur DNA lesions that are nonlethal, largely ignored, but highly mutagenic, appears to play a key role in carcinogenesis. Typically, these lesions are generated by alkylation of DNA or incorporation of base analogues. This tolerance is usually a result of the loss of specific DNA repair processes, most often DNA mismatch repair (MMR). The availability of genetically matched MMR-deficient and -corrected cell systems allows dissection of the consequences of this unrepaired damage in carcinogenesis as well as the elucidation of cell cycle checkpoint responses and cell death consequences. Recent data indicate that MMR plays an important role in detecting damage caused by fluorinated pyrimidines (FPs) and represents a repair system that is probably not the primary system for detecting damage caused by these agents, but may be an important system for correcting key mutagenic lesions that could initiate carcinogenesis. In fact, clinical studies have shown that there is no benefit of FP-based adjuvant chemotherapy in colon cancer patients exhibiting microsatellite instability, a hallmark of MMR deficiency. MMR-mediated damage tolerance and futile cycle repair processes are discussed, as well as possible strategies using FPs to exploit these systems for improved anticancer therapy.

The base excision repair enzyme MED1 mediates DNA damage response to antitumor drugs and is associated with mismatch repair system integrity

Cytotoxicity of methylating agents is caused mostly by methylation of the O6 position of guanine in DNA to form O6-methylguanine (O6-meG). O6-meG can direct misincorporation of thymine during replication, generating O6-meG:T mismatches. Recognition of these mispairs by the mismatch repair (MMR) system leads to cell cycle arrest and apoptosis. MMR also modulates sensitivity to other antitumor drugs. The base excision repair (BER) enzyme MED1 (also known as MBD4) interacts with the MMR protein MLH1. MED1 was found to exhibit thymine glycosylase activity on O6-meG:T mismatches. To examine the biological significance of this activity, we generated mice with targeted inactivation of the Med1 gene and prepared mouse embryonic fibroblasts (MEF) with different Med1 genotype. Unlike wild-type and heterozygous cultures, Med1-/- MEF failed to undergo G2-M cell cycle arrest and apoptosis upon treatment with the methylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Similar results were obtained with platinum compounds' 5-fluorouracil and irinotecan. As is the case with MMR-defective cells, resistance of Med1-/- MEF to MNNG was due to a tolerance mechanism because DNA damage accumulated but did not elicit checkpoint activation. Interestingly, steady state amounts of several MMR proteins are reduced in Med1-/- MEF, in comparison with Med1+/+ and Med1+/- MEF. We conclude that MED1 has an additional role in DNA damage response to antitumor agents and is associated with integrity of the MMR system. MED1 defects (much like MMR defects) may impair cell cycle arrest and apoptosis induced by DNA damage.

MBD4 deficiency reduces the apoptotic response to DNA-damaging agents in the murine small intestine

MBD4 was originally identified through its methyl binding domain, but has more recently been characterized as a thymine DNA glycosylase that interacts with the mismatch repair (MMR) protein MLH1. In vivo, MBD4 functions to reduce the mutability of methyl-CpG sites in the genome and mice deticient in MBD4 show increased intestinal tumorigenesis on an Apc(Min/+) background. As MLH1 and other MMR proteins have been functionally linked to apoptosis, we asked whether MBD4 also plays a role in mediating the apoptotic response within the murine small intestine. Mice deficient for MBD4 showed significantly reduced apoptotic responses 6 h following treatment with a range of cytotoxic agents including gamma-irradiation, cisplatin, temozolomide and 5-fluorouracil (5-FU). This leads to increased clonogenic survival in vivo in Mbd4(-/-) mice following exposure to either 5-FU or cisplatin. We next analysed the apoptotic response to 5-FU and temozolomide in doubly mutant Mbd4(-/-), Mlh1(-/-) mice but observed no additive decrease. The results imply that MBD4 and MLH1 lie in the same pathway and therefore that MMR-dependent apoptosis is mediated through MBD4. MBD4 deficiency also reduced the normal apoptotic response to gamma-irradiation, which we show is independent of Mlh1 status (at least in the murine small intestine), so suggesting that the reliance upon MBD4 may extend beyond MMR-mediated apoptosis. Our results establish a novel functional role for MBD4 in the cellular response to DNA damage and may have implications for its role in suppressing neoplasia.

Apoptosis and mutation in the murine small intestine: loss of Mlh1- and Pms2-dependent apoptosis leads to increased mutation in vivo

The mismatch repair (MMR) protein Msh2 has been shown to function in the apoptotic response to alkylating agents in vivo. Here, we extend these studies to the MutL homologues (MLH) Mlh1 and Pms2 by analysing the apoptotic response within the small intestine of gene targeted strains. We demonstrate significant differences between Msh2, Mlh1 and Pms2 mutations in influencing apoptotic signalling following 50mg/kg N-methyl-nitrosourea (NMNU), with no obvious reliance upon either Mlh1 or Pms2. However, following exposure to 100mg/kg temozolomide or lower levels of NMNU (10mg/kg) both Mlh1- and Pms2-dependent apoptosis was observed, indicating that the apoptotic response at these levels of DNA damage is dependent on the MutL homologues. Given our ability to observe a MutLalpha dependence of the apoptotic response, we tested whether perturbations of this response directly translate into increases in mutation frequency in vivo. We show that treatment with temozolomide or 10mg/kg NMNU significantly increases mutation in both the Mlh1 and Pms2 mutant mice. At higher levels of NMNU, where the apoptotic response is independent of Mlh1 and Pms2, no gene dependent increase in mutation frequency was observed. These results argue that the MutSalpha and MutLalpha are not equally important in their ability to signal apoptosis. However, when MMR does mediate apoptosis, perturbation of this response leads to long-term persistence of mutant cells in vivo.

Methylation-induced G(2)/M arrest requires a full complement of the mismatch repair protein hMLH1

The mismatch repair (MMR) gene hMLH1 is mutated in approximately 50% of hereditary non-polyposis colon cancers and transcriptionally silenced in approximately 25% of sporadic tumours of the right colon. Cells lacking hMLH1 display microsatellite instability and resistance to killing by methylating agents. In an attempt to study the phenotypic effects of hMLH1 downregulation in greater detail, we designed an isogenic system, in which hMLH1 expression is regulated by doxycycline. We now report that human embryonic kidney 293T cells expressing high amounts of hMLH1 were MMR-proficient and arrested at the G(2)/M cell cycle checkpoint following treatment with the DNA methylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), while cells not expressing hMLH1 displayed a MMR defect and failed to arrest upon MNNG treatment. Interestingly, MMR proficiency was restored even at low hMLH1 concentrations, while checkpoint activation required a full complement of hMLH1. In the MMR-proficient cells, activation of the MNNG-induced G(2)/M checkpoint was accompanied by phosphorylation of p53, but the cell death pathway was p53 independent, as the latter polypeptide is functionally inactivated in these cells by SV40 large T antigen.

Dimerization of MLH1 and PMS2 limits nuclear localization of MutLalpha

DNA mismatch repair maintains genomic stability by detecting and correcting mispaired DNA sequences and by signaling cell death when DNA repair fails. The mechanism by which mismatch repair coordinates DNA damage and repair with cell survival or death is not understood, but it suggests the need for regulation. Since the functions of mismatch repair are initiated in the nucleus, we asked whether nuclear transport of MLH1 and PMS2 is limiting for the nuclear localization of MutLalpha (the MLH1-PMS2 dimer). We found that MLH1 and PMS2 have functional nuclear localization signals (NLS) and nuclear export sequences, yet nuclear import depended on their C-terminal dimerization to form MutLalpha. Our studies are consistent with the idea that dimerization of MLH1 and PMS2 regulates nuclear import by unmasking the NLS. Limited nuclear localization of MutLalpha may thus represent a novel mechanism by which cells fine-tune mismatch repair functions. This mechanism may have implications in the pathogenesis of hereditary non-polyposis colon cancer.

Identification of frame-shift intermediate mutant cells

Frame-shift mutations at microsatellites occur as a time-dependent function of polymerase errors followed by failure of postreplicational mismatch repair. A cell-culture system was developed that allows identification of intermediate mutant cells that carry the mutation on a single DNA strand after the initial DNA polymerase errors. A plasmid was constructed that contained 13 repeats of a poly(dC-dA).poly(dG-dT) oligonucleotide immediately after the translation initiation codon of the enhanced GFP (EGFP) gene, shifting the EGFP gene out of its proper reading frame. The plasmid was introduced into human mismatch repair-deficient (HCT116, hMLH1-mutated) and mismatch repair-proficient (HCT116+chr3, hMLH1 wild type) colorectal cancer cells. After frame-shift mutations occurred that restored the EGFP reading frame, EGFP-expressing cells were detected, and two distinct fluorescent populations, M1 (dim cells) and M2 (bright cells), were identified. M1 cell numbers were stable, whereas M2 cells accumulated over time. In HCT116, single M2 cells gave rise to fluorescent colonies that carried a 2-bp deletion at the (CA)(13) microsatellite. Twenty-eight percent of single M1 cells, however, gave rise to colonies with a mixed fluorescence pattern that carried both (CA)(13) and (CA)(12) microsatellites. It is likely that M1 cells represent intermediate mutants that carry (CA)(13).(GT)(12) heteroduplexes. Although the mutation rate in HCT116 cell clones (6.2 x 10(-4)) was 30 times higher than in HCT116+chr3 (1.9 x 10(-5)), the proportion of M1 cells in culture did not significantly differ between HCT116 (5.87 x 10(-3)) and HCT116+chr3 (4.13 x 10(-3)), indicating that the generation of intermediate mutants is not affected by mismatch-repair proficiency.

Conventional and tissue microarray immunohistochemical expression analysis of mismatch repair in hereditary colorectal tumors

Immunohistochemistry (IHC) of mismatch repair (MMR) proteins in colorectal tumors together with microsatellite analysis (MSI) can be helpful in identifying families eligible for mutation analysis. The aims were to determine sensitivity of IHC for MLH1, MSH2, and MSH6 and MSI analysis in tumors from known MMR gene mutation carriers; and to evaluate the use of tissue microarrays for IHC (IHC-TMA) of colon tumors in its ability to identify potential carriers of MMR gene mutations, and compare it with IHC on whole slides. IHC on whole slides was performed in colorectal tumors from 45 carriers of a germline mutation in one of the MMR genes. The TMA cohort consisted of 129 colon tumors from (suspected) hereditary nonpolyposis colorectal cancer (HNPCC) patients. Whole slide IHC analysis had a sensitivity of 89% in detecting MMR deficiency in carriers of a pathogenic MMR mutation. Sensitivity by MSI analysis was 93%. IHC can also be used to predict which gene is expected to harbor the mutation: for MLH1, MSH2, and MSH6, IHC on whole slides would have correctly predicted the mutation in 48%, 92%, and 75% of the cases, respectively. We propose a scheme for the diagnostic approach of families with (suspected) HNPCC. Comparison of the IHC results based on whole slides versus TMA, showed a concordance of 85%, 95%, and 75% for MLH, MSH2, and MSH6, respectively. This study therefore shows that IHC-TMA can be reliably used to simultaneously screen a large number of tumors from (suspected) HNPCC patients, at first in a research setting.

DNA repair defects in colon cancer

Defects in DNA-repair pathways lead to an accumulation of mutations in genomic DNA that result from non-repair or mis-repair of modifications introduced into the DNA by endogenous or exogenous agents or by the malfunction of DNA metabolic pathways. Until recently, only two repair pathways, postreplicative mismatch repair and nucleotide excision repair, have been linked to cancer in mammals, but these have been joined in recent months also by the damage-reversal and base-excision-repair processes, which have been shown to be inactivated, either through mutation or epigenetically, in human cancer.

[Oncogenes and mitotic regulators: a change in perspective of our view of neoplastic processes]

Our vision of the cancer cell has dramatically changed since the discovery of proto-oncogenes, whose deregulation was proposed to mimic normal growth signalling. This notion, linking cancer to cell signalling pathways, has progressively led the way to the concept of the mutator phenotype, in which genetic instability plays an essential role in the onset of cancer. This then transformed cancer into a DNA repair disease. However, as foreseen decades ago by cytogeneticists, point mutations are not sufficient to give a full picture of the whole process. As a result, aneuploidy, rather than gene mutation, has been proposed as the explanation for the complex changes observed in cancer cells. The culprits were found among genes involved in the control of the cell division cycle, and work aimed at understanding the regulation of S phase and mitosis have yielded new insights into our understanding of cancer.

BCR/ABL regulates response to DNA damage: the role in resistance to genotoxic treatment and in genomic instability

BCR/ABL regulates cell proliferation, apoptosis, differentiation and adhesion. In addition, BCR/ABL can induce resistance to cytostatic drugs and irradiation by modulation of DNA repair mechanisms, cell cycle checkpoints and Bcl-2 protein family members. Upon DNA damage BCR/ABL not only enhances reparation of DNA lesions (e.g. homologous recombination repair), but also prolongs activation of cell cycle checkpoints (e.g. G2/M) providing more time for repair of otherwise lethal lesions. Moreover, by modification of anti-apoptotic members of the Bcl-2 family (e.g. upregulation of Bcl-x(L)) BCR/ABL provides a cytoplasmic 'umbrella' protecting mitochondria from the 'rain' of apoptotic signals coming from the damaged DNA in the nucleus, thus preventing release of cytochrome c and activation of caspases. The unrepaired and/or aberrantly repaired (but not lethal) DNA lesions resulting from spontaneous and/or drug-induced damage can accumulate in BCR/ABL-transformed cells leading to genomic instability and malignant progression of the disease. Inhibition of BCR/ABL kinase activity by STI571 (Gleevec, imatinib mesylate) reverses drug resistance and, in combination with standard chemotherapeutics can exert strong anti-leukemia effect.

Emerging concepts in colorectal neoplasia

An understanding of the mechanisms that explain the initiation and early evolution of colorectal cancer should facilitate the development of new approaches to effective prevention and intervention. This review highlights deficiencies in the current model for colorectal neoplasia in which APC mutation is placed at the point of initiation. Other genes implicated in the regulation of apoptosis and DNA repair may underlie the early development of colorectal cancer. Inactivation of these genes may occur not by mutation or loss but through silencing mediated by methylation of the gene's promoter region. hMLH1 and MGMT are examples of DNA repair genes that are silenced by methylation. Loss of expression of hMLH1 and MGMT protein has been demonstrated immunohistochemically in serrated polyps. Multiple lines of evidence point to a "serrated" pathway of neoplasia that is driven by inhibition of apoptosis and the subsequent inactivation of DNA repair genes by promoter methylation. The earliest lesions in this pathway are aberrant crypt foci (ACF). These may develop into hyperplastic polyps or transform while still of microscopic size into admixed polyps, serrated adenomas, or traditional adenomas. Cancers developing from these lesions may show high- or low-level microsatellite instability (MSI-H and MSI-L, respectively) or may be microsatellite stable (MSS). The suggested clinical model for this alternative pathway is the condition hyperplastic polyposis. If colorectal cancer is a heterogeneous disease comprising discrete subsets that evolve through different pathways, it is evident that these subsets will need to be studied individually in the future.

Association of p53 and MSH2 with recombinative repair complexes during S phase

Our previous recombination and biochemical analyses have led to the hypothesis that the tumor suppressor p53 monitors homologous recombination, a function which was previously attributed to the mismatch repair protein MSH2. Here, we show that a certain fraction of p53 is concentrated within discrete nuclear foci of cells synchronized in G1 phase, a pattern which becomes even more pronounced in S phase, especially after gamma-ray treatment. p53 foci show some colocalization with MSH2 within distinct foci during G1 phase, while dots formed by BRCA1 display an independent localization pattern. In S phase nuclei, p53 foci almost completely colocalize with MSH2 foci and associate with the recombination surveillance factor BRCA1 in irradiated S phase cells. These p53 and MSH2 foci also show significant overlaps with foci of the recombination enzymes Rad50 and Rad51, which for the first time unveiled recombination-related functions of p53 in replicating cells. During S phase, p53 and MSH2 are maximally active in binding to early recombination intermediates, and coexist within the same nuclear DNA-protein complexes. Our data suggest that p53 is linked similarly to homologous recombination as MSH2 and provide further evidence for the new concept of a dual role of p53 in the regulation of growth and repair.

HNPCC mutations in hMSH2 result in reduced hMSH2-hMSH6 molecular switch functions

Mutations in the human mismatch repair (MMR) gene hMSH2 have been linked to approximately 40% of hereditary nonpolyposis colorectal cancers (HNPCC). While the consequences of deletion or truncating mutations of hMSH2 would appear clear, the detailed functional defects associated with missense alterations are unknown. We have examined the effect of seven single amino acid substitutions associated with HNPCC that cover the structural subdomains of the hMSH2 protein. We show that alterations which produced a known cancer-causing phenotype affected the mismatch-dependent molecular switch function of the biologically relevant hMSH2-hMSH6 heterodimer. Our observations demonstrate that amino acid substitutions within hMSH2 that are distant from known functional regions significantly alter biochemical activity and the ability of hMSH2-hMSH6 to form a sliding clamp.

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

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

Molecular and genetic defects in colorectal tumorigenesis

Colorectal cancers, whether sporadic or hereditary, are caused by a defined set of molecular events. There are at least two different pathogenetic pathways for colorectal cancer: the chromosomal instability pathway and the microsatellite instability pathway; the two major inherited syndromes, familial adenomatous polyposis (FAP) and hereditary non-polyposis colorectal cancer (HNPCC), are examples of these two mechanisms. These different pathways, however, converge on common pathological entities that have crucial functions in the regulation of normal crypt homeostasis. Preventive strategies aimed at reversing these changes, or therapeutic interventions targeting cell populations with these alterations, should be most efficacious. Genetic testing for inherited syndromes is now available and allows appropriate management of these disorders. Further insight into colorectal tumorigenesis pathways can lead to the development of useful prognostic indicators and target preventive and therapeutic strategies in the management of colorectal cancer.

Mismatch repair and the hereditary non-polyposis colorectal cancer syndrome (HNPCC)

The hereditary non-polyposis colorectal cancer (HNPCC)-syndrome is the most common form of hereditary colorectal cancers, and accounts for 2-7% of the total colorectal cancer burden. Since there are no single clinical features specific for HNPCC, diagnosis is based on family history (Amsterdam or Bethesda criteria) and is confirmed by the detection of a mutation in one of the responsible mismatch repair (MMR) genes. Two types of HNPCC-families can be distinguished. Type I HNPCC tumors are exclusively located in the colon, whereas in Type II HNPCC patients, extracolonic tumors are present in the stomach, endometrium, ovary, and urinary tract. The identification of the human homologues of yeast mismatch repair genes hMSH2, hMSH3, hMSH6, hMLH1, hMLH3, hPMS1 (scMLH2), and hPMS2 (scPMS1) offered the prospect of genetic screening leading to an extensive search for mutations in HNPCC-families. The majority of the alterations have been found in hMSH2 (40%) and hMLH1 (40%) genes. Mutations in the other MMR genes appear rare, absent, and/or associated with atypical families (1-5%). As a result of the mismatch repair deficiency, replication misincorporation errors accumulate, resulting in a mutator phenotype. Diagnosis of HNPCC-associated replication errors is most easily determined by the examination of a panel of the National Cancer Institute (NCI)-recommended simple repeated sequences (microsatellites), combined with immunohistochemical analysis. Although the exact molecular mechanism of the tumor development in these patients remains poorly understood, the identification of tumors that harbor a microsatellite instability has clinical and prognostic implications.

Activation of human MutS homologs by 8-oxo-guanine DNA damage

The DNA lesion 8-oxo-guanine (8-oxo-G) is a highly mutagenic product of the interaction between reactive oxygen species and DNA. To maintain genomic integrity, cells have evolved mechanisms capable of removing this frequently arising oxidative lesion. Mismatch repair (MMR) appears to be one pathway associated with the repair of 8-oxo-G lesions (DeWeese, T. L., Shipman, J. M., Larrier, N. A., Buckley, N. M., Kidd, L. R., Groopman, J. D., Cutler, R. G., te Riele, H., and Nelson, W. G. (1998) Proc. Natl. Acad. Sci. U.S.A. 95, 11915-11920; Ni, T. T., Marsischky, G. T., and Kolodner, R. D. (1999) Mol. Cell 4, 439-444). Here we report the effect of double-stranded DNA oligonucleotides containing a single 8-oxo-G on the DNA binding affinity, ATPase, and ADP right arrow ATP exchange activities of hMSH2-hMSH6 and hMSH2-hMSH3. We found that hMSH2-hMSH6 binds the oligonucleotide DNA substrates with the following affinities: 8-oxo-G/T > 8-oxo-G/G > 8-oxo-G/A > 8-oxo-G/C approximately G/C. A similar trend was observed for DNA-stimulated ATPase and ADP --> ATP exchange activities of hMSH2-hMSH6. In contrast, hMSH2-hMSH3 did not appear to bind any of the 8-oxo-G containing DNA substrates nor was there enhanced ATPase or ADP --> ATP exchange activities. These results suggest that only hMSH2-hMSH6 is activated by recognition of 8-oxo-G lesions. Our data are consistent with the notion that post-replication MMR only participates in the repair of mismatched 8-oxo-G lesions.

Functional interactions and signaling properties of mammalian DNA mismatch repair proteins

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

APC, signal transduction and genetic instability in colorectal cancer

Colorectal cancer arises through a gradual series of histological changes, each of which is accompanied by a specific genetic alteration. In general, an intestinal cell needs to comply with two essential requirements to develop into a cancer: it must acquire selective advantage to allow for the initial clonal expansion, and genetic instability to allow for multiple hits in other genes that are responsible for tumour progression and malignant transformation. Inactivation of APC--the gene responsible for most cases of colorectal cancer--might fulfil both requirements.

BCR/ABL regulates mammalian RecA homologs, resulting in drug resistance

RAD51 is one of six mitotic human homologs of the E. coli RecA protein (RAD51-Paralogs) that play a central role in homologous recombination and repair of DNA double-strand breaks (DSBs). Here we demonstrate that RAD51 is important for resistance to cisplatin and mitomycin C in cells expressing the BCR/ABL oncogenic tyrosine kinase. BCR/ABL significantly enhances the expression of RAD51 and several RAD51-Paralogs. RAD51 overexpression is mediated by a STAT5-dependent transcription as well as by inhibition of caspase-3-dependent cleavage. Phosphorylation of the RAD51 Tyr-315 residue by BCR/ABL appears essential for enhanced DSB repair and drug resistance. Induction of the mammalian RecA homologs establishes a unique mechanism for DNA damage resistance in mammalian cells transformed by an oncogenic tyrosine kinase.

Bias in detection of instability of the (C)8 mononucleotide repeat of MSH6 in tumours from HNPCC patients

Recently, we and others reported instability in the (C)8 repeat in exon 5 of MSH6 as a preferential target for somatic mutations in tumours from MSH6 germline mutation carriers. Here, we report that in 45% of tumours from MLH1, MSH2 and MSH6 germline mutation carriers no sequence change in the (C)8 repeat of MSH6 was found upon DNA sequencing analysis of PCR products with a shift in electrophoresis mobility. Using "standard" PCR primers a high frequency of instability (50-86%) of the (C)8 repeat was found, but using a modified PCR reverse primer, accomplishing modulation of non-templated addition of adenine during in vitro PCR amplification by the Taq polymerase, a markedly lower frequency of instability was found in tumours from MLH1, MSH2 and MSH6 mutation carriers (6, 13 and 40%, respectively). Furthermore, a significant difference of the frequency of instability of the (C)8 repeat in tumours from MSH6 mutation carriers was found compared to MLH1, MSH2 mutation carriers. These results might have important implications for the detection of instability of other short mononucleotide repeats, e.g. TGFbetaRII, BAX, IGFRII, PTEN, BRCA2.

The interaction of DNA mismatch repair proteins with human exonuclease I

Exonucleolytic degradation of DNA is an essential part of many DNA metabolic processes including DNA mismatch repair (MMR) and recombination. Human exonuclease I (hExoI) is a member of a family of conserved 5' --> 3' exonucleases, which are implicated in these processes by genetic studies. Here, we demonstrate that hExoI binds strongly to hMLH1, and we describe interaction regions between hExoI and the MMR proteins hMSH2, hMSH3, and hMLH1. In addition, hExoI forms an immunoprecipitable complex with hMLH1/hPMS2 in vivo. The study of interaction regions suggests a biochemical mechanism of the involvement of hExoI as a downstream effector in MMR and/or DNA recombination.

Adenosine nucleotide modulates the physical interaction between hMSH2 and BRCA1

We have identified the physical interaction between the Breast Cancer susceptibility gene product BRCA1 and the Hereditary Non-Polyposis Colorectal Cancer (HNPCC) and DNA mismatch repair (MMR) gene product hMSH2, both in vitro and in vivo. The BRCA1-hMSH2 association involved several well-defined regions of both proteins which include the adenosine nucleotide binding domain of hMSH2. Moreover, the interaction of BRCA1 with purified hMSH2-hMSH6 appears to be modulated by adenosine nucleotide much like G protein downstream interaction/signaling is modulated by guanosine nucleotide. BARD1, another BRCA1-interacting protein, was also found to interact with hMSH2. In addition, BRCA1 was found to associate with both hMSH3 and hMSH6, the heterodimeric partners of hMSH2. These observations implicate BRCA1/BARD1 as downstream effectors of the adenosine nucleotide-activated hMSH2-hMSH6 signaling complex, and suggest a global role for BRCA1 in DNA damage processing. The functional interaction between BRCA1 and hMSH2 may provide a partial explanation for the background of gynecological and colorectal cancer in both HNPCC and BRCA1 kindreds, respectively.

Evolving responsively: adaptive mutation

A basic principle of genetics is that the likelihood that a particular mutation occurs is independent of its phenotypic consequences. The concept of adaptive mutation seemed to challenge this principle with the discoveries of mutations stimulated by stress, some of which allow adaptation to the stress. The emerging mechanisms of adaptive genetic change cast evolution, development and heredity into a new perspective, indicating new models for the genetic changes that fuel these processes.

Cellular responses to DNA damage

Cells are constantly under threat from the cytotoxic and mutagenic effects of DNA damaging agents. These agents can either be exogenous or formed within cells. Environmental DNA-damaging agents include UV light and ionizing radiation, as well as a variety of chemicals encountered in foodstuffs, or as air- and water-borne agents. Endogenous damaging agents include methylating species and the reactive oxygen species that arise during respiration. Although diverse responses are elicited in cells following DNA damage, this review focuses on three aspects: DNA repair mechanisms, cell cycle checkpoints, and apoptosis. Because the areas of nucleotide excision repair and mismatch repair have been covered extensively in recent reviews, we restrict our coverage of the DNA repair field to base excision repair and DNA double-strand break repair.

Rectified Brownian motion and kinesin motion along microtubules

The mechanism of rectified Brownian movement is used to analyze measured data for kinesin motion along microtubules. A key component of the mechanism is the diffusive movement of the microtubule binding heads of kinesin during the adenosine triphosphate (ATP) cycle. The first-passage time distribution for this step is analyzed in detail and is shown to be responsible for observed load-velocity profiles. The ATPase activity of the kinesin heads is that of a nucleotide switch and not that of a direct chemomechanical energy converter. Experimental data acquisition, rate constants, and alternative explanations are discussed. The mechanism described in this paper is fundamental to the nanobiology of intracellular processes.

Natural selection and the function of genome imprinting: beyond the silenced minority

Most hypotheses of the evolutionary origin of genome imprinting assume that the biochemical character on which natural selection has operated is the expression of the allele from only one parent at an affected locus. We propose an alternative - that natural selection has operated on differences in the chromatin structure of maternal and paternal chromosomes to facilitate pairing during meiosis and to maintain the distinction between homologues during DNA repair and recombination in both meiotic and mitotic cells. Maintenance of differences in chromatin structure in somatic cells can sometimes result in the transcription of only one allele at a locus. This pattern of transcription might be selected, in some instances, for reasons that are unrelated to the original establishment of the imprint. Differences in the chromatin structure of homologous chromosomes might facilitate pairing and recombination during meiosis, but some such differences could also result in non-random segregation of chromosomes, leading to parental-origin-dependent transmission ratio distortion. This hypothesis unites two broad classes of parental origin effects under a single selective force and identifies a single substrate through which Mendel's first and second laws might be violated.

p53 and c-Jun functionally synergize in the regulation of the DNA repair gene hMSH2 in response to UV

The tumor suppresser protein p53 is critical for guarding the genome from incorporation of damaged DNA (Lane, D. P. (1992) Nature 358, 15-16). A relevant stress that activates p53 function is UV light (Noda, A., Toma-Aiba, Y., and Fujiwara, Y. (2000) Oncogene 19, 21-31). Another well known component of the mammalian UV response is the transcription factor c-Jun (Angel, P., and Karin, M. (1991) Biochim. Biophys. Acta 1072, 129-157). We show here that upon UV irradiation p53 activates transcription of the human mismatch repair gene MSH2. Interestingly, this up-regulation critically depends on functional interaction with c-Jun. Hence, the synergistic interaction of a proto-oncogene with a tumor suppresser gene is required for the regulation of the mammalian stress response through activation of expression of MSH2.

The effect of O6-methylguanine DNA adducts on the adenosine nucleotide switch functions of hMSH2-hMSH6 and hMSH2-hMSH3

The human homologs of prokaryotic mismatch repair have been shown to mediate the toxicity of certain DNA damaging agents; cells deficient in the mismatch repair pathway exhibit resistance to the killing effects of several of these agents. Although previous studies have suggested that the human MutS homologs, hMSH2-hMSH6, bind to DNA containing a variety of DNA adducts, as well as mispaired nucleotides, a number of studies have suggested that DNA binding does not correlate with repair activity. In contrast, the ability to process adenosine nucleotides by MutS homologs appears to be fundamentally linked to repair activity. In this study, oligonucleotides containing a single well defined O(6)-methylguanine adduct were used to examine the extent of lesion-provoked DNA binding, single-step ADP --> ATP exchange, and steady-state ATPase activity by hMSH2-hMSH3 and hMSH2-hMSH6 heterodimers. Interestingly, O(6)-methylguanine lesions when paired with either a C or T were found to stimulate ADP --> ATP exchange, as well as the ATPase activity of purified hMSH2-hMSH6, whereas there was no significant stimulation of hMSH2-hMSH3. These results suggest that O(6)-methylguanine uniquely activates the molecular switch functions of hMSH2-hMSH6.