Mammalian DNA mismatch repair

Management of hereditary ovarian cancer

Mutations in BRCA1 and BRCA2 genes account for the majority of hereditary breast and ovarian cancers. Approximately 10% of cases of ovarian cancer are due to germline mutations in BRCA1 and BRCA2. Ovarian cancer associated with BRCA1 and BRCA2 mutations has a distinct histological phenotype. This type of cancer is predominantly of serous or endometrioid histology and is high grade. Patients with BRCA1 or BRCA2 mutations should be offered risk-reducing salpingo-oophorectomy by age 40 years, or when childbearing is complete. Nowadays there are no differences between the treatments provided for sporadic and hereditary ovarian cancer, although there are indications that targeted therapy is effective in women with BRCA1/BRCA2-associated tumors. Retrospective studies reveal a high level of sensitivity to platinum agents in BRCA-associated tumors and initial trials show good efficacy and tolerability for polyADP-ribose polymerase inhibitors in mutation carriers with advanced ovarian cancers. These agents might also potentially be used in chemoprevention. Authors review the current management of hereditary ovarian cancer. Orv. Hetil., 2011, 152, 1596–1608.

BRCA1 is required for hMLH1 stabilization following doxorubicin-induced DNA damage

Human DNA mismatch repair (MMR) is involved in the removal of DNA base mismatches that arise either during DNA replication or are caused by DNA damage. In this study, we show that the activation of the MMR component hMLH1 in response to doxorubicin (DOX) treatment requires the presence of BRCA1 and that this phenomenon is mediated by an ATM/ATR dependent phosphorylation of the hMLH1 Ser-406 residue. BRCA1 is an oncosuppressor protein with a central role in the DNA damage response and it is a critical component of the ATM/ATR mediated checkpoint signaling. Starting from a previous finding in which we demonstrated that hMLH1 is able to bind to BRCA1, in this study we asked whether BRCA1 might be the bridge for ATM/ATR dependent phosphorylation of the hMLH1 molecular partner. We found that: (i) the negative modulation of BRCA1 expression is able to produce a remarkable reversal of hMLH1 stabilization, (ii) BRCA1 is required for post-translational modification produced by DOX treatment on hMLH1 which is, in turn, attributed to the ATM/ATR activity, (iii) the serine 406 phosphorylatable residue is critical for hMLH1 activation by ATM/ATR via BRCA1. Taken together, our data lend support to the hypothesis suggesting an important role of this oncosuppressor as a scaffold or bridging protein in DNA-damage response signaling via downstream phosphorylation of the ATM/ATR substrate hMLH1.

Polymorphisms of MLH1 and MSH2 genes and the risk of lung cancer among never smokers

Mismatch repair (MMR) plays an important role in repairing nucleotide mismatches during DNA replication. Defects in MMR genes are associated with some sporadic tumors. MLH1 and MSH2 are two of the MMR genes. We conducted a case-control study to investigate the associations between the risk of lung cancer and genetic polymorphisms in the MLH1 and MSH2 genes. The SNP genotypes were determined in 730 lung cancer patients and 730 healthy controls that were frequency matched for the age, gender, and smoking status. Among the SNP polymorphisms, -93A>G (rs1800734), which is located in the promoter region of MLH1, was significantly associated with the risk of lung cancer. The GG genotype for MLH1 -93A>G was associated with a significantly increased risk of lung cancer compared with the AA genotype among the never-smoking group (adjusted OR=1.64, 95% CI=1.10-2.44; P=0.013). Consistently, the haplotype of MLH1 with one -93G risk allele was associated with the risk of lung cancer compared with the AA haplotype among the never-smoking group. Furthermore, the risk of MLH1 -93A>G polymorphism in the never-smoking group related to lung adenocarcinoma was modulated by environmental tobacco smoke (ETS) exposure status, with a significant gene-ETS interaction (P=0.042). No evidence was found of the association between MSH2 and the lung cancer risk. In conclusion, our data suggest that the MLH1 -93A>G polymorphism may contribute to the etiology of lung cancer, particularly in never smokers. This study also suggests that MLH1 -93A>G polymorphisms and ETS exposure have a role in the tumorigenesis of lung adenocarcinoma among never smokers.

Phosphorylated hMSH6: DNA mismatch versus DNA damage recognition

DNA mismatch repair (MMR) maintains genomic integrity by correction of mispaired bases and insertion-deletion loops. The MMR pathway can also trigger a DNA damage response upon binding of MutSα to specific DNA lesions such as O(6)methylguanine (O(6)meG). Limited information is available regarding cellular regulation of these two different pathways. Within this report, we demonstrate that phosphorylated hMSH6 increases in concentration in the presence of a G:T mismatch, as compared to an O(6)meG:T lesion. TPA, a kinase activator, enhances the phosphorylation of hMSH6 and binding of hMutSα to a G:T mismatch, though not to O(6)meG:T. UCN-01, a kinase inhibitor, decreases both phosphorylation of hMSH6 and binding of hMutSα to G:T and O(6)meG:T. HeLa MR cells, pretreated with UCN-01 and exposed to MNNG, undergo activation of Cdk1 and mitosis despite phosphorylation of Chk1 and inactivating phosphorylation of Cdc25c. These results indicate that UCN-01 may inhibit an alternative cell cycle arrest pathway associated with the MMR pathway that does not involve Cdc25c. In addition, recombinant hMutSα containing hMSH6 mutated at an N-terminal cluster of four phosphoserines exhibits decreased phosphorylation and decreased binding of hMutSα to G:T and O(6)meG:T. Taken together, these results suggest a model in which the amount of phosphorylated hMSH6 bound to DNA is dependent on the presence of either a DNA mismatch or DNA alkylation damage. We hypothesize that both phosphorylation of hMSH6 and total concentration of bound hMutSα are involved in cellular signaling of either DNA mismatch repair or MMR-dependent damage recognition activities.

Clinical significance of detection of hMLH1 and hPMS2 mRNA expression in gastric cancer by real-time fluorescent quantitative RT-PCR

AIM: To investigate the expression of DNA mismatch repair genes hMLH1/hPMS2 and to analyze their clinical significance in gastric cancer.

METHODS: The expression of hMLH1/hPMS2 mRNAs in 40 specimens of gastric cancer and cancer-adjacent mucosa tissue and 21 specimens of chronic gastritis tissue was detected by real-time fluorescent quantitative reverse transcription-polymerase chain reaction (RT-PCR). The human glyceraldehydes-3-phosphate dehydrogenase (hGAPDH) gene was used for normalization of gene expression levels.

RESULTS: The level of hMLH1 mRNA in gastric cancer was significantly higher than those in cancer-adjacent mucosa tissue and chronic gastritis tissue (7.23 ± 11.91 vs 3.80 ± 5.13 and 2.01 ± 1.25, respectively; both P < 0.05). The level of hMLH1 mRNA was significantly higher in cancer-adjacent mucosa tissue than in chronic gastritis tissue (P < 0.05). The level of hPMS2 mRNA was significantly higher in gastric cancer and cancer-adjacent mucosa tissue than in chronic gastritis tissue (0.43 ± 0.35 and 0.55 ± 0.39 vs 0.32 ± 0.15, respectively; both P < 0.05). No significant difference was noted in the level of hPMS2 mRNA between gastric cancer and cancer-adjacent mucosa tissue. The expression levels of hMLH1/hPMS2 mRNAs in gastric cancer showed no significant correlation with tumor size, infiltration degree, and lymph node metastasis. However, the expression level of hMLH1 mRNA was significantly higher in gastric cancer with lymph node metastasis than in that without lymph node metastasis (all P < 0.05).

CONCLUSION: The expression levels of hMLH1 /hPMS2 gene in gastric cancer and cancer-adjacent mucosa tissue are abnormal when compared with that in chronic gastritis. Abnormal transcription of hMLH1/hPMS2 may be related with the genesis of gastric cancer, but not involved in the progression of the disease.

Oxidized purine nucleotides, genome instability and neurodegeneration

Oxidative DNA damage can be the consequence of endogenous metabolic processes and exogenous insults and several DNA repair enzymes provide protection against the toxic effects of oxidized DNA bases. Here we review the increasing knowledge on the relationship between an oxidized dNTPs pool and genome instability. The review also describes some important progress toward understanding the role of oxidative DNA damage and its repair in neurodegenerative diseases. In particular the hMTH1 hydrolase destroys oxidized nucleic acid precursors to prevent their harmful incorporation into DNA and RNA. Based on results obtained in our transgenic mouse overexpressing hMTH1 in the brain we discussed the mechanisms by which this hydrolase protects against neurodegeneration in Huntington disease models.

Expression of ABC-transport proteins in canine mammary cancer: consequences for chemotherapy

Intrinsic or acquired drug resistance is a major barrier for chemotherapy of cancer. Importantly, the presence of ATP-binding cassette, ABC-transport proteins in tumour cells circumvents an intracellular accumulation of chemotherapeutic drugs. In this study, 103 canine mammary tumour probes were investigated for mRNA expression of seven ABC-transporters by RT-PCR. All tumour samples expressed multidrug resistance-associated protein 1 (MRP1) and breast cancer resistance protein (BCRP). MRP7 was detected in 97.1% of tumour probes, MRP3 in 96.1%, Pgp in 92.2%, MRP5 in 85.4% and MRP6 in 64.1%. More of the half of tumour samples (56.1%) expressed all of the examined ABC-transport proteins. Approximately one-third of the tumour samples (32.7%) were lacking in one transporter and only 11.2% possessed from three to five transporters. The canine transporter cBCRP was functionally analysed in stable transfected Madin-Darby canine kidney-II cells using an MTT viability test. cBCRP transfected cells showed a 5.4-fold resistance to 10 microm doxorubicin. Cell survival in the presence of methotrexate was not affected by cBCRP. In conclusion, absence of efficiency of chemotherapy of canine mammary cancer can be caused by expression of seven various ABC-transport proteins. Because cBCRP is expressed in all examined tumour probes and induces resistance to doxorubicin, the application of doxorubicin for treatment of canine mammary is inappropriate.

3,3',4,4',5,5'-hexahydroxystilbene impairs melanoma progression in a metastatic mouse model

Stilbenes comprise a group of polyphenolic compounds, which exert inhibitory effects on various malignancies. The aim of this study was to evaluate the antitumor effects of a previously unreported stilbene derivative-3,3',4,4',5,5'-hexahydroxystilbene, termed M8-on human melanoma cells. Cell-cycle analysis of the metastatic melanoma cell line M24met showed that M8 treatment induces G(2)/M arrest accompanied with a dose- and time-dependent upregulation of p21 and downregulation of CDK-2 and leads to apoptosis. M8 induces the expression of phosphorylated p53, proteins involved in the mismatch repair machinery (MSH6, MSH2, and MLH1) and a robust tail moment in a comet assay. In addition, M8 inhibited cell migration in Matrigel assays. Shotgun proteomics and western analysis showed the regulation among others of paxillin, integrin-linked protein kinase, p21-activated kinase, and ROCK-1 indicating that M8 inhibits mesenchymal and amoeboid cell migration. These in vitro data were confirmed in vivo in a metastatic human melanoma severe combined immunodeficient (SCID) mouse model. We showed that M8 significantly impairs tumor growth. M8 also interfered with the metastatic process, as M8 treatment prevented the metastatic spread of melanoma cells to distant lymph nodes in vivo. In summary, M8 exerts strong antitumor effects with the potential to become a new drug for the treatment of metastatic melanoma.

Association of common variants in mismatch repair genes and breast cancer susceptibility: a multigene study

Background

MMR is responsible for the repair of base-base mismatches and insertion/deletion loops. Besides this, MMR is also associated with an anti-recombination function, suppressing homologous recombination. Losses of heterozygosity and/or microsatellite instability have been detected in a large number of skin samples from breast cancer patients, suggesting a potential role of MMR in breast cancer susceptibility.

Methods

We carried out a hospital-based case-control study in a Caucasian Portuguese population (287 cases and 547 controls) to estimate the susceptibility to non-familial breast cancer associated with some polymorphisms in mismatch repair genes (MSH3, MSH4, MSH6, MLH1, MLH3, PMS1 and MUTYH).

Results

Using unconditional logistic regression we found that MLH3 (L844P, G>A) polymorphism GA (Leu/Pro) and AA (Pro/Pro) genotypes were associated with a decreased risk: OR = 0.65 (0.45-0.95) (p = 0.03) and OR = 0.62 (0.41-0.94) (p = 0.03), respectively.

Analysis of two-way SNP interaction effects on breast cancer revealed two potential associations to breast cancer susceptibility: MSH3 Ala1045Thr/MSH6 Gly39Glu - AA/TC [OR = 0.43 (0.21-0.83), p = 0.01] associated with a decreased risk; and MSH4 Ala97Thr/MLH3 Leu844Pro - AG/AA [OR = 2.35 (1.23-4.49), p = 0.01], GG/AA [OR = 2.11 (1.12-3,98), p = 0.02], and GG/AG [adjusted OR = 1.88 (1.12-3.15), p = 0.02] all associated with an increased risk for breast cancer.

Conclusion

It is possible that some of these common variants in MMR genes contribute significantly to breast cancer susceptibility. However, further studies with a large sample size will be needed to support our results.

Nuclear import of human MLH1, PMS2, and MutLalpha: redundancy is the key

DNA mismatch repair maintains genomic stability by correcting errors that have escaped polymerase proofreading. Defects on mismatch repair genes lead to an increased mutation rate, microsatellite instability and predisposition to human non-polyposis colorectal cancer (HNPCC). Human MutLalpha is a heterodimer formed by the interaction of MLH1 and PMS2 that coordinates a series of key events in mismatch repair. It has been proposed that nuclear import of MutLalpha may be the first regulatory step on the activation of the mismatch repair pathway. Using confocal microscopy and mismatch repair deficient cells, we have identified the sequence determinants that drive nuclear import of human MLH1, PMS2, and MutLalpha. Transient transfection of the individual proteins reveals that MLH1 has a bipartite and PMS2 has a single monopartite nuclear localization signal. Although dimerization is not required for nuclear localization, the MutLalpha heterodimer is imported more efficiently than the MLH1 or PMS2 monomers. Interestingly, the bipartite localization signal of MLH1 can direct import of MutLalpha even when PMS2 encompasses a mutated localization signal. Hence we conclude that the presence of redundant nuclear localization signals guarantees nuclear transport of MutLalpha and, consequently, efficient mismatch repair.

High mutability of the tumor suppressor genes RASSF1 and RBSP3 (CTDSPL) in cancer

Background

Many different genetic alterations are observed in cancer cells. Individual cancer genes display point mutations such as base changes, insertions and deletions that initiate and promote cancer growth and spread. Somatic hypermutation is a powerful mechanism for generation of different mutations. It was shown previously that somatic hypermutability of proto-oncogenes can induce development of lymphomas.

Methodology/Principal Findings

We found an exceptionally high incidence of single-base mutations in the tumor suppressor genes RASSF1 and RBSP3 (CTDSPL) both located in 3p21.3 regions, LUCA and AP20 respectively. These regions contain clusters of tumor suppressor genes involved in multiple cancer types such as lung, kidney, breast, cervical, head and neck, nasopharyngeal, prostate and other carcinomas. Altogether in 144 sequenced RASSF1A clones (exons 1–2), 129 mutations were detected (mutation frequency, MF = 0.23 per 100 bp) and in 98 clones of exons 3–5 we found 146 mutations (MF = 0.29). In 85 sequenced RBSP3 clones, 89 mutations were found (MF = 0.10). The mutations were not cytidine-specific, as would be expected from alterations generated by AID/APOBEC family enzymes, and appeared de novo during cell proliferation. They diminished the ability of corresponding transgenes to suppress cell and tumor growth implying a loss of function. These high levels of somatic mutations were found both in cancer biopsies and cancer cell lines.

Conclusions/Significance

This is the first report of high frequencies of somatic mutations in RASSF1 and RBSP3 in different cancers suggesting it may underlay the mutator phenotype of cancer. Somatic hypermutations in tumor suppressor genes involved in major human malignancies offer a novel insight in cancer development, progression and spread.

Widespread mismatch repair protein expression in canine cutaneous mast cell tumors

Cutaneous mast cell tumors (MCTs) are common canine neoplasms. Some dog breeds more frequently develop MCTs, suggesting a genetically mediated predisposition. In humans, the most common inherited cancer predisposition is caused by germline defects in the mismatch repair (MMR) genes. To investigate whether inherited defects in the MMR genes predispose some dogs to MCT development, MMR expression in 22 MCTs from young and predisposed breed dogs was compared with MMR expression in 22 MCTs from old dogs of non-MCT-predisposed breeds. MMR expression was investigated immunohistochemically using antibodies against MLH1, MSH2, and MSH6. Mast cells within all MCTs expressed MLH1, MSH2, and MSH6. There were no significant differences in the intensity of immunoreactivity or the percentage of cells expressing MMR proteins between MCTs from the 2 groups of dogs. There were no significant differences in MMR protein expression between grade II and grade III MCTs. These results do not support the hypothesis that inherited MMR defects predispose some dogs to MCT development.

Deficient DNA mismatch repair is common in Lynch syndrome-associated colorectal adenomas

Lynch syndrome is caused by germline mutations in DNA mismatch repair (MMR) genes. Both microsatellite instability (MSI) testing and immunohistochemical analyses (IHC) of colon cancers are valuable diagnostic strategies for Lynch syndrome. We sought to determine whether these markers of MMR deficiency were also detectable in pre-cancerous colorectal adenomas. Fifteen subjects with a germline MMR gene mutation who had 44 adenomas removed during surveillance colonoscopy were identified. MSI testing and IHC for MLH1, MSH2, and MSH6 were performed. MSI was detected in 23 adenomas. There was a significant association between MSI and high-grade dysplasia (P = 0.006) and distal location (P = 0.0008). Loss of MMR protein by IHC was detected in 31 adenomas. A significant association was observed between loss of staining by IHC and high-grade dysplasia (P = 0.04). Among the 40 adenomas in which both MSI tests and IHC were performed, the presence of a germline mutation correlated with an abnormal MSI result in 58% of cases, an abnormal IHC result in 70% of cases, and either an abnormal MSI or IHC result in 73% of cases. The combination of MSI and IHC testing in colorectal adenomas is a sensitive screen for the detection of Lynch syndrome and may be particularly useful when Lynch syndrome is suspected and adenomatous polyps are the only tissues available for analysis.

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

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

Cyclooxygenase-2 and Hypoxia-Inducible Factor-1alpha protein expression is related to inflammation, and up-regulated since the early steps of colorectal carcinogenesis

Chronic mucosal inflammation is considered a risk factor for colorectal cancer. Neutrophils are a major source of oxidants, whereas cyclooxygenase 2 (COX-2) and Hypoxia Inducible Factor-1alpha (HIF-1alpha) protein expression levels are increased in inflammatory and malignant lesions. The main purpose of the present study was to evaluate myeloperoxidase (MPO) positive cell infiltration, COX-2 and HIF-1alpha protein expression in colorectal carcinogenesis, especially in its early phases, using immunohistochemistry and immunofluorescence confocal microscopy techniques. MPO, COX-2 and HIF-1alpha proteins were expressed at higher rates in the normal colorectal mucosa of patients with inflammatory bowel diseases and colorectal tumours than in patients with normal colonoscopy. A gradual increase in COX-2 and HIF-1alpha protein expression was observed in dysplastic aberrant crypt foci, adenomas and carcinomas, showing a strong relation to dysplasia. In conclusion, the present study supports the hypothesis of a key role of inflammation in malignant transformation of colorectal mucosa. The evaluation of some early markers related to inflammation in the mucosa of the large bowel may serve as potential tool for prognosis and therapeutic strategies.

Mouse models of colon cancer

Genetically engineered mice are essential tools in both mechanistic studies and drug development in colon cancer research. Mice with mutations in the Apc gene, as well as in genes that modify or interact with Apc, are important models of familial adenomatous polyposis. Mice with mutations in the beta-catenin signaling pathway have also revealed important information about colon cancer pathogenesis, along with models for hereditary nonpolyposis colon cancer and inflammatory bowel diseases associated with colon cancer. Finally, transplantation models (xenografts)have been useful in the study of metastasis and for testing potential therapeutics. This review discusses what models have been developed most recently and what they have taught us about colon cancer formation, progression, and possible treatment strategies.

Preparation of heteroduplex enhanced green fluorescent protein plasmid for in vivo mismatch repair activity assay

Preparation of heteroduplexes in large quantities with high purity is essential for the measurement of DNA mismatch repair (MMR) activity. Here we report a rapid, less labor-intensive method for the preparation of a heteroduplex plasmid that expresses the enhanced green fluorescent protein (EGFP) if the mismatch is repaired correctly. The method involves the use of a wild-type and a mutated EGFP expression plasmid and a few steps of enzymatic digestion. When the constructed heteroduplex EGFP plasmid was transfected into MMR-proficient and -deficient cell lines, the number of EGFP-expressing cells was much higher in the MMR-proficient cells than in the MMR-deficient cells, suggesting that the heteroduplex can be used for MMR activity assay in live model systems.

Evidence of tumor microsatellite instability in gastric cancer with familial aggregation

About 90% of gastric cancer (GC) cases appear in a sporadic setting. Nonetheless, in high incidence areas high familial aggregation rates have been recently described. Microsatellite instability (MSI) is thought to be an important molecular phenotype both in sporadic GC and in tumors of the HNPCC spectrum. The aim of this study was to assess the frequency of MSI in GC with familial aggregation. Five quasimonomorphic mononucleotide repeats (BAT-26, BAT-25, NR-24, NR-21 and NR-27) were analyzed in 250 GC patients. Seventy-five patients (30%) had at least one-first-degree family member affected by GC and 63 patients (25.2%) showed MSI. The frequency of MSI was significantly higher in patients with a positive family history of GC (38.7%) compared to patients with other tumor types within the family (15.7%) or with a negative oncological familial history (21.9%, P = 0.004). Within cases with a positive familial oncological history, the MSI frequency in families with GC only was similar to the one observed in families with GC and colon cancer (P = 0.96). Nonetheless, in families with GC and lung cancer, the frequency of MSI was significantly lower (5.6%, P = 0.007). MSI occurs in GCs with familial aggregation. Similar MSI rates have been observed in GC patients with other family members affected by GC or colon cancer. The same does not occur in families with other members affected by lung cancer. Our data seem to suggest that familial aggregation for either GC alone or gastric and colon cancer share common etiological factors in contrast to families with gastric and lung cancers.

MicroRNA regulation of DNA repair gene expression in hypoxic stress

Genetic instability is a hallmark of cancer; the hypoxic tumor microenvironment has been implicated as a cause of this phenomenon. MicroRNAs (miR) are small nonprotein coding RNAs that can regulate various cellular pathways. We report here that two miRs, miR-210 and miR-373, are up-regulated in a hypoxia-inducible factor-1alpha-dependent manner in hypoxic cells. Bioinformatics analyses suggested that these miRs could regulate factors implicated in DNA repair pathways. Forced expression of miR-210 was found to suppress the levels of RAD52, which is a key factor in homology-dependent repair (HDR); the forced expression of miR-373 led to a reduction in the nucleotide excision repair (NER) protein, RAD23B, as well as in RAD52. Consistent with these results, both RAD52 and RAD23B were found to be down-regulated in hypoxia, but in both cases, the hypoxia-induced down-regulation could be partially reversed by antisense inhibition of miR-210 and miR-373. Importantly, luciferase reporter assays indicated that miR-210 is capable of interacting with the 3' untranslated region (UTR) of RAD52 and that miR-373 can act on the 3' UTR of RAD23B. These results indicate that hypoxia-inducible miR-210 and miR-373 play roles in modulating the expression levels of key proteins involved in the HDR and NER pathways, providing new mechanistic insight into the effect of hypoxia on DNA repair and genetic instability in cancer.

Detection of hPMS2 mRNA in gastric cancer using real-time fluorescent quantitative RT-PCR and its clinical significance

AIM: To investigate the expression of DNA mismatch repair gene hPMS2 and its clinical significance in gastric cancer.

METHODS: Real-time fluorescent quantitative reverse transcriptase polymerase chain reaction (RT-PCR) was used to detect the level of hPMS2 mRNA respectively in 41 cases of gastric cancer and 37 cancer-adjacent mucosa samples of atrophic gastritis and 25 cases of chronic atrophic gastritis and 20 cases of chronic superficial gastritis. The human glyceraldehydes-3-phosphate dehydrogenase (hGAPDH) gene was used as a reference for normalization of the expression level.

RESULTS: The expression levels of hPMS2 mRNA in gastric cancer, the corresponding cancer-adjacent mucosa, chronic atrophic gastritis and chronic superficial gastritis were 28.33 ± 14.05, 16.88 ± 10.67, 7.25 ± 8.72 and 1.78 ± 1.34, respectively. Significant differences were detected among the four groups (F = 31.82, P = 0.00). The expression level of hPMS2 mRNA was markedly higher in gastric cancer than in the other tissues (P < 0.01), and significantly higher in the corresponding cancer-adjacent mucosa than in nonneoplastic mucosa (P < 0.01), but no significant difference was detected between chronic atrophic gastritis and chronic superficial gastritis. The expression of hPMS2 in gastric cancer had no obviously relationship with the size, the degree of infiltration of tumor nor the metastasis of lymph node.

CONCLUSION: The abnormal transcription of hPMS2 gene may be related to gastric cancer, but not involved in the development of gastric cancer.

Mismatch repair gene polymorphisms and survival in invasive ovarian cancer patients

AIMS

Inherited genetic factors may help partially explain variability of survival length amongst ovarian cancer patients. Of particular interest are genes involved in DNA repair, specifically those involved in mismatch repair (MMR). The aim of this study was to investigate the possible association between the common variants in MMR genes and invasive ovarian cancer overall survival.

METHOD/RESULTS

We examined associations between 44 variants that tag the known common variants (minor allele frequency 0.05) in seven MMR genes (MLH1, MLH3, MSH2, MSH3, MSH6, PMS1 and PMS2) and survival of invasive ovarian cancer patients in three case-control studies from United Kingdom (UK), Denmark and California of United States of America (USA). DNA from up to 1495 women were genotyped. The genotypes of each polymorphism were tested for association with survival using Cox regression analysis stratified by study. A nominally significant association (P=0.04) between genotype and ovarian cancer survival was observed for rs2228006 in PMS2. The per-rare allele hazard ratio (HR 95%CI) was 0.84 (0.71-0.99), however, it was not significant after adjusting for multiple covariants (P=0.47). When the analyses were restricted to serous type ovarian cancer, two SNPs showed marginal significant associations; the per-rare allele HR was 1.3 (1.05-1.6) (P=0.02) for rs1799977 in MLH1 and 1.4 (1.03-1.9) (P=0.04) for rs6151662 in MSH3. Neither was significant after adjusting for multiple covariants.

CONCLUSION

It is unlikely that common variants in the MMR pathways examined have moderate effects on survival after diagnosis with ovarian cancer. Much larger studies would be needed to exclude common variants with small effects.

Selective recognition of pyrimidine-pyrimidine DNA mismatches by distance-constrained macrocyclic bis-intercalators

Binding of three macrocyclic bis-intercalators, derivatives of acridine and naphthalene, and two acyclic model compounds to mismatch-containing and matched duplex oligodeoxynucleotides was analyzed by thermal denaturation experiments, electrospray ionization mass spectrometry studies (ESI-MS) and fluorescent intercalator displacement (FID) titrations. The macrocyclic bis-intercalators bind to duplexes containing mismatched thymine bases with high selectivity over the fully matched ones, whereas the acyclic model compounds are much less selective and strongly bind to the matched DNA. Moreover, the results from thermal denaturation experiments are in very good agreement with the binding affinities obtained by ESI-MS and FID measurements. The FID results also demonstrate that the macrocyclic naphthalene derivative BisNP preferentially binds to pyrimidine–pyrimidine mismatches compared to all other possible base mismatches. This ligand also efficiently competes with a DNA enzyme (M.TaqI) for binding to a duplex with a TT-mismatch, as shown by competitive fluorescence titrations. Altogether, our results demonstrate that macrocyclic distance-constrained bis-intercalators are efficient and selective mismatch-binding ligands that can interfere with mismatch-binding enzymes.

Hereditary ovarian cancer

At least 10% of ovarian tumors are hereditary and associated with highly penetrant, autosomal, dominant genetic predisposition. Three clinical manifestations of hereditary ovarian cancer have been identified: site-specific ovarian cancer, hereditary breast and/or ovarian cancer (HBOC) and hereditary non-polyposis colorectal cancer (HNPCC) syndromes. BRCA germline mutations account for more than 90% of all hereditary epithelial ovarian tumors whereas most of the remaining 10% are caused by MLH1 and MSH2 mutations, which are susceptibility genes of HNPCC. Genetic testing is available for each of the three hereditary syndromes above mentioned. The recommendations for OC surveillance in high-risk women having a strong family history or BRCA mutation carriers include transvaginal pelvic ultrasound with color Doppler and serum CA125 every 6 months. Bilateral salpingo-oophorectomy appears to be effective to reduce the risk of ovarian cancer in BRCA mutation carriers. Hysterosalpingo-oophorectomy should be considered in HNPCC women who undergo surgery for colorectal carcinoma.

Role of c-Abl kinase in DNA mismatch repair-dependent G2 cell cycle checkpoint arrest responses

Current published data suggest that DNA mismatch repair (MMR) triggers prolonged G(2) cell cycle checkpoint arrest after alkylation damage from N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) by activating ATR (ataxia telangiectasia-Rad3-related kinase). However, analyses of isogenic MMR-proficient and MMR-deficient human RKO colon cancer cells revealed that although ATR/Chk1 signaling controlled G(2) arrest in MMR-deficient cells, ATR/Chk1 activation was not involved in MMR-dependent G(2) arrest. Instead, we discovered that disrupting c-Abl activity using STI571 (Gleevec, a c-Abl inhibitor) or stable c-Abl knockdown abolished MMR-dependent p73alpha stabilization, induction of GADD45alpha protein expression, and G(2) arrest. In addition, inhibition of c-Abl also increased the survival of MNNG-exposed MMR-proficient cells to a level comparable with MMR-deficient cells. Furthermore, knocking down GADD45alpha (but not p73alpha) protein levels affected MMR-dependent G(2) arrest responses. Thus, MMR-dependent G(2) arrest responses triggered by MNNG are dependent on a human MLH1/c-Abl/GADD45alpha signaling pathway and activity. Furthermore, our data suggest that caution should be taken with therapies targeting c-Abl kinase because increased survival of mutator phenotypes may be an unwanted consequence.

Preferential loss of mismatch repair function in refractory and relapsed acute myeloid leukemia: potential contribution to AML progression

Acute myeloid leukemia (AML) is an aggressive hematological cancer. Despite therapeutic regimens that lead to complete remission, the vast majority of patients undergo relapse. The molecular mechanisms underlying AML development and relapse remain incompletely defined. To explore whether loss of DNA mismatch repair (MMR) function is involved in AML, we screened two key MMR genes, MSH2 and MLH1, for mutations and promoter hypermethylation in leukemia specimens from 53 AML patients and blood from 17 non-cancer controls. We show here that whereas no amino acid alteration or promoter hypermethylation was detected in all control samples, 18 AML patients exhibited either mutations in MMR genes or hypermethylation in the MLH1 promoter. In vitro functional MMR analysis revealed that almost all the mutations analyzed resulted in loss of MMR function. MMR defects were significantly more frequent in patients with refractory or relapsed AML compared with newly diagnosed patients. These observations suggest for the first time that the loss of MMR function is associated with refractory and relapsed AML and may contribute to disease pathogenesis.

Distinct effects of the recurrent Mlh1G67R mutation on MMR functions, cancer, and meiosis

Mutations in the human DNA mismatch repair (MMR) gene MLH1 are associated with hereditary nonpolyposis colorectal cancer (Lynch syndrome, HNPCC) and a significant proportion of sporadic colorectal cancer. The inactivation of MLH1 results in the accumulation of somatic mutations in the genome of tumor cells and resistance to the genotoxic effects of a variety of DNA damaging agents. To study the effect of MLH1 missense mutations on cancer susceptibility, we generated a mouse line carrying the recurrent Mlh1(G67R) mutation that is located in one of the ATP-binding domains of Mlh1. Although the Mlh1(G67R) mutation resulted in DNA repair deficiency in homozygous mutant mice, it did not affect the MMR-mediated cellular response to DNA damage, including the apoptotic response of epithelial cells in the intestinal mucosa to cisplatin, which was defective in Mlh1(-/-) mice but remained normal in Mlh1(G67R/G67R) mice. Similar to Mlh1(-/-) mice, Mlh1(G67R/G67R) mutant mice displayed a strong cancer predisposition phenotype. However, in contrast to Mlh1(-/-) mice, Mlh1(G67R/G67R) mutant mice developed significantly fewer intestinal tumors, indicating that Mlh1 missense mutations can affect MMR tumor suppressor functions in a tissue-specific manner. In addition, Mlh1(G67R/G67R) mice were sterile because of the inability of the mutant Mlh1(G67R) protein to interact with meiotic chromosomes at pachynema, demonstrating that the ATPase activity of Mlh1 is essential for fertility in mammals.

DNA repair dysfunction in gastrointestinal tract cancers

The DNA repair system surveys the genome, which is always suffering from exposure to both exogenous as well as endogenous mutagens, to maintain the genetic information. The fact that the basis of this DNA repair system is highly conserved, from prokaryote to mammalian cells, suggests the importance of precise genome maintenance mechanisms for organisms. In the past 15 years, considerable progress has been made in understanding how repair processes interact and how disruptions of these mechanisms lead to the accumulation of mutations and carcinogenesis. In 1993, two groups reported that DNA mismatch repair could be associated with hereditary non-polyposis colorectal cancer, indicating a connection between faulty DNA repair function and cancer. More recently, an inherited disorder of DNA glycosylase, which removes mutagenic oxidized base from DNA, has been reported in individuals with a predisposition to multiple colorectal adenomas and carcinomas. This is the first report that directly indicates the role of the repair of oxidative DNA in human inherited cancer. Studies from gene knockout mice have elucidated the principal role of these repair systems in the process of carcinogenesis. Moreover, clinical samples derived from cancer patients have shown the direct involvement. This review focuses on the function of DNA mismatch repair and oxidative DNA/nucleotide repair among various DNA repair systems in cells, both of which are essentially involved in the carcinogenesis of gastrointestinal tract cancer.

The involvement of DNA-damage and -repair defects in neurological dysfunction

A genetic link between defects in DNA repair and neurological abnormalities has been well established through studies of inherited disorders such as ataxia telangiectasia and xeroderma pigmentosum. In this review, we present a comprehensive summary of the major types of DNA damage, the molecular pathways that function in their repair, and the connection between defective DNA-repair responses and specific neurological disease. Particular attention is given to describing the nature of the repair defect and its relationship to the manifestation of the associated neurological dysfunction. Finally, the review touches upon the role of oxidative stress, a leading precursor to DNA damage, in the development of certain neurodegenerative pathologies, such as Alzheimer's and Parkinson's.

Mismatch repair protein expression in ovine intestinal adenocarcinomas

Sheep in New Zealand develop small intestinal adenocarcinomas more frequently than sheep elsewhere in the world. This high rate of neoplasm development could be due to a genetic predisposition or due to an environmental carcinogen. Differentiation between a genetic and an environmental factor is important as, if an environmental carcinogen is present, people could be exposed directly or by consuming sheep meat. In humans, germline defects in the mismatch repair (MMR) genes cause hereditary nonpolyposis colorectal cancer (HNPCC). Affected people are predisposed to neoplasm development, most commonly colonic adenocarcinomas. It was hypothesized that MMR defects are common within the New Zealand sheep flock, and these defects predispose New Zealand sheep to intestinal neoplasia. To investigate this, immunohistochemistry was used to evaluate the expression of the MMR proteins MSH2, MSH6, MLH1, and PMS2 within 49 ovine intestinal adenocarcinomas. Neoplastic cells within all sheep tumors expressed MSH2, MSH6, and MLH1. Expression of PMS2 could not be assessed, most likely because of insufficient affinity of the anti-human PMS2 antibody to ovine PMS2. The consistent expression of MSH2, MSH6, and MLH1 within the ovine intestinal adenocarcinomas does not support the hypothesis that defects in the MMR genes are common in New Zealand sheep.

Mechanisms and functions of DNA mismatch repair

DNA mismatch repair (MMR) is a highly conserved biological pathway that plays a key role in maintaining genomic stability. The specificity of MMR is primarily for base-base mismatches and insertion/deletion mispairs generated during DNA replication and recombination. MMR also suppresses homeologous recombination and was recently shown to play a role in DNA damage signaling in eukaryotic cells. Escherichia coli MutS and MutL and their eukaryotic homologs, MutSalpha and MutLalpha, respectively, are key players in MMR-associated genome maintenance. Many other protein components that participate in various DNA metabolic pathways, such as PCNA and RPA, are also essential for MMR. Defects in MMR are associated with genome-wide instability, predisposition to certain types of cancer including hereditary non-polyposis colorectal cancer, resistance to certain chemotherapeutic agents, and abnormalities in meiosis and sterility in mammalian systems.

Kaposi's sarcoma-associated herpesvirus ori-Lyt-dependent DNA replication: involvement of host cellular factors

Herpesvirus lytic DNA replication requires both the cis-acting element, the origin, and trans-acting factors, including virally encoded origin-binding protein, DNA replication enzymes, and auxiliary factors. Two lytic DNA replication origins (ori-Lyt) of Kaposi's sarcoma-associated herpesvirus (KSHV) have been identified, and two virally encoded proteins, namely, RTA and K8, have been shown to bind to the origins. In this study, we sought to identify cellular factors that associate with ori-Lyt by using DNA affinity purification and mass spectrometry. This approach led to identification of several cellular proteins that bind to KSHV ori-Lyt. They include topoisomerases (Topo) I and II, MSH2/6, RecQL, poly(ADP-ribose) polymerase I (PARP-1), DNA-PK, Ku86/70 autoantigens, and scaffold attachment factor A (SAF-A). RecQL appears to associate with prereplication complexes and be recruited to ori-Lyt through RTA and K8. Topoisomerases, MSH2, PARP-1, DNA-PK, and Ku86 were not detected in prereplication complexes but were present in replication initiation complexes on ori-Lyt. All these cellular proteins accumulate in viral replication compartments in the nucleus, indicating that these proteins may have a role in viral replication. Topo I and II appear to be essential for viral DNA replication as inhibition of their activities with specific inhibitors (camptothecin and ellipticine) blocked ori-Lyt-dependent DNA replication. Furthermore, inhibition of PARP-1 with chemical inhibitors (3-aminobenzamide and niacinamide) resulted in decreased ori-Lyt-dependent DNA replication, whereas hydroxyurea, which raises PARP-1 activity, caused an increase in the DNA replication, suggesting a positive role for PARP-1 in KSHV lytic DNA replication.

Evidence that a mutation in the MLH1 3'-untranslated region confers a mutator phenotype and mismatch repair deficiency in patients with relapsed leukemia

Defects in DNA mismatch repair (MMR) are the molecular basis of certain cancers, including hematological malignancies. The defects are often caused by mutations in coding regions of MMR genes or promoter methylation of the genes. However, in many cases, despite that a hypermutable phenotype is detected in a patient, no mutations/hypermethylations of MMR genes can be detected. We report here a novel mechanism that a mutation in the MLH1 3'-untranslated region (3'-UTR) leads to MMR deficiency. A relapsed leukemia patient displayed microsatellite instability, but no genetic and epigenetic alterations in key MMR genes were identifiable. Instead, a 3-nucleotide (TTC) deletion in the MLH1 3'-UTR was found in the patient's blood sample. The mutant MLH1 3'-UTR was found to significantly reduce the expressions of both a firefly luciferase reporter gene and an ectopic MLH1 gene in model cell lines. Consistent with these observations, a significant reduction in the steady-state level of MLH1 mRNA was observed in white blood cells of the patient. These findings suggest that the mutant MLH1 3'-UTR can cause a severely reduced/defective MMR activity conferring leukemia relapse, likely by down-regulating MLH1 expression at the mRNA level. Although the exact mechanism by which the mutant 3'-UTR down-regulates the MLH1 mRNA is not known, our findings provide a novel marker for cancers with MMR defects.

Mutations affecting a putative MutLalpha endonuclease motif impact multiple mismatch repair functions

Mutations in DNA mismatch repair (MMR) lead to increased mutation rates and higher recombination between similar, but not identical sequences, as well as resistance to certain DNA methylating agents. Recently, a component of human MMR machinery, MutLalpha, has been shown to display a latent endonuclease activity. The endonuclease active site appears to include a conserved motif, DQHA(X)(2)E(X)(4)E, within the COOH-terminus of human PMS2. Substitution of the glutamic acid residue (E705) abolished the endonuclease activity and mismatch-dependent excision in vitro. Previously, we showed that the PMS2-E705K mutation and the corresponding mutation in Saccharomyces cerevisiae were both recessive loss of function alleles for mutation avoidance in vivo. Here, we show that mutations impacting this endonuclease motif also significantly affect MMR-dependent suppression of homeologous recombination in yeast and responses to S(n)1-type methylating agents in both yeast and mammalian cells. Thus, our in vivo results suggest that the endonuclease activity of MutLalpha is important not only in MMR-dependent mutation avoidance but also for recombination and damage response functions.

Microsatellite instability and compromised mismatch repair gene expression during in vitro passaging of monoclonal human T lymphocytes

An age-related accumulation of DNA damage caused by increased insult and/or decreased repair, could contribute to impaired cellular function. DNA mismatch repair (MMR), the main postreplicative correction pathway, can be monitored by assessing microsatellite instability and has been reported to decrease with age. Here, we analyzed the involvement of the MMR system in the accumulation of genetic damage in a cultured monoclonal human T lymphocyte model. We correlated microsatellite instability (MSI) and MMR gene expression, and replicative senescence of CD4+ clones derived from young, old and centenarian individuals or from CD34+ precursors. Cells were analyzed for MSI at five loci (CD4, VWA, Fes, D2S123, and BAT26), for the methylation status of MLH1 and MSH2 gene promoters, and for the expression of the MMR genes MSH2, MSH6, MSH3, MLH1, PMS2, and PMS1. MSI increased with increasing culture passages, particularly in the CD34+ progenitor-derived clones, but also in those from adult T cells. MSI and MMR gene expression were found to correlate, mostly due to a reduced expression of the components of MutL heterodimers, pointing to a role of MMR in the acquisition of DNA damage with in vitro aging.

Accurate homologous recombination is a prominent double-strand break repair pathway in mammalian chromosomes and is modulated by mismatch repair protein Msh2

We designed DNA substrates to study intrachromosomal recombination in mammalian chromosomes. Each substrate contains a thymidine kinase (tk) gene fused to a neomycin resistance (neo) gene. The fusion gene is disrupted by an oligonucleotide containing the 18-bp recognition site for endonuclease I-SceI. Substrates also contain a "donor" tk sequence that displays 1% or 19% sequence divergence relative to the tk portion of the fusion gene. Each donor serves as a potential recombination partner for the fusion gene. After stably transfecting substrates into mammalian cell lines, we investigated spontaneous recombination and double-strand break (DSB)-induced recombination following I-SceI expression. No recombination events between sequences with 19% divergence were recovered. Strikingly, even though no selection for accurate repair was imposed, accurate conservative homologous recombination was the predominant DSB repair event recovered from rodent and human cell lines transfected with the substrate containing sequences displaying 1% divergence. Our work is the first unequivocal demonstration that homologous recombination can serve as a major DSB repair pathway in mammalian chromosomes. We also found that Msh2 can modulate homologous recombination in that Msh2 deficiency promoted discontinuity and increased length of gene conversion tracts and brought about a severalfold increase in the overall frequency of DSB-induced recombination.

Altered dynamics of DNA bases adjacent to a mismatch: a cue for mismatch recognition by MutS

The structural deviations as well as the alteration in the dynamics of DNA at mismatch sites are considered to have a crucial role in mismatch recognition followed by its repair utilizing mismatch repair family proteins. To compare the dynamics at a mismatch and a non-mismatch site, we incorporated 2-aminopurine, a fluorescent analogue of adenine next to a G.T mismatch, a C.C mismatch, or an unpaired T, and at several other non-mismatch positions. Rotational diffusion of 2-aminopurine at these locations, monitored by time-resolved fluorescence anisotropy, showed distinct differences in the dynamics. This alteration in the motional dynamics is largely confined to the normally matched base-pairs that are immediately adjacent to a mismatch/ unpaired base and could be used by MutS as a cue for mismatch-specific recognition. Interestingly, the enhanced dynamics associated with base-pairs adjacent to a mismatch are significantly restricted upon MutS binding, perhaps "resetting" the cues for downstream events that follow MutS binding. Recognition of such details of motional dynamics of DNA for the first time in the current study enabled us to propose a model that integrates the details of mismatch recognition by MutS as revealed by the high-resolution crystal structure with that of observed base dynamics, and unveils a minimal composite read-out involving the base mismatch and its adjacent normal base-pairs.

Escherichia coli mismatch repair protein MutL interacts with the clamp loader subunits of DNA polymerase III

It has been hypothesized that DNA mismatch repair (MMR) is coupled with DNA replication; however, the involvement of DNA polymerase III subunits in bacterial DNA MMR has not been clearly elucidated. In an effort to better understand the relationship between these 2 systems, the potential interactions between the Escherichia coli MMR protein and the clamp loader subunits of E. coli DNA polymerase III were analyzed by far western blotting and then confirmed and characterized by surface plasmon resonance (SPR) imaging. The results showed that the MMR key protein MutL could directly interact with both the individual subunits delta, delta', and gamma and the complex of these subunits (clamp loader). Kinetic parameters revealed that the interactions are strong and stable, suggesting that MutL might be involved in the recruitment of the clamp loader during the resynthesis step in MMR. The interactions between MutL, the delta and gamma subunits, and the clamp loader were observed to be modulated by ATP. Deletion analysis demonstrated that both the N-terminal residues (1-293) and C-terminal residues (556-613) of MutL are required for interacting with the subunits delta and delta'. Based on these findings and the available information, the network of interactions between the MMR components and the DNA polymerase III subunits was established; this network provides strong evidence to support the notion that DNA replication and MMR are highly associated with each other.

Environmental factors and colorectal tumor risk in individuals with hereditary nonpolyposis colorectal cancer

BACKGROUND & AIMS

Individuals with hereditary nonpolyposis colorectal cancer (HNPCC) are at increased risk for colorectal cancer. Environmental factors might play a role in HNPCC-associated carcinogenesis. The aim of this study was to gain insight into the effects of environmental factors on colorectal tumor risk in individuals with HNPCC.

METHODS

We examined associations between dietary factors, cigarette smoking, and HNPCC-associated colorectal tumors in a Dutch case-control study (145 cases, 103 tumor-free controls; all study participants were known or suspected carriers of a germline mutation in one of the DNA mismatch repair genes). We also assessed associations between the various environmental factors and occurrence of adenomatous polyposis coli (APC) mutations in HNPCC-associated polyps in a subset of the study population.

RESULTS

Fruit consumption was inversely associated with ever developing HNPCC-associated colorectal tumors (odds ratio [95% confidence interval] for highest vs lowest tertile, 0.4 [0.2-0.9]; P(trend) = .03); a borderline significant inverse association was observed for dietary fiber intake (0.5 [0.2-1.0]; P(trend) = .06). Cigarette smoking seemed to increase the risk of HNPCC-associated colorectal tumors. Truncating APC mutations were detected in 30 (37.5%) of the 80 available HNPCC-associated polyps; frameshift mutations were most common (73.3%). None of the evaluated environmental factors was distinctively associated with a specific APC status of the polyps.

CONCLUSIONS

Our data suggest that fruit consumption and dietary fiber intake might decrease the risk of colorectal tumors in individuals with HNPCC, whereas cigarette smoking might increase the risk of HNPCC-associated colorectal tumors. The observed associations support the hypothesis that HNPCC-associated outcomes might be modified by environmental factors.

Mutation of msh-2 in Neurospora crassa does not reduce the incidence of recombinants with multiple patches of donor chromosome sequence

The Neurospora homologue msh-2 of the Escherichia coli mismatch repair gene mutS was mutated by repeat-induced point mutation (RIP) of a 1.9-kb duplication covering 1661bp of the coding sequence and 302 bp 5' of the gene. msh-2(RIP-LK1) exhibited a mutator phenotype conferring a 17-fold increase in the frequency of spontaneous mitotic reversion of his-3 allele K458. In msh-2(RIP-LK1) homozygotes, recombination frequency at the his-3 locus increased up to 2.9-fold over that in msh-2(+) diploids. Progeny of crosses homozygous msh-2(RIP-LK1), like those from crosses homozygous msh-2(+) frequently had multiple patches of donor chromosome sequence, suggesting that patchiness in msh-2(+) crosses is not explained by incomplete repair of heteroduplex DNA by MSH-2. These findings are consistent with data from the analysis of events in a Neurospora translocation heterozygote that suggested multiple patches of donor chromosome sequence arising during recombination reflect multiple template switches during DNA repair synthesis.

Nuclease activity of the MutS homologue MutS2 from Thermus thermophilus is confined to the Smr domain

MutS homologues are highly conserved enzymes engaged in DNA mismatch repair (MMR), meiotic recombination and other DNA modifications. Genome sequencing projects have revealed that bacteria and plants possess a MutS homologue, MutS2. MutS2 lacks the mismatch-recognition domain of MutS, but contains an extra C-terminal region called the small MutS-related (Smr) domain. Sequences homologous to the Smr domain are annotated as ‘proteins of unknown function’ in various organisms ranging from bacteria to human. Although recent in vivo studies indicate that MutS2 plays an important role in recombinational events, there had been only limited characterization of the biochemical function of MutS2 and the Smr domain. We previously established that Thermus thermophilus MutS2 (ttMutS2) possesses endonuclease activity. In this study, we report that a Smr-deleted ttMutS2 mutant retains the dimerization, ATPase and DNA-binding activities, but has no endonuclease activity. Furthermore, the Smr domain alone was stable and functional in binding and incising DNA. It is noteworthy that an endonuclease activity is associated with a MutS homologue, which is generally thought to recognize specific DNA structures.

Hypoxia-induced genetic instability--a calculated mechanism underlying tumor progression

The cause of human cancers is imputed to the genetic alterations at nucleotide and chromosomal levels of ill-fated cells. It has long been recognized that genetic instability-the hallmark of human cancers-is responsible for the cellular changes that confer progressive transformation on cancerous cells. How cancer cells acquire genetic instability, however, is unclear. We propose that tumor development is a result of expansion and progression-two complementary aspects that collaborate with the tumor microenvironment-hypoxia in particular, on genetic alterations through the induction of genetic instability. In this article, we review the recent literature regarding how hypoxia functionally impairs various DNA repair pathways resulting in genetic instability and discuss the biomedical implications in cancer biology and treatment.

The T/G mutation in exon 8 of hMSH2 gene in the sporadic colon cancer patients

The DNA mismatch repair (MMR) system guards against genomic instability, therefore the mutations in the human MMR genes cause the majority of the hereditary nonpolyposis colorectal cancer (HNPCC) and a small percentage of the sporadic colon cancer. hMSH2 is one of MMR genes involved in the correction of mispairing during replication and its mutations are associated with both--microsatellite instability and the hereditary and sporadic colon tumourgenesis. The aim of this study was to analyse the T/G mutation (codon 458) in exon 8 of hMSH2 gene in the sporadic colon cancer cells. We also examined the relationship between the T/G mutation of hMSH2 gene, and the selected prognostic factors such as Dukes' stage, histological grade and lymph node metastasis. We analysed samples of tumour from 75 patients with sporadic colorectal cancers. The mutation in the hMSH2 gene ware determined by the RFLP-PCR. We found T/G mutation in exon 8 of hMSH2 gene in 5 patients (6,7%). There was no statistically significant difference between this mutation and selected clinical parameters. The results of our studies revealed that mutations of hMSH2 gene may lead to development of colorectal cancer. No dependence between the mutation of hMSH2 gene and clinical parameters, suggests that the mutation of hMSH2 gene may have a critical significance for the first steps of carcinogenesis in colon epithelial.

Some speculation on the origin of glioblastoma

Glioblastoma, the most common primary brain tumor, is also the most deadly, with median survival of about one year, which is little improved over the last five decades. Its pathogenesis is a vexing problem. Despite extensive basic and clinical scientific research, little is known regarding the cause of this disease, the genetic factors which drive its course, or any strategies which may result in effective treatment. This persistent resistance to understanding suggests to the authors that some of the fundamental assumptions regarding the disease are likely to be flawed, and that a new paradigm must be sought to replace them. This manuscript is a review of some of what is known regarding this disease, and then presents a series of hypotheses which compromise an alternative view of glioblastoma.

The E705K mutation in hPMS2 exerts recessive, not dominant, effects on mismatch repair

The hPMS2 mutation E705K is associated with Turcot syndrome. To elucidate the pathogenesis of hPMS2-E705K, we modeled this mutation in yeast and characterized its expression and effects on mutation avoidance in mammalian cells. We found that while hPMS2-E705K (pms1-E738K in yeast) did not significantly affect hPMS2 (Pms1p in yeast) stability or interaction with MLH1, it could not complement the mutator phenotype in MMR-deficient mouse or yeast cells. Furthermore, hPMS2-E705K/pms1-E738K inhibited MMR in wild-type (WT) mammalian cell extracts or yeast cells only when present in excess amounts relative to WT PMS2. Our results strongly suggest that hPMS2-E705K is a recessive loss-of-function allele.

The relative roles of three DNA repair pathways in preventing Caenorhabditis elegans mutation accumulation

Mutation is a central biological process whose rates and spectra are influenced by a variety of complex and interacting forces. Although DNA repair pathways are generally known to play key roles in maintaining genetic stability, much remains to be understood about the relative roles of different pathways in preventing the accumulation of mutations and the extent of heterogeneity in pathway-specific repair efficiencies across different genomic regions. In this study we examine mutation processes in base excision repair-deficient (nth-1) and nucleotide excision repair-deficient (xpa-1) Caenorhabditis elegans mutation-accumulation (MA) lines across 24 regions of the genome and compare our observations to previous data from mismatch repair-deficient (msh-2 and msh-6) and wild-type (N2) MA lines. Drastic variation in both average and locus-specific mutation rates, ranging two orders of magnitude for the latter, was detected among the four sets of repair-deficient MA lines. Our work provides critical insights into the relative roles of three DNA repair pathways in preventing C. elegans mutation accumulation and provides evidence for the presence of pathway-specific DNA repair territories in the C. elegans genome.