MMR System Research Articles

Effect of Helicobacter pylori infection on p53 expression of gastric mucosa and adenocarcinoma with microsatellite instability.

To investigate the relationship between Helicobacter pylori (H pylori) infection, microsatellite instability and the expressions of the p53 in gastritis, intestinal metaplasia and gastric adenocarcinoma and to elucidate the mechanism of gastric carcinogenesis relating to H pylori infection. One hundred and eight endoscopic biopsies and gastric adenocarcinoma were available for the study including 33 cases of normal, 45 cases of gastritis, 30 cases of intestinal metaplasia, and 46 cases of gastric adenocarcinoma. Peripheral blood samples of these patients were also collected. H pylori infection and p53 expressions were detected by means of streptavidin-peroxidase (SP) immunohistochemical method. Microsatellite loci were studied by PCR-SSCP-CE using the markers BAT-26, D17S261, D3S1283, D2S123, and D3S1611. MSI was defined as the peak shift in the DNA of the gastric tissue compared with that of the peripheral blood samples. Based on the number of mutated MSI markers, specimens were characterized as high MSI (MSI-H) if they manifested instability at two or more markers, low MSI (MSI-L) if unstable at only one marker, and microsatellite stable (MSS) if they showed no instability at any marker. H pylori infection was detected in the samples of gastritis, intestinal metaplasia, and gastric adenocarcinoma and the infection frequencies were 84.4%, 76.7%, and 65.2%, respectively, whereas no H pylori infection was detected in the samples of normal control. There was a significant difference in the infection rates between gastritis and carcinoma samples (P = 0.035). No MSI was detected in gastritis samples, one MSI-H and two MSI-L were detected among the 30 intestinal metaplasia samples, and 12 MSI-H and 3 MSI-L were detected in the 46 gastric carcinomas. In those gastric carcinomas, the MSI-H frequency in H pylori-positive group was significantly higher than that in H pylori-negative group. No p53 expression was detected in the normal and gastritis samples from dyspeptic patients. P53-positive immunohistochemical staining was detected in 13.3% of intestinal metaplasia samples and in 43.5% of gastric carcinoma samples. The levels of p53 in H pylori-positive samples were higher than those in the negative group when the carcinoma samples were subdivided into H pylori-positive and -negative groups (P = 0.013). Eight samples were detected with positive p53 expression out of the 11 MSI-H carcinomas with H pylori infection and no p53 expression could be seen in the H pylori-negative samples. H pylori affect the p53 pattern in gastric mucosa when MMR system fails to work. Mutations of the p53 gene seem to be an early event in gastric carcinogenesis.

MUC Gene Abnormalities in Sporadic and Hereditary Mucinous Colon Cancers with Microsatellite Instability

Aim of this study was verifying whether mucin producing colon cancers (CRCs) could develop through a molecular pathway involving microsatellite instability (MSI) and MUC gene alterations. Out of 49 CRCs expressing variable amounts of mucin, 22 (44.9%) were MSI-H and 5 (10.2%) were MSI-L. MUC genes were analyzed by Southern blotting and extra bands were evident in the Variable Number Tandem Repetition (VNTR) regions of MUC2 (5 cases) and MUC5AC (2 cases), but not MUC1 and MUC4 genes. Since the somatic VNTR abnormalities were detected in 6 MSI-H and in 1 MSI-L tumors, they seem to be peculiar of mismatch repair defective CRCs. Our finding suggests that alteration and/or loss of structurally normal MUC genes may be an important step in the neoplastic molecular pathway of a subset of CRCs and that mutations involving VNTR repetitive sequences may exist in MSI tumors as a direct and/or indirect consequence of an inefficient MMR system.

Microsatellite instability and loss of heterozygosity have distinct prognostic value for testicular germ cell tumor recurrence

Germ cell tumor (GCT) is the most common genitourinary malignancy of men between the ages of 18 and 35. Therapy is ultimately successful in over 90% of patients, however significant morbidity and mortality can be associated with adjuvant treatment and relapse. Molecular markers that predict treatment response and/or poor outcome would have immediate clinical benefit since adjuvant treatment could be selectively reserved for patients at higher risk for relapse and those patients most likely to respond to treatment. In order to identify potential prognostic molecular markers, we evaluated 118 GCT for microsatellite instability (MSI), loss of heterozygosity (LOH) and MSH2 immunostaining to identify tumors associated with relapse and/or poor outcome following initial surgical, medical and/or radiation therapy. MSI in 3 or more markers and/or low MSH2 staining were associated with relapse while LOH in the absence of MSI and/or high MSH2 staining were not. Twenty-five percent of GCT exhibited genetic instability in 3 or more microsatellite markers (MSI+ tumors), 15% exhibited LOH in the absence of MSI (LOH only tumors) and 44% exhibited decreased or absent MSH2 immunostaining (low MSH2 staining tumors). Thirty-six patients (30%) relapsed and 27 of these patients (75%) had MSI+ and/or low MSH2 staining tumors. Only 1 patient (3%) with an LOH only tumor and no patients with high MSH2 staining and LOH only tumors relapsed. Therefore distinct GCT subpopulations identified by detection of MSI, LOH and MMR expression are associated with different clinical outcomes. MMR deficient testicular GCT with increased frequency of MSI had an increased association with tumor recurrence compared to GCT with an intact MMR system and LOH in the absence of MSI.

Mismatch repair ensures fidelity of replication and recombination in the radioresistant organism Deinococcus radiodurans

We have characterized the mismatch repair system (MMR) of the highly radiation-resistant type strain of Deinococcus radiodurans, ATCC 13939. We show that the MMR system is functional in this organism, where it participates in ensuring the fidelity of DNA replication and recombination. The system relies on the activity of two key proteins, MutS1 and MutL, which constitute a conserved core involved in mismatch recognition. Inactivation of MutS1 or MutL resulted in a seven-fold increase in the frequency of spontaneous RifR mutagenesis and a ten-fold increase in the efficiency of integration of a donor point-mutation marker during bacterial transformation. Inactivation of the mismatch repair-associated UvrD helicase increased the level of spontaneous mutagenesis, but had no effect on marker integration--suggesting that binding of MutS1 and MutL proteins to a mismatched heteroduplex suffices to inhibit recombination between non identical (homeologous) DNAs. In contrast, inactivation of MutS2, encoded by the second mutS -related gene present in D. radiodurans, had no effect on mutagenesis or recombination. Cells devoid of MutS1 or MutL proteins were as resistant to gamma-rays, mitomycin C and UV-irradiation as wild-type bacteria, suggesting that the mismatch repair system is not essential for the reconstitution of a functional genome after DNA damage.

Analysis of the human APC mutation spectrum in a saccharomyces cerevisiae strain with a mismatch repair defect.

Somatic APC mutations in colorectal tumors with an RER phenotype reflect excessive frameshift mutations, especially in simple repetition tracts within the coding sequence. Because this type of mutation is characteristic of cells with a deficient DNA MMR system, the APC mutation signature of RER tumors may be attributable to a defect in the MMR system. However, there is little experimental evidence to prove that the spectrum of mutations and the APC gene distribution are directly influenced by MMR system defects. We therefore examined the mutation spectrum of the MCR of the APC gene after transfection into both MMR-proficient and MMR-deficient yeast strains and compared it with a previously reported human APC mutation database. Small insertions or deletions in mono- or dinucleotide repeats were more common in the MMR-deficient than in the MMR-proficient strain (91.2% vs. 38.1%, Fisher's exact test p < 0.0001). Furthermore, the 2 mutation hot spots, 4385-4394(AG)(5) and 4661-4666(A)(6), found in the yeast system corresponded with those in human tumors. Combining our data with those from human tumors, there appears to be hypermutable mutations in specific simple repetitive sequences within the MCR, which are more prevalent in MMR-deficient cells and RER tumors than in MMR-proficient cells and non-RER tumors. We therefore consider that the differences in the spectra of RER and non-RER tumors are attributable at least in part to the MMR system of the host cells.

The mismatch repair system (mutS, mutL and uvrD genes) in Pseudomonas aeruginosa: molecular characterization of naturally occurring mutants.

We have recently described the presence of a high proportion of Pseudomonas aeruginosa isolates (20%) with an increased mutation frequency (mutators) in the lungs of cystic fibrosis (CF) patients. In four out of 11 independent P. aeruginosa strains, the high mutation frequency was found to be complemented with the wild-type mutS gene from P. aeruginosa PAO1. Here, we report the cloning and sequencing of two additional P. aeruginosa mismatch repair genes and the characterization, by complementation of deficient strains, of these two putative P. aeruginosa mismatch repair genes (mutL and uvrD). We also describe the alterations in the mutS, mutL and uvrD genes responsible for the mutator phenotype of hypermutable P. aeruginosa strains isolated from CF patients. Seven out of the 11 mutator strains were found to be defective in the MMR system (four mutS, two mutL and one uvrD). In four cases (three mutS and one mutL), the genes contained frameshift mutations. The fourth mutS strain showed a 3.3 kb insertion after the 10th nucleotide of the mutS gene, and a 54 nucleotide deletion between two eight nucleotide direct repeats. This deletion, involving domain II of MutS, was found to be the main one responsible for mutS inactivation. The second mutL strain presented a K310M mutation, equivalent to K307 in Escherichia coli MutL, a residue known to be essential for its ATPase activity. Finally, the uvrD strain had three amino acid substitutions within the conserved ATP binding site of the deduced UvrD polypeptide, showing defective mismatch repair activity. Interestingly, cells carrying this mutant allele exhibited a fully active UvrABC-mediated excision repair. The results shown here indicate that the putative P. aeruginosa mutS, mutL and uvrD genes are mutator genes and that their alteration results in a mutator phenotype.

Functional significance of concomitant inactivation of hMLH1 and hMSH6 in tumor cells of the microsatellite mutator phenotype.

Genetic or epigenetic inactivation of one of the DNA mismatch repair (MMR) genes in tumor precursor cells causes a profound mutator phenotype, known as the microsatellite mutator phenotype (MMP). This mutator phenotype induces mutations not only in cancer genes that drive tumorigenesis but also in other DNA repair genes. The functional significance of these successive DNA repair gene mutations, however, has not been substantiated. Here we show that the concomitant inactivation of two DNA MMR genes (hMLH1 and hMSH6) increases the mutator phenotype. We isolated cell clones of the SW48 MMP-positive cell line with either active or inactive hMSH6. All of these clones lacked expression of hMLH1 because of promoter hypermethylation. Compared with inactivation of hMLH1 alone, the additional inactivation of hMSH6 produced a higher mutation rate and a different spectrum of mutations in the endogenous hprt gene. These results confirm our model that the mutator phenotype can increase during tumorigenesis by the consecutive inactivation of different members of the DNA MMR system. Thus, a stronger mutator phenotype accelerates the accumulation of mutations in target cancer genes, which, in turn, speeds up tumor progression. The results of this study also have significant impact on our understanding of the mechanism of DNA MMR.

Modulation of irinotecan cytotoxicity by lesions in p53 gene and MMR system

Magrini, R.; Bhonde, M.; Hanski, M. L.; Lenz, M.; Kobalz, U.; Notter, M.; Scherübl, H.; Boland, R.; Zeitz, M.; Hanski, C. Author Information

Expression of hMSH2 and hMLH1 proteins of the human DNA mismatch repair system in ameloblastoma.

The human DNA mismatch repair (hMMR) system plays an important role in reducing mutation and maintaining genomic stability. The MMR system in human cells is composed of at least six genes (hMSH2, hMLH1, hMSH3, hPMS1, hPMS2 and GTBP/hMSH6). In particular, hMSH2 and hMLH1 are expressed in human cells that are undergoing rapid renewal; their reduced expression has been reported in several tumors. We examined the protein expression pattern of hMSH2 and hMLH1 by immunohistochemistry in 25 ameloblastomas. All ameloblastomas expressed hMSH2 and hMLH1 proteins in the outer layer of epithelial cells. The localization of the staining was exclusively nuclear. These data suggest that the development and progression of these tumors do not depend on a defect in the hMMR system.

Ageing and the mismatch repair system

Age-related accumulation of mutations has been extensively documented, and it has been proposed as one of the prominent causes of malignancies in old age. The present review is focused on the particular case of DNA mismatch repair system (MMR), that has drawn increased attention for its possible relevance to malignancy. We also report on our own observations on an age-associated genomic instability that develops with age in the MMR system. Our study was performed on DNA samples that were prepared from peripheral blood cells, obtained at a 10-year interval from young and old human subjects. The two DNA samples from each individual were examined comparatively. The older individuals showed a significantly higher rate of microsatellite instability (MSI) in several loci examined, whereas no difference was found between the paired samples of any of the young subjects. We suggest that this increase in MSI with age may indicate an overall genomic instability in the elderly.

DNA Polymerase III Proofreading Mutants Enhance the Expansion and Deletion of Triplet Repeat Sequences in Escherichia coli

The influence of mutations in the 3' to 5' exonucleolytic proofreading epsilon-subunit of Escherichia coli DNA polymerase III on the genetic instabilities of the CGG.CCG and the CTG.CAG repeats that cause human hereditary neurological diseases was investigated. The dnaQ49(ts) and the mutD5 mutations destabilize the CGG.CCG repeats. The distributions of the deletion products indicate that slipped structures containing a small number of repeats in the loop mediate the deletion process. The CTG.CAG repeats were destabilized by the dnaQ49(ts) mutation by a process mediated by long hairpin loop structures (>/=5 repeats). The mutD5 mutator strain stabilized the (CTG.CAG)(175) tract, which contained two interruptions. Since the mutD5 mutator strain has a saturated mismatch repair system, the stabilization is probably an indirect effect of the nonfunctional mismatch repair system in these strains. Shorter uninterrupted tracts expand readily in the mutD5 strain, presumably due to the greater stability of long CTG.CAG tracts (>100 repeats) in this strain. When parallel studies were conducted in minimal medium, where the mutD5 strain is defective in exonucleolytic proofreading but has a functional MMR system, both CTG.CAG and CGG.CCG repeats were destabilized, showing that the proofreading activity is essential for maintaining the integrity of TRS tracts. Thus, we conclude that the expansion and deletion of triplet repeats are enhanced by mutations that reduce the fidelity of replication.