Efeito da p53 sobre a expressão e atividade da enzima de reparo de DNA Timina-DNA Glicosilase / Effect of p53 on the expression and activity of DNA repair enzyme thymine-DNA glycosylase

O câncer de esôfago é uma malignidade altamente freqüente e letal. Uma característica específica das áreas de alta incidência de câncer de esôfago é a grande proporção de duplas mutações no gene TP53, sendo, ao menos uma delas, uma transição G para A em sítios CpG. Essas transições resultam de malpareamentos G.T causados pela desaminação espontânea da 5-metilcitosina em ilhotas CpG. A enzima de reparo de DNA Timina-DNA Glicosilase (TDG) é responsável pelo primeiro passo na remoção da timina de malpareamentos G.T em CpG. A alta proporção de mutações em sítios CpG em câncer de esôfago das áreas de alta incidência sugere que a via de reparo de DNA iniciada pela TDG pode estar prejudicada. A presença de duplas mutações, sendo ao menos uma delas em CpG, levantou a hipótese de que a primeira mutação no TP53 reduz a atividade da via de reparo iniciada pela TDG, que acarretaria a segunda mutação em sítios CpG. Dessa forma, o objetivo desse trabalho foi analisar o efeito da p53 sobre a expressão e atividade da TDG. Os resultados obtidos mostram que a expressão de TDG é regulada transcricionalmente pela p53 numa gama de linhagens celulares e é induzida pelo dano ao DNA, de forma p53-dependente. Além disto, os resultados apontam um possível papel da proteína p53 ativa na migração nuclear e atividade da TDG. Estes resultados ainda nos levam à conclusão de que o silenciamento de TDG aumenta a sensibilidade à morte celular induzida por MMS quando a p53 é encontrada na forma selvagem, mas não quando esta proteína é mutada, e de que o status mutacional de TP53 parece afetar a expressão de TDG em CEE primários. Juntos esses resultados sugerem que a p53 regula o reparo de DNA mediado pela TDG e que a inativação de p53 em células tumorais pode contribuir para a aquisição de um mutator phenotype

Esophageal squamous cell carcinoma (ESCC) is a highly frequent and fatal malignancy in the world. A peculiar characteristic of the high incidence areas of esophageal cancer is the large proportion of double mutations in TP53 gene, being, at least one of them, a G to A transition at CpG sites. These transitions result from G.T mismatches caused by the spontaneous deamination of 5-methylcytosine at CpG sites. The DNA repair enzyme Thymine-DNA Glycosylase (TDG) is responsible for the first step in the removal of the thymidine from the G.T mismatches at CpG sites. The high proportion of mutations at CpG sites in esophageal tumors in the high incidence areas suggests that the DNA repair pathway initiated by TDG might be impaired. The large number of double mutations, with one being at a CpG site, raised the possibility that the first mutation in TP53 reduces the activity of the TDG base excision repair pathway, increasing the chance of a second mutation event at a CpG site. In this way, the aim of this work was to analyze the effect of p53 on the expression and activity of TDG. The results achieved show that TDG expression is regulated by p53 in a variety of cells lines at the trancriptional level and induced by DNA–damage in a p53-dependent manner. Furthermore, these results point out a possible role of active p53 in the nuclear migration and activity of TDG. The results further support the notion that TDG silencing increases the sensitivity to cell death induced by Methylmethane sulphonate when p53 is found in a wild-type, but not in a mutant form, and that TP53 mutation seems to affect TDG expression in primary ESCC. Together, these results suggest that p53 regulates TDG-mediated repair and that p53 inactivation in cancer cells may contribute to a mutator phenotype through loss of TDG function

"The""modification""Type Microsatellite Change in High Frequency Microsatellite Instability Colorectal Cancer Closely Relating to MLH1 and KRAS Mutation" / 现代生物医学进展

Microsatellite instability(MSI)was defined according to the frequency of positive findings in a panel of MSI markers.High frequency MSI(MSI-H)was the phenotype in which repeat sequences were extraordinarily unstable, and was considered to be the bona fide phenotype of DNA mismatch repair defection. However base substitutions in some well studied oncogenes or tumor suppressors were reported to be uncommon in MSI-H tumors. To explore this obvious contradiction, the relationship between MSI and KRAS gene mutations were studied in a panel of 76 human colorectal carcinomas, the whole exon of MLH1 and MSH2 were sequenced for MSI-H tumors. KRAS gene mutation was confirmed by similar frequencies in tumors of different MSI status. Intriguingly, all of the KRAS mutant MSI-H tumors harbored sequence alterations in MLH1gene, which was a key player in DNA mismatch repair system. This implied that in MSI-H tumors carrying MMR mutations, KRAS mutation were frequently and almost exclusively occurred. Furthermore, these MMR mutants were uniformly carrying a unique modification + jumping type MSI, which was different to MSI-H tumors without MLH1 or MSH2 gene mutations. This study shaded lights on the heterogeneity of MSI-H tumors, and implied the connection between modification type MSI and DNA mismatch defection.

The"modification"Type Microsatellite Change in High Frequency Microsatellite Instability Colorectal Cancer Closely Relating to MLH1 and KRAS Mutation / 现代生物医学进展

Microsatellite instability(MSI)was defined according to the frequency of positive findings in a panel of MSI markers.High frequency MSI(MSI-H)was the phenotype in which repeat sequences were extraordinarily unstable, and was considered to be the bona fide phenotype of DNA mismatch repair defection. However base substitutions in some well studied oncogenes or tumor suppressors were reported to be uncommon in MSI-H tumors. To explore this obvious contradiction, the relationship between MSI and KRAS gene mutations were studied in a panel of 76 human colorectal carcinomas, the whole exon of MLH1 and MSH2 were sequenced for MSI-H tumors. KRAS gene mutation was confirmed by similar frequencies in tumors of different MSI status. Intriguingly, all of the KRAS mutant MSI-H tumors harbored sequence alterations in MLH1gene, which was a key player in DNA mismatch repair system. This implied that in MSI-H tumors carrying MMR mutations, KRAS mutation were frequently and almost exclusively occurred. Furthermore, these MMR mutants were uniformly carrying a unique "modification" + "jumping" type MSI, which was different to MSI-H tumors without MLH1 or MSH2 gene mutations. This study shaded lights on the heterogeneity of MSI-H tumors, and implied the connection between "modification" type MSI and DNA mismatch defection.

The"modification"Type Microsatellite Change in High Frequency Microsatellite Instability Colorectal Cancer Closely Relating to MLH1 and KRAS Mutation / 现代生物医学进展

Microsatellite instability(MSI)was defined according to the frequency of positive findings in a panel of MSI markers.High frequency MSI(MSI-H)was the phenotype in which repeat sequences were extraordinarily unstable, and was considered to be the bona fide phenotype of DNA mismatch repair defection. However base substitutions in some well studied oncogenes or tumor suppressors were reported to be uncommon in MSI-H tumors. To explore this obvious contradiction, the relationship between MSI and KRAS gene mutations were studied in a panel of 76 human colorectal carcinomas, the whole exon of MLH1 and MSH2 were sequenced for MSI-H tumors. KRAS gene mutation was confirmed by similar frequencies in tumors of different MSI status. Intriguingly, all of the KRAS mutant MSI-H tumors harbored sequence alterations in MLH1gene, which was a key player in DNA mismatch repair system. This implied that in MSI-H tumors carrying MMR mutations, KRAS mutation were frequently and almost exclusively occurred. Furthermore, these MMR mutants were uniformly carrying a unique "modification" + "jumping" type MSI, which was different to MSI-H tumors without MLH1 or MSH2 gene mutations. This study shaded lights on the heterogeneity of MSI-H tumors, and implied the connection between "modification" type MSI and DNA mismatch defection.

Clinicopathological Characteristics of Colorectal Cancer with Family History: an Evaluation of Family History as a Predictive Factor for Microsatellite Instability

To determine whether family history of cancer may be a risk factor for the mutator phenotype in colorectal cancer, we recruited 143 consecutive colorectal cancer patients with a family history of accompanying cancers not meeting the Amsterdam criteria. Microsatellite instability (MSI) at 5 markers, hMLH1-promoter methylation, and expression of mismatch repair (MMR) proteins (hMLH1, hMSH2, hMSH6, hMPS1, and hPMS2) were determined. Among the relatives of familial colorectal cancer patients, colorectal cancer was the most common tumor type. Of the proband colorectal cancers, 26 (18.2%) showed high-level MSI (MSI-H); 47 additional tumors with mutator phenotype (32.9%) were identified by hMLH1-promoter methylation and/or loss of MMR protein expression. Mutator phenotype was associated with right-sided colon cancer and the type of accompanying cancer. Family history, which was differentially quantified according to the degree of relatives and the type of accompanying cancers, effectively discriminated MSI-H from microsatellite stable (MSS) and low-level microsatellite instability (MSI-L) and mutator phenotypes. Our findings indicate that familial colorectal cancer may be associated with multiple occurrences of colorectal or accompanying cancers and that family history could be correlated with microsatellite instability.

Identification of Target Genes in the Endometrial Carcinomas with Microsatellite Mutator Phenotype / 대한산부인과학회지

OBJECTIVE: Recent molecular genetic studies have revealed that two major types of genomic instabilities, chromosomal instability and microsatellite instability (MSI), exist in the endometrial carcinomas. Tumors with microsatellite mutator phenotype (MMP) are caused by defects in DNA mismatch repair genes. MMP tumors are believed to progress by accumulating frameshift mutations in coding microsatellite sequences of various cancer related genes including tumor suppressor genes, apoptosis related genes and DNA repair genes. Thus, the identification of the specific target genes in the MMP endometrial carcinomas is important for the elucidation of molecular pathogenesis of endometrial carcinomas. METHODS: We classified the MMP endometrial carcinomas and evaluated the frameshift mutations of the 11 genes containing coding microsatellite sequences by using 34 endometrial carcinomas and 4 MMP endometrial carcinoma cell lines. RESULTS: MSI was found in 6 of 34 endometrial carcinomas. In the endometrial carcinoma tissues, frequent mutations were found in TAF1B (68%), HT001 (50%), SLC23A1 (50%) and ACVR II (50%) in the MMP endometrial carcinoma tissues. The other 7 genes were infrequently mutated. Mutations of these target genes were more frequent in MMP endometrial carcinoma cell lines. CONCLUSION: we identified specific target genes in MMP endometrial carcinomas. These data demonstrate the mechanism of tumor progression in the MMP endometrial carcinomas.

Expression Pattern of DNA Mismatch Repair Genes in Tumors of Microsatellite Mutator Phenotype

Microsatellite mutator phenotype (MMP) tumors were reported in a subset of gastrointestinal carcinomas. The molecular pathogenesis of MMP tumors shows defects in the DNA mismatch repair genes, and also many germline and somatic mutations were reported in the MMP tumors. However, the detection of genetic defects in the MMP tumors is very difficult, mainly because many genes are included in the DNA mismatch repair genes. This study was undertaken to determine the best strategy for detecting defects in the DNA mismatch repair genes in gastrointestinal carcinomas. One of the effective ways for detecting defects in DNA mismatch repair genes is to screen the MMP tumors and evaluate the products of DNA mismatch repair genes by performing the multiplex RT-PCR method. We have screened the MMP tumors by using 5 microsatellite markers in the 12 cancer cell lines, 120 colon carcinomas and 99 gastric carcinomas and found 6 MMP cell lines, 10 MMP colon cancers, and 9 MMP gastric carcinomas. In addition, we evaluated 6 DNA mismatch repair gene products (hMSH2, hMSH3, hMSH6, hMLH1, hPMS1 and hPMS2) by multiplex RT-PCR analysis and found decreased expression of the DNA mismatch repair genes in 5 (hMSH6 in DLD-1 and HCT-15; hMSH2 in LoVo; hMLH1 and hMSH3 in HCT-116; hMLH1 in SNU-638) out of 6 MMP cell lines. We also found a decreased expression of hMLH1 in 3 out of 10 MMP colon carcinomas, and in 6 out of 9 MMP gastric carcinomas. Our results indicate that the expression analysis of the DNA mismatch repair genes by multiplex RT-PCR method can reduce the number of genes subjected to mutational analysis and is convenient for screening the responsible DNA mismatch repair genes.