Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UMLS:C0699790 (colon cancer)
 28,837 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

We have reported that transfer of chromosome 3 (Chr3) containing a single wild-type copy of the hMLH1 gene into HCT116 colon cancer cells, a cell line deficient in DNA mismatch repair (MMR) activity attributable to inactivating hMLH1 mutations, corrects all of the aspects of the MMR repair-deficient phenotype. We inhibited the expression of the wild-type hMLH1 gene using antisense RNA in HCT116+Chr3 cells to determine if this would result in reversion to the MMR-deficient phenotype. Despite profound inhibition of hMLH1 expression, DNA MMR activity and alkylation sensitivity were not impaired in the antisense-transfected HCT116+Chr3 cells. Additionally, arrest of the cell cycle at the G2 phase with alkylation damage occurs in these cells, a phenotype associated with MMR proficiency. These results indicate that even with a reduction in the expression of hMLH1 protein below the limits of detection by Western blotting, DNA MMR activity remained fully functional (by direct DNA MMR activity assay). We would speculate that hMLH1 is expressed in substantially greater abundance than would be minimally necessary for DNA MMR and that minor reductions in the expression of this protein would not be sufficient to permit DNA MMR dysfunction. Alternatively, Chr3 may contain a second hMLH1 homologue that might overlap with the function of hMLH1.
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PMID:Antisense inhibition of hMLH1 is not sufficient for loss of DNA mismatch repair function in the HCT116+chromosome 3 cell line. 1105 Dec 25

Serological tumor markers have proven valuable in the care of individuals with cancer for the early detection of primary cancers, early detection of cancer relapse, monitoring the response of cancers to therapy, and as predictors of cancer prognosis. Recently, the aberrant hypermethylation of the hMLH1 promoter and its consequent transcriptional silencing has been shown to be a common event in the formation of sporadic microsatellite unstable colon cancer. The silencing of hMLH1 expression appears to be controlled by the hypermethylation of a specific region in the hMLH1 promoter. We developed a methylation-specific PCR assay that assesses this region of the hMLH1 promoter. We found that this assay is able to detect methylated hMLH1 promoter DNA in the serum of some patients with microsatellite unstable colon cancers. In a panel of sera from 19 colon cancer cases, 9 with hMLH1 promoter methylation in the tumor primary, the assay proved 33% sensitive and 100% specific. This assay offers a potential means for the serum-based detection and/or monitoring of microsatellite unstable colon cancers.
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PMID:Detection of aberrantly methylated hMLH1 promoter DNA in the serum of patients with microsatellite unstable colon cancer. 1122 78

Members of hereditary nonpolyposis colon cancer (HNPCC) families harboring heterozygous germline mutations in the DNA mismatch repair genes hMSH2 or hMLH1 present with tumors generally two to three decades earlier than individuals with nonfamilial sporadic colon cancer. We searched for phenotypic features that might predispose heterozygous cells from HNPCC kindreds to malignant transformation. hMSH2(+/-) lymphoblastoid cell lines were found to be on average about 4-fold more tolerant than wild-type cells to killing by the methylating agent temozolomide, a phenotype that is invariably linked with impairment of the mismatch repair system. This finding was associated with an average 2-fold decrease of the steady-state level of hMSH2 protein in hMSH2(+/-) cell lines. In contrast, hMLH1(+/-) heterozygous cells were indistinguishable from normal controls in these assays. Thus, despite the fact that HNPCC families harboring mutations in hMSH2 or hMLH1 cannot be distinguished clinically, the early stages of the carcinogenic process in hMSH2 and hMLH1 mutation carriers may be different. Should hMSH2(+/-) colonocytes and lymphoblasts harbor a similar phenotype, the increased tolerance of the former to DNA-damaging agents present in the human colon may play a key role in the initiation of the carcinogenic process.
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PMID:Tolerance of human MSH2+/- lymphoblastoid cells to the methylating agent temozolomide. 1141 1

DNA mismatch repair (MMR) is an efficient system for the detection and repair of mismatched and unpaired bases in DNA. Deficiencies in MMR are commonly found in both hereditary and sporadic colorectal cancers, as well as in cancers of other tissues. Because fluorinated thymidine analogues (which through their actions might generate lesions recognizable by MMR) are widely used in the treatment of colorectal cancer, we investigated the role of MMR in cellular responses to 5-fluorouracil and 5-fluoro-2'-deoxyuridine (FdUrd). Human MLH1(-) and MMR-deficient HCT116 colon cancer cells were 18-fold more resistant to 7.5 microM 5-fluorouracil (continuous treatment) and 17-fold more resistant to 7.5 microM FdUrd in clonogenic survival assays compared with genetically matched, MLH1(+) and MMR-proficient HCT116 3-6 cells. Likewise, murine MLH1(-) and MMR-deficient CT-5 cells were 3-fold more resistant to a 2-h pulse of 10 microM FdUrd than their MLH1(+) and MMR-proficient ME-10 counterparts. Decreased cytotoxicity in MMR-deficient cells after treatment with various methylating agents and other base analogues has been well reported and is believed to reflect a tolerance to DNA damage. Synchronized HCT116 3-6 cells treated with a low dose of FdUrd had a 2-fold greater G(2) cell cycle arrest compared with MMR-deficient HCT116 cells, and asynchronous ME-10 cells demonstrated a 4-fold greater G(2) arrest after FdUrd treatment compared with CT-5 cells. Enhanced G(2) arrest in MMR-proficient cells in response to other agents has been reported and is believed to allow time for DNA repair. G(2) cell cycle arrest as determined by propidium iodide staining was not a result of mitotic arrest, but rather a true G(2) arrest, as indicated by elevated cyclin B1 levels and a lack of staining with mitotic protein monoclonal antibody 2. Additionally, p53 and GADD45 levels were induced in FdUrd-treated HCT116 3-6 cells. DNA double-strand break (DSB) formation was 2-fold higher in MMR-proficient HCT116 3-6 cells after FdUrd treatment, as determined by pulsed-field gel electrophoresis. The formation of DSBs was not the result of enhanced apoptosis in MMR-proficient cells. FdUrd-mediated cytotoxicity was caused by DNA-directed and not RNA-directed effects, because administration of excess thymidine (and not uridine) prevented cytotoxicity, cell cycle arrest, and DSB formation. hMLH1-dependent responses to fluoropyrimidine treatment, which may involve the action of p53 and the formation of DSBs, clearly have clinical relevance for the use of this class of drugs in the treatment of tumors with MMR deficiencies.
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PMID:Role of the hMLH1 DNA mismatch repair protein in fluoropyrimidine-mediated cell death and cell cycle responses. 1143 59

The role of microsatellite instability (MSI) in the pathogenesis of hepatocellular carcinoma (HCC) is incompletely defined. Although high-frequency MSI (MSI-H) is infrequently seen in HCC, some studies have suggested a role for MSI in HCC development. While MSI has been clearly defined for a subset of tumors, in particular colorectal, gastric and endometrial cancers, generally accepted criteria have not been developed for other tumors. Colorectal cancers (CRC) are classified as MSI-H if >30-40% of >5 microsatellite loci analyzed show instability. The MSI-H phenotype is associated with defective DNA mismatch repair (MMR) and is observed in the majority of tumors from patients with hereditary non-polyposis colon cancer (HNPCC) and also in 15% of sporadic CRCs. Inactivating mutations of the hMLH1 or hMSH2 genes lead to defects in MMR in HNPCC. In sporadic CRCs, MMR is usually due to hypermethylation of the hMLH-1 promoter. The role of defective MMR in hepatocellular carcinogenesis is controversial. Immunohistochemistry for hMLH1 and hMSH2 reliably indicates hMLH1 or hMSH2 loss in MSI-H CRC tumors. To investigate the role of defective MMR in HCC carcinogenesis, we performed immunohistochemistry for hMLH1 and hMSH2 on 36 HCCs. BAT26, a microsatellite marker that reliably predicts MSI-H was also examined. All 36 of the tumors stained positively for both hMLH1 and hMSH2, strongly suggesting an absence of either inactivating mutations of hMLH1 and hMSH2 or promoter hypermethylation of hMLH1. None of the tumors showed MSI at the BAT26 locus. These findings suggest that defective MMR does not contribute significantly to hepatocellular carcinogenesis.
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PMID:hMLH1 and hMSH2 expression in human hepatocellular carcinoma. 1149 37

MMR gene mutations and MSI are not found in all clinically diagnosed HNPCC families. We evaluated whether MMR genotyping and tumor MSI analysis could identify distinct clinical subgroups among HNPCC families. Twenty-nine clinical HNPCC families were divided into 3 groups: A, families with hMLH1 or hMSH2 gene mutations; B, MMR gene mutations not present but MSI present in at least 50% of tumors tested; C, mutational and MSI analyses negative. We evaluated tumor spectrum, age at onset, risk of cancer in the follow-up and survival for CRC in the 3 groups. Tumors of the target organs in HNPCC (colon and rectum, endometrium, ovary, small bowel, stomach, renal pelvis and ureter) were more frequent in the first 2 groups than in the latter. Colon cancer was more frequently located in the proximal colon and showed an earlier age at onset in families with MMR gene mutation or with MSI than in families with stable tumors. Comparing the occurrence of tumors in the follow-up, in the first 2 groups patients younger than 50 years had a higher RR, which was particularly marked for CRC (RR = 18.6 for group A vs. group C, RR = 16.7 for group B vs. group C). CRC patients in the first 2 groups had a better clinical prognosis. The results of molecular analysis could distinguish, within clinically defined HNPCC families, different subgroups to which specific programs of surveillance could be addressed.
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PMID:Clinical and biologic heterogeneity of hereditary nonpolyposis colorectal cancer. 1149 33

Microsatellite instability (MSI) is caused by the dysfunction of mismatch repair genes, such as hMLH1, hMSH2. Loss of hMLH1 expression and methylation of CpG sites in hMLH1 promoter are frequently present in sporadic colorectal cancer with MSI. In this study, by transient transfection assay with constructs containing different lengths of hMLH1 promoter and a luciferase reporter gene, we located a proximal region of hMLH1 promoter, which plays a main role in regulating the gene. The fact that luciferase activities were high in all host cell lines regardless of their hMLH1 expression levels indicates that the transcription machinery is intact even in non-expressing cells. When hMLH1 promoter was in vitro methylated before transfection, the luciferase activities in the transfectants were significantly reduced. This observation indicates that methylation causes the inhibition of hMLH1 promoter activity. By electrophoretic mobility shift assay (EMSA), we identified a CCAAT box in this region, which specifically bound transcription factor CBF. Mutations in CCAAT box not only inhibited its binding to CBF factor, but also reduced its ability to drive the expression of luciferase gene. The role of CBF in activating transcription was further substantiated by inhibition of promoter activity with a plasmid expressing a dominant negative CBF-B mutant. Methylation at a CpG site two base pairs upstream of the CCAAT box inhibited the binding of CBF to CCAAT box. We conclude that methylation of an adjacent CpG site inhibits binding of the CBF transcription to the corresponding CCAAT box, and is one of the causes of hMLH1 gene silencing in colon cancer cells.
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PMID:Methylation in hMLH1 promoter interferes with its binding to transcription factor CBF and inhibits gene expression. 1170 38

Hodgkin and Reed-Sternberg (H/RS) cells are characterized by chromosomal instability. Nevertheless, neither specific nor consistent chromosomal alterations could be characterized in H/RS cells. Microsatellite instability (MSI) is another form of genomic instability but its role in the pathogenesis of classical Hodgkin's disease (cHD) has not been investigated so far. We analyzed MSI and mismatch repair (MMR) protein expression in H/RS cells of cHD in order to assess genomic instability in these cells. Using a sensitive single cell approach, MSI-low was detected in a portion of single cells of the H/RS cell line L1236. Mutations of genes encoding for hMSH2 and hMLH1 were excluded by RT-PCR in L1236 cells. An analysis of pooled single H/RS cells of seven primary cases of cHD showed loss of heterozygosity for some allelic markers but absence of MSI in all 7 cases. Owing to a tight correlation between MSI-high, inactivating mutations of MMR genes and MMR protein expression in colon cancer, MMR protein expression commonly is used as a marker for MSI. In order to screen additional primary cases of cHD for MSI, we performed immunohistochemistry for hMSH2 and hMLH1 in 6 of the 7 cases analyzed by single cell PCR and 20 additional cases of cHD. H/RS cells from 25 out of 26 cases showed a nuclear staining pattern for hMSH2 and hMLH1 similar to germinal center B cells of non-malignant lymph nodes. These results indicate a proficient MMR system in most H/RS cells. It is concluded that a defect MMR system is unlikely to contribute to the malignant phenotype and genomic instability of H/RS cells in cHD.
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PMID:Proficient mismatch repair protein expression in Hodgkin and Reed Sternberg cells. 1177 65

Silencing of hMLH1 expression by aberrant hMLH1 promoter methylation accounts for the majority of sporadic colon cancers with microsatellite instability. We have previously shown hMLH1 silencing is biallelic and actively maintained. To study the mechanism of aberrant hMLH1 methylation, we assayed whether an hMLH1 methylated cell could transfer methylation and silencing to an exogenous hMLH1 promoter in somatic cell hybrids between hMLH1 methylated-silenced and hMLH1 unmethylated-expressing colon cancer cells. Conversely, we assayed whether these hybrids could reactivate expression of initially methylated and silenced hMLH1 alleles. Compellingly, within the hybrids each hMLH1 allele remained unchanged, retaining the expression status of its parental cell of origin. This chromosomal autonomy may not be simply determined by DNA methylation, as it is reasserted after experimentally forced demethylation of all hMLH1 alleles in the hybrids. Confirming findings included hMLH1 methylated cells being unable to methylate single transferred exogenous hMLH1 expressing chromosomes or transfected hMLH1 reporter constructs. hMLH1 silencing does not conform to either a dominant or recessive model, and is not determined by trans-acting factors differing between hMLH1 expressing or silenced genomes. We posit that hMLH1 methylation is dependent on and maintained by cis chromosomal marks, whose nature remains to be elucidated.
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PMID:Chromosomal autonomy of hMLH1 methylation in colon cancer. 1185 87

A family history of colorectal cancer has been consistently associated with an increased risk of developing colon cancer. However, there is limited information on the association between family history of colorectal cancer and genetic alterations that occur in colon tumors. In this study, we evaluate the association among genetic alterations of Ki-ras and p53, microsatellite instability and having a family history of colorectal cancer in a study of incident colon cancer cases (n = 1993) and population-based controls (n = 2,410). Although there was a slight nonsignificant increase in risk of having an unstable tumor among those with a family history of colorectal cancer, this increase in risk disappeared after excluding those people with a known mutation in either of the mismatch repair genes hMLH1 or hMSH2. A family history of colorectal cancer was not associated with Ki-ras mutations overall, although those with a G to T mutation of the second base of codon 12 were more likely to have a family history of colorectal cancer than were those without this specific type of Ki-ras mutation. Cases with p53 mutations were less likely to have a family history of colorectal cancer than were cases without a p53 mutation. We believe that, given the general lack of association between having a family history of colorectal cancer and genetic alterations in tumors, these alterations are acquired through disease pathways that involve exposure from diet, lifestyle or other environmental factors.
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PMID:Associations between family history of colorectal cancer and genetic alterations in tumors. 1185 62


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