Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:2.1.1.37 (DNA methyltransferase)
 4,983 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Tumor suppressor gene inactivation is a crucial event in oncogenesis. Gene inactivation mechanisms include events resulting in loss of heterozygosity (LOH), gene mutation, and transcriptional silencing. The contribution of each of these different pathways varies among tumor suppressor genes and by cancer type. The factors that influence the relative utilization of gene inactivation pathways are poorly understood. In this study, we describe a detailed quantitative analysis of the three major gene inactivation mechanisms for a model gene at two different genomic integration sites in mouse embryonic stem (ES) cells. In addition, we targeted the major DNA methyltransferase gene, Dnmt1, to investigate the relative contribution of DNA methylation to these various competing gene inactivation pathways. Our data show that gene loss is the predominant mode of inactivation of a herpes simplex virus thymidine kinase neomycin phosphotransferase reporter gene (HSV-TKNeo) at the two integration sites tested and that this event is significantly reduced in Dnmt1-deficient cells. Gene silencing by promoter methylation requires Dnmt1, suggesting that the expression of Dnmt3a and Dnmt3b alone in ES cells is insufficient to achieve effective gene silencing. We used a novel assay to show that missense mutation rates are also substantially reduced in Dnmt1-deficient cells. This is the first direct demonstration that DNA methylation affects point mutation rates in mammalian cells. Surprisingly, the fraction of CpG transition mutations was not reduced in Dnmt1-deficient cells. Finally, we show that methyl group-deficient growth conditions do not cause an increase in missense mutation rates in Dnmt1-proficient cells, as predicted by methyltransferase-mediated mutagenesis models. We conclude that Dnmt1 deficiency and the accompanying genomic DNA hypomethylation result in a reduction of three major pathways of gene inactivation in our model system.
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PMID:Reduced rates of gene loss, gene silencing, and gene mutation in Dnmt1-deficient embryonic stem cells. 1160 95

Tumor suppressor genes can become inactivated in cancer via hypermethylation of their promoter. The retinoic acid receptor beta (RARbeta) gene is expressed from two distinct promoters, both of which have CpG islands. RARbeta1 is expressed primarily during embryogenesis, whereas RARbeta2 is expressed in adult tissues and hypermethylated in a number of cancer cells. We used combined bisulfite restriction analysis to evaluate their methylation in colorectal mucosa and tumors. Methylation of RARbeta1 was detected, with a mean of 2% in normal colon tissues in young subjects (< 32 years), and 16% in older subjects (> 75 years) (P < 0.001). Using paired normal/tumor tissue samples, we found higher mean methylation rate in tumors than in adjacent normal tissue (mean, 46% versus 16%; P < 0.001) and hypermethylation of RARbeta1 in all eight cell lines examined. By RT-PCR, RARbeta1 was not expressed in normal adult colon tissues and its expression could not be efficiently activated in most cell lines by the DNA methyltransferase inhibitor 5-aza-2'-deoxycytidine (5-Aza-CdR). RARbeta2 methylation was also observed in normal colon tissues and was lower in young individuals than in older ones (mean, 11% versus 23%; P < 0.05). Among paired samples, RARbeta2 methylation was higher in tumor tissue than in normal tissue in 14 cases, vice versa in 7 cases, and equal in 6 cases. All eight cell lines were hypermethylated and did not express RARbeta2, but RARbeta2 expression could be reactivated easily by 5-Aza-CdR. We suggest that the embryonic RARbeta1 isoform is readily hypermethylated in aging colon mucosa and all colorectal cancers because of its lack of expression in normal tissues. The adult RARbeta2 isoform also shows age-related methylation in normal tissues but more variable methylation in colorectal cancer, perhaps because its expression offers continued protection against methylation or its silencing does not provide a selective advantage in the early stages of the disease.
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PMID:Methylation and regulation of expression of different retinoic acid receptor beta isoforms in human colon cancer. 1472 90

Tumor suppressor gene silencing by DNA hypermethylation contributes to tumorigenesis in many tumor types. This aberrant methylation may be due to increased expression and activity of DNA methyltransferases, which catalyze the transfer of methyl groups from S-adenosylmethionine to cytosines in CpG dinucleotides. Elevated expression of the maintenance DNA methyltransferase, DNA methyltransferase 1 (DNMT-1), has been shown in carcinomas of the colon, lung, liver, and prostate. Based on the nearly ubiquitous alterations of both DNA methylation and the retinoblastoma protein (pRb) pathway found in human cancer, we investigated a potential regulatory pathway linking the two alterations in murine and human prostate epithelial cells. Analysis of DNA methyltransferase levels in Rb-/- murine prostate epithelial cell lines revealed elevated Dnmt-1 levels. Genomic DNA sequence analysis identified conserved E2F consensus binding sites in proximity to the transcription initiation points of murine and human Dnmt-1. Furthermore, the Dnmt-1 promoter was shown to be regulated by the pRb/E2F pathway in murine and human cell lines of epithelial and fibroblast origin. In the absence of pRb, Dnmt-1 transcripts exhibited aberrant cell cycle regulation and Rb-/- cells showed aberrant methylation of the paternally expressed gene 3 (Peg3) tumor suppressor gene. These findings show a link between inactivation of the pRb pathway and induction of DNA hypermethylation of CpG island-containing genes in tumorigenesis.
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PMID:Regulation of DNA methyltransferase 1 by the pRb/E2F1 pathway. 1586 57

Tumor suppressor genes such as RASSF1A are often epigenetically repressed by DNA hypermethylation in neuroblastoma, where the MYCN proto-oncogene is frequently amplified. MYC has been shown to associate with DNA methyltransferases, thereby inducing transcriptional repression of target genes, which suggested that MYCN might play a similar mechanistic role in the hypermethylation of tumor suppressor genes in neuroblastoma. This study tested that hypothesis by using co-immunoprecipitation and ChIP to investigate MYCN-DNA methyltransferase interactions, together with MYCN knock-down and over-expression systems to examine the effect of MYCN expression changes on gene methylation, employing both candidate gene and genome-wide assays. We show that MYCN interacts with DNA methyltransferases and is recruited to the promoter region of RASSF1A. However, using four model systems, we showed that long-term silencing of MYCN induces only a small loss of DNA methylation at the RASSF1A promoter in MYCN amplified neuroblastoma cell lines and over-expression of MYCN does not induce any DNA methylation, suggesting that MYCN is not critical for DNA hypermethylation in neuroblastoma.
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PMID:MYCN is recruited to the RASSF1A promoter but is not critical for DNA hypermethylation in neuroblastoma. 2328 Jul 64