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Tumors are caused by the demethylation of DNA sections. But what causes the demethylation?
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In mammalians, demethylation of specific promoter regions often correlates with gene activation; inversely, dense methylation of CpG islands leads to gene silencing, probably mediated by methyl-CpG binding proteins.
Cancer Biol Ther. 2004 Nov 9;3(11)
DNA Hypomethylation Is Prevalent Even in Low-Grade Breast Cancers.
Jackson K, Yu MC, Arakawa K, Fiala E, Youn B, Fiegl H, Muller-Holzner E, Widschwendter M, Ehrlich M.
Hypomethylation of some portions of the genome and hypermethylation of others are very frequent attributes of human cancer. We previously showed that cancer-associated DNA hypomethylation often involves satellite 2 (Sat2), the main DNA component of the large juxtacentromeric (centromere-adjacent) heterochromatin of chromosome 1. In this study, we compared methylation of Sat2 and centromeric satellite DNA (Satalpha) as well as overall genomic methylation in 41 breast adenocarcinomas of known tumor grade and stage, 16 non-neoplastic breast tissues (mostly fibroadenomas), and a variety of normal somatic tissue controls. The cancers were significantly hypomethylated at Sat2 relative to the fibroadenomas or normal somatic tissues and at Satalpha relative to the normal somatic tissues. However, unlike Sat2, Satalpha did not display significant differences in methylation between the cancers and the non-neoplastic breast tissues. Therefore, hypomethylation at Sat2 is a much better marker of breast cancer than is Satalpha hypomethylation. There was a significant association of Sat2 hypomethylation with global DNA hypomethylation in the cancers but not with tumor grade, stage, axillary lymph node involvement, or hormone receptor status. Extensive cancer-associated hypomethylation of juxtacentromeric satellite DNA and global DNA hypomethylation were common even in grade-1 or stage-1 carcinomas, which suggests that demethylation of the genome is an early event in breast carcinogenesis.
J Pharmacol Exp Ther. 2005 Feb;312(2):525-36.
5-aza-Cytidine Is a Potent Inhibitor of DNA Methyltransferase 3a and Induces Apoptosis in HCT-116 Colon Cancer Cells via Gadd45- and p53-Dependent Mechanisms.
Schneider-Stock R, Diab-Assef M, Rohrbeck A, Foltzer-Jourdainne C, Boltze C, Hartig R, Schonfeld P, Roessner A, Gali-Muhtasib H.
Methyltransferase inhibitors commonly used in clinical trials promote tumor cell death, but their detailed cytotoxic action is not yet fully understood. A deeper knowledge about their apotosis-inducing mechanisms and their interaction with DNA methyltransferases (DNMTs) DNMT1, DNMT3a, and DNMT3b might allow the design of more effective drugs with lower cytotoxicity. 5-aza-cytidine (5-aza-CR), a potent inhibitor of DNMT1, is known to induce demethylation and reactivation of silenced genes. In this study, we investigated the p53 dependence of apoptotic, cell cycle, and growth inhibitory effects of 5-aza-CR, as well as the influence on the expression level of DNMT1, DNMT3a, and DNMT3b in the colon cancer cell line HCT-116. Exposure to 5-aza-CR induced the up-regulation of genes promoting cell cycle arrest and DNA repair (p21(WAF1) and GADD45) or apoptosis (p53, RIPK2, Bak1, caspase 5, and caspase 6). In parallel, there was a down-regulation of antiapoptotic Bcl2 protein and the G(2)/M-mediator cyclin B1. Co-incubation with pifithrin-alpha (PFT-alpha), a selective p53 inhibitor, restored GADD45, Bcl2, cyclin B1, and p21(WAF1) expression levels and almost completely reversed the growth inhibitory, cell cycle, and apoptotic effects of 5-aza-CR. 5-aza-CR treatment caused global demethylation and reactivation of p16(INK4) expression. There was a marked decrease in DNMT1 and DNMT3a mRNA expression, with PFT-alpha reversing these effects. However, 5-aza-CR treatment did not modulate DNMT3b expression. Our data demonstrate that 5-aza-CR action in HCT-116 is mediated by p53 and its downstream effectors p21(WAF1) and GADD45. This is the first report to show a link between p53 and regulation of DNMT1 and de novo methyltransferase DNMT3a.
DNA demethylation is a process in which the methyl groups attached to DNA are removed, resulting in changes in gene expression. In tumors, this process can lead to abnormal gene expression, which can contribute to the development and progression of cancer.
The exact cause of DNA demethylation in tumors is not fully understood. However, it is believed that changes in the activity of enzymes that regulate methylation levels, as well as alterations in the tumor microenvironment, can contribute to this process.
Yes, DNA demethylation in tumors is a reversible process. This means that the changes in gene expression caused by demethylation can be reversed by re-methylation, which can potentially be targeted for therapeutic interventions.
DNA demethylation can lead to changes in gene expression, including the activation of oncogenes (genes that promote tumor growth) and the inactivation of tumor suppressor genes (genes that prevent tumor growth). This can promote the growth and spread of cancer cells and contribute to tumor progression.
Yes, DNA demethylation can serve as a potential biomarker for tumor diagnosis and treatment. Changes in methylation patterns have been observed in various types of cancer, and these changes can be detected using specialized techniques. Additionally, targeting DNA demethylation pathways could potentially be used as a therapeutic approach for treating tumors.