DNA hypomethylation

Decitabine (5-aza-deoxycytosine) is an analog of the nucleoside 2 -deoxycytidine. It is believed to exert its antineoplastic effects after phosphorylation and direct incorporation into DNA and by inhibition of the enzyme DNA methyltransferase, causing hypomethylation of DNA and cellular differentiation or apoptosis. DNA hypomethylation is achieved at concentrations below those required to significantly inhibit DNA synthesis, which may promote restoration of function to genes associated with control of cellular differentiation and proliferation. Cytotoxicity in rapidly dividing cells may also result from covalent adducts between DNA methyltransferase and decitabine. 

DIMS was identified as the responsible gene for one of DNA hypomethylation mutants in Neurospora crassa (Tamaru and Selker, 2001). This is the first report that histone (H3K9) methylation regulates DNA methylation. Furthermore, it is reported that this regulation is mediated through the HPl recruitment to the tri-methylated H3K9 loci (Freitag et al, 2004), but it remains to be elucidated whether DNA methylation (DNA methyltransferase) is directly controlled (recruited) by HPl. 

The covalent trapping of the enzyme leads to a depletion of the cellular pool of DNMTs and subsequent DNA hypomethylation. This in turn results in activation with respect to the reactivation of silenced genes. Additionally, the covalently trapped DNMT may inhibit RNA and DNA polymerases, which leads to an inhibition of protein biosynthesis and DNA strand breaks. This may lead to apoptosis and hence cytotoxicity. Thus, it is not easy to dissect the reasons for the clinical efficacy of these inhibitors in terms of real epigenetic and plain cytotoxic effects [81]. 

Castro R, Rivera I, Struys EA, Jansen EE, Ravasco P, Camilo ME, Blom HJ, Jakobs C, Tavares de Almeida I (2003) Increased homocysteine and S-adenosylhomocysteine concentrations and DNA hypomethylation in vascular disease. Clin Chem 49 1292-1296... 

The hallmark and molecular characteristic of progression is karyotype instability. This is due to disruption of mitotic apparatus, alteration of telomere function, DNA hypomethylation, chromosome translocations and recombination, gene amplification, and gene transposition. There is also alteration of mismatch repair genes in some cancers. 

MICROWAVE HEAT-ASSISTED EVALUATION OF GLOBAL DNA HYPOMETHYLATION... 

Hernandez-Blazquez, F. J., Habib, M., Dumollard, J.-M., Barthelemy, C., Benchaib, M., de Capoa, A., and Niveleau, A. 2000. Evaluation of global DNA hypomethylation in human colon cancer tissues by immunohistochemistry and image analysis. Gwi47 689-693. 

Zhao, C.Q., Young, M.R., Diwan, B.A. et al. (1997) Association of arsenic-induced malignant transformation with DNA hypomethylation and aberrant gene expression. Proceedings of the National Academy of Sciences of the United States of America, 94(20), 10907-912. 

Yi P, Melnyk S, Pogribna M, Pogribny IP, Hine RJ, James SJ. 2000. Increase in plasma homocysteine associated with parallel increase in plasma S-adenosylhomocysteine and lymphocyte DNA hypomethylation. J Biol Chem 275 29318-29323. 

In summary, decitabine (1), a deoxycytidine derivative, is a potent antileukemic that is able to induce in vitro gene activation and cellular differentiation by a mechanism involving DNA hypomethylation. It is generally most beneficial in patients with high-risk MDS or in MDS patients with intermediate-2 or high-risk features. Its process synthesis and academic synthetic approaches have been summarized in this chapter. 

Okoji RS, Yu RC, Maronpot RR, Froines JR (2002) Sodium arsenite administration via drinking water increases genome-wide and Ha-ras DNA hypomethylation in methyl-deficient C57BL/6J mice. Carcinogenesis 23(5) 777-785... 

Azacytidine was approved in 2004 for the treatment of patients with myelodysplastic syndrome, a disorder of hematopoietic cell maturation that can progress to acute leukemia. Azacytidine, a cytidine nucleoside analog, causes hypomethylation of DNA. Hypomethylation may normalize the function of genes that control cell differentiation and proliferation, promoting normal cell maturation. ... 

Rosty C, Ueki T, Argani P, et al. Overexpression of S100A4 in pancreatic ductal adenocarcinomas is associated with poor differentiation and DNA hypomethylation. Am J Pathol. 2002 160 45-50. 

A recent report has shown that DNA hypomethylation may contribute to the induction of tumors in mice via an epigenetic mechanism causing chromosomal instability. In addition to facilitating neoplastic transformation, alterations in DNA methylation patterns have been shown to exert effects on embryonic development and acquired drag resistance to cytotoxic agents. Recent data from oiu laboratory have demonstrated the... 

Aberrant gene expression due to DNA hypomethylation has also been observed in some breast cancers,in contrast to other findings that did not reveal any significant changes in DNA methylation patterns. " In another study, global DNA methylation analyses were performed in breast lesions and respective adjacent parenchyma DNA hypomethylation was significantly increased in breast carcinomas and proved to be a causative factor in tumor development. ... 

Alterations in methylation status due to DNA hypomethylation may produce chromosomal instability, rearrangement, and mutations in critical genes, whereas DNA hypermethylation may lead to silencing of... 

Soares, J., Pinto, A.E., Cunha, C.V., Andre, S., Barao, L., Sousa, J.M., and Cravo, M., Global DNA hypomethylation in breast carcinoma correlation with prognostic factors and tumor progression. Cancer, 85 (1), 112-118, 1999. 

Jost, J.P., Sahiz, H.P., McEwan, I. et ed. (1990). Tissue specific expression of avian vitellogenin gene is correlated with DNA hypomethylation and in vivo spedfic-DNA interactions. Philos. Trans. R. Soc. Land, 326B, 231—40. 

Tumorigenidty The epigenetic effects of arsenic in relation to carcinogenesis have been reviewed, including its effects on DNA meth-ylation, global DNA hypomethylation, gene promoter methylation, histone acetylation, histone methylation, histone phosphorylation, and miRNA expression [32 ]. 

Depletion of DNA methyltransferase also causes DNA hypomethylation and induces differentiation of hemopoietic cells [23 ]. Decitabine was approved by the FDA for the treatment of myelodysplastic syndromes in 2006. Decitabine also has activity in the treatment of acute myeloid leukemia in patients aged over 60 years [24 j. 

Wijermans P, Liibbert M, Verhoef G, Bosly A, Ravoet C, Andre M, Ferrant A. Low-dose 5-aza-2 -deoxycytidine, a DNA hypomethylating agent, for the treatment of high-risk myelodysplastic syndrome a multicenter phase II study in elderly patients. J Clin Oncol 2000 18(5) 956-62. 

Niacin Analogs That Induce Differentiation of Friend Erythroleukemia Cells are Able to Cause DNA Hypomethylation... 

Niacin analogs were compared at 10 mM for ejects on induction of differentiation, Hb accumulation, poly(ADP-ribose) synthetase in vitro, and on DNA methylase activity in cell-free assays and in growing cells (Table 1). None of these compounds were active as inhibitors of DNA methylase activity in cell-free assays. In contrast, these compounds were found to have differential effects on poly(ADP-ribose) synthetase activity in isolated nuclei, as measured by a modified procedure of Hayaishi (23). Inhibition of poly(ADP-ribose) synthetase by these compounds did not, as previously hypothesized, correlate with their ability to induce Hb synthesis and differentiation. N -MNA and 6-MNA were not able to inhibit poly(ADP-ribose) synthetase activity in isolated nuclei when present at 5 mM, whereas NAm and 5-MNA both possess inhibitory activity in vitro. When these compounds were incubated at 10 mM in cell culture for 24 hr, all those able to induce differentiation were also able to cause DNA hypomethylation, as judged by their effect on methyl-accepting abilities of DNA isolated from these cells. 

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