DNA methylation, inhibition

Relationships between histone methylation and DNA methylation and histone acetyation and DNA methylation have been reported [191,314,315], A similar relationship may exist between poly(ADP ribosylated) HI and DNA methylation. Inhibition of poly(ADP-ribose) polymerase with 3-aminobenzamide increases the susceptibility of L929 mouse fibroblast nuclei to be methylated by endogenous DNA methyltransferases [316,317], Further, there is evidence that poly(ADP ribosylation) protects CpG islands located at the 5 end of housekeeping genes from methylation [318], Future studies will likely reveal an interesting dynamic relationship between histone methylation, histone acetylation, and histone poly(ADP-ribosylation). 

The connection between DNA methylation and gene expression was first noticed when undifferentiated cells in culture were treated with the methylation inhibitor 5-azacytidine inhibition of methylation led to the appearance of stably inherited differentiated cell types and activation of previously silenced genes [107,108]. Ectopic DNA methylation inhibits gene expression [109], whereas drug-inhibition of methylation induces the expression of certain genes [110]. Transfection experiments... 

Boyes, J. and Bird, A., DNA methylation inhibits transcription indirectly via a methyl-CpG binding protein. Cell, 64 (6), 1123-1134, 1991. 

Moreno FS, S-Wu T, Naves MM, Silveira ER, Oloris SC, Costa MA, Dagli ML and Ong TP. 2002. Inhibitory effects of beta-carotene and vitamin A during progression phase of hepatocarcinogenesis involve inhibition of cell proliferation but not alterations in DNA methylation. Nutr Cancer 44 80-... 

Some xenobiotics may have divergent mechanisms of autoimmune responses. For example, hydralazine demonstrates adduct reactivity as well as inhibition of DNA methylation [68,73], while procainamide inhibits DNA methylation, forms immunogenic NPA, and disrupts clonal selection in the thymus [68, 72, 74], It is this complicated pattern of effects that makes assessment of autoimmune potential in the laboratory for new xenobiotics almost impossible. Animal models can sometimes be recreated to resemble human disease [74], and thus may be useful for therapy considerations, but are difficult to utilize for screening chemicals for hazard potential due to the diverse nature of autoimmunity mechanisms and physiological presentation. While evidence supports many different mechanisms for xenobiotic-induced autoimmune reactions, none have conclusively demonstrated the critical events necessary to lead to the development of autoimmune disease. Therefore, it is difficult to predict or identify xenobiotics that might possess the potential to elicit autoimmune disorders. 

Therefore, there is no doubt that inhibition of PARP leads to DNA hypermethylation in vivo. But what is the molecular mechanism behind this phenomenon To get an insight into this mechanism, PARPs were inhibited at different phases of the cell cycle [41]. Both the mRNA and protein levels of Dnmtl were precociously increased in Gl/S phase. These elevated Dnmtl levels, affecting the normal equilibrium between p21 and Dnmtl in this phase, led to higher than normal levels of the PCNA-Dnmtl complex, and thus, to higher DNA methylation of the sequences replicated in early S phase (CpG islands are early replicating) (see Fig. 3d). 

Transcriptional repression by DNA methylation is achieved by various modes of action. In one such mechanism, DNA methylation simply inhibits the binding of a transcription factor [17[. By a more complex means of action, a number of DNA methyl-binding proteins potentiate transcriptional silencing. In some cases, binding is accompanied by the action of an associated histone-modifying enzyme. 

The potency of zebularine is about 10-fold lower than for the azacytosines [73]. Zebularine also inhibits cytidine deaminase [75] which is involved in nucleoside catabolism and deactivates also for example azacitidine and its desoxy analog [76]. Thus, it increases the concentrations of nucleoside triphosphates for incorporation into DNA, the efficacy of DNA methylation and ultimately the anticancer activity of for example azacitidine [77, 78[. Zebularine is metabolized by aldehyde oxidase and ithasbeen shovm that its activity can be increases if an inhibitor of that enzyme, for example raloxifene is given in combination [79]. One big question about all epigenetic drugs is the origin of the observed selectivity towards cancer cells. For zebularine, it has been shown that much less activation towards triphosphate metabolites that can be incorporated into DNA occurs in normal muscle tissue as compared to cancer tissue [80]. 

Novel approaches to therapeutically downregulate DNA methylation include the DNMTl antisense compound MG98. However, clinical phase trials with the single drug showed no effect (partial or complete remission) or the effects could not be linked to DNMT inhibition [95-97]. 

Temozolomide Methylates DNA and inhibits DNA synthesis and function Brain cancer, melanoma Nausea and vomiting, headache and fatigue Myelosuppression, mild elevation in liver function tests, photosensitivity... 

Procarbazine Methylates DNA and inhibits DNA synthesis and function Flodgkin s and non-Flodgkin s lymphoma, brain tumors Central nervous system depression Myelosuppression, hypersensitivity reactions... 

The extent of methylation of a gene is correlated with its ability to transcribe. Given that DNA methylation usually reduces transcription, two important, closely related questions remain unanswered How is methylation regulated in vivo How does methylation interfere with transcription Since methylation is known not to interfere with the elongation phase of RNA synthesis, it seems likely that methylation blocks initiation. The binding of polymerase and other regulatory proteins at the initiation locus is sensitive to modification of these nucleotides. The precise inhibition mechanisms, however, await further elucidation. 

Cheng JC, Matsen CB, Gonzales FA, Ye W, Greer S, Marquez VE, Jones PA, Selker EU. Inhibition of DNA methylation and reactivation of silenced genes by zebularine. J Natl Cancer Inst 2003 95 399-409. 

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