Methylation of DNA

Two states can be distinguished in chromatin heterochromatin and euchromatin. The two states describe for every cell type a characteristic difference in degree of condensation and transcription activity of DNA. Genes located in the condensed heterochromatin can not be transcribed, whereby genes located in euchromatin are accessible for transcription. 

An essential instrument for the suppression of transcription activity in heterochromatin, as well as for the differential regulation in euchromatin, is the methylation of DNA on the C5 atom of cytidine in the CpG sequence (Fig. 1.43). CpG sequences occur imevenly distributed in the genome. They may be concentrated in CpG islands. Higher eucaryotes possess a characteristic distribution pattern of 5-methyl cytidine (m C), which remains intact upon cell division. Mechanisms must therefore exist to ensure that the methylation pattern is precisely retained in the daughter cells following cell division. A methyl transferase that carries out hemi-methylation in the CpG sequences (Fig. 1.43) is responsible for the inheritance of the methylation pattern. The methyl group is derived from S-adenosyl methionine. The preferential substrates for the hemi-methylation are DNA sequences in which the complementary strand is already methylated. Such a hemi-methylation occurs, for example, shortly after replication of the sequence. 

Based on the following observations, methylation can be viewed as a mechanism of 

A strong correlation exists between the methylation status at CpG sequences and the 

In summary, we now know of three types of epigenetic modifications that are used to maintain a repressed stae of chromatin and to control the flux of information from DNA to RNA (review Richards and Elgin, 2002). These modifications are (Fig. 1.39)  

Transition of active to inactive states of chromatin (and vice versa) is controlled by epigentic modifications including acteylation of N-terminal Lys residues of histones H3 and H4, methylation of Lys9 of H3 and methylation of CpC sequences. CpC methylation induces binding of Methy-C-binding proteins (MeCP and MDB proteins) which in turn leads to recruitment of histone deacetylase activity (HDAC) establishing a repressed state of the chromatin. 

It is assumed that a self-reinforcing network of interactions exists between these modifications that is used in packaging heterochromatin and in stable silencing of euchro-matic genes. 

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Dacarbazine is activated by photodecomposition (chemical breakdown caused by radiant energy) and by enzymatic N-demethylation. Formation of a methyl carbonium ion results in methylation of DNA and RNA and inhibition of nucleic acid and protein synthesis. Cells in all phases of the cell cycle are susceptible to dacarbazine. The drug is not appreciably protein bound, and it does not enter the central nervous system. 

Wiaderkiewicz R, Walter Z, Reimschussel W. 1986. Sites of methylation of DNA bases by the action of organophosphoms insecticides in vitro. Acta Biochim Pol 33 73-85. 

In contrast, little is known about the DNA adducts formed from cyclic nitrosamines. Early studies indicated methylation of DNA by certain cyclic nitrosamines but these were not confirmed (25, 71, 72). Adducts from NPYR and NMOR have been detected but remain mostly uncharacterized (25, 61). The nature of the... 

For patients who do not respond to hydroxyurea, 5-azacytidine and 5-aza-2 -deoxycytidine (decitabine) may be useful. Both induce HbF by inhibiting methylation of DNA, preventing the switch from y- to (i-globulin production. Decitabine appears to be safer and more potent than 5-azacytadine. In a small study in adults refractory to hydroxyurea, decitabine 0.2 mg/kg subcutaneously one to three times weekly was associated with an increase in HbF in all patients. Additionally, RBC adhesion was reduced. Neutropenia was the only significant toxicity reported.6... 

Connell, J.R. and Medcalf, A.S. (1982). The induction of SCE and chromosomal aberrations with relation to specific base methylation of DNA in Chinese hamster cells by N-methyl-n-nitrosourea and dimethyl sulphate. Carcinogenesis 3 385-390. 

Starratt AN, Bond EJ. 1988. In vitro methylation of DNA by the fumigant methyl bromide. J Environ Sci Health [B] 23 513-524. 

Local chromatin-modifying activities Acetylation of histones increases gene expression (many genes) Methylation of DNA silences genes in genetic imprinting (Prader-Willi and Angelman syndromes)... 

The methylation of DNA at CpG islands has also turned out to be an important regulator for cell development, the differentiated proteome and the regulation of cell survival [237,238]. Indeed the implications of this chemical modification have been linked to DNA accessibility, chromatin fluidity and cell transformation [239,240]. DNA methylation is required for genomic stability and believed to act as an inert epigenetic marker in germinal cells and preimplantation embryos [238]. Presumably, DNA methylation is required for the heritable transmission of chromatin structure, which prevents the expression of terminally silenced genes in differentiated tissues, and provides a host-defense mechanism against parasitic transposable elements [241]. 

DNA methylation patterns are heritably propagated in somatic lineage cells, but in some circumstances methylation of DNA is a dynamic process, especially during early embryogenesis. Remodeling of DNA methylation levels and patterns have been observed during development of many vertebrate species [5 9], the most dramatic example being the rapid demethylation of the mouse sperm DNA few... 

DNMT3L connects unmethylated lysine 4 of histone H3 to de novo methylation of DNA. Nature, 448 (7154), 714-717. 

CS-cytosine methylation of DNA mechanistic implications of new ystal structures for HhaL methyitransferase-DNA-AdoHcy complexes. Journal of Molecular Biology, 261 (5), 634-645. 

Fig. 1.43. The methylation of DNA 5-methyl-cytidine and maintenance methylation. a) The methylation of cytidine residues on DNA is catalyzed by a methyl transferase that employs S-ade-nosine methionine as a methyl group donor. The peferable substrate for the methyl transferase are hemi-methylated CpG sequences. 5-aza-cytidine is a specific inhibitor of methyl transferses. b) The methylation pattern of DNA remains intact upon DNA replication and is passed on to the daughter cells. The newly synthesized strands are unmethylated immediately after DNA rephca-tion. The methyltransferase uses the previously methylated parent strand as a matrix to methylate the CpG sequences of the newly synthesized strand.

Teodoridis JM, Hall J, Marsh S et al. CpG island methylation of DNA damage response genes in advanced ovarian cancer. Cancer Res 2005 65 8961-8967. 

In the experiment described in Section 3.1.3, in which Syrian hamsters were given hydrazine sulfate in the drinking-w ater for two years, the levels of methylation of DNA guanine in liver, kidney and lung were measured. Both NI- and Gs-methylguanme were readily detectable at six months of exposure, but only trace amounts of these bases were detected after 12 months of exposure these bases were again detected in liver DNA at exposure times of 18 and 24 months (Bosan et al., 1987). 

Lambert, C.E. Shank, R.C. (1988) Role of formaldehyde hydrazone and catalase in hydrazine-induced methylation of DNA guanine. Carcinogenesis, 9, 65-70... 

In female mammalian cells most of the genes on one of the two X-chromosomes are completely inactivated. DNA methylation plays a major role in this process.244 245 A perfect correlation has been observed between 5 -methylation of cytosines in CpG islands and inactivation of X-chromosome genes.246 Methylation may also play a role in recombination and repair.247 Methylation of DNA decreases with increasing age.248 It increases as a result of oncogenic transformation of cells.249 Some other modifications of DNA largely limited to bacteriophages are discussed on p. 234 247/250... 

The most serious toxicologic effect of hydrazine is its ability to indirectly cause methylation of DNA, leading to cancer. Inhalation of hydrazine has been linked to lung cancer. 

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