The importance of epigenetic gene silencing to the progression of cancer-chromatin events involved

SY19-1 Loss of function for a large and growing list of important tumor suppressor genes, and candidate tumor suppressor genes, is now recognized to result from an association of aberrant promoter DNA methylation and attendant abnormal transcriptional silencing. One of the most important requirements for understanding this phenomenon, from both basic and translational perspectives, is to dissect the molecular mechanisms underlying both the establishment and maintenance of this transcriptional repression. In turn, these questions fit integrally with recent excitement in outlining the role of chromatin in modulating gene expression and how this regulation links to establishing DNA methylation patterns and how this DNA modification participates in transcriptional repression. In this talk, some of the latest understanding in the molecular steps mediating promoter methylation associated transcriptional repression will be reviewed. Currently, the overall scenario for transcriptional silencing in association with dense promoter DNA methylation may be viewed as a multilayered process mediated by recruitment, to methylated regions, of methyl-cytosine binding proteins ( MBP’s), the complexes in which they reside, and especially their involvement with histone de-acetylases ( HDAC’s), and critical modifications in the histone code, marked, particularly, by the methylation status of specific lysines on histone H3. Also, the DNA methyltransferases (DNMT’s) themselves, which catalyze DNA methylation, have the potential to act as transcriptional co-repressors which can recruit HDAC’s and other repressor proteins to sites of gene silencing. Importantly, the DNA methylation may be viewed as a final step which firmly locks the gene silencing into a stable and heritable state and which must be relieved before blocking other components of the “repression machinery” is effective in allowing re-expression of involved genes. For example, inhibiting HDAC activity, alone, in cancer cells does not lead to re-expression of densely DNA hypermethylated genes but will co-operate with initial inhibition of DNA methylation to do so. Specific aspects of the above events are important to stress. First it is essential to understand, for cancer cells, which of the three biologically active DNMT’s, 1, 3a, and 3b, are involved in initiation and maintenance of aberrant promoter hypermethylation. Less is known about initiation but most recent data suggest that chromatin mediation of silencing may precede the DNA methylation and involve histone modifications, such as methylation of lysines 9 and/or 27of histone H3 (H3-K9/K27-methylation), which may target DNA methylation to promoters. As for maintenance, unlike for the situation in mouse embryogenesis where DNMT1 is responsible for perpetuating DNA methylation patterns, many cancer cell types may require intense co-operativity between DNMT’s 1 and 3b to maintain their overall DNA methylation levels and specifically to maintain promoter hypermethylation and attendant gene silencing. Finally, the depth of the gene silencing in cancer cells appears to be extensive as reflected by the transcriptionally repressive histone modification present at the promoters of such genes. Thus, silencing methylation modifications of H3-K9 and H3-K7 are present and reflect characteristics of both facultative and pericenromeric heterochromatin which encompass very densely silenced genes. While reactivation of the genes can be induced by demethylating agents, the gene promoters do not revert to a euchromatin state and transcription appears to be occurring within a retained heterchromatin-like state. All of these above characteristics of epigenetically silenced cancer genes make them fascinating models for study of chromatin controlled regulation of gene expression states. Furthermore, there are many important translational implications of what has been learned so far in terms of the growing interest in targeting reversal of gene silencing as a cancer prevention and/or therapeutic strategy.