Aberrant patterns of DNA methylation, chromatin formation and gene expression in cancer

Gene function in cancer can be disrupted either through genetic alterations, which directly mutate or delete genes, or epigenetic alterations, which alter the heritable state of gene expression. The latter events are mediated by formation of transcriptionally repressive chromatin states around gene transcription start sites and an associated gain of methylation in normally unmethylated CpG islands in these regions. The genes affected include over half of the tumor suppressor genes that cause familial cancers when mutated in the germline; the selective advantage for genetic and epigenetic dysfunction in these genes is very similar. The aberrant methylation can begin very early in tumor progression and mediate most of the important pathway abnormalities in cancer including loss of cell cycle control, altered function of transcription factors, altered receptor function, disruption of normal cell-cell and cell-substratum interaction, inactivation of signal transduction pathways, loss of apoptotic signals and genetic instability. The active role of the aberrant methylation in transcriptional silencing of genes is becoming increasingly understood and involves a synergy between the methylation and histone deacetylase (HDAC) activity. This synergy can be mediated directly by HDAC interaction with DNA methylating enzymes and by recruitment through complexes involving methyl-cytosine binding proteins. In the translational arena, the promoter hypermethylation changes hold great promise as DNA tumor markers and their potentially reversible state creates a target for cancer therapeutic strategies involving gene reactivation.