Stem cell chromatin patterns: an instructive mechanism for DNA hypermethylation?

Epigenetic gene silencing, and associated promoter CpG island DNA hypermethylation, is an alternative mechanism to mutations by which tumor suppressor genes may be inactivated within a cancer cell. These epigenetic changes are prevalent in all types of cancer, and their appearance may precede genetic changes in premalignant cells and foster the accumulation of additional genetic and epigenetic hits. These epigenetically modified genes constitute important categories of tumor suppressor genes including cell cycle regulators, pro-differentiation factors, and anti-apoptotic genes, and many of these genes are known to play a role in normal development. While the silencing of these genes may play an essential role in tumor initiation or progression, the mechanisms underlying the specific targeting of these genes for DNA hypermethylation remains to be determined. The large numbers of epigenetically silenced genes that may be present in any given tumor, and the clustering of silenced genes within single cell pathways, begs the question of whether gene silencing is a series of random events resulting in an enhanced survival of a premalignant clone, or whether silencing is the result of a directed, instructive program for silencing initiation reflective of the cells of origin for tumors. In this regard, the current review stresses the latter hypothesis and the important possibility that the program is linked, at least for silencing of some cancer genes, to the epigenetic control of stem/precursor cell gene expression patterns.