Cancer epigenetics: tumor heterogeneity, plasticity of stem-like states, and drug resistance.

The existence of subpopulations of cells in cancers with increased tumor-initiating capacities and self-renewal potential, often termed "cancer stem cells," is a much discussed and key area of cancer biology. Such cellular heterogeneity is very important because of its impact on therapy and especially states of treatment resistance. A major question is whether there is plasticity for evolution of these cell states during tumorigenesis that can involve movement between cell populations in a reversible fashion. In this review, we discuss the possible role of epigenetic abnormalities as well as genetic alterations in such dynamics and in the creation of cellular heterogeneity in cancers of all types.