Carcinogenesis and the hypothesis of phylogenetic reversion.

Chemoprevention must target early molecular events involved in malignant transformation. The sequence of events leading from a normally functioning interphase cell to an uncontrolled tumor cell is only partially understood, impeding systematic design of chemopreventive agents. The respective roles of mutagenic and epigenetic mechanisms have not been definitively established. Also, traditional models do not appear to incorporate cellular response to events leading to carcinogenesis. A perspective on system response offered by complexity science elucidates the roles of feedback and control in maintaining functional stability during carcinogenesis. Carcinogenesis is seen as a process of epigenetic redifferentiation resulting in a cell behaving like an archetypal karyocyte free of growth restraints (phylogenetic reversion). Genes that evolved during the development of multicellular organisms, restraining uncontrolled growth and regulating intercell communication may be systematically silenced during carcinogenesis. The formation of heterochromatin, which results in epigenetic silencing by hypermethylation in CpG-dense islands, finds expression in the nuclear chromatin pattern. Karyometry is an integrating biomarker of chromatin pattern information that accommodates the possibility of multiple, differently ordered pathways and provides exquisite sensitivity, allowing detection of very early transformation events. Its use can monitor the impact of chemopreventive agents on the earliest events in progression to cancer.