Maintenance of Epigenetic Memory in Cloned Embryos

Abstract

Different cell types have characteristic patterns of gene expression. Once a cell hasdifferentiated, its daughter cells nearly always differentiate in the same way. Themaintenance of cell lineage involves either instructions from a cell’s surroundings orthe inheritance of memory from a parent cell. In normal development, thedifferentiation state of a cell is remarkably stable and irreversible. However thetransplantation of a somatic cell nucleus to an enucleated egg often leads to acomplete reprogramming of gene expression. We summarize here the results of someAmphibian nuclear transfer experiments that reveal a memory of gene expression.This and some other experiments exemplify epigenetic memory that persists throughmany cell divisions. In the case of nuclear transfer experiments, the activelytranscribed state of a gene can be propagated through many cell divisions in theabsence of the stimulus that first induced the activity of this gene. We discuss thepossible basis of these two examples of persistent epigenetic memory, namelychanges at DNA methylation, and histone modifications.