Epigenetic inheritance of cell differentiation status.

Epigenetic modifications influence gene expression pattern and provide a unique signature of a cell differentiation status. Without external stimuli or signalling events, this cell identity remains stable and unlikely to change over many cell divisions. The epigenetic signature of a particular cell fate therefore needs to be replicated faithfully in daughter cells; otherwise a cell lineage cannot be maintained. However, the mechanism of transmission of cellular memory from mother to daughter cells remains unclear. It has been suggested that the inheritance of an active or silent gene state involves different kinds of epigenetic mechanisms, e.g. DNA methylation, histone modifications, replacement of histone variants, Polycomb group (PcG) and Trithorax group (TrxG) proteins. Emerging evidence supports the role of histone variant H3.3 in maintaining an active gene status and in remodelling nucleosomal composition. Here we discuss some recent findings on the propagation of epigenetic memory and propose a model for the inheritance of an active gene state through the interaction of H3.3 with other epigenetic components.