Epigenetic memory: The role of the crosstalk between histone modifications and DNA methylation

Epigenetic memory allows different cells to maintain distinct gene expression patterns despite a common genetic code and plays a role in several biological processes. Chemical modifications to DNA and histones have appeared as critical mediators of epigenetic memory and much attention has gone into characterizing their dynamics. The network of positive feedback loops that these modifications form generates a rich set of dynamics that both recapitulate the traditional binary memory paradigm and also predict a new form of memory that we call analog memory. In this paper, we review models of chromatin modifications and describe how binary or analog memory hinge on the presence or lack of positive feedback loops between repressive histone modifications and DNA methylation. Future research using advanced genetic engineering tools will be able to validate the molecular interactions that dictate different forms of memory, and will thus deepen our understanding of how epigenetic memories form in different biological contexts.