A current view of the epigenome in mouse primordial germ cells

Primordial germ cells (PGCs) are undifferentiated germ line cells in embryos that emerge at early stages of embryonic development, and then differentiate into eggs or sperm in gonads to give rise to individuals of successive generations. During germ cell development, several dynamic changes in epigenetic modifications including DNA methylation and histone modifications occur, and these changes are thought to be reprogramming processes that are required for germ cells to confer totipotency to the zygote. Initially, the epigenetic status of particular gene loci in PGCs was studied, but more recently, genome-wide studies have provided more comprehensive views of the PGC epigenome. Mouse PGCs undergo global DNA demethylation that starts shortly after PGC specification in early embryos. Although the functional importance of global DNA demethylation is not fully understood, demethylation of imprinted genes is crucial for erasure of methylation-based imprinting, and demethylation of PGC-specific genes is crucial for proper transcriptional regulation. PGCs also have unique patterns of histone modification, such as hypomethylation of H3K9 and hypermethylation of H3K27, and experimental evidence suggests that the unique epigenetic modifications of histones are important to the proper development of PGCs.