Examining the Role of DNA Methylation in Naive Pluripotent Stem Cells

The conventional serum-based culture media for mouse embryonic stem cells gives rise to a cell population that is considered metastable and exhibits heterogeneous properties reminiscent of both the naive and primed pluripotent states. These two stages of pluripotency can be represented in vivo by cells of the mouse inner cell mass and mouse post-implantation epiblast, respectively. Adding two inhibitors (2i) to the culture media can stabilize the cells in the naive ground state through inhibition of the MEK/ERK and GSK3-beta pathways. Naive state pluripotent stem cells represent an earlier stage of development and have advantages such as ease of homologous recombination and ability to contribute to chimeras. Epigenetic differences include global hypomethylation in the naive state, as well as downregulation of de novo DNA methyltransferases (Dnmt3a/3b) and upregulation of hydroxymethyltransferases. The proposed mechanism underlying the establishment of hypomethylation in the naive state is Prdm14-mediated repression of de novo Dnmt transcription, but the interdependence of the two major stages of pluripotency on methylation level is unclear. Whether the loss of de novo DNA methyltransferases and subsequent hypomethylation is a requirement for production of naive pluripotency is examined using knock-out and Dnmt3a/3b reconstitution cell lines. Understanding the dynamical control between these stem cell states will be important in realizing the full potential of stem cell therapies.