Epigenetic modifications in the embryonic and induced pluripotent stem cells.

Epigenetic modifications are involved in global reprogramming of the cell transcriptome. Therefore, synchronized major shifts in the expression of many genes could be achieved through epigenetic changes. The regulation of gene expression could be implemented by different epigenetic events including histone modifications, DNA methylation and chromatin remodelling. Interestingly, it has been documented that reprogramming of somatic cells to induced pluripotent stem (iPS) cells is also a typical example of epigenetic modifications. Additionally, epigenetic would determine the fates of almost all cells upon differentiation of stem cells into somatic cells. Currently, generation of iPS cells through epigenetic modifications is a routine laboratory practice. Despite all our knowledge, inconsistency in the results of reprogramming and differentiation of stem cells, highlight the need for more thorough investigation into the role of epigenetic modification in generation and maintenance of stem cells. Besides, subtle differences have been observed among different iPS cells and between iPS and ES cells. Although, a handful of detailed review regarding the status of epigenetics in stem cells has been published previously, in the current review, an abstracted and rather simplified view has been presented for those who want to gain a more general overview on this subject. However, almost all key references and ground breaking studies were included, which could be further explored to gain more in depth knowledge regarding this topic. The most dominant epigenetic changes have been presented followed by the impacts of such changes on the global gene expression. Epigenetic status in iPS and ES cells were compared. In addition to including the issues related to X-chromosome reactivation in the stem cells, we have also included loss of imprinting for some genes as a major drawback in generation of iPS cells. Finally, the overall impacts of epigenetic modifications on different aspects of stem cells has been discussed, including their use in cell therapy.