Induced Tissue-Specific Stem Cells and Epigenetic Memory in Induced Pluripotent Stem Cells.

Induced pluripotent stem (iPS) cells have significant implications for overcoming most of the ethical issues associated with embryonic stem (ES) cells. The pattern of expressed genes, DNA methylation, and covalent histone modifications in iPS cells are very similar to those in ES cells. However, it has recently been shown that, following the reprogramming of mouse/human iPS cells, epigenetic memory is inherited from the parental cells. These findings suggest that the phenotype of iPS cells may be influenced by their cells of origin and that their skewed differentiation potential may prove useful in the generation of differentiated cell types that are currently difficult to produce from ES/iPS cells for the treatment of human diseases. Our recent study demonstrated the generation of induced tissue-specific stem (iTS) cells by transient overexpression of the reprogramming factors combined with tissue-specific selection. iTS cells are cells that inherit numerous components of epigenetic memory from donor tissue and acquire self-renewal potential. This review describes the "epigenetic memory" phenomenon in iPS and iTS cells and the possible clinical applications of these stem cells.