The internationally renowned academic journal Cell Stem Cell published the results, “Excluding Oct4 from Yamanaka Cocktail Unleashes the Developmental Potential of iPSCs”, which are conducted by Hans Schöler of the Max Planck Institute for Molecular Biomedicine in Germany and Guangming Wu, Guangzhou Regenerative Medicine and Health Guangdong Laboratory, on November 7, 2019. The study found that overexpression of Oct4 during reprogramming leads to epigenetic changes, thereby reducing the quality of iPSCs; and clarifying that removing Oct4 from "Yamanaka's reprogramming four factors" not only can be reprogrammed to produce iPSCs which have the same developmental potential as ESCs, and can greatly enhance the ability of iPSCs to produce all-iPSC mice through tetraploid embryo complementation.
Embryonic stem cells (ESCs) are isolated from the inner cell mass of the embryo and have the potential to develop into all cells of the individual. They are widely used in various fields of biological research, and are the center of stem cell and regenerative medicine research. Induced pluripotent stem cells (iPSCs) are somatic cells through the transcription factor or small molecule compound to reverse the terminally differentiated, resulting in reprograming them to obtain development and differentiation potential which is similar to ESCs. Japanese scientist Shinya Yamanaka discovered in 2006 that mouse adult somatic cells (such as skin fibroblasts) can be reprogrammed into iPSCs by expressing four transcription factors – Oct4, Sox2, Klf4 and cMyc (OSKM), and in 2007 it was discovered that OSKM can be applied to the induction of human iPSCs, which was awarded the Nobel Prize in Physiology and Medicine in 2012, opening up a new field of somatic cell reprogramming. iPSCs technology can obtain pluripotent stem cells from specific individuals and can be used for patient-specific regenerative therapy with unparalleled clinical application potential. However, it has been reported that the developmental potential of iPSCs is not completely equivalent to the “gold standard” ESCs. Many iPSC lines obtained by being induced reprogramming by OSKM cannot support effective normal differentiation, indicating that the process of reprogramming is not enough “perfect” and “efficient”. These defects are manifested in changes in iPSCs in terms of apparent modification levels. For example, loss of epigenetic silencing of specific loci in a genome inherited from one of the parents, which is loss of genetic imprinting (LOI). It still not clear that the exact source of these changes or the mechanism of action.
A number of studies on reprogramming factors have shown that Oct4 seems to be an irreplaceable core factor. However, in this work, the authors and their team tried different combinations of transcription factors, in which they deleted Oct4 from the widely used tetO-OKSM polycistronic reprogramming vector to generate a negative control to compare different reprogramming ability of POU family factors. Surprisingly, compared to OSKM, the overexpression of SKM without Oct4 can be efficiently reprogrammed into iPSC produced by fibroblast with only a slight delay while previously people have not successfully obtained iPSCs using SKM. Moreover, the authors found differences from previous studies because Yamanaka and many others used retroviral vectors in their experiments, which were rapidly silenced by reprogramming factors, thereby terminating the reprogramming process.
The founder of iPS technology, Yamanaka Yamanaka, also pay great attention on it and indicates his team will try to establish this reprogramming system in other cell types, especially in human adult cells. He said: "if this works in adult human cells, it will be a huge advantage for the clinical applications of iPS cells."
Oct4, Sox2 and Klf4 are widely considered to be pioneering factors, but their function is highly dependent on cofactors and cellular environment. The data of this study provide a new perspective, which is that Oct4 abnormal gene activation may adversely affect the quality of iPSC. Thus, reprogramming strategies need to be further improved to eliminate cancer risk resulted in epigenetic aberrations caused by overexpression of reprogramming factors and improve the pluripotency of iPSCs.