1.Background of research and development
For the realization of regenerative medicine, such as cell transplantation therapy, the formation of pluripotent stem cells carrying their own genetic information has been attempted to the present day. Some reports have shown successful acquisition of pluripotency by inducing initialization in somatic nuclei by transplanting-nuclei to enucleated unfertilized eggs, from which nuclei from somatic cells have been removed, and by using cell fusion technology for fusing somatic cells and embryonic stem (ES) cells via electrical stimulation and the like. However, there have been no reports of successful nucleus transplants in human cells. In addition, certain ethical issues remain unresolved. Cells produced by means of cell fusion have four times the number of chromosomes, constituting a major issue in terms of clinical application.
Under these circumstances, with the aim of forming ideal pluripotent stem cells that can be used in clinical application, Prof. Shinya Yamanaka, the prizewinner, succeeded in inducing pluripotency by transferring a small number of genes to somatic cells.
The following are five results from his research.
(1) When NANOG, which has been identified as a new gene with ES-cell-specific expression, is knocked-out, the pluripotent differentiation of ES cells is lost, and fatality immediately results to mouse embryos following implantation. In addition, in the absence of cytokine LIF, ES cells with NANOG forcibly expressed were able to maintain undifferentiated stateover the long term. NANOG is believed to play a central role in pluripotent cells maintaining an undifferentiated state. The name NANOG derives from Tir na nog, which means“land of the young.”
(2) By combining four factors that express specificity in ES cells and introducing them into fibroblasts derived from adult and fetal mice, pluripotent stem cells with high proliferative potential and pluripotent differentiation, similar to ES cells, were successfully formed. These cells were named induced pluripotent stem (iPS) cells.
(3) By changing the colony selection indicator from Fbx15 to NANOG, a gene expression pattern that even more closely approximated that of ES cells was demonstrated. Differential ability also compared favorably, and thus formation of the second-generation iPS cells was successfully achieved. These iPS cells also showed germline transmission.
(4) Using the same four factors used for the mice and improving transduction efficiency by means of retroviral vectors, the researcher succeeded in forming human iPS cells from fibroblasts derived from human adult skin, which came outsimilar to human ES cells in terms of morphology, proliferative potential, gene expression, pluripotency and the like.
(5) With three of the four factors-excluding Myc, iPS cells were also created from adult mouse and adult human skin cells. Of the 37 chimeric mice originating from mouse iPS cells that were generated by using the four factors, including Myc, 6 died within the first 100 days of life due to tumor formation. In contrast, none of the 26 chimeric mice originating from iPS cells that were made by introducing the three factors, without Myc, died from a tumor within the first 100 days of life, so it became clear that the safety level was improved by using the method not using Myc.
3.Meaning of the achievements
This series of the results on iPS cell research indicated a possibility of achieving a very simple method for cell nucleus initialization, which had conventionally been deemed impossible in mammals unless ES cells or embryos were used by many researchers. Additionally, it is believed that iPS cells, which can avoide ethical issues and immunological rejection , will substantially accelerate developments in regenerative medicine as innovative pluripotent stem cells. For these reasons, statements were issued to welcome these results by opponents of ES cell research, notably U.S. President George W. Bush and the Vatican. Henceforth, with the iPS cells, created by the Japanese researcher, induction methods will be improved, and safe and stable clones will be selected. It is expected that healthcare innovation will be brought by the use of patient-derived iPS cells to elucidate disease onset mechanisms and applying the iPS cell technology to cell transplantation therapy.
Reasons for Awarding Prize
Prof. Shinya Yamanaka succeeded in using cells he cultivated from structures of human origin to produce induced pluripotent stem (iPS) cells that maintain pluripotency. In doing so, he achieved technological innovation in the field of regenerative medicine.
Yamanaka thought that many of the initialization-inducing factor groups that ES cells have would have an important function in maintaining ES cell pluripotency. As a result of conducting numerous screenings, important factors in maintaining ES cell pluripotency were discovered, one after another. Of the factors that play an important role in maintaining ES cell pluripotency, 24 that expressed it strongly enough in ES cells for it to be termed specificity were discovered as candidate initialization factors. As a result of evaluations, when the candidate factors were administered to mouse fibroblasts one by one, pluripotent stem cells were not induced. However, when the 24 factors were combined and introduced at the same time, pluripotent stem cells were induced. The candidate factors were narrowed down, and 4 specific factors (Oct3/4, Sox2, Klf4 and c-Myc) were combined and administered with a retrovirus. This resulted in the world-leading success in forming induced pluripotent stem (iPS) cells from cultivating fibroblasts derived from fetal and adult mice (as was published in Cell in 2006). Induced pluripotent stem cells can be cultivated over the long term. They are similar to ES cells in terms of cell morphology and proliferative capacity and make differentiation induction possible for a variety of cells, including those of nerves, cartilage, muscle, and gastrointestinal epithelia. Yamanaka named this cell the iPS cell. While investigating the differentiation capacity of iPS cells to a variety of cells in mice, he proved that the iPS cells are equivalent to ES cells. Although the introduction of cancer genes into cells always has the possibility of making cells cancerous, he later also made it clear that even without the use of Myc, which is a cancer gene, human iPS cells can be formed. In addition, his team also showed that mouse iPS cells can be formed from liver and stomach cells, as well (as was published in Science in 2008). Yamanaka also succeeded in forming iPS cells from fibroblasts derived from human adult skin (as was published in Cell in 2007).
In this way, Yamanaka revealed that cells with pluripotency can be formed by introducing specific factors into somatic cells, without ever using embryos or eggs. Conventionally, ethical issues pertaining to the handling of reproductive cells arose constantly, but now those issues can be considered to have been overcome. Moreover, the ability to make iPS cells from the same individual can prevent immunological rejection response, which has been a problem when transplants involve different individuals.
For the above reasons, Shinya Yamanaka has been awarded the Eighth Yamazaki-Teiichi Prize in Biological Science & Technology f“Identification of Factors that Induced and Maintain Pluripotency.”