Scientists cultured the first primate embryonic stem cells in 1998,1 and the achievement was met with equal parts excitement and trepidation. Here was a pluripotent class of cells that could be expanded indefinitely and differentiated at will into cells of the three germ layers. Their capacity to cure degenerative diseases, such as diabetes and Alzheimer’s disease, was potentially limitless. The drawbacks, however, were both ethical and clinical. From an ethical standpoint, many people were uneasy that the source of cells was unused, fertilized eggs generated by in vitro fertilization. From a clinical standpoint, transplanting new, healthy β-cells or neurons into a patient would only work if investigators could find a way to keep the patient from rejecting the transplanted cells. Faced with these obstacles, the research community sought a better way.
The ethical and clinical problems could both be overcome if the somatic cells of a patient could be reprogrammed to generate their own pluripotent stock of cells. In this way, the material for a transplant of, say, β-cells or neurons could be differentiated from the patient’s own somatic cells, alleviating the need for embryonic stem cells and removing the risk of transplant rejection. Importantly, a large body of literature existed to show that an adult, somatic cell could be induced to behave like a pluripotent, embryonic stem cell. Experiments performed in the late 1950s by John Gurdon, Ph.D., showed that a sexually mature, adult frog clone could be generated by transferring the nucleus of an endodermal cell into an oocyte.2 The full implication of this technology was not fully realized, however, until 1996 when the first mammalian clone (Dolly the Sheep) was born.3 These experiments made it abundantly clear that the mammalian oocyte had the power to reprogram a somatic cell nucleus, but the factors responsible for the phenomenon remained a mystery.
Enter the simple yet elegant work of Shinya Yamanaka, M.D., Ph.D., and Kazutoshi Takahashi, Ph.D., who used a reporter cell line to identify the factors in the oocyte that were required for pluripotency. First, they engineered mouse cells to express an antibiotic resistance gene downstream of a gene specifically turned on in pluripotent cells. Then, candidate genes were expressed in the engineered reporter cells using a retrovirus, and the induction of pluripotency was assessed as growth in an antibiotic-containing media. Using this method, they identified four genes, (Oct3/4, Sox2, c-Myc, and Klf4, a.k.a., the Yamanaka factors4) that could turn fibroblasts, or other somatic cells into induced pluripotent stem (iPS) cells. Suddenly, the goal of generating healthy replacement cells for therapeutic use while respecting both ethical and clinical boundaries was within the reach of investigators.
Unfortunately, initial reprogramming of somatic cells relied on retroviral vectors, which made the iPS cells less attractive as a therapeutic tool, because the retrovirus had the potential to integrate into a host genome and produce malignant effects. Within a couple of years, however, researchers found a way to reprogram cells using viruses that could be separated away after reprogramming.5 Currently, investigators are looking to reprogram cells using small molecules, which will eliminate the need for viruses altogether. Thus, a little ingenuity has once again put the goal of generating healthy replacement cells for therapeutic use within our reach.
It should be noted, however, that the ramifications of these experiments reach well beyond cell replacement therapies. For example, investigators can now reprogram somatic cells from patients affected by a genetic disease and expand them for biochemical studies. Studies of this nature have been used to show that neurons differentiated from iPS cells of Rett syndrome patients (an autism spectrum disorder) are less efficient when compared to neurons generated from the iPS cells of healthy control patients. Further, the Rett syndrome iPS cells were used to screen compounds that might improve the synaptic character of the diseased neurons. Furthermore, because the investigators were able to monitor the cells as they differentiated, they found that a developmental window may exist when treatments for this disease might be most effective.6 Thus, iPS cell technology is allowing researchers to examine genetic diseases in multi-dimensional ways that may lead to a deeper understanding of the disease and the rapid development of treatments.
Therefore, the technologies spearheaded by Drs. John Gurdon and Shinya Yamanaka have opened avenues of research and development to the scientific community that were once unimaginable. iPS cells are allowing researchers to understand genetic diseases in new and meaningful ways, and they are providing physiologically relevant experimental platforms for the development of drug and cell-based therapeutic approaches. Taken together, iPS cell technologies are helping translational science advance at a pace that is rapidly turning the hope of a cure for devastating degenerative conditions into a foreseeable reality.