Researchers have achieved the testing goal of generating embryonic stem cells from the cells of an adult primate. The procedure used could provide insights into a variety of diseases, if it can be applied in humans.
Generating patient-specific stem cells holds great promise for medical therapy, drug discovery and basic research. But, quite apart from the associated ethical issues, there are many technical hurdles to achieving this goal for humans. For many species, both adult and fetal cells have been used to generate genetically identical offspring, the most famous of which is probably Dolly the sheep1. This has been achieved by a process known as somatic-cell nuclear transfer (SCNT), which involves replacing the nuclear genetic material of an unfertilized egg with that of a somatic (non-germ) cell. In each case, species-specific modifications to this basic protocol must be made. Yet despite considerable efforts, it has remained impossible to generate primate-derived embryonic stem-cell lines or to clone a primate. Byrne et al.2 (page 497 of this issue) now describe the generation of stem-cell lines from embryos of rhesus macaques using a modified SCNT protocol∗.
Byrne and colleagues' earlier work3 had shown that, in contrast to the situation in other mammals, once the nuclear material of an egg (oocyte) is replaced with that of a primate somatic cell, the oocyte cannot efficiently undergo its essential remodelling to an embryonic state because of delayed and inefficient removal of a protein called lamin A/C. This protein is a component of the nuclear skeleton, and its removal requires a protein complex known as maturation-promoting factor (MPF). The authors hypothesized3 that inappropriate handling of the oocyte might cause premature decay of MPF activity, leading to the persistence of lamin A/C. To test this, they inhibited MPF degradation and found that, indeed, lamin A/C was more efficiently removed. But this treatment resulted in other deleterious effects at later stages of embryonic development, which outweighed its benefits.
Byrne et al.2 have since made several modifications to their SCNT protocol. For example, in conventional SCNT, the oocytes are exposed to a Hoechst stain and/or ultraviolet light to locate and remove their chromosomes. Byrne and colleagues postulated that such treatment might be responsible for preventing the fertilized primate oocyte from reaching the multicellular, blastocyst stage of embryonic development at a desirable rate. They also thought that it might damage mitochondrial DNA, hindering the function of this energy-generating organelle during early embryonic development. To avoid such potential hazards, the authors adopted an alternative method of chromosome detection: they used the Oosight imaging system to visualize chromosomes using polarized light.
Another measure taken was to increase the level of MPF by removing calcium and magnesium from the media in which the oocytes, and subsequently the embryos, were handled. Although calcium is essential for the oocyte's response to the activating effect of sperm during fertilization4, environmental factors can also activate oocytes spontaneously. Thus, the removal of calcium — and maybe magnesium — during SCNT procedures reduces the incidence of spontaneous oocyte activation. This allows a set of uniform oocytes with high levels of MPF to be generated, which can then be activated in a controlled manner. Byrne and colleagues' success2 in generating primate-derived embryonic stem-cell lines followed the introduction of these changes to the SCNT protocol (Fig. 1). However, further work is required to unravel the mechanisms underlying this achievement.
An entirely different approach to SCNT can also lead to embryonic stem-cell lines. This procedure involves reprogramming adult cells directly by inducing the expression of just four gene transcription factors in the cells. Using this method, researchers5,6 have reprogrammed skin cells of adult mice to generate a few cells that have many of the characteristics of embryonic stem cells.
There is so far no sign that this approach could be effective in human cells. But even if it could be applied to generating patient-specific cells, there are other limitations on its use in humans. For example, viral vectors were used to introduce the genes encoding the four transcription factors into the genome of the mice. Because of the potential risks associated with using viral vectors, this procedure would be unacceptable for use in treating humans. Moreover, many of the embryonic stem-cell-like cells generated in this way eventually developed into tumours, probably because misregulation of one of the four introduced genes — Myc — can lead to cancer. But a modified approach to direct reprogramming that does not involve cancer-causing genes or the use of viruses is likely to be the ultimate method of choice for producing human stem cells.
What are the implications of Byrne and colleagues' findings2 for applications in humans? When considering the potential of stem cells derived from adult cells, great emphasis is often placed on the fact that derivatives of such cells, if returned to a patient suffering from a degenerative disease, would not be rejected by their immune system. Realistically, a careful examination of resources and the time required to produce differentiated cells for treatment purposes suggests that large-scale use of stem cells would be impractical.
In our haste to use patient-specific cells in therapy, however, we tend to overlook the fact that they are of great value for basic research and drug discovery. For example, such cells could provide new ways to study inherited diseases. If the diseased tissue cannot easily be recovered from the patient, production of patient-specific cells is the only potential means of obtaining cells with the condition. Such cells could be used to identify causative mutations. Or they could be compared with their healthy counterparts to identify the molecules and molecular mechanisms underlying the disease symptoms. This information could then form the basis for high-throughput screens to identify small-molecule drugs that could prevent such disease-associated changes at a molecular level. Ultimately, this approach might lead to treatments for neurodegenerative diseases, some cancers and psychiatric disorders.
This News & Views article and the paper concerned2 were published online on 14 November 2007.
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Nature Reports Stem Cells (2007)