Human therapeutic cloning requires the reprogramming of a somatic cell by nuclear transfer to generate autologous totipotent stem cells. Emerging embryonic stem cell-based technologies offer the potential for many novel therapeutic modalities. However, clinical implementation requires a definitive resolution of the problem of histocompatibility. The ability to generate totipotent stem cells that carry the nuclear genome of the patient using nuclear transfer (NT) techniques would overcome this last major challenge in transplantation medicine. It would enable the production of virtually all cell and tissue types, all carrying the nuclear genome of the patient. And since gene targeting can modify the starting somatic cell such as fibroblasts, the resulting cells would be modified as well. Clinical application may include the production of cardiomyocytes to replace damaged heart tissue or insulin producing B-cells for patients with diabetes, among many others.
However, the realization of these therapies relies on the generation of early-stage embryos for the purpose of stem cell isolation.
The first demonstration that a somatic cell could be dedifferentiated was shown in cattle when NT-derived blastocysts were generated using cumulus cells. Wilmut et al. later confirmed these results with the cloning of an adult animal from a quiescent mammary gland-derived cell and by Cibelli et al. with actively dividing fetal fibroblasts. Despite these and subsequent studies, protocols for NT using somatic cells are still being developed, and the mechanisms underlying this biological phenomenon have yet to be characterized and understood.
Embryo reconstitution by NT depends upon a number of important technical and biological variables such as egg quality, enucleation and cell transfer procedures, and egg activation. Ultimately, the nucleus of the donor somatic cell must render itself to be reprogrammed by the egg, which in turn must be capable to induce all the epigenetic changes to down-regulate somatic genes and up-regulate embryonic ones.
Although NT as a means of generating stem cells has been achieved in mice and cattle, the cloning of primate embryos, including humans, using somatic donor cells has been problematic and has yet to be reported.