Gene therapy progress and prospects: stem cell plasticity.

With the identification of stem cell plasticity several years ago, multiple reports raised hopes that tissue repair by stem cell transplantation could be within reach in the near future. Krause et al reported that a single purified hematopoietic stem cell not only repopulated the bone marrow of a host animal, but also integrated into unrelated tissues. Lagasse et al demonstrated that in a genetic model of liver disease, purified hematopoietic stem cells can give rise to hepatocytes and rescue fatal liver damage. More recent work by Jiang et al demonstrated that cultured cells can retain their stem cell potential. There are a number of possible mechanisms that could explain these phenomena, and recent experiments have raised controversy about which mechanism is prevalent. One possibility is transdifferentiation of a committed cell directly into another cell type as a response to environmental cues. Transdifferentiation has been shown mainly in vitro, but some in vivo data also support this mechanism. Direct transdifferentiation would clinically be limited by the number of cells that can be introduced into an organ without removal of resident cells. If bone marrow cells could on the other hand give rise to stem cells of another tissue, then they could in theory repopulate whole organs from a few starting cells. This model of dedifferentiation is consistent with recent data from animal models. Genetic analysis of cells of donor origin in vivo and in vitro has brought to light another possible mechanism. The fusion of host and donor cells can give rise to mature tissue cells without trans- or dedifferentiation. The resulting heterokaryons are able to cure a lethal genetic defect and do not seem to be prone to give rise to cancer. All these models will clinically face the problem of accessibility of healthy primary cells for transplantation. This underlines the importance of the recent identification of a population of mesenchymal stem cells (MSCs) with stem cell properties similar to embryonic stem (ES) cells. These cells can be cultured and expanded in vitro without losing their stem cell potential making them an attractive target for cell therapy. Finally, it is still not clear if stem cells for various tissues are present in peripheral blood, or bone marrow and thus can be directly purified from these sources. Identification of putative tissue stem cells would be necessary before purification strategies can be devised. In this review, we discuss the evidence for these models, and the conflicting results obtained to date.