An elegant example comes from studies of the heart. By forcing expression of an appropriate combination of factors—not Oct4, Sox2, Klf4, and Myc, but Gata4, Mef2c, and Tbx5—it is possible to convert heart fibroblasts directly into heart muscle cells. This has been done in the living mouse, using retroviral vectors, and the transformation occurs with high efficiency when the vectors carrying the transgenes are injected directly into the heart muscle tissue itself. Although they occupy only a small fraction of the tissue volume, the fibroblasts in the heart outnumber the heart muscle cells, and they survive in large numbers even where the heart muscle cells have died. Thus, in a typical nonfatal heart attack, where heart muscle cells have died for lack of oxygen, the fibroblasts proliferate and make collagenous matrix so as to replace the lost muscle with a fibrous scar. This is a poor sort of repair. By forcing expression of the appropriate factors in the heart, as described above, it has proved possible, in the mouse at least, to do better than nature and regenerate lost heart muscle by transdifferentiation of heart fibroblasts.
We are still a long way from putting this technique into practice as a treatment for heart attacks in humans, but it shows what the future may hold—not only for this medical problem, but for many others.
eS and ipS Cells are useful for drug discovery and analysis of disease
A large part of the excitement surrounding ES and iPS cells and the technology of transdifferentiation comes from the prospect of using the artificially generated cells for tissue repair. It begins to seem that virtually any type of tissue might be replaceable, allowing treatment of degenerative diseases that have previously had no cure. Research in this area is moving rapidly, but there are many difficulties to be overcome.
treatment transplantation of genetically with drugs matched healthy cells
With the advent of iPS cells and direct transdifferentiation, at least one major hurdle has been surmounted, in principle at least: the problem of immune rejection. ES cells, because they are created from early embryos that generally come from unrelated donors, will never be genetically identical to the cells of the patient receiving the transplant. The transplanted cells and their progeny are therefore liable to rejection by the immune system. Both iPS and transdifferentiated cells, in contrast, can be generated from a small sample of the patient’s own tissue and so should escape immune attack when transplanted back into the same individual.
Tissue repair by transplantation, however, is not the only application for which ES, iPS, and transdifferentiated cells can be used: there are other ways in which they promise to be more immediately valuable. In particular, they can be used to generate large, homogeneous populations of specialized cells of any chosen type in culture; and these can serve for investigation of disease mechanisms and in the search for new drugs acting on a specific cell type (Figure 22–47).
Where a disease has a genetic cause, we can derive iPS cells from sufferers and use these cells to produce the specific cell types that malfunction, to investigate how the malfunction occurs, and to screen for drugs that might help to put it right. Timothy syndrome provides an example. In this rare genetic condition, there is a severe, life-threatening disorder in the rhythm of the heart beat (as well as several other abnormalities), as a result of a mutation in a specific type of Ca2+ channel. To study the underlying pathology, researchers took skin fibroblasts from patients with the disorder, generated iPS cells from the fibroblasts, and drove the iPS cells to differentiate into heart muscle cells. These cells, when compared with heart muscle cells prepared similarly from normal control individuals, showed irregular contractions and abnormal patterns of Ca2+ influx and electrical activity that could be characterized in detail. From this finding, it is a small step to development of an in vitro assay for drugs that might correct the misbehavior of the heart muscle cells.
This approach to drug discovery—where iPS cells are prepared from the individual patient, differentiated into the relevant cell type, and used to test candidate drugs in vitro—would seem to represent a huge advance on the slow, costly traditional methods that involve administration of test compounds to large numbers of people.