Embryonic stem cells might provide the means to heal a variety of physical ailments. However the problem with embryonic stem cells is not necessarily in their use, but in their derivation. In order to make embryonic stem cell lines, human embryos are destroyed.
The following video shows Alice Chen from Doug Melton’s laboratory at Harvard University destroying embryos to make embryonic stem cells: http://www.jove.com/index/details.stp?ID=574.
Now that federal funding is available to not only work with existing embryonic stem cell lines but to MAKE new lines, there is nothing to stop researchers from thawing and (I’m sorry to be so blunt) killing human embryos. Can we have our “cake and eat it too?” Can we have the benefits of embryonic stem cells and not destroy embryos? Perhaps we can.
In 2001, Masako Tada reported the fusion of embryonic stem cells with a connective tissue cell called a fibroblast. This fusion reprograms the fibroblasts so that they behave like embryonic stem cells (Current Biology 11, no. 9 (2001): 1553–8). This suggests that something within embryonic stem cells can redirect the machinery of somatic cells to become more like that of embryonic stem cells. In 2006 Kazutoshi Takahashi and Shinya Yamanaka were able to generate embryonic stem cell lines by introducing four specific genes into mouse skin fibroblasts. These “induced pluripotent stem cells” (iPSCs) shared many of the properties of embryonic stem cells derived from embryos, but when transplanted into mouse embryos, they were not able to participate in the formation of an adult mouse (Cell 126, no. 4 (2006): 663–76). This experiment showed that it is possible to convert adult cells into something that resembles an embryonic stem cell. Could we push adult cells further? In 2007, three different research groups used retroviruses to transfer four different genes (Oct3/4, Sox2, c-Myc and Klf4) into mouse skin fibroblasts and completely transformed them into cells that had all the features and behaviors of embryonic stem cells (Cell Stem Cell 1, no. 1 (2007): 55–70; Nature 448 (2007): 313–7; Nature 448 (2007): 318–24.).
These experiments drew a great deal of excitement, but there were several safety concerns that had to be addressed before iPSCs could be used in human clinical trials. Scientists used engineered retroviruses to introduce genes into adult cells in order to reprogram them into iPSCs (Current Topics in Microbiology and Immunology 261 (2002): 31-52). Retroviruses insert a DNA copy of their genome into the chromosomes of the host cell they have infected. If that viral DNA inserts into a gene, it can disrupt it and cause a mutation. This can have dire consequences (see Folia Biologia 46 (2000): 226-32; Science 302 (2003): 415-9). Fortunately this is not an intractable problem. The conversion of adult cells into iPSCs only requires the transient expression of the inserted genes. Secondly, scientists have created retroviruses that self-inactivate after their initial insertion (Journal of Virology 72 (1998): 8150-7; Virology 261, (1999). One laboratory has also discovered a way to make iPSCs with a virus that does not insert into host cell chromosomes (Science 322 (2008): 945-9). Other researchers have designed ingenious ways to move the necessary genes into adult cells without using viruses (Science 322 (2008): 949-53). Both procedures avoid the dangers associated with the use of retroviruses.
A second concern involves the genes used to convert re-program adult cells into iPSCs. One of these genes, c-Myc, is found in multiple copies in human and animal tumors. Thus increasing the number of copies of the c-Myc gene might predispose such cells to form tumors (Recent Patents on Anticancer Drug Discovery 1 (2006): 305-26; Seminars in Cancer Biology 16 (2006): 318-30). Indeed, the increased ability of iPSCs made by Yamanaka to cause tumors in laboratory animals underscore this concern (Hepatology 46, no 3 (2009): 1049-9). Several groups, however, have succeeded in making iPSCs from adult cells without the use of the c-Myc gene (Science 321 (2008): 699-702; Nature Biotechnology 26 (2008): 101-6; Science 318 (2007): 1917–20), although the conversion is much less efficient. Additionally, several groups have established that particular chemicals, in combination with the addition of a subset of the four genes originally used, can effectively transform particular cells into iPSCs (Cell Stem Cell 2 (2008): 525-8). Thus the larger safety concerns facing iPSCs have been largely solved.
Finally, patient-specific iPSCs have been made in several labs, even though they have not been used in clinical trials to date. Here is a short list of some of the diseases for which patient-specific iPSCs have been made:
Amylotrophic Lateral Sclerosis – Science 321 (2008): 121821.
Spinal Muscular Atrophy – Nature 457 (2009): 27781.
Parkinson’ Disease – Cell 136, no. 5 (2009): 96477.
Adenosine deaminase deficiency-related severe combined immunodeficiency – Cell 134, no. 5 (2008): 87786.
Shwachman-Bodian-Diamond syndrome – Cell 134, no. 5 (2008): 87786.
Gaucher disease – Cell 134, no. 5 (2008): 87786.
Duchenne and Becker muscular dystrophy – Cell 134, no. 5 (2008): 87786.
Huntington disease – Cell 134, no. 5 (2008): 87786.
Juvenile-onset type 1 diabetes mellitus – Cell 134, no. 5 (2008): 87786.
Down syndrome – Cell 134, no. 5 (2008): 87786.
Lesch-Nyhan syndrome –Cell 134, no. 5 (2008): 87786.
Thus iPSCs represent an exciting, embryo-free alternative to embryonic stem cells that provide essentially all of the opportunities for regenerative medicine without destroying embryos.