One of the major hurdles that needs to be overcome in the field of regenerative medicine is the issue of immune rejection, or preventing a patient’s body from
rejecting a tissue transplant from a foreign donor. Consequently, researchers have increasingly focused on ways to regenerate damaged or diseased tissues in a patient by using the patient’s own tissues, which should not trigger an immune response. At this point in time, there are primarily two types of stem cells that hold the greatest promise for use in regenerative medicine where immune rejection is a significant concern: human induced pluripotent stem cells (iPSCs) and cells made through a process called somatic cell nuclear transfer (SCNT). This article will focus on recent SCNT improvements, but we’ll re-visit iPSCs briefly for comparison’s sake.
Human induced pluripotent stem cells: The history and biology of human iPSCs were explored previously in “Induced Pluripotent Stem Cells: A New Stem Cell Line with a Long History.” In essence, iPSCs, which were first created with mouse cells in 2006 (Takahashi and Yamanaka, 2006) and then with human cells in 2007 (Yu et al., 2007; Takahashi et al., 2007), are adult cells that have been “reprogrammed” to an embryonic stem cell (ESC) state. This reprogramming is done by forcing adult cells to express proteins that are essential to the ESC identity (by transducing the adult cells with a retrovirus vector that contains the DNA for the key proteins). Consequently, human iPSCs look and behave nearly indistinguishably from hESCs. Like hESCs, iPSCs are pluripotent (they can become any cell type) and proliferate virtually indefinitely, both features which are important for use in regenerative medicine.
However, while great improvements have been made to make this technology closer to the clinic (such as multiple approaches to create iPSCs that do not have the reprogramming genes randomly integrated into their genomes [Yu et al., 2009; Zhou et al., 2009]), and it may someday be used to generate patient-specific ESC-like cells, the technology is not quite there yet. (Other similar technologies, such as “direct reprogramming,” are also being explored for the generation of patient-specific cells, but, again, this approach also has a ways to go.)
Somatic cell nuclear transfer: SCNT technology significantly predates iPSCs, and in many ways formed the basis for the idea of iPSCs. In SCNT, the nucleus from a somatic cell (an adult cell that is not a sperm or egg, i.e. not the gametes) is implanted into an egg, which already had its own nucleus removed. The egg amazingly reprograms the nucleus to become embryonic again; it’s been found that SCNT causes some 10,000 to 12,000 genes to be expressed (turned into protein) that are normally associated only with embryonic development (Niemann et al., 2008). The newly formed embryo (technically called a blastocyst) can then be implanted into a surrogate mother, and potentially become an adult organism. The organism is a clone of the animal that donated the nucleus. Although nuclear transfer studies have been conducted since the late 1930s (primarily in amphibians using nuclei donated from embryos, not adult tissues) (Spemann, 1938), it wasn’t until 1997 that the first widely-accepted successful use of SCNT was reported: Dolly the sheep was born, and she was the first cloned animal from an adult cell, and the first cloned mammal (Wilmut et al., 1997). Since Dolly, several other animals have been successfully cloned, though many problems still remain (the frequency of successful development is relatively low, as SCNT-derived embryos usually result in about 0 to 10% live births) (Wilmut et al., 1997; Wakayama et al., 1998; Solter, 1998; McKinnell and Di Bernardino, 1999; Gurdon and Byrne, 2003, Beyhan and Cibelli, 2008).
Therapeutic cloning: While SCNT has been long-explored for its ability to create cloned animals like Dolly and others (a practice called “reproductive cloning”), SCNT has other very appealing applications that do not involve the creation of an entire animal, such as “therapeutic cloning.” The goal of therapeutic cloning is to use SCNT technology to create patient-specific embryonic stem cells for medical therapies. These much sought-after cells are being labeled nuclear transfer stem cells (NTSCs), but they are essentially ESCs. While using SCNT to clone an entire animal has been fraught with developmental challenges, using SCNT to create NTSCs may be less difficult because it only requires a very early stage embryo, a blastocyst, to be formed (and NTSCs from a blastocyst may be less compromised by developmental abnormalities than an entire animal would be) (Beyhan and Cibelli, 2008). NTSCs have now been created from many different model animals.