Since their discovery, stem cells have caused a big stir in the biomedical field for a variety of reasons. However, the controversy surrounding embryonic stem cells – cells derived from embryos – is the most fervent. While their totipotency – their ability to become any kind of cell in the human body – makes them an incredibly powerful therapeutic tool, their manufacture involves the destruction of a human embryo. This ethical issue provokes strong feelings in many of its opponents. Induced pluripotent stem (iPS) cells – stem cells capable only of turning into certain cell types – serve as an answer to these ethical issues because they can be derived from normal adult body cells and do not require the destruction of any embryos. However, a lot of research needs to be done before iPS cells can be used frequently and reliably in a clinical setting. That being said, this past year, Derrick Rossi and coworkers revealed their findings regarding a novel way to produce induced pluripotent stem (iPS) cells . The results they discuss are groundbreaking with respect to the reprogramming efficiency and overall quality of the finished iPS cells. This research brings the possibility of iPS cell therapy much closer to reality.
Stem cells have recently been widespread in the media due to the stem cell hearings being held by the Supreme Court regarding the stem cell guidelines established by the National Institutes of Health (NIH) . The primary issue addressed in the hearings is the destruction of the embryos for the purpose of harvesting embryonic stem cells. This was deemed contrary to United States law because of the destruction it causes – which could be considered murder by some – and also because it creates an unfair advantage for embryonic stem cell researchers in competing for government funding compared to those of other stem cell types . This advantage stems from how embryonic stem cells have more potency (ability to transform into more cell types), and thus make for more attractive grant proposals. The ruling was temporarily overturned at the request of President Barack Obama because he considered the ruling to conflict with what Congress intended when it wrote the laws limiting human stem cell research and that it would undercut ongoing medical research . While the latest ethical debates took place in September of this past year, they have been ongoing for years, introducing many obstacles to stem cells’ path to the clinical stage.
To circumvent the roadblocks placed in the way of embryonic stem cell research while the political controversies rage, many researchers have focused on induced pluripotent stem cells. These are normal body cells that are transformed into pluripotent cells by the introduction of genetic material coding for 4 DNA transcription factors: c-Myc, Oct4, Klf4, and Sox2 . Since these cells do not involve the destruction of embryos, people from almost any political background have no qualms about supporting their research and use.
The concept of induced pluripotency is an important way for stem cells to reach the clinic where they can be used to treat many chronic diseases, such as diabetes, Alzheimer’s, and organ failure. Useful pluripotent cells are capable of differentiating into all types of body cells. An additional advantage of pluripotent stem cells is that they are easy to grow. IPS cells also can be custom made for the patient, which reduces the likelihood of tissue rejection at the stage of a transplant. However, they are still difficult, and thus relatively expensive, to produce.
The first researchers to develop a successful protocol for the generation of iPS cells were Yamanaka and coworkers . Their method involved generating viral vectors – a method for introducing foreign DNA to a cell using viruses – using helper cells that serve exclusively to produce those viruses and then introducing the viruses to cultured epidermal tissue cells, like skin cells. These retroviruses introduce the viral RNA into the genetic makeup of the cells, making them express the 4 factors that make them behave like pluripotent stem cells . This method, however, is extremely labor-intensive and time consuming, as over a month is needed before iPS cells can be produced. The use of viral vectors also involves two significant dangers: first, there is the risk of infection of cells other than the target cells, such as those of the researcher. The second danger involves the actual integration of foreign DNA into the genome of the target cells. The DNA contains the coding sequence for the protein factors and their respective promoters – a region of DNA that allows the sequence that follows to be translated into protein. However, should the viral DNA integrate itself next to a gene that regulates the cell cycle or growth controls, it could cause the growth cycle to be deregulated and cancer to form by causing a signaling or transcription factor to be expressed ectopically, or out of control of the cells’ own mechanisms.
Due to the dangers involved in the retroviral method of induction, another laboratory, led by Andras Nagy, reprogrammed somatic body cells by using a protein vector instead of the viral method. Using the piggyBac transposon – a protein capable of moving stretches of DNA from one source to another – the researchers were able to integrate the sequences for the 4 factors into the somatic cells’ genomes . This method not only addresses the infection risk of the viral method, but also limits the potential integration sites in the fibroblasts’ (a type of epithelial cell) genome. However, the oncogenic (cancer-causing) potential is still present because foreign promoter regions are still being added to the cells’ genome.
Most recently, Rossi and coworkers revealed a new method for inducing pluripotency in somatic cells by introducing modified mRNA into the target cells. By encouraging the cells to accept the mRNA via electroporation – the opening of a cell’s membrane by the application of a temporary electrical current – and treating the cells with the signaling molecule interferon, which inhibits the cells’ natural defenses against foreign DNA, the researchers were able to co-opt the cells’ protein-making machinery to generate the necessary transformative factors to induce the cells to pluripotency without any change to their genome . This method was not only faster, as it required only two weeks, but also more efficient, as it presented an approximate 36-fold increase in transformation efficiency compared with the traditional retroviral method . Finally, more of the transformed cells showed the genetic conformation and cell-marking proteins that are specific to embryonic stem cells . This indicates that the induced pluripotent cells formed by this process, even though they require nothing more than epithelial cells from an adult subject, are almost identical to harvested stem cells from embryos, and since the cells’ genomes remain unchanged, the cancer risk associated with retroviral induction is virtually non-existent, a quality unique to this method.
The discovery of this method is incredibly significant to the future use of stem cells in a clinical setting. With faster and more efficient transformations, making custom stem cells for individual patients enters the realm of feasibility. Furthermore, since these stem cells are generated from the patients’ own body cells, they do not run the risk of rejection and integrate seamlessly into the patient’s body upon implantation. Finally, since they are generated from mere skin cells, they present none of the ethical issues that are presently associated with the use of embryonic stem cells. mRNA induced pluripotent stem cells bring regenerative medicine one step closer to the clinic and provide many avenues for further fruitful research.
- Rossi DJ, Warren L, et al. Highly Efficient Reprogramming to Pluripotency and Directed Differentiation of Human Cells with Synthetic Modified mRNA. Cell Stem Cell. 2010;7(1):1-13.
- Vicini J. U.S. appeals court puts on hold stem cell funding ban. Reuters. Sep 9, 2010.
- Pelofsky J. Appeals court to hold hearing on stem cell ban. Reuters. Sep 15, 2010.
- Nagy A, et al. piggyBac Transposition Reprograms Fibroblasts to Induced Pluripotent Stem Cells. Nature. 2009;458: 766-770.
- Yamanaka S, Takahashi K. Induction of Pluripotent Stem Cells from Mouse Embryonic and Adult Fibroblast Cultures by Defined Factors. Cell. 2006;126(4): 663-676
Aleks Penev is a second-year biology and computer science major at the University of Chicago.