Many debilitating diseases, including Parkinson’s disease and Type 1 diabetes, are caused by the failure of certain organs or types of cells in the human body. In theory, it is possible to cure or treat these diseases by replacing the damaged organs or cells with functional ones. However, the supply of functional organs and cells from donors is far less than the current demand. Over 100,000 patients are currently on the national waiting list for organ transplantation and about 18 die every day waiting for transplants (The need is real: Data). As a result, biomedical researchers have been aggressively searching for alternative routes to create new cells and organs. Stem cell therapy, a potential solution to these issues, has generated much excitement and debate during the past decade. While most of us have heard about the controversy surrounding stem cells, fewer of us understand the science behind them, and many myths and misconceptions are still floating around.

What are stem cells?

Stem cells are undifferentiated cells that share two fundamental properties that distinguish them from other types of cells: self-renewal and potency (National Institutes of Health, 2009). Self-renewal refers to the ability of stem cells to replicate and proliferate almost indefinitely. Potency refers to a stem cell’s potential to differentiate into a variety of mature cell types. The ability of some stem cells to differentiate into almost every human cell type has created a lot of optimism about the future of regenerative medicine, since functional cells and organs can potentially be generated to replace defective ones in patients.

What are embryonic stem cells?

The three major types of stem cells are embryonic stem cells, adult stem cells, and induced pluripotent stem cells. Each of these stem cell types has different characteristics and different applications to research in regenerative medicine. Embryonic stem cells are the most widely known and controversial stem cell type. There have been numerous legislative changes in the legal status of embryonic stem cell research and debates about the ethics of using these cells in research. Embryonic stem (ES) cells are cells derived from early stage embryos, when differentiation and cell specialization have not yet begun. These cells are able to propagate themselves indefinitely and they are also Pluripotent—which means that ES cells can later become any cell type except for placental cells (National Institutes of Health, 2009). Therefore, they can theoretically produce an unlimited number of cells for both research purposes and regenerative therapies.

The stem cell debate

The controversy associated with embryonic stem cells arises from their origin. During fertilization, a single cell called a zygote is formed from the union of an egg cell and a sperm cell. The zygote divides into 2 cells, then into 4 cells and so on, forming a small ball of cells. At this stage, all of these cells are still unspecialized, but as the embryo develops, they will eventually differentiate into brain cells, heart cells, blood cells, and all of the other types of cells that make up our organs and tissues.

Embryonic stem cells come from the inner cell mass of a blastocyst, which is a structure containing between 50 and 100 cells that forms approximately 4-5 days after fertilization (Stem Cell Basics, 2013). In harvesting these cells, the entire blastocyst is destroyed. Thus, the process of isolating ES cells for research arouses fundamental questions about when life begins and what it means to be human.

The major controversy regarding stem cell research arises from the question of the moral status of an embryo. Some people believe that human life begins at the moment of conception because every zygote has the potential to develop into a full human being. They argue that an embryo should be considered human, and therefore ES cell research should be outlawed because it destroys human life (Hug, 2011). Others believe that life does not truly begin until birth. There are also many other viewpoints along a continuum, such as the view that life starts with the beginning of the development of the nervous system (2 weeks after fertilization), or at the point when an embryo could survive independently if born prematurely. People who hold these views believe that early-stage embryos should not be considered human beings because they are unable to survive outside of the womb and they lack thoughts, feelings, and other fundamental qualities that define what it means to be human (Hug, 2011). Since ES cells must come from embryos that are only 4-5 days old, these people argue that an embryo in the blastocyst stage should not be considered to be a living human. Thus, if blastocysts are not humans, they should be legally approved for use in research to help save the lives of fully developed humans.

Contrary to what some believe, embryonic stem cells are not, and cannot be created from, aborted fetuses. Instead, almost all embryonic stem cells used in scientific research are derived from extra egg cells that were fertilized via in-vitro fertilization (IVF) that would normally be frozen or discarded (Stem Cell Basics, 2013). Moreover, these leftover embryos can only be used with the consent of the egg and sperm donors (Stem Cell Basics, 2013). Even though these facts do not eliminate the questions about the moral status of an embryo, they clearly distinguish the stem cell debate from the controversy over abortion, and may provide a clearer perspective on the ethics of stem cell research.

Technical challenges

In addition to ethical dilemmas, there are several technical challenges with using human ES cells in regenerative medicine. One major issue is that the mature cells and organs generated from human ES cells cannot readily be transplanted into patients because these ES cells will be rejected by the patient’s immune system (Jones, 2000). This phenomenon occurs because a healthy immune system marks these new organs as foreign objects and attacks them in the same way that it would to defend against bacteria or viruses. Thus, transplant patients must take strong immunosuppressive drugs to prevent an immune response to these new cells. Given the serious side effects of immunosuppressive drugs, this is far from an ideal solution.

The only way to overcome this immune rejection problem would be to use cells generated from a patient’s own ES cell lines, so that the DNA in the patient and the ES cells is identical (Jones, 2000). In this case, the patient’s immune system will treat the ES cell-derived organs as his own and will not attack these cells. Until recently, the only way to produce patient-matched ES cells was to use cloning, which itself is an extremely controversial technology (Jones, 2000).

What are adult stem cells?

Adult stem cells are undifferentiated cells that are found among mature specialized cells in the human body. Adult stem cells are also capable of self-renewal and proliferation, but they are multipotent instead of pluripotent, which means that their capacity for differentiating into different cell types is much more limited than those of ES cells (National Institutes of Health, 2009). The most common functions of adult stem cells are to maintain and repair existing tissues in living patients. Cells in the bone marrow are an example of adult stem cells; they are able to differentiate into different blood cells, but not any other type of cells or tissues in the human body.

Since adult stem cells are derived from tissues of living patients instead of embryos, the use of adult stem cells in research is not considered controversial. In addition, since adult stem cells come from a patient’s own tissues, the immune system will not mark these cells as foreign bodies. Thus, using adult stem cells will also avoid many of the problems associated with immune rejection and immune system suppression in ES cells or transplants.

However, the limited potency of adult stem cells greatly reduces their applications in therapeutic regenerative medicine. While it is easy to grow ES cells in culture, adult stem cells are rare, so it is difficult to isolate these cells from a patient’s tissue samples (NIH, 2009). Moreover, since adult stem cells thrive in the highly regulated environment of a human body, it is much more difficult to provide optimal growth conditions for adult stem cells in a cell culture dish.

What are induced pluripotent stem cells?

We have already seen that both ES cells and adult stem cells have various advantages and disadvantages for use in regenerative medicine. An ideal stem cell would combine the advantages of both ES cells and adult stem cells while eliminating the disadvantages of both types of stem cells.

Another solution was discovered in 2007, when Shinya Yamanaka and his team successfully produced induced pluripotent stem cells (iPS cells) for the first time. iPS cells are adult cells that have been reprogrammed back into a pluripotent state. For example, an adult skin cell can be transformed back into an undifferentiated cell. This new undifferentiated cell would then be able to differentiate into almost any cell type. iPS cells have been shown to express the same genes as embryonic stem cells, which means they have the same potential for self-renewal and differentiation as ES cells (NIH, 2009). However, since iPS cells are derived from adult cells, much of the controversy associated with ES cells can be avoided because no human embryos are destroyed. Equally importantly, since iPS cells are derived from a patient’s own tissues, many of the immune rejection concerns with ES cells are also avoided. Thus, an iPS cell brings us closer to the ideal stem cell, as it combines qualities of ES cells and adult stem cells.

Yamanaka’s team accomplished this revolutionary feat by using viruses to introduce different combinations of stem-cell specific genes into adult fibroblast cells (a type of skin cell) (Takahashi, 2007). High levels of stem-cell specific genes are present in embryonic stem cells, so it makes sense that reprogramming an adult cell to express genes essential to ES cell pluripotency would cause that adult cell to de-differentiate into an undifferentiated and pluripotent state.

Yamanaka’s team first selected 24 candidate transcription factors (reprogramming factors) that were known to play important roles in the development of the early embryo (Takahashi, 2007). Transcription factors are proteins that act as on/off switches for gene expression in a cell. Introducing factors that influence ES cell differentiation would logically activate genes that control pluripotency in an adult cell, causing it to take on properties of ES cells.

After performing hundreds of experiments involving different combinations of these 24 factors, Yamanaka’s team eventually determined that four of these reprogramming factors were essential to inducing and maintaining the pluripotency of cells (Takahashi, 2007). Yamanaka’s approach differed from those of other biologists because he was the first to attempt to introduce multiple transcription factors at the same time, rather than only one factor at a time.

Applications of induced pluripotent stem cells

Yamanaka’s breakthrough discovery of the four transcription factors required to make adult cells revert to an undifferentiated state has opened many new potential applications in the field of regenerative medicine. The two most common applications that are currently being researched are cell therapy and disease modeling.

Cell therapy

Diseases that are caused by the destruction of a specific cell type present an obvious application of iPS cell therapy. For example, Type 1 diabetes is caused by the destruction of beta cells that produce insulin, and Parkinson’s disease is caused by the death of dopaminergic cells, which produce dopamine, an essential hormone that contributes to motor regulation (Wu & Hochedlinger, 2011). In the future, treatments or cures for these diseases could involve differentiating a patient’s iPS cells into these missing cell types and reintroducing them into the patient to restore the original function of these cells.

Another similar application of iPS cell therapy is the treatment of spinal cord injuries. Common cuts and bruises often heal quickly because adult stem cells in the skin and blood replicate quickly and replace these damaged cells. However, most injuries to the spinal cord lead to a lifetime of paralysis because multipotent neural cells are extremely rare and replicate very slowly. Using iPS cell technologies to generate new spinal cord neural cells could potentially restore mobility to victims of spinal cord injuries (Stem cell basics, 2009).

Disease modeling and drug screening

Other important applications of iPS cells that are being widely researched today include disease modeling and drug screening. To ensure the safety of human patients, biomedical researchers must test potential drugs and treatments on animal models, from mice to monkeys. If these animal trials are successful, there are an additional three phases of human clinical trials to prove that a drug is both safe and effective spec (The FDA’s drug review process). These procedures often take several years before a drug can be approved for human use. Even then, no animal is a perfect model of a human, so potentially deadly side effects in humans may be overlooked.

Using iPS cell technology, researchers can harvest cells from patients afflicted with certain diseases and induce pluripotency to create an almost unlimited supply of the diseased cells. Recreating defective cells in culture could greatly increase the efficiency of drug testing because many possible treatments can be tested simultaneously on these cells without risking potentially life-threatening side effects in the patient (Wu & Hochedlinger, 2011). In addition, using iPS cells for drug screening should save a lot of money because the costs associated with raising and caring for laboratory research animals would be greatly reduced or eliminated.

In addition, since all patients have different DNA and genetic markers, even drugs approved by the FDA have varying effects on patients and may not work effectively on all patients with a certain disease. Thus, using a specific patient’s iPS cells to study drug treatments may allow researchers to develop individualized medications that provide the most beneficial effects to each individual patient (Wu & Hochedlinger, 2011).

Limitations and alternative approaches to producing iPS cells

Yamanaka’s method of creating iPS cells using transcription factors was revolutionary and generated enormous excitement among the scientific community. However, there are also several notable limitations to Yamanaka’s original method, and researchers are still working to develop more effective and reliable methods for generating iPS cells.

One important issue with Yamanaka’s original method is that c-Myc, which is one of the four required transcription factors, is an oncogene (Knoepfler, 2009). Oncogenes are genes that increase the chances of a patient developing tumors. Even if iPS cell technologies advance far enough to allow safe and reliable transplantation of cells, increasing the risk of cancer in a patient is clearly not a desirable outcome. Other researchers have discovered transcription factors that could be used in place of c-Myc, but these combinations of transcription factors greatly reduce the efficiency of reprogramming (Marion et al., 2009). A reduced efficiency level means that more skin cells from a human patient would need to be harvested in order to generate a desired number of iPS cells, making the procedure more painful for patients. Thus, there is a clear tradeoff between safety and efficiency when using transcription factors to create iPS cells.

In 2008, Rudolf Jaenisch and his research group developed a method that uses a virus to introduce the reprogramming factors into a cell and then removes the virus and oncogenes from the cell (Kaplan, 2009). Jaenisch’s group accomplished this by adding a short fragment of DNA, called a loxP sequence, to the ends of each transcription factor (Kaplan, 2009). After the reprogramming factors have finished transforming an adult cell into a pluripotent state, a specialized protein recognizes these loxP sequences and removes them and all of the original transcription factors from the genome (Kaplan, 2009). With the oncogenes removed, iPS cells generated using this technique pose no additional risk of cancer to patients. Jaenisch’s methods hold promise for improving the safety of iPS cell therapy without reducing the efficiency rate of producing stem cells.

Where are we now?

iPS cell research has quickly become an extremely hot area of research in biology. Many researchers around the world are collaborating to uncover new techniques that increase the efficiency of iPS cell transformation or reduce the risks associated with introducing these cells back into the human body. Since 2008, scientists have achieved much more efficient methods of reprogramming of adult cells using RNA molecules instead of viruses. As the ease of iPS cell technologies increases and the costs decrease, the increased accessibility of iPS cells will raise new ethical questions. For instance, using a patient’s iPS cells to perform drug-screening tests requires knowledge of the patient’s health records, which makes it much more difficult to respect the cell donor’s privacy (Lehrman, 2010). Moreover, although performing an experiment using a patient’s cells requires the consent of the patient, the potential for iPS cells to constantly renew themselves could allow researchers to use these cells for purposes beyond the original experiment, which the donor may or may not approve of (Lehrman, 2010).

Despite the many advantages of iPS cells, the development of iPS cell technology still has not completely eliminated the need for harvesting ES cells. Embryonic stem cells currently act as a gold standard for verifying that iPS cells were produced correctly, so we cannot yet ignore the ethical dilemmas associated with ES cells.

Biomedical researchers still have a long way to go before being able to generate organ replacements reliably while avoiding important ethical issues. Nevertheless, stem cell research has come a long way during the past decade and each new experiment brings us one step closer to improving the quality of life for millions of patients.

References

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Wu, S., & Hochedlinger, K. (2011). Harnessing the potential of induced pluripotent stem cells for regenerative medicine. Nature Cell Biology, 13, 497-505. doi:10.1038/ncb0511-497