by Francisco Galdos
Suppose you have a year-old laptop that has been working well for you. You begin to notice one day that the computer freezes more frequently, and you continue to have problems. After taking your computer to the engineers, the engineers discover that a few of the small components of the motherboard are faulty, so they decide to replace it. Sounds simple doesn’t it? If we compare the act of replacing a computer part with the feat of replacing a faulty organ in our bodies, we can greatly appreciate the idea of interchangeable parts. Imagine, for example, that someone is born with a defective heart and has had so many surgeries that all that is left is a stiff and scarred heart. If we equate the body to a laptop we could say, “why not replace the organ with a new one?” Why not produce healthy clones of our organs so that we can just replace them when they are defective?
Although a simple idea, scientists and physicians have struggled for more than 50 years to understand how we can manipulate our cells in order to replace or regenerate our bodies. As scientists continue to advance techniques in cloning technologies, we have seen an increase in the number of ethical debates on the future of cloning. Cloning, albeit a straightforward solution to generating new organs, has become taboo in itself, causing the path to duplication to become less linear and more complicated for the scientist. If we would like to see medicine reach an era of curative intervention rather than palliative treatment, it becomes necessary more than ever to fully understand the fundamental scientific questions that cloning has sought to answer. As we look into the history and science of cloning, we find that it reveals a remarkable flexibility in our biology that could allow us to repair many of the problems that often lead to death.
Cloning: A Background
To find the origins of cloning, we need to go back to the 1950s. Embryologists had long been grappling with understanding how the vast diversity of cells in the body could be derived from a single fertilized egg cell. Scientists were puzzled by the concept of cellular differentiation, the ability of a fertilized egg cell to become a unique cell type within the body [1]. Up to this point in history, all that scientists knew was that within the nucleus of a cell there was genetic information, and this nucleus was bathed within the surrounding fluid in the cell, known as the cytoplasm. In order to investigate cellular differentiation, two scientists, Robert Briggs and Thomas King, sought to answer whether there was some type of irreversible change that occurred in the nucleus of the cell, which caused cells early in development to differentiate into the vast array of specialized cells in our tissues and bodies. They pioneered a laboratory technique known as somatic cell nuclear transfer (SCNT)[1]. SCNT involved taking the nucleus of a frog cell that they deemed to be further along in differentiation, and transferring the nucleus into a frog egg cell that had had its nucleus removed. Briggs and King hypothesized that a differentiated cell nucleus that has undergone irreversible genetic changes should have a decreased potential to develop into other cell types, since it would be lacking the genetic information needed to differentiate into all the cells of the body of an animal. Many scientists shared this hypothesis, as well as the idea that some factors within the cytoplasm cause irreversible changes to the genetic material in the nuclei of cells. In 1962, however, a graduate student by the name of John Gurdon conducted SCNT experiments in which he took differentiated frog intestinal cells and transferred their nuclei into enucleated egg cells. Gurdon modified his experimental procedure to conduct serial nuclear transplantations in which he took the already transplanted nuclei, and transplanted them again. In 1966, Gurdon demonstrated that he could effectively obtain adult frog clones with this method [2]. He proposed that if a differentiated cell nucleus could be used to form all the tissues of an entire animal, then the nucleus of a differentiated cell must not have undergone irreversible changes during cellular differentiation [2].
Gurdon’s frog experiment represents the first time in history that anyone had effectively cloned an animal, and also the first time that anyone had experimentally shown that during differentiation, cells do not undergo irreversible genetic changes. In a time when the secrets of molecular genetics were still being experimentally discovered, Gurdon proposed that it was through some type of mechanism that genes were turned on and off rather than lost as the embryo began to differentiate [2]. The idea that you could take a fully mature cell and reprogram it to become a cell capable of becoming any cell in the body led to the coining of the term pluripotency, which is the idea that a cell can become any cell in the body. His idea that all cells in the body maintain the same genome and have the potential to be reprogrammed into pluripotent cells inspired efforts to discover new ways to reprogram cells. To start, scientists began by studying the pluripotent cells—the embryonic stem cells. In 1998 James Thompson derived the first human embryonic stem cells [3]. By deriving these cells, scientists such as Thompson were interested in discovering the various genetic factors responsible for maintaining the pluripotent state. Theoretically, if pluripotent stem cells, i.e. embryonic stem cells, could be derived from a patient’s cells, such as a skin cell, scientists and physicians could use the pluripotent cells to make replacement tissues that are derived from the patient’s own cells.
In 1996, Ian Wilmut effectively derived the first embryonic stem cells using SCNT in a mammal and effectively cloned the first mammal—Dolly the sheep [4]. This lead to widespread fear and resistance that SCNT could be used to clone human beings, and that embryonic stem cells destroyed human life in the process of their derivation. Such fears lead to an 8 year national ban on the use of federal funds for the creation of new embryonic stem cell lines during the Bush administration, which caused massive funding problems for the field of regenerative medicine [5]. Because human embryonic stem cells are both expensive to derive, and ethically controversial to use, they became incredibly inefficient to use for therapeutic purposes. Moreover, as a clinical application, reprogramming was in its infancy since at the time, derivation of human pluripotent stem cells through SCNT had not been done. It was not until 2006 that Shinya Yamanaka and Kazutoshi Takahashi demonstrated the derivation of mouse pluripotent stem cells using the over-expression of four transcription factors [6]. This paper revolutionized the field of cellular reprogramming because it provided an effective alternative to deriving cells equivalent to embryonic stem cells. This effectively took the place of SCNT as a reprogramming technology, as it was accepted as a less controversial form of deriving pluripotent stem cells. SCNT was deemed as controversial because it generates embryos whose embryonic stem cells are then harvested to generate pluripotent cells in a dish. Yamanka’s method bypassed the need to generate the embryo and developed pluripotent embryonic-like stem cells directly from a differentiated cell in the body. Indeed, Yamanaka’s induced pluripotent stem cells (iPSCs) have been derived from human cells and have been used for a variety of purposes. Despite the shift to Yamanaka’s technology, this year, a group of US researchers in Oregon successfully derived the first human embryonic stem cell lines using SCNT, both reviving the scientific discussion of reprogramming and the controversy over human cloning [7]. Looking back, in more than a half-century of research, reprogramming experiments have demonstrated the remarkable flexibility of our cells to be converted into different cell types that can serve as the basis for regenerative therapies.
Therapeutic Hope, The Promise of Cloning
As we saw with the engineer replacing a laptop’s motherboard, we can now see how cloning technologies could be used to achieve such “replacements” in our bodies. Cells, it turns out, can be thought about as computers. The DNA of our cells can be thought of as the motherboard of a computer in that DNA essentially controls all the functions of the machine, our cells. The motherboard controls the entire computer’s functions depending on how it is programmed. Similarly, cells also depend upon how the DNA is programmed to express certain genes that carry out a particular function. Knowing this, we can then try to drive stem cells such as pluripotent stem cells to differentiate into a cell type that we are interested in obtaining. Take for example, a heart attack patient. During a heart attack, the heart muscle often dies off, causing irreplaceable damage in the heart that often puts patients on heart transplant lists [8]. Making heart cells from pluripotent stem cells would allow us to regenerate the damaged heart.
Regenerative technologies do not solely depend upon the generation of pluripotent stem cells. Scientists such as Doug Melton have sought to explore the possibility of bypassing the pluripotent state altogether and directly reprogram one cell type to another. Melton and coworkers showed that a type of cell known as an exocrine cell, located in the pancreas, could be directly reprogrammed into an insulin producing ß-cell by expressing transcription factors that are only present in ß-cells. If Melton’s group can one day make fully mature and functional ß-cells, these cells could effectively be engineered in such a way that they can be transplanted into the pancreas of a patient with type 1 diabetes, which could in theory cure the patient’s diabetes.
The fundamental promise of cloning is that scientists can take a person’s own cells and manipulate the biology of these cells to regenerate injured or diseased tissues. Using Yamanaka’s induced pluripotent cell (iPS) technology, it is even possible to take cells that may have genetic defects, such as defective genes, and genetically engineer the iPS cells derived from a patient such that the defective gene is replaced with the correct gene [8]. For example, consider a patient with muscular dystrophy who has a mutation in the gene called dystrophin. Using iPS technology, we could theoretically take skin cells, make iPS cells, replace the defective dystrophin gene with the correct gene, and make muscle tissue that could be transplanted into the patient to effectively cure his muscular dystrophy [9]. In addition to fixing genetic defects, scientists and physicians such as Harald Ott at the Harvard Stem Cell Institute are pioneering new technologies in what is known as whole-organ assembly [10]. The idea of whole-organ assembly consists of using iPS cells to seed tissue scaffolds that can be assembled to create on-demand replacement organs for patients [10]. Such technology could one day provide patients with fully functional replacement organs made from their own cells.
Breaking Down the Controversy
Despite the incredible promise of these technologies, they continue to find opposition from groups that argue that the use of embryonic stem cells and cloning of human cells into embryonic stem cells devalue human life, and could potentially give rise to the cloning of human beings [11]. The controversy is fueled by questions of right to life and individual determinism [12]. The fact that embryonic stem cells (ESCs) have the potential to give rise to all the cells in the body, and theoretically give rise to human beings, creates vast opposition based on fears that human lives are essentially being killed through the use or creation of these cells [14].
When John Gurdon cloned the first animal, the scientific question he sought to answer was whether cells have some irreversible change in their nuclei as they differentiate. Today, scientists are taking this question a step further towards understanding the molecular and cellular biology of how pluripotent cells undergo cellular differentiation. The Oregon study, which developed SCNT reprogramming of human cells, will serve as a vital study for modifying iPS technologies to make reprogramming more effective, and to remove the inefficiencies of genetic reprogramming that we often see with iPS technologies compared to SCNT [7].
Induced pluripotent stem cells were hailed as ethically acceptable because they bypass the need to use human eggs and human embryos. Although the goal of iPSCs is to replace embryonic stem cells as a away to avoid using human embryos, iPSCs contain many genetic differences that currently make them unsuitable to use for therapeutic purposes [8]. The golden standard for deriving pluripotent cells is in fact an embryonic stem cell derived from an embryo that has been made from the fertilization of an egg. If we are to work out the kinks in the iPS system, the use of embryonic stem cells will be key for making iPSCs suitable for clinical use. Thus, if the controversy arises due to the creation of embryonic stem cells, the following question arises: if we are to perfect iPS technology to effectively derive pluripotent cells that are equivalent to ES cells, should iPS cells be banned as well since our golden standard of comparison must be derived from human blastocysts that have the potential to become a human individual?
If a human life is defined from the moment that a cell has the potential to become a human being (i.e. conception), we find ourselves in an ethical conundrum when thinking about our genome as a whole. We know that all differentiated cells are equivalent in their genomes’ potential to become any cell in our bodies, and to also generate an entirely new adult, as we saw with Gurdon’s frogs and Wilmut’s sheep. Thus, does this indicate that all cells in the body have the potential to form a life and therefore should be considered as such? The beauty of John Gurdon’s, Ian Wilmut’s, the Oregon Group’s, and Yamanaka’s experiments are not that they derived a Brave New World type of technology to institute human cloning, but rather they reveal the inherent flexibility of our biology. If we define life from the moment we make a cell that has the potential to produce an entire individual, then we potentially must begin to categorize everything in our bodies by their own inherent potential to form an individual. It becomes incredibly difficult to make these categorizations. Unfortunately, the biology of our cells cannot be so clearly confined with these strict definitions. We are constantly learning that cells are dynamic systems. One way we could think about the flexibility of our cells is that engineering them for medicine capitalizes on this inherent biological flexibility. We should also keep in mind that when developing an ethical position, we should remember both the incredible life saving potential of these cloning technologies, as well as the historical scientific questions that they have answered.
Towards the Future
Cloning technologies have the potential to drive medicine into an era of regeneration. If we define human life as beginning when a cell has the potential to become a full human being, then we may run into difficulties when we consider that essentially any cell in our bodies has the potential to become a full human being. Many ethical arguments against cloning technologies and embryonic stem cell research argue that doing such research inherently destroys human life. We cannot dismiss these arguments, as they propose a valid question, that is—how do we define a human life? Ideally, for the benefit of both scientists and society, we would set ethical boundaries that would allow cloning technologies to benefit humanity in the best possible way.
Acknowledgements:
I’d like to thank my editor Jennifer Guidera for all of her help and feedback during the writing process.
References
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