Cloning Human Embryos for Therapy
A major technical difficulty looms over the prospects of using currently available stem cells for therapy: the derivatives of these stem cells, if transferred to patients, would likely be rejected by their bodies as foreign and targeted for destruction. Although recent research suggests that human embryonic stem cells and their derivatives are much less susceptible to immune rejection than adult stem cells, it confirms that such cells and derivatives may trigger a rejection response in some patients. To overcome this problem, stem cell scientists are turning to the possibility of using non-reproductive cloning to develop human embryos from which they would derive stem cells that would be immunologically and genetically matched to specific patients.
This use of such cloning would reduce the likelihood of immune rejection and avoid the need for adjunct immunosuppressive therapies. Such non-reproductive cloning is termed research cloning here, rather than therapeutic cloning, as some call it, since this procedure has not yet reached a stage of development at which it could be employed in the treatment of patients. It is also termed ‘‘research cloning’’ to indicate that the embryos developed by this method will not be used for procreative purposes but for research and eventual therapy. ‘‘Research cloning’’, in contrast to reproductive cloning, is carried out in the laboratory in order to generate stem cells that can be cultured to use for tissue replacement in therapy. The embryos developed in this process would not be transferred to any woman’s uterus. Finally, this method is termed ‘‘research cloning’’ instead of somatic cell nuclear transfer for research because the former nontechnical term is more meaningful to those who are not scientists.
Just what is research cloning ? It involves removing the nucleus of a donated human egg, with all its genetic material, and replacing it with the nucleus of an adult somatic (body) cell from an individual patient. This results in an egg that contains the complete complement of the donor cell’s nuclear genetic material. Medical investigators then use an electric impulse or chemicals to cause the somatic cell nucleus and egg to fuse together, resulting in a human embryo. If researchers were to learn how to carry out this sort of cloning for therapeutic purposes, they would take a somatic cell from a specific patient, remove its nucleus, and transfer that nucleus to a donated human egg whose nucleus has been removed. They would clone that egg and at about five days of growth when it reaches the blastocyst stage derive stem cells from it. Next they would direct offshoots of these cells to differentiate into the cell type needed to treat the patient who initially provided the body cell and would then transfer these differentiated cells to that patient. Because these cells would be almost genetically identical to the cells of the patient (except for their mitochondrial DNA, which is found in the material surrounding the nucleus of the egg), it is extremely unlikely that they would be rejected.
Thus, research cloning would provide an additional source of human embryonic stem cells that does not involve the use of fertilized human eggs. For example, investigators engaging in research cloning would remove a skin cell or other body cell from a patient in need of additional heart muscle, insert it into a human egg from which the nucleus had been removed, and stimulate that egg to develop into an embryo. They would then derive stem cells from the inner cell mass of this embryo and establish embryonic stem cell lines from derivatives of these cells. Next they would take one of these lines and direct it to differentiate into heart muscle cells that they would transplant back into the patient. These heart muscle cells would have the same genes and immunological makeup as those of the patient and therefore would not trigger rejection. Remaining heart muscle cells could be stored to have available as replacement cells, so that treatment of the patient could be repeated if necessary.
To treat patients with gene-based diseases, research cloning could also be combined with gene transfer. Embryonic stem cells would be derived by means of research cloning, using the somatic cells of the patient with a genetic disorder, and derivatives of these cells would then be subjected to gene transfer or modification, differentiated, and transplanted back to that patient. The feasibility of this method has been demonstrated in an animal model using mice with a genetic disorder that has a human counterpart. Such research cloning, it is hoped, could enable scientists to identify the origins of such conditions as Parkinson’s disease and Alzheimer’s disease.
Thus, research cloning offers a way to learn what paths certain diseases take as they make headway in the human body. For example, it could enable scientists to identify the origins of such gene-based diseases as amyotrophic lateral sclerosis (Lou Gehrig’s disease) and β-thalassemia and provide new information to use in treating patients with these conditions. Furthermore, research cloning offers a way to develop more accurate models of various human diseases in order to test drugs and to explore the early stages of embryonic development in order to learn how and why embryonic and adult stem cells differentiate into more specialized cells.