Some information on stem cells and their applications.

From: “Stem Cell Research and Application: Monitoring the Frontiers of Biomedical Research”

American Association for the Advancement of Science and Institute for Civil Society
Website: www.aaas.org/spp/sfrl/projects/stem

“Stem cells” is a term to describe precursor cells that can give rise to multiple tissue types. There are important distinctions, however, regarding how developmentally plastic these cells are; that is, how many different paths they can follow and to what portion of a functioning organism they can contribute. Totipotent stem cells are cells that can give rise to a fully functional organism as well as to every cell type of the body. Pluripotent stem cells are capable of giving rise to virtually any tissue type, but not to a functioning organism. Multipotent stem cells are more differentiated cells (that is, their possible lineages are less plastic/more determined) and thus can give rise only to a limited number of tissues. For example, a specific type of multipotent stem cell called a mesenchymal stem cell has been shown to produce bone, muscle, cartilage, fat, and other connective tissues. There are many potential sources for stem cells. Embryonic stem cells are derived from the inner cell mass of a blastocyst (a very early embryo). Embryonic germ cells are collected from fetal tissue at a somewhat later stage of development (from a region called the gonadal ridge), and the cell types that they can develop into may be slightly limited. Adult stem cells are derived from mature tissue. Even after complete maturation of an organism, cells need to be replaced (a good example is blood, but this is true for muscle and other connective tissue as well, and may be true for at least some nervous system cells). Because these give rise to a limited number of cell types, they are perhaps more accurately referred to as multipotent stem cells, as discussed above. Knowledge about stem cell science and potential applications has been accumulating for more than 30 years. In the 1960s, it was recognized that certain mouse cells had the capacity to form multiple tissue types, and the discovery of bona fide stem cells from mice occurred in 1971. Limited types of stem cell therapies are already in use. The most well-known therapy is the stem cell transplant (a form of a bone marrow transplant) for cancer patients. In this therapy, stem cells that can give rise to blood cells (red and white cells, and platelets) are given to patients to restore tissue destroyed by high dose chemotherapy or radiation therapy. But it has been only recently that scientists have understood stem cells well enough to consider the possibilities of growing them outside the body for long periods of time. With that advance, rigorous experiments can be conducted, and the possibility of manipulating these cells in such a way that specific tissues can be grown is real. It is impossible to project when actual treatments or cures might emerge from such research, but the paths this research might take and potential applications have been much discussed. To understand the potential clinical applications, it is critical to understand the research that is taking place now.

Sources and Characterisctics of Human Stem Cells

Human Embryonic Stem Cells. The study of human stem cells has barely begun and what is known is summarized in this section. The vast majority of experimental data discussed here are the results of experiments in mice. ES cells from the mouse have been intensely investigated since their discovery 27 years ago. Therefore, what is said about human ES cells assumes in part that their fundamental properties will resemble those of mouse ES cells. While on the surface this assumption appears to be reasonable it will have to be proven through intensive further investigation. There is an abundance of stem cell lines from mammals including some from human beings. ES cells are valuable scientifically because they combine three properties not found together in other cell lines. First, they appear to replicate indefinitely without undergoing senescence (aging and death) or mutation of the genetic material. They are thus a large-scale and valuable source of cells. Second, ES cells appear genetically normal, both by a series of genetic tests and functionally as shown by the creation of mice with genomes derived entirely from ES cells. In mice these cells are developmentally totipotent; when inserted into an early embryo, they join the host cells to create a normal mouse, differentiating into every cell type of the body (it is this property that earns them the name “stem cell of the body”). ES cells can also differentiate into many cell types in tissue culture, including neurons, blood cells and cardiac and skeletal muscle. The normal embryo has about 100 cells with the properties of ES cells that exist for about one day and then develop into more advanced cell types. The isolation and subsequent growth of ES cells in culture allow scientists to obtain millions of these cells in a single tissue culture flask, making something once rare and precious now readily available to researchers. It is worth noting here the striking parallel to recombinant DNA and monoclonal antibody technologies, both of which have amplified rare and precious biological entities. Like those technologies, ES cell technology may well be transformative in opening scientific arenas that to date have been closed. The isolation, culture, and partial characterization of stem cells isolated from human embryos was reported in November of 1998.1 The ability of the cells to maintain their pluripotent character even after 4 to 5 months of culturing was demonstrated.2 There is concern that this feature of these cells could also lead to cancerous growth. Thus far there are no data indicating the induction of malignant tumours, although there is some evidence for benign hyperproliferation (overgrowth of cells).3

Human Embryonic Germ Cells. Embryonic germ cells are derived from primordial germline cells in early fetal tissue during a narrow window of development. Unlike embryonic stem cells, animal experiments on embryonic germ cells have been limited. In November of 1998, the isolation, culture, and partial characterization of germ cells derived from the gonadal ridge of human tissue obtained from abort uses was reported.4 These experiments showed that these EG cells are capable of forming the three germ layers that make all the specific organs of the body. There are fewer data from animal EG cell experiments than from ES cell experiments, but it is generally assumed that the range of potential fates will be relatively limited compared to ES cells, because the EG cells are much further along in development (5-9 weeks as opposed to 5 days in the published experiments). Fetal tissue may provide committed neural progenitors, but the feasibility of large scale sourcing and manufacturing of products utilizing such cells is questionable. Furthermore, the behaviour of these cells in vivo is not well understood; significant research will be required to avoid unwanted outcomes, including ectopic tissue formation (additional, unwanted tissue), tumour induction, or other abnormal development.5

Human adult stem cells. From post-embryonic development through the normal life of any organism, certain tissues of the body require stem cells for normal turnover and repair. Stem cells that are found in developed tissue, regardless of the age of the organism at the time, are referred to as adult stem cells. The most well-known example of this are the hematopoietic stem cells of blood.6 More recently, mesenchymal stem cells (MSC) required for the maintenance of bone, muscle, and other tissues have been discovered.7 Adult stem cells are multipotent; the number of tissues that they can regenerate compares poorly with the pluripotency of embryonic stem cells and embryonic germ cells. However, the MSC is in fact an excellent example of the potential for use of stem cells in human therapeutic procedures. MSCs are capable of differentiating into bone, cartilage, muscle, fat, and a few other tissue types. Their use for bone and cartilage replacement is undergoing FDA-approved clinical trials at the present time. Adult-derived stem cell therapies will complement, but cannot replace, therapies that may be eventually obtained from ES cells. They do have some advantages. For example, adult stem cells offer the opportunity to utilize small samples of adult tissues to obtain an initial culture of a patient’s own cells for expansion and subsequent implantation (this is called an autologous transplant). This process avoids any ethical or legal issues concerning sourcing, and also protects the patient from viral, bacterial, or other contamination from another individual. With proper manufacturing quality controls and testing, allogeneic adult stem cells (cells from a donor) may be practical as well. Already in clinical use are autologous and allogeneic transplants of hematopoietic stem cells that are isolated from mobilized peripheral blood or from bone marrow by positive selection with antibodies in commercial devices. In general, there is less ethical concern over their initial source. Additionally, since they normally differentiate into a narrower set of cell types, directing them to a desired fate is more straightforward. However, many cells of medical interest cannot, as of yet, be obtained from adult-derived cell types. Production of large numbers of these cells is much more difficult than is the case for ES cells. Based upon our present knowledge base, it appears unlikely that human adult stem cells alone will provide the entire necessary cell types required for the most clinically important areas of research.

The Clinical Potentials for Stem Cell Products

The economic and psychological tolls of chronic, degenerative, and acute diseases in the United States are enormous. It has been estimated that up to 128 million people suffer from such diseases; thus, virtually every citizen is effected directly or indirectly.8 The total costs of treating diabetes, for example is approaching $100 billion in the United States alone.9 As more research takes place, the developmental potential of different kinds of stem cells will become better understood. As the science is understood now, adult stem cells are limited in their potential to differentiate. Embryonic germ cells have a great differentiation capacity, and embryonic stem cells are thought to be able to differentiate into almost any tissue. Thus, different types of stem cells could have different applications. Below is a discussion of potential stem cell applications.

Some Examples of Treatments for Major Diseases

 Type 1 Diabetes in Children. Type 1 diabetes is an autoimmune disease characterized by destruction of insulin producing cells in the pancreas. Current efforts to treat these patients with human islet transplantation in an effort to restore insulin secretory function (obtained from human pancreas) are limited severely by the small numbers of donated pancreas available each year combined with the toxicity of immunosuppressive drug treatments required to prevent graft rejection.10 Pluripotent stem cells, instructed to differentiate into a particular pancreatic cell called a beta cell, could overcome the shortage of therapeutically effective material to transplant. They also afford the opportunity to engineer such cells to effectively resist immune attack as well as graft rejection. Nervous System Diseases. Many nervous system diseases result from loss of nerve cells. Mature nerve cells cannot divide to replace those that are lost. Thus, without a “new” source of functioning nerve tissue, no therapeutic possibilities exist. In Parkinson’s disease, nerve cells that make the chemical dopamine die. In Alzheimer’s disease, cells that are responsible for the production of certain neurotransmitters die. In amyotrophic lateral sclerosis, the motor nerve cells that activate muscles die. In spinal cord injury, brain trauma, and even stroke, many different types of cells are lost or die. In multiple sclerosis, glia, the cells that protect nerve fibers are lost.11 Perhaps the only hope for treating such individuals comes from the potential to create new nerve tissue restoring function from pluripotent stem cells. Remarkably, human clinical experiments have demonstrated the potential effectiveness of this approach to treatment. Parkinson’s patients have been treated by surgical implantation of fetal cells into their brain with some benefit. Although not completely effective, perhaps owing to lack of sufficient numbers of dopamine secreting cells, similar experiments using appropriately differentiated stem cells should overcome those obstacles.12 More complex experiments have already been successfully conducted in rodent models of Parkinson’s.13 Similar approaches could be developed to replace the dead or dysfunctional cells in cortical and hippocampal brain regions that are affected in patients with Alzheimer’s.

Primary Immunodeficiency Diseases. Pluripotent stem cells could be used in treatment of virtually all primary immunodeficiency diseases. Presently, there are more than 70 different forms of congenital and inherited deficiencies of the immune system that have been recognized. These are among the most complicated diseases to treat with the worst prognoses. Included here are diseases such as severe combined immunodeficiency disease (the “bubble boy” disease), Wiskott - Aldrich syndrome, and the autoimmune disease lupus. The immune deficiencies suffered as a result of acquired immune deficiency syndrome (AIDS) following infection with the human immunodeficiency virus are also relevant here. These diseases are characterized by an unusual susceptibility to infection and often associated with anaemia, arthritis, diarrhoea, and selected malignancies. However, the transplantation of stem cells reconstituted with the normal gene could result in restoration of immune function and effective normalization of life span and quality of life for these people.

Diseases of Bone and Cartilage. Stem cells, once appropriately differentiated, could correct many diseases and degenerative conditions in which bone or cartilage cells are deficient in numbers or defective in function. This holds promise for treatment of genetic disorders such as osteogenesis imperfecta and chondrodysplasias. Similarly, cells could be cultivated and introduced into damaged areas of joint cartilage in cases of osteoarthritis or into large gaps in bone from fractures or surgery.

Cancer. At the present time, bone marrow stem cells, representing a more committed stem cell, are used to rescue patients following high dose chemotherapy. Unfortunately, these recovered cells are limited in their capacity to restore immune function completely in this setting. It is hoped that injections of properly-differentiated stem cells would return the complete repertoire of immune response to patients undergoing bone marrow transplantation. Complete and functional restoration will be required if, for example, immune/vaccine anticancer therapy is to work. More importantly, success would permit use of very toxic (and effective) chemotherapeutic regimens that could not currently be utilized for lack of an ability to restore marrow and immune function.

Uses in Research

Much is left to be discovered and understood in all aspects of human biology. What has been frequently lacking are the tools necessary to make the initial discoveries, or to apply the knowledge of discoveries to the understanding of complex systems. These are some of the larger problems in basic and clinical biology where the use of stem cells might be the key to understanding.

A new window on human developmental biology. The study of human developmental biology is particularly constrained by practical and ethical limitations. Human ES cells may allow scientists to investigate how early human cells become committed to the major lineages of the body; how these lineages lay down the rudiments of the body’s tissues and organs; and how cells within these rudiments differentiate to form the myriad functional cell types which underlie normal function in the adult. The knowledge gained will impact many fields. For example, cancer biology will reap an especially large reward because it is now understood that many cancers arise by perturbations of normal developmental processes. The availability of human ES cells will also greatly accelerate the understanding of the causes of birth defects and thus lead directly to their possible prevention.

Models of human disease that are constrained by current animal and cell culture models. Investigation of a number of human diseases is severely constrained by a lack of in vitro models. A number of pathogenic viruses including human immunodeficiency virus and hepatitis C virus grow only in human or chimpanzee cells. ES cells might provide cell and tissue types that will greatly accelerate investigation into these and other viral diseases. Current animal models of neurodegenerative diseases such as Alzheimer’s disease give only a very partial representation of the disease’s process.

Transplantation. Pluripotent stem cells could be used to create an unlimited supply of cells, tissues, or even organs that could be used to restore function without the requirement for toxic immunosuppression and without regard to tissue matching compatibility. Such cells, when used in transplantation therapies, would in effect be suitable for “universal” donation. Bone marrow transplantation, a difficult and expensive procedure associated with significant hazards, could become safe, cost effective, and be available for treating a wide range of clinical disorders, including aplastic anemia and certain inherited blood disorders. This would be especially important in persons who lost marrow function from toxic exposure, for example to radiation or toxic agents. Growth and transplant of other tissues lost to disease or accident, for example, skin, heart, nervous system components, and other major organs, are foreseeable.

Gene Therapy. In gene therapy, genetic material that provides a missing or necessary protein, or causes a clinically-relevant biochemical process, is introduced into an organ for a therapeutic effect. For gene-based therapies (specifically, those using DNA sequences), it is critical that the desired gene be introduced into organ stem cells in order to achieve long-term expression and therapeutic effect. Although techniques for delivering the therapeutic DNA have been greatly improved since the first gene therapy protocol almost 10 years ago, there are as yet no bona fide successes. Besides delivery problems, loss of expression or insufficient expression is an important limiting factor in successful application of gene therapy and could be overcome by transferring genes into stem cells (which presumably will then differentiate and target correctly).

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