INDEX

Chapter 5: Molecular biology and genetics

Molecular biology and genetics

As described in an earlier section, the advances in recombinant DNA technology have enabled the genetic engineering of animals for the purpose of creating models of human disease. Typically mice are used, although rat and rabbit models have been developed. These animals may be engineered to contain a human disease-causing gene in their genome, or alternatively have their native gene (homologous to the human gene) mutated to harbor a human disease-causing mutation. Thereafter, these mice can be bred to transmit the gene of interest, with expression of the mutant protein in succeeding generations. These transgenic animals thereafter become a model for in vivo analysis of the pathogenesis of the disease and to seek novel therapies.

Numerous animal models of human disease have been created as a result of the identification of disease-causing genes. The development of animal models provides the investigator an opportunity to determine the molecular pathways and evaluate novel therapeutic strategies.

See also under "Gene transfer - transgenic animals".

Futuristic therapy utilizing stem cells and nuclear transfer       

There is tremendous interest in stem cells as therapy for certain diseases. The hope is to stimulate stem cells to differentiate into a particular desired cell. Stem cells are thus being explored as a major research interest both in vivo and in vitro together with clinical studies in human beings. While this field of research is still in its infancy, it is very exciting and the potential is perhaps much greater than for gene therapy and likely to yield therapeutic benefit within the next 5 to 10 years.

There are two types of stem cells: embryonic and adult. Embryonic stem cells (ES) are totipotent cells derived from the inner cell mass of the blastocyst that can be replicated indefinitely in an undifferentiated state in cell culture. These ES cells can in vivo be stimulated to differentiate into many cell types. The human being, while thought to have a million trillion cells, has only about 200 different cells as defined by a specific function. It is well recognized that all of these cells come from a single stem cell, namely, the egg and the sperm, and from that probably develops about 20 embryonic undifferentiated stem cells from which are derived the 200 differentiated cells. The use of embryonic stem cells is very much mired into ethical issues that have prohibited this research from proceeding as rapidly as perhaps possible. This has also spurred great interest in so-called adult stems cells. It has become evident from several sources that adult stems cells exist in most organs. The heart is considered to be a terminally differentiated organ and as such does not have stem cells; however, there is increasing evidence to suggest the occasional stem cells may be present in the heart, although data is still not yet convincing. There are considerable data to indicate stem cells exist in the bone marrow, the brain, the liver, and other organs. There are numerous studies to show that, when one has a myocardial infarction, certain progenitor bone marrow cells are attracted to the area and appear to have some therapeutic effect whether it is through release of paracrine factors or other means. Embryonic stem cells could, of course, lead to rejection and in a very small percentage may proliferate into undifferentiated tumors such as teratomas. In contrast, adult stems when utilized from the individual’s bone marrow for example would not be rejected and there are considerable data to show that they are less likely to develop into teratomas. There are many studies ongoing in animals and to a lesser extent in humans utilizing bone marrow adult stem cells with the hope they will regenerate myocardium or induce the formation of blood vessels.

The limitation of adult stem cells is their lack of plasticity since they are already to some extent committed to differentiate into their own cell lineage, in contrast to embryonic stem cells, which are capable of differentiating into any form of cells. Adult stem cells are a fruitful area of research and will, of course, be pursued with great interest over the next decade. The other approach that is receiving some interest but with less success is attempts to reverse the commitment or differentiation and force the cell back into a replicating cell cycle such that it can be stimulated to metamorphosis into the cell of choice. The embryonic stem cell has the most potential with respect to differentiating into the desired cells. Regardless of the potential applications for either the ES or the adult stem cells, research currently must be directed to understanding the mechanisms involved in cell differentiation. Even if ES cells were available, there remains to be determined which factors are required to stimulate their differentiation into a particular cell. It is likely that extensive fundamental research is required to answer these questions before one is likely to achieve therapeutic success in any of these avenues.

The other area of research that is also being pursued but less intensely is that of nuclear transfer. Nuclear transfer, whereby a nucleus of one’s own cells is inserted into a prior enucleated stem cell, also has great promise. This would avoid the problem of rejection as well as that of teratomas associated with ES cells and would indeed regenerate a cell with a genotype identical to that of the individual. This technique has been off limits from an ethical point of view since theoretically one could clone human beings utilizing this technique. However, nuclear transfer is being pursued in countries such as Britain, Russia, and Sweden with the appropriate prohibition not to clone human embryos. These studies are even more primitive than those of stem cells but can be expected to enter into the mainstream as we unravel the fundamental mechanisms and appreciate their true potential as therapeutic weapons.

See also under "Stem cells" and "Cloning".