Xenotransplantation

Over the past 20 years transplantation of heart and kidney has become almost a routine in human but the availability of donors is the major limiting factor and there is a shortage of suitable organs for transplant and many patients die as a result. Transgenic animals can be developed as organ donors to meet this shortfall. Somatic cell cloning will improve the chances of success because it will allow animal to be produced in which the animal proteins responsible for rejection will be removed and replaced by human counterparts. Clones of transgenic bovine embryos and fetal dopamine cells were isolated and transplanted in a patient of Parkinson disease successfully; however, the real effect on recipient has yet to be confirmed. The fact that Dolly was cloned from a cell taken from an adult ewe shows that even specialized cells can be reprogrammed into all the cell types that make up an intact animal. Moreover, there is prospect of using the patient's own cells in such therapies.

The  transplantation of animal organs, tissues or cells into humans, xenotransplantation - is a major practical use to which gene knockout technology in large animals could be applied. Owing to its abundant supply, ease of domestication, anatomical and physiological compatibility, the pig has become the candidate species of choice. However, formidable barriers of cross species reject ion limit xenotransplantation, with the first major hurdle being the phenomenon of hyper acute rejection. The major cause of this rejection is the reaction of antibodies present in human blood to a carbohydrate, galactose-a-1, and 3-galactose. The structures or epitopes of this disaccharide that induce the immune reaction are present on the cell surface of most mammals but not in humans.

Gene knockout technology now opens up the possibility of deleting the a-1,3galactosyltransferase gene, which would allow the production of animals lacking this epitope. This and other targets relating to xenotransplantation has been a major driver to developing pig cloning technology. Concerns, however, have been raised over the possible risk of zoonoses, due to expression of porcine endogenous retroviruses. Gene knockout technology could be used to delete potentially active proviruses from the pig genome, although if there are a large number of active loci this may not be practicable. Nevertheless, if cloning can be made reasonably efficient in the pig, it will provide a method for cloning animals with the appropriate genetic modifications and minimum provirus load which would reduce any risk.

Unfortunately, bovine serum albumin that is synthesised in the liver is secreted across the mammary epithelium into milk. Bovine and human serum albumin is very similar and the high levels of the endogenous protein in the milk poses a problem for the purification of the human protein. One solution to this is to replace the bovine gene with its human counterpart. Thus, the bovine protein would be eliminated without compromising the animals' viability and, indeed, the secretion into milk of the liver- derived human protein would augment that produced in the mammary gland itself.

For the future even more ambitious types of genetic modification can be contemplated. Mice have already been generated in which the major immunoglobulin (Ig) gene families were deleted and replaced by the corresponding light and heavy chain human Ig families. Immunisation of these animals with specific epitopes generates monoclonal antibodies for diagnostic or therapeutic applications that can be produced by means of standard monoclonal antibody technology. Polyclonal antibodies, however, have a greater affinity and broader specificity for their target than monoclonals and, as such, are preferred for therapeutic applications. Notwithstanding the technical difficulties similar modifications in livestock could enable the bulk production of specific human polyclonal antibodies, and so take antibody production technology to the next stage.