If a child is diagnosed to carry a defective gene leading to disability, one may like to get this gene replaced by a normal functional gene. This is gene therapy in theory. One would like to ask that if there is a need and demand for gene therapy, can it be done ? The answer to this question has changed from 'no' to 'yes' in recent years. The possibilities are being explored and treatment by transfusion of cells with functional gene, if not by replacement of defective gene, are being suggested and tried. Gene therapy can be used at two different levels : (i) patient therapy, in which cells with healthy gene may be introduced in the affected tissue, so that the healthy gene overcomes the defect without affecting the inheritance of the patient and (ii) embryo therapy, in which the genetic constitution of embryo at the post-zygotic level is altered, so that the inheritance will be altered. It is believed that in future, gene therapy of both types will possible.
Patient therapy. Patient therapy will involve the following steps : (i) the defective gene should be identified, (ii) normal healthy gene should either be isolated or synthesized, (iii) isolation of cells of the tissue, where the normal healthy gene will need to function, (iv) the normal gene should be placed into a cell, where it can function. The gene will have to be placed into the correct site on the host chromosome, so that the gene may function, or even one may have to delete the defective gene. There are three main problems in this connection. First, the introduced gene may not function, second, that when corrected cells are reintroduced, these may be outnumbered by the non-cured resident cells, and third, there are only few diseases affecting only a single tissue.
Utilizing the above approach, first clinical gene transfer (approved in U.S.A.) was achieved in 1989. It was a marker gene (neomycin resistance = NeoR) introduced into tumor-infiltrating lymphocytes (TIL). These NeoR/TILs were transferred into patients with advanced cancer, to ensure that the approved protocol really works. The first approved gene therapy protocol for correction of adenosine deaminase (ADA) deficiency, however, began in 1990 and by the end of 1992, two dozen active clinical protocols on three continents became available for trials. However, the technology is still very expensive and specialized to be used extensively. Other less expensive techniques (involving delivery of gene through vectors) are being developed.
Embryo therapy. This will involve the following steps, which have been tried in case of mouse or rabbit only, (i) in-vitro fertilization of the egg. (ii) insertion of normal gene into embryo at post zygotic level, either with viruses or directly by microinjection, (iii) integration of inserted gene in host DNA, where it may or may not function. The inserted genes have been found to be inactive generally, but in few animals, genes have been switched on in a tissue specific way but their activity is at a very low level. However, it is not yet possible that the therapeutic newly inserted genes function under normal control in the animal, in time, space or quantity.