The basic principle of gene therapy is to replace defective or faulty genes with normal genes. Although genetic disorders can often be prevented by prenatal diagnosis and genetic counseling but still not possible to cure genetic diseases. Gene therapy refers to the transfer of exogenous DNA into cells in order to fight disease by modifying gene expression. Such modification may benefit the cell directly or may modulate disease activity elsewhere by changing protein secretion by that cell.

Monogenic diseases such as adenosine deminase (ADA) deficiency and cystic fibrosis were the first to be tackled, providing an active gene where both copies of the endogenous genes had been inactivated by inherited mutations. The current trend is aimed at polygenic diseases. Cancer is a major target for gene therapy, with the aim, For example, of transferring ‘suicide genes’ into cancer cells.

Types of gene therapy

Gene therapy can be possible in two ways i.e. through germ line therapy and somatic cell therapy. Successful experiments with mice have shown that gene therapy is possible.

1- Germ line therapy: In this, genes are micro-injected into fertilized eggs with a known genetic disorder and the corrected eggs are reimplanted into the mother. In this method, all cells of the future mouse are normal because they are all derived from a corrected egg. This procedure is known as germ-line therapy and all descendants of the cured animal will be normal.

2- Somatic cell therapy: This involves changing genes of some of the somatic cells (not all) which are non-sex cells of the body. Changes in these can not be inherited. The people treated will therefore be cured but they will still be able to pass the faulty gene onto their offspring. 

It should be noted that for ethical purpose, the principle of gene therapy into the human cells contributing to the germ line has not been accepted, but instead the aim is to offer specific treatment to each individual by transferring DNA into non-germ line somatic cells.

Strategies for Gene therapy

Naked DNA may be transferred to a cell but successful delivery is rare. Some improvement is obtained by transferring DNA in a liposome with a lipid coat but viral based vectors lead to marked improvement in delivery. The following criteria should be carefully viewed for gene therapy.

1)    Efficient DNA delivery into the cells or high efficiency of gene transfer.
2)    Persistent and effective expression of the gene product in the target tissue.
3)    Persistence of the infected cell and its progeny in the face of cellular lifespan and host immune responses.
4)    Stable replication of the introduced gene either as an integrated transgene or as an extra-chromosomal element.
5)    Efficacy of transferred genetic material in correcting disease.
6)    Adequate safety during the gene transfer period and throughout the life of the patient undergoing replacement therapy.

Procedure

In principle, the normal gene is isolated and cloned. A safe and efficient vector is then needed to introduce it into the chosen human cells. These cells may have to be isolated from the body first, corrected and then replaced. This would be relatively easy for blood diseases such as sickle cell anemia because the cells that make blood cells can easily be removed from bone marrow and replaced. The final problem is to make sure that the gene is expressed normally. If it is not switched on and off normally, than it could create more problems than it solves. The situation is more complex when a disease is caused by a dominant gene. Here the dominant gene must also be removed or made ineffective and techniques have not yet been developed for this. Vectors which have been considered for this purpose are viruses, liposomes, microinjection and electroporation.

Cellular uptake of exogenous DNA is enhanced by packing the DNA, for example, in a viral genome from which crucial genes have been deleted.

A case study of Gene therapy

Here the example of Severe Combined Immunodeficiency disease (SCID) is taken into consideration. One form of this disease affects a gene coding for the enzyme ADA (adenosine deaminase). The mutant gene is recessive and makes no enzyme. Heterozygous children are unaffected because their one normal gene makes sufficient ADA. ADA is needed by the white blood cells which is responsible for immunity to infection. Without ADA they die, so suffers of the disease have to live in a completely sterile environment with no direct human contact. Otherwise they normally die by the age of two.

Limitations in gene therapy

1. There is no sure method of introducing genes into human cells in such a way that they are subjected to body’s normal control systems. If gene remains switched off, nothing worthwhile happens. At the same time if it becomes too active, instructing the cell to produce excessive amount of a particular protein, consequently damage as those of disease it was intended to cure.

2. Gene replacement therapy can only be attempted in very few selective genetic diseases. These include single gene defects in which the molecular details of the affected gene are well understood

Gene therapy is a beneficial facility with an important role in the welfare of the society. Gene therapy to cure heredity disease and relive human suffering seems reasonable but any attempt to produce some sort of a superman would destroy humanity’s rich genetic heritage.