What is gene therapy? Name the first clinical case where it was used?

Hint: Gene therapy is a method by which diseases are treated either by modifying the expressions of an individual’s genes or by correction of abnormal genes. It can be done by replacing mutated genes, inactivating a mutated gene, or by introducing a new gene into the body.

Complete answer:
Gene therapy is a method by which diseases are treated either by modifying the expressions of an individual’s genes or by correction of abnormal genes which can be achieved by replacing mutated genes, inactivating mutated genes, or by introducing a new gene into the body. There are two types of gene therapy:
- Somatic cell gene therapy: In this, therapeutic genes are transferred into the somatic cells. It is not inherited to the next generation. Examples: introduction of new genes into bone marrow cells, skin cells, etc.
- Germ line gene therapy: In this, therapeutic genes are transferred into germ cells. It is heritable as genes are introduced in eggs and sperms.
There are two approaches in somatic cell gene therapy:
>In-vivo: Genes are directly inserted into the cells of a particular tissue in the body.
>Ex-vivo: Transfer of genes to cultured cells and then reinsert them in the body.
The first clinical gene therapy was given in 1990 to a four-year girl who was suffering from Adenosine Deaminase (ADA) deficiency. It is an important enzyme for the functioning of the immune system. It is caused due to the deletion of genes for the adenosine deaminase. This disorder can be treated by either bone marrow transplantation or enzyme replacement therapy, in which functional ADA is given to the patient by injection. Although, these are not completely curative. In gene therapy, lymphocytes from the blood of the patients are grown in culture. A functional ADA using retroviral DNA vectors is then introduced into the cultured lymphocytes, which are then reinserted in the patient’s body. However, the patient requires periodic transfusion as the cells are mortal. If in case, this technique is carried out in the embryonic stage, it could be a permanent cure.

Note: Gene therapy has its pros and cons.
Pros: can be used to eradicate disease from future generations, has potential to eliminate and prevent hereditary diseases, possible to cure heart diseases, AIDS, etc.
Cons: long lasting effects cannot be achieved by gene therapy as cells keep rapidly dividing, disorders caused by effects of multiple genes cannot be cured, virus used as vectors may cause infection or toxicity.