How is gene therapy useful for the treatment of various diseases?

(N/A) Gene therapy is a technique used to treat hereditary diseases by correcting a gene defect that has been diagnosed in a child or embryo.
In this process,functional genes are inserted into a person's cells and tissues to treat the underlying cause of a disease.
The first clinical gene therapy was administered in $1990$ to a $4$-year-old girl suffering from adenosine deaminase $(ADA)$ deficiency. This enzyme is crucial for the proper functioning of the immune system,and the disorder is caused by the deletion of the gene responsible for producing it.
While $ADA$ deficiency can sometimes be treated by bone marrow transplantation or enzyme replacement therapy (where functional $ADA$ is injected),these methods are not completely curative.
As a step toward permanent gene therapy,lymphocytes from the patient's blood are grown in a culture outside the body.
$A$ functional $ADA$-$cDNA$ is then introduced into these lymphocytes using a retroviral vector,and the modified cells are returned to the patient. Since these cells are not immortal,the patient requires periodic infusions of these genetically engineered lymphocytes.
However,if the gene isolated from marrow cells producing $ADA$ is introduced into cells at early embryonic stages,it could provide a permanent cure.