Answer:
Gene therapy is the technique of genetic engineering to
replace a faulty gene by a normal healthy functional gene. The first clinical
gene therapy was given in 1990 to a 4 years old girl with adenosine deaminase
deficiency (ADA deficiency). This enzyme is very important for the immune
system to function. Severe combined immunodeficiency (SCID) is caused due to
defect in the gene for the enzyme adenosine deaminase. SCID patient lacks
functional T-lymphocytes and, therefore, fails to fight the infecting
pathogens. Lymphocytes are extracted from the patient's bone marrow and a
normal functional copy of human gene coding for ADA is introduced into these
lymphocytes with the help of retroviral vector. The cells so treated are
reintroduced into the patient's bone marrow. The lymphocytes produced by these
cells contain functional ADA gene which reactivate the victim's immune system.
Though these cells are
not immortal, the patient requires periodic infusion of such genetically
engineered lymphocytes. However, if the gene isolated from marrow cells
producing ADA is introduced into cells at early embryonic stages, it could be a
permanent cure. Steps of gene therapy can be summarised in the given diagram.
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