What is Gene Therapy?
a) There are certain diseases which are not caused by bacteria, viruses, or other organisms. They occur due to defects in genes inside the body. When a defective gene is replaced with the correct gene, it is called gene therapy. It is an attempt that allows correction of a gene defect that has been diagnosed in a child or embryo stage. The correction of a genetic defect involves delivery of a normal gene into the individual or embryo to take over the function of, and compensate for, the non-functional gene.
b) There is an ADA-gene that (present in every cell of the body) expresses itself and instructs the formation of the enzyme, adenosine deaminase. This enzyme is produced in all cells, but the high concentration of this enzyme is present in immune system cells, called lymphocytes. These lymphocytes or white blood cells defend the body against foreign invaders such as viruses and bacteria.
The main function of the ADA enzyme is to remove a toxin called deoxyadenosine, which is formed when DNA is broken down. A high concentration of deoxyadenosine can lead to early cell death. The ADA enzyme converts deoxyadenosine to another molecule called deoxyinosine, which is not harmful at all.
A mutation or deletion of the ADA gene causes Adenosine deaminase (ADA) deficiency. Due to the absence of the adenosine enzyme, the deoxyadenosine toxin is built up that leads to the death of lymphocytes. And lymphocyte formation does not occur. Due to the absence of immune cells, the body becomes susceptible to foreign infections.
How Scientists Developed the Mechanism of Gene Therapy?
The first clinical gene therapy was given in 1990 to a 4-year-old girl with ADA-deficiency. Scientists found three different methods to deal with the defective ADA gene. They are as follows:
- Enzyme replacement therapy: ADA enzyme can be given to patients as injections, but needs to be done periodically. It is not completely curative.
- Bone marrow transplantation: Stem cells from the bone marrow of a healthy individual are injected into the patient. These bone marrow stem cells differentiate and form new lymphocytes.
- Gene therapy: In this method of gene therapy, lymphocytes from the blood of patients are grown in a culture outside the body. A functional ADA cDNA with the help of a retroviral vector is then introduced into these lymphocytes, which are subsequently returned to the patient. Since, these cells are not immortal, the patient requires periodic infusion of such genetically engineered lymphocytes.