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15. An example of gene therapy includes:
SCID (Severe Combined Immune Deficiency) is caused by a lack of an enzyme called ADA. When this enzyme is lacking, healthy lymphocytes (white blood cells) aren't produced by the bone marrow and the immune system can't fight off disease.
SCID is caused by a recessive gene. When a person is homozygous recessive for the trait, the person has SCID.
The gene therapy treatment involves:
1) The dominant (normal) gene for ADA is removed from the DNA of a normal person's cell.
2) The normal ADA gene is inserted into a retrovirus.
3) Stem cells are removed from blood in the infant patient's umbilical cord or placenta immediately upon birth.
4) The stem cells are isolated from the blood.
5) The retroviruses containing the ADA gene are mixed with the stem cells. The retroviruses enter the stem cells and insert their DNA (and the ADA gene) onto the stem cells' DNA.
6) The modified stem cells containing the normal ADA gene are injected into the patient's blood via a vein.
7) The modified stem cells increase in number in the patient. They make T-cells (other white blood cells) that make ADA enzyme. The patient has a normal immune response to disease.
8) Unfortunately, the effectiveness of this gene therapy process declines after several years, and the patient must undergo a new treatment to regain the immune response!
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