Assertion (A): In gene therapy for ADA deficiency, the patient needs periodic infusion of genetically engineered lymphocytes.
Reason (R): If the gene isolated from marrow cells producing ADA is introduced into cells at early embryonic stages, it could be a permanent cure.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are true, but (R) is not the correct explanation of (A).
Gene therapy corrects a gene defect by delivering a normal gene into the patient’s cells. The first clinical gene therapy, in 1990, was for adenosine deaminase (ADA) deficiency, a disorder of the immune system caused by deletion of the gene for ADA.
Assertion (A): In gene therapy for ADA deficiency, the patient needs periodic infusion of genetically engineered lymphocytes. — True. A is about the present treatment and why it must be repeated. Lymphocytes from the patient’s blood are grown outside the body, given a functional ADA cDNA through a retroviral vector, and returned to the patient. This infusion has to be given again and again.
Reason (R): If the gene isolated from marrow cells producing ADA is introduced into cells at early embryonic stages, it could be a permanent cure. — True. R is about a different, possible approach. If the ADA gene from marrow cells is put into cells at an early embryonic stage, it could be a permanent cure.
Why R does not explain A? The reason for the repeated infusions is that the engineered lymphocytes are not immortal; they die, and the patient needs fresh ones. R does not mention this at all. It only says how a permanent cure might be had by treating early embryonic cells, which is a separate method. So both statements are correct, but R is not the explanation of A.
NCERT Extract:
Gene Therapy – “… The first clinical gene therapy was given in 1990 to a 4-year old girl with adenosine deaminase (ADA) deficiency. … As a first step towards gene therapy, lymphocytes from the blood of the patient are grown in a culture outside the body. A functional ADA cDNA (using a retroviral vector) is then introduced into these lymphocytes, which are subsequently returned to the patient. However, as these cells are not immortal, the patient requires periodic infusion of such genetically engineered lymphocytes. However, if the gene isolate from marrow cells producing ADA is introduced into cells at early embryonic stages, it could be a permanent cure.”