Question
Gene therapy was used to treat a child suffering from Adenosine Deaminase (ADA) deficiency. Describe the steps involved in this gene therapy procedure.
(NEET 2024 — Biotechnology and its Applications)
Solution — Step by Step
Blood is drawn from the ADA-deficient child and T-lymphocytes are separated. These are the very cells being destroyed by ADA deficiency — we pull them out before they die, while they still exist in sufficient numbers.
A retrovirus (with its disease-causing genes removed) is loaded with the functional human ADA gene. This recombinant retrovirus is used to infect the isolated lymphocytes in culture — the virus integrates the ADA gene into the lymphocyte’s chromosomal DNA. This integration is permanent, which is why retroviruses are preferred here.
The transformed lymphocytes are allowed to multiply in the lab. We need large numbers of cells — a handful won’t help the patient. This culture phase gives us a therapeutic dose.
The engineered lymphocytes are injected back into the patient’s body. Once inside, they produce functional ADA enzyme, breaking down the toxic deoxyadenosine that was previously killing immune cells.
Because lymphocytes are not immortal (they have a finite lifespan), the procedure must be repeated. Every time the transformed lymphocyte population declines, the patient needs another round of therapy. This is the critical limitation of the current protocol.
Why This Works
ADA (Adenosine Deaminase) converts deoxyadenosine → deoxyinosine. Without this, deoxyadenosine accumulates and becomes toxic — specifically to T-lymphocytes. No T-cells means no adaptive immunity. The child literally cannot fight infections. This is called SCID (Severe Combined Immunodeficiency).
By inserting the functional ADA gene into the patient’s own lymphocytes, we turn those cells into miniature ADA factories. The enzyme gets produced from inside the immune cells — exactly where it’s needed. This is elegant because we’re not trying to deliver the enzyme as a drug to every corner of the body; we’re making the target cells produce it themselves.
This was one of the first successful gene therapy attempts in humans (1990, W. French Anderson’s team). NCERT mentions it as the landmark case. For NEET, know this story cold — it appears almost every year in some form.
Alternative Method
A more permanent fix — not yet routine, but conceptually important for NEET — involves inserting the ADA gene into bone marrow stem cells (haematopoietic stem cells) instead of mature lymphocytes.
Stem cells divide and produce daughter cells for life. If the ADA gene is integrated into a stem cell, every lymphocyte that stem cell ever produces will carry the functional gene. One procedure, lifelong correction. This is the direction gene therapy research is heading, but the challenge is that modifying stem cells carries a higher risk of insertional mutagenesis (accidentally disrupting a tumour-suppressor gene).
Common Mistake
Many students write that “enzyme replacement therapy” and “gene therapy” are the same thing for ADA deficiency. They are not. Enzyme replacement (giving ADA enzyme as a drug, e.g., PEG-ADA injection) treats the symptom — you keep injecting the enzyme. Gene therapy inserts the gene so the patient’s own cells manufacture the enzyme. NEET 2024 specifically tested this distinction. If the question says “gene therapy,” your answer must involve gene insertion into lymphocytes — not enzyme administration.
For quick recall: L-V-C-R — Lymphocytes isolated → Viral vector introduces gene → Culture to multiply → Reinfuse into patient. Four steps, four letters. The fifth point (repetition needed) is what distinguishes it from a permanent cure — examiners love testing that limitation.
The final answer the question is looking for: lymphocytes are extracted from the patient, the functional ADA cDNA is delivered via a retroviral vector, the transformed cells are cultured, and reinfused — but because lymphocytes are mortal, periodic reinfusion is necessary, making this a management strategy rather than a permanent cure.