Q.(a) How does a gene therapy involving direct modification of the cells, in order to achieve a therapeutic goal is used in the treatment of ADA deficiency ? Explain.

(a) In the treatment of ADA deficiency, gene therapy is carried out by isolating lymphocytes from the patient's blood. These cells are then corrected by introducing a functional copy of the ADA gene into them using a retroviral vector. The corrected lymphocytes are multiplied in culture and then returned to the patient's body. This is a direct modification of the patient's own cells, and the therapy needs to be repeated periodically because the corrected cells are not immortal — they have a limited lifespan and must be replenished. …

(a) Gene therapy for ADA deficiency

Adenosine deaminase (ADA) deficiency is a severe inherited disorder that cripples the immune system. Children born with it lack a functional ADA enzyme, leading to a buildup of toxic metabolites that destroy lymphocytes — the very cells that fight infection. Without treatment, even common colds can be fatal.

The first successful gene therapy for ADA deficiency was a landmark in medical history. The approach did not involve editing the patient's genome in every cell. Instead, it was a somatic cell gene therapy — a direct modification of the patient's own blood cells outside the body, followed by returning them.

Here is how the procedure works, step by step:

• Lymphocytes are collected from the patient's blood. These are the immune cells most affected by the enzyme deficiency.
• In the laboratory, a functional copy of the human ADA gene is inserted into these lymphocytes. The vehicle used is typically a retrovirus that has been disabled — it cannot cause disease, but it can still deliver the gene into the cell's DNA.
• The genetically corrected lymphocytes are then cultured (grown in large numbers) to ensure enough cells are available.
• Finally, these modified cells are infused back into the patient's bloodstream. Once inside, they begin producing the missing ADA enzyme, restoring immune function.

The treatment does not cure the genetic defect in every cell of the body. It provides a continuous supply of functional immune cells, allowing the patient to live a near-normal life without the need for enzyme replacement or bone marrow transplant.

(b) Why a host cell must be made competent

Recombinant DNA technology involves taking a piece of foreign DNA (say, a human gene) and inserting it into a host cell — often a bacterium like E. coli. But a cell's plasma membrane is not designed to let large, charged DNA molecules pass through. It is a selective barrier.

If you simply mix foreign DNA with bacterial cells, almost nothing will happen. The DNA cannot cross the membrane on its own. That is why the host cell must be made competent — that is, artificially rendered capable of taking up DNA from its surroundings.

Competence is achieved by altering the cell's membrane permeability. Two common methods are: …