Question

The genetic defect ‘ADA deficiency’ can be permanently cured by which of the following methods?

• A. Injecting functional ADA directly into the patient
• B. Transplanting bone marrow from a suitable donor
• C. Introduction of the functional genes producing ADA into patient’s cells at early embryonic stage
• D. Both a and b

Answer 1

The correct option is C Introduction of the functional genes producing ADA into patient’s cells at early embryonic stage

SCID (Severe Combined Immunodeficiency) can be caused due to mutation of the gene for the enzyme adenosine deaminase (ADA).

ADA (Adenosine deaminase) is involved in purine metabolism. When DNA is broken down, it generates a molecule known as deoxyadenosine which is toxic to lymphocytes. The enzyme ADA is crucial for the immune system to function as it is involved in the conversion of the toxic deoxyadenosine to a non-toxic molecule called deoxyinosine. These patients with ADA deficiency, as a result, would have non-functional lymphocytes and so they cannot mount immune responses against invading pathogens.

Methods used to treat SCID include:

(a) Bone marrow transplantation is done in children if we receive a matching bone marrow.

(b) By enzyme replacement therapy, in which functional ADA is given to the patient by injection.

(c) Lymphocytes from the blood of the patient are grown in a culture outside the body. A copy of human gene encoding a functional ADA cDNA is introduced into the lymphocytes (collected from patients) by using retroviral vectors, which are subsequently returned to the patient.

(d) Gene producing ADA isolated from the donor's bone marrow cells is introduced into the cells at early embryonic stage. The bone marrow cells of embryos are pluripotent stem cells, that means they have the capacity to form the cells of all three germ layers (ectoderm, endoderm and mesoderm). Hence, this method is considered to be a permanent solution for ADA deficiency if identified in the embryonic stage.