Question

(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.

(b) A host cell must be made competent, before it is able to receive an rDNA. Justify.

Answer 1

(a)
Gene therapy is a collection of methods that allows correction of a gene defect that has been diagnosed in a child/embryo. Here, genes are inserted into a person’s cells and tissues to treat a disease.

Adenosine deaminase (ADA) is crucial for the immune system to function. The ADA deficiency disorder is caused due to deletion of the gene for ADA.

In gene therapy, for the treatment of ADA deficiency, a functional ADA cDNA (using a retroviral vector) is
introduced into lymphocytes obtained from blood of the patient which are cultured outside the body.

When these lymphocytes are subsequently returned to the patient, they take over the function and
compensate for the deleted gene for adenosine deaminase.

In children, ADA deficiency can be cured by bone marrow transplantation or enzyme replacement therapy but both of these approaches are not completely curative.

If gene isolated from marrow cells producing ADA is introduced into the cells at early embryonic stages, it
could be a permanent cure.

(b)

• Since DNA is a hydrophilic molecule, it cannot pass through cell membrane as cell membrane is mostly made up of lipids (phospholipids). So, in order to force bacteria to take up the plasmids, the bacterial cells must first be made ‘competent’ to take up DNA.
• This is done by treating them with a specific concentration of a divalent cation, such as calcium which increases the efficiency with which DNA enters the bacterium through pores in its cell wall.
• Recombinant DNA enters the host cells due to heat shock at 42°C.