Two children, A and B aged 4 and 5 years respectively visited a hospital with a similar genetic disorder. The girl A was provided enzyme-replacement therapy and was advised to revisit periodically for further treatment. The girl, B was, however, given a therapy that did not require revisit for further treatment.
(a) Name the ailments the two girls were suffering from ?
(b) Why did the treatment provided to girl A required repeated visits ?
(c) How was the girl B cured permanently ?

Explanation

(a) Two girls are suffering from severe combined immuno deficiency (SCID) which is caused by deficiency of adenosine deaminase (ADA). This disorder is caused due to the deletion of gene for a adenosine deaminase.
(b) Treatment provided to girl A : Girl A, was treated by enzyme replacement therapy in which functional ADA is given to the patient by injection. The approach is not completely curative. The patient requires periodic infusion of enzyme replacement.
(c) The girl B cured permanently : Permanent cure is possible when the lymphocytes from the blood of the patient are grown in a culture outside the boly. A functional ADA cDNA (using a retrovital vector) is then introduced into these lymphocytes. The gene isolate from marrow cells producing ADA is introduced into cells at early embryonic stages. It could be a permanent cure.