Question

Why do children cured by enzyme replacement therapy for adenosine deaminase deficiency need periodic treatment?

Answer 1

Hint: Adenosine deaminase is an enzyme which is involved in purine metabolism. It breaks down the adenosine in food and supplies it as nucleic acids to tissues. ADA is present in all types of cells. Although, its enzyme activity differs considerably in tissues. Deficiency of ADA may present infants, children or adults.

Complete answer:
Adenosine deaminase is an enzyme involved in the purine metabolism. It is highly conserved throughout the phylogeny. Deficiency of ADA destroys the lymphocytes. It is an autosomal recessive disorder. It is also an important enzyme for the functioning of the immune system. It is caused due to the deletion of genes for the adenosine deaminase. This disorder can be cured by gene therapy.
The first clinical gene therapy was given in 1990 to a four-year girl who was suffering from Adenosine Deaminase (ADA) deficiency. This disorder can be treated by either bone marrow transplantation or enzyme replacement therapy, in which functional ADA is given to the patient by injection. Although, these are not completely curative. In gene therapy, lymphocytes from the blood of the patients are grown in culture. A functional ADA using retroviral cDNA vector is then introduced into the cultured lymphocytes, which are then reinserted in the patient’s body. However, the patient requires periodic transfusion as the cells are mortal. If in case, this technique is carried out in the embryonic stage, it could be a permanent cure.
The symptoms of ADA include pneumonia, chronic diarrhea, rashes on skin. In children it can lead to delayed development and growth. Most of the ADA deficiency patients are diagnosed with SCID in six months.

Note: SCID, severe combined immunodeficiency is mostly diagnosed after ADA deficiency in patients. It is also a genetic disorder which is characterized by the disturbed development of T-cells and B-cells. It is also called ‘Bubble Baby’ disease because the infants are protected from bacteria, virus and other microorganisms using a plastic sheet.