Adenosine Deaminase (ADA) Deficiency

What is it?

ADA deficiency is the inability to produce the enzyme adenosine deaminase. Adenosine deaminase catalyzes a reaction which converts toxic deoxyadenosine, a biproduct of DNA break-down, to harmless deoxynosine. A person with ADA deficiency produces abnormal adenosine deaminase enzymes, or does not have any at all. This means that they cannot break down deoxyadenosine. As a result, the toxic chemical collects in their body, killing lymphocytes (white blood cells that are very important in the immune system). This causes babies with ADA deficiency to develop a condition called severe combined immunodeficiency (SCID), which means they cannot fight infections caused by bacteria, viruses, and fungi. Because of this, infants with ADA deficiency usually die within a few months. Occasionaly, an individual will not develop ADA deficiency until later in life. In these cases, the disease is usually not as dangerous. There are also cases of partial ADA deficiency.

How does someone get it?

A person gets ADA deficiency by recieveing a mutated version of the ADA gene, which controls the production of the enzyme adenosine deaminase, from both their mother and their father. The diagram below, from the University of Utah, shows how an individual inherits ADA deficiency. Notice that individuals who recieve only one copy of the mutations are carriers, but do not actually have the disease. Because an individual must get a mutated ADA gene from both their mother and father to show symptoms, ADA deficiency is very rare; the U.S. National Library of Medicine estimates the rate of occurrence at 1 in 200,000 to 1,000,000 newborns worldwide, while the University of Utah says that only about 10-20 children are diagnosed with the disease in the U.S. per year.
This is how someone inherits ADA deficiency. Image from: <http://learn.genetics.utah.edu/units/disorders/whataregd/ada/index.cfm>
This is how someone inherits ADA deficiency. Image from: <http://learn.genetics.utah.edu/units/disorders/whataregd/ada/index.cfm>

The ADA gene is located on chromosome 20 in the region shown below. Mutations on this gene usually cause adenosine deaminase to have an irregular sequence of amino acids, although mutations can occur which prevent production of adenosine deaminase altogether.


The location of the ADA gene in chromosome 20. Image From: <http://ghr.nlm.nih.gov/gene=ada>
The location of the ADA gene in chromosome 20. Image From: <http://ghr.nlm.nih.gov/gene=ada>


What are the treatments?

There are a few treatments of ADA deficiency, some of which are still being developed. Here are some treatments:
  • Bone marrow or stem cell transplants to attempt to restore the immune system
  • Blood transfusions to attempt to restore the immune system
  • Enzyme replacement therapy (giving a patient more adenosine deaminase to break down the toxins)
  • Treating the infections caused by SCID (severe combined immunodeficiency, which results from ADA deficiency)
  • Putting normal DNA into immune cells to correct the mutation (gene therapy)
Click here to check out some studies of new ADA deficiency treatments.

Sources:
http://learn.genetics.utah.edu/units/disorders/whataregd/ada/index.cfm
http://ghr.nlm.nih.gov/condition=adenosinedeaminasedeficiency
http://ghr.nlm.nih.gov/gene=ada
http://www.genetests.org/query?dz=ada