What are the genetic changes related to adenosine deaminase deficiency ?
Adenosine deaminase deficiency is caused by mutations in the ADA gene. This gene provides instructions for producing the enzyme adenosine deaminase. This enzyme is found throughout the body but is most active in specialized white blood cells called lymphocytes. These cells protect the body against potentially harmful invaders, such as bacteria and viruses, by making immune proteins called antibodies or by directly attacking infected cells. Lymphocytes are produced in specialized lymphoid tissues including the thymus, which is a gland located behind the breastbone, and lymph nodes, which are found throughout the body. Lymphocytes in the blood and in lymphoid tissues make up the immune system.  The function of the adenosine deaminase enzyme is to eliminate a molecule called deoxyadenosine, which is generated when DNA is broken down. Adenosine deaminase converts deoxyadenosine, which can be toxic to lymphocytes, to another molecule called deoxyinosine that is not harmful. Mutations in the ADA gene reduce or eliminate the activity of adenosine deaminase and allow the buildup of deoxyadenosine to levels that are toxic to lymphocytes.  Immature lymphocytes in the thymus are particularly vulnerable to a toxic buildup of deoxyadenosine. These cells die before they can mature to help fight infection. The number of lymphocytes in other lymphoid tissues is also greatly reduced. The loss of infection-fighting cells results in the signs and symptoms of SCID.