It accounts for about 15% of all cases of severe combined immunodeficiency (SCID).
ADA deficiency is due to a lack of the enzyme adenosine deaminase coded for by a gene on chromosome 20. There is an accumulation of deoxyadenosine, which causes an increase in S-adenosylhomocysteine; both substances are toxic to immature lymphocytes, which thus fail to mature. As a result, the immune system is severely compromised or completely lacking.
The enzyme adenosine deaminase is important in the purine salvage pathway.
On September 14, 1990,the first gene therapy to combat this disease was performed by Dr. W. French Anderson on a four year old girl, Ashanti DeSilva, at the National Institutes of Health, Bethesda, Maryland, U.S.A.