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SCID due to ADA deficiency accounts for 10-15% of all cases of SCID. Its annual incidence is estimated to be between 1/200,000 and 1/1,000,000 live births. Both males and females are affected.

SCID due to ADA deficiency has a variable clinical presentation. The most common form presents in infancy with severe and recurrent opportunistic infections (including respiratory tract infections and candidiasis), failure to thrive, and usually results in early death. Ten to 15% of patients have a delayed clinical onset by age 6-24 months, and a smaller percentage have a partial form of ADA deficiency with later onset between ages 4 years and adulthood, both types showing less severe infections and gradual immunologic deterioration. Patients may also present with extraimmune manifestations (including neurodevelopmental deficits, behavioral disorders, sensorineural deafness, and skeletal and hepatic abnormalities) as a result of the systemic nature of ADA expression.

SCID due to ADA deficiency is caused by mutations in the ADA gene (20q13.11). The extraimmune manifestations are caused by toxic levels of purine metabolites that result from the deficiency of ADA.

Diagnosis is based on evidence of low or undetectable ADA activity in erythrocytes in combination with evidence of a marked reduction of T, B and NK cell counts when compared to age-matched healthy controls. Diagnosis can be confirmed by raised levels of dATP and reduced S-adenosyl homocysteine hydrolase (SAHH) activity in red cells and elevated amounts of deoxyadenosine in urine.

Differential diagnosis includes all forms of SCID.

Prenatal diagnosis can be carried out through mutation analysis or measurement of enzyme activity in trophoblasts cultured from chorionic villus sampling or in cultured amniocytes.

Transmission is autosomal recessive.

Treatment is based on allogenic hematopoietic stem cell transplantation (HSCT), enzyme replacement therapy with pegylated adenosine deaminase enzyme or gene therapy by infusion of CD34+ marrow cells that have been transduced with an ADA-containing vector.

Prognosis depends on the severity of the disease. Without treatment, SCID due to ADA deficiency that presents in infancy usually results in early death. Survival rates after allogenic hematopoietic stem cell transplantation or gene therapy are high.