Inheritance: autosomal recessive
Gene Mutation: ADA, which affects lymphocyte development and function
Phenotypes in ADA deficiency:
(1) SCID in infants < 6 months
(2) delayed onset in infants > 6 months and young children
(3) adult onset (late onset)
(4) benign (partial deficiency)
Clinical features the SCID phenotype:
(1) failure to thrive
(2) diaper and/or oral Candidiasis
(3) absence of both tonsils
(4) absence of lymph nodes and the thymus
(5) recurrent infections that respond poorly to therapy
(6) serious infections following administration of live-virus vaccines or non-irradiated blood products
(7) Pneumocystis carinii or other opportunistic infections
(8) rib abnormalities with cupping and flaring at the costochondral junctions
(9) variable hepatic abnormalities
(10) variable neurologic abnormalities
Laboratory findings of the SCID phenotype:
(1) lymphopenia, present at birth
(2) flow cytometry shows a marked decrease in B cells, T cells and NK cells (absent cellular immunity)
(3) hypogammaglobulinemia (serum IgG is normal at birth but later declines as maternal antibody disappears)
(4) less than 1% of normal ADA catalytic activity in red blood cell lysates in untransfused patients
(5) less than 1% of normal ADA catalytic activity in other cells (fibroblasts, blood monocytes, etc)
Long-term survival depends on a bone marrow transplant in early infancy.
