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Combined immunodeficiency associated with adenosine deaminase enzyme with altered kinetic properties

About one half of patients with severe combined immunodeficiency also have a hereditary defect in adenosine deaminase (ADA). We report a child with fatal combined immunodeficiency and erythrocyte ADA with altered kinetic properties. The patient developed normally until the age of 6 months. Since then she had chronic candidiasis, recurrent respiratory infections and pneumonias, and her growth retarded. No thymic shadow was seen in chest x-rays at the age of 13 months; slight abnormalities in the bones were noted. Serum levels of IgG, IgA and IgM were low and total lymphocyte counts were at times subnormal (2000-370/mm3). The proportion of T lymphocytes was decreased (24-26%) while that of B lymphocytes was elevated (42%). Several mitogens were used to stimulate the lymphocytes in vitro but no responses were obtained. The patient died of protracted bloody diarrhoea and septic infection at the age of 15 months. In autopsy, the thymic lobuli were small with no Hassal's corpuscles and only scattered thymocytes. The specific activity of ADA in the patient's erythrocyte lysates was 3-10 times the mean of normal values in all three blood samples tested. The apparent Michaelis coefficient (Km) for adenosine was higher than that of ADA of normal individuals (80μM vs. 35-40μM). This kind of enzymatic change might result in increased cellular synthesis of adenine nucleotides, as it happens in the absence of ADA.