Adenosine deaminase deficiency mechanism

The biosynthesis of purines and pyrimidines is stringently regulated and coordinated by feedback mechanisms that ensure their production in quantities and at times appropriate to varying physiologic demand. Genetic diseases of purine metabolism include gout, Lesch-Nyhan syndrome, adenosine deaminase deficiency, and purine nucleoside phosphorylase deficiency. By contrast, apart from the orotic acidurias, there are few clinically significant disorders of pyrimidine catabolism. 

Adenosine deaminase (ADA) and purine nucleoside phosphorylase (PNP) deficiency have been recognized as the primary cause of an associated immune deficiency syndrome. A number of mechanisms have been proposed to explain the predominant effect of these enzyme deficiencies on the development and function of the lymphoid system. One of the mechanisms concerns the phosphorylation of accumulated metabolic compounds i.e. deoxyadenosine (dAdo) in case of ADA-deficiency and deoxyguanosine (dGuo) in case of PNP deficiency in the lymphoid cells and particularly in thymocytes (1). Indeed increased deoxyATP and deoxyGTP levels have been found in the lymphocytes of ADA- and PNP-deficient patients respectively (2,3). These triphosphates may inhibit the enzyme ribonucleotide reductase which leads to a depletion of deoxyCTP and interference with lymphocytic DNA-synthesis (1). 

Evidence that purine metabolism is important in the immune response has been obtained from the observation that markedly reduced or absent adenosine deaminase (ADA) activity in man has been casually associated with an autosomal recessive form of severe combined immunodeficiency disease (3). Recently, ADA levels in lymphocytes from patients with untreated chronic lymphatic leukemia have been found to be consistently lower than in lymphocytes from normal subjects (4). Children with ADA deficiency and immunodeficiency have been shown to have increased levels in plasma, urine, lymphocytes and erythrocytes of adenosine, adenine, deoxy-adenosine, adenine nucleotides, and deoxyadenine (5, 6). Although the exact biochemical mechanism(s) is unknown, elevated levels of adenosine, and/or deoxyadenosine and their metabolites are thought to be selective inhibitors of both differentiation and effector function of lymphocytes (7, 8). Adenosine was known to inhibit the PHA-induced blastogenesis of human peripheral blood lymphocytes (9) even before the discovery of the first ADA-deficient child. In addition, elevated levels of cyclic AMP (cAMP) were known to be inhibitory for lymphocyte-mediated cytotoxicity (7). Since... 

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