The mechanism responsible for the lymphocytotoxicity associated with congenital adenosine deaminase (ADA) deficiency has been ascribed to an accumulation of dATP. Elevated levels of dATP can then lead to inhibition of DNA synthesis by inhibiting ribonucleotide reductase and causing a depletion of the other deoxynucleotide triphosphates (dNTP). This hypothesis was derived principally from studies with murine and human lymphoblastoid cell lines (LCL) and apparently confirmed in a limited number of investigations with lectin-stimulated lymphocytes. Our biochemical studies of lectin-stimulated mouse and human lymphocytes were not consistent with the dATP model and suggested that AdR exerted effects on lymphocyte activation that preceded the initiation of DNA synthesis. In the current studies, we focused on the effects of AdR on the early events in T lymphocyte activation, because we found they were the most sensitive to AdR toxicity. AdR blocked neither the production of T cell growth factor (TCGF) by lectin-stimulated lymphocytes nor the expression of TCGF receptors as detected by the anti-Tac monoclonal antibody that recognizes the human TCGF receptor. AdR did, however, block the early TCGF-dependent events leading to the entry into the cell cycle. By using the metachromatic fluorescence stain acridine orange, we found that AdR blocked the increased synthesis of RNA that characterizes the entry into the G1 phase of the cell cycle from the G0, resting state. Because these early effects were caused by the lowest doses of AdR, and because they preceded the synthesis of DNA by 15 to 20 hr, it suggested that these effects may be principally responsible for the in vivo toxicity associated with ADA deficiency. Furthermore, none of the other proposed biochemical mechanisms, e.g., inhibition of methylation, diminution of ATP levels, or incorporation of AdR into polyadenylated RNA, appeared adequate to explain AdR toxicity during T lymphocyte activation.
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