Possible metabolic basis for the different immunodeficient states associated with genetic deficiencies of adenosine deaminase and purine nucleoside phosphorylase.

Abstract

An inherited deficiency of adenosine deaminase (Ado deaminase; adenosine aminohydrolase, EC 3.5.4.4) causes severe combined immunodeficiency disease in humans. A similar deficiency in purine nucleoside phosphorylase (Puo phosphorylase; purine-nucleoside:orthophosphate ribosyltransferase, EC 2.4.2.1) engenders a selective cellular immune deficit. To elucidate the possible metabolic basis for the contrasting immunologic phenotypes, we compared the toxicity toward mature resting human lymphocytes ofthe Ado deaminase substrates deoxyadenosine and adenosine and the Puo phosphorylase substrate deoxyguanosine. When Ado deaminase was inhibited, micromolar concentrations of deoxyadenosine progressively killed nondividing helper and suppressor-cytotoxic T cells, but not B cells. The toxicity required phosphorylation, with subsequent dATP formation. The deoxyadenosine analogs 2-chlorodeoxyadenosine, 2-fluorodeoxyadenosine, and adenine arabinonucleoside also killed resting T cells. Cell death was unrelated to inhibition of adenosylhomocysteinase (EC 3.3.1. 1) but was preceded by a gradual decline in ATP levels. As much as 1 mM deoxyguanosine did not impair resting lymphocyte viability, despite the synthesis of dGTP. The combination of200 ,M adenosine plus 500 ,uM homocysteine thiolactone killed dividing lymphocytes but had no discernible toxic effect toward resting T cells, which accumulated adenosylhomocysteine over a 4-hr period but thereafter excreted the nucleoside into the culture medium. The different clinical syndromes associated with genetic deficiencies ofAdo deaminase and Puo phosphorylase may be explained by the ability of dATP to kill mature resting T lymphocytes by depleting ATP levels.

2'-deoxyguanosine toxicity for B and mature T lymphoid cell lines is mediated by guanine ribonucleotide accumulation.

“…2'-Deoxyguanosine, guanosine, guanine, hypoxanthine, adenine, and 2'-deoxycytidine were purchased from Sigma Chemical Co. (St. Louis, MO). 8 (11,12). These cells were originally obtained from the peripheral blood or lymph node of patients with cutaneous T cell lymphomas and have surface antigens reactive with OKT4 monoclonal antibodies.…”

“…Growth inhibition and GTP accumulation are prevented by hypoxanthine or adenine, but not by 2'-deoxycytidine. 8 Receivedfor publication 31 January 198.4 and in revisedform 12 July 1984. sine appears to effectively inhibit extracellular PNP activity; thus, it prolongs the extracellular half-life of 2'-deoxyguanosine and guanosine, but does not completely inhibit intracellular PNP activity in these lymphoid cells.…”

“…Thus, accumulation of dGTP leads to depletion of other deoxynucleoside triphosphates, inhibition of DNA synthesis, and cell death (6, 7). Human B lymphoblasts and mature peripheral blood lymphocytes are both relatively resistant to the toxic effects of 2'-deoxyguanosine alone and do not accumulate significant amounts ofdGTP under in vitro culture conditions (8)(9)(10).…”

Antileukemic and immunosuppressive activity of 2-chloro-2'-deoxyadenosine.

“…Notably, in vitro it was exquisitely toxic to normal resting peripheral blood lymphocytes and to slowly dividing malignant T cells. This ability to kill nondividing lymphocytes distinguished CdA from other commonly used chemotherapeutic agents affecting purine and pyrimidine metabolism (18,19). We also tested the anti-proliferative effects of CdA toward more than 40 bone marrow specimens from patients with acute leukemia, as compared to 20 normal bone marrow specimens (18).…”

Inhibition of Deoxynucleoside Kinases in Human Thymocytes Prevents dATP Accumulation and Induction of Apoptosis

“…In the absence of ADA enzyme activity, dAdo is phosphorylated by cellular deoxynucleoside kinases, leading to the formation of accumulated dATP. A number of studies concluded this was likely to be the most relevant biochemical process leading to toxicity in ADA-deficient developing lymphocytes [3,4]. However, some contention was held about the relative role of the two major deoxynucleoside kinases believed to be responsible for the initial phosphorylation of dAdo, deoxycytidine kinase (dCK) and adenosine kinase (AK).…”