deoxyguanosine-triphosphate and Immunologic-Deficiency-Syndromes

The genetic deficiencies of adenosine deaminase and purine nucleoside phosphorylase lead to blocks in the purine pathway. The intracellular accumulation of deoxynucleosides and deoxynucleotides is toxic to both dividing and nondividing lymphocytes via multiple mechanisms. T-lymphocytes are uniquely sensitive to purine-mediated cytotoxicity because of a functional imbalance of phosphorylating and dephosphorylating enzymatic activities. These inborn errors or purine metabolism are rare disorders. The study of these conditions, however, has uncovered unique enzymatic properties of lymphocytes and lymphocyte subclasses. A better understanding of the mechanisms of lymphocytotoxicity in these two purine enzyme defects may lead to better modes of therapeutic manipulation of the immune system.

Topics: Adenosine Deaminase; Cytotoxicity, Immunologic; Deoxyadenine Nucleotides; Deoxyadenosines; Deoxyguanine Nucleotides; Humans; Immunologic Deficiency Syndromes; Lymphocytes; Purine-Nucleoside Phosphorylase; Purine-Pyrimidine Metabolism, Inborn Errors; Purines

Mitochondrial DNA is subject to increased rates of mutations due to its proximity to the source of reactive oxygen species. Here we show that increased MHC class I (MHC I) expression serves to alert the immune system to cells with mitochondrial mutations. MHC I is overexpressed in fibroblasts with mitochondrial dysfunction from patients with mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes and in lymphocytes from purine nucleoside phosphorylase-deficient immune-deficient mice with mitochondrial DNA deletions. Consistent with a role of MHC I in the elimination of cells containing mitochondrial DNA mutations, mice deficient in MHC I accumulate mitochondrial DNA deletions in various tissues. These observations in both mice and humans suggest a role for the immune system in preventing reversion of mitochondrial DNA back into a parasitic state following deleterious mutations affecting mitochondrial oxidative phosphorylation.

Topics: Animals; Deoxyguanine Nucleotides; DNA Damage; DNA-Binding Proteins; DNA, Mitochondrial; Fibroblasts; Histocompatibility Antigens Class I; Humans; Immunologic Deficiency Syndromes; Immunologic Surveillance; Interferon-gamma; MELAS Syndrome; Mice; Mice, Inbred C57BL; Mice, Knockout; Osteosarcoma; Phosphorylation; Purine-Nucleoside Phosphorylase; Reactive Oxygen Species; STAT1 Transcription Factor; Trans-Activators; Tumor Cells, Cultured

Low ATP/ADP ratios have been reported consistently for nucleotide levels of mononuclear cells separated from peripheral blood by conventional techniques. We have established that these low values (mean 2.3:1) were not due to cell damage or poor viability, but resulted from heavy platelet contamination, which is unavoidable when heparinized blood is used. The results reflect the low ATP/ADP ratios (mean 1.6:1) characteristic of platelets. Platelet-free extracts from defibrinated blood had very high ATP/ADP ratios (mean 17.4:1). The initial finding of detectable amounts of deoxy-ATP and deoxy-GTP in mononuclear cells from children with two distinct inherited immunodeficiency disorders [adenosine deaminase (ADA) and purine nucleoside phosphorylase (PNP) deficiency respectively] many have been due to contamination by nucleated erythrocytes as well as platelets in non-defibrinated preparations. Defibrination before nucleotide extraction of mononuclear cells from a patient with T-cell leukaemic/lymphoma treated with the ADA inhibitor deoxycoformycin enabled the demonstration of grossly raised deoxy-ATP levels relative to deoxy-ADP levels (ratio 16.1:1), associated with severe ATP depletion. This reciprocal relationship between ATP and dATP was found by us previously in the erythrocytes in inherited ADA deficiency. These findings underline the importance of extracts uncontaminated by platelets, or nucleated erythrocytes, in the evaluation of lymphocyte nucleotide levels in inherited or acquired immunodeficiency syndromes.

Topics: Adenosine Deaminase; Adenosine Deaminase Inhibitors; Adenosine Diphosphate; Adenosine Triphosphate; Blood Platelets; Cell Separation; Child; Coformycin; Deoxyadenine Nucleotides; Deoxyguanine Nucleotides; Humans; Immunologic Deficiency Syndromes; Lymphocytes; Pentostatin; Purine-Nucleoside Phosphorylase