Disruption of the adenosine deaminase gene causes hepatocellular impairment and perinatal lethality in mice

Ecto-enzyme and signaling functions of lymphocyte CD73

CD73 or ecto-5'-nucleotidase (5'-NT) is a widely expressed ecto-enzyme which catalyzes the dephosphorylation of AMP and other nucleoside monophosphates. CD73 participates in purine salvage through this enzymatic activity, supplying cells with precursors for energy metabolism and nucleic acid biosynthesis. As an enzyme that produces adenosine, CD73 can also regulate adenosine receptor engagement in many tissues. However, CD73 also has functions independent of its enzyme activity. Like many glycosyl phosphatidylinositol (GPI)-anchored molecules, it transmits potent activation signals in T cells when ligated by antibodies. Less compelling evidence suggests that CD73 may function as a cell adhesion molecule. In the human immune system, CD73 is expressed on subsets of T and B cells, on germinal center follicular dendritic cells, and on thymic medullary reticular fibroblasts and epithelial cells. Many challenging areas remain to be explored before the role of CD73 in the immune system will be fully understood. These include an evaluation of the role of adenosine receptors in lymphoid development, the identification of physiological CD73 ligands, a functional assessment of the GPI anchor, and an analysis of the intricate cell-type-specific and developmental regulation of CD73 expression.

Regulation of duodenal specific expression of the human adenosine deaminase gene

Formation of the mammalian gastrointestinal tract is an ordered process of development and differentiation. Yet, the adult small intestine also retains the plasticity to respond to cues both internal and environmental to modulate intestinal function. The components that regulate this development, differentiation, and modulation at the molecular level are only now being elucidated. We have used the human adenosine deaminase (ADA) gene as a model to identify potential cis-regulatory components involved in these processes within the small intestine. In mammals, high levels of ADA in the small intestine are limited specifically to the differentiated enterocytes within the duodenal region. These studies describe the identification of a region of the human ADA gene, completely distinct from the previously identified T-cell enhancer, which is capable of directing the human intestinal expression pattern in the intestine of transgenic mice. The reporter gene expression pattern observed in these transgenic mice is identical to the endogenous gene along both the cephalocaudal and crypt/villus axis of development. Timing of this transgene activation, however, varies from that of the endogenous mouse gene in that the transgene is activated approximately 2 weeks earlier in development. Even so, this precocious activation is also limited to the epithelium of the developing villi strictly within the duodenal region of the small intestine.

Genetically engineered mice demonstrate that adenosine deaminase is essential for early postimplantation development

Adenosine deaminase (ADA) is an essential enzyme of purine metabolism that is enriched at the maternal-fetal interface of mice throughout postimplantation development. During early postimplantation stages Ada is highly expressed in both maternally derived decidual cells and zygotically derived trophoblast cells. For the current study we utilized genetically modified mice to delineate the relative contribution and importance of decidual and trophoblast ADA at the maternal-fetal interface. In females genetically engineered to lack decidual ADA a striking pattern of expression was revealed in giant trophoblast cells that surround the early postimplantation embryo. Embryos within gestation sites lacking both decidual and trophoblast ADA died during the early postimplantation period, whereas expression in trophoblast cells alone was sufficient for survival through this period. Severe disturbances in purine metabolism were observed in gestation sites lacking decidual ADA, including the accumulation of the potentially toxic ADA substrates adenosine and 2′-deoxyadenosine. These experiments provide genetic evidence that Ada expression at the maternal-fetal interface is essential for early postimplantation development in mice.

Clinical expression, genetics and therapy of adenosine deaminase (ADA) deficiency

Adenosine deaminase (ADA) deficiency was the first known cause of primary immunodeficiency. Over the past 25 years the basis for immune deficiency has largely been established. Now it appears that ADA deficiency may also cause hepatic toxicity, raising new questions about its pathogenesis. The ADA gene has been sequenced and the ADA three-dimensional structure solved. The relationship between genotype and phenotype is being analysed, and ADA deficiency has become a focus for novel approaches to enzyme replacement and gene therapy.

Adenosine Deaminase Deficiency in Adults

Adenosine deaminase (ADA) deficiency typically causes severe combined immunodeficiency (SCID) in infants. We report metabolic, immunologic, and genetic findings in two ADA-deficient adults with distinct phenotypes. Patient no. 1 (39 years of age) had combined immunodeficiency. She had frequent infections, lymphopenia, and recurrent hepatitis as a child but did relatively well in her second and third decades. Then she developed chronic sinopulmonary infections, including tuberculosis, and hepatobiliary disease; she died of viral leukoencephalopathy at 40 years of age. Patient no. 2, a healthy 28-year-old man with normal immune function, was identified after his niece died of SCID. Both patients lacked erythrocyte ADA activity but had only modestly elevated deoxyadenosine nucleotides. Both were heteroallelic for missense mutations: patient no. 1, G216R and P126Q (novel); patient no. 2, R101Q and A215T. Three of these mutations eliminated ADA activity, but A215T reduced activity by only 85%. Owing to a single nucleotide change in the middle of exon 7, A215T also appeared to induce exon 7 skipping. ADA deficiency is treatable and should be considered in older patients with unexplained lymphopenia and immune deficiency, who may also manifest autoimmunity or unexplained hepatobiliary disease. Metabolic status and genotype may help in assessing prognosis of more mildly affected patients.

Point mutations at the purine nucleoside phosphorylase locus impair thymocyte differentiation in the mouse

Three point mutations on the Np(b) allele of the purine nucleoside phosphorylase locus in the mouse have been recovered by male germ cell mutagenesis. The mutants were backcrossed, 12-14 generations, and are designated in increasing order of severity of enzyme deficiency and phenotype: B6-NPE, Met-87 --> Lys; B6-NPF, Ala-228 --> Thr; and B6-NPG, Trp-16 --> Arg. A marked decline in total cell numbers per thymus occurs between 2 and 3 months for the more severe B6-NPF and B6-NPG mutants (35% and 52%, respectively) and by 8 months for the less severe B6-NPE mutation. The thymocyte population is thereafter characterized by a 3- or 8-fold expanded precursor, CD4-CD8- double-negative population and 15% or 55% reduced CD4+CD8+ double-positive cells for the B6-NPF and B6-NPG strains, respectively. Spleen lymphocyte Thy-1+ cells are reduced by 50% and spleen lymphocyte response to T cell mitogen and interleukin 2 is reduced by 80%. Increases of thymocyte dGTP pools of 5- and 2.5-fold for B6-NPF and B6-NPG mutants, respectively, are observed. The purine nucleoside phosphorylase-deficient mouse exhibits age-dependent progressive perturbations in thymocyte differentiation, reduced numbers of thymocytes, and reduced splenic T cell numbers and response. The progressive T cell deficit is similar to the human disorder.

Naturally occurring primary deficiencies of the immune system

Naturally occurring genetic disorders of the immune system provide many models for the study of its development and function. In a way, their analysis complements the information provided by the generation of genetic defects in mice created using homologous recombination techniques. In this review, the recent findings made in three areas are focused upon deficiencies in T cell differentiation and in T lymphocyte activation, and on the control process of peripheral immune response.

Neurodegeneration, myocardial injury, and perinatal death in mitochondrial superoxide dismutase-deficient mice

Manganese superoxide dismutase (SOD2) converts superoxide to oxygen plus hydrogen peroxide and serves as the primary defense against mitochondrial superoxide. Impaired SOD2 activity in humans has been associated with several chronic diseases, including ovarian cancer and type I diabetes, and SOD2 overexpression appears to suppress malignancy in cultured cells. We have produced a line of SOD2 knockout mice (SOD2m1BCM/SOD2m1BCM) that survive up to 3 weeks of age and exhibit several novel pathologic phenotypes including severe anemia, degeneration of neurons in the basal ganglia and brainstem, and progressive motor disturbances characterized by weakness, rapid fatigue, and circling behavior. In addition, SOD2m1BCM/SOD2m1BCM mice older than 7 days exhibit extensive mitochondrial injury within degenerating neurons and cardiac myocytes. Approximately 10% of SOD2m1BCM/SOD2m1BCM mice exhibit markedly enlarged and dilated hearts. These observations indicate that SOD2 deficiency causes increased susceptibility to oxidative mitochondrial injury in central nervous system neurons, cardiac myocytes, and other metabolically active tissues after postnatal exposure to ambient oxygen concentrations. Our SOD2-deficient mice differ from a recently described model in which homozygotes die within the first 5 days of life with severe cardiomyopathy and do not exhibit motor disturbances, central nervous system injury, or ultrastructural evidence of mitochondrial injury.

Mice with adenine phosphoribosyltransferase deficiency develop fatal 2,8-dihydroxyadenine lithiasis

Deficiencies in different steps of purine metabolism give rise to a number of human inherited disorders. Lesch-Nyhan syndrome is a severe neurological disorder, caused by a deficiency in the purine salvage enzyme hypoxanthine phosphoribosyltransferase (HPRT). HPRT-deficient mice have been generated, but have proved to be an unsuccessful model of the human disease. We have suggested that this may be due to a greater dependency in rodents on the other purine salvage enzyme, adenine phosphoribosyltransferase (APRT). We have generated an APRT-deficient mouse line by gene targeting, with a phenotype that closely resembled the symptoms of APRT deficiency in man. APRT null mice were viable, but 90% died prematurely before 6 months of age, displaying highly abnormal kidney morphology, with pathology characteristic of tubule obstruction. These mice have elevated urinary levels of adenine and 2,8-dihydroxyadenine, a highly insoluble adenine derivative, plus birefringent crystalline deposits and calculi within tubules throughout the kidney. A standard therapy for APRT-deficient human patients is the administration of the xanthine oxidase inhibitor, allopurinol. This has proved an effective therapy for APRT null mice, preventing accumulation of 2,8-dihydroxyadenine and much of the resultant renal obstruction, allowing us to establish a breeding line. We believe that these mice should provide a useful model for further study of APRT deficiency in humans. Furthermore, by generating APRT and HPRT double mutants, we will be able to test our hypothesis that both genes must be inactivated in mice before a model for Lesch-Nyhan syndrome can be obtained.

Metabolic and immunologic consequences of limited adenosine deaminase expression in mice

Adenosine deaminase (ADA; EC 3.5.4.4) deficiency in humans is an autosomal recessive genetic disorder that results in severe combined immunodeficiency disease. ADA-deficient mice generated by targeted gene disruption die perinatally, preventing postnatal analysis of ADA deficiency. We have recently rescued ADA-deficient fetuses from perinatal lethality by expression of an ADA minigene in the placentas of ADA-deficient fetuses, thus generating postnatal mice admissible to analysis of ADA deficiency. The minigene used also directed ADA expression to the forestomach postnatally, producing adult animals that lacked ADA enzymatic activity in all tissues outside the gastrointestinal tract. Mice with limited ADA expression exhibited profound disturbances in purine metabolism, including thymus-specific accumulations of deoxyadenosine and dATP, and inhibition of S-adenosylhomocysteine hydrolase in the thymus, spleen, and, to a lesser extent, the liver. Lymphopenia and mild immunodeficiency were associated with these tissue-specific metabolic disturbances. These mice represent the first genetic animal model for ADA deficiency and provide insight into the tissue-specific requirements of ADA.

Adenine phosphoribosyltransferase-deficient mice develop 2,8-dihydroxyadenine nephrolithiasis

Adenine phosphoribosyltransferase (APRT) deficiency in humans is an autosomal recessive syndrome characterized by the urinary excretion of adenine and the highly insoluble compound 2,8-dihydroxyadenine (DHA) that can produce kidney stones or renal failure. Targeted homologous recombination in embryonic stem cells was used to produce mice that lack APRT. Mice homozygous for a null Aprt allele excrete adenine and DHA crystals in the urine. Renal histopathology showed extensive tubular dilation, inflammation, necrosis, and fibrosis that varied in severity between different mouse backgrounds. Thus, biochemical and histological changes in these mice mimic the human disease and provide a suitable model of human hereditary nephrolithiasis.

Modelling of purine nucleoside metabolism during mouse embryonic development: relative routes of adenosine, deoxyadenosine, and deoxyguanosine metabolism

The individual activities for adenosine kinase, deoxyadenosine kinase, adenosine deaminase, deoxyguanosine kinase, and purine nucleoside phosphorylase were determined during days 7 to 13 of mouse embryonic development. Adenosine deaminase increased 74-fold between days 7 and 9; deoxyadenosine kinase increased 5.4-fold during the same interval. Adenosine kinase, deoxyguanosine kinase, and purine nucleoside phosphorylase exhibited less than 2-fold changes in activity between days 7 and 13. Using Michaelis constants for each enzyme and the maximal velocities determined from enzyme assay, the relative routes of adenosine and deoxyadenosine metabolism via phosphorylation or deamination were modeled as a function of nucleoside concentration for days 7 through 13. For days 7 and 8, phosphorylation of adenosine is the principle route of metabolism at physiological concentrations. A switch occurred at day 9 and following where deamination is at least 5-fold greater than phosphorylation at all substrate concentrations. Deoxyadenosine phosphorylation was at most 10% of deamination at day 7 and then declined to less than 1% for days 9 to 13. Phosphorolysis was the principle route of deoxyguanosine metabolism through the 7 to 13 day period. Thus catabolism rather than phosphorylation was the principle pathway for purine deoxynucleoside metabolism during this period.

Tissue-specific rescue suggests that placental adenosine deaminase is important for fetal development in mice

Adenosine deaminase (ADA, EC 3.5.4.4) is an essential enzyme of purine metabolism that is expressed at very high levels in the murine placenta where it accounts for over 95% of the ADA present at the fetal gestation site. We have recently shown that ADA-deficient fetuses, which also lack ADA in their adjoining placentas, die during late fetal development in association with profound purine metabolic disturbances and hepatocellular impairment. We have now investigated the potential importance of placental ADA by genetically restoring the enzyme to placentas of ADA-deficient fetuses. This genetic engineering strategy corrected most of the purine metabolic disturbances, prevented serious fetal liver damage, and rescued the fetuses from perinatal lethality. Our findings suggest that placental ADA is important for murine fetal development and illustrate a general strategy for the tissue specific correction of phenotypes associated with null mutations in mice.

Mice lacking ornithine-delta-aminotransferase have paradoxical neonatal hypoornithinaemia and retinal degeneration

Deficiency of ornithine-delta-aminotransferase (OAT) in humans causes hyperornithinaemia and gyrate atrophy (GA), a blinding chorioretinal degeneration. Surprisingly, OAT-deficient mice produced by gene targeting exhibit neonatal hypoornithinaemia and lethality, rescuable by short-term arginine supplementation. Post-weaning, these mice develop hyperornithinaemia similar to human GA patients. Subsequent studies in one human GA infant also showed transient hypoornithinaemia. Thus, the OAT reaction plays opposite roles in neonatal and adult mammals. Over several months, OAT-deficient mice develop a retinal degeneration with involvement of photoreceptors and pigment epithelium. OAT-deficient mice appear to be an excellent model of human GA.

Gene targeting in mouse embryonic stem cells with an adenoviral vector

We examined the ability of an E1, E3-defective adenoviral vector to act as a substrate for homologous recombination with chromosomal DNA by including host chromosomal sequence from the mouse Fgr locus that also contained a selectable marker. After infection of mouse embryonic stem cells, stable integration was selected for neomycin resistance and the efficiency of homologous recombination was evaluated. The adenoviral vector was capable of infecting mouse embryonic stem cells efficiently. Between 30-50% of the input virus reached the nuclei after 24 hours of infection. Surprisingly, even without negative selection, 25-40% of the integration resulted from homologous recombination at m.o.i. 10 and 100, although the absolute efficiency of integration was low. Our results suggest that it is possible to modify the structure of an adenoviral vector to achieve a high gene targeting efficiency, resulting in regulated and long-term expression of an introduced gene.