Deficiency: Unanticipated Benefits from the Study of a Rare

This information is current as Michael R. Blackburn and Linda F. Thompson of September 27, 2021. J Immunol 2012; 188:933-935; ; doi: 10.4049/jimmunol.1103519 http://www.jimmunol.org/content/188/3/933 Downloaded from

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The Journal of Immunology is published twice each month by The American Association of Immunologists, Inc., 1451 Rockville Pike, Suite 650, Rockville, MD 20852 Copyright © 2012 by The American Association of Immunologists, Inc. All rights reserved. Print ISSN: 0022-1767 Online ISSN: 1550-6606. Adenosine Deaminase Deficiency: Unanticipated Benefits from the Study of a Rare Immunodeficiency Michael R. Blackburn* and Linda F. Thompson†

he serendipitous discovery of adenosine deaminase ciated gouty arthritis (2), but this pathway was not thought (ADA) deficiency in two patients with cellular im- to be important for the immune system. Giblett and col- T munodeficiency in 1972 by Dr. Eloise Giblett and leagues proposed that the two patients might have rare mutant colleagues (1) ushered in a new era in the investigation of the alleles for the ADA gene. Alternatively, it was speculated that molecular mechanisms underlying primary immunodeficiency they might have a short chromosomal deletion encompassing disorders. This finding led to the eventual development of the ADA gene and a nearby critical immune response gene. novel therapies not only for ADA deficiency but also for other In either case, Giblett et al. concluded the following: “Since Downloaded from immunodeficiency disorders and certain . ADA anenzymia and the inherited diseases of cellular im- In the early 1970s, several primary immunodeficiency dis- munity are extremely rare, their coexistence in two unrelated eases, including SCID, X-linked agammaglobulinemia, and patients seems very unlikely to be fortuitous.” Wiskott-Aldrich syndrome, were well known to pediatric im- Measurements of purine metabolites in the body fluids of munologists and presumed to be caused by single gene defects ADA-deficient patients showed elevated levels of adenosine based on patterns of inheritance. However, the gene defects (3), one of the two substrates for ADA. Investigators quickly http://www.jimmunol.org/ responsible for these devastating disorders were unknown. showed that adenosine could slow the growth of lymphoid In those days, the only “cure” for severe immunodeficiency cell lines and the mitogen-induced proliferation of primary diseases was transplantation (BMT) from a his- lymphocytes (3). In 1975, Giblett and colleagues (4) reported tocompatible donor. In the case of one of two patients de- a patient with an isolated T cell immunodeficiency who scribed by Giblett et al., routine HLA typing of family lacked activity of purine nucleoside phosphorylase, an enzyme members failed to identify suitable donors. Thus, the patient’s situated between ADA and HPRT in the purine salvage path- physicians sent blood samples to Dr. Giblett at the King way, providing convincing evidence of the critical importance

County Central Blood Bank. It was hoped that she could shed of normal for a functioning immune sys- by guest on September 27, 2021 light on the relationships among the family members of the tem. Although it was originally reported that ATP was ele- patient by examining isozyme patterns for the enzyme ADA. vated in the RBCs of ADA-deficient patients (5), more sen- Much to her surprise, starch gel electrophoresis indicated that sitive HPLC separation schemes in the laboratories of Drs. the RBCs of the patient were completely devoid of ADA Mary Sue Coleman and Amos Cohen (6, 7) revealed that 29- enzyme activity! The parents showed detectable, but reduced, deoxyadenosine 59-triphosphate (dATP) levels were elevated ADA activity, suggesting an autosomal recessive mode of in- as well. This finding confirmed an earlier speculation by Dr. heritance. Subsequently, a second patient with severe cellular Dennis Carson et al. (8) that deoxyadenosine, the other sub- immunodeficiency was studied and also found to be ADA strate of ADA, rather than adenosine, was the toxic metabolite deficient. These were completely unexpected findings, as no in this disease. Subsequent experimentation showed that precedent existed for ADA deficiency in humans or for ADA’s deoxyadenosine is converted first to 29-deoxyadenosine 59- role in either the development or the function of the immune monophosphate and finally to dATP by the high levels of system. deoxynucleoside kinases in the thymus. A likely pathogenic ADA is part of the purine salvage pathway that includes mechanism is dATP-triggered cytochrome c release from mi- the enzyme hypoxanthine-guanine phosphoribosyltransferase tochondria, which triggers an apoptotic cascade, leading (HPRT). Mutations in the HPRT gene were known to cause to failure of T cell development (9). Interestingly, an under- the neurologic disorder Lesch-Nyhan syndrome and its asso- standing of this pathway led to the development of novel and successful chemotherapeutic approaches for treating hairy cell (10). *Department of Biochemistry and Molecular Biology, The University of Texas Med- Both ADA and purine nucleoside phosphorylase are ex- ical School at Houston, Houston, TX 77030; and †Immunobiology and Cancer Pro- pressed in virtually every cell in the body and had been con- gram, Oklahoma Medical Research Foundation, Oklahoma City, OK 73104 sidered “housekeeping” genes. Thus, an immediate question Address correspondence and reprint requests to Dr. Linda F. Thompson, Immunobi- ology and Cancer Program, Oklahoma Medical Research Foundation, 825 NE 13th was why the effects of ADA deficiency were focused upon the Street, Oklahoma City, OK 73104. E-mail address: [email protected] immune system. This question led to a systematic evaluation Abbreviations used in this article: ADA, adenosine deaminase; BMT, bone marrow of the expression of purine-metabolizing enzymes in various transplantation; dATP, 29-deoxyadenosine 59-triphosphate; HPRT, hypoxanthine-gua- human tissues and to the discovery that ADA was found at nine phosphoribosyltransferase; PEG, polyethylene glycol. very high levels in the thymus, suggesting that this organ had Copyright Ó 2012 by The American Association of Immunologists, Inc. 0022-1767/12/$16.00 evolved a mechanism to prevent the buildup of ADA sub- www.jimmunol.org/cgi/doi/10.4049/jimmunol.1103519 934 PILLARS OF IMMUNOLOGY strates. ADA is needed because the high rate of cell death in during fetal development and became ADA deficient only af- the thymus following T cell selection events provides a source ter birth. Surprisingly, they had normal liver function, show- of DNA that is degraded to deoxyadenosine. This, coupled ing that ADA was needed in the liver during fetal develop- with high levels of deoxynucleoside kinases, explains why the ment, but not thereafter. Equally surprising, these mice died thymi of ADA-deficient patients accumulate such high levels of respiratory failure at z3 wk of age (20). However, they of dATP (8). could be maintained on PEG-ADA indefinitely. When ADA In addition to the normal supportive therapy given to was suboptimal, they developed immunodeficiency, as ex- patients with SCID, ADA-deficient patients were initially pected (21). These mice have proved useful for examining the treated with packed RBC transfusions as a sort of “enzyme- mechanisms of ADA-deficient SCID (9). In addition, owing replacement” therapy (5). Many patients showed significant to the accumulation of adenosine, these animals have served as improvement in immune function as a result, especially those a biological screen for disorders associated with aberrant aden- with residual ADA enzyme activity. The breakthrough in the osine receptor signaling (22). In the past 20 y, it has become treatment of these patients came with the development of increasingly apparent that adenosine regulates many impor- polyethylene glycol (PEG)-modified bovine ADA by the bio- tant aspects of physiology through binding to four distinct, technology company Enzon. PEG-ADA (Adagen) was the seven-transmembrane–spanning G protein-coupled adenosine first FDA-approved PEG-modified protein drug. Its use as a receptors (23). Although adenosine is usually immunosup- therapy for ADA-deficient patients was championed by Dr. pressive and anti-inflammatory, work in ADA-deficient mice

Michael Hershfield at Duke (11). Many patients who do not helped uncover novel roles for adenosine in promoting the Downloaded from have suitable bone marrow donors have been able to lead rea- progression of chronic diseases, including asthma, chronic ob- sonably normal lives as a result of treatment with PEG-ADA. structive pulmonary disease, and pulmonary fibrosis (22). In Today, a number of protein-based drugs that are modified by addition, these mice helped to define a novel role for adeno- pegylation to improve stability and decrease immunogenicity sine signaling in certain manifestations of sickle cell disease are on the market. These include Neulasta (Amgen) for the (24). treatment of leukemia, IFN-b for the treatment of chronic In conclusion, the discovery of ADA deficiency as a cause of http://www.jimmunol.org/ hepatitis C, and uricase for the treatment of refractory gout SCID was groundbreaking for several reasons. First, it was the (12). first immunodeficiency disease for which the molecular defect ADA deficiency also played a prominent role in the devel- was identified, making possible both a prenatal and a postnatal opment of gene therapy. It was the perfect disease for this molecular diagnosis. Second, it underscored the importance fledgling field. As mentioned above, it was already known that of normal purine metabolism for the development of the im- patients with SCID could be cured by BMT from a histocom- mune system. Understanding the mechanisms of ADA-de- patibledonor. Itwasalso known that patients with only 10–12% ficient SCID led to the development of ADA inhibitors and

of normal ADA enzyme activity had normal immune systems deoxyadenosine analogs for the treatment of hairy cell leuke- by guest on September 27, 2021 (13). Thus, it was logical to predict that autologous BMT with mia (10). PEG-ADA became the first PEG-modified protein genetically modified bone marrow cells would be of therapeutic to be used as a therapeutic and opened the door for the de- value even if normal levels of gene expression could not be velopment of additional PEG-modified proteins that are in attained. However, initial attempts were unsuccessful because wide clinical use today. ADA deficiency was the first inherited the small numbers of genetically modified cells were not disease to be treated by gene therapy. Finally, ADA-deficient maintained after transplantation (14). Nevertheless, this ap- mice became an invaluable tool for the study of adenosine proach was successful in patients with X-linked SCID because receptor signaling in chronic lung diseases and sickle cell dis- the genetically modified cells had a selective advantage and ease. Thus, the history of investigations of ADA deficiency, eventually overgrew the remaining unmodified cells (15). This initiated by the startling absence of ADA bands on Eloise realization led to the hypothesis that gene therapy for ADA Giblett’s starch gel, illustrates the potential impact of seren- deficiency was unsuccessful because patients were maintained dipitous discoveries in science and medicine and the unantic- on PEG-ADA as a sort of standard of care. This treatment re- ipated rewards that can arise from the study of patients with moved the selective advantage that ADA gene-corrected cells rare diseases. would enjoy in an otherwise ADA-deficient host. Indeed, when treatment protocols were modified to remove the PEG-ADA, gene therapy for this disorder was successful, although it usually Disclosures took a year or more for the number of gene-corrected T cells The authors have no financial conflicts of interest. to reach maximal levels (16). As with many human diseases, immunologists developed mouse models to have an experimental system in which the References 1. Giblett, E. R., J. E. Anderson, F. Cohen, B. Pollara, and H. J. Meuwissen. 1972. consequences of ADA deficiency could be studied and new Adenosine-deaminase deficiency in two patients with severely impaired cellular treatment strategies evaluated. Much to the surprise of the immunity. Lancet 2: 1067–1069. investigators who made ADA-deficient mice, these mice died 2. Seegmiller, J. E., F. M. Rosenbloom, and W. N. Kelley. 1967. Enzyme defect as- sociated with a sex-linked human neurological disorder and excessive purine syn- in the immediate perinatal period—not of immunodeficiency, thesis. Science 155: 1682–1684. but of liver failure (17, 18). At the time of death, the effect of 3. Thompson, L. F., and J. E. Seegmiller. 1980. Adenosine deaminase deficiency and severe combined immunodeficiency disease. In Advances in Enzymology, ADA deficiency on thymus development was relatively mod- Vol. 51. A. Meister, ed. John Wiley and Sons, Chichester, England, p. 167– est. To sidestep this problem, a strain of mice was developed 210. 4. Giblett, E. R., A. J. Ammann, D. W. Wara, R. Sandman, and L. K. Diamond. that was globally ADA deficient except for that controlled 1975. Nucleoside-phosphorylase deficiency in a child with severely defective T-cell with a placenta-specific promoter (19). Thus, they had ADA immunity and normal B-cell immunity. Lancet 1: 1010–1013. The Journal of Immunology 935

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transduced autologous umbilical cord blood CD34 cells in ADA-deficient SCID adenosine in murine penile erectile tissues contributes to priapism via A2B adeno- http://www.jimmunol.org/ neonates. Nat. Med. 4: 775–780. sine receptor signaling. J. Clin. Invest. 118: 1491–1501. by guest on September 27, 2021