Adenosine Deaminase Deficiency: From the Discovery of Its Molecular Pathogenesis to Targeted Therapy


Adenosine deaminase (ADA) is a key enzyme of the purine metabolism pathway whose gene is located on the chromosome 20 in humans. Mutations in the sequence of the ADA gene were found to cause severe combined immunodeficiency (SCID) both in humans and murine disease models.

Because of the ubiquitous nature of ADA expression, ADA deficiency should be recognised as a systemic metabolic disorder that results in multiple organ pathologies. However, since the highest ADA expression is observed in the immune cells, the immunologic defects are the most severe clinical symptoms.

ADA deficiency has also been the first autosomal recessive immunodeficiency disorder for which the molecular mechanism was identified. This led, consequently, to the development of many different therapies for the management of this disease.

Key Concepts

  • ADA‐SCID was the first immunodeficiency for which the molecular mechanism has been identified, thus underscoring the importance of a normal purine metabolism for the development of the immune system.
  • Differences in ADA gene expression in various cell types and maturation stages highlight differences in the rate of degradation (activity) of the ADA enzyme and nucleotide metabolism.
  • The evidence that ADA enzyme is highly expressed in lymphoid tissues suggested that lymphocytes have evolved a mechanism to prevent the build‐up of ADA substrates generated by the massive apoptosis and high proliferation rates of these cells.
  • SCIDs are uniformly fatal but have long‐term survival rates if diagnosed early, stabilised and treated before complications arise, such as infections or organ damages.
  • TREC assay is a sensitive and economical alternative test for the diagnosis of ADA deficiency and most other SCIDs.
  • Gene therapy is a genetic engineering technique used to replace a defective gene of a cell in order to prevent a disease.
  • A clinical trial is a research study in which a cohort of patients is prospectively assigned a new therapeutic treatment, that has shown benefits in animals engaged in preclinical trials, but has not yet been proven effective to humans.
  • ADA‐deficient murine models have been extensively used to understand the pathological bases of ADA‐SCID and to test, in preclinical trials, the efficacy and safety of viral vector‐mediated gene therapy and other therapeutic approaches.

Keywords: adenosine deaminase; purine metabolism; ADA‐SCID; lymphopenia; PEG‐ADA; gene therapy; clinical trial; neonatal screening; mouse model

Figure 1. Timeline highlighting some important milestones of SCID discovery and treatments. Pink boxes are referred to ADA‐SCID disease milestones.
Figure 2. The adenosine deaminase metabolism. ADA is an enzyme of the purine salvage pathway, which catalyses the irreversible deamination of adenosine and deoxyadenosine into inosine and deoxyinosine, respectively. Inosine nucleosides may either be irreversibly catabolised down to hypoxanthine and uric acid or be salvaged back for further use in purine metabolic pathways. Deoxyadenosine is predominantly produced from degradation of DNA, instead, adenosine derives either from endogenous breakdown of ATP and degradation of RNA or is taken up exogenously by ubiquitously expressed nucleoside transporters. In the absence of ADA, the levels of ADA substrates, adenosine and deoxyadenosine, dramatically increase both intracellularly and extracellularly in the body fluids. The excess of ADA substrates may now spill over into additional pathways normally only minimally utilised, thus contributing to the pathogenic mechanisms of the disease. For example, lymphocytes have high levels of kinases which can successively phosphorylate deoxyadenosine to the nucleotides dAMP, dADP and dATP. dATP is the major metabolite that produces the lymphotoxicity that leads to SCID.
Figure 3. Current therapeutic options in ADA‐SCID. Immune reconstitution in ADA deficiency can be achieved by enzyme replacement therapy (ERT), haematopoietic stem cell transplantation (HSCT) and gene therapy (GT). Treatment of choice remains bone marrow transplantation from an HLA‐matched sibling or other family donor. As transplants from alternative donors are associated with high morbidity and mortality, gene therapy is the second option to take into consideration. ADA‐SCID has been the pioneer disease for the development of human gene therapy and it is based on the reinfusion of autologous gene‐corrected HSCs. ADA‐deficient cells are normally transduced with a retroviral vector containing the ADA cDNA. Different from HSCT and GT, enzyme replacement therapy is a noncurative treatment requiring weekly intramuscular injections with PEGylated bovine ADA (Adagen). Variable degrees of immune reconstitution can be achieved by this treatment. However, the effectiveness of PEG‐ADA in promoting clinical well‐being in patients, makes it an important option in the care of patients, especially when the other two treatments are not available. Benefits and potential problems for each available treatment are listed in Table .
Figure 4. Schematic representation of viral vectors and their modifications to improve safety. (a) Conventional gamma retroviral (γRV) vector used for ADA‐SCID clinical trials. The transcription of the therapeutic gene is driven by the promoter/enhancer elements in the U3 region of the LTR. (b) Self‐inactivating (SIN) viral vector. The transcription of the therapeutic gene is driven by the insertion of an internal promoter (IP). Codon‐optimised genes will be useful in models where increased protein production is not an issue. The U3 region of the LTR has been almost completely deleted. (c) SIN vector containing two insulator elements (I) in order to protect the transcriptional cassette against position effects. Abbreviations: U3, unique region 3; R, repeat region; U5, unique region 5; Θ, primer binding site; ψ, packaging signal; SD, splicing donor site; SA, splicing acceptor site; IP, internal promoter; EFS, elongation factor 1α short promoter; PGK, phosphoglycerate kinase promoter; TS promoter, tissue specific promoter; I, insulator element.


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Further Reading

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Moretti, Federico A, and Staal, Frank JT(Jul 2019) Adenosine Deaminase Deficiency: From the Discovery of Its Molecular Pathogenesis to Targeted Therapy. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0027873]