Adenosine Deaminase Deficiency: Metabolic Basis of Immune Deficiency and Pulmonary Inflammation

Michael R. Blackburn; Rodney E. Kellems    Department of Biochemistry and Molecular Biology, University of Texas Health Science Center at Houston, Houston, Texas 77030 USA

1 Introduction

Adenosine deaminase (ADA) is an essential enzyme of purine metabolism (Fig. 1) and is highly conserved throughout phylogeny. The initial clue revealing the importance of ADA to mammalian organisms came with the chance discovery that a form of severe combined immunodeficiency disease (SCID) in humans was associated with ADA deficiency (Giblett et al., 1972). In this serendipitous way ADA deficiency was the first of the immunodeficiency diseases for which the underlying biochemical defect was discovered. Subsequent investigations indicated that ADA deficiency accounts for approximately 20% of cases of human SCID and that it is the most severe of the immunodeficiency diseases, affecting both cell-mediated and humoral immunity (Buckley et al., 1997; Hershfield and Mitchell, 2001). Soon after their discovery that defects in ADA were associated with immunodeficiency, Giblett and colleagues examined other immunodeficient individuals for deficiencies in purine catabolic enzymes and found that defects in purine nucleoside phosphorylase (Fig. 1) also result in immunodeficiency disease (Giblett et al., 1975). These findings demonstrate the importance of purine metabolism in development of the immune system. Deciphering the mechanisms by which defects in purine metabolism lead to abnormal lymphopoiesis has proved a difficult task, and although significant progress in this arena has been made, many questions remain. However, efforts to understand the metabolic basis of the immunodeficiency associated with ADA deficiency have led to advances in the treatment of certain leukemias, and the treatment of ADA deficiency in humans has advanced aspects of enzyme replacement and gene replacement therapies.

The generation of ADA-deficient mice (Blackburn et al., 1998; Migchielsen et al., 1995; Wakamiya et al., 1995) provided the opportunity to examine the pathways by which disturbances in purine metabolism influence physiological systems in the whole animal. ADA-deficient mice develop a combined immunodeficiency similar to that seen in ADA-deficient humans, and experiments in these mice have provided novel mechanistic information about how the accumulation of the ADA substrates adenosine and 2′-deoxyadenosine impact components of the immune system. In addition, by removing the major enzyme that controls adenosine levels in tissues and cells, these mice have served as biological screens for physiological processes sensitive to chronic elevations in adenosine (Blackburn, 2003). In particular, ADA-deficient mice have served as a useful model for deciphering the role of adenosine signaling in chronic inflammatory lung diseases such as asthma and chronic obstructive pulmonary disease (COPD). This review discusses the current understanding of how the accumulation of ADA substrates impacts the immune and pulmonary systems by comparing findings in ADA-deficient humans and mice.

2 ADA Deficiency in Humans

ADA deficiency in humans arises from naturally occurring mutations in the ADA gene that are inherited in an autosomal recessive manner. Most ADA-deficient humans are diagnosed early in life, when they present with marked lymphopenia; failure to thrive; and opportunistic fungal, viral, and bacterial infections (Buckley et al., 1997; Hershfield and Mitchell, 2001). These patients have little to no detectable ADA activity and severe metabolic disturbances associated with the loss of ADA activity. The thymus is absent or small and dysplastic in ADA-deficient individuals (Borzy et al., 1979), and they have severely reduced numbers of peripheral T, B, and natural killer (NK) cells (Buckley et al., 1997). ADA-deficient SCID is the only immunodeficiency in which all three cell types are severely reduced in number. Without intervention, ADA-deficient individuals die from overwhelming infections within the first year of life. A smaller population of ADA-deficient patients presents later in life with a less severe form of immunodeficiency that coincides with less severe loss of ADA enzymatic activity and associated metabolic disturbances (Santisteban et al., 1993). Specific mutations within the ADA gene have been identified for both “early” and “late” onset ADA deficiency (reviewed in Hershfield and Mitchell, 2001), and the severity of the mutations, regarding the loss of ADA enzymatic function, correlates well with the severity of the ensuing disease (Hershfield, 2003).

The most successful treatment for ADA deficiency is histocompatible bone marrow transplantation from an HLA-matched sibling. Because this treatment option is seldom available, alternative treatments have been identified, including T cell–depleted haploidentical bone marrow transplantation from a parent. However, these approaches have met with limited success. A successful biochemical approach for the treatment of ADA deficiency involves the use of enzyme replacement therapy wherein a polyethylene glycol–modified form of bovine ADA (PEG–ADA) is provided to patients by twice weekly intramuscular injection (Hershfield et al., 1993). Polyethylene glycol appears to protect the bovine ADA from proteolytic and immunologic attack, hence increasing the circulating half-life of this exogenous enzyme. ADA replacement therapy is effective in reducing the metabolic impact of ADA deficiency and has prolonged the life of individuals who have in some cases been treated for more than 8 years (Hershfield, 1995). Relatively few complications have been reported with respect to allergic reactions or immunogenicity to PEG–ADA, and it appears to be the best option for the prolonged treatment of ADA-deficient patients who lack an HLA-identical marrow donor.

ADA deficiency has received considerable attention as the testing ground for the development of gene therapy protocols. Several features of ADA deficiency make it an attractive candidate for gene replacement therapy: bone marrow or cord blood stem cells are relatively accessible cell populations; individuals with as little as 5% normal ADA activity have normal immune function, suggesting a high degree of replacement may not be necessary; and evidence exists to suggest that T cells with ADA activity can be selected for and enriched in an ADA-deficient environment. The latter is supported by observations in ADA-deficient individuals where spontaneous clinical remissions occurred in association with mosaicism for ADA expression (Hirschhorn et al., 1994, 1996), which in one instance was associated with the reversion of a mutation in the ADA allele to normal in lymphoid cells (Hirschhorn et al., 1996). Thus, the hope is that efficient transfer of a recombinant ADA gene into hematopoietic cells will result in the outgrowth of a genetically...