ADA SCID

ADA SCID (adenosine deaminase-defective severe combined immunodeficiency) is a fatal genetic disorder caused by defects in the gene that encodes adenosine deaminase (ADA). 

The consequence of ADA deficiency is accumulation of adenosine and 2 -deoxyadenosine, substances toxic to lymphocytes, important cells in the immune response. 2 -Deoxyadenosine is particularly toxic because its presence leads to accumulation of its nucleotide form, dATP, an essential substrate in DNA synthesis. Elevated levels of dATP actually block DNA replication and cell division by inhibiting synthesis of the other deoxynncleoside 5 -triphosphates (see Chapter 27). Accumulation of dATP also leads to selective depletion of cellular ATP, robbing cells of energy. Children with ADA SCID fail to develop normal immune responses and are susceptible to fatal infections, unless kept in protective isolation. 

Aiuti A, Vai S, MorteUaro A, Casorati G, Ficara F, Andolfi G, Ferrari G, Tabucchi A, Carlucci F, Ochs HD, Notarangelo LD, Roncarolo MG, Bordignon C (2002b) Immune reconstitution in ADA-SCID after PBL gene therapy and discontinuation of enzyme replacement. Nat Med 8 423 25... 

Hirschliorn, R., A. Ellenbogen, and S. TzaU, Five missense mutations at the adenosine deaminase locus (ADA) detected by altered restriction fragments and their frequency in ADA—patients with severe combined immunodeficiency (ADA-SCID). Am J Med Genet, 1992. 42(2) 201-7. 

Blaese, R.M. (1995). T-lymphocyte-directed gene therapy for ADA-SCID Initial trial results after 4 years. Science, 270, 475 180. 

ADA-Transduced Hematopoietic Stem Cell Therapy for ADA-SCID... 

San Raffaele Telethon Institute for Gene Therapy is developing an adenosine deaminase transduced hematopoietic stem cell therapy for the potential intravenous treatment of adenosine deaminase deficiency in severe combined immunocompromised individuals (ADA-SCID). 

Adenosine deaminase (ADA) is a ubiquitous enzyme that is essential for the breakdown of the purine base adenosine, from both food intake and the turnover of nucleic acids. ADA hydrolyzes adenosine and deoxyadenosine into inosine and deoxyinosine, respectively, via the removal of an amino group. Deficiency of the ADA enzyme results in the build-up of deoxyadenosine and deoxyATP (adenosine triphosphate), both of which inhibit the normal maturation and survival of lymphocytes. Most importantly, these metabolites affect the ability of T-cells to differentiate into mature T-cells [656430], [666686]. ADA deficiency results in a form of severe combined immunodeficiency (SCID), known as ADA-SCID [467343]. 

Gene transfer of mouse and human CD34+ HSCs, which were genetically modified by a retroviral virus encoding ADA cDNA and the neomycin-resistance marker gene, have been extensively characterized in mouse models without reports of toxicity. Furthermore, none of the preclinical studies for ADA-SCID have demonstrated evidence of cancer in animals treated with this same gene transfer approach [515137], [666652], [666655]. 

Using the optimal protocol combination of T/F/S and IL-3 that was developed from a series of preclinical studies (outlined above), autologous HSC gene therapy for ADA-SCID was performed in combination with nonmyeloablative conditioning (2 mg/kg/day of busulfan) in two patients (aged 7 months [Ptl], and two years and 6 months [Pt2], respectively) [515130], [515136], [607777]. 

The clinical trial was later expanded to include two more ADA-SCID patients, and included treatment follow-ups at 10, 15, 29 and 35 months after therapy [515038], [516040], [643376]. The results from the most recent follow-up matched those found in the initial study of two patients. PBL counts in all patients were 2.5 x 9 , 0.2 x 109/1, 1.3 x 109/1 and 0.7 x 109/1 for Ptl, Pt2, Pt3 and Pt4, respectively. The overall level of myeloid engraftment and the speed and degree of immune reconstitution correlated with both the dose of infused transduced CD34+ cells and the degree of myelosuppression [643376]. 

Pioneering studies have demonstrated the potential of gene therapy for the treatment of inherited hematopoietic diseases [440300], and particularly for ADA-SCID [470017], [470024], [666662], [666664], [666665]. However, vector design, gene transfer protocols and inadequate engraftment and expansion of genetically engineered cells limited the success of earlier studies [206054], [657269], [657273], [668669]. 

In vitro The effects of IL-3 and IL-7 on gene transfer efficiency ADA-SCID BM CD34+ cells IL-3 or IL-7 in a 4-day CD34+ cells culture greatly improved the number of -cell progenitors compared with the T/F/S combination of cytokines, resulting in a 6- and 5-fold increase, respectively, and reached values that did not differ significantly from those of freshly-isolated cells. 643373... 

In vitro/ in vivo Repopulation and differentiation CD34+ cells isolated at day +330 from the BM of an ADA-SCID patient The lymphoid differentia-tion capacity of CD34+ cells was maintained, and genetically corrected HSCs retained their ability to reconstitute lymphopoiesis in a secondary transplant (SCID-hu mice) after infusion. 515130... 

Efficacy Autologous HSC gene therapy and non- myeloablative conditioning in two conditioning in two ADA-SCID patients In both patients, the number of PBLs, serum IgM, IgA and IgG levels, mRNA expression of the ADA vector, intracellular ADA enzymatic activity in PBLs, and erythrocyte enzyme activity indicated a reconstitution of -cell functions, as well as an amelioration of the metabolic pattern. 515130... 

Safety and efficacy Expansion of the trial outlined above to six children affected by early onset ADA-SCID All patients were reported to be alive and healthy in the absence of enzyme replacement therapy. The degree of myelosuppression after conditioning ranged from mild (Pt2, Pt4 and Pt6) to short-term neutropenia (Ptl and Pt5), or more prolonged thrombocytopenia and neutropenia (Pt3). None of the patients experienced severe infections or adverse events 668664... 

Reproduced with permission from The Thomson Corporation and Taupin P ADA-transduced hematopoietic stem cell therapy for ADA-SCID. IDrugs (2006) 9(6) 423-30. Copyright 2006, The Thomson Corporation. 

Correction of ADA-SCID by stem cell gene therapy combined with nonmyeloablative conditioning. Aiuti, A., Slavin, S., Aker, M., Ficara, F., Deola, S., Mortellaro, A., Morecki, S., Andolfi, G., Tabucchi, A., Carlucci, F., Marinello, E. et al. (2002). Science, 296 (5577) 2410-2413. 

Definitive example of successful gene therapy in two patients suffering from ADA-SCID. More patients need to be enrolled in similar trials to confirm the safety, efficacy and optimization of the therapy, and also with regard to the requirement of nonmyeloablative conditioning. 

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