Development of gene therapy for the treatment of adenosine deaminase deficiency.

Adenosine deaminase (ADA) severe combined immunodeficiency (SCED) is a life-threatening condition resulting from lack of the ADA enzyme. Consequences include immunodeficiency and non-immunological symptoms such as neurological abnormalities. Bone marrow transplantation (BMT) from a haploidentical donor usually results in complete restoration of immune function. However, the majority of patients do not have a matched donor and are therefore treated with enzyme replacement therapy (PEG-ADA). This treatment is not always fully effective, it is expensive and needs to be administered throughout life. Gene therapy is an alternative treatment, and previous trials for ADA deficiency have shown that it can significantly improve immunological function. Immune recovery was assessed in three ADA-SCID patients treated with PEG-ADA by analysis of lymphocyte counts and emergence of naive T cells. One patient was not responding well to PEG-ADA and was enrolled in a Phase I clinical gene therapy trial. A gammaretroviral vector encoding ADA was constructed and tested extensively on cell lines and patient cells and a CD34+ cell transduction protocol was optimised. The gene therapy procedure was based on previous successful trials, and involved withdrawal of PEG-ADA prior to treatment to provide selective growth advantage for transduced cells, and mild conditioning to encourage engraftment. Assessments of immune function were then performed in a similar manner to patients treated with PEG-AD A. Recent evidence from studies of ADA deficiency indicates that it is a multi-organ disease. However, gene therapy using CD34+ cells may only correct the immunodeficiency without ameliorating non-immunological symptoms. Hence, studies were performed to develop systemic gene therapy for ADA-SCID, involving the use of CD34+ cells and mesenchymal stem cells (MSCs). MSCs were isolated from bone marrow, and their multipotential nature was assessed prior to and following gene transfer using a cloned ADA lentiviral vector. Transduced MSCs maintained their ability to undergo differentiation and transgene expression was not affected by this. These clinical and preclinical in vitro studies demonstrate that gene therapy holds therapeutic potential for treatment of ADA-SCID.