| Summary: | X-Linked severe combined immunodeficiency (SCID-X1) is caused by mutations in the gene encoding the common cytokine receptor gamma chain, yc, resulting in profound defects in both cellular and humoral immunity. Although allogeneic bone marrow transplantation has proved highly successful, HLA-mismatched procedures are associated with significant morbidity and mortality. This disease is therefore a good candidate for gene therapy and sustained correction of 19 SCID-X1 patients have been reported in two clinical trials. However, the occurrence of severe adverse advents in one trial has highlighted the potential side-effects of retroviral gene transfer and reinforced the need to develop safer gene therapy vectors. A series of self-inactivating (SIN) gammaretroviral and lentiviral vectors for the treatment of SCID-X1 have consequently been developed. To reduce the potential for insertional mutagenesis mediated by the duplicated viral LTR sequences, alternative internal regulatory elements have been incorporated into the vector backbone. These include both endogenous (human elongation factor la - EFS) and viral (spleen focus forming virus - SFFV) promoters. In vitro, the SIN retroviral vectors were able to regulate yc expression on the cell surface of SCID-X1 cell lines and restore the lymphoid differentiation potential of Il2rg" haematopoietic progenitor cells. Functional correction of the immunological defect in the SCID-X1 mouse model was also achieved at similar levels for the both the SIN retroviral vectors and the LTR-regulated clinical vector. To further improve upon safety, lentiviral vectors were developed incorporating the endogenous human IL2RG promoter to regulate physiological expression of yc. In vitro and in vivo analysis of the promoter indicated a degree of haematopoietic tissue specificity and restoration of functional yc-receptor complexes was achieved following transduction of a SCID-X1 T cell line with a lentiviral vector incorporating this promoter however phenotypic correction of the yc-deficient mouse was unsuccessful. These results demonstrate that SIN retroviral vectors for SCID-X1 are effective in restoration of the immune defect in the yc-deficient murine model. The SIN design together with an endogenous (EFS) promoter might provide a potentially less mutagenic but equally effective vector for gene therapy of SCID-X1.
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