Myoblast gene therapy in canine mucopolysaccharidosis. I: Abrogation by an immune response to alpha-L-iduronidase

Abstract

Three dogs with deficiency of the lysosomal enzyme alpha-L-iduronidase were treated by gene replacement therapy targeted at muscle. Direct intramuscular injections of plasmid encoding the alpha-L-iduronidase gene cDNA resulted in no detectable enzyme production, but may have resulted in immunologic sensitization to iduronidase protein, which the dogs lack totally. Myoblasts were grown from skeletal muscle biopsies and transduced with a retroviral vector containing the canine gene under control of the muscle creatine kinase enhancer. Several hundred-fold overexpression of enzyme production occurred in cultured cells; however, following reintroduction of the cultured cells into dogs, enzyme production declined rapidly. Concurrent with the falling enzyme levels, there was production of specific immunoglobulin G (IgG) antibody against iduronidase that was further associated with cellular infiltration of the myoblast injection sites. Most inflammatory cells were lymphocytes and plasma cells, suggesting local humoral and cellular immune responses to the enzyme-producing muscle cells. PCR analysis of tissues collected 2-22 weeks after the final treatment showed the persistence of Neo and canine alpha-L-iduronidase sequences in a progressively decreasing percentage of myoblasts. Results from this study in a canine model of mucopolysaccharidosis I underscore the fact that immunologic reactions to cells producing desirable, normal, but foreign, proteins may be as much an impediment to gene therapy as reactions to the viral vectors used to introduce the foreign gene.