Three dogs with deficiency of the lysosomal enzyme α-l-iduronidase were treated by gene replacement therapy targeted at muscle. Direct intramuscular injections of plasmid encoding the α-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 α-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.
Overview summary
Mucopolysaccharidosis I (MPS I) is an enzyme deficiency disease that may be amenable to gene therapy. When absolute deficiency of an enzyme exists, as is the case in many patients with the severe form of MPS I, it may be necessary to suppress the immune system, in addition to supplying genetically altered cells. Production of beneficial, normal cell products may stimulate an immunologic reaction that results in destruction of transplanted cells or rapid removal of the potentially therapeutic protein.
