Gene transfer into muscle tissue is currently being developed as a method for the production, secretion and delivery of therapeutic proteins. This methodology has been used to produce a variety of physiologically active proteins and may ultimately be applied to the treatment of several diseases. In this review, we consider several applications of this methodology and discuss approaches for modulating therapeutic protein production and secretion from muscle, using growth hormone as an example. In addition, factors limiting the effectiveness of muscle gene transfer are also discussed, as these shall determine the efficacy of muscle gene transfer when applied to humans.
GS MacColl, G Goldspink and PM Bouloux
GS MacColl, FJ Novo, NJ Marshall, M Waters, G Goldspink and PM Bouloux
The production of peptide hormones by skeletal muscle tissue is a promising area of gene therapy. Skeletal muscle myogenesis can be induced in vitro, resulting in the fusion of mononucleate myoblasts to form multinucleate myotubes, and delivery vectors are first tested in vitro. C2C12 myoblasts transfected with pcDNA3-GH, which used the human cytomegalovirus (CMV) promoter, secreted immunoreactive GH with comparable biological activity to pituitary GH. Mouse myeloid leukaemia cells, which express the mouse GH receptor were used for the bioassay, and activation of these cells by GH was measured by a colorimetric microculture tetrazolium assay. Cells were incubated with a tetrazolium salt (MTS) and an intermediate electron acceptor (phenazine methosulphate, PMS), and formazan production was measured as optical density (O.D.) at 490 nm. The efficiencies of several plasmid expression vectors were compared in differentiated and non-differentiated muscle cells, as a function of bioactive GH secreted by the transfected cells. Ten-day differentiated C2C12 myotubes transfected with pcDNA3E-GH, which used the CMV promoter and a rat myosin light chain enhancer element, secreted significantly more biologically active GH than myotubes transfected with pcDNA3-GH (0.82 O.D. units+/-0.06 vs 0.57+/-0.05 respectively, P<0.001). This was consistent with reduced CMV promoter activity in myotubes. Myoblasts transfected with pcDNA3-GH secreted more bioactive GH than 10-day transfected myotubes (1.1+/-0. 1 vs 0.77+/-0.07 respectively). However, the responses were indistinguishable (both 1.0+/-0.09) if both the myotubes and myoblasts had been transfected with pcDNA3E-GH. Substitution of the vector pMHLC-GH, which used a muscle-specific truncated rabbit myosin heavy chain promoter, and the myosin enhancer resulted in a marked decrease in the responses to the conditioned medium from fused myotubes compared with the vectors pcDNA3-GH and pcDNA3E-GH (0. 24+/-0.02 vs 0.57+/-0.05 vs 0.82+/-0.06 respectively). We concluded that the combination of CMV promoter and myosin light chain enhancer in pcDNA3E-GH had the greatest expression efficiency of the several plasmid vectors which we investigated.