GENE-TRANSFER INTO SKELETAL-MUSCLES BY ISOGENIC MYOBLASTS

The best way to overcome immunorejection in heterologous myoblast transfer (HMT) is by the use of immunodeficient and/or highly immunosuppressed mice as hosts. The same may be attained by autologous myoblast transfer (AMT). In this paper, we describe myoblast transfer in mdx and normal mice where the donor myogenic cells originated from highly inbred litter mates that are considered to be isogenic and thus the procedure is analogous to AMT. The myoblasts were marked in vitro with Rous Sarcoma Virus (RSV)-luciferase (Lux) or RSV-beta-galactosidase (LacZ) reporter genes through transduction mediated by an autonomously replication-defective recombinant human adenovirus. This permitted us to follow their fate after transplantation. mdx and normal mice were irradiated with 20 Gray gamma rays; necrosis and regeneration were induced by intramuscular notexin prior to myoblast injection. In both mdx and normal mice, the expression of luciferase rapidly declined after the injection implying that a large portion of the injected myoblasts were lost by 48 hr, due to undetermined cause(s). The surviving, injected myoblasts well-mosaicized large groups of host fibers but only in the immediate vicinity of the injection. Substantial expression of the reporter gene continued up to 1 month post-transplantation in normal mice, but there was a gradual decline and eventual disappearance of the reporter gene expression in mdx mice. This latter phenomenon was due to the ongoing intense necrosis of muscle fibers in mdx. There was no evidence of immunorejection. These experiments indicate that even in the absence of immunorejection, myoblast transfer suffers from important negative features: major loss of myoblasts within 48 hr after the injection and lack of significant spread of the injected cells from the injection site in the host muscle, These factors, plus the limited proliferative and fusion capacity of Duchenne muscular dystrophy (DMD) myoblasts, make them less than an ideal vector for the dystrophin cDNA for dystrophin gene replacement therapy in DMD.