A RETROVIRAL VECTOR CONTAINING A MUSCLE-SPECIFIC ENHANCER DRIVES GENE-EXPRESSION ONLY IN DIFFERENTIATED MUSCLE-FIBERS

Genetically modified myogenic cells have a number of potentially relev ant applications for gene therapy of genetic defects. Retroviral vecto rs proved to be a safe and efficient tool to transfer and express gene s into satellite cells and their differentiated progeny, although musc le-specific regulation of the transferred gene is very difficult to ac hieve in a conventional vector framework. We modified a Moloney murine leukemia virus (MoMLV)-derived retroviral vector containing a bacteri al beta-galactosidase (beta-Gal) reporter gene by inserting a muscle c reatinine kinase (MCK) enhancer element into the U3 region of the vira l long terminal repeat (LTR). The resulting vector (mLBSN) was transfe rred into cells of different histological origin, including undifferen tiated murine and human myogenic cells, which were unable to express t he transgene at detectable levels. Instead, gene expression from the m odified LTR was obtained in a mouse myogenic cell line and in human pr imary satellite cells upon induction of differentiation into myotubes in culture, and correlated with the activation of the muscle different iation program. beta-Gal-negative, mLBSN-transduced human satellite ce lls were also transplanted into the quadricep muscle of immunodeficien t mice, where activation of the, transgene expression was observed in vivo after differentiation and fusion into muscle fibers. These result s show that retroviral vectors carrying LTRs modified in the enhancer sequences may be used to target tissue- and differentiation-specific g ene expression into the muscle. For practical purposes, satellite cell s engineered by muscle-specific retroviral vectors might represent an effective tool to deliver expression of a given gene product specifica lly into the muscle tissue, avoiding undesired protein accumulation in mononucleated cells. More generally, this type of vector might be use ful whenever regulated expression of a transferred gene is necessary i n a target cell or tissue.