Bone-directed expression of Col1a1 promoter-driven self-inactivating retroviral vector in bone marrow cells and transgenic mice

Gene therapy of bone would benefit from the availability of vectors that pr ovide stable, osteoblast-specific expression. This would allow bone-specifi c expression of Col1a1 cDNAs for treatment of osteogenesis imperfecta. In a ddition, such a vector would restrict expression of secreted therapeutic pr oteins to the bone-synthesizing regions of the bone marrow after ex vivo tr ansduction of marrow stromal cells and reintroduction of the cells into pat ients. Retrovirus vectors stably integrate into target cell genomes; howeve r, long-term regulated expression from internal cellular promoters has not been consistently achieved. In some cases this is due to a stem cell-specif ic mechanism for transcriptional repression of retroviruses. We evaluated t he ability of self-inactivating ROSA-derived vectors containing a bone-dire cted 2.3-kb rat Col1a1 promoter to display osteoblast-specific expression. In vitro expression was examined in bone marrow stromal cell cultures induc ed to undergo osteoblastic differentiation. In vivo expression was evaluate d in chimeric mice derived from transduced embryonic stem cells. The result s indicate that self-inactivating retrovirus vectors containing the col1a1 promoter are not permanently inactivated in embryonic stem cells and are sp ecifically expressed in osteoblasts in vivo and in vitro. Thus these vector s should be useful for bone-directed gene therapy.