Osteoblast-specific gene expression after transplantation of marrow cells:Implications for skeletal gene therapy

Somatic gene therapies require targeted transfer of the therapeutic gene(s) into stem cells that proliferate and then differentiate and express the ge ne in a tissue-restricted manner. We have developed an approach for gene th erapy using marrow cells that takes advantage of the osteoblast specificity of the osteocalcin promoter to confine expression of chimeric genes to bon e. Adherent marrow cells, carrying a reporter gene [chloramphenicol acetylt ransferase (CAT)] under the control of a 1.7-kilobase rat osteocalcin gene promoter, were expanded ex vivo. lifter transplantation by intravenous infu sion, engrafted donor cells in recipient mice were detected by the presence of the transgene in a broad spectrum of tissues, How ever, expression of t he transgene was restricted to osteoblasts and osteocytes, as established b y biochemical analysis of CAT activity and immunohistochemical analysis of CAT expression at the single cell level. Our data indicate that donor cells achieved long-term engraftment in various tissues of the recipients and th at the CAT gene under control of the osteocalcin promoter is expressed spec ifically in bone, Thus, transplantation of multipotential marrow cells cont aining the osteocalcin promoter-controlled transgene pro,ides an efficaciou s approach to deliver therapeutic gene expression to osteoblasts for treatm ent of bone disorders or tumor metastasis to the skeleton.