Bone tissue specific transcriptional control: options for targeting gene therapy to the skeleton

The transplantation of multipotential bone marrow cells containing bone tissue specific promoter-controlled transgenes provides an efficacious approach to deliver therapeutic gene expression to osteoblasts for the treatment of patients with bone disorders or tumor metastasis to the skeleton. The specificity of tissue-restricted gene therapy can be refined by utilization of a 31-amino-acid segment of the hematopoietic and osteogenic AML/CBF transcription factors that direct the regulatory proteins to subnuclear sites that support gene expression.

Unfractionated adherent bone marrow cells from transgenic mice constructed with the proximal 1.7 kb of the osteocalcin gene promoter fused to a CAT reporter were transplanted by intravenous infusion. Engraftment and expression at the single-cell level within the context of tissue organization was established by immunohistochemistry using an anti-CAT antibody. Sequences that support the intranuclear trafficking of AML/CBF transcription factors to subnuclear sites that support transcription were determined by the expression and visualization of mutated and epitope tagged AML/CBF proteins.

Immunohistochemical staining of an extensive series of tissue sections from mice posttransplantation using an anti-CAT antibody indicated that CAT-positive osteoblasts and osteocytes were present in bone sections. These findings indicate that donor bone marrow-derived cells engraft in bone tissue in an environment that supports maturation to the developmental stage at which a bone specific osteocalcin promoter is transcriptionally active. Characterization of functional domains in AML/CBF transcription factors has established that there are at least two regulated events that are required for targeting the factors to transcriptionally active nuclear domains: A nuclear localization signal in the amino terminal region controls nuclear import and retention, and a nuclear matrix targeting signal in the carboxyl region controls association with nuclear matrix-linked sites where transcription occurs.

The specificity of hematopoietic and bone phenotypic promoters, together with the additional level of specificity inherent in the AML/CBF family of hematopoietic and osteogenic intranuclear targeting signals, offers viable options for constructing gene therapy regimens that are targeted to the skeleton for the control of metastatic disease. It is realistic to anticipate that, as additional parameters of gene regulatory mechanisms are defined, particularly components of transcriptional control that are operative within a three-dimensional context of nuclear architecture, opportunities for enhancing the effectiveness of treating patients with tumors that metastasize to bone will be extended.