Gene Therapy for the Enhancement of Fracture Healing - Engineered Bone: Tissue Engineering Intelligence Unit - page 128

Figure 1. Human osteoblast like cells retrovirally transduced with LacZ. Transduction was enhanced only
by the cationic polymer Polybrene®. X-gal staining. Approximately 10% blue cells. Micrograph courtesy
of Dr. Axel Baltzer. Reprinted with permission from Baltzer A: Tierexperimentelle Untersuchungen zur
Entwicklung einer Gentherapie für die Behandlung von Knochenerkrankungen. Monograph. Shaker Verlag,
Aachen 2000.
Application of Retroviral Vectors to Bone Healing
There are only a few studies published on gene therapy using retroviral vectors for bone and
fracture healing specifically. Reinecke and coworkers reported an ex vivo approach to gene
transfer to cells of disc, spinal nerve root, and vertebral bone of the lumbar spine of rats. 28 The
cDNA of human interleukin-1 receptor antagonist (IL-1ra) was transferred via retroviral vec-
tor into cells isolated from vertebral bone, resulting in a mean protein expression of 33.5 pg/
ml/10 3 cells/48 hrs (SD 11.1). Baltzer and coworkers were able to transduce human osteoblast
cells in vitro using a retroviral vector. 29 Again, the cDNA of human IL-1ra and the β -galactosi-
dase (LacZ) gene were introduced into the isolated osteoblasts. As previously mentioned, poor
transduction efficiency of target cells may lead to few retroviral particles that actually bind the
appropriate receptors on the target cell surface. Baltzer and coworkers attempted to alleviate
these effects by centrifugation at 500 rpm and lowering of cell culture temperature to room
temperature during the first hour of transduction. By these means it was possible to increase
the initial transduction efficiency from 10% to approximately 60% 29 (Figs. 1 and 2).
Mason, Breitbart and coworkers published several studies using cultured periosteal cells
transduced retrovirally with the BMP-7 gene for bone healing applications. 30,31
Periosteum-derived cells were transduced ex vivo via retroviral vector encoding BMP-7 cDNA
and seeded onto poly-(glycolic acid) (PGA) matrices and implanted to treat an osteochondral
defect in a rabbit knee. Completed or nearly completed bone and articular cartilage regenera-
tion was observed. Since bone was appropriately regenerated only in the subchondral region
and articular cartilage only in the region of overlying cartilage, the authors conclude that BMP-7
may not act alone in signaling during regeneration. Therefore, it may be likely that BMP-7 has
paracrine regulatory effects on local populations of cells. 31
In a similar approach, experiments have been performed on bone marrow-derived cells
which may not only serve as a vehicle for the delivery of therapeutic proteins, but also may
differentiate into osteoprogenitor cells. A fraction of bone marrow-derived cells is considered