Gene therapy for bone healing

Abstract

Clinical problems in bone healing include large segmental defects, spinal fusions, and the nonunion and delayed union of fractures. Gene-transfer technologies have the potential to aid healing by permitting the local delivery and sustained expression of osteogenic gene products within osseous lesions. Key questions for such an approach include the choice of transgene, vector and gene-transfer strategy. Most experimental data have been obtained using cDNAs encoding osteogenic growth factors such as bone morphogenetic protein-2 (BMP-2), BMP-4 and BMP-7, in conjunction with both nonviral and viral vectors using in vivo and ex vivo delivery strategies. Proof of principle has been convincingly demonstrated in small-animal models. Relatively few studies have used large animals, but the results so far are encouraging. Once a reliable method has been developed, it will be necessary to perform detailed pharmacological and toxicological studies, as well as satisfy other demands of the regulatory bodies, before human clinical trials can be initiated. Such studies are very expensive and often protracted. Thus, progress in developing a clinically useful gene therapy for bone healing is determined not only by scientific considerations, but also by financial constraints and the ambient regulatory environment.

Figure 1

Strategies for gene transfer to defects in bone. There are two general strategies: in vivo and ex vivo. For in vivo gene delivery, the vector is introduced directly into the site of the osseous lesion, either as a free suspension (top right) or incorporated into a gene-activated matrix (GAM) (bottom right). For ex vivo delivery, vectors are not introduced directly into the defect. Instead they are used for the genetic modification of cells, which are subsequently implanted. Traditional ex vivo methods (top left) usually involve the establishment of cell cultures, which are genetically modified in vitro. The modified cells are then introduced into the lesion, often after seeding onto an appropriate scaffold. Expedited ex vivo methods (bottom left) avoid the need for cell culture by genetically modifying tissues such as marrow, muscle and fat, intraoperatively and inserting them into the defect during a single operative session.

Figure 2

Healing of a rat femoral segmental defect following in vivo delivery of an adenovirus vector encoding BMP2. Representative radiographic images of segmental bone defects after direct injection of adenoviral vectors encoding human BMP-2 (Ad.BMP-2) or luciferase cDNA (Ad.luc). Most defects that had been treated with Ad.BMP-2 displayed bone formation within the defect by 4 weeks (a) and complete union by 8 weeks (c). Control defects treated with Ad.luc (and untreated defects) did not display appreciable signs of healing within this time (b and d). Images reproduced from Ref. 65.