Gene therapy for spine fusion

Noggin regulation of bone morphogenetic protein (BMP) 2/7 heterodimer activity in vitro

Bone morphogenic proteins (BMPs) are growth factors important for skeletal development and bone growth. Noggin, one of the soluble BMP antagonists, regulates the action of BMPs on mesenchymal precursor cells, partially through a feedback type of inhibition. In this study, we constructed a novel BMP2/7 'fusion gene' that encodes both BMP2 and BMP7 genes in tandem by a linker. Polymerase chain reaction (PCR) and Western blotting showed that the BMP2/7 fusion gene construct led to the production of BMP2/7 heterodimers in A549 'producer' cells. When applied to C2C12 myoblastic cells, BMP2/7 heterodimers increased alkaline phosphatase (ALP) activity and osteocalcin (OCN) expression (markers of osteoblastic differentiation) more effectively than either BMP2 or BMP7 homodimers. Moreover, this heterodimer induced significantly lower levels of Noggin expression in C2C12 cells than respective homodimers at similar doses. The addition of Noggin did not affect the heterodimer's activities in increasing osteoblastic differentiation in C2C12 cells. In contrast, BMP2 and BMP7 homodimers were largely inhibited by Noggin. Our finding suggests that the 'fusion gene' construct led to the production of bioactive BMP2/7 heterodimers, which were not antagonized by Noggin as effectively as it to BMP homodimers. The weaker Noggin antagonism on BMP heterodimers compared to homodimers may contribute to increased osteogenic potency of heterodimers in vitro and in vivo.

Bone Morphogenic Protein-2 Gene Therapy for Mandibular Distraction Osteogenesis

Distraction osteogenesis (DO) requires a long consolidation period and has a low but real failure rate. Bone morphogenic proteins (BMPs) accelerate bone deposition in fractures and critical-sized bone defects, but their effects on mandibular DO are unknown. We investigated the effect of local delivery of adenovirus containing the gene for BMP-2 (Adbmp-2) on mandibular DO in a rat model. Rats (n = 54) were distracted to 3 mm over 6 days. At the start of consolidation (POD 10), Adbmp-2 or adenovirus containing the lacZgene (AdlacZ) was injected directly into the distraction zone. After 1, 2, and 4 weeks of consolidation, mandibles were evaluated for amount of bone deposition. Adbmp-2-treated specimens demonstrated an increased amount of new bone formation by radiographic, histologic, and histomorphometric analysis. This study demonstrates that local, adenovirally-mediated delivery of BMP-2 can increase bone deposition during DO, potentially shortening consolidation and enhancing DO in poorly healing mandibles, such as occurs postirradiation.

Gene therapy applications for spine fusion

OBJECTIVE

This article reviews the potential utilization of various growth factors to enhance spinal fusion and outlines the principles of gene therapy and its application to spinal fusion surgery.

SUMMARY OF BACKGROUND DATA

Gene therapy offers an exciting new way to potentially deliver growth factors locally in a targeted fashion with physiologic doses. In its current definition, gene therapy is defined as the use of nucleic acid transfer, either RNA or DNA, to treat or prevent a disease. The scope of gene therapy has expanded beyond its initial application as a method of replacing genetic defects, and its potential to facilitate spinal fusions is currently being evaluated.

CONCLUSIONS

Gene therapy strategies for spine fusion are appealing because the setting is uniquely suited for genetic manipulation. The intervention is locally applied. Only a short duration of transgene response by the cells is necessary to establish a spine fusion, and a variety of osteoinductive growth factors have been identified and are available for use. Attempts at spine fusion using gene therapy in the lower animals have been successful using both in vivo and ex vivo approaches. Before human clinical trials can be established, further testing is required in more challenging animal models of bone induction such as nonhuman primates. Should a successful clinical program of gene therapy for spine fusion be established, the use of autograft and its associated morbidities could be eliminated. In fact, gene therapy offers the potential for minimally invasive applications that could bypass the need for an open procedure altogether. It is likely that gene therapy will be a powerful therapeutic tool for the spine surgeon in the new millennium.

Gene therapy and tissue engineering in repair of the musculoskeletal system

Historically, surgeons have sought and used different procedures in order to augment the repair of various skeletal tissues. Now, with the completion of the Human Genome Project, many researchers have turned to gene therapy as a means to aid various ailments. In the orthopedic field, many strides have been made toward using gene therapy and tissue engineering in a clinical setting. In this review, several studies are outlined in different areas that gene therapy has or will influence orthopedic surgery. Gene therapy and tissue engineering can aid in fracture healing and spinal fusions by inducing bone formation, ligamentous repairs by increasing the production of connective tissue fibers, intervertebral disc disease by creating potential replacements, and articular cartilage repairs by providing means to improve cartilage. As we continue to see great contributions, such as the few mentioned here, this field will continue to mature and develop.

Bone morphogenetic proteins: relevance in spine surgery

Bone morphogenetic proteins (BMPs) are low molecular weight glycoproteins that play a vital role in the development and maturation of skeletal tissue. Bone morphogenetic protein-induced mesenchymal cell recruitment and differentiation leads to the formation of chondroblasts and osteoblasts leading to the formation of de novo bone. Overwhelming pre-clinical and clinical evidence has suggested a promising role for BMPs for anterior and posterolateral spinal fusion. Strength of this approach lies in the potential ability of these growth factors to reverse inhibitory conditions common in the clinical setting and enabling predictable fusion. However, several issues related to carriers, costs, and dosages still need to be consecutively addressed. Gene therapy techniques producing in vivo osteoinductive factors and utilizing minimally invasive approaches are attractive options being developed for the future.