Gene Therapy and Tissue Engineering in Orthopaedics

Tissue engineering combines three elements to regenerate tissue. CELLS provide the regenerative capacity - mesenchymal stem cells from marrow or adipose tissue (multipotent, can become bone/cartilage/fat) or differentiated chondrocytes/osteoblasts. The SCAFFOLD is a 3D, porous, biodegradable matrix that supports cell attachment and guides new-tissue architecture - natural materials (collagen, hyaluronan, hydrogels) or synthetic polymers/ceramics (PLGA, PCL, calcium phosphates, nanofibres) - ideally osteoconductive and resorbing as new tissue forms. SIGNALS instruct the cells - growth factors such as the BMPs (osteoinductive), TGF-beta (chondrogenic), PDGF and VEGF - together with mechanical stimulation (and bioreactors in the lab). Getting the right combination, and vascularising larger constructs, is the central engineering challenge.

• Autologous chondrocyte implantation (ACI/MACI): the patient's chondrocytes are harvested, expanded and re-implanted into a cartilage defect (in MACI seeded on a collagen membrane) - the established clinical example of cell-based tissue engineering.
• Recombinant BMPs: BMP-2 and BMP-7 delivered on a collagen carrier are used clinically for spinal fusion and tibial nonunion (a signals-plus-scaffold approach using recombinant protein, not a gene).
• Orthobiologics: PRP, bone-marrow aspirate concentrate and demineralised bone matrix are used adjunctively (see our Orthobiologics topic).
• Investigational: engineered bone/cartilage/meniscus/tendon constructs and gene therapy remain largely preclinical or early-clinical.

Gene therapy delivers a gene encoding a therapeutic protein into cells so they produce the protein themselves, giving more sustained local expression than a single dose of recombinant protein. Targets include BMP-2/7 (bone), IGF-1/TGF-beta (cartilage) and anti-inflammatory genes (osteoarthritis).

• Vectors - viral vs non-viral: VIRAL vectors (adenovirus, adeno-associated virus, lentivirus/ retrovirus) are efficient but carry safety concerns (immunogenicity; insertional mutagenesis with integrating retro/lentiviruses). NON-VIRAL vectors (plasmid DNA, lipofection) are safer but less efficient.
• Strategy - in vivo vs ex vivo: IN VIVO delivers the vector directly to the target site; EX VIVO harvests the patient's cells, modifies them genetically, then re-implants them - often on a scaffold as a gene-activated matrix.

• The diffusion limit: cells survive only within roughly 150 to 200 microns of a capillary (the oxygen/nutrient diffusion distance), so beyond that distance the core of a thick construct becomes hypoxic and necrotic. This is why thin, relatively avascular tissues (cartilage) are far easier to engineer than thick, vascular ones (a large bone segment), and why large constructs fail without a blood supply.
• Strategies to overcome it: load the scaffold with angiogenic growth factors (VEGF, with PDGF for vessel maturation); pre-vascularise in vitro by co-culturing endothelial cells to form capillary networks before implantation; design scaffold architecture with channels and high interconnected porosity to invite host-vessel ingrowth; use bioreactor perfusion to nourish the construct during culture; and the in vivo "bioreactor"/prefabrication approach - implant the construct around a vascular pedicle (or in a well-vascularised bed) to vascularise it before transfer, or use a vascularised flap/free transfer surgically.
• The corollary: matching the scaffold's pore architecture and angiogenic signalling to the host's capacity to grow vessels in is as important as the cells and the growth factors themselves.