Review of Components of Gene Therapy

A gene, which is a functional unit of DNA, consists of a DNA sequence that produces a single polypeptide. This gene sequence codes for a specific messenger RNA that goes from nucleus to cytoplasm to translate the amino acid sequence (protein).

Gene therapy is targeted on the production of a selected therapeutic protein to alter a disease process. This protein may enhance (e.g., insulin-like growth factor–1 [IGF-1]) or repress (e.g., IL-1ra) a specific cellular process. The essential components of gene therapy include (1) isolation of a gene (called cloning), (2) manipulation of the gene (engineering), and (3) transfer of the gene into the host cells (transfection/transduction). The gene sequence is initially isolated and characterized and is then engineered by the addition of regulatory elements (promoters) that allow control of protein production.

The essential component of gene transfer is the vector. Gene delivery vectors facilitate the transfer of the therapeutic gene into the nucleus of the target cell. Once in the nucleus, the gene is decoded (expressed) to produce a protein. Viruses, the most commonly used vectors, have the ability to transfer genes into a host cell in an efficient, logical, and easy manner. Viruses used as vectors have been rendered incapable of replicative spread by removal of viral genes and insertion of the therapeutic gene(s). Viral vectors are more efficient than nonviral vectors.

There are two potential methods of gene transfer. Ex vivo gene transfer is an indirect technique where the cells are collected from the joint and grown in the laboratory (e.g., synovial cells). The gene is then transferred (transduced) into cultured cells using a viral vector, and the transduced cells are reimplanted after testing for protein production. Ex vivo transfer is safer but less convenient than the second technique, in vivo transfer. In vivo transfer involves direct transfer of a vector to the target tissues. For example, synovial cells could be transfected by direct transmission of an IL-1ra gene using an adenovirus vector injected into a joint (Figure 63-1). Adenoviruses are the most common vectors proposed for use in human patients with clinical disease² and were used in our equine studies.^3-9 Adenoviral vectors can transduce dividing and nondividing cells and are amenable to in vivo transfer. The adenoviral vector takes up an episomal location (extrachromosomal) in the host nucleus. Second-generation vectors give protein expression for 21 to 40 days.

Fig. 63-1 Schematic drawing representing in vivo gene transfer to the synovium. This is the way in which gene therapy with interleukin-1 receptor antagonist was performed.

(Reproduced with permission from McIlwraith CW: Milne lecture: from arthroscopy to gene therapy—30 years of looking in joints, Proc Am Assoc Equine Pract 51:83, 2005.)

Gene Therapy and Joint Disease

Our investigation of the potential value of gene therapy in the treatment of OA is partially based on limitations of traditional therapy. Currently, most therapeutic agents need to be directly administered intraarticularly, and the half-life of most commonly used agents is short.^10,11

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