Genetic Approaches to Therapy for the Hemoglobinopathies. In: Fairbairn LJ, Testa NG, editors. Blood Cell Biochemistry. Boston: Springer US; 1999. pp. 219-42. [DOI: 10.1007/978-1-4615-4889-8_8]

Stem cell route to neuromuscular therapies

As applied to skeletal muscle, stem cell therapy is a reincarnation of myoblast transfer therapy that has resulted from recent advances in the cell biology of skeletal muscle. Both strategies envisage the reconstruction of damaged muscle from its precursors, but stem cell therapy employs precursors that are earlier in the developmental hierarchy. It is founded on demonstrations of apparently multipotential cells in a wide variety of tissues that can assume, among others, a myogenic phenotype. The main demonstrated advantage of such cells is that they are capable of colonizing many tissues, including skeletal and cardiac muscle via the blood vascular system, thereby providing the potential for a body-wide distribution of myogenic progenitors. From a practical viewpoint, the chief disadvantage is that such colonization has been many orders of magnitude too inefficient to be useful. Proposals for overcoming this drawback are the subject of much speculation but, so far, relatively little experimentation. This review attempts to give some perspective to the status of the stem cell as a therapeutic instrument for neuromuscular disease and to identify issues that need to be addressed for application of this technology.

Myoblast transplantation

Myoblast transplantation was the first quasi-gene therapy to be suggested for Duchenne muscular dystrophy. Animal experiments established the principles that the missing gene could be targeted to muscle by grafting of genetically normal myoblasts that were able to repair the disease-damaged muscle fibres. In the recipient muscle the gene was expressed and the resultant protein provided some functional benefit in protecting the fibres against necrosis. However, these effects were limited to a small region around the injection site and there was some evidence of immunological problems. Human trials provided little evidence of effectiveness probably, in part due to immune rejection, and in part to the inadequacy of the cells implanted. Most work since this time has been directed at preventing immune rejection, improving dispersion of the injected cells, and selecting more 'stem cell-like' myogenic cells which might be more effective at reconstituting large regions of muscle. Most recently, a number of sources of 'stem cell' with myogenic potential have been described, some of which have been found to be dispersed via the blood vascular system but none of which have been very efficient at generating new muscle.

LCR-mediated, long-term tissue-specific gene expression within replicating episomal plasmid and cosmid vectors

Locus control regions (LCRs) are transcriptional regulatory elements, which possess a dominant chromatin remodelling and transcriptional activating capability conferring full physiological levels of expression on a gene linked in cis, when integrated into the host cell genome. Using the human beta-globin LCR (betaLCR) as a model, we show that this class of control element can drive high levels of tissue-specific gene expression in stably transfected cultured cells from within an Epstein-Barr virus-based plasmid REV. Furthermore, a 38-kb betaLCR minilocus-REV cosmid vector was efficiently retained and maintained therapeutic levels of beta-globin transgene expression in the absence of drug selective pressure over a 2-month period of continuous culture equivalent to at least 60 generations. This demonstrates for the first time the feasibility of using REVs for gene therapy of the haemoglobinopathies. Importantly, our results demonstrate that as in the case of integrated transgenes, expression from within REVs is prone to silencing but that the inclusion of the betaLCR prevented this repression of gene function. Therefore, appropriate control elements to provide and maintain tissue-specific gene expression, as well as the episomal status of REVs is a crucial feature in vector design. Our data suggest that LCRs can contribute to this vital function.