Myoblasts
Muscles have their own stem cells, the satellite cells or myoblasts, which, during the development or repair of muscles, fuse together to form myotubes and then long muscle fibers. As the word "myoblast transfer" has been misused, one now often prefers to say myogenic cells instead of myoblasts.
In the years 1990 and 1991, extensive clinical studies with Duchenne boys were performed with myoblasts. This myoblast transfer technique had shown positive results in mdx-mice. The cells used contained the normal dystrophin gene because they were derived from a healthy donor, mostly from the father of the patient. They were applied in multiple injections at 0.5 centimeters distance into some muscles of Duchenne boys with the expectation that muscle cells with normal dystrophin would form. However, these experiments were not successful, because the transplanted cells did not migrate sufficiently inside the muscle, because there were immunological problems, and because almost all of the injected myogenic cells had died after a short time (Karpati, Montreal, and others).
But work on this technique continues in order to determine the reason why far less than 1 % of the transplanted myoblasts survived in the dystrophic muscles. Researchers are now trying to characterize these rare active cells and to find out how they can be isolated from the inactive cells. They are therefore looking for molecular signals, special substances in the muscle cells, which could activate the myoblasts (Partridge, London).
In earlier experiments, only two known drugs, cyclosporin A and cyclophosphamide were used to suppress the immune reactions in Duchenne patients. Now, other substances have been investigated in studies with monkeys. This has shown that the immune inhibitor FK506 alone or in combination with the inhibitor MMF avoids the rejection problems much better for several months. A greater application density with injection distances of only 1 millimeter and a higher number of transplanted cells contributed to the fact that in monkeys, up to 67 % of the muscle cells had taken up the myoblasts, they became hybrid muscle cells (Tremblay, Quebec City).
A clinical study with the modified technique has been started in Canada. Also in other laboratories, work is being done to improve this cell therapy technique. E.g., it was found that m144, a protein of the immune system, avoids the immediate death of the myoblasts after transplantation into mouse muscles (Hodgetts, Crawley, Australia).
To avoid immune problems completely, experiments were performed to isolate the myoblasts from the patient himself and then, in cell culture, to transfer an intact dystrophin gene into the cells, before they are re-injected again. In preliminary experiments an electroperforation technique was used to transfer the gene for a fluorescent marker protein into myoblasts in cell culture. This technique transiently permeabilized the membranes with a single electrical pulse, at e.g. 400 volts across a distance of 4 mm. Under optimal conditions, the marker gene could be transferred into up to 70 % of the myoblasts where it produced the fluorescent protein. The cells maintained their ability to fuse into myotubes, the next stage of muscle development (Bernheim, Geneva).
Go to our document library to download the excellent review article for more information and the above references from Research Approaches towards a Cure for Duchenne Muscular Dystrophy 2003 by Dr Guenter Scheuerbrandt.