The technique, similar to that used in human bone marrow transplants, may some day become an effective way to treat muscular dystrophies affecting some 250,000 Americans. It also offers promise for the systemic delivery of therapeutic cells for other genetic diseases affecting tissues throughout the body, researchers say. The findings are reported in Nature on Thursday.
"This pioneering effort is exciting for two reasons," said Leon Charash, chairman of MDA's Medical Advisory Committee. "First, the work may eventually lead to an unanticipated treatment approach for all the muscles ravaged by neuromuscular disease. Systemic delivery of missing genetic information is a real advantage over the single-muscle delivery currently associated with gene therapy using viral vectors.
"Second, the MDA investigators have added a provocative wrinkle to the promising field of stem cell research," Charash said. "They've successfully used donor stem cells from two living sources - avoiding the ethical questions associated with stem cells from embryonic or fetal tissue."
DMD is a fatal childhood disease that causes generalized muscle wasting and weakness due to the absence of a protein called dystrophin. Other approaches to treatment that involve replacing dystrophin via gene therapy or by transplanting healthy muscle cells from donors have been limited, in part, by the need for multiple injections into every muscle.
The joint research effort between MDA grantees Emanuela Gussoni and Lou Kunkel and by Richard Mulligan, all from Children's Hospital of Boston, Harvard Medical School and the Howard Hughes Medical Institute, demonstrated new potential of highly purified "stem" cells. These cells - which are capable of giving rise to different kinds of mature cell types such as blood or muscle - can migrate from the bloodstream and integrate missing genetic information into all of the voluntary (skeletal) muscles.
When the investigators transplanted stem cells derived from the bone marrow of healthy donors into mice with DMD that had undergone irradiation to destroy their own bone marrow, the cells not only regenerated the bone marrow, but also migrated into the recipient's voluntary muscle.
By 12 weeks after the transplant, researchers found that the amount of dystrophin-positive fibers in the muscles of the mice had increased from less than 1 percent to as much as 10 percent. Because the donor cells were derived from male mice and the recipient mice were female, researchers were able to determine that the dystrophin-positive fibers contained nuclei from donated cells by the presence of the Y chromosome.
"We knew that bone marrow stem cells could reconstitute bone marrow," Kunkel said, "but it was a real surprise that they could also contribute, over time, to the reconstitution of muscle. By fostering dystrophin expression with bone marrow stem cells, we fixed all of the muscle fibers that we got these cells into."
Kunkel says that the results of a parallel experiment, injecting stem cells derived from muscle into irradiated mice with DMD, were an even greater surprise. The muscle-derived cells also regenerated the bone marrow of the recipients and migrated to voluntary muscle.
"This means that these cells derived from muscle had the ability to become something else, and that stem cells isolated from diverse tissues may be more similar than we thought," Kunkel said. "What this work is really doing is opening a whole field of expanding stem cell biology as a potential treatment for genetic disorders, specifically for Duchenne and the other muscular dystrophies."
Although the level of dystrophin-positive fibers resulting from these initial transplants is not high enough to yield clinically significant improvement in the mice, the researchers are working on ways to increase the number of positive fibers.
The group is also working on a way to stimulate donor cell migration into the muscle without having to destroy the bone marrow of the recipient.
"Here's where you have to be cautious," Kunkel said. "We are not going to irradiate children with DMD. What we want to do is find the signal from muscle that says 'I am injured, come help me' to the circulating donor stem cells. Then we can use that signal in some non-toxic way to lure donor stem cells to the muscle.
"This is a long-standing interest of mine," Kunkel said. "Ever since we cloned the dystrophin gene, I've been thinking 'can't we do anything for these kids? Can't we come up with a therapy?'"
Kunkel, whose laboratory identified the dystrophin gene in 1986, was recently awarded the S. Mouchly Small Scientific Achievement Award by MDA during its Jerry Lewis Telethon. The award recognizes an investigator for making significant contributions to MDA research.
MDA is a voluntary health agency working to defeat 40 neuromuscular diseases through programs of worldwide research, comprehensive medical and community services, and far-reaching professional and public health education.
Recognized by the American Medical Association with a Lifetime Achievement Award "for significant and lasting contributions to the health and welfare of humanity," MDA maintains 230 hospital-affiliated clinics that offer families the best in care for progressive neuromuscular diseases.
MDA annually funds some 400 scientific teams worldwide. These investigators have made significant advances toward cures for several muscle-wasting diseases. They also have pioneered breakthroughs that may well lead to therapies for heart disease, cancer, AIDS, Alzheimer's, Huntington's, Parkinson's and cystic fibrosis. For information or referrals to MDA clinics, call (800) 572-1717, or visit the MDA Web site at www.mdausa.org. MDA programs are funded almost entirely by individual private contributors.
