BioWorld International Correspondent
LONDON - Human muscle stem cells, isolated from males with Duchenne's muscular dystrophy and genetically modified to correct the defect that causes the condition, can treat symptoms of muscle weakness in a mouse model of the disease, a study in Italy has shown.
The genetically altered cells were delivered by an injection into the arteries of the mice. The appearance and function of the animals' muscles improved, and the muscles began to produce the protein dystrophin, which is faulty in people with Duchenne's muscular dystrophy.
There are concerns about whether the strategy will be safe, because of the risk that the lentivirus used to achieve the genetic modification could cause cancer in the recipients. Nevertheless, if safety tests go well, the researchers hope to begin treating patients with their own genetically modified muscle stem cells within the next two or three years.
Yvan Torrente, director of the Stem Cell Laboratory at the University of Milan, Italy, told BioWorld International: "This study is important because we are not very far from treating patients in the clinic. Also, this is the first time anyone has combined two different methods - stem cell therapy and gene therapy."
Torrente added that use of the patient's own cells would reduce the risk of implant rejection seen with transplantation of normal muscle-forming cells isolated from healthy subjects.
The work is reported in a paper in the Dec. 12, 2007, issue of Cell Stem Cell titled: "Restoration of Human Dystrophin Following Transplantation of Exon-Skipping-Engineered DMD Patient Stem Cells into Dystrophic Mice."
Duchenne's muscular dystrophy affects about one in 3,500 males. The disease is caused by mutations in the gene that encodes the protein dystrophin.
Many of these mutations result in a premature stop codon, causing production of a truncated protein. In recent years, however, researchers have shown that if they deliver a short piece of antisense RNA to the cell, which will complement the section of the gene containing the mutation and therefore cover up the stop codon, then the cell will make a shorter but still functional version of the dystrophin protein.
Torrente and his colleagues isolated stem cells that had a marker identifying them as future muscle cells. They were able to isolate the cells from both the blood and the muscle of their Duchenne's patients.
They then manipulated the cells, using a lentivirus to ferry in a genetic "cassette" containing the RNA "patch" that would cover over the stop codon. That technique is known as "exon skipping." The researchers then delivered the genetically engineered muscle stem cells into the bloodstream of mice that provide a model of Duchenne's muscular dystrophy.
Tests on the mice showed that those altered muscle stem cells were able to restore the expression of dystrophin protein in the mice, and reduce the amount of muscle damage that the animals normally experience.
"The data strongly suggest," Torrente said, "that cell spreading by this approach can significantly restore skeletal muscle function toward normal."
The team is embarking on a collaboration with a group in Sao Paolo, Brazil, where there is a colony of dogs that provide a model of muscular dystrophy. Torrente said he and his colleagues want to demonstrate that the same strategy as described in the Cell Stem Cell paper can improve the muscle force of larger muscles.
"In addition," he said, "we want to perform studies to find out whether we change the behavior of human dystrophic cells after the exon-skipping treatment. We hope that this will provide evidence of the safety of the method."
Torrente said the paper also had wider implications for the development of new treatments for cancers and other genetic diseases.
"Our work suggests that, using the exon-skipping treatment, it will be possible to skip the mutation that causes the tumor in some people with cancer, or the mutations that cause other types of genetic disease," he said. It may not even be necessary to use lentiviruses to carry out the genetic modification, he added, as some groups already are conducting clinical trials using antisense oligonucleotides on their own.