Small amounts of protein produced from self-corrected genes likely prime immune system.
Researchers found that Duchenne muscular dystrophy patients receiving gene therapy had an immune reaction to the resulting dystrophin protein in a Phase I trial. Additionally the study showed that some patients also mount an immune response even before they have received the gene therapy.
The results suggest that the immune systems of a number of patients, once thought to be completely devoid of the dystrophin protein, are actually primed by the prior existence of tiny amounts of this muscle protein. The trials were conducted at Columbus Children’s Hospital in Ohio and based on a therapy designed by scientists at the University of North Carolina (UNC) at Chapel Hill School of Medicine. Details appear in a paper titled “Dystrophin Immunity in Duchenne’s Muscular Dystrophy” published in The New England Journal of Medicine.
Duchenne muscular dystrophy is a genetic disease that begins in early childhood, causes progressive muscle weakness, and usually leads to death by the age of 20 from respiratory or cardiac muscle failure. The illness, which primarily affects boys, occurs when a gene on the X chromosome fails to make dystrophin.
The use of gene therapy to correct such single-gene disorders has been explored for over two decades and has been met with a number of challenges, the UNC team points out. In the case of muscular dystrophy, the dystrophin gene is far too large to fit into the typical virus used to carry it into patients’ cells. Hence Xiao Xiao, Ph.D., from UNC Eshelman School of Pharmacy engineered a smaller yet functional version of the gene, called a minigene, to place within an adeno-associated virus.
The trial began in 2006, and six boys with muscular dystrophy received the virus containing the dystrophin minigene. The replacement genes were injected into the bicep in one arm, and a placebo was injected into the other arm of each of the patients. However, when the patients were evaluated three months later, long-term production of dystrophin protein from the corrected DMD gene was not detected.
To understand why this therapy failed the researchers measured immune responses against dystrophin. T-cell immunity was a concern going into the study since “parts of the corrected dystrophin protein are clearly foreign because of the patient’s DMD gene deletion, and so unwanted T-cell immunity targeting the repaired muscle cells was a possibility,” explains Christopher M. Walker, Ph.D., study author and director of the Center for Vaccines and Immunity at The Research Institute at Nationwide Children’s Hospital.
They detected that, in one patient with a large DMD gene deletion, T-cell immunity was against foreign segments of the corrected dystrophin protein. However, stronger and faster T-cell immunity was detected in a second patient with a much smaller DMD gene deletion.
“Strong, rapid immunity in the second patient with a very small DMD gene deletion was a surprise,” Dr. Walker notes. “The amount of corrected dystrophin protein that is foreign should also be small and possibly ignored altogether by the T cells.”
The plot thickened when T-cell immunity to dystrophin was found to have been present in this patient even before treatment. Further examination of the muscle revealed that the T cells present before gene therapy recognized dystrophin that was produced in a very small percentage of muscle cells that naturally self-corrected the defective DMD gene. Delivery of the gene therapy vector to the bicep muscle boosted and accelerated this pre-existing immune response.
“We’ve known for a long time that T cells naturally invade muscles of DMD patients,” says Jerry R. Mendell, M.D., director of the Center for Gene Therapy at The Research Institute. “Drugs that suppress immunity can prolong the time until they are confined to a wheel chair, but we never knew how or why this worked.
“This gene therapy study has lead to the new basic discovery that even small amounts of dystrophin naturally produced from self-correcting DMD genes can trigger destructive T cells, and they may target muscle cells in a process that resembles autoimmunity.” Further studies are needed, but this finding suggests that some patients may benefit from immunosuppression prior to receiving gene therapy.
“This study is significant because it documents immunity against a dystrophin protein designed to treat the disease; that may be broadly important to the entire field of gene therapy,” Dr. Mendell also points out.