"Previous attempts at systemic gene therapy for muscle have not been very effective because blood vessel capillaries act much like a mosquito net, blocking the gene drugs from reaching the muscle cells. Fortunately, we found a virus that is just small and sneaky enough to get through this net and deliver the therapeutic gene to both skeletal and cardiac muscle cells," said lead author, Xiao Xiao, Ph.D., associate professor of orthopaedic surgery at the University of Pittsburgh School of Medicine.
The virus used by Dr. Xiao and his colleagues for delivering the corrective gene is known as adeno-associated virus, or AAV, a class of relatively small viruses that do not cause any known disease. In earlier studies, Dr. Xiao's team found that direct intramuscular injection of AAV was effective in transferring a gene into muscle cells in a fairly wide area around the injection site. However, for gene therapy treatments to be successful, particularly for muscular dystrophies where many organs and tissues are affected throughout the body, intramuscular injection is not practical for delivering a corrective gene to the body's more than 600 muscle groups.
Recently, Dr. Xiao's team demonstrated that a type of AAV, known as AAV-8, is particularly efficient at penetrating the capillary barrier, making it a good candidate for whole-body gene delivery. In this study, they tested AAV-8 in an animal model of human muscular dystrophy called limb girdle muscular dystrophy, or LGMD. In human LGMD, defects in a muscle cell membrane protein known as delta-sarcoglycan lead to severe damage and weakness to muscles, particularly around the hips and shoulders--hence the name "limb girdle"-- as well as in the heart. Like humans, hamsters with this particular delta-sarcoglycan gene defect have severe muscle wasting and weakness and significantly shortened lifespans due to cardiac and respiratory failure.
After injecting a very high dose of AAV-8 carrying a normal copy of the delta-sarcoglycan gene intravenously into 10-day-old and adult LGMD hamsters, Dr. Xiao and his colleagues found that it had been systemically incorporated into skeletal, diaphragm and cardiac muscle cells in both groups. More importantly, cardiac and lung muscle cells in both newborn- and adult-treated hamsters were able to express the normal protein almost a year later. There were dramatic biochemical and structural improvements in muscle cells in both groups as well.
This was accompanied by markedly improved skeletal and cardiac muscle functions. Indeed, the newborn-treated hamsters had completely normal hearts, when examined at eight and one-half months after gene therapy. The adult hamsters also showed significant improvements in heart muscle structure. In contrast, untreated hamsters had severe structural and tissue abnormalities of the heart in addition to secondary symptoms of heart failure such as liver problems, swollen lungs and a severe buildup of fluid in the chest and peritoneal cavities.
Perhaps even more impressive was the improvement in endurance and lifespan of the treated versus the untreated hamsters. The AAV-8-treated hamsters were able to run the same distance as normal hamsters before tiring and for much longer than untreated LGMD hamsters. Furthermore, all of the untreated LGMD animals died of heart failure or other complications of muscular dystrophy around 37 weeks, while all of the AAV-8-treated LGMD hamsters survived beyond the 48-week duration of the study.
"When we began the experiment, we anticipated that the treatment would be effective. However, we never imagined it would be so effective, particularly in protecting against or reversing the damage to the heart caused by this mutation and extending lifespan," explained Dr. Tong Zhu, M.D., Ph.D., a research associate in the department of orthopaedic surgery and the first author of the study. "In fact, if this study holds up in human clinical trials, it may have profound implications for the treatment of heart failure."
Dr. Xiao cautioned, however, that human clinical trials of this therapy face several major challenges. Foremost is that effective treatment requires the injection of a large amount of the virus so there is enough to reach every muscle cell. Because 30 percent to 40 percent of the population has antibodies to human AAVs, there is always the possibility that the effectiveness of this form of gene therapy may be blunted by a host immune response. However, Dr. Xiao is optimistic that will not be the case.
"The AAV-8 we used in this study was isolated from monkeys, so we are very hopeful it will be able to deliver the genes before the human immune system produces antibodies to block it. In addition, we used a muscle-specific promoter in the virus, which also should lower the risk of any potential immune response against the gene product. In fact, in hamsters, we did not find any immune response to the human delta-sarcoglycan protein that was encoded by the AAV vector under the control of this promoter," he explained.
In addition to Drs. Xiao and Zhu, other authors of this study are: Liqiao Zhou, V.M.D.; Zhong Wang, M.D., Ph.D.; Chunping Qiao, M.D., Ph.D.; Chunlian Chen, M.D.; and Juan Li, M.D., Molecular Therapy Laboratory, department of orthopaedic surgery, and Satsuki Mori, M.D., and Charles McTiernan, Ph.D., Cardiovascular Institute, all from the University of Pittsburgh School of Medicine; and Daowen Wang, M.D., Ph.D., department of cardiology, Tongi Hospital, Huazhong Science & Technology University, Wuhan, China.
This study is supported by National Institutes of Health grants and a Natural Science Foundation of China grant to Dr. Xiao as well as by a National Institutes of Health grant to Dr. McTiernan.