At Columbus Children’s Hospital in Ohio, six boys with Duchenne muscular dystrophy are receiving injections of replacement genes in the first U.S. human gene therapy trial for the disease. The technique being used to deliver the new genes was developed by Xiao Xiao, PhD, a scientist in the UNC School of Pharmacy’s Division of Molecular Pharmaceutics.

Xiao Xiao, PhD Fred Eshelman Distinguished Professor of Gene Therapy Division of Molecular Pharmaceutics Research Interests Gene therapy for muscular dystrophy, especially Duchenne muscular dystrophy.

Wasting Away
When Xiao began studying a small, harmless virus called adeno-associated virus (AAV) in 1987, he had no idea that one day his research would contribute to a potential breakthrough in treating one of the most prevalent types of muscular dystrophy.

Muscular dystrophies are genetic diseases characterized by progressive muscle wasting. Duchenne muscular dystrophy (DMD) occurs when a genetic mutation prevents the production of dystrophin, an essential muscle protein. Without this protein, individuals with DMD experience progressive loss of muscle strength and usually die in their twenties from respiratory or cardiac failure. There is currently no effective treatment for DMD, which affects approximately one out of every 3,500 boys worldwide.

The boys in the trial are receiving injections of dystrophin-replacement genes in one arm and a placebo in the other arm. At the completion of the trial, the results will be analyzed to determine whether gene therapy for DMD is safe and if it leads to the production of dystrophin in muscle cells.

Big Things in Small Packages
The therapy is based on a combination of groundbreaking research by Xiao and virologist Jude Samulski, director of the UNC Gene Therapy Center. Samulski has extensively researched AAV as a system for gene delivery. Xiao, who worked with Samulski as a graduate student, has focused much of his research on using AAV in muscle gene delivery.

Because the dystrophin gene was too large to be carried in the small virus, Xiao successfully developed a miniaturized version of the gene that would fit into AAV by deleting parts of the DNA that did not compromise the protein’s function.

Xiao and Samulski combined this new “minidystrophin” gene with a technology called biological nanoparticles, which are developed from AAV and are engineered specifically to target and carry the “minidystrophin” gene to muscle cells.

Xiao hesitates to refer to AAV as a virus because of the negative connotation often associated with the term. He emphasizes that AAV is harmless.

“More and more labs are using adeno-associated virus because it’s safe, it’s effective, it does not cause profound or overt immune rejections by the host, and it can be easily injected. It’s a stable virus. It does not cause any disease,” he says.

He also stresses the benefits of using AAV over traditional medications.

“None of the traditional medications can do targeted delivery,” Xiao says. “When you take a pill or get an injection, the drug goes everywhere. This virus actually can deliver its payload to a specific organ,” Xiao says. “It’s like a guided missile.”

A Mind for Business

Samulski and Xiao combined their research and formed Asklepios BioPharmaceutical Inc. in 2003. The company acquired the rights to Samulski’s vector technology and Xiao’s minidystrophin gene. The next year, AskBio received a $1.6 million grant from the Muscular Dystrophy Association (MDA) to develop gene therapy for DMD.

Following extensive laboratory testing to demonstrate the safety and potential benefit of the minidystrophin gene in DMD patients, the U.S. Food and Drug Administration granted approval to AskBio in March 2006 for the human trial to proceed.

Xiao says the results of the first phase are expected in mid-2007, and the trial should enter its second phase sometime in 2008. The second phase will focus on delivering the gene to multiple muscles simultaneously.

In December 2006, AskBio received a $2.5 million grant from MDA for the study’s second phase. This was the largest grant ever awarded by MDA to a for-profit company.

Xiao emphasizes the importance of effective gene therapy, especially in an age where human genome sequencing and personalized medicine have made large strides in informing people about what diseases or mutations they have or are predisposed to have. He says the next step is effective treatment.

“If the mutation or disease is caused by defection in the gene, the best way to treat it is through gene therapy,” he says. “You have a healthy copy of the gene to replace the defective or mutated one. I think that for the long run, gene therapy is going to be a therapy for a lot of diseases—not only for genetic diseases, but also for acquired diseases like AIDS, Parkinson’s, ALS, and hepatitis.”

In addition to studying how AAV can be used to treat DMD, Xiao’s lab is also studying how gene therapy can be used to treat heart failure, hepatitis B, and type 1 and type 2 diabetes.

While Xiao is optimistic that AAV will someday become a mainstream gene delivery vehicle, he cautions that gene therapy as a field is still in its early stage. It could still be fifteen to twenty years before AAV becomes a mainstream treatment.