Researchers from the University of California, Berkeley, and the University of Iowa have turned a relatively benign virus into a highly infectious form that is ideal as a carrier for gene therapy.
In its first gene therapy test, it completely cured human cystic fibrosis lung tissue in culture.
This success with the benign adeno-associated virus (AAV), published this week in the online early edition of the journal Proceedings of the National Academy of Sciences, overcomes a major problem of earlier virus-based gene therapy for cystic fibrosis, and sets the stage for tests in advanced animal models of the disease.
"I think it is worthwhile thinking about clinical therapy at the levels of infection we are achieving," said coauthor David Schaffer, professor of chemical engineering at UC Berkeley.
A new pig model of cystic fibrosis developed last year by Schaffer's colleague, pulmonologist Joseph Zabner of the University of Iowa Hospitals and Clinics in Iowa City, will provide a key test of the virus as a carrier of a gene to replace the mutated gene responsible for the disease.
"If we are able to show that efficient gene transfer can result in gene therapy, if we can cure the lung disease of pigs that have been genetically engineered to have cystic fibrosis lung disease, we should have a real chance of curing cystic fibrosis in humans," Zabner said in an e-mail.
Schaffer's lab is collaborating with groups elsewhere to adapt the virus to gene therapy for other diseases, including Alzheimer's disease and amyotrophic lateral sclerosis (Lou Gehrig's disease).
"Both of those are situations where improvements in the properties of the vehicle can have a significant impact on the success of the therapy," Schaffer said.
Cystic fibrosis (CF) is a common hereditary disease that affects the body's mucus membranes, in particular the lungs, resulting in difficulty breathing and typically in death before the age of 40 from lung or organ failure. One in 4,000 children in the United States is born with CF.
Schaffer and his UC Berkeley colleagues collaborated with Zabner's laboratory to test a technique developed by Schaffer to force the evolution of a virus in ways that make it more effective in gene therapy.
Two years ago, Schaffer and colleagues used the technique to create a variant of AAV that more easily avoids the immune system, allowing the virus to remain in the body long enough to deliver a gene to its intended target.
Using the same technique, the team produced a variant of AAV that is several hundred times more effective at entering lung cells than the natural virus.
Schaffer's technique involves making many mutations in AAV, culturing these variants with cells, and then taking those with specific improved properties – in this case, the ability to infect lung cells – and repeating the process.
"We probably conducted about six rounds of evolution in which we infected the lung epithelial cultures in Iowa, they sent it back to us, we recovered the viral sequences, made new viruses and sent them back again," Schaffer said. "It was iterative rounds of infection and selection for improved infection that finally led to this enhancement of function."