Gene therapy could be the key to curing rare and life-threatening diseases, but high costs and potential negative effects are slowing the fledgling field, experts say.

In gene therapy, normal genetic material replaces missing or defective genes to cure disease. In December, the Food and Drug Administration approved Luxturna, the first commercial gene therapy, which corrects an inherited vision loss, according to an FDA news release.

The FDA has identified gene therapy as an exciting, new field that truly can help people, said Dr. Wilson Bryan of the FDA, which is expediting gene therapy treatments. The number of applications has increased dramatically, to more than 100 last year.

Brian Kaspar, chief scientific officer for AveXis, a company developing a gene therapy treatment for spinal muscular atrophy, or SMA, said other treatments of genetic conditions typically slow progression of a disease, but gene therapy can help patients lead normal lives.

The neuromuscular disease – the leading genetic cause of infant death – causes muscular weakness and paralysis that, at its most severe, manifests as floppy baby syndrome. Children with SMA often don’t live past age 2, Kaspar said.

But his company’s trials have improved motor skills for infants and children with SMA, he said. All of his patients have lived past the life expectancy of the disease, and many can crawl or walk.

“These are things, simply put, that have never been seen,” he said.

Dr. Katherine High, who worked on trials for Luxturna and also spoke at an Association of Health Care Journalists conference in Phoenix in April, said the treatment can dramatically improve vision for patients for whom other treatment has failed.

Bryan told the story of a patient who had always seen a fuzzy light outside her room. She thought it was the moon. After gene therapy treatment, she discovered it was a clock tower.

But he cautioned that gene therapy’s newness comes with questions.

Bryan, of the FDA, agreed: “We don’t know what the safety issues are yet.”

The downsides of certain gene therapy approaches can include genotoxicity, which causes such “untoward events” as tumors in the genetic code, according to the Mayo Clinic Arizona. Another method can cause immunotoxicity, where the body recognizes the new cells as a virus and wreaks havoc with the immune system.

The price of gene therapy also raises concerns. Luxturna costs $425,000 per eye, which is common among gene therapies, High said. She said she hopes to find ways to minimize costs and make the treatment more accessible.

“Gene therapy is arguably one of the most complex treatments,” High said. But Kaspar said its ability to mitigate or cure genetic diseases in a relatively short period of time is unprecedented.

“This is a one-and-done therapy,” he said.