Caroline and Cole Carper have been through more than your typical siblings.
"I can't imagine it being any different than the two of them being together, and going through this together," said their mother, Ashley. "They've had canes together, they've had to learn Braille together, they've gone to school for the blind and visually impaired together."
Early in their lives, they were both diagnosed with a rare genetic retinal disease called Leber congenital amaurosis.
"The doctor really said, 'There's nothing,'" Ashley Carper said. "'There's no medical treatments at this time.'"
Both Caroline, 12 years old, and Cole, 10 years old, were expected eventually to go blind. But their parents found an experimental treatment, and, two years ago, the kids enrolled together in a clinical trial.
It was with Spark Therapeutics, a young biotech company formed out of research from the Children's Hospital of Philadelphia.
"The idea is that we are taking a correct functional copy of a gene and adding it back into the cells that have a dysfunctional, or missing, copy of that gene or blueprint," Spark CEO Jeff Marrazzo explained.
It's called gene therapy, and it's relatively uncharted territory in medicine — though researchers have been working on it for decades. The technology uses a modified virus to deliver a healthy copy of a gene to make up for one that causes disease.
It's been a long slog for science: the field essentially collapsed after the death of a patient in a clinical trial in 1999. But recent years have seen a resurgence, with a better understanding of the science and multiple companies investing in the space.
The first gene therapies are now approaching the market in the U.S., and Spark's is among the most advanced. The healthy genes are delivered through eye surgery. Once they're in place, they're designed to do the work of a normally functioning gene, meaning patients only undergo treatment once.
"What we're doing is different," Marrazzo said. "We're actually adding the blueprint in, and the cell then knows how to do the last step, which is to make the protein or the enzyme that might be missing. Therefore that's what creates the potential for it to be a long-lasting effect."
Spark is so committed to the idea of a single treatment for disease, it chose "ONCE" as its stock ticker.
In trials of 41 patients, Spark's therapy has shown to improve both vision and light sensitivity, with no major side effects.
Caroline and Cole were no exception. Before their surgeries, both kids had trouble seeing in low light. Now, though their vision isn't perfect, they say they're noticing big differences.
"I went outside when it was snowing, and I was like, 'Oh! I can see the snowflakes!'" Caroline said. "It was really cool to actually see something that I've never seen in my life before."
Spark's not alone in developing gene therapies. Companies from GlaxoSmithKline to Biogen to Bluebird Bio are among many others in the space, targeting everything from hemophilia to sickle cell disease to other rare disorders.
Spark has started a rolling application for FDA approval. It would usher in a new paradigm for medicine, as well as likely debates over pricing — a key question for a set of therapies that aim to cure disease rather than treat it chronically.