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When Elizabeth Guardino brought home her newborn from the hospital, he seemed a healthy, bright-eyed boy. But three months later, she realized something was amiss – Christian was unable to track objects placed in front of him and had developed no facial recognition.

Guardino knew something was horribly wrong and she needed to act quickly. He was diagnosed with a rare retinal disease called Leber congenital amaurosis, or LCA, and he spend nearly a decade in the dark.

“People born with this [mutation] have severely poor vision and, as they get older, the disease gets worse because the cells in the back of the eye, where vision begins, die off and the individual becomes completely dependent,” says,” Dr. Jean Bennett, University of Pennsylvania ophthalmologist and Children’s Hospital of Philadelphia (CHOP) physician.

Christian was eventually able to see again, thanks to a breakthrough drug recently approved by the Food and Drug Administration.

The FDA’s approval of Luxturna, the first gene therapy to treat a specific inherited gene mutation, RPE65 – has been deemed a medical milestone. The procedure injects a copy of a healthy gene directly into the retina. This allows proteins to repair tissue, thus gradually restoring the patient’s vision through a one-time injection.

Yet for some groups, the initial enthusiasm surrounding the breakthrough treatment has already worn off. Earlier this month, the company announced that it planned to charge $850,000 per patient for the drug –making it the most expensive medicine sold in the U.S., ranked by the sticker price. The outstanding cost also raises an important question, just how much is eyesight worth?

Spark Therapeutics’ co-founder said the drug is not only a breakthrough for vision, but clinical success reinforces the potential of gene therapy in treating patients with other inherited and challenging diseases.

“This shows that there is a pathway to change a gene into a medicine – and this is the first,” says Dr. Kathy High, co-founder and chief scientific officer of Spark Therapeutics, Inc.

While gene therapy holds this promise– until just a few months ago the field had been plagued with failures. That is perhaps why Luxturna is pushing for an unusual payer model, including installment payments and rebates to address patients who do not have success or experience only partial or temporary success.

Critics are not only concerned about the longevity of the treatment, they also raise skepticism about what this price tag will do for future treatments like it.

“Spark is doing what every drug company does, which is figure out what is the highest rate it can charge and then charging it without any regard to the impact on people or to the impact on our ability to pay as a country,” said David Mitchell, founder and president of Patients for Affordable Drugs.

For the Guardinos, however, life hasn’t been the same since participating in the first clinical trial.

Christian, now 17 and a senior in high school who loves to sing, was a contestant on “America’s Got Talent.” His performance of Jackson 5’s “Who’s Loving You” received a “golden buzzer” pass into the live round of competition and he fulfilled his dream of singing for Simon Cowell.

“The gene therapy treatment gave me confidence. I was a rock on stage, too scared to move because I couldn’t see,” Christian said. “Now, I can see. And I looked Simon Cowell in the eyes.”