CHOP restored eyesight for two children with one injection. It's one more step for gene editing using CRISPR.

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Children's Hospital of Philadelphia physicians restored eyesight in two children by performing a gene therapy technique entirely inside their eyes to correct a genetic mutation causing blindness, a first-of-its-kind treatment for kids.

The 10-year-old and 14-year-old patients had their eyesight improved about 10- to 100-fold after a single injection to their retina, the area in the back of the eye that converts light into electric signals that the brain can process.

The procedure is known as gene editing, a type of therapy used to correct a deficiency in a person's DNA causing disease. It is typically performed outside of the body by taking cells out and then transplanting them again after the genetic correction.

The two children were part of a clinical trial, involving hospitals in five cities, that also included 12 adults. All of the patients had Leber Congenital Amaurosis, a common cause of blindness in children. The results of the clinical trial were published Monday in the New England Journal of Medicine.

"It's the first time that gene editing has been used in the eyes of children, and proved safe and successful," said Tomas Aleman, a retina specialist at CHOP and one of the study's authors.

» READ MORE: Gene therapy at CHOP allowed a deaf boy to hear. But some deaf people object to the treatment.

The study focused on the safety of the procedure, finding the gene-editing injection to the eye to be safe. But researchers also saw vision improvement in half of the patients, including the two children treated at CHOP.

The children still did not leave the hospital with 20/20 eyesight, said Aleman, who is also a professor at Penn Medicine. He hopes that delivering the treatment to children when they are younger will lead to greater improvement in eyesight.

The study is part of an ongoing collaboration that includes CHOP, University of Pennsylvania's Perelman School of Medicine, Harvard Medical School, Oregon Health and Science University, and the University of Michigan.

The research was funded by Editas Medicine, Inc., a Cambridge, Mass.-based gene therapy company.

Spell checking

Gene therapy fixes errors in the body's genetic code using an approach analogous to editing a text, said Eric Kmiec, executive director and chief scientific officer of ChristianaCare's Gene Editing Institute, which was not involved in the study.

A deficiency in a gene is like a misspelled word.

Gene therapies a decade ago could only fix the error by injecting corrected DNA next to it within the cell, like striking out a word and writing it the right way by its side. The newer generation of therapies instead fix the misspelling itself using a technology called CRISPR.

» READ MORE: The FDA just approved the first gene-editing treatment, and CHOP played a key role

Physicians most often apply gene therapy by taking cells out of the body, fixing the genetic error, and then transplanting the cells back into the patient. That's how CRISPR works to cure sickle cell disease: Bone marrow cells are corrected to produce red blood cells in the right shape, and are returned to the body where they multiply.

That approach has a few problems, Kmiec said. The body can reject the corrected cells like any transplantation. And cells in the nervous system don't multiply.

These problems have led scientists to work on ways to fix the mutations in cells while they are inside the body. Kmiec's research focuses on doing that with cancerous tumors. The CHOP team is focusing on editing genes in the eye.

"This is remarkable right? You can correct a mutation in the human gene in the body," Kmiec said.