American health officials recently approved the first gene therapy treatment for an inherited disease – the type that can pass from parent to child.

The treatment improves the sight of patients suffering from a genetic mutation that destroys their ability to see. It is the first gene therapy approved where a corrective gene is given directly to patients.

People with the disease usually start losing their sight before they are 18. This often progresses to total blindness. The gene that causes the disease could suddenly appear when a child is born with a copy of the gene from both parents.

Only a few thousand people in the U.S. are thought to have the condition.

Patients receive the therapy, called Luxturna, through two injections, one for each eye. These replace the problem gene that prevents the retina, a tissue at the back of the eye, from changing light into electronic signals sent to the brain.

Mistie Lovelace received one of the early treatments. She said, "One of the best things I've ever seen since surgery are the stars. I never knew that they were little dots that twinkled."

Drugmaker Spark Therapeutics has not set the price for the treatment. Its own research, however, has put the value of the therapy at around $1 million.

The company says the reason for the high price is due to their prediction that Luxturna will be given only once, with lasting positive effects. To date, the company has researched patients in a study for as long as four years and has not seen their vision worsen.

The Food and Drug Administration has approved three gene therapies since August. The two other treatments are specially designed treatments for forms of blood cancer.

New success for difficult field

The approval marks an important growth for a field of study that has struggled with safety concerns.

In 1999, a patient treated for a rare metabolic disorder at the University of Pennsylvania died. In another case, patients treated for an immune disorder later developed leukemia.

Dr. David Valle said the excitement about the possibilities for genetic medicine create opportunities for research on individual diseases.

University of Pennsylvania researcher Dr. Jean Bennett said she and her husband, Dr. Albert Maguire, first imagined using genetic medicine to treat retinal blindness in the mid-1980s.

But it took many years to develop the science and technology. The first animal tests happened in 2000 and the first human tests in 2007.

"We didn't know what genes caused the disease, we didn't have animal models with those genes, we didn't have the ability to clone genes and deliver them to the retina — so it took time to develop all that," said Bennett.

I'm Phil Dierking.

Matthew Perrone reported this story for the Associated Press. Phil Dierking adapted this story for VOA Learning English. Hai Do was the editor.