The U.S. Food and Drug Administration on Friday said it has approved Biogen Inc's drug to treat spinal muscular atrophy, the leading genetic cause of death in infants.

It is the first FDA-approved medicine for spinal muscular atrophy, a devastating disease that affects about one in 10,000 live births.

The drug, nusinersen, which was discovered by Ionis Pharmaceuticals and licensed to Biogen, will be sold under the brand name Spinraza. The FDA approved it for use across the full range and severity of the disease.

Dr. Billy Dunn, director of the FDA's division of neurology products, said the agency "could not be more pleased to have the first approved treatment for this debilitating disease."

In the most severe form of spinal muscular atrophy, known as type 1, or infantile SMA, babies rarely make it to their second birthday.

Annual Spinraza sales could reach $2 billion, according to RBC Capital Markets analyst Michael Yee.

Spinraza will be the first major drug launch under Biogen's new chief executive, Michel Vounatsos, who will replace George Scangos on Jan. 6.

SMA is caused by deficiency of a protein called smn in the spinal motor neurons. It leads to severe and progressive muscular atrophy and weakness, including in muscles needed for breathing and swallowing. Spinraza, which is injected into the spinal fluid, increases levels of the deficient protein.

In clinical trials, nusinersen led to dramatic improvement in motor milestones and extended survival.

It has been tested with successful results in all severities of the disease, including in infants with the genetic deficiency who have yet to show symptoms of the disease, Biogen said.

Type 2, or childhood onset SMA, affects toddlers and children. Even the least severe form, type 3 SMA, can be disabling as it progresses, leaving people needing wheelchairs, Biogen said.

There are only about 9,000 patients living in the United with SMA. Those numbers are likely to rise with an effective treatment that helps children to live longer.