By Toni Clarke

Feb 14 (Reuters) - The U.S. Food and Drug Administration said on Friday it has approved the first drug to treat Morquio A Syndrome, a rare genetic disorder that causes skeletal malformation and a variety of related lung, eye, ear and heart problems.

The drug, however, will carry a boxed warning, the most serious possible, to include the risk of anaphylaxis, an allergic reaction that threatened the lives of some patients during clinical trials.

The addition of the boxed warning caused shares of BioMarin Pharmaceutical Inc., the drug's maker, to slip 0.2 percent to $75.81 on Friday.

Approval of the drug, Vimizim, follows a positive recommendation in November from an advisory committee to the FDA.

Morquio A Syndrome is caused by a deficiency of an enzyme known as N-acetylgalactosamine-6-sulfatase, which causes excessive storage in the body of long chains of sugars known as glycosaminoglycans. The disease affects about 800 people in the United States, the FDA said.

This build-up can lead to short stature and joint abnormalities that limit mobility and endurance. The disease can also cause hearing loss, eye problems and heart disease. Symptoms often appear before the age of five.

After 24 weeks of treatment, Vimizim, also known as elosulfase alfa, increased patients' six-minute walk distance by 22.5 meters compared with those taking a placebo. Parents told the advisory committee meeting in November that they saw improvements in their children's energy and endurance levels, enabling them to better socialize, learn and function independently.

Morquio A Syndrome is one of a group of lysosomal storage disorders known as mucopolysaccharidoses (MPS). Lysosomal storage disorders are typically chronic and progressive, and involve multiple organs of the body.