(Reuters) - Novartis AG, which this week announced positive interim trial results for its experimental gene therapy for spinal muscular atrophy, on Friday said investigation is underway into whether a second trial death could be related to the treatment.

The logo of Swiss drugmaker Novartis AG is seen at its headquarters in Basel, Switzerland January 25, 2017. REUTERS/Arnd Wiegmann

Novartis has filed for U.S. Food and Drug Administration approval of the gene therapy, Zolgensma, and a decision is expected within weeks. The FDA submission was based on findings from a trial of 15 babies treated with Zolgensma.

But Novartis has expanded its clinical trial program - presenting on Tuesday at an Orlando, Florida meeting of the Muscular Dystrophy Association interim results for 22 babies with Type 1 SMA, the most serious form of the disease. The data showed that Zolgensma treatment resulted in encouraging progress in motor skills such as the ability to sit up. One patient died from respiratory failure, which was deemed by the investigator and an independent monitor to be unrelated to the gene therapy.

SMA, which can lead to paralysis, breathing difficulty and death, is the leading genetic cause of death in infants.

Novartis officials also disclosed that in addition to that death, a 6-month-old patient with Type 1 SMA had recently died after undergoing Zolgensma treatment in the company’s European trial.

“Preliminary findings indicate this occurred in the context of a severe respiratory infection followed by neurological complications in a symptomatic SMA Type 1 patient, and was deemed possibly related to treatment by the investigator,” Novartis spokesman Eric Althoff said in an emailed statement on Friday.

He said an autopsy has been performed and results are pending. Meanwhile, trial investigators and regulatory authorities have been informed.

Gene therapies use engineered viruses to carry healthy genetic material into a person’s cells to replace faulty or mutated genes that cause a disease or condition. Zolgensma

“As we learn more, we will provide further updates,” Althoff said.

Novartis estimates that without treatment, 50 percent of babies with SMA Type 1 will not survive or will need permanent breathing support by the time they are 10.5 months old.

The company has said its price for Zolgensma will be determined in negotiations with health plans, but it believes the gene therapy would be cost effective at $4 million to $5 million as a one-time treatment.