May 7 (Reuters) - A young boy died due to a cardiac arrest in a mid-stage trial that tested Pfizer's experimental gene therapy for a muscle-wasting disorder called Duchenne muscular dystrophy (DMD), the drugmaker told Reuters on Tuesday.

"A fatal serious adverse event was reported as cardiac arrest for a participant in the Phase 2 DAYLIGHT study," a company spokesperson told Reuters in an emailed response.

The trial tested boys 2 years to less than 4 years of age with DMD, a genetic muscle wasting disorder in which most patients lack the protein dystrophin which keeps muscles intact. The disorder affects an estimated one-in-3,500 male births worldwide.

"The patient received the investigational gene therapy, fordadistrogene movaparvovec, in early 2023," as per a statement from a community letter attributed to the drugmaker's DMD gene therapy team and posted by a nonprofit advocacy group.

The company said, together with the independent external data monitoring committee, it is in the process of reviewing the data to understand the potential cause. (Reporting by Pratik Jain in Bengaluru; Editing by Shailesh Kuber)