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The trial included 16 children and young adults born with mutations that cause propionic acidemia, in which cells lack an enzyme that enables them to break down certain proteins and fats.

One such disease is propionic acidemia, caused by mutations in genes encoding one of two subunits of an enzyme that helps mitochondria, cells’ powerhouses, break down certain amino acids and fats.

Physician-scientist Charles Venditti of the National Human Genome Research Institute notes that various mutations can cause propionic acidemia, and he would have liked the study authors to describe the specific mutations participants carried.

“There’s a signal for efficacy, but it's not convincing because there’s no details provided,” says Vendetti, who is working on a gene therapy for propionic acidemia that could insert a new gene into patients’ cells and provide a potentially more permanent fix.

Messenger RNA therapies are also currently being investigated in other rare inherited metabolic diseases -- propionic and methylmalonic acidaemias -- in global clinical trials sponsored by Moderna, including at Great Ormond Street Hospital for Children.