When Rylae-Ann Poulin was a year old, she didn’t crawl or babble like other kids her age. A rare genetic disorder kept her from even lifting her head. Her parents took turns holding her upright at night just so she could breathe comfortably and sleep. Then, months later. doctors delivered gene therapy directly to her brain.

Now the 4-year-old is walking, running, swimming, reading and riding horses - “just doing so many amazing things that doctors once said were impossible,” said her mother, Judy Wei.

Rylae-Ann, who lives with her family in Bangkok, was among the first to benefit from a new way of delivering gene therapy - attacking diseases inside the brain - that experts believe holds great promise for treating a host of brain disorders.

Her treatment recently became the first brain-delivered gene therapy after its approval in Europe and the United Kingdom for AADC deficiency, a disorder that interferes with the way cells in the nervous system communicate. New Jersey drugmaker PTC Therapeutics plans to seek U.S. approval this year.

Meanwhile, about 30 U.S. studies testing gene therapy to the brain for various disorders are ongoing, according to the National Institutes of Health. One, led by Dr. Krystof Bankiewicz at Ohio State University, also targets AADC deficiency. Others test treatments for disorders such as Alzheimer’s, Parkinson’s and Huntington’s.

Challenges remain, especially with diseases caused by more than a single gene. But scientists say the evidence supporting this approach is mounting - opening a new frontier in the fight against disorders afflicting our most complex and mysterious organ.

The most dramatic of those breakthroughs involve Rylae-Ann’s disease, which is caused by mutations in a gene needed for an enzyme that helps make neurotransmitters like dopamine and serotonin, the body’s chemical messengers. The one-time treatment delivers a working version of the gene.

At around 3 months old, Rylae-Ann began having spells her parents thought were seizures - her eyes would roll back and her muscles would tense. Fluid sometimes got into her lungs after feedings, sending her to the emergency room. Doctors thought she might have epilepsy or cerebral palsy.

Around that time, Wei’s brother sent her a Facebook post about a child in Taiwan with AADC deficiency. The extremely rare disorder afflicts about 135 children worldwide, many in that country. Wei, who was born in Taiwan, and her husband, Richard Poulin III, sought out a doctor there who correctly diagnosed Rylae-Ann. They learned she could qualify for a gene therapy clinical trial in Taiwan.

Though they were nervous about the prospect of brain surgery, they realized she likely wouldn’t live past 4 years old without it.

Rylae-Ann had the treatment at 18 months old on Nov. 13, 2019 - which her parents have dubbed her “reborn day.” Doctors delivered it during minimally invasive surgery, with a thin tube through a hole in the skull. A harmless virus carried in a functioning version of the gene.

“It gets put into the brain cells and then the brain cells make the (neurotransmitter) dopamine,” said Stuart Peltz, CEO of PTC Therapeutics.

Company officials said all patients in their clinical trials showed motor and cognitive improvements. Some of them, Peltz said, could eventually stand and walk, and continue getting better over time.

Bankiewicz said all 40 or so patients in his team’s NIH-funded study also saw significant improvements.

Scientists say there are challenges to overcome before this approach becomes widespread for more common brain diseases. For example, the timing of treatment is an issue. Generally, earlier in life is better because diseases can cause a cascade of problems over the years. Also, disorders with more complex causes - like Alzheimer’s - are tougher to treat with gene therapy.

Ryan Gilbert, a biomedical engineer at New York’s Rensselaer Polytechnic Institute, said there can also be issues with the gene-carrying virus, which can potentially insert genetic information in an indiscriminate way.