An experimental gene therapy produced marked clinical improvement in children with aromatic L-amino acid decarboxylase (AADC) deficiency, a rare genetic disorder that affects the synthesis of key neurotransmitters to cause severe developmental and motor disability.
Dr. Krystof Bankiewicz
In an article published July 12, 2021, in Nature Communications, a group of researchers based at the University of California, San Francisco, and Ohio State University, Columbus, described results from seven children ages 4-9 with AADC deficiency who underwent a novel form of surgery to deliver a viral vector expressing the human AADC gene to the midbrain.
Previous trials of this gene therapy in children with AADC deficiency targeted a different region of the brain, the putamen, with only slight clinical improvement. Here, investigators chose two midbrain regions – the substantia nigra pars compacta and the ventral tegmental area – in the hope of restoring healthy AADC enzyme activity in those neurons.
The study’s corresponding author, Krystof Bankiewicz, MD, PhD, professor and vice chair of research at Ohio State University, director of the Brain Health and Performance Center at Ohio State University, and professor emeritus and vice chair for research at UCSF, said in an interview that the brain regions chosen for this trial resulted from years of efforts to identify an ideal target in this disease.
“This particular vector undergoes axonal transport,” he said. “If you inject it into specific regions of the brain it will be transported into the terminals [of the nerve fibers]. And by looking at the imaging of these patients, we found that they still have the wiring in the brain that’s so critical. So we decided to aim at a much more difficult target, going directly to the source of the problem, which is the substantia nigra and the ventral tegmental area. This targets two critical pathways in the brain: one that drives motor responses and another that controls emotions.”
‘Surprising’ improvement seen
The children in the study – four girls and three boys – underwent surgery from 2016 to the end of 2018, and were divided into two dose cohorts, with one receiving three times the amount of vector as the other. Both groups, however, saw similar levels of improvement.
All but one child saw complete resolution of a hallmark symptom of the disease – oculogyric crises, or prolonged spasms of muscles controlling eye movement – within 3 months of surgery. Of the children followed at least 18 months, six attained head control within a year, two became able to eat and drink by mouth, and four gained the ability to sit up unaided in that time. At 18 months one child had learned to speak 50 words using an augmentative communication device.
One child died unexpectedly 7 months after the procedure, Dr. Bankiewicz said in an interview. This death appeared to be caused by cardiac complications of his disease, Dr. Bankiewicz said, which are common in AADC deficiency.
While the investigators are now looking at delivering the AADC gene therapy in younger children – who were excluded from this trial because of safety concerns surrounding the complex procedure – investigators were surprised by the level of improvement seen in older children.
“We initially didn’t believe – at least not all of us – that we could actually make an impact in the older patients, and that is not the case,” said Dr. Bankiewicz, who has since used the same gene therapy on a compassionate-use basis in Europe and seen durable clinical improvement in patients as old as 26. “The fact that we saw a response in that patient tells us something about how incredibly plastic the brain is.”
While the new study does not detail improvements in the children’s social and emotional well-being, Dr. Bankiewicz said these, too, were pronounced. “Kids fall into oculogyric crises in stress-inducing situation. They might be in a stroller being taken for a walk, and something in the environment would stress them. Sometimes they had to be kept in a dark room isolated from stress.” Following the gene therapy, “they’re laughing, they’re social, they can interact with their environment. It’s really touching to see them able to develop a bond now with their caregivers.”