SAN DIEGO—Targeted toward developing an effective treatment for a rare form of autism spectrum disorder, researchers at the University of California, San Diego (UC San Diego) School of Medicine and Sanford Consortium for Regenerative Medicine used lab-grown human brain organoids that mimic Rett syndrome, a malady which affects a child’s speech, movement—and even breathing.
Babies born with this form of the disorder have mutations specifically in the MECP2 gene, causing a severe impairment in brain development that primarily affects girls. Current therapies are aimed at alleviating symptoms, without addressing the root cause. There is still no treatment for Rett.
In a study published Dec. 8, 2020, in EMBO Molecular Medicine, the team identified two drug candidates that counteract the deficiencies caused by lack of the MECP2 gene. These compounds, Nefiracetam and PHA 543613, restored calcium levels, neurotransmitter production, and electrical impulse activity, returning the Rett syndrome brain organoids to near-normal.
“The gene mutation that causes Rett syndrome was discovered decades ago, but progress on treating it has lagged, at least in part because mouse model studies haven’t translated to humans,” says senior author Dr. Alysson R. Muotri, professor of pediatrics and cellular and molecular medicine at UC San Diego School of Medicine. “This study was driven by the need for a model that better mimics the human brain.”
The study’s lab based results provide a compelling argument for advancing Nefiracetam and PHA 543613 into clinical trials for patients with MECP2-deficient neurodevelopmental disorders.
These drugs “could restore the activity in human neurons in our model system,” Muotri told DDN, though he acknowledged more work is needed, noting, “The brain organoid model has several limitations: It is not a full working brain, as there is no vascularization and it is not connected to the body.”
Caused by genetic mutations in the MECP2 gene, the rare syndrome (one in 10,000 girls in United States) “is pretty severe, with regression early in life, loss of speech, stereotype hand movements, and loss of motor coordination,” Muotri says. “Rett in boys is noticeable at birth and in girls it takes approximately 18 months to manifest.”
“We need to know how translational this model is,” he says. “The only way to do that is by testing the drugs directly into people in clinical trials. A positive outcome will teach us how good the model is, and a negative outcome will indicate that we need to further improve these organoids to be closer to the real brain.”
Brain organoids are 3D cellular models that represent aspects of the human brain in the laboratory. These organoids help researchers track human development, unravel the molecular events that lead to disease, and test new treatments.
At UC San Diego, brain organoids have been used to produce the first direct experimental proof that the Brazilian Zika virus can cause severe birth defects and to repurpose existing HIV drugs to treat another rare, inherited neurological disorder. Muotri and his team also sent their brain organoids to the International Space Station to test microgravity’s effect on brain development—and maybe prospects for human life beyond Earth.
Organoids aren’t perfect replicas of the brain, of course, since they lack connections to other organ systems, like blood vessels, Muotri says. Drugs tested on brain organoids are added directly, so they don’t need to get across the blood-brain barrier that keeps the brain largely free of bacteria, viruses, and toxins.
Researchers also find organoids very useful for checking changes in physical structure or gene expression over time or as a result of a gene mutation, virus, or drug, Muotri says, adding that his team recently optimized the brain organoid-building process to match the electrical impulse pattern of premature babies and better resemble real human brains in the population they are targeting for treatment.
In the latest study, researchers applied this new protocol for functional brain organoids, using induced pluripotent stem cells derived from patients with Rett syndrome. To verify their findings, the team also engineered brain organoids that artificially lack the MECP2 gene, and even mixed mutated and control cells to mimic the mosaic pattern typically seen in female patients.
Lack of the MECP2 gene changed many things about the organoids, such as shape, neuron subtypes present, gene expression patterns, neurotransmitter production, and synapse formation. Calcium activity and electrical impulses were also decreased. These changes led to major defects in the emergence of cortical neural oscillatory waves.
First, in an attempt to compensate for the missing MECP2 gene, the team treated the brain organoids with 14 drug candidates that are known to affect various brain cell functions. Nearly all of the molecular and cellular symptoms were resolved when the researchers treated the Rett syndrome brain organoids with Nefiracetam and PHA 543613.
These lab-based results provide a compelling argument for advancing Nefiracetam and PHA 543613 into clinical trials for patients with MECP2-deficient neurodevelopmental disorders, Muotri says.
In the end, the best treatment for Rett syndrome may not be one superdrug, though.
“There’s a tendency in the neuroscience field to look for highly specific drugs that hit exact targets, and to use a single drug for a complex disease,” says Muotri, who is also director of the UC San Diego Stem Cell Program. “But we don’t do that for many other complex disorders, where multipronged treatments are used. Likewise, here, no one target fixed all the problems. We need to start thinking in terms of drug cocktails, as have been successful in treating HIV and cancers.”.
“Our long-term goal is to create the first pharmacological treatment specific for Rett,” Muotri concludes. “If we succeed here, we can repeat the same strategy for other types of autisms.”
