University of California, San Diego (UCSD), School of Medicine scientists used human brain organoids to reveal how a genetic mutation associated with a profound form of autism disrupts neural development. The scientists used gene therapy tools to recover the gene’s function.
Their study is published in Nature Communications.
“Transcription Factor 4 (TCF4) has been associated with autism, schizophrenia, and other neuropsychiatric disorders,” the researchers wrote. “However, how pathological TCF4 mutations affect the human neural tissue is poorly understood. Here, we derive neural progenitor cells, neurons, and brain organoids from skin fibroblasts obtained from children with Pitt-Hopkins Syndrome carrying clinically relevant mutations in TCF4. We show that neural progenitors bearing these mutations have reduced proliferation and impaired capacity to differentiate into neurons.”
Several neurological and neuropsychiatric diseases, including autism spectrum disorders (ASD) and schizophrenia, have been linked to mutations in TCF4, an essential gene in brain development. Transcription factors regulate when other genes are turned on or off, so their presence, or lack thereof, can have a domino effect in the developing embryo. Still, little is known about what happens to the human brain when TCF4 is mutated.
The researchers focused on Pitt-Hopkins Syndrome, an ASD specifically caused by mutations in TCF4.
Existing mouse models of Pitt-Hopkins Syndrome fail to accurately mimic patients’ neural characteristics, so the UCSD team instead created a human research model of the disorder.
“Even without a microscope, you could tell which brain organoid had the mutation,” said senior study author Alysson R. Muotri, PhD, professor at UCSD School of Medicine, director of the UCSD Stem Cell Program, and member of the Sanford Consortium for Regenerative Medicine.
The TCF4-mutated organoids were substantially smaller than normal organoids, and many of the cells were not actually neurons, but neural progenitors. These simple cells are meant to multiply and then mature into specialized brain cells, but in the mutated organoids, some part of this process had gone awry.
A series of experiments revealed that the TCF4 mutation led to downstream dysregulation of SOX genes and the Wnt pathway, two important molecular signals that guide embryonic cells to multiply, mature into neurons, and migrate to the correct location in the brain.
“We were surprised to see such major developmental issues at all these different scales, and it left us wondering what we could do to address them,” said first author Fabio Papes, PhD, associate professor at the University of Campinas and visiting scholar at UCSD School of Medicine, who jointly supervised the work with Muotri. Papes has a relative with Pitt-Hopkins Syndrome, which motivated him to study TCF4.
The team tested two different gene therapy strategies for recovering the functional gene in brain tissue. Both methods effectively increased TCF4 levels.
“The fact that we can correct this one gene and the entire neural system reestablishes itself, even at a functional level, is amazing,” said Muotri.
“For these children and their loved ones, any improvements in motor-cognitive function and quality of life would be worth the try,” Muotri said.
“What is truly outstanding about this work is that these researchers are going beyond the lab and working hard to make these findings translatable to the clinic,” said Audrey Davidow, president of the Pitt-Hopkins Research Foundation. “This is so much more than a stellar academic paper; it’s a true measure of what well-practiced science can accomplish to hopefully change human lives for the better.”