Scientists at the University of California San Diego (UC San Diego) in the United States have used human brain organoids (laboratory-grown human brain tissue) to identify dysfunctions in neurons characteristic of a profound form of autism, which are due to a genetic mutation that interferes with neuronal development, and they have tested a gene therapy on them with which they have managed to reverse the effects of said mutation and recover neuronal structure and function, according to the results of their study, which have been published in Nature Communications.
Several neurological and neuropsychiatric diseases, including autism spectrum disorders (ASD) and schizophrenia, have been linked to mutations in transcription factor 4 (TCF4), a key gene in brain development. Transcription factors regulate when other genes are turned on or off, and so their presence or absence can have a ripple effect on the developing embryo, yet little is known about what happens to the developing embryo. human brain when TCF4 mutates.
The researchers focused on Pitt-Hopkins syndrome, a type of ASD that is directly linked to mutations in TCF4. Children with this genetic alteration have severe cognitive and motor disabilities and do not usually speak. These experts converted the skin cells of the patients into stem cells, which were later developed into three-dimensional brain organoids, or ‘mini-brains’, in which they verified how the development of neurons was interrupted as a result of the genetic mutation and the usefulness of gene therapy tools to recover the function of the affected gene.
They correct the mutated gene and restore the neuronal system
Looking at brain organoids, a large number of structural and functional differences were found between samples with the TCF4 mutation and their controls. “Even without a microscope, you could tell which brain organoid had the mutation,” said Alysson R. Muotri, a professor at the UC San Diego School of Medicine, director of the UC San Diego Stem Cell Program and a member of the Sanford Consortium for Regenerative Medicine and lead author of the study.
“The fact that we can correct this gene and the whole neural system is reset, even at a functional level, is amazing.”
TCF4-mutated organoids were significantly smaller than normal organoids, and many of the cells were not actually neurons, but neural progenitors. These simple cells are destined to multiply and mature into specialized brain cells, but in the mutated organoids part of this process went awry.
Their experiments showed that the TCF4 mutation led to further dysregulation of the SOX genes and the Wnt pathway, two important molecular signals that help embryonic cells multiply, mature into neurons, and migrate to the correct location in the brain. But as a result of this dysregulation, the neuronal progenitors did not multiply correctly and fewer cortical neurons were produced.
In addition, the cells that did develop into neurons were less active than normal and often clumped together rather than organizing themselves into well-tuned neural circuits. This atypical cellular architecture disrupted the flow of neural activity in the mutated brain organoid, and this likely contributed to impaired cognitive and motor function later in life, the authors believe.
The researchers tested two different gene therapy strategies to restore the functional gene to brain tissue, both of which effectively increased TCF4 levels and corrected Pitt-Hopkins syndrome phenotypes at the molecular, cellular, and electrophysiological levels. “The fact that we can correct this gene and the whole neural system is reset, even at a functional level, is amazing,” Muotri said.
Muotri points out that these genetic interventions took place at a prenatal stage of brain development, but in a clinical setting children would be diagnosed and treated a few years later, so clinical trials are first needed to see if a later intervention follows. being safe and effective. The team of scientists is currently optimizing its newly licensed gene therapy tools in preparation for such a trial, in which spinal injections of the gene vector are expected to restore TCF4 function in the brain. “For these children and their loved ones, any improvement in cognitive-motor function and quality of life would be worth trying,” Muotri said.
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