Madrid
Updated:03/05/2022 17:17h
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The use of mini human brain organoids has served to demonstrate how a genetic mutation associated with a profound form of autism alters neuronal development. And now, a team from the University of California at San Diego (USA)
has used gene therapy to restore gene function and rescue neuronal structure and function.
Some neurological and neuropsychiatric diseases, such as autism spectrum disorders (ASD) and schizophrenia have been linked to mutations in transcription factor 4 (TCF4), an essential gene in brain development. Transcription factors regulate when other genes are turned on or off, so their presence, or absence, can have a ripple effect on the developing embryo.
However, little is known about what happens in the human brain when TCF4 is mutated.
To explore this question, the researchers focused on Pitt-Hopkins syndrome, an ASD caused specifically by mutations in TCF4. Children with this genetic condition have profound cognitive and motor disabilities and often lack verbal ability.
Existing mouse models for the syndrome Pitt-Hopkins they can’t exactly mimic the neural characteristics of patients, so the team created a human research model of the disorder.
Using stem cell technology, they converted patients’ skin cells into stem cells, which were then turned into three-dimensional brain organoids, or “mini-brains.”
Initial observations of brain organoids revealed a number of structural and functional differences between the TCF4-mutated samples and their controls.
“Even without a microscope, you could tell which brain organoid had the mutation,” says study lead author Alysson R. Muotri.
The TCF4-mutated organoids were substantially smaller than normal ones, and many of the cells were not actually neurons, but neuronal progenitors. These simple cells are destined to multiply and mature into specialized brain cells, but in the mutated organoids, some part of this process had gone awry.
The researchers then discovered that the TCF4 mutation caused a dysregulation of the genes SOX y of 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.
Due to this dysregulation, the neuronal progenitors did not multiply efficiently and therefore fewer cortical neurons were produced. The cells that matured into neurons were less excitable than normal and often remained clumped together rather than organizing themselves into finely tuned neural circuits.
This atypical cellular architecture disrupted the flow of neuronal activity in the mutated brain organoid, which, according to the authors, probably could contribute to impaired cognitive function and motor in the future.
“We were surprised to see such major development problems at all these different scales, and it made us wonder what we could do to fix them,” says first author Fabio Papes. Papes has a relative with Pitt-Hopkins syndrome, which prompted him to study TCF4.
The team tested two different gene therapy strategies to retrieve the functional gene in brain tissue. Both approaches effectively increased TCF4 levels and, in doing so, corrected Pitt-Hopkins syndrome phenotypes at the molecular, cellular, and electrophysiological scales.
“The fact that we can correct this single gene and the entire neural system is reset, even at a functional level, is amazing,” says Muotri.
Muotri points out that these genetic interventions took place at a prenatal stage of brain development, whereas in a clinical setting, children would receive their diagnosis and treatment a few years later. Thus, clinical trials must first confirm whether a subsequent intervention is still safe and effective.
The team is Currently optimizing your tools recently licensed gene therapy providers to prepare 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 a try,” adds Muotri.
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