Rare Daily Staff

University of Toronto Donnelly Centre researchers have uncovered a genetic network linked to autism. The findings, which may facilitate the development of new therapies, were recently published in the journal Molecular Cell.

The team, led by Benjamin Blencowe, professor at the Donnelly Centre for Cellular and Biomolecular Research,  and postdoctoral fellow Thomas Gonatopoulos-Pournatzis, lead author of the study, uncovered a network of more than 200 genes involved in controlling alternative splicing events that are often disrupted in autism spectrum disorder (ASD). Alternative splicing is a process that functionally diversifies protein molecules in cells. Blencowe’s laboratory previously showed that disruption of this process is closely linked to altered brain wiring and behavior found in autism.

“Our study has revealed a mechanism underlying the splicing of very short coding segments found in genes with genetic links to autism,” said Blencowe, who is also a professor in the Department of Molecular Genetics. “This new knowledge is providing insight into possible ways of targeting this mechanism for therapeutic applications.”

Best known for its effects on social behavior, autism is thought to be caused by mishaps in brain wiring laid down during embryo development. Hundreds of genes have been linked to autism, making its genetic basis difficult to untangle. Blencowe’s team had previously discovered that small gene fragments, called microexons, are disrupted in a large proportion of autistic patients. Alternative splicing of microexons has thus emerged as a rare, unifying concept in the molecular basis of autism.

As tiny protein-coding gene segments, microexons impact the ability of proteins to interact with each other during the formation of neural circuits. They are especially critical in the brain, where they are included into the RNA template for protein synthesis during the splicing process. Splicing enables the utilization of different combinations of protein-coding segments, or exons, as a way of boosting the functional repertoires of protein variants in cells.

And while scientists have a good grasp of how exons, which are about 150 DNA letters long, are spliced, it remained unclear how the much smaller microexons— a mere 3-27 DNA letters long—are utilized in nerve cells.

“The small size of microexons presents a challenge for the splicing machinery and it has been a puzzle for many years how these tiny exons are recognized and spliced,” said Blencowe.

To answer this question, Gonatopoulos-Pournatzis developed a method for identifying genes that are involved in microexon splicing. Using the powerful gene editing tool CRISPR, the team removed from cultured brain cells each of the 20,000 genes in the genome to find out which ones are required for microexon splicing. He identified 233 genes whose diverse roles suggest that microexons are regulated by a wide network of cellular components.

“A really important advantage of this screen is that we’ve been able to capture genes that affect microexon splicing both directly and indirectly and learn how various molecular pathways impinge on this process,” Blencowe said.

Knowing the precise molecular mechanisms of microexon splicing will help guide future efforts to develop potential therapeutics for autism and other disorders. For example, because the splicing of microexons is disrupted in autism, researchers could look for drugs capable of restoring their levels to those seen in unaffected individuals.

The study was supported by research grants from the Canadian Institutes for Health Research, Canada First Research Excellence Fund and the Simons Foundation.

November 5, 2018

Photo: Benjamin Blencowe, professor at the Donnelly Centre for Cellular and Biomolecular Research, University of Toronto

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