Summary: Researchers have identified six genes that appear to modify sensory experiences and may alter brain connectivity in those with synesthesia.
Source: Max Planck Institute.
Some people with synesthesia may see sounds, while others may taste them or feel them as shapes. This kind of sensory cross-talk comes in many forms, and develops during early childhood. It has been known for over a century that synesthesia runs in families, giving a strong hint that inherited factors are important.
“Brain imaging of adults with synesthesia suggests that their circuits are wired a little differently compared to people who don’t make these extra sensory associations. What we don’t know yet is how these differences develop,” said Dr. Amanda Tilot, a geneticist at the Max Planck Institute for Psycholinguistics. “We suspect some of the answers lie in people’s genetic makeup.”
A genetic window into sensory experience
Today, in a report published in the journal Proceedings of the National Academy of Sciences, scientists from the Max Planck Institute for Psycholinguistics and the University of Cambridge report new genetic clues that could help explain the biology of synesthesia. The researchers carefully analysed the DNA of three families in which multiple members, across several different generations, experience colour when listening to sounds.
The team took advantage of advances in genome sequencing, enabling them to identify genetic variants in the synesthesia families and track how they were passed on from one generation to the next. In particular, they focused attention on rare DNA changes that altered the way genes code for proteins, and that perfectly matched the inheritance of synesthesia in each of the three families.
While the highlighted DNA variants differed between the three families, a common theme emerged to connect them: an enrichment for genes involved in axonogenesis and cell migration. Axonogenesis is a key process enabling brain cells to wire up to their correct partners.
Combining families to uncover biological processes
Professor Simon Fisher, Director of the Max Planck Institute, who led the research, said, “We knew from earlier studies by the Cambridge team that no single gene can account for this intriguing trait; even families who experience the same form of synesthesia are likely to differ in terms of specific genetic explanations,” said Fisher. “Our hope was that the DNA data might point to shared biological processes as candidates for involvement in synesthesia.”
Professor Simon Baron-Cohen, Director of the Autism Research Centre, Cambridge University, commented, “This research is revealing how genetic variation can modify our sensory experiences, potentially via altered connectivity in the brain. synesthesia is a clear example of neurodiversity which we should respect and celebrate.”
In search of synaesthetes
To better understand these findings, the team is looking for new families and individuals to join their study. To learn more about their research and take a short test to find out if you experience a common form of synesthesia, go to http://www.mpi.nl/synesthesia.
Source: Katie Willis – Max Planck Institute
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Original Research: Open access research in PNAS.
Rare variants in axonogenesis genes connect three families with sound–color synesthesia
Synesthesia is a rare nonpathological phenomenon where stimulation of one sense automatically provokes a secondary perception in another. Hypothesized to result from differences in cortical wiring during development, synesthetes show atypical structural and functional neural connectivity, but the underlying molecular mechanisms are unknown. The trait also appears to be more common among people with autism spectrum disorder and savant abilities. Previous linkage studies searching for shared loci of large effect size across multiple families have had limited success. To address the critical lack of candidate genes, we applied whole-exome sequencing to three families with sound–color (auditory–visual) synesthesia affecting multiple relatives across three or more generations. We identified rare genetic variants that fully cosegregate with synesthesia in each family, uncovering 37 genes of interest. Consistent with reports indicating genetic heterogeneity, no variants were shared across families. Gene ontology analyses highlighted six genes—COL4A1, ITGA2, MYO10, ROBO3, SLC9A6, and SLIT2—associated with axonogenesis and expressed during early childhood when synesthetic associations are formed. These results are consistent with neuroimaging-based hypotheses about the role of hyperconnectivity in the etiology of synesthesia and offer a potential entry point into the neurobiology that organizes our sensory experiences.