![]() The families were identified from the Cambridge Synesthesia Research Group database and were originally included in a 2009 study using microsatellite markers to look for evidence of genetic linkage across 43 families of varying sizes with forms of auditory–visual synesthesia ( 20). All cases in this study had sound–color synesthesia as confirmed using the established Test of Genuineness ( SI Materials and Methods), although it is possible that some also experience additional, more common types of synesthesia (e.g., colored weekdays). For each family, we obtained exome sequences from four or five verified synesthetes across three generations plus at least one nonsynesthetic family member. We used WES to identify genetic variants within coding parts of the genome for three multigenerational families with sound–color synesthesia ( Fig. An alternative explanation emphasizes how reduced inhibition of specialized cortical regions by control areas could also permit sensory spillover ( 17). It has been argued that synesthesia may result from a breakdown in the neural “modularity” that each sense typically develops ( 16). Importantly, synesthesia mainly occurs in individuals who are otherwise neurotypical. Longitudinal studies support a developmental basis for synesthesia, as the number and strength of these sensory links grows during early childhood ( 15). These results contributed to a major hypothesis in synesthesia research: That such stable, cross-modal sensory experiences arise from alterations to the neural connections between brain regions that process the entwined sensory signals ( 13, 14). Investigations of the neural correlates of synesthetic experiences revealed increased structural and functional connectivity in people with synesthesia compared with nonsynesthetes and differences in functional connectivity when exposed to triggering stimuli ( 9– 12). 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. 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. Consistent with reports indicating genetic heterogeneity, no variants were shared across families. ![]() ![]() We identified rare genetic variants that fully cosegregate with synesthesia in each family, uncovering 37 genes of interest. 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. Previous linkage studies searching for shared loci of large effect size across multiple families have had limited success. ![]() The trait also appears to be more common among people with autism spectrum disorder and savant abilities. 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. Synesthesia is a rare nonpathological phenomenon where stimulation of one sense automatically provokes a secondary perception in another.
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