Synesthesia genes
The DNA of people who "see" sounds carries the same rare genes
The staff of the Institute of Psycholinguistics of the Max Planck Society (Germany), led by the famous British psychologist Simon Baron-Cohen, studied the pedigrees of three families, many of whose members have synesthesia, and analyzed their genomes. In the course of this study, it turned out that all these people carry in their DNA rare variants of genes that control the growth of axons – long processes through which nerve cells transmit electrical signals.
Synesthesia is a condition in which sensations of one modality (for example, auditory) simultaneously involuntarily arise sensations of a different nature (for example, visual). In other words, a person with synesthesia can "see" sounds, or perceive numbers as objects of specific colors, or feel specific tastes after hearing certain words. As a rule, synesthesia does not interfere with normal functioning and therefore is not considered a disease.
The causes of this condition are not exactly clear. It is assumed that the connections between the areas of the cerebral cortex that process information from various sensory organs are normally isolated, in the case of synesthesia they communicate with each other, so the sensations of different modalities are strongly associated with each other. At the same time, almost nothing is known about the molecular and genetic mechanisms of synesthesia.
German psychologists were able to partially fill this major gap in knowledge by studying the manifestation of visual-auditory synesthesia in representatives of three unrelated families. In all of them, one parent (a woman) had visual-auditory synesthesia, that is, involuntarily imagined certain colors while hearing specific sounds. Synesthetes of both sexes were present among the children, grandchildren and great-grandchildren of these women. In 18 out of 25 representatives of these families, the sequences of nucleotides of the genome regions encoding proteins were read (not all DNA is involved in protein synthesis). Then the genes of synesthetes from these families were compared with the genes of their relatives who "do not see" sounds, and with the DNA of people not associated with this study – that is, with control samples.
The comparison showed that those with visual-auditory synesthesia, regardless of their degree of kinship, have the same rare variants of six genes – COL4A1, ITGA2, MYO10, ROBO3, SLC9A6 and SLIT2. It is known that all of them affect the growth of axons – long processes of nerve cells that transmit electrical signals to other neurons, as well as muscles and other organs. These genes are most active in early childhood – precisely when the first unusual associations between sensations of different modalities are formed and manifest.
Thus, it can be assumed that the development of visual-auditory synesthesia is at least partially due to the unusual structure of the genes found. Variations in the sequences of their nucleotides lead to the fact that the amino acid composition of the proteins encoded by these genes differs from the usual one. As a result, the properties of the molecules of these proteins change, and the direction they point to growing axons. Because of this, the ways of transmitting information from the senses, which are independent of each other in ordinary people, are connected, thanks to which synesthetes "see" sounds as colors.
The study is published in the journal Proceedings of the National Academy of Sciences (Tilot et al., Rare variants in axonogenesis genes connect three families with sound–color synesthesia).
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