In a large-scale study, a team of scientists led by the Max Planck Institute for Psycholinguistics in Nijmegen (Netherlands) discovered that genetic variants increasing the likelihood of dyslexia were associated with differences in brain areas involved in motor coordination, vision, and language, as published in 'Science Advances'.
Approximately 5% of school-age children experience severe difficulties in learning to read and/or spell, a condition known as dyslexia. "Dyslexia is partially influenced by genes and is highly hereditary," emphasizes lead author Sourena Soheili-Nezhad.
"However, dyslexia is a complex trait that cannot be explained by changes in a single brain region or a single gene. Studying exactly which genes affect which brain networks can help understand how cognitive functions develop differently in this learning difficulty," she states.
To investigate how genetic contributions to dyslexia relate to brain structure, Soheili-Nezhad and her team conducted a large-scale genetic study. Researchers utilized data from over one million individuals collected by the company 23andMe, revealing many genetic variants that increase the likelihood of an individual having dyslexia.
From a large database (the UK Biobank), the researchers calculated "polygenic scores" for dyslexia for over 30,000 adults and correlated them with brain scans. Although there was no information on which individuals had dyslexia in the UK Biobank database, genetic predisposition to dyslexia varied among adults and could be linked to specific parts of the brain.
A higher genetic probability of dyslexia was associated with a smaller volume in brain areas involved in movement coordination and speech sound processing. Conversely, genetic variants related to dyslexia were associated with a larger volume in the visual cortex.
The researchers also observed differences in a white matter tract deep in the brain, known as the internal capsule. In this area, white matter density was associated with genetic influences not only in dyslexia but also in educational achievement, fluid intelligence, and attention-deficit/hyperactivity disorder (ADHD), traits correlated with dyslexia.
"These results are consistent with the idea that dyslexia is a complex trait that may involve a combination of altered cognitive processes," explains Clyde Francks, the study's senior author. "While our study used data from adults, some of the brain changes are likely related to altered brain development during early life stages, for example, in the fetus or during childhood, which then remain stable throughout life," he asserts.
"Other changes might reflect the brain's responses to decades of altered behavior in individuals with a higher genetic predisposition to dyslexia. For example, years of avoiding reading in personal and professional life could affect the brain's visual system," he adds.
In future studies, the researchers plan to use data from children or adolescents instead of adults to closely examine which brain changes are involved in causing dyslexia, rather than being later consequences of having the trait. "Understanding the brain basis of dyslexia could also help achieve earlier diagnosis and educational intervention in the future, with more specific strategies tailored to each child's profiles," concludes Soheili-Nezhad.