A genetic predisposition to dyslexia may be associated with specific brain structures that could also predispose to several other conditions, according to a major study involving data from the UK Biobank and 23andMe.
The findings implicate brain structures involved in motor, language-related and visual functions in dyslexia, a condition that is partially inherited and affects reading ability.
Predisposing genetic variants had different patterns across brain features with respect to volume and white matter fiber density, and the findings align with the idea that dyslexia is a complex and diverse trait.
“Several of the implicated brain structures were also associated with genetic dispositions to other traits including educational attainment, fluid intelligence, ADHD, and reading-, and language-related performance measures across the population,” the authors reported in the journal Science Advances.
This is consistent with their previously identified links with dyslexia and indicate that some genetically predisposing factors may be shared across these traits.
Sourena Soheili-Nezhad, PhD, from the Max Planck Institute for Psycholinguistics in Nijmegen, The Netherlands, and co-authors noted that nerve fiber density in a deep brain structure called the internal capsule was associated with genetic dispositions to all these traits, including dyslexia.
By contrast, the primary motor cortex volume was only associated with a genetic predisposition to dyslexia. Reduced volume in this region, combined with other brain features, may therefore be relatively specific to dyslexia.
The researchers calculated dyslexia polygenic scores (PGS) for eight traits that were genetically correlated with dyslexia, together with phenotypic information. They then compared this with imaging data of up to 35,231 adults stored on the biobank.
Using past research into genome-wide association studies by 23andMe, the team corelated brain volume differences with 35 dyslexia-associated genetic variants.
Reduced volume in the internal capsule correlated with both with a genetic predisposition to dyslexia, ADHD and lower reading- and language-linked performances.
Only a genetic predisposition to dyslexia was, however, associated with the reduction in the volume of a large continuous brain region along the medial wall, spanning parietal and frontal cortices and peaking within the primary motor cortex.
A combination of reduced primary motor cortex volume and alterations in other regions might therefore particularly dispose individuals to dyslexia.
The team also found that higher dyslexia PGS was associated with lower regional volume in the left temporoparietal junction and left anterior insula part of the brain. This was consistent with the potential involvement of left-lateralized language-related brain regions identified in previous dyslexia studies.
The left temporoparietal junction is involved in the processing of syntactic and semantic domains of language and coding and retrieving speech sounds, the authors noted.
The PGSs for nonword reading and school grades were also associated with temporoparietal junction volume, providing further support for this brain region in phonological decoding ability and educational outcomes.
The authors note that dyslexia emerges through a complex interplay between genes, environmental exposures, and neural adaptations during reading acquisition and is associated with educational and socioeconomic outcomes.
“Some of the structural brain correlates of polygenic disposition in the adult population may therefore be linked with the development of dyslexia as potential causal factors, while others might be consequences of lifestyle differences, for example, time spent reading professionally or personally,” they speculated.
